Diamond anvil cell

Artistic rendition of a diamond anvil cell

A diamond anvil cell (DAC) consists of two opposing diamonds with a sample compressed between the culets. Extreme pressure, which can exceed 1,000,000 atmospheres (100 gigapascals), can be applied to this small volume.

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, metallic hydrogen, and MgSiO₃ perovskite, thought to be the major component of the Earth's mantle.

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 prevent deformation and even breakage of the anvils that apply the force, they must be made from a very hard and virtually incompressible material, such as 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 in the late 1950s 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 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 three 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.
3. Gasket — a 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

Prior to the invention of the diamond anvil cell, static high-pressure apparatus required large hydraulic presses which weighed several metric 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.

References

• S. Block, and G. Piermarini (1976). "The Diamond Cell Stimulates High-Pressure Research". Physics Today. 29: 44.
• A. Jayaraman (1983). "Diamond Anvil Cell and High-Pressure Physical Investigations". Reviews of Modern Physics. 55: 65. doi:10.1103/RevModPhys.55.65.
• A. Jayaraman (1986). "Ultrahigh pressures". Reviews of Scientific Instruments. 57: 1013. doi:10.1063/1.1138654.
• D.J. Dunstan, and I.L. Spain (1989). "The Technology of Diamond Anvil High-Pressure Cells". Journal of Physics E: Scientific Instruments. 22: 913–933. doi:10.1088/0022-3735/22/11/004.
• M.I. Eremets: “High Pressure Experimental Methods”, Oxford Science Publication (1996)

See also

• High pressure physics

External links

• NIST - history of the DAC
• The Diamond-Anvil Cell - Crystallography Laboratory at Virginia Tech.
• easyLab - An Introduction to Diamond Anvil High Pressure Cells
• BETSA - All for very high pressure 50GPa to 200GPa