Alem Teklu - Academia.edu (original) (raw)
Papers by Alem Teklu
SPIE Proceedings, Mar 3, 2005
After a short description of the Ultrasonic Polar Scan Technique for characterizing composites, t... more After a short description of the Ultrasonic Polar Scan Technique for characterizing composites, the paper describes and experimental method, based on the combination of Schlieren photography and a wide bounded ultrasonic beam of frequency 10 MHz, used to detect the fiber direction in composites. Experiments are described on unidirectional and fabric fiber reinforced composites. This method is not based on
Proceedings of SPIE, Mar 3, 2005
ABSTRACT A study by means of the Schlieren technique for visualization of ultrasonic beams, has r... more ABSTRACT A study by means of the Schlieren technique for visualization of ultrasonic beams, has revealed that when leaky Rayleigh waves, propagating along the horizontal edge of a thick, fluid loaded solid plate, are scattered at the extremity of the plate, they travel around the corner and start leaking into the liquid along the Rayleigh angle measured from the normal to the vertical edge of the plate. Furthermore the study reveals that leaky Rayleigh waves are stimulated by the border of an incident ultrasonic bounded beam, more than by the interior of the beam. Comparison with an earlier work shows that the characteristics of the scattering of leaky Rayleigh waves at the edge of the plate is very different from that of Scholte - Stoneley waves.
Bulletin of the American Physical Society, 2018
The Journal of the Acoustical Society of America, 2015
Acousto-Optic Bragg Imaging is a technique that uses the interaction of light with ultrasound to ... more Acousto-Optic Bragg Imaging is a technique that uses the interaction of light with ultrasound to optically image obstructions in acoustical fields. Existing reports of Acousto-Optic Bragg Imaging based on transmission of acoustic fields through obstructions exhibit strong acoustic impedance mismatches manifested by poor image quality and missing details of physical structures of obstructions. In this work, the image quality was improved to exhibit detailed physical structures of an object by using an improved Bragg imaging system. This project investigates the possibility of extending an acoustic Bragg imaging technique in transmission mode to image animal or plant tissues; a small Azalea leaf is used as an illustration in this case. The Bragg image produced clearly shows the veins of the vascular azalea leaf serving as a proof of concept for cost-effective potential application of acoustic Bragg imaging of biological objects in the medical field. Moreover, acousto-optic Bragg imaging is potentially harml...
Physical Review B, 2001
... While there exists no general review, there are a few well documented cases where these two p... more ... While there exists no general review, there are a few well documented cases where these two phenomena are linked; FS Tautz et al., Physica B 206-207, 29 (1995) [INSPEC];[ADS]; T. Terashima et al., Phys. Rev. ... 82, 3669 (1999) [SPIN];[INSPEC];[CAS]; AA Teklu et al., Phys. ...
The Journal of the Acoustical Society of America, 2004
ABSTRACT
Journal of Applied Physics, 2004
ABSTRACT After a short description of the Ultrasonic Polar Scan Technique for characterizing comp... more ABSTRACT After a short description of the Ultrasonic Polar Scan Technique for characterizing composites, the paper describes and experimental method, based on the combination of Schlieren photography and a wide bounded ultrasonic beam of frequency 10 MHz, used to detect the fiber direction in composites. Experiments are described on unidirectional and fabric fiber reinforced composites. This method is not based on Lamb wave phenomena, which can be complicated for interpretation by NDT engineers, but rather is based on relatively simple principles, i.e. inhomogeneity and symmetry of composites.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
The Journal of Physical Chemistry B
Both in critical fluids in microgravity and in nanocolloids on Earth there are long-range correla... more Both in critical fluids in microgravity and in nanocolloids on Earth there are long-range correlations leading to phase separation, respectively, giant concentration-driven fluctuations. Small angle light scattering is commonly used for studying fluctuations and phase separation involving structures that are in the size range up to a hundred nanometers. The static structure factor is the Fourier transformed pair correlation function, which describes the arrangement of the particles in real space. We used experimental data for pure fluids near critical point in microgravity and nanocolloids on Earth to investigate similarities between the effective interaction potentials. For this purpose, the structure factor was determined from twodimensional snapshots of the sample cell. Using OrnsteinZernike approximation, we directly integrated the structure factor and obtained the radial distribution function, which it is a measure of the probability of finding a particle at a distance r away f...
