Composite ceramic preparation method and composite ceramic

Abstract

The invention relates to the technical field of NTC thermistor ceramics, in particular to a composite ceramic preparation method and a composite ceramic. The preparation method of the composite ceramic comprises the following steps: preparation of a spinel phase raw material: weighing oxides of Mn, Cr, Y, Ni, La and Co in proportion, and carrying out ball milling, drying and pre-sintering to prepare primarily densified spinel phase powder; the preparation method comprises the following steps: preparing perovskite phase raw materials: weighing lanthanum nitrate, chromic nitrate and yttrium nitrate in proportion, dissolving in water, adding citric acid, stirring until the mixture is transparent, and preparing a La-Y-Cr mixed solution; formation of a core-shell structure: dispersing the spinel phase powder in water, heating after dispersion, slowly dropwise adding the La-Y-Cr mixed solution and ammonia water, and controlling the pH value to be 8-9 to form a core-shell structure precipitate with spinel as a core and La-Y-Cr hydroxide and citrate as a shell; and after centrifugal washing, drying and high-temperature calcination, the shell in the form of La-Y-Cr hydroxide and citrate is converted into a perovskite type coating layer.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of NTC thermistor ceramics, and particularly relates to a composite ceramic preparation method and composite ceramic. BACKGROUND

[0002] The NTC thermistor, namely negative temperature coefficient thermistor, is a resistor device with a negative temperature coefficient. The performance of the NTC thermistor is mainly determined by two key parameters of resistivity p and thermosensitive constant B. Different p and B combinations are required for different temperature zone applications. The relationship between the resistivity p of the spinel type NTC thermistor ceramic and the temperature T conforms to the Arrhenius equation. The B value usually increases with the increase of the resistivity, and vice versa. In addition, the stability of the traditional spinel type ceramic (Mn-Co-Ni system) becomes poor and the precision decreases when the temperature exceeds 300 DEG C. In order to improve the performance of the NTC material, researchers try to optimize the electrical properties and thermal stability by doping and composite structure design.

[0003] In the prior art, doping Cr, La, Y and other elements can improve the performance of the spinel NTC material, but the single structure material still has limitations. In addition, the perovskite type NTC thermistor ceramic (such as La-Cr-Y system) has good thermal stability and electrical properties, but its preparation process is complex, and the B value is small. It is difficult to meet the application requirements in a wide temperature range.

[0004] Therefore, a new type of composite ceramic material is needed, which can combine the advantages of spinel type and perovskite type, and realize the manufacture of NTC thermistor with wide temperature range and high stability. SUMMARY

[0005] The embodiment of the present application provides a new type of composite ceramic material, which can combine the advantages of spinel type and perovskite type, and realize the manufacture of NTC thermistor with wide temperature range and high stability.

[0006] The present application discloses a composite ceramic preparation method for preparing a composite ceramic of NTC thermistor, comprising the following steps: Preparation of spinel phase raw material: the oxides of Mn, Cr, Y, Ni, La and Co are weighed in proportion, and the spinel phase powder is prepared by ball milling, drying and pre-sintering to obtain a preliminary densification; Preparation of perovskite phase raw material: lanthanum nitrate, chromium nitrate and yttrium nitrate are weighed in proportion, dissolved in water, and stirred to be transparent by adding citric acid to prepare a La-Y-Cr mixed solution; Forming of core-shell structure: the spinel phase powder is dispersed in water, heated after dispersion, slowly drop the La-Y-Cr mixed solution and ammonia, control the pH value between 8-9, form the core-shell structure precipitate with spinel type as core, La-Y-Cr hydroxide and citrate form as shell; centrifugal washing, drying, high temperature calcination, make the shell of La-Y-Cr hydroxide and citrate form into perovskite type coating layer.

[0007] Molding and sintering of composite ceramic: the generated core-shell structure powder is ball milled, granulated, dry pressed to prepare a green body, control the sintering temperature curve and atmosphere, make the spinel type core and perovskite type coating layer further react and densify, form the compact composite ceramic with core-shell structure.

