Jiri Majzner

Nyquist Relation and Its Validity for Piezoelectric Ceramics Considering Temperature

In this paper, we focused on validity of the Nyquist relation for piezoelectric ceramics in temperatures 303 K–393 K. The electrical impedance and noise spectral density were measured and compared for every 10 K in frequency range 100 kHz–1 MHz. The measurements were made in thermal stable condition and under equilibrium conditions in the case of noise measurement.

Noise in Piezoceramics

Thermal noise and polarisation noise are the main sources of voltage or current fluctuations in piezoceramic samples which are used as acoustic emission sensors. Signal to noise ratio plays important role. Noise is related to energy dissipation and due to that conductivity is proportional to imaginary part of susceptibility and to the frequency. The measured noise spectral density is 1/f type, but it is not related to the mobility fluctuations in this case, because no DC current or voltage was applied on the sample. Physical quantity SU/RS shows that in all frequency range SU/RS is a constant in the first approximation and it is very near to 4kT. There is no source of 1/f fluctuations caused by mobility fluctuations.

Noise in amperometric NO2 sensor

Nitrogen dioxide (NO2) is a highly toxic gas harmful to the environment, which can be threat to human health even at low concentrations. To overcome limitations of standard solid NO2 sensors based on inorganic materials, a new sensor with solid polymer electrolytes (SPE) was developed. Our study deals with investigation of fluctuation phenomena in the electrochemical NO2 sensor, which is based on three-electrode topology and solid polymer electrolyte. Experimental result shows that generation-recombination (G-R) noise seems to be main components of current fluctuations in this sensor. The concentration of detected matter affects noise spectral density of sensor. As the concentration increases, G-R noise rises and is supposed to be connected with chemical processes on active layer of sensor. The shift of G-R component is supposed to be caused by increased flux density between active layer and environment.

Comparison of effectiveness of gas sensing by low frequency fluctuations in resistance and microbalance quartz gas sensors

Selectivity and sensitivity of gas sensing can be improved by fluctuation enhanced sensing. This possibility was investigated in resistive Taguchi Gas Sensors (TGS). Research results confirm that such an approach can strongly increase gas detection sensitivity. We suppose that low frequency fluctuations can be utilized to improve gas detection in other types of gas sensors. Fluctuations in the Quartz Crystal Microbalance (QCM) gas sensor for gas detection improving have been investigated. The sensor used in an experiment utilizes an ordinary quartz oscillator with a surface covered by a chemical layer absorbing humidity. The paper presents results of 1/ƒ noise measurements in both types of sensors at different humidity. The prototype QCM sensor and resistive gas sensors of Figaro company were investigated. The results show advantages of this technique for future industrial applications.

Noise in quartz crystal microbalance

There are several mechanisms that are related to fluctuation phenomena in QCM. The aim of our research is oriented to study the sensitivity and influence of different kind of noises on sensor resolution. Our experiments are provided on sensor with sorption layer of polypyrrole which is suitable for detection of water vapor. Based on these experiments, we suppose that 1/f noise caused by quartz internal friction and adoption-desorption (generation-recombination) noise from analyzed gas create the main components of measured noise spectral density. The value of the adoption-desorption noise depends on the physical and chemical parameters of analyzed gas and it is proportional to gas density. Adsorption-desorption kinetics is described by Kolmogorov equation and compared with Wolkenstein and Langmuir equations.

Tantalum and Niobium oxide capacitors: Field crystallization, leakage current kinetics and reliability

The study of the charge carrier transport in Ta and NbO capacitors was performed to analyze the leakage current kinetics at high temperature and high electric field for MnO2 and Conducting Polymer (CP) cathode. Leakage current of Ta and NbO capacitors at the room temperature is driven by the Ohmic and Poole-Frenkel mechanism at the rated voltage. It was found that these capacitors are very stable for temperature below 100°C. High temperature and high voltage applications are considered to be limited by the field crystallization mechanisms and ions diffusion. The leakage current changes in high electric field and at the elevated temperature T = 400 K could be divided into three time intervals: (i) Leakage current is stable (in some samples is slightly decreasing or increasing) during a period of 1 to 10 days. (ii) Leakage current increases with the slope 5 to100 pA/s for time interval about 10 days. (iii) Leakage current is stable or slightly increases with the slope less than 1 pA/s. Activation energy decreases during the ageing period from 0.55 to 0.45 eV. Leakage current variations are partly reversible. Irreversible changes of leakage current appear on about 1% of samples after ageing. Further investigation in this field can lead to the enhancement of reliability and performance of these capacitors.

Noise analysis of infrared detectors

Pyroelectric infrared detectors convert the changes in incoming infrared light to electric signals. Pyroelectric materials are characterized by having spontaneous electric polarization, which is altered by temperature changes as infrared light illuminates the elements. Since our sensor series uses this effect they can be used at ambient temperature even in the presence of thermal noise. By choosing appropriate infrared receiving electrodes, they serve a wide range of applications. Sometimes pyroelectric infrared detectors generate a false alarm. They include thermal resistor and FET device which can be a source of random signals. This paper deals with the measurement of output and transfer characteristics and voltage low noise spectral density. By the help these characteristics we can evaluate detectors and reduce false alarms.

Mathematical Model for Electrical Noise of Piezoelectric Sensor

The aim of the paper is to show how the electrical noise of piezoelectric sensor can be modeled. The calculation of noise spectral density given by geometry and material is required for the design of new sensors. The high sensitivity with wide bandwidth sensors are applied in non‐destructive testing (NDT). The higher signal to noise ratio is used for further signal processing and interpretation. The sensitivity of piezoceramic sensors demands to minimize their noise. For simplicity, only the piezoceramic part of sensor was under study. We investigated samples with the same geometry and material properties. The numerical results are compared with experimental results and good agreement was obtained.

Investigation of adsorption-desorption phenomenon by using current fluctuations of amperometric NO 2 gas sensor

Nitrogen dioxide (NO2) is a highly toxic gas harmful to the environment even at low concentrations. To overcome limitations of standard solid NO2 sensors based on inorganic materials, a fully printed sensor with solid polymer electrolytes (SPE) was developed in Regional Innovation Centre for Electrical engineering in Czech Republic. The amperometric sensor is based on a semi-planar three-electrode topology (reference, working and counter electrode) and solid polymer electrolyte. This paper focuses on adsorption-desorption phenomenon by studying current fluctuations and current mean value of amperometric NO2 gas sensor during its exposition to concentration cycle. Limits of NO2 concentration are from 0 to 6 ppm. Background noise and thermal noise are only apparent for zero concentration, while, noise 1/f becomes main component of current fluctuations for higher NO2 concentrations.

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.