Search Results (35)

Ultrahigh-temperature ceramic composites based on hafnium diboride have a wide range of applications, including as components for high-speed aircraft and energy generation and storage devices. Consequently, developing methodologies for their fabrication and studying their properties are of paramount importance, in particular in using them as an electrode material for energy storage devices with increased oxidation resistance. This study investigates the behavior of ceramic composites based on the HfB₂-HfO₂-SiC system, obtained using 15 vol% Ti₂AlC MAX-phase as a sintering component, under the influence of subsonic flow of dissociated air. It was determined that incorporating the modifying component (Ti₂AlC) altered the composition of the silicate melt formed on the surface during ceramic oxidation. This modification led to the observation of a protective antioxidant function. Consequently, liquation was observed in the silicate melt layer, resulting in the formation of spherical phase inhomogeneities in its volume with increased content of titanium, aluminum, and hafnium. It is hypothesized that the increase in the high-temperature viscosity of this melt prevents it from being carried away in the form of drops, even at a surface temperature of ~1900–2000 °C. Despite the established temperature, there is no sharp increase in its values above 2400–2500 °C. This is due to the evaporation of silicate melt from the surface. In addition, the electrochemical behavior of the obtained material in a liquid electrolyte medium (KOH, 3 mol/L) was examined, and it was shown that according to the value of electrical conductivity and specific capacitance, it is a promising electrode material for supercapacitors. Full article

The chemical and microstructural transformation of the surface of a 31.5 vol.% ZrB₂-31.5 vol.% HfB₂-27 vol.% SiC-10 vol.% C_CNT ultrahigh-temperature ceramic sample (where C_CNT refers to carbon nanotubes) was studied under the influence of a subsonic N₂-plasma flow with the addition of 5 mol% methane, simulating aerodynamic heating in the atmosphere of Titan. As in the case of pure nitrogen flow, it was found that silicon carbide is removed from the surface. Zirconium and hafnium diborides are partially transformed into a Zr-Hf-B-C-N solid solution in the experiment conducted. XRD, Raman spectroscopy, and SEM-EDX analysis show that the presence of C₂ in the N₂-CH₄ plasma flow leads to surface carbonization (formation of a graphite- and diamond-like coating with a high proportion of amorphous carbon), resulting in significant changes in the microstructure and emissivity, potentially affecting the catalytic properties of the surface. Full article

Although the most promising applications of ionogels require their contact with aqueous media, few data are available on the stability of ionogels upon exposure to water. In this paper, a simple, easy-to-setup and precise method is presented, which was developed based on the continuous conductivity measurements of an aqueous phase, to study the washout of imidazolium ionic liquids (IL) from various silica-based ionogels immersed in water. The accuracy of the method was verified using HPLC, its reproducibility was confirmed, and its systematic errors were estimated. The experimental data show the rapid and almost complete (>90% in 5 h) washout of the hydrophilic IL (1-butyl-3-methylimidazolium dicyanamide) from the TMOS-derived silica ionogel. To lower the rate and degree of washout, several approaches were analysed, including decreasing IL content in ionogels, using ionogels in a monolithic form instead of a powder, constructing ionogels by gelation of silica in an ionic liquid, ageing ionogels after sol–gel synthesis and constructing ionogels from both hydrophobic IL and hydrophobic silica. All these approaches inhibited IL washout; the lowest level of washout achieved was ~14% in 24 h. Insights into the ionogels’ structure and composition, using complementary methods (XRD, TGA, FTIR, SEM, NMR and nitrogen adsorption), revealed the washout mechanism, which was shown to be governed by three main processes: the diffusion of (1) IL and (2) water, and (3) IL dissolution in water. Washout was shown to follow pseudo-second-order kinetics, with the kinetic constants being in the range of 0.007–0.154 mol⁻¹·s⁻¹. Full article

