N-type thermoelectric material based on TiNiSb and Fe elemental alloying and preparation method

Abstract

The application provides an N-type thermoelectric material formed based on TiNiSb and Fe element, and the chemical formula is TiNiFe x Sb, and the value range of x in the chemical formula is 0.4<=x<=0.5. The application provides a preparation method of TiNiFe x Sb material, so that the prepared sample has good stability. TiNiFe x Sb is a non-toxic and environment-friendly material, and the raw material cost is low. Compared with a TiNiSb matrix, the thermal conductivity is significantly reduced, and the electrical performance is obviously improved, so the sample has excellent thermoelectric performance in the whole temperature range. Since the sample is an alloy sample, a stable preparation scheme is the key of the application. Therefore, a method combining high-energy ball milling and high-temperature annealing is adopted. The final test of the sample of the application includes judging whether the sample is a pure phase without impurities by using XRD, testing the electrical conductivity and the Seebeck coefficient by using ZEM, testing the thermal conductivity by using LFA-457, and finally obtaining the thermoelectric performance of the sample.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of N-type thermoelectric materials, and particularly relates to an N-type thermoelectric material formed based on TiNiSb and Fe elemental alloying and a preparation method. BACKGROUND

[0002] The basis for thermoelectric materials to realize energy conversion is thermoelectric effect, which can also be referred to as temperature difference electric effect, mainly including Seebeck effect, Peltier effect and Thomson effect, and the interrelation among the three effects is Kelvin relationship. Among them, the Seebeck effect refers to that when two different semiconductors constitute a closed loop, if the two ends of the two semiconductors are at different temperatures, a temperature gradient will be generated, and the carrier will be affected by the temperature gradient and move from the hot end to the cold end, so that an electric potential difference is generated in the loop and an electric current is generated. The Peltier effect can be understood as the inverse effect of the Seebeck effect. When two conductors constitute a loop, when an electric current passes through the loop, the carrier will be affected by the electric current and undertake the work of transferring heat, so that heat release and heat absorption phenomena will occur at the hot end and the cold end of the loop. The Thomson effect refers to that when there is a temperature gradient in the conductor, if an electric current passes through the conductor, the conductor will release or absorb heat. Specifically, when the direction of the electric current is the same as the direction of the temperature gradient, the conductor absorbs heat, and vice versa, the conductor releases heat.

[0003] Although the thermoelectric material has the advantages of green environmental protection and wide application temperature range, it has not been widely applied due to the characteristics of complex preparation process and high production cost. Within the scope of Carnot cycle efficiency, the maximum conversion efficiency of the thermoelectric material is related to the dimensionless thermoelectric figure of merit ZT of the material, wherein 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. It can be seen from the formula that if the performance of the thermoelectric material is to be improved, the electrical conductivity and the Seebeck coefficient of the material need to be improved, and the thermal conductivity of the material needs to be reduced.

[0004] The half-Heusler thermoelectric materials can be classified by the number of valence electrons. For the 17-valence electron thermoelectric materials (such as TiFeSb), the thermoelectric performance can be improved by matching with 19-valence electron thermoelectric materials. For the 18-valence electron thermoelectric materials (such as NbFeSb, TiCoSb, ZrNiSn, etc.), the carrier concentration of the materials can be optimized by doping with different electrons at Y and Z sites to improve the electrical performance. At the same time, the lattice thermal conductivity of the materials can be reduced by doping with equal electrons at X and Y sites to enhance the point defect scattering, thereby optimizing the thermal performance. For the 19-valence electron system (such as TiNiSb, NbCoSb, VCoSb, etc.), the material usually contains many impurities, and the main way to optimize the performance is to stabilize the system by eliminating the impurities. For example, for the TiNiSb system, the valence electron number of the system can be increased to 18 by forming vacancies at the Ti site (i.e. Ti 0.75 NiSb) can eliminate impurities and greatly improve the thermoelectric performance of the material. In addition, we found that filling Fe atoms in the tetrahedral gap 4d site of the TiNiSb lattice can also stabilize the system and optimize the thermoelectric performance, which is a new way to improve the 19-valence electron half-Heusler thermoelectric material. SUMMARY

[0005] The present application provides an N-type thermoelectric material based on TiNiSb and Fe alloying and a preparation method to solve the problems in the background art. The thermoelectric material has good thermoelectric performance compared with the TiNiSb matrix, and the stability of the sample is also greatly improved.

