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A-site Pb doping effect on structural, microstructural and magnetotransport properties of La0.5Sm0.2Ca0.3-xPbxMnO3 (x=0, 0.05, 0.10) manganite
(Elsevier Science SA, 2021) Denbri, Fatih; Mahamdioua, Nabil; Meriche, Faiza; Koç, Nevin Soylu; Terzioğlu, Cabir; Varilci, Ahmet; Altıntaş, Sevgi Polat
Structural, microstructural and magneto-transport properties of polycrystalline La0.5Sm0.2Ca0.3-x PbxMnO3 (x = 0, 0.05 and 0.10) samples, prepared by conventional solid state reaction, were studied. Refinement of the Powder X-ray diffractograms revealed that all the samples crystallize into orthorhombic structure with Pnma space group and the lattice parameters increase with increasing Pb content. The scanning electron microscope (SEM) micrographs show a granular character and the estimated grain sizes ranges between 10 mu m and 20 mu m. The experimental electrical resistivity and the magnetoresistance in the temperature range 20 K-250 K under magnetic field of zero and 1 Ta, have been recorded using four probe technique. The resistivity curves without magnetic field, exhibit a ferromagnetic-metallic (FM) to paramagnetic-insulating (PI) transition at TMI = 125 K, 91 K and 37 K for x = 0, 0.05 and 0.10, respectively. The magnetoresistance reaches 58% for undoped sample and decrease with Pb doping under 1T. The resistivity values increases drastically with Pb doping. These two results were explained mainly by the increase of the disorder sigma 2 and rA TMI) and for x = 0 and 0.05 samples, the resistivity curves is well fitted by a combination of the residual resistivity, weak localization and electron-electron scatterings; in addition to small polaron contribution or electron-phonon interaction respectively. In the high temperature regime T TMI, all our resistivity curves has been analyzed using adiabatic small polaron hopping model (ASPH) above theta D/2 and variable range hopping model (3D-VRH) (TMI < T < theta D/2). An attempt to use the percolation model to describe the resistivity data in the entire temperature range for both x = 0 and 0.05 samples, was established. Finally, density of state, mean hopping distance Rh and mean hopping energy Eh were calculated and discussed.
Anisotropic nature and scaling of thermally activated dissipation mechanism in Bi-2223 superconducting thin film
(Elsevier Science Sa, 2013) Yıldırım, Gürcan; Varilci, Ahmet; Terzioğlu, Cabir
This study deals with the resistive transition of the c-axis oriented Bi-2223 thin film produced on the single crystal MgO (100) substrate using direct current (DC) magnetron reactive sputtering technique at 100 watt in the case of applied field parallel and perpendicular to c-axis up to 3 T, respectively. Peak temperature (T-p) of the sample studied is determined from the variation of dR/dT as a function of temperature plot when the resistance called as R-p at the T-p is deduced from the same curve. It is found that the superconducting translation (onset and offset critical) temperatures reduce with the increment of the applied magnetic field strength and direction. In fact the minimum onset (T-c(onset)) value of 100.94 K and offset (T-c(offset)) value of 55.81 K are observed for 3 T applied field parallel to the c-axis. Moreover, the variation of Delta T-c(T-c(onset) - T-c(offset)) value is found to increase with the enhancement of the field and direction. Similar to the critical transition temperature results, the maximum variation (45.14 K) is observed for the sample in the applied field parallel to c-axis at 3 T. Further, the results obtained show that the dissipative resistivity is well adapted to thermally activated flux motion below the T-p value under the magnetic field and so the R-p values observed are used to examine the temperature and field dependence (mu H-0) of the flux pinning (activation) energy described as the formula U(T, mu H-0) = U-0(1 - T/T-p)H))H-m(-alpha). According to results, while m value is calculated to be about 1.572 and 1.492, alpha value is found to be about 0.546 and 0.498 at H//ab and H//c-axis, respectively, confirming that not only does the film produced show 3D-like behavior but also the thermally activated flux flow is the dominant mechanism on the sample. Additionally, the irreversibility fields (H-irr), upper critical fields (H-c2), penetration depths (lambda) and coherence lengths (xi) are evaluated from the magnetoresistance curves. When the anisotropy ratio of gamma(h) = xi(c)/xi(ab) is estimated to be about 6.42 at 0 K and 7.11 near the transition temperature (R = 0 Omega), the ratio of gamma(lambda) = lambda(c)/lambda(ab) is found to change from 1.97 until 3.28 in the range from 0 K to T-c. Based on the results obtained, it is concluded that both the anisotropy ratios are strongly dependent upon the temperature (particularly the ratio of lambda(lambda)) and the anisotropy ratio lambda(lambda) is more slightly smaller than that of gamma(h). (C) 2012 Elsevier B. V. All rights reserved.
