Gas Atmosphere Research Articles

Study on Al Evaporation during AlV55 Melting and Alloy Preparation

Vacuum induction melting is a commonly used method for preparing AlV55 alloys. However, this method results in high Al evaporation losses, leading to poor cost control. Moreover, the influence of the process parameters on the alloy composition remains unclear. In this study, the evaporation pattern of Al in the melting and preparation processes of AlV55 alloys is studied, and measures for controlling the system pressure are proposed to effectively reduce Al evaporation. The experimental results show that smelting under an Ar gas atmosphere of 2000 Pa can reduce the evaporation loss of Al from 11.48% under vacuum conditions (60 Pa) to 0.58% of the amount of raw material added, effectively improving the metal utilization rate and reducing production costs. The influence of various process parameters on the alloy composition and Al volatilization are investigated to enable the effective control of the compositional uniformity and impurity content of the resulting alloys. Under optimal conditions, the impurity contents of C, O, and N are 0.03%, 0.0073%, and 0.013%, respectively; this reduces the amount of Al lost by evaporation compared to conventional methods, and the alloy produced meets commercial standards.

Quantitative classification of equipment protection level in concept of zone classification with downtime of protective measures

Handling flammable materials has a risk that can create explosive atmosphere. Such locations are classified as hazardous area, depending on the frequency and volume of explosive atmospheres. General electrical equipment is prohibited in the hazardous area and explosion protected equipment is used. These are the spatial isolation of ignition sources within the fire triangle, but even time isolation can prevent explosion hazards as well. It is proposed a method to classify zones from the calculation of downtime of protective measures based on failure rates and repair times of protective measures related to ventilation in explosive gas atmospheres with the Functional Safety under the time isolation concept. On the other hand, according to International Electrotechnical Commission (IEC) System for Certification to Standards Relating to Equipment for Use in Explosive Atmospheres, explosion protected equipment was also classified as an Equipment Protection Level, "EPL". The EPL should be selected according to the risk of the hazardous area. However, in IEC60079–14:2013, users can choose any EPL according to their risk assessment. In this paper, we clarify the quantitative risk difference of EPLs with expansion the idea of classifying hazardous area from the downtime of protective measures and introduce a means to quantitatively classify the EPL of equipment in an explosive gas atmosphere.

Nitrogen-fixing organelle in a marine alga.

Symbiotic interactions were key to the evolution of chloroplast and mitochondria organelles, which mediate carbon and energy metabolism in eukaryotes. Biological nitrogen fixation, the reduction of abundant atmospheric nitrogen gas (N2) to biologically available ammonia, is a key metabolic process performed exclusively by prokaryotes. Candidatus Atelocyanobacterium thalassa, or UCYN-A, is a metabolically streamlined N2-fixing cyanobacterium previously reported to be an endosymbiont of a marine unicellular alga. Here we show that UCYN-A has been tightly integrated into algal cell architecture and organellar division and that it imports proteins encoded by the algal genome. These are characteristics of organelles and show that UCYN-A has evolved beyond endosymbiosis and functions as an early evolutionary stage N2-fixing organelle, or "nitroplast."

Influence of reduction parameters on, weight loss, crack propagation, iron whisker growth, and morphological characteristics of directly reduced iron produced by non-contact direct reduction process

Non-contact direct reduction (NDR) is an alternative technique in iron and steelmaking. Direct reduced iron production (DRI) uses it. To further harness the metallurgical and operational capabilities of the process for its suitability as an alternative feed in the blast furnace process, there is a need to study the effect of the reduction parameters on the weight loss, crack propagation level, gas porosity, iron whisker growth, and morphological characteristics of the DRI. Thus, this paper attempts to utilize the NDR process using commercially acquired goethite-hematite ore in a carbon-monoxide atmosphere from wood charcoal under specified isothermal conditions, with a reduction temperature range from 570, 800, and 1000 °C. The effect of reduction parameters on weight loss, crack propagation, iron whisker growth, and morphological properties of the DRI was investigated using standardized reduction reaction practices under a nitrogen gas atmosphere with a flow rate of 120 mL. Mineralogical and morphological analyses of the direct reduced iron (DRI) and charcoal were performed using XRF and SEM/ED analysis. Proximate and ultimate analyses of the reductant were performed to ascertain their physical and chemical properties. The results show that reduction parameters tremendously influence the weight loss, crack propagation, gas porosity level, and metallurgical quality of the DRI. The reduction degree and swelling extent of the DRI also increase with crack propagation and iron whisker growth. Thus, the overall reduction mechanism still follows the usual stepwise chronological reduction order (Fe2O3 → Fe3O4 → FeO → Fe) regardless of temperature, with ash layer control being the reaction rate control. The NDR technique shows no carbon deposition in the DRI metal matrix. It indicates that this approach can serve as a viable alternative for DRI production in the ironmaking process.

