Paper derived SiC–Si3N4 ceramics for high temperature applications

Biomorphic Si₃N₄–SiC ceramics have been produced by chemical vapour infiltration and reaction technique (CVI-R) using paper preforms as template. The paper consisting mainly of cellulose fibres was first carbonized by pyrolysis in inert atmosphere to obtain carbon bio-template, which was infiltrated with methyltrichlorosilane (MTS) in excess of hydrogen depositing a silicon rich silicon carbide (Si/SiC) layer onto the carbon fibres. Finally, after thermal treatment of this Si/SiC precursor ceramic in nitrogen-containing atmosphere (N₂ or N₂/H₂), in the temperature range of 1300–1450 °C SiC–Si₃N₄ ceramics were obtained by reaction bonding silicon nitride (RBSN) process. They were mainly composed of SiC containing α-Si₃N₄ and/or β-Si₃N₄ phases depending on the nitridation conditions. The SiC–Si₃N₄ ceramics have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and Raman spectroscopy. Thermal gravimetric analysis (TGA) was applied for the determination of the residual carbon as well as for the evaluation of the oxidation behaviour of the ceramics under cyclic conditions. The bending strength of the biomorphic ceramics was related to their different microstructures depending on the nitridation conditions.     

Thermal properties of the nitrogen-rich Ca-α-Sialons

Nitrogen-rich Ca-α-Sialon (Ca_xSi_12−2xAl_2xN₁₆ with x = 0.2, 0.4, and 0.8, 1.2 and 1.6) ceramics were prepared from the mixtures of Si₃N₄, AlN and CaH₂ powders in a hot press at 1800 °C using a pressure of 35 MPa and a holding time of 4 h, and then were investigated with respect to reaction mechanism, phase stability and oxidation resistance. In addition the sample with x = 1.6 was prepared in the temperature range 600–1800 °C using a pressure of 35 MPa and a holding time of 2 h. The α-Sialon phase was first observed at 1400 °C but the α-Si₃N₄ and AlN phases were still present at 1700 °C. Phase pure Ca-α-Sialon ceramics could not be obtained until the sintering temperature reached 1800 °C. The phase pure nitrogen-rich Ca-α-Sialon exhibited no phase transformation in the temperature range 1400–1600 °C. In general, mixed α/β-Sialon showed better oxidation resistance than pure α-Sialon in the low temperature range (1250–1325 °C), while α-Sialons with compositions located at α/β-Sialon border-line showed significant weight gains over the entire temperature range tested (1250–1400 °C). The phases formed upon oxidation were characterized by X-ray, SEM and TEM studies.     

Thermal stability of polymer derived SiBNC ceramics

To study the thermal stability of polymer derived SiBNC ceramics, a polyborosilazane was pyrolyzed in N₂ and NH₃/N₂ atmosphere, respectively. The as-pyrolyzed products were annealed in N₂ atmosphere at 1200–1850 °C for 2 h. The chemical composition and phase structure of the as-pyrolyzed and annealed ceramics were investigated by element analysis, XRD and FT-IR. The results show that all the ceramics exhibit excellent high temperature stability. They are fully amorphous to 1700 °C, and only partial crystallization, giving a mixture of Si₃N₄, BN and SiC phases, is observed upon heating at 1850 °C. The N₂ pyrolyzed products show better stability than the NH₃/N₂ pyrolyzed products and have less tendency to crystalline at higher temperatures. Better retention of nitrogen at high temperatures is also observed for the N₂ pyrolyzed products.     

Low thermal expansion porous SiC–WC composite ceramics

Low thermal expansion porous SiC–WC composite ceramics were prepared by solid state reaction of Si and WC at 1560 °C, with NH₄HCO₃ as a pore generating agent. Phase composition, thermal expansion, flexural strength, and microstructure of the carbide ceramics were examined. Presence of the SiC, WC and WC_1−X phases were detected in the carbide ceramics. As Si content increased from 2 to 14 wt%, the coefficient of thermal expansion first decreased and then increased, with a minimum of 4.11 × 10⁻⁶ °C at 8 wt% Si, whereas the flexural strength decreased gradually, from 143.9 to 82.7 MPa. Pores of SiC–WC ceramics were less than 2 μm in diameter, because of the stacking interstice of carbide particles and volatilization of silicon. However in the presence of NH₄HCO₃, pores of SiC–WC ceramics were bimodally distributed, the stacking interstice of carbide particles loosened from 1 to 4 μm and pores larger than 5 μm were also formed.     

