AU8231987A

AU8231987A - Phosphors and methods of preparing the same

Info

Publication number: AU8231987A
Application number: AU82319/87A
Authority: AU
Inventors: Ernest Kabay; Gabriella Kabay
Original assignee: Sunstone Inc
Current assignee: Sunstone Inc
Priority date: 1986-10-24
Filing date: 1987-10-20
Publication date: 1988-05-25
Also published as: WO1988003161A1

Description

DESCRIPTION PHOSPHORS AND METHODS OF PREPARING THE SAME Technical Field

The present invention relates to phosphors and methods of preparing phosphors. Background Art

Phosphors are materials which absorb energy and release the absorbed energy in the form of electromagnetic radiation, most typically as visible light. Where the phosphor absorbs energy from electromagnetic radiation impinging on the phosphor this radiation may be referred to as "exciting* radiation. Where the absorbed energy is released immediately, the phenomenon is known as "fluorescence." Where the absorbed energy is stored for an appreciable period of time but released spontaneously, the phenomenon is referred to as "phosphorescence." For example, a phosphorescent material may glow with visible light for a considerable period after it is exposed to ultraviolet light, x-rays or the like. The brightness of the glow will gradually decrease as the stored energy is released. The term "luminescence" includes -these and other phenomena involving absorption of energy within a material and release of that energy as electromagnetic radiation, most typically, but not necessarily, as visible light. The term "phosphor" thus includes all luminescent materials.

Phosphors can be categorized in accordance with their behavior as fluorescent, phosphorescent or stimulable. As used in this disclosure, such categories should be understood as based upon the predominant behavior of the phosphor at about room temperature, i.e, at about 20°C. Thus, a phosphorescent phosphor at room temperature will store absorbed energy for an appreciable time but will release the predominant portion of the stored energy spontaneously.

Phosphors can be utilized in a wide variety of scientific and industrial applications. Phosphorescent phosphors may be employed to provide a short-term steady _glow, as an emergency illumination source or the like.

_Although the present invention is not limited^" _by any theory of operation, the behavior of phosphors may be explained in terms of quantum mechanics. A photon is an energetic particle representing a single quantum of electromagnetic radiation. The energy of the photon determines the wavelength, of the radiation. Electrons in solids are regarded as being able to occupy only certain predetermined states having different predetermined energy levels. A solid emits electromagnetic radiation when an electron passes from one state to another state of lower energy; the difference in energy is released as a single photon or quantum of radiation.

The term "active site" as used in this disclosure means a feature of a phosphor which can emit one photon in a specified type of luminescence, regardless of the underlying principle of operation. The properties of an active site are generally believed to be determined by the available energy bands for an electron at the active site. Typically, these include a "ground" state and a "metastable" state or "trap" at a higher energy than, the ground state. There may also be other states as well involved in the transition from the ground state to the metastable state, or from the metastable state back to the ground state, but in general there is believed to be an energy barrier. In an active site which contributes to phosphorescence, there is believed to be a relatively small barrier such that the electron in the metastable state has an appreciable probability of surmounting the barrier at a given moment while the phosphor is maintained at room temperature. In this disclosure, where a phosphor is characteri"^'zed by a single particular type of luminescent activity, and characterized as having a particular number of active sites, that number should be understood as specifying the number of active sites which participate in the particular luminescent activity. Thus, a phosphor characterized as "phosphorescent" and as having a certain number of active sites has that number of active sites capable of participating in phosphorescence. Where a number of active sites is given without any characterization of activity, that number is the total nu-mber of active sites active in all forms of luminescence.

Certain phosphors known heretofore are based upon a crystalline matrix including alkaline earth elements such as strontium, calcium, or combinations thereof, and one or more σhalσogens selected from the group consisting of sulfur and selenium, together with one or more "activator" elements present in minor proportions. These phosphors are referred to herein as "activated alkaline earth/chalcogen phosphors."

A long-sought goal in the phosphor art has been to provide more potent and more efficient phosphors. Thus, it has been a long-standing goal in the phosphor art to increase the quantities of energy which can be absorbed and emitted by a given quantity of the phosphor.

