Advanced engineering ceramics



AFRICA CENTRE OF EXCELLENCE FOR SUSTAINABLE POWER AND ENERGY DEVELOPMENT(ACE-SPED)UNIVERSITY OF NIGERIA, NSUKKACOURSE INFORMATIONCourse Code: MME772Course Title: Industrial Ceramics Topic: Introduction Academic Years: 2019/2020 Semester: Second Instructor: Engr. Prof. V.S.AIGBODION INTRODUCTION TO CERAMICSCeramics are inorganic nonmetallic materials which consist of electropositive/electronegative compounds that are bonded primarily by ionic bonds, sometimes with covalent character. The most common ceramics are composed of oxides, carbides, and nitrides. Silicides, borides, phosphides, tellurides, and selenides also are used to produce ceramics. The crystallinity of ceramic materials ranges from highly oriented to semi-crystalline, vitrified, and often completely amorphous (e.g., glasses). Most often, fired ceramics are either vitrified or semi-vitrified as is the case with earthenware, stoneware, and porcelain. Varying crystallinity and electron composition in the ionic and covalent bonds cause most ceramic materials to be good thermal and electrical insulators (extensively researched in ceramic engineering). With such a large range of possible options for the composition/structure of a ceramic (e.g. nearly all of the elements, nearly all types of bonding, and all levels of crystallinity), the breadth of the subject is vast, and identifiable attributes (e.g. hardness, toughness, electrical conductivity, etc.) are difficult to specify for the group as a whole.General properties such as high melting temperature, high hardness, poor conductivity, high moduli of elasticity, chemical resistance and low ductility are the norm, with known exceptions to each of these rules (e.g. piezoelectric ceramics, glass transition temperature, superconductive ceramics, etc.). Many composites, such as fiberglass and carbon fiber, while containing ceramic materials, are not considered to be part of the ceramic family. The earliest ceramics made by humans were pottery objects (i.e. pots or vessels) or figurines made from clay, either by itself or mixed with other materials like silica, hardened and sintered in fire. Later ceramics were glazed and fired to create smooth, colored surfaces, decreasing porosity through the use of glassy, amorphous ceramic coatings on top of the crystalline ceramic substrates. Ceramics now include domestic, industrial and building products, as well as a wide range of ceramic art. In the 20th century, new ceramic materials were developed for use in advanced ceramic engineering, such as in semiconductors. Comparison of Ceramics with Metals and PolymersIn metals, for example, atoms are relatively weakly bonded (which is why most metals are fairly soft); their electrons are shared between them in a kind of sea that can "wash" right through them, which is (simplistically speaking) why they conduct electricity and heat. A material like rubber, on the other hand, is made of long-chain molecules (polymers) that are very weakly attached to one another; that's why raw, white, latex rubber is so stretchy and why black, vulcanized rubber (like that used in car tires) is harder and stronger, because heat-and-sulfur treatment makes strong cross-links form between the polymer chains, holding them tightly together. All the electrons are locked up in bonds of various kinds (none are free to carry an electric current), and that's why rubber is generally a good insulator. The different between the three engineering materials: ceramics, polymer and metals is displayed in Table 5.1Table 5.1: Comparison of Ceramics with Metals and PolymersProperty Ceramic Metal Polymer Density Low High Lowest Hardness Highest Low Lowest Ductility Low High High Wear resistance High Low Low Thermal conductivity Mostly low High Low Electrical conductivity Mostly low High LowCeramics are different again. Their atoms are ionically bonded (like sodium and chlorine in sodium chloride, common salt), which holds them firmly in place (making ceramics hard and strong) and locks up all their electrons (so, unlike in metals, there are no free electrons to carry heat or electricity). Metals can bend, stretch, and be drawn into wires because their rows of regularly packed atoms will slide past one another. But in a ceramic, there are no rows of atoms; the atoms are either locked in a regularly repeating three-dimensional crystal or randomly arranged to make what's called an amorphous solid (a solid without a neat and tidy, internal crystalline structure).If you whack a lump of metal with a hammer, the mechanical energy you supply is dissipated as layers of atoms jump past one