A surface treatment method for bismuth telluride material

Abstract

The invention relates to the field of thermoelectric technology. In order to solve a series of problems such as the excessively thick Ni layer on the surface of conventional bismuth telluride leading to increased device load, reduced refrigeration temperature difference, complex process, high cost, poor bonding between the Ni layer and bismuth telluride, and insufficient reliability of TEC, the invention proposes a surface treatment technology for bismuth telluride materials. The technology utilizes a composite treatment process of ion implantation and vacuum plating technologies such as multi-arc plating or magnetron sputtering to improve the bonding between the bismuth telluride and the Ni layer, reduce the internal load of TEC, and increase the temperature difference. At the same time, due to the improved bonding between the Ni layer and the bismuth telluride substrate, the reliability of the TEC product is optimized.

Description

Technical Field

[0001] The invention relates to the field of thermoelectric technology, and in particular to a surface treatment method for bismuth telluride materials. Background Art

[0002] At present, the main application market of thermoelectric technology is solid-state refrigeration near room temperature. The main principle is that the carriers in the semiconductor material drift under the action of the electric field and interact with the lattice, absorbing and releasing heat at the material joints. This requires the semiconductor material to have a higher carrier mobility to reduce the generation of Joule heat, a stronger Peltier effect to produce a larger temperature difference, and a lower thermal conductivity to maintain the generated temperature difference. Since the thermal conductivity, electrical conductivity and Seebeck coefficient of the material all change with temperature, the thermoelectric properties of the material are also related to temperature. Bismuth telluride is currently the best thermoelectric material near room temperature and the only thermoelectric material for large-scale commercial applications.

[0003] Commercial thermoelectric coolers (TEC) use a structure in which N and P type thermoelectric arms are connected in series, usually by connecting bismuth telluride to a metal conductive layer through solder. Bismuth telluride is an intermetallic compound with poor wettability to solder. At the same time, tin atoms easily diffuse into the bismuth telluride matrix at high temperatures, deteriorating the thermoelectric properties of the material. Therefore, a layer of Ni is usually plated on the surface of bismuth telluride to increase wettability with tin and hinder the diffusion of Sn atoms. Usually, in order to enhance the bonding strength between the coating and the substrate, it is necessary to obtain sufficient surface roughness of the bismuth telluride wafer in advance, generally using a sandblasting process. Furthermore, in order to form a metallurgical bond between the Ni layer and the bismuth telluride layer and form an IMC layer, a preliminary Ni layer needs to be applied in the form of thermal spraying. However, it is difficult to evenly coat the Ni layer with the thermal spraying process. In order to ensure the compactness of this Ni layer, it is generally necessary to make this Ni layer reach about 100um, and then electroplate a layer of Ni of about 10um to improve the compactness of the overall Ni layer, thereby improving the reliability of the TEC during service, and finally electroplate a layer of Sn of about 10um to prevent oxidation of the Ni layer. Most domestic manufacturers use a surface treatment process of electric spark wire cutting-thermal spraying Ni-electroplating Ni-electroplating Sn for process cost reasons, eliminating the sandblasting step. However, this conventional bismuth telluride surface has a series of problems such as increased device load and reduced cooling temperature difference due to the excessively thick Ni layer, and the process is complicated and costly, and the bonding force between the Ni layer and bismuth telluride is insufficient, and the reliability of TEC is not high enough. Summary of the invention

[0004] In order to solve a series of problems on the conventional bismuth telluride surface, such as the excessively thick Ni layer leading to increased device load, reduced cooling temperature difference, complex process, high cost, poor bonding between the Ni layer and bismuth telluride, and insufficient TEC reliability, the present invention proposes a bismuth telluride material surface treatment technology, which utilizes a composite treatment process of ion implantation and vacuum plating technologies such as multi-arc plating or magnetron sputtering to improve the bonding between bismuth telluride and the Ni layer, reduce the internal load of TEC, and increase the temperature difference. At the same time, due to the improved bonding between the Ni layer and the bismuth telluride matrix, the reliability of the TEC product is optimized. It is difficult for a general vacuum coating process to form a metallurgical bond on the bismuth telluride surface, and a long annealing period is required to form an IMC layer. However, long annealing has a significant effect on the intrinsic point defects of bismuth telluride, especially the P type, so the industry cannot use the vacuum Ni plating method. In the early stage of the present invention, ion implantation was used to modify the surface of the bismuth telluride semiconductor, and a high bonding effect could be achieved without forming an IMC layer. The implanted ion concentration needs to be strictly controlled. If it is too high, it will affect the thermoelectric properties of the bismuth telluride matrix. If it is too low, the "pinning" effect will not be achieved and the coating bonding requirements cannot be met.

