Evanescent microwave probe study on dielectric properties of materials

Measurement of the permittivity and loss of high-loss materials using a Near-Field Scanning Microwave Microscope

“…The window material used was ultrathin glass 3 . This has a high quality surface finish 4 , and is thinner and stiffer than the PEEK window used in previously reported work [23] (which had a tendency to creep during measurements). Using a Linear Variable Differential Transformer (LVDT) system and gauge blocks, the thickness was measured as 0.031 mm with a population standard deviation of 0.001 mm (for pieces from the same batch).…”

“…Two main types of Near-Field Scanning Microwave Microscope (NSMM) [1] are described in the literature: Instruments that are based on the perturbation of the resonance of a cavity that is coupled to a probe tip [2][3] [4][5] [6] [7] [8], and instruments that are modified Atomic Force Microscopes (AFMs). AFM-based instruments [9][10] [11] [12][13] use a Vector Network Analyser (VNA) to measure reflection coefficient data at a reference plane at a tip attached to an AFM cantilever.…”

High-Resolution Detection of Rock-Forming Minerals by Permittivity Measurements with a Near-Field Scanning Microwave Microscope

“…The increase in the size of the probe and the geometry of this tip has allowed the use of Equation (6) with the complex resonance parameter (Equation (9)) to model the response of this new probe with the improved sensitivity of Q. However, the parameters minimized showed certain dependence with frequency or dielectric constant, as reported in previous works [ 48 , 49 ] for the parameters determined in the dielectric loss equation. Figure 5 shows the dependence of A with the logarithm of the dielectric constant of some reference materials (Rexolite, Acetal, PVC, Alumina, SiC, and Temex E5980).…”

“…For both configurations, the dielectric losses were determined to be related to the fr and Q shifts, as follows: where B is a constant to be minimized and tan δ = ε″/ε′. Over the years, some scientific works have tried to improve the accuracy of this model, especially concerning dielectric losses, adding new constants or quadratic terms to Equation (8) [ 48 , 49 , 50 ]. Finally, Gregory et al [ 40 ] reported the use of Equation (6) with the complex resonant frequency ( f s * in Equation (9)) to avoid the use of Equation (8).…”

“…The cavity perturbation technique has also been widely used, since the electromagnetic fields inside the sample are much smaller than the fields in the resonator. The theory developed by Gao and Xiang [ 47 ] is one of the most used approaches for quantitative microscopy of complex permittivity [ 40 , 48 , 49 , 50 ].…”