Meltem Sezen

3D electron microscopy investigations of human dentin at the micro/nano-scale using focused ion beam based nanostructuring

In this study, high resolution electron microscopy techniques, such as Focused Ion Beam (FIB), Scanning Electron Microscopy (SEM) and High Resolution Transmission Electron Microscopy (HRTEM) revealed micro and nano features within human dentin with high definition and accuracy. The samples were prepared using FIB based advanced nanostructuring techniques in a dual-beam instrument. The related secondary electron (SE) image tomographs were acquired by means of stacking the images from FIB slice-series for monitoring micro-sized dentinal tubules, whereas FIB-structured pin-like samples were investigated at the TEM to observe the collagen fibrils at the nanoscale. The complimentary analysis helped to reveal the microstructure and morphology of human dentin in three dimensions in detail.

Increasing the stability of nanofluids with cavitating flows in micro orifices

One of the most critical challenges for nanofluids in practical applications is related to their stability and reusability since a gradual agglomeration of nanoparticles in nanofluids occurs with time and is accelerated by heating. In this study, we propose a technique to maintain the performance and stability of nanofluids with the use of cavitating flows through micro orifices to prevent agglomeration and sedimentation of nanoparticles, which will increase the durability of the nanofluids. γ-Al2O3 (gamma-alumina) nanoparticles with a mean diameter of 20 nm suspended in water were utilized. In the current approach, a flow restrictive element induces sudden pressure, which leads to cavitation bubbles downstream from the orifice. The emerging bubbles interact with the agglomerated structure of nanoparticles and decrease its size through hitting or shock waves generated by their collapse, thereby increasing the stability and reusability of nanofluids. The method does not involve any use of expensive surfact...

Production of Carbon Nanotubes over Fe-FSM-16 Catalytic Material: Effect of Acetylene Flow Rate and CVD Temperature

In this article, a high-yield synthesis of high-quality carbon nanotubes (CNTs) using Fe catalysts trapped within channels of Folded Sheet Mesoporous Materials, FSM-16, by chemical vapor deposition (CVD), using acetylene as a hydrocarbon source, is reported. The effect of reaction temperature and acetylene flow rate on the formation of CNTs was investigated. It was found that the yield, diameter and quality of CNTs synthesized strongly depend on reaction temperature during CVD. The resulting materials were characterized by scanning electron microscopy, Raman spectroscopy and thermogravimetric analysis. Our research found that carbon deposition, diameter and quality of the CNTs strongly depend on CVD temperature. However, the acetylene flow rate did not have any significant effect on diameter distribution. Raman measurement indicated that the synthesized products were multi-walled carbon nanotubes (MWCNTs). High-resolution transmission electron micrographs of samples reveal the multi-layer sidewalls of individual MWCNTs with a diameter of 40 nm, in which hollow and tubal structures were observed.

Hydrogen Chemical Configuration and Thermal Stability in Tungsten Disulfide Nanoparticles Exposed to Hydrogen Plasma

The chemical configuration and interaction mechanism of hydrogen adsorbed in inorganic nanoparticles of WS2 are investigated. Our recent approaches of using hydrogen activated by either microwave or radiofrequency plasma dramatically increased the efficiency of its adsorption on the nanoparticles surface. In the current work we make an emphasis on elucidation of the chemical configuration of the adsorbed hydrogen. This configuration is of primary importance as it affects its adsorption stability and possibility of release. To get insight on the chemical configuration, we combined the experimental analysis methods with theoretical modeling based on the density functional theory (DFT). Micro-Raman spectroscopy was used as a primary tool to elucidate chemical bonding of hydrogen and to distinguish between chemi- and physisorption. Hydrogen adsorbed in molecular form (H2) was clearly identified in all the plasma-hydrogenated WS2 nanoparticles samples. It was shown that the adsorbed hydrogen is generally stable under high vacuum conditions at room temperature, which implies its stability at the ambient atmosphere. A DFT model was developed to simulate the adsorption of hydrogen in the WS2 nanoparticles. This model considers various adsorption sites and identifies the preferential locations of the adsorbed hydrogen in several WS2 structures, demonstrating good concordance between theory and experiment and providing tools for optimizing of hydrogen exposure conditions and the type of substrate materials.

