P-type high-performance thermoelectric material with reversible phase change, and preparation method therefor

Abstract

The present invention relates to a P-type high-performance thermoelectric material featuring reversible phase change, and a preparation method therefor. The thermoelectric material has a chemical composition of Cu.sub.2Se.sub.1-xI.sub.x, wherein 0<x0.08. The method comprises: weighing elemental copper metal, elemental selenium metal, and cuprous iodide according to the molar ratio (2x):(1x):x, and packaging them in a vacuum; raising the temperature to 1150-1170 C. in stages and performing a melting treatment for 12-24 hours; lowering the temperature to 600-700 C. in stages and then performing an annealing treatment for 5-7 days, the substances being cooled to room temperature in a furnace after the annealing treatment; and performing pressure sintering at 400-500 C.

Claims

1. A method for preparing the P-type high-performance thermoelectric material with the reversible phase transition, characterized in that a chemical composition of the thermoelectric material is Cu.sub.2Se.sub.1-xI.sub.x, wherein 0<x0.08, and wherein the thermoelectric material has a sandwich-shape layered structure with a thickness of 2050 nm, the method comprising: weighing elemental copper metal, elemental selenium metal, and cuprous iodide at a mole ratio of (2x):(1x):x as raw materials and encapsulating the raw materials under a vacuum; gradiently heating the raw materials by first heating the raw materials to 650700 C. at a heating rate of 2.55 C./min and keeping them thereat for 12 hours, then further heating the raw materials to 11501170 C. at a heating rate of 0.82 C./min and keeping them thereat for 1224 hours for melting; gradiently cooling the melt to 600700 C. and keeping it thereat for 57 days for annealing treatment; furnace-cooling the melt to room temperature; and heating the room temperature melt to 400450 C. for pressure sintering.

2. The method according to claim 1, characterized in that a process of encapsulating the raw materials under a vacuum is carried out by plasma encapsulation or flame gun encapsulation under protection of an inert gas.

3. The method according to claim 1, characterized in that a method for pressure sintering is spark-plasma sintering, a pressure of the sintering is 5065 Mpa, and a sintering duration is 510 minutes.

4. A method for preparing the P-type high-performance thermoelectric material with the reversible phase transition, characterized in that a chemical composition of the thermoelectric material is Cu.sub.2Se.sub.1-xI.sub.x, wherein 0<x0.08, and wherein the thermoelectric material has a sandwich-shape layered structure with a thickness of 2050 nm, the method comprising: weighing elemental copper metal, elemental selenium metal, and cuprous iodide at a mole ratio of (2x):(1x):x as raw materials and encapsulating the raw materials under a vacuum; gradiently heating the raw materials to 11501170 C. and keeping them thereat for 1224 hours for melting; gradiently cooling the melt by first cooling the melt to 10001120 C. at a cooling rate of 510 C./min and keeping it thereat for 1224 hours, then further cooling the melt to 600700 C. at a cooling rate of 510 C./min and keeping it thereat for 57 days for annealing treatment; furnace-cooling the melt to room temperature; and heating the room temperature melt to 400450 C. for pressure sintering.

5. The method according to claim 4, characterized in that a process of encapsulating the raw materials under a vacuum is carried out by plasma encapsulation or flame gun encapsulation under protection of an inert gas.

6. The method according to claim 4, characterized in that a method for pressure sintering is spark-plasma sintering, a pressure of the sintering is 5065 Mpa, and a sintering duration is 510 minutes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view showing a preparation flow of an exemplary thermoelectric material of the invention.

(2) FIG. 2 is a schematic view of a single-pair thermoelectric device consisting of the P-type Cu.sub.2Se.sub.1-xI.sub.x provided herein and N-type Yb.sub.0.3Co.sub.3Sb.sub.12.

(3) FIG. 3A shows a scanning electron microscopy image of the Cu.sub.2Se compounds in example 1.

(4) FIG. 3B shows a high resolution scanning electron microscope image of the Cu.sub.2Se compounds in example 1.

