Semi-closed method for preparing active metal compound thermoelectric material

Abstract

The invention relates to a semi-closed preparation method of an active metal compound thermoelectric material, and belongs to the technical field of metal compound preparation. A graphite tank with a cover is selected as a heating container, graphite paper is placed between the cover and a tank body, a semi-closed state is formed through pressurization, the graphite tank is heated in a closed / semi-closed cavity with a protective atmosphere, heat preservation is conducted at a preset temperature, and finally the active metal compound thermoelectric material is obtained. Due to the semi-closed state, the problems of oxidation and volatilization of active metal in the reaction process are solved to a great extent, and the purity of a sample is improved; the graphite tank and the graphite paper are compatible with a direct-current / alternating-current rapid heating technology, so that the material production efficiency is improved; according to the semi-closed preparation method, various one-time sealing processes are avoided, the process is remarkably simplified, and the cost is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of metal compound preparation, and in particular relates to a controllable preparation method suitable for forming active metal compound thermoelectric materials. Background Art

[0002] Active metals often include alkali metals (lithium, sodium, potassium), alkaline earth metals (magnesium, calcium, strontium, barium), lanthanide rare earth metals, and group III elements such as aluminum, scandium, and yttrium. These metal elements have low electronegativity, are chemically active, and are easy to react with other elements. The narrow bandgap semiconductors or semimetals formed by the reaction of active metals with other highly electronegative elements are typical thermoelectric materials, such as magnesium-based thermoelectric materials Mg3(Sb,Bi)2, MgAgSb, Mg2(Si,Sn), 112-type thermoelectric materials EuZn2Sb2, CaZn2Sb2 and their solid solutions with Mg3Sb2, typical high-temperature thermoelectric materials La3Te4, Yb 14 Sb 11 , SmS, etc.

[0003] Because these metal elements are highly chemically active and have relatively low melting points, the preparation of their compounds usually requires a controlled environment:

[0004] (1) The synthesis needs to be carried out in a vacuum or inert atmosphere to prevent oxygen in the air from oxidizing the material and causing synthesis failure and deterioration of material properties;

[0005] (2) Since some low-melting-point components are easily volatilized at high temperatures, the final composition may be different from the initial composition, so the corresponding materials need to be sealed in a container to ensure that their relative composition remains unchanged;

[0006] (3) Due to the high activity of these active metals, they can react with quartz tubes or corundum crucibles at high temperatures. Therefore, it is also necessary to use targeted crucibles (such as zirconia crucibles) and take protective measures.

[0007] Therefore, the conventional preparation method of active metal compound thermoelectric materials requires that the material be placed in a zirconia crucible first to prevent the active metal from corroding the common alumina crucible, and then the crucible is sealed with a metal tantalum tube to prevent the active metal vapor from corroding the quartz tube. Finally, a layer of quartz tube is placed outside to prevent the tantalum tube from being oxidized by air during the heating process and to ensure the integrity of the sealing system. Although the use of this tantalum tube / quartz tube double-layer sealing tube and zirconia crucible can ensure the purity and performance of the sample, it also brings another problem: (4) The traditional tantalum tube / quartz tube double-layer sealing tube plus zirconia crucible synthesis process is complicated and costly, and is not suitable for large-scale production.

[0008] Therefore, finding an alternative large-scale synthesis method that can effectively prevent oxidation and component volatilization during the synthesis of active metal compound thermoelectric materials while simplifying the synthesis process and reducing production costs is a key issue that needs to be solved urgently. Summary of the invention

[0009] The technical solution adopted by the present invention realizes the large-scale controllable preparation of different active metal compound materials in a fast and low-cost method, and solves the following problems existing in the synthesis process of this type of material: (1) Active metal compounds are easily oxidized during the synthesis process, resulting in phase impurity and decreased performance; (2) Active metals are easily reacted with crucibles, corroding the crucible and reducing product purity; (3) Some components of the compound have low melting points and are easily volatilized at high temperatures, resulting in a shift in the final components; (4) The traditional double-layer tube sealing method is complex and costly, which is not conducive to large-scale industrial preparation.

[0010] To solve the above problems, the present invention adopts the following technical solution: a semi-closed method for preparing active metal compound thermoelectric materials, wherein a covered graphite can is selected as a heating container in a protective atmosphere cavity, graphite paper is placed between the graphite can lid and the can body to form a semi-closed state, the graphite can is quickly heated to a preset temperature, and the raw materials in the graphite can or the crucible placed in the graphite can react when heated to obtain the active metal compound thermoelectric material.

[0011] Furthermore, the active metal compound thermoelectric material contains one or more of alkali metal elements, alkaline earth metal elements, Group III elements, and lanthanide rare earth elements, and the compound is a narrow bandgap semiconductor or semimetal.