[0005] The present invention is realized by the following technical scheme: a surface treatment method of bismuth telluride material is to plate a Ni barrier layer after Ni ions are injected into bismuth telluride.

[0006] Ion implantation has the advantages of precision, controlled energy and dosage, good doping uniformity, high purity, low-temperature doping, and no influence from the injected material. Ni ion implantation surface treatment technology uses the physical and chemical changes caused by the high-energy Ni ion beam injected into the surface of the bismuth telluride material to improve the surface properties of the bismuth telluride material. The ion implantation layer is a new surface layer formed by a series of physical and chemical interactions between the ion beam and the substrate surface, and there is no peeling problem between it and the substrate.

[0007] The concentration range of the injected Ni ions is 1×10 17 -9×10 18 cm -3 The Ni ion injection voltage is 100 KeV. To avoid the high injection ion concentration affecting the performance of the bismuth telluride thermoelectric material (the general carrier concentration of bismuth telluride material is 1×10 19 -5×10 19 cm -3 ).

[0008] Preferably, the bismuth telluride surface is pretreated before Ni ion injection, and the pretreatment includes one or more of ultrasonic cleaning, plasma cleaning, mechanical polishing, electrolytic polishing, chemical degreasing, and acid activation to remove surface impurities and damaged layers and enhance the bonding strength between the subsequent coating and the substrate.

[0009] The thickness of the Ni barrier layer is 2-20um. Due to the thinning of the Ni layer, the intrinsic load of the TEC device is reduced, the heat loss caused by Joule heat is reduced, and the temperature difference is improved compared to conventional surface treatment processes.

[0010] Preferably, the Ni barrier layer is plated by a vacuum plating process, wherein the vacuum plating process includes multi-arc plating, magnetron sputtering, and the like.

[0011] When the Ni barrier layer is plated by vacuum plating technology such as multi-arc plating or magnetron sputtering, the temperature of the bismuth telluride substrate material is ≤180°C to prevent the coating from falling off due to the large difference in thermal expansion coefficients between the Ni layer and the bismuth telluride layer under a large temperature difference. Preferably, a water cooling circulation device is used to reduce the temperature of the bismuth telluride substrate material.

[0012] Multi-arc ion plating uses arc discharge, which has the characteristics of high film density, good strength and durability, and good adhesion strength. Generally, an acceleration voltage of 50KeV is used. Depending on the number of target materials used, the time to reach the required coating thickness is also different. For efficiency considerations, multi-arc plating technology is preferred in vacuum plating technology.

[0013] The present invention utilizes the Ni ions injected into the surface of bismuth telluride to produce a bridging pinning effect on the Ni-plated layer, and can obtain better bonding force in the case of a thinner Ni layer (2-20um), which is greater than the bonding force of the conventional electric spark wire cutting-thermal spraying Ni-electroplating Ni process and improves the temperature difference.

[0014] Compared with the prior art, the present invention has the following beneficial effects: improving the bonding strength between bismuth telluride and the Ni layer, reducing the internal load of TEC, and increasing the temperature difference. At the same time, due to the improved bonding strength between the Ni layer and the bismuth telluride matrix, the reliability of the TEC product is optimized. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a process flow chart of the present invention;

[0016] Figure 2 Schematic diagram of the binding force of Examples 1-3. DETAILED DESCRIPTION

[0017] The present invention is further described in detail below through the accompanying drawings and examples, but the content of the present invention is not limited to the following examples. The raw materials used in the examples can be purchased commercially or prepared by conventional methods.

[0018] Example 1

[0019] A method for surface treatment of bismuth telluride material comprises the following steps: Figure 1 As shown:

[0020] (1) firstly cutting the bismuth telluride crystal rod into 1.5 mm wafers, and then performing two surface pretreatments of polishing with 2000 grit sandpaper and plasma cleaning on the surface of the bismuth telluride wafers;

[0021] (2) After pretreatment, the bismuth telluride wafer was injected with an accelerating voltage of 100 KeV and a Ni ion concentration of 1×10 17 cm -3 ;

[0022] (3) A 20 μm Ni barrier layer is plated on the surface of the bismuth telluride wafer using a multi-arc ion plating method with an acceleration voltage of 50 KeV, and a water cooling circulation device is used to control the temperature of the bismuth telluride matrix material to not exceed 170 degrees Celsius, thereby obtaining a surface-treated bismuth telluride material.

