THERMOELECTRIC MATERIAL FOR LOW TEMPERATURE USE AND METHOD OF MANUFACTURING THE SAME

Abstract: This invention relates to a method of manufactur­ ing thermoelectric material which has the steps of quenching a thermoelectric alloy in a molten state at a quenching rate higher than 10³°C/sec into a membrane or powdery form and subjecting the membrane or powder to cold-forming or sintering. The thermoelectric alloy is a Bi-Sb series alloy having a composition represented by {(Bi 100-x ·Sb x ) 100-y ·E II y } 100-z ·E I z where E I represents a group III or group IV element, E II represents a group IV or group VI element, x represents a number of 5 - 20, y represents an integer of 0 - 20 and z represents a number of 0.05 - 10, respectively.

Description: Bi-Sb THERMOELECTRIC MATERIAL FOR LOW TEMPERATURE USE AND METHOD OF MANUFACTURING THE SAME BACKGROUND OF THE INVENTION Field of the Invention: This invention concerns a Bi-Sb series thermo­ electric material showing high performance at low tempera­ ture (77-200°K), as well as a method of manufacturing such material. More specifically, the invention relates to a thermoelectric material composition of a p-type Bi-Sb series alloy which is useful as a leg member for an electronic cooling module utilizing the Peltier effect, or a leg member for use in a cold heat source power generating medium utilizing the Seebeck effect that has not yet been obtained, as well as a method of manufacturing the same. Description of the Prior Art: Heretofore, a process of subjecting powder to molding and sintering (hereinafter referred to as a press sinter method) or to a hot press method has been used for the manufacture of thermoelectric material. The characteristics of the thermoelectric material are represented by the figure of merit Z: Z ≡ α² x σ/K where α: Seebeck constant σ: electroconductivity K: heat conductivity While α is not substantially effected by the crystal grain size, σ and K become smaller generally as the crystal grain size becomes finer. It has been reported in the literature for a Si-Ge series thermoelectric material (Rowe, Sinter Theory Pract. p.487-495, 1982) that if the grain size becomes finer exceeding a certain extent, only K becomes smaller while σ remains unchanged. Accordingly, since the figure of merit Z becomes greater as the crystal grain size becomes finer, the press sinter method or the hot press method using a powder of fine crystal grain size has generally been used. Further, it has been generally known that the Bi-Sb series alloy forms an n-type semiconductor having a band gap of about 0.015 eV within a restricted range at a low temperature region (for instance Bi 95 Sb₅-Bi 80 Sb 20 at 4.2°K) and that an excellent Peltier effect is shown in the low temperature region (refer, for example, to Japanese Patent Publication No. Sho 38-15421). In fact, the n-type Bi-Sb alloy is an intrinsic semiconductor, in which electrons and positive holes are present substantially in an equal number. However, it has been said that since the mobility of the electrons is greater than that of the positive holes, it shows an n-type conduction (refer, for example, to T. Aono and S. Aizawa in "Study on Thermal Gap of Bi-Sb Alloys", Tokyo Denki Univ.). A structural rearrangement of the valence band, and the concentration dependence of the thermal emf coefficient α for x = 0-20, in Bi 100-x Sb x alloys with increasing Sb concentration, and with an increasing role of impurity holes, were found (Y.N. Saraev et al. Chem. Abstr. Vol. 79 No. 18 (1973), p.292, abstract no. 109261t). The effects of the addition of up to 10 mol% Si, Ge, Sn, Pb, Ga, Sb and InSb to Bi 0,88 Sb 0,12 on the figure of merit were determined (V.V. Leonov, Chem. Abstr. Vol 101 (1984), p.587; abstract no. 64a365j). Also, the effect of plastic deformation on transfer processes in Bi-12 at% Sb-Sn alloys was studied at 77-300°K. The plastic deformation increased the electrical resistance at all temperatures, whereas thermal emf increased at low temperatures, and decreased at higher temperatures. Decreasing the Sn content increased the power factor (V. Prokoshin et al, Chem. Abstr. Vol. 102 (1985) p.268; abstract No. 10951z). Further, it has also been reported that while single crystals of Bi-Sb in which group IV elements, Sn, Pb, etc. are solid-solubilized by several hundreds ppm show a p-type conduction in a cryogenic, so-called, impurity region, they turn to the n-type as the temperature increases (refer, for example, to W. Yim and A. Amith, Solid-State Electronics, 1972, Vol. 15, p.1141-1165). Accordingly, it is impossible to manufacture Bi-Sb series alloys which form a p-type from the cryogenic tempera­ ture near the room temperature by a Bridgeman or zone melting method intending for the manufacture of single crystals and, therefore, such p-type Bi-Sb alloys have not yet been found and manufactured. In the following descriptions, only Bi-Sb alloys