A method for improving the strength and long-term service performance of a brazed joint in a skutterudite thermoelectric device using a cobalt-based barrier layer

Abstract

A method for improving the strength and long-term service performance of brazed joints in skutterudite thermoelectric devices using a cobalt-based barrier layer belongs to the technical field of thermoelectric material connection. In order to solve the problems of poor long-term service performance and low strength of brazed joints of the existing skutterudite and electrode, a cobalt-based barrier layer is added between the skutterudite and the electrode, which is cobalt and refractory metal. The refractory metal has high melting point, large bond energy and high diffusion activation energy, thereby inhibiting the diffusion of Sb element through the barrier layer to the electrode. Therefore, the composition of the cobalt-based barrier layer can be flexibly adjusted so that the thermal expansion coefficient of the barrier layer is easily matched with that of the skutterudite. Moreover, the barrier layer powder and the skutterudite powder are integrated by hot-pressing sintering, and a staggered interface is formed between the powder and the powder, which can well relieve the interface stress. The barrier layer of the present application can better inhibit the diffusion of Sb element because it contains fewer grain boundaries. The shear strength of the brazed joint of the skutterudite and the electrode obtained by the present application reaches 33.6 MPa.

Description

Technical Field

[0001] This invention belongs to the field of thermoelectric material connection technology, specifically relating to a method for improving the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices. Background Technology

[0002] Thermoelectric devices are semiconductor energy conversion devices with enormous application potential, capable of converting heat energy into electrical energy. Cobaltite thermoelectric devices, with an operating temperature of 300℃ to 600℃, are representative of medium-temperature thermoelectric devices. In the aerospace field, cobaltite thermoelectric devices are commonly used in deep space exploration. For example, the Curiosity Mars rover and the Cassini Saturn probe both carried cobaltite thermoelectric devices. These devices convert the heat generated by nuclear reactors into electrical energy, thus enabling the long-term power supply of spacecraft using nuclear energy.

[0003] In the manufacturing process of cobaltite thermoelectric devices, electrodes and multiple cobaltite thermoelectric legs need to be brazed together before being packaged into a device. Existing brazing technology between cobaltite and electrodes presents two problems. First, long-term service performance degradation. Cobaltite thermoelectric devices are often used in environments requiring long-term operation, such as deep space exploration. Continuous elemental diffusion between the cobaltite and the electrode leads to the continuous growth of the interface reaction layer, increasing contact resistance, and a gradual decline in thermoelectric performance. Second, low brazed joint strength. During service, cobaltite thermoelectric devices are subjected to repeated thermal shocks, causing varying degrees of expansion and contraction between the cobaltite and the electrode, resulting in significant stress at the interface. If the brazed bond strength between the cobaltite and the electrode is insufficient, the device may crack or even experience an open circuit.

[0004] In summary, obtaining high-strength cobaltite thermoelectric devices capable of long-term service places two demands on the brazing of the cobaltite ore and the electrodes: first, the interfacial reaction layer must be as thin as possible, which requires blocking element diffusion; second, the joint strength must be high, which requires alleviating interfacial stress. Ensuring that the brazed joint meets these two requirements is a crucial prerequisite for the large-scale application of cobaltite thermoelectric devices. Summary of the Invention

[0005] In order to solve the problems of poor long-term service performance and low brazing joint strength of existing cobalt ore to electrode brazing, this invention proposes a method to improve the brazing joint strength and long-term service performance of cobalt ore thermoelectric devices by using a cobalt-based barrier layer.

[0006] The present invention utilizes a cobalt-based barrier layer to improve the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices, and the method comprises the following steps:

[0007] Step 1: Put cobalt powder and refractory metal powder into a ball mill for ball milling, and mix them evenly to obtain barrier layer powder;

[0008] The barrier layer powder is composed of a mixture of cobalt powder and refractory metal powder, and the mass fraction of cobalt powder in the barrier layer powder is 50% to 98%.

[0009] The refractory metal powder is one or more of tungsten powder, molybdenum powder, niobium powder, and tantalum powder;

[0010] Step 2: Pour cobaltite powder into a graphite mold, then spread a barrier layer powder on the cobaltite powder, and then transfer it to a hot press furnace for integrated hot pressing sintering to obtain cobaltite with a pre-set barrier layer.

