Copyright (c) 2012 University of Pennsylvania All rights reserved. http://repository.upenn.edu/cbe_papers Recent documents in Departmental Papers (CBE) en-us Sat, 28 Jan 2012 01:38:42 PST 3600 http://repository.upenn.edu/cbe_papers/155 http://repository.upenn.edu/cbe_papers/155 Thu, 26 Jan 2012 10:07:47 PST An energy-storage concept is proposed using molten Sb as the fuel in a reversible solid-oxide electrochemical cell (SOEC). Because both Sb and Sb₂O₃ are liquids at typical SOEC operating temperatures, it is possible to flow Sb from an external tank and use it as the fuel under fuel-cell conditions and then electrolyze Sb₂O₃ during recharging. This concept was tested using a button cell with a Sc-stabilized zirconia electrolyte at 973 K by measuring the impedances under fuel-cell and electrolyzer conditions for a range of stirred Sb-Sb₂O₃ compositions. The Sb-Sb₂O₃ electrode impedances were found to be on the order of 0.15 Ωcm² for both fuel-cell and electrolyzer conditions, for compositions up to 30% Sb and 70% Sb₂O₃. The open circuit voltages (OCV) were 0.75 V, independent of oxygen composition. Some features of using molten Sb as an energy-storage medium are discussed.

]]> Ashay Javadekar et al. http://repository.upenn.edu/cbe_papers/154 http://repository.upenn.edu/cbe_papers/154 Tue, 10 Jan 2012 13:03:57 PST Complex coacervation, a liquid-liquid phase separation that occurs when two oppositely charged polyelectrolytes are mixed in a solution, has the potential to be exploited for many emerging applications including wet adhesives and drug delivery vehicles. The ultra-low interfacial tension of coacervate systems against water is critical for such applications, and it would be advantageous if molecular models could be used to characterize how various system properties (e.g., salt concentration) affect the interfacial tension. In this article we use field-theoretic simulations to characterize the interfacial tension between a complex coacervate and its supernatant. After demonstrating that our model is free of ultraviolet divergences (calculated properties converge as the collocation grid is refined), we develop two methods for calculating the interfacial tension from field-theoretic simulations. One method relies on the mechanical interpretation of the interfacial tension as the interfacial pressure, and the second method estimates the change in free energy as the area between the two phases is changed. These are the first calculations of the interfacial tension from full field-theoretic simulation of which we are aware, and both the magnitude and scaling behaviors of our calculated interfacial tension agree with recent experiments.

]]> Robert A. Riggleman et al. http://repository.upenn.edu/cbe_papers/153 http://repository.upenn.edu/cbe_papers/153 Thu, 13 Oct 2011 11:14:42 PDT A detailed analysis of homogeneous melting in crystalline materials modeled by empirical interatomic potentials is presented using the theory of inherent structures.We show that the homogeneous melting of a perfect, infinite crystalline material can be inferred directly from the growth exponent of the inherent structure density-of-states distribution expressed as a function of formation enthalpy. Interestingly, this growth is already established by the presence of very few homogeneously nucleated point defects in the form of Frenkel pairs. This finding supports the notion that homogeneous melting is appropriately defined in terms of a one-phase theory and does not require detailed consideration of the liquid phase. We then apply this framework to the study of applied hydrostatic compression on homogeneous melting and show that the inherent structure analysis used here is able to capture the correct pressure-dependence for two crystalline materials, namely silicon and aluminum. The coupling between the melting temperature and applied pressure arises through the distribution of formation volumes for the various inherent structures.

]]> Alex M. Nieves et al. http://repository.upenn.edu/cbe_papers/152 http://repository.upenn.edu/cbe_papers/152 Tue, 19 Jul 2011 09:29:57 PDT We study the stability of polymer-shelled bubbles with controlled dimensions generated from air-in-oil-in-water (A/O/W) compound bubbles. We show that the ratio of the shell thickness to bubble radius is critical in generating un-deformed polymer-shelled bubbles from A/O/W compound bubbles. In addition, the effects of shell stiffness and encapsulated gas on bubble stability are also investigated.

