Microwave-induced Temperature Research Articles

Computational characterisation of microwave heating of fibre preforms for CVI of SiCf/SiC composites

Silicon carbide fibre reinforced silicon carbide matrix composites (SiCf/SiC) are known as materials with high-performance mechanical properties for the aerospace industry. Microwave-enhanced (ME) chemical vapour infiltration (CVI) heating of ceramic matrix composites is potentially an energy efficient production technique capable of yielding near fully dense SiCf/SiC composites in a much shorter time span. This paper reports on the output of computational analysis of electromagnetic (EM) and thermal characteristics of the ME CVI process occurring with thin circular SiC fibre preform in a Labotron microwave system from SAIREM. Computer simulation is performed with the use of the finite-difference time-domain technique implemented in QuickWave computational environment. Multiple puzzling phenomena observed in the earlier experimental work are illuminated in the present study and the causes for the formation of microwave-induced temperature fields are clarified. With the use of the developed EM model, resonant and non-resonant frequencies of the Labotron system for different temperatures of the processed samples are analysed to explain the differences and variability in heating rates. This showed that when microwave processing of small SiC samples, energy coupling is extremely sensitive to frequency: a change of the reflection coefficient from 0.05 (absorbing) to 0.75 (reflective) could be made by a drift as small as 0.003–0.005 GHz, respectively, indicating the importance of scaling the microwave cavity to the sample size and the ability to precisely control the frequency of the microwave source. Moreover, energy coupling is temperature-dependent: low reflections produces very high heating rates (greater than 550 ℃ min-1); the opposite is true for high reflections where heating rates are significantly slower. Temperature fields in the SiC fibre preforms are computed with the coupled EM-thermal model at different frequencies. It is shown that while being highly non-uniform in the beginning of the process, temperature patterns evolve to being fairly homogeneous by its end. Overall, the results suggest a means for better control of the equipment to pave the way to more efficient, controlled, and repeatable implementations of the ME CVI technology to produce high quality SiCf/SiC composites.

Effects of microwave combined with ultrasound treatment on the pasteurization and nutritional properties of bottle gourd (Lagenaria siceraria) juice

This present study revealed the effectiveness of a combined microwave-ultrasonic pasteurization system on Listeria monocytogenes in bottle gourd juice (BGJ). Response surface methodology based on the central composite design was employed for optimization with microwave power (W), microwave-induced temperature (MT) (°C), ultrasound amplitude (UA) (%), and ultrasound exposure time (UT) (min) as the independent factors. Microbial cell viability, total phenolic content, total terpenoid content, and antioxidant activity were considered as the dependent factors. MT, UA percentage as well as ultrasonic exposure time were found to be most effective in reducing L. monocytogenes count. The ideal condition was 70°C temperature; 750 W microwave yield power, 80% amplitude, and 15 min of UT in which about 5 log decrease of L. monocytogenes count was observed. Furthermore, microwave and ultrasound processed juice showed superior antioxidant activity, vitamin content, storage activity, and physicochemical parameters as compared to conventionally processed juice. Practical applications The combined treatment of microwave and ultrasound turned out to be a very effective technique for pasteurization of BGJ. The technique was efficient enough to reduce heat-resistant bacteria such as Listeria monocytogenes. The technique can be further scaled up for pilot plant followed by industrial use as it proved to be better than the conventional methods of pasteurization. The functional parameters of the juice were very much retained in the sample pasteurized by the combined technique.

Uniform, Scalable, High-Temperature Microwave Shock for Nanoparticle Synthesis through Defect Engineering

Summary Here we demonstrate a thermal shock synthesis method triggered by microwave irradiation for the rapid synthesis of nanoparticles on reduced graphene oxide (RGO) substrate. With properly controlled reduction, RGO has high electrical conductivity while maintaining functional groups, leading to an extremely efficient microwave absorption of ∼70%. The high utilization of microwaves results in the ability to raise the temperature to 1,600 K in just 100 ms, which is followed by rapid quenching to room temperature. The defects on the RGO are crucial for achieving this record-high microwave-induced temperature as these defects play a fundamental role in absorbing the radiation as well as the self-quenching mechanism. By loading precursors onto RGO, we can utilize rapid temperature change to synthesize nanoparticles. The nanoparticles are ∼10 nm with uniform distribution. This facile, rapid, and universal synthesis technique has the potential to be employed in large-scale production of nanomaterials and suggests a new direction for nanosynthesis.

