This glossary is being created to facilitate the understanding of interface complexions, a new concept in kinetic engineering, est. 2007.
See also, Table 2, Terminology related to complexion transitions and methods of categorizing complexions in P.R.Cantwell, M.Tang, S.J.Dillon, J.Luo, G.S.Rohrer and M.P.Harmer, Grain Boundary Complexions,” Acta Materialia, 62 (2014), page 38.
Glossary of Terms associated with MURI_Interphase complexions
Activation energy -- energy that must be overcome in order for a chemical reaction to occur. Activation energy may also be defined as the minimum energy required to start a chemical reaction. The activation energy of a reaction is usually denoted by Ea, and given in units of kilojoules per mole. Activation energy can be thought of as the height of the potential barrier (sometimes called the energy barrier) separating two minima of potential energy (of the reactants and products of a reaction). For a chemical reaction to proceed at a reasonable rate, there should exist an appreciable number of molecules with energy equal to or greater than the activation energy.
Anisotropy -- exceptions, or inequalities in isotropy, are frequently indicated by the prefix an, hence anisotropy. Anisotropy is also used to describe situations where properties vary systematically, dependent on direction.
Complexion -- Interfacial material or strata that is in thermodynamic equilibrium with its abutting phase(s) and has a stable, finite thickness that is typically on the order of 0.2 to 2nm. A complexions cannot exist independently of the abutting phases and its average composition and structure need not be the same as the abutting phases. Complexions can be made to transform into different complexions with vastly different properties by chemistry and heat treatment, thereby enabling the engineering control of material properties on a level not previously realizable.
Entropy -- a thermodynamic property that can be used to determine the energy not available for work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when converting energy to work. During this work, entropy accumulates in the system, which then dissipates in the form of waste heat. Entropy is an expression of disorder or randomness.
Grain Boundary - a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material. The high interfacial energy and relatively weak bonding in most grain boundaries often makes them preferred sites for the onset of corrosion and for the precipitation of new phases from the solid. They are also important to many of the mechanisms of creep. On the other hand, grain boundaries disrupt the motion of dislocations through a material, so reducing crystallite size is a common way to improve strength, as described by the Hall–Petch relationship.
Grain Boundary Energy -- the energy of a low-angle boundary is dependent on the degree of misorientation between the neighbouring grains up to the transition to high-angle status. The situation in high-angle boundaries is more complex. Although theory predicts that the energy will be a minimum for ideal CSL configurations, with deviations requiring dislocations and other energetic features, empirical measurements suggest the relationship is more complicated. Some predicted troughs in energy are found as expected while others missing or substantially reduced. Surveys of the available experimental data have indicated that simple relationships such as low Σ are misleading:
It is concluded that no general and useful criterion for low energy can be enshrined in a simple geometric framework. Any understanding of the variations of interfacial energy must take account of the atomic structure and the details of the bonding at the interface.[1]
Grain Boundary Mobility or Migration -- The movement of grain boundaries (HAGB) has implications for recrystallization and grain growth while subgrain boundary (LAGB) movement strongly influences recovery and the nucleation of recrystallization. A boundary moves due to a pressure acting on it. It is generally assumed that the velocity is directly proportional to the pressure with the constant of proportionality being the mobility of the boundary. The mobility is strongly temperature dependent and often follows an Arrhenius type relationship:
The apparent activation energy (Q) may be related to the thermally activated atomistic processes that occur during boundary movement. However, there are several proposed mechanisms where the mobility will depend on the driving pressure and the assumed proportionality may break down.
It is generally accepted that the mobility of low-angle boundaries is much lower than that of high-angle boundaries. The following observations appear to hold true over a range of conditions:
- The mobility of low-angle boundaries is proportional to the pressure acting on it.
- The rate controlling process is that of bulk diffusion
- The boundary mobility increases with misorientation.
Since low-angle boundaries are composed of arrays of dislocations and their movement may be related to dislocation theory. The most likely mechanism, given the experimental data, is that of dislocation climb, rate limited by the diffusion of solute in the bulk.[2]
The movement of high-angle boundaries occurs by the transfer of atoms between the neighbouring grains. The ease with which this can occur will depend on the structure of the boundary, itself dependent on the crystallography of the grains involved, impurity atoms and the temperature. It is possible that some form of diffusionless mechanism (akin to diffusionless phase transformations such as martensite) may operate in certain conditions. Some defects in the boundary, such as steps and ledges, may also offer alternative mechanisms for atomic transfer.
Isotropy -- Isotropy is uniformity in all orientations; it is derived from the Greek iso (equal) and tropos (direction). An isotropic field exerts the same action regardless of how the test particle is oriented.
Quantum confinement - the effect describes the phenomenon resulting from electrons and electron holes being squeezed into a dimension that approaches a critical quantum measurement, called the exciton Bohr radius. In current application, a quantum dot such as a small sphere confines in three dimensions, a quantum wire confines in two dimensions, and a quantum wellconfines only in one dimension. These are also known as zero-, one- and two-dimensional potential wells, respectively. In these cases they refer to the number of dimensions in which a confined particle can act as a free carrier.
STEM - HAADF -- high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). By using a STEM detector with a large inner radius, a HAADF detector, electrons are collected which are not Bragg scattered. As such HAADF images show little or no diffraction effects, and their intensity is approximately proportional to Z2. This imaging technique proves ideal for tomographic reconstruction as it generates strong contrast that has a fully monotonic relationship with thickness.