Mechanochemical synthesis of NaNbO3

Article abstract of DOI:10.1016/j.materresbull.2004.10.018

The mechanochemical synthesis of NaNbO₃ is studied. It is shown that NaNbO₃ can be prepared by milling the constituents, i.e. Na ₂CO₃ and Nb₂O₅ in the planetary mill. After 40 h of mechanochemical treatment NaNbO₃ nanoparticles in the range of 10-20 nm are obtained. Furthermore, the high-energy milling leads to the mechanochemically-triggered carbonate decomposition, which has been observed for a few cases in the open literature.

Full text of DOI:10.1016/j.materresbull.2004.10.018

Materials Research Bulletin 40 (2005) 341–345
www.elsevier.com/locate/matresbu
Mechanochemical synthesis of NaNbO3
*
Tadej Rojac , Marija Kosec, Barbara Malic, Janez Holc
ˇ
Jozef Stefan Institute, Electronic Ceramics Department, Jamova 39, 1000 Ljubljana, Slovenia
Received 28 July 2004; accepted 21 October 2004
Abstract
The mechanochemical synthesis of NaNbO3 is studied. It is shown that NaNbO3 can be prepared by milling the
constituents, i.e. Na2CO3 and Nb2O5 in the planetary mill. After 40 h of mechanochemical treatment NaNbO3
nanoparticles in the range of 10–20 nm are obtained. Furthermore, the high-energy milling leads to the
mechanochemically-triggered carbonate decomposition, which has been observed for a few cases in the open
literature.
# 2004 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides; C. Electron microscopy; C. X-ray diffraction
1. Introduction
Sodium niobate (NaNbO3) is of great interest because it exhibits a number of phase transitions
between paraelectric, antiferroelectric and ferroelectric phases [1–4]. When mixed with a small amount
of KNbO3 (0.6 at.% of K or K0.006Na0.994O3), NaNbO3 becomes ferroelectric and shows an improvement
in its piezoelectric properties [5]. A broad peak in the piezoelectric properties was also observed close to
the K0.5Na0.5NbO3 (KNN) composition [6,7]. Furthermore, it has been shown that the addition of
LiNbO3 to NaNbO3 produces a ferroelectric phase [8,9]. For this reason, NaNbO3 has shown an
increasing interest due to its role in the production of lead-free piezoelectric ceramics.
The conventional processing of alkaline niobate powders usually involves multiple steps of phase-
forming calcination at high temperatures, which can lead to the volatilisation of alkaline species and, as a
result, the formation of undesirable secondary phases. Furthermore, agglomerates are often formed
* Corresponding author. Tel.: +386 1 477 38 34; fax: +386 1 426 31 26.
E-mail address: tadej.rojac@ijs.si (T. Rojac).
0025-5408/$ – see front matter # 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2004.10.018

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during heating, which can adversely affect the sinterability of the resulting powder. In order to avoid such
drawbacks, a mechanochemical route can be employed to synthesize the NaNbO3 using alkaline
carbonates and niobium (V) oxide as the starting compounds.
Mechanochemical synthesis was originally developed for the production of alloys and intermetallic
compounds in 1966 [10]. Its major advantage is that the solid-state reaction is activated by the
mechanical energy rather than the calcination at elevated temperatures. Since the mechanochemical
technique has shown an increasing interest for the synthesis of complex ceramic oxides, great efforts
have been made in order to understand the basic principles and the mechanisms involved. Recently,
attention has been paid to the influence of different starting compounds, i.e., oxides, hydroxides and
carbonates, on the course of the mechanochemical reactions during milling. It has been shown to be
possible to synthesize a variety of feroelectric materials, such as PbTiO3 (PT) [11], Pb(Mg1/3Nb2/3)O3
(PMN) [12], Pb(Sc0.5Ta0.5)O3 (PST) [13] and SrBi4Ti4O15 (SBIT) [14] using a single-step mechan-
ochemical technique and starting from oxide precursors. In addition, a novel soft mechanochemical
procedure has been developed by Senna [15] using a mixture of oxides and hydroxides as the starting
compounds. In this case the mechanochemical reaction proceeds via a superficial acid/base mechanism,
which allows a reduction in the subsequent synthesis temperature to a lower expence of mechanical
energy.
Several attempts have been made to synthesize various complex oxides using mechanochemical
synthesis starting from a powder mixture of carbonates and oxides, such as BaTiO3 [16–20], SrTiO3 [21],
Sr2TiO4 [21], Sr3Ti2O7 [22], SrFeO2.5 [23], LiFeO2 [24], (BaxSr1Àx)TiO3 (0 < x <1) [25] and BaAl2-
Si2O8 [26]. No formation of the final products was observed during the mechanochemical treatment.
However, in all the cases the mechanical activation of the starting compounds enhances the reactivity of
ˇ
the as-milled powders and decreases the synthesis temperature. In contrast, Milosevic and co-workers
[27,28] reported the mechanochemical formation of Na2SeO3 starting from a mixture of Na2CO3 and
SeO2. Similar observations were made for LiMn2O4 synthesis, where LiCO3 reacts with MnO2 almost
completely [29]. On the other hand, no observable interaction between Mn2O3 and MnO with Li2CO3
occurs, showing the importance of the oxidation state of manganese on the course of the mechan-
ochemical reaction.
Recently, a new synthesis route based on high-energy ball-milling has been applied to obtain NaNbO3
from a starting powder mixture of Na2CO3 and Nb2O5 [30]. However, NaNbO3 was not formed during
milling. A prolonged milling (30 days) leads to the amorphization of the Na2CO3 and a reduction of the
crystallite size. This in turn leads to the lowering of the temperature required for the completion of the
reaction between Na2CO3 and Nb2O5 from 750 8C for the non-activated sample to 600 8C for the 30 days
mechanochemically activated sample.
Here, we present the study of the mechanochemical synthesis of NaNbO3 using Na2CO3 and Nb2O5 as
the starting compounds. The aim of the work is to follow the course of the mechanochemical reaction
between Na2CO3 and Nb2O5. The possibility to prepare NaNbO3 by mechanochemical synthesis is
discussed.
2. Experimental details
Na2CO3 (Alfa, 99.95–100.05%) and Nb2O5 (Alfa, 99.5%) were used as the starting compounds for the
mechanochemical synthesis of NaNbO3. Sodium carbonate powder was dried at 200 8C for 1 h prior to

