Effect of Niobium – Doping on the

Anais do 44º Congresso Brasileiro de Cerâmica 2920

31 de maio a 4 de junho de 2000 - São Pedro – S.P.

EFFECT OF NIOBIUM – DOPING ON THE MICROSTRUCTURAL DEVELOPMENT OF BARIUM TITANATE CERAMICS

E. Brzozowski ¹, M.S.Castro¹ and B.Stojanovic²

¹ Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA)

Avda. J.B.Justo 4302 (7600) Mar del Plata, Argentina.

[e-mail]

[e-mail]
²UNESP Araraquara – IQ

Rua Francisco Degni s/n° caixa Postal 355 Araraquara – SP / CEP 14801-970

[e-mail]

ABSTRACT
It has been shown that the use of dopants is an effective approach for the production of fine-grained BaTiO₃ needed for electronic applications. When the dopant concentration exceeds the limit of solubility, secondary phases are formed. This phenomena affect strongly the sintering behaviour of BaTiO₃, and usually a high content of secondary phases results in the deterioration of electrical properties of BaTiO₃. In this work, the effect of niobium addition on the microstructural development of BaTiO₃ is studied. In order to detect and identify secondary phases, EDAX and SEM analysis were carried out on niobium-doped barium titanate. We found that donor concentrations greater than 0.15% mol, produce a fine-grained BaTiO₃ free of abnormal grain growth. Also, small precipitates of a titanium-rich phase were detected. It has been determinated that titanium content in the precipitates increases with the Nb₂O₅ addition.

Keywords: Barium Titanate, microstructure, SEM, secondary phases.
INTRODUCTION
It is known that optimization of the electrical properties of BaTiO₃ requires high-density microstructures and homogeneous granulometry with grain sizes nearby to 1µm. ^(1,2)

In order to characterize the structure of defects of BaTiO₃ and the sintering behaviour of this material, several investigations have been carried out.⁽³⁾ It has been shown that the addition of dopants could modify the microstructure and the electrical properties of the BaTiO₃ ceramics.^(1,2,4) In fact, Kutty et al⁽⁵⁾ observed microstructures in this materials characterized by small needles among the grains of doped BaTiO₃. In the same way, Hennings et al⁽⁶⁾ reported that the Nb/Co ratio had great influence on the grain growth and the electrical properties in the system BaTiO₃ - Nb₂O₅ - Co₃O₄.

In particular, it was determined that the niobium addition at low concentration could produce a charge compensation mechanism by electron mobility⁽⁷⁾, according to:
2BaO + Nb₂O₅  2 Ba_Ba + 2 Nb_Ti^· + 6 O_o + ½ O₂ (g) + 2e’ (A)
As a consequence, a reduction in the titanium valence (Ti⁺⁴ to Ti⁺³) is produced, and structures like Ba⁺²(Ti_1-2x⁺⁴Nb_x⁺⁵Ti_x⁺³)O₃ could appear.⁽⁸⁾ On the other hand, when the niobium concentration increases, a charge compensation mechanism by ionic defects is produced, according to:
BaO + Nb₂O₅  Ba_Ba + V_Ba” + 2 Nb_Ti^· + 6 O_o (B)

6 BaO + 3 Nb₂O₅  6 Ba_Ba + V_Ba” + 6 Nb_Ti^· + V_Ti”” + 21 O_o (C)

Besides the above equilibrium mechanisms, the segregation of titanium-rich phase with incorporated niobium can be expectable in BaTiO₃ based ceramics.⁽⁹⁾ In spite of secondary phases could be an undesirable phenomena for many electronic applications, the technology of boundary layer capacitors provides a good example of how knowledge of the microstructural characteristics permits to develop new products. In fact, insulating layers grown due to the presence of intergranular phases are necessary for the application of BaTiO₃ as grain boundary barrier layer capacitor (GBBL).

In this work, a study of the effect of the niobium addition on barium titanate is presented. The influence of the niobium addition on the microstructure development and on the electrical properties of BaTiO₃ was studied.

