Heat transfer in microwave heating

Heat transfer is considered as one of the most critical issues for design and implement of large-scale microwave heating systems, in which improvement of the microwave absorption of materials and suppression of uneven temperature distribution are the two main objectives. The present work focuses on the analysis of heat transfer in microwave heating for achieving highly efficient microwave assisted steelmaking through the investigations on the following aspects: (1) characterization of microwave dissipation using the derived equations, (2) quantification of magnetic loss, (3) determination of microwave absorption properties of materials, (4) modeling of microwave propagation, (5) simulation of heat transfer, and (6) improvement of microwave absorption and heating uniformity. Microwave heating is attributed to the heat generation in materials, which depends on the microwave dissipation. To theoretically characterize microwave heating, simplified equations for determining the transverse electromagnetic mode (TEM) power penetration depth, microwave field attenuation length, and half-power depth of microwaves in materials having both magnetic and dielectric responses were derived. It was followed by developing a simplified equation for quantifying magnetic loss in materials under microwave irradiation to demonstrate the importance of magnetic loss in microwave heating. The permittivity and permeability measurements of various materials, namely, hematite, magnetite concentrate, wüstite, and coal were performed. Microwave loss calculations for these materials were carried out. It is suggested that magnetic loss can play a major role in the heating of magnetic dielectrics. Microwave propagation in various media was predicted using the finite-difference time-domain method. For lossy magnetic dielectrics, the dissipation of microwaves in the medium is ascribed to the decay of both electric and magnetic fields. The heat transfer process in microwave heating of magnetite, which is a typical magnetic dielectric, was simulated by using an explicit finite-difference approach. It is demonstrated that the heat generation due to microwave irradiation dominates the initial temperature rise in the heating and the heat radiation heavily affects the temperature distribution, giving rise to a hot spot in the predicted temperature profile. Microwave heating at 915 MHz exhibits better heating homogeneity than that at 2450 MHz due to larger microwave penetration depth. To minimize/avoid temperature nonuniformity during microwave heating the optimization of object dimension should be considered. The calculated reflection loss over the temperature range of heating is found to be useful for obtaining a rapid optimization of absorber dimension, which increases microwave absorption and achieves relatively uniform heating. To further improve the heating effectiveness, a function for evaluating absorber impedance matching in microwave heating was proposed. It is found that the maximum absorption is associated with perfect impedance matching, which can be achieved by either selecting a reasonable sample dimension or modifying the microwave parameters of the sample.

Chemical reactions of double bonds in activated carbon:microwave and bromination methods

The reaction of localised C=C bonds on the surface of activated carbons has been shown to be an effective method of chemical modification especially using microwave-assisted reactions.

Quantification of free auxins in semi-hardwood plant cuttings and microshoots by dispersive liquid–liquid microextraction/microwave derivatization and GC/MS analysis

Several studies have suggested that differences in the natural rooting ability of plant cuttings could be attributed to differences in endogenous auxin levels. Hence, during rooting experiments, it is important to be able to routinely monitor the evolution of endogenous levels of plant hormones. This work reports the development of a new method for the quantification of free auxins in auxin-treated Olea europaea (L.) explants, using dispersive liquid–liquid microextraction (DLLME) and microwave assisted derivatization (MAD) followed by gas chromatography/mass spectrometry (GC/MS) analysis. Linear ranges of 0.5–500 ng mL 1 and 1–500 mg mL 1 were used for the quantification of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA), respectively. Determined by serial dilutions, the limits of detection (LOD) and quantification (LOQ) were 0.05 ng mL 1 and 0.25 ng mL 1, respectively for both compounds. When using the calibration curve for determination, the LOQ corresponded to 0.5 ng mL 1 (IAA) and 0.5 mg mL 1 (IBA). The proposed method proved to be substantially faster than other alternatives, and allowed free auxin quantification in real samples of semi-hardwood cuttings and microshoots of two olive cultivars. The concentrations found in the analyzed samples are in the range of 0.131–0.342 mg g 1 (IAA) and 20–264 mg g 1 (IBA).

Poly(2-oxazoline) hydrogels for controlled fibroblast attachment

Currently there is a lack of choice when selecting synthetic materials with the cell-instructive properties demanded by modern biomaterials. The purpose of this study was to investigate the attachment of cells onto hydrogels prepared from poly(2-oxazoline)s selectively-functionalized with cell adhesion motifs. A water-soluble macromer based on the microwave-assisted cationic ring-opening polymerization of 2-methyl-2-oxazoline and 2-(dec-9-enyl)-2-oxazoline was functionalized with the peptide CRGDSG or controls using thiol-ene photochemistry followed by facile crosslinking in the presence of a dithiol crosslinker. The growth of human fibroblasts on the hydrogel surfaces was dictated by the structure and amount of incorporated peptide. Controls without any peptide showed resistance to cellular attachment. The benignity of the crosslinking conditions was demonstrated by the incorporation of fibroblasts within the hydrogels to produce three-dimensional cell-polymer constructs.

