Effect of carbon content on electrical,thermal, and mechanical properties of porous SiC ceramics with B4C and C additives

The electrical, thermal, and mechanical properties of porous SiC ceramics with B₄C-C additives were investigated as functions of C content and sintering temperature. The electrical resistivity of porous SiC ceramics decreased with increases in C content and sintering temperature. A minimal electrical resistivity of 4.6 × 10^?2 Ω·cm was obtained in porous SiC ceramics with 1 wt% B₄C and 10 wt% C. The thermal conductivity and flexural strength increased with increasing sintering temperature and showed maxima at 4 wt% C addition when sintered at 2000 °C and 2100 °C. The thermal conductivity and flexural strength of porous SiC ceramics can be tuned independently from the porosity by controlling C content and sintering temperature. Typical electrical resistivity, thermal conductivity, and flexural strength of porous SiC ceramics with 1 wt% B₄C-4 wt% C sintered at 2100 °C were 1.3 × 10^?1 Ω·cm, 76.0 W/(m·K), and 110.3 MPa, respectively.     

Thermal,electrical, and mechanical properties of addition-type liquid silicone rubber co-filled with Al2O3 particles and BN sheets

The addition-type liquid silicone rubber (ALSR) co-filled with spheroidal Al₂O₃ and flaky BN was prepared by the mechanical blending and hot press methods to enhance the thermal, electrical, and mechanical properties for industrial applications. Morphologies of ALSR composites were observed by scanning electron microscopy (SEM). It was found that the interaction and dispersion state of fillers in the ALSR matrix were improved by the introduction of BN sheets. Thermal, electrical, and mechanical performances of the ALSR composites were also investigated in this work. The result indicated that the thermal conductivity of ALSR can reach 0.64 W m⁻¹ K⁻¹ at the loading of 20 wt% Al₂O₃/20 wt% BN, which is 3.76 times higher than that of pure ALSR. The addition of Al₂O₃ particles and BN sheets also improve the thermal stability of ALSR composites. Moreover, pure ALSR and ALSR composites showed relatively lower dielectric permittivity (1.9–3.1) and dielectric loss factor (<0.001) at the frequency of 10³ Hz. The insulation properties including volume resistivity and breakdown strength were improved by the introduction of flaky BN in the ALSR matrix. The volume resistivity and characteristic breakdown strength E₀ are 6.68 × 10¹⁵ Ω m and 93 kV/mm, respectively, at the loading of 20 wt% Al₂O₃/20 wt% BN. In addition, the mechanical characteristics including elongation at break and tensile strength of ALSR composites were also enhanced by co-filled fillers. The combination of these improved performances makes the co-filled ALSR composites attractive in the field of electrical and electronic applications.     

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.     

Simultaneously enhanced thermal conductivity and dielectric properties of borosilicate glass-based LTCC with AlN and h-BN additions

Microwave devices with reduced dielectric loss and electronic components with increased integration density necessitate the higher performance of electronic packaging materials. The h-BN/AlN/CaCO₃-MgO-B₂O₃-SiO₂-Li₂CO₃ glass composites were prepared via tape-casting and then sintered by pressureless and hot-pressing, respectively. The thermal conductivity of pressureless sintered composite was increased to 6.55 W/(m·K) by incorporating 3 wt% h-BN, and the thermal expansion of 4.47 ppm/K was achieved along with low dielectric constant of 5.76 and dielectric loss of 7.02 × 10⁻⁴ at 24 GHz. In contrast, the hot-pressing sintered composite containing 4 wt% h-BN exhibited higher thermal conductivity of 10.3 W/(m·K) and lower dielectric loss of 4.77 × 10⁻⁴. The microstructure characterization indicated the construction of heat conduction networks, and XRD analysis illustrated the formation of crystallization in the glass. Such low-temperature co-fired ceramic (LTCC) with high thermal conductivity and low dielectric loss would be a promising candidate for electronic packaging and 5G communication applications.     

