Indirect Microwave Research Articles

A kinetics study of microwave pyrolysis of sewage sludge and corn stalk mixture

AbstractFast pyrolysis of sewage sludge (SS), corn stalk (CS), and their mixture under both direct and indirect microwave heating was carried out in a prototype microwave thermogravimetric reactor at high heating rates, 179 and 203 K/min, respectively. It was found that the samples could be heated up rapidly by direct microwave heating, while there is a temperature lag of the sample to the heated reactor wall in indirect heating. The Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS) methods were used to analyze the thermogravimetric data of SS, CS, and their mixture under direct microwave heating. When the FWO method was used, the average activation energies of SS, CS, and their mixture were obtained as 6.0, 18.9, and 39.8 kJ/mol, respectively. When the KAS method was used, the average activation energies of CS and mixture were 12.6 and 31.6 kJ/mol, respectively. Those values are much lower than reported under conventional heating, and likely result from the interaction of microwave irradiation and the biomass samples. The reaction kinetics of directly microwave‐heated biomass pyrolysis from this study provide essential data for modelling, scaling, and designing the microwave‐assisted in‐situ catalytic pyrolysis reactors with mixed biomass and catalyst particles.

Assessment of microwave susceptors for optimum temperature rise using parametric numerical simulation

Microwave hybrid heating or indirect microwave heating has been an important development in the processing of metals using microwave energy. In this method of heating, a “susceptor” is used to absorb and convert microwave energy into thermal energy which is utilized for materials processing. Few of these processing applications are sintering, joining, cladding, casting, and composite development. However, the precise selection of a susceptor for a given process is challenging and requires a thorough understanding of the microwave heating phenomenon. In addition, predicting temperature rise in a microwave cavity is a complex task. Numerical simulation can serve to solve these two significant problems. In this work, parametric simulation of microwave heating of three major susceptor materials; silicon carbide, alumina, and coal has been carried out using a multiphysics approach. Initially, a model was developed to validate the simulation procedure. The results were in good agreement with the available data in published literature. Thereafter, the effect of microwave power, position of susceptor block, and time of irradiation on temperature rise in these materials were investigated. On comparing these susceptor blocks after altering the parametric combinations, coal exhibited the formation of hot spots and uneven temperature field with a maximum temperature of 901°C. On the contrary, the temperature attained in silicon carbide (529°C) and alumina (616°C) were lower but distributed in a relatively uniform field across the block. Regression models have also been built to correlate maximum temperature achieved with the parametric variation of parameters. It has been found that microwave power and the time of microwave irradiation have a significant coactive effect on the maximum temperature reached by the material. While the rise in the position of the susceptor block from the base leads to an adverse effect on the temperature rise. The study demonstrates that indirect microwave heating can be harnessed using the proper selection of susceptors as per the temperature requirement of the process. The regression models can be used for the accurate prediction of temperature attainment for given power input, position, and time.

Electronic transport and electrochemical performance of MnCo2O4 synthesized using the microwave-assisted sonochemical method for potential supercapacitor application

MnCo2O4, a cubic structured spinel compound, was successfully synthesized (at 550 °C/4h) using a novel sonochemical route utilizing the power of indirect microwave heating for phase formation. MnCo2O4 displayed good supercapacitive performance, exhibiting a specific capacitance of 190.2 F/g at 2 mVs, with energy and power densities of 2.7 Wh/kg and 540 W/kg, respectively. A semiconducting to metallic phase transition was found from its temperature-dependent conductivity behaviour at 423 K, with the activation energy of 0.156 eV, and band conduction at higher temperatures. Also, MCO exhibited excellent conductivity ranging between 7 × 10−3 to 1.1 × 101 Scm−1, significantly higher than the earlier reported.

Electronic transport properties of NASICON structured NaFe2(PO4)(SO4)2: A potential cathode material for Na-ion batteries, synthesized using ultrasound-assisted, indirect microwave heating technique

Low-temperature synthesis and reduced reaction time for fabrication of nanocrystalline, NaFe2PO4(SO4)2, a NASICON structured potential cathode material for the sodium-ion battery, is reported here. For this, we have used a novel ultrasound-assisted wet chemical approach with further assistance from efficient microwave heating. We were able to synthesize this NASICON compound at 450 °C in 6h using indirect microwave heating. Using broadband dielectric spectroscopy, we measured its complex permittivity and conductivity. A non-Debye type relaxation process was observed for this compound, with conducting charge carriers and polaron hopping primarily responsible for overall dielectric losses in the sample.

Accelerated thermal reaction kinetics by indirect microwave heating of a microwave-transparent substrate.

