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Piezoelectricity, pyroelectricity and ferroelectricity in glass ceramics based on PbTiO3
344
Journal of Non-Crystalline Solids 84 (1986) 344-351 North-Holland, Amsterdam
PIEZOELECTRICITY, PYROELECTRICITY AND FERROELECTRICITY IN GLASS CERAMICS BASED ON PbTiO3
W U M i a n x u e a n d Z H U Peinan East China Institute of Chemical Technology, Shanghai, PRC
Piezoelectric, pyroelectric and ferroelectric glass ceramics of the PbO-TiO2-AI203-SiO2 system with the addition of other oxides have been studied. A piezoelectric resonance in a freely-suspended sample according to its thickness extension vibration mode is observed. The dielectric, piezoelectric, electromecbanical coupling and other parameters, including d33 = 40 × 10-12C/N, c3r3/c0= 60, g33 = 8 0 ) < 1 0 - 3 m . V / N , k t = 0.225, are measured. The pyroelectric responses, with the characteristics p = (10-22)× 10 - 9 C / / c m 2 K , have good linearities from room temperature to nearly 200°C. The existence of ferroelectricity, which gives the most important precondition of the piezoelectricity and pyroelectricity after poling, is verified through: (1) the hysteresis loops by Sowyer-Tower circuit, (2) the ferroelectric domains by TEM, (3) the perovskite-type of ferroelectric crystalline phase PbTiO3 solid solution by XRD, (4) the abnormality of transition between ferroelectric phase and paraelectric phase, as well as (5) the normalized mode based on an IR absorption band referred to the ferroelectric soft mode of Ti-O l bond stretching vibration. This material is different either from the corresponding ceramics and single crystals or from non°ferroelectricpolar glass ceramics and isotropic glass ceramics containing ferroelectric crystalline phases. The new characters and features have also been discussed.
1. Introduction Glass ceramics, a type of polycrystalline material u n d e r controlled crystallization of glass, were invented as a kind of material whose mechanical properties were superior to those of their parent glasses. Most of the subjects in the glass-ceramic field used to be focused o n the mechanical a n d thermal properties. Gradually, the subjects have been extended to their dielectric, optical and other properties, which have also gained m a n y applications. Till now, however, there is no sufficient investigation of their functional properties or coupling behaviors, such as piezoelectricity, pyroelectricity and ferroelectricity. The studies on glass ceramics based on P b T i O 3 have lasted a long time. As early as 1960, Bergeron [1] a n d Russell [2] investigated respectively the crystallization of glasses based on P b O - T i O 2 - S i O 2 a n d P b O - T i O 2 - P 2 0 3 system by D T A , X R D and EM. K o k u b o et al. [3,4] reported studies on glass-forming regions, dielectric properties and s p o n t a n e o u s distortion in crystal grains of t r a n s p a r e n t glass ceramics based on P b T i O 3. All those glass 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Wu Mianxue, Zhu Peinan / Glass ceramics based on Pb TiOs
345
ceramics mentioned above contained a perovskite-type ferroelectric phase but had no piezoelectricity or pyroelectricity. Borrelli et al. [5], who developed some electro-optic glass ceramics, pointed out the difficulties in researching ferroelectric glass ceramics. The electro-optic properties correlated well with the polarization. Where the optical transparency in the glass ceramics approaches that of ordinary glasses, the ferroelectricity is practically absent. Kokubo [6] believed the piezoelectric effect in a glass ceramic would be extremely weak because the ferroelectric crystalline grains were frozen and cramped by glass matrix in surroundings. Layton et al. [7] reported that ferroelectricity in glass ceramics with small grains failed to be detected by means of piezoelectric and hysteresis loop measurements. On the other hand, some inconvenient methods, such as directional crystallization under a very large thermal gradient, have been used to prepare non-ferroelectric polar glass ceramics including Li2SiO 5, Li2Si206 and Ba2Ge2TiOs systems [8-10]. Nevertheless, their application was limited due to the intricate preparation and low piezoelectric or pyroelectric properties compared to those of the other materials. The purpose of this work was to obtain the piezoelectricity, pyroelectricity and ferroelectricity in glass ceramics based on PbTiO 3 through a convenient method - a conventional heat-treatment process followed by the usual poling technique. We reported the preparation and some piezoelectric and pyroelectric behavior of the material in our previous papers [11,12]. In this paper, we systematically study the ferroelectricity and its appearance after poling, piezoelectricity and pyroelectricity. We concentrate on the comparison between glass ceramics and other materials.
