Eect of NiO addition on the sintering and grain growth behaviour
of BaTiO
3
W.H. Tzing, W.H. Tuan*
Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 10764 Received 24 November 1997; accepted 16 January 1998
Abstract
The sintering and grain growth behaviour of NiO-doped BaTiO3in air are investigated in the present study. The solubility of
NiO in BaTiO3is determined by measuring the lattice constant of BaTiO3as a function of NiO content. The solubility is around
0.13 wt% as sintering temperature ranges from 1250 to 1330C. The densi®cation of BaTiO
3is retarded due to the solution of NiO.
Possible mechanism is proposed. The existence of residual NiO inclusions also reduces the densi®cation rate. The adding of NiO inhibits the grain growth of BaTiO3. For the BaTiO3doped with 0.13 wt% NiO, no abnormal grains are observed. # 1998 Elsevier
Science Limited and Techna S.r.l. All rights reserved
Keywords: A. sintering; A. grain growth; D. BaTiO3
1. Introduction
Barium titanate (BaTiO3) is widely used as a dielectric material for its high dielectric constant. Due to the capacitance is increased with increasing area, multilayer structure BaTiO3is developed to ful®l the requirement on size reduction. The material frequently used as the inner electrode for the multilayer capacitors is Pd/Ag alloys. Due to the cost of Pd is high, nickel is considered as an alternative [1]. The nickel may be oxidised during binder pyrolysis in air. Nickel oxide then tends to dif-fuse into BaTiO3to change its dielectric properties [2]. The capacitance is also increased with decreasing thickness. However, the thickness of the dielectric is reduced, the in¯uence of Ni solutes on the dielectric properties can be more noticeable.
The solubility of NiO in the BaTiO3lattice is reported as 0.6 to 1.0 wt% [3±5]. The valence state of Ni ions in BaTiO3 lattice is proved as Ni2+[4,6]. The Ni ions may occupy the Ba2+ sites [5,7] or Ti4+ sites [3,6]. The dominated sintering mechanism for BaTiO3is reported as grain boundary diusion [8,9]. The rate control spe-cies for diusion as proposed by Lewis et al. is Ti ion vacancy [10]. Polycrystalline BaTiO3readily experiences abnormal grain growth during sintering. Considerable studies have focused on the grain growth behaviour of
BaTiO3. The addition of Dy2O3, [11] Nb2O5 [12] or Sc2O3 [13] inhibits the grain growth of BaTiO3. The
presence of NiO can also reduce the size of BaTiO3
grains [14].
To the knowledge of the present authors, the eect of NiO on the sintering behaviour of BaTiO3has not been determined. In the present study, the nickel oxide parti-cles are mixed intimately with BaTiO3 powder before sintering. The solubility of NiO in BaTiO3is determined ®rst. The eect of Ni solutes or undissolved NiO inclu-sions, on the sintering behaviour of BaTiO3can then be determined.
2. Experimental procedures
The BaTiO3 powder (no. 219-6, Ferro Co., USA)
having a Ba/Ti ratio of 0.995 was mixed with various amounts of nickel nitrate (Johnson Matthey Chem. Co., USA). The powder mixtures were milled together in ethyl alcohol for 4 h. The grinding media used was zir-conia balls. The slurry was dried using a rotary eva-porator. The dried lumps were crushed and passed through a plastic sieve. The sieved powder was calcined at 500C in air for 1 h. The temperature was higher than
the decomposition temperature of nickel nitrate. The calcined powder was then formed by uniaxially pressing at 25 MPa into disks (10 mm in diameter and about 0272-8842/98/$Ðsee front matter #1998 Elsevier Science Limited and Techna S.r.l. All rights reserved
PII: S0272-8842(98)00003-0 * Corresponding author.
5 mm in thickness). The green density of specimens was measured by the geometric method. The pore size dis-tribution of green compacts was characterised by mer-cury porosimetry (Autopore II 9220, Micromeritics Instrument Co., USA).
