Mechanical properties of the SiCf/SiC composites reinforced with KD-I and KD-II fibers fabricated assisted by a microwave heating method

SiC_f/SiC composites using KD-I and KD-II SiC fibers braided preforms as the reinforcements were fabricated by applying the polymer impregnation and pyrolysis (PIP) technique with a microwave heating assistance. The microwave heating temperature was 1100 °C, 1200 °C, 1300 °C, and 1400 °C, respectively. Microstructures, flexure properties, and fracture behaviors of the composites were investigated. The KDIISiC_f/SiC composites exhibited higher flexure properties and improved non-brittle fracture characteristics than those of the KD-ISiC_f/SiC composites. The differences in the flexural properties, fracture behaviors and microstructures between the KD-I and KDIISiC_f/SiC composites were discussed based on the tensile properties of the SiC filaments and the interfacial bonding statues in the composites.     

ZrC/C aerogel with high compressive strength by a carbothermic process

The ZrC/C aerogel is successfully prepared through copolymerization sols combined with carbothermal reduction method, using the ZrOC and phenol formaldehyde (PF) as the sols, and hexamethylenetetramine (HMTA) as cross-linker, respectively. The effects of heating treatment temperatures on physical and thermal properties of the aerogels are also investigated. XRD, SEM, and XPS measurements were adopted to characterize the morphology and microstructure of aerogels. Aerogels displayed low bulk density (0.262?0.379 g/cm³), relatively low thermal conductivity (0.0896?0.1064 W/m?K), and high compressive strength (0.87–4.42 MPa). XRD results show that the aerogel is composed of ZrC and ZrO₂ phases at 1400 °C and ZrC at 1500 °C or even high temperature, respectively, indicating that phase transformation of ZrO₂ into ZrC. XPS results demonstrated that the element Zr, C, and O are based on ZrOC, ZrC, and ZrO valence band binding, respectively. ZrC/C aerogels with excellent physical and thermal properties may be used in high-temperature field soon.     

Preparation of NiW/TiNY2O3 composite ceramic coating for metallic parts protection by direct current deposition

A novel NiW/TiNY₂O₃ composite ceramic coating has been synthesized by direct current deposition for metallic parts protection. The structural, morphology, hardness and anti-corrosion properties of the NiW/TiNY₂O₃ coating have been evaluated by SEM, EDS, TEM, XRD and EIS methods. Results indicated that the samples have uniform and compact nodular structure without defects. It demonstrated that the TiN and Y₂O₃ nanoparticles had been uniformly distributed in the composites. The incorporation of TiN and Y₂O₃ in NiW matrix could improve the hardness and anti-corrosion properties. The crystallite size was in the diameter of 13–16 nm. The electrochemical results illustrated that 6-8 Adm⁻² and 30 min were beneficial to the improvement of anti-corrosion behaviors of the produced composite coating. After immersed 168 h in 3.5 wt% NaCl aqueous solution, the coating prepared at 30 min and 2 A dm⁻² owns better anti-corrosion properties. The embedded TiN and Y₂O₃ nanoparticles in NiW matrix could decrease the electrochemical activity and enhance the protective properties.     

Effect of nano-AlN addition on thermal diffusivity and mechanical strength of porous AL2O3 ceramic composites

In a properly made porous abrasive composite, the vitrified bond should ideally cover the grains and form a continuous network of bridges, and thus part of the heat energy from the grinding process is also transferred to the vitrified bond. Until recently, most studies on the design of composite properties have focused mainly on improving their mechanical strength and wear resistance, but increasingly the very important aspect of their thermal properties is noticed. The vitrified Al₂O₃ composites were made from Al₂O₃ grains, vitrified bond of Na₂OK₂OAl₂OB₂O₃SiO₂ and AlN nanopowder. The increase in porosity in the tested composites is the effect of the AlN decomposition reaction. Crystalline phases were identified in both composites - α-Al₂O₃ and NaAl₁₁O₁₇, but with a different percentage share in individual composites. In composites doped with AlN nanopowder, the proportion of NaAl₁₁O₁₇ crystalline phase decreases, due to its high susceptibility to reduction by Al, obtained from the AlN decomposition reaction. The product of the redox reaction is also Na⁺ ions, which may participate in the formation of the glass phase and thus increasing the fraction of the residual glass phase. As a result of the partial reduction of NaAl₁₁O₁₇ phase, an increase in α-Al₂O₃ content is observed. A higher proportion of α-Al₂O₃ phase with high thermal conductivity can be a factor that increases the rate of heat removal from the work zone.     

