Adrian S. Sabau

Transfer-matrix formalism for the calculation of optical response in multilayer systems: from coherent to incoherent interference.

We present a novel way to account for partially coherent interference in multilayer systems via the transfer-matrix method. The novel feature is that there is no need to use modified Fresnel coefficients or the square of their amplitudes to work in the incoherent limit. The transition from coherent to incoherent interference is achieved by introducing a random phase of increasing intensity in the propagating media. This random phase can simulate the effect of defects or impurities. This method provides a general way of dealing with optical multilayer systems, in which coherent and incoherent interference are treated on equal footing.

Material properties for predicting wax pattern dimensions in investment casting

Abstract An important factor in determining tooling allowances in investment casting is the dimensional change of the wax pattern. Dimensional changes between a pattern die and its wax pattern occur as a result of complex phenomena such as thermal expansion–contraction and hot deformation (elastic, plastic, and creep). Thus, the wax pattern dimensions are determined by the wax’s thermophysical and thermomechanical properties, restraint of geometrical features by the metal die, and process parameters such as die temperature, platen temperature, injection pressure, injection temperature, and dwell time. In this paper, constitutive equations of material behavior that determine dimensional changes associated with the wax system are provided. Material property measurements were conducted in this study for Cerita™ 29-51 wax with the aim of obtaining a complete set of data that can be used as input in computer simulation software for predicting wax pattern dimensions.

Development of Strain in Oxides Grown in Steam Tubes

In this study, the foundation is being developed for the numerical simulation of the processes that determine the oxide scale exfoliation behavior of the steam-side surfaces of superheater and reheater tubes in a steam boiler. Initially, the assumptions concerning the base state for calculating oxide strains also were critically examined. The state of stress-strain of an oxide growing on the inside surface of an externally-heated tube was considered for the conditions experienced in a boiler during transition from full- to partial-load operation. Since the rate at which the oxide grows is an important consideration, it was necessary to determine the appropriate temperature to use in the oxidation rate calculations. The existence of a temperature gradient through the tube, and the cyclic nature of the boiler operation (temperature and pressure) were considered; the growth temperature of the oxide was taken to be the oxide surface temperature. It was determined that the commonly-used approach for accounting for geometrical effects when calculating stress-strain development in a growing oxide scale of using the analogy of an infinitelylong flat plate gave sufficiently different results than when using a cylindrical geometry, that the latter was adopted as the preferred calculation procedure. Preliminary calculation of strains developed in multilayered oxides formed on alloy T22 as a function of boiler operating conditions indicated the magnitude of the strains in each layer; the large strain gradients between the layers inferred the importance of the detailed scale morphology in determining the mode of exfoliation.

Consolidation Process in Near Net Shape Manufacturing of Armstrong CP-Ti/Ti-6Al-4V Powders

This paper summarizes our recent efforts to develop the manufacturing technologies of consolidated net-shape components by using new low-cost commercially pure titanium (CP-Ti) and Ti-6Al-4V alloy powders made by the Armstrong process. Fabrication processes of net shape/ near net shape components, such as uniaxial die-pressing, cold isostatic pressing (CIP), sintering, roll compaction and stamping, have been evaluated. The press-and-sinter processing of the powders were systematically investigated in terms of theoretical density and microstructure as a function of time, pressure, and temperature. Up to 96.4% theoretical density has been achieved with the press-and-sinter technology. Tensile properties of the consolidated samples exhibit good ductility as well as equivalent yield/ultimate tensile strengths to those of fully consolidate materials, even with the presence of a certain amount of porosity. A consolidation model is also under development to interpret the powder deformation during processing. Net shape components made of the Armstrong powder can successfully be fabricated with clearer surface details by using press-and-sinter processing.

Organic Fluids in a Supercritical Rankine Cycle for Low Temperature Power Generation

This paper presents a performance analysis of a supercritical organic Rankine cycle (SORC) with various working fluids with thermal energy provided from a geothermal energy source. In the present study, a number of pure fluids (R23, R32, R125, R143a, R134a, R218, and R170) are analyzed to identify the most suitable fluids for different operating conditions. The source temperature is varied between 125 C and 200 C, to study its effect on the efficiency of the cycle for fixed and variable pressure ratios. The energy and exergy efficiencies for each working fluid are obtained and the optimum fluid is selected. It is found that thermal efficiencies as high as 21% can be obtained with 200 C source temperature and 10 C cooling water temperature considered in this study. For medium source temperatures (125 150 C), thermal efficiencies higher than 12% are obtained.

