Introduction to glass

What is Glass?

• Altogether: Fusion product of inorganic materials which have been cooled to a rigid condition without crystallizing.
• Is silica an essential requirement for a glass?
• Is melting a perquisite to glass formation?
• Glasses are always inorganic and non-metallic?
• Property of glass-formation is not an atomic or molecular property but rather one of a state of aggregation: The word glass is a generic term similar to crystals, liquids, gases etc.
• Definition of a glass circle around two facts:
• Absence of a long range periodic atomic arrangement
• Time dependent glass transformation behavior
• A glass can thus be defined as “an amorphous solid completely lacking in long range, periodic atomic structure, and exhibiting a region of glass transformation behavior.”
• Any material, inorganic, organic, or metallic, formed by any technique, which exhibits glass transformation behavior is a glass.

Radial distribution function

• Glassy form of matter combines the 'short- time' rigidity characteristic of the crystalline state with the 'long-time‘ fluidity of the liquid state.
• Glasses, like liquids are isotropic.

In statistical mechanics, the radial distribution function, (or pair correlation function) in a system of particles (atoms, molecules, colloids, etc.), describes how density varies as a function of distance from a reference particle.

Amorphous nature of glasses

Diffraction pattern from a thin film of amorphous carbon

Glass transformation behavior

• Enthalpy (H) is a thermodynamic property that measures the total heat content of a system. It is defined as the sum of the system's internal energy (U) and the product of its pressure (P) and volume (V): H = U + PV
• The temperature where the enthalpy departs from the equilibrium curve is controlled by the viscosity of the liquid, i.e. by kinetic factors.
• Use of a slower cooling rate will allow the enthalpy to follow the equilibrium curve to a lower temperature.
• The glass transformation region will shift to lower temperatures and the formation of a completely frozen liquid, or glass, will not occur until a lower temperature.

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• The glass obtained will have a lower enthalpy than that obtained using a faster cooling rate.
• The atomic arrangement will be that characteristic of the equilibrium liquid at a lower temperature than that of the more rapidly cooled glass.
• Time dependent process by which the glass reaches a more stable condition is known as stabilization.
• As a result of the existence of stabilization effects, the properties of a glass depend to a certain extent on the rate at which it has been cooled, particularly through the transformation range

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Fictive temperature

• Although the glass transformation actually occurs over a temperature range, it is convenient to define a term which allows us to express the difference in thermal history between these two glasses.
• If we extrapolate the glass and supercooled liquid lines, they intersect at a temperature defined as the fictive temperature.
• The structure of the glass is considered to be that of the equilibrium liquid at the fictive temperature.
• Although the fictive temperature concept is not a completely satisfactory method for characterizing the thermal history of glasses, it does provide a useful parameter for discussion of the effect of changes in cooling rate on glass structure and properties.

Glass transition temperature

• The glass transformation occurs over a range of temperatures and cannot be characterized by any single temperature.
• It is, however, convenient to be able to use just such a single temperature as an indication of the onset of the glass transformation region during heating/cooling of a glass.
• This temperature, which is termed either the glass transformation temperature, or the glass transition temperature, is rather vaguely defined by changes in either thermal analysis (DSC) curves or thermal expansion (DTA) curves.
• The values obtained from these two methods, while similar, are not identical.
• The value obtained for T_g, is also a function of the heating rate used to produce these curves.

Glass transition temperature, T_g

• T_g is a function of
• Method of determination
• Heating/Cooling rate
• Since T_g is a function of both the experimental method used for the measurement and the heating rate used in that measurement, it cannot be considered a true property of the glass.
• T_g is a useful indicator of the approximate temperature where the supercooled liquid converts to a solid on cooling, or conversely of which the solid begins to behave as a viscoelastic solid on heating.

DTA vs. DSC

Change in physical properties as indicator of T_g

• Glucose, a familiar substance, is an example of a material which readily undercools to form a glass.
• It melts at 414K and, once molten, can be kept below this temperature for a long time without crystallization.
• As the melt is cooled below about 300K (T_g), its specific heat decreases almost by a factor of two.
• The specific volume and enthalpy show no analogous change, but they do show a slight discontinuity.
• There is no volume change or latent heat at this transition but the thermal expansivity decreases by a factor of four.
• Glucose stays optically transparent and there is no change in refractive index at this temperature, although the temperature coefficient of the refractive index suddenly decreases.

Is temperature essential for glass formation?

• Liquids can also be transformed into glassy state by the application of pressure.
• The melting point of Selenium (Se) under one atmosphere pressure is 493 K.
• The volume of selenium at 313 K changes non-linearly with pressure; near 11 kbar there is a discontinuity in the curve.
• The compressibility, obtained from the slope of the curve, decreases by about 40 per cent at 11 kbar.
• At higher pressures, the compressibility of liquid Selenium is close to that of the crystalline phase.
• The pressure at which there is a sudden decrease in compressibility is known as the pressure of glass transition, P_g.
• Selenium at a pressure above 11 kbar is in the glassy state.

Phenomenological rather than a�generic definition of the glassy state

• A glass can thus be defined as “an amorphous solid completely lacking in long range, periodic atomic structure, and exhibiting a region of glass transformation behavior.”
• Glassy form of matter combines the 'short- time' rigidity characteristic of the crystalline state with the 'long-time‘ fluidity of the liquid state.
• A glass is a state of matter which maintains the energy, volume and atomic arrangement of a liquid, but for which the changes in energy and volume with temperature and pressure are similar in magnitude to those of a crystalline solid.

Order of glass transformation

• For glucose: At the glass transition the liquid and glass differ in the second derivative of the free energy, with respect to temperature and pressure but not in the free energies themselves, or in their first derivatives.
• It indicate that the glass transition has more or less the characteristics specified for a second-order thermodynamic transition (at least for Glucose).
• Whether or not it is a true thermodynamic transition (for every kind of glass) is a question that has not yet been satisfactorily answered