Potential-current characteristics in SiC/ZrB2 composite ceramics

ZrB₂/SiC composite ceramics were fabricated to improve the electrical conductive properties of SiC matrix. The debinding and sintering temperatures were determined by computation of Gibbs free energy. As a result, all the samples have the relative density above 99%, and have excellent mechanical and electrical properties. The effects of ZrB₂ content on the microstructure, mechanical and electrical properties were systematically studied. With increasing ZrB₂ content, as-prepared composites show great improvement in their mechanical properties. Importantly, the introduction of ZrB₂ weakened varistor nonlinear characteristic of composite and reduced its resistivity. The reason is the evolution of grain boundary in conductive paths. The sharp decrease of resistivity indicates the formation of percolation paths. The percolation threshold at 1?mA?cm^?2 obtained via percolation model is 10.7963?vol% (19.7098?wt%) ZrB₂. This value is much less than conventional composites, because the percolation path originates from grain boundary breakdown other than continuous conductor chains.     

Combined role of SiC particles and SiC whiskers on the characteristics of spark plasma sintered ZrB2 ceramics

In this research work, the effects of silicon carbide (SiC) as the most important reinforcement phase on the densification percentage and mechanical characteristics of zirconium diboride (ZrB₂)-matrix composites were studied. In this way, a monolithic ZrB₂ ceramic (as the baseline) and three ZrB₂ matrix specimens each of which contains 25 vol% SiC as reinforcement in various morphologies (SiC particulates, SiC whiskers, and a mixture of SiC particulates/SiC whiskers), have been processed through spark plasma sintering (SPS) technology. The sintering parameters were 1900 °C as sintering temperature, 7 min as the dwell time, and 40 MPa as external pressure in vacuum conditions. After spark plasma sintering, a relative density of ~96% was obtained (using the Archimedes principles and mixture rule for evaluation of relative density) for the unreinforced ZrB₂ specimen, but the porosity of composites containing SiC approached zero. Also, the assessment of sintered materials mechanical properties has shown that the existence of silicon carbide in ZrB₂ matrix ceramics results in fracture toughness and microhardness improvement, compared to those measured for the monolithic one. The simultaneous addition of silicon carbide particulates (SiC_p) and whiskers (SiC_w) showed a synergistic effect on the enhancement of mechanical performance of ZrB₂-based composites.     

Elevated temperature thermal properties of ZrB2-B4C ceramics

The elevated temperature thermal properties of zirconium diboride ceramics containing boron carbide additions of up to 15 vol% were investigated using a combined experimental and modeling approach. The addition of B₄C led to a decrease in the ZrB₂ grain size from 22 µm for nominally pure ZrB₂ to 5.4 µm for ZrB₂ containing 15 vol% B₄C. The measured room temperature thermal conductivity decreased from 93 W/m·K for nominally pure ZrB₂ to 80 W/m·K for ZrB₂ containing 15 vol% B₄C. The thermal conductivity also decreased as temperature increased. For nominally pure ZrB₂, the thermal conductivity was 67 W/m·K at 2000 °C compared to 55 W/m·K for ZrB₂ containing 15 vol% B₄C. A model was developed to describe the effects of grain size and the second phase additions on thermal conductivity from room temperature to 2000 °C. Differences between model predictions and measured values were less than 2 W/m·K at 25 °C for nominally pure ZrB₂ and less than 6 W/m·K when 15 vol% B₄C was added.     

Electrical percolation and infrared emissivity of pressureless sintered SiC-MoSi2 composites tailored by sintering temperature

SiC-MoSi₂ composites with low electrical resistivity and high infrared emissivity were fabricated via pressureless sintering. The relationship between microstructure evolution and electrical behaviors along with infrared emission properties of the resulting composites is investigated at various sintering temperatures. The electrical resistivity undergoes two significant drops with increasing sintering temperature. Pore elimination bears responsible for the initial decrease in electrical resistivity. Transmission electron microscopy (TEM) observation manifests that the thinned amorphous layers at SiC/MoSi₂ interface decrease grain boundary resistivity and allow for electrical percolation to occur when sintering temperature further rises. Additionally, increasing sintering temperature leads to a higher infrared emissivity owing to the formation of Mo_4.8Si₃C_0.6 and the decreased boundaries. The lowest electrical resistivity of 7.2 Ω cm and the highest infrared emissivity of 0.721 are recorded for composite sintered at 2000 ℃. Overall, SiC-MoSi₂ composites exhibit a promising prospect as infrared source elements that must endure harsh environments.     

