A kind of Cu 8 GeSe 6 Preparation method of thermoelectric crystal based thereon

Abstract

The present invention relates to the technical field of crystal growth, and discloses a preparation method of a Cu8GeSe6-based thermoelectric crystal, comprising the following steps: mixing and melting elemental substances to form a Cu8GeSe6-based polycrystal; placing the Cu8GeSe6-based polycrystal in a crucible with a thermal conductivity higher than 100 W·m ‑1 ·K ‑1 , vacuum-sealing and then putting it into a vertical growth furnace; a temperature zone I, a temperature zone II and a temperature zone III with gradually decreasing temperatures are arranged from top to bottom in the vertical growth furnace; after melting the crystal of the crucible containing the Cu8GeSe6-based polycrystal in the temperature zone I, moving it down to the temperature zone II for crystal growth, and then moving it down to the temperature zone III for annealing. The preparation method of the present invention can reduce the pores in the Cu8GeSe6-based thermoelectric crystal, thereby improving its mechanical strength, and at the same time can also make the Cu8GeSe6-based thermoelectric crystal have a lower thermal conductivity in the temperature range of 300-500 K.

Description

Technical Field

[0001] The present invention relates to the technical field of crystal growth, and in particular to a method for preparing a Cu 8 GeSe 6 -based thermoelectric crystal. Background Art

[0002] Thermoelectric conversion technology is based on the Seebeck effect, Peltier effect, and Thomson effect, and can achieve the mutual conversion between electrical energy and thermal energy. It can be used to convert low-grade thermal energy into electrical energy and improve the energy utilization efficiency. It is an energy conversion technology with broad development prospects. In thermoelectric conversion technology, the core of energy conversion is thermoelectric materials, and how to improve the performance of thermoelectric materials is the research focus of thermoelectric conversion technology.

[0003] Cu 8 GeSe 6 -based thermoelectric crystal is a new type of thermoelectric material. The existence of two independent structural units endows Cu 8 GeSe 6 with an extremely low lattice thermal conductivity: in the non-fast ion conductor phase, the weak chemical bond between copper ions and the [GeSe 6 framework can introduce a large number of low-frequency optical phonon branches of anharmonic vibrations in the material, hindering the propagation of normal phonons; in the fast ion conductor phase, copper ions have a strong liquid-like effect, which can strongly scatter phonons and soften some transverse phonons. The extremely low thermal conductivity enables Cu 8 GeSe 6 -based thermoelectric crystal to still have a high zT value under poor electrical properties.

[0004] In the prior art, Cu 8 GeSe 6 -based thermoelectric crystals are prepared by the method of melting-quenching-annealing-hot press sintering. For example, in the literature "Cu 8 GeSe 6 -based thermoelectric materials with an argyrodite structure" (Jiang B, Qiu P, Eikeland E, et al. Cu 8 GeSe 6-based thermoelectric materials with an argyrodite structure[J]. Journal of Materials Chemistry C, 2017, 5(4):943-952. DOI:10.1039 / c6tc05068a.). Since Cu 8 GeSe 6 The thermal conductivity of Cu-based thermoelectric crystals is very low. When preparing large-sized crystals by this method, a large number of pores are easily generated, resulting in low mechanical strength of the crystals and easy breakage during the processing, and the processing performance is poor. Summary of the Invention

[0005] In order to solve the above technical problems, that is, the poor mechanical properties of Cu 8 GeSe 6 -based thermoelectric crystals prepared by the existing method, the present invention provides a method for preparing Cu 8 GeSe 6 -based thermoelectric crystals. This preparation method can reduce the pores in the Cu 8 GeSe 6 -based thermoelectric crystals, thereby improving their mechanical strength. At the same time, it can also make the Cu 8 GeSe 6 -based thermoelectric crystals have lower thermal conductivity in the direction perpendicular to the axis within the temperature range of 300-500 K.

[0006] The specific technical solution of the present invention is as follows:

[0007] In the first aspect, the present invention provides a method for preparing Cu 8 GeSe 6 -based thermoelectric crystals, comprising the following steps:

[0008] S1: Mix and melt the elements to make Cu 8 GeSe 6 -based polycrystals;

[0009] S2: Place the Cu 8 GeSe 6 -based polycrystals in a crucible with a thermal conductivity higher than 100 W·m -1 ·K -1 After vacuum sealing, put it into a vertical growth furnace; there are temperature zones I, II, and III with gradually decreasing temperatures from top to bottom in the vertical growth furnace;

[0010] S3: Place the crucible containing Cu 8 GeSe 6After the crucible of the matrix polycrystal melts the crystal in temperature zone I, it moves down to temperature zone II for crystal growth, and then moves down to temperature zone III for annealing to obtain Cu 8 GeSe 6 matrix thermoelectric crystal.

[0011] For the Cu 8 GeSe 6 matrix thermoelectric crystal, the temperature inside the crystal is prone to exhibit a gradient distribution characteristic under the influence of the temperature field. By using a crucible with high thermal conductivity in the present invention, it helps to reduce the temperature difference between different regions inside the crystal, making it easier to nucleate and grow along the axial direction under the temperature gradient in the vertical growth furnace, and facilitating the discharge of internal micro-pores. As a result, the finally prepared Cu 8 GeSe 6 matrix thermoelectric crystal has fewer pores, which can endow the Cu 8 GeSe 6 matrix thermoelectric crystal with higher mechanical strength, making it not easy to break during the processing, thus achieving better processing performance.

