Shape-memory effects

This chapter is an overview of the synthesis and properties of PVA/ nanotube composites. Various films and fibers have been processed from carbon nanotube and PVA dispersions. Compared to other polymers, PVA exhibit particularly strong interaction with single-walled as well as multiwalled carbon nanotubes. This leads to unique properties which are not observed in other nanotube polymer nanocomposites. In particular, this literature review confirms 

Beyond the usual mechanical and electrical performances, this review also points out the emergence of other original properties, like the remarkable capability of some nanotube/PVA composites to absorb mechanical energy and shape memory phenomena that differ from traditional behaviors of other polymers. These features are opening new investigation fields, in which several fundamental questions will have to be solved. But they also offer new opportunities for a variety of applications like smart or protective clothing, helmets, bullet proof vests, or active composites. 

Dresselhaus, and M.S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, 1998. 

Krenchel, Fibre reinforcement, Akademisk Forlag, Copenhagen, Denmark, 1964. 

Sakurada, International Fiber Science and Technologies Series 6,1985. 

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Cu-Zn system, shape-memory effect in, 22 343. See also Copper entries CVD-deposited films, properties of,... 

Shape-memory materials are those materials that return to a specific shape after being exposed to specific temperatures. In other words, these materials are able to remember their initial shape. This process of changing the shape of the material can be repeated several times. The shape-memory effect has been observed in different materials, such as metallic alloys, ceramics, glasses, polymers and gels. 

Shape-memory alloys (e.g. Cu-Zn-Al, Fe-Ni-Al, Ti-Ni alloys) are already in use in biomedical applications such as cardiovascular stents, guidewires and orthodontic wires. The shape-memory effect of these materials is based on a martensitic phase transformation. Shape memory alloys, such as nickel-titanium, are used to provide increased protection against sources of (extreme) heat. A shape-memory alloy possesses different properties below and above the temperature at which it is activated. Below this temperature, the shape of the alloy is easily deformed due to its flexible structure. At the activation temperature, the alloy can be changed by applying a force, but the structure resists this deformation and returns back to its initial shape. The activation temperature is a function of the ratio of nickel to titanium in the alloy. In contrast with Ni-Ti, copper-zinc alloys are capable of a two-way activation, and therefore a reversible variation of the shape is possible, which is a necessary condition for protection purposes in textiles used to resist changeable weather conditions. 

The discovery of the shape memory effect in TiNi by Buehler et al. at the Naval Ordinance Labs occurred during an investigation of the alloy for possible use as a corrosion-resistant knife for underwater activities. The investigators called the alloy nitinol for Nickel, Titanium, and Naval Ordinance Labs. 

J. D. Harrison and D. E. Hodgson. Shape Memory Effects in Alloys. New York Plenum Press, 1975. 

Ferromagnetic materials exhibiting shape memory effects and superelasticity under action of a magnetic field turn out to have a great potential for applications. This problem was discussed in a talk by A. Vasiliev (Shape memory alloys of the Ni-Mn-Ga type). 

Figure 10.5. The one-way shape memory effect illustrated pictorially.

Stdckel, D. The Shape Memory Effect Phenomenon, Alloys, and Applications, Nitinol Devices Components, Inc., Freemont, CA, 2000. 

The shape-memory effect is observed when the temperature of a piece of alloy is cooled to below that required to form the martensite phase Ms (initial martensite formation) until Mf (martensite formation complete), as seen in Figure 3.25. Upon heating the martensitic material, a reformation of austenite begins to occur at Hg... 

Figure 3.24. Photographs of the shape-memory effect at varying temperatures for a Ni-Ti wire. Reproduced with permission from the real-time video clip made by Rolf Gotthardt (rolf.gotthardt epfl.ch) -found onhne at http //www.msmcam.ac.uk/phasetrans/2002/memory.gif...

In Japan, several commercial projects have been reported in the literature. For example, at the National Research Institute for Metals, the NiTi shape-memory alloy is produced by combustion synthesis from elemental powder for use as wires, tubes, and sheets. The mechanical properties and the shape-memory effect of the wires are similar to those produced conventionally (Kaieda et ai, 1990b). Also, the production of metal-ceramic composite pipes from the centrifugal-thermite process has been reported (Odawara, 1990 see also Section III,C,1). 

Theory of Martensitic Microstructure and the Shape-Memory Effect by Kaushik Bhattacharya, unpublished (1998) (a huge pity ) - available from author bhatta caltech.edu. Bhattacharya s article gives a complete and thorough discussion of the many ideas that have been brought to bear on the problem of microstructure in martensites. 

Bhattacharya K., Theory of Martensitic Micro structure and the Shape-Memory Effect (unpublished) (1998) - available from author bhatta caltech.edu Bhattacharya K., Wedge-like Microstructure in Martensites, Acta Metall. Mater. 39, 2431 (1991). Binder K., Ordering of the Face-Centered-Cubic Lattice with Nearest-Neighbor Interaction, Phys. Rev. Lett., 45, 811 (1980). 

The high thermal conductivity of SiC may turn out to facilitate new process designs for highly exothermic or endothermic reactions in which heat transfer must be carefully controlled. Furthermore, because the SiC can be formed from carbon in any shape or porosity due to the shape memory effect , a catalyst with good activity and selectivity in a particular reaction can be formed closer to a commercially acceptable form without an extensive development project. For all these reasons it is expected that research will accelerate rapidly in order to understand, improve and develop this novel material. 

It is well known that the martensitic transformation of Al-deficient NiAl (see Sec. 4.3.2) is thermoelastic and produces the shape memory effect. Consequently materials developments have been started which aim at applications as shape memory alloys (Furukawa et al., 1988 Kainuma et al., 1992b, c). The martensitic transformation temperature can be varied within a broad temperature range up to 900 °C, and thus the shape memory efftct can be produced at high temperatures which allows the development of high-temperature shape memory alloys. The problem of low room temperature ductility of NiAl has been overcome by alloying with a third element - in particular Fe - to produce a ductile second phase with an f.c.c. structure. 

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