Multi-anvil press

The multi-anvil press is a type of device designed to produce extremely high pressures in a relatively small volume. This type of anvil press is used in materials science and geology for the synthesis and study of solid phase materials under extreme pressure, as well as for the industrial production of valuable minerals, especially synthetic diamonds. These instruments allow the simultaneous compression and heating of millimeter size solid phase samples such as rocks, minerals, ceramics, glasses, composite materials, or metal alloys and are capable of reaching pressures above 25 GPa and temperatures exceeding 2500 °C. This allows mineral physicists and petrologists studying the Earth’s interior to experimentally reproduce the conditions found throughout the lithosphere and upper mantle, to a depth of 700 km (citation, figure 1,2). Diamond anvil cells and light-gas guns can access even higher pressures, but the multi-anvil apparatus can accommodate much larger samples, which simplifies sample preparation and improves the precision of measurements and the stability of the experimental parameters (citation needed).

The 6-8 multi-anvil apparatus was introduced by Kawai and Endo (1970) using a split steel sphere suspended in pressurized oil, later modified to use the hydraulic ram. In 1990, Walker et al. simplified the first compression stage by introducing the removable hatbox design, allowing ordinary machine presses to be converted into multi-anvil systems. A variety of assembly designs have been introduced and standardized including the Walker castable, and the COMPRES assemblies. Recent advances have focused on in-situ measurements (link to next section), and standardizing materials and calibrations.

A typical Kawai cell 8-6 multi-anvil apparatus uses air pumps to pressurize oil, which drives a vertical hydraulic ram to compress a cylindrical cavity known as a hatbox. This cavity is filled with six steel anvils, three facing and three facing down, that converge on a set of eight tungsten carbide cubes. The interior corners of these cubes truncated to fit an octahedral assembly. These octahedra range from 8 mm to 25 mm on edge and are typically composed of MgO or another material that deforms ductilely over the range of experimental conditions, to make sure the experiment is under hydrostatic stress. As this assembly is compressed, it extrudes out between the cubes, forming a gasket. A cylinder is drilled out between two opposite faces to accommodate the experiment. Experiments that require heating are surrounded by a cylindrical graphite or LaCrO3 cylinder furnace, which can produce considerable heat by electrical resistance (figure 4). However, the graphite furnace can be troublesome at higher pressures due to its tendency to transform into diamond. The DIA multi-anvil is the main alternative to the Kawai cell: it uses six anvils to compress a cubic sample.

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