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Патент USA US3073892

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gtates atent
?ice
3,@73,88Z
Patented Jan. 15, 1953
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2
simple method which increases the ?gure of merit of such
3,073,882
THERVIGELECTRIC MATERIAL
Samuel W. Kurnick and Lee D. La Grange, San Diego,
Robert L. Fitzpatrick, La Mesa, and Marshal F. Mer
riam, San Diego, Calif., assignors to General Dynamics
Corporation, New York, N.Y., a corporation of Dela
semiconductor elements to a substantial extent.
Further objects and advantages of the present invention
will be apparent from a study of the following detailed
description.
Ware
The method of the present invention generally com
prises adding to a high temperature semiconductor material
No Drawing. Filed June 19, 1%1, Ser. No. 117,772
15 Claims. ((31. 136—5)
such as cerium sul?de, a ?gure of merit-improving concen
tration of a substance which dissolves in the semiconductor
The present invention generally relates to thermoelectric
10 and which stretches the crystal lattice, i.e., the lattice spac
material and more particularly relates to semiconductor
ing of the crystal structure, e.g., with consequent improve
ments in the thermoelectric properties of the semicon
elements having improved thermoelectric ef?ciency and to
ductor.
methods of making the same.
In order to obtain a high conversion e?iciency in a ther
moelectric device, both the ?gure of merit of each dis
similar semiconductor and the difference in temperature
between the hot and cold junction should be high as pos
sible. The ?gure of merit is equal to the square of the
The thermoelectric ef?ciency of a high temperature
15 semiconductor such as ceriumsul?de can be substantially
increased without changing the basic 'Ih3P4 type crystal
structure of the cerium sul?de. This involves the addition,
in solid solution, of a crystal lattice-stretching amount of
a barium compound such as barium sul?de.
Seebeck coe?icient times the speci?c electrical conductivity 20' Now referring more particularly to the steps of the
of the semiconductor divided by the thermal conductivity
method of the present invention, a major proportion of the
of the semiconductor.
above-mentioned high temperature semiconductor ele
Recently, increased attention has been directed towards
ment, cerium sul?de, is combined with a minor proportion
the development of semiconductors having a high ?gure
of barium sul?de. The semiconductor material is rela;
of merit at relatively high temperatures, for example, 25 tively stable at high temperatures of about 800° K. ‘and
800° K. to l600° K. Di?iculties have been encountered
above, Cerium sul?de having a formula of Ce2S3 can
inasmuch as the ?gure of merit of most semiconductors
be prepared by reacting approximately stoichiometric
seriously depreciates as temperatures increase beyond
amounts of CeO2 and H28 together in the presence of
moderate limits. However, it has been found that certain
carbon. The Ce2S3_ can then be melted‘ down to provide
semiconductors are more or less suitable for use at the 30 an optimal lattice arrangement, the sulfur concentration
indicated high temperatures, although they have a rela
therein being adjusted during the melting'operation. That
tively low ?gure of merit. Cerium sul?de is a primary
is, a desired amount of sulfur can be removed by boiling
example of such semiconductors, particularly of the In
oil? from .themolten cerium- sul?de so as to provide the
type.
desired conductivity of the semiconductor. Thus, the sul7
Such a high temperature semiconductor has the desira 35 fur concentration can be adjusted to provide cerium sul
ble high temperature properties, including low crystal
lattice thermal conductivity and high chemical stability at
elevated temperatures of the order of 800° K. or more. It
is believed that the conduction mechanism in the indicated
?de having the formula-CeS1_33_L50, i.e., between 05283
and Ce3S4. The product can then be allowed to cool and
solidify.
,
_
_
.
‘ Barium sul?de is added to the cerium sul?de, in ac
high temperature semiconductor is similar to that of the 40 cordance with the present invention. The addition of
polaron model wherein the electrons proceed simultane
barium sul?de is controlled to provide the desired results.
ously with the polarization of the surrounding medium.