forming a p-type from the cryogenic to room temperature are referred to as the p-type Bi-Sb alloy. In this way, although Bi-Sb alloys are widely known as thermoelectric materials showing high performance at low temperature, since only the n-type material could be prepared, it has not at present been put to pratical use as the leg member for an electronic cooling module. Further, although the powder used for the press sinter method or the hot press method as described above is generally prepared by mechanical pulverization, since the amount of impurities incorporated is increased during pulverization over a long period of time, those having an average grain size of about several microns are used. As a result, the crystal grain size of the obtained sintering product is also of about several microns and it is impos­ sible to manufacture thermoelectric material having a finer crystal grain size. SUMMARY OF THE INVENTION This invention has been achieved in view of the foregoing situation and the object thereof is to provide a method of manufacturing thermoelectric material having an excellent figure of merit Z by quenching a thermoelectric alloy in a molten state to prepare membrane or powdery products having a sub-micron order of crystal grain size, and cold-molding and sintering them thereby obtaining a thermo­ electric material having a sub-micron order of crystal grain size. Another object of this invention is to provide a Bi-Sb alloy forming a p-type from cryogenic to room temperature, as well as a method of manufacturing the same. A further object of this invention is to provide a Bi-Sb series thermoelectric material showing high perfor­ mance at a low temperature, for example, 77-200°K which is useful as a leg member for an electronic cooling module or leg member for a cold heat source generating module, etc. as well as a method of manufacturing such material. For attaining the various objects as described above, the invention provides a method of preparing a thermoelectric alloy characterized in that a p-type Bi-Sb-thermoelectric alloy having a composition represented by where E I represents a group III or group IV element, E II represents a group IV or group VI element, x represents a number of 5-20, y represents an integer of 0-20 and z represents a number of 0,05-10, respectively, is quenched from a molten state at a quenching rate higher than 10³°C/sec and capable of producing the non-equilibrium phase into a membrane or powdery form, and in that the so formed membrane or powder is subjected to cold-forming or sintering. In a preferred embodiment of the invention, E I represents a group IV element and y = 0. Advantageous and preferred embodiments of the invention are defined in the subclaims. The above and other advantages, features and additional objects of the invention will become manifest to those versed in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principles of the present invention are shown by way of illustrative example. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic sectional view illustrating one embodiment of a device for practicing the method of manufacturing a thermoelectric material according to this invention; Figure 2 is a photograph of the texture observed under a transmission electron microscope of 30,000 magni­ fying ratio of a thin film of thermoelectric material obtained in Example 1 according to this invention, Figure 3 is a photograph of the texture observed under an optical microscope of 1000 magnifying ratio of sintering products of thermoelectric material obtained in Comparative Example 1: Figure 4 is a graph illustrating the temperature-­ dependent change of the Seebeck constant for a p-type Bi-Sb alloy membrane obtained in Example 2 according to this invention; Figure 5 is a graph illustrating the temperature-­ dependent change of the Seebeck constant for a conventional n-type Bi-Sb alloy membrane obtained in Comparative Example 2; Figure 6 is a graph illustrating the temperature-­ dependent change of the Seebeck constant for a Bi-Sb element prepared by a Bridgeman method, and Figures 7, 8 and 9 are graphs illustrating the temperature-dependent change of the Seebeck constant of a p-type Bi-Sb alloy membrane prepared in Examples 3, 4 and 5 respectively according to this invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention will further be described more in detail while referring to the accompanying drawings. In the method of obtaining thermoelectric material having a sub-micron order of crystal grain size, thermoelectric alloy 3 is charged in a molten pool 4 and heated by high frequency coils 2, and the thermoelectric alloy 3 is rendered into a molten state as shown in Figure 1. While rotating a metal roll 1 (about 200 mm diameter and 20 mm width) at a circumferential speed from 5 to 40 m/sec, molten metal is jetted out by an inert gas (pressure: 0.1 kg/cm²-4 kg/cm²) from the molten