A review is presented of the current research and development of shape-memory materials, including shape-memory alloys, shape-memory ceramics and shape-memory polymers. The shape-memory materials exhibit some novel performances, such as sensoring (thermal, stress or field), large-stroke actuation, high damping, adaptive responses, shape memory and superelasticity capability, which can be utilized in various engineering approaches to smart systems. Based on an extensive literature survey, the various shape-memory materials are outlined, with special attention to the recently developed or emerged materials. The basic phenomena in the materials, that is, the stimulus-induced phase transformations which result in the unique performance and govern the remarkable changes in properties of the materials, are systematically lineated. The remaining technical barriers, and the challenges to improve the present materials system and develop a new shape memory materials are discussed.

Shape-memory materials and hybrid composites for smart systems: Part I Shape-memory materials

Thin film shape memory alloys (SMAs) have the potential to become a primary actuating mechanism for mechanical devices with dimensions in the micron-to-millimeter range requiring large forces over long displacements. The work output per volume of thin film SMA microactuators exceeds that of other microactuation mechanisms such as electrostatic, magnetic, thermal bimorph, piezoelectric, and thermopneumatic, and it is possible to achieve cycling frequencies on the order of 100 Hz due to the rapid heat transfer rates associated with thin film devices. In this paper, a quantitative comparison of several microactuation schemes is made, techniques for depositing and characterizing Ni-Ti-based shape memory films are evaluated, and micromachining and design issues for SMA microactuators are discussed. The substrate curvature method is used to investigate the thermo-mechanical properties of Ni-Ti-Cu SMA films, revealing recoverable stresses up to 510 MPa, transformation temperatures above 32/spl deg/C, and hysteresis widths between 5 and 13/spl deg/C. Fatigue data shows that for small strains, applied loads up to 350 MPa can be sustained for thousands of cycles. Two micromachined shape memory-actuated devices-a microgripper and microvalve-also are presented.

Thin film shape memory alloy microactuators

http://www.aem.umn.edu/~james/research/Publications_Files/PDF/67.pdf

Martensitic transformations and shape-memory materials ☆

Theoretical models for small ferroelectric particles predict a progressive decrease of the Curie temperature, spontaneous lattice strain, and polarization until the critical size corresponding to t ...

High dielectric constant and frozen macroscopic polarization in dense nanocrystalline BaTiO3 ceramics

Dependence of the sheet resistance of indium-tin-oxide thin films on grain size and grain orientation determined from X-ray diffraction techniques

We studied the size dependence of the ferroelectric domain structure in unique free-standing ${\mathrm{PbTiO}}_{3}$ thin film, composed of grains 60--1000 nm in size, with transmission-electron microscopy. With such samples, we showed that the apparent dependence of electrical properties on the thickness of polycrystalline thin films stems from their grain-size dependence. We found that a domain-structure transition, from predominantly multidomain to predominantly single domain occurs at a grain size of \ensuremath{\sim}150 nm (corresponding to a thickness 200--300 nm). Based on these experimental results, the drastic change of coercive field and dielectric permittivity below a thickness 200--300 nm was reasonably explained as resulting from a domain-structure transition. \textcopyright{} 1996 The American Physical Society.

Size-related ferroelectric-domain-structure transition in a polycrystalline PbTiO 3 thin film

The internal friction in potassium dihydrogen phosphate (KDP) crystal and the dielectric loss in KDP and triglycine sulfate (TGS) crystals are measured near T c in the kHz frequency range. An internal-friction peak (P M2 ) and a dielectric loss peak (P D2 ) appear at about several degrees below T c , which are found to be associated with the viscous movement of domain walls

Internal friction and dielectric loss related to domain walls.

The temperature dependence of the dielectric constant and dissipation in potassium dihydrogen phosphate (KDP), its deuterated compound (DKDP), triglycine sulfate (TGS), and TGS doped with \ensuremath{\alpha}-alanine (LATGS) has been studied at various frequencies. It is found that the relaxation time of domain freezing in KDP and DKDP in the kHz range can be described by the Vogel-Fulcher relation. Evidence of domain freezing in TGS is presented through an analysis of relaxation time related to domain walls and a comparison between TGS and LATGS. Studies of internal friction and compliance show preliminary evidence of domain freezing in CuAlZnNi alloy. A domain-freezing model is proposed based upon the collective pinning of randomly distributed pinning centers to domain walls. Some key experiments related to domain freezing, such as (1) the Vogel-Fulcher relation for relaxation time; (2) the size effect of domain freezing; (3) two kinds of relaxation in low- and high-frequency ranges, respectively; and (4) the dependence of ${\mathrm{T}}_{\mathrm{F}}$ on defect density and applied field, etc., are explained.

https://hub.hku.hk/bitstream/10722/43203/1/25258.pdf

Domain freezing in potassium dihydrogen phosphate, triglycine sulfate, and CuAlZnNi

The ferroelectric domain structure and its evolution with external stress in free-standing polycrystalline ${\mathrm{PbTiO}}_{3}$ very thin films (VTF's) (thickness t200 nm) and thicker films (tg200 nm), have been studied by in situ transmission electron microscopy. It is found that the domain structure of a VTF and its dynamic response to strain are remarkably different from those of a thicker film. The VTF is composed of nanosized grains, mostly single domained. These single-domained grains manifested a strong resistance against domain formation even under high stress. On the contrary, the thicker film is composed of larger grains, mostly multidomained. The domain structure in the thicker film changes significantly with external stress. The high stability of single domains in VTF's gives a satisfactory explanation to the abnormal electrical properties of VTF's compared with those of thicker films.

Huimin Shen

Papers

Size-related ferroelectric-domain-structure transition in a polycrystalline PbTiO 3 thin film

Internal friction and dielectric loss related to domain walls.

Domain freezing in potassium dihydrogen phosphate, triglycine sulfate, and CuAlZnNi

In situ study of the evolution of domain structure in free-standing polycrystalline PbTiO 3 thin films under external stress

Thermal cycling stability and two-way shape memory effect of Ni–Cu–Ti–Hf alloys