The eect of microstructure on the electrical properties of
NiO-doped BaTiO
3
W.H. Tzing
a, W.H. Tuan
a,*, H.L. Lin
baInstitute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan bDepartment of Materials Engineering, Tatung Institute of Technology, Taipei, Taiwan
Received 21 January 1998; accepted 16 May 1998
Abstract
A small amount of NiO may dissolve into BaTiO3during the co-®ring of BaTiO3based dielectrics and Ni internal electrodes. In
the present study, the eect of NiO addition on the microstructure and the electrical properties of BaTiO3 is investigated. The
microstructure is observed by scanning electron microscopy and transmission electron microscopy. The formation of eutectic liquid phase may degrade the relative permittivity at room temperature of undoped BaTiO3. The presence of NiO solute reduces both the
size of BaTiO3grains and the width of 90domain. The relative permittivity at room temperature is increased with the decrease of
BaTiO3grain size. The electrical resistivity also increases with the decrease of grain size. It is due to the enhancement of grain
boundary resistivity by the Ni solute. The NiO inclusions can inhibit the grain growth of BaTiO3. However, the relative permittivity
and electrical resistivity of NiO are low; the relative permittivity and electrical resistivity of NiO-doped BaTiO3is thus decreased
with the increase of NiO inclusions. # 1999 Elsevier Science Limited and Techna S.r.l. All rights reserved.
Keywords: B. Microstructure; C. Electrical properties; D. BaTiO3
1. Introduction
The electrical properties of BaTiO3 depend strongly
on its microstructure. For example, the relative permit-tivity of BaTiO3reaches its highest value as its grain size
is around 1 mm [1]. The high permittivity was related to the decrease 90 domain amount [2] or to the decrease
of 90 domain width [1]. The electrical resistivity, Curie
temperature and the relative permittivity at Curie tem-perature of BaTiO3 are all aected by its
micro-structure.
The microstructure of BaTiO3 can be controlled by
two approaches. One approach uses novel processing techniques to tailor the microstructure [3,4]. Another approach uses grain growth inhibitors to prohibit the grain growth. The second approach has been proved to be a useful one [5±9]. For example, a small addition of Dy2O3 [5] or Nb2O5 [6] or SC2O3 [7] or Ta2O5 [8] or
ZnO [9] can all prohibit the abnormal grain growth. BaTiO3 and its related compounds are frequently
used as high-permittivity capacitor materials. For multi-layer ceramic capacitors, Ag±Pd alloys are usually used
as internal electrodes. Ni is used recently to replace the Ag±Pd alloys for the cost of the Ag±Pd alloys is high [10]. Nickel may be oxidized during the powder processing and the subsequent co-®ring with BaTiO3
in a protective atmosphere. A recent study indicated that a small amount of NiO can dissolve into BaTiO3
as it is ®red with NiO at 1100C in air for 1 min [11].
It is thus important to investigate the eect of NiO on the electrical properties of BaTiO3. In the present study,
NiO is mixed intimately with BaTiO3 and sintered in
air. The relationships between the microstructure and electrical properties of the NiO-doped BaTiO3 are
investigated. 2. Experimental
Barium titanate powder (no. 216-9, Ferro Co., USA) and various amount of nickel nitrate (Johnson Matthey Chem. Co., USA) were tumble milled together in ethyl alcohol for 4 h. The Ba/Ti ratio of the BaTiO3powder is
0.995 as reported by the manufacturer. The grinding media used was zirconia balls. The slurry of the powder mixtures was dried using a rotary evaporator. The dried
0272-8842/99/$20.00 # 1999 Elsevier Science Limited and Techna S.r.l. All rights reserved. PII: S0272-8842(98)00058-3
lumps were then crushed and passed through a plastic sieve. As-sieved powder was calcined in air at 500C for
1 h. The powder was formed into disks by pressing uni-axially at 25 MPa. The size of the discs is 10 mm in diameter and about 5 mm in thickness. Sintering was performed in air at 1290 to 1370C for 2 h with a mue
furnace (Lindberg/Blue Co., USA). The heating and cooling rates were 3C/min.