another; in other words, the metal bends out of shape. If you whack a ceramic such as glass, there's nowhere for that energy to go—no way for the glass to deform and soak up the blow—so it shatters instead. This explains why ceramics are both hard and brittle.Graphite is soft because it's made of layers of carbon atoms that will slide and shear (that's why a graphite pencil leaves lines on paper); diamond is hard because it has a much more rigid crystalline structure. Clay dug from the ground is soft and pliable because, like graphite, its atoms are made of flat sheets that can slip past one another, held together only by weak bonds. When you add water to clay, the polar water molecules (positively charged at one end, negative at the other end) help to pull those bonds apart, making the clay even more malleable. When you fire clay, the water evaporates and the aluminum, silicon, and oxygen atoms lock into a rigid structure made from aluminum silicate, bonded together by silicate glass—and that's why fired clay is so hard. Displayed Atomic movement of metals and CeramicsWhy do ceramics and metals behave differently? 1) You can bend metals because the atoms inside them can slide past one another fairly easily. 2) In a ceramic, the atoms are tightly bonded. If you apply too much force, the only thing a ceramic can do is break apart: the energy has nowhere else to go. 3) In metals, there are free electrons (blue) to carry heat and electricity. That's why metals are good conductors. 4) In a ceramic, the electrons are all "busy" binding atoms together and there are none spare for carrying electricity and heat. That's why ceramics tend to be good insulators (non-conductors).Classification of ceramics materialsCeramics greatly differ in their basic composition. The properties of ceramic materials also vary greatly due to differences in bonding, and thus found a wide range of engineering applications. Below is the classification of ceramics . Ceramics can be classified in diverse ways i.e. there are number of ways to classify the ceramic materials. Most commonly, the ceramics can be classified on the following basis:Classification based on compositionGlasses: Based on SiO2 + additives Traditional Ceramics(clay)Porous ceramics (bricks, pottery, china) Clay: Al2O3? SiO2?H2OCompact ceramics (porcelain, earthware) Silica: SiO2Refractory ceramics Feldspar: K2O? Al2O3 6SiO2Classification based on applicationsRefractory ceramics (SiC, Al2O3, ZrO2, BeO, MgO).Piezoelectrics and Ferroelectrics: BaTiO3, SrTiO3Electro-optics: LiNbO3Abrasive ceramics: nitrides and carbides Si3N4 ,SiCMolecular membranesSuperconductive ceramics (YBa2Cu3O7)Biomaterials : Hydroxyapatite Classification of Ceramic Materials. Composition base classification of ceramic materials Classification based on compositionOxide ceramicsOxide ceramics include alumina, zirconia, silica, aluminium silicate, magnesia and other metal oxide based materials. These are non-metallic and inorganic compounds by nature that include oxygen, carbon, or nitrogen. Oxide ceramics possess the following properties:(a) High melting points(b) Low wear resistance(c) An extensive collection of electrical propertiesThese types of ceramics are available with a variety of special features. For example, glazes and protective coatings seal porosity, improved water or chemical resistance, and enhanced joining to metals or other materials. Oxide ceramics are used in a wide range of applications, which include materials and chemical processing, radio frequency and microwave applications, electrical and high voltage power applications and foundry and metal processing. Aluminium oxide (Al2O3) is the most important technical oxide ceramic material. This synthetically manufactured material consists of aluminium oxide ranging from 80 % to more than 99 %. (Figure 5.4a and 5.4b). (a) Aluminium oxide (b) Structure of aluminium oxideSilicate ceramicsSilicates are materials generally having composition of silicon and oxygen (Figure 5.5). Four large oxygen (o) atoms surround each smaller silicon (Si) atom as shown in Figure 5.5b. The main types of silicate ceramics are based either on alumosilicates or on magnesium silicates. Out of these two, the former include clay-based ceramics such as porcelain, earthenware, stoneware, bricks etc. while the latter consists of talc-based technical ceramics such as steatite, cordierite and forsterite ceramics. Silicate ceramics are traditionally categorized into coarse or fine and, according to water absorption, into dense (< 2 % for fine and < 6 % for coarse) or porous ceramics (> 2% and > 6 %, respectively). (a) Silicate