Macroscopically homogeneous mixtures of p-nitroanisole (pNA) and mesitylene (MES) can be selectively heated using microwave (MW) energy. The pNA solutes agglomerate into distinct phase domains on the attoliter-scale (1 aL = 10-18 L), and these agglomerates can be MW-heated selectively to temperatures that far exceed the boiling point of the surrounding MES solvent. Here, a 1 : 20 mixture of pNA : MES is used as a mixed solvent for aryl Claisen rearrangement of allyl naphthyl ether (ANE). ANE itself does not heat effectively in the MW, but selective MW heating of pNA allows for transfer of thermal energy to ANE to accelerate rearrangement kinetics above what would be expected based on Arrhenius kinetics and the measured bulk solution temperature. This focused study builds on prior work and highlights 1 : 20 pNA : MES as a mixed solvent system to consider for strategically exploiting MW-specific thermal effects.

Indirect microwave holography with resolution enhancement in metallic imaging

The development of compact indirect microwave holographic set-up by the implementation of low cost, specifically designed directive antennae as transmitter and receiver is proposed. Microwave holograms are recorded by 2D scanning over a plane using motorized translation stage. The recorded interference pattern i.e. holograms are then processed numerically to reconstruct the amplitude and phase information employing the angular spectrum diffraction method. The quality of the reconstructed amplitude image is further enhanced through the deep neural network, in order to combat with the low resolution of reconstructed images. The qualitative experimental results exploit the possibility of developing the miniaturized, and low cost indirect microwave holographic system for near field applications.

Early detection of cancerous tissues in human breast utilizing near field microwave holography

AbstractThis work demonstrates an application of near field indirect microwave holography for the detection of malignant tissues in human breast in an effective way. The holograms are recorded by two directive antennas aligned along each other's boresight while performing a raster scan over a 2D plane utilizing XY‐linear motorized translation stage and a uniform reference wave. The whole information that is, amplitude and phase of an object has been provided by indirect holography at microwave frequencies. The extracted phase values are used to determine the dielectric permittivity values which are further utilized for the identification and validating the positions of malignant tissues in the breast phantom. The experimental evaluations performed on the in‐house designed and developed tissue mimicking 3D printed breast phantoms. The experimental results demonstrate the ability of microwave holography using directive antennas in detecting and identifying the tumors upto the minimum size of 4 mm and maximum depth of 25 mm in fabricated phantom. The quantitative results present the potential of the Near Field Indirect Holographic Imaging (NFIHI) in order to develop an efficient and economical tool for breast cancer detection.

Indirect microwave holography and Through-Wall imaging

In this paper, a review of indirect microwave holography for through-wall imaging is presented. Indirect microwave holography is an imaging technique, enabling the complex object scattered fields (amplitude and phase) to be mathematically recovered from intensity-only, scalar microwave measurements. By removing the requirement to use vector measurement equipment to directly measure the complex fields, indirect microwave holography significantly reduces the cost of the imaging system and simplifies the hardware implementation. The application of a back-propagation algorithm enables the reconstructed amplitude and phase images to be obtained at the plane of the concealed object. In order to demonstrate the validity of the reviewed approach, experimental work is carried out on a metallic gun concealed under a 5 cm thick plywood wall and it is demonstrated that the indirect microwave holographic through wall imaging can produce good resolution amplitude and phase images when back-propagation is applied. Through wall imaging of a concealed dielectric box representing non-metallic ordnance is also performed to demonstrate the ability of the technique to reconstruct through-wall images of concealed dielectric objects. An investigation of the resolution characteristics of the system suggests diffraction limited resolution can be achieved.

Indirect Microwave Curing Process Design for Manufacturing Thick Multidirectional Carbon Fiber Reinforced Thermoset Composite Materials

Recently, indirect microwave curing technology was developed to process multidirectional carbon fiber reinforced composite materials with high efficient and energy saving purpose. The aim of this paper is to solve the problem of large mid-plane heat generation in manufacturing this kind of materials with large thickness. The difference between the traditional thermal curing and indirect microwave curing processes was analyzed. A Multiphysics model was constructed to reflect the composite curing behavior in a cost-effective manner, which agreed well with the experimental results. On this basis, a new indirect microwave curing cycle was designed, and relevant process parameters were determined. Compared with the manufacturer’s recommended cycle, the degree of mid-plane heat generation was greatly relieved, and a reduction of 34.6% in composite residual strains was achieved. Moreover, the curing cycle was reduced by 38%, while the interlaminar shear strength of the composite was improved by 1.38 times. Corresponding reinforcement mechanisms were explored through the observation of composite cross-sections with optical microscopes.

