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Shape memory alloys (SMAs) are a unique class of smart materials exhibiting extraordinary properties with a wide range of applications in engineering, biomedical, and aerospace technologies. In this study, an advanced, efficient, low-cost, and highly scalable laser-assisted imprinting method with low environmental impact to create thermally controllable surface patterns is reported. Two different imprinting methods were carried out mainly on Ni50Ti50 (at. %) SMAs by using a nanosecond pulsed Nd:YAG laser operating at 1064 nm wavelength and 10 Hz frequency. First, laser pulses at selected fluences were directly focused on the NiTi surface, which generated pressure pulses of up to a few gigapascal (GPa), and thus created micro-indents. Second, a suitable transparent overlay serving as a confining medium, a sacrificial layer, and a mesh grid was placed on the NiTi sample, whereafter the laser was focused through the confinement medium, ablating the sacrificial layer to create plasma and pressure, and thus pushing and transferring the grid pattern onto the sample. Scanning electron microscope (SEM) and laser profiler images show that surface patterns with tailorable sizes and high fidelity could be obtained. The depth of the patterns was shown to increase and later level off with the increase in laser power and irradiation time. Upon heating, the depth profile of the imprinted SMA surfaces changed where the maximum depth recovery ratio of 30 % was observed. Recovery ratio decreased and saturated at about 15 % when the number of pulses were increased. A numerical simulation of the laser irradiation process was performed showing that considerably high pressure and temperature could be generated depending on the laser fluence. The stress wave closely followed the rise time of the laser pulse to its peak value and followed by the rapid attenuation and dispersion of the stress through the sample.

Shape memory polymers (SMPs) have been candidate materials for morphing applications. However, the SMPs have not been fully tested to work in relevant environments required for Air Force missions. In this study, an epoxy-based SMP was separately exposed to moisture, lubricating oil and UV radiation, which are simulated service environments designed to be reflective of anticipated performance requirements. The thermomechanical properties and shape memory effects were studied by using novel high-temperature nanoindentation technique. Results show that environmental conditions have affected the glass transition temperature and mechanical properties of the SMPs. In most cases, the conditioned SMPs exhibited higher elastic moduli than the unconditioned SMP. The shape recovery ability of the SMP was assessed by creating an indent and then observing the corresponding recovery according to the standard shape memory cycle. It was found that the deformation was mostly recovered for both conditioned and unconditioned SMP samples on heating the material above its glass transition temperature.

Made available in DSpace on 2015-05-08T15:00:01Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 1709663 bytes, checksum: 35ce26ec1cf65f8e0507ea2d1c7adfd0 (MD5) Previous issue date: 2009-10-13 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Cu-Al-Ni and Cu-Al-Be shape memory alloys were cast under ambient atmosphere, and characterized thought optical microscopy, X-ray diffraction, and differential scanning calorimetry. Superelasticity, and shape recovery were evaluated using mechanical tensile test. Thermomechanical properties of shape memory alloys used were obtained under different temperature range, in which was possible to conclude that Cu-Al-Ni alloy can be used as mechanical actuators for temperatures above 90ºC, as well as this alloy in particular should not be considered for application in temperatures below 30ºC. Differently of CuAlBe alloy that can be used in low temperatures, as well as in temperature superiors to 0ºC, as, for example, mechanical actuators. / Ligas Cu-Al-Ni e Cu-Al-Be forma elaboradas sob atmosfera ambiente e caracterizadas por microscopia, difração de R-X e Calorimetria Diferencial de varredura. Foram realizados nestas ligas ensaios de tração, recuperação de forma e superelasticidade. As propriedades termomecânicas destas ligas realizadas em diferentes temperaturas permitiu concluir que as ligas Cu-Al-Ni podem ser utilizadas como atuadores mecânicos para temperaturas acima de 90°C e que estas ligas não devem ser empregadas em temperaturas abaixo de 30°C. Podemos concluir também que as ligas Cu-Al-be podem ser utilizadas como atuadores mecânicos em temperaturas superiores a 0°C.

京都大学 / 新制・課程博士 / 博士(情報学) / 甲第23543号 / 情博第773号 / 新制||情||132(附属図書館) / 京都大学大学院情報学研究科知能情報学専攻 / (主査)准教授 延原 章平, 教授 西野 恒, 教授 西田 眞也, 教授 佐藤 いまり(国立情報学研究所) / 学位規則第4条第1項該当 / Doctor of Informatics / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(農学) / 甲第21277号 / 農博第2293号 / 新制||農||1062(附属図書館) / 学位論文||H30||N5141(農学部図書室) / 京都大学大学院農学研究科森林科学専攻 / (主査)教授 杉山 淳司, 教授 吉村 剛, 教授 渡邊 隆司, 教授 高妻 洋成 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DFAM