Generation of nitrogen acceptors in ZnO using pulse thermal processing

Bipolar doping in wide bandgap semiconductors is difficult to achieve under equilibrium conditions because of the spontaneous formation of compensating defects and unfavorable energetics for dopant substitution. In this work, we explored the use of rapid pulse thermal processing for activating nitrogen dopants into acceptor states in ZnO. Low-temperature photoluminescence spectra revealed both acceptor-bound exciton (AX0) and donor-acceptor pair emissions, which present direct evidence for acceptors generated after pulse thermal processing of nitrogen-doped ZnO. This work suggests that pulse thermal processing is potentially an effective method for p-type doping of ZnO.

ADVANCED MANUFACTURING TECHNOLOGIES UTILISING HIGH DENSITY INFRARED RADIANT HEATING

Abstract Oak Ridge National Laboratory has developed a unique rapid heating capability utilising a high density infrared (HDI) radiant plasma arc lamp. Power densities ≤3.5 W cm-2 are achievable over an area 35 x 3.175 cm. The power output of the lamp is continuously variable over a range from 1.5% to 100% of available power, and power changes can occur in <20 ms. Processing temperatures ≤3000°C can be obtained in a wide variety of processing environments, making HDI a flexible processing tool. Recently, this newly developed heating method was used to investigate selective softening, i.e. hardness reduction of 6063-T6 aluminium alloy. By changing the incident power and exposure time, the percentage reduction in hardness and softened zone size can be varied. It is shown that computer modelling can be used to predict the thermal history and the resulting heat affected zone during HDI processing. In the present work, a 50% reduction in hardness was achieved and confirmed by mechanical testing and microstructural investigation. Micrographs of softened aluminium show that Mg2Si precipitates had dissolved back into solution. This new approach allows materials to be engineered for a predetermined response to dynamic loading or other environmental situations. SE/S282

On the estimation of thermal strains developed during oxide growth

This paper presents results for the strains and stresses in oxide scales under the conditions of temperature and pressure expected in typical steam boiler operation. These conditions are radically different from those typically encountered in laboratory testing and include features such as a thermal gradient across the tube wall, significant internal (steam) pressure, and cycling of both steam temperature and pressure. Critical examination of the assumptions of flat-plate geometry, which is usually made in calculating stresses and strains in oxide scales, indicated that only the component of the hoop strain that generates stress must be reported for the cylindrical case, and that the use of simple plane-strain is adequate for the system studied. Calculations were made for alloy T22 with a hypothetical, single-layered oxide with appropriate properties. Typical conditions associated with transition of the boiler from full to partial load involve a decrease in both steam temperature and pressure, and these t...

Morphological evolution of oxide scales grown on ferritic steels in steam

Abstract This study is concerned with the thermally-grown oxide scales formed inside tubes in a steam boiler during normal operation, and the influence of the morphological evolution of those scales on how strains are developed and accommodated. Understanding such details is an important consideration in modeling the tendency for scale spallation or exfoliation. Overall, the scales formed on ferritic steels in steam can be simplistically described as a layer of essentially pure magnetite in contact with the steam, and a second layer of magnetite containing alloying elements (especially Cr) next to the alloy. Hematite also can develop as a third, outermost oxide layer that may be non-uniform in thickness, and penetrate inwards along grain boundaries in the magnetite layer. A peculiar variant of this morphology often reported on low-alloy ferritic steels after boiler service (but not on laboratoryoxidized specimens) has a relatively thick outer magnetite layer and multiple ‘inner layers’ described as decreasing in thickness as the alloy surface is approached. These inner layers appear to be repeating sets of double layers of ‘pure’ magnetite and magnetite containing alloying elements. The results of detailed examination of such multi-layered morphology and the implications for the mode of scale growth and failure are discussed.

Current Status of Ti PM: Progress, Opportunities and Challenges

Utilization of titanium components made by powder metallurgy methods has had limited acceptance largely due to the high cost of titanium (Ti) powder. There has been renewed interest in lower cost economical powders and several Ti reduction methods that produce a particulate product show promise. This talk summarizes work done at Oak Ridge National Laboratory to consolidate these economical powders into mill products. Press and sinter consolidation, hot isostatic pressing (HIP) and direct roll consolidation to make sheet have been explored. The characteristics of the consolidated products will be described as a function of the consolidation parameters.