Browsing by Author "Gültekin S."

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    Dielectric properties of polystyrene/alumina composites for microelectronic devices
    (European Conference on Composite Materials, ECCM, 2016) Baştürk S.B.; Yalamaç E.; Gültekin S.
    The availability/lightness of electronic equipments in automotive, telecommunication and computer sectors have great importance in terms of the cost decrease and usability in high speed applications. Printed circuit boards (PCBs) are the main components of these systems and composed of two basic elements: substrate and passive electronic devices. Microelectronic segments having low dielectric permittivity (dielectric constant) and loss tangent minimize the capacitive coupling effects and decrease the signal attenuation. Ceramic particle filled polymer composites combine the desired dielectric properties and have been employed in electronic industry. In this study, 5%, 15% and 30% wt. alumina (Al2O3) loaded polystyrene (PS) composites were produced. The size of the PS granules was reduced below 100 microns by ball milling process. The Al2O3 and PS powders were mixed by vibratory disc mill and subjected to injection molding for manufacturing the circular specimens. The dielectric permittivity (real and imaginary parts) and loss tangent parameters of the composites were measured via dielectric analyzer up to 1 MHz at room temperature. Based on dielectric measurements, there was no pronounced difference among the samples' real part permittivities along the frequency variation. As expected, the ceramic content increase led to the increase of permittivity and 30%Al2O3 wt. loaded structure showed the maximum values. This behavior was attributed to the effect of polarization which appears in heterogeneous media consisting of phases with different dielectric constant and conductivity. © 2016, European Conference on Composite Materials, ECCM. All rights reserved.
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    X-ray irradiated thermo- and radioluminescence, structural and thermal characterization of septarian (powder&bulk) from Madagascar
    (Elsevier B.V., 2018) Keskin İ.; Katı M.İ.; Türemiş M.; Gültekin S.; Üstün S.; Çetin A.; Kibar R.
    The luminescence properties of septarian have been investigated for the first time in this study, which has been the subject of many studies in both geological and geophysical fields. This sediment with a calcium carbonate structure exhibited high luminescence properties with X-ray excitation. The radioluminescence (RL) and thermoluminescence (TL) properties were investigated as well as their structural (FT-IR, XRD), morphological (SEM), thermal (TG-DTA) and absorption (UV-Vis-NIR) properties of this sediment. The broad RL peak of septarian was observed at 640 nm. There was a significant increase in the RL intensities of the sifted samples compared with the bulk sample. The TL glow curves of septarian irradiated with X-rays exhibited intense main TL glow peak having the maximum temperatures at 116 °C and about 390 °C with a heating rate of 2 °C/s. Also; the TL kinetic parameters were reported; activation energy (E), frequency factor (s) and the order of kinetics (b) of the first peak have been determined in detail by using peak shape (PS) and Computerized Glow Curve Deconvolution (CGCD) methods. SEM and EDS analysis were performed for the two different layers of septarian. The light colored side with the amorphous structure of the sample has more impurities (such as Fe, Al, Zr, K) than the crystallized and darker side. © 2018 Elsevier B.V.
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    Structural and optical properties of SrAl2O4: Eu2+/Dy3+ phosphors synthesized by flame spray pyrolysis technique
    (Elsevier B.V., 2019) Gültekin S.; Yıldırım S.; Yılmaz O.; Keskin İ.Ç.; Katı M.İ.; Çelik E.
    In this study Sr0.985Al2O4:0.005Eu2+/0.01Dy3+ luminescent materials with long afterglow and high intensity were synthesized by flame spray pyrolysis (FSP) method for the first time. Precursor solution was divided into two separate part. One of the parts was used FSP process, the other one was dried to simulate sol-gel process. Thermal properties and organic content of FSP derived particles and dried gel were compared using DTA-TG and FTIR devices. FSP derived particles were sintered in argon medium at 1200 oC. Phase and elemental analysis of the final phosphors were carried out by X-Ray Diffraction (XRD) and X-Ray Photo electron Spectroscopy (XPS) measurements. Scanning electron microscopy (SEM) images were obtained to observe particle morphology. Optical properties of phosphor were determined by Photoluminescence (PL), thermoluminescence (TL) and radioluminescence (RL) measurements. The emission spectra of Sr0.985Al2O4:0.005Eu2+/0.01Dy3+ were composed by a green band at 525 nm at room temperature when it was excited by 350 nm light, assigned to anomalous luminescence originated by Eu2+ and Dy3+ in the host lattice. There were estimated peaks at around 90, 130, 170 and 200 oC under the main glow curve in the sample irradiated by X-ray and He-Ne laser (λexc = 632 nm). RL spectrum of Sr0.985Al2O4:0.005Eu2+/0.01Dy3+ powders have four distinct emission peaks at around 527 nm, 576 nm, 667 nm, 753;nm and a comparatively weak emission band at about 846 nm. © 2018 Elsevier B.V.