]]> Myung Han Lee et al. http://repository.upenn.edu/cbe_papers/151 http://repository.upenn.edu/cbe_papers/151 Thu, 23 Jun 2011 09:36:32 PDT The growth and structure of Pd films on Zn(O
0001) were investigated using high resolution electron energy loss spectroscopy, x-ray photoelectron spectroscopy, and low energy electron diffraction. Vapor deposited Pd films at 300 K were found to follow a two-dimensional (
2D) island growth mode, in which 2D metal islands are formed up to a critical coverage at which point growth occurs primarily in a layer-by-layer fashion on top of the islands. Heating to only 350 K was found to be sufficient to induce partial agglomeration of Pd films into three-dimensional particles. In addition to causing further agglomeration into particles, heating to 700 K resulted in partial reduction of the ZnO surface and the formation of a PdZn alloy.

]]> Parthasarathi Bera et al. http://repository.upenn.edu/cbe_papers/150 http://repository.upenn.edu/cbe_papers/150 Thu, 12 May 2011 08:41:30 PDT A new approach to the design of ceramic anodes that uses a thin catalytically active functional layer that has only modest electronic conductivity sandwiched between the electrolyte and a non-catalytic elecronically conducting ceramic layer that is used as the current collector is described. The anode design is flexible and allows various materials to be used in the functional and current collector layers. Results are presented for anodes with thin functional layers (12 µm) consisting of a porous CeO₂/YSZ composite impregnated with 1 wt% Pd to optimize catalytic activity and a 100 µm thick layer of porous La_0.3Sr_0.7TiO₃ (LST) as the current collector. Low anode impedances and excellent overall performance were obtained with cells with these anodes while operating on both humidified hydrogen and hydrocarbon fuels.

]]> Michael D. Gross et al. http://repository.upenn.edu/cbe_papers/149 http://repository.upenn.edu/cbe_papers/149 Wed, 11 May 2011 08:18:21 PDT The properties of solid oxide fuel cell (SOFC) anode functional layers prepared by impregnation of ceria and catalytic metals into porous yttria-stabilized zirconia (YSZ) have been examined for operation at 973 K. By varying the thickness of the functional layer, the conductivity of the ceria-YSZ composite was determined to be only 0.015–0.02 S/cm. The initial performance of anodes made with ceria loadings of 40 or 60 wt % were similar but the anodes with lower loadings lost conductivity above 1073 K due to sintering of the ceria. The addition of dopant levels of catalytic metals was found to be critical. The addition of 1 wt % Pd or Ni decreased the anode impedances in humidified H₂ dramatically, while the improvement with 5 wt % Cu was significant but more modest. Pd doping also decreased the anode impedance in dry CH₄ much more than did Cu doping; however, addition of either Pd or Cu led to similar improvements for operation in n-butane. Based on these results, suggestions are made for ways to improve SOFC anode functional layers.

]]> Michael D. Gross et al. http://repository.upenn.edu/cbe_papers/148 http://repository.upenn.edu/cbe_papers/148 Wed, 11 May 2011 08:18:17 PDT We present an overview of mathematical models and their large-scale numerical solution for simulating different phenomena and scales in melt and solution crystal growth. Samples of both classical analyses and state-of-the-art computations are presented. It is argued that the fundamental multi-scale nature of crystal growth precludes any one approach for modeling, rather successful crystal growth modeling relies on an artful blend of rigor and practicality.

]]> Jeffrey J. Derby et al. http://repository.upenn.edu/cbe_papers/147 http://repository.upenn.edu/cbe_papers/147 Wed, 11 May 2011 08:18:11 PDT In this study, the effect of the morphology and ionic conductivity of the electrolyte material in SOFC composite cathodes is systematically studied. The specific surface area of prous yttria-stabilized zirconia (YSZ) scaffolds was varied by almost two orders of magnitude using different pore formers and surface treatment with hydrofluoric acid (HF). The effect of ionic conductivity on the performance of SOFC cathodes was studied for electrodes prepared by infiltration of 35 wt % LSF into 65% porous scandia-stabilized zirconia (ScSZ), YSZ, or yttria-alumina co-stabilized zirconia (YAZ) scaffolds of identical microstructure cathodes.

