A coating for inhibiting surface oxidation and volatilization of half-Heusler thermoelectric materials, a preparation method thereof, and an application thereof

Abstract

The present invention relates to a coating for inhibiting surface oxidation and volatilization of a half-Heusler thermoelectric material, a preparation method thereof and an application. The coating for inhibiting surface oxidation and volatilization of the half-Heusler thermoelectric material sequentially includes an AO2 and B-X mixed layer formed in-situ on the surface of the half-Heusler thermoelectric material, a B-X compound layer, and an oxide layer of A, B and X; the chemical general formula of the half-Heusler thermoelectric material is ABX, where A is at least one of Ti, Zr, Hf, Sc, Y, V, Nb, Ta, B is at least one of Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, and X is at least one of Sn, Sb, Bi.

Description

Technical Field

[0001] The present invention relates to a coating for the surface of a half-Heusler thermoelectric material, a preparation method thereof, and an application thereof, belonging to the field of thermoelectric materials. Background Art

[0002] Half-Heusler materials are thermoelectric materials in the medium and high temperature regions, with the chemical general formula ABX, having a face-centered cubic MgAgAs structure and a space group of F-43m. A and X atoms form a NaCl structure, and B atoms occupy half of the 4c positions in the face-centered cubic lattice. Half-Heusler materials have excellent thermoelectric properties, good mechanical properties, and non-toxic constituent elements, etc., and have good application prospects.

[0003] However, during the high-temperature service process of thermoelectric devices, the thermoelectric materials will experience surface volatilization and oxidation, resulting in the deterioration of material properties. For ordinary thermoelectric materials, surface coatings can be prepared to inhibit the oxidation and volatilization of thermoelectric materials. The coating materials include metals, ceramics, glasses, polymers, and composite materials, etc. The preparation techniques of surface coatings are mainly magnetron sputtering, sol-gel method, dip coating method, etc. For example, Chinese Patent 1 (Publication No. CN102514282A) deposits a W / ZrO2-Y2O3 mixture on the surface of a CoSb3-based skutterudite by electron beam physical vapor deposition. The coating has a relatively high bonding strength with the substrate (greater than 5 MPa), but the preparation process is harsh and the cost is high, which is not suitable for large-scale preparation. Chinese Patent 2 (Publication No. CN103146301A) uses a mixture of organosilane-modified silica sol and glass powder as a protective coating for thermoelectric materials. The coating has good chemical stability, but the thermal expansion coefficient does not match that of the substrate, and cracks are likely to occur. Chinese Patent 3 (Publication No. CN103531704A) coats a glass protective layer on the surface of a thermoelectric element during electric welding, but the wettability of the glass coating with the substrate material is poor and the bonding strength is low. Chinese Patent 4 (Publication No. CN104890325A) coats a ceramic composite / porous glass on the surface of a thermoelectric material, but the preparation process of the composite coating is complex, and the softening temperature of the glass layer (673 - 973K) is low. Therefore, the existing surface protection coating technologies for other thermoelectric material systems are not applicable to half-Heusler materials. Moreover, there is no publicly reported research on the surface protection coating technology for half-Heusler materials internationally currently. Summary of the Invention

[0004] In view of the above problems, the present invention provides a coating, a preparation method and an application thereof, which have a simple process and can effectively inhibit the surface oxidation and element volatilization of half-Heusler thermoelectric materials. The present invention develops surface protective coating materials suitable for half-Heusler materials and their preparation technologies. Inhibiting the surface oxidation and element volatilization of materials and meeting the long-term working conditions of the materials in the medium and high temperature regions are the keys to improving the performance and stability of half-Heusler thermoelectric devices, and are of great significance for promoting the practical application of half-Heusler devices.

