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Phase formation in phosphorus doped BaTiO3
April 1998
Materials Letters 35 Ž1998. 72–77
Phase formation in phosphorus doped BaTiO 3 A.C. Caballero ) , J.F. Fernandez, C. Moure, P. Duran ´ ´ Departamento de Electroceramica, Instituto de Ceramica y Vidrio, Crta. de Valencia Km 24,3, 28500 Arganda del Rey, Madrid, Spain ´ ´ Received 12 May 1997; revised 3 September 1997; accepted 5 September 1997
Abstract Residual phosphorus cations left by phosphate ester form a surface layer covering the BaTiO 3 particles. During the sintering step, this layer reacts with BaTiO 3 to form BaO-rich compounds as Ba 2TiP2 O 9 and Ba 3ŽPO4 . 2 . BaO enrichment of the surface of the BaTiO 3 particles may be the origin of the grain growth inhibition observed in BaTiO 3 doped with small amounts of P2 O5. Moreover, these results support the hypothesis that the reported 4BaOP 3TiO 2 P P2 O5 compound actually represents two binary compounds; Ba 3ŽPO4 . 2 and a TiO 2 rich ‘polytitanate’. q 1998 Elsevier Science B.V. Keywords: Barium titanate; Phosphorus; Doping; Reactions; Phases
1. Introduction The most important application of BaTiO 3 is as dielectric layers in multilayer ceramic manufacturing. The multilayer ceramic capacitor industry is continuing intensive efforts to reduce component size, to decrease dielectric layer thickness and precious-metal use, and to improve component reliability w1x. Phosphate ester is an excellent dispersing agent and has been successfully used in BaTiO 3 slips w2x. However, previous works w3–5x have shown that phosphate ester leaves a certain amount of residual phosphorus which remains adsorbed onto the surfaces of the BaTiO 3 particles, i.e., the material became phosphorus doped. Due to the adsorption characteristics of the phosphate ester on the BaTiO 3 ) Corresponding author. Tel.: q91-871-1800; fax: q91-8700550.
particles w6x, the residual phosphorus forms an homogeneous adsorbed layer which covers the BaTiO 3 particles as a ‘surface doping’. This doped material showed better porosity removal in the first sintering stage and higher shrinkage rate than undoped BaTiO 3 . Therefore, lower sintering temperatures were needed for phosphorus doped BaTiO 3 ceramics. BaTiO 3 was the only phase detected and homogeneous fine grained microstructures were observed. Consequently, the material showed excellent dielectric properties Ždielectric constant around 4000 at RT and 1 kHz and dielectric losses below 1%.. Grain growth control in these samples may be understood in terms of solute drag mechanism, yet the marked change of the porosity removal in the first sintering stage seems to points out to more complex mechanisms. In fact, the results presented in this paper show that residual phosphorous located at the surface of the particles react with BaTiO 3 during sintering to form BaO-rich compounds.
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 7 . 0 0 2 2 6 - 7
A.C. Caballero et al.r Materials Letters 35 (1998) 72–77
2. Experimental procedure The BaTiO 3 raw material is a commercial grade ŽELMIC BT100, Rhone ˆ Poulenc. with BarTis 1000. Sr - 0.05 wt% is the main impurity detected, the average particle size is 0.84 m m and the specific surface is 2.7 m2 gy1 . Phosphate ester is composed of 60% C 4 H 11O4 P and 40% C 8 H 19 O4 P and shows a density value of 1.13 grcc. BaTiO 3 was dispersed in an isopropyl alcohol solution of phosphate ester by stirring with a high speed turbine Ž6000 rpm. for 10 min. The mixture was oven dried below 1008C. Assuming that all the phosphorous present in the phosphate ester remains as P2 O5 , then the amount of P2 O5 is 5 wt% with respect to the BaTiO 3 . Differential thermal analysis ŽDTA-TG, Netzsch STA 409. of the dried doped powder was carried out with 58Crm heating rate. Crystalline phases present in samples treated at different temperatures for 2 h
73
were determined by powder X-ray diffraction ŽXRD, Siemmens D5000.. Transmission electron microscopy ŽTEM, JEOL 2000. characterisation was carried out on several powder samples treated at different temperatures. Semiquantitative SEM-EDS analysis ŽZeiss DSM 950. of the samples treated at high temperature allowed to identify the morphology and distribution of different phases.
