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DESIGN OF THERMO-ELECTRIC GENERATOR MODULES USING SILICON-GERMANIUM ALLOY
39 DESIGN OF THERMO-ELECTRIC GENERATOR MODULES USING SILICON-GERMANIUM ALLOY W.
THORP
Ferranti Limited, Manchester, 22, England ABSTRACT Si/Ge alloy is becoming accepted as the most useful high temperature thermo-electric material for fabrication of Thermo-Electric Generators. This paper gives a survey of possible applications for thermo-electric generators together with the required generator operating conditions. The properties of Si/Ge alloy will be described together with various methods of material preparation and generator fabrication including the single stage deposition of multi-couple generators from the vapour phase. The advantages of Si/Ge over the other thermo-electric materials will be discussed and certain immediate applications analysed showing which Si/Ge fabrication technique will most satisfactorily meet the requirements. APPLICATIONS
A survey of applications for thermo-electric generators shows an extensive range of uses from weather stations and space vehicles, to improving the efficiency of domestic heaters. Remote Weather Stations ( 1 ) have been installed in the Arctic Circle and Antarctica by the United States Weather Bureau to enhance the accuracy of its long-range predictions. T h e stationing of personnel in these locations for extended periods of time could be both unpleasant and expensive. Therefore remote weather stations have been constructed to transmit temperature, wind velocity, and precipitation on a 6 hour schedule. These units draw their power from 40,000 curies of strontium 90 whose decay heat is converted directly into electricity by a thermo-electric generator, trickle charging a 32 volt nickel-cadmium battery. T h e radio-active source and generator are buried in the ground in containers designed to prevent accidental contamination of men or animals. These units are designed so that they need not be visited by maintenance personnel more often than about every 10 years. Similar units fuelled with bottled propane have been designed to power V H F - F M radio relay transmitters located in inaccessible terrain which would otherwise be radio dead spots. T o raise the productivity in under-developed countries small self-contained 613
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irrigation pumping systems have been developed and powered from thermoelectric generators. These are 50 watt to 200 watt units powered from the sun, whose energy is gathered by means of a collector. The 200 watt output system uses an 8 ft parabolic mirror to focus the sun's rays onto the converter module, which is about 8 in. square by 2 in. deep. The power from this unit can pump to a height of 20 ft enough water to irrigate 4 acres at a rate corresponding to 24 in. of natural rainfall per year, assessing 250 days of sunshine per year with 10 hours sunshine per day. Considerable development has also been carried out on the construction and operating characteristics of a combustion chamber that would accept almost any type of solid fuel, i.e. coal, green wood, charcoal etc. The purpose of this unit is to provide a burner that could operate almost anywhere in the world on an indigenous fuel and supply sufficient heat energy to a thermo-electric generator to give an output of 150 watts. A potentially large market for thermo-electric generators exists in the domestic gas appliance field, using domestic gas supply to heat the generator. In 1959, a gas company in the U.S.A. (2) demonstrated a bathroom heater in which a thermo-electrically driven circulating fan was used. The fan driven by power generated from 24 lead telluride thermo-couples, delivered about 14^ cu. ft. of air per minute. Most of the heat delivered by the unit, however, was by means of direct radiation. A prototype space heater, described in 1960, also used a thermo-electrically operated blower. Thermo-electric modules, located in a portion of the combustion chamber wall, operated with hot junction temperature of 230°G and cold junction temperature of 50°C. The blower delivered 300 to 400 cubic feet of air per minute, and was driven by a HOOr.p.m., d.c. motor, operating at 2-3 volts and 3*5 amps. The thermo-electric generators operated at an efficiency of about 4 per cent. Researchers pointed out that only 10 per cent of the gas input rate to a 100,000 Btu per hour boiler would have to pass through such a device to provide the necessary 120 watts for a circulating pump in a hot water heating system. In 1961, a gas furnace used a thermo-electric generator of four 8 J in. by 4 \ in. modules inserted in the combustion chamber walls, to produce 130 watts at 9 volts and operate an electrically driven air blower. In 1961, a co-operative developmental program was begun in the U.S.A. by a manufacturer and 11 gas utilities to develop a preproduction prototype of a furnace with a thermo-electrically operated blower. These units were subjected to field tests evaluations by participating utilities and the A.G.A. Laboratories. Thermo-electrically operated furnace blowers provide an all-gas furnace. They also have an additional operating advantage from a viewpoint of comfort heating. The blower speed is automatically modulated through thermo-electric power output as the furnace heats up and cools down. Thus, closer control of delivered air temperature is possible than with an unmodulated furnace.
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Low wattage thermo-electric generation can also be applied to oceanographic data collection buoys, sea beacons, undersea electronic equipment and cathodic protection of pipe lines. Fail-safe flame detectors operating gas appliance protection devices could be more compact or give greater electrical output using thermo-electric devices in place of thermopiles; power for the operation of electronic control circuits in jet engines can be obtained by locating thermo-electric generators against the hot waste gas stream. T h e most publicized and investigated applications for the thermo-electric devices have been in space. In space two power sources are available, i.e., solar energy and nuclear reactions ; if solar energy were used thermo-electric devices should have a longer radiation belt life than solar cells. However, the emphasis to date has been on nuclear reactors which give a larger power output than that practicable from solar sources. Both the Russian Romashka and the U.S.A. Snap 10A nuclear reactor thermo-electric generators use Si-Ge alloy as the thermo-electric material. These systems give an electrical output of around 0-6 kilowatts. I n the case of SNAP 10A this output is derived from a temperature differential of 305 °F along the thermo-electric element from a hot junction of 935°F to a cold radiator temperature of 600°F. T h e Romashka has a temperature differential of approximately 700°F from 1700°F to a radiator temperature of 1000°F. T H E R M O - E L E C T R I C GENERATOR DESIGN Therefore T E generators from the above applications divide into two basic groups, that is, thermo-electric generators and fuel systems designed together, and thermo-electric generators designed for use with existing equipment and generally making use of waste or diverted heat from that equipment. In the first case the design is limited by cost, space and available fuel. Assuming that the thermo-electric device can use m a x i m u m fuel temperature, this would be around 900°C for a fossil fuel with a 70 per cent to 80 per cent burner combustion efficiency from a regenerative burner. For nuclear energized applications the hot junction temperature could be 900°C using an isotope heat source. In the second case where waste heat or diverted heat is used the source is normally a hot gas or domestic gas b u r n e r ; both these sources give hot junction temperatures around a m a x i m u m of 700°C and will have cold junction temperatures between 100°C and 400°C. At this stage we can consider the best thermo-electric generation material to use. I n this selection of a practical thermo-electric material its stability, inertness, mechanical strength, should be considered together with its thermoelectric characteristics. T h e cost of the material is normally a fraction of the overall thermo-electric generator fabricating costs and therefore is not the main consideration.
