Conductive silicon carbide-based composite ceramic and preparation method thereof

Abstract

The embodiment of the invention relates to the technical field of ceramics, in particular to conductive silicon carbide-based composite ceramic and a preparation method thereof. The conductive silicon carbide-based composite ceramic comprises the following components in parts by weight: 75-95 parts of silicon carbide and 5-25 parts of niobium diboride, the niobium diboride is generated by reaction of boron powder and niobium powder, and the mass ratio of the boron powder to the niobium powder is 1: 4. The conductivity and fracture toughness of the silicon carbide-based ceramic are improved.

Description

Technical Field

[0001] The embodiments of the present application relate to the field of ceramic technology, and specifically to a conductive silicon carbide-based composite ceramic and a preparation method thereof. Background Art

[0002] Silicon carbide-based ceramics are high-temperature-resistant, highly stable inorganic non-metallic materials widely used in aerospace, metallurgy, semiconductors, and chemical industries. Their electrical conductivity is particularly crucial in electrical discharge machining (EDM), semiconductor devices, and high-temperature heating elements. In these applications, the excellent electrical conductivity of conductive silicon carbide-based composite ceramics can reduce energy loss, improve processing and thermal efficiency, and directly impact the performance of semiconductor devices. Therefore, improving the electrical conductivity of silicon carbide-based ceramics is a pressing issue. Summary of the Invention

[0003] In view of the above problems, the embodiments of the present application provide a conductive silicon carbide-based composite ceramic and a preparation method thereof, which improve the conductivity of the silicon carbide-based ceramic.

[0004] According to one aspect of an embodiment of the present application, a conductive silicon carbide-based composite ceramic is provided, comprising, by weight, 75-95 parts of silicon carbide and 5-25 parts of niobium diboride, wherein the niobium diboride is produced by reacting boron powder and niobium powder, wherein the mass ratio of boron powder to niobium powder is 1:4.

[0005] In an optional manner, in parts by weight, the boron powder is 1-5 parts, and the niobium powder is 4-20 parts.

[0006] In an optional manner, in parts by weight, the silicon carbide is 75-90 parts, and the niobium diboride is 10-25 parts.

[0007] In an optional manner, in parts by weight, the silicon carbide is 75-85 parts, and the niobium diboride is 15-25 parts.

[0008] In an optional manner, the conductive silicon carbide-based composite ceramic further comprises, by weight, 8 parts of a binder, 2 parts of a dispersant, and 5 parts of a lubricant.

[0009] In an optional manner, the conductive silicon carbide-based composite ceramic further comprises 2 parts of boron carbide in parts by weight.

[0010] According to another aspect of an embodiment of the present application, a method for preparing a conductive silicon carbide-based composite ceramic is provided, comprising the following steps:

[0011] S100, using silicon carbide powder, boron powder, niobium powder, a binder, a dispersant, and a lubricant as raw materials and anhydrous ethanol as a solvent, ball milling in a ball mill to obtain a slurry, wherein, by weight, the silicon carbide powder is 75-95 parts, the boron powder is 1-5 parts, the niobium powder is 4-20 parts, the binder is 8 parts, the dispersant is 2 parts, and the lubricant is 5 parts. The mass of the anhydrous ethanol is 1.2 times the mass of the raw materials;

[0012] S200, drying, crushing, sieving and granulating the slurry to obtain ceramic powder;

[0013] S300, dry-pressing the ceramic powder to obtain a green body;

[0014] S400, sintering the green body in a sintering furnace to obtain a conductive silicon carbide-based composite ceramic.

[0015] In an optional manner, the particle size of the silicon carbide powder is 0.5-0.7 microns, the particle size of the boron powder is 0.5-1 microns, and the particle size of the niobium powder is 1 micron.

[0016] In an optional manner, based on weight, the silicon carbide is 75-85 parts, the boron powder is 3-5 parts, and the niobium powder is 12-20 parts.

[0017] In an optional manner, in S100, 2 parts by weight of boron carbide are further added during ball milling in the ball mill jar.

[0018] This application enhances the conductivity of the prepared conductive silicon carbide-based composite ceramic by adding conductive niobium diboride to the first phase material of silicon carbide. The niobium diboride is generated by reacting boron powder and niobium powder. This improves the performance stability of the conductive silicon carbide-based composite ceramic and enhances the flexural strength and fracture toughness of the conductive silicon carbide-based composite ceramic. Moreover, compared with directly adding niobium diboride, the niobium diboride generated by the reaction of boron powder and niobium powder is more evenly distributed in the silicon carbide matrix and can exert a better conductive effect.