3. Results and discussion 3.1. Low temperature regime (- 5008C) DTA and TG results are shown in Fig. 1. DTA curve shows one endothermic peak between 100 and 1208C which may be related to the loss of residual water. In this case, the 0.8% weight loss measured between room temperature and 1908C is mostly
Fig. 1. DTA-TG of 5 wt% P2 O5 doped BaTiO 3 powder.
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A.C. Caballero et al.r Materials Letters 35 (1998) 72–77
residual water. A wide endothermic peak is detected between 1908C and 4208C. Actually this peak seems to be the result of the convolution of three peaks which correlates with the progressive burnt out of the phosphate ester. The 6.8% weight loss Žobserved in the TG curve. associated to this peak is in excellent agreement with the expected value of 6.9%,
assuming that the residual phosphorus remains as P2 O5 . On the other hand, DRX Žnot shown. of powder samples treated at 5008C and slowly cooled only revealed the presence of crystalline BaTiO 3 . However, this result can be understood taking into account the phosphorus location in the material. Since residual phosphorus is adsorbed on the surface
Fig. 2. DRX spectra of 5 wt% P2 O5 doped BaTiO 3 powder treated at different temperatures. BaTiO 3 peaks Žunmarked. are always detected. In order to keep the figure as clear as possible, Ba 3 ŽPO4 . 2 peaks are unmarked in the samples treated above 9008C.
A.C. Caballero et al.r Materials Letters 35 (1998) 72–77
of the BaTiO 3 particles w6x, P2 O5 is formed as a surface phase. As it is well known, DRX is more sensitive to the coherent scattering from the grains interior, consequently, small amounts of phases located at the particles surface could remain undetected even if they are well crystallised.
3.2. Intermediate temperature regime (500–10008C) No significant weight loss was detected above 4208C. At 5208C, the DTA base line begins to decrease which points out to an energy consuming process. This temperature is very close to the reported melting temperature for P2 O5 Ž535–5858C.. Therefore, this could reflect the formation of a liquid on the surface of the BaTiO 3 particles. Also, reaction between P2 O5 and BaO can drive to liquid formation for this range of temperatures according to the BaO–P2 O5 phase diagram w7x. In any case, it seems reasonable to assume that at these temperatures the BaTiO 3 particles are covered by a not well crystallised P2 O5 rich layer which tends to react to form a BaO-rich compound. DTA exothermic peak detected at 6508C indicates the formation of a new compound. DRX ŽFig. 2. of powder samples treated at 7008C for 2 h showed different results depending on the cooling rate. Only crystalline BaTiO 3 was detected in fast cooled samples, however the presence of Ba 2TiP2 O 9 compound Žpattern a 36-1467. was detected in slow cooled samples. Formation of this compound leaves a certain TiO 2 excess:
75
Ba 2TiP2 O 9 actually can act as a control mechanism of the amount and characteristics of a P2 O5-rich glass phase during heating. DTA curve shows another exothermic peak at 8758C related to the formation of a new compound. XRD of powder sample treated at 9008C for 2 h shows clearly the presence of three crystalline compounds: BaTiO 3 , Ba 3 ŽPO4 . 2 Žpattern a 25-0028. and BaTi 4 O 9 Žpattern a 34-0070.. At this temperature the reaction can be expressed in the following way: 4BaTiO 3 q P2 O5 ´ Ba 3 Ž PO4 . 2 q BaTi 4 O 9 . This result is in disagreement with reported data on the system BaO–TiO 2 –P2 O5 w9x in which a 4BaOP 3TiO 2 P P2 O5 compound was reported to be at equilibrium with BaTiO 3 and Ba 3 ŽPO4 . 2 for this range of temperatures. However, our data seem to support the hypothesis proposed by O’Bryan Jr. w10x which suggested that the ternary formula actually represents Ba 3 ŽPO4 . 2 and a TiO 2-rich polytitanate. As in the case of the sample treated at 7008C, both resulting
BaTiO 3 q x P P2 O5´ x P Ba 2TiP2 O 9 q Ba 1y 2 xTi 1yxO 3y4 x . Due to the low solubility of TiO 2 in BaTiO 3 , the presence of a TiO 2-rich polytitanate is also expected w8x, although it was not detected by DRX. Since the above reaction takes place on the surface of the BaTiO 3 particles, TiO 2-rich polytitanate would be formed as a surface layer which remains undetected by DRX as previously discussed for residual P2 O5 . The Ba 2TiP2 O 9 phase melts congruently above 11908C w9x which indicates that its formation must eliminate the presence of a phosphorus-rich glass-like phase. Therefore, the formation of small amounts of
Fig. 3. SEM micrograph of a 5 wt% P2 O5 doped BaTiO 3 sample treated at 12508C. The liquid phase is clearly observed in areas in where Ba 3 ŽPO4 . 2 platelets are formed.