[0005] In the first aspect, the present invention provides a coating for inhibiting the surface oxidation and volatilization of half-Heusler thermoelectric materials. The chemical general formula of the half-Heusler thermoelectric materials is ABX, where A is at least one of Ti, Zr, Hf, Sc, Y, V, Nb, Ta; B is at least one of Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt; and X is at least one of Sn, Sb, Bi.

[0006] The coating sequentially includes an AO2 and B-X mixed layer, a B-X compound layer, and an oxide layer of A, B, and X, which are in-situ formed on the surface of the half-Heusler thermoelectric material.

[0007] In the present invention, an oxide coating is in-situ generated on the surface of the half-Heusler thermoelectric material. This coating is directly oxidized from the half-Heusler thermoelectric material, which can ensure good bonding between the coating and the matrix material. At the same time, this coating is formed by the reaction of the half-Heusler thermoelectric material matrix, without using complex coating preparation processes, which not only reduces the preparation cost but also ensures the stability of the coating structure.

[0008] Preferably, the half-Heusler thermoelectric material is TiNiSn, ZrNiSn, HfNiSn, TiCoSb, ZrCoSb, HfCoSb, TiFeSb, NbFeSb, or HfFeSb.

[0009] Preferably, the oxides of A, B, and X are ZrO2, HfO2, CoSb2O6, NiO, SnO2, NbO2, or Fe2O3; the B-X compounds are CoSb, Ni3Sn2, NbSb2, FeSb.

[0010] Preferably, the thickness of the oxide layer of A, B, and X is 1-250 μm.

[0011] Preferably, the thickness of the B-X compound layer is 1-50 μm.

[0012] Preferably, the thickness of the AO2 and B-X mixed layer is 1-200 μm.

[0013] Preferably, the total thickness of the coating is 3 μm to 500 μm, and preferably 5 μm to 50 μm.

[0014] In a second aspect, the present invention provides a method for preparing a coating for suppressing surface oxidation and volatilization of a half-Heusler thermoelectric material. The surface of the half-Heusler thermoelectric material bulk is processed to be flat and clean, and then it is placed in an oxygen-containing atmosphere and heat-treated at 400°C to 900°C to complete the preparation of the coating. After the surface of the half-Heusler thermoelectric material bulk is processed, it is placed in a low-oxygen environment and heat-treated at a certain temperature. An in-situ coating with a multi-layer structure will be formed on the surface of the half-Heusler material. This coating can effectively inhibit the diffusion of oxygen in the environment and the volatilization of low-melting-point elements in the matrix material, and can improve the stability of the half-Heusler material.

[0015] Preferably, at least one of polishing, buffing, and cleaning is used to make the surface of the half-Heusler thermoelectric material bulk flat.

[0016] Preferably, the oxygen-containing atmosphere is an oxygen atmosphere, an air atmosphere, or a mixed atmosphere of oxygen and an inert gas with an oxygen partial pressure of 1 Pa to 1 MPa.

[0017] Preferably, the heat treatment holding time is 5 minutes to 100 hours.

[0018] Preferably, the heating rate of the heat treatment is 50°C / minute to 200°C / minute.

[0019] In a third aspect, the present invention provides a half-Heusler thermoelectric device containing a coating for suppressing surface oxidation and volatilization of a half-Heusler thermoelectric material.

[0020] In a fourth aspect, the present invention provides an application of a coating for suppressing surface oxidation and volatilization of a half-Heusler thermoelectric material in the service of a half-Heusler thermoelectric material in an oxygen-containing environment.

[0021] In a fifth aspect, the present invention provides an application of a coating for suppressing surface oxidation and volatilization of a half-Heusler thermoelectric material in the service of a half-Heusler thermoelectric material in an oxygen-free environment.

