Zdenek Chobola

Dielectric Properties of Concrete Specimens after Heat Stress

Concrete quality depends on various stress factors. The impedance spectroscopy method, as one of the non-destructive testing method group, was used to characterize concrete specimens and track the changes in the concrete spectrum after heat stress. The temperature range was between 0°C and 1200°C. Variances in the tan δ (f) and Im Z(f) or Re Z(f) of the specimens under investigation have been observed. The specimen quality has been described by means of the loss type prevailing in the material. The results of this study are expected to provide information about the correlation between the relative permittivity and changes in the structure for heat stress samples.

Low-frequency noise, microplasma, and electroluminescence measurements as faster tools to investigate quality of monocrystalline-silicon solar cells

Abstract. Two sets of c-Si solar cells varying in front side phosphorus doped emitters were produced by standard screen printing techniques. The first group of samples, 3121, was prepared by a combination of standard washing and a bath with a highly dilute HF before diffusion of n+-emitter. The second group of samples, 3122, was treated only with standard washing. A comparison of solar cell conversion efficiency and results from a noise spectroscopy, microplasma, and electroluminescence presence are presented. As was already shown in previous publications noise spectral density reflects the quality of solar cells, and thus represents an alternative advanced cell diagnostic tool. Our results confirm this relationship and moreover bring clear evidence for the maximum spectral noise voltage density being related to the emitter structure. The best results were reached for the group of solar cells in sample 3122, which was treated only with standard washing.

Low-Frequency Noise and Microplasma Analysis for c-Si Solar Cell Characterization

This paper brings the comparison of solar cell conversion efficiency and results from a noise spectroscopy and microplasma presence to evaluate the solar cell technology. Three sets of monocrystalline silicon solar cells (c-Si) varying in front side phosphorus doped emitters were produced by standard screen-printing technique. From the measurements it follows that the noise spectral density related to defects is of 1/f type and its magnitude. It has been established that samples showing low noise feature high-conversion efficiency. The best results were reached for a group solar cells with selective emitter structure prepared by double-phosphorus diffusion process.

Low-frequency noise measurements used for semiconductors light active devices

Three different sets of semiconductors light active devices were by low frequency noise diagnostic described. In the first set the low frequency noise of 2.3 μm CW GaSb based Laser Diodes was measured, in set II the noise characteristic of forward biased silicon monocrystalline solar cells were measured and in set III the noise characteristic of forward biased Si:H amorphous solar cells were measured. The results of noise measurement in all systems were compared.

Application of Acoustic Emission Method and Impact Echo Method to Structural Rehabilitation

The paper is concerned with the technical aspects of the appraisal and retrofitting process of fire damaged reinforced concrete structures. The assessment of fire damaged structures is carried out along lines similar to those of the appraisal of existing structures. In practice, constructions are most often assessed by destructive tests in-situ and on core bore specimens. In addition to destructive tests, damaged structures are also assessed by non-destructive ones. The present paper shows the use of non-destructive methods of measurement using the acoustic-emission and impact-echo methods. Acoustic emission provides valuable data on the structural integrity of a material. This method has a significant potential to be used for in-situ monitoring and evaluation of the current state of structures. An impact-echo method is based on impact-generated stress waves that propagate through concrete and are reflected by internal flaws and external surfaces. Impact-echo can be used to determine the location and extent of flaws such as crack delamination, voids, honeycombing and deboning in plain, reinforced, and post-tensioned concrete structures. The paper presents a possible rehabilitation plan based on the potential results obtained by these non-destructive methods.

Comparison of Ultrasonic Methods for Thermally Damaged Concrete Nondestructive Testing

Behaviour of concrete under elevated temperatures is very complex. There is a change of mechanical and physical parameters with temperature. In this paper we study the relations of thermal damage processes in concrete and parameters obtained by different ultrasonic methods. The concrete specimens were heated in programmable laboratory furnace. Selected temperature (200°C, 400°C, 600°C, 800°C, 1000°C and 1200°C) were maintained for 60 minutes. The first ultrasonic measurement technique in this paper was Ultrasonic Pulse Velocity method. The pulse velocity in a concrete depends on its density and its elastic properties. Therefore, it is possible to deduce the quality and the compressive strength of the concrete from the ultrasonic pulse velocity. The second ultrasonic measurement technique in this paper uses broadband pulse-compression signal, with variable amplitude to measure the change of fundamental frequency. This method is based on Nonlinear Elastic Wave Spectroscopy. Nonlinear Elastic Wave Spectroscopy methods takes advantage of the fact, that nonlinearities in material manifest themselves as a resonant frequency shifts and harmonics or dumping coefficients changes. The progress of nondestructive testing parameters was confirmed by results from the destructive tests.