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sulphoaluminate cement composites
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 135–139 www.elsevier.com/locate/compscitech
Effect of forming pressures on electric properties of piezoelectric ceramic/sulphoaluminate cement composites Huang Shifeng, Ye Zhengmao, Hu Yali, Chang Jun, Lu Lingchao, Cheng Xin
*
School of Materials Science and Engineering, Jinan University, Jinan 250022, China Received 1 August 2005; received in revised form 7 March 2006; accepted 21 March 2006 Available online 24 July 2006
Abstract Cement-based piezoelectric composites with 0-3 connectivity are fabricated from 0.08Pb(Li1/4Nb3/4)O3 Æ 0.47PbTiO3 Æ 0.45PbZrO3(PLN) and sulphoaluminate cement by compressing technique. Piezoelectric and dielectric properties of the composites under different forming pressures are measured and investigated. The results show that the composites prepared by compression technique exhibit excellent piezoelectric and dielectric properties. With increasing of forming pressure, the piezoelectric strain coefficient and dielectric constant increase, but the forming pressure has insignificant influence on electromechanical coupling property of the composites. The dielectric constant decreases sharply with increasing frequency from 40 kHz to 100 kHz, which is mainly due to interfacial polarization in the composite and polarization in the cement matrix. At high frequency, the composites show excellent dielectric frequency stability. The composite prepared at high pressure is favorable to the polarization of the ion at low frequency. 2006 Elsevier Ltd. All rights reserved. Keywords: 0-3 cement-based piezoelectric composites; Forming pressure; Piezoelectric properties; Dielectric properties; Electromechanical coupling properties
1. Introduction With the modern development of civil engineering, the intelligent material and structure is being introduced. In a smart structure, sensors and actuators are essential components for sensing and controlling purposes [1]. Different types of sensors and actuators have been designed and used according to different applications [2,3]. However, in civil engineering, cement-based materials are the most commonly used structural material. It is well known that the hydration of cement is a long process and can last for decades [4]. The change of the water state during hydration will cause shrinkage of concrete in general [5]. Therefore the intelligent materials suitable for application in the other engineering fields may not be applicable in civil engineering due to the differences in the properties between the intelli-
*
Corresponding author. Tel.: +86 531 82767017; fax: +86 531 82767977. E-mail address: [email protected] (X. Cheng).
0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.03.035
gent materials and the concrete, such as acoustic impedance, volume stability, temperature coefficient and interfacial adhesion etc. To meet the requirement of development for smart or intelligent structures in civil engineering, a new kind of function composites, cement-based piezoelectric composite has been developed in recent years [6–8]. Cement-based piezoelectric composite has good compatibility with civil engineering’s main structural material—concrete. It can be prepared in any shape or size with great ease and at lower cost. It not only has sensing function, but also actuating property, which is very suitable for application in civil engineering fields, such as high-rise buildings, long-span bridges and some buildings whose failure would cause disasters, including nuclear waster containment structures, dams, and bridge decks. Therefore, research and development of the cementbased piezoelectric composite play an extremely important role in advancing all kinds of civil engineering structure to be intelligent [9–11].
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S. Huang et al. / Composites Science and Technology 67 (2007) 135–139
There were many pores in the cement-based piezoelectric composites prepared by the routine slip casting technique. In order to eliminate influence of pores on the properties of the composites, in this study, compressing technique is adopted to fabricate cement-based piezoelectric composites. The effect of different forming pressures on the properties of composites is studied in details. 2. Experimental procedure In this study, PLN and sulphoaluminate cement were used to fabricate the 0-3 cement-based piezoelectric composites through the following procedure: initially, two raw materials were ball-milled for 30 min with ethyl alcohol in a resin mill. After drying, a little water was added to the mixed materials. The mixed materials were pressed into disks of 15 mm diameter and 1.5 mm thickness. The pressures are 32 MPa, 48 MPa, 96 MPa, 128 MPa, respectively. The specimens were put in a curing room with a temperature of 20 C and relative humidity of 100% for 3 d before measurements. After curing, the surfaces of the disks were polished and coated with a low temperature silver paint, then the specimens were poled by applying an electric field of 4 kV/mm for 30 min at 100 C in a stirred silicone oil bath. The content of PLN in the composite was 80 wt%. After poling, the composites were aged for 24 h prior to the measurements. The piezoelectric strain factor d33 was directly measured using a Model ZJ-3A d33 piezometer. The frequency of dynamic force is fixed at 110 Hz. At least six measurements were made over the surface of the sample in order to obtain an acceptable average d33 value. Capacitance (C) was measured at 1 kHz with an Agilent 4294A Impedance Phase Analyzer. The dielectric permittivity er and piezoelectric voltage factor g33 of each specimen were calculated as er ¼
Ct ; Ae0
g33 ¼
d 33 er e0
where t and A are the specimen thickness and electrode area, respectively. e0 is the vacuum dielectric constant
Fig. 1. Piezoelectric constant of the composites as a function of the forming pressures.
