Thermal and Mechanical Properties of SiC–TiC0.5N0.5 Composites

SiC–TiC_0.5N_0.5 composites were fabricated from β‐SiC and TiN powders with 2 vol% equimolar Y₂O₃–Sc₂O₃ additives by conventional hot pressing. Thermal and mechanical properties of the SiC–TiC_0.5N_0.5 composites were investigated as a function of initial TiN content. Relative densities of ≥98.9% were achieved for all samples. The addition of a small amount of TiN increased thermal conductivity, flexural strength, and fracture toughness of SiC ceramics. However, further addition of TiN in excess of 10 and 20 vol% deteriorated both thermal conductivity and flexural strength of the composites, respectively. In contrast, the fracture toughness of the composites increased continuously from 4.2 to 6.2 MPa?m^1/2 with increasing initial TiN content from 0 to 35 vol%, due to crack deflection by TiC_0.5N_0.5. The maximum values of thermal conductivity and flexural strength were 224 W/m K for a 2 vol% TiC_0.5N_0.5 and 599 MPa for a 10 vol% TiC_0.5N_0.5 composite.     

Polymer composites with enhanced thermal conductivity via oriented boron nitride and alumina hybrid fillers assisted by 3-D printing

Herein, oriented boron nitride (BN)/alumina (Al₂O₃)/polydimethylsiloxane (PDMS) composites were obtained by filler orientation due to the shear-inducing effect via 3-D printing. The oriented BN platelets acted as a rapid highway for heat transfer in the matrix and resulted in a significant increase in the thermal conductivity along the orientation direction. Extra addition of spherical Al₂O₃ enhanced the fillers networks and resulted in the dramatic growth of slurry viscosity. This, together with filler orientation induced the synergism and provided large increases in the thermal conductivity. A high orientation degree of 90.65% and in-plane thermal conductivity of 3.64 W/(m∙K) were realized in the composites with oriented 35 wt% BN and 30 wt% Al₂O₃ hybrid fillers. We attributed the influence of filler orientation and hybrid fillers on the thermal conductivity to the decrease of thermal interface resistance of composites and proposed possible theoretical models for the thermal conductivity enhancement mechanisms.     

Tuning the electronic and thermal transport behaviors of metal-dispersed Ti2O3 composites derived by metal–insulator transition

Thermal management requires an understanding of the relations among the thermal energy transfer, electronic properties, and structures of thermoconductive materials. Here, we enhanced the metal–insulator transition (MIT)-induced effect on the thermal conductivities of microstructure-controlled Ti₂O₃ composites containing W as a thermal conductive filler at approximately 450 K. To change the electronic and thermal transport properties, we varied the particle radii of the conductive phases in the raw material. The change in the calculated electronic thermal conductivity relative to the electrical conductivity of the W_x(Ti₂O₃)_1?x composite was enhanced by compounding the material. When x was reduced from 50 vol% to 20 vol% and the W particle diameter was reduced from 150 μm to 5 μm, the variation in the estimated electronic thermal conductivity of the W_x(Ti₂O₃)_1?x composite was increased by a factor of 2.01. The total thermal conductivity was also changed by the MIT. At x = 50 vol% and a W particle diameter of 5 μm, the maximum thermal conductivity change was 6.34 times larger than that of pure Ti₂O₃. The detailed relation between the MIT-induced changes in thermal transport and the microstructure were elucidated in classical effective medium approximations.     

Electrical and thermal properties of SiC-Zr2CN composites sintered with Y2O3-Sc2O3 additives

SiC-Zr₂CN composites were fabricated from β-SiC and ZrN powders with 2 vol% equimolar Y₂O₃-Sc₂O₃ additives via conventional hot pressing at 2000 °C for 3 h in a nitrogen atmosphere. The electrical and thermal properties of the SiC-Zr₂CN composites were investigated as a function of initial ZrN content. Relative densities above 98% were obtained for all samples. The electrical conductivity of Zr₂CN composites increased continuously from 3.8 × 10³ (Ωm)⁻¹ to 2.3 × 10⁵ (Ωm)⁻¹ with increasing ZrN content from 0 to 35 vol%. In contrast, the thermal conductivity of the composites decreased from 200 W/mK to 81 W/mK with increasing ZrN content from 0 to 35 vol%. Typical electrical and thermal conductivity values of the SiC-Zr₂CN composites fabricated from a SiC-10 vol% ZrN mixture were 2.6 × 10⁴ (Ωm)⁻¹ and 168 W/m K, respectively.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Fabrication and characterization of highly transparent Er:Y2O3 ceramics with ZrO2 and La2O3 additives

