Sabri Altıntaş | Bogazici University (original) (raw)

Papers by Sabri Altıntaş

Research paper thumbnail of Microwave assisted synthesis of hydroxyapatite nano strips

Crystal Research and Technology, 2010

Synthesis of hydroxyapatite (HAP) nano strips was carried out by chemical precipitation method fo... more Synthesis of hydroxyapatite (HAP) nano strips was carried out by chemical precipitation method followed by microwave irradiation. The microwave assisted reactions proceed at fast rates. It is found that the presence of the complex reagent EDTA plays an important role in the morphological changes of nanostructure hydroxyapatite. EDTA acts as a hexadentate unit by wrapping itself around the Ca2+ metal ion with, four oxygen and two nitrogen atoms and forms several five member chelate rings. The relative specific surface energies associated with the facets of the crystal determines the shape of the crystal. Scanning electron microscopy revealed the presence of hydroxyapatite nano strips with the range 50‐100 nm in EDTA influenced HAP powders. Fourier transform‐infrared spectroscopy (FT‐IR) result combined with the X‐ray diffraction (XRD) indicates the presence of amorphous hydroxyapatite (HAP) in the as‐prepared material. X‐ray patterns collected on the powder after heat‐treatment at 11...

Research paper thumbnail of Effects of Sintering Parameters on Initial Magnetic Permeability of Ni-Zn Ferrite

Key Engineering Materials, May 1, 2004

Ni-Zn ferrites with a nominal composition of Ni1-xZnxFe2O4 for varying x values were prepared by ... more Ni-Zn ferrites with a nominal composition of Ni1-xZnxFe2O4 for varying x values were prepared by cold pressing and sintering. Sintering was performed for various sintering times and temperatures. Initial permeability values were measured for different compositions using toroidal shape specimens. The density and initial permeability values obtained in this study showed good agreement with the results given in recent literature. Between the temperatures, 1000°C and 1150°C, the density and the initial permeability increased with increasing sintering time and sintering temperature. Introduction The importance of the soft ferrites, especially nickel-zinc ferrite, in the transformer field is well known and during the past few years, the processing of good-quality ferrites has received a great deal of attention. However, the aspect of the industrial processing of nickel-zinc ferrites still ask some significant questions in the area of how the processing conditions affect the microstructure and how the microstructure influences the magnetic performance. For this purpose, in this study we have investigated a relation between the sintering conditions and initial permeability of Ni-Zn ferrite. Because transformers utilize an alternating field, the core material is continually cycled through the hysteresis loop. Therefore the relative area within the hysteresis loop must be small to have low hysteresis energy losses. To lower the hysteresis energy losses, the ferrite transformer core must have a high initial permeability. Experimental Procedure Mixture Preparation and Compaction. The mixture consisted of NiO, ZnO and Fe2O3 powders. By using the composition formula (NiO)1-x (ZnO)x Fe2O3 in which x is a fraction lying between 0 and 1.0, four mixtures (x:0.2, 0.3, 0.4, 0.5) were prepared. These mixtures were calcined in air at 900°C for 4 h. The ferrite powder obtained after calcination was mixed with 5 % by weight of polyvinyl alcohol as binder and pressed into toroidal die with 10 mm inside diameter, 20 mm outside diameter and 8 mm height at a pressure of 100 MPa [1-2]. The pressure was maintained a definite period of time (3 min.), dwell time [3]. Sintering. The toroidal green compacts were subsequently sintered in air atmosphere for varying durations of time from 1h to 6h at each temperature in the range from 1000 to 1150°C at intervals of 50°C. After the completion of the sintering period, cooling of the samples was carried out at an approximate rate of 180°C per hour for 2 h from the peak temperatures, and subsequently the furnace was switched off and the samples were allowed to cool naturally to 60°C [4]. The heating rate of the sintering furnace was chosen to be 4°C/min. Measurements. The densities of the ferrite specimens were measured according to the ASTM D792 standard. The inductance values of toroids were measured by Phillips PM6303 LCR-Meter. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 1277-1280 doi:10.4028/www.scientific.net/KEM.264-268.1277 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,18:14:25) The sintered toroids were wound with 20 turns (N) of insulated copper wire. The initial permeability was calculated according to the formula [5]; μ i= L C1/ 4 π N 2 (1) where L is the inductance (H), μ i is the initial permeability, N is number of turns of the copper wire, and C1 is the core constant [5]. The core constant C1 is calculated by the formula, C1 = 2 π / h ln (r2/r1), (2) in the literature [6]. From the fractured surfaces of the samples sintered at 1150°C for 6 hours, ESEM image were obtained. Results and Discussion Sintering. In order to find the optimum sintering parameters for the ferrite particles, preliminary experiments were conducted varying the sintering time from one hour to two hours and sintering temperature from 1000°C to 1150°C. It was observed that the specimens sintered below 1100°C did not show enough density. Although the average sintering time in many applications of ferrite production is around four hours [9], in some extreme cases it can be as short as one hour and as long as 24 hours. It was shown that after 24 hours of sintering no further change in the density and magnetic properties occurred [4]. Table 1 shows the variation in density with increasing sintering time for Ni1-x Znx Fe2O4 at 1150°C. Table 1. Changes in resulting density [g cm -3 ] by changing the sintering time for various fraction at 1150°C. Fig. 1. ESEM image of Ni0,8 Zn0,2 Fe2O4 at 1150°C for six hours sintering time. Fig. 2. ESEM image of Ni0,7 Zn0,3 Fe2O4 at 1150°C for six hours sintering time 1278 Euro Ceramics VIII

