Analysis of pulsed microwave processing of polymer slabs supported with ceramic plates

A detailed theoretical analysis has been carried out to study the role of ceramic supports (alumina and SiC) and pulsed microwave heating of polymer (Natural Rubber, NR, and Nylon 66) slabs due to various distributions of microwave incidences. Ceramic plates are typical representations as they withstand high temperature without any deformation. It is found that ceramic plates influence the heating processes significantly and local hot spots within samples are governed by specific type of ceramic plates for various sample thicknesses and distributions of microwave incidence (one side or both sides). Optimized pulsing of microwave incidence has been employed to minimize the thermal runaway or hot spots in order to achieve uniform temperature distribution and pulsing is introduced based on two parameters: setpoint (ΔT_S) and on-off constraint (T^′). Detailed spatial distributions of power and temperature are illustrated for a few representative length scales to demonstrate the role of local maxima in power and temperature on heating rate as well as thermal runaway with or without pulsing. Pulsing ratio (PR) has been defined as PR=t_off/t_p, where t_off is power-off time and t_p is the total processing time such that smaller PR denotes large processing rates. It is found that one side incidence gives smaller values of PR for both the ceramic plates whereas SiC plate may be suitable for both sides incidence with large sample thicknesses of NR samples. It is also found that larger values of setpoints also minimize PR. The setpoints along with the on-off constraint play critical role to select the heating strategy as a function of ceramic plates and types of incidence. Pulsing may not be important for smaller thicknesses of Nylon samples and SiC or alumina plates may be recommended for processing larger thicknesses of Nylon samples in presence of pulsing. Current study recommends the efficient microwave heating methodologies for polymer processing attached with ceramic plates by means of optimized pulsing for the first time.     

Role of lateral and radial irradiations on efficient microwave processing of food cylinders

A detailed theoretical analysis has been carried out to assess the role of lateral and radial irradiations on microwave heating of 2D cylinders for beef and oil samples. The lateral irradiation may represent the sample incident at one direction and the radial irradiation refers to sample exposed to uniform microwave intensities at all directions. For both lateral and radial irradiations, the effective microwave incidence from the source is assumed to be identical. A preliminary analysis on microwave heating of samples has been shown via average power within a sample vs sample radius for beef and oil samples. The samples with smaller radius are found to have larger average power with radial irradiation for both beef and oil samples. The hot spot formation within a sample is found to be a strong function of lateral/radial irradiation for various food dielectrics. The radial irradiation is found to be not favorable especially for large oil samples where the hot spot occurs at the center resulting in larger thermal gradient that contrasts earlier works which established that the radial irradiation minimizes the thermal gradient irrespective of sample size. The present work provides guideline on heating policy based on either lateral or radial irradiation depending on sample size and food dielectric properties.     

Energy efficient and controlled flow processing under microwave heating by using a millireactor–heat exchanger

A continuously operated microwave heated millireactor setup has been developed for performing reactions of highly microwave absorbing media in a controlled and energy efficient manner. The setup consists of a tubular reactor integrated with a heat exchanger. A microwave transparent liquid was used as coolant to extract the excess heat from the reaction mixture, thus controlling the temperature of the reaction mixture by avoiding overshoots and subsequent boiling. A reactor‐heat exchanger shell and tube unit with a diameter of the inner tube of 3·10^?3 m and a shell of 7·10^?3 m inner diameter has been manufactured in quartz. The unit size was defined based on simulation with a heat‐transfer model for the microwave cavity part. Microwave heating was incorporated as a volumetric heating source term using the temperature‐dependent dielectric properties of the liquid. Model predictions were validated with measurements for a range of 0.167·10^?6 to 1.67·10^?6 m³/s flow rates of coolant. The outlet temperature of both the reaction mixture and the coolant, were predicted accurately (tolerance of 3 K), and the process window was determined. The model for the reactor part provided the required length of the reactor for a hetero‐geneously catalyzed esterification reaction. The predicted conversions, based on the obtained temperature profile in the reactor packed with the catalyst bed, known residence times and kinetics of the esterification reaction, were found to be in good agreement with the experimental results. Efficient utilization of microwave energy with heat recovery up to 20% of the total absorbed microwave power and heating efficiencies up to 96% were achieved. It has been demonstrated that the microwave heating combined with millireactor flow processing provides controlled and energy efficient operation thus making it a viable option for a fine chemical production scale of 1 kg/day (24 h period). © 2011 American Institute of Chemical Engineers AIChE J, 58: 3144–3155, 2012     

