Low field ac study of PZT/PVDF nano composites ...

Low field ac study of PZT/PVDF nano composites

Sara Aftab, D. A. Hall, M. A. Aleem & M. Siddiq

Journal of Materials Science: Materials in Electronics ISSN 0957-4522 J Mater Sci: Mater Electron DOI 10.1007/s10854-012-0861-z

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Author's personal copy J Mater Sci: Mater Electron DOI 10.1007/s10854-012-0861-z

Low field ac study of PZT/PVDF nano composites Sara Aftab • D. A. Hall • M. A. Aleem M. Siddiq

•

Received: 14 May 2012 / Accepted: 3 August 2012 Ó Springer Science+Business Media, LLC 2012

Abstract Composites of nanocrystalline Pb0.96Sr0.04 (Zr0.53,Ti0.47)O3 (PZT) and a-phase PVDF have been developed using solution casting technique. Characterization of the composites has been done using XRD, FEGSEM, DSC and impedance analysis. XRD and FEGSEM determined the size range of PZT as 22–40 nm. XRD shows the successful incorporation of PZT into PVDF matrix and also confirms that no new phase is developed. DSC of the nanocomposites showed decrease in crystallinity with increasing PZT content. Broadband impedance analysis has been carried out to study the effect of the addition of PZT on the low field ac electrical properties of PVDF. Room temperature dielectric permittivity measurement of the PZT-PVDF composites at 1 kHz determined using impedance analyzer gives values of permittivity 2–4 times higher as compared to neat PVDF. It is found that dielectric permittivity values at the lower frequency edge are affected by space charges while the higher frequencies show the influence of relaxation effects in the materials. It is suggested that PZT/PVDF composites are the preferred materials for high temperature and high frequency applications. However, for low frequency use at higher temperatures, these composites do not offer any specific advantage. At room temperature, the composites are again the better choice in the 1 mHz–1 MHz frequency range.

S. Aftab D. A. Hall School of Materials, Materials Science Center, Manchester University, Grosvenor Street, Manchester M1 7HS, UK S. Aftab M. Siddiq (&) Department of Chemistry, Quaid-e-Azam University, Islamabad 45320, Pakistan e-mail: [email protected] M. A. Aleem PIEAS, P.O. Nilore, Islamabad, Pakistan

1 Introduction Capacitors for integrated electronic devices require high dielectric permittivity materials. Ceramics such as lead zirconate titanate (PZT) and barium titanate (BT) have high dielectric permittivity and are, therefore, the most appropriate choice for capacitors; but because of high manufacturing temperatures these are costly to process. Also, their high density, hardness and stiffness make them unsuitable for use in certain applications e.g. piezo film sensors, piezo electric cables, ultrasonic imaging devices etc. Polymers are well known for their low density, flexibility, high break down strength and ease of processing as well as low processing costs. PVDF (poly vinylidene fluoride) is a semi crystalline polymer and has a dielectric permittivity higher than other polymers and is, thus, preferred over other polymers for dielectric applications. Ceramic polymer composites have been engineered to obtain the combined properties of both constituents’ phases. A great deal of research has been carried out on PZT/ PVDF composites [1–8] because of the dielectric, ferroelectric, piezoelectric and pyroelectric properties of both PZT and PVDF. The simplest composite has a 0–3 connectivity pattern [9]. Wei et al. [10] investigated the frequency dependent behavior of 0–3 PZT/PVDF composites using micron sized PZT particles and reported a broad peak at 1 kHz which they attributed to the a-relaxation of the polymer. Sinha et al. [11] studied the dc conductivity mechanism of PZT/PVDF composites. In a separate work [12], they determined the conductivity behavior of the same composites and correlated their results to IR and hysteresis studies, and attributed the major contribution of conductivity to the polymer phase. Thonsanitgarn et al. [13] carried out research on the electrical and mechanical properties of PZT/PVDF composites. They reported an