The ®nal density was determined by the water dis-placement method. The polished specimens were pre-pared by grinding with SiC particles and polishing with Al2O3particles. The grain boundary and domain
struc-ture were revealed by etching with a dilute solution of HCl and HF. The microstructure was observed by scanning electron microscopy (SEM). The grain size was determined by using the line intercept method. Samples for transmission electron microscopy (TEM) observa-tion were dimpled and ion-milled to form a thin secobserva-tion. Phase identi®cation was performed by X-ray dif-fractometry (XRD). The dielectric properties were measured by a LCZ meter (BP 4272A, Hewlett Packard Co., USA) with a 1 V signal at 1 k Hz. Silver paste was applied as electrodes. The testing temperature was var-ied from room temperature to 165C. The electrical
resistivity was measured by using a two-probe method with a constant voltage of 50 volts at room temperature. 3. Results and discussion
3.1. Microstructure characterization
The XRD analysis reveals only tetragonal BaTiO3in
the doped BaTiO3containing less than 1 wt% NiO. It is
due to a small amount NiO is dissolved into BaTiO3[11]
Fig. 1. The relative density of NiO-doped BaTiO3 as a function of
NiO content.
Fig. 2. TEM micrographs of an (a) undoped and a (b) 0.13 wt% NiO-doped BaTiO3. The sintering temperature is 1330C. The liquid phase
is indicated with an arrow.
Fig. 3. The grain size of NiO-doped BaTiO3 as a function of NiO
and 1 wt% is below the detection limit of the XRD technique. Cubic NiO is found in the doped BaTiO3
containing more than 1 wt% NiO. A monoclinic phase, Ba6Til7O40, is found in the specimens sintered at
1370C. The presence of Ba6Ti17O40 phase indicates
the presence of eutectic liquid phase during sintering [12].
The relative density of the NiO-doped BaTiO3 is
shown as a function of NiO content in Fig. 1. The microstructures of undoped BaTiO3and 0.13 wt%
NiO-doped BaTiO3are shown in a Fig. 2. The liquid phase
can also be observed in the specimens sintered at 1330C, Fig. 2(a). It is due to the eutectic temperature
for TiO2-rich region of the BaO±TiO2system is around
1312C [12]. The presence of the liquid phase enhances
the densi®cation; the density of the specimens sintered at 1330 and 1370C is thus higher than that of the
specimens sintered at 1290C (Fig. 1). Since the amount
of Ba6Til7O40 phase is small, there is no Ba6Til7O40
phase detected by XRD analysis in the specimens sin-tered at 1330C. The NiO second phase inclusions are
presented in the doped BaTiO3 containing more than
0.13 wt% NiO [11]. The presence of second phase inclusions can inhibit the densi®cation [13]. The density of the NiO-doped BaTiO3 sintered at 1290C is thus
decreased with the increase of NiO content. To compare
Fig. 4. The relative permittivity (K) as a function of temperature and NiO content. The specimens are sintered at (a) 1290C, (b) 1330C and
the electrical properties of NiO-doped BaTiO3 in a
relatively narrow density range, only the specimens with relative density higher than 94% are investigated in the present study.
The grain size of NiO-doped BaTiO3 is shown as a
function of NiO content in Fig. 3. Both the Ni solute and NiO second phase can inhibit the grain growth of BaTiO3. In summary, specimens with the density
rang-ing from 94 to 99% are obtained in the present study. The grain size of the specimens varies from 1 to 100 mm. For most of the specimens, there is a liquid phase located at the grain boundaries.
3.2. Electrical propertiesÐmicrostructure relationships The permittivity±temperature curves for the NiO-doped BaTiO3are shown in Fig. 4. Each point in the
permittivity±temperature curves was obtained by hold-ing the specimens at each temperature for 9 min. The Curie temperature decreases with the increase of NiO content as the NiO content is below the solubility limit. The decrease can be related to the solution of Ni in BaTiO3[14,15]. As the NiO content is above the
solu-bility limit, the relative permittivity is less sensitive to the change of temperature. The ¯at permittivity±
Fig. 6. The relative permittivity at room temperature (Kr) of NiO-doped BaTiO3as a function of NiO content.
Fig. 8. The Km/Kr ratio of NiO-doped BaTiO3as a function of NiO
content. Fig. 5. The relative permittivity at Curie temperature (Km) as a
func-tion of grain size.
Fig. 7. The relative permittivity at room temperature (Kr) as a func-tion of grain size. The sintering temperature is between 1290 and 1370C.
temperature curve indicates that the diusion phase transition (DPT) phenomenon exists in the NiO inclu-sions containing BaTiO3.