(5) FIG. 3C shows a scanning electron microscopy image of the thermoelectric material of example 2 of the invention.

(6) FIG. 3D shows a high resolution scanning electron microscopy image of the thermoelectric material of example 2 of the invention.

(7) FIG. 4 shows temperature dependence of a figure of merit ZT of the compound Cu.sub.2Se of example 1 in the phase transition region.

(8) FIG. 5 shows temperature dependence of the figure of merit ZT of the compound Cu.sub.2Se.sub.0.96I.sub.0.04 of example 2 in the phase transition region.

(9) FIG. 6 shows temperature dependence of the figure of merit ZT of the compound Cu.sub.2Se.sub.0.92I.sub.0.08 of example 3 in the phase transition region.

(10) FIG. 7 shows the refrigeration performance of the single-pair thermoelectric device consisting of the P-type Cu.sub.2Se provided herein and N-type Yb.sub.0.3Co.sub.3Sb.sub.12.

(11) FIG. 8 shows refrigeration performance of the single-pair thermoelectric device consisting of the P-type Cu.sub.2Se.sub.1-xI.sub.x provided herein and N-type Yb.sub.0.3Co.sub.3Sb.sub.12.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(12) The present invention will be further described with the following embodiments below with reference to the drawings. It should be understood that the drawings and the following embodiments are only used for explaining this invention, and do not limit this invention.

(13) Hereinafter, the preparation of a new reversible phase transition thermoelectric material compound Cu.sub.2Se.sub.1-xI.sub.x (0<x0.08) will be described as an example.

(14) The compound synthesized by the present invention is Cu.sub.2Se.sub.1-xI.sub.x, consisting of copper, selenium, and iodine, wherein 0<x0.08.

(15) The preparation process of the present invention comprises vacuum packaging, melting, slow cooling, and annealing, as described by FIG. 1 which shows a schematic preparation flow of the thermoelectric material of the invention. In the invention, pure copper metal, pure selenium metal, and iodine compounds (for example cuprous iodide) are used as starting materials. Pure elemental copper (99.999%), pure elemental selenium (99.999%), and cuprous iodide compound (99.98%) as employed are abundant in source and easy to obtain. First, elemental copper metal, elemental selenium metal, and cuprous iodide are weighed at a mole ratio of (2x):(1x):x and encapsulated under a vacuum. The vacuum encapsulation may be carried out in a glove compartment under protection of an inert gas such as argon, etc., or in a condition of an external vacuum. The vacuum encapsulation may be plasma encapsulation or flame gun encapsulation. During the encapsulation, the quartz tube is vacuumized so as to maintain the internal pressure at 1-10000 Pa. Copper and selenium can be directly encapsulated in the quartz tube under vacuum, or be placed in a pyrolytic boron nitride crucible (PBN), followed by being encapsulated in the quartz tube.

(16) Then, the starting materials are subjected to a high temperature melting treatment. The melting process of the present invention can be carried out in a box-type furnace. Firstly, the starting materials are heated to 650-700 C. at a heating rate of 2.5-5 C./min, and kept at the temperature for 1-2 hours; then heated to 1150-1170 C. at a heating rate of 0.8-2 C./min, and kept at the temperature for 12-24 hours for melting. The melt is slowly cooled down to 1000-1120 C. at a cooling rate of 5-10 C./min, and kept at the temperature for 12-24 hours; then slowly cooled down to 600-700 C. at a cooling rate of 5-10 C./min, and kept at the temperature for 57 days for annealing; and finally cooled down to room temperature by the manner of natural furnace cooling.

(17) Finally, the annealed ingots are ground into a powder, then sintered under pressure. Spark plasma sintering (SPS) is chosen as the sintering method, wherein a size of the graphite mold with a size of 10 mm is used, and the inner wall and pressure head are sprayed with BN for insulation; the sintering temperature is 450 C.-500 C., the sintering pressure is 50-65 MPa, and the sintering duration is 5-10 minutes. A dense bulk is obtained by the sintering. It is observed under a scanning electron microscope that the compounds prepared exhibit a sandwich-shape layered structure with a thickness of a few tens of nanometers (2050 nm) at room temperature. High-resolution electron microscopy observation validates that this material mainly comprises small nanocrystallines, and nano defects such as dislocations, twins, etc. (see FIGS. 3C and 3D).

