RU2607299C1 - Method of producing composite branch of thermoelement operating in range of temperatures from room to 900 °c - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: invention relates to thermoelectric energy conversion and can be used during manufacturing of thermoelectric composite thermoelement branches, meant for making electric power generators with high coefficient of conversion. Proposed method of making composite thermoelement branches connection sections of low-, medium- and high-temperature thermoelectric materials includes preliminary application of metal coatings on ends of joined sections, selected from: nickel, indium, iron, molybdenum, tungsten, forming binding layer between sections, installation of all sections in series at increasing operating temperature of butt joint so, that between low and medium temperature sections, as well as between medium- and high-temperature sections binding layers are formed of layer stack Ni/In/N. Assembled mold is placed in plant of spark plasma sintering and heated to temperature of 450–460 °C. It is kept at this temperature for 5–8 minutes at pressure of 3–5 Mpa, in vacuum, until dissolution of boundary part of nickel in molten indium. Then it is cooled to room temperature and isothermal annealing is made in electric furnace with controlled temperature in atmosphere containing 97 % of argon and 3 % of hydrogen, at temperature of 420±2 °C at pressure of 0.5–1.0 MPa for 6–10 hours to produce high-temperature intermetallic compound Ni₂In₃. After diffusion welding process mold is gradually cooled to room temperature.

EFFECT: technical result is reliable and strong connection of sections of composite thermoelement branches.

6 cl, 2 dwg, 1 tbl

Description

The invention relates to the field of thermoelectric energy conversion, in particular to the production of thermoelectric composite branches of a thermoelement intended for the manufacture of power generators with a high conversion coefficient.

Thermoelectric generators (TEG) can be used in systems for the utilization of "waste" heat (the volume of which in the world makes up more than 50% of all generated energy) of both technogenic and natural origin. For example, the conversion of waste gas heat from waste incineration plants into household electricity can significantly reduce its cost to consumers. Broad prospects for their use in road and rail transport, in metallurgical processes, as well as in space, etc. open up for TEGs.

However, the expansion of the fields of use of thermoelectricity in modern energy is constrained by a relatively small value of efficiency, which is associated with the low quality factor of thermoelectric materials (ZT), which does not always exceed unity. Accordingly, the TEG efficiency does not exceed ~ 5% for single-stage and ~ 8-10% for two-stage modules. It is believed that with an efficiency of more than 10%, TEG production becomes cost-effective, which will lead to a significant expansion in the scale of their use.

No thermoelectric material can be used in a wide temperature range, for example, from room temperature to 900 ° C. Thermoelectric materials can be divided into three groups: low temperature (operating up to 250-300 ° C), where Bi ₂ Te _3-x Se _x and Bi _y Sb _2-y Te ₃ solid solutions are non-alternative, medium temperature (for temperatures of 300-600 ° C), among which PbTe, GeTe and CoSb ₃ -based alloys stand out, and high-temperature (600-1000 ° C), where n- and p-type solid solutions, as well as n-La _3-, come to the fore _x Te ₄ and p-Yb ₁₄ MnSb ₁₁ . Until now, commercial TEGs are mainly made from individual thermoelectric materials that operate in relatively narrow temperature ranges, where they have optimal efficiency. At the same time, the calculations, which are confirmed by practice, show that the efficiency of TEGs increases significantly (up to> 10%) when sections of low-, medium-, and high-temperature materials with better properties are combined sequentially into composite branches of a thermoelement operating at a large temperature difference.

The technical task of this invention is to provide a method for manufacturing a composite branch of a thermocouple operating in the temperature range: from room temperature to 900 ° C.

In addition to choosing thermoelectric materials with the best properties for sections, the formation of a compound branch by connecting the sections together plays a very important role. As a rule, sections of selected thermoelectric materials are connected through contact layers, which are created by applying metal coatings to the end surfaces of each of them (for example, W, Mo, Fe, Co, Ni, etc.). These contact layers further facilitate the connection of the sections with each other, as well as with heat transfer and electrical wires. Moreover, depending on the functionality (for example, thermal expansion coefficient compatibility (KTP), adhesion ability, electrical resistivity, anti-diffusion protective properties, etc.), metal coatings can be single-layer and multi-layer.

Depending on the heat source and the behavior of the molten binding agent (solder), two main groups of soldering and welding methods are known that are used in practice for these specific purposes.

