Materials Science and Engineering/List of Topics/Engineered Glasses

Factors that Enhance Glass formation
 * High viscosity of liquid
 * Complex crystal structure
 * High cooling rate

Difficult to find proper lattice sites

Particular form of glass: silica
 * Silica is inorganic and covalent

In crystalline silicon, all the atoms are the same
 * In silica, there is silicon and oxygen that form long chains and is polymeric
 * The molecules entangle and must disentangle
 * Drop temperature, bonds must twist
 * Quick solidification - fail to achieve crystallinity
 * Quench in disorder
 * Silicon: all four bonds are fixed in space and fully specified
 * Oxygen: bonds free to rotate and there is one degree of freedom

Perfect alignment - create crystalline structure

X-ray spectra
 * Crystobalite - several peaks
 * Amorphous silica - one peak
 * The peak is due to short-range order
 * There is no long-range order

Energetics of glass formation - when are more bonds formed?
 * Crystalline solid is at lower energy
 * There is a higher bond density
 * More compact structure
 * More bonds per unit volume
 * Volume is a macroscopic measure of disorder

Volume as function of temperature
 * Liquid turns to solid and contracts
 * Most substances pack more tightly in solid state
 * Volume decreases as cool
 * Jump to liquid state

Mercury in glass bulb - change in liquid unit volume is larger than solid

A glass is formed by cooling so quickly that act as liquid below melting point
 * Glass transition temperature
 * Viscosity is highly dependent on temperature
 * Knee in curve is function of cooling rate
 * When cool slowly, there is more time of the constituents of the system to rearrange position
 * The excess volume is not as large in comparison to crystal

Solidification is determined by knee in curve of volume versus temperature
 * Coefficient of thermal expansion is less

Difference between solidication to form crystal and form glass
 * Crystal - abrupt change in V versus T
 * Glass - no abrupt change in V versus T at glass transition temperature

Liquid to crystalline solid
 * Melting point independent of cooling rate

Glass transition
 * Supercooled liquid - cooled below melting point
 * The substance is a glassy solid
 * $$T_g$$ is function of the cooling rate
 * More or less free volume determined by cooling rate

There are other systems in addition to silica
 * Other glass forming oxides:
 * $$GeO_2$$
 * $$B_2O_3$$: Borate
 * $$P_2O_5$$
 * $$As_2O_5$$
 * $$Sb_2O_5$$
 * Volume of glass far in excess between glass and crystal
 * Form covalent bonds of metal to metal via bridging oxygen
 * Three dimensional covalent bonds

Properties of oxide glass
 * Chemically inert
 * Electrically insulating
 * Strong bonds hold electrons
 * Mechanically brittle
 * Very directional, no possibility of glide
 * Optically transparent
 * Strong covalent bonds, energy levels far part
 * Visually arresting
 * Don't form sharp edges
 * Color from dopant
 * High melting temperature

Source:

MIT - Course 3.091 - Professor Sadoway - Fall 2004 - Lecture 21
 * Content accessed through MIT OpenCourseWare (http://ocw.mit.edu/)