Infrared window materials and their fabrication

Process for making densified ternary sulfide ceramics as infrared window erials using sulfide compounds MLn.sub.2 S.sub.4 belonging to the Th.sub.3 P.sub.4, CaFe.sub.2 O.sub.4 and spinel structure types. These refractory sulfides show good sinterability especially when fired in flowing hydrogen sulfide, and they can be densified by a combination of hot-pressing and hot-isostatic pressing into ceramic pieces approaching their theoretical density with closed pores and which have good transmission characteristics in the infrared region.

BACKGROUND OF THE INVENTION 
This invention is related to ceramic window materials and, more 
particularly, to densified ternary sulfides to be used as infrared window 
materials and fabrication thereof. 
Infrared transmitting materials particularly for the 8-14 .mu.m region (1 
.mu.m is equal to 10.sup.-6 meters) are necessary for sensor windows and 
domes on satellites, missiles and other similar devices. These materials 
must be chemically stable, abrasion resistant, have low coefficients of 
thermal expansion, high melting temperatures and good optical transmission 
characteristics in the desired region of the infrared. The materials used 
in the past are alkali halides, zinc sulfide and zinc selenide. However, 
these materials do not have the hardness and abrasion resistance required 
for most of the above-enumerated applications. Furthermore, the optical 
transmission of the zinc sulfide and zinc selenide materials is deficient 
at the long wavelength end of the infrared region of interest. It is also 
desirable that any window material must be a theoretically dense compact 
of high purity in order to fulfill the criteria listed above. Single 
crystals are considered ideal for subject applications, but it is 
extremely difficult to prepare single crystals in the sizes required for 
window applications. Polycrystalline ceramics are suitable as windows, 
provided that densification is complete, scattering centers are kept to a 
sufficiently low level, and the ceramics are made from cubic compounds to 
avoid optical anisotropy from the randomly oriented grains. It is thus 
desirable to have some infrared transmitting materials having the 
above-mentioned characteristics. 
SUMMARY OF THE INVENTION 
Suitable infrared-transmitting materials according to the teachings of 
subject invention have been identified in the densified ternary alkaline 
earth-rare earth sulfides. A novel densification procedure has been 
developed which produces optical quality material by a two step process. 
First, uniaxial vacuum hot-pressing is used to close porosity in the 
samples of the ternary sulfides and then hot isostatic pressing (HIP) is 
used to remove residual porosity. The fabrication of ternary sulfides is 
accomplished by placing a few grams of the ternary sulfide powder into a 
one-inch diameter punch and die assembly machined from graphite cylinders. 
The die walls and plunger faces are coated with a slurry of boron nitride 
and ethanol (ethyl alcohol) which when dry acts as a reaction barrier. The 
die and sample are placed into a uniaxial vacuum hot-pressing which is 
evacuated to a pressure of 10.sup.-5 torr (1 torr=1 m.m of Hg) and is 
heated at a rate of 15.degree. C. per minute through the use of graphite 
heating elements. At 600.degree. C., the die is fully loaded to a pressure 
of 6,000 psi (6,000 per square inch) or 40 MPa (1 MPa=1 mega 
pascal=10.sup.6 newtons/meter.sup.2). Temperature is increased to 
1400.degree. C. and maintained for 5 minutes. Temperature is then lowered 
slowly at a rate of 15.degree. per minute while pressure is maintained at 
40 MPa. Pressure is relieved at 600.degree. C. and the sample is cooled to 
ambient conditions. The vacuum is released and the sample is extracted 
after which the sample surfaces are ground to remove any boron nitride. 
The hot-pressed sample is wrapped in platinum foil and placed in a 
hot-isostatic press. A uniform temperature gradient is maintained by using 
ceramic barriers such as alumina crucibles placed around the furnace to 
minimize heat loss. The system is sealed and then evacuated to a high 
vacuum. The sample chamber is then backfilled with argon to a pressure of 
20 MPa and is heated at a rate of 12.degree. C. per minute to 1400.degree. 
C. The pressure is increased to 24 MPa and held for 90 minutes. The sample 
is then cooled slowly at the rate of 12.degree. C. per minute to ambient 
conditions with the pressure maintained at 3500 psi. Pressure is then 
relieved and the HIP sample is removed. This process produces final 
specimens as discs, 2 centimeters in diameter and 0.3 centimeters thick. 
