基础材料和有机材料 (19).pdf

Lecture 6 The typical glass1.Silicate glassSilicate glass is widely used due to the low cost,good chemical stability,high hardness,and simple production process.A.Structural parametersWith the introduction of alkaline oxides,the glass system has a high ratio of the oxygen-to-silicon,and the skeleton of the three-dimensional network breaks down,which is vary from a frame to a layer,a chain,and even an island with the increasing oxygen-to-silicon ratio.R=O/Si,the ratio of the oxygen atoms to the network forming ionsX=the average number of non-bridge oxygen atoms in the polyhedronY=the average number of bridge oxygen atoms in the polyhedronZ=the average number of oxygen atoms in the polyhedronX+Y=ZX+Y/2=RX=2R-ZY=2Z-2R The percentage of non-bridge=X/(X+Y/2)The percentage of bridge oxygen=Y/2/(X+Y/2)(1)SiO2Z=4 R=2 X=224=0Y=2(42)=4(2)Na2O2SiO2 Z=4 R=5/2 X=25/24=1Y=2(45/2)=3(3)Na2OSiO2Z=4 R=3 X=2Y=2(4)2Na2OSiO2 Z=4 R=4 X=4Y=0(5)10molNa2O8%molCaO82%molSiO2 Z=4 R=(10+8+822)/82=2.22 X=0.44Y=3.56(6)10molNa2O8%molAl2O382%molSiO2Z=4 R=(10+24+822)/(82+82)=2.02 X=0.04 Y=3.96If the intermediate such as Al2O3exists inthe glass,we should further judge whetherthe Al3+cations act as the network former.The criterion is as follows:(R2O+RO)/Al2O31，Al3+cations act asthe network former(R2O+RO)/Al2O31，Al3+cations act asthe network modifier(R2O+RO)/Al2O31，Al3+cations act asthe network formerY is an important parameter for the glass.Many properties of the glass are controlled by the value of Y.Silicate glass cannot be formed if Y is less than 2.If the glass has a higher value of Y,the strengths and viscosity are higher,but the thermal expansion coefficient and electrical conductivity are lower.On the contrary,the structure of the glass becomes open and the modifying cations move easily.As a result,the glass has lower viscosity,higher thermal expansion coefficient and electrical conductivity.B.Significance of structural parameterYEffects of Y on the glass propertiesC.Differences between silicate glass and silicate crystalsThe SiO4 arrangement is strictly order in the crystal,but it is disorder in the glass.R+or R2+cations occupy fixed sites in the crystal lattices,while they are statistically distributed in the voids of the glass network to balance the negative charge of non-bridge oxygen anions.The cations with similar radius can be replaced each other in the crystal.However,the replacement of cations is much easier in the glass regardless of the ion radius as long as the valence is balanced,owing to the fact that the network is easy to deform in the glass.The composition of the crystal is fixed and satisfies the stoichiometric ratio(except for the solid solution).But the oxides can be mixed in any ratio to form glass as long as they are within the composition range of the glass.The chemical composition and structure of glass have greater variability and tolerance than crystal,so the properties of the glass can be adjusted in a larger range with abundant species and wide uses.Conclusion2.Borate glassThe structure of B2O3glass:It can be regarded as a network developed randomly by BO3 triangles(flat triangular pyramids,almost triangles)in two-dimensional space.The layers in the glass are connected by Van Der Waals force which is a weak bond.B2O3glass has a low softening temperature(450oC),low surface tension,poor chemical stability(easy to deliquesce in air),and high thermal expansion coefficient.B2O3glass has little practical value.But B2O3is the only oxide glass that can be used to effectively absorb slow neutrons,which is irreplaceable by other materials.The borate glass is more stable with the addition of R2O,RO,etc.,which can be used in many fields.Furthermore,when R2O and RO were added to B2O3,a boron anomaly appeared.Na2O%ObTg43B OOB On b The application of borate glass Borate glass can effectively absorb slow neutrons and has important uses in the nuclear industry.Borate glass is stable to alkali metal(Na,Cs)steam,and can be used as a lampshade material for discharge lamps containing Na and Cs.Borate glass has high transmittance coefficient of X-rays,which can be used as a window material for X-ray tube.Borate glass has good electrical insulating properties and low soften temperature,which can be used as glass flux or adhesive.The phase separation of borate glass can make the glass opalescent.