Thermal stability and mechanical properties of SiC particulate-reinforced Si–C–N composites after heating at 2000 °C

A SiC particulate-reinforced Si–C–N ceramic composite was fabricated using the precursor impregnation and pyrolysis method, and its thermal and mechanical properties were analyzed. The weight loss of the composite was 5% after a heating at 2100 °C in Ar. The pores of the composite enlarged at and above 1700 °C in Ar due to the decomposition of the Si–C–N matrix. However, the composite retained mechanical properties such as strength and hardness after heating at 1700 °C. 88% of the original strength was remained after heating at 2000 °C for 10 h although the fabrication temperature was 1350 °C. The weight gain of the composite was 3.2% after an oxidation at 1450 °C for 30 min in air. The inner oxidation of the particulate-reinforced composites (PRC) was suppressed above 1400 °C due to the closure of the open pores by SiO₂. Consequently, the composite possessed excellent creep resistance at 1400 °C in air. The SiC/Si–C–N composite is a challenging candidate for the application at high temperature.     

A simple and effective process for fabrication of Gd0.1Ce0.9O1.95 solid electrolyte ceramics

Ce_0.9Gd_0.1O_1.95 ceramics were prepared using a simple and effective process in this study. Without any prior calcination, the mixture of raw materials was pressed and sintered directly. The reaction of the raw materials occurred during the heating up period by passing the calcination stage in the conventional solid-state reaction method. More than 99.5% of theoretical density was obtained for Ce_0.9Gd_0.1O_1.95 sintering at 1500–1600 °C. Fine grains (<1 μm) formed in pellets sintered at 1450 °C. The homogeneity of grains increased with the sintering temperature. The grains grew to >4.5 μm in pellets sintered at 1600 °C. The reactive-sintering process is proved to be a simple and effective method in preparing Ce_0.9Gd_0.1O_1.95 ceramics for solid electrolyte application.     

Improvement of the high temperature properties of the LiYO2–Si3N4 system by removing residual Li

Effects of the vaporization of residual Li on the microstructure, oxidation behavior and high temperature properties of a low-temperature pressureless sintered Si₃N₄ using LiYO₂ additive were investigated. The oxidation and creep resistance of the Si₃N₄ was improved after an annealing at 1650 °C because residual Li, which deteriorated the high temperature properties of the Si₃N₄, could be mostly removed. The high temperature deformation of the Si₃N₄ was strongly suppressed after the annealing treatment. The annealed specimens retained 64% of the room temperature strength at 1300 °C in air. The present investigation reports a method to improve the high temperature properties of Si₃N₄.     

Influence of the heat treatment on mechanical properties and oxidation resistance of SiC–Si3N4 composites

The basic mechanical properties and oxidation behaviour of liquid-phase-sintered SiC–Si₃N₄ composites were investigated as a function of the heat treatment at oxidising condition at 1350 °C/0–204 h. The results were compared to those obtained for a reference silicon carbide material, prepared by the same fabrication route. The heat treatment at higher temperature had a positive effect on fracture toughness values but no changes of hardness were observed. It was shown that the oxidation resistance increases with increasing temperature of heat treatment from 1650 °C to 1850 °C. Oxidation always followed parabolic rate law indicating diffusion as the rate limiting mechanisms. The addition of silicon nitride content had no significant influence on oxidation resistance of SiC–Si₃N₄ composites.     

Thermal shock resistance of in situ formed Si2N2O–Si3N4 composites by gelcasting

Thermal shock resistance of Si₂N₂O–Si₃N₄ composites was evaluated by water quenching and subsequent three-point bending tests of strength diminution. Si₂N₂O–Si₃N₄ composites which was prepared with in situ liquid pressureless sintering process using Yb₂O₃ and Al₂O₃ powders as sintering additives by gelcasting showed no macroscopic cracks and the critical temperature difference (ΔT_c) could be up to 1400 °C. A mass of pores existed in the sintered body and the irregular shaped fibers extended from the pores increased the thermal shock property.     

