Boron nitride silicone rubber composite foam with low dielectric and high thermal conductivity

Silicone rubber（SR） is widely used in the field of electronic packaging because of its low dielectric properties. In this work, the porosity of the SR was improved, and the dielectric constant of the SR foam was reduced by adding expanded microspheres（EM）. Then, the thermal conductivity of the system was improved by combining the modified boron nitride（f-BN）. The results showed that after the f-BN was added, the dielectric constant and dielectric loss were much lower than those of pure SR. Micron-sized modified boron nitride（f-m BN） improved the dielectric and thermal conductivity of the SR foam better than that of nano-sized modified boron nitride（f-n BN）, but f-n BN improved the volume resistivity,tensile strength, and thermal stability of the SR better than f-m BN. When the mass ratio of f-m BN and fn BN is 2:1, the thermal conductivity of the SR foam reaches the maximum value of 0.808 W·m^-1·K^-1,which is 6.5 times that before the addition. The heat release rate and fire growth index are the lowest,and the improvement in flame retardancy is mainly attributed to the high thermal stability and physical barrier of f-BN.     

Hot-pressing sintered BN-SiO2 composite ceramics with excellent thermal conductivity and dielectric properties for high frequency substrate

High thermal conductivity and low dielectric constant are the more and more important properties for high-frequency substrate materials to enhance their heat radiation and reduce signal delay. In this work, a series of BN-SiO₂ composite ceramics for high frequency application were successfully synthesized by hot-pressing sintering method. And their structures, thermal and dielectric properties were systematically studied. According to the results, the excellent thermal conductivity with low dielectric constant and low dielectric loss has been obtained in the BN-SiO₂ ceramic. Compared to the pure SiO₂, the sample with 50?wt% BN addition sintered at 1650?℃ exhibited excellent physical properties, including a high thermal conductivity of 6.75?W/m?K which is almost five times higher than that of pure SiO₂ and a low dielectric constant of 3.73. The achieved high thermal conductivity and appropriate dielectric property of the BN-SiO₂ composite ceramic make it a promising candidate for high-frequency substrate application.     

Highly porous Y2SiO5 ceramic with extremely low thermal conductivity prepared by foam‐gelcasting‐freeze drying method

Foam‐gelcasting‐freeze drying method is developed to fabricate porous Y₂SiO₅ ceramic with ultrahigh porosity of 92.2%‐95.8% and isotropous multiple pore structures. As prepared porous samples have quite low shrinkages of 0.8%‐1.9% during demolding and drying processes, lightweights of 0.19‐0.35 g/cm³, and extremely low thermal conductivities of 0.054‐0.089 W·(m·K)^?1. Our approach combines the merits of foam‐gelcasting method and freeze drying method. It is a simple and effective method to fabricate porous ceramics with very high porosity and extremely low thermal conductivity through low shrinkage of green body and near net complex shape forming.     

Preparation of porous Y2SiO5 ceramics with relatively high compressive strength and ultra-low thermal conductivity by a TBA-based gel-casting method

Porous Y₂SiO₅ ceramics with relative high compressive strength (as high as 24.45 MPa) and ultra-low thermal conductivity (～0.08 W/m K) were successfully fabricated by a tert-butyl alcohol based gel-casting method. The formation mechanism of the 3D interconnected pores and the properties of the green body are discussed. The porosity, pore size, compressive strength and thermal conductivity could be controlled by varying the initial solid loading and the sintering temperature. When regulating the initial solid loading (from 20 to 50 wt%) and sintering temperature (from 1200 to 1500 °C), the porosity can be controlled between 47.74% and 73.93%, and the compressive strength and the thermal conductivity of porous Y₂SiO₅ ceramics varied from 3.34 to 24.45 MPa and from 0.08 to 0.55 W/m K, respectively. It should be noted that the porous Y₂SiO₅ ceramics with 30 wt% solid loading and sintering at 1400 °C had an open porosity of 61.80%, a pore size of 2.24 μm, a low room-temperature thermal conductivity of 0.17 W/m K and a relatively high compressive strength of 13.91 MPa, which make this porous Y₂SiO₅ ceramics suitable for applications in high-temperature thermal insulators.     

