Okan Ozkok - Academia.edu (original) (raw)
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Papers by Okan Ozkok
Ultrasonics, Sep 1, 2014
A mathematical model is developed and presented to capture the effect of viscoelastic nature of a... more A mathematical model is developed and presented to capture the effect of viscoelastic nature of a material on modulated ultrasound (US) pulses. The model is established by considering perturbation of material elements subject to modulated US pulses and by introducing the exponential relaxation of the perturbed fluid elements with a spectrum of time constants. Both the model and experimental findings revealed that consecutive perturbation of a material via the modulated US pulses enabled to probe the relaxation times of similar order of magnitudes to the frequency of the US modulation while filtering out the impact of other relaxation times on the US measurement. The US experimental results were verified by those of a conventional rheometer. Hence carrying out measurements at different US modulation frequencies in the Hz ranges seems to allow one to obtain the relaxation time spectrum of the investigated material in the time scales of milliseconds to seconds.
Ultrasonic methods for material characterization have increasingly been used for the last decades... more Ultrasonic methods for material characterization have increasingly been used for the last decades thanks to advances in electronics and digital technologies since conventional methods accommodate several disadvantages like being time consuming. Advanced technology has brought highly accurate measurements with reasonable confidence level, and flexible ultrasonic testing parameters. The aim of this work is to carry out material characterization by combining modeling study and outputs of the ultrasonic device. This study, being both theoretical and experimental, is divided into three subsections; characterization of non-linear viscoelastic parameters, concentration measurement of solutions and particle size measurement by ultrasonic methods. Viscoelastic parameters can be determined by the use of ultrasonic methods. In order to evaluate the output of the ultrasonic measurements properly, these outputs should be related to the material properties. Hence, a proper modeling of sound propa...
Ultrasonics, 2014
Microchannel plates (MCPs) are widely utilized as key device components in various photomultiplie... more Microchannel plates (MCPs) are widely utilized as key device components in various photomultipliers; however, the performance of MCPs cannot be further improved by traditional processing. Atomic layer deposition (ALD) is a promising route to prepare a composite conductive layer and secondary electron emission (SEE) layer structure on the inner wall of the MCP. Moreover, ZnO is an essential component of a composite conductive layer, which is located at the bottom of the SEE layer and significantly influences the SEE coefficient, which, in turn, affects the gain performance of MCPs. Herein, ALD is used to deposit different thicknesses of ZnO films (1-50 nm) on an Si substrate, resulting in an ZnO/Si double-layer film structure. The relationship between the SEE coefficient and the primary electron energy of ZnO films with different thicknesses was established. The maximum secondary electron yield value of 2.04 is achieved at a film thickness of 30 nm. Moreover, Dionne's SEE model and theory of semiconductors are used to simulate and verify the experimental results. These results provide useful guidelines for the development of ALD-MCPs.
Ultrasonics, Sep 1, 2014
A mathematical model is developed and presented to capture the effect of viscoelastic nature of a... more A mathematical model is developed and presented to capture the effect of viscoelastic nature of a material on modulated ultrasound (US) pulses. The model is established by considering perturbation of material elements subject to modulated US pulses and by introducing the exponential relaxation of the perturbed fluid elements with a spectrum of time constants. Both the model and experimental findings revealed that consecutive perturbation of a material via the modulated US pulses enabled to probe the relaxation times of similar order of magnitudes to the frequency of the US modulation while filtering out the impact of other relaxation times on the US measurement. The US experimental results were verified by those of a conventional rheometer. Hence carrying out measurements at different US modulation frequencies in the Hz ranges seems to allow one to obtain the relaxation time spectrum of the investigated material in the time scales of milliseconds to seconds.
Ultrasonic methods for material characterization have increasingly been used for the last decades... more Ultrasonic methods for material characterization have increasingly been used for the last decades thanks to advances in electronics and digital technologies since conventional methods accommodate several disadvantages like being time consuming. Advanced technology has brought highly accurate measurements with reasonable confidence level, and flexible ultrasonic testing parameters. The aim of this work is to carry out material characterization by combining modeling study and outputs of the ultrasonic device. This study, being both theoretical and experimental, is divided into three subsections; characterization of non-linear viscoelastic parameters, concentration measurement of solutions and particle size measurement by ultrasonic methods. Viscoelastic parameters can be determined by the use of ultrasonic methods. In order to evaluate the output of the ultrasonic measurements properly, these outputs should be related to the material properties. Hence, a proper modeling of sound propa...
Ultrasonics, 2014
Microchannel plates (MCPs) are widely utilized as key device components in various photomultiplie... more Microchannel plates (MCPs) are widely utilized as key device components in various photomultipliers; however, the performance of MCPs cannot be further improved by traditional processing. Atomic layer deposition (ALD) is a promising route to prepare a composite conductive layer and secondary electron emission (SEE) layer structure on the inner wall of the MCP. Moreover, ZnO is an essential component of a composite conductive layer, which is located at the bottom of the SEE layer and significantly influences the SEE coefficient, which, in turn, affects the gain performance of MCPs. Herein, ALD is used to deposit different thicknesses of ZnO films (1-50 nm) on an Si substrate, resulting in an ZnO/Si double-layer film structure. The relationship between the SEE coefficient and the primary electron energy of ZnO films with different thicknesses was established. The maximum secondary electron yield value of 2.04 is achieved at a film thickness of 30 nm. Moreover, Dionne's SEE model and theory of semiconductors are used to simulate and verify the experimental results. These results provide useful guidelines for the development of ALD-MCPs.