Preparation of metal sulfides

Metal sulfides are prepared by reacting a compound of the metal and an oxygen-containing anion, with a source of carbonyl sulfide. The resulting metal sulfide is not contaminated by hydrogen, as in the form of hydroxides, and is suitable for use in photoluminescence and electroluminescence applications. The starting material is preferably a metal oxalate, which may be appropriately doped, and the source of the carbonyl sulfide is preferably a mixture of carbon monoxide and sulfur dioxide (to produce a reducing mode) or a mixture of carbon dioxide and carbon disulfide (to produce an oxidizing mode). Oxyanions that decompose to a produce a nascent oxygen anion, as does the oxalate, are preferred, as they can be reacted to achieve a high conversion rate to the sulfide.

BACKGROUND OF THE INVENTION 
This invention relates to the preparation of metal sulfides, and, more 
particularly, to the preparation of high purity sulfides and doped 
sulfides suitable for luminescense applications. 
Luminescence occurs when certain materials are exposed to light or other 
energy, termed photoluminescence, or excited with an applied electric 
field, termed electroluminescence. Luminescence is widely used in a 
variety of commercial and scientific applications. For example, 
conventional television screens and cathode ray tubes are coated with 
photoluminescent phosphors which produce light when irradiated with 
electrons directed against the back side of the screen by the electron gun 
of the picture tube. Luminescent materials and devices also have 
application in detectors, instruments, and displays, among other things. 
Not all materials can produce luminescence, and the identification and 
development of improved luminescent materials is ongoing. The most widely 
used luminescent materials to date have been rare earth oxysulfides doped 
with other rare earth elements. As an example, Y.sub.2 O.sub.2 S doped 
with less than one percent of europium is widely used as the red light 
emitting phosphor in television tubes. As in most luminescent materials, 
there is a host material in which is dopant species resides. The dopant 
actually undergoes the electronic transitions that produce light, but the 
character of the host is important in creating an environment for the 
luminescing species so that luminescence can occur and not be quenched by 
impurities. 
Other classes of materials are under consideration for their luminescence, 
such as metal sulfides doped with metallic or rare earth cations. These 
materials have the advantage that they can be excited or driven to higher 
light outputs than doped rare earth oxysulfides, with high light output 
efficiency and without a breakdown of the material. Additionally, metal 
sulfides are less expensive than rare-earth-based hosts. 
The key to the effectiveness of any luminescent material or device is its 
ability to produce a high light output under a particular level of 
excitation, its efficiency, and to be driven to high light output without 
degradation of the material. A principal obstacle to reaching these goals 
is the presence of certain types of impurities in the host lattice. 
Specifically, hydrogen-containing impurities in the lattice of doped metal 
sulfides have inhibited their widespread use in many applications, because 
the impurities interfere with the production of light by the dopant and 
reduce the threshold of radiation damage of the material. Consequently, 
this class of luminescent materials has not been widely exploited in 
devices for which it is otherwise ideal, in spite of its potential 
advantages. 
There are a number of techniques now used for producing metal sulfides and 
doped metal sulfides commercially. In one method, sulfur is boiled in a 
thick aqueous suspension of calcium hydroxide. A metal sulfide is 
precipitated, but the ratio of metal to sulfide ions varies widely from 
batch to batch, and is not readily controlled. Moreover, hydroxide ions 
are inherently present to contaminate the solid. In another method, solid 
state conversion of metal sulfates is accomplished with reduction by 
carbon. This approach is used to prepare metal sulfides for industrial 
applications where high purity is not required, such as insecticides and 
depilatories. The resulting impurity content of the resulting metal 
sulfide is simply too high for its use in luminescence. 
In another process, the metal oxide is fused with sodium carbonate in the 
presence of an excess of sulfur. The metal sulfide is precipitated, with 
evolution of sulfur dioxide. The free energy of the reaction is positive, 
but the reaction can be coaxed along by continuously removing the 
products. This process does not avoid the hydrogen-containing impurities, 
as oxide in the reaction and carbonate present in the flux can react with 
any available moisture to produce hydroxide. 
