MSE 352 Engineering Ceramics II

Transcription

1 Kwame Nkrumah University of Science & Technology, Kumasi, Ghana MSE 352 Engineering Ceramics II 3 Credit Hours Ing. Anthony Andrews (PhD) Department of Materials Engineering Faculty of Mechanical and Chemical Engineering College of Engineering Website: Course Objectives Understanding the fundamentals of glass technology the thermodynamics and kinetics conditions necessary for the formation of glass (and glass ceramics) the structure of glass, and the relationship between composition, structure, and properties Understanding cement chemistry and concrete technology Mineral aggregates, properties and testing Portland cement composition, manufacturing process, hydration, properties and testing Mechanical properties and testing of hardened concrete Prerequisite Engineering Ceramics I (MSE 351) Phase Transformation (MSE 260) Recommended Books Materials science and engineering: An introduction; 2010 W. D. Callister Jr. and D. G. Rethwisch The science and engineering of materials; D.R. Askerland, P. P. Fulay, and W. J. Wright Fundamentals of Inorganic Glasses; A. K. Varshneya Physical Ceramics; 1997 Y.-T Chiang, D. P. Birnie, W. D. Kingery Advances in Ceramics, Volume 18-Commercial Glasses; D. C. Boyd and J. F. MacDowell (Editors) Week Course Content Topic 1 Introduction 2-3 Glass formation - Thermodynamics and kinetics considerations 4-5 Glass manufacturing composition and batch calculations 6 Glass products 7 Mid-semester exams 8 Glass toughening 9-10 Introduction to cement technology Introduction to concrete technology Forms of Assessments Quizzes 10 Assignments 5 Mid-Semester Exam 15 End Semester Exam 70 Total 100 1

2 Quiz 1 1. What are the three major classifications of ceramic materials? 2. What is crystallization? 3. List two properties that may be improved by crystallization? 4. What is the major component in a glass? Introduction: Classification of Engineering Materials 1. Metals and alloys inorganic materials composed of one or more metallic elements. 2. Ceramic and glasses inorganic materials composed of both metallic and nonmetallic elements for which inter atomic bonding is ionic or covalent, and which are generally formed at high temperatures. 3. Polymers organic materials composed of long molecular chains and networks containing carbon. 4. Composites materials where two or more of the above materials are brought together on a macroscopic scale. Introduction: Classification of Ceramics Abrasive Ceramics? Properties? Characteristics of Ceramic Materials Usually a compound between metallic and nonmetallic elements Desirable properties achieved through a high temperature heat treatment process (firing). Bonds are partially or totally ionic, and can have combination of ionic and covalent bonding Generally hard and brittle Generally electrical and thermal insulators Traditional ceramics based on clay (china, bricks, tiles, porcelain), glasses. Ceramic Crystal Structures Similar to metal crystal structures but with one important difference In ceramics, the lattice sites are occupied by IONS CATIONS: positively charged ions (the smaller of the two) electron loss TO the more electronegative atom cations are usually metals, from the left side of the periodic table ANIONS: negatively charged ions (the larger of the two) electron gain FROM the more electropositive atom anions are usually non-metals, from the right side of the periodic table Ceramic Crystal Structures 1. Charge neutrality the bulk ceramic must remain electrically neutral this means the NET CHARGE must sum to ZERO 2. Coordination number (CN) CN number of nearest-neighbor atoms as r c /r a increases, the cation s CN also increases 2

3 Coordination Number What s so important about CN? CN determines the possible crystal structures Coordination Number The stability of the crystal structure is influenced by the ion contact. Since CN determines crystal structure, and crystal structure determines physical properties, therefore, CN determines physical properties The most common coordination numbers for ceramics are 4, 6, and 8. Coordination Number Larger coordination numbers correspond to larger cation ions. Rationale?? Silicate Ceramics Composed mainly of silicon and oxygen, the two most abundant elements in earth s crust (rocks, soils, clays, sand) Basic building block: SiO 4 4- tetrahedron Si-O bonding is largely covalent, but overall SiO 4 block has charge of 4 Various silicate structures different ways to arrange SiO 4 4- blocks Silicate Structures Silicates are classified on the basis of Si-O polymerism [SiO 4 ] 4- Isolated tetrahedra Nesosilicates Examples: olivine garnet [Si 2 O 7 ] 6- Paired tetrahedra Sorosilicates Examples: lawsonite Silica = silicon dioxide = SiO 2 Every oxygen atom is shared by adjacent tetrahedra. Regular and orderly arrangement of tetrahedra units produce crystalline structure. n[sio 3 ] 2- n = 3, 4, 6 Ring silicates Cyclosilicates Examples: benitoite BaTi[Si 3 O 9 ] axinite Ca 3 Al 2 BO 3 [Si 4 O 12 ]OH Fused Silica or Vitreous Silica 3

