ELE 1102 Electrical Engineering Materials INTRODUCTION • Any material can be physically characterized by several measurable quantities such as electrical conductivity, magnetic permeability, dielectric constant of the material etc. • While considering the different characteristics of material, one should keep in mind that all materials are constituted of atoms and atoms consist of nuclei and electrons. • The physical and chemical properties of the atom indicate that each atom consists of a positively charged particle – the nucleus and negatively charged particles – the electrons with their number corresponding to the atomic number of the given element in the Periodic Table. • A hydrogen atom is the simplest type of atom so that it can be considered as a model while discussing the properties of atoms and their relationship with the materials. The periodic Table Beginning of the Material Science • People began to make tools from stone – Start of the Stone Age about two million years ago. • Natural materials: stone, wood, clay, skins, etc. • The Stone Age ended about 5000 years ago with introduction of Bronze in the Far East. Bronze is an alloy (a metal made up of more than one element), copper + < 25% of tin + other elements. • Bronze: can be hammered or cast into a variety of shapes, can be made harder by alloying, corrode only slowly after a surface oxide film forms. • The Iron Age began about 3000 years ago and continues today. Use of iron and steel, a stronger and cheaper material changed drastically daily life of a common person. Age of Advanced materials • Throughout the Iron Age many new types of materials have been introduced (ceramic, semiconductors, polymers, composites…). Understanding of the relationship among structure, properties, processing, and performance of materials. • Intelligent design of new materials. Evolution of Materials • A better understanding of structure composition- properties relations has lead to a remarkable progress in properties of materials. Properties • Properties are the way the material responds to the environment and external forces. • Mechanical properties – response to mechanical forces, strength, etc. • Electrical and magnetic properties - response electrical and magnetic fields, conductivity, etc. • Thermal properties are related to transmission of heat and heat capacity. • Optical properties include to absorption, transmission and scattering of light. • Chemical stability in contact with the environment – corrosion resistance. Structure of materials • Subatomic Level: Electronic structure of individual atoms that define interaction among atoms. • Atomic Level: 3-D arrangements of atoms in materials (for the same atoms can have different properties, eg. Diamond and graphite). • Microscopic Structure: Arrangement of small grains of materials that can be identified by microscopy. • Macroscopic Structure: Structural elements that can be viewed by naked eye. Chemical classification • bonding THE ATOMIC BONDS • The Ionic Bond • The ionic bond is due to the attraction between a positive and a negative charge. Group I and II elements of the periodic table have one or two electrons in the outer shells respectively. • So these elements release the one or two electrons to get a more stable outer shell completely filled with electrons. The atoms which loose electrons and become positive ions are called electropositive atoms. (a) Sodium Atom with One Electron in the Outer Shell (b) Inert Gas Structure Formed by Sodium after Loosing Electron • There should be some negative charges also for combining with this positive charge. • Elements from groups VI and VII elements of the periodic table have six or seven electrons in the outer shells. These elements will accept the electrons released from the group I and II elements and become stable. Thus the atoms will become negative ions and are called electronegative atoms. (a) Chlorine Atom with Seven Electrons in the Outer Shell (b) Inert Gas Structure Formed by Sodium after Receiving One Electron Formation of Sodium Chloride Molecule by Ionic Bonding • Since, the electropositive atoms tend to give up electrons and the electronegative atoms always tend to receive them, they will easily become ions if brought together. • For example, sodium, which is an electropositive element, has a single electron in its outer shell and chlorine, which is an electronegative element, has seven electrons in its outer shell. • Now when they would be brought together, the sodium atom will become a positively charged ion loosing the single electron from its outer shell and the chlorine atom will be negatively charged by gaining an extra electron. • Since these ions are of opposite charge they will attract each other such that the sodium ion will move closer to the chloride ion until the electrons in the outer shells of the two atoms begin to repel each other. Formation of Sodium Chloride Molecule by Ionic Bonding • At a certain value of the interionic separation, there will be a balance in between the attractive force and the repulsive force and at this point the atom will become stable. • This interionic separation is called the equilibrium separation, d0. Relation between Interionic Separation and Force The Covalent Bond • In groups III, IV and V, there are three or more electrons in the outer shell and so the energy required to form the ions is usually very large. So these elements cannot form an ionic bond, instead of this they fill their outer shells by sharing electrons from other atoms. • For example, carbon, a group IV element, has four electrons in the outer shell. So to become stable it should gain four more electrons in the outer shell. It is well known that the distribution of charge in an unfilled shell is not spherically symmetric. Hence, the electron clouds in carbon atom will be pointing out to some particular direction depending on the position of the largest probability and the wave function. Structure of CH4 Atom Now if four hydrogen atoms are brought closer to these valence electrons then the electron orbits of the hydrogen atom will begin to overlap until the electrons are being shared by the hydrogen and the carbon. Bonding can only take place in the same direction as of the electron clouds. The covalent bonded molecules have a bond of greater strength between the constituent atoms. But, since the atoms