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Effect of grain size and external stress on dielectric temperature characteristics of dysprosium modified barium titanate
MATERIAIS SCIENCE & ENGINEERING ELSEVIER
Materials Science and Engineering B47 (1997) 28-32
B
Effect of grain size and external stress on dielectric temperature characteristics of dysprosium modified barium titanate Yung Park a,*, Se Ahn Song b a Department
of Ceramics, Korea Institute of Science and Technology, b Samswtg Advunced Institute of Technology, P.O.
P.O. Box 131, Cheongrynng, Seoul, South Box 111, Swan, South Korea
Korea
Received 19 September 1996; accepted 30 October 1996
Abstract Various particle sizes of starting barium titanate were used to examine effect of grain sizeand external stresson the dielectric temperature characteristics of dysprosium modified barium titanate. The grain size was somewhat proportional to the particle size of starting barium titanate. The dielectric temperature characteristics of the small grains ( < 4 pm) was less sensitive to the grain size in comparison with those of the large grains (> 4 pm), which was attributable to the internal stress. Moreover it was suggested that stress is one of the origins of diffuse phase transition (DPT). 0 1997 Elsevier Science S.A. Keywords:
Barium titanate; Grain
size; Dysprosium;
Stress
1. Introduction Recently, multilayer capacitors with high capacitance values have been manufactured fsom rare earth metal oxide modified barium titanate [1,2]. Pure barium titanate displays three dielectric anomalies associated with phase transitions at 130°C 0°C and - 90°C (T,, T, and T,, respectively) and these anomalies accompany high dielectric constants around phase transitions [3]. Therefore, the ceramic capacitor formulation can be chemically and physically modified in order to meet the EIAs specification of dielectric temperature characteristics [4]. The dielectric temperature characteristics can originate both from controlling the stress [5-71 and from chemical features [8 - lo]. The chemical features of grain consists of two types: one is the chemically inhomogeneous grain which has been analyzed in the ferroelectric grain core of unreacted barium titanate, in gradient regions with a varying dopant concentration and in paraelectric grain shell of barium titanate severely doped with isovalent or aliovalent cations. The other is the chemically homogeneous grain which was associated with the fact that dopant was evenly distributed in grain. Through a transmission electron microscopy (TEM) and energy * Corresponding 0921-5107/97/%17.00 PIISO921-5107(96)02028-4
tanate.
author. 0 1997 Elsevier
dispersive spectroscopy (EDS), Park et al. proved that cerium-modified barium titanate and samariummodified barium titanate shows the chemically inhomogeneous grain with core-shell structure and chemicalIy homogeneous grain, respectively [ll- 131(Y, Park and Y. Kim, unpublished results). Now, although the origin of diffuse phase transition (DPT) is still open to questions, it has been apparent since the TEM study, that DPT relates to the chemical inhomogeneity and internal stress. This formulation of rare metal oxide modified barium titanate has four strong points in comparison with [Ca,Sn]O-modified barium titanate: (i) the high flexure strength, (ii) excellent d.c. bias characteristics, (iii) long term reliability, resulting from grain size refinement; and (iv) a dielectric-constant vs. temperature ratio easily controlled by the amount of rare earth metal oxide [13]. The ratio of rare earth metal oxide to barium titanate was small, but excess titanate dioxide increased the solubility of rare earth metal oxide in barium titanate. Compared with other candidates of rare earth metal oxide-cerium, neodymium, gadolinium and samarium, adding dysprosium to barium titanate improved the d.c. bias characteristics [4]. The object of this study was to examine the effect of grain size and external stress on the dielectric temperature characteristics of dysprosium-modified barium ti-
Science
S.A. All rights
reserved.
