Mühendislik Fakültesi / Faculty of Engineering

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    The Capacitance/Conductance And Surface State Intensity Characteristics Of The Al/(CMAT)/p-Si Structures
    (PHYSICA SCRIPTA, 2024-01-24) Cetinkaya, H. G.; Bengi, S.; Sevgili, O.; Altindal, S.
    To determine the Al/(CMAT)/p-Si structure's admittance analysis, capacitance/conductance versus frequency (C/G-V-f) data was obtained in the 3 kHz-3 MHz and -2/4 V ranges at room temperature. The powder form of CeMgAl11O19: Tb (CMAT) was thermally evaporated onto the front of p-Si wafer at 10-6 Torr as interfacial layer. From the Nicollian and Brews method, voltage-dependent spectra of Rs were derived for various frequencies. The parallel conductance and low-high frequency capacitance (CLF-CHF) techniques, respectively, were used to determine the voltage and frequency dependent spectra of Nss and their lifetime (tau). Surface states (Nss), which are identified by admittance measurements, emerge at the M/S interlayer because of high capacitance and conductance values at low frequencies. This can also be explained by the Nss's ability to track ac signals well at lower frequencies. The normalized parallel conductance versus frequency (Gp/omega-f) plot under various biases shows a peak because of Nss existence. x-ray diffractometer (XRD) was used for structural investigation and the average crystal size (D) of the nanocrystals (CMAT) was found to be less than 0.34 nm by using the Debye-Scherer's equation.
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    Material Gradation Effects On Twisting Statics Of Bi-Directional Functionally Graded Micro-Tubes
    (AIP ADVANCES, 2024-03-04) Aghazadeh, Reza; Rafighi, Mohammad; Kumar, Raman; Al Awadh, Mohammed
    This study aims to characterize the twisting behavior of bi-directional functionally graded (FG) micro-tubes under torsional loads within the modified couple stress theory framework. The two material properties involved in the torsional static model of FG small-scale tubes, i.e., shear modulus and material length scale parameter, are assumed to possess smooth spatial variations in both radial and axial directions. Through the utilization of Hamilton's principle, the governing equations and boundary conditions are derived, and then, the system of partial differential equations is numerically solved by using the differential quadrature method. A verification study is conducted by comparing limiting cases with the analytical results available in the literature to check the validity of the developed procedures. A detailed study is carried out on the influences of the phase distribution profile and geometric parameters upon twist angles and shear stresses developed in FG micro-tubes undergoing external distributed torques.
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    Exploring the Critical Risk Factors of Public-Private Partnership City Hospital Projects in Turkey
    (BUILDINGS, 2024-03-14) Dogan Erdem, Tugba; Birgonul, Zeynep; Bilgin, Gozde; Akcay, Emre Caner
    Governments face challenges in delivering essential public services due to their limited funds. This has led to an increasing reliance on the Public-Private Partnership (PPP) model, an alternative financing model involving a long-term collaboration between the private and public sectors to provide public services. Turkey, as a developing country facing financial limitations, has embraced the PPP model to address urgent public needs. Over the past decade, the Turkish Government has extensively utilized the PPP model, particularly in executing city hospital projects. However, investors have faced challenges in project execution due to various risk factors. Therefore, the main objective of this study is to explore the critical risk factors associated with PPP city hospital projects in Turkey. In this context, a comprehensive literature review was conducted to identify potential risks related to PPP city hospital projects. A questionnaire survey was implemented to assess the probability of occurrence and the severity of the impact of these risk factors. The collected data underwent analysis to determine the priority of these risk factors. The findings revealed that the top five most critical risk factors in PPP city hospital projects in Turkey are "foreign exchange rate fluctuations", "inflation rate volatility", "high finance costs", "fiscal issues", and "economic crises". Conversely, "unavailability of equipment" was identified as the least significant risk factor. The insights gained from this research can offer valuable guidance for prospective investors interested in participating in PPP city hospital projects in Turkey and other developing countries with similar conditions.
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    Accelerating The Environmental Biodegradation Of Poly-3-Hydroxybutyrate (Phb) Via Plasma Surface Treatment
    (BIORESOURCE TECHNOLOGY REPORTS, 2024-09-02) Akdogan, Ebru; Sirin, Hasret Tolga; Sahal, Gulcan; Deniz, Zulkuf; Kaya, Ayberk; Serdaroglu, Dilek Cokeliler
    The surface of poly-3-hydroxybutyrate (PHB) was modified using a low-pressure plasma system with air as the process gas to accelerate its biodegradation rate in soil. The water contact angle of PHB was reduced from 98 degrees to 57 degrees after plasma treatment, rendering the surface hydrophilic and also induced an increase in the surface free energy. Etching on the surface was observed after the plasma treatment without a significant change in the surface crystallinity. AFM imaging showed that the plasma treatment increased the surface roughness by about 10 folds and created diverse surface structures. The soil burial test showed an approximately 1.5-fold increase in the biodegradation rate for the plasma-treated sample. Initial microbial attachment and biofilm formation were higher on the modified surface. This study demonstrated that the surface morphology created by plasma treatment promoted initial colonization and subsequent biofilm formation on the PHB surface, facilitating and accelerating its biodegradation in soil.