Research paper thumbnail of Production of hydroxylapatite reinforced polymer composites for biomedical applications

In this study, hydroxylapatite (HAp) reinforced polymer composites were produced and evaluated to... more In this study, hydroxylapatite (HAp) reinforced polymer composites were produced and evaluated to be used in bone implants. The aim was to produce a material that has similar modulus of elasticity (E) and other mechanical properties to those of bone in order to achieve mechanical compatibility in the body. Hydroxylapatite was mixed with high density polyethylene (HDPE) and polypropylene (PP) in different amounts. The resulting composite materials were injection molded into standard tensile and impact test specimens and their mechanical properties were examined. Stiffness and hardness of the composites were increased toughness, percent elongation and impact energy were decreased with increasing HAp content. No significant change was observed in ultimate tensile strength while an increase was significant in the yield strength related to HAp content. PP composites were found to have better impact properties in spite of more ductile behavior of HDPE composites under tension. Although the problem of mechanical compatibility in bone implants has not totally been solved, hydroxylapatite reinforced polymer composites are suggested as promising alternatives to traditional implant materials.

Research paper thumbnail of Green Manufacturing of Core-Shell Polyvinyl Alcohol-Gelatin Electrospun Nanofiber Scaffolds

Proceedings of the 2nd World Congress on Recent Advances in Nanotechnology, Apr 1, 2017

In tissue engineering, natural/synthetic polymer based fibrous composite scaffolds obtained via e... more In tissue engineering, natural/synthetic polymer based fibrous composite scaffolds obtained via electrospinning method were shown to support the cell adhesion and tissue regeneration. However, electrospinning of natural polymers requires the use of toxic solvents that are negatively affecting the cell proliferation and biocompatibility of the produced scaffolds in addition to the usage of acidic solvents which will result in massive biodegradation inside the body[1] .Here, a method was proposed that is higher safety for the patient and even for the experimentalists who are using harmful and highly volatile solvents. Two types of polymers were used in the synthesis of the scaffolds by mimicking the key features of the tissue extracellular matrix which contains gelatin and coaxially organized nanofibers. Coaxial electrospinning technique was used to obtain core(PVA)-shell (gelatin) nanofibers. While material in the shell provides recognition sites for the tissue cells, core material provides mechanical endurance. Different from conventional methods, proposed work aims to lower the steps of application of the scaffold to the harmed tissue by using only deionized water as solvent. Instead of dissolving PVA and gelatin in toxic and acidic solvents, they were dissolved in the deionized water above the gelation temperature. Later, coaxial electrospinning generated increased cell spread and mechanical stiffness. The samples were characterized by scanning and transmission electron microscopy. Based on the experimental results it is concluded that electrospun fibers obtained from the 8% concentrated gelatin solution had a beaded structure, whereas the coaxially fabricated PVA and gelatin from the same concentration solutions did not show any beaded morphology. Also core-shell fibers have diameters down to 180 nm. This result showed that PVA aids to the uniform gelatin fiber formation which, may give higher mechanical stability. The electron microscopy analysis leading to these results has received support by the

Research paper thumbnail of Deformation behavior of an idealized crystal

The deformation behavior of an idealized crystal made by stacking of parallel slip planes is stud... more The deformation behavior of an idealized crystal made by stacking of parallel slip planes is studied. Each slip plane is assumed to contain active sources of dislocations leading to a constant density of non-interacting dislocations in the plane which glide through randomly distributed localized point obstacles, representing small precipitates. The dislocation is assumed to have a constant line tension and the dislocation-obstacle interaction is taken to have a simple step form. Using results of computer simulation of thermally activated glide through random arrays of point obstacles the deformation is modeled as a function of temperature and applied stress, determining the strain rate and the morphological characteristics of slip. Stress-strain rate and flow stress-temperature relations are discussed. * Let the dislocation under the applied stress T encounter a configuration of point obstacles denoted by i (Figure 1). Between two adjacent

Research paper thumbnail of PLASTIC DEFORMATION OF CRYSTALS: ANALYTICAL AND COMPUTER SIMULATION STUDIES OF DISLOCATION GLIDE - eScholarship

