Mustafa Tepe

Fiber optic sodium and potassium sensing by using a newly synthesized squaraine dye in PVC matrix.

In recent years squarines received attention as fluorescent labels. Their very promising spectral properties such as long wavelength absorption and emission, high extinction coefficients and quantum yields could lead novel sensing technologies. In this work newly synthesized fluoroinophores named bis[4-N-(1-aza-4,7,10,13-tetraoxacyclopentadecyl)-3,5-dihydroxyphenyl]squaraine, azacrown-1 and 2 bis[4-N-(1-aza-4,7,10,13,16-pentaoxacyclooctadecyl)-3,5-dihydroxyphenyl]squaraine, azacrown-2 have been used for sodium and potassium sensing in plasticized PVC matrix. The squaraine derivatives exhibited fluorescence emission based optical responses to sodium and potassium with a detection limit of 1.10(-9) M. The sensor compositions exhibited wide response ranges between 10(-9) and 10(-5) M Na(+)or K(+), and, therefore, may be an alternative method to flame emission spectroscopy. The sensor is fully reversible within the dynamic range and the response time is 3 min under batch conditions. Cross sensitivity to pH is negligible in the pH range of 6.2-7.3. Throughout fiber optic based studies a relative signal change of 54-56% has been achieved. The azacrown dyes have the advantage that they can be excited with long wavelength light and, are, therefore, LED compatible. The cross sensitivity of azacrown-1 and -2 to Ba(2+), Ca(2+) and NH(4)(+)were also tested in separate solutions.

Surface-induced textures in lyotropic liquid crystalline mesophases

In the present work the investigation of the effect of YBa2Cu3O7−x complex compounds thin films on the mesomorphic, morphologic and orientational properties of lyotropic nematics have been carried out. The thin films of YBa2Cu3O7−x lead to an increase in number of morphologies of investigated nematics and promote the obtaining of tilted oriented and homeotropic textures of these nematics. The coating of YBa2Cu3O7−x thin films by inverted cylindrical DC magnetron sputtering system has been shown in this work to be a useful method for obtaining oriented textures of lyotropic nematics.

Effect of YBa2Cu3O7-x thin films on the textures and orientational properties of liquid crystals

Abstract.In the present work, the effect of thin films of YBa2Cu3O7-x complex compound on the mesomorphic and orientational properties of thermotropic nematic liquid crystals has been studied. Homogeneous, stable and reproducible homeotropic and tilted oriented textures of nematic liquid crystals were obtained. The effect of YBa2Cu3O7-x thin films on the morphologic, orientational and optical properties of thermotropic nematic liquid crystals are discussed.

Surface-induced and oriented textures of smectic liquid crystals: effect of thin films

In this work, the effect of YBa2Cu3O7−x complex compound thin films on the morphological, thermotropical, optical and orientational properties of smectic liquid crystals have been studied. The peculiarities of surface-induced textures and dynamics of their temperature transformations have been investigated. Stable, homogeneous and reproducible oriented textures of smectic liquid crystals have been obtained.

Investigation of the effect of thermal cycling on the device performance of YBa2Cu3O7−δ DC-SQUIDs

We investigated the effect of thermal cycling on the operational performance of YBa2Cu3O7?? (YBCO) direct current superconducting quantum interference devices (DC-SQUIDs) fabricated onto 24??SrTiO3 (STO) bicrystal substrates. The devices under investigation consist of directly coupled DC-SQUID magnetometer configurations. Thin films having 200?nm thicknesses were deposited by dc-magnetron sputtering and device patterns were made by a standard lithography process and chemical etching. The SQUIDs having 4??m-wide grain boundary Josephson junctions (GBJJs) were characterized by means of critical currents, peak-to-peak output voltages and noise levels, depending on the thermal cycles. In order to achieve a protective layer for the junctions against the undesired effects of thermal cycles and ambient atmosphere during the room temperature storage, the devices were coated with a 400?nm thick YBCO layer at room temperature. Since the second layer of amorphous YBCO is completely electrically insulating, it does not affect the operation of the junctions and pick-up coils of magnetometers. This two-layered configuration ensures the protection of the junctions from ambient atmosphere as well as from the effect of water molecules interacting with the film structure during each thermal cycle.

