Browsing by Author "Mertdinc-Ulkuseven, Siddika"

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A Few Layers Graphene Encapsulated Fe-Based Nanoparticles Synthesized from Ferrocene Containing Precursors: CVD Optimization and Evaluation for Possible Nanocatalyst Performance towards H2 Production
(Pergamon-Elsevier Science Ltd, 2026) Demirbas, Derya; Kutluay, Sinan; Agaogullari, Duygu; Suzer-Cicek, Layda; Mertdinc-Ulkuseven, Siddika; Padberg, Gero; Felderhoff, Michael
This study focuses on optimizing the synthesis of a few-layer graphene-encapsulated iron-based nanoparticles (Fe/Fe3C@C), prepared through spray drying, chemical vapor deposition (CVD), and leaching processes using ferrocene-based precursors, and their application as nanocatalysts for hydrogen (H2) production via sodium borohydride (NaBH4) methanolysis. Ferrocene-impregnated silica powders were prepared by spray drying them from a solution containing ferrocene, fumed silica, and ethanol. Then, these prepared powders, known as precursor powders, were subsequently introduced into the CVD system. Both the reduction of ferrocene and the encapsulation of Fe-based nanoparticles by graphene layers occurred in-situ during the CVD process. CVD temperature and the flow rates of CH4 and H2 gases are critical parameters that effects of the microstructural, thermal, and magnetic properties of synthesized nanoparticles. The CVD system was performed at temperatures ranging from 850 to 1000 degrees C, with variable gas flow rates of 50 or 100 mL/min. Additionally, acid leaching with hydrofluoric (HF) and hydrochloric (HCl) acids ensured the synthesis of pure powders free from silica and uncoated Fe, confirming the chemical stability of the nanoparticles. The presence of graphene in all synthesized samples within these parameter ranges were confirmed by Raman spectroscopy. Phase identifications were carried out using X-ray diffraction (XRD) and Mo & uml;ssbauer spectroscopy, revealing the Fe and trace amount Fe3C as core phases. Transmission electron microscopy (TEM) revealed the core-shell structure of the nanoparticles with a few layers of graphene coatings. Based on the coercivity and magnetic saturation values obtained from vibrating sample magnetometry (VSM), synthesized core-shell nanoparticles exhibited soft magnetic properties (Ms = 22.4-33.5 emu/g, Hc = 82.3-278.3 Oe). Fe/Fe3C@C nanoparticles obtained under optimum conditions achieved very high H2 production rate (HPR = 54200 mLH2 gcat h- 1) values, with low activation energy (Ea = 20.08 kJ mol- 1) value, highlighting their potential as an efficient and promising candidate catalyst for industrial-scale H2 production via the NaBH4 methanolysis reaction. In addition, it was found that the Fe/Fe3C@C nanoparticles retained 48% and 71% of their initial activity after 5 consecutive cycles, as measured by the HPR and TOF values, respectively.