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Author's personal copy J Mater Sci: Mater Electron

increase in the dielectric permittivity and tand of composites having higher ceramic volume fraction and also observed a jump in ferroelectric properties at 0.9 volume fraction composite. Tripathi et al. [14] reported an increase in the conductivity of the composites as the amount of PZT increased. With the advent of nanotechnology, the use of nanocrystalline PZT in composites has added a new dimension to this area. Bozena et al. [15] reported on the dielectric response of the nano PZT/PVDF composites over a wide range of frequency and temperature and also discussed the effect of glass transition temperature and dielectric relaxations of the polymer on the dielectric properties of the polymer. Zak et al. [16] prepared PZT/PVDF nanocomposites thin films by spin coating technique onto a glass substrate and determined its frequency dependence over a wide range (100 Hz–30 MHz) at room temperature. Zak also employed various theoretical approaches to predict and compare the dielectric permittivity of their films. Suresh et al. [17] studied the high field properties of spin coated nanocomposite films of PZT/PVDF. Mendes et al. [18] determined the effect of ceramic grain size and concentration on the dielectric behavior of PZT/PVDF composites with PVDF in a and b phase. It was submitted that the dielectric properties are mainly affected by the amount of the ceramic particles and concluded that the temperature dependence of dielectric response is a result of low temperature relaxation and high temperature conductivity. Gregorio et al. [19] studied the dielectric behavior of thin films of b phase PZT/PVDF composites in the frequency range 100 Hz–13 MHz and applied the Yamada model to their results. The temperature dependence of the composites and electrical properties were, however, not discussed. Detailed investigation into the electrical behavior of PZT/PVDF nanocomposites in a low ac field for a broad frequency range at different temperatures has not been reported so far. Impedance analysis enables investigation of properties over a broad frequency range. It also includes the separation of the real and imaginary components of electrical parameters so that the true nature of the material can be ascertained. The dielectric permittivity of PZT may be raised and its dielectric loss lowered by doping with appropriate dopants [20] .In this study Sr-doped PZT has been used as the filler in the composites. The interpretation of the low field electrical properties of 0–3 PZT/PVDF nanocomposites has thus been done by determining the effect of both frequency and temperature on the dielectric parameters. The variation in capacitance, impedance and conductivity with changing frequency (1 mHz–1 MHz) and temperature (room temperature to 160 °C) has also been investigated to ascertain the nature of the conductive mechanisms in the composites. The ultimate aim of this study is to give a thorough understanding of the electrical behavior of such composites and to

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determine appropriate frequencies and temperatures for their use as capacitors in microelectronics. 2 Experimental Composite samples were prepared by solution casting followed by hot compression at a load of 25 MPa. Nanocrystalline Sr-doped PZT was prepared by the sol gel method [21]. The composite films obtained were ground finely and the powders spread evenly in a die mould for hot compression. The hot pressed disks were 200–300 microns thick. Square samples 9 9 9 mm2 were punched for testing. Composite samples of different volume fractions (0.20, 0.30, 0.40 and 0.50) were prepared in this way. Thermal study of the samples was carried out from -100 to 200 °C under an Argon atmosphere using a heating/ cooling rate of 10 °C in a Netschz STA 409 analyzer. DSC/ TG is used to investigate phase changes, percentage crystallinity, glass transition (Tg) and material stability. The principle of the technique is the measurement of the exothermicity and endothermicity accompanying the physical or chemical processes in a specified atmosphere [22]. Structural and phase analysis was done at room temperature by XRD (JEOL JDX-9C) equipped with reflection geometry and affixed anode X-ray generator of Cu Ka radiation. Cryofractured samples were gold sputtered to avoid charging before investigating the morphology and homogeneity of the composites in a FEGSEM (XL 30 PHILIPS). Samples for electrical measurements were electroded with Ag paint (thickness *200 microns) to ensure good electrical contact. Broadband impedance measurements were carried out in the frequency range 1 mHz–1 MHz using a Solartron 1260 frequency response analyzer at temperatures of 20, 80 and 160 °C. The electrical impedance of a piezoelectric material can be written as Z = ZR ? jZC where ZR = R and ZC = 1/xC. Resistance and reactance mutually contribute to the magnitude and phase of impedance through the following relations: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z ¼ ZR2 þ ZC2 1 ZC u ¼ tan ZR u = 90° for an ideal capacitor. Also, we know the expression for admittance: Y ¼ Y 0 þ jY 00 ¼ G þ jB Y¼