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A.C. Caballero et al.r Materials Letters 35 (1998) 72–77
compounds were reported to be solid at 9008C indicating again that reaction with BaTiO 3 tends to consume the P2 O5-rich glassy phase. 3.3. High temperature regime () 10008C) DRX of samples treated at 11008C showed the presence of BaTiO 3 , Ba 3 ŽPO4 . 2 , and Ba 6Ti 17 O40 Žpattern a 35-0817.. When the treatment temperature increases, the amount of Ba 3 ŽPO4 . 2 also increases and the TiO 2-rich polytitanate phase changes. It is clear that the formation of Ba 3 ŽPO4 . 2 shifts the system to the TiO 2 rich region and several polytitanates can be detected depending on the treatment temperature. DTA curve shows an endothermic peak centred around 13108C which correlates well with the eutectic temperature of the system BaTiO 3 –TiO 2 w11x. However, for samples treated at 12508C, the presence of liquid phase is already detected by SEM ŽFig. 3. in regions in which the Ba 3 ŽPO4 . 2 platelets are located. DRX analysis of powdered samples treated above the eutectic temperature of the system BaTiO 3 –TiO 2 , showed three crystalline phases: BaTiO 3 , Ba 3 ŽPO4 . 2 and BaTi 2 O5 Žpattern a 34-
0133.. Moreover, SEM-EDS ŽFig. 4. of pellets treated at this temperature revealed also the presence of small crystals of Ba 6Ti 17 O40 . Obviously the presence of different polytitanates indicates that the material is far from equilibrium conditions.
4. Conclusions When phosphate ester doped-BaTiO3 is heated up, a residual phosphorus layer remains covering the BaTiO 3 particles. This layer is not well crystallised and tends to react forming BaO rich compounds at the surface of the particles above the P2 O5 melting temperature. According to the phase diagram of the system BaO–P2 O5 w7x, a P2 O5 rich liquid could be formed, however reaction of this liquid with TiO 2 to form a 2BaO P TiO 2 P P2 O5 solid compound prevents the formation of large amounts of liquid phase. As temperature increases, BaO enrichment of the surface layer progress. Above 9008C, a certain amount of solid Ba 3 ŽPO4 . 2 starts to form. Ba 3 ŽPO4 . 2 formation shifts the system to the TiO 2 rich region and, therefore, large amounts of liquid phase are formed above the eutectic temperature of the system BaTiO 3 –TiO 2 . Regarding the reported sintering behaviour of P 5q-doped BaTiO 3 w4x, this sequence of reactions play a determinant role. The presence of phosphorus causes a Ba2q enrichment of the surfaces of the BaTiO 3 particles when the temperature increases. Consequently, a surface layer is formed which avoids mass transport between adjacent grains. Therefore porosity coalescence and removal are initially favoured. On the other hand, Ba 3 ŽPO4 . 2 formation at higher temperatures breaks the covering layer leaving TiO 2 rich surfaces of the BaTiO 3 particles. As it is well known in TiO 2 rich-BaTiO 3 , liquid phase formation and exaggerated grain growth are expected for temperatures close to the eutectic one.
Acknowledgements Fig. 4. SEM micrograph of the surface of a 5 wt% P2 O5 doped BaTiO 3 sample treated at 13708C. The marked area indicates the presence of Ba 6Ti 17 O40 detected by EDS analysis.
The authors would like to thank to the CICYT ŽMAT94-807. for the financial support.
