RU2628677C1 - Thermoelectric generator manufacturing method - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: manufacturing method of a thermoelectric generator includes puncturing of a plane-parallel plate (3) from the SmS sulphide ingot, the samarium applying(2) to the surface of the first plane current contact (1) made of refractory metal, applying of the plane-parallel plate (3) to the samarium (2), annealing of the obtained structure in a vacuum at a temperature rate of 1072-1200°C for a time t determined from a predetermined ratio, and the following formation of the second current contact (4) on the surface of the plane-parallel plate.

EFFECT: improved stability and reproducibility of the electric signal of the generator.

6 cl, 7 dwg

Description

The invention relates to the field of conversion of thermal energy into electrical energy, in particular, to the creation of thermoelectric generators based on semiconductor structures, and can be used, for example, in the nuclear industry, in medicine.

A known method of manufacturing a thermoelectric generator (see patent RU 2303834, IPC H01L 37/00, published July 27, 2007), comprising applying a discrete evaporation in vacuum of a polycrystalline layer of a semiconductor material based on an element of group III onto a heated substrate and attaching current contacts to it. As a starting material for application, samarium sulfide powder SmS is taken. The substrate is made with a metal surface, which is simultaneously the first current contact. In the process of applying a polycrystalline layer by discrete evaporation in vacuum, the temperature of the substrate monotonically increases from the initial to the final value, selected from the temperature range from 250 to 600 ° C, and the second current contact is attached to the surface of the obtained polycrystalline layer.

A thermoelectric generator made in a known manner can operate in the absence of a temperature gradient, but generates a small signal power, up to 10 μW. The known method does not allow to create sufficiently thick layers of a semiconductor with a composition gradient, since the deposition of thick layers occurs rather slowly (about 1 μm in 5 minutes), and the composition is aligned in volume due to thermal diffusion. A small volume of the working substance (a layer with a thickness of not more than ~ 1 μm) causes a low power of the generated signal. The main disadvantage of this method is the instability and poor reproducibility of the generated signal manufactured thermoelectric generators.

A known method of manufacturing a thermoelectric generator (see patent RU 2548062, IPC H01L 37/00, publ. 04/10/2015), coinciding with this decision for the largest number of essential features and adopted for the prototype. The known method includes puncturing a flat parallel plate from an ingot of Samarium sulfide SmS, depositing on the upper surface of the plate a solution containing excess samarium atoms and atoms belonging to the lanthanide family, namely gadolinium Gd, or cerium Ce, or europium Eu, or ytterbium Yb, diffusion of these atoms deep into the plate using high-temperature annealing in vacuum. The temperature and annealing time are selected based on the condition that the concentration of atoms belonging to the lanthanide family after annealing in the plate is Gd y≤0.15 for gadolinium, Ce y≤0.15 for cerium, Yb y≤0 for ytterbium, 2, and the concentration x of excess Sm samarium atoms in the plate after annealing is ≤0.2.

The disadvantage of the known prototype method is that after diffusion on the surface of the Samarium sulfide plate SmS there remains a layer containing the remains of undecomposed SmCl ₂ , oxides and samarium oxysulfide (Sm ₂ O ₃ , Sm ₂ O ₂ S). In addition, the surface of the sample becomes rough. All this makes it impossible to create a good ohmic contact between the current output and the SmS sample. Sanding the layer is not possible, since SmS during polishing passes from the semiconductor to the metallic state and loses its working capacity.

The objective of the present invention is to develop a method of manufacturing a thermoelectric generator that would generate an electrical signal of increased stability and reproducibility.

The problem is solved in that the method of manufacturing a thermoelectric generator includes puncturing a flat parallel plate from an ingot of samarium sulfide SmS, depositing samarium on the surface of the first flat current contact made of refractory metal, applying said plane-parallel plate to samarium, annealing the resulting structure in vacuum at a temperature of 1072- 1200 ° C for a time t determined from the ratio:

where d is the thickness of the plane-parallel plate, cm;

K = 2.5⋅10 ⁶ - empirical coefficient, s / cm ² ,

and the subsequent formation on the surface of the plane-parallel plate of the second current contact.

Samarium can be applied to the surface of the first flat current contact in the form of chips, or in the form of foil, or in the form of lumpy material.

The second current contact can be formed by applying a metal layer to the surface of a plane-parallel plate or by pressing.

