Pavel Tofel

A Game Changer: A Multifunctional Perovskite Exhibiting Giant Ferroelectricity and Narrow Bandgap with Potential Application in a Truly Monolithic Multienergy Harvester or Sensor

An ABO3 -type perovskite solid-solution, (K0.5 Na0.5 )NbO3 (KNN) doped with 2 mol% Ba(Ni0.5 Nb0.5 )O3-δ (BNNO) is reported. Such a composition yields a much narrower bandgap (≈1.6 eV) compared to the parental composition-pure KNN-and other widely used piezoelectric and pyroelectric materials (e.g., Pb(Zr,Ti)O3 , BaTiO3 ). Meanwhile, it exhibits the same large piezoelectric coefficient as that of KNN (≈100 pC N-1 ) and a much larger pyroelectric coefficient (≈130 µC m-2 K-1 ) compared to the previously reported narrow-bandgap material (KNbO3 )1-x -BNNOx . The unique combination of these excellent ferroelectric and optical properties opens the door to the development of multisource energy harvesting or multifunctional sensing devices for the simultaneous and efficient conversion of solar, thermal, and kinetic energies into electricity in a single material. Individual and comprehensive characterizations of the optical, ferroelectric, piezoelectric, pyroelectric, and photovoltaic properties are investigated with single and coexisting energy sources. No degrading interaction between ferroelectric and photovoltaic behaviors is observed. This composition may fundamentally change the working principles of state-of-the-art hybrid energy harvesters and sensors, and thus significantly increases the unit-volume energy conversion efficiency and reliability of energy harvesters in ambient environments.

Electromagnetic and Acoustic Emission Signals Real-Time Measurement, Processing and Data Evaluation

This paper describes our new improved set-up for EME and AE signals measurement and includes some experimental results. This new measurement system is based on the PXI platform, which is one of the most extended modular platforms for measurement and automation systems. The complex program package in the LabVIEW graphical programming environment was developed for the continual multi-channel EME and AE signals measurement, real time processing and evaluation. It offers the possibility to observe the stressed materials response to applied mechanical load in real time.

Cracks Detection in Mg Alloy by Electro-Ultrasonic Spectroscopy

New non-destructive testing (NDT) method is based on the effect of the ultrasonic vibrations on the electron transport in samples with macroscopic defects as cracks or defect centers affecting electrical conductivity. On the frequency given by the subtraction of exciting frequencies new intermodulation signal appears. Its value is given by electric resistance modulation by the defects and un-homogeneities in the sample structure. In our experiment we used the ultrasonic actuator with frequency fU when the period of wave is longer than the dielectric relaxation time in analyzed sample. In this case the effects of electron bunching by ultrasonic wave are negligible. The ultrasonic wave length is much larger than the electrons mean free path and the wave period is much longer than the mean free time among the electrons collision with scattering centers and defects. Then the electron transport is described by the quasi-steady state transport equation in one-electron approximation. Because of the requirement of charge neutrality, no net AC electric current with the ultrasonic wave frequency fU can be carried by the wave. Similar situation exists for samples excited by standing ultrasonic waves. The electrical conductivity varies with time due to that the cracks geometry is changed with frequency of the ultrasonic vibration. The sample conductivity is affected mainly by the presence of cracks and defects boundaries perpendicular to the electric current density vector. In our experiment the amplitude of ultrasonic vibration is so low that no new cracks are generated and then the proposed testing method belongs to NDT.

Thermal stability of gallium arsenide solar cells

This article summarizes a measurement of gallium arsenide (GaAs) solar cells during their thermal processing. These solar cells compared to standard silicon cells have better efficiency and high thermal stability. However, their use is partly limited due to high acquisition costs. For these reasons, GaAs cells are deployed only in the most demanding applications where their features are needed, such as space applications. In this work, GaAs solar cells were studied in a high temperature range within 30-650 °C where their functionality and changes in surface topology were monitored. These changes were recorded using an electron microscope which determined the position of the defects; using an atomic force microscope we determined the roughness of the surface and an infrared camera that showed us the thermal radiated places of the defected parts of the cell. The electrical characteristics of the cells during processing were determined by its current-voltage characteristics. Despite the occurrence of subtle changes on the solar cell with newly created surface features after 300 °C thermal processing, its current-voltage characteristic remained without a significant change.

