COMBINED CONVECTIVE-MICROWAVE DRYING OF AGAR GELS: INFLUENCE OF MICROWAVE POWER ON DRYING KINETICS

The use of microwave energy in the drying of deformable material such as gel considerably reduces drying time and enables the control of retraction in the sample. A further advantage is that no hot spots are produced, allowing a dry product of superior quality to be obtained.The aim of this work has been to determine the kinetics of the convective-microwave drying process of agar gel plates. For this purpose, we developed a pilot closed loop, computer-controlled apparatus of convective-microwave drying, that enables the drying air conditions to be changed and the microwave power to be supplied over a wide value range. The equipment also records the sample surface temperature by means of an infrared thermometer. The drying curves obtained for plane geometry present four different drying phases: an initial phase where a rapid increase in the drying rate and in the surface temperature can be observed, as well as a constant rate phase that ends in the so-called convective critical moisture content, a first falling rate phase that concludes in the microwave critical moisture point, and finally a second falling rate phase. Combined convective-microwave drying enables a considerable reduction in drying time compared to convective drying, the time required being inversely proportional to the microwave power supplied. The empirical equation that best represents the kinetics is of the Page type. The absorbed volumetric power in terms of the moisture content was experimentally estimated, with the experimental data fitting an empirical equation.     

Local doping of silicon by a point-contact microwave applicator

The feasibility of local doping in silicon by an open-end coaxial applicator with a tip made of the doping material, e.g. aluminum or silver, is studied in this paper. In these experiments, localized microwave power of 100-350 W at 2.45 GHz was applied for ∼1 min to obtain doped regions of ∼1-mm width and ∼0.3-μm depth. Independent measurements of secondary ion mass spectroscopy (SIMS) and junction built-in potential measured by atomic-force microscopy (AFM) were used to estimate the activated doping concentrations in the order of 10¹⁹ and 10²² cm⁻³ for aluminum and silver doping, respectively. Potential barriers (pn junctions) of 0.5-0.7 V were measured across the aluminum-doped regions, and I-V characteristics were observed. The doping experiments were conducted in air atmosphere, hence oxidation effects were observed as well. The localized-microwave doping concept presented here could be useful in small-scale semiconductor processes, integrated optics, and MEMS applications.     

复合的手征介质材料

手征介质是近年来受到特别关注的科学,本文对左手手征介质材料的工作原理、构成和在微波、辐射波等方面的应用作了讨论.     

微波晶体管振荡器的计算机优化设计

本文用负阻概念分析了微波晶体管振荡器的基本工作原理及设计方法，着重讨论了微波振荡器的电路特性，给出了精确可靠的数学模型，并编写出最优化设计程序，从而提高了微波振荡电路的设计效率和精度。     

Recent inversion procedures for microwave imaging in biomedical, subsurface detection and nondestructive evaluation applications

Systems for electromagnetic imaging at microwave frequencies are of great interest in many applications ranging from industrial diagnostics to subsurface inspection and medical imaging. The development of efficient inverse-scattering-based procedures represents a critical aspect, due to the difficulties inherent in this nonlinear ill-posed problem. In this paper, some of the recently proposed inversion methods (both nonlinear and linearized) for two-dimensional imaging are reviewed. Concerning industrial imaging, nondestructive evaluation and medical diagnostic, tomographic approaches are presented, whereas for the inspection of buried inhomogeneities, tomographic and borehole-configurations are considered. In particular, deterministic and stochastic approaches are discussed and the most specific features of these approaches are pointed out. Finally, the issue of data collection, which is another key point of microwave imaging system, is briefly addressed, with particular emphasis on approaches based on the modulated scattering technique, which is a promising technique strongly related to electromagnetic scattering concepts.     

Microwave sensitivity maximization of disbond characterization in conductor backed dielectric composites

A swept frequency microwave nondestructive technique for detection of disbond in a dielectric composite backed by a conducting plate is discussed. The theoretical development is based on an incident plane wave illuminating such a medium, and then formulating the characteristics of the reflected wave. This way the effective reflection coefficient of the medium is found. The phase of this reflection coefficient changes in the presence of a disbonded medium. With the determination of an optimally sensitive frequency range using the theoretical model and the known properties of the composite dielectric material, it is shown that disbonds on the order of a few microns can be accurately detected at relatively low microwave frequencies. Several experimental results are also presented.     

