Influence of various shapes of annular metallic support on microwave heating of 2D cylinders

A detailed analysis has been shown to illustrate the influence of various shapes of annular metallic support on microwave heating of 2D cylinders for beef samples. The beef samples are chosen to represent typical food materials with high dielectric loss. A preliminary analysis on microwave heating of samples has been shown via average power within a sample vs cylinder radius diagram in the absence of support and three regimes have been identified such that regimes I and III denote the local minima in average power and regime II corresponds to a maxima in average power. In addition, regime I corresponds to a spatial maxima in power at the unexposed face, regime II corresponds to maxima in power at both the exposed and unexposed faces and regime III has two maxima at the exposed face and the center of the sample. The influence of support has been illustrated with average power distribution vs aspect ratio (φ) diagram. Three types of cross sections of annular supports such as circular (type I), horizontal square (type II) and inclined square (type III) are considered. It has been observed that the average power is enhanced due to the metallic support for φ?0.3. Type I support would correspond larger power distributions and heating rates, for regimes I and II whereas for regime III, the square shaped supports correspond to greater heating rates at larger aspect ratios. The location of spatial maxima in power or heating rate is found to be shifted due to metallic support with high aspect ratios. The shape of the support plays important role on localized heating for the samples corresponding to regime III.     

Properties of ceramic foam catalyst supports: mass and heat transfer

Mass and heat transport properties have been determined for 30 PPI -Al₂O₃ ceramic foam containing 6 wt.% γ-Al₂O₃ washcoat. The foam was loaded with 5 wt.% platinum and the rate of carbon monoxide oxidation measured for a 0.3 cm cylindrical segment of the foam operating with mass transfer controlling at 550 °C. This gave a mass transfer factor versus Reynolds number correlation that was equivalent to a packed bed of particles.

A correlation for the radial heat transfer coefficient in a bed of ceramic foam was determined by measuring outlet temperatures achieved when air at varying flow rates and inlet temperatures was passed through a bed of foam pellets. Correlation parameters of a 1D model were fitted from 700 to 1000 °C using a Simplex optimization routine. Radial heat transfer coefficients were two to five times higher than those predicted from packed bed correlations.     

Manufacture and optimization of tubular ceramic membrane supports

The manufacturing and optimization of centrifugally casted ceramic membrane supports is presented. For the optimization, the effect of three different powder sizes (0.25, 0.31 and 0.61 μm) and a sintering temperature range between 1050 and 1400 °C was investigated. The ceramic tubes were characterized according to tube dimensions, mercury porosimetry, water permeability, SEM and mechanical strength. It was shown that the centrifugal casting technique delivers highly reproducible support properties. A novel strength testing apparatus was developed to determine the mechanical strength of the ceramic tubes. It was found that the strength varied between 3300 MPa and 300 MPa, depending on the porosity of the supports. With increased sintering temperature, water permeability and porosity decreased, while strength and linear shrinkage increased. The pore diameter of the supports produced by the 0.31 and 0.61 μm powders decreased, while that of the 0.25 μm powders remained constant (72 nm) with increasing sintering temperature. The 0.61 μm powder sintered at the lowest sintering temperature resulted in the support with the highest porosity, pore diameter and porosity, but the lowest mechanical strength and linear shrinkage. An overall improvement concerning pure water permeability was seen when the support in this study was compared to our own previous studies and similar studies in literature.     

The deposition of γ-Al2O3 layers on ceramic and metallic supports for the preparation of structured catalysts

Deposition of γ-Al₂O₃ washcoats onto Aluminium and FeCrAlloy^® supports in the form of slabs and onto -Al₂O₃ tubes was performed according to a two-step procedure involving: (a) deposition of a bohemite primer, (b) deposition of the γ-Al₂O₃ layer, both by dip coating into powder dispersions in HNO₃ aqueous solutions. We present herein data concerning the effects of the major preparation variables (HNO₃ and H₂O concentrations in the dispersion, withdrawal velocity, drying temperature, number of dipping cycles, calcination temperature) on the deposited coating load and on the adherence of the washcoat. Based on a rheological characterisation of the γ-Al₂O₃ dispersions, we propose also a correlation between their apparent viscosity and the washcoat load. Finally, we briefly illustrate the activity in catalytic combustion reactions of structured Pd-based catalysts prepared according to the investigated washcoating methods.     

