A nickel-chromium alloy material with a negative Seebeck coefficient, its preparation method and application

Abstract

This invention discloses a nickel-chromium alloy material with a negative Seebeck coefficient, its preparation method, and its applications. The chemical formula of the nickel-chromium alloy material with a negative Seebeck coefficient is Ni-xCr (x = 10~20 wt.%). By precisely controlling the discharge plasma sintering process parameters, namely through the coupling effect of rapid heating, short-term holding, and external pressure, combined with the immediate removal of the pressure applied to the powder after the sintering reaction, densification is achieved at a lower temperature while suppressing complete equilibrium solid solution. Under non-equilibrium conditions, the solid solution behavior of the alloy can be controllably adjusted, constructing a composite phase microstructure composed of a solid solution phase and an incompletely solidified phase. This material can be paired with traditional nickel-chromium alloys with positive Seebeck coefficients to construct self-compensating or differential thermocouple systems, which has significant application value in the fields of thermoelectric temperature measurement, thermoelectric conversion, and precision temperature control.

Description

Technical Field

[0001] This invention relates to the fields of functional metal materials, thermoelectric materials and thermocouples, specifically a nickel-chromium alloy material with a negative Seebeck coefficient, its preparation method and application. Background Technology

[0002] Nickel-chromium alloys (Ni-Cr alloys), as a type of functional metallic material with excellent comprehensive properties, occupy an extremely important and irreplaceable position in the field of thermocouples. These alloys possess good electrical conductivity, high-temperature mechanical stability, and excellent oxidation resistance, maintaining stability in chemical composition and microstructure over a wide temperature range, thereby outputting stable and reliable thermoelectric potential signals.

[0003] In existing technologies, nickel-chromium alloys are widely used in standardized thermocouple systems such as Type K (nickel-chromium-nickel-silicon) and Type E (nickel-chromium-constantan) due to their large Seebeck coefficient (S) and good linearity, for measuring temperatures ranging from low temperatures to over 1000°C. Their long service life, moderate cost, and good electrochemical compatibility with various reference electrode materials make them a key material in industrial temperature detection and control systems. Simultaneously, nickel-chromium alloys are also widely used in thermoelectric sensing, energy conversion, and temperature calibration of high-precision instruments.

[0004] The temperature measurement principle of thermocouples is closely related to the magnitude and sign of the Seebeck coefficient. The Seebeck coefficient reflects the direction and amplitude of the thermoelectric potential generated by a material under the action of a temperature gradient, and determines the polarity and sensitivity of the thermocouple output signal. Existing research shows that nickel-chromium alloys prepared with conventional compositions and processes exhibit positive or near-zero Seebeck coefficients near room temperature, and their stability and repeatability are significantly affected by factors such as alloy composition, impurity content, grain size, and heat treatment history.

[0005] However, there are no published research reports on nickel-chromium systems with stable negative Seebeck coefficients. If negative Seebeck coefficients can be controlled in similar nickel-chromium systems, it would not only enable the internal reversal design of thermocouple polarity, but also allow for the construction of self-compensating temperature detection or novel differential thermocouple structures, thus providing new ideas and technical approaches for the expansion of thermocouple material systems and the development of high-precision thermoelectric measurement technology. Summary of the Invention

[0006] Therefore, the technical problem to be solved by the present invention is to provide a nickel-chromium alloy material with a negative Seebeck coefficient, its preparation method and application, which together with the traditional nickel-chromium alloy with a positive Seebeck coefficient to construct a self-compensating temperature detection or a novel differential thermocouple structure, so as to realize the reversal of thermoelectric potential polarity or signal compensation during temperature measurement.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0008] A nickel-chromium alloy material with a negative Seebeck coefficient, the chemical formula of which is Ni-xCr, where x represents the mass percentage of Cr in the nickel-chromium alloy, x = 10~20 wt.%;

[0009] The phase structure of the nickel-chromium alloy material with a negative Seebeck coefficient is a composite phase structure consisting of a main phase and a secondary phase; wherein, the main phase is a face-centered cubic solid solution formed by Ni as the matrix and Cr atoms dissolved in the Ni matrix; the secondary phase is a body-centered cubic structure composed of Cr elements that are not dissolved in the main phase.