[0022] Beneficial effects:

[0023] The present invention provides a suitable coating and preparation method for suppressing surface oxidation and element volatilization of a half-Heusler thermoelectric material. The coating has good bonding strength with the half-Heusler matrix material, can improve the oxidation resistance of the half-Heusler thermoelectric material and reduce the surface element volatilization rate, and can effectively alleviate the problem that the output performance of the half-Heusler thermoelectric device decreases due to material surface oxidation and element volatilization. At the same time, the present invention also provides an in-situ oxidation preparation method for the coating suitable for the half-Heusler thermoelectric material. The surface coating prepared by this method has a controllable thickness, good wettability with the matrix thermoelectric material, high bonding strength, and a simple process and low cost, and can be used for the large-scale preparation of the half-Heusler thermoelectric material coating. Description of the Drawings

[0024] Figure 1 Schematic diagram of the coating structure with oxidation and volatilization inhibition generated on the surface of the half-Heusler thermoelectric material by the in-situ oxidation method of the present invention;

[0025] Figure 2a Backscattered electron pattern of the ZrCoSb sample in Example 1 and the coating generated on its surface by the in-situ oxidation method;

[0026] Figure 2b Elemental surface distribution map of the ZrCoSb sample in Example 1;

[0027] Figure 3 Backscattered electron pattern of the surface of the ZrCoSb sample with an oxidation coating in Example 1 after oxidation in air at 600 °C for 15 days;

[0028] Figure 4 Comparison chart of the oxidation rates of the ZrCoSb sample with an in-situ generated oxidation coating at 750 °C for 6 hours and the ZrCoSb sample without a coating after being kept in air at 600 °C for 15 days in Example 1;

[0029] Figure 5a Backscattered electron pattern of the NbFeSb sample in Example 2 and the coating generated on its surface by the in-situ oxidation method;

[0030] Figure 5b Elemental surface distribution map of the NbFeSb sample in Example 2;

[0031] Figure 6 Backscattered electron pattern of the NbFeSb sample with an in-situ generated oxidation coating in Example 2 after being kept at 850 °C in vacuum for 7 days;

[0032] Figure 7 Backscattered electron pattern of the NbFeSb sample without an oxidation coating in Example 2 after being kept in vacuum for 7 days. Detailed implementation manners

[0033] The present invention is further illustrated by the following implementation manners. It should be understood that the following implementation manners are only used to illustrate the present invention and do not limit the present invention.

[0034] In the present disclosure, a coating prepared by in-situ oxidation of a half-Heusler material in air is selected as the coating for suppressing oxidation and element volatilization during the service of the half-Heusler thermoelectric material, and this coating is located on the surface of the half-Heusler thermoelectric material. The half-Heusler thermoelectric material is TiNiSn, ZrNiSn, HfNiSn, TiCoSb, ZrCoSb, HfCoSb, TiFeSb, NbFeSb or HfFeSb. The in-situ oxidation coating is a multi-layer structure, and the components of each layer are mainly an AO2 and B-X mixed layer, a B-X compound layer, and an oxide layer of A, B, and X.

[0035] In the present invention, the multi-layer structure oxidation coating generated in-situ is directly generated by the oxidation of the half-Heusler thermoelectric material and has good bonding. At the same time, the coating raw materials are generated from the half-Heusler thermoelectric material substrate, and there is no need to separately prepare the coating by using complex processes. Moreover, the process for preparing the coating in the present invention is simple and has low cost.

[0036] In the present invention, first, the surface of the sample is treated by means such as sandblasting, grinding, and polishing to ensure that the sample surface is flat and clean. Then, the surface-treated sample is placed in a muffle furnace, a tube furnace, or an atmosphere furnace, etc. for heat treatment. Among them, an in-situ oxidation coating is generated by oxidizing for 5 minutes to 100 hours under the conditions of an oxygen partial pressure of 1 Pa to 1 MPa and a temperature of 400 to 900 °C.

