Preparation of the novel B4C–SiC composite aerogel with high compressive strength and low thermal conductivity

Journal of Porous Materials - A novel B4C–SiC composite aerogel is synthesized using nano boron carbide suspension, 3-aminopropyltriethoxysilane (APTES), and resorcinol–formaldehyde...     

ZrSi2–MgO as novel additives for high thermal conductivity of β-Si3N4 ceramics

A novel ZrSi₂–MgO system was used as sintering additive for fabricating high thermal conductivity silicon nitride ceramics by gas pressure sintering at 1900°C for 12 hours. By keeping the total amount of additives at 7 mol% and adjusting the amount of ZrSi₂ in the range of 0-7 mol%, the effect of ZrSi₂ addition on sintering behaviors and thermal conductivity of silicon nitride were investigated. It was found that binary additives ZrSi₂–MgO were effective for the densification of Si₃N₄ ceramics. XRD observations demonstrated that ZrSi₂ reacted with native silica on the Si₃N₄ surface to generate ZrO₂ and β-Si₃N₄ grains. TEM and in situ dilatometry confirmed that the as formed ZrO₂ collaborated with MgO and Si₃N₄ to form Si–Zr–Mg–O–N liquid phase promoting the densification of Si₃N₄. Abnormal grain growth was promoted by in situ generated β-Si₃N₄ grains. Consequently, compared to ZrO₂-doped materials, the addition of ZrSi₂ led to enlarged grains, extremely thin grain boundary film and high contiguity of Si₃N₄–Si₃N₄ grains. Ultimately, the thermal conductivity increased by 34.6% from 84.58 to 113.91 W·(m·K)⁻¹ when ZrO₂ was substituted by ZrSi₂.     

Novel synthesis and thermal property analysis of MgO–Nd2Zr2O7 composite

MgO-Nd₂Zr₂O₇composites with ratios of 50–70 vol% MgO were produced via a one-pot combustion synthesis. A suite of characterization techniques, including X-ray diffraction, scanning and transmission electron microscopy were employed to investigate the structural properties while dilatometry, simultaneous thermal analysis and laser flash analysis were used to characterize the thermal properties of the composites. Dense pellets were produced after sintering at 1400 °C with grain sizes between 200 and 500 nm for both phases. The thermal properties of the composites are similar to those produced using standard methods. The composite with 70 vol% MgO was found to have the highest thermal conductivity below 1000 °C, while above this temperature the thermal conductivity was found to be similar and independent of MgO content. This novel synthesis route produces materials which show significant improvements in homogeneity with smaller particle sizes when compared to current standard synthesis techniques without significantly reducing thermal conductivity.     

Fabrication of high strength Li-rich 2Li2TiO3–Li4SiO4 composite breeding ceramics at low-temperature by two-step sintering

In this study, Li-rich 2Li₂TiO₃–Li₄SiO₄ composite breeding ceramics were prepared for the first time through a two-step low-temperature sintering route to meet the increasingly stringent performance requirements of tritium breeding materials. The effects of excess Li addition and sintering atmosphere on the phase composition, mechanical properties, and microstructure of the composite breeding ceramics were systematically studied. A two-step low-temperature sintering method was proposed to integrate the advantages of different sintering environments. The results show that moderate Li addition can significantly enhance the crushing load of composite ceramics while maintaining the nanostructure. However, further increasing the amount of Li addition does not continue to increase the crushing load, and will even affect the structural uniformity of the composite breeding ceramics. Most importantly, the S2.5 (2Li_2.5TiO₃–Li₄SiO₄, molar ratio) composite breeding ceramics sintered at 750 °C in an air-vacuum environment exhibited a grain size of ～86 nm and a crushing load of ～66.9 N, which has not been achieved by previous studies.     

Fabrication of Al2O3–ZrO2 ceramics with high porosity and strength

This study describes the synthesis of ceramics, in which a micrometre-sized Al₂O₃–ZrO₂ nanopowder was used as an oxide base for the hardening of the materials. To a suspension of this mixed metal oxide, the pore-forming crystallisation additives camphor and carbamide were added to produce ceramics with thin permeable pores. Camphor crystallised in the oxide suspension in the form of single pentagonal stars and сarbamide crystallised in the form of thin elongated needles. The use of the different crystallisation additives allowed the formation of ceramics after sintering that have both permeable and complex pore morphologies, where anisotropic properties were observed using carbamide as an additive but not when camphor was used. The total porosity of the resulting ceramics was 51.3%, with a compressive strength in the range of 17.3–92.3 MPa.     