Thus, a suf?cient, amount of barium sul?de is mixed with
In cerium sul?de this type of conduction mechanism is
cerium sul?de to provide a concentration ofbarium in
characterized by a rise in Seebeck potential with increas—
the crystal lattice of cerium sul?de of from about 4.to
ing temperature, a slowly decreasing electrical conductivity
about 14 atom'percent. Thus, at least about 4 atom
with increasing temperatures and an extremely small Hall
percent of barium is usually necessary to provide any
coe?icient. This semiconductor is, therefore, attractive
appreciable- increase in the parameter of thesemicondum
for high temperature use.
tor crystal lattice. Furthermore, the limit of solubility
It would be desirable to substantially enhance the ?gure
50 of the barium in the cerium sul?de is approximately 14
of merit of such a high temperature semiconductor at the
maximum temperature of the heat source so as to improve
its utility in thermoelectric generators and the like. In
other words, the product zT, Where z is the ?gure of merit
and T is equal to the temperature of the hot junction plus
the temperature of the cold junction divided by 2, should
be as high as possible.
It has now been discovered that the ?gure of merit of
high temperature semiconductors can be suf?ciently opti
percent.
It has further been found that for desired over
. all results, a concentration of barium of about 7 atom
percent is preferred.
Addition of the barium in a concentration within the
55 indicated range of from about 4 to about 14 atom percent
stretches the cerium sul?de crystal lattice to a substan
tial extent and improves the ?gure of merit for the semi
conductor material. However, no changes occur in the
basic type of lattice. The cerium sul?de crystal lattice,
mized to provide zT values substantially above 0.1 and in
after as well as before introduction of barium sul?de, is
. many instances of the order of about 1.0. Such optimiza 60 athorium phosphide (Th3P4) type crystal lattice. The
tion of zT values can be provided with a minimum of
initial cerium sul?de crystal lattice parameter without
difficulty in accordance with the method of the present
invention.
Accordingly, it is the principal object of the present
invention to provide improved semiconductors for thermo
barium addition is about 8.6 angstroms. It can be
stretched to about 8.82 angstroms by the addition of the
barium, the extent of increase depending upon the con
centration of barium added to the cerium sul?de crystal
electric conversion.
lattice.
It is also an object of the present
invention to provide a method of making improved semi
conductor elements operable at high temperatures in ex
cess of 800° K. with improved thermoelectric efficiency.
<
In order- to be effective in stretching the cerium sul?de
crystal lattice, the barium must be dissolved in the cerium
sul?de and present in the crystal lattice thereof. Any un
It is a further object of the present invention to provide 70 dissolved barium which may be present is without e?ect
improved high temperature semiconductor elements by a
in stretching the cerium sul?de crystal lattice and, more
.
8,073,882
4
3
lated to be approximately 1.0 for element A in contrast
to a maximum zT of under 0.5 for the untreated cerium
sul?de (element B). Throughout a temperature range
between about 800° K. and 1300° K. element A contain
ing the barium sul?de had a [F between about 2 times
and about 4 times that of element B not containing the
barium sul?de. This remarkable increase in zT was
over, may have undesirable effects on other properties
of the semiconductor. Accordingly, all barium present
should be in solution in the cerium sul?de.
Another important factor regarding'the addition of
barium sul?de to ‘cerium, sul?de is the adjustment necessary
with respect to the sulfur ratio to the total metal, i.e., ceri
um plus barium. Thus, the total sulfur, represented by the
sulfur of the barium sul?de plus the sulfur of the cerium
I wholly unexpected.
sul?de should be controlled to provide a desired con
The improved semiconductor elements are particularly
ductivity for the semiconductor material. In this regard, 10 important for high temperature use in thermoelectric gen
the total sulfur ratio to the total metal should be be
tween about 1.3:l and about 1.5 :l,'preferably approxi
mately 1.4:1. It is desirable to provide the semiconduc
erators and the like, for example, generators for use in
nuclear reactors, solar generators, etc. Heretofore, even
- minor improvements in thermoelectric ef?ciency have
tor with a conductivity somewhere between the two ex
been obtained with dif?culty. Now, in accordance with
tremes of an insulator and a metal. The conductivity 15 the present invention, the thermoelectricv ef?ciency of
of the cerium sul?de-barium sul?de mixture can be read;
high temperature semiconductor elements can be im
ily adjusted, as desired, ,as by boiling off excess sulfur
proved to a substantial extent with a minimum of di?i
from the molten mixture, etc.
culty. Other advantages of the present invention are set
In carrying out the method of the present invention,
after the barium sul?de and semiconductor are mixed 20
together, the barium sul?de is alloyed with the cerium
forth in the foregoing.
