Ag2se-based thermoelectric crystal material and preparation method thereof

Abstract

The application relates to an Ag2Se-based thermoelectric crystal material and a preparation method thereof, and the preparation method comprises the steps of vacuum packaging, polycrystal pre-melting and crystal growth, specifically as follows: taking silver and selenium as raw materials, and according to the chemical formula Ag2Se 1+x Ingredients are prepared, loaded into a reactor (1), vacuumized and then packaged; the reactor (1) is placed into a high-temperature swing furnace, heated and kept at room temperature, and when the highest temperature is reached, the swing is started, the reactor (1) is taken out after the pre-melting is finished and naturally cooled; the reactor (1) is placed into a Bridgman furnace, heated and kept at room temperature, and the crystal growth process is started to be executed, and the furnace is cooled after the growth is finished. Compared with the prior art, the application optimizes the growth temperature, temperature gradient, descending speed and other parameters of the Bridgman growth furnace, and obtains the Ag2Se-based thermoelectric crystal material with obvious crystal face orientation, wherein the Ag2Se 1.02 The thermoelectric dimensionless merit value of the crystal material can reach 0.58 at 375K, and the Ag2Se-based thermoelectric crystal material is a kind of good near-room-temperature thermoelectric material.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy materials, and relates to an Ag2Se-based thermoelectric crystal material and a preparation method thereof. BACKGROUND

[0002] With the intensification of global energy crisis and the increasingly serious environmental pollution, it is an important strategy to find clean and renewable energy. Thermoelectric energy conversion material is a clean energy conversion technology, which can realize the direct mutual conversion of heat and electricity based on the Seebeck effect or Peltier effect of materials, and has unique advantages such as no emission, no emission and no pollution, and is widely used in thermoelectric power generation and thermoelectric refrigeration.

[0003] The conversion efficiency of thermoelectric conversion technology is the dimensionless thermoelectric figure of merit zT = S 2 σT / κ, where T is the absolute temperature, S is the Seebeck coefficient, σ is the electrical conductivity, PF = S 2 σ is the power factor, and κ is the thermal conductivity. Ideal thermoelectric materials require materials to have high electrical conductivity and low thermal conductivity. In recent years, the performance of typical thermoelectric materials has been improved, and the development of new thermoelectric materials has made rapid progress. However, thermoelectric materials are mostly brittle materials, which are prone to cracking and breaking during actual application processing and use, which degrades or even invalidates the conversion of thermoelectric materials. Ag2(S, Se, Te) material has high electron mobility and intrinsic low lattice thermal conductivity, and is a kind of relatively potential thermoelectric material. In addition, it has been reported that this kind of material has good plastic mechanical properties, and can be processed into materials with specific shapes, providing new possibilities for customized thermoelectric material scenario application.

[0004] Ag2Se thermoelectric material is one of the typical thermoelectric materials, but existing researches are mainly based on the preparation and thermoelectric performance research of polycrystalline materials. This is because the material has obvious phase transition, and the volume change of the crystal material is prone to occur during slow cooling in the growth process, which causes the rupture of the quartz tube, so the growth and performance research of single crystal materials is difficult to achieve.

[0005] Patent CN113353896A discloses an Ag2Se nano ultrafine crystal thermoelectric material with superplasticity. The phase structure of the material is single low-temperature orthorhombic phase, the grain morphology is uniform equiaxed crystal, and the average grain size is between 0-200 nm, and the size fluctuation range is within 50 nm. The Ag2Se nano ultrafine crystal thermoelectric material with such characteristics can be obtained by grinding-ball milling-cold pressing or grinding-heat treatment-cold pressing process, but the Ag2Se nano ultrafine crystal thermoelectric material sintered by cold pressing in the patent is a polycrystalline material without obvious crystal face orientation.

[0006] Patent CN107792838A discloses a method for preparing Ag2X (X=Se, Te) bulk thermoelectric material at super speed, which takes silver powder and selenium or tellurium powder as raw materials, first mixes the raw materials uniformly, puts the obtained mixed raw materials into a graphite mold, and places the graphite mold in a plasma activated sintering device to prepare a dense Ag2X bulk thermoelectric material in a plasma activation stage of a plasma activated sintering process. However, the Ag2X bulk thermoelectric material prepared by plasma activated sintering in the patent is also a polycrystalline material without obvious crystal face orientation. SUMMARY

[0007] The purpose of the present application is to overcome the defects of the prior art and provide an Ag2Se-based thermoelectric crystal material and a preparation method thereof. The present application grows Ag2Se crystal material with obvious crystal face preferential orientation by using the Bridgman method, wherein the grown Ag2Se 1.02 The dimensionless thermoelectric value of the crystal material can reach 0.58 at 375K, which is a good near room temperature thermoelectric material.

