Shape memory ceramics

Shape memory ceramics (SMCs) have also been studied. The principal drawback of the SMC materials is their small recovery strain, much smaller than those of metal alloys, due to the intrinsic fragile behaviour and the microfractures that ceramics tend to produce in their structure. However, it is possible to classify these materials in terms of their shape memory mechanisms (Wei et al, 1998) ... 

Schurch, K.E. and K.H.G. Ashbee (1977), Near perfect shape-memory ceramic material. Nature, 266(5604) pp. 706-707. 

In addition, the copper industry s market development activities have resulted in appHcations such as clad ship hulls, sheathing for offshore platforms, automotive electrical systems including electric vehicles, improved automobde radiators, solar energy, fire sprinkler systems, parts for fusion reactors, semiconductor lead frames, shape memory alloys, and superconducting ceramics (qv) containing copper oxides. 

Such transformations have been extensively studied in quenched steels, but they can also be found in nonferrous alloys, ceramics, minerals, and polymers. They have been studied mainly for technical reasons, since the transformed material often has useful mechanical properties (hard, stiff, high damping (internal friction), shape memory). Martensitic transformations can occur at rather low temperature ( 100 K) where diffusional jumps of atoms are definitely frozen, but also at much higher temperature. Since they occur without transport of matter, they are not of central interest to solid state kinetics. However, in view of the crystallographic as well as the elastic and even plastic implications, diffusionless transformations may inform us about the principles involved in the structural part of heterogeneous solid state reactions, and for this reason we will discuss them. 

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. 

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). 

There are a number of displacive transitions mentioned in this book. The order-disorder transformation of hydrogen atoms in hydrogen bonds in ferroelectric ceramics (Section 11.3.5) is one example. Displacive transitions that involve a change from an ordered arrangement of atoms to a random arrangement are commonly found in alloys. A subgroup of such order-disorder transitions, martensitic transitions, which can be used to produce shape-memory alloys, are considered in Sections 8.3.2 and 8.3.3. 

The electromechanical devices that convert electric energy into mechanical energy are generally made from ferroelectric ceramics, shape memory alloys, and EAPs, typically... 

In fact, not only alloys have shape memory properties, some polymers, ceramics, and even biological systems also possess such properties. For example, bacteriophages can use the shape... 

As mentioned in Chapter 8, some vegetable oil-based polymers exhibit shape memory properties. The discovery of smart materials is one of the revolutionary steps in the held of active materials research. Among different types of smart materials (metals, alloys and ceramics), shape memory polymers (SMPs) have attracted considerable research interest in last few years because of their advantages over other categories of materials, as discussed in Chapter 1. Shape memory polymers which are deformed and... 

A reduction of the required energy could be reached by the incorporation of conductive fillers such as heat conductive ceramics, carbon black and carbon nanotubes [103-105] as these materials allowed a better heat distribution between the heat source and the shape-memory devices. At the same time the incorporation of particles influenced the mechanical properties increased stiffness and recoverable strain levels could be reached by the incorporation of microscale particles [106, 107], while the usage of nanoscale particles enhanced stiffness and recoverable strain levels even more [108, 109]. When nanoscale particles are used to improve the photothermal effect and to enhance the mechanical properties, the molecular structure of the particles has to be considered. An inconsistent behavior in mechanical properties was observed by the reinforcement of polyesterurethanes with carbon nanotubes or carbon black or silicon carbide of similar size [3, 110]. While carbon black reinforced materials showed limited Ri around 25-30%, in carbon-nanotube reinforced polymers shape-recovery stresses increased and R s of almost 100% could be determined [110]. A synergism between the anisotropic carbon nanotubes and the crystallizing polyurethane switching segments was proposed as a possible... 

With regard to applications, these include mixing, aero- and hydrodynamics, heat dissipation and chemical reactions. One could use the features to, for example, change the location of transition from laminar to turbulent flow, or change the drag characteristics of a surface. Curved protrusions could be used to create swirl, and the holes could function as micro-injectors - even for chemicals - or to increase cooling. Surfl-sculpt could also function as a mechanical interlock. Shape memory alloys could also be improved in their functionality. Many materials could be processed in this manner metals, polymers, ceramics and glass are all feasible. The time to process 5 cm of material is a few seconds, and the equipment needed includes an electron beam machine and a vacuum chamber. 

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