A P-type thermoelectric material based on Cu2Se and BaCu2Se2 composite and its preparation method

Abstract

This invention provides a p-type thermoelectric material based on a composite of Cu₂Se and BaCu₂Se₂, wherein the value of x ranges from 0 ≤ x ≤ 0.04. This experiment also provides (Cu₂Se) (1‑x) (BaCu2Se2) x The material is non-toxic and exhibits lower thermal conductivity compared to pure Cu₂Se, while maintaining good thermoelectric properties within its operating temperature range. As it is a composite sample, the focus is on the two-phase mixing process, which was achieved using mechanical ball milling. The final performance testing of the sample involved comparing the material's stability through cyclic testing, measuring conductivity using ZnEM with multiple cycles for comparison, and then determining whether ions migrated between the two phases using high-temperature XRD.

Description

Technical Field

[0001] This invention belongs to the field of P-type thermoelectric materials technology, specifically relating to a P-type thermoelectric material based on Cu2Se and BaCu2Se2 composite. Background Technology

[0002] Thermoelectric materials have garnered worldwide attention for their ability to directly convert heat and electricity using charge carriers within solids. The working principle of thermoelectric materials is primarily based on three effects: the Seebeck effect, the Peltier effect, and the Thomson effect. The Seebeck effect, discovered by the German scientist Seebeck, describes how connecting two different metallic conductors to form a current loop, maintaining different temperatures at the junctions, generates a thermoelectric electromotive force due to the temperature difference in the circulation. The Peltier effect, discovered by the French scientist Peliter, describes how when current flows through a circuit composed of different conductors, in addition to irreversible Joule heating, heat absorption and release occur at the junctions of the different conductors depending on the direction of the current. Based on these principles, various commercial thermoelectric devices and materials have emerged. These devices also possess unique advantages, such as small size, light weight, simple structure, robustness, durability, and the absence of moving parts.

[0003] Currently, due to the low thermoelectric properties of thermoelectric materials, the low conversion efficiency of devices, and the high manufacturing cost, they have not yet reached the economic demand range and therefore have not been widely used. Within the Carnot cycle efficiency range, the maximum conversion efficiency of thermoelectric materials is only related to the dimensionless thermoelectric figure of merit ZT of the material. Where S represents the Seebeck coefficient of the material, σ represents the electrical conductivity of the material, and κ represents the thermal conductivity of the material. Long-term research has found that in order to achieve high thermoelectric properties, it is necessary to improve the electrical properties of the material and reduce its thermal conductivity.

[0004] Generally, thermoelectric materials are classified into near-room temperature, mid-temperature, and high-temperature thermoelectric materials based on their temperature range. Currently, due to severe automotive emissions, some thermoelectric technologies have been applied in this field. These emissions produce high-temperature gases, thus requiring mid- to high-temperature materials for device fabrication. Typical mid- to high-temperature materials include Cu-based fast ion conductors, Ag-based fast ion conductors, cobaltite, PbTe, and Half-Hesuler alloys. However, these materials also have significant disadvantages. For example, Pb in PbTe is highly toxic, resulting in weak mechanical strength. Cobaltite exhibits low high-temperature stability. While fast ion conductors such as copper chalcogenides also possess ion release characteristics, numerous methods have been developed to mitigate this property. For fast ion conductors, the liquid-like properties of metal ions allow for fluidity at high temperatures and directional migration under external fields, resulting in extremely low intrinsic thermal conductivity. On the other hand, at high temperatures, copper chalcogenides are cubic phases, and the highly symmetrical crystal structure gives the system a high band degeneracy. Therefore, the material has a large effective mass of the density of states, resulting in a high Seebeck coefficient. This leads to Cu2Se having a very high thermoelectric figure of merit, which is very consistent with the thermoelectric characteristics of high-quality thermoelectric materials. Summary of the Invention

[0005] Purpose of the invention: In order to solve the problems mentioned in the background art, the present invention provides a P-type thermoelectric material based on Cu2Se and BaCu2Se2 composite and its preparation method, which has appropriate thermoelectric properties, low thermal conductivity and high stability, thereby enabling efficient preparation of Cu2Se-based composite thermoelectric materials with thermoelectric properties.

[0006] Technical solution: To achieve the above objectives, this invention provides a P-type thermoelectric material based on a composite of Cu2Se and BaCu2Se2, with the chemical formula (Cu2Se). (1-x) (BaCu2Se2) x , where the range of x is 0≤x≤0.04.

