Diffusion in Materials

7 Key excerpts on "Diffusion in Materials"

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  eBook - PDF

  The Science and Engineering of Materials, Enhanced, SI Edition

  • Donald Askeland, Wendelin Wright, Donald Askeland(Authors)
  • 2020(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  5-1 Applications of Diffusion Diffusion Diffusion refers to the net flux of any species, such as ions, atoms, electrons, holes (Chapter 19), and molecules. The magnitude of this flux depends upon the concentration gradient and temperature. The process of diffusion is central to a large number of today’s important technologies. In materials processing technologies, control over the diffusion of atoms, ions, molecules, or other species is key. There are hundreds of applications and technologies that depend on either enhancing or limiting diffusion. The following are just a few examples. Carburization for Surface Hardening of Steels Let’s say we want a surface, such as the teeth of a gear, to be hard; however, we do not want the entire gear to be hard. The carburization process can be used to increase surface hardness. In carburization, a source of carbon, such as a graphite powder or gaseous phase containing carbon, is diffused into steel components such as gears (Figure 5-1). The increased carbon concentration on the surface of the steel increases the steel’s hardness because the carbon atoms in interstitial sites hinder dislocation motion. Similar to the introduction of carbon, we can also use a process known as nitriding, in which nitrogen is introduced into the surface of a metallic material. Diffusion also plays a central role in the control of the phase transformations needed for the heat treatment of metals and alloys, the processing of ceramics, and the solidification and joining of materials (see Section 5-9). Dopant Diffusion for Semiconductor Devices The entire microelectronics industry, as we know it today, would not exist if we did not have a very good understanding of the diffusion of different atoms into silicon or other semiconductors. The creation of the p-n junction (Chapter 19) involves diffusing dopant atoms, such as phosphorus, arsenic, antimony, boron, aluminum, etc., into precisely defined regions of silicon wafers.

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  The Physical Chemistry of Materials

  Energy and Environmental Applications

  • Rolando Roque-Malherbe(Author)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  219 5 Diffusion in Materials 5.1 INTRODUCTION Diffusion is the random movement of molecules or small particles taking place due to the motion caused by thermal energy [1–20]. It is a general property of matter linked with the propensity of systems to occupy all accessible states [20]. In a more simple way, diffusion is a spontaneous tendency of all systems to equalize concentration, if any external influence does not impede this process. Specifically, atoms, molecules, or any particle chaotically moves in the direction where less elements of its own type are located. Diffusion in dense materials is a significant area of materials science. Diffusion plays a crucial role in the kinetics of numerous processes taking place during the processing of dense materials, such as phase transformation, high-temperature oxidation, permeation, precipitation, ion conduction, sintering, and other processes. Besides, diffusion of gases in porous materials is an important topic as well, because this process is crucial in catalysis, gas chromatography, and gas separation. In this chapter, diffusion in solid materials, that is, metals, oxides, and nanoporous crystalline, ordered, and amorphous materials is discussed. We first study diffusion in a phenomenological, general form; afterward the diffusion of atoms in crystals by means of knowledge obtained from studies of diffusion in metals is discussed. Thereafter, those phenomena that are exclusive to oxides are separately discussed. Finally, diffusion in nanoporous materials is described. 5.2 FICK’S LAWS The quantitative study of diffusion started in 1850–1855 with the works of Adolf Fick and Thomas Graham. From the conclusion of his studies, Fick understood that diffusion obeys a law isomorphic to the Fourier law of heat transfer [17].

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  Materials Science and Engineering

  An Introduction

  • William D. Callister, Jr., David G. Rethwisch(Authors)
  • 2018(Publication Date)
  • Wiley

    (Publisher)

  The process by which atoms of one metal diffuse into another is termed interdiffusion, or impurity diffusion. Interdiffusion may be discerned from a macroscopic perspective by changes in con- centration that occur over time, as in the example for the Cu–Ni diffusion couple. There is a net drift or transport of atoms from high- to low-concentration regions. Diffusion also occurs for pure metals, but all atoms exchanging positions are of the same type; this is termed self-diffusion. Of course, self-diffusion is not normally subject to observation by noting compositional changes. diffusion Tutorial Video: What Is Diffusion? interdiffusion impurity diffusion self-diffusion 5.1 INTRODUCTION 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 can often be predicted by using the mathematics of diffusion and appropriate diffusion constants. The steel gear shown on page 135 (top) has been case hardened (Section 8.10)—that is, its hardness and resistance to failure by fatigue have been enhanced by diffusing excess carbon or nitrogen into the outer surface layer. 122 • 5.2 Diffusion Mechanisms • 123 From an atomic perspective, diffusion is just the stepwise migration of atoms from lattice site to lattice site. In fact, the atoms in solid materials are in constant motion, rapidly chang- ing positions. For an atom to make such a move, two conditions must be met: (1) there must be an empty adjacent site, and (2) the atom must have sufficient energy to break bonds with its neighbor atoms and then cause some lattice distortion during the displacement.