Effect of background gas composition on the stoichiometry and lithium ion conductivity of pulse laser deposited epitaxial lithium lanthanum tantalate (Li3xLa1/3−xTaO3)

Lithium lanthanum tantalate (Li3xLa1/3−xTaO3, x = 0.075) thin films were grown via pulsed laser deposition using background gas atmospheres with varying partial pressures of oxygen and argon. The background gas composition was varied from 100% to 6.6% oxygen, with the pressure fixed at 150 mTorr. The maximum ion conductivity of 1.5 × 10−6 S/cm was found for the film deposited in 100% oxygen. The ion conductivity of the films was found to decrease with reduced oxygen content from 100% to 16.6% O2 in the background gas. The 6.6% oxygen background condition produced ion conductivity that approached that of the 100% oxygen condition film. The lithium transfer from the target to the film was found to decrease monotonically with decreasing oxygen content in the background gas but did not account for all changes in the ion conductivity. The activation energy of ion conduction was measured and found to correlate well with the measured ion conductivity trends. Analysis of x-ray diffraction results revealed that the films also exhibited a change in the lattice parameter that directly correlated with the ion conduction activation energy, indicating that a primary factor for determining the conductivity of these films is the changing size of the ion conduction bottleneck, which controls the activation energy of ion conduction.

Effect of Ar pressure on phase transition characteristics and charge transport mechanism in VO2 films grown by RF sputtering of V2O5

In this study, we report the growth and characterization of VO2 films deposited on YSZ (001) substrate employing RF magnetron sputtering of vanadium pentoxide (V2O5) target in pure Ar gas ambient. The VO2 film growth has been carried out at ∼700 °C for ∼15 min at ∼100 W RF power with a flow rate of ∼20 sccm at Ar gas deposition pressure of ∼3, ∼6, ∼20, and ∼40 mTorr. x-ray diffractometry and Raman spectroscopy show that the nearly pure VO2 phase achieved at lower Ar pressure, e.g., ∼3 and ∼6 mTorr transform into a multiphase V-O system at ∼20 and ∼40 mTorr. This pattern is also supported by the electrical transport measurements through the occurrence of hysteretic IMT in films grown at ∼3 and ∼6 mTorr and the absence of this signature in these films deposited at ∼20 and ∼40 mtorr Ar pressure. The most pronounced with the strongest hysteresis is seen in the Y6 film, and therefore, the optimum growth pressure in the present study is ∼6 mTorr. The suppression of IMT in Y20 and Y40 is attributed to the appearance of other V-O phases, which smear out the phase transition. The activation energy of the insulating phase is estimated from the Arrhenius fit to the ρ-T data is found to be ∼0.223 eV at ∼3 mTorr Ar pressure which increases to ∼0.311 eV for ∼6 mTorr film in the cooling cycle. The low temperature (120K≤T≤300K) transport conduction in all VO2 films is governed by Efros-Shklovskii’s variable range hopping (ES-VRH) mechanism with a systematic relation between growth conditions and phase transition characteristics. Thus, Argon gas pressure plays a critical role in growth and brings out the feasibility of VO2 films growth by RF sputtering of oxide V2O5 target under Argon ambient only.