[0011] The integrated hot pressing sintering step is as follows: the hot press furnace is evacuated to a pressure of less than 1 kPa, then argon gas is introduced into the hot press furnace until the argon gas pressure is 50-100 kPa, then a pressure of 50-70 MPa is applied, the furnace is heated to 700-800°C and held for 5-20 minutes, and finally cooled to 100-200°C.

[0012] Step 3: Grind, polish and clean the surface of the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode in sequence. Then place brazing filler metal between the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode for brazing.

[0013] The solder is a Cu-based solder;

[0014] The brazing process is as follows: the vacuum pressure of the vacuum furnace is less than 5 × 10⁻⁶. -3 Pa, heat to 600-700℃ at a heating rate of 5-20℃ / min for 5-20 min, and then cool to 100-200℃ at a rate of 2-10℃ / min.

[0015] The principle and beneficial effects of this invention are as follows:

[0016] 1. The element diffusion between cobaltite and the electrode is mainly the diffusion of Sb element from cobaltite into the electrode. In this invention, after adding a cobalt-based barrier layer between cobaltite and the electrode, the cobalt-based barrier layer is composed of cobalt and refractory metal. Refractory metal has a high melting point, large bond energy and high diffusion activation energy, which inhibits the diffusion of Sb element through the barrier layer into the electrode.

[0017] 2. In this invention, the cobalt-based barrier layer is composed of cobalt and a refractory metal. Cobalt has a higher coefficient of thermal expansion than cobaltite, while the refractory metal has a lower coefficient of thermal expansion than cobaltite. The cobalt-based barrier layer is formed by powder sintering, thus allowing for flexible adjustment of its composition. The coefficient of thermal expansion of the barrier layer easily matches that of cobaltite. Furthermore, the barrier layer powder and cobaltite powder are integrally hot-pressed and sintered, forming a staggered interface between the powders, effectively alleviating interfacial stress.

[0018] Existing methods for connecting cobaltite thermoelectric materials and electrodes use a CoMo foil as the barrier layer, which is then hot-pressed and sintered with cobaltite powder. The surface of the barrier layer is also roughened by etching grooves to avoid stress concentration and increase the path of crack propagation along the interface. This invention integrates the barrier layer powder and cobaltite powder through hot-pressing and sintering, directly obtaining a low-stress interface between the barrier layer and cobaltite, eliminating the need for surface roughening. Regarding the suppression of Sb diffusion, the CoMo foil is obtained through an arc melting process, which has a rapid cooling rate, resulting in a grain size of tens to hundreds of nanometers. In contrast, the grain size of the barrier layer in this invention is comparable to the powder particle size, ranging from a few micrometers to tens of micrometers. Therefore, the barrier layer of this invention contains fewer grain boundaries. Since the diffusion rate of atoms at grain boundaries is much higher than that within grains, the barrier layer of this invention, with fewer grain boundaries, can better suppress Sb diffusion.

[0019] 3. This invention enables the production of high-strength cobaltite thermoelectric devices capable of long-term service. Regarding long-term service performance, this invention is the first to demonstrate the feasibility of long-term service through annealing experiments. The interfacial reaction thickness between cobaltite and the cobalt-based barrier layer is only 15 μm, and after annealing at 550℃ for 10 days, the reaction layer thickness only increases to 39 μm. In terms of strength, the brazed joint between cobaltite and the electrode obtained by this invention achieves a shear strength of 33.6 MPa. Attached Figure Description

[0020] Figure 1 Yb in the brazed joint obtained in Example 1 0.3 Co4Sb 12 Photograph of the interfacial structure with the CoNb barrier layer;

[0021] Figure 2 Yb after annealing in Example 1 0.3 Co4Sb 12 Photograph of the interfacial structure with the CoNb barrier layer;

[0022] Figure 3 Yb in the brazed joint obtained in Example 3 0.3 Co4Sb 12 Photograph of the interface structure with the CoMo barrier layer. Detailed Implementation

[0023] The technical solution of the present invention is not limited to the specific embodiments listed below, but also includes any reasonable combination of the specific embodiments.

[0024] Specific Implementation Method 1: This implementation method for improving the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices using a cobalt-based barrier layer is carried out according to the following steps:

[0025] Step 1: Put cobalt powder and refractory metal powder into a ball mill for ball milling, and mix them evenly to obtain barrier layer powder;

[0026] The barrier layer powder is composed of a mixture of cobalt powder and refractory metal powder, and the mass fraction of cobalt powder in the barrier layer powder is 50% to 98%.