[0019] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 XRD patterns of SiC-based composite ceramics with different NbB2 contents;

[0021] Figure 2 The morphology of SiC-based composite ceramic materials with different NbB2 contents after corrosion;

[0022] Figure 3 The morphology of the sample after N3 corrosion and the EDS element distribution map;

[0023] Figure 4 This is the fracture morphology of sample N4, i.e., Example 4;

[0024] Figure 5 The resistivity variation diagram of SiC-based composite ceramics with different NbB2 contents;

[0025] Figure 6 The relationship between current density and electric field intensity of samples with different NbB2 contents;

[0026] Figure 7 The AC impedance diagram of samples with different NbB2 contents;

[0027] Figure 8 A rough schematic diagram of the two conduction mechanisms: electroosmosis and electron tunneling;

[0028] Figure 9 Schematic diagram of topological transformation of SiC composite ceramic structure;

[0029] Figure 10 is the topological theoretical resistivity of samples with different NbB2 contents;

[0030] Figure 11 The density and compactness of samples with different NbB2 contents are changed;

[0031] Figure 12 is the flexural strength (σ) and elastic modulus (ε) of samples with different NbB2 contents;

[0032] Figure 13 is the change in fracture toughness of samples with different NbB2 contents;

[0033] Figure 14 is a diagram of the toughening mechanism of the sample. DETAILED DESCRIPTION

[0034] The following embodiments of the technical solution of the present application are described in detail. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only used as examples and are not intended to limit the scope of protection of the present application.

[0035] Silicon carbide-based ceramics are advanced inorganic non-metallic materials typically produced through high-temperature sintering processes. The crystal structure of silicon carbide-based ceramics allows them to remain stable in many extreme environments. Silicon carbide-based ceramics have a wide range of applications, including the manufacture of high-temperature structural components and engine parts in the aerospace industry, as lining materials for high-temperature furnaces in the metallurgical industry, as substrates or heat sinks for semiconductor devices, as materials for cutting tools and abrasives in machining, and for corrosion- and wear-resistant containers and pipelines in the chemical industry.

[0036] In some applications, such as electrical discharge machining (EDM), semiconductor devices, and high-temperature heating elements, SiC-based ceramics require electrical conductivity. For example, in EDM, conductive SiC-based ceramics can be used as tool electrodes, leveraging their electrical conductivity and high-temperature resistance for precision machining. In semiconductor devices, conductive SiC-based ceramics can serve as substrates. In high-temperature heating elements, conductive SiC-based ceramics can withstand high temperatures and effectively conduct current, enabling heating and temperature control.

[0037] In these applications, the excellent electrical conductivity of conductive silicon carbide-based ceramics can enhance performance. For example, in fields such as electrical discharge machining (EDM), the excellent electrical conductivity of silicon carbide-based ceramics can reduce resistance, minimize energy loss, and improve machining efficiency. In high-temperature heating elements, the excellent electrical conductivity of silicon carbide-based ceramics helps reduce heat loss and improve thermal efficiency. In semiconductor devices, the electrical conductivity of silicon carbide-based ceramics directly affects the device's electrical performance, such as switching speed and power handling capability.

[0038] Therefore, how to improve the conductivity of silicon carbide-based ceramics is an urgent problem to be solved.

[0039] Based on this, the present application provides a conductive silicon carbide-based composite ceramic. Based on silicon carbide as the first phase material, niobium diboride, a second phase material with excellent conductive properties, is added. This enhances the conductivity of the prepared conductive silicon carbide-based composite ceramic, improves the performance stability of the conductive silicon carbide-based composite ceramic, and improves the flexural strength and fracture toughness of the conductive silicon carbide-based composite ceramic. Furthermore, the niobium diboride is generated by reacting boron powder and niobium powder. Compared with directly adding niobium diboride, the generated niobium diboride is more evenly distributed in the silicon carbide matrix, which can achieve a better conductive effect.

[0040] An embodiment of the present application provides a conductive silicon carbide-based composite ceramic, which comprises 75-95 parts of silicon carbide (SiC) and 5-25 parts of niobium diboride (NbB2) by weight.

[0041] Among them, silicon carbide is the first phase material and the base material of silicon carbide-based ceramics.

[0042] Niobium diboride is a second-phase material with excellent electrical conductivity. By doping niobium diboride into silicon carbide, the electrical conductivity of silicon carbide-based ceramics can be significantly improved.

[0043] Preferably, niobium diboride is produced by reacting boron powder and niobium powder, wherein the mass ratio of boron powder to niobium powder is 1:4.

[0044] Preferably, in parts by weight, silicon carbide is 75-90 parts and niobium diboride is 10-25 parts. The conductive silicon carbide-based composite ceramic with the above ratio has better electrical conductivity.

[0045] More preferably, in parts by weight, silicon carbide accounts for 75-85 parts and niobium diboride accounts for 15-25 parts. The conductive silicon carbide-based composite ceramic with the above ratio has better electrical conductivity.

