Core-shell structure carbon ceramic resistor material, preparation method and resistor

Abstract

The invention belongs to the technical field of electrical ceramics, and discloses a core-shell structure carbon ceramic resistor material, a preparation method and a resistor. The material takes a ceramic matrix as a main body, a carbon conductive phase is uniformly distributed on the surfaces of ceramic particles in a coating layer form to form ceramic core-carbon shell structural units with the thickness of 5-100nm, the structural units are mutually communicated through sintering to form a dual-scale continuous conductive network, and meanwhile, a gradient pore structure which is gradually reduced from the center to the surface is constructed in the material. The preparation method comprises the steps of preparation of a core-shell precursor, bidirectional unequal-pressure forming and segmented controllable sintering, and accurate regulation and control of a carbon phase form and a conductive network are realized through oriented coating and topological optimization sintering of an interface regulating agent assisted carbon source. The resistor material is small in resistance fluctuation, high in uniformity, excellent in large-current impact bearing capacity and long in thermal impact cycle life, can be used for a high-reliability closing resistor, and remarkably improves the stability and prolongs the service life of a resistor element under an extreme working condition.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electrotechnical ceramics, and particularly relates to a core-shell structure carbon ceramic resistance material, a preparation method and a resistance. BACKGROUND

[0002] In power electronic devices, high-voltage switches and power transmission and distribution systems, closing resistors or current limiting resistors are often arranged to suppress transient impact current and stabilize the operation state of the power grid. Carbon ceramic resistance materials are widely used as the base materials of such resistance elements due to their high heat resistance, good mechanical strength and certain electrical conductivity. The related resistance elements need to withstand large current density impact and frequent thermal cycle conditions during operation, and the performance stability thereof depends largely on the distribution form of the conductive phase in the carbon ceramic composite material and the bonding state thereof with the ceramic matrix. Therefore, how to obtain carbon ceramic composite materials with stable electrical conductivity and reliable structure has been a technical direction that the field has been continuously concerned about.

[0003] In the prior art, patent document CN115376769A discloses a carbon composite ceramic resistance sheet and a preparation method thereof. The technology forms a relatively uniform composite structure of the conductive phase in the ceramic matrix by mixing and forming an organic carbon source with a ceramic dielectric phase and a bonding system and then performing heat treatment, so as to obtain a carbon composite resistance material with certain electrical and mechanical properties. Such materials are usually prepared by mixing ceramic powder with carbon source, pressing and sintering. The conductive phase is generated from the carbon source during heat treatment and is distributed in the dielectric phase. The material as a whole has a porous structure and can meet the use requirements under certain power conditions.

[0004] However, in actual applications, the traditional carbon ceramic composite material still has some problems to be further improved. On the one hand, the existence form of the carbon conductive phase in the ceramic matrix is mostly random dispersion or local connection, and the uniformity of the distribution and the stability of the connection in the microscale are still affected by the process factors such as mixing, molding and sintering, and it is difficult to form a stable conductive network in the entire material volume. When the material works under the conditions of large current impact or rapid on-off cycle, the unevenness of the local conductive path is easy to cause current density concentration, and then cause local temperature rise and stress concentration, resulting in resistance value fluctuation or performance attenuation.

[0005] On the other hand, in the existing carbon ceramic resistance material, the controllability of the interface structure and the pore morphology of the carbon phase and the ceramic matrix is limited. In the traditional process, the pores formed are randomly distributed, and the size and spatial distribution lack effective regulation, which is not conducive to the uniform diffusion of current and heat in the material. Under the condition of high-energy pulse or frequent thermal shock, the heat in the material is not easy to release uniformly, which may aggravate the local structural damage, thereby affecting the reliability and service life of the resistance element. In addition, the migration, agglomeration or rearrangement of the conductive phase during sintering may also cause the resistance of the material to increase in dispersion and decrease in stability during long-term service.

