Electrical,thermal, and mechanical properties of porous SiC-nitride composites

The effects of nitride (AlN, BN, TiN) addition on the electrical, thermal, and mechanical properties of porous SiC-nitride composites were investigated within a porosity range of 40–74 %. The electrical conductivity was predominantly controlled by chemistry rather than porosity, whereas the thermal conductivity was more susceptible to changes in porosity. These results suggest that the electrical conductivity of porous SiC ceramics can be tuned independently from the thermal conductivity by nitride addition. At constant thermal conductivity (∼5 Wm⁻¹ K^-1), the electrical conductivity of the baseline specimen (6.3 × 10^-3 Ω^-1 cm^-1) could be: (1) increased by an order of magnitude (8.3 × 10^-2 Ω^-1 cm^-1) by adding AlN and (2) decreased by an order of magnitude (7.0 × 10^-4 Ω^-1 cm^-1) by adding BN. Typical electrical conductivity and thermal conductivity values of the porous SiC-10 vol% TiN composite were 5.3 × 10^-1 Ω^-1 cm^-1 and ∼14.0 Wm⁻¹ K^-1, respectively, at 51 % porosity.     

Enhanced thermal conductivity and dimensional stability of flexible polyimide nanocomposite film by addition of functionalized graphene oxide

Polyimide (PI) nanocomposites with both enhanced thermal conductivity and dimensional stability were achieved by incorporating glycidyl methacrylate‐grafted graphene oxide (g‐GO) in the PI matrix. The PI/g‐GO nanocomposites exhibited linear enhancement in thermal conductivity when the amount of incorporated g‐GO was less than 10 wt%. With the addition of 10 wt% of g‐GO to PI (PI/g‐GO‐10), the thermal conductivity increased to 0.81 W m^?1 K^?1 compared to 0.13 W m^?1 K^?1 for pure PI. Moreover, the PI/g‐GO‐10 composite exhibited a low coefficient of thermal expansion (CTE) of 29 ppm °C^?1. The values of CTE and thermal conductivity continuously decreased and increased, respectively, as the g‐GO content increased to 20 wt%. Combined with excellent thermal stability and high mechanical strength, the highly thermally conducting PI/g‐GO‐10 nanocomposite is a potential substrate material for modern flexible printed circuits requiring efficient heat transfer capability.     

Thermal conductivity and permeability of consolidated expanded natural graphite treated with sulphuric acid

The thermal conductivity and permeability of consolidated expanded natural graphite treated with sulphuric acid (ENG-TSA) were measured both parallel and perpendicular to the direction of compression used to produce the samples. Results showed that the thermal conductivity and permeability were highly anisotropic. The thermal conductivity perpendicular to the direction of compression was 50 times higher than that parallel to the direction of compression and the permeability was 200 times higher. The maximum thermal conductivity measured was 337 W m⁻¹K⁻¹ at a bulk density of 831 kg m⁻³. The permeability perpendicular to the direction of compression varied in the range of 10⁻¹¹ to 10⁻¹⁶ m² as the density increased from 111 to 539 kg m⁻³. The specific heat was measured, and the average value is 0.89 kJ kg⁻¹K⁻¹ in the temperature range 30–150 °C. As a type of heat transfer matrix the thermal diffusivity was about five times higher than that of, for example, pure aluminium due to the combination of improved thermal conductivity with comparatively low density and reasonable specific heat.     

Thermal and electrical properties of additive-free rapidly hot-pressed SiC ceramics

The thermal and electrical properties of newly developed additive free SiC ceramics processed at a temperature as low as 1850 °C (RHP0) and SiC ceramics with 0.79 vol.% Y₂O₃-Sc₂O₃ additives (RHP79) were investigated and compared with those of the chemically vapor-deposited SiC (CVD-SiC) reference material. The additive free RHP0 showed a very high thermal conductivity, as high as 164 Wm⁻¹ K⁻¹, and a low electrical resistivity of 1.2 × 10⁻¹ Ω cm at room temperature (RT), which are the highest thermal conductivity and the lowest electrical resistivity yet seen in sintered SiC ceramics processed at ≤1900 °C. The thermal conductivity and electrical resistivity values of RHP79 were 117 Wm⁻¹ K⁻¹ and 9.5 × 10⁻² Ω cm, respectively. The thermal and electrical conductivities of CVD-SiC parallel to the direction of growth were ∼324 Wm⁻¹ K⁻¹ and ∼5 × 10⁻⁴Ω⁻¹ cm⁻¹ at RT, respectively.     

