G. Brodieand, G. Torgovnikov. EXPERIMENTAL STUDY OF MICROWAVE SLOW WAVE COMB AND CERAMIC APPLICATORS FOR SOIL TREATMENT AT FREQUENCY 2.45 GHZ
Authors: Torgovnikov Grigory , Brodie Graham 
Published: October 2019

EXPERIMENTAL STUDY OF MICROWAVE SLOW WAVE COMB AND CERAMIC APPLICATORS FOR SOIL TREATMENT AT FREQUENCY 2.45 GHZ. G. Brodie and G. Torgovnikov University of Melbourne, 4 Water St, Creswick, Victoria 3363, Australia; e-mail: grigori@unimelb.edu.au   Keywords: ceramic applicator, comb applicator, microwave, slow wave, soil microwave treatment In many cases in industry it is required to heat or treat surface layers of different material (soil, timber, concrete, plastics and so on) with microwaves (MW). Traditional MW irradiators (antennas) cannot provide heating only in the surface areas and energy penetrates deep into the material, where it decays exponentially due to normal attenuation. Therefore, energy losses, if a heating depth of 20 - 40 mm (for example to heat soil for killing weed seeds) is all that is required, are very significant. Therefore, it is required to develop special MW applicators for surface treatment to increase process efficiency. To address this problem, a slow wave (which is sometimes called a "surface wave" applicator) comb and ceramic structures, was studied. The main property of slow waves is that the energy concentration is very near impedance electrode – comb or ceramic plate surface.  Previously, slow wave structures were used mostly as delay lines and as interaction circuits in MW vacuum devices, and their properties were explored only for these specific applications. The work objectives of this study were: design slow wave, ceramic and comb structure applicators for soil treatment at frequency 2.45 GHz;experimentally study the energy distribution from slow wave applicators in the soil;study of opportunities to use slow wave structures for surface soil layer heating; andrecommendations for practical use of new slow wave applicators. Comb and ceramic slab applicators for frequency 2.45 GHz operation were designed for the soil treatment on the bases of theoretical studies and computer modelling. The comb applicator was made from aluminium and the ceramic slab applicator was made from alumina (DC=9.8, loss tangent=0.0002). A 30 kW (2.45 GHz) microwave generator was used for experiments. Containers with soil were placed on the applicator surface. An auto tuner was used in MW system to provided good impedance matching of the generator and applicators (with soil on top). This resulted in practically no power reflection. The soil “Potting Mix Hortico”, with moisture content range 32-174% and density range 590-1070 kg/m3, was used for the experiments. Energy distribution in the soil was determined by temperature measuring in the soil using thermocouples, after MW heating. Distribution of temperature measuring points covered the whole volume of the soil along and across the applicator. Results of the experiments showed that the comb applicator provides maximum energy release in soil in the central vertical plane. The ceramic alumina applicator forms two temperature maximums in two vertical planes at a distance of about 40 mm from the central applicator plane and a minimum in the applicator central plane.  The ceramic applicator provides better uniformity of energy distribution across the width of the applicator due to the two temperature maximums. It reduces overheating of the soil surface and energy losses. The depth of energy penetration provided by ceramic applicator is lower compared with the comb applicator. It means that the ceramic applicator provides better energy localization and more energy absorption in the soil surface layers compared with the comb applicator. To provide better uniformity of energy distribution across the ceramic applicator it is recommended to use ceramics with higher dielectric constants, such as in the range of 15-25, which will allow more energy to be released closer to the applicator surface. It will increase efficiency of MW energy use. The ceramic applicator is more effective for MW treatment of the soil surface areas and is recommended for practical use in machines for thermal treatment and sterilization of surface layers of the soil and other materials.