[0012] In addition, the thermal conductivity of the Cu 8 GeSe 6 matrix thermoelectric crystal is comprehensively affected by various factors such as the internal pore structure, grain orientation, Umklapp process of phonon collision, multi-phonon effect, and intrinsic excitation of the crystal. Moreover, for the Cu 8 GeSe 6 matrix thermoelectric crystal, the temperature environment it is in will act on its Umklapp process, multi-phonon effect, and intrinsic excitation. Therefore, the environmental temperature will also have a complex impact on the thermal conductivity. The same operation may have completely opposite effects on the thermal conductivity of this thermoelectric crystal in different temperature ranges. These factors are intertwined and intricate, and some principles are even still unexplained so far. Therefore, regarding the influence of the preparation process on the thermal conductivity of the Cu 8 GeSe 6 matrix thermoelectric crystal in a specific temperature range, its predictability is very poor, which brings great difficulties to the research. The research team of the present invention found that by using the preparation method of the present invention, in addition to reducing the pores in the Cu 8 GeSe 6 matrix thermoelectric crystal, it can also reduce its thermal conductivity in the direction perpendicular to the axis within the temperature range of 300 - 500 K, which will help to improve the thermoelectric performance of the Cu 8 GeSe 6 matrix thermoelectric crystal in this temperature range, making it more suitable for some application occasions that require low thermal conductivity in the near-room-temperature environment.

[0013] Preferably, in step S2, the crucible is a SiC crucible or a graphite crucible.

[0014] Preferably, in step S3, the temperature of heating zone I is 1000 - 1150 °C, and the heat preservation time of the crucible in heating zone I is 12 - 20 h; the temperature in heating zone II decreases from top to bottom at a gradient of 15 - 20 °C / cm, and the crucible stays at a position with a temperature of 850 - 1000 °C in heating zone II for 100 - 120 h; the temperature of heating zone III is 700 - 850 °C, and the heat preservation time of the crucible in heating zone III is 20 - 24 h.

[0015] Preferably, in step S3, the speed of the crucible descending from heating zone I to heating zone II is 0.3 - 0.5 mm / h.

[0016] Preferably, in step S3, the speed of the crucible descending from heating zone II to heating zone III is 0.1 - 0.5 mm / h.

[0017] Preferably, in step S3, the steps of vacuum sealing include: evacuating the crucible and then filling it with inert gas, repeating this process multiple times, and finally filling the inert gas until the air pressure in the crucible is 4×10 -4 ~6×10 -4 Pa, and then sealing it with a flame.

[0018] When the speeds of the crucible descending from heating zone I to heating zone II and from heating zone II to heating zone III, and the air pressure after the last filling of inert gas before sealing are not properly controlled, it will cause the thermal conductivity of the prepared Cu 8 GeSe 6 -based thermoelectric crystals to be relatively large in the direction perpendicular to the axis within the temperature range of 300 - 500 K.

[0019] Preferably, in step S3, the Cu 8 GeSe 6 -based thermoelectric crystal is a Cu 8-x A x Ge 1-y B y Se 6-z C z crystal; in step S1, the Cu 8 GeSe 6 -based polycrystal is a Cu 8-x A x Ge 1-y B y Se 6-z C z polycrystal; Cu 8-x A x Ge 1-y B y Se 6-z C zAmong them, A is Ag and / or Au, B is Si and / or Sn, C is S and / or Te, 0 ≤ x ≤ 8, 0 ≤ y ≤ 1, 0 ≤ z ≤ 6.

[0020] Preferably, in step S1, the process of melting the elemental mixture is carried out in a vertical melting furnace or a rocking furnace; in the rocking furnace, the rocking time is controlled to be 0.5 - 3 h, the preparation temperature is 950 - 1200 °C, the heating rate is 4 - 10 °C / min, and the rocking rate is 10 - 30 r / min.

[0021] Preferably, in step S2, the bottom of the crucible is conical, and the taper is 10° - 70°.

[0022] Preferably, in step S2, the length of the crucible is 5 - 50 cm, and the diameter of the cylindrical part is 8 - 100 mm.

[0023] Preferably, in step S3, the Cu 8 GeSe 6 -based thermoelectric crystal has a diameter of 8 - 100 mm and a length of 1 - 100 mm.

[0024] In a second aspect, the present invention provides the application of the zone melting method in reducing the thermal conductivity of Cu 8 GeSe 6 -based thermoelectric crystals at 300 - 500 K. The preparation method of the Cu 8 GeSe 6 -based thermoelectric crystal includes the following steps:

[0025] S1: Melting the elemental mixture to form a Cu 8 GeSe 6 -based polycrystal;

[0026] S2: Placing the Cu 8 GeSe 6 -based polycrystal in a crucible with a thermal conductivity higher than 100 W·m -1 ·K -1 After vacuum sealing, it is placed in a vertical growth furnace; inside the vertical growth furnace, there are temperature zones I, II, and III with gradually decreasing temperatures from top to bottom;

[0027] S3: After melting the crystal of the crucible containing the Cu 8 GeSe 6 -based polycrystal in temperature zone I, moving it down to temperature zone II for crystal growth, and then moving it down to temperature zone III for annealing to obtain a Cu 8 GeSe 6 -based thermoelectric crystal.

[0028] In a third aspect, the present invention provides a method for reducing Cu8 GeSe 6 Method for the thermal conductivity of a Cu-based thermoelectric crystal at 300 - 500 K, said Cu 8 GeSe 6 The preparation method of the Cu-based thermoelectric crystal comprises the following steps:

[0029] S1: Mix and melt the elements to make a Cu 8 GeSe 6 -based polycrystal;

[0030] S2: Place the Cu 8 GeSe 6 -based polycrystal in a crucible with a thermal conductivity higher than 100 W·m -1 ·K -1 After vacuum sealing, put it into a vertical growth furnace; Inside the vertical growth furnace, there are temperature zones I, II, and III with decreasing temperatures from top to bottom;

[0031] S3: After melting the crystal of the crucible containing the Cu 8 GeSe 6 -based polycrystal in temperature zone I, move it down to temperature zone II for crystal growth, and then move it down to temperature zone III for annealing to obtain a Cu 8 GeSe 6 -based thermoelectric crystal.