Macro-porous ceramic supports for membranes prepared from quartz sand and calcite mixtures

A new type of porous ceramic supports for membranes has been designed. The new supports have been fabricated from polycrystalline quartz sand and calcite raw materials. In this work, two configurations of support (tubular and flat) have been produced using extrusion method. The open porosity, the pore size distribution, the average pore size (APS), the strength and the permeability of sintered supports have been found to depend mainly on the weight ratio of calcite (CaCO₃) additive. The results showed that with the addition of 15–35 wt.% of calcite and sintering temperature of about 1375 °C for 1 h the best characteristics of sintered supports could be obtained. The developed tubular ceramic supports with the APS 6.3–12 μm, open porosity 42–55%, the water permeability (16–68 m³/h m² bar) and flexural strength 8–18 MPa hopefully offer many perspectives for a wide use in membranes technology.     

A generalized analysis on material invariant characteristics for microwave heating of slabs

A generalized dimensionless formulation has been developed to predict the spatial distribution of microwave power and temperature. The ‘dimensionless analysis’ is mainly based on three numbers: wave number, ; free space wave number, ; and penetration number, , where is the ratio of sample thickness to wavelength of microwaves within a material, is based on wavelength within free space and is the ratio of sample thickness to penetration depth. The material dielectric properties and sample thicknesses form the basis of these dimensionless numbers. The volumetric heat source due to microwaves can be expressed as a combination of dimensionless numbers and electric field distributions. The spatial distributions of microwave power for uniform plane waves can be obtained from the combination of transmitted and reflected waves within a material. Microwave heating characteristics are obtained by solving energy balance equations where the dimensionless temperature is scaled with respect to incident microwave intensity. The generalized trends of microwave power absorption are illustrated via average power plots as a function of , and . The average power contours exhibit oscillatory behavior with corresponding to smaller for smaller values of . The spatial distributions of dimensionless electric fields and power are obtained for various and . The spatial resonance or maxima on microwave power is represented by zero phase difference between transmitted and reflected waves. It is observed that the number of spatial resonances increases with for smaller regimes whereas the spatial power follows the exponential decay law for higher regimes irrespective of and . These trends are observed for samples incident with microwaves at one face and both the faces. The heating characteristics are shown for various materials and generalized heating patterns are shown as functions of , and . The generalized heating characteristics involve either spatial temperature distributions or uniform temperature profiles based on both thermal parameters and dimensionless numbers ().     

Synthesis of high-area chemically modified electrodes using microwave heating

The synthesis of dimensionally stable carbon-based chemically modified electrodes (CMEs) is a challenge. The synthesis method, the electrode composition, and the heating used influences various properties of the electrodes. Here, we show an innovative, low cost, and efficient heating method for the CMEs production. We deposited ternary ruthenium, antimony, and tantalum or bismuth oxides on carbon-felts by the Pechini method focusing on the influence of different heating methods (conventional and microwave) on their electrochemical and physical properties. The oxides synthesized using microwave heating coated completely the carbon-felts fibers, displaying homogeneous morphology, while maintaining their three-dimensional character. XRD and XRF analyses confirm the presence of the desired oxides on the coating surfaces and the experimental metal load values close to the nominal (87–91% for Ru, 3–6% for Sb, and 4–7% for Ta or Bi, respectively). The microwave-assisted method yields CMEs with electrochemical active areas 136-fold (for tantalum-based) and 153-fold (for bismuth-based) higher than the unmodified carbon felt. The service lifetime of the CMEs is almost twice as high when using microwave heating than when using conventional heating, which is attributed to the complete and homogeneous coating of the fibers obtained using this heating method. In contrast, the conventional heating led to the incomplete covering of the fibers and to CMEs with low stability. Therefore, the use of microwave heating for the CMEs synthesis reduces the production time at around 60%, thus decreasing the production costs and producing CMEs with improved quality and stability when compared to both conventional-made CMEs and unmodified electrodes.     

Fabrication of ceramic bioscaffolds from fly ash cenosphere by susceptor-assisted microwave sintering

Cenospheres (CS) are ceramic hollow microspheres and have been used to prepare composite foams for applications such as medical implants. However, its potential standalone application in the biomedical field is not fully explored. Herein, a susceptor-assisted microwave (SMW) sintering approach was used for producing CS foam scaffolds. Owing to the hybrid heating mechanism offered by the SMW process, sintering of the low-dielectric cenospheres was realized. We found that sintering was initiated at a lower temperature (1100 °C) compared to conventional heating (1250 °C) as reported in the literature, probably due to the lower activation energy required by SMW sintering. The physical and compositional properties of the sintered CS specimens were examined, and in vitro studies were performed. The as-fabricated CS foam possessed minimal effect on cell viability. Cells migrated and adhered well within the pores of the specimens, which indicates the potential of the CS as scaffold materials for cell engineering applications.     