[0027] The refractory metal powder is one or more of tungsten powder, molybdenum powder, niobium powder, and tantalum powder;

[0028] Step 2: Pour cobaltite powder into a graphite mold, then spread a barrier layer powder on the cobaltite powder, and then transfer it to a hot press furnace for integrated hot pressing sintering to obtain cobaltite with a pre-set barrier layer.

[0029] The integrated hot pressing sintering step is as follows: the hot press furnace is evacuated to a pressure of less than 1 kPa, then argon gas is introduced into the hot press furnace until the argon gas pressure is 50-100 kPa, then a pressure of 50-70 MPa is applied, the furnace is heated to 700-800°C and held for 5-20 minutes, and finally cooled to 100-200°C.

[0030] Step 3: Grind, polish and clean the surface of the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode in sequence. Then place brazing filler metal between the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode for brazing.

[0031] The solder is a Cu-based solder;

[0032] The brazing process is as follows: the vacuum pressure of the vacuum furnace is less than 5 × 10⁻⁶. -3 Pa, heat to 600-700℃ at a heating rate of 5-20℃ / min for 5-20 min, and then cool to 100-200℃ at a rate of 2-10℃ / min.

[0033] This embodiment has the following beneficial effects:

[0034] 1. The element diffusion between cobaltite and the electrode is mainly the diffusion of Sb element from cobaltite into the electrode. In this embodiment, after adding a cobalt-based barrier layer between cobaltite and the electrode, the diffusion of Sb element through the barrier layer into the electrode is inhibited because the cobalt-based barrier layer is composed of cobalt and refractory metal. Refractory metal has a high melting point, large bond energy and high diffusion activation energy.

[0035] 2. In this embodiment, the cobalt-based barrier layer is composed of cobalt and a refractory metal. Cobalt has a higher coefficient of thermal expansion than cobaltite, while the refractory metal has a lower coefficient of thermal expansion than cobaltite. The cobalt-based barrier layer is formed by powder sintering, thus allowing for flexible adjustment of its composition. The coefficient of thermal expansion of the barrier layer is easily matched with that of cobaltite. Furthermore, the barrier layer powder and the cobaltite powder are integrally hot-pressed and sintered, forming a staggered interface between the powders, which effectively alleviates interfacial stress.

[0036] Existing methods for connecting cobaltite thermoelectric materials and electrodes use a CoMo foil as the barrier layer, which is then hot-pressed and sintered with cobaltite powder. The surface of the barrier layer is also roughened by etching grooves to avoid stress concentration and increase the path of crack propagation along the interface. This embodiment integrates the barrier layer powder and cobaltite powder through hot-pressing and sintering, directly obtaining a low-stress interface between the barrier layer and cobaltite, eliminating the need for surface roughening. Regarding the suppression of Sb diffusion, the CoMo foil is obtained through an arc melting process, which has a rapid cooling rate, resulting in a grain size of tens to hundreds of nanometers. In contrast, the grain size of the barrier layer in this embodiment is comparable to the powder particle size, ranging from a few micrometers to tens of micrometers. Therefore, the barrier layer in this embodiment contains fewer grain boundaries. Since the diffusion rate of atoms at grain boundaries is much higher than that within grains, the barrier layer in this embodiment, with fewer grain boundaries, can better suppress Sb diffusion.

[0037] 3. This embodiment can obtain a high-strength cobaltite thermoelectric device capable of long-term service. Regarding long-term service performance, this embodiment is the first to demonstrate the feasibility of long-term service through annealing experiments. The interfacial reaction thickness between cobaltite and the cobalt-based barrier layer is only 15 μm, and after annealing at 550°C for 10 days, the reaction layer thickness only increases to 39 μm. In terms of strength, the shear strength of the brazed joint between cobaltite and the electrode obtained in this embodiment reaches 33.6 MPa.

[0038] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the ball milling step in step one is as follows: Cobalt powder, refractory metal powder, grinding balls, and anhydrous ethanol are loaded into a ball milling jar, so that the anhydrous ethanol just covers the powder and grinding balls, and then ball milling is performed for 6 to 24 hours under atmospheric conditions. After ball milling, the jar is placed in a drying oven to dry.

[0039] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that the chemical formula of the cobaltite mentioned in step two is R. x Co y Fe 4-y Sb 12 R is one or more of alkali metals, alkaline earth metals, rare earth metals, and Group III elements, where 0 < x ≤ 1 and 0 ≤ y ≤ 4.

