CA2062750A1 - Electroluminescent phosphors with long life and method for making the same - Google Patents
Electroluminescent phosphors with long life and method for making the sameInfo
- Publication number
- CA2062750A1 CA2062750A1 CA 2062750 CA2062750A CA2062750A1 CA 2062750 A1 CA2062750 A1 CA 2062750A1 CA 2062750 CA2062750 CA 2062750 CA 2062750 A CA2062750 A CA 2062750A CA 2062750 A1 CA2062750 A1 CA 2062750A1
- Authority
- CA
- Canada
- Prior art keywords
- phosphor
- firing
- brightness
- stressing
- electroluminescent
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Classifications
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B33/00—Electroluminescent light sources
- H05B33/12—Light sources with substantially two-dimensional radiating surfaces
- H05B33/14—Light sources with substantially two-dimensional radiating surfaces characterised by the chemical or physical composition or the arrangement of the electroluminescent material, or by the simultaneous addition of the electroluminescent material in or onto the light source
-
- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
- C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
- C09K11/08—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials
- C09K11/58—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing copper, silver or gold
- C09K11/582—Chalcogenides
- C09K11/584—Chalcogenides with zinc or cadmium
-
- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
- C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
- C09K11/08—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials
- C09K11/88—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing selenium, tellurium or unspecified chalcogen elements
- C09K11/881—Chalcogenides
- C09K11/883—Chalcogenides with zinc or cadmium
Abstract
A mixture of predominantly zinc sulfide and minor amounts of a copper activator compound and fluxing agents is first-fired at about 1200 ·C to form particles of a hexagonal crystalline structure.
The first-fired mixture is mortared or otherwise mechanically stressed for variable periods of time beyond its maximum brightness stress period. This material is fired a second time at about 750 ·C and cooled to produce the phosphor powder. A number of electroluminescent test lamps (Fig. 1) were made incorporating the various experimental phosphors (30) and each of the different mortaring times. For each phosphor made, the brightness rose, peaked and decreased as mortaring time increased (35), but the life characteristic remained a positive function of mortaring time for a substantially longer time than did the brightness (36). An excellent combination of brightness and life is obtained by mortaring beyond the time required for maximum brightness.
The first-fired mixture is mortared or otherwise mechanically stressed for variable periods of time beyond its maximum brightness stress period. This material is fired a second time at about 750 ·C and cooled to produce the phosphor powder. A number of electroluminescent test lamps (Fig. 1) were made incorporating the various experimental phosphors (30) and each of the different mortaring times. For each phosphor made, the brightness rose, peaked and decreased as mortaring time increased (35), but the life characteristic remained a positive function of mortaring time for a substantially longer time than did the brightness (36). An excellent combination of brightness and life is obtained by mortaring beyond the time required for maximum brightness.
Description
WO~t/1~7~2 PCTtVS91/02~0 20~27~a ELECTROLUMINESCE~T PHOSPHORS WITX LONG LIFE
AND METHOD FOR ~AKING THE SAME
.;'.", FIELD OF THE INVENTION
This invention relates to electroluminescent phcsphors and a method for making such phosphors. In particular, this invention relates to long life phosphors, preferably zinc sul~ide based phosphors, obtained by a specially controlled ~echanical deformation of an initially fired crystalline zinc sulfid~ phosphor precursor material. While the phosphors of the present invention may be controlled to emit light in a broad spectrum of colors, the present .~;
invention is particularly suited for the manu~acture of phosphors e~ittin~ in the blue, blue-green, green a~d yellow-o.range portions o~ the spect~lm.
This invention also relates to electroluminescent .:~
lamps and a method for making such lamps. In ~
particular, the present invention relates to ;; :
electroluminescent lamps having unexpectedly long life -~
and retained high brightness.
, ., . ~,, . :: . :-BACKGROUND OF TH INVENTION ~ :
;,;, Electrolu~inescent (EL) phosphors, particularly .- ;.~.,.
,~
W~9l/16722 P~T/US91/02640 2~5?~7~ ~ ' ~inc sulfide phosphors, and method fox their manufacture are well-known. While such phosphors can be controlled to emit light in a broad spectrum, especially common phosphors emit light in the green and blue portions of the spectrum.
Blue EL phosphors arP Xnown for their poor life.
The half life (time at which the brightness o~ the phosphor is one-half the initial brightness~ for ~any blue EL phosphors is in the range of from about 100 to 300 hours when the phosphors are excited at 115 volts and 400 Hz. The half lives are even shorter when these phosphors are used at higher voltages and frequencies.
Green EL phosphors have longer half lives than blue EL phosphors, typically in the range of about lO-1000 hours at 115 volts, 400 Hz; but they also exhibit shortened half life when sub~ectPd to higher voltages and/or frequencies. the foregoing data is based upon typical EL lamp constructions wherein the blue phos?hor typically emits light in the 15 ft-~ range and the -green EL phosphor typically emits in the 25 ft-L range.
Blue EL phosphors are normally prepared by firing zinc sul~ide, a cupper activatar, and one or more halide ~luxes at high temperatures. Phosphors such as ZnS:Cu:X (X=Cl or Br) containing mo-e than about 0.05 weight % Cu are typically blue-green rather ~han blue.
It is known that ~s the copper content increases, there is a gradual shift in ~he light emitted from blue to .
green. Similarly, a yellow-orange shift in emitted -light can be achieved by adding manganese to conventional ZnS:Cu phosphor materials. West German ¦
Patent No. 3,011,815 (Fischer) claims a long life blue - :: . - . , ,. . . .. : .~- : ,: : .. .
WO~ 722 P~T/U~91/02~
20~27~0 phosphor consisting o~ zinc-cadmium sulfide containing magnesium sulfide "to give a blue shift," however no absolute values for "long li~e" are provided in the I ~-patent to veri~y this claim.
For many years, various techniques have been proposed for improving the brightness and half life of zinc sulfide phosphors activated with copper. O.ten, these techniques have involved the addition of one or more coactivators such as halide (e.g. Cl, Br, I) or trivalent ion (aluminum, gallium, indium, etc.) compounds or complexes. See, for example, U.S. Patent Nos. 3,082,344 (Thornton), 3,152,995 (Strock) and Peters, et al. (J.-~D1. Phys., 34:2210-2215, tl963~) which describe various method for manufacturing such phosphors with the objective of improving their brightness.
Zinc sulfide electroluminescent phosphors of the prior art have typically been prepared via the s,eps -of: (a) preparing a mixture of predominantly zinc sulfide, about one weight percent of an electroluminescent activator (e.g., a copper compound such as CuSO4) and a few weight percent of a halide coactivator fluxing compound; (b) ~iring this mix~ure, I
e.g., at from about 1000C to 1300C; (c) a~lowin~ ! :
this ~irst-fired material to return to room tempera~ure , !
and washing with water: (d) mechanically stressing ~he first-fired material te.g., by milling); (e) ~iring (baking) the de~ormed powder at an elevated , temperature, e.g., from about 600C to 950C; and ~
tf) bringing this second-fired`powder material back to room temperature (e.g., either by removing it from the over and allowing it to cool slowly, or by cooling ."',:
;,~',.' , .. ... , . , .. . .. . . , ,, . ,. , . .- - - -,. :.. .~. ~ : ~ . . - . . -; . . : - - . . - : . - -.. - ; - - .. - .. . . - .
WO91/16722 PCT/US91/~2~0 . ..~, ~Q~ r,ia slowly and then quenching).
In the process described above, the first firing causes a tran~formation of the initially predominantly cubic structure of the phosphor precursor material to a predominantly hexagonal conformation. Upon the subsequent cooling, therè is some reversion of the hexagonal conformation to the cubic conformation;
however, such reversion is minimal. The mechanical stressing step further reverts more phosphor to the cubic conformation and introduces "faults" within the phosphor particle. The second firing and cooling cycle results in the further transformation of the phosphor particles back to a predominantly cubic structure.
It is generally accepted in the phosphor art that the reversion of the phosphor back to a predominantly CUhiC structure or conformation is necessary to attain brightness and life. In following therewith, it is recognized that the parameters of a mechanical stressing step are critical for promoting or attaining a sufficient level of reversion to the cu~ic struct~re to obtain a commercially useful phosphor. Such deformation is typically accomplished by milling or mulling the first-fired mat~rial for a standard period of time, i.e., that which by prior experiments and/or experience has affor~ed the manufacturer with maximum phosphor brightness.
.. ..
While the art teaches that various stressing steps may be employed for fracturing the first-fired zinc sulfide crystals, none of these processes have resulted in zinc sulfide phosphors having both the high initial brightness and the long half life with the retained :., .'' ' , ' . - -W0~1/167~2 PCr/US91/02~0 _ 5 _ high brightness of the phosphors of the present invention.
SUMMARY OF T~E INVENTION
As described above, the present invention relates to electroluminescent phosphors and to a method for making such phosphors. In particular, the present invention relates to phosphors having long life obtained by a specially controlled mechanical deformation or an initially fired crystalline zinc sulfide phosphor precursor material.
It is known in the phosphor art that duration is a key parameter of the mechanical stressing step in order to attain maximum brightness. Generally, it has been ;~
shown that the initial brightness of a phosphor product increases during the early periods of mechanical i~
stressing to a maximum brightness level and thereafter decreases with continued stressing. See Peters, et al., supra. It has now been unexpectedly discovered that the charac~eristic time to half brightness of the electroluminescent phosphor product (i.e., the hal~
life) is a positive function of mechanical stressing time, not only up to the time that brightness peaks, but beyond that time as well. This phenomenon has been ~ound to exist for all variations of parameters employed in the processes used ~or making electroluminescent phosphors thus far expiored.
..~ ~, . . . . .
Furthermore,-it has been discovered that the rate at which ~he phosphor brightness tends to decrease with continued stressing after peak brightnes~ has b-en ''`'' ','. ' ' ''~ ~"' W091~167~2 PCT/US91/01~0 ~a ~ t~ - 6 -achleved is substantially slower than the rate at which brightness increases with stressing before peak ~rightness ls reached. Thus, the present invention is based upon the discovery that extreme mechanical stressing of a first-fired phosphor precursor material, i.e., stressing for a period of time beyond that at which peak brightness is normally realized, yields an electroluminescent phosphor product having unexpectedly superior properties in tarms of both brightness and useful life (i.e., improved half life with little sacrifice in briqhtness).
The critical factor for obtaining the improved properties in electroluminescent phosphors in accordance with the present invention is the performance of the mechanical stressing of the first-fired phosphor precursor material. As noted, it is critical that this mechanical stressing be per~ormed for a period of time which exceeds that at which maximum brightness is attained for the given phosphor material. The specific parameters chosen for the mechanical stressing step will depend upon the starting materials, the type of mechanical stressing imparted upon the phosphor, the efficiency of the apparatus or methodology employed in stressing the phosphor precursor, as well as the brightness and li~e characteristics de~ired of the ~inal phosphor product.