Research paper thumbnail of Computer simulation of dislocation glide—II. Comparison with experiments

Acta Metallurgica, May 1, 1986

We consider the glide of a single dislocation through randomly distributed obstacles of given pro... more We consider the glide of a single dislocation through randomly distributed obstacles of given properties. Using a model we studied the critical resolved shear stress for athermal glide and the velocity of thermally activated glide at finite temperature as functions of the nature and strength of the obstacles. Results obtained from computer simulation are compared with the available experimental data and good agreement is found. Zusammenfassung-Die Gleitung einer einzigen Versetzung durch eine zuf5llige Anordnung von Hindernissen mit vorgegebenen Eigenschaften wird behandelt. Anhand eines Modelles wird die kritische FlieBspannung fiir athermische Gleitung und die Geschwindigkeit der thermisch aktivierten Gleitung bei endlicher Temperatur in Abhlngigkeit von Natur und StLke der Hindernisse untersucht. Die aus der Computersimulation erhaltenen Ergebnisse werden mit den verfiigbaren experimentallen Daten verglichen; es ergibt sich gute iibereinstimmung.

Research paper thumbnail of Production of Hydroxylapatite from Animal Bone

Key Engineering Materials, May 1, 2004

... 1300 2.48 ± 0.08 65.01 ± 41.57 130.68 ±17.90 Fig.3 ESEM micrograph of HAp with PVA binder by ... more ... 1300 2.48 ± 0.08 65.01 ± 41.57 130.68 ±17.90 Fig.3 ESEM micrograph of HAp with PVA binder by 1%, sintered at 1200 o C, 4h. ... [4] AC Tas, F. Korkusuz, M. Timucin and N. Akkas: Journal of Materials Science: Materials in Medicine, Vol. 8 (1997), p. 91. ...

Research paper thumbnail of Computer simulation of dislocation glide—I. Comparison with statistical theories

Acta Metallurgica, May 1, 1986

Research paper thumbnail of Effects of squeeze casting on the properties of Zn-Bi monotectic alloy

Metallurgical and Materials Transactions A, 1997

In composite production, the shortest route is via an in situ composite in which a melt dissociat... more In composite production, the shortest route is via an in situ composite in which a melt dissociates simultaneously into two rather different solid phases. The monotectic alloys can be included in this group. The present work was aimed at extending our recent squeeze casting experience on the Zn-Bi monotectic alloy in order to increase its cast quality and mechanical properties. A squeeze casting unit was built, and its die and punch were machined. The molten monotectic alloy was squeezed in this unit under pressures up to 120 MPa in its freezing range until it solidified completely. It was found that an increase in squeeze casting pressure provided increases in density, tensile strength, and Vickers hardness, which resulted in decreases in chip length and electrical resistivity. Before the squeeze casting practice, the freezing characteristics of this monotectic were estimated using basic solidification principles.

Research paper thumbnail of Effects of squeeze casting on the wide freezing range binary alloys

Materials Science and Engineering: A, 1993

Research paper thumbnail of Hot Torsion Experiments on Aluminium 1100 and 2024

Journal of the mechanical behavior of materials, Mar 1, 1993

In this study, the effects of temperature and strain rate on the deformation behavior of aluminiu... more In this study, the effects of temperature and strain rate on the deformation behavior of aluminium 1100 and aluminium 2024 are investigated. Specimens are tested at different temperatures and strain rates under the effect of torsional loading. The data are evaluated using critical radius and effective length concepts. For both materials, as a result of the interaction between strain hardening and thermal softening mechanisms, flow curves pass through a peak value after which aluminium alloy tends to fracture while aluminium 1100 approaches to a steady state deformation. The strain rate sensitivity of materials obtained from torsion testing increases with temperature. The length of aluminium specimens subjected to free-end hot torsion is observed to change with both temperature and strain rate.

Research paper thumbnail of A System of Cracks in Bonded Half Planes

Springer eBooks, 1987

The structural strength of materials generally depends on the materials properties, the shape and... more The structural strength of materials generally depends on the materials properties, the shape and size of defects as well as the orientation of such flaws in the medium. In the application of fracture mechanics other kinds of imperfections due to manufacturing are particularly important such as flat cavities which develop during casting, small cracks resulting from residual stresses in welded materials, and fatigue cracks under fluctuating external loads. Thus in a design dealing with such materials, it is necessary to have a good estimate of the stress state disturbed by the existence of these flaws.

