Determination of Piezoelectric and Pyroelectric Coefficients and Thermal Diffusivity of 1-3 PZT/Epoxy Composites Y.Phermpornsakul,S.Muensit Material Physics Research Unit Department of Physics, Prince of Songkla university I.L.Guy Department of Physics Macquarie University

Abstract  PZT/epoxy composites with 1-3 connectivity were prepared using the dice-and-fill technique.  The samples were poled with an electric field of 10 MV/m for 30minutes at room temperature.  From the laser interferometric measurements, Piezoelectric d 33 coefficients were independent of the volume fraction and averaged (190±10)pm/V.  The Pyroelectric coefficient increased with the ceramic content and reached values as large as 54μC/m 2 ℃.  The thermal diffusivity was determined using a technique based on the measurement of the phase retardation of a thermal wave passing through the material. The average was (2.15±0.05)×10 -7 m 2 /s.

1. Introduction  For 1-3 type PZT/epoxy, both the piezoelectric and pyroelectric activities depend on the polarization of the ceramic phase.  Inhomogeneous displacement profiles under an electric field for a single rod of ceramic embedded in an epoxy matrix have been analyzed and the effective piezoelectric coefficients derived  Piezoelectric coefficient (d) = S: strain, E: electric field  A pyroelectric material exhibits a spontaneous polarization in the absence of an electric field.  A variety of new pyroelectric materials can be made by combining a ferroelectric ceramic with a passive polymer phase.  By selection of the ceramic, polymer, proportions, and spatial scales, these material properties can be tailored to specific device requirements such as in infrared sensors and thermal imaging devices.  Pyroelectric coefficient (p) =  p can be measured either by measuring the current generated for a known rate of change of temperature, or by measure the charge generated for a known temperature change. Q: charge, T: temperature A: electrode area perpendicular to polar axis

2. Sample Preparation 1  PZT (supplied by Ultra-sonic Powders Ltd.., type PKI 502)  Epoxy (Araldite type, LY51382/Hy5138)  PZT: diameter 10.5mm disk,Thickness:1.5mm, sintered at 1285 ℃  The disks diced with a diamond saw to create a set of groove which were then the filled with the epoxy. (dice-and-fill technique)  Each diced ceramic disk was placed in glass petri dish in a low pressure chamber.  A solution of the epoxy was poured over the disk.( left 15min&50~60 ℃ at 30 min)  The second set of grooves was cut normal to the first and also filled with the epoxy  The ceramic content in the epoxy was varied as shown in table 1  The final finishing of the composites to the desired thickness was done by polishing using abrasive paper of various grades.  check the separation of the ceramic rods  SEM, electron micrographs (fig 1)  To enhance the piezoelectric and pyroelectric activities, each sample had electrodes put on the top and bottom and poled with an electric field of 10MV/m at room temperature for 15 minutes

2. Sample Preparation 2. Figure 1. SEM micrograph ( X 60) for 1-3 PZT/epoxy composite with ceramic volume fraction a, 0.4 ; b, 0.6, Volume fraction of ceramicCeramic width (mm)Epoxy width (mm) Table 1. Ceramic and polymer parameters used in the composite fabrication

3. Piezoelectric Measurement 1  The piezoelectric d 33 coefficients was measured using a single-beam Michelson interferometer  A sinusoidal driving voltage was applied to the sample to produce a change in sample thickness, which in turn resulted in a change in the light intensity of the interference pattern at the detector  The component of the detector voltage related to the displacement of the sample surface was measured by a SR530 Lock-in Amplifier.  For a small displacement d ac of the sample surface, the piezoelectric coefficient d ii, d ii =d ac /V V: driving voltage. (compensator) (Beam Splitter) (Fixed Mirror) (Movable Mirror) Uinphase 1135P He-Ne Laser Probe beam Sample’s surface (made reflective By a moralized coating) Reference beam Reference mirror (mounted on a piezo- electric transducer) Pin photodiode detector

3. Piezoelectric Measurement 2  All of the responses showed a linear relation  d 33 (freq 1-5kHz): PZT(360±10) pm/V, a (0.4): 190 pm/V, b(0.6):188 pm/V -1~5kHz: avoid problems arising from heat generation -This difference may be due to incomplete poling of PZT or to heat accumulation in epoxy  The dielectric constants were measured using an HP 4268B LCR meter and HP 16451B Dielectric Test Fixture.  increased linearly with increasing ceramic content  The piezoelectric g 33 =d 33 /ε 0 ε 3  a(0.4): 43×10 -3 m 2 /C, PZT: 22×10 -3 m 2 /C Fig2. Piezoelectric responses for PZT(PKI 502) Observed at a driving signal of 1kHz Fig 3 Piezoelectric responses at 1kHz for 1-3 PZT/epoxy with ceramic volume fraction. A, 0.4; b, 0.6.