[0046] In some embodiments, the conductive silicon carbide-based composite ceramic further includes 8 parts of a binder, 2 parts of a dispersant, and 5 parts of a lubricant, based on weight.

[0047] Binder can reduce the friction between powders, improve powder fluidity, make material distribution uniform, and strengthen the formability of powder, prevent powder agglomeration, and can also improve the intensity of green blank, prevent the blank from breaking or deforming when carrying or processing. In addition, binder can also promote sintering, reduce sintering temperature, shorten sintering time, thereby improving production efficiency. Binder can adopt polymers such as phenolic resin, polyvinyl alcohol, ammonium polyacrylate, preferably adopt phenolic resin.

[0048] Adding a dispersant prevents powder particles from agglomerating and ensures uniform dispersion within the slurry, thereby improving the uniformity of the green body and the final properties of the ceramic. Dispersants can be surfactants such as sodium polyacrylate, polyoxyethylene ether, sodium dodecyl sulfate (SDS), organic polymers such as polyacrylic acid, polyvinyl alcohol, and polyethylene glycol, or high molecular weight polymers such as polyethyleneimine and polyacrylamide, with polyethylene glycol being preferred.

[0049] Lubricants reduce friction between powders and improve powder fluidity, facilitating uniform mixing and compaction, while minimizing cracks and defects during the molding process. Lubricants can be paraffin wax, stearic acid, polyvinyl alcohol, and oleic acid, which form a lubricating film on the surface of powder particles, reducing interparticle adhesion. Polyvinyl alcohol is preferred.

[0050] In some embodiments, the conductive silicon carbide-based composite ceramic further includes 2 parts by weight of boron carbide (B4C).

[0051] Boron carbide reacts with silicon carbide at high temperatures to form a liquid phase. This liquid phase helps lower the sintering temperature, promotes interparticle bonding, and increases material density, thereby achieving densification, improving sintering efficiency, and reducing energy consumption. The formation of the liquid phase also helps fill interparticle gaps, reducing porosity, and increasing the density and strength of the ceramic. Boron carbide can exist as a tiny second phase in the silicon carbide matrix, helping to inhibit crack propagation. Therefore, its addition can also improve the mechanical properties of silicon carbide ceramics, such as flexural strength and fracture toughness. Boron carbide also has a certain degree of oxidation resistance, which can protect the performance of silicon carbide ceramics in high-temperature environments and reduce oxidation losses. In certain applications, it may be necessary to adjust the electrical conductivity of silicon carbide ceramics. The addition of boron carbide can modify the conductive properties of the ceramic, making it more suitable for specific applications, such as electrical discharge machining.

[0052] It should be noted that excessive addition of boron carbide may increase the brittleness of the ceramic or reduce other properties. In the embodiment of the present application, the amount of boron carbide added is 2 parts, which can achieve a better performance balance.

[0053] The method for preparing the conductive silicon carbide-based composite ceramic of the embodiment of the present application comprises the following steps:

[0054] S100 uses silicon carbide powder (particle size 0.5-0.7 μm), boron powder (particle size 0.5-1 μm), niobium powder (particle size 1 μm), a binder, a dispersant, and a lubricant as raw materials, and anhydrous ethanol as the solvent. The materials are ball-milled in a jar to produce a slurry. The slurry comprises 75-95 parts silicon carbide powder, 1-5 parts boron powder, 4-20 parts niobium powder, 8 parts binder, 2 parts dispersant, and 5 parts lubricant, with the mass of anhydrous ethanol being 1.2 times the mass of the silicon carbide powder, boron powder, niobium powder, binder, dispersant, and lubricant (hereinafter referred to as the raw materials).

[0055] In this step, silicon carbide grinding balls are added during ball milling, and anhydrous ethanol (1.2 times the mass of the raw material) is added for wet milling. The ball mill can be a polyurethane mill. The weight of the silicon carbide grinding balls is twice that of the raw material, and the particle size is 5 mm. The ball mill speed is 220 rpm, and the milling time is 48 hours.

[0056] S200, drying, crushing, sieving and granulating the slurry to obtain ceramic powder.

[0057] In this step, the product can be dried in a constant temperature drying oven at 78° C. for 7 h to form dry cracked blocks, which are then ground and granulated through an 80-mesh sieve.

[0058] S300, dry-pressing the ceramic powder to obtain a green body.

[0059] In this step, the sieved ceramic powder is sealed and aged for 24 hours before being pressed at a pressure of 100 MPa to obtain a green blank with uniform size and appropriate density. The aging process allows for sufficient contact between the powder particles, improving their forming properties.

[0060] S400, sintering the green body in a sintering furnace to obtain a conductive silicon carbide-based composite ceramic.

