Diamond anvil cell

File:DiaAnvCell1.jpg

Schematics of the core of a diamond anvil cell. The diamond size is a few millimeters at most

A diamond anvil cell (DAC) is a device used in scientific experiments. It allows compressing a small (sub-millimeter sized) piece of material to extreme pressures, which can exceed 3,000,000 atmospheres (300 gigapascals).

The device has been used to recreate the pressure existing deep inside planets, creating materials and phases not observed under normal conditions. Notable examples include the non-molecular ice X, polymeric nitrogen and metallic xenon (in inert gas at lower pressures).

A DAC consists of two opposing diamonds with a sample compressed between the culets. Pressure may be monitored using a reference material whose behavior under pressure is known. Common pressure standards include ruby fluorescence, and various structurally simple metals, such as copper or platinum. The uniaxial pressure supplied by the DAC may be transformed into uniform hydrostatic pressure using a pressure transmitting medium, such as argon, xenon, hydrogen, helium, paraffin oil or a mixture of methanol and ethanol. The pressure-transmitting medium is enclosed by a gasket and the two diamond anvils. The sample can be viewed through the diamonds and illuminated by X-rays and visible light. In this way, X-ray diffraction and fluorescence; optical absorption and photoluminescence; Mossbauer, Raman and Brillouin scattering; positron annihilation and other signals can be measured from materials under high pressure. Magnetic and microwave field can be applied externally to the cell allowing nuclear magnetic resonance, electron paramagnetic resonance and other magnetic measurements. Attaching electrodes to the sample allows electrical and magnetoelectrical measurements as well as heating up the sample to a few thousand degrees. Much higher temperatures (up to 7000 K) can be achieved with laser-induced heating, and cooling down to millikelvins has been demonstrated.

Principle

The operation of the diamond anvil cell relies on a simple principle:

\[p=\frac{F}{A}\]

where P is the pressure, F the applied force, and A the 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 minimize deformation and failure of the anvils that apply the force, they must be made from a very hard and virtually incompressible material, such as diamond.

History

File:First diamond anvil cell.jpg

The first diamond anvil cell in the NIST museum of Gaithersburg. Shown is the part which compresses the central assembly shown in the top figure.

Percy Williams Bridgman, the great pioneer of high-pressure research during the first half of the 20th century, revolutionized the field of high pressures with his development of 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 invention of the diamond anvil cell in the late 1950s at the National Bureau of Standards (NBS) by Weir, Lippincott, Van Valkenburg, and Bunting further refined the process. 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 following decades DACs have been successively refined, the most important innovations 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 estimated pressures at the Earth’s center (~360 GPa).

Components

There are many different DAC designs but all have four main components:

1. The force-generating device — relies on the operation of either a lever arm, tightening screws, or pneumatic or hydraulic 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 weigh 1/8 to 1/3 carat (25 to 70 mg). 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. Specially selected anvils are required for specific measurements—for example, low diamond absorption and luminescence is required in corresponding experiments.
3. Gasket — a foil of ~0.2 mm thickness (before compression) 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. Standard gasket materials are hard metals and their alloys, such as stainless steel, Inconel, rhenium, iridium or tungsten carbide. They are not transparent to X-rays, and thus if X-ray illumination through the gasket is required then lighter materials, such as beryllium, boron nitride, boron or diamond are used as a gasket.
4. Pressure-transmitting medium — homogenizes the pressure. Methanol:ethanol 4:1 mixture is rather popular because of ease of handling. However, above ~20 GPa it turns into a glass and thus the pressure becomes nonhydrostatic. Argon, hydrogen and helium are usable up to the highest pressures, and ingenious techniques have been developed to seal them in the cell.

Uses

Prior to the invention of the diamond anvil cell, static high-pressure apparatus required large hydraulic presses which weighed several tons and required large specialized laboratories. The simplicity and compactness of the DAC meant that it could be accommodated in a wide variety of experiments. Some 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 spectroscopic experiments and for crystallographic studies using hard X-rays.

A variant of the diamond anvil, the hydrothermal diamond anvil cell (HDAC) is used in experimental petrology/geochemistry for the study of aqueous fluids, silicate melts, immiscible liquids, mineral solubility and aqueous fluid speciation at geologic pressures and temperatures. The HDAC is sometimes used to examine aqueous complexes in solution using the synchrotron light source techniques XANES and EXAFS. The design of HDAC is very similar to that of DAC, but it is optimized for studying liquids.

See also

• Anvil press
• High pressure
• Pressure experiment
• Fluid statics
• Material properties of diamond

Books and reviews

• M.I. Eremets: "High Pressure Experimental Methods" Oxford Science Publication (1996)
• M. Yousuf Semiconductors and Semimetals: eds. T. Suski and W. Paul, Academic Press, Sun Diego, 1998, 55, 381.
• N. V. Chandra Shekar et al. "Laser-heated diamond-anvil cell (LHDAC) in materials science research" J. Mater. Sci. Techn. 19 (2003) 518.
• Keith Brister "X-ray diffraction and absorption at extreme pressures" Rev. Sci. Instrum. 68 (1997) 1629
• N.Subramanian et al. "Development of laser-heated diamond anvil cell facility for synthesis of novel materials"Current Science, 91 (2006) 175.

External links

• NIST - history of the DAC
• The Diamond-Anvil Cell - Crystallography Laboratory at Virginia Tech.
• "Putting the Squeeze on Materials". http://www.llnl.gov/str/December04/Weir.html. Retrieved 2009-05-05.de:Diamantstempelzelle