Papers by Muhammet S Toprak
Applied Surface Science, Nov 1, 2023
Applied Thermal Engineering, Apr 1, 2014
This study investigates the fabrication, thermal conductivity and rheological characteristics eva... more This study investigates the fabrication, thermal conductivity and rheological characteristics evaluation of nanofluids consisting of copper nanoparticles in diethylene glycol base liquid. The fabri ...
Open Chemistry, Dec 30, 2014
A novel synthesis route for the fabrication of p-type nanostructured skutterudite, FexCo1-xSb3 in... more A novel synthesis route for the fabrication of p-type nanostructured skutterudite, FexCo1-xSb3 in large quantity is reported. This scalable synthesis route provides nano-engineered material with less impact on the environment compared to conventional synthesis procedures. Several Fe substituted compositions have been synthesized to confirm the feasibility of the process. The process consists of a nano-sized precursor fabrication of iron and cobalt oxalate, and antimony oxides by chemical co-precipitation. Further thermochemical processes result in the formation of iron substituted skutterudites. The nanopowders are compacted by Spark Plasma Sintering (SPS) technique in order to maintain nanostructure. Detailed physicochemical as well as thermoelectric transport properties are evaluated. Results reveal strongly reduced thermal conductivity values compared to conventionally prepared counterparts, due to nanostructuring. P-type characteristic was observed from the Seebeck measurements while electrical conductivity is high and shows metallic behavior. The highest TE figure of merit of 0.25 at 800 K has been achieved, which is strongly enhanced with respect to the mother compound CoSb 3. This suggests the promise of the utilized method of fabrication and processing for TE applications with improved performance.
Journal of Cleaner Production, 2016
A nanoparticle-based cooling-lubricating fluid (nCLF), designed and fabricated by suspending engi... more A nanoparticle-based cooling-lubricating fluid (nCLF), designed and fabricated by suspending engineered nanoparticles (ENPs) in biodegradable vegetable-based fluids, has been developed for integration into material processing technologies. This new product exhibits tribological properties superior to those of conventional metalworking fluids. The major innovation is the ability to create a stable nCLF through the modification of ENP surfaces. Functionalized MoS2 nanotube ENPs were successful used as low friction additives. The experimental work, required for the proof-of-concept and technology validation, was carried out on three different levels to quantify the improved tribological behavior of nCLF. These experiments include standard tribological tests, mock-up tests to simulate machining, and actual machining tests. It is demonstrated that the specific properties of ENPs, fluid design and composition, as well as specific lubrication mechanisms, exhibit superior performance of nCLF in terms of friction and wear. The objective of this paper is to demonstrate how recent nanotechnology developments support innovation needed for transitioning to sustainable production via new product development and integrated industrial applications.
Stability of nanofluids is one of the critical assessments for the efficient systems that the sol... more Stability of nanofluids is one of the critical assessments for the efficient systems that the solid content of nanofluid should be stable and well dispersed for longer time. Production of a homogen ...
Meeting abstracts, 2016
Advanced substrate engineering is gaining new applications for nano-scaled devices where such sub... more Advanced substrate engineering is gaining new applications for nano-scaled devices where such substrates offer unique possibility to tailor the strain and the carrier transport. In general, XOI is a notation for wafers with a semiconductor-on-insulator where X is any alloy from SiGeSnC materials or III-V compounds. In special, strained XOI substrates (s-XOI) are widely used to manufacture nanowire transistors where the carrier mobility is enhanced by strain. So far, wafer bonding or condensation methods have been applied to manufacture XOI wafers. In case of SiGe-on-insulator (SGOI), a thin SiGe layer is grown on SOI and subsequently oxidized to create a single SiGe layer on oxide. During oxidation the Si atoms are preferably consumed and Ge atoms are driven toward the oxide box acting as a diffusion barrier. Therefore SGOI wafers suffer from point defects which have a strong influence on the carrier mobility, thermal conductivity and