The coefficient of thermal expansion of highly enriched 28Si

Abstract

For the new definition of the SI unit of mass based on a fundamental constant, a redetermination of Avogadro's constant is the goal of an international collaboration of numerous national laboratories and universities. Since a relative uncertainty of about 2 × 10−8 is aimed at, the macroscopic density, the isotopic composition and the volume of the unit cell of a silicon single crystal have to be measured with high precision. One step to improve the precision was the production of a silicon crystal of highly enriched 28Si. This paper addresses the effect of thermal expansion of that material in order to account for a possible discrepancy between the coefficient of thermal expansion (CTE) of natural silicon and that of 28Si. The results of two independent CTE measuring methods are presented and compared in this paper.