Structural, Dielectric and Piezoelectric Properties of PZT/PVDF Composites Prepared by Hot Press Method

Structural, Dielectric and Piezoelectric Properties of PZT/PVDF Composites Prepared by Hot Press Method

Do Phuong Anp,*, Do Viet On1, Dao Anh Quang2, Nguyen Van Thinp, Nguyen Thi Anh Tuyet4, Vo Thanh Tung1, Truong Van Chuong1

1College of Sciences, Hue University, Vietnam

2Institute of Reasearch and Development, Duy Tan University, Danang, Vietnam 3University of Technology and Education, University of Danang, Vietnam 4Faculty of Natural Sciences, Quy Nhon University, Binhdinh, Vietnam

Abstract – The 0-3 PZT/PVDF piezoelectric composites are prepared separately by hot-press. The effects of the PZT content and the shaping-process on the composites are studied. The experimental results indicate that composites with 70% PZT nanopowders prepared by the hot-press method exhibit excellent piezoelectric and dielectric properties. This is mainly attributed to the favourable coupling of the two materials in the process of the hot press and the formation of the -type PVDF, which possesses better electric properties.

Keywords – PVDF; PZT; composites; electrospinning; ferroelectric; piezoelectric; 0-3 composite.

1. INTRODUCTION

   In recent years, the lead zirconate titanate (PZT) ceramicsmhas been widely used in sensors, actuators, hydrophones and ultrasonic transducers because of its excellent piezoelectric properties. As a result, the ferroelectric ceramic-polymer composites are the promising materials for applications in high-pressure sensors, hydrophones and shock accelerometers [1, 5, 8, 11, 12, 14,

   16, 26, 28, 30, 31].

   The simplest ceramic-polymer composite is that which consists of ceramic particles randomly dispersed in a polymer matrix, which is well known as 0-3 connectivity. Then, the PZT ceramic particles with excellent piezoelectric properties were evenly dispersed in a three-dimensional connected flexible polymer matrix. This type of composite has excellent comprehensive properties. Hot press technique that combines solution and melt processing was found to be the better method for the preparation of ceramic-polymer composites. Consequently, the 0-3 polymer-ceramic composites, especially of the ferroelectric ceramics such as PZT and PMN-PT and the ferroelectric polymer PVDF (polyvinylidene fluoride), have been the subject of a lot of research work, and detailed studies were reported on their piezoelectric, dielectric and pyroelectric properties [2, 3, 5, 9, 12, 14, 15, 18, 24, 27, 29, 34].

2. EXPERIMENTAL SECTION

   1. Materials

      PVDF of number-average molecular weight 534,000 supplied by Aldrich chemistry, France, The PZT ceramic (d33 = 317 pC/N, r = 1800), which was prepared via the conventional solid-state reaction method, PZT powers are prepared with a mortar and pestle, the mean particle size of the PZT powders

      is about 200 500 nm (Figure 1). Dimethyl formamide (DMF) supplied by Merck and acetone (Merck, 99.7%), India, were used in this study.

      1. (b)

         Figure 1. P-E hysteresis loop of PZT ceramic sample and the d33 values of PZT disc in the Model YE2730 meter

   2. Devices

      The crystalline structure analysis was performed at room temperature using an X-ray diffraction system (XRD, Bruker D8 Advance, Germany) with Cu K radiation (= 0.154 nm). The surface morphology was examined using a scanning electron microscopy (SEM, Nova NanoSEM 450-FEI) operated at an accelerating voltage of 1020 kV)

      The polarization-electric field (PE) hysteresis loops were measured with a HIOKI 3532 (Radiant Technologies) which testing unit connected with a high-voltage interface.

      Dielectric properties of the materials were obtained together using an impedance analyzer (Agilent 4396B, Agilent Technologies, America, HIOKI3532) by measuring the capacitance and phase angle of the specimens from room temperature to 150oC.

   3. Hot press method combines solution and melt processing techniques. PVDF was dissolved in DMF/Acetone solution and the solution is concentrated to get an optimum viscosity for the loading of ceramic powder. PZT powder was added and stirred well to get a uniform distribution of the filler. The prepared slurry of PZT/PVDF was then coagulated by the addition of nonsolvent and dried, the obtained powders were dried and pressed into disks specimens with a diameter of 12 mm and a thickness of 1.2 mm only by uniaxial hot-pressing

   at 140oC with a pressure of 100 Mpa. The composite samples were prepared for 0.3 to 0.7 weight fractions, Table 1.