4 Glass Structure Glass Structure Basic Unit: 4- SiO4 tetrahedron Si4+ O 2- Quartz is crystalline SiO2: Glass is noncrystalline (amorphous) Fused silica is SiO 2 to which no impurities have been added Other common glasses contain impurity ions such as Na +, Ca 2+, Al 3+, and B 3+ Na + Si 4+ O 2- (soda glass) 2-D of: (a) crystalline structure of silica; (b) amorphous structure of silica glass; (c) soda silica glass. Differences between crystalline and amorphous solids 1. Structure of atoms 2. Phase transformation behaviour Characteristics of Glasses Glass is an amorphous solid with the structure of a liquid. Glass has no distinct melting or freezing point - similar to that of amorphous alloys and amorphous polymers. Most glasses are produced by adding other oxides (e.g. CaO, Na 2 O) whose cations are incorporated within SiO 4 network. Phase transformation of crystalline and amorphous solids on heating Some other oxides (TiO 2, Al 2 O 3 ) substitute for silicon and become part of the network. Composition of Glasses All glasses contain at least 50% silica, which is known as a glass former. Other network formers - B 2 O 3, GeO 2 The composition and properties of glasses can be modified greatly by the addition of various other elements. The additives act as intermediates (TiO 2, Al 2 O 3 ) or modifiers (CaO, Na 2 O). Additives role? Examples of Glasses 1. Soda Lime Glass General purpose glass Lowest cost E.g window glass 2. Borosilicate Glass Very resistant to chemical attack Easy to cut High luminous transmission E.g. touch control panels, LCD, solar cells 4

5 Examples of Glasses 3. Lithium Potash Borosilicate Glass Relatively high operating temperature Low coefficient of thermal expansion Excellent sealing characteristics Microwave window applications 4. Glass Ceramics Have a high crystalline component to their microstructure. They have a near-zero coefficient of thermal expansion. They are strong because of the absence of the porosity found in conventional ceramics. Uses are cookware, heat exchangers, gas turbine engines Glass State: Solid or Liquid? Usually when a liquid is cooled to below its melting point, crystals form and it solidifies. Molecular arrangement in a crystal Glass State: Solid or Liquid? If the viscosity rises enough as it is cooled further, it may never crystallize. The molecules then have a disordered arrangement, but sufficient cohesion to maintain some rigidity. molecular arrangement in a glass Glass State: Solid or Liquid? Glass is a liquid which does not crystallize at lower temperatures below its melting point (metastable state). Glasses are ceramic materials with the amorphous structure of a frozen liquid. Thus any material including metals and water can be made into a glass when cooled rapidly. Therefore is glass a solid or a liquid?? Upon sufficiently fast cooling to a low enough temperature nearly any material can form a glass. sufficiently fast to ensure that while traversing the T m to T g temperature range, there is insufficient time to crystallize. low enough to ensure that the sample is brought to T< T g Vitrification Glass Formation Specific volume (1/ρ) vs Temperature (T): Specific volume Supercooled Liquid Glass (amorphous solid) Crystalline (i.e., ordered) Tg Tm Glass Formation Liquid (disordered) solid T Crystalline materials: -- crystallize at melting temp, T m -- have abrupt change in spec. vol. at T m Glasses: -- do not crystallize -- change in slope in spec. vol. curve at T g -- transparent - no grain boundaries to scatter light 5

6 Glass Formation 1. From the vapour state by condensing the vapour of the material onto a cold substrate. 2. From the liquid state quenching from the melt. The rate of quenching required depends on the material. 3. In the solid state by severely deforming the crystals, amorphous state can be obtained. Ball milling has been one of the popular examples for this method (for alloys). 6