SMART/INTELLIGENT MATERIALS STRUCTURAL APPLICATIONS Nonli near ferromagnetic effect Pyrosensftive effect Electroplastic effect ACOUSTICAL APPLICATIONS OPTICAL APPLICATIONS Magnetostrictive effect (Kerr effect and Pockel's effect) Electrochromic effect Reman active effect FIGURE 58.2 Applicatio ific classification of smart/intelligent materials effect(SME) [Jackson et al., 1972]. It is a memory mechanism that is the result of a martensitic transformation taking place during heating Alhough the exact mechanism by which the shape-memory effect occurs is still under study, the process by which the original shape is regained is associated with a reverse transformation of the deformed martensitic phase to the higher temperature austenite phase. A group of nickel-titanium alloys(referred to as Nitinol)of proper composition exhibit the shape-memory property and are widely used in smart material applications [Jackson et al., 1972] Electrorheological Property Electrorheological property is the property exhibited by certain fluids that are capable of altering their flow haracteristics depending on an external applied electric field. These fluids have a fast response time, only a few milliseconds. Once the external field is applied, there is a form of progressive gelling of the fluid proportional to the applied field strength. Without the applied field, the fluid flows freely. If the electrified electrorheological (ER)fluid is sheared by an applied force larger than a certain critical value, it flows. Below this critical value of applied shear force, the electrified fluid remains in the gel phase [gandhi and Thompson, 1989] An electrorheological fluid requires particles(1 to 100 mm in diameter)dispersed in a carrier fluid. Sometimes a surfactant is also added to help the dispersion of particles in the fluid. The surfactant is used to prevent particle interaction that could otherwise result in a tendency for the particulates to clump together when the fluid is allowed to stand still over a stretch of time. The tendency of the particles to clump together is referred to as settling The applied electric field to perceive the electrorheological phenomenon is usually in the order of 4 kv/mm. When the electric field is applied, the positive and negative charges on the suspended particles are separated, forming a dipole of charges. These dipoles then align(polarize)themselves by mutual forces of attraction and repulsion to other similar dipoles, resulting in unique flow characteristics. In the absence of an electric there is no dipole separation of charges, and hence the fluid returns to its normal flow An ideal electrorheological fluid is one that has a low viscosity in the absence of an applied field and that which transforms into a high-viscosity gel capable of withstanding high shear stresses when the field is on. Further, it must also have a low power consumption. The first reported ER fluid consisted of finely dispersed suspensions of starch or silica gel in mineral oil nearly 40 years ago. Nonlinear Electro-optic Properties In certain materials that are optically transparent when subjected to an external electric field, the refractive index of the material changes. Invariably the electric field versus optical effect thus experienced is nonlinear, c 2000 by CRC Press LLC
© 2000 by CRC Press LLC effect (SME) [Jackson et al., 1972]. It is a memory mechanism that is the result of a martensitic transformation taking place during heating. Alhough the exact mechanism by which the shape-memory effect occurs is still under study, the process by which the original shape is regained is associated with a reverse transformation of the deformed martensitic phase to the higher temperature austenite phase.A group of nickel-titanium alloys (referred to as Nitinol) of proper composition exhibit the shape-memory property and are widely used in smart material applications [Jackson et al., 1972]. Electrorheological Property Electrorheological property is the property exhibited by certain fluids that are capable of altering their flow characteristics depending on an external applied electric field. These fluids have a fast response time, only a few milliseconds. Once the external field is applied, there is a form of progressive gelling of the fluid proportional to the applied field strength. Without the applied field, the fluid flows freely. If the electrified electrorheological (ER) fluid is sheared by an applied force larger than a certain critical value, it flows. Below this critical value of applied shear force, the electrified fluid remains in the gel phase [Gandhi and Thompson, 1989]. An electrorheological fluid requires particles (1 to 100 mm in diameter) dispersed in a carrier fluid. Sometimes a surfactant is also added to help the dispersion of particles in the fluid. The surfactant is used to prevent particle interaction that could otherwise result in a tendency for the particulates to clump together when the fluid is allowed to stand still over a