Citation - WoS: 3
Citation - Scopus: 3
Graphene Encapsulated Fe-Based Nanoparticles Synthesized From Iron(ii) Sulfate Heptahydrate Containing Precursors: Influence of Chemical Vapor Deposition Parameters
(Elsevier, 2024) Öveçoğlu, Mustafa Lutfi; Ağaoğulları, Duygu; Felderhoff, Michael; Winkelmann, Frederik; Demirbaş, Derya; Mertdinç Ülküseven, Sıddıka; Mertdinc-Ulkuseven, Siddika
Importance of process parameters on thermal, microstructural, and magnetic properties of synthesized core/shell nanoparticles was investigated during their production via chemical vapor deposition (CVD). Herein, iron(II) sulfate heptahydrate and fumed silica powders were mixed in ethanol, and the solution was used for precursor preparation by utilizing spray dryer. These prepared precursors were treated in the CVD process under methane/ hydrogen (CH4/H2) 4 /H 2 ) gas flow to synthesize graphene-encapsulated core/shell nanoparticles. CVD studies were performed at various temperatures (900-1000 degrees C), holding times (60, 90 min), and gas flow rates (100, 200 mL/ min). After CVD studies, purification was applied to remove uncoated nanoparticles, and remaining fumed silica phases originated from the precursor via selective acid leaching using hydrofloric acid (HF) and hydrochloric acid (HCl) solutions. X-ray diffractometry, Raman and Mossbauer spectroscopy, Zeta potential measurement, thermogravimetry combined with differential scanning calorimetry, scanning and transmission electron microscopy/energy-dispersive spectroscopy, and vibrating sample magnetometry (VSM) results yielded the optimized CVD parameters as 950 degrees C, 60 min, CH4/H2: 4 /H 2 : 1/1 and 50 mbar. The characterization results proved that multilayer graphene (d-spacing: 0.34 nm) encapsulated Fe/Fe3C 3 C nanoparticles (average core size: 46.9 nm, shell thickness: 16.6 nm) can be successfully synthesized by using CVD process followed by a leaching treatment. VSM results revealed that synthesized nanoparticles had soft ferromagnetic properties (Ms: s : 90.6-185 emu/ g; Hc: c : 255.4-301.6 Oe). Characterization results deepen the understanding of process parameters of CVD system on characteristics of core/shell nanoparticles.
Citation - WoS: 11
Citation - Scopus: 9
Graphene Nanoplatelet Reinforced Al-Based Composites Prepared From Recycled Powders Via Mechanical Alloying and Pressureless Sintering
(Elsevier Science Sa, 2024) Süzer, İlayda; Hayırcı, Sena Burcu; Boyacı, Ege; Deniz, Ayşe; Mertdinç Ülküseven, Sıddıka; Öveçoğlu, Mustafa Lutfi; Ağaoğulları, Duygu; Mertdinc-Ulkuseven, Siddika
This study reports on the powder metallurgy preparation and characterization of aluminum-graphene nanoplatelet (Al-GNP) composites synthesized using recycled Al powders. Recycled Al and GNP powders (0.1-1 wt%) were mechanically alloyed (MA'd) for 4 h, followed by cold pressing (at 450 MPa) and pressureless sintering at 590 degrees C for 2 h. Starting powders were analyzed using an optical emission spectrometer (OES) and a Raman spectrometer. Also, MA'd powders and sintered samples were characterized using an X-ray diffractometer (XRD), a scanning electron microscope/energy dispersive spectrometer (SEM/EDS), and a differential scanning calorimeter (DSC). Particle size analyses, pycnometer, and Archimedes' densities, Vickers microhardness, dry-sliding wear, and compression tests were also conducted. The Al4C3 formation was observed in the XRD patterns of sintered compositions. The highest and lowest relative densities were measured for the 1 wt% and 0.1 wt% GNP reinforced samples as 97 % and 92 %, respectively. The highest hardness value was obtained as approximately 1.31 GPa for 1 wt% GNP reinforced. With the addition of reinforcement GNP, the wear rate developed to approximately 0.00225 mm3/Nm. The compressive strength increased from nearly 70 MPa to 162 MPa.
Powder Metallurgical Synthesis, Thermochemical Calculations and Characterization Studies of HfB2 Powders
(Springer India, 2025) Akbari, Amir; Suzer-Cicek, Ilayda; Mertdinc-Ulkuseven, Siddika; Gokce, Hasan; Ovecoglu, M. Lutfi; Agaogullari, Duygu
This study reports on the thermochemical calculations, mechanochemical synthesis, purification process, and characterization studies of the HfB2 powders by using native sources. Firstly, HfO2, native B2O3, and Mg starting powders were prepared with a multi-axial vibratory ball mill (NanoMultimix) in stoichiometric and excess amounts. The milling process was optimized by varying the time (2, 4, 6, 8, 10, 12 h). Then, unwanted by-products (HfO2, MgO) were removed by leaching with 4 and 6 M HCl. Phase and Rietveld analysis, microstructure investigations with scanning electron microscopy/energy dispersion spectroscopy and transmission electron microscopy, and particle size measurement were conducted. The purest HfB2 was obtained in the powders milled for 8 h in stoichiometric ratios and leached with 6 M HCl. The resulting optimum powder has an average particle size of 135 nm. Oxidation kinetics (500, 600, 700, 800, and 900 degrees C) were also investigated. As the temperature increased, the amount of oxidation increased based on the TG result. As a result of the characterization studies, the synthesis of single-phase, high-purity HfB2 was achieved using domestic resources.