2. Piezoelectricity The observation of piezoelectric resonance was performed based on the thickness extension vibration mode of a freely suspended sample, which was
Fig. 1. Piezoelectric resonance of a sample.
Wu Mianxue, Zhu Peinan / Glass ceramics based on PbTiO~
346
200 l~"0
6o
4o
I
,I
Ig °
II
O0
IJ
2o
1~3
105
[07
Fig. 2. Dielectric frequency spectrum.
subjected to an excitation voltage generated by a pulse signal generator. The result is shown in fig. 1, which illustrates the coupling between electric and mechanical (acoustic) energies. A cOoperative action of the direct and converse piezoelectric effects was accomplished in each oscillation period of the vibration. From the slowly damped oscillation, a high mechanical quality factor may be predicted. Another evidence concerning the piezoelectricity was found in measuring the dielectric frequency spectrum, as is shown in fig. 2. When the operating frequencies were applied to about 3.6 MHz, the value of which is a function of the sample thickness, both c33 and tan 8 showed abnormality, corresponding to the piezoelectric resonance just mentioned. When the applied frequencies are higher than a certain value, (33 is abruptly lowered, thus reflecting the difference between the free state and cramped state of a vibrator, whereas tan 8 appears as a singular point because of a damping during the transformation between electric and mechanical (acoustic) energies. The piezoelectric and other related parameters of a typical sample (C25) are summarized in table 1, in which those of other materials are also listed in
Table 1 Piezoelectric and other related parameters of some materials c3r3/Co tan 8 PbTiO 3 glass ceramic PbTiO 3 ceramic [13] non-ferroelectric polar [9] glass ceramic
kp
k31
kt
k33
d3] d33 (10 -12 C / N )
0.225
0.23
-2.9
42
- 5.4
80.4
0.40
-4.2
39
-3.2
33
59
0.008
0.05
0.046
133
0.011
0.07
0.044
0.090.14
3-6
g31 g33 (10 -3 V - m / N )
Wu Mianxue, Zhu Peinan / Glass ceramics based on PbTiO 3
347
order to make a comparison. Obviously, the ferroelectric glass ceramic has advantages, such as low dielectric constant, high ratio of kt/kp, high value of g33, etc., which may be suitable for some applications like sensors and filters.
3. Pyroelectrieity The pyroelectric responses of some samples recorded as continuous functions of temperature on a X-Y recorder, showed a linear range from room temperature up to near 200°C [11,12]. The pyroelectric coefficients p = (1.0-2.2) × 10 -8 C / c m 2 K within this range kept constant respectively. Table 2 is a summary of the pyroelectric and other parameters of the ferroelectric glass ceramic sample (C15) and other materials. We can see that the low dielectric constant c r and specific heat c' result in the high pyroelectric coefficient p and high figure of merit for infrared detector F~, F v and F D. The sample C15 was used in making infrared detectors on a trial basis. In spite of no blackening and vacuum sealing, the detectivity D* (500 K, 12.5 Hz, 1 Hz) could reach 0.8 × 108 c m - Hz '2 • W - 1 under the size ~1 × 0.03 mm. Although the detector should be improved to the comparable with that of single crystals like LiNbO 3 or TGS, the ferroelectric glass ceramic may have potential prospects for applications because of its convenient preparation and low cost.
