|H Bureau, D. Massare|
In situ SR-XRF measurements performed in an externally heated Bassett-type diamond anvil cell, µFID, ESRF, Grenoble.
Micro-experimental studies require the use of diamond anvil cells, allowing observation and in situ characterization of the studied phase equilibrium using classical spectroscopy techniques (Infrared, Raman, X-Ray Fluorescence, X-Ray diffraction…).
In the Bassett’s cell, the sample is contained in a rhenium gasket compressed by two type I diamonds of 0.21 carat with 1 mm flats. The initial diameter of the borehole in the gasket is 0.5 mm. Heating is achieved by passing a high electrical current through the molybdenum wires around the tungsten carbide seats below the diamonds. During operation, the cell is flushed with an argon-hydrogen (2 %) mixture to prevent oxidation of the diamonds and the molybdenum heater. Temperature is measured to ± 2˚C by two type K thermocouples attached to the diamonds. The temperature in the sample chamber is calibrated against these thermocouples by measuring the melting points of NaNO3, CsCl, NaCl, sulfur and ice in the cell at room pressure.
Pressure medium in all experiments is water. Pressure at a given temperature is calculated from the equation of state of water. This is based on the observation that the sample cavity behaves as an isochoric system after several cycles of heating and cooling. Accordingly, the gasket is first filled with water and an air bubble to give the desired bulk density and cycled through the relevant temperature range several times, until the homogenization temperature of the fluid is virtually constant. After that the cell is loaded with the sample (ships of glass, crystals…), water (or aqueous solution) and an air bubble, then it is heated to the desired temperature. After cooling, the bulk density of the pressure medium is determined by measuring the homogenization temperature, if a bubble was present, or the melting point of ice, if there is no bubble. From this bulk density, the pressure is then calculated for the temperatures prevailing during the experiment.
Observation and measurements are performed in situ at high pressure (up to 2 GPa) and high temperature (up to 1100°C) through the diamonds along the compression axis.
See also : http://mplmac.geo.cornell.edu/HDAC/.
Maj : 28/10/2008 (368)