The potentially most satisfactory of the prior methods for producing metal 
sulfides is the conversion of metal chlorides or oxides with hydrogen 
sulfide or ammonium sulfide. Different variations of this approach are 
used, some in aqueous solution and others at elevated temperature with the 
chloride or oxide in the solid state. When accomplished in aqueous 
solution, there is an inevitable contamination of the metal sulfide with 
hydrogen in the form of incorporated hydroxide and bisulfide ions. Each 
solid state reaction has a positive free energy change, and elevated 
temperature or removal of products are used to encourage the reaction. 
However, the product remains contaminated with a level of the chloride or 
oxide, due to the incomplete reaction. In turn, due to the ubiquitous 
presence of water vapor these impurities form the hydroxide ions that 
remain to contaminate the metal sulfide. 
To summarize, there has been publicly proposed no alternate approach for 
producing metal sulfides and doped metal sulfides of suitably high purity 
for use in luminescence applications, although several supplier companies 
apparently utilize proprietary processing. As a result, this potentially 
attractive class of luminescent materials has not been exploited. There 
therefore exists a need for a process for economically and commercially 
preparing metal sulfides and doped metal sulfides of sufficiently high 
purity for use in luminescence. Any such process should specifically 
result in low oxygen and hydroxide contamination of the sulfide. The 
present invention fulfills this need, and further provides related 
advantages. 
SUMMARY OF THE INVENTION 
The present invention provides a method of preparing metal sulfides and 
doped metal sulfides having a low content of hydroxide and other damaging 
impurities. The resulting purified sulfides can be used directly as 
luminescent materials, without further purification, in both 
photoluminescent and electroluminescent applications. The process can be 
applied with the sulfide remaining in the solid state, so that handling is 
simplified and relatively conventional apparatus can be used. Formation of 
the sulfide is quantitative and economical. 
In accordance with the invention, a process for preparing a metal sulfide 
comprises the steps of furnishing a starting compound of the metal cation 
with an anion containing oxygen, and heating the starting compound in the 
presence of a source of carbonyl sulfide, whereupon the starting compound 
is converted to a metal sulfide. 
Using carbonyl sulfide, COS, the reaction with the oxide anion is direct 
and without intermediaries. Significantly, hydrogen impurity in the 
sulfide product is avoided, as the carbonyl sulfide reacts with water to 
produce gaseous carbon dioxide and hydrogen sulfide, which are removed in 
the gas stream. The resulting metal sulfide is free of the principal 
contaminant that limited its use in luminescence applications, when 
prepared by prior methods. 
Carbonyl sulfide is an expensive, poisonous gas, when obtained in that 
form, and can be furnished by a combination of inexpensive gases that 
react together at elevated temperature to produce carbonyl sulfide. 
Moreover, the use of a combination of gases permits the preparation of the 
metal sulfide to be conducted either in a reducing mode or an oxidizing 
mode. To provide a reducing mode, the carbonyl sulfide is furnished by 
reacting together carbon monoxide and sulfur dioxide, preferably in a mole 
ratio of about 10 moles carbon monoxide to 1 mole sulfur dioxide. To 
provide an oxidizing mode, the carbonyl sulfide is furnished by reacting 
together carbon dioxide and carbon disulfide, preferably in a mole ratio 
of about 4 moles carbon dioxide to 1 part carbon disulfide. 
The preferred oxygen-containing anion is an oxalate, as oxalates can be 
readily doped so that the dopant is carried into and incorporated into the 
metal sulfide. Other oxygen-containing anions, such as hydroxide, nitrate, 
sulfate and carbonate can also be used. 
Accordingly, in a preferred aspect of the invention, a process for 
preparing a metal sulfide, comprises the steps of furnishing a starting 
compound consisting of the metal cation with an oxalate anion, and heating 
the starting compound in the presence of a gas selected from the group 
consisting of carbonyl sulfide, a mixture of carbon monoxide and sulfur 
dioxide, and a mixture of carbon dioxide and carbon disulfide. 
Another feature of oxygen-containing anions is that, under proper reaction 
conditions, they can be decomposed to produce the oxide anion in a nascent 
or highly active state. When in the nascent state, the oxide is more 
highly reactive than is an oxide furnished as a starting material, leading 
to a faster and more complete conversion to the sulfide. A procedure has 
been developed for achieving nascent state processing to the sulfide of 
the oxygen-containing anion, for the metal oxalates. As previously noted, 
oxalates are otherwise preferred because they can be readily doped with 
desirable dopants prior to the conversion reaction, so that the resulting 
sulfides have the desired dopants and dopant levels. 
