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Öğe Boron-Doped Strontium-Stabilized Bismuth Cobalt Oxide Thermoelectric Nanocrystalline Ceramic Powders Synthesized via Electrospinning(SPRINGER, 2014) Kocyigit, Serhat; Aytimur, Arda; Cinar, Emre; Uslu, Ibrahim; Akdemir, AhmetBoron-doped strontium-stabilized bismuth cobalt oxide thermoelectric nanocrystalline ceramic powders were produced by using a polymeric precursor technique. The powders were characterized by using x-ray diffraction (XRD), scanning electron microscopy (SEM), and physical properties measurement system (PPMS) techniques. The XRD results showed that these patterns have a two-phase mixture. The phases are face-centered cubic (fcc) and body-centered cubic (bcc). Values of the crystallite size, dislocation density, and microstrain were calculated by using the Scherrer equation. The lattice parameters were calculated for fcc and bcc phases. The SEM results showed that needle-like grains are formed in boron-undoped composite materials, but the needle-like grains changed to the plate-like grains with the addition of boron. The distribution of the nanofiber diameters was calculated and the average diameter of the boron-doped sample is smaller than the boron-undoped one. PPMS values showed that the electrical resistivity values decreased, but the thermal conductivity values, the Seebeck coefficients, and figure of merit (ZT) increased with increasing temperature for the two samples.Öğe The effect of synthesis technique on thermoelectric properties of nanocrystalline NaCo2O4 ceramics(SPRINGER, 2014) Erdal, Mehmet Okan; Koyuncu, Mustafa; Uslu, IbrahimSodium cobalt oxide (NaCo2O4) nanocrystalline thermoelectric ceramic powders were synthesized using conventional polymerized complex sol-gel and electrospinning techniques and then consolidated into bulk ceramics. We have investigated the differences in the microstructure and thermoelectric properties of the samples prepared using electrospinning and sol-gel methods. The crystal structures of the powders and nanofibers were characterized using X-ray diffraction and scanning electron microscopy before and after calcination at different temperatures. Nanofibers prepared by electrospinning have a diameter of approximately 300 nm, while the diameter of the grains of sol-gel powders ranges from 1 to 5 mu m. Thermoelectric properties of the bulk ceramics were measured by physical properties measurement system in the temperature range of 15-300 K. The calculated values of dimensionless figure of merit at 300 K are 6 x 10(-7) and 4 x 10(-5) for sintered powders from sol-gel and nanofibers, respectively. While electrospinning production technique increase thermoelectric power approximately 15 % and it fold electrical conductivity average 20 times, it decreased thermal conductivity by 90 %.Öğe Fabrication and characterization of bismuth oxide-holmia nanofibers and nanoceramics(ELSEVIER SCIENCE BV, 2013) Aytimur, Arda; Kocyigit, Serhat; Uslu, Ibrahim; Durmusoglu, Senol; Akdemir, AhmetIn this article, a novel and simple method to produce both boron doped and undoped holmia stabilized bismuth oxide nanoceramic materials has been put forward. Boron doped and undoped poly ( vinyl alcohol)/bismuth-holmia acetate nanofibers were produced using the electrospinning technique and were calcined at 850 degrees C afterward in order to obtain nanopowder. The characteristics of the nanofibers were investigated with FT-IR, XRD, and SEM. XRD analyses showed that boron undoped holmia stabilized bismuth oxide nanopowders have the face-centered cubic structure (delta-phase), and that the incorporation of boron atoms into the composite prevents the nucleus formation and turns the structure into a more amorphous glassy form. The SEM micrographs of the fibers showed that the addition of boron results in the formation of cross-linked bright-surfaced fibers. The average fiber diameters for electrospun boron doped and undoped PVA/Bi-Ho acetate nanofibers were calculated using the ImageJ software as 102 nm and 171 nm, respectively. (C) 2012 Elsevier B.V. All rights reserved.