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1.
E. Zapata-Solvas D.D. Jayaseelan P.M. Brown W.E. Lee 《Journal of the European Ceramic Society》2013,33(15-16):3467-3472
Thermal properties of La2O3-doped ZrB2- and HfB2-based ultra high temperature ceramics (UHTCs) have been measured at temperatures from room temperature to 2000 °C and compared with SiC-doped ZrB2- and HfB2-based UHTCs and monolithic ZrB2 and HfB2. Thermal conductivities of La2O3-doped UHTCs remain constant around 55–60 W/mK from 1500 °C to 1900 °C while SiC-doped UHTCs showed a trend to decreasing values over this range. 相似文献
2.
First‐principles investigations on elevated temperature elastic and thermodynamic properties of ZrB2 and HfB2
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Huimin Xiang Zhihai Feng Zhongping Li Yanchun Zhou 《Journal of the American Ceramic Society》2017,100(8):3662-3672
As promising candidates for ultrahigh temperature applications, high‐temperature properties, which are quite rare and fragmentary, have great significance to ZrB2 and HfB2. In this work, thermodynamic and mechanical properties of ZrB2 and HfB2 from 0 K to 2000 K were investigated by a combination of first principles calculations and quasi‐harmonic approximations. The ground‐state properties, including lattice parameters, elastic constants, phonon dispersion, and mode‐Grüneisen parameters are calculated. The theoretical thermal expansion, elastic and thermodynamic properties at elevated temperatures show good agreement with experiments. By discussing Grüneisen parameters anisotropy, the mechanism for the thermal expansion anisotropy of ZrB2 and HfB2 is uncovered. The influence of direction‐dependent sound velocities on the anisotropy of thermal conductivity is also discussed. 相似文献
3.
E. Zapata-Solvas D.D. Jayaseelan H.T. Lin P. Brown W.E. Lee 《Journal of the European Ceramic Society》2013,33(7):1373-1386
Flexural strengths at room temperature, at 1400 °C in air and at room temperature after 1 h oxidation at 1400 °C were determined for ZrB2- and HfB2-based ultra-high temperature ceramics (UHTCs). Defects caused by electrical discharge machining (EDM) lowered measured strengths significantly and were used to calculate fracture toughness via a fracture mechanics approach. ZrB2 with 20 vol.% SiC had room temperature strength of 700 ± 90 MPa, fracture toughness of 6.4 ± 0.6 MPa, Vickers hardness at 9.8 N load of 21.1 ± 0.6 GPa, 1400 °C strength of 400 ± 30 MPa and room temperature strength after 1 h oxidation at 1400 °C of 678 ± 15 MPa with an oxide layer thickness of 45 ± 5 μm. HfB2 with 20 vol.% SiC showed room temperature strength of 620 ± 50 MPa, fracture toughness of 5.0 ± 0.4 MPa, Vickers hardness at 9.8 N load of 27.0 ± 0.6 GPa, 1400 °C strength of 590 ± 150 MPa and room temperature strength after 1 h oxidation at 1400 °C of 660 ± 25 MPa with an oxide layer thickness of 12 ± 1 μm. 2 wt.% La2O3 addition to UHTCs slightly reduced mechanical performance while increasing tolerance to property degradation after oxidation and effectively aided internal stress relaxation during spark plasma sintering (SPS) cooling, as quantified by X-ray diffraction (XRD). Slow crack growth was suggested as the failure mechanism at high temperatures as a consequence of sharp cracks formation during oxidation. 相似文献
4.
The effect of oxidation on room temperature (RT) flexure strength degradation in SiC-reinforced ultra-high temperature ceramics (UHTCs) and La2O3-doped UHTCs has been characterised in the temperature range 1400–1600°C for oxidation times of up to 32?h. Flaw healing was identified for oxide scale thicknesses 50 μm, although second phase agglomerates limit the flaw healing effect to scale thicknesses of 20 μm. A linear degradation of RT strength with oxide scale thickness was observed for oxide scale thickness >50 μm. Two oxide scale configurations have been proposed to minimise RT strength degradation. The most promising is the scale with a porous layer containing non-interconnected porosity (85–90% dense) of either MeLa2O7 or MeOxCy (Me?=?Zr or Hf). 相似文献
5.