A.C. Caballero et al.r Materials Letters 35 (1998) 72–77
References w1x J.M. Wilson, Minerals review: Barium titanate, Am. Ceram. Soc. Bull. 74 Ž6. Ž1995. 106–110. w2x K. Mikeska, W.R. Cannon, Dispersant for tape-casting pure barium titanate, in: J.A. Mangels ŽEd.., Adv. in Ceram., vol. 9, Forming of Ceramics, Am. Ceram. Soc. OH, 1984, pp. 164–183. w3x J.F. Fernandez, A.C. Caballero, P. Duran, ´ ´ C. Moure, Improving sintering behaviour of BaTiO 3 by small doping additions, J. Mater. Sci. 31 Ž1996. 975–981. w4x A.C. Caballero, J.F. Fernandez, C. Moure, P. Duran, ´ ´ Effect of residual phosphorus left by phosphate ester on BaTiO 3 ceramics, Mater. Res. Bull. 32 Ž2. Ž1997. 221–229. w5x J.F. Fernandez, P. Duran, ´ ´ C. Moure, Microstructure development of tape casting BaTiO 3 ceramics, Ferroelectrics 127 Ž1992. 65–70.
77
w6x N. Le Bars, D. Tinet, A.M. Fangere, H. van Damme, P. Levitz, Adsorption mechanism of phosphate ester on barium titanate in organic medium. Preliminary results on the structure of the adsorbed layer, J. Phys. III 1 Ž1991. 707–718. w7x R.A. McCauley, F.A. Hummel, Phase relationships in a portion of the system BaO–P2 O5 , Trans. Br. Ceram. Soc. 67 Ž1968. 619–628. w8x R.K. Sharma, N.H. Chan, D.M. Smyth, Solubility of TiO 2 in BaTiO 3 , J. Am. Ceram. Soc. 64 Ž8. Ž1981. 448–451. w9x D.E. Harrison, The system BaO–TiO 2 –P2 O5 : Phase relations, fluorescence, and phosphorous preparation, J. Electrochem. Soc. 107 Ž3. Ž1960. 217–221. w10x H.M. O’Bryan Jr., Identification of surface phases on BaTiO 3 –TiO 2 ceramics, Am. Ceram. Soc. Bull. 66 Ž4. Ž1987. 677–680. w11x P.P. Phule, H.S. Risbud, Low temperature synthesis and processing of electronic materials in the BaO–TiO 2 system, J. Mater. Sci. 25 Ž1990. 1169–1183.
Materials Letters 35 Ž1998. 72–77
Phase formation in phosphorus doped BaTiO 3 A.C. Caballero ) , J.F. Fernandez, C. Moure, P. Duran ´ ´ Departamento de Electroceramica, Instituto de Ceramica y Vidrio, Crta. de Valencia Km 24,3, 28500 Arganda del Rey, Madrid, Spain ´ ´ Received 12 May 1997; revised 3 September 1997; accepted 5 September 1997
Abstract Residual phosphorus cations left by phosphate ester form a surface layer covering the BaTiO 3 particles. During the sintering step, this layer reacts with BaTiO 3 to form BaO-rich compounds as Ba 2TiP2 O 9 and Ba 3ŽPO4 . 2 . BaO enrichment of the surface of the BaTiO 3 particles may be the origin of the grain growth inhibition observed in BaTiO 3 doped with small amounts of P2 O5. Moreover, these results support the hypothesis that the reported 4BaOP 3TiO 2 P P2 O5 compound actually represents two binary compounds; Ba 3ŽPO4 . 2 and a TiO 2 rich ‘polytitanate’. q 1998 Elsevier Science B.V. Keywords: Barium titanate; Phosphorus; Doping; Reactions; Phases
1. Introduction The most important application of BaTiO 3 is as dielectric layers in multilayer ceramic manufacturing. The multilayer ceramic capacitor industry is continuing intensive efforts to reduce component size, to decrease dielectric layer thickness and precious-metal use, and to improve component reliability w1x. Phosphate ester is an excellent dispersing agent and has been successfully used in BaTiO 3 slips w2x. However, previous works w3–5x have shown that phosphate ester leaves a certain amount of residual phosphorus which remains adsorbed onto the surfaces of the BaTiO 3 particles, i.e., the material became phosphorus doped. Due to the adsorption characteristics of the phosphate ester on the BaTiO 3 ) Corresponding author. Tel.: q91-871-1800; fax: q91-8700550.