Neck formation and role of particle-particle contact area in the thermal conductivity of green and partially sintered alumina ceramics

Sintering ceramics involves neck formation, densification and eventually grain growth. A simplified model is developed to describe the effects of pore fraction, average grain size and the contact area between particles due to neck formation on the thermal conductivity of the green or partially sintered ceramic. Laser flash measurements on green bodies of alumina powders with different average particle sizes reveal similar thermal conductivity values close to 0.5 Wm^?1K^?1, corresponding to thermal resistances for equivalent planes of contacts in the range 10^-7 to 2 10^-6 m²KW^?1. BET specific surface area measurements were then used to estimate the contact area due to neck formation in partially sintered alumina ceramics fired from 400 °C to 1200 °C. As predicted by the model, there is a strong increase in thermal conductivity. Such information is relevant as input data for numerical modelling of the green body behaviour during thermal treatment.     

Effect of γ-radiation on the thermal conductivity of polypropylene

The effect of ⁶⁰Co γ-radiation on the thermal conductivity of polypropylene (PP) has been studied over the temperature range 0–160°C. for radiation doses of 600 and 1800 Mrad. The conductivity of unirradiated specimens rises from 4.5 × 10^?4 cgs units (cal./cm.-sec.-°C.) at 0°C. to 4.8 × 10^?4cgs units at 80°C. and subsequently decreases with temperature to a value of about 3.1 × 10^?4cgs units at 160°C. Upon irradiation to 600 Mrad the thermal conductivity is lowered over the 0–150°C. temperature range. Above 90°C. the conductivity decreases with temperature and becomes relatively constant at 3.4 × 10^?4 cgs units from 120 to 160°C. Differential scanning calorimeter (DCS) measurements from 30 to 200°C. show that irradiation to 600 Mrad lowers the energy associated with crystalline melting and shifts the endotherm melting peak from about 160 to 105°C. Irradiation to 1800 Mrad results in additional lowering of the thermal conductivity over the 50–160°C. range, a further decrease in area of the endothermic peak and a shift of its maximum peak position to about 75°C. The effects of radiation on the thermal conductivity of polypropylene are compared and correlated with the observed effects of radiation on the dynamic mechanical behavior.     

Thermal expansivity and conductivity of pure and silicon-alloyed pyrocarbons

Measurements were made of the thermal expansivity (22°–1000°C) and the thermal conductivity (22°–800°C) of isotropic pyrocarbons deposited in fluidized beds and containing up to 34 wt% silicon in the form of β-silicon carbide particles. The thermal expansivity of the pure carbons was proportional to their density, while that of the silicon-alloyed carbons decreased with increasing silicon content, falling from 6·3 × 10^?°C^?1 for material containing 4 wt% silicon to 4·6 × 10^?6°C^?1 for material with 34 wt% silicon. The thermal conductivity of both pure and silicon-alloyed pyrocarbons increased with increasing temperature up to about 500°C. The room-temperature thermal conductivity of the pure carbons increased with increasing apparent crystallite height (L_c), and the conductivity of the silicon-alloyed carbons was significantly lower than that of pure pyrocarbon with the same L_c. The results suggest that silicon entering substitutionally into the carbon lattice may reduce the conductivity.     

An Evaluation of the Thermophysical Properties of Stoichiometric CeO2 in Comparison to UO2 and PuO2

The thermal conductivity of stoichiometric CeO₂ was determined through measurement of thermal expansion from 313 to 1723 K, thermal diffusivity from 298 to 1473 K, and specific heat capacity from 313 to 1373 K. The thermal conductivity was then calculated as the product of the density, thermal diffusivity, and specific heat capacity. The thermal conductivity was found to obey an (A + BT)^?1 relationship with A = 6.776×10^?2 m·K·W^?1 and B = 2.793 × 10^?4 m·W^?1. Extrapolations of applied models were made to provide suggested data for the specific heat capacity, thermal diffusivity, and thermal conductivity data up to 1723 K. Results of thermal expansion and heat capacity measurements agreed well with the limited low‐temperature data available in the literature. The thermal conductivity values provided in the current study are significantly higher than the only high‐temperature data located for CeO₂. This is attributed to the tendency of CeO₂ to rapidly reduce at elevated temperatures given the available partial pressure of O₂ in air at ambient pressure. The CeO₂ data are compared to literature values for UO₂ and PuO₂ to evaluate its suitability as a surrogate in nuclear fuel systems where thermal transport is a primary criterion for performance     