Gel reactive melt infiltration: A new method for large-sized complex-shaped C/C components ceramic modification

To widen the applications of conventional reactive melt infiltration (RMI) in large-sized complex-shaped C/C components, an ingenious process of gel-RMI (GRMI) was proposed in this study. The arching C/CSiC composite was prepared successfully using GRMI method with polycarbosilane (PCS)Si90Zr10 (Si: 90 at.%; Zr:10 at.%) sol. The porosity rate of the C/C preform decreased from 18.5% to 2.9%, while the density was raised from 1.40 g·cm⁻³ to 2.05 g·cm⁻³ after GRMI. The reason why C/C preform has been significantly densified is as follow: the PCS in PCS-Si90Zr10 sol formed SiC aerogel skeleton after pyrolysis, and then the Si90Zr10 powders were melted and released from the SiC aerogel into the C/C preform body when the temperature reached the melting point of Si90Zr10 alloy. The obtained C/CSiC composite showed a pseudo-ductile rupture characteristic distinguished from that of the C/C preform, and its bending strength was significantly improved from 104.2 MPa of the C/C preform to 258.8 MPa. The C/CSiC composite had a far lower mass ablation rate of 0.75 mg·s⁻¹ than that of C/C preform, 23.30 mg·s⁻¹. Moreover, the GRMI was preliminarily applied in ceramic modifying nozzle-like C/C preform, and the result showed that the nozzle-like C/C preform was successfully densified from 1.3 g cm⁻³ to 1.96 g cm⁻³. The GRMI process has great potential in ceramic modifying large-sized complex-shaped C/C components.     

The formation of chemical/structural gradients in strong covalent bonded SiBN fibers under active nitrogen atmosphere

In this work, SiBN fibers with chemical/structural gradients under the radial direction were fabricated by one-step treatment at 1700 °C under active nitrogen atmosphere. The formation of chemical gradient should be contributed to the boron atoms diffusion from the fiber interior to the surface, under the driven force of BN bond-breaking in Si–N–B networks to form stable BN bonds in the BNB networks at the fiber surface. Meanwhile, the structural gradients with graded phase separation of Si–N–B networks and crystallization behavior of Si₃N₄ phase could form under the effect of graded boron content distribution. The SiBN fibers with chemical/structural gradients showed better high-temperature stability due to the possibly synergistic effect of the decomposition resistant of boron-rich surface and the optimization of graded boron distribution. We expect our finding as a universal method to fabricate chemical/structural gradients on other boron-containing multinary ceramics with various shapes.     

A comparative process simulation study of CaCu looping involving post-combustion CO2 capture

This work presents a simulation study of several CaCu looping variants with CO₂ capture, aiming at both parameter optimization and exergy analysis of these CaCu looping systems. Three kinds of CaCu looping are considered: 1) carbonation–calcination/reduction–oxidation; 2) carbonation–oxidation–calcination/reduction and 3) carbonation/oxidation–calcination/reduction. A conventional Ca looping is also simulated for comparison. The influences of the calcination temperature on the mole fractions of CO₂ and CaO at the calciner outlet, the CaCO₃ flow rate on the carbonator performance and the Cu/Ca ratio on the calciner performance are analyzed. The second kind of CaCu looping has the highest carbonation conversion. At 1 × 10⁵ Pa and 820 °C, complete decomposition of CaCO₃ can be achieved in three CaCu looping systems, while the operation condition of 1 × 10⁵ Pa, 840 °C is required for the conventional Ca looping system. Furthermore, the Cu/Ca molar ratio of 5.13–5.19 is required for the CaCu looping. Exergy analyses show that the maximum exergy destruction occurs in the calciner for the four modes and the second CaCu looping system (i.e., carbonation–oxidation–calcination/reduction) performs the highest exergy efficiency, up to 65.04%, which is about 30% higher than that of the conventional Ca looping.     

