For a rational number q, a rationalD(q)-n-tuple is a set of n distinct nonzero rationals {a1,a2,…,an} such that aiaj+q is a square for all 1⩽i<j⩽n. For every q we find all rational m such that there ...exists a D(q)-quadruple with product a1a2a3a4=m. We describe all such quadruples using points on a specific elliptic curve depending on (q,m).
Achieving macroscale superlubricity on engineering steel by utilizing aqueous green lubricants has gained growing interest, given its substantial potential to reduce energy consumption and carbon ...footprint. However, maintaining superlubricity under diverse sliding conditions over a prolonged duration is a major obstacle for real-scale applications. Herein, we report that a robust and durable tribofilm enabled by a unique lubrication mechanism based on carboxylated graphene quantum dots (CGQDs) in aqueous glycerol triggers macroscale superlubricity in self-mated steel contacts. A dedicated intermittent test was designed to show the superlubricity's robustness and the ability of the tribofilm to adapt to a variety of relevant sliding conditions. Moreover, the boundary film provides an average coefficient of friction of around 0.007 and up to 69 % wear reduction (compared to the base lubricant), resulting in the maintenance of superlubricity at a real final contact pressure of 123 MPa, which increases the upper limit of the contact pressure compared to current aqueous-lubricated steel contacts. The new superlubricity mechanism was enabled by the chemical adsorption of the CGQDs onto the worn metal surface, coupled with the tribo-induced structural degradation and transformation of the CGQDs into layered graphitic structures that generate an adaptable low-shear interface. This work provides new insights into the role of chemical adsorption and structural transformation of CGQDs in achieving superlubricity and is an important step forward for implementing energy-efficient and green lubrication technologies for industrial applications.
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This article provides an overview of the main characteristics of SiCf/SiC that suggest the use of this SiC-based composite as a structural material for the blanket in future fusion reactors, a brief ...description of its structure and the role of its main constituents. The relevant fabrication processes and their ability to produce a material with the required properties are also summarised. The main part of the paper is devoted to an assessment of the state-of-the-art materials, and the basic requirements for the target material are discussed in terms of the achieved properties. The key issues and areas of uncertainty are described and suggestions for overcoming them are presented.
Manufacturing of flexible electrochromic (EC) devices requires the use of low-temperature films for deposition on electronically conductive foils. With this aim, low-temperature V-oxide films were ...spin-coated from a dispersion prepared by milling of crystalline V2O5 powder in vanadium(V) tri-n-propoxide oxide/isopropanol sol as dispersant. TEM images showed that the milling procedure enables the introduction of some V2O5 nanocrystallites into the matrix of the V-oxide pigmented film prepared at 150 °C. The electrochromic properties were analyzed using in situ UV–visible absorbance spectroelectrochemistry. The low transmittance values of as-prepared pigmented films revealed the presence of a greater amount of reduced V4+ centers compared to the crystalline sol-gel V2O5 model films. Significant bleaching of pigmented V-oxide films occurred during subsequent 521 cycles in the safe potential range.
Three different types of vibrational spectroscopies were applied to examine the powdered V-oxide film, and the results were compared to the band changes for the sol-gel crystalline V2O5 films. For initial films, the modes in the IR reflection-absorption spectra were systematically shifted to higher wavenumbers with regard to the modes in the IR absorbance spectra. Both IR techniques confirmed the reversible shift of the vanadyl V-OA band to lower frequencies and the disappearance of the bridging V-OB-V mode upon intercalation. Raman spectra showed the appearance of two vanadyl stretching modes at 986 and 961 cm−1 that occurred simultaneously with the decrease in the intensity of the bands in the 750–400 cm−1 spectral region. After the deintercalation, the Raman spectra of the low-temperature films showed broad bands that remained after the second bleaching. Finally, a flexible NiO1-x/ormolyte/V-oxide EC device was constructed. The device was composed of two different pigmented films and demonstrated the practical applicability of the low-temperature pigmented films.
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•Low-temperature pigmented V-oxide films with nanocrystalites of V2O5 were fabricated.•V4+ was also present in V-oxide films that were bleached during the cycling.•Ex situ IR reflection-absorption, IR absorption and Raman spectroscopy were applied.•Vibrational properties of pigmented V-oxide were compared to those of crystalline V2O5.•A flexible Ni1-xO/ormolyte/V-oxide EC device was fabricated using two types of pigmented films.
