The purpose of placing a metal gasket between the two diamonds is to provide a chamber in which liquids and suspensions can be contained.
A metal gasket between the two diamonds is used to confine the sample under high pressure. The gasket is initially indented between the two diamond anvils and then drilled at the center of the indenting.
Gasket is a metal foil, typically 200 mm thick, which provides great yield strength, a high friction coefficient on diamonds (necessary to hold the sample in its position) along with a certain degree of ductility. The materials typically employed are high-strength stainless steel T301 or T304, but at ultrahigh pressures it is advisable to use rhenium, as it provides greater yield strength.
The following part describes the procedure for the preparation of the sample compression chamber, formed by the cylindrical hole in the gasket and by the diamonds’ culets. First, the gasket has to be fixed over one of the two diamonds by means of two small dowel pins fitting into the gasket holes. Then, the cell is closed and force is applied on diamonds to create an indented region with the desired final thickness.
The indented gasket can then be removed and a hole can be drilled by our Spark Eroder in the center of the indented region. The gasket is then placed in its exact position using again the two pins, and a small ruby chip is inserted into the hole. The cell is now ready to be closed and loaded with gases, liquids or solids..
Indentation produces deformation of the foil, which acquires the typical shape of a crater; the extruded material provides a lateral support, important for the attainment of high pressures.
Dimension of the circular hole drilled in the gasket, the gasket material, the original and final thickness of the indented region as well as the dimensions of the diamond culets are some of the most important factors determining the maximum reachable pressure in a DAC. Though a few empirical rules are followed, low values for all mentioned parameters are usually required when dealing with extreme pressures.
The size of DAC samples is of necessity very small. This presents some logistical challenges loading them into the cell. At this size, the electrostatic attraction of surfaces is much greater than gravitational attraction. Thus samples can be manipulated by adhering to the tip of a fine needle or fiber. However, if it encounters static charges en route to the cell, the sample can also suddenly fly off, never to be seen again. Loading a DAC requires a good binocular microscope and a very steady hand.