Microanalysis and Piezoelectric Properties Study of Piezoelectric Film Sensors Based on Heat Treatment Process

Purchased piezoelectric ceramic particles have a high surface activity, a low dielectric constant, and a low ferroelectric performance. The piezoelectric activity of piezoelectric ceramic particles was further improved by improving the interface bonding state between the piezoelectric ceramic particles and the polymer matrix. In this study, the piezoelectric ceramic particles were treated at a high temperature under nitrogen protection. Crystal forms of the piezoelectric ceramic particles were analyzed via X-ray diffraction (XRD) and scanning electron microscopy (SEM) technologies. The dielectric and piezoelectric properties of the piezoelectric film, which were influenced by the heat treatment process, were the focus of this study. The heat treatment temperature of the piezoelectric ceramic particles was determined, and piezoelectric film sensors were prepared. The applied research of the piezoelectric film sensors was performed via a cantilever beam experiment. The results showed that the heat treatment resulted in the decomposition of strong groups at the piezoelectric ceramic particle surface, the surface activity of the particles decreased, the interface compatibility of the piezoelectric ceramic particles and the polymer matrix was enhanced, the reasonable heat treatment temperature was 1200 °C, and the piezoelectric film sensors that were used for the real-time monitoring of structures were effective.


Introduction
The preparation of piezoelectric ceramic particles is typically divided into four steps: collection of ingredients, presintering, molding, and high-temperature sintering. (1)(2)(3) The block broken method is the most common method used in production, as it reduces the production cost. This method results in the surface damage of the particles, the multidomain, and the dielectric, as well a reduction in the ferroelectric properties. (4) The surface activity of the piezoelectric ceramic particles significantly increased during the preparation process, which leads to the piezoelectric ceramic particle surface absorbing some organic impurities or the water in the air in order to produce hydroxyl groups (-OH). (5) The compatibility between the piezoelectric ceramic particles and the polyvinylidene fluoride (PVDF) matrix is poor, owing to the existence of strong groups, which causes additional defects between the two phases. (6)(7)(8) In this study, the piezoelectric ceramic particles were treated at a high temperature under nitrogen protection. The interface bonding state between the piezoelectric ceramic particles and the polymer matrix was improved. Crystal forms of the piezoelectric ceramic particles were analyzed via X-ray diffraction (XRD) and scanning electron microscopy (SEM) technologies. The dielectric and piezoelectric properties of the film, which were influenced by the heat treatment process, were the focus of this study.

Materials
• Functional phase: Lead zirconate titanate (PZT) ceramic particles were used for the functional phase, which were produced by Bailing Ceramic Limited Company. The molecular symbol for piezoelectric ceramic is Pb(Zr 0.52 Ti 0.48 )O 3 . • Matrix phase: PVDF powders were used for the matrix phase, which were produced by Shanghai 3F New Materials Co. Ltd. The major parameters of the PVDF powders are shown in Table 1. • Conductive phase: Carbon nanotubes were used for the conductive phase, which were generated by Shenzhen Nanotech Port Co. Ltd. The major parameters of carbon nanotubes are shown in Table 2.

Heat treatment of PZT ceramic particles
Piezoelectric ceramic particles were layered in the corundum crucible and heat treated under nitrogen protection. The heat treatment device for PZT powders is shown in Fig. 1. The heat treatment temperatures were 600, 800, 1000, 1100, 1150, 1200, and 1250 ℃.

Fabrication of piezoelectric film sensors
The piezoelectric films (0-3 type) that were doped with carbon nanotubes were prepared by flowing. The fabrication process is shown in Fig. 2. The volume content of the PZT ceramic particles was 50%. That of the carbon nanotubes was 50%. A silver slurry was then coated on the surface of the piezoelectric film after polarization. Wires were attached to the surface of the sample. Insulation films were used to insulate the film. The piezoelectric film sensors are shown in Fig. 3.

Cantilever beam experiment
The piezoelectric film sensors and strain gauge were attached to the surface of the equalstrength cantilever steel beam. A vibrator was used to vibrate the beam. DSPACE was used to collect the signals from the piezoelectric film sensors and strain gauge concurrently. The experimental device is shown in Fig. 4.

