What is low expansion glass - Zerodur, CLEARCERAM-Z, and ULE?

Low expansion optical ceramics contains a specific formulation that is extremely resistant to expansion from temperature changes resulting in low coefficient of thermal expansion (CTE) and high resistance to thermal shock. Due to their chemical stability, they have lower levels of bubbles, striae, and excellent homogeneity. The most common low expansion materials are Ultra Low Expansion glass (ULE®) by Corning, Zerodur® by Schott, and CLEARCERAM™-Z from Ohara. These offerings all have very similar CTE required in space and semiconductor applications, among others.

Near-zero thermal expansion material is essential for any space application to withstand both thermal shock and sub-zero temperatures of space. Whether that application is a coated mirror or part of an optical assembly, even the smallest expansion could lead to failure or improperly working equipment. All three materials are manufacturable using traditional methods and can be polished to an extreme degree of flatness. When machining, material can be removed from the underside of the optic to reduce its weight. For example, a space application mirror may have a honeycomb underside with only narrow bands of material holding the structure together. 

Within each product brand of glass, there are different grades available which pertain to the number of inclusions for the overall size of the blank. For example, Corning ULE 7972 comes in premium, mirror, and standard, and Schott's Zerodur allows for a tailored formulation of their glass to meet customers' needs. Ohara also offers 3 versions based on thermal expansion differences including CLEARCERAM™-Z (Regular), CCZ HS and CCZ EX. While Regular offers lower thermal expansion across a wide temperature range, CCZ-HS offers lower thermal expansion near room temperature, and CCZ EX features extremely low temperature dependence of thermal expansion in 0 to 50º C. 

We work with all three providers and can assist customers in their choices based on their applications. For customers seeking prototype or volume optics, additional considerations exist from a manufacturing standpoint.  First, material availability and processing times from these suppliers can impact your schedule. Schott provides Zerodur in blank form, which means they process the material near the final size of your order saving machining time; however, delivery lead times can be longer depending on demand to that supplier. Corning ULE is generally sold in solid boules which are shipped on a pallet and processed by the manufacturer. Ohara's CLEARCERAM-Z is also sold in raw glass bulk, but a provider can procure various sized pieces. It is always a good practice to inquire with your manufacturer to ask what they may have on-hand and whether alternate material works in your application. Also, ULE glass is a clear transparent glass while Zerodur and CLEARCERAM-Z have a yellowish tint. Lastly, ULE also has the highest working temperature of the three and their refractive indices vary slightly. 

While there are many similarities between these products, there are also many variables.  Having an extensive conversation with your manufacturer in advance of procurement will ensure a successful outcome. Connect with our sales engineers today to see how we can bring your project to the next step. sales@escooptics.com