[0008] Optionally, the preparation step of the spinel phase raw material is specifically: According to the chemical composition of spinel type ceramic Mn 0.8～1.0 Co 1.8～2.0 Ni 0.03～0.07 Cr 0.03～0.07 La 0.05～ 0.08 Y 0.04～0.06 O4 atomic molar ratio, MnO2, CrO3, Y2O3, NiO, La2O3, Co3O4 are weighed, and the initially densified spinel phase powder is prepared by ball milling, drying and pre-sintering.

[0009] Optionally, the preparation step of the perovskite phase raw material is specifically: According to the chemical composition of perovskite type ceramic La 0.93～0.97 Cr 0.03～0.07 Y 0.95～1.05 O3 atomic molar ratio, La(NO3)3·6H2O, Cr(NO3)3·9H2O and Y(NO3)3·6H2O are weighed and dissolved in deionized water, citric acid is added and stirred until transparent to prepare a La-Y-Cr mixed solution.

[0010] Optionally, in the forming step of the core-shell structure, the spinel phase powder is dispersed in water and heated to 60℃ after dispersion.

[0011] Optionally, the molding and sintering step of the composite ceramic further comprises the following steps: The sintered composite ceramic is ultrasonically cleaned, ground and polished to remove burrs and uneven parts on the surface.

[0012] Optionally, the molding and sintering step of the composite ceramic is specifically: The generated core-shell structure powder is ball-milled, granulated, and dry-pressed to prepare a green blank, which is then placed in a sintering furnace for sintering. The sintering temperature curve is: room temperature → 600°C (10°C / min) → 600°C (maintained for 2 hours) → 1150°C (15°C / min) → 1150°C (maintained for 10 minutes) → 1350°C (5°C / min) → 1350°C (maintained for 30 minutes) → natural cooling. The sintering atmosphere is nitrogen. During the sintering process, the spinel core and the perovskite coating layer further react and densify to form a composite ceramic with a tight core-shell structure.

[0013] Optionally, the forming pressure of the dry pressing is 200 MPa.

[0014] Optionally, in the steps of forming and sintering the composite ceramic, the dry pressing forming is specifically: The core-shell structure powder is loaded into the metal mold cavity, and pressure is applied by the pressing head to rearrange and deform the core-shell structure powder particles in the mold cavity and compact them to form a green billet with a certain strength and shape. The formed green billet is then taken out of the mold.

[0015] Optionally, in the step of preparing the perovskite phase raw material, the ratio of the total amount of La, Y, and Cr to citric acid is 1:0.8 to 1:1.2.

[0016] The invention also discloses a composite ceramic used for an NTC thermistor, which is prepared by the above-mentioned preparation method.

[0017] The beneficial effects of the composite ceramic preparation method provided by the embodiment of the present invention are: the spinel and perovskite composite ceramic material prepared by the present invention combines the advantages of spinel and perovskite types, and has the following excellent properties: the resistivity ρ is about 2000Ω·cm at 25°C, which is suitable for temperature measurement in a wide temperature range; the thermal constant B value is 3000-4000K, which has good temperature sensitivity; and good stability at high temperature (400°C). BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments, in which: Figure 1 1 is a graph showing the change in resistivity of the composite ceramic material according to an embodiment of the present invention as a function of temperature.

[0019] Figure 2 1 is a graph showing the change of the thermal sensitivity constant of the composite ceramic material according to an embodiment of the present invention with respect to temperature.

[0020] Figure 3 FIG. 4 is a graph showing the aging rate of the composite ceramic material according to an embodiment of the present invention at 400° C. DETAILED DESCRIPTION

[0021] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments in this application can be combined with each other. Now, in conjunction with the accompanying drawings, the preferred embodiments of the present invention will be described in detail.