Lightweight porous silica-alumina ceramics with ultra-low thermal conductivity

Thermal protection has always been an important issue in the energy, environment and aerospace fields. Porous ceramics produced by the particle-stabilized foaming method have become a competitive material for thermal protection because of their low density and low thermal conductivity. However, the study of porous ceramics for composite systems using particle-stabilized foaming method was relatively rare. Here, silica-alumina composite porous ceramics were prepared by particle-stabilized foaming method, which was achieved by tailoring the surface charges of silica and alumina through adjustment of the pH. Porous ceramics exhibited porosity as high as 97.49% and thermal conductivity (25 °C) as low as 0.063 W m^?1 K^?1. The compressive strength of porous ceramics sintered at 1500 °C with a solid content of 30 wt% could reach 0.765 MPa. Based on the light weight and excellent thermal insulation properties, the composite porous ceramic could be used as a potential thermal insulation material in the spacecraft industry.     

Analysis and modeling of the pore size effect on the thermal conductivity of alumina foams for high temperature applications

Analytical and finite element analyses were carried out to investigate the influence of the pore sizes on the effective thermal conductivity, which is the main physical property related to the ceramic microstructure insulating capacity at high temperatures. Thermal conductivity was estimated by analytical models using Litovsky's and Rosseland's approaches for a monodisperse pore distribution, whereas via finite element analysis a high porosity microstructure with three different pore sizes was investigated. Based on this, an ideal pore size range (0.5–3.0 µm) was found that optimizes the reduction of thermal energy transmission in the 1000–1700 °C range. Furthermore, the ideal pore size range seems to be independent of the ceramic foam material. When considering a pore size distribution, the ideal range is narrowed due to less effective thermal radiation scattering by sub-micron and large pores. The results obtained showed that nanopores (< 0.1 µm) are not the best option to reduce thermal conductivity at high temperatures. This statement is supported by experimental data on nanopore aerogels, which show a significant thermal conductivity increase at the high temperature range.     

Highly porous nano-SiC with very low thermal conductivity and excellent high temperature behavior

Highly porous nano-SiC is fabricated by partial sintering and decarburizing process using SiC nano-powders as starting materials and graphite flakes as pore forming agents. The prepared porous nano-SiC ceramics possess multiple pore structures, including well-distributed meso-pores in the skeleton and interconnected flakelike micro-pores. The samples prepared at 1800 °C have relatively low thermal conductivities of 5.61 ～ 0.25 W m^?1 K^?1 with porosities of 55.5–76.1%. While the samples sintered at 1500 °C with porosities between 54.0% to 76.3% show very low thermal conductivities of 0.74 ～ 0.14 W m^?1 K^?1, which is attributed to the integrated nano-scale phonon-scattering mechanisms and duplex pore structures. Porous nano-SiC ceramics also show good retention of elastic stiffness up to 1350 °C and low thermal conductivity at 1400 °C. Our results shed light on porous nano-SiC as a promising thermal insulator used in extreme thermal and chemical environments.     

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.     

Theoretical consideration of the effect of porosity on thermal conductivity of porous materials

The thermal conductivity of porous materials is theoretically studied in connection with nanoporous materials used in recent semiconductor devices. The effects of porosity and pore size on the thermal conductivity are discussed. The thermal conductivity of insulating materials is determined by the heat capacity of phonons, the average phonon velocity and the phonon mean free path. We investigate the porosity dependence of these quantities, especially by taking into account phonon scatterings by pores, and present an expression for the thermal conductivity as a function of porosity. Our model consideration predicts that the thermal conductivity of nanoporous materials depends on the ratio of the pore size R_p to the phonon mean free path for zero-porosity, l₀. The thermal conductivity for l₀/R_p > 1 decreases steeply with increasing porosity because of effective phonon scatterings by pores. On the other hand, the thermal conductivity for l₀/R_p < 0.1 decreases moderately with increasing porosity because phonon scatterings by pores are no longer effective. On the basis of the present theoretical consideration, we discuss the principal factor dominating the porosity dependence of thermal conductivity in nanoporous materials. We also discuss how one can design nanoporous materials with lower or higher thermal conductivity.     

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.     

Effect of humidity on the thermal conductivity of porous zirconia ceramics

This study focuses on the role of the water content on the effective thermal conductivity of porous ceramics placed in different conditions of relative humidity. Fully stabilized zirconia samples with variation in the capacity to take up water were prepared by varying the temperature of the thermal treatment. The pore volume fraction of the dried samples decreases from 56% down to 30%. Thermal conductivity measurements were made on samples placed in a chamber where the relative humidity was fixed between 3% and 99%. For all samples, the experimental values of the effective thermal conductivity increase significantly with the water content. Experimental results agree closely to analytical predictions based on the upper limit of the Hashin and Shtrikman expressions for calculating the thermal conductivity of the pores (constituted by air and water) and Landauer's effective medium expression for calculating the effective thermal conductivity of the material.     