We investigated the H2 sensing performance of pure α-Fe2O3 and Pt/α-Fe2O3 samples, where platinum was mechanochemically dispersed on reducible α-Fe2O3 supports by ball miling, at concentrations of 1, ...3, 5 and 10 mol%. The hematite nature of the supports was confirmed by Mössbauer spectroscopy. Raman spectroscopy reveals signature lines of hematite as well as “forbidden” longitudinal optical modes indicating stress-induced asymmetry. Scanning and transmission electron micrographs show nanometer-sized Pt dots dispersed on loosely aggregated α-Fe2O3 crystallites with a diameter of ⪆ 50 nm.
Pt/α-Fe2O3 powder was suspended in ethanol and drop-cast onto glass substrates with interdigitated electrodes. The samples were exposed to H2 concentrations from 0 – 500 ppm at different temperatures from 293 K to 553 K.
The electrical resistance of the samples with Pt decreased with increasing H2 concentration even at room temperature, indicating sensitivity to H2. The sensitivity and response time improved significantly at higher temperatures. The response and response time at 100 ppm H2 ranged from ∼-10% and ⪆75 s at 298 K to ∼-50% and ∼5 s at 553 K. No dependence of the sensitivity on the Pt loading was observed, but a Pt loading of at least 1 mol% is essential for the sensors to function. The behavior of the sensors suggests a reversible sensing mechanism based on the Pt-mediated interaction of H2 with adsorbed oxygen. These results indicate a possible use of Pt/α-Fe2O3 as a sensor for H2 with a linear response at concentrations of 0–100 ppm and encourage further research.
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•We have studied the H2 sensing performance of pure α−Fe2O3 and Pt/α−Fe2O3 samples.•The samples were exposed to H2 concentrations (0 – 500 ppm) at different temperatures.•Samples with Pt were sensitive to H2 and their sensor response improved at higher temperatures.•Results suggest a potential use of Pt/α−Fe2O3 as a sensor for H2 with a linear response at concentrations of 0–100 ppm and encourage further research.
Water interaction with mineral surfaces is a complex living system decisive for any photocatalytic process. Resolving the atomistic structure of mineral–water interfaces is thus crucial for ...understanding these processes. Fibrous rutile TiO2, grown hydrothermally on twinned rutile seeds under acidic conditions, is studied in terms of interface translation, atomic structure, and surface chemistry in the presence of water, by means of advanced microscopy and spectroscopy methods combined with structure modeling and density functional theory calculations. It is shown that fibers while staying in stable separation during their growth, adopt a special crystallographic registry that is controlled by repulsion forces between fully hydroxylated and protonated (110) surfaces. During relaxation, a turbulent proton transfer and cracking of O─H bonds is observed, generating a strong acidic character via proton jump from bridge ─OHb to terminal ─OHt groups, and spontaneous dissociation of interfacial water via a transient protonation of the ─OHt groups. It is shown, that this specific interface structure can be implemented to induce acidic response in an initially neutral medium when re‐immersed. This is thought to be the first demonstration of quantum‐confined mineral–water interface, capable of memorizing its past and conveying its structurally encoded properties into a new environment.
Surface mineral–water interactions are decisive for any photocatalytic process. In this work, fibrous rutile homoepitaxially grown on twinned rutile seeds with a specific interface structure that displays the ability to spontaneously dissociate water are investigated. This is thought to be the first example of an interface with mnemonic behavior capable of transmitting structurally encoded properties to a new environment.
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•Graphene quantum dots (GQDs) nano-additives in aqueous glycerol provide super-low friction (µ ≈ 0.012) between self-mated steel contacts.•The super-low friction is observed under a ...contact pressure as high as 316.5 MPa in the boundary lubrication regime (rarely observed for steel/steel contacts).•A 98% improvement in wear performance is observed compared to pure aqueous glycerol without GQDs nano-additives.•A more realistic lubrication mechanism is proposed based on the friction-induced structural degradation of GQDs.
Reducing friction is a promising strategy to decrease material losses and energy consumption in industrial systems. However, in aqueous-lubricated steel contacts, the contact pressure rarely exceeds 50 MPa during super-low friction due to excessive wear. This work demonstrates that even in steel/steel contacts, by combining graphene quantum dots (GQDs) with aqueous glycerol, it is possible to maintain super-low friction (µ ≈ 0.012) under a contact pressure as high as 316.5 MPa. Moreover, the use of GQDs improved the wear performance by 98 % compared to pure aqueous glycerol due to the formation of a tribochemical film, resulting from the electrostatic adsorption of GQDs on the positively charged sites on the worn surface. In particular, the exfoliation of graphene sheets within GQDs, the shearing of graphene layers inside the GQDs, and the OH–OH repulsion between the asperities shortens the running-in period and consequently reduces the friction and wear. At the same time, the formation of a chemically adsorbed tribofilm containing friction-induced structurally degraded GQDs protects the surface from wear and facilitates the maintenance of super-low friction at high contact pressures by improving the load-carrying capacity. This study suggests that green nano-lubricants based on GQDs have immense potential in sustainable engineering.