Target visualisation and microwave hyperthermia monitoring using nanoparticle-enhanced transmission ultrasound (NETUS)

Purpose: The aim of this study was to examine the feasibility of using nanoparticle-enhanced transmission ultrasound (NETUS) as an image-based monitoring modality for microwave hyperthermia treatment.Methods: A dedicated transmission ultrasound imaging system was used to obtain acoustic projections and ultrasound computed tomography images. Initially, speed-of-sound based images were used to non-invasively monitor temperature changes in in vitro and ex vivo specimens, induced by a microwave needle-type applicator. Next, the hyperthermia acceleration ability of two ultrasound nanoparticles based contrast agents (iron oxide and copper oxide) was examined and visualised. Finally, a two-step image guided microwave therapeutic procedure using NETUS was investigated in a realistic breast mimicking phantom. First, the pathology simulating region borders were detected. Then, a microwave-induced temperature elevation was non-invasively monitored.Results: The transmission ultrasound scanning system was able to detect temperature changes with a resolution of less than 0.5 °C, both in vitro and ex vivo. In accordance with previous studies, it was visually demonstrated that iron oxide nanoparticles expedite the heating process (p < 0.05). Copper oxide nanoparticles, however, did not alter the hyperthermia profile significantly. In the breast mimicking phantom, NETUS yielded accurate detection of the target region as well as thermal monitoring of the microwave heating procedure.Conclusions: NETUS can combine enhanced target visualisation with non-invasive thermometry and accelerated heating effect. Quantitative feedback, however, requires a tissue-specific calibration-curve. A proof of concept for microwave hyperthermia treatment monitoring using NETUS was established. The suggested methodology may potentially provide a non-invasive cost-effective means for monitoring thermal treatment of the breast.

Computational Dosimetry of the Human Head Exposed to Near-Field Microwaves Using Measured Blood Flow

Computational human thermal modeling studies have suffered from lack of reliable data regarding tissue blood flow. In this study, we report the measured results of blood flow on the surface of the head at multiple sites in human subjects (12 healthy young male subjects, 22-24 years), using a laser Doppler blood-flow method. We then incorporate these measured blood flow in a computational bioheat thermal model in anatomical human head models and investigate the effects of individual variations in blood flow on a microwave-induced temperature elevation in the frequency range from 1 to 12 GHz. The measurements show variations in blood flow at different sites and depths from the skin surface. However, computational bioheat modeling indicates that the typical variability of peak temperature elevation in the head due to individual and regional variations in the blood flow is less than ±15%. The presented data will aid in the development of more accurate and consistent human thermal models and strengthen the thermophysiological rationale of guidelines and standards for human radio-frequency near-field exposure.

Metal-assisted and microwave-accelerated evaporative crystallization: an approach to rapid crystallization of biomolecules

Our laboratory has recently introduced and demonstrated the use of the metal-assisted and microwave-accelerated evaporative crystallization (MA-MAEC) technique for rapid crystallization of biomolecules (e.g., amino acids, peptides and proteins). The MA-MAEC technique utilizes metal nanoparticles (silver, gold, copper, nickel, iron oxide, indium tin oxide), which are deposited on to iCrystal plates or glass surfaces to serve as selective nucleation sites and microwave-transparent medium to generate microwave-induced temperature gradients. In this highlight article, we will chronicle the effectiveness of the MA-MAEC technique as a rapid, efficient and easy to use technique for crystallization of biomolecules in potential applications in the pharmaceutical industry.

