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Effect of WO3 doping on dielectric temperature characteristics of lead complex perovskite ferroelectric ceramics
Materials Science and Engineering B83 (2001) 66 – 69 www.elsevier.com/locate/mseb
Effect of WO3 doping on dielectric temperature characteristics of lead complex perovskite ferroelectric ceramics Yong Zhang a,*, Zhilun Gui b, Longtu Li b, Jiemo Tian a a
Beijing Fine Ceramics Laboratory, State Key Laboratory of New Ceramics and Fine Processing, Institute of Nuclear Energy Technology, Tsinghua Uni6ersity, Beijing 100084, People’s Republic of China b State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua Uni6ersity, Beijing 100084, People’s Republic of China Received 3 January 2000; accepted 28 November 2000
Abstract The effect of WO3 doping on the dielectric temperature characteristics and the microstructure of a ternary ceramic system were investigated. Both the increase of WO3 doping level and the increase of sintering temperature led to a decrease in dielectric constant and the flatting of dielectric curves. The presence of a liquid phase and the pyrochlore phase cause this decrease. Also, microstructural and elemental analyses of the ceramics revealed the structure within one grain consists of three parts: the core, the shell and in-between one. This inhomogeneous composition distribution contributes to flat dielectric temperature dependence. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Relaxor ferroelectrics; Dielectric temperature characteristics; Doping; Inhomogeneous distribution
1. Introduction With the development of surface mounting technology for miniaturization of electronic devices, multilayer ceramic capacitors (MLCCs) with a high capacitance per unit volume have become more important today. MLCCs with temperature-stable characteristics such as satisfying X7R specification in the Electronic Industries Association (EIA) standard have been required extensively. The X7R specification requires the change of capacitance should be less than 15% over the temperature range from − 55 to + 125°C. Therefore, the ceramic materials used for MLCC must have a high dielectric constant with small temperature dependence. A tremendous amount of research has been devoted to developing this kind of dielectric ceramics [1 – 3]. Conventionally, barium titanate-based ceramics, which were chemically and physically modified with several oxides have been used as X7R-type MLCCs [4]. However, the sintering temperature was very high for * Corresponding author. Tel.: + 86-10-62770238-419. E-mail address: zhangyong – [email protected] (Y. Zhang).
such ceramics, an expensive noble metal such as Pd has to be used for internal electrodes. The inner electrodes account for approximately 60% of the total cost of such MLCCs. Although base metal systems such as Ni and Cu have been proposed as replacement recently [5], MLCCs with these base metal system electrodes have to be fired in a reducing environment to suppress electrode oxidation. Moreover, with the adopting of thinner dielectric layer, the barium titanate-based dielectrics face two serious problems because the dc and ac electric fields applied to the dielectric layer become large. One is that capacitance decrease with increasing dc bias field because the dielectric constant strongly depends on dc bias. Another is the dissipation factor increases with increasing intensity of ac oscillation signals. Lead-based dielectrics called relaxor have been attracting attention because of their relatively low sintering temperatures, excellent dc bias characteristic and large dielectric constant. Since conventional relaxors have only one dielectric maximum unlike barium titanate display three dielectric anomalies, it is difficult to obtain a high dielectric constant over a wide temperature range. Therefore, many approaches were taken to meet the X7R specification. A temperature stable
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 8 0 0 - X
Y. Zhang et al. / Materials Science and Engineering B83 (2001) 66–69
dielectric material was obtained using the mixed-sintering method [6] from composite ceramics with two phases in which one has the higher Curie temperature and the other has the lower Curie temperature. The difficulty of this method is the sintering temperature must be suitable so that two phases can coexist. Recently, a new device called composite multilayer ceramic capacitor using multiple, co-fired dielectrics separated by electrodes was developed. This new approach offers manufactures the ability to design any device response by adding dielectric components [7], but the manufacturing process is complicated. In the present paper we developed a solid solution system of relaxor as dielectric material.
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of excess WO3 was 0, 1, 2, 3, 4, and 8 mol.%, respectively. The calcination condition was 8000C for 2h. An additional ball-milling step was to ensure a fine particle size before sintering. The dried powders were then pressed as disks (10 mm in diameter and 1 mm thick) under the application of 100 MPa pressure. The pressed pellets were sintered at various temperatures from 1050 to 1150°C for 1 h. Samples were buried in the same composition powder and fired in a covered alumina crucible. The sintered disks were polished the faces with SiC grinding paper. The dimensions were measured before electroding with silver paste. Finally, the silvercoated specimens were heat-treated at 600°C.