Comparative study of nano-sized particles CoFe2O4 effects on superconducting properties of Y-123 and Y-358
(Elsevier, 2014) Slimani, Yassine; Hannachi, Essia; Ben Salem, M. K.; Hamrita, A.; Varilci, Ahmet
The effects of nano sized CoFe2O4 particles (10 nm) addition on the structural and the normal state resistivity of YBa2Cu3O7 (noted Y-123) and Y3Ba5Cu8O18 (noted Y-358) polycrystalline were systematically studied. Samples were synthesized in oxygen atmosphere using a standard solid state reaction technique by adding CoFe2O4 up to 2 wt%. Phases, microstructure and superconductivity have been systematically investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrical measurements rho(T). XRD results reveal that the lattice parameters change for both Y-123 and Y-358 phases. SEM observations reveal that the grain size is reduced with increasing the content of CoFe2O4. The measurements for the resistivity dependence of temperature show that the depression in superconducting temperature is more pronounced for CoFe2O4 addition in Y-358 compound than in Y-123 one. These results may be attributed to the existence of much more disorder due to a greater number of Cu sites to be substituted by Fe and Co in Y-358 compared to Y-123. Published by Elsevier B.V.
Current-voltage characteristics of nano whisker ZnO/Si heterojunction under UV exposition
(Elsevier Science Sa, 2022) Koç, Nevin Soylu; Altıntaş, Sevgi Polat; Gökçen, Muharrem; Doğruer, Musa; Altuğ, Cevher; Varilci, Ahmet
In/ZnO/p-Si heterojunction diode was produced to investigate the photo-responsivity and electrical features under ultraviolet (UV) light. A hydrothermal synthesis technique was used to coat the ZnO layer on the p-Si single crystal as nanowhisker/rods. The formation of surface and nanowhisker properties of the ZnO layer were investigated by scanning electron microscope (SEM). The I-V (current-voltage) analysis of the In/ZnO/p-Si diode was realized in dark and under UV (290-400 nm) illumination. Further, the main electrical parameters of the diode; such as reverse bias saturation current (I-V), ideality factor (n), zero bias barrier height (Phi(Bo)), resistance (R) and interface state density (N-ss) were obtained from the experimental I-V measurements by thermionic emission (TE) and Card and Rhoderick's function. Also, the power law of the photocurrents (I-PC), photoresponsivity (PR) and response time were extracted. Photo-responsivity and response time values of In/ZnO/p-Si heterojunction diode were obtained as 2.0 A/W and (rise/decay) 160/200 ms, respectively.