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    Mechanical Behavior İnvestigation Of Fused Deposition Modeling Joints By Using Different Bonding Geometry With Variable Adhesive Thickness
    (RAPID PROTOTYPING JOURNAL, 2024-01-22) Kemiklioglu, Ugur; Demir, Sermet; Yuksel, Caner
    PurposeAdhesively bonded joints are used in many fields, especially in the automotive, marine, aviation, defense and outdoor industries. Adhesive bonding offers advantages over traditional mechanical methods, including the ability to join diverse materials, even load distribution and efficient thermal-electrical insulation. This study aims to investigate the mechanical properties of adhesively bonded joints, focusing on adherends produced with auxetic and flat surfaces adhered with varying adhesive thicknesses.Design/methodology/approachThe research uses three-dimensional (3D)-printed materials, polyethylene terephthalate glycol and polylactic acid, and two adhesive types with ductile and brittle properties for single lap joints, analyzing their mechanical performance through tensile testing. The adhesion region of one of these adherends was formed with a flat surface and the other with an auxetic surface. Adhesively bonded joints were produced with 0.2, 0.3 and 0.4 mm bonding thickness.FindingsResults reveal that auxetic adherends exhibit higher strength compared to flat surfaces. Interestingly, the strength of ductile adhesives in auxetic bonded joints increases with adhesive thickness, while brittle adhesive strength decreases with thicker auxetic bonds. Moreover, the auxetic structure displays reduced elongation under comparable force.Originality/valueThe findings emphasize the intricate interplay between adhesive type, bonded surface configuration of adherend and bonding thickness, crucial for understanding the mechanical behavior of adhesively bonded joints in the context of 3D-printed materials.
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    Improving The Quality Of Micro Holes Drilled With A Current Detection Plasma Arc Device
    (JOURNAL OF THE BRAZILIAN SOCIETY OF MECHANICAL SCIENCES AND ENGINEERING, 2024-10) Ic, Yusuf Tansel; Kocum, Cengiz; Atalay, Kumru Didem; Serdaroglu, Dilek Cokeliler; Akar, Gurel; Polat, Isil Yanki; Samsun, Berk; Caliskan, Sevde; Atmaca, Dicle Naz; Karayalcin, Serkan
    Today, microholes play a crucial role in many sectors. Microholes are used in fields such as aerospace, computer systems, and electronic and mechanical industries. In this paper, we improve the performance of the current sensing plasma arc device to create holes within the same diameter, appropriate delamination, and surface quality. In addition, we aim to improve the processing quality of the current sensing plasma arc device, which drills holes from the micron level to the millimeter level and creates microstructures on hard, durable, and inert materials such as glass and quartz. Additionally, we presented a multiobjective optimization model to reach the optimal factor levels to obtain the minimum hole diameter with minimum delamination. For this objective, we propose a design of an experiment-integrated goal programming model in this study. The optimal levels are 90 Watt, 18 Hz, 2.73 ms, and 11.6 cm for the parameter values Power, Frequency, Lead time, and Distance between the probes, respectively, to reach the optimal diameter (183 mu m) and delamination values (1.025).
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    Multi-Objective Teaching-Learning-Based Optimizer for a Multi-Weeding Robot Task Assignment Problem
    (TSINGHUA SCIENCE AND TECHNOLOGY, 2024-10) Kang, Nianbo; Miao, Zhonghua; Pan, Quan-Ke; Li, Weimin); Tasgetiren, M. Fatih
    With the emergence of the artificial intelligence era, all kinds of robots are traditionally used in agricultural production. However, studies concerning the robot task assignment problem in the agriculture field, which is closely related to the cost and efficiency of a smart farm, are limited. Therefore, a Multi-Weeding Robot Task Assignment (MWRTA) problem is addressed in this paper to minimize the maximum completion time and residual herbicide. A mathematical model is set up, and a Multi-Objective Teaching-Learning-Based Optimization (MOTLBO) algorithm is presented to solve the problem. In the MOTLBO algorithm, a heuristic-based initialization comprising an improved Nawaz Enscore, and Ham (NEH) heuristic and maximum load-based heuristic is used to generate an initial population with a high level of quality and diversity. An effective teaching-learning-based optimization process is designed with a dynamic grouping mechanism and a redefined individual updating rule. A multi-neighborhood-based local search strategy is provided to balance the exploitation and exploration of the algorithm. Finally, a comprehensive experiment is conducted to compare the proposed algorithm with several state-of-the-art algorithms in the literature. Experimental results demonstrate the significant superiority of the proposed algorithm for solving the problem under consideration.