[0010] This invention achieves controllable regulation of the solid solution behavior of an alloy under non-equilibrium conditions by precisely controlling the Ni to Cr composition ratio (x = 10–20 wt.%) and by precisely controlling the process parameters of spark plasma sintering (SPS). This constructs a composite phase microstructure consisting of a solid solution phase and a Cr incompletely dissolved phase. Furthermore, it may affect the resonance scattering of the alloy material, thereby producing a nickel-chromium alloy material with a negative Seebeck coefficient. The nickel-chromium alloy material with a negative Seebeck coefficient does not undergo a complete Seebeck coefficient reversal compared to pure nickel, and retains the advantages of high-temperature stability and oxidation resistance of nickel-chromium alloys.

[0011] By controlling the rapid heating rate, final heating temperature, short holding time, and applied pressure to the powder during the SPS sintering process, and simultaneously removing the applied pressure immediately after the sintering reaction and allowing natural cooling to room temperature, densification is achieved at a lower sintering temperature, and complete equilibrium solid solution is suppressed, resulting in only partial solid solution of Cr in the Ni matrix. While forming a face-centered cubic (FCC) solid solution with Ni as the matrix and Cr atoms dissolved in the Ni matrix as the main phase, a certain amount of undissolved Cr in the body-centered cubic (BCC) structure is retained in the material. At the same time, the undissolved Cr phase does not disrupt the continuous conductive network of the matrix, thus constructing a composite phase structure with adjustable solid solution degree and phase ratio. Therefore, within the same material system, the Seebeck coefficient sign is reversed (an extension from the traditional positive polarity nickel-chromium alloy system to the negative polarity system), causing the original output thermoelectric potential direction to reverse, providing a new material basis for the controllable design of thermocouple output polarity.

[0012] The aforementioned nickel-chromium alloy material with a negative Seebeck coefficient has a Seebeck coefficient of -18 to -5 μV / K and an electrical conductivity of 1.113 × 10⁻⁶ in the Ni-xCr range of 50–600 °C. 6 ~3.598×10 6S / m, power factor is 0.090~0.478mW / (m·K) 2 ).

[0013] The aforementioned nickel-chromium alloy material with a negative Seebeck coefficient, x = 10wt.%, 15wt.%, or 20wt.%.

[0014] A method for preparing a nickel-chromium alloy material with a negative Seebeck coefficient involves mixing Ni powder as a matrix and Cr powder as a solute, then grinding them to obtain a mixed powder; and subjecting the mixed powder to discharge plasma sintering to obtain the aforementioned nickel-chromium alloy material with a negative Seebeck coefficient.

[0015] The method for preparing a nickel-chromium alloy material with a negative Seebeck coefficient described above includes the following discharge plasma sintering conditions: In a vacuum environment, at a pressure of 20-40 MPa, the temperature is raised to 700-900℃ at a heating rate of 100-120℃ / min, and held at that temperature and pressure for 5-10 minutes. After the sintering reaction is completed, the pressure applied to the powder is immediately removed, and the material is allowed to cool naturally to room temperature before the vacuum state is terminated, resulting in a nickel-chromium alloy material with a negative Seebeck coefficient. Under these conditions of 20-40 MPa pressure, 700-900℃ heating temperature, and 5-10 minutes of holding time, not only is powder contact and diffusion bonding promoted, but complete solid solution is also inhibited. When the pressure is less than 20 MPa, the heating temperature is less than 700℃, or the holding time is less than 5 minutes, the sample may not be sufficiently densified, resulting in an uneven microstructure and discontinuous conductive paths. When the pressure exceeds 40 MPa, the heating temperature exceeds 900 °C, or the holding time exceeds 10 min, Cr may completely dissolve in the Ni matrix, leading to the inability to form a nickel-chromium alloy material and a reversal of thermoelectric polarity. Furthermore, in this application, the pressure applied to the powder is removed immediately after the sintering reaction. If the pressure is removed after cooling, continuous Cr diffusion will occur during the cooling process, resulting in an increase in solid solubility. Simultaneously, natural cooling to room temperature helps release the internal thermal stress generated during sintering, contributing to the stability of the formed microstructure and preventing defects caused by a sudden temperature drop.

