Papers by Author: Pavel Koktavý

Abstract: Laboratory tests on a wide range of solid materials shoved that the electromagnetic emission (EME) signals are generated during the samples mechanical stress. EME anomalies have been observed also under natural conditions in association to fracture processes, tectonic loading, stress redistribution and crack propagation prior to earthquake or in relation to deep-seated gravitational mass movements. This paper describes a first prototype of the Emission data logger, which was specially developed for the continual EME monitoring in field conditions. Our equipment has been installed and tested in Obir Caves (Austria) at an active tectonic fault. The pilot long-term EME measurement results from this location are also presented in this paper.

Abstract: Solar cells of common sizes contains many of these defects and it is not easy to determine the influence of particular defects on the characteristics of the whole solar cell. Therefore, in our research we use samples of size of square centimeter at which we can disentangle the influence of the defect. We localize the defect by using a CCD camera, we measure the electrical, thermal and optical properties of the sample and then study it by means an electron microscope, we find the damaged structure and put it to focused ion beam. We expect the change in electrical, thermal and optical properties of the sample.

Abstract: Composite materials such as concrete or polymer fiber composites contain a large number of inhomogeneities. It is possible to use the method of partial discharges to detect existing inhomogeneities. Tested materials covered with metal electrodes on both sides represent a parallel plate capacitor. Partial discharges have an impact on conductivity at a high electric field and they are important sources of current noise and acoustic emission signals. The concrete samples were measured before and after mechanical loading by the method of partial discharges and the correlation between them was studied. This method may be used for the diagnosis of inhomogeneities and cracks in composite materials.

Abstract: 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.

Abstract: Presented research is involved in excess electrical currents created when the silicon material contains cracks and fractures. We performed transport characteristics measurements and electrical noise measurement as well as sample visible and deep infra-red imaging. It turns out that mechanical induced defects are followed by specific electric characteristics. We observe crack-related local breakdowns and local overheating. It is also followed by the electrical current fluctuation in the 1/f form. All regions are thermally but also electrically stressed and the irreversible sample degradation originates. It could be pointed out that our detection methods are very sensitive and they could be also used for analyses of different materials.

Abstract: Conducted loading tests of plain concrete specimens are briefly introduced in the paper together with description of the performed numerical simulations of these tests in ATENA 3D software. The simulation results of concrete failure are analysed in detail and compared with the experimental results recording failure processes with the help of acoustic emission. The simulation results are in good agreement with the recorded data.

Abstract: The paper is focused on the utilization of electromagnetic emission (EME) and acoustic emission (AE) phenomena within the non-destructive assessment of damage and fracture in cementitious composites. In the first part of the paper, the EME and AE responses from tests performed in two testing configurations differing considerably in stress distribution are investigated. Typical EME and AE response waveforms appear not to differ from each other for either of the loading configu-rations. However, a significant difference is observed in the frequency distribution of the time lag of the acoustic signal behind the electromagnetic one, which results from the different distribution of the fracture events over the specimen’s volume. In the second part of the paper, the failure process within the specimens in the used testing configurations is assessed from the fracture-mechanical point of view. A numerical 3D analysis of the stress distribution and crack propagation within the specimens was performed by using ATENA non-linear FEM code based on a fracture- and damage-mechanics, and plasticity approach. The predictions of the numerical model are in good agreement with the results obtained by AE/EME experimental measurements. The comparison of the AE/EME records from the tests with the results of performed numerical simulation can help in the understanding, interpretation and exploitation of AE/EME in research into damage to and failure of non-electric quasi-brittle silicate-based building materials.

Abstract: PN junction is one of the most important parts of solar cells. Its quality affects lifetime and efficiency of solar cells. Local defects which appear in PN junctions during the manufacture process are very important from this point of view. These are caused by localized areas with high donor or acceptor doping agents, impurities, dislocations or other mechanisms which effect in lower breakdown voltage of PN junction in reverse bias. Several base methods can be used for solar cells nondestructive diagnostics. Measuring methods of low-band noise current effective value with reverse bias junction were used in this paper. This method allows detection of local defects and volume degradation in PN junctions of solar cells and it can be used for detection of microplasma noise. This noise is an impulse noise and it is caused by local avalanche breakdowns in small area of the junction. It can be recognized by two or more level random square current pulses with constant height, random appearance time and random pulse length. Information about these effects can be used in noise diagnostics of structural defects of PN junctions and then it can be used for quality and lifetime estimation of samples with these parameters.

Abstract: 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.

Abstract: This article discusses the issue of noise measurements application for the quality assessment of the solar cells themselves and production technology alike. The main focus of our research is the random n-level (in most case just two-level) impulse noise, usually referred to as microplasma noise. This noise was found to be in a direct consequence of local breakdowns in micro-sized regions and brings about a reduction of lifetime or a destruction of the pn junction. Non-destructive measurement methodology as presented here is suitable for testing of a large number of various semiconductor devices not only for solar cells. In this paper experimental measurement of noise signals in the frequency and time domain is presented. Furthermore the microplasma noise behaviour and defect geometry is discussed.