Chong Li Yaran Niu Tao Liu Xin Zhong Xiaohui Pan Yi Zeng Xuebin Zheng Chuanxian Ding 《Ceramics International》2019,45(6):6648-6654
Bubble phenomenon is common for ultra-high temperature ceramics (UHTCs) during oxidation or ablation processes, which will impair the oxidation/ablation resistant properties. This work is aiming to illuminate the formation and rupture processes of bubbles. In this work, ZrB2-SiC-WB composite coatings were prepared via vacuum plasma spray technique and oxidized at 1500?°C for different durations. Obvious bubble phenomenon was observed. The morphology and distribution of bubbles were characterized. The formation mechanism of bubbles was calculated and analyzed based on thermal dynamics. The results showed that B2O3 gas played a key role in affecting the bubble behaviors. Bubbles tended to nucleate near the interface between the solid and liquid oxide layers. Small bubbles aggregated to large bubbles near the outmost liquid layer. Large bubbles near the surface were easy to rupture. The calculated results were consistent with the observed results. 相似文献
6.
《Ceramics International》2017,43(2):2170-2173
HfB2-x vol%CNTs (x=0, 5, 10, and 15) composites are prepared by spark plasma sintering. The influence of CNTs content and sintering temperature on densification, microstructure and mechanical properties is studied. Compared with pure HfB2 ceramic, the sinterability of HfB2-CNTs composites is remarkably improved by the addition of CNTs. Appropriate addition of CNTs (10 vol%) and sintering temperature (1800 °C) can achieve the highest mechanical properties: the hardness, flexural strength and fracture toughness are measured to be 21.8±0.5 GPa, 894±60 MPa, and 7.8±0.2 MPa m1/2, respectively. This is contributed to the optimal combination of the relative density, grain size and the dispersion of CNTs. The crack deflection, CNTs debonding and pull-out are observed and supposed to exhaust more fracture energy during the fracture process. 相似文献
7.
Work has been done to develop and optimize a procedure for the preparation of electrically conductive exfoliated-graphite filled unsaturated polyester composites. The exfoliated graphite flakes impart excellent electrical properties to the polyester composite. EMI (electromagnetic interference) shielding effectiveness, comparable to that of exfoliated graphite alone, has been observed over a frequency range of 0 to 1000 MHz with composite samples containing as little as 4 weight percent of exfoliated graphite. The resistivities approached the value for exfoliated graphite in the c-direction (0.03 μ-cm) at a graphite loading of 11 weight percent. It has been observed that the mechanical integrity of the composite remained good up to graphite loadings of some 17 weight percent. The polyester readily wets the graphite and penetrates most of the graphite open pore volume during processing. Flow visualization measurements have been made in a transparent compression molding apparatus. No effect of the flow conditions on the apparent electrical properties has been found. 相似文献
8.
Tao Zhu Lin Xu Xinghong Zhang Wenbo Han Ping Hu Ling Weng 《Journal of the European Ceramic Society》2009,29(13):2893-2901
ZrB2–SiCw composites were prepared through hot-pressing at a low temperature of 1800 °C, and Al2O3 plus Y2O3 were added as sintering aids. Analysis revealed that additives may react with impurities (i.e. surface oxygen impurities and residual metallic impurities) to form a transient liquid phase, thus promote the sintering and densification of ZrB2–SiCw composites. The content of additives was found to have a significant influence on the sinterability, microstructure and mechanical properties of ZrB2–SiCw composites. ZrB2–SiCw composite prepared with a small amount of additives (3 vol.%) provided the optimal combination of microstructure (relative density of 98.3%) and excellent properties, including flexural strength of 783 MPa and fracture toughness of 6.7 MPa m1/2. With further addition of additives, SiC whiskers were inclined to gather together and be enveloped by excessive liquids to form core-rim-like structures, which lead to little decrease in mechanical properties. 相似文献
9.
