Makine Mühendisliği Bölümü Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.11779/1944
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Article 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; Süzer-Çiçek, İlaydaThis 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.Article Increasing and Other Subsequence Problems for Random Interval Sequences(Elsevier, 2026) Arslan, Ilker; Islak, UmitVarious relations for comparison of intervals of real numbers are introduced, and the expected length of the corresponding longest increasing subsequence is analyzed. When intervals are randomly generated by taking the minimum and maximum of two independent uniform random variables, we prove that the expected length of the longest increasing subsequence grows on root the order of 3 n. We also investigate the asymptotic behavior of the expected length under alternative comparison relations and random interval models. Discussions on other subsequence problems for interval sequences are included.Article 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, SiddikaMo-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.Article Nbmovta Refractory High-Entropy Alloy Incorporated Wni Matrix Composite as a Future Plasma-Facing Material: Evaluation of Mechanical Properties and Helium Ion Irradiation Behavior(Elsevier Sci Ltd, 2025) Boztemur, Burcak; Filiz, Kaan; Karaguney, Zahide; Gokaydin, Eyupcan; Bozkurt, Yasin; Ozbasmaci, Ceren; Ovecoglu, M. LutfiRefractory high-entropy alloys (RHEAs) have gained attention in the last decades with their high mechanical strength, self-healing mechanism, and high irradiation resistance. These materials are evaluated to have a high potential as plasma-facing materials for fusion reactors. In this study, helium ion irradiation and mechanical behaviors of the RHEA-reinforced WNi matrix composites were investigated based on this perspective. Equimolar molybdenum, niobium, tantalum and vanadium powders were mechanically alloyed for 6 h to produce NbMoVTa RHEA with a single BCC phase. Then, different amounts (10, 20, 30, and 40 wt%) of RHEA were added into the W1Ni (containing 99 wt% W and 1 wt% Ni) matrix by planetary ball milling for 72 h. Consolidation was conducted by spark plasma sintering technique (1410 degrees C, 1 min). X-ray diffraction, scanning electron microscopy coupled with energy dispersion spectroscopy, and Archimedes' density analyses were performed on the composites. Moreover, wear and hardness properties of the composites were examined, and the lowest specific wear rate (0.59 mm3/N.m x 10- 5) and the highest hardness value (10.10 GPa) were found for the W1Ni-40RHEA composite. Helium irradiation was exposed to the composites to observe their irradiation resistance. It was observed that the lowest increment and the least deformation were obtained with the SPS'ed W1Ni-40RHEA composite. With the analysis of He+ irradiation, it was determined that the effect of radiation on mechanical properties is irrelevant. Also, it was observed that the addition of RHEA into the W matrix can create a high potential for using plasma-facing material. Moreover, it decreased the problems of tungsten against He+ irradiation.Article Citation - WoS: 3Citation - Scopus: 3Powder metallurgy processing of seven/eight component multi-phase (HfTiZr-Mn/Mo/W/Cr/Ta)B<sub>2</sub> 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. LutfiThis 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.Article Citation - WoS: 12Citation - Scopus: 13Computational Alloy Design, Synthesis, and Characterization of Wmonbvcrx Refractory High Entropy Alloy Prepared by Vacuum Arc Melting(Elsevier Ltd, 2024) Alkraidi, A.B.N.; Mansoor, M.; Boztemur, B.; Gökçe, H.; Kaya, F.; Yıldırım, C.; Öveçoğlu, M.L.Prior investigations have demonstrated enhanced mechanical properties, such as hardness and wear resistance, through high-entropy alloy designs that contain refractory metals. We propose the WMoNbVCrx alloy phase space as a single-phase BCC-structured, hard, and refractory high-entropy alloy for the first time. The WMoNbVCrx alloy (x = 0, 0.25, 0.5, 0.75, and 1) system is investigated computationally through CALPHAD and DFT for the equimolar and non-equimolar compositional phase spaces and synthesized through vacuum arc melting. The DFT calculations demonstrated the excellence of specific non-equimolar compositional spaces. It was found that stoichiometries rich in W and poor in V are exceptionally hard, while those rich in V and poor in W demonstrate unprecedented toughness, as determined by the ductility descriptor (Pugh's Ratio). The computational analysis shows the significance of microstructures that contain both (W-rich and W-poor) solid solution, where a synergy between hardness and toughness is created. Our experimental synthesis using vacuum arc melting demonstrated the possibility of successfully producing these alloys with W-rich (dendritic) and W-poor (interdendritic) solid solution regions, starting from elemental powders. The introduction of chromium (Cr) resulted in enhanced microhardness and wear resistance. The peak microhardness was attained when 0.5 moles of Cr were added, reaching 7.03 ±0.24 GPa, accompanied by the least wear volume loss. The produced alloys were found to align with the computationally predicted-designed alloys in terms of the hardness and Young's modulus trends that they follow. This comprehensive investigation underscores the