Abstract
Recent improvements in precursor chemistry, reactor geometry and run conditions extend the manufacturing capability of traditional flame aerosol synthesis of oxide nanoparticles to metals, alloys and inorganic complex salts. As an example of a demanding composition, we demonstrate here the one-step flame synthesis of nanoparticles of a 4-element non-oxide phosphor for upconversion applications. The phosphors are characterized in terms of emission capability, phase purity and thermal phase evolution. The preparation of flame-made beta-NaYF4 with dopants of Yb, Tm or Yb, Er furthermore illustrates the now available nanoparticle synthesis tool boxes based on modified flamespray synthesis from our laboratories at ETH Zurich. Since scaling concepts for flame synthesis, including large-scale filtration and powder handling, have become available commercially, the development of industrial applications of complex nanoparticles of metals, alloys or most other thermally stable, inorganic compounds can now be considered a feasible alternative to traditional top-down manufacturing or liquid-intense wet chemistry.
Highlights
Nanoparticle powder technology is a widely applied industrial process for the preparation of advanced functional materials (Hosokawa, 2008)
The relative upconversion luminescence was measured according to the following procedure: Powders of the UC phosphors were filled in glass tubes of 1 mm inner and 1.5 mm outer diameter and fixed in a sample holder
The medium 5/5 l min-1 feeding rate led to a mixed product with hexagonal(β) and cubic(α) phases of sodium yttrium fluoride, see Fig. 2a
Summary
Nanoparticle powder technology is a widely applied industrial process for the preparation of advanced functional materials (Hosokawa, 2008). Gas-phase synthesis stands aside as an independent process to fabricate nanopowders, with aerosol methods This technique allows a multiscale and cheap production of nanoparticles. The established materials, namely spherical oxide nanoparticles, a straightforward product of gas-phase synthesis, stand at the origin of the graph. This group typically includes titanium, silicon, and aluminum oxides. Er3+ and Tm3+, as well as U3+, and several d-element ions embedded into specific matrices (Auzel, 2004) Upconversion phosphors such as β-NaYF 4: Yb, Tm or Yb, Er are regularly synthesized as bulk microcr ystalline materials by high-temperature solidstate synthesis: NaF + (1–x-y)YF3 + xErF3 + yYbF3. Flame pyrolysis is used to prepare nanoparticles of non-oxidic, doped rare earth fluorides. The cubic-to-hexagonal phase transition of NaYF4 will be examined for various syntheses and thermal treatment conditions
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