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The end column in Table 1 grades materials into the ease with which they may be fabricated into TE generators, grade 1 being the best, with respect to : (i) Protection of the thermo-electric material from oxidation at high temperatures. (ii) Ease of fabricating couples if the material can have doped N or P conductivity. If the material is available in " N " and " P " type conductivity form, the couples may be joined without being subject to differing thermal expansion coefficient which would cause built-in stress at the hot junction. (iii) The ability to alloy the material to a heat sink. (iv) Volatilization of a component element in the thermo-electric material, i.e. Te from PbTe. TABLE 1.
Material BiSb4Te7.5 BiaTe2Se PbTe GeTe(+Bi) GeS FeSi2 Ge.3Si.7
MP °C
—
904
1225
CHARACTERISTICS OF VARIOUS TE MATERIALS
Max. Operating Temp. °G 175 325 625 625 1125 900 925
Density cm8/gram 6-8 7-5 8-15 5-5 5-27
—
3-3
Type P N NorP P N NorP NorP
Max. Figure of Merit X10-* at °C 33 23 12 16 4 2 7
25 25 25 525 925 650 775
Feasibility Grade for TE Fabrication
— 2 2 2 1 1
From the above applications and consideration of thermo-electric materials, it will be seen that Ge.3Si.7 alloy has the preferred characteristics. Its operating temperature is in line with the generator temperatures mentioned, it has the highest efficiency of the high temperature materials, it can be oxidized to produce a S i 0 2 film for protection during operation in air at high temperature, it can be alloyed to other materials, and is mechanically strong. It can also be vapour phase deposited to produce thermo-electric generator modules in one processing stage. FeSi2, can be fabricated by powder metallurgy techniques(3) but the low efficiency of the FeSi2 means that larger generators are required to give comparable output to Ge.3Si.7. Also the Ge.3Si.7 has the higher operating temperature. Preparation of Germanium-Silicon Alloy The Ge-Si system was originally studied by Stohr and Klemm who prepared their specimens by alternately powdering and sintering the alloys
Design of Thermo-electric Generator Modules
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several times, with annealing times ranging u p to seven months. This technique is completely unsuitable for quantity preparation a n d during 1955 C. C. W a n g and B. H . Alexander ( 4 ) prepared homogenous alloys by the isothermal solidification method. This technique has been the basic method of preparing Ge-Si for thermo-electric devices but is now superseded by other more convenient methods. In preparing Ge-Si alloy by techniques involving freezing from the melt it will be seen from the phase diagram Fig. 1 that if an alloy of a certain composition solidifies under normal conditions, the first particles crystallizing out will be richer in silicon t h a n the original melt. As solidification proceeds, the composition of both the liquid and the freezing solid becomes richer in germanium. Since the diffusion rate in the solid is extremely low the individual grains in the final melt will have a heterogeneous structure I400 |C| L.
LIQ ου
a 2
SOL/
Ι200
Id
/V"' /
SOL
·" MOO
c2
IOOO 1
9 0 Ge 0
20
1
1
1
1
1
40 60 70 ΘΟ Si ATOMIC7o SI
FIG. 1. Ge-Si alloy phase diagram.
varying from a silicon rich core at the centre to a germanium shell at the grain boundaries. T h e isothermal solidification technique carried out at a constant temperature obviates this " coring " difficulty. Figure 2 represents the solidification process at a temperature Tx with composition of liquid and solid phases so adjusted to correspond to Cx and C 2 . T o achieve these conditions in practice, the process may be carried out in a similar m a n n e r to zone melting, but with the molten zone corresponding to Cx and the solid sections to C 2 . If the charge is now drawn through the hot zone of a furnace, a solid phase of constant concentration C 2 precipitates out behind the liquid zone whilst the liquid continuously dissolves the solid charge in front of it. I n this way the liquid is prevented from being depleted of silicon and is maintained at a constant composition C±.
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The success of this technique depends on how closely equilibrium conditions are achieved and maintained. If non-equilibrium conditions exist, local dendrites form and some coring m a y result. I n order to approach equilibrium conditions as closely as possible, the important conditions are the rate of zone travel, the w i d t h of zone, the temperature gradient at the liquid/solid interface. For thermo-electric work the germanium-silicon alloy should have a composition of Ge. 3 -Si. 7 and should have between 5 X 10 1 8 a t / c m - 3 and 5 x 10 20 a t / c m " 3 of added impurity, either phosphorus for " N " type conductivity or boron for the " P " type conductivity. Decreasing the impurity concentration increases the Seebeck voltage b u t decreases the overall efficiency. T h e alloy should also be homogeneous a n d mechanically strong.
\B
y
RFCOIL ~2
o MOLTEN ZONE TRAVERSE DIRECTION
INITIAL CONDITION CAST ALLOY 1 / QUARTZ ENVELOPE
v
GROWTH CONDITION F2|TE.ALLOY|C|
\f
FIG. 2. Isothermal solidification method.
T o achieve these conditions in practice the silicon and germanium is predoped and broken down to small granules. These are then mixed in the proportions to give Ge. 3 -Si. 7 (C 2 ) and Ge e - S i . ^ C j ) . Ingots of each concentration are cast in boats of the same shape as the final isothermal solidification boat. T h e ingots are then cut and assembled in a quartz boat for the isothermal solidification traverse, that is a one inch section of (Cj) is placed after a \ in. section of (C 2 ) and followed by a length of (C 2 ) filling the remainder of the quartz boat. T h e length of the (C x ) section should be sufficient to be completely melted by the heater. T o avoid a mechanically strained alloy the walls of the quartz boats are either coated with SiC or lined with mica. T o achieve the homogeneous alloy the growth rate is to be no faster than 10~ 3 inches per minute. This slow traverse rate allows complete solution of the high melting constituents in the solid charge and the narrow molten zone allows easier transport of silicon from the melting to the freezing interface. Also the rate of zone travel has to be limited to a certain value to avoid any constitutional under-cooling at the growth interface which could lead to a cellular structure
Design of Thermo-electric Generator Modules
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or dendritic growth. I n our apparatus the molten zone is produced by passing the ingot through a R . F . heater coil directly coupled to the alloy, this direct R . F . coupling helping the transport of silicon across the zone by its stirring effects. T h e slow traverse is controlled by a fixed oil leak in a hydraulic r a m system, the hydraulic system being immersed in a constant temperature oil bath. T h e operation is carried out in an atmosphere of pure argon, and vapour doping techniques m a y be used to compensate for the loss of phosphorus from the melt during the long traverse period. T h e measurements carried out on the growth section of ingot are : (1) Resistivity : although these ingots are polycrystalline they are also of high conductivity so that 2 probe, or 4 probe techniques can be used. (2) Seebeck voltage and conductivity type. (3) Alloy concentration: this is measured by hydrostatic weighing methods relating density to alloy concentration, or X-ray diffraction photographs showing lattice parameter which is related to alloy concentration. Chemical analysis and flotation techniques m a y also be used. (4) T h e r m a l conductivity and measured.
