Jason H. Nadler

Acoustic absorption calculation in irreducible porous media: A unified computational approach

A critical task in predicting and tailoring the acoustic absorption properties of porous media is the calculation of the frequency-dependent effective density and compressibility tensors, which are explicitly related to the micro-scale permeability properties. Although these two quantities exhibit strong sensitivity to physics occurring at complex micro-scale geometries, most of the existing literature focuses on employing very limited in-house and oftentimes multiple numerical analysis tools. In order to predict these parameters and acoustic absorption efficiently and conveniently, this article synthesizes multiple disparate approaches into a single unified formulation suitable for incorporation into a commercial analysis package. Numerical results computed herein for four close-packed porous media are compared to similar results available in the literature. These include simple cubic, body-centered cubic, and face-centered cubic structures, and also hexagonal close-packed, which has not appeared in the literature. Together with critical comparisons of a hybrid versus direct numerical approaches, the close agreement demonstrates the capabilities of the unified formulation to analyze and control the acoustic absorption properties at the microscopic level.

Transparent oxyhalide glass and glass ceramics for gamma-ray detection

Nuclear radiation detection is continuously becoming more important for todays society. Conventional scintillator based gamma-ray detectors use single crystal materials such as NaI:Tl, LaBr3:Ce, which provide excellent radiation detection properties, but suffer from their environment-related fluctuation, high cost and size limitation. The incorporation of nanophosphors or quantum dots (QD) into a transparent host matrix has been studied recently as a cost-saving alternative that may solve the scalability and stability problems while still providing considerable optical performance. In this work, a new glass based detecting material with promising gamma-ray detection performance is reported. Transparent alumino-silicate glasses containing cerium-doped gadolinium halide nanocrystals were prepared by a melt-quench method followed by annealing to form nanocrystal precipitates. Samples were cast and polished for optical and radiation characterization. The preliminary results indicated a similar gamma-ray detection efficiency compared to a conventional NaI:Tl detector and a gamma-ray peak resolution of ~27% at 662 KeV from some of these samples. By controlling elemental composition and ratio of the in-situ precipitated nanoparticles, radiation detection performance is expected to be improved.

Remora fish suction pad attachment is enhanced by spinule friction.

ABSTRACT The remora fishes are capable of adhering to a wide variety of natural and artificial marine substrates using a dorsal suction pad. The pad is made of serial parallel pectinated lamellae, which are homologous to the dorsal fin elements of other fishes. Small tooth-like projections of mineralized tissue from the dorsal pad lamella, known as spinules, are thought to increase the remoras resistance to slippage and thereby enhance friction to maintain attachment to a moving host. In this work, the geometry of the spinules and host topology as determined by micro-computed tomography and confocal microscope data, respectively, are combined in a friction model to estimate the spinule contribution to shear resistance. Model results are validated with natural and artificially created spinules and compared with previous remora pull-off experiments. It was found that spinule geometry plays an essential role in friction enhancement, especially at short spatial wavelengths in the host surface, and that spinule tip geometry is not correlated with lamellar position. Furthermore, comparisons with pull-off experiments suggest that spinules are primarily responsible for friction enhancement on rough host topologies such as shark skin. Summary: The geometry of remora fish suction disc spinules increases passive friction on rough host surfaces such as shark skin.

Composition optimization of scintillating rare-earth nanocrystals in oxide glass-ceramics for radiation spectroscopy.

Glass-ceramic nanocomposites comprising GdBr₃/CeBr₃ loaded sodium-aluminosilicate glasses in which scintillating crystallites are precipitated in situ from a host glass matrix were studied. This materials system shows promise as an alternative to single-crystal scintillators, with potential to be fabricated into a wide variety of sizes, shapes, and compositions. Batch compositions containing 15-18 mol. % GdBr₃ and 3-4 mol. % CeBr₃ were prepared and analyzed for photoluminescent light yield. Light yield peaked with rare-earth content of 15 mol. % GdBr₃ and 4 mol. % CeBr₃. Preliminary ceramization studies on this composition found that the precipitated phase more closely matched a Gd₂O₃-CeO₂ mixture rather than the GdBr₃(Ce) that was targeted.

Nanocomposites for radiation sensing

The use of light emitting nanoparticles in polymer and glass matrices was studied for the detection of radiation. These nanocomposite scintillators were produced by various approaches including quantum dot/polymer, fluoride nanophosphor/epoxy and halide nanophosphor containing glass-ceramic composites. The synthesis and characterization of these nanoparticles as well as their incorporation into composites is discussed. Further, the application of these composites for radiation detection and spectroscopy is described.

Theoretical and computational fluid dynamics of an attached remora (Echeneis naucrates).