[0006] Therefore, in view of the problems of insufficient stability of the conductive phase distribution, easy influence of the conductive network connectivity by the process, and large resistance fluctuation of the material under the conditions of large current and thermal cycle in the existing carbon ceramic resistance material, it is necessary to provide a carbon ceramic composite material capable of realizing uniform distribution of the conductive phase at the microscale and constructing a stable conductive structure, so as to improve the resistance stability and long-term use reliability of the material under the conditions of high current density and complex working conditions. SUMMARY

[0007] The purpose of the present application is to solve the problem that, in the existing carbon ceramic resistance material, the conductive phase can form a continuous structure, but the stability of the conductive path is insufficient under the conditions of large current impact and thermal cycle, and the local current and heat distribution is uneven, thereby causing large resistance fluctuation and insufficient reliability.

[0008] The purpose of the present application is achieved by adopting the following technical solutions: A core-shell structure carbon ceramic resistance material, comprising a ceramic matrix and a carbon conductive phase distributed in the ceramic matrix; the carbon conductive phase is in the form of a coating layer covering the surface of the ceramic particles to form a ceramic core-carbon shell structure unit, the thickness of the carbon coating layer is 5-100 nm, and the standard deviation of the thickness distribution of the carbon coating layer is not more than 20% of the average value.

[0009] Preferably, a plurality of the ceramic core-carbon shell structure units are connected to each other by sintering to form a double-scale continuous conductive network consisting of a primary conductive network composed of the carbon coating layer on the surface of the particles and a secondary percolation network composed of the interconnected carbon coating layers in the material.

[0010] Preferably, the pores in the material are gradiently distributed, and the average pore diameter gradually decreases from 1-5 μm in the interior of the material to 0.1-0.5 μm on the surface.

[0011] Preferably, in the current density range of 10-1000 A / cm 2 The fluctuation range of the resistance of the material is not more than ±10%.

[0012] Preferably, the carbon coating layer comprises an amorphous carbon or graphitized carbon layer formed by carbonization and graphitization treatment of a soluble carbon source.

[0013] Preferably, the ceramic matrix is a ceramic material formed of alumina, magnesia, zinc oxide or a combination thereof.

[0014] Based on the same inventive concept, the application further provides a preparation method of the core-shell structure carbon ceramic resistor material, characterized by comprising the following steps: S1, dispersing ceramic powder in a solution containing a soluble carbon source and an interface control agent, preferentially precipitating the carbon source on the surface of the ceramic powder by evaporation drying, and obtaining a core-shell structure precursor powder; S2, mixing the core-shell structure precursor powder with a pore-forming agent and a binder, and pressing a green body by using bidirectional unequal pressure forming, so that the green body forms a density gradient along the thickness direction; S3, segmenting and sintering the green body under an inert atmosphere, including a low-temperature carbonization section, a medium-temperature topology optimization section and a high-temperature densification section, to form the core-shell structure carbon ceramic resistor material.

[0015] Preferably, the interface control agent in step S1 is one or more of polyvinylpyrrolidone, sodium dodecyl benzene sulfonate or polyethylene glycol, and the addition amount is 0.1% to 1% of the mass of the ceramic powder.

[0016] Preferably, in the bidirectional unequal pressure forming process in step S2, the pressure difference applied by the upper punch and the lower punch is 10% to 50%.

[0017] Preferably, in step S3: the temperature of the low-temperature carbonization section is 400 to 600℃; the temperature of the medium-temperature topology optimization section is 800 to 1100℃ and the holding time is 1 to 4 hours; the temperature of the high-temperature densification section is 1350 to 1500℃.

[0018] Based on the same inventive concept, the application further provides a resistor comprising a resistor body and at least two electrodes arranged on the resistor body, wherein the resistor body is made of the core-shell structure carbon ceramic resistor material.

[0019] Preferably, the resistor is a closing resistor or a current limiting resistor used in a power system or a high-voltage switching device.

