Abstract
It is well established that gas/humidity-sensing properties of spinels are markedly influenced by their stoichiometry and microstructure. In this work nucleation and spinel phase development in the Zn-Cr-O system were investigated from the viewpoint of structural and morphological phenomena occurred during nanophased particle synthesis through aerosol reaction. The aerosol was generated from nitrates precursor solution using ultrasonic atomizer operated at 1.7 MHz. The influence of different decomposition schedules on the particle chemical content and morphology was determined by adjusting the processing parameters (aerosol droplet density 3.9x106 droplets/cm3, droplet velocity 0.035m/s, max. temperature 900oC and residence times 3, 6 and 9s). A composite particle structure comprised of primary crystallites sized from 22 to 44nm is revealed by SEM and TEM analysis. XRD structural analysis (crystallite size, microstrains, unit cell and ionic occupancies) is performed in accordance with procedure based on Koalariet-Xfit program. A certain degree of non-stochiometry is characteristic for all powders. Homogenous distribution of the constituting elements and Zn/Cr ratio of about 0.68 are proved by EDAX mapping analysis in 470nm sized as-prepared particles. After additional treatment at 1000oC octahedral crystals form with the (111) surface dominat. Evaluated spinel non-stochiometry (Zn/Cr=0.58) is a result of the ZnO dissolving (1.9%wt) in the stoichiometric ZnCr2O4. Determination of the way by which the ZnO is incorporated into the spinel lattice is performed according to the procedure based on calculation of both formation and attachment energies.
Highlights
Amongst the various types of solid-state humidity sensors that have been proposed in the literature the simplest are sensors which use solid electrolytes or semiconductors as the active elements [1,2]
In this work nucleation and spinel phase development in the Zn-Cr-O system were investigated from the viewpoint of structural and morphological phenomena occurred during nanophased particle synthesis through aerosol reaction
A composite particle structure comprised of primary crystallites sized from 22 to 44nm is revealed by SEM and TEM analysis
Summary
Amongst the various types of solid-state humidity sensors that have been proposed in the literature the simplest are sensors which use solid electrolytes or semiconductors as the active elements [1,2]. In order to understand how a ceramic material interacts with its environment, it is necessary to determine the atomic structure at its surface [7]. A detailed investigation of the microstructure evolution of the nanophased spinel ZnCr2O4 powder prepared by aerosol synthesis is reported in this article. Three charges of powders were collected with different droplet/particle residence time - after the first heated zone (powder charge 1), the second one (powder charge 2) and from the collecting chamber (powder charge 3). In the case of powder charge 1, the aerosol residence time was 3s at Tmax. Powder charge 3 had a residence time of 9s at Tmax. Compositional homogeneity and morphology of obtained particles were determined in accordance to scanning/transmission electron microscopy (SEM S-4500), TEM (JEOL JEM-200) and HORIBA Xray Microanalyzer EMAX-7000. Mapping of the constitutive elements was proceeded for the purpose of inner particle uniformity confirmation
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