An intracranial aneurysm can be a serious, life-threatening condition which may go undetected until the aneurysm ruptures causing hemorrhaging within the brain. The typical treatment method for large aneurysms is by embolization using platinum coils. However, in about 15% of the cases treated by platinum coils, the aneurysm eventually re-opens. The solution to the problem of aneurysm recurrence may be to develop more bio-active materials, including certain polymers, to use as coil implants. In this research, a shape memory polymer (SMP) was investigated as a potential candidate for aneurysm coils. The benefit of a shape memory polymer is that a small diameter fiber can be fed through a micro-catheter and then change its shape into a three-dimensional configuration when heated to body temperature. The SMP was tested to determine its thermo-mechanical properties and the strength of the shape recovery force. In addition, composite specimens containing tantalum filler were produced and tested to determine the mechanical effect of adding this radio-opaque metal. Thermo-mechanical testing showed that the material exhibited a shape recovery force a few degrees above Tg. The effects of the metal filler were small and included depression of Tg and recovery force. SMP coils deployed inside a simulated aneurysm model demonstrated that typical hemodynamic forces would not hinder the shape recovery process. The x-ray absorption capability the tantalum-filled material was characterized using x-ray diffractometry and clinical fluoroscopy. Diffractometry revealed that x-ray absorption increased with tantalum concentration, however, not as the rule of mixtures would predict. Fluoroscopic imaging of the composite coils in a clinical setting verified the radio-opacity of the material.

NiTiCu is a shape memory alloy that regains its original shape after large amount of shape changing deformation when heated above a critical temperature called reverse martensitic trans-formation temperature( Af). When external load is applied on the sample in twinned martensite phase at low temperature, it deforms by detwinning, accommodating large amount of strains. When it is heated above Af, the shape recovers by transformation of the martensite to austenite phase. However, the amount of shape recovery degrades over time due to internal factors such as precipitates, residual strains and thermal history as well as external factors such as stresses. Severe localized stresses induced by contacts result in plastic deformation that affect the reverse martensitic transformation and hence the shape recovery. In this work, we study how varying levels of contact stresses induced in NiTiCu thin film affect its shape recovery. NiTiCu thin films of six different compositions are deposited on Si(100) wafer by co-sputtering from elemental targets. After deposition, the films are annealed at 500 C for 4 h to make them crystalline. The composition of the films varied linearly with applied power to the targets. Uniformity in composition over a 4 inch substrate area is achieved by substrate rotation. All the films show ne grain microstructure after annealing. The subsurface of the Ni-rich films is columnar. Ni-rich films have annealing cracks and the crack width increases with Ni composition in the films. The roughness of as-deposited films is found to be more for Ni-rich films compared to Ti-rich films. The roughness of the Ni-rich and Ti-rich films increased after annealing. From the X-ray diffraction studies, it was observed that the films are nanocrystalline. Indentation is carried out using a Berkovich diamond indenter with spherical apex, at nine different locations with loads ranging from 0.25 mN to 25 mN. A predefined array is chosen for indentation such that the larger indents act as a guide to precisely locate minute indents generated at lower loads, with residual depth as small as 10 nm, for imaging in high-resolution microscopes like Scanning Electron Microscope as well as in Atomic Force Microscope . In Ti60 (a Ti-rich) lm, the residual indents generated at loads greater than 10 mN show radial cracks originating at corners. Average crack length increases with the maximum load used for generating the indent. Sequential sectioning of Ti48 (a Ni-rich) lm using Focused Ion Beam microscope, revealed that the cracks originate at the lm-substrate interface and reach the surface. In Ti48 lm, residual indents do not show any indentation cracks. The indentation stresses are accommodated by breaking of the columnar structure and the voids between them. Delamination of the film from the substrate is observed on either sides of the indent in both the Ti60 and Ti48 films. The hardness of the films is high at low loads and decrease as the load increases. The deformation by indentation at lower loads is mainly due to detwinning as only the apex of the indenter, which is nearly spherical, is in contact with the sample and the resulting stresses are low. As the load increases, the deformation starts getting accommodated through dislocations along with detwinning as the stress beneath the indenter increases. Spherical cavity model extended to SMA shows that inner hemisphere near the tip contains dislocations where stresses are very high, surrounded by detwinned region with stresses that are relatively low. When the sample is heated above reverse martensitic transformation temperature to induce shape recovery in the indents, only the detwinned region recovers to the original shape. Recovery ratio, quantification of shape recovery, is calculated from the depth of the indents before and after heating. Recovery ratio in Ti60 films is found to be large at low loads and decreases with increase in load. The decrease in shape recovery in Ti60 is attributed to the increase in the amount of plastic deformation at the expense of detwinning. Three-dimensional mapping of the surfaces shows that the recovery ratio is high at the apex of the indent at the maximum depth and reduces towards the edges of the indent. There is no evident recovery in Ti48 films. The shape recovery of SMAs can be achieved by Joule heating. When electric current is passed through the material, it heats up by Joule heating because of the intrinsic resistivity. The resistivity and hence the resistance would get effected by the dislocation based plastic deformation induced by the contact. This might result in shape recovery through resistive heating. Towards understanding this, the effect of contact stresses on electrical contact resistance is studied. Experimental setup is designed, developed and calibrated for studying the variation of electrical contact resistance of the NiTiCu thin films as a function of load. Electrical contact resistance is found to decrease with increase in applied load. Contact stresses in sub-micron NiTiCu thin films are simulated by carrying out nanoindentation at different loads. The recovery ratio is high when the stresses induced by the contact is less, at lower loads. The shape recovery ratio is reduced when the induced contact stresses in-creases. There is no shape recovery at the sharp edges of the indentation where contact stresses are very high. Hence, by carefully designing the features to reduce the stress concentrations, the performance of the device can be improved.