Thermal Analysis Research Articles

Chalcopyrite CuFeS2: Solid-State Synthesis and Thermoelectric Properties

The optimal conditions for synthesizing a pure chalcopyrite CuFeS2 phase were thoroughly investigated through the combination of mechanical alloying (MA) and hot pressing (HP) processes. The MA process was performed at a rotational speed of 350 rpm for durations ranging from 6 to 24 h under an Ar atmosphere, ensuring proper mixing and alloying of the starting materials. Afterward, MA-synthesized chalcopyrite powder was subjected to HP at temperatures between 723 K and 823 K under a pressure of 70 MPa for 2 h in a vacuum. This approach aimed to achieve phase consolidation and densification. A thermal analysis via differential scanning calorimetry (DSC) revealed distinct endothermic peaks at the range of 740–749 K and 1169–1170 K, corresponding to the synthesis of the chalcopyrite phase and its melting point, respectively. An X-ray diffraction (XRD) analysis confirmed the successful synthesis of the tetragonal chalcopyrite phase across all samples. However, a minor secondary phase, identified as Cu1.1Fe1.1S2 (talnakhite), was observed in the sample hot-pressed at the highest temperature of 823 K. This secondary phase could result from slight compositional deviations or local phase transformations at elevated temperatures. The thermoelectric properties of the CuFeS2 samples were evaluated as a function of the HP temperatures. As the HP temperature increased, the electrical conductivity exhibited a corresponding rise, likely due to enhanced densification and reduced grain boundary resistance. However, this increase in electrical conductivity was accompanied by a decrease in both the Seebeck coefficient and thermal conductivity. The reduction in the Seebeck coefficient could be attributed to the higher carrier concentration resulting from improved electrical conductivity, while the decrease in thermal conductivity was likely due to reduced phonon scattering facilitated by the grain boundaries. Among the samples, the one that was hot-pressed at 773 K displayed the most favorable thermoelectric performance. It achieved the highest power factor of 0.81 mWm−1K−1 at 523 K, indicating a good balance between the Seebeck coefficient and electrical conductivity. Additionally, this sample achieved a maximum figure-of-merit (ZT) of 0.32 at 723 K, a notable value for chalcopyrite-based thermoelectric materials, indicating its potential for mid-range temperature applications.

Extending interfacial toughness and thermal cycling lifetime of thermal barrier coatings via interfacial 3D pattern

AbstractWeak interface bonding is a critical factor that compromises the long‐term durability of the thermal barrier coatings. To mitigate this, we introduce an innovative interface‐strengthening approach by integrating four types of 3D patterns at the top coat/bond coat interface using the laser cladding technique. Tensile tests demonstrate a significant enhancement in interfacial bonding strength, increasing by over 139.2% from 21.6 ± 1.7 MPa for conventional thermal barrier coatings without patterns to 51.67 MPa for the patterned coatings. Moreover, the thermal cycling lifetime has been extended by 41%, from 1026 ± 25 cycles for the conventional coating to 1936 ± 164 cycles for the patterned coating. This enhancement in interfacial toughness and thermal cycling lifetime is primarily because of the blocking and deflection effects of 3D patterns on interfacial crack propagation. Besides, the geometric parameters of the patterns play a crucial role in determining the failure behavior of the thermal barrier coatings. This strategy presents a practical solution for the development of durable thermal barrier coatings and provides the valuable insights for the optimization of other heterogeneous interfaces.

Thermal Analysis of a Simplified Railway Brake Model with Numerical Simulation

The braking system is the most important safety device in all vehicles. When braking, the kinetic energy of the vehicle is converted into thermal energy. The use of friction brakes can cause problems in the long term, as the brake can heat up, reducing its coefficient of friction. Heating and cooling have a harmful effect on all frictional elements, causing material fatigue. As a result of the negative effects, the hot brake cannot reduce the velocity of the vehicle at the correct rate. In this paper, the propagation of frictional heat is analysed in a test specimen made of cast iron material using different numerical simulations. The computational results are validated with the results obtained during measurement. With proper validation, the brake discs are able to be modelled at different designs and materials without having to manufacture a test specimen for each case. In the study, it is important to point physical quantities to carry out the simulation.