Upon annealing the obtained structure in vacuum at a temperature of 1072–1200 ° С for a time t, the diffusion of samarium atoms into the plane-parallel SmS plate occurs with the formation of a gradient of excess samarium ions in the plane-parallel SmS plate. Regions with a gradient of excess samarium ions correspond to the formula Sm _{1 + x} S with a varying samarium content (x) in a direction perpendicular to the boundary surface of a plane-parallel plate. In this case, the concentration of samarium changes monotonically (decreases) in the direction perpendicular to the surface of the plane-parallel plate, its gradient in this direction, thus, ensures the emergence of EMF on opposite boundary surfaces of the plane-parallel plate when the thermoelectric generator is heated. The effect of EMF generation is associated with a change in the valency of samarium ions in the system of Sm ²⁺ ions (located outside the regular lattice sites of the plane-parallel SmS plate). EMF is created due to the occurrence of temperature gradients in a plane-parallel SmS plate due to absorption and release of the Mott type phase transition energy, as well as to the appearance of conduction electron concentration gradients due to changes in the valence of samarium ions (Sm ²⁺ ↔ Sm ³⁺ + e ^- ) due to distribution gradients of defective samarium ions that form donor impurities over the volume of a plane-parallel SmS plate. The generated EMF (U) obeys the relation:

where n ₁ and n ₂ are the concentrations of conduction electrons in the region of current contacts at the generation temperature, cm ^-3 ;

T is the difference between the generation temperature and before heating, K;

e is the electron charge, C;

k is the Boltzmann constant J / K.

U does not depend on the volume of the sample and its geometric configuration, but depends only on the difference in the values of n ₁ and n ₂ at a given temperature T, which is determined by the gradient of the donor impurity (Sm).

At the same time, samarium serves as a solder for reliable attachment and the creation of good ohmic contact between the plane-parallel SmS plate and the first flat current contact.

The choice of annealing parameters is due to the following. The temperature of 1072 ° C is the melting point of metallic samarium. At temperatures above 1200 ° C, samarium begins to evaporate intensively, scatters and settles on the entire surface of the structure and the walls of the working chamber. The minimum annealing time is limited to 250 seconds, since with a shorter annealing time, as established experimentally, diffusing samarium atoms pass only a distance of less than 100 microns. This determines the thickness of the working layer (layer with a gradient of excess Sm). However, in this case, the use of bulk samples to increase the lasing power loses practical sense, since such thin working layers can be obtained in thin and thick film samples. The experimentally established maximum annealing time determined by relation (1) is due to the passage of a diffusing flux of samarium atoms through the entire thickness of the plane-parallel plate SmS, and a further diffusion process leads to a decrease in the generated emf, since the difference between n ₁ and n ₂ in the relation ( 2).

This technical solution allows to obtain working thermoelectric generators in the form of bulk samples with a thickness of several millimeters in the presence of a gradient of excess samarium ions in them, which diffuse from the layer of metal samarium.

The present method of manufacturing a thermoelectric generator is illustrated by drawings, where:

in FIG. 1 is a cross-sectional view of a thermoelectric generator manufactured by the present method (1 — first flat current contact, 2 — samarium, 3 — plane-parallel plate SmS, 4 — second current contact);

in FIG. 2 shows the dependences of the generated voltage (5) and temperature (6) of heating a thermoelectric generator manufactured by the present method on time;

in FIG. 3 shows the results of measurements of the internal resistance of the same generator manufactured by the present method, obtained in various measurement cycles;

in FIG. 4 shows the results of measurements of the internal resistance of the same generator made by the prototype method, obtained in various measurement cycles;

in FIG. 5 shows the results of measurements of the electrical voltage generated at T = 175 ° C by the same thermoelectric generator manufactured by the present method, obtained in various measurement cycles;

in FIG. 6 shows the results of measurements of the electric voltage generated at T = 175 ° C by the same thermoelectric generator manufactured by the prototype method, obtained in various measurement cycles;

in FIG. 7 shows the results of measuring the internal resistance for 5 generators made by the proposed method (square dots), and 5 generators made by the prototype method (diamond-shaped dots).