The art heretofore has not been able to provide phosphors, and specifically, activated alkaline earth/chalcogen phosphors, with the desired number of active sites. Thus, although it is generally believed that the formation of active sites in activated alkaline earth/chalcogen phosphors, and the nature of the active sites so formed relate to the presence of activators in the system, merely increasing the amounts of activators present does not always result in an increase in the number of active sites or in the stimulation quantum efficiency of the phosphor. The activated alkaline earth/chalcogen phosphors available heretofore have incorporated no more than about 10¹⁷ active sites per cm ₃.

In particular there has been a need heretofore for improved phosphorescent phosphors. Disclosure of Invention

The present invention provides phosphorescent alkaline earth/chalσogen phosphors having activators including both europium and copper. Preferred phosphors according to this aspect of the invention are more potent than comparable phosphors of the prior art.

Phosphors according to this aspect of the present invention comprise a crystalline matrix including sulphur or a combination of sulphur and selenium, and strontium, and/or a combination of strontium and calcium. The phosphor also includes first and second activators consisting essentially of europium and copper, respectively, dispersed in the matrix. The activators and the matrix cooperatively define active sites. At room temperature, the active sites are capable of storing energy upon exposure of the phosphor to visible or ultraviolet exciting light and emitting the stored energy in the form of visible light after

• cessation of the exciting light. Each activator preferably is present in the matrix in an amount equal to between about 5 and about 500 parts per million by weight based on the weight of the matrix, and more preferably between about 10 and about 300 parts per million by weight. The preferred phosphorescent phosphors according to this aspect of the present invention typically have absorption spectra extending from about 220nm to about 540nm, with peaks at about 300-350nm and about 400-450nm. The emission spectrum typically is between about 500 to about 700nm, most preferably about 510 to 630nm, with an emission peak at about 550nm to about 600nm, most preferably at about 560-570nm. The emission spectrum can be controlled by controlling the composition of the phosphor. The preferred phosphors have other valuable properties such as stability, ability to withstand intense radiation without damage, and the like. Preferred phosphors according to this aspect of the present invention can absorb and store intense light typically in excess of 10⁵ lux, without damage, and can withstand infrared or ultraviolet radiation of about 6.4MW/cm² without damage. Thus these phosphors are rugged and well adapted to store energy from intense sources such as powerful lasers and the like. Preferred phosphors according to this aspect of the present invention include at least about 5 x 10¹⁷ active sites per cm³, and more preferably at least about

10¹⁸ active sites per cm³. Thus, the preferred phosphorescent phosphors according to this aspect of the present invention can emit by phosphorescence more than

5xl0¹⁷, typically more than 10¹⁸ photons/cm³.

A method of making phosphors according to the invention preferably includes the step of forming the crystalline matrix comprising S or S and Se together and an alkaline earth selected from the group consisting of Sr and combinations of Sr and Ca, and dispersing the Eu and Cu activators in the matrix. The matrix is exposed to a treating atmosphere comprising S vapor and/or Se vapor with any remainder most preferably consisting essentially of inert gas. This exposure occurs at an elevated exposure temperature sufficient to cause release of S and/or Se from the matrix and replacement thereof by components from the vapor. Following the exposing step, the exposed matrix is cooled to provide the finished phosphor. Preferably, the step of forming the matrix and dispersing the activator in the matrix includes the step of firing a mass of a starting material comprising a matrix-forming material which may include one or more mixtures of alkaline-earth sulfides and selenides, the starting material preferably also including the activators and one or more fluxes. In this case, the exposing step is performed during the firing step, and the firing step includes the step of heating the starting material to the exposure temperature. In processes according to these preferred methods, the firing step serves to convert the matrix- forming material in the mass into the final matrix by recrystallization, and the matrix is exposed to the treating atmosphere while it is formed by this recrystalliza ion process.

In the most preferred processes, appreciable concentrations of both S vapor and Se vapor are maintained in the treating atmosphere in contact with the matrix during the exposing step. The vapors in contact with the matrix may include at least some S and/or Se vapbr released from the mass, i.e., from the matrix or from the starting material. These vapors may be retained in contact with the matrix by keeping the mass in a closed or "semi-closed" vessel during firing. The term "semi-closed" vessel as used herein means a vessel which, although it allows some escape of vapors from the space within the vessel, nonetheless retards such escape. Chalcogen may be supplied from an exogenous source, i.e., a source other than the starting material or matrix. The exogenous source of sulphur and/or selenium may be disposed within the vessel along with the starting material.

