5 Atom and Ion Movements in Materials CBE4010 Introduction to Materials Science for Chemical Engineers 5 Atom and Ion Movements in Materials Objectives Examine the principles and applications of diffusion in materials. Discuss, how diffusion is used in the synthesis and processing of advanced materials as well as manufacturing of components using advanced materials. 5.1 Applications of Diffusion 5.2 Stability of Atoms and Ions 5.3 Mechanisms for Diffusion 5.4 Activation Energy for Diffusion 5.5 Rate of Diffusion (Fick’s First Law) 5.6 Factors Affecting Diffusion 5.7 Permeability of Polymers 5.8 Composition Profile (Fick’s Second Law) 5.9 Diffusion and Materials Processing

5.1 Applications of Diffusion Carburization for Surface Hardening of Steels p-n junction - Dopant Diffusion for Semiconductor Devices Manufacturing of Plastic Beverage Bottles-PET(polyethylene terephthalate) Oxidation of Aluminum Thermal Barrier Coatings for Turbine Blades in an aircraft engine nickel superalloy-based turbine blades/ceramic oxide(yttria stabilized zirconia(YSZ)) 5.2 Stability of Atoms and Ions Atoms and ions in their normal position in the crystal structures are not stable or at rest. The rate of atom or ion movement is related to temperature or thermal energy by the Arrhenius equation Rate = C0 exp(-Q/RT) Svante August Arrhenius (1859–1927, Sweden). Nobel Prize in Chemistry in 1903 for research on electrolytic theory of dissociation.

5.3 Mechanisms for Diffusion Self-diffusion - The random movement of atoms within an essentially pure material. By using radioactive tracers. Vacancy diffusion - Diffusion of atoms when an atom leaves a regular lattice position to fill a vacancy in the crystal. Interstitial diffusion - Diffusion of small atoms from one interstitial position to another in the crystal structure. Inter-diffusion - In an alloy, atoms tend to migrate from regions of large concentration Time  Self diffusion

5.4 Activation Energy for Diffusion Diffusion couple - A combination of a atom of a given element diffusing in a host material

5.5 Rate of Diffusion (Fick’s First Law) Fick’s first law - The equation relating the flux (atoms/cm2 sec) of atoms by diffusion to the diffusion coefficient and the concentration gradient. Diffusivity, Diffusion coefficient (D, cm2/sec) - A temperature-dependent coefficient related to the rate at which atoms, ions, or other species diffuse. Concentration gradient (dc/dx; atoms/cm3.cm or at%/cm) - The rate of change of composition with distance in a nonuniform material. Figure 5.8 The flux during diffusion is defined as the number of atoms passing through a plane of unit area per unit time Figure 5.9 Illustration of the concentration gradient

Adolf Eugen Fick (1829–1901) was a German physiologist. He earned his doctorate in medicine at Marburg in 1851. In 1855 he introduced Fick's law of diffusion, which governs the diffusion of a gas across a fluid membrane. In 1870 he was the first to devise a technique for measuring cardiac output, called the Fick principle. Fick managed to double-publish his law of diffusion, as it applied equally to physiology and physics. His work led to the development of the direct Fick method for measuring cardiac output. Fick also invented the tonometer, work that influenced his nephew (Adolf Gaston Eugen Fick (1852-1937), ) who invented the contact lens.

Example 5.4 Diffusion of Nickel in Magnesium Oxide (MgO) A 0.05 cm layer of magnesium oxide (MgO) is deposited between layers of nickel (Ni) and tantalum (Ta) to provide a diffusion barrier that prevents reactions between the two metals. At 14000C, nickel ions are created and diffuse through the MgO ceramic to the tantalum. Determine the number of nickel ions that pass through the MgO per second. The diffusion coefficient of nickel ions in MgO is 9  10-12 cm2/s, and the lattice parameter of nickel at 14000C is 3.6  10-8 cm. SOLUTION The composition of nickel at the Ni/MgO interface is 100% Ni, or The composition of nickel at the Ta/MgO interface is 0% Ni. Thus, the concentration gradient is: The total number of nickel atoms crossing the 2 cm  2 cm interface per second is: Total Ni atoms per second = (J)(Area) = (1.54  1013 atoms/cm2.s) (2 cm)(2 cm) = 6.16  1013 Ni atoms/s The flux of nickel atoms through the MgO layer is:

5.6 Factors Affecting Diffusion Temperature and the Diffusion Coefficient :

Types of Diffusion - volume diffusion grain boundary diffusion Surface diffusion Time Remarkable structures and properties are obtained by preventing diffusion. quenching - annealing process in steel metallic glasses: cooling rate ~ 106 °C/sec

Dependence on Bonding and Crystal Structure - Activation energies are usually lower for atoms diffusing through open structures than for closed-packed crystal structures. - Because the activation energy depends on the strength of atomic bonding, it is higher for diffusion of atoms in materials with a high melting temperature. Dependence on Concentration of Diffusing Species and Composition of Matrix Figure. The dependence of diffusion coefficient of Au on concentration.