[0012] Furthermore, the protective atmosphere cavity is a vacuum-enclosed cavity that can be opened and closed multiple times, or a semi-enclosed cavity with a shielding curtain protected by an inert gas.

[0013] Furthermore, the shape of the graphite tank with a cover can be cylindrical, cubic, etc., and the internal volume can be 1cm3 to 300L to meet the reaction requirements of different magnitudes; the side wall has a blind hole for inserting a thermocouple thermometer.

[0014] Furthermore, the graphite paper is placed between the graphite can lid and the can body to form a semi-closed state, which means that the annular graphite paper is covered between the top of the graphite can and the lid, and pressure is applied to the lid and the bottom of the graphite can to form a semi-sealed state between the upper edge of the can body and the lid.

[0015] Furthermore, the rapid heating includes AC induction heating, pulsed DC heating directly applied to the graphite tank by resistance heating, or conduction or radiation heating of the graphite tank by other heating bodies.

[0016] Furthermore, the method comprises the following steps:

[0017] S1: weighing the raw materials of the active metal compound according to the stoichiometric ratio of each element and sequentially loading them into a graphite crucible;

[0018] S2: placing the graphite crucible in a cylindrical graphite can, placing a ring of graphite paper on the contact surface between the graphite can and the lid, and then placing all the devices in a protective atmosphere chamber; or placing them in a vacuum-sealed chamber that can be opened and closed multiple times or in an appropriate position in a semi-sealed chamber with a shielding curtain;

[0019] S3: Apply pressure from top to bottom of the graphite tank to make the graphite tank form a semi-sealed state; the applied pressure is 10-20000N.

[0020] S4: Under the control of the temperature control system, the graphite tank is quickly heated to the required temperature and kept warm so that the raw materials are fully mixed and reacted; a large current is passed through the graphite tank to directly heat it or high-frequency induction is used to heat the graphite tank, and a thermocouple or infrared temperature detector is turned on for real-time temperature detection and control;

[0021] The real-time temperature monitoring method of the temperature control system is a thermocouple or an infrared temperature detector. Specifically, SPS can be used for high-current direct heating, and an infrared temperature detector can be used for real-time temperature detection.

[0022] The heating temperature is specifically 200°C-1500°C, and the insulation time is 1min-48h;

[0023] S5: After the reaction is finished, the temperature is lowered, the pressure on the graphite tank is removed and the sample is taken out. The cooling method is cooling with the furnace or cooling at different rates.

[0024] Furthermore, if the protective atmosphere chamber uses a vacuum-sealed chamber that can be opened and closed multiple times, the vacuum-sealed chamber is evacuated to below 100 Pa; if the protective atmosphere chamber uses a semi-closed chamber with a shielding curtain, flowing inert gas (argon or nitrogen) is introduced to replace the atmosphere in the semi-closed chamber with an inert atmosphere.

[0025] Compared with the prior art, the present invention has at least the following advantages:

[0026] 1. The present invention adopts a semi-closed method to prepare thermoelectric materials of different active metal compounds. By simplifying the preparation process conditions, the synthesis time is shortened to about 50 minutes, the preparation time is short, and the operation is simple; compared with the traditional high-temperature melting-annealing process, the oxidation and volatilization of active metals during the reaction process can be reduced, and the risks in the preparation process can be reduced.

[0027] 2. The present invention uses a graphite can with graphite paper as a reaction container and heating medium. Its conductivity makes it compatible with a variety of rapid heating technologies; its inertness prevents the reaction between the container and the active raw materials; its semi-sealed design can also control the volatilization rate through the internal and external pressure difference, thereby improving the synthesis quality.

[0028] 3. The semi-sealed cavity with a sealing curtain used in the present invention is compatible with continuous production processes, avoiding various one-time sealing processes, and is conducive to improving production efficiency.

[0029] 4. The present invention is not only simple and effective, but also highly safe and low-cost, and is easy to scale up, and the technology is expected to be widely used in the preparation of various active metal materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the XRD diagram of the α-MgAgSb thermoelectric material prepared in Example 1 of the present invention.

[0031] Figure 2 This is the XRD diagram of the Mg2(Si,Sn) thermoelectric material prepared in Example 2 of the present invention.

[0032] Figure 3 The Cu prepared in Example 3 of the present invention 16 XRD pattern of Mg6Si7 thermoelectric material. DETAILED DESCRIPTION

[0033] The present invention is described in further detail below.