particles w6x, the residual phosphorus forms an homogeneous adsorbed layer which covers the BaTiO 3 particles as a ‘surface doping’. This doped material showed better porosity removal in the first sintering stage and higher shrinkage rate than undoped BaTiO 3 . Therefore, lower sintering temperatures were needed for phosphorus doped BaTiO 3 ceramics. BaTiO 3 was the only phase detected and homogeneous fine grained microstructures were observed. Consequently, the material showed excellent dielectric properties Ždielectric constant around 4000 at RT and 1 kHz and dielectric losses below 1%.. Grain growth control in these samples may be understood in terms of solute drag mechanism, yet the marked change of the porosity removal in the first sintering stage seems to points out to more complex mechanisms. In fact, the results presented in this paper show that residual phosphorous located at the surface of the particles react with BaTiO 3 during sintering to form BaO-rich compounds.
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 7 . 0 0 2 2 6 - 7
A.C. Caballero et al.r Materials Letters 35 (1998) 72–77
2. Experimental procedure The BaTiO 3 raw material is a commercial grade ŽELMIC BT100, Rhone ˆ Poulenc. with BarTis 1000. Sr - 0.05 wt% is the main impurity detected, the average particle size is 0.84 m m and the specific surface is 2.7 m2 gy1 . Phosphate ester is composed of 60% C 4 H 11O4 P and 40% C 8 H 19 O4 P and shows a density value of 1.13 grcc. BaTiO 3 was dispersed in an isopropyl alcohol solution of phosphate ester by stirring with a high speed turbine Ž6000 rpm. for 10 min. The mixture was oven dried below 1008C. Assuming that all the phosphorous present in the phosphate ester remains as P2 O5 , then the amount of P2 O5 is 5 wt% with respect to the BaTiO 3 . Differential thermal analysis ŽDTA-TG, Netzsch STA 409. of the dried doped powder was carried out with 58Crm heating rate. Crystalline phases present in samples treated at different temperatures for 2 h
73
were determined by powder X-ray diffraction ŽXRD, Siemmens D5000.. Transmission electron microscopy ŽTEM, JEOL 2000. characterisation was carried out on several powder samples treated at different temperatures. Semiquantitative SEM-EDS analysis ŽZeiss DSM 950. of the samples treated at high temperature allowed to identify the morphology and distribution of different phases.
3. Results and discussion 3.1. Low temperature regime (- 5008C) DTA and TG results are shown in Fig. 1. DTA curve shows one endothermic peak between 100 and 1208C which may be related to the loss of residual water. In this case, the 0.8% weight loss measured between room temperature and 1908C is mostly
Fig. 1. DTA-TG of 5 wt% P2 O5 doped BaTiO 3 powder.
74
A.C. Caballero et al.r Materials Letters 35 (1998) 72–77
residual water. A wide endothermic peak is detected between 1908C and 4208C. Actually this peak seems to be the result of the convolution of three peaks which correlates with the progressive burnt out of the phosphate ester. The 6.8% weight loss Žobserved in the TG curve. associated to this peak is in excellent agreement with the expected value of 6.9%,
assuming that the residual phosphorus remains as P2 O5 . On the other hand, DRX Žnot shown. of powder samples treated at 5008C and slowly cooled only revealed the presence of crystalline BaTiO 3 . However, this result can be understood taking into account the phosphorus location in the material. Since residual phosphorus is adsorbed on the surface
Fig. 2. DRX spectra of 5 wt% P2 O5 doped BaTiO 3 powder treated at different temperatures. BaTiO 3 peaks Žunmarked. are always detected. In order to keep the figure as clear as possible, Ba 3 ŽPO4 . 2 peaks are unmarked in the samples treated above 9008C.
A.C. Caballero et al.r Materials Letters 35 (1998) 72–77
of the BaTiO 3 particles w6x, P2 O5 is formed as a surface phase. As it is well known, DRX is more sensitive to the coherent scattering from the grains interior, consequently, small amounts of phases located at the particles surface could remain undetected even if they are well crystallised.