Analysis of the combinative effect of ultrasound and microwave power on Saccharomyces cerevisiae in orange juice processing

Abstract High temperature in conventional method for juice pasteurization causes adverse effects on nutrients and nutritional value of food. The objective of this study was to examine the effect of microwave output power, temperature, ultrasound power, and ultrasonic exposure time on Saccharomyces cerevisiae in orange juice. Based on our findings, microwave output power, ultrasound power, ultrasonic exposure time orange juice temperature were the most effective factors to reduce S. cerevisiae . The results showed that the quadratic model included was the best model for account. The model showed that regarding decrease of S. cerevisiae account microwave-induced temperature was more effective than microwave output power. Also, compared to microwave power, the ultrasound power was more effective on S. cerevisiae reduction. The optimum processing condition was 350 W microwave power, 35 °C temperature, 778.2 W ultrasonic power, and 11 min of exposure. Based on our result, the consumption energy was 142.77 J/mL with no remaining of S. cerevisiae . The results showed that the given scores by panelists to the combinative and conventional methods for color and flavor indices were significant ( P Industrial Relevance In order to reduce the adverse effects (loss of vitamins, flavor, and non-enzymatic browning) of the thermal pasteurization method, other methods capable of inactivation of microorganisms can be applied. In doing so, non-thermal methods are of interest, including pasteurization using high hydrostatic pressure processing (HPP), electric fields, and ultrasound waves. The ultrasound technology has been the main focus of studies in recent years. However, the main challenge facing the non-thermal technologies in food processing is the inactivation of pathogenic microorganisms and food spoilage agents, which can be achieved by various methods. The aim of present research was examined simultaneous effect of ultrasonic and microwave to remove microorganism. This research introduces new, innovative, and combined method for fruit juice pasteurization, and this method can benefit the food industry.

Microwave-induced temperature fields in cylindrical samples of graphite powder – Experimental and modeling studies

Microwave heating of graphite powder is the key process of a number of promising applications and emerging technologies ranging from kiln crucibles to synthesis of graphene. However, this process is typically studied in terms of structural parameters of the resulting products, whereas the prime electromagnetic and associated temperature characteristics remain insufficiently studied. In this paper, we investigate microwave heating of graphite powder in a batch (non-resonant) and waveguide (resonant) reactors using multiphysics (electromagnetic–thermal) modeling and experimentation. Temperature-dependent material parameters of graphite power are determined from measurements and appropriate physical models. Non-uniform temperature fields and their trend for relatively quick homogenization are shown to be conditioned by high values of the loss factor and thermal conductivity. These parameters are also found to be responsible for the effect of post-microwave heating. It is demonstrated that processing of graphite power in a resonant reactor may be convenient for controlling the heating rate and the level of temperature uniformity.

Microwave propagation and absorption and its thermo-mechanical consequences in heterogeneous rocks

A numerical analysis in a two-component model rock is presented including the propagation and absorption of a microwave beam as well as the microwave-induced temperature and stress distributions in a consistent way. The analyses are two-dimensional and consider absorbing inclusions (discs) in a non-absorbing matrix representing the model of a heterogeneous rock. The microwave analysis (finite difference time domain — FDTD) is performed with values of the dielectric permittivity typical for hard rocks. Reflections at the discs/matrix interfaces and absorption in the discs lead to diffuse scattering with up to 20% changes of the intensity in the main beam compared to a homogeneous model rock. The subsequent thermo-mechanical finite element (FE) analysis indicates that the stresses become large enough to initiate damage. The results are supported by preliminary experiments on hard rock performed at 2.45GHz.

