Sawyer-Tower circuit

To characterize ferroelectric materials usually the dependence of the polarization on the applied voltage is measured by means of a Sawyer-Tower circuit or by recording the current response to a voltage step. The / (V/)-hys(crcsis curve is used to determine the remanent polarization and coercive voltage, respectively coercive field. These two parameters are of critical importance to the design of external circuits of FeRAMs. 

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... 

Figure 3. Modified Sawyer-Tower circuit for hysteresis loop measurement.

Another distinguishing feature of ferroelectric behavior is the polarization versus electric field P—B) hysteresis loop. The hysteresis loop results from the domain reorientation which occurs as the electric field direction is varied. The size and shape of the loop is determined by the magnitude of the dipole moment of the unit cell and the domain-switching characteristics of the material. Hysteresis loop behavior is measured using either a Sawyer—Tower circuit or a Diamant—Pepinsky bridge. Details of the construction and operation of a Sawyer—Tower circuit are given in Reference 24. Thin film properties have also been measmed with these two devices, and in addition, a commercially available measurement system has been widely used. ... 

A Sawyer-Tower circuit (Sawyer and Tower 1930), with a 9.8/jF series capacitance, was used to measure the ferroelectric hysteresis at room temperature. Figures 3 and 4 respectively show the electric displacement of PLZT (9.5/65/35) and PLZT (9.0/65/35) ceramics as a function of a dc bias electric field. The field was first increased from zero to +1.7 MV.m-i, back down to -1.7 MV.m-i, and finally up to zero. This cycle lasted 50 seconds and was repeated 3 consecutive times. Typical relaxor ferroelectric hysteretic curves were observed for these two compositions. 

Qiu X, Hollander L, Wirges W, Gerhard R, Basso HC (2013) Direct hysteresis measurements on ferroelectret films by means of a modified Sawyer-Tower circuit. J Appl Phys 113 224106 Qiu X, Wirges W, Gerhard R (2014) Polarisation and hysteresis in tubular-channel fluoro-ethylenepropylene-copolymer ferroelectrets. Ferroelectrics 472 100-109... 

Apart from nonlinearities in their D(E) characteristics, the dielectric displacement D of ferroelectric materials also depends on the electric field history, a phenomenon known as ferroelectric hysteresis. The common approach to measure D(E) or P(E) hysteresis curves utilizes the Sawyer-Tower circuit (Sawyer and Tower 1930) (cf Fig. 9a) that operates in the voltage-voltage mode. 

Fig. 9 (a) Principle of a Sawyer-Tower circuit in voltage-voltage mode. 

Figure 1.13 Experimental Sawyer-Tower Circuit. Reprinted with permission from Ok et al., Chem. Soc. Rev., 35, 710 (2006). Copyright (2006) Royal Society of Chemistry...

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. 

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. 

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... 

Figure 22-7. Circuit of Sawyer-Tower bridge for observation of P-E loop (from Sawyer and Tower, 1930, withpermission).

Measurement techniques are based on a circuit developed by Sawyer and Tower in 1930 [19]. For PVDF and the VDF TrFE copolymers, typical values of Pj. and are in the region of 5 juCm and SOkVmm" respectively. Furukawa et al [20] have shown that for PVDF, switching times of a few microseconds can be obtained at room temperature with an applied field of a few hundred kVmm Further investigations into VDF TrFE [21] have found that for a 65 35 molar composition at 20 °C, tg decreases from 1 ms to 100 ns as the field strength increases from 80 to 400kVmm . ... 

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