Severe dehydration is lethal for most biological species, however there are a number of organisms which have evolved mechanisms to avoid damage during dehydration. One of these mechanisms is the accumulation of small solutes (e.g. sugars), which have been shown to preserve membranes by inhibiting deleterious phase changes at low hydration. Specifically, sugars reduce the gel to fluid phase transition temperatures of model lipid/water mixtures. However, there is debate about the precise mechanism, the resolution of which hinges on the location of the sugars. An experimental investigation into the effects of small solutes on the phase behaviour of phospholipid membranes is presented in order help identify the mechanisms by which solutes facilitate desiccation tolerance. Differential Scanning Calorimetry (DSC) was used to determine the first comprehensive phase diagram for the synthetic phospholipid DPPC over a wide range of hydration and solute molar ratios between 0.1 and 1.0 sugars per lipid. Over the same range of hydrations and solute concentrations Small Angle X-Ray Scattering (SAXS) was used to measure the structural parameters of the membrane bilayers necessary to determine both the phase of the membrane lipids and the location of the solutes. SAXS was also used to conduct the first comprehensive study of the effect of solutes on the kinetics of the fluid - gel transition of DPPC over a range of both hydration and solute concentration. Finally, contrast variation Small Angle Neutron Scattering (SANS) was used to quantitatively determine the location of the solutes. Data from these complimentary techniques are presented which show a monotonic relationship between both transition temperature and repeat spacing with respect to solute concentration. This relationship exists between solute:lipid molar ratios between 0.1 to approximately 0.5, after which higher concentrations of solute are shown to have no further effect on either the bilayer repeat spacing or transition temperature. It is proposed that the exclusion of small solutes into sugar/water micro-phases external to the bilayer can account for this behaviour. A theoretical model previously used to describe membrane phase behaviour at low hydrations is modified to account for the presence of solutes between membrane bilayers. This model is shown to be in quantitative agreement with the experimental data up until approximately 0.5 sucrose molecules per lipid, the point of solute exclusion. Once exclusion is taken into account, the model is quantitative over the whole range of sugar concentrations.

No description available.

The lattice-gas model of sorption in disordered porous media is studied for a variety of settings, using existing, updated and newly developed numerical techniques. Firstly, we construct an efficient algorithm to calculate the exact partition function for small lattice-gas systems. The exact partition function is used for detailed analysis of the core features exhibited by such systems. We proceed to develop an interactive Monte Carlo (MC) simulation engine, that simulates sorption in a porous media sample and provides real-time visual data of the state space projection and the 3d view of the sample among other parameters of interest, as the external fields are manipulated. The use of such tool provides a more intuitive understanding of the system behaviour. The MC simulations are employed to study sorption in several porous solids: silica aerogel, Vycor glass and soil. We investigate how the phenomena depend on the microstructure of the original samples, how the behaviour varies with the external conditions, and how it is reflected in the paths that the system takes across its state space. Secondly, we develop two methods for estimation of the relative degeneracy (the number of microstates that have the same value of some macroscopic variables) in the systems that are too large to be handled exactly. The methods, based on a restricted infinite temperature sampling, obtain equidegenerate surfaces and the degeneracy gradient across the state space. Combined with the knowledge of an internal energy of a microstate, it enables us to construct the free energy map and thus the equilibrium probability distribution for the studied projection of the state space. Thirdly, the jump-walking Monte-Carlo algorithm is revisited and updated to study the equilibrium properties of systems exhibiting quasi-ergodicity. It is designed for a single processing thread as opposed to currently predominant algorithms for large parallel processing systems. The updated algorithm is tested on the Ising model and applied to the lattice-gas model for sorption in aerogel and Vycor glass at low temperatures, when dynamics of the system is significantly slowed down. It is demonstrated that the updated jump-walking simulations are able to produce equilibrium isotherms which are typically hidden by the hysteresis effect characteristic of the standard single-flip simulations. As a result, we answer the long standing question about the existence of the first-order phase transitions in Vycor. Finally, we investigate sorption in several distinct topology network representations of soil and aerogel samples and demonstrate that the recently developed analytical techniques for random networks can be used to achieve a qualitative understanding of the phenomena in real materials.