1 Z

where G and B are conductance and susceptance, respectively. By calculating conductance (G) and


capacitance (C) using G = 1/ZR and C = 1/xZC respectively, the dielectric permittivities (e0r and e00r ) can be calculated using the following expressions: e0r ¼

Cl Aeo

e00r ¼

Gl Axeo

The relative permittivity and ac conductivities are obtained as follows: er ðxÞ ¼ e0r ðxÞ þ je00r ðxÞ rac ¼ xeo e00o where A/l refers to sample dimensions, x is the angular frequency and eo is the permittivity of free space (eo = 8.85 9 10-11 Fm-1).

3 Results and discussion Nomenclature used for the samples is A2, A3, A4 and A5 where suffices indicate the PZT volume fraction 0.2, 0.3, 0.4 and 0.5, respectively. The XRD pattern for the neat polymer shows the presence of predominant a phase of PVDF. The addition of PZT into PVDF causes the appearance of major peaks at 21.66°, 30.9°, 38.12°, 44.38° and 55.14° as shown in Fig. 1a. The PVDF peak at 19.86° cannot be seen since it is masked by PZT. The particle size of Sr-doped PZT determined from XRD using the Scherrer equation is 22 nm. HRSEM images of the powder suggests a size range of 30–40 nm as shown in Fig. 2. The morphology of the PZT/PVDF composites was determined by HRSEM. The presence of the two phases may readily be discerned from the micrographs shown in Fig. 3. The micrographs also show the dispersion of the ceramic phase in the polymer matrix for two compositions (A2 and A5). The dispersion at higher volume fraction (A5) is to an extent heterogeneous wherein PZT aggregates are readily seen. Firstly, this may be due to the lesser volume available to the ceramic particles and secondly, as a result of inadequate mixing at the higher volume fractions so that some of the ceramic particles tend to agglomerate with a possible decrease of the 0–3 connectivity in the composite [23]. Figure 4 shows three isothermal scans of log |Z| versus frequency plots for the neat polymer and the composite samples. |Z| falls with rise in frequency at the two lower temperatures. The trend at both isotherms is the same; the results for measurements carried out at high temperature (160 °C) are however, different. At the lower frequency, the lowest |Z| is manifested by the neat polymer and the

Fig. 1 XRD scan showing a a-phase of neat PVDF, b incorporation of PZT into PVDF in nanocomposite A2

Fig. 2 FEGSEM image of nanocrystalline PZT

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Fig. 3 FEGSEM images of PZT/PVDF composites. a Even dispersion of PZT in PVDF is observed in A2. b Microstructure of A5 shows the relatively lesser homogeneity of A5, also seen are the aggregates of PZT particles

highest by A5, but at higher frequencies this trend is reversed because the role of increased space charges due to increased interfaces vanishes and the composite samples are then influenced solely by the intrinsic nature of PZT. All composite samples then follow an order consistent to the ceramic volume fractions. Also, at the lower frequency, the |Z| of the polymer decreases from 10 ohms at room temperature to 7 ohms at 160 °C. The |Z| of the composites also decreases with the rise in temperature but to a smaller extent. The greater loss in |Z| may again be attributed to the greater number of space charges in neat PVDF. Figure 5 depicts the capacitive trends at three temperatures in terms of phi (u) versus frequency. At lower frequencies (less than 1 kHz), the value for the polymer is the maximum at all temperatures thus indicating the least capacitive nature of the polymer. With an increase in temperature, the spread in the values increases so that the u values for all samples merge at a relatively higher frequency. So that, at 160 °C, the frequency at which the capacitance of all samples merges is shifted to 1 MHz .This may be explained in terms of the effect of temperature on the dipoles in the samples. At high temperatures and frequencies, the dipoles lose their orientation which thus