(e0 = 8.85 · 1012 F/m). The pore structures of the specimens are measured by using Quanta Chrome POREMASTER-60 Automatic Pore Size Analyzers. 3. Results and discussion 3.1. Piezoelectric properties Fig. 1 shows the variation of piezoelectric constant of the composites as a function of the forming pressure. It can be seen that with decreasing pressure, the d33 values decreases gradually. This is mainly attributed to the effect of pores. It is believed that pores play the role of a stress absorber in the composites. The reduced local stress was transferred to the piezoelectric ceramic particles. Thus the piezoelectric property was remarkably diminished by the pores. This can be further supported by the pore structures analysis. Dependence of the porosity of the composites on the pressure is showed in Fig. 2. It can be seen that the porosity gradually increases with decreasing pressure, which make microstructure of the composites more porous. This is one of reasons that the piezoelectric properties of
Fig. 2. Analysis of pore structure of the composites prepared at various pressures.
S. Huang et al. / Composites Science and Technology 67 (2007) 135–139
the composites prepared at lower pressure are lower than that of the composites prepared at higher pressure. On the other hand, there are more pores in the composites prepared at lower pressure, especially interfacial pores. When the external electric field acts on the composites, the weak 3+ conductive ions (such as OH, Ca2+, SO2 4 and Al ) of cement matrix will accumulate in interfacial pores, which generate depolarization field, making a shielding on the external electric field. Thus the external electric fields which act on piezoelectric ceramic particles are weakened greatly, which is insufficient for carrying out the poling of the composite. It can also be seen that g33 decreases with increasing pressure. The main reason is that upon increasing pressure, the increase of the dielectric constant e is much faster than that of d33, while g33 is calculated as g33 = d33/ee0. 3.2. Dielectric properties The variation of dielectric constant of the composites with forming pressure is shown in Fig. 3. It is clear that
200
Dielectric constant / εr
160
120
80
40
0 0
32
64 96 Pressure / MPa
128
Fig. 3. Dielectric constant of the composites as a function of the forming pressures.
137
the dielectric constant increase gradually with increasing pressure. Fig. 4 shows the variation of dielectric constant with frequency for the composites prepared at various pressures. From Fig. 4, it can be seen that the dielectric constant decreases sharply with increasing frequency in the range of about 40–100 kHz. This is mainly attributed to interface polarization of the composites and polarization in the cement matrix. It is well known that the cement is a porous material with a complicated microstructure. In brief, it is composed of an amorphous phase, crystallites in the micrometer range and bound water [12]. The various types of polarization are excited in the presence of an applied electric field on the cement. When external electrical field acts on the cement matrix, besides hydrated products of cements, water and unreacted cement particles etc. give rise to polarization of electron, iron and dipole, many weak conducting ions (such as Ca2+, 3+ OH, SO2 and so on) in the sulphoaluminate 4 and Al cement matrix [13] begin to migrate. After the cement is mixed with PLN piezoelectric ceramic, there are many interfaces between the cement and PLN, at the same time, there are also many defects in cement matrix, such as impurities, lattice distortion and phase boundary etc. [14]. When the weak conducting ions encounter these interface and defects, the movement of the ions gets slower, which leads to the accumulation of the ions in the interface and defect areas, causing the polarization of space charge (interfacial polarization). All the polarizations can follow the change of the electrical field at low frequency. Hence the dielectric constant values of the composites are higher at low frequency. With increasing frequency, some polarizations, especially interfacial polarizations cannot follow the change of electric field because of the long time for the construction of space charge polarization. Therefore, the dielectric constant of the composites is lower at high frequency. About 120 kHz, some dielectric peaks appear in the dielectric constant-frequency curve of the composites. These peaks are mainly contributed by the dipole
Dielectric constant / εr
150
128MPa
96MPa
48MPa
32MPa
110
70
30 0
200
400
600
800
1000
Frequence / kHz Fig. 4. Variation of dielectric constant with frequency for the composites prepared at various pressures.
