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 Scanning Calorimetry (DSC). Two types of PS samples (p(-) and p(+) type) with different pore size and morphology were used (with spherical pores with a radius of about 1.5 nm and cylindrical shape with a radius of about 4 nm respectively). The DSC results clearly show that the smaller the pores are, the 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 small particles and confined fluids is presented and used to interpret and discuss our DSC results. The effects of the chemical dissolution as well as the influence of anodization time are presented, showing that thick p(+) type porous silicon layers are non-homogeneous. The DSC technique which was used for the first time to investigate fluids confined in porous silicon, enables us to deduce original information, such as the pore size distribution, the decrease in the freezing temperature of confined water, and the thickness of non-freezing liquid layer at the pore wall surface.