BaTiO3grains. This critical amount of Ni (i.e. 0.7 mol%) corresponds to the solubility of Ni in BaTiO3. The exaggerated growth of the plate-like
grains in BaTiO3relates strongly to the formation of oxygen vacancies due to the presence of Ni acceptors.
© 2007 Elsevier B.V. All rights reserved.
Keywords: BaTiO3; NiO; Pressureless sintering; Grain growth; Solubility
1. Introduction
Barium titanate (BaTiO3) is a ferroelectric material with high
permittivity. The phase of BaTiO3 at room temperature is a
tetragonal phase, then transforms to cubic phase at 130◦C[1]. The phase transformation from cubic phase to tetragonal phase has received wide attention for its correlation with the ferroelec-tric characteristics of BaTiO3. Apart from cubic and tetragonal
phases, BaTiO3can also be present as other crystalline forms.
For example, a hexagonal phase is stable at a temperature higher than 1460◦C[1]. However, the attention given to the phase is relatively little.
Several studies indicated that the h-phase could be formed by sintering BaTiO3in a reducing atmosphere[2]; or by doping
acceptors, such as Mg, Al, Cr, Mn, Fe, Co, Zn, Ga, Ni, In, Cu, etc.[3–7]. The vacancy concentration within BaTiO3increases
with the decrease of oxygen partial pressure in the sintering atmosphere[8]. The addition of acceptors also induces the for-mation of vacancies[9]. To increase the concentration of oxygen vacancy is the key to the formation of hexagonal phase. A recent study indicated that polycrystalline h-BaTiO3could be used as
microwave component[10]. However, the microwave charac-teristics of h-BaTiO3single crystal are not yet available. It is
therefore of interest to prepare the h-BaTiO3single crystal.
∗Corresponding author. Tel.: +886 2 23659800; fax: +886 2 23634562.
E-mail address:tuan@ccms.ntu.edu.tw(W.H. Tuan).
Tetragonal BaTiO3tends to form large grains with equiaxed
features at elevated temperatures, especially when a small amount of Ti-rich phase is present[11]. Many researchers had taken the advantage of this behavior to produce large BaTiO3
single crystal by using pressureless sintering. The dimensions of t-BaTiO3single crystal can reach 10 mm[12].
The shape of h-BaTiO3 grains is plate-like[13]. However,
the size of such h-phase grains depends strongly on the sintering atmosphere and the amount of acceptors. To the best knowledge of the present authors, the size of the h-BaTiO3grains can be
produced by pressureless sintering in previous studies is less than 150m.
The objective of the present study is to produce large h-BaTiO3grains by using pressureless sintering in air. In order
to achieve this target, a small amount of NiO is added into BaTiO3. The processing conditions to produce such large
h-phase grains are reported here. The electrical properties of the h-BaTiO3single crystal will be discussed in a separate report.
2. Experimental procedure
Barium titanate powder (NEB, Product No. 52909, Ferro Co., USA) and various amount of nickel nitrate (ACROS Organics Co., USA) were tumble milled together in ethyl alcohol for 4 h. The Ba/Ti ratio of the BaTiO3powder
as reported by the manufacturer was 1.000± 0.002. The total amount of other impurities (SrO∼ 150 ppm, CaO ∼ 20 ppm, Fe2O3∼ 70 ppm, SiO2∼ 75 ppm,
Al2O3∼ 75 ppm) was less than 400 ppm. The grinding media used was zirconia
balls. The slurry of the powder mixtures was dried using a rotary evaporator. The dried lumps were then crushed and passed through a plastic sieve. The powder was calcined in air at 500◦C for 2 h to remove the nitrate; then the 0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
Fig. 1. Morphology of BaTiO3(a and b) and Ni-doped BaTiO3(c) specimens after sintering at 1400◦C in air for 2 h. The BaTiO3in (b) was sintered in a flowing
H2/N2.
powder was sieved again. The powder was formed into disks by pressing uni-axially at 25 MPa. The size of the discs was 10 mm in diameter and about 3 mm in thickness. The sintering was performed in air at 1330–1400◦C for 2 h. The heating and cooling rates were 3◦C min−1. For comparison purpose, several BaTiO3specimens were also prepared and sintered in a flowing gas mixtures of
nitrogen (95%) and hydrogen (5%).