2.2. Characterization of the sample 2. Experimental procedure
2.1. Preparation of sample The composition used in this experiment was as follows: 0.25PMW(Pb(Mg1/2W1/2)O3 – 0.35PT(PbTiO3)– 0.40PNN(Pb(Ni1/3Nb2/3)O3. The samples were prepared by the two-stage calcination method. The first stage was the formation of columbite NiNb2O6 (NN), stoichiometric amounts of analytical reagent (AR) grade Ni(CH3COO)2, Nb2O5 were first ball-milled in a resin pot with silica media for 24 h, The dried powder was then calcined at 1000°C for 6 h in a closed alumina crucible. X-ray analysis at this stage indicated a singlephase columbite. The second stage was the formation of wolframite MgWO4 (MW) starting from reagent grade MgCO3Mg(OH)2 · 6H2O and WO3. The wolframite was synthesized at 1000°C for 2 h by using the same procedure as the columbite. Thirdly, Pb3O4, TiO2 and excess WO3 were added to a stoichiometric mixture of the calcined precursors, NN and MW, and then ball-milled, dried and calcined as before. The amount
Fig. 1. Temperature dependence of dielectric constant at three frequencies for WO3-doped specimens fired at 1000°C for 1 h. Doping levels are indicated.
Phase analysis was made by X-ray diffraction using a Rigaku D/max-RA type X-ray diffractometer with CuKa radiation. The microstructure was observed by JEOL JSM 6301F scanning electron microscope (SEM). Compositional distribution or gradient was observed by Energy Dispersive X-ray Spectrometer (EDS) and backward electron image (BEI) of secondary electron microscopy. Dielectric measurements were on an automated system, whereby an impedance analyzer (model HP4192A) and an automatic temperature chamber (Delta 2300) were interfaced with a desk-top computer. The temperature dependence of dielectric constant and dissipation factor was measured over a temperature ranging from − 60 to + 130°C at three frequencies and 1 Vrms.
3. Results and discussion
3.1. Effect of WO3 doping le6els A series of dielectric constant temperature curves measured at three frequencies for the composition ceramics with various WO3 doping levels, sintered at 1000°C for 1 h, were shown in Fig. 1. Two important features can be noted from these curves. One is that the dielectric constant decreases and the dielectric temperature curve become flatter with increasing WO3 doping level. In particular, WO3 doping at 8 mol.% causes the dielectric constant maximum to fall significantly. The other is the dielectric constant was slightly increased when the doping level was 1 mol.%. The reason for this is that the increasing W6 + might induce the increase of ordering degree of B-site, thereby resulting in the increase of dielectric constant [8]. The temperature of dielectric maximum increases from 18 to 38°C with the doping level increasing from 1 to 2 mol.%. However, the temperature of dielectric maximum is almost constant as the doping level exceeds 2 mol.%.
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Y. Zhang et al. / Materials Science and Engineering B83 (2001) 66–69
from stoichiometric to PbO-deficient. Thus, the molar ratio of all elements at A-sites in the perovskite structure to those at B-sites falls below 1.00, and the pyrochlore phase is formed.
3.2. Effect of sintering temperatures
Fig. 2. XRD patterns of WO3-doped specimens fired at 1000°C for 1 h. Marked peaks belong to pyrochlore phase.
Fig. 3. Dielectric constant versus temperature at various sintering temperatures: (a) 2 mol.% doped sample; and (b) 4 mol.% doped sample.
