Makine Mühendisliği Bölümü Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.11779/1944
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Article A Distance-Dependent Random Graph Model and Its Analysis(Taylor & Francis Inc, 2026) Arslan, İlker; Işlak, ÜmitLet W-1,..., Wn be non-negative random variables. We consider an undirected random graph model on the node set {1,. ..,n}, where two nodes i < j are adjacent if W-i < W-j. In our setting, the Wi's are independent but not necessarily identically distributed, resulting in a model that generalizes the classical random permutation graphs. The model exhibits a certain dependence among the edges. Moreover, when nodes have physical interpretations- such as points on the real line R with node i located at position x = i-the model gains spatial structure and becomes, in particular, distance-dependent. We derive theoretical results on degree distributions, the number of isolated vertices, and the number of close neighbors. Simulation-based observations are also provided for the average clustering and the global efficiency.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 - Scopus: 4Investigation of the Motion of a Spherical Object Located at Soft Elastic and Viscoelastic Material Interface for Identification of Material Properties(Academic Enhancement Department, King Mongkut's University of Technology North Bangkok, 2024) Körük, Hasan; Pouliopoulos, A.N.Measuring the properties of soft viscoelastic materials is challenging. Here, the motion of a spherical object located at the soft elastic and viscoelastic material interface for the identification of material properties is thoroughly investigated. Formulations for different loading cases were derived. First, the theoretical models for a spherical object located at an elastic medium interface were derived, ignoring the medium viscosity. After summarizing the model for the force reducing to zero following the initial loading, we developed mathematical models for the force reducing to a lower non-zero value or increasing to a higher non-zero value, following the initial loading. Second, a similar derivation process was followed to evaluate the response of a spherical object located at a viscoelastic medium interface. Third, by performing systematic analyses, the theoretical models obtained via different approaches were compared and evaluated. Fourth, the measured and predicted responses of a spherical object located at a gelatin phantom interface were compared and the viscoelastic material properties were identified. It was seen that the frequency of oscillations of a spherical object located at the sample interface during loading was 10–15% different from that during unloading in the experimental studies here. The results showed that different loading cases have immense practical value and the formulations for different loading cases can provide an accurate determination of material properties in a multitude of biomedical and industrial applications. © 2023 King Mongkut’s University of Technology North Bangkok. All Rights Reserved.Article Citation - WoS: 1Mechanochemical Synthesis and Characterization of Nanostructured Erb4 and Ndb4 Rare-Earth Tetraborides(John Wiley and Sons Inc, 2024) Boztemur, B.; Kaya, F.; Derin, B.; Öveçoğlu, M.L.; Li, J.; Ağaoğulları, D.Rare-earth borides have become very popular in recent decades with high mechanical strength, melting point, good corrosion, wear, and magnetic behavior. However, the production of these borides is very challenging and unique. The production of ErB4 and NdB4 nanopowders via mechanochemical synthesis (MCS) is reported in this study first time in the literature. Er2O3 or Nd2O3, B2O3, and Mg initial powders are mechanically alloyed for different milling times to optimize the process. Rare-earth borides with MgO phases are synthesized, then MgO is removed with HCl acid. The nanostructured rare-earth tetraboride powders are analyzed using X-ray diffraction (XRD). Based on the XRD, ErB4 powders are produced successfully at the end of the 5 h milling. However, the NdB4 phase does not occur as the stoichiometric ratio, so the B2O3 amount is decreased to nearly 35 wt%. When the amount of B2O3 is decreased to 20 wt%, NdB4 and NdB6 phases are 50:50 according to the Rietveld analysis. However, a homogenous NdB4 phase is obtained with 30 wt% loss of B2O3. The average particle sizes of ErB4 and NdB4 powders are nearly 100.4 and 85.6 nm, respectively. The rare-earth tetraborides exhibit antiferromagnetic-to-paramagnetic-like phase transitions at 18 and 8.53 K, respectively. © 2024 The Author(s). Advanced Engineering Materials published by Wiley-VCH GmbH.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.Article Citation - WoS: 6Citation - Scopus: 6A New Approach for Measuring Viscoelastic Properties of Soft Materials Using the Dynamic Response of a Spherical Object Placed at the Sample Interface(Springer, 2023) Besli, Ayça; Koç,Ömer Hayati; Körük,Hasan; Yurdaer, Berk Salih; Koc, H. O.; Yurdaer, S. B.; Pouliopoulos, A. N.Background: There are several techniques to characterize the mechanical properties of soft materials, such as the indentation method and the method based on the application of a spherical object placed inside the sample. The indentation systems usually yield the elastic properties of materials and their mathematical models do not consider the inertia of the sample involved in motion and radiation damping, while placing an object inside the sample is not practical and this procedure can alter the mechanical properties of the sample for the method based on the application of a bubble/sphere placed inside the sample. Objective: A new approach for the identification of the viscoelastic properties of soft materials using the dynamic response of a spherical object placed at the sample interface was proposed. Methods: The spherical object placed at the sample interface was pressed using an electromagnet and the dynamic response of the spherical object was tracked using a high-speed camera, while the dynamic response of the spherical object placed at the sample interface was estimated using a comprehensive analytical model. The effects of the shear modulus, viscosity, Poisson’s ratio and density of the soft sample, the radius and density of the spherical object and the damping due to radiation were considered in this mathematical model. The shear modulus and viscosity of the soft sample were determined by matching the experimentally identified and theoretically estimated responses of the spherical object. Results: The shear moduli and viscosities of the three phantoms with the gelatin mass ratios of 0.20, 0.25 and 0.29 were measured to be 3450, 4300 and 4950 Pa and 12.5, 14.0 and 15.0 Pa⋅s, respectively. The shear modulus and viscosity of the phantom increases as the gelatin mass ratio increases. The frequency of oscillations of the hemisphere placed at the phantom interface increases as the gelatin mass ratio increases due to stiffness increase. Conclusions: After matching the experimental and theoretical steady-state displacements and amplitudes of oscillations of the hemisphere at the sample interface, the comparison of the experimentally identified and theoretically predicted frequency of oscillations further confirmed the identified material properties of the samples. The approach presented here is expected to provide valuable information on material properties in biomedical and industrial applications.