Yazar "Akyildiz, Hasan" seçeneğine göre listele

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    Effect of Synthesis Conditions on the Size and Morphology of Magnetic Nickel Particles
    (Wiley-VCH GmbH, 2025) Kivrak, Burak; Akyildiz, Hasan; Aka, Cemal; Akyol, Mustafa
    This study compares the properties of nickel (Ni) particles synthesized through the chemical-reduction-reactions (CRRs) under open-atmosphere and pressurized (solvothermal) conditions. The primary objective was to produce Ni particles with sizes below 100 nm with smooth surfaces, single-phase composition, highly crystalline, mono-disperse, and magnetic which are essential for constructing nanostructured Ni suitable for diverse applications. CRRs were conducted in a glass beaker for ambient pressure synthesis and a solvothermal reactor for pressurized conditions. The structural, morphological, and magnetic characteristics of the samples were investigated in detail. The findings demonstrated that the manipulation of the synthesis conditions in both methods allowed the formation of Ni particles with a wide range of morphologies, crystallinity, and purity. Besides, particle sizes varied from nanometer to micrometer scale, depending on the synthesis approach and processing conditions. Moreover, the relation between magnetic characteristics and morphological features was discussed in detail. Therefore, this research provides a comparative evaluation of two different synthesis methods, offering insight into the optimization of conditions for producing phase-pure, magnetic, and equiaxed Ni nanoparticles (NPs). By addressing the challenge of achieving mono-disperse Ni NPs, this study contributes to the understanding of synthesis techniques and elucidating the mechanisms underlying the formation of magnetic Ni NPs.
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    Interface engineering in honeybee-leg-like TiO2@NiCo2O4 nanocomposites: A novel platform for high-performance microwave absorbers
    (Elsevier, 2025) Kivrak, Burak; Kaya, Ismail Cihan; Akyol, Mustafa; Akyildiz, Hasan
    This study reports fabrication of novel TiO2@NiCo2O4 nanocomposites featuring a honeybee-leg-inspired hierarchical architecture, in which radially aligned NiCo2O4 nanoneedles grow on TiO2 fibers to mimic the branched morphology of honeybee legs. This unique architecture generates abundant heterointerfaces and multi-level scattering centers, which directly contribute to enhanced interfacial polarization, impedance matching, and microwave attenuation. The nanocomposites were constructed by electrospinning dual-phase TiO2 nanofibers, followed by the hydrothermal growth of radially aligned NiCo2O4 nanoneedles. Structural and morphological characterization via XRD, SEM, and TEM revealed the formation of a heterostructure with well-defined interfaces. Microwave absorption properties were examined between 2-12 GHz considering reflection loss (RL), impedance matching, complex permittivity and permeability, Cole-Cole plots, Eddy current loss, and attenuation constant. Results demonstrated that the TiO2@NiCo2O4 nanocomposite achieved a minimum reflection loss (RLmin) of-21.30 dB at 9.35 GHz with a 4 mm thickness, and an effective absorption bandwidth (EAB) of 4.27 GHz (7.50-11.77 GHz), covering 94.3 % of the X-band. Additionally, with 5 mm thickness, it reached an RLminof-20.51 dB at 7.31 GHz and an EAB of 3.93 GHz (5.38-9.31 GHz), corresponding to 65.5 % C-band and 32.8 % Xband coverage. These superior absorption capabilities are derived from the bio-inspired hierarchical design, which synergistically integrates dielectric and magnetic losses with morphology-assisted multiple-scattering. The findings demonstrate the significance of bio-inspired design and interface engineering in the development of next-generation high-performance microwave absorbing materials.
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    Phase (1T/2H) dependent electromagnetic wave absorbing performance of flower-like MoS2 nanosheets
    (Elsevier, 2023) Aka, Cemal; Kivrak, Burak; Teksen, Fikret Alpay; Akyildiz, Hasan; Akgol, Oguzhan; Karaaslan, Muharrem; Akyol, Mustafa
    A study was carried out to determine the phase-dependent electromagnetic wave (EMW) absorption performance of MoS2 particles via systematically diversified hydrothermal synthesis conditions such as time, temperature, precursor molarity, and oxalic acid concentration. The formation of mixed-phase structure was verified depending on the presence of the characteristic diffraction peaks belong to 1T (trigonal, P3?m1 space group) and 2H (hexagonal, P63/mmc space group) phases in the X-ray diffraction (XRD) patterns. Phase-pure 2H-MoS2 was obtained at 200 degrees C after 12 h of reaction and using 5 mmol oxalic acid. Microscopic examinations indicate that MoS2 particles formed by assembling several nanosheets to create a flower-like morphology. The size, shape, and the distance between the nanosheets were observed to change with increasing temperature, time, and acid concentration. The band gap was found to be modified depending on the phase distribution and decreased from 1.54 eV (2H) to a minimum of 0.64 eV (1T/2H) with the formation of the 1T (metallic) phase. Finally, the minimum reflection loss was measured as -67.73 dB (99.9999 % of absorption at 9.05 GHz) with an effective absorption bandwidth (EAB) of 3.52 GHz for the sample composed of 1T/2H mixed-phased structure.
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    Polyaniline-Functionalized Nanosized Cobalt Ferrite-Decorated MoS2 Composites for Broadband Electromagnetic Wave Absorption
    (Amer Chemical Soc, 2024) Kivrak, Burak; Akyildiz, Hasan; Akgol, Oguzhan; Karaaslan, Muharrem; Akyol, Mustafa
    This study presents the synthesis of polyaniline (PANI)-coated cobalt ferrite-decorated molybdenum disulfide (MoS2@CoFe2O4); a composite architecture engineered to combine unique morphological, magnetic, and dielectric properties for the electromagnetic wave/radar absorption applications. MoS2 and CoFe2O4 were produced separately using a one-step hydrothermal process. Then, the CoFe2O4 nanoparticles were integrated into the flower-like MoS2 nanosheets using the sonication method. Finally, PANI was synthesized through in situ polymerization of aniline on the surface of MoS2@CoFe2O4 to create the final design as well as to boost the dielectric performance. Structural analysis confirmed the existence of the 2H phase and a minor trace of the 1T phase in MoS2, while CoFe2O4 displayed a cubic structure without any detectable impurities. Scanning electron microscopy verified the successful distribution of CoFe2O4 particles within the MoS2 nanosheets, aligning with the design's intended configuration. The electromagnetic wave (EMW) characteristics of the composites were analyzed by using a vector network analyzer. The MoS2@CoFe2O4 demonstrated a broad effective absorption bandwidth, spanning from the X-band to the Ku-band with a minimum matching thickness of just 2 mm. The reflection loss minimum value (RLmin) reached -22.82 dB at 8.96 GHz, corresponding to a 99.47% absorption of the incident EMWs. The addition of PANI at 50% by weight further increased the RLmin value to approximately 99.999% absorption (-50 dB), indicating enhanced impedance matching and absorption efficiency.