Combustion synthesis of high-temperature ZrB2-SiC ceramics

The work is dedicated to researching into combustion kinetics and mechanism as well as the stages of the chemical transformations during self-propagating high-temperature synthesis of ZrB₂-SiC based ceramics. Dependences of the combustion temperature and rate on the initial temperature (T₀) have been studied. It has been shown that the stages of the chemical reactions of ZrB₂ diboride and SiC carbide formation do not change within the range of T_0?=?298–700?К. The effective activation energy of the combustion process amounted to 170–270?kJ/mol, from which it has been concluded that chemical interaction through the melt plays a leading role. The stages of the chemical transformations in the combustion wave have been studied by dynamic X-ray diffraction. First, ZrB₂ phase forms from Zr-Si melt saturated with boron, and SiC phase is registered later. The SHS method has successfully been used in order to obtain ZrB₂-SiC composite powders and compact ceramics with a silicon carbide content of 25–75%. The ceramics are characterized by a residual porosity of 1.5%, hardness up to 25?GPa, the elastic modulus of 318?±?21?GPa, elastic recovery of 36% and thermal conductivity of 54.9?W/(m?×?K) at T_room.     

Effect of SiC addition on the formation of ZrB2 through reaction of ZrO2, boric acid (H3BO3) and carbon black

ZrB₂-SiC powders with different amounts of SiC (10–30 wt%) were in-situ synthesized at 1600 °C for 90 min in Ar atmosphere. Effects of SiC addition on the formation of ZrB₂ via carbothermal reduction of ZrO₂, H₃BO₃ and carbon black were investigated. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and transmission electron microscope (TEM). The grain size of ZrB₂ in final powders decreased with adding SiC. Columnar ZrB₂ and granular SiC were combined interactively when the SiC content was 25 wt%. Layer-like hexagonal SiC was obtained in the product containing 30 wt% SiC, whereas the ZrB₂ grain growth was strongly inhibited. Furthermore, the growth mechanisms of ZrB₂ and SiC were studied.     

Synthesis of SiC nanowires by a simple chemical vapour deposition route in the presence of ZrB2

The SiC nanowires (NWs) were fabricated by a simple chemical vapour deposition (CVD) method at high temperature using Si, phenolic resin, and ZrB₂ powder. The morphologies of the fabricated SiC NWs included SiC/SiO₂ chain-beads and straight wires with core-shell structures. The fabricated SiC NWs were micrometre-to-millimetre in length, with chains 100–300 nm in diameter and beads with diameters of less than 1 μm. The core-shell-structured SiC NWs consisted of crystalline SiC cores and thin amorphous SiO₂ shells. SiC crystals grew in the [111] direction governed by a vapour-solid (VS) mechanism. The added ZrB₂ promotes the generation of gaseous species at higher gas pressures, which contributes to the formation of SiC NWs by CVD. The fabricated SiC NWs exhibited good photoluminescence properties due to many stacking faults and the presence of amorphous SiO₂.     

Thermal and electrical properties of additive-free rapidly hot-pressed SiC ceramics

The thermal and electrical properties of newly developed additive free SiC ceramics processed at a temperature as low as 1850 °C (RHP0) and SiC ceramics with 0.79 vol.% Y₂O₃-Sc₂O₃ additives (RHP79) were investigated and compared with those of the chemically vapor-deposited SiC (CVD-SiC) reference material. The additive free RHP0 showed a very high thermal conductivity, as high as 164 Wm⁻¹ K⁻¹, and a low electrical resistivity of 1.2 × 10⁻¹ Ω cm at room temperature (RT), which are the highest thermal conductivity and the lowest electrical resistivity yet seen in sintered SiC ceramics processed at ≤1900 °C. The thermal conductivity and electrical resistivity values of RHP79 were 117 Wm⁻¹ K⁻¹ and 9.5 × 10⁻² Ω cm, respectively. The thermal and electrical conductivities of CVD-SiC parallel to the direction of growth were ∼324 Wm⁻¹ K⁻¹ and ∼5 × 10⁻⁴Ω⁻¹ cm⁻¹ at RT, respectively.     