[0032] Compared with the prior art, the present invention has the following advantages:

[0033] (1) In the process of preparing the Cu 8 GeSe 6 -based thermoelectric crystal, by using a vertical growth furnace and cooperating with a crucible with high thermal conductivity, the internal pores of the obtained Cu 8 GeSe 6 -based thermoelectric crystal can be reduced, thereby improving its mechanical strength, making it not easy to break during the processing process, and achieving better processing performance.

[0034] (2) In the present invention, through the comprehensive design of the following three aspects: ① using a vertical growth furnace with three temperature zones for crystal growth + ② the air pressure after the last filling of inert gas before crucible sealing + ③ the speed of the crucible descending from temperature zone I to temperature zone II and from temperature zone II to temperature zone III, the obtained Cu 8 GeSe 6 -based thermoelectric crystal has a lower thermal conductivity in the direction perpendicular to the axis within the temperature range of 300 - 500 K, which will help improve the thermoelectric performance of the Cu 8 GeSe 6 -based thermoelectric crystal within this temperature range. Description of the Drawings

[0035] Figure 1 The Cu 8 GeSe 6 -based thermoelectric crystal appearance obtained in Example 1.

[0036] Figure 2 The Cu 8 GeSe 6 -based thermoelectric crystal X-ray diffraction pattern obtained in Example 1.

[0037] Figure 3 The optical photograph of the fracture of the Cu 8 GeSe 6 -based thermoelectric crystal fragments obtained in Comparative Example 2.

[0038] Figure 4 The relationship between the temperature dependence of the thermal conductivity perpendicular to the axis of the Cu 8 GeSe 6 -based thermoelectric crystals prepared in Example 1 and Comparative Example 1. Specific embodiments

[0039] The present invention will be further described below in conjunction with the embodiments.

[0040] A method for preparing a Cu 8 GeSe 6 -based thermoelectric crystal, comprising the following steps:

[0041] S1: Mix and melt the elemental substances to form a Cu 8 GeSe 6 -based polycrystal;

[0042] S2: Place the Cu 8 GeSe 6 -based polycrystal in a crucible with a thermal conductivity higher than 100 W·m -1 ·K -1 ; vacuum seal it and put it into a vertical growth furnace; inside the vertical growth furnace, there are temperature zones I, II, and III with gradually decreasing temperatures from top to bottom;

[0043] S3: After melting the crystal of the crucible containing the Cu 8 GeSe 6 -based polycrystal in temperature zone I, move it down to temperature zone II for crystal growth, and then move it down to temperature zone III for annealing to obtain a Cu 8 GeSe 6 -based thermoelectric crystal.

[0044] In some specific embodiments, in step S3, the Cu 8 GeSe 6 -based thermoelectric crystal is Cu 8-x Ax Ge 1-y B y Se 6- z C z Crystal; in step S1, the Cu 8 GeSe 6 -based polycrystal is Cu 8-x A x Ge 1-y B y Se 6-z C z polycrystal; Cu 8-x A x Ge 1-y B y Se 6- z C z wherein, A is Ag and / or Au, B is Si and / or Sn, C is S and / or Te, 0 ≤ x ≤ 8, 0 ≤ y ≤ 1, 0 ≤ z ≤ 6.

[0045] In some specific embodiments, the process of step S1 specifically includes: placing the elemental substances in a quartz crucible, vacuum-sealing it, and then putting it into a vertical melting furnace or a rocking furnace for mixed melting to obtain a Cu 8 GeSe 6 -based polycrystal; in the rocking furnace, controlling the rocking time to be 0.5 - 3 h, the preparation temperature to be 950 - 1200 °C, the heating rate to be 4 - 10 °C / min, and the rocking rate to be 10 - 30 r / min.

[0046] In some specific embodiments, in step S2, the crucible is a SiC crucible or a graphite crucible.

[0047] In some specific embodiments, in step S2, the bottom of the crucible is conical, with a taper of 10° - 70°; the length of the crucible is 5 - 50 cm, and the diameter of the cylindrical part is 8 - 100 mm.

[0048] In some specific embodiments, in step S3, the step of vacuum sealing includes: evacuating the crucible to a pressure not higher than 10 -4 Pa and then filling it with an inert gas, repeating this multiple times, and finally filling the inert gas until the pressure in the crucible is 4×10 -4 ~6×10 -4 Pa, and then sealing it with a flame.

[0049] In some specific embodiments, in step S3, the temperature of the temperature zone I is 1000~1150 °C, and the heat preservation time of the crucible in the temperature zone I is 12~20 h; the temperature in the temperature zone II decreases from top to bottom at a gradient of 15~20 °C / cm, and the crucible stays at a position with a temperature of 850~1000 °C in the temperature zone II for 100~120 h; the temperature of the temperature zone III is 700~850 °C, and the heat preservation time of the crucible in the temperature zone III is 20~24 h; the descending speed of the crucible from the temperature zone I to the temperature zone II is 0.3~0.5 mm / h, and the descending speed from the temperature zone II to the temperature zone III is 0.1~0.5 mm / h.

[0050] In some specific embodiments, in step S3, the Cu 8 GeSe 6 -based thermoelectric crystal has a diameter of 8~100 mm and a length of 1~100 mm.

[0051] The present invention will be described below through specific examples. It should be understood that these examples are only used to illustrate the present invention and not to limit the scope of the present invention. Without departing from the spirit and scope of the inventive concept, the changes and advantages that can be conceived by those skilled in the art are included in the present invention, and the scope of protection of the present invention is defined by the appended claims and any equivalents thereof.