In this study, we report highly transparent Er:Y₂O₃ ceramics (0–10 at% Er) fabricated by a vacuum sintering method using compound sintering additives of ZrO₂ and La₂O₃. The transmittance, microstructure, thermal conductivity and mechanical properties of the Er:Y₂O₃ ceramics were evaluated. The in-line transmittance of all of the Er:Y₂O₃ ceramics (1.2 mm thick) exceeds 83% at 1100 nm and 81% at 600 nm. With an increase in the Er doping concentration from 0 to 10 at%, the average grain size, microhardness and fracture toughness remain nearly unchanged, while the thermal conductivity decreases slightly from 5.55 to 4.89 W/m K. A nearly homogeneous doping level of the laser activator Er up to 10 at% in macro-and nanoscale was measured along the radial direction from the center to the edge of a disk specimen, which is the prominent advantage of polycrystalline over single-crystal materials. Based on the finding of excellent optical and mechanical properties, the compound sintering additives of ZrO₂ and La₂O₃ are demonstrated to be effective for the fabrication of transparent Y₂O₃ ceramics. These results may provide a guideline for the application of transparent Er:Y₂O₃ laser ceramics.     

Thermal conductivity of porous Alumina-20 wt% zirconia ceramic composites

Effect of porosity and temperature on thermal conductivity of the porous Alumina-20 wt% Zirconia (3 mol.% Y₂O₃) ceramic composites with and without niobia were investigated. The ceramic powders were synthesized by the sol-gel route using alkoxide precursors. The porosity in the composites was maintained in the range of 9.5–65 vol% using starch as a space holder material. After processing, samples were compacted uniaxially and sintered at 1873 K for 3 h. The thermal conductivity of porous ceramic composites with and without niobia dopant was measured at three different temperatures of 300, 473, and 673 K using laser-flash technique. The thermal conductivity of the samples was reduced with increasing temperature and porosity. At temperature of 300 K, the thermal conductivity value of 11 W/m.K was obtained for the undoped sample S0 with 17 vol% residual porosity, dropped to 2 W/mK for the sample S40 containing 65 vol% porosity, and for the same sample it was further reduced to the lowest value of 0.68 W/m.K at 673 K. The measured conductivity values were used to determine the grain boundary thermal resistance value (R) of the samples which exhibited an ascending trend with the porosity. The obtained thermal conductivity for the different porous composites was verified and formulated with the Maxwell-Eucken and Ticha models. The results showed that the experimentally measured conductivity values follow a descending order with the models while at the higher-porosity level (57–65 vol%), it fits well with the Ticha equation with only 9% and 4.6% deviation for undoped and doped samples, respectively. Results also revealed that the addition of niobia significantly reduced thermal conductivity at the lower porosity levels, but at higher porosity level the effect of porosity was more dominant.     

Electrical and Thermal Properties of SiC Ceramics Sintered with Yttria and Nitrides

The electrical and thermal properties of SiC ceramics containing 1 vol% nitrides (BN, AlN or TiN) were investigated with 2 vol% Y₂O₃ addition as a sintering additive. The AlN‐added SiC specimen exhibited an electrical resistivity (3.8 × 10¹ Ω·cm) that is larger by a factor of ~10² compared to that (1.3 × 10^?1 Ω·cm) of a baseline specimen sintered with Y₂O₃ only. On the other hand, BN‐ or TiN‐added SiC specimens exhibited resistivity that is lower than that of the baseline specimen by a factor of 10^?1. The addition of 1 vol% BN or AlN led to a decrease in the thermal conductivity of SiC from 178 W/m·K (baseline) to 99 W/m·K or 133 W/m·K, respectively. The electrical resistivity and thermal conductivity of the TiN‐added SiC specimen were 1.6 × 10^?2 Ω·cm and 211 W/m·K at room temperature, respectively. The present results suggest that the electrical and thermal properties of SiC ceramics are controllable by adding a small amount of nitrides.     

