JPH0379693A - High-performance luminiscent material for conversion to increased optical intensity and its manufacture - Google Patents

Abstract

PURPOSE: To obtain the subject material with enough electron trap depth and density by providing a base material such as strontium sulfide, the first dopant being samarium and the second dopant being europium compd.

CONSTITUTION: The objective material is obtd. by including a base material (A) formed of a mixture of strontium sulfide and calcium sulfide, the first dopant (B) being samarium and the second dopant (C) being a europium compd. selected pref. from a group of europium oxide, europium fluoride, europium chloride and europium sulfide.

COPYRIGHT: (C)1991,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention This invention relates to electron trapping optical materials and methods of making and using such materials.

[Conventional technology]

It is instructive to review its history in order to define related material groups, especially as there is sometimes confusion about academic terminology. Term luminescence
e) It is important to start with the ability of certain solid bodies to emit light under different conditions.

Luminescence is a long-known natural phenomenon dating back to ancient history. Seebeck and Beffler observed a momentary visible afterglow in some materials. 188
In 1999, Kratt and Leonard also observed some effects with infrared radiation. During this period, terms such as "phosphor" and "luminescence" appeared. In 1904, Daamus made a clear distinction between "stlmulatlon" J, meaning to induce or stop afterglow, and "quenching" J, meaning to induce or stop afterglow.

Much of the latter work is associated with Leonard, who
He won the 1905 Nobel Prize in physics for his work on cathode rays. He researched different phosphors until at least 1918.

The latter study spanned from 1000 to 1934 and was discovered by Urbach. These old scientific sonic scientists were basically looking at very small luminescence effects.

In 1941, a program was set up by the National Defense Committee for the development of luminescent phosphors. The study started at Rotinister University and other institutes participated. However, this project ended with World War II.

The following technical papers were published on this research between 1946 and 1949.

B O'Br1en, r
"Development of Infrared Phosphors", Optical Society of America (J, op.
t, Soc.

or Am), Volume 38, Published July 1946, i
Page 369: F. Urbach
et al., “Infrared Sensitive Sac, of A+a) + Volume 36, Published July 1948, Page 372; G. Ponda, ``Production and Properties of Infrared Sensitive Zinc Sulfide'', Optical Society of America (J .

Opt, Soc, of' Am), No. 88@, 1
94 [Published July 1, p. 382; N. L. Smith (A, L, Sm1th),
``Production and properties of strontium selenide as a base material for phosphors stimulated by infrared radiation''.

Journal or the American Chemical Society
Am, Chew.

Soc, ) + Volume 69, Published 1947, No. 172
Page 5; “Fabrication and Properties of Solid Luminescent Materials” 1 Editor Niji R. Ponda (G, R, Ponda)
and F. Seitz (F, 5e1tz), Joan Riley and Sanz, Inc. (
John Wllley & 5ons, Inc.), New York, 1948.

These papers provide historical stories about the materials studied. After a few decades, its purpose was forgotten by most physicists. The focus was solely on research in the field of cathodoluminescence of cathode ray tube screens and fluorescent lamps.

Accordingly, the field of luminescence is broad and concerns the ability of certain substances or materials to emit light when driven by an external energy source. When the driving energy source is light,
The correct term is photoluminescence.

The most relevant class of materials was published in ``Visible Luminescence under Infrared Excitation'', Vol. 8, International Conference on Luminescence, Leningrad, August 1972.
On page B, J.L. Samadjik ('',■,.

Susmerdlgk) and Ni Prill (A, Br.
1ll) is a material in which electrons can be stored in a "trap" for various lengths of time. In the case of a deep trap, the trapped electrons are It can later be released by a photon with energy similar to that of the photon.

Heat release is negligible for deep traps. In these situations, the light is "stored" for later use.
ored) J, and it is likely that visible light emission can be excited by infrared radiation. In the case of narrow traps, spontaneous emission occurs at room temperature because thermal motion is sufficient to excite the electrons from the trap. These materials are currently called electron trapping optical materials.

The principle of electron trapping material is as follows. A host crystal is a wide bandgap semiconductor (II-Vl) compound that does not always have any specific value. However, these crystals are doped with many impurities to create new energy levels and bands. Impurities from the lanthanide (rare earth) system are some elements that can be accommodated within the lattice to form "communication" bands and trapping levels. The new communication band provides an energy band in which untrapped electrons can interact. The still low energy trapping level causes non-communication (n
on-co+gmunlcating) location.

