Background of the Invention

This invention relates to cathodoluminescent phosphors and, more particularly, to yttrium aluminum garnet (YAG; Y ₃ A1 ₅ 0 ₁₂ ) phosphors.

Several techniques have been developed for the deposition of powder phosphor material onto the faceplates of cathode ray tubes (CRTs) . These techniques can be classified as settling, dusting, spraying and electrophoresis . Phosphor powder layers prepared by these techniques contain randomly distributed and randomly oriented contact areas between neighboring phosphor particles and between the particles 3nd the substrate. When the phosphor deposit is used in a CRT, heat generated by the electron beam is essentially dissipated by thermal conduction through these contact areas.

The thermal conduction process limits the brightness of the CRT. More specifically, in order to increase the brightness of a CRT display, the electron current density is increased, which in turn increases th'e temperature of the phosphor layer. Unfortunately, the cathodoluminescence of powder phosphors exhibit a thermal quenching effect at high temperatures. For example, in a YAG phosphor the cathodoluminescence is significantly reduced above a quench temperature of ~300°C. Thus, the amount of electron beam energy that can be usefully deposited in the ohosphor layer is limited by its thermal conductivity. In order to achieve CRTs of greater luminosity, therefore, phosphor layers of higher thermal conductivity are needed.

Summary of the Invention

A high temperature reaction is performed between YAG particles and sulfuric acid. The reaction produces sulfate compounds which serve as a binder between YAG particles themselves as well as between the

YAG particles and the substrate.

Detailed Description

We have found that YAG particles react with sulphuric acid at elevated temperatures to form sulphate compounds which bind the YAG particles to one another and to the substrate. As a result, the packing density of the YAG phosphor layer is increased; that is, the contact area between the particles and the substrate is increased, thereby enhancing the thermal conductivity of the phosphor layer. The cathodoluminescence is about

10% higher than that of YAG phosphors prepared by prior art settling techniques.

In general, our technique entails providing a

-suitable substrate, treating phosphor YAG particles with sulphuric acid, depositing the treated particles on the substrate, and heating to an elevated temperature in an oxygen ambient. In a preferred embodiment, the treated phosphor is preheated on the substrate at a lower temperature in air until dry before being subjected to the last heating step.

The YAG particles constitute a phosphor; that is, they contain a suitable activator (dopant) to generate luminescence when impacted by an electron beam.

Illustrative activators are Eu (red) and Ce (green) . Suitable substrates comprise transparent materials such as quartz, non-browning glass, and sapphire. The preheating may take place at temperatures ranging from about 100°C to 200°C. On the other hand, the last heating step should take place at higher temperatures effective to form the desired sulphate compounds.

Temperatures from 320°C to 780°C for time periods of 1 h to 2 h are acceptable. Similarly, a relatively wide

range of sulphuric acid solutions are acceptable: concentrated H ₂ so ₄ : to H ₂ 0 volume ratios can range from about 0.5 to 2 and diluted sulphuric acid to YAG weight ratios can range from about 0.25 to 1. Example I_

This example describes the fabrication of a YAG phosphor using, as a starting material, YAG phosphor particles designated P53 which is a powder commercially available from United Mineral and Chemical Co., N.Y., M.Y. (manufactured by Derby Luminescents, Middlesex,

England) . Electronic grade concentrated sulphuric acid ⁽ 95-97% H_so ₄ ) was diluted with deionized H ₂ 0 in a volume ratio of 1:1. The diluted H ₂ so ₄ solution was mixed with the phosphor particles in a weight ratio of 1:2 (i.e., 0.5) . The mixture was deposited uniformly on different transparent substrates, e.g., a quartz plate, non-browning glass and sapphire. The coated substrates were preheated to 150°C in air to dry off water, and then heated in an 0 ₂ ambient to a variety of elevated temperatures for 1 h to 2 h to complete the reaction. X-ray diffraction (XRD) analyses were performed on the treated phosphor powder to study the phase composition. Thermogravimetric analyses (TGA) ^' were made to study the weight changes during heating. TGA was performed using a commercially available thermal analysis system. Approximately 30 mg of the treated and dried phosphor YAG powders was loaded into a standard platinum boat, which was heated from 20°C to 900°C at a constant rate of 20°C per minute in a flowing air atmosphere. Weight loss measurements were recorded at 3 sec intervals. At the end of each run, computer analysis of the data gave the temperature and weight loss during the transition. p canning electron microscopy was used to study the icrostructures of the phosphor layer.