[0052] Unless otherwise defined, all technical terms and scientific terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the art to which the present disclosure belongs. The raw materials and equipment used in the present invention are conventional raw materials and equipment in the art and can be obtained from conventional commercial channels without special instructions; the methods used in the present invention are conventional methods in the art without special instructions.

[0053] Example 1

[0054] The Cu 8 GeSe 6 -based thermoelectric crystal is prepared through the following steps:

[0055] S1: Prepare Cu 8 GeSe 6 polycrystals

[0056] Weigh high-purity (purity 99.999%) Cu particles, Ge particles and Se particles according to the stoichiometric ratio of 8:1:6, put them into a quartz crucible, evacuate to a pressure of 10 -4 Pa and then fill with argon, repeat 3 times, and then seal with a hydrogen-oxygen flame. Put the sealed quartz crucible into a rocking furnace, raise the temperature to 1100 °C, keep it warm for 1 hour, and rock at a rocking speed of 20 r / min during this period to completely melt the raw materials and fully mix and react to obtain Cu8 GeSe 6 Polycrystalline

[0057] S2: Prepare Cu 8 GeSe 6 Based thermoelectric crystal

[0058] In the vertical growth furnace used in this step, there are temperature zones I (high temperature zone), II (medium temperature zone), and III (low temperature zone) with decreasing temperatures from top to bottom. The length of temperature zone I in the vertical direction is 20 cm, and the temperature is set at 1150 °C; the length of temperature zone II in the vertical direction is 10 cm, and the set temperature decreases downward at a gradient of 15 °C / cm; the length of temperature zone III in the vertical direction is 20 cm, and the temperature is set at 700 °C.

[0059] Take the Cu 8 GeSe 6 Polycrystalline obtained in step S1, put it into a graphite crucible. The bottom of the graphite crucible is conical with a taper of 60°, the diameter of the cylindrical part is 15 mm, and the total length of the cylindrical part and the conical part is 10 cm. Vacuum the inside of the graphite crucible to a pressure of 10 -4 Pa and then fill it with argon, repeat this process 3 times, and finally fill it with argon to a pressure of 5×10 - 4 Pa, and then use a hydrogen-oxygen flame for sealing.

[0060] Put the sealed graphite crucible into the vertical growth furnace, stay in the middle of temperature zone I for 20 hours to melt the polycrystalline and stabilize the solid-liquid interface. Then start the lifting device, set the descending speed at 0.5 mm / h, move the crucible down into temperature zone II, and stay at the position with a set temperature of 950 °C for 120 hours until the crystal growth is completed and all the melt has crystallized. Then start the lifting device again, set the descending speed at 0.1 mm / h, move the crucible down to the middle of temperature zone III, and stay for 20 hours to complete annealing. Finally, cool it to room temperature to obtain the Cu 8 GeSe 6 Based thermoelectric crystal (Cu 8 GeSe 6 Crystal).

[0061] The Cu 8 GeSe 6 Based thermoelectric crystal obtained in this example has an appearance as Figure 1 shown, consisting of two parts: a cone and a cylinder, with a total length of 6 cm, a taper of 60° for the cone, and a diameter of 17 mm for the cylinder.

[0062] The Cu 8 GeSe 6The X-ray diffraction pattern of the base thermoelectric crystal is as follows Figure 2 as shown Figure 2 indicating that the crystal obtained in this example is single-phase.

[0063] Example 2

[0064] The Cu 8 GeSe 6 base thermoelectric crystal is prepared through the following steps:[[]]

[0065] S1: Preparation of Cu 8 GeSe 6 polycrystals

[0066] Weigh high-purity Cu particles, Ge particles, and Se particles according to the stoichiometric ratio of 8:1:6, put them into a quartz crucible, evacuate to a pressure of 10 -4 Pa and then fill with argon, repeat 3 times, and then use a hydrogen-oxygen flame for sealing. Put the sealed quartz crucible into a rocking furnace, raise the temperature to 1100 °C, keep it warm for 1 hour, and rock at a rocking speed of 30 r / min during this period to completely melt the raw materials and fully mix and react to obtain Cu 8 GeSe 6 polycrystals.

[0067] S2: Preparation of Cu 8 GeSe 6 base thermoelectric crystal

[0068] In the vertical growth furnace used in this step, there are temperature zones I (high-temperature zone), II (medium-temperature zone), and III (low-temperature zone) with gradually decreasing temperatures from top to bottom. The length of temperature zone I in the vertical direction is 20 cm, and the temperature is set at 1000 °C; the length of temperature zone II in the vertical direction is 15 cm, and the set temperature decreases downward at a gradient of 20 °C / cm; the length of temperature zone III in the vertical direction is 10 cm, and the temperature is set at 750 °C.

[0069] Take the Cu 8 GeSe 6 polycrystals prepared in step S1, put them into a graphite crucible. The bottom of the graphite crucible is conical with a taper of 10°, the diameter of the cylindrical part is 8 mm, and the total length of the cylindrical part and the conical part is 15 cm. Evacuate the graphite crucible to a pressure of 10 -4 Pa and then fill with argon, repeat 3 times, and finally fill with argon to a pressure of 6×10 -4 Pa, and then use a hydrogen-oxygen flame for sealing.

[0070] Put the sealed graphite crucible into the vertical growth furnace and stay in the middle of temperature zone I for 20 hours to melt the polycrystal and stabilize the solid-liquid interface. Then start the lifting device, set the descending speed to 0.5 mm / h, move the crucible down into temperature zone II, and stay at the position where the set temperature is 850 °C for 110 hours to grow the crystal until all the melt crystallizes. Then start the lifting device again, set the descending speed to 0.5 mm / h, move the crucible down to the middle of temperature zone III, and stay for 24 hours to complete annealing. Finally, cool it to room temperature to obtain Cu 8 GeSe 6 -based thermoelectric crystal (Cu 8 GeSe 6 crystal), which consists of two parts: a cone and a cylinder, with a total length of 10 cm, a taper of 60° for the cone, and a diameter of 8 mm for the cylinder.