3.2. Intermediate temperature regime (500–10008C) No significant weight loss was detected above 4208C. At 5208C, the DTA base line begins to decrease which points out to an energy consuming process. This temperature is very close to the reported melting temperature for P2 O5 Ž535–5858C.. Therefore, this could reflect the formation of a liquid on the surface of the BaTiO 3 particles. Also, reaction between P2 O5 and BaO can drive to liquid formation for this range of temperatures according to the BaO–P2 O5 phase diagram w7x. In any case, it seems reasonable to assume that at these temperatures the BaTiO 3 particles are covered by a not well crystallised P2 O5 rich layer which tends to react to form a BaO-rich compound. DTA exothermic peak detected at 6508C indicates the formation of a new compound. DRX ŽFig. 2. of powder samples treated at 7008C for 2 h showed different results depending on the cooling rate. Only crystalline BaTiO 3 was detected in fast cooled samples, however the presence of Ba 2TiP2 O 9 compound Žpattern a 36-1467. was detected in slow cooled samples. Formation of this compound leaves a certain TiO 2 excess:
75
Ba 2TiP2 O 9 actually can act as a control mechanism of the amount and characteristics of a P2 O5-rich glass phase during heating. DTA curve shows another exothermic peak at 8758C related to the formation of a new compound. XRD of powder sample treated at 9008C for 2 h shows clearly the presence of three crystalline compounds: BaTiO 3 , Ba 3 ŽPO4 . 2 Žpattern a 25-0028. and BaTi 4 O 9 Žpattern a 34-0070.. At this temperature the reaction can be expressed in the following way: 4BaTiO 3 q P2 O5 ´ Ba 3 Ž PO4 . 2 q BaTi 4 O 9 . This result is in disagreement with reported data on the system BaO–TiO 2 –P2 O5 w9x in which a 4BaOP 3TiO 2 P P2 O5 compound was reported to be at equilibrium with BaTiO 3 and Ba 3 ŽPO4 . 2 for this range of temperatures. However, our data seem to support the hypothesis proposed by O’Bryan Jr. w10x which suggested that the ternary formula actually represents Ba 3 ŽPO4 . 2 and a TiO 2-rich polytitanate. As in the case of the sample treated at 7008C, both resulting
BaTiO 3 q x P P2 O5´ x P Ba 2TiP2 O 9 q Ba 1y 2 xTi 1yxO 3y4 x . Due to the low solubility of TiO 2 in BaTiO 3 , the presence of a TiO 2-rich polytitanate is also expected w8x, although it was not detected by DRX. Since the above reaction takes place on the surface of the BaTiO 3 particles, TiO 2-rich polytitanate would be formed as a surface layer which remains undetected by DRX as previously discussed for residual P2 O5 . The Ba 2TiP2 O 9 phase melts congruently above 11908C w9x which indicates that its formation must eliminate the presence of a phosphorus-rich glass-like phase. Therefore, the formation of small amounts of
Fig. 3. SEM micrograph of a 5 wt% P2 O5 doped BaTiO 3 sample treated at 12508C. The liquid phase is clearly observed in areas in where Ba 3 ŽPO4 . 2 platelets are formed.
76
A.C. Caballero et al.r Materials Letters 35 (1998) 72–77
compounds were reported to be solid at 9008C indicating again that reaction with BaTiO 3 tends to consume the P2 O5-rich glassy phase. 3.3. High temperature regime () 10008C) DRX of samples treated at 11008C showed the presence of BaTiO 3 , Ba 3 ŽPO4 . 2 , and Ba 6Ti 17 O40 Žpattern a 35-0817.. When the treatment temperature increases, the amount of Ba 3 ŽPO4 . 2 also increases and the TiO 2-rich polytitanate phase changes. It is clear that the formation of Ba 3 ŽPO4 . 2 shifts the system to the TiO 2 rich region and several polytitanates can be detected depending on the treatment temperature. DTA curve shows an endothermic peak centred around 13108C which correlates well with the eutectic temperature of the system BaTiO 3 –TiO 2 w11x. However, for samples treated at 12508C, the presence of liquid phase is already detected by SEM ŽFig. 3. in regions in which the Ba 3 ŽPO4 . 2 platelets are located. DRX analysis of powdered samples treated above the eutectic temperature of the system BaTiO 3 –TiO 2 , showed three crystalline phases: BaTiO 3 , Ba 3 ŽPO4 . 2 and BaTi 2 O5 Žpattern a 34-
0133.. Moreover, SEM-EDS ŽFig. 4. of pellets treated at this temperature revealed also the presence of small crystals of Ba 6Ti 17 O40 . Obviously the presence of different polytitanates indicates that the material is far from equilibrium conditions.