Glass powders and reactive silicone binder: Interactions and application to additive manufacturing of bioactive glass-ceramic scaffolds

A novel concept for the additive manufacturing of three-dimensional glass-ceramic scaffolds, to be used for tissue engineering applications, was based on fine glass powders mixed with a reactive binder, in the form of a commercial silicone. The powders consisted of ‘silica-defective glass’ specifically designed to react, upon firing in air, with the amorphous silica yielded by the binder. By silica incorporation, the glass was intended to reach the composition of an already known CaONa₂OB₂O₃SiO₂ system. Silica from the binder provided up to 15 wt% of the total silica. With the same overall formulation, silicone-glass powder mixtures led to nearly the same phase assemblage formed by the reference system, crystallizing into wollastonite (CaSiO₃) and Ca-borate (CaB₂O₄). Samples from silicone-glass powder mixtures exhibited an excellent shape retention after firing, which was later exploited in highly porous reticulated scaffolds, obtained by means of direct ink writing (DIW).     

A family of functional oxides of titanosilicates: A2TiSi2O8 (A= Ba,Sr) with temperature insensitive ultrahigh breakdown strength

Discovering new materials with high breakdown strength is of great significance for scientific research and industrial applications. Here, we combine experiments and density functional theory (DFT) calculations to report a systematic study of two titanosilicates - A₂TiSi₂O₈ (A = Br, Sr) as a family of functional materials with wide band gaps, ultrahigh breakdown strength, and high thermal stability. The A₂TiSi₂O₈ materials possess ultrahigh electric breakdown (E_b >200 kVmm⁻¹, at 200 ℃), and good temperature stability (the variation of capacitance 3 %, from −100 to 200 ℃). DFT results show that electrons are likely localized in the SiO and TiO polyhedrons. The electron difference density map and density of states (DOS) results elucidate the wide band gaps in titanosilicate family due to the strong SiO and TiO bonds, indicating that the semiconductive behaviors are similar. The temperature-dependent structural evolution (from −100 to 500 ℃) is investigated via in-situ Raman spectroscopy.     

Structure,bond characteristics and microwave dielectric properties of new A0.75Ti0.75Ta1.5O6 (ANi,Co, Zn and Mg) ceramics based on complex chemical bond theory

Tri-rutile structured A_0.75Ti_0.75Ta_1.5O₆ (ANi, Co, Mg, Zn) ceramics were synthesized using traditional solid reaction method. The crystal structures were studied by X-ray diffraction in conjunction with Rietveld refinement analysis. Based on the complex chemical bond theory and crystallographic data, some principle chemical bond characteristics such as bond ionicity, lattice energy, bond energy and coefficient of thermal expansion of complex A_0.75Ti_0.75Ta_1.5O₆ ceramics were obtained through quantitative calculation. The calculated results provided useful information to clarify the correlations between chemical bond characteristics and microwave dielectric properties of A_0.75Ti_0.75Ta_1.5O₆ ceramics. The dielectric constant was closely associated with the ionicity of TaO bond, and the Q × f values were correlated with the lattice energy of TaO bond. The τ_f values were affected by the bond energy of TaO bond and the coefficient of thermal expansion of AO bond.     

Static and dynamic oxidation behaviour of silicon carbide at high temperature

SiC has extensive applications in high-temperature oxidation environments. However, few studies have investigated the differences between the static and dynamic oxidation behaviour. In this study, the static and dynamic oxidation of SiC were investigated in air and in plasma wind tunnels, respectively. The results demonstrated that the activation energy of static oxidation was ～68.02 kJ/mol at 1300–1600 ℃, which was approximately ten times that of dynamic oxidation ～7.05 kJ/mol at 1290–1534 ℃. The observed Si-O-C transition layer located at the SiO₂/SiC interface, and its thickness after dynamic oxidation for 300 s was thicker than that after static oxidation for 30 h. In dynamic oxidation, high-speed flowing atomic oxygen reacted directly with SiC, whereas molecular oxygen needed extra energy to break the OO bond and react with SiC in static oxidation. Atomic oxygen also migrated easier in the amorphous SiO₂ coating, contributing to a thicker Si-O-C layer and lower activation energy.     

Ablative property of ZrCSiC multilayer coating for PIP-C/SiC composites under oxy-acetylene torch

To improve ablation resistance of PIP-C/SiC composites, ZrCSiC multilayer coating was prepared on surface of PIP-C/SiC composites by chemical vapor deposition and slurry. The coating shows dense surface and outstanding anti-ablation ability. Compare with uncoated PIP-C/SiC, the linear and mass ablation rates of the coated PIP-C/SiC decrease by 59.5% and 50.3%, respectively, after ablation for 30 s. Large amounts of heat can be taken away by the gas generated during ablation, which is also helpful for protection for the composites.     