[0008] The purpose of the present application can be achieved by the following technical solutions.

[0009] One of the technical solutions of the present application is to provide a preparation method of Ag2Se-based thermoelectric crystal material, which comprises the following steps:

[0010] (1) Vacuum packaging: taking silver and selenium elements with a purity of more than 99.99% as raw materials, dosing according to the stoichiometric ratio of the chemical formula Ag2Se 1+x , mixing uniformly, and then loading into a reactor, vacuumizing and packaging;

[0011] (2) Polycrystalline pre-melting: placing the reactor loaded with raw materials into a high-temperature rocking furnace, slowly heating from room temperature and keeping warm, allowing the raw materials to uniformly react in a high-temperature molten state, opening the rocking when reaching the highest temperature, taking out the reactor after pre-melting and naturally cooling, and obtaining a pre-melting polycrystalline ingot;

[0012] (3) Crystal growth: placing the reactor loaded with the pre-melting polycrystalline ingot into a Bridgman furnace, slowly heating from room temperature and keeping warm, starting the crystal growth process, and cooling the furnace after growth to obtain Ag2Se-based thermoelectric crystal material.

[0013] Further, the value of x in step (1) is in the range of 0≤x≤0.02, i.e. the selenium element is 0-2% excessive, to compensate for the loss of volatile selenium element in the preparation process.

[0014] Further, the vacuum degree of vacuumizing in step (1) is 10 -3 -10 -1 Pa, and the time is 20-40 min.

[0015] Further, the temperature of heating in steps (2) and (3) is 1173-1373K, and the temperature rate is 100-200K / h.

[0016] As a preferred technical solution, the temperature of heating in step (3) is 1273K, and the temperature rate is 200K / h.

[0017] Further, the holding time in steps (2) and (3) is 5-12h.

[0018] Further, the shaking rate in step (2) is 10-30rpm.

[0019] Further, the Bridgman furnace in step (3) comprises a heating furnace body and a heating body, an insulation layer, a lifting mechanism, a support frame and a thermocouple arranged inside the heating furnace body, the insulation layer is arranged at the top and bottom of the heating body, the lifting mechanism is connected to the support frame, the support frame supports the reactor with a sharp lower end, the lifting mechanism drives the raw material in the reactor to move at a certain lifting speed, the heating body heats the raw material into a molten part, the heated part cools into a grown crystal, and the thermocouple is arranged at the upper part of the support frame.

[0020] Further, the descending speed of the lifting mechanism is 0.5-2.0mm / h.

[0021] As a preferred technical solution, the descending speed of the lifting mechanism is 1.0mm / h.

[0022] Further, the temperature gradient of the Bridgman furnace is 20-40K / cm.

[0023] As a preferred technical solution, the temperature gradient of the Bridgman furnace is 30K / cm.

[0024] One of the technical solutions of the present application is to provide an Ag2Se-based thermoelectric crystal material, which is prepared by the preparation method.

[0025] Current research on Ag₂Se-based thermoelectric materials mainly focuses on polycrystalline materials, typically employing high-temperature melting and powder sintering to prepare dense bulk samples for further research. However, Ag₂Se exhibits a non-cubic crystal structure at low temperatures, and research on thermoelectric materials with distinct orientations is currently lacking. Therefore, this study proposes growing Ag₂Se materials using the Bridgman method. The Bridgman method has long been widely used for the growth of various crystalline materials. It involves slowly lowering a growth crucible in a high-temperature furnace with a temperature gradient, allowing the melt to nucleate and crystallize at the bottom of the crucible, resulting in a crystalline material. Although the Bridgman method has a simple growth principle, selecting appropriate growth temperature, temperature gradient, descent rate, and initial chemical composition is crucial for growing crystalline materials with distinct crystal orientations. However, the specific growth conditions vary for different materials. Therefore, optimizing the growth temperature, temperature gradient, and descent rate of Ag₂Se thermoelectric crystal materials is a key technical point for obtaining Ag₂Se thermoelectric crystal materials with distinct crystal orientations.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] (1) The present invention can achieve compositional uniformity of pre-melted polycrystalline material by turning on a high-temperature swing furnace during the pre-melting process;

[0028] (2) By optimizing parameters such as growth temperature, temperature gradient, and descent rate of the Bridgman furnace, this invention can obtain Ag2Se thermoelectric crystal material with obvious crystal orientation.