[0020] Compared with the prior art, the application has the following beneficial effects: The core-shell structure carbon ceramic resistance material of the present application forms a carbon coating layer with a thickness of 5-100 nm and controlled uniformity on the surface of ceramic particles, so that the carbon conductive phase no longer exists in the form of traditional random filling or agglomeration, but is continuously distributed on a microscale in the form of "ceramic core-carbon shell" structural units. This structure makes each ceramic particle be coated with a conductive carbon layer, so that a continuous conductive channel is formed by the interconnection of the particles and the carbon layer after sintering. The principle is that the nanoscale carbon coating layer can provide a stable carrier migration path, and by controlling the uniformity of the coating layer thickness, it can avoid excessive enrichment or lack of local conductive phase, so that the current is dispersed and transmitted in a multi-path way in the material body. When the standard deviation of the thickness distribution is not more than 20% of the average value, the resistance characteristics of the carbon layer on the surface of each particle tend to be consistent, thereby significantly reducing the dispersion of the macroscopic resistance value, and further obtaining the technical effects of small resistance fluctuation and high stability. The core-shell structure essentially changes the conductive network construction mechanism from "bulk phase filling" to "interface continuous coating", making the conductive path more stable and controllable, and being able to maintain uniform current distribution under high current impact conditions, reduce the risk of local overheating, and improve the reliability of the material.

[0021] On the basis of the above-mentioned core-shell structure, a primary conductive network composed of carbon coating layers on the surfaces of particles and a secondary percolation network formed by the mutual connection between the carbon layers are formed in the material by sintering, forming a double-scale continuous conductive network. The principle is that the primary network ensures the conductive continuity at the single particle level, and the secondary network establishes multi-directional current conduction paths at the particle group level, so that the current forms a network-like shunt structure in the material. When the material is subjected to high current density or pulse energy, the current can diffuse along multiple parallel paths, thereby reducing the local current density peak value and dispersing heat, avoiding local thermal stress concentration. The double-scale network structure has more topological redundancy than a single-scale conductive path, so that even if a local area produces microcracks or local degradation of the carbon phase due to thermal shock, the overall conductive network can still maintain connectivity, so that the material can maintain a stable state with smaller resistivity fluctuation in the range of 10-1000 A / cm 2 The formation mechanism of the network structure is derived from the migration, rearrangement and graphitization process of the carbon layer in the medium temperature sintering stage, which grows on the surface of the ceramic particles and forms a bridging structure in the particle contact area, thereby realizing the stable locking of the topological structure of the conductive network.

[0022] In addition, the application forms a gradient pore structure with gradually decreasing pore diameters from the center to the surface in the material by the two-way unequal pressure forming and the segmented sintering process. The principle is that the density gradient is converted into a pore distribution gradient in the sintering process, so that the material has a large-pore area in the interior to relieve thermal stress and accommodate thermal expansion, and a small-pore area in the surface to improve the structural density and mechanical strength. The gradient pore structure can form a heat diffusion channel that gradually conducts from the interior to the exterior when the current passes through, promote the rapid release of heat, reduce the local temperature rise, and thus improve the thermal shock resistance of the material under the conditions of thermal cycling and pulse current. At the same time, the pore gradient can also adjust the overall resistance path length and heat diffusion path of the material, so that the current and heat distribution are more uniform, further stabilizing the resistance value.

[0023] In the preparation method of the application, the preferential deposition of the carbon source on the surface of the ceramic particles is assisted by the interfacial regulator, ensuring that the carbon coating layer has high uniformity and continuity in the early stage of formation; the graphitization and network rearrangement of the carbon layer are promoted in the medium-temperature topological optimization sintering stage, so that the conductive path is more stable; and the carbon network structure is locked and excessive migration is prevented in the high-temperature short-time densification stage. The above process steps and material structure formation mechanisms form a synergistic effect, so that the carbon phase morphology, conductive network topology and pore structure are all in a controlled state, thereby macroscopically exhibiting the comprehensive technical effects of high resistance stability, strong impact bearing capacity and long service life. In summary, through the synergistic design of the core-shell structure, the double-scale conductive network and the gradient pore structure, the application realizes the logical closed loop from microstructure regulation to macroscopic performance stability, and significantly improves the reliability and stability of the carbon ceramic resistor material under the conditions of large current and thermal shock. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The electron microscope photo of the core-shell structure carbon ceramic resistor material obtained in Example 1 of the application; wherein: Print Mag represents the print magnification, FOV represents the field of view, WD represents the working distance, Det represents the detector type, Scan Mode represents the scanning mode, Energy represents the acceleration voltage; "10 μm" represents the scale of 10 microns, "1.00 kx" represents the magnification of 1000 times, "300 pA" represents the beam current of 300 picoamperes, "ET-T" is the type of secondary electron detector, "ANALYSIS" represents the analysis mode, and "15 kV" represents the acceleration voltage of 15 kilovolts.