Sintering was performed at a temperature varied from
1250±1370C in air. The dwelling time was 2 h. The
heating and cooling rates were 3C minÿ1. Shrinkage kinetics was determined by a dierential dilatometer (Theta Co., USA). Phase identi®cation was performed by X-ray diractometry (XRD). A very slow scanning rate (0.01 degree 2 /s) was also used to determine the lattice constant ratio of c over a by using the (002) and (200) re¯ections. The size of NiO crystals in the calcined powder was determined by measuring the line broad-ening of the NiO XRD peaks. The ®red density was determined by the water displacement method. The polished specimens were prepared by grinding with SiC particles and by polishing with Al2O3 particles. The polished surfaces were chemically etched with a HCl± HF solution to reveal the grain boundaries. The micro-structure was observed with scanning electron micros-copy (SEM). The volume fraction of abnormal grains was determined by measuring the area fraction of abnormal grains in SEM micrographs. The size of
BaTiO3 and NiO grains was determined by using the
linear intercept technique. Samples for transmission electron microscopy (TEM, Jeol 10OCXII) were mechanically thinned to about 50 mm. The thinned samples were subsequently dimpled to a thickness of 20 mm in the centre and then ion-milled to perforation of the central disk region. These samples were observed at the voltage of 100 kV.
3. Results and discussion
There are only tetragonal BaTiO3 and cubic NiO
detected in the calcined powder by the XRD analysis. The morphology of the calcined powder is shown in Fig. 1. On the surface of the BaTiO3 particles, small rectangular NiO particles are observed, Fig. 1(b). The size of NiO crystals as determined by the XRD line broadening technique is around 30 nm. The pore size distribution of green bodies is shown in Fig. 2 Bimodal pore size distribution is noted in all green compacts. It suggests that the powder is strongly agglomerated [15]. The strong agglomerates are survived after the die-pressing stage. The mean pore size within the agglom-erates (intra-agglomerate) is about 0.18 mm. The amount of intra-agglomerate pores is little. Most pores are located at agglomerate sites. The size of
inter-agglomerate pores for pure BaTiO3 is 0.26 mm. As
0.13 wt% NiO is added, the size of inter-agglomerate pores is virtually unchanged. However, as NiO content is higher than 1.3 wt%, the size of inter-agglomerate
pores increases with increasing NiO content. The sur-face of BaTiO3particles is coated with small NiO parti-cles. The presence of small NiO particles can push the
BaTiO3 particles further apart. The size of
inter-agglomerate pore is thus increased.
The green density is shown as a function of NiO content in Fig. 3. As the NiO content is below 1.3 wt%, the green density is around 53%. As the NiO content is higher than 1.3 wt%, the green density is decreased due to the size of pores is enlarged, Fig. 2. The XRD pat-terns of the sintered specimens are shown in Fig. 4. Only
tetragonal BaTiO3 phase and cubic NiO phase are
detected as sintering temperature is below 1330C
(Fig. 4(a)). There is no NiO detected in the BaTiO3± 1.3 wt% NiO specimen. However, due to the detection limit of the XRD analysis, the XRD analysis can only suggest that the solubility of NiO in BaTiO3 is below 1.3 wt%. A monoclinic phase, Ba6Ti17O40 phase, is found as the sintering temperature is 1370C (Fig. 4(b)).
Fig. 1. The morphology of the calcined powders which contain (a) 0 and (b) 12.4 wt% NiO.
The presence of the Ba6Ti17O40phase suggests the pre-sence of an eutectic liquid during sintering.
Fig. 5 shows typical XRD patterns obtained by using the slow scanning rate. The splitting of (002) and (200) re¯ections of tetragonal phase decreases with increasing NiO content. The lattice constant ratio of c over a of BaTiO3is shown as a function of NiO content in Fig. 6 Two factors aect the c/a ratio. The factors are the
grain size and the solution of NiO in BaTiO3. Alrt
reported that c/a ratio is reduced with decreasing grain size as the grain size is smaller than 1 mm [16]. The c/a ratio is constant for the specimens with grain size above 1.5 mm. To be shown latter, the grain size measured for
the specimens investigated in the present study is larger than 1.2 mm. The major factor to reduce the c/a ratio is thus the solution of NiO in BaTiO3lattice. The c/a ratio is decreased as NiO content is increased (Fig. 6). As the NiO content is higher than a certain value, the values of c/a ratio are stabilized. The solubility of NiO in BaTiO3 can thus be determined by using Fig. 6. As the sintering temperature ranges from 1250 to 1330C, the solubility
of NiO in BaTiO3is about 0.13 wt%. As the sintering temperature is 1370C, the solubility is higher than
0.13 wt% and lower than 1.3 wt%. It may be due to the presence of the eutectic liquid that enhances the solu-bility. The solubility limit obtained in the present study is lower than the values reported by others [3±5]. The discrepancy may result from the dierent raw material used.