Research paper thumbnail of Dynamic simulation of a pinned dislocation in an atomistic model

Materials Science and Engineering, Feb 1, 1981

Three different models for a pinned dislocation under an applied stress are compared in this pape... more Three different models for a pinned dislocation under an applied stress are compared in this paper: (i) the continuous string model of Koehler, Granato and Liicke; (ii) a twodimensional lattice model which represents the dislocation in its slip plane; (iii) a discrete string model which is obtained as an analytical approximation to model (ii). Good agreement is obtained for the quantities investigated for the three models. Comparison of models (i) and (iii) therefore provides an atomistic interpretation for the parameters appearing in the defining equation of the continuous string model. One result of interest is that these parameters cannot be expressed solely in terms of the characteristics of the interatomic law for atoms in the vicinity of their equilibrium sites for the perfect crystal but must also depend on aspects of the interatomic force law for atoms in configurations such as those which occur near the dislocation.

Research paper thumbnail of On the Application Possibility of Thermal Barrier Coatings in Space Practices

Recently, application of ceramic coatings to space vehicles to generate thermal barriers upon re-... more Recently, application of ceramic coatings to space vehicles to generate thermal barriers upon re-entry is being considered as a challenge for research. Conventionally, thermal barrier coatings (TBC) are utilized to increase the gas turbine engine components' thermal and oxidation resistance values in order to get higher power and efficiency values. In this study, various superalloys and stainless steels were thermal barrier coated using the atmospheric plasma spraying method. One goal is to determine the effect of the substrate on the performance of the coating. Coating morphology was determined using microscopy analysis. Various heat exposure procedures were applied to the specimens and the changes in the mechanical properties of the top coat were analyzed using indentation techniques. Durability of the coatings after various periods at 1100degC was examined. Microstructures, coatings and reaction products were characterized by optical microscopy and image analysis. Effects of TBC thickness variations were also analyzed in order to achieve a model for determining the optimum coating thickness.

Research paper thumbnail of Control of mechanical and wear properties of a commercial Al-Si eutectic alloy

Journal of Materials Science, 1992

The properties examined as a function of microstructural modification were ultimate tensile stren... more The properties examined as a function of microstructural modification were ultimate tensile strength, fracture elongation, Vickers hardness and wear resistance. The microstructural modification was achieved by rapid cooling and additions of small amounts of strontium and lithium master alloys into the eutectic melt. In all experiments the commercial ETIAL 140 alloy was cast instead of a high-purity aluminium-silicon eutectic. This allowed determination of the effect of modification treatment, both on silicon and intermetallic phases. It was found that the slowly cooled and unalloyed castings which contained coarse silicon flakes showed highest wear resistance and lowest ultimate tensile strength, fracture elongation and Vickers hardness values. Rapid cooling and also additions of strontium and lithium master alloys reduced the eutectic interphase spacing and refined the silicon phase. This usually corresponded to a significant increase in all properties except the wear resistance. It was noted, however, that the size of the intermetallic phase particles increased abruptly above 0.04% Sr content which resulted in a sharp reduction in all mechanical properties. Unlike the strontium effect, the lithium addition did not influence the intermetallic size significantly and, therefore, the mechanical properties were not impaired. In addition, the wear resistance also remained relatively unaffected because lithium solid solution hardened the primary aluminium dendrites appeared in the modified alloys.

Research paper thumbnail of Fabrication of Al matrix composites reinforced with high contents of Si particles

Journal of Materials Science Letters, 1996

Research paper thumbnail of Production of Hypereutectic Al-Si Alloys by P/M Route

Materials Science Forum, May 1, 1996

Research paper thumbnail of Plastic deformation of crystals: analytical and computer simulation studies of dislocation glide

The plastic deformation of crystals is usually accomplished through the motion of dislocations. T... more The plastic deformation of crystals is usually accomplished through the motion of dislocations. The glide of a dislocation is impelled by the applied stress and opposed by microstructural defects such as point defects, voids, pre cipitates and other dislocations.

Research paper thumbnail of The microstructural control of cast and mechanical properties of zinc-aluminium alloys

Journal of Materials Science, 1993

The zinc-aluminium alloys containing 8, 1 2, and 27% aluminium are finding increasing application... more The zinc-aluminium alloys containing 8, 1 2, and 27% aluminium are finding increasing applications in the casting industry. These alloys are stronger than most aluminium alloys. In addition, they possess high wear resistance and bearing properties. However, surface sinks and shrinkage defects are observed on the bottom faces of such castings, contrary to general foundry practice. In the present investigation, this problem observed in the Zn-8%AI, Zn-12%AI, Zn-27%AI alloys was tackled by controlling various casting parameters and also by additions of the master alloys of strontium and lithium into the molten alloys. It was found that the underside shrinkage problem was influenced by the aluminium content of the alloy, melt superheat, casting size and cooling conditions'. The strontium and lithium additions were found to be beneficial in reducing the underside shrinkage problem. The ultimate tensile strength, fracture elongation and Vickers hardness were all increased with aluminium concentration and lithium addition. It was found also that the most problematical Zn-27%AI alloy, which provided the highest mechanical properties, was very suitable for the squeezecasting technique. The mechanical properties were increased sharply in these squeeze-cast bars.

