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Adjustable tunneling barrier in bi-based high-Tc cross-whisker junctions
(Iop Publishing Ltd, 2015) Kizilaslan, O.; Simsek, Y.; Aksan, M. A.; Koval, Y.; Yakinci, M. E.; Mueller, P.
We present a study of cross-whisker junctions with electronically modified tunneling barriers. Cross-whisker junctions were successfully prepared by annealing two crossed Bi-based whisker bars at temperatures of around 840 degrees C. In addition to the artificially produced junction at the interface, intrinsic Josephson junctions in the respective bars of the cross were observed. The artificial junction exhibited reproducible and almost ideal junction characteristics. The tunneling barrier/interface properties were controlled by carrier injection in c-axis direction. Using this process, we were able to reduce the tunneling resistance from 408 Omega to 30 Omega. At the same time, the critical current did rise by a factor of 4. Subsequently, the critical current was doubled while the tunneling resistance stayed constant. We interpret this observation in terms of the counter-play between transparency of the barrier and the carrier concentration of the electrodes. In this sense, we can consider the current injection procedure as an electronic gluing process of the two cross-junction whisker bars.
Crystallization of glass-ceramic Bi3Sr2Ca2Cu3Oy superconducting system
(Elsevier Science Bv, 2013) Kizilaslan, O.; Aksan, M. A.
In this study, the nucleation and the crystallization of the material with nominal composition of Bi3Sr2Ca2Cu3O delta have been investigated, depending on the heat treatment conditions. It was found that 4 and 8-fold branched nuclei on the samples occurred with increasing the heat treatment temperature. Finally, crystallization occurred with further increase of the temperature. The needle-like crystals (whiskers) perpendicular to the sample surface was formed above 800 degrees C, which consisted of the Bi-2212 phase. The results clearly showed that the an isotropy of surface tension is them ostim portant parameter indetermining the shape of the nucleus. (C) 2013 Elsevier B.V. All rights reserved.
Current-Voltage Measurements on a Bi2Sr2CaCu2Ox Whisker and Investigation of Its Vortex Dynamics in Self Field
(Springer, 2013) Altin, S.; Aksan, M. A.; Yakinci, M. E.; Bernstein, P.
We report current-voltage measurements carried out on a high quality Bi2Sr2CaCu2Ox whisker in a large temperature range below the critical temperature. When a perpendicular field is applied, the current-voltage curves are typical of the flux flow regime at low fields and resemble flux creep characteristics at high fields. This change, related to what is described as the critical current peak effect is associated to a transition between different vortex phases. In this contribution, we focus on the dynamics of the vortices in self field. We show that the critical current in the flux flow regime can be calculated straightforwardly taking into account inter-vortex forces only, supposing that free vortices run between pinned vortices.
Decrease of Ca3Co4O9+? thermal conductivity by Yb-doping
(Elsevier Sci Ltd, 2015) KIirat, G.; Aksan, M. A.; Rasekh, Sh.; Madre, M. A.; Diez, J. C.; Sotelo, A.
In this study, the effect of Yb-substitution on the structural, electrical and thermal transport properties of the Ca3Co4O9+delta system has been investigated in the low temperature region (between 10 and 300 K). The resistivity of samples increases with raising the Yb-concentration in the system. All the samples show a metal-semiconductor transition below 85 K. In the analysis based on Strongly Correlated Fermi Liquid Model, an increased bandwidth and a reduced electronic correlation are found. It has also been found that the energy gap, E-0, value also decreases with Yb-substitution. The samples show positive thermoelectric power, indicating that dominant charge carriers are holes in all the samples. The thermoelectric power value decreases with Yb-substitution. From Mott equation, it is determined that the Fermi energy and hole concentration decrease in the Yb-susbtituted samples, compared to the undoped ones. Thermal conductivity, kappa, is decreased in about 50% of the measured in undoped samples for the 0.01 and 0.03 Yb-doped samples at 300 K. Highest figure of merit, ZT, value is found to be 5.4 10(-3) at 300 K for the unsubstituted sample and the ZT value decreases by the substitution. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Effect of Bi deficiency on grain alignment of BiPb-2223 thin film fabricated using rf sputtering process and on critical current density properties
(Taylor & Francis Ltd, 2011) Aksan, M. A.; Altin, S.; Yakinci, M. E.; Guldeste, A.; Balci, Y.
