CC BY
19
https://doi.org/10.29013/AJT-20-1.2-21-25
Azimov Toolanboy Ma'rifjonovich, Physics department Ferghana State University of Ferghana, Uzbekistan Onarkulov Karimberdi Egamberdiyvich, Physics department Ferghana State University of Ferghana, Uzbekistan Gaynazarova Kizlarhon Isroilovna, Physics department Ferghana State University of Ferghana, Uzbekistan E-mail: tmazimov@mail.ru
EFFECT OF COMMUTATION SOLDER ON THE OPERATING CHARACTERISTICS OF COOLING ELEMENTS BASED ON BISMUTH AND ANTIMONY CHALCOGENIDES
Abstract. The article investigates the near-contact regions of cooling thermo elements based on Bi and Sb chalcogenides commutated by BiSb and PbSb solders and clarifies the nature of the chemical interaction of component solders with the materials of thermo element branches and current-carrying buses, in addition to studying their microstructure, extensive X-ray spectral analyzes, and their characteristics.
Keywords: metal, solder, semiconductor, diffusion layer, annealing, tires, thermo element, contact, switching, concentration.
Introduction: A large number of solders for semiconductor devices are known, but the implementation of soldering with these alloys does not provide a strong contact with thermoelectric materials [1]. Many of them contain precious metals, high melting point and wide crystallization interval and cannot be taken as a basis in the development of solders, and used for switching in cooling thermocouples.
For successful implementation of solder switching, it is necessary to have such solders, which must meet the following basic requirements [2];
1. Do not form compounds with the semiconductor material that have high ohmic resistance.
2. When penetrating a semiconductor, its electrical, mechanical and thermal properties arenot significantly altered.
3. In the molten state, it is good to wet the semiconductor and metal, current supply and be liquid flowing.
4. Have a small difference in the temperatures of the beginning and end of melting, a small difference between the liquid solidus lines and us.
5. To have a sufficient strength.
It is quite obvious that to pick up the solder meeting all listed requirements, it is rather difficult. This is one of the reasons that lead to a large percentage of marriage thermoelectric batteries, both in the manufacture and in their operation. Therefore, the further search for new more effective and more hardened solder compositions is very relevant.
Material and Methods: Использование термобатарей в условиях вакуума требует их предварительной вакуумной термообработки. At
the same time, there is a sharp deterioration of the parameters of the thermal coolers and in some cases (with prolonged processing) their destruction. This is due to the interaction of the solder with the materials of the branches of the thermo element, and with the metal of the current-carrying bus. The rate of contact failure depends on the temperature and time of heat treatment on the one hand, and the type of solder on the other. For switching oflow-temperature and cooling thermo batters by a soldering method solders on the basis of bismuth and tin though, as it is known, solders on the basis of lead possess higher physical and chemical stability and compatibility with soldered materials in comparison with bismuth solders which in thermoelectricity has not found wide application are used mainly. In this regard, the study of physical and chemical compatibility of lead-based solders with structural materials that make up the thermo element is ofparticular interest. It is also necessary to consider the state of the surface of the branches before switching them. The near-surface layer of branches made of materials by directed crystallization, during its cutting there are mechanical disturbances of the surface, which penetrate to a significant depth - up to 10 microns. In addition, in the process of deposition, heat stroke occurs, and structural disturbances are observed, penetrating into the body of the branch of the thermo element.
Among semiconductor materials, the best thermoelectric properties in the range of 200 ^ 600 °C are the compound Bi2 Te3 and Sb2 Te3 solid solutions based on them [3]. In the Bi Te system, the chemical compound Bi2 Te3 is formed with an open maximum at 858 °C and forming eutectic with tellurium
[4]. The compound Bi2 Te3, Bi2 Se3, Sb2 Te3 have a rhombohedra lattice of the tetra ide type, which is based on a nine-layer packing of halogen atoms
[5], with two-thirds of the octahedral voids occupied by antimony or bismuth atoms, and the layers are arranged according to the cubic packing law. The atoms of a single layer are the same and form a flat hexagonal lattice.
As p-type materials for cooling devices, solid solutions close in composition to BiQ5 Sb15 Te3 are used, in which a part of the tellurium atoms is replaced by selenium [3; 6]. Under such conditions, to obtain high values of thermoelectric q-factor a ^ * t
Z =-, additional tellurium up to 3^4 weight%
x
is introduced into the solid solution of the composition. The action of excess tellurium is to suppress the acceptor ability of excess antimony, formed due to the displacement of stoichiometry. As a result, on a solid solution ofthe composition 74 mol% Sb2Te3-26 mol% Bi2Te3 with an admixture of Te grown from the
3 (3 - 3.3 )-10-3 melt, a K value Z = --- at room temperature is achieved. It is important to note that the excess tellurium does not dissolve, but is released as a second phase, which also affects the efficiency of the material. It is known that among the n - type materials, alloys based on solid solutions Bi2Te3Bi2Se3 containing Bi2Se3 in an amount of 4 ^ 10 mol% doped with halides have the highest efficiency.
In the process of switching the thermocouples of the contact area, a layer of switching material of uncertain composition and structure is formed. Therefore, the aim of the work is to study and study the contact areas of cooling thermo elements based on Bi and Sb chalcogenides commuted by BiSb and PbSb solders [7].
