Nese Orbey

Determination of the relaxation spectrum from oscillatory shear data

In order to use either a linear or nonlinear model of viscoelasticity to calculate the stress response of a material to various deformations, it is usually necessary to have available an explicit equation for the linear relaxation modulus G(t). The most popular procedure is to use the data from a small‐amplitude oscillatory shear experiment to determine the parameters of a generalized Maxwell model. However, this is an ill‐posed problem and is not at all a straightforward curve‐fitting operation. We compare three procedures for determining a set of relaxation times and discrete moduli that can then be used as empirical fitting parameters in fluid mechanics computations. These are linear regression, with and without regularization, and nonlinear regression. Nonlinear regression is found to give a good fit of the data with a minimum number of parameters.

Chemical reaction analysis of copper indium selenization

A chemical reaction analysis of the selenization of copper indium layers to form copper indium diselenide is presented. Time-progressive selenization reactions were carried out in a laminar flow tubular reactor in a dilute H2Se atmosphere at 400°C. Copper, indium and copper-indium thin films were reacted for 1–60 min. The reacted films are analyzed by x-ray diffraction and atomic absorption spectrophotometry to identify the chemical species present in the reacted films. A reaction network for film formation is proposed and data from time-progressive selenizations were analyzed to obtain species composition profiles. Rate expressions are postulated and a mathematical model for the selenization is developed. The behavior of the model is compared with the experimentally determined species compositions to obtain specific reaction rate constants.

Inferring meaningful relaxation spectra from experimental data

The continuous spectrum is a unique representation of the linear viscoelastic behavior of a polymer that reveals aspects of its behavior that may not be obvious in plots of the storage and loss moduli. A spectrum can in principle be inferred from experimental data, but because data are always corrupted by random error, this is one of the several ill-posed problems that arise in rheology [Honerkamp, J., Rheol. Acta 28, 363–371 (1989) and Malkin, A. Ya., Rheol. Acta 29, 512–518 (1990)], and this throws into doubt the possibility of obtaining a unique relaxation spectrum. A number of methods have been proposed to overcome the ill-posedness of the problem to arrive at a unique continuous spectrum that is a characteristic of the material. Wider use of these methods has been limited by the fact that many of them are the work of mathematicians or physicists who use languages and symbols not readily intelligible to rheologists and that the codes required to implement them are not readily available. For these reas...

Reaction analysis of the formation of CuInSe2 films in a physical vapor deposition reactor

The reaction kinetics for the formation of CuInSe2 films by reacting Cu/In layers with elemental selenium are compared with those for H2Se. The species mole fractions as a function of time in a single Se-source physical vapor deposition (PVD) reactor are found to be essentially the same as those obtained in a chemical vapor deposition (CVD) reactor with flowing H2Se, indicating that the same chemical equation representation can be used in both cases. The chemical engineering reaction analysis model developed previously by us is shown to predict adequately the experimental data in both reactors. The model is employed to predict three-source behavior. The effects of rate of species delivery and substrate temperature on the time to make CuInSe2 is presented quantitatively.

Effect of temperature on copper indium selenization

The reaction kinetics of copper indium diselenide formation is studied by measuring species composition as a function of time at 250°C and 325°C in a tubular chemical vapor deposition reactor. This extends our previous modeling and experimental study at 400°C. The initial copper–indium alloy is analyzed at the reaction temperatures using high-temperature X-ray diffraction measurements. This modifies the understanding of the chemistry of the copper indium growth kinetics and a new set of model equations is presented. The specific reaction rate constants and activation energies for the chemical reactions are obtained, enabling one to calculate the holding time for the reaction.