Curing multidirectional carbon fiber reinforced polymer composites with indirect microwave heating

Microwave curing technologies have many advantages in manufacturing fiber reinforced polymer composite materials used in aerospace products, compared with traditional autoclave curing technologies. However, multidirectional carbon fiber reinforced polymer composites can hardly be penetrated and heated by microwave directly, which has become a major obstacle in industrial application worldwide. In this paper, an indirect microwave curing method was proposed to solve this problem. The microwave absorption performance of the indirect microwave heating medium was systematically optimized by evaluating its dielectric properties and reflection loss according to the transmission line theory. On this basis, the microwave susceptive mold was carefully designed and manufactured. Subsequently, the multidirectional carbon fiber/epoxy composite was successfully cured with indirect microwave heating, which was demonstrated by the observation of the curing process with infrared thermal imager and differential scanning calorimetry analysis of the final products. Compared with the traditional thermal curing method, the curing cycle and energy consumption were reduced by 42.1% and 75.9% respectively. Results of further characterization experiments indicated that the mechanical properties of indirect microwave cured specimens were slightly higher than those of the thermally cured counterparts.

Microwave imaging using indirect holographic techniques

This work describes how indirect holographic techniques, previously applied to the determination of antenna radiation patterns, can be adapted for the imaging of passive objects. It provides details of how complex scattered field values can be obtained in a simple and inexpensive manner from sampled scalar intensity measurements taken over a single scanning aperture. This work provides a brief outline of the basic theory of indirect microwave holography, and how the transformation of the holographic intensity pattern into the Fourier domain enables the isolation of the terms required for complex field reconstruction to be isolated from the remaining terms. The work is supported by a range of experimental results, illustrating the reconstructed complex fields for a number of simple test objects. Back-propagation techniques have also been included to reconstruct complex fields at the position of the scattering objects.

INDIRECT MICROWAVE HOLOGRAPHIC IMAGING OF CONCEALED ORDNANCE FOR AIRPORT SECURITY IMAGING SYSTEMS

In this paper, indirect microwave holographic imaging of concealed ordnance is demonstrated. The proposed imaging technique difiers from conventional microwave imaging methods in that it does not require the direct measurement of the complex fleld scattered from the imaged object but mathematically recovers it from intensity-only scalar microwave measurements. This brings the advantages of simplifying the hardware implementation and signiflcantly reducing the cost of the imaging system. In order to demonstrate the ability of the proposed technique to reconstruct good quality images of concealed ordnance, indirect microwave holographic imaging of a metallic gun concealed in a pouch is carried out for airport security imaging applications. It is demonstrated that good resolution amplitude and phase images of concealed objects can be recovered when back-propagation is applied.

Investigations of resolution limits for indirect microwave holographic imaging

This work describes how indirect holography which has previously been applied to the determination of antenna radiation patterns can be adapted for the imaging of passive objects. It provides details of how complex scattered field values can be obtained in a simple and inexpensive manner from sampled scalar intensity measurements taken over a single scanning aperture. This work uses indirect holographic techniques to image a number of simple objects including a rectangular metallic plate, a small metal plate covered by a dielectric sheet and a small metallic circular annulus. This work demonstrates that good quality images can be reconstructed from simple scalar intensity patterns. It demonstrates that clear outlines can be obtained in particular from reconstructed phase patterns and that good images can be obtained from objects with dimensions of the order of a half wavelength. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2013.

Indirect and direct microwave regeneration of Na-ETS-10

Indirect and direct microwave regeneration were assessed as potential techniques for desorbing a CO2/CH4 mixture from Na-ETS-10. Indirect microwave regeneration consists of desorption with water followed by microwave drying, while direct microwave regeneration consists of constant power microwave heating. Five adsorption–desorption cycles were completed to determine and compare the swing capacity, net energy consumption, and gas recovery obtained with these two regeneration techniques. During indirect microwave regeneration the swing capacity of Na-ETS-10 was stable at 0.3mmol/g, with the exception of the first cycle where it reached 1.6mmol/g. The first cycle swing capacity decreased however, by fivefold during the subsequent desorption cycles. During direct microwave regeneration, the swing capacity remained stable at 0.7mmol/g over all five consecutive cycles. On average, 2.46KJ and 0.32KJ were consumed during regeneration of 1g of Na-ETS-10 using indirect and direct microwave regeneration, respectively. With the exception of the first cycle, gas recovery for indirect microwave regeneration was only 20%. In contrast, an average of 50% gas recovery was achieved by direct microwave regeneration.

