James G. Serdy

Dislocation density reduction in multicrystalline silicon solar cell material by high temperature annealing

We propose and demonstrate a method to remove performance-limiting dislocations from multicrystalline silicon (mc-Si) solar cell material, appropriate for wafers or bricks. Dislocation density reductions of >95% are achieved in commercial mc-Si via high temperature annealing with an impurity diffusion barrier, with controlled ambient and time-temperature profiles. The dislocation density reduction follows temperature-dependent models developed by Kuhlmann [Proc. Phys. Soc., London, Sect. A 64, 140 (1951)] and Nes [Acta Metall. Mater. 43, 2189 (1995)]. It is believed that higher annealing temperatures (>1170 °C) allow dislocation movement unconstrained by crystallographic glide planes, leading to dislocation annihilation within minutes.

Design, modeling and fabrication of a constant flow pneumatic micropump

This paper characterizes a bi-directional pneumatic diaphragm micropump and presents a model for performance of an integrated fluidic capacitor. The fluidic capacitor is used to convert pulsatile flow into a nearly continuous flow stream. The pump was fabricated in acrylic using a CNC mill. The stroke volume of the pump is ~1 µL. The pump is self-priming, bubble tolerant and insensitive to changes in head pressure and pneumatic pressure within its operating range. The pump achieves a maximum flow rate of 5 mL min−1 against zero head pressure. With pneumatic pressure set to 40 kPa, the pump can provide flow at 2.6 mL min−1 against a head pressure of 25 kPa. A nonlinear model for the capacitor was developed and compared with experimental results. The ratio of the time constant of the capacitor to the cycle time of the pump is shown to be an accurate indicator of capacitor performance and a useful design tool.

Perfused Microreactors for Liver Tissue Engineering

We developed scalable microreactors that foster the development of 3D microscopic pieces of tissue. By integrating microreactors, reservoirs, and pumps in the multiwell cell culture plate format, we created a high throughput cell culture system. However, in contrast to commonly used 2D static cell culture in multiwell plates, our new system allows 3D perfused cell culture. The system provides a means to conduct assays for toxicology and metabolism and can be used as a model for human diseases such as hepatic diseases, exposure-related pathologies, and cancer

The rheology of aqueous solutions of ethyl hydroxy-ethyl cellulose (EHEC) and its hydrophobically modified analogue (hmEHEC)

Cellulose derivatives containing associating hydrophobic groups along their hydrophilic backbone are used as rheology modifiers in the formulation of water-based spray paints, medicinal sprays, cosmetics and printable inks. Jetting and spraying applications of these materials involve progressive thinning and break-up of a fluid column or sheet into drops. Strong extensional kinematics develop in the thinning fluid neck. In viscous Newtonian fluids, inertial and viscous stresses oppose the surface tension-driven instability. In aqueous solutions of polymers such as Ethyl Hydroxy-Ethyl Cellulose (EHEC), chain elongation provides additional elastic stresses that can delay the capillary-driven pinch-off, influencing the sprayability or jettability of the complex fluid. In this study, we quantify the transient response of thinning filaments of cellulose ether solutions to extensional flows in a Capillary Break-up Extensional Rheometer (CaBER) and in a forced jet undergoing break-up using Rayleigh Ohnesorge Jetting Extensional Rheometry (ROJER). We also characterize the steady state molecular deformations using measurements of the flow-induced birefringence and excess pressure drop in an extensional stagnation point flow using a Cross-Slot Extensional Rheometer (CSER). We show that under the high extension rates encountered in jetting and spraying, the semi-dilute solutions of hydrophobically modified ethyl hydroxy-ethyl cellulose (hmEHEC) exhibit extensional thinning, while the unmodified bare chains of EHEC display an increase in extensional viscosity, up to a plateau value. For both EHEC and hmEHEC dispersions, the low extensibility of the cellulose derivatives limits the Trouton ratio observed at the highest extension rates attained (close to 10(5) s(-1)) to around 10-20. The reduction in extensional viscosity with increasing extension rate for the hydrophobically modified cellulose ether is primarily caused by the disruption of a transient elastic network that is initially formed by intermolecular association of hydrophobic stickers. This extensional thinning behavior, in conjunction with the low extensibility of the hydrophobically modified cellulose ether additives, makes these rheology modifiers ideal for controlling the extensional rheology in formulations that require jetting or spraying, with minimal residual stringiness or stranding.

Two-Step Annealing Study of Cuprous Oxide for Photovoltaic Applications

The properties of large grain cuprous oxide (Cu2O) foils are explored after the implementation of a controlled postgrowth annealing process. P-type foils with a wide range of carrier density are demonstrated, enabling a promising processing window for wide bandgap solar cell devices. Hall measurements at room temperature show increased majority carrier concentration after nitrogen annealing and a reduction in mobility. The progressive change in resistivity with annealing temperature is shown, with values approaching 100 Ω·cm. Carrier recombination, measured by microwave photoconductance decay, shows a discrete change upon annealing.