[0006] Technical scheme: To achieve the above-mentioned purpose, the present application provides an N-type thermoelectric material based on TiNiSb and Fe alloying, which has a chemical formula of TiNiFe x Sb, wherein the value of x is in the range of 0.4≤x≤0.5.

[0007] Further, x = 0.4, 0.45, 0.5.

[0008] In addition, the present application also provides a preparation method of the above-mentioned thermoelectric material, which comprises the following steps:

[0009] Step a, weigh the Ti sheet, Ni sheet and Sb particles with a purity of more than 99.99% according to the ratio of 1:1:1 and put them into a high-energy ball mill pot, then weigh the Fe sheet with a purity of more than 99.99% according to the value of x (0.4, 0.45, 0.5) in the chemical formula and put it into the high-energy ball mill pot, then tighten the high-energy ball mill pot with a wrench to ensure that there is no air leakage;

[0010] Step b, the screwed high-energy ball milling tank is placed into a high-energy ball mill, the ball milling speed is set to 8000 rpm, the ball milling time is set to 20 hours, and the high-energy ball milling tank is taken out after the ball milling is completed;

[0011] Step c, the alloy powder in the high-energy ball milling tank is scraped out, the powder with a weight of 1.2 g is placed into a graphite mold (with a diameter of 12 mm), and then the graphite mold is subjected to spark plasma sintering, the temperature is increased to 850 DEG C at a rate of 80 DEG C / min at room temperature, the sintering pressure is 50 MPa, the temperature is kept at 850 DEG C for 10 min, and then the temperature is cooled to room temperature, and the sheet-shaped TiNiFe x Sb thermoelectric material is obtained.

[0012] Step d, the obtained sheet-shaped TiNiFe x Sb thermoelectric material is vacuum sealed, and then the tube is placed into a muffle furnace for annealing treatment, the annealing temperature is 800 DEG C, and the annealing time is 14 days.

[0013] Further, in step a, the operation of placing the raw materials into the high-energy ball milling tank is carried out in an argon glove box, so that the sample raw materials are not oxidized in the subsequent high-energy ball milling process.

[0014] Further, in step b, the single high-energy ball milling process is set to 4 hours, the interval time is 2 minutes, the process is cycled 5 times, and then the process is ended.

[0015] Further, in step c, during the ball milling process, after the ball milling t time, the effective collision times N of the milling balls are represented as: N=KC r t / R, wherein R is the radius of the milling balls in the ball milling tank, C r is the ball-to-material ratio, K is a constant, and t is the ball milling time.

[0016] Further, in step c, the processes of scraping the sample powder and loading the powder into the graphite mold are carried out in an argon glove box, so that the sample powder is not oxidized under the protection of an argon atmosphere.

[0017] Further, in step d, before annealing using the muffle furnace, it is ensured that the pressure in the tube is less than 3 Pa, the single annealing time is set to 7 days, the annealing is ended after being cycled twice, and then the cooling process is set to furnace cooling.

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

[0019] (1) The method of introducing high-energy ball milling to prepare the sample can better alloy the sample raw materials at room temperature, and avoids the volatilization of Sb elements at high temperatures and the safety hazards caused by high temperatures.

[0020] (2) The sample after discharge plasma sintering is annealed, so that the sample lattice is further grown, the performance is optimized, and the impurities are avoided from being adsorbed in the grain boundary, so that the sample itself is more pure.