Shear-induced anisotropy in thermal conductivity of a polyethylene melt

A rotating concentric-cylinder thermal conductivity cell for polymer liquids is described. Thermal conductivity can be measured at temperatures approaching 200°C and at strain rates up to 400 s^?1, The transient heat flux probe (with inner cylinder as heat source and temperature probe) method is used to permit the separation of the viscous heating effect from the probe heating effect. A polyethylene melt was studied and showed that at 50 s^?1, a 2 percent increase in thermal conductivity occurs, followed by a gradual decrease until a value 10 percent less than the no-shear thermal conductivity was found at 400 s^?1. This effect is due to molecular orientation.     

High thermal conductive epoxy molding compound with thermal conductive pathway

The epoxy molding compound (EMC) with thermal conductive pathways was developed by structure designing. Three kinds of EMCs with different thermal conductivities were used in this investigation, specifically epoxy filled with Si₃N₄, filled with hybrid Si₃N₄/SiO₂, and filled with SiO₂. Improved thermal conductivity was achieved by constructing thermal conductive pathways using high thermal conductivity EMC (Si₃N₄) in low thermal conductivity EMC (SiO₂). The morphology and microstructure of the top of EMC indicate that continuous network is formed by the filler which anticipates heat conductivity. The highest thermal conductivity of the EMC was 2.5 W/m K, reached when the volume fraction of EMC (Si₃N₄) is 80% (to compare with hybrid Si₃N₄/SiO₂ filled‐EMC, the content of total fillers in the EMC was kept at 60 vol %). For a given volume fraction of EMC (Si₃N₄) in the EMC system, thermal conductivity values increase according to the order EMC (Si₃N₄) particles filled‐EMC, hybrid Si₃N₄/SiO₂ filled‐EMC, and EMC(SiO₂) particles filled‐EMC. The coefficient of thermal expansion (CTE) decreases with increasing Si₃N₄ content in the whole filler. The values of CTE ranged between 23 × 10^?6 and 30 × 10^?6 K^?1. The investigated EMC samples have a flexural strength of about 36–39 MPa. The dielectric constant increases with Si₃N₄ content but generally remains at a low level (<6, at 1 MHz). The average electrical volume resistivity of the EMC samples are higher than 1.4 × 10¹⁰ Ω m, the average electrical surface resistivity of the EMC samples are higher than 6.7 × 10¹⁴ Ω. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Measurement of through-plane effective thermal conductivity and contact resistance in PEM fuel cell diffusion media

An experimental study was performed to determine the through-plane thermal conductivity of various gas diffusion layer materials and thermal contact resistance between the gas diffusion layer (GDL) materials and an electrolytic iron surface as a function of compression load and PTFE content at 70 °C. The effective thermal conductivity of commercially available SpectraCarb untreated GDL was found to vary from 0.26 to 0.7 W/(m °C) as the compression load was increased from 0.7 to 13.8 bar. The contact resistance was reduced from 2.4×10⁻⁴ m²°C/W at 0.7 bar to 0.6×10⁻⁴ m²°C/W at 13.8 bar. The PTFE coating seemed to enhance the effective thermal conductivity at low compression loads and degrade effective thermal conductivity at higher compression loads. The presence of microporous layer and PTFE on SolviCore diffusion material reduced the effective thermal conductivity and increased thermal contact resistance as compared with the pure carbon fibers. The effective thermal conductivity was measured to be 0.25 W/(m °C) and 0.52 W/(m °C) at 70 °C, respectively at 0.7 and 13.8 bar for 30%-coated SolviCore GDL with microporous layer. The corresponding thermal contact resistance reduced from 3.6×10⁻⁴ m²°C/W at 0.7 bar to 0.9×10⁻⁴ m²°C/W at 13.8 bar. All GDL materials studied showed non-linear deformation under compression loads. The thermal properties characterized should be useful to help modelers accurately predict the temperature distribution in a fuel cell.     