[0037] In an alternative embodiment, the sample can be oxidized under pure oxygen or air conditions. It can also be oxidized under a low oxygen partial pressure mixed atmosphere condition (the oxygen partial pressure can be 1 Pa to 1 MPa) composed of oxygen and an inert gas. An in-situ multi-layer structure oxidation coating is generated. The present invention realizes the in-situ generation of an oxidation coating on the surface of the half-Heusler material, and this coating is directly generated by the oxidation of the surface elements of the half-Heusler material, and the coating has good bonding with the material.

[0038] In an alternative embodiment, the thickness of the oxidation coating generated under the oxidation time can be 5 to 50 micrometers. It should be noted that when the thickness of the in-situ oxidation coating generated is relatively thick (greater than several hundred micrometers), it will affect the thermoelectric performance of the half-Heusler alloy material. When the thickness of the in-situ oxidation coating generated is relatively thin, the function of the in-situ oxidation coating in suppressing oxidation and element volatilization is not obvious.

[0039] The following exemplarily illustrates the method for in-situ generating an oxidation coating on the surface of the half-Heusler thermoelectric material.

[0040] In an alternative embodiment, first, the sample is treated by means such as sandblasting, grinding, and polishing to ensure that the sample surface is flat and clean.

[0041] Then, the sample is placed under high-temperature oxygen-containing conditions, and an in-situ oxidation coating is generated on the surface of the half-Heusler alloy by using the high-temperature oxidation method.

[0042] In an alternative embodiment, a high-temperature furnace such as a muffle furnace, a tube furnace, or an atmosphere furnace can be used for the oxidation treatment. Among them, the sintering atmosphere environment can be pure oxygen, air, or a low-oxygen partial pressure mixed gas composed of oxygen and an inert gas.

[0043] In an alternative embodiment, the oxidation temperature (heat treatment temperature) of the in-situ oxidation coating can be 400 - 900 °C. The oxidation time (heat treatment time) of the in-situ oxidation coating can be 5 minutes to 100 hours. The heating rate of the heat treatment can be 50 - 200 °C / minute.

[0044] The above method for preparing the in-situ oxidation coating can also be applied to half-Heusler thermoelectric devices.

[0045] The in-situ oxidation coating provided by the present invention has the same elemental composition as the half-Heusler thermoelectric material, a similar coefficient of thermal expansion, and a large bonding strength. The method for preparing the in-situ oxidation coating suitable for suppressing the surface oxidation and element volatilization of the half-Heusler thermoelectric material can not only be applied to the research of surface protection coating technology for other thermoelectric material systems, but also simplify the preparation process of thermoelectric components with a coating for suppressing the surface oxidation and element volatilization of thermoelectric materials.

[0046] The following further exemplifies embodiments to illustrate the present invention in detail. It should also be understood that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention fall within the protection scope of the present invention. The specific process parameters and the like in the following examples are only an example within a suitable range, that is, those skilled in the art can make selections within a suitable range through the description herein, rather than being limited to the specific values in the following examples.

[0047] Example 1

[0048] In this Example 1, the oxide layer formed on the surface of the ZrCoSb sample in high-temperature air is used as a material protection coating to reduce the oxidation rate of the ZrCoSb sample in air, including the following steps:

[0049] (1) The surface of the ZrCoSb block is treated. The material surface treatment uses sandblasting, grinding, and polishing to ensure the surface of the sample is clean.

[0050] (2) The size and mass of the ZrCoSb sample after surface treatment are measured, and then it is placed in a furnace at 750 °C and oxidized in air for 6 hours.

[0051] (3) The oxidized sample is taken out, weighed again, and then polished to observe the in-situ formed oxidation coating.

[0052] The samples with in-situ formed oxidation coatings were cleaned with alcohol for surface cleaning. After measuring the sample size and mass, they were placed in a furnace at 600 °C and oxidized in air for 15 days. After taking out the oxidized samples and weighing them, they were polished to observe the oxidation degree of the sample surface, analyze the oxidation resistance effect of the in-situ formed oxidation coating, and compare it with the oxidation rate of the ZrCoSb sample without the oxidation coating at 600 °C to analyze and obtain the oxidation resistance effect of the in-situ formed oxidation coating. The following is the analysis of the oxidation resistance effect.