]]> Rainer Küngas et al. http://repository.upenn.edu/cbe_papers/146 http://repository.upenn.edu/cbe_papers/146 Wed, 27 Apr 2011 13:30:44 PDT A mathematical model has been developed to understand the performance of electrodes prepared by infiltration of La_0.8Sr_0.2FeO₃
(LSF) and La_0.8Sr_0.2MnO₃ (LSM) into yttria-stabilized zirconia (YSZ). The model calculates the resistances for the case where
perovskite-coated, YSZ fins extend from the electrolyte. Two rate-limiting cases are considered: oxygen ion diffusion through the
perovskite film or reactive adsorption of O₂ at the perovskite surface. Adsorption is treated as a reaction between gas-phase O₂ and
oxygen vacancies, using equilibrium data. With the exception of the sticking probability, all parameters in the model are experimentally
determined. Resistances and capacitances are calculated for LSF-YSZ and there is good agreement with experimental
values at 973 K, assuming adsorption is rate limiting, with a sticking probability between 10^-3 and 10^-4 on vacancy sites. According
to the model, perovskite ionic conductivity does not limit performance so long as it is above ~10^-7 S/cm. However, the structure
of the YSZ scaffold, the ionic conductivity of the scaffold, and the slope of the perovskite redox isotherm significantly impact
electrode impedance. Finally, it is shown that characteristic frequencies of the electrode cannot be used to distinguish when diffusion
or adsorption is rate-limiting.

]]> Fred Bidrawn et al. http://repository.upenn.edu/cbe_papers/145 http://repository.upenn.edu/cbe_papers/145 Wed, 27 Apr 2011 13:30:38 PDT Solid oxide fuel cell (SOFC) cathodes were prepared by infiltration of 35 wt % La_0.8Sr_0.2FeO₃ (LSF) into porous scaffolds of three, zirconia-based electrolytes in order to determine the effect of the ionic conductivity of the electrolyte material on cathode impedances. The electrolyte scaffolds were 10 mol % Sc₂O₃-stabilized zirconia (ScSZ), 8 mol % Y₂O₃-stabilized zirconia (YSZ), and 3 mol % Y₂O₃- 20 mol % Al₂O₃-doped zirconia (YAZ), prepared by tape casting with graphite pore formers. Each electrolyte scaffold was 65% porous, with identical pore structures as determined by scanning electron microscopy (SEM). Both symmetric cells and fuel cells were prepared and tested between 873 and 1073 K, using LSF composites that had been calcined to 1123 or 1373 K. Literature values for the electrolyte conductivities were confirmed using the ohmic losses from the impedance spectra. The electrode impedances decreased with increasing electrolyte conductivity, with the dependence being between to the power of 0.5 and 1.0, depending on the operating temperature and LSF calcination temperature.

]]> Rainer Küngas et al. http://repository.upenn.edu/cbe_papers/144 http://repository.upenn.edu/cbe_papers/144 Tue, 29 Mar 2011 13:55:33 PDT A mathematical model has been developed to understand the performance of electrodes prepared by infiltration of La_0.8Sr_0.2FeO₃ (LSF) and La_0.8Sr_0.2MnO₃ (LSM) into yttria-stabilized zirconia (YSZ). The model calculates the resistances for the case where perovskite-coated, YSZ fins extend from the electrolyte. Two rate-limiting cases are considered: oxygen ion diffusion through the perovskite film or reactive adsorption of O₂ at the perovskite surface. Adsorption is treated as a reaction between gas-phase O₂ and oxygen vacancies, using equilibrium data. With the exception of the sticking probability, all parameters in the model are experimentally determined. Resistances and capacitances are calculated for LSF-YSZ and there is good agreement with experimental values at 973 K, assuming adsorption is rate limiting, with a sticking probability between 10^-3 and 10^-4 on vacancy sites. According to the model, perovskite ionic conductivity does not limit performance so long as it is above ~10^-7 S/cm. However, the structure of the YSZ scaffold, the ionic conductivity of the scaffold, and the slope of the perovskite redox isotherm significantly impact electrode impedance. Finally, it is shown that characteristic frequencies of the electrode cannot be used to distinguish when diffusion or adsorption is rate-limiting.

