Moisture insensitive electroluminescent phosphor

Abstract

A phosphor particle has thereon a moisture resistant treatment of a metallic nitride. By moisture resistant is meant a condition allowing the phosphor particle to function in a humid atmosphere for a significantly longer period of time than an untreated particle. The method of making such phosphors comprises the steps of introducing an inert gas into a reaction vessel; charging phosphor particles into the reaction vessel; heating the reaction vessel to a reaction temperature; introducing a nitride coating precursor into the reaction vessel; introducing a co-reactant into the reaction vessel; and maintaining the inert gas flow, co-reactant flow and precursor supply for a time sufficient to moisture-proof the phosphor particles. The nitride treated phosphor particles produced by this method, which can include the deposition of a nitride coating on the particles, have excellent efficacy ratings and strong luminance values in lamps after 100 hours use in high humidity (i.e., >95%).

Description

TECHNICAL FIELD

This invention relates to electroluminescent phosphors and more particularly to electroluminescent phosphors that have been treated to be moisture resistant. More particularly, this invention relates to electroluminescent phosphors having greatly reduced moisture absorption and greatly increased life and efficacy.

BACKGROUND ART

Treated phosphors are known from U.S. Pat. Nos. 4,585,673; 4,825,124; 5,080,928; 5,118,529; 5,156,885; 5,220,243; 5,244,750; and 5,418,062. It is known from some of the just-mentioned patents that a coating precursor and oxygen can be used to apply a protective coating. See, for example, U.S. Pat. Nos. 5,244,750 and 4,585,673. The treatment processes in several of the others of these patents employ chemical vapor deposition to apply a protective coating by hydrolysis. It also has been reported that chemical vapor deposition, at atmospheric pressure, can be used to deposit thin films of aluminum nitride coatings from hexakis(dimethylamido)dialuminum and ammonia precursors upon silicon, vitreous carbon and glass substrates. See, for example, “Atmospheric pressure chemical vapor deposition of aluminum nitride films at 200-250° C.”, Gordon, et al., Journal Material Resources, Vol. 6, No. 1, January 1991; and “Chemical vapor deposition of aluminum nitride thin films”, Gordon, et al., Journal Material Resources, Vol. 7, No. 7, July 1992. See, also, U.S. Pat. Nos. 5,139,825 and 5,178,911, Gordon, which also disclose transition metal nitrides and other metallic nitrides such as gallium and tin, respectively. U.S. Pat. No. 5,856,009 discloses a high temperature process (i.e., 300 to 700° C.) for applying a silicon nitride coating over a previously applied heat resistant coating on phosphor particles. It would be an advance in the art to provide a process for providing moisture resistant electroluminescent phosphors. It would be a further advance if that process operated in the absence of water or water vapor. It would be a further advance in the art to increase the efficacy and the life of such phosphors manufactured by such a process. It would be a still further advance in the art to provide a process that did not rely upon oxygen. It would be a still further advance in the art to provide an electroluminescent phosphor with a non-oxide coating such, for example, as a metallic nitride coating that is applied directly to the phosphor particles at a low temperature, i.e., about 100° C., so that the phosphor performance is not degraded.

DISCLOSURE OF INVENTION

It is, therefore, an object of the invention to obviate the disadvantages of the prior art.

It is another object of the invention to enhance the operation of moisture-resistant phosphors.

Yet another object of the invention is the provision of a method for providing moisture resistant phosphors that does not employ water or water vapor, or oxygen.

These objects are accomplished, in one aspect of the invention, by the provision of a phosphor particle having thereon a coating of a metallic nitride. The coating may be conformal to the particle surface. By conformal is meant a coating that follows the surface contours of the individual particles.

The objects additionally are accomplished by a process of preparing moisture resistant particles of electroluminescent phosphor, comprising the steps of: introducing an inert gas into a reaction vessel that is charged with phosphor particles; heating the reaction vessel to a reaction temperature; introducing a nitride coating precursor into the reaction vessel; introducing a co-reactant into the reaction vessel; and maintaining the inert gas flow, co-reactant flow and precursor supply for a time sufficient to make the phosphor particles moisture resistant.