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Fig. 4 Plots showing the dependence of |Z| on frequency at a room temperature, b 80 °C and c 160 °C

results in a lower capacity to withhold charge. Also, the difference in u values between the polymer and composite samples increases. The initial sharp decrease in capacitance as a result of mobile space charges is followed by a relatively constant capacitance region which shows the maximum capacitive nature of the composites in direct relationship to the PZT amounts. This dependence of the capacitive nature of the composites on PZT may be

Author's personal copy J Mater Sci: Mater Electron Table 1 Comparison of e0r of PVDF and composite samples measured with an LCR meter and impedance analyzer Sample

PVDF

A2

A3

A4

A5

LCR

9.2

18.1

23.0

30.7

40.3

Impedance

7.3

17.2

22.7

27.2

37.4

Fig. 6 Effect of increasing temperature on e0r of the neat polymer and composite samples at 1 kHz

Fig. 5 Plots of u versus frequency at a room temperature, b 80 °C and c 160 °C

because of an increase in the number of capacitors formed as a result of PZT/PVDF [24]. Table 1 presents the room temperature e0r for the neat polymer and the composite samples measured using the LCR meter at 1 kHz. The average e0r of the samples measured under the same conditions using impedance analyzer is also listed alongside for comparison purposes.

A plot of e0r versus temperature at constant frequency (Fig. 6), shows that the e0r of all samples increases with an increase in the temperature. Furthermore, the e0r value of the polymer increases from 7.3 to 22.7 for A3 at 1 kHz (see impedance data in Table 1). Similar results are reported by Chanmal et al. [25]. Plots of dielectric loss e00r versus temperature reveal several distinct relaxation processes as shown in Fig. 7. As the temperature rises, the emergence of various types of molecular mobilities results in the orientation of their dipoles. A detailed study of such relaxations requires investigation of the dependence of e0r and e00r on frequency at constant temperature. Figure 8 shows e00r versus frequency temperature plots of neat PVDF and A2 at three different frequencies. T J Moon [26] has reported the observation of three relaxations in the dielectric spectra of PVDF measured at 1 kHz. These relaxations designated as a, b and c were observed near 80, -20 and -80 °C, respectively. The a relaxation is related to the crystalline regions of the polymer. The change in trend of e00r at all frequencies close to this temperature, in both samples is evident in the plots and indicates the onset of the a relaxation peak. The frequency and temperature range of the equipment used pose constraints on the full observation of these peaks. However, the presence of these relaxations is clearly seen in A2 and it is also obvious that the magnitude of the peak is the maximum at 1 kHz, and decreases with the rise in frequency. It is also seen that at this frequency,

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Fig. 7 Plots of e00r versus temperature at three different frequencies. a Neat PVDF, b A2

the e00r for A2 is about 40 % lower than for the neat polymer suggesting that the observed loss peaks have their origin in the crystalline regions of the polymer. Furthermore, a relaxation peaks in A2 appear at a relatively lower temperature as compared to neat PVDF. This implies that the onset of mobility in the polymer main chain occurs at a higher temperature which is attributed to the greater crystallinity of the neat polymer [27]. This is also supported by the percentage crystallinity obtained from DSC study, according to which the amorphous nature of the polymer increases with increasing amounts of PZT as shown in Table 2. The dispersion at the lower frequency edge is a result of the space charges, while the variation at the high frequency side is because of relaxation effects. The initial shoulders of the peak in Fig. 8a, at the higher frequency edge indicate a range of relaxation frequencies. This shifting of e00r towards the higher frequency side with rise in temperature also indicates the presence of some sort of temperature dependent relaxation phenomenon [28]. The variation in loss with temperature is greater in the neat polymer for all isotherms. Figure 9 shows the frequency dependence of e0r at three temperatures for neat PVDF and A2. It is obvious from the

