A thermosyphon-driven hydrothermal flow-through cell for in situ and time-resolved neutron diffraction studies

A flow-through cell for hydrothermal phase transformation studies by in situ and time-resolved neutron diffraction has been designed and constructed. The cell has a large internal volume of 320 ml and can operate at temperatures up to 573 K under autogenous vapor pressures (ca 8.5 × 10⁶ Pa). The fluid flow is driven by a thermosyphon, which is achieved by the proper design of temperature difference around the closed loop. The main body of the cell is made of stainless steel (316 type), but the sample compartment is constructed from non-scattering Ti-Zr alloy. The cell has been successfully commissioned on Australia's new high-intensity powder diffractometer WOMBAT at the Australian Nuclear Science and Technology Organization, using two simple phase transformation reactions from KAlSi₂O₆ (leucite) to NaAlSi₂O₆·H₂O (analcime) and then back from NaAlSi₂O₆·H₂O to KAlSi₂O₆ as examples. The demonstration proved that the cell is an excellent tool for probing hydrothermal crystallization. By collecting diffraction data every 5 min, it was clearly seen that KAlSi₂O₆ was progressively transformed to NaAlSi₂O₆·H₂O in a sodium chloride solution, and the produced NaAlSi₂O₆·H₂O was progressively transformed back to KAlSi₂O₆ in a potassium carbonate solution.