As described above, the WO3 doping caused the dielectric maximum fall sharply. This reduction could be attributed to the formation of a liquid phase, consisting of the doped WO3 and PbO, with a lower dielectric constant. The reduction also results from presence of a pyrochlore phase. Fig. 2 shows X-ray diffraction patterns for the samples with various doping levels also fired at 1000°C for 1 h. It can be seen that doping of more than 1 mol.% WO3 initiates the formation of pyrochlore phase. Since PbO is consumed to react with excess WO3, the matrix composition changes
The dielectric constant temperature curves as a function of sintering temperature for samples with different doping levels are illustrated in Fig. 3. As sintering temperature increases, the temperature of dielectric constant maximum can be seen to be increased, and the dielectric constant can be seen to be decreased. It is well known that PbO segregates easily from a matrix into grain boundaries and forms an amorphous phase with some oxides. In this experiment, excess WO3 was added in order to obtain flat temperature dependence of the dielectric constant. The reaction between PbO and WO3 can form a liquid phase Pb2WO5 during the sintering process. This may lead to the occurrence of the pyrochlore phase and compositional fluctuation of the matrix which induces the change of the temperature of dielectric maximum. The decrease in perovskite formation ratio, which represent the relative amount of the pyrochlore and perovskite phase, and the increase in liquid phase are responsible for the reduction of dielectric constant. In the case of the samples doped with 4 mol.%WO3, examination of dielectric properties showed a high dielectric constant of 5161 at room temperature, and flat temperature characteristics of capacitance satisfying X7R specifications in the EIA standard. The flat dielectric curve is attributed to core-shell compositional distribution confirmed by SEM analysis.
3.2.1. Microstructural and elemental analyses Both BEI observation and EDS analysis results were performed using a specimen which was prepared by polishing and coating with sputtered gold. From Fig. 4, it can be seen that almost all of grains exhibit clear core-shell structure. In BEI of secondary electron microscopy, the dark and bright contrast represents different compositional distribution. Three parts including the core, the shell and the intermediate one revealed by the BEI correspond to compositional gradient in the EDS line analysis. The concentration of W became highest in the shell, but that of Nb, Ti and Pb became highest in the intermediate part. And the intensities of W, Nb, Ti, Pb remain moderate in the core. The compositional variation in the three parts results in different Curie temperature distribution. Therefore, the overlapped dielectric temperature dependence shows flat dielectric properties. As shown in Fig. 3, at lower sintering temperature this solid-solution system displays relaxor ferroelectric behavior characterized by one dielectric maximum
Y. Zhang et al. / Materials Science and Engineering B83 (2001) 66–69
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mechanism of the core-shell structure, it is believed that after saturation of the liquid phase melt with main composition, a solution-reprecipitation process starts whereupon smaller particles dissolve and larger ones grow by epitaxially seeding the liquid phase reprecipitation of perovskite from the melt. Large particles are rounded because of the dissolving process and contain the original composition, thereby forming the core. These are embedded by the intermediate part that contains the highest concentration of Nb, Ti and Pb. Finally W-rich shell was reprecipitated. Thus an inhomogeneous microstructure composed of three parts was formed.
4. Conclusions .
Various WO3 doping levels and sintering temperatures were used to examine the dielectric temperature characteristics of PMW-PT-PNN relaxor ferroelectric ceramics. It was found that both the increase of doping level and the increase of sintering temperature result in the reduction of the dielectric constant. The presence of a liquid phase and the pyrochlore phase contribute to this reduction. Microstructural and elemental analyses of the sample satisfying X7R specification revealed the structure within one grain consists of three parts: the core, the shell and in-between one, indicating an inhomogeneous compositional distribution. Further examination of element fluctuation allows a mechanism to be proposed for the formation of the core-shell structure.
References Fig. 4. BEI micrograph and EDS line analysis across two grains.
within the measuring temperature range. While at higher sintering temperatures, the dielectric constant exhibits flat temperature dependence. In conjunction with the dielectric temperature curves shown in Fig. 1, it can be assumed that both sintering temperature and WO3 doping level play an important role in the required temperature-stable behavior. For the formation
[1] Y. Park, S.A. Song, Mater. Sci. Eng. B47 (1997) 28. [2] F. Uchikoba, K. Sawamura, J. Am. Ceram. Soc. 77 (1994) 1345. [3] A.C. Caballero, J.F. Fernandez, C. Moure, J. Am. Ceram. Soc. 81 (4) (1998) 939. [4] H. Kishi, Y. Okino, M. Honda, et al., Jpn. J. Appl. Phys. 36 (9B) (1997) 5954. [5] T. Nomura, N. Kawano, J. Yamamatsu, et al., Jpn. J. Appl. Phys. 34 (9B) (1995) 5389. [6] L.J. Ruan, L.T. Li, Z.L. Gui, J. Mater. Sci. 16 (1997) 1020. [7] B. Robert, Ceramic Industry, December 29, 1997. [8] N. Setter, L.E. Cross, J. Appl. Phys. 51 (8) (1980) 4356.
