Taku Okada

High-pressure science with a multi-anvil apparatus at SPring-8

Since first opening its doors to public research in 1997, SPring-8 has seen the accomplishment of many important studies in a wide variety of fields through its stable operation and cutting edge technology. High-pressure experiments have been carried out on a number of beamlines using a diamond anvil cell or a multi-anvil press. Here, we review the multi-anvil presses installed on the SPring-8 beamlines and a few research projects currently utilizing this technology. The significant difference in post-spinel boundary between multi-anvil experiments and diamond anvil studies will also be discussed.

Nucleation and growth in bulk metallic glass under high pressure investigated using in situ x-ray diffraction

The effects of pressure up to 8.3 GPa on nucleation and growth in a Zr57Nb5Cu15.5Ni12.5Al10 bulk glass-forming alloy during heating, isothermal annealing, and cooling processes are investigated using in situ x-ray diffraction with synchrotron radiation. The metallic glass shows markedly different crystallization behaviors for different heat treatments under high pressure. Heating under high pressure can stabilize the supercooled liquid state. Isothermal high pressure annealing leads to nanocrystallization. The glass forming ability of the alloy can be enhanced by high pressure during cooling process. The effects of pressure on the nucleation and growth are discussed based on the nucleation theory.

Kinetics of diamond spontaneous crystallization from the melt of the Fe-Al-C system at 6.5 GPa

Abstract Kinetics of diamond spontaneous crystallization from the Fe–Al–C system melt in equilibrium with graphite has been studied in situ at 6.5 GPa in the range 1720–1920 K using diffraction of synchrotron radiation. The process under consideration is controlled by carbon diffusion in the melt at 1720 and 1820 K. Kinetics data might be best fitted by a model that assumes instantaneous nucleation. At 1920 K the mechanism of diamond crystallization changes; during growth, continuous nucleation is observed and the kinetics of the processes on the surface of growing crystals becomes the stage that controls the crystallization. The rate of spontaneous crystallization in the Fe–Al–C system decreases with increasing temperature as the graphite–diamond equilibrium line is approached.

Determination of low-pressure crystalline–liquid phase boundary of SnI4

The location of the liquidus in the low-pressure crystalline phase of SnI(4) was determined utilizing in situ x-ray diffraction measurements under pressures up to approximately 3.5 GPa. The liquidus is not well fitted to a monotonically increasing curve such as Simons equation, but breaks near 1.5 GPa and then becomes almost flat. The results are compared to those from molecular dynamics simulations. Ways to improve the model potential adopted in the simulations are discussed.

Formation and crystallization of CuZrHfTi bulk metallic glass under ambient and high pressures

The formation of CuZrHfTi bulk metallic glasses (BMGs) and the crystallization of the typical Cu60Zr20Hf10Ti10 BMG under ambient conditions and high pressure have been investigated by differential scanning calorimetry, x-ray diffraction (XRD) and in situ synchrotron radiation XRD. The effects of high pressure on crystallization and formation of the CuZrHfTi alloy are discussed. The Kauzmann temperature, TK, where the entropy of the undercooled liquid equals that of the crystal, is also determined to be 724 K. The TK is compared with the experimentally observed rate-dependent glass transition, Tg. The kinetic study of the crystallization shows that the Cu-based BMG has much larger activation energies obtained using Kissinger analysis and is markedly different in crystallization kinetic behaviour compared to that of other BMGs with better glass forming abilities. An apparent correlation between crystallization temperature and activation energy is found in various metallic glasses. The correlation is discussed and connected with the thermal stability of metallic glasses.

In situ x-ray diffraction of graphite–diamond transformation using various catalysts under high pressures and high temperatures

In situ x-ray diffraction studies of graphite–diamond transitions with various solvent catalysts under high pressures and high temperatures at the Photon Factory and SPring-8 are reviewed. By combining synchrotron radiation and a large-volume multi-anvil high-pressure apparatus, real-time observations have been successful in the diamond formation process with the help of various catalysts, such as transition metals, carbonates and aqueous fluids. The experimental procedures and the technical details are described. The diffraction data with various catalysts are shown and the problems and limitations of this method are discussed.

Melting and crystallization of Nd60Al10Fe20Co10 bulk metallic glass under high pressure

Crystallization, melting and structural evolution upon crystallization in Nd60Al10Fe20Co10 bulk metallic glass (BMG) are in situ investigated by x-ray diffraction with synchrotron radiation under high pressure. It is found that the crystallization is pressure promoted, while the melting is inhibited. The crystallization and melting process are also changed under high pressure. The features of the crystallization and melting under high pressure are discussed.

In situ x-ray observations of the diamond formation process in the C-H2O-MgO system

The diamond formation process in aqueous fluid catalyst under high-pressure and high-temperature conditions has been observed for the first time. Quench experiments and in situ x-ray diffraction experiments using synchrotron radiation have been performed upon a mixture of brucite (Mg(OH)2) and graphite as the starting material. It was confirmed that brucite decomposed into periclase and H2O at 3.6 GPa and 1050°C while its complete melting occurred at 6.2 GPa and 1150°C, indicating that the solubility of MgO in H2O greatly increases with increasing pressure. The conversion of carbon from its graphite to its diamond form in aqueous fluid was observed at 7.7 GPa and 1835°C.