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Effect of sintering atmosphere on the dielectric properties of barium titanate based capacitors
Pergamon
Materials Research Bulletin 36 (2001) 905–913
Effect of sintering atmosphere on the dielectric properties of barium titanate based capacitors N. Halder, D. Chattopadhyay, A. Das Sharma, D. Saha, A. Sen*, H.S. Maiti Electroceramics Division, Central Glass & Ceramic Research Institute, Calcutta 700 032, India (Refereed) Received 13 June 2000; accepted 23 August 2000
Abstract It has been found that BaTiO3 based capacitor compositions containing lithium tetraborate (flux) and bismuth oxide (flux and Curie peak suppressor) can be fired in an inert (argon) atmosphere without sacrificing densification. However, the inert atmosphere firing can lead to a dramatic increase of both the dielectric constant and dissipation factor of the samples containing bismuth oxide which can be suppressed by adding a small amount of manganese dioxide in the compositions. The temperature coefficient of capacitance of such samples shows the desirable flattened response when fired in argon atmosphere. The observations have been explained by considering the formation of defects at elevated temperatures in bismuth containing samples when fired in an inert atmosphere. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; A. Electronic materials; D. Dielectric properties; D. Ferroelectricity; D. Defects
1. Introduction Multilayer ceramic capacitors (MLCCs continue to be one of the most important and widely used passive components in all electronic circuitry today. The explosive growth of personal computer, the PC-TV hybrid and the telecommunication industry will undoubtedly drive the usage of MLCCs to record high levels [1]. In the quest for reducing the cost of MLCCs, many “low-fire” (low temperature processed) compositions based on BaTiO3 have been developed which can be co-fired with 70:30 Ag-Pd electrode at temperatures less than 1150°C. Efforts are also being made (predominantly in Japan * Corresponding author. Fax: ⫹91-33-4730957. 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 5 4 0 - 2
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Table 1 Composition of different batches (weight %)* Batch 1 Batch 2 Batch 3
BaTiO3 ⫹ 0.65% Li2B4O7 BaTiO3 ⫹ 0.65% Li2B4O7 ⫹ 0.40% Bi2O3 BaTiO3 ⫹ 0.65% Li2B4O7 ⫹ 0.40% Bi2O3 ⫹ 0.40% MnO2
* Weight percent of all the additives are with respect to BaTiO3
[2]) to reduce the cost further by developing BaTiO3 based composition which can be sintered in inert/reducing atmospheres thus enabling the use of cheap base metal (e.g., nickel) electrodes. Generally, to modify the temperature variation of capacitance, additives [3–5] like CaZrO3, Bi2O3, Nb2O5 (Curie peak suppressors) and Li2O, CuO, PbO (fluxes) are used with BaTiO3. However, information regarding the behaviour of such complex mixtures during sintering in inert/reducing atmosphere (so that nickel electrodes can be used) is lacking and hence such process warrants detailed studies. Based on our early experience [6 –7], we have selected three basic compositions: (a) barium titanate and lithium tetraborate where the latter is a well-known flux (b) barium titanate, lithium tetraborate and bismuth oxide where the last is a Curie-peak suppressor as well as a flux; and (c) barium titanate, lithium tetraborate, bismuth oxide and manganese dioxide where MnO2 being an acceptor [3,4] dopant, lowers dissipation factor and improves long term stability. The dielectric properties of the aforesaid compositions were studied after sintering the batches in atmospheres containing different percentages of oxygen and argon. 2. Experimental Three different batches having compositions as shown in Table 1 were studied in this investigation. High purity BaTiO3 powder (average particle size ⬃1.2 m) was prepared by oxalate coprecipitation technique from a mixed aqueous solution of BaCl2 (Merck) and TiCl4 (Riedel). The details of the process have been described elsewhere [8]. Other raw materials used in this investigation are reagent grade Bi2O3 (Merck), Li2B4O7 (S.D. Fine) and MnO2 (S.D. Fine). The raw materials for each batch were mixed under acetone in an agate mortar and a pestle for 30 minutes and then dried. Pellets were made from the dried powder mix under 50 MPa pressure using 1.5 wt% ethyl cellulose binder. The green pellets were then sintered at 1150°C for 3 h in atmospheres containing different percentages of oxygen and argon. The bulk densities of the sintered pellets were measured geometrically. A sample containing BaTiO3 and relatively high amount of Bi2O3 (50 wt%) was sintered in argon at atmosphere at 1150°C/3 h and studied by X-ray diffraction (Philips, PW 1730 diffractometer) to find out any phase change of bismuth oxide after sintering in 100% argon atmosphere in presence of BaTiO3. For dielectric measurements, the surfaces of the sintered pellets were ground and polished followed by application of silver conducting paste on the two opposite surfaces. The samples were then cured at 350°C for 1 h. The capacitance (C) and dissipation factor (DF) with respect to temperature (from room temp. to 130°C) were measured by an LCZ meter (HP-4276A) at 1 kHz frequency under 1 V rms.