The objects are further accomplished by the provision of a method of making moisture-resistant phosphors which comprises the steps of introducing an inert gas into a reaction vessel; charging phosphor particles into the reaction vessel; heating the reaction vessel to a reaction temperature; introducing a nitride coating precursor into the reaction vessel; introducing a co-reactant into the reaction vessel; and maintaining the inert gas flow, co-reactant flow and precursor supply for a time sufficient to coat the phosphor particles.

The nitrided phosphor particles produced by this method had excellent efficacy ratings and strong luminance values in lamps after 100 hours use in high humidity (i.e., >95%).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the precursor transport rate vs. reciprocal temperature;

FIG. 2 is a graph of the same data expressed as a vapor pressure curve; and

FIG. 3 is a graph of the precursor transport rate vs. the flow rate of the carrier gas.

BEST MODE FOR CARRYING OUT THE INVENTION

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.

In a preferred embodiment of the invention the coating reaction was carried out in a gas-fluidized bed reaction vessel that comprised a one inch O.D. (2.54 cm) glass tube with a coarse porosity, fitted glass disk as the gas distributor. The phosphor employed was a Type 723 electroluminescent phosphor (ZnS:Cu) available from Osram Sylvania Inc., Towanda, Pa. and the phosphor was fluidized by the injection of an inert gas such as nitrogen. The nitride coatings (which can contain amounts of hydrogen as well as the aluminum nitride) were formed via the reaction of ammonia with hexakis(dimethylamido)dialuminum (Al 2 (N(CH 3 ) 2 ) 6 ). However, there is no reason to believe that other organo-metallic nitrides would not work as well, particularly, for example those containing gallium or tin. The aluminum nitride precursor was obtained from Strem Chemicals, Newburyport, Mass., and contained within a stainless steel bubbler. The bubbler was maintained at 100° C. and the precursor was transported to the reaction vessel by a carrier of purified nitrogen. The precursor-entrained nitrogen was flowed upwards through the fritted glass distributor through lines that were maintained 20 to 30° C. above the temperature of the bubbler. The anhydrous ammonia coreactant, which was obtained from Matheson Chemicals, Gloucester, Mass., was passed through a Unit mass flow controller prior to entering the fluidized bed via a central glass tube having a fritted glass tip. The ammonia was diluted with purified nitrogen prior to entering the bed. Additionally, the nitrogen carrier was purified by passing through a Centorr purifier followed by a Matheson Nanochem gas purifier. The ammonia, also, was passed through a Nanochem purifier.

The gas handling system was constructed from stainless steel tubing and fittings. Glass-to-metal seals were employed between the glass reactor parts and the gas lines.

Four coating runs were made on a well-sealed system. The phosphor weight was 40 grams and the bubbler temperature was 110° C. in each run. The coating temperatures (i.e., the reaction vessel temperature), times and gas flows are shown in Table I.

TABLE I
	Temp.	Time	N 2 Carrier	NH 3	N 2 Diluent
Run No.	(° C.)	(hours)	Flow (sccm)	Flow (sccm)	Flow (sccm)
L2503-12	200	4.5	1000	200	300
L2503-13	150	5.0	500	100	150
L2503-14	150	20.0	250	200	100
L2503-16	225	12	500	100	150

Prior to the coating runs, the vapor pressure of the nitride precursor was determined at temperatures between 95 and 120° C. via transport measurements using as a carrier gas highly purified nitrogen flowing at 1000 sccm (standard cubic centimeter). Then, with a 100° C. bubbler temperature, the transport rate was determined with carrier flows ranging between 10 and 1000 sccm. The results are shown in FIG. 1 . FIG. 2 contains the same data expressed as a vapor pressure curve. The transport data obtained as a function of carrier flow with a 100° C. bubbler temperature are shown in FIG. 3 . The Figs. illustrate that the vapor pressures are high enough to make the use of a bubbler a practical means of delivering the chemical to the fluidized bed reaction vessel. The linearity of the transport data versus the flow curve, over two orders of magnitude (between 10 and 1000 sccm N 2 ), also indicates the suitability of this mode of precursor delivery.

The aluminum content, expressed as a percentage of total sample weight (% Al), B.E.T. surface area (S.A.(m 2 /gm)), percent coverage (% coverage) from Electron Spectroscopy for Chemical Analysis (ESCA) and approximate coating thickness, from Sputtered Neutral Mass Spectroscopy (SNMS) measurements vs. SiO 2 as a reference material, are shown in Table II.

