Search results for "Shape"
showing 10 items of 585 documents
Shape memory NiTi thin films deposited at low temperature
1999
Abstract NiTi shape memory alloy (SMA) thin films have the potential to become high performance actuators for micro-electromechanical systems. Low temperature crystallized NiTi films would ensure a good compatibility with microelectronic processes and polymers. To avoid the drawbacks induced by annealing, we have tried to obtain low temperature crystallized RF sputtered NiTi films by optimising deposition parameters. We have found that NiTi films containing an excess of Ti (∼52%) were crystallized when deposited on Si(100) substrates heated up to only 473 K. NiTi/Si(n) Schottky diodes I–V characteristics showed a temperature dependence indicating structural transition in the NiTi electrode.…
Gradient-enhanced model and its micromorphic regularization for simulation of Lüders-like bands in shape memory alloys
2018
Abstract Shape memory alloys, notably NiTi, often exhibit softening pseudoelastic response that results in formation and propagation of Luders-like bands upon loading, for instance, in uniaxial tension. A common approach to modelling softening and strain localization is to resort to gradient-enhanced formulations that are capable of restoring well-posedness of the boundary-value problem. This approach is also followed in the present paper by introducing a gradient-enhancement into a simple one-dimensional model of pseudoelasticity. In order to facilitate computational treatment, a micromorphic-type regularization of the gradient-enhanced model is subsequently performed. The formulation empl…
Optical Forging of Graphene into Three-Dimensional Shapes
2017
Atomically thin materials, such as graphene, are the ultimate building blocks for nanoscale devices. But although their synthesis and handling today are routine, all efforts thus far have been restricted to flat natural geometries, since the means to control their three-dimensional (3D) morphology has remained elusive. Here we show that, just as a blacksmith uses a hammer to forge a metal sheet into 3D shapes, a pulsed laser beam can forge a graphene sheet into controlled 3D shapes in the nanoscale. The forging mechanism is based on laser-induced local expansion of graphene, as confirmed by computer simulations using thin sheet elasticity theory. peerReviewed
Fibrin-chitosan composite substrate for in vitro culture of chondrocytes
2013
The aim of this study was to develop a biocompatible monolayer substrate based on fibrin and chitosan for in vitro culture of chondrocytes. Fibrin-chitosan composite substrates combined the proved cell adhesion properties of fibrin with the hydrophilicity and poor adhesion capacity of chitosan. Chitosan microspheres were produced by coacervation method, agglomerated within a fibrin network and subsequently crosslinked with genipin. The composite substrate was stable for 28 days of culture due to the high crosslinking density. Human chondrocytes cultured on the composite substrate were viable during the culture period. At the end of culture time (28 days) the composite substrate showed low c…
Friction Stir Processing of NiTi Shape Memory Alloy: Microstructural Characterization
2010
Shape Memory Alloys (SMA) are metal materials that, after being strained, come back to their original shape at a designated temperature. Welding NiTi alloys is not simple because when the material is melted, due to the high reactivity of the alloy elements, the typical shape memory properties may disappear. The solid state welding process, such as friction stir welding, is thus attractive for SMA joining and it exhibits potentials for achieving welded joints affected by microstructural changes that preserve the shape memory properties. The present study investigates the feasibility of friction stir welding process to join NiTi shape memory alloys; in order to analyse the welding process, it…
Epitaxial films of the magnetic shape memory material
2007
Abstract By DC-sputtering from a stoichiometric target onto sapphire substrates we prepared epitaxial films of Ni 2 MnGa . X-ray diffraction in two circle geometry shows only the Ni 2 MnGa peaks corresponding to a (1 1 0) oriented growth on the (1 1 0) oriented sapphire substrate. Rocking curve scans shows that the out of plane orientation has a width of only 1 . 1 ∘ . By four circle X-ray diffraction we find an in-plane aligned growth. The Curie temperature as determined by SQUID magnetometry is 368 K. On cooling down the low field magnetization drops at around 260–280 K indicating the austenite to martensite transition. The presence of two different phases at high and low temperature also…
Real time polarization imaging of weld pool surface
2015
The search for an efficient on-line monitoring system focused on the real-time analysis of arc welding quality is an active area of research. The topography and the superficial temperature field of the weld pool can provide important information which can be used to regulate the welding parameters for depositing consistent welds. One difficulty relies on accessing this information despite the bright dazzling welding arc. In the present work, Stokes polarimetry and associated shape-from-polarization methods are applied for the analysis of the weld pool through its 810 nm-wavelength infrared emissions. The obtained information can provide a better understanding of the process, such as the usa…
Structural and Magnetic Properties of Epitaxial Ni<sub>2</sub>MnGa Thin Films
2009
We report on the preparation and investigation of epitaxial thin films of the magnetic shape memory alloy Ni2MnGa. For samples close to the stoichiometric composition we find that the phase transformation temperature is affected by the crystallographic orientation. Changes in the crystal structure due to the transformation are observed using temperature-dependent X-ray diffraction. Films with higher manganese content are in the martensitic state at room temperature. Those samples on Al2O3(11-20) reveal the 7-layered orthorhombic structure that allows strains up to 10 %. To avoid blocking of magnetostrictive effects by the substrate, free-standing films are prepared using water-soluble NaCl(…
Threshold Voltage Variability of NROM Memories After Exposure to Ionizing Radiation
2012
Threshold voltage (V-th) behavior of nitride readonly memories (NROMs) was studied after irradiation with photons (gamma-and X-rays), light and heavy ions. Both programmed and nonprogrammed single cells were investigated. The data suggest that two main physical phenomena are contributing to V-th variation and that the V-th loss and the variability can be modeled by a Weibull statistics with a shape parameter k similar to 2.2 regardless of the irradiation species and total dose. The same peculiarities were found in large memory arrays, confirming the results from single-cell studies but with significantly larger statistics. Hence, once the irradiation dose is known, the V-th loss distributio…
Microstructure of freestanding single-crystalline Ni2MnGa thin films
2011
Abstract The complex crystal structure and variant distribution of single-crystalline freestanding Ni–Mn–Ga films was studied in detail using X-ray diffraction in two- and four-circle geometry. Selective chemical etching of a chromium buffer layer facilitates the release of the rigid MgO substrate that would inhibit magnetically induced reorientation (MIR) of variants. The substrate-constrained as well as the freestanding films possess identical crystal structure featuring a seven-layered (pseudo-)orthorhombic modulation (7 M/14 M). Of the 12 different variants observed before and after releasing the film from the substrate, four are predominantly represented. These have the short c -axis a…