Evidence that Eddington ratio depends upon a supermassive black hole's mass and redshift: Implications for radiative efficiency

Abstract Presently, it is unclear whether the Eddington ratio (λ) and radiative efficiency (ε) depend on a supermassive black hole's (SMBH's) redshift z and mass MBH. We attempt to resolve this issue using published data for 132,000 SMBHs with MBH ≥107 Msun (solar masses) at ∼ 0.1< z <2.4 covering ∼10 billion years of cosmic time, with MBH determined using Mg-II lines and bolometric luminosities Lbol based on a weighted mean of Lbol from 2 or more monochromatic luminosities and a single uniformly applied correction factor. The SMBHs are sorted into 7 MBH bins separated from each other by half an order of magnitude. The λ and z data in each bin are subjected to spline regression analysis. The results unambiguously show that for similar-size SMBHs, λ decreases as z decreases and that for a given redshift, larger SMBHs have a lower λ. These findings require that either a SMBH's accretion rate and/or its radiative efficiency be a function of z and MBH and, in the context of the Bondi accretion model, imply that radiative efficiency is an inverse function of z and MBH. These findings suggest that SMBHs become less efficient (higher ε) in accreting gases as the ambient gas density decreases with z and that larger SMBHs are more efficient (lower ε) than smaller ones. The results leave little doubt that the current widespread practice of assigning ε a standard value is untenable and gives erroneous estimates of accretion rates and growth times of SMBHs.

A Comparative Study of the Life Cycle Inventory of Thermally Sprayed WC-12Co Coatings

In this research, a life cycle inventory (LCI) is developed for tungsten carbide–cobalt (WC-Co) coatings deposited via atmospheric plasma spray (APS), high-velocity oxy-fuel (HVOF), and cold gas spray (CGS) techniques. For the APS process, a mixture of Ar/H2 was used, while the HVOF process was fueled by H2. The carrier gas for CGS was N2. This study aims to determine and quantify the inputs (consumption of inputs and materials) and outputs (emissions to air, soil, water, and waste generation) that could be used in the life cycle analysis (LCA) of these processes. The dataset produced will allow users to estimate the environmental impacts of these processes using WC-Co feedstock powder. To obtain a complete and detailed LCI, measurements of electrical energy, gas, WC-CO powder, and alumina powder consumption were performed (the use of alumina was for sandblasting). Furthermore, emissions like carbon dioxide (CO2), carbon monoxide (CO), and noise were also measured. This practice allowed us to determine the input/output process quantities. For the first time, it was possible to obtain LCI data for the APS, HVOF, and CGS deposition processes using WC-12Co as a feedstock powder, allowing access to the LCI data to a broader audience. Comparisons were made between APS, HVOF, and CGS processes in terms of consumption and emissions. It was determined that the APS process consumes more electrical energy and that its deposition efficiency is higher than the other processes, while the HVOF process consumes a large amount of H2, which makes the process costlier. CGS has comparatively low electricity consumption, high N2 consumption, and low deposition efficiency. The APS, HVOF, and CGS processes analyzed in this study do not emit CO, and CO2 emissions are negligible.

Origin of extended visible light absorption in nitrogen-doped CuTa2O6 perovskites: the role of copper defects

Abstract The optical band gap of the semiconductor CuTa2O6, synthesised via solid-state reaction, can be greatly reduced by annealing in ammonia, which leads to a significant red-shift of the visible light absorption. Using X-ray photoelectron spectroscopy (XPS), we have shown that this absorption extension does not result from the incorporation of nitrogen, but can be attributed to copper defects formed under the reducing conditions of ammonia treatment. Photocatalytic hydrogen evolution experiments were used to investigate the influence of these defects on the photocatalytic performance. We have further shown that CuTa2O6 with similar increased visible light absorption can be prepared by annealing with an organic reducing agent – sodium citrate – in inert gas atmosphere.

Investigation of thermodynamic state evolution, phase transition and mixing of the hydrocarbon droplet in convective supercritical environments

Liquid alkane droplets transit to supercritical fluids under supercritical conditions. However, the subsequent thermodynamic state evolution, phase transition, and mixing in this case, which directly influences combustion in actual diesel engines, need further investigation. We studied the thermodynamic state evolution, phase transition, and mixing of n-heptane droplets in supercritical laminar nitrogen streams through a set of conservation equations. Non-ideal thermodynamic and transport properties were calculated to incorporate the dramatic variation of physical properties in the supercritical regime. We focus on the effects of Reynolds number, ambient pressure, and droplet size on the thermodynamic state evolution, phase transition and mixing of droplets. Results showed that increasing the Reynolds number can accelerate the evolution of thermodynamic states, which leads to the faster transition of droplets from the liquid phase and supercritical fluids to gas-phase mixtures. With the increased Reynolds number, the phase transition rate is increased and the mixing homogeneity is improved due to the enhanced mass transfer. Caused by the negative correlation between mass transfer and ambient pressure, in higher pressure, droplets after the supercritical transition require more time to transition from supercritical fluids to vapor-phase mixtures. Increasing ambient pressure inhibited the droplet phase transition process and deteriorated the mixing homogeneity, but no significant influence was shown on the droplet dynamic evolution process. Smaller droplets transition faster from liquid phase and supercritical fluids to vapor-phase mixtures. The faster thermodynamic state evolution and faster dynamic evolution process of the smaller droplet leads to its more rapid homogeneous mixing with ambient gasses, but the mode of dynamics is independent of the droplet size.