The density was determined by the water displacement method. The pol-ished specimens were prepared by grinding with SiC particles and polishing with Al2O3particles. The grain boundary was revealed by etching with a dilute
solution of HCl and HF or by etching thermally at a temperature 100–120◦C below the sintering temperature for 30 min. The microstructure was observed by an optical microscope (OM) or a scanning electron microscope (SEM). The grain size was determined by applying an image analysis technique on the photos taken from OM or SEM. Phase identification was performed by X-ray diffractometry (XRD) at a scanning rate of 0.05◦2 s−1. A very slow scanning rate (0.002◦2 s−1) was also used to determine the lattice constants of c and a by using the (0 0 2) and (2 0 0) reflections. The c/a ratio was then obtained.
3. Results
XRD analysis reveals only tetragonal BaTiO3and NiO in the
powder after the calcination at 500◦C.Fig. 1(a) and (b) show the surface of the BaTiO3samples after sintering at 1400◦C in air.
The color of BaTiO3specimen,Fig. 1(a), is light brownish after
sintering. However, the color of the BaTiO3specimen sintered
in an oxygen-lean environment (flowing H2/N2) is dark blue
(seeFig. 1(b)). The color of Ni-doped BaTiO3specimen is also
very dark after sintering in air (Fig. 1(c)). Several large grains are long enough to spread across the entire cross-section of the disc. The maximum length of such large grains is around 15 mm, which is close to the diameter of the disc.
Fig. 2 shows the XRD patterns of BaTiO3 and Ni-doped
BaTiO3 specimens after sintering at 1330◦C and 1400◦C in
air. Only tetragonal phase is found in the BaTiO3 specimens.
After the addition of 0.7 mol% Ni, the tetragonal phase remains the only phase present in the specimen sintered at 1330◦C. However, a hexagonal phase is found as the major phase in the Ni-doped BaTiO3specimen as the sintering temperature is
raised to 1400◦C. Furthermore, a small amount of Ba6Ti17O40
(B6) phase is also found.
Fig. 2. XRD patterns of BaTiO3and Ni-doped BaTiO3specimens after sintering
at 1330◦C or 1400◦C.
The values of c/a ratio are shown as a function of Ni content inFig. 3. The ratio drops from 1.010 to a lower value of 1.004 as the Ni content is higher than 0.7 mol%, then stabilize at 1.004 with the further increase of Ni content. It indicates that Ni2+ion
Fig. 4. Relative density of Ni-doped BaTiO3 specimens as a function of Ni
content.
is capable to dissolve into BaTiO3crystals and the solubility of
Ni2+ion in BaTiO3is 0.7 mol%.
The relative densities of the Ni-doped BaTiO3specimens are
shown as a function of Ni content inFig. 4. As the Ni content is lower than the solubility, the presence of Ni solute reduces the density of the specimens. The density reaches its lowest value as the Ni content is close to its solubility in BaTiO3. As
the Ni content is higher than the solubility, the density of all the specimens increases to a value of 98%, which shows little dependence on the Ni amount added.
Fig. 5shows the microstructures of the BaTiO3specimens
after sintering at 1330◦C and 1400◦C for 2 h in air. The BaTiO3specimen sintered at 1330◦C shows a typical bimodal
microstructure for pure barium titanate specimens. The large grains inFig. 5(a) are twinned BaTiO3 grains and are larger
than the untwined grains (the twin plane can be seen as a straight
like large grains are formed after sintering at 1400◦C (Fig. 7). The length of such plate-like grains reaches a value over 10 mm. As Ni content increases to 3.6 mol%, the size of BaTiO3grains
remains small even when the sintering temperature is increased to 1400◦C (Fig. 8).Table 1summarizes the dimensions of the BaTiO3grains in the BaTiO3and Ni-doped BaTiO3specimens.