]]> Fred Bidrawn et al. http://repository.upenn.edu/cbe_papers/143 http://repository.upenn.edu/cbe_papers/143 Thu, 27 Jan 2011 08:53:40 PST Composites formed by infiltration of 45 wt % La_0.3Sr_0.7TiO₃ (LST) into 65% porous yttria-stabilized zirconia (YSZ) were examined for application as solid oxide fuel cell (SOFC) anodes. Although LST does not react with YSZ, the structure of the LST deposits was strongly affected by the calcination temperature. At 1373 K, the LST formed loosely packed, 0.1 µm particles that filled the YSZ pores. The conductivity of this composite depended strongly on the pretreatment conditions but was greater than 0.4 S/cm after heating to 1173 K in humidified (3% H,₂O) H₂. Following calcination at 1573 K, the LST had sintered significantly, decreasing the conductivity of the composite by a factor of approximately 5. The addition of a catalyst was critical for achieving reasonable electrochemical performance, with the addition of 0.5 wt % Pd and 5 wt % ceria increasing the power density of otherwise identical cells from less than 20 to 780 mW/cm² for operation in humidified (3% H₂O) H₂ at 1073 K. Electrodes prepared from LST deposits calcined at 1373 K were found to exhibit a much better performance than those prepared from LST deposits calcined at 1573 K, demonstrating that the structure of the composite is critical for achieving high performance.

]]> Shiwoo Lee et al. http://repository.upenn.edu/cbe_papers/142 http://repository.upenn.edu/cbe_papers/142 Tue, 11 Jan 2011 12:50:15 PST The use of patterned stress fields to direct phase separation in thin film alloys is investigated computationally with Monte Carlo simulations in which atomic interactions are represented by a Lennard-Jones potential.We show that careful design of annealing schedules based on consideration of the system phase diagram can lead to vastly enhanced patterning kinetics. In particular, by avoiding the low temperature formation of highly stable nuclei within the entire system, the kinetics of patterning are accelerated by rapid monomer diffusion, rather than classical Ostwald ripening in which small precipitates must dissolve to feed larger ones.

]]> Alex M. Nieves et al. http://repository.upenn.edu/cbe_papers/141 http://repository.upenn.edu/cbe_papers/141 Tue, 11 Jan 2011 12:50:11 PST The role of entropy in the thermodynamic properties of small interstitial clusters in crystalline silicon is investigated using an empirical potential. It is shown that both vibrational and configurational entropies are potentially important in setting the properties of small silicon interstitial clusters and, in particular, contribute to the formation of “magic” sizes that exhibit special stability, which have been inferred by experimental measurements of dopant diffusion. The results suggest that a competition between formation energy and entropy of small clusters could be linked to the selection process between various self-interstitial precipitate morphologies observed in ion-implanted crystalline silicon.

]]> Sumeet Kapur et al. http://repository.upenn.edu/cbe_papers/140 http://repository.upenn.edu/cbe_papers/140 Tue, 11 Jan 2011 12:50:07 PST Amphiphilic block copolymers of suitable proportions can self-assemble into surprisingly long and stable worm-like micelles, but the intrinsic polydispersity of polymers as well as polymer blending efforts and the increasing use of degradable chains all raise basic questions of curvature–composition coupling and morphological stability of these high curvature assemblies. Molecular simulations here of polyethylene glycol (PEG) based systems show that a systematic increase in the hydrated PEG fraction, in both monodisperse and binary blends, induces budding and breakup into spherical and novel ‘dumbbell’ micelles—as seen in electron microscopy images of degradable worm-like micelles. Core dimension, d, in our large-scale, long-time dissipative particle dynamics (DPD) simulations is shown to scale with chain-length, N, as predicted theoretically by the strong segregation limit (d ≈ N^2/3), but morphological transitions of binary mixtures are only crudely predicted by simple mixture rules. Here we show that for weakly demixing diblock copolymers, the coupling between local interfacial concentration and mean curvature can be described with a simple linear relationship. The computational methods developed here for PEG-based assemblies should be useful for many high curvature nanosystems.

]]> Loverde M. Sharon et al. http://repository.upenn.edu/cbe_papers/139 http://repository.upenn.edu/cbe_papers/139 Thu, 02 Dec 2010 07:40:51 PST Atomic force microscopy and electron microscopy with energy dispersive X-ray analysis was used to characterize changes in teh structure and composition of La_0.8Sr_0.2MnO₃ (LSM) nanoparticles supported on single crystal YSZ (100) (yttria-stabilized zirconia) and SrTiO₃(100) surfaces as a function of temperature and exposure to oxidizing and reducing environments. On YSZ(100), LSM particles were found to decompose into Mn- and La-rich phases and spread over the surface upon calcination in air at temperatures above 1123 K. The Mn-rich phase was observed to have a higher mobility and spread more rapidly. In contrast to YSZ(100), on SrTiO₃(100) the LSM particles underwent agglomeration via an Ostwald ripening mechanism upon calcination at temperatures above 1123 K, resulting in an increase in the particle size. Phase separation was not observed on this substrate.