4. Ferroelectricity The classical and direct evidence of ferroelectricity in matter is considered as a hysteresis loop. By means of the Sowyer-Tower circuit, we obtained the
Table 2 Pyroelectric and other related parameters of some materials
PbTiO 3 glass ceramic PbTiO 3 crystal [14] PbTiO 3 ceramic PVF 2 polymer [ 1 5 ] Non-ferroelectric polar glass ceramic [8]
tan ~
C'
Fi
Fv
Fo
65
0.008
2.0
0.95
1.4
1.3
2.7
142
0.024
3.2
0.84
0.59
0.45
6
200
3.1
1.80
0.94
2.4
0.10
0.91
T~
p
350
1.9
492 470 150°C softening
%
0.24
11
0.025
0.1
15
0.002
* T,., °C; p, 1 0 - S C / c m 2 K; C', J / c m 3 K; F i = p / C ", 10 - s C - c m / J ; F , , = p / C % , C-cm/J; FD= p/C'
(% tan 8)'2, 10 - s C . c m / J .
0.19
10 - l °
348
Wu Mianxue, Zhu Peinan / Glass ceramics based on PbTiO 3
P
t
E
Fig. 3. Hysteresis loops of a sample (P, 3.6 # C / c m 2 / d i v ;
E, 1.8 K V / m m / d i v . ) .
loops under different operating frequencies. The representative results are shown in fig. 3. The loops show some non-saturation since the glass ceramic has high coercivity both in lattice structure and in microstructure. By high coercivity in lattice structure, we mean the coercivity caused by a high axial ratio c/a. By that in microstructure, we refer to some factors including crystallinity, grain size, position and connection, which affect the difficulty of polarization switching during a poling process. Table 3 summarizes the data collected from experiments on the loops of several materials, through which we can see the E c of the PbTiO 3 based glass ceramic is between those of ceramic and thin film while the Pr value is near that of the BaTiO 3 ceramic, about ½ that of the BaTiO 3 crystal. Ferroelectric domains have long been observed in single crystals and polycrystalline ceramics as strong evidence to confirm their ferroelectricity, but there has been no information in the literature about those in glass ceramics. By using electronic microscopy with replica technique, we observed domain patterns in a glass ceramic, which provide the material a structural
Table 3 Parameters of hysteresis loops in some materials
BAT!0 3 crystal [16] PbTiO 3 crystal [17] PbTiO 3 ceramic [18] PbTiO 3 thin film [19] PbTiO 3 thin film (0.6 #) [201 PbTiO 3 glass ceramic N a N b O 3 glass ceramic (1/z) [51 N a N b O 3 glass ceramic (0.2/~) [5]
Electric field (KV/cm)
Polarization (/~ C / c m 2 )
Ec = 1 E c = 3-7 E = 40 (unsaturated) E c = 57-87 Ec = 250-300 Ec > 70
Ps = P~ = Pr = P~ = Pr = Pr > Pr = Pr =
26' 75 42 2.2-4.8 10 7.2 0.8 0.2
Wu Mianxue, Zhu Peinan / Glass ceramics based on PbTiO~
349
Fig. 4. Ferroelectric domain patterns (EM, 7000x).