Determination of solid solubility level of Ho nanoparticles in Y-123 superconducting matrix and strong Cu-1 site preference of nanoparticles
(Elsevier Science Sa, 2014) Sarıtekin, Namık Kemal; Zalaoğlu, Yusuf; Yıldırım, Gürcan; Doğruer, Musa; Terzioğlu, Cabir; Varilci, Ahmet; Görür, Osman
This comprehensive study reports the effect of the Ho inclusions on the microstructural, electrical, mechanical and superconducting characteristics of YBa2Cu3O7-delta ceramic superconductors with the aid of standard characterization methods including the bulk density, dc resistivity (rho-T), transport critical current density (J(c)), X-ray diffraction (XRD), electron dispersive X-ray (EDX), scanning electron microscopy (SEM) and Vickers microhardness (H-V) investigations. The experimental results such as the degree of granularity, hole (filling) localization effect, room temperature resistivity, onset-offset critical transition temperature, degree of the broadening, thermodynamic fluctuations (spin-gap opening temperature), crystallinity, crystal plane alignments (texturing), crystal structure, grain size, phase purity and lattice parameters, appearance of flux pinning centers, grain boundary weak-links (interaction between the superconducting grains), surface morphologies (grain size distribution), real (load independent) microhardness values, elemental compositions and distributions belonging to the pure and Y-site Ho substituted Y-123 superconducting samples are discussed in detail for the first time. Moreover, mechanical characterization enables us to theoretically determine the elastic (Young's) modulus and yield strength being in charge of the potential mechanical applications. Additionally, the load dependent microhardness values of the Y-site Ho substituted Y-123 materials, have not been modeled by the available theoretical methods (Hays-Kendall and indentation-induced cracking approach) up to the present. All the experimental findings show that the microstructural, electrical, mechanical and superconducting properties improve regularly with the increment in the Ho concentration level at the Y-site in the superconducting matrix until a certain value of x = 0.100 (optimum) beyond which the characteristics tend to retrograde rapidly. This is attributed to the fact that excess (0.100 < x) penetration of the homovalent Ho-sites on the Y-sites damages the crucial properties given above. In other words, from the Ho content level of x = 0.100 onwards in the Y-123 matrix, the oxygen content in the Cu-O chains begins to increase considerably and new induced oxygen atoms reorder the unit cell structure (rapid increment of the a axis length), so the structural phase transits from the orthorhombic to pseudotetragonal, being one of the most striking points deduced from this paper. Another vital discussion relies on the fact that the degradation of the mobile hole concentration (hole localization effect) in the Cu-sites leads to transition from optimally doped to the underdoped position in the crystal structure. Besides, the rapid degradation in the mechanical properties due to the increase of the specimen cracking/porosity, grain boundary weak-links and irregular grain orientation distribution confirms the relationships between the microstructure and mechanical properties of the Y-123 superconducting materials. Furthermore, all the samples exhibit the typical Reverse Indentation Size Effect (RISE) behavior under the applied indentation test load. As for the theoretical modeling of the hardness evidences, the calculations performed by IIC model are quite closer to the values of the plateau region as compared to those of HK approach. Based on the results, the IIC model is found to be superior to HK approach for the description of the real microhardness values belonging to all the superconducting samples. (C) 2014 Elsevier B.V. All rights reserved.
Diffusion coefficient, activation energy and the effect of Ag-doping on some physical properties of silver doped Bi1.8Pb0.4Ca2.2Sr2Cu3Ox
(Elsevier Science Bv, 2010) Akdoğan, Mustafa; Terzioğlu, Cabir; Varilci, Ahmet; Belenli, İbrahim
The aim of the present study is to investigate the effect of diffusion-doped silver on some physical properties of Bi1.8Pb0.4Ca2.2Sr2Cu3Ox superconducting samples and to calculate the diffusion coefficient and the activation energy of silver. The present work consists of three parts: (a) optimization of annealing temperature, (b) the effect of Ag diffusion-doped on microstructure and superconducting properties of Bi(Pb)CaSrCuO, and (c) calculation of diffusion coefficient of silver. First, to investigate the optimum annealing temperature we prepared Bi1.8Pb0.4Ca2.2Sr2Cu3Ox ceramic superconductors annealed at 830, 835, 840, 845, and 850 degrees C using the solid state reaction method. The investigations consist of XRD, SEM, dc resistivity, and transport critical current density measurements. The highest T-c and J(c) values were observed for the sample annealed at 840 degrees C for 48 h (B840). Large grain size, denser surface, and high volume fraction of the high-T-c phase were obtained for the sample B840. The diffusion doping of Bi1.8Pb0.4Ca2.2Sr2Cu3Ox by silver increased the critical current density from 123 to 696 A/cm(2) and the transition critical temperature by about 3 K compared with the undoped sample. Ag-doping increased the amount of high-T-c phase and improved the surface morphology. It also caused an increase of the lattice parameter c by an amount of 0.15%. The temperature dependence of the silver diffusion coefficient in the range 600-800 degrees C is described by D=2.9 x 10(-4)exp(-1.05 eV/k(B)T). Possible reasons for the observed improvement in the structural and superconducting properties of the samples due to silver diffusion are discussed. (C) 2010 Elsevier B.V. All rights reserved.