S. Huang et al. / Composites Science and Technology 67 (2007) 135–139
a
30
0 24
Impedance / kΩ
25
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-10
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-20
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15 115.038kHz
10
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120
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Phase / (˚)
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400
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800
Frequence / kHz 32MPa
30 -68
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25
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115
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Impedance / kΩ
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30
Impedance / kΩ
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25
18
-70 130.037kHz
20 13
15 127.537kHz
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10
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-20 -30 -40
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0 0
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600
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c
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Frequence / kHz 96MPa
0 -10
30 16
25
142.537kHz
-78
-20 -30 -40
-83
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-60 -70 -80 -90 1000
-73
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15 140.037kHz
10
6 115
135
155
Frequence / kHz
5 0
-68
0
200
400
600
800
Phase / (˚)
Impedance / kΩ
d
Frequence / kHz 128MPa
Fig. 5. Impedance magnitude and the phase spectra of the composites pressed at various pressures.
S. Huang et al. / Composites Science and Technology 67 (2007) 135–139
orientation polarization of PLN piezoelectric ceramic particles. It can also be observed that the higher the pressure is, the steeper is the rate of variation of the dielectric constant with increasing frequency in the range of 40–100 kHz. While the dielectric constants are almost unaffected by frequency at high frequency. This indicates that the composite prepared at high pressure is favorable to the polarization of the ions at low frequency. 3.3. Electromechanical coupling properties Fig. 5 shows the impedance magnitude and the phase spectra of the composites prepared at various pressures. It can be seen that some peaks appear in all phase curves. This means that the addition of PLN brings an electromechanical coupling behavior to the composites, which is caused by the piezoelectric effect and inverse piezoelectric effect. At the same time, the series resonance frequency and the parallel resonance frequency increase as the pressures increase, which causes change of the electromechanical coupling property. The thickness electromechanical coupling coefficient Kt and the mechanical quality factor Qm were calculated from the impedance measurements according to the following formula [15]: p fs p fp fs K 2t ¼ tan 2 fp 2 fp Qm ¼ 1=2pfs RCðfp2 fs2 Þ=fp2 where fs and fp are the series frequency and the parallel resonance frequency, respectively. They can be approximated replaced by the frequency at which the impedance magnitude reaches minimum and maximum electric impedance, respectively. R is the magnitude of the electrical impedance at fs, and C is the capacitance measured at 1 kHz. The planar electromechanical coupling coefficient Kp can approximately be evaluated using the curve of Kp versus Df/fs. The electromechanical coupling coefficients of the composites prepared at various pressures are summarized in Table 1. It can be seen that the Kt and Kp do not show significant change at various pressures. This indicates that the forming pressure has insignificant influence on electromechanical coupling properties of the composites. The Qm ranged between 16.61 and 23.66.