thermo-electric power m a y also
be
Thermo-electric generators can be fabricated from Ge. 3 Si. 7 ingots by two routes. T h e ingots can be cast to the required cross-sectional area and m a d e u p directly into couples, or the ingot can be cut into bars or slices and these slices m a d e into couples. T h e output from a highly efficient Ge. 3 -Si. 7 thermo-electric couple, with a cross-sectional area of 0-44 sq. cm. for each leg is 0*2 V at 11 ·0 A for a temperature drop of 850°C. ( 5 ) This illustrates a main factor in the use and design of T E generators, that is the low voltage high current electrical output. Therefore in practice the output voltage has to be increased either by connecting thermo-electric couples in series, or by use of electronic convenors. Both techniques give a drop in efficiency of approximately 20 per cent. However, for long-term reliability the series connection of couples is preferable. If we consider the first method of fabricating generators, using the as grown ingots as legs in the thermo-couple, we find that the commercial supply of Ge. 3 -Si. 7 shows reduced price per gram as the ingot cross-section increases. Taking a 2 cm diameter ingot, i.e. 3*2 sq. cm. area, the output of a single couple could be 0*2 volt at 80 amps giving a power output of 16 watts for a temperature difference of 850°C. Therefore this type of construction is of use mainly in high power output applications in the 200 to 500 watt region. These generators are constructed by brazing the Ge. 3 -Si. 7 alloy to silicon molybdenum alloy plates which serve as the heat collecting surface, cold end connection being m a d e by alloying the Ge-Si to tungsten pads
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located on electrical connectors and then alumina plate insulators are arranged to give the required electrical interconnections.(6) However, a large potential market exists for low power generators of 1 to 30 watts at output voltages of 1 -5 V d.c. to 6 V d.c. These applications can be met using the second method previously mentioned. Devices in this range are relatively low current devices therefore the cross-section of Ge.3-Si.7 in the couple legs should be small but multi-couple assemblies are required. HEAT IN
S!02 COATING
TUNGSTEN PAD
COPPER iCONNECTION G«7-Si.3 (toP IOOO C)
!
HEAT OUT
HEAT OUT
FIG. 3. Diagrammatic view of low power stacked generator.
These multi-couple assemblies can be constructed by stacking alternate slices of " P " and " N " doped Ge-Si alloy (Fig. 3). The slices are oxidized for electrical insulation and the oxide preferentially removed at opposite edges of the slice to allow the slices to be fused together using the connecting strips of Ge.7-Si.3. After the fusing operation the slices, now in electrical series connection, are oxidized in a high water content gas stream at 1000°G and tungsten pads alloyed to the outer legs for electrical output connection. We will now consider the use of this technique to meet an application
Design of Thermo-electric Generator Modules
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requiring 2 V d.c. at 2 amps for a temperature differential of 500°C from 750°C to 250°C. Experimentally we have recorded outputs of 2 V d.c. from couples of \ in. square cross-section legs for a temperature difference of 1000°C. However, our repeatable outputs have been 1 -5 V d.c. under the above conditions. From the typical output curve Fig. 4 a temperature gradient between ALLOY = Ge. 3 _Si. 7 RESISTIVITY = 0.001 Ücm COUPLE AREA = 0-125"X0-25" COUPLE IMPEDANCE = 0· 15Ω CURRENT
30 2 0 AMPS l-O
200 40O 600 ΘΟΟ x IOOO A T ° C (COLD END 60°C)
VOLTAGE
OS 0-3 VOLTS Ol 200
400 600 ΘΟΟ IOOO A T ° C (COLD END 60°C)
FIG. 4. Output curve for typical TE couple Ge. 3 -Si. 7 .
250°C and 750°C will give 0-33 volt, 2-3 amps, off load. I n the design of a series connected generator the current is determined by the cross-section area of each leg and the voltage by the n u m b e r of couples allowing a 20 per cent voltage drop due to contact resistance, this 20 per cent being an experimentally derived figure. Therefore to achieve 2 V d.c. a total of 8 couples are required. T h e current of 2 A requires a leg cross-section of 0*014 in 2 , with 16 legs the total cross-sectional area of the device is 0-224 in 2 . Therefore a square section generator would have 0-47 in. sides, the individual
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couple legs would be slices of Ge-Si alloy 0-06 in. thick. The length of the generator along the temperature gradient would be determined by the conditions available for heat dissipation but typically would be 0-5 in. to 1-0 in. The outputs quoted are measured under " no load " condition so that this generator would in practice be used at 1 -0 V d.c. Also in terms of material cost the larger the ingot cross-section the lower the cost per gram, therefore it would be more economical to have a long narrow generator possibly 1 in. Χθ·22 in. in which case the slices would be 0*025 in. thick. A further consideration in the design is the increasing possibility of generator failure with the increasing number of alloy connections. In this case there are 15 connections including hot and cold junctions. These can be reduced by using Ge-Si alloy with a higher Seebeck voltage (see Fig. 5). However, to make effective use of this characteristic of Ge-Si alloys the generator would have to run at a lower radiator temperature, if possible 100°C, although the higher temperature could be reduced to 600°G. Under these conditions an output of 0-5 V per couple would be achieved;
IOO
300 500 700 9 0 0 IOOO ΔΤ °C (COLD END 60°C)
FIG. 5. Change in voltage output for change in resistivity.