Research paper thumbnail of Microwave assisted synthesis of hydroxyapatite nano strips

Crystal Research and Technology, 2010

Synthesis of hydroxyapatite (HAP) nano strips was carried out by chemical precipitation method fo... more Synthesis of hydroxyapatite (HAP) nano strips was carried out by chemical precipitation method followed by microwave irradiation. The microwave assisted reactions proceed at fast rates. It is found that the presence of the complex reagent EDTA plays an important role in the morphological changes of nanostructure hydroxyapatite. EDTA acts as a hexadentate unit by wrapping itself around the Ca2+ metal ion with, four oxygen and two nitrogen atoms and forms several five member chelate rings. The relative specific surface energies associated with the facets of the crystal determines the shape of the crystal. Scanning electron microscopy revealed the presence of hydroxyapatite nano strips with the range 50‐100 nm in EDTA influenced HAP powders. Fourier transform‐infrared spectroscopy (FT‐IR) result combined with the X‐ray diffraction (XRD) indicates the presence of amorphous hydroxyapatite (HAP) in the as‐prepared material. X‐ray patterns collected on the powder after heat‐treatment at 11...

Research paper thumbnail of Effects of Sintering Parameters on Initial Magnetic Permeability of Ni-Zn Ferrite

Key Engineering Materials, May 1, 2004

Ni-Zn ferrites with a nominal composition of Ni1-xZnxFe2O4 for varying x values were prepared by ... more Ni-Zn ferrites with a nominal composition of Ni1-xZnxFe2O4 for varying x values were prepared by cold pressing and sintering. Sintering was performed for various sintering times and temperatures. Initial permeability values were measured for different compositions using toroidal shape specimens. The density and initial permeability values obtained in this study showed good agreement with the results given in recent literature. Between the temperatures, 1000°C and 1150°C, the density and the initial permeability increased with increasing sintering time and sintering temperature. Introduction The importance of the soft ferrites, especially nickel-zinc ferrite, in the transformer field is well known and during the past few years, the processing of good-quality ferrites has received a great deal of attention. However, the aspect of the industrial processing of nickel-zinc ferrites still ask some significant questions in the area of how the processing conditions affect the microstructure and how the microstructure influences the magnetic performance. For this purpose, in this study we have investigated a relation between the sintering conditions and initial permeability of Ni-Zn ferrite. Because transformers utilize an alternating field, the core material is continually cycled through the hysteresis loop. Therefore the relative area within the hysteresis loop must be small to have low hysteresis energy losses. To lower the hysteresis energy losses, the ferrite transformer core must have a high initial permeability. Experimental Procedure Mixture Preparation and Compaction. The mixture consisted of NiO, ZnO and Fe2O3 powders. By using the composition formula (NiO)1-x (ZnO)x Fe2O3 in which x is a fraction lying between 0 and 1.0, four mixtures (x:0.2, 0.3, 0.4, 0.5) were prepared. These mixtures were calcined in air at 900°C for 4 h. The ferrite powder obtained after calcination was mixed with 5 % by weight of polyvinyl alcohol as binder and pressed into toroidal die with 10 mm inside diameter, 20 mm outside diameter and 8 mm height at a pressure of 100 MPa [1-2]. The pressure was maintained a definite period of time (3 min.), dwell time [3]. Sintering. The toroidal green compacts were subsequently sintered in air atmosphere for varying durations of time from 1h to 6h at each temperature in the range from 1000 to 1150°C at intervals of 50°C. After the completion of the sintering period, cooling of the samples was carried out at an approximate rate of 180°C per hour for 2 h from the peak temperatures, and subsequently the furnace was switched off and the samples were allowed to cool naturally to 60°C [4]. The heating rate of the sintering furnace was chosen to be 4°C/min. Measurements. The densities of the ferrite specimens were measured according to the ASTM D792 standard. The inductance values of toroids were measured by Phillips PM6303 LCR-Meter. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 1277-1280 doi:10.4028/www.scientific.net/KEM.264-268.1277 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,18:14:25) The sintered toroids were wound with 20 turns (N) of insulated copper wire. The initial permeability was calculated according to the formula [5]; μ i= L C1/ 4 π N 2 (1) where L is the inductance (H), μ i is the initial permeability, N is number of turns of the copper wire, and C1 is the core constant [5]. The core constant C1 is calculated by the formula, C1 = 2 π / h ln (r2/r1), (2) in the literature [6]. From the fractured surfaces of the samples sintered at 1150°C for 6 hours, ESEM image were obtained. Results and Discussion Sintering. In order to find the optimum sintering parameters for the ferrite particles, preliminary experiments were conducted varying the sintering time from one hour to two hours and sintering temperature from 1000°C to 1150°C. It was observed that the specimens sintered below 1100°C did not show enough density. Although the average sintering time in many applications of ferrite production is around four hours [9], in some extreme cases it can be as short as one hour and as long as 24 hours. It was shown that after 24 hours of sintering no further change in the density and magnetic properties occurred [4]. Table 1 shows the variation in density with increasing sintering time for Ni1-x Znx Fe2O4 at 1150°C. Table 1. Changes in resulting density [g cm -3 ] by changing the sintering time for various fraction at 1150°C. Fig. 1. ESEM image of Ni0,8 Zn0,2 Fe2O4 at 1150°C for six hours sintering time. Fig. 2. ESEM image of Ni0,7 Zn0,3 Fe2O4 at 1150°C for six hours sintering time 1278 Euro Ceramics VIII