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•Fenton-like AOP with Cu-Mn porous silica supported photocatalysts was performed Catalysts were evaluated at neutral pH for methylene blue model dye degradation•Adsorption of model ...dye on photocatalysts strongly depends on Mn.•Cu addition reduces Mn leaching from the silica support.•Mn and Cu act as separate active sites.
Fenton and Fenton-like Advanced Oxidation Processes (AOP) have obtained large applicative use for the removal of organic pollutants from wastewater. Herein, we discuss a novel bimetal Cu-Mn porous silica-supported Fenton-like AOP catalyst with interparticle mesoporosity working under neutral pH and UV-VIS irradiation (CuMnKIL with Mn/Si = 0.01, Mn/Cu = 0.06). The photocatalyst was prepared via solvothermal synthesis (Mn incorporation into porous silica support) followed by incipient wetness impregnation (Cu loading into Mn containing porous silica support) and was tested for the decomposition of methylene blue as a model dye. Copper addition significantly reduced Mn leaching from the porous silica support (from 60 to 30 %) and functioned also as additional surface adsorption site. CuMnKIL (Mn/Si = 0.01, Mn/Cu = 0.06) showed 20 % higher adsorption and 20 % lower efficiency if compared with MnKIL (Mn/Si = 0.01) in Fenton-like AOP system, however, addition of irradiation (photo-Fenton-like system) improved efficiency of Cu/MnKIL (Mn/Si = 0.01, Mn/Cu = 0.06) for 10 % if compared to MnKIL (Mn/Si=0.01) due to the additional •OH radicals produced by the decomposition of H2O2 by the light. No synergistic effects between Cu and Mn were recorded, meaning that Cu and Mn sites were acting as separate sites, which was confirmed with AR-TEM imaging as well as UV spectroscopy studies.
Thermal annealing processes for supported Pt-based nanoparticles are usually developed based on iterative empirical findings resulting from ex-situ characterization of pre- and post-annealed samples. ...Such an approach, however, offers limited insight into processes occurring during the heating step. In this work, we first exemplify typical findings that are accessible by ex-situ investigation using typical conventional techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), and thin film – rotating disc electrode (TF-RDE). As a model system we select a well-researched Pt-Cu alloy which, as demonstrated, offers exciting new insights into the dynamics occurring during heat treatment on the nano-to-atomic scale. This dynamics can be viewed by upgrading the ex-situ findings with a high resolution TEM imaging in combination with carefully designed in-situ heating protocol. This way one can directly observe the particle growth mechanisms during heat treatment. Such direct observations, in turn, provide new understanding of morphology-performance correlations in alloys. For example, it is shown that the enhanced activity of the present PtCu3/C electrocatalyst is due to Cu enrichment during heat treatment. This enrichment, however, is only possible due to the presence of relatively large excess CuO needle-like particles left over from the previous double passivation galvanic displacement step. Very importantly, we further show that the mechanism of Cu enrichment at elevated temperatures involves migration of Cu single atoms via the carbon support. At moderate temperatures (up to 500 °C), other effects have also been observed such as reshaping into a sphere-like shape as well as ordering of the crystal lattice which could not occur without enrichment of the initial Pt-Cu nanoparticles with Cu. In that region, Cu enrichment is also responsible for the initial growth of PtCu nanoparticles. By contrast, upon heating till 800 °C, the growth is mainly due to coalescence. Ostwald ripening, on the other hand, does not seem to play a significant role in the increase in the nanoparticle size. The new general insights can be readily extended to various other similar alloy systems.
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•In-situ heating TEM provides detailed insight into the process of alloy formation during the heating step.•The present Pt-Cu system was found to be a good model system for the study of dynamics on the nano-to-atomic scale.•Nanoparticle growth mechanisms were studied both in-situ and at identical location.•Evaporation of the less noble metal was observed upon in-situ heating to 800 °C or higher under TEM conditions.
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