Separation of polyaromatic hydrocarbons from contaminated soils using microwave heating

The microwave treatment of soils contaminated with heavy- and light-hydrocarbons was investigated. The soils were characterised to determine the total organic liquid content and PAH contents, and the dielectric properties of the soils were measured across a range of temperatures. The heavy- and light-contaminated soils behaved very differently in a microwave environment, with bulk soil temperatures limited to 100 °C for the light-contaminated soil. Microwave treatment is shown to remove PAHs from both the heavy- and light-contaminated soils, and it is demonstrated that 95%+ PAH removal can be achieved under moderate processing conditions. Complete remediation of the soils is possible at high microwave powers or long residence times. It is shown that PAH removal can take place at bulk temperatures well below the boiling point of those compounds and a number of explanations are proposed for this behaviour. The mechanisms of PAH removal are investigated for both the heavy- and light-contaminated soils and thermal desorption, selective heating and entrainment mechanisms can all be exploited.This is the first step in the development of a continuous microwave treatment process for the removal of PAHs from contaminated soil on an industrial scale.     

A novel closed-form analysis on asymptotes and resonances of microwave power

This paper presents a comprehensive material invariant analysis on asymptotes and resonances of absorbed power distribution. A closed-form solution for absorbed power in the presence of distributed microwave source has been derived from the first principle, which shows the existence of two distinct asymptotic regimes: thin and thick samples. Within thin samples, absorbed power is shown to be weak function of position, while it decays exponentially within thick sample. In between thick and thin samples, absorbed power is shown to exhibit resonances, where a detailed analysis has been performed to reveal various resonating characteristics of average power. The main purpose of the analysis is to predict the occurrence of resonances and establish correlations for a priori prediction of resonating sample dimensions in the presence of various distributed microwave incidences. The closed-form analysis has been shown to be useful in forecasting the heating characteristics: hot spot vs uniform heating. It has been shown that the heating characteristics can be tuned by suitable distribution of microwave source towards optimal material processing.     

Facile preparation of thermal insulation materials by microwave sintering of ferronickel slag and fly ash cenosphere

A novel approach for preparing thermal insulation materials by microwave sintering of ferronickel slag (FNS) in the presence of fly ash cenosphere (FAC) was proposed and evaluated. The study showed that during microwave radiation, the contact interface between FNS and FAC would preferentially form magnesium iron chromate spinel and magnesium iron aluminate spinel particles as hot spots by absorbing microwave vigorously, promoting decomposition and transformation of the raw materials into the thermal insulation phases, mainly cordierite and enstatite. After sintering at 900 °C by microwave for only 20 min with the addition of 25 wt% FAC, a thermal insulation material with thermal conductivity of 0.41 W/(m·K), bulk density of 1.46 g/cm³, compressive strength of 30.72 MPa, water absorption of 21.07%, and linear shrinkage of 7.06% was obtained. Compared with the conventional sintering method, the temperature was reduced by 300 °C, with the sintering time shortened by 6 times. This study represents a good example for clean and efficient value-added utilization of FNS, FAC and other relavent solid wastes.     

Impact of microwave processing on porcelain microstructure

Microstructural evolution on sintering of porcelain powder compacts using microwave radiation was compared with that in conventionally sintered samples. Using microwaves sintering temperature was reduced by ~ 75 °C and dwell time from 15 min to 5 min while retaining comparable physical properties i.e. apparent bulk density, water absorption to conventionally sintered porcelain. Porcelain powder absorbed microwave energy above 600 °C due to a rapid increase in its loss tangent. Mullite and glass were used as indicators of the microwave effect: mullite produced using microwaves had a nanofibre morphology with high aspect ratio (~ 32 ± 3:1) believed associated with a vapour-liquid-solid (VLS) formation mechanism not previously reported. Microwaves also produced mullite with different chemistry having ~ 63 mol% alumina content compared to ~ 60 mol% alumina in conventional sintered porcelain. This was likely due to accelerated Al⁺³ diffusion in mullite under microwave radiation. Liquid glass was observed to form at relatively low temperature (~ 900–1000 °C) using microwaves when compared to conventional sintering which promoted the porcelains ability to absorb them.     