Thin-Film Solar Cells on Polymer Substrates for Space Power

Photovoltaic arrays have played a key role in power generation in space. The current technology will continue to evolve but is limited in the important mass specific power metric (MSP or power/weight ratio) because it is based on bulk crystal technology. Solar cells based on thin-film materials offer the promise of much higher MSP and much lower cost. However, for many space applications, a 20% or greater AM0 efficiency (eta) may be required. The leading thin-film materials, amorphous Si, CuInSe, and CdTe have seen significant advances in efficiency over the last decade but will not achieve the required efficiency in the near future. Several new technologies are herein described to maximize both device eta and MSP. We will discuss these technologies in the context of space exploration and commercialization. One novel approach involves the use of very lightweight polyimide substrates. We describe efforts to enable this advance including materials processing and device fabrication and characterization. Another approach involves stacking two cells on top of each other. These tandem devices more effectively utilize solar radiation by passing through non-absorbed longer wavelength light to a narrow-bandgap bottom cell material. Modeling of current devices in tandem format indicates that AM0 efficiencies near 20% can be achieved with potential for 25% in the near future. Several important technical issues need to be resolved to realize the benefits of lightweight technologies for solar arrays, such as: monolithic interconnects, lightweight array structures, and new ultra-light support and deployment mechanisms. Recent advances will be stressed.

Reaction analysis of the formation of CIS at temperatures from 250 to 400/spl deg/C

A chemical reaction analysis of the selenization of copper indium layers to form copper indium diselenide is presented. Time progressive selenizations were carried out in a tubular laminar flow reactor in a dilute H/sub 2/Se atmosphere at 250, 325 and 400/spl deg/C. The reacted films were analyzed by X-ray diffraction and atomic absorption spectroscopy. The chemical species present in the reacted films were identified and a reaction network for film formation is proposed. The data from time progressive selenizations were analyzed to obtain species concentration profiles. Rate expressions were postulated and a mathematical model for the selenization was developed. The behavior of the model is compared with the experimentally determined species concentrations to obtain specific reaction rate constants at each temperature and the activation energies. This information is needed for the design and process control of commercial scale selenization reactors.

On the use of continuous relaxation spectra to characterize model polymers

There have been many more papers published on methods for inferring a continuous relaxation spectrum from linear viscoelastic data than have been published on the use of such methods for polymer characterization. But a continuous spectrum can reveal key details of the relaxation process that are not evident from plots of the storage and loss moduli, particularly in the case of model polymers prepared for research. In particular there are sharp peaks in the spectrum where Rouse relaxation is complete and further relaxation is inhibited by entanglements in the plateau zone and at the entry to the terminal zone where molecules have escaped their tubes. Examples of continuous spectra that reveal these relaxation phenomena are presented.

A chemical kinetic analysis of the formation of CuInSe/sub 2/ at different temperatures [solar cell modules]

Understanding the mechanism of the growth of CuInSe/sub 2/ to obtain the pertinent kinetic data is essential for the effective design and real time control of processing equipment to produce CuInSe/sub 2/ solar cell modules. Species concentrations as a function of time and temperature are obtained from experiments in a tubular chemical vapor deposition selenization reactor and a physical vapor deposition selenization reactor. In the present work, a chemical reaction analysis is carried out for both reactors. Using this information, the numerical solution to the set of mass balance equations, estimates of the rate constants and the activation energies are presented. The authors believe that this reaction analysis can be used to interpret reactions which take place during formation of CuInSe/sub 2/ in a three source physical vapor deposition (PVD) reactor.

AN EMPIRICAL VIRIAL-LIKE CUBIC EQUATION OF STATE FOR PHASE EQUILIBRIUM CALCULATIONS

An empirical cubic equation of state(EOS) was obtained by truncating the virial expansion in reciprocal of molar volume after the third term. The constants of the EOS was generalized in terms of critical temperature, critical pressure and Pitzers acentric factor. In pure component applications the EOS exhibited a performance comparable to Peng-Robinson (1976) EOS in the reduced temperature range of 0.5 to 1. The present EOS tends to predict better saturation liquid volumes at reduced temperatures below 0.8, and better estimations for second virial coefficient at high reduced temperatures. The EOS was successfully employed for vapor liquid equilibrium calculations for some mixtures of normal or slightly polar fluids with traditional one binary parameter mixing rule at moderately high pressures. At low reduced temperatures, where conventional one adjustable parameter applications of the cubic equations compare unfavorably with dual methods based on excess Gibbs energy functions for the liquid phase, a new ...