.
Various of the features of the present invention are
as set forth in the appended claims.
sul?de. ' This can readily be accomplished by heat treat
What is claimed is:
1. A method of making an improved semiconductor
depending on the particular alloy required. Thus, for
element, which. method comprises the step of alloying a
example, ?ne grained Cease and BaS (prepared from 25 major proportion of cerium sul?de semiconductor ma
BaCO3 by treatment at elevated temperatures with HZS)
terial, chemically stable at temperatures in excess of
may be compressed in dies and annealed. atv about 1500°
800° K., with a minor proportion of a barium compound.
C. for about an hour, so as to minimize loss of BaS at
which. enters into solution in. the cerium sul?de and
ing the mixture at any suitable temperature,- pressure, etc.,
the melting point of the alloy. The sintered mixture
stretches the crystal lattice-thereof, said proportion of said
in each case may then be melted down and zone leveled. 30 compound being su?icient to substantially stretch the
The mixture of barium sul?de and cerium sul?de can
crystal lattice of said cerium sul?de, all of said compound
also be formed by treatingv a mixture of BaCOa and 'CeOz
with H28 in the presence ofcarbon at gradually increas
ing temperatures so that both.v carbon and oxygen are
dissolving in’ said cerium sul?de, said compound improv
ing the ?gure ofv merit of said semiconductor material.
2. A method of making an improved semiconductor
eliminated and an alloy havingv the overall composition 35 element. which. method comprises the steps of mixing to‘
of Ce2BaS4 is formed.
gether a major proportion of cerium. sul?de semiconduc
Following solidi?cation of the mixture, the formed n-i
tor material, chemically stable at temperatures in excess
type semiconductor elementis ready for incorporation. in
of 800“ K. and a minor proportion of barium sul?de,
a thermoeelctric generator or the. like in combination
sufficient to stretch the crystal lattice of said cerium sul
with a p-type semiconductor element. A conventional 40 ?de, and improve the ?gure of merit of the cerium sul-'
type method of joining the n-type and p-type semiconduc
?de, dissolving substantially all of said barium sul?de in
tor elements in the electrical apparatus may be used and,
said cerium sul?de, and forming an improved solid semi
accordingly, is not described hereinafter.
conductor element therefrom.
A number of tests have been performed on cerium
3. A method of making an improved semiconductor
sul?de n-type semiconductor elements containing barium 45 element which method comprises the steps of mixing to
sul?de in solid solution therein to, determine, thermo
gether cerium sul?de semiconductor material and a minor
electric properties thereof.
In one test, two semiconduc
proportion of barium. sul?de to provide a concentration
of from about 4 to about 14 atom percent of barium,
based‘ on the cerium, dissolving said barium sul?de in
temperature (22° C.) and 1000° C. The two semi 50 said cerium; sul?de and adjusting the atom ratio of the
conductor elements were substantially identical in that
total sulfur in the resulting solution to the total metal‘,
each comprised cerium sul?de, except that one element
that is, cerium plus barium, present to a value‘ between
tor elements were tested to determine their expansion co
e?icients, over a range of temperatures between room
(A) was prepared in accordance with the method of
about 1.33:1 and about 1.50:1, thereafter cooling said
the present invention and contained approximately 7
solution to form a solid semiconductor element therefrom,
atom percent of barium. The other element (B) .con-v 55 said barium in the crystal lattice of said cerium sul?de
tained no addends (barium sul?de, etc.).
causing stretching of said lattice and improvement of the
Element A was prepared by grinding together cerium
?gure of merit of the semiconductor element.
