Sawyer Tower method

The Sawyer Tower measurement circuit is based on a charge measurement method which relies on a reference capacitor in series with the ferroelectric capacitor [2], The voltage drop 

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The spontaneous polarization of FLCs can be measured by the pyroelectric technique using the temperature dependence of the pyroelectric coefficient 7 = dPsfdT [27], and by the conventional capacity Sawyer-Tower method [28] or by integrating the time dependence of the repolarization current ip (5 is the electrode area) [175, 176]... 

With respect to the equivalent circuit in Figure 3.3, an evaluation of the known methods for hysteresis measurements will be given, in view of the effective parasitic capacitance and the influence of reflection. Well known methods to record the hysteresis loop of ferroelectric capacitors by measuring the current response are Sawyer Tower, Virtual Ground, and Shunt measurement as shown in Figure 3.4. 

Figure 3.4 Schematic comparison of (a) Sawyer Tower, (b) Virtual Ground, and (c) Shunt hysteresis measurement method.

The Sawyer-Tower circuit, which is the standard method for observing polarization as a function of applied field in solid ferroelectrics, was modified slightly as shown in Figure 3 to adapt it better to the properties... 

There are several methods for measurement of pyroelectric coefficients, but most can be grouped into two kinds. The objective in the first case is to directly measure the pyroelectric current and at the same time change the temperature. In the second method measurements of polarization or charge can be estimated either by the integration of pyroelectric current during the continuous heating of the sample or recording the hysteresis loop at various temperatures by the Sawyer-Tower technique. 

Sawyer-Tower Bridge Method. As we mentioned in earlier, a ferroelectric material must have a P-E hysteresis loop. Usually, P-E loop may be observed by using a Sawyer-Tower bridge (Fig. 22-7) or a modified Sawyer-Tower bridge (Johnson, 2002). For the measurement, a ferroelectric thin filmsanple was put into a sandwich structure, i.e., upper electrode/thin film/bottom electrode. The bottom electrode is a conductor (for example, a Pt-film deposited on a silicon substrate), on which a ferroelectric film having thickness... 

Usually, when ferroelectric thin film samples formed as metal-electrode/film/metal-electrode sandwich structures, we can observe a symmetric P-E loop. The ferroelectric P-E hysteresis loop of PZT thin film samples fabricated by the sol-gel method were measiued by using a modified Sawyer-Tower bridge. The PZT films were coated on titanium foil-substrates and were heat-treated at 700°C and 400°C, respectively. The samples were prepared as a sandwich structure of metal electrode (sputtered Au)/PZT film/titanimn-electrode (substrate). The observed P-Eloops ofthese samples are shown in Figure22-8(a) and (b). Similarly, a parallel study was conducted for crystalline and amorphous BaTiOs thin films fabricated by the sol-gel method. A P-E hysteresis loop was observed on the amorphous BaTiOs thin film (Xu and Mackenzie, 1994). 

A convenient method for determining the phase transition temperatures is to measure the relative permittivity k as a function of temperature, using a capacitance bridge. The ratio of the capacitance of an electrical condenser with and without the dielectric between the condenser plates gives the permittivity. The value of k approaches a maximum at the transition temperature, where it changes discontinu-ously. Hysteresis loops, spontaneous polarization, and coercive fields are obtained with a Sawyer-Tower circuit and displayed with an oscilloscope. 

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