A known method of connecting the branches of TEM, where as a solder for the formation of the junction of n- and p-type PbTe, for example, with heat transitions from Fe or Ni, a SnTe compound with an additive of 1% of the mass was selected. Ti. The shear strength of such a compound at 25 ° C was for n-PbTe, in the range of 800-1800 psi (5.6-12.7 MPa). In this case, metallization on test samples of lead telluride was absent. NASA Report [A.L. Eiss, "Thermoelectric bonding study", NASA CR-369, January 1966].

A known method of manufacturing two-component branches from sections of PbTe and SiGe by hot pressing, where tungsten is selected as the transition metal layer, which is recommended for coating a Si-Ge solid solution due to the proximity of the thermal expansion coefficients (CTE). The indicated thermoelectric materials are of the greatest interest for space power systems. Each of these materials shows a rather high efficiency in its operating temperature range, namely ~ 5-7% for PbTe in the temperature range of 25-500 ° C and 500-900 ° C for SiGe. In this case, special attention is paid to the preparation of surfaces of articulated materials. The connection of the n-type sections is carried out at a temperature of 850-865 ° C, and the p-type sections at 840-850 ° C in an atmosphere of high-purity Ar (O ₂ content did not exceed 1.5 ppm) for a duration of 25 minutes Data on the strength of such a compound are not available in the work (US Patent No. 3452423, HO1L 7/4, 7/16, publ. 07/01/1969).

The disadvantages of this technology include the use of hot pressing to obtain a compound based on a refractory metal, the strength of which cannot compete with a compound obtained by plasma or ion-plasma sputtering of tungsten. In addition, due to the large difference in the CTE between W (~ 4.5⋅10 ^-6 ° C ^-1 ) and PbTe (~ 18⋅10 ^-6 ° C ^-1 ) microcracks can occur at the boundary during cooling after heating.

A known method of manufacturing composite branches of a thermocouple for operation in the temperature range of 100-700 ° C by hot pressing in a graphite mold using graphite punches at a temperature of 500-550 ° C, for 1 h in an atmosphere of Ar. For the p-type branch between sections of scutterudite, Zn ₄ Sb ₃ and Bi _0.4 Sb _1.6 Te ₃ , as well as for the n-type branch between sections of CoSb ₃ and Bi ₂ Te _2.95 Se _0.05 in Ti was used as a metal coating. In this case, the sections were joined on the cold side by brazing with a low-temperature BiSn alloy (operating temperature below 138 ° C, which forms a very unstable soldered joint, which is especially affected by thermal cycling), and on the hot side by medium-temperature alloy Cu ₂₂ Ag ₅₆ Zn ₁₇ Sn ₅ (operating temperature over 650 ° C). The specific contact resistance was measured between the heat transmission of Nb and threefold branches n- and p-type, which was less than 5 mkOm⋅sm ^2. On the layout of a thermocouple consisting of an n-branch (Bi ₂ Te _2.95 Se _0.05 / CoSb ₃ ) and a p-branch (Bi _0.4 Sb _1.6 Te ₃ / β-Zn ₄ Sb ₃ / CeFe ₄ Sb ₁₂ ), an efficiency of ~ 10% was achieved (US Patent No. 6,673,996, IPC H01L 35/04, published on January 6, 2004).

The connection of the branches by soldering is not reliable, for example, when using a solder in the form of a foil during the formation of a solder joint, part of the liquid solder is squeezed out of the contact zone of the sections, and when using a powder solder, the solder joint is porous, resulting in increased contact resistance.

A known method of manufacturing two-component branches of low-temperature TEM Bi ₂ Te ₃ and medium-temperature TEM PbTe, which also applies hot pressing in the mold from Mo. Moreover, as a means to level the difference in the linear expansion coefficients, it is proposed to introduce a transition layer between the TEM and the protective layer Fe on the hot side of PbTe, consisting of a powder mixture of 25 wt%. Fe (with a particle size of -44 μm) + 75% of the mass. PbTe (-400 μm), and on the cold side of Bi ₂ Te ₃ - from a powder mixture of 90% of the mass. Fe + 10% of the mass. Bi ₂ Te ₃ . If hot pressing was carried out in vacuum at a temperature of 600 ° C, then the load was 7000 psi (49.2 MPa), the duration of the process was 1 h; and at a temperature of 500 ° C the load is 6000 psi (42.2 MPa), the duration of the process is 2 hours or more (US Application No. 2012/0103381, IPC H01L 35/30, publ. 03.05.2012).

