August 2006
Conversion of SiO2 Diatom Frustules
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temperature (Tc 5 105 K), the room-temperature XRD pattern
shows a thermodynamically stable cubic form. It is not known
whether a tetragonal phase will form even below T 5 105 K.
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IV. Summary
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The conversion of silica frustules to TiO2, BaTiO3, and SrTiO3
using a gas–solid displacement and solid–liquid reaction was
studied. Conversion of pyrolyzed frustules to TiO2 was realized
in two stages: in the first step, frustules reacted with TiF4 to form
TiOF2, which then converted to TiO2 (anatase) by heat treat-
ment in air. Morphology and microstructural details of the orig-
inal frustules have been retained through these processes. The
titania frustules were then converted to BaTiO3 hydrothermally
with Ba(OH)2 ꢂ 8H2O at 1201C for 10 h. Conversion to BaTiO3
was nearly complete and it is believed that higher temperature
and/or longer reaction times will lead to complete conversion.
The BaTiO3 formed in this process showed a cubic crystal struc-
ture, which is attributed to nanoscale crystallite size (B70 nm).21
Extensive coarsening of the BaTiO3 frustules to a particle size of
190 nm or greater would possibly result in tetragonal BaTiO3 at
room temperature. However, this may lead to loss of original
frustule morphology.
Conversion to SrTiO3 was performed using the same proce-
dure as the one used for BaTiO3. Like BaTiO3, nearly complete
conversion of SiO2 frustules to cubic SrTiO3 while retaining the
original frustule shape was achieved. As the normal tetragonal
to cubic transition is below room temperature, observation of
the cubic phase is not surprising.
The present work demonstrated that with further experimen-
tal refinement, solid–gas displacement reactions may be a viable
method of microscaled piezoelectric component synthesis.
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Acknowledgment
The assistance provided by Dr. Ozan Ugurlu in TEM study is greatly appre-
ciated.
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