[0071] Example 3

[0072] Prepare Cu 8 GeSe 6 -based thermoelectric crystal through the following steps:

[0073] S1: Prepare Cu 8 GeSe 6 polycrystal

[0074] Weigh high-purity Cu particles, Ge particles and Se particles according to the stoichiometric ratio of 8:1:6, put them into a quartz crucible, evacuate to a pressure of 10 -4 Pa and then fill it with argon, repeat this process 3 times, and then seal it using a hydrogen-oxygen flame. Put the sealed quartz crucible into a rocking furnace, raise the temperature to 950 °C, keep it warm for 3 hours, and rock it at a rocking speed of 10 r / min during this period to completely melt the raw materials and fully mix and react to obtain Cu 8 GeSe 6 polycrystal.

[0075] S2: Prepare Cu 8 GeSe 6 -based thermoelectric crystal

[0076] In the vertical growth furnace used in this step, there are temperature zones I (high-temperature zone), II (medium-temperature zone) and III (low-temperature zone) with decreasing temperatures from top to bottom. The length of temperature zone I in the vertical direction is 20 cm, and the temperature is set to 1150 °C; the length of temperature zone II in the vertical direction is 15 cm, and the set temperature decreases downward at a gradient of 15 °C / cm; the length of temperature zone III in the vertical direction is 10 cm, and the temperature is set to 850 °C.

[0077] Take the Cu 8 GeSe6 The polycrystal is placed in a graphite crucible. The bottom of the graphite crucible is conical with a taper of 70°, the diameter of the cylindrical part is 100 mm, and the total length of the cylindrical part and the conical part is 50 cm. The air in the graphite crucible is evacuated to a pressure of 10 -4 Pa and then filled with argon gas, and this process is repeated 3 times. Finally, the argon gas is filled to a pressure of 5×10 - 4 Pa, and then it is sealed using a hydrogen-oxygen flame.

[0078] The sealed graphite crucible is placed in a vertical growth furnace and stays in the middle of temperature zone I for 20 hours to melt the polycrystal and stabilize the solid-liquid interface. Then, the lifting device is started, and the descending speed is set to 0.3 mm / h to move the crucible down into temperature zone II and stay at a position with a set temperature of 1000 °C for 100 hours until the crystal growth is completed and all the melt has crystallized. Then, the lifting device is started again, and the descending speed is set to 0.3 mm / h to move the crucible down to the middle of temperature zone III and stay for 20 hours to complete annealing. Finally, it is cooled to room temperature to obtain a Cu 8 GeSe 6 -based thermoelectric crystal (Cu 8 GeSe 6 crystal), which consists of two parts, a cone and a cylinder, with a total length of 12 cm, a taper of 60° for the cone, and a diameter of 100 mm for the cylinder.

[0079] Comparative Example 1

[0080] In this comparative example, the method in the literature "Cu 8 GeSe 6 -based thermoelectric materials with an argyrodite structure" is used to prepare a Cu 8 GeSe 6 -based thermoelectric crystal (Cu 8 GeSe 6 crystal).

[0081] Comparative Example 2

[0082] In this comparative example, the Cu 8 GeSe 6 -based thermoelectric crystal (Cu 8 GeSe 6 crystal) is prepared according to the method in Example 1. The only difference from Example 1 is that in step S2, the graphite crucible is replaced with a quartz crucible. The rest of the process is the same as in Example 1.

[0083] Specifically, in this comparative example, the Cu 8 GeSe 6The steps for the base thermoelectric crystal are as follows:

[0084] S1: Prepare Cu 8 GeSe 6 polycrystals

[0085] Weigh high-purity (purity 99.999%) Cu particles, Ge particles and Se particles according to the stoichiometric ratio of 8:1:6, put them into a quartz crucible, evacuate to a pressure of 10 -4 Pa and then fill with argon, repeat 3 times, and then use a hydrogen-oxygen flame for sealing. Put the sealed quartz crucible into a rocking furnace, raise the temperature to 1100 °C, keep it warm for 1 hour, and rock at a rocking speed of 20 r / min during this period to completely melt the raw materials and fully mix and react to obtain Cu 8 GeSe 6 polycrystals.

[0086] S2: Prepare Cu 8 GeSe 6 base thermoelectric crystal

[0087] In the vertical growth furnace used in this step, there are temperature zones I (high-temperature zone), II (medium-temperature zone) and III (low-temperature zone) with gradually decreasing temperatures from top to bottom. The length of zone I in the vertical direction is 20 cm, and the temperature is set at 1150 °C; the length of zone II in the vertical direction is 10 cm, and the set temperature decreases downward at a gradient of 15 °C / cm; the length of zone III in the vertical direction is 20 cm, and the temperature is set at 700 °C.

[0088] Take the Cu 8 GeSe 6 polycrystals prepared in step S1, put them into a quartz crucible. The bottom of the quartz crucible is conical with a taper of 60°, the diameter of the cylindrical part is 15 mm, and the total length of the cylindrical part and the conical part is 10 cm. Evacuate the quartz crucible to a pressure of 10 -4 Pa and then fill with argon, repeat 3 times, and finally fill with argon to a pressure of 5×10 - 4 Pa, and then use a hydrogen-oxygen flame for sealing.

