Study On Microwave And Millimeter Wave Nondestructive Testing Of Dielectric Materials

With the increasing development of industrial technology in recent years,there is a growing demand for NDT techniques.Based on the properties of microwave millimeter waves,the study of NDT of dielectric materials using microwave millimeter waves has become an important research area to describe the properties of materials.For example,for a dielectric material,there will be many types of defects such as porosity,surface defects,embedded structures,etc.Designing and developing probes that can nondestructively detect these defects and characterize them at subwavelength sizes is a current research hotspot.In addition,the technique can be used to characterize the dielectric constant of dielectric materials and enable accurate measurement of dielectric constant under non-damaging conditions of dielectric materials.This thesis focuses on the use of the near-field principle of microwave millimetre waves for the non-destructive testing of dielectric materials.As near-field microwaves can penetrate dielectric materials,they can reflect the internal properties of the sample and quantitatively measure the dielectric properties of the sample,such as the dielectric constant,without touching the sample during detection.In this thesis,we design a mi-crowave millimetre-wave near-field probe for non-destructive testing based on the above principles,and use HFSS simulation software to simulate the electromagnetic parameters of the probe,such as resonant frequency,phase,etc.At the same time,we consider the properties of dielectric materials,and improve and optimise the designed near-field probe.Based on the above principles,this thesis investigates the use of microwave near-field probes based on open resonant ring structures excited by microstrip transmission lines for measuring the properties of dielectric materials,and designs two microwave probe struc-tures based on square split resonant rings and rectangular split resonant rings,respectively.Both microwave probes are designed with a band-resistive design,offering a higher quality factor and higher resolution at high frequencies,and the split resonant ring is excited by a microstrip transmission line with a small size and low sample shape requirement.Based on the theory,this thesis analyses the performance of microwave near-field probes based on the resonance characteristics of the frequency shift,amplitude and phase of the S parame-ter in detail.After an in-depth analysis of the theory and combined with electromagnetic software simulations,two microwave probes are designed and processed,and the results of the final experimental measurements show that the microwave near-field probes based on the split resonant ring structure are highly sensitive and can measure the dielectric constant of the sample and perform non-destructive detection of defects.In addition,a preliminary study of millimetre wave near-field probes based on open waveguide structures is carried out in this thesis,and simulations and experimental measurements are carried out.