Change in Diorite Microwave Dielectric Property at the Free End Upon Compressive Stress Application to the Other End

The application of compressive stress on minerals and rocks can reduce dielectric constant (
$\varepsilon '$
) at microwave frequency, which is of considerable significance to geophysics, mineralogy, petrology, and microwave remote sensing. However, the stress near the surface (e.g., Earth surface) is generally on the order of several MPa only, and its increment is supposed to be excessively slight to influence rock
$\varepsilon '$
. The cubic- and conical-shaped diorite specimens were specially produced in this study to investigate experimentally the variations of rock
$\varepsilon '$
and tan
$\boldsymbol {\delta }$
at the stress-free end as the other end is subjected axially to compressive stress by using an open coaxial resonator probe working at 2.0–18.3 GHz. Surprisingly, the rock
$\varepsilon '$
at the free end decreased by approximately −13% until loading to fracturing, and rock tan
$\boldsymbol {\delta }$
demonstrated frequency-dependent variations. This particular phenomenon is supposed to be induced by stress-activated positive holes (h•) that are produced inside the loaded rock volume bearing with mineral peroxy defects, flown out to slightly stressed regions, and, finally, accumulated beneath the free rock surface. Consequently, these phenomena lead to the decrease in the electronic and ionic polarizability and the increase in the local field around dipoles, eventually changing the rock
$\varepsilon '$
and tan
$\boldsymbol {\delta }$
at the stress-free end. This study implies that the microwave dielectric property of the surface rock might decrease accordingly with deep stress enhancement, which is an important factor to be considered in the application of radar investigation and microwave remote sensing.