A novel inverse transmission-line method for the complex permittivity determination of arbitrary shaped materials is presented. Complex permittivity is inferred by using an inverse calculation procedure, which is based on a combined optimization strategy of both genetic algorithms and the gradient descent method. The optimization procedure matches the measured and simulated complex scattering parameters' frequency behavior of materials within a WR340 waveguide. High accuracy and practical suitability are validated through experimental tests. The dielectric properties of PTFE and epoxy resin mixed with iron-oxide-doped fiberglass have been measured for different shapes and positions. Dielectric multilayer structures have been used to demonstrate that this technique is able to measure the individual permittivity of each element of the structure. Both twoand three-dimensional approaches have been carried out and their advantages and drawbacks discussed.
11The lack of dielectric properties data has often been named as one of the 12 reasons that has hampered the simulation of microwave processing of 13 biomass feedstock and process design. In this work, the dielectric behavior 14 of an organic fraction from municipal solid wastes during pyrolysis has been 15 monitored as a function of temperature. Furthermore, the effect of the 16 addition of a microwave absorbent material (carbonaceous char) to the raw 17 biowaste upon the dielectric properties has been investigated for the first 18 time.
19The efficiency of the conversion of microwave energy to heat, measured by 20 means of the tan δ parameter, is shown in this study to be nearly 20 times 21 higher when the absorbent char is added to the reaction bulk at room 22 temperature and this gap is even greater in the 600 -800 ºC range. 23Nevertheless, the results suggest that the addition of increasing amounts of 24 microwave absorbent (up to ca. 40%) impairs microwave penetration, which 25gives rise to a less homogeneous heating of the bulk. There is therefore an 26 optimum proportion that balances heat conversion and penetration depth.
27The results of this study lend support to the use of char as a means to 28 induce thermochemical treatments by microwaves and reduce energy 29 consumption in the process. 30 31
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