[0016] The above-mentioned method for preparing a nickel-chromium alloy material with a negative Seebeck coefficient involves mixing Ni powder and Cr powder at a mass ratio of (4~9):1; after mixing, anhydrous ethanol is added for grinding, and the mixed powder is then dried. Dry grinding may cause frictional heating of the particle surface, exposing fresh metal surfaces that are easily oxidized by air. Adding anhydrous ethanol for grinding ensures more uniform mixing of the two powders, prevents powder agglomeration, and reduces the risk of oxidation. The purity of the Ni powder and Cr powder are 99.99% and 99.99%, respectively.

[0017] In the above-mentioned method for preparing a nickel-chromium alloy material with a negative Seebeck coefficient, the particle size of the mixed powder is 120-500 mesh. If the particle size is too large, the diffusion distance may be too long, making it difficult to form an effective solid solution during preparation, resulting in insufficient local sintering. In addition, coarse particles will make densification difficult. On the other hand, if the particle size is too fine, the diffusion will be too fast, which will make it easy for Cr to completely dissolve in the Ni matrix during preparation.

[0018] The above-mentioned method for preparing a nickel-chromium alloy material with a negative Seebeck coefficient yields a nickel-chromium alloy material with a negative Seebeck coefficient that is a disc-shaped block with a relative density greater than 95%.

[0019] Application of a nickel-chromium alloy material with a negative Seebeck coefficient in thermocouples, using the aforementioned nickel-chromium alloy material with a negative Seebeck coefficient.

[0020] The aforementioned application of a nickel-chromium alloy material with a negative Seebeck coefficient in thermocouples involves pairing a nickel-chromium alloy material with a negative Seebeck coefficient with one with a positive Seebeck coefficient to construct a self-compensating thermocouple structure or a differential thermocouple system. A self-compensating thermocouple refers to a thermocouple composed of two materials from the same source with matched physical properties. The additional thermoelectric potential caused by environmental changes is symmetrically distributed in the circuit, thus being automatically compensated for and directly used for temperature measurement, improving measurement accuracy. A differential thermocouple, on the other hand, measures the difference between two temperatures of the materials, rather than the absolute temperature. Its structure involves two thermocouples sharing a common reference junction, thereby measuring the temperature difference between the two hot junctions and improving sensitivity.

[0021] The technical solution of the present invention achieves the following beneficial technical effects:

[0022] (1) The Seebeck coefficient of pure nickel is negative, while that of conventional nickel-chromium alloys is positive. This is because the introduction of the transition metal element Cr forms a quasi-bound state, which leads to resonant scattering and changes its Seebeck coefficient. That is, the amount of Cr dissolved and the form of Ni and Cr directly affect this change. This invention achieves controllable adjustment of the solid solution behavior of the alloy under non-equilibrium conditions by precisely controlling the composition ratio of Ni and Cr (x = 10-20 wt.%) and by precisely controlling the process parameters of spark plasma sintering (SPS), thus constructing a composite phase structure with adjustable Cr solid solution degree and phase ratio; thereby potentially affecting the resonant scattering of the alloy material, resulting in a nickel-chromium alloy material with a negative Seebeck coefficient. Nickel-chromium alloys with negative Seebeck coefficients do not undergo a complete Seebeck coefficient reversal compared to pure nickel, and retain the advantages of high-temperature stability and oxidation resistance of nickel-chromium alloys. They can be paired with traditional nickel-chromium alloys with positive Seebeck coefficients to construct thermocouple structures or differential thermocouple systems with complementary polarities, significantly enhancing the effective Seebeck coefficient difference, improving temperature response sensitivity and measurement stability, and providing new design ideas for thermocouple technology, energy conversion, and intelligent temperature control.

[0023] (2) This invention controls the rapid heating rate, the final heating temperature, the short holding time, and the pressure applied to the powder during the SPS sintering process. At the same time, it removes the pressure applied to the powder immediately after the sintering reaction and allows it to cool naturally to room temperature. This achieves densification and suppresses complete equilibrium solid solution at a lower sintering temperature, so that Cr only partially dissolves in the Ni matrix. While forming a face-centered cubic (FCC) solid solution with Ni as the matrix and Cr atoms dissolved in the Ni matrix as the main phase, a certain amount of undissolved Cr phase with body-centered cubic (BCC) structure is retained in the material. At the same time, the undissolved Cr phase does not destroy the continuous conductive network of the matrix, thereby constructing a composite phase structure with adjustable solid solution degree and phase ratio. Within the same material system, the Seebeck coefficient sign is reversed (the extension of the traditional positive polarity nickel-chromium alloy system to the negative polarity system), causing the original output thermoelectric potential direction to be reversed, providing a new material basis for the controllable design of thermocouple output polarity. Furthermore, the discharge plasma sintering technology used in this invention can complete the densification process at a relatively low temperature (700-900℃) and in a short time (5-10min), which can effectively reduce energy consumption, shorten the preparation cycle, and has good industrial scalability and repeatability.