Fig. 3. The relative density of NiO-doped BaTiO3 specimens as a
function of NiO content.
Fig. 2. The pore size distribution for the green bodies containing dierent NiO content.
Fig. 4. The XRD patterns of undoped and 29.8 wt% NiO-doped BaTiO3specimens sintered at (a) 1330C and (b) 1370C.
When the coordination number is 6, the ionic radius of the Ti4+ and Ni2+ is 0.61 and 0.69AÊ, respectively. The ionic radius of Ba2+ion is 1.59 AÊ as the coordina-tion number is 12. Although the valence of Ni ion is the same as that of Ba ion, the ionic radius of Ni2+is much smaller than that of Ba2+. The Ni2+is more likely to replace the Ti4+ for their similar ionic size. This has been proved recently [17]. Tzing and Tuan found that n-type semiconducting BaTiO3is obtained as sintering is taken place in a reducing atmosphere. The presence of Ni ion increased the electrical resistivity. It suggested that Ni ion acts as electron acceptor in BaTiO3lattice.
The replacement of Ti4+by Ni2+may accompany the
formation of oxygen vacancy as [6] NiO ÿÿÿÿ!BaTiO3
NiTi00 V o Oo 1
The BaTiO3crystal is distorted due to the solution of Ni. The fraction of the increase in a-axis is higher than that in c-axis. The c/a ratio is thus reduced. The low solubility may result from dierent valence state between Ti4+and Ni2+.
The relative density of sintered specimens is also shown in Fig. 3. The relative density of the undoped BaTiO3is higher than 95%. As NiO is added, the rela-tive density is decreased with the increase of NiO
con-tent as sintering temperature is below 1290C. It
indicates that the addition of NiO retards the densi®-cation of BaTiO3. The shrinkage is isotropic for all the specimens prepared in the present study. The relative density during sintering can thus be calculated by using the linear shrinkage data from dilatometer. The densi®-cation kinetics for the NiO-doped BaTiO3are shown in Fig. 7. A constant heating rate, 3C minÿ1 is used dur-ing sinterdur-ing in the dilatometer. The densi®cation rate can be estimated by dierentiating the relative density with temperature (Fig. 8). Two peaks are found in the densi®cation rate curves. There is a similarity between the densi®cation rate and the pore size distribution curves, (Figs. 2 and 8). It implies that the ®rst peak in the densi®cation rate curves is resulted from the shrink-age of the small intra-agglomerate pores. The second peak is resulted from the shrinkage of the inter-agglomerate pores.
The densi®cation is mainly started roughly from 1150C (Fig. 8). It is interesting to examine whether the
solution of NiO in BaTiO3 or the densi®cation takes place ®rst. A 0.13 wt% NiO-doped BaTiO3is sintered at 1100C for 1 min. The c/a ratio for the doped specimen
is 1.0095 which is less than the value for the undoped BaTiO3. It thus indicates that NiO starts to dissolve into BaTiO3before 1100C. The solution of NiO in BaTiO3 takes place before the densi®cation. When the added
Fig. 6. The lattice constant c/a ratio of NiO-doped BaTiO3specimens
as a function of NiO content.
Fig. 5. The (002) and (200) diraction peaks for NiO-doped BaTiO3
specimens. The specimens are sintered at 1330C for 2 h.
