3.1 Microstructure and Phase characteristics
Figure 1 shows the detailed pore morphology and interconnection of porous ceramics and cement-based piezoelectric composites. As can be seen in Fig. 1(a), macropores with an average size of approximately 226 µm were formed in the porous PZT ceramics, and interconnected structure of open-cells were resembled, which was helpful for the cement paste to fill into the inner parts of porous ceramics. Figure 1(b) demonstrates the fracture surfaces of cement-based piezoelectric composites without being modified by PVDF yet, while the PZT ceramic particles were surrounded by the cement hydrated products such as calcium silicate hydrate (C-S-H) and calcium hydroxide (Ca(OH)2), the PZT-PC composites were formed with significant enhancement of strength. However, there still existed some pores with an average size of 5 µm in the hydrated cement and at the interface binding between PZT particles and hydrate cement, which would generate leakage current during the poling process and deteriorate the polarization effect. The microstructure of PVDF-modified 3–3 type cement-based piezoelectric composites with 100 mg/ml of PVDF is shown in Fig. 1(c), it can be found that not only the PZT piezoelectric ceramic particles were embraced by hydrated cement matrix, the PVDF polymer also infiltrated into the inner domain of composites as a connecting third phase and occupied the pore space between PZT ceramics and hydration products of cement. The existence of PVDF phase succeeded in reducing the defects in the composites and contributed to the improvement of poling process.
Figure 2 shows the density of PVDF-modified cement-based piezoelectric composites with different concentration of PVDF solution. It can be observed that with the increase in the concentration of PVDF solution, the density of PZT-PC composites was in the range from 3.43 g/cm3 to 3.61 g/cm3, which means an increase of ~ 5% and more PVDF occupying the pore space of cement-based piezoelectric composites. This result also indicates that the density of composites varied and could be easily tailored in a range to possess excellent piezoelectric properties by adjusting the concentration of PVDF solution.
Figure 3(a) reveals X-ray diffraction patterns of PVDF-modified cement-based piezoelectric composites with different concentration of PVDF solution. It is noted that the characteristic peaks appearing in the XRD traces of all PZT-PC composites were attributed to PZT ceramics where the peaks resembled those of a PZT ceramic matching JCPDS file no. 33–0784. Besides that, the main crystal hydrated products of tricalcium silicates (3CaO.SiO2 or C3S) in cement such as calcium hydroxide (Ca(OH)2) and the amorphous glassy phase of calcium silicate hydrates (C-S-H) were detected, but they were not as dominant as those of PZT due to both the preferred orientation of PZT and the relatively greater quantity of PZT ceramics. Calcium carbonate (CaCO3), the reaction product of CO2 and Ca(OH)2 due to the sample exposure to air, was also detected. More importantly, the characteristic peaks corresponding to 2θ values around 18.37° and 19.93° revealed the existence of α and γ phase of PVDF. In general, the amplified diffraction peaks can be used to characterize the phase variation of PVDF modified cement-based piezoelectric composites. Therefore, the correspondingly amplified XRD patterns in the 2θ range of 17–21° are presented in Fig. 3(b). With increasing concentration of PVDF solution, the intensity of diffraction peaks nearby 18.37° and 19.93° increased gradually, which implies the increasing combined quantity of PVDF with the PZT-PC composites and is constant with the variation trends of density in Fig. 2.
3.2 Dielectric and piezoelectric properties
Figure 4 shows the relative permittivity (εr) of the PVDF-modified cement-based piezoelectric composites in the range of 100 Hz-2 MHz. As expected, the relative permittivity of all PVDF-modified cement-based piezoelectric composites decreased with increasing frequency and demonstrated ordinary ferroelectric property, it could be explained as follows: the dipole relaxation connected with domain walls motion of the ionic particles have made it hard for the electron hopping to follow the alternative field, for another, the inhomogeneities of PZT-PC composites gave rise to a frequency dependence of conductivity for the accumulating charge carries at the boundaries of less conducting regions, which led to the interfacial polarization and the frequency shift of dipole lagging behind the electric field. As far as to the effect of PVDF concentration on the relative permittivity, it is noted that the εr increased remarkably with the increasing concentration of PVDF solution, and the εr values at a frequency of 1 KHz were in the range from 360 to 406. It could be attributed to the infiltration of PVDF into the micron-sized pore space, the substitution of PVDF for air-filler pores has reduced the leakage current chiefly caused by the interface pore and cement matrix pore, and was in favor of high capacitance and the corresponding relative permittivity of PZT-PC composites.
The effect of PVDF concentration on the dielectric loss (tanδ) of PVDF-modified cement-based piezoelectric composites is shown in Fig. 5. The dielectric loss factor demonstrated a peaking behavior for all PZT-PC composites, which could be attributed to the dielectric relaxation phenomenon for many ferroelectrics. Moreover, owing to the insulating effect of the PVDF phase, the tanδ values at a frequency of 1 KHz decreased from 0.04 to 0.02 with the increasing PVDF concentration.
