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Ferrite by reconstructive transformation
Published in Harshad K. D. H. Bhadeshia, Theory of Transformations in Steels, 2021
Cahn's model assumed the diffusion coefficient to be a function of the distance from the centre of the boundary, approaching the value of the bulk diffusivity at sufficiently large distances from the interface, and a grain boundary value as the centre of the interface is approached. The diffusivity could therefore increase typically by a factor of 106 towards the core of the boundary [155], but this may exaggerate the real situation because grain boundary diffusion coefficients refer to transport along the boundary rather than across it. The mobility of atoms within an interface must in general be anisotropic, reflecting the nature of its defect structure. These problems are compounded because the theories generally treat the boundary width δb to be several interatomic distances. This is necessary because Gs may be non-zero over that range, but the diffusivity is expected to rapidly reach that of the defect-free lattice. It has also been proposed that the diffusion coefficient of a moving boundary may be larger than that of one which is stationary [156, 157]. It is not obvious how these complications can be resolved in practice.
2 widegap chalcopyrite semiconductors
Published in R D Tomlinson, A E Hill, R D Pilkington, Ternary and Multinary Compounds, 2020
Shigefusa Chichibu, Sho Shirakata, Hisayuki Nakanishi
The lattice parameter c of CuGaS2 grown on GaAs or GaP substrates is smaller than that of strain-free bulk crystals, as shown in Fig. 3. This suggests the increase of biaxial tensile stress. The strain-free lattice parameter c is closer to that of CuGaS2/GaAs than to that of CuGaS2/GaP, although the lattice mismatch for the former is larger than that for the latter. If we consider only the lattice mismatch assuming the coherent growth, the lattice parameter of CuGaS2/GaP would be closer to the bulk value. However, the result indicated otherwise. Therefore we calculated the c value of tensile-strained CuGaS2 assuming the thermal stress model [5]; the epilayer grows free standingly and the strain is induced during the post-growth cooling. Since αepi, is closer to αsub of GaAs than that of GaP, the c value of CuGaS2/GaAs (1.0438 nm) is estimated to be closer to the bulk value than that of CuGaS2/GaP (1.0417 nm), and these values are close to the experimental ones. Therefore the majority of the lattice strain is considered to be induced by the difference between αepi and αsub.
Materials—Flexible 1D Electronics
Published in Muhammad Mustafa Hussain, Nazek El-Atab, Handbook of Flexible and Stretchable Electronics, 2019
A key characteristic of nanotubes is the presence of delocalized π electrons, which provides excellent electrical conductivity to the nanotubes. Early studies have shown the resistivity of metallic CNTs to be 90 μΩ-cm (Kane et al. 1998). Because of the presence of a relatively defect-free lattice, the π electrons can achieve near ballistic conduction under an applied electric field. Even in the case of semiconducting nanotubes, most MWNTs tend to be semimetallic due to the inter-tube coupling. Hence, many current and potential applications are related to exploiting this high conductivity in CNT forests. However, these characteristics pertain to single SWNTs or MWNTs, and the electrical properties of a bulk sample or a large thin film of nanotubes depend greatly on the fabrication process, sample quality, and alignment. Along with high electrical conductance, the presence of π electrons also accords phenomenal thermal conductivity to CNTs. Reported values range widely, again pertaining to the quality of the sample, however, single CNTs have been reported to have theoretical thermal conductivity of more than 6000 W/mK (Berber et al. 2000).
Effects of pressure-temperature protocols on the properties of crystals and ageing effects – an analogy with glasses
Published in Philosophical Magazine, 2022
There are also experimentally observed distinctions between the kinetic and also between the thermodynamic behaviour of a defect-disordered crystal from those of an ultraviscous liquid. These are listed as follows: (i) defect-disordered crystals show Arrhenius-type temperature variation of D, but liquids generally show super-Arrhenius temperature variation of self diffusion coefficient, (ii) defect-disordered crystals show no localised motions of the JG relaxation but such localised motions are characteristics of viscous liquids and glasses [50,51], (iii) crystals have a higher density than the liquid, still there is no indication of co-operativity of atomic, molecular or ionic motions in their structure. (This co-operative motion is different from the collective modes of phonons in a defects-free lattice structure) and, (iv) defect-disordered crystals do not show a distribution of relaxation rates, but liquids generally show such a distribution. Thermodynamic and dynamic features of ODICs [52] are similar to those of liquids and glasses. Therefore, properties of defect-disordered crystals are also distinct from those of ODICs.
Numerical investigation of transient responses of triangular fins having linear and power law property variation under step changes in base temperature and base heat flux using lattice Boltzmann method
Published in Numerical Heat Transfer, Part A: Applications, 2021
Abhishek Sahu, Shubhankar Bhowmick
In this section, modeling of lattice Boltzmann solver has been discussed. Lattice Boltzmann is a mesoscopic scale-based method which is derived from the application of kinetic theory and statistical mechanics. Here, a set of particles having a velocity within a certain range known as distribution function is located instead of tracking the behavior of each molecule, since microscopic properties of each molecule is not significant in study of fluid dynamics and heat transfer problems. These distribution functions are related to macroscopic variables under consideration and then it is studied in discretized space and time. The execution of lattice Boltzmann solver consists of local streaming and collision of distribution functions at each lattice site, hence due to local operation it provides massive advantage over the conventional macroscopic based method when used to solve the complex boundary condition problems, complex geometry problems and parallel computing. Distribution functions stream toward its neighboring lattice at different directions with certain weightage corresponding to different velocity set which are designated as DnQm (n = problem dimension, m = number velocity set), different velocity sets available for one-dimensional problem being shown in Figure 2. Selection of velocity set depends on the type of problem, accuracy required, and memory available. The force free lattice Boltzmann equation [24,25] is given as:
Synthesis, characterization, crystal structure, and electrochemical study of zinc(II) metal-organic framework
Published in Inorganic and Nano-Metal Chemistry, 2019
B. M. Omkaramurthy, G. Krishnamurthy
TGA for the compound measured under a N2 atmosphere at a heating rate of 20 °C/min without pretreatment before the TG measurement is shown in Figure 10. The Zn-MOF shows the weight loss at two points. The first weight loss observed between the temperature range of 300 °C and 450 °C and the second weight loss was observed between region 460 °C and 650 °C. The first weight loss (30%) could be due to the loss of free lattice DMF molecule. The second weight loss of 44% can be attributed to the removal of the phenyl ring and carboxylate group. After this, no such weight loss observed till 650 °C. Previously, all solvent molecules are disappeared. The framework starts to collapse and gives amorphous material as a final product. There is no functional group peak of the figure showing the collapse of the compound because of evaporation of solvent molecules at 1000 °C. The Zn-MOF is stable in the temperature range of to 460 °C–650 °C.