Improved mechanical properties and directional heat transfer performance of h-BN matrix multilayer composites with alternately stacked untextured/textured layers

The h-BN matrix multilayer composites with alternately stacked untextured/textured layers were fabricated by hot-pressing from the alternately stacked “fine h-BN powders” layers and “plate-like h-BN powders + 3Y₂O₃–5Al₂O₃” layers. During the hot-pressing process, Y₂O₃ and Al₂O₃ reacted, forming Y₃Al₅O₁₂, which provided a liquid phase environment for h-BN plates to be readily rotated and oriented under the action of the uniaxial pressure. The residual compressive stress in textured layers, which was caused by the mismatch of thermal expansion between textured layers and untextured layers, resulted in crack arrest at the first textured layer in the multilayer composite with 49 vol% textured layers, which improved its flexural strength and fracture toughness by 33.1% and 23.4% compared with the textured monolith, respectively. The multilayer composite with 35 vol% textured layers behaved a much better directional heat transfer performance than the textured and untextured monoliths, making it a promising candidate as thermal management devices in electronics.     

The effects of microstructure on mechanical and electrical properties of W dispersed Al2O3 ceramics

This work aims to enhance the fracture toughness of brittle Al₂O₃ ceramics and apply insulated Al₂O₃ ceramics with electrical conductivity by dispersing second tungsten (W) metal particles. In order to investigate the effects of W dispersion on mechanical and electrical properties, Al₂O₃–W composites with various amounts of W (ranging from 5 vol% to 20 vol%) were fabricated by the hot-press sintering method at various sintering temperatures. Microstructure analysis revealed submicron Al₂O₃ matrix grains and W particles. The existence of three phases of Al₂O₃, W, and AlWO₄ was confirmed by X-ray diffraction patterns. All Al₂O₃–W composites showed higher fracture toughness than monolithic Al₂O₃. The toughening mechanism was attributed to crack deflection and crack bridging. Transgranular fracture was visible in all composites. Electrical resistivity dramatically lowered from 2.9 × 10¹² Ω cm of monolithic Al₂O₃ to 4.1 × 10² Ω cm of the composite with 20 vol% W addition. The percolation threshold is calculated as 18.5%. With the increase in sintering temperature, the amount of W particles was decreased and Al₂O₃ grains became large, leading to the reduced number of conductive pathways formed by the dispersed W particles. As a result, electrical conductivity was decreased.     

Study on properties of Ca0.9La0.067TiO3-0.01Al2O3 ceramics reinforced PSAE/fiber composite substrate with high εr

A “self-permeation” method was used to fabricate a Ca_0.9La_0.067TiO₃-0.01Al₂O₃ (CLT)-reinforced polysilylaryl-enyne/fiber multilayer board with different volume fractions of CLT (0–40 vol%). The microstructure, dielectric, mechanical, and thermal properties were fully studied. The results showed that the composite with optimal dielectric properties (ε_r∼8.75, tanδ∼0.0043) could be obtained when the volume fraction of CLT reached 30 vol%. Meanwhile, the thermal conductivity reached a high level of 0.644 W/(m·K) and 0.762 W/(m·K) at Z and X/Y directions, respectively. Due to the high decomposition temperature of PSAE, T_d5 (the temperature corresponding to 5% weight loss) of composite loading with 30 vol% CLT was higher than 900℃, which indicates an excellent thermal resistance. And the bending strength could reach 115.3 MPa indicating excellent mechanical property. The novel polysilylaryl-enyne/fiber/Ca_0.9La_0.067TiO₃-0.01Al₂O₃ multilayer board with excellent performance is expected to be a candidate material for print circuit boards and widely used in the microwave communication field.     

Effects of Y2O3/MgO ratio on mechanical properties and thermal conductivity of silicon nitride ceramics

In this work, the effects of Y₂O₃/MgO ratio on the densification behavior, phase transformation, microstructure evolution, mechanical properties, and thermal conductivity of Si₃N₄ ceramics were investigated. Densified samples with bimodal microstructure could be obtained by adjusting the ratio of Y₂O₃/MgO. It was found that a low Y₂O₃/MgO ratio facilitated the densification of Si₃N₄ ceramics while a high Y₂O₃/MgO ratio benefited the phase transformation of Si₃N₄ ceramics. Best mechanical properties (flexural strength of 875 MPa, and fracture toughness of 8.25 MPa·m^1/2, respectively) and optimal thermal conductivity of 98.04W/(m·K) were achieved in the sample fabricated with Y₂O₃/MgO ratio of 3:4 by sintering at 1900°C for 4 h.     