Preparation and characterization of calcium-doped graphene oxide-chitosan Nanocarrier to enhance the gene delivery in MCF-7 cell line

Gene delivery has emerged as a novel and effective method in the treatment of malignancies within medical interventions by applying nanotechnology. Consequently, the development of appropriate nanocarriers is a key focus of this research. Dynamic light scattering (DLS), fourier transform infrared (FT-IR) spectroscopy, x-ray diffraction (XRD), and thermal gravimetric analysis (TGA) were employed for the characterization of the synthesized nanocarrier. Furthermore, to assess the gene transfer capability of the nanocarrier, various techniques such as gel retardation assay, nuclease resistance assay, cytotoxicity assay, flow cytometry, and transfection were employed. The average particle size and zeta potential of the GO-CS@Ca nanocarrier were obtained as 319.8 nm and + 92.8 mv, respectively. In the gel retardation test, it was observed that pDNA was effectively condensed by the GO-CS@Ca nanocarrier. The results of the MTT assay indicated that both GO-CS@Ca nanocarrier and the GO-CS@Ca/pDNA nanoplex with low toxicity. In flow cytometry analysis, it was observed that the complexation of pDNA with the GO-CS@Ca nanocarrier resulted in effective gene delivery to the MCF-7 cell line and consequently increased apoptosis induction.

Electrochemical and Thermal Analysis of Lithium‐Ion Battery Pack With Different Cell Configurations

ABSTRACTThe primary purpose of this research is to analyze and evaluate the effects of various discharge rates and cell configurations on the electrochemical and thermal behavior of a Li‐ion battery pack that is exposed to ambient air throughout the discharge process. The three‐dimensional numerical model is designed to accomplish this purpose and discusses two different cases. While the discharge rate is changed from 0.5 C to 2 C (stepping by 0.5 C) for each cell configuration considered in the first case, the numerical solutions are obtained for the various cell configurations (6S4P and 8S3P) by keeping the discharge rate constant at 1 C. The results obtained from these solutions show that the discharge rate affects a considerable amount of the battery performances and discharge times of the battery packs, activation, and ohmic losses occurring inside each battery cell. Moreover, 6S4P discharges over a longer period (about 25%) than 8S3P. While both activation and ohmic losses decrease with the increase of discharge rate, these losses remain almost constant at 0.5 C discharge rate in all analyzed conditions. As a result, having a battery pack with a long discharge time while maintaining low temperatures is useful and desired. With this in mind, while evaluating battery packs, the 6S4P battery pack looks to have the best arrangement.

Electroosmotic mechanism of Ellis fluid with Joule heating, viscous dissipation and magnetic field effects in a pumping microtube

Abstract The dynamics of electro-osmotically generated flow of biological viscoelastic fluid in a cylindrical geometry are investigated in this paper. This flux is the result of walls contracting and relaxing sinusoidally in a magnetic environment. The rheology of the fluid is accurately captured with the Ellis fluid (blood) model. Both Joule heating and viscous dissipation are accounted for during thermal analysis. The electric potential induced in the EDL is obtained by applying the Debye-Huckel linearization to the non-linear Poisson-Boltzmann equation. Mathematical modelling is incorporated in cylindrical coordinates in wave frame of reference. Assuming a long wavelength characterized by a low Reynolds number, the Ellis fluid model's governing equations are simplified. Subsequently, the differential equations that result are computed numerically utilizing the NDSolve utility that is integrated into Mathematica. Graphical representations are utilized to visually and comprehensively assess the thermal characteristics, flow features, heat transfer coefficient, and skin friction coefficient. Various factors are taken into consideration, including the impact of Ellis fluid parameters, electric double layer, magnetic field, Brinkman number, and Ohmic dissipation. Ellis fluid's axial velocity boosts with a rise of the electroosmotic parameter and power-law index while decreasing with an increase in the Hartmann number and material fluid parameter. The fluid temperature is directly proportional to EDL parameter and parameters of Ohmic and viscous dissipation. The current model may be used in clinical scenarios involving the gastrointestinal system and capillaries, electro-hydrodynamic therapy, delivery of drugs in pharmacological, and biomedical devices.