(18) P-type Cu.sub.2Se.sub.1-xI.sub.x and N-type Yb-filled skutterudite (Yb.sub.0.3Co.sub.3Sb.sub.12) thermoelectric materials are connected in a -shape (See FIG. 2) to form a refrigeration test device with a single-pair of legs, wherein a nickel sheet with a thickness of 0.2 mm is selected as the guide plate, 10 mm10 mm6 mm copper blocks are selected as the hot junction heat absorbing electrodes, the size of the P-type thermoelectric material Cu.sub.2Se.sub.1-xI.sub.x is 3 mm3 mm1 mm, the size of N-type thermoelectric material Yb.sub.0.3Co.sub.3Sb.sub.12 is 1 mm1 mm1 mm, and the surfaces of the P-type Cu.sub.2Se.sub.1-xI.sub.x and N-type Yb.sub.0.3Co.sub.3Sb.sub.12 samples are treated by nickel electroplating and are connected to the guide plate and the hot junction heat absorbing electrodes by tin soldering. FIG. 8 shows the refrigeration performance of the single-pair thermoelectric device consisting of the P-type Cu.sub.2Se.sub.1-xI.sub.x provided herein and N-type Yb.sub.0.3Co.sub.3Sb.sub.12. As shown in FIG. 8, the temperature difference of refrigeration in the phase transition region is higher than that after the phase transition and that at room temperature.

(19) The preparation method of the invention has the advantages of simple raw material, low cost, simple process flow, high controllability, and good repeatability. The thermoelectric materials provided herein have a high Seebeck coefficient, a high electrical conductivity, and a low thermal conductivity.

(20) Hereinafter, the present invention will be better described with the following representative examples. It is understood that the following examples are only used to explain this invention and do not limit the scope of this invention, and any non-essential improvements and modifications made by a person skilled in the art based on this invention all fall into the protection scope of this invention. The specific parameters below such as temperature and time are only exemplary, and a person skilled in the art can choose proper values within an appropriate range according to the description of this article, and are not restricted to the specific values cited below.

Example 1: Preparation and Thermoelectric Properties of Cu2Se

(21) As raw materials, pure metal Cu and Se are weighed at a mole ratio of 2:1, placed into a PBN crucible, and then put into a quartz tube. The quartz tube is vacuumized and supplied with protective Ar gas for 3 times, and then sealed by plasma flame or gas flame in a glove box. A small amount of Ar gas is pumped into the quartz tube as an inert atmosphere to protect the raw material. The raw materials are heated to 650-700 C. at a heating rate of 2.5 5 C./min, and kept at this temperature for 1-2 hours; then heated to 1150-1170 C. at a heating rate of 0.8-2 C./min, and kept at the temperature for 12-24 hours for melting; then slowly cooled down to 1000-1120 C. at a cooling rate of 5-10 C./min, and kept at this temperature for 1224 hours; then slowly cooled down to 600-700 C. at a cooling rate of 5-10 C./min, and kept at this temperature for 5-7 days; and finally cooled down to room temperature by the manner of natural furnace cooling. The resulting ingots are ground into powder, then sintered by spark plasma sintering at 400-450 C. under a pressure of 50-65 MPa for 5-10 minutes, to give a dense bulk with a density of 97% or more. The field emission electron microscope image shows that the Cu.sub.2Se at room temperature exhibits a sandwich-shape layered structure with a thickness of a few tens of nanometers. The transmission electron microscopy (TEM) images show that there are no large crystalline grains but many nanocrystallines and nano defects such as dislocations and twins in the materials (as shown in FIGS. 3A and 3B). Such a complex structure can further enhance the thermoelectric properties. Measurements of thermoelectric performance show that the material undergoes a phase transition at around 400 K, which is a reversible phase transition. The material has a very high Seebeck coefficient, an excellent electrical conductivity, and a good power factor in the phase transition region. At the same time, the thermal conductivity of the material is very low in the phase transition region. According to the measurement and calculation, the ZT value of the material is about 0.2 at room temperature, and can reach 2.3 in the phase transition region (around 400 K) (see FIG. 4).