[0089] Put the sealed quartz crucible into the vertical growth furnace, stay in the middle of zone I for 20 hours to melt the polycrystals and stabilize the solid-liquid interface. Then start the lifting device, set the descent speed at 0.5 mm / h, lower the crucible into zone II, and stay at the position with a set temperature of 950 °C for 120 hours to grow the crystal until all the melt has crystallized. Then start the lifting device again, set the descent speed at 0.1 mm / h, lower the crucible to the middle of zone III, and stay for 20 hours to complete annealing. Finally, cool to room temperature to obtain Cu8 GeSe 6 Cu-based thermoelectric crystal (Cu 8 GeSe 6 crystal).

[0090] Comparative Example 3

[0091] In this comparative example, the Cu 8 GeSe 6 -based thermoelectric crystal (Cu 8 GeSe 6 crystal) was prepared according to the method in Example 1, and the difference from Example 1 was only that: in step S2, before sealing the graphite crucible with a hydrogen-oxygen flame, argon was filled for the last time until the air pressure reached 10 -3 Pa. The remaining processes were the same as those in Example 1.

[0092] Specifically, the steps for preparing the Cu8GeSe6-based thermoelectric crystal in this comparative example are as follows:

[0093] S1: Preparation of Cu 8 GeSe 6 polycrystalline

[0094] According to the stoichiometric ratio of 8:1:6, high-purity (purity 99.999%) Cu particles, Ge particles, and Se particles were weighed and placed in a quartz crucible. After evacuating to a pressure of 10 -4 Pa, argon was filled, and this was repeated 3 times. Then, it was sealed with a hydrogen-oxygen flame. The sealed quartz crucible was placed in a rocking furnace, the temperature was raised to 1100 °C, and it was kept warm for 1 hour. During this period, it was rocked at a rocking speed of 20 r / min to completely melt the raw materials and fully mix and react to obtain Cu 8 GeSe 6 polycrystalline.

[0095] S2: Preparation of Cu 8 GeSe 6 -based thermoelectric crystal

[0096] In the vertical growth furnace used in this step, there are temperature zones I (high-temperature zone), II (medium-temperature zone), and III (low-temperature zone) with gradually decreasing temperatures from top to bottom. The length of temperature zone I in the vertical direction is 20 cm, and the temperature is set at 1150 °C; the length of temperature zone II in the vertical direction is 10 cm, and the set temperature decreases downward at a gradient of 15 °C / cm; the length of temperature zone III in the vertical direction is 20 cm, and the temperature is set at 700 °C.

[0097] Take the Cu 8 GeSe 6The polycrystal is placed in a graphite crucible. The bottom of the graphite crucible is conical with a taper of 60°, the diameter of the cylindrical part is 15 mm, and the total length of the cylindrical part and the conical part is 10 cm. The inside of the graphite crucible is evacuated to a pressure of 10 -4 Pa and then filled with argon three times. The last time it is filled with argon to a pressure of 10 -3 Pa, and then sealed using a hydrogen-oxygen flame.

[0098] The sealed graphite crucible is placed in a vertical growth furnace and stays in the middle of temperature zone I for 20 hours to melt the polycrystal and stabilize the solid-liquid interface. Then the lifting device is started, and the descending speed is set to 0.5 mm / h to move the crucible down into temperature zone II and stay at a position with a set temperature of 950 °C for 120 hours until the crystal growth is completed and all the melt has crystallized. Then the lifting device is started again, and the descending speed is set to 0.1 mm / h to move the crucible down to the middle of temperature zone III and stay for 20 hours to complete annealing. Finally, it is cooled to room temperature to obtain a Cu 8 GeSe 6 -based thermoelectric crystal (Cu 8 GeSe 6 crystal). It consists of two parts, a cone and a cylinder, with a total length of 6 cm, a taper of 60° for the cone, and a diameter of 17 mm for the cylinder.

[0099] Comparative Example 4

[0100] This comparative example prepares a Cu 8 GeSe 6 -based thermoelectric crystal (Cu 8 GeSe 6 crystal) in the same method as in Example 1, except that in step S2, before sealing the graphite crucible using a hydrogen-oxygen flame, no argon is filled after the last evacuation (i.e., the pressure is 10 -4 Pa). The rest of the process is the same as in Example 1.

[0101] Specifically, the steps for preparing the Cu8GeSe6-based thermoelectric crystal in this comparative example are as follows:

[0102] S1: Prepare Cu 8 GeSe 6 polycrystals

[0103] According to the stoichiometric ratio of 8:1:6, weigh high-purity (purity 99.999%) Cu particles, Ge particles, and Se particles and place them in a quartz crucible. Evacuate to a pressure of 10 -4After charging argon at [Pa], repeat 3 times, and then use a hydrogen-oxygen flame for sealing. Place the sealed quartz crucible in a rocking furnace, raise the temperature to 1100 °C, hold for 1 hour, and rock at a rocking speed of 20 r / min during this period to completely melt the raw materials and fully mix and react to obtain Cu 8 GeSe 6 polycrystalline.

[0104] S2: Preparation of Cu 8 GeSe 6 based thermoelectric crystal

[0105] In the vertical growth furnace used in this step, there are temperature zones I (high-temperature zone), II (medium-temperature zone), and III (low-temperature zone) with decreasing temperatures from top to bottom. The length of temperature zone I in the vertical direction is 20 cm, and the temperature is set at 1150 °C; the length of temperature zone II in the vertical direction is 10 cm, and the set temperature decreases downward at a gradient of 15 °C / cm; the length of temperature zone III in the vertical direction is 20 cm, and the temperature is set at 700 °C.

