Eliminating chemical effects from thermal expansion coefficient measurements

A new approach for measuring coefficient of thermal expansion (CTE) in materials that exhibit chemical expansion as well is introduced. This allows separating the expansion that is induced by temperature changes alone from the total expansion, which is induced by temperature and chemical changes in the material. Combining this with measurements that are done at constant temperature and in controlled oxygen environment can yield better understanding of the subject of solid expansion in oxides. In order to understand whether the oxygen partial pressure has an influence on the CTE or not, measurements were carried out on three different oxide materials at several oxygen activities. A comparison between SrTiO₃, SrZrO₃ and Al₂O₃ was done by a single push-rod dilatometer equipped with a system that allows purging with different gas mixtures and a pO₂ monitoring device. In order to make the experiment within a reasonable timeframe, very porous samples have been used. The experiments were done at three different temperatures: 722, 822 and 1022 °C. The pO₂ was controlled by CO–CO₂–Ar gas mixture, and monitored with an in house built zirconia sensor. The main results reveal that the CTE of the perovskites is influenced by the pO₂ while that of alumina, as expected, is not. The CTE of SrTiO₃ is more sensitive to changes in the pO₂ than that of SrZrO₃. In some of the measurements there exists a clear change of the behavior of the CTE vs. log(pO₂) at a certain oxygen partial pressure.