Particle Size Control , Sinterability and Piezoelectric Properties of BaTiO 3 Prepared by a Novel Composite-Hydroxide-Mediated Approach

The size of BaTiO3 particles was controlled by adjusting the molar ratio of the starting materials (BaCl2 + TiO2) to mineralizer (NaOH + KOH) during a composite-hydroxide-mediated approach using a novel hydrothermal reaction apparatus with a rolling system. The mean particle diameter decreased from 500 to 50 nm with a decrease in the (BaCl2 + TiO2)/(NaOH + KOH) molar ratio from 0.44 to 0.04. The powders were sintered by normal one-step sintering at 1200 ̊C for 5 h and two-step sintering in which temperature was raised to 1200 ̊C at first and then decreased to 1100 ̊C and kept at 1100 ̊C for 5 h. The BaTiO3 particles prepared with the (BaCl2 + TiO2)/(NaOH + KOH) molar ratio of 0.32 and 0.22 showed excellent sinterability and could be sintered to almost full theoretical density by both method. The sintered bodies obtained by both methods showed similarly excellent dielectric and piezoelectric properties.


Introduction
The excellent dielectric and ferroelectric properties of barium titanate make it attractive material in the field of electrceramic and microelectronics.However, the conventional synthetic methods for BaTiO 3 involves calcination of a BaCO 3 and TiO 2 powder mixture above 1200˚C, and thus often results in polydispersity and the introduction of impurities and the powders prepared by this method consist of non-uniform and coarse particles [1][2][3].To overcome these deficiencies, a number of innovative synthesis methods and chemical prcedures have been developed [4][5][6][7].Among them, the composite-hydroxidemediated (CHM) approach has been considered as an effective method to synthesize high purity BaTiO 3 [8][9][10].
In the present research a novel ball-milling assisted hydrothermal reaction was used to promote the diffusion of chemical species for a higher reaction rate as well as the uniformity of the product [11][12].By combining with the addition of a Li 2 CO 3 and V 2 O 5 mixed sintering additive [13][14][15][16], BaTiO 3 could be sintered to almost full theoretical density at a low sintering temperature of 1200˚C for 5 h.
The tendency of the electronic industry towards miniaturization leads to high interest in fabricating some materials with nanometer-scale structure.The conventional sintering prcesses are generally accompanied by rapid grain growth, which has greatly hampered efforts to produce dense materials with nanometer-scale structure.Two-step sintering is a promising approach to obtain dense nanograin ceramics because it suppresses grain growth in the final stage of sintering.The two-step sintering prcedure consists of the following two steps: (1) at first step, sintering at a constant rapid heating rate until a normal sintering temperature; (2) lowering the temperature by ca.100˚C afterwards followed by sintering at a lowered temperature until it is fully dense.In this paper, two-step sintering approach is used to densify the prepared BaTiO 3 ceramics at low temperature [17][18][19].

Synthesis Techniques
A mixture of 460 mmol of anhydrous hydroxides (NaOH/ KOH molar ratio = 51.5:48.5)was put into a Teflon ® -3 Composite-Hydroxide-Mediated Approach lined stainless steel autclave with 100 cm 3 of internal volume and a 5.5 cm outer diameter, followed by the addition of 100, 75, 50, 30, 10 mmol of BaCl 2 and TiO 2 (anatase) with ten Teflon ® balls, 1.1 cm in diameter.Then, the autclave was sealed and placed in an electric oven and heated at 200˚C for 24 h with a rotation speed of 100 rpm during the reaction.After the reaction, the autclave was taken out to allow to cool down to room temperature.The product was dispersed in deionized water to remove the hydroxide on the surface of the particles, then centrifuged and rinsed with ethanol, deionized water and acetone, three times, respectively.Finally, the obtained powders were dried overnight at 60˚C in a vacuum oven.
The prepared BaTiO 3 powders and commercial Ba-TiO 3 powders (Sakai Chemical Industry Co., BT-05, which is considered the best commercial pure BaTiO 3 having excellent properties) were precalcined at 800˚C for 1 h followed by mixing well with 0.3 wt% Li 2 CO 3 and 0.04 wt% V 2 O 5 powder by a wet-ball milling with acetone and ZrO 2 balls of 10 mm in diameter in a Teflon ® container at a rolling rate of 90 rpm for 12 h, respectively.Then, the powders were collected, dried and uniaxially pressed at 20 MPa in a steel die to form pellets, 20 mm in diameter and 3 mm thick, and then isostatically pressed at 200 MPa.The pellets were then sintered by normal one-step sintering at 1200˚C for 5 h which was designated as (1200/5) with a heating rate of 10˚C/min.The two-step sintering designated as (1200/1100/5) was also conducted, in which the sample was heated at first from room temperature to the first sintering temperature T 1 (1200˚C) with a heating rate of 10˚C/min, then the temperature was held at 1200˚C for 1 min, after that rapidly decreased to T 2 (1100˚C), and held at T 2 for 5 h, as illustrated in Figure 5 (insert).The densities of the sintered pellets were measured by the Archimedes' methods.The rectangular bars of 2 mm × 4 mm × 12 mm were prepared for piezoelectric measurements.Gold paste was fired on both sides of the disks at 800˚C as electrode.The specimens were poled in a stirred silicon oil at 80˚C by applying DC electrical fields of 2 -3 kV/mm for 30 min.About 24 h later of the poling, the dielectric and piezoelectric properties were measured using an Agilent 4294 A precision impedance analyzer at a frequency of 100 kHz.