Ces travaux de thèse, réalisés dans le cadre du programme de recherche ASZTech, portent sur l’étude de deux réfractaires électrofondus : un matériau Alumine-Zircone-Silice (AZS) et un produit à Très Haute Teneur en Zircone (THTZ), destinés à l’industrie verrière. La zircone étant non stabilisée, ces réfractaires subissent donc, lors de l’étape de recuisson (refroidissement), la transformation quadratique (Q) à monoclinique (M) de la zircone qui, de par sa forte expansion volumique, est une source d’endommagement. L’objectif de cette étude a consisté à caractériser, au cours de traitements thermiques représentatifs, le comportement thermomécanique de ces matériaux et à établir des corrélations avec les évolutions microstructurales rencontrées. En étudiant leurs comportements sous sollicitations thermiques par des techniques acoustiques, l’impact de la transformation Q-M de la zircone sur le développement de l’endommagement a été mis en évidence. En s’intéressant particulièrement au comportement mécanique au passage de cette transformation, la présence d’un phénomène de plasticité de transformation (TRIP) a été confirmée. Outre le phénomène de TRIP qui participe à la relaxation des contraintes au passage de la transformation de phase, un phénomène d’endommagement diffus, offrant également une capacité d’accommodation, a été identifié pour le matériau THTZ. Au final, ces données ont permis d’améliorer le modèle de simulation numérique décrivant l’étape de recuisson de blocs industriels. En outre, la robustesse de ce modèle pourra maintenant être validée par confrontation à des mesures expérimentales de contraintes résiduelles réalisées au cours de cette étude. / This thesis, carried out under the research program ASZTech, focuses on the study of two fused cast refractories: an Alumina-Zirconia-Silica (AZS) product and a product with very High content of Zirconia (HZ), used in glass industry. These refractories containing non-stabilized zirconia are therefore subjected, during the annealing step (cooling), to the tetragonal (T) to the monoclinic (M) phase transition of zirconia associated with a very high volumetric expansion which is a source of stresses. This study aims at characterizing, during representative thermal treatments, the thermo-mechanical behavior of these materials, in correlation with the detected microstructural evolutions. Thanks to the analysis of their behavior under thermal stresses by acoustic techniques, the effect of the T-M transition of zirconia on the development of damage has been highlighted. In particular, the occurrence of a TRansformation Induced Plasticity (TRIP) phenomenon during the zirconia phase transition has been identified and characterized thanks to cooling tests under mechanical stress. In addition to this TRIP phenomenon which allows to relax the mechanical stresses induced during the phase transition, a diffuse damage phenomenon has been also identified for HZ product. Eventually, these data have led to an improvement of the numerical model which simulates the annealing step of industrial blocks. Moreover, the reliability of this model will be now checked by comparison to experimental measurements of residual stresses performed during this study.

The magnetocaloric and thermomagnetic properties of the MnNiGe1-xAlx, Ni50Mn35(In1−xCrx)15 and (Mn1−xCrx)NiGe1.05 systems have been studied by x-ray diffraction, differential scanning calorimetry (DSC), and magnetization measurements. Partial substitution of Al for Ge in MnNiGe1−xAlx results in a first-order magnetostructural transition (MST) from a hexagonal ferromagnetic to an orthorhombic antiferromagnetic phase at 186 K (for x = 0.09). A large magnetic entropy change of ∆SM = -17.6 J/kg K for ∆H = 5 T was observed in the vicinity of TM = 186 K for x = 0.09. This value is comparable to those of well-known giant magnetocaloric materials, such as Gd5Si2Ge2, MnFeP0.45As0.55, and Ni50Mn37Sn13 [1]. The values of the latent heat (L = 6.6 J/g) and corresponding total entropy changes (∆ST = 35 J/kg K) have been evaluated for the MST using DSC measurements. Large negative values of ∆SM of -5.8 and -4.8 J/kg K for ∆H = 5 T in the vicinity of TC were observed for x = 0.09 and 0.085, respectively. Partial substitution of Cr for Mn in(Mn1−xCrx)NiGe1.05 results in a MST from a hexagonal paramagnetic to an orthorhombic paramagnetic phase near TM ~ 380 K (for x = 0.07). Partial substitution of Cr for In in Ni50Mn35(In1−xCrx)15 shifts the magnetostructural transition to a higher temperature (TM ~ 450 K) for x = 0.1. Large magnetic entropy changes of ∆SM = -12 (J/kgK) and ∆S = -11 (J/kgK), both for a magnetic field change of 5 T, were observed in the vicinity of TM for (Mn1−xCrx)NiGe1.05 and Ni50Mn35(In1−xCrx)15, respectively. The concentration-dependent (T-x) phase diagram of transition temperatures (magnetic, structural, and magnetostructural) has been generated using magnetic, XRD, and DSC data. The role of magnetic and structural changes on transition temperatures are discussed.

