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.     

Improvement of electrical conductivity of silicon nitride/carbon nano-fibers composite using magnesium silicon nitride and ytterbium oxide as sintering additives

In order to find out the influence of sintering additives on the electrical conductivity of Si₃N₄-based ceramics composites with dispersed carbon nano-fibers (CNFs) two different mixtures of sintering additives were tested – Al₂O₃/Yb₂O₃ and MgSiN₂/Yb₂O₃, respectively. Optimization of hot-pressing conditions was performed for each mixture. The results show that the electrical conductivity can be effectively increased up to 1315 S/m by replacement of traditional sintering aid – alumina, with magnesium silicon nitride, while the mechanical properties remained on the same level. Other advantages of using MgSiN₂ instead of alumina are the preservation of higher amounts of CNFs in the ceramic composite and lower densification temperature (1500 °C) compared to samples sintered with alumina-based sintering aids (1550 °C).     

Method for fabricating microwave absorption ceramics with high thermal conductivity

In this work, a high-performance microwave absorption ceramic together with high thermal conductivity was proposed through the introduction of phenolic resin as the carbon source into AlN ceramic substrate. The phenolic resin was initially mixed with AlN powder and sintering additives in ethanol to form homogeneous slurries, followed by drying, pyrolysis, dry pressing and pressureless sintering to develop nano carbon in situ in AlN substrate. The well controlled microstructure with homogeneous distribution of nano sized carbon lead to the high thermal conductivity and excellent microwave absorption properties. by adjusting the phenolic resin content, the ceramic showed a minimal reflection coefficient of about ?30 dB with an effective bandwidth of about 2 GHz together with a high thermal conductivity of about 135.1 W/m K.     

Lightweight porous silica ceramics with ultra-low thermal conductivity and enhanced compressive strength

In recent years, the need for robust thermal protection for reusable spacecraft and vehicles has spurred strong demand for high-performance lightweight thermal insulation materials that exhibit high strength. Herein, we report silica porous ceramics prepared via the direct foaming technique with lightweight, ultra-low thermal conductivity and enhanced compressive strength. Silica particles (particle size: 500 nm and 2 μm) were used as the raw materials. The nano-sized silica particles were easily sintered, thereby improving the compressive strength of the ceramics, whereas the micro-sized silica particles maintained the pore structure integrity without deformation. The addition of nano-silica enhanced the compressive strength by 764% (from 0.039 to 0.337 MPa). In addition, the thermal conductivity of the ceramics was as low as 0.039 W m^?1 K^?1. Owing to these outstanding characteristics, these porous silica ceramics are expected to be employed as thermal insulation material in diverse fields, especially aerospace and space where weight is an important constraint.     

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.     

Effect of MgF2 addition on mechanical properties and thermal conductivity of silicon nitride ceramics

Dense silicon nitride (Si₃N₄) ceramics were prepared using Y₂O₃ and MgF₂ as sintering aids by spark plasma sintering (SPS) at 1650 °C for 5 min and post-sintering annealing at 1900 °C for 4 h. Effects of MgF₂ contents on densification, phase transformation, microstructure, mechanical properties, and thermal conductivity of the Si₃N₄ ceramics before and after heat treatment were investigated. Results indicated that the initial temperature of liquid phase was effectively decreased, whereas phase transformation was improved as increasing the content of MgF₂. For optimized mechanical properties and thermal conductivity of Si₃N₄, optimum value for MgF₂ content existed. Sample with 3 mol.% Y₂O₃ and 2 mol.% MgF₂ obtained optimum flexural strength, fracture toughness and thermal conductivity (857 MPa, 7.4 MPa m^1/2 and 76 W m⁻¹ K⁻¹, respectively). It was observed that excessive MgF₂ reduced the performance of the ceramic, which was caused by the presence of excessive volatiles.     

Fabrication of high thermal conductivity silicon nitride ceramics by pressureless sintering with MgO and Y2O3 as sintering additives

The fabrication of silicon nitride (Si₃N₄) ceramics with a high thermal conductivity was investigated by pressureless sintering at 1800 °C for 4 h in a nitrogen atmosphere with MgO and Y₂O₃ as sintering additives. The phase compositions, relative densities, microstructures, and thermal conductivities of the obtained Si₃N₄ ceramics were investigated systemically. It was found that at the optimal MgO/Y₂O₃ ratio of 3/6, the relative density and thermal conductivity of the obtained Si₃N₄ ceramic doped with 9 wt% sintering aids reached 98.2% and 71.51 W/(m·K), respectively. EDS element mapping showed the distributions of yttrium, magnesium and oxygen elements. The Si₃N₄ ceramics containing rod-like grains and grain boundaries were fabricated by focused ion beam technique. TEM observations revealed that magnesium existed as an amorphous phase and that yttrium produced a new secondary phase.     