Appearances of PZT ceramic particles
The appearances of the PZT ceramic particles treated at various temperatures are shown in Fig. 5. The initial powders were gray. As the heat treatment temperature increased, the PZT ceramic particles became light yellow, and finally, dark yellow. When the heat treatment temperature was over 1000 ℃, the PZT ceramic particles began to condense with the loss of mass. When the temperature reached 1200 ℃, high-hardness PZT ceramic particles were formed. When the temperature reached 1250 ℃, massive PZT ceramic particles were formed.

XRD patterns of PZT ceramic particles
The piezoelectric ceramic is a multiphase coexistence material. The main ingredients are the tetragonal and cubic phases. The tetragonal phase (TP) has a high performance, which could provide additional piezoelectric properties. The XRD of the PZT ceramic particles that    were heated to various temperatures is shown as Fig. 6. When the heat treatment temperature increased, a diffraction peak appeared at a 44° diffraction angle. This diffraction peak was determined to be the characteristic peak of the TP. The diffraction peak reached the maximum intensity when the heat treatment temperature reached 1200 ℃. The relative contents of the tetragonal and cubic phases were calculated using the following equation: where M T is the volume content of the tetragonal phase, I T (002) and I T (200) are the intensities of the tetragonal phase's two characteristic peaks T(002) and T(200), and I R (200) is the intensity of the cubic phase's characteristic peak R(200). (9) The characteristic peak change of the PZT ceramic particles before and after heating is shown in Fig. 7. The solid line is the XRD patterns of the initial powders and the dotted line is the XRD patterns of the powders heated at 1200 ℃. T(002) and T(200) are the two diffraction peaks of the tetragonal phase and R(200) is the diffraction peak of the cubic phase. When the temperature reached 1200 ℃, the intensities of T(002) and T(200) increased and the intensity of R(200) decreased. Equation (1) was used to determine that when the heat treatment temperature reached 1200 ℃, the relative content of the tetragonal phase increased from 62 to 85%. The heat treatment of the PZT ceramic particles improved the tetragonal phase content, which improved the piezoelectric activity of the piezoelectric ceramic particles.

Thermogravimetry-differential scanning analysis (TG-DSC) of PZT ceramic particles
The TG-DSC of PZT ceramic particles is shown in Fig. 8. When the temperature increased from 0-600 ℃, the PZT ceramic particles went through an endothermic process, and the weight of the particles decreased. The water and the low-molecular-weight organic matter adsorbed  onto the surface of the volatile particles or the breakdown resulted in the loss of weight. (10)(11)(12) When the temperature increased from 600-1000 ℃, the PZT ceramic particles went through an exothermic process, and the weight of the particles decreased. (13) The oxidative decomposition of the strong groups (such as -OH, -C=O, and -COOH) adsorbed onto the surface of the particles results in the loss of weight. When the temperature was over 1000 ℃, the PZT ceramic particles went through an endothermic process, where the weight of the particles decreased. The volatility of the PbO and the phase change of the internal particles resulted in this phenomenon.