Bulletin of the American Physical Society, 2017
Bulletin of the American Physical Society, 2018
Bulletin of the American Physical Society, 2016
The Journal of Physical Chemistry A, 2021
Two-photon absorption (TPA) enables the excitation of molecules by comparatively lower energy pho... more Two-photon absorption (TPA) enables the excitation of molecules by comparatively lower energy photons with longer penetration depth and higher spatial precision control, which advances the uses of organic molecules in various applications. In this work, we report two simple all-organic molecules C42H33N (compound 3) and C138H168N4 (compound 14) with strong TPA and fluorescent emission activity. Density functional theory calculations show that the enhanced oscillator strengths could be responsible for improved TPA and emission activity for compound 14 compared to those for 3. The degradation of C138H168N4 under focused laser illumination without circulation is almost negligible within 5 h, making it a candidate for laser dyes. Solid-state measurements confirm the presence of a direct band gap for electron transition that determines the channel to release the absorbed energy and functionality of the studied molecules. This work emphasizes that a high TPA cross-section and selectable energy relaxation (fluorescent emission or heat dissipation) are equally important to the design of advanced functional TPA molecules.
Advances in Condensed Matter Physics, 2019
Nanoindentation coupled with Atomic Force Microscopy was used to study stiffness, hardness, and t... more Nanoindentation coupled with Atomic Force Microscopy was used to study stiffness, hardness, and the reduced Young’s modulus of reduced graphene oxide. Oxygen reduction on the graphene oxide sample was performed via LightScribe DVD burner reduction, a cost-effective approach with potential for large scale graphene production. The reduction of oxygen in the graphene oxide sample was estimated to about 10 percent using FTIR spectroscopic analysis. Images of the various samples were captured after each reduction cycle using Atomic Force Microscopy. Elastic and spectroscopic analyses were performed on the samples after each oxygen reduction cycle in the LightScribe, thus allowing for a comparison of stiffness, hardness, and the reduced Young’s modulus based on the number of reduction cycles. The highest values obtained were after the fifth and final reduction cycle, yielding a stiffness of 22.4 N/m, a hardness of 0.55 GPa, and a reduced Young’s modulus of 1.62 GPa as compared to a stiffn...
Journal of Electrostatics, 2017
A prototype direct current Triboelectric Generator (DC-TEG) has been demonstrated to generate dc ... more A prototype direct current Triboelectric Generator (DC-TEG) has been demonstrated to generate dc voltages by converting rotational mechanical energy into electrical energy. The generator primarily consists of four rotating wheels (two copper electrodes and two triboelectric wheels) utilizing polytetrafluoroethylene (PTFE) tape and nylon fabric as the triboelectric materials in its design. Once subjected to mechanical rotation, the interaction between the two triboelectric materials generates equal, but opposite charges and deposits them on the copper electrodes resulting in a dc voltage across an external load resistor. The power performance of the generator, with changes in angular frequency, has been measured. The DC-TEG can generate a dc voltage of about 1.5 mV across an external load of 15.14 kU at a rotational speed of 10 rev/sec. The dependences of current and power on the angular frequency have also been measured. Preliminary results suggest that the relationship between the dc voltage and angular frequency of rotation is quadratic, however further measurements at relatively higher angular frequencies are needed before a proper mathematical model may be established. The work here presents a new, cost-effective DC-TEG design for power scavenging from any naturally or artificially occurring rotational mechanical motion facilitated by either wind or flowing water for selfpowered systems.
Two experiments were performed using shadowgraph visualization technique in order to study the dy... more Two experiments were performed using shadowgraph visualization technique in order to study the dynamics of concentration fluctuations. Our experimental setup includes an objective attached to the CCD camera to increase the field of view. Using two colloidal suspensions, one with gold and the other with silica colloid, we extracted both the structure factors and the correlation time during the intermediate to late stages of concentration fluctuations during a free diffusion experiment. The temporal evolution of fluctuations is qualitatively investigated using recursive plots and spatial-temporal sections of fluctuating images. Our experiment reveals significant differences between concentration fluctuations induced in silica and gold colloids. The difference is explained not only in terms of the particle size but due to the possible plasmonic interaction between gold nanoparticle and the incident light.
Physica B: Condensed Matter, 2001
In this paper, we brie#y review some recent de Haas}van Alphen experiments on two contrasting typ... more In this paper, we brie#y review some recent de Haas}van Alphen experiments on two contrasting types of heavy fermion/Kondo systems for varying concentrations x of the impurity ion, Ce or U. In both systems the Fermi surface is observed to vary continuously with x: in Ce V La \V B , the Ce ions behave as Kondo impurities while in U V Th \V Be they do not. In both systems, the f electrons are also found to contribute only partially to the Fermi surface volume, therefore raising questions concerning the applicability of Fermi liquid theory.