Novel-structured plasma-sprayed thermal barrier coatings with low thermal conductivity,high sintering resistance and high durability

The microstructure of the ceramic topcoat has a great influence on the service performance of thermal barrier coatings (TBCs). In this study, conventional layered-structure TBCs, nanostructured TBCs, and novel-structured TBCs with a unique microstructure were fabricated by air plasma spraying. The relationship between the microstructure and properties of the three different TBCs was analysed. Their thermal insulation ability, sintering resistance, and durability were systematically evaluated. Additionally, their failure modes after being subjected to two kinds of thermal shock tests were analysed. The results revealed that the novel-structured TBCs had remarkably superior performances in all the examined aspects. The thermal conductivity of the novel-structured TBCs was significantly lower than those of the conventional and nanostructured TBCs both in the as-sprayed state and after thermal treatment for 500 h at 1100 °C. The macroscopic elastic modulus of the novel-structured TBCs after sintering at 1300 °C for 100 h was similar to those of the conventional and nanostructured TBCs in the as-sprayed state. During both a burner rig thermal shock test and a furnace cyclic oxidation test, the thermal shock lifetime of the novel-structured TBCs was much longer than those of the conventional and nanostructured TBCs. This study has demonstrated novel-structured plasma-sprayed TBCs with high thermal insulation ability and high durability.     

Improved thermal conductivity of sintered reaction-bonded silicon nitride using a BN/graphite powder bed

Sintered reaction-bonded silicon nitride (SRBSN) with improved thermal conductivity was achieved after the green compact of submicron Si powder containing 4.22 wt% impurity oxygen and Y₂O₃-MgO additives was nitrided at 1400 °C for 6 h and then post-sintered at 1900 °C for 12 h using a BN/graphite powder bed. During nitridation, the BN/10 wt% C powder bed altered the chemistry of secondary phase by promoting the removal of SiO₂, which led to the formation of larger, purer and more elongated Si₃N₄ grains in RBSN sample. Moreover, it also enhanced the elimination of SiO₂ and residual Y₂Si₃O₃N₄ secondary phase during post-sintering, and thus induced larger elongated grains, decreased lattice oxygen content and increased Si₃N₄-Si₃N₄ contiguity in final SRBSN product. These characteristics enabled SRBSN to obtain significant increase (∼40.7%) in thermal conductivity from 86 to 121 W ∙ m⁻¹ ∙ K⁻¹ without obvious decrease in electrical resistivity after the use of BN/graphite instead of BN as powder bed.     

Simulation of structured catalytic reactors with enhanced thermal conductivity for selective oxidation reactions

The replacement of conventional-packed beds of pellets with “high conductivity” honeycomb catalysts in industrial externally cooled multitubular fixed-bed reactors is investigated by modeling and simulation for the oxidation of methanol to formaldehyde and for the epoxidation of ethylene to ethylene oxide, which involve a consecutive and a parallel reaction scheme, respectively. Results suggest that near-isothermal operation of the fixed-bed reactors can be achieved using monolithic catalyst supports based on relatively large volume fractions of highly conductive materials. Pressure drops are reduced to less than 1%. The selectivity is favored by the excellent control of the intraporous diffusional resistances resulting from the thin catalytic washcoats. Reactor designs based on larger tubes are feasible at the expense of greater volume fractions of catalyst support. A critical aspect is represented by the restrictions on the specific load of catalyst per reactor volume resulting from the poor adhesion of very thick catalyst layers onto metallic surfaces. Such a difficulty can be circumvented by maximizing the geometric surface area of the monolith (e.g. minimizing the honeycomb pitch), enhancing the catalytic activity (e.g. increasing the load of active components), and increasing the coolant temperature (if the selectivity is not adversely affected).     

