Global insight into microwave stoneware firing: Macro and microstructural changes

Industrial competition and environmental concerns lead to the exploration of alternative and energy-efficient technologies for ceramic materials processing. The main objective of this work was to present microwave heating as a viable option for stoneware processing. Stoneware functional properties are presented and discussed, with emphasis on impact strength, water absorption, porosity, and color. Microstructure analyses show that microwave- and gas-fired samples have higher densifications than electrically fired samples. A relevant finding for processing conditions is that microwave firing requires temperatures approximately 100°C lower than those required by conventional firing. Microwave-fired samples’ rupture energies are approximately twice (0.57 ± 0.06 (J)) those of the reference samples (0.26 ± 0.03 (J)), and their water absorptions are approximately one-half (1.5% at 1170°C and 0.8% at 1190°C) of those of the reference samples (2.0%), whereas the water absorption of electrically fired samples at 1180°C has been estimated to be 7.5%. Color analysis also evidences a shift to lower microwave firing temperatures, what is attributed to the enhanced transformations promoted by microwave heating when comparing with the transformations promoted by conventional (gas or electric) heating.     

Importance of Site III Cation of Zeolite A in Microwave Heating

Properties of dehydrated Na_{12 – 2x} Ca _x -A zeolites with x = 0, 1.1 and 4 in microwave heating were experimentally and theoretically investigated and compared with one another. Final heating temperatures of zeolites by microwave radiation were measured against radiation time and the easiness of heating was in the order of Na₁₂-A Na₁₀Ca₁-A > Na₄Ca₄-A for every irradiating condition studied. Microwave (relative) absorption efficiencies of the zeolites were calculated as a function of temperature with a simple method by using dielectric properties of the dehydrated states. In every temperature calculated, the order of absorption efficiency was Na₁₂-A Na₁₀Ca₁-A Na₄Ca₄-A. From these results, it was clearly verified that Na⁺ ion on the site III played the most important role in absorbing microwaves because Na₁₂-A has one Na⁺ ion on the site III but Na₁₀Ca₁-A and Na₄Ca₄-A have no cations on the site.     

微波加热用于活性炭的制备、再生和改性

从微波加热的应用着手,阐述了微波辐照的热效应、微波在活性炭的制备、再生和改性过程中的作用机理;指出了微波加热处理活性炭有待进一步研究的问题。     

可随机成型的透波性胶凝材料的微波加热特性

研究了硬化铝酸钙材料在不同煅烧温度（700,800,900,1000,1100,1200,1300,1400和1500℃）下的吸波能力、抗压强度、体积收缩率和质量损失率,采用XRF和XRD对不同煅烧温度（800,1000,1300和1500℃）下的试样进行表征,利用铝酸钙材料制备一种简易杯状容器并对其进行微波冲击特性测试。结果表明：硬化铝酸钙材料的吸波能力随煅烧温度的升高而降低,超过1000℃后,吸波能力趋于稳定;材料抗压强度随煅烧温度的升高先升高,在800℃左右达到最高,然后逐渐降低,并在1300℃达到最低,再经过小幅地升高后趋于稳定;同时,材料煅烧后体积收缩率和质量损失率都随着温度的升高而增加;微波冲击特性测试表明硬化铝酸钙材料在长时间微波辐照下强度基本不发生变化,微波-金属放电试验证明该胶凝材料硬化后的高温稳定性好,具有优异的抗高温冲击能力。综上所述,1000℃煅烧后的硬化铝酸钙材料具有透波性能好、力学性能优异、高温稳定性好的特点,非常适用于微波加热领域。     

微波技术在水处理领域的应用

对当前微波辐射技术在水处理领域的应用和研究状况进行了综述,着重介绍了微波加热原理与特点及在水处理领域的应用,讨论了应用中存在的一些问题,并展望了微波技术在水处理领域的应用前景。     

Ultra-fast firing: Effect of heating rate on sintering of 3YSZ,with and without an electric field

It has recently been reported that ceramics can be sintered in a few seconds with the aid of an electric field (“flash sintering”). This investigation tests the possibility that the accelerated sintering is a consequence of the rapid heating rate involved rather than a direct effect of the electric field on mass transport. The sintering of 3YSZ powder compacts at a temperature of ∼1300 °C was compared (i) in flash sintering, (ii) with rapid heating rates produced without the application of an electric field, and (iii) with conventional heating rates. The results show that rapid heating can accelerate sintering by over 2 orders of magnitude compared with heating to the same temperature at conventional rates, even without the application of an electric field. It is concluded that the rapid densification in flash sintering of 3YSZ is at least partly a consequence of the rapid heating involved. Possible explanations are discussed.     