Radically different effects on materials by separated microwave electric and magnetic fields

Using a 2.45 GHz wave-guided cavity, in a single mode TE₁₀₃ excitation, we were able to physically locate compacted 5 mm pellets of samples separately at the H (magnetic) node (where the E field is nearly zero), or the E (electric) node (where H field is nearly zero). A preliminary survey of a variety of metals, (Cu, Fe, Co..) ceramics (ZnO, etc.), and composites, (WC-Co, ZnO-Co) showed remarkable differences in their heating behaviors. The results establish conclusively that the magnetic field interaction contributes greatly to microwave heating of common materials in a manner, previously neglected in most theories of microwave heating, albeit still to be understood. Received: 15 June 2001 / Accepted: 15 June 2001     

Hierarchical porous silica membranes with dispersed Pt nanoparticles

Multifunctional silica membranes with a hierarchical porosity and containing dispersed Pt nanoparticles were prepared by an original one pot microwave-assisted sol–gel route. These membranes exhibit three porosity levels: interconnected micropores (<2 nm) in silica walls, isolated ordered mesopores (4 nm), and isolated macropores (70 nm). They were directly coated on tubular macroporous alumina supports without any intermediate mesoporous layer contrary to conventional membrane architectures. The isolated macropores and mesopores enable to increase the membrane permeability whereas the interconnected microporosity defines the membrane cut-off. The catalytic Pt nanoparticles (4 nm) mainly hosted in the mesoporous volume, are stabilised against undesirable agglomeration under working conditions. A first series of multifunctional membranes were prepared, in which preferential adsorption of hydrocarbon gases and efficient propene oxidation were evidenced. In order to avoid any possible interconnection between macropores, several strategies were investigated, which prevented sol infiltration in the macroporous support during the deposition process. These original multifunctional membranes are potentially attractive for gas separation and catalytic reactor applications.     

Tunable photoluminescence of nanostructured LaPO4:Eu3+/Tb3+ synthesized via a microwave-assisted ethylene glycol route

Nanostructured phosphors LaPO₄:Eu³⁺/Tb³⁺ were prepared via a combined approach using microwave heating and a bifunctional solvent (ethylene glycol) acting as both microwave absorber and capping agent. The synthesized samples were characterized by X-ray diffraction (XRD), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), transmission electron microscopy (TEM) and photoluminescence (PL) measurements. All samples crystallized in a monoclinic monazite-type structure. Electron microscopy analysis revealed a hierarchical organization of self-assembled seed crystals of lanthanum phosphate into nanoparticles that, in turn, gave rise to sponge-like aggregates. The co-doped samples exhibited self-activated blue luminescence from the host matrix, as well as red and green emissions due to the presence of Eu³⁺ and Tb³⁺ ions, respectively. Furthermore, the spectroscopic analysis indicated energy transfer from terbium to europium ions.The synthetic route described here is efficient to prepare nanomaterials with advanced optical properties, which exhibit a potential for applications in photonics, sensing and biolabelling.     

Desorption and re-adsorption of carbon nanotubes: comparisons of sodium hydroxide and microwave irradiation processes

This study evaluates exhausted carbon nanotubes regenerated by desorption processes using 1mM NaOH and microwave irradiation processes. Kinetic analyses of re-adsorption were performed using pseudo first- and second-order models. Regression results revealed that a pseudo first-order model accurately captured re-adsorption kinetics. The regeneration efficiency was 28, 30, 35 and 44% at 18, 28, 38 and 48 degrees C using desorption agent of 1mM NaOH for 24h, respectively. Microwave power was considered the most important factor in regeneration experiments, as the temperature reached by exhausted CNTs was directly related to microwave power in this study. Additionally, microwave regeneration was more effective than the desorption agent of NaOH. The most effective conditions for regenerating exhausted CNTs were a microwave power input of 1000 W for 20 min.