A rapid and high efficient microwave promoted multicomponent domino reaction for the synthesis of spirooxindole derivatives

A simple, efficient and rapid method has been developed for synthesis of 6,6-dimethyl-4-phenyl-6,7-dihydro-1H-spiro[furo[3,4-b]quinoline-9,3′-indoline]1,2′,8(3H,4H,5H)-trione derivatives. These heterocycles were prepared through domino one-pot and multicomponent condensation reactions of isatins, dimedone, and anilinolactones in the presence of alum (15 mol%) as an inexpensive, nontoxic, convenient, and available Lewis acid catalyst under microwave irradiation. The corresponding products have been obtained in excellent isolated yields between 78% and 90%, with high purity, in short reaction times about 10–12 min and easy work up.     

微波加热用透波材料的研究进展

微波加热技术因其绿色环保、体积加热、选择性加热等优势,已被广泛应用于化工强化、金属冶炼、陶瓷烧结、食品加工等众多领域,但微波在反应器内普遍存在透波效果差、微波利用率低等问题。随着微波加热技术的不断发展,微波加热设备中透波材料的选用越来越受到大家的关注。本文主要针对透波材料在微波加热领域中的应用现状进行综述,对透波材料的种类进行简要介绍,分别从微波加热用容器和保温材料两方面进行论述。详细介绍了氧化物、氮化物、硅酸盐、磷酸盐等高温透波材料及聚四氟乙烯、玻纤增强树脂基、环氧树脂等中、低温透波材料的研究进展,并具体论述了目前微波加热常用纤维棉、纤维毯和纤维板等各种陶瓷纤维制品的介电特性和透波性能,最后指出了目前微波加热用透波材料普遍存在的问题,并对透波材料的应用和发展作出了展望。     

Fabrication of hierarchical PANI@W-type barium hexaferrite composites for highly efficient microwave absorption

Hexagonal barium ferries is a promising and efficient microwave (MW) absorbing material, but the low dielectric loss and poor conductivity have limited their extensive applications. In this work, a simple tactic of coating conductive polymer PANI on hexaferrite BaCo₂Fe₁₆O₂₇ is presented, wherein the dielectric properties are customized, and more significantly, the electromagnetic loss is greatly enhanced. As displayed from structural characterizations, PANI were coated equably on the surface of hexaferrite grains by an in-situ polymerization process. The outcomes exhibit the as-prepared PANI@hexaferrite composite has remarkable electromagnetic wave absorption capacity. When the thickness is 6.0 mm, the minimal RL of ?40.4 dB was achieved at 2.9 GHz. The effective absorption bandwidth (RL < ?20 dB) of 0.65 GHz, 0.53 GHz, 0.65 GHz, 0.52 GHz, 0.46 GHz and 0.39 GHz was achieved separately when the thickness ranges from 4 to 9 mm. The highly efficient MW absorbing performance of PANI@hexaferrite composite were the consequence of multiple loss mechanisms and perfect impedance matching. It is demonstrated that the PANI@hexaferrite composite with excellent MW absorption performance is expected to be potential MW absorbers for extensive applications.     

Preparation of zeolite A membranes by microwave heating

Molecular sieve membranes consisting of NaA zeolite crystals have been successfully synthesized on symbol -Al₂O₃ substrate by means of microwave heating. In this way, the reaction time is greatly reduced to only 15–20 min and the membranes obtained are very stable and dense. Moreover, the thickness of the membranes can be easily controlled by varying the amount of the reaction mixture. To study the formation process of the membranes, the samples in various reaction stages have been characterized by field emission scanning electron microscopy and X-ray diffraction.     

A novel concept for improved thermal management of the planar SOFC

A new design for the solid oxide fuel cell (SOFC) planar stack is proposed to minimise the thermal gradients in the cell. This design involves including a secondary air channel with flow in the counter direction to the cathodic air channel. The effectiveness of the new design is tested by means of a tank in series reactor (TSR) model of the SOFC. It is found that the new design is capable of reducing the steady state temperature difference across the cell to less than 2 K over a range of voltages, while satisfying the requirements on fuel utilisation (FU) and cell average temperature. This is achieved by manipulating the primary air channel inlet flow rate and the secondary air channel inlet temperature. More modelling and experimental studies are required to further investigate the proposed design.     