Effect of sintering atmosphere on the grain growth and hardness of SiC/polysilazane ceramic composites

Silicon carbide (SiC) monoliths were synthesised using nano-size SiC powder mixed with/without polysilazane by hot pressing at 1750°C for 1?h under an applied pressure of 20?MPa in N₂ or Ar atmosphere. The effects of polysilazane and sintering atmosphere on the microstructure and hardness of SiC were examined. The grain sizes of the SiC ceramics sintered in N₂ atmosphere with and without the polysilazane were 161 and 605?nm, while the density for those samples were 96.5 and 98.1%, respectively. It was shown that Si₂N₂O was formed for the SiC/polysilazane composite and sintered in N₂. In addition, the sample mixed with polysilazane followed by sintering in N₂ atmosphere revealed a quite high hardness in spite of its relatively low density. It was suggested that Si₂N₂O phase played an important role for the inhibition of grain and subsequent high hardness.     

Low-temperature firing and microwave dielectric properties of 16CaO–9Li2O–12Sm2O3–63TiO2 ceramics with V2O5 addition

The sintering behaviors and microwave dielectric properties of the 16CaO–9Li₂O–12Sm₂O₃–63TiO₂ (abbreviated CLST) ceramics with different amounts of V₂O₅ addition had been investigated in this paper. The sintering temperature of the CLST ceramic had been efficiently decreased by nearly 100 °C. No secondary phase was observed in the CLST ceramics and complete solid solution of the complex perovskite phase was confirmed. The CLST ceramics with small amounts of V₂O₅ addition could be well sintered at 1200 °C for 3 h without much degradation in the microwave dielectric properties. Especially, the 0.75 wt.% V₂O₅-doped ceramics sintered at 1200 °C for 3 h have optimum microwave dielectric properties of Kr = 100.4, Q × f = 5600 GHz, and TCF = 7 ppm/°C. Obviously, V₂O₅ could be a suitable sintering aid that improves densification and microwave dielectric properties of the CLST ceramics.     

A ternary compound additive for vacuum densification of β-silicon carbide at low temperature

The thermal decomposition behavior of a ternary carbide compound (Al₄SiC₄) was investigated under vacuum conditions. Decomposition of Al₄SiC₄ occurred above 1450 °C, resulting in the formation of SiC and carbon phases in the matrix, with some losses of Al. To simultaneously obtain the densification and refinement of SiC, the potential of the compound as a sintering additive for low-temperature sintering of SiC was evaluated and compared to cases of SiC with Al₄C₃ and Al₂O₃ additives. SiC that was almost entirely densified with fine and elongated grains was successfully formed using a 10 wt% Al₄SiC₄ additive by hot pressing at 1700 °C for 2 h in a vacuum. During the densification, the decomposition behavior of the Al₄SiC₄ was strongly related to the densification behavior of the SiC.     

Abrasive wear of Al2O3–SiC and Al2O3–(SiC)–C composites with micrometer- and submicrometer-sized alumina matrix grains

The response of Al₂O₃, Al₂O₃–SiC–(C) and Al₂O₃–C nanocomposites to grinding was investigated in terms of changes of quality of ground surfaces and of the weight losses with time. The study used monolithic polycrystalline aluminas as references, and alumina-based composites with nanosized SiC and C inclusions and with alumina matrix grain size varying from submicrometer to approximately 4 μm. The studied materials can be roughly divided into two groups. Materials with submicrometer alumina matrix grains (Group 1) wear predominantly by plastic deformation and grooving. Coarse-grained materials (Group 2) wear by mixed wear mechanism involving crack initiation and interlinking accompanied by grain pull-out, plastic deformation and grooving. The wear rate of composites increases with increasing volume fraction of SiC. The Group 2 materials wear much faster then those with submicron microstructure. In all cases (with one exception) the wear resistance of composites was higher than that of pure aluminas of comparable grain sizes used as reference materials.     