this would reduce the number of alloy connections to 9. The figure of merit for this alloy would be lower (Fig. 6) therefore the cross-sectional area of the legs would have to be increased to maintain the output current, thus increasing the material costs. In the above generator fabrication the Ge.3-Si.7 has first to be prepared in an homogeneous form before assembly of the generator can begin. A more sophisticated preparation technique would be to produce the final multicouple generator during the alloy preparation in a single stage automatic process. This can be achieved by use of the vapour phase deposition method. By mixing the tetrachlorides of silicon and germanium in the vapour state and passing the mixture over a hot surface in the presence of hydrogen, the
Design of Thermo-electric Generator Modules
623
tetrachlorides are decomposed to Ge-Si and HC1, the Ge-Si being deposited on the hot surface. 1200°C
SiCl 4 + G e C l 4 + 4 H 2 ► Ge-Si+8HC1 Dopants in the form of halides can be added to the reaction so that layers of " N " or " P " type impurity can be deposited. By replacing the G e C l 4 a n d hydrogen by carbon dioxide, layers of silica can be deposited between the Ge-Si alloy layers. In this way a sandwich of insulated layers of thermoelectric material can be built u p (Fig. 7). This Ge-Si alloy is stronger and more homogeneous than Ge-Si alloy from the isothermal solidification method. Although the linear deposition rate for the vapour phase method is 0-2 X 10~ 3 in. per min, the deposition carries on at this rate over the whole hot surface so that the volume growth rate can be faster than from the
o o UJ Q
er
UJ
2
A
0
u. O UJ
O u.
2 O
200 400 TEMP °C
600
800
FIG. 6. Variation of figure of merit for change in resistivity.
isothermal solidification method. Typical ingots could be 1 in. X 36 i n . ; the thickness by present heating methods may be limited to 0-5 in. due to the deposition temperature limits and the thermal resistivity of the alloy. T E generator modules cut from these ingots will have a voltage output determined by the number of layers in the ingot and the current output determined by the module length cut from the ingot. Various methods of attaching electrical connections have been investigated, the most successful technique being to alloy tungsten to the Ge-Si alloy, this operation being carried out at 1200°C in vacuum or inert gas. However, tungsten will oxidize at 400°C and this temperature is the highest radiator temperature for operation in air. In conclusion there are applications for large numbers of thermo-electric w
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generators in the 0 to 200 watt range. There are also applications for larger output generators but these may be ultimately taken over by fuel cells or nuclear thermionic generators. The quantity production will depend on the long-term reliability and cost of the generator. Germanium-silicon alloy is a material with long-term stability at the higher temperatures required for efficient use of thermo-electric generators. WASTE GASES
REACTION CHAMBEI Gc Si ALLQS=
^ A
DEPOSITION RESISTANCE
f
■>
\
SECTION A-A
V
(FLUSHING
SUBSTR| ATE HEATED / N Ge3Si7
-Si02 PG«3Si7
)
SUBSTRATE
LINE)
FIG. 7. Ge-Si vapour phase deposition process. REFERENCES 1. ANGRIST, STANLEY W. Direct Energy Conversion. Allyn and Bacon, Boston, 1965. 2. A.G.A. Research Bulletin 101, 46. 3. WARE, R. M. and MCNEILL, D. J . Iron disilicide as a thermo-electric generator material, Proc. IEE Vol. I l l , No. 1, January 1964. 4. WANG, G. C. and ALEXANDER, B. H. Bureau of Ships Contract No. NObsr. 63180, February 17th 1955. U. S. Navy Bureau of Ships, Washington D.C. 5. ABELES, B. and COHEN, R. W. Ge-Si thermo-electric power generator, Journal of Abb Physics Vol. 35, No. 1, 247-248, January 1964. 6. VAN HEYST, H. P. and CUNNINGHAM, T. M. Silicon germanium thermo-electric power modules, Proc. 18/A Annual Power Sources Conference, May 1964.
Design of Thermo-electric Generator Modules
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DISCUSSION B. J. LEIGH (J. Stone (Deptford) Ltd.) : Some 2^ to 3 years ago, some experiments were done at Bell Telephone in which thermo-electric units, in this case they were Peltier units rather than Seebeck units described in your paper, were placed in a magnetic field, and the results were that semiconductor units which normally were not quite as good as the best type of semiconductor unit, became about equal to the best, with a Z of about 3 x 10~3 at ordinary room temperature for thermo-electric units which were heating slightly above room temperature and cooling slightly below. Have any experiments been done putting your high temperature units in a magnetic field ? Answer: We certainly have not done any work of this sort, and I have not come across any reference to this technique applied to silicon-germanium. Si/Ge is a material compatible with our normal work, and I think that using thermo-electric material in a magnetic field would complicate the general use of the generators. The applications that I am really interested in are low-powered generators for gas appliances. I think this is where these thermo-electric units have a positive use. P. W. HAAIJMAN (Philips Research Laboratories, Holland) : Are there bad effects from working with a polycrystalline material? Would it be advantageous to try the mono-crystalline material as is always done in other applications of these semi-conductors ? Answer: I think it would be advantageous to use the single crystal alloy, but at 70% silicon this is rather difficult to obtain by normal preparation techniques; by pulling techniques one can get something like 90% single crystal Si/Ge alloy. One may be able to get single crystal out of the zone traverse technique at extremely slow traverse rates, but I haven't really heard about this. I think the vapour deposition technique will come the closest to giving single crystal Si/Ge alloy at 70% atomic silicon. P. W. HAAIJMAN : You are intending then to start from a single crystal wire ? Answer: No, I think that it is not an economic feasibility to go after single crystal material. I think that the efficiency will be improved vastly by the vapour deposition technique giving a better molecular mixture, an absence of coring and the troubles associated with working from the melt. P. W. HAAIJMAN : Do you think that the p-n junctions will be sharp enough in the deposition technique ? Answer: They may be too sharp. P.JONES (A.E.I.) : I believe that in other spheres of thermo-electric material technology powder metallurgy has been used to prepare specimens. Have you tried this with your Si/Ge? Answer: We have done a little work on this, but not very much. Si/Ge modules have been prepared by powder metallurgy techniques. I think that the powder metallurgy technique has similar drawbacks to the melt technique. Powder metallurgy is very useful, when you can get the correct particle size. G. B. NEWNS (G.P.O. Research Stn.) : Could you say what techniques you use for obtaining oxide film ? Answer: This is done by passing in a mixture of carbon dioxide, hydrogen and silicon tetrachloride, and breaking this down to form an oxide layer. We are not reacting with the silicon germanium alloy, but are actually putting down an oxide layer.