[0061] In this step, the formed green blank can be subjected to an isostatic pressing treatment (pressure of 120MPa) to further improve the density and uniformity of the green blank. Subsequently, the green blank is placed in a vacuum sintering and debonding integrated furnace, using argon (Ar) as a protective gas to prevent oxidation. The sintering temperature is set to 2100℃ and the holding time is 1h. Through the above sintering process, the densification process is completed, and finally a conductive silicon carbide-based composite ceramic is obtained.

[0062] In some embodiments, 2 parts of boron carbide powder may be added for ball milling in step S100. Boron carbide as a sintering aid can increase the material density, improve the efficiency of the sintering process, and increase the density and strength of the ceramic.

[0063] The present application will be further described in detail below with reference to specific examples. Table 1 shows the components and contents of the silicon carbide-based ceramics of various examples and comparative examples, with the units in the table being parts by weight.

[0064] Table 1

[0065]

[0066]

[0067] Example 1:

[0068] S100, using silicon carbide powder (particle size 0.6 μm), boron powder (particle size 0.5 μm-1 μm), niobium powder (particle size 1 μm), boron carbide powder (particle size 1 μm), phenolic resin binder, polyethylene glycol dispersant and polyvinyl alcohol lubricant with the components and contents shown in Table 1 Example 1 as raw materials, using anhydrous ethanol (1.2 times the mass of the raw materials) as solvent, adding silicon carbide grinding balls (particle size 5 mm) (2 times the mass of the raw materials) in a polyurethane ball mill jar for wet ball milling at a ball mill speed of 220 r / min and a ball milling time of 48 h to obtain a slurry.

[0069] In step S200, the slurry is dried in a constant temperature drying oven at 78° C. for 7 h to form dry blocks, which are then ground and granulated through an 80-mesh sieve to obtain ceramic powder.

[0070] S300: After the ceramic powder is sealed and aged for 24 hours, it is pressed into shape by a hydraulic press at a pressure of 100 MPa to obtain a green body with uniform size and appropriate density.

[0071] S400, the green body is subjected to an isostatic pressing treatment (pressure of 120 MPa) and sintered in an ultra-high temperature vacuum sintering furnace at a sintering temperature of 2100°C and kept warm for 1 hour to obtain a conductive silicon carbide-based composite ceramic sample, sample numbered N1.

[0072] Example 2:

[0073] The raw materials are shown in Example 2 in Table 1, the preparation steps are the same as those in Example 1, and the sample number is N2.

[0074] Example 3:

[0075] The raw materials are shown in Example 3 in Table 1, the preparation steps are the same as in Example 1, and the sample number is N3.

[0076] Example 4:

[0077] The raw materials are shown in Example 4 in Table 1, the preparation steps are the same as in Example 1, and the sample number is N4.

[0078] Example 5:

[0079] The raw materials are shown in Example 5 in Table 1, the preparation steps are the same as in Example 1, and the sample number is N5.

[0080] Comparative Example:

[0081] The raw materials are shown in Table 1. The preparation steps are the same as those in Example 1. The sample number is 0.

[0082] First, the phase compositions of the above samples N1 to N5 are analyzed. Figure 1 The following are XRD patterns of SiC-based composite ceramics with different NbB2 contents. XRD (X-ray Diffraction) is an analytical technique based on the interaction between X-rays and crystalline materials. When X-rays irradiate a crystal, they diffract at specific angles, forming characteristic diffraction peaks. The position and intensity of these diffraction peaks are closely related to the crystal's lattice structure. By analyzing the XRD pattern, information such as the material's crystal structure, phase composition, lattice parameters, and crystallinity can be determined.

[0083] like Figure 1As shown in the figure, XRD patterns of the NbB2 / SiC-based composite ceramic material reveal that its main phases are 6H-SiC, 4H-SiC, and NbB2, with a small amount of C. 4H-SiC is converted from 6H-SiC at high temperatures (greater than 2000°C), while C is a residual C phase obtained by the decomposition of phenolic resin and other organic matter. The figure shows that as the NbB2 content gradually increases, the diffraction peak positions of the main component phases remain unchanged, with no left or right shifts. This indicates that SiC and NbB2 are well-matched, do not dissolve in each other, and lack significant stress within the material, maintaining structural integrity and performance stability even in high-temperature environments.

[0084] Next, the microstructures of the above samples N1 to N5 are analyzed. Figure 2 The morphology of SiC-based composite ceramic materials with different NbB2 contents after corrosion, specifically the surface morphology after the material was polished and then corroded with sodium hydroxide at 600℃. In the figure, (a) is sample 0, (b) is sample N4, and (c) is sample N5. Figure 2 The microscopic morphology reveals cavities left by the eroded grains, and the long, columnar shape of the sintered SiC grains. Grain size analysis using nanomeasurer software reveals a significant grain refinement after the addition of the conductive phase (4.19 μm for sample 0 and 3.16 to 3.43 μm for samples N4 and N5).