Electrical and thermophysical properties of mechanochemically obtained lanthanide hafnates
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Francisco A. López‐Cota Nayeli M. Cepeda‐Sánchez José A. Díaz‐Guillén Oscar J. Dura Marco A. López de la Torre Miroslaw Maczka Maciej Ptak Antonio F. Fuentes 《Journal of the American Ceramic Society》2017,100(5):1994-2004
This contribution presents the synthesis and thermophysical characterization of seven lanthanide hafnates Ln2Hf2O7 (Ln=Sm3+, Eu3+, Gd3+, Dy3+, Y3+, Ho3+, Yb3+); the title samples were prepared at room temperature by mechanically milling stoichiometric mixtures of the corresponding elemental oxides. Irrespective of the lanthanide ion involved, milling promotes the formation of highly disordered fluoritelike materials. Postmilling thermal treatments facilitate the formation of the fluorite ordered derivative, the pyrochlore structure, but only for the larger lanthanides (Sm3+, Eu3+, Gd3+). Impedance spectroscopy measurements revealed that these materials show a moderate‐to‐good oxygen ion conductivity at high temperatures; furthermore, those adopting the pyrochlore structure give higher σdc and lower Edc than their fluorite analogues (σdc at 750°C>10?3 S·cm?1 vs <5·10?4 S·cm?1, respectively). The same trend also holds for the thermal resistivity at high temperatures; the highest thermal resistivity and thus, lowest κ was obtained for Eu2Hf2O7 (κ~1.3·W·m?1·K?1 at 800°C). Therefore, Ln2Hf2O7 phases might be attractive component materials for electrochemical devices and thermal insulating coatings. 相似文献
10.
Mesophase pitch based graphite foams with different thermal properties and cell structures were infiltrated with glass by pressureless infiltration to prepare potential alternative composites for cooling electronics. Microstructure, thermal diffusivity and coefficient of thermal expansion (CTE) of the obtained composites were investigated. It was demonstrated that there was excellent wettability of the graphite foam by molten glass, and the foam framework was retained well after infiltration, which could facilitate good heat transfer throughout the composites. The highest thermal diffusivity of the composites reached 202.80 mm2/s with a density of 3.81 g/cm3. And its CTE value was 4.53 ppm/K, much lower than the corresponding calculated result (7.46 ppm/K) based on a simple “rule of mixtures” without considering the space limitations of the graphite foams. Thus, the mechanical interlocking within the space limitations of the graphite network played a crucial role in limiting the thermal expansion of the glass. The CTEs of the graphite foam/glass composites varied from 4.53 to 7.40 ppm/K depending on the graphite foam density which varied from 0.82 to 0.48 g/cm3. The CTEs were a good match to those of semiconductor chips and packaging materials. 相似文献
11.
To improve the fracture properties of ZrB2-based composites, SiC nanowires (SiCnw) were introduced into the powder mixtures via an in-situ growth method and the composites were fabricated by hot pressing. Microstructure observations found that the SiCnw with a diameter of less than 100?nm presented twisted morphology and homogeneously distributed among ceramic particles. It was also found that the SiCnw could inhibit the grain growth of ZrB2-based composites. Compared to that of composites without SiCnw, the fracture toughness and the work of fracture for the composites with SiCnw were 7.17?MPa?m1/2 and 205?J?m?2, which increased by 61.1% and 91.6%, respectively. The main enhancing mechanisms could be associated with the obvious crack deflection, crack branching and pull out of SiCnw. The SiCnw could refrain the crack opening and reduce the stress intensity in front of crack tips, which absorbed more energy and improved the fracture properties. 相似文献
12.