The relative permittivity at the Curie temperature, Km, strongly depends on the size of BaTiO3 grains as
observed in Fig. 5. The ®gure shows that the Km decreases with the decrease of grain size. Martirena et al. [16] proposed that the stresses due to phase trans-formation become more dicult to release as the grain size decreases. The suppression of Km for the ®ne-grained specimens is resulted from the increase of inter-nal stresses. The presence of non-ferroelectric NiO phase can also reduce the Km value. The Km is thus decreased with increasing amount of NiO and with decreasing of grain size.
The relative permittivity at room temperature, Kr, is shown as a function of NiO content in Fig. 6. The Kr of undoped BaTiO3is decreased as the sintering
tempera-ture is higher than the eutectic temperatempera-ture (1312C).
The formation of eutectic liquid and the precipitation of Ba6Ti17O40 phase degrade the Kr. The relative
permit-tivity decreases slightly with the increase of NiO content as the content is below the solubility limit. However, the Kr value increases signi®cantly as the NiO content approaches the solubility limit. The Kr value then decreases as NiO inclusions are presented. The Kr values are shown as a function of grain size in Fig. 7. The Kr shows little dependence on grain size as the grain size is larger than 20 mm. However, there is an obvious increase of Kr as the size of grains approaches 2 mm. This phenomenon is similar to the results for BaTiO3. It was related to the decrease of 90 domain
width by Arlt et al. [1]. The size of BaTiO3 grains is
signi®cantly reduced as NiO is added (Fig. 3). Further-more, the width of the 90 domain in NiO-doped
BaTiO3 is also decreased as Ni is soluble in BaTiO3
(Fig. 2). The increase of Kr (the right-hand side of Fig. 7 may be resulted from the decrease of 90
domain width. For the left-hand side of Fig. 7, the relative permittivity of doped BaTiO3 decreases with
the increase of the amount of non-ferroelectric NiO inclusions. It is due to the low relative permittivity of NiO phase.
For most of the specimens, the dissipation factor at room temperature is less than 2%. The dissipation fac-tor increases with the increase of porosity. This phe-nomenon is caused by the adsorption of moisture on the pore surface [17]. On the other hand, the dissipation factor increases signi®cantly as NiO content is high. It may be due to the dissipation factor of non-ferroelectric NiO phase is high.
The Km/Kr ratio is shown as a function of NiO con-tent in Fig. 8. The increase of Km/Kr ratio in undoped BaTiO3 sintered at a temperature higher than 1312C
may be due to the formation of the eutectic liquid. However, the Km/Kr ratio drops signi®cantly when NiO content changes from 0.09 to 0.13 wt%. These values correspond to the reported solubility limit of NiO in BaTiO3[11]. The solubility limit of NiO in BaTiO3has
been determined by measuring the lattice parameter c/a ratio with a XRD analysis technique [11]. The curves in Fig. 8 are surprisingly similar to the curves of lattice parameter c/a ratio. It implies that the Km/Kr ratio can also be used to determine the solubility limit of NiO in BaTiO3. The NiO content for dropping the Km/Kr
ratio is increased with the increase of sintering tem-perature. Fig. 8 suggests that the solubility limit of NiO in BaTiO3 is increased with the increase of sintering
temperature. The ®gure also shows that the Km/Kr ratio approaches unity as NiO second phase is presented. The
Fig. 9. The electrical resistivity of NiO-doped BaTiO3as a function of
low value of Km/Kr ratio is typical for the materials with diuse phase transition (DPT).
The electrical resistivity of NiO-doped BaTiO3 is
shown as a function of NiO content in Fig. 9. The resistivity is increased as the NiO content is below the solubility limit. The resistivity as a function of grain size is shown in Fig. 10. The right-hand side of the ®gure shows the results for BaTiO3 and BaTiO3 with
Ni solute. The resistivity increases with the decrease of grain size as the NiO content is below the solubility. In other words, the resistivity is enhanced as the grain boundary area is increased. The Ni solute may segre-gate at the grain boundaries of BaTiO3, the grain
boundary resistivity may thus be increased. The resis-tivity of doped BaTiO3 decreases with the increases
of the amount of NiO inclusions (the left-hand side of Fig. 10). The reported value for the electrical resistivity of NiO is relatively low [18]. The presence of NiO sec-ond phase can thus reduce the resistivity of the doped BaTiO3.