The process can also be performed without using a closed or semi-closed vessel. Thus, where the appropriate treating atmosphere is maintained within a furnace, as by passing a gas containing the appropriate chalcogen vapor or vapors through the furnace, the starting material and/or matrix may. simply be disposed within the furnace. The treating atmosphere can be provided in the furnace by passing an inert gas through the furnace and placing chalcogen sources as aforesaid within the furnace, typically at or upstream of the starting material or matrix. Best Mode of Carrying Out Invention

Apparatus employed in one process according to the present invention includes an electrical furnace equipped . with a horizontal quartz tube. A first crucible and a second crucible fitted upside down within the first crucible cooperatively define a "semi-closed" vessel, i.e., a vessel which although not hermetically sealed nonetheless substantially retards escape of vapors from within the vessel. The chalcogen source is disposed in the bottom of the first crucible, whereas a pellet of starting material is supported within the vessel, above the chalcogen source, by a refractory tube, so that the starting material or pellet is out of contact with the chalcogen source. The first crucible is supported within the furnace on a refractory slab, the entire assemblage being disposed within the tube of the furnace.

In this process, the crucible, with the pellet of starting material and the chalcogen source are loaded into the furnace while the same is cool, typically at about room temperature. The furnace tube is evacuated and then purged with an inert gas before firing. In the firing step, the furnace tube is heated at a gradual rate to the desired exposure temperature, maintained substantially at the exposure temperature for a predetermined dwell time, and then rapidly^, cooled back , to about room temperature. During these steps, a gentle flow of the dry inert gas through the furnace tube is maintained. The furnace is arranged so that the entire boat and pellet are maintained within the "hot zone" of the furnace and hence maintained at substantially equal temperatures during the process.

As the pellet of starting material and the chalcogen source are heated to and maintained at the exposure temperature, processes of crystal growth and diffusion occur within the mass. These processes typically do not involve melting of the alkaline earth chalcogenides in the pellet. The pellet as a whole typically remains in a solid or semi-solid form. Crystal growth phenomena occurring during the firing step convert the original crystalline matrix of the alkaline earth chalcogenides into a new crystalline matrix. As the new matrix is formed, the activators originally present in the starting material are dispersed within the new matrix. These processes of crystal growth and diffusion continue from the time the temperature reaches a sufficient value during the _heating cycle, during the dwell period at the exposure temperature and during the initial portion of the cooling step, until they are substantially arrested as the temperature of the sample declines. Concomitantly with these processes, appreciable quantities of chalcogen vapor are released from the mass, i.e., from the new starting material or from the crystalline matrix as the same is formed. Release of chalcogens from the mass continues while the pellet is at or above the dissociation temperature of the alkaline earth chalcogenides. Such release tends to create vacancies within the crystal lattice of the newly-formed matrix. As the temperature increases during the firing step, the chalcogen source also begins to liberate significant quantities of chalcogen vapor. Thus, where this source is a substantially pure chalcogen, the rate of vapor liberation rises with increasing vapor pressure as the 'furnace temperature increases. Where the chalcogen source is a chalcogen compound such as alkaline earth chalcogenide, the liberation of chalcogen vapor from the source proceeds by -dissociation. A flux which melts at or below the exposure temperature may be added to an alkaline-earth chalcogenide to promote release of chalcogen vapors.

Chalcogen vapors liberated from the pellet of starting material and the chalcogen source mingle with one another in the inert gas thereby bathing the pellet in a treating atmosphere including a mixture of these vapors. Thus, as the new crystalline matrix is forming, it is in contact with substantial amounts of the vapors evolved both from the matrix-forming materials in the matrix and from the exogenous chalcogen source. The matrix accordingly absorbs chalcogen-containing species from the vapors, thereby filling the vacancies created by release of chalcogens from the mass. Stated another way, chalcogens from the matrix are replaced by the vapors.

