Microstructural evolution of carbon fibers by silicon vapor deposition and its effect on mullite-corundum castables

Carbon fibers (CFs) can be introduced to castables, due to the relative higher thermal conductivity and stronger damping properties. In this research, microstructural evolution of carbon fibers (CFs) in the presence of mixture of silicon and silica powders under the protection of carbon black was studied in the temperature range of 1000–1300 °C by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that some small amorphous SiO_x globules were formed on the surface of CFs at the temperature below 1200 °C. With the increase of the treated temperature, the size of globules became bigger and reached maximum at 1200 °C. The growth of SiO_x globules can be controlled by vapor-solid mechanism. In addition, the effects of these CFs on properties and microstructure of mullite-corundum castables were studied. The results showed that the CFs with SiO_x globules (CFs/SiO_X) could significantly improve the mechanical properties of the castables because the interfacial bonding strength between CFs/SiO_x and matrix is much stronger than the untreated CFs.     

Effects of residual radicals on compositional and structural stability of silicon nitride fibers

In this study, amorphous silicon nitride fibers were prepared through the nitridation of cured polycarbosilane fibers. It was observed that their composition and properties can be controlled by adjusting the flow of NH₃ during the nitridation process. Based on their compositional and structural stability, the samples could be divided into two classes: stable fibers and unstable fibers. As indicated by the electron spin resonance spectra, the amount of the residual free radicals in the unstable fibers was significantly higher than that in the stable fibers. Because of the reactions of the radicals with the moisture in the air, the oxygen content of the unstable fibers increased day by day until the residual radicals were exhausted. Thus, to develop silicon nitride fibers with both low oxygen content and low carbon content, the amount of NH₃ used should be optimized to eliminate the free radicals. These results suggest that it is possible to tailor the nitridation conditions for preparing high-purity silicon nitride materials so that they exhibit desirable properties, such as compositional and structural stability, good mechanical properties, and high electrical resistivity.     

Formation and thermal stability of dual glass phases in the h-BN/SiO2/Yb-Si-Al-O composites

The glass phases formation and microstructural evolution in the h-BN/SiO₂/Yb-Si-Al-O composite were investigated. Owing to the introduction of Al₂O₃, SiO₂ could transform into amorphous silica at a lower temperature (∼1600 °C). On the other hand, with increasing temperature, Al³⁺ gradually dissolved into ytterbium silicate (Yb₂Si₂O₇) and then accelerated the formation of amorphous Yb-Si-Al-O glass phase. Up to 1880 °C, fine spherical Yb-Si-Al-O glass particles and irregular amorphous silica distributed uniformly, which contributes to the excellent mechanical properties of the composite. The thermal stability study disclosed that more [AlO₄] units could effectively inhibit crystallization of the Yb-Si-Al-O glass phase, but mechanical properties of composite still decreased slowly with increasing the heat treatment temperature. For example, the flexural strength of the composite with 1.5 wt.% Al₂O₃ decreased from 297 ± 30 to 284 ± 22 MPa as the treatment temperature rose from 800 to 1200 °C.     

Effect of oxygen on the strength of reaction-bonded silicon nitride

Conclusions When obtaining reaction-bonded Si₃N₄, drying of the silicon powder or the charge at 400°C leads to an increased oxygen content in it and can cause deterioration of the mechanical properties of the ceramics after sintering.Addition of small quantities of SiO₂ to the reaction-bonded Si₃N₄ improves the high-temperature strength to some extent although it affects the room temperature strength adversely.Translated from Ogneupory, No. 5, pp. 13–14, May, 1991.     

Effects of microstructure and intergranular glassy phases on thermal conductivity of silicon nitride

In this study, the binary sintering additives Y₂O₃-Sc₂O₃, were first applied to the Si₃N₄ system to investigate their effects on microstructure and thermal conductivity. The microstructure and thermal conductivity of both sintered silicon nitride (SSN) and sintered reaction-bonded silicon nitride (SRBSN) were found to be significantly dependent on the additive composition. Among various combinations of Y₂O₃ and Sc₂O₃, 1 mol% Y₂O₃−3 mol% Sc₂O₃ prominently enhanced thermal conductivity, and the enhancement could not be attributed to any difference in microstructure or lattice defects. TEM observation revealed that this composition was more liable to devitrify the glassy phase with a lower degree of stress accumulation, and to possibly produce a grain boundary that was cleaner or with a higher order of atomic arrangement. A microstructure model for thermal conductivity was proposed which took the thermal resistance of the grain boundaries into account. The grain boundary state exerted a remarkable influence on the thermal conductivity of fine microstructures, and the experimentally measured thermal conductivity values were consistent with those given by the proposed model.     