[0021] (3) Fe atoms are introduced in the 4d position of the lattice, which can effectively reduce the thermal conductivity, and by adjusting the content of Fe, the TiNiFe 0.5 The ZT value of Sb reaches 0.25 at 973K, which is greatly improved compared with the TiNiSb matrix, and also provides a new idea for the stable method of the 19-electron system half-Heusler material.

[0022] Drawings

[0023] Figure 1 TiNiFe x XRD image of Sb thermoelectric material;

[0024] Figure 2 TiNiFe x Variation rule of lattice constant of Sb thermoelectric material;

[0025] Figure 3 TiNiFe x Variation image of electrical conductivity of Sb thermoelectric material with temperature;

[0026] Figure 4 TiNiFe x Variation image of Seebeck coefficient of Sb thermoelectric material with temperature;

[0027] Figure 5 TiNiFe x Variation image of power factor of Sb thermoelectric material with temperature;

[0028] Figure 6 TiNiFe x Variation image of total thermal conductivity of Sb thermoelectric material with temperature;

[0029] Figure 7 TiNiFe x Variation image of dimensionless thermoelectric optimal value ZT of Sb thermoelectric material with temperature. DETAILED DESCRIPTION

[0030] The preparation method of the TiNiSb-based thermoelectric composite material provided in the technical solution will be described in detail below with reference to the drawings.

[0031] The application provides an N-type thermoelectric material formed by alloying TiNiSb and Fe, which has a chemical formula of TiNiFe x Sb, wherein the value range of x is 0.4<=x<=0.5.

[0032] Further, x = 0.4, 0.45, 0.5.

[0033] In addition, the application further provides a preparation method of the thermoelectric material, and the method comprises the following steps:

[0034] In step a, Ti pieces, Ni pieces and Sb particles with a purity greater than 99.99% are weighed according to a ratio of 1:1:1 and then placed into a high-energy ball milling jar, then Fe pieces with a purity greater than 99.99% are weighed according to the value of x (0.4, 0.45, 0.5) in the chemical formula and then placed into the high-energy ball milling jar, and then the high-energy ball milling jar is tightened by using a wrench to ensure that there is no air leakage.

[0035] In step b, the tightened high-energy ball milling jar is placed into a high-energy ball mill for ball milling, the ball milling speed is set to 8000 rpm, the ball milling time is set to 20 hours, and then the high-energy ball milling jar is taken out after the ball milling is completed.

[0036] In step c, the alloy powder in the high-energy ball milling jar is scraped out, 1.2 g of the powder is weighed and placed into a graphite mold (with a diameter of 12 mm), and then the graphite mold is subjected to discharge plasma sintering, the temperature is raised to 850 ℃ at a rate of 80 ℃ / min under room temperature, the sintering pressure is 50 MPa, the temperature is kept at 850 ℃ for 10 min, and then the temperature is cooled to room temperature, so that a sheet-shaped TiNiFe x Sb thermoelectric material is obtained.

[0037] In step d, the obtained sheet-shaped TiNiFe x Sb thermoelectric material is subjected to vacuum sealing, and then the tube is placed into a muffle furnace for annealing treatment, the annealing temperature is 800 ℃, and the annealing time is 14 days.

[0038] Further, in step a, the operation of placing raw materials into the high-energy ball milling jar is performed in an argon glove box, so that the sample raw materials cannot be oxidized in the subsequent high-energy ball milling process.

[0039] Further, in step b, the single high-energy ball milling process is set to 4 hours, the interval time is 2 minutes, the process is cycled for 5 times, and then the process is ended.

[0040] Further, in step c, in the ball milling process, after the ball milling t time, the effective collision times N of the milling balls are represented as: N = KC r t / R, wherein R is the radius of the milling balls in the ball milling jar, C r is the ball-to-material ratio, K is a constant, and t is the ball milling time.

[0041] Further, in step c, the processes of scraping the sample powder and placing the powder into the graphite mold are performed in an argon glove box, so that the sample powder cannot be oxidized under the protection of the argon atmosphere.