A hybrid microreactor/microwave high-pressure flow system of a novel concept design and its application to the synthesis of silver nanoparticles

This article reports on a microreactor/microwave high-pressure flow hybrid apparatus of a novel concept design, which includes both the microreactor and a spiral reactor, and its efficient use in the synthesis of silver nanoparticles of relatively uniform sizes (4.3 ± 0.7 nm) under microwave irradiation. By contrast, under otherwise identical experimental conditions but with conventional heating, the nanoparticle size was non-uniform (8.3 ± 2.7 nm) and the spiral reactor walls were covered with a silver mirror deposit. Formation of the nanoparticles was monitored by UV–visible spectroscopy (plasmonic absorption band; LSPR), TEM and by small-angle X-ray scattering (SAXS). Both the spiral microreactor and the spiral quartz reactor of the hybrid system played an important role in the synthesis, with the microreactor providing the environment wherein mixing of the aqueous solution of [Ag(NH₃)₂]⁺ and the solution of glucose (the reducing agent) and poly(N-vinyl-2-pyrrolidone) (PVP; stabilizer/dispersing agent) occurred. The microwaves provided the thermal energy to effect a uniform growth of the silver nanoparticles at temperatures above 120 °C. Mixing the two solutions by conventional methods (no microreactor) failed to yield such nanoparticles even under microwave irradiation and no formation of a silver mirror occurred in the inner walls of the spiral reactor.     

Design principles of microwave applicators for small-scale process equipment

In this work, we bridge fundamental electromagnetics and chemical process engineering with the aim to develop tailor-made (microwave or high frequency radiowave) applicators for heating of micro- and small-structured process equipment. In this context, two simple configurations with well-defined single mode field patterns, namely a cylindrical and a rectangular cavity both containing a homogeneous cylindrical load were analyzed either analytically or numerically. We present design charts that illustrate how important operating, geometric and materials parameters relate with each other. It was found that load size, heating uniformity and desired frequency mutually constrain one another. The required cavity volume increases with increasing heating uniformity or with increasing load permittivity for a given heating uniformity requirement. At the popular frequency of 2.45 GHz the load is restricted to a small size, compared to the cavity size, in order to achieve high heating uniformity. Opting for lower resonance frequencies allows for bigger load volumes to be heated uniformly. Furthermore, we show that the relations found for the operating, structural and material properties on the basis of these simple configurations can provide design guidelines and first approximations for more realistic process equipment geometries.     

A study on tangential force laws applicable to the discrete element method (DEM) for materials with viscoelastic or plastic behavior

The discrete element method is a widely used particle orientated simulation approach for modeling granular systems. It is based on tracking each particle's movement and its interactions with the surroundings over time. The motion of a particle is given by a system of coupled ordinary differential equations which are solved numerically. Therefore, models for the forces acting between particles in contact need to be specified. In the past, detailed investigations dealing with the accuracy of tangential force-displacement models have been very limited, with sparse experimental data considered and the frequent restriction of including only fully elastic materials. In large scale discrete element simulations, on the other hand, viscoelastic or plastic material behavior is often assumed for normal contacts and combined with arbitrary tangential models. To address this situation a number of tangential force-displacement models are reviewed including linear models by Cundall and Strack [1979. A discrete numerical model for granular assemblies, Geotechnique 29, 47-65], Di Maio and Di Renzo [2004. Analytical solution for the problem of frictional-elastic collisions of spherical particles using the linear model. Chemical Engineering Science 59(16), 3461-3475], Brendel and Dippel [1998. Lasting contacts in molecular dynamics simulations. In: Herrmann, H.J., Hovi, J.-P., Luding, S. (Eds.), Physics of Dry Granular Media, Dordrecht. Kluwer Academic Publishers, pp. 313], Walton and Braun [1986. Viscosity, granular temperature and stress calculations for shearing assemblies of inelastic, frictional disks. Journal of Rheology 30, 949] and simple non-linear models by Brilliantov et al. [1996. Model for collisions in granular gases. Physical Review E 53(5), 5382-5392], Tsuji et al. [1992. Lagrangian numerical simulation of plug flow of cohesionless particles in a horizontal pipe. Powder Technology 71, 239-250] and Di Renzo and Di Maio [2005. An improved integral non-linear model for the contact of particles in distinct element simulations. Chemical Engineering Science 60(5), 1303-1312]. Whereas for fully elastic materials the parameters of the tangential force-displacement models can be derived directly from mechanical properties a scaling approach is proposed for the estimation of the parameters in the non-elastic case. The effect of different normal force-displacement models is analyzed. For all model combinations macroscopic final collision properties are derived and compared to experimental results by Foerster et al. [1994. Measurements of the collision properties of small spheres. Physics of Fluids 6(3), 1108-1115], Lorenz et al. [1997. Measurements of impact properties of small, nearly spherical particles. Experimental Mechanics 37(3), 292-298], Gorham and Kharaz [2000. The measurement of particle rebound characteristics. Powder Technology 112(3), 193-202] and Dong and Moys [2003. Measurement of impact behaviour between balls and walls in grinding mills. Minerals Engineering 16(6), 543-550; 2006. Experimental study of oblique impacts with initial spin. Powder Technology 161(1), 22-31].     

