Eliminating chemical effects from thermal expansion coefficient measurements
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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.