This study presents the fabrication possibilities of ultra-high-temperature ceramics of ZrB₂-30 vol.%SiC and (ZrB₂-HfB₂)-30 vol.% SiC composition using the reaction spark plasma sintering of composite powders ZrB₂(HfB₂)-(SiO₂-C) under two-stage heating conditions. The phase composition and microstructure of the obtained ceramic materials have been subjected to detailed analysis, their electrical conductivity has been evaluated using the four-contact method, and the electron work function has been determined using Kelvin probe force microscopy. The thermal analysis in the air, as well as the calcination of the samples at temperatures of 800, 1000, and 1200 °C in the air, demonstrated a comparable behavior of the materials in general. However, based on the XRD data and mapping of the distribution of elements on the oxidized surface (EDX), a slightly higher oxidation resistance of the ceramics (ZrB₂-HfB₂)-30 vol.% SiC was observed. The I-V curves of the sample surfaces recorded with atomic force microscopy demonstrated that following oxidation in the air at 1200 °C, the surfaces of the materials exhibited a marked reduction in current conductivity due to the formation of a dielectric layer. However, data obtained from Kelvin probe force microscopy indicated that (ZrB₂-HfB₂)-30 vol.% SiC ceramics also demonstrated enhanced resistance to oxidation. Full article

The short-term (5 min) exposure to the supersonic flow of carbon dioxide plasma on ultrahigh-temperature ceramics of HfB₂-30vol.%SiC composition has been studied. It was shown that, when established on the surface at a temperature of 1615–1655 °C, the beginning of the formation of an oxidized layer takes place. Raman spectroscopy and scanning electron microscopy studies showed that the formation of a porous SiC-depleted region is not possible under the HfO₂-SiO₂ surface oxide layer. Numerical modeling based on the Navier–Stokes equations and experimental probe measurements of the test conditions were performed. The desirability of continuing systematic studies on the behavior of ultrahigh-temperature ZrB₂/HfB₂-SiC ceramics, including those doped with various components under the influence of high-enthalpy gas flows, was noted. Full article

Polymer nanocomposites filled with carbon nanoparticles (CNPs) are a hot topic in materials science. This article discusses the current research on the use of these materials as interfacial electron transfer films for solid contact potentiometric membrane sensors (SC-PMSs). The results of a comparative study of plasticized poly (vinyl chloride) (pPVC) matrices modified with single-walled carbon nanotubes (SWCNTs), fullerenes-C60, and their hybrid ensemble (SWCNTs-C60) are reported. The morphological characteristics and electrical conductivity of the prepared nanostructured composite films are reported. It was found that the specific electrical conductivity of the pPVC/SWCNTs-C60 polymer film was higher than that of pPVC filled with individual nanocomponents. The effectiveness of this composite material as an electron transfer film in a new potentiometric membrane sensor for detecting phenylpyruvic acid (in anionic form) was demonstrated. Screening for this metabolic product of phenylalanine in body fluids is of significant diagnostic interest in phenylketonuria (dementia), viral hepatitis, and alcoholism. The developed sensor showed a stable and fast Nernstian response for phenylpyruvate ions in aqueous solutions over the wide linear concentration range of 5 × 10⁻⁷–1 × 10⁻³ M, with a detection limit of 10^−7.2 M. Full article

The development of scientific and technological foundations for the creation of high-performance energy storage devices is becoming increasingly important due to the rapid development of microelectronics, including flexible and wearable microelectronics. Supercapacitors are indispensable devices for the power supply of systems requiring high power, high charging-discharging rates, cyclic stability, and long service life and a wide range of operating temperatures (from −40 to 70 °C). The use of printing technologies gives an opportunity to move the production of such devices to a new level due to the possibility of the automated formation of micro-supercapacitors (including flexible, stretchable, wearable) with the required type of geometric implementation, to reduce time and labour costs for their creation, and to expand the prospects of their commercialization and widespread use. Within the framework of this review, we have focused on the consideration of the key commonly used supercapacitor electrode materials and highlighted examples of their successful printing in the process of assembling miniature energy storage devices. Full article