[0024] (3) This invention achieves a stable negative Seebeck coefficient in the Ni-Cr system for the first time. In the range of 50–600 °C, the Seebeck coefficient of Ni-xCr is -18 to -5 μV / K, and the conductivity is 1.113 × 10⁻⁶. 6 ~3.598×10 6S / m, power factor is 0.090~0.478mW / (m·K) 2 This technology successfully achieved thermoelectric polarity reversal characteristics that are difficult to obtain in traditional nickel-chromium systems. Attached Figure Description

[0025] Figure 1 X-ray diffraction patterns of Ni-xCr prepared in the smelting furnaces shown in Comparative Examples 1-3;

[0026] Figure 2 The X-ray diffraction patterns of Ni-xCr prepared by SPS as shown in Examples 1-3;

[0027] Figure 3 The Seebeck coefficient curves of the Ni-xCr alloy materials shown in Comparative Examples 1-3 and Examples 1-3 as a function of temperature are shown.

[0028] Figure 4 The electrical conductivity of the Ni-xCr alloy materials shown in Comparative Examples 1-3 and Examples 1-3 as a function of temperature is illustrated.

[0029] Figure 5 The power factor curves of the Ni-xCr alloy materials shown in Comparative Examples 1-3 and Examples 1-3 as a function of temperature are shown.

[0030] Figure 6 The image shows a scanning electron microscope (SEM) image of the Ni-20Cr alloy material prepared by SPS as shown in Example 3.

[0031] Figure 7 The energy dispersive spectroscopy (EDS) diagram of Ni element in the Ni-20Cr alloy material prepared by SPS as shown in Example 3;

[0032] Figure 8 The image shows the energy dispersive spectroscopy (EDS) analysis of Cr in the Ni-20Cr alloy material prepared by SPS as shown in Example 3.

[0033] Figure 9 This is a schematic diagram of the density distribution of pure Ni states.

[0034] Figure 10 This is a schematic diagram of the density of states distribution of a Ni-Cr alloy with a positive Seebeck coefficient. Detailed Implementation

[0035] Comparative Example 1

[0036] Ni particles and Cr particles were weighed out at a mass ratio of 9:1. The mass of Cr particles was 0.5% more than the total mass of the nickel particles and chromium particles, which was based on the 9:1 mass ratio of nickel particles to chromium particles. This was to offset the loss of chromium during the high-temperature refining stage.

[0037] Place the weighed Ni and Cr particles in a clean container, spray with a small amount of anhydrous ethanol, and stir for 2–3 minutes.

[0038] After stirring, place in an 80–120℃ hot air oven for 30–60 minutes (until surface dry and no alcohol smell), then remove from the oven and set aside for later use.

[0039] After drying, the nickel and chromium particles are loaded into a crucible. Initially, about 30-50% of the total Ni mass is loaded with nickel particles, and all the chromium particles and the remaining nickel particles are evenly distributed on top of it.

[0040] Before loading the crucible containing the raw materials into the furnace, first evacuate the empty furnace cavity of the vacuum induction melting furnace to ≤1Pa, dry it at 100–200℃ for 5–10 minutes to remove adsorbed water, and then load it into the crucible after returning it to normal pressure.

[0041] Close the furnace door and evacuate the furnace. Then, purge the furnace with argon gas three times. First, evacuate the furnace to ≤1 Pa and then purge with argon gas to 0.05 MPa. Second, evacuate the furnace to ≤1 Pa and purge with argon gas to 0.04 MPa. Third, evacuate the furnace to ≤1 Pa and purge with argon gas to 0.03 MPa.

[0042] Apply current to raise the temperature to 1460-1570℃ at a uniform rate until the target substance is completely dissolved. Lower the graphite stirring rod and stir for 5 minutes. Then turn off the current.