Latent luminescent materials often contain one or more types of sites in which electrons are trapped in an energized state.

When applying energizing radiation of the appropriate wavelength, such as visible light or X-rays, such locations become filled with electrons.

The energizing radiation results in electrons being energized up through the communication band, through which transfers such as absorption and recombination take place. Upon removal of the energizing radiation, the electrons either become trapped at a higher energy level than their original ground state, or fall back to their original ground state. The number of trapped electrons is highly dependent on the composition of the photoluminescent material and the dopants used therein.

If the trapping level is sufficiently below the level of the communication band, the electrons within them will be separated from each other and remain trapped for a long time and will not be affected by normal ambient temperature.

In fact, if the trap depth is sufficient, electrons will remain trapped almost indefinitely unless they are excited by a certain light energy or by a thermal energy much higher than room temperature. .

The electrons are exposed to light or other radiation that is sufficient to cause them to escape from the trap and to release visible light photons, bringing them back up to the level of the communication band where transfer takes place in the form of recombination. It remains trapped until it is released. The material must be such that room temperature thermal energy is insufficient to cause any significant portion of trapped electrons to escape from their traps. As used in this application, "opHcal energy" includes visible light, infrared light and ultraviolet light; unless otherwise specified, photoluminescent materials have the above-mentioned properties. It is a material that has

Although various photoluminescent materials have been known, their properties have been less than desirable.

For example, photoluminescent materials have been used to position infrared beams by outputting visible light when placing the material within the infrared beam, but previous such photoluminescent materials have Not sensitive enough to detect low levels of infrared radiation. The visible light output by such materials is often at such low levels that visible light is difficult to detect. Furthermore, since such materials generally have electron traps with insufficient depth and/or a relatively low density of electron traps,
It is difficult to maintain trapped electrons for a long time. The ratio of optical input energy to optical output energy for such materials is often very high. That is, a relatively large amount of energy must be applied to the material to provide a given output of light energy. The development of photoluminescent materials that avoid or minimize the above disadvantages would open up many other applications for many materials.

[Problem to be solved by the invention]

The main object of the invention is to provide new photoluminescent materials which avoid or minimize the above-mentioned disadvantages.

A more particular object of the present invention is to provide a photoluminescent material having sufficient electron trap depth and sufficient density of electron traps to be useful as an optical memory material.

Yet another object of the present invention is to provide a photoluminescent material that emits visible light when exposed to infrared radiation.

Another object of the invention is to provide a photoluminescent material that outputs high intensity visible light.

Yet another object of the present invention is to provide a new and improved method for making photoluminescent materials.

[Means to solve the problem]

The above and other objects of the present invention, which will become more apparent as the description progresses, are such that two base materials, a first dopant, a second dopant, preferably a third dopant, and an optional soluble salt are provided. High performance electron trapping is realized by essentially consisting of photoluminescent materials. More particularly, the photoluminescent material comprises two base metals selected from the group of alkaline earth metal sulfides and alkaline earth metal selenides; a first dopant of samarium; europium oxide, europium fluoride. a second dopant selected from the group of , europium chloride, europium sulfide; a third dopant of similar composition of cerium; and a soluble salt of up to 10 parts for every 100 parts of the Heath material.

An important improvement in photoluminescent materials results from the unique use of a base material comprising a mixture of two base materials. Additional improvements result from the use of a third dopant.

The invention further comprises a photoluminescent material as described above disposed on a substrate. Optionally, the photoluminescent material is
Applied onto the substrate by the use of a transparent binder.

Mixing the photoluminescent material with a transparent binder does not significantly affect the optical properties of the photoluminescent material. In this application, "a base material consisting essentially of a base material, a first dopant, a second dopant, a third dopant,
References to ``consisting of soluble salts'' should be construed to include these materials alone or in combination with transparent binders.

The present invention is also suitable for depositing in thin film form on a suitable substrate such as sapphire or alumina, and
The photoluminescent material described above is provided heated to a moderate temperature, such as °C. The surface crystals so developed display the photoluminescent properties mentioned above. In this application, regarding "consisting essentially of a base material, a first dopant, a second dopant, a third dopant, and a soluble salt", it refers to "consisting essentially of a base material, a first dopant, a second dopant, a third dopant, and a soluble salt". shall be construed as including combinations of the above crystalline forms.

The method of the present invention comprises mixing two base materials such as strontium sulfide and calcium sulfide, a first dopant of samarium, a second dopant of europium compound and a third dopant of cerium compound; heating the resulting mixture to a temperature sufficient to melt and allow diffusion of the different dopants. This first heating step takes place in an inert atmosphere and in a furnace at a temperature of 950° C. or higher.