[0029] (3) The present invention uses the Ag2Se thermoelectric crystal material grown to characterize and analyze the Seebeck coefficient, electrical conductivity, electrical power factor, thermal conductivity and thermoelectric properties of the material. Its thermoelectric dimensionless figure of merit can reach 0.58 at 375K, which is a good near-room temperature thermoelectric material. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the Bridgeman furnace in an embodiment of the present invention;

[0031] Figure 2 In the embodiment of the present invention, Ag2Se 1.02 Comparison of X-ray diffraction (XRD) patterns, powder XRD patterns, and standard XRD patterns of crystalline samples;

[0032] Figure 3 In the embodiment of the present invention, Ag2Se 1.02 The Seebeck coefficient (S) of a crystalline sample versus temperature;

[0033] Figure 4In the embodiment of the present invention, Ag2Se 1.02 The relationship between the electrical conductivity (σ) of a crystalline sample and temperature;

[0034] Figure 5 In the embodiment of the present invention, Ag2Se 1.02 The relationship between the electrical power factor (PF) and temperature of a crystal sample;

[0035] Figure 6 In the embodiment of the present invention, Ag2Se 1.02 The relationship between thermal conductivity (κ) and temperature of a crystalline sample;

[0036] Figure 7 In the embodiment of the present invention, Ag2Se 1.02 The thermoelectric properties (zT) of the crystal sample versus temperature.

[0037] Explanation of markings in the diagram:

[0038] 1—Reactor, 2—Heating element, 3—Insulation layer, 4—Lifting mechanism, 5—Raw material, 6—Melting section, 7—Growing crystal, 8—Support frame, 9—Thermocouple. Detailed Implementation

[0039] The present invention will now be described in detail with reference to specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0040] Unless otherwise specified, the equipment used in the following embodiments is conventional equipment in the art; unless otherwise specified, the reagents used are commercially available products or prepared by conventional methods in the art. In the following embodiments, unless otherwise described in detail, conventional experimental methods in the art can be used.

[0041] The Bridgeman furnace used in this embodiment, such as Figure 1 As shown, the furnace includes a heating furnace body and a heating element 2, an insulation layer 3, a lifting mechanism 4, a support frame 8, and a thermocouple 9 inside the heating furnace body. The insulation layer 3 is located at the top and bottom of the heating element 2. The lifting mechanism 4 is connected to the support frame 8, which supports the reactor 1 with a sharp lower end. The lifting mechanism 4 drives the raw material 5 inside the reactor 1 to move at a certain lifting speed. The heating element 2 heats the raw material 5 into a molten part 6, and the heated part is cooled into a grown crystal 7. The thermocouple 9 is located on the upper part of the support frame 8.

[0042] Example:

[0043] An Ag2Se-based thermoelectric crystal material and its preparation method, comprising the following steps:

[0044] (1) Vacuum encapsulation: using Ag and Se elements with a purity greater than 99.99% as raw materials, according to the chemical formula Ag₂Se 1.02 The ingredients were weighed and mixed thoroughly according to the stoichiometric ratio, then poured into a quartz tube with a diameter of 25 mm. A vacuum was then applied for 30 minutes until the vacuum level reached 10. -1 At Pa, the quartz tube is encapsulated at high temperature using an oxyhydrogen flame gun.

[0045] (2) Polycrystalline premelting: The raw material quartz tube containing elemental Ag and Se is placed in a high-temperature oscillating furnace and heated from room temperature at a rate of 200 K / h, slowly increasing the temperature to 1273 K and holding it at that temperature for 5 hours. At 1273 K, the oscillating is turned on at a oscillating rate of 20 rpm to allow the raw material to react uniformly in a high-temperature molten state. Then the quartz tube is taken out and allowed to cool naturally at room temperature to obtain a premelted polycrystalline ingot.

[0046] (3) Crystal growth: The quartz tube containing the pre-melted polycrystalline ingot was placed in the Bridgman furnace and heated from room temperature at a rate of 200 K / h. The temperature was slowly increased to 1273 K and held for 12 h before crystal growth was started. The growth temperature gradient was 30 K / cm and the growth descent rate was 1.0 mm / h. After the growth was completed, the furnace was cooled naturally. The quartz tube was then removed and broken open to obtain Ag2Se-based thermoelectric crystal material with crystal orientation.

[0047] like Figure 2 As shown, by comparing the XRD pattern of the obtained crystal material with the powder XRD pattern and the standard XRD pattern, it can be found that the XRD diffraction peaks of the Ag2Se powder sample are consistent with the standard XRD results of the Ag2Se compound with space group P212121. In addition, the obtained crystal material has obvious dissociation surfaces, which show that the preferred orientation of the obtained Ag2Se crystal material is in the (201) direction.