[0085] Figure 3 The following are the morphology and EDS (Energy Dispersive Spectroscopy) element distribution patterns of sample N3 after corrosion. Specifically, the surface morphology and EDS element distribution patterns of sample N3 (15 parts by weight of NbB2 content) SiC-based composite ceramic material after polishing and then 600°C sodium hydroxide corrosion. In the figure, (a) is the surface morphology, (b) is the Si element distribution spectrum, (c) is the C element distribution spectrum, (d) is the Nb element distribution spectrum, and (e) is the B element distribution spectrum. Surface scanning analysis of sample N3 using an EDS spectrometer revealed that Si and C are the main elements, accounting for the highest proportion, and Nb and B elements are evenly distributed in the matrix, indicating that the conductive phase is well dispersed in the matrix.

[0086] Figure 4 This is the fracture morphology of sample N4, i.e., Example 4. Figure 4 As shown in the figure, from the cross-sectional morphology, it can be found that the fracture surfaces are relatively smooth, which belongs to the brittle fracture mode. Obvious step texture features can be observed, and the grain boundaries are blurred, with the main fracture mode being transgranular.

[0087] The electrical properties of the samples are tested using a four-probe instrument. Figure 5 The resistivity change diagram of SiC-based composite ceramics with different NbB2 contents is shown in Figure 2. Figure 5 As shown in the figure, with the continuous increase of NbB2 content, the resistivity of the material changes from a sharp decrease to a slow decrease. The resistivity of the SiC ceramic material (sample 0) without adding any conductive phase is about 5.87×10 6 Ω·cm, which is comparable to the resistivity of SiC materials prepared by pressureless solid-phase sintering and lower than that of undoped SiC ceramics prepared by liquid-phase sintering. When the NbB2 content increases from 5 parts by weight (sample N1) to 10 parts by weight (sample N2), the sample's conductivity is greatly improved, with a change of four orders of magnitude, indicating that the conductive phase particles may bridge each other and a current network transmission channel begins to form (electroosmosis). As the content continues to increase, the sample's conductivity continues to strengthen, tending to a stable conductive state. At a content of 15 parts by weight (sample 3), the resistivity is approximately 65.4Ω·cm, which meets the requirements of EDM.

[0088] SiC is a typical semiconductor material. Because it is polycrystalline, the Schottky barriers at its grain boundaries significantly hinder electron conduction. Generally speaking, the resistance of SiC ceramics primarily originates from the grains and grain boundaries, with the grain boundaries contributing the majority. The dynamics of electrons flowing through the material can be analyzed using IV and AC impedance methods. Due to its wide bandgap and high Schottky barrier, SiC ceramics exhibit a varistor effect. This means that when the applied voltage does not reach a certain threshold, the varistor exhibits extremely high electrical impedance, with virtually no current flowing. However, once the applied voltage exceeds this threshold, electrons gain sufficient potential energy to cross the barrier, causing the electron flow per unit time and volume to increase exponentially. This results in a sharp drop in resistivity and current conduction, as seen in the macroscopic material. Further increases in applied voltage lead to a sharp increase in current intensity, exhibiting a nonlinear behavior. This nonlinear volt-ampere characteristic indirectly confirms the presence and height of the Schottky barrier. Ceramics exhibiting varistor characteristics can be characterized by the degree of nonlinearity using Equation 1.

[0089]

[0090] Where U1 is the voltage when the current is 0.1mA, U2 is the voltage when the current is 1mA, and α is the nonlinear coefficient. The larger α is, the more obvious the nonlinear characteristic is. When the value of α approaches 1, the nonlinear characteristic gradually turns into a linear characteristic.

[0091] Figure 6 The graph shows the relationship between current density and electric field intensity of samples with different NbB2 contents. Figure 6 It can be seen that sample 0 (no NbB2 addition) and sample N1 have nonlinear characteristics, and the nonlinear characteristics of sample 0 are significant, indicating that the height of the grain boundary barrier inside the material is relatively high. As the content of the NbB2 conductive phase gradually increases, the nonlinear characteristics weaken and change towards linear characteristics. When the content is 15 parts by weight (sample N3), the material's pressure-sensitive characteristics completely disappear and turn into ohmic characteristics. The main reason is that the SiC-SiC grain boundary gradually decreases and is replaced by the SiC-NbB2 grain boundary, forming a continuous distribution such as SiC-NbB2-SiC. The SiC-SiC grain boundary barrier is much higher than the SiC-NbB2 grain boundary barrier. When the conductive phase content is sufficient, the NbB2-NbB2 grain boundary begins to appear, and the NbB2 grains are in ohmic contact. There is no barrier between the grain boundaries, so the material's pressure-sensitive characteristics disappear.