   The silver pastes were fired at 120oC for 30 minutes on both sides of these sintered bulks as electrodes for electrical measurements.

   1. Crystalline phase analysis

      • Surface morphology

   (2)

   Samples		Composite compound ratio	(g/cm3)
   No.	Name of products
   1	F16	PVDF 16 wt% + 0wt% PZT	1.74
   2	F-P30	PVDF 16 wt% + 30wt% PZT	3.41
   3	F-P40	PVDF 16 wt% + 40wt% PZT	4.10
   4	F-P50	PVDF 16 wt% + 50wt% PZT	4.70
   5	F-P60	PVDF 16 wt% + 60wt% PZT	5.30
   6	F-P70	PVDF 16 wt% + 70wt% PZT	5.87
   7	PZT	PVDF 0 wt% + 100wt% PZT	7.63

   Samples		Composite compound ratio	(g/cm3)
   No.	Name of products
   1	F16	PVDF 16 wt% + 0wt% PZT	1.74
   2	F-P30	PVDF 16 wt% + 30wt% PZT	3.41
   3	F-P40	PVDF 16 wt% + 40wt% PZT	4.10
   4	F-P50	PVDF 16 wt% + 50wt% PZT	4.70
   5	F-P60	PVDF 16 wt% + 60wt% PZT	5.30
   6	F-P70	PVDF 16 wt% + 70wt% PZT	5.87
   7	PZT	PVDF 0 wt% + 100wt% PZT	7.63

   Table 1. Samples of PZT/PVDF

3. RESULTS AND DISCUSSION

   The microstructure of the 0-3 PZT/PVDF piezoelectric composite with 30 – 70 wt% PZT is shown in Fig. 3. It shows the typical characteristic of particle connection. Quite a few PVDFs exist by the shaping of PZT grains. Furthermore, the SEM image also shows that the PZT grains are wrapped by the PVDF, which indicates the connection characteristic of wrap and curl. The crevice is much less in the pellets of 0-3 PZT/PVDF piezoelectric composite materialprepared by hot- press process, the connection between the grains of piezoelectric composite is much closer, and the grain size is also bigger [1-4, 9, 15, 16, 28-34, 37, 38].

   A. Density and porosity

   The experimental density values of the composite were measured and the theoretical density values were calculated using the formula (1), Fig 2a

   (1)

   where x represents the volume fraction of the ceramic phase and c, 1 and 2 are the densities of the composite, polymer and ceramic respectively [6, 18, 19, 21].

   The densities of the composite samples were determined by weighing samples of known sizes accurately using a

   F16

   F-P50

   F-P30

   F-P60

   F-P40

   F-P70

   digital balance of accuracy 0.01 g resolution. Fig. 2 shows the variation in the experimental and calculated density values for the PVDF-PZT composites with the volume fraction of PZT. cal increases linearly with increasing volume fraction of PZT, since the density of PZT (7630 kg/m3) is higher than that of PVDF (1740 kg/m3). Obviously the experimental density exhibits a linear increase with increasing volume fraction of PZT and especially for the volume fraction from

   1. to 0.7, it stays unchanged with the theoretically calculated values.

      Figure 3. SEM images and grain size distribution for PZT/PVDF composites with different PZT content.

      Figure 4 shows the schematic illustrations of the microstructures for the PZT/PVDF composites prepared by hot-press process. It explains the influence of the shaping- processes on the morphologies of the PZT/PVDF. For the low PZT content, the PVDF exists in the grain boundary of the PZT particles, which hold much more voids and porosities and are detrimental for polarization of the

      Calculated Density Experimental Density

      Calculated Density Experimental Density

      (a)

      6,0k

      Density (kg/m3)

      Density (kg/m3)

      5,5k

      5,0k

      4,5k

      4,0k

      3,5k

      (b)

      Porosity (vol.%)

      Porosity (vol.%)

      4

      3

      2

      1

      0

      composites and the movement of ferroelectric domains. Accordingly, the electric properties are much lower.

