Volume variation in a thermochemical material- An experimental study

The research focuses on swelling and shrinkage during cycling of a thermochemical material. Potassium carbonate has been cycled and the change in size has been monitored over subsequent cycles with the help of in-situ measurement in the micro-climate chamber. The experiments have been performed for different operating conditions and the resultant images were processed to calculate the equivalent diameter of the salt grains. Micro -CT scans were performed for both the samples to compare the two-dimensional results from in-situ experiments to a complete three-dimensional analysis

Characterization and modelling of K2CO3 cycles for thermochemical energy storage applications

Thermochemical heat storage in salt hydrates is a promising concept to bridge the gap between supply and demand of solar thermal energy in the built environment. Using a suitable thermochemical material (TCM), a heat battery can be created to supply low-temperature thermal energy during colder time periods. The principle is based on a reversible hydration-dehydration reaction with water vapour. The TCM can be charged (dehydrated) at a temperature of 120°C by using solar thermal collectors. Conversely, the discharge (hydration) occurs at room temperature using a constant water vapour pressure of 12 mbar. Previous studies have indicated that potassium carbonate (K2CO3) is a good candidate to fulfil the role of TCM in built environment applications. To generate adequate power from a heat battery for hot tap water or space heating, the kinetics of the TCM need to be sufficiently fast. It is hypothesized that the kinetics of the material improve over multiple charge and discharge cycles due to crack formation and volume increase of the grains. The aim of this work is to evaluate the kinetics of 500-700 µm K2CO3 grains using thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), and to quantify the improvement in kinetics over multiple charge and discharge cycles. The kinetics serve as input for an existing nucleation and growth model, simulating the fractional conversion at grain level. In the TGA/DSC experiments, the material was charged and discharged numerous times under a constant water vapour pressure of 12 mbar. The cycling temperature varies from room temperature to a maximum temperature of 120°C. The conversion time of each cycle was monitored. Additionally, using an optical microscope, cycling experiments of K2CO3 were performed in a micro climate chamber with the same conditions as in the TGA/DSC experiments. This allows tracking of the apparent surface area of the grains and the observation of crack formation for each cycle. The existing nucleation and growth model is enhanced by incorporating grain growth and crack formation observed from the optical experiments. Thermal characterization by means of TGA/DSC has indicated that indeed the kinetics of the material improve over multiple cycles. Typical conversion rates are increased by a factor 10 comparing the first and the 12th cycle. Preliminary optical microscope experiments show an increase of the apparent grain surface area of approximately 55%. Additionally, crack formation is observed over multiple hydration and dehydration cycles leading to increased inter-particle porosity, likely adding to the improved kinetics

Magnetic behavior of bulk and nanostructured MnxTaS2

At its base, material science research aims to categorize specific materials by their various attributes, such as structure, integrity, electronic properties, magnetic properties, and others. By categorizing materials in this way, it becomes easier to generalize the application of a specific material to those within a broader category. The interest in materials that exhibit useful characteristics at small scales derives directly from the technology industry’s need for smaller and smaller devices. Two-dimensional materials are of great interest for this reason. Two-dimensional materials are comprised of many single layers, or planes, stacked together to create a crystal. Each layer may be composed of single or multiple elements. The layers interact weakly with one another; consequently, the properties of the material may be largely determined by the characteristics of the layers. The electronic properties of these materials were researched in detail within the last decades. The result of this research was the categorization of specific two-dimensional materials as insulators, semimetals, superconductors, metals, and semiconductors (Ajayn, Kim, & Banerjee, 2016). Two-dimensional materials that are chemically similar to any of the specific materials exhibiting these properties quickly become candidates for similar behavior. The research that produced the results detailed within this work was completed with the above results in mind. The material described in this work is composed of layers of tantalum sulfide between which manganese was deposited. The number of manganese atoms per one tantalum is called the concentration, x. In contrast to the research that led to the categorizations described above, the magnetic properties of this material were explored. Specifically, this project aimed to characterize the magnetic phase transitions of bulk and nanostructured samples of manganese intercalated tantalum disulfide (MnxTaS2) using several well documented analysis methods such as those used by Anthony Arrott and John Noakes (Arrott & Noakes, 1967). Determining and comparing these magnetic characteristics will provide both novel results and a basis for subsequent projects

Dielectric Properties of Isolated Adrenal Chromaffin Cells Determined by Microfluidic Impedance Spectroscopy

Knowledge of the dielectric properties of biological cells plays an important role in numerical models aimed at understanding how high intensity ultrashort nanosecond electric pulses affect the plasma membrane and the membranes of intracellular organelles. To this end, using electrical impedance spectroscopy, the dielectric properties of isolated, neuroendocrine adrenal chromaffin cells were obtained. Measured impedance data of the cell suspension, acquired between 1 kHz and 20 MHz, were fit into a combination of constant phase element and Cole-Cole models from which the effect of electrode polarization was extracted. The dielectric spectrum of each cell suspension was fit into a Maxwell-Wagner mixture model and the Clausius-Mossotti factor was obtained. Lastly, to extract the cellular dielectric parameters, the cell dielectric data were fit into a granular cell model representative of a chromaffin cell, which was based on the inclusion of secretory granules in the cytoplasm. Chromaffin cell parameters determined from this study were the cell and secretory granule membrane specific capacitance (1.22 and 7.10 mu F/cm(2), respectively), the cytoplasmic conductivity, which excludes and includes the effect of intracellular membranous structures (1.14 and 0.49 S/m, respectively), and the secretory granule milieu conductivity (0.35 S/m). These measurements will be crucial for incorporating into numerical models aimed at understanding the differential poration effect of nanosecond electric pulses on chromaffin cell membranes. (C) 2017 Elsevier B.V. All rights reserved