basis of ferroelectricity and then piezoelectricity and pyroelectricity after poling. One of these micrographs is shown in fig. 4. There appear several patterns in the domains, including 90 ° and 180 ° domain walls as well as herringbone patterns. The domain patterns of the glass ceramic have their own features such as polydomains in a single grain, reasonably small regions, etc. According to X R D analysis; the dominant crystalline phase of the glass ceramic is a ferroelectric tetragonal phase of perovskite-type solid solution because of the fact that the d values of the diffraction have some displacement by the standards of data of PbTiO a pure crystal. By calculating the d values of the sample C25 we can obtain its lattice parameters with a = 3.90A, c---4.05,~ and c / a = 1.04 whereas~ those of pure PbTiO 3 and a = 3.904, c = 4.125 and c / a = 1.06. The content of rutile, a secondary crystalline phase, is very low. Dielectric anomaly in ferroelectric-paraelectric phase transition, another common character of ferroelectricity, found in the glass ceramics. Some results from dielectric measurement can be presented in fig. 5. It indicates that the sample which possesses a higher dielectric constant in room temperature has a lower peak value and a stronger tendency of diffusion phase transition. The dielectric peak value of PbTiO 3 single crystal reaches about 10000 [21] while that of the glass ceramic is below 1000. The reason seems that the glass ceramic contains a certain amount of A1203 and SiO 2 in a continuous position of the matrix, which results in a diffused and low dielectric peak. The IR absorption spectrum of a typical sample (C25) can be seen in fig. 6. There are two absorption bands, ~'1 = 555 cm-1 and ~2 = 365 cm-1. According to the experiment on the IR spectra of perovskite-type single crystals by Last [22], 1,1 = 590 c m - 1 and ~2 = 405 c m - 1 in PbTiO 3 while ~'1 = 540 c m - l and ~2 = 360 cm i in CaTiO 3. As a result of our data, we may conclude that ul and P2 in fig. 6 are corresponding to the characteristic absorption bands of (Pbl_xCax)TiO 4 crystalline solid solution. Last [22], Perry [23] and other
350
Wu Mianxue, Zhu Peinan / Glass ceramics based on PbTiO 3
Z
4m
2~ ~oo •
0
too
!
2o0
l
500
I
ao0
5O0
Fig. 5. Variation of dielectric constants with temperature in some samples.
a u t h o r s suggested that v I was caused b y a n o r m a l i z e d m o d e b a s e d on stretching v i b r a t i o n of T i - O I b o n d changing its length a n d that k"2 c a m e f r o m a n o t h e r n o r m a l i z e d m o d e owing to its b e n d i n g v i b r a t i o n of c h a n g i n g O ] i - T i - O I b o n d angle. Those m o d e s are shown in fig. 6. In fact, /11 is b e a r i n g o n a ferroelectric soft mode, a n o r m a l i z e d m o d e of long wave o f transverse
Fig. 6. IR absorption spectrum and its normalized modes of a sample.
Wu Mianxue, Zhu Peinan / Glass ceramics based on PbTiO~
351
o p t i c a l b r a n c h , a c t i v a t e d b y i n f r a r e d light. T h e " s o f t e n i n g " of the m o d e will a p p e a r w h e n the f e r r o e l e c t r i c - p a r a e l e c t r i c t r a n s i t i o n occurs.
5. Conclusions T h e research we h a v e d o n e leads us to the f o l l o w i n g c o n c l u s i o n s : (1) T h e glass c e r a m i c s b a s e d o n P b T i O 3 h a v e n o t a b l e piezoelectric a n d p y r o e l e c t r i c effects. T h e p r o p e r t i e s c a n b e u s e d for s o m e a p p l i c a t i o n fields. (2) T h e existence of ferroelectricity i n the glass c e r a m i c s m e n t i o n e d a b o v e has b e e n c o n f i r m e d b y their hysteresis loops, ferroelectric d o m a i n s , dielectric a n o m a l y , X R D a n d I R analyses.