Effect of WO3 doping on dielectric temperature characteristics of lead complex perovskite ferroelectric ceramics Yong Zhang a,*, Zhilun Gui b, Longtu Li b, Jiemo Tian a a
Beijing Fine Ceramics Laboratory, State Key Laboratory of New Ceramics and Fine Processing, Institute of Nuclear Energy Technology, Tsinghua Uni6ersity, Beijing 100084, People’s Republic of China b State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua Uni6ersity, Beijing 100084, People’s Republic of China Received 3 January 2000; accepted 28 November 2000
Abstract The effect of WO3 doping on the dielectric temperature characteristics and the microstructure of a ternary ceramic system were investigated. Both the increase of WO3 doping level and the increase of sintering temperature led to a decrease in dielectric constant and the flatting of dielectric curves. The presence of a liquid phase and the pyrochlore phase cause this decrease. Also, microstructural and elemental analyses of the ceramics revealed the structure within one grain consists of three parts: the core, the shell and in-between one. This inhomogeneous composition distribution contributes to flat dielectric temperature dependence. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Relaxor ferroelectrics; Dielectric temperature characteristics; Doping; Inhomogeneous distribution
1. Introduction With the development of surface mounting technology for miniaturization of electronic devices, multilayer ceramic capacitors (MLCCs) with a high capacitance per unit volume have become more important today. MLCCs with temperature-stable characteristics such as satisfying X7R specification in the Electronic Industries Association (EIA) standard have been required extensively. The X7R specification requires the change of capacitance should be less than 15% over the temperature range from − 55 to + 125°C. Therefore, the ceramic materials used for MLCC must have a high dielectric constant with small temperature dependence. A tremendous amount of research has been devoted to developing this kind of dielectric ceramics [1 – 3]. Conventionally, barium titanate-based ceramics, which were chemically and physically modified with several oxides have been used as X7R-type MLCCs [4]. However, the sintering temperature was very high for * Corresponding author. Tel.: + 86-10-62770238-419. E-mail address: zhangyong – [email protected] (Y. Zhang).
such ceramics, an expensive noble metal such as Pd has to be used for internal electrodes. The inner electrodes account for approximately 60% of the total cost of such MLCCs. Although base metal systems such as Ni and Cu have been proposed as replacement recently [5], MLCCs with these base metal system electrodes have to be fired in a reducing environment to suppress electrode oxidation. Moreover, with the adopting of thinner dielectric layer, the barium titanate-based dielectrics face two serious problems because the dc and ac electric fields applied to the dielectric layer become large. One is that capacitance decrease with increasing dc bias field because the dielectric constant strongly depends on dc bias. Another is the dissipation factor increases with increasing intensity of ac oscillation signals. Lead-based dielectrics called relaxor have been attracting attention because of their relatively low sintering temperatures, excellent dc bias characteristic and large dielectric constant. Since conventional relaxors have only one dielectric maximum unlike barium titanate display three dielectric anomalies, it is difficult to obtain a high dielectric constant over a wide temperature range. Therefore, many approaches were taken to meet the X7R specification. A temperature stable
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 8 0 0 - X
Y. Zhang et al. / Materials Science and Engineering B83 (2001) 66–69
dielectric material was obtained using the mixed-sintering method [6] from composite ceramics with two phases in which one has the higher Curie temperature and the other has the lower Curie temperature. The difficulty of this method is the sintering temperature must be suitable so that two phases can coexist. Recently, a new device called composite multilayer ceramic capacitor using multiple, co-fired dielectrics separated by electrodes was developed. This new approach offers manufactures the ability to design any device response by adding dielectric components [7], but the manufacturing process is complicated. In the present paper we developed a solid solution system of relaxor as dielectric material.
67
of excess WO3 was 0, 1, 2, 3, 4, and 8 mol.%, respectively. The calcination condition was 8000C for 2h. An additional ball-milling step was to ensure a fine particle size before sintering. The dried powders were then pressed as disks (10 mm in diameter and 1 mm thick) under the application of 100 MPa pressure. The pressed pellets were sintered at various temperatures from 1050 to 1150°C for 1 h. Samples were buried in the same composition powder and fired in a covered alumina crucible. The sintered disks were polished the faces with SiC grinding paper. The dimensions were measured before electroding with silver paste. Finally, the silvercoated specimens were heat-treated at 600°C.
