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Antifreeze proteins (AFPs) help cold-adapted organisms survive below 0 ◦C by binding to and inhibiting the growth of ice crystals. In this way, AFPs depress the freezing point of aqueous fluids below the melting point of ice (thermal hysteresis; TH). They also have the ability to inhibit ice recrystallization in the frozen state (ice recrystallization inhibition; IRI). Some AFPs show an order of magnitude higher TH activity than others, and are termed ‘hyperactive’. One of the objectives of this thesis was to see if IRI activities of the hyperactive AFPs are also an order of magnitude higher than the moderately active AFPs. Using a capillary-based assay for IRI, the activities of three hyperactive and three moderately active AFPs were determined. There was no apparent correlation between hyperactivity in TH and high IRI activity. However, mutations of residues on the ice-binding face (IBF) of both types of AFP reduced IRI and TH activities to a similar extent. In this way, the use of IBF mutant AFPs showed that the IBF responsible for an AFP’s TH activity is also responsible for its IRI activity. Analysis of the diverse AFP structures solved to date indicate that their IBFs are relatively flat, occupy a significant proportion of the protein’s surface area and are more hydrophobic than other surfaces of the protein. The IBFs also often have repeating sequence motifs and tend to be rich in alanine and/or, threonine. The de novo design of an ice-binding protein was undertaken using these features to verify the underlying physicochemical requirements necessary for a protein’s interaction with ice. Using site-directed mutagenesis, a total of sixteen threonine substitutions were made on one of the four faces of a cyanobacterial protein with no endogenous TH activity. The inclusion of eight paired threonines on one face of this quadrilateral helix gave the engineered protein low levels of TH activity, but at the cost of destabilizing the structure to some extent. The results of this study have validated some of the properties needed for the ice-binding activity of AFPs. / Thesis (Master, Biochemistry) -- Queen's University, 2010-01-29 17:37:24.322

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Made available in DSpace on 2015-03-03T15:15:24Z (GMT). No. of bitstreams: 1 MariaGDS_DISSERT1.pdf: 887413 bytes, checksum: 6a4faf603ae54b731858d5bcc34833b8 (MD5) Previous issue date: 2010-02-24 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / We report a theoretical investigation of thermal hysteresis in magnetic nanoelements. Thermal hysteresis originates in the existence of meta-stable states in temperature intervals which may be tuned by small values of the external magnetic field, and are controlled by the systems geometric dimensions as well as the composition. Two systems have been investigated. The first system is a trilayer consisting of one antiferromagnetic MnF2 film, exchange coupled with two Fe lms. At low temperatures the ferromagnetic layers are oriented in opposite directions. By heating in the presence of an external magnetic field, the Zeeman energy induces a gradual orientation of the ferromagnets with the external field and the nucleation of spin- op-like states in the antiferromagnetic layer, leading eventually, in temperatures close to the Neel temperature, to full alignment of the ferromagnetic films and the formation of frustrated exchange bonds in the center of the antiferromagnetic layer. By cooling down to low temperatures, the system follows a different sequence of states, due to the anisotropy barriers of both materials. The width of the thermal hysteresis loop depends on the thicknesses of the FM and AFM layers as well as on the strength of the external field. The second system consists in Fe and Permalloy ferromagnetic nanoelements exchange coupled to a NiO uncompensated substrate. In this case the thermal hysteresis originates in the modifications of the intrinsic magnetic / Relatamos um estudo de histerese t?rmica em sistemas magn?ticos nanoestruturados. A histerese t?rmica se origina da exist?ncia de estados meta-est?veis em intervalos de temperatura que s?o control?veis pelas dimens?es f?sicas e composi??o do sistema magn?tico e pelo valor do campo magn?tico externo. Dois sistemas s?o investigados. O primeiro sistema consiste de uma tricamada contendo um filme antiferromagn?tico de MnF2 com intera??o de troca de interface com dois filmes ferromagn?ticos de Fe. Em baixa temperatura os dois filmes ferromagn?ticos t?m magnetiza??o em dire??es opostas. Ao aquecer o sistema em presen?a de campo magn?tico externo a energia Zeeman se sobrep?e a ordem magn?tica do filme de MnF2 induzindo uma orienta??o progressiva dos filmes ferromagn?ticos com o campo externo, e a forma??o de estados com spins fora da dire??o f?cil (spin flop) no material antiferromagn?tico, evoluindo para alinhamento dos filmes ferromagn?ticos em temperaturas ao redor da temperatura de N?el, com a forma??o de liga??es com frustra??o da energia de troca no centro do filme antiferromagn?tico. Ao resfriar o sistema segue uma sequ?ncia diferente de estados devido a barreira de anisotropia dos