Browsing by Author "Altintas, A"

Now showing 1 - 9 of 9
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    Analysis Of Thin Dielectric Cylindrical Reflector Having An Arbitrary Conic Section Profile Illuminated By Complex Line Source: H-Polarization Case
    Oguzer, T; Kuyucuoglu, F; Avgin, I; Altintas, A
    Arbitrary conic section profile and thin dielectric reflector is analyzed by using the Method of Analytical Regularization (MAR) technique based on Riemann-Hilbert problem and Fourier inversion procedures. The reflector surface is assumed to be illuminated by an H-polarized complex line source type feed antenna. The convergence of the solution is verified and some changes in the radiation patterns are obtained especially for rather thicker cases.
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    Analysis of a thin, penetrable, and non-uniformly loaded cylindrical reflector illuminated by a complex line source
    Oguzer, T; Kuyucuoglu, F; Avgin, I; Altintas, A
    A thin, penetrable, and cylindrical reflector is illuminated by the incident field of a complex source point. The scattered field inside the reflector is not considered and its effect is modelled through a thin layer generalised boundary condition (GBC). The authors formulate the structure as an electromagnetic boundary value problem and two resultant coupled singular integral equation system of equations are solved by using regularisation techniques. The GBC provides us to simulate the thin layer better than the resistive model which is applicable only for very thin sheets. Hence, the more reliable data can be obtained for high-contrast and low-loss dielectric material. The scattering and absorption characteristics of the front-fed and offset reflectors are obtained depending on system parameters. Also, the effects of the edge loading are examined for both E- and H-polarisations. The convergence and the accuracy of the formulation are verified in reasonable computational running time.
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    The Effect of Graphene Nanoplatelets on the Wear Properties of High-Frequency Induction Sintered Alumina Nanocomposites
    Altintas, A; Çavdar, U; Kusoglu, IM
    Alumina nanoparticles mixed with up to 1wt% graphene nanoplatelets (GNPs) were mixed by high-speed ball milling for 2 h at 350 rpm and sintered in a graphite die at 1650 degrees C for 20 min in an ultra-high-frequency induction heated system (UHFIHS) under uniaxial load in a vacuum chamber. The effect of different GNPs ratios on the microstructure, hardness and wear rate of alumina nanocomposites was examined. The results showed that the mechanical properties of Al2O3-GNPs nanocomposites gradually improved with GNP additions of 0.1 wt% to 0.6 wt%.
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    In-situ compaction and sintering of Al2O3 - GNP nanoparticles using a high-frequency induction system
    Çavdar, U; Kusoglu, IM; Altintas, A
    In-situ compaction and sintering of nanopowders are of great interest with respect to industrial production because they save time and energy. As a key study, Al2O3-1 wt.-% GNP composites were mixed in a V-type mixer for 2 hours at 35 cycle/min to obtain homogeneous mixtures. Mixed graphene nanoplatelets and alumina nanoparticles were compacted and sintered in-situ by applying uni-axial pressure in a graphite die, heated and generated by the ultra high-frequency induction system (UHFIHS). The graphite die was heated to 1500, 1550, 1600 and 1650 degrees C for 5, 10 and 20 minutes under a vacuum. The effect of sintering temperature and time on density, surface roughness, hardness and the microstructure of in-situ sintered nano graphene-alumina compacts were investigated.
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    The effects of graphene nanoplatelet addition to in situ compacted alumina nanocomposites using ultra-high frequency induction sintering system
    Kusoglu, IM; Çavdar, U; Altintas, A
    The aim of this study was to compact and sinter in situ graphene nanoplatelets (GNPs) with the addition of alumina (Al2O3) nanoparticles to form nanocomposites by using an ultra-high frequency induction sintering system with the assistance of applying a uniaxial load. To obtain the effect of the addition of GNPs to the compaction and to the mechanical properties of the nanocomposite, 1 to 5 wt.% GNPs was mixed to alumina nanoparticles by high-speed ball milling for 2 h at 350 rpm. Mixed Al2O3-GNP compositions were compacted in situ and sintered in a graphite die by induction-assisted heating at 1650 degrees C for 20 min with a uniaxial load applied in a vacuum chamber. The microstructure of as-sintered nanocomposites was observed using a scanning electron microscope (SEM) before and after thermal etching. Hardness and wear tests were carried out to determine mechanical properties. The results were compared with the properties of a pure alumina nanocompact. It was found that minimum grain size and maximum density, hardness, and wear resistance can be obtained by the addition of 1 wt.% GNPs to alumina nanoparticles. Higher amounts of GNP addition gradually decreased the density, hardness, and wear rates and increased the grain size of the alumina matrix.