In the present study, thin films prepared as a function of the Bi concentration in the BiPbSrCaCuO system were synthesised. Thin films were fabricated using radio frequency sputtering method. Crystal structure of the films fabricated was determined from X-ray diffraction measurements. The crystal orientation was analysed by X-ray pole figure and in-plane alignment. Both X-ray diffraction and pole figure analysis revealed that crystallinity in the films decreased significantly with decreasing Bi concentration in the system. A systematic decrease in the superconducting transition temperature and hole concentration per CuO plane was obtained with decreasing Bi concentration. The J(c) values of the films were calculated using Bean formula. It exhibits a significant dependence on the magnetic field and temperature. It was found that J(c) decreased sharply with increasing applied magnetic field. The highest J(c) value was found to be 1.06 x 10(6) A cm(-2) at 10 K, which corresponded to the best flux pinning among the films fabricated.
Effect of Bi2O3 Addition on the Single-Crystal BiSrCaCuO Whisker Growth
(Springer, 2011) Altin, S.; Aksan, M. A.; Altin, E.; Balci, Y.; Yakinci, M. E.
Bi-2212 superconducting whiskers have been fabricated by annealing a melt-quenched precursor using suitable heat-treatment cycles for Bi3+x Sr2Ca2Cu3O8+delta where x=1 and 2. Approximately 1-3 mm long whiskers were fabricated for x=1 and x=2 and their physical, electrical and magnetic properties were investigated. Crystallization activation energies of the materials were calculated according to Kissinger and Augis-Bennet method using the DTA data. Microstructural changes from glass to heat-treated glass-ceramic materials at different temperatures were investigated with SEM, EDX and XRD analyses. It was observed that the structure of the whiskers has high c-axis oriented single-crystalline Bi-2212 phase. The whiskers fabricated for x=1 and 2 showed very smooth surface without cracks and dislocations. T (c) of the whiskers for x=1 and 2 were obtained to be 83.2 K and 79.8 K respectively. The change of magnetization with applied magnetic fields (M-H) was investigated at three different temperatures, 10, 20 and 40 K, and J (c mag) was determined using the Bean model.
Effect of Ce substitution on superconducting properties of Bi2Sr2Ca2Cu3O10+? system fabricated by glass-ceramic technique
(Springer, 2011) Ozturk, R.; Aksan, M. A.; Altin, S.; Yakinci, M. E.; Balci, Y.
In this study, the samples with nominal composition of Bi2-x Ce (x) Sr2Ca2Cu3O10+delta , where x=0, 0.05, 0.1, 0.2, 0.4 and 0.6, are synthesized by using conventional glass-ceramic technique. Effects of the Ce substitution on the structural/microstructural and electrical properties of the Bi-2223 system have been investigated in detail. Ce has a solid solubility limit on glassification, which is found to be x=0.2. It is seen that glassification is destroyed for the Ce substitutions above x=0.2. Further, XRD investigations showed that main phase in all the heat-treated samples is identified to be Bi-2212. From SEM-EDX investigations of heat-treated samples, it is revealed that the highly Ce-substituted samples had complex and deformed structure. It is also found that electrical properties degraded with increasing the Ce concentration. Especially in high Ce substitution level, superconductivity is destroyed and an insulating behavior is observed.
Effect of Co addition on the critical current density of MgB2 superconductor
(Elsevier, 2014) Kirat, G.; Kizilaslan, O.; Aksan, M. A.; Yakinci, M. E.
In this study, the (MgB2),Co-1-x(x), samples, where x-0.0. 0.1, 0.3, 0.6 and 0.8, were prepared by using the solid-state reaction technique. Effect of the Co-addition on the structural, magnetic, critical current density. J(c), and flux pinning properties of the superconducting MgB2 system was investigated. A magnetic transition was observed with increasing the Co-concentration in MgB2. For the pure MgB2, J(c), was obtained to be 2.78 x 10(4) A cm(-2) at 5 K and 1.7 kOe, but J(c) was suppressed by addition of Co to the system. It was found that a large amount of Co in MgB2 did not act as pinning centers which improves J(c) The magnetic behavior observed by the Co-addition originates from the magnetic properties of cobalt itself. (C) 2014 Elsevier B.V. All rights reserved.
The effect of Mo addition on the microstructure and Jc properties of MgB2 tapes fabricated by PIT method
(Elsevier Science Sa, 2008) Bayazit, E.; Altin, S.; Yakinci, M. E.; Aksan, M. A.; Balci, Y.