sul?de with barium sul?de, melting the mixture to place
4. A method of making an improved semiconductor
the barium in solution in the cerium, sul?de lattice, and
element which method comprises the steps of mixing to
adjusting the sulfur to total metal ratio (.Ce-l-Ba) to 60 gether cerium sul?de semiconductor material and a‘ su?i~
about 1.4:1v by boilingot‘f excess sulfur. The mixture
cient amount of barium sul?de to provide a concentration
was then slowly cooledv and‘. solidi?ed to a ?nished prod
of barium of about 7 atom percent, said amount being
uct..
sufficient to stretch the crystal lattice of said cerium sul
Element A was found to exhibit a higher thermal ex
?de and improve the ?gure of merit of said semiconduc
pansion coefficient, approximately 4 percent. higher than 65 tor material, melting the mixture to alloy the barium sul
the element B (which was free of barium sul?de). This
?de and cerium sul?de, and controlling ‘the temperature
result indicated higher anharmonicities with respect to,
of the melted mixture to'remove any excess sulfur and
lattice vibrations, and lower thermal conductivity. It
adjust the atom ratio of total sulfur in the melt to total‘
was found that the crystal lattice parameter of cerium
sul?de had, increased from about 8.6 A. (with no, barium 70 metal, that is, cerium plus barium, in the melt to about
11.421, thereafter cooling said melt su?iciently slowly to
sul?de present) to about 8.8 A. (with the barium sul?de
solidify said semiconductor element without substantial
present as indicated).
cracking thereof, whereby an improved‘ semiconductor
' By assuming the thermal conductivity of elements A
and B to be approximately 0.010 watt per cm.-degree, the
element is provided.
.
zT at hot junction temperature of‘ 1300° K. was calcu 75
5'. An improved‘ semiconductor element, stable at tem
5
3,073,882
6
peratures in excess of 800° K. and having an improved
?gure of merit, said element comprising a major propor
ing therefrom a solid semiconductor element having an
improved ?gure of merit.
tion of cerium sul?de and a minor proportion of a
11. A method of making an improved semiconductor
barium compound in solid solution in said cerium sul?de,
element which method comprises the steps of mixing
the metal of said compound being in su?’icient concentra 5 together a major proportion of cerium sul?de semicon
tion in the crystal lattice of said cerium sul?de, so that
ductor material and a minor proportion of barium sul?de
the lattice parameter is increased, said element having an
to provide a concentration of from about 4 to about 14
improved ?gure of merit.
atom percent of barium, compressing said mixture in a
6. An improved semiconductor element chemically
die and annealing the mixture at about 1500“ C. for a
stable at temperatures in excess of 800° K., said element
time su?icient to provide a sintered mixture, whereby loss
comprising a major proportion of cerium sul?de semi
of barium sul?de during subsequent melting is minimized,
conductor and a minor proportion of barium sul?de in
thereafter melting the sintered mixture and zone leveling
solid solution in said cerium sul?de, the concentration of
the same, thereby dissolving the barium sul?de in the
barium present in the crystal lattice of said cerium sul?de
cerium sul?de, the atom ratio of the total sulfur in the
being su?iciently large so that the lattice parameter is 15 resulting solution to the total metal being adjusted to a
increased, and the ?gure of merit of said element is im
value between about 1.33:1 and about 1.50:1, thereafter
proved.
cooling said solution to form therefrom a solid semicon
7. A method of making an improved semiconductor ele
ductor element having an improved ?gure of merit.
ment which method comprises the steps of mixing together
12. A method of making an improved semiconductor
a major proportion of a cerium-containing compound and 20 element which method comprises the steps of mixing to
a minor proportion of a barium-containing compound and
gether a major proportion of cerium sul?de semiconductor
treating the resulting mixture with a sulfur-containing
material and a minor proportion of barium sul?de to pro
compound so as to form in situ an alloy of cerium sul—
vide a concentration of about 7 atom percent of barium,
compressing the mixture in a die and annealing it at
?de and barium sul?de, the barium sul?de being present
in an amount su?icient to stretch the crystal lattice of the 25 about 1500“ C. for a time suf?cient to provide a sintered
cerium sul?de, and improve the ?gure of merit of the
cerium sul?de, substantially all of said barium sul?de dis
solving in said cerium sul?de.