[0043] The target solution is slowly cooled to room temperature in an argon atmosphere. The ingot is then removed, polished with a polishing machine to remove surface impurities, and cut into the required sample size using a wire cutting machine.

[0044] Referring to Figure 1, phase and crystal structure analysis was performed using a HAOYUAN DX-27MINI X-ray diffraction (XRD) instrument. In this study, the XRD test used a Cu-Kα target with a characteristic X-ray wavelength of λ = 0.15406 nm. The scanning range was set to 20°–80° (2θ), and the test parameters were maintained in the instrument's high-resolution mode to ensure the reliability of the diffraction peak shape and intensity.

[0045] The results show that the diffraction peaks in Comparative Example 1 mainly exhibit a face-centered cubic (FCC) structure with Ni as the matrix. This result is in high agreement with the characteristic peak positions and relative intensities of the international standard diffraction data card PDF#01-074-5730 (Ni0.9Cr0.1), indicating that the crystal structure of the sample is in good consistency with its theoretical composition.

[0046] Referring to Figure 3, the Seebeck coefficient of the samples was tested using a Corio CTA-3 thermometer. The Seebeck coefficient of the Ni-10Cr prepared in Comparative Example 1 showed a generally positive value in the range of 50–600 °C, with a relatively stable trend. As the temperature increased, the Seebeck coefficient showed a trend of remaining positive and slightly increasing or stabilizing, indicating that the charge carrier type was always predominantly hole-type, and the conductivity was p-type. Simultaneously, this result also shows that the band structure of the material did not change significantly within this temperature range, and thermal excitation had little impact on its charge carrier type, verifying the good stability of the thermoelectric properties of the Ni-10Cr alloy prepared by the conventional melting method.

[0047] Referring to Figure 4, the conductivity of Comparative Example 1 Ni-10Cr remained relatively stable within the temperature range of 50–600 °C, with a value ranging from 0.880 to 1.061 × 10⁻⁶. 6 S / m. The conductivity decreases slightly with increasing temperature, but the overall change is small, exhibiting the typical temperature-dependent behavior of metallic solid solutions.

[0048] Referring to Figure 5, the power factor of Comparative Example 1 Ni-10Cr is 0.353~0.638 mW / (m·K) in the range of 50–600 °C. 2 The power factor gradually decreases with temperature and then tends to stabilize, which is related to the positive and stable increase of the Seebeck coefficient and the slight change in conductivity.

[0049] Comparative Example 2

[0050] Ni particles and Cr particles were weighed at a mass ratio of 8.5:1.5. The mass of Cr particles was 0.5% more than the total mass of the nickel particles and Cr particles at the mass ratio of 8.5:1.5. The weighed nickel and Cr particles were prepared in the same way as in Comparative Example 1 to obtain the final Ni-15Cr sample.

[0051] Referring to Figure 1, the diffraction peaks of Comparative Example 2 mainly exhibit a face-centered cubic (FCC) structure with Ni as the matrix. This result is in high agreement with the characteristic peak positions and relative intensities of the international standard diffraction data card PDF#01-074-5730 (Ni0.9Cr0.1), indicating that the crystal structure of the sample is in good consistency with its theoretical composition.

[0052] Referring to Figure 3, the Seebeck coefficient of Comparative Example 2 Ni-15Cr is consistently positive within the temperature range of 50-600℃, and its trend remains relatively stable with increasing temperature. This demonstrates that the Ni-15Cr prepared by the conventional smelting method has a positive Seebeck coefficient and is a P-type conductor.

[0053] Referring to Figure 4, the conductivity of Comparative Example 2 Ni-15Cr remained relatively stable in the temperature range of 50–600℃, with a value range of 0.899~1.030×10⁻⁶. 6 S / m.

[0054] Referring to Figure 5, the power factor of Comparative Example 2 Ni-15Cr is 0.358~0.496 mW / (m·K) in the range of 50–600 °C. 2 ).

[0055] Comparative Example 3

[0056] Ni particles and Cr particles were weighed at a mass ratio of 8:2. The mass of Cr particles was 0.5% more than the total mass of the nickel particles and Cr particles, based on the mass ratio of nickel particles to Cr particles of 8:2. The weighed nickel and Cr particles were prepared in the same way as in Comparative Example 1 to obtain the final Ni-20Cr sample.

