Manufacturing and characterization of welded, sintered condensers for a loop heat pipe

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 55-56).A manufacturing process plan was developed for a welded condenser utilizing a sintered wick. Electronic devices have progressed to the point where new designs are limited by the thermal management system used to ensure safe operating temperatures. Coupling the effects of high surface area of multiple condensers, the low thermal resistance of loop heat pipes, and the increased dissipation rates with an integrated fan system, a high efficiency heat exchanger has been previously designed. The multiplecondenser loop heat pipe required reliably manufactured condensers, with specified internal features composed of sinter wick material in order to prevent flooding and flash vaporization; key challenges faced by having multiple condensers in a loop heat pipe. The implementation of a functional sintered bond used to assemble the condenser and a welded flange design for the creation of a hermetic seal, resulted in a more reliable condenser, which functioned in a similar manner to previous designs. Additionally, the performance of the condenser under air restriction was addressed.by Jay D. Sircar.S.B

Clustering in a precipitate free GeMn magnetic semiconductor

We present the first study relating structural parameters of precipitate free Ge0.95Mn0.05 films to magnetisation data. Nanometer sized clusters - areas with increased Mn content on substitutional lattice sites compared to the host matrix - are detected in transmission electron microscopy (TEM) analysis. The films show no overall spontaneous magnetisation at all down to 2K. The TEM and magnetisation results are interpreted in terms of an assembly of superparamagnetic moments developing in the dense distribution of clusters. Each cluster individually turns ferromagnetic below an ordering temperature which depends on its volume and Mn content.Comment: accepted for publication in Phys. Rev. Lett. (2006). High resolution images ibide

Interplay between the electrical transport properties of GeMn thin films and Ge substrates

We present evidence that electrical transport studies of epitaxial p-type GeMn thin films fabricated on high resistivity Ge substrates are severely influenced by parallel conduction through the substrate, related to the large intrinsic conductivity of Ge due to its small bandgap. Anomalous Hall measurements and large magneto resistance effects are completely understood by taking a dominating substrate contribution as well as the measurement geometry into account. It is shown that substrate conduction persists also for well conducting, degenerate, p-type thin films, giving rise to an effective two-layer conduction scheme. Using n-type Ge substrates, parallel conduction through the substrate can be reduced for the p-type epi-layers, as a consequence of the emerging pn-interface junction. GeMn thin films fabricated on these substrates exhibit a negligible magneto resistance effect. Our study underlines the importance of a thorough characterization and understanding of possible substrate contributions for electrical transport studies of GeMn thin films.Comment: 9 pages, 9 figure

Magnetic and structural properties of GeMn films: precipitation of intermetallic nanomagnets

We present a comprehensive study relating the nanostructure of Ge_0.95Mn_0.05 films to their magnetic properties. The formation of ferromagnetic nanometer sized inclusions in a defect free Ge matrix fabricated by low temperature molecular beam epitaxy is observed down to substrate temperatures T_S as low as 70 deg. Celsius. A combined transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) analysis of the films identifies the inclusions as precipitates of the ferromagnetic compound Mn_5Ge_3. The volume and amount of these precipitates decreases with decreasing T_S. Magnetometry of the films containing precipitates reveals distinct temperature ranges: Between the characteristic ferromagnetic transition temperature of Mn_5Ge_3 at approximately room temperature and a lower, T_S dependent blocking temperature T_B the magnetic properties are dominated by superparamagnetism of the Mn_5Ge_3 precipitates. Below T_B, the magnetic signature of ferromagnetic precipitates with blocked magnetic moments is observed. At the lowest temperatures, the films show features characteristic for a metastable state.Comment: accepted for publication in Phys. Rev. B 74 (01.12.2006). High resolution images ibide