Example 5.5 Design of an Iron Membrane An impermeable cylinder 3 cm in diameter and 10 cm long contains a gas that includes 0.5  1020 N atoms per cm3 and 0.5  1020 H atoms per cm3 on one side of an iron membrane. Gas is continuously introduced to the pipe to assure a constant concentration of nitrogen and hydrogen. The gas on the other side of the membrane includes a constant 1  1018 N atoms per cm3 and 1  1018 H atoms per cm3. The entire system is to operate at 700oC, where the iron has the BCC structure. Design an iron membrane that will allow no more than 1% of the nitrogen to be lost through the membrane each hour, while allowing 90% of the hydrogen to pass through the membrane per hour.

Example 5.6 Tungsten Thorium Diffusion Couple Consider a diffusion couple setup between pure tungsten and a tungsten alloy containing 1 at.% thorium. After several minutes of exposure at 20000C, a transition zone of 0.01 cm thickness is established. What is the flux of thorium atoms at this time if diffusion is due to (a) volume diffusion, (b) grain boundary diffusion, and (c) surface diffusion? (See Table 5.2.)

5.7 Permeability of Polymers Permeability is expressed in terms of the volume of gas or vapor that can permeate per unit area, per unit time, or per unit thickness at a specified temperature and relative humidity. Example. Design of Carbonated Beverage Bottles You want to select a polymer for making plastic bottles that can be used for storing carbonated beverages. What factors would you consider in choosing a polymer for this application? SOLUTION First, since the bottles are to be used for storing carbonated beverages, a plastic material with a small diffusivity for carbon dioxide gas should be chosen. The bottles should have enough strength so that they can survive a fall of about six feet. This is often tested using a ‘‘drop test.’’ The surface of the polymer should also be amenable to printing of labels or other product information. The effect of processing on the resultant microstructure of polymers must also be considered.

5.8 Composition Profile (Fick’s Second Law) Fick’s second law – the dynamic or nonsteady state, diffusion of atoms. The partial differential equation that describes the rate at which atoms are redistributed in a material by diffusion. Species Diffusion Equation on a Mass Basis: nanocm.sogang.ac.kr

The solution to the equation depends on the boundary conditions for a particular situation. nanocm.sogang.ac.kr

Case 1. One dimensional, a constant concentration of the diffusing atoms at the surface of the material (Cs). C0: The initial uniform concentration of the diffusing atoms in the material Cx: the concentration of at location x below the surface after time t.

Example 5.7 Design of a Carburizing Treatment The surface of a 0.1% C steel gears is to be hardened by carburizing. In gas carburizing, the steel gears are placed in an atmosphere that provides 1.2% C at the surface of the steel at a high temperature (Figure 5.1). Carbon then diffuses from the surface into the steel. For optimum properties, the steel must contain 0.45% C at a depth of 0.2 cm below the surface. Design a carburizing heat treatment that will produce these optimum properties. Assume that the temperature is high enough (at least 900oC) so that the iron has the FCC structure. Figure 5.1 Furnace for heat treating steel using the carburization process. (Courtesy of Cincinnati Steel Treating).

Example 5.8 Design of a More Economical Heat Treatment We find that 10 h are required to successfully carburize a batch of 500 steel gears at 900oC, where the iron has the FCC structure. We find that it costs $1000 per hour to operate the carburizing furnace at 900oC and $1500 per hour to operate the furnace at 1000oC. Is it economical to increase the carburizing temperature to 1000oC? What other factors must be considered?

Example 5.9 Silicon Device Fabrication Devices such as transistors (Figure) are made by doping semiconductors with different dopants to generate regions that have p- or n-type semiconductivity. The diffusion coefficient of phosphorus (P) in Si is D = 6.5  10-17 m2/s at a temperature of 11000C. Assume the source provides a surface concentration of 1029 atoms/m3 and the diffusion time is one hour. Assume that the silicon wafer contains no P. Calculate the depth at which the concentration of P will be 1024 atoms/cm3. State any assumptions you have made while solving this problem. . Figure. Schematic of a n-p-n transistor. Diffusion plays a critical role in formation of the different regions created in the semiconductor substrates.

5.9 Diffusion and Materials Processing Melting and Casting Sintering – A high-temperature treatment used to join small particles. Mainly ceramics and some metallic materials. Powder metallurgy - A processing route by which metal powders are pressed and sintered into dense, monolithic metallic parts.

Grain growth - Movement of grain boundaries by diffusion in order to reduce the amount of grain boundary area. Must be carefully controlled to avoid excessive grain growth, because the strength of a metallic material will decrease with increasing grain size. Diffusion bonding - A joining technique in which two surfaces are pressed together at high pressures and temperatures. Figure. Grain growth occurs as atoms diffuse across the grain boundary from one grain to another