[0034] The present invention provides a "semi-enclosed" method to achieve low-cost, large-scale and controllable preparation of active metal compound materials. Its main features and advantages are: (1) using a graphite can container with a cover and graphite paper / gasket, so as to be compatible with large current direct heating methods (such as spark plasma sintering and high-frequency induction heating, etc.), avoid the reaction between the raw materials and the crucible, and easily obtain a large reaction volume, which is conducive to industrial production; (2) using the graphite paper between the graphite can and the cover as a "semi-enclosed" medium to form a differential air pressure inside and outside the can, thereby greatly suppressing the volatilization of low-melting point components and ensuring the composition of the final product; (3) the outermost layer uses an openable and closable sealed cavity or a multi-layer "semi-enclosed" shielding curtain to ensure that the reaction is in a vacuum atmosphere or an inert gas atmosphere.

[0035] Example 1: A method for preparing Mg in a discharge plasma chamber 1.05 Ag 0.97 Sb 0.99 The method for preparing a thermoelectric material comprises the following specific steps:

[0036] 1) In a glove box, a Mg block with a purity of not less than 99.9%, a Sb block with a purity of not less than 99.999%, and an Ag block with a purity of not less than 99.99% were mixed into an α-Mg 1.05 Ag 0.97 Sb 0.99 The stoichiometric ratio of each atom in the mixture is weighed, and the Mg block, Sb block and Ag block are sequentially placed into a cylindrical graphite crucible with a diameter of 2 cm and a height of 6 cm;

[0037] 2) Place the graphite crucible in a cylindrical graphite can with a diameter of 6 cm and a height of 8 cm, place an annular graphite pad under the lid of the graphite can, take them out of the glove box and put them into the plasma spark discharge sintering furnace cavity, and start the vacuum system to evacuate to 8 Pa;

[0038] 3) Adjust the pressure button to apply 2000N of pressure on the graphite tank, and turn on the infrared temperature detector to record the real-time temperature;

[0039] 4) Adjust the current button to increase the temperature to 900 degrees at a rate of 50 degrees / minute;

[0040] 5) Raise the temperature to 950°C at a rate of 20°C / min;

[0041] 6) Raise the temperature to 1000°C at a rate of 10°C / min and keep at 1000°C for 30 min;

[0042] 7) After the insulation is completed, the temperature is lowered at a rate of 30 degrees / minute, and vacuum is continuously drawn during this period;

[0043] 8) After the temperature is lowered, adjust the pressure button to 0N, fill with argon to normal pressure, and open the chamber door to take out the sample. At this time, the sample is a mixed phase of γ-MgAgSb phase and α-MgAgSb phase;

[0044] 9) After densification and sintering, seal the tube and anneal at 290 degrees for one week to transform the γ-MgAgSb phase into the α-MgAgSb phase. The XRD pattern of the obtained product is shown in Figure 1 .

[0045] Example 2: A method for preparing Mg2X(Si, Sn)-based thermoelectric materials in a semi-enclosed hot pressing chamber with a sealing curtain, the specific steps comprising:

[0046] 1) Lower the sealing curtain of the semi-sealed hot pressing chamber and open the argon valve to replace the air in the chamber with an argon environment;

[0047] 2) In a glove box, place Mg blocks with a purity of not less than 99.9%, Sn particles with a purity of not less than 99.999%, and Si blocks with a purity of not less than 99.99% in a Mg 2.07 Sn 0.7 Si0.3 The stoichiometric ratio of each atom in the graphite crucible is weighed, and the Mg block, the Sn block and the Si block are sequentially placed into the graphite crucible;

[0048] 3) Place the graphite crucible in a cylindrical graphite can with a diameter of 15 cm and a height of 10 cm, place a ring-shaped graphite pad under the lid of the graphite can, take it out of the glove box, and use an automatic sampler to put it in the middle of the AC induction heating coil of the hot press cavity. The graphite can passes through three layers of flexible sealing curtains on the way in, which can basically ensure that the inert atmosphere of the hot press cavity is not destroyed;

[0049] 4) Adjust the pressure button to apply 1500N of pressure on the graphite tank, and turn on the thermocouple thermometer to record the real-time temperature;

[0050] 5) Start the AC induction heating device and raise the temperature to 800 degrees at a rate of 30 degrees per minute;

[0051] 6) Raise the temperature to 850°C at a rate of 20°C / min;

[0052] 7) Raise the temperature to 900°C at a rate of 10°C / min and keep warm for 30 min;

[0053] 8) After the insulation is completed, the temperature is lowered at a rate of 20 degrees / minute, and vacuum is continuously drawn during this period;

[0054] 9) After the cooling is completed, adjust the pressure button to 0N and take out the graphite tank with an automatic sampler. Figure 2 .