The dwell time at the exposure temperature can be varied, with larger samples or pellets of starting material generally requiring greater dwell times. Typically, dwell times between about 10 and about 200 minutes can be employed, and dwell times between about 30 and about 90 minutes, most preferably about 60 minutes, are generally useful. These preferred dwell times are particularly useful with about one gram of starting material. Exposure temperatures above about 900^βC, preferably between about 900 and about 1200 " C can be employed, temperatures between about 950 and about 1150"C being more preferred, and about 975 to about 1075 " C being most preferred. Rapid cooling from the exposure temperatures at the end of the dwell period is highly desirable. Typically, the furnace tube, with the crucible inside, is moved out of the furnace after the dwell period and cooled by contact with the air, preferably with forced convection. The cooling rate desirably brings the crucible temperature from the exposure temperature to 600^βC in about one minute, to 300°C in about five minutes, and to about room temperature in about 20 minutes, in each case as measured from the end of the exposure temperature dwell period and start of the cooling period.

Any gas which is substantially inert to the starting material and to the resulting phosphor at the temperatures employed in the process may be used. Thus, nitrogen, argon, helium and the like may be employed.

The starting materials employed in the processes as described above include alkaline earth metals selected from the group consisting of strontium^' and mixtures of strontium and calcium and one or more chalcogens selected from the group consisting of sulphur and mixtures of sulphur and selenium. The preferred starting materials include one or more alkaline earth chalcogenides, i.e., SrS, SrSe, CaS and CaSe or combinations thereof. The strontium compounds are generally preferred.

In the preferred processes of the present^" invention, the crystalline matrix is formed by reσrystallization at temperatures below the melting points of the alkaline earth chalcogenides. In these processes, the starting material should include a flux. The flux preferably is substantially non-reactive with the alkaline earth chalcogenides and with the activators and also preferably is substantially insoluble in the crystalline matrix formed during the process. That is, the flux should have relatively little tendency to form solid solutions with the crystalline matrix during the recrystallization operation. Particular fluxes which are preferred in procedures according to the present invention include CaF₂, SrF₂ LiF, MgF₂/ NaCl, SrS04, SrCl₂, SrO and SrS0₃ and mixtures of these. Of these, fluxes which incorporate strontium or calcium cations are preferred. CaF₂ may be employed successfully either as a single flux or in conjunction with another flux such as SrS0₄.

The quantity of flux preferably is about 1%- to about 8% by weight, and more preferably, about 1% to about 6% by weight, based on the total weight of alkaline earth chalcogenides in the starting material. Depending on its composition, the flux may react during the firing step, as by reaction with the chalcogen vapors, or may be ionized and dispersed in the matrix to some extent. The term "flux residue" as used in this disclosure includes both a flux in its original state and the products of these decomposition processes.

The europium and copper activators are incorporated in the starting material as salt. Preferably, the europium is provided as the chloride or sulfate salt and the copper is provided as the chloride salt. The term "activator pair" as used herein refers to a system of two activators used together in a single phosphor. It is believed that the two activators in such a pair coact with one another and with the crystalline matrix to define active sites. Typically, in a given crystalline matrix, one activator, referred to herein as the^" "dominant" activator, appears to have a predominant influence on the emission wavelength exhibited by the active sites and hence by the phosphor as a whole whereas the other activator, referred to herein as the "auxiliary" activator, ordinarily has a lesser influence on the absorption and emission wavelengths but a greater influence on the probability of eventual photon emission at room temperature.

The combination of Eu and Cu activators, preferably without other activators, provides a phosphorescent phosphor. It is also possible, although generally not desired, to prepare phosphors with mixed infrared-stimulable and phosphorescent activity, as by including Cu in a mixture with Sm and/or Bi in an activator system which also includes Eu.

Preferred concentrations for the activators are about 5 to about 500 parts per million by weight of each activator based upon the total weight of alkaline earth chalcogenides in the starting material. Concentrations of about 10 to about 300 parts per million by weight, on the same basis, are particularly preferred.

