ABSTRACT
The high pressure-high temperature structural stability of Zeolite A (ZA) has been studied using the X-ray diffraction (XRD) method. Structural studies at high temperatures show a reduction in the oxygen occupancy, belonging to the water molecule, indicating thermal dehydration and subsequent expulsion of water molecules from the pores of the structure. ZA does not undergo structural phase transition with temperature. However, structural transitions are observed in in situ XRD studies at high pressure and high temperature. At 1.3 GPa and 300 °C, the cubic ZA concomitantly transformed to cubic sodalite (SOD) and tetragonal zeolite NaP (ZNP). This transition was completely forbidden at 2.7 GPa, where a temperature-induced amorphization was favored at 250 °C. The thermal studies at higher pressure reveal the marginal influence of pressure on the thermal expansion coefficients of hydrated ZA. Pressure evolution of the high pressure-high temperature phases indicates no further phase transitions up to 5.9 GPa. The equation of state fit to the pressure-volume data of these phases show that ZNP is less compressible, followed by SOD and ZA. In contrast to the behavior at 0.1 MPa, SOD shows a pressure-induced negative thermal expansion (NTE) at 5.9 GPa. On the other hand, the positive thermal expansion (PTE) observed along the direction of c axis is compensated by the NTE along the a axis leading to a negligible volume thermal expansion for the ZNP structure. The bulk moduli and thermal expansion coefficients of all of the observed phases are reported. The outcomes of this study have been consolidated as a pressure-temperature phase diagram, which provides an insight into the technological and industrial applications of ZA at extreme conditions.
ABSTRACT
A simple system for loading argon fluid at cryogenic temperatures in a Mao-Bell-type diamond anvil cell (DAC) has been developed. It is done in a two step process in which the piston-cylinder assembly alone is submerged in the cryogenic chamber for trapping the liquefied inert gas. Liquid nitrogen is used for condensing the argon gas. This system is now being efficiently used for loading liquid argon in the DAC for high pressure-high temperature experiments. The success rate of trapping liquefied argon in the sample chamber is about 75%. The performance of the gas loading system is successfully tested by carrying out direct conversion of pyrolitic graphite to diamond under high pressure-high temperature using laser heated DAC facility.