[0048] like Figure 3 As shown, within the test temperature range, the Seebeck coefficient is negative, indicating that the dominant conductive carrier type is holes. The absolute value of the Seebeck coefficient decreases with increasing temperature; it is 144 μV / K at 300 K and drops to 90 μV / K at 475 K. Near 400 K, the temperature dependence of the Seebeck coefficient changes, corresponding to the phase transition from the orthorhombic β-Ag₂Se to the cubic α-Ag₂Se.

[0049] like Figure 4 As shown, the dependence of conductivity on temperature also changed during the phase transition near 400 K, with conductivity rapidly decreasing from 1350 S·cm to 800 S·cm, which is consistent with the Seebeck coefficient as a function of temperature.

[0050] like Figure 5 As shown, the low-temperature phase β-Ag₂Se exhibits a high electrical power factor due to the combined effects of a high Seebeck coefficient and low electrical conductivity, with the highest electrical power factor reaching 22.5 μW / cm⁻¹K at 350 K. 2 The electrical power factor of the high-temperature phase α-Ag₂Se is 12.0 μW / cm⁻¹K at 425 K. 2 .

[0051] like Figure 6 As shown, Ag2Se 1.02 The thermal conductivity of the crystal is 1.3 W / mK at 300 K, gradually increases to 1.6 W / mK at 400 K, and then decreases to 1.2 W / mK at 450 K after the phase transition.

[0052] like Figure 7 As shown, the thermoelectric performance of β-Ag2Se reaches a peak of 0.58 at 375K at low temperature, and the thermoelectric performance of α-Ag2Se is 0.35 at high temperature of 425K. It exhibits a high average thermoelectric performance of 0.54 in the temperature range of 300-375K.

[0053] Characterization results of Ag2Se crystal materials show that the thermoelectric properties of the low-temperature phase Ag2Se are better than those of the high-temperature phase Ag2Se. The thermoelectric properties of Ag2Se reach a peak of 0.58 at 375K. The results indicate that Ag2Se is a near-room temperature thermoelectric material with good thermoelectric properties.

[0054] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for preparing an Ag₂Se-based thermoelectric crystal material, characterized in that, The preparation method comprises the following steps: (1) Vacuum packaging: with silver and selenium as raw materials, the stoichiometric ratio of chemical formula Ag2Se 1+x is prepared, mixed uniformly into the reactor (1), vacuumized and then packaged; (2) Polycrystal pre-melting: the reactor (1) containing raw materials is placed in a high-temperature rocking furnace, heated from room temperature and kept warm, the rocking is started when the highest temperature is reached, the reactor (1) is taken out after the pre-melting is completed, and the pre-melted polycrystal ingot is obtained after natural cooling; (3) Crystal growth: the reactor (1) containing the pre-melted polycrystal ingot is placed in a Bridgman furnace, heated from room temperature and kept warm, the crystal growth process is started, the furnace is cooled after the growth is completed, and the Ag2Se-based thermoelectric crystal material is obtained; The value range of x in step (1) is 0≤x≤0.02; In step (3), the heating temperature is 1173-1373 K, the crystal growth descending speed is 0.5-2.0 mm / h, and the temperature gradient is 20-40 K / cm.

2. The method of claim 1, wherein the Ag2Se-based thermoelectric crystal material is prepared by the following steps of: The vacuum degree in step (1) is 10 -3 -10 -1 Pa, for 20-40 min. ​ 3. The method of claim 1, wherein the Ag2Se-based thermoelectric crystal material is prepared by the following steps of: In step (2), the heating temperature is 1173-1373 K, and the heating rate is 100-200 K / h; ​ In step (3), the heating rate is 100-200 K / h.

4. The method of claim 1, wherein the Ag2Se-based thermoelectric crystal material is prepared by the following steps of: In steps (2) and (3), the keeping warm time is 5-12 h. ​ 5. The method of claim 1, wherein the Ag2Se-based thermoelectric crystal material is prepared by the following steps of: In step (2), the rocking speed is 10-30 rpm. ​ 6. The method of claim 1, wherein the Ag2Se-based thermoelectric crystal material is prepared by the following steps of: In step (3), the Bridgman furnace comprises a heating furnace body, a heating body (2), an insulation layer (3), a lifting mechanism (4), a support frame (8) and a thermocouple (9) inside the heating furnace body, the insulation layer (3) is arranged at the top and bottom of the heating body (2), the lifting mechanism (4) is connected to the support frame (8), the support frame (8) supports the reactor (1) with a sharp lower end, and the thermocouple (9) is arranged at the upper part of the support frame (8). ​ 7. An Ag2Se-based thermoelectric crystal material, characterized by, The thermoelectric crystal material is prepared by the preparation method in any one of claims 1-6.

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