Research paper thumbnail of Production of hydroxylapatite reinforced polymer composites for biomedical applications

In this study, hydroxylapatite (HAp) reinforced polymer composites were produced and evaluated to... more In this study, hydroxylapatite (HAp) reinforced polymer composites were produced and evaluated to be used in bone implants. The aim was to produce a material that has similar modulus of elasticity (E) and other mechanical properties to those of bone in order to achieve mechanical compatibility in the body. Hydroxylapatite was mixed with high density polyethylene (HDPE) and polypropylene (PP) in different amounts. The resulting composite materials were injection molded into standard tensile and impact test specimens and their mechanical properties were examined. Stiffness and hardness of the composites were increased toughness, percent elongation and impact energy were decreased with increasing HAp content. No significant change was observed in ultimate tensile strength while an increase was significant in the yield strength related to HAp content. PP composites were found to have better impact properties in spite of more ductile behavior of HDPE composites under tension. Although the problem of mechanical compatibility in bone implants has not totally been solved, hydroxylapatite reinforced polymer composites are suggested as promising alternatives to traditional implant materials.

Research paper thumbnail of Green Manufacturing of Core-Shell Polyvinyl Alcohol-Gelatin Electrospun Nanofiber Scaffolds

Proceedings of the 2nd World Congress on Recent Advances in Nanotechnology, Apr 1, 2017

In tissue engineering, natural/synthetic polymer based fibrous composite scaffolds obtained via e... more In tissue engineering, natural/synthetic polymer based fibrous composite scaffolds obtained via electrospinning method were shown to support the cell adhesion and tissue regeneration. However, electrospinning of natural polymers requires the use of toxic solvents that are negatively affecting the cell proliferation and biocompatibility of the produced scaffolds in addition to the usage of acidic solvents which will result in massive biodegradation inside the body[1] .Here, a method was proposed that is higher safety for the patient and even for the experimentalists who are using harmful and highly volatile solvents. Two types of polymers were used in the synthesis of the scaffolds by mimicking the key features of the tissue extracellular matrix which contains gelatin and coaxially organized nanofibers. Coaxial electrospinning technique was used to obtain core(PVA)-shell (gelatin) nanofibers. While material in the shell provides recognition sites for the tissue cells, core material provides mechanical endurance. Different from conventional methods, proposed work aims to lower the steps of application of the scaffold to the harmed tissue by using only deionized water as solvent. Instead of dissolving PVA and gelatin in toxic and acidic solvents, they were dissolved in the deionized water above the gelation temperature. Later, coaxial electrospinning generated increased cell spread and mechanical stiffness. The samples were characterized by scanning and transmission electron microscopy. Based on the experimental results it is concluded that electrospun fibers obtained from the 8% concentrated gelatin solution had a beaded structure, whereas the coaxially fabricated PVA and gelatin from the same concentration solutions did not show any beaded morphology. Also core-shell fibers have diameters down to 180 nm. This result showed that PVA aids to the uniform gelatin fiber formation which, may give higher mechanical stability. The electron microscopy analysis leading to these results has received support by the

Research paper thumbnail of Deformation behavior of an idealized crystal

The deformation behavior of an idealized crystal made by stacking of parallel slip planes is stud... more The deformation behavior of an idealized crystal made by stacking of parallel slip planes is studied. Each slip plane is assumed to contain active sources of dislocations leading to a constant density of non-interacting dislocations in the plane which glide through randomly distributed localized point obstacles, representing small precipitates. The dislocation is assumed to have a constant line tension and the dislocation-obstacle interaction is taken to have a simple step form. Using results of computer simulation of thermally activated glide through random arrays of point obstacles the deformation is modeled as a function of temperature and applied stress, determining the strain rate and the morphological characteristics of slip. Stress-strain rate and flow stress-temperature relations are discussed. * Let the dislocation under the applied stress T encounter a configuration of point obstacles denoted by i (Figure 1). Between two adjacent

Research paper thumbnail of PLASTIC DEFORMATION OF CRYSTALS: ANALYTICAL AND COMPUTER SIMULATION STUDIES OF DISLOCATION GLIDE - eScholarship

