Lanthanum sulfide is maintained in the stable cubic phase form over a temperature range of from 500 C to 1500 C by adding to it small amounts of calcium, barium. or strontium. This novel compound is an excellent thermoelectric material

Elsner, Norbert B.

Reynolds, George H.

Shearer, Clyde H.

NASA Technical Reports Server

United States Patent [1 9] 
Reynolds et al. 
[54] STABILIZED LANTHANU\I SL:LPHL'R 
COl\IPOli~DS 
[75] In ventors: George H. Reynolds, San ~larcos : 
~orbert B. Elsner, La Jolla: Clyde H. 
Shearer, San D iego. all of Cal if. 
[7J] Assignee: The United States of America as 
represented by the United States 
:"Iational Aeronautics and Space 
Administration, Washingto n. D .C. 
[21] Appl. :-.10.: 469,863 
[22] 
[51] 
[52] 
[58] 
Filed : Feb. 25, 1983 
Int. 0 .... ........ ..... ......... ......... ............. COIF 17/ 00 
U.S. O . .................................... 423/263; 423/265; 
423/266; 423/ 275 
Field of Search ... ..... .. .. .... 423/263, 265, 266, 275 
[56] 
[IIJ Patent Number: 
[45] Date of Patent: 
References Cited 
4,545,967 
Oct. 8, 1985 
U.S. PATENT DOCUMENTS 
3.009.9'77 11 / 1961 HouslOn ..................... ......... 423/263 
3,086.925 4/ 1963 D idchenko el at. ................ 4231263 
3.174.939 3/ 1965 Suchow ............................... 423/203 
3.247.022 4/ 1966 Didchc:nko .......................... 423/263 
OTHER PUBLICATIONS 
Kalinina et aI. , "Chern. Abstracts", vol. 93. 1930. 
#124l 65x. 
Primary Examiner-H . T. Carter 
Allorney. Agel/I. or Firm-Paul F . McCaul : John R. 
Manning: Thomas H . Jones 
[5 7] ABSTRACT 
Lanthan um sulfide is maintained in the stable cubic 
phase form over a temperature range of from 500° C. to 
15000 C. by adding to it small amounts of calcium, bar-
ium. or stront ium. This novel compound is an e.1(ceJlent 
thermoelectric material. 
3 Claims, ~o Drawings 
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4,545,967 
2 1 
STABILIZED LA:-';THA:-.;t.:\l SULPH UR 
CO\IPOL~DS 
Alternately. the start ing metallic ingredients may be 
reacted with sulphur vapor at a temperature of 
600' _1 200' C. in a closed reaction vessel. The propor-
tion of slarting materials is such that the finished mate-
BACKGROUND OF THE I:--1VENTION 
I . . Field of the Invention 
ThIs inven tion relates to lanthanum sulphur com-
pounds which are stabilized by calcium. barium. or 
strontium and have desirable pro perties for use in ther-
5 rial has the above formul a. Also, a mixture of the com-
pound lanthanum sulfide and compounds barium sul-
fide . calcium sulfide, and/ or strontium sulfide may be 
heated to 1100'- 1200' C. in vacuum for a time sufficient 
moelectric energy conversion. 10 
2. Background Discussion 
to form the desired material. 
In some instances it may be desirable to heat the 
st3.rting ingredients to temperatures in excess of 1200' 
C. to melt the ingredient. The finished product is cooled 
and then crushed or ground into powder and then fabri-
cated into a thermoelectric device by known ;lowder 
metallurgical techniques such as vacuum hot pressing. 
The following example illustrates the preferred way 
of making the compound of the present invention. 
EXAMPLE I 
A mi xture of 138 .9 grams of lanthanum powder, 58. 1 
gr.arns o f sulphur powder, and 7.9 grams of calcium 
po wder were blended together in an inert, protective 
atmosphere and loaded into a quartz reaction vessel. 
Thermoelectric materials 3. re ubstances whi ch. when 
subjected to a temperatu re different ial across the mate-
rial. produce a voltage drop and a current flow thro ugh 
the material. Such materi3.ls are used in remote power 15 
generation. for example. in a spacecraft as a source of 
electrical power. One end of the material is exposed to 
heat. for example. fro m a radioisotope heat source, and 
the other end is at a su bslanl ially lower temperalure 
caused by intentional hea l removal such as radiative 20 
coaling, subjecling Ihe material to a thermal gradient 
\vhich causes power to be generated by the material. 
Thus. heat energy is direct ly convened to electrical 
energy to provide power for the spacecraft. 
In a material suc h as lanthanum sulfide, certain crys-
tallographic phases may have desirable thermoelectric 
properties. while other phases may be electrical insula-
tors. For example. the cubic phase form of lanthanum 
sulfide exhibits desi rable thermoelectric properties over 
the temperature range of from about 1200' C. to about 30 
1500' C. At temperatures below about 1200' C. the 
crystal structure of the lanthanum sulfide changes fro m 
the cubic phase. which provides the desirable thermo-
electric properties, to an orthorhombic form which 
lacks the desired thermo-electric propenies. Such phase 35 
transformation can also produce a britt le, fragile mate-
rial with insufficient strength to be used as a thermo-
electric power conversion device. 
5 The vessel was evacuated and sealed and slowly heated 
2 • to 1100' C. for several days to homogenize the mixture. 
The reacted mixt ure was removed from the quartz reac-
tion vessel, ground to powder, and vacuum hot pressed 
