Glass: An Overview

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Glasses are not stoichiometric compounds but variable mixtures of substances, some of which can independently form glasses while others cannot. This variability allows for a vast range of compositions, offering diverse properties that can be tailored to specific technological applications. Many properties of glass, such as thermal expansion and elasticity, are additive functions of their constituent materials.

In addition to their amorphous nature, both inorganic and organic glasses exhibit a transformation or glass transition temperature range. Unlike crystalline materials, glasses do not solidify or crystallize at a distinct melting point. Instead, a glass-forming liquid transitions gradually over a range of temperatures without a sharp change in viscosity.

1. General Properties of Glass

The properties of glass vary widely depending on its composition and application. Key properties include:

• Crystallization: The tendency of glasses to crystallize when heated is known as devitrification. This process is influenced by the glass composition and thermal treatment applied.
• Viscosity: Viscosity, which is crucial for glass production and processing, varies with composition and temperature.
• Mechanical Strength: The mechanical strength of glass depends on its surface condition. Compressive strength is approximately ten times greater than tensile strength. Surface treatments, such as toughening, can significantly increase the strength.
• Thermal Expansion: The coefficient of thermal expansion, which varies with composition, is critical for applications like flat glass or optical components. For example, quartz glass has a low thermal expansion coefficient (3.2 x 10^-6/K), while heavy lead flint glass has a much higher one (8.0 x 10^-6/K).
• Chemical Durability: Glasses are often chemically durable, resisting concentrated acidic and alkaline solutions. Some glasses are also soluble, a property that can be exploited for specific applications.
• Electrical Properties: Generally considered an electrical insulator at high temperatures, glasses can conduct electricity. Soda lime glass, for instance, has a resistivity of around 10¹² ohms at room temperature compared to 5-50 ohms at 1400 °C.

Learn More: What are the Electrical Properties of Glass?

2. Types of Glass and Their Composition and Properties

Vitreous Silica – 'Quartz' Glass

Vitreous silica is produced by melting ground quartz under vacuum at around 2000 °C to remove gas bubbles. It has a low thermal expansion coefficient (6.7x10^-7/K) and remains thermally stable up to around 1000 °C, with devitrification to cristobalite occurring between 1150 °C and 1200 °C.

Some vitreous silicas are produced at lower temperatures or without vacuum, resulting in an opaque material due to many small bubbles within the glass. Modifications can be made to produce highly reactive forms of silica.

Sodium Silicate – 'Water Glass'

Sodium silicate is produced by melting quartz and soda at 1400 °C. The resulting glass granulate is then dissolved at high temperatures in pressure autoclaves. Sodium silicate is widely used as a binder solution in various ceramic systems, making it an effective adhesive material.

The SiO₂ content in the initial glass typically ranges from 66 % to 76 % by weight, while the composition of sodium silicate in liquid form varies depending on the grade.

Potassium silicate, a similar material, is also manufactured for specialized applications, such as acid-resistant cements.

Sheet and Container Glass (Soda-Lime-Silica)

The basic composition of soda-lime glass is similar across flat and container (holloware) applications. It typically consists of approximately 72 % SiO₂, 14 % Na₂O (or K₂O), 9 % CaO, 2-4 % MgO, and 1-2 % Al₂O₃.

Due to their high alkali content, these glasses exhibit relatively high thermal expansion coefficients (8.0-9.0 x 10^-6/K) and low viscosity at elevated temperatures, attributed to the low Al₂O₃ content.

‘Crystal’ Glasses (K₂O-CaO-SiO₂, K2O-PbO- SiO₂)

Lead and potassium oxides are key components of glasses known as 'crystal.' K₂O and PbO are also widely used in optical, sealing, and other technical glasses. The term "crystal" specifically refers to high-grade, clear, colorless glass with high gloss and optical transmission. Conventionally, only glass containing more than 24 % PbO and a refractive index exceeding 1.545 is classified as crystal.

K₂O and PbO contribute to the brilliant appearance and optical properties of crystal glass. Glasses can be formed with up to 65 % K₂O in the K₂O-SiO₂ system and up to 80 wt.% PbO in the PbO-SiO₂ system. In practice, industrially produced crystal glasses are often more complex than these basic ternary systems and may include additional components such as Na₂O, BaO, ZnO, B₂O₃, and MgO.

Lead crystal glasses typically contain 24-32 wt.% PbO. Typical compositions for K₂O and PbO types are provided in Table 1.