However, the resulting pores during the hot pressing of the powder mixture also increase the contact resistance, as well as microcracks arising from differences in the linear coefficients between the TEM and the metal coating.

U.S. Patent Application No. 2013/0152990, publ. 07/20/2013 (IPC H01L 35/08) a method for assembling a single-section thermoelectric module using the process of mutual diffusion during heating at the joints Ag / Sn / Ag, Ni / Sn / Ni and Cu / Sn / Cu, where tin is a low-melting component, is declared. Corresponding layers with a thickness of 1-10 μm are formed either by galvanic or electrolytic processes, or in the processes of evaporation and spraying in vacuum, as well as chemical vapor deposition. During thermocompression treatment (temperature - 250-350 ° C, pressure - 2 MPa, duration of the process - 3-60 min), intermetallic compounds are formed at the interface: at the junction of the Ag / Sn / Ag compound Ag ₃ Sn is formed (T _PL = 480 ° C), at the junction of Ni / Sn / Ni, three compounds Ni ₃ Sn ₄ (T _pl = 796 ° C), Ni ₃ Sn ₂ (T _pl = 1267 ° C) and NiSn (T _pl = 1169 ° C) can form ), and finally, at the junction of Cu / Sn / Cu - two compounds Cu ₆ Sn ₅ (T _pl = 415 ° C) and Cu ₃ Sn (T _pl = 640 ° C). The thermoelectric modules assembled after such thermocompression processing using the Ag / Sn / Ag joint can be operated in the temperature range 232–480 ° C, with the Ni / Sn / Ni joint the temperature range is 232–796 ° C, and finally, using junction Cu / Sn / Cu - in the temperature range 232-415 ° C. Such an assembly of generator modules is not suitable for operation at temperatures from room temperature to 900 ° C, since the choice of elements for the adjacent joint and the modes of thermal compression processing are made for specific purposes of creating medium temperature current sources.

A known method of diffusion welding for the manufacture of three- and two-segment branches of thermocouples from sections n- (Bi ₂ Te ₃ ) ₉₀ (Bi ₂ Se ₃ ) ₁₀ and p- (Bi ₂ Te ₃ ) ₂₅ (Sb ₂ Te ₃ ) ₇₅ , n- and p-PbTe and p-TAGS-85 (whose composition (GeTe) _0.85 (AgSbTe ₂ ) _0.15 ) using Cu / In / Cu joints. For this, a nickel coating was preliminarily chemically applied to samples of Bi and Sb chalcogenides, and nickel was electrolytically applied to PbTe samples. In both cases, the thickness of the Ni layer was 6–7 μm. On the hot side of p-TAGS-85, a contact layer was first created by pressing Mo powder with a thickness of ~ 50 μm, which is due to the strong interaction between TAGS and Ni. Only after that was Ni electroplated 6–7 μm thick. Then, Cu layers 8–9 μm thick from an acidic copper solution were deposited onto the connected surfaces by electrolysis, onto which a 9–10 µm thick In electrolytic layer was deposited from a sulfate solution at a current density of 10 mA / cm ² . Then, the sections to be joined were fastened together with clamps providing a load of 3-4 MPa and good gas and heat transfer during the melting of the low-temperature In layer. Initially, multiple connections of sections in the branches of the thermocouple occurred simultaneously during the passage for 9-12 minutes of assembly on a belt-pull mechanism through a furnace heated to a temperature exceeding _Tm (In) = 156.6 ° C. After that, the hot assembly was transferred to a furnace with a nitrogen atmosphere heated to 200 ° C for 14–20 h to complete the thermal diffusion process. Thermoelectric properties were measured on n-type composite branches at temperatures from room temperature to 200 ° C. The specific contact resistance on n-PbTe samples was estimated by the authors of the work to be less than 40 μO⋅cm ² . In this case, the influence on the resistance of the sample thickness, which in the experiment was 1, 3, and 5 mm, was noticed. The same effect was observed on the Seebeck coefficient (J. Sharp et al. "Electric Resistivity and Seebeck Coefficient of Segmented Thermoelements", International Conference on Thermoelectrics, 2006). The method adopted for the prototype.