[0007] Furthermore, x = 0, 0.01, 0.02, 0.03, 0.04.

[0008] Furthermore, the present invention also proposes a method for preparing the above-mentioned thermoelectric material, the method comprising the following steps:

[0009] Step a: Weigh elemental Cu and Se with a purity greater than 99.99% at a ratio of 2:1 and place them into a boron nitride crucible. Put the prepared Cu-Se powder into a pit furnace and heat it to 650-680℃ at a rate of 2.8-3℃ / min, hold it for two hours, then heat it to 1000-1050℃ at a rate of 1.5-1.7℃ / min and hold it for about 12 hours for the first quenching. After cooling to room temperature, take out the sample and grind it thoroughly to form powder. Put the powder into a boron nitride crucible and vacuum seal it. Put the vacuum-sealed boron nitride crucible back into the pit furnace and heat it to 650℃ at a rate of 5-6℃ / min. After annealing for 72 hours, perform a second quenching. Take out the boron nitride crucible and grind the obtained ingot into Cu2Se powder in a mortar.

[0010] Step b, the solid-state sintered Cu2Se powder and BaCu2Se2 powder are mixed according to (Cu2Se) (1-x) (BaCu2Se2) x The values ​​of x are used to determine the proportions;

[0011] Step c: Place the powder from step b into a ball mill jar and ball mill it in a glove box with argon protection. Ball mill it for a certain time at a preset rate to obtain a mixed powder sample.

[0012] Step d: The ball-milled powder sample is loaded into a graphite mold and subjected to spark plasma sintering. The temperature is increased to 480-490℃ at a rate of 25℃ / min at room temperature, held for 4 minutes, and then cooled to room temperature to obtain flake-like (Cu2Se). (1-x) (BaCu2Se2) x Thermoelectric materials.

[0013] Furthermore, in step b, the first quenching is performed using cold water quenching.

[0014] Furthermore, in step c, the mass of the grinding balls is set to be between 80g and 110g, and the ball-to-material ratio is set to 13-17.

[0015] In this step, considering that the ball milling time and the quality of the grinding balls will affect the fineness of the powder and whether the final tablets are formed, after multiple experiments, the ball-to-powder ratio was determined to be between 13 and 17. After testing, it was found that the powder with a ball-to-powder ratio of 16 had the best density, so it was set to 16.

[0016] Furthermore, in step c, during the ball milling process, after a milling time t, the effective number of collisions N of the grinding balls is expressed as: N = KC r t / R, where R is the radius of the grinding balls inside the milling jar; C r The ratio of balls to material is given, and K is a constant.

[0017] Furthermore, in step c, the ball milling time is 1-2 hours and the ball milling rate is 500 r / min.

[0018] Beneficial effects: Compared with the prior art, the technical solution of the present invention has the following beneficial technical effects:

[0019] (1) The ball milling time for the two-phase mixture is very short, only 1 hour. Combined with SPS technology, the preparation of composite samples does not require annealing, and the samples are relatively pure as can be seen from the XRD images.

[0020] (2) Both types of P-type thermoelectric semiconductor materials have intrinsically low thermal conductivity. Due to the characteristics of fast ions, there will be ion migration during the heating and cooling process, which will regulate the carrier concentration of the material.

[0021] (3) By controlling the content of the second phase BaCu2Se2, the conductivity of the optimized composition was repeatedly tested and the concentration of internal charge carriers was stabilized. The electrical and thermal properties were cycled and found to be stable even at high temperatures. After the thermoelectric properties were adjusted, the ZT value reached 1.5 at 873 K, and it remained stable around this value after multiple tests. These results lay the foundation for the application of Cu2Se-based thermoelectric materials.

[0022] (4) First, Cu2Se is prepared by solid-state sintering, and then the optimal grain size for grain refinement is explored according to the ball-to-material ratio to obtain high density while reducing the probability of fragmentation.

