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1 – 10 of over 3000A.S. Tonkoshkur, A.B. Glot and A.V. Ivanchenko
The purpose of this paper is to develop the models of the dielectric permittivity dispersion of heterogeneous systems based on semiconductors to a level that would allow to apply…
Abstract
Purpose
The purpose of this paper is to develop the models of the dielectric permittivity dispersion of heterogeneous systems based on semiconductors to a level that would allow to apply effectively the method of broadband dielectric spectroscopy for the study of electronic processes in ceramic and composite materials.
Design/methodology/approach
The new approach for determining the complex dielectric permittivity of heterogeneous systems with semiconductor particles is used. It includes finding the analytical expression of the effective dielectric permittivity of the separate semiconductor particle of spherical shape. This approach takes into account the polarization of the free charge carriers in this particle, including capturing to localized electron states. This enabled the authors to use the known equations for complex dielectric permittivity of two-component matrix systems and statistical mixtures.
Findings
The presented dispersion equations establish the relationship between the parameters of the dielectric spectrum and electronic processes in the structures like semiconductor particles in a dielectric matrix in a wide frequency range. Conditions of manifestation and location of the different dispersion regions of the complex dielectric heterogeneous systems based on semiconductors in the frequency axis and their features are established. The most high-frequency dispersion region corresponds to the separation of free charge carriers at polarization. After this region in the direction of reducing of the frequency, the dispersion regions caused by recharge bulk and/or surface localized states follow. The most low-frequency dispersion region is caused by recharging electron traps in the boundary layer of the dielectric matrix.
Originality/value
Dielectric dispersion models are developed that are associated with: electronic processes of separation of free charge carriers in the semiconductor component, recapture of free charge carriers in the localized electronic states in bulk and on the surface of the semiconductor and also boundary layers of the dielectric at the polarization. The authors have analyzed to situations that correspond applicable and promising materials: varistor ceramics and composite structure with conductive and semiconductor fillers. The modelling results correspond to the existing level of understanding of the electron phenomena in matrix systems and statistical mixtures based on semiconductors. It allows to raise efficiency of research and control properties of heterogeneous materials by dielectric spectroscopy.
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Alexander Sergeevich Tonkoshkur and Alexander Vladimirovich Ivanchenko
– The purpose of this paper is modeling the effect of negative capacitance in the capacitance-voltage characteristic of the intergranular potential barrier of varistor structure.
Abstract
Purpose
The purpose of this paper is modeling the effect of negative capacitance in the capacitance-voltage characteristic of the intergranular potential barrier of varistor structure.
Design/methodology/approach
The modeling of the capacitance-voltage characteristic of the intergranular barrier in metal oxide varistor ceramics is based on the development of the algorithm. It includes all the known mechanisms of electrotransfer in a wide range of voltages and currents, and also takes into account the voltage drop on the intergranular interlayer of intergranular potential barrier.
Findings
The models and algorithms for calculating the capacitance-voltage characteristics of a single intergranular potential barrier with the use of the most established understanding used at the interpretation of the nonlinear conductivity intergranular barrier are developed. The results of the capacitance-voltage characteristics modeling correspond to the existing understanding of the electrical properties on the ac current varistor ceramics are based on zinc oxide. The model allows to predict the behavior of varistors on the alternating current (voltage).
Originality/value
It is established that the recharge of the surface localized states occurs when a voltage is applied to the varistor structure, it can lead to a relaxation decrease in the width of the potential barrier overcome by tunneling electrons in the field emission from the conduction band of the one crystallite in the conduction band of the other crystallite and thus to the current backlog of applied voltage on the phase (i.e. the expression of the negative capacitance effect).
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Alexander S. Tonkoshkur and Alexander V. Ivanchenko
The purpose of this paper is to develop a generalized model of the nonlinear conductivity of varistor ceramic suitable for solving problems of prediction and control of ceramic…
Abstract
Purpose
The purpose of this paper is to develop a generalized model of the nonlinear conductivity of varistor ceramic suitable for solving problems of prediction and control of ceramic nonlinearity, stability of varistor properties.
Design/methodology/approach
The modeling of current-voltage characteristic of the intergranular barrier in metal oxide varistor ceramics is based on the development of the algorithm. It includes all the known mechanisms of electrotransfer in a wide range of voltages and currents of the current-voltage characteristics, and also takes into account the deviation of the barrier form the Schottky barrier.
Findings
The models of double Schottky barrier and double barrier of arbitrary form, as well as the algorithms for calculating the current-voltage characteristics of a single intergranular potential barrier and a separate “microvaristor” with the use of the most well-established understanding of the main mechanisms of electrical are developed. The results of current-voltage characteristics modeling correspond to the existing understanding of the nonlinear electrical conductivity varistor ceramics are based on zinc oxide. The model of double barrier of arbitrary form takes into account the deviation of the barrier form the Schottky barrier which is important in predicting the deformation of the current-voltage characteristics of the varistor products in the process of degradation.