D. Halder et al. / Materials Research Bulletin 36 (2001) 905–913
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Fig. 1. Variation of sintered densities for the samples of different batches as a function of oxygen concentration (rest is argon) of the sintering atmosphere.
3. Results and discussion It is evident from Fig. 1 that the densities of the samples of the three batches remain almost constant during sintering in different atmospheres. The dielectric constant (K) and dissipation factor (DF) values of batch 1 samples (Fig. 2) remain unaffected even in 100% Ar atmosphere though it is expected [9] that, as given below, n type defects should form in low pO2 atmosphere leading to increased conductivity with consequent high DF: O o 3 1/ 2 O 2 ⫹ Vo•⫹e⬘
(1)
O o 3 1/ 2 O 2 ⫹ V o•• ⫹ 2e⬘
(2)
Ti 4⫹ ⫹ e⬘ 3 Ti 3⫹
(3)
Probably, the oxygen loss from BaTiO3 (and hence n type defect formation) is inhibited owing to the formation of a glass like coating of molten Li2B4O7 (melting point 930°C) on BaTiO3 grains.
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Fig. 2. Variation of dielectric constant and dissipation factor for the samples of different batches with oxygen concentration of the sintering atmosphere.
In the case of batch 2 samples containing Bi2O3, it is expected [10] that Bi3⫹ is doped as a donor in BaTiO3 substituting for Ba2⫹ in the lattice as given below: Bi 2O 3 3 2Bi •Ba ⫹ 2O o ⫹ 1/ 2 O 2 ⫹ 2e⬘
(4)
However, it is to be noted that, specifically at a higher concentration (and also depending on the atmosphere and presence of other additives) the defect chemistry can switch over from the electronic to the nonelectronic type (either Ba or Ti vacancy) [11–13]. Bi 2O 3 3 2Bi •Ba ⫹ V ⬙Ba ⫹ 3O o
(5)
2Bi 2O 3 3 4Bi •Ba ⫹ VTi ⫹ 6O o
(6)
VBa (or VTi) and Vo are related as Schottky-type point defects. V ⬙Ba 共or V Ti兲 ⫹ V o•• 共or 2V o••兲 3 null
(7)
However, titanium vacancy (VTi) has generally been considered to be an unlikely defect [14] because of its high effective charge that corresponds to a major disruption of the chemical bonding of the crystals. From Fig. 2, it is evident that the K values of batch 2 samples (when sintered in atmospheres containing more than 25% oxygen), are lower than those of batch 1 samples. This is expected, as, additives, specifically during substitution, lower [15] the K values. However, batch 2 samples, when sintered in an atmosphere containing less than 25% oxygen, showed a dramatic increase in both the K and DF values (Fig. 2). To understand this
D. Halder et al. / Materials Research Bulletin 36 (2001) 905–913
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Fig. 3. X-ray diffractogram of BaTiO3 powder containing 50 wt% Bi2O3 after sintering in argon.
