Thermal-tempering analysis of bulk metallic glass plates using an instant-freezing model

The viscoelastic nature of bulk metallic glasses (BMGs), their low thermal conductivity, and the fast cooling used in their processing subject them to thermal tempering. This process leads to a residual stress state in which compression on the surface is balanced by tension in the interior. For the first time, we have calculated such stresses in metallic glasses by adapting an analytical instant-freezing model previously developed for silicate glasses. This model has been demonstrated to be reasonably accurate in predicting the final residual stresses, although, due to its very nature, it neglects transient effects. For an infinite plate geometry and employing processing parameters often used for metallic glasses, we predict that significant residual stresses can be generated in these materials during thermal tempering. Preliminary measurements conducted using the layer-removal method yield compressive residual stress values close to model predictions.     

Development of a Four-Channel Potentiostat for Amperometric Detection in Flow Injection Analysis

Abstract

A four-channel multipotentiostat has been developed for simultaneous voltammetric measurements with microelectrode arrays. To reduce the complexity of electrochemical cell, only one reference and one auxiliary electrode are used. The potential ranges of the four working electrodes may be independently controlled, but they are synchronously scanned by only one DAC. This is accomplished through a microcomputer interfaced with a PCL-711B data acquisition board, which allows the continuous monitoring of the currents and actual potentials of the working electrodes.

The system has been applied to the simultaneous determination of ascorbic and uric acids in synthetic samples. The measurements were carried out with gold microelectrodes modified by deposition of noble metals, which enhance the electrocatalysis and provide an anticipation and a better definition of the oxidation potentials. Recuperation of 98.2% and 97.8% for ascorbic and uric acids, respectively, was obtained by using amperometry.     

Stability of the chromia layer on steels in flowing nonequilibrium H2-H2O-CH4-CO mixtures at 950 °C

The stability of the protective oxide layer formed on Fe-12.5 pct Cr, Fe-18 pct Cr-10 pct Ni, and Fe-25 pct Cr-20 pct Ni was investigated in a flowing nonequilibrium H2-H2O-CH4-CO mixture at 950 °C. The samples were preoxidized in H2-H2O, and subsequently the carbon activity was raised in stepwise manner by adding methane. CO formation occurred on the sample surface by the catalytic reaction CH4 + H2O → CO + 3H2. The conversion of oxide to carbide commenced at the end of the sample wherePco has its highest andPH2O its lowest value. Oxidation, conversion, and catalytic reaction were monitored by gravimetric measurements and CO-determination during the experiments. Partial equilibria can be calculated for (ac)1 =K ₁ P _{ch 4}/(Ph2)2, (ac)2 =K2(PcoPh2)ph{2o}and(Po2)1\2 = K3Ph2o/Ph2. The conversion of oxide to carbide started when the steady state ofac and(Po2)1/2 on the surface approached the value for the equilibrium of chromium oxide and chromium carbide. The results show that this steady state value is much nearer to the point(ac)2-(Po2)1/2 in the thermochemical stability diagram than to the point (ac)1 — (Po ₂)^1/2- The oxide layers were stable even at very high values of (ac)₁ which demonstrates that there is only very little interaction of CH₄ with the oxide surface. P. C. Prasannan formerly Research Scientist at the Max-Planck-Institut für Eisenforschung This paper is based on a presentation made at the “Stephen R. Shatynski Memorial Symposium on Surfaces and Interfaces” held at the 114th annual AIME meeting in New York, February 24-28, 1985, under the auspices of the ASM-MSD Thermodynamic Activity Committee.     

Laser-induced desorption of small molecules from oxide surfaces: A first-principles study

Recent efforts in the theoretical simulation of laser-induced desorption of small molecules from surfaces are summarized. As a representative example, photodesorption of CO molecules from a Cr₂O₃(0001) surface is investigated since detailed quantum state resolved experimental results are available for this system. In particular, vectorial properties such as the alignment of the desorbing species are considered. Furthermore, the influence of surface temperature as a control parameter is investigated, and lateral velocity distributions are calculated and compared with experimental results. All simulations presented in the present study are based on ab initio potential energy surfaces (PESs) for the electronic ground state as well as electronically excited states involved in the desorption process. These PESs provide the prerequisite for extensive high-dimensional quantum mechanical simulations of the dynamics of nuclear motion based on a stochastic wave packet scheme. These wave packet calculations allow for a detailed microscopic understanding of experimental results and provide a perspective for future experiments.     

Intentional ρ-type doping by carbon in metalorganic MBE of GaAs

We report on the intentional ρ-type doping of GaAs layers grown in an UHV system from molecular beams of arsine (AsH₃) and mixtures of frimethyl gallium (TMG) and friethyl gallium (TEG). The entire doping range between 10¹⁴ cm^-3 (growth from pure TEG) and 10²⁰ cm^-3 (growth from pure TMG) can be covered by using mixtures of TMG and TEG. As revealed by SIMS and photoluminescence (PL) carbon is the dominant acceptor in the layers. Comparison of the Hall mobility and of the PL spectra shows that the quality of our films equals that of the best LPE and MBE grown ρ-type GaAs layers.