Preparation of high purity silicate-containing phosphors

A method of preparing a silicate-containing phosphor is provided. The method includes combining a mixture of metal or metalloid compounds with a gaseous silicon-containing in an amount sufficient to convert the compounds to silicates, and heating the silicates under conditions effective to form a phosphor.

FIELD OF THE INVENTION 
The present invention is directed to display devices, such as field 
emission display devices, particularly phosphors used in display devices 
and methods of preparation of these phosphors. 
BACKGROUND OF THE INVENTION 
Display devices, such as desk-top computer screens and direct view and 
projection television sets, include electron excited fluorescent display 
devices such as cathode ray tubes. Cathode ray tubes (CRTs) function as a 
result of a scanning electron beam from an electron gun impinging on 
phosphors on a relatively distant glass screen. The phosphors absorb the 
energy from the electron beam and subsequently emit a portion of the 
energy, which is typically in the visible region of the electromagnetic 
spectrum. This visible emission is then transmitted through the glass 
screen to the viewer. Other display devices, such as field emission 
displays for use in flat panel display screens, which include cold cathode 
emission devices, and vacuum fluorescent displays for use in handheld 
calculators, which include hot cathode emission devices, also function as 
a result of electrons exciting phosphors on a screen. 
Phosphors are inorganic or organic luminescent materials that typically 
include "activator" atoms that can modify the emitted radiation, such that 
the emission is in the visible region, as well as modify the emission 
intensity and the persistence of the image. Phosphors should preferably be 
capable of maintaining luminescence (e.g., fluorescence) under excitation 
for a relatively long period of time to provide superior image 
reproduction. This is particularly important for high resolution CRTs, for 
example. With a CRT phosphor screen of a short persistence phosphor, 
flickering can occur on the screen, particularly when the phosphor screen 
scanning speed is relatively slow. Thus, phosphors have been developed 
that exhibit a long "after glow period" (i.e., the time required for the 
emission luminance to decrease to 10% of the emission luminance under 
excitation after the excitation of the phosphor is stopped), which is also 
referred to as the "persistence time". 
A number of silicate-containing phosphors are known that possess a 
relatively long persistence time. For example, manganese- and 
arsenic-activated zinc silicate phosphor (P39 phosphor), 
manganese-activated zinc silicate phosphor (P 1 phosphor), as well as 
other zinc silicate phosphors containing manganese as the main activator 
are green emitting phosphors with long persistence times and large 
emission intensities. Other silicate-containing phosphors are also of 
commercial significance, such as yttrium silicates (e.g., cerium-activated 
yttrium silicate or P47 phosphor), magnesium silicates (e.g., 
manganese-activated magnesium silicate or P13 phosphor), calcium silicates 
(e.g., lead- and manganese-activated calcium silicate or P25 phosphor), 
and the like, because of various desirable properties. 
Silicate-containing phosphors are typically prepared using silicic acid 
(H.sub.3 SiO.sub.3). For example, Zn.sub.2 SiO.sub.4 :Mn (P1 phosphor) is 
prepared by the reaction of ZnO, hydrated SiO.sub.2 (i.e., H.sub.3 
SiO.sub.3), MnCO.sub.3, and other additives. Silicic acid, however, tends 
to contain impurities, such as sodium, potassium, calcium, etc. For 
example, highly pure silicic acid suitable for the preparation of 
phosphors can include as much as 450 ppm sodium, 180 ppm calcium, 50 ppm 
magnesium, 450 ppm aluminum, and 60 ppm iron. Although these elements can 
be used in a wide variety of phosphors for certain applications, they are 
generally undesirable in phosphors used in high resolution displays, such 
as field emission displays. Typically, this is because mobile ions such as 
Na.sup.+ and Ca.sup.2+ can degrade the silicon circuits. 
Also, in general, it is desirable to produce highly pure phosphors to 
increase absorption of the available excitation energy by the activator 
that emits the required radiation, rather than being consumed by other 
impurities or "killer" centers, which would result in lower luminescence 
and lower efficiency. In some cases, the presence of impurities may also 
result in reduced phosphor lifetime. Thus, what is needed is a method of 
preparing silicate-containing phosphors that are more pure than many 
conventional silicate-containing phosphors. 
SUMMARY OF THE INVENTION 
The present invention provides a method for preparing a silicate containing 
phosphor. This method includes the steps of: providing a mixture of metal 
or metalloid compounds; providing a gaseous silicon compound selected from 
the group consisting of a silicon hydride, a silicon halide, and a silicon 
hydride halide; combining the mixture of metal or metalloid compounds with 
the gaseous silicon compound in an amount sufficient to convert the 
compounds to silicates; and heating the silicates under conditions 
effective to form a phosphor. As used herein, the phrase "mixture of metal 
or metalloid compounds" includes mixtures of metal compounds, metalloid 
compounds, or metal compounds and metalloid compounds. 
