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Metal Crystals—III Energies and Processes
Published in Alan Cottrell, An Introduction to Metallurgy, 2019
Vacancies occur naturally in crystals at high temperatures. This is because their energies of formation are relatively small. The argument at the start of § 12.6 is directly applicable here. The formation of one vacancy, by a thermodynamically reversible process, at a given site in a crystal requires a work w, essentially to take the atom from this site and place it on the surface of the crystal. The chance of this site being empty is thus exp (− w/kT). A crystal of N atoms provides N sites where this can happen. Hence the number n of vacancies in the crystal is given by n=Ne−(w/kT)=Ne−(W/RT)
General Properties and Characterization Methods of Biomaterials
Published in Yaser Dahman, Biomaterials Science and Technology, 2019
Aside from the phases and their spatial distribution in biomaterials, there are also possible crystal imperfections that need to be examined. These imperfections can have effects on other properties, depending on their level of concentration. Even when this concentration is low, these defects have a significant effect on optical and electrical properties, and a dominant effect on mechanical properties (Bhat, 2006). For example, in the case of metals, crystalline defects have an impact on their ductility. The imperfections are classified into point defect and linear defect, depending on geometry and dimension of the defect. Point defect is associated with vacancies and self-interstitials; the former refers to vacant lattice sites from which an atom is missing, while the latter refers to an atom that occupies a small space that normally is not occupied (Callister and Rethwisch, 2006). On the other hand, linear defects are related to edge or screw dislocations occurring on the planes of atoms. Furthermore, there are defects related to the chemical nature of materials, which are called impurities; impurities can also modify the properties, whether they are there by accident or have been added intentionally.
3 Thin Film as Photoanode by Anodic Oxidation on Iron
Published in Kuan Yew Cheong, Two-Dimensional Nanostructures for Energy-Related Applications, 2017
Monna Rozana, Atsunori Matsuda, Go Kawamura, Wai Kian Tan, Zainovia Lockman
Introduction of vacancies (for example oxygen vacancies) also can help in the improvement of the conductivity of the photoanode. Vacancies can be introduced by doping or by selecting a fabrication process that would induce their formation. Creation of vacancies has many different advantages including intentionally adding electrons and the state level possessed by the vacancy can act as electron traps (Rocket 2008). One way to include oxygen vacancies in α-Fe2O3 is by fabricating the oxide in condition with lower pressure of oxygen. Anodic oxidation, due to the concentration gradient of oxygen can also be thought as an effective way in oxygen vacancy creation. In fact more importantly, oxidation produces surface oxide which depend on the anodization parameters that can be easily nanostructured.
Effect of weld geometry on irradiation damage resistance of 316L stainless steel weld at 400 ℃
Published in Journal of Nuclear Science and Technology, 2023
Yongfeng Qiao, Yucheng Lei, Yiqiang Yao, Xinyi Zhao, Zeyu Wang
Vacancy clusters were also observed in the three irradiated types of weld metal, as shown in Figure 8. The distribution of vacancy clusters is the same as that observed by Yang et al. under TEM [25]. However, the irradiation damage amount selected by Yang et al. is 3.6 dpa, so the defect diameter is larger. Figure 8(a–c) is the morphology of vacancy clusters observed under TEM, and Figure 8(d–f) is the statistical diagram of vacancy cluster size. The average size of vacancy clusters in V-groove welds is 3.46 nm, the average size of vacancy clusters in U-groove welds is 3.28 nm, and the average size of vacancy clusters in double U-groove welds is 3.12 nm. As the groove shape changes from V-groove to U-groove and then to double U-groove, the number of vacancy clusters with size of 1.0 nm − 3.0 nm increases, while the number of vacancy clusters with size of 3.5 nm − 6.0 nm decreases. Similar to the surface void size, the double U-groove weld with smaller grain size can better inhibit the growth of vacancy clusters. In the process of irradiation, the target atoms are bombarded, resulting in dislocation damage and the formation of vacancy and other point defects. Under the influence of atomic motion, some vacancies are re-occupied by the target atoms and the other vacancies gather and nucleate into vacancy clusters, which eventually grow into voids. The voids formed are not spherical. They appear in the shape of a regular octahedron with {111} plane as the surface. However, the vertex of the octahedron is truncated by the {110} plane to form a shape similar to that of a sphere [14].
On the nature of trapped states in an MoS2 two-dimensional semiconductor with sulfur vacancies
Published in Molecular Physics, 2019
Gabriela Ben-Melech Stan, Maytal Caspary Toroker
Even a small amount of vacancies may be created and have a dramatic effect on the electronic structure of a material and can cause severe consequences on performance. Some vacancies occur naturally in a material due to low thermodynamic formation energies [4]. But there are methods to control their concentration during the fabrication procedure by changing the gas partial pressure and the temperature [5]. The advantage of increasing vacancies is that they increase the number of charge carriers and therefore they may improve electronic conductivity. However, if the vacancies also generate low-energy states then those charge carriers will be trapped. In MoS2, the electronic conductivity is often referred to as n-type [6] as a result of sulfur (S) vacancies that form under reducing fabrication conditions [7,8]. S vacancies can also be generated through argon plasma exposure or electrochemical desulfurization [9].
Evaluation of diffusion mechanism and structural characterizations of ZnS compound during chemical reaction of Zn and S in liquid phase
Published in Journal of Sulfur Chemistry, 2018
Ehsan Karami, Majid Tavoosi, Ali Ghasemi, Gholam Reza Gordani
In fact, the formation of Kirkendall voids near the ZnS/S interface can be related to the higher diffusion coefficient of sulfur atoms in the ZnS layer than zinc atoms. If the diffusion phenomenon takes place on a large scale, then a flux of atoms will occur in one direction and a flux of vacancies in the other [20]. The Kirkendall effect arises when two distinct materials (such as S and Zn) are placed next to each other and diffusion is allowed to take place between them. Generally, the diffusion coefficients of the two materials in each other are not the same (DS > DZn) [20]. In the Zn/S diffusion couple, the net movement of atoms will be from sulfur toward zinc. In this condition, vacancies will diffuse in the ZnS phase from zinc toward the sulfur layer. One important result derived from increasing the vacancies’ density is the formation of pores in the tZnS/S interface [28]. The formed voids act as sinks for the vacancies, and when enough accumulate they can become substantial and expand in an attempt to restore equilibrium [20,28]. It is important to note that, the formation of Kirkendall voids has been reported in other diffusional couples such as Al/Ti [29], Cu/Sn [30] and Ni/Al [31].