Preparation of bioactive TiO2–MgO composite ceramics with bone-promoting properties

In the field of hard tissue repair, titanium-based materials have excellent mechanical properties and magnesium-based materials have good bioactivity, but their shortcomings are that titanium-based materials do not have good bioactivity, while magnesium-based materials are limited in application due to their rapid degradation rate. In order to give full play to the advantages of these two materials, the TiO₂–MgO composite ceramic materials were prepared by combining the two elements and sintering at high temperature. By changing sintering temperature and MgO content, the structure composition and bioactivity of composite ceramic materials can be controlled. The surface morphology, mineralization ability in vitro, cytotoxicity and bone-promoting properties of composite ceramic materials were studied. The experimental results show that high MgO content composite ceramic materials will bring too strong alkalinity to the environment, which will accelerate the mineralization ability of materials, but is not conducive to the survival of cells. Composite ceramic materials with suitable sintering temperature and MgO content have good bioactivity and bone-promoting performance, while the porous structure produced by MgO degradation is beneficial to cell spreading and can form a good combination between the material and bone tissue at an early stage. Porous structure and Mg²⁺ can adjust the bone-promoting properties of materials together. Through the above experimental research, it is found that TiO₂–MgO composite ceramic material is a new type of material which is used in the field of hard tissue repair due to its good bioactivity.     

Dual phase nano-particulate AlN composite — A kind of ceramics with high strength and ductility

Ceramics are widely used in many fields due to their excellent properties. However, the brittle fracture is a short board restricting their applications. To understand their deformation mechanism and explore a way to enhance both the strength and ductility, we investigated the mechanical behaviour of dual-phase AlNs composed of amorphous AlN matrix and crystalline nanoparticles under compression via molecular dynamics simulations. The stress concentration exists at the interface of nanocomposite AlN, where the particles and matrix are in the tensile and compressive states of stress, respectively. Strain hardening occurs when crystalline nanoparticle fraction f_v ≥ 40.9%, attributed to the intersection between shear bands. The phase transformation from wurtzite structure (B4) to graphene-like structure (GL) is observed in the crystalline phase, as a result of high hydrostatic stress. After phase transformation, the particle might be cut into half during further compression along with the recovery of the GL structure to the wurtzite structure that could still bear load. The investigation of the effects of the volume fraction, surface-to-volume ratio, distribution pattern of the crystalline nanoparticles indicates that the dual-phase AlN nanocomposite with f_v ≥ 40.9% and triangle distribution of particles would possess both higher strength and ductility.     

Temperature stability and high-Qf of low temperature firing Mg2SiO4–Li2TiO3 microwave dielectric ceramics

In this work, a series of low-temperature-firing (1−x)Mg₂SiO₄–xLi₂TiO₃–8 wt% LiF (x = 35–85 wt%) microwave dielectric ceramics was prepared through conventional solid state reaction. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that the Li₂TiO₃ phase was transformed into cubic phase LiTiO₂ phase and secondary phase Li₂TiSiO₅. Partial substitution of Mg²⁺ ions for Ti³⁺ ions or Li⁺Ti³⁺ ions increased the cell volume of the LiTiO₂ phase. The dense microstructures were obtained in low Li₂TiO₃ content (x ≤ 65 wt%) samples sintered at 900 °C, whereas the small quantity of pores presented in high Li₂TiO₃ content (x ≥ 75 wt%) samples sintered at 900 °C and low Li₂TiO₃ content (x = 45 wt%) sintered at 850 and 950 °C. Samples at x = 45 wt% under sintering at 900 °C for 4 h showed excellent microwave dielectric properties of ε_r = 10.7, high Q × f = 237,400 GHz and near-zero τ_f = − 3.0 ppm/°C. The ceramic also exhibited excellent chemical compatibility with Ag. Thus, the fabricated material could be a possible candidate for low temperature co-fired ceramic (LTCC) applications.     

