As represented by long-period stacking ordered structure (LPSO), long-period phase based on hcp-Mg lattice are received attention as new kind strengthening phase of Mg alloy. We have researched formation process of LPSO and new types of long period phase based on hcp-Mg lattice using high pressure experiment technique. For the formation process of LPSO, we found hcp-Mg lattice transformed to LPSO after the lattice expansion of hcp due to invasion of Y. This result was interpreted together with the result of the first-principles calculation, and then a model of the formation process of LPSO was proposed. On the other hand, we worked on development for long-period superlattices based on hcp-Mg lattice other than LPSO. As the results, two types of long-period superlattices (LPSLs) based on the hcp-Mg lattice were found in Mg97Yb2Zn1 after subjected at 5 GPa: one LPSL is the Mg14Yb3Zn1 phase, whose unit cell dimensions are described as √3×√3×3 with respect to those of the original α-Mg lattice; the other LPSL is the Mg17Yb4Zn3 phase, whose unit cell dimensions are described as √3×√3×2 with respect to those of the original hcp-Mg lattice. These findings using high pressure technique can use in the development of Mg alloy at ambient pressure.
Negative thermal expansion (NTE) materials are expected to be utilized for control of the thermal expansion of structure materials in the fields of semiconductor manufacturing, optical devices and precise machining where precise positioning is required. In this review, giant NTEs in Pb1－xBixVO3 induced by ferroelectric-paraelectric transition and in BiNi1－xFexO3 induced by intermetallic charge transfer will be discussed.
High-pressure metathesis reaction is a promising method for synthesizing novel nitride compounds, e.g. multi-component semiconductors and hard materials. The strong nitridation ability under high pressure makes it possible to realize such compounds. In this article, we introduce our attempts to synthesize 5d nitrides (ReN2, WN and W2.25N3) and ternary nitride semiconductors (ZnSnN2 and MgSnN2) using the metathesis reaction. This method contributes not only to improvement of crystallinity but also to the synthesis of metastable phases. The high-pressure metathesis reaction may open a way for realizing materials designed by data driven computational approach.
Basic approaches of first-principles calculations for high-pressure synthesis of novel materials were discussed. Stabilities of materials under high-pressure conditions were often explained using enthalpies H, which are estimated with total energies in electronic structure calculations, volumes of materials and pressure. First-principles calculations reveal that the pressure dependence of enthalpies is mainly composed of that of the product of pressure and volumes rather than that of the total energies not only in hard materials but also in soft materials such as molecular crystals. Therefore, the volumes of materials play important roles for high-pressure phase transitions and chemical reactions under high-pressure conditions. Phase transitions during decompression process were also investigated using first-principles methods. Since the phase transitions are caused by mobile ions, configurations of immobile ions have the key to recovery of high-pressure phases.
Transparent nanopolycrystalline cubic silicon nitride (γ-Si3N4) was fabricated under high pressure and temperature using Kawai-type apparatus. Vickers indentation tests were carried out and the hardness value of 35 GPa was obtained. Fracture toughness was estimated based on length of crack generated from the Vickers indentation prints and the estimated value is 3.5 MPa m1/2, which is similar to that of polycrystalline alumina. Elastic moduli of this material were measured by Brillouin spectroscopy. These results show that cubic silicon nitride is one of the third hardest materials. We confirmed excellent thermal metastability of this material in air at temperatures up to 1,460℃. Low-temperature heat capacities of β- and γ-Si3N4 were measured and the equilibrium phase boundary between these two phases were calculated.
Laser-driven dynamic compression is used to study matters under extreme pressure and temperature conditions of dynamical pressure and temperature conditions, moreover, the application of this technique to material science researches has been accelerated. A high-strain rate over 10－8 s－1 is a unique and critical feature of the compression in the material science applications. X-ray free electron laser (XFEL) is a vital tool to directly observe the structure under such ultrafast, high-strain rate compression. Here we present the latest observations taking advantage of the ultrafast compression at the XFEL platform; 1) ultrafast uniaxial compression of highly oriented pyrolytic graphite, 2) nano-crystallization of crystalline ceramics under ultrafast deformation.
I have been applying the Finite Element Method (FEM) to high-temperature and high-pressure experiments since 2007. FEM is a tool for numerically solving phenomena described in partial differential equations. FEM divides an object into meshes (finite elements), which enables us to handle complex geometry easily. I have been conducting FEM by using COMSOL commercial software, and was invited to give a talk at the COMSOL conference in 2019. At the conference, I presented the FEM applications which I have conducted. Based on the opportunity, I summarize the contents of the presentation in this document.
The Eulerian finite strain of an elastically isotropic body is defined by taking the state after compression as the reference state and expanding the squared length. The second-, third- and fourth-order Birch-Murnaghan equations of state are plainly derived based on the Eulerian finite strain. The key for the plain derivation is no use of differenital or tensor because of isotropic, uniform and finite change in length. For better understanding, the finite strain in the Lagrangian scheme is defined by taking the state before compression as the reference state, and the Lagrange equations of state are derived in this scheme. In this scheme, pressure increases less significantly with compression than the Eulerian scheme. The different Eulerian strains are also defined by expanding the linear and cubed lengths instead of the squared length, and the first- and third-power Eulerian equations of state are derived in these schemes. Fitting of pressure-scale-free data to these equations indicates that the Lagrangian scheme is inappropriate to describe P-V-T relations of MgO, whereas three Eulerian equations of state have equivalent significance, and the Birch-Murnaghan equations of state does not have special meaning compared to the other Eulerian equations of state.