The effect of ag diffusion on properties of YBa2CU3O7-x thin films produced by electron beam deposition techniques
(Elsevier Science Bv, 2005) Görür, Osman; Küçükömeroğlu, Tayfur; Terzioğlu, Cabir; Varilci, Ahmet; Altunbaş, Mustafa
Superconducting YBa2Cu3O7-x thin films were prepared on pure MgO and Ag/MgO substrates (without and with Ag buffer layer) using an electron beam evaporation technique. The effects of isothermal annealing temperature and Ag diffusion on the crystalline structure and some superconducting properties were investigated by X-ray diffraction, scanning electron microscopy, critical temperature, critical current density and room temperature resistivity measurements. The optimum annealing conditions causing a high degree of preferential orientation with the c-axis perpendicular to the substrates were found to be the isothermal annealing at 930 degreesC for 5 h. Annealing of films on Ag/MgO substrates is accompanied by Ag diffusion from the buffer layer into YBCO films. The higher rate of crystallization of the YBCO films, the higher degree of c-axis orientation, the higher dense surface morphology, the increased lattice parameter c (by approximate to0.1%), the reduced room temperature resistivity (2-3 times), the slightly enchanced critical temperature (T-c = 92 K at R = 0) and the critical current density (J(c) = 4.2 x 10(5) A/cm(2) at 77 K) were observed for the Ag-doped films (on Ag/MgO substrates) in comparison with those for the undoped films (on MgO substrates). The temperature dependence of the Ag diffusion coefficient in YBCO films in the range 600-800 degreesC was described by the relation D = 1.9 X 10(-6) exp(-0.73 eV/kT).
Effect of annealing temperature on some physical properties of MgB2 by using the hall probe ac-susceptibility method
(Elsevier Science Bv, 2009) Varilci, Ahmet; Yeğen, Dinçer; Tassi, M.; Stamopoulos, D.; Terzioğlu, Cabir
A commercially available powder of MgB2 is used as starting material for the examination of the influence of the annealing temperature on the properties of this intermediate-T-c superconductor. We performed scanning electron microscopy (SEM) and Hall ac-susceptibility measurements as a function of temperature and ac-field amplitude on samples annealed at 650, 750, 850 and 950 degrees C. The imaginary part of ac-susceptibility measurements is used to calculate both the inter-granular critical current density, J(c)(T-p) and density of pinning force, alpha(j)(0). It was observed that all T-c, J(c)(T-p) and alpha(j)(0) exhibit a non-monotonic behavior on the annealing temperature range studied in this work. T-c is measured to be 39.85 +/- 0.02 K and J(c)(T-p) is estimated to be as high as 60 A/cm(2) at 39.2 K for the sample annealed at 850 degrees C. The peak temperature, T-p, in the imaginary part of the ac-susceptibility curves shifts to lower temperatures with both decreasing the annealing temperature and increasing the amplitude of the ac-magnetic fields. A comparison of the experimental ac-susceptibility data with theoretical critical-state models that are currently available is performed. SEM investigations showed that the grain size increases, and the grain connectivity improves when the annealing temperature increases up to 850 degrees C. The possible reasons for the observed changes in transport, microstructure and magnetic properties due to annealing temperature are discussed. (C) 2009 Elsevier B.V. All rights reserved.
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.