Crystallizing Solders

Crystallizing solders retain their glassy characteristics until the soldering temperature is reached. At this point, they transform into a glassy-crystalline structure, creating an irreversible joint.

For applications requiring soldering at temperatures above 550 °C, zinc-borate and silicon borate glasses can be used, with typical compositions such as 50-65 % ZnO, 0-15 % SiO₂, and 20-35 % B₂O₃.

Phosphate Glasses

Calcium phosphate glass is a significant component of bone china. Phosphate glasses can be produced using elements such as Zn, Mg, Ca, Ba, or Na, with thermal expansion coefficients ranging from 7.0 to 13.0 x 10^-6/K (e.g., Ca-P glass has a coefficient of 8.0 x 10^-6/K). Adding aluminum (Al) to the composition improves resistance to water vapor and reduces thermal expansion.

Alkali phosphate glasses are water-soluble, which can be advantageous for specific applications. A typical calcium phosphate glass can be produced by melting a mixture of 49.6 % P₂O_5, 49.6 % CaO, and 0.8 % SiO₂ at 1300 °C.

Chalcogenide Glasses

This group of glasses is formed from chalcogen elements such as sulfur (S), selenium (Se), and tellurium (Te). They may also include elements like germanium (Ge), silicon (Si), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi), and occasionally titanium (Ti), cadmium (Cd), and lead (Pb).

Chalcogenide glasses are typically simple in composition, containing 2-4 elements, and are used in specialized optical applications. Glasses in the As-Se, As-S, and As-Se(S)-Te systems form around 200 °C, while other chalcogenide glasses require formation temperatures above 500 °C.

3. Glass Production Techniques

3.1. Standard Glass Production

Glass can be produced through various methods, ranging from small batch operations yielding a few kilograms to large-scale continuous processes producing tonnes per day.

Several glass production techniques are used to create different types of glass, including the float glass process, blowing, pressing, drawing, and casting.

3.1.1. Float Glass Process

In the float glass process, recycled glass is combined with materials lime, soda, lime, and silica, and heated to approximately 1600°C to create molten glass. This molten glass is then poured onto a bed of molten tin, producing flat, smooth glass with uniform thickness.¹ This method is widely used for making flat glass sheets.

3.1.2. Blowing

Blowing is a traditional technique used for the small-scale production of decorative or custom glassware. In this process, molten glass is inflated using a blowpipe and then shaped by rolling it on a metal surface. Due to the cooling effect of the air, the glass must be reheated periodically to maintain its workability.²

3.1.3. Pressing

Pressing involves placing a lump of molten glass into a metal mold, where a metal plunger presses it into shape. This technique is commonly used for creating glassware with intricate interior designs and patterns. ³

3.1.4. Flat Drawn Process

In the drawing method, molten glass is pulled vertically from a bath and air-cooled to form long, thin sheets or fibers. This process is typically employed in the production of continuous glass fibers.⁴

3.1.5. Casting

Casting involves pouring molten glass into a mold, allowing it to cool and solidify into the desired shape. This method is ideal for producing thick, detailed glass components, such as sculptures or specialized industrial parts.⁵

3.2. Innovations in Glass Production

3.2.1. Sol-Gel Process

The sol-gel process is a glass manufacturing technique that transitions a liquid solution into a solid gel, offering precise control over the final properties of the glass. Key parameters like hydrolysis ratio, aging time, drying, and calcination temperature influence the structure of the resulting glass.

This method provides advantages over traditional melt-quenching, producing glasses with higher surface areas and porosity, which enhance bioactivity. These features support faster degradation and improved interaction with biological tissues.⁶

3.3.2. Novel Abrasive Machining Technique for Glass

Glass is a brittle material, making it challenging to drill without causing micro-cracks or damage, which can compromise its structural integrity and performance. This issue is particularly problematic for applications like gaseous electron multipliers (THGEMs), where creating high-density hole patterns is difficult using traditional methods.

