We have developed a combined extreme condition of very low-temperature down to 50mK and ultrahigh pressure up to 200GPa ; using a 3He/4He dilution refrigerator and a diamond-anvil cell (DAC). Producing techniques for ultrahigh pressure and measuring techniques for electrical resistance are developed especially for studying the pressure-induced superconductivity of simple systems such as element. For example, the pressure-induced superconductivity of calcium is reviewed which is the highest superconducting temperature in the elements.
Macroscopic and microscopic physical properties of Fe1+δSexTe1-x were investigated through the magnetic susceptibility, electrical resistivity, heat capacity and nuclear magnetic resonance (NMR) measurements. The mother compound Fe1.14Te shows an antiferromagnetic phase transition accompanied by the structural change at 61.5K. The antiferromagnetic transition temperature of Fe1+δSexTe1-x decreases with increasing x, then the superconducting transition appears above x = 0.2. The superconductivity with clean limit occurs when δ is small. In such a compound, the temperature dependence of the nuclear spin - lattice relaxation rate and the electron contributed specific heat reveals presence of the nodal superconducting gap structure, suggesting that the superconductivity occurs in an unconventional mechanism. From a systematic investigation of the nuclear spin - lattice relaxation rate in Fe1+δSexTe1-x, it was found that the antiferromagnetic quantum critical point lies at x ∼0.03. The superconductivity occurs in the vicinity of the antiferromagnetic quantum critical point, resulting in that the superconductivity in Fe1+δSexTe1-x is mediated by the antiferromagnetic spin fluctuations.
Diamond-SiC composites were produced from diamond and Si powders by reaction synthesis using a hot isostatic pressing (HIP) technique. At an HIP condition of 1450°C and 100MPa, which pressure is much lower than that of the diamond stability field, diamond powders react with molten Si to form well-sintered diamond-SiC composites. Using the cubes of the composites thereby fabricated, an application to the second stage anvils in the Kawai-type multi-anvil high-pressure apparatus was attempted. Because the diamond-SiC composites are transparent to X rays, the present anvils can be employed not only for conventional energy dispersive X-ray diffraction studies but also for angle dispersive diffraction and radiographic studies that need a larger window for X-ray images. In this paper, our recent advances in the HIP production of diamond-SiC composites and their applications to high-pressure anvils for the Kawai-type multi-anvil high-pressure apparatus were reported.
Single-phase (binderless) super-hard polycrystalline cBN and polycrystalline diamond have been successfully synthesized by direct conversion sintering from high-purity hBN and graphite, respectively, under static high pressure and at high temperature. The polycrystalline cBN synthesized at ≥7.7GPa and 2200-2400°C consists of homogeneous fine grains of <0.5μm. The polycrystalline diamond synthesized at ≥15GPa and 2200-2400°C reveals a homogeneous fine structure comprised of very small diamond grains of several tens of nanometers. The polycrystalline cBN and polycrystalline diamond have considerably higher hardnesses than those of their single crystals. The transverse rupture strengths (TRSs) are much higher than those of conventional polycrystalline cBN and diamond containing binder and show a positive temperature dependence above 800-1000°C where the TRSs of conventional polycrystals are rather deteriorated. The fine microstructure without any secondary phases and excellent mechanical properties of the polycrystalline cBN and diamond thus fabricated are promising for next-generation high-precision and high-efficiency cutting tools.
