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1.
Minimum lattice thermal conductivities and mechanical properties of polymorphous MPO4 (M = Al, Ga) are investigated by first principles calculations. The theoretical minimum thermal conductivities are found to be 1.02 W (m K)?1 for α-AlPO4, 1.20 W (m K)?1 for β-AlPO4, 0.87 W (m K)?1 for α-GaPO4 and 0.88 W (m K)?1 for β-GaPO4. The lower thermal conductivities in comparison to YSZ can be attributed to the lattice phonon scattering due to the framework of heterogeneous bonds. In addition, the low shear-to-bulk modulus ratio for both β-AlPO4 (0.38) and β-GaPO4 (0.30) is observed. Our results suggest their applications as light-weight thermal insulator and damage-tolerant/machinable ceramics.  相似文献   

2.
A rapid method for the synthesis of Ca3Co4O9+δ powder is introduced. The procedure is a modification of the conventional citric-nitrate sol–gel method where an auto-combustion process is initiated by a controlled thermal oxidation–reduction reaction. The resulting powders inherit the advantages of a wet chemical synthesis, such as morphological and compositional homogeneity, and fine, well-defined particle sizes coming from the controlled nature of the auto-combustion. Optimized spark plasma sintering (SPS) processing conditions were determined and used to fabricate dense and highly c-axis oriented samples. The microstructure and thermoelectric transport properties were determined both parallel (||) and perpendicular (⊥) to the SPS pressure axis in order to investigate any possible anisotropy variations in the transport properties. At 800 °C, power factors of 506 μW/m K2 (⊥) and 147 μW/m K2 (||), thermal conductivities values of 2.53 W/m K (⊥) and 1.25 W/m K (||), and resulting figures-of-merit, ZT, of 0.21 (⊥) and 0.13 (||) were observed.  相似文献   

3.
Using a naphthalene-derived mesophase pitch as a starting material, highly oriented ribbon-shaped carbon fibers with a smooth and flat surface were prepared by melt-spinning, oxidative stabilization, carbonization, and graphitization. The preferred orientation, morphology, and microstructure, as well as physical properties, of the ribbon-shaped carbon fibers were characterized. The results show that, the ribbon-shaped fibers possessed uniform shrinkage upon heat treatment, thereby avoiding shrinkage cracking commonly observed in round-shaped fibers. As heat treatment progressed, the ribbon-shaped graphite fibers displayed larger crystallite sizes and higher orientation of graphene layers along the main surface of the ribbon-shaped fiber in comparison with corresponding round-shaped fibers. The stability of the ribbon-shaped graphite fibers towards thermal oxidation was significantly higher than that of K-1100 graphite fibers. The longitudinal thermal conductivity of the ribbon fibers increased, and electrical resistivity decreased, with increasing the heat treatment temperatures. The longitudinal electrical resistivity and the calculated thermal conductivity of the ribbon-shaped fibers graphitized at 3000 °C are about 1.1 μΩ m and above 1100 W/m K at room temperature, respectively. The tensile strength and Young’s modulus of these fibers approach 2.53 and 842 GPa, respectively.  相似文献   

4.
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.  相似文献   

5.
《Ceramics International》2017,43(12):8564-8571
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.  相似文献   

6.
Ribbon-shaped carbon fibers have been prepared from mesophase pitch by melt-spinning, oxidative stabilization and further heat treatment. The internal graphitic layers of ribbon-shaped carbon fibers graphitized at 2800 °C show a highly preferred orientation along the longitudinal direction. Parallel stretched and unidirectional arranged ribbon-shaped carbon fibers treated at about 450 °C were sprayed with a mesophase pitch powder grout, and then hot-pressed at 500 °C and subsequently carbonized and graphitized at various temperatures to produce one-dimensional carbon/carbon (C/C) composite blocks. The shape and microstructural orientation of ribbon fibers have been maintained in the process of hot-pressing and subsequent heat treatments and the main planes of the ribbon fibers are orderly accumulated along the hot-pressing direction. Microstructural analyses indicate that the C/C composite blocks have a typical structural anisotropy derived from the unidirectional arrangement of the highly oriented wide ribbon-shaped fibers in the composite block. The thermal conductivities of the C/C composites along the longitudinal direction of ribbon fibers increase with heat-treatment temperatures. The longitudinal thermal conductivity and thermal diffusivity at room temperature of the C/C composite blocks graphitized at 3100 °C are 896 W/m K and 642 mm2/s, respectively.  相似文献   

7.
A carbon block with ultra-high anisotropy was produced from a commercial graphite paper as the thermal reinforcement and a thermosetting phenolic resin as the binder. Hot-pressing at a maximum temperature of 200 °C was used to densify and integrate the graphite paper stacks. It has been found that the graphite paper blocks have high thermal conductivities in the paper direction and low ones perpendicular. An anisotropy of 98.8% and a thermal conductivity of 197.8 W m?1 K?1 in the paper direction were achieved when the density was 1.1 g cm?3. The thermal conductivity increased to 284.8 W m?1 K?1 with a decrease of anisotropy to 98.3% with a density of 1.56 g cm?3.  相似文献   

