A high-pressure/high-temperature cell for interface-sensitive X-ray scattering experiments of supercritical fluids

We present a high-pressure/high-temperature cell design suitable for in situ X-ray scattering experiments at pressures up to 100 MPa and temperatures up to 773 K. The high energy X-ray beam is guided through two opposing sapphire windows to enable X-ray reflectivity studies for a wide range of momentum transfer allowing access to atomic length scales. The sample cell is heated by four heating cartridges inserted into the cell body. By measuring the temperature directly in the cell volume, high temperature stability of up to 0.1 K can be achieved. A Bridgman sealing system ensures leak tightness of the sample cell even when corrosive and supercritical fluids are used. Custom-built sample holders accommodate solid substrates of various dimensions up to 12x20 mm2 (W × L). The performance of the cell is demonstrated by visual observation of the supercritical phase transition of H2O and an X-ray reflectivity study of octadecyltrichlorosilane coated silicon wafers in contact with sub- and supercritical carbon dioxide. Here, formation of a liquid CO2 layer in the subcritical regime is observed that vanishes at supercritical conditions.