The role of pressure medium is to transmit pressure to the sample. Weak solids and liquids are typically used. “Gases” such as He, Ne, Ar, or N2 are a highly desirable pressure media.
The role of confining media is to transmit pressure to the sample while minimizing the shear stress transmitted to the sample. Weak solids and liquids are typically used. “Gasses” such as Ne, Ar, or N2 are a highly desirable confining media. However, in all cases they are loaded into the cell in a liquid state since the high compressibility and low density of material in a gaseous state renders true gasses useless as a confining media. Depending on whether the confining media is in a liquid or solid state they are more or less successful at protecting samples from stress. Liquids provide true hydrostatic environment. However, many fluid media crystallize as pressure increases; thus a media that works well at lower pressure may crystallize and introduce error at high pressures. Common confining media include LN2, Ar, He, ethanol, methanol and NaCl.
Since diamonds are quite inert, a wide range of substances may be examined. As mentioned above, the simplest sample consists of a powder trapped between the two anvils. Once compressed, such a sample will generally become a circular film about 30μm thick. The center of the sample will be at high pressure. However, because the edges of the sample are at 1 atmosphere, there is a tremendous pressure gradient within the sample. The pressure at the center of the sample is supported by extreme shear stress within the sample and friction between the sample and the diamond face. This arrangement does not work well for samples that cannot support shear σ (e.g. liquids) or for research questions where a sample needs to be in a quasi hydrostatic stress state or if the defect microstructure is important. Since many physical properties are impacted by defect microstructure and the state of stress, many early studies were marred by this drawback. To alleviate the stress in the sample and to preserve the defect state of sample, gaskets and confining media are now frequently used.
For this reason, it is recommended that solid samples be placed inside the gasket’s hole and immersed in a gas or a liquid before compression. The pressure-transmitting medium – which provides a pressure as close as possible to perfect hydro staticity – A gas-loading system has been constructed to load diamond-anvil cells with various kinds of gases at room temperature. A pressure remote-control is necessary during cryo-loading of the DAC at low temperatures. This technique allows DACs to be loaded with condensed gases such as nitrogen, argon, oxygen, etc. The advantage over gas loading is the higher initial density that determines a smaller volume reduction under compression.