Synthesis of MgSiO3 ceramics using natural desert sand as SiO2 source

High-quality inorganic minerals are being increasingly consumed in the fabrication of ceramics. In the present work, desert sand was adopted as the source of SiO₂ to synthesize MgSiO₃ and MgSiO₃SiC composite ceramics in an endeavor to economize on mineral resources and improve the desert ecosystem through the industrial application of desert sand. Experimental results show that the use of drift sand enabled the formation of glass phase at lower temperatures, which promoted protoenstatite transformation, prevented sintering cracking, and densified the ceramic bodies. The composites consisted of MgSiO₃ (protoenstatite), SiC, glass phase, and small amounts of forsterite and SiO₂. The addition of SiC particles caused the green bodies to resist densification, but this was improved by increasing the sintering temperature. The composite ceramic containing 30 wt % of SiC and sintered at 1350 °C had the highest bending strength, whereas that containing 50 wt% of SiC and sintered at 1400 °C had the highest hardness and the lowest coefficient of thermal expansion among all the samples.     

Effect of BaOB2O3 composite sintering aid on sinterability and electrical property of BaZr0.85Y0.15O3-δ ceramic

Yttrium-doped barium zirconate (BZY) has been extensively studied as a promising electrolyte material for protonic ceramic fuel cells. However, inferior sinterability is a major barrier in BZY development. In our research, the effect of BaOB₂O₃ composite sintering aid on the sintering behavior and electrical property of BaZr_0.85Y_0.15O_3-δ (BZY15) are examined. BaOB₂O₃ addition reduces the sintering temperature of BZY15 by approximately 200 °C via the liquid-phase sintering mechanism. The corresponding bulk and grain boundary conductivities are prominently improved (<3 wt% BaOB₂O₃ addition), whereas the further addition of sintering aid decreases the grain boundary conductivity. Notably, the scanning electron microscope (SEM) and electrochemical impedance spectroscopy (EIS) analyses suggest that the enhanced conductivity may be related to the temperature dependence of Ba evaporation.     

Reactive wetting of amorphous silica by Sn0.3Ag0.7Cu-xTi (x = 1 and 3 wt.%) alloys at 800–900 °C

The wetting of SiO₂ by molten Sn_0.3Ag_0.7Cu(SAC)-xTi (x = 1 and 3 wt.%) was studied by using the modified sessile drop method at 800–900 °C under a high vacuum. The small addition of Ti into SAC alloy can improve wettability, significantly. Because the SnTi intermetallics with high melting point covered Ti addition and blocked further dissolution of Ti, and thus Ti cannot be dissolved completely when the nominal concentration of Ti was 3 wt.%. The reaction products at liquid/solid interface are Ti₅Si₃ and TiO. The spreading dynamics can be described by reaction product control model. The spreading may be coupled firstly with the precipitation of Ti₅Si₃ and TiO together meanwhile be affected by the dissolution of Ti addition, and then the precipitation of TiO alone. The final wettability was determined both by the wetting character of reaction products and also TiO adsorption at interface.     

Effect of SiO2 deposition on thermal stability of Al2O3-SiO2 aerogel

The SiO₂-deposited Al₂O₃-SiO₂ aerogel (Si-ASA) with high thermal stability and mechanical properties was synthesized by silica deposition method during aging. Compared with the ASA without silica deposition, the Si-ASA maintains higher specific surface areas after being calcined at 1000 °C (384.0), 1100 °C (245.5), and 1200 °C (124.2 m²/g). When the heating temperature exceeds 1000 °C, the mullite phase begins to appear, and the crystallization activation energy of the Si-ASA (1019.24 kJ/mol) is higher than that of the ASA (975.95 kJ/mol). The results show that silica deposition can restrain viscous flow between particles and increase skeleton strength by particles growth and skeleton coarsening, and it can restrain the growth of γ-Al₂O₃ by forming more SiOAl bonds in the system. This work is of great significance to the design of super thermal insulation materials with high thermal stability.     