4. Conclusions
The relationships between the microstructure and electrical properties of NiO-doped BaTiO3 are
in-vestigated. The solubility limit of NiO in BaTiO3 is
around 0.13 wt%. The formation of eutectic liquid phase and Ba6T17O40 precipitates degrades the relative
permittivity at room temperature. However, the size of BaTiO3 grains is decreased as NiO is dissolved
into BaTiO3. The relative permittivity at room
tem-perature of the Ni-solute containing BaTiO3 is thus
better than that of the undoped one. The relative permittivity at the Curie temperature is decreased instead due to the decrease of grain size. The Ni solute can also enhance the grain boundary resistivity. The relative permittivity and resistivity of NiO are low. The relative permittivity and resistivity of BaTiO3
reach its highest values as NiO content approaches its solubility limit. Although the existence of NiO inclu-sions reduces the relative permittivity, their presence also decreases the temperature sensitivity of the doped BaTiO3.
References
[1] G. Arlt, D. Hennings, G. de With, Dielectric properties of ®ne-grained barium titanate ceramics, J. Appl. Phys 58 (1985) 1619±1625.
[2] W.R. Buessem, L.E. Cross, A.K. Goswami, Phenomenological theory of high permittivity in ®ne-grained barium titanate, J. Am. Ceram. Soc. 49 (1966) 33±36.
[3] H. Mostaghaci, R.J. Brook, Microstructure development and dielectric properties of fast-®red BaTiO3Ceramics, J. Mater. Sci.
21 (1986) 3575±3580.
[4] H.G. Graham, N.M. Tallan, K.S. Mazdiyasni, Electrical proper-ties of high-purity polycrystalline barium titanate, J. Am. Ceram. Soc. 54 (1971) 548±553.
[5] A. Yamaji, Y. Enomoto, K. Kinoshita, T. Mrakami, Prepara-tion, characterizaPrepara-tion, and properties of Dy-doped small-grained BaTiO3Ceramics, J. Am. Ceram. Soc. 60 (1977) 97±101.
[6] M. Kahn, Preparation of small-grained and large-grained ceramics from Nb-doped BaTiO3, J. Am. Ceram. Soc. 54 (1971) 452±454.
[7] K.S. Mazdiyasni, L.M. Brown, Microstructure and electrical properties of Sc2O3-doped, rare-earth-oxide-doped, and undoped
BaTiO3, J. Am. Ceram. Soc. 54 (1971) 539±543.
[8] D.A. Payne, H.U. Anderson, Inhibition of grain growth in bar-ium titanate, J. Am. Ceram Soc. 50 (1967) 491±503.
[9] A.C. Caballero, J.C. Fernandez, C. Moure, P. Duran, ZnO-doped BaTiO3: microstructure and electrical properties, Journal
of the European Ceramic Society 17 (1997) 513±523.
[10] Y. Sakabe, Dielectric materials for base-metal multilayer ceramic capacitors, Am. Ceram. Soc. Bull. 66 (1987) 1338±1341. [11] W.H. Tzing, W.H. Tuan, Eect of NiO addition on the sintering
and grain growth behaviours of BaTiO3, Ceramics International
25 (1) (1999) 69±75.
[12] H.M. O'Bryan, J. Thomson, Phase equilibria in the TiO2-rich region
of the system BaO±TiO2, J. Am. Ceram. Soc. 57 (1974) 522±526.
[13] W.H. Tuan, E. Gilbart, R.J. Brook, Sintering of heterogeneous ceramic compacts, 1. Alumina/alumina, J. Mater. Sci. 24 (1989) 1062±1068.
[14] H. Ihrig, The phase stability of BaTiO3as a function of doped 3d
elements: an experimental study, J. Phys. C: Solid State Phys. 11 (1978) 819±827.
[15] H. Emoto, J. Hojo, Sintering and dielectric properties of BaTiO3
-Ni composite ceramics, J. Ceram. Soc. Japan, Inter. Ed. 100 (1992) 553±557.
[16] H.T. Martirena, J.C. Burfoot, Grain-size eects on properties of some ferroelectric ceramics, J. Phys. C: Solid State Phys. 7 (1974) 3182±3192.
[17] T.T. Fang, H.L. Hsieh, F.S. Shiau, Eects of pore morphology and grain size on the dielectric properties and tetragonal-cubic phase transition of high-purity barium titanate., J. Am. Ceram. Soc. 76 (1993) 1205±1211.
[18] W.D. Kingery, H.K. Brown, D.R. Uhlmann, Electrical con-ductivity, in: Introduction to Ceramics, John Wiley and Sons, New York, 1976, pp. 867±869.