Reactive hot pressing of ZrB2-based composites with changes in ZrO2/SiC ratio and sintering conditions. Part I: Densification behavior

ZrB₂–SiC–ZrO₂ composites were hot pressed in order to investigate the effects of adding nano-sized ZrO₂ particles as well as the hot pressing parameters on the densification behavior of ZrB₂–SiC composites. An L9 orthogonal array of the Taguchi method was employed to study the significance of each parameter such as the sintering temperature, time, the applied external pressure, and ZrO₂/SiC volume ratio on the densification process. The statistical analyses revealed that among the mentioned parameters, the hot pressing temperature had a great influence over the densification. By being hot pressed at 1850 °C for 90 min under 16 MPa, fully dense ZrB₂-based composites were obtained. The relative density of the composites decreased at first and then enhanced as a function of ZrO₂/SiC ratio. Microstructural investigation of the fracture surfaces of the composites, which was carried out using the SEM analysis, showed the formation of new phases on the surfaces of SiC grains. The EDS and XRD analyses identified the ZrC as the newly formed interfacial phase due to the reaction between nano-ZrO₂ and SiC. The ZrC acted as an adhesive interphase between the ZrB₂/SiC grains, which could assist the sintering process.     

Mechanical properties of sintered ZrB2-SiC ceramics

ZrB₂ ceramics containing 10-30 vol% SiC were pressurelessly sintered to near full density (relative density >97%). The effects of carbon content, SiC volume fraction and SiC starting particle size on the mechanical properties were evaluated. Microstructure analysis indicated that higher levels of carbon additions (10 wt% based on SiC content) resulted in excess carbon at the grain boundaries, which decreased flexure strength. Elastic modulus, hardness, flexure strength and fracture toughness values all increased with increasing SiC content for compositions with 5 wt% carbon. Reducing the size of the starting SiC particles decreased the ZrB₂ grain size and changed the morphology of the final SiC grains from equiaxed to whisker-like, also affecting the flexure strength. The ceramics prepared from middle starting powder with an equiaxed SiC grain morphology had the highest flexure strength (600 MPa) compared with ceramics prepared from finer or coarser SiC powders.     

Influence of sintering atmosphere and BN additives on microstructure and properties of porous SiC ceramics

The effects of the boron nitride (BN) content on the electrical, thermal, and mechanical properties of porous SiC ceramics were investigated in N₂ and Ar atmospheres. The electrical resistivity was predominantly controlled by the sintering atmosphere and secondarily by the BN concentration, whereas the thermal conductivity and flexural strength were more susceptible to changes in the porosity and necking area between the SiC grains. The electrical resistivities of argon-sintered porous SiC ceramics (6.3 × 10⁵ – 1.6 × 10⁶ Ω·cm) were seven orders of magnitude higher than those of nitrogen-sintered porous SiC ceramics (1.5 × 10⁻¹ – 6.0 × 10⁻¹ Ω·cm). The thermal conductivity and flexural strength of the argon-sintered porous SiC ceramics increased from 8.4–11.6 W·m⁻¹ K⁻¹ and from 9.3–28.2 MPa, respectively, with an increase in the BN content from 0 to 1.5 vol%, which was attributed to the increase in necking area and the decrease in porosity.     

Influence of small amount of sintering additives on unlubricated sliding wear properties of SiC ceramics

Fully densified SiC ceramics were developed from commercially available β-SiC powders using small amount (3 wt%) of AlN-Sc₂O₃ or AlN-Y₂O₃ additives by hot pressing at 2050 °C for 6 h in nitrogen atmosphere, and their wear properties were investigated by subjecting to self-mated sliding at different loads (1, 6 and 13 N) under unlubricated conditions. SiC ceramics prepared with 3 wt% AlN-Y₂O₃ additives consisted of mostly large equi-axed grains with amorphous grain boundary phase of ∼1.2 nm thickness, whereas SiC ceramics sintered with 3 wt% AlN-Sc₂O₃ additives showed duplex microstructure of elongated and fine equi-axed grains with clean grain boundary. As the load was increased, the steady state coefficient of friction reduced from ∼0.6 to ∼0.2, and wear rate increased from 10⁻⁶ to 10⁻⁵ mm³/N·m. It was observed that the friction did not depend on the additive composition, while less wear was observed for the SiC ceramics sintered with 3 wt% AlN-Sc₂O₃ additives consisting of clean grain boundary. The material loss was increased with the increased amount of sintering additive to 10 wt%. The worn surface morphology revealed that the material was primarily removed via surface grooving and microcracking at 1 N load, while tribochemical wear dominated at 6 and 13 N loads.     