Structural,thermal and mechanical properties of aluminum nitride ceramics with CeO2 as a sintering aid

AlN ceramics have been prepared with CeO₂ as a sintering aid at a sintering temperature of 1900 °C. The effect of CeO₂ contents on the microstructure, density, thermal conductivity and hardness was investigated. Addition of CeO₂ exerted a significant effect on the densification of AlN ceramics and hence on the microstructure. Thermal conductivity of AlN ceramics increased with CeO₂ content and was greater than that of Y₂O₃-doped AlN ceramics at a similar sintering temperature. The resulting AlN ceramics with 1.50 wt% of CeO₂ had the highest relative density of 99.94%, thermal conductivity of 156 W m⁻¹ K⁻¹ and hardness of 72.46 kg/mm².     

Powder injection molding of complex-shaped aluminium nitride ceramic with high thermal conductivity

Aluminum nitride (AlN) is a promising material for heat sinks and microelectronic applications because of the advantages of high theoretical thermal conductivity, high mechanical strength, good electrical insulation, low dielectric constant and low thermal expansion coefficient. However, the difficulties in shaping complex-shaped parts with a high thermal conductivity have retarded the wide applications of AlN ceramic. Herein, we design a new binder system containing resin components and adopt the powder injection molding technology to fabricate complex-shaped AlN parts. After the debinding process, the special binder system would produce residual carbon, which could react with Al₂O₃ and result in decreasing oxygen impurity and forming the yttrium-rich aluminates. The yttrium-rich aluminates can accelerate the densification of AlN ceramic and fasten the oxygen on the triangular grain boundary, leaving the clean grain boundary beneficial for high thermal conductivity. The as-prepared AlN parts with complex shape possess a high thermal conductivity of 248 W m⁻¹ K⁻¹.     

Anchoring carbon nanotubes to boron nitride microrods for enhancing the thermal conductivity of polystyrene

The development of high-performance thermally conductive fillers is crucial for the thermal management of polymer-based composites. Herein, a facile precursor pyrolysis strategy was adopted to fabricate boron nitride@multiwalled carbon nanotubes (BN@MWCNTs) fillers, wherein uniformly distributed MWCNTs were firmly anchored on BN microrods. Benefiting from the unique structure, the BN@MWCNTs act as fillers in the design of polystyrene (PS)/BN@MWCNTs composites via in situ polymerization. As a result, a 7.74-fold higher thermal conductivity (TC) (9.55 W/m·K) was achieved for the 10 wt% BN@MWCNTs, as compared to native PS/BN/MWCNTs prepared by the conventional melt-mixing method. More importantly, the PS/BN@MWCNTs composites exhibited satisfactory electrical insulation owing to the isolation effect of BN. Overall, this work provides a promising frontier for the design of polymer-based thermally conducting materials for applications in thermal management.     

A novel strategy for c-axis textured silicon nitride ceramics by hot extrusion

Highly c-axis textured β-silicon nitride (β-Si₃N₄) ceramic with fine grains was prepared by a new method of hot extrusion for the first time. The (002) pole figure on the section plane vertical to extruding direction showed a characteristic of center rotational symmetry. The average cline angle between elongated β-Si₃N₄ grains and hot extruding direction was about 14.4°. The degree of c-axis texturing by hot extrusion was comparable to that achieved by rotating magnetic field. The hardness and toughness anisotropy in different direction was apparent and relatively higher hardness was achieved in the present work mainly due to the finer grain size. Therefore, many different compositions of c-axis aligned Si₃N₄-based ceramics with tailored mechanical properties could be achieved by the strategy of hot extrusion.     

Vacuum sintered lunar regolith simulant: Pore-forming and thermal conductivity

Lunar regolith molding technologies are receiving an increasing attention in lunar exploration program. Most studies are carried out in the air on earth, ignored the effects of high vacuum (~?10^-12 mbar) on the lunar surface. This paper presents the results of a study aimed at assessing the effect of vacuum on the sintering of low Ti basalt type lunar simulant CLRS-1. The results show that porous sample with density of 1.19?g?cm^-3 can be obtained by sintering at 1100?°C under vacuum, which has much lower thermal conductivity (0.265?W?m^-1 K^-1) than concrete and other lunar resource derived structural materials. It could potentially be applied as the thermal insulation material for the Moon base construction. The pore-forming mechanism in vacuum was investigated. It was found that evaporation of the products of solid dissolved mineral crystals led to a great deal of weight loss and inhibited the densification during sintering.     

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.     