References [1] C.G. Bergeron, Dissertation Abstracts 22 (1961) 1535. [2] C.K. Russell, Dissertation Abstracts 28 (1965) 4600. [3] T. Kokubo et al., Yogyo-KyoKai-Shi 77 (1969) 293. [4] T. Kokubo et al., Bull. Inst. Chem. Res. Kyoto Univ. 54 (1976) 301. [5] N.F. Borrelli et al., J. Non-Cryst. Solids 6 (1971) 197. [6] T. Kokubo, Denshi-zaryo 9 (1969) 68. [7] M.M. Layton et al., J. Am. Ceram, Soc. 58 (1975) 435. [8] A. Halliyal et al., J. Mat. Sci. 16 (1981) 1023. [9] A. Halliyal et al., J. Appl. Phys. 53 (1982) 2871. [10] A. Halliyal et al., J. Mat. Sci. 17 (1982) 295. [11] Zhu Peinan et al., Glass and Enamel 13 (2) 1 (1985). [12] Wu Mianxue et al., Chinese Phys. Lett. 2 (1985) 235. [13] I. Ueda, Japan, J. Appl. Phys. 11 (1972) 450. [14] H.P. Beerman, Infrared Phys. 15(1975) "225. [15] R.J. Phelan et at., Ferroelectrics 7 (1974) 375. [16] B. Jaffe et al., Piezoelectric ceramics (Academic Press, New York, 1971) p. 78. [17] V.G. Gavrilyachenko et al., Soy. Phys. Sol. St. 12 (1970) 1023. [18] Xu Yuhuan et al., Physics 11 (1982) 29. [19] H. Hamada et al., Oyo-Buturi 48 (1980) 783. [20] G. Okuyama et al., Denshi-zaryo 21 (1982) 67. [21] V.G. Bhide et al., Physica 28 (1962) 871. [22] J.T. Last, Phys. Rev. 105 (1957) 1740. [23] C.H. Perry et al., Phys. Rev. 138 (1965) A1537.
Journal of Non-Crystalline Solids 84 (1986) 344-351 North-Holland, Amsterdam
PIEZOELECTRICITY, PYROELECTRICITY AND FERROELECTRICITY IN GLASS CERAMICS BASED ON PbTiO3
W U M i a n x u e a n d Z H U Peinan East China Institute of Chemical Technology, Shanghai, PRC
Piezoelectric, pyroelectric and ferroelectric glass ceramics of the PbO-TiO2-AI203-SiO2 system with the addition of other oxides have been studied. A piezoelectric resonance in a freely-suspended sample according to its thickness extension vibration mode is observed. The dielectric, piezoelectric, electromecbanical coupling and other parameters, including d33 = 40 × 10-12C/N, c3r3/c0= 60, g33 = 8 0 ) < 1 0 - 3 m . V / N , k t = 0.225, are measured. The pyroelectric responses, with the characteristics p = (10-22)× 10 - 9 C / / c m 2 K , have good linearities from room temperature to nearly 200°C. The existence of ferroelectricity, which gives the most important precondition of the piezoelectricity and pyroelectricity after poling, is verified through: (1) the hysteresis loops by Sowyer-Tower circuit, (2) the ferroelectric domains by TEM, (3) the perovskite-type of ferroelectric crystalline phase PbTiO3 solid solution by XRD, (4) the abnormality of transition between ferroelectric phase and paraelectric phase, as well as (5) the normalized mode based on an IR absorption band referred to the ferroelectric soft mode of Ti-O l bond stretching vibration. This material is different either from the corresponding ceramics and single crystals or from non°ferroelectricpolar glass ceramics and isotropic glass ceramics containing ferroelectric crystalline phases. The new characters and features have also been discussed.
1. Introduction Glass ceramics, a type of polycrystalline material u n d e r controlled crystallization of glass, were invented as a kind of material whose mechanical properties were superior to those of their parent glasses. Most of the subjects in the glass-ceramic field used to be focused o n the mechanical a n d thermal properties. Gradually, the subjects have been extended to their dielectric, optical and other properties, which have also gained m a n y applications. Till now, however, there is no sufficient investigation of their functional properties or coupling behaviors, such as piezoelectricity, pyroelectricity and ferroelectricity. The studies on glass ceramics based on P b T i O 3 have lasted a long time. As early as 1960, Bergeron [1] a n d Russell [2] investigated respectively the crystallization of glasses based on P b O - T i O 2 - S i O 2 a n d P b O - T i O 2 - P 2 0 3 system by D T A , X R D and EM. K o k u b o et al. [3,4] reported studies on glass-forming regions, dielectric properties and s p o n t a n e o u s distortion in crystal grains of t r a n s p a r e n t glass ceramics based on P b T i O 3. All those glass 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Wu Mianxue, Zhu Peinan / Glass ceramics based on Pb TiOs
