Phase transitions of fluids confined in porous silicon: A differential calorimetry investigation

The phase transitions of non-polar organic fluids and of water, confined in the pores of porous silicon samples, were investigated by Differential Sca nning Calorimetry (DSC). Two types of PS samples (p(-) and p(+) type) with different pore size and morphology were used (with spherical pores with a r adius of about 1.5 nm and cylindrical shape with a radius of about 4 nm res pectively). The DSC results clearly show that the smaller the pores are, th e larger is the decrease in the transition temperature. Moreover, a larger hysteresis between melting and freezing is observed for p(+) type than for p(-) type samples. A critical review of the thermodynamical properties of s mall particles and confined fluids is presented and used to interpret and d iscuss our DSC results. The effects of the chemical dissolution as well as the influence of anodization time are presented, showing that thick p(+) ty pe porous silicon layers are non-homogeneous. The DSC technique which was u sed for the first time to investigate fluids confined in porous silicon, en ables us to deduce original information, such as the pore size distribution , the decrease in the freezing temperature of confined water, and the thick ness of non-freezing liquid layer at the pore wall surface.