Influence of the Latent Heat of Transformation and Thermomechanical Coupling on the Performance of Shape Memory Alloy Actuators

In this work, the influence of the latent heat of transformation and heat transfer on the performance of shape memory alloy (SMA) actuators is numerically explored. A 1D analytical model is first considered and used to perform parametric studies on the effects of geometry and heat transfer conditions on SMA wire responses. In order to consider the response of SMA structures, a recent SMA constitutive model is expanded to include the effects of the latent heat of transformation. The enhanced model is then implemented in a 3D finite element framework to solve the coupled and transient thermomechanical problems. The resultant model is used to explore the isothermal and adiabatic assumptions commonly used for quasi-static SMA modeling by considering the response of SMA structures. The responses of an axial SMA actuator heated from one end is evaluated and it is shown that the generation of latent heat during forward transformation and its absorption during reverse transformation decreases the actuation response when compared to a case neglecting thermal effects. It is concluded that the latent heat of transformation must be considered for the design of SMA components unless their operation can reasonably be approximated as isothermal.

Structural, Thermal and Magnetic Properties of Ga Excess Ni-Mn-Ga

The martensitic transition and the ferro- to paramagnetic transition have been studied in a series of Ga excess Ni-Mn-Ga specimens [Ni2-xMnGa1+x (0.4≤ x≤ 0.9)] by differential scanning calorimetry and magnetization measurements. The martensitic transition exhibits a hysteresis whose width is similar to Ni2MnGa, indicating that the transition is thermoelastic. The latent heat of transformation is comparable with other Ni-Mn-Ga alloys. A substantial increase in the martensitic transition temperature is observed due to Ga doping. Interestingly, the x-ray diffraction pattern of all the compositions studied show a modulated martensitic structure in the martensitic phase.

Phase Transformation of Hexacelsians Doped with Li, Na and Ca

A series of hexacelsians with different compositions were synthesized by thermally induced transformation of a LTA zeolite. Various forms of hexacelsians have been characterized by XRPD, IR, Raman and 29Si MAS NMR method. The phase transformation ab hexacelsian was investigated by DSC method. This phase transformation is sensitive to synthesis conditions, doping and thermal treatment. The peak maximum temperature, Tm, varies from 302 up to 353 oC. Heat of transformation changes from 0.42 to 1.77 kJ/mol.

“Burst-like” Characteristics of the δ/α′ Phase Transformation in Pu-Ga Alloys

ABSTRACTThe δ to α′ phase transformation in Pu-Ga alloys is intriguing for both scientific and technological reasons. On cooling, the ductile fcc δ-phase transforms martensitically to the brittle monoclinic α′-phase at approximately −120°C (depending on composition). This exothermic transformation involves a 20% volume contraction and a significant increase in resistivity. The reversion of α′ to δ involves a large temperature hysteresis and begins just above room temperature. In an attempt to better understand the underlying thermodynamics and kinetics responsible for these unusual features, we are examining the δ/α' transformations in a Pu-0.6 wt% Ga alloy using differential scanning calorimetry (DSC) and resistometry. Both techniques indicate that the martensite start temperature is −120°C and the austenite start temperature is 35°C. The heat of transformation is approximately 3 kJ/mole. During the α ′ → δ reversion, “spikes” and “steps” are observed in DSC and resistometry scans, respectively. These spikes and steps are periodic, and their periodicity with respect to temperature does not vary with heating rate. With an appropriate annealing cycle, including a “rest” at room temperature, these spikes and steps can be reproduced through many thermal cycles of a single sample.

A Laser Triggered Self-Sustaining Metals-Compound Semiconductor Transition for AlSb

ABSTRACTThe preparation of AlSb thin films by pulsed laser annealing of Al/Sb sandwiches is studied in order to resolve some past controversy about the temperature rise induced by the laser pulse. Using 1000 Ȧ thick two layer films supported by TEM grids, we investigate the energy threshold for complete transformation as a function of pulse duration from 15 nsec to 100 msec, and of ambient temperature from −100°C to 250°C.We thence calculate the temperature effect directly induced by the laser to be about 930°C, or approximately the melting point of the metals, whereas inert gas furnace anneals of comparable films show transformation at this temperature occuring only in about 100 sec. We discuss the isoenergetic nature of the system for short laser pulses and the role of the heat of transformation, and thus conclude that the reaction is thermally triggered by the laser pulse but is to some extent self-sustaining via the heat of transformation locally distributed. This model is also shown to have equal validity for the systems CdTe, CdSe and AlAs.