345
ceramics mentioned above contained a perovskite-type ferroelectric phase but had no piezoelectricity or pyroelectricity. Borrelli et al. [5], who developed some electro-optic glass ceramics, pointed out the difficulties in researching ferroelectric glass ceramics. The electro-optic properties correlated well with the polarization. Where the optical transparency in the glass ceramics approaches that of ordinary glasses, the ferroelectricity is practically absent. Kokubo [6] believed the piezoelectric effect in a glass ceramic would be extremely weak because the ferroelectric crystalline grains were frozen and cramped by glass matrix in surroundings. Layton et al. [7] reported that ferroelectricity in glass ceramics with small grains failed to be detected by means of piezoelectric and hysteresis loop measurements. On the other hand, some inconvenient methods, such as directional crystallization under a very large thermal gradient, have been used to prepare non-ferroelectric polar glass ceramics including Li2SiO 5, Li2Si206 and Ba2Ge2TiOs systems [8-10]. Nevertheless, their application was limited due to the intricate preparation and low piezoelectric or pyroelectric properties compared to those of the other materials. The purpose of this work was to obtain the piezoelectricity, pyroelectricity and ferroelectricity in glass ceramics based on PbTiO 3 through a convenient method - a conventional heat-treatment process followed by the usual poling technique. We reported the preparation and some piezoelectric and pyroelectric behavior of the material in our previous papers [11,12]. In this paper, we systematically study the ferroelectricity and its appearance after poling, piezoelectricity and pyroelectricity. We concentrate on the comparison between glass ceramics and other materials.
2. Piezoelectricity The observation of piezoelectric resonance was performed based on the thickness extension vibration mode of a freely suspended sample, which was
Fig. 1. Piezoelectric resonance of a sample.
Wu Mianxue, Zhu Peinan / Glass ceramics based on PbTiO~
346
200 l~"0
6o
4o
I
,I
Ig °
II
O0
IJ
2o
1~3
105
[07
Fig. 2. Dielectric frequency spectrum.
subjected to an excitation voltage generated by a pulse signal generator. The result is shown in fig. 1, which illustrates the coupling between electric and mechanical (acoustic) energies. A cOoperative action of the direct and converse piezoelectric effects was accomplished in each oscillation period of the vibration. From the slowly damped oscillation, a high mechanical quality factor may be predicted. Another evidence concerning the piezoelectricity was found in measuring the dielectric frequency spectrum, as is shown in fig. 2. When the operating frequencies were applied to about 3.6 MHz, the value of which is a function of the sample thickness, both c33 and tan 8 showed abnormality, corresponding to the piezoelectric resonance just mentioned. When the applied frequencies are higher than a certain value, (33 is abruptly lowered, thus reflecting the difference between the free state and cramped state of a vibrator, whereas tan 8 appears as a singular point because of a damping during the transformation between electric and mechanical (acoustic) energies. The piezoelectric and other related parameters of a typical sample (C25) are summarized in table 1, in which those of other materials are also listed in
Table 1 Piezoelectric and other related parameters of some materials c3r3/Co tan 8 PbTiO 3 glass ceramic PbTiO 3 ceramic [13] non-ferroelectric polar [9] glass ceramic
kp
k31
kt
k33
d3] d33 (10 -12 C / N )
0.225
0.23
-2.9
42
- 5.4
80.4
0.40
-4.2
39
-3.2
33
59
0.008
0.05
0.046
133
0.011
0.07
0.044
0.090.14
3-6
g31 g33 (10 -3 V - m / N )
Wu Mianxue, Zhu Peinan / Glass ceramics based on PbTiO 3
347
order to make a comparison. Obviously, the ferroelectric glass ceramic has advantages, such as low dielectric constant, high ratio of kt/kp, high value of g33, etc., which may be suitable for some applications like sensors and filters.