Highly electrically and thermally conductive silicon carbide-graphene composites with yttria and scandia additives

Dense silicon carbide/graphene nanoplatelets (GNPs) and silicon carbide/graphene oxide (GO) composites with 1 vol.% equimolar Y₂O₃–Sc₂O₃ sintering additives were sintered at 2000 °C in nitrogen atmosphere by rapid hot-pressing technique. The sintered composites were further annealed in gas pressure sintering (GPS) furnace at 1800 °C for 6 h in overpressure of nitrogen (3 MPa). The effects of types and amount of graphene, orientation of graphene sheets, as well as the influence of annealing on microstructure and functional properties of prepared composites were investigated. SiC-graphene composite materials exhibit anisotropic electrical as well as thermal conductivity due to the alignment of graphene platelets as a consequence of applied high uniaxial pressure (50 MPa) during sintering. The electrical conductivity of annealed sample with 10 wt.% of GNPs oriented parallel to the measuring direction increased significantly up to 118 S·cm⁻¹. Similarly, the thermal conductivity of composites was very sensitive to the orientation of GNPs. In direction perpendicular to the GNPs the thermal conductivity decreased with increasing amount of graphene from 180 W·m⁻¹ K⁻¹ to 70 W·m⁻¹ K⁻¹, mainly due to the scattering of phonons on the graphene – SiC interface. In parallel direction to GNPs the thermal conductivity varied from 130 W·m⁻¹ K⁻¹ up to 238 W·m⁻¹ K⁻¹ for composites with 1 wt.% of GO and 5 wt.% of GNPs after annealing. In this case both the microstructure and composition of SiC matrix and the good thermal conductivity of GNPs improved the thermal conductivity of composites.     

Influence of sintering atmosphere and BN additives on microstructure and properties of porous SiC ceramics

The effects of the boron nitride (BN) content on the electrical, thermal, and mechanical properties of porous SiC ceramics were investigated in N₂ and Ar atmospheres. The electrical resistivity was predominantly controlled by the sintering atmosphere and secondarily by the BN concentration, whereas the thermal conductivity and flexural strength were more susceptible to changes in the porosity and necking area between the SiC grains. The electrical resistivities of argon-sintered porous SiC ceramics (6.3 × 10⁵ – 1.6 × 10⁶ Ω·cm) were seven orders of magnitude higher than those of nitrogen-sintered porous SiC ceramics (1.5 × 10⁻¹ – 6.0 × 10⁻¹ Ω·cm). The thermal conductivity and flexural strength of the argon-sintered porous SiC ceramics increased from 8.4–11.6 W·m⁻¹ K⁻¹ and from 9.3–28.2 MPa, respectively, with an increase in the BN content from 0 to 1.5 vol%, which was attributed to the increase in necking area and the decrease in porosity.     

Grain-growth-induced high electrical conductivity in SiC–BN composites

SiC–BN composites were fabricated by conventional hot-pressing from β-SiC and h-BN nanopowders with 2?vol% yttria as a sintering additive. Electrical and thermal properties of the composites were investigated as a function of initial BN content. Owing to the nanosize of the starting powders, the grain-growth-assisted N-doping of the SiC lattice was significantly enhanced during liquid-phase sintering, yielding the highest-reported electrical conductivity of ~124 (Ω?cm)^?1 for a SiC–4-vol% BN composite. The typical values of electrical resistivity and thermal conductivity of the SiC–4-vol% BN composite at room temperature were 8.1?×?10^?3 Ω?cm and 92.4?W?m^?1 K^?1, respectively.     

Improved dielectric and mechanical properties of Ti3C2Tx MXene/silicone rubber composites achieved by adding a few boron nitride nanoplates

MXenes have attracted great attentions in the fabrication of dielectric polymer composites because of their excellent electrical conductivity. However, the high dielectric loss tangent would suppress the application of such polymer-based composites. Incorporating insulating fillers might be a solution. Herein, Ti₃C₂T_x MXene/silicone rubber (SR) composites incorporated with boron nitride (BN) nanoplates were prepared. The homogeneous distribution of fillers was obtained in the composites, which was also thermally stable up to 400 °C. Dielectric constant of 7.06 (2.54 times of pure SR) and dielectric loss tangent of 0.00131 were achieved when the filling contents of MXene and BN in SR composite were 1.2 wt% and 5 wt%, respectively. The improved dielectric constant can be ascribed to the enhanced interfacial polarization and the formation of conductive network, while the low dielectric loss tangent can be due to the insulating interlayers of BN which could inhibit the transfer of free electrons from conductive fillers to the insulating polymer matrices. BN/MXene/SR composites displayed improved mechanical properties (tensile stress of 671 kPa and elongation at break of 353%) and good flexibility (elastic modulus of 540 kPa) due to the low filling content of fillers. This work is promising for preparing dielectric polymer composites in applications of electronic devices.     

