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Glass-Ceramics: Properties, Production, and Applications

• 0:00 Introduction to Glass-Ceramics: Glass-ceramics combine properties of both glass and ceramics, offering tailored characteristics for specific uses.
• 0:14 Types and Applications:
  • ClearCeram and ULE: Engineered for minimal thermal expansion, ideal for precision optics and measurement equipment.
  • Ceran, Robax, and Corningware: Designed for thermal shock resistance, commonly used in cooktops, fireplace shields, and ovenware.
• 1:06 Discovery: Accidental discovery by Donald Stookey at Corning. A misset oven on an experimental photosensitive glass (containing lithium oxide) at 900°C, rather than the intended 600°C. This resulted in a milky-white material with extreme thermal shock resistance.
• 2:12 Thermal Shock Resistance Demonstration:
  • Soda-Lime Glass: Shatters upon rapid cooling (quenching in cold water) due to high thermal stress.
  • Borosilicate Glass (Pyrex): More resistant, but still develops cracks under rapid temperature changes.
  • Glass-Ceramic (Robax): Withstands extreme temperature changes, even when heated to red-hot and then quenched, without any visible damage.
• 3:48 Glass vs. Ceramic Structure:
  • Glass: Amorphous, disordered structure, like a "frozen liquid."
  • Ceramic: Crystalline, ordered structure, usually formed by sintering powdered materials.
  • Glass-Ceramic: Combines both, containing crystalline regions within an amorphous glass matrix.
• 4:45 Robax Properties: Robax is a transparent glass-ceramic that combines clarity with exceptional thermal shock resistance.
• 5:02 Thermal Stress and Expansion: Thermal shock resistance is due to a low Coefficient of Thermal Expansion (CTE). Materials expand/contract with temperature changes. Large CTE creates stress gradients, leading to cracks.
• 6:42 Coefficient of Thermal Expansion (CTE) Explained: CTE quantifies the change in length per degree Celsius (or Fahrenheit) temperature change. Units are ppm/°C (parts per million per degree Celsius).
• 7:30 CTE Examples:
  • Polymers: High CTE (up to 200 ppm/°C).
  • Aluminum: ~23 ppm/°C.
  • Soda-Lime Glass: 7.5-9.5 ppm/°C.
  • Borosilicate Glass: ~3.3 ppm/°C.
  • Fused Silica (Quartz): ~0.55 ppm/°C.
  • Glass-Ceramics (Robax, Ceran, ClearCeram): Near zero CTE (0 ± 0.15 ppm/°C).
• 8:33 Fused Silica Demonstration: Fused silica (low CTE) also survives thermal shock (quenching from red-hot).
• 10:14 CTE Measurement Setup: A custom-built tool uses ClearCeram (low CTE) spacers and a dial gauge to measure length changes of sample materials when heated.
• 12:53 CTE Measurement Results:
  • Aluminum: 23.7 ppm/°C (close to expected range).
  • Soda-Lime Glass: 10.8 ppm/°C (slightly high).
  • Borosilicate Glass: 3.3 ppm/°C (matches datasheet).
  • Fused Silica: 0.6 ppm/°C (close to literature value).
  • Robax: -0.1 ppm/°C (within datasheet specification).
  • Ceran: -0.3 ppm/°C (within datasheet specification).
  • ClearCeram: +0.2 ppm/°C (slightly outside specified tolerance, possibly due to setup limitations).
• 16:42 Negative CTE Advantage: A slightly negative CTE can be beneficial in reducing thermal stress during rapid cooling (quenching), as it counteracts the tensile stress.
• 17:11 Glass Ceramic Composition Relies in part of Lithium and Aluminium oxide, with crystals that have an extremely low or negative coefficient of thermal expansion.
• 18:37 Chemical Composition of Crystals: Different crystal compositions (e.g., β-quartz, β-spodumene, β-eucryptite) have highly anisotropic CTEs, meaning they expand/contract differently along different crystal axes.
• 19:13 Zero-Expansion Material Design: Achieved by combining a glassy matrix (positive CTE) with a controlled volume fraction of crystals (negative CTE) to achieve a net CTE of zero.
• 20:05 Critical Factors in Production: Precise control of oxide concentrations, nucleation agents, and temperature profiles are crucial for successful glass-ceramic production.
• 20:58 Nucleation and Crystal Growth: Nucleation (formation of initial crystal seeds) and crystal growth rates vary with temperature. A two-step heat treatment process is used.
• 22:25 Glass-Ceramic Production Process:
  • Melting: Oxides are mixed and melted at high temperatures (e.g., 1650°C).
  • Forming: Molten glass is formed into the desired shape (e.g., sheets).
  • Rapid Cooling: Quick cooling prevents significant crystal formation, resulting in a predominantly glassy material.
  • Nucleation: The glass is reheated to a specific temperature to promote the formation of crystal nuclei.
  • Crystallization: Temperature is increased further to promote crystal growth around the nuclei.
• 24:48 Complexity of Development: Glass-ceramic development is complex due to the many variables involved (composition, temperature profiles, nucleation agents).
• 25:10 Crystalline Structure Visualization: Etching the surface of polished Ceran reveals small crystals (4-7 microns) embedded in the glass matrix.
• 26:02 Zerodur Crystal Size: Crystals in Zerodur are smaller (~1 micron) than those in Ceran.
• 26:15 Transparency: The dark color of Ceran is due to additives. Glass ceramics are relatively transparant.
• 27:29 Recommended Reading: "Glass-Ceramic Technology" by Wolfram Höland and George Beall.