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Proceedings of the 4th International Conference on Nanostructures (ICNS4)
12-14 March, 2012, Kish Island, I.R. Iran
Synthesis of a New NASICON Structure by Pechini-Type Sol-Gel Method
F. Ejehia*, S. P. H. Marashia, D. Haghshenasa, M. Ghaanib, A. Nekahia
a
Department of Mining and Metallurgical Engineering, Amirkabir University of Technology,
Tehran, 15875-4413, Iran
b
Department of Material Science, University of Milano Bicocca,
Milano, 20125, Italy
*[email protected]
Abstract: In this research, a new NASICON-type (Na-Super Ionic Conductor) nanostructure, ZrNb(PO4)3 was
synthesized through Pechini-type sol-gel method for the first time. The effect of different heat treatment procedures
was studied on the purity of the produced structure. X-Ray diffraction patterns (XRD) were obtained to analyze the
developed phases and simultaneous thermal analysis (STA) was performed to investigate the phase evolution of the
product during heat treatment and to identify the appropriate heating temperatures. The procedure of synthesis was
optimized to obtain the maximum purity and consequently the maximum ionic conduction in the final product. The
obtained structure can be used as separators, ion-exchange membranes, and sensors.
Keywords: NASICON; Zirconium; Pechini method; heat treatment.
Introduction
+
NASICON (Na Super Ionic CONductor) structure is a
kind of solid electrolyte which shows high ionic
conductivity. In NASICON type structure, with general
formula of AM(PO4)3, the mobile ions have enough space
to migrate through the surrounding anions.
Rhombohedral NASICON structure consists of a threedimensional rigid framework with MO6 octahedra and
PO4 tetrahedra sharing common corners [1]. The
interstitial spaces among these sites provide appropriate
channels for migration of the mobile ions. The choice of
an appropriate atom (M) in NASICON structures
significantly improves the conduction of mobile ion,
especially at higher valences. In the case of tetravalent
Zr4+ ion, M should be a penta- (or more) valent ion to
make a higher electrostatic interaction with anions and to
enhance Zr4+ ion mobility, and accordingly, Nb5+ and
Ta5+ can be suitable choices. ZrNb(PO4)3 [2] and
ZrTa(PO4)3 [3] structures were first synthesized via solid
state method.
Among different types of sol-gel routes, Pechini has been
favorably employed due to the very stable citrate/metal
complexes formed during this process [4]. Moreover,
there is no precipitate in the sol which is a very common
problem in other methods [5]. In the present work, the
effect of single-pass and multiple-pass heat treatment on
the synthesis of ZrNb(PO4)3 NASICON structure based
on Pechini sol-gel method was studied.
Experimental
In order to synthesize ZrNb(PO4)3 structure the following
precursors with analytical grade were used; ZrOCl2.8H2O
(Sigma-Aldrich), NbCl5 (Alfa Aesar), NH4H2PO4 (Sigma-
504
Aldrich), citric acid (Merck), ethylene glycol (Merck),
ammonia (Merck), and hydrogen peroxide (Merck).
Niobium solution was prepared by dissolving desired
amount of NbCl5 in diluted ammonia; thereafter, citric
acid (CA) and hydrogen peroxide were added and the
mixture was stirred and heated at about 70°C for an hour.
The obtained transparent solution was mixed with the
water solution of NH4H2PO4. Then, the solution of
ZrOCl2.8H2O (containing CA) and ethylene glycol (EG)
was added to the sol. The final solution was heated to
obtain a gel at 96°C and dried at 150°C in oven for 5
hours. Then, the dry gel was heated at 700°C, 1000°C,
and 1200°C as single-pass heat treatments. The structure
of the obtained white powders was studied by X-ray
powder diffraction (XRD) using CuKα radiation (Bruker
D8). The 2θ range in the XRD analysis was between 5
and 70̊ with a step size of 0.031̊. Simultaneous thermal
analysis (STA) including thermogravimetric and
differential thermal analyses (TG-DTA) was performed
on dry gel with a NETZSCH STA 409 PC/PG instrument
to improve the heat treatment based on the obtained
results. Therefore, the multiple-pass heat treatment was
applied at 300°C, 950°C, and 1300°C for 12 hours at each
temperature on the dry gel in order to obtain an
appropriate structure.