This prototype method has a number of significant drawbacks: firstly, the composite branches interconnected by diffusion welding using a Cu / In / Cu metal coating system are not suitable for operation at temperatures up to 900 ° C, since, according to the Cu phase diagram -In, the maximum temperature of the intermetallic compound in this system is limited to ~ 600 ° C. Selection of other systems for high temperature joints is required. Secondly, the disadvantages of the method include the use of Cu in the metallization of sections of thermoelectric materials, because, despite the presence of an anti-diffusion protective layer of Ni, rapidly diffusing Cu atoms can fall (for example, as a result of surface diffusion) into TEMs at high temperatures and cause to its degradation. Perhaps this is due to the relatively high values of specific contact resistance.

As follows from the analysis of the above examples, soldering is a relatively fast process. At the same time, hot-pressed brazing methods are often used to connect sections into composite branches of a thermocouple, as well as to connect branches with heat transitions. By soldering, relatively flat and not very flat surfaces are joined together, because during the joining of the sections, the solder forms a liquid phase during heating, which spreads over the surface, filling in irregularities. However, the main disadvantage of the solder joint is that it cannot withstand higher temperatures than the temperature of the solder after it is completed. Liquid solder metal does not sufficiently react with articulated materials, which requires a change in its melting point; however, it still has the same melting point as before before soldering and the solder joint at the junction will be melted if it is exposed to a temperature approaching the soldering temperature. Therefore, other more reliable methods of joining sections of compound branches are needed. At the same time, the developers of Marlow Industries (USA), as well as the authors of patent application US 2013/0152990 (Taiwan), showed the possibility of obtaining welded joints between sections of composite branches of a thermoelement by diffusion welding with an intermediate low-melting metal.

The technical result of the invention is to obtain a reliable and durable connection of the sections of the composite branches of the thermocouple, ensuring the operation of the branches in the temperature range from room temperature to 900 ° C.

The technical result is achieved by the fact that in the method of manufacturing a composite branch of a thermoelement operating in the temperature range from room temperature to 900 ° C, by joining sections of low, medium and high temperature thermoelectric materials, comprising pre-coating the ends of the joined sections of metal coatings forming a bonding layer between sections, and their subsequent diffusion welding under pressure and during heating, characterized in that the quality of the metal coating selected metals from the group: nickel, ind Before the diffusion welding process, all the sections are sequentially installed in the graphite mold so that the working temperature increases, so that between the low- and medium-temperature sections, as well as between the medium- and high-temperature sections, binder layers form a layer package Ni / In / Ni, the assembled mold is placed in a spark plasma sintering unit and heated to a temperature of 450-460 ° C, maintained at this temperature for 5-8 minutes under a pressure of 3-5 MPa in vacuum until dissolved the border part of nickel in molten indium, then it is cooled to room temperature, after which isothermal annealing is carried out in an electric furnace with temperature controlled in an atmosphere containing 97% argon and 3% hydrogen, at a temperature of 420 ° C ± 2 ° C, at a pressure of 0, 5-1.0 MPa, lasting 6-10 hours until the formation of the high-temperature intermetallic compound Ni ₂ In ₃ , and after completion of the diffusion welding process, the mold is smoothly cooled to room temperature, while as a section of low-temperature thermoelectric The tertiary material is preferably used n-Bi ₂ Te _2.85 Se _0.15 and p-Bi _0.4 Sb _1.6 Te ₃ obtained by hot extrusion, on the "hot" end of which a layer of metallic nickel is applied; As a section of a medium-temperature thermoelectric material, n-PbTe and p-GeTe obtained by hot pressing are preferably used, on the ends of which are first applied a layer of metallic iron or molybdenum, and then successively layers of metallic nickel and indium, with the ratio of the thickness of the nickel layer to the thickness the indium layer is not less than 10: 1, and the thickness of the indium layer does not exceed 2.0 microns; as a section of high-temperature thermoelectric material, nanostructured n-Si _0.8 Ge _0.2 and p-Si _0.8 Ge _0.2 obtained by spark plasma sintering (IPS) are preferably used; on the “cold” end of which is first applied a layer of metallic tungsten, and then a layer of metallic nickel, while metal layers are applied by plasma spraying to the ends of the sections.

The invention consists in the following.