[0040] Specific Implementation Method Four: This implementation method differs from one of the specific implementation methods one to three in that the heating rate in the integrated hot pressing sintering step described in step two is 80-150℃ / min, and the cooling rate is 5-50℃ / min.

[0041] Specific Implementation Method 5: This implementation method differs from one of the specific implementation methods 1 to 4 in that the mass of the barrier layer powder in step 2 is 5% to 25% of the cobaltite powder.

[0042] Specific Implementation Method Six: This implementation method differs from one of Specific Implementation Methods One to Five in that the electrode described in step three is a metal electrode or an alloy electrode.

[0043] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Method Six in that the metal electrode material is Cu, Ni, Fe, Co, or Cr.

[0044] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Method Six in that the alloy electrode is a Cu-based alloy, Ni-based alloy, Fe-based alloy, Co-based alloy, or Cr-based alloy.

[0045] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Method Six in that: the Cu-based alloy is CuW or CuMo; the Ni-based alloy is NiW, NiMo, or NiCr; the Fe-based alloy is FeW, FeMo, FeCr, or FeCoNi; the Co-based alloy is CoW, CoMo, CoCr, CoNi, or CoFe; and the Cr-based alloy is CrW, CrMo, CrCoNi, or CrFeNi.

[0046] Specific Implementation Method 10: This implementation method differs from Specific Implementation Methods 1 to 9 in that: the grinding method in step 3 is to grind with 800-3000 grit sandpaper in stages; the polishing method is to polish with diamond polishing agent with a particle size of 0.5-2.5μm on a polishing cloth; the cleaning method is to rinse with acetone or anhydrous ethanol, then place in acetone or anhydrous ethanol for ultrasonic cleaning for 5-30 minutes, and finally air dry.

[0047] Example 1

[0048] This embodiment utilizes a cobalt-based barrier layer to improve the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices, and the method is carried out according to the following steps:

[0049] Step 1: Put Co powder and Nb powder into a ball mill for ball milling, and mix them evenly to obtain barrier layer powder;

[0050] The mass ratio of Co powder to Nb powder is 8:2;

[0051] The ball milling step is as follows: Co powder, Nb powder, grinding balls, and anhydrous ethanol are loaded into a ball milling jar, and the anhydrous ethanol is just enough to cover the powder and grinding balls. Then, the jar is ball milled for 12 hours under atmospheric conditions. After the ball milling is completed, the jar is placed in a drying oven to dry.

[0052] Step 2: Yb 0.3 Co4Sb 12 Pour the powder into a graphite mold, and then Yb 0.3 Co4Sb 12 A barrier layer powder is spread on the powder, and then transferred to a hot press furnace for integrated hot pressing sintering to obtain cobaltite with a pre-placed barrier layer.

[0053] The integrated hot pressing sintering step is as follows: the hot press furnace is evacuated to a pressure of less than 0.5 kPa, then argon gas is introduced into the hot press furnace until the argon gas pressure is 80 kPa, then a pressure of 60 MPa is applied, the furnace is heated to 750°C and held for 10 min, and finally cooled to 150°C.

[0054] The heating rate in the integrated hot pressing sintering step is 120℃ / min, and the cooling rate is 20℃ / min.

[0055] The mass of the barrier layer powder is 20% of the cobaltite powder;

[0056] Step 3: Grind, polish and clean the surface of the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode in sequence. Then place brazing filler metal between the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode for brazing.

[0057] The electrode is a Cu electrode;

[0058] The grinding method involves grinding sequentially with 800-grit, 1500-grit, and 3000-grit sandpaper; the polishing method involves polishing with a diamond polishing agent with a particle size of 2.5 μm on a polishing cloth; the cleaning method involves rinsing with anhydrous ethanol, then ultrasonically cleaning in anhydrous ethanol for 10 minutes, and finally air drying.

[0059] The solder is CuSnP solder; CuSnP solder is a commercially available solder with a liquidus temperature of 600-700℃.

[0060] The brazing process is as follows: the vacuum pressure of the vacuum furnace is less than 5 × 10⁻⁶. -3 Pa was heated to 675°C at a rate of 10°C / min for 10 min, and then cooled to 200°C at a rate of 5°C / min.