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use because of its hygroscopic nature. The Nb2O5 was used as received. 1.42 g of Na2CO3 and 3.58 g of
Nb2O5 (5 g in total), corresponding to a molar ratio of Na/Nb = 1:1, were weighed and placed in the vial.
The high-energy ball-milling was carried out in a planetary ball mill (Retsch PM 400), specially equipped
for mechanical alloying, operated at 300 rpm. The ratio between the vial rotational speed and the
supporting disk rotational speed was 2:1. A 125 ml YSZ vial and 40 YSZ balls (YTZ Grinding Media,
Nikkato Corporation) with diameters of 10 mm were used as the milling equipment. The mass of the 40
YSZ balls was 132 g, corresponding to a ball-to-powder weight ratio of 26:1. The powders were
mechanochemically treated for various times ranging from 0 to 40 h.
X-ray powder diffraction (XRD) analysis with Cu Ka radiation (Bruker AXD-D4 Endeavor) was used
to follow the changes in the phase composition during the mechanochemical treatment. The powder
morphology after high-energy ball-milling was characterized using transmission electron microscopy
(TEM, JEOL 2000FX).
3. Results and discussion
Fig. 1 shows the XRD patterns of the initial mixture of Na2CO3 and Nb2O5 and the same mixture after
the mechanochemical treatment for 5, 10, 20 and 40 h. The starting powder mixture is composed of
monoclinic Na2CO3 [31] and monoclinic Nb2O5 [32]. After 5 h of milling the peaks corresponding to the
Na2CO3 disappear, whereas those of the Nb2O5 are still present but with a reduced intensity. Furthermore,
the broadening of the Nb2O5 peaks indicates a reduction of the crystallite size and a degree of
amorphization, which results from the mechanical energy. New peaks at 2# ꢀ 23.88, 32.48 were
observed after 5 h of milling, and they were assigned to the strongest NaNbO3 peaks [33]. This indicates
Fig. 1. XRD patterns of the initial mixture of Na2CO3 and Nb2O5 and after mechanochemical treatment for 5, 10, 20 and 40 h
(notation: & , Na2CO3; ^, Nb2O5; *, NaNbO3; and Al, aluminium).

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that the mechanochemical reaction between Na2CO3 and Nb2O5, which yields NaNbO3, is triggered by
mechanical energy. The peak denoted with ‘‘Al’’ is due to the sample holder.
The intensities of the NaNbO3 peaks increase with increasing mechanochemical treatment time, from
5 to 10 h, at the expense of the Nb2O5 peaks. In the 20-h mechanochemically treated powder, NaNbO3 is
˚
the predominant phase detected by XRD. A small peak with an interplanar spacing of d = 2.96 A is
observed after 20 and 40 h of milling. This peak could be attributed to ZrO2 [34], and it is most probably
due to a minor contamination from the milling equipment.
Fig. 2. TEM BF (a) and DF (b) images of NaNbO3 powder after 40 h of high-energy milling.
TEM BF and DF images of NaNbO3 formed after 40 h of high-energy milling are shown in Fig. 2.
From the BF image (Fig. 2a) it is seen that the powder consists of agglomerates of 100–300 nm in size.
However, the TEM DF image of the same area (Fig. 2b) shows that the agglomerates are composed of
smaller particles of a size between 10 and 20 nm.
4. Conclusions
From the results we were able to conclude that NaNbO3 can be prepared by mechanochemical
synthesis starting from a mixture of Na2CO3 and Nb2O5, in contrast with Castro [30], where only an
increased reactivity of the starting compounds was observed. This implicates advantages offered by
mechanochemical synthesis in terms of the possibility to omit the currently employed multiple steps of
phase-forming calcinaton at elevated temperatures. We also showed that NaNbO3 nanoparticles in the
range 10–20 nm can be prepared by a single-step mechanochemical treatment. In addition, the results of
the XRD analysis show that an appropriate energy input leads to the mechanochemically-triggered
carbonate decomposition, which has been reported in occasions in the open literature [27–29].

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Acknowledgement
The work was supported by the Ministry of Education, Science and Sport of the Republic of Slovenia.
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