Effect of the diluent on combustion synthesis of silicon nitride

Si₃N₄ powders were prepared by combustion synthesis with 1- and 3-μm α-Si₃N₄, β-Si₃N₄ diluent and BN inert diluent. The maximum temperatures of samples with boron nitride (BN) as a diluent are about 1500–1600°C lower than that of samples with α-Si₃N₄ and β-Si₃N₄ as diluents are about 1600–1800°C. Moreover, the newly formed α-Si₃N₄ contents in the synthesized products with BN as diluent over 90 wt% are much higher than those with α-Si₃N₄ and β-Si₃N₄ as diluent about 20–40 wt%. The strip-like α-Si₃N₄, rod-like β-Si₃N₄ grains, and radiative shaped grains can be observed in the synthesized products. Finally, the effect of the diluent on the α-phase content of combustion synthesized Si₃N₄ is discussed, which provides key guidance for preparing Si₃N₄ powders with high α-phase content.     

Bioactive silicon nitride by surface thermal treatment

Silicon nitride-based ceramics with SiO₂, CaO and Ca₃(PO₄)₂ as sintering additives, have been prepared in order to study the bioactivity. Dense ceramic bodies were oxidized by an oxy-acetylene flame at approx. 1475 °C for 60 s, in order to modify the surface in terms of bioactivity enhancement and the formation of optimal porosity for cell viability. During oxidation two concurrent processes occurred on the ceramic body surface: (i) formation of thin glassy layer with a composition close to that of grain boundary phase in ceramic body, and (ii) partial decomposition of silicon nitride matrix. The latter one resulted in the formation of gases (N₂ and SiO), which formed bubbles in the viscous surface glassy phase, resulting in porosity required for cell adhesion (small pores) and tissue ingrowth (large pores). The best bioactivity was obtained for oxy-acetylene flame treated Si₃N₄ ceramics with Ca₃(PO₄)₂ sintering additive.     

Effect of seeding on the thermal conductivity of self-reinforced silicon nitride

Thermal conductivity of Si₃N₄ containing large β-Si₃N₄ particles as seeds for grain growth was investigated. Seeds addition promotes growth of β-Si₃N₄ grains during sintering to develop the duplex microstructure. The thermal conductivity of the material sintered at 1900 °C improved up to 106 W m⁻¹ K⁻¹, although that of unseeded material was 77 Wm⁻¹ K⁻¹. Seeds addition leads to reduction of the sintering temperature with developing the duplex microstructure and with improving the thermal conductivity, which benefits in terms of production cost of Si₃N₄ ceramics with thermal conductivity. ©     

Surface defect healing effect of silica coatings on silicon nitride fibers

In this study, silica coatings with different thickness were prepared on silicon nitride fibers by a continuous dip-coating method. The effects of the coatings on the mechanical properties of the silicon nitride fibers were investigated. The SiO₂ coatings with uniform thickness were prepared from a sol solution with a concentration of 0.75 wt% and then heat-treated at 400 °C, and the strength of the fibers was improved by the treated coating. The tensile strength of a coated fiber was approximately 26% higher than that of an uncoated fiber because the thin coating healed the surface defects. Our study also confirmed that the size of sol particles must match that of the flaws on the fiber surface before these flaws could be effectively repaired. Finally, a probable mechanism will be proposed here to explain this effect. The present results demonstrate that the strength of silicon nitride fibers can be enhanced by coating them through the sol–gel process, and the findings are expected to provide guidelines for repairing strength-limiting flaws in other fibers.     

