Mechanism of piezoelectric effect on dielectric constant change of quartz minerals under pressure

Stressing-induced dielectric response of quartz is much important for understanding the basic dielectric property of quartz-bearing rocks. Previous studies have illustrated the correlation between quartz dielectric variations and additional compressive stress; however, the physical mechanism governing this process is still unclear. In this study, an open coaxial resonant microwave testing system was utilized to measure the dielectric constants (2 GHz) of quartz specimen, which is composed entirely of pure quartz minerals, during uniaxial cyclic loading processes. The experimental results demonstrate two different types of features: (1) the dielectric constants of quartz minerals were inversely proportional to the stress variations, namely decreased with the stress increasing and increased with the stress decreasing; (2) the dielectric constants exhibited a sustained increasing trend throughout the stressing processes, including both loading and unloading processes. Based on the theories of piezoelectric effect, for the quartz mineral crystals that were subject to the additional compressive stress, the piezoelectric effect would induce the additional electric field for some crystals. For a given quartz mineral or crystal, the direction of such additional electric field was definitely determined by the orientations of its mechanical axis or the electric axis. Thus, the local electric field intensity was enhanced or weakened by such additional electric field correspondingly, with which, based on the theory of dielectric physics, the dielectric constant of the quartz mineral or crystal was changed accordingly. These findings provide a mechanistic basis for interpreting quartz dielectric changes under additional stress, which is of great significance in exploring the petroleum and geological, and in under-standing the precursors of rock engineering failure and geohazards occurrence.