With resp2ct to tha starting materials, it is found that the addition o~ high levels of the copper activator, besides resulting in a more green phosphor, results in phosphors having lonyer ~ife and higher brightness as compared to similar phosphors employing less copper activator. Furthermore, it is also found ~ -that a small quantity of zinc selenide added to the WO91t~7~ PCT/VS~I/O~O
_ 7 _ 20~27~
starting materi~l will also contribute in a bene~icial way to th~ quality o~ the EL phosphor so produced, particularly with respect to the production of hi~h brightness and excellent long life green phosphors.
The process of the present invention thus comprises the following steps:
(a) preparing a mixture of a predominant amount of zinc sulfide and a minor amount of an activator compound, preferably a copper-containing activator compound; ;~
- (b) firing the mixture (first fire) at a temperature sufficient to form particles having a predominantly (>50%) hexagonal crystalline structure;
(c) mechanically stressing the first-~ired mixture, e.g., by mortaring, mulling or milling, for period of time exceeding that at which maximum brightness would be achieved;
(d) firing the mechanically stressed material for a second time (second fire), to complete the reversion of the crystalline structure to a predominantly cubic -conformation; and (e) subsequently cooling the twice-fired material to yield the desired high brightness, long half life electroluminescent phosphor products.
'~ ..
The present invention is generally applicable ~o all elactroluminescent phosphors, regaxdless of color emittance, but is particularly applicable to phosphor materials emitting in the blue, green:and yellow-orange portions of the spectrum. Ultimate coloration of the , finished phosphor is dependent, at least in ~art, upon the starting materials employed. Variations in coloration ~ay be effected by varying the constituents '.
WO91/16722 PCT/US91/02~0 ~ ~ 8 and relati~e amoun~s o~ the constituents in said s~arting materials~ as is well-known by those of ordinary skill in the phosphor arts. For example, Ihe brightness and color may be controlled by shifting the copper and/or zinc selenide content.
In any event, many improvements and attributes are realized from the phosphors produced in accordance with the practice of the present invention as compared with typical, comparable phosphors. For example, phospnors made in accordance with the practice of the present invention which emit in the yreen portion of the sDectrum have halr liv~s in excess of 2000 hours to over lO,000 hours, as compared with its standard counterparts which typically have half lives of only about lO00 hours. Similarly, blue electroluminescent phosphors produced in accordance with the practice of the present invention typically have half lives 3 to 5 times longer than those of traditional blue phosphors.
In addition to the improved phosphors and improv~d process for making phosphor as described abovP, the present invention also relates to electroluminescent lamp elements having superior lifa and brightness characteristics. The improved electroluminescent lamps of the present invention result from tha incorporation of the phosphors produced in accordance with the practice of the present invention into typical electroluminescent lamps. The construction and method of manufacture of.these EL lamps is well-known and well-described in the patent literature.
. :
~0~l~l67~? PCT/US91/02~0 .
_ 9 - 2~27~ :
BRIEF DESCRIPTION OF THE DRAWINGS
.
Figure 1 shows a side sectional view of the i--electroluminescent lamp used for testing phosphors of this invention.
Figures 2 through 6 are plotted curves, corresponding to each of a number of different phosphor compositions which were incorporated into test electroluminescent lamps, showin~ how the characteristic brightness and life of each phosphor product changes as a function of the duration of ~echanical stressing during processing.
Figure 7 shows the brightness and life characteristics versus stressing time for a phosphor composition wherein the stressing is brought about by mulling.
Figure 8 shows the life characteristics of a zinc sulfide:copper phosphor stressed for 20 minutes by --~ortaring, wherein the retained Cu content was sequentially elevate~ from 0.054% through 0.091%, and, for comparative purposes, the life characteristics of a comparable commPrcial green EL phosphor. The ~ -commercial phosphor has a retained Cu content of 0~08%
and showed shorter life than the 0.054~ Cu content phosphor ~rom the present invention~
.
, . _ . .
. .
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The process of the present invention is generally applicable to any electroluminescent phosphor precursor w~ gl/167~2 Pcr/ussl/n26~0 ~aterial or co~position. Such compositions are ~ell-known and readily recognizable by those of ordinary skill in the art.
Typical phosphor precursor materials generally comprise zinc sulfide, alone or in combination with zinc selenide, an activator therefore, and one or more coactivators. In preferred phosphor precursor compositionsr a combination of zinc sulfide-and zinc selenide is employed. This zinc sulfoselenide mixture generally comprises at least 80~, preferably go~, and ~ost preferably, at least 96% by weight zinc sulfide, -~ith the remainder comprising the zinc selenide.
Activator compounds which may be used in the phosphor precursor compositions of the instant invention include, but are not limited to, copper, silver, gold, phosphorus, arsenic, vanadium, antimony, lead, tin, manganese, iron, sodium, lithium, gallium, tellurium, scandium, indium, and rare earth compounds.
The preferred activator comprises a copper compound, alone or in combination with one or more of the other aforementioned ackivator compounds. Preferred copper co~pounds include copper sulfate, copper nitrate, copper chloride, copper bromi~e, copper acetate, copper fluoride, copp~r iodide and copper carbonate: with copper sulfate being most preferred. Gene~ally, the activator will be present in an amount of less than ~%, pre~erably less than or equal to about 1~ by weight, based upon the aforementioned zinc sulfide or zinc sulfoselenide materials.
'' :' ' ' Coactivators that may be employed in the practice of the present invention include compounds or complexes ~ -~
',''" .
':
WO 9 1 / ] 67 ? 2 PCT/US91/0~40 2~27~
o~ halide (Cl, Br, I) and trivalent ions (Al, Ga, In, etc.)~ Prefe~red coactivators include complexes based on halides, most preferably halide fluxes. Suitable coactivators are well-known to those of ordinary skill in the art. Exemplary of suitable coactivators there may be given barium bromide, magnesium bromide, sodium bromide, magnesium chloride, ammonium bromide, sodium chloride, barium chloride, magnesium iodide, ammonium iodide, barium iodide, sodium iodide, and the like.
Generally, the coactivator is pr~sent in iks hydrated form, e.g., magnesium chloride (hydrated) (MgC12.6H2O). The coactivator component may comprise one or more coactivators and is generally e~.ployed in an amount of 20% by weight or less, preferably 10~ by weight or less, and, most preferably, about 8~ by weight or less, based upon the weight of ~-the zinc sulfide or zinc sulfoselenide material.
As noted, additional constituents may be added cr :
incorporated into the phosphor precursor mixture or composition as desired. Such additional constituents include zinc sulfate, zinc oxide, sulfur and the like, and will be readily known to those sXilled in the art.
While the exact reasons for the improved results obtained by Applicant's process are not fully undexstQod, the results are quite clear. The present invention provides a zinc sulfide phosphor which, when emitting in the green portion o~ the spectrum, has a half li~e in excess of 2000 hours and,-when emittins in the blue portion of the spectrum,- has a half life in excess of 1000 hours~ both of which represen~ drama~ic improvements over existing comparable emitting phosphors.
. J~
WO91/167~ PCT/US~1!02~0 1~ _ Briefly, the present invention starts with the aforedescribed phosphor precursor materials or compositions. Preferably, said mixture comprises a mixture of cubic or predominantly cubi^ zinc sulfide and a copper activator,-e.g., copper sulfate, and at least one coacti~ator such as a halide flux (e.g., a chloride or bromide flux). This precursor composition is fired at a temperature and for a period of time sufficient to convert the cubic conformation of the zinc sulfide ~o a predominantly (> 50~) hexagonal conformation. Appropriate temperatures and time for this firing will be readily apparent to those of ordinary sXill in the art. Typically, the first firing will be conducted at a temperature of at least about 1000C, preferably of at least 1100C, most prererably of at least about 1200C. However, the maximum firing temperature will generally not exceed 1400C. The period of time for which the phosphor precurso_ material is subject to said firing will depend upon the precursor materials and the temperature of the firing itself. Generally, the first firing ~ill be for a period of time of from about 1 hours to 6 hours, preferably from about 2 hours to about 4 hours.
It should be noted that all reference within this specification, including the examples, to ~iring time, is to ~e understood as the time of the actual firing of the phosphor precursor or phosphor material ~rom when the temperature of the material achieves the ~iring temperature until the firing is completed. Thus, the actual residence tim in the furnace will be somewhat~;
longer than the firing times so as to allow the phosphor matarials to attain the desired firing temperature. ~ ;
. ~.
...
WO~1~167~2 PCT/VS91/02~0 - 13 _ 2 ~ ~2 ~
~ fter firing, the ~irst-ired materials are allowed to cool to allaw the same to be further processed.
During said cooling cycle, which may be an ambient cooling or a Eorced coo}ing, some of the crystalline zinc sulfide will revert from the hexagonal ,' conformation to the cubic conformation.
The first-fired materials are then milled or '',,'~
mortared in a mortar-grinder, for example in a Model ~o mortar-grinder made by Retsch GmbH of West Germany, f~r a period of time in excess of that at which maximum ~rightness is achieved (Tb). Tb is easily aetermined through simple and undue experimentation for any given phosphor composition and/or is well-known in ~
the ~rt for standard phosphor compositions. The, ,',' purpose of the extended milling process is to produce a highly fractured zinc sulfide crystal.
. !
While it is known that milling will induce faults , or fractures into the crystalline structure of the zinc sulfide particles, because phosphors havP typically been rated on krightness, it was believed to be detrimental to mill for exten~ed periods of time and ~;-thus induce a degree of faulting or fracturing beyond that at which maximum brightness was attained. ,i Unexpectedly, however, it has now been found that continued milling and, thus, the inducement of higher ' ~'' levels or degrees of ~aulking or fracturing in the zinc ', sul~ide crystals results in phosphors having far superior lives with only minimal loss in brightness. , ~, - ~ .
In addition,to produoing a more highly fractured or faulted ~inc sulfide crystal, the milling also causes l the conversion of some of the ~inc sulfide crystals '~ ', :
WOgl~l67~' PCT/~S91/02~0 ~7J~ 14 -from the he~agonal conformation to the cubic conformation and, perhaps more importantly, app~ars to ~acilitate the subsequent and more extensive crystalline phase transition ~rom hexagonal to cubic conformation caused by the second firing step. It has also been found that continued milling may lead to a more uniform and smaller particle size, which from the standpoint of phosphors useful in the manufacture of electroluminescent lamps may be desirable, but is not otherwise believed to be critical.
The specific length of time for which the first-fired materials are mechanically stressed is depe~dent upon the materials themselves, the desired final properties and characteristics, and the mechanical stressing technique or methodology employed. Exemplary of the method by which-stressing can be accomplished include mortaring, mix-mulling, ball milling, fluid-energy milling, jet pulverization, grinding, vibratory milling, cen~rifugal milling, hydrostatic pressure, uniaxial or biaxial pressure and the like.
Furthermore,this invention is also intended to encompa s mechanical stressing by means other than directly applied external forces as exemplified above.
Although not yet attempted, such other methods include subjecting the material to thermal shocks or other shocks that would most c~rtain~y entail a mechanical stressing o~ the .individual particles o~ the in-process phosphor. -.
Concurrent with or subsequent to the mechanicalstressing steps, additional activators, coactivators, or other co-constituents may be added to the in-process phosphor mixture. Such additives or dopants, as they -' .
wn ~I/1fi7~ PCT/US91/02~0 .
