Coupled reaction-diffusion transport into a core-shell geometry.

The coupled reaction-diffusion transport of glucose and oxygen into a core-shell geometry is modeled using dimensionless parameterization. The consumption of oxygen and glucose are coupled by a metabolic modulation function. The results are fit to a Bayesian-Ridge model that can be easily applied to generate large data sets spanning the parameter space. The non-linear correlation between the oxygen and glucose concentrations are presented and the effect of system parameters is explored. The maximum radius of a viable system is shown to decrease by up to 13.8% with high glucose saturation.

Modeling of reaction-diffusion transport into a core-shell geometry.

Fickian diffusion into a core-shell geometry is modeled. The interior core mimics pancreatic Langerhan islets and the exterior shell acts as inert protection. The consumption of oxygen diffusing into the cells is approximated using Michaelis-Menten kinetics. The problem is transformed to dimensionless units and solved numerically. Two regimes are identified, one that is diffusion limited and the other consumption limited. A regression is fit that describes the concentration at the center of the cells as a function of the relevant physical parameters. It is determined that, in a cell culture environment, the cells will remain viable as long as the islet has a radius of around 142 µm or less and the encapsulating shell has a radius of less than approximately 283 µm. When the islet is on the order of 100 µm it is possible for the cells to remain viable in environments with as little as 4.6×10-2 mol/m-3 O2. These results indicate such an encapsulation scheme may be used to prepare artificial pancreas to treat diabetes.

Oriented polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) films by Langmuir-Blodgett deposition: a synchrotron X-ray diffraction study.

Langmuir-Blodgett films of polyvinylidene fluoride trifluoroethylene - P(VDF-TrFE)-copolymers possess substantially improved electrocaloric and pyroelectric properties, when compared with conventionally spin-cast films. In order to rationalize this, we prepared single-layered films of P(VDF-TrFE) (70 : 30) using both deposition techniques. Grazing incidence wide-angle X-ray scattering (GIWAXS), reveals that Langmuir-Blodgett deposited films have a higher concentration of the ferroelectric ß-phase crystals, and that these films are highly oriented with respect to the substrate. Based on these observations, we suggest alternative means of deposition, which may substantially enhance the electrocaloric effect in P(VDF-TrFE) films. This development has significant implications for the potential use of P(VDF-TrFE) in solid-state refrigeration.

Complex borides based on AlLiB14 as high-temperature thermoelectric compounds.

AlLiB14 is examined as a potential high-temperature thermoelectric material. First-principles methods are used to investigate the thermoelectric behavior and it is found to have a band gap of 2.13 eV, and an electronic dispersion with characteristic indicative of having a high Seebeck coefficient. Semiclassical Boltzmann transport theory predicts that AlLiB14 will have a Seebeck coefficient greater than 200 µV K(-1), at temperatures near 1000 K and carrier concentrations around 1 × 10(20) cm(-3). Using a elasticity based expression for the thermal conductivity, the thermoelectric figure of merit is approximated to be 0.45 × 10(-3) T at moderate doping levels.

Substitutional C on B sites in AlLiB14.

The effect of C substitution in the AlLiB14 lattice is examined using first-principles methods. The inter-icosahedra B site is found to be the most favorable B site for C substitution and the formation energy is predicted to be 1.7 eV in B-rich conditions. Substituting C does not affect the band gap, nor does it introduce defect states to the gap. An ideal brittle cleavage model is used to study the impact of C doping on the mechanical properties of AlLiB14, and it is concluded that introducing C to the crystal decreases the ideal fracture strength by 3.3 GPa, which is about a 12% reduction in overall strength.

Fracture strength of AlLiB14.

The orthorhombic boride crystal family XYB14, where X and Y are metal atoms, plays a critical role in a unique class of superhard compounds, yet there have been no studies aimed at understanding the origin of the mechanical strength of this compound. We present here the results from a comprehensive investigation into the fracture strength of the archetypal AlLiB14 crystal. First principles, ab initio, methods are used to determine the ideal brittle cleavage strength for several high-symmetry orientations. The elastic tensor and the orientation-dependent Young's modulus are calculated. From these results the lower bound fracture strength of AlLiB14 is predicted to be between 29 and 31 GPa, which is near the measured hardness reported in the literature. These results indicate that the intrinsic strength of AlLiB14 is limited by the interatomic B-B bonds that span between the B layers.

Non-Arrhenius ionic conductivities in glasses due to a distribution of activation energies.

Previously observed non-Arrhenius behavior in fast ion conducting glasses [J. Kincs and S. W. Martin, Phys. Rev. Lett. 76, 70 (1996)] occurs at temperatures near the glass transition temperature, T(g), and is attributed to changes in the ion mobility due to ion trapping mechanisms that diminish the conductivity and result in a decreasing conductivity with increasing temperature. It is intuitive that disorder in glass will also result in a distribution of the activation energies (DAE) for ion conduction, which should increase the conductivity with increasing temperature, yet this has not been identified in the literature. In this Letter, a series of high precision ionic conductivity measurements are reported for 0.5Na(2)S + 0.5[xGeS(2) + (1-x)PS(5/2)] glasses with compositions ranging from 0 ≤ x ≤ 1. The impact of the cation site disorder on the activation energy is identified and explained using a DAE model. The absence of the non-Arrhenius behavior in other glasses is explained and it is predicted which glasses are expected to accentuate the DAE effect on the ionic conductivity.

Distortion and segregation in a dislocation core region at atomic resolution.

The structure of an isolated, Ga terminated, 30 degree partial dislocation in GaAs:Be is determined by high resolution transmission electron microscopes and focal series reconstruction. The positions of atomic columns in the core region are measured to an accuracy of better than 10 pm. A quantitative comparison of the structure predicted by an ab initio electronic structure total energy calculation to the experiment indicates that theory and experiment agree to within 20 pm. Further analysis shows the deviations between theory and experiment appear to be systematic. Electron energy loss spectroscopy establishes that defects segregate to the core region, thus accounting for the systematic deviations.