Discussion: In order to clarify the nature of the chemical interaction of the solder components BiSb and PbSb with the materials of the branches of the thermo element and current carrying tires, experimental samples we remade according to the following schemes:
1. BiQ5Sbh sTe3+BiSb+Cu;
2. BiQTe2,88Se3l12+BiSb + CU;
3. BiQ Sb, sTe3+PbSb+Cu;
4. Bi 2Te 2,88Se Q,12+PbSb+ Cu.
To activate the diffusion processes on the metal-solder-semiconductor contact, the samples were subjected to isothermal annealing in vacuum at a temperature of200 °C for 100, 150, 500, 1000 hours.
Microstructural analysis was carried out on the prepared sections and the distribution of elements was studied by means of local x-ray spectral analysis, as well as the electrical resistance along their length was measured.
Microstructural analysis was carried out on the prepared sections and the distribution of elements was studied by means of local x-ray spectral analysis,
as well as the electrical resistance along their length was measured.
Immediately after soldering, on the contact of the switching solder with the semiconductor and the copper conductor bus, diffusion layers with a thickness of 16 microns ("p" + BiSb), 1 microns (Cu + BiSb) and 15 microns ("n" + BiSb) are detected (See Fig. 1. a, b).
P- type a)
P- type c)
flw type
b)
Figure 1.
a) Bi05Sb15Te3 + BiSb + Cu; before annealing
b) Bi2Te
c) BiQ5Sb15Te3 + BiSb + Cu; annealing200 °C, 150h.
288Se012 + BiSb + Cu; before annealing
Local X-ray spectral analysis of the contact zones "p" + BiSb and Cu+BiSb detects tellurium diffusion into the solder. The chemical interaction of Sb and Cu at elevated tellurium concentration is detected at the solder-copper interface. Apparently in the soldering process Te and Sb due to the concentration gradient diffuse into the solder and concentrated at the copper boundary. As a result of diffusion of the above elements, they Deplete the contact layer of the semiconductor. Measurements of electrical resistance along the length of these samples did not reveal the presence of areas with high transient resistances (Fig. 2 a). After vacuum heat treatment for 150 hours, the thickness of the depleted layer in the p-type semiconductor increases to 61 microns (Fig. 1 c.). On the copper side, there is a crack, which shows an abrupt increase in resistance on contact with copper (Fig. 2 b).
Apparently this is a consequence of the high level of mechanical: stresses on the Cu+ BiSb contact arising during the formation of a new phase, which oc-
curs due to the difference in the volume expansion coefficients of the solder, copper and formed phase.
Figure 2. Resistance distribution in the contact area Bi0 5Sb15Te3+BiSb+Cu; a) before annealing, b) after annealing at 200 °C for 150 hours
The reason for the formation of intermediate layers in the contact area is, apparently diffusion caused
by the concentration gradient of the elements entering the contacting materials under the action of temperature. Studies of the microstructure of soldered half-elements with Pb-Sb solder showed that there is no formation of intermediate phases between thermoelectric materials of p - and n - type and Pb-Sb solder after soldering (Fig. 3 a, b).
Figure 3.
a)"p"+PbSb+Cu; before annealing
b) "n'+PbSb+Cu; before annealing
c) "p"+PbSb+Cu; annealing200 °C, 100 h.
d) "n'+PbSb+Cu; annealing 200 °C, 100 h.
At the Cu+PbSb boundary, local x-ray spectral analysis revealed a diffusion zone, ~2 microns thick, containing copper and antimony. The concentration of antimony at the border abruptly increases. The increase in the concentration of antimony atoms at the junction boundary immediately after soldering, apparently depends on the activity of the latter in the liquid solder. Antimony in solder is an
adhesive-active component, has a great influence on wettability and adhesion in the semiconductor - solder - metal system, as well as the formation of a transition layer between them. After 100 hours of annealing, the diffusion layer between the copper and the solder formed a gray zone, taking the form of "grooves" directed into the solder, the size of which is 26 microns (Fig. 3 c, d). According to x ray spectral analysis the gray phase formed mainly consists of Cu and Sb atoms with a small amount of Pb. As is known, antimony is soluble in copper in the solid state up to 0.2 weight% at a temperature of 200 °C and has a negative effect on the ductility of copper. A Cu dissolves in Sb at room temperature. Therefore, the transition region is formed mainly by mutual diffusion of Sb and Cu atoms.
Result: I t is established that direct contact of thermoelectric materials based on bismuth and antimony chalcogenides and Bi-Sb with solders Bi-Sb and Pb-Sb leads to chemical interaction in the contact region between them in the temperature range 100^200 °C. Therefore, one of the ways to reduce the intensity of reactive diffusion at switching transitions is the introduction of an anti-diffusion layer of metals between contacting materials.
Conclusion:
Microstructural analysis of metallization of branches and current carrying buses of thermoelements is carried out;
The possibility of using the alloy eutectic Pb-Sb, having a melting point of 525 K, as a solder for single-layer switching of thermal batteries operating in the temperature range (213 ^ 423) °C in vacuum is studied.
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