Nanofibers reinforced silica aerogel composites having flexibility and ultra-low thermal conductivity

For the sake of enhancing the performance of flexible silica aerogel in practical applications, flexible SiO₂/SnO₂ nanofibers (SSNF) reinforced flexible silica aerogel composites (abbreviated as SiO₂-SSNF) were successfully prepared. Firstly, the SiO₂/SnO₂ nanofibers with fine diameter (~320 nm) and excellent flexibility were prepared by electrospinning technology. Then the aerogel composites were synthesized by adding the flexible SSNF to the silica solution and through the sol-gel method and ethanol supercritical drying technology. The effects of different content of the nanofibers on thermal conductivity and Yong's modulus of SiO₂-SSNF aerogel composites were investigated. The SiO₂/SnO₂ nanofibers were randomly dispersed in the flexible silica aerogel and the great integrity of the material result in smaller linear shrinkage, better thermal protection, and mechanical properties compared with those pure SiO₂ aerogels. The final SiO₂-SSNF aerogel composites possess excellent thermal conductivity (0.025-0.029 W/(m∙K)) and higher Yong's modulus (70 kPa), which was twice than that of the pure silica aerogel. This prepared SiO₂-SSNF aerogel composites can be better used in thermal insulation due to its excellent flexible and thermal insulation property.     

Novel monolithic yttria-alumina aerogel with low thermal conductivity made from inorganic precursors

A novel monolithic yttria-alumina aerogel was successfully prepared using inorganic precursors through the sol-gel method. The yttria-alumina aerogel possesses a three-dimensional network structure composed of numerous nanoscale flake like particles and nanoscale pores. In addition, the aerogel is mainly made up of nano Y₂O₃ grains, and Al atoms are dissolved in the Y₂O₃ grains to form the solid solution. The aerogel with a low density of 0.203 g/cm³ exhibits low thermal conductivity of 0.016 W/(m·K). Therefore, the yttria-alumina aerogel shows promise for application as a thermal insulation material.     

Design and preparation of low-filled magnetic oriented BN@Fe3O4/EP insulating composite with improved thermal conductivity and thermal stability

As an excellent two-dimensional insulating material with high thermal conductivity, high temperature stability and high hardness, hexagonal boron nitride(h-BN) is widely applied in semiconductor manufacturing, aerospace, metallurgical manufacturing and other cutting-edge fields. However, the unique surface structure of h-BN leads to poor lubricity and easy agglomeration, which limits the application of h-BN especially in the field of electronic packaging. To address key issues boosted above, this study designed and prepared the BN@Fe₃O₄ magnetic insulating particles and doped it into the reduced viscosity epoxy resin to prepare the composites. By selecting appropriate external magnetic field strength and BN@Fe₃O₄ particles’ content, a novel 3D structure of fillers like dominoes in epoxy resin composite was successfully constructed. The microstructure of the BN@Fe₃O₄ particles and composites were analyzed, the thermal conductivity, the mechanical and the electrical properties of composites were simultaneously tested. Results manifested that the core-shell structures with BN as core and Fe₃O₄ as shell was successfully prepared, linking through the PDA middle layer between the BN core and Fe₃O₄ shell. Under the influence of magnetic orientation, the BN@Fe₃O₄ magnetic particles were preferred an out-of-plane oriented in the epoxy resin composites, resulted an enormously enhanced on thermal conductivity of composites. At a magnetic field strength of 60 mT and 25 vol% BN@Fe₃O₄ content, the thermal conductivity of BN@Fe₃O₄/EP composites is as lofty as 1.832 W/(m K), which is 1023.46% higher than that of pure epoxy resin. Meanwhile, the thermal stability has also been slightly improved, the elastic modulus and insulation performances remain at the same level.     

High temperature performances of tri-layer environmental barrier coating with Si bond layer modified by Yb2O3

Environmental barrier coatings (EBCs) have been expected to be applied on the surface of ceramic matrix composites (CMCs). However, the oxidation and propagation cracking of the silicon bond layer are the most direct causes to induce the failure of EBCs under high temperature service environment. The modification of silicon bond layer has become an important method to prolong the service life of EBCs. In this work, the Yb₂O₃ have been introduced to the silicon bond layer, and three kinds of tri-layer Yb₂SiO₅/Yb₂Si₂O₇/(Si-xYb₂O₃) EBCs with modified Si bond layer by different contents of Yb₂O₃ (x = 0, 10 vol%, 15 vol%) were prepared by vacuum plasma spray technique. The thermal shock performance and long-term oxidation resistance of the EBCs at 1350 °C were investigated. The results showed that the addition of appropriate amount of Yb₂O₃ (10 vol%) can improve the structural stability and reduce the cracks of the mixed thermal growth oxide (mTGO) layer by forming the oxidation product of Yb₂Si₂O₇ during long-term oxidation. The excessive addition of Yb₂O₃ increased the stress during thermal shock as well as accelerated the oxygen diffusion during long-term oxidation, leading to the failure of EBCs. Moreover, the distribution uniformity of Yb₂O₃ deserves further consideration and improvement.     