Y. Park,
S.A. Song/Materials
Science
and Engineering
B47 (1997)
28-32
29
Table 1 Pre-sintering conditions, milling conditions, average particle size and standard deviation of BaTiO, Type of BaTQ
Pre-sintering time (h)
Pre-sintering temperature (“C)
Milling time of fluid energy mill (h)
Average particle size of BaTiO, (w)
S.D. (pm)
BTA BTB BTC BTD BTE
2 2 2 2 2
1400 1400 1400 1400 1400
4.0 1.5 1.0 0.5 0.2
0.2 1.0 1.5 3.0 5.0
0.13 0.25 0.50 1.50 2.32
Table 2 Experimental compositions, starting BaTiO,, sintering condition, average grain size and standard deviation Specimen
Composition (mol)
Sintering condition (temperature (“C)/holding
A
0.914BTA-O.O21Dy,O, -0.065Ti0, 0.91BTB-O.O21Dy,O, -0.065Ti0, 0.914BTC-O.O21Dy,O, -0.065Ti0, 0.91BTD-O.O21Dy,O, -0.065Ti0, 0.914BTD-0.21Dy,O, -0.065Ti0,
B C D E
Average grain size (w)
SD. of grain size (ml
1300/2
1.80
0.90
1300/2
2.40
1.35
1300/2
3.50
2.10
1300/2
5.84
3.20
1300/2
7.10
4.01
2. Experimental details The high purity ( > 99.6%) barium titanate (HP3, Kyoritsu Company, Japan) powders used in this study had a BaO/TiO, mol ratio of 1.002 and had been synthesized by a conventional solid-state reaction process. The starting barium titanates was pressed and then pre-sintered at 1400°C for 2 h. The pre-sintered barium titanate was milled with a fluid energy mill to prevent contamination. The controlled milling time made it possible to obtain various particle sizes of barium titanates, ranging from 0.2 to 5 pm. The milling conditions, average particle sizes of barium titanates and standard deviations are described in Table 1. The particle size of the BTA, BTB, BTC, BTD and BTE increased in the sample order. The average particle size of those barium titanates was obtained by using a light intensity fluctuation type of particle size analyzer (Autosizer 2C, Malvern Instrument, Southborough, MA). All of the additives used were of reagent-grade purity. The compositions chosen in this study are shown in Table 2. Appropriate mixtures of the additives for the barium titanates were premilled, mixed with barium titanate powders, pressedinto disk form and sintered at 1300°C for 2 h. Sintered ceramics were crushed to an average particle size of 40 pm to achieve an optimal random orientation
time (h))
for X-ray diffraction analysis. Powder X-ray diffractions were conducted using an X-ray diffractometer equipped with a temperature-controlled attachment and with a pressure-controlled attachment. Especially, highpressure X-ray diffraction measurements were made on powder samples contained in a diamond-anvil high pressure cell [14]. Diffraction angles were corrected by a silicon internal standard. More than five diffractions were collected from 20 to 110” (Cu K,, 28) in order to calculate the lattice parameters by using a least-square determination. Foil for analysis by TEM (H9000NA analytical transmission electron microscope, fitted with a Kevex EDS detector, Hitachi, Japan) were prepared by dimple grinding samplesto a thickness of 25 kernand ion-beam thinning to perforation. The transmission electron microscope was operated at 300 kV, and energy dispersive spectroscopy (EDS) microanalysis was performed, using a scanning transmission electron microscope (STEM), at equidistant spots 10 nm in diameter. The relative concentrations of barium, titanium and dysprosium were evaluated from the measured intensities of the BaL,, TiK,, and DyL, lines using a standardless quantitative analysis programs (on a Delta 5 analyzer, Kevex Corporation, Foster City, CA). The temperature dependence of the dielectric constant was measured over a temperature ranging from
Y. Pntk,
S.A. Song/Marerinls
Science
and Engineering
B47 (1997)
G,, z ::
03
28-32
E specimen
Scan length (urn)
Fig. 1. (a) TEM image of A ceramic, (b) TEM image of E ceramic and (c) STEM microanalysis (EDS) for Dy/(Ba + Ti); schematic diagram of grainbased on the result of (b).
- 130 to + 150°C using an LCR meter (Model 4294A, Hewlett Packard, Palo Alto, CA) at 1 kHz and 1 V tms. The samples were compressed between two sapphire plates [15]. The upper sapphire plate connected with a phosphor bronze was pressedby a stainless-steel rod. The compressive stresswas generated by tightening the screw between the rod and a stainless-steelcylinder. The block around the sample was made of phosphorbronze. The applied stress was measured by a calibrated strain gauge in a load cell. The error in measurement of stressis within about + 0.1 kbar. The relationship between grain size and starting barium titanates was studied on the as-fired surfaces of the
sintered ceramics, using both optical microscopy and scanning electron microscope (SEM) (Modei JXA-840, JEOL, Tokyo, Japan). The grain size was calculated from the average grain-boundary intercept distance of 100 grains,
3. Results and discussion TEM images of A ceramic and E ceramic in Fig. l(a) and (b) shows that all of the grain was full of ferroelectric domain, which was associatedwith the fact that all grains are of ferroelectric phase. The results of EDS
Y. Pa&,
S.A.