Preferably, the silicon compound is gaseous at room temperature and the 
metal or metalloid compounds include metals or metalloids that provide a 
phosphor lattice having a band gap of at least about 2.5 eV. In certain 
particularly preferred embodiments, the metal or metalloid compounds are 
salts and the silicon compound is a silane optionally substituted with 
iodine, chlorine, and bromine. 
The method of the present invention can be carded out in solution. For 
example, the method can involve providing an aqueous solution of the 
mixture of metal or metalloid salts; and passing the gaseous silicon 
compound through the aqueous solution to form the silicates. The solution 
of the mixture of metal or metalloid salts can be prepared by dissolving a 
mixture of metal or metalloid oxides in an acidic solution. With this 
solution method, the silicates are collected by precipitating them out of 
solution prior to the subsequent step of heating, which preferably occurs 
at a temperature of about 1000.degree.-1500.degree. C. 
The method of the present invention can also involve reaction at a 
solid-vapor interface. That is, the gaseous silicon compound can be passed 
over a mixture of the metal or metalloid compounds, which are in the solid 
state. Preferably, the gaseous silicon compound is mixed with a carrier 
gas and oxygen. Alternatively, the method of the present invention can 
involve reaction in the gas phase. That is, the method can include a step 
of volatilizing the mixture of metal or metalloid compounds prior to 
combining it with the gaseous silicon compound. Prior to the step of 
volatilizing, this mixture of metal or metalloid compounds can be in the 
solid state or the compounds can be dissolved in an organic solvent. 
The metal or metalloid compounds preferably include metals of Groups IA and 
IIA, the transition metals, the lanthanides and actinides, the metals and 
metalloids of Groups IIIA, IVA, and VA, although they more preferably 
include the transition metals, the lanthanides and actinides, and the 
metals and metalloids of Groups IIIA, IVA, and VA. For certain 
applications, the phosphors made by the methods of the present invention 
include less than about 10 ppm of impurities selected from the group 
consisting of alkali metals and alkaline earth metals. 
Also provided are phosphors preparable by the method of the present 
invention and display devices, such as field emission display devices, 
that include these phosphors.

DETAILED DESCRIPTION 
The present invention provides a method for making silicate-containing 
phosphors that are generally more pure than conventionally synthesized 
silicate-containing phosphors. That is, the present invention provides a 
method for making silicate-containing phosphors that typically have fewer 
undesirable elements than similar phosphors made using silicic acid, for 
example. Preferably, the method of the present invention provides 
silicate-containing phosphors having less than about 1 part per million 
(ppm) each of undesirable impurities. As used herein, an "undesirable 
impurity" refers to a element, ion, or compound that is not specifically 
added as a reactant, but is added as a result of its presence as an 
impurity in one of the reactants (e.g., silicic acid). 
Typically, undesirable impurities are the alkali metals and alkaline earth 
metals, although other elements, such as iron and aluminum, for example, 
can also be undesirable, if they are not added as a specific reactant to 
the reaction mixture for the phosphor. Thus, more preferably, the method 
of the present invention provides silicate-containing phosphors having a 
total of less than about 10 ppm of impurities selected from the group 
consisting of alkali metals and alkaline earth metals. It is understood, 
however, that any of these elements, even the alkali metals and alkaline 
earth metals, can be purposely added as a specific reactant and, 
therefore, not be considered an "undesirable impurity". Therefore, the 
present invention provides a more controllable method of forming 
silicate-containing phosphors. 
This method involves combining a mixture of metal or metalloid compounds 
(e.g., Zn(NO.sub.3).sub.2 and Ce(NO.sub.3).sub.3) with a gaseous silicon 
compound, preferably a silicon hydride compound optionally substituted 
with one or more halide atoms per molecule (e.g., the silanes SiH.sub.4 
and Si.sub.2 H.sub.6, and the silicon hydride halides SiH.sub.3 Cl, 
SiH.sub.3 Br, and SiH.sub.2 Cl.sub.2). This reaction can be carried out in 
solution, at a solid-vapor phase interface, or in the gas phase. That is, 
the metal or metalloid salts can be dissolved (partially or completely) in 
an aqueous solution, for example, and the gaseous silicon compound passed 
through the solution, either by bubbling or vigorously stirring under an 
atmosphere of the gas, for example. Alternatively, the solid metal or 
metalloid salts can be placed in an atmosphere of the gaseous silicon 
compound. Finally, the metal or metalloid salts, either in the solid state 
or solution (e.g., acetone solution), can be volatilized, as in a chemical 
vapor deposition process, and combined with the gaseous silicon compound, 
in a tube furnace, for example. Furthermore, the silicon compound can be a 
solid, liquid, or gas at room temperature and atmospheric pressure, as 
long as under the conditions of the reaction it is in the gas phase. 