stretch of time. The tendency of the particles to clump together is referred to as settling. The applied electric field to perceive the electrorheological phenomenon is usually in the order of 4 kV/mm. When the electric field is applied, the positive and negative charges on the suspended particles are separated, forming a dipole of charges. These dipoles then align (polarize) themselves by mutual forces of attraction and repulsion to other similar dipoles, resulting in unique flow characteristics. In the absence of an electric field, there is no dipole separation of charges, and hence the fluid returns to its normal flow. An ideal electrorheological fluid is one that has a low viscosity in the absence of an applied field and that which transforms into a high-viscosity gel capable of withstanding high shear stresses when the field is on. Further, it must also have a low power consumption. The first reported ER fluid consisted of finely dispersed suspensions of starch or silica gel in mineral oil nearly 40 years ago. Nonlinear Electro-optic Properties In certain materials that are optically transparent when subjected to an external electric field, the refractive index of the material changes. Invariably the electric field versus optical effect thus experienced is nonlinear, FIGURE 58.2 Application-specific classification of smart/intelligent materials
with the result that a time-varying electric field will modulate the refractive index, and hence a phase shift is experienced by the light passing through the medium. In materials that have a central symmetry, this phenomenon is called the Kerr effect; in noncentrosymmetric materials, it is referred to as Pockel's effect [ Kaminow, 1965] Nonlinear Electroacoustic Properties Electroacoustic synergism is experienced in certain classes of materials in which the mechanical atomic vibrations re influenced by the electronic polarizability, with the result that nonlinear interaction between the atomic dis- placements versus the electric field causes modulation effects resulting in the generation of new sideband frequencies uch sidebands(labeled Raman frequencies)and the response function of a Raman active medium have the form H(u)=A1E(u)+A2E2(u)+A3E3(o)+ Pyrosensitive properties The pyrosensitive property is governed by a class of materials known as solid electrolytes. On thermally energizing such materials, they exhibit superionic electric conduction(also known as fast ion conduction) with the result that the medium, which is dielectric under cold conditions, becomes conducting at elevated temperatures. Correspondingly, the media that are embedded with solid electrolytes show different extents of electromagnetic reflection/transmission characteristics at low and high temperatures and hence can be manip- ulated thermally (Neelakanta et al., 1992 e Typical solid electrolytes that can be adopted for such pyrosensitive applications are, for example, Agl and rbAgIs. materials like B-Agl and B-alumina show increasing conductivity with increasing temperature. The compound B-AgI exhibits superionic conductivity, with an abrupt transition at a temperature close to 147"C. This transition is known as the B-to a-phase transition, and there are a host of other materials that exhibit this phenomenon. For example, the material RbAg I, has a high electrical conductivity even at room temperature. It has also been observed that solid electrolytes provide sufficiently high electrical conductivity in the a-phase even when included in low volume fractions in a mixture with a nonsolid-electrolyte host[Neelakanta et al., 1992] Nonlinear Electromagnetic Properties Basically, the nonlinear electromagnetic properties can manifest as two subsets of material characteristics, namely, nonlinear dielectric properties and nonlinear magnetic properties Nonlinear Dielectric Properties are referred to as active or nonlinear dielectrics. Such materials demonstrate very high values of permittivity (in the order of several thousand), pronounced dependence of dielectric parameters on the temperature, and a loop of electric hysteresis under the action of an alternating voltage. Ferroelectrics are the most typical example of nonlinear dielectrics. Rochelle's salt(potassium sodium artrate)was the first substance in which nonlinearity was discovered. All ferroelectrics, however, possess nonlinear properties only within a definite temperature range. The temperature transition points over which the ferroelectric materials gain or lose their ferroelectric properties are referred to as Curie points. The arsenates and dihydrogen phosphates of alkali metals are also examples of ferroelectric materials. Piezoelectrics also fall under the category of active dielectrics. Electrets, which are capable of preserving an electric charge for a long period of time(hence regarded analogous to permanent magnets), exhibit highly nonlinear dielectric properti Nonlinear Magnetic Properties Ferromagnetic materials are materials in which the permanent magnetic dipoles align themselves parallel to each other. These materials have a characteristic temperature below and above which their properties differ greatly. This temperature is referred to as the Curie temperature. Above the Curie temperature they behave as paramagnetic materials, while below it they exhibit the well known hysteresis B versus H curves. Examples of such ferromagnetic materials are iron, Mu-metal, and Supermalloy Ferrimagnetic materials are similar in their hysteresis properties to ferromagnetic materials but differ from them in that their magnetic dipoles aligi c 2000 by CRC Press LLC