8.
《Ceramics International》2016,42(13):14843-14848
A novel fibrous porous mullite network with a quasi-layered microstructure was produced by a simple vacuum squeeze moulding technique. The effects of organic binder content, inorganic binder and adsorbent on the microstructure and the room-temperature thermal and mechanical properties of fibrous porous mullite ceramics were systematically investigated. An anisotropy microstructure without agglomeration and layering was achieved. The fibrous porous mullite ceramics reported in this study exhibited low density (0.40 g/cm3), low thermal conductivity (~0.095 W/(m K)), and high compressive strength (~2.1 MPa in the x/y direction). This study reports an optimal processing method for the production of fibrous porous ceramics, which have the potential for use as high-temperature thermal insulation material.  相似文献   

9.
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.  相似文献   

10.
The electrical resistivity of CNT yarns of diameters 10–34 μm, spun from multi-walled carbon nanotube arrays, have been determined from 2 to 300 K in magnetic fields up to 9 T. The magnetoresistance is large and negative at low temperatures. The thermal conductivity also has been determined, by parallel thermal conductance, from 5 to 300 K. The room-temperature thermal conductivity of the 10 μm yarn is (60 ± 20) W m?1 K?1, the highest measured result for a CNT yarn to date. The thermal and electrical conductivities both decrease with increasing yarn diameter, which is attributed to structural differences that vary with the yarn diameter.  相似文献   

11.
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.  相似文献   

12.
This work provides a comprehensive investigation on the spectral phonon properties in graphene nanoribbons (GNRs) by the normal mode decomposition (NMD) method, considering the effects of edge chirality, width, and temperature. We find that the edge chirality has no significant effect on the phonon relaxation time but has a large influence to the phonon group velocity. As a result, the thermal conductivity of around 707 W/(m K) in the 4.26 nm-wide zigzag GNR at room temperature is higher than that of 467 W/(m K) in the armchair GNR with the same width. As the width decreases or the temperature increases, the thermal conductivity reduces significantly due to the decreasing relaxation times. Good agreement is achieved between the thermal conductivities predicted from the Green–Kubo method and the NMD method. We find that optical phonons dominate the thermal transport in the GNRs while the relative contribution of acoustic phonons to the thermal conductivity is only 10.1% and 13% in the zigzag GNR and the armchair GNR, respectively. Interestingly, the ZA mode is found to contribute only 1–5% to the total thermal transport in GNRs, being much lower than that of 30–70% in single layer graphene.  相似文献   

13.
A thermally conductive and electrically insulating composite filler was produced by surfactant assisted sol–gel coating of amorphous silica on flake graphite. Amorphous silica-coated graphite (a-Si coated grp) obtained using a cationic surfactant showed the best enhancement of the insulating coating. The resulting a-Si coated grp/boehmite/polybutylene terephthalate polyester resin composite exhibited a high volume resistivity, exceeding 1.0 × 1014 Ω cm at an applied voltage of 500 V, and a thermal conductivity of 3.3 W/m K at 22.9 vol.% a-Si coated grp loading. The heat releasing performance of the developed resin composite in actual light-emitting diodes bulb housings was compared with conventionally used thermally and electrically conductive resin. This comparison revealed that the new composite released heat more effectively. This innovative technology, which may solve the trade-off between material properties and cost, will be available for a broad range of thermally conductive resin applications that simultaneously require thermal conduction and electrical insulation.  相似文献   

14.
An Off-Lattice Monte Carlo model was developed to investigate effective thermal conductivities (Keff) and thermal transport limitations of polymer composites containing carbon nanotubes (CNTs) and inorganic nanoparticles. The simulation results agree with experimental data for poly(ether ether ketone) (PEEK) with inclusions of CNTs and tungsten disulfide (WS2) nanoparticles. The developed model can predict the thermal conductivities of multiphase composite systems more accurately than previous models by taking into account interfacial thermal resistance (Rbd) between the nanofillers and the polymer matrix, and the nanofiller orientation and morphology. The effects of (i) Rbd of CNT–PEEK and WS2–PEEK (0.0232–115.8 × 10−8 m2K/W), (ii) CNT concentration (0.1–0.5 wt%), (iii) CNT morphology (aspect ratio of 50–450, and diameter of 2–8 nm), and (iv) CNT orientation (parallel, random and perpendicular to the heat flux) on Keff of a multi-phase composite are quantified. The simulation results show that Keff of multiphase composites increases when the CNT concentration increases, and when the Rbd of CNT–PEEK and WS2–PEEK interfaces decrease. The thermal conductivity of composites with CNTs parallel to the heat flux can be enhanced ∼2.7 times relative to that of composites with randomly-dispersed CNTs. CNTs with larger aspect ratio and smaller diameter can significantly improve the thermal conductivity of a multiphase polymer composite.  相似文献   