The present method of manufacturing a thermoelectric generator is as follows. A plane-parallel plate 3 is punctured from the samarium sulfide ingot SmS. 3. Samarium 2 is placed, for example, in the form of chips, or foil, or lump material on the surface of the first flat current contact 1 made of refractory metal, for example, molybdenum or tantalum, or tungsten, or cobalt. A plane-parallel plate 3 is applied to samarium 2 and the structure thus prepared is placed in a vacuum chamber, in which a reduced pressure is created (usually not more than 10 ^-3 mm Hg). Then the structure is heated with a resistive or induction furnace to a temperature of 1072-1200 ° C and maintained at this temperature for a time determined by the ratio (1). A wire is welded to the first flat current contact 1, or a pressure contact is created. The second current contact 4 is created either by applying a layer of metal on the surface of a plane-parallel plate 3 opposite to the first flat current contact 1, or the second current contact 4 is pressed.

Example 1. A plane-parallel plate with dimensions of 3.5 × 2 × 2 mm ³ was punctured from an ingot of samarium sulfide SmS. On the first flat current contact with dimensions of 9 × 7 × 1.5 mm ^2, molybdenum was filled with metallic samarium in the form of 0.2 g sawdust. A punctured plane-parallel SmS plate was placed on top of the sawdust. The structure obtained in this way was placed in a vacuum chamber in which a pressure of 10 ^-4 mm Hg was created, gradual (within 12 s) induction heating of the obtained structure to 1190 ° C was carried out, the temperature was controlled using a tungsten-rhenium thermocouple with accuracy ± 3 ° С, samarium melting process was monitored visually through the transparent wall of the working chamber. At T = 1190 ° C, heating continued for 250 seconds. After heating, the resulting structure was removed from the chamber. A second clamping current contact was established on the surface of the plane-parallel plate SmS. When testing the action of a thermoelectric generator, it was placed on a massive copper plate heated by a resistive type electric stove. The temperature of the copper plate and structure was measured using a copper-constantan thermocouple embedded in a copper plate. The signal from the current contacts using the clamping contacts and the signal from the thermocouple were fed to two channels of the ADC of a personal computer and recorded during heating. The thermoelectric generator was heated in the range from 25 ° C to 175 ° C. The voltage U produced by the thermoelectric generator was 24 mV, and the power W at a load of 1 ohm was 106 μW. The results of the measurements are presented in FIG. 2, where the dependences of the voltage U and temperature Τ on time are shown (curve 5 and curve 6, respectively). The “dips” on curve 5 correspond to the time intervals during which the power W was measured when the signal was shorted with a resistance of 1 Ω, it was 91 μW at T = 150 ° C and 106 μW at T = 172 ° C. Measurements were repeated 10 times at time intervals of the order of a week. Each time before the measurement, the pressure contacts were opened and set again. In this case, the internal resistance of the thermoelectric generator, R _i , was measured. The measurement results are presented in FIG. 3. The mean square deviation of the results was calculated by the formula:

where n is the number of measurements;

- среднее значение сопротивления, Ом.

- average value of resistance, Ohm.

In all experiments, the value of the generated voltage was also measured at T = 175 ° C. The measurement results are presented in FIG. 5. For comparison, a thermoelectric generator was manufactured according to the prototype method, according to which a layer of SmCl ₂ salt was applied to a plane-parallel SmS plate. This structure was heated to T = 850 ° C for one hour. The output signals from the fabricated thermoelectric generator were recorded in the same manner as from the thermoelectric generator manufactured by the present method. Both contacts were clamped. Measurements were repeated 10 times. The results of measuring the internal resistance are shown in FIG. 4, and the results of measuring the generated voltage in FIG. 6. As can be seen from the measurement results, the standard deviation σ for the measured resistance and the generated voltage in the manufacture of the thermoelectric generator by the present method is respectively: 0.15 Ohm and 1.08 mV, while for the thermoelectric generator manufactured by the prototype method, these values were 3.05 ohms and 5.16 mV, respectively. Thus, the reproducibility of the parameters of the thermoelectric generator manufactured by the present method exceeds the reproducibility of the parameters of the thermoelectric generator manufactured by the prototype method by at least 5 times.

Example 2. A thermoelectric generator was made in the same way as in example 1, but upon induction heating of the obtained structure for 240 seconds at a temperature of 1070 ° C. As a result, the plane-parallel SmS plate did not solder to the first plane current contact.