2.2. Characterization of the sample 2. Experimental procedure
2.1. Preparation of sample The composition used in this experiment was as follows: 0.25PMW(Pb(Mg1/2W1/2)O3 – 0.35PT(PbTiO3)– 0.40PNN(Pb(Ni1/3Nb2/3)O3. The samples were prepared by the two-stage calcination method. The first stage was the formation of columbite NiNb2O6 (NN), stoichiometric amounts of analytical reagent (AR) grade Ni(CH3COO)2, Nb2O5 were first ball-milled in a resin pot with silica media for 24 h, The dried powder was then calcined at 1000°C for 6 h in a closed alumina crucible. X-ray analysis at this stage indicated a singlephase columbite. The second stage was the formation of wolframite MgWO4 (MW) starting from reagent grade MgCO3Mg(OH)2 · 6H2O and WO3. The wolframite was synthesized at 1000°C for 2 h by using the same procedure as the columbite. Thirdly, Pb3O4, TiO2 and excess WO3 were added to a stoichiometric mixture of the calcined precursors, NN and MW, and then ball-milled, dried and calcined as before. The amount
Fig. 1. Temperature dependence of dielectric constant at three frequencies for WO3-doped specimens fired at 1000°C for 1 h. Doping levels are indicated.
Phase analysis was made by X-ray diffraction using a Rigaku D/max-RA type X-ray diffractometer with CuKa radiation. The microstructure was observed by JEOL JSM 6301F scanning electron microscope (SEM). Compositional distribution or gradient was observed by Energy Dispersive X-ray Spectrometer (EDS) and backward electron image (BEI) of secondary electron microscopy. Dielectric measurements were on an automated system, whereby an impedance analyzer (model HP4192A) and an automatic temperature chamber (Delta 2300) were interfaced with a desk-top computer. The temperature dependence of dielectric constant and dissipation factor was measured over a temperature ranging from − 60 to + 130°C at three frequencies and 1 Vrms.
3. Results and discussion
3.1. Effect of WO3 doping le6els A series of dielectric constant temperature curves measured at three frequencies for the composition ceramics with various WO3 doping levels, sintered at 1000°C for 1 h, were shown in Fig. 1. Two important features can be noted from these curves. One is that the dielectric constant decreases and the dielectric temperature curve become flatter with increasing WO3 doping level. In particular, WO3 doping at 8 mol.% causes the dielectric constant maximum to fall significantly. The other is the dielectric constant was slightly increased when the doping level was 1 mol.%. The reason for this is that the increasing W6 + might induce the increase of ordering degree of B-site, thereby resulting in the increase of dielectric constant [8]. The temperature of dielectric maximum increases from 18 to 38°C with the doping level increasing from 1 to 2 mol.%. However, the temperature of dielectric maximum is almost constant as the doping level exceeds 2 mol.%.
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Y. Zhang et al. / Materials Science and Engineering B83 (2001) 66–69
from stoichiometric to PbO-deficient. Thus, the molar ratio of all elements at A-sites in the perovskite structure to those at B-sites falls below 1.00, and the pyrochlore phase is formed.
3.2. Effect of sintering temperatures
Fig. 2. XRD patterns of WO3-doped specimens fired at 1000°C for 1 h. Marked peaks belong to pyrochlore phase.
Fig. 3. Dielectric constant versus temperature at various sintering temperatures: (a) 2 mol.% doped sample; and (b) 4 mol.% doped sample.
