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.     

Evaluation of thermal conductivity of the constituent layers in TRISO particles using Raman spectroscopy

The thermal conductivity of individual layer in the tristructural-isotropic fuel particle was evaluated using Raman spectroscopy. In this method, laser acted simultaneously as an excitation source and a heating source. A three-dimensional point-heating model was developed to estimate the local temperature rise in the probing volume of the laser. The thermal conductivity can be evaluated based on the dependences of the Raman peak position on the temperature and laser power. The calculated thermal conductivities were 8.9 ± 0.2 W/m °C, 13.9 ± 1.5 W/m °C and 11.9 ± 0.9 W/m °C for the buffer, the inner and the outer PyC layers, respectively. Contrastly, the thermal conductivity of the SiC layer was 4.1 W/m °C, which is much lower than the reference value, e.g. 168 W/m °C reported by López-Honorato et al. (J. Nucl. Mater. 378(1) 35–39, 2008). The uncertainty of employing Raman spectroscopy to determine thermal conducitvity was discussed.     

Enhancement of superhydrophobicity and conductivity of carbon nanofibers-coated glass fabrics

This paper explores the role of carbon nanofibers (CNFs) on its potential to produce surperhydrophobic and conductive surfaces of glass fiber (GF) fabrics when processed by the catalytic chemical vapor deposition. Large-area helical CNFs were prepared over GF surfaces by the pyrolysis of acetylene. CNFs/GFs composites were characterized by XPS, SEM, and contact angle measurements. The results indicate the CNFs/GF fabrics surface exhibited excellent superhydrophocity and electroconductivity due to the grown CNFs The contact angle and volume resistivity of CNFs decorating the GF fabrics was equal to 152° and 1.13 × 10⁻³ Ω cm, respectively.     

Thermal conductivity of MWCNT/epoxy composites: The effects of length,alignment and functionalization

Carbon nanotubes (CNTs) show great promise to improve composite electrical and thermal conductivity due to their exceptional high intrinsic conductance performance. In this research, long multi-walled carbon nanotubes (long-MWCNTs) and its thin sheet of entangled nanotubes were used to make composites to achieve higher electrical and thermal conductivity. Compared to short-MWCNT sheet/epoxy composites, at room temperature, long-MWCNT samples showed improved thermal conductivity up to 55 W/mK. The temperature dependence of thermal conductivity was in agreement with κ ∝ Tⁿ (n = 1.9–2.3) below 150 K and saturated around room temperature due to Umklapp scattering. Samples with the improved CNT degree of alignment by mechanically stretching can enhance the room temperature thermal conductivity to over 100 W/mK. However, functionalization of CNTs to improve the interfacial bonding resulted in damaging the CNT walls and decreasing the electrical and thermal conductivity of the composites.     

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

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

Preparation and characterization of the one-piece wall ceramic board by using solid wastes

For the purpose of building energy-saving, a novel one-piece wall ceramic board was prepared by using fly ash and ceramic waste as the main raw materials for its matrix part and foam part, respectively. The effects of raw material composition, sintering temperature on the macro and micro properties were systematically investigated. The optimum parameter for the matrix part was obtained at 1220 °C with 70 wt% fly ash and 4 wt% quartz, while that for the foam part was 1220 °C with 97 wt% ceramic waste and 3 wt% silicon carbide. For the matrix sample, the highest rupture modulus reaches 53.97 MPa, and the corresponding water absorption capacity and thermal conductivity are 1.08% and 0.54396 W/(m K), respectively. For the foam part, the best bulk density and thermal conductivity are 443 kg/m³ and 0.10528 W/(m K), respectively. Subsequently, the optimal matrix and foam samples were introduced into the co-fired process (1220 °C), and the results show that the new method for the preparation of one-piece wall ceramic board was fully acceptable. Furthermore, the simulated results indicate that the proposed one-piece wall ceramic board can efficiently reduce the thermal bridges and exerts excellent energy conservation effect.     

Effect of Nd-doping on structural,thermal and electrochemical properties of LaFe0.5Cr0.5O3 perovskites

