Glasses- Vitreous Silica Flashcards
What natural processes have produced glassy silica?
Lightning strikes
Meteoric impacts on deserts
Melting point of silica
1723°C
How was vitreous silica made in early 20th century?
Quartz melted electrically or used oxyfuel flames. Made high viscosity liquid which tended to trap bubbles and only simple shapes could be made. Made translucent forms.
How was original way of making vitreous silica improved?
Melted under low pressure so bubbles expanded and rose to surface faster and burst. Finish process at high pressure to encourage gas solution. Made transparent forms
How does chemical vapour deposition work (CVD)?
SiCl4 is low boiling point liquid easily purified by distillation. React with O2 to make SiO2 and 2Cl2. SiO2 formed as fine soot, deposited on substrate and sintered to full density at 1200°C (>Tg) so no trapped bubbles. Cl2 recycled. Ideal for optical quality production like for optical fibres
Vycor process
Make a sodium borosilicate glass (easier to melt than silica). Heat treatment causes phase separation into a silica rich matrix containing a fully connected, water-soluble sodium borate phase on nm scale. Leach to remove sodium borate. Sinter resulting porous glass at 1200°C. Final glass about 96.4% silica
How is vitreous silica made via the sol gel method?
Start with purified liquid organic derivatives of SiO2 e.g Si(OC2H4)4. Under controlled hydrolysis this produces a silica gel. Dry to remove organics (ethanol) and water. Singer at 1200°C. Allows manufacture of coatings and complex shapes.
Describe the structure of silica
Strongly bonded 3D network based on short Si4+ to O2- bonds. Building units are SiO4 tetrahedra. The corners of each tetrahedron (O) are shared with other tetrahedra
Physical properties of silica glasses
Low thermal expansion coefficient 0.4x10^-6 C^-1 so excellent thermal shock resistance. High YM 70GPa so stiff. High viscosity melt >10^7 poise at melting point 1723°C.
What can diffuse through silica and why?
Holes/interstices in the structure where the atoms don’t fill all of the space can allow small molecules to diffuse (slowly) through like H2 or He
Optical properties of silica glass
Transparent because there are no crystal boundaries to scatter light. Wide optical transmission from 180nm to 4.5μm as a result of bonding
Electrical properties of silica glasses
Filled bands with no free electrons and wide band gap. Is an insulator
Chemical properties of silica glasses
Excellent chemical durability in water, acids (except HF). Alkalis May dissolve it though
Other common glass forming oxides
P2O5 based on tetrahedral networks. B2O3 based on tetrahedral or triangular units. Borosilicates, borophosphates
What conditions and composition are best for easiest glass formation?
Formed at eutectics on the Na2O SiO2 phase diagram. The liquidus temperature is lowest and sub-liquidus melt viscosities are higher
How do modifier ions work?
Na2O or CaO. The extra Os break up the network so that not all corners of the tetrahedra are shared with other tetrahedra. The modifier ions (Na+) sit in the interstices of the structure. The addition of Na2O to a bridging SiO2 produces 2SiO- and 2Na+
Effects of modifier ions
Lower viscosity at given temperature. Increase TEC. Increase density (by filling holes). Lower liquidus temperatures. Ions can diffuse within rigid network influencing chemical durability, conductivity, dielectric constant. Still good insulation at RT. Free electrons in metals for charge transport
What are conditional glass formers?
Oxides that can adopt forming role in the structure but will only form glasses with a second component present. Examples are PbO, Al2O3
Common commercial glasses
Borosilicates- Pyrex (low TEC), B2O3 is flux.
Lead crystal- PbO-K2O-SiO2 (Pb now replaced by Ba due to PbO volatilisation), soft to cut, easy to melt, high r.i, used in glass art and lighting, high elec resistance.
E glass- calcium boroaluminosilicate, low alkali content so high chemical durability for composites, high elec resistance, hard to melt
Variety of glasses and properties
Many systems (some non-oxides) can be made as glasses with different properties. In any one system can have wide range of compositions so range of properties. Systems multi-component so can tailor several properties simultaneously
Lost of glass types
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