[0025] Figure 2 The element distribution map of the core-shell structure carbon ceramic resistor material obtained in Example 1 of the application; the scale of each small graph is 2 μm; wherein, CH1: channel 1; C: carbon, O: oxygen, Al: aluminum, Si: silicon, Na: sodium, K: potassium, Ti: titanium, Fe: iron. DETAILED DESCRIPTION

[0026] The technical solutions are further described below in conjunction with the drawings and specific embodiments to help understand the content of the present application.

[0027] Embodiment 1 A core-shell structure carbon ceramic resistance material, comprising a ceramic matrix and carbon conductive phase distributed in the ceramic matrix; the carbon conductive phase forms a ceramic core-carbon shell structure unit in the form of a coating layer on the surface of ceramic particles, the thickness of the carbon coating layer is 5-100 nm, and the standard deviation of the thickness distribution of the carbon coating layer is not more than 20% of the average value.

[0028] Preferably, a plurality of the ceramic core-carbon shell structure units are connected to each other by sintering to form a double-scale continuous conductive network of a primary conductive network composed of the carbon coating layer on the surface of the particles and a secondary percolation network composed of the interconnected carbon coating layers in the material.

[0029] Preferably, the pores in the material are gradiently distributed, and the average pore diameter gradually decreases from 1-5 μm in the interior of the material to 0.1-0.5 μm on the surface.

[0030] Preferably, the fluctuation range of the resistivity of the material is not more than ±10% in the current density range of 10-1000 A / cm².

[0031] Preferably, the carbon coating layer comprises an amorphous carbon or graphitized carbon layer formed by carbonization and graphitization treatment of a soluble carbon source.

[0032] Preferably, the ceramic matrix is a ceramic material formed of alumina, magnesia, zinc oxide or a combination thereof.

[0033] A typical preparation process comprises the following steps: Take 400 g of kaolin, 500 g of alpha phase alumina, 100 g of gamma phase alumina as the ceramic matrix, add 800 mL of deionized water solution containing 35 g of sucrose and 5 g of polyvinylpyrrolidone (PVP, molecular weight 40000) into the planetary ball mill at a speed of 300 r / min for 4 hours to form a uniform suspension; then the suspension is evaporated and dried at 85°C under stirring, and the evaporation rate is controlled to make the sucrose and PVP work together to uniformly adsorb on the surface of each alumina particle to obtain a composite powder with the structure of "alumina core-organic carbon source shell". After dry mixing the composite powder with 15 g of polymethyl methacrylate (PMMA) pore-forming agent (particle size 5 μm), 10 g of polyvinyl alcohol binder, it is placed in a mold and formed by two-way unequal pressure molding: the upper punch pressure is 120 MPa, the lower punch pressure is 80 MPa, and the pressure is maintained for 120 seconds to press the green body into a diameter of 50 mm and a thickness of 10 mm, so that the density of the green body forms a gradient from the center to the surface. The green body is placed in a sintering furnace under flowing argon protection, first heated to 550°C at a rate of 2°C / min and held for 2 hours to make the sucrose and PMMA pyrolyze and form an amorphous carbon coating layer on the surface of the alumina particles and create initial pores; then heated to 950°C at a rate of 5°C / min and held for 3 hours, during which the amorphous carbon undergoes structural rearrangement and preliminary graphitization, and migrates and connects between particles to optimize the topological connectivity of the conductive network; finally, heat to 1420°C at a rate of 10°C / min and sinter for 30 minutes to densify the ceramic matrix and lock the carbon network structure, then cool in the furnace to obtain the high uniformity carbon ceramic composite material. The thickness of the carbon coating layer in the material is about 15±2 nm, and the pore size in the cross-section shows a gradient distribution from the inside to the outside (1.5 μm to 0.3 μm), and the double-scale continuous conductive network makes the resistance fluctuation rate less than ±8%.