Characterizations
The X-ray diffraction (XRD) analysis of the obtained powder samples was carried out using CuK radiation with a pyrolytic graphite monchromator mounted on a powder diffractometer (Shimadzu XD-D1).Thermogravimetric analysis (TG-DTA, Rigaku, TG8101D) was performed for the powders from room temperature to 1200˚C with a heating rate of 10˚C/min in air.The particle morphology and the microstructures of the sintered bodies were observed by a scanning electron microscope (SEM; Hitachi S-4100).

Crystalline Phase
Figure 1 shows the XRD patterns of the BaTiO 3 powders synthesized by the composite-hydroxide-mediated approach at 200˚C for 24 h with the molar ratio of starting materials (BaCl 2 + TiO 2 ) to mineralizer (NaOH + KOH) of (A) 0.44, (B) 0.32, (C) 0.22, (D) 0.12, (E) 0.04.The peaks of ( 002) and ( 200) around 2θ = 45.5˚ were enlarged in the insert to compare the powder tetragonality.All peaks of the synthesized powders were consistent with the single perovskite phase.The slight peak splitting around 2θ = 45.5˚ to (002) and ( 200) suggested the formation of the tetragonal BaTiO 3 .The tetragonality (c/a) decreased from 1.0055 to 1.0038, 1.0032, 1.0025 and 1.0013 with a decrease of the (BaCl 2 + TiO 2 )/(NaOH+ KOH) molar ratio from 0.44 to 0.32, 0.22, 0.12 and 0.04, respectively, which probably due to the difference in the particle size, since the tetragonality (c/a) of barium titanate is strongly dependent on particle size.

Thermal Analysis
The TG-DTA curves of the samples are shown in Figure 2. All samples showed three steps of weight losses.The weight loss up to 200˚C, 200˚C -800˚C and above 800˚C may be due to the elimination of water adsorbed on the surface, dehydration from the OH incorporated in the lattice and elimination of CO 2 from the contaminated BaCO 3 , respectively.The weight losses of the present samples were much smaller than those prepared by conventional hydrothermal reactions [24], especially the samples with the (BaCl 2 + TiO 2 )/(NaOH + KOH) molar ratio of 0.32 and 0.22 showed quite little weight losses of ca.1.05and 1.30%, respectively, indicating formation of high purity stoichiometric BaTiO 3 powder.According to the TG-DTA curves, the powders were calcined at 800˚C for 1 h prior to the sintering.0.22, 0.12 and 0.04 (Figure 3 A, B, C, D and E), respectively.The samples A and B showed broad size distribution from 50 to 500 nm.In contrast, the samples C, D and E showed relatively narrower size distribution of 30 to 100 nm.Generally, in the solution prcess the formation of nuclei and growth of crystals cur.When the (BaCl 2 + TiO 2 )/(NaOH + KOH) molar ratio is low, the Therefore, the product consisted of larger crystals with a large particle distribution.These results indicate that the lower (BaCl 2 + TiO 2 )/(NaOH + KOH) molar ratio seems to be beneficial to obtain a narrower size distribution as well as a smaller particle size.Comparing with the commercial BaTiO 3 particles showing irregular phase with average particle size of ca.400 -500 nm (Figure 3. F), BaTiO 3 powders prepared in the present study exhibited a smaller average particle size (Sample C, D and E) and a regular cubical shape.

