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Chapter 6: Diffusion

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**Important Concepts Applications of Diffusion**

Activation Energy for Diffusion Mechanisms for Diffusion Rate of Diffusion (Fick’s First Law) Factors Affecting Diffusion Composition Profile (Fick’s Second Law)

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**Diffusion How does diffusion occur?**

Why is diffusion an important part of processing? How can the rate of diffusion be predicted for some simple cases? How does diffusion depend on structure and temperature?

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**Applications of Diffusion**

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**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.

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Case Hardening 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.” 6 6

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**©2003 Brooks/Cole, a division of Thomson Learning, Inc**

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. 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.

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**Hot-dip galvanizing is a form of galvanization**

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.

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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.

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Doping by Diffusion 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).

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**Processing Using Diffusion**

• Doping silicon with phosphorus for n-type semiconductors: • Process: magnified image of a computer chip 0.5 mm light regions: Si atoms light regions: Al atoms 1. Deposit P rich layers on surface. silicon 2. Heat. 3. Result: Doped semiconductor regions. silicon 11 11

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Diffusion Diffusion - Mass transport by atomic motion. Diffusion 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. Mechanisms Brownian motion is the seemingly random movement of particles suspended in a liquid or gas. Solids – vacancy diffusion or interstitial diffusion.

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Interdiffusion • Interdiffusion (impurity diffusion): In an alloy, atoms tend to migrate from regions of high concentration to regions of low concentration. After some time Initially

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Self-Diffusion • Self-diffusion: In an elemental solid, atoms also migrate. specific atom movement After some time A B C D A B C D

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Diffusion Mechanisms Atoms in solid materials are in constant motion, rapidly changing positions. For an atom to move, 2 conditions must be met: There must be an empty adjacent site, and The atom must have sufficient (vibrational) energy to break bonds with its neighboring atoms and then cause lattice distortion during the displacement. At a specific temperature, only a small fraction of the atoms is capable of motion by diffusion. This fraction increases with rising temperature. There are 2 dominant models for metallic diffusion: Vacancy Diffusion Interstitial Diffusion

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**Vacancy Diffusion Vacancy Diffusion: • atoms exchange with vacancies**

• applies to substitutional impurity atoms • rate depends on: -- number of vacancies -- activation energy to exchange. increasing elapsed time

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**Interstitial Diffusion**

Interstitial diffusion – smaller atoms (H, C, O, N) can diffuse between atoms. this is essentially mechanism in amorphous materials such as polymer membranes More rapid than vacancy diffusion due to more mobile small atoms and more empty interstitial sites.

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**Diffusion How do we quantify the rate of diffusion?**

M = mass diffused time Measured empirically Make thin film (membrane) of known surface area Impose concentration gradient Measure how fast atoms or molecules diffuse through the membrane J slope

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**Steady-state diffusion across a thin plate**

c06f04 Rate of diffusion is independent of time; the diffusion flux does not change with time. The concentration profile shows the concentration (C) vs the position within the solid (x); the slope at a particular point is the concentration gradient.

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**Steady-State Diffusion**

Flux proportional to concentration gradient = C1 C2 x x1 x2 Fick’s first law of diffusion D diffusion coefficient

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**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: C1 = 0.44 g/cm3 C2 = 0.02 g/cm3

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**Example 1 (cont). Solution – assuming linear conc. gradient Data:**

glove C1 paint remover skin D = 110 x 10-8 cm2/s C2 = 0.02 g/cm3 C1 = 0.44 g/cm3 x2 – x1 = 0.04 cm Data: C2 x1 x2

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**Diffusion and Temperature**

• Diffusion coefficient increases with increasing T. D = Do exp æ è ç ö ø ÷ - Qd R T = 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] Activation energy - energy required to produce the movement of 1 mole of atoms by diffusion.

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**Factors that influence 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.

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**What is the diffusion coefficient at 350ºC?**

Example 2: At 300ºC the diffusion coefficient and activation energy for Cu in Si are: D(300ºC) = 7.8 x m2/s Qd = 41,500 J/mol What is the diffusion coefficient at 350ºC? transform data D Temp = T ln D 1/T D = Do exp æ è ç ö ø ÷ - Qd R T

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**Example 2 (cont.) T1 = 273 + 300 = 573 K T2 = 273 + 350 = 623 K**

D2 = 15.7 x m2/s

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**Nonsteady State Diffusion**

The concentration of diffusing species is a function of both time and position C = C(x,t). More likely scenario than steady state. In this case, Fick’s Second Law is used. In the non-steady state the concentration profile develops with time. Fick’s Second Law

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**Processing – Ex 6.3 • Copper diffuses into a bar of aluminum.**

pre-existing concentration Co of copper atoms Surface concentration C of Cu atoms bar s • 10 hours at 600˚C gives C(x). • How many hours would it take to get the same C(x) if processed at 500˚C? 115.5 hrs • Answer:

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**Non-steady State Diffusion**

Example 3: An FCC iron-carbon alloy initially containing 0.20 wt% C is carburized at an elevated temperature and in an atmosphere that gives a surface carbon concentration constant at 1.0 wt%. If after 49.5 h the concentration of carbon is 0.35 wt% at a position 4.0 mm below the surface, determine the temperature at which the treatment was carried out. Solution: use Eqn. 6.5

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**Example 3 Solution (1): t = 49.5 h x = 4 x 10-3 m**

Cx = 0.35 wt% Cs = 1.0 wt% Co = 0.20 wt% erf(z) =

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Example 3 Solution (2): We must now determine from Table 6.1 the value of z for which the error function is An interpolation is necessary as follows z erf(z) z z = 0.93 Now solve for D

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D = Do exp æ è ç ö ø ÷ - Qd R T Example 4 Solution (3): To solve for the temperature at which D has the calculated value, we use a rearranged form of Equation (6.9a); from Table 6.2, for diffusion of C in FCC Fe Do = 2.3 x 10-5 m2/s Qd = 148,000 J/mol T = 1300 K = 1027°C

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