Browsing by Author "Nikitin, Timur"

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Open Access
Crystallization Kinetics: Relationship Between Crystal Morphology and the Cooling Rate-Applications for Different Geological Materials
(MDPI, 2023-01-24) Aysal, Namık; Kurt, Yiğit; ILDIZ, GÜLCE ÖĞRÜÇ; Öztürk, Hüseyin; Yeşiltaş, Mehmet; Laçin, Davut; Öngen, Sinan; Nikitin, Timur; FAUSTO, RUI
Crystal morphology is controlled by several physicochemical parameters such as the temperature, pressure, cooling rate, nucleation, diffusion, volatile composition, and viscosity. The development of different crystal morphologies is observed as a function of the cooling rate in many different rock types (i.e., glassy volcanic rocks, and archeometallurgical slags). Crystallization is a two-stage kinetic process that begins with the formation of a nucleus and then continues with the accumulation of ions on it. The shapes of the crystals depend on the degree of undercooling (& UDelta;T), and euhedral crystals, having characteristic forms that reflect their crystallographic internal structure, that grow just below their liquidus temperature. In this study, crystal morphologies in different minerals (e.g., quartz, sanidine, olivine, pyroxene, magnetite, etc.) that had developed in silicic volcanic rocks (spherulites) and slags from ancient mining were investigated and characterized using optical microscopy, X-ray diffraction, and Fourier-transform infrared (FTIR), Raman, and scanning electron microscope-energy dispersive X-ray fluorescence (SEM-EDX) spectroscopic techniques. Depending on the increase in the cooling rate, quartz, feldspar, olivine, pyroxene, and magnetite minerals were found to crystallize in subhedral, skeletal, dendritic, spherical, bow-tie and fibrous forms in glassy volcanic rocks and archeometallurgical slags.
Open Access
Graphene-Assisted Chemical Stabilization of Liquid Metal Nano Droplets for Liquid Metal Based Energy Storage
(Wiley, 2024) Sanati, Afsaneh L.; Nikitin, Timur; FAUSTO, RUI; Majidi, Carmel; Tavakoli, Mahmoud
Energy storage devices with liquid-metal electrodes have attracted interest in recent years due to their potential for mechanical resilience, self-healing, dendrite-free operation, and fast reaction kinetics. Gallium alloys like Eutectic Gallium Indium (EGaIn) are appealing due to their low melting point and high theoretical specific capacity. However, EGaIn electrodes are unstable in highly alkaline electrolytes due to Gallium oxide dissolution. In this letter, this bottleneck is addressed by introducing chemically stable films in which nanoscale droplets of EGaIn are coated with trace amounts of graphene oxide (GO). It is demonstrated that a GO to EGaIn weight ratio as low as 0.01 provides enough protection for a thin film formed by GO@EGaIn nanocomposite against significantly acidic or alkaline environments (pH 1-14). It is shown that GO coating significantly enhances the surface stability in such environments, thus improving the energy storage capacity by over 10x. Microstructural analysis confirms GO@EGaIn composite stability and enhanced electrochemical performance. Utilizing this, a thin-film supercapacitor is fabricated. Results indicate that when coating the EGaIn with GO to EGaIn ratio of 0.001, the areal capacitance improves by 10 times, reaching 20.02 mF cm(-2). This breakthrough paves the way for advanced liquid metal-based thin-film electrodes, promising significant improvements in energy storage applications.
Open Access
Micro-Raman Spectroscopy and X-ray Diffraction Analyses of the Core and Shell Compartments of an Iron-Rich Fulgurite
(MDPI, 2022) KARADAĞ, AHMET; Kaygısız, Ersin; Nikitin, Timur; Öngen, Sinan; ILDIZ, GÜLCE ÖĞRÜÇ; Aysal, Namık; Yılmaz, Ayberk; Fausto, Rui
Fulgurites are naturally occurring structures that are formed when lightning discharges reach the ground. In this investigation, the mineralogical compositions of core and shell compartments of a rare, iron-rich fulgurite from the Mongolian Gobi Desert were investigated by X-ray diffraction and micro-Raman spectroscopy. The interpretation of the Raman data was helped by chemometric analysis, using both multivariate curve resolution (MCR) and principal component analysis (PCA), which allowed for the fast identification of the minerals present in each region of the fulgurite. In the core of the fulgurite, quartz, microcline, albite, hematite, and barite were first identified based on the Raman spectroscopy and chemometrics analyses. In contrast, in the shell compartment of the fulgurite, the detected minerals were quartz, a mixture of the K-feldspars orthoclase and microcline, albite, hematite, and goethite. The Raman spectroscopy results were confirmed by X-ray diffraction analysis of powdered samples of the two fulgurite regions, and are consistent with infrared spectroscopy data, being also in agreement with the petrographic analysis of the fulgurite, including scanning electron microscopy with backscattering electrons (SEM-BSE) and scanning electron microscopy with energy dispersive X-ray (SEM-EDX) data. The observed differences in the mineralogical composition of the core and shell regions of the studied fulgurite can be explained by taking into account the effects of both the diffusion of the melted material to the periphery of the fulgurite following the lightning and the faster cooling at the external shell region, together with the differential properties of the various minerals. The heavier materials diffused slower, leading to the concentration in the core of the fulgurite of the iron and barium containing minerals, hematite, and barite. They first underwent subsequent partial transformation into goethite due to meteoric water within the shell of the fulgurite. The faster cooling of the shell region kinetically trapped orthoclase, while the slower cooling in the core area allowed for the extensive formation of microcline, a lower temperature polymorph of orthoclase, thus justifying the prevalence of microcline in the core and a mixture of the two polymorphs in the shell. The total amount of the K-feldspars decreases only slightly in the shell, while quartz and albite appeared in somewhat larger amounts in this compartment of the fulgurite. On the other hand, at the surface of the fulgurite, barite could not be stabilized due to sulfate lost (in the form of SO2 plus O-2 gaseous products). The conjugation of the performed Raman spectroscopy experiments with the chemometrics analysis (PCA and, in particular, MCR analyses) was shown to allow for the fast identification of the minerals present in the two compartments (shell and core) of the sample. This way, the XRD experiments could be done while knowing in advance the minerals that were present in the samples, strongly facilitating the data analysis, which for compositionally complex samples, such as that studied in the present investigation, would have been very much challenging, if possible.