煅烧温度对MgO-CaO-SiO_2材料性能的影响

为了提高MgO-CaO耐火材料的抗水化性能,在白云石粉(≤0.037 4 mm)掺加10%(w)的石英粉(≤0.045 mm)合成MgO-CaO-SiO2耐火材料,并着重探究了煅烧温度分别为1 400、1 500、1 600、1 650℃时对MgO-CaO-SiO2耐火材料致密性及抗水化性能的影响。结果表明:随着煅烧温度的升高,试样的致密度增大,抗水化性增强,但煅烧温度达到1 650℃时出现过烧现象;在煅烧过程中,SiO2与CaO反应生成了3CaO·SiO2,但仍有游离态CaO存在,可净化钢液。     

关于日用瓷吸水率测试方法的商榷

针对目前日用瓷吸水率的测定方法，论述了引起测试误差的原因并提出了减小方法误差和偶然误差的方法。这种新方法要求制作试块，以石蜡填充开口气孔，借助密度计（精度０．００１ｃｍ３）测试体积密度、表观密度和真密度．用单位面积吸水质量或气孔率（真气孔率）表示陶瓷制品的瓷化程度。     

微波加热法脱除炼焦剩余氨水氨氮的工艺研究

采用微波加热法脱除炼焦剩余氨水中氨氮。结果表明,静态实验中,在加碱量为1.0008 g.(100 mL)-1、微波辐射时间为5 min、微波功率为900 W的条件下,氨氮脱除率为98.72%,剩余氨水中氨氮含量降到68 mg.L-1;动态实验中,当空气流量为0.3~0.7 m3.h-1、剩余氨水流量为4~12 mL.min-1、微波功率为90~900 W、加碱量与剩余氨水的体积比为(0.09~0.21)∶1时,对应氨氮脱除率分别为75.96%~94.36%、93.91%~75.96%、72.88%~93.50%和84.67%~93.01%。通过正交实验,确定最优工艺条件为:空气流量0.6 m3.h-1、剩余氨水流量4 mL.min-1、微波功率720 W、加碱量与剩余氨水的体积比0.15∶1,此时,氨氮脱除率达到94.58%,剩余氨水中氨氮含量降到287 mg.L-1。     

Dielectric properties of polymers at microwave frequencies: a review

A review of the dielectric loss spectra of polymers at microwave frequencies has been carried out. While the main focus of attention is the frequency range from 100 MHz to 100 GHz, loss spectra outside this region are also reviewed because variations in temperature can cause a shift of dielectric loss into or out of the microwave range. A large volume of data for low loss polymers (polyethylene, polypropylene and poly(tetrafluoroethylene)), which are used in the communications industry, was available for review. Other polymers, for which data were available and which have significant loss at microwave frequencies are: polystyrene, poly(vinylchloride), poly(vinylidene chloride), poly(vinylidene fluoride), poly(methyl-methacrylate), poly(methyl acrylate), poly(oxymethylene), poly(ethylene oxide), poly(propylene oxide), polyacetylene, and poly(sulphur nitride). Also, the microwave dielectric properties of engineering thermoplastics such as poly(phenylene oxide), polycarbonate and polysulphane have been reviewed. The origins of microwave dielectric loss in polymers are categorized as: (a) dipolar absorption dispersions in both crystalline and amorphous polymers; (b) dipolar losses due to impurities, additives or fillers in a polymer material; (c) microwave absorption in conducting polymers (polyacetylene and poly(sulphur nitride)) for which the current carriers are electrons; and (d) photon-phonon absorption spectra corresponding to the density of states in amorphous regions of a polymer material.     

Effect of conventional and microwave heating on the structural and physicochemical properties of silver-doped hydroxyapatite nanopowders

ABSTRACT

This paper discussed the influence of duration and modes of heating on structural and physicochemical properties of silver-doped hydroxyapatite (Ag-HAP) nanopowders (NPs). Conventional electric furnace and microwave (MW) heating modes were employed during the synthesis. MW mode of synthesis was relatively an efficient method as it prepared monolithic HAP NP in just one minute of heating. With the increase in duration of heating in both modes of heating; lattice parameters, crystal size, lattice strain, Ca/P ratio, and degree of crystallinity of HAP phase increased. The morphology of particles was rod-shaped having aspect ratio between 2 and 3. EDX confirmed the presence of Ag and corroborated the formation of apatite. The hydrodynamic diameter of Ag-HAP NPs was significantly bigger than the particle sizes calculated using XRD, FESEM, and TEM. Thus, an overall examination concluded MW as an efficient mode of synthesis, able to produce Ag-HAP NPs in a possible minimum time.     