[0042] Furthermore, in step d, before using the muffle furnace for annealing, ensure that the pressure inside the tube is less than 3 Pa, set the single annealing time to 7 days, end the annealing after two cycles, and then set the cooling process to furnace-based cooling.

[0043] Example 1:

[0044] Using Ti flakes, Fe flakes, Ni flakes, and Sb particles as raw materials (x = 0.4 in this example), the raw materials were prepared according to the chemical formula in an argon glove box. The prepared raw materials were then placed in a high-energy ball mill jar, which was tightened and placed in a high-energy ball mill for ball milling. After 20 hours of ball milling, the jar was removed, and the milled sample powder was scraped out in the argon glove box. 1.2 g of powder was weighed and placed into a graphite mold. The mold was then subjected to spark plasma sintering, with the temperature increased from room temperature to 850°C at a rate of 80°C / min, and the sintering pressure was 50 MPa. After holding at 850°C for 10 minutes, the temperature was cooled to room temperature to obtain flake-shaped TiNiFe. x Sb thermoelectric material, using sheet-like TiNiFe x The Sb thermoelectric material was vacuum-sealed, and then the tube was placed in a muffle furnace for annealing at 800℃ for 14 days. After annealing, TiNiFe was obtained. x Sb disc-shaped thermoelectric material.

[0045] Example 2:

[0046] This embodiment is the same as Embodiment 1, except that the value of x is 0.45 in this embodiment.

[0047] Example 3:

[0048] This embodiment is the same as Embodiment 1, except that the value of x is 0.5 in this embodiment.

[0049] Example 4:

[0050] This embodiment is the same as Embodiment 1, except that the value of x is 0 in this embodiment.

[0051] TiNiFe prepared by this invention x XRD diffraction pattern of Sb thermoelectric material as follows Figure 1 As shown in Examples 1 to 4, as the Fe content gradually increases, the diffraction peaks gradually shift to the left. This is because Fe atoms occupy the 4d position of the crystal lattice, and as the amount of Fe atoms increases, the unit cell volume also gradually increases. According to... Figure 2 It can be seen that the variation law of lattice constant obtained by XRD refinement can also verify this phenomenon, indicating that Fe atoms effectively occupy the 4d position of the lattice.

[0052] TiNiFe prepared by this invention x The changes in the conductivity and Seebeck coefficient of Sb with temperature are as follows: Figure 3 , Figure 4 As shown, the electrical conductivity decreases significantly compared to the matrix after introducing Fe atoms at the 4d position of the lattice. An intuitive explanation might be that filling the 4d position with Fe atoms increases the concentration of minority carriers (holes), thus reducing the electron concentration and consequently decreasing the conductivity. However, the actual situation may be more complex, as adding Fe atoms could alter the material's band structure. The Seebeck coefficient, while significantly increased, remains negative, indicating that the sample is still electronically conductive. Furthermore, the Seebeck coefficient initially increases and then decreases with increasing temperature. Due to the small band gap of the sample, bipolar diffusion occurs, affecting the electrical performance at high temperatures.

[0053] Figure 4 It can be seen from TiNiFe 0.5 The Sb sample achieved a Seebeck coefficient of -152 μV / K at 710 K, indicating that this type of material has excellent potential as an n-type thermoelectric material. According to... Figure 5 It can be seen that, compared to the TiNiSb matrix, the sample with introduced Fe interstitial atoms shows a significant improvement in power factor. Although the electrical conductivity decreases, the power factor is still higher according to the formula PF = S. 2 σ indicates that the increase in Seebeck coefficient is sufficient to offset the effect of decreased conductivity, verifying the positive effect of interstitial Fe atoms on the electrical properties of the material.