[0022] An embodiment of the present invention provides a method for preparing a composite ceramic for an NTC thermistor, comprising the steps of: Preparation of spinel phase raw materials: Mn, Cr, Y, Ni, La, and Co oxides are weighed in proportion, and a preliminary densified spinel phase powder is prepared by ball milling, drying, and pre-calcining; Preparation of perovskite phase raw materials: Weigh lanthanum nitrate, chromium nitrate, and yttrium nitrate in proportion and dissolve them in water. Add citric acid and stir until transparent to prepare a La-Y-Cr mixed solution. Formation of core-shell structure: Spinel phase powder is dispersed in water, heated after dispersion, and La-Y-Cr mixed solution and ammonia water are slowly added dropwise, and the pH value is controlled between 8-9 to form a core-shell structure precipitate with spinel type as the core and La-Y-Cr hydroxide and citrate as the shell; after centrifugal washing, drying, and high-temperature calcination, the La-Y-Cr hydroxide and citrate shell are converted into a perovskite coating layer.

[0023] Molding and sintering of composite ceramics: The generated core-shell structure powder is ball-milled, granulated, and dry-pressed to prepare a green body. The sintering temperature curve and atmosphere are controlled to allow the spinel core and perovskite coating to further react and densify, forming a composite ceramic with a tight core-shell structure.

[0024] The spinel-perovskite composite ceramic material prepared by the present invention combines the advantages of both spinel and perovskite types and has the following excellent properties: the resistivity ρ is about 2000Ω·cm at 25°C, which is suitable for temperature measurement in a wide temperature range; the thermal constant B value is 3000-4000K, which has good temperature sensitivity; and good stability at high temperature (400°C).

[0025] Specifically, from Figure 1 It can be seen that Figure 1 The resistivity changes of the composite ceramic at different temperatures are shown. When the temperature is 25°C, its resistivity is about 2000Ω·cm, indicating that the material has stable resistance characteristics in a wide temperature range. Figure 2 It can be seen that Figure 2 The stability of the B value in different temperature ranges is demonstrated, and the range of the B value is 3000-4000K, indicating stable high temperature performance. Figure 3 The performance changes of the material after long-term aging at high temperature (400℃) are shown. Figure 3It can be seen that with the increase of aging time, the aging rate gradually increases slowly. When the aging time is 1000h, the aging rate is only about 4.3%. The aging rate is lower than 5%, which proves that the material is stable at high temperature.

[0026] Specifically, citric acid can form stable water-soluble complexes with metal ions La³⁺, Cr³⁺, and Y³⁺, ​​preventing uneven precipitation due to excessive local concentrations. It also inhibits the premature hydrolysis of metal ions through complexation, keeping the solution transparent and facilitating subsequent slow dripping and uniform coating of the core-shell structure. In subsequent calcination, citric acid can serve as an organic fuel to promote the decomposition of the precursor and the crystallization of the perovskite phase at low temperatures.

[0027] Compared with other organic acids such as oxalic acid and acetic acid, citric acid has a stronger chelating ability for polyvalent metal ions, and its pH adjustment range of 8-9 is suitable for coprecipitation reaction. Its decomposition temperature is moderate (200-400℃), which can avoid the volatilization or oxidation of metal ions at high temperatures (such as Cr 3+ →Cr 6+ ).

[0028] The pH is controlled at 8-9. First, the core-shell structure is completely precipitated. The hydroxides of La³⁺, Y³⁺, ​​and Cr³⁺ are almost completely precipitated at pH ≥ 8 (the solubility product Ksp is extremely small), avoiding ion residue. Second, the spinel core is prevented from dissolving. The spinel phase may partially dissolve under strong acidity (pH < 4) or strong alkalinity (pH > 10). Third, the core-shell structure is uniform. Too high a pH (> 9) will lead to too fast a precipitation rate and the formation of loose agglomerates, while a pH of 8-9 will allow the precipitation to be slow and uniformly coated. Fourth, under alkaline conditions, Cr³⁺ is more stable and avoids oxidation to soluble CrO4. 2- (pH < 9.5 is required.) Therefore, pH = 8-9 is the optimal range for balancing precipitation efficiency, core-shell coating quality, and component stability.