[0061] Figure 1 Yb in the brazed joint obtained in Example 1 0.3 Co4Sb 12Interfacial structure of the CoNb barrier layer; Yb 0.3 Co4Sb 12 The brazed joint exhibits excellent interfacial bonding with CoNb, free from defects such as cracks, indicating a good match in thermal expansion coefficients. The surface reaction layer thickness is only 15 μm, demonstrating the excellent diffusion-blocking effect of the CoNb barrier layer. Annealing the brazed joint at 550℃ for 1, 2, 4, and 10 days yielded… Figure 2 The Yb shown after annealing treatment 0.3 Co4Sb 12 The microstructure of the interface with the CoNb barrier layer is shown in the photograph. The thicknesses of the interface reaction layer corresponding to annealing the brazed joint at 550℃ for 1, 2, 4, and 10 days are 22 μm, 30 μm, 34 μm, and 39 μm, respectively. Annealing at 550℃ was used to simulate the service environment of cobaltite. Based on the thickness corresponding to different annealing times, it can be observed that the thickness increases more slowly over time and approaches a limit. According to relevant theories of non-equilibrium thermodynamics, the predicted limit thickness of the interface reaction layer of the joint obtained in this embodiment is <100 μm, meeting the application requirements. This indicates that the cobalt-based barrier layer can effectively suppress the problems of continuous element diffusion and continuous reaction layer growth during long-term service, and is a feasible method to obtain cobaltite thermoelectric devices that can operate for a long time.

[0062] Testing revealed that the Yb obtained using the CoNb barrier layer in this embodiment... 0.3 Co4Sb 12 The shear strength of the joint in the thermoelectric device is 33.6 MPa, which meets the mechanical performance requirements of the thermoelectric device.

[0063] Example 2

[0064] This embodiment utilizes a cobalt-based barrier layer to improve the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices, and the method is carried out according to the following steps:

[0065] Step 1: Put Co powder and Nb powder into a ball mill for ball milling, and mix them evenly to obtain barrier layer powder;

[0066] The mass ratio of Co powder to Nb powder is 7:3;

[0067] The ball milling step is as follows: Co powder, Nb powder, grinding balls, and anhydrous ethanol are loaded into a ball milling jar, and the anhydrous ethanol is just enough to cover the powder and grinding balls. Then, the jar is ball milled for 12 hours under atmospheric conditions. After the ball milling is completed, the jar is placed in a drying oven to dry.

[0068] Step 2: Yb 0.3 Co4Sb 12 Pour the powder into a graphite mold, and then Yb 0.3 Co4Sb 12A barrier layer powder is spread on the powder, and then transferred to a hot press furnace for integrated hot pressing sintering to obtain cobaltite with a pre-placed barrier layer.

[0069] The integrated hot pressing sintering step is as follows: the hot press furnace is evacuated to a pressure of less than 0.5 kPa, then argon gas is introduced into the hot press furnace until the argon gas pressure is 80 kPa, then a pressure of 60 MPa is applied, the furnace is heated to 750°C and held for 10 min, and finally cooled to 150°C.

[0070] The heating rate in the integrated hot pressing sintering step is 120℃ / min, and the cooling rate is 20℃ / min.

[0071] The mass of the barrier layer powder is 20% of the cobaltite powder;

[0072] Step 3: Grind, polish and clean the surface of the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode in sequence. Then place brazing filler metal between the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode for brazing.

[0073] The electrode is a Cu electrode;

[0074] The grinding method involves grinding sequentially with 800-grit, 1500-grit, and 3000-grit sandpaper; the polishing method involves polishing with a diamond polishing agent with a particle size of 2.5 μm on a polishing cloth; the cleaning method involves rinsing with anhydrous ethanol, then ultrasonically cleaning in anhydrous ethanol for 10 minutes, and finally air drying.

[0075] The solder is CuSnP solder; CuSnP solder is a commercially available solder with a liquidus temperature of 600-700℃.

[0076] The brazing process is as follows: the vacuum pressure of the vacuum furnace is less than 5 × 10⁻⁶. -3 Pa was heated to 675°C at a rate of 10°C / min for 10 min, and then cooled to 200°C at a rate of 5°C / min.

[0077] Testing revealed that the Yb obtained using the CoNb barrier layer in this embodiment... 0.3 Co4Sb 12 The brazed joint of the thermoelectric device has a shear strength of 28.4 MPa, which meets the mechanical performance requirements of the thermoelectric device.