Song /Materials
Science
analysis, shown in Fig. l(c), indicate that dysprosium was uniformly distributed in grain. In contrast with the additives’ modified barium titanate which exhibit the core-shell structured grain, dysprosium modified barium titanate do not reveal the chemical inhomogeneity-core-shell structured grain. So we call dysprosium-modified barium titanate the chemically homogeneous ceramic. As shown in Table 2, the grain size of A-E increases with the sample order and the sintered grain size was nearly proportional to the particle size of starting barium titanate. The temperature dependence of dielectric constant for A, C, D and E ceramics is shown in Fig. 2. E ceramic with the sintered grain size of 7.10 pm shows the typical dielectric temperature characteristics whose shapes are similar to those of pure barium titanate. Compared with the transition temperatures (T,, T, and T2 with the temperature of 130, 0 and - 9O’C) of pure barium titanate, the addition of dysprosium to barium titanate shifts the ferroelectric transition peak (T,) for cubic-tetragonal form to a lower temperature, while temperatures for tetragonal-orthorhombic and or-
lOOO( I-
A ceramic Grain size= 1.80pm
.
0 Kbar
500()-
c 1oooc
*... 0 Kbar - -D.-. 10 Kbar -lOOKbar
5 z z
C ceramic Grain size = 3 50pm
5ooc
E .g ‘5 2 c al ‘5 10000
i
D ceramic Grain sire = 5.84pm
b
5000
0 E ceramic Grain size = 7. IOpm
10000
5000
0
..,. D... 0 Kbar we.-,- 10 Kbar n 100 Kbar
-100
-50
0
Temperature
50
100
(“C)
Fig. 2. Dielectric constant of A, C, D and E ceramics as a function temperature. Grain sizes and external stress are indicated.
of
and Engineering
4.02
B47 (1997)
t
28-32
31
A ceramic
._.-_.10 Kbar -
100 Kbar
'0
Temperature(“C) Fig. 3. Lattice parameters of A and E ceramics as a function temperature. Grain sizes and external stress are indicated.
of
thorhombic-rhombohedral (T, and T,, respectively) to a higher value. As shown in Fig. 2, we find two common characteristics of temperature dependence of dielectric constant at 0 kbar: (i) increase of the sintered grain size caused the ferroelectric transition peak (T,) to a higher temperature, while the transition temperatures for the orthorhombic-tetragonal and the rhombohedral-orthorhombic forms (Tl and T,, respectively) shifted to lower value; (ii) increase of the sintered grain size decreasedthe dielectric base intensity at low temperatures, whereas increased the dielectric peak intensity at higher temperatures. In particular, decrease of the grain size made the shapes of the curves broad and symmetrical. The grain size dependence of dielectric temperature characteristics is in good agreement with that of pure barium titanate with various grain size [6]. The effect of external stress on the temperature dependence of dielectric constant and the transition temperatures (T,, T, and T2) is similar to that of grain size refinement. Fig. 2 shows that the increase of external stress depressed the dielectric peak intensity at higher temperature and decreased the ferroelectric transition temperature (T,), while it increased the dielectric base intensity at lower temperatures and the transition temperatures for the orthorhombic-tetragonal and the rhombohedral-orthorhombic forms (T, and T,, respectively). Moreover, increase of the external stress made the transition peak broaden, which suggestedthat stress is one of the origins of diffuse phase transition (DPT). The dielectric temperature characteristics of small grain of A and C ceramic ( < 4 ltm) are in contrast with
32
Y. Park,
S.A. Song /Materials
Science