GF/ACF电路屏复合材料吸波性能研究

将一定尺寸的玻璃纤维片(GF-sheet)填充至矩形活性碳纤维(Activated Carbon Fiber,ACF)电路屏空穴中,制备了GF/ACF电路屏复合材料,研究了GF-sheet尺寸对ACF电路屏复合材料电磁波反射及吸收特性的影响。结果表明,在均匀分布的ACF电路屏空穴中填充相应尺寸的GF-sheet能有效降低复合材料对电磁波的反射特性,增强吸波能力;GF水平排布的反射性能优于垂直排布;当在正方形ACF电路屏空穴中填充边长l=2 cm的GF-sheet时,最低反射值达到-29.8 d B,其吸波性能较未填充GF-sheet的复合材料,其-10 d B以下的最低反射衰减提高了60.4%,达到-21.5 d B,有效带宽有所拓宽;当在长方形ACF电路屏空穴中填充l=2 cm,w=1 cm的GF-sheet,反射衰减最低,最低反射衰减为-16.1 d B,有效带宽为12.1 GHz。     

Microwave heating of cottonseed: A pilot plant study

Two cottonseed samples (50 kg) were exposed to microwaves at 45 KW and 2450 MHz for 4 min in an air temperature of 94°C. The final temperature of the seed was 76°C, and the treatment caused a 20% reduction in seed moisture. Examination of the seed immediately after microwave heating (MWH) indicated no differences in the total or soluble protein content of the meal or the content, quality or color of the oil as compared with unheated seed. There was some reduction in the quality of the oil from unheated seed during storage, but only minimal reduction in the quality of the oil from the MWH samples. Protein quality deteriorated in all samples; however, the deterioration was reduced in the microwave-heated seed.     

化学交联法改善氧化淀粉胶粘剂性能的研究

以硼砂、甲苯二异氰酸酯(TDI)、环氧氯丙烷和三聚氰胺-甲醛树脂等分别作为氧化淀粉的交联改性剂,研究了交联改性剂类型对氧化淀粉胶粘剂耐水性、干燥速率等影响。结果表明:经交联改性后,氧化淀粉胶粘剂的耐水性及干燥速率均明显提高;当交联改性剂为三聚氰胺-甲醛树脂时,相应胶粘剂的吸水率(58.9%)相对最低、干燥时间(700 s)相对最短,说明其改性效果相对最好;当交联改性剂为硼砂时,相应胶粘剂的吸水率(144.9%)相对最高、干燥时间相对最长,说明其改性效果相对最差。用三聚氰胺-甲醛树脂交联改性淀粉胶粘剂制成的瓦楞纸板,在常温水中浸泡48 h后不开胶,完全满足包装纸箱用相关标准的耐水性指标要求。     

A review: production of activated carbon from agricultural byproducts via conventional and microwave heating

Production of activated carbon (AC) from agricultural byproducts is a research field that has gained increased interest in recent years because of its potential for the disposal of agro‐residues. At the same time, a beneficial byproduct that can be used in a number of environmental applications is produced. This paper surveys the developments in the production processes of AC from agricultural byproducts in the past 7 years from 2005 to 2012 via conventional and microwave heating. Emphasis is placed on the applied methodology and the influences of activating conditions, such as carbonization temperature, retention time, and impregnation ratio. From the review of AC production processes, agricultural wastes produced by a chemical method with microwave heating can be a source of AC with relatively higher surface area than that produced via conventional heating. © 2013 Society of Chemical Industry     

Investigation of Structural Changes of Alkaline-extracted Wood Using X-ray Microtomography and X-ray Diffraction: A Comparison of Microwave versus Conventional Method of Extraction

Abstract

The effect of extraction of wood components on wood anatomy and cellulose crystallinity was studied using X-ray computed microtomography (micro CT) and X-ray crystallography. Micro CT of the xylem vessels of birch wood samples was used for the quantitative determination of the wood porosity after conventional and microwave extraction. The method was also used as an indirect means for the determination of temperature generated inside the fibers. Original porosity of birch wood was 18.5 ± 1.5%, and porosity increased with both extractions. The increase in wood porosity after 10 minutes of microwave extraction was double that of wood after conventional method of extraction at 90°C for two hours (42% vs. 26%), indicating a sudden rupture of the wood structure due to the volumetric heating effect produced by the microwave heating. Comparison of porosities of the conventionally extracted samples at different temperatures for the same duration indicated that the temperature generated inside the fibers during 10 minutes of microwave extraction is around 120°C. Crystallinity of cellulose did not change after both extractions, suggesting that the extraction did not affect the strength of the fibers.     

