One of the qualities that distinguishes living systems from inanimate matter is the ability to adapt to changes in the environment. Smart materials have the ability to perform both sensing and actuating functions and are, therefore, capable of imitating this rudimentary aspect of life. Poled piezoelectric ceramics, for instance, are capable of acting as both sensor and actuator. External forces are detected through the direct piezoelectric effect, and a response is elicited through the converse piezoelectric effect, in which a voltage of suitable phase, frequency, and amplitude is applied to the same ceramic.

In this special issue, emphasis is placed on actuators, with articles on piezoelectric, electrostrictive, magnetostrictive, and shape memory materials. This is not to say that sensor materials are any less important; it is simply a matter of space. Optical fiber sensors, chemical sensors, thermistors, micromachined semiconductors, and other smart materials deserve special issues of their own.

Smart materials can be conveniently subdivided into passively smart materials that respond to external change without assistance, and actively smart materials that utilize a feedback loop enabling them to both recognize the change and initiate an appropriate response through an actuator circuit.

Zinc oxide varistors are passively smart materials capable of self-protection against high voltage breakdown. When struck by lightning, the ceramic varistor loses most of its electrical resistance, and the current is bypassed to ground. The resistance change is reversible, and acts as a standby protection phenomenon.

The field of electronic ceramics includes components as diverse as Mn-Zn ferrites, lead zirconate titanate transducers, ZnO varistors, and Al2O3 or cordierite-based packages for integrated circuits. An overview of electroceramics is given in Figure 1, which depicts schematically the types of electronic and ionic phenomena of interest. Many of the materials pictured utilize some bulk property of the ceramic, such as the presence of ferroelectric of ferrimagnetic dipoles, electronic or ionic conductivity, or a phase transition to provide a useful function. In other materials, extraordinary responses can be engineered by suitably tailoring the thickness and properties of a grain boundary phase. Varistors, barrier layer capacitors, and PTC thermistors, for example, all rely on the preparation of semiconducting grains and insulating grain boundaries to create properties that cannot be found in single crystals. In a similar way, some of the porous ceramics utilized in humidity sensors rely on surface properties rather than on a bulk response to detect changes in the ambient humidity (Kulwicki, 1992). Finally, in materials for insulators or substrates, all of the otherwise interesting phenomena must be eliminated. With the exception of the last category, all of the phenomena depicted in Figure 1 can be utilized in ceramic sensors. While sensing and actuating materials are currently a fraction of the overall multibillion-dollar electroceramics market, the percentage is expected to grow as more and more devices are made “smart.”

As discussed in the introduction to this issue, smart materials are capable of sensing a change in the environment and responding in a useful way.

Smart materials combine sensors, intelligence, and actuators to allow a material to respond to its environment. Magnetostrictive materials can be used as both the sensors and actuators in such materials. High-power magnetostrictive actuators can deliver forces greater than 50 MPa with strains of up to 0.6%, while other magnetostrictive sensor materials can provide hundreds of times the sensitivity of semiconductor strain gages. Magnetoelastic materials also have adaptable elastic moduli which may be varied by external magnetic fields.

Magnetostriction is the change in any dimension of a magnetic material caused by a change in its magnetic state. In this article we concentrate on ferromagnetic materials exhibiting Joule magnetostriction, which is a change in linear dimension parallel to an applied magnetic field (see Figure 1), and the reciprocal effect in which the material changes its magnetic state under the influence of applied stress.

The phenomenon of magnetostriction has been known for well over a century, since Joule discovered in 1847 the change in length of an iron rod when magnetized. The modern era of magnetostrictive materials began in 1963 with the measurement of nearly 1% magnetostrictive strains at low temperatures in the basal planes of Dy and Tb. A search for magnetostrictive materials with high magnetostriction at room temperature led to the alloying of rare earths with transition metals, culminating in the discovery in 1971 of giant room-temperature magnetostriction in the Laves phase compound TbFe2.

Piezoelectric and electrostrictive actuators, capable of moving something electromechanically, are forming a new field between electronic and structural ceramics. Application fields are classified into three categories: positioners, motors, and vibration suppressors. The manufacturing precision of optical instruments such as lasers and cameras, and the positioning accuracy for fabricating semiconductor chips, which must be adjusted using solidstate actuators, is of the order of 0.1 μm. Regarding conventional electromagnetic motors, tiny motors smaller than 1 cm3 are often required in office or factory automation equipment and are rather difficult to produce with sufficient energy efficiency. Ultrasonic motors whose efficiency is insensitive to size are superior in the minimotor area. Vibration suppression in space structures and military vehicles using piezoelectric actuators is also a promising technology.

New solid-state displacement transducers controlled by temperature (shape memory alloy) or magnetic field (amorphous magnetostrictive alloy) have been proposed, but are generally inferior to the piezoelectric/electrostrictive actuators because of technological trends aimed at reduced driving power and miniaturization.

Numerous metallic alloys are now known to exhibit a shape memory effect through which an article deformed at a lower temperature will regain its original undeformed shape when heated to a higher temperature. This behavior is basically a consequence of a martensitic phase transformation. When compared, the various shape memory materials are found to have common characteristics such as atomic ordering, a thermoelastic martensitic transformation that is crystallographically reversible, and a martensite phase that forms in a self-accommodating manner. The explanation of the shape memory phenomenon is now universal and well in hand. In addition to the familiar “one-way” memory, shape memory alloys also exhibit a “two-way” memory as well and a “mechanical” shape memory resulting from the formation and reversal of stressinduced martensite.

Fundamental to the shape memory effect (SME) is the occurrence of a martensitic phase transformation and its subsequent reversal Basically, a shape memory alloy (SMA) is deformed in the martensitic condition (martensite), and the shape recovery occurs during heating when the specimen undergoes a reverse transformation of the martensite to its parent phase. This is the essence of the shape memory effect. Materials that exhibit shape memory behavior also show a two-way shape memory, as well as a phenomenon called superelasticity. These are also discussed.

The shape memory response after deformation and thermal stimulation constitutes “smart” behavior, i.e., Stimulated Martensite-Austenite Reverse Transformation.