Thermal and irreversibility analysis of non-Newtonian fluid within trapezoidal cavity containing heated cylinder

In this article, the mixed convection transport of the Carreau fluid in the presence of a magnetic field and a heated circular cylinder within a trapezoidal cavity is examined. Finite element analysis is used via COMSOL Multiphysics software to simulate the two-dimensional flow. The heat equation is formulated by incorporating the effects of viscous dissipation and Joule heating. The upper and lower surfaces of the cavity are adiabatic, while the left and right sides are assumed to be cold. The temperature distribution and flow field within the cavity are visualized through the plotting of isotherms, streamlines, and total entropy generation. The graphical representations are performed to examine the impacts of physical parameters on temperature, flow field, and total entropy generation, including the Reynolds number , Weissenberg number ( 1.0 ≤ We ≤ 6.0 ) , Power law index ( 0.1 ≤ n ≤ 5.0 ) , Hartman number ( 1.0 ≤ M ≤ 50 ) , and Richardson number ( 0.2 ≤ Ri ≤ 5.0 ) and ( 0.2 ≤ Rc ≤ 0.8 ) . It is found that the Joule heating and Richardson number are the main reasons to increase the thermal performance within the cavity. The r ( 100 ≤ Re ≤ 500 ) ising values of power law index and Weissenberg numbers caused to reduce the velocity field, irreversible effects and circulation within cavity. While the entropy is augmented by elevating the Eckert and Hartmann numbers. Furthermore, heat transfer improved with the increased magnetic field and Eckert number.

Repetitive corrugation and straightening (RCS) of NiTi shape memory alloy strip product

ABSTRACT Repetitive corrugation and straightening (RCS) is a novel method of severe plastic deformation in bulk metallic materials. In this study, the effect of RCS on the properties of NiTi shape memory alloy (SMA) is investigated. Cold-rolled (CR) Ni49.8–Ti50.2 (at. %) strips with varying degrees of cold-work were subjected to RCS and evaluated for transformation, mechanical and functional properties. The study demonstrated that RCS can be used effectively to improve the strength and ductility of the CR SMA strips, while its transformation and functional properties are largely preserved. The ultimate tensile strength (UTS) and elongation to failure of the 30% CR strip post RCS increased from 950 MPa to 1130 MPa and ~19% to ~26%, respectively. The microstructural evaluation confirmed that RCS resulted in notable grain refinement and re-distribution of Ti2Ni second phase in the matrix. Consequent to its high strength, the cyclic response of the RCS strip on stress-free thermal cycling was found to be better than the CR strip.

Multi-scale inhomogeneity and anomalous mechanical response of nanoscale metallic glass pillar by cryogenic thermal cycling

The mechanical responses and structure variations of Ta80Co20 nanoscale metallic glass (MG) film samples upon cryogenic thermal cycling (CTC) treatment were studied. The simultaneous improvements of strength and deformation ability bring about a super-high strength of 4.5 GPa and a large plastic strain of about 80% after CTC treatment. The significant increase in inter-element bonding and hardness makes the activation and percolation of shear transformation zones to be more difficult and delays the yielding event, leading to the ultra-high strength. Although the TaCo MG pillar reaches a relaxation energy state, the micro- and nanoscale inhomogeneities remain induced by the local densely packed units along with crystal-like ordering embedded in the matrix. The multi-scale inhomogeneity can effectively hinder the sliding of the shear bands and improve their propagation stability, which is considered to be the origin of its excellent plasticity. Our study reveals another prospect of CTC treatment on nanoscale MG samples of constructing an anomalous inhomogeneous structure and obtaining simultaneous enhancement of strength and plasticity. Graphical abstract