[0054] Figure 6 TiNiFe x The change in total thermal conductivity of Sb samples in TiNiFe x In Sb, the occupancy of Fe atoms at the 4d position can scatter additional phonons, thereby significantly reducing the lattice thermal conductivity. Figure 6 It can be seen that after introducing interstitial Fe atoms, TiNiFe [performs better] across the entire temperature range. x The total thermal conductivity of the Sb sample is significantly lower than that of the TiNiSb matrix, even reaching 3.35 W / m² at 973 K. -1 K -1 It is comparable to state-of-the-art thermoelectric materials (such as IV-VI compounds, Ge-Te alloys, and filled cobaltite), while the total thermal conductivity of a typical half-Heusler semiconductor is usually as high as 10 W / m². -1 K -1 TiNiFe x The exceptionally low thermal conductivity of the Sb sample is essential for improving the thermoelectric properties of the half-Heusler compound, and this experiment verifies that significantly reducing thermal conductivity by filling the tetrahedral interstitial 4d positions in the crystal lattice is a sufficiently feasible and effective approach. According toFigure 7 It can be seen that, thanks to the improved electrical properties and significantly reduced thermal conductivity, TiNiFe x The dimensionless thermoelectric figure of merit ZT of the Sb sample was significantly improved compared to the TiNiSb matrix, among which TiNiFe 0.5 The ZT value of Sb reached 0.27 at 710K.

[0055] In summary, TiNiFe was prepared by alloying TiNiSb with Fe. x The Sb sample has a very good ZT value and a wide application temperature range, making it a promising new thermoelectric material. It also provides a new approach and method for stabilizing the inherently unstable 19-electron half-Heusler system.

Claims

1. An N-type thermoelectric material based on the alloying of TiNiSb and Fe, characterized in that, Its chemical formula is TiNiFe x Sb, where x takes values ​​in the range of 0.4≤x≤0.5, and Fe atoms fill the tetrahedral interstitial 4d sites of the TiNiSb lattice.

2. The N-type thermoelectric material based on the alloying of TiNiSb and Fe elemental alloys according to claim 1, characterized in that, x＝0.4，0.45，0.5。 3. A method for preparing an N-type thermoelectric material based on the alloying of TiNiSb and Fe as described in claim 1 or 2, characterized in that, The method includes the following steps: Step a: Weigh Ti flakes, Ni flakes, and Sb particles with a purity greater than 99.99% in a 1:1:1 ratio and place them into the ball mill jar. Weigh Fe flakes with a purity greater than 99.99% according to the value of x in the chemical formula and place them into the ball mill jar. Tighten the ball mill jar with a wrench to ensure that it is airtight. Step b: Place the tightened grinding jar into the ball mill for grinding. Set the grinding speed to 8000 rpm and the grinding time to 20 hours. Remove the grinding jar after grinding is complete. Step c: Scrape out the alloy powder from the ball mill jar, weigh out 1.2g of powder and place it into a graphite mold with a diameter of 12mm. Perform spark plasma sintering on the graphite mold, heating it to 850℃ at a rate of 80℃ / min from room temperature, with a sintering pressure of 50MPa. Hold at 850℃ for 10min and then cool to room temperature to obtain flake-shaped TiNiFe. x Sb thermoelectric materials; Step d, the obtained sheet-like TiNiFe x The Sb thermoelectric material is vacuum-sealed and then placed in a muffle furnace for annealing at 800°C for 14 days.

4. The preparation method according to claim 3, characterized in that, In step a, the process of placing the raw materials into the ball mill jar is carried out in an argon glove box.

5. The preparation method according to claim 3, characterized in that, In step b, a single ball milling process is set to 4 hours, with a 2-minute pause. This process is repeated 5 times before ending.

6. The preparation method according to claim 3, characterized in that, In step b, 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, K is a constant, and t is the ball milling time.

7. The preparation method according to claim 3, characterized in that, In step c, the process of scraping the sample powder and loading the powder into the graphite mold is carried out in an argon glove box to ensure that the sample powder will not be oxidized under the protection of the argon atmosphere.

8. The preparation method according to claim 3, characterized in that, In step d, before using the muffle furnace for annealing, ensure that the pressure inside the tube is less than 3 Pa, set the single annealing time to 7 days, end the annealing after two cycles, and set the cooling process to furnace-in-flight cooling.

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