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Carbon fiber-containing refractory high-entropy ceramic matrix composites (C/RHECs) were fabricated through a reaction with carbon powders, transition metal carbides, and Zr–Ti alloys as a novel heat resistant material used for components of hypersonic vehicles cruising at Mach 7–10. With the infiltration of alloys at 1750 °C into a composite preform containing carbon and carbide powders for 15 min, a high-entropy matrix was successfully formed in situ. Arc-jet tests were conducted in the temperature range of 1800–1900 °C. Results showed the formation of an oxidized region composed of complex oxides, such as (Zr, Hf)O2, (Nb, Ta)2(Zr, Hf)6O17, (Zr, Hf)TiO4, and Ti(Nb, Ta)2O7, with an average thickness of ~600 μm, under which an unoxidized region remained. The porous oxidized region resulted from the evolution of CO(g) during oxidation, while a dense oxide region formed as the outermost region. This indicates that the dense oxide region acted as a barrier to oxygen diffusion for the unoxidized region during oxidation.

Herein, we prepared 3D-networked porous carbon materials (TNPCs) (porosity ≈ 70%, average pore size ≈ 10 μm, density ≈ 0.37 g cm–3) composed of glassy carbon via pyrolysis of porous phenolic precursors. A compressive test for TNPCs shows that the Young’s moduli and compressive strength of TNPCs ranged from 0.7 to 1.5 GPa and 11 to 30 MPa, respectively, which increased with increasing pore diameter. Although these moduli could not be accurately predicted using conventional periodic unit cell models, the homogenized Young’s moduli predicted using a 3D image-based model acquired by X-ray computed tomography and focused ion beam scanning electron microscopy were in good agreement with the experimental values. These results indicate that the method can be used to reliably investigate the microstructures and evaluate the Young’s modulus using 3D image-based modeling.

The thermal protection performances of polymethylmethacrylate-impregnated porous carbon ablators with three different densities were examined in an arc wind-tunnel test. The more impregnated ablato...

The effect of stress concentration on the compressive failure behavior of a carbon fiber composite laminate with an open hole is experimentally and numerically studied. The aspect ratio of the elli...

Recent studies on carbon fiber-reinforced ultra-high temperature ceramic matrix (C/UHTC) composites fabricated by hot-pressing, chemical vapor infiltration, polymer impregnation and pyrolysis, and melt infiltration (MI) are reviewed. Various efforts have been made to improve these preparation processes and to combine two or more of these because they have both the advantages and disadvantages in terms of the processing time, operating temperature, and the porosity of the resulting C/UHTC composites. In addition, the parameters governing the fracture toughness, thermal conductivity, and recession behavior (in oxidizing environments) of these composites have been discussed. This review demonstrates that C/UHTC composites with Zr- or Hf-based UHTC matrices fabricated via MI are potential candidates for advanced heat-resistant structural materials.

In the present study, strain-rate and temperature dependence of the transverse tensile failure mode of unidirectional heat-resistant carbon fiber-reinforced plastics is numerically simulated by finite element analyses. In the analyses, interface failure and matrix failure are represented by cohesive zone modeling and continuum damage mechanics, respectively. For the continuum damage mechanics, Christensen's failure criteria of multi-axial stress states for each strain rate are applied to the matrix properties. Interfacial properties which are obtained by microbond test are introduced into cohesive zone modeling. A time-temperature superposition principle approach is applied in order to translate the difference in temperature as the difference in strain rate. The damage initiation depends on strain rate and temperature, while the cohesive zone modeling is assumed to be temperature- and time-independent. The initial damage starting points and the failure mode are predicted in numerical analysis. The transverse tensile strengths in analysis results are compared with the three-point bending testing results.

Si-rich ytterbium silicide was fabricated by through an arc-melting technique for applying use as a bond coat material in an environmental barrier coating system. Evaluation of its potential was accomplished through oxidation tests in dry air and an inert atmosphere. The experimental results showed that the changes in weight and morphologies of ytterbium silicide observed after the tests depended on the oxygen partial pressure. Extensive oxidation and weight gain occurred after oxidation in air. In order to apply this material for hot structures, improvement of the oxidation resistance is needed.

Inter- and intragranular SiC-nanoparticle/Mg2Si composites were fabricated by plasma activated sintering, and the fracture toughness and thermoelectric properties were characterized. The toughness of the intragranular composite was 60% higher than that of pure Mg2Si. The electrical conductivity became lower compared with pure Mg2Si, however, it was independent of SiC content up to 5 vol%. Incorporation of a large amount of SiC nanoparticles within the Mg2Si grains is therefore an effective way to simultaneously improve toughness and thermoelectric properties.