1 S ~ L~ ~ ~
are sometimes referred, are as previously defined. In general, it is preferred to add additional activators, preferably copper activakors, alone or in combination with other co-constituents. A preferred combination of dopants to be used at this point include copper sulfate and zinc sulfate. Generally, the amount of activator added to the in-process phosphor will be from about 0.5~ to about 5~, preferably from about 1% to about 3%, by weight based upon the total weight of the in-process phosphor. While lower amounts may be employed, such amounts are less likely to provide as much improvement in brightness and life in the final compositionO
However, said compositions will still have properties superior to con~entional phosphoxs. Similarly, while `
higher amounts of the activator may be employed, it is belie~ed that amounts of the activator may be empioyed, it is believed that amounts in excess of the 5~ by weight are beyond the saturation point for the composition and, thus, generally do not provide any further improvement in brightness and/or life. -With respect to the co-constituent, the co-constituent is preferably employed in an amount of from about 1% to about 20~, preferably from about 1~ to about 16~, by weight base~ upon the total weight of the in-process phosphor. A~ noted previously, the preferred combination of dopants added during or following the ~irst firing comprises copper sulfate and zinc sulfate.
The mixture is then refired at a temperature lower than the initial firing, generally less than about 1050C, preferably less than about 900C. The most preferred second firing temperatures range from about . .
.
. : - : , , . ~ :. :. ~ . : . . - , -: - : . . ~ , . ...... .
W~ 67~2 P~l/US~1/02~0 16 ~
7~0C to about 800c. The length of time for which ~he sacond firing is c~nducted will generally depend upon the ~iring te~perature, the materials themselves and the desired characteristics and properties of the final phosphor materials. Generally, the time of firing is that time which is sufficient to complete reversion of the conformation of the crystalline structure from a hexagonal conformation to a predominantly cubic conformation. This time is generally on the order of one-half hour to 3 hours or more. Preferxed firing tim~s for the second firi~g are -on the order of one-half hour to 2 hours, more specifically 1 hour to about 2 hours.
Following the second firing, the phosphor materials are allowed to cool, either atmospherically or by induced cooling or quenching. Thereafter, the phosphor particles are treated or washed to remove excess activato-, e.g., copper compounds, from the surface of the zinc sulfide crystals. Such washings may be accomplished by any methods known in the art.
Exemplary of such methods include a multi-step wasning -~
employing, in sequence, hydrochloric acid (4N), water and a 4~ sodium cyanide solution. Following washing, the phosphor materials are dried, .
After drying, the phosphor is preferably sieved through a 325 mesh screen so as to remove all axcessively large phosphor particles. Although the -unsieved phosphors do provide the attributes of long life with high brightness, they are less desirable for use in the manufacture of electroluminescent lamps -which have preferred parti~le si~e tolerances for insuring a good phosphor layer in its construction.
wn 91/lfi722 PCT/VS91/02~0 - 17 - 2~ ~2 ~ ~
In addition to the ~oregoing steps, other steps or pr~cesses may be conducted at various times in the process as described above. For example, following ~he initial firing, it may be desirable to wash the in-process phosphor materials with water so as to remove the excess coactivators, including any ions thereof that may have formed, especially any excess halide ions. Such washing may be accomplish~d with ;
repeated treatments with hot, deionized water until the conductivity of the wash water is below certain levels, preferably less than about 50 micromho cm-l Additionally, it may be desirable to sieve the first-fired materials to remove any excessively large~ -particles. Generally, at this stage, it is preferred to employ a lO0 mesh screen for sieving purposes.
. , .
Although not intending to be bound by theory, it is believed that in the practice ~f the instant process the initial high temperature firing, combined with the excessively long period of mechanical stressing, leads to phosphor particles having a higher degree of faults or fractures in the crystalline structure as well as to phosphor compositions having a higher level or percentage of particles having the cubic as opposed to the hexagonal conformation. It is believed that the increased number of fractuxes or faults in the zinc sulfide particle provides additional sites for activator, copper compol~nd, deposition during the second firing.
. . ....- . . : . ~ . ............................ -, The doping of the ~irst-fired material with-additional copper activator tends to increase the likelihood of saturation of the numerous faults or fractures in the crystalline structures. Obviously, -- .. . . ... ..... . .
WO91~167~ PCT/US91/02640 ~ 18 -with very low levels of copper activator in the in-p~ocess mixture, the opportunity or likelihood o~ -the copper compound to infiltrate th~ fracture or fault is less likely than in instances where high levels of copper activator are employed. In general, the amount of initial copper and added copper is maintained sufficiently high during the two firing steps such that the total retained copper content of the final product after washing is at least 0.02~, preferably at least 0.06%, by weight. In general, the total retained copper content will preferably be in the range of 0.0~%
to about 0.25%, m~st preferably about 0.06~ to about 0.2%, by weight of the phosphor mixture. Compositions within the foregoing limitation and manufactured in accordance with practice of the present invention generally provide phosphors having high initial brightness and a long h~lf life.
To determine the effect of mortaring time on phosphor performance, several series of ~-elPctroluminescent phosphors were manufactured and electroluminescent test lamps produced with said phosphors. Each series differs from one another by some variation in the method ~or making electroluminescent phosphor and/or the composltional makeup of the phosphor precursor materials. Within each series, the phosphor and its method of manufacturing were held constant while the phosphor mortaring time was varied. Each phosphor sample was then incorporated into five electroluminescent lamps ~nd the lamps tested.
.. : . . . .
The EL test lamps (illustrated in Figure l~ were-prepared as follow~. A paste was formed by dispersing . .
WO91~1~722 PCT/US91/02~0 .
-- 19 -- ' ~e electroluminescent phosphor powder in a A7elQctric ~
resin. A layer of the phosphor powder paste 30 and a . .-layer of a barium titanate paste 34 are sand~iched between an opaque metallic foil (aluminum) servicing as a rear electrode 20 and a transparen~ ~ron~ electrode (indium tin oxide) 22. A layer 24 of a desiccant is : ::
formed over the front transparent electrodP 22. A pair .
of lead wires 26 and 28 are attached, respectively, to the front and rear electrodes 22 and 20. A polymeric protective film 32 encapsulat~s this assembly, but with :
the lead wires 26 and 28 extending away therefrom, ~
provide physical support and a moisture barrier. The : .
phosphor layer 30 has a thickness of 0.04 millimeters. .
The barium titanate layer 34 is 0.015 millimeters thick.
A more detailed description of the construction of an electroluminescent lamp is provided in U.S. Patent - :
No. 4,104,555 (Fleming), the disclosure of which is hereby incorporated herein by reference.
The general meihod for making the phosphor as àescribed above will be referred to hereinafter as the reference method.
In the ~ollowing examples, the brightness of each test lamp was measured with excitation at 115 volts and 400 H7. The brightness and life date points for each phosphor.produced were determined by averaging the measured brightness and li~e for each of the five test lamps per.group. By convention, the half life o~ an electroluminescent phosphor.(or-lamp-containing the -same). is defined as the time re~uired for the initial brightness (Bo) to decrease to one-half of its ,, ., .. - , . , ,, . ... - - , , . ,, , ; , ~ . . . . ~ . . - .
~V~91/167~ PCT/~S91/02~10 - ~0 -initial ~alue. S.ince this time may be several thousand ~o~rs, an accelerated li~e test is a near necessity.
One such test was devised by British Aerospace and consists o~ operating la~ps at elevated temperature and fre~uency. This method is described in R.H. Marion et al. "Analysis of the Lifetime and Powder Electroluminescent Phosphors" in Shionoya, S. and Kobayashi, H., Ed., Springer-Verlag, P. 332, 1939.
The accompanying figures represent the brightness and life data realized from evaluations of various ~-phosphors made in accordance wîth the practice of the present invention as set forth in the following examples. Figures 2 through 6 are graphic representations of the life and brightness data versus mortaring time for compositions made in accordance with the practice of the present invention. ~igure 7 illustra~es the effect on brightness and life of mulling rather than mortaring. Figure 8 represents data demonstrating the effect on life of various levels of retained copper in the phosphor. Figures 2, 3 and 6 are based on actual life test data. All other figures represent life data which are based on accelerated life test results. Figure 2 sets forth the actual data points arrived at, whereas Figures 3 throu~h 7 present the brightness and life data in a normalized fashion, i.e., the data points representing the greatest level of brightness and life, as measured, are each set to a value of 1.0~ ~ -''' - :
The following examples are-intended to aid in the understanding of the present invention, but are not to be construed as limitations thereof. Unless otherwise : :'.
.
.
W~91~67~ P~T/~ /02~0 - 21 - 2 ~2 ~.
indicated, all a~ounts with respect to the various constituents o~ the phosphor precursor materials and the in-process additives are in parts by weight~ All temperatures are expressed in degrees Celsius.
:
~ series of phosphor compositions emitting in the green portion of the visible light spectrum were made according to the following procedure:
, 98 parts zinc sulfide, 2 parts zinc selenide, 0.3 parts copper sulfate (anhydrous~, 3 parts barium bromide (hydrated), 3 parts magnesium bromide (hydrated) and 2 parts sodium bromide were dry blended together in a V-blender for at least one-half hour. The blended materials were then placed in a quartz crucible, covered and fired at 1200C for 3 hours. After 3 hours firing, the crucible was brought out of the furnace and cooled rapidly with cold air blowing on the bottom of the cruciDle. Once the phosphor was cooled, it was washed free of ions, especially bromide ions, with hot deionized water at least s.ix times, or until the conductivity of the wash water was less than 50 micromho cm l. The phosphor was then filtered, dried and sieved through a lOO mesh scre~n. This step of the process yields a material that is referred to as the first-fired material.
.. . .
, . . ... . . .
Individual 200 gram aliquots~ of the first- ~-fired material were then mortared for various -lengths of time to demonstrate the effect thereof.
.. . . . . . . ......... . . . . . ........ ... . . . . .. .. ..
.. . . . . . ,, : . ., ; : . , : - ~ - . ..
- . . ,~. ~ : - ~: ., . . :
. - - . . , . ~ . . . .. .. . .. .. . . .
WOg~/~672~ PCr/US91!02~0 In this example, mortaring times of 0, 5, 10, 20, 30 and 40 minutes were employedO
Each ~ortared sample was then doped with 5 grams of copper sulfat2 (CuSO4) and 32 grams of zinc sulfate (ZnSO4), blended and fired at 800C for 1 hour in a covered quartz crucible.
Following this second firing, the phosphor was then cooled and sequenti~lly washed with hydrochloric acid (4N), water, and finally a 4% NaCN solution to remoYe any excess copper found on the surface o~
the phosphor particles. The phosphor was then dried, sieved through a 400 mesh screen and processed into electroluminescent lamps.
The data representing the brightness levels and half lives attained for the various samples made in accordance with this example are depicted in Figure 2 above. As shown, brightness (curve 35) peaked at a mortaring time of about 5 minutest whereas the half life (curYe 36) continues to increase even up to 30 ~ ;
minutes. Furthermore, although brightness diminishes with continued mortaring beyond the time at which peak brightness is attained, its rate of diminution is much slower as compared to its rate of enhancement in the early stages of milling. Based on ~his data, it is clearly advant~geous to mortar the phosphor beyond the 5 minutes at which peak brightness is obtained, thereby obtaining a signi~icant increase in life at the expense of a minimal loss in brightness. These same plots are depicted in a normalized fashion in Figure 3 as per ~~~ curves 41 and 42, respectively.