[0023] Example 2

[0024] A method for surface treatment of bismuth telluride material comprises the following steps: Figure 1 As shown:

[0025] (1) cutting the bismuth telluride crystal rod into 2 mm wafers, then ultrasonically cleaning the surface of the bismuth telluride wafers, wiping with alcohol and drying;

[0026] (2) After pretreatment, the bismuth telluride wafer was injected with an accelerating voltage of 100 KeV and a Ni ion concentration of 1×10 18 cm -3 ;

[0027] (3) A 10 μm Ni barrier layer is plated on the surface of the bismuth telluride wafer using a multi-arc ion plating method with an acceleration voltage of 50 KeV, and a water cooling circulation device is used to control the temperature of the bismuth telluride matrix material to 150 degrees Celsius to obtain a surface-treated bismuth telluride material.

[0028] Example 3

[0029] A method for surface treatment of bismuth telluride material comprises the following steps: Figure 1 As shown:

[0030] (1) cutting the bismuth telluride crystal rod into 1.5 mm wafers, then ultrasonically cleaning the surface of the bismuth telluride wafers, and then soaking them in a mixed alkaline solution of NaOH and NaCO3 in a molar ratio of 1:3 to remove oil;

[0031] (2) After pretreatment, the bismuth telluride wafer was implanted with Ni ions at a concentration of 9×10 18 cm -3 ;

[0032] (3) A 2 μm Ni barrier layer is plated on the surface of the bismuth telluride wafer using a multi-arc ion plating method with an acceleration voltage of 50 KeV, and a water cooling circulation device is used to control the temperature of the bismuth telluride matrix material to 160 degrees Celsius to obtain a surface-treated bismuth telluride material.

[0033] Comparative Example

[0034] A method for surface treatment of bismuth telluride material comprises the following steps:

[0035] (1) cutting the bismuth telluride crystal rod into 1.5 mm wafers, and then ultrasonically cleaning the surface of the bismuth telluride wafers and drying them;

[0036] (2) thermally spraying a 100 μm Ni layer on the surface of the bismuth telluride wafer;

[0037] (3) Then electroplate a 3-4um Ni layer.

[0038] Test Case

[0039] The bonding strength between the Ni layer and the bismuth telluride substrate obtained by using a surface treatment method of bismuth telluride material in Examples 1-3 and a surface treatment method of bismuth telluride material in Comparative Example is as follows: Figure 2 As shown, it is explained that the bonding force between the Ni layer and the bismuth telluride substrate is increased by 50% by using the surface treatment method of the present invention.

[0040] The average temperature difference of TECs of the same specifications obtained by using a bismuth telluride material surface treatment method in Examples 1-3 and a bismuth telluride material surface treatment method currently commonly used in the comparative example is shown in Table 1:

[0041] Table 1

[0042] Comparative Example 62.2K Example 1 62.9K Example 2 63.5K Example 3 64.2K

[0043] As shown in Table 1, the temperature difference increased by up to 2K.

[0044] The present invention can obtain better bonding force with a thinner Ni layer (2-20um), which is more than 50% greater than the bonding force of the conventional electric spark wire cutting-thermal spraying Ni-electroplating Ni process, and the temperature difference is increased by up to 2K, so that the reliability of TEC products is optimized.

Claims

1. A method for surface treatment of bismuth telluride material, characterized in that: After Ni ions are implanted on the surface of bismuth telluride, a Ni barrier layer is plated. The concentration of the implanted Ni ions is in the range of 1×10 17 -9×10 18 cm -3 ; The thickness of the Ni barrier layer is 2-20 μm; When the Ni barrier layer is plated, the temperature of the bismuth telluride substrate material is ≤180°C.

2. The method for surface treatment of bismuth telluride material according to claim 1, characterized in that: The bismuth telluride surface is pretreated before Ni ion injection, and the pretreatment includes ultrasonic cleaning, plasma cleaning, mechanical polishing, electrolytic polishing, chemical degreasing, and acid activation.

3. The method for surface treatment of bismuth telluride material according to claim 1, characterized in that: The Ni barrier layer was deposited by vacuum plating process.

4. The method for surface treatment of bismuth telluride material according to claim 1, characterized in that: A water cooling circulation device is used to reduce the temperature of the bismuth telluride matrix material.

5. The method for surface treatment of bismuth telluride material according to claim 3, characterized in that: Vacuum plating includes multi-arc plating and magnetron sputtering.

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