Understanding Diffusion: A Process Involving Atomic Motion and Material Transport, Study Guides, Projects, Research of Engineering

Download Understanding Diffusion: A Process Involving Atomic Motion and Material Transport and more Study Guides, Projects, Research Engineering in PDF only on Docsity! 10/2/2015 1 Art Ian G. Bautista ,ECE, ECT  A steel gear that has been ‘‘case hardened.’’ The outer surface layer was selectively hardened by a high temperature heat treatment during which carbon from the surrounding atmosphere diffused into the surface. The ‘‘case’’ appears as the dark outer rim of that segment of the gear that has been sectioned.  Materials of all types are often heat treated to improve their properties. The phenomena that occur during a heat treatment almost always involve atomic diffusion. Often an enhancement of diffusion rate is desired; on occasion measures are taken to reduce it. Heat- treating temperatures and times, and/or cooling rates are often predictable using the mathematics of diffusion and appropriate diffusion constants.  The steel gear shown on this page has been case hardened; that is, its hardness and resistance to failure by fatigue have been enhanced by diffusing excess carbon or nitrogen into the outer surface layer. Applications of Diffusion • Furnace for heat treating steel using carburization. • Carburizing is the addition of carbon to the surface of low- carbon steels at temperatures ranging from 1560°F to 1740°F. • Hardening is achieved when a high carbon martensitic case with good wear and fatigue resistance is superimposed on a tough, low-carbon steel core. 10/2/2015 2 7 • Case hardening or surface hardening is the process of hardening the surface of a metal, often a low carbon steel, by diffusing elements into the material's surface, forming a thin layer of a harder alloy. • Carbon atoms diffuse into the iron lattice atoms at the surface. • This is an example of interstitial diffusion. • The C atoms make iron (steel) harder. “Carbide band saw blade can cut through case hardened materials.” Schematic of the microstructure of the Co-Pt-Ta-Cr film after annealing. Most of the chromium diffuses from the grains to the grain boundaries after the annealing process. This helps improve the magnetic properties of the hard disk. © 2 0 0 3 B ro ok s/ C o le , a d iv is io n o f T h o m so n L ea rn in g , I n c. T h o m so n L ea rn in g ™ is a t ra d em ar k u se d h er ei n u nd er li ce n se . • Hot-dip galvanizing is a form of galvanization. It is the process of coating iron, steel, or aluminum with a thin zinc layer, by passing the metal through a molten bath of zinc at a temperature of around 860 °F (460 °C). • When exposed to the atmosphere, the pure zinc (Zn) reacts with oxygen (O2) to form zinc oxide (ZnO), which further reacts with carbon dioxide (CO2) to form zinc carbonate (ZnCO3), a dull grey, fairly strong material. • In many environments, the steel below the coating will be protected from further corrosion. •Galvanized steel is widely used in applications where rust resistance is needed. A hot-dip galvanizing 'kettle' with fume hood Galvanized steel and coils popular for applications in industrial goods, automobile components, precision tubes, consumer durable and many more. Galvanized i-beams. 10 Thermal barrier coatings (TBC) with a ceramic topcoat are widely used for protecting highly loaded gas turbine components against overheating. For example, on internally cooled turbine blades the ceramic topcoat maintains a high temperature difference between the outer surface and the underlying metallic substrate.  Integrated circuits (ICs), found in numerous electronic devices have been fabricated using doping techniques.  The base material for these ICs is silicon that has been “doped” with other materials.  More precisely, controlled concentrations of impurities have been diffused into specific regions of the device to change the properties (improve electrical conductivity). 12 • Doping silicon with phosphorus for n-type semiconductors: • Process: 3. Result: Doped semiconductor regions. silicon magnified image of a computer chip 0.5 mm light regions: Si atoms light regions: Al atoms 2. Heat. 1. Deposit P rich layers on surface. silicon 10/2/2015 5 25 Example 1: Chemical Protective Clothing (CPC) • Methylene chloride is a common ingredient of paint removers. Besides being an irritant, it also may be absorbed through skin. When using this paint remover, protective gloves are worn. If butyl rubber gloves (0.04 cm thick) are used, what is the diffusive flux of methylene chloride through a glove? Data: – diffusion coefficient for butyl rubber: D = 110 x10-8 cm2/s – surface concentrations: C2 = 0.02 g/cm 3 C1 = 0.44 g/cm 3 26 scm g 10 x 16.1 cm) 04.0( )g/cm 44.0g/cm 02.0( /s)cm 10 x 110( 2 5- 33 28-   J Solution: 12 12- xx CC D dx dC DJ    D tb 6 2  glove C1 C2 skinpaint remover x1 x2 assuming linear conc. gradient D = 110x10-8 cm2/s C2 = 0.02 g/cm 3 C1 = 0.44 g/cm 3 x2 – x1 = 0.04 cm Data: 27 Diffusion and Temperature • Diffusion coefficient increases with increasing T. D  Do exp       Qd RT = pre-exponential [m2/s] = diffusion coefficient [m2/s] = activation energy [J/mol or eV/atom] = gas constant [8.314 J/mol-K] = absolute temperature [K] D Do Qd R T Activation energy - energy required to produce the movement of 1 mole of atoms by diffusion. • The diffusing species, host material and temperature influence the diffusion coefficient. • For example, there is a significant difference in magnitude between self-diffusion and carbon interdiffusion in α iron at 500 °C. Factors that influence diffusion 29 At 300ºC the diffusion coefficient and activation energy for Cu in Si are: D(300ºC) = 7.8 x 10-11 m2/s Qd = 41,500 J/mol What is the diffusion coefficient at 350ºC? Example 2: 30              350 0350 300 0300 1 lnln and 1 lnln TR Q DD TR Q DD dd        300350300 350 300 350 11 lnlnln TTR Q D D DD d D  Do exp       Qd R T Solution: 10/2/2015 6 31                 K 573 1 K 623 1 K-J/mol 314.8 J/mol 500,41 exp /s)m 10 x 8.7( 2112D              12 12 11 exp TTR Q DD d T1 = 273 + 300 = 573 K T2 = 273 + 350 = 623 K D2 = 15.7 x 10 -11 m2/s 32 Non-steady State Diffusion The diffusion flux and the concentration gradient at some particular point in a solid vary with time, with a net accumulation or depletion of the diffusing species resulting. Concentration profiles for nonsteady-state diffusion taken at three different times, t1 , t2 , and t3 . Thank you… 1. A plate of iron is exposed to a carburizing (carbon- rich) atmosphere on one side and a decarburizing (carbon-deficient) atmosphere on the other side at 700°C (1300°F). If a condition of steady state is achieved, calculate the diffusion flux of carbon through the plate if the concentrations of carbon at positions of 5 and 10 mm beneath the carburizing surface are 1.2 and 0.8 kg/m3, respectively. Assume a diffusion coefficient of 3x10-11 m2/s at this temperature. 2. Using the data in Table 6.2, compute the diffusion coefficient for magnesium in aluminum at 550C.