The features of oxidation of ultra-high-temperature ceramic material HfB₂-30 vol.%SiC modified with 1 vol.% graphene as a result of supersonic flow of dissociated CO₂ (generated with the use of high-frequency induction plasmatron), as well as under the influence of combined heating by high-speed CO₂ jets and ytterbium laser radiation, were studied for the first time. It was found that the addition of laser radiation leads to local heating of the central region from ~1750 to ~2000–2200 °C; the observed temperature difference between the central region and the periphery of ~300–550 °C did not lead to cracking and destruction of the sample. Oxidized surfaces and cross sections of HfB₂-SiC-C_G ceramics with and without laser heating were investigated using X-ray phase analysis, Raman spectroscopy and scanning electron microscopy with local elemental analysis. During oxidation by supersonic flow of dissociated CO₂, a multilayer near-surface region similar to that formed under the influence of high-speed dissociated air flows was formed. An increase in surface temperature with the addition of laser heating from 1750–1790 to 2000–2200 °C (short term, within 2 min) led to a two to threefold increase in the thickness of the degraded near-surface area of ceramics from 165 to 380 microns. The experimental results indicate promising applications of ceramic materials based on HfB₂-SiC as part of high-speed flying vehicles in planetary atmospheres predominantly composed of CO₂ (e.g., Venus and Mars). Full article

The thermal reduction of the copper(II) complexes [Cu^II(N₂H₄)₃][B₁₀H₁₀]·nH₂O (I·nH₂O) and [Cu^II(NH₃)₄][B₁₀H₁₀]·nH₂O (II·nH₂O) has been studied in an argon atmosphere at 900 °C. It has been found that the annealing of both compounds results in a Cu@BN boron-containing copper composite. It has been shown that this process leads to the formation of a boron nitride matrix doped with cubic copper(0) nanoparticles due to the copper(II)→copper(I)→copper(0) thermal reduction. The phase composition of annealing products I⁹⁰⁰ and II⁹⁰⁰ has been determined based on powder X-ray diffraction, IR spectroscopy and thermal analysis data. The morphology, average particle size and composition of the composite have been determined by TEM and high-resolution TEM + EDS. The average particle size has been found to be about 81 nm and 52 nm for samples I⁹⁰⁰ and II⁹⁰⁰, respectively. Comparison of the results obtained using physicochemical studies has shown the identity of the composition of the products of annealing I⁹⁰⁰ and II⁹⁰⁰. The electrical properties of a coating based on an I⁹⁰⁰ sample modified with Cu⁰→Cu₂O in situ during deposition on a chip at 300 °C in air have been studied. As a result, with increasing temperature, an increase in the electrical conductivity characteristic of semiconductors has been observed. Full article

The goal of this work was the synthesis study of the [WO₂(C₅H₇O₂)₂] complex and its application as a precursor for the growth of WO₃ films in hydrothermal conditions, as well as evaluating the microstructural features and electrochromic properties of the formed materials. Dioxotungsten acetylacetonate was synthesized in an aqueous medium and purified. It was found that during hydrothermal treatment of the alcohol solution of the complex, acetylacetonate ligands undergo partial destructive substitution by alkoxyl groups, intensifying at temperatures above 140 °C. Considering these data and using a [WO₂(C₅H₇O₂)₂] solution, WO₃ films were grown on glass and glass/ITO substrates. The resulting films had different microstructures according to scanning electron microscopy (SEM) and atomic force microscopy (AFM): the former consisted of submicron spheres (~500 nm), distinct nanoparticles (60–160 nm), and submicron- and micron-sized ridges, while the latter consisted of 1D structures (length 350 ± 25 nm, width 110 ± 25 nm). Using Kelvin probe force microscopy (KPFM), the electron work function was determined for the film on glass/ITO substrate (4.77 eV). It was found that the electrochemical coloration process of the obtained WO₃ film can proceed in two stages, and the optical contrast is about 17.5% (at the wavelengths of 600–1100 nm). The results obtained show the prospects of applying the proposed approach to obtaining WO₃ electrochromic films with a hierarchical microstructure with the hydrothermal method using the [WO₂(C₅H₇O₂)₂] complex as a precursor. Full article

The formation of a cellular hierarchically organized NiO film on a carbon paper substrate under hydrothermal conditions using triethanolamine as a base has been studied. The thermal behavior of the carbon paper substrate with the applied semi-product shell was studied using synchronous thermal analysis (TGA/DSC) and it was demonstrated that such modification of the material surface leads to a noticeable increase in its thermal stability. Using scanning electron microscopy (SEM), it was shown that the NiO film grown on the carbon fiber surface is characterized by a complex cellular morphology, organized by partially layered individual nanosheets of about 4–5 nm thickness and lateral dimensions up to 1–2 μm, some edges and folds of which are located vertically relative to the carbon fiber surface. The surface of the obtained material was also examined using atomic force microscopy (AFM), and the electronic work function of the oxide shell surface was evaluated using the Kelvin probe force microscopy (KPFM) method. The electrochemical parameters of the obtained flexible NiO/CP electrode were analyzed: the dependence of the specific capacitance on the current density was determined and the stability of the material during cycling was studied, which showed that the proposed approach is promising for manufacturing hierarchically organized electrodes for flexible supercapacitors. Full article

