Microwave plasma torch operating at a low pressure for material processing

Most operations involving the use of a microwave plasma torch are carried out at atmospheric pressure as an open system. A microwave plasma torch operated in a closed and isolated environment may provide an opportunity for the mass production of chemically active radicals for various chemical and biological processes. This study presents a microwave plasma torch operating at a low pressure in an isolated chamber. A microwave torch operating at a low pressure may be economical in terms of capital costs, maintenance costs, and operational costs compared to other plasma devices used in material processing operations. The properties of the torch plasma at a low pressure were investigated. It is found that the plasma profile at a low pressure is asymmetric with higher density on the incoming side of the microwaves. This behavior of the torch inhibits high-power operation of the microwave plasma torch at a low pressure. However, the asymmetry of the plasma profile disappears under a high gas flow rate. It was also found that a microwave plasma torch used at a low pressure can efficiently produce an abundance of chemical radicals.     

Concept of autonomous processing of liquid radioactive waste from ship nuclear power installations

The concept of autonomous processing of liquid radioactive waste from ship nuclear power installations is substantiated, and its implementation in the form of a module membrane sorption installation consisting of micro- and ultrafiltration, reverse-osmosis, and ion-exchange modules is suggested. Data on treatment of liquid radioactive waste of complex physicochemical composition using this installation are reported.     

Bias frequency dependence of pn junction charging damage induced by plasma processing

Plasma-induced charging damage to pn junction in metal-oxide-semiconductor field-effect transistors (MOSFETs) was studied by using an inductively coupled plasma (ICP) reactor with Ar gas. The junction leakage current (I_leak) was measured for various source/drain to well in MOSFETs and simple diode structures. Two different rf bias frequencies, 13.56 MHz and 400 kHz, were utilized to investigate the effect of frequency on an increase in I_leak. The I_leak of n⁺/p and p⁺/n junctions was found to increase by a plasma treatment. In particular, more severe damage was observed for n⁺/p junction in both n-channel MOSFETs and the diodes. These observed results imply that plasma plays a role primarily as a positive current source. With regard to the rf bias frequency effects, samples exposed to the 400-kHz plasma were found to suffer from larger I_leak than those to the 13.56 MHz. From capacitance-voltage (C-V) measurement of junction capacitance changes, we also clarified that the observed increase in I_leak was attributed to the defect density at pn junction created by the plasma charging damage.     

Techno-economic appraisal of waste cellulose processing

Clean Technologies and Environmental Policy - Huge quantities of waste cellulose fibres are being produced in textile, food and particularly paper industries. Their incineration without a costly...     

Microwave processing: fundamentals and applications

In microwave processing, energy is supplied by an electromagnetic field directly to the material. This results in rapid heating throughout the material thickness with reduced thermal gradients. Volumetric heating can also reduce processing times and save energy. The microwave field and the dielectric response of a material govern its ability to heat with microwave energy. A knowledge of electromagnetic theory and dielectric response is essential to optimize the processing of materials through microwave heating. The fundamentals of electromagnetic theory, dielectric response, and applications of microwave heating to materials processing, especially fiber composites, are reviewed in this article.     

Use of a plasma reactor with a three-jet mixing chamber for processing of liquid toxic waste materials

We present a brief review and show the outlook for processing of toxic waste materials by a plasma method. We carried out experimental investigations of the processing of liquid toxic organic and organochlorine waste materials in a plasma reactor with a three-jet mixing chamber. Translated from InzhenernoFizicheskii Zhurnal, Vol. 70, No. 1, pp. 87–92, January–February, 1997.     

Importance measures for nuclear waste repositories

Several importance measures are identified for possible use in the performance assessment of a high-level nuclear waste repository. These importance measures are based on concepts of importance used in system reliability analysis, but the concepts are modified and adapted to the special characteristics of the repository and similar passive systems. In particular, the importance measures proposed here are based on risk (in comparison to traditional importance measures which are based on frequency of failure) and are intended to be more suitable to systems comprised of components whose behavior is most easily and naturally represented as continuous, rather than binary. These importance measures appear to be able to evaluate systems comprised of both continuous-behavior and binary-behavior components. Three separate examples are provided to illustrate the concepts and behavior of these importance measures. The first example demonstrates various formulations for the importance measures and their implementation for a simple radiation safety system comprised of a radiation source and three shields. The second example demonstrates use of these importance measures for a system comprised of components modeled with binary behavior and components modeled with continuous behavior. The third example investigates the use of these importance measures for a proposed repository system, using a total system model and code currently under development. Currently, these concepts and formulations of importance are undergoing further evaluation for a repository system to determine to what degree they provide useful insights and to determine which formulations are most useful.     

Microwave plasma removal of sulphur hexafluoride

Sulphur hexafluoride (SF(6)) gas is a common pollutant emitted during the plasma etching of thin films and plasma cleaning chemical vapor deposition (CVD) production processes used in the semiconductor industry. In this paper a method using microwave (2.45GHz frequency) plasmas sustained at atmospheric pressure for the abatement of SF(6) is investigated experimentally for various gas mixture constituents and operating conditions, with respect to its ability to decompose SF(6) to less harmful molecules. The destruction and removal efficiencies (DRE) of plasma abatement of SF(6) at concentrations between 1.7 and 5% in nitrogen in the presence of water vapor were studied as a function of the total gas flow rate and microwave power. Water vapor proved to be an effective source of free radical species that reacts with the radicals and ions resulting from SF(6) fragmentation in the plasma and also, it proved to reduce the process by-products. It was measured that approximately 25% of the initial SF(6) is converted to SO(2). Destruction and removal efficiencies of SF(6) up to 99.9% have been achieved.     

