Extreme conditions allow us to reveal unusual material properties. At the same time an experimental approach is di-cult under such conditions. Capabilities of a theoretical approach based on simplied models are limited. This explainsa wide application of computer simulations at extreme conditions. My thesis is concerned with computer simulations undersuch a conditions. I address such problems as melting, solidsolid phase transitions, shockwave impact on material properties and chemical reactions under extreme conditions. We addressed these problems to facilitate simulations of phase transitions to provide some interpretation of experimental data andexplain enigmatic phenomena in interior of the Earth. / <p>QC 20160615</p>

Abstract In this paper, we study the nature of phase transition of the two-dimensional six-state Potts model under the external random magnetic field. The six-state Potts model exist temperature-dependent first-order phase transition. When the external random field is applied, the nature of phase can be altered from first-order to second-order.By employing the Monte Carlo simulation method, we inspected the energy histogram and Binder parameter of the six-state Potts model under the external random magnetic field. According to our analyses, the evidences reveal that the phase transition does not change until the external magnetic field is greater then 0.02

La doped HoMn2O5 in studying the influence of the local structure distortion on the magnetic transition has been investigated systematically. It is found that by doping of La ions in a proper percentage, 0.1≤x≤0.2, the formation of the possible impurity RMnO3 (113) phase could be suppressed; single phases of LaxHo1-xMn2O5 (0.1≤x≤0.2) can be formed in one atmosphere of flowing oxygen. For x=0.2, an weak ferromagnetic transition on top of the paramagnetic background appears at 150K and saturated at 75K, which implies that the compound undergoes a ferromagnetic to antiferromagnetic transition. In addition, the lattice didn¡¦t appear a huge phase transition at low temperature, it¡¦s observed local distortion behavior within 100~150K, and back to normal structure as high temperature ones. It¡¦s convinced that will be associated with the magnetic properties, which need further examination and experiment.

No description available.

The Project in Canada to Search for Supersymmetric Objects (PICASSO) searches for cold dark matter through the direct detection of Weakly Interacting Massive Particles (WIMPs) via spin-dependent and spin independent interactions with 19F nuclei. The detection principle is based on the superheated droplet technique; the detectors consist of a gel matrix with millions of liquid droplets of superheated fluorocarbon (C4F10) dispersed in it. In order to reduce the background, it is essential to distinguish the signature of different background particles interacting in the detector. A dedicated setup was developed in order to study the response of the C4F10 droplets in the presence of different backgrounds. The main objectives of this research are to identify the actual size (diameter) of the droplet increases due to phase transition and to check and establish the correlation between the droplet size and the maximum amplitude of the signal. In addition, the alpha-neutron discrimination was studied by observing each event’s image frames and the associated acoustic signal to get the amplitude distribution. The mean ratio of bubble size to droplet size was found to be 5.48, independent of temperature and type of interacting particle. Furthermore, no correlation was found between the droplet size and the maximum amplitude of the signal. As for the discrimination studies, the analysis of the signal events has confirmed that alphas generated outside the active liquid in the gel are much more difficult to discriminate from neutron than when alphas are generated inside the active liquid.

<p>Phase change materials (PCMs) has emerged as advanced functional materials for efficient thermal energy storage and release. Compared to other organic and inorganic PCMs, oxide-based PCMs have attracted growing interest because of small volume expansion, minor leakage issue, and moderate latent heat. In this dissertation, two special cases of oxide-based PCMs is discussed, i.e., vanadium dioxide (VO₂), and Bi-based perovskite/supercell structures. Specifically, VO₂ emerges as a focus of research because of its well-known semiconductor-to-metal transition (SMT) upon heating close to 68 °C. The intrinsic coupling of SMT and R-M1 structural change makes VO₂ a favorite material both scientifically interesting and technologically important for potential sensor and memory device applications. On the other hand, BFMO supercell structure originates from the double-perovskites, while the substrate-induced epitaxial strain induces the stacking and commensurate modulations of Fe/Mn double layers. The significantly enhanced multiferroic response is attributed to its non-centrosymmetric structure.</p> <p>In this dissertation, a comprehensive study on the FM integration and novel approaches to achieve broad range transition temperature (<em>T</em>_<em>c</em>) tuning is explored in VO₂ thin films. Specifically, three novel metal/VO₂ nanocomposite designs are discussed, i.e., Pt/VO₂, Ni/VO₂ and Li/VO₂, with different morphology and Tc tuning mechanisms. First, by reconstructing the energy band structure at the metal/VO₂ interface, bidirectional <em>T</em>_<em>c</em> tuning in Pt/VO₂ nanocomposites can be achieved owing to the size dependent work function of Pt NPs. Next, by engineering the morphology by lattice matching, diffusion kinetics, and interfacial mixing, the exploration on Ni/VO₂ nanocomposites achieve the combined goals of Tc tuning and magnetic incorporation/magneto-optical (MO) coupling. Finally, by varying Li concentration during the metal-ion intercalation, <em>T</em>_<em>c</em> of both VO₂(B) and VO₂(M1) thin films can be systematically tailored because of structural deformation and the change in charge carrier density. The demonstration of metal/VO₂ nanocomposite thin films reveals a promising approach to fulfill various working environments for VO₂-based novel electronics, photonics, and spintronics. Furthermore, the microstructure evolution of the ultrathin BFMO supercell structure as well as its physical properties is first studied. The robust ferromagnetic and ferroelectric response is preserved in the ultrathin structure less than 10 nm, making it an attractive candidate for next-generation spintronics based on 2D materials.</p>

no abstract provided. / Thesis / Master of Science (MSc)

