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    Effect of annealing time and temperature on microstructural and superconducting properties of (Bi,Pb)-2212 thin films produced by magnetron reactive sputtering
    (Springer, 2012) Yıldırım, Gürcan; Bal, Sevgi; Varilci, Ahmet
    This study deals with the role of various annealing time (1 h, 2 h, 4 h, 6 h and 8 h) and temperature (840 and 850 degrees C) on the microstructural and superconducting properties of thin films with the aid of scanning electron microscopy (SEM), X-Ray analysis (XRD), electron dispersive X-Ray (EDX), resistivity and transport critical current density (J(c)) measurements. The T-c, J(c), variation of transition temperatures, hole-carrier concentration, grain size, phase purity, lattice parameter, surface morphology, element distribution, crystallinity and resistivity (at room temperature) values of the films prepared are compared with each other. Critical transition temperatures (T-c) of the samples are deduced from the dc resistivity measurement while critical current density values are estimated from the critical current and total cross-sectional area values. It is found that maximum T-c of 79.7 K and Jc of 1520 A/cm(2) are observed for the film annealed at 840 degrees C for 6 h as against 54.9 K and 30 A/cm(2) (minimum values), respectively, for the film annealed at 840 degrees C for 4 h. Moreover, SEM images indicate that the former has the best crystallinity, grain connectivity and largest grain size. Based on these results, T-c and J(c) values of the samples studied are found to depend strongly on the microstructure. Additionally, EDX results show that the elements used for the preparation of all the samples are observed to distribute homogeneously. As for the XRD results, all the samples exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines. According to the refinement of cell parameters done by considering the structural modulation, the largest lattice parameter a and c are obtained for the film annealed at 840 degrees C for 8 h. To sum up, the aim of the present study is not only to investigate the changes of microstructural and superconducting properties of the samples fabricated in the varied time and temperature but to determine the best ambient for the film fabrication and show the feasibility of obtaining Bi-2212 film with tailored structure, as well.
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    Effect of magnetic field direction on magnetoresistivity, activation energy, irreversibility and upper critical field of Bi-2212 thin film fabricated by DC sputtering method
    (Springer, 2012) Yıldırım, Gürcan; Bal, Sevgi; Varilci, Ahmet
    This study aims to investigate the effect of magnetic field direction on superconducting properties of Bi-2212 thin film fabricated on MgO (100) substrate using the direct current (DC) magnetron reactive sputtering technique at 100 watt with the aid of magnetoresistivity measurements. The zero resistivity transition temperatures (T-c), irreversibility fields (mu H-0(irr)) and upper critical fields (mu H-0(c2)) are deduced from the magnetoresistivity versus temperature curves under DC magnetic fields (parallel and perpendicular to c-axis) up to 5 T. Moreover, thermally activated flux flow (TAFF) model is studied for activation energy (U-0) values of the sample. It is found that the T-c value decreases from 76.4 K to 39.1 K for the applied magnetic field perpendicular to c-axis (mu H-0 perpendicular to c-axis); likewise, the T-c reduces towards 28.8 K with the increase in the applied field parallel to c-axis (mu H-0 parallel to c-axis). Furthermore, the U-0 values are found to decrease considerably with increasing applied magnetic field. In fact, the U-0 of 134.5 K is obtained to be smallest at 5 T field parallel to the c-axis. Additionally, both the mu H-0(irr) and mu H-0(c2) values determined are also observed to reduce with the increase of the applied magnetic field. At absolute zero temperature (T = 0 K), the extrapolation of the mu H-0(irr)(T) and mu H-0(c2)(T) curves is used to obtain the mu H-0(irr)(0) and mu H-0(c2)(0) values of the film, respectively. The inner is found to be about 22.216 T (19.046 T) for the applied field perpendicular (parallel) to c-axis whereas the latter is determined to be about 54.095 T (126.522 T) for the applied field parallel (perpendicular) to c-axis, respectively, as a result of anisotropic behavior of the film prepared. On the other hand, penetration depths (lambda) and coherence lengths (xi) inferred from mu H-0(irr)(0) and mu H-0(c2)(0) values are obtained to be about 38.519 angstrom (41.601 angstrom) and 16.147 angstrom (24.685 angstrom) in the case of applied field perpendicular (parallel) to c-axis, respectively. Based on all the results, the change of the superconducting properties as a function of the magnetic field direction presents the anisotropy of the sample produced. X-ray diffraction (XRD) and scanning electron microscopy (SEM) examinations are also conducted for microstructural and phase analyses of the film.