Open Access
Nanoscale Study of the Polar and Electronic Properties of a Molecular Erbium(III) Complex Observed via Scanning Probe Microscopy
(MDPI, 2023) Ivanov, Maxim; Grempka, Arkadiusz; Buryakov, Arseniy; Nikitin, Timur; Justino, Licinia L. G.; FAUSTO, RUI; Vilarinho, Paula M.; Paixao, Jose A.
We successfully synthesized millimeter-sized single crystals of the molecular erbium(III) complex Er(acac)(3)(cphen), where acac = acetylacetonate and cphen = 5-chloro-1,10-phenanthroline. The novelty of this work stems from the exhaustive examination of the polar and electronic properties of the obtained samples at the macro-, micro-, and nanoscale levels. The single crystal X-ray diffraction method demonstrates the monoclinic (noncentrosymmetric space group P2(1)) crystallographic structure of the synthesized samples and scanning electron microscopy exhibits the terrace-ledge morphology of the surface in erbium(III) crystals. By using the piezoelectric force microscopy mode, the origin of the polar properties and the hyperpolarizability in the synthesized samples were assigned to the internal domain structure framed by the characteristic terrace-ledge topography. The direct piezoelectric coefficient (similar to d33) was found to be intensely dependent on the local area and was measured in the range of 4-8 pm/V. A nanoscale study using the kelvin probe force and capacitance force (dC/dz) microscopy modes exposed the effect of the Er ions clustering in the erbium(III) complex. The PFM method applied solely to the Er ion revealed the corresponding direct piezoelectric coefficient (similar to d33) of about 4 pm/V. Given the maximum piezoelectric coefficient in the erbium(III) complex at 8 pm/V, we highlight the significant importance of the spatial coordination between the lanthanide ion and the ligands. The polar coordination between the lanthanide ion and the nitrogen and oxygen atoms was also corroborated by Raman spectroscopy supported by the density functional theory calculations. The obtained results can be of paramount importance for the application of molecular erbium(III) complex crystals in low-magnitude magnetic or electric field devices, which would reduce the energy consumption and speed up the processing switching in nonvolatile memory devices.
Open Access
Solid-Liquid Phase Equilibrium of the n-Nonane plus n-Undecane System for Low-Temperature Thermal Energy Storage
(Springer/Plenum Publishers, 2024) Sequeira, Maria C. M.; Nikitin, Timur; Caetano, Fernando J. P.; Diogo, Herminio P.; Fareleira, Joao M. N. A.; FAUSTO, RUI
The current article presents an exploration of the solid-liquid phase diagram for a binary system comprising n-alkanes with an odd number of carbon atoms, specifically n-nonane (n-C-9) and n-undecane (n-C-11). This binary system exhibits promising characteristics for application as a phase change material (PCM) in low-temperature thermal energy storage (TES), due to the fusion temperatures of the individual components, thereby motivating an in-depth investigation of the solid-liquid phase diagram of their mixtures. The n-nonane (n-C-9) + n-undecane (n-C-11) solid-liquid phase equilibrium study herein reported includes the construction of the phase diagram using Differential Scanning Calorimetry (DSC) data, complemented with Hot-Stage Microscopy (HSM) and low-temperature Raman Spectroscopy results. From the DSC analysis, both the temperature and the enthalpy of solid-solid and solid-liquid transitions were obtained. The binary system n-C-9 + n-C-11 has evidenced a congruent melting solid solution at low temperatures. In particular, the blend with a molar composition x(undecane) = 0.10 shows to be a congruent melting solid solution with a melting point at 215.84 K and an enthalpy of fusion of 13.6 kJmol(-1). For this reason, this system has confirmed the initial signs to be a candidate with good potential to be applied as a PCM in low-temperature TES applications. This work aims not only to contribute to gather information on the solid-liquid phase equilibrium on the system n-C-9 + n-C-11, which presently are not available in the literature, but especially to obtain essential and practical information on the possibility to use this system as PCM at low temperatures. The solid-liquid phase diagram of the system n-C-9 + n-C-11 is being published for the first time, as far as the authors are aware.