[0092] Figure 7 The AC impedance diagram of samples with different NbB2 contents is shown in Figure 2. Figure 7 As shown in the figure, when the NbB2 content does not exceed 5 parts by weight (sample 0 and sample N1), due to the limitation of instrument specifications, the graph presents an incomplete semicircle, which is mainly caused by the interface diffusion effect. The equivalent circuit is composed of equivalent grains, grain boundary resistance elements and conventional circuit elements (CPE) used to fit the descending semicircle. The equivalent circuit contains two elements, resistance and capacitance, indicating that the resistance of the sample comes from both the grains and the grain boundaries. The AC impedance spectrum of conventional SiC ceramic materials typically has two semicircles, located at low and high frequencies, respectively. The sizes of the semicircles at high and low frequencies represent the resistance of the material's grains and grain boundaries, respectively. In this experiment, the semicircles in the low-frequency portion of the AC impedance spectrum of Samples 0 and N1 almost cover the entire portion, indicating that the resistance of the high-resistivity samples is almost entirely contributed by the grain boundaries, while the grain resistance is almost negligible in comparison. As the content of the conductive phase NbB2 gradually increases, the grain boundary resistance of the material decreases because the contact between NbB2 and SiC can reduce the Schottky barrier height. When the conductive phase content reaches a certain amount, the distance between the conductive phase particles shortens or they directly contact each other, and the SiC-NbB2 grain boundary and NbB2-NbB2 grain boundary become the main body, and the overall grain boundary resistance of the material decreases rapidly. The main source of resistance for the entire sample begins to shift from the grain boundary to the grain. When the content is increased to 25 parts by weight (sample 5), an inductive reactance signal characteristic appears. Faraday's law of electromagnetic induction can be used to explain that the conductive percolation path formed by NbB2 in SiC is a tortuous route.

[0093] By using Zview software to perform in-depth fitting analysis on the AC impedance diagram of the samples, the grain and grain boundary fitting data in Table 2 were obtained. Table 2 shows the equivalent resistance and nonlinear coefficient of samples with different NbB2 contents.

[0094] Table 2

[0095]

[0096] As shown in Table 2, when no conductive phase is added, the grain boundary resistance of SiC ceramics is as high as 5.08×10 6 Ω·cm, and the grain resistance is only 226.1Ω·cm. The low grain resistance is due to the B element in the B4C sintering aid replacing Si atoms on the SiC, creating vacancies and forming a P-type semiconductor. The increase in carrier concentration leads to a decrease in SiC grain resistance. Furthermore, with increasing NbB2 content, the grain resistance decreases gradually, while the grain boundary resistance changes from a rapid decrease to a slow decrease, roughly consistent with the magnitude of the resistivity change.

[0097] According to the analysis of the fitted grain resistance changes in the table, the source of the grain resistance may be the gradual transformation from the initial single SiC grain resistance to the SiC-NbB2 composite grain resistance and then to the NbB2 grain resistance. At the same time, according to Table 2, it was found that the electro-osmosis phenomenon occurred when the NbB2 conductive phase content was between 5 and 10 weight parts (2.31 vol.% to 4.74 vol.%). At this time, the grain boundary resistance was greatly reduced to 342.3 Ω·cm. This percolation value is much lower than the theoretical value (16 vol.% to 33 vol.%) of a complete continuous channel formed due to good contact between the conductive phase particles.

[0098] Figure 8 This is a rough schematic diagram of the two conduction mechanisms of electroosmosis and electron tunneling. Even if there is no direct contact between the conductive phase particles, when they are close enough (usually less than 10nm), the volatility of the conductive phase particles themselves may produce quantum effects, with a certain probability that the potential barrier will penetrate and tunnel to form a non-direct contact conductive path, such as Figure 8 As shown, (a) is electroosmosis and (b) is electron tunneling.

[0099] Therefore, the occurrence of this electroosmosis phenomenon is likely due to the synergistic effect between the reduction of the grain boundary barrier height and the electron tunneling effect induced by the applied electric field. When the conductive phase content reaches 25 parts by weight (12.9 vol.%), the grain boundary resistance is 0.036Ω·cm, which is lower than the grain resistance, and basically achieves good ohmic contact. This indicates that at this time, most of the electron conduction is stable along the NbB2-NbB2 grain channel. There are almost no or only a few SiC grains in the channel. The percolation channel has undergone a qualitative transformation from initial formation to full formation.