Combustion synthesis of high flexural strength,low linear shrinkage and machinable porous β-Si3N4 ceramics

Porous β-Si₃N₄ ceramics were prepared by combustion synthesis using Si, α-Si₃N₄ and Y₂O₃ powders as raw materials. The effects of α-Si₃N₄ diluent content in pellet on nitriding rate, shrinkage, porosity and flexural strength of porous ceramics were investigated. The results show that porous β-Si₃N₄ ceramic with porosity of 49 % and flexural strength of 151 MPa can be obtained by combustion synthesis when the content of α-Si₃N₄ diluent in raw materials is equal to that of Si, and the linear shrinkage of porous ceramic is only 2.8 %. In addition, the porous ceramic can be drilled and turned by WC drill and turning tool respectively.     

BaMg1/3Nb2/3O3–Mg4Nb2O9 composite microwave ceramics with high Q-factor and low sintering temperature

Revised thermodynamic equilibrium in the BaO–MgO–Nb₂O₅ pseudo-ternary system has lead to development of a novel composite dielectric material with dielectric constant, ?′ = 25.5, efficacy factor, Q × f = 160 THz, and temperature coefficient of the resonant frequency, τ_f = +0.5 ppm/K. The material shows one of the highest Q-factors among the Ta-free microwave dielectric resonators. It also does not contain volatile Zn and Co elements. Other important property of the title compound is low sintering temperature of 1320 °C which significantly reduces the processing cost.     

Optimization of SiC–ZrC high emissivity composite flexible coating for thermal protection with high interfacial bond strength and temperature resistance

Aluminum silicate fiber fabric (ASFF) has been widely used in the outer surface of flexible insulation felt on the leeward side of aerospace vehicle. In order to improve the temperature resistance of ASFF, a kind of SiC–ZrC composite coating was prepared on the surface of fiber fabric via spraying method with SiC as emittance agent and ZrC as additive. The surface morphology and mechanical properties of the coating were studied. Compared with the single-component SiC coating, the composite coating could effectively avoid coating spalling and improve the surface integrity at high temperature. After thermal treatment at 1100 °C for 2 h, the interface bond strength of the composite coating/substrate was 52.41% higher than that of SiC coating/substrate. The tensile strength of fiber fabric with SiC–ZrC composite coating could reach 91.75 MPa, which was 101.76% higher than that of raw ASFF. Therefore, the SiC–ZrC coating could greatly improve the temperature resistance of ASFF, and has an attractive application prospect in the field of thermal protection system.     

Core–shell NTC materials with low thermal constant and high resistivity for wide-temperature thermistor ceramics

Core–shell structures have been proposed to improve the electrical properties of negative-temperature coefficient (NTC) thermistor ceramics. In this work, Al₂O₃-modified Co_1.5Mn_1.2Ni_0.3O₄ NTC thermistor ceramics with adjustable electrical properties were prepared through citrate-chelation followed by conventional sintering. Co_1.5Mn_1.2Ni_0.3O₄ powder was coated with a thin Al₂O₃ shell layer to form a core–shell structure. Resistivity (ρ) increased rapidly with increasing thickness of the Al₂O₃ layer, and the thermal constant (B) varied moderately between 3706 and 3846 K. In particular, Co_1.5Mn_1.2Ni_0.3O₄@Al₂O₃ ceramic with 0.08 wt% Al₂O₃ showed the increase of ρ double, and the change in its B was less than 140 K. The Co_1.5Mn_1.2Ni_0.3O₄@Al₂O₃ NTC ceramics showed high stability, and their grain size was relatively uniform due to the protection offered by the shell. The aging coefficient of the ceramic was less than 0.2% after aging for 500 hours at 125°C. Taken together, the results indicate that as-prepared Co_1.5Mn_1.2Ni_0.3O₄@Al₂O₃ NTC ceramics with a core–shell structure may be promising candidates for application as wide-temperature NTC thermistor ceramics.     