those of large grain of D and E (> 4 pm). With increase of external stress, first peak near 130°C for the large grain of D and E ( > 4 pm) was shifted to a lower temperature, whereas transition temperatures of T1 and T2 to a higher value. Finally, at the pressure of 100 kbar. for D ceramic with the grain size of 5.84 pm, the three peaks merged into one peak. On the other hand, the temperature dependence of dielectric constant of C ceramics with the small grain size of 3.50 pm ( < 4 ktm) is lessdependent of external stressthan that of the large grain samples ( > 4 pm). That is to say, the dielectric temperature characteristics of D and E ceramics of large grain size ( > 4 ,um) are sensitive to external stress, whereas those of A and C ceramics with small grain size ( < 4 pm) are insensitive. In effect, it was ascribed to the internal stress, which originated from the small grain size, that the external stresshas little influence on the temperature dependence of dielectric constant. Fig. 3 shows the lattice parameters for A and E as a function of temperature. For large grain of E ceramic (7.10 pm), the transition temperatures for T,, T, and T2 in Fig. 2 were in good agreement with those of X-ray results in Fig. 3. With decrease of grain size and with increase of external stress, the ferroelectric transition peak (TJ was shifted to a lower temperature, the transition temperatures (T, and r,) shifted to a higher temperature, and tetragonality (c/n ratio) decreased. The lattice parameters vs. temperature of A ceramic ( < 4 pm) were less sensitive to external stress than those of large grain ( > 4 pm). On the basis of external stress and grain size, the effect of external stresson the temperature dependence of dielectric constant and the transition temperatures
and Etzgineering
B47 (1997)
28-32
T, and r,) is similar to that of decreaseof grain size. External stresshas influence on the dielectric temperature characteristics of the large grain (> 4 pm) more than those of the small grain ( < 4 ltm), which was attributable to the internal stress. Actually, the internal stress, induced by decrease of grain size, reduces the effect of external stress on the dielectric temperature characteristics. (T,,
References VI Y. Park, I.Y. Song and KS. Ahn,
Korean
Patent
90-13543,
1990. PI Y. Park and C.I. Kim, Korean Patent 91-l 1699, 1991. Ceramics, [31 B. Zaffe, W.R. Cook and H. Jaffe, Piezoelectric Academic Press, London, 1971, pp. 53-57. SAIT, 1990. [41 Y. Park, Report EBA9009053, [51 G. Alrt, D. Henning and G. de With, J. Appl. Plzyys., 58 (1985) 1619. [61 A. Yamaji, Y. Enomoto, K. Kinoshita and T. Murakami, J. Am. Ceram. Sot., 64 (1977) 97. 171 T.M. Markulich, J. Magder, MS. Vukasovich and R.J. Lockhart, J. Am. Ceram. Sot., 49 (1966) 295. 181T.R. Armstrong and R.C. Buchanan, J. Am. Ceram. Sot., 73 (1990) 1268. PI H.Y. Lu, J.S. Bow and W.H. Deng, J. Am. Ceram. Sot., 73 (1990) 3562. [lOI B.S. Rawal, M. Kahn and W.R. Buessem, in L.M. Levinson (ed.), Advances in Ceramics, Vol. 1, American Ceramic Society, Columbus, OH, 1981, pp. 172-188. U11Y. Park and Y. Kim, J. Mater. Res., 10 (1995) 2770. [121C.J. Choi and Y. Park, Ceramic Trans., 8 (1990) 148. 380. N31 Y. Park and S.A. Song, J. Mater. Sci. Mater. Elec., 6(1995) [I41 R.W. Lynch, J. Chem. Phys., 47(1967) 5180. u51 M. Wada, H. Shichi, A. Sawada and Y. Ishibashi, J. Phys. Sot. Jpn., 51 (1982) 3245.
Materials Science and Engineering B47 (1997) 28-32
B
Effect of grain size and external stress on dielectric temperature characteristics of dysprosium modified barium titanate Yung Park a,*, Se Ahn Song b a Department
of Ceramics, Korea Institute of Science and Technology, b Samswtg Advunced Institute of Technology, P.O.