[0034] Figure 1 The scanning electron microscope (SEM) image of the core-shell structure carbon ceramic resistor material obtained in this embodiment shows a typical three-dimensional structure characteristic of multi-level particle aggregation and pore coexistence, which is consistent with the expected morphology of the conductive network and gradient pore system of the core-shell structure carbon ceramic resistor material after sintering. A large number of ceramic particle skeletons with a size of 1-3 μm can be observed in the image, and these particles are not completely sintered but form a continuous support structure through neck connection, and there is a relatively fine secondary particle or thin layer of material covering the surface of the particles and the inter-particle region. From the contrast and morphology, these fine and small undulating structures attached to the surface of the ceramic particles can correspond to the carbon coating layer formed after sintering and the carbon network bridging region formed by local rearrangement. This kind of interface layer usually migrates and restructures at the medium temperature graphitization stage, forming a continuous conductive channel between particles and thus building a stable double-scale conductive network.

[0035] Further observation shows that there is a clear pore connectivity structure inside the material, with pore size mainly concentrated in the sub-micron to several micron range, and the pores are not isolated but interconnected. This pore morphology is conducive to heat diffusion and stress release during power-on and thermal cycling. From the structure, larger pores are usually located inside the particle agglomeration region, while the pores on the surface of the particles and in the outer region are relatively fine, and the overall trend is to gradually refine from the inside to the surface, which is consistent with the density gradient-pore gradient structure formed by bidirectional unequal pressure molding and segmented sintering. Such gradient pores can provide a thermal buffer zone when the current is impacted, allowing heat to diffuse along multiple paths, reducing local temperature rise, and thus improving the stability of the material under high current density conditions.

[0036] From the particle interface characteristics, the connection area between the ceramic particles does not appear a clear large-size molten or glass phase continuous coating, but mainly neck connection and small particle bridging, indicating that the high-temperature densification stage realizes the stable combination of the matrix skeleton, while the topological structure of the conductive phase network is preserved. This structure is conducive to maintaining the continuity of the conductive path, while avoiding excessive densification leading to thermal stress concentration. In the local area, relatively smooth or sheet-like attached structures can be seen, which may correspond to the rearrangement area of the carbon phase after graphitization at high temperature. Such structures are usually helpful to reduce the interface resistance and improve the uniformity of electrical conductivity.

[0037] Combining the image information, it can be judged that the material forms a three-dimensional network system at the microscale, with ceramic particles as the skeleton, carbon coating layer and its bridging structure as the conductive path, and pores as the stress buffer and heat dissipation channel. Such structure is conducive to current dispersion and uniform heat transfer during power-on, thereby reducing the local current density peak and thermal stress concentration. Combined with the material performance test results (such as lower resistance fluctuation rate and higher thermal shock cycle life), the microstructure reflected by the electron microscope image is consistent with the expected core-shell structure, double-scale conductive network and gradient pore structure, indicating that the process parameters have achieved a relatively ideal structural effect in regulating the carbon phase distribution and the stability of the conductive network.