~ ' ~ ''~ ':' ' :
. .,, . 1- .
- : - - .. ---.-: . , , . . .. ; , . . .-- , . . . .
W~9~/167~2 PCTtUS~I/02~0 . .
- 23 - 2052~
EX~MPLE 2 A second phosphor composition was made in accordance with the procedure of Example 1 with the exception that the second firing was at 750c instead of 800C. This phosphor composition was slightly different from that set forth in -Example 1 in that zinc selenide was eliminated from the formulation and that magnesium chloride (hydrated) was substituted on a weight for weight basis for the magnesium bromide of Example 1. The brightness and life data for this sample of phosphor is depicted in a normalized fasAion in curves 45 and 46, respectively, of Figure 3. .
A phosphor composition was made in accordance with the procedure of Example 1 with the exception that the amount of copper su}fate starting material was increased from 0.30 parts by weight to 0.40 parts by weight. Brightness and life data, as a function of mortaring time, for this phosphor composition is presented in Figure 4 by curves 51 and 52, respectively. ~ :
:' ~XAMP~
., A phosphor composition emittlng in the blue portion - of the spectrum was made in accordance with the procedure of Example 1 except that the level of ccppër sulfate was reduced-from 0.30 parts by weight-to Q;15 parts by weight, 1 part by weight of ammonium bromide ,.. . . . .. . .... . . . .
~O~1/1672 PCT/US~/0~0 ~ Q - 24 -was substituted for the 1 ~luxes (barium bromide (hydrated), ~agnesium bromide (hydrated) and sodium bromide)), and 1 part by wei~ht of sulfur was added as a co-constituent. The brightness and life data for the phosphor so made is depicted in Figure 4 by curves 55 and 56, respectively.
EXAMPT ~ 5 An additional series or phosphors was made in accordance with the procedure of Example 1 except that the starting materials were blended through a wet mixing effect2d by the addition of water, which was subsequently removed by heating and drying, and the second firing was conducted at a temperature of 750C. Additionally, the composition of Example 1 was modified in that the magnesium chloride (hydrated) was substituted on a weight for weight basis for the magnesium bromide. The data points detDrmined for the brightness and life of the phosphor so produced are depicted by curves 61 and 62 7 respectively, in Figure 5.
~!
. .
A series a phosphor compositions were manu~actured in accordance with the general procedure ~f Example 1 with the exception that the firing time-was varied as set ~orth below and that a magnesium chloride (hydrated) flux was substituted for the magnesium bromide (hydrated) flux on a weight for weight basisO
The first series of phosphors as represented by curves I ' ..
~... i . -~V~ 91/16722 PCr/VS9~/02640 2~6~7~
71 and 72 (~igure 6), for brightness and life, xespectively, were ~ired for 2 hours in both the first and second firings. On the other hand, curves 75 and 7~ correspond to the brightness and life behavior, respectively, of similar series of phosphors with the exception that the first firing was co~ducted for a period of 2 hours and the second firing for a period of 4 hours. Once again, as shown in ~igure 6, the advantages of mortaring beyond the time at which brightness becomes greatest is clearly evident for all o~ these changes in the method or making the phosphor.
':
A series of phosphor compositions as described in Example 1 were prepared in accordance with the method -:;
of Example 1 with the exception that the starting materials, following the first firing, were mulled, rather than mortared, and the second firing was conducted at 750C. The mulling machine employed was a Simpson Mix-Muller, Model LF, Style UD, made by the :;
National Engineering Company o~ Chicago, Illinois.
This device consists of two heavy crusher wheels mounted at the ends of a horizontal axle that rotates in a horizontal plane about the axle center. Sweepers direct the material being mulled under the wheels.
As shown in Figure 7, the phosphors produced in accordance with the above teachiny mani~ested a maximum brightness after 30 minutes of mulling (curve 813.
Continued mulling of the phosphor composition beyond the 30 minute peak period resulted in the development o~ phosphors having a longer life perfo~mance with a ~ ~.
~ ' ' ' ' '' ' '' ' ' .. ' '~ ' ... . . . .
WO~1/167~? PCT/US9l/02~0 ~ 26 -minor loss in brightness (curve 82). This example demonstrates the fact that alternative methods for mechanically stressing the phosphors may be emploved in the practice of the present invention. It is recognized that certain mechanical stressing methodologies may require longer periods of stressing ln order to achieve the same result. Nevertheless, the practice of the present invention still provides improved phosphor compositions and, subsequently, electroluminescent lamps having enhanced brightness and longevity.
A series of phosphor compositions was prepared in ;;,;
accordance with the methodology of Example 1 and of the formulation of Example 1 with the exception that the amount of copper added initially was varied so as to provide phosphor compositions having a formulated initial copper content of 0.06%, 0.08%, 3.10% and 0.12%
by weight, respectively. The actual lamp brightness and half life data, as function of copper content and mortaring time, is depicted in Table I.
;:
-: ~ ' ' . - ~ ' ' , .
- . . -. : .. , . . - -~ - . -.:. , .
W~91~167~' Pcr/ uss ~ !02b40 - 27 - 20~27~3 ~ABLE I
Relationship of Brightness and Life to Initial CooDer Content and Mortarinq_Tlme Mortaring Initial Half Life CuTime fMin.)Brightness (Ft-LltHours) : :
~ ' -.
0.06 0 18.9 150 23.2 1900 0.08 0 9.6 200 :
20.1 4gO0 0.10 0 9.4 250 24.6 6150 - 0.12 0 9.5 . 500 27.5 3200 25.3 5300 22.5 5650 : .
19.3 7700 17~5 7600 .:
! :`~
I
, i~
WO 91/1~727 PCI/US91/0264~) 2 ~ -The mortared samples always showed higher initial brightness and better half life than the unmortared samples. The hal~ life also improved with increasing copper content. The 30 minute mortared sample, for the 0.12% Cu, showed a slight decrease in brightness, but continued to show very good half life.
The relationship o~ brightness and life to copper i' content is shown in Table II and depicted in Pigure 8.
Also shown in Figure 8 is the plot of a typical commercial green phosphor which has a retained copper content of 0.08%. After 600 hours operation, the :
brightness of the EL lamp was only 66~ of its initial .
brightness level.
`.
.
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WO91/1~72~ PCr/US~ 2~10 The present invention has been described in detall, including the preferred embodiments thereof. However, it will be appreciated that those skilled in the 2-~, upon consideration of the present disclosure, may make ~:
modiflcations and/or improvements of this invention and still be within the scope and spirit of this invention :
as set forth in the following claims.
'`'.''"~'" ' -
AND METHOD FOR ~AKING THE SAME
.;'.", FIELD OF THE INVENTION
This invention relates to electroluminescent phcsphors and a method for making such phosphors. In particular, this invention relates to long life phosphors, preferably zinc sul~ide based phosphors, obtained by a specially controlled ~echanical deformation of an initially fired crystalline zinc sulfid~ phosphor precursor material. While the phosphors of the present invention may be controlled to emit light in a broad spectrum of colors, the present .~;
invention is particularly suited for the manu~acture of phosphors e~ittin~ in the blue, blue-green, green a~d yellow-o.range portions o~ the spect~lm.
This invention also relates to electroluminescent .:~
lamps and a method for making such lamps. In ~
particular, the present invention relates to ;; :
electroluminescent lamps having unexpectedly long life -~
and retained high brightness.
, ., . ~,, . :: . :-BACKGROUND OF TH INVENTION ~ :
;,;, Electrolu~inescent (EL) phosphors, particularly .- ;.~.,.
,~
W~9l/16722 P~T/US91/02640 2~5?~7~ ~ ' ~inc sulfide phosphors, and method fox their manufacture are well-known. While such phosphors can be controlled to emit light in a broad spectrum, especially common phosphors emit light in the green and blue portions of the spectrum.
Blue EL phosphors arP Xnown for their poor life.
The half life (time at which the brightness o~ the phosphor is one-half the initial brightness~ for ~any blue EL phosphors is in the range of from about 100 to 300 hours when the phosphors are excited at 115 volts and 400 Hz. The half lives are even shorter when these phosphors are used at higher voltages and frequencies.
Green EL phosphors have longer half lives than blue EL phosphors, typically in the range of about lO-1000 hours at 115 volts, 400 Hz; but they also exhibit shortened half life when sub~ectPd to higher voltages and/or frequencies. the foregoing data is based upon typical EL lamp constructions wherein the blue phos?hor typically emits light in the 15 ft-~ range and the -green EL phosphor typically emits in the 25 ft-L range.
Blue EL phosphors are normally prepared by firing zinc sul~ide, a cupper activatar, and one or more halide ~luxes at high temperatures. Phosphors such as ZnS:Cu:X (X=Cl or Br) containing mo-e than about 0.05 weight % Cu are typically blue-green rather ~han blue.
It is known that ~s the copper content increases, there is a gradual shift in ~he light emitted from blue to .
green. Similarly, a yellow-orange shift in emitted -light can be achieved by adding manganese to conventional ZnS:Cu phosphor materials. West German ¦
Patent No. 3,011,815 (Fischer) claims a long life blue - :: . - . , ,. . . .. : .~- : ,: : .. .
WO~ 722 P~T/U~91/02~
20~27~0 phosphor consisting o~ zinc-cadmium sulfide containing magnesium sulfide "to give a blue shift," however no absolute values for "long li~e" are provided in the I ~-patent to veri~y this claim.
For many years, various techniques have been proposed for improving the brightness and half life of zinc sulfide phosphors activated with copper. O.ten, these techniques have involved the addition of one or more coactivators such as halide (e.g. Cl, Br, I) or trivalent ion (aluminum, gallium, indium, etc.) compounds or complexes. See, for example, U.S. Patent Nos. 3,082,344 (Thornton), 3,152,995 (Strock) and Peters, et al. (J.-~D1. Phys., 34:2210-2215, tl963~) which describe various method for manufacturing such phosphors with the objective of improving their brightness.
Zinc sulfide electroluminescent phosphors of the prior art have typically been prepared via the s,eps -of: (a) preparing a mixture of predominantly zinc sulfide, about one weight percent of an electroluminescent activator (e.g., a copper compound such as CuSO4) and a few weight percent of a halide coactivator fluxing compound; (b) ~iring this mix~ure, I
e.g., at from about 1000C to 1300C; (c) a~lowin~ ! :
this ~irst-fired material to return to room tempera~ure , !
and washing with water: (d) mechanically stressing ~he first-fired material te.g., by milling); (e) ~iring (baking) the de~ormed powder at an elevated , temperature, e.g., from about 600C to 950C; and ~
tf) bringing this second-fired`powder material back to room temperature (e.g., either by removing it from the over and allowing it to cool slowly, or by cooling ."',:
;,~',.' , .. ... , . , .. . .. . . , ,, . ,. , . .- - - -,. :.. .~. ~ : ~ . . - . . -; . . : - - . . - : . - -.. - ; - - .. - .. . . - .
WO91/16722 PCT/US91/~2~0 . ..~, ~Q~ r,ia slowly and then quenching).