even optical properties. Post annealing may improve the layer quality and majority of the defects could be disappeared. The wafer bonding is more complicated and sensitive method. This relates to the requirements e.g. surface flatness, layer quality of the semiconductor and the amount of exerting bonding force. In manufacturing of Ge-on-insulator wafers, a high quality Ge layer is grown at 700-750 °C on a low quality Ge (grown at 300-400 °C). The Ge layers are transferred and bonded to oxide wafers by using a uniform force of few kNs. Afterwards, the wafers are annealed at 500-600 °C before etch back step (see Fig.1). It is important to mention here that the annealing step has to be decreased for bonding of GeSn-on-insulator wafers in order to avoid strain relaxation. In this work, manufacturing methods for different sizes of XOI and s-XOI wafers are presented and an attention is paid for using the manufactured XOI wafers for the thermoelectric application, where lateral nanowires are formed. As an example, for SiGe nanowires the seebeck coefficient is enhanced in presence of defects, however, any kind of defects in XOI wafers degrade the channel mobility in the processed MOSFETs. Characterization techniques e.g. high-resolution reciprocal maps using synchrotron x-ray beam, high resolution transmission electron microscopy, and photoluminescence were applied to detect the defects and estimate the strain relaxation. The experimental data is linked to explain the electrical properties of XOI wafers and finally to nanowires and MOSFETs. Figure 1
MRS Proceedings, 2001
The synthesis and characterisation of gold-coated cobalt nanoparticles, as well as their chemical... more The synthesis and characterisation of gold-coated cobalt nanoparticles, as well as their chemically-and magnetically-induced self-organisation have been studied. Metallic core-shell nanoparticles were prepared using two different experimental techniques: bulk reductive precipitation, with average particles size ~15 nm, and microemulsion confining method, with average particle size of ~6 nm. The self-assembly of prepared nanoparticles on flat substrates was achieved by derivatising the substrate and particle surfaces with bifunctional organic molecules that attaches to both particles and substrates. Examination of the self-assembled systems was carried out by a number of characterisation techniques including transmission electron microscopy (TEM), UV-visible spectrophotometry (UV-VIS), and atomic force microscopy (AFM). EXPERIMENTAL Reagents All the salts used were purchased from Aldrich with ACS grade. As solvents c-Hexane (cHex, +99.5%), n-decane (C 10 , 97%), and n-octane (C 8 , 97%) were used as obtained without any further purification. Triton-X 100 (TX100), n-Hexanol (HxOH), and n-Butanol (BuOH) were purchased from Fluka and were used as obtained. 3-aminopropyl trimethoxy silane (APTMS) and 3-mercaptopropyl trimethoxy silane (MPTMS) were purchased from Aldrich. High purity water was used with a specific resistivity of 18 MΩ•cm (Milli-Q, Millipore Inc.).
Environmental Science: Nano
An effective, eco-friendly and easily scalable nanosilver-based technology offers affordable and ... more An effective, eco-friendly and easily scalable nanosilver-based technology offers affordable and broad-spectrum antimicrobial solutions against SARS-CoV-2 and Escherichia coli.
Materials Chemistry and Physics
Journal of Materials Research and Technology, 2021
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Scientific Reports, 2021
Bioconversion of organic materials is the foundation of many applications in chemical engineering... more Bioconversion of organic materials is the foundation of many applications in chemical engineering, microbiology and biochemistry. Herein, we introduce a new methodology to quantitatively determine conversion of biomass in viral infections while simultaneously imaging morphological changes of the host cell. As proof of concept, the viral replication of an unidentified giant DNA virus and the cellular response of an amoebal host are studied using soft X-ray microscopy, titration dilution measurements and thermal gravimetric analysis. We find that virions produced inside the cell are visible from 18 h post infection and their numbers increase gradually to a burst size of 280–660 virions. Due to the large size of the virion and its strong X-ray absorption contrast, we estimate that the burst size corresponds to a conversion of 6–12% of carbonaceous biomass from amoebal host to virus. The occurrence of virion production correlates with the appearance of a possible viral factory and morph...