Rapid and Sensitive Detection of p53 Based on DNA-Protein Binding Interactions Using Silver Nanoparticle Films and Microwave Heating

Tumor detection can be carried out via the detection of proteins, such as p53, which is known to play vital role in more than 50% of all cancers affecting humans. Early diagnosis of tumor detection can be achieved by decreasing the lower detection limit of p53 bioassays. Microwave-accelerated bioassay (MAB) technique, which is based on the use of circular bioassay platforms in combination with microwave heating, is employed for the rapid and sensitive detection of p53 protein. Direct sandwich ELISA was constructed on our circular bioassay platforms based on DNA-protein binding interactions. Colorimetric and fluorescence based detection methods were used for room temperature bioassay (control bioassay; total bioassay time is 27 hours) and bioassay using microwave heating (i.e., the MAB technique; total bioassay time is 10 minutes). In the colorimetric based detection, a very high background signal due to the non-specific binding of proteins for the bioassay carried out at room temperature and a LLOD of 0.01 ng/mL for p53 was observed using the MAB technique. The LLOD for the fluorescence-based detection using the MAB technique was found to be 0.01 ng/mL. The use of circular bioassay platforms in the MAB technique results in microwave-induced temperature gradient, where the specific protein binding interactions are significantly accelerated; thereby reducing the background signal and the lower limit of detection of p53 protein.

Crystallization of Amino Acids on A 21-Well Circular PMMA Platform Using Metal- Assisted and Microwave-Accelerated Evaporative Crystallization

We describe the design and the use of a circular poly(methyl methacrylate) (PMMA) crystallization platform capable of processing 21 samples in Metal-Assisted and Microwave-Accelerated Evaporative Crystallization (MA-MAEC). The PMMA platforms were modified with silver nanoparticle films (SNFs) to generate a microwave-induced temperature gradient between the solvent and the SNFs due to the marked differences in their physical properties. Since amino acids only chemisorb on to silver on the PMMA platform, SNFs served as selective and heterogeneous nucleation sites for amino acids. Theoretical simulations for electric field and temperature distributions inside a microwave cavity equipped with a PMMA platform were carried out to determine the optimum experimental conditions, i.e., temperature variations and placement of the PMMA platform inside a microwave cavity. In addition, the actual temperature profiles of the amino acid solutions were monitored for the duration of the crystallization experiments carried out at room temperature and during microwave heating. The crystallization of five amino acids (L-threonine, L-histidine, L-leucine, L-serine and L-valine HCl) at room temperature (control experiment) and using MA-MAEC were followed by optical microscopy. The induction time and crystal growth rates for all amino acids were determined. Using MA-MAEC, for all amino acids the induction times were significantly reduced (up to ~8-fold) and the crystal growth rates were increased (up to ~50-fold) as compared to room temperature crystallization, respectively. All crystals were characterized by Raman spectroscopy and powder x-ray diffraction, which demonstrated that the crystal structures of all amino acids grown at room temperature and using MA-MAEC were similar.

An Efficient Empirical Model for Microwave-Induced Average Temperature of Liquid Cylindrical Reactants

Microwave-assisted chemical reactions have become very popular in preparative chemistry due to many advantages such as accelerated reaction rate, higher chemical yield and lower energy use. In dedicated equipment, however, the microwave units operate as “black boxes” keeping the role of the thermal effects in microwave-assisted chemical processes somewhat obscure. To address this issue, in this paper, we propose a simple mathematical model for computing microwave-induced temperature in a three-media cylindrical structure representing a core element of a typical microwave reactor with the reactant assumed to be stirred by convection flows. The model determines the average temperature of the reactant for the known absorbed microwave power and heating time. To illustrate its functionality, the model is used to compute time-temperature characteristics of water, ethanol, and methanol heated in the batch reactor MiniFlow 200SS. The curve calculated for water appears to be in an excellent agreement with an experiment. This confirms the hypothesis on temperature homogenization in liquid reactants in batch reactors due to convection and suggests that modeling can be helpful in clarifying and quantifying the details of microwave-assisted chemical processes.