The structural, thermal and electrochemical properties of the perovskite-type compound La_1−xNd_xFe_0.5Cr_0.5O₃ (x=0.10, 0.15, 0.20) are investigated by X-ray diffraction, thermal expansion, thermal diffusion, thermal conductivity and impedance spectroscopy measurements. Rietveld refinement shows that the compounds crystallize with orthorhombic symmetry in the space group Pbnm. The average thermal expansion coefficient decreases as the content of Nd increases. The average coefficient of thermal expansion in the temperature range of 30–850 °C is 10.12×10⁻⁶, 9.48×10⁻⁶ and 7.51×10⁻⁶ °C⁻¹ for samples with x=0.1, 0.15 and 0.2, respectively. Thermogravimetric analyses show small weight gain at high temperatures which correspond to filling up of oxygen vacancies as well as the valence change of the transition metals. The electrical conductivity measured by four-probe method shows that the conductivity increases with the content of Nd; the electrical conductivity at 520 °C is about 4.71×10⁻³, 6.59×10⁻³ and 9.62×10⁻³ S cm⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. The thermal diffusivity of the samples decreases monotonically as temperature increases. At 600 °C, the thermal diffusivity is 0.00425, 0.00455 and 0.00485 cm² s⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. Impedance measurements in symmetrical cell arrangement in air reveal that the polarization resistance decreases from 55 Ω cm⁻² to 22.5 Ω cm⁻² for increasing temperature from 800 °C to 900 °C, respectively.     

Microstructure and thermophysical properties of B4C/graphite composites containing substitutional boron

B₄C/graphite composites (BGC) containing substitutional boron were fabricated by pressureless sintering of powder mixtures of petroleum coke, coal tar pitch and B₄C. After sintering at 900 °C and graphitizing at 2200 °C, the microstructure of BGC was characterized by SEM, TEM, XRD, Raman spectroscopy and optical microscopy. XPS measurements revealed the formation of BC₃, and the matrix carbon contained around 6 wt.% substitutional boron. The thermal conductivity of the BGC at room temperature is 52.7 W/m K and the flexural strength is up to 35.1 MPa. The bulk density and electrical resistivity are 1.72 g/cm³ and 13.4 μΩ m, respectively. The correlation between microstructure and properties was investigated. The results showed that the microstructure improvement of the BGC has obvious effect on the thermal conductivity, flexural strength, and electrical resistivity.     

High thermal conductivity in pressureless densified SiC ceramics with ultra-low contents of additives derived from novel boron–carbon sources

In order to attain high thermal conductivity, SiC was doped with ultra-low amounts of B and C as sintering additives using boric acid together with d-fructose as boron–carbon sources. The contents of in situ generated B and C were both tailored as low as 0.4 wt.%, which can significantly reduce the impurities induced phonon scattering effect. The SiC ceramics were pressureless densified at 2150 °C for 1 h, and some samples experienced subsequent annealing at 1950 °C for 4 h. High thermal conductivities of 180.94 W/(m K) for the as-sintered SiC ceramics and 192.17 W/(m K) for the annealed specimens at room temperature were achieved. The reasons for the high thermal conductivity in the polycrystalline SiC ceramics were specified, based on the close correlation with microstructure.     

Superoleophobic and conductive carbon nanofiber/fluoropolymer composite films

A solution-based, large-area coating procedure is developed to produce conductive polymer composite films consisting of hollow-core carbon nanofibers (CNFs) and a fluoroacrylic co-polymer available as a water-based dispersion. CNFs (100 nm dia., length ～130 μm) were dispersed by sonication in a formic acid/acetone co-solvent system, which enabled colloidal stability and direct blending of the CNFs and aqueous fluoroacrylic dispersions in the absence of surfactants. The dispersions were sprayed on smooth and microtextured surfaces, thus forming conformal coatings after drying. Nanostructured composite films of different degrees of oil and water repellency were fabricated by varying the concentration of CNFs. The effect of substrate texture and CNF content on oil/water repellency was studied. Water and oil static contact angles (CAs) ranged from 98° to 164° and from 61° to 164°, respectively. Some coatings with the highest water/oil CAs displayed self-cleaning behavior (droplet roll-off angles <10°). Inherent conductivity of the composite films ranged from 63 to 940 S/m at CNF concentrations from 10 to 60 wt.%, respectively. Replacement of the long CNFs with shorter solid-core carbon nanowhiskers (150 nm dia., length 6–8 μm) produced stable fluoropolymer–nanowhisker dispersions, which were ink-jetted to generate hydrophobic, conductive, printed line patterns with a feature size ～100 μm.     