In the process described above, the first firing causes a tran~formation of the initially predominantly cubic structure of the phosphor precursor material to a predominantly hexagonal conformation. Upon the subsequent cooling, therè is some reversion of the hexagonal conformation to the cubic conformation;
however, such reversion is minimal. The mechanical stressing step further reverts more phosphor to the cubic conformation and introduces "faults" within the phosphor particle. The second firing and cooling cycle results in the further transformation of the phosphor particles back to a predominantly cubic structure.
It is generally accepted in the phosphor art that the reversion of the phosphor back to a predominantly CUhiC structure or conformation is necessary to attain brightness and life. In following therewith, it is recognized that the parameters of a mechanical stressing step are critical for promoting or attaining a sufficient level of reversion to the cu~ic struct~re to obtain a commercially useful phosphor. Such deformation is typically accomplished by milling or mulling the first-fired mat~rial for a standard period of time, i.e., that which by prior experiments and/or experience has affor~ed the manufacturer with maximum phosphor brightness.
.. ..
While the art teaches that various stressing steps may be employed for fracturing the first-fired zinc sulfide crystals, none of these processes have resulted in zinc sulfide phosphors having both the high initial brightness and the long half life with the retained :., .'' ' , ' . - -W0~1/167~2 PCr/US91/02~0 _ 5 _ high brightness of the phosphors of the present invention.
SUMMARY OF T~E INVENTION
As described above, the present invention relates to electroluminescent phosphors and to a method for making such phosphors. In particular, the present invention relates to phosphors having long life obtained by a specially controlled mechanical deformation or an initially fired crystalline zinc sulfide phosphor precursor material.
It is known in the phosphor art that duration is a key parameter of the mechanical stressing step in order to attain maximum brightness. Generally, it has been ;~
shown that the initial brightness of a phosphor product increases during the early periods of mechanical i~
stressing to a maximum brightness level and thereafter decreases with continued stressing. See Peters, et al., supra. It has now been unexpectedly discovered that the charac~eristic time to half brightness of the electroluminescent phosphor product (i.e., the hal~
life) is a positive function of mechanical stressing time, not only up to the time that brightness peaks, but beyond that time as well. This phenomenon has been ~ound to exist for all variations of parameters employed in the processes used ~or making electroluminescent phosphors thus far expiored.
..~ ~, . . . . .
Furthermore,-it has been discovered that the rate at which ~he phosphor brightness tends to decrease with continued stressing after peak brightnes~ has b-en ''`'' ','. ' ' ''~ ~"' W091~167~2 PCT/US91/01~0 ~a ~ t~ - 6 -achleved is substantially slower than the rate at which brightness increases with stressing before peak ~rightness ls reached. Thus, the present invention is based upon the discovery that extreme mechanical stressing of a first-fired phosphor precursor material, i.e., stressing for a period of time beyond that at which peak brightness is normally realized, yields an electroluminescent phosphor product having unexpectedly superior properties in tarms of both brightness and useful life (i.e., improved half life with little sacrifice in briqhtness).
The critical factor for obtaining the improved properties in electroluminescent phosphors in accordance with the present invention is the performance of the mechanical stressing of the first-fired phosphor precursor material. As noted, it is critical that this mechanical stressing be per~ormed for a period of time which exceeds that at which maximum brightness is attained for the given phosphor material. The specific parameters chosen for the mechanical stressing step will depend upon the starting materials, the type of mechanical stressing imparted upon the phosphor, the efficiency of the apparatus or methodology employed in stressing the phosphor precursor, as well as the brightness and li~e characteristics de~ired of the ~inal phosphor product.
With resp2ct to tha starting materials, it is found that the addition o~ high levels of the copper activator, besides resulting in a more green phosphor, results in phosphors having lonyer ~ife and higher brightness as compared to similar phosphors employing less copper activator. Furthermore, it is also found ~ -that a small quantity of zinc selenide added to the WO91t~7~ PCT/VS~I/O~O
_ 7 _ 20~27~
starting materi~l will also contribute in a bene~icial way to th~ quality o~ the EL phosphor so produced, particularly with respect to the production of hi~h brightness and excellent long life green phosphors.
The process of the present invention thus comprises the following steps:
(a) preparing a mixture of a predominant amount of zinc sulfide and a minor amount of an activator compound, preferably a copper-containing activator compound; ;~
- (b) firing the mixture (first fire) at a temperature sufficient to form particles having a predominantly (>50%) hexagonal crystalline structure;
(c) mechanically stressing the first-~ired mixture, e.g., by mortaring, mulling or milling, for period of time exceeding that at which maximum brightness would be achieved;
(d) firing the mechanically stressed material for a second time (second fire), to complete the reversion of the crystalline structure to a predominantly cubic -conformation; and (e) subsequently cooling the twice-fired material to yield the desired high brightness, long half life electroluminescent phosphor products.
'~ ..
The present invention is generally applicable ~o all elactroluminescent phosphors, regaxdless of color emittance, but is particularly applicable to phosphor materials emitting in the blue, green:and yellow-orange portions of the spectrum. Ultimate coloration of the , finished phosphor is dependent, at least in ~art, upon the starting materials employed. Variations in coloration ~ay be effected by varying the constituents '.
WO91/16722 PCT/US91/02~0 ~ ~ 8 and relati~e amoun~s o~ the constituents in said s~arting materials~ as is well-known by those of ordinary skill in the phosphor arts. For example, Ihe brightness and color may be controlled by shifting the copper and/or zinc selenide content.
In any event, many improvements and attributes are realized from the phosphors produced in accordance with the practice of the present invention as compared with typical, comparable phosphors. For example, phospnors made in accordance with the practice of the present invention which emit in the yreen portion of the sDectrum have halr liv~s in excess of 2000 hours to over lO,000 hours, as compared with its standard counterparts which typically have half lives of only about lO00 hours. Similarly, blue electroluminescent phosphors produced in accordance with the practice of the present invention typically have half lives 3 to 5 times longer than those of traditional blue phosphors.
In addition to the improved phosphors and improv~d process for making phosphor as described abovP, the present invention also relates to electroluminescent lamp elements having superior lifa and brightness characteristics. The improved electroluminescent lamps of the present invention result from tha incorporation of the phosphors produced in accordance with the practice of the present invention into typical electroluminescent lamps. The construction and method of manufacture of.these EL lamps is well-known and well-described in the patent literature.
. :
~0~l~l67~? PCT/US91/02~0 .
_ 9 - 2~27~ :
BRIEF DESCRIPTION OF THE DRAWINGS
.
Figure 1 shows a side sectional view of the i--electroluminescent lamp used for testing phosphors of this invention.
Figures 2 through 6 are plotted curves, corresponding to each of a number of different phosphor compositions which were incorporated into test electroluminescent lamps, showin~ how the characteristic brightness and life of each phosphor product changes as a function of the duration of ~echanical stressing during processing.
Figure 7 shows the brightness and life characteristics versus stressing time for a phosphor composition wherein the stressing is brought about by mulling.
Figure 8 shows the life characteristics of a zinc sulfide:copper phosphor stressed for 20 minutes by --~ortaring, wherein the retained Cu content was sequentially elevate~ from 0.054% through 0.091%, and, for comparative purposes, the life characteristics of a comparable commPrcial green EL phosphor. The ~ -commercial phosphor has a retained Cu content of 0~08%
and showed shorter life than the 0.054~ Cu content phosphor ~rom the present invention~
.
, . _ . .
. .
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The process of the present invention is generally applicable to any electroluminescent phosphor precursor w~ gl/167~2 Pcr/ussl/n26~0 ~aterial or co~position. Such compositions are ~ell-known and readily recognizable by those of ordinary skill in the art.
Typical phosphor precursor materials generally comprise zinc sulfide, alone or in combination with zinc selenide, an activator therefore, and one or more coactivators. In preferred phosphor precursor compositionsr a combination of zinc sulfide-and zinc selenide is employed. This zinc sulfoselenide mixture generally comprises at least 80~, preferably go~, and ~ost preferably, at least 96% by weight zinc sulfide, -~ith the remainder comprising the zinc selenide.
Activator compounds which may be used in the phosphor precursor compositions of the instant invention include, but are not limited to, copper, silver, gold, phosphorus, arsenic, vanadium, antimony, lead, tin, manganese, iron, sodium, lithium, gallium, tellurium, scandium, indium, and rare earth compounds.
The preferred activator comprises a copper compound, alone or in combination with one or more of the other aforementioned ackivator compounds. Preferred copper co~pounds include copper sulfate, copper nitrate, copper chloride, copper bromi~e, copper acetate, copper fluoride, copp~r iodide and copper carbonate: with copper sulfate being most preferred. Gene~ally, the activator will be present in an amount of less than ~%, pre~erably less than or equal to about 1~ by weight, based upon the aforementioned zinc sulfide or zinc sulfoselenide materials.
'' :' ' ' Coactivators that may be employed in the practice of the present invention include compounds or complexes ~ -~
',''" .
':
WO 9 1 / ] 67 ? 2 PCT/US91/0~40 2~27~
o~ halide (Cl, Br, I) and trivalent ions (Al, Ga, In, etc.)~ Prefe~red coactivators include complexes based on halides, most preferably halide fluxes. Suitable coactivators are well-known to those of ordinary skill in the art. Exemplary of suitable coactivators there may be given barium bromide, magnesium bromide, sodium bromide, magnesium chloride, ammonium bromide, sodium chloride, barium chloride, magnesium iodide, ammonium iodide, barium iodide, sodium iodide, and the like.
Generally, the coactivator is pr~sent in iks hydrated form, e.g., magnesium chloride (hydrated) (MgC12.6H2O). The coactivator component may comprise one or more coactivators and is generally e~.ployed in an amount of 20% by weight or less, preferably 10~ by weight or less, and, most preferably, about 8~ by weight or less, based upon the weight of ~-the zinc sulfide or zinc sulfoselenide material.
As noted, additional constituents may be added cr :
incorporated into the phosphor precursor mixture or composition as desired. Such additional constituents include zinc sulfate, zinc oxide, sulfur and the like, and will be readily known to those sXilled in the art.
While the exact reasons for the improved results obtained by Applicant's process are not fully undexstQod, the results are quite clear. The present invention provides a zinc sulfide phosphor which, when emitting in the green portion o~ the spectrum, has a half li~e in excess of 2000 hours and,-when emittins in the blue portion of the spectrum,- has a half life in excess of 1000 hours~ both of which represen~ drama~ic improvements over existing comparable emitting phosphors.
. J~
WO91/167~ PCT/US~1!02~0 1~ _ Briefly, the present invention starts with the aforedescribed phosphor precursor materials or compositions. Preferably, said mixture comprises a mixture of cubic or predominantly cubi^ zinc sulfide and a copper activator,-e.g., copper sulfate, and at least one coacti~ator such as a halide flux (e.g., a chloride or bromide flux). This precursor composition is fired at a temperature and for a period of time sufficient to convert the cubic conformation of the zinc sulfide ~o a predominantly (> 50~) hexagonal conformation. Appropriate temperatures and time for this firing will be readily apparent to those of ordinary sXill in the art. Typically, the first firing will be conducted at a temperature of at least about 1000C, preferably of at least 1100C, most prererably of at least about 1200C. However, the maximum firing temperature will generally not exceed 1400C. The period of time for which the phosphor precurso_ material is subject to said firing will depend upon the precursor materials and the temperature of the firing itself. Generally, the first firing ~ill be for a period of time of from about 1 hours to 6 hours, preferably from about 2 hours to about 4 hours.