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  Callister's Materials Science and Engineering

  • William D. Callister, Jr., David G. Rethwisch(Authors)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  The process by which atoms of one metal diffuse into another is termed interdiffusion, or impurity diffusion. Interdiffusion may be discerned from a macroscopic perspective by changes in con- centration that occur over time, as in the example for the Cu–Ni diffusion couple. There is a net drift or transport of atoms from high- to low-concentration regions. Diffusion also occurs for pure metals, but all atoms exchanging positions are of the same type; this is termed self-diffusion. Of course, self-diffusion is not normally subject to observation by noting compositional changes. diffusion interdiffusion impurity diffusion self-diffusion 5.1 INTRODUCTION 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 can often be predicted by using the mathematics of diffusion and appropriate diffusion constants. The steel gear shown on page 129 (top) has been case hardened (Section 8.10)—that is, its hardness and resistance to failure by fatigue have been enhanced by diffusing excess carbon or nitrogen into the outer surface layer. 130 • 5.2 Diffusion Mechanisms • 131 From an atomic perspective, diffusion is just the stepwise migration of atoms from lattice site to lattice site. In fact, the atoms in solid materials are in constant motion, rapidly chang- ing positions. For an atom to make such a move, two conditions must be met: (1) there must be an empty adjacent site, and (2) the atom must have sufficient energy to break bonds with its neighbor atoms and then cause some lattice distortion during the displacement.

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  Essentials of Materials Science and Engineering, SI Edition

  • Donald Askeland, Wendelin Wright(Authors)
  • 2018(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  There are situations under which these conditions may not be met and hence the concentration profile evolution will not be predicted by the error-function solution given by Equation 5-7. If the boundary conditions are different from the ones we assumed, different solutions to Fick’s second law must be used. 5-9 Diffusion and Materials Processing We briefly discussed applications of diffusion in processing materials in Section 5-1. Many important examples related to solidification, phase transformations, heat treat-ments, etc., will be discussed in later chapters. In this section, we provide more informa-tion to highlight the importance of diffusion in the processing of engineered materials. Diffusional processes become very important when materials are used or processed at elevated temperatures. Melting and Casting One of the most widely used methods to process metals, alloys, many plastics, and glasses involves melting and casting of materials into a desired shape. Diffusion plays a particularly important role in solidification of metals and alloys. During the growth of single crystals of semiconductors, for example, we must ensure that the differences in the diffusion of dopants in both the molten and solid forms are accounted for. This also applies for the diffusion of elements during the casting of alloys. Similarly, diffusion plays a critical role in the processing of glasses. In inorganic glasses, for instance, we rely on the fact that diffusion is slow and inorganic glasses do not crystallize easily. We will examine this topic further in Chapter 9. Sintering Although casting and melting methods are very popular for many manufactured materials, the melting points of many ceramic and some metallic materials are too high for processing by melting and casting. These relatively refractory materials are manufactured into useful shapes by a process that requires the consolidation of small particles of a powder into a solid mass (Chapter 15).

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  Fundamentals of Materials Science and Engineering

  An Integrated Approach

  • William D. Callister, Jr., David G. Rethwisch(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  WHY STUDY Diffusion? Learning Objectives After studying this chapter, you should be able to do the following: 1. Name and describe the two atomic mechanisms of diffusion. 2. Distinguish between steady-state and nonsteady- state diffusion. 3. (a) Write Fick’s first and second laws in equation form and define all parameters. (b) Note the kind of diffusion for which each of these equations is normally applied. 4. Write the solution to Fick’s second law for diffu- sion into a semi-infinite solid when the concen- tration of diffusing species at the surface is held constant. Define all parameters in this equation. 5. Calculate the diffusion coefficient for a material at a specified temperature, given the appropriate diffusion constants. 6. Note one difference in diffusion mechanisms for metals and ionic solids. Many reactions and processes that are important in the treatment of materials rely on the transfer of mass either within a specific solid (ordinarily on a microscopic level) or from a liquid, a gas, or another solid phase. This is necessarily accomplished by diffusion, the phenomenon of material transport by atomic motion. This chapter discusses the atomic mechanisms by which diffusion occurs, the mathematics of diffusion, and the influence of temperature and diffusing species on the rate of diffusion. The phenomenon of diffusion may be demonstrated with the use of a diffusion couple, which is formed by joining bars of two different metals together so that there is intimate contact between the two faces; this is illustrated for copper and nickel in Figure 6.1a, which includes schematic representations of atom positions and composition across the interface. This couple is heated for an extended period at an elevated temperature (but below the melting temperatures of both metals) and cooled to room temperature.

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  eBook - PDF

  Fundamentals of Materials Science and Engineering

  An Integrated Approach

  • William D. Callister, Jr., David G. Rethwisch(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  This is necessarily accomplished by diffusion, the phenomenon of material transport by atomic motion. This chapter discusses the atomic mechanisms by which diffusion occurs, the mathematics of diffusion, and the influence of temperature and diffusing species on the rate of diffusion. The phenomenon of diffusion may be demonstrated with the use of a diffusion couple, which is formed by joining bars of two different metals together so that there is intimate contact between the two faces; this is illustrated for copper and nickel in Figure 6.1, which includes schematic representations of atom positions and composition across the interface. This couple is heated for an extended period at an elevated temperature (but below the melting temperatures of both metals) and cooled to room temperature. Chemical analysis reveals a condition similar to that represented in Figure 6.2—namely, pure copper and nickel at the two extremities of the couple, separated by an alloyed diffusion 6.1 INTRODUCTION Tutorial Video: What Is Diffusion? 188 • Chapter 6 / Diffusion region. Concentrations of both metals vary with position as shown in Figure 6.2c. This result indicates that copper atoms have migrated or diffused into the nickel, and that nickel has diffused into copper. The process by which atoms of one metal diffuse into another is termed interdiffusion, or impurity diffusion. Interdiffusion may be discerned from a macroscopic perspective by changes in con- centration that occur over time, as in the example for the Cu–Ni diffusion couple. There is a net drift or transport of atoms from high- to low-concentration regions. Diffusion also occurs for pure metals, but all atoms exchanging positions are of the same type; this is termed self-diffusion. Of course, self-diffusion is not normally subject to observation by noting compositional changes.

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