Still other processing methods include grinding the obtained solid material into a powder, and after grinding, the obtained powder is heated to a temperature lower than the first temperature, but
reheating crystal defects within the powder to a second temperature high enough to repair, thereby creating an electron trapping optical material without melting the powder into a mass. This milling step produces a powder with a particle size of less than 100 microns. The method further includes mixing the optical material and a transparent binder and applying the same to the substrate.

Other processing methods involve creating an initial chunk of fused material.
), which can be deposited on a smooth foreign substrate (fore.
reheating the resulting film to a temperature below the first temperature but high enough to form a surface crystalline film, thereby forming an electron-trapping optical film with high optical resolution. including the step of producing a material. The thickness of this film may range from 2 to 10 microns.

These and other features of the invention will be more readily understood when considering the following detailed description in conjunction with the accompanying drawings. In these figures, similar characters all represent similar parts in the several figures.

〔Example〕

FIG. 1 shows the principle of operation of the invention. The basic polycrystalline photoluminescent material used has a full electronic valence span in the ground state.
) has G. This material receives visible light that serves to energize certain electrons within the valence spunt G. The electron shown on the left is initially in the valence punt G,
Then, it receives visible charged light. Electrons absorb photons and increase their energy level to communication band E, where communication with other energized electrons occurs and transfer occurs. As a result of the energizing light, the electrons either fall back down to the trapping level T or return to the valence spunt G depending on the composition of the material and the available trapping locations. When the electron is at the trapping level T, it remains isolated from other electrons and is trapped until sufficient external energy is applied to the electron to raise it back up to the communication band E.

As shown on the right side of Figure 1, an electron may be stimulated by infrared electromagnetic energy to bring it back up to communication band E, where it interacts with other electrons and It then descends to band G again, and in this process outputs photons of visible light. The material of the present invention operates according to the principle illustrated by FIG. The number of trapping locations, the depth of the trap, and the possibility of transitions occurring within the communication band all depend on the composition of the photoluminescent material used. There is.

As mentioned above, the photoluminescent material of the present invention comprises two base material compositions, a first dopant, a second dopant ε, and a third dopant. Soluble salts, such as lithium fluoride, are useful in that they promote melting of the material quickly and at low temperatures.

The base material may be selected from the group of alkaline earth metal sulphides and alkaline earth metal selenides. Sulfides such as strontium sulfide and calcium sulfide are
It is preferred over selenide due to its toxicity. The use of a mixture of two alkaline earth metal sulfides as the base material significantly improved the light output, as discussed below.

The first dopant is samarium, the second dopant is selected from the group of europium oxide, europium fluoride, europium chloride, europium sulfide, and the m3 dopant is cerium oxide, cerium fluoride, cerium chloride, It may be cerium sulfide. Preferred dopants are europium oxide and cerium sulfide.

These dopants provide the best results and work together most easily. The use of a third dopant is not absolutely essential; however, it provides a performance improvement of about 10% over materials with only the other two dopants.

The photoluminescent material for outputting orange light is made from a mixture having the following composition: strontium sulfide 69 parts calcium sulfide
25 parts samarium 150p, p
, ■Europium oxide 500p, p, mono cerium sulfide 500p, p, m lithium fluoride
5.7 Parts As used above and throughout this application, "parts" and rp, p, s J refer to parts of weight unless specified otherwise.

One of the important discoveries of this composition arises from the fact that the light output and therefore the performance is greatly improved by the correct mixing of the two sulfides, as shown in FIG. The two sulfides clearly form new crystals (ternary compounds). This fundamental change in the base crystal significantly improves the performance of the optical material. This improvement is evident when the samarium/europium impurity pair is used without the cerium compound. Adoption of cerium compounds further improves performance.

The concentration of soluble salts, such as lithium fluoride, is not critical. Soluble salts can be completely omitted if high melting temperatures and long melting times are accepted.

The mixture is placed under a dry nitrogen atmosphere (obtained from a liquid source) or
Placed in a graphite crucible in a furnace filled with another dry active atmosphere such as argon, the temperature between 1100°C and 1400°C (preferably 1200°C) is such that the melt is formed.
℃ for 30 minutes to 1 hour).

For long heating times, the melt is formed at temperatures below 950°C. To form such a melt in a short time, temperatures above 2000° C. can be used. The use of lithium fluoride has a significant effect on melting time and temperature.