It should be noted that all reference within this specification, including the examples, to ~iring time, is to ~e understood as the time of the actual firing of the phosphor precursor or phosphor material ~rom when the temperature of the material achieves the ~iring temperature until the firing is completed. Thus, the actual residence tim in the furnace will be somewhat~;
longer than the firing times so as to allow the phosphor matarials to attain the desired firing temperature. ~ ;
. ~.
...
WO~1~167~2 PCT/VS91/02~0 - 13 _ 2 ~ ~2 ~
~ fter firing, the ~irst-ired materials are allowed to cool to allaw the same to be further processed.
During said cooling cycle, which may be an ambient cooling or a Eorced coo}ing, some of the crystalline zinc sulfide will revert from the hexagonal ,' conformation to the cubic conformation.
The first-fired materials are then milled or '',,'~
mortared in a mortar-grinder, for example in a Model ~o mortar-grinder made by Retsch GmbH of West Germany, f~r a period of time in excess of that at which maximum ~rightness is achieved (Tb). Tb is easily aetermined through simple and undue experimentation for any given phosphor composition and/or is well-known in ~
the ~rt for standard phosphor compositions. The, ,',' purpose of the extended milling process is to produce a highly fractured zinc sulfide crystal.
. !
While it is known that milling will induce faults , or fractures into the crystalline structure of the zinc sulfide particles, because phosphors havP typically been rated on krightness, it was believed to be detrimental to mill for exten~ed periods of time and ~;-thus induce a degree of faulting or fracturing beyond that at which maximum brightness was attained. ,i Unexpectedly, however, it has now been found that continued milling and, thus, the inducement of higher ' ~'' levels or degrees of ~aulking or fracturing in the zinc ', sul~ide crystals results in phosphors having far superior lives with only minimal loss in brightness. , ~, - ~ .
In addition,to produoing a more highly fractured or faulted ~inc sulfide crystal, the milling also causes l the conversion of some of the ~inc sulfide crystals '~ ', :
WOgl~l67~' PCT/~S91/02~0 ~7J~ 14 -from the he~agonal conformation to the cubic conformation and, perhaps more importantly, app~ars to ~acilitate the subsequent and more extensive crystalline phase transition ~rom hexagonal to cubic conformation caused by the second firing step. It has also been found that continued milling may lead to a more uniform and smaller particle size, which from the standpoint of phosphors useful in the manufacture of electroluminescent lamps may be desirable, but is not otherwise believed to be critical.
The specific length of time for which the first-fired materials are mechanically stressed is depe~dent upon the materials themselves, the desired final properties and characteristics, and the mechanical stressing technique or methodology employed. Exemplary of the method by which-stressing can be accomplished include mortaring, mix-mulling, ball milling, fluid-energy milling, jet pulverization, grinding, vibratory milling, cen~rifugal milling, hydrostatic pressure, uniaxial or biaxial pressure and the like.
Furthermore,this invention is also intended to encompa s mechanical stressing by means other than directly applied external forces as exemplified above.
Although not yet attempted, such other methods include subjecting the material to thermal shocks or other shocks that would most c~rtain~y entail a mechanical stressing o~ the .individual particles o~ the in-process phosphor. -.
Concurrent with or subsequent to the mechanicalstressing steps, additional activators, coactivators, or other co-constituents may be added to the in-process phosphor mixture. Such additives or dopants, as they -' .
wn ~I/1fi7~ PCT/US91/02~0 .
1 S ~ L~ ~ ~
are sometimes referred, are as previously defined. In general, it is preferred to add additional activators, preferably copper activakors, alone or in combination with other co-constituents. A preferred combination of dopants to be used at this point include copper sulfate and zinc sulfate. Generally, the amount of activator added to the in-process phosphor will be from about 0.5~ to about 5~, preferably from about 1% to about 3%, by weight based upon the total weight of the in-process phosphor. While lower amounts may be employed, such amounts are less likely to provide as much improvement in brightness and life in the final compositionO
However, said compositions will still have properties superior to con~entional phosphoxs. Similarly, while `
higher amounts of the activator may be employed, it is belie~ed that amounts of the activator may be empioyed, it is believed that amounts in excess of the 5~ by weight are beyond the saturation point for the composition and, thus, generally do not provide any further improvement in brightness and/or life. -With respect to the co-constituent, the co-constituent is preferably employed in an amount of from about 1% to about 20~, preferably from about 1~ to about 16~, by weight base~ upon the total weight of the in-process phosphor. A~ noted previously, the preferred combination of dopants added during or following the ~irst firing comprises copper sulfate and zinc sulfate.
The mixture is then refired at a temperature lower than the initial firing, generally less than about 1050C, preferably less than about 900C. The most preferred second firing temperatures range from about . .
.
. : - : , , . ~ :. :. ~ . : . . - , -: - : . . ~ , . ...... .
W~ 67~2 P~l/US~1/02~0 16 ~
7~0C to about 800c. The length of time for which ~he sacond firing is c~nducted will generally depend upon the ~iring te~perature, the materials themselves and the desired characteristics and properties of the final phosphor materials. Generally, the time of firing is that time which is sufficient to complete reversion of the conformation of the crystalline structure from a hexagonal conformation to a predominantly cubic conformation. This time is generally on the order of one-half hour to 3 hours or more. Preferxed firing tim~s for the second firi~g are -on the order of one-half hour to 2 hours, more specifically 1 hour to about 2 hours.
Following the second firing, the phosphor materials are allowed to cool, either atmospherically or by induced cooling or quenching. Thereafter, the phosphor particles are treated or washed to remove excess activato-, e.g., copper compounds, from the surface of the zinc sulfide crystals. Such washings may be accomplished by any methods known in the art.
Exemplary of such methods include a multi-step wasning -~
employing, in sequence, hydrochloric acid (4N), water and a 4~ sodium cyanide solution. Following washing, the phosphor materials are dried, .
After drying, the phosphor is preferably sieved through a 325 mesh screen so as to remove all axcessively large phosphor particles. Although the -unsieved phosphors do provide the attributes of long life with high brightness, they are less desirable for use in the manufacture of electroluminescent lamps -which have preferred parti~le si~e tolerances for insuring a good phosphor layer in its construction.
wn 91/lfi722 PCT/VS91/02~0 - 17 - 2~ ~2 ~ ~
In addition to the ~oregoing steps, other steps or pr~cesses may be conducted at various times in the process as described above. For example, following ~he initial firing, it may be desirable to wash the in-process phosphor materials with water so as to remove the excess coactivators, including any ions thereof that may have formed, especially any excess halide ions. Such washing may be accomplish~d with ;
repeated treatments with hot, deionized water until the conductivity of the wash water is below certain levels, preferably less than about 50 micromho cm-l Additionally, it may be desirable to sieve the first-fired materials to remove any excessively large~ -particles. Generally, at this stage, it is preferred to employ a lO0 mesh screen for sieving purposes.
. , .
Although not intending to be bound by theory, it is believed that in the practice ~f the instant process the initial high temperature firing, combined with the excessively long period of mechanical stressing, leads to phosphor particles having a higher degree of faults or fractures in the crystalline structure as well as to phosphor compositions having a higher level or percentage of particles having the cubic as opposed to the hexagonal conformation. It is believed that the increased number of fractuxes or faults in the zinc sulfide particle provides additional sites for activator, copper compol~nd, deposition during the second firing.
. . ....- . . : . ~ . ............................ -, The doping of the ~irst-fired material with-additional copper activator tends to increase the likelihood of saturation of the numerous faults or fractures in the crystalline structures. Obviously, -- .. . . ... ..... . .
WO91~167~ PCT/US91/02640 ~ 18 -with very low levels of copper activator in the in-p~ocess mixture, the opportunity or likelihood o~ -the copper compound to infiltrate th~ fracture or fault is less likely than in instances where high levels of copper activator are employed. In general, the amount of initial copper and added copper is maintained sufficiently high during the two firing steps such that the total retained copper content of the final product after washing is at least 0.02~, preferably at least 0.06%, by weight. In general, the total retained copper content will preferably be in the range of 0.0~%
to about 0.25%, m~st preferably about 0.06~ to about 0.2%, by weight of the phosphor mixture. Compositions within the foregoing limitation and manufactured in accordance with practice of the present invention generally provide phosphors having high initial brightness and a long h~lf life.
To determine the effect of mortaring time on phosphor performance, several series of ~-elPctroluminescent phosphors were manufactured and electroluminescent test lamps produced with said phosphors. Each series differs from one another by some variation in the method ~or making electroluminescent phosphor and/or the composltional makeup of the phosphor precursor materials. Within each series, the phosphor and its method of manufacturing were held constant while the phosphor mortaring time was varied. Each phosphor sample was then incorporated into five electroluminescent lamps ~nd the lamps tested.
.. : . . . .
The EL test lamps (illustrated in Figure l~ were-prepared as follow~. A paste was formed by dispersing . .
WO91~1~722 PCT/US91/02~0 .
-- 19 -- ' ~e electroluminescent phosphor powder in a A7elQctric ~
resin. A layer of the phosphor powder paste 30 and a . .-layer of a barium titanate paste 34 are sand~iched between an opaque metallic foil (aluminum) servicing as a rear electrode 20 and a transparen~ ~ron~ electrode (indium tin oxide) 22. A layer 24 of a desiccant is : ::
formed over the front transparent electrodP 22. A pair .
of lead wires 26 and 28 are attached, respectively, to the front and rear electrodes 22 and 20. A polymeric protective film 32 encapsulat~s this assembly, but with :
the lead wires 26 and 28 extending away therefrom, ~
provide physical support and a moisture barrier. The : .
phosphor layer 30 has a thickness of 0.04 millimeters. .
The barium titanate layer 34 is 0.015 millimeters thick.
A more detailed description of the construction of an electroluminescent lamp is provided in U.S. Patent - :
No. 4,104,555 (Fleming), the disclosure of which is hereby incorporated herein by reference.
The general meihod for making the phosphor as àescribed above will be referred to hereinafter as the reference method.
In the ~ollowing examples, the brightness of each test lamp was measured with excitation at 115 volts and 400 H7. The brightness and life date points for each phosphor.produced were determined by averaging the measured brightness and li~e for each of the five test lamps per.group. By convention, the half life o~ an electroluminescent phosphor.(or-lamp-containing the -same). is defined as the time re~uired for the initial brightness (Bo) to decrease to one-half of its ,, ., .. - , . , ,, . ... - - , , . ,, , ; , ~ . . . . ~ . . - .
~V~91/167~ PCT/~S91/02~10 - ~0 -initial ~alue. S.ince this time may be several thousand ~o~rs, an accelerated li~e test is a near necessity.
One such test was devised by British Aerospace and consists o~ operating la~ps at elevated temperature and fre~uency. This method is described in R.H. Marion et al. "Analysis of the Lifetime and Powder Electroluminescent Phosphors" in Shionoya, S. and Kobayashi, H., Ed., Springer-Verlag, P. 332, 1939.