EXPERIMENTAL

Niobium - doped BaTiO₃ materials were prepared by the oxide route, from a BaTiO3 powder. Mixtures of BaTiO₃ (TAM Ceramics Inc.) and Nb₂O₅ (Fluka AG, Buchs SG) were homogenized in isopropyl alcohol medium by stirring with a high speed turbine at 5000 rpm for 5 minutes. After drying, powders were granulated and sieved through a 100 m mesh. Finally, powders were uniaxially pressed at 200 MPa and sintered at temperatures between 1300 - 1400ºC for 2 hours.

The microstructures of sintered compacts was studied on polished and etched samples by Scanning Electron Microscopy (SEM) (Philips 505), while elemental analysis of the grains and secondary phases by EDAX (Topcon SM-300 and PGT digital spectrometer) were performed.

RESULTS AND DISCUSSION
Figure 1 shows the SEM microphotographs corresponding to samples sintered at 1300°C during 2 hours. Pure barium titanate and samples with a Nb₂O₅ content smaller than 0.15% showed a non-homogeneous development of the grain growth. Actually, pure BaTiO₃ and BaTiO₃ doped with 0.05% Nb₂O₅ containing grains of approximately 200 µm inside a matrix of fine grain microstructure with grain size of 1-2 m was observed.

A

B



Figure 1. SEM microphotographs corresponding to BaTiO₃ (A) pure and (B) doped with 0.05% mol Nb₂O₅ and sintered at 1300ºC for 2 hours. Bar 100 m.
On the contrary, the barium titanate doped with higher concentration of dopant and sintered at 1300°C, exhibited a homogeneous microstructure with small grain size. A small quantity of secondary phase in BaTiO₃ doped with a concentration of dopant higher than 0.15 % mol was detected. In Figure 2 SEM microphotographs of the samples with 0.15 and 0.60% Nb₂O₅ are shown.



A



B



Figure 2. SEM microphotographs corresponding to BaTiO₃ doped with (A) 0.15 % mol Nb₂O₅ and (B) 0.60 % mol Nb₂O₅ and sintered at 1300ºC for 2 hours. Bar 100 m.
As it has been reported, small concentrations of dopant oxide incorporated to the BaTiO₃ powder usually implies a faulty in the dopant distribution into the system of BaTiO₃ particles.⁽¹⁰⁾ This effect leads to an abnormal grain growth and when the dopant concentration increases, the additive is able to diffuse into the BaTiO₃ grains and secondary phases can be generated.

The presence of a small quantity of secondary phases with vitreous characteristics favours the diffusion of the dopant through the grain boundaries. Moreover, it has been verified that the liquid formation at high temperature contributes to the redistribution of the dopant, obtaining an effective homogenisation of the additive.⁽¹¹⁾ It has been reported^{(12, 13)} that the incorporation of a dopant in the lattice of BaTiO₃ leads to the displacement of the titanium ions outside of the grains according to equations A-C. Thus, the presence of a liquid phase at temperatures lower than 1332°C is observed. In fact, the existence of the secondary phase in form of needles has been confirmed by the SEM microphotographs.

When the sintering temperature is 1350°C or 1375°C, the content of secondary phases increases respect to the content at 1300°C. In all the investigated cases the secondary phases are generated only in samples with more than 0.15% mol of Nb₂O₅. Figure 3 shows microphotographies corresponding to samples containing these secondary phases. As we can see in this figure, the secondary phases exhibit morphology of needles. Analysis EDAX of Ba and Ti carried out on these phases showed the same composition along the needles.

The elemental analysis by EDAX showed a Ti/Ba ratio with an excess of Ti in the needles. Samples with 0.15, 0.30, and 0.60% Nb₂O₅ (sintered at 1350°C) contained a Ti/Ba relation of 2.30 (70% Ti), 2.46 (71% Ti) and 2.55 (72% Ti) respectively. It is not possible to detect the niobium presence.

A

B





Figure 3. SEM microphotographs corresponding to BaTiO₃ doped with (A) 0.15 % mol Nb₂O₅ and (B) 0.60 % mol Nb₂O₅ and sintered at 1350ºC for 2 hours. Details of secondary phases. Bar 100 m.
In order to explain these results, we must consider that during the sintering process Nb⁵⁺ ions diffuse toward the grains of BaTiO₃. Indeed, as the Nb₂O₅ concentration increases, the number of niobium ions susceptible of entering to the BaTiO₃ lattice and the number of Ti displaced outside of the grain increase. Therefore, Ti/Ba ratio in the secondary phases increases with the Nb₂O₅ concentration used.