[0023] Instruction manual illustrations

[0024] Figure 1 (Cu2Se) (1-x) (BaCu2Se2) x XRD images of composite thermoelectric materials;

[0025] Figure 2 (Cu2Se) (1-x) (BaCu2Se2) x Graph showing the change in electrical conductivity of composite thermoelectric materials with temperature;

[0026] Figure 3 (Cu2Se) (1-x) (BaCu2Se2) x Graph showing the change in total thermal conductivity of composite thermoelectric materials with temperature;

[0027] Figure 4 (Cu2Se) (1-x) (BaCu2Se2) x Graph of dimensionless thermoelectric figure of merit ZT of composite thermoelectric materials as a function of temperature;

[0028] Figure 5(Cu2Se) 0.99 (BaCu2Se2) 0.01 Repeatability of electrical conductivity in composite thermoelectric materials;

[0029] Figure 6 The individual lattice constant variations are for the composite samples BaCu2Se2 and Cu2Se. Detailed Implementation

[0030] The following detailed description, in conjunction with the accompanying drawings, illustrates a method for preparing a Cu2Se-based thermoelectric composite material according to this technical solution.

[0031] This invention provides a p-type thermoelectric material based on a composite of Cu₂Se and BaCu₂Se₂, with the chemical formula (Cu₂Se). (1-x) (BaCu2Se2) x , where the range of x is 0≤x≤0.04.

[0032] Furthermore, x = 0, 0.01, 0.02, 0.03, 0.04.

[0033] Furthermore, the present invention also proposes a method for preparing the above-mentioned thermoelectric material, the method comprising the following steps:

[0034] Step a: Weigh elemental Cu and Se with a purity greater than 99.99% at a ratio of 2:1 and place them into a boron nitride crucible. Put the prepared Cu-Se powder into a pit furnace and heat it to 650-680℃ at a rate of 2.8-3℃ / min, hold it for two hours, then heat it to 1000-1050℃ at a rate of 1.5-1.7℃ / min and hold it for about 12 hours for the first quenching. After cooling to room temperature, take out the sample and grind it thoroughly to form powder. Put the powder into a boron nitride crucible and vacuum seal it. Put the vacuum-sealed boron nitride crucible back into the pit furnace and heat it to 650℃ at a rate of 5-6℃ / min. After annealing for 72 hours, perform a second quenching. Take out the boron nitride crucible and grind the obtained ingot into Cu2Se powder in a mortar.

[0035] Step b: Mix the solid-state sintered Cu2Se powder and BaCu2Se2 powder according to the value of x in (Cu2Se)(1-x)(BaCu2Se2)x;

[0036] Step c: Place the powder from step b into a ball mill jar and ball mill it in a glove box with argon protection. Ball mill it for a certain time at a preset rate to obtain a mixed powder sample.

[0037] Step d: The ball-milled powder sample is loaded into a graphite mold and subjected to spark plasma sintering. The temperature is increased to 480-490℃ at a rate of 25℃ / min at room temperature, held for 4 minutes, and then cooled to room temperature to obtain flake-like (Cu2Se). (1-x) (BaCu2Se2) x Thermoelectric materials.

[0038] Furthermore, in step b, the first quenching is performed using cold water quenching.

[0039] Furthermore, in step c, the grinding ball mass is set to between 80g and 110g, and the ball-to-powder ratio is set to 13-17. In this step, considering that the ball milling time and grinding ball mass will affect the fineness of the powder and whether the final tablet is formed, after multiple experiments, the ball-to-powder ratio range was determined to be between 13 and 17. After testing, it was found that the powder with the ball-to-powder ratio of 16 has the best density, so it was set to 16.

[0040] Furthermore, in step c, during the ball milling process, after a milling time t, the effective number of collisions N of the grinding balls is expressed as: N = KC r t / R, where R is the radius of the grinding balls inside the milling jar; C r Where is the ball-to-material ratio, and K is a constant.

[0041] Furthermore, in step c, the ball milling time is 1-2 hours and the ball milling rate is 500 r / min.

[0042] Example 1:

[0043] Using Cu powder and Se powder as raw materials (x = 0.01 in this example), the raw material ratio was determined according to the chemical formula Cu₂Se. The prepared raw materials were placed in a boron nitride crucible, then sealed with glass, and placed in a pit furnace for high-temperature sintering. The temperature was increased from room temperature to 650℃ at a rate of 2.8-3.0℃ / min, held for 2 hours, then increased to 1050℃ at a rate of 1.5℃ / min, held for 12 hours for quenching, cooled to room temperature, and then annealed at 650-700℃ at a rate of 5-6℃ / min for 3 days, followed by a second quenching to obtain Cu₂Se ingots. The ingots were then ground into initial powder, which was then ball-milled with BaCu₂Se₂ in a ball mill jar. The ball-to-material ratio was 16, and the mass of grinding balls was provided according to the actual sample mass. The mixture was ball-milled at 500 r / min for 1 hour to obtain ball-milled powder. The ball-milled powder is loaded into a graphite mold and subjected to spark plasma sintering under vacuum at a temperature of 480-490℃, a pressure of 50 MPa, and a holding time of 4 min to obtain (Cu₂Se). (1-x) (BaCu2Se2) x Circular thermoelectric material.

