Glass Lec 1

• The earliest glasses used by man were found in nature, such as obsidians, which allowed the production of knives, arrow heads, and other cutting tools
• Naturally occurring glasses, which result from the cooling of molten rock or lava, contain a wide variety of components, including alkali, alkaline earths, and transition metal oxides, but silica is the major constituent
• The earliest man-made glasses were also silicates, and very few non-silicate glasses were known prior to 1900
• In recent years, we have recognized the existence of a vast number of non-silicate glasses, including polymers and metals, as well as a large number of non-oxide, inorganic compositions
• We now recognize that virtually any material can be formed as a glass
• Structural Theories of Glass Formation


  Theories that assume some unique feature of certain melts leads to glass formation, while the lack of these features prevents the formation of glasses from other materials
• Kinetic Theory of Glass Formation


  A new approach that focuses on the control of glass formation by changes in processing, rather than the control of glass formation by selection of materials
• Goldschmidt's theory


  Glasses of the general formula R^m O^n form most easily when the ionic radius ratio of the cation, R, to the oxygen ion lies in the range 0.2 to 0.4, as this tends to produce cations surrounded by four oxygen ions in the form of tetrahedra
• Zachariasen's theory


  The formation of a vitreous network is necessary for glass formation, and this requires that: 1) the material contains a high proportion of cations surrounded by either oxygen triangles or oxygen tetrahedra, 2) these polyhedra are connected only by their corners, and 3) some oxygen atoms are linked to only two such cations and do not form additional bonds with other cations
• Zachariasen's ideas have become the basis for the most widely used models for glass structures, known as the Random Network Theory
• Smekal's theory


  Glasses are only formed from melts which contain bonds that are intermediate in character between purely covalent and purely ionic bonds
• Stanworth's theory


  Oxides can be classified into three groups based on the fractional ionic character of the cation-anion bond: network formers, intermediates, and modifiers
• Sun's theory


  Strong bonds prevent reorganization of the melt structure into the crystalline structure during cooling, and thus promote glass formation
• Rawson's theory


  The ease of glass formation should be improved for compositions near eutectics in binary and ternary systems, due to the "liquidus temperature effect"
• Nucleation


  The first step in the crystallization process, where nuclei are formed, either homogeneously within the melt or heterogeneously at a pre-existing surface
• Classical nucleation theory


  1. Addresses the process of homogeneous nucleation, where nuclei are formed with equal probability throughout the bulk of the melt

  2. The nucleation rate, I, is determined by dividing the concentration of nuclei by the total time of the isothermal heat treatment at the nucleation temperature
• Nucleation rate equation


  I = A exp[-(W* + ΔG^0) / kT], where A is a constant, W* and ΔG^0 are the thermodynamic and kinetic free energy barriers to nucleation, respectively, k is the Boltzmann constant, and T is the absolute temperature
• Energy change in a system

  When a nucleus is formed
• Kinetic barrier


  Result of the requirement that mass be moved or rearranged in space, to allow the growth of an ordered particle (a crystal) from a disordered liquid
• A


  Constant in nucleation rate equation
• Metastable zone of undercooling exists where no nuclei form despite temperature being below melting point
• Once temperature passes lower limit of metastable zone, nucleation rate increases rapidly with decreasing temperature
• Viscosity also increases rapidly with decreasing temperature, increasing kinetic barrier and causing nucleation rate to eventually decrease
• No metastable zone for crystal growth - growth can occur at any temperature below melting point if nuclei are present
• Time-temperature-transformation (TTT) curve shows combinations of time and temperature that yield a specified volume fraction of crystals
• TTT diagrams are models to aid understanding, not experimental tools - crystal content criteria is arbitrary
• TTT diagram must be considered as a model for aiding our understanding of the glass formation process, rather than as an experimental tool
• At this time, TTT diagrams have been produced for very few materials
• Critical cooling rate


  The slowest cooling rate which produces a glass
• Determining critical cooling rate

  1. Heat melt to predetermined temperature

  2. Hold for specified time to dissolve residual crystals and nuclei

  3. Cool at linear rate until solid forms

  4. Examine solid to determine if glass

  5. Repeat with slower cooling rate until sample fails to form glass

  6. Slowest cooling rate that produces glass is critical cooling rate
• Glass forming ability


  Resistance to crystallization of a melt during cooling
• Glass stability


  Resistance to crystallization of a glass during heating
• Glass forming ability is most important during processes requiring production of an initial glass, while glass stability is most important during processes involving re-forming of an existing glass
• Poor glass forming ability does not automatically lead to poor glass stability, and vice versa
• Tg


  Onset of glass transformation region
• Tx


  Occurrence of crystallization
• Typical thermal spectra may contain one or more exothermic peaks due to crystallization of different phases, but only the lowest temperature peak is considered in discussing glass stability
• Glasses which are relatively stable, including almost all common commercial compositions, crystallize so slowly that they will not exhibit a crystallization exotherm during heating at the usual 10 or 20 K min-1 used in these studies
• Determining stability of highly resistant glasses

  1. Carry out isothermal heat treatments to determine conditions under which stable glasses crystallize to a detectable extent

  2. Create a TTT diagram

  3. Determine crystal growth rates for related glasses as a function of temperature
• Virtually any material can be formed as a glass under the proper experimental conditions