THORP
Ferranti Limited, Manchester, 22, England ABSTRACT Si/Ge alloy is becoming accepted as the most useful high temperature thermo-electric material for fabrication of Thermo-Electric Generators. This paper gives a survey of possible applications for thermo-electric generators together with the required generator operating conditions. The properties of Si/Ge alloy will be described together with various methods of material preparation and generator fabrication including the single stage deposition of multi-couple generators from the vapour phase. The advantages of Si/Ge over the other thermo-electric materials will be discussed and certain immediate applications analysed showing which Si/Ge fabrication technique will most satisfactorily meet the requirements. APPLICATIONS
A survey of applications for thermo-electric generators shows an extensive range of uses from weather stations and space vehicles, to improving the efficiency of domestic heaters. Remote Weather Stations ( 1 ) have been installed in the Arctic Circle and Antarctica by the United States Weather Bureau to enhance the accuracy of its long-range predictions. T h e stationing of personnel in these locations for extended periods of time could be both unpleasant and expensive. Therefore remote weather stations have been constructed to transmit temperature, wind velocity, and precipitation on a 6 hour schedule. These units draw their power from 40,000 curies of strontium 90 whose decay heat is converted directly into electricity by a thermo-electric generator, trickle charging a 32 volt nickel-cadmium battery. T h e radio-active source and generator are buried in the ground in containers designed to prevent accidental contamination of men or animals. These units are designed so that they need not be visited by maintenance personnel more often than about every 10 years. Similar units fuelled with bottled propane have been designed to power V H F - F M radio relay transmitters located in inaccessible terrain which would otherwise be radio dead spots. T o raise the productivity in under-developed countries small self-contained 613
614
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irrigation pumping systems have been developed and powered from thermoelectric generators. These are 50 watt to 200 watt units powered from the sun, whose energy is gathered by means of a collector. The 200 watt output system uses an 8 ft parabolic mirror to focus the sun's rays onto the converter module, which is about 8 in. square by 2 in. deep. The power from this unit can pump to a height of 20 ft enough water to irrigate 4 acres at a rate corresponding to 24 in. of natural rainfall per year, assessing 250 days of sunshine per year with 10 hours sunshine per day. Considerable development has also been carried out on the construction and operating characteristics of a combustion chamber that would accept almost any type of solid fuel, i.e. coal, green wood, charcoal etc. The purpose of this unit is to provide a burner that could operate almost anywhere in the world on an indigenous fuel and supply sufficient heat energy to a thermo-electric generator to give an output of 150 watts. A potentially large market for thermo-electric generators exists in the domestic gas appliance field, using domestic gas supply to heat the generator. In 1959, a gas company in the U.S.A. (2) demonstrated a bathroom heater in which a thermo-electrically driven circulating fan was used. The fan driven by power generated from 24 lead telluride thermo-couples, delivered about 14^ cu. ft. of air per minute. Most of the heat delivered by the unit, however, was by means of direct radiation. A prototype space heater, described in 1960, also used a thermo-electrically operated blower. Thermo-electric modules, located in a portion of the combustion chamber wall, operated with hot junction temperature of 230°G and cold junction temperature of 50°C. The blower delivered 300 to 400 cubic feet of air per minute, and was driven by a HOOr.p.m., d.c. motor, operating at 2-3 volts and 3*5 amps. The thermo-electric generators operated at an efficiency of about 4 per cent. Researchers pointed out that only 10 per cent of the gas input rate to a 100,000 Btu per hour boiler would have to pass through such a device to provide the necessary 120 watts for a circulating pump in a hot water heating system. In 1961, a gas furnace used a thermo-electric generator of four 8 J in. by 4 \ in. modules inserted in the combustion chamber walls, to produce 130 watts at 9 volts and operate an electrically driven air blower. In 1961, a co-operative developmental program was begun in the U.S.A. by a manufacturer and 11 gas utilities to develop a preproduction prototype of a furnace with a thermo-electrically operated blower. These units were subjected to field tests evaluations by participating utilities and the A.G.A. Laboratories. Thermo-electrically operated furnace blowers provide an all-gas furnace. They also have an additional operating advantage from a viewpoint of comfort heating. The blower speed is automatically modulated through thermo-electric power output as the furnace heats up and cools down. Thus, closer control of delivered air temperature is possible than with an unmodulated furnace.
Design of Thermo-electric Generator Modules
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Low wattage thermo-electric generation can also be applied to oceanographic data collection buoys, sea beacons, undersea electronic equipment and cathodic protection of pipe lines. Fail-safe flame detectors operating gas appliance protection devices could be more compact or give greater electrical output using thermo-electric devices in place of thermopiles; power for the operation of electronic control circuits in jet engines can be obtained by locating thermo-electric generators against the hot waste gas stream. T h e most publicized and investigated applications for the thermo-electric devices have been in space. In space two power sources are available, i.e., solar energy and nuclear reactions ; if solar energy were used thermo-electric devices should have a longer radiation belt life than solar cells. However, the emphasis to date has been on nuclear reactors which give a larger power output than that practicable from solar sources. Both the Russian Romashka and the U.S.A. Snap 10A nuclear reactor thermo-electric generators use Si-Ge alloy as the thermo-electric material. These systems give an electrical output of around 0-6 kilowatts. I n the case of SNAP 10A this output is derived from a temperature differential of 305 °F along the thermo-electric element from a hot junction of 935°F to a cold radiator temperature of 600°F. T h e Romashka has a temperature differential of approximately 700°F from 1700°F to a radiator temperature of 1000°F. T H E R M O - E L E C T R I C GENERATOR DESIGN Therefore T E generators from the above applications divide into two basic groups, that is, thermo-electric generators and fuel systems designed together, and thermo-electric generators designed for use with existing equipment and generally making use of waste or diverted heat from that equipment. In the first case the design is limited by cost, space and available fuel. Assuming that the thermo-electric device can use m a x i m u m fuel temperature, this would be around 900°C for a fossil fuel with a 70 per cent to 80 per cent burner combustion efficiency from a regenerative burner. For nuclear energized applications the hot junction temperature could be 900°C using an isotope heat source. In the second case where waste heat or diverted heat is used the source is normally a hot gas or domestic gas b u r n e r ; both these sources give hot junction temperatures around a m a x i m u m of 700°C and will have cold junction temperatures between 100°C and 400°C. At this stage we can consider the best thermo-electric generation material to use. I n this selection of a practical thermo-electric material its stability, inertness, mechanical strength, should be considered together with its thermoelectric characteristics. T h e cost of the material is normally a fraction of the overall thermo-electric generator fabricating costs and therefore is not the main consideration.