[0055] Example 3: Preparation of Cu in a vacuum hot pressing chamber with resistance heating 16 The method of Mg6Si7-based thermoelectric material comprises the following specific steps:

[0056] 1) In a glove box, place Mg blocks with a purity of not less than 99.9%, Cu powder with a purity of not less than 99.999%, and Si blocks with a purity of not less than 99.99% in a vacuum oven. 16 Mg 6.07 The stoichiometric ratio of each atom in Si7 is weighed, and the Mg block, Cu rod and Si block are sequentially placed in a graphite crucible;

[0057] 2) Place the graphite crucible in a cylindrical graphite can with a diameter of 6 cm and a height of 8 cm, place an annular graphite pad inside the lid of the graphite can, take it out of the glove box, and put it into the hot pressing furnace cavity together; close the hot pressing chamber door;

[0058] 3) Apply a pressure of 4000N on the graphite tank and turn on the infrared thermometer to record the real-time temperature;

[0059] 4) Start the resistance heating device and raise the temperature to 950 degrees at a rate of 10 degrees / minute;

[0060] 5) Raise the temperature to 980°C at a rate of 5°C / min;

[0061] 6) Raise the temperature to 1000°C at a rate of 3°C / min and keep warm for 60 min;

[0062] 7) After the heat preservation is completed, the temperature is lowered at a cooling rate of 10 degrees / minute;

[0063] 8) After the cooling is completed, fill with argon gas to an atmospheric pressure, open the hatch, and take out the sample. The XRD pattern of the obtained product is shown in Figure 3 .

Claims

1. A semi-closed method for preparing active metal compound thermoelectric materials, characterized in that: The method comprises the following steps: selecting a graphite can with a cover as a heating container in a protective atmosphere cavity, placing graphite paper between the graphite can cover and the can body to form a semi-closed state, rapidly heating the graphite can to a preset temperature, and reacting the raw materials in the graphite can or a crucible placed in the graphite can when heated to obtain an active metal compound thermoelectric material.

2. A semi-closed method for preparing active metal compound thermoelectric materials according to claim 1, characterized in that: The active metal compound thermoelectric material contains one or more of alkali metal elements, alkaline earth metal elements, group III elements, and lanthanide rare earth elements, and the compound is a narrow bandgap semiconductor or a semimetal.

3. A semi-closed method for preparing active metal compound thermoelectric materials according to claim 1, characterized in that: The protective atmosphere cavity is a vacuum-enclosed cavity that can be opened and closed multiple times or a semi-enclosed cavity with a shielding curtain protected by an inert gas.

4. A semi-closed method for preparing active metal compound thermoelectric materials according to claim 1, characterized in that: The side wall of the graphite tank with a cover is provided with a blind hole for inserting a thermocouple thermometer.

5. The semi-closed method for preparing active metal compound thermoelectric materials according to claim 1, characterized in that: The graphite paper is placed between the graphite can lid and the can body to form a semi-closed state, which means that the annular graphite paper is covered between the top of the graphite can and the lid, and pressure is applied to the lid and the bottom of the graphite can to form a semi-sealed state between the upper edge of the can body and the lid.

6. A semi-closed method for preparing active metal compound thermoelectric materials according to claim 1, characterized in that: The rapid heating includes a resistance heating method directly applied to the graphite can, or a conduction or radiation heating method of the graphite can through other heating bodies.

7. The semi-closed method for preparing active metal compound thermoelectric materials according to claim 1, characterized in that: The method comprises the following steps: S1: weighing the raw materials of the active metal compound according to the stoichiometric ratio of each element and sequentially loading them into a graphite crucible; S2: placing the graphite crucible in a cylindrical graphite can, placing a ring of graphite paper on the contact surface between the graphite can and the lid, and then placing the whole into a protective atmosphere chamber; S3: applying pressure up and down on the graphite tank to make the graphite tank form a semi-sealed state; S4: Under the control of the temperature control system, the graphite tank is quickly heated to the required temperature and kept warm so that the raw materials are fully mixed and reacted; a large current is passed through the graphite tank to directly heat it or high-frequency induction is used to heat the graphite tank, and a thermocouple or infrared temperature detector is turned on for real-time temperature detection and control; S5: After the reaction is completed, cool down, remove the pressure on the graphite tank and take out the sample.

8. A semi-closed method for preparing active metal compound thermoelectric materials according to claim 7, characterized in that: If the protective atmosphere chamber uses a vacuum-sealed chamber that can be opened and closed multiple times, the vacuum-sealed chamber is evacuated to below 100 Pa; if the protective atmosphere chamber uses a semi-enclosed chamber with a shielding curtain, flowing inert gas is introduced to replace the atmosphere in the semi-enclosed chamber with an inert atmosphere.

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