materiais. A largura da histerese t?rmica depende da espessura dos filmes e da intensidade do campo magn?tico externo. O segundo sistema estudado consiste de nanoelementos de Fe e Permalloy em substratos n?o-compensados de NiO. Nesse caso a histerese t?rmica se origina nas modifica??es, impostas pelo acoplamento de troca na interface, na ordem magn?tica intr?nseca do nanolemento ferromagn?tico. Ao aquecer al?m da temperatura de N?el, o nanoelemento se ajusta gradualmente ao padr?o magn?tico imposto pelo seu pr?prio campo dipolar e, no processo de resfriamento pode seguir uma sequ?ncia diferente de fases magn?ticas devido a barreira imposta por sua alta anisotropia de forma. Mostramos que histerese t?rmica ? mais prov?vel em nanoelementos de Fe, devido ao valor elevado da magnetiza??o de satura??o, e para nanoelementos de base quadrada, com dimens?es laterais ao redor de 100nm, devido a possibilidade de nuclea??o de v?rtices. Comentamos no poss?vel impacto de histerese t?rmica no funcionamento de c?lulas de tunelamento, usadas em mem?rias magn?ticas de acesso aleat?rio

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Made available in DSpace on 2015-05-08T14:59:43Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 3419531 bytes, checksum: 47a7d7cdb7277fb0c7f884e30a96df42 (MD5) Previous issue date: 2012-09-14 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work was evaluated the influence of multiple quenching in the peak temperatures of phase transformation (PTPT) in the alloy Cu-11.8Al-0.6Be-0.5Nb-0.27Ni (%wt), as well as the influence that deformation applied in temperatures above Ms, at nominal composition Cu-11.8Al-0.6Be-0.5Nb-0.27Ni, Cu-11.8Al-0.55Be-0.5Nb-0.27Ni and Cu-11.8Al-4Nb-2.16Ni-0.5Be, would have on the residual deformation. The alloys were melted, homogenized during 12h by 850ºC and machined using wire electroerosion. Then, the samples were quenched in water at room temperature and subsequently analyzed by optical microscopy, scanning electron microscopy, using energy dispersive x-ray, differential scanning calorimetric analysis and x-ray diffractometry. For samples quenched successively, it was found a large change in PTPT after 22 quenching, there is no reverse transformation in this range. From the 34th quenching, the PTPT remained constant around 420ºC and severe changes in your micrographs were detected. Nevertheless, there was no change in Cu/Al able to change the PTPT. Alloys with containing nominal composition 0.4% and 0.2%Be indicated strong influence of the Be in the PTPT. When analyzed by x-ray diffractometry, the sample with 0.2Be indicated the presence of β' and γ' phases, when aged by 530ºC. For quantifying the residual deformations, the samples were subjected to static tensile and loading/unloading tests. When subjected large deformation and temperature near Ms, the results showed a great residual deformation, whereas small deformations with temperatures above Af showed not to be viable. The alloy Cu-11.8Al-4Nb-2.16Ni-0.5Be when tractioned, showed excessive weakness even after treatment of solubilization. / Neste trabalho avaliou-se a influência de têmperas múltiplas, nas temperaturas de pico da transformação de fase (TPTF) e na microestrutura da liga Cu-11,8Al-0,6Be-0,5Nb-0,27Ni (% em peso), bem como a influência que deformações aplicadas, em temperaturas a partir de Ms, às ligas de composição nominal Cu-11,8Al-0,6Be-0,5Nb-0,27Ni, Cu-11,8Al-0,55Be-0,5Nb-0,27Ni e Cu-11,8Al-4Nb-2,16Ni-0,5Be, teriam nas deformações residuais. As ligas foram fundidas, homogeneizadas durante 12h a 850ºC e usinadas via eletroerosão à fio. Em seguida, os corpos de prova foram temperados em água a temperatura ambiente sendo posteriormente analisadas via microscopia óptica, microscopia eletrônica de varredura, utilizando-se energia dispersiva de raios-x (EDS), análise calorimétrica diferencial de varredura (DSC) e difratometria de raios-X. Para as amostras cicladas termicamente, verificou-se que após 22 têmperas uma mudança significativa nas TPTF ocorre, não havendo a partir deste intervalo transformação reversa. A partir da 34ª têmpera, as TPTF permaneceram constantes em torno de 420ºC e as micrografias indicaram mudanças severas em suas microestruturas. Não obstante, não se verificou alteração na relação Cu/Al capaz de alterar as TPTF. As amostras contendo composição nominal de 0,4% e 0,2% de Be indicaram que as ligas estudadas são fortemente influenciadas pela presença do Be. Quando analisada por difratometria de raios-x, a amostra com 0,2Be indicou a presença das fases β e γ , quando submetida a tratamento de envelhecimento a 530ºC. Para a quantificação das deformações residuais, os corpos de prova foram submetidos aos ensaios de tração estática e de carregamento/descarregamento. As amostras submetidas a deformações próximas as de ruptura e com temperatura de ensaio próximo a Ms mostraram resultar em deformações residuais de maiores intensidades, enquanto quedeformações de pequena magnitude, com temperaturas acima de Af, mostraram não serem viáveis. A liga de composição nominal Cu-11,8Al-4Nb-2,16Ni-0,5Be, quando ensaiada sob tração, mostrou fragilidade excessiva mesmo após tratamento térmico de solubilização.