Matching Analysis of Technical Parameters and Safety Standards for Nuclear Replacement of Coal-Fired Units

In the context of the growing share of renewable energy and the impending decommission of a large number of coal-fired units, nuclear energy is the only green energy that can replace coal power for a stable, clean, efficient, and large-scale power supply. This article compares the differences between coal power and nuclear power in terms of thermal system, thermal cycle, turbine parameters, and safety. It discusses the possibility of replacing the boiler of a coal power plant with nuclear power, that is, replacing the boiler of a coal power plant with a nuclear reactor for generation/heating/cogeneration. For coal-fired units with similar capacity that do not use a reheat cycle (at or below high pressure) and nuclear power units (such as a high-temperature gas-cooled reactor), as well as coal-fired units with a reheat cycle (ultra-high pressure and above) and nuclear power units (such as pressurized water reactors), there are great differences in steam parameters. In terms of steam turbines, the size of nuclear power units is relatively larger, requiring additional dehumidification measures. In addition, the safety factors and management methods considered in the site selection, construction, and operation of nuclear power plants are more stringent and complex, and comprehensive analysis and evaluation are needed in aspects such as waste treatment and accident emergency response. Except for the relevant provisions of the American Society of Mechanical Engineers code for pressure vessels, nuclear power units are not compatible with coal-fired units in terms of safety standards. Therefore, considering comprehensively, it is believed that the scheme of nuclear power replacing coal-fired units for power generation/heating/cogeneration program is not feasible at present.

Properties of thermoplastic polyurethane synthesized from bio‐based diisocyanate for FDM 3D printing

AbstractBio‐based polymeric materials have recently gained popularity due to their unique properties, including environmental friendliness, biodegradability, and sustainability. In this study, the bio‐based TPUs were successfully synthesized by one‐shot polymerization method, utilizing 100% bio‐based polytrimethylene ether glycol (PO3G) as polyols, 71% bio‐based 1,5‐pentamethylene diisocyanate (PDI) as isocyanates, and 100% bio‐based 1,4‐butanediol BDO as chain extenders. The as‐prepared TPUs, which contained up to 92% bio‐based material were investigated using a variety of analytical methods, including morphological investigations, mechanical testing, thermal analysis, rheological behavior, docking analysis, and cytotoxicity studies. For PPB 3 (1:3:2), PPB 4 (1:4:3), PPB 5 (1:5:4), and PPB 7 (1:7:6), the initial modulus values were 78, 151, 194, and 314 GPa, and the shore‐A hardness values were 92, 93, 93, and 94. Additionally, a notable variation in the degree of phase separation (DPS) of 0.575, 0.647, 0.716, and, 0.738 between hard segment (HS) and soft segment (SS) was noticed among synthesized bio‐based TPUs and an increase in DPS with higher molar ratios corresponded to a higher content of HS. Besides, the bio‐based TPU proved outstanding cell viability results, representing its potential appropriateness for various biomedical applications. Eventually, docking simulations were shown in silico to evaluate the interaction of bio‐based TPU with the DNA gyrase enzyme. Furthermore, the results of bio‐based TPUs demonstrated excellent applications in the production of 3D printing using FDM. We effectively prepared 3D printing to provide a viable answer to environmental concerns.