The cathodoluminescent efficiency of the deposited phosphor layers was also evaluated in a scanning electron microscope. The electron gun produced a beam density on the order of 10 ⁹ W/m ² . Electron energies were varied between 5 kV and 30 kV. The electron beam scanned the samples in a single line or in a 256 line raster having a 66 μsec period line. Phosphor layers deposited in accordance with our invention and other layers deposited by a conventional settling technique were measured. The layers prepared by both procedures had a thickness of about 75 μm. The phosphor layers were covered with a 120 nm thick Al coating to prevent charging of the phosphor and also to enhance the luminous output. Light generated in the phosphor layers was transmitted through the substrate and was focused by a lens to a photomultipler. The light intensity was measured with a commercially available radiometer.

Some of the phosphor layers were removed mechanically from the substrate so that their flexural strength could be measured by a well-known three point bending technique using a commercially available universal testing machine. Furthermore, P53 YAG powder was compacted at 10,000 psi (69 MPa) without any binder, and the flexural strength of these powder compacts was measured for comparison with our H-go ₄ _treated Y _AG phosphor layers.

XRD patterns were measured on a number of our

^H S0 -treated YAG phosphor layers, each heated 1.25 h in 0 _{2 a} t different temperatures: 1080°C, 997°C, 642°C, and 330°C. In contrast with the as-received commercial YAG powder, which exhibited a pattern characteristic only of its garnet structure, XRD data showed that our H-so - treated YAG phosphor layers had many extra non-garnet peaks with high intensities in samples heat-treated at 330°C and 642°C. We believe that these extra peaks are due to the sulfate compounds created by high temperature

reactions between the garnet phase and H-,go ₄ .

However, the sulfate compounds decomposed at higher temperatures greater than about 1000°C. For example, the sulfate peaks disappeared in samples heated at 1080°C. Samples heated at this temperature showed the garnet peaks only, and their ,XRD patterns became identical to those of the as-received YAG powder.

TGA analyses of our H ₂ so ₄ -treated YAG phosphor layers was done at temperatures between 25°C and 950°C. The specimens were dried in air at 150°C for 2 days prior to the TGA measurements. A significant amount of weight loss was detected between 240°C and 310°C, probably due to the dehydration of the H ₂ so ₄ . Little or no change in weight was detected between 314°C and 783°C. Sulfate compounds formed at this temperature range were rather stable. However, decomposition of the sulfate compounds between 780°C and 840°C led to a significant weight change at this temperature range. From the TGA data the weight loss at this temperature range was about 36% of the average weight between 400°C and 700°C. If we assume that part of the garnet particles reacted with H ₂ so ₄ to form xY ₃ Al ₅ (S0 ₄ ) ₁₂ . (1- ^χ)Y 3Al5θ _] _ at a temperature ranging from 400 ^c to 700 C, and at a higher temperature of >_ 900°C the sulfate compounds decomposed to the garnet phase, calculations show that x = 0.348 gives a weight loss of 36%. Thus, approximately 35% of the YAG particles reacted with

^H S0 ₄ to form some sulfate compounds. Such a high concentration of sulfate compounds in sample heat at 300°C to 700°C is consistent with the high XRD peak intensities of the sulfate compounds. The latter can take on forms other than Y ₃ 1 ₅ (S0 ₄ ) ₁₂ , such as Y ₂ (S0 ₄ ) ₃ and A1 ₂ (S0 ) ₃ , including hydrated forms thereof.

SEM micrograph were taken of YAG phosphor layer deposited by our technique on sapphire substrates and heat at 550°C for 1.25 h in 0-,. _0ne micrograph showed enhanced contact area between the phosphor layer

and the substrate, which allowed for efficient heat dissipation. Another micrograph of the same specimen, but with a tilt angle of 68.5 degree showed large contact area between phosphor particles. Yet another micrograph of the cross-section of the phosphor layer illustrated the 2 μm phosphor particles held together in a continuous network by some sulfate binder phases.

In contrast, when the H- _S o ₄ -treated Y _A G phosphor layer was heated to a higher temperature (e.g., 1100°C for 1.25 h in 0 ₂₎ _ _{a SEM} micrograph showed that the layer morphology became irregular and the particle packing lost its cohesion, probably due to the decomposition of the sulfate binder phases at temperatures above 783°C. Cathodoluminescence measurements of 75 μm thick layers at electron beam voltage of 15 keV and 30 keV showed about a 10% higher cathodoluminescence in the phosphor layer prepared by our technique as compared to layers prepared by the conventional settling technique.

Flexural strength measurements showed that our phosphor layers were also mechanically much stronger than those prepared by the conventional settling method. Example II We also reacted YAG particles with other acids, e.g., HNO- _f H ₃ Pθ4, HC1 and CH ₃ COOH at high temperatures using a technique similar to that described earlier. However, the use of these acids did not lead to the formation of coherent YAG layers on glass and sapphire substrates. Furthermore, the reaction of these acids with YAG particles decreased the cathodoluminescent efficiency of the phosphor layer.