in the molecule have all been filled by sharing electrons, the molecules themselves are not attracted to each other. The Metallic Bond • Generally, all metals have a small number of electrons in their outer shell, which can easily be removed to form a metal ion. Now depending upon the wave function and the probability functions, these metallic ions are bonded together to form a solid. The van der Waals Bond • The atoms consist of a positive charged nucleus and the electrons around the nucleus. • Sometimes the electrons change their position, and so the centers of positive and negative charges will not always coincide. Thus the atoms can be regarded as fluctuating dipoles. • If atom X has a dipole moment, then it will induce an opposite dipole moment on atom Y. On average there will be an attractive force, since the tendency described always leads to attraction. • This attraction is called a van der Waals bond. Such bonds are responsible for the formation of organic crystals. Mixed Bond • Many materials usually form molecules and solids with a combination of primary and secondary bonds. For example, in case of hydrogen atom mostly covalent bonds form organic compounds and sometimes ionic bonds. • Group I atoms tend to loose electrons while group VII atoms do not loose electrons. So, the group I atoms have lower ionization potential than the group VII atoms. • The ionization potential is the energy required to move an electron from the outer shell of an atom. The ionization potential normally increases with the group number and in the same group it decreases with the increase in the atomic number. Metals • Examples iron (Fe), copper (Cu), aluminum (Al), nickel (Ni), titanium (Ti). Non metallic elements such as carbon (C), nitrogen (N) and oxygen (O) may also be contained in metallic materials. • Metals usually are good conductors of heat and electricity. Metals have a crystalline structure in which the atoms are arranged in an orderly manner. • Also, they are quite strong but malleable and tend to have a lustrous look when polished. Metallic Materials Ceramics: • They are generally compounds between metallic and nonmetallic elements chemically bonded together and include such compounds as oxides, nitrides, and carbides. • Ceramic materials can be crystalline, non-crystalline, or mixtures of both. • Typically they have high hardness and high-temperature strength but they tend to have mechanical brittleness. They are usually insulating and resistant to high temperatures and harsh environments. • Ceramics can be divided into two classes: traditional and advanced. • Traditional ceramics include clay products, silicate glass and cement; while advanced ceramics consist of carbides (SiC), pure oxides (Al2O3), nitrides (Si3N4), non-silicate glasses and many others. Ceramic materials Plastics • Plastics or polymers are substances containing a large number of structural units joined by the same type of linkage. • These substances often form into a chain-like structure and are made of organic compounds based upon carbon and hydrogen. • Usually they are low density and are not stable at high temperatures. • Polymers already have a range of applications that far exceeds that of any other class of material. Current applications extend from adhesives, coatings, foams, and packaging materials to textile and industrial fibers, composites, electronic devices, biomedical devices, optical devices, and precursors for many newly developed high-tech ceramics. Today, the polymer industry has grown to be larger than the aluminum, copper and steel industries combined. Plastic Materials Semiconductors (Electronic Materials) • Semiconductors are materials which have a conductivity between conductors (generally metals) and nonconductors or insulators (such as most ceramics). Semiconductors can be pure elements, such as silicon or germanium, or compounds such as gallium arsenide or cadmium selenide. In a process called doping, small amounts of impurities are added to pure semiconductors causing large changes in the conductivity of the material. • Due to their role in the fabrication of electronic devices, semiconductors are an important part of our lives. Semi-conductor materials Composites • Composites consist of a mixture of two or more materials. Most composite materials consist of a selected filler or reinforcing material and a compatible resin binder to obtain the specific characteristics and properties desired. • Usually, the components do not dissolve in each other and can be physically identified by an interface between the components. • Fiberglass, a combination of glass and a polymer, is an example. • Concrete and plywood are other familiar composites. • Many new combinations include ceramic fibers in metal or polymer matrix. Biomaterial • A biomaterial is "any substance (other than drugs) or combination of substances synthetic or natural in origin, which can be used for any period of time, as a whole or as a part of a system which treats, augments, or replaces any tissue, organ, or function of the body". • Biocompatibility— The ability of a material to perform with an appropriate host response in a specific application • Host Response— The response of the host organism (local and systemic) to the implanted material or device. Biomaterial Future of Materials Science • Design of materials having specific desired characteristics directly from our knowledge of atomic structure. • Miniaturization • Smart materials • Environment-friendly materials • Learning from Nature • Materials for lightweight batteries with high storage densities, for turbine blades that can operate at 2500°C, room-temperature superconductors? Chemical sensors (artificial nose) of extremely high sensitivity. • Moore’s Law: Computer chips (processors, memory, etc.) will double their complexity every 12-24 months. Smart materials • Smart materials are those that respond to environmental stimuli in a timely manner with particular changes in some variables. • These are materials that receive, transmit or process a stimulus and respond by producing a “useful” reversible effect. • The piezoelectric effect is: 1. the production of a voltage when a crystal plate is subjected to mechanical pressure or when it is physically deformed by bending. 2. The physical deformation of the crystal plate (bending) when it is subjected to a voltage.