Effect of the heat treatment on the physicochemical characteristics of rubberwood: Results of thermal analysis and FTIR spectroscopy

Heat treatment is an environmentally friendly method used to improve properties of rubberwood. In this work, the changes in the chemical composition, thermal behavior and thermal degradation kinetics of heat-treated Hevea brasiliensis (rubber tree) were evaluated using thermogravimetry, differential scanning calorimetry, and Fourier transform infrared spectroscopy. The rubberwood samples were exposed to temperatures of 180 °C and 220 °C in air under atmospheric pressure for durations of 15 25 and 35 h. Thermal analysis revealed degradation of hemicelluloses, an increase in the relative proportions of cellulose and lignin in heat-treated rubberwood. The thermal decomposition of rubberwood heat-treated at 220 °C started at a higher temperature compared to untreated wood. A shift in the position of peaks on differential thermogravimetry and differential scanning calorimetry curves of heat-treated samples was observed, indicating changes in the structure of wood polymers. The temperature of heat treatment had a stronger effect on the chemical composition of rubberwood than duration. Significant changes in the chemical composition of rubberwood occurred after the treatment duration of 15 h at both 180 °C and 220 °C. The duration of 25 h and 35 h had no further substantial effect. The isoconversional method of Flynn-Wall-Ozawa was used to determine the kinetics of thermal degradation of untreated and heat-treated rubberwood. It is found that the average values of activation energy in the conversion degree range of 0,05 - 0,65 (the thermal degradation of polysaccharides) increased with increasing treatment temperature and duration. Fourier transform infrared spectra demonstrated alterations in wood polymers.

Artificial intelligence-enhanced infrared thermography as a diagnostic tool for thyroid malignancy detection

Introduction Thyroid nodules are common, and investigation is crucial for excluding malignancy. Increased intranodular vascularity is frequently observed in malignant tumors, which can be detected through increased skin surface temperatures using noninvasive infrared thermography. We aimed to develop a diagnostic tool for thyroid cancer using infrared thermal images combined with an artificial intelligence (AI) algorithm. Methods We conducted a prospective cross-sectional study involving participants with thyroid nodules undergoing thyroid surgery. Infrared thermal images were collected using a thermal camera on the day prior to surgery. In combination with the final thyroid pathological reports, we utilized a machine learning model based on the pre-trained ResNet50V2 model, a convolutional neural network, to evaluate diagnostic accuracy for malignancy diagnosis. Results The study included 98 participants, 58 with malignant thyroid nodules and 40 with benign thyroid nodules, as determined by pathological results. The AI-enhanced infrared thermal image analyses demonstrated good performance in distinguishing between benign and malignant thyroid nodules, achieving an accuracy of 75% and a sensitivity of 78%. These parameters were slightly lower than those of the AI-model predictor that integrated current practice using preoperative thyroid ultrasound findings and cytological results, yielding an accuracy of 81% and a sensitivity of 84%. Conclusions The infrared thermal images, assisted by an AI model, exhibit good performance in distinguishing thyroid malignancy from benign nodules. This imaging modality has great potential to be used as a noninvasive screening tool for adjunct evaluation of thyroid nodules.

POST-OCCUPATION EVALUATION OF A BUILDING IN A SEA CONTAINER FROM THE PERSPECTIVE OF THERMAL PERFORMANCE IN PALMAS – TO

This study aims to evaluate the environmental, comfort, functional and construction aspects of a container residence located in a rural area in the city of Palmas - TO. The methodology combines NBR 15.220/2005, 15.220/2023 and NBR 15.575/2013, Post-Occupancy Evaluation of the building, regarding its thermal performance in the housing process, data collection and field research, thermal analysis equipment was used, in order to understand the ways in which the material acts in the face of the city's climate. Based on the results of the post-occupancy research, it was possible to analyze the possibilities of use, generate possible solutions for these structures in order to contribute to the development of the field of Architecture and Urbanism.

