Oxidation of Ceramic Materials Based on HfB2-SiC under the Influence of Supersonic CO2 Jets and Additional Laser Heating.

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-CG 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).

Gas-Sensitive Properties of ZnO/Ti2CTx Nanocomposites.

At present, a new class of 2D nanomaterials, MXenes, is of great scientific and applied interest, and their application prospects are very broad, including as effective doping components for receptor materials of MOS sensors. In this work we have studied the influence on the gas-sensitive properties of nanocrystalline zinc oxide synthesized by atmospheric pressure solvothermal synthesis, with the addition of 1-5% of multilayer two-dimensional titanium carbide Ti2CTx, obtained by etching Ti2AlC with NaF solution in hydrochloric acid. It was found that all the obtained materials have high sensitivity and selectivity with respect to 4-20 ppm NO2 at a detection temperature of 200 °C. It is shown that the selectivity towards this compound is best for the sample containing the highest amount of Ti2CTx dopant. It has been found that as the MXene content increases, there is an increase in nitrogen dioxide (4 ppm) from 1.6 (ZnO) to 20.5 (ZnO-5 mol% Ti2CTx). reactions which the responses to nitrogen dioxide increase. This may be due to the increase in the specific surface area of the receptor layers, the presence of MXene surface functional groups, as well as the formation of the Schottky barrier at the interface between the phases of the components.

Low Temperature Chemoresistive Oxygen Sensors Based on Titanium-Containing Ti2CTx and Ti3C2Tx MXenes.

The chemoresistive properties of multilayer titanium-containing Ti2CTx and Ti3C2Tx 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 Ti2CTx and Ti3C2Tx 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 Ti2CTx and a high and selective response to oxygen for Ti3C2Tx 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.

Obtaining of ZnO/Fe2O3 Thin Nanostructured Films by AACVD for Detection of ppb-Concentrations of NO2 as a Biomarker of Lung Infections.

ZnO/Fe2O3 nanocomposites with different concentration and thickness of the Fe2O3 layer were obtained by two-stage aerosol vapor deposition (AACVD). It was shown that the ZnO particles have a wurtzite structure with an average size of 51-66 nm, and the iron oxide particles on the ZnO surface have a hematite structure and an average size of 23-28 nm. According to EDX data, the iron content in the films was found to be 1.3-5.8 at.%. The optical properties of the obtained films were studied, and the optical band gap was found to be 3.16-3.26 eV. Gas-sensitive properties at 150-300 °C were studied using a wide group of analyte gases: CO, NH3, H2, CH4, C6H6, ethanol, acetone, and NO2. A high response to 100 ppm acetone and ethanol at 225-300 °C and a high and selective response to 300-2000 ppb NO2 at 175 °C were established. The effect of humidity on the magnitude and shape of the signal obtained upon NO2 detection was studied.

Application of Titanium Carbide MXenes in Chemiresistive Gas Sensors.

The titanium carbide MXenes currently attract an extreme amount of interest from the material science community due to their promising functional properties arising from the two-dimensionality of these layered structures. In particular, the interaction between MXene and gaseous molecules, even at the physisorption level, yields a substantial shift in electrical parameters, which makes it possible to design gas sensors working at RT as a prerequisite to low-powered detection units. Herein, we consider to review such sensors, primarily based on Ti3C2Tx and Ti2CTx crystals as the most studied ones to date, delivering a chemiresistive type of signal. We analyze the ways reported in the literature to modify these 2D nanomaterials for (i) detecting various analyte gases, (ii) improving stability and sensitivity, (iii) reducing response/recovery times, and (iv) advancing a sensitivity to atmospheric humidity. The most powerful approach based on designing hetero-layers of MXenes with other crystals is discussed with regard to employing semiconductor metal oxides and chalcogenides, noble metal nanoparticles, carbon materials (graphene and nanotubes), and polymeric components. The current concepts on the detection mechanisms of MXenes and their hetero-composites are considered, and the background reasons for improving gas-sensing functionality in the hetero-composite when compared with pristine MXenes are classified. We formulate state-of-the-art advances and challenges in the field while proposing some possible solutions, in particular via employing a multisensor array paradigm.

Investigation of the Effect of Supersonic Flow of Dissociated Nitrogen on ZrB2-HfB2-SiC Ceramics Doped with 10 vol.% Carbon Nanotubes.

