Browsing by Subject "SMA"
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Item Analysis of oscillating flow cooled SMA actuator(Texas A&M University, 2005-11-01) Pachalla Seshadri, RajagopalShape Memory Alloys (SMA) are a group of metallic alloys that have the ability to return to some previously defined shape or size when subjected to an appropriate thermal cycling procedure. In recent years there has been a lot of research on the development of small, light and, yet, powerful actuators for use in areas like robotics, prosthetics, biomimetics, shape control and grippers. Many of the miniaturized conventional actuators do not have sufficient power output to be useful and SMAs can be used advantageously here. The widespread use of SMAs in actuators is limited by their low bandwidth. Use of SMAs in two-way actuators requires that they undergo thermal cycling (heating and cooling). While SMAs can be heated quickly by resistive heating, conventional convection cooling mechanisms are much slower as the exothermic austenitic to martensitic phase transformation is accompanied by the release of significant amount of latent heat. While a number of cooling mechanisms have been studied in SMA actuator literature, most of the cooling mechanisms involve unidirectional forced convection. This may not be the most effective method. Oscillating flow in a channel can sometimes enhance heat transfer over a unidirectional flow. One possible explanation for this heat transfer enhancement is that the oscillatory flow creates a very thin Stokes viscous boundary-layer and hence a large time-dependent transverse temperature gradient at the heated wall. Therefore heat transfer takes place at a large temperature difference, thereby enhancing the heat transfer. In this work, the heat transfer from an SMA actuator under an oscillating channel is investigated and is compared to steady, unidirectional flow heat transfer. Oscillating flow is simulated using a finite volume based method. The resulting velocity field is made use of in solving the heat transfer problem using a finite difference scheme. A parametric study is undertaken to identify the optimal flow conditions required to produce the maximum output for a given geometry of the SMA actuator. The latent heat of transformation of the SMA is accounted for by means of a temperature dependent specific heat.Item Motor Neuron-Specific Restoration of SMN in Two SMA Mouse Models: Insights into the Role of Motor Neurons in Spinal Muscular Atrophy(2014-06-02) Paez, XimenaSpinal muscular atrophy (SMA) results from ?-motor neuron loss in the spinal cord due to low levels of the survival of motor neuron (SMN) protein, required for proper spliceosome assembly. The reduced levels of SMN cause muscle atrophy and ultimately death in the most severe cases. Although mouse models of SMA recapitulate many features of the human disease, it is still unclear whether their phenotypes are primarily due to motor neuron deficits. If so, motor neuron-selective restoration of normal SMN levels should have a great positive impact on SMA mice. To test this, we first exogenously raised normal human SMN in severe SMA mice that die perinatally, by driving its expression selectively in motor neurons with an Hb9 promoter. We found no extension of survival. We detected motor neuronal-SMN protein expression in Hb9-SMN transgenic mice from mid embryogenesis to postnatal day 6. However, mRNA for transgenic SMN was undetected by late embryogenesis. These results suggest that expression of Hb9-SMN declines before SMN levels are most needed perinatally. Second, we increased endogenous motor neuronal-SMN expression following embryonic Hb9-dependent Cre recombination of a conditional hybrid mutant allele (Smn^(res)) in another severe SMA mouse model (SMA?7-like). Cre recombination irreversibly transforms the Smn^(res) allele to WT. We confirmed that recombination of Smn^(res) occurred exclusively in the spinal cord. Yet, unlike a previous study that used choline acetyltransferase (ChAT) as a driver on the same mice, we found no improvement in survival, weight, motor behavior or presynaptic neurofilament accumulation. However, like in ChAT^(Cre+) SMA mice, we detected rescue of endplate size and mitigation of neuromuscular junction (NMJ) denervation status. Real time-PCR showed that the expression of spinal cord SMN transcript was sharply reduced in Hb9^(Cre+) SMA mice relative to ChAT^(Cre+) SMA mice. This suggests that our lack of overall phenotypic improvement was most likely due to an unexpectedly poor recombination efficiency driven by Hb9^(Cre). Nonetheless, the low levels of SMN were sufficient to rescue two NMJ structural parameters indicating that these motor neuron cell-autonomous phenotypes are very sensitive to changes in motor neuronal-SMN levels.Item Shape memory response of ni2mnga and nimncoin magnetic shape memory alloys under compression(2009-05-15) Brewer, Andrew LeeIn this study, the shape memory response of Ni2MnGa and NiMnCoIn magnetic shape memory alloys was observed under compressive stresses. Ni2MnGa is a magnetic shape memory alloy (MSMA) that has been shown to exhibit fully reversible, stressassisted magnetic field induced phase transformation (MFIPT) in the I X-phase transformation because of a large magnetostress of 7 MPa and small stress hysteresis. The X-phase is a recently discovered phase that is mechanically induced, however, the crystal structure is unknown. To better understand the transformation behavior of Ni2MnGa single crystal with [100] orientation, thermal cycling and pseudoelasticity tests were conducted with the goal of determining the Clausius-Clapeyron relationships for the various phase transformations. This information was then used to construct a stresstemperature phase diagram that illustrates the stress and temperature ranges where MFIPT is possible, as well as where the X-phase may be found. NiMnCoIn is a recently discovered meta-magnetic shape memory alloy (MMSMA) that exhibits unique magnetic properties. The ferromagnetic parent phase and the paramagnetic martensite phase allow the exploitation of the Zeeman energy. To gain a better understanding of the transformation behavior of NiMnCoIn, thermal cycling and pseudoelasticity tests were conducted on single crystals from two different batches with crystallographic orientations along the [100](011), [087], and [25 7 15] directions. A stress-temperature phase diagram was created that illustrates the Clausius- Clapeyron relationships for each orientation and batch. SQUID tests revealed the magnetic response of the alloy as well as the suppression of the martensite start temperature with increasing magnetic field. Pseudoelasticity experiments with and without magnetic field were conducted to experimentally quantify the magnetostress as a function of magnetic field. For the first time, it has been shown that NiMnCoIn is capable of exhibiting magnetostress levels of 18-36 MPa depending upon orientation, as well as nearly 6.5% transformation strain in the [100] direction. The results of this study reveal increased actuation stress levels in NiMnCoIn, which is the main limitation in most MSMAs. With this increased blocking stress, NiMnCoIn is a strong candidate for MFIPT.