Pavel Mokrý

Restricted domain growth and polarization reversal kinetics in ferroelectric polymer thin films

A combination of microscopic and macroscopic methods enables an insight into polarization reversal in the copolymer of vinylidene fluoride and trifluoroethylene. Piezoforce microscopy together with switching charge measurements suggests that the polarization reversal is impeded by the restricted geometry of the domain growth. This effect limits the applicability of the conventional switching model of Kolmogorov-Avrami to the first stage of the polarization reversal. The interface-adjacent passive layer is found to impact the switching properties considerably resulting in a retardation of the polarization reversal. Furthermore, an extraordinary dielectric constant increase is observed in the films with a passive layer due to an additional domain wall contribution.

Noise shielding system utilizing a thin piezoelectric membrane and elasticity control

The description and theoretical analysis of a noise shielding system are presented. In this system, the noise and/or sound are transmitted through the piezoelectric curved membrane, which is connected to an external feedback circuit. Using the principle of elasticity control, i.e., utilization of both direct and inverse piezoelectric effects simultaneously, the movement of the membrane as well as the sound pressure of the transmitted wave can be controlled to a large extent. Transmission loss of an audible sound through the membrane in such a system is expressed as a function of a sound frequency, geometrical properties of the membrane, and parameters of the feedback circuit. It is used for the comparison of theoretical predictions and experimental data. Using this technique, the increase of the transmission loss of about 60 dB in a narrow frequency range or about 7 dB in the broad frequency range has been achieved. The performance of this system is discussed.

Sound absorbing system as an application of the active elasticity control technique

Active elasticity control has recently become a promising method used in acoustics to suppress the vibration of mechanical systems or to increase the noise transmission loss. The principle of this technique is based on the simultaneous utilization of both the direct and inverse piezoelectric effects, which is realized by connecting the piezoelectric sample to an external feed back circuit. The action of the feedback circuit results in an essential change of the vibrational response of the system, and a significant change of the resonant frequency in the mechanical system. This leads to the modification of the acoustic properties of the system. For that reason, this technique offers a very general approach to solve vibration and noise problems. To demonstrate this method, the design and theoretical analysis of an experimental realization of a sound absorbing system are presented. The system consists of a curved piezoelectric membrane and a passive sound absorber, which are attached to a rigid backing wall....

Displacements of 180° domain walls in electroded ferroelectric single crystals: The effect of surface layers on restoring force

Abstract Macroscopic properties of ferroelectric samples, including those in form of thin films, are, to large extent, influenced by their domain structure. In this paper the free energy is calculated for a plate-like sample composed of nonferroelectric surface layers and ferroelectric central part with antiparallel domains. The sample is provided with electrodes with a defined potential difference. The effect of applied field and its small changes on the resulting domain structure is discussed. This makes it possible to determine the restoring force acting on domain walls which codetermines dielectric and piezoelectric properties of the sample. Calculations of the potential and free energy take into account interactions of opposite surfaces and are applicable also to thin films

Patterning of dielectric nanoparticles using dielectrophoretic forces generated by ferroelectric polydomain films

A theoretical study of a dielectrophoretic force, i.e., the force acting on an electrically neutral particle in the inhomogeneous electric field, which is produced by a ferroelectric domain pattern, is presented. It has been shown by several researchers that artificially prepared domain patterns with given geometry in ferroelectric single crystals represent an easy and flexible method for patterning dielectric nanoobjects using dielectrophoretic forces. The source of the dielectrophoretic force is a strong and highly inhomogeneous (stray) electric field, which exists in the vicinity of the ferroelectric domain walls at the surface of the ferroelectric film. We analyzed dielectrophoretic forces in the model of a ferroelectric film of a given thickness with a lamellar 180° domain pattern. The analytical formula for the spatial distribution of the stray field in the ionic liquid above the top surface of the film is calculated including the effect of free charge screening. The spatial distribution of the diel...