Studies of particle dispersion in plasticized poly(vinyl chloride)/montmorillonite nanocomposites

Poly(vinyl chloride)(PVC) and dioctyl phthalate (DOP) were mixed with 5 and 10 wt % of Cloisite Na⁺, Cloisite 30B or Cloisite 93A. The obtained nanocomposites were characterized by thermal analysis using a thermogravimetric analyzer which showed that addition of 5 wt % of nanoclay to PVC increased its thermal stability in the sequence: Cloisite Na⁺< Cloisite 93A< Cloisite 30B. The electrical conductivity of these composites was studied as a function of temperatures and showed that the conductivity of PVC was enhanced upon using 5 wt % of nanoclay in the sequence: Cloisite Na⁺< Cloisite 30B < Cloisite 93A. The activation energy of interaction of PVC with nanoclay was found to be lowest for the composite containing 5 wt % of nanoclay in the same sequence. The tensile strength, elongation (%), and Young's modulus were considerably enhanced upon increasing the clay content to 5 wt % in the sequence: Cloisite Na⁺< Cloisite 93A < Cloisite 30B. X‐ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study these nanocomposite structures, and it was found that the organoclay layers are homogeneously dispersed in the PVC matrix when 5 wt % of Cloisite 30B or Cloisite 93A was used. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Enhanced out-of-plane thermal conductivity of polyimide composite films by adding hollow cube-like boron nitride

Boron nitride nanosheet (BNNS) is widely used in electronic thermal management due to its excellent planar thermal conductivity and insulating properties. However, it is challenging to improve the out-of-plane thermal conductivity of BNNS-doped composites due to the anisotropy of the thermal conductivity of BNNS. Therefore, the BNNS in the matrix must be oriented to obtain composites with high out-of-plane thermal conductivity. In this study, BNNS powders with directional structures were synthesized directly using sodium chloride templates. The as-obtained BNNS powders have a unique hollow cube-like structure with an ultra-low density of 2.67 × 10⁻² g/cm³ and nearly 8 times the volume of the same mass of two-dimensional (2D) BNNS, making it easy to form the out-of-plane thermal conductivity paths in the polymer matrix. In addition, the high out-of-plane thermal conductivity of 4.93 W m⁻¹ K⁻¹ at 23.3 wt% loadings was obtained by doping it into a polyimide (PI) matrix. This value is 9.7 times higher than that of 2D BNNS-doped PI at the same loadings, 17.6 times higher than pure PI, and 6.1 times higher than the thermally conductive PI film sold by DuPont. Therefore, the prepared composite film has great potential for application in electronic thermal management.     

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.     

Improving tribological properties of phenolic resin/carbon fiber composites using m-Si3N4@PANI core–shell particles

Phenolic resin/carbon fiber (PF/CF) composites have good tribological properties; however, their extensive applications are limited because of the poor thermal conductivity of the phenolic resins. In this work, core‑shell particles of polyaniline-coated (3-aminopropyl) triethoxysilane-modified β-Si₃N₄ (m-SiN@PANI) were used to enhance the tribological, electrical, and thermal conductivity properties of a PF/CF composite. A core‑shell particle, consisting of m-SiN@PANI, was characterized by Fourier Transform Infrared Spectrometry, X-Ray Diffraction, Scanning Electron Microscope, and Transmission Electron Microscope. The friction, thermal, and electrical properties of the composites were characterized by multifunctional vertical friction testing, wear measurement testing, thermogravimetric analysis, thermal constant analysis, and electrical conductivity testing. Remarkably, the test results showed that compared with the wear surface of the PF/CF composite, that of the phenolic resin/(2.0 wt % m-SiN@PANI)/carbon fiber composite exhibited a smoother morphology. The results indicated that the addition of m-SiN@PANI effectively improved the thermal conductivity, electrical conductivity, friction coefficient, and wear rate of the composites, which were 3.164 Wm⁻¹ K⁻¹, 5.33 × 10⁻⁶ S/m, 0.1681 and 1.13 × 10⁻⁸ mm³/Nm, respectively. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47785.     

Highly anisotropic thermal conductivity of mesogenic epoxy resin film through orientation control

To develop insulating materials with a high thermally conductive anisotropy, planarly aligned mesogenic epoxy (ME) resin film was fabricated by uniaxial coating on a hydrophobic polyethylene terephthalate substrate. Grazing incidence small-angle X-ray scattering (GISAXS) and transmission SAXS measurements exhibited that the films spontaneously formed uniaxially aligned monodomain-like smectic structures by curing on the hydrophobic substrate. Then, an in- and out-of-plane thermal conductivity of 10 and 0.048 W m⁻¹ K⁻¹ and outstanding thermal conductivity anisotropy of 208 have been confirmed, respectively. The ME resin films with high thermal conductivity can be applied as insulating materials for multiple-layer electrical and electronic devices.     