Electrical,thermal, and mechanical properties of porous silicon carbide ceramics with a boron carbide additive

The effects of the boron carbide (B₄C) content and sintering atmosphere on the electrical, thermal, and mechanical properties of porous silicon carbide (SiC) ceramics were investigated in the porosity range of 58.3%–70.3%. The electrical resistivities of the nitrogen-sintered porous SiC ceramics (∼10^–1 Ω·cm) were two orders of magnitude lower than those of argon-sintered porous SiC ceramics (∼10¹ Ω·cm). Both the thermal conductivities (3.3–19.8 W·m^–1·K^–1) and flexural strengths (8.1–32.9 MPa) of the argon- and nitrogen-sintered porous SiC ceramics increased as the B₄C content increased, owing to the decreased porosity and increased necking area between SiC grains. The electrical resistivity of the porous SiC ceramics was primarily controlled by the sintering atmosphere owing to the N-doping from the nitrogen atmosphere, and secondarily by the B₄C content, owing to the B-doping from the B₄C. In contrast, the thermal conductivity and flexural strength were dependent on both the porosity and necking area, as influenced by both the sintering atmosphere and B₄C content. These results suggest that it is possible to decouple the electrical resistivity from the thermal conductivity by judicious selection of the B₄C content and sintering atmosphere.     

Effect of Ni content and its particle size on electrical resistivity and flexural strength of porous SiC ceramic sintered at low-temperature using clay additive

A low-temperature sintered porous SiC-based clay-Ni system with controlled electrical resistivity (2.54 × 10¹⁰ Ω cm to 2 Ω cm), and thermal conductivity (3.5 W/m. K to 12.6 W/m. K) was successfully designed. Clay (20 wt% kaolin) was used as a sintering additive in all the compositions. The electrical resistivity, and thermal conductivity was controlled by varying the Ni content (0–25 wt%) in the samples. The electrical resistivity was recorded as low as 2 Ω cm with 25 wt% Ni that was sintered at 1400 °C in argon. The interface reaction between Ni and SiC formed conductive nickel silicide (Ni₂Si), while the transformation of kaolin to mullite strengthened the mechanical properties. Submicron-sized Ni (0.3 μm) was more effective than micron-sized Ni (3.5 μm) in reducing the electrical resistivity, and increasing the thermal conductivity along with flexural strength. A comparative study of sintering temperatures showed that 1400 °C resulted in the lowest electrical resistivity (2 Ω cm) and the highest thermal conductivity of 12.6 W/m. K with flexural strength of 54 MPa at 32% porosity in the SiC-kaolin-Ni system.     

Effects of cerium doping on the microstructure,mechanical properties,thermal conductivity,and dielectric properties of ZrP2O7 ceramics

A two-step method, combined with cold isostatic pressing, was used to prepare CeO₂-doped ZrP₂O₇ ceramics, and their microstructure, mechanical properties, thermal conductivities, and dielectric properties were determined. It was found that CeO₂ doping could increase the Zr–P and P–O bond lengths, which in turn decreased the thermal conductivity of the ZrP₂O₇ matrix. Doping with 12 wt% CeO₂ simultaneously reduced the sintering temperature and improved the mechanical properties of the ZrP₂O₇ ceramics, while retaining its low thermal conductivity and good dielectric properties. The maximum cold modulus of rupture of a sample at 1250 °C was 75.91 MPa, which met most conditions for use at room temperature. A COMSOL model was used to predict the thermal conductivity, based on the microstructure, with a relatively high degree of accuracy. The thermal conductivity of the CeO₂-doped samples was lower than 1.083 W/(m·K). The dielectric constant was in the range of 5.93–6.52 at 20–40 GHz, and the dielectric loss was less than 4 × 10^?3. The ZrP₂O₇-doped ceramics have potential for application in millimetre wave technology, satellite communication, and vehicle radar fields, because they can meet the high thermal insulation requirements for these applications.     

Liquid crystal epoxy composites based on functionalized boron nitride: Synthesis and thermal properties

A composite was prepared by in-situ polymerization of liquid crystal epoxy (LCE₄) with a low dielectric and high thermal conductivity boron nitride (BN) filler, which the filler (f-BN) was surface-functionalized by γ-glycidoxypropyltrimethoxysilane (KH560) and aminopropylisobutyl polyhedral oligomeric silsesquioxane (NH₂-POSS). The surface-functionalized BN was more uniformly dispersed in LCE₄, which improved the interfacial compatibility between inorganic and organic phases. Compared with pure LCE₄, KH560, and NH₂-POSS modified f-BN/LCE₄ composites exhibited a higher glass transition temperature, better thermal stability, and higher thermal conductivity. For example, when the f-BN content reached 30 wt%, the energy storage modulus of the composite increased to 2580 MPa, and the glass transition temperature was 103°C. The thermal conductivity of this 30 wt% f-BN composite was 0.48 W m⁻¹ K⁻¹, 128.6% higher than that of pure LCE₄. In addition, thermal stability, low hygroscopicity, and dielectric properties of the composites were characterized and analyzed to explore the application prospects of f-BN/LCE₄ composites in electronic packaging and in high-performance microelectronic devices.     