In accordance with this aspect of the invention, a process for preparing a 
metal sulfide comprises the steps of furnishing the oxalate of the metal 
cation as a starting material, heating the metal oxalate in an 
oxygen-containing atmosphere to form a nascent metal carbonate, and 
heating the metal carbonate in an atmosphere having a source of carbonyl 
sulfide to a temperature whereat the metal carbonate decomposes to a 
nascent metal oxide, whereupon the nascent metal oxide reacts with the 
carbonyl sulfide to produce the metal sulfide, the step of heating the 
metal carbonate to continuously follow the step of heating the metal 
oxalate. 
This most preferred processing operation is accomplished by heating the 
metal oxalate or doped metal oxalate in oxygen to a temperature sufficient 
to convert the oxalate to a carbonate, with the evolution of carbon 
monoxide. This reaction is typically accomplished at a temperature of 
about 500.degree. C. The oxygen atmosphere is replaced with a gaseous 
source of carbonyl sulfide by discontinuing the oxygen flow, flowing 
nitrogen to purge the system of oxygen, and then introducing the source of 
carbonyl sulfide. Preferably, the source of carbonyl sulfide is a mixture 
of carbon monoxide and sulfur dioxide, or a mixture of carbon dioxide and 
carbon disulfide, as discussed previously. 
The metal carbonate is further heated in the source of carbonyl sulfide to 
a temperature whereat the carbonate decomposes to the oxide and carbon 
dioxide gas. The oxide is a highly reactive nascent oxide, which in turn 
reacts with the carbonyl sulfide to produce the metal sulfide. The 
decomposition of the carbonate typically begins at 600-700.degree. C., 
although this temperature can vary depending upon the cation. The optimal 
reaction temperature for the decomposition of the carbonate in the source 
of carbonyl sulfide is the temperature at which the rate of the 
decomposition of the carbonate to the oxide is about equal to the rate of 
reaction of the oxide with the carbonyl sulfide to produce the metal 
sulfide. This temperature is, for example, about 720.degree. C. for a 
starting material of calcium oxalate and about 900.degree. for a starting 
material of strontium oxalate. The optimal temperature can be determined 
by established analytical techniques, to be described subsequently. 
A variety of metal sulfides can be prepared by the techniques described 
herein, including, for example, cadmium, strontium, zinc, and calcium 
sulfides. The resulting sulfides are of high purity, and particularly have 
the low hydrogen and oxygen impurity levels that are suitable for 
preparing luminescent materials. The sulfides can be doped either after 
preparation, or, most conveniently, during preparation by doping the 
oxygen-containing anion prior to formation of the sulfide. The dopant is 
then carried into the sulfide. Dopants include, for example, rare earth 
elements, silver, manganese, and copper. 
It will be appreciated that the present approach provides a convenient, 
solid state method for preparing metal sulfides and doped metal sulfides 
of sufficiently low impurity content to be used in luminescence 
applications. The method is safe to use, economical, and operable with 
readily available laboratory or commercial purification apparatus. Other 
features and advantages of the invention will be apparent from the 
following more detailed description of the invention, taken in conjunction 
with the accompanying drawings, which illustrate, by way of example, the 
principles of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the preferred embodiment, an undoped or doped metal oxalate is converted 
to the corresponding undoped or doped metal sulfide in a flowing gaseous 
apparatus 10 of conventional design. The metal oxalate is preferably 
furnished in a finely divided form such as a powder 12 of 600 mesh. The 
use of the finely divided form reduces the diffusion distances for 
complete reaction with a gaseous phase, reducing the reaction time to 
produce the sulfide. 
The finely divided metal oxalate powder 12 is placed into a suitable 
reaction boat 14 made of fused silica. The boat 14 is placed into a 
reaction tube 16 made of fused silica. The reaction tube 16 is surrounded 
by a furnace 18 capable of heating the tube 16, boat 14, and oxalate 
powder 12 to a sufficiently high temperature to accomplish the reactions, 
typically 1000.degree. C. or less. A gas flow of oxygen is initiated in 
the reaction tube 16 from an oxygen source 20, and the furnace 18 is 
turned on to heat the apparatus 10. 
The oxalate is heated in the oxygen atmosphere to a temperature at which 
the oxalate decomposes to the carbonate plus carbon monoxide. The carbon 
monoxide is evolved and removed with the flowing oxygen stream. The 
decomposition temperature may depend upon the particular oxalate being 
processed, but has been found to be about 500.degree. C. for the 
processing of calcium oxalate and strontium oxalate. 