Example 2: Preparation and Thermoelectric Properties of Cu2Se0.96I0.04

(22) As raw materials, pure metal Cu, Se, and compound cuprous iodide are weighed at a mole ratio of 1.96:0.96:0.04, placed into a PBN crucible, and then put into a quartz tube. The quartz tube is vacuumized and supplied with protective Ar gas 3 times, then sealed by plasma flame or gas flame in a glove box. A small amount of Ar gas is pumped into the quartz tube as inert atmosphere to protect the raw material. The raw materials are heated to 650-700 C. at a heating rate of 2.5-5 C./min, and kept at this temperature for 1-2 hours; then heated to 1150-1170 C. at a heating rate of 0.8-2 C./min, and kept at the temperature for 12-24 hours for melting; then slowly cooled down to 1000-1120 C. at a cooling rate of 5-10 C./min, and kept at this temperature for 12-24 hours; then slowly cooled down to 600-700 C. at a cooling rate of 5-10 C./min, and kept at this temperature for 5-7 days; and finally cooled down to room temperature by the manner of natural furnace cooling. The resulting ingots are ground into powder, then sintered by spark plasma sintering at 400-450 C. under a pressure of 50-65 MPa for 5-10 minutes to give a dense bulk with a density of 97% or more. The field emission electron microscope image shows that the Cu.sub.2Se at room temperature exhibits a sandwich-shape layered structure with a thickness of a few tens of nanometers. The TEM images show that there are no large crystalline grains but many nanocrystallines and nano defects such as dislocations and twins in the material (as shown in FIGS. 3C and 3D). Measurements of thermoelectric performance show that the material undergoes a phase transition at around 380 K, which is a reversible phase transition. The material has a very high Seebeck coefficient, an excellent electrical conductivity, and a good power factor in the phase transition region. At the same time, the thermal conductivity of the material is very low in the phase transition region. According to the measurement and calculation, the ZT value of the material is about 0.2 at room temperature, and can reach 1.1 in the phase transition region (around 380 K) (see FIG. 5).

Example 3: Preparation and Thermoelectric Properties of Cu2Se0.92I0.08

(23) As raw materials, pure metal Cu, Se, and compound cuprous iodide are weighed at a mole ratio of 1.92:0.92:0.08, and placed into a PBN crucible, and then put into a quartz tube. The quartz tube is vacuumized and supplied with protective Ar gas 3 times, and then sealed by plasma flame or gas flame in a glove box. A small amount of Ar gas is pumped into the quartz tube as inert atmosphere to protect the raw material. The raw materials are heated to 650-700 C. at a heating rate of 2.5-5 C./min, and kept at this temperature for 1-2 hours; then heated to 1150-1170 C. at a heating rate of 0.8-2 C./min, and kept at the temperature for 12-24 hours for melting; then slowly cooled down to 1000-1120 C. at a cooling rate of 5-10 C./min, and kept at this temperature for 12-24 hours; then slowly cooled down to 600-700 C. at a cooling rate of 5-10 C./min, and kept at this temperature for 5-7 days; and finally cooled down to room temperature by the manner of natural furnace cooling. The resulting ingots are ground into powder, then sintered by spark plasma sintering at 400-450 C. under a pressure of 50-65 MPa for 5-10 minutes, to give a dense bulk with a density of 97% or more. Measurements of thermoelectric performance show that the material undergoes a phase transition at around 360 K, which is a reversible phase transition. The material has a very high Seebeck coefficient, an excellent electrical conductivity, and a good power factor in the phase transition region. At the same time, the thermal conductivity of the material is very low in the phase transition region. According to the measurement and calculation, the ZT value of the material is about 0.2 at room temperature, and can reach 0.8 in the phase transition region (around 360 K) (see FIG. 6).