Low temperature growth mechanisms of vertically aligned carbon nanofibers and nanotubes by radio frequency-plasma enhanced chemical vapor deposition

Using radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD), carbon nanofibers (CNFs) and carbon nanotubes (CNTs) were synthesized at low temperature. Base growth vertical turbostratic CNFs were grown using a sputtered 8 nm Ni thin film catalyst on Si substrates at 140 °C. Tip growth vertical platelet nanofibers were grown using a Ni nanocatalyst in 8 nm Ni films on TiN/Si at 180 °C. Using a Ni catalyst on glass substrate at 180 °C a transformation of the structure from CNFs to CNTs was observed. By adding hydrogen, tip growth vertical multi-walled carbon nanotubes were produced at 180 °C using FeNi nanocatalyst in 8 nm FeNi films on glass substrates. Compared to the most widely used thermal CVD method, in which the synthesis temperature was 400–850 °C, RF-PECVD had a huge advantage in low temperature growth and control of other deposition parameters. Despite significant progress in CNT synthesis by PECVD, the low temperature growth mechanisms are not clearly understood. Here, low temperature growth mechanisms of CNFs and CNTs in RF-PECVD are discussed based on plasma physics and chemistry, catalyst, substrate characteristics, temperature, and type of gas.     

Effect of grain growth on the thermal conductivity of liquid-phase sintered silicon carbide ceramics

The effect of grain growth on the thermal conductivity of SiC ceramics sintered with 3 vol% equimolar Gd₂O₃-Y₂O₃ was investigated. During prolonged sintering at 2000 °C in an argon or nitrogen atmosphere, the β → α phase transformation, grain growth, and reduction in lattice oxygen content occurs in the ceramics. The effects of these parameters on the thermal conductivity of liquid-phase sintered SiC ceramics were investigated. The results suggest that (1) grain growth achieved by prolonged sintering at 2000 °C accompanies the decrease of lattice oxygen content and the occurrence of the β → α phase transformation; (2) the reduction of lattice oxygen content plays the most important role in enhancing the thermal conductivity; and (3) the thermal conductivity of the SiC ceramic was insensitive to the occurrence of the β → α phase transformation. The highest thermal conductivity obtained was 225 W(m K)⁻¹ after 12 h sintering at 2000 °C under an applied pressure of 40 MPa in argon.     

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.     

Preparation and electrochemical properties of carbon nanofibers derived from polybenzimidazole/polyimide precursor blends

Carbon nanofibers (CNFs) were fabricated by thermal treatment of electrospun nanofibers obtained from precursor blends of polybenzimidazole (PBI) and Matrimid®. The microcarbon structures of CNFs obtained from PBI, and the 50:50 and 75:25 blends were studied using XRD and Raman spectra. Nitrogen adsorption/desorption measurements revealed that surface area and porosity of CNFs increased with an increase in Matrimid® content. Electrochemical performance of these CNF electrodes was studied for their application in energy storage devices. The CNFs from the PBI/Matrimid® (75:25)-precursor blend showed the lowest electrochemical impedance, and highest specific capacitance (111 F/g) and energy and power densities of 24 and 6 kW/kg, respectively. Steam activation and annealing further enhanced the performance resulting in a specific capacitance of 126 F/g, and energy and power densities of 49 and 7 kW/kg, respectively.     

High thermal conductive diamond/Ag–Ti composites fabricated by low-cost cold pressing and vacuum liquid sintering techniques

Diamond/Ag–Ti composites were fabricated by a low-cost liquid sintering technique. The Ti addition can effectively improve wetting and promote penetration in composite pores during liquid sintering. The interface structure of the diamond/Ag–Ti composite was identified as Ag/TiC/Ag–Ti/diamond. A high thermal conductivity of 719 W/mK was obtained for the 50 vol.% diamond/Ag-1 at.% Ti composite. Using a bimodal mixture (60 vol.% 150 μm + 10 vol.% 50 μm diamond/Ag-2 at.% Ti composite), a low coefficient of thermal expansion of 6.3 × 10^− 6/K still with high thermal conductivity of 687 W/mK was achieved. These composites have potential applications for thermal management of high integration electronic devices.     

Measuring the thermal conductivity of residue-free suspended graphene bridge using null point scanning thermal microscopy

Despite the importance of the accurate measurement of the thermal conductivity of graphene, deviations in previous data are still quite large due to the low signal-to-noise ratio in the measurement of graphene temperature, the uncertainties in the measurement of the heat dissipation, and the influence of the polymeric residues. Herein, we improve signal-to-noise ratio by using null point scanning thermal microscopy, which profiles temperature quantitatively with nanoscale spatial resolution (∼50 nm), independently of both the heat flux through the air and the variation of the sample surface properties. Also, we control and monitor the heat generation rate accurately by heating the suspended graphene bridge electrically. Furthermore, we prevent the disturbance of the thermal conductivity caused by the polymeric residues by using polydimethylsiloxane stamping method, which leaves much less residue than using polymethylmethacrylate. The thermal conductivity values of graphene, whose length and width are 3.6 and 5.52 μm, respectively, were measured as 2430 ± 190, 2150 ± 170, and 2100 ± 160 W/mK at the peak temperatures of 335, 361, and 366 K, respectively, with much smaller error range compared to the previously reported values. The measured values exceed the highest value (∼2000 W/mK at room temperature) obtained for graphite.     