Synthesis of Isoflavone Conjugates

During this study different approaches were studied to obtain isoflavone sulphates, glucuronides and sulphoglucuronides. Three isoflavone disulphates (daidzein-di-O-sulphate, genistein-di-O-sulphate and glycitein-di-O-sulphate) and three isoflavonoid disulphates (dihydrodaidzein-di-O-sulphate, dihydrogenistein-di-O-sulphate and equol-di-O-sulphate) were synthesised in moderate yields by using in situ prepared pyridine sulphur trioxide complex, made from chlorosulphonic acid and pyridine. These disulphated compounds can be used to develop analytical procedures and study the biological activity of disulphated products. As the use of the HPLC-MS methods in the field of isoflavones has increased its popularity, deuterated isoflavone disulphates were synthesised. A new microwave assisted deuteration method, using CF3COOD, was developed for this purpose. Three polydeuterated isoflavone disulphates (daidzein-d6-di-O-sulphate, genistein-d4-di-O-sulphate and glycitein-d6-di-O-sulphate) were obtained in moderate yields with high isotopic purity. A synthetic method was developed for daidzein sulphoglucuronide (daidzein-7-O-b-D-glucuronide-4´-O-sulphate), which is a major metabolite in rat bile. By using protection/deprotection steps, the desired product was finally obtained in moderate yield. The method developed can be used in further studies of synthesis of isoflavonoid mixed conjugates. As a part of this study, the structure of naturally occurring daidzein-4´-O-b-glucoside was verified. Different glycosidation methods are reviewed and possible factors affecting the stereoselectivity are discussed. The study of the selective chlorination of isoflavones was a consequence of the observed unexpected chlorination during the synthesis of isoflavone acid chlorides by thionyl chloride. This fascinating phenomenon was investigated further with various isoflavones and as a result a method for producing isoflavone chlorides (8-chlorogenistein, 6,8-dichlorogenistein and 6,8-dichlorobiochanin A) was developed. Protecting groups played a great role during this study, which led to an intensive study on them. A regioselective protection method was developed by using direct introduction of the protecting group (Benzyl and Benzoyl) to positions 7-O or 4´-O in daidzein, genistein and glycitein with t-BuOK as a base in DMF in moderate yields. The possibility of exploiting the transesterification was also investigated. It was observed that by using K2CO3 as a base in DMF, daidzein, genistein and glycitein could be benzoylated at position 4´-O selectively, in the presence of the more acidic 7 hydroxy group. Transesterification also proved to be useful in the glycosidation of isoflavones at position 7-O, starting from 7-O-benzoylated isoflavones. Different carboxylic acid derivatives were synthesised for use either in the development of radioimmunoassay (7-O-carboxymethylglycitein and 4´-O-carboxymethylglycitein) or synthesis of daunorubicin isoflavone derivative for biological testing (7-O-carboxypropylbiochanin A and 7-O-carboxypropylgenistein).

Uncoupled surface spin induced exchange bias in α-MnO 2 nanowires

We have studied the microstructure, surface states, valence fluctuations, magnetic properties, and exchange bias effect in MnO2 nanowires. High purity α-MnO 2 rectangular nanowires were synthesized by a facile hydrothermal method with microwave-assisted procedures. The microstructure analysis indicates that the nanowires grow in the [0 0 1] direction with the (2 1 0) plane as the surface. Mn3+ and Mn2+ ions are not found in the system by X-ray photoelectron spectroscopy. The effective magnetic moment of the manganese ions fits in with the theoretical and experimental values of Mn4+ very well. The uncoupled spins in 3d3 orbitals of the Mn 4+ ions in MnO 6 octahedra on the rough surface are responsible for the net magnetic moment. Spin glass behavior is observed through magnetic measurements. Furthermore, the exchange bias effect is observed for the first time in pure α-MnO2 phase due to the coupling of the surface spin glass with the antiferromagnetic α-MnO2 matrix. These α-MnO2 nanowires, with a spin-glass-like behavior and with an exchange bias effect excited by the uncoupled surface spins, should therefore inspire further study concerning the origin, theory, and applicability of surface structure induced magnetism in nanostructures.

Synthesis of inorganic solids using microwaves

Use of microwaves in the synthesis of materials is gaining importance. Microwave-assisted synthesis is generally much faster, cleaner, and more economical than the conventional methods. A variety of materials such as carbides, nitrides, complex oxides, silicides, zeolites, apatite, etc. have been synthesized using microwaves. Many of these are of industrial and technological importance. An understanding of the microwave interaction with materials has been based on concepts of dielectric heating and of the resonance absorption due to rotational excitation. This review presents a summary of recent reports of microwave synthesis of inorganic materials. Various observations regarding microwave interaction with materials are also briefly discussed.