ceramics Structure of silicate ceramicsGlass ceramicsThese are basically polycrystalline material manufactured through the controlled crystallization of base glass. Glass-ceramic materials share many common characteristics with both glasses and ceramics. Glass-ceramics possess an amorphous phase and more than one crystalline phases. These are produced by a controlled crystallization procedure. Glass-ceramics holds the processing advantage of glass and has special characteristics of ceramics.Glass-ceramics yield an array of materials with interesting properties like zero porosity, fluorescence, high strength, toughness, low or even negative thermal expansion, opacity, pigmentation, high temperature stability, low dielectric constant, machinability, high chemical durability, biocompatibility, superconductivity, isolation capabilities and high resistivity. These properties can be altered by controlling composition and by controlled heat treatment of the base glass.Non-Oxide ceramicsThe use of non-oxide ceramics has enabled extreme wear and corrosion problems to be overcome, even at high temperature and severe thermal shock conditions. These types of ceramics find its application in different spheres such as pharmaceuticals, oil and gas industry, valves, seals, rotating parts, wear plates, location pins for projection welding, cutting tool tips, abrasive powder blast nozzles, metal forming tooling etc.Classification based on applicationsAbrasivesExtremely hard materials used for cutting, grinding, and polishing other materials. Due to high heat generated during the process, these materials also must have some refractory properties.Examples are diamond, SiC, WC, Corundum, and silica sandCementsCLAY +LIME-bearing minerals at 1400℃calicination→clinker →ground into a very fine powder to which is added a small amount of gypsum (CaSO4- 2H2O) to retard the setting process. The product is portland cement.Hydration reaction 2CaO-SiO2＝2CaOSiO2-XH2OMaterials that form a paste when mixed with water and then hardened by chemical reactionsExamples include cement, plaster of paris and limeNote: Concrete is a composite material made up of cement, large aggregates (gravel), and small aggregates (sand). The cement in concrete is similar to the glassy phase in ceramics. It bonds the other phases together.RefractoriesMaterials that can withstand high temperatures without melting, or decomposing.They are able to maintain insulative properties at high temperatures.They remain inert and unreactive at high temperatures.The four main categories: fireclay, silica (acid), basic and special.A decrease in the porosity of refractories will: increase strength, increase corrosion resistance and increase load-bearing capacity TYPES OF CERAMICSTraditional ceramicsTraditional ceramics refers to ceramic products that are produced from unrefined clay and combinations of refined clay and powdered or granulated non-plastic minerals. Often, traditional ceramics is used to refer to ceramics in which the clay content exceeds 20 percent. Some of the traditional ceramics in Nigeria are displayed below.Clay samples showing colour variations: (a) Afuze (orange-brown), (b) Ihitte (pink), (c) Kutigi (white), (d) Minna (orange-yellow), (e) Nsu (light brown), (f) Oboro (grey-white) and (g) Obowo (yellow- brown).Traditional ceramics are silicon -based ceramics and their example arePottery, bricks, tiles, earthenware, china, and porcelain are common examples of ceramics. These materials are well-known for use in building, crafting, and art. They are described below:Pottery is sometimes used as a generic term for ceramics that contain clay and are not used for structural, technical, or refractory purposes.Whiteware refers to ceramic ware that is white, ivory, or light gray in color after firing. Whiteware is further classified as earthenware, stoneware, chinaware, porcelain, and technical ceramics.Earthenware is defined as glazed or unglazed non-vitreous (porous) clay-based ceramic ware. Applications for earthenware include artware, kitchenware, ovenware, tableware, and tile.Stoneware is vitreous or semi-vitreous ceramic ware of fine texture, made primarily from nonrefractory fire clay or some combination of clays, fluxes, and silica that, when fired, has properties similar to stoneware made from fire clay. Applications for stoneware include artware, chemical-ware, cookware, drainpipe, kitchenware, tableware, and tile.Chinaware is vitreous ceramic ware of zero or low absorption after firing that are used for nontechnical applications. Applications for chinaware include artware, ovenware, sanitaryware, and tableware.Porcelain