Öğe Fabrication and Characterization of Boron Doped Ni/Zn Acetate Nanofibers(TAYLOR & FRANCIS INC, 2009) Uslu, Ibrahim; Öztürk, M. Kemal; Aksu, Mehmet Levent; Gökmeşe, FarukThis study is related to the preparation of boron doped Nickel/Zinc metal fibers prepared by the use of electrospinning technique. The originality of this study is the addition of boron to metal acetates. The addition of boron did not only increase the thermal stability of the fibers, but also increased their diameters, which gave stronger fibers. The FT-IR spectra of the fibers were in good accordance with literature. The differential scanning calorimetry (DSC) results indicate that the glass transition (Tg) and melting temperatures (Tm) showed great change with the addition of boron and boron doped fibers were observed to degrade at higher temperatures. The SEM appearance of the fibers showed that the addition of boron resulted in the formation of cross linked bright surfaced fibers.Öğe The Illumination Intensity and Applied Bias Voltage on Dielectric Properties of Au/Polyvinyl Alcohol (Co, Zn-Doped)/n-Si Schottky Barrier Diodes(WILEY-BLACKWELL, 2011) Uslu, Habibe; Altındal, Şemsettin; Tunç, Tuncay; Uslu, Ibrahim; Mammadov, Tofig S.The Au/polyvinyl alcohol (PVA) (Co, Zndoped)/ n-Si Schottky barrier diodes (SBDs) were exposed to various illumination intensities. Illumination effect on the dielectric properties has been investigated by using capacitance- voltage (C-V) and conductance-voltage (G/omega-V) characteristics at 1 MHz and room temperature. The values of dielectric constant (epsilon'), dielectric loss (epsilon ''), loss tangent (tan delta), electric modulus (M' and M ''), and AC electrical conductivity (sigma(AC)) were found strongly intensity dependent on both the illumination levels and applied bias voltage especially in depletion and accumulation regions. Such bias and illumination dependency of these parameters can be explained on the basis of Maxwell-Wagner interfacial polarization and restructuring and reordering of charges at interface states. In addition, the epsilon'-V plots also show an intersection feature at similar to 2.8 V and such behavior of the epsilon'-V plots appears as an abnormality compared with the conventional behavior of an ideal SBD. The obtained results revealed that illumination intensity enhances the conductivity of Au/PVA(Co, Zndoped)/ n-Si SBD. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 322-328, 2011Öğe Polymer-derived yttria stabilized bismuth oxide nanocrystalline ceramics(ELSEVIER SCI LTD, 2014) Aytimur, Arda; Uslu, Ibrahim; Durmusoglu, Senol; Akdemir, AhmetBoron doped and undoped Bi2O3-Y2O3 nanofibers were synthesized by the electrospinning method. The nanofibers were then calcined to obtain nanocrystalline ceramics. The synthesized nanofibers and nanocrystalline ceramics were characterized using XRD, FT-IR, SEM and XPS. According to the XRD results the undoped Bi2O3-Y2O3 nanocrystalline ceramic has a face-centered cubic structure. The XPS results show that nanocrystalline ceramics were pure Bi2O3, and there were no peaks related to either bivalent or tetravalent or pentavalent states in Bi2O3. The XPS results also show that the crystallinity of the boron doped nanocrystalline ceramic was decreased because of the network former property of the boron. The average fiber diameters for electrospun boron doped and undoped PVA/Bi-Y acetate nanofibers were calculated as 179 nm and 96 nm, respectively. The SEM micrographs of the nanocrystalline ceramics show that the undoped Bi2O3-Y2O3 ceramic has needle-like crystalline structure. However, the crystallinity of the boron doped Bi2O3-Y2O3 ceramic decreased because of boron doping. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.