Pressure inside the chamber was reduced by a vacuum pump (Emmecom, Model AL50M50, Italy) connected to a 1.5-kW motor,stabilized by a buffer pressure tank and controlled by a pressure control system. The sample temperature was measured with a precision of ±0.8◦C by a fibre optic sensor (Omega FOBS-2) and recorded on a computer via a fibre optic thermometer(Omega, model FOB104, Omega Engineering, Canada).