]]> Ju-Sik Kim et al. http://repository.upenn.edu/cbe_papers/138 http://repository.upenn.edu/cbe_papers/138 Tue, 30 Nov 2010 09:24:32 PST Coated microbubbles, unlike tissue are able to scatter sound subharmonically. Therefore, the subharmonic behavior of coated microbubbles can be used to enhance the contrast in ultrasound contrast imaging. Theoretically, a threshold amplitude of the driving pressure can be calculated above which subharmonic oscillations of microbubbles are initiated. Interestingly, earlier experimental studies on coated microbubbles demonstrated that the threshold for these bubbles is much lower than predicted by the traditional linear viscoelastic shell models. This paper presents an experimental study on the subharmonic behavior of differently sized individual phospholipid coated microbubbles. The radial subharmonic response of the microbubbles was recorded with the Brandaris ultra high-speed camera as a function of both the amplitude and the frequency of the driving pulse. Threshold pressures for subharmonic generation as low as 5 kPa were found near a driving frequency equal to twice the resonance frequency of the bubble. An explanation for this low threshold pressure is provided by the shell buckling model proposed by Marmottant et al. [J. Acoust. Soc. Am. 118, 3499–3505 (2005)]. It is shown that the change in the elasticity of the bubble shell as a function of bubble radius as proposed in this model, enhances the subharmonic behavior of the microbubbles.

]]> Jeroen Sijl et al. http://repository.upenn.edu/cbe_papers/137 http://repository.upenn.edu/cbe_papers/137 Tue, 30 Nov 2010 09:24:28 PST To stabilize solid oxide fuel cells cathodes prepared by infiltration of La_0.8Sr_0.2CoO₃ (LSCo) into porous yttria-stabilized zirconia (YSZ), a coating of Sm-doped ceria (SDC) was first deposited onto the YSZ scaffold. The dense SDC coating was prepared by infiltration with aqueous solutions of SM(NO₃)₃ and Ce(NO₃)₃, followed by calcination to 1473 K. The SDC coating prevented ~ 20 mΩ cm², at 973 K, with acceptable degradation after heating to 1373 K.

]]> Rainer Küngas et al. http://repository.upenn.edu/cbe_papers/136 http://repository.upenn.edu/cbe_papers/136 Thu, 04 Nov 2010 09:54:33 PDT Determination of the patient-specific response to antiplatelet agents facilitates proper dosing for both acute and chronic prophylaxis. "Closed" systems (with or without flow) may fail to predict pharmacological potency in situations where platelets rapidly accumulate under flow conditions at the site of thrombosis ("Open" systems). Using an 8-channel microfluidic flow assay of human whole blood with corn trypsin inhibitor (± PPACK) perfused over focul zones of collagen, dose-response curves were measured for pharmacological agents at a wall shear rate of 210 s^-1. The P2Y₁ inhibitor MRS 2179 (IC₅₀ = 0.233 ± 0.132 µM) and P2Y₁₂ inhibitor 2-MeSAMP (IC₅₀ = 2.558 ± 0.799 µM) were potent blockers of secondary platelet accumulation under flow, while the P2X₁ inhibitor (NF 449) and apyrase failed to reduce platelet accumulation. MRS 2179 and 2-MeSAMP and undetectable effects on initial platelet adhesion to collagen. Numerical simulation of convective-diffusive transport and apyrase-mediated catalytic degradation of ADP indicated that ultra-high concentrations of apyrase (~ 2000 U mL^-1) would be required to have the same effect under flow as much lower concentrations (1 U mL^-1) currently used in closed systems (aggregometry or cone-and-plate viscometer). This is the first evaluation of IC₅₀ values for P2Y₁₂ and P2Y₁ antagonists under controlled flow conditions. Evaluation of antiplatelet agents in open flow systems demonstrates that inhibition of either ADP by apyrase or antagonism of P2X₁ signaling had no inhibitory effect on platelet accumulation. This technique provides a platform for rapidly investigating effects of antithrombotic therapies simultaneously in a model injury system.

]]> Maloney et al.