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The end column in Table 1 grades materials into the ease with which they may be fabricated into TE generators, grade 1 being the best, with respect to : (i) Protection of the thermo-electric material from oxidation at high temperatures. (ii) Ease of fabricating couples if the material can have doped N or P conductivity. If the material is available in " N " and " P " type conductivity form, the couples may be joined without being subject to differing thermal expansion coefficient which would cause built-in stress at the hot junction. (iii) The ability to alloy the material to a heat sink. (iv) Volatilization of a component element in the thermo-electric material, i.e. Te from PbTe. TABLE 1.
Material BiSb4Te7.5 BiaTe2Se PbTe GeTe(+Bi) GeS FeSi2 Ge.3Si.7
MP °C
—
904
1225
CHARACTERISTICS OF VARIOUS TE MATERIALS
Max. Operating Temp. °G 175 325 625 625 1125 900 925
Density cm8/gram 6-8 7-5 8-15 5-5 5-27
—
3-3
Type P N NorP P N NorP NorP
Max. Figure of Merit X10-* at °C 33 23 12 16 4 2 7
25 25 25 525 925 650 775
Feasibility Grade for TE Fabrication
— 2 2 2 1 1
From the above applications and consideration of thermo-electric materials, it will be seen that Ge.3Si.7 alloy has the preferred characteristics. Its operating temperature is in line with the generator temperatures mentioned, it has the highest efficiency of the high temperature materials, it can be oxidized to produce a S i 0 2 film for protection during operation in air at high temperature, it can be alloyed to other materials, and is mechanically strong. It can also be vapour phase deposited to produce thermo-electric generator modules in one processing stage. FeSi2, can be fabricated by powder metallurgy techniques(3) but the low efficiency of the FeSi2 means that larger generators are required to give comparable output to Ge.3Si.7. Also the Ge.3Si.7 has the higher operating temperature. Preparation of Germanium-Silicon Alloy The Ge-Si system was originally studied by Stohr and Klemm who prepared their specimens by alternately powdering and sintering the alloys
Design of Thermo-electric Generator Modules
617
several times, with annealing times ranging u p to seven months. This technique is completely unsuitable for quantity preparation a n d during 1955 C. C. W a n g and B. H . Alexander ( 4 ) prepared homogenous alloys by the isothermal solidification method. This technique has been the basic method of preparing Ge-Si for thermo-electric devices but is now superseded by other more convenient methods. In preparing Ge-Si alloy by techniques involving freezing from the melt it will be seen from the phase diagram Fig. 1 that if an alloy of a certain composition solidifies under normal conditions, the first particles crystallizing out will be richer in silicon t h a n the original melt. As solidification proceeds, the composition of both the liquid and the freezing solid becomes richer in germanium. Since the diffusion rate in the solid is extremely low the individual grains in the final melt will have a heterogeneous structure I400 |C| L.
LIQ ου
a 2
SOL/
Ι200
Id
/V"' /
SOL
·" MOO
c2
IOOO 1
9 0 Ge 0
20
1
1
1
1
1
40 60 70 ΘΟ Si ATOMIC7o SI
FIG. 1. Ge-Si alloy phase diagram.
varying from a silicon rich core at the centre to a germanium shell at the grain boundaries. T h e isothermal solidification technique carried out at a constant temperature obviates this " coring " difficulty. Figure 2 represents the solidification process at a temperature Tx with composition of liquid and solid phases so adjusted to correspond to Cx and C 2 . T o achieve these conditions in practice, the process may be carried out in a similar m a n n e r to zone melting, but with the molten zone corresponding to Cx and the solid sections to C 2 . If the charge is now drawn through the hot zone of a furnace, a solid phase of constant concentration C 2 precipitates out behind the liquid zone whilst the liquid continuously dissolves the solid charge in front of it. I n this way the liquid is prevented from being depleted of silicon and is maintained at a constant composition C±.
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The success of this technique depends on how closely equilibrium conditions are achieved and maintained. If non-equilibrium conditions exist, local dendrites form and some coring m a y result. I n order to approach equilibrium conditions as closely as possible, the important conditions are the rate of zone travel, the w i d t h of zone, the temperature gradient at the liquid/solid interface. For thermo-electric work the germanium-silicon alloy should have a composition of Ge. 3 -Si. 7 and should have between 5 X 10 1 8 a t / c m - 3 and 5 x 10 20 a t / c m " 3 of added impurity, either phosphorus for " N " type conductivity or boron for the " P " type conductivity. Decreasing the impurity concentration increases the Seebeck voltage b u t decreases the overall efficiency. T h e alloy should also be homogeneous a n d mechanically strong.
\B
y
RFCOIL ~2
o MOLTEN ZONE TRAVERSE DIRECTION
INITIAL CONDITION CAST ALLOY 1 / QUARTZ ENVELOPE
v
GROWTH CONDITION F2|TE.ALLOY|C|
\f
FIG. 2. Isothermal solidification method.
T o achieve these conditions in practice the silicon and germanium is predoped and broken down to small granules. These are then mixed in the proportions to give Ge. 3 -Si. 7 (C 2 ) and Ge e - S i . ^ C j ) . Ingots of each concentration are cast in boats of the same shape as the final isothermal solidification boat. T h e ingots are then cut and assembled in a quartz boat for the isothermal solidification traverse, that is a one inch section of (Cj) is placed after a \ in. section of (C 2 ) and followed by a length of (C 2 ) filling the remainder of the quartz boat. T h e length of the (C x ) section should be sufficient to be completely melted by the heater. T o avoid a mechanically strained alloy the walls of the quartz boats are either coated with SiC or lined with mica. T o achieve the homogeneous alloy the growth rate is to be no faster than 10~ 3 inches per minute. This slow traverse rate allows complete solution of the high melting constituents in the solid charge and the narrow molten zone allows easier transport of silicon from the melting to the freezing interface. Also the rate of zone travel has to be limited to a certain value to avoid any constitutional under-cooling at the growth interface which could lead to a cellular structure
Design of Thermo-electric Generator Modules
619
or dendritic growth. I n our apparatus the molten zone is produced by passing the ingot through a R . F . heater coil directly coupled to the alloy, this direct R . F . coupling helping the transport of silicon across the zone by its stirring effects. T h e slow traverse is controlled by a fixed oil leak in a hydraulic r a m system, the hydraulic system being immersed in a constant temperature oil bath. T h e operation is carried out in an atmosphere of pure argon, and vapour doping techniques m a y be used to compensate for the loss of phosphorus from the melt during the long traverse period. T h e measurements carried out on the growth section of ingot are : (1) Resistivity : although these ingots are polycrystalline they are also of high conductivity so that 2 probe, or 4 probe techniques can be used. (2) Seebeck voltage and conductivity type. (3) Alloy concentration: this is measured by hydrostatic weighing methods relating density to alloy concentration, or X-ray diffraction photographs showing lattice parameter which is related to alloy concentration. Chemical analysis and flotation techniques m a y also be used. (4) T h e r m a l conductivity and measured.