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Fig. 8 Isotherms of e00r versus frequency at three different temperatures. a Neat PVDF, b nanocomposite A2 Table 2 Melting temperature (Tm), heat of fusion (DH) and %age crystallinity (crystalline contents) of PVDF and composites using DSC analysis Melting temperature Tm (°C)

Heat of fusion DH (J/g)

%Age crystallinity

PVDF

158.4

-26.78

30.43

A2

158.5

-10.86

13.303

A3

156.4

-8.70

8.872

A4

156.1

-6.94

6.070

A5

155.9

-2.73

3.198

Sample

plots that the e0r of both samples decreases with an increase in frequency at all temperatures. However, the effect is more dominant at 160 °C and is negligible at room temperature. It is also observed that the e0r for all temperature becomes frequency independent at approximately 10 kHz for neat PVDF and 100 kHz for composite sample A2. This behavior is readily understood in terms of the inability of the dipoles to maintain their orientation at high temperatures, as well as to align more dipoles in a rapidly changing electric field. The nature of ac conductivity r0 in neat PVDF and the composite samples has been investigated by measurement


Fig. 9 Frequency dependent isotherms of e0r at three different temperatures for a neat PVDF and b nanocomposite A2

of its frequency and temperature dependence. Figure 10a shows the frequency dependence of r0 at room temperature. It is apparent that r0 increases with rise in frequency for all samples. The odd behavior of A5 may be attributed to its heterogeneous nature as already discussed. This variation in r0 with frequency is more obvious in the three isotherms for A2 (see Fig. 10b). The behavior of the conductivity r0 of the polymer and the composite samples is further investigated by determining its Arrhenius dependence on temperature (see Fig. 11). A plot of log r0 versus 1/T for A2 shows an overall non-linear behavior. A sharp decrease in r0 at approximately 110 °C results an inflection point separating two conductive mechanisms in the sample. The Ea values determined from the slopes are 0.02 and 0.122 eV, for temperatures above and below 110 °C, respectively. Thus, Ea in the low temperature region is higher than that at higher temperatures, where conductivity attains asymptotic values. It may readily be inferred that the conductive processes at higher temperatures are more viable than those having the higher Ea. The approximate temperature at which the change over in the conductive nature is observed may correspond to some structural changes in the composites.

Fig. 10 a Isotherms for r0 as a function of frequency for A2. b Frequency dependence of r0 for neat PVDF and all nanocomposites at 80 °C

Fig. 11 Arrhenius plot for A2 showing the different temperature dependence of r0 in the low temperature and high temperature zones

4 Conclusions Nanocomposites of Sr-doped PZT with PVDF (a phase) have been developed and their low field ac properties studied. According to XRD and FEGSEM the size of the nanocrystalline PZT is between 22 and 40 nm. FEGSEM also depicts the homogenous distribution of nanocrystalline PZT in PVDF for low volume fraction composites (up to

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30 vol% PZT). The e0r increases with addition of PZT and is 2–4 times higher than the neat PVDF. e00r falls with increasing frequency, the higher values at the lower frequency edge are attributed to the presence of space charges. The temperature dependence reveals the presence of two separate conductive mechanisms in the samples. Space charges also affect the conductivity measurements at low frequencies while relaxation effects in the samples are responsible for the variations at higher frequencies. The a relaxation frequency is affected by the volume fractions as well as the variation in frequencies. This relaxation is attributed to the crystalline phase of the polymer since DSC reflects a decrease in the crystallinity of the composite samples with increasing amounts of PZT. For capacitor applications at higher temperatures (80 °C and above) and frequencies greater than 1 kHz, composite samples are the perfect choice because of their higher capacitance. For room temperature applications of capacitors in the frequency range 10 Hz–100 kHz, PZT/PVDF nanocomposites are again the preferred materials.

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