Effect of Ce addition on the magnetoresistivity, irreversibility field, upper critical field and activation energies of Bi-2212 superconducting ceramics
(Springer, 2012) Yıldırım, Gürcan; Doğruer, Musa; Öztürk, Özgür; Varilci, Ahmet; Terzioğlu, Cabir; Zalaoğlu, Yusuf
This study aims to analyze the effect of Ce addition on the microstructural, superconducting and physical properties of Bi1.8Sr2.0Ce (x) Ca1.1Cu2.1O (y) ceramics with x=0, 0.001, 0.003, 0.005, 0.01, 0.03, 0.05 and 0.1 via X-Ray analysis (XRD), scanning electron microscopy (SEM), electron dispersive X-Ray (EDX) and magnetoresistivity measurements. The ceramics produced in this work are prepared using the standard solid-state reaction method. The zero resistivity transition temperatures (T (c)), activation energies (U (0)), irreversibility fields (mu (0) H (irr)) and upper critical fields (mu (0) H (c2)) are determined from the resistivity versus temperature (R-T) curves under dc magnetic fields up to 7 T. The results show that T (c) and U (0) values of the samples are found to decrease dramatically with the increase in the Ce-content and applied magnetic field. Moreover, XRD results indicate that all the samples contain the Bi-2212 phase only and exhibit the polycrystalline superconducting phase with less intensity of diffraction lines with the increase of the Ce addition. As for the results of SEM images, the texturing, crystallinity, grain size distribution, layered grain growth and grain connectivity are observed to degrade with the increase of the Ce doping. Besides, the irreversibility fields and upper critical fields are found to degrade as Ce doping increases. Penetration depths (lambda) and coherence lengths (xi) are also discussed.
Effect of diffusion-annealing time (0.5 h <= t <= 2 h) on the mechanical and superconducting properties of Cu-diffused bulk MgB2 superconductors by use of experimental and different theoretical models
(Springer, 2013) Doğruer, Musa; Zalaoğlu, Yusuf; Yıldırım, Gürcan; Varilci, Ahmet; Terzioğlu, Cabir
This study reports the effect of different annealing time (0.5 h a parts per thousand currency sign t a parts per thousand currency sign 2 h) on the electrical, physical, microstructural, mechanical and superconducting properties of Cu-diffused bulk magnesium diboride (MgB2) system by means of dc resistivity, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and microhardness measurements (H (v) ). The room temperature resistivity (at 300 K), critical transition (T (c) (offset) and T (c) (onset) ) temperature, variation of transition temperature, grain size, phase purity, lattice parameter, texturing, surface morphology, crystallinity and Vickers microhardness values of the samples are evaluated and compared with each other. The resistivity results obtained reveal that the (T (c) (offset) and T (c) (onset) ) values of the samples produced ascend with the enhancement in the annealing time up to 1 h beyond which these values start to reduce systematically and in fact the smallest T (c) (onset) of 38.1 K and T (c) (offset) of 36.2 K are observed for the sample annealed for 2 h. Similarly, the SEM micrographs display that the surface morphology, crystallinity and grain connectivity improve until a certain diffusion-annealing time (1 h), and after this point, all the properties obtained start to degrade with the increase of the annealing time. Furthermore, the peak intensities, grain sizes and lattice parameters deduced from the XRD measurements illustrate that a systematic elongation in the a and c axis lengths is detected with the annealing time until 1 h beyond which a regular contraction in the lattice parameters is observed for the samples. Likewise, the peak intensities belonging to MgB2 phase enhance with the increment of the annealing time up to 1 h after which they reduce slowly; however, a new peak belonging to Mg2Cu phase appears in the sample annealed for the duration of 2 h, confirming both the reduction of the grain size and degradation of the critical temperature. Additionally, we have focused on the microhardness measurements for the examination of the mechanical properties of the samples studied. Experimental results of microhardness measurements are estimated using the various models such as Meyer's law, proportional sample resistance model, modified proportional sample resistance model and Hays-Kendall (HK) approach. Based on the simulation results obtained, the Hays-Kendall (HK) approach is determined as the most suitable model describing the mechanical properties of samples prepared.