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    Structural, optical, luminescence properties and energy transfer mechanism of Tb3+/Eu3+co-doped SrLa2(MoO4)4 phosphors produced by sol-gel process
    (Elsevier Ltd, 2019) Keskin İ.Ç.; Gültekin S.; Katı M.İ.; Türemiş M.; Ay K.; Arslanlar Y.; Çetin A.; Kibar R.
    Tb3+/Eu3+co-doped SrLa2(MoO4)4 have been synthesized using the sol-gel process. The as-produced light-emitting phosphors were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA)/thermogravimetric analysis (TG), optical absorption spectra, radioluminescence (RL), and photoluminescence (PL) spectra. The effect of Tb3+and Eu3+doping concentrations on the RL and PL intensity of host material was analyzed in details. Furthermore, the energy transfer mechanism based on RL spectra were determined of Tb3+/Eu3+ co-doped SrLa2(MoO4)4 phosphors. When the samples excited by X-ray, the SrLa2(MoO4)4 phosphors exhibit characteristic emissions of Tb3+ (489, 544, 586, 620 and 675 nm) and Eu3+ (545, 592, 614, 650 and 700 nm). Also, similar emissions are observed in the results of PL spectra, when the samples excited by 488 nm. The host material has excellent energy transfer efficiency for lanthanide ions. In addition, the luminescence spectra indicated that the energy transfer is also effective between from Tb3+ to Eu3+ and vice versa. The CIE parameters are calculated to characterize the color emission. Under UV excitation, the synthesized materials display different color from green to red. Based on the results, the SrLa2(MoO4)4:Eu3+,Tb3+ phosphors may be potential candidates for light-emitting diode. © 2019 Elsevier B.V.
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    Detailed luminescence (RL, PL, CL, TL) behaviors of Tb3+ and Dy3+ doped LiMgPO4 synthesized by sol-gel method
    (Elsevier B.V., 2020) Keskin İ.Ç.; Türemiş M.; Katı M.İ.; Gültekin S.; Tuncer Arslanlar Y.; Çetin A.; Kibar R.
    The un-doped, Tb3+ and, Dy3+ doped LiMgPO4 phosphors were successfully synthesized by the sol-gel method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to characterize the light-emitting phosphors. The XRD spectra verify the phase formation of samples. The luminescence properties were analyzed by radioluminescence (RL), photoluminescence (PL), cathodoluminescence (CL) and, thermoluminescence (TL) spectra. Also, the effect of Tb3+, Dy3+ ions in LiMgPO4 was determined on the luminescence intensity in detail. The RL spectrum of un-doped LiMgPO4 consists of two broad emission bands in the 310–450 nm and 575–850 nm range. The maximum intensities of these bands were observed at 360 nm and 655 nm. The emission spectrum of Dy3+ doped LiMgPO4 under X-ray excitation show characteristic emissions of Dy3+ corresponds to 4F9/2→6Hj transitions. In addition to the 5D4→7Fj (j:6–1) transitions of Tb3+ doped LiMgPO4 phosphor, strong emissions derived from 5D3→7Fj (j:6–3) transitions were observed in the blue region. Also, these blue emissions were seen at RL, PL and, CL spectra. The optical absorption spectra were recorded and the energy band gaps were calculated 3.62 eV, 3.30 eV and, 4,01 eV for un-doped, Dy3+ and, Tb3+ doped LiMgPO4 phosphors, respectively. The phosphors irradiated with UV (254 nm) light. The main TL glow peak was observed at 288 °C for the Dy3+ doped phosphor, and the peaks at 198, 283 °C were observed for the Tb3+ doped phosphor. © 2020 Elsevier B.V.
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    Assessing of Photoluminescence and Thermoluminescence Properties of Dy3+ Doped White Light Emitter TTB-Lead Metatantalate Phosphor
    (Springer, 2020) İlhan M.; Keskin İ.Ç.; Gültekin S.