Correlation of the fatigue impact resistance of bilayer and nanolayered PVD coatings with their cutting performance in machining Ti6Al4V

Titanium alloys are widely employed in aerospace and biomedical applications. During the cutting process of Ti6Al4V alloy, the material characteristics of high specific strength and poor thermal conductivity lead to a stress concentration occurred at the tool/chip interface, which may cause severe failure of the coated tools. Besides, the titanium alloy gives rise to serrated chips, whose thickness varies regularly in the cycle mode, which lead to the variation of cutting force and tool stress. So a cyclic impact with high frequency is applied on the coated tool surface, and this induces the fatigue fracture of coating. In this study, repetitive contact between the coating and Vickers diamond indenter through the high frequency cyclic impact tester was developed by employing the ultrasonic actuator device. High frequency cyclic impact testing has been used to study the impact resistance of the TiSiN/TiAlN bilayer coating and TiSiN/TiAlN nanolayered coating. Besides, stress concentration resulting in the failure of coatings can be generated by using the sharp Vickers indenter. The crack propagation in coatings was investigated by the scanning electron microscope (SEM), and the fracture processes of coatings were studied from the force measurement data acquisition system. In addition, the cutting performance of cemented carbide tools coated with these two coatings was evaluated in turning Ti6Al4V titanium alloy. The correlation between the fatigue impact resistance of coatings at high frequency and the wear behavior of coated tools under different cutting conditions was studied. It can be concluded that the TiSiN/TiAlN nanolayered coating showed better properties to resist fatigue impact at the lower impact load, but worse properties to resist fatigue impact at the higher impact load.     

Ablation behavior and mechanism of C/C–ZrC–SiC composites under an oxyacetylene torch at 3000 °C

C/C–ZrC–SiC composites with continuous ZrC–SiC ceramic matrix were prepared by a multistep technique of precursor infiltration and pyrolysis process. Ablation properties of the composites were tested under an oxyacetylene flame at 3000 °C for 120 s. The results show that the linear ablation rate of the composites was about an order lower than that of pure C/C and C/C–SiC composites as comparisons, and the mass of the C/C–ZrC–SiC composites increased after ablation. Three concentric ring regions with different coatings appeared on the surface of the ablated C/C–ZrC–SiC composites: (i) brim ablation region covered by a coating with layered structure including SiO₂ outer layer and ZrO₂–SiO₂ inner layer; (ii) transition ablation region, and (iii) center ablation region with molten ZrO₂ coating. Presence of these coatings which acted as an effective oxygen and heat barrier is the reason for the great ablation resistance of the composites.     

Study on physicochemical properties of Al2O3SiCC castable for blast furnace

The physicochemical properties of a new Al₂O₃SiCC castable (ASCC) material were systematically investigated in this study. The results indicate that different heat treatment (HT) temperatures can lead to the formation of different phases, generating performance changes. A high HT temperature of 1773 K can create the mullite phase, serving as a strength-enhancer and shrinkage-compensator. The coexistence of carbon and corundum phases can effectively impede the corrosion caused by iron and slag, respectively, while the low carbon content in the ASCC can strengthen the iron attack resistance. The compressive strength of the ASCC showed a greater than 40% decrease after impurity element (K, Na, and Zn) corrosion experiments, which is greater than the change of the breaking strength. The larger atomic diameter of alkali metal could cause local relaxation and volume expansion of the aluminosilicate structure. The alkali metal scatters on the substrate; however, zinc exists in the form of ZnS and is primarily concentrated in the voids of the ASCC.     

Synthesis of cyano-polycarbosilane and investigation of its pyrolysis process

Polycarbonsilane (PCS) is an important precursor of silicon carbide (SiC) fibers and ceramics. The ceramic yield of PCS is relatively low, about 60 %, which may bring some deficiencies in its applications. In this work, a novel precursor cyano-polycarbosilane (PCSCN) is synthesized by hydrosilylation reaction between PCS and acrylonitrile using a rhodium-containing catalyst, although acrylonitrile is generally not easy for hydrosilylation. After introducing tiny amounts of cyano (-C≡N) groups into the PCS molecules, the ceramic yield of PCSCN can increase largely to over 80 %. The ceramization mechanism of PCSCN is investigated by FTIR, TG, XPS, ESR, NMR, Raman and XRD analyses. It is found that some crosslinking structures in PCSCN are formed between SiH bonds and CN groups from about 200 ℃, which can be responsible for the high ceramic yield. The existence of a little more N, O and free C elements in the pyrolysis products may inhibit the growth of crystalline β-SiC. Moreover, the PCSCN precursor can also be melt-spun into continuous fibers by tailoring its molecular weight and softening point. The oxidized PCSCN fiber with relatively low oxygen content can be pyrolyzed without melting, and the final SiC fiber with an oxygen content as low as 8.5 % is obtained.