P.O. Box 131, Cheongrynng, Seoul, South Box 111, Swan, South Korea
Korea
Received 19 September 1996; accepted 30 October 1996
Abstract Various particle sizes of starting barium titanate were used to examine effect of grain sizeand external stresson the dielectric temperature characteristics of dysprosium modified barium titanate. The grain size was somewhat proportional to the particle size of starting barium titanate. The dielectric temperature characteristics of the small grains ( < 4 pm) was less sensitive to the grain size in comparison with those of the large grains (> 4 pm), which was attributable to the internal stress. Moreover it was suggested that stress is one of the origins of diffuse phase transition (DPT). 0 1997 Elsevier Science S.A. Keywords:
Barium titanate; Grain
size; Dysprosium;
Stress
1. Introduction Recently, multilayer capacitors with high capacitance values have been manufactured fsom rare earth metal oxide modified barium titanate [1,2]. Pure barium titanate displays three dielectric anomalies associated with phase transitions at 130°C 0°C and - 90°C (T,, T, and T,, respectively) and these anomalies accompany high dielectric constants around phase transitions [3]. Therefore, the ceramic capacitor formulation can be chemically and physically modified in order to meet the EIAs specification of dielectric temperature characteristics [4]. The dielectric temperature characteristics can originate both from controlling the stress [5-71 and from chemical features [8 - lo]. The chemical features of grain consists of two types: one is the chemically inhomogeneous grain which has been analyzed in the ferroelectric grain core of unreacted barium titanate, in gradient regions with a varying dopant concentration and in paraelectric grain shell of barium titanate severely doped with isovalent or aliovalent cations. The other is the chemically homogeneous grain which was associated with the fact that dopant was evenly distributed in grain. Through a transmission electron microscopy (TEM) and energy * Corresponding 0921-5107/97/%17.00 PIISO921-5107(96)02028-4
tanate.
author. 0 1997 Elsevier
dispersive spectroscopy (EDS), Park et al. proved that cerium-modified barium titanate and samariummodified barium titanate shows the chemically inhomogeneous grain with core-shell structure and chemicalIy homogeneous grain, respectively [ll- 131(Y, Park and Y. Kim, unpublished results). Now, although the origin of diffuse phase transition (DPT) is still open to questions, it has been apparent since the TEM study, that DPT relates to the chemical inhomogeneity and internal stress. This formulation of rare metal oxide modified barium titanate has four strong points in comparison with [Ca,Sn]O-modified barium titanate: (i) the high flexure strength, (ii) excellent d.c. bias characteristics, (iii) long term reliability, resulting from grain size refinement; and (iv) a dielectric-constant vs. temperature ratio easily controlled by the amount of rare earth metal oxide [13]. The ratio of rare earth metal oxide to barium titanate was small, but excess titanate dioxide increased the solubility of rare earth metal oxide in barium titanate. Compared with other candidates of rare earth metal oxide-cerium, neodymium, gadolinium and samarium, adding dysprosium to barium titanate improved the d.c. bias characteristics [4]. The object of this study was to examine the effect of grain size and external stress on the dielectric temperature characteristics of dysprosium-modified barium ti-
Science
S.A. All rights
reserved.
Y. Park,
S.A. Song/Materials
Science
and Engineering
B47 (1997)
28-32
29
Table 1 Pre-sintering conditions, milling conditions, average particle size and standard deviation of BaTiO, Type of BaTQ
Pre-sintering time (h)
Pre-sintering temperature (“C)
Milling time of fluid energy mill (h)
Average particle size of BaTiO, (w)
S.D. (pm)
BTA BTB BTC BTD BTE
2 2 2 2 2
1400 1400 1400 1400 1400
4.0 1.5 1.0 0.5 0.2
0.2 1.0 1.5 3.0 5.0
0.13 0.25 0.50 1.50 2.32
Table 2 Experimental compositions, starting BaTiO,, sintering condition, average grain size and standard deviation Specimen
Composition (mol)
Sintering condition (temperature (“C)/holding
A
0.914BTA-O.O21Dy,O, -0.065Ti0, 0.91BTB-O.O21Dy,O, -0.065Ti0, 0.914BTC-O.O21Dy,O, -0.065Ti0, 0.91BTD-O.O21Dy,O, -0.065Ti0, 0.914BTD-0.21Dy,O, -0.065Ti0,
B C D E
Average grain size (w)
SD. of grain size (ml
1300/2
1.80
0.90
1300/2
2.40
1.35
1300/2
3.50
2.10
1300/2
5.84
3.20
1300/2
7.10
4.01