It demonstrates that the exaggerated grain growth is found only within a narrow composition range and above a certain sintering temperature. Since the sintering temperature is higher than the eutectic temperature of BaTiO3and TiO2[1]; a liquid phase is
found in the Ni-doped BaTiO3specimens after sintering at about
1385◦C. The liquid phase mainly locates at the boundaries of BaTiO3grains.
4. Discussion
There are many techniques available to produce single crys-tals; however, the pressureless sintering is the most economically
Fig. 5. Micrographs of BaTiO3specimens after sintering at 1330◦C (a) and 1400◦C (b).
Fig. 7. Micrographs of 0.7 mol% Ni-doped BaTiO3specimens after sintering at 1330◦C (a) and 1400◦C (b).
Fig. 8. Micrographs of 3.6 mol% Ni-doped BaTiO3specimens after sintering at 1330◦C (a) and 1400◦C (b).
viable process. Nevertheless, the size of the single crystal can be prepared by applying this technique is relatively small, less than 10 mm, as reported in the previous studies[4,5,13]. It is interesting to note that the size of the plate-like BaTiO3grains
prepared in the present study seems to depend strongly on the size of the specimen. We could always observe some large grains grow from one end of the disc to another end. As the diameter of disc is 8 mm, the maximum length of the large grain is close to 8 mm; as the diameter of the disc is 15 mm, the largest grain can then reach 15 mm (seeFig. 1(c)).
Several techniques have been employed to determine the sol-ubility of various dopants in BaTiO3[14–16]. The most popular
analysis techniques involve the measurement of the Curie tem-perature and lattice parameter. However, the Curie temtem-perature of dielectric is affected by many factors, such as the presence of internal stress, external stresses and microstructural characteris-tics[16]. To determine the solubility by measuring the lattice parameters has thus received wide popularity. In the present study, the c/a ratio is used instead of the lattice parameters of c and a. The two peaks, (0 0 2) and (2 0 0), used in the present study
to determine the c/a ratio are very close to each other, therefore a calibration is no longer needed to ensure the accuracy for the c/a values.
In the present study, the hexagonal BaTiO3 phase starts to
form at a temperature as low as 1385◦C. This temperature is lower than the temperature used in the previous study to prepare h-BaTiO3. In their study, a minimum temperature of 1400◦C
was used[5]. Though the temperature used in the present study is relatively low, the addition of a certain amount of Ni, around 0.7 mol%, is essential for the formation of hexagonal phase. This critical amount is close to the solubility of Ni in BaTiO3.
The solubility of a dopant in BaTiO3depends not only on
the charge but also on the radius of the dopant[14]. Though the charge of Ni2+ion is the same as that of Ba2+, the radius of Ni2+ ion (0.069 nm) is close to that of Ti4+ion (0.0605 nm) instead
[17]. Therefore, Ni2+ion is possible to substitute Ti4+ion, such as the reaction shown below:
NiBaTiO−→ Ni3 Ti+ Vo•• (1)
Table 1
Size of BaTiO3grains in BaTiO3and Ni-doped BaTiO3specimens after sintering at the indicated temperatures in air for 2 h
1330◦C (m) 1370◦C (m) 1385◦C (m) 1400◦C (m) 0 mol% Ni N: 0.98± 0.47(t), A: 106± 41(t) 250± 138(t) 180± 66(t) 160± 56(t) 0.4 mol% Ni 0.61± 0.16(t) 170± 76(t) 210± 74(t) 200± 55(t) 0.7 mol% Ni 0.42± 0.18(t) 0.35± 0.18(t) L: 4780± 2500(h), W: 370± 160(h), N: 35 ± 19(t) L: 3130± 2600(h), W: 650± 350(h), N: 33 ± 16(t) 3.6 mol% Ni 0.36± 0.18(t) 0.47± 0.22(t) 0.40± 0.18(t) 0.40± 0.19(t)
BaTiO3modification is indicated in parenthesis. The diameter of the specimen is 8 mm. Note: N: size of normal grains; A: size of abnormal grains; L: length of
of oxygen vacancy.
