In-situ construction of plasma-pretreated CNT networks endow the LPDS dual-layer coating with advanced thermal management and anti-static properties

Carbon nanotubes (CNTs) for temperature regulation have been proven to be promising for passive radiative heat dissipation. However, it remains a considerable challenge to assemble a CNTs layer in situ while simultaneously achieving horizontal electrical conductivity, vertical electrical insulation, and radiative heat dissipation. Herein, plasma pretreatment was employed to functionalize CNTs in an aqueous solution, thus improving their dispersion capability. Subsequently, a liquid-plasma-assisted particle deposition and sintering (LPDS) technology was proposed to prepare a dual-layer coating with an Al₂O₃-P₂O₅-SiO₂-In₂O₃ glass system embedded with crystalline indium tin oxide (ITO) as the porous bottom layer and ITO-CNTs as the top layer on the aluminum alloy surface. The results show that plasma pretreatment significantly increases the deposition amount of ITO nanoparticles and functionalized CNTs on the coating surface, resulting in the transition from a single-layer composite coating to a dual-layer coating. The surface micro-/nano hierarchical structure favors strong absorptance/emittance, exhibiting a high infrared emittance of 0.94 (3-20 μm) and high solar absorptance of 0.92 (0.2-2.5 μm). Meanwhile, the surface balance temperature of the dual-layer coating is about 392 K, which is 146 K lower than that of aluminum alloy. Furthermore, the top conductive ITO-CNTs layer contributes to the low surface resistivity of 3.46×10² Ω, while the glass phase of the bottom layer ensures vertical electrical insulation with a volume resistance of 4.20×10⁷ Ω. The process provides a new path for preparing thermal control coatings with anti-static function.     

Characterization of carbon fiber-reinforced ultra-high temperature ceramic matrix composites fabricated via Zr-Ti alloy melt infiltration

Carbon fiber-reinforced ultra-high temperature ceramic matrix composites (C/UHTCMCs) were fabricated via Zr-Ti alloy melt infiltration (Zr-Ti MI) using carbon-carbon composite (C/C) preforms and alloys with three different compositions. Alloys were successfully infiltrated into C/C to form solid solutions of TiC and ZrC, with melting temperatures > 2900 °C. Notably, residual alloys were not observed after MI occurred at 1750 °C. Bending strength and fracture toughness of the C/UHTCMCs at room temperature and 1500 °C in air/Ar revealed that mechanical properties of the composites were similar to those of the C/C preform. During arc wind tunnel tests at 2000 °C, a recession of C/UHTCMCs fabricated using Ti-rich alloys was observed; however, this behavior was not observed for the composites prepared using Zr-rich alloys owing to the formation of a ZrO₂ solid solution. Accordingly, Zr-Ti MI is a viable method for preparing C/UHTCMCs without degrading the mechanical properties of the C/C preform, while increasing the ablation resistance.     

Densification,microstructures, and mechanical properties of (Zr,Ti)(C,N) ceramics fabricated by spark plasma sintering

Dense (Zr, Ti) (C, N) ceramics were fabricated by spark plasma sintering (SPS) at 1900–2000 °C using ZrC, TiCN and ZrH₂ powders as raw materials. A single Zr-rich (Zr, Ti)(C, N) solid solution was formed in Zr_0.95Ti_0.05C_0.975N_0.025 and Zr_0.80Ti_0.20C_0.90N_0.10 ceramics (nominal composition). A Ti-rich solid solution appears in Zr_0.50Ti_0.50C_0.75N_0.25 ceramics. The coaddition of TiCN and ZrH₂ promoted the densification of (Zr, Ti) (C, N) ceramics by forming solid solutions and carbon vacancies, which could reduce critical resolved shear stress (CRSS) and promote carbon and metal atom diffusion. ZrC-45 mol% TiCN-10 mol% ZrH₂ (raw powder composition) possesses good comprehensive mechanical properties (Vickers hardness of 24.5 ± 0.9 GPa, flexural strength of 503 ± 51 MPa, and fracture toughness of 4.3 ± 0.2 MPa·m^1/2), which reach or exceed most ZrC-based (Zr, Ti) C and (Zr, Ti) (C, N) ceramics in previous reports.     

In-situ characterization on crack propagation behavior of SiCf/SiC composites during monotonic tensile loading

The microstructure and crack propagation path of 2.5D SiC_f/SiC composites were observed by synchrotron radiation x-ray computed micro tomography (SR-μCT) equipped with in-situ tensile device. The results showed that the pore morphologies of the SiC_f/SiC composites are mainly divided into three types in three-dimension space: interconnected pores, isolated pores and micro pores in fiber bundles. The crack initiation occurred at the root of the defects under in-situ tensile load and the crack was perpendicular, parallel to the stress axis or mixed mode to propagate. At the interface scale between fiber and matrix, the crack deflection will be controlled by physical parameters such as fracture energy release rate and the modulus of elasticity. At the fiber bundle scale, the crack is easy to shear propagate along the interface between weft and warp fiber bundles due to the existence of the mechanical bonding and residual tensile stress.     

MAS-content dependence of the texture and fracture behavior of h-BN-MAS composite ceramics

Texturing was employed to tailor the mechanical properties of h-BN ceramic with MAS as liquid texturing aid. Effects of MAS content on the texture behavior, densification behavior, bending strength, fracture toughness and the fracture behavior of the h-BN ceramics were studied. Moderate MAS of 30 wt% can significantly facilitate the orientation of h-BN grains in the direction perpendicular to the hot-pressing direction. MAS is also in favor of the densification behavior of h-BN as sintering aid. In-situ mechanical testing shows that texturing can greatly improve the mechanical properties and avoid the catastrophic fracture of the h-BN ceramics, and the textured h-BN ceramics exhibit distinct rising R-curve behavior, which is primarily attributed to the effect of parallel-aligned h-BN grains to the crack propagation under loading.     