2. Experimental details The high purity ( > 99.6%) barium titanate (HP3, Kyoritsu Company, Japan) powders used in this study had a BaO/TiO, mol ratio of 1.002 and had been synthesized by a conventional solid-state reaction process. The starting barium titanates was pressed and then pre-sintered at 1400°C for 2 h. The pre-sintered barium titanate was milled with a fluid energy mill to prevent contamination. The controlled milling time made it possible to obtain various particle sizes of barium titanates, ranging from 0.2 to 5 pm. The milling conditions, average particle sizes of barium titanates and standard deviations are described in Table 1. The particle size of the BTA, BTB, BTC, BTD and BTE increased in the sample order. The average particle size of those barium titanates was obtained by using a light intensity fluctuation type of particle size analyzer (Autosizer 2C, Malvern Instrument, Southborough, MA). All of the additives used were of reagent-grade purity. The compositions chosen in this study are shown in Table 2. Appropriate mixtures of the additives for the barium titanates were premilled, mixed with barium titanate powders, pressedinto disk form and sintered at 1300°C for 2 h. Sintered ceramics were crushed to an average particle size of 40 pm to achieve an optimal random orientation
time (h))
for X-ray diffraction analysis. Powder X-ray diffractions were conducted using an X-ray diffractometer equipped with a temperature-controlled attachment and with a pressure-controlled attachment. Especially, highpressure X-ray diffraction measurements were made on powder samples contained in a diamond-anvil high pressure cell [14]. Diffraction angles were corrected by a silicon internal standard. More than five diffractions were collected from 20 to 110” (Cu K,, 28) in order to calculate the lattice parameters by using a least-square determination. Foil for analysis by TEM (H9000NA analytical transmission electron microscope, fitted with a Kevex EDS detector, Hitachi, Japan) were prepared by dimple grinding samplesto a thickness of 25 kernand ion-beam thinning to perforation. The transmission electron microscope was operated at 300 kV, and energy dispersive spectroscopy (EDS) microanalysis was performed, using a scanning transmission electron microscope (STEM), at equidistant spots 10 nm in diameter. The relative concentrations of barium, titanium and dysprosium were evaluated from the measured intensities of the BaL,, TiK,, and DyL, lines using a standardless quantitative analysis programs (on a Delta 5 analyzer, Kevex Corporation, Foster City, CA). The temperature dependence of the dielectric constant was measured over a temperature ranging from
Y. Pntk,
S.A. Song/Marerinls
Science
and Engineering
B47 (1997)
G,, z ::
03
28-32
E specimen
Scan length (urn)
Fig. 1. (a) TEM image of A ceramic, (b) TEM image of E ceramic and (c) STEM microanalysis (EDS) for Dy/(Ba + Ti); schematic diagram of grainbased on the result of (b).
- 130 to + 150°C using an LCR meter (Model 4294A, Hewlett Packard, Palo Alto, CA) at 1 kHz and 1 V tms. The samples were compressed between two sapphire plates [15]. The upper sapphire plate connected with a phosphor bronze was pressedby a stainless-steel rod. The compressive stresswas generated by tightening the screw between the rod and a stainless-steelcylinder. The block around the sample was made of phosphorbronze. The applied stress was measured by a calibrated strain gauge in a load cell. The error in measurement of stressis within about + 0.1 kbar. The relationship between grain size and starting barium titanates was studied on the as-fired surfaces of the
sintered ceramics, using both optical microscopy and scanning electron microscope (SEM) (Modei JXA-840, JEOL, Tokyo, Japan). The grain size was calculated from the average grain-boundary intercept distance of 100 grains,
3. Results and discussion TEM images of A ceramic and E ceramic in Fig. l(a) and (b) shows that all of the grain was full of ferroelectric domain, which was associatedwith the fact that all grains are of ferroelectric phase. The results of EDS
Y. Pa&,
S.A.
Song /Materials
Science
analysis, shown in Fig. l(c), indicate that dysprosium was uniformly distributed in grain. In contrast with the additives’ modified barium titanate which exhibit the core-shell structured grain, dysprosium modified barium titanate do not reveal the chemical inhomogeneity-core-shell structured grain. So we call dysprosium-modified barium titanate the chemically homogeneous ceramic. As shown in Table 2, the grain size of A-E increases with the sample order and the sintered grain size was nearly proportional to the particle size of starting barium titanate. The temperature dependence of dielectric constant for A, C, D and E ceramics is shown in Fig. 2. E ceramic with the sintered grain size of 7.10 pm shows the typical dielectric temperature characteristics whose shapes are similar to those of pure barium titanate. Compared with the transition temperatures (T,, T, and T2 with the temperature of 130, 0 and - 9O’C) of pure barium titanate, the addition of dysprosium to barium titanate shifts the ferroelectric transition peak (T,) for cubic-tetragonal form to a lower temperature, while temperatures for tetragonal-orthorhombic and or-
lOOO( I-
A ceramic Grain size= 1.80pm
.