Results and Discussion
The dry gel contains the main ions in NASICON
structure (i.e. zirconium, niobium, and phosphate) and a
lot of additional materials which must be eliminated.
Therefore, there are two purposes in applying heat
treatment: 1) elimination of additional material, 2)
crystallization of NASICON structure. In this regard, two
types of single-pass and multiple-pass heat treatment
Proceedings of the 4th International Conference on Nanostructures (ICNS4)
12-14 March, 2012, Kish Island, I.R. Iran
were employed. The single-pass procedure was applied at
3 different temperatures of 700°C (Fig 1.), 1000°C (Fig.
2), and 1200°C (Fig. 3). The XRD pattern of 700°C is
revealed an amorphous structure which means that more
heating is required. According to Fig. 2, a crystalline
phase was produced by heating at 1000°C, however, it
needed heating at higher temperatures. As can be seen in
Fig. 3, the obtained structure after heating at 1200°C is
completely crystalline and the main peaks of NASICON
structure are observable even though not very clear.
Additionally, there are some impurity phase peaks which
probably belong to niobium and zirconium phosphates.
Fig. 1. The XRD pattern of the gel dried at 96°C and
heated at 700°C.
To obtain a better structure, simultaneous thermal
analysis (STA) was carried out on dried gels (Fig. 4). The
first weight loss at 25-190°C range observed on TG
diagram is due to the water elimination of dry gel [6]. The
second weight loss at 190-930°C is related to the
decomposition of polymer to CO2 and H2O [7]. The third
decrease in weight of the sample heat treated at 9301020°C is attributed to the decomposition of starting
materials via organic precursor chemical reactions. The
last small weight loss at 1020-1370°C in the TG curve is
ascribed to the elimination of oxidized carbon from the
decomposed organic compounds.
DTA curve confirms the above mentioned
interpretation. The first endothermic peak at 307°C is due
to the decomposition of the polymer to CO2 and H2O [7].
The second peak at 993°C is related to the decomposition
of the starting materials with organic precursor chemical
reactions between the precursors, which resulted in the
formation of a crystalline phase [8]. The third peak at
1344°C shows the sintering temperature which is not
accompanied by a noticeable weight loss in TG curve.
TG
DTA
Fig. 4. The STA curves of the dried gel at 96°C.
Fig. 2. The XRD pattern of the gel dried at 96°C and
heated at 1000°C.
Based on the above mentioned temperatures at which
considerable weight loss happened, multiple-pass heat
treatment was chosen at 300°C, 950°C, and 1300°C for
12 hours to give enough time to eliminate carbon and
complete the crystallization. Fig. 5. Shows the XRD
pattern of the powder heated at 300°C and 950°C. The
crystalline structure indicates that heating in two stages
decreases the temperature at which the amorphous phase
disapears.
Fig. 3. The XRD pattern of the gel dried at 96°C and
heated at 1200°C for 12 hours. The impurity peaks are
marked.
505
Proceedings of the 4th International Conference on Nanostructures (ICNS4)
12-14 March, 2012, Kish Island, I.R. Iran
influence of the heat treatment on the purity of the
product and the discussed evolutions during the heating
can help to improve the procedures of synthesis.
Increasing the purity of structure would result in
enhancement of ionic conduction and improvement of the
performance of ZrNb(PO4)3 structure.
Fig. 5. The XRD pattern of the gel dried at 96°C and
heated at 300°C, 950°C for 12 hours.
The result related to the final product implies that a pure
NASICON phase with sharp and distinct peaks (Fig. 6) is
formed which perfectly matches the previous reported
work [2].
Fig. 6. The XRD pattern of the gel dried at 96°C and
heated at 300°C, 950°C, and 1300°C for 12 hours.
By calculating the lattice parameters of pure powder for
hexagonal system, the values of “a” and “c” were
obtained as 0.89 nm and 2.24 nm, respectively. The
crystallite size of this sample has a mean value of 60 nm.
Conclusions
ZrNb(PO4)3 NASICON-type structure was synthesized by
modified Pechini type method for the first time. The
method was used to obtain more homogeneity and purity
in the final solid electrolyte. The results confirm the
506
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