Relation of Hydrostatic Pressure to Both Ionic Conductivity and Permittivity of Nanophase CaF2

A nanophase solid CaF2 with a clean interface and an average grain size of 14 nm was prepared by evaporating in inert gas and compacting in situ in high vacuum. Under different hydrostatic pressures ranging from 0.1 MPa to 2.20 GPa, the ionic conductivity () and permittivity () of the resulting product were measured to determine their relation to pressure. The results show: (1) The change caused by the pressure in the ionic migration channel (equal to, greater or smaller than the optimum) is the main factor that affects the peak of the ionic conductivity-pressure curve. (2) When pressure goes up from 0.66 GPa, lg drops linearly along 3 line segments, which may be ascribed to three kinds of free volumes in the nanometer-sized crystalline materials. (3) Polarization of space charge on the boundary layer is the main cause of the high relative permittivity of nanophase CaF2, which illustrates the effect of pressure on permittivity. In order to obtain a compact bulk with higher fluorine ionic conductivity, a higher pressure of 0.66 GPa is recommended in the preparation of the nanophase material.