A technology that has the potential to liquefy hydrogen and natural gas efficiently is an Active Magnetic Regenerative Liquefier (AMRL). An AMRL exploits the magnetocaloric effect displayed by magnetic materials whereby a reversible temperature change is induced when the material is exposed to a magnetic field. This effect can be used to produce cooling. By using the magnetic materials in a regenerator as the heat storage medium and as the means of work input, one creates an Active Magnetic Regenerator (AMR). Because the adiabatic temperature change is a strong function of temperature for most materials, to span a large temperature range such as that needed to liquefy hydrogen, a number of different materials may be needed to make up one or more regenerators. Single material AMRs have been proven, but layering with more than one material has not. This thesis is a study of AMRs using magnetic refrigerants displaying second-order paramagnetic to ferromagnetic ordering. An analysis of AMR thermodynamics is performed and results are used to define properties of ideal magnetic refrigerants. The design and construction of a novel test apparatus consisting of a conduction-cooled superconducting solenoid and a reciprocating AMR test apparatus are described. A numerical model is developed describing the energy transport in an AMR. Experiments using Gd are performed and results are used to validate the model. A strong relationship between flow phasing is discovered and possible reasons for this phenomenon are discussed. Simulations of AMRs operating in unconventional modes such as at temperatures greater than the transition temperature reveal new insights into AMR behaviour. Simulations of two-material layered AMRs suggest the existence of a jump phenomenon occurring regarding the temperature span. These results are used to explain the experimental results reported by other researchers for a two-material AMR. / Graduate

Made available in DSpace on 2014-06-12T18:06:41Z (GMT). No. of bitstreams: 2 arquivo945_1.pdf: 1972275 bytes, checksum: faf69ec63b01c315fa88949dbaf22651 (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2009 / Sandoildo Freitas Tenório, Antônio; Domingues Coutinho Filho, Maurício. Phase transitions and thermodynamics of quasione- dimensional quantum rotor and spin systems. 2009. Tese (Doutorado). Programa de Pós-Graduação em Física, Universidade Federal de Pernambuco, Recife, 2009.

No / The two known crystalline phases of resorcinol and their phase transitions are of considerable interest. The crystals exhibit pyro- and piezo-electricity and, remarkably, the higher temperature phase is the denser phase. Furthermore, crystals of the phase, by virtue of having a polar axis, have played a crucial role in investigating fundamental issues of crystal growth. We report an optimized force field for the molecular simulation of crystalline phases of resorcinol. The hydroxyl groups of the resorcinol molecule have a torsional degree of freedom and the molecule adopts a different conformation in each of the two phases of resorcinol. The torsional barrier, therefore, was considered to be critical and has been characterized using ab initio methods. Although the atomic partial charges showed some dependence on the molecular conformation, a single set of partial charges was found to be sufficient in describing the electrostatic potential for all conformations. The parameters for the van der Waals interactions were optimized using sensitivity analysis. The proposed force field reproduces not only the static structures but also the stability of the crystalline phases in extended molecular dynamics simulations.

No description available.

Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: We present a theoretical approach that allows for the description of trapped Bose-Fermi mixtures with a tunable interspecies interaction in the vicinity of a Feshbach resonance magnetic field.The many-body physics of the system is treated at equilibrium using the well-established mean-field and local density approximations. This reduces the physics locally to that of a homogeneous system. We observe a rich local phase structure exhibiting both first and second order phase transitions between the normal and BEC phases. We also consider the global properties of the mixture at a fixed number of particles and investigate how the density profiles and the populations of the various particle species depend on the detuning and trap profile. / AFRIKAANSE OPSOMMING: Ons beskou ’n teoretiese beskrywing van gevangde Bose-Fermi mengsels met ’n verstelbare interspesie wisselwerking in die teenwoordigheid van ’n magneties-geïnduseerde Feshbach resonansie. Die veeldeeltjiefisika van die sisteem word by ekwilibrium binne die welbekende gemiddelde-veld en lokale-digtheid benaderings hanteer. Sodoende word die fisika lokaal tot die van ’n homogene sisteem gereduseer. Ons neem ’n ryk fase-struktuur waar met beide eerste- en tweede-orde fase-oorgange tussen die normale en BEK fases. Ons beskou ook die globale eienskappe van die mengsel by ’n vaste totale aantal deeltjies en ondersoek hoe die digtheidsprofiele en deeltjiegetalle van die afstemming en die profiel van die val afhang.