Measuring Method And Experimental Study On The Permittivity Of Complex Media

With the development of electromagnetic technology, microwave energy is widely used in various fields, e.g. chemical industry, agriculture, health, medicine, chemical engineering and etc. But the application of microwave energy is seriously handicapped by some problems, one of which is the mechanism of microwave radiation on the medias has not been fully explored. Hence further researches on the interaction between microwave energy and complex media are quite necessary.The permittivity, which indicates the macro-characteristics of media responding to microwave radiation, is a key parameter in the application of microwave energy.Complex media are with diversified geometry and composed of various components, e.g. biological tissues, chemical reaction systems. Using suitable methods to measure the permittivity of complex media is a fundamental work in the application of microwave energy. From simple media to complex media, this paper discussed the conception of the permittivity and measurement methods for the permittivity. Research works in this paper will be a help for advancing the application of microwave energy.The content and innovation in this paper can be summarized as follows:(1) A method that is able to overcome the ill-posedness existing in the short-ended coaxial line method is proposed. In the measurement process of the short-ended coaxial line method, a function composed of the reflection coefficient and the permittivity must be solved. Due to one-time measurement can not provide enough information, the function is usually ill-posed and has multi-solutions. Because the permittivity usually varies in a narrow range if the measurement frequency band is narrow, this paper proposed a new method, which calculates the curves of the reflection coefficient using the solutions of the function in a narrow frequency band, and compared calculated curves with measured one, then the solution whose reflection coefficient curve is most close to the measured curve is selected as the true solution of the function.This paper analyzes the measurement error in the open-ended coaxial line method, and validates the feasibility of employing the open-ended coaxial line method for measuring the permittivity of a sample with little volume.(2) A new type of open-slotted coaxial line sensor was designed and fabricated. Using this sensor, a new permittivity measurement method based on the Genetic Algorithm (GA) and the Finite Element method was proposed. Results of sample numerical examples validate the feasibility and precision of the proposed method. In comparison with traditional permittivity measurement method, it has many advantages, for example, it can do a measurement without the phase data and a standard calibration medium, is suitable be used in a broadband or high power permittivity measurement.(3) By experimental measuring the effective permittivity of a few organic solutions and biphase mixed solutions with different ratio of components' volume, an interesting characteristics for the biphase organic solution was discovered for the first time, i.e. when the real or imaginary part of components is close, the effective permittivity of mixed media may has a peak value with respect to the fraction of volume, where the real or imaginary part of effective permittivity of the mixed media is larger than that of any components in the mixed media. The characteristics of mixed media, which is quite different from the traditional conception, can be explained by the symmetric Bruggrman's theory.(4) Once the three-dimensional size of media is determined by the molecular model, the classical ellipsoid and spherical mixing theories can be used to forecast the effective permittivity of the mixture with respect to the fraction of volume. The comparison of the calculation and measurement results, especially their peak values, validates the applicability of the traditional mixing theories predicting the peak value of mixed solutions' permittivity at microwave frequency. One of the traditional mixing theories, the symmetrical Bruggeman's theory is able to predict the permittivity of a mixture with small molecules in a high precision. Though Maxwell Garnett formula and Coherent potencial formula are nonsymmetrical, they are able to obtain better results for predicting the permittivity of some mixtures with long molecules.(5) Chemical reactions are non-equilibrium systems which could not be simply considered as mixture of various reactants. Up to now, except for a method proposed by Kama Huang, no method can describe the variation of the effective permittivity of a chemical reaction with the reaction time. This paper investigates the liquid chemical reactions, and considers the change of the number of molecules per unit volume with temperature, then proposes an improved polynomial formula based on existing empirical formula to calculate the effective permittivity for chemical reaction with temperature and reaction time by means of the experimental results. The calculated results of effective permittivity of the reactions are good agreement with the measured results. Eventually, the effective complex permittivity of a chemical reaction at any reaction time and temperature can be obtained by employing the interpolation method, reaction kinetic equation and experiment results.