With the Eu.Cu activator pairs, the emission radiant energy is in the visible range, within the range from orange to green. With this activator system, the emission color can be shifted towards the green end of the spectrum by increasing the selenium content of the phosphor, and towards the orange by ^* increasing the sulphur content. A phosphor between the orange and green extremes, having a yellow emission color and an emission peak at about 560-575nm is preferred, inasmuch as these wavelengths correspond substantially to the peak sensitivity of the dark-adapted human eye. With the activator systems, where the phosphor is prepared with strontium as the sole alkaline earth and without the use of an exogenous chalcogen ^" source, starting materials including about 20-40 weight % (25.8-48.1 mole %) strontium sulphide and 80-60 weight % (74.2-51.9 mole^* %) strontium selenide, are preferred, about 30 weight % (37.4 mole %) strontium sulphide and 70 weight % (62.6 mole %) strontium selenide being particularly preferred. Similar proportions, by mole %, are employed where calcium is included along with strontium. All of these percentages are based on the total alkaline earth chalcogenides. The molar ratio of sulphur to selenium in the finished phosphor typically will differ from the molar ratio in the starting material, particularly where an exogenous chalcogen source is employed. With the Eu:Cu activator system, the S:Se molar ratio in the finished phosphor preferably is at least about 1:10. In general, substitution of calcium for strontium in phosphor having the preferred activator systems, as by substituting calcium selenide for strontium selenide or calcium sulphide for strontium sulphide results in displacement of the emission spectrum towards longer wavelengths. With the Eu:Cu activator, amounts of calcium less than 90% by weight are preferred, less than 50% being more preferred, 0% calcium (100% strontium) being most preferred. As in other phosphor preparations, contaminants in minor amounts can affect the results achieved. Therefore, it is important to use the purest available starting reagents and to avoid contamination during the process. Reagents available commercially under purity grades "TMI 10" and "TMI 25" from Spex Industries, Inc. of Metuchen, N.J., USA generally are suitable for use in the process. The implements which contact the starting materials and/or the finished phosphor during the process should be formed from materials which are substantially inert to the system and which do not release any appreciable contaminants into the system. Alumina implements generally provide satisfactory results. • The inert gas used in the process should likewise be selected to avoid contamination.

Nitrogen containing no more than 2 ppm oxygen by volume and no more than 3 ppm water vapor by volume as supplied to the experimental apparatus typically is satisfactory. The solid reagents utilized in preparation of the phosphor preferably are ground, as by grinding in a ball mill with acetone and then evaporating the acetone.

After grinding, the reagents preferably are dried under an atmosphere of dried inert gas at a temperature of at least about 100^βc for several days, so as to assure that the reagents do not contain any appreciable moisture at the commencement of the firing step. The phosphors themselves are hygroscopic and can be damaged by moisture absorbed from the atmosphere. Accordingly, as ,with other alkaline earth/chalcogen phosphors, phosphors according to the present invention should be protected from atmospheric moisture, as by packaging in sealed containers, preferably with dessicants. Also, if the phosphors or the reagents are handled in the .laboratory atmosphere, the humidity of the laboratory desirably is_. minimized.

The examples set forth below illustrate certain features of the present invention. Unless otherwise stated, each of the precautions mentioned in the preceding paragraph were applied in all of these examples. Examples

The optical properties of the phosphors in the examples are measured using a Fluorolog instrument with a Scamp microprocessor control (both as supplied by Spex, Inc., Metuchen, N.J.), a 450 watt mercury vapor lamp light source, a thermoelectrically cooled Ga As photomultiplier tube with a dark count less than 30 photons per second and having a measurement range of 180-860nm. A GaAs infrared emitting diode with a mechanical shutter is employed as the stimulation source. The absorption spectrum is measured by exposing the sample to exciting radiant energy of lOnm bandwidth in a preselected region of the 200-nm-800-nm range. As t_he phosphor absorbs the exciting radiant energy, it typically starts to emit radiant energy by fluorescence at wavelengths close to its stimulated emission wavelengths. When the phosphor has absorbed all of the energy it can store, the brilliance of this fluorescence reaches a substantially constant value, indicating that the phosphor Is fully "charged." All samples are exposed to the exciting radiant energy for a time sufficient to reach this steady-state value, typically five -minutes. Example 1