We investigated the effect of Mo addition to MgB2/Ag tapes fabricated by the ex situ PIT method. The heat treatments were carried out at 600 degrees C for 24 h under At atmosphere. The structural development, microstructures, superconducting properties and critical current density were characterized by means of XRD, SEM, M-T and M-H measurements. XRD and SEM studies indicated that Mo, not reacting with matrix of MgB2, was formed. The T-c of the tapes decreased in all the addition levels. The maximum J(c) value was obtained to be 1.2 x 10(5) A cm(-2) at 10 K for pure MgB2 tape. Decrease of J(c) was observed with increasing the Mo concentration. The J(c)-H characteristics of the tapes were decreased by the addition of Mo. This is considered to be due to the reduced flux-pinning force in the material. (C) 2007 Elsevier B.V. All rights reserved.
The Effect of Ru Substitution on the Thermal, Structural and Magnetic Properties of Bi3Sr2Ca2Cu3O ? Superconducting System
(Springer, 2010) Aksan, M. A.; Yakinci, M. E.; Kadowaki, K.
We have investigated the effect of Ru substitution on the thermal, structural/microstructural and magnetic properties of Bi-3223 system prepared using solid-state technique. The crystallization studies were made using DSC data with four different uniform heating rates. The formation temperature of Bi-2212 did not change significantly with increasing the Ru concentration; however, it was found that the thermal decomposition temperatures decreased. The activation energy for crystallization, E (a) , decreased by increasing the Ru concentration. X-ray diffraction and SEM measurements showed that substitution of Ru for Bi caused significant changes in the crystal structure of the BSCCO material. A distorted and multiphase crystalline structure was observed. The M--T measurement exhibited that superconducting onset temperature (T (c) ) decreased with increasing the Ru concentration in the system. The substitution of Ru for Bi caused the formation of ferromagnetic clusters in the materials, which destroyed the superconductivity in the system. Symmetric M(H) loops were obtained for lower Ru substitution, but in the case of high level substitution of Ru, a typical weak ferromagnetic-like hysteresis loop was found. It is believed that the Ru ions in the system are responsible for ferromagnetic behavior in the highly substituted materials.
Effect of Secondary Annealing Process on Critical Current Density in Highly Textured Bi-2212 Superconducting System
(Springer, 2015) Aksan, M. A.; Madre, M. A.; Rasekh, Sh.; Constantinescu, G.; Torres, M. A.; Diez, J. C.; Sotelo, A.
Bi-2212 samples prepared by a solid-state reaction technique have been grown from the melt using the laser floating zone method. After annealing the as-grown bars, the samples showed a good grain alignment and a high transport critical current density. Secondary annealing processes were performed on the annealed samples with the aim of producing Bi-2212 phase controlled decomposition. Hence, the Bi-2201 phase and the secondary phases, which act as effective pinning centers, were obtained with the secondary annealing process. After these thermal treatments, the transport critical current densities of samples significantly increased, when compared to the annealed ones. The maximum critical current density was achieved when the samples were subjected to secondary annealing at 680A degrees C for 168 h with an improvement of similar to 80%, compared to the annealed ones. Moreover, it was found that magnetization of the secondarily annealed samples was also increased. The magnetic critical current densities in these secondary annealed samples were about 3 times higher than the values obtained for the annealed ones. These results clearly indicate that the secondary annealing processes lead to the formation of effective pinning centers in the bulk material.
The effect of Sn content on mechanical, magnetization and shape memory behavior in NiMnSn alloys
(Elsevier Science Sa, 2016) Aydogdu, Y.; Turabi, A. S.; Kok, M.; Aydogdu, A.; Yakinci, Z. D.; Aksan, M. A.; Yakinci, M. E.
The effects of the composition alteration on the magnetization and shape memory behavior of Ni50Mn40-xSn10+x (x = 0, 1, 2, 3) alloys were systematically investigated by thermal, magnetic and mechanical experiments. Phase transformation from ferromagnetic austenite to weakly magnetic martensite was observed during thermal cycling under magnetic field and transformation temperatures were decreased with increasing Sn (or decreasing Mn). Moreover, the saturation magnetization of martensite increased with Sn content. Ni50Mn40Sn10 showed the recoverable strain of 2.1% during thermal cycling under 300 MPa and the reversible superelastic strain of 2% at 190 degrees C. Mechanical experiments exhibited that the largest compressive deformation of about 7% at 650 MPa occurred in Ni50Mn39Sn11 while Ni50Mn40Sn10 had fractured at 4% and 390 MPa at room temperature. (C) 2016 Elsevier B.V. All rights reserved.
Effect of Substrates on Superconducting Properties of MgB2 Thick Films Fabricated by Spraying Method
(Springer, 2011) Ozabaci, M.; Aksan, M. A.; Yakinci, M. E.