Interlaminar debonding (delamination) is a major problem for carbon fiber-reinforced plastic (CFRP) when it is used in thermal protection systems subjected to rapid heating, such as in a re-entry situation. In this study, the possibility of avoiding delamination by controlling material porosity was investigated experimentally and numerically. CFRP specimens having various initial porosities (mid-density CFRP) were prepared and rapid heating tests (∼6 MW/m2) were performed, and the specimens were examined to determine whether delamination had occurred. To predict the delamination, heat transfer and pyrolysis gas-induced internal gas pressure growth were simulated numerically as a function of heating time. The parameters regarding temperature-dependent chemical reaction rate, orthotropic coefficients for gas permeability, and flatwise tensile strength were determined by individual experiments using the CFRP specimens. The coupled analysis of thermal transfer and pyrolysis gas flow following Darcy’s law was conducted by finite-difference form, employing the experimentally obtained parameters. When the gas pressure exceeds the flatwise tensile strength of CFRP, delamination is assumed to occur. The numerical results show good agreement with the experimental results, and it is concluded that delamination can be eliminated by using mid-density CFRP.

To further enhance the thermoelectric (TE) capability of Mg- and Mn-silicides in order to realize a practical TE power generator (TEG), sufficiently low resistance ohmic contacts at the interface between the electrode material and the Mg- and Mn-silicides are needed. In this chapter, we report a method to obtain thermally stable electrodes on Mg- and Mn-silicides. We also have examined the interface profile, the thermal diffusion, and alloy formation and discussed these observations in terms of the behavior of the electrical resistance. TE devices need to be as long lasting as possible during operation over a mid-temperature range of 600–900 K. This requires an improvement in the mechanical properties. By incorporating SiC nanoparticles, we have substantially improved these, especially the fracture toughness of the material. The process of introducing the nanoparticles into the silicide matrix and the amount introduced have been optimized to minimize any deterioration in the TE properties. Using first-principles calculations, we can predict the physical properties of silicide materials, such as their electric and thermoelectric properties, based on the calculation of their electronic states. The calculations are now feasible even for systems with complicated microscopic structures containing impurities, defects, and interfaces.

Accepted: 2018-05-30, 資料番号: SA1180328000

A porous carbon material with a three-dimensional network and open-cell structure was applied to preform new lightweight ablators. The materials impregnated into the porous carbon were polyimide, p...

The oxidation behavior and oxidation mechanisms of monolithic ZrB2 and particulate-ZrB2 matrix composites were reviewed. Dispersion of SiC particles into ZrB2 was found to be an effective way to prevent extensive oxidation. However, the formation of a SiC-depleted layer can become a critical problem because it can lead to spallation and delamination of the protective surface layer. The addition of ZrC in conjunction with rapid heating to temperatures higher than 2000 °C effectively reduced the porosity of the SiC-depleted layer. The formation of a dense surface layer was attributed to large volumetric expansion during the conversion from ZrC to ZrO2. The effect of the ZrC addition depended on the temperature, heating rate, and composition. This review showed that material design for specific applications is required for high-temperature applications to maximize the oxidation resistance of ZSZ composites.

Accepted: 2018-02-03, 資料番号: SA1170294000

Accepted: 2017-11-10, 資料番号: SA1170255000

Accepted: 2017-10-05, 資料番号: SA1170186000

Accepted: 2016-10-31, 資料番号: SA1160360000

The interfacial strength between carbon fiber and phenolic resin is studied using molecular dynamics simulations to demonstrate that carbon fiber-reinforced carbon matrix composites (C/C composites) have improved tensile strength. Simulations are performed using two carbon fiber models, one of which has only carbon atoms and the other has carbon atoms and some fluorinated carbon groups. The carbon fiber models are regarded as two-layer graphite, and the phenolic resin model is treated as cross-linked structures. All force field parameters are based on the Dreiding force field. The tensile stress and interfacial fracture energy are calculated for the estimation of the interfacial strength. The results show that the model including the fluorinated carbon groups has lower interfacial strength than the model having only carbon atoms, up to a certain coating ratio of fluorinated carbon groups. Similarly, within the limits of the coating ratio, the interfacial fracture energy of the fluorinated carbon fiber mod...

Ytterbium (Yb)- and silicon (Si)-rich ytterbium silicides (Yb5Si3, and Yb3Si5) were successfully fabricated by arc-melting and heat treatment. The degradation behavior of both silicides was evaluated by means of oxidation tests under three different conditions: Air, air-H2O, and steam. The results showed that the degradation of these Yb-silicides in air was accelerated relative to that in air–H2O and in steam. Although Yb3Si5 formed an un-oxidized region in air–H2O and in steam, a complex phase transformation occurred as a result of the decrease in the amount of Yb in Yb3Si5, caused by exposure to heat, while the liquid phase evolved below the melting point of Si. The liquid phases did not form upon exposure of Yb5Si3 to heat, whereas Yb5Si3 was more oxidized and the amount of SiO2 formed during heat exposure in air-H2O and steam is higher than Yb3Si5.

A method for connecting a driving shaft to a three-dimensional composite disk via a polymer ring was examined to achieve stable rotation at high rotation speeds. Polyoxymethylene (POM) was adopted as the polymer ring material. Compression test was conducted to determine the limit of linear viscoelasticity, and creep resistance of POM was examined to evaluate its long-term durability. Structural design using finite element analysis including long-term creep resistance has shown that this method was predicted to be effective up to a tip speed of 1210 m/s. A three-dimensional composite disk was reinforced to three directions in accordance with the cylindrical coordinates. A trial composite disk with outer and inner diameters of 304 and 41 mm, respectively, was manufactured and tested up to a tip speed of 908 m/s. Vibration amplitudes were measured using gap and laser sensors. The vibration phenomenon and methods of suppressing vibration were discussed for higher rotation speeds.

