Structure, Bonding, and Properties
Rule 4: Linking of polyhedra having different cations
1.14. STRUCTURE AND PROPERTY
1.15.1.4. SELECTIVITY, SENSITIVITY, REPRODUCIBILITY, AND RECOVERABILITY
Functional materials with chemical sensitivity are mandatory for response to a specific
f f
(a)
r--
~
I
u u I u u
I
-1 --l
-1 ---l
(b) f
__ ~ __ ~ __ +-~ ____ ~~u
A
Figure 1.27. (a) Noncontinuous and (b) continuous hysteresis loops. Both have memory effect.
change in its chemical environment, such as a specific type of molecules. An important characteristic of these compounds is their sensitivity to detect the species in small quantities. Selectivity is the ability to distinguish two or more similar chemical species, and specificity is the ability to quantitatively measure the same unique property for any case. Reproducibility is the ability to obtain high accurate measurement consistently and repeatedly. Recoverability is needed to perform reproducibility so that the material can be
"reset" at the same "ground state" for each measurement. Sensing and actuating the chemical environment demand that the compound have a memory effect, i.e., a chemical hysteresis loop.
1.15.1.5. SPEED OF RESPONSE, STABILITY, AND INTEGRABILITY. For practical applications, a functional material is also judged by its speed of response, structure stability, and integrability. The material is required to quickly respond and quickly recover its status when a change in its environment is introduced and withdrawn. The criterion is vitally important for device applications. Structure stability determines the life-time of the device and its fatigue behavior. A hurdle in the memory application of ferroelectric BaTi03, for example, is the fatigue of the domain switching. Finally, integrability of the materials with semiconductors is required to link the sensing, actuating, and feedback systems to the logic system. This imposes some requirements on the crystallography, structure compatibility, and processing temperature of the materials.
A large amount of research is needed in these areas.
1.15.2. STRUCTURAL EVOLUTION AND FuNCTIONALITY
A hysteresis loop of a material characterizes its tunablity and switchability. The tunable or switchable property comes from the structural tunable or switchable evolution
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STRUCTURE, BONDING, AND PROPERTIES
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CHAPTER 1
(e.g., phase transformation). Therefore, the behavior of structural evolution is closely related to the functionality of the material. We now use the following examples to show the importance of phase transformation in functional materials.
Four of the most widely used smart materials are piezoelectric Pb(Zr,Ti)03, magnetostrictive (Tb,Dy)Fe2, electrostrictive Pb(Mg,Nb)03' and the shape memory alloy NiTi. The former three are ferric with active domain walls, and the fourth has a martensitic phase transformation. The domain wall movement and the martensitic transition help tune the properties of these materials. Pb(Zr,Ti)03 is a ferroelectric ceramic which is cubic at 250-480°C. At room temperature (RT), the highest piezoelectric coefficients can be obtained at a rhombohedral-tetragonal phase boundary (called a morphotropic boundary).
Terfeno-D, (Tbx,DYl-x)Fe2, is a Laves phase compound that undergoes a rhombohedral-tetragonal transition at RT, which enhances its magnetostrictive coefficients. Since TbFe2 and DyFe2 both have large positive magnetostrictive coefficients L111 , they posses opposite signs of magnetic anisotropy, K, and have different directions of easy magnetization. Since Llil and K are temperature dependent, the largest magnetostriction will occur when the anisotropy compensation (at which
K ~ 0) occurs at x
=
0.27 in TbxDYI_xFe2. Therefore, Terfenol-D has the largest magnetostriction and magnetization at RT.Pb(Mg,Nb )03 is perovskite in a disorder state, but a face-centered cubic when Mg and Nb are ordered. The order-disorder transition occurs over a wide temperature range, called the Curie range. The ordered domains are Nb deficient and the disordered ones are Nb-rich, and both have different Curie temperatures and can give a broad range of phase transitions. When cooled from high temperature, these composition domains undergo a paraelectric-ferroelectric phase transition and create the local polar microdomain (a few nanometers in size) that results in the maximum Curie temperature depending on electric field frequency and temperature. These are the typical characteristics of the so-called relaxor ferroelectrics (Cross et al., 1980). If we use the relaxor PMN and PT (lead titanate) to form a solid solution, it has a morphotropic phase boundary near 35%(mole) PT. Therefore, PMN-PT relaxors are a unique family of materials with a remarkable set of properties such as (a) electrostrictive strains comparable to that of the best conventional piezoelectric ceramics, (b) nonhysteresis behavior responsible for excellent positional reproducibility, and (c) the large magnitude of piezoelectric coefficients. On further cooling, Pb(Mg,Nb)03 passes through a diffuse phase transformation at RT where it exhibits very large dielectric and electrostrictive coefficients. Just below RT, it transforms to a ferroelectric rhombohedral phase.
The shape memory alloy NiTi is based on the transformation from the austenitic bcc (or B2 type or CsCI type) structure to the monoclinic (B 19 type) structure. The former is a high-temperature phase and the latter is a low-temperature phase. When the austenitic phase transforms to martensitic, deformation occurs. If the phase transition reverses, the deformation will disappear. Therefore, it is easily deformed in the martensitic state but recovers its original shape when reheated to austenite.
All these examples have illustrated the key role of phase transformation (or structural evolution) in determining the functionality and performance of these materials.
Landau's theory of phase transitions uses an order parameter to represent the degree of ordering in a system. The order parameter connects the thermodynamic functions to the structural configurations of the atoms. Therefore, structural evolution determines the order parameter and eventually affects the material's properties.