Do smallholders have a role to play in atmospheric greenhouse gas removal? Insights from western Kenya

Despite consensus by the international community on the need for setting targets for reducing emissions or enhancing removal of greenhouse gases, comprehensive strategies for enumerating smallholders’ roles are yet to be designed and tested. Moreover, although smallholders’ role as net greenhouse gas removers has been speculated, evidence of this fact is necessary. A survey of 380 smallholders involved in maize-farming system in four sub-counties of western Kenya representing heterogeneity in agro-ecological zones and postulated biomass cooking energy access and demand was undertaken. The objectives of the study included to i). determine the smallholders’ net role in GHGs’ removals; ii). assess the presence of differences in smallholders’ emission status for an identical system but in dissimilar agro-ecological zones iii). relate atmospheric GHG removers to households' socio-economic characteristics and iv). enumerate a cost-effective approach of estimating atmospheric greenhouse gases emissions and removal. Greenhouse gas emissions and removal activity data, default emission factors and outcomes of allometric equations based on existing secondary sources were incorporated into the primary database from the survey to derive smallholders’ roles as net GHG removers. Economic models were used to derived the drivers of GHG removal. Most smallholders were net CO2e removers with differences among them and the sampled sub-counties associated with intensity in adoption of the maize-agroforestry system and levels of biomass utilization. Net CO2e removers significantly (p < 0.05) reared more livestock, utilized more fertilizer at both planting and topdressing stages, reported higher maize yields and had planted more trees than those who were net dischargers. Factors influencing the probability (p < 0.05) of smallholder being a net carbon remover included household size, maize yield, land owned and adoption of energy saving cooking stoves. Emission mitigation practices reported were compatible with food security, agricultural commercialization and the welfare enhancing production operations. The approach adopted in enumerating greenhouse gases emission and removal could highly enhance developing countries reporting the National GHG's Inventories and pin-point intervention options.

Acceleration of daily land temperature extremes and correlations with surface energy fluxes

Assessment of climate reanalysis data for land (ECMWF Re-Analysis v5; ERA5-Land) covering the last seven decades reveals regions where extreme daily mean temperatures are rising faster than the average rate of temperature rise of the 6 months of highest background warmth. However, such extreme temperature acceleration is very heterogeneous, occurring only in some places including regions of Europe, the western part of North America, parts of southeast Asia and much of South America. An ensemble average of Earth System Models (ESMs) over the same period also shows acceleration across land areas, but this enhancement is much more spatially uniform in the models than it is for ERA5-Land. Examination of projections from now to the end of the 21st Century, with ESMs driven by the highest emissions Shared Socio-economic Pathway scenario (SSP585) of future changes to atmospheric greenhouse gases, also reveals larger warming during extreme days for most land areas. The increase in high-temperature extremes is driven by different processes depending on location. In northern mid-latitudes, a key driver is often a decrease in the evaporative fraction of the available energy, consistent with soil drying. By contrast, the acceleration of high-temperature extremes in tropical Africa is primarily due to increased available energy. These two drivers combine via the surface energy balance to equal the sensible heat flux, which we find is often strongly correlated with the areas where the acceleration of high-temperature extremes is largest.

GeV γ-ray emission in the field of young massive star cluster RCW 38

ABSTRACT We report the detection of γ-ray emission by the Fermi Large Area Telescope towards the young massive star cluster RCW 38 in the 1–500 GeV photon energy range. We found spatially extended GeV emission towards the direction of RCW 38, which is best modelled by a Gaussian disc of 0.23° radius with a significance of the extension of ∼11.4σ. Furthermore, the spatial correlation with the ionized and molecular gas content favours the hadronic origin of the γ-ray emission. The γ-ray spectrum of RCW 38 has a relatively hard photon index of 2.44 ± 0.03, which is similar to other young massive star clusters. We argue that the diffuse GeV γ-ray emission in this region likely originates from the interaction of accelerated protons in the stellar cluster with the ambient gas.

