Ceramics based on tin oxide (SnO₂) and zinc oxide (ZnO) are important materials with many applications (varistors, electrodes, gas sensors, catalysts). The sintering behavior of tin oxide is very specific because it has a high vapor pressure at relatively low temperatures, and therefore evaporation-condensation is a dominant non-densifying sintering mechanism beside surface diffusion. On the other hand, in the case of zinc oxide, which also has a high vapour pressure, the dominant sintering mechanism is a densifying one, which means that it is much easier to obtain highly dense materials by conventional sintering. Composites of these two materials could have interesting properties,  since zinc oxide improves the sinterability, while tin oxide enhances other properties (such as elastic moduli).
In this contribution we present experimental results obtained for tin oxide and zinc oxide ceramics as well as composites of SnO₂ and ZnO in different weight ratios. All samples were prepared by uniaxial pressing at 50 MPa from commercial nanopowders with initial particle sizes of about 80 nm. The as-pressed samples were conventionally sintered in air at temperatures ranging from 500 to 1400 °C with different dwell times. After sintering, the Archimedes technique was used to characterize the open porosity and bulk density of the as-sintered samples. In the case of tin oxide samples, the porosity remained constant around 50 % for all sintering temperatures, whereas in zinc oxide samples the porosity varied from 2 to 30 %. X-ray diffraction was used to check the purity of the samples (mainly the SnO₂-ZnO composites). To characterize the microstructure evolution, stereology-based image analysis was performed on micrographs obtained by scanning electron microscopy. It was found that the grain size significantly increased in both type of ceramics, from 80 nm to 2 µm in the case of SnO₂ sintered at 1400 °C and to 25 µm in the case of ZnO sintered at 1300 °C.  Characteristic microstructural differences were found with respect to the pore size, which grows significantly with increasing sintering temperature in the case of SnO₂, whereas it exhibits the usual decrease in the case of ZnO.
The elastic properties were measured by the impulse excitation technique at room temperature, and both its temperature dependence and its evolution during sintering has been determined during heating and cooling. It is shown that both tin oxide and zinc oxide ceramics show a decrease of Young’s modulus with temperature and an increase of the latter during sintering, but in contrast to ZnO ceramics, where this increase is related to a reduction of porosity, in SnO₂ ceramics it occurs without any significant changes in porosity, which indicates the importance of other microstructural descriptors (beyond porosity). Based on the experience with the end members (SnO₂ and ZnO), the behavior of SnO₂-ZnO composites is discussed.  
 
References:
[1] Šimonová P., Gregorová E., Pabst W.: J. Eur. Ceram. Soc. 2021, 41, 7816.
[2] Šimonová P., Pabst W., Cibulková J.: Ceram. Int. 2021, 47, 35333.

Acknowledgement: Project GA22-25562S, funded by the Czech Science Foundation (GA?R), and project A2_FCHT_2023_019 (specific university research).