Table 1 The electromechanical coupling properties of the composites prepared at various pressures Pressure (MPa)
fs (kHz)
fp (kHz)
R (kX)
Df (kHz)
Kp (%)
Kt (%)
Qm
32 48 96 128
115.038 120.038 127.537 140.037
117.538 122.538 130.037 142.537
18.14 14.75 9.36 8.91
2.5 2.5 2.5 2.5
23.16 22.64 22.11 21.01
22.66 22.20 21.56 20.44
17.43 16.61 23.66 19.51
139
4. Conclusions (1) The forming pressures have significant influence on the properties of the cement-based piezoelectric composites. The piezoelectric strain factor and dielectric constant increase as the forming pressures increase. (2) The higher the pressure is, the steeper the rate of variation of the dielectric constant with increasing frequency in the range of 40–100 kHz. This indicates that the composite prepared at high pressure is favorable to the ionic polarization at low frequency. (3) The series resonance frequency and the parallel resonance frequency of the composites increase as the pressure increases, while Kt and Kp values are almost unaffected by the forming pressures.
Acknowledgements This work is supported by National Natural Science Foundation of China (50502017) and Natural Science Foundation of Shandong Province (Y2005F08). References [1] Li Zongjin, Zhang Dong, Wu Keru. Cement-based 0-3piezoelectric composites. J Am Ceram Soc 2002;85(2):305–13. [2] Banks HT, Smith RC, Wang Y. Smart material structures: modeling, estimation, and control. New York: Wiley; 1996. [3] Tao BQ. Smart/intelligent materials and structures. Beijing: Defense Industry Press; 1997. [4] Taylor HFW. Cement chemistry. London: Academic Press; 1990. [5] Neville AM. Properties of concrete. New York: John Wiley & Sons; 1996. [6] Zhang Dong, Wu Keru, Li Zongjin. Feasibility study of cement based piezoelectric smart composites. J Build Mater 2002;5(2):141–6 (in Chinese). [7] Huang Shifeng, Chang Jun, Cheng Xin, et al. Piezoelectric properties of 0-3 PZT/sulphoaluminate cement composites. Smart Mater Struct 2004;13:270–4. [8] Cheng Xin, Huang Shifeng, Chang Jun, et al. Piezoelectric and dielectric properties of piezoelectric ceramic–sulphoaluminate cement composites. J Eur Ceram Soc 2005;25(13):3223–8. [9] Li Zongjin, Dong Biqin, Zhang Dong. Influence of polarization on properties of 0-3 cement-based PZT composites. Cement Concrete Compos 2005;27(1):27–32. [10] Huang Shifeng, Chang Jun, Cheng Xin, et al. Dielectric and piezoelectric properties of 0-3 piezoelectric ceramic/sulphoaluminate cement composites [J]. Acta Materiae Compos Sinica 2005;22(2):87–90 (in Chinese). [11] Huang Shifeng, Chang Jun, Cheng Xin, et al. Piezoelectric and dielectric properties of piezoelectric ceramic–sulphoaluminate cement composites [J]. J Chinese Ceram Soc 2004;32(9):1082–7. [12] Taylor HFW. Cement Chemistry. 2nd ed. London: Thomas Telford Publishing; 1998. [13] Wang Yanmou, Su Muzhen, Zhang Liang. Sulphoaluminate cement. Beijing: Beijing University of Technology Press; 1999. p. 12. [14] Junda Wang, Muhua Tan, Keru Wu. Dielectric properties of macrodefect-free cement based composites. J Tongji Univ 2000;28(2):168–71. [15] An American National Standard IEEE Standard on Piezoelectricity, ANSI/IEEE Std 1987;176:227–273.
