Microwave Processing of Materials

Rigorous design of industrial microwave processing
systems requires in-depth knowledge of the
dielectric properties of the materials to be processed. These
values are not easy to measure, particularly when a material
is multi-layered containing multiple phases, when one
phase has a much higher loss than the other and the
application is based on selective heating. This paper
demonstrates the ability of the Clausius–Mossotti (CM)
model to predict the dielectric constant of multi-layered
materials. Furthermore, mixing rules and graphical
extrapolation techniques were used to further evidence our
conclusions and to estimate the loss factor. The material
used for this study was vermiculite, a layered aluminasilicate
mineral containing up to 10 % of an interlayer
hydrated phase. It was measured at different bulk densities
at two distinct microwave frequencies, namely 934 and
2143 MHz. The CM model, based on the ionic polarisability
of the bulk material, gives only a prediction of the
dielectric constant for experimental data with a deviation of
\5 % at microwave frequencies. The complex refractive
index model, Landau, Lifshitz and Looyenga, Goldschmidt,
Bo¨ttcher and Bruggeman–Hanai model equations
are then shown to give a strong estimation of both
dielectric constant and loss factor of the solid material
compared to that of the measured powder with a deviation
of\1 %. Results obtained from this work provide a basis
for the design of further electromagnetic processing
systems for multi-layered materials consisting of both high
loss and low loss components.

Titanium dioxide (TiO2) nanostructures (nanorods and nanoparticles) were prepared using a low-cost microwave irradiation method from a polyol medium of glycerol. Titanium glycerolate and TiO2 powders were obtained in the glycerol medium for the first time with four different power densities (240 W, 480 W, 720 W, 960 W) of irradiation using a domestic microwave oven of 2.45 GHz, to understand the impact of power on morphology tuning. The structural and morphological features of the titanium glycerolate and TiO2 powders were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and Raman spectra analysis. The TiO2 was successfully used in the fabrication of photovoltaic devices and as a proof-of-concept binder free paste was prepared and successfully employed for photo-anode using screen printing on the fluorine-doped tin oxide substrate.

In the present work, polycrystalline ferromagnetic ZIlo.9SCoo.osO samples have been synthesized in a TM oll single mode cylindrical microwave resonant cavity operating at 2.45 GHz. The synthesized specimen is analyzed for its crystallographic and electronic structure along with magnetic and electron transport properties. The X-ray diffractogram and photoelectron spectroscopy of ZIlo.9SCoo.osO establish the incorporation of cobalt atoms into ZnO lattice. The room temperature magnetization study confirms the ferromagnetic ordering in the microwave processed highly resistive samples; which can be attributed to bound magnetic polarons model.

The low energy nuclear transmutation reactions were investigated within a compact flourescent lamp (CFL), which was irradiated by intense microwave energy. A modified microwave owen operated as a source of intense electromagnetic field. Solids like glasses or CFLs absorbed efficiently microwave energy (E+H field both) resulting arcing, ionization and plasma state. We observed and measured possible elemental transmutation of the P to Si and S in CFL-s during intense microwave (MW) irradiation. Since this phenomenon seems to be very complex, therefore it requires multidisciplinary approaches.

In the present work, Si 0.95 Ge 0.05 alloy nanowires were synthesised in a single-mode resonant cavity operated in H 0 1 1 mode in ambient at 2.45 GHz in a short duration (5 min) and at a temperature (900 • C) lower than the equilibrium alloy-phase formation temperature. The photoluminescence (PL) studies were carried out at room temperature. The intense PL peaks, in the energy range of 2.9-3.6 eV, can be ascribed to radiative recombination of the generated carriers. The low temperature (100-250 K) dc electrical-transport is investigated; and the activation energy is estimated to be lower by an order than that of polycrystalline samples. The lower activation energy is possibly due to electronic/structural defects introduced by the non-equilibrium growth process and high aspect ratio of the grown nanostructure. With these improved characteristics, the alloy nanowires can be exploited for device fabrication.

Graphene has recently attracted great interest among the researcher in the field of material science and nanotechnology. Some of unique properties of graphene with new device concepts and nanotechnology can overcome some of the main limitations of traditional radio frequency electronics in terms of maximum frequency, linearity, and power dissipation. However, graphene devices may not be ideal for all electronics, but may have advantages over silicon in many applications. Numerous experts already see it as the successor of silicon. Many groups around the different parts of the world are fabricating graphene transistors, and graphene MOSFETs with record cutoff frequencies 300 GHz have been reported. In this paper, we highlight the recent research done on graphene. We also attempt to discuss the possible use of graphene in future electronics applications.