behaviour, we have to first appreciate that Bi2O3 has a strong tendency [16] to get reduced at high temperature and specifically at low partial pressure of oxygen. To find out the valence state of Bi2O3 (in presence of BaTiO3 when fired in an inert atmosphere) we studied the X-ray diffractogram (Fig. 3) of a BaTiO3 sample containing high amount (50 wt%) of Bi2O3 (sintered in 100% Ar at 1150°C/3 h) and found the presence of BiO phase [17], along with BaTiO3 and Bi2O3. Hence in 100% Ar atmosphere and under our specified firing conditions, reduction of some Bi2O3 and formation of electronic defects as shown in equation (4) are much more likely to occur. Such electronic defects in sufficient concentration can give rise to high DF. At the same time, due to the tendency of bismuth ions to form Bi2⫹ state in the Ar atmosphere, some (BiBa ⫺ e) defect pairs are likely to form. It is to be noted [18,19] that, closely coupled pairs of defects of opposite sign, in the limit of two-centre hopping, are physically indistinguishable from the dipolar situation and can give rise to high dielectric constant. Our conjecture about the defect formation is supported by the fact that the manganese ions, being well-known acceptors [20] can consume the charge carriers as given below: Mn 4⫹ ⫹ e 3 Mn 3⫹
(8)
Mn 3⫹ ⫹ e 3 Mn 2⫹
(9)
Hence the K and DF values of batch 3 samples (containing MnO2) under identical conditions are much lower than those of batch 2 samples.
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Fig. 4. Variation of percentage TCC with temperature for batch 1 samples.
Interestingly, the temperature coefficients of capacitance (TCC) for bismuth containing samples (batches 2 & 3) fired in oxygen and argon atmospheres show a remarkably different behaviour. Figs. 4 – 6 depict the percent TCC for batch 1, 2 and 3 samples. The percent TCC is defined as (C25 ⫺ CT)/C25 * 100 where C25 is the capacitance at 25°C and CT is the capacitance at a temperature T. In contrast with batch 1 samples, both batch 2 and batch 3 samples show much flattened TCC response [at around the Curie temperature (120 –130°C)] when fired in argon atmosphere. To understand such diffuse phase transition behaviour of argon-atmosphere-fired bismuth containing samples, we have to first appreciate that if by some means, random local fields can be generated, they can influence the critical behaviour (in our case, ferroelectric to paraelectric phase transition) of the system [21,22]. Such random fields can modify the domains and with the increasing concentration of the random-field generating defects, the domain size may become comparable to the correlation length for spatial fluctuations of the order parameter and ferroelectric state may eventually give way to a dipolar glass state [21]. Such dipolar states [in materials like (Ba,Sr)TiO3, (K,Li)TaO3] can give rise to smeared ferroelectric transition which is somewhat different from that of relaxor materials like Pb(Mg1/3Nb2/3)O3 in the sense that the latter shows strong frequency dispersion of dielectric properties [23,24]. As discussed earlier, in
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Fig. 5. Variation of percentage TCC with temperature for batch 2 samples.
bismuth containing samples (batches 2 & 3) an appreciable concentration of closely coupled defects are formed and such defects can give rise to random local fields with consequent diffuse phase transition. Manganese, by forming ions of different valancies, can add up to the random local field leading to more suppression of the Curie peak in batch 3 samples.
4. Conclusion 1. BaTiO3 based capacitor compositions containing lithium borate, bismuth oxide and manganese dioxide do not show any appreciable variation in densification during firing in atmospheres containing different percentages of oxygen and argon. 2. The bismuth containing composition without MnO2 when fired in atmospheres containing less than 25% oxygen, shows a dramatic increase in both the K and DF values due to the formation of n-type electronic defects. Manganese ions being well-known acceptors can annihilate the defects and inhibit the dramatic increase of both the K and DF values of bismuth containing samples.
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Fig. 6. Variation of percentage TCC with temperature for batch 3 samples.
3. The flattened response of temperature coefficient of capacitance of argon-atmospheresintered samples can be explained by considering the generation of random local fields due to formation of closely coupled defects and ions of variable valences leading to a smeared ferroelectric transition.