When the conversion of the oxalate to the carbonate is completed, typically 
about 0.5 hours for about 0.2 moles of oxalate, the oxygen gas flow is 
discontinued and replaced by a nitrogen gas flow from a nitrogen source 
22, to purge the system of oxygen. The nitrogen gas flow is continued for 
about 20 minutes. 
After the system is purged completely of oxygen, a flowing source of 
carbonyl sulfide is introduced. The source can be carbonyl sulfide itself, 
although the pure compound is expensive to purchase and does not permit 
the process control possible when a mixture of gases is used to produce 
the carbonyl sulfide. Preferably, the source of the carbonyl sulfide is a 
mixture of carbon monoxide and sulfur dioxide, most preferably in a mole 
ratio of about 10 moles CO to 1 mole SO.sub.2, if a reducing mode is 
needed. Alternatively, the source of the carbonyl sulfide is a mixture of 
carbon dioxide and carbon disulfide, most preferably in a mole ratio of 
about 4 moles CO.sub.2 to 1 mole CS.sub.2, if an oxidizing mode is needed. 
These gases used in the two mixtures are relatively inexpensive, and are 
furnished in appropriate sources 24 and 26, that are provided separately 
and simultaneously flowed into the tube 16. Both of these mixtures of 
gases react at the elevated temperature within the furnace 18 to produce 
carbonyl sulfide, COS, but the associated conditions can vary. The results 
of using these alternative mixtures will be discussed in more detail in 
relation to the examples. 
After the gaseous flow of the source of carbonyl sulfide is established, 
the temperature of the reaction tube, boat, and metal carbonate is 
increased so that the carbonate decomposes to nascent oxide ions and 
carbon dioxide gas, according to the relation 
EQU CO.sub.3.sup.-2 (s)=O.sup.-2 (s) +CO.sub.2 (g). 
The carbon dioxide is evolved and removed in the flowing gas stream. The 
temperature at which this decomposition reaction begins, upon heating, is 
typically about 600-700.degree. C. 
The nascent oxide ions are highly reactive, and react with the carbonyl 
sulfide to produce the sulfide and carbon dioxide, according to the 
reaction: 
EQU O.sup.-2 (s)+COS(g)=S.sup.-2 (s) +CO.sub.2 (g). 
The carbon dioxide is evolved and carried away in the flowing gas stream. 
The reactions are conducted by continuously heating the powder from the 
temperature of the first reaction to the temperature of the second 
reaction. As used herein, "continuous" does not require that the 
temperature be continuously increased, as there can be and usually are 
pauses for purging the system with nitrogen. However, the nascent state of 
the oxide may be lost if the compound containing the nascent oxide 
produced by carbonate decomposition is cooled to a low temperature or 
exposed to contaminants. "Continuous" is therefore used to mean that no 
process interruption is permitted that would interfere with the 
maintenance of the nascent state of the oxide ions. 
The sulfide anion remains in the solid state with the metallic cation, 
which is unaffected by the various reactions. Dopants present in small 
quantities in the oxalate, if any, are also unaffected by the reactions 
and are carried through to the final metal sulfide. 
The use of the carbonyl sulfide provides an important benefit not 
obtainable with prior sources of the sulfide. Hydrogen and oxygen present 
as contaminants, as in the form of water vapor, react with the excess 
carbonyl sulfide, to produce carbon dioxide and hydrogen sulfide, both of 
which are gases that are carried away in the flowing gas stream. That is, 
the hydrogen and oxygen impurities are reacted and carried away, so that 
they cannot contaminate the final metal sulfide. 
The most preferred temperature for the reaction of the nascent oxide and 
the carbonyl sulfide, according to the second equation, is the temperature 
at which the rate of this reaction is about equal to the rate of 
decomposition of the carbonate to the nascent oxide, according to the 
first reaction. The value of this temperature depends upon the cation 
being used, and is as indicated for the following preferred cases: calcium 
sulfide, 720.degree. C., strontium sulfide, 900.degree. C. 