thermo-electric power m a y also
be
Thermo-electric generators can be fabricated from Ge. 3 Si. 7 ingots by two routes. T h e ingots can be cast to the required cross-sectional area and m a d e u p directly into couples, or the ingot can be cut into bars or slices and these slices m a d e into couples. T h e output from a highly efficient Ge. 3 -Si. 7 thermo-electric couple, with a cross-sectional area of 0-44 sq. cm. for each leg is 0*2 V at 11 ·0 A for a temperature drop of 850°C. ( 5 ) This illustrates a main factor in the use and design of T E generators, that is the low voltage high current electrical output. Therefore in practice the output voltage has to be increased either by connecting thermo-electric couples in series, or by use of electronic convenors. Both techniques give a drop in efficiency of approximately 20 per cent. However, for long-term reliability the series connection of couples is preferable. If we consider the first method of fabricating generators, using the as grown ingots as legs in the thermo-couple, we find that the commercial supply of Ge. 3 -Si. 7 shows reduced price per gram as the ingot cross-section increases. Taking a 2 cm diameter ingot, i.e. 3*2 sq. cm. area, the output of a single couple could be 0*2 volt at 80 amps giving a power output of 16 watts for a temperature difference of 850°C. Therefore this type of construction is of use mainly in high power output applications in the 200 to 500 watt region. These generators are constructed by brazing the Ge. 3 -Si. 7 alloy to silicon molybdenum alloy plates which serve as the heat collecting surface, cold end connection being m a d e by alloying the Ge-Si to tungsten pads
620
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located on electrical connectors and then alumina plate insulators are arranged to give the required electrical interconnections.(6) However, a large potential market exists for low power generators of 1 to 30 watts at output voltages of 1 -5 V d.c. to 6 V d.c. These applications can be met using the second method previously mentioned. Devices in this range are relatively low current devices therefore the cross-section of Ge.3-Si.7 in the couple legs should be small but multi-couple assemblies are required. HEAT IN
S!02 COATING
TUNGSTEN PAD
COPPER iCONNECTION G«7-Si.3 (toP IOOO C)
!
HEAT OUT
HEAT OUT
FIG. 3. Diagrammatic view of low power stacked generator.
These multi-couple assemblies can be constructed by stacking alternate slices of " P " and " N " doped Ge-Si alloy (Fig. 3). The slices are oxidized for electrical insulation and the oxide preferentially removed at opposite edges of the slice to allow the slices to be fused together using the connecting strips of Ge.7-Si.3. After the fusing operation the slices, now in electrical series connection, are oxidized in a high water content gas stream at 1000°G and tungsten pads alloyed to the outer legs for electrical output connection. We will now consider the use of this technique to meet an application
Design of Thermo-electric Generator Modules
621
requiring 2 V d.c. at 2 amps for a temperature differential of 500°C from 750°C to 250°C. Experimentally we have recorded outputs of 2 V d.c. from couples of \ in. square cross-section legs for a temperature difference of 1000°C. However, our repeatable outputs have been 1 -5 V d.c. under the above conditions. From the typical output curve Fig. 4 a temperature gradient between ALLOY = Ge. 3 _Si. 7 RESISTIVITY = 0.001 Ücm COUPLE AREA = 0-125"X0-25" COUPLE IMPEDANCE = 0· 15Ω CURRENT
30 2 0 AMPS l-O
200 40O 600 ΘΟΟ x IOOO A T ° C (COLD END 60°C)
VOLTAGE
OS 0-3 VOLTS Ol 200
400 600 ΘΟΟ IOOO A T ° C (COLD END 60°C)
FIG. 4. Output curve for typical TE couple Ge. 3 -Si. 7 .
250°C and 750°C will give 0-33 volt, 2-3 amps, off load. I n the design of a series connected generator the current is determined by the cross-section area of each leg and the voltage by the n u m b e r of couples allowing a 20 per cent voltage drop due to contact resistance, this 20 per cent being an experimentally derived figure. Therefore to achieve 2 V d.c. a total of 8 couples are required. T h e current of 2 A requires a leg cross-section of 0*014 in 2 , with 16 legs the total cross-sectional area of the device is 0-224 in 2 . Therefore a square section generator would have 0-47 in. sides, the individual
622
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couple legs would be slices of Ge-Si alloy 0-06 in. thick. The length of the generator along the temperature gradient would be determined by the conditions available for heat dissipation but typically would be 0-5 in. to 1-0 in. The outputs quoted are measured under " no load " condition so that this generator would in practice be used at 1 -0 V d.c. Also in terms of material cost the larger the ingot cross-section the lower the cost per gram, therefore it would be more economical to have a long narrow generator possibly 1 in. Χθ·22 in. in which case the slices would be 0*025 in. thick. A further consideration in the design is the increasing possibility of generator failure with the increasing number of alloy connections. In this case there are 15 connections including hot and cold junctions. These can be reduced by using Ge-Si alloy with a higher Seebeck voltage (see Fig. 5). However, to make effective use of this characteristic of Ge-Si alloys the generator would have to run at a lower radiator temperature, if possible 100°C, although the higher temperature could be reduced to 600°G. Under these conditions an output of 0-5 V per couple would be achieved;
IOO
300 500 700 9 0 0 IOOO ΔΤ °C (COLD END 60°C)
FIG. 5. Change in voltage output for change in resistivity.
