Chemistry of Engineering Materials PDF

THE CHEMISTRY OF ENGINEERING MATERIALS Classification of Solids according to the types of bonding interactions: The physical properties...

THE CHEMISTRY OF ENGINEERING MATERIALS Classification of Solids according to the types of bonding interactions: The physical properties as well as the structures of solids are dictated by the types of bonds that hold the atoms in place: Metallic Solids Ionic Solids Covalent Network Solids Molecular Solids METALLIC SOLIDS Extended networks of atoms held together by metallic bonding Properties of Metallic Solids: Electrons are mobile and no individual electron is confined to any particular metal ion high thermal conductivity - the presence of mobile electrons malleability and ductility - metal atoms form bonds with many neighbors delocalized “sea” of collectively shared valence electrons conduct electricity strong without being brittle Metallic solids are materials made up of metal atoms. Imagine a sea of free-moving electrons holding metal ions together. This unique structure gives metals their distinct properties: Conductivity: Metals conduct electricity and heat well because their free electrons can move easily. Malleability: You can hammer or roll metals into thin sheets without them breaking. Ductility: Metals can be stretched into wires without snapping. Luster: They have a shiny appearance, often reflecting light well. High melting and boiling points: Metals generally require high temperatures to change state due to strong metallic bonds. METALLIC BONDING: ELECTRON-SEA MODEL pictures the metal as an array of metal cations in a “sea” of valence electrons The electrons are confined to the metal by electrostatic attractions to the cations, and they are uniformly distributed throughout the structure.  IONIC SOLIDS Extended networks of ions held together by ion-ion interactions Properties of Ionic Solids electrostatic attraction between cations and anions do not conduct electricity well and are brittle high melting and boiling points typically electrical insulators tend to be brittle Ionic solids are materials made from positive and negative ions. Imagine a giant 3D lattice of charged particles. Here’s what makes them stand out: High melting and boiling points: Their ionic bonds are strong, requiring a lot of energy to break. Brittleness: When you apply force, they tend to shatter rather than bend because like-charged ions repel each other. Electrical conductivity in molten state: They don’t conduct electricity as solids, but when melted or dissolved in water, the ions can move freely and conduct electricity. Solubility: Many ionic solids dissolve in water because water molecules can separate the ions from each other. IONIC BONDING the electrostatic attraction between cations and anions: ionic bonds the strength of an ionic bond depends on the charges and sizes of the ions the attractions between cations and anions increase as the charges of the ions increase valence electrons are confined to the anions, rather than being delocalized, ionic compounds are ionic and metallic solids both have high melting and boiling point COVALENT-NETWORK SOLIDS Extended networks of atoms held together by covalent bonds Properties of Covalent-Network Solids extended network of covalent bonds extremely hard materials, like diamond, and it is also responsible for the unique properties of semiconductors Covalent-network solids are materials where atoms are connected by a vast network of covalent bonds, forming one big molecule. Think of a giant, unbreakable web of atoms. Their key properties are: Very high melting points: The strong covalent bonds require immense energy to break. Hardness: They are incredibly hard due to their solid structure. Brittleness: Despite their hardness, they can be brittle and crack under stress. Electrical conductivity: Usually, they don’t conduct electricity well, except for some like graphite, which has free-moving electrons. Diamond and silicon carbide are prime examples, where each carbon or silicon atom is bonded to multiple others in a robust, repeating pattern. COVALENT-NETWORK BONDING atoms held together in large networks by covalent bonds covalent bonds are much stronger than intermolecular forces These solids are much harder and have higher melting points than molecular solids EXAMPLES OF COVALENT-NETWORK SOLIDS: Properties of Diamond: carbon atom is bonded tetrahedrally to four other carbon atoms The strength and directionality of these bonds make diamonds the hardest known material. Diamond is one of the best-known thermal conductors not electrically conductive. has a high melting point, 3550 °C. Properties of Graphite: carbon atoms form covalently bonded layers - held together by intermolecular forces Each carbon is covalently bonded to three other carbons in the same layer