Example 3. A thermoelectric generator was made in the same way as in example 1, but upon induction heating of the obtained structure for 250 seconds at a temperature of 1205 ° C. As a result, intense evaporation of samarium occurred, followed by its deposition on the walls of the working chamber. In this case, the flow of samarium vapor destroyed the structure prepared for soldering (shifted the plane-parallel plate SmS).

Example 4. A thermoelectric generator was made in the same way as in example 1, but upon induction heating of the obtained structure for 2400 seconds at a temperature of 1080 ° C. Next, measurements were made of the voltage U and power generated by the thermoelectric generator. At a thermoelectric generator temperature of 172 ° C, U = 23 mV, W = 121 μW.

Example 5. A thermoelectric generator was made in the same way as in example 1, but upon induction heating of the obtained structure for 10,000 seconds at a temperature of 1100 ° C. Next, measurements were made of the voltage U and power generated by the thermoelectric generator. At a thermoelectric generator temperature of 172 ° C, U = 25 mV, W = 123 μW.

Example 6. A thermoelectric generator was made in the same way as in example 1, but upon induction heating of the obtained structure for 24000 seconds at a temperature of 1080 ° C. Next, measurements were made of the voltage U and power generated by the thermoelectric generator. At a thermoelectric generator temperature of 172 ° C, U = 24 mV, W = 126 μW.

Example 7. A thermoelectric generator was made in the same way as in example 1, but the plane-parallel plate SmS had dimensions 3 × 2 × 1 mm ³ and heating was carried out at a temperature of 1072 ° C for a time determined on the right-hand side of expression (1), and within 25,000 seconds (6 hours 57 minutes). Next, measurements were made of the voltage U and power generated by the thermoelectric generator. At a thermoelectric generator temperature of 172 ° C, U = 24 mV, W = 120 μW.

Example 8. A thermoelectric generator was made in the same way as in example 7, but the fabricated structure was heated at T = 1072 ° C for another 40 minutes, i.e. diffusion proceeded for 25000 + 2400 = 27400 seconds, i.e. more than 25000 seconds. Next, measurements were made of the voltage U and power generated by the thermoelectric generator. At a thermoelectric generator temperature of 172 ° C, U = 20 mV, W = 95 μW. Thus, when the annealing time is longer than on the right-hand side of expression (1), there is a decrease in both the output signal U of the thermoelectric generator and its power W.

Example 9. Made 5 thermoelectric generators in the same way as in example 1. Each generator was measured internal resistance, R _i . The measurement results are presented in FIG. 7 (square dots). The root-mean-square deviation of the results was calculated by the formula (3), which turned out to be equal to σ = 0.61 Ohm. For comparison, 5 thermoelectric generators were manufactured by the prototype method. The measurement results R _{i of} these generators are shown in FIG. 7 (diamond-shaped dots). The mean square deviation calculated by formula (3) turned out to be equal to σ = 3.12 Ohms. Thus, the reproducibility of the internal resistance of thermoelectric generators made by the present method exceeds the reproducibility of the internal resistance of thermoelectric generators made by the prototype method by about 5 times. It should be noted that internal resistance is one of the main parameters that determine other operational parameters of the device. Its stability is the key to the stability of other parameters.

Claims (10)

1. A method of manufacturing a thermoelectric generator, comprising puncturing a samarium SmS sulfide ingot of a plane-parallel plate, depositing samarium on the surface of the first plane current contact made of refractory metal, applying said plane-parallel plate to samarium, annealing the resulting structure in vacuum at a temperature of 1072-1200 ° C during time t determined from the relation: 250≤t≤K⋅d ² , s; where d is the thickness of the plane-parallel plate, cm; K = 2.5⋅10 ⁶ - empirical coefficient, s / cm ² ; and the subsequent formation on the surface of the plane-parallel plate of the second current contact. 2. The method according to p. 1, characterized in that the samarium is applied to the surface of the first flat current contact in the form of chips. 3. The method according to p. 1, characterized in that the samarium is applied to the surface of the first flat current contact in the form of a foil. 4. The method according to p. 1, characterized in that the samarium is applied to the surface of the first flat current contact in the form of bulk material. 5. The method according to p. 1, characterized in that the second current contact is formed by applying a layer of metal to the surface of a plane-parallel plate. 6. The method according to p. 1, characterized in that the second current contact is made clamped.

Images