Coupling additive manufacturing and microwave sintering: A fast processing route of alumina ceramics

In this paper, a quick and efficient route to produce complex-shape alumina is reported. Alumina pieces are shaped by additive manufacturing (stereolithography) and densified by microwave sintering. Two raw powders are investigated in terms of both printing and microwave sintering: one alumina grade appropriate for additive manufacturing but not for microwave sintering, and vice-versa for the other grade. The mixture of the two raw powders allows both stereolithography printing and microwave sintering. Alumina parts processed by additive manufacturing followed by microwave sintering were successfully prepared and exhibited relative densities of about 93%, elastic modulus up to 236 GPa and Vickers hardness up to 12 GPa. Notwithstanding the part properties, the as-proposed coupling resulted in a time saving of 30%.     

Influences of pre-forming on preparation of SiC by microwave heating

Silicon carbide (SiC) crystals were synthesized by microwave sintering using coal and tetraethoxysilane (TEOS) as raw materials. A sol-gel method was carried out to coat coal mineral particles with silicon dioxide (SiO₂). The mixed raw powders were pre-formed by uniaxial pressing into cylindrical pellets in dimension of ~ 30?×?3?mm². The pre-forming pressure was selected at 0?MPa, 1?MPa, 2?MPa, 3?MPa, 4?MPa and 5?MPa respectively, which led to different apparent density of the green pellets. The influence of apparent density of green pellets on microwave heating behavior was investigated. Different microwave thermal effects were analyzed. Techniques of XRD、SEM were carried out to characterize samples. It was found that pre-forming pressure showed crucial influences on microwave thermal effects and electric field (E-field) intensification. No SiC crystal could be formed without pre-forming pressure. Pre-forming pressure might be the prerequisite for synthesis of SiC by microwave heating. Five consecutive and indispensable heating stages including accumulation of residual air, microwave plasma generation, complex chemical reactions, nucleation and grain growth of SiC crystallites could be distinguished for samples under pre-forming pressure. Different pre-forming pressure leads to changes in heating behavior as well as morphologies of SiC crystals. ~ 4?MPa might be the optimized pre-forming pressure for both microwave plasma effects and E-field intensification.     

Evidence for non-thermal microwave effect in processing of tailing-based glass-ceramics

Tailing-based glass-ceramics were crystallized via conventional and microwave heating at 720 °C for 30 min and 820 °C without holding and compared in order to obtain evidence for a non-thermal microwave effect. The comparative analytical results showed that the microstructural uniformity was greatly enhanced and the crystallization activation energy was significantly reduced by microwave processing compared to conventional heating, suggesting accelerated grain growth during crystallization. Microwave radiation affected the crystal orientation and induced corresponding changes in the SiO bond lengths and SiOSi angles, thus enhancing the formation of the diopside crystal structure. In addition, the samples under microwave processing exhibited superior physicochemical performance, including greater relative density, bending strength, microhardness, and resistance to acids and alkali, compared to conventionally processed samples. All these results provided positive evidence supporting the existence of a genuine microwave non-thermal effect, allowing deepened understanding for fine control over microwave-assisted metallurgy.     

A novel microwave absorption material of Ni doped cryptomelane type manganese oxides

Cryptomelane type manganese oxide α-MnO₂ and Ni doped KMn₈O₁₆ nanostructures were synthesized by water-bathing methods at 80 °C for 24 h using NiSO₄·H₂O as the dopant sources. The structures, morphologies and physical properties were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The results show that the products are Ni doped KMn₈O₁₆ nanorods after the introduction of NiSO₄·H₂O during the reaction process. The electromagnetic characteristics and microwave absorption properties of the materials were carried out with a vector network analyzer (VNA) and the transmission line (TML) theory. The dielectric loss and microwave absorption properties of the cryptomelane materials are improved after Ni doping. The thickness dependent reflection loss shows that the peak frequency and effective absorption bandwidth all decrease with the increasing material thickness. With the increase of Ni doping concentration, the peak frequency shifts to higher frequency bands and the effective absorption bandwidth increases. The electromagnetic performance of cryptomelane can be attributed to its unique tunnel structures and the improvement of Ni doping can be due to the enhanced electromagnetic polarization.