Dielectric properties of Si3N4–SiCN composite ceramics in X-band

Si₃N₄–SiCN composite ceramics were successfully fabricated through precursor infiltration pyrolysis (PIP) method using polysilazane as precursor and porous Si₃N₄ as preform. After annealed at temperatures varying from 900 °C to 1400 °C, the phase composition of SiCN ceramics, electrical conductivity and dielectric properties of Si₃N₄–SiCN composite ceramics over the frequency range of 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, the content of amorphous SiCN decreases and that of N-doped SiC nano-crystals increases, which leads to the increase of electrical conductivity. After annealed at 1400 °C, the average real and imaginary permittivities of Si₃N₄–SiCN composite ceramics are increased from 3.7 and 4.68 × 10^?3 to 8.9 and 1.8, respectively. The permittivities of Si₃N₄–SiCN composite ceramics show a typical ternary polarization relaxation, which are ascribed to the electric dipole and grain boundary relaxation of N-doped SiC nano-crystals, and dielectric polarization relaxation of the in situ formed graphite. The Si₃N₄–SiCN composite ceramics exhibit a promising prospect as microwave absorbing materials.     

Effect of Ar or N2 sintering atmosphere on the high-temperature oxidation behaviour of pressureless liquid-phase-sintered α-SiC in air

The effect of the Ar or N₂ sintering atmosphere on the oxidation behaviour of pressureless liquid-phase-sintered (PLPS) α-SiC was studied. PLPS α-SiC specimens processed under Ar or N₂ atmospheres were isothermally oxidized at 1100–1450 °C in air for up to 500 h, and their oxidation kinetics, activation energy, and rate-controlling mechanisms were compared. It was found that, regardless of the sintering atmosphere, the oxidation is passive due to the formation of oxide scales. In addition, below 1350 °C the oxidation is protective, with a kinetics that follows initially the arctan-rate law and then the parabolic-rate law. However, from 1350 °C onwards the oxidation becomes only semi-protective, with a kinetics that obeys the arctan-rate law briefly and then the paralinear-rate law. Furthermore, the activation energies and rate-controlling mechanisms are similar for the arctan and paralinear oxidations, but different for the parabolic oxidation. It was also observed that the N₂-processed material oxidizes more slowly than the Ar-processed material below 1200 °C due to a greater crystallization of its oxide scale, whereas above 1200 °C the Ar-processed material is more oxidation-resistant due to greater viscosity of its oxide liquid. Implications concerning the optimization of the processing route of PLPS SiC for high-temperature applications in air are discussed.     

Effect of sintering temperature on varistor properties and aging characteristics of ZnO–V2O5–MnO2 ceramics

The microstructure, electrical properties, dielectric characteristics, and DC accelerated aging behavior of the ZVM-based varistors were investigated for different sintering temperatures of 800–950 °C. The microstructure of the ZVM-based ceramics consisted of mainly ZnO grain and secondary phase Zn₃(VO₄)₂, which acts as liquid-phase sintering aid. The Zn₃(VO₄)₂ has a significant effect on the sintered density, in the light of an experimental fact, which the decreases of the Zn₃(VO₄)₂ distribution with increasing sintering temperature resulted in the low sintered density. The breakdown field exhibited the highest value (17,640 V/cm) at 800 °C in the sintering temperature and the lowest value (992 V/cm) at 900 °C in the sintering temperature. The nonlinear coefficient exhibited the highest value, reaching 38 at 800 °C and the lowest value, reaching 17 at 850 °C. The varistor sintered at 900 °C exhibited not only high nonlinearity with 27.2 in nonlinear coefficient, but also the highest stability, in which %ΔE_1 mA = −0.6%, %Δα = −26.1%, and %Δ tan δ = +21.8% for DC accelerated aging stress of 0.85 E_1 mA/85 °C/24 h.     

Sintering of aluminum nitride by using alumina crucible and MoSi2 heating element at temperatures of 1650 °C and 1700 °C

Aluminum nitride (AlN) ceramics, prepared with Y₂O₃ and CaO sintering additives, have been densified in an Al₂O₃ crucible at temperatures of up to 1650 °C and 1700 °C using a conventional MoSi₂ heating element furnace. The results of this study show that relative densities in excess of 99% of theoretical and a relatively high-thermal conductivity of 147 W m⁻¹ K⁻¹ have been achieved for feedstock materials prepared with combined addition of 1 wt.% Y₂O₃ and 1 wt.% CaO. All of the phases in sintered samples have been shown to be crystalline AlN and minor amount of secondary phases, were detected such as enriched Y- and Ca-aluminates by the XRD patterns, back-scattered imagery and microprobe analysis. The advantage of using the particular experimental system and sintering condition is considered to be amenable to lower production cost and enhance the feasibility of mass production. Critical temperature for AlN densification to obtain the highest density is about 1650 °C.     