Originality/value
The relation between the form of the current-voltage characteristic and the distribution profile of the donor concentration in the surface regions of the semiconductor crystallites constituting the intergranular potential barrier is established. The accumulation of donors in the space charge region leads to the increase in the current on the prebreakdown region of the current-voltage characteristic and the reduction of voltage corresponding to the breakdown region beginning of the current-voltage characteristic. The significant role of the interlayer in the formation of current-voltage characteristic of the intergranular potential barrier is shown.
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Abdollah Afshar, Mohsen Shirazi, Masoud Rahman and Esmaeil Fakheri
The purpose of this paper is to evaluate the galvanic corrosion of nitinol orthodontic wires with six dental alloys in artificial saliva and consider the effect of initiated…
Abstract
Purpose
The purpose of this paper is to evaluate the galvanic corrosion of nitinol orthodontic wires with six dental alloys in artificial saliva and consider the effect of initiated localized corrosion and real surfaces of anode and cathode on galvanic current.
Design/methodology/approach
Linear polarization and cyclic polarization curves for each alloy in de‐aerated Duffo and Castillo's artificial saliva are obtained. Galvanic corrosion investigation is conducted by polarization curve intersection and mixed potential theory methods. In order to verify the initiation of localized corrosion, scanning electron microscopy is used.
Findings
Initiation of localized corrosion on the anode increases the galvanic current up to 45 times and therefore considering the effect of localized corrosion on galvanic corrosion is necessary. Placing stainless steel brackets or Aristaloy amalgam in direct contact to nitinol arch wire is not recommended.
Originality/value
In order not to underestimate the galvanic corrosion between two alloys, it is recommended to consider the effects of localized corrosion and anode/cathode surface area ratio. In this paper, an electrochemical method for estimating these factors is proposed.
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Jing Liu, Zhifeng Shi and Yimin Shao
Combined defects in ball bearings may be caused during the use or manufacturing process, which can significantly affect their vibration characteristics. The previous defect models…
Abstract
Purpose
Combined defects in ball bearings may be caused during the use or manufacturing process, which can significantly affect their vibration characteristics. The previous defect models in the literature can only describe single defects such as the surface waviness and localized defect. This paper aims to propose an in-depth understanding of radial vibrations of a ball bearing with the combined defect.
Design/methodology/approach
A dynamic model for a ball bearing with the combined defect including the surface waviness and localized defect on its races is proposed. The effects of the combined defect sizes on the radial bearing vibrations are investigated. The results from the proposed model considering the combined defect are compared with the available results from the previous methods considering the single defects.
Findings
The acceleration amplitude is significantly affected by the surface waviness, localized defect and the combined defect on its races. The effect of the combined defect on the acceleration amplitude is larger than that of the single defect. The amplitude and peak frequency of the spectrum of acceleration for the combined defect increases with the defect sizes. The RMS value of the accelerations for the combined defect increases with the combined defect sizes.
Originality/value
Consequently, the proposed model can predict more accurate and in-depth understanding of the radial vibrations caused by the combined defect in the ball bearing.
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Qian Hu, Jing Liu and Feng Huang
The purpose of this paper was to clarify the influence of H2PO4-, HCO3-, pH increase and phosphate coating on corrosion rate and localized corrosion tendency of AZ31 magnesium…
Abstract
Purpose
The purpose of this paper was to clarify the influence of H2PO4-, HCO3-, pH increase and phosphate coating on corrosion rate and localized corrosion tendency of AZ31 magnesium alloy.
Design/methodology/approach
The corrosion behavior of AZ31 magnesium alloy in physiological environments was investigated by hydrogen evolution collection measurements, electrochemical techniques and by use of a three-dimensional digital microscope.
Findings
H2PO4- and HCO3- have corrosion inhibition effects on AZ31 magnesium alloy in normal saline solutions. After immersing for 54 h, the surface undulations decrease from 100 to about 60 μm and 45 μm. The average corrosion rate decreased with increasing pH value. The localized corrosion tendency, however, increased significantly. CaHPO4·2H2O [dicalcium phosphate dehydrate (DCPD)] coating could decrease the initial icorr of AZ31 substrate in Hank’s solution. With partial dissolution of the coating, localized corrosion was readily evident on the AZ31 substrate surface, and a large corrosion pit with depth of over 350 μm appeared. The combined effect of the presence of inhibited ions, the increase in pH during corrosion process and the DCPD coating caused the decrease in the average corrosion rate while enhancing the localized corrosion tendency, resulting in the observed localized attack.
Originality/value
The paper provides an essential insight into the localized corrosion mechanism of AZ31 magnesium alloy in physiological environments.