Cyclic thermal shock resistance of h-BN composite ceramics with La2O3–Al2O3–SiO2 addition

The effects of La₂O₃–Al₂O₃–SiO₂ addition on the thermal conductivity, coefficient of thermal expansion (CTE), Young's modulus and cyclic thermal shock resistance of hot-pressed h-BN composite ceramics were investigated. The samples were heated to 1000 °C and then quenched to room temperature with 1–50 cycles, and the residual flexural strength was used to evaluate cyclic thermal shock resistance. h-BN composite ceramics containing 10 vol% La₂O₃–Al₂O₃ and 20 vol% SiO₂ addition exhibited the highest flexural strength, thermal conductivity and relatively low CTE, which were beneficial to the excellent thermal shock resistance. In addition, the viscous amorphous phase of ternary La₂O₃–Al₂O₃–SiO₂ system could accommodate and relax thermal stress contributing to the high thermal shock resistance. Therefore, the residual flexural strength still maintained the value of 234.3 MPa (86.9% of initial strength) after 50 cycles of thermal shock.     

Mechanical properties and microstructure of porous BN–SiO2–Si3N4 composite ceramics

Porous silicon nitride (Si₃N₄) ceramics incorporated with hexagonal boron nitride (h-BN) and silica (SiO₂) nanoparticles were fabricated by pressureless-sintering at relatively low temperature, in which stearic acid was used as pore-making agent. Bending strength at room and high temperatures, thermal shock resistance, fracture toughness, elastic modulus, porosity and microstructure were investigated in detail. The mechanical properties and thermal shock resistance behavior of porous Si₃N₄ ceramics were greatly influenced by incorporation of BN and SiO₂ nanoparticles. Porous BN–SiO₂–Si₃N₄ composites were successfully obtained with good critical thermal shock temperature of 800 °C, high bending strength (130 MPa at room temperature and 60 MPa at 1000 °C) and high porosity.     

Porous (Ce0.2Zr0.2Ti0.2Sn0.2Ca0.2)O2-δ high-entropy ceramics with both high strength and low thermal conductivity

Single-phase (Ce_0.2Zr_0.2Ti_0.2Sn_0.2Ca_0.2)O_2-δ porous high-entropy ceramics have been in-situ fabricated by foam-gelcasting-freeze drying method at different temperatures. The microstructure, phase composition, and properties of the obtained ceramics were investigated. The results indicate that compared with other porous ceramics reported in the literatures, this type of ceramics exhibits excellent performance. The sample prepared at 1350 °C shows high porosity (88.6 %), low thermal conductivity (0.023 W m^-1 K^-1), and high compressive strength (1.48 MPa). The current study suggests that porous (Ce_0.2Zr_0.2Ti_0.2Sn_0.2Ca_0.2)O_2-δ high entropy ceramics are promising candidates for thermal insulation applications.     

Novel self-reinforcing ZrO2–SiO2 aerogels with high mechanical strength and ultralow thermal conductivity

Novel self-reinforcing ZrO₂–SiO₂ aerogels with high mechanical strength and ultralow thermal conductivity are fabricated by impregnating hydrolyzed ZrO₂–SiO₂ sol into wet gel matrix and drying. The ZrO₂–SiO₂ sol fills the macropores and defects of ZrO₂–SiO₂ aerogel matrix generating during the gelation process, which contributes to the improvement of the mechanical properties of the ZrO₂–SiO₂ aerogel matrix. The mechanical and thermal properties of the as-prepared ZrO₂–SiO₂ aerogel are investigated and discussed. The results show that the mechanical strength of the self-reinforcing aerogels obviously increases from 0.51 to 3.11?MPa with the increase of impregnation times, while the thermal conductivity of the aerogels slightly increases from 0.0235 to 0.0306?W?m^?1 K^?1. The novel self-reinforcing ZrO₂–SiO₂ aerogel could have interesting applications in aerospace and energy because of its outstanding mechanical and thermal properties.     