Microscopy and Microanalysis, 2018
Frontiers in Microbiology, 2019
Background: Colistin is a polypeptide antibiotic drug that targets lipopolysaccharides in the out... more Background: Colistin is a polypeptide antibiotic drug that targets lipopolysaccharides in the outer membrane of Gram-negative bacteria. Inactivation of the mgrB-gene is a common mechanism behind colistin-resistance in Klebsiella pneumoniae (Kpn). Since colistin is a cyclic polypeptide, it may exhibit cross-resistance with the antimicrobial peptide LL-37, and with other innate effector mechanisms, but previous results are inconclusive. Objective: To study potential cross-resistance between colistin and LL-37, as well as with other innate effector mechanisms, and to compare virulence of colistin-resistant and susceptible Kpn strains. Materials/Methods: Carbapenemase-producing Kpn from Oman (n = 17) were subjected to antimicrobial susceptibility testing and whole genome sequencing. Susceptibility to colistin and LL-37 was studied. The surface charge was determined by zeta-potential measurements and the morphology of treated bacteria was analyzed with electron microscopy. Bacterial survival was assessed in human whole blood and serum, as well as in a zebrafish infection-model. Results: Genome-analysis revealed insertion-sequences in the mgrB gene, as a cause of colistin resistance in 8/17 isolates. Colistin-resistant (Col-R) isolates were found to be more resistant to LL-37 compared to colistin-susceptible (Col-S) isolates, but only at concentrations ≥50 µg/ml. There was no significant difference in surface charge between the isolates. The morphological changes were similar in both Col-R and Col-S
ECS Journal of Solid State Science and Technology, 2017
In this work, a novel CMOS compatible process for Si-based materials has been presented to form S... more In this work, a novel CMOS compatible process for Si-based materials has been presented to form SiGe nanowires (NWs) on SiGe On Insulator (SGOI) wafers with unprecedented thermoelectric (TE) power factor (PF). The TE properties of SiGe NWs were characterized in a back-gate configuration and a physical model was applied to explain the experimental data. The carrier transport in NWs was modified by biasing voltage to the gate at different temperatures. The PF of SiGe NWs was enhanced by a factor of >2 in comparison with bulk SiGe over the temperature range of 273 K to 450 K. This enhancement is mainly attributed to the energy filtering of carriers in SiGe NWs, which were introduced by imperfections and defects created during condensation process to form SiGe layer or in NWs during the processing of NWs.
Journal of Materials Science: Materials in Electronics, 2019
This article presents the impact of epitaxial quality, contact resistance and profile of Ge PIN p... more This article presents the impact of epitaxial quality, contact resistance and profile of Ge PIN photodetectors (PDs) on dark current and responsivity. The PD structures were processed with either selectively grown Ge with integrated waveguides on SOI wafer or globally grown Ge on the entire wafer. The contact resistance was lowered by introducing NiGe layer prior to the metallization. The n-type doped Ge PIN structure was formed by ion implantation and the contact resistivity was estimated to 2.6 × 10 −4 Ω cm 2. This value is rather high and it is believed to be due to fomation of defects during implantation. The results show a minor difference in dark currents for selectively and globally grown PDs but in both types, it depends on detector area and the epitaxial quality of Ge. For example, the threading dislocation density (TDD) in non-selectively grown PDs with thickness of 1 µm was estimated to be 10 6 cm −2 yielding relatively low dark currents while it dramatically changes for PDs with thinner Ge layers where TDD increases to 10 8 cm −2 and the dark current levels increase almost by 1.5 magnitude. Surprisingly, for selectively grown PDs with Ge thickness of 500 nm, TDD was still low resulting in low dark currents. The dark current densities at − 1 V bias of non-selectively and selectively grown PDs with optimized profile were measured to be 5 mA/cm 2 and 47 mA/cm 2 , respectively, while the responsivity of these detectors were 0.17 A/W and 0.46 A/W at λ ~ 1.55 µm, respectively. Excellent performance for selectively grown PD shows an appropriate choice for detection of 1.55 µm wavelength.
Journal of Nanoscience and Nanotechnology, 2017
Higher manganese silicides (HMS), represented by MnSi x x = 1.71-1.75), are promising p-type cand... more Higher manganese silicides (HMS), represented by MnSi x x = 1.71-1.75), are promising p-type candidates for thermoelectric (TE) energy harvesting systems at intermediate temperature range. The materials are very attractive as they may replace lead based compounds due to their nontoxicity, low cost of starting materials, and high thermal and chemical stability. Dense pellets were obtained through fast reactive sintering by spark plasma sintering (SPS). The addition-or nanoinclusion, of Al and Mg permitted the figure of merit enhancement of the material obtained with this technique, reaching the highest value of 0.40 at 600 C. Morphology, composition and crystal structure of the samples were characterized by electron microscopies, energy dispersive X-ray spectroscopy, and X-ray diffraction analyses, respectively.
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Papers by Muhammet S Toprak