Oxyfluoride glass-silica ceramic composite for low temperature co-fired ceramics

Glass/ceramic composite materials based on CaF₂–AlF₃–SiO₂ oxyfluoride glass and silica ceramic filler were prepared. The sintering behavior, phase composition and dielectric property of oxyfluoride glass/silica ceramic composites, as well as its compatibility with Ag electrode, were investigated. The results show that the glass/ceramic composite system can be sintered at 825 °C. When the amount of SiO₂ increased from 0 to 20 wt.%, the shrinkage decreased from 17.0 to 14.5%, and the dielectric constant decreased from 5.9 to 5.4, while the thermal expansion coefficient (20–200 °C) increased from 6.0 to 10.1 ppm/°C. The sintered samples had low dielectric losses less than 0.002 and high flexural strengths. This novel glass/ceramic composite system exhibits good sintering compatibility with silver paste, which makes it a promising candidate for low temperature co-fired ceramic application.     

Porous SiC–Si3N4 composite ceramics with excellent EMW absorption property prepared by gelcasting using DMAA

The porous SiC–Si₃N₄ composite ceramics with good EMW absorption properties were prepared by combination of gelcasting and carbothermal reduction. The pre-oxidation of Si₃N₄ powders significantly improved the rheological properties of slurries (0.06 Pa s at 103.92 s⁻¹) and also suppressed the generation of NH₃ and N₂ from Si₃N₄ hydrolysis and reaction between Si₃N₄ and initiator APS, thereby reducing the pore defects in green bodies and enhancing mechanical properties with a maximum value of 42.88 MPa. With the extension of oxidation time from 0 h to 10 h, the porosity and pore size of porous SiC–Si₃N₄ composite ceramics increased from approximately 41.86% and 1.0–1.5 μm to 46.33% and ~200 μm due to the production of CO, N₂ and gaseous SiO, while the sintering shrinkage decreased from 16.24% to 10.50%. With oxidation time of 2 h, the Si₂N₂O fibers formed in situ by the reaction of Si₃N₄ and amorphous SiO₂ effectively enhanced the mechanical properties, achieving the highest flexural strength of 129.37 MPa and fracture toughness of 4.25 MPa m^1/2. Compared with monolithic Si₃N₄ ceramics, the electrical conductivity, relative permittivity and dielectric loss were significantly improved by the in-situ introduced PyC from the pyrolysis of three-dimensional network DMAA-MBAM gel in green bodies and the SiC from the carbothermal reduction reaction between PyC and SiO₂ and Si₃N₄. The porous SiC–Si₃N₄ composite ceramics prepared by the unoxidized Si₃N₄ powders demonstrated the optimal EMW absorption properties with reflection loss of −22.35 dB at 8.37 GHz and 2 mm thickness, corresponding to the effective bandwidth of 8.20–9.29 GHz, displaying great application potential in EMW absorption fields.     

Effect of in situ synthesis of Si2N2O on microstructure and the mechanical properties of fused quartz ceramics

Si/SiO₂ composite billets were prepared using a low-toxicity gel system, and the resulting billets were sintered at high temperature in nitrogen to synthesize Si₂N₂O in the central position of the fused silica ceramic matrix. The influences of in situ synthesized Si₂N₂O on the microstructure and mechanical properties of fused silica ceramics were studied. The results show that Si/SiO₂ composite billets can be used to synthesize spike-like and fibrous Si₂N₂O in situ in nitrogen at 1450 °C. Si₂N₂O synthesized in situ can improve the mechanical properties and microstructure of quartz ceramics. When the Si/SiO₂ composite billet is sintered in nitrogen at 1450 °C for 2 h, the volume density and bending strength of the quartz ceramics can reach 2.36 g/cm³ and 114.37 MPa, respectively.