[0029] In the above-mentioned preparation method, the overall reaction process of the composite ceramic molding and sintering steps is as follows: ① Spinel phase formation: During the pre-sintering stage (750°C → 1250°C), the Mn / Co / Ni oxides undergo a solid-phase reaction to form a spinel structure, and the dopant elements (Cr / La / Y) are incorporated into the crystal lattice. ② Perovskite phase formation: After co-precipitation, the precursor (La-Y-Cr hydroxide and a citrate shell) decomposes into a LaCrO-YCrO3 solid solution via calcination (850°C). Further densification mechanism: During the sintering stage (1350°C), the core-shell powder softens at high temperature, and the particles diffuse to fill the voids. The perovskite phase melts (approximately 1200°C) to form a liquid phase, promoting the densification of the spinel particles. Ion exchange occurs between the spinel and perovskite at the interface (Mn²⁺ + La³⁺ → vacancies + defects), forming a chemical bond, thus forming a compact core-shell composite ceramic.

[0030] The composite ceramic structure features: ① Micro-morphology is that spinel particles are wrapped by continuous perovskite phase, and La / Mn / Cr enrichment zone is formed at the grain boundary; ② Performance optimization is that the perovskite shell layer inhibits high-temperature element volatilization (such as Mn / Co) of the spinel phase, and the interface defect (such as oxygen vacancy) optimizes the carrier migration path, balances the resistivity p and B value.

[0031] Precipitation reaction of core-shell structure: + +3 → / L / (The equation is not balanced). Wherein " / " represents a parallel relationship, La(OH)3, Cr(OH)3, Y(OH)3 three hydroxides precipitate at the same time, but they are independent compounds of each other. The precipitation reaction generates hydroxide precipitate, which adheres to the surface of the spinel particles, forming a core-shell precursor, i.e. La-Y-Cr hydroxide and citrate shell.

[0032] Side reaction of ammonia PH adjustment: , the reaction provides Promote the precipitation of metal ions La³⁺, Y³⁺, Cr³⁺.

[0033] Further, the forming and sintering steps of the composite ceramic further include the steps of: ultrasonic cleaning, grinding and polishing the sintered composite ceramic to remove surface burrs and uneven parts. Ultrasonic cleaning removes surface residual particles, diamond grinding wheel 200 mesh grinding reduces surface roughness, and polishing cloth polishing 30 min further refines the surface.

[0034] Specifically, in the preparation step of the perovskite phase raw material, the ratio of the total amount of La, Y, and Cr to citric acid is 1:0.8 to 1:1.2. If the ratio is less than 1:0.8, the citric acid is insufficient, which may lead to incomplete complexation of metal ions and uneven precipitation; if the ratio is greater than 1:1.2, the citric acid is excessive, which may introduce too much organic matter and affect the subsequent calcination effect. Specifically, this ratio can ensure that the La, Y, and Cr ions are fully complexed with citric acid, so that the nitrate dissolved in water and the citric acid form a transparent La-Y-Cr mixed solution. This stable complex state helps prevent excessive aggregation or premature precipitation of metal ions, ensures the uniformity and stability of the La-Y-Cr mixed solution, and provides a good precursor solution foundation for the subsequent formation of a uniform core-shell structure, so that when it is subsequently added dropwise to the spinel phase powder dispersion, it can be more evenly wrapped on the surface of the spinel phase particles, thereby facilitating the formation of a uniform and dense perovskite coating layer. The 1:1 ratio also helps the La-Y-Cr hydroxide and citrate shells to uniformly coat the spinel core during the subsequent core-shell structure formation process. During high-temperature calcination, this uniform coating more smoothly transforms into a perovskite-type coating, resulting in a more complete and dense perovskite phase, thereby enhancing the integrity and stability of the core-shell structure and improving the overall performance of the composite ceramic material.

[0035] Furthermore, the 1:1 ratio facilitates better control of the pH value between 8 and 9 during the core-shell structure formation process when reacting with ammonia. Because the amount of citric acid added can affect the solution's acid-base buffering capacity, the 1:1 ratio allows for more precise pH adjustment when adding ammonia, ensuring a smooth precipitation reaction and the quality of the core-shell structure formation. It also contributes to the stability and repeatability of the entire process, facilitating large-scale production and quality control.