TABLE II
Run No.	% Al	S.A. (m 2 /g)	% Coverage	Thickness (Å)
L2503-12	2.9	0.07	99	2700
L2503-13	1.5	0.05	98	800
L2503-14	2.5	0.06	99	2200
L2503-16	3.3	0.05	100	4300

Comparing the data in Tables 1 and 2 and FIG. 1 , it will be seen that substantially all of the precursor reacts within the fluidized bed to form a coating which covers practically all of the phosphor particles. X-ray photoelectron spectroscopy (XPS) surface analysis shows a surface oxygen concentration, a result that is in agreement with the well known surface reactivity of CVD-deposited aluminum nitride. However, SNMS analyses of the coated phosphors has indicated no apparent correlation between the relatively low oxygen signal levels and those of Zn, S, Al, and N, suggesting a relatively constant oxygen background that is not specifically associated with the aluminum nitride coating. Further, as shown in Table III, EDS analyses indicated relative oxygen concentrations comparable to that found in a sample of pure AlN.

TABLE III
Atomic Composition From EDS (%)
	Run No.	Al	N	O	Zn	S
	L2503-12	16	71	5.1	4.3	2.6
	L2503-14	18	70	3.6	4.5	3.3
	L2503-16	20	70	2.8	4.1	1.8
	Pure AlN	28	67	4.8	—	—

Electroluminescent lamps were made containing uncoated phosphor as well as coated phosphors from each of the Runs. The lamps were packaged in Mylar, a water-permeable material, so that the moisture sensitivity of the various materials could be determined and compared. Identical lamps were operated at 100 V and 400 Hz in two environments; with less than 10% relative humidity and with more than 95% relative humidity. The efficacy (in lumens per watt) was also determined. These results are summarized in Table IV.

	TABLE IV
		Luminance (fL)		Luminance (fL) with
	Efficacy	with <10% R.H.		>95% R.H.
Run No.	(Lumens/W)	0 hr.	24 hr.	100 hr.	0 hr.	24 hr.	100 hr.
L2503-12	5.26	22.6	22.0	20.0	23.0	23.6	17.6
L2503-13	4.23	26.0	24.9	22.0	26.2	26.4	6.1
L2503-14	4.26	22.6	22.1	19.4	22.4	21.9	17.5
L2503-16	5.90	22.8	21.7	19.5	22.9	23.5	21.9
Uncoated	1.75	29.9	31.6	24.9	30.5	10.0	3.7

The lamp performance data clearly show the advantages of the metallic nitride coating when properly applied. The comparison with the uncoated phosphor, whose performance falls off drastically in a humid environment after 100 hours, and that of the adequately coated materials, such as L2503-12, L2503-14 and L2503-16, is readily apparent. Even a coated material (L2503-13) without an adequate cover (note from Table II that this latter material has only 98% coverage and a thickness of 800 Å) does not fare well in the harsh environment.

Accordingly, there is here provided an electroluminescent phosphor that has good efficacy, long life and a suitability for use in a humid environment.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.

Claims

1. In a process of preparing moisture resistant particles of electroluminescent phosphor, the steps comprising:introducing an inert gas into a reaction vessel that is charged with phosphor particles; heating said reaction vessel to a reaction temperature; introducing a nitride precursor into said reaction vessel; introducing a co-reactant into said reaction vessel; and maintaining said inert gas flow, co-reactant flow and precursor supply for a time sufficient to make said phosphor particles moisture resistant wherein said nitride precursor includes a metal-nitrogen bond.

2. A method of encapsulating phosphor particles comprising:providing a bed of activated zinc sulfide electroluminescent phosphor particles, each of which exhibits humidity-accelerated decay; providing one or more precursors wherein at least one precursor has a metal-nitrogen bond; and exposing the bed of phosphor particles to the precursors such that the precursors chemically react and encapsulate the phosphor particles with a coating, wherein said coating is sufficiently encapsulating to provide the phosphor particles with reduced sensitivity to humidity-accelerated decay as compared to the uncoated phosphor.

3. A method as set forth in claim 2 wherein said coating includes metal- nitrogen bonds.

4. A method as set forth in claim 2 wherein said at least one precursor with a metal-nitrogen bond provides at least a single source of metal and nitrogen for said coating.