[0038] Figure 2The element distribution map of the core-shell structure carbon ceramic resistor material obtained in the embodiment is shown in Figure 6. As can be seen from the element distribution map, the spatial distribution of C, O, Al, Si, Na, K, Ti, Fe and other elements in the material has obvious hierarchical and interface characteristics on the microscale, which is consistent with the formation mechanism of the core-shell structure carbon ceramic resistor material described in the present application. First, observe the C element distribution map. The carbon element is in a continuous and dispersed covering state in the entire field of view, and forms a relatively uniform ring-shaped distribution at the boundaries of ceramic particles and between particles, rather than being concentrated in isolated particles or agglomerated blocks. This distribution pattern shows that the carbon phase is mainly attached to the surface of the ceramic particles in the form of a coating, forming a bridge between the particles to build a continuous conductive channel. Combined with the original SEM morphology map, it can be inferred that the high-density region of the carbon element corresponds to the outer surface profile of the ceramic particles, indicating that the carbon source is preferentially deposited on the surface of the ceramic particles during the precursor stage, and carbonizes and graphitizes during the subsequent heat treatment process to form a coating layer. This phenomenon is consistent with the formation characteristics of the "ceramic core-carbon shell" structural unit, that is, an interface-type conductive structure with ceramic particles as the core and carbon phase as the shell.

[0039] In addition, the C element forms a continuous network or bridge-like distribution between the particles, interwoven with the particle skeleton formed by Al and O, indicating that the carbon coating layer not only exists on the surface of single particles after sintering, but also connects in the particle contact area to form a multi-path conductive network. This structural feature is an important basis for building a double-scale continuous conductive network: on the one hand, the carbon shell on the surface of single particles constitutes a primary conductive path; on the other hand, the carbon layer between particles constitutes a secondary percolation network. The continuity and uniformity of the carbon signal in the element distribution map indicate that the conductive phase has not appeared obvious agglomeration or local absence, thereby facilitating the uniform dispersion of current when a large current passes through and reducing the local current density concentration.

[0040] By combining the element distribution maps, it can be judged that the material forms a composite system on the microscale, with an Al and O-based ceramic particle core structure, a continuous coating layer formed by C elements on the outside, and a conductive network formed by carbon bridges between particles. This element distribution characteristic is consistent with the technical route of the present application, which forms a core-shell structure and a double-scale conductive network by interface control deposition of carbon source and staged sintering, confirming the "ceramic core-carbon shell" structural unit of the core-shell structure carbon ceramic resistor material of the present application. Such interface-type conductive structure can improve the stability of the conductive network and reduce the resistance fluctuation while maintaining the mechanical strength of the ceramic matrix, thereby providing a structural basis for stable operation of the material under high current impact and thermal cycling conditions.

[0041] The present application effectively realizes nanoscale dispersion of carbon conductive phase in the ceramic matrix by constructing ceramic core-carbon shell structure units and precisely controlling the thickness (5-100 nm) and distribution uniformity of the carbon coating layer, and avoids local uneven conduction. In combination with the construction of a double-scale continuous conductive network and internal gradient pore distribution, the connectivity and structural stability of the conductive path are significantly enhanced. The material exhibits excellent resistance linearity in a wide current density range of 10-1000 A / cm², with a resistance fluctuation of not more than ±10%, and has high current resistance and high reliability.

[0042] The present application realizes nanoscale uniform coating of carbon source on the surface of alumina particles through synergistic interface regulation of sucrose and PVP and controlled evaporation drying process, obtains a highly consistent carbon shell layer with a thickness of only 15±2 nm, and effectively eliminates the resistance difference caused by uneven distribution of conductive phase; in combination with bidirectional unequal pressure forming and segmented sintering process including a "medium temperature topology optimization section", a gradient pore structure with decreasing pore diameter from inside to outside and a stable double-scale continuous conductive network are constructed, which significantly improves the connectivity and stability of the conductive network while ensuring the densification of the ceramic matrix, and finally makes the material resistance fluctuation rate less than ±8%, showing excellent electrical performance consistency.

[0043] Embodiment 2-7 Based on the same inventive concept, the present application also provides a preparation method of the core-shell structure carbon ceramic resistor material according to Embodiment 1, characterized in that it comprises the following steps: S1, dispersing ceramic powder in a solution containing soluble carbon source and interface regulator, and making the carbon source preferentially precipitate and coat on the surface of the ceramic powder through evaporation drying to obtain a core-shell structure precursor powder; S2, mixing the core-shell structure precursor powder with a pore former and a binder, and pressing into a green body by bidirectional unequal pressure forming, so that the green body forms a density gradient along the thickness direction; S3, performing segmented sintering on the green body in an inert atmosphere, including a low-temperature carbonization section, a medium-temperature topology optimization section and a high-temperature densification section, to form the core-shell structure carbon ceramic resistor material.

