Glasses- Vitreous Silica

Question 1

What natural processes have produced glassy silica?

Answer 1

Lightning strikes

Meteoric impacts on deserts

Question 2

Melting point of silica

Answer 2

1723°C

Question 3

How was vitreous silica made in early 20th century?

Answer 3

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.

Question 4

How was original way of making vitreous silica improved?

Answer 4

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

Question 5

How does chemical vapour deposition work (CVD)?

Answer 5

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

Question 6

Vycor process

Answer 6

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

Question 7

How is vitreous silica made via the sol gel method?

Answer 7

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.

Question 8

Describe the structure of silica

Answer 8

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

Question 9

Physical properties of silica glasses

Answer 9

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.

Question 10

What can diffuse through silica and why?

Answer 10

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

Question 11

Optical properties of silica glass

Answer 11

Transparent because there are no crystal boundaries to scatter light. Wide optical transmission from 180nm to 4.5μm as a result of bonding

Question 12

Electrical properties of silica glasses

Answer 12

Filled bands with no free electrons and wide band gap. Is an insulator

Question 13

Chemical properties of silica glasses

Answer 13

Excellent chemical durability in water, acids (except HF). Alkalis May dissolve it though

Question 14

Other common glass forming oxides

Answer 14

P2O5 based on tetrahedral networks. B2O3 based on tetrahedral or triangular units. Borosilicates, borophosphates

Question 15

What conditions and composition are best for easiest glass formation?

Answer 15

Formed at eutectics on the Na2O SiO2 phase diagram. The liquidus temperature is lowest and sub-liquidus melt viscosities are higher

Question 16

How do modifier ions work?

Answer 16

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+

Question 17

Effects of modifier ions

Answer 17

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

Question 18

What are conditional glass formers?

Answer 18

Oxides that can adopt forming role in the structure but will only form glasses with a second component present. Examples are PbO, Al2O3

Question 19

Common commercial glasses

Answer 19

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

Question 20

Variety of glasses and properties

Answer 20

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

Question 21

Lost of glass types

Answer 21

Page 18 lecture 2b