GB2050437A

GB2050437A - Sintering spherical granules of thermoelectric alloys

Info

Publication number: GB2050437A
Application number: GB7919373A
Authority: GB
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1979-06-04
Filing date: 1979-06-04
Publication date: 1981-01-07
Also published as: GB2050437B

Abstract

Sintered alloy articles, particularly thermoelectric elements of bismuth selenide telluride or bismuth antimony tellurides are made from spherical particles up to 2.5 mm diameter which are produced by forcing a melt of the alloy 14 through through apertures 6, the particles falling into liquid paraffin 10. <IMAGE>

Description

SPECIFICATION Method of manufacturing articles by powder metallurgy The invention relates to a method of manufacturing an article from an alloy by a modified powder metallurgy technique, and to athermoelectric element manufactured by such a method.

Powder metallurgy techniques have been widely used in making various materials, for example nickel alloys and thermoelectric materials, which are used in the electronics industry, instead of fusion and casting, or fusion and crystallisation techniques.

When it is necessary to produce thermoelectric elements consisting of a material in which segregation occurs during crystallisation from a melt, a more homogeneous product can be obtained by compacting powder formed bycomminution of an ingot formed from a melt of the material, and sintering the compacted powder, than can be obtained by casting articles from the melt or by growing crystals from the melt.

When processing expensive thermoelectric materials, powder metallurgy methods are significantly cheaper than crystallisation methods because it is normally possible to directly produce elements of a desired size by powder metallurgy techniques, whereas considerable machining is required when producing elements from an ingot or from a grown crystal, with the attendant cost of machining and production of considerable quantities of scrap material.

There are two main methods of making thermoelectric materials such as combinations of Sb2Te3 and Bi2Te3 or of Bi2Te3 and Bi2Se3. A first method starts from a mixture of the elemental powders which is compacted and sintered. This method usually involves liquid phase sintering since the melting-point of the lowest melting-point element or compound is usually exceeded during sintering. A more common method is to start with a homogeneous alloy powder and to sinter it to produce a finished element. F. Wendler and H. Schreiner describe a method of the latter type in Metallkunde, 56,583(1965), used to makep-type material consisting of (Sb2Te3)0.74(Bi2Te3)0.26 doped with 0.2 to 0.3% by weight Pb.This material was prepared by melting the constituent elements, casting the melt to form an ingot, pulverizing the ingot and then finely grinding the material to a grain size of < 0.05 mm. This pow derwascompacted at4tons/cni, and the com- pacted material was sintered for 5 hours in hydrogen at 370"C, the sintered product having a density of 6.5 g/cm3. The thermoelectric figure of meritz for this material was in the range from 2.1 to 2.45 x 10-3 per Celsius degree, wherez = ot21(K, a being the Seeback coefficient in ,uV/Celsius degree, being the resistivity of the material in ohm.cm. and K being the thermal conductivity of the material. Microscopic examination of material produced by a similar method showed that the material was not homogeneous.It appears that this was due to the powder used to form the compacted material was not homogeneous in composition, due to segregation occurring as the melt solidified.

An object of the invention is to provide a method of preparing homogeneous materials by a modified powder metallurgy technique.

The invention provides a method of manufacturing an article from an alloy by a modified powder metallurgy technique, the method comprising the steps of preparing a melt of the alloy, processing the melt to form drops of the alloy which solidify into substantially spherical particles of the alloy having diameters of up to 2.5 mm, pressing the said particles so as to form a compact, and sintering the compact to form the said article.