PLGA/Nano-ZnO Composite Particles for Use in Biomedical Applications

Copolymer poly DL-lactide-co-glycolide PLGA is extensively investigated for various biomedical applications such as controlled drug delivery or carriers in the tissue engineering. In addition, zinc oxide ZnO is widely used in biomedicine especially for materials like dental composites, as a constituent of creams for the treatment of a variety of skin irritations, to enhance the antibacterial activity of different medicaments and so on. Uniform, spherical ZnO nanoparticles nano-ZnO have been synthesized via microwave synthesis method. In addition to obtaining nano-ZnO, a further aim was to examine their immobilization in the PLGA polymer matrix PLGA/nano-ZnO and this was done by a simple physicochemical solvent/nonsolvent method. The samples were characterized by X-ray diffraction, scanning electron microscopy, laser diffraction particle size analyzer, differential thermal analysis, and thermal gravimetric analysis. The synthesized PLGA/nano-ZnO particles are spherical, uniform, and with diameters below 1 µm. The influence of the different solvents and the drying methods during the synthesis was investigated too. The biocompatibility of the samples is discussed in terms of in vitro toxicity on human hepatoma HepG2 cells by application of MTT assay and the antimicrobial activity was evaluated by broth microdilution method against different groups of microorganisms Gram-positive bacteria, Gram-negative bacteria, and yeast Candida albicans.

A STUDY OF MICROSTRUCTURE AND PHASE TRANSFORMATIONS OF MEDIUM CARBON DUAL PHASE STEELS

Microstructure development in medium-carbon dual-phase steels depends on various phase transformations depending on initial microstructure and cooling. Dual-phase steels may contain small amounts of retained austenite and bainite as well austenite and ferrite. In this study, intermediate quenching applied to medium-carbon hot rolled steel. The microstructures were characterized using field emission scanning electron microscopy and focused ion beam scanning electron microscopy. Microstructural investigations show that two different types of martensite morphology can be found in microstructure as martensite island and martensite fibers. During the annealing, the nucleation of austenite may occur along the lath boundaries, in martensite laths, blocks and packets as well as prior austenite grain boundary. It is concluded that martensite islands were nucleated at prior austenite grain boundary, while martensite fibers nucleated at martensite laths. Granular bainite was also observed at the boundaries between martensite islands. DICTRA software was applied to understand formation of granular bainite by means of elemental distribution.

Stress evolution of Ge nanocrystals in dielectric matrices

Germanium nanocrystals (Ge NCs) embedded in single and multilayer silicon oxide and silicon nitride matrices have been synthesized using plasma enhanced chemical vapor deposition followed by conventional furnace annealing or rapid thermal processing in N2 ambient. Compositions of the films were determined by Rutherford backscattering spectrometry and x-ray photoelectron spectroscopy. The formation of NCs under suitable process conditions was observed with high resolution transmission electron microscope micrographs and Raman spectroscopy. Stress measurements were done using Raman shifts of the Ge optical phonon line at 300.7 cm-1. The effect of the embedding matrix and annealing methods on Ge NC formation were investigated. In addition to Ge NCs in single layer samples, the stress on Ge NCs in multilayer samples was also analyzed. Multilayers of Ge NCs in a silicon nitride matrix separated by dielectric buffer layers to control the size and density of NCs were fabricated. Multilayers consisted of SiN y :Ge ultrathin films sandwiched between either SiO2 or Si3N4 by the proper choice of buffer material. We demonstrated that it is possible to tune the stress state of Ge NCs from compressive to tensile, a desirable property for optoelectronic applications. We also observed that there is a correlation between the stress and the crystallization threshold in which the compressive stress enhances the crystallization, while the tensile stress suppresses the process.