The present materials were yellow and translucent to visible light. The 
grain size of the final product was of the order of 12 .mu.m and electron 
microscope images when scanned showed no evidence for pore space in the 
final products. 
An object of the subject invention is to fabricate infrared transmitting 
window materials. 
Another object of the subject invention is to fabricate infrared window 
materials which are chemically stable and are abrasion resistant. 
Still another object of the subject invention is to fabricate infrared 
window materials which have low coefficient of thermal expansion. 
Still another object of the subject invention is to fabricate window 
materials which are free of porosity and are dense to their theoretical 
limit with closed pores.

DESCRIPTION OF A PREFERRED EMBODIMENT 
According to the teachings of subject invention a new family of window 
materials for the 8-14 .mu.m (1 .mu.m=micrometer=10.sup.-6 meter) infrared 
band have been fabricated. The materials are densified ternary sulfides 
with a generalized formula MLn.sub.2 S.sub.4 of the many structural 
families of ternary sulfides. The compounds with the cubic Th.sub.3 
P.sub.4 structure have been fabricated wherein the M-cation of Th.sub.3 
P.sub.4 -structure ternary sulfides is a large alkaline earth ion such as 
Ba.sup.+2, Sr.sup.+2, or Ca.sup.+2. The Ln-cation is one of the light 
lanthanides, La.sup.3+ through Gd.sup.3+. Densified ternary sulfide 
CaLa.sub.2 S.sub.4 is a typical member of the group of ternary sulfide 
having Th.sub.3 P.sub.4 structure and isotropic optical properties. The 
Th.sub.3 P.sub.4 structure is cubic, space group I43d, with 4 formula 
units in the unit cell. Both divalent and trivalent cations occupy the 
same 8-coordinated site. Table 1 indicates the densified ternary sulfides 
synthesized according to the teachings of the subject invention and from 
the powder diffraction data revised unit cell parameters calculated are 
also known in Table 1 on the following page. 
TABLE 1 
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Parameters for Some Th.sub.3 P.sub.4 Structure Type Compounds 
Melting Lattice Parameter 
Band Gap 
Compound Point (.degree.C.) 
(.ANG.) (eV .+-. 0.05 eV) 
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BaLa.sub.2 S.sub.4 
NA 8.917 2.85 
CaLa.sub.2 S.sub.4 
1810 .+-. 25.degree. C. 
8.687 2.70 
CaNd.sub.2 S.sub.4 
NA 8.533 2.70 
CaPr.sub.2 S.sub.4 
1850 8.578 2.90 
CaSm.sub.2 S.sub.4 
1830 8.472 2.05 
CaGd.sub.2 S.sub.4 
1990 8.423 2.55 
SrLa.sub.2 S.sub.4 
NA 8.790 2.85 
SrNd.sub.2 S.sub.4 
1825 8.649 2.45 
SrPr.sub.2 S.sub.4 
1890 8.682 2.70 
SrSm.sub.2 S.sub.4 
1880 8.595 NA 
SrGd.sub.2 S.sub.4 
1980 8.551 NA 
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*NA = Not Available 
The ternary sulfides were prepared in powder form by reacting cabonates of 
the alkaline earth elements with either oxides or hydroxides of the 
lanthanide elements at 100.degree. C. in an atmosphere of flowing H.sub.2 
S. The overall reaction is given by: 
EQU CaCO.sub.3 +2La(OH).sub.3 +4H.sub.2 S.revreaction.CaLa.sub.2 S.sub.4 
+7H.sub.2 O+CO.sub.2 
The powder of the starting materials were mixed in the correct 
stoichiometric ratio, placed in boats of pyrolytic graphite and inserted 
into silica-glass furnace tubes. Typical reaction times were from 3 to 7 
days. Since it was found difficult to achieve the exact stoichiometric 
required by the ternary sulfide compounds, the end product contained CaS 
as an impurity. CaLa.sub.2 S.sub.4 is stable in the presence of water but 
CaS is not and was thus removed by washing. The washing procedure, 
however, caused some hydrolysis of grain surfaces which was removed by a 
second firing at 800.degree. C. in H.sub.2 S for a few hours. Any needed 
grinding or powder processing was also done before the second firing step. 