Lead-free piezoelectric materials and composites for high frequency medical ultrasound transducer applications

Lead-free ceramics based on the (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-BCT) system have been reported to exhibit piezoelectric properties comparable to lead zirconate titanate ceramics, and are thus being considered as replacement materials for some applications. In this work, the possibility of fabricating BZT-BCT based 1-3 piezocomposites for high frequency ultrasound imaging has been explored. An investigation of the interdependence of synthesis, processing and sintering conditions on the physical, structural, microstructural and functional properties of piezoelectric compositions in the BZT-BCT system has been carried out. Ceramic powders with a composition of 0.5BZT-0.5BCT and a particle size of 1 μm were chosen for the fabrication of the ceramic segments which were subsequently sintered at 1425°C. A novel randomised pattern that has recently demonstrated its advantages in suppressing undesirable spurious resonances was adopted for the lead-free composites and a micro-moulding technique based on gel casting ceramic processing has been used for the fabrication of the composites. A 0.5BZT-0.5BCT based randomised composite operating at ~30 MHz with an electromechanical coupling factor of 0.63 has been achieved.

NDT of Conducting Solids by Electro‐Ultrasonic Spectroscopy

The paper presents new NDT method of conducting solids. Tested sample is excited by harmonic electrical and ultrasonic signals with different frequencies. On the defect caused non linearity new harmonic signal is created with frequency given by subtraction of exciting frequencies fE and fU. It was found that the amplitude of the intermodulation component Ui is linear function of electrical excitation and quadratic function of ultrasonic excitation. High sensitivity of this method follows from the fact that the signal giving information on tested sample quality has frequency different from exciting signals. The signal to noise ratio and high sensitivity for NDT analyses is based on the application of special electrical filters. Experimental verification of this method was performed on the samples of aluminum and dural plates and on the thick conducting films, both without and with cracks prepared artificially.

Grain size dependence of the microstructures and functional properties of (Ba0.85 Ca0.15-x Cex) (Zr0.1 Ti0.9) O3 lead-free piezoelectric ceramics.

ABSTRACT By the solid-state reaction route, (Ba0.85Ca0.15-x Cex) (Zr0.1Ti0.9) O3 (BCCeZT) lead-free piezoelectric ceramics were prepared. The powder was processed at 1250 ºC for 2 h, and compacted green bodies were then sintered at various sintering temperatures. X-ray diffraction analysis and Raman spectra confirmed the rhombohedral-tetragonal phase coexistence of x = 0–0.00131. Addition of CeO2 facilitated development of grain sizes greater than 10 μm at low sintering temperatures. The effects of grain size on the ferroelectric and piezoelectric properties were studied systematically and it was found that a ~ (10–12) μm grain size is critical for the processing of high-performance lead-free BCCeZT ceramics. CeO2 substitution at the A site may bring down the sintering temperature by about 200 °C without significant loss of properties. The best properties were obtained for x = 0.00131 at a low sintering temperature of 1350°C for 4 h exhibiting d33 = 501 ± 10 pC/N, kp = 38.5 ± 1.92 %, Pr = 12.19 μC/cm2, TC = 108.1°C, and a large strain of 0.14%. These results show that BCCeZT ceramics could be promising candidates for the fabrication of lead-free devices.

Fractal Analysis of the 3-D surface Topography of GaAs Solar Cells

This article is devoted to study of Atomic Force Microscopy (AFM) images of solar cells based on gallium arsenide (GaAs). Mathematical processing of data involves obtaining additional information about topography. The analysis was carried out using AFM data for GaAs solar cells, before and after temperature treatment. The state of the surface affects the reflectivity, so the structuring of the surface is of particular interest. Optimized textures are needed to improve the optical properties of surfaces. Fractal analysis allows quantifying the condition of the morphology of the surface on the basis of AFM data.

Phase transitions and dielectric, ferroelectric and piezoelectric properties of Bi 0.5 (Na 0.82 K 0.18 ) 0.5 TiO 3 -doped (Ba 0.85 Ca 0.15 )(Zr 0.1 Ti 0.9 )O 3 ceramics

This paper reports an investigation of (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ (50BCZT) piezoelectric ceramics doped with Bi_0.5(Na_0.82K_0.18)_0.5TiO₃ (18BNKT). For compositions between 1-5 wt% 18BNKT, a perovskite phase was observed in the sintered ceramics, with d₃₃ values of approximately 370 pC/N measured at room temperature, which is about 80 % of that of pure 50BCZT. Also, the doped materials exhibit a remanent polarization of 2-5 μC/cm² at temperatures above the Curie temperature (> 90 °C) of the pure 50BCZT composition. This research shows a promising route to improve the working temperature range whilst maintain good piezoelectric properties of the 50BCZT ceramics.

Fluctuations of ultrasonic transducer vibration measurement quadrature interferometer for transducer vibration measurement

Langevin type transducers are being used by reason of the high transmitting power capability for various purposes. The peak-to-peak value of the transducer mechanical oscillation is in order of tens of the nanometers (for the resonant frequency and two ring transducer). Transducer vibration parameters depend considerably on excitation voltage and frequency. Beyond the resonant frequency some parasitic resonance frequencies are produced. Precise measurement of vibration with nanometer accuracy in the range of the ultrasonic frequencies can be done with using optical interferometry. Results show high distortion of the transducer displacement waveforms and poor amplitude stability.