[0044] Preferably, the interface regulator in step S1 is one or more of polyvinylpyrrolidone, sodium dodecylbenzenesulfonate or polyethylene glycol, and the addition amount is 0.1%-1% of the mass of the ceramic powder.

[0045] Preferably, in the bidirectional unequal pressure forming process in step S2, the pressure difference applied by the upper punch and the lower punch is 10%-50%.

[0046] Preferably, in step S3: The temperature of the low-temperature carbonization section is 400-600℃; The temperature of the medium temperature topological optimization section is 800-1100°C and the temperature is kept for 1-4 hours; The temperature of the high temperature densification section is 1350-1500°C.

[0047] The parameters and test data of each of Examples 2-7 are shown in Table 1 below, and other necessary technical contents not mentioned are consistent with Example 1 or are prior art known in the art, and thus will not be described again.

[0048] Table 1 Process parameters and test data of Examples 2-7

[0049] From the test results of each example, the thermal shock cycle life shows a certain difference between about 3500 times and 6800 times, which has obvious correlation with the combination of process parameters such as the thickness of the carbon coating layer, the sintering system, the pressing density gradient and the pore structure used in each example. Through the microstructure observation and electrical performance test of each example material, it can be known that when the thickness of the carbon coating layer is in the middle range and the uniformity is good, the continuous carbon shell formed on the surface of the ceramic particles can construct a stable conductive network after sintering, so that the current is distributed in multiple paths in the material, thereby reducing the local current density and the degree of heat accumulation. Under this condition, the thermal stress generated in the process of repeated on-off current or temperature cycle can be more evenly dispersed, and the microcrack initiation and expansion speed is relatively slow, and the corresponding thermal shock cycle life usually shows a high level.

[0050] By comparing different examples, it can be found that when the thickness of the carbon coating layer is low or the local uniformity is slightly poor, the connectivity of the conductive network on the microscale is relatively weakened, and the material is more prone to local conductive path interruption or thermal stress concentration during multiple thermal cycles, resulting in accelerated resistance change speed, thereby the thermal shock life is near the lower limit of the interval. On the other hand, when the sintering temperature and holding time are optimized, the densification degree of the ceramic matrix is improved, and at the same time, the connection structure between the carbon coating layers is fully rearranged at the medium temperature stage and is stably locked at the high temperature stage, forming a relatively stable double-scale conductive network. In this case, the material can maintain a relatively stable current distribution and heat diffusion path under high current density impact, and the thermal shock cycle life is correspondingly improved.

[0051] In addition, the density gradient formed by the two-way unequal pressure forming of the embodiments is converted into a gradient pore structure after sintering, which has a certain influence on the thermal stress relief and heat dissipation capacity of the material. The relatively large pore size inside helps to absorb thermal expansion stress, while the small pore size on the surface improves the overall structural strength and uniformity of heat conduction. Experimental results show that when the pore gradient distribution is reasonable, the temperature gradient generated by the material in the on-off cycle is relatively small, which is conducive to delaying the expansion of micro-cracks, thereby showing a high thermal shock cycle life. If the pore gradient is slightly weak or the density distribution is biased towards a single value, the local stress relief capacity of the material in the thermal cycle is relatively low, and the corresponding life is in a lower range.

[0052] In summary, the difference in the values of the thermal shock life in the embodiments can be understood as the result of the synergistic effect of various process parameters. The thickness and uniformity of the carbon coating layer, the sintering regime, the density gradient, and the pore structure, etc. factors jointly affect the stability of the conductive network and the thermal stress distribution, thereby showing a certain range of life differences in experimental tests. This difference reflects the performance variation trend of the material under different process combinations, and the overall variation range is still within a reasonable range of the same technical system, without abnormal fluctuations that are obviously inconsistent with the material structure and physical mechanism. Therefore, it can be considered that the difference in the relevant performance parameters has experimental rationality and interpretability.

