Eutectic colony formation: A phase field study

Eutectic two-phase cells, also known as eutectic colonies, are commonly observed during the solidification of ternary alloys when the composition is close to a binary eutectic valley. In analogy with the solidification cells formed in dilute binary alloys, colony formation is triggered by a morphological instability of a macroscopically planar eutectic solidification front due to the rejection by both solid phases of a ternary impurity that diffuses in the liquid. Here we develop a phase-field model of a binary eutectic with a dilute ternary impurity and we investigate by dynamical simulations both the initial linear regime of this instability, and the subsequent highly nonlinear evolution of the interface that leads to fully developed two-phase cells with a spacing much larger than the lamellar spacing. We find a good overall agreement with our recent linear stability analysis [M. Plapp and A. Karma, Phys. Rev. E 60, 6865 (1999)], which predicts a destabilization of the front by long-wavelength modes that may be stationary or oscillatory. A fine comparison, however, reveals that the assumption commonly attributed to Cahn that lamella grow perpendicular to the envelope of the solidification front is weakly violated in the phase-field simulations. We show that, even though weak, this violation has an important quantitative effect on the stability properties of the eutectic front. We also investigate the dynamics of fully developed colonies and find that the large-scale envelope of the composite eutectic front does not converge to a steady state, but exhibits cell elimination and tip-splitting events up to the largest times simulated.Comment: 18 pages, 18 EPS figures, RevTeX twocolumn, submitted to Phys. Rev.

Response of an Energy Foundation to Temperature Fluctuations

This paper focuses on the evaluation of the thermo-mechanical response of a 15.2 m-long energy foundation in dry sandstone over a 4-month period. Although the foundation was not actively heated or cooled using a heat pump, the fluctuations in the temperature of the heat exchange fluid occurred due to seasonal variations in the mechanical room temperature. Although these fluctuations only led to small changes in foundation temperature (from -1 to 1°C), these were sufficient to evaluate the mobilized coefficient of thermal expansion of the foundation. Further, the trends in the thermal axial stress and strain with depth in the foundation were evaluated, along with the mobilized side shear stress and axial displacement

Identifying best practice, installation, laboratory testing, and field testing

This paper summarizes recommendations for best practice associated with the installation of geothermal loops within foundations to form thermopiles, based on experience gained over the past 10 years in the UK. The issue of paramount importance in constructing a successful thermopile installation is the early stage coordination with all parties that will encounter, install or test the geothermal loops. Several lessons learned from the installation and construction of thermopiles are described to help ensure a smooth installation process. As long as there is early coordination, the installation of geothermal heat exchange tubing is relatively simple and will have very little or no impact on typical deep foundation installation procedures. This, coupled with the fact that there are additional costs and implications associated with other geothermal heat exchange approaches, implies that thermopiles are an ideal economic solution to access a renewable energy source

Near-infrared fluorescence lifetime assay for serum glucose based on allophycocyanin-labeled concanavalin A

We describe an assay scheme for glucose based on fluorescence resonance energy transfer (FRET) between concanavalin A (con A), labeled with the nearinfrared fluorescent protein allophycocyanin (APC) as donor, and dextran labeled with malachite green (MG) as acceptor. Glucose competitively displaces dextran- MG and leads to reduction in FRET, assessed by time-domain fluorescence lifetime measurements using time-correlated single-photon counting. The assay is operative in the glucose concentration range 2.5-30 mM, and therefore suitable for use in monitoring diabetes control. Albumin and serum inhibit FRET but the interference can be prevented by removal of high molecular weight substances by membrane filters. APC shows promise for incorporation in an implanted glucose sensor which can be interrogated from outside the body