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    An Effective Optimization Method for Integrated Scheduling of Multiple Automated Guided Vehicle Problems
    (TSINGHUA SCIENCE AND TECHNOLOGY, 2024-10) Sang, Hongyan; Li, Zhongkai; Tasgetiren, M. Fatih
    Automated Guided Vehicle (AGV) scheduling problem is an emerging research topic in the recent literature. This paper studies an integrated scheduling problem comprising task assignment and path planning for AGVs. To reduce the transportation cost of AGVs, this work also proposes an optimization method consisting of the total running distance, total delay time, and machine loss cost of AGVs. A mathematical model is formulated for the problem at hand, along with an improved Discrete Invasive Weed Optimization algorithm (DIWO). In the proposed DIWO algorithm, an insertion-based local search operator is developed to improve the local search ability of the algorithm. A staggered time departure heuristic is also proposed to reduce the number of AGV collisions in path planning. Comprehensive experiments are conducted, and 100 instances from actual factories have proven the effectiveness of the optimization method.
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    The Cerium Oxide Nanoparticles Toxicity Induced Physiological, Histological And Biochemical Alterations In Freshwater Mussels, Unio Crassus
    (JOURNAL OF TRACE ELEMENTS IN MEDICINE AND BIOLOGY, 2024-02-13) Turkmen, Ezgi Uluer; Arslan, Pinar; Erkoc, Figen; Gunal, Aysel Caglan; Duran, Hatice
    Introduction: Releasing of cerium oxide nanoparticles (nano-CeO2) to the nature has increased due to the widespread use in many fields ranging from cosmetics to the food industry. Therefore, nano-CeO2 has been included in the Organization for Economic Co-operation and Development's (OECD) priority list for engineering nanomaterials. In this study, the effects of nano-CeO2 on the freshwater mussels were investigated to reveal the impact on the freshwater systems on model organism. Methods: First, the chemical and structural properties of nano-CeO2 were characterized in details. Second, the freshwater mussels were exposed to environmentally relevant concentrations of nano-CeO2 as 10 mg, 25 mg and 50 mg/L during 48-h and 7-d. Third, after the exposure periods, hemolymph and tissue samples were taken to analyse the Total Hemocyte Counts (THCs) histology and oxidative stress parameters (total antioxidant status, glutathione, glutathione-S-transferase, and advanced oxidative protein products). Results: Significant decrease of the THCs was observed in the nano-CeO2 exposed mussels compared to the control group (P < 0.05). The histological results showed a positive association between nano-CeO2 exposure concentration in the water and level of tissue damage and histopathological alterations were detected in the gill and the digestive gland tissues. Oxidative stress parameters were slightly affected after exposure to nano-CeO2 (P > 0.05). In conclusion, this study showed that acute exposure of freshwater mussels to nano-CeO2 did not pose significant biological risk. However, it has been proven that mussels are able to accumulate nano-CeO2 significantly in their bodies. Conclusion: This suggests that nano-CeO2 may be a potential risk to other organisms in the ecosystem through trophic transfer in the food-web based on their habitat and niche in the ecosystem.
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    Optimization Of The Redundancy Allocation Problem: Genetic Algorithm And Monte Carlo Simulation With Discrete Events
    (JOURNAL OF THE FACULTY OF ENGINEERING AND ARCHITECTURE OF GAZI UNIVERSITY, 2023-09-26) Sahin, Merve Uzuner; Dengiz, Orha; Dengiz, Berna
    The reliability optimization of a system with various problem-specific constraints is an important problem. The Redundancy Allocation Problem (RAP) is the design of new systems with higher reliability using redundant components in parallel arrangement. While improving the system's reliability, the cost is also on the upswing. It has been ensured that system designs with higher reliability at lower costs, where failure and repair are considered, can be obtained (Table A). The reliability of the system with nonrepairable components is lower than the reliability of the system with repairable components. Furthermore, the cost of the system with nonrepairable components is higher than the cost of the system with repairable components.Purpose:The aims of this study are modeling the dynamic behavior of a system considering increasing failure and repair rates, and finding the optimal repairable system design.Theory and Methods:This paper presents a Discrete Event Simulation (DES) model to estimate the system reliability considering increasing failure and repair rates, and a Genetic Algorithm (GA) to find the optimal repairable system design.Results:According to the results, system designs with higher reliability at lower costs, where failure and repair are considered, can be obtained. It has been found that systems with repairable components are more reliable and cheaper than systems with nonrepairable components. Conclusion:It is obtained that the optimal repairable system design with higher reliability at lower cost than the nonrepairable system design.