[0106] Take the Cu 8 GeSe 6 polycrystalline obtained in step S1, put it into a graphite crucible. The bottom of the graphite crucible is conical with a taper of 60°, the diameter of the cylindrical part is 15 mm, and the total length of the cylindrical part and the conical part is 10 cm. Vacuum the inside of the graphite crucible to a pressure of 10 -4 Pa, then charge argon, repeat 3 times, and finally vacuum to a pressure of 10 -4 Pa without charging argon, and then use a hydrogen-oxygen flame for sealing.

[0107] Place the sealed graphite crucible in the vertical growth furnace, stay in the middle of temperature zone I for 20 hours to melt the polycrystalline and stabilize the solid-liquid interface. Then start the lifting device, set the descending speed at 0.5 mm / h, lower the crucible into temperature zone II, and stay at a position with a set temperature of 950 °C for 120 hours to grow the crystal until all the melt crystallizes. Then start the lifting device again, set the descending speed at 0.1 mm / h, lower the crucible to the middle of temperature zone III, and stay for 20 hours to complete annealing. Finally, cool to room temperature to obtain the Cu 8 GeSe 6 -based thermoelectric crystal (Cu 8 GeSe 6 crystal), which consists of two parts, a cone and a cylinder, with a total length of 6 cm, a taper of 60° for the cone, and a diameter of 17 mm for the cylinder.

[0108] Comparative Example 5

[0109] This comparative example prepares Cu 8 GeSe 6 -based thermoelectric crystal (Cu 8 GeSe 6 crystal) in the same method as in Example 1, and the difference from Example 1 is only that: in step S2, the speed of the crucible descending from temperature I to temperature zone II and from temperature zone II to temperature zone III is both 1 mm / h. The remaining processes are the same as those in Example 1.

[0110] Specifically, the steps for preparing the Cu8GeSe6-based thermoelectric crystal in this comparative example are as follows:

[0111] S1: Prepare Cu 8 GeSe 6 polycrystalline

[0112] Weigh high-purity (purity 99.999%) Cu particles, Ge particles and Se particles according to the stoichiometric ratio of 8:1:6, put them into a quartz crucible, evacuate to a pressure of 10 -4 Pa and then fill with argon, repeat 3 times, and then use a hydrogen-oxygen flame for sealing. Put the sealed quartz crucible into a rocking furnace, raise the temperature to 1100 °C, keep it warm for 1 hour, and rock at a rocking speed of 20 r / min during this period to completely melt the raw materials and fully mix and react to obtain Cu 8 GeSe 6 polycrystalline.

[0113] S2: Prepare Cu 8 GeSe 6 -based thermoelectric crystal

[0114] In the vertical growth furnace used in this step, there are temperature zones I (high-temperature zone), II (medium-temperature zone) and III (low-temperature zone) with decreasing temperatures from top to bottom. The length of temperature zone I in the vertical direction is 20 cm, and the temperature is set at 1150 °C; the length of temperature zone II in the vertical direction is 10 cm, and the set temperature decreases downward at a gradient of 15 °C / cm; the length of temperature zone III in the vertical direction is 20 cm, and the temperature is set at 700 °C.

[0115] Take the Cu 8 GeSe 6 polycrystalline prepared in step S1, put it into a graphite crucible. The bottom of the graphite crucible is conical, with a taper of 60°, the diameter of the cylindrical part is 15 mm, and the total length of the cylindrical part and the conical part is 10 cm. Evacuate the graphite crucible to a pressure of 10 -4 Pa and then fill with argon, repeat 3 times, and finally fill with argon to a pressure of 5×10 - 4 Pa, and then use a hydrogen-oxygen flame for sealing.

[0116] Put the sealed graphite crucible into the vertical growth furnace and stay in the middle of temperature zone I for 20 hours to melt the polycrystal and stabilize the solid-liquid interface. Then start the lifting device, set the descending speed to 1 mm / h, move the crucible down into temperature zone II, and stay at the position with a set temperature of 950 °C for 120 hours to grow the crystal until all the melt has crystallized. Then start the lifting device again, set the descending speed to 1 mm / h, move the crucible down to the middle of temperature zone III, and stay for 20 hours to complete annealing. Finally, cool to room temperature to obtain Cu 8 GeSe 6 -based thermoelectric crystal (Cu 8 GeSe 6 crystal), which consists of two parts, a cone and a cylinder, with a total length of 6 cm, a taper of 60° for the cone, and a diameter of 17 mm for the cylinder.

[0117] Test example

[0118] Take the Cu 8 GeSe 6 -based thermoelectric crystals prepared in each example and comparative example, and test their porosity, mechanical strength, and thermal conductivity in the direction perpendicular to the axis at different temperatures (unit: W·K -1 ·m -1 ). The results are shown in Table 1 and Table 2. Among them, the mechanical strength is detected by the vacuum pumping method, and the test air pressures are 10 -4 Pa and 10 -3 Pa. "Broken under the air pressure of 5×10 -3 Pa" in Comparative Example 2 means that the Cu 8 GeSe 6 -based thermoelectric crystal was broken during the preparation process, and the optical photograph of the fracture is as shown in Figure 3 .

[0119] Table 1 Detection results of porosity and mechanical strength of Cu 8 GeSe 6 -based thermoelectric crystals

[0120]

[0121] Table 2 Detection results of thermal conductivity of Cu 8 GeSe 6 -based thermoelectric crystals (unit of thermal conductivity: W·K -1 ·m -1 )

[0122]

[0123] Analyzing the detection results in Table 1 and Table 2, it can be seen that:

[0124] (1) By adopting the method of the present invention and combining with the design of reasonable crystal growth process parameters, a complete Cu 8 GeSe 6 -based thermoelectric crystal with large size and low thermal conductivity at 300 - 500 K can be obtained.