[0044] Example 2:

[0045] This embodiment is the same as Embodiment 1, except that the value of x is 0.02 in this embodiment.

[0046] Example 3:

[0047] This embodiment is the same as Embodiment 1, except that the value of x is 0.03 in this embodiment.

[0048] Example 4:

[0049] This embodiment is the same as Embodiment 1, except that the value of x is 0.04 in this embodiment.

[0050] Example 5:

[0051] This embodiment is the same as Embodiment 1, except that the value of x is 0 in this embodiment.

[0052] The composite material (Cu2Se) prepared in this invention (1-x) (BaCu2Se2) x XRD diffraction images as follows Figure 1 As shown, according to Examples 1 to 5, different contents of BaCu2Se2 were obtained, and diffraction peaks of the second phase BaCu2Se2 could be observed. As the content of the second phase BaCu2Se2 increased, the diffraction peaks shifted to the left, and the lattice constant increased. This indicates that the second phase played a role in hindering the migration of Cu ions during the recombination process. The higher the content, the more obvious the effect of inhibiting the migration of Cu ions, which to some extent indicates that the stability is improved. On the other hand, the diffraction peak intensities in the XRD diffraction pattern also changed, with the peak intensity of the α phase significantly weakened. This indicates that there may be ion rearrangement inside. This is because the main difference between the α phase (monoclinic) and the β phase (cubic) is the number of Cu ions. Due to the introduction of the second phase, Cu ions enter the BaCu2Se2 lattice and do not have time to migrate out during the cooling process, resulting in a decrease in the overall Cu ion content in Cu2Se. Therefore, the α phase content decreases. It is speculated that ion migration at high temperatures causes a change in the ratio of cubic and monoclinic phases. Since Cu ions in the cubic phase are mobile at high temperatures, the monoclinic phase undergoes phase deformation to become the cubic phase during the heating process. Furthermore, since some Cu ions do not return to the Cu2Se crystal in time, the monoclinic phase content ultimately decreases.

[0053] The (Cu₂Se) prepared by this invention (1-x) (BaCu2Se2) x The change in conductivity is as follows Figure 2As shown, the conductivity decreases with increasing temperature. Cu₂Se is a p-type semiconductor, with holes as the primary transport carriers. At room temperature, the conductivity of several samples does not change significantly. This may be because the addition of the second phase reduces the carrier concentration and increases the mobility, resulting in minimal conductivity changes at room temperature. With increasing temperature, the conductivity decreases, exhibiting metallic behavior. However, there is no clear pattern to the conductivity change with varying second-phase content. This may be because the migration and concentration of ions cannot be determined during the heating process.

[0054] Figure 3 The change in thermal conductivity of the composite sample shows a significant decrease compared to the pure sample, indicating that the introduced second phase has a significant phonon scattering effect. The low thermal conductivity mainly stems from the strong phonon scattering effect at the two-phase interface. As mentioned in Example 1, the ball milling process used refines the grains, leading to enhanced multi-scale phonon scattering. Combined with... Figure 4 By comparing the ZT values, it was found that when x = 0.01, the optimization of electrical performance and the reduction of lattice thermal conductivity lead to high thermoelectric performance (Cu2Se). (1-x) (BaCu2Se2) x Thermoelectric materials have a thermoelectric figure of merit as high as around 1.5, such as... Figure 3 As shown.