[0053] It can be seen from Figure 2a the backscattered electron spectrum that there are no cracks and obvious pores at the interface between the in-situ formed multi-layer structure oxidation coating and the ZrCoSb material, indicating good interfacial bonding. Figure 2b The elemental surface distribution map of Figure 2a shows that the in-situ formed oxidation coating is divided into three layers, and the elements in each layer are evenly distributed. There is no obvious diffusion and chemical reaction at the interface. The oxidation layer from the inside to the outside is a ZrO2 and CoSb mixed layer with a thickness of 20 - 30 μm, a CoSb compound layer with a thickness of 4 - 8 μm, and a ZrO2, Co3O4, and CoSb2O6 oxide mixed layer with a thickness of 2 - 3 μm. It can be seen from Figure 2b and Figure 3 that the thickness of the oxidation layer of the oxidation coating on the ZrCoSb surface hardly changes before and after oxidation at 600 °C for 15 days. Figure 4 shows a comparison chart of the oxidation weight gain rate of ZrCoSb materials with in-situ formed oxidation coatings and without oxidation coatings at 600 °C. It can be seen that the oxidation weight gain rate of the ZrCoSb sample with the oxidation coating is much lower than that of the sample without the coating structure, indicating that the in-situ formed oxidation coating has good oxidation resistance.

[0054] Example 2

[0055] In this Example 2, the oxidation layer formed on the surface of NbFeSb under high temperature and low oxygen partial pressure conditions is used as a coating to inhibit the volatilization of elements on the material surface to reduce the volatilization rate of elements on the material surface. It includes the following steps:

[0056] (1) The surface of the NbFeSb material block was treated by means such as sandblasting, grinding, and polishing to ensure the surface of the sample is flat and clean.

[0057] (2) After measuring the size and mass of the surface-treated sample, it was placed in a furnace at 800 °C and oxidized in an atmosphere with a low oxygen partial pressure (∼200 Pa) for 2 days.

[0058] (3) After taking out the sample, it was weighed again, and polished to observe the in-situ formed oxidation coating.

[0059] The samples with in-situ oxidation coatings are cleaned with alcohol, and their sizes and masses are measured. Then they are sealed in a long glass tube under vacuum and placed in a furnace at 850 °C for a 7-day aging test. After taking out the samples after the aging test, they are weighed again, polished, and the surface changes of the samples are observed to analyze the effect of the in-situ oxidation coating on inhibiting the surface element volatilization of NbFeSb materials. The following is the analysis of the volatilization inhibition effect.

[0060] It can be seen from Figure 5a the backscattered electron spectrum that there are no cracks and obvious voids at the interface between the in-situ generated multi-layer structure oxidation coating and the NbFeSb material, indicating good bonding between the coating and the material. Figure 5b It can be seen from the surface element distribution map of the sample that the in-situ generated oxidation coating is divided into two layers. The elements in each layer are evenly distributed, and there is no obvious element aggregation. Moreover, there is no obvious diffusion and chemical reaction at the interface. The oxidation layer is composed of an FeSb compound layer with a thickness of 10 - 20 μm from the inside to the outside, and an NbO2 and Fe2O3 oxide mixed layer with a thickness of 20 - 40 μm. It can be seen from Figure 5a , Figure 5b and Figure 6 that the thickness of the volatilization layer of the in-situ oxidation coating on the surface of the NbFeSb material hardly changes before and after aging at 850 °C for 7 days. It can be seen from Figure 6 and Figure 7 that the backscattered electron images of the surfaces of NbFeSb materials with in-situ generated oxidation coatings and without oxidation coatings after aging at 850 °C for 7 days show that the thickness of the volatilization layer of the NbFeSb with the in-situ oxidation coating is significantly smaller than that of the sample without the oxidation coating, indicating that the in-situ oxidation coating has a good effect on inhibiting the surface element volatilization of the half-Heusler material.