The method of fabricating dense ultra-high temperature ceramic materials ZrB2−HfB2−SiC−CCNT was developed using a combination of sol-gel synthesis and reaction hot pressing approaches at 1800 °C. It was found that the introduction of multilayer nanotubes (10 vol.%) led to an increase in the consolidation efficiency of ceramics (at temperatures > 1600 °C). The obtained ZrB2−HfB2−SiC and ZrB2−HfB2−SiC−CCNT materials were characterized by a complex of physical and chemical analysis methods. A study of the effects on the modified sample ZrB2−HfB2−SiC−CCNT composition speed flow of partially dissociated nitrogen, using a high-frequency plasmatron, showed that, despite the relatively low temperature established on the surface (≤1585 °C), there was a significant change in the chemical composition and surface microstructure: in the near-surface layer, zirconium−hafnium carbonitride, amorphous boron nitride, and carbon were present. The latter caused changes in crucial characteristics such as the emission coefficient and surface catalyticity.

Atmospheric Pressure Solvothermal Synthesis of Nanoscale SnO2 and Its Application in Microextrusion Printing of a Thick-Film Chemosensor Material for Effective Ethanol Detection.

The atmospheric pressure solvothermal (APS) synthesis of nanocrystalline SnO2 (average size of coherent scattering regions (CSR)-7.5 ± 0.6 nm) using tin acetylacetonate as a precursor was studied. The resulting nanopowder was used as a functional ink component in microextrusion printing of a tin dioxide thick film on the surface of a Pt/Al2O3/Pt chip. Synchronous thermal analysis shows that the resulting semiproduct is transformed completely into tin dioxide nanopowder at 400 °C within 1 h. The SnO2 powder and the resulting film were shown to have a cassiterite-type structure according to X-ray diffraction analysis, and IR spectroscopy was used to establish the set of functional groups in the material composition. The microstructural features of the tin dioxide powder were analyzed using scanning (SEM) and transmission (TEM) electron microscopy: the average size of the oxide powder particles was 8.2 ± 0.7 nm. Various atomic force microscopy (AFM) techniques were employed to investigate the topography of the oxide film and to build maps of surface capacitance and potential distribution. The temperature dependence of the electrical conductivity of the printed SnO2 film was studied using impedance spectroscopy. The chemosensory properties of the formed material when detecting H2, CO, NH3, C6H6, C3H6O and C2H5OH, including at varying humidity, were also examined. It was demonstrated that the obtained SnO2 film has an increased sensitivity (the sensory response value was 1.4-63.5) and selectivity for detection of 4-100 ppm C2H5OH at an operating temperature of 200 °C.

Microextrusion Printing of Hierarchically Structured Thick V2O5 Film with Independent from Humidity Sensing Response to Benzene.

The process of V2O5 oxide by the combination of sol-gel technique and hydrothermal treatment using heteroligand [VO(C5H7O2)2-x(C4H9O)x] precursor was studied. Using thermal analysis, X-ray powder diffraction (XRD) and infra-red spectroscopy (IR), it was found that the resulting product was VO2(B), which after calcining at 300 °C (1 h), oxidized to orthorhombic V2O5. Scanning electron microscopy (SEM) results for V2O5 powder showed that it consisted of nanosheets (~50 nm long and ~10 nm thick) assembled in slightly spherical hierarchic structures (diameter ~200 nm). VO2 powder dispersion was used as functional ink for microextrusion printing of oxide film. After calcining the film at 300 °C (30 min), it was found that it oxidized to V2O5, with SEM and atomic force microscopy (AFM) results showing that the film structure retained the hierarchic structure of the powder. Using Kelvin probe force microscopy (KPFM), the work function value for V2O5 film in ambient conditions was calculated (4.81 eV), indicating a high amount of deficiencies in the sample. V2O5 film exhibited selective response upon sensing benzene, with response value invariable under changing humidity. Studies of the electrical conductivity of the film revealed increased resistance due to high film porosity, with conductivity activation energy being 0.26 eV.

Printing Technologies as an Emerging Approach in Gas Sensors: Survey of Literature.