Effect of forming pressures on electric properties of piezoelectric ceramic/sulphoaluminate cement composites Huang Shifeng, Ye Zhengmao, Hu Yali, Chang Jun, Lu Lingchao, Cheng Xin
*
School of Materials Science and Engineering, Jinan University, Jinan 250022, China Received 1 August 2005; received in revised form 7 March 2006; accepted 21 March 2006 Available online 24 July 2006
Abstract Cement-based piezoelectric composites with 0-3 connectivity are fabricated from 0.08Pb(Li1/4Nb3/4)O3 Æ 0.47PbTiO3 Æ 0.45PbZrO3(PLN) and sulphoaluminate cement by compressing technique. Piezoelectric and dielectric properties of the composites under different forming pressures are measured and investigated. The results show that the composites prepared by compression technique exhibit excellent piezoelectric and dielectric properties. With increasing of forming pressure, the piezoelectric strain coefficient and dielectric constant increase, but the forming pressure has insignificant influence on electromechanical coupling property of the composites. The dielectric constant decreases sharply with increasing frequency from 40 kHz to 100 kHz, which is mainly due to interfacial polarization in the composite and polarization in the cement matrix. At high frequency, the composites show excellent dielectric frequency stability. The composite prepared at high pressure is favorable to the polarization of the ion at low frequency. 2006 Elsevier Ltd. All rights reserved. Keywords: 0-3 cement-based piezoelectric composites; Forming pressure; Piezoelectric properties; Dielectric properties; Electromechanical coupling properties
1. Introduction With the modern development of civil engineering, the intelligent material and structure is being introduced. In a smart structure, sensors and actuators are essential components for sensing and controlling purposes [1]. Different types of sensors and actuators have been designed and used according to different applications [2,3]. However, in civil engineering, cement-based materials are the most commonly used structural material. It is well known that the hydration of cement is a long process and can last for decades [4]. The change of the water state during hydration will cause shrinkage of concrete in general [5]. Therefore the intelligent materials suitable for application in the other engineering fields may not be applicable in civil engineering due to the differences in the properties between the intelli-
*
Corresponding author. Tel.: +86 531 82767017; fax: +86 531 82767977. E-mail address: [email protected] (X. Cheng).
0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.03.035
gent materials and the concrete, such as acoustic impedance, volume stability, temperature coefficient and interfacial adhesion etc. To meet the requirement of development for smart or intelligent structures in civil engineering, a new kind of function composites, cement-based piezoelectric composite has been developed in recent years [6–8]. Cement-based piezoelectric composite has good compatibility with civil engineering’s main structural material—concrete. It can be prepared in any shape or size with great ease and at lower cost. It not only has sensing function, but also actuating property, which is very suitable for application in civil engineering fields, such as high-rise buildings, long-span bridges and some buildings whose failure would cause disasters, including nuclear waster containment structures, dams, and bridge decks. Therefore, research and development of the cementbased piezoelectric composite play an extremely important role in advancing all kinds of civil engineering structure to be intelligent [9–11].
136
S. Huang et al. / Composites Science and Technology 67 (2007) 135–139
There were many pores in the cement-based piezoelectric composites prepared by the routine slip casting technique. In order to eliminate influence of pores on the properties of the composites, in this study, compressing technique is adopted to fabricate cement-based piezoelectric composites. The effect of different forming pressures on the properties of composites is studied in details. 2. Experimental procedure In this study, PLN and sulphoaluminate cement were used to fabricate the 0-3 cement-based piezoelectric composites through the following procedure: initially, two raw materials were ball-milled for 30 min with ethyl alcohol in a resin mill. After drying, a little water was added to the mixed materials. The mixed materials were pressed into disks of 15 mm diameter and 1.5 mm thickness. The pressures are 32 MPa, 48 MPa, 96 MPa, 128 MPa, respectively. The specimens were put in a curing room with a temperature of 20 C and relative humidity of 100% for 3 d before measurements. After curing, the surfaces of the disks were polished and coated with a low temperature silver paint, then the specimens were poled by applying an electric field of 4 kV/mm for 30 min at 100 C in a stirred silicone oil bath. The content of PLN in the composite was 80 wt%. After poling, the composites were aged for 24 h prior to the measurements. The piezoelectric strain factor d33 was directly measured using a Model ZJ-3A d33 piezometer. The frequency of dynamic force is fixed at 110 Hz. At least six measurements were made over the surface of the sample in order to obtain an acceptable average d33 value. Capacitance (C) was measured at 1 kHz with an Agilent 4294A Impedance Phase Analyzer. The dielectric permittivity er and piezoelectric voltage factor g33 of each specimen were calculated as er ¼
Ct ; Ae0
g33 ¼
d 33 er e0
where t and A are the specimen thickness and electrode area, respectively. e0 is the vacuum dielectric constant
Fig. 1. Piezoelectric constant of the composites as a function of the forming pressures.