The accompanying figures represent the brightness and life data realized from evaluations of various ~-phosphors made in accordance wîth the practice of the present invention as set forth in the following examples. Figures 2 through 6 are graphic representations of the life and brightness data versus mortaring time for compositions made in accordance with the practice of the present invention. ~igure 7 illustra~es the effect on brightness and life of mulling rather than mortaring. Figure 8 represents data demonstrating the effect on life of various levels of retained copper in the phosphor. Figures 2, 3 and 6 are based on actual life test data. All other figures represent life data which are based on accelerated life test results. Figure 2 sets forth the actual data points arrived at, whereas Figures 3 throu~h 7 present the brightness and life data in a normalized fashion, i.e., the data points representing the greatest level of brightness and life, as measured, are each set to a value of 1.0~ ~ -''' - :
The following examples are-intended to aid in the understanding of the present invention, but are not to be construed as limitations thereof. Unless otherwise : :'.
.
.
W~91~67~ P~T/~ /02~0 - 21 - 2 ~2 ~.
indicated, all a~ounts with respect to the various constituents o~ the phosphor precursor materials and the in-process additives are in parts by weight~ All temperatures are expressed in degrees Celsius.
:
~ series of phosphor compositions emitting in the green portion of the visible light spectrum were made according to the following procedure:
, 98 parts zinc sulfide, 2 parts zinc selenide, 0.3 parts copper sulfate (anhydrous~, 3 parts barium bromide (hydrated), 3 parts magnesium bromide (hydrated) and 2 parts sodium bromide were dry blended together in a V-blender for at least one-half hour. The blended materials were then placed in a quartz crucible, covered and fired at 1200C for 3 hours. After 3 hours firing, the crucible was brought out of the furnace and cooled rapidly with cold air blowing on the bottom of the cruciDle. Once the phosphor was cooled, it was washed free of ions, especially bromide ions, with hot deionized water at least s.ix times, or until the conductivity of the wash water was less than 50 micromho cm l. The phosphor was then filtered, dried and sieved through a lOO mesh scre~n. This step of the process yields a material that is referred to as the first-fired material.
.. . .
, . . ... . . .
Individual 200 gram aliquots~ of the first- ~-fired material were then mortared for various -lengths of time to demonstrate the effect thereof.
.. . . . . . . ......... . . . . . ........ ... . . . . .. .. ..
.. . . . . . ,, : . ., ; : . , : - ~ - . ..
- . . ,~. ~ : - ~: ., . . :
. - - . . , . ~ . . . .. .. . .. .. . . .
WOg~/~672~ PCr/US91!02~0 In this example, mortaring times of 0, 5, 10, 20, 30 and 40 minutes were employedO
Each ~ortared sample was then doped with 5 grams of copper sulfat2 (CuSO4) and 32 grams of zinc sulfate (ZnSO4), blended and fired at 800C for 1 hour in a covered quartz crucible.
Following this second firing, the phosphor was then cooled and sequenti~lly washed with hydrochloric acid (4N), water, and finally a 4% NaCN solution to remoYe any excess copper found on the surface o~
the phosphor particles. The phosphor was then dried, sieved through a 400 mesh screen and processed into electroluminescent lamps.
The data representing the brightness levels and half lives attained for the various samples made in accordance with this example are depicted in Figure 2 above. As shown, brightness (curve 35) peaked at a mortaring time of about 5 minutest whereas the half life (curYe 36) continues to increase even up to 30 ~ ;
minutes. Furthermore, although brightness diminishes with continued mortaring beyond the time at which peak brightness is attained, its rate of diminution is much slower as compared to its rate of enhancement in the early stages of milling. Based on ~his data, it is clearly advant~geous to mortar the phosphor beyond the 5 minutes at which peak brightness is obtained, thereby obtaining a signi~icant increase in life at the expense of a minimal loss in brightness. These same plots are depicted in a normalized fashion in Figure 3 as per ~~~ curves 41 and 42, respectively.
~ ' ~ ''~ ':' ' :
. .,, . 1- .
- : - - .. ---.-: . , , . . .. ; , . . .-- , . . . .
W~9~/167~2 PCTtUS~I/02~0 . .
- 23 - 2052~
EX~MPLE 2 A second phosphor composition was made in accordance with the procedure of Example 1 with the exception that the second firing was at 750c instead of 800C. This phosphor composition was slightly different from that set forth in -Example 1 in that zinc selenide was eliminated from the formulation and that magnesium chloride (hydrated) was substituted on a weight for weight basis for the magnesium bromide of Example 1. The brightness and life data for this sample of phosphor is depicted in a normalized fasAion in curves 45 and 46, respectively, of Figure 3. .
A phosphor composition was made in accordance with the procedure of Example 1 with the exception that the amount of copper su}fate starting material was increased from 0.30 parts by weight to 0.40 parts by weight. Brightness and life data, as a function of mortaring time, for this phosphor composition is presented in Figure 4 by curves 51 and 52, respectively. ~ :
:' ~XAMP~
., A phosphor composition emittlng in the blue portion - of the spectrum was made in accordance with the procedure of Example 1 except that the level of ccppër sulfate was reduced-from 0.30 parts by weight-to Q;15 parts by weight, 1 part by weight of ammonium bromide ,.. . . . .. . .... . . . .
~O~1/1672 PCT/US~/0~0 ~ Q - 24 -was substituted for the 1 ~luxes (barium bromide (hydrated), ~agnesium bromide (hydrated) and sodium bromide)), and 1 part by wei~ht of sulfur was added as a co-constituent. The brightness and life data for the phosphor so made is depicted in Figure 4 by curves 55 and 56, respectively.
EXAMPT ~ 5 An additional series or phosphors was made in accordance with the procedure of Example 1 except that the starting materials were blended through a wet mixing effect2d by the addition of water, which was subsequently removed by heating and drying, and the second firing was conducted at a temperature of 750C. Additionally, the composition of Example 1 was modified in that the magnesium chloride (hydrated) was substituted on a weight for weight basis for the magnesium bromide. The data points detDrmined for the brightness and life of the phosphor so produced are depicted by curves 61 and 62 7 respectively, in Figure 5.
~!
. .
A series a phosphor compositions were manu~actured in accordance with the general procedure ~f Example 1 with the exception that the firing time-was varied as set ~orth below and that a magnesium chloride (hydrated) flux was substituted for the magnesium bromide (hydrated) flux on a weight for weight basisO
The first series of phosphors as represented by curves I ' ..
~... i . -~V~ 91/16722 PCr/VS9~/02640 2~6~7~
71 and 72 (~igure 6), for brightness and life, xespectively, were ~ired for 2 hours in both the first and second firings. On the other hand, curves 75 and 7~ correspond to the brightness and life behavior, respectively, of similar series of phosphors with the exception that the first firing was co~ducted for a period of 2 hours and the second firing for a period of 4 hours. Once again, as shown in ~igure 6, the advantages of mortaring beyond the time at which brightness becomes greatest is clearly evident for all o~ these changes in the method or making the phosphor.
':
A series of phosphor compositions as described in Example 1 were prepared in accordance with the method -:;
of Example 1 with the exception that the starting materials, following the first firing, were mulled, rather than mortared, and the second firing was conducted at 750C. The mulling machine employed was a Simpson Mix-Muller, Model LF, Style UD, made by the :;
National Engineering Company o~ Chicago, Illinois.
This device consists of two heavy crusher wheels mounted at the ends of a horizontal axle that rotates in a horizontal plane about the axle center. Sweepers direct the material being mulled under the wheels.
As shown in Figure 7, the phosphors produced in accordance with the above teachiny mani~ested a maximum brightness after 30 minutes of mulling (curve 813.
Continued mulling of the phosphor composition beyond the 30 minute peak period resulted in the development o~ phosphors having a longer life perfo~mance with a ~ ~.
~ ' ' ' ' '' ' '' ' ' .. ' '~ ' ... . . . .
WO~1/167~? PCT/US9l/02~0 ~ 26 -minor loss in brightness (curve 82). This example demonstrates the fact that alternative methods for mechanically stressing the phosphors may be emploved in the practice of the present invention. It is recognized that certain mechanical stressing methodologies may require longer periods of stressing ln order to achieve the same result. Nevertheless, the practice of the present invention still provides improved phosphor compositions and, subsequently, electroluminescent lamps having enhanced brightness and longevity.
A series of phosphor compositions was prepared in ;;,;
accordance with the methodology of Example 1 and of the formulation of Example 1 with the exception that the amount of copper added initially was varied so as to provide phosphor compositions having a formulated initial copper content of 0.06%, 0.08%, 3.10% and 0.12%
by weight, respectively. The actual lamp brightness and half life data, as function of copper content and mortaring time, is depicted in Table I.
;:
-: ~ ' ' . - ~ ' ' , .
- . . -. : .. , . . - -~ - . -.:. , .
W~91~167~' Pcr/ uss ~ !02b40 - 27 - 20~27~3 ~ABLE I
Relationship of Brightness and Life to Initial CooDer Content and Mortarinq_Tlme Mortaring Initial Half Life CuTime fMin.)Brightness (Ft-LltHours) : :
~ ' -.
0.06 0 18.9 150 23.2 1900 0.08 0 9.6 200 :
20.1 4gO0 0.10 0 9.4 250 24.6 6150 - 0.12 0 9.5 . 500 27.5 3200 25.3 5300 22.5 5650 : .
19.3 7700 17~5 7600 .:
! :`~
I
, i~
WO 91/1~727 PCI/US91/0264~) 2 ~ -The mortared samples always showed higher initial brightness and better half life than the unmortared samples. The hal~ life also improved with increasing copper content. The 30 minute mortared sample, for the 0.12% Cu, showed a slight decrease in brightness, but continued to show very good half life.
The relationship o~ brightness and life to copper i' content is shown in Table II and depicted in Pigure 8.
Also shown in Figure 8 is the plot of a typical commercial green phosphor which has a retained copper content of 0.08%. After 600 hours operation, the :
brightness of the EL lamp was only 66~ of its initial .
brightness level.
`.
.
;::
;' ' '`~'" ' `"'' ' ' ., .
, -: :- - .. . . . . . . . . ,, , . . " .. , . ,, , . , . . .. , . - - : -WO 91/167~2 PfCr/US91/02f640 ~ f~ ~2 ~
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l m f~l¦ Ul t` N 0 f~
O
~1 ~ o n o~ I f f f ~1 ~ ~ ~7 0 t~
O, t~ f~7 ~1 1~ 7 . f t~ ~ 1~ 7 ~ Clf O f~l ~7 ~ ~ O ~ f7 ~ f f~
~3 ~ f ~ -n ~ ~¦ . ~ n f~ f~ f~
U3 f~ I C:~ 0 C~ a~
H f_ ~ _ ' f~ 7 f7 ~ ~ l o ~ 0 O e ~ ~ '` '` f`
~ o o o o ~ ~ r 1~ ~
~_~ O f ,1 ,. ,_f _1 :
~, C ~¦ ". fC ~ frf .