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.     

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.     

多孔氧化铝支撑体的制备与表征

以α-Al2O3粉末为骨料,采用塑性挤压成型技术和固态粒子烧结法制备Al2O3多孔陶瓷支撑体。研究了粘结剂和成孔剂的种类对支撑体性能的影响。研究结果表明:通过选用合适的粘结剂和成孔剂,可制得孔径分布窄、孔隙率高的支撑体。     

Effects of microwave heating on dielectric and piezoelectric properties of PZT ceramic tapes

Commercial lead zirconate titanate (PZT) perovskite powders were used to fabricate ceramic tape and then sintered by microwave and conventional methods. Both dielectric and piezoelectric properties of PZT ceramic tapes were studied in terms of sintering process. X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM) show the PZT perovskite phase with smaller grain size and dense microstructure can be obtained at a lower sintering temperature by microwave process. It was also observed that shrinkage ratio and bulk density of the tapes sintered at 800 °C were obtained about 19% and 7.46 g/cm³ by the microwave heating method, respectively, that is corresponding to those values of sintered PZT tapes at 950 °C by conventional process. Moreover, the dielectric constant and maximum permittivity are increased about 30% as compared with conventional processing method. The experimental results demonstrated that the characteristics of the PZT tapes could be significantly improved by microwave heating method. These results demonstrate that such a simple approach can upswing the piezoelectric and dielectric properties of these tapes by using microwave process with a short heating time.     

Design of double-layer ceramic absorbers for microwave heating

We propose a guide for designing double-layer ceramic absorbers in microwave heating by optimizing the thickness based on the analysis of reflection loss (RL) of a double-layer absorber consisting of a high-loss SiC layer and a low-loss Al₂O₃ layer. The calculated reflection losses for individual layers of SiC and Al₂O₃ show that the former with a thickness of 0.0054 m has the maximum microwave absorption while the latter in the thickness range up to 0.1 m is identified as a poor microwave absorbing material with RL larger than −0.4 dB. By using a 0.0054-m-thick SiC layer as the susceptor, the absorption in the Al₂O₃ layer and of the entire double-layer absorber increases significantly. The results demonstrate that high microwave absorption throughout the heating process can only be achieved in a sample with a small thickness in which a slight absorption peak shift during heating (less than one eighth-wavelength in the medium) occurs.     

Reliability prediction of a microwave sintered Si3N4-based composite ceramic tool

The wear life reliability prediction model of microwave sintered Si₃N₄/(W,Ti)C/Y₂O₃/MgO/Al₂O₃ composite ceramic tools based on the random distribution characteristics of hardness and fracture toughness of ceramic tool material was established. It showed that the Vickers hardness of ceramic tool materials followed a normal distribution and the fracture toughness followed a lognormal distribution. Distribution law of wear life can be determined by the joint distribution of hardness and fracture toughness. Experimental research on tool reliability of continuous dry cutting quenched high quality carbon steel T10A was carried out and the applicability of the tool reliability prediction model was verified. The results showed that the error between the theoretical reliable life and the actual life of the ceramic tool was less than 5% under the same reliability when the reliability was above 0.5.     

Bond characteristics and microwave dielectric ceramic of rare-earth tantalite NdTaO4 ceramic

A type of rare-earth tantalite ceramic NdTaO₄ was synthesized by the solid-state reaction method and investigated for the relationship between the structure and microwave dielectric properties for the first time. X-ray diffraction and Rietveld refinement confirmed that an M-fergusonite phase was formed. The dense micromorphology and uniform grain distribution obtained at 1500 °C are advantageous for the microwave dielectric properties. Based on the different bonding properties in the crystal, we investigated the dependence between the crystal structure and the dielectric properties of NdTaO₄, where the ionicity of Nd–O bond in the crystal structure mainly determines the dielectric constant compared with Ta–O bond, and the lattice energy of Ta–O contributes significantly to the quality factor, the TaO₄ and NdO₈ polyhedron contributes collectively to the thermal stability. The optimum microwave dielectric properties were obtained for 1500 °C - sintered samples: ε_r = 18, Q × f = 13,000 GHz (at 8.9 GHz), τ_f = ?21 ppm/^°C.     

