In this study, Mo0.9Cr0.1AlB solid solution ceramic bulks were prepared from the element powder mixtures using hot pressing sintering method. Compared with MoAlB ceramics, the grains of as-prepared Mo0.9Cr0.1AlB were refined obviously. The lattice constants of Mo0.9Cr0.1AlB were confirmed to be a = 3.205 Å, b = 13.999 Å and c = 3.098 Å. The density of Mo0.9Cr0.1AlB was lower than that of MoAlB due to the incorporation of Cr element. In addition, the effect of doping Cr element on the comprehensive mechanical properties was studied as well. The hardness and compressive strength were improved significantly. In comparison with MoAlB ceramic, the improvement of mechanical properties could be attributed to solid solution strengthening and grain refinement. 相似文献
For the first time, dense continuous carbon fiber (Cf) reinforced (TiZrHfNbTa)C high-entropy ceramic (Cf/HEC) composites were rapidly prepared via in-situ reactive melt infiltration (RMI). A TiZrHfNbTa high-entropy alloy served as the cation source and carbon in Cf reinforced carbon matrix (Cf/C) preforms served as the anion source, and a (TiZrHfNbTa)C high-entropy ceramic phase with a near equimolar ratio was successfully formed. The results revealed that most of the TiZrHfNbTa high-entropy alloy reacted with the carbon matrix, and the harvested Cf/HEC composites exhibited an excellent bending strength (612.6 MPa) and low ablation rates. High reaction rates caused by ultra-high temperature and homogeneous distribution of elements in the high-entropy TiZrHfNbTa alloy significantly reduced the difference in reactivity with C among Ti, Zr, Hf, Nb, and Ta are considered to be the reasons for successful formation of (TiZrHfNbTa)C high-entropy ceramic with a near equimolar ratio in Cf/HEC composites. 相似文献
A novel combined molten carbonate fuel cell – steam turbine based system is proposed herein. In this cycle, steam is produced through the recovery of useful heat of an internal reforming MCFC and operates as work fluid in a Rankine cycle. Exergoeconomic analysis was performed, in order to verify the technical feasibility, including which components could be improved for greater efficiencies, as well as the cost of the power generated by the plant. A 10 MW MCFC was initially proposed, when the system reached 54.1% of thermal efficiency, 8.3% higher than MCFC alone, 11.9 MW of net power, 19% higher than MCFC alone, and an energy cost of 0.352 $/kWh. A sensitivity analysis was carried out and the parameters that most influenced on the cost were pointed out. The analysis pointed to the MCFC generation as the most impactful factor. By manipulating these values, it could be noted a significant power cost decrease, reaching satisfactory values to become economically feasible. The concept of economy of scale could be noticed in the proposed system, proving that a large-scale plant could be the focus of investment and public policies. 相似文献
Diamond possesses a unique combination of excellent optical, thermal, and mechanical properties, and is therefore an ideal transparent ceramic material for harsh and extreme environments. Due to its important applications in technology, transparent diamond ceramic (TDC) has been explored and prepared by chemical vapor deposition (CVD) or direct conversions of non-diamond carbon precursors at high pressure and high temperature (HPHT), but the preparation of large-size TDC with high mechanical strength remains a challenge. Here, we report for the first time, a transparent polycrystalline diamond ceramic from diamond powder with a transmittance of ~60 % at wavelengths of 400–1600 nm. The analyses of phase composition, residual stress and microstructure evolution of the sintered samples with different sintering conditions indicate that compression at high temperatures (>2000 ?C) facilitates the deformation of diamond grains, allowing for densification and diamond-diamond bonding formation. The sintering pressure of the diamond powders with an optimized particle size distribution was dramatically reduced from 16 GPa to 10 GPa. Our results, based on successfully preparing centimeter-sized TDC, set the standard and the precedent for the large-scale preparation of larger TDC in proximity to industrial conditions. 相似文献
The electrical properties of C/SiC composites could be used for online and in-situ damage monitoring. To investigate alternating current (AC) impedance response to damage in the C/SiC composites, monotonic and incremental cyclic tensile tests were performed. Both AC impedance and acoustic emission (AE) techniques were applied to clarify the damage evolution during the tests. The relationship between damage and electrical impedance response was investigated and validated via macroscopic equivalent circuit models. The effects of longitudinal deformation and damage on AC impedance characteristics, including impedance magnitude and phase angle, were obtained from the models. Results showed that the longitudinal deformation increases the impedance magnitude and the phase angle, and the damage causes the impedance magnitude to increase and the phase angle to decrease. The phase angle is significantly sensitive to fiber breakage, which makes the AC-based method more suitable for online damage monitoring and final failure warning. 相似文献
Carbon/Carbon (C/C) composites are expected to serve as structural materials over 2800 ℃. Experiments under ultra-high temperatures (UHT) are critical and demanding. In this paper, we established the UHT compressive experiment technique using simultaneous Joule heating and compressive loading fixtures. The specimen was designed and validated to achieve uniform temperature and strain at the gauge section. Compressive strengths and failure behaviors of three-directional (3D) needled, 3D woven, and four-directional (4D) woven C/C composites under UHT up to 3100 ℃ were investigated. The failure modes and mechanism of strength differences were illustrated through mesoscopic surface morphologies. Results showed that the dog-bone-shaped specimen avoided crushing at loading ends and exhibited failure at gauge sections. Temperatures with peak compressive strengths for 3D and 4D woven C/C composites were determined. Differences between the C/C composites were related to heat treatment temperatures. The sublimation phenomenon was observed for 4D woven C/C composites over 3000 ℃, degrading the compressive strengths by over 50%. 相似文献
In order to overcome the roughness of the previously proposed micromechanical model [Acta Mech. Sin. (2011) 382], an enhanced multiscale analytical model was thus developed based on the rule of mixture, shear-lag theory and statistical approach to forecast the load carrying capacity of the prestressed ceramic matrix composites (CMCs) subjected to high-temperature oxidation. For comprehensive characterization of the mechanical degradation mechanisms, the oxidation induced fiber necking (or embrittlement) and fiber-matrix interface weakening were both taken into account. The suggested model was then applied to 2D-C/SiC composites. The influences of interface friction resistance, interface recession length, fiber necking factor and oxidation duration upon the residual mechanical property were investigated. Parametric analysis demonstrates that the modified formulations are much more reasonable than the previous model. The predicted residual tensile modulus and strength for the 2D-C/SiC composite agree well with the experimental data and furthermore the microscopic damage mechanisms were correlated properly with the macroscopic fracture morphologies. 相似文献
Recently, a new i-MAX phase with in-plane chemical order has been discovered. In this study, the i-MAX phase (Mo2/3Sc1/3)2AlC and (Mo2/3Lu1/3)2AlC ceramics were synthesized by spark plasma sintering (SPS) with purity up to 96.45 wt% and 95.46 wt%, respectively. Relative densities were up to 98.29 % (M = Sc) and 98.23 % (M = Lu). Microstructure, physical and mechanical properties of (Mo2/3M1/3)2AlC (M = Sc and Lu) ceramics were systematically investigated. Grain sizes of (Mo2/3M1/3)2AlC (M = Sc and Lu) were also counted. The measured coefficients of thermal expansion (CTE) were 8.95 × 10−6 K−1 (M = Sc) and 8.97 × 10−6 K−1 (M = Lu), from 25 °C to 1200 °C. Regarding mechanical properties, the Vickers hardness of (Mo2/3Lu1/3)2AlC was much higher than that of (Mo2/3Sc1/3)2AlC, while the differences in flexural strength, fracture toughness and compressive strength were small. It indicates that both ceramics have good application prospects. 相似文献