Microwave plasma synthesis of silver nanopowders

Silver nanopowder was synthesized by the microwave plasma synthesis method. The effect of feeding rate of precursor raw material to the average particle size and crystallinity were studied. The microwave plasma unit can be modified to prepare silver nanopowder with polymeric coating. The silver nanopowder was characterized by means of transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDAX) and X-ray diffraction (XRD).     

Plasma-thermal processing of toxic halogen-containing organic waste

Results of experimental plasma-thermal processing of halogen-containing organic waste at temperatures of 4000-5000 K are reported. Analyses of exhaust gases of waste processing by chemicoanalytical and mass-spectrometric methods show complete thermal decomposition of the waste.     

Microwave processing of WC-Co composities and ferroic titanates

 The innovations in microwave processing of ceramics have been dominated to date by serendipitous discovery, because the interaction between such radiation as delivered via available tools and the materials of widely varying properties, sizes, and shapes is so complex that it has defied quantitative analysis. For over 10 years a wide variety of inorganic ceramic and semiconducting materials have been synthesized, sintered, and reacted in our own labs, including microwave hydrothermal synthesis of metals, ferrites, and electroceramic phases. These local results are summarized and used as the reference point for reporting on two different new advances: sintering of WC-Co composite tool bits and other similar objects in under 15 min, while retaining extremely fine microstructures, without any grain growth inhibitors; using reduced TiO₂ or Ta₂O₅ for the synthesis of phases such as BaTiO₃, Ba3MgTa₂O9, and Pb(Zr.Ti)O₃ in a few minutes in a 2.45 GHz field at the astonishing temperatures of 300–700 oC. Received: 2 January 1997 / Accepted: 27 March 1997     

Microwave emission from explosive plasma jets

Microwave emission from explosive plasma jets excited by RF pulses has been observed. Plasma jets excited by frequency-modulated signals have been studied, and the distributions of the number and duration of emission pulses over the microwave frequency range have been determined.     

Accelerated chemical aging of crystalline nuclear waste forms

Nuclear waste disposal is a significant technological issue, and the solution of this problem (or lack thereof) will ultimately determine whether nuclear energy is deemed environmentally friendly, despite significantly lower carbon emissions than fossil fuel energy sources. A critical component of any waste disposal strategy is the selection of the waste form that is tasked with preventing radionuclides from entering the environment. The design of robust nuclear waste forms requires consideration of several criteria, including: radiation tolerance, geological interaction and chemical durability; all of these criteria ensure that the radionuclides do not escape from the waste form. Over the past 30 years, there have been numerous and thorough studies of these criteria on candidate waste forms, including radiation damage and leaching. However, most of these efforts have focused on the performance of the candidate waste form at t = 0, with far less attention paid to the phase stability, and subsequent durability, of candidate waste forms during the course of daughter product formation; that is, the chemical aging of the material. Systematic understanding of phase evolution as a function of chemistry is important for predictions of waste form performance as well as informing waste form design. In this paper, we highlight the research challenges associated with understanding waste form stability when attempting to systematically study the effects of dynamic composition variation due to in situ radionuclide daughter production formation.     

Modeling cation exchange in zeolitic nuclear waste form

Zeolites are currently being considered as an encapsulation material for high-level nuclear wastes. As a first step toward understanding the mechanisms of radionuclide migration in such molecular crystals, we apply an atomistic simulation approach to the study of ion exchange, which is assumed to be the underlying chemical process for the release of radioactive cations. Specifically, we investigate the Cs-Na cation exchange isotherm for dehydrated sodalite waste form. Our calculations indicate significant relaxation effects of the sodalite cage, in the form of displacement of cation adsorption sites and cage distortion. This revised version was published online in July 2006 with corrections to the Cover Date.     

Microwave plasma synthesis of carbon-supported ultrafine metal particles

Microwave plasma decomposition of metal carbonyls has been used to synthesize a series of carbon-supported monometallic (Fe, Co) and bimetallic (Co-Mo) materials. The average metal particle diameters in all cases were less than 10 nm. By using 10% H₂/Ar instead of pure Ar as a carrier gas, the mean particle diameters could be decreased to less than 2 nm. Although the distribution of particles is slightly broader than those generated by other nanoparticle synthesis methods, the mean diameter of the particles generated using 10% H₂/Ar is as small as any previously reported for Fe-, Co-, Ni-, or Mo-containing compounds. The ultrafine metal particles were dispersed on moderate surface area amorphous carbon support matrixes derived from the concomitant microwave decomposition of the toluene solvent.     

Flash calcines of kaolinite: Effect of process variables on physical characteristics

The physical transformation of powdered kaolinite associated with rapid dehydroxylation during flash calcination has been followed using pycnometry, thermogravimetry, electron microscopy, X-ray powder diffraction and infrared spectroscopy. 116 partially dehydroxylated kinetically-frozen calcines were produced in a laboratory flash calciner covering the following ranges of process variables: calciner temperature 700–1000 °C, rate of heating to calciner temperature 4700–15000 Ks^–1, residence time at the calciner temperature 0.1–1.5 s, He or N₂ calciner atmosphere. From the analyses of these calcines a picture of the changes in physical characteristics caused by flash heating of kaolinite has emerged and is described.