LECTURE 9.2

LECTURE OUTLINE Weekly Reading Weekly Reading Practice Quiz #9: Feedback Practice Quiz #9: Feedback Deformation of Materials Deformation of Materials

CHAPTER XXVIII: ELECTRONIC PROPERTIES I Chapter 28 presents the basic concepts necessary to understand the differences among an electrical insulator, an electrical conductor, and a semi-conductor. Building on concepts first introduced in Chapters 14 and 17, it is shown that, in solids, electricity is “carried” by electrons. Hence, metals with their high density of free, mobile electrons are good electrical conductors, while non-metals are typically electrical insulators. A series of Gedankenexperiments is used to draw the connection between such apparently diverse phenomena as heat flow, fluid flow, and electrical currents. It is demonstrated that all of these “fluxes” occur “down” a gradient in pressure. Chapter 28 presents the basic concepts necessary to understand the differences among an electrical insulator, an electrical conductor, and a semi-conductor. Building on concepts first introduced in Chapters 14 and 17, it is shown that, in solids, electricity is “carried” by electrons. Hence, metals with their high density of free, mobile electrons are good electrical conductors, while non-metals are typically electrical insulators. A series of Gedankenexperiments is used to draw the connection between such apparently diverse phenomena as heat flow, fluid flow, and electrical currents. It is demonstrated that all of these “fluxes” occur “down” a gradient in pressure.

CHAPTER XXVIII: ELECTRONIC PROPERTIES I A form of Ohm's Law is presented, but one that emphasizes the relationship between the flow of electricity and a material dependant parameter, called the conductivity. It is demonstrated that for metallic conductors, the magnitude of the current depends on A form of Ohm's Law is presented, but one that emphasizes the relationship between the flow of electricity and a material dependant parameter, called the conductivity. It is demonstrated that for metallic conductors, the magnitude of the current depends on the number of free electrons per atom, the number of free electrons per atom, the drift velocity of the electron. the drift velocity of the electron. The concept of an ionization energy (Chapter 14) is reintroduced, and it is shown how this energy is related to the “band-gap” energy. The concept of an ionization energy (Chapter 14) is reintroduced, and it is shown how this energy is related to the “band-gap” energy. As a prelude to Chapter 29, the electrical conductivities of metals are discussed.

CHAPTER XXIX: ELECTRONIC PROPERTIES II The subject matter of this chapter is, potentially, the most conceptually difficult to grasp. Electrical phenomena always seem obscure, and never more so than when semiconductors are mentioned. In the current chapter, the major goal is to present the fundamental science of electronics in a simple yet coherent fashion. The chapter describes the creation of a free and mobile electron through the promotion of an outer-shell electron. The subject matter of this chapter is, potentially, the most conceptually difficult to grasp. Electrical phenomena always seem obscure, and never more so than when semiconductors are mentioned. In the current chapter, the major goal is to present the fundamental science of electronics in a simple yet coherent fashion. The chapter describes the creation of a free and mobile electron through the promotion of an outer-shell electron. It is also noted that when an outer-shell, or valence, electron is removed, an electron “vacancy” must be left in the valence shell. Conventionally, this electron vacancy is called an “electron hole.” Any atom that possesses an electron hole in its outer shell is in a high-energy state and will try to fill this vacancy by stealing an outer-shell electron from a neighboring atom. Hence, the electron hole can move from atom to atom. The “motion” of this electron hole actually creates an electronic current. The motion of both free electrons and electron holes in semiconductors is explored.

CHAPTER XXIX: ELECTRONIC PROPERTIES II The energy that is needed to create an “electron hole pair” is described with the aid of both the ionization energy (Chapter 14) and the band-gap energy (Chapter 28). The energy that is needed to create an “electron hole pair” is described with the aid of both the ionization energy (Chapter 14) and the band-gap energy (Chapter 28). The reader is introduced to the concept of extrinsic semiconductors through the intentional “doping” of the Group IV elements silicon (Si) and germanium (Ge) with Group III or Group V elements. Most importantly, the use of semiconductors in devices such as “light meters,” rectifiers, and solar cells is described.