In Japan, scallop shells are considered to be industrial waste. Thus far, attempts for reusing these shells have been mainly limited to the commercial production of CaCO3. Nevertheless, there are no clear economic benefits associated with the use of scallop shells as a source of CaCO3. Hence, we are attempting to investigate a new value-added use for scallop shells as an advanced functional material. In this regard, we focused our attention on nuclear wastewater, which contains radioactive Sr and Cs. Sr, which tends to accumulate in bones, is believed to cause bone cancer. Hence, it is highly desirable to develop a method for removing Sr from contaminated water. In this study, we investigate whether scallop shells demonstrate the ability to remove Sr from a solution. From the results obtained, scallop shells can remove Sr solutes from a solution; furthermore, as compared to CaCO3, they demonstrate superior ability for removing Sr.

A rotating disk often undergoes severe vibration at high rotation speeds because of unstable joining between the disk and a drive shaft. As described herein, three connection methods between a driving shaft and an annular rotation disk made of three-dimensionally carbon-fiber reinforced composite were discussed to achieve stable rotation at high rotation speeds by changing the hub material and joining geometry: the connecting device. In two of the three methods, the vibration amplitude increased at a tip speed higher than 500 m/s. Key factors that caused the vibration were analyzed. Results show that the third method by which a resin ring (hub) was inserted between the disk and rotation shaft maintained stable joining, even at high-speed rotation.

The mechanical properties (the elastic moduli, hardness, and fracture toughness) were investigated for polycrystalline Ba8Al15Si31 clathrate as one of useful thermoelectric materials on a phonon glass and electron crystal concept. Samples were prepared by combining arc melting and spark plasma sintering techniques. The Young’s modulus E = 96.88 GPa, shear modulus G = 38.61 GPa, bulk modulus K = 65.77 GPa, and Poisson’s ratio ν = 0.2545 were calculated from longitudinal sound velocity v L = 6105 m/s and transverse sound velocity v T = 3503 m/s, measured by an ultrasonic test. The elastic constants c 11 = 78.64 GPa, c 12 = 40.03 GPa, and c 44 = 38.61 GPa were calculated from E and G. The Vickers hardness HV and Young’s modulus E were simultaneously determined to be 634 and 109.7 GPa, respectively, by a nanoindentation test. The fracture toughness K IC was determined to be 1.1 MPa m1/2, which was comparable to typical values 1.1–1.2 MPa m1/2 for thermoelectric silicide Mg2Si, by a Vickers indentation fracture test.

Annular disks made of three-dimensionally carbon-fiber-reinforced composites were optimized using numerical calculations, targeting high rotation speed and high energy density in a rotor system. This study examined two disk reinforcement patterns. One had added radial bundles at locations requiring high strength. The other had no added radial bundles. Two disk thicknesses were examined: uniform thickness and varied thickness, becoming thinner with distance from the disk center. Disk failure rotation speeds were estimated using the maximum stress criterion. The maximum burst tip speed was calculated as 1376 m/s for the uniform thickness no-bundles-added model and 1797 m/s for the varied thickness no-bundles-added model. Maximum energy densities including the aluminum alloy hub were calculated, respectively, as 124 and 151 Wh/kg.

A ternary alloy with an Yb-Gd-Si composition was designed using thermodynamic calculation software. The ternary alloy with two different steady-state phases (Yb:Gd:Si = 25:18:57 and 19:23:58 at%) was successfully fabricated using arc melting. Oxidation tests were carried out at 1200 °C for 1, 2, 4, and 8 h in air. Thermogravimetric and microstructural analyses indicated that weight gain during oxidation at 1200 °C was suppressed compared with that for the Yb–Si alloy (Yb3Si5) owing to the formation of complex oxides in the Yb2O3–Gd2O3–SiO2 system. In addition, the phase stability of Yb-Gd-Si was higher than that of Yb3Si5 because the growth of a eutectic phase in Yb-Gd-Si during oxidation was prevented compared with Yb3Si5. The addition of Gd in Yb–Si alloy was effective in improving phase stability and oxidation behavior.

High-speed rotation tests of a three-dimensionally reinforced annular disk were conducted with specimens, but were suspended at a speed below that predicted for disk fracture by high-amplitude vibrations. As described herein, the vibration source was assumed to be disk stiffness degradation . This study examined the validity of this mechanism. First, laser devices were used to measure elastic displacement at the disk periphery and to estimate global disk stiffness. Second, static ring burst tests using circular ring specimens machined from composite disks were conducted to evaluate the circumferential stiffness. Compression tests were done using rectangular parallelepiped specimens for the radial stiffness. Results show that the circumferential stiffness was 67% of the prediction by the rule of mixture. The radial stiffness was 85% of that. Locally wavy carbon fiber bundles were responsible for stiffness degradation. Stiffness degradation effects on the shaft–disk connection were specifically examined based on those results.