References [1] R.B. Nami, Ceram. Ind. 12 (1997) 28. [2] S.L. Swartz, T.R. Shrout, T. Takenaka, Bull. Am. Ceram. Soc. 76 (1997) 59. [3] M. Kahan, D.P. Burks, I. Burn, W.A. Schulze, in: L.M. Levinson (Ed.), Electronic Ceramics—Properties, Devices and Applications, Marcel Dekker, New York, 1988, p. 202. [4] G. Goodman, in: R.C. Buchanan (Ed.), Ceramic Materials for Electronics—Processing, Properties and Applications, Marcel Dekker, New York, 1986, p. 131. [5] I. Burn, J. Mat. Sc. 1398 (1982) 17. [6] N. Haldar, A. Das Sharma, S.K. Khan, A. Sen, H.S. Maiti, Mat. Res. Bull. 34 (1999) 545. [7] A. Das Sharma, N. Haldar, S.K. Khan, A. Sen, H.S. Maiti, J. Mat. Sc. Lett. 100 (1998) 157.
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[8] S. Bhattacharya, M.K. Paria, H.S. Maiti, Ceram. Int. 16 (1990) 211. [9] J. Nowotny, M. Rekas, in: J. Nowotny (Ed.), Electronic Ceramic Materials, Trans Tech., Switzerland, 1992, p. 12. [10] J. Qi, W. Chen, Y. Wu, L. Li, J. Am. Ceram. Soc. 81 (1998) 437. [11] D.C. Hill, H.L. Tuller, in: R.C. Buchanan (Ed.), Ceramic Materials for Electronics—Processing, Properties and Applications, Marcel Dekker, New York, 1986, p. 336. [12] F.D. Morrison, D.C. Sinclair, J.M.S. Shakle, A.R. West, J. Am. Ceram. Soc. 81 (1998) 957. [13] L. Zhou, P.M. Vilarinho, J.L. Bapista, J. Am. Ceram. Soc. 82 (1999) 1064. [14] N.H. Chan, D.M. Smyth, J. Am. Ceram. Soc. 67 (1984) 285. [15] I. Burn, Bull. Amer. Ceram. Soc. 50 (1971) 501. [16] H. Remy, in: J. Kleinberg (Ed.), Treatise on Inorganic Chemistry, Vol I, Translated by J.S. Anderson, Elsevier, New York, 1956, p. 677. [17] Powder Diffraction File—Inorganic Phases, Compiled by JCPDS, International Centre for Diffraction Data, 1986, p. 28. [18] Y. Zhi, A. Chen, P.M. Vilarinho, P.Q. Mantas, J.L. Bapista, J. Europ. Ceram. Soc. 18 (1998) 1621. [19] A.K. Jonscher, J. Mater. Sci. 16 (1981) 2037. [20] A.J. Moulson, J.M. Herbert, in: Electroceramics—Materials, Properties and Applications, Chapman & Hall, London, 1990, p. 247. [21] Y. Imri, S. Ma, Phys. Rev. Lett. 35 (1975) 1399. [22] D.P. Belanger, A.R. King, V. Jaccarino, Phys. Rev. Lett. 48 (1982) 1050. [23] V.S. Tiwari, N. Singh, D. Pandey, J. Phys. Condens. Matter. 7 (1995) 1441. [24] U.T. Hochli, K. Knorr, A. Loidl, Adv. Phys. 39 (1990) 599.