A novel aerogels/porous Si3N4 ceramics composite with high strength and improved thermal insulation property

A novel ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite with high strength, low density, good dielectric properties and low thermal conductivity was synthesized by filling ZrO₂-SiO₂ aerogels into the porous Si₃N₄ ceramics through vacuum sol-impregnating. The effects of aerogels on the microstructure and properties of composite were discussed. The results show that aerogels could form a mesoporous structure and significantly decrease the thermal conductivity from 9.8 to 7.3 W m^?1 K^?1. Meanwhile, the density, mechanical and dielectric properties of the porous Si₃N₄ ceramics could not be affected after introducing ZrO₂-SiO₂ aerogels. The composite exhibits high porosity (62.6%), high flexural strength (53.86 MPa) and low dielectric constant (2.86). The ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite shows great potential as radome materials applied in the fields of aerospace.     

Porous anorthite ceramics with ultra-low thermal conductivity

Porous anorthite ceramics with an ultra-low thermal conductivity of 0.018 W/m K have been fabricated by hydrous foam-gelcasting process and pressureless sintering method using γ-alumina, calcium carbonate and silica powders as raw materials. Microstructure and phase composition were analyzed by SEM and XRD respectively. Properties such as porosity, pore size distribution and thermal conductivity were measured. High porosity (69–91%) and low thermal conductivity (0.018–0.13 W/m K) were obtained after sintering samples with different catalyst additions at 1300–1450 °C. Porosity, pore size, pore structure and grain size had obvious effect on heat conduction, resulting in the low thermal conductivity. The experimental thermal conductivity data of porous anorthite ceramics were found to be fit well with the computed values derived from a universal model.     

Preparation of Ni-doped ZnO ceramics for thermoelectric applications

Zn_1−xNi_xO dense ceramics were prepared from Zn_1−xNi_xO nanoparticles with x varying from 0 to 0.06. These nanoparticles were synthesized by liquid route. In the sintered samples, the solubility limit of Ni in the Zn_1−xNi_xO wurtzite structure was found to be 0.03. The increase of x until 0.03 led to a significant raise in both electrical conductivity (σ) and absolute value of Seebeck coefficient (|S|). Ni-richer samples (x > 0.03) contained in addition a small amount of Ni rich secondary phase (Zn_yNi_zO) with a cubic structure similar to NiO. The thermoelectric properties of all samples were investigated from room temperature to 1000 K. All doped samples showed a n-type semiconducting conductivity. For Ni contents higher than x = 0.03, the increase of the secondary phase content induced a decrease in σ and |S|. The highest power factor (0.6 mW m⁻¹ K⁻²) and ZT (0.09) were found for Zn_0.97Ni_0.03O at 1000 K.     

Dielectric relaxation and electrical conductivity in Ca5Nb4TiO17 ceramics

The temperature dependence of dielectric properties and electrical conduction of Ca₅Nb₄TiO₁₇ ceramics were characterized in a broad temperature range. A dielectric anomaly with strong frequency dispersion was detected in the temperature range 700–1010 °C. This dielectric relaxation could be almost removed completely by annealing in an oxidizing atmosphere. Complex impedance analysis confirmed the electrical inhomogeneity of the ceramics with different contributions from the bulk and grain boundaries. This suggests that the main mechanism for the observed relaxation is the Maxwell–Wagner polarization. ac conductivity results revealed the variation of conduction mechanism with increasing temperatures from localized hopping to long-range motion of the doubly ionized oxygen vacancies.     

Influence of pore distribution on the equivalent thermal conductivity of low porosity ceramic closed-cell foams

The microstructures of porous alumina materials with different porosities were established by introducing the departure factor of pore position and acentric factor of pore diameter to describe the distribution of pores in space and in size, respectively. The contribution of radiation and influence of pore distribution on the equivalent thermal conductivity were discussed based on numerical simulations by the finite volume method (FVM) considering both thermal conduction and radiation. When the pore diameter was less than 10?µm, the radiation component was less than 2%, and radiation could be neglected. Radiative heat transfer played a dominant role for materials with high porosity and large pore size at high temperatures. For micro pore materials (<?100?µm), broad pore size and non-uniform pore space distribution decreased the thermal conductivity across the entire temperature range. For materials with macro pores (>1?mm), broad pore distribution decreased the thermal conductivity at low temperatures and increased it at high temperatures. The basic prediction model of effective thermal conductivity for a two-component material, the Maxwell–Eucken model (ME1) and its modified model were corrected by introducing the pore structure factor. The results from experiments prove that the numerical values were satisfactory.     

