SYNTHESIS of PURE
and Gd-DOPED CaZrO3 POWDERS
by
SELF-PROPAGATING COMBUSTION SYNTHESIS
and
ACID-BASE TITRATION VIA EDTA
I. Erkin Gonenli
and A. Cuneyt
TAS
Dept. of Metallurgical and
* I. E. Gonenli
and A. C. TAS, "Chemical Synthesis of Pure and Gd-doped
CaZrO3 Powders,"
Journal of The European Ceramic Society, Vol. 19 (13-14),
2563-2567 (1999). (----> download
cazro3.pdf)
* I. E. Gonenli
and A. C. TAS, "Chemical Synthesis of Pure and Gd-doped
CaZrO3 Powders,”
"Processing and Characterization of Electrochemical Materials and
Devices," Ceramic Transactions, Vol. 109, pp. 153-162, (Eds.) P.N. Kumta, A. Manthiram, S.K. Sundaram, and Y-M. Chiang, 2000, The
American Ceramic
* IV. Ceramics Congress,
Aqueous
solutions of calcium chloride (CaCl2.2H2O) and zirconium oxychloride
(ZrOCl2.8H2O), in appropriate volumetric amounts, were used as the starting
chemicals in the synthesis of phase-pure CaZrO3 powders. Rare earth element
doping (upto 25 at%) was
performed by using the aqueous chloride solutions of either gadolinium (Gd). Formation of the CaZrO3 powders were achieved by two
chemical synthesis techniques: (a) self-propagating combustion synthesis, and
(b) precipitation in the presence of EDTA by the technique of acid-base
titration. Phase characterization was performed by powder XRD (X-ray
diffraction).
INTRODUCTION
There
is a growing interest in calcium zirconate-based
oxides for potential sensor/device applications at elevated temperatures. In
particular, several studies have been reported on the use of calcium zirconate-based systems for monitoring oxygen (1-3),
humidity and hydrogen (4-8). In these studies, sintered polycrystalline samples
were used to characterize carrier types and the concentrations of ionic (proton
or oxygen) and electronic charge carriers as a function of temperature,
impurity distribution, and oxygen and/or water vapor partial pressures. Pretis et al. (9) reported that undoped
calcium zirconate (CaZrO3) is
a p-type semiconductor in air. When doped with oxides such as Al2O3, Y2O3 and MgO or with a small excess of ZrO2 or CaO,
it becomes predominantly an oxygen-ion conductor (1, 2, 7).
For a sample doped with trivalent cations such as
indium, scandium and gallium, it may become predominantly a proton conductor when
exposed to a hydrogen-containing atmosphere (steam) at temperatures ranging
from 600-1000°C (4, 5). The protonic conduction,
however, tends to diminish at higher temperatures and can be replaced by
electronic (hole) conduction, especially in a dry air
atmosphere (4).
CaZrO3
has also been studied for its potential use as high-temperature thermistor material (8). The electrical response of calcium
zirconate (prepared by the solid state reactive
firing of CaCO3 and ZrO2 powders at 1400°C) was found to be sensitive to
methane, but was practically unaffected by humidity and and
carbon monoxide. The use of a calcium zirconate-based
thermistor is, therefore, limited to atmospheres
without methane and/or possibly other hydrocarbon gases. The dramatic response
to methane, however, makes CaZrO3 a potential candidate material for
hydrocarbon sensing.
In
this paper, we report results of two different chemical powder preparation
techniques for synthesis of fine powders of pure and Gd-doped
(5-15 at%) CaZrO3. This study, to our knowledge, has
been the first attempt in the relevant literature for the synthesis of calcium zirconate powders by chemical means, rather than
conventional solid state firing and calcination
practices.
EXPERIMENTAL PROCEDURE
Starting
chemicals used in this study were reagent-grade CaCl2.2H2O (Riedel de Haen), ZrOCl2.8H2O (Merck), Gd2O3 (+99.9%, Ames Laboratory,
IA, USA), EDTA (C10H14N2Na2O8.2H2O, Merck), NaOH
(Merck), and Urea (CH4N2O, Riedel de Haen). GdCl3
stock solutions were prepared by reacting the oxide
powders in an HCl solution of correct stoichiometry. Two different synthesis techniques were
employed to produce pure and Gd-doped calcium zirconate powders.
In
the first technique, i.e., “self-propagating
combustion synthesis (SPCS) (10, 11),” appropriate
amounts of Ca- and Zr-chloride salts (in the case of Gd-doping, proper aliquots of Gd-chloride
solutions were added) were first dissolved in distilled water. Urea of proper
amount (which serves as the fuel / oxidant in the combustion reaction) was then
added to this solution. The solution, following its transfer into a Pyrex
beaker, was placed into an electric furnace pre-heated to 500 ± 10°C. The
combustion reaction was completed in less than 15 minutes yielding an amorphous
and foam-like powder body. This powder body was then ground, and calcined for crystallization in a stagnant air atmosphere
in 17 h at 1200°C.
In
the second technique, again appropriate amounts of Ca-, Zr-
and/or Gd-chloride salts/solution were dissolved in
distilled water. EDTA was used as a chelating agent. The cation
solution at room temperature was added in a drop-wise manner into a
concentrated NaOH solution. The formed precipitates
were filtered, washed with distilled water, dried at 90°C, and later calcined at 1200°C for 17 h.
Phase characterization of the calcium zirconate powders were achieved by powder XRD (X-ray
diffraction). The diffractometer (Rigaku
Corp., Model: D/Max-B,
Download another XRD chart
here
Synthesis of Pure and Gd-doped
CaZrO3 Powders (in the presence of EDTA)
SEM micrograph of phase-pure CaZrO3 powders produced in the
presence of EDTA in the precipitation solutions
(powders calcined at 1200°C in loose form
for 17 h)
Phase-pure CaZrO3 powders
produced by the EDTA-precipitation technique were sub-micron,
and had the lattice parameters of a = 8.0159, b = 5.7516, c = 5.5954 Å,
with an orthorhombic (space group: Pnma (62))
unit cell of the volume of 257.97 Å3.
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