(e0 = 8.85 · 1012 F/m). The pore structures of the specimens are measured by using Quanta Chrome POREMASTER-60 Automatic Pore Size Analyzers. 3. Results and discussion 3.1. Piezoelectric properties Fig. 1 shows the variation of piezoelectric constant of the composites as a function of the forming pressure. It can be seen that with decreasing pressure, the d33 values decreases gradually. This is mainly attributed to the effect of pores. It is believed that pores play the role of a stress absorber in the composites. The reduced local stress was transferred to the piezoelectric ceramic particles. Thus the piezoelectric property was remarkably diminished by the pores. This can be further supported by the pore structures analysis. Dependence of the porosity of the composites on the pressure is showed in Fig. 2. It can be seen that the porosity gradually increases with decreasing pressure, which make microstructure of the composites more porous. This is one of reasons that the piezoelectric properties of
Fig. 2. Analysis of pore structure of the composites prepared at various pressures.
S. Huang et al. / Composites Science and Technology 67 (2007) 135–139
the composites prepared at lower pressure are lower than that of the composites prepared at higher pressure. On the other hand, there are more pores in the composites prepared at lower pressure, especially interfacial pores. When the external electric field acts on the composites, the weak 3+ conductive ions (such as OH, Ca2+, SO2 4 and Al ) of cement matrix will accumulate in interfacial pores, which generate depolarization field, making a shielding on the external electric field. Thus the external electric fields which act on piezoelectric ceramic particles are weakened greatly, which is insufficient for carrying out the poling of the composite. It can also be seen that g33 decreases with increasing pressure. The main reason is that upon increasing pressure, the increase of the dielectric constant e is much faster than that of d33, while g33 is calculated as g33 = d33/ee0. 3.2. Dielectric properties The variation of dielectric constant of the composites with forming pressure is shown in Fig. 3. It is clear that
200
Dielectric constant / εr
160
120
80
40
0 0
32
64 96 Pressure / MPa
128
Fig. 3. Dielectric constant of the composites as a function of the forming pressures.
137
the dielectric constant increase gradually with increasing pressure. Fig. 4 shows the variation of dielectric constant with frequency for the composites prepared at various pressures. From Fig. 4, it can be seen that the dielectric constant decreases sharply with increasing frequency in the range of about 40–100 kHz. This is mainly attributed to interface polarization of the composites and polarization in the cement matrix. It is well known that the cement is a porous material with a complicated microstructure. In brief, it is composed of an amorphous phase, crystallites in the micrometer range and bound water [12]. The various types of polarization are excited in the presence of an applied electric field on the cement. When external electrical field acts on the cement matrix, besides hydrated products of cements, water and unreacted cement particles etc. give rise to polarization of electron, iron and dipole, many weak conducting ions (such as Ca2+, 3+ OH, SO2 and so on) in the sulphoaluminate 4 and Al cement matrix [13] begin to migrate. After the cement is mixed with PLN piezoelectric ceramic, there are many interfaces between the cement and PLN, at the same time, there are also many defects in cement matrix, such as impurities, lattice distortion and phase boundary etc. [14]. When the weak conducting ions encounter these interface and defects, the movement of the ions gets slower, which leads to the accumulation of the ions in the interface and defect areas, causing the polarization of space charge (interfacial polarization). All the polarizations can follow the change of the electrical field at low frequency. Hence the dielectric constant values of the composites are higher at low frequency. With increasing frequency, some polarizations, especially interfacial polarizations cannot follow the change of electric field because of the long time for the construction of space charge polarization. Therefore, the dielectric constant of the composites is lower at high frequency. About 120 kHz, some dielectric peaks appear in the dielectric constant-frequency curve of the composites. These peaks are mainly contributed by the dipole
Dielectric constant / εr
150
128MPa
96MPa
48MPa
32MPa
110
70
30 0
200
400
600
800
1000
Frequence / kHz Fig. 4. Variation of dielectric constant with frequency for the composites prepared at various pressures.