SPIE Proceedings, Mar 3, 2005
After a short description of the Ultrasonic Polar Scan Technique for characterizing composites, t... more After a short description of the Ultrasonic Polar Scan Technique for characterizing composites, the paper describes and experimental method, based on the combination of Schlieren photography and a wide bounded ultrasonic beam of frequency 10 MHz, used to detect the fiber direction in composites. Experiments are described on unidirectional and fabric fiber reinforced composites. This method is not based on
Proceedings of SPIE, Mar 3, 2005
ABSTRACT A study by means of the Schlieren technique for visualization of ultrasonic beams, has r... more ABSTRACT A study by means of the Schlieren technique for visualization of ultrasonic beams, has revealed that when leaky Rayleigh waves, propagating along the horizontal edge of a thick, fluid loaded solid plate, are scattered at the extremity of the plate, they travel around the corner and start leaking into the liquid along the Rayleigh angle measured from the normal to the vertical edge of the plate. Furthermore the study reveals that leaky Rayleigh waves are stimulated by the border of an incident ultrasonic bounded beam, more than by the interior of the beam. Comparison with an earlier work shows that the characteristics of the scattering of leaky Rayleigh waves at the edge of the plate is very different from that of Scholte - Stoneley waves.
Bulletin of the American Physical Society, 2018
The Journal of the Acoustical Society of America, 2015
Acousto-Optic Bragg Imaging is a technique that uses the interaction of light with ultrasound to ... more Acousto-Optic Bragg Imaging is a technique that uses the interaction of light with ultrasound to optically image obstructions in acoustical fields. Existing reports of Acousto-Optic Bragg Imaging based on transmission of acoustic fields through obstructions exhibit strong acoustic impedance mismatches manifested by poor image quality and missing details of physical structures of obstructions. In this work, the image quality was improved to exhibit detailed physical structures of an object by using an improved Bragg imaging system. This project investigates the possibility of extending an acoustic Bragg imaging technique in transmission mode to image animal or plant tissues; a small Azalea leaf is used as an illustration in this case. The Bragg image produced clearly shows the veins of the vascular azalea leaf serving as a proof of concept for cost-effective potential application of acoustic Bragg imaging of biological objects in the medical field. Moreover, acousto-optic Bragg imaging is potentially harml...
Physical Review B, 2001
... While there exists no general review, there are a few well documented cases where these two p... more ... While there exists no general review, there are a few well documented cases where these two phenomena are linked; FS Tautz et al., Physica B 206-207, 29 (1995) [INSPEC];[ADS]; T. Terashima et al., Phys. Rev. ... 82, 3669 (1999) [SPIN];[INSPEC];[CAS]; AA Teklu et al., Phys. ...
The Journal of the Acoustical Society of America, 2004
ABSTRACT
Journal of Applied Physics, 2004
ABSTRACT After a short description of the Ultrasonic Polar Scan Technique for characterizing comp... more ABSTRACT After a short description of the Ultrasonic Polar Scan Technique for characterizing composites, the paper describes and experimental method, based on the combination of Schlieren photography and a wide bounded ultrasonic beam of frequency 10 MHz, used to detect the fiber direction in composites. Experiments are described on unidirectional and fabric fiber reinforced composites. This method is not based on Lamb wave phenomena, which can be complicated for interpretation by NDT engineers, but rather is based on relatively simple principles, i.e. inhomogeneity and symmetry of composites.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
The Journal of Physical Chemistry B
Both in critical fluids in microgravity and in nanocolloids on Earth there are long-range correla... more Both in critical fluids in microgravity and in nanocolloids on Earth there are long-range correlations leading to phase separation, respectively, giant concentration-driven fluctuations. Small angle light scattering is commonly used for studying fluctuations and phase separation involving structures that are in the size range up to a hundred nanometers. The static structure factor is the Fourier transformed pair correlation function, which describes the arrangement of the particles in real space. We used experimental data for pure fluids near critical point in microgravity and nanocolloids on Earth to investigate similarities between the effective interaction potentials. For this purpose, the structure factor was determined from twodimensional snapshots of the sample cell. Using OrnsteinZernike approximation, we directly integrated the structure factor and obtained the radial distribution function, which it is a measure of the probability of finding a particle at a distance r away f...