0 Kbar
500()-
c 1oooc
*... 0 Kbar - -D.-. 10 Kbar -lOOKbar
5 z z
C ceramic Grain size = 3 50pm
5ooc
E .g ‘5 2 c al ‘5 10000
i
D ceramic Grain sire = 5.84pm
b
5000
0 E ceramic Grain size = 7. IOpm
10000
5000
0
..,. D... 0 Kbar we.-,- 10 Kbar n 100 Kbar
-100
-50
0
Temperature
50
100
(“C)
Fig. 2. Dielectric constant of A, C, D and E ceramics as a function temperature. Grain sizes and external stress are indicated.
of
and Engineering
4.02
B47 (1997)
t
28-32
31
A ceramic
._.-_.10 Kbar -
100 Kbar
'0
Temperature(“C) Fig. 3. Lattice parameters of A and E ceramics as a function temperature. Grain sizes and external stress are indicated.
of
thorhombic-rhombohedral (T, and T,, respectively) to a higher value. As shown in Fig. 2, we find two common characteristics of temperature dependence of dielectric constant at 0 kbar: (i) increase of the sintered grain size caused the ferroelectric transition peak (T,) to a higher temperature, while the transition temperatures for the orthorhombic-tetragonal and the rhombohedral-orthorhombic forms (Tl and T,, respectively) shifted to lower value; (ii) increase of the sintered grain size decreasedthe dielectric base intensity at low temperatures, whereas increased the dielectric peak intensity at higher temperatures. In particular, decrease of the grain size made the shapes of the curves broad and symmetrical. The grain size dependence of dielectric temperature characteristics is in good agreement with that of pure barium titanate with various grain size [6]. The effect of external stress on the temperature dependence of dielectric constant and the transition temperatures (T,, T, and T2) is similar to that of grain size refinement. Fig. 2 shows that the increase of external stress depressed the dielectric peak intensity at higher temperature and decreased the ferroelectric transition temperature (T,), while it increased the dielectric base intensity at lower temperatures and the transition temperatures for the orthorhombic-tetragonal and the rhombohedral-orthorhombic forms (T, and T,, respectively). Moreover, increase of the external stress made the transition peak broaden, which suggestedthat stress is one of the origins of diffuse phase transition (DPT). The dielectric temperature characteristics of small grain of A and C ceramic ( < 4 ltm) are in contrast with
32
Y. Park,
S.A. Song /Materials
Science
those of large grain of D and E (> 4 pm). With increase of external stress, first peak near 130°C for the large grain of D and E ( > 4 pm) was shifted to a lower temperature, whereas transition temperatures of T1 and T2 to a higher value. Finally, at the pressure of 100 kbar. for D ceramic with the grain size of 5.84 pm, the three peaks merged into one peak. On the other hand, the temperature dependence of dielectric constant of C ceramics with the small grain size of 3.50 pm ( < 4 ktm) is lessdependent of external stressthan that of the large grain samples ( > 4 pm). That is to say, the dielectric temperature characteristics of D and E ceramics of large grain size ( > 4 ,um) are sensitive to external stress, whereas those of A and C ceramics with small grain size ( < 4 pm) are insensitive. In effect, it was ascribed to the internal stress, which originated from the small grain size, that the external stresshas little influence on the temperature dependence of dielectric constant. Fig. 3 shows the lattice parameters for A and E as a function of temperature. For large grain of E ceramic (7.10 pm), the transition temperatures for T,, T, and T2 in Fig. 2 were in good agreement with those of X-ray results in Fig. 3. With decrease of grain size and with increase of external stress, the ferroelectric transition peak (TJ was shifted to a lower temperature, the transition temperatures (T, and r,) shifted to a higher temperature, and tetragonality (c/n ratio) decreased. The lattice parameters vs. temperature of A ceramic ( < 4 pm) were less sensitive to external stress than those of large grain ( > 4 pm). On the basis of external stress and grain size, the effect of external stresson the temperature dependence of dielectric constant and the transition temperatures
and Etzgineering
B47 (1997)
28-32
T, and r,) is similar to that of decreaseof grain size. External stresshas influence on the dielectric temperature characteristics of the large grain (> 4 pm) more than those of the small grain ( < 4 ltm), which was attributable to the internal stress. Actually, the internal stress, induced by decrease of grain size, reduces the effect of external stress on the dielectric temperature characteristics. (T,,
References VI Y. Park, I.Y. Song and KS. Ahn,
Korean
Patent
90-13543,
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