The method according to the invention affords several advantages. The composition of the substantially spherical particles is inevitably identical with the melt composition from which the particles are formed. This contrasts with the situation when crystallizing a melt in which significant segregation occurs, the crystals produced having non-uniform compositions. It was found when using the method according to the invention to produce unsintered compacts from alloys consisting of mixtures of Bi2Te3 and Sb2Ti3 having densities which were 96% or 97% of the theoretical density of the material when the particles had been cold-pressed or hotpressed respectively.Since it is possible to obtain compacts having such high green densities, it is possible to make deeper compacts by the method according to the invention than would be possible by conventional powder metallurgy techniques without having undue porosity in the middle of the compacts. This result was quite unexpected. When using conventional powder metallurgy techniques to make articles from finely-divided metal powders, the unsintered compacts formed normally have much lower densities, which are only of the order of 60% of the theoretical density of the relevant metal. Since the method according to the invention does not include any grinding steps or need to use any fine powders, toxicity problems which occur when conventional powder metallurgy methods are used to process toxic material, for example materials containing selenium, antimony or tellurium, are avoided.

The substantially spherical particles of the alloy may be formed by a "shot tower" technique, that is to say by forcing the melt through apertures in an apertured plate so as to form drops of the alloy, and allowing the drops formed to fall through a reducing or neutral gaseous atmosphere. By using a "shot tower" having a sufficient height, at least a surface layer of each drop will have solidified by the time the drops have reached the bottom of the tower. In order to prevent conglomeration of the particles formed from these drops, the particles are preferably collected in a bath of a liquid, for example glycerol, water or medicinal paraffin, which is inert to the alloy.

The method according to the invention may include the steps of forming the alloy by melting the constituent materials in a closed vessel, quenching the melt, placing the quenched material in a crucible having an apertured bottom, heating the material in the crucible to a temperature not more than 30 Cel sius degrees above the melting-point of the alloy, and forcing the molten alloy through the apertures in the bottom of the crucible so as to form drops of the alloy, and allowing the drops to solidify into substantially spherical particles.

The alloy processed by the method according to the invention may consist of Bi2Te3. Thermoelectric articles may be made by a method according to the invention in which the melt consists of a mixture of from 20 to 30% by weight Bi2Se3 and from 80 to 70% by weight Bi2Te3, together with one or more n-type dopants. The melt may consist of a mixture of from 20 to 30% by weight Bi2Te3 and from 80 to 70% by weight Sb2Te3, together with one or more p-type dopants.

An embodiment of the invention will now be described with reference to the following Example and to the drawing, in which: Figure lisa schematic longitudinal section of a rocking furnace used to prepare a melt of an alloy, Figure 2 is a schematic side sectional elevation of an apparatus in which substantially spherical parti clesofan alloy were produced from a melt of the alloy, Figure 3 is a plan view of a graphite crucible forming part of the apparatus shown in Figure 2, and Figure 4 is a schematic side sectional elevation of a mould assembly used to form a compact from substantially spherical particles of the alloy.

Example A mixture was prepared which consisted of 379 Sb2Te3, 13g Bi2Te3, 1.5g Te and 0.05 Pb. This mixture was loaded into a fused silica tube 1 (Figure 1) having an internal diameter of lOmm. The tube 1 was filled with 10 mbar of hydrogen and was then sealed.