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f5 f3~ ~ ~ 7 f,~
~-~1 r` ~ fn cl:~ .o f~ f.~7. frl r~ S 01 o ~7 .~J . r~ . ..... ... ... ... ... ' ~ ~1 :
ul o o ~ o 7 0 ~ ~
o~ O O O 0 3 d~ O f ~ O O : :
' ~' ~
,.
' ;,~
WO91/1~72~ PCr/US~ 2~10 The present invention has been described in detall, including the preferred embodiments thereof. However, it will be appreciated that those skilled in the 2-~, upon consideration of the present disclosure, may make ~:
modiflcations and/or improvements of this invention and still be within the scope and spirit of this invention :
as set forth in the following claims.
'`'.''"~'" ' -
Claims (23)
1. A method for making an electroluminescent phosphor having long life, said method comprising:
(a) preparing a mixture of phosphor precursor materials comprising predominantly zinc sulfide and minor amount of a copper activator compound;
(b) first firing said mixture to form particles of a predominantly hexagonal crystalline structure;
(c) mechanically stressing said first-fired material to produce a crystalline powder, some portion of which is, by said stressing, converted to a cubic structure, said mechanical stressing being continued for a period of time beyond that at which maximum brightness of said finished phosphor would have been realized;
(d) subsequently firing said stressed crystalline powder to revert the structure thereof to a predominantly cubic structure; and (e) cooling said subsequently fired powder.
(a) preparing a mixture of phosphor precursor materials comprising predominantly zinc sulfide and minor amount of a copper activator compound;
(b) first firing said mixture to form particles of a predominantly hexagonal crystalline structure;
(c) mechanically stressing said first-fired material to produce a crystalline powder, some portion of which is, by said stressing, converted to a cubic structure, said mechanical stressing being continued for a period of time beyond that at which maximum brightness of said finished phosphor would have been realized;
(d) subsequently firing said stressed crystalline powder to revert the structure thereof to a predominantly cubic structure; and (e) cooling said subsequently fired powder.
2. The method of claim 1 wherein said stressing time amounts to at least 1.25 times that at which said maximum brightness of said finished phosphor would have been realized.
3. The method of claim 1 wherein additional copper activator, alone or in combination with coactivator or other additive, is added to the mixture during or subsequent to the mechanical stressing step.
4. The method of claim 3 wherein an additive is employed and the additive is zinc sulfate.
5. The method of claim 1 wherein said first firing is conducted at a temperature of at least 1100°C and said second firing is conducted at a temperature of 900°C or less.
6. The method of claim 1 wherein said first firing is conducted at about 1200°C or more and said second firing is conducted at about 800°C or less.
7. The method of claim 1 wherein the amount of copper activator compound is about 1% by weight or less based upon the weight of the zinc sulfide.
8. The method of claim 1 wherein the phosphor precursor materials further comprise a coactivator.
9. The method of claim 8 wherein the coactivator is a halide flux.
10. The method of claim 8 further including the step of washing the first-fired material so as to remove any excess coactivator for coactivator ions.
11. The method of claim 1 wherein the phosphor precursor material further comprises zinc selenide.
12. A powdered electroluminescent phosphor made in accordance with the method of claim 1.
13. A powdered electroluminescent phosphor made in accordance with the method of claim 8.
14. A method for making an electroluminescent lamp, said method comprising:
(a) preparing a mixture of phosphor precursor materials comprising predominantly zinc sulfide and a minor amount of a copper activator compound;
(b) first firing said mixture to form particles of a predominantly hexagonal crystalline structure;
(c) mechanically stressing said first-fired material to produce a crystalline powder, some portion of which is, by said stressing, converted to a cubic structure, said mechanical stressing being continued for a period of time beyond that at which maximum brightness of said finished phosphor would have been realized;
(d) subsequently firing said stressed crystalline powder to revert the structure thereof to a predominantly cubic structure;
(e) cooling said subsequently fired crystalline powder;
(f) forming a stack with layer of said second-fired crystalline powder dispersed in a dielectric resin sandwiched between a rear electrode and a light transparent front electrode; and (g) attaching a pair of conductive leads respectively to said rear and front electrodes.
(a) preparing a mixture of phosphor precursor materials comprising predominantly zinc sulfide and a minor amount of a copper activator compound;
(b) first firing said mixture to form particles of a predominantly hexagonal crystalline structure;
(c) mechanically stressing said first-fired material to produce a crystalline powder, some portion of which is, by said stressing, converted to a cubic structure, said mechanical stressing being continued for a period of time beyond that at which maximum brightness of said finished phosphor would have been realized;
(d) subsequently firing said stressed crystalline powder to revert the structure thereof to a predominantly cubic structure;
(e) cooling said subsequently fired crystalline powder;
(f) forming a stack with layer of said second-fired crystalline powder dispersed in a dielectric resin sandwiched between a rear electrode and a light transparent front electrode; and (g) attaching a pair of conductive leads respectively to said rear and front electrodes.
15. The method of claim 14 wherein said mechanical stressing time amount to at least 1.25 times that at which said maximum brightness of said finished phosphor would have been realized.
16. The method of claim 14 wherein additional copper activator, alone or in combination with a coactivator or another additive, is added to the mixture during or subsequent to the mechanical stressing step.
17. The method of claim 16 wherein an additive is employed and the additive is zinc sulfate.
18. The method of claim 14 wherein said first firing is conducted at a temperature of at least 1100°C and said second firing is conducted at a temperature of 900°C or less.
19. The method of claim 14 wherein said first firing is conducted at a temperature of about 1200°C
or more and said second firing is conducted at a temperature of about 800°C or less.
or more and said second firing is conducted at a temperature of about 800°C or less.
20. The method of claim 14 wherein the amount of copper activator compound is about 1% by weight or less based upon the weight of the zinc sulfide.
21. The method of claim 14 wherein the phosphor precursor materials further comprise a coactivator.
22. An electroluminescent lamp comprising:
(a) a rear electrode;
(b) a light transparent front electrode;
(c) an electroluminescent phosphor dispersed in a layer of a dielectric resin, said phosphor comprising a copper doped zinc sulfide which has been prepared in accordance with the method of claim 1, said phosphor containing dielectric resin layer being sandwiched between said rear electrode and said front electrode; and (d) a pair of conductive leads attached respectively to said rear and front electrodes.
(a) a rear electrode;
(b) a light transparent front electrode;
(c) an electroluminescent phosphor dispersed in a layer of a dielectric resin, said phosphor comprising a copper doped zinc sulfide which has been prepared in accordance with the method of claim 1, said phosphor containing dielectric resin layer being sandwiched between said rear electrode and said front electrode; and (d) a pair of conductive leads attached respectively to said rear and front electrodes.
23. The electroluminescent lamp of claim 22 wherein said copper doped electroluminescent phosphor has been mechanically stressed for a period of time of at least 1.25 times that at which said maximum brightness of said finished phosphor would have been realized.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US50985690A | 1990-04-17 | 1990-04-17 | |
| US509,856 | 1990-04-17 | ||
| US61361290A | 1990-11-15 | 1990-11-15 | |
| US613,612 | 1990-11-15 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2062750A1 true CA2062750A1 (en) | 1991-10-18 |
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ID=27056681
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2062750 Abandoned CA2062750A1 (en) | 1990-04-17 | 1991-04-17 | Electroluminescent phosphors with long life and method for making the same |
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| Country | Link |
|---|---|
| EP (1) | EP0477354A4 (en) |
| JP (1) | JPH05501584A (en) |
| CA (1) | CA2062750A1 (en) |
| WO (1) | WO1991016722A1 (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5643496A (en) * | 1996-04-04 | 1997-07-01 | Osram Sylvania Inc. | Small size electroluminescent phosphor |
| US5702643A (en) * | 1996-04-24 | 1997-12-30 | Osram Sylvania Inc. | ZnS:Cu electroluminescent phosphor and method of making same |
| US5711898A (en) * | 1996-04-24 | 1998-01-27 | Osram Sylvania Inc. | Improved blue-green emitting ZnS:Cu,Cl electroluminescent phosphor and method of making same |
| DE19953924A1 (en) * | 1999-11-10 | 2001-06-07 | Bundesdruckerei Gmbh | Zinc sulfidic electroluminophores and process for their production |
| US7815124B2 (en) | 2002-04-09 | 2010-10-19 | L-1 Secure Credentialing, Inc. | Image processing techniques for printing identification cards and documents |
| WO2003055684A2 (en) | 2001-12-24 | 2003-07-10 | Digimarc Id Systems, Llc | Laser engraving methods and compositions |
| US20040007692A1 (en) | 2002-05-31 | 2004-01-15 | Chenot Charles F. | Method of making electroluminescent phosphor using sonochemistry |
| US6682664B1 (en) | 2002-09-05 | 2004-01-27 | Osram Sylvania Inc. | High brightness orange-yellow-emitting electroluminescent phosphor and method of making |
| GB0413121D0 (en) * | 2004-06-11 | 2004-07-14 | Pelikon Ltd | Improved displays |
| US8698979B2 (en) | 2004-06-11 | 2014-04-15 | Mflex Uk Limited | Electroluminescent displays |
| US7291291B2 (en) | 2005-03-30 | 2007-11-06 | Osram Sylvania Inc. | Electroluminescent phosphor powder with D50 value of less than 12 μm and method of making |
| US7288216B2 (en) | 2005-03-30 | 2007-10-30 | Osram Sylvania Inc. | Method of making electroluminescent phosphors with small particle sizes and powder with D50 value of no more than 10 micrometers |
| JP5151980B2 (en) * | 2006-07-05 | 2013-02-27 | 宇部興産株式会社 | Sialon oxynitride phosphor and method for producing the same |
| CN112213661A (en) * | 2020-10-23 | 2021-01-12 | 江苏辉皓正电气有限公司 | LED lamp aging detection system and detection method thereof |
Family Cites Families (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2957830A (en) * | 1959-03-19 | 1960-10-25 | Sylvania Electric Prod | Process for producing electroluminescent phosphors |
| JPS513311B2 (en) * | 1972-09-09 | 1976-02-02 | ||
| US4104555A (en) * | 1977-01-27 | 1978-08-01 | Atkins & Merrill, Inc. | High temperature encapsulated electroluminescent lamp |
| JPS61296085A (en) * | 1985-06-25 | 1986-12-26 | Toshiba Corp | Production of fluorescent substance |
| US4859361A (en) * | 1988-05-13 | 1989-08-22 | Gte Products Corporation | Process for producing electroluminescent phosphors of improved brightness |
| US5009808A (en) * | 1988-12-27 | 1991-04-23 | Gte Products Corporation | Process for producing electroluminescent yellow zinc sulfide phosphors |
-
1991
- 1991-04-17 CA CA 2062750 patent/CA2062750A1/en not_active Abandoned
- 1991-04-17 WO PCT/US1991/002640 patent/WO1991016722A1/en not_active Application Discontinuation
- 1991-04-17 EP EP19910908777 patent/EP0477354A4/en not_active Withdrawn
- 1991-04-17 JP JP50838491A patent/JPH05501584A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| EP0477354A4 (en) | 1993-07-28 |
| JPH05501584A (en) | 1993-03-25 |
| EP0477354A1 (en) | 1992-04-01 |
| WO1991016722A1 (en) | 1991-10-31 |
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