ZrO2 ceramics containing 25mol% Al2O3 have been fabricated by sintering cubic or tetragonal ZrO2 (t-ZrO2) solid solution (ss) nanoparticles prepared via a sol-gel process with a piston-cylinder type high-pressure apparatus at 850~950°C for 10 to 60min under 1GPa. Dense t-ZrO2(ss)/Al2O3 composite ceramics with a small amount of monoclinic ZrO2 (m-ZrO2) sintered at 900°C for 30min using the calcined powder prepared at 1000°C for 1 h in air were composed of around 55nm grains with a high relative density of 99.9%. They showed extreme high bending strength (σb : 1125MPa) and high fracture toughness (KIC : 15.8MPa·m1/2) simultaneously. Their mechanical properties were investigated from viewpoints of the microstructure of ceramics by high-resolution TEM observation and XRD analysis. It was clear that high mechanical properties were much related with high relative density, high ratio of t-ZrO2 phase to m-ZrO2, homogeneous distribution of α-Al2O3 particles in the nanometer-size grain matrix of t-ZrO2 ; these might be achieved by adopting both the the sol-gel technique for preparation of solid solution powders containing Al2O3 and high-pressure sintering at low temperatures.
Process-induced deformation in composite parts affects their qualities and demands a lot of efforts for the compensation. In this work, the strength reduction after structural assembly due to process-induced deformation was investigated with L-shaped bending tests. The strengths of cured specimens after enforced shape compensations depended on the number of ply and the spring-in angle. Failure criteria of the L-shaped specimen were evaluated and the interlaminar tensile stress at the curved location was dominant for the thick specimens. However, an effect of the in-plane compression stress was found for the thin specimens. Pull-off tests of skin-stringer structures cured by normal tools and shape compensated tools were also conducted to valid the effects of the tool shape compensation.
In this study, a numerical method to estimate the mechanical behaviors of woven fabric composite under off-axis loading is proposed. Although FEM has been widely used for the estimation of mechanical behaviors or optimization of the FRP structure to save designing costs, mechanical behaviors, especially damage developments of fibrous composites under off-axis loading haven't been estimated by FEM because of the complex boundary conditions. As the FE method for woven fabric composite, unit cell with periodic boundary conditions has been used. In order to loading to off-axis direction of the unit cell, periodic boundary conditions based on relative displacement vectors are employed. The unit model is rotated so that loading direction become horizontal. The length of the relative displacement vectors can be calculated by the size of unit cell and loading angle. Poisson's effect is also considered in the method. The mechanical behaviors of arbitrary in-plane off-axis loading directions can be estimated by the proposed method. FE analysis based on damage mechanics was carried out and the damage behaviors of woven fabric composites under several loading directions were characterized. As the results, when the angle between loading direction and principal axis is larger, stress when initial damage occur decreased and damage of fiber bundle dominated by shear stress occurred. The proposed method will also contribute to analyze off-axis direction of other FRP with periodic boundary conditions such as braded composites or non-crimp fabric composites.
This study considered the behavior of tensile and fatigue damage, and the effect of reinforcement of the interface strength given to a damage process by observing the microscopic damage to the fiber interface of glass fiber reinforcement polycarbonate. As a result, the following thing became clear. 1) In both tensile and fatigue process, damage was initiated from the fiber end in the early stages, and the debonding propagated along the fiber interface. 2) The GF/PC interface strength improved by adding the epoxy resin. 3) The debonding along the fiber interface was inhibited by improving interface strength.
For automotive applications of composites to secure safety and reliability of its structure and functionality are critical issues. Carbon fiber reinforced polymer-matrix composites (CFRP) are widely used under vibrating load condition in various automotive and aerospace applications as well as in structural engineering. Today, more application of CFRP is strongly desired for automotive lightweighting. In this paper damage development of non-crimp fabric carbon fiber reinforced polyamide 6 (NCF-CF/NY6) composite plates that are orthotropic laminates has been investigated through a lock-in thermography system under cyclic constant amplitude loading. The instrument system can offer thermoelastic stress image (TSA image) of materials based on an algorithm. We demonstrate that damages in notched CF/NY6 laminate are clearly obtained as image information (TDA image) by post-processing the TSA images obtained before and after loading: it provides the feature of damage. The specific morphology derives identification of the various type of the damage, i.e. transverse cracks, notch induced splitting and delamination. It is found that the results offer the development of damage in [90°/0°]s and [0°/90°]s laminates as distinctly different modes, respectively. Intensity information of TDA images is also discussed to be corresponded to damage density and distribution feature based on scanning electron microscopy examination.