15.
Buckypapers, the thin sheets made from an aggregate of carbon nanotubes (CNTs), have demonstrated promising electrical and thermal conductivities. However, the high in-plane to perpendicular anisotropy makes its application as thermal interface materials difficult. In order to increase the perpendicular electrical and thermal conductivities, copper nanowires (Cu NWs) were introduced into buckypapers. The Cu NWs stuck into the empty spaces between CNTs, connected them perpendicularly, and even induced a certain perpendicular CNT alignment. The electrical conductivity increased continuously with increasing the Cu content, while the smallest anisotropy was observed at the 50 wt.% Cu filling because an in-plane Cu network formed and improved much more the in-plane conductivity above this filling. On the contrary, as CNTs are more thermally conducting than Cu, the loading of Cu NWs over 50 wt.% decreased the thermal conductivity. Our measurement showed a high perpendicular conductivity of 10.1 W/m K at the 50 wt.% loading, more than quadruple and double as compared with the ones for a pure buckypaper and the one filled with 67–75 wt.% Cu NWs.  相似文献   

16.
《Ceramics International》2016,42(12):13491-13496
Two kinds of novel Ln2LaTaO7 (Ln=Er and Yb) ceramics were prepared via high-temperature solid reaction method. The phase composition, micro-morphology and thermophysical properties were investigated. Results indicate that pure Ln2LaTaO7 ceramics with single fluorite-type structure are synthesized successfully. The thermal conductivities of Er2LaTaO7 and Yb2LaTaO7 are in the range of 1.22–1.43 W/m K and 1.17–1.51 W/m K, respectively, which are much lower than that of YSZ. The lower thermal conductivity can be attributed to the phonon scattering caused by oxygen vacancies and the substituting atoms. The average thermal expansion coefficients of Yb2LaTaO7 and Er2LaTaO7 are 9.94×10−6/K and 9.63×10−6/K, respectively. As compared with Yb2LaTaO7, the higher thermal expansion coefficient of Er2LaTaO7 can be ascribed to its lower ionic-bond strength between cations at sites A and B.  相似文献   

17.
Graphite foams were prepared from a coal tar pitch that was partially converted into mesophase. Expandable graphite was used instead of an inert gas to “foam” the pitch. The resulting foam was subjected to a series of heat treatments with the objective of first crosslinking the pitch, and thereafter carbonizing and graphitizing the resulting foam. XRD confirmed that the graphitization at 2600 °C resulted in a highly graphitic material. The porosity of this foam derives from the loose packing of the vermicular exfoliated graphite particles together with their internal porosity. During the foaming process the pitch tends to coat the outside surface of the expanding graphite flakes. It also bonds them together. The graphite foam prepared with 5 wt.% expandable graphite had a bulk density of 0.249 g cm−3, a compressive strength of 0.46 MPa and a thermal conductivity of 21 W m−1 K−1. The specific thermal conductivity (thermal conductivity divided by the bulk density) of this low-density carbon foam was 0.084 W m2 kg−1 K−1 which is considerably higher than that of copper metal (0.045 W m2 kg−1 K−1) traditionally used in thermal management applications.  相似文献   

18.
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.  相似文献   

19.
The electrical and thermal conductivities of bulk barium-added silicon oxycarbide (SiOC-Ba) ceramics are investigated. The SiOC-Ba ceramics exhibited improved electrical and thermal conductivities upon increasing the sintering temperature from 1450 °C to 1650 °C. Precipitation of graphitic carbon clusters observed by Raman spectroscopy and high-resolution transmission electron microscopy is attributed to the phase separation during the fabrication process. The increase in the electrical conductivity can be rationalized in terms of an increase in the density of the sp2 CC bonds within the carbon clusters. The increase in the thermal conductivity is mainly attributed to the formation of interconnected graphitic clusters in the SiOC matrix and SiC embedded in the clusters. The electrical and thermal conductivities of the SiOC-Ba ceramics sintered at 1650 °C are 14.0 Ω?1 cm?1 and 5.6 W/m K, respectively, at room temperature. The electrical conductivity of SiOC-Ba sintered at 1550 °C is 5.3 Ω?1 cm?1 and 7.0 Ω?1 cm?1 at 2 and 300 K, respectively.  相似文献   

20.
Chemically-derived ultralarge graphene oxide (UL-GO) sheets are synthesized from natural graphite (NG) flakes based on the modified Hummers method. Three different approaches are adopted and the effects of ultrasonication, thermal shock expansion, degree of oxidation and precursor NG flake size are specifically studied on the quality and size of GO sheets produced. Results show that the use of large-size NG flakes as precursors does not necessarily produce large GO sheets. Optimal processing conditions are identified to be thermal shock exfoliation with the addition of moderate oxidation, i.e. with an expanded graphite to KMnO4 weight ratio = 1:7 for 24 h, and avoiding ultrasonication during the oxidation process. The resulting UL-GO sheets have a maximum area over 10,000 μm2 with a mean area 3400 μm2 at a yield of 39.8% for GO sheets larger than 2500 μm2, which are considered quite sufficient as precursors for many multifunctional applications, including transparent conductive films, optoelectronic devices and aligned graphene composites.  相似文献   

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