S. Huang et al. / Composites Science and Technology 67 (2007) 135–139
a
30
0 24
Impedance / kΩ
25
-65
-10
-70
-20
-75
-30
-80
-40
-85
-50
-90 140
-60
117.538kHz
20 20
15 115.038kHz
10
16 100
120
-70
Frequence / kHz
5
Phase / (˚)
138
-80
0 0
200
400
600
-90 1000
800
Frequence / kHz 32MPa
30 -68
19
25
-73
122.538kHz
20 15
120.038kHz
13 100
10
115
-30 -40
-88
-50
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130
-70 -80
Frequence / kHz
5
-90 1000
0 0
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Impedance / kΩ
b
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30
Impedance / kΩ
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25
18
-70 130.037kHz
20 13
15 127.537kHz
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10
130
-80
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-50
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0 0
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400
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c
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Frequence / kHz 96MPa
0 -10
30 16
25
142.537kHz
-78
-20 -30 -40
-83
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-60 -70 -80 -90 1000
-73
20 11
15 140.037kHz
10
6 115
135
155
Frequence / kHz
5 0
-68
0
200
400
600
800
Phase / (˚)
Impedance / kΩ
d
Frequence / kHz 128MPa
Fig. 5. Impedance magnitude and the phase spectra of the composites pressed at various pressures.
S. Huang et al. / Composites Science and Technology 67 (2007) 135–139
orientation polarization of PLN piezoelectric ceramic particles. It can also be observed that the higher the pressure is, the steeper is the rate of variation of the dielectric constant with increasing frequency in the range of 40–100 kHz. While the dielectric constants are almost unaffected by frequency at high frequency. This indicates that the composite prepared at high pressure is favorable to the polarization of the ions at low frequency. 3.3. Electromechanical coupling properties Fig. 5 shows the impedance magnitude and the phase spectra of the composites prepared at various pressures. It can be seen that some peaks appear in all phase curves. This means that the addition of PLN brings an electromechanical coupling behavior to the composites, which is caused by the piezoelectric effect and inverse piezoelectric effect. At the same time, the series resonance frequency and the parallel resonance frequency increase as the pressures increase, which causes change of the electromechanical coupling property. The thickness electromechanical coupling coefficient Kt and the mechanical quality factor Qm were calculated from the impedance measurements according to the following formula [15]: p fs p fp fs K 2t ¼ tan 2 fp 2 fp Qm ¼ 1=2pfs RCðfp2 fs2 Þ=fp2 where fs and fp are the series frequency and the parallel resonance frequency, respectively. They can be approximated replaced by the frequency at which the impedance magnitude reaches minimum and maximum electric impedance, respectively. R is the magnitude of the electrical impedance at fs, and C is the capacitance measured at 1 kHz. The planar electromechanical coupling coefficient Kp can approximately be evaluated using the curve of Kp versus Df/fs. The electromechanical coupling coefficients of the composites prepared at various pressures are summarized in Table 1. It can be seen that the Kt and Kp do not show significant change at various pressures. This indicates that the forming pressure has insignificant influence on electromechanical coupling properties of the composites. The Qm ranged between 16.61 and 23.66.
Table 1 The electromechanical coupling properties of the composites prepared at various pressures Pressure (MPa)
fs (kHz)
fp (kHz)
R (kX)
Df (kHz)
Kp (%)
Kt (%)
Qm
32 48 96 128
115.038 120.038 127.537 140.037
117.538 122.538 130.037 142.537
18.14 14.75 9.36 8.91
2.5 2.5 2.5 2.5
23.16 22.64 22.11 21.01
22.66 22.20 21.56 20.44
17.43 16.61 23.66 19.51
139
4. Conclusions (1) The forming pressures have significant influence on the properties of the cement-based piezoelectric composites. The piezoelectric strain factor and dielectric constant increase as the forming pressures increase. (2) The higher the pressure is, the steeper the rate of variation of the dielectric constant with increasing frequency in the range of 40–100 kHz. This indicates that the composite prepared at high pressure is favorable to the ionic polarization at low frequency. (3) The series resonance frequency and the parallel resonance frequency of the composites increase as the pressure increases, while Kt and Kp values are almost unaffected by the forming pressures.
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