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Fiber takviyeli polimerik kompozitlerin ara yüzlerine elektrosprey kaplama yöntemiyle su bazlı karbon nanotüp ekleme
The utilization of fiber reinforced polymeric composites (FRPCs) has been broadening in recent years, especially in aerospace, automobile and marine industries, sports goods and many other high-performance applications, all of which demand enhanced thermal, electrical and mechanical properties. The ultimate performance of FRPCs can be enhanced by improving the fiber-matrix interface. Using nanophase reinforcements; tailoring fiber-matrix interface with carbon nanotubes (CNTs) or other carbon nanomaterials has shown significant improvements in properties of the composite. This thesis focuses on the deposition of CNTs onto carbon fabric (CF) surface by means of electrospray deposition and airbrush coating. Unlike the state-of-the-art methods to deposit carbon nanomaterials onto fiber surfaces, this study reports the deposition of CNTs from a waterborne dispersion, eliminates the use of organic volatile solvents and offers a method that is environmentally friendly and easily adaptable to large scale composite manufacturing processes. The hybrid CF-CNT structures prepared by surface deposition were used for the manufacturing of FPRCs by the vacuum infusion process (VIP) to assess the influence of CNTs on the stress transfer between the fiber-matrix interface. The surface morphology of the hybrid CNT-CF structures was characterized using scanning electron microscopy to verify homogeneous dispersion of CNTs on CF fabrics. CNTs deliberately placed at the fiber-matrix interface are expected to serve as stress transfer bridges between the fiber and the matrix and contribute to the enhancement of interlaminar shear strength and flexural properties. As by measured Mode I and Mode II interlaminar fracture testing experiment, CNT deposition on the CF surface strengthens the attachment of the laminate plies
Vanadyum oxi-fosfat malzemelerinin florinasyonu: Yüksekenerji Li iyon pili katod malzemeleri
The requirement for sustainable high energy density materials for next generations of Li-ion batteries is driving the research to develop new materials with enhanced properties. The thesis work was focused on fluorination of vanadium oxy-phosphate cathode materials with an aim of increasing their energy density. The synthesis, structural and chemical characteristics including electrochemical properties of the vanadium oxy-phosphates and their fluorinated counterparts were investigated. β-VPO4O, ε-LiVPO4O and β-LiVPO4O phases were synthesized by solid state synthesis method. The fluorination process was carried out using a special stainless steel reactor under argon atmosphere at high temperatures. Lithium fluoride (LiF) and Polytetrafluoroethylene (PTFE) compounds were used as the fluorine resources. The heat-treatment of the powder mixes of vanadium oxy-phosphate and F-containing compounds resulted in incorporation of F into the structure of materials. The ε-LiVPO4O phase preserved the main framework structure after the fluorination by LiF, but it changed to LiVPO4F-type framework by the use of PTFE as the F source. The β-VPO4O phase formed a LiVPO4F-type structure after the incorporation of LiF. All of the fluorinated materials had a Tavorite-type crystal structure, composed of VO6 octahedra interconnected through corners to PO4 tetrahedra. The operating potential of all the precursor vanadium oxy-phosphates increased after fluorination. This was due to the higher ionicity of the V-O/F ligands brought by the inductive effect of F in the structure. The synthesized materials were subjected to a systematic characterization at both micro- and nano-scale using various methods such as XRD, NMR, SEM, STEM, EDS and EELS, in addition to the electrochemical characterization.
Grafit ve grafen esaslı hibrit katkı maddelerinin silanizasyon yoluyla sentezi ve bu katkı maddelerinin Sığ Jeotermal sistemlerde kullanılan çimentolu harçların ısıl iletkenliği üzerindeki etkileri
The thermal conductivity of grout backfilling boreholes and pipes has been considered as an important issue for the improvement of the efficiency of shallow geothermal systems. Especially preserving the heat through the boreholes in the ground without temperature difference is achieved by formulating grout composition having high thermal conductivity. In this thesis, the main objective is to develop hybrid silica-carbon additives to enhance the thermal conductivity of the grout and thus increase the effectiveness of the heat transmission and prevent the aggregation of treated hybrid additives in grout mixture. Three main materials, graphene from waste tire, expanded graphite and rice husk ash, were hybridized by using silane coupling agents and building chemical bridges with silica particles in grout composites. This functionalization provided to enhance the dispersion and solubility of carbon materials and adjust their water uptake during grout mixing since there is a close relation between water demand and thermal conductivity. According to optimization study on the formulation development of grout mixtures, as the amount of graphene-based hybrid additive increased from 3 to 5 wt%, water uptake increased from 660 to 725 g resulting in the reduction of thermal conductivity by 20.6%. Furthermore, the highest thermal conductivity of 2.656 W/mK was achieved by adding 5 wt% expanded graphite-based hybrid additive compared to reference grout having thermal conductivity of 2.373 W/mK. Consequently, this study shows noticeable potential of hybrid additives produced from virgin and recycled sources to be used in the grouts of geothermal heat-exchange boreholes. This research will be more discernible since renewable energy sources come into prominence by ever-increasing energy-demand and global pollution.