[0100] The quantitative topological relationship is used to calculate the resistivity of the composite material. This method comprehensively considers the microstructure distribution inside the composite material and the influence of the volume content and state of each phase. The results calculated by this method are consistent with the actual measured values. Therefore, according to this theory, the structure of SiC-based composite ceramics can be simplified into a three-unit structure of A, B, and C, where the volume fractions of SiC and conductive phase particles are V and V, respectively. a 、V b Unit A is a continuous silicon carbide phase with a volume fraction of V a m ; Unit B is a continuous NbB2 conductive phase with a volume fraction of V b n ; Unit C is a dispersed phase of SiC and NbB2, with a volume fraction of V c . Figure 9 Schematic diagram of topological transformation of SiC composite ceramic structure, as shown in Figure 9 As shown in the figure, a is the microstructure of SiC composite material, and b is the schematic diagram of structural topological transformation.

[0101] The total resistivity ρ of the composite ceramic can be expressed by distributing the three units parallel to the current direction and simplifying it using the following formula 2:

[0102]

[0103] Where ρ1 is the resistivity of SiC, ρ2 is the resistivity of the conductive phase NbB2, and m and n are constants. When the conductive phases in the composite ceramic are completely and continuously bridged and evenly distributed with the SiC phase, the resistivity of the composite ceramic is the minimum, that is, V a m =V a , V b n =V b , then the resistivity ρ of the multiphase ceramic is:

[0104]

[0105] When the conductive phase is completely dispersed in the SiC phase, that is, V a m =0, V b n =0, at this time the resistivity of the multiphase ceramic is the largest, then:

[0106] ρ=ρ a V a +ρ b V b (4)

[0107] The actual measured value of the sample resistivity should be between the maximum and minimum values predicted by the topological theory. When the conductive phase particles are in good contact with each other to form a stable continuous bridge structure, the measured value will gradually approach the minimum value and move away from the maximum value. Substitute the sample content into the above formula to obtain the predicted maximum and minimum resistivity curves and combine them with the actual measured values to obtain Figure 10 , Figure 10 The topologically theoretical resistivity of samples with varying NbB2 contents is shown in Table 2. Furthermore, when the conductive phase content reaches 10 parts by weight (4.8 vol.%) or more, the experimentally measured value approaches the topologically theoretical minimum, but there is still a significant gap between the two values. This combined characterization and demonstration clearly demonstrates that the initial occurrence of conductive percolation is due to a change in barrier height and electron tunneling, rather than bridging of conductive particles. As the content increases further, a continuous and stable conductive channel forms, containing only a small number of SiC grains.

[0108] Next, we analyze the mechanical properties of the samples. Material density has always been a crucial aspect of material performance characterization, indirectly reflecting the material's overall mechanical strength. Density also applies to different industrial sectors. For example, aerospace requires the lowest possible material density, as low density reduces overall shell weight and allows for greater internal carrying capacity. Figure 11 The density and compactness of samples with different NbB2 contents are measured using the Archimedes drainage method. The main instrument is an analytical balance, and a density meter ST-300A can also be used. Figure 11 As shown in the figure, with the increase of NbB2 content, the density of the sample also increases, and the relative density first increases and then decreases. This is mainly because the density of NbB2 itself is 6.97g / cm 3 , greater than the density of SiC itself, and the relative density is around 96%. For solid-phase sintering, the density is relatively good, so the density of the composite ceramic sample increases with the increase of the high-density second phase content. NbB2 can promote the sintering process of SiC within a certain range. The incorporation of more impurities can cause an increase in the material's lattice defects. The increase in defect energy can make the mass transfer process more efficient. However, due to the incomplete consistency of the thermal expansion coefficients of NbB2 and SiC, the thermal mismatch between the two increases when the content exceeds a certain amount. The mutual antagonism between thermal mismatch and defect energy causes the density to first increase and then decrease.

[0109] Flexural strength and elastic modulus can well reflect the ability of ceramic materials to withstand bending loads and resist deformation in practical applications. Both determine the mechanical behavior, reliability and service life of ceramic materials in industrial applications. Figure 12The flexural strength (σ) and elastic modulus (ε) of samples with different NbB2 contents were measured using a universal testing machine, Instron 3369. The trend shown in the figure shows that increasing NbB2 content can improve the flexural strength and elastic modulus of SiC-based composite ceramics, while the elastic modulus does not change significantly and remains essentially stable (392 GPa to 428 GPa). The elastic modulus is primarily related to the density of the material, internal stress, and the elastic modulus of the different phases themselves. The relative density of the composite ceramic is relatively stable, and the elastic modulus of NbB2 is higher than that of SiC, so the elastic modulus of the composite material changes steadily. The flexural strength shows a clear increase followed by a decrease with increasing second phase content. This indicates that the flexural strength and elastic modulus of SiC-NbB2 composite ceramics are superior to those of SiC ceramics. The addition of NbB2 can significantly refine the size of SiC grains and inhibit their growth. According to the Hall-Petch formula, the bending strength of a material is inversely proportional to the grain size. The close bonding between fine grains can significantly enhance the bending strength of the material. This phenomenon is called fine grain strengthening. At the same time, thermal mismatch also exists. The thermal expansion coefficients of the two phases do not match, resulting in stress during the cooling process. Therefore, as the NbB2 content increases, the interface bonding strength defects between the grains are continuously amplified. The two compete with each other, causing the bending strength of the sample to increase. Figure 12 High flexural strength allows the material to withstand high mechanical stress, extend its service life, and improve its stability in high temperature environments; high elastic modulus allows the material to maintain good dimensional stability, helping it maintain structural integrity and performance stability under thermal shock conditions.