Öğe Preparation and characterization of PVA/Zinc acetate/boron composite fibers(WALTER DE GRUYTER GMBH, 2007) Uslu, Ibrahim; Baser, Burcu; Yayli, Ahmet; Aksu, Mehmet LeventThe objective of this work was to synthesize boron incorporated PVA/Zinc acetate nano precursor composite polymer fibers using electrospinning technique. Produced fibers, are useful reagents in organic synthesis of zinc oxide nano structures which have gained wide interest because of their potential applications in the fabrication of microelectronic devices. Boron incorporated PVA/ Zinc acetate not only improve the electrical properties of ZnO films but also has an influence on the crystal structure of ZnO films. The fibers were measured and characterized by conductiometer ( with four- point probe), Fourier transform infrared spectroscopy ( FT- IR), scanning electron microscopy ( SEM), differential scanning calorimetry ( DSC) and thermogravimetric analysis ( TGA). The formation of boron incorporated PVA/ Zinc acetate hybrid polymer was confirmed by FT- IR spectra. SEM micrographs clearly reveal that there are random- growth oriented fibers on aluminum foil with diameters ranging from 0.5 to 1 mu m. According to the TGA analysis decomposition of zinc acetate from the PVA chain was observed around 273 degrees C, 368 degrees C and 447 degrees C. Boron incorporated PVA/ Zinc acetate nano composite polymer fibers have been successfully synthesized for the first time using electrospinning technique and characterized in detail as precursor material for ZnO. Electrospinning process has been a widely used, simple and versatile method and in future may be used for large- scale preparation of nanofibers of ZnO.Öğe Synthesis and characterization of boron doped bismuth-calcium-cobalt oxide nanoceramic powders via polymeric precursor technique(ELSEVIER SCI LTD, 2013) Aytimur, Arda; Uslu, Ibrahim; Cinar, Emre; Kocyigit, Serhat; Ozcan, Fatih; Akdemir, AhmetIn this study, boron doped calcium stabilized bismuth cobalt oxide nanocrystalline ceramic powders were successfully prepared from aqueous boric acid containing calcium bismuth cobalt acetate/poly(vinyl alcohol) hybrid precursor polymer solutions. Then, obtained ceramic powders were characterized via FT-IR. XRD, and SEM techniques. According to X-ray results, fcc and bcc phases coexist in the samples of the nanocrystalline ceramic powders. fcc peaks became sharper and bcc peak decreased with increasing boron content. Structural parameters for face centered cubic structure were calculated using the Scherrer equation. Moreover, dislocation densities and microstrain values were calculated for the nanocrystalline powder samples. (c) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.Öğe Synthesis and characterization of boron-doped Bi2O3-La2O3 fiber derived nanocomposite precursor(SPRINGER, 2011) Durmusoglu, Senol; Uslu, Ibrahim; Tunc, Tuncay; Keskin, Selda; Aytimur, Arda; Akdemir, AhmetBoron doped poly(vinyl) alcohol/ bismuth - lanthanum acetate (PVA/Bi-La) nanofibers were prepared by electrospinning using PVA as a precursor. The effect of boron doping was investigated in terms of solution properties, morphological changes and thermal characteristics. The fibers were characterized by FT-IR, XRD, SEM and BET. The addition of boron did not only increase the thermal stability of the fibers, but also their diameters, which yielded stronger fibers. XRD analyses showed that boron doping increased the peak intensities and indicated that the boron doping enhanced the crystallite size. Moreover, no shifts were noticed in diffraction angles for boron doped and undoped samples. Therefore, boron doping did not significantly alter the lattice spacing. The SEM micrograph of the fibers showed that the addition of boron resulted in the formation of cross linked bright surfaced fibers. Also, grain diameters of boron doped and undoped nanocrystalline sintered powders were measured as 170 nm and 120 nm respectively. The BET results show that boron undoped and doped Bi2O3-La2O3 nanocrystalline powder ceramic structures sintered at 800 A degrees C have surface areas of 20.44 m(2)/g and 12.93 m(2)/g, respectively.