      0,3 0,4 0,5 0,6 0,7

      Volume fraction of PZT

      0,3 0,4 0,5 0,6 0,7

      Volume fraction of PZT

      Figure 4. Schematic illustration of the microstructures for the PZT/PVDF

      Figure 2. (a) Experimental and calculated density, (b) Porosity of composites at different volume fractions of PZT.

      Furthermore the experimental density is smaller than calculated one at fixed volume fraction of the PZT, which may result from the presence of pores in composites. The porosity P, of composites can be calculated from the experimental and calculated density of composites using equation (2), Fig 2b.

      composites prepared hot-press process with an increase in the PZT content

      However, for the hot-press method with higher PZT content, the PZT grains are wrapped by the PVDF, indicating the connection characteristic of wrap and curl. The particles of PZT are much closer and the density of the composite is also higher, which are in favour of the polarization of the PZT/PVDF composites and will enhance the electric

      properties. Moreover, the formation of the -type PVDF are also important to the improvement of the electric properties.

      • XRAY Analysis

   molecules causing a steeper increase in the dielectric constant with temperature. A similar report has been presented by several authors [1, 2, 4, 6, 10, 13, 15-24, 28, 29, 33-38].

   Figure 5b shows the XRD patterns of the PZT nanopowders, 50 wt% PZT/PVDF composites and PVDF prepared by the

   (a)

   500

   Dielectric Constant

   Dielectric Constant

   F-70P

   300

   (b)

   F-70P

   hot-press method, respectively. The single and sharp intensity

   peaks indicate the formation of the perovskite phase of the

   400

   F-60P F-50P

   Dielectric Constant r

   Dielectric Constant r

   250

   F-60P F-50P

   PZT/PVDF composite. Moreover, the composite is free from the pyrochlore phase, which is considered unwanted in the PZT system. Compared with the XRD pattern of the PVDF, it also can be observed the presence of the peak of the PVDF between 17.3 – 20.7o. It is therefore presumed the formation

   300 F-40P

   F-30P

   200

   100

   0

   200

   150

   100

   50

   F-40P

   1. 0P

      of the -type crystalline, which is considered possesses of

      50 100 150 200 250

      Temperature (oC)

      0,0 200,0k 400,0k 600,0k 800,0k 1,0M

      Frequency (Hz)

      better electric properties [1, 3, 7, 14, 17, 19, 20, 23, 25, 28-

      30, 35, 39, 40].

      1. (b)

   Intensity (a.u.)

   Intensity (a.u.)

   (010)

   (010)

   (110)

   (110)

   (111)

   (200)

   (210)

   (211)

   (111)

   (200)

   (210)

   (211)

   (220)

   (220)

   PZT

   10 20 30 40 50 60 70

   2 (degree)

   Figure 5. (a) SEM photograph and XRD pattern of the PZT nanopowders

   Figure 7. (a) Plot between dielectric constant with temperature, (b) variation in dielectric constant with frequency of PZT/PVDF composite

   • Ferroelectric properties

   A series of PE hysteresis loops for PZT/PVDF composite measured at room temperature were shown in Fig. 8. The results showed that the composites with PZT volume fraction

   1. to 0.7 exhibited non-saturated PE hysteresis loop. It seems that the applied field of 30kV/cm was the main cause for the observed non-saturated hysteresis behavior in this study. It is also be noted that when increasing the PZT content to beyond 0.7 volume fraction, values of remnant polarization and coercive field increases as shown in table 2.

      10

      calcined at 1050oC for 2 h, (b) XRD patterns of PZT ceramic, PZT/PVDF composites and PVDF powders.

      C

      C

      C. The electrical properties of PZT/PVDF composites

      1. (b)

         P(C/cm2)

         P(C/cm2)

         5

         0

         -40 -20 0 20 40

         (c)

         • Dielectric constant

      18000	PZT 337oC	350 Furukawa Model
      15000		300 Hot Press Method
      12000		250
      9000		200
      6000 3000 0		150 100 50

      18000	PZT 337oC	350 Furukawa Model
      15000		300 Hot Press Method
      12000		250
      9000		200
      6000 3000 0		150 100 50

      (a)

      Permittivity,

      Permittivity,

      r

      r

      400

      (b)

      (d)

      -5

      -10

      E (kV/cm)

      1. (f)

         Dielectricconstant r

         Dielectricconstant r

         0 100 200 300 400

         Temperature (oC)