The process of obtaining composite branches of a thermoelement by diffusion welding with a bonding layer between sections of contact metal coating of nickel (Ni) and low melting indium metal (In) proceeds in two stages: preliminary welding of sections by spark plasma sintering (IPS) and subsequent diffusion isothermal treatment of welded junctions In this case, the coatings are for TEM, and the In layer plays the role of a low-melting metal (T _PL = 156.6 ° C), which is a solvent for Ni in the molten state. The choice of nickel is related to its advantages over for other metals, since it has high strength characteristics, ductility in a wide temperature range, good adhesion to most TEMs, anti-diffusion protective properties and good wettability with various solders.In combination with nickel from a number of low-melting metals Ga, In, Bi, For various reasons, Sn and Pb were chosen indium.In the phase equilibrium diagram in the In-Ni system, there are several intermetallic compounds (Fig. one). On the Indian side, two areas can be distinguished in the diagram. At temperatures below 411 ° C, crystallization of the In ₇₂ Ni ₂₈ phase can be expected, and above 411 ° C, the In ₃ Ni ₂ phase, which undergoes peritectic transformation at temperatures above 870 ° C with the formation of a δ phase with a melting point of ~ 943 ° C. Due to the need to expand the operating range for the branch up to 900 ° C, the system under consideration is the most suitable for our case. At the first stage of diffusion welding (Fig. 2), for its implementation it is necessary to quickly go over the region of formation of a low-temperature intermetallic compound (In ₇₂ Ni ₂₈ ). For this, the method of spark plasma sintering (IPS) is used. In the case of IPS, in the volume of sections from TEM and between adjacent sections, Joule heat is generated due to spark discharges that occur in a pulsed mode, which allows under small pressure (3-5 MPa), without binding additives, to quickly heat up TEM sections to a predetermined temperature of 450-460 ° C exceeding the temperature of formation of a low-temperature intermetallic compound (In ₇₂ Ni ₂₈ ). Then, at the In / Ni boundary, a part of the near-boundary solid Ni dissolves and its subsequent saturation with the In melt forms active interaction centers. After a temporary exposure (5-8 min), the system is cooled to room temperature. A decrease in temperature causes supercooling of liquid In saturated with nickel, which is accompanied by the formation of nuclei of the high-temperature intermetallic phase In ₃ Ni ₂ at the In / Ni interface. In the second stage of the process, the mold with the samples is transferred to a chamber-type electric furnace heated to 420 ° C ± 2 ° C, i.e. already lower than the previous temperature, but higher than the temperature of In ₇₂ Ni ₂₈ formation, where under isothermal conditions: in an atmosphere containing 97% argon and 3% hydrogen, at a temperature of 420 ° C ± 2 ° C, under a pressure of 0.5-1.0 MPa, lasting 6–10 h, the diffusion process of embryo growth continues due to an increase in the concentration of dissolved Ni with the formation of protrusions of a new phase, which grow together with each other. In this case, exactly the same process occurs simultaneously on the opposite side of the Ni / In / Ni interface, which naturally speeds up the formation of a weld joint. As a result, free indium disappears from the weld junction area. In addition, at this stage, the junction composition is homogenized (IM phase in Fig. 2), i.e. bringing it into a more equilibrium state and relieving residual stresses throughout the structure. In the end, the obvious boundary of a sharp change in properties (primarily the specific contact resistance) may disappear, which is typical for traditional soldering. So that the Ni layers do not lose their protective properties during the process, their thickness must be at least ten times the thickness of the In layer, in which it is ~ 2.0 μm. A distinctive feature of this welding method is that the liquid phase (indium in our case) exists temporarily, namely, only during the formation of the weld junction, and it does not require the use of high pressure, fluxes and solvents, while at the same time it allows to obtain reliable and durable (without large pores, voids and microcracks) welded joints of sections from various thermoelectric materials in one package. In addition, the possibility of changing the properties of the materials being joined is excluded, the quality and reliability of the composite branches of the thermocouple increases.

An example implementation of the method.

The following compatible combination of low-, medium-, and high-temperature thermoelectric materials was used to fabricate a composite branch of the electronic type of conductivity, represented by the increase in the working temperature range: n-Bi ₂ Te _2.85 Se _0.15 (20-250 ° C) + n- PbTe (250-600 ° C) + n-Si _0.8 Ge _0.2 (600-900 ° C). The brackets indicate the temperatures of the cold and hot sides, respectively.

For the low-temperature section, a core was taken from n-Bi ₂ Te _2.85 Se _0.15 material, manufactured by hot extrusion, the dimensionless efficiency of which ZT _cp = 0.75.

For the medium temperature section, a core of n-PbTe material obtained by hot pressing was taken, ZT _cp ~ 1.05.

