Editing Diamond anvil cell (section)

==Gas loading==

===Principle===
The pressure transmitting medium is an important component in any high-pressure experiment. The medium fills the space within the sample 'chamber' and applies the pressure being transmitted to the medium onto the sample. In a good high-pressure experiment, the medium should maintain a homogeneous distribution of pressure on the sample. In other words, the medium must stay hydrostatic to ensure uniform compressibility of the sample. Once a pressure transmitting medium has lost its hydrostaticity, a pressure gradient forms in the chamber that increases with increasing pressure. This gradient can greatly affect the sample, compromising results. The medium must also be inert, as to not interact with the sample, and stable under high pressures. For experiments with laser heating, the medium should have low thermal conductivity. If an optical technique is being employed, the medium should be optically transparent and for x-ray diffraction, the medium should be a poor x-ray scatterer – as to not contribute to the signal.

Some of the most commonly used pressure transmitting media have been sodium chloride, silicone oil, and a 4:1&nbsp;methanol-ethanol mixture. Sodium chloride is easy to load and is used for high-temperature experiments because it acts as a good thermal insulator. The methanol-ethanol mixture displays good hydrostaticity to about 10&nbsp;GPa and with the addition of a small amount of water can be extended to about 15&nbsp;GPa.<ref name="jay" />

For pressure experiments that exceed 10&nbsp;GPa, noble gases are preferred. The extended hydrostaticity greatly reduces the pressure gradient in samples at high pressure. Noble gases, such as helium, neon, and argon are optically transparent, thermally insulating, have small X-ray scattering factors, and have good hydrostaticity at high pressures. Even after solidification, noble gases provide quasihydrostatic environments.

Argon is used for experiments involving laser heating because it is chemically insulating. Since it condenses at a temperature above that of liquid nitrogen, it can be loaded cryogenically. Helium and neon have low X-ray scattering factors and are thus used for collecting X-ray diffraction data. Helium and neon also have low shear moduli; minimizing strain on the sample.<ref name="riv">{{cite journal |author1=Rivers, M. |author2=Prakapenka, V.B. |author3=Kubo, A. |author4=Pullins, C. |author5=Holl, C. |author6=Jacobson, S. |title=The COMPRES/GSECARS gas-loading system for diamond anvil cells at the Advanced Photon Source |year=2008 |journal=High Pressure Research |volume=28 |issue=3 |pages=273–292 |bibcode=2008HPR....28..273R |doi=10.1080/08957950802333593|s2cid=11986700 }}</ref> These two noble gases do not condense above that of liquid nitrogen and cannot be loaded cryogenically. Instead, a high-pressure gas loading system has been developed that employs a gas compression method.<ref name="Uch">{{cite journal |author1=Uchida, T. |author2=Funamori, N. |author3=Yagi, T. |title=Lattice strains in crystals under uniaxial stress field |year=1996 |journal=Journal of Applied Physics |volume=80 |issue=2 |page=739 |bibcode=1996JAP....80..739U |doi=10.1063/1.362920}}</ref>

===Techniques===
In order to load a gas as a sample or pressure transmitting medium, the gas must be in a dense state, as to not shrink the sample chamber once pressure is induced. To achieve a dense state, gases can be liquefied at low temperatures or compressed. Cryogenic loading is a technique that uses liquefied gas as a means of filling the sample chamber. The DAC is directly immersed into the cryogenic fluid that fills the sample chamber. However, there are disadvantages to cryogenic loading. With the low temperatures indicative of cryogenic loading, the sample is subjected to temperatures that could irreversibly change it. Also, the boiling liquid could displace the sample or trap an air bubble in the chamber. It is not possible to load gas mixtures using the cryogenic method due to the different boiling points of most gases. Gas compression technique densifies the gases at room temperature. With this method, most of the problems seen with cryogenic loading are fixed. Also, loading gas mixtures becomes a possibility. The technique uses a vessel or chamber in which the DAC is placed and is filled with gas. Gases are pressurized and pumped into the vessel with a compressor. Once the vessel is filled and the desired pressure is reached the DAC is closed with a clamp system run by motor driven screws.

===Components===
* High-pressure vessel: Vessel in which the diamond anvil cell is loaded.
* Clamp device seals the DAC; which is tightened by closure mechanism with motor driven screws.
* [[Programmable logic controller|PLC]] (programmable logic controller): Controls air flow to the compressor and all valves. The PLC ensures that valves are opened and closed in the correct sequence for accurate loading and safety.
* Compressor: Responsible for compression of the gas. The compressor employs a dual-stage air-driven diaphragm design that creates pressure and avoids contamination. Able to achieve 207&nbsp;MPa of pressure.
* Valves: Valves open and close via the PLC to regulate which gases enter the high-pressure vessel.
* Burst disks: Two burst disks in the system – one for the high-pressure system and one for the low-pressure system. These disks act as a pressure relief system that protects the system from over-pressurization
* Pressure transducers: A pressure sensor for the low- and high-pressure systems. Produces a 0–5&nbsp;V output over their pressure range.
* Pressure meters: Digital displays connected to each pressure transducer and the PLC system.
* Vacuum pump and gauges: Cleans the system (by evacuation) before loading.
* Optical system: Used visual observation; allowing in situ observations of gasket deformation.
* Ruby fluorescence system: Pressure in the sample chamber can be measured during loading using an online ruby fluorescence system. Not all systems have an online ruby fluorescence system for in situ measuring. However, being able to monitor the pressure within the chamber while the DAC is being sealed is advantageous – ensuring the desired pressure is reached (or not over-shot). Pressure is measured by the shift in the laser induced luminescence of rubies in the sample chamber.