Sinterablity
The sample powder was sintered at 1200˚C for 5 h with a heating rate of 10˚C .min -1 with a 0.3 wt% Li 2 CO 3 and 0.04 wt% V 2 O 5 complex sintering additive.The relative densities and grain sizes of the sintered bodies are shown in Figure 4 as a function of the (BaCl 2 + TiO 2 )/ (NaOH + KOH) molar ratio.The average grain size changed as 10 μm, 25 μm, 20 μm, 8 μm and 5μm for the samples with the (BaCl 2 + TiO 2 )/(NaOH + KOH) molar ratio of 0.44, 0.32, 0.22, 0.12 and 0.04.The samples with the (BaCl 2 + TiO 2 )/(NaOH + KOH) molar ratio of 0.32 and 0.22 could be sintered to almost full theoretical density (> 98%), but the density of other samples were less than 94%.From Figure 4, it is seen that the powder prepared with the (BaCl 2 + TiO 2 )/(NaOH + KOH) molar ratio of 0.22 possessed the highest sinterability in the present samples.These results agreed with the results that the sample showed the high purity, small particle size and uniform particle size distribution (Figures 2 and  3).Morphology of Ceramics Bodies Figure 5 showed the scanning electron micrographs of the fracture surfaces of the sintered bodies by normal one-step sintering (1200/5) and two-step sintering (1200/1100/5) using the powders with the (BaCl 2 + TiO 2 ) /(NaOH + KOH) molar ratio of 0.22 without and with a 0.3 wt% Li 2 CO 3 and 0.04 wt% V 2 O 5 complex sintering additive.For comparison the morphology of sintered bodies using the commercial BaTiO 3 powder are also shown.Although it was difficult to densify the comercial powder without sintering aid by both sintering prcess (Figure 5 E and G), where the mean grain sizes of the sintered bodies by two-step sintering and one-step sintering were 0.5 and 1 μm, respectively, the prepared BaTiO 3 could be densified without sintering aid (Figure 5 A and C), where the mean grain sizes of the sintered bodies by two-step sintering and one-step sintering were 1 and 8 1μm, respectively.It can be observed that the average grain sizes of the samples prepared by the two-step sintering were smaller than those by normal one-step sintering.Additionally, the addition of a 0.3 wt% Li 2 CO 3 and 0.04 wt% V 2 O 5 complex sintering additive greatly increased the average grain size and decreased the porosity, especially in the case of the commercial powder.

Dielectric Properties of Ceramics Bodies
The relative density, grain size, dielectric constant (ε), dielectric loss (tan δ), piezoelectric constant (d 31 , d 33 , k 31 , S 11 ), mechanical quality factor (Q m ) and electr˚Chemical coupling factor (k p ) of the BaTiO 3 samples prepared with the (BaCl 2 + TiO 2 )/(NaOH + KOH) molar ratio of 0.22 and commercial BaTiO 3 sintered by the normal sintering (1200/5) and two-step sintering (1200/1100/5), respectively, are listed in Table 1.All samples could be sintered to almost full theoretical density using a 0.3 wt% Li 2 CO 3 and 0.04 wt% V 2 O 5 complex sintering additive, and the dielectric properties of the sample prepared by the one-step sintering and two-step sintering did not change so much, indicating that the two-step sintering is useful to decrease the sintering temperature without loss of the dielectric properties.It is notable that the samples using prepared BaTiO 3 powders showed lower dielectric constant and larger piezoelectric constant than those prepared using the commercial powder, this result can be illustrated by the grain size effect and the dielectric constant at room temperature decreased with increasing mean grain size from grain size of 1μm (GS > 1 μm), but below 1 μm, dielectric constant at room temperature decreased with decreasing the mean grain size for the same sample.Comparing with the samples sintered without sintering additive, it can be seen that the addition of sintering additive decreased the dielectric constant at the room temperature, increased dielectric loss and changed the piezoelectric properties for the specimens sintered by both one-step sintering and two-step sintering.

Conclusions
The size of BaTiO 3 particles was controlled by adjusting the (BaCl 2 + TiO 2 )/(NaOH + KOH) molar ratio during a composite-hydroxide-mediated approach using a novel hydrothermal reaction apparatus with a rolling system.The mean particle diameter decreased from 500 nm to 50 nm with the decrease of the (BaCl 2 + TiO 2 )/(NaOH+KO H)molar ratio from 0.44 to 0.04, where the sample with the molar ratio of 0.22 exhibited the excellent sinterabilty and dielectric properties due to the small particle size, uniform morphology and narrow size distribution.The two-step sintering was useful to decrease the sintering temperature and to obtain the mean grain size without loss of the dielectric properties.

Figure 3 Figure 1 .
Figure3shows the morphologies of the prepared Ba-TiO 3 powders together with those of commercial BaTiO 3 powders.All samples showed high quality crystal characteristics of cubes or tetrahedrons.The average particle size of BaTiO 3 prepared in the present study decreased as 400, 200, 100, 60 and 50 nm with a decrease in the (BaCl 2 + TiO 2 )/(NaOH + KOH) molar ratio as 0.44, 0.32,