3. Pyroelectrieity The pyroelectric responses of some samples recorded as continuous functions of temperature on a X-Y recorder, showed a linear range from room temperature up to near 200°C [11,12]. The pyroelectric coefficients p = (1.0-2.2) × 10 -8 C / c m 2 K within this range kept constant respectively. Table 2 is a summary of the pyroelectric and other parameters of the ferroelectric glass ceramic sample (C15) and other materials. We can see that the low dielectric constant c r and specific heat c' result in the high pyroelectric coefficient p and high figure of merit for infrared detector F~, F v and F D. The sample C15 was used in making infrared detectors on a trial basis. In spite of no blackening and vacuum sealing, the detectivity D* (500 K, 12.5 Hz, 1 Hz) could reach 0.8 × 108 c m - Hz '2 • W - 1 under the size ~1 × 0.03 mm. Although the detector should be improved to the comparable with that of single crystals like LiNbO 3 or TGS, the ferroelectric glass ceramic may have potential prospects for applications because of its convenient preparation and low cost.
4. Ferroelectricity The classical and direct evidence of ferroelectricity in matter is considered as a hysteresis loop. By means of the Sowyer-Tower circuit, we obtained the
Table 2 Pyroelectric and other related parameters of some materials
PbTiO 3 glass ceramic PbTiO 3 crystal [14] PbTiO 3 ceramic PVF 2 polymer [ 1 5 ] Non-ferroelectric polar glass ceramic [8]
tan ~
C'
Fi
Fv
Fo
65
0.008
2.0
0.95
1.4
1.3
2.7
142
0.024
3.2
0.84
0.59
0.45
6
200
3.1
1.80
0.94
2.4
0.10
0.91
T~
p
350
1.9
492 470 150°C softening
%
0.24
11
0.025
0.1
15
0.002
* T,., °C; p, 1 0 - S C / c m 2 K; C', J / c m 3 K; F i = p / C ", 10 - s C - c m / J ; F , , = p / C % , C-cm/J; FD= p/C'
(% tan 8)'2, 10 - s C . c m / J .
0.19
10 - l °
348
Wu Mianxue, Zhu Peinan / Glass ceramics based on PbTiO 3
P
t
E
Fig. 3. Hysteresis loops of a sample (P, 3.6 # C / c m 2 / d i v ;
E, 1.8 K V / m m / d i v . ) .
loops under different operating frequencies. The representative results are shown in fig. 3. The loops show some non-saturation since the glass ceramic has high coercivity both in lattice structure and in microstructure. By high coercivity in lattice structure, we mean the coercivity caused by a high axial ratio c/a. By that in microstructure, we refer to some factors including crystallinity, grain size, position and connection, which affect the difficulty of polarization switching during a poling process. Table 3 summarizes the data collected from experiments on the loops of several materials, through which we can see the E c of the PbTiO 3 based glass ceramic is between those of ceramic and thin film while the Pr value is near that of the BaTiO 3 ceramic, about ½ that of the BaTiO 3 crystal. Ferroelectric domains have long been observed in single crystals and polycrystalline ceramics as strong evidence to confirm their ferroelectricity, but there has been no information in the literature about those in glass ceramics. By using electronic microscopy with replica technique, we observed domain patterns in a glass ceramic, which provide the material a structural
Table 3 Parameters of hysteresis loops in some materials
BAT!0 3 crystal [16] PbTiO 3 crystal [17] PbTiO 3 ceramic [18] PbTiO 3 thin film [19] PbTiO 3 thin film (0.6 #) [201 PbTiO 3 glass ceramic N a N b O 3 glass ceramic (1/z) [51 N a N b O 3 glass ceramic (0.2/~) [5]
Electric field (KV/cm)
Polarization (/~ C / c m 2 )
Ec = 1 E c = 3-7 E = 40 (unsaturated) E c = 57-87 Ec = 250-300 Ec > 70
Ps = P~ = Pr = P~ = Pr = Pr > Pr = Pr =
26' 75 42 2.2-4.8 10 7.2 0.8 0.2
Wu Mianxue, Zhu Peinan / Glass ceramics based on PbTiO~
349
Fig. 4. Ferroelectric domain patterns (EM, 7000x).