Bulletin of the American Physical Society, 2017
Bulletin of the American Physical Society, 2018
Bulletin of the American Physical Society, 2016
The Journal of Physical Chemistry A, 2021
Two-photon absorption (TPA) enables the excitation of molecules by comparatively lower energy pho... more Two-photon absorption (TPA) enables the excitation of molecules by comparatively lower energy photons with longer penetration depth and higher spatial precision control, which advances the uses of organic molecules in various applications. In this work, we report two simple all-organic molecules C42H33N (compound 3) and C138H168N4 (compound 14) with strong TPA and fluorescent emission activity. Density functional theory calculations show that the enhanced oscillator strengths could be responsible for improved TPA and emission activity for compound 14 compared to those for 3. The degradation of C138H168N4 under focused laser illumination without circulation is almost negligible within 5 h, making it a candidate for laser dyes. Solid-state measurements confirm the presence of a direct band gap for electron transition that determines the channel to release the absorbed energy and functionality of the studied molecules. This work emphasizes that a high TPA cross-section and selectable energy relaxation (fluorescent emission or heat dissipation) are equally important to the design of advanced functional TPA molecules.
Advances in Condensed Matter Physics, 2019
Nanoindentation coupled with Atomic Force Microscopy was used to study stiffness, hardness, and t... more Nanoindentation coupled with Atomic Force Microscopy was used to study stiffness, hardness, and the reduced Young’s modulus of reduced graphene oxide. Oxygen reduction on the graphene oxide sample was performed via LightScribe DVD burner reduction, a cost-effective approach with potential for large scale graphene production. The reduction of oxygen in the graphene oxide sample was estimated to about 10 percent using FTIR spectroscopic analysis. Images of the various samples were captured after each reduction cycle using Atomic Force Microscopy. Elastic and spectroscopic analyses were performed on the samples after each oxygen reduction cycle in the LightScribe, thus allowing for a comparison of stiffness, hardness, and the reduced Young’s modulus based on the number of reduction cycles. The highest values obtained were after the fifth and final reduction cycle, yielding a stiffness of 22.4 N/m, a hardness of 0.55 GPa, and a reduced Young’s modulus of 1.62 GPa as compared to a stiffn...
Journal of Electrostatics, 2017
A prototype direct current Triboelectric Generator (DC-TEG) has been demonstrated to generate dc ... more A prototype direct current Triboelectric Generator (DC-TEG) has been demonstrated to generate dc voltages by converting rotational mechanical energy into electrical energy. The generator primarily consists of four rotating wheels (two copper electrodes and two triboelectric wheels) utilizing polytetrafluoroethylene (PTFE) tape and nylon fabric as the triboelectric materials in its design. Once subjected to mechanical rotation, the interaction between the two triboelectric materials generates equal, but opposite charges and deposits them on the copper electrodes resulting in a dc voltage across an external load resistor. The power performance of the generator, with changes in angular frequency, has been measured. The DC-TEG can generate a dc voltage of about 1.5 mV across an external load of 15.14 kU at a rotational speed of 10 rev/sec. The dependences of current and power on the angular frequency have also been measured. Preliminary results suggest that the relationship between the dc voltage and angular frequency of rotation is quadratic, however further measurements at relatively higher angular frequencies are needed before a proper mathematical model may be established. The work here presents a new, cost-effective DC-TEG design for power scavenging from any naturally or artificially occurring rotational mechanical motion facilitated by either wind or flowing water for selfpowered systems.
Two experiments were performed using shadowgraph visualization technique in order to study the dy... more Two experiments were performed using shadowgraph visualization technique in order to study the dynamics of concentration fluctuations. Our experimental setup includes an objective attached to the CCD camera to increase the field of view. Using two colloidal suspensions, one with gold and the other with silica colloid, we extracted both the structure factors and the correlation time during the intermediate to late stages of concentration fluctuations during a free diffusion experiment. The temporal evolution of fluctuations is qualitatively investigated using recursive plots and spatial-temporal sections of fluctuating images. Our experiment reveals significant differences between concentration fluctuations induced in silica and gold colloids. The difference is explained not only in terms of the particle size but due to the possible plasmonic interaction between gold nanoparticle and the incident light.
Physica B: Condensed Matter, 2001
In this paper, we brie#y review some recent de Haas}van Alphen experiments on two contrasting typ... more In this paper, we brie#y review some recent de Haas}van Alphen experiments on two contrasting types of heavy fermion/Kondo systems for varying concentrations x of the impurity ion, Ce or U. In both systems the Fermi surface is observed to vary continuously with x: in Ce V La \V B , the Ce ions behave as Kondo impurities while in U V Th \V Be they do not. In both systems, the f electrons are also found to contribute only partially to the Fermi surface volume, therefore raising questions concerning the applicability of Fermi liquid theory.