Vladimir Holcman

Study of electric field distribution and low frequency noise of CdZnTe radiation detectors

Polarization phenomena in a metal-semiconductor-metal (M-S-M) structure of metallic Schottky contacts deposited on CdZnTe radiation detectors were studied. We evaluate the distribution of the electric field along the biased M-S-M structure by Pockels measurements. The results show that almost all the electric field is developed across the depletion layer of the reverse-biased contact. The noise measurements of the CdZnTe detectors studied show that the dominant noise is 1/fm noise. The 1/fm noise, with the parameter m close to one, is present at frequencies below 100 Hz and its bandwidth decreases in the course of the polarization process. At higher frequencies, we observed an increase of the m parameter to 2, which indicates a strengthened effect of the generation-recombination processes. In the frequency band of dominating 1/fm = 1 noise, the increase of magnitude of the noise spectral density was proportional to the power of 6, in relation to the current through the detector. This high value is explained as a result of a screening effect of the space charge buildup during the polarization.

Noise of cold emission cathode

The paper deals with a spectroscopic method intended for characterization of virgin oxide and resin-coated cold emission cathodes. The noise spectroscopy in time and frequency domain is based on measuring cathodes voltage noise, in ultrahigh vacuum conditions ∼ 2 · 10−5 Pa. These conditions are required in order to avoid environment interaction with ions, which are present in the vacuum chamber, and also to reduce the influence of the electron-induced desorption and adsorption, which poses one of the significant noise sources. For these experiments, cold cathode with epoxy-resin coating was prepared. A comprehensive investigation was carried out to determine particular noise sources and to compare clean cathodes with the resin-coated one, in order to describe influence of the oxide and dielectric epoxy layer on to the emission current stability. Spectra obtained, for various emission currents at different voltages, are described and explained. The results suggest that the resin-layer changes cathode performance and extends its durability.

Dielectric relaxation in glycerol at long-time exposure to low temperatures

This paper deals with the dielectric relaxation in glycerol at different temperatures (down to 25 K) for protracted periods of time. Two relaxation maxima were observed. The peak at lower frequencies was ascribed to the relaxation alpha-process of glycerol proper; the peak at higher frequencies differed in samples from different manufacturers, so that it represents rather a relaxation peak associated with contaminants and not the true beta-process in glycerol. The alpha-process was considerably masked by the conductivity component. During a long-time exposure of glycerol to a low constant temperature, the relaxation spectrum shifts with increasing time to lower frequencies, reaching a steady position after about 20 hours.

Dielectric characteristics of composite systems

This paper deals with the dielectric and electric characteristics of a polymer - Ni composite. Permittivity (dielectric constant) and loss factor are analyzed as functions of frequency and filling factor, and internal resistivity as a function of the Ni filling factor. The analyzed system is the composite of the ethylene-vinyl-acetate copolymer with the Ni powder.

Relaxation time in CdTe single crystals

Bulk resistance decay of two cadmium telluride single crystals was investigated. Each CdTe crystal has four golden contacts, two current contacts and two voltage contacts. That allows us to distinguish between bulk resistance and contact area. The bulk resistance of each CdTe single crystal was measured during long time interval with an applied voltage U = 16 V. Detectors were placed into a cryostat. That allowed to hold the temperature constant during the measurements and eliminate the illumination influence. The temperature of the samples was 300 K at first and after some period of time (approximately 1 day) it was sharply raised to 390 K. We observed the resistance slow decreasing with time with temperature T = 300 K and T = 390 K. All the samples have very high value of relaxation time. The presented samples must have not one but four acceptor or donor levels, some of them are deep levels. We have discovered that the interactions between the valence band and deep acceptor levels or between the conductivity band and deep donor levels cause this long value of relaxation time.

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.

P1-approximation for radiative transfer: application to SF6 + Cu arc plasmas

Abstract The objective of the paper consists of a theoretical prediction of radiative heat transfer in arc plasmas of SF6 (sulphur hexafluoride) with various admixtures of copper vapours. The P1-approximation was used as a mathematical tool. Due to the very complicated frequency dependence of absorption coefficients, the Planck and Rosseland mean absorption coefficients have been derived from the calculated absorption spectrum. The main radiation quantities (radiation intensity, radiation flux density and its divergence – net emission) have been determined in cylindrical arc plasmas for several model temperature profiles. Contribution to the net emission of copper admixtures is discussed. Conclusions have been made concerning validity and utilization of various absorption means. Graphical Abstract

Analysis of excess noise sources in concrete composite materials and impact on electromagnetic emission measurement

This paper deals with an excess noise sources in concrete materials. The theoretical description is put forward as well as supporting experiments of real samples. Physical variables under consideration are spectral densities of voltage and/or current fluctuation for different sample dimension and electrical excitation. It managed to get information about Johnson-Nyquist noise, 1/f noise and Polarization noise contributions. These individual components define detection limit or ability to recognize electromagnetic emission signal when mechanical load is applied and micro-cracks are created. Described investigations also pointed out real requirement on noise sensing equipment and sample preparation methodology.

Noise and local diagnostics of laser notches on silicon solar cells

The present paper deals with diagnostics of crystalline silicon solar cells with structures prepared by application of laser technologies. These laser-generated structures should help to isolate the edges of solar cells and also should isolate bulk defects. At the present we are trying to find the optimal parameters of laser processing. Laser parameters influencing how deep the laser notch is realized. We tried to determine what depth of the notch seems to be an ideal. Noise diagnostic methods are used for our study. These methods are based on the study of electrically measurable fluctuation quantities, especially electrical voltage and current, complemented by the transport characteristics. Electrical models describing behaviour of laser-prepared structures were put forward. The study of optical near-field and far-field will be also realized. Measurement of local radiation from solar cells during electrical excitation were carried out.

Experimental Studies of Excess Noise Sources in Concrete Based Materials as a Limiting Factor for Electromagnetic Emission Measurement

This paper deals with excess noise sources in dielectric materials. We focus especially on the concrete samples that are frequently tested to ensure information about the reliability and level of degradation. Nevertheless, the testing methods are limited mainly by the proper contact creation, signal detection and noise defined sensitivity. Our efforts are directed to the noise properties assessment. It turns out that the Johnson-Nyquist noise and the 1/f (flicker) noise are generated in the different regions with the different response to the internal or external electric field. In addition the noise analysis is affected by the internal polarization phenomena and the material residual humidity. This issue in connection with the sample geometrical properties and the dielectric noise measurement methodology take part in this paper.