Research paper thumbnail of Computer simulation of dislocation glide—II. Comparison with experiments

Acta Metallurgica, May 1, 1986

We consider the glide of a single dislocation through randomly distributed obstacles of given pro... more We consider the glide of a single dislocation through randomly distributed obstacles of given properties. Using a model we studied the critical resolved shear stress for athermal glide and the velocity of thermally activated glide at finite temperature as functions of the nature and strength of the obstacles. Results obtained from computer simulation are compared with the available experimental data and good agreement is found. Zusammenfassung-Die Gleitung einer einzigen Versetzung durch eine zuf5llige Anordnung von Hindernissen mit vorgegebenen Eigenschaften wird behandelt. Anhand eines Modelles wird die kritische FlieBspannung fiir athermische Gleitung und die Geschwindigkeit der thermisch aktivierten Gleitung bei endlicher Temperatur in Abhlngigkeit von Natur und StLke der Hindernisse untersucht. Die aus der Computersimulation erhaltenen Ergebnisse werden mit den verfiigbaren experimentallen Daten verglichen; es ergibt sich gute iibereinstimmung.

Research paper thumbnail of Production of Hydroxylapatite from Animal Bone

Key Engineering Materials, May 1, 2004

... 1300 2.48 ± 0.08 65.01 ± 41.57 130.68 ±17.90 Fig.3 ESEM micrograph of HAp with PVA binder by ... more ... 1300 2.48 ± 0.08 65.01 ± 41.57 130.68 ±17.90 Fig.3 ESEM micrograph of HAp with PVA binder by 1%, sintered at 1200 o C, 4h. ... [4] AC Tas, F. Korkusuz, M. Timucin and N. Akkas: Journal of Materials Science: Materials in Medicine, Vol. 8 (1997), p. 91. ...

Research paper thumbnail of Computer simulation of dislocation glide—I. Comparison with statistical theories

Acta Metallurgica, May 1, 1986

Research paper thumbnail of Effects of squeeze casting on the properties of Zn-Bi monotectic alloy

Metallurgical and Materials Transactions A, 1997

In composite production, the shortest route is via an in situ composite in which a melt dissociat... more In composite production, the shortest route is via an in situ composite in which a melt dissociates simultaneously into two rather different solid phases. The monotectic alloys can be included in this group. The present work was aimed at extending our recent squeeze casting experience on the Zn-Bi monotectic alloy in order to increase its cast quality and mechanical properties. A squeeze casting unit was built, and its die and punch were machined. The molten monotectic alloy was squeezed in this unit under pressures up to 120 MPa in its freezing range until it solidified completely. It was found that an increase in squeeze casting pressure provided increases in density, tensile strength, and Vickers hardness, which resulted in decreases in chip length and electrical resistivity. Before the squeeze casting practice, the freezing characteristics of this monotectic were estimated using basic solidification principles.

Research paper thumbnail of Effects of squeeze casting on the wide freezing range binary alloys

Materials Science and Engineering: A, 1993

Research paper thumbnail of Hot Torsion Experiments on Aluminium 1100 and 2024

Journal of the mechanical behavior of materials, Mar 1, 1993

In this study, the effects of temperature and strain rate on the deformation behavior of aluminiu... more In this study, the effects of temperature and strain rate on the deformation behavior of aluminium 1100 and aluminium 2024 are investigated. Specimens are tested at different temperatures and strain rates under the effect of torsional loading. The data are evaluated using critical radius and effective length concepts. For both materials, as a result of the interaction between strain hardening and thermal softening mechanisms, flow curves pass through a peak value after which aluminium alloy tends to fracture while aluminium 1100 approaches to a steady state deformation. The strain rate sensitivity of materials obtained from torsion testing increases with temperature. The length of aluminium specimens subjected to free-end hot torsion is observed to change with both temperature and strain rate.

Research paper thumbnail of A System of Cracks in Bonded Half Planes

Springer eBooks, 1987

The structural strength of materials generally depends on the materials properties, the shape and... more The structural strength of materials generally depends on the materials properties, the shape and size of defects as well as the orientation of such flaws in the medium. In the application of fracture mechanics other kinds of imperfections due to manufacturing are particularly important such as flat cavities which develop during casting, small cracks resulting from residual stresses in welded materials, and fatigue cracks under fluctuating external loads. Thus in a design dealing with such materials, it is necessary to have a good estimate of the stress state disturbed by the existence of these flaws.

