LEE- Fizik Mühendisliği Lisansüstü Programı

Bu topluluk için Kalıcı Uri

http://hdl.handle.net/11527/19275

Gözat

Yazar "Aktaş Kaya, Demet" ile LEE- Fizik Mühendisliği Lisansüstü Programı'a göz atma
  • Öge
    Structural and upconversion luminescence properties of polyethyl methacrylate (PEMA) polymers doped with rare earths ions
    (Graduate School, 2024-04-18) Bahuar, Thami ; Aktaş Kaya, Demet ; 509122112 ; Physics Engineering
    This thesis aims to analyze the spectral characteristics of nanocrystal powders containing [CdNb2O6:Er3+] and [CdNb2O6: Er3+/Yb3+] by using polyethyl methacrylate (PEMA) crosslinked networks as the host matrix. The first study involved the incorporation of CdNb2O6:Er2O3(%1,5 Er3+) powders doped with Er3+ into both bulk linear polyethyl methacrylate (PEMA) and bulk PEMA crosslinked networks (gels) with varying amounts of crosslinker. The polymerization process was achieved through free-radical crosslinking copolymerization using 0.1 EMA (weight %) at a temperature of 60 °C. The X-ray diffraction (XRD) method was used to analyze the structures of Er3+ ions contained in bulk linear PEMA and PEMA gels. The crystalline grain size was determined using the Pielazsek grain distribution and the Scherrer equation techniques. By increasing the crosslinker amounts, the average-crystalline grain sizes reduced from 75 nm to 12.50 nm for CdNb2O6:Er2O3 (%1,5 Er3+) crystalines. Thus, it was shown that the distribution grew more evenly distributed. The Fourier transform infrared (FT-IR) spectra were used to monitor the functional groups present in both linear PEMA and PEMA gel samples. The obtained results were found to be in agreement with the XRD results for the two cases. The second part of this thesis included the synthesis of linear polyethyl methacrylate (PEMA) and crosslinked PEMA gels, which were doped with CdNb2O6: Er3+/Yb3+ nanocrystal powders. The synthesis was achieved using the process of free radical crosslinking copolymerization. The X-ray diffraction method was used to analyze the architectures of Er3+/Yb3+ ions contained in PEMA polymers. The average sizes of the crystalline particles were determined by using the Pielazsek particle distribution and the Scherrer equation. The particle sizes exhibited a reduction from 60 nm to 17 nm for CdNb2O6: Er3+/Yb3+ powders upon their incorporation into PEMA polymers. The morphological changes of polymer samples doped with CdNb2O6: Er3+/Yb3+ powders were observed using transmission electron microscopy and scanning electron microscopy. Luminescence spectra were measured at room temperature to explore the optical characteristics of polymer materials. The Er3+/Yb3+ ions, which was excited with a 975 nm diode laser, produced upconversion (UC) emissions using two-photon absorptions in the visible range. The intensities of UC (upconversion) and the quantities of absorbed photons were enhanced by increasing the quantity of crosslinker. The study examined how changes in the structure and morphology of the host polymer material affected color tuning, color coordinates, and color quality.
  • Öge
    Structural, optical and mechanical properties of polyacrylamide hydrogels doped with multiwalled carbon nanotubes
    (Graduate School, 2022) Öztürk, Mert Can ; Aktaş Kaya, Demet ; 725977 ; Physics Engineering Programme
    Hydrogels are two or multi-components systems made up of a three-dimensional network of polymer chains and water fillings the gap between macromolecules. Depending on the characteristics of the polymers utilised, and the type and density of the network joints, such structures in equilibrium can comprise varying quantities of water; in the swollen state, the weight percentage of water in a hydrogel is generally considerably larger than the weight percentage of polymer. On the other hand, carbon nanotubes (CNT) are one of the most intriguing novel materials in the last three decades. CNTs are hollow carbon tubes in nanometer sizes. They create a new form of carbon equivalent in configuration to a graphite sheet wrapped in a nanometer-sized hollow tube. It can be synthesized in sizes ranging from a few microns to several nanometers and in the thickness of many carbon layers from single-walled (SWCNT) to multi-walled (MWCNT) structures. The unique structure of these nanotubes gives them properties such as electrical and thermal