Made available in DSpace on 2014-12-17T15:15:01Z (GMT). No. of bitstreams: 1 MariaGDS_TESE.pdf: 10253325 bytes, checksum: a11a4b9893c49f999607f55737b5aded (MD5) Previous issue date: 2014-07-28 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The effect of confinement on the magnetic structure of vortices of dipolar coupled ferromagnetic nanoelements is an issue of current interest, not only for academic reasons, but also for the potential impact in a number of promising applications. Most applications, such as nano-oscillators for wireless data transmission, benefit from the possibility of tailoring the vortex core magnetic pattern. We report a theoretical study of vortex nucleation in pairs of coaxial iron and Permalloy cylinders, with diameters ranging from 21nm to 150nm, and 12nm and 21nm thicknesses, separated by a non-magnetic layer. 12nm thick iron and Permalloy isolated (single) cylinders do not hold a vortex, and 21nm isolated cylinders hold a vortex. Our results indicate that one may tailor the magnetic structure of the vortices, and the relative chirality, by selecting the thickness of the non-magnetic spacer and the values of the cylinders diameters and thicknesses. Also, the dipolar interaction may induce vortex formation in pairs of 12nm thick nanocylinders and inhibit the formation of vortices in pairs of 21nm thick nanocylinders. These new phases are formed according to the value of the distance between the cylinderes. Furthermore, we show that the preparation route may control relative chirality and polarity of the vortex pair. For instance: by saturating a pair of Fe 81nm diameter, 21nm thickness cylinders, along the crystalline anisotropy direction, a pair of 36nm core diameter vortices, with same chirality and polarity is prepared. By saturating along the perpendicular direction, one prepares a 30nm diameter core vortex pair, with opposite chirality and opposite polarity. We also present a theoretical discussion of the impact of vortices on the thermal hysteresis of a pair of interface biased elliptical iron nanoelements, separated by an ultrathin nonmagnetic insulating layer. We have found that iron nanoelements exchange coupled to a noncompensated NiO substrate, display thermal hysteresis at room temperature, well below the iron Curie temperature. The thermal hysteresis consists in different sequences of magnetic states in the heating and cooling branches of a thermal loop, and originates in the thermal reduction of the interface field, and on the rearrangements of the magnetic structure at high temperatures, 5 produce by the strong dipolar coupling. The width of the thermal hysteresis varies from 500 K to 100 K for lateral dimensions of 125 nm x 65 nm and 145 nm x 65 nm. We focus on the thermal effects on two particular states: the antiparallel state, which has, at low temperatures, the interface biased nanoelement with the magnetization aligned with the interface field and the second nanoelement aligned opposite to the interface field; and in the parallel state, which has both nanoelements with the magnetization aligned with the interface field at low temperatures. We show that the dipolar interaction leads to enhanced thermal stability of the antiparallel state, and reduces the thermal stability of the parallel state. These states are the key phases in the application of pairs of ferromagnetic nanoelements, separated by a thin insulating layer, for tunneling magnetic memory cells. We have found that for a pair of 125nm x 65nm nanoelements, separated by 1.1nm, and low temperature interface field strength of 5.88kOe, the low temperature state (T = 100K) consists of a pair of nearly parallel buckle-states. This low temperature phase is kept with minor changes up to T= 249 K when the magnetization is reduced to 50% of the low temperature value due to nucleation of a vortex centered around the middle of the free surface nanoelement. By further increasing the temperature, there is another small change in the magnetization due to vortex motion. Apart from minor changes in the vortex position, the high temperature vortex state remains stable, in the cooling branch, down to low temperatures. We note that wide loop thermal hysteresis may pose limits on the design of tunneling magnetic memory cells / Os efeitos de confinamento e o forte acoplamento dipolar na estrutura de v?rtices de nano-elementos ferromagn?ticos ? um tema de interesse atual, n?o apenas pelo valor puramente acad?mico, mas tamb?m pelo impacto em