As described above, the WO3 doping caused the dielectric maximum fall sharply. This reduction could be attributed to the formation of a liquid phase, consisting of the doped WO3 and PbO, with a lower dielectric constant. The reduction also results from presence of a pyrochlore phase. Fig. 2 shows X-ray diffraction patterns for the samples with various doping levels also fired at 1000°C for 1 h. It can be seen that doping of more than 1 mol.% WO3 initiates the formation of pyrochlore phase. Since PbO is consumed to react with excess WO3, the matrix composition changes
The dielectric constant temperature curves as a function of sintering temperature for samples with different doping levels are illustrated in Fig. 3. As sintering temperature increases, the temperature of dielectric constant maximum can be seen to be increased, and the dielectric constant can be seen to be decreased. It is well known that PbO segregates easily from a matrix into grain boundaries and forms an amorphous phase with some oxides. In this experiment, excess WO3 was added in order to obtain flat temperature dependence of the dielectric constant. The reaction between PbO and WO3 can form a liquid phase Pb2WO5 during the sintering process. This may lead to the occurrence of the pyrochlore phase and compositional fluctuation of the matrix which induces the change of the temperature of dielectric maximum. The decrease in perovskite formation ratio, which represent the relative amount of the pyrochlore and perovskite phase, and the increase in liquid phase are responsible for the reduction of dielectric constant. In the case of the samples doped with 4 mol.%WO3, examination of dielectric properties showed a high dielectric constant of 5161 at room temperature, and flat temperature characteristics of capacitance satisfying X7R specifications in the EIA standard. The flat dielectric curve is attributed to core-shell compositional distribution confirmed by SEM analysis.
3.2.1. Microstructural and elemental analyses Both BEI observation and EDS analysis results were performed using a specimen which was prepared by polishing and coating with sputtered gold. From Fig. 4, it can be seen that almost all of grains exhibit clear core-shell structure. In BEI of secondary electron microscopy, the dark and bright contrast represents different compositional distribution. Three parts including the core, the shell and the intermediate one revealed by the BEI correspond to compositional gradient in the EDS line analysis. The concentration of W became highest in the shell, but that of Nb, Ti and Pb became highest in the intermediate part. And the intensities of W, Nb, Ti, Pb remain moderate in the core. The compositional variation in the three parts results in different Curie temperature distribution. Therefore, the overlapped dielectric temperature dependence shows flat dielectric properties. As shown in Fig. 3, at lower sintering temperature this solid-solution system displays relaxor ferroelectric behavior characterized by one dielectric maximum
Y. Zhang et al. / Materials Science and Engineering B83 (2001) 66–69
69
mechanism of the core-shell structure, it is believed that after saturation of the liquid phase melt with main composition, a solution-reprecipitation process starts whereupon smaller particles dissolve and larger ones grow by epitaxially seeding the liquid phase reprecipitation of perovskite from the melt. Large particles are rounded because of the dissolving process and contain the original composition, thereby forming the core. These are embedded by the intermediate part that contains the highest concentration of Nb, Ti and Pb. Finally W-rich shell was reprecipitated. Thus an inhomogeneous microstructure composed of three parts was formed.
4. Conclusions .
Various WO3 doping levels and sintering temperatures were used to examine the dielectric temperature characteristics of PMW-PT-PNN relaxor ferroelectric ceramics. It was found that both the increase of doping level and the increase of sintering temperature result in the reduction of the dielectric constant. The presence of a liquid phase and the pyrochlore phase contribute to this reduction. Microstructural and elemental analyses of the sample satisfying X7R specification revealed the structure within one grain consists of three parts: the core, the shell and in-between one, indicating an inhomogeneous compositional distribution. Further examination of element fluctuation allows a mechanism to be proposed for the formation of the core-shell structure.
References Fig. 4. BEI micrograph and EDS line analysis across two grains.
within the measuring temperature range. While at higher sintering temperatures, the dielectric constant exhibits flat temperature dependence. In conjunction with the dielectric temperature curves shown in Fig. 1, it can be assumed that both sintering temperature and WO3 doping level play an important role in the required temperature-stable behavior. For the formation
[1] Y. Park, S.A. Song, Mater. Sci. Eng. B47 (1997) 28. [2] F. Uchikoba, K. Sawamura, J. Am. Ceram. Soc. 77 (1994) 1345. [3] A.C. Caballero, J.F. Fernandez, C. Moure, J. Am. Ceram. Soc. 81 (4) (1998) 939. [4] H. Kishi, Y. Okino, M. Honda, et al., Jpn. J. Appl. Phys. 36 (9B) (1997) 5954. [5] T. Nomura, N. Kawano, J. Yamamatsu, et al., Jpn. J. Appl. Phys. 34 (9B) (1995) 5389. [6] L.J. Ruan, L.T. Li, Z.L. Gui, J. Mater. Sci. 16 (1997) 1020. [7] B. Robert, Ceramic Industry, December 29, 1997. [8] N. Setter, L.E. Cross, J. Appl. Phys. 51 (8) (1980) 4356.