Kawai-type apparatus

Kawai-type apparatus
Name	Kawai-type apparatus
Caption	A modern Kawai-type multi-anvil press at a synchrotron facility.
Classification	High-pressure apparatus
Inventor	Nobuhiko Kawai
Manufacturer	Various, including Sumitomo Heavy Industries
Related	Diamond anvil cell, Bridgman anvil, Paris-Edinburgh press

Kawai-type apparatus. It is a prominent class of multi-anvil press used extensively in high-pressure geophysics and materials science to simulate conditions within planetary interiors. The design utilizes a set of multiple anvils arranged to compress a sample volume within a solid pressure-transmitting medium, enabling the generation of static pressure exceeding 25 gigapascals. Developed in Japan, it has become a cornerstone instrument at major facilities like the Advanced Photon Source and SPring-8.

Design and components

The core assembly consists of a cubic or octahedral pressure medium, typically made of magnesium oxide or a semi-sintered alumina ceramic, which contains the experimental sample. This assembly is compressed by a set of six or eight hardened tungsten carbide anvils, each driven by a separate hydraulic ram within a large, robust frame. The anvils are often truncated to form guide blocks that precisely align the system, with the entire stack contained within a gasket system to contain the pressurized materials. Key ancillary components include internal furnace assemblies for high-temperature synthesis and electrical feedthroughs for in situ measurements.

Operating principles

The apparatus operates on the principle of multi-anvil shear compression, where synchronized hydraulic rams drive the anvils uniformly inward onto the pressure medium. This quasi-hydrostatic compression creates a large-volume pressure chamber where the sample experiences near-uniform stress, a significant advantage over simpler uniaxial compression devices. The solid medium transmits pressure to the sample while also providing thermal and electrical insulation. During operation, in situ X-ray diffraction using synchrotron radiation is commonly employed to monitor crystal structure and phase transitions under extreme conditions.

Development and history

The apparatus was pioneered in the late 1960s by Japanese physicist Nobuhiko Kawai at Osaka University, building upon earlier concepts like the Bridgman anvil and the Drickamer apparatus. Kawai's innovation of using multiple anvils to achieve truly multiaxial stress represented a major advance in high-pressure technology. Subsequent refinements by researchers such as Eiji Ito and teams at the University of Tokyo and the Bayerisches Geoinstitut greatly enhanced its capabilities. Its adoption at international synchrotron user facilities in the 1990s, like the National Synchrotron Light Source, cemented its role in modern experimental science.

Scientific applications

This apparatus is fundamental for studying the mineral physics of Earth's mantle and core, having been used to determine the properties of key phases like ringwoodite, bridgmanite, and post-perovskite. It enables synthesis of novel materials, including superhard materials like polycrystalline diamond and potentially superconducting hydrides. Researchers at institutions like the Carnegie Institution for Science and the University of Bayreuth employ it to investigate planetary differentiation, magma ocean crystallization, and the behavior of icy moons like Ganymede. It is also crucial for calibrating pressure scales used in other devices like the diamond anvil cell.

Technical specifications and capabilities

Modern systems can routinely achieve pressures of 25–30 GPa on sample volumes of several cubic millimeters, with simultaneous temperatures exceeding 2500 Kelvin using resistive heating or laser heating systems. The largest variants, such as those at SPring-8, can pressurize samples up to ~50 GPa. The apparatus allows for precise control of the pressure-temperature path, facilitating studies of kinetics and reaction rates. Integration with energy-dispersive X-ray diffraction and radiographic imaging provides real-time data on density, elasticity, and melting curves of geological and industrial materials.

Comparison with other high-pressure devices

Compared to the diamond anvil cell, the Kawai-type apparatus offers a much larger sample volume, enabling more accurate bulk property measurements and synthesis of recoverable samples, but at a lower maximum pressure. It generates more hydrostatic conditions than the simpler Paris-Edinburgh press, though the latter is faster for neutron scattering experiments. Unlike the rotating anvil cell or dynamic compression techniques using gas guns or lasers, it provides sustained, static conditions ideal for equilibrium phase studies. Its multi-anvil approach is more complex than the belt apparatus but offers superior pressure uniformity and in situ analytical access.

Category:High-pressure apparatus Category:Geophysical instrumentation Category:Materials science equipment