metal pool 4 onto the roll 1 to prepare a membrane or powder. Thermoelectric material is prepared by cold-­ molding (or cold-forming) or sintering the membrane or powder. In the manufacturing method for the thermoelectric material as described above, since crystallization occurs everywhere upon quenching, a membrane or powdery thermoelectric material having a sub-micron order of crystal grain size can be obtained. Then, the thermoelectric material having a sub-micron order of crystal grain size can be obtained by cold-molding or sintering at a temperature below the crystal grain size coarsing temperature. As a result, it is possible to manufacture a thermo­ electric material having a figure of merit Z more excellent than the thermoelectric material prepared by the conven­ tional press sinter method or hot press method. Example 1 (outside the scope of claim 1) A thermoelectric alloy having a Bi 88 Sb 12 composi­ tion was heated to about 600°C into a liquid phase. The molten alloy was jetted to a Cu roll rotating at a circum­ ferential speed of about 10 m/sec which is substantially equal to the circumferential speed of a 200 mm diameter roll when rotated at 500 rpm, at a gas jetting pressure of about 1.0 km/cm² to prepare a membrane of about 20 mm length, about 2 mm width and about 30 µm thickness. When the membrane was observed under a transmis­ sion electron microscope of 30,000 X, it had a texture shown by the photograph in Figure 2. Comparative Example 1 A thermoelectric alloy having a Bi 88 Sb 12 composi­ tion was heated to about 600°C into a liquid phase. An ingot after cooling in the furnace from the liquid state was pulverized in a ball mill for about 5 hours and then sintered in an Ar atmosphere at about 180°C and 300 kg/cm² of pressure for 10 minutes. When the sintering product was observed under 1000 X optical microscope, it has a texture shown by the photograph in Figure 3. Comparative Example 1 shows one example of the thermoelectric material prepared by the hot press method which is the prior art as described above. The crystal grain size of the sintering product is several microns, Although a sintering product of finer crystal grain size can be obtained by using ultra-fine grains as the powder, it is not practical since the cost for the thermoelectric material is increased. As shown in Example 1, a quenched membrane having a sub-micron order of crystal grain size could easily be obtained by the method according to this invention. By bundling the membranes together and subjecting them to cold working, and supplying an electrical current in the direction perpendicular to the direction of the film thickness, it can be used as thermoelectric material having excellent performance. Quenched powder having a sub-micron order of crystal grain size can be obtained by changing the number of the roll rotation and the jetting pressure upon quenching. In the case of preparing a sintering product by hot pressing the quenched powder, the production work is carried out below a temperature at which the crystal grains become coarser. In the quenching method, that is, in the method according to this invention, the shape of the quenching product changes from the membrane to the powdery state as the number of the roll rotation is increased and as the jetting pressure is lowered. If the number of the roll rotation is below the circumferential speed of 5 m/sec and the jetting pressure increases higher than 4 kg/cm², disadvantages are brought about, such as an increase in the thickness of the membrane to reduce the quenching rate and winding of the membrane around the roll. On the other hand, if the number of the roll rotation increases to greater than 40 m/sec of the circumferential speed and the jetting pressure goes lower than 0.1 kg/cm², the powder becomes finer to worsen the recovery rate. Similar effects can also be obtained by the method other than the quenching method using the roll shown in Example 1, that is, an atomizing method and spin-desk method, if the powder or membrane is prepared at a quenching speed of greater than 10³°C/sec and then subjecting them to cold-forming or sintering. Further, it has been found by the study of the present inventors that the following composition is required for obtaining a p-type Bi-Sb alloy. where E I represents a group III or a group IV element, E II represents a group IV or a group VI element, x represents 5-20, y represents 0-20 and z represents 0.05-10, in which it may sometimes be necessary for obtaining the alloy composition described above to prepare a forced solid-solution by using a quench roll method from the state where a complete 1-liquid phase is formed. That is, a Bi-Sb series thermoelectric material according to this invention is prepared by using Bi 100-x Sb x (where x = 5-20) forming an intrinsic semiconductor as the Bi-Sb series matrix alloy, adding the group III or IV element by 0.05-10 at% as