The Background to High Pressure Research
Opposed Anvil Devices
Anvils are scientific devices used to pressurise very small material samples at extremely high pressures – sometimes into the megabar pressure range. The main principal of the anvil apparatus is to use two identical so-called ‘anvils’ of hard strong material and place the sample to be pressurised between their opposing faces, together with a metal marker or test ruby. The two anvils are then squeezed together by the surrounding apparatus to creature extreme pressures on the sample. Anvils work on the simple principle of the following equation;
P=F/A
where P is pressure measured in Pascals, F is force measured in Newtons and A is the area measured in mm2. This equations explains why, by using such a small sample (in the region of a few micrometers) and thus a tiny pressurised area, a very high pressure can be achieved by using only a moderate force. While under compression the exact pressure at which the sample is being subjected is usually measured by one of two methods; the equation of state of the metal marker measured by x-ray diffraction or, laser excited ruby fluorescence (a view-port through the support must be included to allow for these methods to be executed).
P=F/A
where P is pressure measured in Pascals, F is force measured in Newtons and A is the area measured in mm2. This equations explains why, by using such a small sample (in the region of a few micrometers) and thus a tiny pressurised area, a very high pressure can be achieved by using only a moderate force. While under compression the exact pressure at which the sample is being subjected is usually measured by one of two methods; the equation of state of the metal marker measured by x-ray diffraction or, laser excited ruby fluorescence (a view-port through the support must be included to allow for these methods to be executed).
History of the Anvil![]() Photograph of the original diamond anvil cell on display in the NIST Gaithersberg Museum courtesy of Piermarini (2001). |
The first anvil device was developed by Percy Williams Bridgman and consisted of two small flat plates of tungsten carbide pressed together. This early anvil changed high-pressure research completely as it allowed huge pressures of a few gigapascals could be achieved. The anvil device was further developed by a group of scientists working at the NBS laboratories in Washington in the late 1950s. Their anvil cell worked on a much similar method to Bridgman’s, however, for the anvil material they used single crystal diamond (for further information click here). These pressure cells were found to achieve extremely high pressures - as large as 4.6Mbar to date (Moss et al, 1986) This again revolutionised research possibilities by allowing useful experiments to be done such as those done on NaNO3, KNO3, AgNO3, ferrocene, ice, and CaCO3 that showed there are pressure-dependent shifts and changes in intensity of infrared absorption bands. The Diamond Anvil Cell (DAC) is widely known today as it has been able to create the highest static pressures in a laboratory. DACs are relatively simple and small so have the advantages of low cost and portability compared to other high-pressure apparatus. These days anvils are usually of gem-quality and most commonly single crystal diamond as it is the hardest material known to man. Despite their hardness, diamonds, and other gems, have specific material advantages that suit them well for anvil use such as their transparency to x-rays for pressure calculation. |
Modern Diamond Anvil Cell![]() Schematic of the modern diamond anvil cell. Image courtesy of Steve Jacobson, The Department of Earth and Planetary Physics, Northwestern University. |
The DAC consists of 3 main components - two opposed diamonds, a pressure cell and a gasket in between containing the sample. The cut shape of the diamond amplifies the pressure even further. Diamonds also possess the ability of transparency to x-rays required for pressure calibration. |