For the high-temperature section, n-Si _0.8 Ge _{0.2 was} taken, the average dimensionless efficiency of which ZT _cp = 0.96.

The end surfaces of the obtained samples of n-Bi ₂ Te _2.85 Se _0.15 , n-PbTe and n-Si _0.8 Ge _0.2 were first treated with diamond paste 14/10 to remove the outer deformed layer, polished on a grinding and polishing machine first using silicon carbide paper with a grit of P2500, and then matting paste, after which they were degreased with acetone, isopropyl alcohol and thoroughly washed with deionized water, and at the end of the preparatory operation they were dried under vacuum.

Then, on the ends of each TEM section, taking into account the thermal expansion coefficients (KTR), metal layers were applied by plasma spraying:

- for n-Bi ₂ Te _2.85 Se _0.15 , in which KTP = ~ (12-15) ⋅10 ^-6 ° C ^-1 , Ni (13.3⋅10 ^-6 ° C ^-1 ) was used;

- on the surface of the end face of n-PbTe (KTP = 18⋅10 ^-6 ° C ^-1 ), a layer of the antidiffusion barrier was first applied, iron was used as it (KTP = (~ 15⋅10 ^-6 ° C ^-1 ), since the atoms Ni diffused into the volume of lead telluride at temperatures above 400 ° C. Instead of iron, in some cases, Mo can be used (5-6⋅10 ^-6 ° C ^-1 );

- for n-Si _0.8 Ge _0.2 (KTP ~ 4.8⋅10 ^-6 ° C ^-1 ), W (~ 4.5⋅10 ^-6 ° С ^-1 ) was used as the anti-diffusion metal barrier, which precipitated nickel.

For deposition of metal coatings on the prepared end surfaces of sections from TEM, spraying was used using low-temperature plasma on a UGNP-7/2025 plasmatron, since this method made it possible to avoid warping of the base. So, on the “hot” side of the n-Bi ₂ Te _2.85 Se section, a layer of Ni (Fig. 2, pos. 1) with a thickness of ~ 15–20 μm was deposited by _0.15 plasma spraying. Then, on both sides of the n-PbTe section, first, buffer layers of Fe (pos. 3) with a thickness of ~ 5.0 μm were sprayed, then successively deposited layers of Ni (pos. 1) with a thickness of ~ 15-20 μm and In (pos. 2) ~ 2.0 μm thick. Finally, on the “cold” side of the n-Si _0.8 Ge _0.2 section, a protective layer W (pos. 4) with a thickness of ~ 5.0 μm was first sprayed, and then Ni (pos. 1) with a thickness of ~ 15-20 microns. Control assessments of the tensile strength of metal coatings at room temperature were carried out on Ni / n-Bi ₂ Te _2.85 Se _0.15 and Ni / Fe / n-PbTe samples, as well as Ni / W / n-Si _0.8 Ge _{0 , 2} using a standard dynamometer type DNU. To do this, samples were cut in the form of columns with a cross section of 4 × 4 mm ² and a metal wire was soldered to the Ni surface on the samples. Tensile loads were created by a helical mechanism providing longitudinal tension of the wire. The ultimate tensile strength was fixed on a dynamometer at the time of destruction of the sample or metal coating. The test results showed that the gap mainly occurred on the material n-Bi ₂ Te _2.85 Se _0.15 and n-PbTe. Thus, the tensile strength for n-Bi ₂ Te _2.85 Se _0.15 was ~ 13-15 MPa, and for the n-PbTe sample, the tensile strength was 12-14 MPa, in some cases, separation was observed along the layer. On Ni / W / n-Si _0.8 Ge _0.2 samples, cracking was observed at the interface between the metal layer and the TEM at loads of 20-22 MPa.

The prepared coated sections were loaded into a graphite mold with graphite punches in the sequence as shown in Fig. 2, and installed on the installation of spark plasma sintering (IPA) type (SPS Syntex, Japan). The working chamber of the installation was evacuated to a residual pressure of ~ 0.13 Pa, after which the IPA process was carried out according to a given program. Upon heating, In melted and active centers of interaction arose at the interface with Ni with the formation of a new intermetallic phase, i.e. there was an active preliminary articulation by welding sections between themselves. After a short period of time (5 min), the mold was cooled and transferred to a temperature-controlled chamber-type electric furnace. The working chamber of the furnace was pumped out, and then filled with a gas mixture of Ar (97%) + Н ₂ (3%) under a pressure of 0.8 ⋅ P _ata and heated to a temperature of 415 ° C ± 5 ° C. The static load on the samples was small and amounted to ~ 0.5-1.0 MPa. In this mode, diffusion isothermal treatment of welds lasting 8-10 hours was carried out in order to homogenize their composition by recrystallization with the formation of an intermetallic phase (Fig. 2, item IM). After the end of the annealing process, in order to avoid the occurrence of residual stresses, the mold was cooled to room temperature at a rate of 1-2 ° C / min.

