Influence of the Starting Solution on the Growth and Morphology of Rare Earth-Doped Yttrium Oxide Spherical Particles By the Urea Precipitation Method
Since the start of this millennium, the need for amorphous submicron spherical rare earth element (REE) precursors that after subsequent heat treatment (~1000°C) forms crystalline REE-doped yttrium or gadolinium oxide (Y2O3:REE or Gd2O3:REE) phosphors has generated an active area of research.1 The original impetus was for a variety of display applications such as cathode ray tubes for high definition television and field emission devices.2 Later uses have included X-ray imaging and X–ray computed tomography (after converting these spherical REE phosphor particles to oxysulfide phosphor particles using sulfur and further heat treatment whereby they retained their former size and morphology).3 These spherical REE phosphor particles have the ability to close pack to form a thin emissive layer.2
These REE phosphor particles are produced via a urea homogeneous precipitation method and its modifications. In this work REE nitrate stock solutions which yielded spherical REE-doped hydroxycarbonate precursor particles.1 It was assumed due to the ‘boring’ similarity of the chemical properties of REEs that all the spherical phosphor particles precipitated using different dopants would have identical particle sizes and spherical morphologies. Our observations have found this not to be the case, and that their chemistry is much more interesting. For example precipitated Y2O3:Eu spherical particles are larger than Y2O3:Tb precursor particles, this is also the case after annealing at 980°C where the original morphology is retained (see Figure 1).
Also, it has been found that highly viscous Y2O3:REE and Gd2O3:REE nitrate precursor solutions used for the preparation of three-dimensional photonic structures such as inverse photonic band gap crystals4 and bio-templates5vary in stability (non-precipitation) for each dopant.
The reason behind these findings will be discussed in terms of the pH of the precursor solutions, electron configuration, oxidation state (preferred) and stoichiometry.
Figure 1. Annealed spherical phosphor particles of (a) Y2O3:Eu and (b) Y2O3:Tb.
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