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    Effect of Ekabor 2 powder on the mechanical properties of pure iron powder metal compacts
    Altintas, A; Sarigün, Y; Çavdar, U
    Ekabor 2 powder of different weights, used by V type powder mixture was mixed with pure iron powder. Specimens were created by dry pressing into die followed by cold isostatic press at a pressure of 100 bar for purpose of resizing. The conventional sintering process at 900 degrees C and 950 degrees C was carried out for 4 h using conventional furnaces. Specimens were gradually cooled by air. The sintered pellets were examined in terms of density, wear abrasion, surface roughness and hardness at 900 degrees C and 950 degrees C. Microstructure of pellets was observed with optic microscope. The results show that the hardness gradually increased with additions of 0 wt. % to 10 wt. % of Ekabor 2 however, after 10 wt. %, this hardness tended to decrease.
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    In-situ compaction and sintering of Al2O3-B4C composites by using a High-Frequency Induction System
    Çavdar, U; Altintas, A; Karaca, B
    Alumina powders containing varying ratios of B4C were mixed in a V-type mixer for 30 min with 35 cycle/min to obtain a homogeneous mixture. Six different chemical compositions were prepared and sintered at 1600 degrees C for 20 min using an Ultra-High-Frequency Induction Heated System. The samples were cooled by air. The density, surface roughness, hardness and microstructure of samples were investigated. The results showed that when the amount of B4C increased, the values of hardness increased and the values of surface roughness decreased.
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    A Turkish Gynecologic Oncology Group study of fertility-sparing treatment for early-stage endometrial cancer
    Dursun, P; Erkanli, S; Güzel, AB; Gultekin, M; Tarhan, NC; Altundag, O; Demirkiran, F; Bese, T; Yildirim, Y; Bozdag, G; Yarali, H; Simsek, T; Ozcelik, B; Ortaç, F; Taskin, S; Guvenal, T; Ozgul, N; Haberal, A; Vardar, MA; Dede, M; Yenen, M; Altintas, A; Arvas, M; Ayhan, A
    Objective: To analyze the results of fertility-sparing treatment of early-stage endometrial cancer (EC) in patients treated at Turkish gynecologic oncology centers, and to present a review of the literature. Methods: Thirteen healthcare centers in Turkey were contacted to determine if they were eligible to participate in the study. Centers that were eligible and agreed to participate were sent a database form to record the demographic characteristics, clinicopathologic findings, and follow-up results for their EC patients. Results: Eleven Turkish healthcare centers provided data on 43 EC patients. Mean duration of treatment was 5 months and mean follow-up was 49 months. In total, 35 (81.4%) patients were tumor free following primary progesterone therapy. Mean time from the end of progesterone therapy to pregnancy was 10.6 +/- 4.3 months (range, 3-18 months). Two patients had tumor recurrence during follow-up. The pregnancy rate among the 31 women who actively sought pregnancy was 41.9% (n = 13). Conclusion: Conservative management of early-stage EC in women of reproductive age using oral progestins was effective and did not compromise oncological outcome. Pregnancy in the study patients was achieved spontaneously and artificially. (C) 2012 International Federation of Gynecology and Obstetrics. Published by Elsevier Ireland Ltd. All rights reserved.
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    CLINICOPATHOLOGIC AND SURGICAL ANALYSIS OF 1090 PATIENTS WITH BORDERLINE OVARIAN TUMORS: A TURKISH SOCIETY GYNECOLOGIC ONCOLOGY (TRSGO) MULTI-INSTITUTIONAL RETROSPECTIVE TRIAL
    Guvenal, T; Demirkiran, F; Simsek, E; Yalcin, Y; Khatib, G; Gokulu, SG; Simsek, T; Dolapcioglu, K; Onan, A; Ozgul, N; Ortac, F; Yuksel, H; Altintas, A; Boso, B; Seyfettinoglu, S; Celik, C; Ureyen, I; Yalcin, I; Gungorduk, K; Senturk, B; Ayhan, A