Differential volumetric analysis combined with monitoring of differential ultra-sound travel time, an approach for tracing fine-structural compression behaviour of solids: a study on MgO / periclase and polyethylene / PE up to 1.5 GPa at room temperature

The study addresses the topic of fine-structural compression behaviour of solids. MgO / periclase and polyethylene / PE are studied as exemplary materials by volumetric and ultra-sound techniques at room temperature up to 1.5 GPa. The experiments conducted with a high-pressure device of the piston cylinder type rely on: (a) measurements of the piston displacement performed under continuously changing uniaxial load in combination with (b) simultaneous monitoring of the travel time of ultra sound. Both methods base on differential technique implying that all data are of relative nature. The volume-density relation (Delta rho = M-const / Delta V) is the fundamental relation which provides information on compressional as well as on acoustic behaviour of the sample. It serves, additionally, as mutual control of the observations. The data evaluation bases on the method of difference calculation: the data obtained from a run with sample [sample + (vessel + piston assembly)] are subtracted from those of a calibration run [(vessel + piston assembly)]. In most cases the differences retrieved are small and may be presented best in form of derivatives with respect to pressure. The compression behaviour of MgO under hydrostatic conditions is reported to be quasi-linear up to about 2 GPa. Under the uniaxial load exerted by the piston cylinder apparatus MgO reveals a compression behaviour characterized by two volumetric anomalies at 400 and at 1000 MPa. The ultrasonic measurements suggest that shear plays an important role. Comparative measurements on the visco-elastic material polyethylene corroborate that interpretation.