GHz Microwave Applicator Research Articles

Uniform microwave heating system design and evaluation with an orthogonally slot‐loaded array waveguide

AbstractIn this article, we propose a 1‐kW 2.45 GHz microwave applicator with an orthogonally slot‐loaded array waveguide to improve the heating uniformity. In the conventional microwave applicator with rectangular waveguides, the heating uniformity is low due to various hot and cold spots. Using the proposed slot‐loaded array waveguide having horizontal and vertical slots, the proposed microwave applicator realizes a uniform temperature variation in the heating material and achieves an improved uniformity of up to 77.4% at 2.45 GHz compared to the conventional microwave applicator.

Microwave digestion of gibbsite and bauxite in sodium hydroxide

It was hypothesized that bauxite digestion may be improved by using microwave heating as it has been shown in literature that some material processes have improved efficiency. To test this hypothesis, a set of digestion experiments were conducted using gibbsite, one of the major minerals in bauxite. Gibbsite was digested at various temperatures (50, 75, and 95 °C) in either 1 M or 6 M sodium hydroxide solutions for 30 min using either a convection oven or a 2.45 GHz microwave applicator. Results show that microwave heating provided an increase of 5–7% in the digestion after 30 min and required around 1/10th the time to heat the solutions compared to conventional heating. Electromagnetic simulations show that preferential heating occurs at the solution surface creating a temperature gradient within the solution. Although vigorous stirring of the solution was used to minimize the temperature gradient, it could still be responsible for the observed difference in digestion. Digestion of bauxite itself yielded similar results to the gibbsite.

Microwave ablation of ex vivo human liver and colorectal liver metastases with a novel 14.5 GHz generator

Purpose: This study assessed the relationship between time, power and ablation size using a novel high-frequency 14.5 GHz microwave applicator in ex vivo human hepatic parenchyma and colorectal liver metastases. Previous examination has demonstrated structurally normal but non-viable cells within the ablation zone. This study aimed to further investigate how ablation affects these cells, and to confirm non-viability.Materials and methods: Ablations were performed in ex vivo human hepatic parenchyma and tumour for a variety of time (10–180 s) and power (10–50 W) settings. Histological examination was performed to assess cellular anatomy, whilst enzyme histochemistry was used to confirm cellular non-viability. Transmission electron microscopy was used to investigate the subcellular structural effects of ablation within these fixed cells. Preliminary proteomic analysis was also performed to explore the mechanism of microwave cell death.Results: Increasing time and power settings led to a predictable and reproducible increase in size of ablation. At 50 W and 180 s application, a maximum ablation diameter of 38.8 mm (±1.3) was produced. Ablations were produced rapidly, and at all time and power settings ablations remained spherical (longest:shortest diameter <1.2). Routine histological analysis using haematoxylin-eosin (H&E) confirmed well preserved cellular anatomy despite ablation. Transmission electron microscopy demonstrated marked subcellular damage. Enzyme histochemistry showed complete absence of viability in ablated tissue.Conclusions: Large spherical ablation zones can be rapidly and reproducibly achieved in ex vivo human hepatic parenchyma and colorectal liver metastases using a 14.5 GHz microwave generator. Despite well preserved cellular appearance, ablated tissue is non-viable.

Microwave ablation of ex vivo human liver and colorectal liver metastases with a novel 14.5 GHz generator

This study assessed the relationship between time, power and ablation size using a novel high-frequency 14.5 GHz microwave applicator in ex vivo human hepatic parenchyma and colorectal liver metastases. Previous examination has demonstrated structurally normal but non-viable cells within the ablation zone. This study aimed to further investigate how ablation affects these cells, and to confirm non-viability.

Ni-Nb-Sn Bulk Metallic Glass Matrix Composites Fabricated by Microwave-Induced Sintering Process

Using a gas-atomized Ni59.35Nb34.45Sn6.2 metallic glassy alloy powder blended with Sn powder of various contents, Ni-Nb-Sn bulk metallic glassy matrix composites were fabricated by a microwave (MW)–induced sintering process in a single-mode 2.45 GHz MW applicator in a separated magnetic field. The Ni59.35Nb34.45Sn6.2 glassy alloy powder and its mixed powders containing Sn particles could be heated well in the magnetic field. The addition of Sn particles promoted densification of the sintered Ni59.35Nb34.45Sn6.2 metallic glassy powder. Bulk samples without crystallization of the glassy matrix and with good bonding state among the particles were achieved at a sintering temperature of 833 K.