Role of intermediate water in alleviating postsurgical intrapericardial adhesion

Abstract Purpose Various polymers have been used as postsurgical antiadhesive materials; however, the mechanisms underlying their efficacy remain unclear. Intermediate water has been found to prevent the adhesion between polymer molecules and proteins or cells. The present study investigated the role of intermediate water retained in the polymer in alleviating postsurgical pericardial adhesion. Methods Hydrophobic fabrics were prepared using biodegradable polyglycolic acid. To add intermediate water, the fabric fibers were coated with poly(oxyethylene)oleyl ethers. Intermediate water in the hydrated state was detected by a thermal analysis for each material, and cell attachment to the fibers with or without coating was observed in vitro. Using a canine model of postsurgical pericardial adhesion, the severity of adhesion was examined along with a histological assessment during treatment, with or without fabric coating. Results Intermediate water was detected in the coating materials but not in polyglycolic acid. Coating significantly reduced the cell attachment to the fibers. Coating also alleviated adhesion by reducing inflammation in the fibrous layer and replacing the fabric and granulomas that develop around the surgical sutures in the pericardial space. Conclusions Intermediate water in the hydrated polymer of anti-adhesives may play an important role in alleviating postoperative pericardial adhesion.

Chemical and Thermal Changes in Mg3Si2O5 (OH)4 Polymorph Minerals and Importance as an Industrial Material

Serpentine (Mg3Si2O5(OH)4), like quartz, dolomite and magnesite minerals, is a versatile mineral group characterized by silica and magnesium silicate contents with multiple polymorphic phases. Among the phases composed of antigorite, lizardite, and chrysotile, lizardite and chrysotile are the most prevalent phases in the serpentinites studied here. The formation process of serpentinites, which arise from the hydrothermal alteration of peridotites, influences the ratio of light rare earth elements (LREE) to heavy rare earth elements (HREE). In serpentinites, the ratio of light rare earth elements (LREE)/heavy rare earth elements (HREE) provides insights into formation conditions, geochemical evolution, and magmatic processes. The depletion of REE compositions in serpentinites indicates high melting extraction for fore-arc/mantle wedge serpentinites. The studied serpentinites show a depletion in REE concentrations compared to chondrite values, with HREE exhibiting a lesser degree of depletion compared to LREE. The high ΣLREE/ΣHREE ratios of the samples are between 0.16 and 4 ppm. While Ce shows a strong negative anomaly (0.1–12), Eu shows a weak positive anomaly (0.1–0.3). This indicates that fluid interacts significantly with rock during serpentinization, and highly incompatible elements (HIEs) gradually become involved in the serpentinization process. While high REE concentrations indicate mantle wedge serpentinites, REE levels are lower in mid-ocean ridge serpentinites. The enrichment of LREE in the analyzed samples reflects melt/rock interaction with depleted mantle and is consistent with rock–water interaction during serpentinization. The gradual increase in highly incompatible elements (HIEs) suggests that they result from fluid integration into the system and a subduction process. The large differential thermal analysis (DTA) peak at 810–830 °C is an important sign of dehydration, transformation reactions and thermal decomposition, and is compatible with H2O phyllosilicates in the mineral structure losing water at this temperature. In SEM images, chrysotile, which has a fibrous structure, and lizardite, which has a flat appearance, transform into talc as a result of dehydration with increasing temperature. Therefore, the sudden temperature drop observed in DTA graphs is an indicator of crystal form transformation and CO2 loss. In this study, the mineralogical and structural properties and the formation of serpentinites were examined for the first time using thermo-gravimetric analysis methods. In addition, the mineralogical and physical properties of serpentinites can be recommended for industrial use as additives in polymers or in the adsorption of organic pollutants. As a result, the high refractory nature of examined serpentine suggests that it is well-suited for applications involving high temperatures. This includes industries such as metallurgy and steel production, glass manufacturing, ceramic production, and the chemical industry.