Research paper thumbnail of Dynamic simulation of a pinned dislocation in an atomistic model

Materials Science and Engineering, Feb 1, 1981

Three different models for a pinned dislocation under an applied stress are compared in this pape... more Three different models for a pinned dislocation under an applied stress are compared in this paper: (i) the continuous string model of Koehler, Granato and Liicke; (ii) a twodimensional lattice model which represents the dislocation in its slip plane; (iii) a discrete string model which is obtained as an analytical approximation to model (ii). Good agreement is obtained for the quantities investigated for the three models. Comparison of models (i) and (iii) therefore provides an atomistic interpretation for the parameters appearing in the defining equation of the continuous string model. One result of interest is that these parameters cannot be expressed solely in terms of the characteristics of the interatomic law for atoms in the vicinity of their equilibrium sites for the perfect crystal but must also depend on aspects of the interatomic force law for atoms in configurations such as those which occur near the dislocation.

Research paper thumbnail of On the Application Possibility of Thermal Barrier Coatings in Space Practices

Recently, application of ceramic coatings to space vehicles to generate thermal barriers upon re-... more Recently, application of ceramic coatings to space vehicles to generate thermal barriers upon re-entry is being considered as a challenge for research. Conventionally, thermal barrier coatings (TBC) are utilized to increase the gas turbine engine components' thermal and oxidation resistance values in order to get higher power and efficiency values. In this study, various superalloys and stainless steels were thermal barrier coated using the atmospheric plasma spraying method. One goal is to determine the effect of the substrate on the performance of the coating. Coating morphology was determined using microscopy analysis. Various heat exposure procedures were applied to the specimens and the changes in the mechanical properties of the top coat were analyzed using indentation techniques. Durability of the coatings after various periods at 1100degC was examined. Microstructures, coatings and reaction products were characterized by optical microscopy and image analysis. Effects of TBC thickness variations were also analyzed in order to achieve a model for determining the optimum coating thickness.

Research paper thumbnail of Control of mechanical and wear properties of a commercial Al-Si eutectic alloy

Journal of Materials Science, 1992

The properties examined as a function of microstructural modification were ultimate tensile stren... more The properties examined as a function of microstructural modification were ultimate tensile strength, fracture elongation, Vickers hardness and wear resistance. The microstructural modification was achieved by rapid cooling and additions of small amounts of strontium and lithium master alloys into the eutectic melt. In all experiments the commercial ETIAL 140 alloy was cast instead of a high-purity aluminium-silicon eutectic. This allowed determination of the effect of modification treatment, both on silicon and intermetallic phases. It was found that the slowly cooled and unalloyed castings which contained coarse silicon flakes showed highest wear resistance and lowest ultimate tensile strength, fracture elongation and Vickers hardness values. Rapid cooling and also additions of strontium and lithium master alloys reduced the eutectic interphase spacing and refined the silicon phase. This usually corresponded to a significant increase in all properties except the wear resistance. It was noted, however, that the size of the intermetallic phase particles increased abruptly above 0.04% Sr content which resulted in a sharp reduction in all mechanical properties. Unlike the strontium effect, the lithium addition did not influence the intermetallic size significantly and, therefore, the mechanical properties were not impaired. In addition, the wear resistance also remained relatively unaffected because lithium solid solution hardened the primary aluminium dendrites appeared in the modified alloys.

Research paper thumbnail of Fabrication of Al matrix composites reinforced with high contents of Si particles

Journal of Materials Science Letters, 1996

Research paper thumbnail of Production of Hypereutectic Al-Si Alloys by P/M Route

Materials Science Forum, May 1, 1996

Research paper thumbnail of Plastic deformation of crystals: analytical and computer simulation studies of dislocation glide

The plastic deformation of crystals is usually accomplished through the motion of dislocations. T... more The plastic deformation of crystals is usually accomplished through the motion of dislocations. The glide of a dislocation is impelled by the applied stress and opposed by microstructural defects such as point defects, voids, pre cipitates and other dislocations.

Research paper thumbnail of The microstructural control of cast and mechanical properties of zinc-aluminium alloys

Journal of Materials Science, 1993

The zinc-aluminium alloys containing 8, 1 2, and 27% aluminium are finding increasing application... more The zinc-aluminium alloys containing 8, 1 2, and 27% aluminium are finding increasing applications in the casting industry. These alloys are stronger than most aluminium alloys. In addition, they possess high wear resistance and bearing properties. However, surface sinks and shrinkage defects are observed on the bottom faces of such castings, contrary to general foundry practice. In the present investigation, this problem observed in the Zn-8%AI, Zn-12%AI, Zn-27%AI alloys was tackled by controlling various casting parameters and also by additions of the master alloys of strontium and lithium into the molten alloys. It was found that the underside shrinkage problem was influenced by the aluminium content of the alloy, melt superheat, casting size and cooling conditions'. The strontium and lithium additions were found to be beneficial in reducing the underside shrinkage problem. The ultimate tensile strength, fracture elongation and Vickers hardness were all increased with aluminium concentration and lithium addition. It was found also that the most problematical Zn-27%AI alloy, which provided the highest mechanical properties, was very suitable for the squeezecasting technique. The mechanical properties were increased sharply in these squeeze-cast bars.