One of the most attractive phenomena in the microstructural evolution of Nb₂O₅-doped BaTiO₃ appears in samples with abnormal grain growth. As it has already been reported⁽¹⁰⁾, this microstructural development is common when the dopant is not homogeneously distributed among the particles of BaTiO₃. Then, in dopant-rich areas the inhibition of the grain growth due to the dopant presence is generated. On the contrary, those regions with low or zero dopant concentration do not exhibit a grain growth control. On the other hand, a secondary phase with dendrite morphology was only observed on the fine grains region. This behaviour is due to a greatly incorporation of niobium and an important segregation of titanium in those areas of fine grains.

According to the phase diagram of BaTiO₃-TiO₂,⁽¹⁴⁾ the needle composition would correspond to the non-stoichiometric couple BaTiO₃-Ba₆Ti₁₇O₄₀ with a small percentage of Nb⁵⁺ incorporated. Secondary phases are in low concentration and then, only BaTiO₃ was detected by XRD analysis.
CONCLUSIONS
From the obtained results it is possible to conclude that:

• 
  Due to the non-satisfactory distribution of dopant among the BaTiO₃ particles, BaTiO₃ doped with a concentration of Nb₂O₅ less than 0.15% mol exhibits a microstructure with abnormal grain growth. On the contrary, the content of 0.15% mol of Nb₂O₅ is enough to stimulate the satisfactory distribution of dopant in barium titanate and in consequence, the microstructure control in doped material is successfully achieved.
  
• 
  It has been confirmed that the presence of dopant stimulates the formation of the titanium-rich phase in barium titanate at temperature below the established ones for the binary system BaTiO₃-TiO₂.
  
• 
  The amount of titanium in the secondary phases increases according to the niobium content increases in the system. This fact implies the niobium incorporation in the BaTiO₃ grains as well as the rejection of titanium out of the BaTiO₃ lattice.
  

REFERENCES

1. 
   H. Martirena, J. Burfoof, J. Phys. C: Solid State Phys. 7 (1974) 3182.
   
2. 
   B. Begg, D. Vance, D. Cassidy, J. Nowotny, S. Blavis, in Grain boundaries and interfacial phenomen in electronic ceramics, Ceramic Transactions, The Am. Ceram. Soc., ed. by Levinson and Hirano, Ohio, Vol. 41, pp. 169-176 (1994).
   
3. 
   G. Jonker, E. Havinga, Mat.res. Bull. 17 (1982) 345-350.
   
4. 
   N. Hari, T.R.N. Kutty, J. Mat. Sic. 33 (1998) 3275-3284.
   
5. 
   T.R.N. Kutty, N.S. Hari, Mat. Lett. 34 (1998) 43-49.
   
6. 
   D.F.K. Hennings, B.S. Schreinemacher, J. Eur.Ceram.Soc. 14 (1994) 463 - 471.
   
7. 
   H. Chan, M. Harmer, D. Smyth, J.Am.Ceram.Soc., 69 (1986) 507-510.
   
8. 
   J.P. Bonsack, Am. Ceram. Soc. Bull., 50(1971)488-492.
   
9. 
   B. Hoffmann, A. Janitgki, Ver. Dt. Keram. Ges., 55 (1978) 317-319.
   
10. 
    J.F.F. Fernández, Doctoral thesis, Universidad Autónoma de Madrid, Madrid, Spain, 1990.
    
11. 
    I. Zajc, M. Drofenik, J. Mat. Res. 13 (1998)660-664.
    
12. 
    N. Chan, D. Smyth, J. Am. Ceram.Soc., 67 (1984) 285-288.
    
13. 
    J. Nowotny, M. Rekas, Ceram. Int., 20 (1994) 265-275.
    
14. 
    K. Kirby, B. Wechsler, J. Am. Ceram. Soc., 74 (1991) 1841-1847.