conductivity, strength, hardness and toughness. CNT's mechanical properties have been extensively researched, both theoretically and experimentally. Measurements were taken using a 'nanostressing stage' inside a scanning electron microscope (SEM) indicating that the Young's modulus of MWCNT's outermost layers ranged from 270 to 950 GPa. Single nanotube tensile strengths were also determined, with values ranging from 11 to 63 GPa. CNT dispersion is critical for manufacturing high-quality nanocomposites because nanotube dispersion has such a large impact on suspension quality. CNTs readily agglomerate, bundle, and entangle, resulting in a plethora of defect sites in composites and a reduction in CNT efficiency. When all accessible CNTs are evenly dispersed in the host matrix, a good distribution is achieved. Mechanical parameters that fall short of theoretical expectations are mostly due to poor distribution and limited interfacial load transmission among CNTs and the polymer matrices. The mechanical characteristics of nanocomposites are influenced by interfaces in particular. Mechanical interlocking, chemical bonding, and the van der Waals force are three interfacial load transfer techniques that have been identified. Inadequate load transference across CNT and polymer chains may result in interfacial slippage and low mechanical strength and stiffness unless the interface is carefully built. When there is no interfacial load transmission, nonlinear stress–strain behavior is expected. CNTs have been widely explored as a novel nanomaterial because of their remarkable electrical conductivities and mechanical qualities. The inclusion of CNTs in the hydrogel matrix could result in a resilient and electrically conductive hydrogel due to their nano-reinforcement and intrinsic conductivity. The preparation procedure, however, is extremely complex due to the CNTs' inherent hydrophobic nature, restricting their application and even reducing the hydrogel's tensile strength. Studies continue on hydrophobic linkages and π-π interactions to better disperse CNTs in the hydrogel matrix. For example, adding oxidized multi-walled carbon nanotubes (oxCNTs) to the PAAm hydrogel results in a hydrogel with high sensitivity, self-recovery, mechanical strength, and stretchability. Gelatin was employed to functionalize the oxCNTs via hydrogen bonding among carboxyl groups on the oxCNTs and hydroxyl, carboxyl groups in the gelatin chain to minimize oxCNT aggregation in the hydrogel mesh. The PAAm-oxCNTs hydrogel was then created by polymerizing AAm free-radically in the presence of an initiator and a chemical crosslinker. The hydrogel's backbone was chemically cross-linked PAAm, and the oxCNTs successfully increased the mechanical characteristics via the nano-enhancement effect. Furthermore, the hydrogel's mechanical characteristics were improved by physical interplays amongst oxCNTs, gelatin, and PAAm. It has increased stretchability and tensile strength and exhibits rapid self-recovery behavior. Furthermore, the hydrogel's mechanical characteristics were improved by physical interactions between oxCNTs, gelatin, and PAAm. The oxCNTs in the hydrogel also contributed to the strain sensing activity by generating conducting routes in the hydrogel. Unlike the literature, in this thesis study, homogeneous distribution of CNTs in pure PAAm hydrogels was observed for the first time. These results are very important in terms of literature for the development of new high quality, conductive and mechanically durable composite systems. The universality class, which shares critical exponents and other universal properties, encompasses a wide range of systems and models with continuous phase transitions in composite systems. Monomers are thought to locate to the corners of a periodic lattice in percolation theory. The ends connecting these corners at any instant, namely chemical bonds, are given with the probability p. The gel point is defined by the percolation threshold p_c at the thermodynamic limit where the infinite set begins to form. Around the percolation threshold, many percolation properties obey the scaling law regardless of network structure and microscopic details. Thus, the scaling laws for the gel fraction, G(p) and mean cluster size, S(p) around the threshold value are obtained. G(p)≈(p-p_c )^β , p>p_c S(p)≈(p_c-p)^(-γ) , p