4,570 research outputs found

    Analysis and calculation of macrosegregation in a casting ingot. MPS solidification model. Volume 1: Formulation and analysis

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    The physical and numerical formulation of a model for the horizontal solidification of a binary alloy is described. It can be applied in an ingot. The major purpose of the model is to calculate macrosegregation in a casting ingot which results from flow of interdendritic liquid during solidification. The flow, driven by solidification contractions and by gravity acting on density gradients in the interdendritic liquid, was modeled as flow through a porous medium. The symbols used are defined. The physical formulation of the problem leading to a set of equations which can be used to obtain: (1) the pressure field; (2) the velocity field: (3) mass flow and (4) solute flow in the solid plus liquid zone during solidification is presented. With these established, the model calculates macrosegregation after solidification is complete. The numerical techniques used to obtain solution on a computational grid are presented. Results, evaluation of the results, and recommendations for future development of the model are given. The macrosegregation and flow field predictions for tin-lead, aluminum-copper, and tin-bismuth alloys are included as well as comparisons of some of the predictions with published predictions or with empirical data

    MPS solidification model. Analysis and calculation of macrosegregation in a casting ingot

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    Work performed on several existing solidification models for which computer codes and documentation were developed is presented. The models describe the solidification of alloys in which there is a time varying zone of coexisting solid and liquid phases; i.e., the S/L zone. The primary purpose of the models is to calculate macrosegregation in a casting or ingot which results from flow of interdendritic liquid in this S/L zone during solidification. The flow, driven by solidification contractions and by gravity acting on density gradients in the interdendritic liquid, is modeled as flow through a porous medium. In Model 1, the steady state model, the heat flow characteristics are those of steady state solidification; i.e., the S/L zone is of constant width and it moves at a constant velocity relative to the mold. In Model 2, the unsteady state model, the width and rate of movement of the S/L zone are allowed to vary with time as it moves through the ingot. Each of these models exists in two versions. Models 1 and 2 are applicable to binary alloys; models 1M and 2M are applicable to multicomponent alloys

    Quantum Hall effect in exfoliated graphene affected by charged impurities: metrological measurements

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    Metrological investigations of the quantum Hall effect (QHE) completed by transport measurements at low magnetic field are carried out in a-few-μm\mu\mathrm{m}-wide Hall bars made of monolayer (ML) or bilayer (BL) exfoliated graphene transferred on Si/SiO2\textrm{Si/SiO}_{2} substrate. From the charge carrier density dependence of the conductivity and from the measurement of the quantum corrections at low magnetic field, we deduce that transport properties in these devices are mainly governed by the Coulomb interaction of carriers with a large concentration of charged impurities. In the QHE regime, at high magnetic field and low temperature (T<1.3KT<1.3 \textrm{K}), the Hall resistance is measured by comparison with a GaAs based quantum resistance standard using a cryogenic current comparator. In the low dissipation limit, it is found quantized within 5 parts in 10710^{7} (one standard deviation, 1σ1 \sigma) at the expected rational fractions of the von Klitzing constant, respectively RK/2R_{\mathrm{K}}/2 and RK/4R_{\mathrm{K}}/4 in the ML and BL devices. These results constitute the most accurate QHE quantization tests to date in monolayer and bilayer exfoliated graphene. It turns out that a main limitation to the quantization accuracy, which is found well above the 10910^{-9} accuracy usually achieved in GaAs, is the low value of the QHE breakdown current being no more than 1μA1 \mu\mathrm{A}. The current dependence of the longitudinal conductivity investigated in the BL Hall bar shows that dissipation occurs through quasi-elastic inter-Landau level scattering, assisted by large local electric fields. We propose that charged impurities are responsible for an enhancement of such inter-Landau level transition rate and cause small breakdown currents.Comment: 14 pages, 9 figure

    Cancelled Utility and Traditional Ratemaking Theories: Are Either Used and Useful

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    This Comment reviews and analyzes recent regulatory proceedings and decisions on cancelled plant losses and the means by which regulatory agencies have attempted to apply traditional ratemaking theories to allocate the losses between ratepayers and shareholders. The author provides a background on the investor owned electric utility industry and its regulatory aspects, as well as some of the ratemaking theories. The author concludes by providing recommendations for improving the application of traditional ratemaking theories to cancelled plant losses

    Development and fabrication of insulator seals for thermionic diodes

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    Eight different types of cermet seals for thermionic diodes were investigated: (1) 1 micron Al2O3 with Nb spheres; (2) 200 A Al2O3 with Nb spheres; (3) 1 micron Al2O3 with Nb 1% Zr spheres; (4) 200 A Al2O3 with Nb 1% Zr spheres; (5) Pure Y2O3 with Nb 1% Zr spheres; (6) Y2O3 3% ZrO2 with Nb 1% Zr spheres; (7) Y2O3 10% ZrO2 with Nb 1% Zr spheres; and (8) ZrO2 12% Y2O3 with Nb 1% Zr spheres. Investigations were made to determine the most favorable fabrication techniques and the effect of the bonding cycle, (length of bonding time and shutdown sequences). The analysis of the seals included tensile test, vacuum test, electrical test and metallurgical examination. At the conclusion of the development phase, 36 seals were fabricated for delivery for evaluation
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