Investigating the effect of SPRM on mechanical strength and thermal conductivity of highly porous alumina ceramics

For recycling waste refractory materials in metallurgical industry, porous alumina ceramics were prepared via pore forming agent method from α-Al₂O₃ powder and slide plate renewable material. Effects of slide plate renewable material (SPRM) on densification, mechanical strength, thermal conductivity, phase composition and microstructure of the porous alumina ceramics were investigated. The results showed that SPRM effectively affected physical and thermal properties of the porous ceramics. With the increase of SPRM, apparent porosity of the ceramic materials firstly increased and then decreased, which brought an opposite change for the bulk density and thermal conductivity values, whereas the bending strength didn’t decrease obviously. The optimum sample A2 with 50 wt% SPRM introducing sintered at 1500 °C obtained the best properties. The water absorption, apparent porosity, bulk density, bending strength and thermal conductivity of the sample were 31.7%, 62.8%, 1.71 g/cm³, 47.1 ± 3.7 MPa and 1.73 W/m K, respectively. XRD analysis indicated that a small quantity of silicon carbide and graphite in SPRM have been oxidized to SiO₂ during the firing process, resulting in rising the porous microstructures. SEM micrographs illustrated that rod-like mullite grains combined with plate-like corundum grains to endow the samples with high bending strength. This study was intended to confirm the preparation of porous alumina ceramics with high porosity, good mechanical properties and low thermal conductivity by using SPRM as pore forming additive.     

Influence of the de-waxing atmosphere on the properties of AlN ceramics processed from aqueous media

The influence of binder burnout atmosphere (air or N₂) on surface chemistry of thermo-chemically treated AlN powders processed in aqueous media, and on the final properties of AlN ceramics was studied. The surface chemistry after de-waxing was accessed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD), SEM, high-resolution transmission electron microscopy (HR-TEM), were used to identify the phase assemblage and for microstructural analysis. The effects of the residual carbon and oxygen at the surface on the thermal conductivity and sintered density of AlN samples were investigated. The surface C/O ratios were observed to correlate with the sintering behaviour, the composition and distribution of secondary phases, and grain-boundary composition, as well as thermal conductivity of AlN samples. Thermal conductivities of about 140 W/mK were obtained for the aqueous processed AlN samples de-waxed in nitrogen atmosphere and sintered for 2 h at 1750 °C in the presence of 4 wt.% YF₃ + 2 wt.% CaF₂ as sintering additives.     

Enhanced thermal conductive 3D-SiC/Al-Si-Mg interpenetrating composites fabricated by pressureless infiltration

A high thermal conductive 3D-SiC/Al-Si-Mg interpenetrating composite (IPC) with three dimensional mutually interpenetrated structure was fabricated by mold-forming and pressureless infiltration method. Al-15Si-10Mg was used as the infiltration aluminum alloy. The obtained composite was treated with a T6 procedure. The composed phases, microstructure, thermal conductivity, mechanical strength and fractography of the prepared 3D-SiC/Al-Si-Mg IPC were either analyzed or measured with X-ray diffraction (XRD), optical metallography, laser thermal conductivity instrument, universal testing machine, field emission electron scanning microscopy (SEM) with energy dispersive spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), and etc. The results showed that both SiC ceramic and aluminum alloy phases distribute evenly and form a three-dimensional mutually interpenetrated structure in the obtained IPC. No clear brittle and harmful Al₄C₃ phase was found in the composite. The obtained IPC contains a SiC volume fraction of 67 vol% and has the properties of a density of 3.02 g/cm², a thermal conductivity of 233.6 W/(m °C), a thermal expansion coefficient (RT~300 °C) of 7.03×10⁻⁶/°C and a bending strength of 288 MPa.     

Preparation of nanoporous alumina superinsulator with ultra-low thermal conductivity and improved heat resistance up to 1200 °C

Nanoporous alumina superinsulator (NanoASI) with ultra-low thermal conductivity and excellent thermal stability has been prepared by a low-cost and simple dry pressing method. The thermal conductivity of the NanoASI is as low as 0.11 W/m K at 1200 °C and the linear shrinkage is less than 2% after heating at 1200 °C for 1 h. These values are superior to that of previous reported nanoporous insulation materials. Thermal conductivities of this material in the temperature range of 25–1200 °C and pressure range of 10–10⁵ Pa were firstly measured by the transient hot-plane method. The mechanism that improves the heat resistance of the NanoASI is discussed and found that the stabilization of the alumina nanoparticles contributes significantly to the thermal stability of the NanoASI.