© 2000 by CRC Press LLC with the result that a time-varying electric field will modulate the refractive index, and hence a phase shift is experienced by the light passing through the medium. In materials that have a central symmetry, this phenomenon is called the Kerr effect; in noncentrosymmetric materials, it is referred to as Pockel’s effect [Kaminow, 1965]. Nonlinear Electroacoustic Properties Electroacoustic synergism is experienced in certain classes of materials in which the mechanical atomic vibrations are influenced by the electronic polarizability, with the result that nonlinear interaction between the atomic displacements versus the electric field causes modulation effects resulting in the generation of new sideband frequencies. Such sidebands (labeled Raman frequencies) and the response function of a Raman active medium have the form H(v) = A1E(v) + A2E 2(v) + A3E 3(v) + · · · Pyrosensitive Properties The pyrosensitive property is governed by a class of materials known as solid electrolytes. On thermally energizing such materials, they exhibit superionic electric conduction (also known as fast ion conduction), with the result that the medium, which is dielectric under cold conditions, becomes conducting at elevated temperatures. Correspondingly, the media that are embedded with solid electrolytes show different extents of electromagnetic reflection/transmission characteristics at low and high temperatures and hence can be manipulated thermally [Neelakanta et al., 1992]. Typical solid electrolytes that can be adopted for such pyrosensitive applications are, for example,AgI and RbAg4I5. The materials like b-AgI and b-alumina show increasing conductivity with increasing temperature. The compound b-AgI exhibits superionic conductivity, with an abrupt transition at a temperature close to 147°C. This transition is known as the b- to a-phase transition, and there are a host of other materials that exhibit this phenomenon. For example, the material RbAg4I5 has a high electrical conductivity even at room temperature.It has also been observed that solid electrolytes provide sufficiently high electrical conductivity in the a-phase even when included in low volume fractions in a mixture with a nonsolid-electrolyte host [Neelakanta et al., 1992]. Nonlinear Electromagnetic Properties Basically, the nonlinear electromagnetic properties can manifest as two subsets of material characteristics, namely, nonlinear dielectric properties and nonlinear magnetic properties. Nonlinear Dielectric Properties Dielectric materials whose permittivity has a distinct dependence on the intensity of the applied electric field are referred to as active or nonlinear dielectrics. Such materials demonstrate very high values of permittivity (in the order of several thousand), pronounced dependence of dielectric parameters on the temperature, and a loop of electric hysteresis under the action of an alternating voltage. Ferroelectrics are the most typical example of nonlinear dielectrics. Rochelle’s salt (potassium sodium tartrate) was the first substance in which nonlinearity was discovered. All ferroelectrics, however, possess nonlinear properties only within a definite temperature range. The temperature transition points over which the ferroelectric materials gain or lose their ferroelectric properties are referred to as Curie points. The arsenates and dihydrogen phosphates of alkali metals are also examples of ferroelectric materials. Piezoelectrics also fall under the category of active dielectrics. Electrets, which are capable of preserving an electric charge for a long period of time (hence regarded analogous to permanent magnets), exhibit highly nonlinear dielectric properties. Nonlinear Magnetic Properties Ferromagnetic materials are materials in which the permanent magnetic dipoles align themselves parallel to each other. These materials have a characteristic temperature below and above which their properties differ greatly. This temperature is referred to as the Curie temperature. Above the Curie temperature they behave as paramagnetic materials, while below it they exhibit the well known hysteresis B versus H curves. Examples of such ferromagnetic materials are iron, Mu-metal, and Supermalloy. Ferrimagnetic materials are similar in their hysteresis properties to ferromagnetic materials but differ from them in that their magnetic dipoles align