Claims

1. A coating for suppressing surface oxidation and volatilization of half-Heusler thermoelectric materials, characterized in that, The chemical general formula of the half-Heusler thermoelectric material is ABX, where A is at least one of Ti, Zr, Hf, Sc, Y, V, Nb, Ta; B is at least one of Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt; and X is at least one of Sn, Sb, Bi. The coating sequentially includes, from the inside to the outside, an AO2 and B-X mixed layer formed in situ on the surface of the half-Heusler thermoelectric material, a B-X compound layer, and a mixed layer formed by oxides of A, oxides of B, and oxides of X.

2. The coating according to claim 1, wherein The half-Heusler thermoelectric material is TiNiSn, ZrNiSn, HfNiSn, TiCoSb, ZrCoSb, HfCoSb, TiFeSb, NbFeSb, or HfFeSb.

3. The coating according to claim 1, wherein The oxide of A is ZrO2, HfO2, or NbO2; the oxide of B is NiO or Fe2O3; the oxide of X is SnO2; and the B-X compound is CoSb, Ni3Sn2, NbSb2, or FeSb.

4. The coating according to claim 1, characterized in that, The thickness of the mixed layer formed by oxides of A, oxides of B, and oxides of X is 1 - 250 μm; the thickness of the B-X compound layer is 1 - 50 μm; and the thickness of the AO2 and B-X mixed layer is 1 - 200 μm.

5. The coating according to claim 4, characterized in that, The total thickness of the coating is 3 μm to 500 μm.

6. The coating according to claim 5, characterized in that, The total thickness of the coating is 5 μm to 50 μm.

7. A method for preparing a coating for suppressing surface oxidation and volatilization of a half-Heusler thermoelectric material according to any one of claims 1-6, characterized in that, The surface of the half-Heusler thermoelectric material block is processed to be flat and clean, and then placed in an oxygen-containing atmosphere and heat-treated at 400°C to 900°C to complete the preparation of the coating.

8. According to the preparation method described in claim 7, characterized in that, The surface treatment of the half-Heusler thermoelectric material block to be flat is achieved by at least one of polishing, grinding, and cleaning; the oxygen-containing atmosphere is an oxygen atmosphere, an air atmosphere, or a mixed atmosphere of oxygen and inert gas with an oxygen partial pressure of 1 Pa to 1 MPa; and the holding time of the heat treatment is 5 minutes to 100 hours.

9. According to the preparation method described in claim 8, characterized in that, The heating rate of the heat treatment is 50°C / minute to 200°C / minute.

10. A coating for suppressing surface oxidation and volatilization of a half-Heusler thermoelectric material, characterized in that, The chemical general formula of the half-Heusler thermoelectric material is ABX, where A is at least one of Ti, Zr, Hf, Sc, Y, V, Nb, Ta; B is at least one of Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt; and X is at least one of Sn, Sb, Bi. The coating sequentially includes, from the inside to the outside, a ZrO2 and CoSb mixed layer formed in situ on the surface of the half-Heusler thermoelectric material, a CoSb compound layer, and a mixed layer of ZrO2, Co3O4, and CoSb2O6 oxides.

11. A half-Heusler thermoelectric device comprising the coating according to any one of claims 1 - 6, 10 for inhibiting surface oxidation and volatilization of the half-Heusler thermoelectric material.

12. Use of the coating according to any one of claims 1 - 6, 10 for inhibiting surface oxidation and volatilization of the half-Heusler thermoelectric material in the service of the half-Heusler thermoelectric material in an oxygen-containing environment.

13. Use of the coating according to any one of claims 1 - 6, 10 for inhibiting surface oxidation and volatilization of the half-Heusler thermoelectric material in the service of the half-Heusler thermoelectric material in an oxygen-free environment.

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