[0057] Referring to Figure 1, the diffraction peaks of Comparative Example 3 mainly exhibit a face-centered cubic (FCC) structure with Ni as the matrix. This result is in high agreement with the characteristic peak positions and relative intensities of the international standard diffraction data card PDF#01-074-5730 (Ni0.9Cr0.1), indicating that the crystal structure of the sample is in good consistency with its theoretical composition.

[0058] Referring to Figure 3, the Seebeck coefficient of Comparative Example 3 Ni-20Cr is consistently positive within the temperature range of 50-600℃, and its trend remains relatively stable with increasing temperature. This demonstrates that the Ni-20Cr prepared by the conventional smelting method has a positive Seebeck coefficient and is a P-type conductor.

[0059] Referring to Figure 4, the electrical conductivity of Comparative Example 3 Ni-20Cr remained relatively stable in the range of 50–600 °C, with a value range of 0.903~1.072×10⁻⁶. 6 S / m.

[0060] Referring to Figure 5, the power factor of Comparative Example 3 Ni-20Cr is 0.267~0.394 mW / (m·K) in the range of 50–600 °C. 2 ).

[0061] Example 1

[0062] Ni powder and Cr powder were mixed at a mass ratio of 9:1, and an appropriate amount of anhydrous ethanol was added (enough to wet all the powder and form a flowing slurry). The mixture was then ground in an agate crucible for 1 hour. The ground mixture was dried at 80°C for 2 hours to obtain a mixed powder with a particle size of 120-500 mesh.

[0063] The mixed powder is loaded into a graphite mold, with three layers of 0.2-0.3mm graphite paper placed on the top and bottom of the powder, and three layers of graphite paper placed between the graphite pads. The purpose is to prevent the sintered block from sticking to the mold, facilitate demolding, and protect the mold from damage.

[0064] Place the graphite mold containing the powder into the SPS chamber and evacuate it to ≤60Pa. Adjust the pressure to 30MPa.

[0065] Turn on the frequency converter power supply and apply current to heat the graphite mold to 800℃ at a uniform heating rate of 100℃ / min, and hold at that temperature for 5 minutes. Immediately after the sintering reaction is complete, remove the pressure applied to the powder and allow it to cool naturally to room temperature. Then, end the vacuum state to obtain a sample of nickel-chromium alloy material with a negative Seebeck coefficient.

[0066] Remove the graphite mold, grind the sample with a polishing machine to remove surface impurities, and cut the sample to the required size with a wire cutting machine.

[0067] Referring to Figure 2, phase and crystal structure analysis was performed using a HAOYUAN DX-27MINI X-ray diffraction (XRD) instrument. In this study, the XRD test used a Cu-Kα target with a characteristic X-ray wavelength of λ = 0.15406 nm. The scanning range was set to 20°–80° (2θ), and the test parameters were maintained in the instrument's high-resolution mode to ensure the reliability of the diffraction peak shape and intensity.

[0068] The obtained diffraction patterns show that the Ni-10Cr prepared by SPS in Example 1 mainly exhibits a face-centered cubic (FCC) structure with Ni as the matrix, and a small amount of body-centered cubic (BCC) Cr coexists.

[0069] Referring to Figure 3, the Seebeck coefficient of the samples was tested using a Corio CTA-3. The Seebeck coefficient of Ni-10Cr in Example 1 showed a generally negative value in the 50–600 °C range, indicating that the charge carrier type was consistently electron-dominated, and the conductivity was N-type. This contrasts sharply with Comparative Example 1, where the Seebeck coefficient sign of the Ni-10Cr prepared by SPS was reversed compared to that prepared by conventional smelting. With increasing temperature, the Seebeck coefficient of Ni-10Cr in Example 1 showed a trend of first decreasing, then increasing, and then decreasing again; this result may be related to the intrinsic properties of the material.

[0070] Referring to Figure 4, the electrical conductivity of Ni-10Cr in Example 1, within the temperature range of 50–600 °C, shows a decreasing trend at low temperatures and a tendency to stabilize at high temperatures, compared to Comparative Examples 1–3. Its value ranges from 1.337 to 3.598 × 10⁻⁶. 6 S / m.

[0071] Referring to Figure 5, the power factor of Ni-10Cr in Example 1 is 0.148 to 0.478 mW / (m·K²) in the range of 50–600 °C. The power factor shows a trend of first increasing, then decreasing and then increasing again with temperature.

