Grain Morphology Memory: Yttria as a Study Case

Abstract

This work shows how it is possible to orientate powder grain morphology characteristics through synthesis conditions both for as-synthesized powders (T = 115°C) and for calcined powders (T = 800 or 1100°C). The chosen example deals with the synthesis of fine yttria powders, using chemical methods. As a first point of interest, the preparation of the as-synthesized powder is discussed. A sol-gel method has been developed in which the chemical nature of the yttrium chelating agent and the [chelating agent/Y] relative concentration are the two main parameters. Depending on these conditions, very different grain morphologies were obtained, needles, platelets, gel-like, in relation to the chemical and structural composition of these precursor powders. The second point of interest deals with the morphology of the yttria powder obtained after thermal treatment of these precursors at high temperature. In some cases, the original morphologies were maintained all over the calcining treatment whereas chemical composition changes were large. Thermodynamics consideration are also proposed to explain the final grain evolution at 1100°C. This yttria study case shows that a "memory effect" can actually be observed all over a large temperature range when the synthesis conditions constrain enough the precursor grain formation from a chemical and structural point of view.

Preparation and properties of uncommon Cd‐Mn carboxylate complexes—per se and as precursors for CdMn2O4‐based ceramics

“…On a smaller scale, a further structuring occurs, yielding single framework of aggregated 200‐ to 500‐nm particles with regular slit‐like pores (Figure 8a). One can thus conclude that a certain grain morphology memory occurs upon thermal decomposition of 2 [ 29 ] under mentioned conditions. This effect is apparently due to gradual loss of EtOH ligands from the entire particle of 2 upon slow heating combined to comparatively small amount of gaseous products evolving during further oxidative degradation of “[Cd 2 (OOCCym) 4 ]” (Cym = C 5 H 4 Mn(CO) 3 ), that is, of desolvated 2 .…”

“…As it is shown on Figure 8b,c, both grain morphology memory effect and regular slit‐like pores almost disappear in both the samples annealed at 900°C for 3 h with heating rate of 7°C/min, not to mention that annealed at 1000°C for 48 h with heating rate of 30°C/min (Figure 8d). The morphology of samples prepared via annealing for 3 and 4 h is typical for spinel oxides prepared by precursor routes with strictly followed M n+ to organic ligands stoichiometry, [ 30a ] and exhibits stronger aggregation and larger average grain size comparing to ones prepared by sol–gel. [ 30b–d ] Compared to the latter case, differences in morphology of oxides produced from 2 are apparently due to comparatively small amounts of gaseous products upon combustion of “[Cd 2 (OOCCym) 4 ]” versus those upon combustion of gels with the excess of organic component.…”