If the solution method is used, the silicates are precipitated out of 
solution and then heated in a firing step to form the phosphor. If either 
the solid-vapor method or the gas phase method is used, the silicates are 
prepared in situ and further heated during a firing step to form the 
phosphor. The firing process used is that typically used in conventional 
processes for the preparation of phosphors, although lower temperatures 
(e.g., about 100.degree.-400.degree. C. lower) are possible. 
Using either the solution, solid-vapor, or gas phase methods, the silicate 
structure can be built from ionic species, rather than resulting from the 
break down of large silicate tetrahedral structures, which occurs in many 
conventional methods. Thus, the method of the present invention is more 
efficient. That is, the reactions typically occur faster and at lower 
temperatures. Thus, the method of the present invention is more energy 
efficient. 
The metal or metalloid compounds include, but are not limited to, nitrates, 
carbonates, sulfates, oxalates, halides (preferably, chlorides, bromides, 
and iodides), oxides, sulfides, nitrides, acetates, acetylacetonates 
("acac"), or mixtures thereof. Preferably, the metal or metalloid compound 
is a salt (i.e., the compound formed when the hydrogen of an acid is 
replaced by a metal or its equivalent). More preferably, the metal or 
metalloid salt is a water soluble salt, such as a nitrate, for the 
solution phase method, or an organic soluble salt, such as an 
acetylacetonate, for the gas phase reaction. 
Many of the metal or metalloid compounds are commercially available, and 
can be used in various combinations. That is, for the preparation of a 
phosphor containing manganese and zinc, the manganese can be present as a 
nitrate salt and the zinc as a sulfate salt. Preferably, however, because 
it is often easier and more economical to produce these salts in situ, 
they all have the same counterion. For example, oxides of the metals or 
metalloids can be combined and dissolved in nitric acid to form the 
nitrates. Typically, the nitric acid is of an appropriate molarity to 
effectively dissolve all of the oxides. Typically, this occurs using a 2 
molar solution, although 1-10 molar solutions can be used, depending on 
the metal oxide. Other acids, such as hydrochloric acid, for example, can 
also be used, depending on the compound to be dissolved. 
As used herein, the metal or metalloid compounds include metals and 
metalloids that produce a phosphor lattice having a band gap of at least 
about 2.5 eV, preferably a band gap within a range of about 2.5-3.8 eV. 
These typically include the metals of Group IA (i.e., Group 1) and Group 
IIA (i.e., Group 2) of the Periodic Table, the transition metals (i.e., 
those elements with their outermost electrons in "d" orbitals), the 
lanthanides and actinides (i.e., those elements with their outermost 
electrons in "f" orbitals), the metals and metalloids of Group IIIA (i.e., 
Group 13, the metals Al, Ca, In, and TI, and the metalloid B), the metals 
and metalloids of Group IVA (i.e., Group 14, the metals Sn and Pb, and the 
metalloids Si and Ge), and the metals and metalloids of Group VA (i.e., 
Group 15, the metal Bi, and the metalloids As and Sb). Preferably, for 
display devices such as field emission displays, the metal and metalloid 
compounds include the transition metals, the lanthanides and actinides, as 
well as the metals and metalloids of Groups IIIA, IVA, and VA. The alkali 
metals and alkaline earth metals are not desired for the phosphors used in 
such display devices generally because they can be detrimental to the 
silicon circuitry. 
Typically, silicate-containing phosphors include zinc, magnesium, 
manganese, calcium, beryllium, titanium, lead, arsenic, cerium, yttrium, 
lanthanum, and cadmium. Preferably, the silicate-containing phosphors made 
by the method of the present invention include zinc, manganese, titanium, 
lead, arsenic, cerium, yttrium, lanthanum, and cadmium. Examples of 
silicate-containing phosphors that can be made using the method of the 
present invention include Zn.sub.2 SiO.sub.4 :Mn (P1), MgSiO.sub.3 :Mn 
(P13), CaMgSiO.sub.4 :Ti (P18), CaBeSiO.sub.4 :Mn, CaSiO.sub.3 :Pb,Mn 
(P25), Zn.sub.2 SiO.sub.4 :Mn,As (P39), Y.sub.2 SiO.sub.5 :Ce (P47), 
Zn.sub.2 SiO.sub.4 :Ti (P52), and mixtures thereof. Preferably, the 
silicate-containing phosphors that can be made using the method of the 
present invention include Zn.sub.2 SiO.sub.4 :Mn (P1), Zn.sub.2SiO.sub.4 
:Mn,As (P39), Y.sub.2 SiO.sub.5 :Ce (P47), Zn.sub.2 SiO.sub.4 :Ti (P52), 
and mixtures thereof. As used herein, "silicate-containing phosphors" or 
simply "silicates" include both silicates and orthosilicates. 