Materials Research Bulletin 36 (2001) 905–913
Effect of sintering atmosphere on the dielectric properties of barium titanate based capacitors N. Halder, D. Chattopadhyay, A. Das Sharma, D. Saha, A. Sen*, H.S. Maiti Electroceramics Division, Central Glass & Ceramic Research Institute, Calcutta 700 032, India (Refereed) Received 13 June 2000; accepted 23 August 2000
Abstract It has been found that BaTiO3 based capacitor compositions containing lithium tetraborate (flux) and bismuth oxide (flux and Curie peak suppressor) can be fired in an inert (argon) atmosphere without sacrificing densification. However, the inert atmosphere firing can lead to a dramatic increase of both the dielectric constant and dissipation factor of the samples containing bismuth oxide which can be suppressed by adding a small amount of manganese dioxide in the compositions. The temperature coefficient of capacitance of such samples shows the desirable flattened response when fired in argon atmosphere. The observations have been explained by considering the formation of defects at elevated temperatures in bismuth containing samples when fired in an inert atmosphere. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; A. Electronic materials; D. Dielectric properties; D. Ferroelectricity; D. Defects
1. Introduction Multilayer ceramic capacitors (MLCCs continue to be one of the most important and widely used passive components in all electronic circuitry today. The explosive growth of personal computer, the PC-TV hybrid and the telecommunication industry will undoubtedly drive the usage of MLCCs to record high levels [1]. In the quest for reducing the cost of MLCCs, many “low-fire” (low temperature processed) compositions based on BaTiO3 have been developed which can be co-fired with 70:30 Ag-Pd electrode at temperatures less than 1150°C. Efforts are also being made (predominantly in Japan * Corresponding author. Fax: ⫹91-33-4730957. 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 5 4 0 - 2
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D. Halder et al. / Materials Research Bulletin 36 (2001) 905–913
Table 1 Composition of different batches (weight %)* Batch 1 Batch 2 Batch 3
BaTiO3 ⫹ 0.65% Li2B4O7 BaTiO3 ⫹ 0.65% Li2B4O7 ⫹ 0.40% Bi2O3 BaTiO3 ⫹ 0.65% Li2B4O7 ⫹ 0.40% Bi2O3 ⫹ 0.40% MnO2
* Weight percent of all the additives are with respect to BaTiO3
[2]) to reduce the cost further by developing BaTiO3 based composition which can be sintered in inert/reducing atmospheres thus enabling the use of cheap base metal (e.g., nickel) electrodes. Generally, to modify the temperature variation of capacitance, additives [3–5] like CaZrO3, Bi2O3, Nb2O5 (Curie peak suppressors) and Li2O, CuO, PbO (fluxes) are used with BaTiO3. However, information regarding the behaviour of such complex mixtures during sintering in inert/reducing atmosphere (so that nickel electrodes can be used) is lacking and hence such process warrants detailed studies. Based on our early experience [6 –7], we have selected three basic compositions: (a) barium titanate and lithium tetraborate where the latter is a well-known flux (b) barium titanate, lithium tetraborate and bismuth oxide where the last is a Curie-peak suppressor as well as a flux; and (c) barium titanate, lithium tetraborate, bismuth oxide and manganese dioxide where MnO2 being an acceptor [3,4] dopant, lowers dissipation factor and improves long term stability. The dielectric properties of the aforesaid compositions were studied after sintering the batches in atmospheres containing different percentages of oxygen and argon. 2. Experimental Three different batches having compositions as shown in Table 1 were studied in this investigation. High purity BaTiO3 powder (average particle size ⬃1.2 m) was prepared by oxalate coprecipitation technique from a mixed aqueous solution of BaCl2 (Merck) and TiCl4 (Riedel). The details of the process have been described elsewhere [8]. Other raw materials used in this investigation are reagent grade Bi2O3 (Merck), Li2B4O7 (S.D. Fine) and MnO2 (S.D. Fine). The raw materials for each batch were mixed under acetone in an agate mortar and a pestle for 30 minutes and then dried. Pellets were made from the dried powder mix under 50 MPa pressure using 1.5 wt% ethyl cellulose binder. The green pellets were then sintered at 1150°C for 3 h in atmospheres containing different percentages of oxygen and argon. The bulk densities of the sintered pellets were measured geometrically. A sample containing BaTiO3 and relatively high amount of Bi2O3 (50 wt%) was sintered in argon at atmosphere at 1150°C/3 h and studied by X-ray diffraction (Philips, PW 1730 diffractometer) to find out any phase change of bismuth oxide after sintering in 100% argon atmosphere in presence of BaTiO3. For dielectric measurements, the surfaces of the sintered pellets were ground and polished followed by application of silver conducting paste on the two opposite surfaces. The samples were then cured at 350°C for 1 h. The capacitance (C) and dissipation factor (DF) with respect to temperature (from room temp. to 130°C) were measured by an LCZ meter (HP-4276A) at 1 kHz frequency under 1 V rms.