Micro and nanostructural analysis of a human tooth using correlated focused ion beam (FIB) and transmission Electron microscopy (TEM) investigations

In this study, natural molar human tooth specimens were investigated for determining their micro- and nanoscale structural morphology, chemistry and crystallinity. The differences were tracked comparatively for both enamel and dentin layers and at their interfaces. Although dental material structures are hard and tough and the cross-sectioning of these materials using mechanical methods is challenging, FIB-SEM dual-beam instruments serve for preparing ultra-thin homogenous lamella sections. In this work, both FIB-SEM and TEM based advanced characterization methods were applied to reveal different morphological characteristics of dental tissue via complementary imaging and diffraction analysis. In addition, SEM-EDS and Raman spectroscopy techniques provided additional information about the elemental distribution and the chemical composition differences of the dental tissues. According to electron microscopy examinations at the intersection between the enamel and the dentin layers, it was shown that the enamel was denser and polycrystalline, while the dentin layer was porous, fibrillar and of negligible long-range order, due to its tubular structure and organic components. In particular, EDS mapping and linescan analyses showed almost no differences in the elemental distribution. Raman results confirmed that both tissues had similar chemical composition except dentin showed spectral background effects in the spectrum due to its tubular structure and organic components.

Analysis of Deterioration Phenomena in a Koran by Nineteenth Century Ottoman Calligrapher Mehmed Şevki

Abstract A Koran written by Mehmed Şevki is the subject of this comparative analysis. This manuscript displayed an extensive degree of deterioration in 28 of the folios, while the rest of the manuscript is in considerably better condition. The aforementioned deteriorated sections of the book proved to be brittle and much darker in colour compared to the rest of this manuscript. The possible cause for this condition was ascertained using complementary techniques including the utilisation of micro-chemical (spot) tests, SEM-EDS, FTIR and Raman spectroscopy. The Micro-chemical Herzberg test indicated rag fibres in the paper used for the Koran, while the Raspail test #1 revealed rosin only in the sizing material of the deteriorated folios. SEM-EDS identified the presence of iron in all layers of the deteriorated folios but not in the stable folios. These results led subsequently to the conclusion that the deterioration of the folios was in fact related to the acid-catalysed hydrolysis due mainly to the presence of rosin and oxidation processes promoted by iron ions. This finding was also supported by FTIR analysis. The analyses indicated that different sizing materials had been used in the papermaking process of the two types of folios. In addition, Raman, EDS and HPLC techniques were also applied for the characterization of the inks and dyes. The results showed that inks and dyes were consistent throughout the entire text, thus suggesting calligrapher Mehmed Şevki and illuminator Hüseyin processed the book continually. It is plausible that calligrapher Mehmed Şevki changed his paper supply during the writing process which is why today we see a variety of differences in the condition of the papers.

Structural and chemical analysis of hydroxyapatite (HA)-Boron nitride (BN) nanocomposites sintered under different atmospheric conditions

Calcium phosphate derivatives have been widely employed in medical and dental applications for hard tissue repair, as they are the main inorganic constitution of hard tissue; such as bones and teeth. Owing to their excellent osteoconductive and bioactive properties, hydroxyapatite- (HA) based ceramics are the best candidates of this group for medical, bioscience, and dental applications. However, when replacing a bone or tooth, HA is not able to sustain similar mechanical properties. In this study, to improve the mechanical properties, nanoscale hexagonal boron nitride with different compositional percentages was added to the nano HA to form composites. The effect of compositional changes and sintering parameters on microstructural and morphological properties of the ceramic composites was comparatively investigated. Detailed chemical characterization of the composite materials was carried out using X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, and energy-dispersive X-ray spectroscopy, whereas scanning electron microscopy and atomic force microscopy investigations were employed to monitor morphological and surface features. Additional transmission electron microscopy investigations were carried out to reveal the nanostructure and crystal structure of the composites.