basis of ferroelectricity and then piezoelectricity and pyroelectricity after poling. One of these micrographs is shown in fig. 4. There appear several patterns in the domains, including 90 ° and 180 ° domain walls as well as herringbone patterns. The domain patterns of the glass ceramic have their own features such as polydomains in a single grain, reasonably small regions, etc. According to X R D analysis; the dominant crystalline phase of the glass ceramic is a ferroelectric tetragonal phase of perovskite-type solid solution because of the fact that the d values of the diffraction have some displacement by the standards of data of PbTiO a pure crystal. By calculating the d values of the sample C25 we can obtain its lattice parameters with a = 3.90A, c---4.05,~ and c / a = 1.04 whereas~ those of pure PbTiO 3 and a = 3.904, c = 4.125 and c / a = 1.06. The content of rutile, a secondary crystalline phase, is very low. Dielectric anomaly in ferroelectric-paraelectric phase transition, another common character of ferroelectricity, found in the glass ceramics. Some results from dielectric measurement can be presented in fig. 5. It indicates that the sample which possesses a higher dielectric constant in room temperature has a lower peak value and a stronger tendency of diffusion phase transition. The dielectric peak value of PbTiO 3 single crystal reaches about 10000 [21] while that of the glass ceramic is below 1000. The reason seems that the glass ceramic contains a certain amount of A1203 and SiO 2 in a continuous position of the matrix, which results in a diffused and low dielectric peak. The IR absorption spectrum of a typical sample (C25) can be seen in fig. 6. There are two absorption bands, ~'1 = 555 cm-1 and ~2 = 365 cm-1. According to the experiment on the IR spectra of perovskite-type single crystals by Last [22], 1,1 = 590 c m - 1 and ~2 = 405 c m - 1 in PbTiO 3 while ~'1 = 540 c m - l and ~2 = 360 cm i in CaTiO 3. As a result of our data, we may conclude that ul and P2 in fig. 6 are corresponding to the characteristic absorption bands of (Pbl_xCax)TiO 4 crystalline solid solution. Last [22], Perry [23] and other
350
Wu Mianxue, Zhu Peinan / Glass ceramics based on PbTiO 3
Z
4m
2~ ~oo •
0
too
!
2o0
l
500
I
ao0
5O0
Fig. 5. Variation of dielectric constants with temperature in some samples.
a u t h o r s suggested that v I was caused b y a n o r m a l i z e d m o d e b a s e d on stretching v i b r a t i o n of T i - O I b o n d changing its length a n d that k"2 c a m e f r o m a n o t h e r n o r m a l i z e d m o d e owing to its b e n d i n g v i b r a t i o n of c h a n g i n g O ] i - T i - O I b o n d angle. Those m o d e s are shown in fig. 6. In fact, /11 is b e a r i n g o n a ferroelectric soft mode, a n o r m a l i z e d m o d e of long wave o f transverse
Fig. 6. IR absorption spectrum and its normalized modes of a sample.
Wu Mianxue, Zhu Peinan / Glass ceramics based on PbTiO~
351
o p t i c a l b r a n c h , a c t i v a t e d b y i n f r a r e d light. T h e " s o f t e n i n g " of the m o d e will a p p e a r w h e n the f e r r o e l e c t r i c - p a r a e l e c t r i c t r a n s i t i o n occurs.
5. Conclusions T h e research we h a v e d o n e leads us to the f o l l o w i n g c o n c l u s i o n s : (1) T h e glass c e r a m i c s b a s e d o n P b T i O 3 h a v e n o t a b l e piezoelectric a n d p y r o e l e c t r i c effects. T h e p r o p e r t i e s c a n b e u s e d for s o m e a p p l i c a t i o n fields. (2) T h e existence of ferroelectricity i n the glass c e r a m i c s m e n t i o n e d a b o v e has b e e n c o n f i r m e d b y their hysteresis loops, ferroelectric d o m a i n s , dielectric a n o m a l y , X R D a n d I R analyses.
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