is defined as glazed or unglazed vitreous ceramic ware used primarily for technical purposes. Applications for porcelain include artware, ball mill balls, ball mill liners, chemical-ware, insulators, and tableware.Engineering ceramics include vitreous ceramic whiteware used for such products as electrical insulation, or for chemical, mechanical, structural, or thermal applications. Engineering ceramic products are made from highly refined natural or synthetic compositions and designed to have special properties and they are referred to as advanced ceramics. Advanced engineering ceramics Advanced ceramics are ones that have been engineered (mostly since the early 20th century) for highly specific applications. For example, silicon nitrides and tungsten carbides are designed for making exceptionally hard, high-performance cutting tools—though they do have other uses as well. Most modern engineered ceramics are metal oxides, carbides, and nitrides, which means they're compounds made by combining atoms of a metal with oxygen, carbon, or nitrogen atoms. So, for example, we have tungsten carbide, silicon carbide, and boron nitride, which are hard, cutting-tool ceramics; aluminum oxide (alumina) and silicon dioxide are used in making integrated circuits ("microchips"); and lithium-silicon oxide is used to make the heat-protective nose cones on space rockets. High-temperature superconductors are made from crystals of yttrium, barium, copper, and oxygen.Not all high-tech ceramic materials are simple compounds. Some are composite materials, in which the ceramic forms a kind of background material called the matrix, which is reinforced with fibers of another material (often carbon fibers, or sometimes fibers of a totally different ceramic). A material like this is known as a ceramic matrix composite (CMC). Examples include silicon carbide fibers in a silicon carbide matrix (SiC/SiC) with boron nitride at the interface between them—a material used in cutting-edge gas-turbine jet enginesCrystal Structure of Ceramics materials.The crystal structures of ceramics materials for which the atomic bonding is predominantly ionic are determined by the charge magnitude and the radius of each kind of ion. Percentage of ionic and covalent character of the bond for some ceramic materials determines by the crystalline structure. Two types of bonds are found in ceramics: ionic and covalent. The ionic bond occurs between a metal and a nonmetal, in other words, two elements with very different electronegativity. Electronegativity is the capability of the nucleus in an atom to attract and retain all the electrons within the atom itself, and depends on the number of electrons and the distance of the electrons in the outer shells from the nucleus. In an ionic bond, one of the atoms (the metal) transfers electrons to the other atom (the nonmetal), thus becoming positively charged (cation), whereas the nonmetal becomes negatively charged (anion). The two ions having opposite charges attract each other with a strong electrostatic force.Covalent bonding instead occurs between two nonmetals, in other words two atoms that have similar electronegativity, and involves the sharing of electron pairs between the two atoms. Although both types of bonds occur between atoms in ceramic materials, in most of them (particularly the oxides) the ionic bond is predominant.There are two other types of atomic bonds: metallic and the Van der Waals. In the first one, the metal cations are surrounded by electrons that can move freely between atoms. Metallic bonds are not as strong as ionic and covalent bonds. Metallic bonds are responsible for the main properties of metals, such as ductility, where the metal can be easily bent or stretched without breaking, allowing it to be drawn into wire. The free movement of electrons also explains why metals tend to be conductors of electricity and heat.Van der Waals bonds consist of weak?electrostatic forces between atoms that have permanent or induced polarization. An example of Van der Waal bond is the hydrogen bond between hydrogen and oxygen, which is responsible for many properties of water. Some relatively simple ceramic structures are: Sodium Chloride structure (NaCl) Cesium Chloride structure (CsCl) Zinc Blende structure (ZnS and Compound Semiconductors) Fluorite structure (CaF2)Perovskite structure (CaTiO3) Diamond Cubic structure (Carbon and Elemental Semiconductors)graphite (Carbon) Fullerenes (Carbon) Cristobalite structure (SiO2) Corundum Structure (Al2O3) Spinel Structure (MgAl2O4)/Inverse Spinel ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download