Effect of doping by low content of yttrium at Ca and Sr sites of Bi(Pb)-2212 superconducting ceramics
(Elsevier, 2011) Amira, Abderrezak; Boudjadja, Y.; Saoudel, A.; Varilci, Ahmet; Akdoğan, Mustafa; Terzioğlu, Cabir
In this work, a comparative study of the effect of doping bylaw content of Y3+ between (Ca site) and out of (Sr site) the CuO2 planes of Bi(Pb)-2212 phase is presented. Ceramics of Bi1.6Pb0.4Sr2Ca1-xYxCu2O8+d (called CY series) and Bi1.6Pb0.4Sr2-xYxCaCu2O8+d (called SY series) with x=0, 0.025, 0.05, 0.075 and 0.1 are elaborated in air by conventional solid state reaction. They are characterized by X-ray diffraction (XRD), Scanning Electronic Microscopy (SEM), density, Vickers microhardness and resistivity measurements. The refinement of cell parameters is done by considering the structural modulation. In comparison with the undoped sample (x=0), the cell parameters a, b and c are reduced by the doping for both series while the b component of the modulation vector increases. A good correlation between the variations of the bulk density and the Vickers microhardness with x is obtained. For both series, the SEM analysis shows that the doped samples exhibit a reduced grain size than that of the undoped one. The variation of resistivity with temperature shows that all samples exhibit a metallic-like character in the normal state. For all doping levels, the CY series presents higher onset critical transition temperature than that of the undoped sample, which is equal to 85.43 K. The opposite is obtained for SY series. The highest value of this temperature is obtained for x=0.075 in the doped samples and is about 92.15 and 79.96 K for CY and SY series, respectively. These values may correspond to a near optimally doped state since the slope (d rho/dT) of resistivity shows a maximum at the same value of x=0.075. For both series, when Y3+ is introduced into the samples, the residual resistivity decreases first for x=0.025 and increases gradually after this value until x=0.1. (C) 2010 Elsevier B.V. All rights reserved.
Effect of Eu doping on structural and magneto-electrical properties of La0.7Ca0.3MnO3 manganites
(Elsevier Science Sa, 2011) Altıntaş, Sevgi Polat; Amira, A.; Mahamdioua, N.; Varilci, Ahmet; Terzioğlu, Cabir
We have investigated the influence of europium (Eu) doping on structural and magneto-electrical properties of La0.7Ca0.3MnO3 compounds. In order to analyze the physical mechanism of a spin disorder system and study their relative evolutions, two samples of nominal compositions (La0.7-xEux)Ca0.3MnO3 (x = 0.0 and 0.1) have been elaborated and characterized. In addition to structural changes seen by this kind of doping, the magnetoresistance (MR) and resistivity are increased while the metal-insulator transition temperature is decreased. Comparing the experimental data with the theoretical models shows that in the metal-ferromagnetic region, the electrical behaviour of both samples is quite well described by a theory based on Kondo-like spin dependent scattering, electron-electron scattering, electron-phonon and electron-magnon scatterings. For the high temperature paramagnetic insulating regime, the adiabatic small polaron hopping model is found to fit well the experimental curves. (c) 2011 Elsevier B.V. All rights reserved.