Thermal properties of field-assisted-sintered SiCN–Y2O3 composites

Polymer-derived amorphous SiCN has excellent high-temperature stability and properties. To reduce the shrinkage during pyrolysis and to improve the high-temperature oxidation resistance, Y₂O₃ was added as a filler. In this study, polymer-derived SiCN–Y₂O₃ composites were fabricated by mixing a polymeric precursor of SiCN with Y₂O₃ submicron powders in different ratios. The mixtures were cross-linked and pyrolyzed in argon. SiCN–Y₂O₃ composites were processed using field-assisted sintering technology at 1350°C for 5 min under vacuum. Dense SiCN–Y₂O₃ composite pellets were successfully made with relative density higher than 98% and homogeneous microstructure. Due to low temperature and short time of the heat-treatment, the grain growth of Y₂O₃ was substantially inhibited. The Y₂O₃ grain size was ∼1 μm after sintering. The composites’ heat capacity, thermal diffusivity, and thermal expansion coefficients were characterized as a function of temperature. The thermal conductivity of the composites ceramics decreased as the amount of amorphous SiCN increased and the coefficient of thermal expansion (CTE) of the composites increased with Y₂O₃ content. However, the thermal conductivity and CTE did not follow the rule of mixture. This is likely due to the partial oxidation of SiCN and the resultant impurity phases such as Y₂SiO₅, Y₂Si₂O₇, and Y_4.67(SiO₄)₃O.     

Novel high-temperature-resistant Y2SiO5 aerogel with ultralow thermal conductivity

A novel Y₂SiO₅ ternary aerogel was prepared from tetraethoxysilane and yttrium chloride hexahydrate via the sol-gel method followed by high-temperature calcination. The effects of different calcination temperatures on the microstructure, mechanical and thermal stability of the Y₂SiO₅ aerogels were investigated. The aerogels exhibited low densities of 0.33-0.62 g/cm³, low thermal conductivities of 0.029-0.05 W/(m·K), and a relatively high strength of 0.16-56.47 MPa. Moreover, compared with the Al₂O₃–SiO₂ aerogel, the Y₂SiO₅ aerogel has higher thermal stability and more excellent high-temperature insulation, which has potential applications as a thermal protection material in hypersonic vehicles.     

Novel Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> aerogel/porous zirconia composite with ultra-low thermal conductivity

Highly porous zirconia fibers networks with a quasi-layered microstructure were successfully fabricated using vacuum squeeze moulding. The effects of inorganic binder content on the microstructure, room-temperature thermal and mechanical properties of fibrous porous zirconia ceramics were systematically investigated. Al₂O₃–SiO₂ aerogel was impregnated into fibrous porous ceramics, and the microstructures, thermal and mechanical properties of Al₂O₃–SiO₂ aerogel/porous zirconia composites were also studied. Results show that the Al₂O₃–SiO₂ aerogel/porous zirconia composites exhibited higher compressive strength (i.e., 1.22 MPa in the z direction) and lower thermal conductivity [i.e., 0.049 W/(m/K)]. This method provides an efficient way to prepare high-temperature thermal insulation materials.     

The effect of carbon nanotubes on the sintering behaviour of zirconia

The sintering behaviour and activation energy of Y₂O₃ partially stabilised ZrO₂ and ZrO₂–CNT (0.5 and 2 vol%) composites was determined using spark plasma sintering (SPS) under isothermal conditions. The sintering activation energy for the Y₂O₃ partially stabilised ZrO₂ was found to be 456 kJ/mol. The addition of 2 vol% CNTs reduced the sintering activation energy to 172 kJ/mol. The significant reduction of the activation energy with the addition of only 2 vol% CNTs is attributed to the formation of a percolating network of CNTs providing a lower energy diffusion pathway. The sintering mechanism was found to be grain boundary diffusion for all samples suggesting that the presence of CNTs does not change the sintering mechanism but does lower the activation energy for the rate limiting step in the sintering process.