Two samples of CaLa.sub.2 S.sub.4 ; HP-49/HIP-7 and HP-54/HIP-7 were 
prepared by a two stage process of hot-processing followed by 
hot-isostatic pressing. Compaction by hot-pressing produced materials of 
near theoretical density after long pressing times but reaction between 
the ternary sulfide and the graphite dies of the hot-pressing caused a 
sulfur deficiency and corresponding electronic absorption leading to lower 
wavelength cut-off. A 15-minute densification in the hot-press produced 
CaLa.sub.2 S.sub.4 discs that ranged from 85 to 90% theoretical density 
with closed pores. Further densification was obtained by insertion of 
these discs into the hot isostatic press with preparation conditions shown 
in Table 2 below and some of their transmission characteristics are shown 
in FIG. 2. 
TABLE 2 
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Preparation Conditions for CaLa.sub.2 S.sub.4 Optical Ceramics 
Hot-Pressing 
Hot Isostatic Pressing 
Powder P P 
Preparation 
T (.degree.C.) 
(MPa) t (hr) 
T (.degree.C.) 
(MPa) t (hr) 
______________________________________ 
HP-49/HIP-7 
1450.degree. C. 
20 0.25 1400.degree. C. 
24 2.0 
HP-54/HIP-7 
1450.degree. C. 
41 0.25 1400.degree. C. 
24 2.0 
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The final specimens were discs, 2 cm in diameter and 0.3 cm thick. The 
materials so obtained were yellow and translucent to visible light. The 
grain size of the final product was of the order of 12 .mu.m. Scanning 
electron microscope images showed no evidence for pore space in the final 
products. 
As shown in FIG. 1, curves 10, 12, and 14 graphically indicate optical 
absorption edge measured by standary diffuse reflectance techniques. As 
can be shown from FIG. 1, these ternary sulfides are opaque to radiation 
of wavelength less than 500 nm (1 nm is equal to 10.sup.-9 meter). 
FIG. 2 shows the infrared transmission spectra of the samples HP-49/HIP-7, 
and HP-54/HIP-7 of CaLa.sub.2 S.sub.4. Curves 16 and 18 are the infrared 
transmission spectra for the two samples without undergoing hot-isotropic 
pressing (HIP). Curves 20 and 22 represent the infrared transmission 
spectra without the use of any sample and curves 24 and 26 represent the 
infrared transmission spectra using the final products of the samples Run 
HP-54/HIP-7 and Run HP-49/HIP-7, respectively. FIG. 3 graphically 
represents the absorption spectrum of SrNdS.sub.4 showing available window 
region as depicted by curve 30. As can be seen from FIG. 3, the left hand 
portion of curve 30 represents the absorption edge of the window resulting 
from vibration absorption of the radiant energy and the right hand edge of 
curve 30 indicates the electronic excitation resulting in the right hand 
edge of the infrared window. FIG. 4 represents diagramitically 
microhardness (Hv-kg/mm.sup.2) for CaLa.sub.2 S.sub.4 processed in 
different ways. It can be seen from FIG. 4 that a sample which is 
hot-pressed and HIP processed has the highest microhardness as shown by 
dot 40. The densified ternary sulfides fabricated using the teachings of 
subject invention were found to be stable indefinitely in contact with the 
ambient atmosphere. There was no evidence for fogging or frosting on 
polished surfaces when exposed to atmospheric water vapor for periods of 
many months. 
The high temperature stability of the samples was tested by boiling chips 
of CaLa.sub.2 S.sub.4 ceramics in a Soxhlet extractor for several weeks. 
According to the teachings of subject invention new infrared window 
materials in the form of densified ternary sulfides have been fabricated 
using a two-step process of hot-pressing and hot-isostatic pressing of the 
ternary sulfides. The densified ternary sulfides so obtained are 
chemically stable, abrasion resistant and have low coefficients of thermal 
expansion and high melting temperatures. Furthermore, these materials have 
good optical transmission in the infrared region of 8-14 .mu.m. 
Obviously, many modifications and variations of the present invention are 
possible in the light of the above teachings. As an example, the 
densification of the ternary sulfides so as to reach their theoretical 
density can be achieved by changing various parameters such as pressure 
and temperature. Furthermore, the densified ternary sulfides can also be 
prepared by using chemical reaction other than the one described above. 
It is, therefore, understood that within the scope of the appended claims, 
the invention may be practiced otherwise than as specifically described.