[0053] Embodiment 8 Based on the same inventive concept, the application further provides an electric resistance, comprising an electric resistance body and at least two electrodes arranged on the electric resistance body, wherein the electric resistance body is made of the core-shell structure carbon ceramic electric resistance material according to any one of embodiments 1-7.

[0054] Preferably, the electric resistance is a closing resistance or a current-limiting resistance used in a power system or a high-voltage switching device.

[0055] The above is only an embodiment of the application and is not intended to limit the application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the application shall be included in the scope of the claims of the application.

Claims

1. A core-shell structured carbon ceramic resistive material, characterized in that, It includes a ceramic matrix and a carbon conductive phase distributed in the ceramic matrix; the carbon conductive phase is coated on the surface of ceramic particles in the form of a coating layer to form a ceramic core-carbon shell structure unit, the thickness of the carbon coating layer is 5 to 100 nm, and the standard deviation of the thickness distribution of the carbon coating layer is not greater than 20% of its average value.

2. The core-shell structured carbon ceramic resistive material according to claim 1, characterized in that, Multiple ceramic core-carbon shell structural units are sintered and interconnected to form a two-scale continuous conductive network inside the material, consisting of a primary conductive network composed of a carbon coating layer on the particle surface and a secondary percolation network composed of interconnected carbon coating layers.

3. The core-shell structured carbon ceramic resistive material according to claim 2, characterized in that, The material has a gradient distribution of pores, with the average pore size gradually decreasing from 1 to 5 μm to 0.1 to 0.5 μm from the interior to the surface.

4. The core-shell structured carbon ceramic resistive material according to claim 2, characterized in that, 10–1000 A / cm 2 Within the current density range, the fluctuation range of the material resistivity does not exceed ±10%.

5. The core-shell structured carbon ceramic resistive material according to claim 1, characterized in that, The carbon coating layer includes an amorphous carbon or graphitized carbon layer formed by carbonization and graphitization of a soluble carbon source.

6. The core-shell structured carbon ceramic resistive material according to claim 1, characterized in that, The ceramic matrix is ​​a ceramic material formed from alumina, magnesium oxide, zinc oxide, or a combination thereof.

7. A method for preparing the core-shell structured carbon ceramic resistive material according to any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Ceramic powder is dispersed in a solution containing a soluble carbon source and an interface modifier. The carbon source is preferentially precipitated and coated on the surface of the ceramic powder by evaporation and drying to obtain a core-shell structure precursor powder. S2. The core-shell structure precursor powder is mixed with a pore-forming agent and a binder, and then pressed into a green body using bidirectional unequal pressure molding, so that the green body forms a density gradient along the thickness direction. S3. The green body is sintered in an inert atmosphere in stages, including a low-temperature carbonization stage, a medium-temperature topology optimization stage, and a high-temperature densification stage, to form the core-shell structured carbon ceramic resistive material.

8. The preparation method according to claim 7, characterized in that, The interface regulator mentioned in step S1 is one or more of polyvinylpyrrolidone, sodium dodecylbenzenesulfonate, or polyethylene glycol, and its addition amount is 0.1% to 1% of the mass of ceramic powder.

9. The preparation method according to claim 7, characterized in that, In step S2, during the bidirectional unequal pressure forming process, the pressure difference applied by the upper punch and the lower punch is 10% to 50%.

10. The preparation method according to claim 7, characterized in that, In step S3: The temperature of the low-temperature carbonization section is 400–600℃; The temperature of the intermediate-temperature topology optimization section is 800–1100℃ and held for 1–4 hours; The temperature of the high-temperature densification section is 1350–1500℃.

11. A resistor, characterized in that, It includes a resistor body and at least two electrodes disposed on the resistor body, wherein the resistor body is made of the core-shell structured carbon ceramic resistive material as described in any one of claims 1 to 6.

12. The resistor according to claim 11, characterized in that, The resistor is a closing resistor or a current-limiting resistor used in power systems or high-voltage switchgear.

Images