[0110] Figure 13 The fracture toughness of samples with different NbB2 contents changes. The fracture toughness is tested using a Vickers hardness tester (using the Vickers hardness indentation method). From the figure, it can be seen that with the increase of the conductive phase (NbB2) content, the fracture toughness of the material continues to increase. The fracture toughness of the composite material reaches its peak at 4.38MPa·m when the second phase content reaches about 25 parts by weight. 1 / 2 , compared with sample 02.59MPa·m 1 / 2 The increase was 69%.

[0111] Figure 14 is a diagram of the sample toughening mechanism, where (a) is particle bridging and (b) is crack deflection. Figure 14As shown in Figure 2, observations of crack extension in SiC-based composite ceramics fabricated with NbB2 reveal several toughening mechanisms: significant deflection of the crack through the second-phase particles, as well as the presence of voids left by particle bridging and pullout. These mechanisms significantly consume the energy required for fracture propagation, representing the primary toughening mechanism for SiC ceramics when NbB2 is used as a reinforcing phase. Residual stress toughening also occurs due to internal stress generated by the different thermal expansion coefficients of the two phases and the mismatch with the matrix.

[0112] It should be noted that, unless otherwise specified, the technical terms or scientific terms used in the embodiments of the present application should have the common meanings understood by technicians in the field to which the embodiments of the present application belong.

[0113] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, rather than to limit them. Although this application has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of this application, and they should all be included in the scope of the claims and description of this application. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions that fall within the scope of the claims.

Claims

1. A conductive silicon carbide-based composite ceramic, characterized in that: In parts by weight, it comprises 75-95 parts of silicon carbide and 5-25 parts of niobium diboride, wherein the niobium diboride is produced by reacting boron powder and niobium powder, wherein the mass ratio of the boron powder to the niobium powder is 1:

4.

2. The conductive silicon carbide-based composite ceramic according to claim 1, characterized in that: In parts by weight, the boron powder is 1-5 parts, and the niobium powder is 4-20 parts.

3. The conductive silicon carbide-based composite ceramic according to claim 2, characterized in that: In parts by weight, the silicon carbide is 75-90 parts, and the niobium diboride is 10-25 parts.

4. The conductive silicon carbide-based composite ceramic according to claim 3, characterized in that: In parts by weight, the silicon carbide is 75-85 parts, and the niobium diboride is 15-25 parts.

5. The conductive silicon carbide-based composite ceramic according to claim 1, characterized in that: In parts by weight, the conductive silicon carbide-based composite ceramic further comprises 8 parts of a binder, 2 parts of a dispersant and 5 parts of a lubricant.

6. The conductive silicon carbide-based composite ceramic according to claim 1, characterized in that: In parts by weight, the conductive silicon carbide-based composite ceramic further comprises 2 parts of boron carbide.

7. A method for preparing a conductive silicon carbide-based composite ceramic, characterized in that: The steps include: S100, using silicon carbide powder, boron powder, niobium powder, a binder, a dispersant, and a lubricant as raw materials and anhydrous ethanol as a solvent, ball milling in a ball mill to obtain a slurry, wherein, by weight, the silicon carbide powder is 75-95 parts, the boron powder is 1-5 parts, the niobium powder is 4-20 parts, the binder is 8 parts, the dispersant is 2 parts, and the lubricant is 5 parts. The mass of the anhydrous ethanol is 1.2 times the mass of the raw materials; S200, drying, crushing, sieving and granulating the slurry to obtain ceramic powder; S300, dry-pressing the ceramic powder to obtain a green body; S400, sintering the green body in a sintering furnace to obtain a conductive silicon carbide-based composite ceramic.

8. The method according to claim 7, characterized in that The particle size of the silicon carbide powder is 0.5-0.7 microns, the particle size of the boron powder is 0.5-1 microns, and the particle size of the niobium powder is 1 micron.

9. The method according to claim 7, characterized in that In parts by weight, the silicon carbide is 75-85 parts, the boron powder is 3-5 parts, and the niobium powder is 12-20 parts.

10. The method according to claim 7, characterized in that In the S100, 2 parts by weight of boron carbide is added during ball milling in the ball mill jar.

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