The tube 1 was inserted in a rocking furnace 2 and was maintained at 900"C for 2 hours so as to form a melt 3 ofp-type (Sb2Te3)0.,4(Bi2Te3)0.26:Pb. The tube 1 was cooled to 700"C over a period of one hour, and then the contents of the tube 1 were quenched by placing the tube 1 in a molten tin bath (not shown) maintained at 350"C. The tube 1 containing an ingot was removed from the molten tin bath, the tube was opened and removed from the ingot The ingot was broken into sections which were loaded into a graphite crucible 4 (Figure 2) having inside and outside diameters of 35 and 48 mm respectively. The bottom 5 of the graphite crucible 4 had a number of 0.7mm diameter apertures 6 (Figure 3).The graphite crucible 4 fitted in an asbestos collar 7 disposed near the top of a fused silica tube 8 having an internal diameter of 50mm. A dish 9 containing medicinal paraffin 10 was placed at the bottom of the tube 8, the bottom 5 of the crucible 4 being approximately 1 metre above the surface of the med icinal paraffin 10. An atmosphere consisting of a mixture of 90 volumes of N2 and 10 volumes of H2 was passed into the tube 8 both above and below the crucible 4. A steady stream of this mixture was pas sed through an inlettube 11 and flowed out through an outlet tube 12. The crucible 4was heated by means of an H.F. coil 13, until the ingot sections had melted in the crucible 4to form a melt 14. The temp erature of the melt 14 was maintained at620 C which was 20"C above the melting-point of this alloy.The surface tension of the melt 14wassufficientatthis temperature to prevent the melt 14 from flowing through the apertures 6 in the bottom 5 of the crucible 4 until an excess gas pressure had been created in the tube 8 above the crucible 4 by increasing the pressure in this volume by passing the nitrogenhydrogen mixture through a tube 15 at the top of the tube 8, an excess pressure of approximately 14kPa being sufficientforthis purpose. The melt 14 was thereby forced through the apertures 6 in the bottom 5 of the crucible 4 and formed drops 16 which solidified at least superficially while falling from the crucible 4 before entering the medicinal paraffin 10 in the dish 9 as substantially spherical particles 17 having diameters in the range from 0.3 to 1.5 mms.

The particles 17 were removed from the liquid paraffin, and were drained, washed with acetone and were air dried.

The dry particles were placed in a 0.7 x 0.7 mm section floating cavity mould 18 having a base portion 19 and were compressed by means of a plunger 20 which exerted a pressure of 5 tons/sq.cm. to form a compact 21 having the dimensions 50 x 0.7 x 0.7 mm. The compact was then sintered in hydrogen for 2 hours at 380 C. The value ofzofthe sintered compact was 2.4 x 10-3 per Celsius degree, and its conductance was 1400 LSlcm. After annealing, the value ofz was 3 x 10-3 per Celsius degree, and the conduc- tance was 1200 S/cm.

Claims

1. A method of manufacturing an article from an alloy by a modified powder metallurgical technique, the method comprising the steps of preparing a melt of the alloy, processing the melt to form drops of the alloy which solidify into substantially spherical particles of the alloy having diameters upto 2.5 mm, pressing the said particles so as to form a compact, and sintering the compact to form the said article.

2. A method as claimed in Claim 1, wherein the substantially spherical particles of the alloy are formed by forcing the melt through apertures in an apertured plate so as to form drops of the alloy and allowing the drops formed to fall through a reducing or neutral gaseous atmosphere.

3. A method as claimed in Claim 1 or Claim 2, including the steps of forming the alloy by melting the constituent materials in a closed vessel, quenching the melt, placing the quenched material in a crucible having an apertured bottom, heating the mater ial in the crucible to a temperature not more than 30 Celsius degrees above the melting-point of the alloy, forcing the molten alloy through the apertures in the bottom of the crucible so as to form drops of the alloy and allowing the drops to solidify into substantially spherical particles.

4. A method as claimed in any preceding Claim, wherein the compact is formed by pressing substantially spherical particles having diameters in the range from 0.3 to 1.5 mm.

5. A method as claimed in any preceding Claim, wherein the alloy consists of Bi2Te3.

6. A method as claimed in any of Claims 1 to 4, wherein the melt consists of a mixture of from 20 to 30% by weight Bi2Se3 and from 80 to 20% by weight Bi2Te3,togetherwith one or more n-type dopants.

7. A method as claimed in any of Claims 1 to 4 wherein the melt consists of a mixture of from 20 to 30% by weight Bi2Te3 and from 80 to 70% by weight Sb2Ti3, together with one or more p-type do pants.

8. A method of manufacturing an article from an alloy by a modified powder metallurgical technique, substantially as herein described with reference to the drawing.

9. Athermoelectric element manufactured by a method as claimed in any preceding Claim.