Microwave-induced sintering of NiNbTiPt metallic glass blended with Sn powders using a single-mode applicator

Microwave (MW) processing is emerging as an innovative and highly effective material processing method offering many advantages over conventional methods especially for sintering application. In the present study, using a mixed powder of the Ni55Nb25Ti15Pt5metallic glassy powder prepared by an argon gas atomization process blended with 50 vol.% Sn powder, the MW-induced heating and sintering were carried out by a single-mode 2.45 GHz MW applicator in a magnetic field maximum. The structure and thermal stability of the sintered glassy composite specimens were investigated. The addition of Sn particles enhanced densification of the sintered Ni-based metallic glassy alloy powders.

Heating of metallic powders by microwaves: Experiment and theory

It is known that bulk metallic samples reflect microwaves while powdered samples can absorb such radiation and be heated efficiently. In the present work we studied heating mechanisms of metallic powders in a multimode 2.45 GHz microwave applicator. The present paper shows direct evidence of penetration of a layer of metallic powder by microwave radiation and provides theoretical explanation of such behavior.The most effectively heated powder is Fe because both eddy current loss (in alternating H-field) and magnetic reversal loss (in alternating E-field) mechanisms act in case of such metal. Diamagnetic metals Sn and Cu are heated better than paramagnetic Ti while Au is also only slightly heated. Cu- and Ni-based metallic glassy powders are also moderately heated. Weak heating of Au powder (which is a noble metal) can be explained by the absence on the particles of the oxide layer (shell), which allows eddy currents flowing through larger area compared to other metals, and reflection mechanism works much better in such case.

Fabrication of Ni-Nb-Sn Metallic Glassy Alloy Powder and its Microwave-Induced Sintering Behavior

In the present study, we prepared Ni59.35Nb34.45Sn6.2 metallic glassy alloy powder by an argon gas atomization process. Microwave (MW)-induced heating and sintering was carried out by a singlemode 2.45 GHz MW applicator in the separated magnetic field or electric field using the obtained glassy powders. The structure and thermal stability of the sintered glassy alloy specimens were investigated.

Evidence for a Specific Microwave Radiation Effect on the Green Fluorescent Protein

We have compared the effect of microwave irradiation and of conventional heating on the fluorescence of solution-based green fluorescent protein. A specialized near-field 8.5 GHz microwave applicator operating at 250 mW input microwave power was used. The solution temperature, the intensity, and the spectrum of the green fluorescent protein fluorescence 1), under microwave irradiation and 2), under conventional heating, were measured. In both cases the fluorescence intensity decreases and the spectrum becomes red-shifted. Although the microwave irradiation heats the solution, the microwave-induced changes in fluorescence cannot be explained by heating alone. Several possible scenarios are discussed.

Role of Specimen Insulation on Densification and Transformation during Microwave Sintering of Silicon Nitride.

Silicon nitride ceramics, with Y2O3, Al2O3 and MgO as sintering aids, have been sintered in a 28GHz microwave applicator using a number of sample insulation techniques. The sintering characteristics in terms of densification, α→β transformation, microstructural development and power requirement were studied and compared to identical samples sintered conventionally. All of the microwave sintered materials could be sintered to near theoretical density and a full α→β transformation obtained at temperatures around 200°C lower than the materials sintered conventionally. In addition, the microstructural development showed important differences, with the selective development of elongated β-grains being observed from the very early stage of transformation. Samples sintered using a powder bed insulation technique, achieved full α→β transformation at the lowest temperature, but a tendency to thermal runaway using this method of insulation meant the samples were susceptible to cracking. Samples sintered using silicon carbide plates as a low temperature microwave absorber reached full density and onset of transformation earlier than the other materials. In addition, the maximum power requirement for these samples was around 1/3 of that required for other insulation techniques, and more uniform heating meant that these samples were free from cracking.

Temperature Gradients and Residual Porosity in Microwave Sintered Zinc Oxide

AbstractZnO samples were sintered in an overmoded 2.45 GHz microwave applicator. In-situ differential temperature measurements were made to allow comparison of surface and core temperatures during heating. At intermediate temperatures, near 600°C, the sample core was measured to be more than 250°C hotter than the sample surface. As the core temperature approached 1100°C, however, the difference between the surface and core temperatures diminished. Post-sintering scanning electron microscopy (SEM) showed spatial variations in the residual porosity which were consistent with the measured temperature differential. For samples sintered to intermediate temperatures, where large temperature differences persisted, there were significant gradients in the residual porosity. For samples sintered to higher temperatures, there was little residual porosity and no observable porosity gradient. Local density versus temperature behavior was obtained by correlating porosity levels measured from the micrographs with temperature measurements made during sintering. These data demonstrate a significantly lower activation energy for microwave sintering than for conventional sintering.