Carbon-ceramic composite materials, which seamlessly integrate the exceptional mechanical strength, high-temperature tolerance, corrosion resistance, and thermal shock resistance of ceramics with the superior electrical conductivity of carbon-based materials, exhibit an immense application potential in various fields. They are particularly critical in the manufacturing of closing resistors for ultra-high voltage (UHV) circuit breakers. Under extreme service environments characterized by high temperatures, high pressures, intense electromagnetic fields, and the corrosive byproducts of SF₆ decomposition, these key components face stringent performance requirements, including high mechanical strength, controllable and stable resistivity, a low temperature coefficient of resistance (TCR), and remarkable thermal shock resistance. Conventional conductive ceramics predominantly utilize nano-carbon fillers such as carbon black, carbon nanotubes, carbon fibers, and graphene. However, these nano-sized carbon materials typically suffer from severe agglomeration, poor interfacial wettability with the ceramic matrix, and high susceptibility to oxidation during the high-temperature sintering process, severely limiting their structural integrity and functional stability. To overcome these bottlenecks, this study was to introduce a pre-synthesized carbon nanofiber/calcium aluminate composite powder (CNFs/CAC) as an innovative conductive filler. The implementation of CNFs/CAC could effectively mitigate the agglomeration of nano-sized carbons. Furthermore, the unique microstructural architecture of CNFs/CAC could facilitate the construction of a robust three-dimensional carbon conductive network and leverage calcium aluminate as an efficient sintering aid to promote particle rearrangement and matrix densification, thereby lowering the required sintering temperature. Crucially, the CAC shell encapsulating the carbon nanofibers significantly retarded their oxidation at elevated temperatures. Sintering temperature acts as a paramount thermodynamic variable governing densification kinetics, phase transformations, and microstructural evolution. Inadequate temperatures fail to provide a sufficient driving force for grain boundary migration, resulting in excessive porosity and deteriorated mechanical properties. Conversely, excessive temperatures provoke abnormal grain growth, disruption of the percolation network, and pronounced oxidation of the carbon phase. Therefore, this study was to investigate the evolution mechanisms of phase composition, microstructure, mechanical strength, and electrical properties of CNF/ceramic composites at a sintering temperature range of 1300-1500 ℃, to optimize processing parameters and provide a robust theoretical foundation for the fabrication of high-performance composite conductive ceramics.

The ceramic matrix raw materials comprised bauxite, kaolin, and potassium feldspar, while the conductive filler was a laboratory-synthesized CNFs/CAC powder. The introduction of CaO via the CAC into the Al₂O₃-SiO₂ system deliberately promoted the formation of anorthite, subsequently generating a liquid phase that substantially enhanced the liquid-phase sintering process. The preparation commenced with mixing the raw materials (i.e., 60% bauxite, 22% kaolin, 18% potassium feldspar, and 1.5% CNFs/CAC) by dry ball milling in a ball-to-material mass ratio of 2∶1 at 320 r/min for 9 h. The homogeneous mixture was then granulated through a 20-mesh sieve to obtain granules of approximately 0.85 mm in diameter. These granules were uniaxially pressed into cylindrical green bodies (20 mm in diameter and 20 mm in height) at 50 MPa, followed by cold isostatic pressing (CIP) at 200 MPa for 300 s to ensure initial uniformity. The compacts were dried at 300 ℃ for 2 h and subsequently sintered in a carbon-embedded graphite crucible atmosphere. The sintering profile was systematically varied from 1300 ℃ to 1500 ℃. The phase compositions were analyzed by X-ray diffraction (XRD), and microstructural morphology alongside elemental carbon distribution was determined by field emission scanning electron microscopy (FESEM) with energy dispersive spectrometry (EDS). The macroscopic physical properties, including apparent porosity and bulk density, were measured by the Archimedes method. The compressive strength was tested by a universal testing machine, while the volume resistivity and TCR were evaluated by a precise digital LCR meter coupled with a controlled electro-thermal drying oven.

The sintering temperature fundamentally dictates the structural evolution and functional performance of carbon nanofiber/ceramic composite resistive materials at 1300-1500 ℃. Elevated temperatures facilitate a liquid-phase formation, drastically reducing apparent porosity from 31% to 23% and boosting bulk density from 2.11 g/cm³ to 2.31 g/cm³. Nevertheless, exceeding 1400 ℃ exacerbates carbon depletion via the Boudouard reaction, irreversibly disrupting the conductive pathways and spiking resistivity. The material achieves its optimal comprehensive properties precisely at 1400 ℃, yielding a compressive strength of 189 MPa, a minimal resistivity of 476 Ω·cm, a highly stable NTC behavior with a TCR of -0.14%/℃, and a lowest I_D/I_G ratio of 0.85, reflecting an exceptional graphitic ordering. The incorporation of the pre-synthesized CNFs/CAC filler serves a dual purpose. It establishes a highly uniform and continuous conductive percolating network at minimal doping levels, and it synergistically has fracture toughness via inducing sophisticated crack deflection and bridging mechanisms, offering a robust paradigm for advanced composite electro-ceramics.