An oxidation treatment, often termed "pre-oxidation", is performed on austenitic stainless steel prior to joining to alkali barium silicate glass to produce hermetic seals. The resulting thin oxide acts as a transitional layer and a source of Cr and other elements which diffuse into the glass during the subsequent bonding process. Pre-oxidation is performed in a low pO2 atmosphere to avoid iron oxide formation and the final oxide is composed of Cr2O3, MnCr2O4 spinel, and SiO2. Significant heat-to-heat variations in the oxidation behavior of 304L stainless steel have been observed, which result in inconsistent glass-to-metal (GTM) seal behavior. The objectives of this work were to characterize the stainless steel pre-oxidized layer and the glass/oxide/304L interface region after glass sealing. The 304L oxidation kinetics were determined by thermogravimetric (TG) analysis and the glass/metal seals characteristics were studied using sessile drop tests, in which wetting angles were measured and glass adhesion was analyzed. The pre-oxidized layers and glass/metal interface regions were characterized using metallography, focused ion beam (FIB) sectioning, scanning and transmission electron microscopy, and electron probe microanalysis (EPMA). The results show that poor glass sealing behavior is associated with a more continuous layer of SiO 2 at the metal/oxide interface.
This research involves the measurement of the electrical conductivity (K) for the ESR (electroslag remelting) slag (60 wt.% CaF{sub 2} - 20 wt.% CaO - 20 wt.% Al{sub 2}O{sub 3}) used in the decontamination of radioactive stainless steel. The electrical conductivity is measured with an improved high-accuracy-height-differential technique that requires no calibration. This method consists of making continuous AC impedance measurements over several successive depth increments of the coaxial cylindrical electrodes in the ESR slag. The electrical conductivity is then calculated from the slope of the plot of inverse impedance versus the depth of the electrodes in the slag. The improvements on the existing technique include an increased electrochemical cell geometry and the capability of measuring high precision depth increments and the associated impedances. These improvements allow this technique to be used for measuring the electrical conductivity of highly conductive slags such as the ESR slag. The volatilization rate and the volatile species of the ESR slag measured through thermogravimetric (TG) and mass spectroscopy analysis, respectively, reveal that the ESR slag composition essentially remains the same throughout the electrical conductivity experiments.
The availability of polysilicon feedstock has become a major issue for the photovoltaic (PV) industry in recent years. Most of the current polysilicon feedstock is derived from rejected material from the semiconductor industry. However, the reject material can become scarce and more expensive during periods of expansion in the integrated-circuit industry. Continued rapid expansion of the PV crystalline-silicon industry will eventually require a dedicated supply of polysilicon feedstock to produce solar cells at lower costs. The photovoltaic industry can accept a lower purity polysilicon feedstock (solar-grade) compared to the semiconductor industry. The purity requirements and potential production techniques for solar-grade polysilicon have been reviewed. One interesting process from previous research involves reactive gas blowing of the molten silicon charge. As an example, Dosaj et all reported a reduction of metal and boron impurities from silicon melts using reactive gas blowing with 0{sub 2} and Cl{sub 2}. The same authors later reassessed their data and the literature, and concluded that Cl{sub 2}and 0{sub 2}/Cl{sub 2} gas blowing are only effective for removing Al, Ca, and Mg from the silicon melt. Researchers from Kawasaki Steel Corp. reported removal of B and C from silicon melts using reactive gas blowing with an 0{sub 2}/Ar plasma torch. Processes that purify the silicon melt are believed to be potentially much lower cost compared to present production methods that purify gas species.
Decontamination of radioactive contaminated stainless steel using the ESR process is investigated by conducting thermophysical and thermochemical laboratory studies on the slag. The ESR base slag investigated in this research project is 60wt%CaF{sub 2}-20wt%CaO-20wt%Al{sub 2}O{sub 3}. In this report, we present the data obtained to date on relevant slag properties, capacity to incorporate the radioactive contaminant (using CeO{sub 3}) as surrogate, simulant for PUO{sub 2} and UO{sub 2}, slag-metal partition coefficient, volatilization rate and volatile species, viscosity, electrical conductivity and surface tension as a function of temperature. The impact of these properties on the ESR decontamination process is presented.
Approximate probabilities of inclusion survival through an electron beam melting hearth are computed from nitride dissolution rates, flotation velocities, and residence times. Dissolution rates were determined by measuring shrinkage rates of pure TiN and nitrided sponge in small pools of molten titanium in an electron beam melting hearth. Flotation velocities were calculated using correlations for fluid flow around spheres, and show that particles sink or float unless their densities are extremely close to that of molten titanium. Flow field characteristics which lead to effective inclusion removal are discussed in terms of heat flux pattern required to produce them, based on the electron beam`s unique ability to impart a nearly arbitrary heat flux pattern to the melt surface.
Experimental results and a mathematical model are presented to describe differential evaporation rates in electron beam melting of titanium alloys containing aluminum and vanadium. Experiments characterized the evaporation rate of commercially pure titanium, and vapor composition over titanium with up to 6% Al and 4.5% V content as a function of beam power, scan frequency and background pressure. The model is made up of a steady-state heat and mass transport model of a melting hearth and a model of transient thermal and flow behavior near the surface. Activity coefficients for aluminum and vanadium in titanium are roughly estimated by fitting model parameters to experimental results. Based on the ability to vary evaporation rate by 10-15% using scan frequency alone, we discuss the possibility of on-line composition control by means of intelligent manipulation of the electron beam.