Citation - WoS: 3
Citation - Scopus: 3
Powder metallurgy processing of seven/eight component multi-phase (HfTiZr-Mn/Mo/W/Cr/Ta)B₂ high entropy diboride ceramics
(Elsevier, 2024) Suzer, Ilayda; Akbari, Amir; Ates, Semih; Bayrak, Kuebra Gurcan; Mertdinc-Ulkuseven, Siddika; Arisoy, C. Fahir; Agaogullari, Duygu; Öveçoğlu, M. Lutfi
This study aims to show the possibility of synthesizing seven- and eight-component high entropy diboride (HEB) ceramics using high energy ball milling-assisted spark plasma sintering (SPS). Metal boride powders, synthesized in laboratory conditions from metal oxide-boron oxide-magnesium powder blends, were combined equimolarly as seven and eight components containing systems. Afterwards, hybridized powders were mechanically alloyed (MA) for 6 h and subjected to spark plasma sintering (SPS) at 2000 degrees C and under 30 MPa. Detailed phase analysis and physical, microstructural, and mechanical characterization of the samples were performed. in the sintered products, the main phase belongs to the HEB, and also low amounts of Hf/Zr oxides and secondary phases (W or Ti-rich) occurred. The highest hardness was observed at the (HfTiZrMoWCrTa)B-2 with 25 GPa, and the lowest hardness was seen at the (HfTiZrMnCrMoWTa)B2 with 17 GPa. Also, the highest wear resistance was calculated for the (HfTiZrMnCrMoTa)B-2 as 6.05 x 10(-7) mm(3)/Nm. Additionally, (HfTiZrMnMoWTa)B-2 and (HfTiZrMnMoCrTa)B-2 have the highest and lowest Archimedes' densities, with 7.94 g/cm(3) and 6.91 g/cm(3), respectively.
Room-Temperature Synthesis of Refractory Borides: a Case Study on Mechanochemistry and Characterization of Mo-Borides and W-Borides
(Elsevier Sci Ltd, 2025) Süzer, İlayda; Akbari, Amir; Kaya, Faruk; Mertdinç Ülküseven, Sıddıka; Derin, Bora; Öveçoğlu, M. Lütfi; Ağaoğulları, Duygu; Mertdinc-Ulkuseven, Siddika
Mo-boride and W-boride powders were produced from native boron oxide, magnesium, and related metal oxide starting materials by mechanochemical synthesis (MCS) followed by an purification treatment. The reaction formation mechanisms and the products were predicted with the FactSageTM thermochemical simulation program. Different conditions were tested to determine the optimum synthesis parameters. MCS was conducted at stoichiometric ratios and different milling durations, using excess reactant amounts over the determined optimum time. After MCS, unwanted phases were removed by HCl acid leaching. Detailed phase analyses of the final powders were obtained by X-ray diffractometer (XRD), whereas detailed microstructure characterization was conducted by scanning electron microscope/energy dispersion spectrometer (SEM/EDS), transmission electron microscope (TEM) and particle size analyzer. Among the utilized parameters, the ideal composition chosen for Mo boride synthesis was 6 h milled and leached MoO3-100 wt% B2O3-50 wt% Mg (1.41 mu m), including alpha-MoB, beta-MoB, MoB2, Mo2B, Mo2B5, and Mo phases. For the synthesis of W boride, the proper composition was found as WO3-100 wt% B2O3-50 wt% Mg (0.37 mu m) containing W2B5, WB, beta-WB, WB4, W2B, and W phases after milling for 20 h and leaching. Besides, as a result of the oxidation resistance measurements at 700 and 800 degrees C, phases belonging to MoO2 and WO2 were found along with boride phases.