Low thermal conductivity in porous SiC–SiO2–Al2O3–TiO2 ceramics induced by multiphase thermal resistance

The introduction of multiple heterogeneous interfaces in a ceramic is an efficient way to increase its thermal resistance. Novel porous SiC–SiO₂–Al₂O₃–TiO₂ (SSAT) ceramics were fabricated to achieve multiple heterogeneous interfaces by sintering equal volumes of SiC, SiO₂, Al₂O₃, and TiO₂ compacted powders with polysiloxane as a bonding phase and carbon as a template at 600 °C in air. The porosity could be controlled between 66% and 74% by adjusting the amounts of polysiloxane and the carbon template. The lowest thermal conductivity (0.059 W/(m·K) at 74% porosity) obtained in this study is an order of magnitude lower than those (0.2–1.3 W/(m·K)) of porous monolithic SiC, SiO₂, Al₂O₃, and TiO₂ ceramics at an equivalent porosity. The typical specific compressive strength value of the porous SSAT ceramics at 74% porosity was 3.2 MPa cm³/g.     

Microwave sintering of IR-transparent Y2O3–MgO composite ceramics

A method for preparation of dense Y₂O₃–MgO composite ceramics by the microwave sintering was developed. The initial powders were obtained by glycine-nitrate self-propagating high-temperature synthesis (SHS) with different oxidant-to-fuel ratio. Density and IR-transmission of microwave sintered Y₂O₃–MgO ceramics increase with respect to dispersity of the SHS-powders and reach its maximum values for the powder prepared in a 20% fuel excess. The sintering behavior of Y₂O₃–MgO compacts was investigated by optical dilatometry and measuring an electric conductivity upon heating. Significant microwave radiation power surges at temperatures of 900–1000 °C, caused by the decomposition of magnesium carbonate, have been found. As a result of matching the conditions for the synthesis of powders and sintering modes, a transmission of composite ceramics of 78% at a wavelength of 6 μm was achieved at a maximum processing temperature of 1500 °C.     

Structural and thermal characterization of CaO–MgO–SiO2–P2O5–CaF2 glasses

The influence of varying the CaO/MgO ratio on the structure and thermal properties of CaO–MgO–SiO₂–P₂O₅–CaF₂ glasses was studied in a series of eight glass compositions in the glass forming region of diopside (CaMgSi₂O₆)–fluorapatite [Ca₅(PO₄)₃F]–wollastonite (CaSiO₃) ternary system. The melt-quenched glasses were characterized for their structure by infrared spectroscopy (FTIR) and magic angle spinning (MAS)-nuclear magnetic resonance (NMR) spectroscopy. Silicon is predominantly present as Q² (Si) species, while phosphorus tends to coordinate in orthophosphate environment. The sintering and crystallization parameters of the glasses were obtained from differential thermal analysis (DTA) while crystalline phase fractions in the sintered glass–ceramics were analyzed by X-ray diffraction adjoined with Rietveld refinement. Diopside, fluorapatite, wollastonite and pseudowollastonite crystallized as the main crystalline phases in all the glass–ceramics with their content varying with respect to variation in CaO/MgO ratio in glasses. The implications of structure and sintering behaviour of glasses on their bioactivity were discussed.     

Broad sintering temperature range and stable microwave dielectric properties of Mg2TiO4–CeO2 composite ceramics

There has been extensive research on microwave dielectric materials considering their application in 5G and 6G communication technologies. In this study, the sintering temperature range of Mg₂TiO₄–CeO₂ (MT-C) ceramics was broadened using a composite of CeO₂ and Mg₂TiO₄ ceramics, and their microwave dielectric performance was stabilized. Low-loss MT-C composite ceramics were prepared using the solid-phase reaction method, and their microwave dielectric properties, microscopic morphologies, and phase structures were investigated. The proposed MT-C ceramics contained Mg₂TiO₄ and CeO₂ phases; their average grain size was maintained at 2–4 μm in the sintering temperature range of 1275–1425 °C, and the samples were uniformly dense without porosity. The cross-distribution of Mg₂TiO₄ and CeO₂ grains in the samples inhibited the growth of ceramic grains, providing uniform and dense surfaces. The dielectric loss of MT-C ceramics remained constant in the temperature range of 1300–1425 °C at 9 × 10^?4 (8.45 ≤ f ≤ 8.75 GHz). As opposed to the base material, MT-C ceramics are advantageous owing to their wide sintering temperature range and the stable microwave dielectric properties, and there are suitable substrate materials for further industrial applications.