[0072] Example 2

[0073] Ni powder and Cr powder were mixed at a mass ratio of 8.5:1.5. The mixed nickel-chromium powder was then prepared using the same method as in Example 1 to obtain the final Ni-15Cr sample.

[0074] Referring to Figure 1, the X-ray diffraction pattern shows that the Ni-15Cr prepared by SPS in Example 2 mainly exhibits a face-centered cubic (FCC) structure with Ni as the matrix, and a small amount of body-centered cubic (BCC) Cr coexists.

[0075] Referring to Figure 3, the Seebeck coefficient test shows that the Seebeck coefficient of Ni-15Cr in Example 2 is negative throughout the 50–600 °C range, indicating that the charge carrier type is always electron-dominated and the conductivity is N-type. This can be compared with Comparative Example 2, where the Seebeck coefficient of Ni-15Cr prepared by SPS and Ni-15Cr prepared by conventional smelting methods shows an inversion of the sign. As the temperature increases, the Seebeck coefficient of Ni-15Cr in Example 2 exhibits a trend of first decreasing, then increasing, and then decreasing again.

[0076] Referring to Figure 4, the conductivity of Ni-15Cr in Example 2, within the temperature range of 50–600 °C, shows a decreasing trend at low temperatures and a tendency to stabilize at high temperatures, compared to Comparative Examples 1–3. Its value ranges from 1.113 to 2.805 × 10⁻⁶. 6 S / m.

[0077] Referring to Figure 5, the power factor of Ni-10Cr in Example 2 is 0.090 to 0.285 mW / (m·K²) in the range of 50–600 ℃. The power factor shows a trend of first increasing, then decreasing and then increasing again with temperature.

[0078] Example 3

[0079] Ni powder and Cr powder were mixed at a mass ratio of 8:2. The mixed nickel-chromium powder was prepared in the same way as in Example 1 to obtain the final Ni-20Cr sample.

[0080] Referring to Figure 1, the X-ray diffraction pattern shows that the Ni-20Cr prepared by SPS in Example 3 mainly exhibits a face-centered cubic (FCC) structure with Ni as the matrix, and a small amount of body-centered cubic (BCC) Cr coexists.

[0081] Referring to Figure 3, the Seebeck coefficient test shows that the Seebeck coefficient of Ni-20Cr in Example 3 is negative throughout the 50–600 °C range, indicating that the charge carrier type is always electron-dominated and the conductivity is N-type. This can be compared with Comparative Example 3, where the Seebeck coefficient of Ni-20Cr prepared by SPS and Ni-20Cr prepared by conventional smelting methods shows an inversion of the sign. As the temperature increases, the Seebeck coefficient of Ni-20Cr in Example 3 exhibits a trend of first decreasing, then increasing, and then decreasing again.

[0082] Referring to Figure 4, the conductivity of Ni-20Cr in Example 3, within the temperature range of 50–600 °C, shows a decreasing trend at low temperatures and a tendency to stabilize at high temperatures compared to Comparative Examples 1–3, with a value range of 1.297–3.300 × 10⁻⁶. 6 S / m.

[0083] Referring to Figure 5, the power factor of Ni-20Cr in Example 3 is 0.112–0.391 mW / (m·K) in the range of 50–600 °C. 2 The power factor shows a trend of first increasing, then decreasing, and then increasing again with temperature.

[0084] See appendix Figures 6-8 As shown in the SEM image of the Ni-20Cr alloy prepared in Example 3, the sample exhibits regular grain morphology, clear grain boundaries, and a uniform equiaxed crystal structure, indicating good crystallinity. EDS analysis results show that by rationally controlling the discharge plasma sintering process parameters, the Ni-20Cr alloy can form a face-centered cubic (FCC) solid solution main phase with Ni as the matrix and Cr atoms dissolved within it; simultaneously, a certain amount of body-centered cubic (BCC) Cr phase also exists in the material. These two phases together constitute the composite phase structure of the Ni-20Cr alloy, and its phase composition can be adjusted by the sintering process parameters.