Microstructure and properties of aluminum nitride ceramics prepared by aqueous gelcasting method with nontoxic curdlan as gelling agent

Aluminum nitride ceramics were prepared by aqueous gelcasting method and pressureless sintering technique in N₂ atmosphere using Y₂O₃ as sintering additives with nontoxic curdlan as gel system. The solidification mechanism of curdlan was studied. The effects of curdlan content and solid content on the microstructure, relative density and flexural strength of green bodies were investigated. The influences of Y₂O₃ content and sintering soaking time on the microstructure and properties of sintered bodies were also studied. The results show that, as the temperature increases to 80 °C, the ceramic powders solidify through three-dimensional gel networks of curdlan during gelling process. The green bodies can be successfully fabricated through aqueous gelcasting method with modified powder as original materials. Suitable curdlan content and solid content contribute to fabricating green body with uniform microstructures and high flexural strength. The relative density and flexural strength of sintered bodies enhance as the Y₂O₃ content and soaking time increase. The flexural strength and thermal conductivity are about 107.5∼172.3 MPa and 75.2∼112.5 W/(m·K), respectively. The sintered body with 4 wt% Y₂O₃ soaking for 3 h exhibits the highest thermal conductivity because of appropriate relative density, uniform microstructure and reasonable intergranular phase distribution. The mechanical property and thermal conductivity of sintered bodies can be improved by optimizing the gelcasting process parameter, Y₂O₃ content, and soaking time. The nontoxic gelling system will have wide application for aqueous gelcasting ceramic with complex shape.     

Evaluation of mesoporous silicon thermal conductivity by electrothermal finite element simulation

ABSTRACT: The aim of this work is to determine the thermal conductivity of mesoporous silicon (PoSi) by fitting the experimental results with simulated ones. The electro-thermal response (resistance versus applied current) of differently designed test lines integrated onto PoSi / Silicon substrates and the bulk were compared to the simulations. The PoSi thermal conductivity was the single parameter used to fit the experimental results. The obtained thermal conductivity value were compared with that determined from Raman scattering measurements and a good agreement between both methods was found. This methodology can be used to easily determine the thermal conductivity value for various porous silicon morphologies.     

Alkylated modified boron nitride nanosheets/polyimide composite films with advanced thermal conductivity and low dielectric constant

Owing to the rapid development of the latest micro-electronic devices, polymer composite materials that combine high thermal conductivity and low permittivity have aroused the interest of researchers. However, it is a huge challenge to balance the above parameters. In this work, hexagonal boron nitride (h-BN) powder was ultrasonically exfoliated to obtain alkylated boron nitride nanosheets (Alkyl-BNNS). Then, a series of polyimide (PI) composites were synthesized with different amounts of Alkyl-BNNS. Attributed to more robust interfacial non-covalent interactions between Alkyl-BNNS and polymer chains to inhibit interfacial polarization, Alkyl-BNNS can be scattered well in PI substrate. Thus, the obtained PI composite behaved a high thermal conductivity of 6.21 W/(mK) and a low dielectric constant (3.23) under the load of 20 wt%. Besides, Alkyl-BNNS/PI composites have efficient thermal management capability, low water absorption, favorable electrical resistance, and prominent tensile strength. Importantly, these composite films are expected to be excellent candidates in the field of microelectronics.     

Through thickness changes to stiffness and thermal conductivity in thermal barrier coatings subjected to gradient exposure

Sintering-induced increases in the elastic modulus and thermal conductivity of thermally sprayed Thermal Barrier Coatings (TBCs) under constraint by a bond-coated substrate due to thermal gradient exposure have been measured. The results suggest the degree of sintering due to this constrained thermal exposure to be significantly lower when compared to free standing exposed specimens. Overall, a ∼30% increase in elastic modulus and ∼12% increase in thermal conductivity have been measured in a ∼ 450 µm thick TBC after 12 h of gradient exposure. Here, the top surface of the TBC was kept at 1250 °C, and the gradient was determined based on a constant backside cooling air flow rate. The elastic modulus, measured across the thickness of coating, reveals that in the region near the top surface, the elastic modulus increased by ∼ 50%. Contrastingly, no significant increase in elastic modulus was observed near the metal/coating interface. The dynamic evolution of thermal conductivity and sintering under gradient exposure and constraint was calculated using an analytical model which was then used to rationalize the experimental observations.