SEM image of piezoelectric film
The SEM image of the piezoelectric film made with the initial powders is shown in Fig.  9. The SEM image of the piezoelectric film made with the PZT powders heated at 1200 ℃ is shown in Fig. 10. These figures show that the piezoelectric ceramic particles were uniformly distributed in the polymer matrix. The system was a 0-3 type. The film made with the PZT powders heated at 1200 ℃ had superior interface compatibility, as well as less defects between the ceramic particles and the matrix. These results indicated that the heat treatment leads to the decomposition of the strong groups at the surface of the piezoelectric ceramic particles, the surface activity of the particles was reduced, and the interface compatibility of the piezoelectric ceramic particles and the polymer matrix was enhanced. Figure 11 shows the conductivity of the piezoelectric film at room temperature and 1 kHz. The conductivity of the piezoelectric film slowly increased when the temperature was increased. When the heat treatment temperature reached 1200 ℃, the conductivity of the piezoelectric film was 86 × 10 −7 S/m, which was 1.09 times higher than that of the untreated film. This result indicated that the heat treatment increased the conductivity of the piezoelectric film, but the effect was small.    Figure 12 shows the dielectric constant and dielectric loss of the piezoelectric film influenced by the heat treatment temperature at room temperature and 1 kHz. The dielectric constant of the piezoelectric film slowly increased as the temperature increased. The dielectric loss of the piezoelectric film slowly decreased as the temperature increased. When the heat treatment temperature reached 1200 ℃, the dielectric constant of the piezoelectric film reached 106, which increased by approximately 10%. The dielectric loss of the piezoelectric film reached 84 mU, which was a 9% decrease. These results indicated that the heat treatment could improve the dielectric properties of the piezoelectric film. Figure 13 shows the d 33 and g 33 of the piezoelectric film, which were influenced by the heat treatment temperature. When the temperature was below 1000 ℃, the d 33 of the piezoelectric film slowly increased as the temperature increased. When the temperature was over 1000 ℃, the d 33 increased rapidly as the heat treatment temperature increased. When the heat treatment temperature was over 1200 ℃, the d 33 decreased as the temperature increased. The PbO of piezoelectric ceramic particles was volatile when the heat treatment temperature was over 1200 ℃. The internal structure of the particles changed, which led to the decrease in piezoelectric activity. When the heat treatment temperature was 1200 ℃, the d 33 of the piezoelectric film reached 45 pC/N, which was 1.8 times higher than that of the untreated film. The g 33 of the film was 44 mV·m/N, which was 1.4 times higher than that of the untreated film. The change rules of the piezoelectric constant verified the conclusions obtained in Sects. 3.2 and 3.3. A reasonable heat treatment temperature could improve the tetragonal phase content, which could increase the piezoelectric activity.  Figure 14 shows the K p and K t of the piezoelectric film, which were influenced by the heat treatment temperature. The K p and K t of the piezoelectric film slowly increased as the temperature increased. When the temperature was below 1000 ℃, the K p and K t increased as the temperature increased. When the heat treatment temperature was greater than 1200 ℃, the K p and K t decreased as the temperature increased. When the heat treatment temperature was 1200 ℃, the K p of the piezoelectric film reached 0.212, which was 1.45 times higher than that of the untreated film. The K t of the film was 0.208, which was 1.44 times higher than that of the untreated film. The change rules of the electromechanical coupling coefficients verified the conclusions obtained in Sect. 4.4. Figure 15 shows the Q m of the piezoelectric film, which was influenced by the heat treatment temperature. When the heat treatment temperature was below 1000 ℃, the Q m of the piezoelectric film slowly decreased as the temperature increased. When the heat treatment temperature increased from 1000 to 1200 ℃, the Q m decreased rapidly as the heat treatment temperature increased. When the heat treatment temperature was over 1200 ℃, the Q m increased as the temperature increased. When the heat treatment temperature was 1200 ℃, the Q m of the piezoelectric film reached 0.349, which was 1.45 times higher than that of the untreated film. The K t of the film was 0.208, which decreased 30% more than that of the untreated film.

Results of Cantilever Beam Experiment
In this experiment, the cantilever beam was vibrated by 1 and 20 Hz sinusoidal loads. The responses of the piezoelectric film sensors and strain gauge sensors under the sinusoidal load are shown in Fig. 16. The response of the strain gauge sensors was associated with the voltage  value of the external power, so the signals seen in Fig. 16 were homogenized. The Y-axis of Fig.  16 was the measured value divided by the maximum measured value of each data set. The responses of the piezoelectric film sensors and strain gauge sensors under a sinusoidal load are denoted as the standard sine curve in Fig. 16. These results indicated that under a low   frequency load, the piezoelectric film sensors and strain gauge sensors correctly reflected the deformation of the structure. The signals of the strain gauge sensors were slightly delayed from the other two sensors. This showed that the synchronicity and sensitivity of the piezoelectric film sensors were higher than those of the strain gauge sensors. The piezoelectric film sensors have value in both research and application.

Conclusions
The effect that the heat treatment had on the performance of the piezoelectric film was examined. The analysis and comparisons resulted in the following conclusions: • The heat treatment of the PZT ceramic particle improved the content of the tetragonal phase, which improved the piezoelectric activity of the piezoelectric ceramic particles. • The heat treatment resulted in the decomposition of strong groups at the piezoelectric ceramic particle surface, the surface activity of the particles decreased, and the interface compatibility of the piezoelectric ceramic particles and the polymer matrix was enhanced. • The enhanced interface compatibility will lead to defects between the ceramic particles decreased; the polarization voltage was increased, and finally, the piezoelectric properties were improved. • The heat treatment improved the piezoelectric activity of the piezoelectric film. The reasonable heat treatment temperature was 1200 ℃. • The synchronicity and sensitivity of the piezoelectric film sensors were higher than those of the strain gauge sensors. Piezoelectric film sensors have great value in both research and application. The piezoelectric composite film sensors were effective for use in the real-time monitoring of structure stress.