Computational modeling of temperature elevation and thermoregulatory response in the brains of anesthetized rats locally exposed at 1.5 GHz

The dominant effect of human exposures to microwaves is caused by temperature elevation (‘thermal effect’). In the safety guidelines/standards, the specific absorption rate averaged over a specific volume is used as a metric for human protection from localized exposure. Further investigation on the use of this metric is required, especially in terms of thermophysiology. The World Health Organization (2006 RF research agenda) has given high priority to research into the extent and consequences of microwave-induced temperature elevation in children. In this study, an electromagnetic-thermal computational code was developed to model electromagnetic power absorption and resulting temperature elevation leading to changes in active blood flow in response to localized 1.457 GHz exposure in rat heads. Both juvenile (4 week old) and young adult (8 week old) rats were considered. The computational code was validated against measurements for 4 and 8 week old rats. Our computational results suggest that the blood flow rate depends on both brain and core temperature elevations. No significant difference was observed between thermophysiological responses in 4 and 8 week old rats under these exposure conditions. The computational model developed herein is thus applicable to set exposure conditions for rats in laboratory investigations, as well as in planning treatment protocols in the thermal therapy.

Microwave-electrochemical formation of colloidal zinc oxide at fluorine doped tin oxide electrodes

Colloidal ZnO is obtained during microwave-enhanced electrochemical deposition experiments from an aqueous solution containing 0.1 M Zn(NO 3) 2 and 0.02 M H 2O 2 via repetitive negative going potential cycles from 0.3 to −0.8 V vs. SCE. The effects of temperature and temperature gradients on ZnO electro-formation at fluorine doped tin oxide (FTO) electrodes are investigated with both a conventional thermostated bath system (isothermal) and an in situ microwave electrochemistry system (non-isothermal). Mainly electrodeposition of ZnO is observed in uniformly heated stagnant solution and predominantly the electro-formation of ZnO colloid is observed in the presence of microwave-induced temperature gradients in a flowing solution. For the ZnO colloid prepared via microwave activation, SAXS data suggests an average particle radius of ca. 18 nm. The increase of ZnO nanoparticle concentration during repetitive potential scans is followed by photoluminescence spectroscopy. A possible mechanism for ZnO colloid formation during electrochemical reduction of H 2O 2 is suggested.

Unprecedented Stereocontrol of β-Lactam Formation Derived from N-Cinnamylidenearylamine

Formation of β-lactams by the reaction of acid chloride, N-cinnamylidenearylamine and a tertiary amine seems to involve multiple pathways. Microwave-induced irradiation and high temperature lead to preferential formation (95-100 %) of trans (3-lactams derived from conjugated imines.

Kinetics of the lamellar and hexagonal phase transitions in phosphatidylethanolamine. Time-resolved x-ray diffraction study using a microwave-induced temperature jump

The kinetics of the thermotropic lamellar gel (L beta')/lamellar liquid crystal (L alpha) and L alpha/inverted hexagonal (HII) phase transitions in fully hydrated dihexadecylphosphatidylethanolamine (DHPE) have been studied. Measurements were made by using time-resolved x-ray diffraction (TRXRD) to monitor progress of the transitions. In these studies microwave energy at 2.5 GHz was used to increase the sample temperature rapidly and uniformly through the phase transition regions. The L beta'/L alpha and L alpha/HII transitions of DHPE were examined under active microwave heating and passive cooling. The transitions were found to be repeatable and reversible, and to have an upper bound on the time required to complete the transition of less than 3 s. Regardless of the direction of the transition, both phase transitions appeared to be two-state with no accumulation of intermediates to within the sensitivity limits of the TRXRD method. The rate and amplitude of the temperature jump can be controlled by regulating microwave radiation input power. A temperature jump rate of 29 degrees C/s was obtained at a final microwave power setting of 120 W. Comparisons between previously reported fluid flow (Caffrey, M. 1985. Biochemistry. 24:4826-4844) and microwave heating studies suggest that the determination of limiting transit times will require faster heating.

Quantitative assessment of impedance tomography for temperature measurements in microwave hyperthermia.