Microwave assisted flow synthesis: Coupling of electromagnetic and hydrodynamic phenomena

This article describes the results of a modeling study performed to understand the microwave heating process in continuous‐flow reactors. It demonstrates the influence of liquid velocity profiles on temperature and microwave energy dissipation in a microwave integrated milli reactor‐heat exchanger. Horizontal cocurrent flow of a strong microwave absorbing reaction mixture (ethanol + acetic acid, molar ratio 5:1) and a microwave transparent coolant (toluene) was established in a Teflon supported quartz tube (i.d.: 3 × 10^?3 m and o.d.: 4 × 10^?3 m) and shell (i.d.: 7 × 10^?3 m and o.d.: 9 × 10^?3 m), respectively. Modeling showed that the temperature rise of the highly microwave absorbing reaction mixture was up to four times higher in the almost stagnant liquid at the reactor walls than in the bulk liquid. The coolant flow was ineffective in controlling the outlet reaction mixture temperature. However, at high flow rates it limits the overheating of the stagnant liquid film of the reaction mixture at the reactor walls. It was also found that the stagnant layer around a fiber optic temperature probe, when inserted from the direction of the flow, resulted in much higher temperatures than the bulk liquid. This was not the case when the probe was inserted from the opposite direction. The experimental validations of these modeling results proved that the temperature profiles depend more on the reaction mixture velocity profiles than on the microwave energy dissipation/electric field intensity. Thus, in flow synthesis, particularly where a focused microwave field is applied over a small tubular flow reactor, it is very important to understand the large (direct/indirect) influence of reactor internals on the microwave heating process. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3824–3832, 2014     

Reusable,Polymer‐Supported,Palladium‐Catalyzed,Atom‐ Efficient Coupling Reaction of Aryl Halides with Sodium Tetraphenylborate in Water by Focused Microwave Irradiation

A rapid and efficient cross‐coupling reaction of sodium tetraphenylborate with aryl bromides was carried out in water at 120 °C in the presence of a polymer‐supported palladium catalyst and potassium carbonate under focused microwave irradiation. All four phenyl groups of sodium tetraphenylborate participated in the reaction and produced polyfunctional biaryls in excellent yields. The polymeric catalyst can be easily separated from the reaction mixture and reused more than 10 times without showing any decrease in activity.     

Microwave irradiated and thermally heated olive stone activated carbon for nickel adsorption from synthetic wastewater: A comparative study

In attempt to compare the removal efficiency and yield of the activated carbon prepared using the conventional and microwave‐assisted heating is the focus of this work. Toward this olive stone (a biomass precursor) is activated using the popular activating agent potassium hydroxide. The process optimization exercise is carried out by using the standard full factorial statistical design of experiments (response surface methodology). The activated carbons prepared under the optimized conditions are compared based on the adsorption capacity and yield. The adsorption capacity was found higher using microwave heating as compared with conventional heating. The microwave heating requires significantly lesser holding time as compared to conventional heating method to produce activated carbon of comparable quality, with higher yield. The BET surface area of carbon using microwave heating is significantly higher than the conventional heating. Although the mesopore surface area of carbon is not vary significantly, the activation time, power, and nitrogen gas consumption are significantly lower than the conventional heating rendering that the activation process via microwave is more economical than that via conventional heating. The adsorption isotherm data fitted the Langmuir isotherm well and the monolayer adsorption capacity was found to be 12.0 and 8.42 mg/g for microwave and thermally heated activated carbon, respectively. Regeneration studies showed that microwave‐irradiated and thermally heated olive stone could be used several times by desorption with an HCl reagent. Both carbons can be used for the efficient removal of Ni²⁺ (>99%) from contaminated wastewater. © 2013 American Institute of Chemical Engineers AIChE J, 60: 237–250, 2014     

添加剂对铜渣还原尾渣微晶玻璃析晶行为及性能的影响

铜渣还原尾渣与CaO-Al₂O₃-SiO₂系微晶玻璃的化学组成相似,采用熔融法以铜渣还原尾渣为主要原料制备微晶玻璃可为铜渣的高效利用提供一种有效途径。通过DSC、XRD和SEM-EDS等技术研究了不同添加剂(B₂O₃、CaF₂、TiO₂和Cr₂O₃)对微晶玻璃物相组成、抗折强度、体密度、吸水率和显气孔率等性能的影响。结果表明:无论添加哪种添加剂,微晶玻璃均能析出力学性能较好的钙长石和钙铝黄长石,且能使晶体析晶方式转化为整体析晶;添加B₂O₃可使微晶玻璃性能最优,其次是TiO₂与CaF₂。