D. Halder et al. / Materials Research Bulletin 36 (2001) 905–913
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Fig. 1. Variation of sintered densities for the samples of different batches as a function of oxygen concentration (rest is argon) of the sintering atmosphere.
3. Results and discussion It is evident from Fig. 1 that the densities of the samples of the three batches remain almost constant during sintering in different atmospheres. The dielectric constant (K) and dissipation factor (DF) values of batch 1 samples (Fig. 2) remain unaffected even in 100% Ar atmosphere though it is expected [9] that, as given below, n type defects should form in low pO2 atmosphere leading to increased conductivity with consequent high DF: O o 3 1/ 2 O 2 ⫹ Vo•⫹e⬘
(1)
O o 3 1/ 2 O 2 ⫹ V o•• ⫹ 2e⬘
(2)
Ti 4⫹ ⫹ e⬘ 3 Ti 3⫹
(3)
Probably, the oxygen loss from BaTiO3 (and hence n type defect formation) is inhibited owing to the formation of a glass like coating of molten Li2B4O7 (melting point 930°C) on BaTiO3 grains.
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Fig. 2. Variation of dielectric constant and dissipation factor for the samples of different batches with oxygen concentration of the sintering atmosphere.
In the case of batch 2 samples containing Bi2O3, it is expected [10] that Bi3⫹ is doped as a donor in BaTiO3 substituting for Ba2⫹ in the lattice as given below: Bi 2O 3 3 2Bi •Ba ⫹ 2O o ⫹ 1/ 2 O 2 ⫹ 2e⬘
(4)
However, it is to be noted that, specifically at a higher concentration (and also depending on the atmosphere and presence of other additives) the defect chemistry can switch over from the electronic to the nonelectronic type (either Ba or Ti vacancy) [11–13]. Bi 2O 3 3 2Bi •Ba ⫹ V ⬙Ba ⫹ 3O o
(5)
2Bi 2O 3 3 4Bi •Ba ⫹ VTi ⫹ 6O o
(6)
VBa (or VTi) and Vo are related as Schottky-type point defects. V ⬙Ba 共or V Ti兲 ⫹ V o•• 共or 2V o••兲 3 null
(7)
However, titanium vacancy (VTi) has generally been considered to be an unlikely defect [14] because of its high effective charge that corresponds to a major disruption of the chemical bonding of the crystals. From Fig. 2, it is evident that the K values of batch 2 samples (when sintered in atmospheres containing more than 25% oxygen), are lower than those of batch 1 samples. This is expected, as, additives, specifically during substitution, lower [15] the K values. However, batch 2 samples, when sintered in an atmosphere containing less than 25% oxygen, showed a dramatic increase in both the K and DF values (Fig. 2). To understand this
D. Halder et al. / Materials Research Bulletin 36 (2001) 905–913
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Fig. 3. X-ray diffractogram of BaTiO3 powder containing 50 wt% Bi2O3 after sintering in argon.
behaviour, we have to first appreciate that Bi2O3 has a strong tendency [16] to get reduced at high temperature and specifically at low partial pressure of oxygen. To find out the valence state of Bi2O3 (in presence of BaTiO3 when fired in an inert atmosphere) we studied the X-ray diffractogram (Fig. 3) of a BaTiO3 sample containing high amount (50 wt%) of Bi2O3 (sintered in 100% Ar at 1150°C/3 h) and found the presence of BiO phase [17], along with BaTiO3 and Bi2O3. Hence in 100% Ar atmosphere and under our specified firing conditions, reduction of some Bi2O3 and formation of electronic defects as shown in equation (4) are much more likely to occur. Such electronic defects in sufficient concentration can give rise to high DF. At the same time, due to the tendency of bismuth ions to form Bi2⫹ state in the Ar atmosphere, some (BiBa ⫺ e) defect pairs are likely to form. It is to be noted [18,19] that, closely coupled pairs of defects of opposite sign, in the limit of two-centre hopping, are physically indistinguishable from the dipolar situation and can give rise to high dielectric constant. Our conjecture about the defect formation is supported by the fact that the manganese ions, being well-known acceptors [20] can consume the charge carriers as given below: Mn 4⫹ ⫹ e 3 Mn 3⫹
(8)
Mn 3⫹ ⫹ e 3 Mn 2⫹
(9)
Hence the K and DF values of batch 3 samples (containing MnO2) under identical conditions are much lower than those of batch 2 samples.