8. A method of making an improved semiconductor ele
ment which method comprises the steps of mixing together 30
a major proportion of cerium oxide and a minor propor
tion of barium carbonate and reacting the resulting mix
ture at elevated temperatures with hydrogen sul?de in the
presence of carbon, whereby an alloy of cerium sul?de and
mixture, whereby loss of barium sul?de during subsequent
melting is minimized, thereafter melting the sintered mix~
ture and zone leveling the same, whereby the barium sul
?de is dissolved in the cerium sul?de, the atom ratio of
total sulfur in the resulting solution to total metal being
about 1.4: 1, thereafter cooling said solution to form there
from a solid semiconductor element having an improved
?gure of merit.
13. An improved semiconductor element chemically
barium sul?de is formed and carbon and oxygen are elim
35 stable at temperatures in excess of 800° K., said element
inated from the mixture, said barium of said barium sul
comprising cerium sul?de containing in solid solution an
?de being present in an amount su?icient to stretch the
amount of barium sol?de su?icient to provide in the
crystal lattice of said cerium sul?de, and improve the
crystal lattice of said cerium sul?de a concentration of
?gure of merit of said cerium sul?de, substantially all of
barium of between about 4 and about 14 atom percent,
said barium sul?de dissolving in said cerium sul?de, and 40 whereby the crystal lattice of said cerium sul?de is
forming an improved solid semiconductor element there
stretched, said element having an improved ?gure of
merit.
from.
9. A method of making an improved semiconductor ele~
14. An improved semiconductor element, chemically
ment which method comprises the steps of mixing to
stable at temperature in excess of 800° K., said element
gether a major proportion of cerium oxide and a minor 45 comprising cerium sul?de containing in solid solution a
proportion of barium carbonate, said barium carbonate
su?icient amount of barium sul?de to provide in the
being in an amount su?icient to provide a concentration
of barium in the crystal lattice of cerium sul?de which
stretches said lattice, reacting the mixture at elevated
temperatures with hydrogen sul?de in the presence of
carbon to form an alloy of cerium sul?de and barium sul
crystal lattice of said cerium sul?de a concentration of
barium of between about 4 and about 14 atom percent,
the total sulfur present in said semiconductor element
being in a ratio to the total metal, that is, cerium plus
barium, present of between about 1.33:1 and about
?de, substantially all of the barium sul?de dissolving in
the cerium sul?de, with the barium disposed in the crystal
1.50:1, said cerium sul?de crystal of said semiconductor
element having an increased lattice parameter and said
lattice of the cerium sul?de and With total sulfur in an
semiconductor element having an improved ?gure of
atom ratio to total metal of about 1.33:1, thereafter melt 55 merit.
ing and cooling said sul?de to form a solid semiconductor
15. An improved semiconductor element, chemically
element having an improved ?gure of merit.
stable at temperatures in excess of 800° K., said element
10. A method of making an improved semiconductor
comprising cerium sul?de containing in solid solution a
element which method comprises the steps of mixing to
\su?icient amount of barium sul?de to provide in the
gether a major proportion of cerium sul?de semiconductor
crystal lattice of said cerium sul?de a concentration of
material and a minor proportion of barium sul?de suf
about 7 atom percent of barium, said crystal lattice
?cient to stretch the crystal lattice of said cerium sul?de,
thereby having an increased parameter, said element
all of said barium sul?de being dissolvable in said cerium
having an improved thermoelectric e?’iciency, the total
sul?de, compressing the mixture in a die and annealing
amount of sulfur present in said semiconductor element
the same to a sintered mass, whereby loss of barium sul
being in a ratio to the total amount of metal, that is, the
?de during subsequent melting is minimized, thereupon
cerium plus barium in said semiconductor of about 1.4:1.
melting the sintered mass and zone leveling the same to
dissolve the barium sul?de in the cerium sul?de, and form
No references cited.
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