The chemoresistive properties of multilayer titanium-containing Ti₂CT_x and Ti₃C₂T_x MXenes, synthesized by etching the corresponding MAX phases with NaF solution in hydrochloric acid, and the composites based on them, obtained by partial oxidation directly in a sensor cell in an air flow at 150 °C, were studied. Significant differences were observed for the initial MXenes, both in microstructure and in the composition of surface functional groups, as well as in gas sensitivity. For single Ti₂CT_x and Ti₃C₂T_x MXenes, significant responses to oxygen and ammonia were observed. For their partial oxidation at a moderate temperature of 150 °C, a high humidity sensitivity (T, RH = 55%) is observed for Ti₂CT_x and a high and selective response to oxygen for Ti₃C₂T_x at 125 °C (RH = 0%). Overall, these titanium-containing MXenes and composites based on them are considered promising as receptor materials for low temperature oxygen sensors. Full article

The hydrothermal synthesis of a nanosized NiCo₂O₄ oxide with several levels of hierarchical self-organization was studied. Using X-ray diffraction analysis (XRD) and Fourier-transform infrared (FTIR) spectroscopy, it was determined that under the selected synthesis conditions, a nickel-cobalt carbonate hydroxide hydrate of the composition M(CO₃)_0.5(OH)·0.11H₂O (where M–Ni²⁺ and Co²⁺) is formed as a semi-product. The conditions of semi-product transformation into the target oxide were determined by simultaneous thermal analysis. It was found by means of scanning electron microscopy (SEM) that the main powder fraction consists of hierarchically organized microspheres of 3–10 μm in diameter, and individual nanorods are observed as the second fraction of the powder. Nanorod microstructure was further studied by transmission electron microscopy (TEM). A hierarchically organized NiCo₂O₄ film was printed on the surface of a flexible carbon paper (CP) using an optimized microplotter printing technique and functional inks based on the obtained oxide powder. It was shown by XRD, TEM, and atomic force microscopy (AFM) that the crystalline structure and microstructural features of the oxide particles are preserved when deposited on the surface of the flexible substrate. It was found that the obtained electrode sample is characterized by a specific capacitance value of 420 F/g at a current density of 1 A/g, and the capacitance loss during 2000 charge–discharge cycles at 10 A/g is 10%, which indicates a high material stability. It was established that the proposed synthesis and printing technology enables the efficient automated formation of corresponding miniature electrode nanostructures as promising components for flexible planar supercapacitors. Full article

The hydrothermal synthesis of a hierarchically organized nanocomposite based on nickel–cobalt carbonate hydroxide hydrate of composition M(CO₃)_0.5(OH)·0.11H₂O (where M is Ni²⁺ and Co²⁺) and nickel–cobalt layered double hydroxides (NiCo-LDH) was studied. Using synchronous thermal analysis (TGA/DSC), it was determined that the material retained thermal stability up to 200 °C. The crystal structure of the powder and the set of functional groups in its composition were determined by X-ray diffraction analysis (XRD) and Fourier transform infrared spectroscopy (FTIR). The resulting hierarchically organized nanopowder was employed as a functional ink component for microplotter printing of an electrode film, which is an array of miniature planar structures with a diameter of about 140 μm, on the surface of a nickel-plated steel substrate. Using scanning electron microscopy (SEM), it was established that the main area of the electrode “pixels” represents a thin film of individual nanorods with periodic inclusions of larger hierarchically organized spherical formations. According to atomic force microscopy (AFM) data, the mean square roughness of the material surface was 28 nm. The electrochemical properties of the printed composite film were examined; in particular, the areal specific capacitance at different current densities was calculated, and the electrochemical kinetics of the material was studied by impedance spectroscopy. It was found that the electrode material under study exhibited relatively low R_s and R_ct resistance, which indicates active ion transfer at the electrode/electrolyte interface. Full article