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Mhd Anwar Orabi, Jin Qiu, Liming Jiang and Asif Usmani
Reinforced concrete slabs in fire have been heavily studied over the last three decades. However, most experimental and numerical work focuses on long-duration uniform exposure to…
Abstract
Purpose
Reinforced concrete slabs in fire have been heavily studied over the last three decades. However, most experimental and numerical work focuses on long-duration uniform exposure to standard fire. Considerably less effort has been put into investigating the response to localised fires that result in planarly non-uniform temperature distribution in the exposed elements.
Design/methodology/approach
In this paper, the OpenSees for Fire framework for modelling slabs under non-uniform fire exposure is presented, verified against numerical predictions by Abaqus and then validated against experimental tests. The thermal wrapper developed within OpenSees for Fire is then utilised to apply localised fire exposure to the validated slab models using the parameters of an experimentally observed localised fire. The effect of the smoke layer is also considered in this model and shown to significantly contribute to the thermal and thus thermo-mechanical response of slabs. Finally, the effect of localised fire heat release rate (HRR) and boundary conditions are studied.
Findings
The analysis showed that boundary conditions are very important for the response of slabs subject to localised fire, and expansive strains may be accommodated as deflections without severely damaging the slab by considering the lateral restraint.
Originality/value
This work demonstrates the capabilities of OpenSees for Fire in modelling structural behaviours subjected to non-uniform fire conditions and investigates the damage pattens of flat slabs exposed to localised fires. It is an advancing step towards understanding structural responses to realistic fires.
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V.S. Khandetskyi and Yury A. Tonkoshkur
The purpose of this paper is to explore and develop specific models of the kinetics of isothermal depolarization currents (IDC) and the corresponding methods for the diagnostics…
Abstract
Purpose
The purpose of this paper is to explore and develop specific models of the kinetics of isothermal depolarization currents (IDC) and the corresponding methods for the diagnostics of the physical parameters of localized electronic states (LES) in heterogeneous materials and corresponding polycrystalline semiconductor materials and heterogeneous insulators with a conductive phase.
Design/methodology/approach
Analysis of the kinetics of isothermal depolarization on the basis of the models allowed the authors to establish a sufficient level of their information content. This also allowed the possibility of applying for research and testing of heterogeneous structures of electronic technique.
Findings
Optimal conditions (full charge of LES on one side of the object and full discharge on the other side) and the correction factors, allowed the researchers to find concentration of these states using the developed models.
Originality/value
This paper uses a particular method to determine and test the parameters of LES, including operations of determining the time constant of IDC signal from its frequency spectrum, finding the ionization energy and the capture coefficient of electrons from the temperature dependence of this time constant, determining the concentration based on the integration of the time dependence of current density of IDC in the time interval that boundaries are determined from the limited range of frequencies of the signal IDC spectrum has been proposed, validated and verified by numerical experiments.
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D. Brancherie and A. Ibrahimbegovic
The purpose of this paper is to present a finite element model capable of describing both the diffuse damage mechanism which develops first during the loading of massive brittle…
Abstract
Purpose
The purpose of this paper is to present a finite element model capable of describing both the diffuse damage mechanism which develops first during the loading of massive brittle structures and the failure process, essentially due to the propagation of a macro‐crack responsible for the softening behaviour of the structure. The theoretical developments for such a model are presented, considering an isotropic damage model for the continuum and a Coulomb‐type criterion for the localized part.
Design/methodology/approach
This is achieved by activating subsequently diffuse and localized damage mechanisms. Localized phenomena are taken into account by means of the introduction of a displacement discontinuity at the element level.
Findings
It was found that, with such an approach, the final crack direction is predicted quite well, in fact much better than the prediction made by the fracture mechanics type of models considering combination of only elastic response and softening.
Originality/value
The presented model has the potential to describe complex damage phenomena in a cyclic and/or non‐proportional loading program, such as crack closing and re‐opening, cohesive resistance deterioration due to tangential sliding, by using only a few parameters compared to the traditional models for cyclic loading.
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A.A. El Warraky, A.M. El‐Aziz and Kh.A. Soliman
The paper aims to examine the enrichment of Cu, and the type of attack occurring during the dissolution of Al‐3.84%Cu alloy in 0.5 M NaCl.
Abstract
Purpose
The paper aims to examine the enrichment of Cu, and the type of attack occurring during the dissolution of Al‐3.84%Cu alloy in 0.5 M NaCl.
Design/methodology/approach
The approach is to use scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopy (EDX) and solution analyses using atomic absorption spectrophotometry.
Findings
Examination of the surface after five days of immersion showed that Cu clusters had concentrated around the edges of the specimens and two types of localized corrosion were evident, pitting corrosion and the intergranular attack, which were concentrated mainly around the Cu clusters and in the regions where a higher concentration of Cu was evident. After 80 days of immersion, the higher concentration of Cu was distributed over nearly the entire surface and its concentration differed from one site to another.
Originality/value
The analysis results of the study confirmed the mechanism of simultaneous dissolution.