As the Ni content is lower than the solubility, the presence of the Ni2+ solutes slows down the densification of BaTiO3;
nevertheless, enhances its grain growth. The grain size of the Ni-doped BaTiO3specimens sintered at 1330◦C is small; it may
be resulted from their low density. Raising the sintering temper-ature can compensate the decrease of densification. However, the density of the 0.7 mol% Ni-doped BaTiO3 specimen after
sintering at 1400◦C is still low (seeFig. 4). It is contributed by the formation of many cracks (seeFig. 7), which may correlate to the formation of hexagonal phase.
As the Ni content is higher than the solubility limit, the growth of large BaTiO3grains is prohibited. As stated above,
the addition of fine NiO particles introduces many vacancies into BaTiO3. However, the increase of NiO particles may also
increase the number of nuclei of large grains. The number of these nuclei may be too high to allow their growth. The size of the BaTiO3grains is thus small after sintering. The transformation
from tetragonal to hexagonal phase is also not observed. There is a strong connection between the presence of large grain and the formation of hexagonal phase. It seems that the formation of large grain triggers the phase transformation. The possible reason needs further investigation.
The formation of a liquid phase (B6) seems also helpful to the growth of large grains. The solution of Ni ion into BaTiO3
pushes a little bit amount Ti ion out of the crystal into grain boundary area as the reaction shown below:
tor and induces the increase of oxygen vacancy concentration. The vacancy concentration reaches its highest value as the Ni addition approaches its solubility. The large amount of oxygen vacancy helps the formation of h-BaTiO3and large plate-like
grains.
References
[1] K.W. Kirby, B.A. Wechsler, J. Am. Ceram. Soc. 74 (1991) 1841. [2] A. Reˇcnik, D. Kolar, J. Am. Ceram. Soc. 79 (1996) 1015. [3] R.M. Glaister, H.F. Kay, Proc. Phys. Soc. 76 (1960) 763.
[4] H.T. Langhammer, T. M¨uller, K.H. Felgner, H.P. Abicht, J. Am. Ceram. Soc. 83 (2000) 605.
[5] M.H. Lin, H.Y. Lu, Philos. Mag. A 81 (2001) 181.
[6] H.T. Langhammer, T. M¨uller, R. Bottcher, H.P. Abicht, Solid State Sci. 5 (2003) 965.
[7] G.M. Keith, M.J. Rampling, K. Sarma, N.Mc. Alford, D.C. Sinclair, J. Eur. Ceram. Soc. 24 (2004) 1721.
[8] N.H. Chan, R.K. Sharma, D.M. Smyth, J. Am. Ceram. Soc. 64 (1981) 556. [9] N.H. Chan, R.K. Sharma, D.M. Smyth, J. Am. Ceram. Soc. 65 (1982) 167. [10] S.F. Wang, Y.C. Hsu, J.P. Chu, C.H. Wu, Appl. Phys. Lett. 88 (2006)
042909.
[11] Y.K. Cho, S.J.L. Kang, D.Y. Yoon, J. Am. Ceram. Soc. 83 (2004) 119. [12] H.Y. Lee, J.S. Kim, D.Y. Kim, J. Eur. Ceram. Soc. 20 (2000) 1595. [13] D. Kolar, U. Kunaver, A. Reˇcnik, Phys. Stat. Sol. A 166 (1998) 219. [14] D. Makovec, Z. Samardzija, M. Drofenik, J. Am. Ceram. Soc. 87 (2004)
1324.
[15] S.J. Shih, W.H. Tuan, J. Am. Ceram. Soc. 87 (2004) 401.
[16] H.J. Hwang, T. Nagai, T. Ohji, M. Sando, M. Toriyama, K. Niihara, J. Am. Ceram. Soc. 81 (1998) 709.