Remora fishes have a unique dorsal suction pad that allows them to form robust, reliable, and reversible attachment to a wide variety of host organisms and marine vessels. Although investigations of the suction pad have been performed, the primary force that remoras must resist, namely fluid drag, has received little attention. This work provides a theoretical estimate of the drag experienced by an attached remora using computational fluid dynamics informed by geometry obtained from micro-computed tomography. Here, simulated flows are compared to measured flow fields of a euthanized specimen in a flow tank. Additionally, the influence of the hosts boundary layer is investigated, and scaling relationships between remora features are inferred from the digitized geometry. The results suggest the drag on an attached remora is similar to that of a streamlined body, and is minimally influenced by the hosts viscous boundary layer. Consequently, this evidence does not support the hypothesis that remoras discriminate between attachment locations based on hydrodynamic considerations. Comparison of the simulated drag with experimental friction tests show that even at elevated swimming speeds it is unlikely that remoras are dislodged by drag alone, and furthermore that larger remoras may be more difficult to dislodge than smaller remoras indicating that they become more suited to attachment as they mature.

Transitioning glass-ceramic scintillators for diagnostic x-ray imaging from the laboratory to commercial scale

This study sought to mitigate risk in transitioning newly developed glass-ceramic scintillator technology from a laboratory concept to commercial product by identifying the most significant hurdles to increased scale. These included selection of cost effective raw material sources, investigation of process parameters with the most significant impact on performance, and synthesis steps that could see the greatest benefit from participation of an industry partner that specializes in glass or optical component manufacturing. Efforts focused on enhancing the performance of glass-ceramic nanocomposite scintillators developed specifically for medical imaging via composition and process modifications that ensured efficient capture of incident X-ray energy and emission of scintillation light. The use of cost effective raw materials and existing manufacturing methods demonstrated proof-of-concept for economical viable alternatives to existing benchmark materials, as well as possible disruptive applications afforded by novel geometries and comparatively lower cost per volume. The authors now seek the expertise of industry to effectively navigate the transition from laboratory demonstrations to pilot scale production and testing to evince the industry of the viability and usefulness of composite-based scintillators.

Evaluating Thermal Composites Using Asymptotic Homogenization

Thermal management is of paramount importance in high powered electronic devices. Although composites can provide new materials with improved thermal performance, major hurdles exist in their implementation. Here some principles from asymptotic homogenization analysis are used to demonstrate how a reinforced epoxys properties can be tailored to design specifications, and to evaluate the feasibility of processing such a composite. Two simulations were carried out to predict the bulk thermal conductivity and CTE of a diamond filled, heat curable epoxy. Additionally, the local thermal stresses that develop during the curing process were also found. All simulations were performed using commercially available finite element software.

Parametric Studies on the Processing Parameters of a Thermally Wicking Material using Image Analysis

A heat pipe is a device that transports heat against gravity using a wicking material and evaporation-condensation cycle..In these systems a thermal wick moves fluid from the cool region of a heat pipe to the hot region, where evaporative cooling occurs. Due to the operating demands of a thermal wick, several microstructural features are integral to the performance of the wick: capillary radii, specific surface area and permeability. Measuring these properties of a thermal wick (capillary radii, specific surface area and permeability) is difficult, therefore image analysis methods of quantification of the critical properties of a thermal wick has been developed . However, the microstructure of a thermal wick contains semicontinuous pores, therefore connectivity of pores cannot be assumed during quantification of the critical properties.. Two processing parameters, sacrificial template particle size and sintering temperature, were varied during the thermal wick synthesis. Quantification of the critical properties of the thermal wick was performed using the newly developed method. The newly developed method was able to detect the an increase in the pore connectivity as the sintering temperature decreased, and an increase in the connectivity as the sacrificial template particle size decreased. The newly developed method was also able to describe the size distribution of individual pores as well as the hydraulic resistance and orientation of individual pores as well as estimate the porosity and true specific surface area of the different samples.

Synthesis and thermal characterization of sodium borosilicate/carbon nanofiber composites

Sodium borosilicate (NBS) glass/annealed pyrolytic graphite (APG) fiber composites have been fabricated through a sol-gel process. A glass composition of 80wt% SiO2, 15wt% B2O3, and 5wt% Na2O was chosen, similar to commercial NBS glasses such as Pyrex. A variety of gels with increasing weight percentages of APG were produced and subsequently heat treated and sintered in a uniaxial hot-press. Sol-gel derived NBS glass composites have been prepared with up to 40wt% of APG homogeneously distributed throughout the glassy matrix. Increased loading of the sol-gel derived sodium borosilicate glass with SiO2-coated APG fibers has shown to decrease its coefficient of thermal expansion (CTE) below that of traditional melt-quenched NBS glasses of similar composition while keeping the bulk density relatively constant. The high aspect ratio of APG fibers encourages the formation of percolating networks within the glassy matrix, increasing the thermal conductivity of the composite material beyond that of a conventional NBS glass. The resulting thermal conductivity of the composites has been found to be directional, with a much higher conductivity value in the direction perpendicular to the axis of the hot press (“in-plane”) than in its parallel direction (“through-thickness”), indicating of orientation of the fiber axes perpendicular to the direction of applied pressure during processing.