Fig. 7. The densi®cation curves for NiO-doped BaTiO3specimens as a
NiO content is below the solubility limit, higher tem-perature is needed to reach maximum densi®cation rate. It indicates the densi®cation of BaTiO3is retarded due to the presence of Ni solute. The solution of NiO into BaTiO3 increases the concentration of VoÈ Eq. (1). As Schottky-type, defect is predominant in BaTiO3[18], the increase of the concentration of VoÈ reduces the con-centration of VTi0000. The Ti vacancy is the rate control-ling species because it has the highest activation energy of diusion in BaTiO3[10]. The solution of NiO thus retards the densi®cation of BaTiO3.
As the NiO content is higher than the solubility limit, residual NiO inclusions can be observed, (Fig. 9). The
presence of NiO inclusions between BaTiO3 grains
increases the inter-diusion distance. The densi®cation rate and ®nal density are thus reduced (Figs. 3 and 8).
According to the BaO±TiO2 phase diagram [19], an
eutectic liquid phase is formed above 1312C in the
TiO2excess system. The liquid phase is observed in the specimens sintered at 1330C (Fig. 10). The presence of
the liquid phase also enhances the densi®cation of NiO-doped BaTiO3specimens (Fig. 3). As the sintering tem-perature is 1370C, the relative density of the 30 wt%
NiO-doped BaTiO3is higher than 95%.
Typical microstructures of pure BaTiO3 and
NiO-doped BaTiO3are Shown in Fig. 11. The size of abnor-mal and sabnor-mall grains is shown in Fig. 12. The volume fraction of abnormal grain is shown in the bracket in the ®gure. Abnormal grains can be observed in the BaTiO3specimen sintered at 1250C. This temperature is lower than the BaTiO3±Ba6Ti17O40 eutectic tempera-ture [19]. As shown in Fig. 2, agglomerates are pre-sented in the green compacts. The shrinkage of the intra-agglomerate pores takes place in the very begin-ning of the sintering (Fig. 8). The abnormal grains may nucleate from the dense agglomerates. Chemical inho-mogeneity can reduce the liquid formation temperature. The possible formation of liquid phase may also play some role on the formation of abnormal grains. The volume fraction of abnormal grains, the size of abnor-mal and of sabnor-mall grains increase with increasing
sinter-ing temperature. The size of BaTiO3 grains in the
undoped BaTiO3 specimens grow to a size of 50 mm as
they are sintered above 1330C, (Fig. 11(b)). Many
pores are trapped in the abnormal grains. It leads to the decrease of relative density.
The microstructure of a NiO-doped BaTiO3sintered
at 1330C is shown in Fig. 11(c). The added NiO
con-tent is 0.13 wt%. The relative density is about 98.2%. No abnormal grains are observed due to the existence of some NiO inclusions. The size of NiO inclusions in all
the NiO-doped BaTiO3 is smaller than 0.7 mm. When
NiO content is below the solubility limit, the inhibition of grain growth can be attributed to the retardation of Fig. 10. The liquid phase observed in the undoped BaTiO3specimen
sintered at 1330C.
Fig. 9. A TEM micrograph of the 1.3 wt% NiO-doped BaTiO3
speci-men. The specimen is sintered at 1330C.
Fig. 8. The densi®cation rate of NiO-doped BaTiO3specimens as a
densi®cation [20]. When NiO content is higher than the solubility limit, the presence of NiO inclusions prohibits the movement of grain boundaries (Fig. 9). The micro-structure of these specimens is re®ned despite the pre-sence of liquid phase.
4. Conclusions
The following conclusions can be made from the pre-sent study:
1. Around 0.13 wt% NiO is dissolved into BaTiO3as the sintering temperature varies from 1250 to 1330C. The presence of the eutectic liquid
increa-ses the solubility.
2. When the added NiO content is below the solubi-lity limit, higher temperature is needed to reach maximum densi®cation rate. The densi®cation of
BaTiO3 is retarded due to the presence of Ni
solute.
3. The presence of residual NiO inclusions reduces the sintered density. Raising the sintering tem-perature to form an eutectic liquid enhances the sinterability.
4. Both the Ni solute and NiO inclusions reduce the
grain size of BaTiO3. The abnormal grains are
totally suppressed as the NiO content is higher than the solubility limit.
Acknowledgements
The comments given by Dr. Simmon H-P. Li, National Taiwan University, were very helpful.
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