Akdeniz kalamarından esinlenilmiş tekrar eden proteinlerın çok ölçekli modellenmesi
Squid ring teeth (SRT) proteins are structural proteins with repetitive amino acid sequences. They comprise two regions which are crystal forming and tie-chain regions. The mechanical properties of the protein known however the exact mechanism for the aggregation of the protein between the previously mentioned segments are still unknown. SRT proteins have unknown folding behavior and the size of those synthesized to date vary between 140 to 875 for a single chain. Considering these factors, we used Dissipative Particle Dynamics (DPD) simulations as our primary method of simulations rather than only using Molecular Dynamics (MD) simulations since MD simulations would be computationally expensive. So, in this study, we propose a method, which was previously used in polymers, of parameterizing the SRT proteins via multiscale simulations. To parameterize the system, we initially used binary MD simulations of each bead pair in the system at the atomistic detail. Then, we coarse-grained all the molecules into beads, and using the cohesive energy density values from the MD simulations, we constructed Flory-Huggins interaction parameters for all pairs in our system. We used four varying sizes of SRT proteins, n4, n7, n11, and n25 and two different solvents which were the good solvent HFIP and the hypothetical poor solvent P. Radial distribution function and structure factor calculations were used to characterize the structure of the SRT proteins in specified solvents. The results show that the SRT proteins swell in HFIP and they have no long-range order, but they cluster and form ordered structures in solvent P which show that the computational results agree with the experimental data.
Grafen polimerik gerinim ölçer dizininin CMOS uyumlu ölçeklenebilir mikrofabrikasyonu
Over the years, microelectromechanical systems (MEMS) have been utilized widely in sensing applications due to their characteristics such as small form-factor, ultra-high sensitivity, low-cost and scalability. Among the various sensing principles, piezoresistive effect has proved to be critical for strain sensing applications, owing to several advantages including compatibility with standard microelectronic fabrication techniques, ability for either monolithic or heterogeneous integration with readout circuitry which have rendered widespread use of piezoresistive sensors in various fields like structural and environmental monitoring. However, the sensitivity of strain gauges otherwise referred to as the gauge factor (GF) is limited to single digits (~ 2) for commercial metal-foil gauges on polymeric substrates. Single crystal silicon or polysilicon strain gauges achieve much higher GF values but at the expense of smaller ultimate strains and need for moderate to high levels of doping translating into additional process steps and higher device costs. On the other hand, graphene, a two-dimensional (2D) honeycomb structure of sp2 hybridized carbon atoms has vast potential for strain sensing applications due to its distinctive mechanical and electrical properties, provided that it can be integrated into standard semiconductor process flows. This thesis reports on the microfabrication of graphene strain gauges in arrayed format on flexible, polymeric structural layers where SU-8 was selected due its stable chemical and mechanical properties. Experimental characterization results show that, the fabricated graphene strain gauges achieve more than two orders of magnitude increase in GF values of up to 300, along with Raman results verifying successful integration of graphene layers into device format based on well-defined, scalable and IC-compatible processes.
Deneysel analoglar kullanılarak altıgen bal peteği yapıların düzlem içi tek eksenli basma modülünün belirlenmesi
Cellular solids have been utilized in many engineering applications for thermal insulation, their high specific out-of-plane compressive strengths and stiffnesses, their sieving capabilities, and in-plane energy absorption properties. With the advances in additive manufacturing, numerous novel 2D cellular solid designs have emerged. In-plane properties of 2D cellular solids have attracted attention for their intriguing behaviour under compressive, tensional and shear loads. As structures deviate from common geometries such as square, triangular, or hexagonal, analytical and numerical methods to predict effective elastic properties get dramatically more convoluted. Thus, analytical models in particular have been limited to the simpler designs. Moreover, validating and/or characterizing experimental analyses of novel geometries are often limited in scope due to size effects and inconsistent constraints among the test specimens and practical structures. This study presents a new approach that amalgamates virtual and real-life static analysis of cellular structures of repeating cells. Representative equivalent structures for testing, i.e. analogue test specimens are determined using parametric FEM analysis. Analogues for hexagonal honeycomb arrays are manufactured and tested under compression. Compressive moduli of the selected analogues exhibit great consistency between numerical and experimental analyses. The approach sets a framework for future research in using analogues for determination of in-plane properties of numerous other 2D cellular solid designs.