[0085] The Seebeck coefficient of pure nickel is negative, while that of conventional nickel-chromium alloys is positive. This is because the introduction of the transition metal element Cr creates quasi-bound states, leading to resonant scattering and altering the Seebeck coefficient. First-principles calculations confirm that the introduction of Cr does indeed induce quasi-bound states, and a scattering resonance peak appears near the Fermi level in the density of states (e.g., ...). Figure 9 , Figure 10Through precise control techniques, it was discovered that the solid solution content of Cr and the presence forms of Ni and Cr directly affect this change. Therefore, this invention achieves controllable regulation of the solid solution behavior of the alloy under non-equilibrium conditions by precisely controlling the composition ratio of Ni and Cr (x = 10~20 wt.%) and by precisely controlling the process parameters of spark plasma sintering (SPS), thus constructing a composite phase microstructure with adjustable Cr solid solution degree and phase ratio; this may further affect the resonance scattering of the alloy material, thereby obtaining a nickel-chromium alloy material with a negative Seebeck coefficient.

[0086] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. A nickel-chromium alloy material with a negative Seebeck coefficient, characterized in that, The chemical formula of nickel-chromium alloys with negative Seebeck coefficients is Ni-xCr, where x represents the mass percentage of Cr in the nickel-chromium alloy, x = 10~20 wt.%; The phase structure of the nickel-chromium alloy material with a negative Seebeck coefficient is a composite phase structure consisting of a main phase and a secondary phase; wherein, the main phase is a face-centered cubic solid solution formed by Ni as the matrix and Cr atoms dissolved in the Ni matrix; the secondary phase is a body-centered cubic structure composed of Cr elements that are not dissolved in the main phase.

2. The nickel-chromium alloy material with a negative Seebeck coefficient according to claim 1, characterized in that, Within the temperature range of 50–600 °C, the Seebeck coefficient of Ni-xCr is -18 to -5 μV / K, and the conductivity is 1.113 × 10⁻⁶. 6 ~3.598×10 6 S / m, power factor is 0.090~0.478mW / (m·K) 2 ).

3. The nickel-chromium alloy material with a negative Seebeck coefficient according to claim 1, characterized in that, x = 10wt.%, 15wt.%, or 20wt.%.

4. A method for preparing a nickel-chromium alloy material with a negative Seebeck coefficient, characterized in that, Ni powder, as the matrix, and Cr powder, as the solute, are mixed and ground to obtain a mixed powder; the mixed powder is then subjected to discharge plasma sintering to obtain a nickel-chromium alloy material with a negative Seebeck coefficient as described in any one of claims 1 to 3.

5. The method for preparing a nickel-chromium alloy material with a negative Seebeck coefficient according to claim 4, characterized in that, The conditions for the discharge plasma sintering are as follows: under vacuum, at a pressure of 20-40 MPa, the temperature is increased to 700-900℃ at a heating rate of 100-120℃ / min, and the temperature and pressure are maintained for 5-10 minutes. After the sintering reaction is completed, the pressure applied to the powder is immediately removed, and the powder is allowed to cool naturally to room temperature before the vacuum state is ended, thus obtaining a nickel-chromium alloy material with a negative Seebeck coefficient.

6. The method for preparing a nickel-chromium alloy material with a negative Seebeck coefficient according to claim 4, characterized in that, Ni powder and Cr powder are mixed at a mass ratio of (4~9):1; after mixing, anhydrous ethanol is added for grinding, and the mixed powder is dried after grinding.

7. A method for preparing a nickel-chromium alloy material with a negative Seebeck coefficient according to claim 4 or 6, characterized in that, The particle size of the mixed powder is 120 to 500 mesh.

8. The method for preparing a nickel-chromium alloy material with a negative Seebeck coefficient according to claim 4, characterized in that, The obtained nickel-chromium alloy material with a negative Seebeck coefficient is a disc-shaped block with a relative density greater than 95%.

9. The application of a nickel-chromium alloy material with a negative Seebeck coefficient in thermocouples, characterized in that, Use the nickel-chromium alloy material with a negative Seebeck coefficient as described in any one of claims 1 to 3.

10. The application of a nickel-chromium alloy material with a negative Seebeck coefficient according to claim 9 in thermocouples, characterized in that, By pairing nickel-chromium alloy materials with negative Seebeck coefficients with nickel-chromium alloy materials with positive Seebeck coefficients, self-compensating thermocouple structures or differential thermocouple systems can be constructed.

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