Feedback Control of Piezoelectric Actuator Elastic Properties in a Vibration Isolation System

The design of a vibration isolation system utilizing a method to control the effective elastic stiffness of piezoelectric materials has been realized. The vibration isolation system consists of a piezoelectric actuator and an active analog circuit with an operational amplifier. The problem of a low stability of this system is analyzed and the method for the automatic stabilization and adjustment of the analog circuit by an additional feedback control is introduced. The impact of the proposed method on the transmissibility of vibrations through the vibration isolation system is experimentally verified on a preliminary device. It is shown that using the proposed method, it is possible to decrease the transmissibility of vibrations by an additional 20 dB compared to the transmissibility values achieved by a manual adjustment of the analog circuitry.

Elastic aspects of domain quadruplets in ferroics

Coexistence of ferroelectric ferroelastic domains in some materials plays an important role in practical area of domain engineering. Here the problem is discussed theoretically from the point of view of elastic aspects. Domain quadruplets are considered, i.e., systems of four ferroelastic domains coinciding along one intersection line. Conditions at which the elastic energy of quadruplets is zero are specified; they would allow for the existence of permissible quadruplets. The problem has been solved for the species m3¯m-mxmy2z, m3¯m-2xymx¯ymz, m3¯m-3m, and m3¯m-4mm. It is shown that, in general, no mechanically compatible (stress-free) permissible quadruplets exist. The results make it possible to specify those conditions for spontaneous strain components which must be complied to allow for energetically acceptable quadruplets. These conditions are fulfilled in the orthorhombic phase of KNbO3. The theoretical approach offered in this paper can be used for specific discussions of the existence of multidom...

Sound reflection in an acoustic impedance tube terminated with a loudspeaker shunted by a negative impedance converter

A system for the absorption of sound at the termination of an acoustic tube using an electrodynamic loudspeaker connected to a shunt circuit is presented. A theoretical model of the electrodynamic loudspeaker in the acoustic tube is used for the calculation of the frequency dependence of the ideal shunt circuit impedance, which yields perfect sound absorption in broad frequency range. It is shown that both the real and imaginary parts of the shunt circuit impedance must be negative in the considered system. The required negative values of the shunt circuit impedance are achieved using a negative impedance converter. Frequency dependences of the reflection coefficient were measured in the acoustic impedance tube using the two-microphone transfer function method. Greatly reduced values of sound absorption coefficient were achieved in a narrow frequency range. The stability, applicability of the sound absorption system, and broadening its frequency range are discussed.

Measurement of mechanical and electrical energy flows in the semiactive piezoelectric shunt damping system

The analysis of the mechanical and electric energy flows in the semiactive vibration control system, which is made of a piezoelectric actuator shunted by a negative capacitor, is presented. The vibration control system is designed to suppress the vibration transmission in a narrow frequency range. Using a proper adjustment of the electronic circuit elements, the transmissibility of vibrations has been suppressed by 35 dB at the frequency of 1.5 kHz. The results of our measurements indicate that the average apparent electrical power in the electric part of the vibration control device is about 100 times larger than the mechanical power, which is supplied to the system from the source of vibrations. The rules for designing the highly efficient and low-power electronic parts of semiactive vibration control systems are discussed.

Finite element analysis of the macro fiber composite actuator: macroscopic elastic and piezoelectric properties and active control thereof by means of negative capacitance shunt circuit

The finite element method (FEM) model of a piezoelectric macro fiber composite (MFC) is presented. Using a specially developed numerical model, the complete set of macroscopic values of elastic compliance and piezoelectric tensors is computed. These values are useful in numerical FEM simulations of more complex systems such as noise and vibration suppression devices or active acoustic metamaterials, where the MFC actuator can be approximated by a plate-like uniform piezoelectric material. Using this approach, a great reduction of the FEM model complexity can be achieved. The computed numerical macroscopic values of the MFC actuator are compared with MFC manufacturerʼs data and with data obtained using different computational methods. A demonstration of active tuning of effective elastic constants of the piezoelectric MFC actuator by means of a shunt electric circuit is presented. The effective material constants are computed using the FEM model developed. The effect of the shunt circuit capacitance on the effective anisotropic Youngʼs moduli is analyzed in detail. A method for finding the proper shunt circuit adjustment that yields the maximum values of the MFC actuator Youngʼs modulus is shown. Possible applications to noise and vibration suppression are discussed.