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Fig. 4. Variation of percentage TCC with temperature for batch 1 samples.
Interestingly, the temperature coefficients of capacitance (TCC) for bismuth containing samples (batches 2 & 3) fired in oxygen and argon atmospheres show a remarkably different behaviour. Figs. 4 – 6 depict the percent TCC for batch 1, 2 and 3 samples. The percent TCC is defined as (C25 ⫺ CT)/C25 * 100 where C25 is the capacitance at 25°C and CT is the capacitance at a temperature T. In contrast with batch 1 samples, both batch 2 and batch 3 samples show much flattened TCC response [at around the Curie temperature (120 –130°C)] when fired in argon atmosphere. To understand such diffuse phase transition behaviour of argon-atmosphere-fired bismuth containing samples, we have to first appreciate that if by some means, random local fields can be generated, they can influence the critical behaviour (in our case, ferroelectric to paraelectric phase transition) of the system [21,22]. Such random fields can modify the domains and with the increasing concentration of the random-field generating defects, the domain size may become comparable to the correlation length for spatial fluctuations of the order parameter and ferroelectric state may eventually give way to a dipolar glass state [21]. Such dipolar states [in materials like (Ba,Sr)TiO3, (K,Li)TaO3] can give rise to smeared ferroelectric transition which is somewhat different from that of relaxor materials like Pb(Mg1/3Nb2/3)O3 in the sense that the latter shows strong frequency dispersion of dielectric properties [23,24]. As discussed earlier, in
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Fig. 5. Variation of percentage TCC with temperature for batch 2 samples.
bismuth containing samples (batches 2 & 3) an appreciable concentration of closely coupled defects are formed and such defects can give rise to random local fields with consequent diffuse phase transition. Manganese, by forming ions of different valancies, can add up to the random local field leading to more suppression of the Curie peak in batch 3 samples.
4. Conclusion 1. BaTiO3 based capacitor compositions containing lithium borate, bismuth oxide and manganese dioxide do not show any appreciable variation in densification during firing in atmospheres containing different percentages of oxygen and argon. 2. The bismuth containing composition without MnO2 when fired in atmospheres containing less than 25% oxygen, shows a dramatic increase in both the K and DF values due to the formation of n-type electronic defects. Manganese ions being well-known acceptors can annihilate the defects and inhibit the dramatic increase of both the K and DF values of bismuth containing samples.
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Fig. 6. Variation of percentage TCC with temperature for batch 3 samples.
3. The flattened response of temperature coefficient of capacitance of argon-atmospheresintered samples can be explained by considering the generation of random local fields due to formation of closely coupled defects and ions of variable valences leading to a smeared ferroelectric transition.
References [1] R.B. Nami, Ceram. Ind. 12 (1997) 28. [2] S.L. Swartz, T.R. Shrout, T. Takenaka, Bull. Am. Ceram. Soc. 76 (1997) 59. [3] M. Kahan, D.P. Burks, I. Burn, W.A. Schulze, in: L.M. Levinson (Ed.), Electronic Ceramics—Properties, Devices and Applications, Marcel Dekker, New York, 1988, p. 202. [4] G. Goodman, in: R.C. Buchanan (Ed.), Ceramic Materials for Electronics—Processing, Properties and Applications, Marcel Dekker, New York, 1986, p. 131. [5] I. Burn, J. Mat. Sc. 1398 (1982) 17. [6] N. Haldar, A. Das Sharma, S.K. Khan, A. Sen, H.S. Maiti, Mat. Res. Bull. 34 (1999) 545. [7] A. Das Sharma, N. Haldar, S.K. Khan, A. Sen, H.S. Maiti, J. Mat. Sc. Lett. 100 (1998) 157.
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