Effects of oxygen concentration on the electrical properties of ZnO films

In this paper, electrical characteristics by various oxygen content in ZnO films were studied. To control the oxygen content of ZnO films, post-thermal annealing was performed in N₂ and air ambient, led to improve crystallinity and optical properties of ZnO films. The oxygen concentration was measured by Auger electron spectroscopy. The ZnO films having the deficiency of oxygen showed the electron concentrations between 10²¹ and mid 6 × 10¹⁷ cm⁻³ and resistivity at 10⁻³–10⁻¹ Ω cm. On the other hand, when the oxygen concentration of the ZnO films was up to the stoichiometry with Zn, the ZnO films showed low electron concentration at −10¹⁷ cm⁻³ and resistivity at 10 Ω cm.     

Determining the oxygen content in partly oxidized silicon nitride

Conclusions The vacuum-extraction method is suitable for determining with adequate accuracy the content of oxygen (oxynitride) in partly oxidized silicon nitride and in silicon-carbide refractories with a nitride and silicon oxynitride bond.     

Fracture toughness of silicon nitride balls via thermal shock

A new method for fracture toughness determination of ceramic balls is presented. The starter crack is introduced into the surface of the ball by a Knoop indentation followed by grinding off the deformed zone. The loading through surface tensile stresses is realized by water quenching, i.e. dropping the heated ball into water. The temperature difference is stepwise increased to find the critical temperature difference for the initiation of crack growth. The geometric factor is calculated in a parametric finite element study, whereas the temperature distribution in the ball was previously determined by using the Biot concept. Combining experimentally measured critical temperature differences for different cracksizes and ball diameters with numerical results of the geometric factor, the fracture toughness of the silicon nitride balls is evaluated. For the evaluation, the knowledge of several material properties (e.g. the CTE) and other parameters is necessary, which have influence on the precision of the measurement. The overall measurement uncertainty is estimated to be about ± 10 %, what roughly corresponds to the value determined with standard measurement procedures. There is an excellent agreement with published fracture toughness results of these balls determined by the modified Surface Crack in Flexure procedure.     

Preparation of silicon carbide–silicon nitride fibers by the controlled pyrolysis of polycarbosilazane precursors

Experiments to produce polycarbosilazane resin and high-strength silicon carbide–silicon nitride (Si_xN_yC_z) fibers as well as resin/fiber characteristics are reported. Polycarbosilazane resin was drawn into fibers from the melt and subsequently treated and pyrolyzed into Si_xN_yC_z fibers. These materials are characterized by high tensile modulus (29 × 10⁶ psi for 0.4-mil diameter) and high electrical resistivity (6.9 × 10⁸ Ω·cm for 0.6-mil diameter).     

Aluminum-doped ceria-zirconia solid solutions with enhanced thermal stability and high oxygen storage capacity

A facile solvothermal method to synthesize aluminum-doped ceria-zirconia (Ce_0.5Zr_0.5-xAl_xO_2-x/2, x = 0.1 to 0.4) solid solutions was carried out using Ce(NH₄)₂(NO₃)₆, Zr(NO₃)₃·2H₂O Al(NO₃)₃·9H₂O, and NH₄OH as the starting materials at 200°C for 24 h. The obtained solid solutions from the solvothermal reaction were calcined at 1,000°C for 20 h in air atmosphere to evaluate the thermal stability. The synthesized Ce_0.5Zr_0.3Al_0.2O_1.9 particle was characterized for the oxygen storage capacity (OSC) in automotive catalysis. For the characterization, X-ray diffraction, transmission electron microscopy, and the Brunauer-Emmet-Teller (BET) technique were employed. The OSC values of all samples were measured at 600°C using thermogravimetric-differential thermal analysis. Ce_0.5Zr_0.3Al_0.2O_1.9 solid solutions calcined at 1,000°C for 20 h with a BET surface area of 18 m² g⁻¹ exhibited a considerably high OSC of 427 μmol-O g⁻¹ and good OSC performance stability. The same synthesis route was employed for the preparation of the CeO₂ and Ce_0.5Zr_0.5O₂. The incorporation of aluminum ion in the lattice of ceria-based catalyst greatly enhanced the thermal stability and OSC.     

