MSE 352 Engineering Ceramics II

Transcription

1 Kwame Nkrumah University of Science & Technology, Kumasi, Ghana MSE 352 Engineering Ceramics II Ing. Anthony Andrews (PhD) Department of Materials Engineering Faculty of Mechanical and Chemical Engineering College of Engineering Website: 1. Release of gases Gases expand to large volumes: CaCO 3 CaO + CO 2 One mole limestone: 37 cm 3 CaO; 22,400 cm 3 CO 2 Gases produce a considerable stirring/homogenization effect Gases lead to the formation of large number of bubbles Hygroscopic components 2. Formation of liquid phases by Direct melting of batch components Melting of eutectic mixtures Melting of Cullet Liquid phase reactions are much faster than solid-state reactions; addition of cullet, fluxes accelerate various batch reactions 2. Formation of liquid phases Viscosity effect Dissolution of refractory particles increases with increasing temperature Viscosity increases with increasing rate of dissolution Final stage of melting process occurs much more slower due to high viscosity Batch-free time: time required to completely dissolve the original batch 3. Volatilization of melt components Oxide (liquid) Oxide (gas) Alkali oxides (Li<Na<K<Rb<Cs) Pb, B, P, halides have relatively high vapor pressures at high temperatures Glass composition can vary from that expected from the batch 1

2 Molar Volume 4. Fining Reactions: removing bubbles Trapping of atmospheric gases by fine particles Gases created by batch decomposition reactions Gases created by refractory corrosion reactions, etc. Bubbles created by precipitation from melts with supersaturated gases Homogenizing of Melt The fluid produced during the initial batch decomposition process is very heterogeneous. Heterogeneity is reduced by stirring action of rising bubbles. Removal of bubbles by buoyancy effects Rate of bubble rise, V b (gδρr 2 )/3η g is gravitational constant; Δρ is density difference between gas and melt; r is bubble radius η is viscosity Homogenizing of Melt Use of fining agents creates large bubbles sweeping smaller ones Increasing the melt temperature reduces the oxide: As 2 O 5 As 2 O 3 + O 2 more bubbles are swept away Reducing the process temperature reverses the equilibrium: As 2 O 3 + O 2 As 2 O 5 : pulls O 2 out of remaining bubbles Melting and Refining Continuously operated tank furnaces are used for glass melting. Heating uses oxy-fuel or air-fuel burners, or direct electrical (Joule) heating, or a combination of both. Natural gas commonly used. The use of oxygen to replace combustion air helps to reduce NO x emissions. Cullet Cullet is recycled glass that is added to the raw material batch. The energy savings may be quite significant. Supply of quality cullet requires crushing, cleaning, sorting and transportation. The color separation of cullet is essential to optimize its use for each type of glass product Glass Formation is Controlled by Kinetics Liquid may have a high viscosity that makes it difficult for atoms of the liquid to diffuse (rearrange) into the crystalline structure Liquid maybe cooled so fast that it does not have enough time to crystallize glass liquid Temperature 2

3 Viscosity of Glass Forming Melts Glasses are most easily formed if either a) the viscosity is very high at the melting temperature of the crystalline phase which would form from the melt, or b) if the viscosity increases very rapidly with decreasing temperature. Viscosity determines the melting conditions necessary for bubble-free and homogeneous melts temperature of annealing upper use of any glass object and devitrification conditions Viscosity Definition Relates shear stress ( ) and velocity gradient (dv/dy) Measure of fluid resistance to gradual deformation by shear stress and tensile stress t glass η has units of (Pa-s) or Poise 1 Pa.s = 10 P t dv dv dy dy velocity gradient dv / dy Simulation of substances with different viscosities Viscosity dependence on nucleation and growth rate of homogeneous nucleation: Viscosity and bubble rise rate of crystal growth (m/s): v = rate of rise of bubbles r = bubble radius ρ = density of melt log Pa.s log P Melting Pt Working Pt Softening Pt 4x10 6 4x10 7 Annealing Pt Straining Pt 3x x

4 Properties of glass required for blowing 1. Chemical structure of glass itself. 2. Viscosity of the molten glass. 3. Surface tension Effects of Composition on Viscosity Viscosity is determined by: 1. Molecular attractive forces, especially associated with glass-forming oxides 2. Number of non-bridging oxygen in structure Alkali oxide additions reduce viscosity Water (-OH) and fluorine reduce viscosity 3. Coordination number of the cation SiO 2 and B 2 O 3 Glass Formers Silica (SiO 2 ) and Boric oxide (B 2 O 3 ) form glasses readily SiO 2 and B 2 O 3 Glass Formers Even though SiO 2 and B 2 O 3 have extreme properties, addition of modifiers broadens usefulness The properties of these glasses is strongly tied to the strength of the metal-oxygen bonds (Si-O or B-O) making them up. Si-O bonds are notably stronger than B-O bonds. Network modifiers affect the properties of a pure SiO 2 glass as follows: Thermal Expansion Increases Hardness Lowers Chemical Durability Lowers Density Increases Effects of composition on viscosity Electrical Conductivity Charge transport in a glass may be due to the motions of ions or electrons. Common silicate glasses exhibit ionic conductivity due to alkali cations like Na + or Li + Fluoride glasses are usually anionic conductors by means of F - motions. Divalent ions like Ca 2+ have very low mobility and they will contribute very little to the overall ionic conductivity of a glass in the presence of alkali ions 4

5 Chemical Durability Most non-silicate glasses are quite susceptible to dissolution in water. Glasses which exhibit excellent chemical durability in water may be readily dissolved in highly acidic or highly basic solutions. Silicate glasses, despite their excellent resistance to neutral water, are readily attacked by HF. Weathering Weathering refers to the interaction of a glass with water vapor. When a glass surface is exposed to water vapor, ion exchange can occur. The ions leached from the glass remain on its surface and react with the surrounding atmosphere forming hydroxides In neutral solutions, additions of alumina or CaO increase the chemical durability of alkali silicate glasses, while zirconia usually improves durability at high ph values. 2 NaOH + CO 2 = Na 2 CO 3 + H 2 O 5