Influence of ball milling contamination on properties of sintered AlN substrates

The AlN substrate was fabricated by the tape casting process, and its thermal conductivity and electrical conductivity were investigated for various ball milling times and types of milling media. The oxygen content was measured after ball milling, de-binding process, and sintering. The oxygen content after the de-binding process was 1.2–2.3 wt%, similar to that after milling. After the de-binding process, the specimens were sintered at 1850 ℃ for 3 h in nitrogen atmosphere. The thermal conductivity of the sintered specimens decreased from 158 W m⁻¹ K⁻¹ to 100 W m⁻¹K⁻¹ with increasing milling time. Simultaneously, the electrical conductivity decreased from approximately 10⁻⁷ Ω⁻¹ cm⁻¹ to 10⁻⁹ Ω⁻¹ cm⁻¹ at 500 °C when Al₂O₃ or ZrO₂ balls were used, whereas the electrical conductivity did not decrease when Si₃N₄ balls were used.     

Modification of polyimide coatings by high energy ion bombardment

Exposure of 25-μm films of polyimide and polyamideimide to high doses (> 10¹⁵/cm²) of energetic ions (energy ≥ 100 keV) resulted in physical and chemical modification of the film surface. Cross-section microscopy revealed damaged layers extending beyond the projected ion range; conductivity in this damaged layer was found to be as high as 10³ω^?1 cm^?1. Surface conductivity was found to be a function of ion energy and dose, with an exponential energy dependence from 200 to 900 keV. The temperature dependence and thermal stability of the surface conductivity were determined.     

Investigation on thermal conductivity and mechanical properties of Si3N4 ceramics via one-step sintering

High thermal conductivity Si₃N₄ ceramics were fabricated using a one-step method consisting of reaction-bonded Si₃N₄ (RBSN) and post-sintering. The influence of Si content on nitridation rate, β/(α+β) phase rate, thermal conductivity and mechanical properties was investigated in this work. It is of special interest to note that the thermal conductivity showed a tendency to increase first and then decrease with increasing Si content. This experimental result shows that the optimal thermal conductivity and fracture toughness were obtained to be 66 W (m K)^-1 and 12.0 MPa m^1/2, respectively. As a comparison, the nitridation rate and β/(α+β) phase rate in a static pressure nitriding system, i.e., 97% (MS10), 97% (MS15), 97% (MS20) and 8.3% (MS10), 8.3% (MS15), 8.9% (MS20), respectively, have obvious advantages over those in a flowing nitriding system, i.e., 91% (MS10), 91% (MS15), 93% (MS20) and 3.1% (MS10), 3.3% (MS15), 3.3% (MS20), respectively. Moreover, high lattice integrity of the β-Si₃N₄ phase was observed, which can effectively confine O atoms into the β-Si₃N₄ lattice using MgO as a sintering additive. This result indicates that one-step sintering can provide a new route to prepare Si₃N₄ ceramics with a good combination of thermal conductivity and mechanical properties.     

Conductive and radiative heat transfer inhibition in YSZ photonic glass

A high temperature stable ceramic photonic structure is demonstrated with low thermal conductivity and suppressed external radiative heat transfer. The structure is based on a disordered arrangement of yttria-stabilized zirconia (YSZ) microparticles, called photonic glass (PhG). The prepared YSZ-PhG film exhibits low thermal conductivity of 0.03 Wm⁻¹K⁻¹ comparable to that of the air. The small point contacts of the adjacent YSZ particles are the main cause of such low thermal conductivity. After annealing at 1400 °C for 5 h, the solid thermal conductivity increased to 0.3 Wm⁻¹K⁻¹ at room temperature due to the thermally induced neck formation, associated with an increased contact area between adjacent particles. This thermal conductivity is still much lower than that of conventional YSZ thermal barrier coatings (TBCs) with approximately 1 Wm⁻¹K⁻¹. At the same time, the PhG structure is an efficient scatterer for thermal radiation in the wavelength range between 1 and 6 μm. In an only 100 μm thick structure an average reflection of 84% was obtained. At 1400 °C, the effective thermal conductivity is 0.2 Wm⁻¹K⁻¹. The presented structure is applicable to other oxides with even lower bulk thermal conductivity and can be considered for future TBCs.