In the present work, polycrystalline samples of ZnO doped with cobalt have been prepared in a H 011 single mode cylindrical microwave (MW) resonant cavity at a temperature of nearly 1000 o C in 40 minutes. The x-ray diffractograms of the microwave processed samples establish the incorporation of cobalt in ZnO lattice in the MW processed samples. Room temperature magnetic (M-H) measurements of the starting mixture and Co-substituted samples indicate that the nature of magnetic interaction changes on incorporation of Co into ZnO lattice.

The factors affecting the microwave bleaching of sunflower oil and the interaction between them were investigated and optimized by response surface methodology using a three-factor five-level central composite rotatable design. Microwave power, time and the amount of bleaching clay were selected as independent variables studied in the range of 70-120 W, 2-15 min, and 0.01-0.5%. The dependent variables that measure the bleaching efficiency and oil quality were evaluated as hue angle, chroma and totox value. Optimization was carried out by minimizing totox and chroma and maximizing hue angle. Hue angle, chroma and totox were found as 96.91, 37.66 and 23.31 under optimal conditions. Optimal microwave bleaching was successfully performed by using less bleaching clay (0.4%) and a shorter time (8 min) compared to the current industrial application without any adverse effect on oil quality. Hence, microwave bleaching is thought to be an alternative method for the bleaching of edible oils.

Polycrystalline Si 0.9 Ge 0.1 alloy has been synthesized by microwave processing in a H 011 single mode cylindrical resonant cavity (2.45 GHz, 450 W) at 9508 8 8 8 8C in about 4 minutes. The X-ray diffractograms of mixture recorded before and after the exposure to microwaves confirm the formation of Si-Ge alloy. IR absorption spectrum also confirms Si-Ge alloy formation along with the presence of silicon and germanium oxide in the sample. Band gap estimated from the electrical conductivity versus temperature measurement comes 1.08 eV. With an increase in heating time from 4 to 12 minutes, the grain size marginally increases from 94 to 104 nm.

In the present work, polycrystalline ferromagnetic ZIlo.9SCoo.osO samples have been synthesized in a TM oll single mode cylindrical microwave resonant cavity operating at 2.45 GHz. The synthesized specimen is analyzed for its crystallographic and electronic structure along with magnetic and electron transport properties. The X-ray diffractogram and photoelectron spectroscopy of ZIlo.9SCoo.osO establish the incorporation of cobalt atoms into ZnO lattice. The room temperature magnetization study confirms the ferromagnetic ordering in the microwave processed highly resistive samples; which can be attributed to bound magnetic polarons model.

Titanium dioxide (TiO 2) nanostructures (nanorods and nanoparticles) were prepared using a low-cost microwave irradiation method from a polyol medium of glycerol. Titanium glycerolate and TiO 2 powders were obtained in the glycerol medium for the first time with four different power densities (240W, 480W, 720W, 960W) of irradiation using a domestic microwave oven of 2.45 GHz, to understand the impact of power on morphology tuning. The structural and morphological features of the titanium glycerolate and TiO 2 powders were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and Raman spectra analysis. The TiO 2 was successfully used in the fabrication of photovoltaic devices and as a proof-of-concept binder free paste was prepared and successfully employed for photo-anode using screen printing on the fluorinedoped tin oxide substrate.

The paper reports the synthesis, structural and high frequency dielectric properties of Ba(Zr x Ti 1-x ) O 3, BZT, nanopowders where x = 0, 0.1, 0.2, 0.3. These powders were synthesized using both microwave assisted and conventional heating, with the former requiring lower temperature and shorter times compared to the latter, viz., 700°C for 30 min versus 900°C for 5 h. The synthesized nanopowders were characterized using X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy, BET surface area analysis, differential scanning calorimetry and high frequency dielectric measurements. All the microwave synthesized BZT compositions were found to have well crystallized, finer nanoparticles with less agglomeration and higher dielectric permittivity compared to the conventionally prepared powders. The rapidity and less demanding processing conditions associated with the microwave assisted method augers well for the general applicability of the technique for the production of nanocrystalline powders.

The present paper describes our results on the phase transformation (from crystalline to amorphous) of Mtype hard barium hexaferrite, and consequently its conversion into soft ferrite by the application of magnetic energy at microwave frequency (2.45 GHz). The rapid volumetric heating of the conventionally prepared polycrystalline hexaferrite by microwave energy at a temperature of 950°C in a short duration of only 5 min resulted in a decrystallized phase. The magnetic studies (M-H loops) of the resistively heated and MW processed samples have revealed a lowering in coercivity (from 188 to 2 mT) in the latter samples, thereby establishing the formation of soft ferrite. An attempt has also been made to understand the observed transformation from hard to soft ferrite.