chap.4-1材料概论第四章

Chapter 4 The Glass Science玻璃科学14.1 the World of Glasses 4.1.1 Natural Glasses天然玻璃 The volcanic outflow of the molten magma from the earth: get cooled and form glasses.在从地球的熔融岩浆火山流出：冷却和形成得到玻璃。 Impactites 冲击岩 : formed from crystalline结晶 materials rendered 致使 amorphous无定形 by the shock休克 of meteoric流星 impact. 2 Glasses: formed from impacted melts are known as tektites 玻殒石 . Tektites: over the world Australia, Indian Ocean, Europe, Libyan desert Composition: alumino-silicate glasses At least 700,000 years old Micro-tektites: diameter 90% : is based on oxides and in particular silica6 B2O3 and borate glasses P2O5 and phosphate glasses Complex oxide glasses Chalcogenide 硫化物 Glasses:In the importance, chalcogenides are next to oxide glasses. v Use: for infrared transmission74.2 Preparation Technologies of Glasses Glass: a solid obtained by supercooling a liquid, it is X-ray amorphous. Tg: glass transition temperatureAt Tg, second derivative thermodynamic properties, namely heat capacity, thermal expansivity and compressibility undergo more or less sudden changes.8 Tm: melting temperature crystalline product: when a liquid is cooled to its Tm, it generally solidifies to a crystalline product. not crystallize: when it is cooled rapidly. supercooling process9Dependence of rate of crystallization of an undercooled liquid on temperature 104.2.1 Preparation of Glasses The required rate of cooling: is remarkably low because the maximum crystallization velocities in these materials are themselves very low. High rates of quenching : it appears that any liquid can be quenched into a glassprovided the required high degree ofquenching rate is achieved. 114.2.2 Microwave Heating replacing conventional heating, rapid volumetric or bulk heating temperature of melt remain constant short duration of time , homogenizing 124.2.3 Sol-gel Process 溶胶凝胶 Sol: is a suspension 悬浮液 of colloidal particles, which are of sub-micron or nanometer size. Sol-gel procedure: provides a convenient route to make glasses. Catalyst: is needed, otherwise the process is very slow .134.2.3 Chemical Nature of Glass Forming Materials Glasses types: covalent, ionic, molecular, metallic and hydrogen bonded materials. Made from: elements, simple chemical compounds, complex organic molecules, salt mixtures and alloys. 14 key factor: is just how fast the melt can be quenched. other factors: structural aspects154.2.4 Structural Approach to Glass Formation Rules for glass formation (glassAmOn) : oxygen atom may be linked to less than two A atoms. the number of oxygen atoms surrounding A atoms must be small.16 the oxygen polyhedra 多面体 share only coners with each other, neither edges nor faces. at least three coners in each polyhedron must be shared.174.3 Structure and Properties of Glasses Knowledge: from experimental measurements Properties: are essentially composition-dependent, controlled by the first neighbor shell of the network formers or properties that are the result of generalized disorder. 184.3.1 Continuous Random Network Model Basic unit: SiO2 glass Character: continuous random network (crn model) The primary polyhedral unit remains the same in glass as in its crystalline phase in the crn model. 19 Short range order (SRO): a local or short range configuration with a length scale defined by the first neighbour distance. Loss of periodicity: Extension of bond lengths and bond angles within reasonable limits in combination with rotation around single bonds over long distances. 20 Intermediate range order (IRO) / medium range order (MRO): the length scale over which a reasonable semblance to some positional ordering is still retained defines. Long range order (LRO): positional ordering over long distances. It is a characteristic of a crystalline state. Its absence defines the glassy state. 21 One of the requirements to apply crn model: the glass forming compound must have strong and highly covalent directional bonds. Metallic glasses: obtained by rapid quenching from their melts require a different structural model known as random close packing, rcp, (of spheres) model. 224.3.2 Random Close Packing(RCP) Model Non-directional bonding: ionic, metallic or van der Waals CRN & RCP models: describe two rather distinct classes of glass structures, but there appears to be a hidden logical relation between them. Weakness of structural approaches: the nature of bonding is not clearly taken into consideration 234.3.3 Bonding Theories An essential requirement for glass formation: mixed ionic and covalent bonding Reasons: purely covalent bonding: induce crystalline order. a completely ionic bonding: long range potentials induce crystalline order. 24 Rules for glass formation: high bond energy and covalency: help glass formation. tendency towards formation of polymeric species: introduces glass formation tendency. the nature of bonding: determines structure + melting temperatures + the rheological 流 变学的 properties, which determine glass formation.254.3.4 Kinetic Criteria for Glass Formation Melting: is a thermodynamic first order transition Tm: crystallization takes place rapidly at some temperature. The structural relaxation time of the supercooled liquid keeps the melt from crystallizing viscosity. Additives: increase the viscosity of the system. 264.3.5 Thermal Behaviour of Glasses Glass transition: the second important characteristic of the glassy state, to distinguish glasses from other amorphous materials. Glass transition region: marked by changes in several properties of the supercooled melt