4. Conclusions When phosphate ester doped-BaTiO3 is heated up, a residual phosphorus layer remains covering the BaTiO 3 particles. This layer is not well crystallised and tends to react forming BaO rich compounds at the surface of the particles above the P2 O5 melting temperature. According to the phase diagram of the system BaO–P2 O5 w7x, a P2 O5 rich liquid could be formed, however reaction of this liquid with TiO 2 to form a 2BaO P TiO 2 P P2 O5 solid compound prevents the formation of large amounts of liquid phase. As temperature increases, BaO enrichment of the surface layer progress. Above 9008C, a certain amount of solid Ba 3 ŽPO4 . 2 starts to form. Ba 3 ŽPO4 . 2 formation shifts the system to the TiO 2 rich region and, therefore, large amounts of liquid phase are formed above the eutectic temperature of the system BaTiO 3 –TiO 2 . Regarding the reported sintering behaviour of P 5q-doped BaTiO 3 w4x, this sequence of reactions play a determinant role. The presence of phosphorus causes a Ba2q enrichment of the surfaces of the BaTiO 3 particles when the temperature increases. Consequently, a surface layer is formed which avoids mass transport between adjacent grains. Therefore porosity coalescence and removal are initially favoured. On the other hand, Ba 3 ŽPO4 . 2 formation at higher temperatures breaks the covering layer leaving TiO 2 rich surfaces of the BaTiO 3 particles. As it is well known in TiO 2 rich-BaTiO 3 , liquid phase formation and exaggerated grain growth are expected for temperatures close to the eutectic one.
Acknowledgements Fig. 4. SEM micrograph of the surface of a 5 wt% P2 O5 doped BaTiO 3 sample treated at 13708C. The marked area indicates the presence of Ba 6Ti 17 O40 detected by EDS analysis.
The authors would like to thank to the CICYT ŽMAT94-807. for the financial support.
A.C. Caballero et al.r Materials Letters 35 (1998) 72–77
References w1x J.M. Wilson, Minerals review: Barium titanate, Am. Ceram. Soc. Bull. 74 Ž6. Ž1995. 106–110. w2x K. Mikeska, W.R. Cannon, Dispersant for tape-casting pure barium titanate, in: J.A. Mangels ŽEd.., Adv. in Ceram., vol. 9, Forming of Ceramics, Am. Ceram. Soc. OH, 1984, pp. 164–183. w3x J.F. Fernandez, A.C. Caballero, P. Duran, ´ ´ C. Moure, Improving sintering behaviour of BaTiO 3 by small doping additions, J. Mater. Sci. 31 Ž1996. 975–981. w4x A.C. Caballero, J.F. Fernandez, C. Moure, P. Duran, ´ ´ Effect of residual phosphorus left by phosphate ester on BaTiO 3 ceramics, Mater. Res. Bull. 32 Ž2. Ž1997. 221–229. w5x J.F. Fernandez, P. Duran, ´ ´ C. Moure, Microstructure development of tape casting BaTiO 3 ceramics, Ferroelectrics 127 Ž1992. 65–70.
77
w6x N. Le Bars, D. Tinet, A.M. Fangere, H. van Damme, P. Levitz, Adsorption mechanism of phosphate ester on barium titanate in organic medium. Preliminary results on the structure of the adsorbed layer, J. Phys. III 1 Ž1991. 707–718. w7x R.A. McCauley, F.A. Hummel, Phase relationships in a portion of the system BaO–P2 O5 , Trans. Br. Ceram. Soc. 67 Ž1968. 619–628. w8x R.K. Sharma, N.H. Chan, D.M. Smyth, Solubility of TiO 2 in BaTiO 3 , J. Am. Ceram. Soc. 64 Ž8. Ž1981. 448–451. w9x D.E. Harrison, The system BaO–TiO 2 –P2 O5 : Phase relations, fluorescence, and phosphorous preparation, J. Electrochem. Soc. 107 Ž3. Ž1960. 217–221. w10x H.M. O’Bryan Jr., Identification of surface phases on BaTiO 3 –TiO 2 ceramics, Am. Ceram. Soc. Bull. 66 Ž4. Ž1987. 677–680. w11x P.P. Phule, H.S. Risbud, Low temperature synthesis and processing of electronic materials in the BaO–TiO 2 system, J. Mater. Sci. 25 Ž1990. 1169–1183.