Example 4: Preparation for the Single-Pair Device Consisting of P-Type Cu2Se and N-Type Yb0.3Co3Sb12 and Performance Measurements Therefor

(24) P-type Cu.sub.2Se and N-type Yb.sub.0.3Co.sub.3Sb.sub.12 are selected for preparation of the single-pair device. A Cu.sub.2Se sample of 3 mm3 mm1 mm and a Yb.sub.0.3Co.sub.3Sb.sub.12 sample of 1 mm1 mm1 mm are prepared by cutting. After surface grinding, the samples are soaked in a mixture of nitric acid and hydrofluoric acid (HNO.sub.3:HF:H.sub.2O=3:1:6) for 1-3 minutes, then ultrasonically cleaned in deionized water, then treated by electroplating, wherein the electric current is 0.05-0.08 A, the samples are first pre-electroplated in a nickel chloride solution of 1 mol/L for 1-3 minutes, and then electroplated in a nickel sulfamate solution of 200 g/L at 40 C. for 3-5 minutes. After polishing the surrounding electroplated nickel, the resulting samples are briefly washed in deionized water. Then the samples are welded between the copper electrodes and the heat conducting plate by tin soldering. The measurement is carried out under vacuum of 1-20 Pa, with the test current being 0.25-4 A. The relationship between the maximum temperature difference of refrigeration and the current are measured before the phase transition (300 K, 370 K), in the phase transition region (about 395 K) and after the phase transition (420 K), respectively. According to the test results, when the current is 4 A, the maximum temperature difference of refrigeration of the device in phase transition region is 24.3% higher than that in the normal phase after phase transition and 79% higher than that at the room temperature phase (FIG. 7).

Example 5: Preparation for the Single-Pair Device Consisting of P-Type Cu2Se0.96I0.04 and N-Type Yb0.3Co3Sb12 and Performance Measurements Therefor

(25) P-type Cu.sub.2Se.sub.0.96I.sub.0.04 and N-type Yb.sub.0.3Co.sub.3Sb.sub.12 are selected for preparation of the single-pair device. A Cu.sub.2Se.sub.0.96I.sub.0.04 sample of 3 mm3 mm1 mm and a Yb.sub.0.3Co.sub.3Sb.sub.12 sample of 1 mm1 mm1 mm are prepared by cutting. After surface grinding, the samples are soaked in a mixture of nitric acid and hydrofluoric acid (HNO.sub.3:HF:H.sub.2O=3:1:6) for 1-3 minutes, then ultrasonically cleaned in deionized water, then treated by electroplating, wherein the electric current is 0.05-0.08 A, the samples are first pre-electroplated in a nickel chloride solution of 1 mol/L for 1-3 minutes, and then electroplated in a nickel sulfamate solution of 200 g/L at 40 C. for 3-5 minutes. After polishing the surrounding electroplated nickel, the resulting samples are briefly washed in deionized water. Then the samples are welded between the copper electrodes and the heat conducting plate by tin soldering. The measurement is carried out under vacuum of 1-20 Pa, with the test current being 0.25-4 A. The relationship between the maximum temperature difference of refrigeration and the current are measured before the phase transition (300 K, 340 K), in the phase transition region (about 380 K) and after the phase transition (400 K), respectively. According to the test results, when current is 4 A, the maximum temperature difference of refrigeration of the device in phase transition region is 25.7% higher than that in the normal phase after phase transition and 83.3% higher than that in the room temperature phase (FIG. 8).

INDUSTRIAL APPLICABILITY

(26) The thermoelectric material provided herein has a simple chemical composition, a low dimensional layered structure, and a high ZT value, and thus can be developed as a new type of thermoelectric material. The method of the present invention is simple, feasible, low cost, and suitable for large-scale production.