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    Morphological, microstructural and electrical examinations on ZnO film on p-Si wafer
    (Springer, 2012) Gökçen, Muharrem; Bal, Sevgi; Yıldırım, Gürcan; Gülen, Mahir; Varilci, Ahmet
    This study reports not only main electrical and dielectric characteristics of Ag/ZnO/p-Si heterostructure with the aid of the experimental admittance measurements at room temperature and theoretical approaches but also the microstructure and surface morphology of the heterostructure by means of X-ray diffraction, scanning electron microscopy and atomic force microscopy measurements. The results obtained show that the sample, obtaining Wurtzite structure with the (002) preferred orientation, has a fine crystalline microstructure consisting of micro-sized hexagonal rods growing uniformly in large scale on the film surface. When the diameters of the rods are found to vary from 0.5 mu m to 1.5 mu m, thickness values are observed to be about 2 mu m. Further, series resistance (R-s) and some other electronic parameters of the heterostructure are obtained by the capacitance-voltage (C-V), conductance-voltage (G-V) and C-2-V measurements. Moreover, voltage (V) and frequency (f) dependence of dielectric parameters such as dielectric constant (epsilon'), dielectric loss (epsilon aEuro(3)), dielectric loss tangent (tan delta), real and imaginary parts of electric modulus (IeaEuro(2) and IeaEuro(3)) are determined and discussed. It is found that both electrical and dielectric parameters of the heterostructure prepared in this work depend strongly on the applied bias voltage and frequency.
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    Role of cerium addition on structural and superconducting properties of Bi-2212 system
    (Springer, 2012) Bal, Sevgi; Doğruer, Musa; Yıldırım, Gürcan; Varilci, Ahmet; Terzioğlu, Cabir
    This study examines the significant changes in the structural and superconducting properties of cerium (Ce) doped Bi-2212 superconductors via X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), electron dispersive X-ray (EDX), electrical resistance and transport critical current density (J (c) ) measurements. Ce concentration is varied from x=0.0 until 0.1 in a general stoichiometry of Bi1.8Sr2.0Ce (x) Ca1.1Cu2.1O (y) . Zero resistivity transition temperatures () of the samples produced by the conventional solid-state reaction method are deduced from the dc resistivity measurements. Furthermore, the phase fractions and lattice parameters are determined from XRD measurements when the microstructure, surface morphology and element composition analyses of the samples are investigated by SEM and EDX measurements, respectively. The results show that and J (c) at self-field of the samples reduced gradually with the increase in the Ce addition. Maximum of 79.7 K and J (c) of 356.8 Aa <...cm(-2) at 77 K are obtained for pure sample as against 44.6 K and 18.7 Aa <...cm(-2), respectively, for the sample doped with 0.1 wt.% Ce. According to the refinement of cell parameters done by considering the structural modulation, the Ce doping is confirmed by both an increase of the lattice parameter a and a decrease of the cell parameter c of the samples in comparison with that of the pure sample. As for SEM measurements, it is found that not only do the surface morphology and grain connectivity degrade but the grain size of the samples also decreases with the increase of the Ce addition. Moreover, EDX images indicate that the elements used for the preparation of samples distribute homogeneously and the Ce atoms enter into the crystal structure by replacing Cu atom. In addition, the variation of Delta T (c) () is investigated for the presence of impurities and weak links between superconducting grains of the samples. The possible reasons for the degradation in microstructural and superconducting properties are also interpreted.
  • Küçük Resim Yok
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    Some physical and magnetic properties of dc sputtered Bi(Pb)SrCaCuO thin film
    (Bolu Abant İzzet Baysal Üniversitesi, 2011) Bal, Sevgi; Varilci, Ahmet
    Bu tezde, tek kristal yapıya sahip olan magnezyum oksit üzerine 100 Watt'da standart doğru akım (DC) reaktif püskürtme yöntemi ile üretilen Bi(Pb)SrCaCuO (Bi-2212) ince filmler için en uygun tavlama koşulları araştırıldı. Ayrıca, 840 C ve 850 C' de değişik tavlama sürelerinde (1sa, 2sa, 4sa, 6sa and 8sa) tavlanan ince filmlerin bazı fiziksel ve manyetik özellikleri incelendi. Deneysel verileri elde etmek için DC püskürtme cihazı, kriyostat sistemi, X-ışınları kırınım (XRD) cihazı, taramalı elektron mikroskopu (SEM) ve enerji dağılımı spektrum analizi (EDS) kullanıldı. İlk olarak bu çalışmada üretilen ince filmlerin hedef malzemesi katıhal tepkime yöntemiyle hazırlandı. Doğru akım püskürtme cihazında on adet Bi(Pb)SrCaCuO (Bi-2212) ince film magnezyum oksit üzerine üretildi. Özdirenç ölçümleri ile değişik tavlama sürelerinin Bi(Pb)SrCaCuO ince filmlerin süperiletken kritik sıcaklığı (Tc) üzerine etkisi incelendi. Bu ölçümler 30 ile 120 K sıcaklığı aralığında 1 mA' lik doğru akım altında helyum gaz bağlantılı kriyostat tarafından yapıldı. Ayrıca film yüzeyine paralel ve dik olarak uygulanan 0, 0.5, 1, 3 and 5 T'lık manyetik alan kuvvetleri altında sadece 8400C'de 2 saat tavlanan film için sıcaklığa bağlı özdirenç ölçümleri yapıldı. X-ışınları kırınım grafikleri elde edildi ve bu grafiklerin üzerindeki zirve noktaları literatür yardımı ile belirlendi. Ayrıca Bi-2212 ve Bi-2223 hacim oranları ve kafes parametreleri hesaplandı. Sonra yüzey morfolojisini incelemek için taramalı elektron mikroskobu görüntüleri alındı. Son olarak da Bi(Pb)SrCaCuO ince filmlerin enerji dağılımı spektrum analizleri alındı ve elementlerin yüzde ağırlığı hesaplandı.