MgB2 thick films have been prepared on Al2O3, MgO and SrTiO3 ceramic substrates using the spray method with two different heat-treatment cycles, 925 A degrees C with 10 min (Group A) and 610 A degrees C with 24 h (Group B). The structural/microstructural (XRD, SEM) and transport (R-T, MR-T) properties of the films prepared were investigated. XRD analysis showed that the films in Groups A and B consisted mainly of MgB2, but the peaks originating from the substrates were also observed in films fabricated on the Al2O3 substrate. From scanning electron microscopy, it was seen that the surface of MgB2 thick films prepared is not sufficiently homogeneous. Some cracks and heaps with different sizes were observed. The best electrical results were obtained for films in Group A on Al2O3 substrate. The T (c) value for films in Group A was found to be 36.1 K, 36 K and 35 K, for Al2O3, MgO and SrTiO3 substrates, respectively. No superconducting state was reached for films in Group B on the SrTiO3 substrate. In all the films which showed a superconducting state, the magnetic field strongly affected T (c) . A tail in the resistance curves was observed with increasing magnetic field for films in Group A on the MgO and SrTiO3 substrates. At magnetic fields above 1 T for films in Group A on the SrTiO3 substrate and above 2 T for films in Group A on the MgO substrate, the zero-resistance temperature, T (0), was not obtained.
Effect of the Er-substitution on critical current density in glass-ceramic Bi2Sr2Ca2(Cu3-xErx)O10+? superconducting system
(Elsevier Sci Ltd, 2016) Kirat, G.; Kizilaslan, O.; Aksan, M. A.
Bi2Sr2Ca2Cu3-xErxO10+delta (x=0.0, 0.5 and 1.0) samples were manufactured using conventional glass ceramic technique and their structural, magnetic and flux pinning properties were studied. The onset temperature of diamagnetic signal decreased by the Er-substitution. The x=0.5 Er-substituted sample showed perfect diamagnetism, which is attributed to secondary phases acting as pinning centers in the crystal structure. J(c) of the samples was calculated by using Bean's equation. The highest J(c) value was obtained for x=0.5 Er-substitution. The results suggest that the Er ions in the host material act as pinning centers which lead to the increase of J(c). (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Effects of In-situ and Ex-situ Heat-Treatment Procedures on the Transport Properties of the MgB2 Superconducting Thin Films Fabricated by Ultrasonic Spray Pyrolysis (USP) System
(Springer, 2011) Yakinci, M. E.; Aydogdu, Y.; Aksan, M. A.; Balci, Y.; Altin, S.
Superconducting MgB2 thin films have been prepared using Ultrasonic Spray Pyrolysis (USP) system. 2.4 MHz USP system and various solutions which contain different Mg, B and de-ionized water and LAPSA concentrations and gas atmospheres were used to obtain 500 nm to 1.0 mu m thick MgB2 films. Some of the films produced were heat treated in-situ in the spraying chamber during deposition and some of them were annealed ex-situ in the tube furnace under Mg vapor. T (c) and T (zero) of the samples were obtained to be 39.5 K and 37.4, 39.5 and 37.0 for ex-situ and in-situ prepared films, respectively. The highest critical current density was obtained to be 4.12x10(6) A cm(-2) for ex-situ annealed films and 4.01x10(6) A cm(-2) for in-situ produced films. The result obtained indicated that the ex-situ preparation method provides improvement in the transport and magnetic properties.
Effects of SnO on growth and physical properties of BSCCO whiskers
(Maney Publishing, 2010) Altin, S.; Aksan, M. A.; Balci, Y.; Yakinci, M. E.
Superconductor whiskers doped with SnO have been fabricated by annealing a melt quenched (Bi2Sn1)-223 precursor using suitable heat treatment cycles. Approximately 5 mu m thick, 90 mu m wide and 5 mm long whiskers were fabricated, and their physical, electrical and magnetic properties were investigated. Crystallisation activation energies of glass phase fabricated were calculated to be 390 kJ mol(-1) using Kissinger method based on the differential thermal analysis data. The T-c value of the whiskers was found to be 94 K from M-T measurement. The magnetisation of whiskers before superconducting transition increased with decreasing temperature, and after superconducting transition, the magnetisation of whiskers decreased, from positive to negative, due to the diamagnetic nature of superconductivity. The change on magnetisation dependence of applied magnetic fields (M-H) showed that whiskers have paramagnetic-diamagnetic multiphase structure.
Enhancement of the critical current of intrinsic Josephson junctions by carrier injection
(Iop Publishing Ltd, 2015) Kizilaslan, O.; Simsek, Y.; Aksan, M. A.; Koval, Y.; Mueller, P.