[0078] Example 3

[0079] This embodiment utilizes a cobalt-based barrier layer to improve the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices, and the method is carried out according to the following steps:

[0080] Step 1: Put Co powder and Mo powder into a ball mill for ball milling, and mix them evenly to obtain barrier layer powder;

[0081] The mass ratio of Co powder to Mo powder is 7:3;

[0082] The ball milling step is as follows: Co powder, Mo powder, grinding balls, and anhydrous ethanol are loaded into a ball milling jar, and the anhydrous ethanol just covers the powder and grinding balls. Then, the jar is ball milled for 12 hours under atmospheric conditions. After the ball milling is completed, the jar is placed in a drying oven to dry.

[0083] Step 2: Yb 0.3 Co4Sb 12 Pour the powder into a graphite mold, and then Yb 0.3 Co4Sb 12 A barrier layer powder is spread on the powder, and then transferred to a hot press furnace for integrated hot pressing sintering to obtain cobaltite with a pre-placed barrier layer.

[0084] The integrated hot pressing sintering step is as follows: the hot press furnace is evacuated to a pressure of less than 0.5 kPa, then argon gas is introduced into the hot press furnace until the argon gas pressure is 80 kPa, then a pressure of 60 MPa is applied, the furnace is heated to 750°C and held for 10 min, and finally cooled to 150°C.

[0085] The heating rate in the integrated hot pressing sintering step is 120℃ / min, and the cooling rate is 20℃ / min.

[0086] The mass of the barrier layer powder is 20% of the cobaltite powder;

[0087] Step 3: Grind, polish and clean the surface of the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode in sequence. Then place brazing filler metal between the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode for brazing.

[0088] The electrode is a Cu electrode;

[0089] The grinding method involves grinding sequentially with 800-grit, 1500-grit, and 3000-grit sandpaper; the polishing method involves polishing with a diamond polishing agent with a particle size of 2.5 μm on a polishing cloth; the cleaning method involves rinsing with anhydrous ethanol, then ultrasonically cleaning in anhydrous ethanol for 10 minutes, and finally air drying.

[0090] The solder is CuSnP solder; CuSnP solder is a commercially available solder with a liquidus temperature of 600-700℃.

[0091] The brazing process is as follows: the vacuum pressure of the vacuum furnace is less than 5 × 10⁻⁶. -3 Pa was heated to 675°C at a rate of 10°C / min for 10 min, and then cooled to 200°C at a rate of 5°C / min.

[0092] Figure 3 Yb in the brazed joint obtained in Example 3 0.3 Co4Sb 12 Photograph of the interface structure with the CoMo barrier layer. The CoMo barrier layer also provides a joint with good barrier effect and no cracks or pores, demonstrating the universality of this invention in the field of thermoelectric material bonding technology.

[0093] Testing revealed that the Yb obtained using the CoMo barrier layer in this embodiment... 0.3 Co4Sb 12 The brazed joint of the thermoelectric device has a shear strength of 32.0 MPa, which meets the mechanical performance requirements of the thermoelectric device.

[0094] Example 4

[0095] This embodiment utilizes a cobalt-based barrier layer to improve the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices, and the method is carried out according to the following steps:

[0096] Step 1: Put Co powder and Mo powder into a ball mill for ball milling, and mix them evenly to obtain barrier layer powder;

[0097] The mass ratio of Co powder to Mo powder is 6:4;

[0098] The ball milling step is as follows: Co powder, Mo powder, grinding balls, and anhydrous ethanol are loaded into a ball milling jar, and the anhydrous ethanol just covers the powder and grinding balls. Then, the jar is ball milled for 12 hours under atmospheric conditions. After the ball milling is completed, the jar is placed in a drying oven to dry.

[0099] Step 2: Yb 0.3 Co4Sb 12 Pour the powder into a graphite mold, and then Yb 0.3 Co4Sb 12 A barrier layer powder is spread on the powder, and then transferred to a hot press furnace for integrated hot pressing sintering to obtain cobaltite with a pre-placed barrier layer.

[0100] The integrated hot pressing sintering step is as follows: the hot press furnace is evacuated to a pressure of less than 0.5 kPa, then argon gas is introduced into the hot press furnace until the argon gas pressure is 80 kPa, then a pressure of 60 MPa is applied, the furnace is heated to 750°C and held for 10 min, and finally cooled to 150°C.