Effect of fluorine doping on phase formation and properties of Bi(Pb)-2223 ceramics
(Elsevier Science Bv, 2011) Amira, A.; Saoudel, A.; Boudjadja, Y.; Amirouche, L.; Mahamdioua, N.; Varilci, Ahmet; Akdoğan, Mustafa; Terzioğlu, Cabir
Superconducting ceramics of Bi1.6Pb0.4Sr2Ca2Cu3OyFx (x = 0-0.6) are prepared in air by conventional solid state reaction and characterized. The study shows that the melting point of the samples decreases as fluorine content increases. As a consequence, the grain size increases with the doping level and for x = 0.6, the sample is completely deformed and presents a concave shape making impossible the measurements on it. The Vickers microhardness reaches its maximum for x = 0.2. The analysis of the X-ray diffraction results reveals that all the samples are composed of only Bi(Pb)-2212 and Bi(Pb)-2223 phases. The highest proportion of the high T-c phase (Bi(Pb)-2223) is also observed for x = 0.2 and is about 67.32%. The refinement of cell parameters is done by considering the structural modulation. The results show that the doping leads to a reduction of cell volume as well as the a axis component of modulation. Fran resistivity versus temperature measurements, it is shown that the doped phases exhibit higher onset critical transition temperatures than the undoped one. The residual resistivity increases with fluorine content suggesting that the doping introduces structural defects and disorder into the samples. The obtainal critical current density at 77 K under zero magnetic field also increases with fluorine doping. (C) 2011 Elsevier B.V. All rights reserved.
Effect of Gd addition on the activation energies of Bi-2223 superconductor
(Elsevier Science Bv, 2011) Erdem, M.; Öztürk, Özgür; Yücel, E.; Altıntaş, Sevgi Polat; Varilci, Ahmet; Terzioğlu, Cabir; Belenli, İbrahim
We have investigated the effect of addition of Gd in Bi1.8Pb0.35Sr1.9Ca2.1Cu3GdxOy superconductor with x=0, 0.1, 0.2, 0.3, 0.4 and 0.5. The samples were prepared using the standard solid-state reaction method. The activation energies, irreversibility fields (H-irr), upper critical fields (H-c2) and coherence lengths at 0 K (xi(0)) were calculated from the resistivity versus temperature (R-T) curves under DC magnetic fields up to 7 T. The superconducting transition temperature, T-c, and activation energy, U-0, were found to decrease with increase in Gd concentration and with increase in applied magnetic field. The offset transition temperature of the pure (Gd00) sample without applied magnetic field is 108 K, whereas for Gd05 sample, the offset transition temperature drops to 5 K with 7 T applied magnetic field. The activation energy of the Gd00 sample without applied magnetic field is 34,980 K, and for Gd05 sample with 7 T applied field it is 98 K. H-irr and H-c2 values also decrease with increase in Gd addition. The possible reasons for the observed degradation in microstructural and superconducting properties due to Gd addition were discussed. (C) 2010 Elsevier B.V. All rights reserved.
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.
Effect of Mn addition on structural and superconducting properties of (Bi, Pb)-2223 superconducting ceramics
(Springer, 2012) Yıldırım, Gürcan; Bal, S.; Yücel, Ersin; Doğruer, Musa; Akdoğan, Mustafa; Varilci, Ahmet; Terzioğlu, Cabir
This study deals with the effect of Mn addition on the structural and superconducting properties of Bi1.8Pb0.4Sr2Mn (x) Ca2.2Cu3.0O (y) ceramics with x=0,0.03,0.06,0.15,0.3 and 0.6 by means of X-ray analysis (XRD), scanning electron microscopy (SEM), electron dispersive X-ray (EDX), resistivity, and transport critical current density (J (c)) measurements. Zero-resistivity transition temperatures (T (c)) of the samples produced via the standard solid-state reaction method are estimated from the dc resistivity measurements. Moreover, the phase fraction and lattice parameters are determined from XRD measurements while the microstructure, surface morphology and element composition analyses of the samples are investigated by SEM and EDX measurements, respectively. It is found that T (c) values are obtained to decrease from 109 K to 85 K; likewise, J (c) values are observed to reduce from 3200 A/cm(2) to 125 A/cm(2) with increasing Mn addition. According to the refinement of cell parameters done by considering the structural modulation, the Mn addition 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 (Mn0). SEM measurements show that not only the surface morphology and grain connectivity are seen to degrade but the grain sizes of the samples are found to decrease with the increase of the Mn addition as well. The EDX results reveal that the elements used for the preparation of samples distribute homogeneously and the Mn atoms enter into the crystal structure by replacing Sr and Cu atoms. The possible reasons for the obtained degradation in microstructural and superconducting properties are also interpreted.