Öğe Synthesis and characterization of boron-doped bismuth oxide-erbium oxide fiber derived nanocomposite precursor(SAGE PUBLICATIONS LTD, 2014) Aytimur, Arda; Kocyigit, Serhat; Uslu, Ibrahim; Durmusoglu, Senol; Akdemir, AhmetIn this study, boron doped and undoped Bi2O3-Er2O3 nanocomposite fibers were produced via electrospinning technique. Obtained fibers were turned into ceramics via calcination process. Obtained nanocomposite fibers and ceramics were characterized by Fourier transform infrared, x-ray diffraction, and scanning electron microscopy techniques. X-ray diffraction results show that boron undoped Bi2O3-Er2O ceramic consisted of face-centered cubic Bi2O3-Er2O phase. However, boron doped Bi2O3-Er2O ceramic consisted of orthorhombic phase. Crystallite sizes of the ceramics were evaluated using Scherrer's equation. Crystallite sizes of boron doped and undoped ceramics were calculated as 50 and 17 nm, respectively. The average fiber diameters for boron doped and undoped poly vinyl alcohol/Bi-Er acetate nanofibers were calculated as 79 nm and 96 nm, respectively. The Brunauer Emmett teller results show that boron undoped and doped Bi2O3-Er2O nanocrystalline powder ceramic structures sintered at 800 degrees C have surface area of 20.44 and 12.93 m(2)/g, respectively.Öğe Synthesis, Characterization, and Thermoelectric Properties of Electrospun Boron-Doped Barium-Stabilized Bismuth-Cobalt Oxide Nanoceramics(SPRINGER, 2014) Cinar, Emre; Kocyigit, Serhat; Aytimur, Arda; Uslu, Ibrahim; Akdemir, AhmetIn this study, the boron-doped barium-stabilized bismuth cobalt oxide thermoelectric nanocrystalline ceramic powders were produced by the polymeric precursor technique. The powders were characterized by X-ray diffraction, scanning electron microscopy, and the physical properties measurement system. The X-ray diffraction results showed that these patterns have mixture of two phases as face-centered cubic and body-centered cubic. Values of the crystallite size, the dislocation density, and the microstrain were calculated by the Scherrer equation. According to these values, the crystallite size decreased from 60 to 51 nm with the boron addition in the boron-undoped and boron-doped samples, respectively. The scanning electron microscope results showed that nanograins are rarely seen in the boron-undoped samples, but nanograins turn into needle-like and layered structures with boron addition. The diameters distribution of nanofibers was calculated. The average diameter of the boron-doped sample is smaller than the boron-undoped sample. The physical properties measurement system values showed that the electrical and thermal conductivity, the Seebeck coefficient, and the figure of merit increased with the temperature rise for both samples. The boron-doping effect increased the electrical and thermal conductivity, decreased the Seebeck coefficient, and decreased the figure of merit.Öğe Thermoelectric Properties of Nickel and Boron Co-Substituted NaCo2O4 Prepared by Electrospinning Technique(TRANS TECH PUBLICATIONS LTD, 2018) Erdal, Mehmet Okan; Koyuncu, Mustafa; Aksu, Mehmet Levent; Uslu, Ibrahim; Koçyiğit, SerhatIn this study nickel and boron doped sodium cobalt oxide NaCo2-xNixByO4 (0 <= x <= 0.3, 0 <= y <= 0.1) nanocrystalline thermoelectric ceramic powders were synthesized using electrospinning techniques and then consolidated into bulk ceramics. The differences in the microstructure and thermoelectric properties of the samples as a result of doping effect have been investigated. The crystalline structures of the powders and nanofibers were characterized using Xray diffraction and scanning electron microscopy and BET Analysis before and after the calcination process at different temperatures. Nanofibers prepared by the use of electrospinning technique, have a diameter of approximately 300 nm, and the diameter of the grains of calcined powders was observed to range between 150 to 500 nanometers. Thermoelectric properties of the bulk ceramics were measured by physical properties measurement system (Lot-Oriel PPMS) in a temperature range of 15-300 K. The calculated values of dimensionless figure of merit at 300 K are 4.25 x 10(-5), 5.3 x 10(-6), 8.6 x 10(-5) and 9 x 10(-6) for sintered powders from undoped, Ni and B doped powders, respectively.