A starting material comprising lOg SrSe, 0.3g calcium fluoride flux and activator compounds including 0.005g Eu₂(So₄)₃ and 0.002g CuCl₂ is prepared and blended ^" with 0.5g sulphur as an exogenous sulphur source. Pellets of the blended material each 13mm in diameter and weighing about 0.8g, are pressed under 3-4 tons pressure. The pellets thus contain both a selenide-based starting material and a sulphur exogenous chalcogen source. Each pellet is fired in a semi-closed vessel. The pellet is supported within the vessel by several alumina rods. The firing regime includes rapid heating at about 100^βC/min, followed by a 60 minute dwell at an exposure temperature of 1150"C with subsequent cooling to room temperature, likewise at a rapid rate so that the sample is cooled in about 15 minutes. Nitrogen flow through the furnace is continued throughout the heating, dwell and cooling periods. The resulting phosphor is phosphorescent. It has an absorption spectrum extending from about 220nm to about 540nm with peaks at 300-350nm in the UV and 400-450nm in the blue visible range. After exposure to exciting radiant energy within its absorption range, the material emits visible phosphorescence in the yellow region of the spectrum, with peak intensity between 560-570nm. The light sum of the material is measured by exposing the phosphor to intense exciting ultraviolet radiation for a time sufficient to fully saturate the phosphor, typically for at least about five minutes, and then measuring the total light emitted by the phosphor. The^" light sum of the phosphorescent emissions corresponds to about 10¹⁸ photons per cubic centimeter of phosphor, and hence indicates the presence in the phosphor of about 10¹⁸ active sites per cm³. Example 2

A procedure generally similar to Example 1 is followed, but using a different vessel. The starting material is prepared by emitting a total of 20 grams SrS and/or SrSe with 0.9 grams calcium fluoride and 0.9 grams strontium sulphate as fluxes with the same activator (0.005g Eu₂(So₄)₃ and 0.002g CuCl₂) as employed in Example 1. These ingredients are ground in a ball mill for 60 minutes, and a 750 gram aliquot is pressed into an 18 mm long 5 mm inside diameter alumina vessel which is capped and placed in the furnace and fired as in Example 1. The proportions of SrS to SrSe are varied.

With less than about 5 weight percent SrS, the material shows very weak phosphorescence and no detectable sti ulatability. With about ten weight percent SrS and 90% SrSe, there is a bright green phosphorescence. As the content of SrS is increased, the phosphorescence shifts to yellow at about 30 weight percent SrS and shifts progressively to orange as the proportion of SrS further increases. Notably, this activator system provides a vivid orange phosphorescence even at 100% SrS, where no Se is present in the system either as vapors or as part of the starting material. Industrial Applicability

Phosphors based on a crystalline matrix of an alkaline earth and a chalcogen together with an activator system are prepared by a process which preferably includes exposure to both S and Se vapors. The phosphors have enhanced light output. Eu:Cu activators provide phosphorescent phosphors. -1.6- Phosphorescent phosphors may be employed to provide a short-term steady glow, as an emergency illumination source or the like.

Claims

Cl ims

1. A phosphorescent phosphor comprising a crystalline matrix including sulphur or a combination of sulfur and selenium, one or more metals including Sr or combination of Sr and Ca, and first and second activators consisting essentially of Eu and Cu, respectively, said activators being dispersed in said matrix, said activators and said matrix cooperatively defining active sites capable of storing energy upon exposure of the phosphor to visible or ultraviolet exciting light and emitting said stored energy in the form of visible light after cessation of said exposure of said phosphor to said exciting visible light.

2. A phosphor as claimed in Claim 1 having at least about 5 x 10¹⁷ of said active sites per cm³.

3. A phosphor as claimed in Claim 2 having at least about 10¹⁸ of said active sites per cm³.

4. A phosphor as claimed in Claim 1 wherein said phosphor includes between about 5 and about 500 parts per million by weight of each of said activators based on the weight of said matrix.

5. A phosphor as claimed in Claim 4 wherein the molar ratio of Eu to Cu is about 1:1 to 5:1.

6. A phosphor as claimed in Claim 5 wherein said molar ratio is between about 2.5:1 and about 3:1.

7. A phosphor as claimed in Claim 1 further comprising a flux residue selected from the group consisting of CaF-, SrF₂, LiF, MgF₂, NaCl, SrS0₄, SrCl₂, SrO, SrS0₃, combinations thereof and ionization and reaction products thereof.

8. A phosphor as claimed in Claim 7 wherein said flux residue is present in said phosphor in an amount corresponding to between about 1% and about 8% by weight of said matrix.

9. A phosphor as claimed in Claim 1 wherein said one or more metals consists essentially of Sr.

10. A phosphor as claimed in Claim 9 wherein the molar ratio of S to Se is more than about 1:10.