Enhanced mechanical properties,thermal shock resistance and ablation resistance of Si2BC3N ceramics with nano ZrB2 addition

The mechanical properties, thermal shock resistance, and ablation resistance of nano ZrB₂ modified Si₂BC₃N ceramics were investigated. The results show that ZrB₂ stimulated microstructure evolution obviously. Therefore, the maximum strength and fracture toughness reach 559.6 MPa and 6.77 MPa·m^1/2, which are improved by 61.0% and 29.4%, respectively. Furthermore, the residual strengths of 10 wt% ZrB₂ containing composites tested at 1000 ℃ retain 363.6 MPa, which is much higher than 97.7 MPa of pristine Si₂BC₃N ceramics. Besides, the ablation resistance of ZrB₂ modified Si₂BC₃N ceramics at 3000 ℃ is enhanced remarkably and the linear and mass ablation rates of ZrB₂-10 are only 0.009 mm/s and 1.91 mg/s, respectively. The ablation in the ultra-high temperature zone is totally dominated by the ZrB₂ component, and the thermochemical erosion is determined by the oxidation resistance of ZrB₂ in the thermal affected zone.     

Corrosion behavior and strength degradation of Ti3SiC2 exposed to a eutectic K2CO3 and Li2CO3 mixture

The corrosion of polycrystalline Ti₃SiC₂ was studied in the eutectic Li₂CO₃ (68 at.%) and K₂CO₃ (32 at.%) mixture at 650–850 °C. Ti₃SiC₂ exhibited better corrosion resistance at 650 °C. However, the mass loss was fast when temperature was above 700 °C. It was demonstrated that the surface chemical reaction-controlled shrinking core model could be applied to describe the relationship between the degree of the corrosion and reaction time for the corrosion of Ti₃SiC₂ in the 700–850 °C temperature range. The corresponding apparent activation energy was 206 kJ/mol. Corrosion resulted in roughness of specimen surface. The fracture strength of the corroded samples was evaluated by a three-point bending test. The results showed that the degradation of the fracture strength was about 25% of the original values for the corroded specimens up to 10% weight loss. The mechanism of the strength degradation was discussed based on the analysis of the microstructure and composition of the corroded sample.     

Improvement of high-temperature mechanical properties of heat treated Cf/geopolymer composites by Sol-SiO2 impregnation

Unidirectional carbon fiber reinforced geopolymer composite was prepared by ultrasonic-assisted slurry infiltration method and heat treated at 1100 °C. Then it was impregnated with Sol-SiO₂ to seal the cracks and pores formed during heat treatment. The ambient strength of composite after impregnation was enhanced by 35.6% due to the increase relative density from the starting 79% to 93.6%. Composites both before and after impregnation fractured in a non-brittle manner at both ambient and high temperatures. Over an elevated temperature range from 700 to 900 °C, the strength of the two composites showed anomalous gains and reached their maximum values at 900 °C, 322.1 and 425.1 MPa, respectively. These values were 19.8% and 16.8% higher than their ambient ones. When the temperature was further increased to 1100 °C, the impregnated composite showed superior high-temperature properties, which was attributed to the improved fiber integrity due to the Sol-SiO₂ sealing effect.     

Refractory metal silicides reinforced by in-situ formed Nb2O5 fibers and mullite nanoclusters

There is keen interest in the use of refractory metal silicides as structural materials or thermal barrier coatings for a high temperature environment. However, a long-standing problem for these materials is their poor thermal shock property. To address this challenge, Nb-Al-SiC elements were introduced into the MoSi₂ matrix and consolidated by in-situ hot pressing. We find that this treatment leads to improved performance of MoSi₂ composites in high temperature thermal shock resistance and bending strength. After in-situ HPing, the Nb, Al₂O₃ particles, and SiC nanoclusters were uniformly dispersed in the MoSi₂ matrix and inhibited the movement of dislocation, resulting in a strengthening effect. During the thermal shock process, the fragmentized oxide layer present in the surface of the pure MoSi₂ alloy disappeared completely, and a dense multi-component oxide layer was formed in-situ on the surface of the MoSi₂ composites. The dense multi-component oxide layer was composed of SiO₂ glass, fiber-structured Nb₂O₅, and nano-sized mullite phases. The fiber structured and nano-sized oxide phases play an important role in strengthening the oxide layer.     

Effect of the starting AlN content on the phase formation and property of the novel in-situ fabricated X-SiAlON/BN composites

Novel in-situ X-SiAlON reinforced BN composites were first fabricated via the procedure of mechanical alloying plus hot press sintering. The effects of the starting AlN content (0∼25 vol%) on phase formation, evolution and microstructure were carefully investigated. XRD results indicated that AlN content was the crucial factor in the phase composition and evolution in the composites, for instance, excess AlN leading to the transformation from X-SiAlON to β-SiAlON. The relationships of AlN content with mechanical, thermal and dielectric properties of the composites were also involved in this study. The composite with the 15 vol% AlN, mainly consisting of X-SiAlON and BN, exhibited the best mechanical properties (flexural and fracture strength were 337.5 MPa and 4.15 MPa.m^1/2, respectively), low thermal conductivity as well as the excellent dielectric properties (ε < 5.71), which enabled it to be a promising candidate for the application of high-temperature structural/ functional materials.