After cooling, the material can be used with infrared-sensing cards, optical screens that require moderate resolution (opttcatviewin
g5clean), optical signal upconversion, or - when used in applications where numerically, resolution is not critical, the melt can be converted from IO to 1 using standard techniques.
It is ground into a fine powder with a particle size of 0.00 microns.

After milling, the powder material is heated to about 300°C to 700°C (preferably 800°C) in a graphite crucible in a nitrogen or other inert atmosphere furnace. This second heating is below the melting temperature of the material (approximately 700°C) and is between IO and 80°C.
(preferably 30 minutes).

This second heating step relieves internal stresses and restores damage done to the crystal during the milling step.

After the second heating, the material is cooled and the powdered material is then mixed with a suitable binder or medium such as acrylic, polypropylene or other organic polymer.

After the material is mixed with a clear binder, it is applied to the substrate as a thin coating. The coating of photoluminescent material on the substrate is preferably between 1 and 500 microns thick. Depending on how the material is used, the substrate can be transparent plastic, aluminum oxide, glass, paper, or almost any other solid material.

Special mention should be made of the melting of particles on certain substrates at high temperatures.

In this respect, aluminum oxide (alumina) and its crystalline form, sapphire, are of particular importance. For example, dispersing photosensitive particles onto a sapphire substrate and then heating the composition to about 1000° C. in a dry atmosphere further improves the efficiency of the optical material and increases its usability. Alumina or sapphire substrates also allow conduction of luminescence from both sides of the substrate. Conversely, if fusion is performed with a quartz substrate, the optical properties deteriorate. - within the membrane;
Substrates containing silicon tend to destroy the efficiency of the material above about 800°C.

Milling is not used for thin film applications. Here, the initially fused material is deposited in chunks as a deposition source, for example by vapor deposition or sputtering.
used for.

The dopants samarium, europium and cerium in the above mixture are used to establish communication bands and electron trapping levels. Preferably, 150 p, p, +g of samarium is used, but the samarium can optionally be between 50 p, p, +g and 500 p, p, m, depending on the particular application. The concentration of europium and cerium compounds may be between 100 p, p, - and 1000 p, p, l, preferably between 200 p, p, gg and 600 p, p, w, preferably 500 p, p is the optimal value.

The mixture obtained from the above method provides an electron trap depth of about 1.1 to 1.2 electron volts below the communication band and has an output vector as shown in FIG. The figure shows that the center frequency of the output has a wavelength of about 820 nanometers, corresponding to orange light.

FIG. 2 shows the relative performance of optical materials as a function of strontium sulfide to calcium sulfide ratio.

Although the examples given describe specific ratios,
Functional operating characteristics derived from precise numbers provide further improved optical performance. It is the basic teaching of this disclosure that by mixing strontium sulfide and calcium sulfide, performance is improved over a wide range of ratios. At 100% strontium sulfide, the intensity is about 70% of the peak intensity experienced in the example. Therefore, the range of strontium sulfide can be between about 45% and 100%.

FIG. 4 shows photoluminescent material deposited onto a substrate forming device IO.

As shown, device 10 includes a substrate 12 and a substrate 12.
A card shown in longitudinal section with a photoluminescent material 14 coated with a transparent binder. Substrate 12 may be paper, aluminum oxide, plastic such as PvC, or other solid material. The substrate may be transparent or opaque. A reflective surface or bright color is preferred if it is opaque.

Material 14 establishes plane 1B. An optically transparent coating 18 may surround material 14 and substrate 12.

The card or device 10 of FIG. 4 is useful for arranging scientific and industrial metrology operations with infrared beams. In operation, card 10 is "charged up" by exposure of photoluminescent material 14 to a visible light source such that electrons are excited and trapped.
J is done. The card 10 is then placed in front of an infrared light source. The photoluminescent material 14 outputs visible light where the infrared beam hits the material, allowing the user to accurately identify where other invisible beams of infrared radiation are located. Accordingly, card 10 is used to test scientific or industrial instruments.

Advantageously, the materials of the embodiments discussed in this application and other materials provide relatively high light output when stimulated with infrared light so that the card can be used with a visible light background. The light output due to free electrons within the photoluminescent material 14 is visible despite the background light.

Screens that display infrared information as visible light patterns can be made by the same method.