Sticky surface: sphere - sphere adhesion dynamics

Special Issue: Selected Papers from the The Fourth International Conference on Mathematical Modeling and Analysis of Populations in Biological Systems (ICMA IV), Texas Tech University, Lubbock, Texas, USA, 4-6 October 2013We present a multi-scale model to study the attachment of spherical particles with a rigid core, coated with binding ligands and suspended in the surrounding, quiescent fluid medium. This class of fluidimmersed adhesion is widespread in many natural and engineering settings, particularly in microbial surface adhesion. Our theory highlights how the micro-scale binding kinetics of these ligands, as well as the attractive/repulsive surface potential in an ionic medium affects the eventual macro-scale size distribution of the particle aggregates (flocs). The bridge between the micro-macro model is made via an aggregation kernel. Results suggest that the presence of elastic ligands on the particle surface lead to the formation of larger floc aggregates via efficient inter-floc collisions (i.e. non-zero sticking probability, g). Strong electrolytic composition of the surrounding fluid favours large floc formation as well. The kernel for the Brownian diffusion for hard spheres is recovered in the limit of perfect binding effectiveness (g -> 1) and in a neutral solution with no dissolved salts.Sarthok Sircara, John G.Younger and David M. Bort

Gas Adsorption in Novel Environments, Including Effects of Pore Relaxation

Adsorption experiments have been interpreted frequently with simplified model geometries, such as ideally flat surfaces and slit or cylindrical pores. Recent explorations of unusual environments, such as fullerenes and metal-organic-framework materials, have led to a broadened scope of experimental, theoretical and simulation investigations. This paper reviews a number of such studies undertaken by our group. Among the topics receiving emphasis are these: universality of gas uptake in pores, relaxation of a porous absorbent due to gas uptake and the novel phases of gases on a single nanotube, all of which studies have been motivated by recent experiments

Eroding market stability by proliferation of financial instruments

We contrast Arbitrage Pricing Theory (APT), the theoretical basis for the development of financial instruments, with a dynamical picture of an interacting market, in a simple setting. The proliferation of financial instruments apparently provides more means for risk diversification, making the market more efficient and complete. In the simple market of interacting traders discussed here, the proliferation of financial instruments erodes systemic stability and it drives the market to a critical state characterized by large susceptibility, strong fluctuations and enhanced correlations among risks. This suggests that the hypothesis of APT may not be compatible with a stable market dynamics. In this perspective, market stability acquires the properties of a common good, which suggests that appropriate measures should be introduced in derivative markets, to preserve stability.Comment: 26 pages, 8 figure

Soot and Spectral Radiation Modeling for High-Pressure Turbulent Spray Flames

A transported probability density function (PDF) method and a photon Monte Carlo/line-by-line (PMC/LBL) spectral model are exercised to generate physical insight into soot processes and spectral radiation characteristics in transient high-pressure turbulent n-dodecane spray flames, under conditions that are relevant for compression-ignition piston engines. PDF model results are compared with experimental measurements and with results from a locally well-stirred reactor (WSR) model that neglects unresolved turbulent fluctuations in composition and temperature. Computed total soot mass and soot spatial distributions are highly sensitive to the modeling of unresolved turbulent fluctuations. To achieve reasonable agreement between model and experiment and to capture the highly intermittent nature of soot in the turbulent flame, it is necessary to accurately represent mixing and the low diffusivity of soot particles. This is accomplished in the PDF framework using a mixing model that enforces locality in the gas-phase composition space, while not mixing the transported soot variables. The results suggest that mixing is at least as important as kinetics in controlling soot formation and evolution in high-pressure turbulent flames. Regarding radiation, radiant fractions and global influences of radiation in these flames are relatively small. Nevertheless, an examination of spectral radiative heat transfer provides valuable insight into the nature and modeling of radiation in high-pressure turbulent combustion systems. There are complex spectral interactions that are revealed using PMC/LBL. CO2 dominates the total radiative emission and reabsorption, but most of the emitted CO2 radiation is reabsorbed before reaching the walls. On the other hand, most of the emitted soot radiation reaches the walls, but soot radiation is a small contribution overall; H2O dominates the radiation that reaches the walls. Global turbulence–radiation interactions (TRI) effects are small, but radiative emission from soot increases by approximately a factor two when TRI are considered. Radiative transfer contributes both to energy redistribution in the vessel and to wall heat losses. The results suggest that a simple model that considers soot radiation and the principal CO2 and H2O spectral bands might be sufficient to capture the key influences of radiation in engine CFD. It is expected that these findings will contribute to the development of truly predictive CFD models for engines and other high-pressure turbulent combustion systems