synergistic application of CALPHAD and DFT techniques in the tailored design of novel high-entropy alloys, explaining their synthesis, structural correspondence, and the pivotal role of Cr in enhancing the mechanical properties of these alloys. © 2024 Elsevier B.V.Book Part Citation - Scopus: 4Jute and Luffa Fibers: Physical, Acoustical, and Mechanical Properties(Elsevier, 2022) Yüce, Hüseyin; Genç, Garip; Körük, HasanIn this study, the physical, acoustical, and mechanical properties of jute and luffa fibers are presented. It should be noted that, although the main materials of jute and luffa fibers are cellulose, hemicellulose, and lignin, the mechanical properties of jute and luffa fibers and the acoustical properties of jute and fiber samples can be quite different. It is worth noting that jute fibers are bidirectional while luffa fibers are random-oriented. Furthermore, the diameters and lengths of these two fibers can be quite different. One problem with these natural fibers is that very different values for their diameter, length, and density have been reported in the literature. Therefore, the diameter, length, and density of a huge number of jute and luffa fibers were measured using precise equipment to determine their average values and deviations in this study. In addition to the results of these measurements, the results of a comprehensive literature review on the physical, acoustical, and mechanical properties of jute and fibers and their green samples (i.e., fiber samples without any resin) were presented in this study. © 2023 Elsevier Ltd. All rights reserved.Book Part Citation - Scopus: 3Effects of Machining on the Acoustic and Mechanical Properties of Jute and Luffa Biocomposites(Elsevier, 2023) Genç, Garip; Körük, HasanAfter their production, biocomposite structures do not always have the final shape or dimensions required for their purpose, hence, they need machining. However, the effects of machining on the acoustic and mechanical properties of many biocomposites are still not well known. The effects of machining on the acoustic and mechanical properties of jute and luffa biocomposites are revealed in this chapter. To do this, the sound absorption coefficients (or SACs) and transmission losses (or TLs) of jute and epoxy and luffa and epoxy composite samples, with and without a turning process, are determined using the impedance tube method. The loss factors and Young’s moduli of the jute and epoxy and luffa and epoxy composite samples, with and without a milling process, are identified using experimental and theoretical modal analyses. The results show that, when the samples are machined, the sound absorption coefficients reduce by 3%-7%, the transmission loss levels increase by 6-11dB, and the damping levels and Young’s moduli reduce by 0.1%-0.5% and 3%-4%, respectively. © 2023 Elsevier Ltd. All rights reserved.Book Part Citation - Scopus: 1Prediction of the Sound Absorption Performance of Porous Samples Including Cellulose Fiber-Based Structures(Elsevier, 2022) Körük, HasanThe mathematical models for predicting the sound absorption coefficients (SACs) of porous samples are first presented, then they are used to predict the SACs of some porous structures, and their performances are evaluated. First of all, the parameters needed for the calculation of the SACs of a porous sample are briefly introduced. After that, the mathematical models for the prediction of acoustic properties are presented. These models include (i) simple empirical models such as Delany-Bazley and its modified versions, (ii) rigid-frame models such as Johnson-Champoux-Allard and Johnson-Champoux-Allard-Lafarge, and (iii) deformable-frame models such as Biot-Allard. After that, the estimation of the parameters needed in the mathematical models is presented. Then, the aforementioned models are used to predict the SACs of some porous samples including cellulose fiber-based structures, and their performances are evaluated in detail. © 2023 Elsevier Ltd. All rights reserved.Article Citation - WoS: 8Citation - Scopus: 9On the Gas Pressure Inside Cavitation Bubbles(Aip Publishing, 2023) Pasinlioğlu, Şenay; Delale, Can F. F.The validity of the reduced order [Delale and Pasinlioglu, Acoustic cavitation model based on a novel reduced order gas pressure law, AIP Adv. 11, 115309 (2021)] and of classical polytropic gas pressure laws during the response of a bubble to variations in the pressure of the surrounding liquid is investigated. In particular, from the exact expression of the gas pressure coupled to the thermal behavior of gas bubbles, we identify the conditions where the reduced order gas pressure law and the classical polytropic law hold. We then distinguish three regimes for the change of state of the bubble during its nonlinear oscillations as the nearly isothermal, transition, and nearly adiabatic regimes, depending on the value of the polytropic index, and we determine the mean value of the polytropic index in each regime by averaging over a parameter, which is a function of the Peclet number based on the characteristic thermal diffusion time. The results of the temporal evolution of the bubble radius, the bubble wall temperature, and the partial gas pressure inside the bubble are presented using an acoustic cavitation model based on the reduced order gas pressure law for both constant and variable interface properties.
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