Papers by Keyword: Lifetime

Abstract: Thermo-fluctuation model of failure of one-dimensional nanocrystals is offered for wide range of change in temperature, time and applied loading. Unlike the classic models, which are based on the theory of reaction rates, in present model the failure initiation is related to the instability of atomic interaction. It enables to account for the effect of both temperature and applied loading on the lifetime of crystal within a framework of the unified approach. The model proposed was employed to predict the temperature dependence of both strength and lifetime of carbyne nanoconductor (CNC). Obtained temperature dependences agree well with the MD-simulation findings. Within the framework of the above model, CNC lifetime was estimated. It is exhibited that for temperatures not higher than 600K, CNC have a long-lasting lifetime, so, they can be used in nanodevices.

Abstract: Accelerated tests of the polyethylene pipes are necessary for the pipe lifetime calculations. Accepted methodology for prediction of slow crack grow rate in these materials is fatigue testing of CRB (cracked round bar) specimens. This paper deals with the FEM modelling of the crack propagation during the CRB test under the influence of residual stresses. The crack growth is described based on the stress intensity factor and Paris-Erdogan law. The purpose of this research is to determine, if the residual stress influences crack behavior during CRB test.

Abstract: Three fatigue regimes in accordance with three scale levels are considered and mechanisms of fatigue crack origination subsurface discussed. Surface hardening procedure can be used for the transition area of the crack origination from surface to subsurface in High-Cycle-Fatigue (meso-scale level) regime. In this case subsurface cracking characterized the same fatigue curve that was constructed for the Very-High-Cycle-Fatigue (micro-scale level) regimes due to the same mechanism of metals cracking. This effect was considered for Al-and Fe-based alloys. Subsurface crack origination can be received because of introduction of residual compressive stresses in the bulk material in the Low-Cycle-Fatigue (macro-scale-level) regime. The unified or master fatigue curve can be constructed for all scale levels for metals if subsurface crack origination will be realized because of hardening procedures for meso-and macro-scale levels. Subsurface crack origination in Very-High-Cycle-Fatigue regime for Ti-based alloy in the case of torsion is also discussed and three scale levels for fatigue regimes are demonstrated.

Abstract: During their use, optical fibers are subject to harsh installation and environmental conditions. To evaluate more precisely the lifetime of an optical fiber, it is necessary to study the mechanical behavior of optical fibers under extreme conditions, in particular under mechanical and thermal stress.This paper presents the results of new silica optical fibers aged in hot water between 20°C and 70°C and subjected to mechanical static bending stresses from 3 GPa to 3.5 GPa. Thermal dependence of the time to failure was observed. This dependence can be described by the Arrhenius model, where the activation energy is one of the main physical characteristic.

Abstract: Kelvin-probe surface voltage mapping, SVM, on epitaxial SiC, charged with corona into deep depletion, reveals SV defects manifested as spots with decreased surface voltage. For 150μm thick epi-layer, SV defects coincide with low carrier lifetime spots revealed by microwave detected photoconductance decay, μPCD. In the photoluminescence image, these defects are seen as triangular dark spots, described in literature as stacking-fault related triangular defects. For thin epi-layers (2.2μm), defects are visible only in SVM. In this case, high resolution SVM performed with Kelvin Force Microscopy identifies a triangular defect shape. Two mechanisms are proposed, accounting for SV defects. For high intensity defects exhibiting large magnitude fast decreasing voltage, the probable mechanism is defect related leakage; causing neutralization of corona surface ions. Low intensity defects can be explained considering deep level emission. The latter mechanism has been investigated using SV transient and spectral analysis analogous to isothermal DLTS and Laplace DLTS.

Abstract: In this contribution, we report on the electrical characterization of point defects in 4H-SiC p⁺in diodes. Ten electrically active levels have been detected in the base region of the devices, by employing Deep Level Transient Spectroscopy (DLTS) and Minority Carrier Transient Spectroscopy (MCTS). Of these ten levels, six are majority carrier traps, in the 0.1-1.7 eV energy range below the conduction band edge, and four were minority carrier traps located in the 0.13-0.4 eV energy range above the valence band edge. We found that, during DLTS measurements, both majority and minority carrier traps can be detected and we explain this by considering the behavior of the quasi-Fermi levels. At last, we studied the impact of proton irradiation on the minority charge carrier lifetime.

Abstract: The temperature dependence of the bulk and periphery forward current components of vertical 4H-SiC p⁺-i-n diodes, with Al⁺ implanted circular anodes of diameters in the range 150-1000 mm, have been obtained from the analysis of the results of forward current-voltage measurements in the temperature range 30-290°C. The zero voltage bulk and periphery current densities at different temperatures have been used to compute the temperature dependence of: (i) the effective carrier lifetime in the space charge region, (ii) the minority carrier diffusion coefficient to lifetime ratio in the base region, and (iii) a surface quality parameter.

Abstract: In this paper we have studied the connection between crystal quality and electrical transport in 4H-SiC by simultaneous micro-photoluminescence (μPL) and photocurrent (PC) measurements. We have used a focused HeCd laser at 325 nm (i.e. above bandgap) to measure with a spatial resolution of few microns both the μPL spectra and the I-V characteristics in 4H-SiC/Ni Schottky diodes. We found that the PC signal acquired along a defect can give information on its spatial distribution in depth. The minority carrier lifetime has been also estimated and its dependence on the emission wavelength has been determined for several stacking faults.

Abstract: The influence of thickness of atomic layer deposited Al₂O₃ films on nanotextured fluorescent 6H-SiC passivation is investigated. The passivation effect on the light emission has been characterized by photoluminescence and time-resolved photoluminescence at room temperature. The results show that 20nm thickness of Al₂O₃ layer is favorable to observe a large photoluminescence enhancement (25.9%) and long carrier lifetime (0.86ms). This is a strong indication for an interface hydrogenation that takes place during post-thermal annealing. These result show that an Al₂O₃ layer could serve as passivation in fluorescent SiC based white LEDs applications.

Abstract: Commercial 4H-SiC p⁺n structures with an uncompensated donor concentration (N_d-N_a) of ~1.5∙10¹⁵ cm^-3 in the n-type epitaxial layer are studied. The measurement of the photocurrent, electron beam induced current and transient switching characteristics (from forward to reverse voltage), altogether showed that the value of the hole diffusion length, about 2 μm at room temperature, increases to at least 7 μm at 620 K.