4. Pyroelectric Measurement 1  At a reduced pressure of Pa in order to minimize the humidity  Sample temperature was varied using a Peltier element  Heating and cooling were achieved by varying the magnitude and direction of the current passing through the Peltier elements.  Temperature was measured with a Pt-100 resistance thermometer and converted into an equivalent voltage by mean of a linearized RTD module Fig 4. Schematic diagram of the pyroelectric measurement system Fig 5. Typical temperature pattern upon heating and cooling the composites with different current applied to the Peltier elements

4. Pyroelectric Measurement 2  To ensure uniform temperature in the sample, vary the temperature slowly  To check the performance of the system, measurements were made on a piece of commercial LiNbO 3 wafer  Charge variation upon heating & cooling was measured by an electrometer  The slope of the polarization-temperature plot gave a pyroelectric coefficient of 115 μC/m2, slightly higher than that reported in the literature.  The difference could be due to the better quality of material used  a(0.4) : 44 μC/m 2 ℃, b(0.6): 74 μC/m 2 ℃ PZT: 74 μC/m 2 ℃

5. Thermal Diffusivity Measurement 1  A method has been developed by Muensit and Lang.  The sample was attached by a very thin layer of thermally conductive glue to a pyroelectric detector, in this case, LiTaO 3.  A Lisa HL25 diode laser was used and the laser beam was modulated at a number of different frequencies and allowed to impinge on a blackened area on the top surface of the sample.  The computer package (Mathematica) was used to find an analytic solution to the one -dimensional heat conduction equation with appropriate boundary conditions.  The unknown thermal diffusivity of the sample was found by fitting the experimental frequency - phase lag data to theory by a nonlinear regression algorithm. Fig 6 Simulation showing the attenuation and phase lag of the thermal waves for the composite with ceramic volume fraction 0.4.

5. Thermal Diffusivity Measurement 2  Lag between the phase of modulated laser beam and that of the pyroelectric current from the LiTaO 3 was measured for different modulation frequencies in the range of 1-10Hz  To calculate, it know the physical properties such as the dimensions, density and heat capacity of the sample  Heat capacity: using a Differential Scanning Calorimeter  Experimental phase lag and pyroelectric current data : fig 7  a(0.4):2.2×10 -7 m 2 /s b(0.6):2.1×10 -7 m 2 /s Fig 7 Variations of phase lag and amplitude of the pyroelectric current measured on the composite with ceramic volume fraction 0.4.a, phase lag ;b, amplitude

6. Discussion and Conclusion 1  Table2. Results for the 1-3 PZT/epoxy composites  Density and heat capacities correspond to volumetric averages  Thermal diffusivity can be modeled as polymer and ceramic elements in parallel  d 33 value will be higher than that predicted from a volumetric average  elastic compliance of the polymer is higher than that of the ceramic  d 31 (not measured) would have a smaller absolute value than predicted from a volumetric average.  This would increase the pyroelectric coefficient because of a larger contribution due to piezoelectricity (secondary pyroelectric effect) Composites Density 10 3 (kg/m 3 ) Dielectric constant (1kHz) Heat capacity (J/kg ℃ ) d 33 ( m/V) g 33 (10 -3 m 2 /C) p (μC/m 2 ℃ ) α (10 -7 m 2 /s) 0.0(epoxy) (PZT)

6. Discussion and Conclusion 2  g 33 value increased considerably with decreasing volume fraction of PZT  a large decrease in the permittivity  Qualitative differences between measurement and theoretical models may be due to incomplete poling of the composites or heat accumulation in the epoxy  The composites were mechanically flexible  The relatively high piezoelectric and pyroelectric coefficients combined with the flexibility  piezoelectric transducers and pyroelectric detectors.  However,it is important that the structural designs of the transducers or detectors using the composites should take into account the dissipation of the heat generated by either an ac field or sharp temperature changes.