at 1-+00' C. for one hour to form a solid thermoelectric 
element. 
The above description presen ts the best mode con-
templated of carrying out the present invention. The 
compounds of this in ven tion are, however, susceptible 
to modifications and alternate ways of preparing them. 
Co nseq uently, it is not the intention to limit this inven-
tion to the particular example disclosed. On the con-
trary, the invention is to cover all modifications anc 
alternate fo rms of the compo unds falling within the 
spirit and scope of the invention as expressed in the 
THE lNVE~TION 40 appended claims. 
We have discovered that the desirable cubic phase of 
lanthanum sulfide can be stabilized over a broad tem-
perature range by dissolving calcium, barium. or stro n-
tium in the lanthanum sulfide. This material may be 
represented by the fo llowing formula: 45 
(La.\i )SI ~_ ~, 
where x is greater than 0 and less than 0.1, and M is 
selected from the group consisting of calcium, barium, 50 
strontium, and mixtures thereof. The amount of cal-
cium, barium, and stront ium used should be effec tive to 
maintain the desired cubic phase of the lanthanum sul-
fide at temperatures of 500' C. or lower. Typicall y. the 
amount of calcium, barium. or strontium ranges be- 55 
tween about 0.1 and abou t 5.0 weight percent of the 
compound. and preferably is in the range from about 0.2 
to about 5 weight percent. to stabilize the cubic phase 
and provide a useful thermoelectric material whic h is 
usable in the temperat ure ran ge of from about 500' to 60 
about 1500' C. 
The thermoelectric mate rial of this invention may be 
made by simply blendin g together a t room tempe rature 
powders of the elemental ingredients (La. S, Ba. Ca, Sr) 
in the correct proportions. This blend is slowly heated 65 
in vacuum to a temperature of approxi mately 
1100'-1200' C. and at this temperature fo r a sufficient 
amount of time for chemical homogenization to occur. 
What is claimed is: 
1. The method of formi ng a thermoelectric material 
of lanthanum sulfide which exhibi ts a stable phase form 
over a temperature range from about 500' C. to 1500' C. 
wherein thermoelectric properties exist, said method 
comprising the steps of: 
blending at room temperature lanthanum sulfide and 
an ingredient selected from calcium, barium or 
strontium to form a lanthanum sulfide compound 
having the following formula: 
(La. M)S U_x 
where x is greater than 0 and less than 0.1 and M is 
selec ted from the group co nsisting of calcium, barium 
o r stro ntium, and M is present in an amount sufficient to 
stabil ize the cubic phase o f said material at temperatures 
as low as abo ut 500' c.; 
select in g M in an amount rangin g from 0.1 to 5.0 
weight percent; 
slowly heating the blended ingredients in a vacuum to 
a temperature of abou t 1100' to ~ 1200' c.; 
allowing a chemical homogenization to occur; and 
fabricating said thermoelectric material from the ho-
mogenized compound. 
2. The method of fo rming a thermoelectric material 
o f lanthanum sulfide which exhibits a stable c ubic phase 
fo rm over a temperature range from about 500' C. to 
4,545,967 
3 
15()()· C. whaein thermoelectric pro pert ies e.,ist. said 
method comprisi ng the steps of: 
reacting lanthanum and an ingredient selected from 
calcium. barium or st rontium with a sulfur va po r at 
a temperature of about 6()()· C. to 12()()· C. in a 
closed reaction vessel to fo rm a lanthanum sulfide 
compound havi ng the fo llowing formula : 
where x is greater than a and less than 0. 1 and M is 
selected from the group consisting of calcium, barium 
10 
o r strontium. and M is present in an amount ranging 
from 0. 1 to 5 weight percent sufficient to stabilize the 
cubic phase of said material at temperatures as low as IS 
about 5()()· c.; 
selecting M in an amount ranging from 0.1 to 5.0 
weight percent ; 
allowing a c hemical homogenization to occur in the 
reaction vesse l; and 20 
fabricating said thermoelec tric material from the ho-
mogenized compound. 
3. The method of forming a thermoelectric material 
of lanthanum sulfide which exhibits thermoelectric ' 
25 
30 
35 
40 
45 
50 
55 
60 
65 
4 
pro perties over a temperature range from about 5()()· C. 
to 15()()· C. by exhibiting a stable cubic phase form over 
said temperature range, said method comprising the 
steps o f: 
ind ividuall y heating to about l2()()· C. the ingredients 
of lanthanum, sulphur and one ingredient selected 
from calcium, barium, or strontium to form a lan-
thanum sulfide compound having the following 
formula : 
(La. M)SU _ z 
where x is greater than a and less than 0. 1 and M is 
selected from the group consisting of calcium, barium, 
and strontium. and M is present in an amount ranging 
from 0.1 to 5 weight percent sufficient to stabilize the 
composition in it's cubic crystall ine phase at tempera-
tures as low as about 5()()· c.; 
cooling the heated compo und; 
c rushing the compo und into powder; and 
fabricating said thermoelectric material from said 
powder. 
• • • • • 


Stabilized Lanthanum Sulphur Compounds

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