Engr 313, Week 7 Notes
Engr 313, Week 7 Notes Engr 313
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This 3 page Class Notes was uploaded by Andres Rodriguez on Monday October 10, 2016. The Class Notes belongs to Engr 313 at University of Mississippi taught by Dr. Amrita Mishra in Fall 2016. Since its upload, it has received 15 views. For similar materials see Introduction to Materials Science in General Engineering at University of Mississippi.
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Date Created: 10/10/16
Chapter 5: Atom and Ion Movements in Materials Diffusion: Mass transport by atomic motion, which is a consequence of the constant thermal motion of atoms, molecules, and particles that results in material moving from areas of high to low concentration. A great example is the water flow from a mountain to the sea in order to minimize the gravitational potential energy. In this process, atoms and ions tend to move in a predictable fashion in order to eliminate any concentration differences, which generates homogeneous compositions that make the material thermodynamically stable. Mechanisms: Brownian Motion: Random movement of particles in a liquid or gas as a result of continuous bombardment from molecules of the surrounding medium. Solids: Vacancy diffusion and interstitial diffusion. Diffusion Applications: Case/Surface Hardening: A source of carbon (often low carbon steel) is to be carburized, which consists in the diffusion of this carbon source into steel components. This process will form a thin layer of harder alloy. (Example of interstitial diffusion). Thermal Barrier Coatings: Consisting of a ceramic topcoat, they are widely used to protect highly loaded gas turbine components from overheating. Creation of Plastic Bottles: Not always the purpose is to make diffusion occur, sometimes we want to minimize it. In this application, we want to minimize the diffusion of carbon dioxide by using polyethylene terephthalate to extend the duration of the fizz in carbonated beverages. Dopant: Many electronic devices consist of integrated circuits that are fabricated by using doping methods. Techniques that are based on the diffusion of controlled impurities’ concentrations into specific regions of the device with the purpose of improving electrical conductivity by changing the properties. Doping is also used to diffuse different atoms (dopant atoms) into semiconductors such as silicon. Stability of Atoms and Ions: Arrhenius Equation: Used to predict the rate −Q Rate=c eop? ( ) RT co= Constant R = Gas constant, 1.987 cal/mol K T = Absolute temperature in K Q = Activation energy Mechanisms for Diffusion: Self-Diffusion: Jump of atoms from one lattice position to another that occurs in materials containing vacancies. It can be detected by using radioactive tracers. Interdiffusion: Diffusion of different atoms in different directions. Vacancy Diffusion: Process where an atom leaves its lattice site to fill a nearby vacancy, which creates a new vacancy at the original lattice site. Now, there is going to be counterflows of atoms and vacancies. The number of vacancies, which increases with temperature, has an important influence in the extent of self-diffusion and diffusion of substitutional atoms. Interstitial Diffusion: Process where a small interstitial atom or ion moves from one interstitial site to another, where vacancies are not required. Since there are more interstitial sites than vacancies, this process is more likely to happen than vacancy diffusion. Check Figure 5-6 for illustrations of some of these mechanisms. Activation Energy of Diffusion: Energy barrier that must be overcome in order to move an atom from its initial low-energy location to a new location. Since less energy is required to squeeze an interstitial atom past the surrounding atoms, activation energy is going to be lower for interstitial diffusion than for vacancy diffusion. Low activation energy means easy diffusion The term diffusion couple is used to describe the combination of given element’s atom diffusing in a host material. In self-diffusion, activation energy is the energy required to create a vacancy and to cause the movement of an atom. Table 5-1 shows the typical activation energy values of diffusion couples. Rate of Diffusion: Fick’s First Law: Explains the net flux of atoms dc J=−D dx J = Flux 2 D = Diffusivity or diffusion coefficient (cm /s) dc/dx = Concentration gradient (atoms/cm cm) 3 Concentration will be expressed in atom, weigh, or mole percent depending on the situation. It can also be expressed as atom or mole fractions. Therefore, the units of concentration gradient will change accordingly. The negative sign indicates that the flux of diffusing species is from higher to lower concentrations. Flux is proportional to concentration gradient. If linear dc/dx = ∆c/∆x = (c 2c 1/(x2-x1). Flux is defined as the number of atoms passing through a plane of unit area per unit time. Types of Diffusion: Volume Diffusion: Atoms move through the crystal from one regular or interstitial site to another. It has large activation energy and low diffusion rate because of the surrounding atoms. It is the slowest method. Grain Boundary Diffusion: Since atoms can also diffuse along boundaries, interfaces, and surfaces in the material, they can easily diffuse by using this technique because of the disordered and less dense atom packing in the grain boundaries. Low activation energy because it is easier for atoms to squeeze through the grain boundary. Surface Diffusion: It is the easiest (fastest) out of the three method since there is even less constraint on the diffusing atoms at the surface. Factors Affecting Diffusion: Temperature and Diffusion Coefficient: The higher the temperature of a material, the higher the diffusion coefficient. D=D exp? (−Q ) o RT Types of Diffusion: The speed of the diffusion is going to change according to the method that you use. Volume diffusion is the slowest one, grain boundary is faster that volume diffusion, and surface diffusion is the faster out of the three. Time: Longer time will be required as the number of atom that must diffuse to create an uniform structure increases. Bonding and Crystal Structure: Lower activation energies for atoms diffusing through open crystal structures than for close-packed structures. Since activation energy depends on the atomic bonding strength, it is higher for diffusion of atoms in materials with higher melting point. Cations have higher diffusion coefficients than anions.
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