A study introduced a novel manufacturing process known as abrasive machining, which enables the precise formation of sub-millimeter holes in non-ductile materials like glass. This technique improves the performance of THGEMs by allowing for the customization of substrate and electrode materials.⁷

3.3.3. Casting Complex Glass Components with 3D-printed Sand Moulds

In a 2020 study, researchers explored innovative methods for casting complex glass components using 3D-printed sand molds and adjustable steel molds, testing this approach for glass casting for the first time. The 3D-printed sand molds provided a cost-effective, high-precision solution for creating intricate glass shapes.⁸

The researchers successfully tested various binders and coatings for high-temperature durability. They also developed adjustable steel molds to cast components of different sizes using the same mold, offering potential applications in flexible manufacturing. The findings of this study provide a method that reduces costs and enhances flexibility in glass casting.⁸

4. Novel Glass Types

4.1. Bioactive Glass

Bioactive glass is primarily used in bone regeneration due to its unique ability to bond with living tissue and stimulate biological responses. It typically comprises a combination of oxides, including silica (SiO₂), sodium oxide (Na₂O), calcium oxide (CaO), and phosphorus pentoxide (P₂O₅), which are carefully selected and blended to achieve specific properties.⁶

The key attribute of bioactive glass is its biocompatibility; it is well-tolerated by the body and does not cause adverse reactions. Upon implantation, bioactive glass undergoes a series of chemical reactions that lead to the formation of a hydroxyapatite layer—a mineral naturally found in human bones and teeth. This property makes it highly compatible with living tissue.

Additionally, bioactive glass is osteoconductive, meaning it can support the growth and attachment of bone cells, further enhancing its effectiveness in bone regeneration.⁶

4.2. Smart Glasses

Smart glasses, also known as switchable or dynamic glass, have gained significant attention due to their ability to control light transmission by switching between transparent and opaque states. This functionality makes them highly valuable in various industries, including architecture, automotive, and interior design.⁹

Smart glasses are categorized into two main types: active and passive. Active smart glasses, such as Polymer Dispersed Liquid Crystal (PDLC) and Suspended Particle Device (SPD) technologies, use electricity to adjust transparency.

In contrast, passive smart glasses, like photochromic and thermochromic glass, respond to environmental changes such as light or temperature. These glasses are commonly used in windows, partitions, and electronic displays, providing both privacy and shading solutions.⁹

5. The Future of Glass

Future developments in glass technology suggest that smart glasses will become increasingly important in architectural designs and energy-efficient buildings, offering solutions for privacy and aesthetic enhancement.⁹

Advances in material science are expected to support this growth. Additionally, as environmental concerns rise, researchers are focusing on reducing the carbon footprint of glass manufacturing, making eco-friendly production methods an emerging trend that is likely to shape the future of glass.

References and Further Reading

1. Achintha, M. (2016). Sustainability of glass in construction. Sustainability of construction materials. https://doi.org/10.1016/B978-0-08-100370-1.00005-6
2. Schreiber, BA. (n.d). Glassblowing. [Online] Britannica. Available at: https://www.britannica.com/technology/glassblowing (Accessed on 25 September 2024)
3. Corning Museum of Glass. (n.d.) Pressed glass. [Online] Corning Museum of Glass. Available at: https://allaboutglass.cmog.org/definition/pressed-glass (Accessed on 25 September 2024)
4. Pilkington. (n.d). Flat drawn process. [Online] Pilkington. Available at: https://www.pilkington.com/en-gb/uk/about/heritage/flat-drawn-process (Accessed on 25 September 2024)
5. Jockimo Architectural Glass Products (2022). What Is Cast Glass? [Online] Jockimo Architectural Glass Products. Available at: https://www.jockimo.com/about/jockimo-blog/what-is-cast-glass (Accessed on 25 September 2024)
6. Deshmukh, K., Kovářík, T., Křenek, T., Docheva, D., Stich, T., & Pola, J. (2020). Recent advances and future perspectives of sol–gel derived porous bioactive glasses: a review. RSC advances. https://doi.org/10.1039/D0RA04287K
7. Lowe, A., Majumdar, K., Mavrokoridis, K., Philippou, B., Roberts, A., & Touramanis, C. (2021). A novel manufacturing process for glass THGEMs and first characterisation in an optical gaseous argon TPC. Applied Sciences. https://doi.org/10.3390/app11209450
8. Oikonomopoulou, F., Bhatia, IS., van der Weijst, FA., Damen, JTW., Bristogianni, T. (2020). Rethinking the cast glass mould: an exploration on novel techniques for generating complex and customized geometries. CGC 7: Challenging Glass Conference (Webinar): 7th International Conference on the Architectural and Structural Application of Glass. https://doi.org/10.7480/cgc.7.4662
9. Gauzy. (n.d). Everything You Want to Know About Smart Glass. [Online] Gauzy. Available at: https://www.gauzy.com/smart-glass-everything-you-want-to-know/ (Accessed on 22 September 2024)