Sensors, Actuators, and Smart Materials
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ensors, Actuators, and Smart Materials
S. Trolier-McKinstry and R.E. Newnham Introduction The field of electronic ceramics includes components as diverse as Mn-Zn ferrites, lead zirconate titanate transducers, ZnO varistors, and A12O3 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. Many types of sensing and actuating functions can be envisaged, as shown in Figure 2. This definition encompasses materials that are both passively and actively smart, where the distinction is made on the basis of
MRS BULLETIN/APRIL 1993
whether a feedback loop is used to trigger the actuation. A passively smart material responds automatically to a change in the environment. This is particularly useful when there is only one correct response to make to a situation, as is the case for the self-repair mechanism in aluminum electrolytic capacitors. Capacitors of this type are prepared by electrolytically forming thin alumina layers on the surfaces of a
Ferroelectric Domains: Capacitors and Piezoelectrics PTC Thermistor
high-purity aluminum foil. The coated foil is then separated from a second aluminum sheet by porous paper impregnated with an electrolyte solution. During operation, the thin A12O3 layer acts as the dielectric separating the anode (the Al substrate for the coating) and the cathode (the electrolyte solution). Minor breakdowns in the layer can thus be repaired by reforming the oxide overcoat from the electrolyte.1 This self-healing makes the aluminum electrolytic capacitor the electrical analog to stainless steel, another passively smart material with
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