Alternatively, the listed materials (within the ranges specified for the examples above) may be used by physical and chemical vapor deposition (vapo).
r dcpositon) (such as evaporation or sputtering), gaseous expansion, ion beam deposition, molecular beam deposition, and electron beam deposition onto aluminum oxide substrates of various shapes. The materials and substrate are placed in a furnace and fused under example conditions. This material may also be fused without the lithium fluoride of the example. Since the photoluminescent material adheres well to aluminum oxide, there is no need to use a separate binder or medium.

The construct resulting from the fusion step may optionally be wrapped in transparent plastic to realize the device IO of FIG. 4. No milling or additional reheating is necessary. This method is also applied to aluminum oxide disks to provide erasable optical memories, which can be coated with transparent plastic or with various optical layers.

The substrate can be aluminum oxide in sapphire or ceramic form. In this case, the layer of optical memory can be thinner than 0.5 microns, which is obtained with optical materials of fine quality type.

The optical materials of the present invention may be used as memory by using infrared sensing, photography and/or their electronic trapping properties. This material may be used in powder form (thick film) and in fine form (thin III).

Although various specific details have been discussed herein, it is to be understood that these are for purposes of illustration. Various modifications and changes will be apparent to those skilled in the art. The scope of the invention should, therefore, be determined by reference to the claims appended hereto.

[Brief explanation of drawings]

1 is a schematic diagram illustrating the principle of operation of the invention; FIG. 2 is a graphical representation of radiance as a function of the percentage of strontium sulfide in the base material; and FIG. , a graphical illustration showing the spectrum of light output by an optical material according to the present invention. FIG. 4 is a cross-sectional view of a photoluminescent material deposited on a substrate. 10...Card t2...Base 14
... Photoluminescent material 16 ... Plane 18 ...
・Coating procedure correction type (Method) (Patent Office Examiner 1, Case Indication 1999 2, Title of Invention Patent No. 111939 High performance photoluminescent material for optical upconversion and method for making the same 3゜4゜Relationship name with the person making the amendment

Claims (16)

[Claims] (1) An electron-trapping photoluminescent material, comprising: a base material formed of a mixture of strontium sulfide and calcium sulfide; a first dopant of samarium; and a second dopant of a europium compound. photoluminescent material. 2. The photoluminescent material of claim 1 further comprising a third dopant of a cerium compound. (3) The photoluminescent material of claim 1, further comprising a soluble salt. (4) Europium and cerium compound concentration is 200p
．． p. m and 600p. p. 3. The photoluminescent material of claim 2, wherein the photoluminescent material is between m. 5. The photoluminescent material of claim 1, wherein said material is deposited onto a substrate. (6) The photoluminescent material according to claim 5, wherein the substrate is alumina. (7) A method of making a photoluminescent material, comprising: mixing a base material comprising a mixture of strontium sulfide and calcium sulfide; a first dopant of samarium; a second dopant of a europium compound; A method characterized in that it comprises the steps of: heating the resulting mixture to a first temperature in order to form a mixture; and cooling the melt such that the mixture is in crystalline form. (8) milling the resulting crystalline form into a powder; reheating the resulting powder to a second temperature below the first temperature but high enough to restore the crystalline edges of the powder; 8. The method of claim 7, further comprising the steps of: producing an electron trapping optical material without melting the powder into a mass. 9. The method of claim 8, further comprising the step of: (9) mixing the optical material with a transparent binder. 10. The method of claim 9, further comprising the step of: (10) encasing the substrate and optical material with a transparent coating. (11) depositing an optical material onto the alumina substrate;
9. The method of claim 8, further comprising the step of: and heating the substrate to about 1000<0>C in a dry atmosphere. 8. The method of claim 7, further comprising: (12) depositing the optical material onto the alumina substrate using evaporation or sputtering techniques to form a thin film. 8. The method of claim 7, further comprising the step of: (13) including in the mixture a third dopant of a cerium compound. (14) the base material is formed of a mixture of strontium sulfide and calcium sulfide, and the percentage of strontium sulfide in the mixture is between 45 percent and 100 percent, and the second dopant is europium oxide. and the third dopant comprises cerium sulfide. (15) A method of providing a photoluminescent material on an alumina substrate comprises mixing a base material comprising a mixture of strontium sulfide and calcium sulfide; a first dopant of samarium; and a second dopant of a europium compound. depositing the mixed material onto an alumina substrate; heating the resulting mixture and the substrate to a first temperature sufficient to melt the mixture into a crystalline form on the substrate; How to characterize it. 16. The method of claim 15, further comprising the step of: (16) including in the mixture a third dopant of a cerium compound.