A Concurrent Enhancement of Both In-Plane and Through-Plane Thermal Conductivity of Injection Molded Polycarbonate/Boron Nitride/Alumina Composites by Constructing a Dense Filler Packing Structure

In this work, a facile strategy is proposed to concurrently enhance both in-plane and through-plane thermal conductivity of injection molded polycarbonate (PC)-based composites by constructing a dense filler packing structure with planar boron nitride (BN) and spherical alumina (Al₂O₃) particles. The state of orientation of BN platelets is altered with the presence of Al₂O₃, which is favorable for improving both in-plane and through-plane thermal conductivity of subsequent moldings. Rheological analysis showed that the formation of intact thermal conductive pathways is crucial to the overall enhancement of thermal conductivity. Both in-plane and through-plane thermal conductivity of PC/BN(20 wt%)/Al₂O₃(40 wt%) composites reached as high as 1.52 and 1.09 W mK⁻¹, which are 485% and 474% higher than that of pure PC counterparts, respectively. Furthermore, the prepared samples demonstrated excellent electrical insulation and dielectric properties which show potential application in electronic and automotive industries.     

Dielectric and EMW absorbing properties of PDCs-SiBCN annealed at different temperatures

Polymer derived SiBCN ceramics (PDCs-SiBCN) were prepared from polyborosilazane and then annealed at 1200–1800 °C in N₂. Effects of annealing temperature on the microstructure, phase composition, dielectric and wave-absorbing properties of ceramics were investigated. Results showed that nano-sized SiC grains were formed in amorphous SiBCN after annealing and the content and crystallization degree of SiC gradually intensified with annealing temperature increasing. The permittivity, dielectric loss and electrical conductivity of PDCs-SiBCN gradually increased as the temperature rose due to the formation of conductivity network of SiC grains and the increase of nano-grain boundary. The increased content of SiC (as the dipole) and interface between SiC nano-grains and amorphous SiBCN phase led to a higher polarization ability and higher dielectric loss. The RC gradually decreased with the annealing temperature increasing, demonstrating the annealed ceramics had the superior wave-absorbing ability and high annealing temperature was conducive to the improvement of wave-absorbing property.     

Electrically conductive SiC ceramics processed by pressureless sintering

Polycrystalline SiC ceramics with 10 vol% Y₂O₃-AlN additives were sintered without any applied pressure at temperatures of 1900-2050°C in nitrogen. The electrical resistivity of the resulting SiC ceramics decreased from 6.5 × 10¹ to 1.9 × 10⁻² Ω·cm as the sintering temperature increased from 1900 to 2050°C. The average grain size increased from 0.68 to 2.34 μm with increase in sintering temperature. A decrease in the electrical resistivity with increasing sintering temperature was attributed to the grain-growth-induced N-doping in the SiC grains, which is supported by the enhanced carrier density. The electrical conductivity of the SiC ceramic sintered at 2050°C was ~53 Ω⁻¹·cm⁻¹ at room temperature. This ceramic achieved the highest electrical conductivity among pressureless liquid-phase sintered SiC ceramics.     

Mechanical and thermal properties of spark plasma sintered Al2O3-graphene-SiC hybrid composites

Monolithic Al₂O₃ and Al₂O₃-graphene-SiC hybrid composites were prepared by spark plasma sintering (SPS) under vacuum atmosphere. The results show that the hybrid composites were almost completely dense (>97%). SiC content has a significant effect on the microstructure of the composites. With the increase of SiC content, the average grain size of alumina decreased gradually. The addition of SiC to alumina changed fracture mode from inter-granular fracture to mixed fracture mode of inter-granular fracture and trans-granular fracture. The Al₂O₃-0.4 wt%graphene-5 wt% SiC hybrid composite has the highest bending strength and hardness, which were 57% and 19.22% higher than those of the monolithic alumina, respectively. The room temperature (RT) thermal conductivity of the monolithic Al₂O₃ (25.5 W/m·K) was the highest. The thermal conductivity and thermal diffusivity coefficient of the composites decreased with the increase in temperature, while the specific heat of monolithic alumina and composites increased with the increase in temperature and additives. These properties were related to the microstructure of materials and the possible transport mechanisms were discussed.     