The preferred temperature can be determined from a TGA thermogram for any 
particular starting material, an example of which is presented in FIG. 2 
for conversion of calcium oxalate to calcium oxide. The calcium oxalate is 
provided in a hydrated form, CaC.sub.2 O.sub.4 . H.sub.2 O. The water of 
hydration is lost during the first portion 28 of the heating curve, 
extending to a temperature of about 200.degree. C. The oxalate is stable 
from about 200.degree. C. to about 400.degree. C., and becomes unstable to 
form carbonate with the evolution of carbon monoxide from about 
400.degree. C. to about 500.degree. C., in the second portion 30. The 
carbonate decomposes to the nascent oxide, with the loss of carbon 
dioxide, in the third portion 32, extending from about 650.degree. C. to 
about 780.degree. C. The oxide is nascent in this range, being of lower 
concentration and greater reactivity toward the low end of the range and 
of higher concentration but lesser reactivity toward the high end of the 
range. The optimal operating temperature is chosen to be slightly higher 
than the knee of the curve in the third portion 32, at about 720.degree. 
C. For other starting materials involving intermediate production of a 
nascent anion whose presence aids in achieving a rapid conversion and is 
therefore desirable, the temperatures for the conversion steps upon 
heating can be determined similarly. 
The following examples are intended to illustrate aspects of the invention, 
and should not be taken as limiting the invention in any respect. 
EXAMPLE 1 
Undoped calcium sulfide was prepared from calcium oxalate in the apparatus 
and with the processing previously described in relation to FIG. 1. The 
decomposition of the oxalate to the carbonate according to the first 
equation is accomplished in flowing oxygen at about 500.degree. C. The 
decomposition of the carbonate to the nascent oxide in a source of 
carbonyl sulfide is accomplished at about 720.degree. C., as established 
by the technique of FIG. 2. The source of carbonyl sulfide was a mixture 
having a mole ratio of 10 moles carbon monoxide to 1 part sulfur dioxide. 
The resulting calcium sulfide was a powder having the same general 
appearance as the calcium oxalate starting material. Complete conversion 
of 20 grams of calcium oxalate to calcium sulfide required about 12 hours. 
An X-ray diffraction pattern was taken to verify the presence of calcium 
sulfide with a 1:1 atomic ratio of the calcium and the sulfur atoms. 
EXAMPLE 2 
Example 1 was repeated, except that strontium sulfide was produced from 
strontium oxalate. The operating conditions were the same as in Example 1, 
and the results were the same, except that the temperature of the reaction 
of the nascent oxide to the sulfide was selected to be 900.degree. C. This 
selection was made based upon a TGA thermogram of strontium oxalate, 
similar to that of FIG. 2. 
EXAMPLE 3 
Impure zinc sulfide, doped with silver, was prepared by reacting an aqueous 
solution of zinc chloride having silver ions therein, with ammonium 
sulfide, to precipitate zinc sulfide doped with silver. The precipitate 
was washed with methyl alcohol. The precipitate, while being primarily 
zinc sulfide doped with silver, also had a substantial impurity level. 
The impure zinc sulfide doped with silver was processed in CO and SO.sub.2 
as in Example 1, to produce a purified zinc sulfide doped with silver. 
EXAMPLE 4 
Example 3 was repeated, except that the zinc sulfide doped with 0.2 atomic 
percent silver was purified in an oxidizing mode from the impure zinc 
sulfide doped with silver. The oxidizing mode was achieved by using, a 
source of carbonyl sulfide, a gaseous mixture having molar ratios of 4 
moles of carbon dioxide to 1 mole of carbon disulfide. 
Examples 3 and 4 demonstrate that the process of the invention can be used 
to prepare doped sulfides, and also can be used with the impure sulfide as 
the starting material. 
EXAMPLE 5 
The doped zinc sulfides of Examples 3 and 4 were measured to obtain their 
luminescence spectra, when excited by a mercury-arc lamp. The spectra are 
shown in FIG. 3. The material produced under oxidizing conditions, Example 
4, has a major peak in the blue light range. The material produced under 
reducing conditions, Example 3, has a peak in the green light range. The 
peak luminescence can therefore be influenced by the source of the 
carbonyl sulfide. 
Thus, the present invention provides a method for producing high purity 
metal sulfides and doped metal sulfides suitable for use in luminescence 
applications. Although particular embodiments of the invention have been 
described in detail for purposes of illustration, various modifications 
may be made without departing form the spirit and scope of the invention. 
Accordingly, the invention is not to be limited except as by the appended 
claims.