the p-type dopant and further adding 0-20 at% of group IV, VI elements as required for improving the performance of the thermoelectric material upon practical use. In the case of adding the group IV element as the p-type dopant, its not necessary to add the group IV, VI element. The p-type Bi-Sb alloy can be obtained in accord­ ance with the method of this invention by solidifying a Bi-Sb series alloy in a molten state at a quenching speed capable of forming a non-equilibrium phase. Specifically, a Bi-Sb series alloy 3 is charged to a molten alloy pool 4 in the device as shown in Figure 1 and heated by the high frequency coil 2 to thereby render the Bi-Sb series alloy into a molten state. While on the other hand, the metal roll 1 (about 200 mm and 20 mm width) is rotated at 500-4000 rpm, while the molten alloy is jetted out under an inert gas pressure (0.5-4 kg/cm²) onto the roll to be quenched and solidified. It may be possible to prepare the p-type Bi-Sb alloy, not by using the quenching roll method, but by a method of rapid solidification (for example a quenched powder) capable of adding a greater amount of p-dopant than in the equilibrium solidification. Further, since no quenched membrane of good quality can be obtained in the quenching roll method des­ cribed above unless the production conditions are set within the range for the number of roll rotation from 500 to 4000 rpm and the gas jetting pressure from 0.5 to 4 kg/cm², the conditions are preferably set within the above-mentioned range. While it is possible to add the p-type dopant only by an amount solid-solubilized in the equilibrium solidifi­ cation (about several hundreds ppm) in the conventional Bridgeman method or zone melting method, it is possible to add the p-dopant by an amount more than that in the equili­ brium solidification and, as a result, the p-type Bi-Sb alloy that could not yet be manufactured can be made. That is, as has been described above for the prior art, while the Bi-Sb alloy incorporated with the group IV element by several hundreds of ppm in the equilibrium solidification turns from p to n type as the temperature increases (refer to Figure 6), a Bi-Sb alloy showing the p-type conduction at 77°K - room temperature can be obtained by adding from 0.05 to 10 at% of the group III or group IV element as the p-type dopant to the intrinsic semiconductor of Bi 100-x Sb x (x = 5-20) (refer to Figures 4, 7, 8 and 9). In the case of the Bi-Sb alloy in which the p-type dopant Sn is added by an amount less than that for equili­ brium coagulation by the conventional method, although the alloy forms a p-type since the positive hole density is higher than the electron density due to the p-type dopant Sn at low temperature, it forms an intrinsic conduction region where the concentrations of positive holes and the electrons are substantially equal to each other as the temperature increases. Accordingly, electrons of greater mobility become predominant for the conduction and the conduction type turns to the n-type (refer to Figure 6). A similar phenomenon is also reported for the group IV element Pb, etc. other than Sn (for example, refer to G.E. Smith and R. Wolfe, Journal of Applied Physics, Vol. 33, 841 (1962)). On the contrary, it is considered that in the case of adding from 0.05 to 10 at% of the p-dopant which is greater than the equilibrium solidification amount as in this invention, since the positive hole concentration is still higher than the electron concentration near the room temperature due to the group III element (Al, Tl, etc.) or group IV elements (Sn, Pb, etc.) added, it shows p-type conduction. If the addition amount of the group III element or group IV element is less than 0.05 at%, the p-type conduction is no longer shown in the vicinity of the room temperature. On the other hand, it is not practical to increase the addition amount of the element by greater than 10 at% (for the practical use, the carrier concentration is controlled to about 10 19 -10 20 ). Further, in the p-type Bi-Sb alloy according to this invention, group IV and group VI elements may be added within a range not impairing the p-type conduction for lowering the heat conductivity and improving the performance of the thermoelectric material upon practical use. As a matter of fact, an addition of the group IV, VI elements is not necessary. Addition of the group IV, VI elements in excess of 20 at% is not desirable since the thermoelectric perform­ ance as the Bi-Sb series alloy is impaired. Example 2 Sn was added by 1 at% as the p-type dopant to a Bi-Sb alloy having the composition: Bi 88 Sb 12 and heated to about 600°C into a uniform liquid phase in an Ar atmos­ phere at about 1 atm. The molten alloy was jetted out from the state onto a Cu roll rotating at about 1000 rpm under a gas jetting pressure of about 1.0 kg/cm² to prepare a membrane of about 20 mm length, about 2 mm width and about 30 µm