Li-ion pillerde uygulanan disprosyum katkılı Li7La3Zr2O12 katı elektrolitin iyonik iletkenliğinde artış
In this investigation, a novel Li-stuffed garnet type solid electrolyte with enhanced properties was fabricated. For this purpose, different concentrations of Dy ranging from 0.1 to o.8 atoms per formula unit (pfu) were doped into the Li7La3Zr2O12 to stabilize the cubic structure and, therefore, tailor the ionic conductivity. Furthermore, fundamental studies were performed through X-ray diffraction and Rietveld refinement to develop crystal structure of the Dy doped LLZO and determine the site preference of Dy. On another attempt in this study, Density Functional (DFT) total energy computations were applied to investigate the convergence of Dy at different Wyckoff sites energetically and to further validate the results of experiments. Additionally, 7Li and 6Li solid-state MAS NMR was performed to reveal the chemical coordination of Li at different sites. The results of this thesis project indicated that Dy ions probably substitute for Zr site and Li ions prefer tetrahedral (24d) and octahedral (48g) atomic sites. Additionally, our novel solid electrolyte demonstrated the highest ionic conductivity (2.03×10-3 S.cm-1) reported for LLZOs.
Reaktif ve reaktif olmayan partiküllerin reoloji kontrolü
Controlling the shear response of colloidal systems is critical in the ceramics industry as it affects the final properties of the product. In this thesis, reactive cementitious particles and nonreactive ceramic particles were studied and characterized by a colloidal science point of view. In the first part, the formulation of an extrudable, lightweight, high-strength cement-based material that utilizes industrial waste products was achieved. A random grafted copolymer that capitalizes electrostatic repulsion and steric hindrance was the key to control the flow of the particles. In the optimized mixture, high performance was maintained in terms of the compressive strength at 80 MPa and the dry hardened density at 1900 kg/cm3. According to the American Concrete Institute, this formulation is classified as high-strength (>55 MPa) and lightweight (320–1920 kg/m3). In the second part, the green machinability of yttria-stabilized zirconia was attained by employing a linear random copolymer. The copolymer was designed to coagulate the system by polymer-polymer bridging and facilitated an innovative ceramic gelcasting approach. The optimized system resulted in a shrinkage of 22.8%, a flexural strength of 149 MPa, and a density of 5.93 g/cm3. Compared to the values given in the literature; the shrinkage is 20–30%, the flexural strength is 150 MPa, and the theoretical density is 6.07 g/cm3.
Peridynamic modelling of internal features and interfaces for material toughening
Metals and ceramics are the two widely used materials in naval, aerospace, and structural engineering due to their high stiffness/weight ratio and design flexibility. However, their vulnerability to the occurrence of micro/macro cracks limits their potential and usage for the critical engineering applications. One way to improve the capabilities of metals and ceramics against crack occurrences is the implantation of the local weak zones into the material. Nevertheless, numerical analysis of such domains become quite challenging due to the simultaneous interactions of multiple interfaces, discontinuities, and phase changes. This study aims to systematically analyze the effects of different local weak zones on the behavior of the crack and the global toughness of homogeneous and graded materials. The realms of this thesis are assessed in two articles with an improved peridynamic formulation for precise modeling of interfaces. In the first paper, traditional peridynamic formulation is used to simulate the effects of the shape and locations of stop-holes on crack dynamics in homogeneous materials. Various combinations of stop-holes are analyzed under tensile and shear loadings while comparing their toughening effects. In the second paper, an improved formulation of peridynamic is proposed for graded composites by considering the interface and multiscale effects, through introducing the dominancy rate parameter. Overall, this study provides a unique contribution to the existing state of the art in terms of proposing a novel peridynamic methodology which can handle modelling of sharp transitions in material properties as well as suggesting numerically validated new toughening configurations for different materialsç
Buzfobik yüzeyler ve hidrodinamik kavitasyon için LbL tekniği ile kaygan sıvı doldurulmuş gözenekli yüzeylerin tasarımı ve mühendisliği