High-temperature microstructural evolution of SiCN fibers derived from polycarbosilane with different C/N ratios

Research into the high-temperature microstructural evolution of SiCN ceramic fibers is important for the aerospace application of advanced ceramic matrix composites in harsh environments. In this work, we studied the microstructural evolution of SiCN fibers with different C/N ratios that derived from polycarbosilane fibers at the annealing temperature range of 1400∼1600 °C. These results showed that the phase separation of SiC_xN_y phase and the two-dimension grain growth process of free carbon nanoclusters could be processed at the researched temperature range. As the annealing temperature increased to 1600 °C, the crystallization of amorphous SiC and Si₃N₄ could be detected. SEM and Raman analysis showed that the decomposition and carbothermal reduction of the Si₃N₄ phase at high temperatures played primary roles in contributing to the fiber strength degradation. Thus, a higher C/N ratio, which is beneficial for inhibiting the decomposition of amorphous Si₃N₄, helps SiCN fibers retain high tensile strength at high temperatures.     

Wet-chemical coating of silicon carbide fibers with hexagonal boron nitride layers

Hexagonal BN fiber coatings and BN powders were prepared by pyrolysis of the raw materials boric acid and urea in an atmosphere consisting of hydrogen and nitrogen. The powders were used to determine the appropriate mixing ratio of the raw materials to produce BN with the desired composition and crystal structure. The pyrolysis of boric acid and urea in a molar mixing ratio of 1:2 resulted in a BN that was hexagonal and had a near-stoichiometric composition.To prepare a solution for the coating of fibers, boric acid and urea were dissolved in an ethanol-water mixture. The coating was then applied to SiC filaments using a continuous roll-to-roll dip-coating process. It could be shown by SEM/EDS that BN layers were applied to the fibers. No significant bridging in the fiber bundle was found. Furthermore, it could be demonstrated by grazing incidence x-ray diffraction that the layers were crystalline.     

Electromagnetic wave absorption and mechanical properties of silicon carbide fibers reinforced silicon nitride matrix composites

It is difficult for ceramic matrix composites to combine good electromagnetic wave (EMW) absorption properties (reflection coefficient, RC less than -7 dB in X band) and good mechanical properties (flexural strength more than 300 MPa and fracture toughness more than 10 M P·m^1/2). To solve this problem, two kinds of wave-absorbing SiC fibers reinforced Si₃N₄ matrix composites (SiC_f/Si₃N₄) were designed and fabricated via chemical vapor infiltration technique. Effects of conductivity on EM wave absorbing properties and fiber/matrix bonding strength on mechanical properties were studied. The SiC_f/Si₃N₄ composite, having a relatively low conductivity (its conduction loss is about 33% of the total dielectric loss) has good EMW absorption properties, i.e. a relative complex permittivity of about 9.2-j6.4 at 10 GHz and an RC lower than ?7.2 dB in the whole X band. Its low relative complex permittivity matches impedances between composites and air better, and its strong polarization relaxation loss ability help it to absorb more EM wave energy. Moreover, with a suitably strong fiber/matrix bonding strength, the composite can transfer load more effectively from matrix to fibers, resulting in a higher flexural strength (380 MPa) and fracture toughness (12.9 MPa?m^1/2).     

Anisotropic thermal conductivity of silicon nitride ceramics containing carbon nanostructures

Silicon nitride (Si₃N₄) composites containing carbon nanotubes (CNTs) or graphene nanoplateles (GNPs) are of great relevance in the electronic and aerospace industries where the search for new materials with enhanced and anisotropic thermal conductivity to work in harsh environments is a strategic guideline. Here we study thermal conduction in Si₃N₄ composites with different amounts of carbon nanostructures. The effects of the nanostructure orientation respect the heat flux, the testing temperature and the α/β Si₃N₄ phase ratio are analyzed. The addition of CNTs and GNPs leads to an anisotropic thermal response, decreasing the through-thickness thermal conductivity of the Si₃N₄ composites and raising the in-plane thermal conductivity, especially for GNPs that enhance it up to twice that of the monolithic Si₃N₄. This effect is related to the preferred orientation of the nanostructures that gives a less resistive network in the in-plane direction and the intrinsic anisotropy of their thermal conductivity.