[0055] To investigate the reproducibility of the materials, the best-performing (Cu2Se) was selected. 0.99 (BaCu2Se2) 0.01 Repeatability tests were performed with pure Cu₂Se, such as... Figure 5 As shown. However, at around 500 degrees Celsius, Cu₂Se segregated, causing sample testing to be interrupted, indicating poor repeatability at high temperatures. But for (Cu₂Se)... 0.99 (BaCu2Se2) 0.01 In this regard, the conductivity remained almost unchanged at 600℃, indicating improved stability. Ultimately... Figure 4 The results showed that the reproducibility of the two batches of samples was improved compared to the pure samples, with improvements in thermoelectric performance and stability. Figure 6 The changes in lattice constants between the two phases within the composite sample are shown, where 6a represents the composite (Cu2Se). 0.99 (BaCu2Se2) 0.01The lattice constant of pure Cu₂Se was analyzed. The results show a decrease in lattice constant, and the rate of decrease diminishes with increasing temperature. This indicates that the second phase, BaCu₂Se₂, enhances the ion-blocking effect at high temperatures. Although the Cu ion migration rate increases at high temperatures, the high potential barrier at the two-phase interface prevents excessive ions from migrating in, thus slowing the change in lattice constant. High-temperature XRD analysis of pure BCS was also performed to investigate the effect of temperature on the lattice constant. Due to thermal expansion, the diffraction peaks shifted to the left. However, calculations of the changes in the composite sample and pure BCS showed that… Figure 6 As shown in b, the composite sample showed a larger change, which suggests that ion migration into the crystal lattice may increase the lattice size.

[0056] In summary, composite engineering improved the thermoelectric performance and high-temperature stability of Cu2Se-based thermoelectric materials. Furthermore, experimental data confirmed the phenomenon of ion migration between the two phases, further verifying that BaCu2Se2 acts as an ion "reservoir," enabling Cu2Se to have a wider operating temperature range.

Claims

1. A method for preparing a p-type thermoelectric material based on a composite of Cu₂Se and BaCu₂Se₂, characterized in that, The method includes the following steps: Step a: Weigh elemental Cu and Se with a purity greater than 99.99% at a ratio of 2:1 and place them into a boron nitride crucible. Put the prepared Cu-Se powder into a pit furnace and heat it to 650-680℃ at a rate of 2.8-3℃ / min, hold it for two hours, then heat it to 1000-1050℃ at a rate of 1.5-1.7℃ / min, hold it for 12 hours, and perform the first quenching. After cooling to room temperature, take out the sample and grind it thoroughly to form powder. Then put the powder into a boron nitride crucible and vacuum seal it. Put the vacuum-sealed boron nitride crucible back into the pit furnace and heat it to 650℃ at a rate of 5-6℃ / min. After annealing for 72 hours, perform the second quenching. Take out the boron nitride crucible and grind the obtained ingot into Cu2Se powder in a mortar. Step b, the solid phase sintered Cu2Se powder and BaCu2Se2 powder are proportioned according to (Cu2Se) (1-x) (BaCu2Se2) x The value of x is selected according to the proportioning Step c: Place the powder from step b into a ball mill jar and ball mill it in a glove box with argon protection. Ball mill it for a certain time at a preset rate to obtain a mixed powder sample. Step d, the ball-milled powder sample was loaded into a graphite die and subjected to spark plasma sintering, the temperature was raised to 480-490°C at a rate of 25°C / min, the sintering pressure was 50Mpa, and the sample was kept at this temperature for 4min before cooling to room temperature, obtaining a sheet of (Cu2Se) (1-x) (BaCu2Se2) x thermoelectric material.

2. The preparation method according to claim 1, characterized in that, In step b, the first quenching is performed using cold water quenching.

3. The preparation method according to claim 1, characterized in that, In step c: set the grinding ball mass to 80g to 110g and the ball-to-material ratio to 13-17.

4. The preparation method according to claim 3, characterized in that, Set the ball-to-material ratio to 16.

5. The preparation method according to claim 1, characterized in that, In step c, during the ball milling process, after a milling time t, the effective number of collisions N of the grinding balls is expressed as: N = K t / R, where R is the radius of the grinding balls inside the grinding jar; Where is the ball-to-material ratio, and K is a constant.

6. The preparation method according to claim 1 or 3, characterized in that, In step c, the ball milling time is 1-2 hours and the ball milling rate is 500 r / min.

7. The P-type thermoelectric material based on Cu₂Se and BaCu₂Se₂ composite prepared by the method according to any one of claims 1-6, characterized in that, having the chemical formula (Cu2Se) (1-x) (BaCu2Se2) x wherein x has a value in the range of 0 < x < 0.

04.

8. The P-type thermoelectric material based on Cu₂Se and BaCu₂Se₂ composite as described in claim 7, characterized in that, x=0，0.01，0.02，0.03，0.04。

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