Zhihui Lü Dongliang Jiang Jingxian Zhang Qingling Lin Zhengren Huang 《Journal of the European Ceramic Society》2012,32(7):1435-1439
The oxidation behavior for ZrB2–20 vol% SiC (ZS20) and ZrB2–30 vol% SiC (ZS30) ceramics at 1500 °C was evaluated by weight gain measurements and cross-sectional microstructure analysis. Based on the oxidation results, laminated ZrB2–30 vol% SiC (ZS30)/ZrB2–25 vol% SiC (ZS25)/ZrB2–30 vol% SiC (ZS30) symmetric structure with ZS30 as the outer layer were prepared. The influence of thermal residual stress and the layer thickness ratio of outer and inner layer on the mechanical properties of ZS30/ZS25/ZS30 composites were studied. It was found that higher surface compressive stress resulted in higher flexural strength. The fracture toughness of ZS30/ZS25/ZS30 laminates was found to reach to 10.73 MPa m1/2 at the layer thickness ratio of 0.5, which was almost 2 times that of ZS30 monolithic ceramics. 相似文献
13.
HfB2 and HfB2–10 vol% HfC fine powders were synthesized by carbo/boronthermal reduction of HfO2, which showed high sinterability. Using the as-synthesized powders and commercially available SiC as starting powders, nearly full dense HfB2–20 vol% SiC (HS) and HfB2–8 vol% HfC–20 vol% SiC (HHS) ceramics were obtained by hot pressing at 2000 °C/30 MPa. With the incorporation of HfC, the grain size of HHS was much finer than HS. As well, the fracture toughness and bending strength of HHS (5.09 MPa m1/2, 863 MPa) increased significantly compared with HS (3.95 MPa m1/2, 654 MPa). Therefore, it could be concluded that the incorporation of HfC refined the microstructure and improved the mechanical properties of HfB2–SiC ceramics. 相似文献
14.
Shu-Qi Guo 《Journal of the European Ceramic Society》2009,29(6):995-1011
This study reviews densification behaviour, mechanical properties, thermal, and electrical conductivities of the ZrB2 ceramics and ZrB2-based composites. Hot-pressing is the most commonly used densification method for the ZrB2-based ceramics in historic studies. Recently, pressureless sintering, reactive hot pressing, and spark plasma sintering are being developed. Compositions with added carbides and disilicides displayed significant improvement of densification and made pressureless sintering possible at ≤2000 °C. Reactive hot-pressing allows in situ synthesizing and densifying of ZrB2-based composites. Spark plasma sintering displays a potential and attractive way to densify the ZrB2 ceramics and ZrB2-based composites without any additive. Young's modulus can be described by a mixture rule and it decreased with porosity. Fracture toughness displayed in the ZrB2-based composites is in the range of 2–6 MPa m1/2. Fine-grained ZrB2 ceramics had strengths of a few hundred MPa, which increased with the additions of SiC and MoSi2. The small second phase size and uniform distribution led to higher strengths. The addition of nano-sized SiC particles imparts a better oxidation resistance and improves the strength of post-oxidized ZrB2-based ceramics. In addition, the ZrB2-based composites showed high thermal and electrical conductivities, which decreased with temperature. These conductivities are sensitive to composition, microstructure and intergranular phase. The unique combinations of mechanical and physical properties make the ZrB2-based composites attractive candidates for high-temperature thermomechanical structural applications. 相似文献
15.