[0036] Specifically, the steps for preparing the spinel phase raw material are as follows: according to the chemical composition of spinel ceramics Mn 0.8～1.0 Co 1.8～2.0 Ni 0.03～0.07 Cr 0.03～0.07 La 0.05～0.08 Y 0.04～0.06 The atomic molar ratio of O4 is 2.5:1. MnO2, CrO3, Y2O3, NiO, La2O3, Co3O4, and the primary densified spinel phase powder is prepared by ball milling, drying, and pre-calcining. Specifically, the chemical composition of spinel ceramics is Mn 0.9 Co 1.9 Ni 0.05 Cr 0.05 La 0.06 Y 0.05 O4.

[0037] Among them, Mn is the main lattice element, providing NTC characteristics, stabilizing the spinel structure, and affecting the resistivity; Co adjusts the resistivity and B value, improving the thermal stability and electrical properties of the material; Ni is a doping element, optimizing the electrical properties, reducing the resistivity, and improving the temperature sensitivity; Cr is a doping element, enhancing high-temperature stability and inhibiting the volatilization of Mn / Co at high temperatures; Lar is a doping element, stabilizing the grain boundary, reducing the grain boundary resistance, and improving the density; Yr is a doping element, inhibiting lattice defects, enhancing high-temperature stability, and optimizing the B value.

[0038] According to the specific atomic molar ratio (Mn 0.8～1.0 Co 1.8～2.0 Ni 0.03～0.07 Cr 0.03～0.07 La 0.05～0.08 Y 0.04～ 0.06 O4) Various metal oxides are weighed to prepare spinel phase powder. Precise chemical composition control ensures the spinel phase has stable performance. The ratio of each element directly determines its crystal structure and electronic properties, which in turn affects the resistivity, thermal constant and other key performance parameters of the final composite ceramic, enabling the material to perform well in wide temperature range applications after subsequent compounding.

[0039] After ball milling, drying and pre-firing, the spinel phase powder can achieve initial densification, which is beneficial to improving the mechanical strength and structural stability of the spinel phase itself, so that it can better maintain its own structural integrity during the subsequent formation of core-shell structure and compounding and sintering with other phases, reduce the performance degradation caused by defects such as internal pores, and lay a good foundation for the high-quality molding of the final composite ceramic material.

[0040] The preparation steps of the perovskite phase raw material are as follows: according to the chemical composition of the perovskite ceramic La 0.93～ 0.97 Cr 0.03～0.07 Y 0.95～1.05 The atomic molar ratio of O3 was adjusted by weighing La(NO3)3·6H2O, Cr(NO3)3·9H2O, and Y(NO3)3·6H2O and dissolving them in deionized water. Citric acid was added and stirred until transparent to prepare a La-Y-Cr mixed solution.

[0041] In perovskite ceramics, La is the main lattice element, stabilizing the perovskite structure; Cr provides electrical conductivity and enhances high-temperature stability; Y adjusts the lattice parameters and optimizes the thermal expansion coefficient.

[0042] In the step of forming the core-shell structure, the specific process of forming the core-shell structure is as follows: spinel powder (Mn 0.8～1.0 Co 1.8～ 2.0 Ni 0.03～0.07 Cr0.03～0.07 La 0.05～0.08 Y 0.04～0.06 O4) as the "core" and dispersed in the solution; after adding the La-Y-Cr mixed solution, under the condition of pH = 8-9, the metal ions precipitated on the surface of the spinel particles in the form of hydroxide and citrate, forming a "shell". After high temperature treatment (850℃), the precursor shell was transformed into the perovskite phase (La 0.93～0.97 Cr 0.03～0.07 Y 0.95～ 1.05 O3), and finally formed a core-shell structure of "spinel core and perovskite shell".

[0043] Specifically, the chemical composition of perovskite ceramics is La 0.95 Cr 0.05 Y1O3.

[0044] In the core-shell structure forming step, the spinel phase powder is dispersed in water and heated to 60°C after dispersion. The forming pressure of the dry pressing is 200 MPa. In the forming and sintering steps of the composite ceramic, the dry pressing specifically comprises: placing the core-shell structure powder into a metal mold cavity, applying pressure through a pressing head, causing the core-shell structure powder particles in the mold cavity to rearrange and deform and be compacted to form a green body with a certain strength and shape, and then removing the formed green body from the mold.