The objective of this study is a non-invasive assessment of the thermal dose in microwave hyperthermia. We intend to monitor the induced temperature rise via the change in the resistivity of body tissue, and hence control the microwave power during treatment. An initial feasibility study using electrical impedance tomography in vivo has indicated that a microwave induced temperature difference of a suitable magnitude can be mapped from a knowledge of the temperature coefficient of conductivity for tissue (typically 2% per degrees C). The accuracy of the currently employed reconstruction algorithm is assessed from data simulated by finite element prediction methods. A circular arrangement of electrodes surrounding a conductive sheet is used to evaluate the voltage distribution on the boundary of the sheet. A range of regions inside the circular field are chosen and the conductivity is changed uniformly by steps of 1% up to 10%. Images of these changes are produced. It is noticed that the algorithm underestimates the values of resistivity change for the small areas and overestimates the change for the larger areas. We are studying results for a variety of shapes of surface regions of the body that undergo resistivity change with microwave heating applied. Further work is necessary to account for three-dimensional current paths. Preliminary results are also presented of experimental investigations of the microwave-induced temperature rise in layers of carbon-loaded paper sheet.

Broadband, Non-Thermal Millimeter-Wave Influence on Giant Chromosomes

A non-thermal influence of low-intensity millimeter-wave radiation on the puffing of giant chromosom es from salivary glands of larvae of the midge A cricotopus lucidus (Diptera, Chironomidae) is reported. The effect is manifested as a strong regression in size of a specific puff that expresses genes for a secretory protein (Fig. 1). While millimeter-wave irradiation leads to an about tenfold increase of the regression probability com pared to controls, simulation of the small microwave-induced temperature increase in the sample does not result in a significant effect, thus confirming the non-thermal nature of the microwave irradiation effect. This finding could be of importance for the understanding of the interaction between microwave radiation and living systems and hence for the establishment of safety standards in that frequency regime.

The Influence of Low-intensity Millimeter-Wave Radiation on the Growth of Cress Roots

Abstract An inhibition of growth in cress roots (Lepidium sativum L.) by irradiation with low-intensity millimeter-waves was found using a computer controlled optical system which is capable of measuring nearly continuously the length of the roots to an accuracy of ± 2 jam. The effect is reversible and, for a power density of 6 mWcm-2, results in completely halting the root growth. It occurs within about 100 s after the onset of irradiation. The microwave-induced temperature increase at the surface of the root tip was found to be less than 0.3 °C at this power density. The effect did not show a sharp frequency dependence however it depended strongly on the polariza­tion of the microwaves with respect to the root orientation. The sensitivity of the root growth to the ambient temperature was examined. Only a weak temperature dependence was found which could not explain the observed effects. However simulating the microwave-induced temperature increase at the surface of the root tip with (incoherent) far-infrared light (λ≥20 μm) resulted in similar effects as with microwaves. Hence one can conclude that the observed effects are primarily caused by the small local irradiation induced thermal gradients across the surface of the root tip.

The Effects of Microwaves on Cell Survival at Elevated Temperatures

Since microwaves are used in human cancer therapy, information on specific biological effects of microwaves at elevated temperatures is important. To help supply this information, we exposed mammalian cells (CHO) and bacteria (Serratia marcescens) to hyperthermal temperatures (43, 44, and 45/sup o/C for CHO and 48, 49, and 50/sup o/C for the bacteria) with and without microwave irradiation. Temperature control was maintained by a refrigeration-reheat system and high-velocity water recirculation. The 2450-MHz microwave source was operated in a pulsed mode with power density up to 500 mW/cm/sup 2/. As expected, the survival curve slopes for both cell types increased rapidly with temperature, doubling for each degree Celsius. Microwave irradiation produced no significant change in extrapolation number for either cell type. However, survival curves of CHO cells which received microwaves were steeper by a factor of 1.25 than their sham-irradiated controls. No significant effect on slope was seen with the bacteria. Liquid crystal thermometry revealed a microwave-induced temperature elevation of 0.3/sup o/C in the glass microcapillary exposure tubes. This temperature elevation closely corresponded to the observed difference in survival curve slopes for the CHO cells and suggests a simple thermal origin for that difference.