this would reduce the number of alloy connections to 9. The figure of merit for this alloy would be lower (Fig. 6) therefore the cross-sectional area of the legs would have to be increased to maintain the output current, thus increasing the material costs. In the above generator fabrication the Ge.3-Si.7 has first to be prepared in an homogeneous form before assembly of the generator can begin. A more sophisticated preparation technique would be to produce the final multicouple generator during the alloy preparation in a single stage automatic process. This can be achieved by use of the vapour phase deposition method. By mixing the tetrachlorides of silicon and germanium in the vapour state and passing the mixture over a hot surface in the presence of hydrogen, the
Design of Thermo-electric Generator Modules
623
tetrachlorides are decomposed to Ge-Si and HC1, the Ge-Si being deposited on the hot surface. 1200°C
SiCl 4 + G e C l 4 + 4 H 2 ► Ge-Si+8HC1 Dopants in the form of halides can be added to the reaction so that layers of " N " or " P " type impurity can be deposited. By replacing the G e C l 4 a n d hydrogen by carbon dioxide, layers of silica can be deposited between the Ge-Si alloy layers. In this way a sandwich of insulated layers of thermoelectric material can be built u p (Fig. 7). This Ge-Si alloy is stronger and more homogeneous than Ge-Si alloy from the isothermal solidification method. Although the linear deposition rate for the vapour phase method is 0-2 X 10~ 3 in. per min, the deposition carries on at this rate over the whole hot surface so that the volume growth rate can be faster than from the
o o UJ Q
er
UJ
2
A
0
u. O UJ
O u.
2 O
200 400 TEMP °C
600
800
FIG. 6. Variation of figure of merit for change in resistivity.
isothermal solidification method. Typical ingots could be 1 in. X 36 i n . ; the thickness by present heating methods may be limited to 0-5 in. due to the deposition temperature limits and the thermal resistivity of the alloy. T E generator modules cut from these ingots will have a voltage output determined by the number of layers in the ingot and the current output determined by the module length cut from the ingot. Various methods of attaching electrical connections have been investigated, the most successful technique being to alloy tungsten to the Ge-Si alloy, this operation being carried out at 1200°C in vacuum or inert gas. However, tungsten will oxidize at 400°C and this temperature is the highest radiator temperature for operation in air. In conclusion there are applications for large numbers of thermo-electric w
624
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generators in the 0 to 200 watt range. There are also applications for larger output generators but these may be ultimately taken over by fuel cells or nuclear thermionic generators. The quantity production will depend on the long-term reliability and cost of the generator. Germanium-silicon alloy is a material with long-term stability at the higher temperatures required for efficient use of thermo-electric generators. WASTE GASES
REACTION CHAMBEI Gc Si ALLQS=
^ A
DEPOSITION RESISTANCE
f
■>
\
SECTION A-A
V
(FLUSHING
SUBSTR| ATE HEATED / N Ge3Si7
-Si02 PG«3Si7
)
SUBSTRATE
LINE)
FIG. 7. Ge-Si vapour phase deposition process. REFERENCES 1. ANGRIST, STANLEY W. Direct Energy Conversion. Allyn and Bacon, Boston, 1965. 2. A.G.A. Research Bulletin 101, 46. 3. WARE, R. M. and MCNEILL, D. J . Iron disilicide as a thermo-electric generator material, Proc. IEE Vol. I l l , No. 1, January 1964. 4. WANG, G. C. and ALEXANDER, B. H. Bureau of Ships Contract No. NObsr. 63180, February 17th 1955. U. S. Navy Bureau of Ships, Washington D.C. 5. ABELES, B. and COHEN, R. W. Ge-Si thermo-electric power generator, Journal of Abb Physics Vol. 35, No. 1, 247-248, January 1964. 6. VAN HEYST, H. P. and CUNNINGHAM, T. M. Silicon germanium thermo-electric power modules, Proc. 18/A Annual Power Sources Conference, May 1964.
Design of Thermo-electric Generator Modules
625
DISCUSSION B. J. LEIGH (J. Stone (Deptford) Ltd.) : Some 2^ to 3 years ago, some experiments were done at Bell Telephone in which thermo-electric units, in this case they were Peltier units rather than Seebeck units described in your paper, were placed in a magnetic field, and the results were that semiconductor units which normally were not quite as good as the best type of semiconductor unit, became about equal to the best, with a Z of about 3 x 10~3 at ordinary room temperature for thermo-electric units which were heating slightly above room temperature and cooling slightly below. Have any experiments been done putting your high temperature units in a magnetic field ? Answer: We certainly have not done any work of this sort, and I have not come across any reference to this technique applied to silicon-germanium. Si/Ge is a material compatible with our normal work, and I think that using thermo-electric material in a magnetic field would complicate the general use of the generators. The applications that I am really interested in are low-powered generators for gas appliances. I think this is where these thermo-electric units have a positive use. P. W. HAAIJMAN (Philips Research Laboratories, Holland) : Are there bad effects from working with a polycrystalline material? Would it be advantageous to try the mono-crystalline material as is always done in other applications of these semi-conductors ? Answer: I think it would be advantageous to use the single crystal alloy, but at 70% silicon this is rather difficult to obtain by normal preparation techniques; by pulling techniques one can get something like 90% single crystal Si/Ge alloy. One may be able to get single crystal out of the zone traverse technique at extremely slow traverse rates, but I haven't really heard about this. I think the vapour deposition technique will come the closest to giving single crystal Si/Ge alloy at 70% atomic silicon. P. W. HAAIJMAN : You are intending then to start from a single crystal wire ? Answer: No, I think that it is not an economic feasibility to go after single crystal material. I think that the efficiency will be improved vastly by the vapour deposition technique giving a better molecular mixture, an absence of coring and the troubles associated with working from the melt. P. W. HAAIJMAN : Do you think that the p-n junctions will be sharp enough in the deposition technique ? Answer: They may be too sharp. P.JONES (A.E.I.) : I believe that in other spheres of thermo-electric material technology powder metallurgy has been used to prepare specimens. Have you tried this with your Si/Ge? Answer: We have done a little work on this, but not very much. Si/Ge modules have been prepared by powder metallurgy techniques. I think that the powder metallurgy technique has similar drawbacks to the melt technique. Powder metallurgy is very useful, when you can get the correct particle size. G. B. NEWNS (G.P.O. Research Stn.) : Could you say what techniques you use for obtaining oxide film ? Answer: This is done by passing in a mixture of carbon dioxide, hydrogen and silicon tetrachloride, and breaking this down to form an oxide layer. We are not reacting with the silicon germanium alloy, but are actually putting down an oxide layer.