[0045] The forming and sintering steps of the composite ceramic are specifically as follows: ball milling, granulating, and dry pressing the generated core-shell structure powder to prepare a green blank, and placing the green blank in a sintering furnace for sintering. The sintering temperature curve is: room temperature → 600°C (10°C / min) → 600°C (maintained for 2 hours) → 1150°C (15°C / min) → 1150°C (maintained for 10 minutes) → 1350°C (5°C / min) → 1350°C (maintained for 30 minutes) → natural cooling. The sintering atmosphere is nitrogen. During the sintering process, the spinel core and the perovskite coating layer are further reacted and densified to form a composite ceramic with a tight core-shell structure.

[0046] The present invention employs a high-temperature sintering method at 1350°C and a controlled heating curve. This process promotes grain growth and densification, and the introduction of a nitrogen protective atmosphere reduces voids caused by oxidation, further optimizing density. Mechanical strength is highly correlated with density; higher density results in greater strength. Furthermore, the core-shell structure enhances interfacial bonding, reduces crack propagation, and further optimizes strength. The composite ceramic material has a dense structure and high mechanical strength, along with excellent surface finish, making it suitable for the manufacture of precision electronic components. The composite ceramic exhibits excellent electrical conductivity and electrochemical stability, making it suitable for functional materials such as high-temperature fuel cells and sensors.

[0047] Finally, the composite ceramic spinel phase of the present invention: Mn0.8～1.0 Co 1.8～2.0 Ni 0.03～0.07 Cr 0.03～0.07 La 0.05～ 0.08 Y 0.04～0.06 O4 (main crystal phase, providing NTC characteristics), perovskite phase: La 0.93～0.97 Cr 0.03～0.07 Y 0.95～1.05 O3 (coating phase, improving high temperature stability), the overall structure is: the surface of the spinel particles is coated with a perovskite layer, ( ) x -( ) 1−x , where x=0.7.

[0048] The invention also discloses a composite ceramic used for an NTC thermistor, which is prepared by the above-mentioned preparation method.

[0049] The present invention further discloses detailed embodiments: ①First, MnO2, CrO3, Y2O3, NiO, La2O3, Co3O4 are used as spinel raw materials, according to the chemical composition of spinel ceramics (Mn 0.9 Co 1.9 Ni 0.05 Cr 0.05 La 0.06 Y 0.05 The raw materials were weighed into an agate ball mill, and 5% La2O3 and 5% Y2O3 were added to compensate for high-temperature volatilization. Zirconia balls were used as ball milling media and anhydrous ethanol was used as dispersion medium. The ball milling was carried out according to the ratio of zirconia balls to spinel raw materials = 5:1 and the liquid-solid ratio = 3:1. The ball milling was carried out alternately in clockwise and counterclockwise directions with an interval of 30 minutes at a speed of 300 r / min for 12 hours to obtain a mixed slurry. ② The milled slurry is dried with hot air at 80°C, then passed through a 200-mesh sieve to obtain a dry powder. The powder is placed in a sintering furnace at a pre-burning temperature of 750°C for 3 hours, and argon is introduced to suppress the volatilization of La / Y. The temperature is then set to 950°C and kept for 2 hours to promote the formation of the spinel phase. Finally, the furnace is cooled to room temperature. The pre-burning process helps remove organic impurities, initially densify the powder particles, initially form the spinel phase, and avoid the loss of La / Y volatilization. ③ Ball milling is performed again, with the same steps as ①, but the time is shortened to 8 hours. The ball-milled powder is calcined at high temperature, heating to 1000°C at 5°C / min, holding for 2 hours, then heating to 1250°C at 3°C / min, holding for 3 hours, holding for 8 hours, and finally cooling to room temperature at 3°C / min to complete the spinel phase formation. The calcined powder is ground and passed through a 400-mesh sieve to obtain a uniform spinel phase powder; ④Ultrasonic dispersion of spinel powder in 0.05 mol / L HNO3 for 15 min to remove surface impurities, and vacuum dry at 80°C for 12 h before use; ⑤ Using La(NO3)3·6H2O, Cr(NO3)3·9H2O, and Y(NO3)3·6H2O as perovskite raw materials, according to the chemical composition of perovskite ceramics (La 0.95 Cr 0.05 The atomic molar ratio of La(NO3)3·6H2O:Cr(NO3)3·9H2O:Y(NO3)3·6H2O=0.95:0.05:1) was dissolved in deionized water, and citric acid (metal ions La³⁺, Y³⁺, ​​Cr³⁺: citric acid=1:1) was added and stirred until transparent to prepare a mixed metal nitrate solution, i.e., La-Y-Cr mixed solution; ⑥ The pretreated spinel phase powder was dispersed in deionized water and ultrasonicated for 30 minutes. The spinel phase powder suspension was heated to 60°C and stirred continuously. The La-Y-Cr mixed solution and ammonia water were slowly added dropwise at 1 ml / min. The pH value was controlled between 8 and 9. After the reaction, stirring was continued for 2 hours to form a core-shell structure precipitate. The precipitate after the reaction was aged for 24 hours to promote the growth of the precipitated particles and the improvement of the structure. ⑦ Use deionized water to centrifuge and wash until neutral to remove NH4 + / NO3 - The residue was vacuum dried at 80℃ for 20h to obtain the coated precursor powder, which was placed in a sintering furnace for high-temperature calcination. The temperature was raised to 400℃ and kept for 2h (to decompose organic matter) → 850℃ and kept for 6h (to crystallize perovskite) → rapid cooling (to inhibit Cr 3+ Oxidation). Convert the La-Y-Cr precursor into perovskite-type La 0.95 Cr 0.05 YO3 coating layer.