Subsequently, the obtained three-sectional sample was cut into branches with a cross section of 4 × 4 mm ² and the specific contact resistance was estimated by scanning with a potential probe along the length of the composite branch. Then, tests were carried out to determine the mechanical strength of the welds between the sections at break. The obtained test results in comparison with the prototype are presented in the table, which implies the obvious advantage of the proposed method of connecting sections into composite branches of the thermocouple. According to experts, it is assumed that in order to achieve a generator efficiency of 14%, it is necessary to have a specific contact resistance of no more than ~ 1.0⋅10 ^-5 Ohm⋅cm on composite branches. The values obtained by the claimed method are close to this value.

Example 2

For the manufacture of the composite branch of the hole type of conductivity was taken:

As low-temperature thermoelectric materials p-Bi _0,4 Sb _1,6 Te ₃ (ZT _cp = 1,09), made by hot extrusion; medium temperature p-GeTe (ZT _cp = 1.27) obtained by hot pressing; high-temperature p-SiGe (ZT _cp = 0.70) created by spark plasma sintering.

Next, the manufacture of the composite branches was carried out as in example 1.

The results of tests of junctions for strength and the results of measuring contact resistance are presented in table 1.

Thus, the claimed method allows to obtain reliable and durable connection sections of the composite branches of the thermocouple, providing the branch in the temperature range from room to 900 ° C.

Claims (6)

1. A method of manufacturing a composite branch of a thermocouple by joining sections of low, medium and high temperature thermoelectric materials, comprising pre-applying metal coatings on the ends of the joined sections to form a bonding layer between the sections, and subsequent diffusion welding them under pressure and when heated, characterized in that metals from the group are selected as a metal coating: nickel, indium, iron, molybdenum, tungsten, before the diffusion welding process, all sections are sequentially to increase the operating temperature, they are installed end-to-end in a graphite mold so that between the low and medium temperature sections, as well as between the medium and high temperature sections, binder layers from the Ni / In / Ni layer package are formed, the assembled mold is placed in a spark plasma sintering unit and heated to a temperature of 450-460 ° C, kept at this temperature for 5-8 minutes under a pressure of 3-5 MPa, in vacuum, to dissolve the border part of nickel in molten indium, then cooled to room temperature, after why carry out isothermal annealing in an electric furnace with a controlled temperature in the atmosphere containing 97% argon and 3% hydrogen, at a temperature of 420 ° C ± 2 ° C, under a pressure of 0.5-1.0 MPa, lasting 6-10 hours until formation high-temperature intermetallic compound Ni ₂ In ₃ , and after completion of the diffusion welding process, the mold is smoothly cooled to room temperature. 2. The method according to p. 1, characterized in that as a section of a low-temperature thermoelectric material, n-Bi ₂ Te _2.85 Se _0.15 and p-Bi _0.4 Sb _1.6 Te ₃ obtained by hot method are preferably used extrusion, on the "hot" end of which a layer of metallic nickel is applied. 3. The method according to p. 1, characterized in that as a section of medium-temperature thermoelectric material, n-PbTe and p-GeTe obtained by hot pressing are preferably used, on the end of which is first applied a layer of metallic iron or molybdenum, and then successively layers of metallic nickel and indium. 4. The method according to p. 1, characterized in that the nanostructured n-Si _0.8 Ge _0.2 and p-Si _0.8 Ge _0.2 obtained by the method of spark plasma sintering are preferably used as a section of high-temperature thermoelectric material IPS), on the “cold” end of which they first apply a layer of metallic tungsten, and then a layer of metallic nickel. 5. The method according to p. 3, characterized in that the ratio of the thickness of the nickel layer to the thickness of the indium layer is not less than 10: 1, while the thickness of the indium layer does not exceed 2.0 microns. 6. The method according to any one of paragraphs. 2, 3, 4, characterized in that the metal layers on the ends of the sections are applied by plasma spraying.

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