Microwave heating of ceramic laminates

The microwave heating of a ceramic laminate composed of three layers is modeled and analyzed. Two materials with widely disparate effective electrical conductivities comprise the laminate. The ratio of these conductivities is exploited as a small parameter in the development of an asymptotic theory. Two physically distinct situations are considered. In the first, a low-loss ceramic is surrounded by lossy material. Here, the asymptotic theory yields simplified equations which are reduced to a nonlinear Volterra integral equation. The integral equation is amenable to analysis, and the results are physically interpreted. In the second situation, a lossy material is surrounded by a low-loss material. In this case the asymptotic theory yields simplified equations which are analyzed through numerical techniques. Again, the results are physically interpreted to attain insight into the dynamics and parameter dependence of the microwave heating of ceramic laminates.     

Microwave heating of laminate panels

The use of microwaves to heat laminate panels occurs in a variety of industrial processes, from chemical vapor infiltration (CVI) systems to the curing of adhesives in laminate panels. The electrical conductivity of the materials used in these systems is typically temperature-dependent. Characteristically, the thickness of the laminate panel is on the order of wavelength of the incident microwave, but the thickness of the laminate sheet is much smaller. This allows us to apply asymptotic techniques to find averaged wave and heat equations when the direction of the incident microwave is normal to, or tangent to, the laminates. These equations are analyzed in the small-Biot-number limit and are numerically approximated using finite differences. The results are in excellent agreement for small Biot numbers. More importantly, heating trends are observed for a wide variety of volume fractions for two particular CVI applications. In addition, the effect of the incident polarization on the heating process are also established. In particular, the use of a TE polarized incident microwave is shown to be inefficient in certain CVI applications, but produces a more favorable temperature gradient.     

Microwave conductivity from magnetic impurities in an s-wave superconductor

We calculate the optical conductivity in an s-wave superconductor interacting with a low density of magnetic impurities. Interactions are treated beyond the Born approximation so that an impurity band develops inside the superconducting gap. We find that the contribution of these states to the low frequency conductivity is large, and that they have a clear spectroscopic signature which would would make them observable in conventional superconductors.     

Microwave ferrite materials and devices

The crystal structure of ferrites including garnets and their methods of preparation are first reviewed. This is followed by an examination of microwave ferrite devices and their application to actual systems. Sufficient theoretical details as well as an extensive though not exhaustive list of references are included for ready reference. The most widely used microwave ferrite device is the isolator, a device which possesses nonreciprocal properties. The function of this device in various configurations is therefore studied in some detail. Then, ferrite-phase shifters, including the latching variety, circulators, modulators, power limiters, switches, amplifiers, delay lines, and filters are examined in turn. Microwave ferrite devices, in particular isolators and circulators, are commonly used in the laboratory. Other devices, such as modulators, limiters, and switches, are used respectively in microwave systems in carrier modulation, receiver protection, and antenna switching, etc. Much of the ferrite device research and development in recent years has been concerned with theY-junction circulator and the coincidence power limiter.     

Microwave rapid heating for the synthesis of gold nanorods

A microwave heating system with a sealed reaction chamber and a precise temperature-controlled module was employed to assess the synthesis of tetradecylammonium bromide (TOAB) cation surfactant-stabilized gold nanorods. Scanning electron microscopy (SEM) was employed to characterize the size and the shape of particles. It was found that both TOAB concentrations and reaction temperature programs played an important role in the formation of gold nanorods. By using higher TOAB concentration and faster rising reaction temperature, gold nanorods with higher aspect ratio could be produced. This finding demonstrated the combination of TOAB surfactant and microwave system to be a useful approach for the preparation of gold nanorods.     

Microwave dielectric heating to disassemble a modified cementitious joint

According to existing theory on the deterioration of cement and concrete, a cement hardener undergoes a hydrate disassembly reaction at high temperatures of 300 °C or above; this increases the internal void volume and decreases the residual strength. This weakness mechanism has already been applied to manufacturing recycled aggregates and has reached the stage of practical application for removing adhered mortar from the aggregate surface. However, heating using an external heat source consumes a considerable amount of energy, and greenhouse gases are emitted during the recycling process because of low energy efficiency. In this study, a susceptor with outstanding microwave heating efficiency was selected, and the temperature elevation characteristics from the microwave heating of cementitious material containing the susceptor were analyzed. Through measurement, the bond strength of a cementitious joint including a modifier with the susceptor was found to weaken after microwave heating; thus, a new cementitious joint that can be reused or recycled as construction material and members is proposed. If the susceptor can selectively make cement material vulnerable in a short time by absorbing microwaves at the cementitious joint, the combined waste from the cementitious joint can be separated into single materials.     