We present a study of the doping effect by carrier injection of high-T-c superconducting Bi-based whiskers. The current was injected in the c-axis direction, i.e., perpendicular to the superconducting planes. Superconducting properties were investigated systematically as a function of the doping level. The doping level of one and the same sample was changed by current injection in very small steps from an underdoped state up to a slightly overdoped state. We have observed that T-c versus log (j(c)) exhibits a dome-shaped characteristic, which can be fitted by a parabola. As T-c versus carrier concentration has a parabolic form, too, it can be concluded that the critical current density j(c) increases exponentially with the doping level. The electron-trapping mechanism is interpreted in the framework of Phillips' microscopic theory. In addition, the Joule heating effect in the intrinsic Josephson junction (IJJ) was controlled by carrier injection, and the effect of the non-equilibrium quasiparticle on the I-V curves of the IJJs was also discussed.
Fabrication of BSCCO Films on SrTiO3 Substrates Using Ultrasonic Spray Pyrolysis (USP) Technique
(Springer, 2013) Turkoglu, S.; Aksan, M. A.
In this study, the BiSrCaCuO (Bi-2212) films on SrTiO3 substrates were fabricated using an ultrasonic spray pyrolysis technique (USP). Structural, electrical, magnetic, and critical current density, J(c), properties of the films fabricated were investigated under different heat treatment conditions. XRD analysis showed that the films mainly consisted of the Bi-2212 phase, but the Bi-2223 phase was also detected. T-c values of the films were found between 81 K and 88 K, depending on the heat treatment conditions. J(c) values of the films were calculated using the Beans' equation. Highest J(c) value was found to be 2.93 x 10(5) A cm(-2) at 5 K and 0 T for Film C. The results obviously showed that USP method is a very effective technique for fabrication of the HTc films having high J(c) values as well as its simplicity, low cost, and easily coating.
Fabrication of NdCeCuO and effects of binding agents on the growth, micro-structural and electrical properties
(Elsevier Science Bv, 2011) Altin, S.; Aksan, M. A.; Turkoglu, S.; Yakinci, M. E.
NdCeCuO superconducting samples were fabricated using ethyl alcohol, acetone and ethylenediaminetetraacetic acid (EDTA) as binding agents. For evaporation of binding agents, the samples were heat treated at 1050 degrees C for 24 h and then at 950 degrees C for 6-48 h under argon atmosphere to obtain the superconducting phase. The best superconducting performance was found in the sample heat treated at 1050 degrees C for 24 h and then 950 degrees C for 12 h which was fabricated by using acetone as binding agent. The T(c) and T(0) value was found to be similar to 25 K and 23.4 K, respectively. Grain size in the samples fabricated was calculated using Scherer equation and SEM data. It was found that grain size strongly depends on the binding agents and heat treatment conditions. Some cracks and voids on the surface of the samples were observed, which influences the superconducting and electrical transport properties of the samples. (C) 2011 Elsevier B.V. All rights reserved.
Fabrication of single crystalline Bi-2212 whisker with addition of Sb2O3 into the Bi2Sr2Ca2Cu3Ox system and their thermal, structural and superconducting properties
(Elsevier, 2011) Altin, S.; Aksan, M. A.; Yakinci, M. E.
Single crystalline Bi-based superconducting whiskers have been fabricated from a melt quenched Bi2SbSr2Ca2Cu3Ox. precursor using suitable heat treatment cycles. Whiskers fabricated in this study have 6-15 mu m thickness, 60-95 mu m width and 1-3 mm length. Crystallization activation energy of the material fabricated was calculated to be 609 kJmol(-1) by Kissinger method. Structural and micro-structural changes with the heating treatment cycles were analyzed by XRD and SEM-EDX. It was found that whiskers have pure c-axis oriented single crystalline Bi2Sr2CaCu2Ox (Bi-2212) phase with very smooth surface and without cracks and dislocations. Sb ions did not diffuse directly to the structure of whiskers. But Sb ions added to the system caused to decrease the number of whiskers. Temperature dependence of resistivity was analyzed along both the a- and c-axis. The superconducting transition temperature. To of the whiskers was found to be 93.8 K for the a-axis resistivity and 90.9 K for the c-axis resistivity, respectively. The transport critical current density, integral(trans)(c) was found as 2.6 x 10(5) A/cm(2) at 10 and 1.1 x 10(5) at 70 K. Magnetization hysteresis (M-H) was investigated at three different temperatures (10, 20 and 30 K). Magnetic critical current densities, integral(mag)(c), of whiskers were determined using the Bean's Model. (C) 2011 Elsevier Masson SAS. All rights reserved.

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