Energy-efficient synthesis of ferrite powders and films

In recent years, there has been significant effort in the synthesis of nanocrystalline spinel ferrites due to their unique properties. Among them, zinc ferrite has been widely investigated for countless applications. As traditional ferrite synthesis methods are energy- and time-intensive, there is need for a resource-effective process that can prepare ferrites quickly and efficiently without compromising material quality. We report on a novel microwave-assisted soft-chemical synthesis technique in the liquid medium for synthesis of ZnFe2O4 powder below 100 °C, within 5 min. The use of β-diketonate precursors, featuring direct metal-to-oxygen bonds in their molecular structure, not only reduces process temperature and duration sharply, but also leads to water-soluble and non-toxic by-products. As synthesized powder is annealed at 300 °C for 2 hrs in a conventional anneal (CA) schedule. An alternative procedure, a 2-min rapid anneal at 300 °C (RA) is shown to be sufficient to crystallize the ferrite particles, which show a saturation magnetization (MS) of 38 emu/g, compared with 39 emu/g for a 2-hr CA. This signifies that our process is efficient enough to reduce energy consumption by ∼85% just by altering the anneal scheme. Recognizing the criticality of anneal process to the energy budget, a more energy-efficient variation of the reaction process was developed, which obviates the need for post-synthesis annealing altogether. It is shown that the process also can be employed to deposit crystalline thin films of ferrites.

Low-thermal-budget solution processing of thin films of zinc ferrite and other complex oxides

Further miniaturization of magnetic and electronic devices demands thin films of advanced nanomaterials with unique properties. Spinel ferrites have been studied extensively owing to their interesting magnetic and electrical properties coupled with stability against oxidation. Being an important ferrospinel, zinc ferrite has wide applications in the biological (MRI) and electronics (RF-CMOS) arenas. The performance of an oxide like ZnFe2O4 depends on stoichiometry (defect structure), and technological applications require thin films of high density, low porosity and controlled microstructure, which depend on the preparation process. While there are many methods for the synthesis of polycrystalline ZnFe2O4 powder, few methods exist for the deposition of its thin films, where prolonged processing at elevated temperature is not required. We report a novel, microwave-assisted, low temperature (<100°C) deposition process that is conducted in the liquid medium, developed for obtaining high quality, polycrystalline ZnFe2O4 thin films on technologically important substrates like Si(100). An environment-friendly solvent (ethanol) and non-hazardous oxide precursors (β-diketonates of Zn and Fe in 1:2 molar ratio), forming a solution together, is subjected to irradiation in a domestic microwave oven (2.45 GHz) for a few minutes, leading to reactions which result in the deposition of ZnFe2O4 films on Si (100) substrates suspended in the solution. Selected surfactants added to the reactant solution in optimum concentration can be used to control film microstructure. The nominal temperature of the irradiated solution, i.e., film deposition temperature, seldom exceeds 100°C, thus sharply lowering the thermal budget. Surface roughness and uniformity of large area depositions (50x50 mm2) are controlled by tweaking the concentration of the mother solution. Thickness of the films thus grown on Si (100) within 5 min of microwave irradiation can be as high as several microns. The present process, not requiring a vacuum system, carries a very low thermal budget and, together with a proper choice of solvents, is compatible with CMOS integration. This novel solution-based process for depositing highly resistive, adherent, smooth ferrimagnetic films on Si (100) is promising to RF engineers for the fabrication of passive circuit components. It is readily extended to a wide variety of functional oxide films.

Synthesis and characterisation of novel alkoxycyanobiphenyl-substituted rufigallols

Microwave-assisted synthesis of novel alkoxycyanobiphenyl-substituted rufigallols are reported by systematically replacing one, two, four, five or six cyanobiphenyl-tethered alkoxy chains. The synthesis of the target compounds was challenging since classical reactions failed to produce these hybrids. Chemical structures of the hybrids were determined by H-1 nuclear magnetic resonance (NMR), C-13 NMR, infrared, ultraviolet spectroscopy and elemental analysis. The thermotropic liquid crystalline properties of the new compounds were investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffractometry.

Pristine nanomaterials: synthesis, stability and applications

Capping-free and linker-free nanostructures/hybrids possess superior properties due to the presence of pristine surfaces and interfaces. In this review, various methods for synthesizing pristine nanomaterials are presented along with the general principles involved in their morphology control. In wet chemical synthesis, the interplay between various reaction parameters results in diverse morphology. The fundamental principles behind the evolution of morphology including nanoporous aggregates of metals and other inorganic materials, 2D nanocrystals of metals is elucidated by capping-free methods in aqueous medium. In addition, strategies leading to the attachment of bare noble metal nanoparticles to functional oxide supports/reduced graphene oxide has been demonstrated which can serve as a simple solution for obtaining thermally stable and efficient supported catalysts with free surfaces. Solution based synthesis of linker-free oxide-semiconductor hybrids and capping-free metal nanowires on substrates are also discussed in this context with ZnO/CdS and ultrathin Au nanowires as examples. A simple and rapid microwave-assisted method is highlighted for obtaining such hybrids which can be employed for high-yield production of similar materials.

