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1、 4.1 Fundamental characteristicsThis chapter first identifies characteristics that distinguish smart materials from other materials, and then systematically reviews many of the more widely used ones. We begin by noting that the five fundamental characteristics that were defined as distinguishing a s
2、mart material from the more traditional materials used in architecture were transiency, selectivity, immediacy, self-actuation and directness. If we apply these characteristics to the organization of these materials then we can group them into:1 Property change capability 2 Energy exchange capabilit
3、y 3 Discrete size/location 4 ReversibilityThese features can potentially be exploited to either optimize a material property to better match transient input conditions or to optimize certain behaviors to maintain steady state conditions in the environment. As we begin to explore these distinguishing
4、 characteristics, we will see that the reasons why smart materials exhibit these and other traits is not easy to explain without recourse to thinking about both the material science precepts noted in the last chapter and the specific conditions surrounding the placement and use of the material. Of p
5、articular importance is the concept of the surrounding energy or stimulus field that was discussed in Chapter 3. We recall that energy fields can be constructed of many types of energy potential, electrical, thermal, mechanical, chemical, nuclear, kinetic all of which can be exchanged or converted a
6、ccording to the First Law of Thermodynamics (the law of the conservation of energy). The physical characteristics of smart materials are deter- mined by these energy fields and the mechanism through which this energy input to a material is converted. If the mechanism affects the internal energy of t
7、he material by altering either the materials molecular structure or micro- structure then the input results in a property change of the material. If the mechanism changes the energy state of theTypes and characteristics of smart materials794Types and characteristics of smart materialsmaterial compos
8、ition, but does not alter the material, then the inputresultsinanexchangeofenergyfromoneformtoanother. A simple way of differentiating between the two mechan- isms is that for property change type, the material absorbs the input energy and undergoes a change, whereas for the energy exchange type, th
9、e material stays the same but the energy undergoesachange.Weconsiderbothofthesemechanismsto operate at the micro-scale, as none will affect anything larger than the molecule, and furthermore, many of the energy- exchanges take place at the atomic level. As such, we cannot see this physical behavior
10、at the scale at which it occurs.Property change The class of smart materials with the greatest number of potential applications to the field of architecture is the property-changing class. These materials undergo a change in a property or properties chemical, thermal, mechanical, magnetic, optical o
11、r electrical in response to a change in the conditions of the environment of the material. The conditions of the environment may be ambient or may be produced via a direct energy input. Included in this class are all color- changing materials, such as thermochromics, electrochro- mics, photochromics
12、, etc., in which the intrinsic surface or molecular spectral absorptivity of visible electromagnetic radiation is modified through an environmental change (incident solar radiation, surface temperature) or a direct energy input to the material (current, voltage).Energy exchange The next class of mat
13、erials that is expected to have large penetration into the field of architecture is the energy- exchanging class. These materials, which can also be called First Law materials, change an input energy into another form to produce an output energy in accordance with the First Law of Thermodynamics. Al
14、though the energy conver- sion efficiency for smart materials such as photovoltaics and thermoelectrics is typically much less than for more conven- tional technologies, the potential utility of the energy is much greater. For example, the direct relationship between input energy and output energy r
15、enders many of the energy- exchanging smart materials, including piezoelectrics, pyro- electrics and photovoltaics, as excellent environmental sen- sors. The form of the output energy can further add direct actuation capabilities such as those currently demonstrated byelectrostrictives,chemoluminesc
16、entsandconducting polymers.Smart Materials and New Technologies80Types and characteristics of smart materialsReversibility/directionality Many of the materials in the two above classes also exhibit the characteristic either of reversibility or of bi-directionality. Several of the electricity converting materials can reverse their input and output energy forms. For example, some piezoelectric materials can produce a current with an applied strain or can deform with an applied curren