Solubility and magnetic properties of Ag−Co−Si alloy synthesized by mechanical alloying and annealing

AbstractThis present work concerns the effect of ternary addition of non‐metallic and diamagnetic silicon on the metastable solubility of the immiscible silver‐cobalt system during the course of ball milling, annealing of the ball milled samples and the corresponding magnetic properties. It should be noted that silicon has a smaller goldsmidt radius (111 pm) than silver (144 pm) and silicon+4 has more valence electrons than silver+1. Keeping the objectives of the current study in mind, silicon may thus be considered as an option for ternary addition, in contrast to metallic components. An attempt has been made to examine the phase changes that occurred in the 50 silver‐40 cobalt‐10 silicon (atom percent) system during ball milling and the subsequent isothermal annealing of the ball milled product. Phase evolution during mechanical alloying and isothermal annealing is identified by means of x‐ray diffraction, differential thermal analysis, high resolution transmission electron microscopy and magnetic measurements. The addition of silicon facilitates cobalt‘s formation of a metastable alloy with copper after 50 hours of ball milling. Annealing significantly improves the magnetic characteristics of materials that have been ball milled; the optimal combination of magnetic properties was obtained after an hour of annealing at 550 °C.

TiO2 and down-conversion phosphors to enhance UV protection of solar cells

In recent years, demand for solar generators for LEO (Low Earth Orbit) applications has been growing, and much research has focused on the use of less expensive and thinner (<90 μm). silicon-based solar cells that can be integrated on flexible Photovoltaic Assemblies (PVAs). These solar cells must be protected from space UV radiations and also withstand more than 50,000 thermal cycles in LEO. The solution advocated here involves the incorporation of UV-absorbing particles into a spatial polymer, combined with lanthanide ions based inorganic phosphors (Y2O3:Eu3+ and Y2O2S:Eu3+) to achieve the down-conversion process. Embedded into a silicone-based polymer matrix with a thickness close to 100 μm, these particles are then deposited on 180 μm thick Ga-doped heterojunction silicon cells (30x30 mm2). UV tests are carried out on ONERA's SEMIRAMIS platform at two doses: 425 esh and 1005 esh. A series of 1000 thermal cycles is carried out at the CEA. The first spatial UV analysis revealed a maximum loss of almost 5 % in short-circuit current (Isc) for PV devices after 1005 esh. Comparing results in open circuit voltage (Voc), the bare cell degrades as the dose increases (−2 % at 425 esh and −5 % at 1005 esh). One positive point is that the addition of TiO2 particles protects the solar cell. These initial results point out that it is possible to produce a protective coating to limit the effects of degradation of Silicon cells under space UV flux, with a focus on producing flexible PVAs.

Different Magnitudes of Second-Order Jahn-Teller Effect in Isostructural NaMO2F2 (M = Nb5+, Ta5+) Oxyfluorides

The structures of the ordered and isotype oxyfluorides NaMO2F2 (M = Nb, Ta) were thoroughly investigated by combining powder X-Ray Diffraction (PXRD), 19F and high-field 23Na and 93Nb solid-state NMR, and DFT calculations. The structures, derived from Rietveld refinement of the PXRD data, exclusively consist of cis-[MO4F2]5– octahedra, in which cations are displaced from their ideal centered positions toward an oxide face. The NMR parameters were calculated for both the experimental (ES) and the atomic positions optimized (APO) structures, the latter exhibiting, as is often the case, the best agreement with the experimental data. Nb5+ and Ta5+ cations having the same size, niobium and tantalum isotypes have usually very close cell parameters. However, those of NaNbO2F2 and NaTaO2F2, particularly c, differ in unusual proportions. This difference in c parameters is due to stronger second-order Jahn-Teller effect (SOJTE) for the cis-[NbO4F2]5– than for the cis-[TaO4F2]5– octahedra, further confirmed by band structure and projected density of states calculations. Furthermore, by optimizing the synthesis conditions of these compounds using thermal analysis, a very low amplitude endothermic event, upon heating, was observed only for NaNbO2F2. An extensive analysis of the variable temperature (VT) PXRD data revealed that this event is related to a deviation from linearity of the cell parameters evolution and that structural features of these two isotypes evolve differently with temperature.

Numerical Thermal Modeling and Analysis of Dry Sliding Contacts between Copper-Graphite and Bronze-Graphite

Numerical Thermal Modeling and Analysis of Dry Sliding Contacts between Copper-Graphite and Bronze-Graphite