Carbonaceous nickel oxide nano-composites: as electrode materials in electrochemical capacitor applications

Carbonaceous nickel oxide powder samples have been synthesized from an adducted nickel beta-ketoester complex used as a ``single source precursor'' through a solution-based microwave-assisted chemical route. Comprehensive analysis of the resulting powder material has been carried out using various characterization techniques. These analysis reveal that, depending on the solvent used, either NiO/C or Ni/NiO/C composites are formed, wherein Ni and/or NiO nanocrystals are enveloped in amorphous carbon. As the components emerge from the same molecular source, the composites are homogeneous on a fine scale, making them promising electrode materials for supercapacitors. Electrochemical capacitive behavior of these oxide composites is studied in a three-electrode configuration. With a specific capacitance of 113 F g(-1), Ni/NiO/C is superior to NiO/C as capacitor electrode material, in 0.1 M Na2SO4 electrolyte. This is confirmed by impedance measurements, which show that charge-transfer resistance and equivalent series resistance are lower in Ni/NiO/C than in NiO/C, presumably because of the presence of metallic nickel in the former. The cyclic voltammograms are nearly rectangular and the electrodes display excellent cyclability in different electrolytes: Na2SO4, KOH and Ca(NO3)(2)center dot 4H(2)O. Specific capacitance as high as 143 F g(-1), is measured in Ca(NO3)(2)center dot 4H(2)O electrolyte.

Influence of CeO2 morphology on the catalytic activity of CeO2-Pt hybrids for CO oxidation

Ceria, because of its excellent redox behavior and oxygen storage capacity, is used as a catalyst for several technologically important reactions. In the present study, different morphologies of nano-CeO2 (rods, cubes, octahedra) were synthesized using the hydrothermal route. An ultrafast microwave-assisted method was used to efficiently attach Pt particles to the CeO2 polyhedra. These nanohybrids were tested as catalysts for the CO oxidation reaction. The CeO2/Pt catalyst with nanorods as the support was found to be the most active catalyst. XPS and IR spectroscopy measurements were carried out in order to obtain a mechanistic understanding and it was observed that the adsorbed carbonates with lower stability on the reactive planes of nanorods and cubes are the major contributor to this enhanced catalytic activity.

Dispersible crystalline nanobundles of YPO4 and Ln (Eu, Tb)-doped YPO4: rapid synthesis, optical properties and bio-probe applications

Undoped and Ln(3+) (Eu and Tb)-doped crystalline nanobundles of YPO4 were prepared by a facile microwave-assisted route with water as a solvent and without using any surfactant. TEM investigations reveal that the as-prepared powder consists of lenticular-shaped nanobundles (similar to 100 nm in diameter) made of very small nanorods with diameter less than 10 nm and length varying from 20 to 50 nm. Each nanorod in turn is single crystalline, as revealed by HRTEM imaging. The as-prepared nanobundles are easily dispersible in various solvents, especially water, without any surface functionalization, which is critical for various bio-probe applications like cell and tissue imaging. The Eu- and Tb-doped YPO4 nanobundles show good photoluminescence properties and were further evaluated for their use as fluorescent biolabels. Our results show that HeLa cells labelled with Eu- and Tb-doped YPO4 nanobundles show bright red (Eu) and green (Tb) intracellular luminescence under a confocal microscope. Concentration-and time-dependent MTT cell viability assays show that the nanobundles show low toxicity towards cells which makes them promising in bioimaging field.

Flower-like porous cobalt(II) monoxide nanostructures as anode material for Li-ion batteries

In the present study, a microwave-assisted, solution-based route has been employed to obtain porous CoO nano structures. Detailed characterization reveals that the flower-like nanostructures comprise petal-like sheets, each of which is made of an ordered, porous arrangement of crystallites of CoO measuring about 6 nm. TEM analysis shows that each ``petal'' is an oriented aggregate of CoO nanocrystals, such aggregation promoted by the hydroxyl moieties derived from the solution. The structure provides a large specific area as well as the porosity desirable in electrodes in Li-ion batteries. Electrochemical measurements carried out on electrodes made of nanostructured CoO show excellent Li ion-storing capability. A specific capacitance of 779 mAh g(-1) has been measured at a specific current of 100 mA g(-1). Measurements show also excellent cyclability and coulombic efficiency. Impedance spectroscopy provides evidence for charge transfer occurring in the porous networks. (C) 2015 Elsevier B.V. All rights reserved.