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issue: October 2009 APPLIANCE Magazine

APPLIANCE Engineer - Materials Technology
Glass That Takes the Heat

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Glass ceramics with near-zero thermal expansion are opening up new design possibilities.

Available in various coatings and shapes, Robax has widened both functional and aesthetic options for designers. Pictured is a fireplace from Stove Builder International (Quebec, Canada) featuring Robax.

Cooking appliance designers are no strangerto Ceran, a glass ceramic material from SCHOTT North America (Louisville, KY, U.S.; www.us.schott.com) widely used in cooktop panel production. But Resistan and Robax, two of SCHOTT’s glass ceramics that also boast low thermal expansion and high heat resistance, are quickly finding their niches in other high-temperature household applications such as ovens and fireplaces.

The extremely low thermal expansion that characterizes glass ceramics from SCHOTT is achieved through a special manufacturing process. Formed initially as a glass, the material is heated to obtain a crystal structure similar to a ceramic. The resulting material contains both a glass structure with a crystal structure nested inside. “The crystal structure has a negative thermal expansion—it gets smaller when heated—and the glass a positive thermal expansion. The two materials then offset, keeping the coefficient of thermal expansion near zero,” says Ted Wegert, director of applications for SCHOTT North America, Home Tech. “For users, this means that even though the glass might get hot in some areas and not in others, they can be assured that the glass can withstand the temperature differential without breaking.”

Wall oven windows made of Resistan can stretch from the oven cavity surface to the cold zone at the door’s edge, as shown in Electrolux’s ICON oven.

The move to use transparent glass ceramic in inner wall oven doors is a new trend Wegert has observed. “With Resistan, designers and engineers can get very large windows allowing for a greater look inside the oven, an aesthetically pleasing glass door, and easy cleanup on typically messy inner doors.”

Wegert says if incorporated correctly into the door, Resistan can withstand “nearly any thermal gradient a residential oven can throw at it.” Thanks to its transparency and chemical durability, Wegert adds, some Resistan materials are also suitable for residential or commercial microwave or infrared grill applications.

With equally high heat resistance, SCHOTT’s transparent Robax is used in fireplace and hearth panels, which allow users to enjoy the fire view and the warmth while being protected from flying sparks and other fire dangers. “Robax is an exceptional infrared heat transmitter from a fire. So, in a high-heat fireplace, the heat generated inside the firebox can be comfortably felt inside the room while still having a protective buffer from the fire,” Wegert tells APPLIANCE.

Robax also elevates the combustion efficiency by allowing for a higher-temperature fire, which is a desired goal of many designs. “Other glasses would break in this environment due to high thermal gradients across the glass panel. Cool edges of the glass combined with an intense flame proximal to the glass center cause most glasses to fail, but Robax can handle up to 700°C (1292°F) temperature gradients across the panel,” Wegert says.

SCHOTT has designed systems for fireplace use that tap into the thermal robustness of Robax by adding an infrared reflecting coatings into a glass package. “These systems provide a cooler external surface so a fireplace is safer to touch, and also increase the internal combustion temperature of the fireplace,” Wegert explains. “This increased combustion temperatures aid in more efficient use of the fuel used, either wood or gas.”

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