grande n?mero de dispositivos da ?rea de spintr?nica. Muitos dispositivos, como nano-osciladores para transmiss?o de dados sem fio, podem tirar grande proveito da possibilidade de controlar o padr?o magn?tico do n?cleo do v?rtice magn?tico. Relatamos um estudo te?rico da nuclea??o de v?rtices em um par de cilindros coaxiais de ferro e de Permalloy, com di?metros desde 21nm at? 150nm e espessuras de 12nm e de 21nm, separados por uma fina camada n?o-magn?tica. Cilindros isolados de ferro e Permalloy com espessura de 12nm n?o permitem a forma??o de v?rtices, enquanto que cilindros de espessura de 21nm possuem v?rtices quando isolados em reman?ncia. Nossos resultados indicam que ? poss?vel controlar a estrutura magn?tica dos v?rtices, bem como a chiralidade e polaridade relativa dos dois v?rtices, pela escolha apropriada dos valores dos di?metros e da separa??o dos dois cilindros ferromagn?ticos. Dependendo do valor da separa??o entre os cilindros, a intera??o dipolar pode induzir a forma??o de v?rtices em pares de cilindros de espessura de 12nm e inibir a forma??o de v?rtices em pares de cilindros de 21nm de espessura. Al?m disso, mostramos que a rota de prepara??o do estado magn?tico em campo nulo, pode ser usada para determinar a chiralidade e polaridade relativa dos dois v?rtices. Por exemplo: partindo da satura??o da magnetiza??o de um par de cilindros de ferro com di?metro de 81nm e espessura de 21nm, na dire??o do eixo f?cil da anisotropia uniaxial do ferro, resulta um par de v?rtices com n?cleo de 36nm, mesma chiralidade e mesma polaridade. Partindo do estado saturado em uma dire??o no plano e perpendicular ao eixo de anisotropia uniaxial, resulta um par de v?rtices com n?cleo de 30nm de di?metro, com chiralidade e polaridade opostas. Relatamos tamb?m um estudo te?rico do impacto de v?rtices magn?ticos na histerese t?rmica de um par de nanoelementos el?pticos de ferro, de 10nm de espessura, separados por um espa?ador n?o-magn?tico e acoplados com um substrato antiferromagn?tico por energia de 3 troca. Nossos resultados indicam que h? histerese t?rmica em temperatura ambiente (muito menor do que a temperatura de Curie do ferro), se o substrato for uma superf?cie n?o compensada de NiO. A histerese t?rmica consiste na diferen?a da sequ?ncia de estados magn?ticos nos ramos de aquecimento e resfriamento de um ciclo t?rmico, e se origina na redu??o do valor do campo de interface em altas temperaturas, e na reestrutura??o das fases magn?ticas impostas pela intera??o dipolar forte entre os dois nanoelementos de ferro. A largura da histerese t?rmica varia entre 500K ? 100K para dimens?es laterais de 125nm x 65nm e 145nm x 65nm. Focamos nos ciclos t?rmicos de dois estados especiais: o estado antiparalelo, com o nanoelmento em contato com o substrato alinhado na dire??o do campo de interface e o outro nanoelemento alinhado em dire??o oposta; e o estado paralelo em que os dois nanoelementos est?o alinhados com o campo de interface em temperaturas baixas. Esses s?o os dois estados magn?ticos b?sicos de c?lulas de mem?rias magn?ticas de tunelamento. Mostramos que a intera??o dipolar confere estabilidade t?rmica ao estado antiparalelo e reduz a estabilidade t?rmica do estado paralelo. Al?m disso, nossos resultados indicam que um par de cilindros com dimens?es de 125nm x 65nm, separados por 1.1nm, com campo de interface de 5.88kOe em temperatura de 100K, est? no estado paralelo. Essa fase se mant?m at? 249K, quando h? uma redu??o de 50% da magnetiza??o devido ? nuclea??o de um v?rtice no nanoelemento com superf?cie livre. Pequenas varia??es da magnetiza??o, devidas ao movimento do v?rtice, s?o encontradas no ramo de aquecimento, at? 600K. O estado encontrado em 600K se mant?m ao longo do ramo de resfriamento, com pequenas mudan?as na posi??o do v?rtice. A exist?ncia de histerese t?rmica pode ser um s?rio limite de viabilidade de mem?rias magn?ticas de tunelamento