Effect of ZrB2 powders on densification,microstructure, mechanical properties and thermal conductivity of ZrB2-SiC ceramics

With the view to improve the densification behaviour and mechanical properties of ZrB₂-SiC ceramics, three synthesis routes were investigated for the production of ZrB₂, prior to the fabrication of ZrB₂-20 vol. % SiC via spark plasma sintering (SPS). Two borothermal reduction routes, modified with a water-washing stage (BRW) and partial solid solution of Ti (BRS), were utilised, alongside a boro/carbothermal mechanism (BRCR) were utilised to synthesise ZrB₂, as a precursor material for the production of ZrB₂-SiC. It was determined that reduction in the primary ZrB₂ particle size, alongside a diminished oxygen content, was capable of improving densification. ZrB₂-SiC ceramics, with ZrB₂ derived from BRW synthesis, exhibited a favorable combination of high relative density (98.6%), promoting a marked increase in Vickers hardness (21.4 ± 1.7 GPa) and improved thermal conductivity (68.7 W·m^-1K^-1).     

Sustainable nitrogen-doped carbon latexes with high electrical and thermal conductivity

Nitrogen-doped carbon particles were produced using the hydrothermal carbonization of a nitrogen-containing carbohydrate, namely glucosamine, under mild temperature conditions (180 °C) followed by further calcination under a stream of inert gas at 750 °C. The resulting materials contain significant amounts of nitrogen doping within their structure, mainly as quaternary N involved in an aromatized/graphitized carbon structure according to X-ray photoelectron spectroscopy. This nitrogen-doped material was dispersed with nanolatexes having a high affinity for carbon. The resulting hybrid dispersions could be conveniently cast into dense and stable films for thermal and electrical conductivity measurements. The conductivities were commensurate with technical carbon nanotube latex-based films. A morphological analysis of the dispersing mechanism suggests that the potential for high performance materials realized in this contribution is very competitive, but still far from being fully exploited.     

New approach to low thermal conductivity of thermal barrier protection with improved mechanical integrity

Layered ceramic systems with designed stacks of dense and porous layers were investigated as alternative for thermal barrier protection system (TBPs). This approach gives the possibility to obtain low thermal conductivity with the impact protection of dense external layers whilst maintaining the relatively high mechanical properties. Different stacking configurations have been proposed utilizing in total a combination of up to 30 dense/porous layers. Porous layers were produced with two different nominal porosities 20 vol% and 40 vol%. For comparison uni-axial pressed samples with the same porosity level have been prepared. Thermal and mechanical characterization was performed on samples of tape cast (with different stacking designs) and uni-axial pressed fully stabilized zirconia TBPs. The layered fully stabilized zirconia (8YSZ) has 15–30 % lower thermal conductivity in comparison with the uni-axial pressed samples, nevertheless by the same Young`s modulus value. The results of the thermal and mechanical observation shows, that such an approach can be beneficial as an alternative for future thermal barrier protection systems.     

Dielectric relaxation and ac conductivity in magnetoelectric YCrO3 ceramics: A temperature dependent impedance spectroscopy analysis

Here, we report on the temperature and frequency dependent electrical conduction and dielectric behaviour of YCrO₃ ceramics. Dielectric studies reveal a peak in the dielectric constant ～230?K, suggesting presence of spin-charge coupling. Also, an additional broad peak found at ～450?K is reminiscent of a relaxor like behaviour for YCrO₃, attributed to a diffused phase transition. The nature of dc conductivity is of Arrhenius type and shows an abrupt change in the activation energy at ～230?K and ～450?K. The activation energy suggests that the polaronic hopping mechanism stabilizes at low temperature while, at higher temperatures, the process is associated with the diffusion of double ionized oxygen vacancies. However, ac conductivity suggests that the overlapping large polaron tunnelling conduction mechanism drives the ac conduction below 300?K and above 300?K, the conduction behaviour is consistent with the correlated barrier hopping conduction mechanism.