In-situ chemical vapor deposition (CVD) of aluminum on vapor grown carbon fibers (VGCF) was investigated for the fabrication of Al-VGCF composites. Reaction of aluminum powder with iodine was used to produce the aluminum vapor source. Aluminum powder, VGCF and iodine powder were placed into quartz tubes in various molar ratios and the quartz tubes were sealed in vacuum. Then they were annealed in the furnace. Annealing temperature was 673K or 773K. Obained powder was observed using a field emission transmission electron microscope (FETEM). Coated layer was successfully formed on VGCF using a molar ratio of VGCF : aluminum : iodine of 1 : 1 : 0.1 under an annealing temperature of 773K for 1.728 × 105s. The coated layer was proved to be aluminum by characterization of crystal and chemical structure using X-ray diffraction, Fourier transform-infrared spectroscopy, and X-ray photoelectron spectroscopy. As a result of high resolution transmission electron microscope observation, mild reaction of VGCF with the aluminum coated layer at the interface, accompanied with formation of the interlayer (reaction layer) was evident. Detailed observation of the graphitic layers of VGCF revealed that graphitic layers on VGCF surface are slightly consumed due to reaction with aluminum. Crystal structure of the interlayer was determined to be aluminum oxycarbides (Al4O4C, Al2OC), by measurement of lattice spacing. It was also made clear that the interface (interlayer) formation process is strongly dominated by the surface structure of the VGCF.
Carbon fiber reinforced carbon composites (C/C composites) are fabricated from PAN- and pitch-based carbon fibers. The carbon matrix of the composites is prepared from phenol resin. In addition to this, the pitch matrix precursor is re-impregnated by HIP into the carbonized composite. Finally, the re-impregnated composites are graphitzed. The mechanical property of the composites is then evaluated using a 4-point bending test at ambient temperature, 2000°C and 2200°C. The morphology of the C/C composites following the bending test at different temperatures is observed under OM and SEM. The bending strength of C/C composites at 2000°C is ca. 1.5 times higher than that at ambient temperature, being independent of type of carbon fiber. The composite from PAN-based composites shows a lower modulus with the elevated test temperature from 2000°C to 2200°C. On the other hand, the modulus of the composites from pitch-based carbon fiber shows almost no change at high temperatures. For the C/C Composite from pitch-based carbon fiber, it is observed that some buckling fractures of carbon fiber tows and plastic deformation of the carbon fibers is observed after the bending test above 2000°C. However, such fractures and deformation are rarely almost observed in the composite from PAN-based carbon fiber.
This paper proposes an effective technique to improve bending strength and fracture toughness of C/C composite by adding carbonized Micro Fibrillated Cellulose (CMFC). In this study, conventional CFRP was carbonized at 1,700°C with inert atmosphere to fabricate the C/C composites in which char matrix was made from phenolic resin. In order to fabricate the carbon particles with 3-dimensional web-like structures, the absorbed water of market available MFC was eliminated by freeze-drying technique, then MFC was carbonized at 800°C with inert atmosphere. The improvement in the bending strength was appeared at the condition of 1.0wt% of addition, while the excessive addition of CMFC was not effective to improve the bending strength. The morphology evaluation of fracture surface was revealed that the fiber braking was observed at the condition of 1.0wt% of addition, while debonding was appeared on that of unmodified, 3.0 and 5.0wt% of CMFC addition. The interfacial shear strength and fracture toughness were also improved when matrix was modified with appropriate contents of CMFC. The effect of CMFC addition on bending strength and fracture toughness of carbon matrix was also investigated. The local fracture toughness evaluated by IF(Indentation Fracture) test was increased with increase of CMFC contents. However, the bending strength of carbon matrix was increased when 1.0wt% of CMFC addition, while the excessive addition of CMFC was not effective to improve the bending strength of carbon matrix. The agglomeration of CMFC was appeared due to the increment of viscosity of carbon precursor, when the excessive content of CMFC was added.