Les solutions solides (1-x)Na0,5Bi0,5TiO3 (NBT) – xCaTiO3 (CT) ont été étudiées par diffraction des rayons X, spectroscopie Raman, microscopie électronique à balayage, spectroscopie d’impédance et DSC. Ce sont des matériaux présentant la structure cristalline pérovskite. L’étude révèle la complexité mais aussi la richesse des phénomènes physiques dans cette famille de composés : les séquences des transitions de phases, l’influence du dopant Ca2+ sur les propriétés physico-chimiques du matériau, la relation étroite entre propriétés diélectriques et caractéristiques structurales. Des solutions solides (1-x)NBT – xCT, avec 0 ≤ x ≤ 1,00, ont été synthétisées par voie solide classique puis frittées selon une procédure spécifique dans un milieu confiné pour éviter toute perte de sodium et de bismuth. Les caractéristiques cristallines des solides obtenus imposent clairement de distinguer trois domaines suivant les valeurs de x. En effet, pour les valeurs croissantes de x et à la température ambiante, on observe un premier domaine (Région I, pour x ≤ 0,07) dans lequel le solide obtenu est une solution solide de structure cristalline, de groupe d’espace R3c, identique à celle de NBT pur. Pour les valeurs les plus élevées de x (Région II, pour x ≥ 0,15), le solide obtenu est une solution solide de structure cristalline, de groupe d’espace Pnma, identique à celle de CT pur. Enfin, entre ces deux domaines (Région III, 0,09 ≤ x ≤ 0,13), les solides obtenus sont biphasés, R3c + Pnma, en se limitant aux appellations des groupes d’espacé des phases formées. Dans la région I, lors du chauffage, la séquence des transitions de phases R3c → P4bm → Pm3m est mise en évidence; les températures des transitions se déplacent vers les plus basses températures quand la concentration en Ca2+ augmente. Les solides sont ferroélectriques à l’ambiante puis développent un caractère relaxeur, par coexistence de deux phases, avec l’augmentation de la température. Dans la région II, les solides révèlent un comportement relaxeur dès l’ambiante. Une transition de phase diffuse au sein de la phase orthorhombique Pnma est toutefois mise en évidence ; le solide passe d’un état relaxeur à un état paraélectrique tout en conservant, a priori, la même structure cristalline. Le phénomène de relaxation dans ces composés est expliqué par la formation de micros ou nanorégions polaires. La région III, quant à elle, est caractérisée par l’apparition d’une hystérésis thermique mise en évidence pour la première fois ; elle est expliquée par la relation entre la microstructure cristalline et les propriétés diélectriques. Enfin, l’ensemble de nos résultats a été regroupé dans un diagramme de phase original en composition et en température. / The solid solutions (1-x)Na0,5Bi0,5TiO3 (NBT) – xCaTiO3 (CT) were studied by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, impedance spectroscopy and DSC. These materials have a perovskite crystalline structure. This study reveals not only the complexity but also the richness of physical phenomena in these compounds: phases transitions sequences, the Ca2+ effect on the physical-chemistry properties and the relation between dielectric properties and crystalline structure. Thereby, (1-x)NBT – xCT solid solutions (0 ≤ x ≤ 1.00) were synthesized by chemical solid route, then they were sintered by a particular procedure in order to avoid sodium and bismuth volatilization. The solid crystalline characteristics obtained prove clearly the necessity to distinguish three fields as a function of x values. First of all, for increasing x at room temperature, there is a first region so called region I (x ≤ 0.07), wherein the crystalline structure of solid solutions obtained has a space group R3c identical to that of pure NBT. For the highest values of x, (Region II, x ≥ 0.15), the solid obtained has a space group Pnma, identical to that of pure CT. Finally, between these two regions, (0.09 ≤ x ≤ 0.13), the solid solutions obtained are biphasic, R3c + Pnma, limited to appellations of the space groups formed phases. In region I, upon heating, phase transition sequence R3c → R3c + Pnma → Pnma was determined; the corresponding transition temperatures move to low values with increasing Ca2+ concentration. These solids are ferroelectric at room temperature and then develop a relaxor character, by coexistence of two phases, with increasing temperature. In region II, these solids reveal a relaxor behavior at room temperature. However, a diffuse phase transition within the orthorhombic phase Pnma has been identified; the solid changes from relaxor to paraelectric while maintaining the same crystal structure. This phenomenon was explained by the formation of micro or nano-polar regions. Region III, demonstrated for the first time, is characterized by thermal hysteresis, and explained by the relation between crystalline microstructure and dielectric properties. Finally, all our results were assembled in an original phase diagram as a function of concentration of Ca2+ dopant and temperature.

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