[0125] (2) The Cu 8 GeSe 6 crystal prepared in Comparative Example 1 was broken under a lower vacuum degree (atmospheric pressure 10 -3 Pa), while the Cu 8 GeSe 6 crystal prepared in Example 1 remained intact under a higher vacuum degree (atmospheric pressure 10 -4 Pa), indicating that the latter has higher mechanical strength. Moreover, the porosity of the crystal prepared in Example 1 is lower than that in Comparative Example 1. The reason for analysis is that: compared with the melting - quenching - annealing - hot - pressing sintering method in the prior art, under the method of the present invention, the crystal is more likely to nucleate and grow along the axial direction, which helps to discharge the internal micro - pores, so that the finally prepared Cu 8 GeSe 6 - based thermoelectric crystal has fewer pores inside.

[0126] (3) Compared with Comparative Example 2, the crystal prepared in Example 1 has a lower porosity and higher mechanical strength. The reason for analysis is that: for the low - thermal - conductivity Cu 8 GeSe 6 - based thermoelectric crystal, the temperature inside the crystal is prone to show a gradient distribution characteristic under the influence of the temperature field. By adopting a crucible with high thermal conductivity in the present invention, it helps to reduce the temperature difference between different regions inside the crystal, and makes it easier to nucleate and grow along the axial direction under the temperature gradient in the vertical growth furnace, thereby reducing the pores in the Cu 8 GeSe 6 - based thermoelectric crystal.

[0127] (4) By comparing the thermal conductivity detection results of Example 1 and Comparative Example 1, it can be seen that: at 300 - 500 K, the thermal conductivity of the crystal prepared in Example 1 in the direction perpendicular to the axis is lower than that in Comparative Example 1 (for a more intuitive display of the difference in thermal conductivity between the two, the relationship between the thermal conductivity of Example 1 and Comparative Example 1 and temperature is as Figure 4 shown). The above results show that compared with the melting - quenching - annealing - hot - pressing sintering method in the prior art, by adopting the method of the present invention, the thermal conductivity of the prepared Cu 8 GeSe 6 - based thermoelectric crystal in the direction perpendicular to the axis at 300 - 500 K can be reduced.

[0128] (5) Compared with Comparative Examples 3 to 5, the crystal prepared in Example 1 has a higher thermal conductivity perpendicular to the axis direction at 300 - 500 K. The above results show that when the cooling rate of the crucible from Temperature Region I to Temperature Region II and from Temperature Region II to Temperature Region III, as well as the gas pressure control after the last inert gas filling before sealing are improper, it will cause the Cu 8 GeSe 6 - based thermoelectric crystal to have a larger thermal conductivity perpendicular to the axis direction within the temperature range of 300 - 500 K.

[0129] The above are only the preferred embodiments of the present invention and do not impose any limitations on the present invention. Any simple modifications, changes, and equivalent transformations made to the above embodiments according to the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing a Cu8GeSe6-based thermoelectric crystal, characterized in that: The following steps are involved: S1: Mix and melt the single substances to form Cu8GeSe6-based polycrystals; S2: Place the Cu8GeSe6-based polycrystalline in a thermal conductivity higher than 100 W·m -1 ·K -1 The crucible is filled with inert gas after vacuuming the crucible, and this process is repeated several times. The last time the pressure in the crucible is filled with inert gas to 4×10 -4 ~6×10 -4 After Pa, it is sealed and placed in a vertical growth furnace; the vertical growth furnace is provided with temperature zones I, II and III with decreasing temperatures from top to bottom; S3: After melting the crucible containing Cu8GeSe6-based polycrystals in temperature zone I, the crucible is moved down to temperature zone II at a speed of 0.3-0.5 mm / h for crystal growth, and then moved down to temperature zone III at a speed of 0.1-0.5 mm / h for annealing to obtain a Cu8GeSe6-based thermoelectric crystal.

2. The preparation method according to claim 1, characterized in that: In step S2, the crucible is a SiC crucible or a graphite crucible.

3. The preparation method according to claim 1, characterized in that In step S3, the temperature of the temperature zone I is 1000-1150° C., and the holding time of the crucible in the temperature zone I is 12-20 h.

4. The preparation method according to claim 1, characterized in that In step S3, the temperature in the temperature zone II decreases from top to bottom at a gradient of 15-20°C / cm, and the crucible stays at a temperature of 850-1000°C in the temperature zone II for 100-120 hours.

5. The preparation method according to claim 1, characterized in that In step S3, the temperature of the temperature zone III is 600-850° C., and the holding time of the crucible in the temperature zone III is 20-24 h.

6. The preparation method according to claim 1, characterized in that: In step S3, the sealing method is flame sealing.

7. The preparation method according to claim 1, characterized in that: In step S3, the Cu8GeSe6-based thermoelectric crystal is Cu 8-x A x Ge 1-y B y Se 6-z C z In step S1, the Cu8GeSe6-based polycrystalline is Cu 8-x A x Ge 1-y B y Se 6-z C z Polycrystalline; Cu 8-x A x Ge 1-y B y Se 6-z C z In which, A is Ag and / or Au, B is Si and / or Sn, C is S and / or Te, 0≤x≤8, 0≤y≤1, 0≤z≤6.

8. The preparation method according to claim 1, characterized in that In step S1, the process of mixing and melting the single substances is carried out in a vertical melting furnace or a swinging furnace; in the swinging furnace, the swinging time is controlled to be 0.5-3 h, the preparation temperature is 950-1200°C, the heating rate is 4-10°C / min, and the swinging rate is 10-30r / min.

9. The preparation method according to claim 1, characterized in that: In step S2, the bottom of the crucible is conical, and the taper is 10° to 70°.

10. The preparation method according to claim 1, characterized in that In step S2, the length of the crucible is 5-50 cm, and the diameter of the cylindrical part is 8-100 mm.

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