Electrical conductivity and infrared radiation performance of SiC-CNT composite ceramics

SiC ceramic is an excellent infrared source material that can be used in a wide range of fields, like infrared heating, night vision and communication, but its poor electrical properties limit it. In this work, carbon nanotubes (CNTs) were selected as conductive phase filler, and SiC-CNT composite ceramics were prepared by SPS method. The effects of CNT content on the microstructures, electrical properties and infrared radiation performance of the composites were studied. The introduction of CNT effectively reduced the height of Schottky barrier at grain boundary, thus weakening the grain boundary effect, reducing the grain boundary resistance, further weakening the nonlinear characteristics and bulk resistivity of the composite ceramics. When the content of CNT was 1 wt%, electrical percolation was achieved, and the bulk resistivity of SiC ceramics dropped by nearly 3 orders of magnitude. The preferred orientation distribution of CNT made the bulk resistivity perpendicular to the pressure direction R_⊥ always lower than that parallel to the pressure direction R_//. The sample with 5 wt% CNT assumed linear conductivity characteristics, with bulk resistivity in different direction of 16.5 Ω cm (R_//) and 11.8 Ω cm (R_⊥), respectively. CNT addition slightly increased the infrared radiation performance of SiC ceramics, and the sample with 5 wt% CNT possessed the highest total emissivity of 0.675. The excellent electrical conductivity and infrared radiation performance of SiC-CNT composite ceramic confirmed this class as a promising infrared source material.     

Effects of porosity on electrical and thermal conductivities of porous SiC ceramics

The effects of porosity on the electrical and thermal conductivities of porous SiC ceramics, containing Y₂O₃–AlN additives, were investigated. The porosity of the porous SiC ceramic could be controlled in the range of 28–64 % by adjusting the sacrificial template (polymer microbead) content (0–30 wt%) and sintering temperature (1800–2000 °C). Both electrical and thermal conductivities of the porous SiC ceramics decreased, from 7.7 to 1.7 Ω⁻¹ cm⁻¹ and from 37.9 to 5.8 W/(m·K), respectively, with the increase in porosity from 30 to 63 %. The porous SiC ceramic with a coarser microstructure exhibited higher electrical and thermal conductivities than those of the ceramic with a finer microstructure at the equivalent porosity because of the smaller number of grain boundaries per unit volume. The decoupling of the electrical conductivity from the thermal conductivity was possible to some extent by adjusting the sintering temperature, i.e., microstructure, of the porous SiC ceramic.     

Thermal conductivity and dielectric property of hot-pressing sintered AlN–BN ceramic composites

A study was conducted of the effects of sintering temperature and CaF₂ additives on densification, microstructure, dielectric property and thermal conductivity of AlN–BN composites. Increasing sintering temperature and CaF₂ contents help to improve the densification, thermal conductivity, and purification of the grain boundaries. Thermal conductivity value reached 110 W m⁻¹ K⁻¹ for AlN–BN composites with 3 wt.% CaF₂ and sintered at 1850 °C. Increasing sintering temperature decreases relative dielectric constant and tan δ. The increase in CaF₂ content increases relative dielectric constant and decreases tan δ. Relative dielectric constants values were between 7.29 and 7.64 and dielectric loss tangent values ranged from 6.36 to 7.83 × 10⁻⁴ at 1 MHz.     

Heat conduction mechanisms in hot pressed ZrB2 and ZrB2–SiC composites

Thermal diffusivity and conductivity of hot pressed ZrB₂ with different amounts of B₄C (0–5 wt%) and ZrB₂–SiC composites (10–30 vol% SiC) were investigated experimentally over a wide range of temperature (25–1500 °C). Both thermal diffusivity and thermal conductivity were found to decrease with increase in temperature for all the hot pressed ZrB₂ and ZrB₂–SiC composites. At around 200 °C, thermal conductivity of ZrB₂–SiC composites was found to be composition independent. Thermal conductivity of ZrB₂–SiC composites was also correlated with theoretical predictions of the Maxwell–Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB₂ and ZrB₂–SiC composites at room temperature were confirmed by Wiedemann–Franz analysis by using measured electrical conductivity of these materials at room temperature. It was found that electronic thermal conductivity dominated for all monolithic ZrB₂ whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB₂–SiC composites.