The human brain is considered to be the most complex and powerful information-processing device in the known universe. The fundamental concepts behind the physics of complex systems motivate scientists to investigate the human brain as a collective property emerging from the interaction of thousand agents. In this dissertation, I investigate the emergence of cooperation-induced properties in a system of interacting units. I demonstrate that the neural network of my research generates a series of properties such as avalanche distribution in size and duration coinciding with the experimental results on neural networks both in vivo and in vitro. Focusing attention on temporal complexity and fractal index of the system, I discuss how to define an order parameter and phase transition. Criticality is assumed to correspond to the emergence of temporal complexity, interpreted as a manifestation of non-Poisson renewal dynamics. In addition, I study the transmission of information between two networks to confirm the criticality and discuss how the network topology changes over time in the light of Hebbian learning.

The major goal of the work is to develop effective additives for U-10Zr (wt.%) metallic fuel to mitigate the fuel-cladding chemical interactions (FCCIs) due to fission product lanthanides and to optimize the fuel phase mainly by lowering the gamma-onset temperature. The additives Sb, Mo, Nb, and Ti have been investigated. Metallic fuels with one or two of the additives and with or without lanthanide fission products were fabricated. In this study, Ce was selected as the representative lanthanide fission product. A series of tests and characterizations were carried out on the additive-bearing fuels, including annealing, diffusion coupling, scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and differential scanning calorimetry (DSC). Sb was investigated to mitigate FCCIs because available studies show its potential as a lanthanide immobilizer. This work extends the knowledge of Sb in U-10Zr, including its effect in the Zr-free region. Sb forms precipitates with fuel constituents, either U or Zr. However, it combines with the lanthanide fission product Ce when Ce is present. Those Sb-precipitates are found to be stable upon annealing, and are compatible with the cladding. The additive does not change the phase transition of U-10Zr. Mo, Nb, and Ti have been investigated for phase optimization based on the known characteristics shown in the binary phase diagrams. The quaternary alloys, i.e., two Mo-bearing alloys and two Nb-bearing alloys, were investigated. Compared to U-10Zr, a few weight percentages of Zr are replaced by those additives in the quarternary alloys. The solid-state phase transitions were determined (alpha and U2Ti transfer into gamma). The transition temperature varies depending on the compositions. The Mo-bearing alloys have lower -onset temperatures than the Nb-bearing alloys. All of them have lower gamma-onset temperatures than that of U-10Zr. Since low gamma-onset temperature is favorable, the results indicate that the fuel phase can be optimized by the replacement of a few weight percentages of Zr into those additives. All the experiments were out-of-pile tests. Therefore, in-pile experiments will be necessary to fully evaluate the performance of the additives in the future. / Doctor of Philosophy / Fuel is the "heart" of a nuclear reactor, and fuel development is a key to improving the performance and reliability of a nuclear reactor. This study investigated the effects of metallic fuel additives in a sodium-cooled fast reactor (SFR). SFRs are an advanced reactor design. Metallic fuel, e.g., U-10Zr (wt.%), is one of the common candidates for SFR fuel. The aim of this study is to develop effective additives for U-10Zr metallic fuel to improve fuel performance. The study has two main objectives. The first one is to mitigate the fuel-cladding chemical interactions (FCCIs), while the second one is to optimize the fuel phase. Four additives, i.e., Sb, Mo, Nb, and Ti have been investigated. The study is a pioneer for the application, thus, the experiments were performed without considering the irradiation effect. Metallic fuels with one or two additives were fabricated, with a series of tests being performed at a laboratory scale. The additive, Sb, was used to mitigate the FCCIs, since FCCIs are a limitation of fuel utilization (i.e., burnup). Lanthanides are produced during fuel operation and attack cladding, being one of the reasons for FCCIs. It is known that the additive Sb has the potential to bind lanthanides into stable precipitates. This work brings the investigation a step further, providing more evidence to demonstrate the stability of the precipitates and the compatibility with cladding. The results are favorable as they demonstrate that the lanthanides will not attack the cladding if they can be caught by the additive Sb in the fuel. The additives Mo, Nb, and Ti were investigated to optimize the phase. One of the favorable phase properties is the gamma-onset temperature - the lower the better. For example, the gamma-onset temperature is 776°C in pure U, while it is 680°C in U-10Zr (meaning that 10 wt.% Zr lowers the gamma-onset temperature by 96°C). In this work, the exploration moves forward by replacing a few percentages of Zr with Mo+Ti, or Nb+Ti. After the change, the gamma-onset temperatures are further decreased, with the temperatures decreasing more in the Mo-bearing fuels than in the Nb-bearing fuels. The significance of this work is twofold. Firstly, it extends the knowledge of Sb as an additive for mitigating FCCIs; secondly, it shows that Mo, Nb, and Ti can optimize the fuel to achieve a favorable phase property. The results provide strong reasons for additional irradiation tests in the future.

We examine a model of solid-solid phase transitions that includes thermo-elastic effects and an order parameter. The model is derived as a special case of the Gurtin-Fried model posed in one space dimension with a symmetric triple-well free energy in which the relative heights of the wells vary with temperature. We examine the temperature independent case, showing existence of a unique classical solution of a regularized system of partial differential equations using semigroup theory. This is followed by numerical study of a finite element algorithm for the temperature independent model. Finally, we present computational material concerning the temperature dependent model. / Ph. D.