Significantly enhanced sinterability and temperature stability of Li3Mg4NbO8-based microwave dielectric ceramics with LiF and Ba3(VO4)2 addition

Li₃Mg₄NbO₈-basic composite ceramics were elaborated via the solid-state reaction process, in which LiF and Ba₃(VO₄)₂ were utilized as a sintering aid and reinforcement phase, respectively. The sinterability, phase assemblage, microstructures, and microwave dielectric performances of Li₃Mg₄NbO₈–LiF–Ba₃(VO₄)₂ composite ceramics were thoroughly researched. The co-addition of LiF–Ba₃(VO₄)₂ can simultaneously lower the sintering temperature and improve the thermal stability of Li₃Mg₄NbO₈-basic ceramics. Solid state activated sintering is responsible for the low-temperature densification of the present ceramics. The coexistence of rock-salt structural Li₃Mg₄NbO₈/Li₄Mg₄NbO₈F and hexagonal structural Ba₃(VO₄)₂ phases was demonstrated by the combinational XRD and SEM-EDS analysis results. The 0.65(Li₃Mg₄NbO₈–LiF)-0.35Ba₃(VO₄)₂ ceramics fired at 825 °C/5 h exhibited promising microwave dielectric performances: τ_f = 0.5 ppm/°C along with ε_r = 13.8 and Qxf = 68500 GHz. The good compatibility of the developed ceramics with Ag demonstrates it potential for use in LTCC technology.     

苯加氢重沸器加热工艺的比较与分析

介绍了苯加氢蒸馏塔重沸器采用蒸汽加热和导热油加热2种工艺流程,针对蒸汽加热存在的问题进行分析并提出解决办法,对2种工艺流程的经济性和安全性进行比较,可根据实际情况确定需采用的加热工艺。     

Preparation and characterization of Ir-based catalysts on metallic supports for high-temperature steam reforming of methanol

Ir-based catalysts on heat-resisting foil supports with different washcoats were investigated for hydrogen production by high-temperature steam reforming of methanol. Al₂O₃, Ce_0.8Zr_0.2O₂–Al₂O₃, Ce_0.8Zr_0.2O₂/Al₂O₃ and Ce_0.8Zr_0.2O₂ coatings were prepared on the metallic supports and iridium was deposited on them as the active component. The samples were characterized by X-ray powder diffraction (XRD), ultrasonic vibration test, scanning electron microscope (SEM) and temperature-programmed reduction (TPR). The performance of the catalysts for steam reforming of methanol was evaluated with a fixed-bed reactor. It was found that the phase structure, the shape of the surface particles and the coating adherence were different from each other for the four kinds of coatings. The activities, selectivities and stabilities of these Ir-based catalysts on metallic supports were compared to select the optimal one for use in high-temperature methanol steam reforming. The results indicated that the Ir/Ce_0.8Zr_0.2O₂/Al₂O₃/FeCrAl catalyst showed better performance than the other catalysts, which is a promising candidate for hydrogen production via the methanol steam reforming process in Pd membrane reactors.     

Influencing factors on the performance of tubular ceramic membrane supports prepared by extrusion

Inorganic ceramic membrane has the advantages of excellent thermal stability, mechanical strength, chemical resistance and fouling resistance. And it has been widely studied and applied in flue gas moisture recovery, gas purification and wastewater treatment. The support is the basis of preparation and application of inorganic ceramic membrane, and its performance determines the performance of the ceramic membrane to a certain extent. At present, extrusion is a common method used in the ceramic industry to prepare supports, which has the advantages of high production efficiency and good product uniformity. In this paper, the processes of preparing support by extrusion including mixing, vacuum pugging, aging, extruding and sintering are introduced in detail. In preparing support by extrusion, the product performance is easily affected by many factors, including the type of raw materials, raw material particles, extrusion parameters, pore-forming agents, sintering temperature, and sintering aids. Therefore, this paper also conducts a comparative and comprehensive study on the process parameters and influencing factors in the process of preparing ceramic membrane support by extrusion to obtain the support with the best performance. The main influencing factors that this paper focuses on are the types of raw materials, powder particles, water content, pore-forming agents, binders, sintering temperature, and sintering aids. The trends and mechanisms of these factors affecting the performance of the support are analyzed in detail.     

氧化铝多孔支撑体的制备工艺

叙述了用挤出成型法制备无机膜支撑体的工艺,研究了原料粒径、成孔剂用量和烧结温度对所制得多孔氧化铝支撑体性能的影响。研究结果表明,用粒径小于10μm的-αA l2O3粉体,以7%碳粉为成孔剂,烧结温度为1300℃,可以成功制得孔径分布较窄、平均孔径为2.1μm、孔隙率为48.9%的多通道无机膜支撑体。