Patent ID: 8328417

Claim:
A method for determining the CTE zero crossover temperature Tzc of a material, said method comprising: providing an apparatus having a top and a bottom block of a high thermal conductivity material, and elements for independently heating and/or cooling each of said blocks, a source of polarized light and a detector for measuring changes in the polarization of said light, and said apparatus being within a chamber; providing a sample of a material having a selected length, width and height, opposing top and bottom faces, and a plurality of side faces, said material being transparent to light passing through the material and said material having a zero crossover temperature in its expansivity versus temperature curve; positioning said sample between said top and bottom blocks such that the top face of the sample is in thermal contact with the top block and bottom face is in contact with the bottom block; independently heating or cooling the top and bottom faces of the sample to a selected temperature, wherein the selected top face temperature T t is different from the selected bottom face temperature T b , and maintaining the selected temperature of the sample top and bottom faces for a time sufficient to establish a thermal gradient between the top and bottom faces; measuring the stress distribution within the sample using a photoelastic technique consisting of measuring the changes in polarization state of polarized light traversing the sample in a direction parallel to the isothermal planes within the sample established by the thermal gradient, to determine the horizontal sample plane having the highest tensile stress; measuring the temperature profile of the sample along the vertical axis; and determining Tzc from the temperature value of the plane having the maximum tensile stress; wherein said material is selected from the group consisting of glasses and glass-ceramics having a CTE zero crossover temperature Tzc.