The gaseous silicon compound suitable for conversion of the metal or 
metalloid compounds to silicates are silicon hydrides (i.e., silanes), 
silicon halides, or silicon hydride halides. Thus, suitable silicon 
compounds include, but are not limited to, silanes such as SiH.sub.4 and 
Si.sub.2 H.sub.6, silicon hydride halides such as SiH.sub.3 Cl, SiH.sub.3 
Br, and SiH.sub.2 Cl.sub.2, and silicon halides, including mixed halides, 
such as SiCl.sub.4, SiBr.sub.4, and SiFCl.sub.2. Preferably, the gaseous 
silicon compound is a silicon hydride optionally substituted with one or 
more halide atoms per molecule. More preferably, the gaseous silicon 
compound is a silicon hydride (i.e., silane) optionally substituted with 
one or more iodine, chlorine, or bromine atoms per molecule. Most 
preferably, the silicon compound is a silane, of which the most suitable 
is SiH.sub.4. The silicon compound is preferably gaseous under ambient 
conditions, although it only needs to be gaseous under the conditions of 
the reaction (e.g., under the conditions of the vapor phase reaction). As 
used herein, ambient conditions refer to room temperature (i.e., about 
20.degree.-25.degree. C.) and atmospheric pressure (i.e., about 1 
atmosphere). 
The metal or metalloid compounds are typically combined in the appropriate 
stoichiometric mounts for the desired phosphor, although the silicon 
compound can be included in an excess amount. If in the form of water 
soluble salts, the mixture of metal or metalloid compounds can be 
dissolved (at least partially) in an aqueous solution, or if in the form 
of oxides, they can be dissolved in an acidic aqueous solution, and an 
appropriate amount of the gaseous silicon compound passed through the 
solution to form silicates. This can be done by bubbling the gas through 
the solution or stirring the solution rapidly under an atmosphere of the 
gas, for example. A source of oxygen can also be supplied to the reaction 
mixture, such as a peroxide, to allow for higher yields of the silicates. 
The silicates typically have low solubility in water and thus tend to fall 
out of solution, although the reaction mixture may need to be cooled to 
cause the silicates to precipitate out of solution. The precipitate can 
then be collected, washed, and dried before subsequent firing. 
Alternatively, a mixture of solid metal or metalloid compounds can be 
placed in a tube furnace, for example, and the gaseous silicon compound 
combined with a carder gas (e.g., argon, helium, neon, xenon, and 
nitrogen) and passed over the solid mixture. This may or may not require 
external heating, depending on how exothermic is the silicate forming 
reaction. The metal or metalloid compounds can also be subjected to 
conditions to cause them to volatilize while in the tube furnace, thereby 
allowing for a gas phase reaction with the gaseous silicon compound. This 
can be done under standard chemical vapor deposition techniques (e.g., 
temperatures of about 500.degree.-1000.degree. C. and pressures of about 
1-100 millitorr). For example, the metal or metalloid compounds can be in 
solution (e.g., Zn(acac).sub.2 and Ce(acac).sub.2 in an acetone solution) 
or the solid state and volatilized by exposing them to reduced pressures, 
for example. Similarly, the silicon compound can also be in solution or 
the solid state and volatilized. Whether the reaction occurs at the 
interface of a solid with a gas, or completely in the gas phase, a source 
of oxygen can also be supplied to the reaction mixture, such as oxygen gas 
mixed with the gaseous silicon compound, to allow for higher yields of the 
silicates. The silicates can then be collected, or they can be exposed to 
subsequent firing while in the tube furnace. 
These methods produce silicate materials having a generally small particle 
size (i.e., less than about 5 micrometers). Whereas conventional methods 
typically produce silicates having an average particle size of about 5 
micrometers, the solution method of the present invention can produce 
silicates having an average particle size of no greater than about 2 
micrometers. Furthermore, the solid-vapor and gas phase methods can 
produce submicron particle size silicates. Such fine particle size 
materials are typically desirable for high resolution displays. 