Studies on structural and magnetic properties of melt textured growth YBa2Cu3O7-x superconducting bulk materials

In this study, production methods and the structural and magnetic properties of melt-textured growth of YBa2Cu3O7-x bulk superconductors are discussed. During the research; various temperatures for melt texturing are tried for the optimization of the production of bulk YBCO. With the help of AC magnetic susceptibility measurements, some insights for the produced samples are handed out including critical phase transition temperatures and the effects of crystal growth inside the YBCO samples. XRD measurements showed bigger and greater number of crystal growth in a correlation with the increasing temperature and the microphotographs of the samples verified successive growth of YBCO crystals inside the samples. Sharper phase transition is reached with the increasing texturing temperature. Top seeding for the selected crystal orientation has also been tried at the 1070oC for the texturing temperature, but the resulting melt textured sample was not different with the only-textured sample in the means of magnetic properties. Optimization over the texturing temperature found out to be between 1070oC and 1080oC while this result only depends on magnetic and structural properties.

Dependence of the structural, electrical and magnetic properties of the superconductive YBCO thin films on the deposition rate

In this study, YBCO thin films on single crystal LaAlO3 (100) substrates have been grown using DC inverted cylindrical magnetron sputtering technique and the effect of the deposition rate on these films is investigated. Three different deposition rates are used to produce superconducting YBCO thin films with 150 nm of thickness on (100) LaAlO3 single crystal substrate at 780 0C. The samples are analyzed in detail by means of XRD, R-T, χ-T, M-H and AFM characterizations and also the critical current densities (Jc) are derived from the magnetic hysteresis curves using the modified Bean formula [1]. The critical current density at 50 K was found to be in the range of 3.107 A/m2 to 8. 107 A/m2 with a deposition rate between 2nm/min and 1.2nm/min. A correlation has been obtained so that as the film deposition rate increases, the surface smoothness and crystalline quality of the films significantly deteriorate, resulting in a significant decrease in Jc.

Dependence of Josephson Junction Critical Current on The Deposition Rate of YBa2Cu3O7−δ Thin Films

We have reported the effect of YBa2Cu3O7−δ (YBCO) thin film deposition rate on the 24 and 30 degree STO bicrystal Josephson junctions critical currents by fabricating series of junctions with different deposition rates. Dependence of YBCO thin film structures on the deposition rate was investigated. We have observed that the critical currents of junctions are strongly affected by the thin film deposition rate.

Selection of the Best Proper DC-SQUIDs in a Multi-SQUID Configuration

We have carried out experimental investigation of multi-DC-SQUID magnetometer configuration fabricated on thin films onto 24 degree bicrystal substrates by directly coupling the pick-up loop to DC-SQUIDs. The layout of the magnetometer pick-up loop was chosen as a square washer configuration by maximizing loop effective area and minimizing loop inductance. We have used Dc-Magnetron Sputtering technique for deposition of the films and chemical etching process for patterning the Josephson junctions having 4 widths. The use of multi-SQUID configuration is related to the selection of the best proper junctions for SQUID to improve the chip sensitivity with selectivity option of choosing the squid junctions rather than multichannel operation. Selection of the best junctions compared to each other depending on the junction critical currents and noise levels caused by the fabrication process and placements of the junctions on the grain boundary enable having an increased output signal of the DC-SQUID.

Developing a trilayer processing technique for superconducting Y Ba2Cu3O7−δ thin films by using Ge ion implantation

For making trilayer superconducting devices based on Y Ba2Cu3O7?? (YBCO) thin film processing, we developed a new technique by employing Ge ion implantation. A YBCO thin film of 150?nm thickness having high c-axis orientation and a transition temperature, Tc, of 90?K was implanted with 80?keV, 1 ? 1016?Ge?ions?cm?2 at room temperature. By the result of TRIM calculation, Ge ions were found to penetrate into the YBCO thin film approximately 60 nm below the surface of the film, thus leaving the lower part of the film as a superconductor. Upon implantation with Ge ions, the implanted upper part of the sample lost its electrical conductivity and diamagnetism while its original crystalline structure was preserved. The implanted ions we found did not alter the overall crystal structure of the YBCO thin film; this allowed us to grow an epitaxial superconducting upper layer of YBCO on top of the implanted area, leaving no need to use any buffer layer. The superconducting properties of the upper layer were similar to those of the pure YBCO base layer with an increased room temperature resistivity and a lowered Tc (88?K). This process provides an effective method for fabrication of a trilayer HTS device structure.