thickness. When the Seebeck constant of the thus obtained membrane was measured, the results shown in Figure 4 were obtained. Comparative Example 2 A quenched membrane having the composition: Bi 88 Sb 12 was prepared in the same method procedures as in Example 1 and the Seebeck constant thereof was measured to obtain results as shown in Figure 5. Comparative Example 3 An alloy having the same composition as that in Example 2 was formed into a uniform liquid phase at 600°C, solidified by the Bridgeman method at a temperature slope of about 40°C/cm and a solidifying rate of 0.76 mm/hr to prepare a Bi-Sb device of 10 mm diameter and 150 mm length. When the Seebeck constant at the central portion of the device was measured, the results as shown in Figure 6 were obtained. As is apparent from Figure 4, the Seebeck constant of the alloy membrane prepared in Example 2 (membrane obtained by solidifying the molten alloy having the composition: (Bi 88 Sb 12 ) 99 Sn₁ by the quenching method) was positive as far as 77°K - room temperature, that is, it shows the p-type conduction. While on the other hand, as shown in Figure 5, the Seebeck constant of the alloy having the composition: Bi 88 Sb 12 in Comparative Example 1 is negative as far as till 77°K - room temperature, that is, it shows the n-type conduction. The situation is the same for the alloys of Bi 100-x Sb x , x = 5-20, not containing the p-type dopant. While on the other hand, as shown in Figure 6, the Seebeck constant of the device prepared from the molten alloy having the composition: (Bi 88 Sb 12 ) 99 Sn₁ in Comparative Example 3 by the Bridgeman method (since only about 0.03 at% of Sn can be solid-solubilized into Bi 88 Sb 12 if prepared by the Bridgeman method, it is actually impossible to prepare the composition: (Bi 88 Sb 12 ) 99 Sn₁ and it only forms single crystals containing 0.03 at% Sn to Bi 88 Sb 12 ) turns from positive to negative along with an increase of the tempera­ ture as shown in Figure 6, that is, it turns from the p-type to the n-type. Example 3 A quenched membrane having the composition: {(Bi 88 Sb 12 ) 95 (PbSe)₅} 99 Ga₁ was prepared in the same procedures as in Example 1 and when the Seebeck constant thereof was measured, the results as shown in Figure 7 were obtained. Example 4 A quenched membrane having the composition: {(Bi 88 Sb 12 ) 95 (PbTe)₅} 99 Tl₁ was prepared in the same procedures as in Example 1 and when the Seebeck constant thereof was measured, the results as shown in Figure 8 were obtained. Example 5 A quenched membrane having the composition: {(Bi 88 Sb 12 ) 94 (Pbse)₆} 99 Al₁ was prepared in the same procedures as in Example 2 and when the seebeck constant thereof was measured, the results as shown in Figure 9 were obtained. As has been explained above according to this invention, thermoelectric material having a sub-micron order of crystal grain size can be obtained, by quenching the thermoelectric alloy in a molten state, crystallization occurs everywhere to produce a membrane or powdery product having a sub-micron order of crystal grain size and, by subjecting them to cold-forming or sintering.

Claims: 1. A method of preparing a thermoelectric alloy, characterized in that a p-type Bi-Sb-thermoelectric alloy having a composition represented by where E I represents a group III or group IV element, E II represents a group IV or group VI element, x represents a number of 5-20, y represents an integer of 0-20 and z represents a number of 0,05-10, respectively, is quenched from a molten state at a quenching rate higher than 10³°C/sec and capable of producing the non-equilibrium phase into a membrane or powdery form, and in that the so formed membrane or powder is subjected to cold-forming or sintering. 2. A method as defined in claim 1, wherein E I represents a group IV element and y = 0. 3. A method as defined in claims 1 or 2, wherein the Bi-Sb series alloy in the molten state is quenched to solidify by jetting out the molten alloy onto a metal roll of 200 mm diameter and 20 mm width rotating at 500-4000 rpm under a jetting pressure of from 0.5 to 4 kg/cm². 4. A method as defined in any of claims 1 to 3, wherein the membrane or powdery form of the thermoelectric alloy having a crystal grain size of a sub-micron order of magnitude is cold-formed or sintered at a temperature below the crystal grain size coarsening temperature, 5. A method as defined in any of claims 1 to 4 wherein said Bi-Sb series alloy is prepared by using Bi 100-x ·Sb x (where x = 5-20) forming an intrinsic semiconductor as the Bi-Sb series matrix alloy, adding the group III or group IV ele­ ment as the p-type dopant within the stated range; and adding the group IV, VI elements as required for group III dopant.

Inventors: OKUMURA, TAKUJI K.K. KOMATSU SEISAKUSHO
Applicants: KABUSHIKI KAISHA KOMATSU SEISAKUSHO
Application year: 1987
Main classification: C22C1
IPC codes: C22C12, H01L35, C22C1