The silicates formed by the method of the present invention can 
subsequently be fired by conventional methods. The firing can be conducted 
from one to a few times (e.g., 3-4 times). Typically, this is done in air 
or other oxidizing atmosphere, although an inert or reducing atmosphere 
can be used. Firing conditions for conventional methods of forming the 
silicates include temperatures of about 1300.degree.-1400.degree. C. at 
atmospheric pressure (i.e., about 1 atmosphere). The silicates formed by 
the method of the present invention can be fired at a temperature of about 
1000.degree.-1500.degree. C., and preferably at a temperature of about 
1200.degree.-1300.degree. C., at least in part because of their finer 
particle size. The firing time may be varied depending on the types of 
silicates, the firing temperature, and the method used in producing the 
silicates. That is, if the silicates are formed in a tube furnace with 
extemally applied heat, for example, they may be somewhat prefired. Thus, 
a shorter firing time can be used. Typically, however, firing times are on 
the order of about 0.5-6 hours. 
The phosphors produced by the method of the present invention can be used 
alone or in combination. They can also be used with other phosphors made 
by conventional methods. Furthermore, they can be deposited by a variety 
of standard techniques, such as precipitation coating, spin coating, 
PVA-AD slurry, electrophoresis, brushing, and electrostatic coating. 
Although they are particularly suitable for use in computer display 
devices, particularly field emission displays, they can be used in a 
variety of other applications and electron excited fluorescent display 
devices, such as a standard CRT used in television sets. They can also be 
used in virtual reality screens, book video screens, head-mounted display 
devices, and the like. 
The highly pure silicate-containing phosphors made by the method of the 
present invention are particularly useful in field emission displays. 
Field emission displays typically include a display panel having a 
transparent gas-tight envelope, and two main planar electrodes arranged 
within the gas-tight envelope parallel with each other. One of the two 
main electrodes is a cold cathode with a grid, and the other is an anode. 
The anode may consist of a transparent glass plate, a transparent 
electrode formed on the transparent glass plate, and a phosphor layer 
coated on the transparent electrode. Devices such as this are further 
disclosed in U.S. Pat. Nos. 5,210,472 and 5,372,973, for example. 
A field emission display employing a cold cathode is shown in FIG. 1, for 
example. Referring to this figure, substrate 11 can be made of glass, for 
example, or a variety of other suitable materials. Preferably, a single 
crystal silicon layer serves as substrate 11 onto which a conductive 
material layer 12, such as doped polycrystalline silicon, has been 
deposited. At a field emission site location, a conical micro-cathode 13 
has been constructed on top of substrate 11. Surrounding the micro-cathode 
13, is a low potential anode gate structure 15. When a voltage 
differential, through source 20, is applied between the cathode 13 and the 
gate 15, a stream of electrons 17 is emitted toward a phosphor coated 
screen 16. Screen 16 is an anode and includes phosphors made by the method 
of the present invention. The electron emission tip 13 is integral with 
the single crystal semiconductor substrate 11, and serves as a cathode 
conductor. Gate 15 serves as a low potential anode or grid structure for 
its respective cathode 13. A dielectric insulating layer 14 is deposited 
on the conductive cathode layer 12, and has an opening at the field 
emission site location. 
The invention will be further described by reference to the following 
detailed example. This example is offered to further illustrate the 
various specific and illustrative embodiments and techniques. It should be 
understood, however, that many variations and modifications may be made 
while remaining within the scope of the present invention. 
EXPERIMENTAL EXAMPLE 
Preparation of Y.sub.2 SiO.sub.5 :Ce 
Y.sub.2 O.sub.3 and Ce.sub.2 O.sub.3 are mixed in the desired amounts and 
dissolved in 2M nitric acid to form nitrates. When cooled, a 30% aqueous 
solution of H.sub.2 O.sub.2 is added to the mixture. The requisite amount 
of silane gas is bubbled through this mixture with vigorous stirring to 
convert the nitrates to silicates. The reaction mixture is then cooled 
using external means, such as an ice bath to precipitate the silicates. 
The precipitated silicates are filtered and washed with copious quantities 
of deionized water. The filtrate is dried at 125.degree. C. for 12 hours. 
The dried filtrate is then fired under conventional conditions. 
The complete disclosure of all patents, patent documents, and publications 
cited herein are incorporated by reference. The foregoing detailed 
description and example have been given for clarity of understanding only. 
No unnecessary limitations are to be understood therefrom. The invention 
is not limited to the exact details shown and described, for variations 
obvious to one skilled in the art will be included within the invention 
defined by the claims.