PRACTICE QUIZ #9: FEEDBACK

Q1-3. A polymer rod has been elongated elastically by 200 mm by a 1,000 kg weight. The original length was 1,000 mm and the cross sectional area was 2 mm 2. The stress developed in the rod is (1)______MPa. The strain is (2)_______. The Young’s modulus of the rod is (3) _____ MPa. (1)a) 250 b) 500 c) 1,000 d) 2,000 (2)a) 0.1 b) 0.2 c) 0.5 d) 1.0 (3)a) 1,000 b) 1,500 c) 2,000 d) 2,500

Stress is defined as: Stress = Load = 1,000 = 500 MPa Area 2 (Note that the area is given as 2mm 2.) Strain is defined as: Strain = Elongation Original Length (Note that the elongation is given, not the final length.) Or Strain = 200 = 0.2 1,000

Young's modulus is defined as: Young's modulus = Stress = 500 = 2,500MPa Strain 0.2 Answers: (1) b(2) b(3) d

Q4-6. The above figure is a stress-strain curve for a metallic material. You may assume that the material was tested to failure. The yield strength is (4)______MPa. The elastic strain is (5)_____MPa. The Ultimate Tensile Strength is (6)______Mpa. (4) a) 17.5 b) 20 c) 22.5 d) 25 (5) a) 2 b) 4 c) 6 d) 8 (6) a) 17.5 b) 20 c) 22.5 d) 25

The yield stress (or strength) is given by the stress at which the stress-strain curve first departs from linearity. The elastic strain is also given by the strain at the yield stress. The Ultimate Tensile Strength (or stress) is given by the last “datum point” (i.e., the stress at which the material breaks). Answers: (4) b(5) b(6) d

Q7-9. This figure is the stress-strain curve for a metallic material. Select the letters a, b, c, or d to indicate which part or point of the curve matches the description. (7) The region in which the material obeys Hooke’s law. (8) The region within which the alloy is plastically deforming. (9) The region within which the alloy behaves elastically.

The initial portion of the stress-strain curve, region a, corresponds to elastic deformation— the region within which stress is proportional to strain and the material obeys Hooke’s law. Beyond point b (i.e., in region c), the material deforms plastically. Answers: (7) a(8) c(9) a

Q The figure on the left is a histogram of the Young’s moduli of a series of materials, while the figure on the right depicts schematic stress-strain curves for (in no particular order) tungsten, rubber, diamond, quartz, and molybdenum. Select the letters A, B, C, D or E to indicate which stress- strain curve corresponds to the following: (10) quartz (11) molybdenum (12) tungsten

The materials, in decreasing order for Young’s modulus, are diamond, tungsten, molybdenum, quartz, and rubber. Because the initial slope of the stress-strain curve is numerically equal to Young’s modulus, each stress-strain curve can be associated with one of the materials, such that E = diamond, D = tungsten, C = molybdenum, B = quartz, and A = rubber. Answers: (10) B(11) C(12) D

DEFORMATION OF MATERIALS Plastic Deformation of Metals Plastic Deformation of Metals The Brittle Nature of Ceramics The Brittle Nature of Ceramics The Dislocation The Dislocation The “Carpet-Ruck” Analogy The “Carpet-Ruck” Analogy

PLASTIC DEFORMATION OF A METAL I

PLASTIC DEFORMATION OF A METAL 2

THE DEFORMATION OF A COVALENT CERAMIC

THE DEFORMATION OF AN IONIC CERAMIC

VARIOUS DEFECTS, AND THE DISLOCATION I

THE DISLOCATION II

THE DISLOCATION III

THE DISLOCATION IV

THE DISLOCATION V

THE DISLOCATION VI

THE DISLOCATION VII