High temperature erosion behavior of spark plasma sintered ZrB2-SiC composites

Damage of structural components of hypersonic vehicles by atmospheric particles demands thorough understanding on their wear behavior. In the present work, dense ZrB₂-SiC (10, 20, and 30 vol%) composites are prepared by spark plasma sintering at 55 MPa in two stages: 1400 °C for 6 min followed by 1600 °C for 2 min. With increase in SiC content, microstructures of sintered composites reveal strongly bonded ZrB₂ grains with SiC particles. A combination of maximum hardness of 23 GPa, elastic modulus of 398 GPa and fracture toughness of 5.4 MPa m^1/2 are obtained for the composite containing 30 vol% SiC particles. It is found that cracks are bridged or deflected by SiC particles in the composites. When the composites are subjected to SiC particle erosion at 800 °C, a 14% decrease in erosion rate is obtained with increase in SiC content from 10 to 30 vol%. The formation of large extent of boro-silicate rich viscous surface on eroded surfaces is attributed to reduced fracture or removal of ZrB₂ grains of the composites with increased SiC content.     

Pursuing enhanced oxidation resistance of ZrB2 ceramics by SiC and WC co-doping

The oxidative degradation of ZrB₂ ceramics is the main challenge for its extensive application under high temperature condition. Here, we report an effective method for co-doping suitable compounds into ZrB₂ in order to significantly improve its anti-oxidation performance. The incorporation of SiC and WC into ZrB₂ matrix is achieved using spark plasma sintering (SPS) at 1800?°C. The oxidation behavior of ZrB₂-based ceramics is investigated in the temperature range of 1000?°C–1600?°C. The oxidation resistance of single SiC-doped ZrB₂ ceramics is improved due to the formation of silica layer on the surface of the ceramics. As for the WC-doped ZrB₂, a dense ZrO₂ layer is formed which enhances the oxidation resistance. Notably, the SiC and WC co-doped ZrB₂ ceramics with relative density of almost 100% exhibit the lowest oxidation weight gain in the process of oxidation treatment. Consequently, the co-doped ZrB₂ ceramics have the highest oxidation resistance among all the samples.     

Influence of SiC on phase and microstructure of ZrB2 powders synthesized via carbothermal reduction at different temperatures

ZrB₂ powders were successfully prepared via carbothermal reduction of ZrO₂ with H₃BO₃ and carbon black under flowing argon. By introducing SiC species into reaction mixtures, the effects of SiC addition on phase composition and morphology of ZrB₂ powders thermally treated at different temperatures were investigated. The resultant samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). The highly pure ZrB₂ with the mean size of 5?µm could be obtained at 1600?°C for 90?min and the grains presented columnar shapes. After addition of SiC, ZrB₂ revealed relatively better crystallinity and finer particle size. Regular columnar ZrB₂ grains ranging from 1 to 2?µm were seen existing after reaction at 1500?°C for 90?min.     

Reactive hot-pressing of ZrB2-ZrC-SiC ceramics via direct addition of SiC

A series of ZrB₂-ZrC-SiC composites with various SiC content from 0 to 20 vol% were prepared by reactive hot-pressing using Zr, B₄C and SiC as raw materials. Self-propagating high-temperature synthesis (SHS) occurred, and ZrC grains connected each other to form a layered structure when the SiC content is below 20 vol%. The evolution of microstructure has been discussed via reaction processes. The composite with 10 vol% SiC presents the most excellent mechanical properties (four-point bending strength: 828.6±49.9 MPa, Vickers hardness: 19.9±0.2 GPa) and finest grain size (ZrB₂: 1.52 µm, ZrC: 1.07 µm, SiC: 0.79 µm) among ZrB₂-ZrC-SiC composites with various SiC content from 0 to 20 vol%.     