Microwave heating in multiphase systems: evaluation of series solutions

The 1D electric field and heat-conduction equations are solved for a slab where the dielectric properties vary spatially in the sample. Series solutions to the electric field are obtained for systems where the spatial variation in the dielectric properties can be expressed as polynomials. The series solution is used to obtain electric-field distributions for a binary oil-water system where the dielectric properties are assumed to vary linearly within the sample. Using the finite-element method temperature distributions are computed in a three-phase oil, water and rock system where the dielectric properties vary due to the changing oil saturation in the rock. Temperature distributions predicted using a linear variation in the dielectric properties are compared with those obtained using the exact nonlinear variation.     

Problems regarding the transient heating of heat-shielding materials

An analysis of problems regarding the transient heating and breakdown of heat-shielding materials is presented. The limiting heating rates for which allowance for the processes taking place in the interior of the heat-shielding materials becomes decisive are established.     

Microwave synthesis of electrode materials for lithium batteries

A novel microwave method is described for the preparation of electrode materials required for lithium batteries. The method is simple, fast and carried out in most cases with the same starting material as in conventional methods. Good crystallinity has been noted and lower temperatures of reaction has been inferred in cases where low temperature products have been identified     

Microwave initiated self-propagating high temperature synthesis of materials

Abstract

The use of microwave energy to initiate self-propagating, high temperature synthesis (SHS) reactions has been reviewed. Microwave initiation usually results in ignition occurring at the centre of the body, with the combustion wavefront propagating radially outwards. This leads to a number of differences compared with conventionally ignited SHS reactions. These include ignition in both weakly exothermic systems and denser green bodies, crude control of the wavefront propagation, and the generation of different microstructures owing to dissimilar time-temperature and spatial temperature profiles. The technology also extends the range of materials and compositions that can be produced in a self-propagating manner. Commercially important developments are likely to be those that utilise these features to produce tailored microstructures for niche applications     

Perovskite-type oxides as susceptor materials in dielectric heating

Dielectric properties of La_1−x Ce _x MnO₃ perovskites are investigated in order to assess the heating behaviour in the wider context of the use of perovskite coatings in microwave assisted soot filter regeneration. Dielectric permittivities in the radio and microwave region for these perovskites were determined at room temperature. The dielectric constant and dielectric loss are related to ionic conduction at low frequencies, while at microwave frequencies storage and loss mainly proceed through reorientation of molecular dipoles. The dielectric constant rises for a higher degree of La substitution by Ce, which is explained by an increase of the number of cation/oxygen vacancies. Concurrently the mean perovskite crystallite size decreases, which is possibly related to defect formation. The dielectric constant declines for x ≥ 0.3, along with the formation of a separate, low dielectric permittivity CeO₂ phase. The La–Ce–Mn perovskites are further shown to exhibit a high thermal stability during repeated heating/cooling cycles.     

Porous materials prepared by heating derivatives from halloysite

Halloysite clay was subjected to serious treatment by trimethylsilylation and hydrochloric acid. The resulting derivatives were heated below 1473 Kin nitrogen or air to prepare thermally stable porous materials including larger pores than those in zeolite. The trimethylsilylated halloysite resulted in a thermally stable porous material, in an amorphous state, including extremely uniform pores of 1.7 nm. Another derivative also resulted in a thermally cable porous material but consisted of very heterogeneous pores. The thermal degradation behaviour of both derivatives is discussed in detail.     

Microwave heating synthesis of HgS and PbS nanocrystals in ethanol solvent

A novel route was developed to prepare PbS and HgS nanocrystals in ethanol solvent in the presence of sodium hydroxide by microwave heating method. PbS and HgS nanocrystals were obtained with an average size of approximately 10 and 6 nm, respectively. In the reaction, mercury acetate and lead acetate were used as mercury and lead source; sulfur powder was employed as chalcogenide source. The products were characterized by X-ray powder analysis (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV–Vis absorption spectroscopy. The probable mechanism was presented.