[0050] ⑧ The generated core-shell structure was ball-milled using anhydrous ethanol as the medium, a ball-to-material ratio of 4:1, a rotation speed of 250 r / min, and ball-milling for 4 h. The ball-milled powder was dried at 80°C and passed through a 100-mesh sieve to obtain a core-shell structure powder; ⑨ Granulation was performed using PVA as a binder, with the addition amount of PVA being 3wt% of the powder, to obtain a mixed core-shell structure powder with uniform particle size, good fluidity, and good formability; ⑩ Using dry pressing, the core-shell powder is placed into a metal mold cavity at a pressure of 200 MPa. The core-shell powder is compacted by applying pressure through the indenter, causing the core-shell powder particles in the mold cavity to rearrange and deform, forming a green body with a certain strength and shape. The formed green body is then removed from the mold.

[0051] ⑪ Place the green blank into a sintering furnace for sintering. The sintering temperature curve is: room temperature → 600℃ (10℃ / min) → 600℃ (hold for 2h) → 1150℃ (15℃ / min) → 1150℃ (hold for 10min) → 1350℃ (5℃ / min) → 1350℃ (hold for 30min) → natural cooling. The sintering atmosphere is nitrogen. During the sintering process, the spinel core and the perovskite coating layer further react and densify to form a composite ceramic with a tight core-shell structure.

[0052] After sintering, the composite ceramics were post-processed. First, they were cleaned with an ultrasonic cleaner for 20 minutes. After cleaning, they were ground with a diamond grinding wheel with a particle size of 200 mesh and a speed of 150 r / min. Polishing was performed with a polishing cloth for 30 minutes to remove burrs and uneven parts on the surface and improve the surface quality and dimensional accuracy of the composite ceramics.

[0053] It should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Those skilled in the art may modify the technical solutions described in the above embodiments, or make equivalent replacements for some of the technical features therein; and all these modifications and replacements should fall within the scope of protection of the claims attached to the present invention.