The effect of Nd (2) O (3) addition on superconducting and structural properties and activation energy calculation of Bi-2212 superconducting system
(Springer, 2014) Öztürk, Özgür; Aşıkuzun, Elif; Coşkunyürek, Murat; Soylu, Nevin; Hancerlioğulları, Aybaba; Varilci, Ahmet; Terzioğlu, Cabir
The effect of Nd (2) O (3) addition on the micro-structural and the superconducting properties of Bi-2212 superconductor ceramics, prepared by solid state reaction method, was analyzed by performing X-ray diffraction (XRD), scanning electronic microscope (SEM), energy dispersive spectroscopy (EDS) and dc Resistivity (rho-T) measurements. The magnetoresistivity of the samples was measured for different values of the applied magnetic field strengths (0-7 T). Also, the activation energies were calculated using the Arrhenius equation. According to these results, the T (c) (offset) value of the undoped sample was decreased from 79 to 42 K with the growth of magnetic field. In the same way, the activation energy (U (o) ) values were significantly diminished by the increasing of magnetic field. A similar situation was observed in other doped samples. Activation energy for 0.05 % Nd (2) O (3) doped sample under 7 T magnetic field was 550 J/mol the least. In addition, lattice parameter c, calculated by analysis of XRD data, was decreased with doping while lattice parameter a was increased. SEM analysis shows that particles were shrinking with the addition. When compared with other elements for EDS analyses, it was analyzed an important decrease in the percentage of Sr with the increasing of Nd contribution.
Effect of Re doping on structure and magneto-electrical properties of La1.2Re0.2Ca1.6Mn2O7 manganites
(Elsevier, 2013) Mahamdioua, Nabil; Amira, Abderrezak; Altıntaş, Sevgi Polat; Varilci, Ahmet; Terzioğlu, Cabir
In this paper, the structural and magneto-electrical properties of the double layered La12Re0.2Ca1.6Mn2O7 manganites doped with Re=La, Pr, Eu, Y and Gd are presented. The results show that the decreasing of the ionic radii of the doping element decreases the lattice parameters. The grain size of the samples also decreases in the same order. Except for Pr, the doping decreases the metal-insulator transition temperature (T-MI) and increases the resistivity. The doped samples exhibit higher magnetoresistance (MR) than the undoped one on a large domain of temperature. The highest value is seen for the Eu doped sample and is about 91.96% at 139.4 K and 5 T. This sample exhibits a crossover from positive to negative MR for temperatures greater than T-MI. The obtained minimum of MR is about - 100.7% at 185.4 K. (C) 2013 Elsevier B.V. All rights reserved.
The effect of Sm substitution on properties of Bi1.6Pb0.4Sr2Ca2-xSmxCu3Oy superconductors
(Springer, 2007) Yılmazlar, Mustafa; Aydın, Hüseyin; Varilci, Ahmet; Terzioğlu, Cabir
The effect of the partial substitution of Ca by Sm in the Bi-2223 superconducting samples have been investigated in terms of X-ray diffraction (XRD), EDXRF (Energy Dispersive X-ray Fluorescent), magnetoresistivity, critical temperature, transport critical current density, and ac susceptibility measurements. The samples were prepared by the conventional solid-state reaction method. XRD patterns are used to calculate lattice parameters and phase ratio of the Bi-2223 samples. The volume fraction was determined from the intensities of Bi-2223 and Bi-2212 peaks. The room temperature XRD patterns of the samples showed the presence of Bi-2223 phase decreases with increasing the Sm content. We estimated the transition temperature of the samples from the resistivity versus temperature measurements in dc magnetic fields up to 0.6 T. We observed that transition temperature, T (c) , and transport critical current density, J(c)(trans) depend on the Sm substitution. They both decrease with increasing the Sm substitution. We extracted the peak temperature, T (p) , and the pinning force density from our previous ac susceptibility measurements. The pinning force density decreased with increasing the Sm content. The possible reasons for the observed decreases in critical temperature and critical current density due to Sm substitution were discussed.