[0101] The heating rate in the integrated hot pressing sintering step is 120℃ / min, and the cooling rate is 20℃ / min.

[0102] The mass of the barrier layer powder is 20% of the cobaltite powder;

[0103] Step 3: Grind, polish and clean the surface of the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode in sequence. Then place brazing filler metal between the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode for brazing.

[0104] The electrode is a Cu electrode;

[0105] The grinding method involves grinding sequentially with 800-grit, 1500-grit, and 3000-grit sandpaper; the polishing method involves polishing with a diamond polishing agent with a particle size of 2.5 μm on a polishing cloth; the cleaning method involves rinsing with anhydrous ethanol, then ultrasonically cleaning in anhydrous ethanol for 10 minutes, and finally air drying.

[0106] The solder is CuSnP solder; CuSnP solder is a commercially available solder with a liquidus temperature of 600-700℃.

[0107] The brazing process is as follows: the vacuum pressure of the vacuum furnace is less than 5 × 10⁻⁶. -3 Pa was heated to 675°C at a rate of 10°C / min for 10 min, and then cooled to 200°C at a rate of 5°C / min.

[0108] Testing revealed that the Yb obtained using the CoMo barrier layer in this embodiment... 0.3 Co4Sb 12 The shear strength of the welded joint of the thermoelectric device is 26.9 MPa, which meets the mechanical performance requirements of the thermoelectric device.

[0109] Example 5

[0110] This embodiment utilizes a cobalt-based barrier layer to improve the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices, and the method is carried out according to the following steps:

[0111] Step 1: Put Co powder and Mo powder into a ball mill for ball milling, and mix them evenly to obtain barrier layer powder;

[0112] The mass ratio of Co powder to Mo powder is 7:3;

[0113] The ball milling step is as follows: Co powder, Mo powder, grinding balls, and anhydrous ethanol are loaded into a ball milling jar, and the anhydrous ethanol just covers the powder and grinding balls. Then, the jar is ball milled for 12 hours under atmospheric conditions. After the ball milling is completed, the jar is placed in a drying oven to dry.

[0114] Step 2: Yb 0.3 Co4Sb 12 Pour the powder into a graphite mold, and then Yb 0.3 Co4Sb 12 A barrier layer powder is spread on the powder, and then transferred to a hot press furnace for integrated hot pressing sintering to obtain cobaltite with a pre-placed barrier layer.

[0115] The integrated hot pressing sintering step is as follows: the hot press furnace is evacuated to a pressure of less than 0.5 kPa, then argon gas is introduced into the hot press furnace until the argon gas pressure is 80 kPa, then a pressure of 60 MPa is applied, the furnace is heated to 750°C and held for 10 min, and finally cooled to 150°C.

[0116] The heating rate in the integrated hot pressing sintering step is 120℃ / min, and the cooling rate is 20℃ / min.

[0117] The mass of the barrier layer powder is 20% of the cobaltite powder;

[0118] Step 3: Grind, polish and clean the surface of the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode in sequence. Then place brazing filler metal between the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode for brazing.

[0119] The electrode is a Cu electrode;

[0120] The grinding method involves grinding sequentially with 800-grit, 1500-grit, and 3000-grit sandpaper; the polishing method involves polishing with a diamond polishing agent with a particle size of 2.5 μm on a polishing cloth; the cleaning method involves rinsing with anhydrous ethanol, then ultrasonically cleaning in anhydrous ethanol for 10 minutes, and finally air drying.

[0121] The solder is CuSnP solder; CuSnP solder is a commercially available solder with a liquidus temperature of 600-700℃.

[0122] The brazing process is as follows: the vacuum pressure of the vacuum furnace is less than 5 × 10⁻⁶. -3 Pa was heated to 650°C at a rate of 10°C / min for 10 min, and then cooled to 200°C at a rate of 5°C / min.

[0123] Testing revealed that the Yb obtained using the CoMo barrier layer in this embodiment... 0.3 Co4Sb 12 The brazed joint of the thermoelectric device has a shear strength of 30.1 MPa, which meets the mechanical performance requirements of the thermoelectric device.