粉体超微化对碳化硅陶瓷显微结构的影响

研究了粉体超微化后碳化硅陶瓷材料显微结构的变化特征 ,分析了超微化处理对碳化硅陶瓷烧结过程及显微结构的影响机理。经超微化处理后 ,粉体颗粒尺寸减小 ,分布范围变窄 ,硬团聚解聚 ,颗粒形状均匀分布 ;素坯均匀性提高 ,坯体烧结温度降低 ,促进了坯体烧结过程 ;陶瓷材料表面显微结构缺陷明显减少 ,获得了理想的显微结构和良好的力学性能     

In-situ oxide scale investigation of Sm-doped ZrB2/SiC Billets

Samarium-doped zirconium diboride/silicon carbide (Sm-ZBS) ceramics possess an emittance of 0.9 at 1600 °C and develop oxide scales that have excellent ablation performance. This study investigates the oxide scale development of 3 mol% doped Sm-ZBS which contains 80 vol% ZrB₂ and 20 vol% SiC when exposed to temperatures in excess of 1800 °C in an oxidizing atmosphere. Samples were prepared via chemical infiltration of samarium nitrate into spray-dried powders of 80 vol.% ZrB₂/20 vol.% SiC; powders were then pressed into billets and sintered without pressure. Samples cut from these billets were then oxidized for 10, 60, and 300 s, respectively, using an oxyacetylene torch. A Sm-depletion region was observed and believed to form due to glass transport to the surface. X-ray diffraction was used to determine the sequence of oxidation of Sm-ZBS, beginning with the formation of ZrO₂ and Sm₂O₃. The final oxide scale was determined to be c₁-Sm_0.2Zr_0.8O_1.9, with a melting temperature exceeding 2500 °C. SEM and EDS were also used to investigate microstructural formation due to the bursting of convection cells.     

助烧剂和造孔剂对真空烧结SiC多孔陶瓷性能的影响

以粒度≤0.063mm的SiC为主要原料,分别加入30%(质量分数)的Al2O3-Y2O3与10%的Al2O3-高岭土复合助烧剂,并外加不同量(分别为12.8%、26.3%、30.0%和36.4%)的造孔剂羧甲基纤维素钠(CMC),制样后首先在空气炉中经过300℃2h或1100℃4h的预烧,然后在真空炉中于1550℃4h真空烧结而制备成SiC多孔陶瓷,并研究了助烧剂种类以及造孔剂CMC外加量对SiC多孔陶瓷显微组织、显气孔率及抗折强度的影响。结果显示:采用Al2O3-Y2O3作为助烧剂的SiC多孔陶瓷比Al2O3-高岭土作助烧剂的具有较高的抗折强度,显气孔率稍有减小;随着羧甲基纤维素钠量的增加,加入两种助烧剂的SiC多孔陶瓷均表现为显气孔率增加,抗折强度降低。     

Influence of cobalt and sintering temperature on structure and electrical properties of BaZr0.05Ti0.95O3 ceramics

Pure and Co-modified BaZr_0.05Ti_0.95O₃ ceramics were fabricated by the traditional solid state reaction technique. The influence of cobalt and sintering temperature on structure, dielectric, ferroelectric properties and diffuse phase transition of BZT ceramics were investigated systematically. 1300 °C was the optimal sintering temperature for BZT ceramics. The solid solubility limit of Co ions in BZT matrix was determined to be 0.4 mol%. The introduction of a moderate amount of Co ions was believed to benefit the microstructure development and make the grain size more uniform. Compared with undoped counterparts, 0.4 mol% Co-modified ceramics showed equivalent ferroelectric properties with a high remnant polarization (P_r=9.6 μC/cm²) and a low coercive field (E_c=0.21 kV/mm). Besides these, a relative high dielectric coefficient (ε_r=2030) and a low dielectric loss (tan δ=1.85%) were also obtained on this composition. The degree of diffuse phase transition was enhanced by the addition of Co ions. The related mechanism of the diffused phase transition behavior was discussed.