Claims

1. A method for preparing a composite ceramic for preparing an NTC thermistor, characterized in that: Including steps: Preparation of spinel phase raw materials: Mn, Cr, Y, Ni, La, and Co oxides are weighed in proportion, and a preliminary densified spinel phase powder is prepared by ball milling, drying, and pre-calcining; Preparation of perovskite phase raw materials: Weigh lanthanum nitrate, chromium nitrate, and yttrium nitrate in proportion and dissolve them in water. Add citric acid and stir until transparent to prepare a La-Y-Cr mixed solution. Formation of a core-shell structure: Spinel phase powder is dispersed in water, heated after dispersion, and a La-Y-Cr mixed solution and ammonia water are slowly added dropwise, with the pH value controlled between 8 and 9, to form a core-shell structure precipitate with a spinel core and La-Y-Cr hydroxide and citrate as the shell; after centrifugal washing, drying, and high-temperature calcination, the La-Y-Cr hydroxide and citrate shells are converted into a perovskite coating layer; Molding and sintering of composite ceramics: The generated core-shell structure powder is ball-milled, granulated, and dry-pressed to prepare a green body. The sintering temperature curve and atmosphere are controlled to allow the spinel core and perovskite coating to further react and densify, forming a composite ceramic with a tight core-shell structure.

2. The method for preparing composite ceramics according to claim 1, wherein: The preparation steps of the spinel phase raw material are specifically as follows: According to the chemical composition of spinel ceramics Mn 0.8～1.0 Co 1.8～2.0 Ni 0.03～0.07 Cr 0.03～0.07 La 0.05～0.08 Y 0.04～ 0.06 The atomic molar ratio of O4 was 1.5, and MnO2, CrO3, Y2O3, NiO, La2O3, and Co3O4 were weighed and the initially densified spinel phase powder was prepared by ball milling, drying, and pre-calcining.

3. The method for preparing composite ceramics according to claim 1, wherein: The preparation steps of the perovskite phase raw material are specifically as follows: According to the chemical composition of perovskite ceramics La 0.93～0.97 Cr 0.03～0.07 Y 0.95～1.05 The atomic molar ratio of O3 was adjusted by weighing La(NO3)3·6H2O, Cr(NO3)3·9H2O, and Y(NO3)3·6H2O and dissolving them in deionized water. Citric acid was added and stirred until transparent to prepare a La-Y-Cr mixed solution.

4. The method for preparing composite ceramics according to claim 1, wherein: In the step of forming the core-shell structure, the spinel phase powder is dispersed in water and then heated to 60° C.

5. The method for preparing composite ceramics according to claim 1, wherein: The composite ceramic forming and sintering steps further include the following steps: The sintered composite ceramic is subjected to ultrasonic cleaning, grinding and polishing to remove burrs and uneven parts on the surface.

6. The method for preparing composite ceramics according to claim 1, wherein: The steps of forming and sintering the composite ceramic are specifically as follows: The generated core-shell structure powder is ball-milled, granulated, and dry-pressed to prepare a green blank, which is then placed in a sintering furnace for sintering. The sintering temperature curve is: room temperature → 600°C (10°C / min) → 600°C (maintained for 2 hours) → 1150°C (15°C / min) → 1150°C (maintained for 10 minutes) → 1350°C (5°C / min) → 1350°C (maintained for 30 minutes) → natural cooling. The sintering atmosphere is nitrogen. During the sintering process, the spinel core and the perovskite coating layer further react and densify to form a composite ceramic with a tight core-shell structure.

7. The method for preparing composite ceramics according to claim 1, wherein: The forming pressure of the dry pressing is 200 MPa.

8. The method for preparing composite ceramics according to claim 7, characterized in that: In the steps of forming and sintering the composite ceramic, the dry pressing forming is specifically as follows: The core-shell structure powder is loaded into the metal mold cavity, and pressure is applied by the pressing head to rearrange and deform the core-shell structure powder particles in the mold cavity and compact them to form a green billet with a certain strength and shape. The formed green billet is then taken out of the mold.

9. The method for preparing composite ceramics according to claim 1, wherein: In the step of preparing the perovskite phase raw material, the ratio of the total amount of La, Y, and Cr to citric acid is 1:0.8 to 1:1.

2.

10. A composite ceramic for an NTC thermistor, characterized in that: Prepared by the preparation method according to any one of claims 1 to 9.

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