In this thesis a phenomenon that had been observed in nature and has been explained by fluid dynamics and surface engineering, was mimicked to study its properties and potential applications. The slippery liquid-infused porous surfaces (SLIPS) technology, which is inspired by pitcher plant, has been developed using Layer-by-Layer (LbL) assembly technique. The roughness of the surface was provided by deposition of a thin film of silica nanoparticles on a substrate and then the porosities of the surface was filled by a lubricant to have a non-stick, ultra-repellent, self-healing, icephobic and hydrophobic SLIPS. The charged silica nanoparticles with a diameter range of 40 to 80nm were synthesized using Stöber method and their size and surface charge were adjusted by controlling the TEOS/Ammonia ratio. The synthesized silica nanoparticles were deposited on the surface of the substrate using LbL assembly technique via dip coating and fluidic coating methods. The SEM, AFM, UV-Vis and ellipsometry results confirmed the deposition of a rough coating with root mean square roughness of 30 to 15nm, young modules of 5.3Gpa, 98% transparency in visible region and thickness of 100 to 200 nm. The icephobic porosities of the assembled thin films, which were filled by a lubricant were evaluated using a homemade ice adhesion strength measurement setup in an environmental chamber. The ice adhesion strength of the prepared SLIPS was measured as less than 5kPa. The cycling and aging tests, which were carried out on the SLIPS showed 35% reduction in the icephobicity of the SLIPS after 100 days and the ice adhesion strength of the coatings was about 5 times lower than untreated samples even after 50 icing deicing cycles. Surface topography and properties have an important influence on the generation of cavitating flow in microscale. For studying the effect of SLIPS and the surface roughness on the cavitating flow, the designed SLIPS structure was layer-by-layer assembled using fluidic method on the hydrodynamic cavitation microchips with various hydraulic diameters. The microfluidic devices were exposed to upstream pressures varying from 1 to 7.23 MPa and it has been observed that the inception of the cavitating flow and supercavitation condition have been occurred at much lower pressures in comparison with non-treated microfluidic devices. Introducing the cellulose nanofiber-stabilized perfluoropentane droplets to the SLIPS assembled micro channels, reduced the upstream pressure down to 1.7 MPa for generation of the supercavitation flow pattern within the device. The cellulose nanofibers were assessed by AFM after the cavitation process and it was observed that they were left undamaged during the cavitation process due to the lower upstream pressure, which in turn, increased the regeneration potential of the droplets for closed-loop applications.
Alüminosilicat malzemelerin activasyonu
Supplementary Cementitious Materials (SCMs) are the proper partial substitute for Ordinary Portland Cement (OPC) to reduce the production of carbon dioxide and fossil fuel consumption. Sufficient clay containing schist type materials, therefore, are promising substitutes. The schist-type raw powders contain clays, feldspars, carbonates, and some minor chemical phases. Recent studies described the effectiveness of pure calcined clay materials (kaolinite) as a proper partial replacement for cement. This study explores and illustrates the possibility of activating and using schist type materials as a more accessible source for partial replacement of cement. Furthermore, the positive effect of calcined calcium carbonates in the pozzolanic reactions of SCM substituted OPC was also illustrated in this study. Activated carbonates were effective in accelerating the pozzolanic reactions and forming additional Calcium Alumino Silicate Hydrate (CASH). Five schist minerals containing various types and amounts of clays were mineralogically augmented with additional calcite. The total carbonate amount was adjusted to 15, 22.5, and 30 wt% of the resource. Virgin and augmented resources were activated through a thermal or mechanical activation process. The proper activation method for schist type materials depends on their phase composition and the portion of clay and feldspar. The thermal activation includes a calcination process up to 80%, 100%, and 110% of the respective potentials by heat treatment and air quenching. This study claims that an efficient calcination process for specific schist can be designed by calculating the amount of energy that is needed for de-stabilizing the activatable aluminosilicate and carbonate phases. Phase content and decomposition behavior of the active components were investigated by Thermo Gravimetric Analysis (TGA), X-Ray Diffraction (XRD), and Scanning Electron Microscopy (SEM). Blended cement paste samples were prepared with 30 wt% replacement of OPC by activated schist SCMs. Cement paste samples from this blend were hydrated for 2, 7, 28, 50, and 90 days. The mechanical strength of the samples was measured at the end of the determined curing times by compression testing. Cement paste prepared with activated calcium augmented SCMs reached strength values that are better than 90% of the compressive strength of pure OPC.