《Ceramics International》2017,43(5):4372-4378
A simple method for introducing ZrB2 using sol-gel processing into a SiBCN matrix is presented in this paper. Zirconium n-propoxide (ZNP), boric acid and furfuryl alcohol (C5H6O2) (FA) were added as the precursors of zirconia, boron oxide and carbon forming ZrB2 dispersed in a SiBCN matrix. SiBCN/ZrB2 composites with different contents of ZrB2 (5, 10, 15, and 20 wt%) were formed at 2000 °C for 5 min by spark plasma sintering (SPS). The microstructures were carefully studied. TEM analysis showed that the as formed ZrB2 grains were typically 100–500 nm in size and had uniform distribution. HRTEM revealed clean grain boundaries between ZrB2 and SiC, however, a separation of C near the SiC boundary was observed. The flexural strength, fracture toughness, Young's modulus and Vicker's hardness of composites all improved with the ZrB2 contents and SiBCN matrix containing 20 wt% of ZrB2 could reach 351±18 MPa, 4.5±0.2 MPa m1/2, 172±8 GPa and 7.2±0.2 GPa, respectively. The improvement in fracture toughness can be attributed to the tortuous crack paths due to the presence of reinforcing particles. 相似文献
16.
Composite materials based on undoped conjugated polymers and conductive filler were synthesized and their electrophysical and electrochemical properties were investigated. Conjugated polymers such as polyaniline (PANI), polyacetylene (PA) and typical polymer dielectric, polypropylene (PP) were used as matrices. Graphite and single-walled nanotubes were used as conductive fillers. The investigation of the dependencies of conductivity on filler concentration show that in contrast to PP and PANI, PA becomes conductive due to injected charge carriers from filler particles. This conclusion is supported by investigated dependencies of composite conductivity on temperature and time during aging and on cyclic voltammetry data. It was shown that a conjugated polymer matrix allows composite materials with new electrophysical and electrochemical properties to be produced. 相似文献
17.
R. Licheri R. Orrù C. Musa A. M. Locci G. Cao 《International Journal of Self-Propagating High-Temperature Synthesis》2009,18(1):15-24
The combination of the SHS technique and the Spark Plasma Sintering (SPS) technology was adopted in this work for the fabrication
of fully dense MB2-SiC and MB2-MC-SiC (M = Zr, Hf) Ultra High Temperature Ceramics (UHTCs). Specifically, Zr or Hf, B4C, Si, and (for the cases of ternary systems) graphite powders were first reacted by SHS to successfully form the desired
composites. The resulting powders were then subjected to consolidation by SPS. In particular, by setting a dwell temperature
level of 1800°C, a mechanical pressure of 20 MPa, and a non-isothermal heating time of 10 min, products with relative densities
≥98.5% were obtained for the all systems investigated within 30 min of total processing time. The characteristics of the resulting
dense UHTCs, i.e. hardness, fracture toughness, and oxidation resistance, are similar to, and in some cases superior than,
those related to analogous products synthesized by alternative, less rapid, methods.
The article is published in the original. 相似文献
18.
《应用陶瓷进展》2013,112(5):308-312
ZrB2 based composites containing 10 vol.-% carbon nanotubes (CNTs) are synthesised by spark plasma sintering at temperatures ranging from 1600 to 18008C and at an applied pressure of 25?MPa. The effects of sintering temperature on densification behaviour, microstructural evolutions and mechanical properties are presented. Results indicate that ZrB2-CNT composites fabricated at 16508C have the optimal combination of dense microstructure and properties. The fracture toughness is sensitive to the temperature change and reaches 7.2?MPa m1/2 for the CNT toughened ZrB2 ceramics, which is higher than the measured result for monolithic ZrB2 (3.3?MPa m1/2). The crack deflection and CNT pullout are the dominant toughening mechanisms. 相似文献
19.
Conducting composites consisting of polybithiophene and based on porous crosslinked polystyrene as host polymer have been synthetized in the vicinity and above the percolation threshold by oxidative polymerization with FeCl3. Electrical conductivity and thermoelectric power measurements for different degrees of doping have been carried out in the temperature range 80–300 K. The electrical conductivity variations are weakly thermally activated while the thermoelectric power has metallic magnitude with positive sign and increases with temperature. Conduction mechanisms are interpreted on the basis of an hopping model involving bipolaronic clusters. © 1996 John Wiley & Sons, Inc. 相似文献