Broadband Dielectric Spectroscopy on Human Blood

Dielectric spectra of human blood reveal a rich variety of dynamic processes. Achieving a better characterization and understanding of these processes not only is of academic interest but also of high relevance for medical applications as, e.g., the determination of absorption rates of electromagnetic radiation by the human body. The dielectric properties of human blood are studied using broadband dielectric spectroscopy, systematically investigating the dependence on temperature and hematocrit value. By covering a frequency range from 1 Hz to 40 GHz, information on all the typical dispersion regions of biological matter is obtained. We find no evidence for a low-frequency relaxation (alpha-relaxation) caused, e.g., by counterion diffusion effects as reported for some types of biological matter. The analysis of a strong Maxwell-Wagner relaxation arising from the polarization of the cell membranes in the 1-100 MHz region (beta-relaxation) allows for the test of model predictions and the determination of various intrinsic cell properties. In the microwave region beyond 1 GHz, the reorientational motion of water molecules in the blood plasma leads to another relaxation feature (gamma-relaxation). Between beta- and gamma-relaxation, significant dispersion is observed, which, however, can be explained by a superposition of these relaxation processes and is not due to an additional delta-relaxation often found in biological matter. Our measurements provide dielectric data on human blood of so far unsurpassed precision for a broad parameter range. All data are provided in electronic form to serve as basis for the calculation of the absorption rate of electromagnetic radiation and other medical purposes. Moreover, by investigating an exceptionally broad frequency range, valuable new information on the dynamic processes in blood is obtained.Comment: 17 pages, 9 figure

Perioperative antiplatelet therapy: the case for continuing therapy in patients at risk of myocardial infarction

Recent clinical data show that the risk of coronary thrombosis after antiplatelet drugs withdrawal is much higher than that of surgical bleeding if they are continued. In secondary prevention, aspirin is a lifelong therapy and should never be stopped. Clopidogrel is regarded as mandatory until the coronary stents are fully endothelialized, which takes 3 months for bare metal stents, but up to 1 yr for drug-eluting stents. Therefore, interruption of antiplatelet therapy 10 days before surgery should be revised. After reviewing the data on the use of antiplatelet drugs in cardiology and in surgery, we propose an algorithm for the management of patients, based on the risk of myocardial ischaemia and death compared with that of bleeding, for different types of surgery. Even if large prospective studies with a high degree of evidence are still lacking on different antiplatelet regimens during non-cardiac surgery, we propose that, apart from low coronary risk situations, patients on antiplatelet drugs should continue their treatment throughout surgery, except when bleeding might occur in a closed space. A therapeutic bridge with shorter-acting antiplatelet drugs may be considered

Testing Models of Sheaths and Instabilities with Particle-in-cell Simulations

Sheaths and presheaths represent the response of a plasma to boundaries and are an instance of plasma self-organization. They are commonly utilized in plasma technologies and reduced models of plasmas across a range of gas pressures. This thesis leverages the particle-in-cell method to explain discrepancies between models and measurements of ion temperature at low pressures, test untested models of high pressure sheaths, and explore a novel electron plasma wave instability driven by an ambipolar electric field. Simulations reveal that ion-acoustic instabilities excited in presheaths can cause significant ion heating. Ion-acoustic instabilities are excited by the ion flow toward a sheath when the neutral pressure is small enough and the electron temperature is large enough. A series of 1D simulations were conducted in which electrons and ions were uniformly sourced with an ion temperature of 0.026 eV and different electron temperatures (0.1 - 50 eV). Ion heating was observed when the electron-to-ion temperature ratio exceeded the minimum value predicted by linear response theory to excite ion-acoustic instabilities at the sheath edge (T_e/T_i ~ 28). When this threshold was exceeded, the temperature equilibriation rate between ions and electrons increased near the sheath so that the local temperature ratio did not exceed the threshold for instability. This resulted in significant ion heating near the sheath edge, which also extended back into the bulk plasma because of wave reflection from the sheath. The instability heating was found to decrease for higher pressures, where ion-neutral collisions damp the waves and ion heating is instead dominated by inelastic collisions in the presheath. Simulations using the direct simulation Monte Carlo method were used to study how neutral pressure influences plasma properties at the sheath edge. The high rate of ion-neutral collisions at pressures above several mTorr were found to cause a decrease in the ion velocity at the sheath edge (collisional Bohm criterion), a decrease in the edge-to-center density ratio, and an increase in the sheath width and sheath potential drop. A comparison with existing analytic models generally indicates favorable agreement, but with some distinctions. One is that models for the edge-to-center density ratio need to be made consistent with the collisional Bohm criterion. With this and similar corrections, a comprehensive fluid-based model of the plasma boundary was constructed that compares well with the simulations. Ambipolar electric fields are commonplace in plasmas and affect transport by driving currents and in some cases instabilities. Simulations demonstrate that an instability, named the electron-field instability, can be driven by an ambipolar strength electric field. The instability excites waves of 30 Debye-lengths and has a growth-rate that is proportional to the electric field strength. Unlike other instabilities, the electron-field instability only requires that the electrons interact with the field and does not result from the relative drift between electron populations (beam instability) or electrons and ions (ion-acoustic instability). In fact, the instability occurs near the electron plasma frequency which is much higher than most drift instabilities. Low-temperature and space-based plasmas are found to be likely systems where the instability may be excited. We find that our simulations and linear theory agree until a non-linear state is reached in the simulations. These results demonstrate that low pressure sheaths are susceptible to instabilities that can significantly affect plasmas properties, while fluid model accurately capture collisional effects at higher pressures.PhDApplied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/176525/1/lbeving_1.pd