Measurement of dielectric properties of liquid polymers with their temperature dependence at 2.45 GHz

In recent years, there is lot of interest to employ microwave heating for various material processing applications. However, in order to obtain efficient heating using microwave energy for different applications, the knowledge of dielectric parameters of the test specimen is quite important. The penetration of microwaves inside the material under test, and its selective heating compared to surroundings are quite strongly dependent upon its dielectric properties. For measuring the microwave dielectric properties of different slabs, we have recently proposed the waveguide based two-step method. The basic idea behind this method was to combine an analytical approach with a numerical optimization algorithm in order to obtain stable reconstructions of the dielectric parameters at 2.45 GHz. In this paper, our aim is to propose a similar kind of two-step approach for measuring the temperature dependent dielectric properties of different kinds of liquids commonly used in the polymer industry for various applications. However, there are certain problems in using our earlier proposed conventional waveguide approach for characterizing liquids. The problem mainly arises from the fact that most of these polymer liquids are highly viscous, and hence we have to employ an extra piece of liquid container or the standard glass tube so that the waveguide sample holder is not contaminated and the whole process remains nondestructive. But the inclusion of this glass tube in the experimental setup introduces a partially filled multi-layered structure, and hence the analytical expressions derived earlier for the completely filled waveguide arrangement can not be used any more. Here, we have made certain approximations in order to derive simple analytical relationships, which provide approximate values of the dielectric parameters of the liquid specimen in a multi- layered partially filled waveguide arrangement. These approximate values are then used as the initial guess in a numerical opt- - imization approach, which is employed to reconstruct more accurate values of the dielectric parameters of liquid specimens. For measuring the temperature- dependent dielectric parameters of the liquid under test, the glass tube filled with the liquid specimen is heated externally and is accordingly placed inside the waveguide sample holder at regular intervals. The initial guess of the dielectric parameters of the liquid specimen at the higher temperature is obtained from its value computed at the previous lower temperature.