Diamond anvil cell
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A diamond anvil cell (DAC), is a device used by physicists to exert extreme pressures on a material. It consists of two opposing cone-shaped diamonds squeezed together by a lever arm. The resultant high pressures - of as much as a million atmospheres - are produced when force is applied to small areas of the opposing diamond faces.
The device has been used to simulate the extreme pressures existing in the heart of planets, often creating new substances in the process. Notable examples are the production of metallic hydrogen, found in the gas giant planets, and perovskite, thought to be the major component of the Earth's mantle.
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Principle
The operation of the diamond anvil cell relies on a simple principle:
- Pressure = Force / Area
Therefore high pressure can be achieved by applying a moderate force on a sample with a small area, rather than applying a large force on a large area. In order to prevent deformation and even breakage of the anvils that apply the force, they must be made from a very hard and virtually incompressible material - diamond.
History
Percy Williams Bridgman - the great pioneer of high-pressure research during the first half of the 20th century - developed an opposed anvil device with small flat areas that were pressed one against the other with a lever-arm. The anvils were made of a tungsten-carbon alloy (WC). This device could achieve pressure of a few gigapascals, and was used in electrical resistance and compressibility measurements. The revolution in the field of high pressures came with the development of the diamond anvil cell (DAC) in the late 1950's in the National Bureau of Standards (NBS) by Weir, Lippincott, Van Valkenburg, and Bunting [1]. The principles of the DAC are similar to the Bridgman anvils but in order to achieve the highest possible pressures without breaking the anvils, they were made of the hardest known material – a single crystal diamond. The first prototypes were limited in their pressure range and there was not a reliable way to calibrate the pressure. During the next decades many investigators have used DACs and following their experience many innovative improvements were introduced, the most important being the use of gaskets and the ruby pressure calibration. The DAC evolved to be the most powerful lab device for generating static high-pressure. The range of static pressure attainable today extends to the pressures prevailing in the earth’s center (~360 GPa).
Components
There are many different DAC designs but all of them have three main components:
(1) The force-generating device - relies on the operation of either a lever arm, tightening screws, or gas pressure applied to a membrane. In all cases the force is uniaxial and is applied to the tables (bases) of the two anvils
(2) Two opposing diamond anvils - made of high gem quality, flawless diamonds, usually with 16 facets. They typically weight 1/8 to 1/3 carat. The culet (tip) is ground and polished to a hexadecagonal surface parallel to the table. The culets of the two diamonds face one another, and must be perfectly parallel in order to produce uniform pressure and to prevent dangerous strains.
(3) Gasket - a hard metal foil that separates the two culets. It has an important role: to contain the sample with a hydrostatic fluid in a cavity between the diamonds, and to prevent anvil failure by supporting the diamond tips, thus reducing stresses at the edges of the culet.
Uses
Its relative simplicity and compactness makes the DAC a diverse tool that can be accommodated to a variety of experiments. Some of the contemporary DACs can easily fit into a cryostat for low-temperature measurements, and for use with a superconducting electromagnet. In addition to being hard, diamonds have the advantage of being transparent to a wide range of the electromagnetic spectrum - from infrared to gamma rays, with the exception of the far ultraviolet and soft X-rays. This makes the DAC a perfect device for various spectroscopic experiments.
External links
- NIST - history of the DAC
- The Diamond-Anvil Cell - Crystallography Laboratory at Virginia Tech.
References
- S. Block, and G. Piermarini: "The Diamond Cell Stimulates High-Pressure Research”, Physics Today 29, p. 44 (1976)
- A. Jayaraman: “Diamod Anvil Cell and High-Pressure Physical Investigations”, Reviews of Modern Physics 55, p. 65 (1983)
- A. Jayaraman: "Ultrahigh pressures", Reviews of Scientific Instruments 57, p. 1013 (1986)
- D.J. Dunstan, and I.L. Spain: “The Technology of Diamond Anvil High-Pressure Cells”, Journal of Physics E: Scientific Instruments 22, p. 913-933 (1989)
- M.I. Eremets: “High Pressure Experimental Methods”, Oxford Science Publication (1996)