Claims

1. A method for improving the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices using a cobalt-based barrier layer, characterized in that: The method for improving the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices using a cobalt-based barrier layer is carried out according to the following steps: Step 1: Put cobalt powder and refractory metal powder into a ball mill for ball milling, and mix them evenly to obtain barrier layer powder; The barrier layer powder is composed of a mixture of cobalt powder and refractory metal powder, and the mass fraction of cobalt powder in the barrier layer powder is 50% to 98%. The refractory metal powder is one or more of tungsten powder, molybdenum powder, niobium powder, and tantalum powder; Step 2: Pour cobaltite powder into a graphite mold, then spread a barrier layer powder on the cobaltite powder, and then transfer it to a hot press furnace for integrated hot pressing sintering to obtain cobaltite with a pre-set barrier layer. The integrated hot pressing sintering step is as follows: the hot press furnace is evacuated to a pressure of less than 1 kPa, then argon gas is introduced into the hot press furnace until the argon gas pressure is 50-100 kPa, then a pressure of 50-70 MPa is applied, the furnace is heated to 700-800°C and held for 5-20 minutes, and finally cooled to 100-200°C. Step 3: Grind, polish and clean the surface of the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode in sequence. Then place brazing filler metal between the barrier layer in the pre-set barrier layer of cobalt ore and the welding surface of the electrode for brazing. The solder is a Cu-based solder; The brazing process is as follows: the vacuum pressure of the vacuum furnace is less than 5 × 10⁻⁶. -3 Pa, heat to 600-700℃ at a heating rate of 5-20℃ / min for 5-20 min, and then cool to 100-200℃ at a rate of 2-10℃ / min.

2. The method for improving the brazed joint strength and long-term service performance of cobalt-based thermoelectric devices using a cobalt-based barrier layer according to claim 1, characterized in that: The ball milling step described in step one is as follows: Cobalt powder, refractory metal powder, grinding balls, and anhydrous ethanol are loaded into a ball mill jar, and the anhydrous ethanol is just enough to cover the powder and grinding balls. Then, the mixture is ball milled for 6 to 24 hours under atmospheric conditions. After the ball milling is completed, the mixture is placed in a drying oven to dry.

3. The method for improving the brazed joint strength and long-term service performance of cobalt-based thermoelectric devices using a cobalt-based barrier layer according to claim 1, characterized in that: The chemical formula of the cobaltite mentioned in step two is R. x Co y Fe 4-y Sb 12 R is one or more of alkali metals, alkaline earth metals, rare earth metals, and Group III elements, where 0 < x ≤ 1 and 0 ≤ y ≤ 4.

4. The method for improving the brazed joint strength and long-term service performance of cobaltite thermoelectric devices using a cobalt-based barrier layer according to claim 1, characterized in that: In step two, the heating rate in the integrated hot pressing sintering process is 80–150 °C / min, and the cooling rate is 5–50 °C / min.

5. The method for improving the brazed joint strength and long-term service performance of cobalt-based thermoelectric devices using a cobalt-based barrier layer according to claim 1, characterized in that: The mass of the barrier layer powder in step two is 5% to 25% of the cobaltite powder.

6. The method for improving the brazed joint strength and long-term service performance of cobalt-based barrier layers in cobaltite thermoelectric devices according to claim 1, characterized in that: The electrode described in step three is a metal electrode or an alloy electrode.

7. The method for improving the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices using a cobalt-based barrier layer according to claim 6, characterized in that: The metal electrode is made of Cu, Ni, Fe, Co, or Cr.

8. The method for improving the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices using a cobalt-based barrier layer according to claim 6, characterized in that: The alloy electrode is a Cu-based alloy, Ni-based alloy, Fe-based alloy, Co-based alloy, or Cr-based alloy.

9. The method for improving the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices using a cobalt-based barrier layer according to claim 6, characterized in that: The Cu-based alloy is CuW or CuMo; the Ni-based alloy is NiW, NiMo or NiCr; the Fe-based alloy is FeW, FeMo, FeCr or FeCoNi; the Co-based alloy is CoW, CoMo, CoCr, CoNi or CoFe; and the Cr-based alloy is CrW, CrMo, CrCoNi or CrFeNi.

10. The method for improving the strength and long-term service performance of brazed joints in cobaltite thermoelectric devices using a cobalt-based barrier layer according to claim 1, characterized in that: The grinding method described in step three involves grinding with 800-3000 grit sandpaper in stages; the polishing method involves polishing with diamond polishing agent with a particle size of 0.5-2.5μm on a polishing cloth; the cleaning method involves rinsing with acetone or anhydrous ethanol, then ultrasonically cleaning in acetone or anhydrous ethanol for 5-30 minutes, and finally air drying.

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