Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
1998-10-13
2002-10-01
Chu, John S. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C428S403000, C428S404000, C428S690000, C428S917000, C313S512000, C313S503000, C257S100000, C427S066000
Reexamination Certificate
active
06458512
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electroluminescent phosphor particles, particularly, to phosphor particles which are encapsulated in a moisture resistant coating and exhibit high electroluminescent brightness and, even more particularly to such an electroluminescent phosphor particle encapsulated with a metal oxynitride protective coating having improved electrical, chemical, thermal-mechanical, or surface characteristics. The present invention also relates to a method for making such encapsulated phosphor particles.
BACKGROUND
Phosphor particles are used in a variety of applications such as flat panel displays and decorations, cathode ray tubes, and fluorescent lighting fixtures. Luminescence or light emission by phosphor particles may be stimulated by application of various forms of energy including electric fields (electroluminescence). Electroluminescent (“EL”) phosphors have significant commercial importance. The luminescent brightness of such phosphors and the “maintenance” of this, brightness are two criteria typically used to characterize phosphor particles.
Luminescent brightness is typically reported as a quantity of light emitted by the subject phosphor when excited. Because of the sensitivity of phosphor emission brightness to varying conditions of excitement, it is often useful to report the brightness of phosphors as relative brightness rather than as absolute brightness. “Maintenance” refers to the rate at which phosphors lose brightness (i.e., decay) with operating time. The rate of decay is substantially increased if the phosphor particles are subjected to conditions of high humidity while being operated. This effect of moisture or high humidity has been referred to as “humidity-accelerated decay”.
Particulate EL phosphors are most commonly used in thick film constructions. These devices typically include a layer of an organic material. having a high dielectric constant which forms a matrix for a load of phosphor particles. Such layers are typically coated on a plastic substrate having a transparent front electrode. A rear electrode is typically applied to the backside of the phosphor layer, with a dielectric layer sandwiched there between. When an electric field is applied across the electrodes, the proximate portions of the layer emit light as the phosphor particles therein are excited.
Organic matrices and substrate materials, as well as organic coatings applied to individual particles, have typically been ineffective in preventing the decay of brightness caused by the diffusion of water vapor to the phosphor particles. For this reason, thick film electroluminescent devices have been encased in relatively thick envelopes, e.g., 25 to 125 microns, of moisture-resistant polymeric materials. However, such envelopes are typically expensive, result in unlit borders, and have the potential of delaminating, for example, under heat.
To improve their moisture resistance, phosphor particles have been encapsulated in an inorganic coating, such as a coating of a metal oxide. Inorganic coating techniques have been employed with varying degrees of success. Hydrolysis-based processes for encapsulating EL phosphor particles in an inorganic coating, e.g., hydrolysis-based chemical vapor deposition (CVD), have typically been the most successful. In hydrolysis-based CVD processes, water and oxide precursors are used to form the protective coating. Such hydrolysis-based CVD processes have been able to produce moisture insensitive encapsulated phosphor particles, while minimizing process related phosphor damage and retaining a high initial luminescent brightness.
Thus, existing coatings on phosphors provide protection against humidity accelerated decay, but there is a continuing need for new coating materials which provide protection from moisture, and which provide improved electrical, chemical, thermal-mechanical, or surface characteristics.
SUMMARY OF THE INVENTION
The present invention provides novel encapsulated phosphor particles, each having a substantially transparent metal oxynitride coating. The encapsulated phosphors exhibit a reduced sensitivity to humidity-accelerated decay of luminescent brightness. Additionally, the present invention involves a method which results in the encapsulated particles.
Each encapsulated particle of the present invention includes a phosphor particle of an electroluminescent phosphor material which exhibits humidity-accelerated decay in the presence of moisture. The phosphor particle is coated with a substantially transparent metal oxynitride. The metal oxynitride sufficiently encapsulates the phosphor particle to provide the particle with reduced sensitivity to humidity-accelerated decay.
The phosphor particles are generally made of at least one of a zinc sulfide-based phosphor, a calcium sulfide-based phosphor, a zinc selenide-based phosphor, a strontium sulfide-based phosphor or a combination of the phosphor compounds. The phosphor particles are sensitive to humidity-accelerated decay and to thermal degradation if exposed to high; temperatures.
In accordance with the present invention, a metal oxynitride coating sufficiently encapsulates the phosphor particle to limit exposure of the phosphor to moisture or water. The metal oxynitride coating includes one or more layers of a single metal oxynitride, a mixed metal oxynitride, or a combination of such layers. The one or more layers are generally applied such that the total thickness of the metal oxynitride coating is in the range of about 0.03 microns to about 1.0 microns. The metal component in the metal oxynitride is preferably selected from aluminum, boron, silicon, titanium, zirconium, or a combination of the preferred metals. Preferably, the oxynitride coating of the present invention has a nitrogen to oxygen molar ratio in the range of about 4:1 to about 1:4.
The metal oxynitride coating of the present invention exhibits reduced sensitivity to chemical degradation caused by exposure to condensed moisture or otherwise liquid water (i.e., greater resistance to corrosion in a liquid water environment). It is desirable for the present metal oxynitride coating to be sufficiently non-porous. The non-porous coating provides a phosphor particle that exhibits reduced sensitivity to humidity. Preferably, the coating is sufficiently non-porous and is also sufficiently resistant to chemical degradation (i.e., corrosion) from water such that the encapsulated particle can survive immersion in a 0.1 molar silver nitrate aqueous solution, with substantial resistance to darkening. Such a silver nitrate test has typically been used to check the permeability of a phosphor coating. Being more resistant to water induced corrosion enables the present metal oxynitride coating to survive for longer periods in a liquid water environment. Additionally, the encapsulated particle of the present invention preferably has an initial electroluminescent brightness of about 50% or greater of the initial electroluminescent brightness of the phosphor particle with no coating.
The present invention also provides a novel method for making such encapsulated phosphor particles. The method comprises providing a bed of phosphor particles, each of which exhibits humidity-accelerated decay in the presence of moisture; providing one or more precursors comprising a vapor phase metal containing precursor, a vapor phase nitrogen containing precursor, and a vapor phase oxygen containing precursor; and exposing the bed to the precursors such that the precursors chemically react and encapsulate each phosphor particle with a metal oxynitride coating. The one or more precursors utilized in the present invention could include compounds in which the metal component and the nitrogen component are present in a single precursor. Additionally, a single precursor containing the metal component, nitrogen and oxygen could be utilized to form the metal oxynitride coating of the present invention. The resulting coating is substantially transparent and sufficiently encapsulating to provide the phosphor particle with redu
Budd Kenton D.
David Moses M.
Lieder Stephen L.
Moh Kyung H.
3M Innovative Properties Company
Chu John S.
Knecht III Harold C.
Lee Sin J.
Szymanski Brian E.
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