Glass technology: During glass manufacture, which additive is most effective for reducing the coefficient of thermal expansion (CTE) and improving thermal shock resistance?

Introduction / Context:
Thermal expansion control is crucial in glass design for cookware, laboratory ware, and high-temperature service. Borosilicate glasses are renowned for low coefficient of thermal expansion (CTE) and superior thermal shock resistance compared with soda–lime glass.

Given Data / Assumptions:

• Goal: lower CTE to resist cracking during rapid temperature changes.
• Common oxides in glass: network formers (SiO2, B2O3), intermediates (Al2O3, ZnO), and modifiers (Na2O, K2O, CaO, PbO).
• Borosilicate formulations (e.g., ~80% SiO2, ~13% B2O3) demonstrate very low CTE.

Concept / Approach:

B2O3 acts as a network former that increases network connectivity and reduces mobility of modifier cations, thereby decreasing thermal expansion. In contrast, alkali oxides (Na2O, K2O) depolymerize the network and increase CTE; PbO often raises density and CTE while enhancing optical properties.

Step-by-Step Solution:

Verification / Alternative check:

Comparative data show borosilicate CTE values roughly one-third to one-half of soda–lime glass, confirming B2O3 as the key variable. Alumina can also help but was not among the provided options.

Why Other Options Are Wrong:

Na2O/K2O: Increase CTE by breaking the silica network. PbO: Typically increases density and often CTE; not preferred for thermal shock resistance.

Common Pitfalls:

Assuming any oxide addition lowers CTE; overlooking the role of network formers versus modifiers.

Final Answer:

B2O3 (boric oxide)