to form interconnected hexagonal rings Electrons move freely through the delocalized orbitals, making graphite a good electrical conductor along the layers. the layers readily slide past one another when rubbed, giving graphite a greasy feel. used as a lubricant and as the “lead” in pencils. MOLECULAR SOLIDS Discrete molecules held together by intermolecular forces intermolecular forces dispersion forces dipole-dipole interactions hydrogen bonds molecular solids tend to be soft and have low melting points  MOLECULAR BONDING held together by dipole-dipole, forces, dispersion forces, and/or hydrogen bonds molecular solids are soft and have relatively low melting points (usually below 200 °C) Classification Of Solids - Types Of Bonding Interactions Structural Features of Solids The CRYSTAL LATTICE and THE UNIT CELL CRYSTAL LATTICE – The collection of points that forms a regular pattern called a crystal lattice UNIT CELL – The smallest portion that gives the crystal if it is repeated in all directions. the solid state of majority of metallic elements, some covalent compounds, and many ionic compounds occur as cubic lattices. A key parameter of any lattice is the coordination number, the number of nearest neighbors of a particle. In a cubic system, there are three types of cubic unit cells THE THREE CUBIC UNIT CELLS SIMPLE CUBIC One unit cell (1 of 3) touches the other, with no gaps, a particle at the corner or face is shared by adjacent cells. in the cubic cells, the particle at each corner is part of eight adjacent cells one-eight of each particle belongs to each cell. COORDINATION NUMBER: 6 ATOMS/UNIT CELL: 1/8 x 8 = 1 BODY-CENTERED CUBIC COORDINATION NUMBER: 8 ATOMS/UNIT CELL: (1/8 x 8)+1=2 FACE-CENTERED CUBIC COORDINATION NUMBER: 12 ATOMS/ UNIT CELL: (1/8 x 8) + (1/2 x 6)=4 (photos to study) Amorphous Solid non-crystalline solids have small somewhat ordered regions interspersed among large disordered regions Examples: charcoal, rubber, and glass ADVANCED MATERIALS Material Science has grown from solid chemistry, physics, and engineering objects that were once considered futuristic fantasies are becoming realities ultrasmall machines constructed by manipulating individual atoms and molecules Electronic Materials Perfectly ordered crystals are formed when grown very slowly and carefully under controlled conditions Crystal defects are formed when grown more rapidly planes of particles are misaligned, out of place, or missing entirely, and foreign particles replace those that belong in the lattice defects usually weaken a substance but are introduced intentionally to improve materials to increase strength, hardness, or conductivity Processes of Introducing Defects: Welding and Alloying ▪ welding causes the two types of metal atoms to intermingle and fill each other's vacancies - welded metals are stronger than pure metal ▪ alloying introduces some atoms of a second metal to occupy lattice sites of the first - alloy is harder than pure metal: e.g., brass, an alloy of copper and zinc ▪ stronger and harder since the second metal contributes additional valence electrons for metallic bonding Doping Semiconductors ▪ increasing the conductivity of semiconductors ▪ semiconductors - conducts poorly at room temperature because an energy gap separates its filled valence band from each conduction band ▪ doping - enhance conductivity adding small amounts of other elements to increase or decrease the number of valence electrons in the bands. Crystal structures and band representations of doped semiconductors. Ceramic Materials nonmetallic, nonpolymeric solids hardened by high temperature clay ceramic consist of silicate microcrystals suspended in a glassy cementing medium For example: aluminum silicate clay(kaolinite) heated to 1500oC, during heating the structure rearranges to an extended network of Si-centered and Alcentered tetrahedra of O atoms  Bricks, porcelain, glazes, and other clay ceramics are hard and resistant to heat and chemicals high-tech ceramics such as zinc oxide (ZnO) composites have been developed to have unusual electrical behavior - ceramics doped with ZnO produces a variable resistor: at low voltage conducts poorly, but at high voltage it conducts well Polymeric Materials Polymer - consists of a covalently linked chain of smaller molecules called monomers - a repeat unit of the polymer. - have hundreds to hundreds of thousands of repeat unit Synthetic polymers created in the laboratory Natural polymers (or biopolymers) created within organisms Nanotechnology: Designing Materials Atom by atom Nanotechnology the science and engineering of nanoscale system, whose sizes range from 1 to 100 nm behave neither like atoms, which are smaller atoms (1x101 nm), nor like crystals which are at least 1x105 nm For example: 5-nm particle has about half of its atoms on its surface