Indirect microwave heating to pharmaceutical excipients: Lactose hydrate

Research and study of crystal structure of materials provide valuable information for physical–chemical characterization. The indirect microwave heating (IMH) which is applied in inorganic materials with poor dielectric properties was optimized to achieve the dehydration of two types of αL.H2O (Granulac 200 and Pharmatose DCL 11) in a microwave field with Pyrex glass as an indirect heating medium. Lactose monohydrate was chosen as a model substance to the study because of its polymorphic properties. The IMH produces dehydration of αL.H2O but needs to overcome a power threshold and can be accelerated by increasing the microwave field. The IMH process is not a selective method to produce a specific lactose anomer. Structural changes in lactose were characterized by the Rietveld method which is mainly used in the characterization of mixtures of inorganic materials. With this method, it was possible to quantify differences in the contribution of anhydrous lactose anomers. This methodology (IMH and Rietveld method) might be used for different hydrates or solvates of a given compound, which opens the possibility, at least theoretically, of characterization of samples with different anomeric stoichiometries or new molecular packing.

Microwave pretreatment of a double refractory gold ore

Double refractory gold ores, such as Barrick’s Goldstrike ore, contain both pyrite and carbonaceous matter and the latter results in the loss of gold from solution during cyanide leaching. One possible remedy is to roast the ore and thus oxidize the pyrite and remove the organic carbon. In this research, microwave roasting of a double refractory gold ore was investigated as an alternative method and the results were compared to those obtained by conventional roasting. The compositional changes of the ore during roasting were determined by thermogravimetric analysis (TGA). Also, both the real and the imaginary permittivities, which determine the amount of energy absorbed by the ore and the heating rate of the ore respectively, were evaluated. In addition, the microwave heating behaviour was studied. Conventional and both direct and indirect microwave roasting tests were performed and in all the cases, the pyrite was readily converted into hematite. Direct microwave roasting could not remove the organic carbon. Indirect microwave roasting was conducted using magnetite as a susceptor and preg-robbing was eliminated when about 94% of the organic carbon was removed. For both conventional and indirect microwave roasting, gold recoveries of over about 98% were achieved after cyanide leaching. For microwave roasting, both the total carbon removal rates and the heating rates were higher and the specific energy consumptions were lower than the corresponding values for conventional roasting.

Dehydration of sodium carbonate monohydrate with indirect microwave heating

In this study, dehydration of sodium carbonate monohydrate (Na 2CO 3·H 2O) (SCM) in microwave (MW) field with silicon carbide (SiC) as an indirect heating medium was investigated. SCM samples containing up to 3% free moisture were placed in the microwave oven. The heating experiments showed that SCM is a poor microwave energy absorber for up to 6 min of irradiation at an 800 W of microwave power. The heat for SCM calcination is provided by SiC which absorbs microwave. The monohydrate is then converted to anhydrous sodium carbonate on the SiC plate by calcining, i.e. by removing the crystal water through heating of the monohydrate temperatures of over 120 °C. The calcination results in a solid phase recrystallization of the monohydrate into anhydrate. In the microwave irradiation process, dehydration of SCM in terms of indirect heating can be accelerated by increasing the microwave field power.

Imaging dielectric objects using a novel synthetic off-axis holographic technique

Original results are presented for the imaging of dielectric objects using an indirect planar microwave imaging system that employs an off-axis holographic concept. The ability of this intensity only planar recording technique to recover phase information about the object under test proves to be significant in the imaging of dielectrics. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1957–1961, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21826

Experimental investigation of breast tumor imaging using indirect microwave holography

A novel indirect holographic technique for the early detection of breast cancer, which offers considerable benefits in terms of simplicity and expense, is presented and discussed. It is shown how this approach, usually employed at optical frequencies, can be adapted to image breast tumors at microwave frequencies. Practical results, using a simulated phantom, successfully demonstrate the validity of this technique

Materials processed by indirect microwave heating in a single-mode cavity

We report in this paper the synthesis, the sintering and the properties of La 0.8Sr 0.2MnO 3 (LSMO) and BaZrO 3 (BZ) compound using microwave heating. The starting raw material was prepared by solid state reaction. Synthesis and sintering were carried out using an indirect microwave heating process. SiC susceptor tube was utilized to produce the indirect heating in a microwave symmetrical single mode cavity TE10p. The LSMO and BZ phases have been successfully sintered in a microwave cavity in a very short time. Scanning electron microscopy (SEM), X-ray diffraction (XRD), magnetic or dielectric and electrical properties were carried out for both processing conditions, i.e., using classical and microwave heating. The results and advantages of this microwave process to synthesize many materials with various physicals properties are discussed.