Herein, we review printing technologies which are commonly approbated at recent time in the course of fabricating gas sensors and multisensor arrays, mainly of chemiresistive type. The most important characteristics of the receptor materials, which need to be addressed in order to achieve a high efficiency of chemisensor devices, are considered. The printing technologies are comparatively analyzed with regard to, (i) the rheological properties of the employed inks representing both reagent solutions or organometallic precursors and disperse systems, (ii) the printing speed and resolution, and (iii) the thickness of the formed coatings to highlight benefits and drawbacks of the methods. Particular attention is given to protocols suitable for manufacturing single miniature devices with unique characteristics under a large-scale production of gas sensors where the receptor materials could be rather quickly tuned to modify their geometry and morphology. We address the most convenient approaches to the rapid printing single-crystal multisensor arrays at lab-on-chip paradigm with sufficiently high resolution, employing receptor layers with various chemical composition which could replace in nearest future the single-sensor units for advancing a selectivity.

Pen plotter printing of ITO thin film as a highly CO sensitive component of a resistive gas sensor.

In2O3-10%SnO2 (ITO) thin films on various substrates have been obtained by pen plotter printing using a solution of hydrolytically active heteroligand complexes [M(C5H7O2)x(C4H9O)y] (where М = In3+ and Sn4+) as a functional ink. According to XRD and Raman spectroscopy, it has been established that the film has a bixbyite structure (space group Ia3/Th7), consists of particles with an average size of about 20 nm (according to SEM and AFM) and has a band gap of 3.57 eV. In order to obtain the ITO film, the temperature dependence of resistivity characterised by a minimum at 150 °C has been determined, and its gas-sensitive properties have been studied. It has been shown that the greatest resistive response is observed to carbon monoxide at 200 °C, and the film has a high sensitivity to low concentrations of CO. Two possible models describing the dependence of the sensory response on the CO concentration have been suggested. The mechanisms of defect formation in the ITO film structure and CO detection, including in a humid environment, have been considered in detail.

Microplotter-Printed On-Chip Combinatorial Library of Ink-Derived Multiple Metal Oxides as an "Electronic Olfaction" Unit.

Information about the surrounding atmosphere at a real timescale significantly relies on available gas sensors to be efficiently combined into multisensor arrays as electronic olfaction units. However, the array's performance is challenged by the ability to provide orthogonal responses from the employed sensors at a reasonable cost. This issue becomes more demanded when the arrays are designed under an on-chip paradigm to meet a number of emerging calls either in the internet-of-things industry or in situ noninvasive diagnostics of human breath, to name a few, for small-sized low-powered detectors. The recent advances in additive manufacturing provide a solid top-down background to develop such chip-based gas-analytical systems under low-cost technology protocols. Here, we employ hydrolytically active heteroligand complexes of metals as ink components for microplotter patterning a multioxide combinatorial library of chemiresistive type at a single chip equipped with multiple electrodes. To primarily test the performance of such a multisensor array, various semiconducting oxides of the p- and n-conductance origins based on pristine and mixed nanocrystalline MnOx, TiO2, ZrO2, CeO2, ZnO, Cr2O3, Co3O4, and SnO2 thin films, of up to 70 nm thick, have been printed over hundred µm areas and their micronanostructure and fabrication conditions are thoroughly assessed. The developed multioxide library is shown to deliver at a range of operating temperatures, up to 400 °C, highly sensitive and highly selective vector signals to different, but chemically akin, alcohol vapors (methanol, ethanol, isopropanol, and n-butanol) as examples at low ppm concentrations when mixed with air. The suggested approach provides us a promising way to achieve cost-effective and well-performed electronic olfaction devices matured from the diverse chemiresistive responses of the printed nanocrystalline oxides.

H2O2 induced formation of graded composition sodium-doped tin dioxide and template-free synthesis of yolk-shell SnO2 particles and their sensing application.

Sodium peroxostannate nanoparticles with graded composition were produced from aqueous hydrogen peroxide-sodium hydroxostannate solution. The uniform particles were converted to composition graded sodium stannate by mild thermal treatment for peroxide decomposition and yielded yolk-shell tin dioxide particles by dilute acid treatment. The mechanism of formation of the graded sodium concentration is explained in view of the solubility of peroxostannate in H2O2-H2O solution and based on 119Sn NMR, XRD, dynamic light scattering (DLS) and electron microscopy studies. Initial studies illuminating sensitive hydrogen sensing by yolk-shell tin oxide particles are presented.