    Trivalent Dy doped TTB (tetragonal tungsten bronze) lead tantalate (PbTa2O6) phosphors were synthesized by the solid-state reaction route. Structural and spectroscopic properties have been carried out by x-ray diffraction (XRD), scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS), photoluminescence (PL) and thermoluminescence (TL) analyses. XRD patterns of samples revealed the presence of TTB symmetry which continued up to 10 mol.%. SEM analysis showed the transformation of grain morphology from the shapeless-rounded to elongate. Strong PL emissions of PbTa2O6:Dy3+ phosphor were monitored at 480.5 nm and 578.5 nm. PL emission increased up to 7 mol.% and then decreased because of the concentration quenching. Decay times decreased with the increase of concentration and the end of the process (10 mol.%) fell to 355 μm. The energy transfer efficiency (ηET) energy transfer rate (W) and quantum efficiency (ηQE) of the whole process were found as 56.25%, 1.581 ms–1, and 20.27%, respectively. The CIE (Commission Internationale de l’Éclairage) coordinates of all phosphors have located close to the standard white light center, also the CIE coordinates (x, y) and CCT (correlated color temperature) value for 7 mol.% phosphor were found as (0.310, 0.295) and 6238 K, respectively. TL emissions of PbTa2O6:Dy3+ phosphor were monitored by four glow peaks in the range between 50°C and 400°C after x-ray irradiation. TL kinetic parameters were determined by the deconvolution of the 5 mol.% sample which has the highest glow curve intensity. © 2020, The Minerals, Metals & Materials Society.
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    Outcomes of high-risk breast lesions diagnosed using image-guided core needle biopsy: results from a multicenter retrospective study
    (Galenos Publishing House, 2023) Oktay A.; Aslan Ö.; Taşkın F.; Tunçbilek N.; İçten S.G.E.; Balcı P.; Arıbal M.E.; Çelik L.; Örgüç İ.Ş.; Demirkazık F.B.; Gültekin S.; Aydın A.M.; Durmaz E.; Kul S.; Binokay F.; Çetin M.; Emlik G.D.; Akpınar M.G.; Voyvoda S.N.K.; Polat A.V.; Akın I.B.; Yıldız Ş.; Poyraz N.; Özsoy A.; Öztekin P.S.; Elverici E.; Bayrak İ.K.; İkizceli T.; Dinç F.; Sezgin G.; Gülşen G.; Tunçbilek I.; Yalçın S.R.; Çolakoğlu G.; Ağlamış S.; Yılmaz R.; Rona G.; Durhan G.; Güner D.C.; Yabul F.Ç.; Karabekmez L.G.; Tutar B.; Göktaş M.; Buğdaycı O.; Suner A.; Özdemir N.
    Purpose: The clinical management of high-risk lesions using image-guided biopsy is challenging. This study aimed to evaluate the rates at which such lesions were upgraded to malignancy and identify possible predictive factors for upgrading high-risk lesions. Methods: This retrospective multicenter analysis included 1.343 patients diagnosed with high-risk lesions using an image-guided core needle or vacuum-assisted biopsy (VAB). Only patients managed using an excisional biopsy or with at least one year of documented radiological follow-up were included. For each, the Breast Imaging Reporting and Data System (BI-RADS) category, number of samples, needle thickness, and lesion size were correlated with malignancy upgrade rates in different histologic subtypes. Pearson's chi-squared test, the Fisher-Freeman-Halton test, and Fisher's exact test were used for the statistical analyses. Results: The overall upgrade rate was 20.6%, with the highest rates in the subtypes of intraductal papilloma (IP) with atypia (44.7%; 55/123), followed by atypical ductal hyperplasia (ADH) (38.4%; 144/375), lobular neoplasia (LN) (12.7%; 7/55), papilloma without atypia (9.4%; 58/611), flat epithelial atypia (FEA) (8.7%; 10/114), and radial scars (RSs) (4.6%; 3/65). There was a significant relationship between the upgrade rate and BI-RADS category, number of samples, and lesion size Lesion size was the most predictive factor for an upgrade in all subtypes. Conclusion: ADH and atypical IP showed considerable upgrade rates to malignancy, requiring surgical excision. The LN, IP without atypia, pure FEA, and RS subtypes showed lower malignancy rates when the BI-RADS category was lower and in smaller lesions that had been adequately sampled using VAB. After being discussed in a multidisciplinary meeting, these cases could be managed with follow-up instead of excision. © 2023, Galenos Publishing House. All rights reserved.