Figure 6 shows the correlations of longitudinal piezoelectric strain coefficient (d33) and longitudinal piezoelectric voltage coefficient (g33) with concentration of PVDF solution. The d33 values of PZT-PC composites increased almost linearly from 270 to 289 pC/N in the PVDF concentration range of 0-200 mg/ml. As is known, when the external electric field acts on the PZT-PC composites during the poling process, the weakly conducting ions (such as Ca2+, OH− and Al3+) in cement phase tend to generate depolarization and make a shielding electric field to weaken the effect of external electric field. Furthermore, the effect of leakage current that arises from micropores in the PZT-PC composites should not be neglected. Under the combined action of the two factors, the degree of poling process was brought down and the d33 values were reduced. When the PVDF phase was combined with the PZT-PC composites, it acted as an insulator with optimum effect and less conducting path connecting the porosity or conducting ion in the system, which resulted in greater electrical current flow to PZT phase and increasing values of d33. The g33 value is defined as the ratio of d33 to εr·ε0, so the change in the g33 value depends on both the d33 and εr values. When the PVDF concentration ranged from 0 to 150 mg/ml, the g33 value decreased from 84.7 to 80.7 mV·m/N, and then increased when the PVDF concentration went higher. It is believed that the descending trend of g33 values upon increasing the PVDF concentration to 150 mg/ml was mainly due to the faster increasing trend of εr than that of d33. When exceeding the PVDF concentration of 150 mg/ml, the d33 value increased at a more rapid rate than εr, so the g33 value increased appropriately.
The ferroelectric hysteresis (P-E) loops were characterized to investigate the effect of PVDF concentration on the piezoelectric properties of PZT-PC composites. From Fig. 7, it is shown that all specimens exhibited typical P-E hysteresis loops at room temperature. The addition of PVDF exerted a significant influence on the remanent polarization Pr and coercive field Ec. With increasing the concentration of PVDF solution, the remanent polarization Pr and coercive field Ec increased. When the PVDF concentration increased from 0 to 200 mg/ml, the remanent polarization Pr increased from 1.05 to 5.91 µC/cm2, and the coercive field Ec increased from 1.29 to 6.01 kV/mm. Obviously, the leakage current caused by defects in the composites promoted the hysteresis loop to be less developed during the poling process and degraded the ferroelectric properties. Therefore, both the remnant polarization (Pr) and the longitudinal piezoelectric strain coefficient (d33) of the 3–3 type cement-based piezoelectric composites could be improved as a function of PVDF modification. It should also be noticed that when the PVDF concentration was between 0 and 100 mg/ml, there was an obvious gap at the direction of the negative remanent polarization, which is still deduced to be caused by the leakage current. When the PVDF concentration increased to 150 mg/ml, the infiltration of PVDF into the pores reduced the leakage current and prevented the distortion of the hysteresis loops.
3.3 Electromechanical and acoustic properties
Figure 8 shows the impedance magnitude of the PZT-PC composites with different PVDF concentration. It can be seen that the impedance of the composites increased with PVDF concentration for the insulating property of PVDF phase. Meanwhile, there also appear some resonance peaks in all curves, which means that the PZT-PC composites possessed an electromechanical coupling behavior from the piezoelectric effect and inverse piezoelectric effect. The planar resonance peak of the composites was at the frequency zones between 50 kHz and 80 kHz, while the thickness resonance peak appeared around 200 kHz. With the increase of PVDF concentration, both the series resonance frequency and the parallel resonance frequency decreased accordingly, which resulted in the change of electromechanical coupling property.
In this study, the thickness electromechanical coupling coefficient Kt of the PVDF-modified cement-based piezoelectric composites were calculated from the impedance measurements according to the following formula:
where fs and fp are the series resonant frequency and the parallel resonant frequency, respectively, which can be replaced by the frequencies at the minimum and maximum impedances (fm and fn) in the fundamental resonant region of impedance spectrum. The planar electromechanical coupling coefficient Kp can approximately be evaluated using the curve of Kp versus △f/fs.
The acoustic impedance (Z) of the composites can be obtained by the following equation:
where d and ρc are the thickness and density of the composites respectively.
The electromechanical coupling coefficients of the PZT-PC composites with different PVDF concentration are summarized in Table I. It is observed that the thickness electromechanical coupling coefficient Kt increased from 30.83–42.02%, and the planar electromechanical coupling coefficient Kp increased from 22.53–32.25% with the combination of PVDF phase, which indicated higher resolution factor and thickness electromechanical transformation ability for the PVDF-modified cement-based piezoelectric composites to be used as the transducer. The acoustic impedance (Z) of PZT-PC composites decreased from 7.65 to 6.89 MRayls with the increasing PVDF concentration for the gradual introduction of the PVDF phase. The lower value of Z was close to that of concrete structures (~ 6.9 ~ 11.23 MRayls), which is beneficial in improving acoustic matching and helpful for application in civil engineering.