         0,3 0,4 0,5 0,6 0,7

         Volume fraction of PZT

         Figure 8. The PE hysteresis loops for F16(a) and F16-x%PZT composites

         with different concentrations of PZT, x= 30 (b), 40 (c), 50 (d), 60 (e), 70 (f) wt%

         Figure 6. (a) The temperature dependence of dielectric constant measured at 1 kHz for the PZT ceramics.(b) Influence of PZT content on the dielectric

         Table 2. Ferroelectric properties of the PZT/PVDF composites

         compound

         ratio

         	Samples	Composite	Remnant	Coercive field
         No.	Name of products	Polariation (Pr) µC/cm2	(EC) kV/cm
         1	F16	0	2.1	9.3
         2	F-P30	30	5.1	5.2
         3	F-P40	40	5.4	5.1
         4	F-P50	50	6.3	4.9
         5	F-P60	60	7.7	5.0
         6	F-P70	70	8.2	5.0

         	Samples	Composite	Remnant	Coercive field
         No.	Name of products	compound ratio	Polariation (Pr) µC/cm2	(EC) kV/cm
         1	F16	0	2.1	9.3
         2	F-P30	30	5.1	5.2
         3	F-P40	40	5.4	5.1
         4	F-P50	50	6.3	4.9
         5	F-P60	60	7.7	5.0
         6	F-P70	70	8.2	5.0

         of the PZT/PVDF composites.

         Variation of dielectric constant with temperature for various volume fractions of the composite poled at 30 kV/cm is shown in Fig. 6b. A steady dielectric constant values are observed in case of volume fraction from 0.3 to 0.7 till 140 oC and a steeper change is observed for volume fractions 0.6 and 0.7. Hence it is observed that the rate of variation of dielectric constant with temperature steadily increases with the volume fraction of PZT. This may be due to the fact that the poled PZT particles generate an internal field which favors the orientation of the PVDF molecules. At higher temperatures the molecules are more mobile and a higher concentration of PZT dipoles enables easier orientation of the

         The results thus suggested that distribution of PZT granules in PVDF matrix played a significant role in controlling of ferroelectric behavior [5, 7, 15-20, 25, 35].

         • Piezoelectric measurements

      An attempt has been made to plot the predicted and experimentally observed variation in d33 values with the volume fraction of PZT and it is illustrated in Fig. 9a. The d33 values were calculated by using the equation (3) which is given by the Furukawa model [2, 9, 11-13, 18-23, 32-

      34, 36].

      (3)

      The values of piezoelectric strain coefficient, voltage coefficient and ferroelectric behavior obtained in this work revealed that it may be more suitable for piezoelectric devices, sensors, force transducer, and so on with technological applications [8, 22, 26, 27, 30, 32, 36, 39, 40].

      ACKNOWLEDGMENT

      This work was carried out in the framework of the National Project in Physics Program until 2020 under No. ÐTÐLCN.10/18.

      Thank you also for the technical support by Faculty of Electrical, & Electronics and Materials Technology (Hue University of Science, Hue, Vietnam) and Institute of Research and Development (Duy Tan University, Danang,

      Furukawa Model

      Hot Press Method

      Furukawa Model

      Hot Press Method

      (a)

      80

      9,3k

      (b)

      103.9

      -80

      Vietnam).

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      d33 (pC/N)

      d33 (pC/N)

      Impedance(Ohm)

      Impedance(Ohm)

      60 9,0k

      40

      20 8,7k

      100.8

      -82

      Phase angle(o)

      Phase angle(o)

      -84

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4. CONCLUSION

In this study, PZT-PVDF composites with 03 connectivity was successfully fabricated from a series of PZT volume fractions from 0.3 to 0.7 by hot press method. The heat treatment method can obviously enhance the content of tetragonal phase crystalline in piezoelectric ceramics granular, decrease the defect between ceramic phase and polymer phase and reduce the porosity. Experimental density values of the composites were found to be harmonized with calculated density values. SEM observations revealed a homogeneous mixture of PZT-PVDF phases.The dielectric constant values obtained in this work were found to be analogous to Furukawa model for the PZT volume fraction up to 0.7. The piezoelectric strain coefficient has been incremented with a raise in volume fractions of PZT but the Furukawa model fails to consider the experimental variations at higher volume fractions.

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