Assessing transient changes in the ocean carbon cycle during the last deglaciation through carbon isotope modeling

Abstract. Atmospheric carbon dioxide concentration (pCO2) has increased by approximately 80 ppm from the Last Glacial Maximum (LGM) to the early Holocene. The change in this atmospheric greenhouse gas is recognized as a climate system response to gradual change in insolation. Previous modeling studies suggested that the deglacial increase in atmospheric pCO2 is primarily attributed to the release of CO2 from the ocean. Additionally, it has been suggested that abrupt change in the Atlantic meridional overturning circulation (AMOC) and associated interhemispheric climate changes are involved in the release of CO2. However, understanding remains limited regarding oceanic circulation changes and the factors responsible for changes in chemical tracers in the ocean during the last deglaciation and their impact on atmospheric pCO2. In this study, we investigate the evolution of the ocean carbon cycle during the last deglaciation (21 to 11 ka BP) using three-dimensional ocean fields from the transient simulation of the MIROC 4m climate model, which exhibits abrupt AMOC changes similar to those observed in reconstructions. We investigate the reliability of simulated changes in the ocean carbon cycle by comparing the simulated carbon isotope ratios with sediment core data, and we examine potential biases and overlooked or underestimated processes in the model. Qualitatively, the modeled changes in atmospheric pCO2 are consistent with ice core records. For example, during Heinrich Stadial 1 (HS1), atmospheric pCO2 increases by 10.2 ppm, followed by a reduction of 7.0 ppm during the Bølling–Allerød (BA) period and then by an increase of 6.8 ppm during the Younger Dryas (YD) period. However, the model underestimates the changes in atmospheric pCO2 during these events compared to values derived from ice core data. Radiocarbon and stable isotope signatures (Δ14C and δ13C) indicate that the model underestimates both the activated deep-ocean ventilation and reduced efficiency of biological carbon export in the Southern Ocean and the active ventilation in the North Pacific Intermediate Water (NPIW) during HS1. The relatively small changes in simulated atmospheric pCO2 during HS1 might be attributable to these underestimations of ocean circulation variation. The changes in Δ14C associated with strengthening and weakening of the AMOC during the BA and YD periods are generally consistent with values derived from sediment core records. However, although the data indicate continuous increase in δ13C in the deep ocean throughout the YD period, the model shows the opposite trend. It suggests that the model either simulates excessive weakening of the AMOC during the YD period or has limited representation of geochemical processes, including marine ecosystem response and terrestrial carbon storage. Decomposing the factors behind the changes in ocean pCO2 reveals that variations in temperature and alkalinity have the greatest impact on change in atmospheric pCO2. Compensation for the effects of temperature and alkalinity suggests that the AMOC changes and the associated bipolar climate changes contribute to the decrease in atmospheric pCO2 during the BA and the increase in atmospheric pCO2 during the YD period.

Trialkoxysilane-Induced Iridium-Catalyzed para-Selective C–H Bond Borylation of Arenes

An ideal approach for the construction of aryl boron compounds is to selectively replace a C–H bond in arenes with a C–B bond, and controlling regioselectivity is one of the most challenging aspects of these transformations. Herein, we report an iridium-catalyzed trialkoxysilane protecting group-assisted regioselective C–H borylation of arenes, including derivatives of benzaldehydes, acetophenones, benzoic acids, benzyl alcohols, phenols, aryl silanes, benzyl silanes, and multi-functionalized aromatic rings are all well tolerated and gave the para -selective C–H borylation products in a short time without the requirement of inert gases atmosphere. The site-selective C–H borylation can be adjustable by installing the developed trialkoxysilane protecting group on different functional groups on one aromatic ring. Importantly, the preparation process of the trialkoxychlorosilane is efficient and scalable. Mechanistic and computational studies reveal that the steric hindrance of the trialkoxysilane protecting group plays a key role in dictating the para-selectivity.

Turbulent Fluxes and Evaporation/Sublimation Rates on Earth, Mars, Titan, and Exoplanets

AbstractTurbulent fluxes of heat, momentum, and humidity in the atmospheric boundary layer are pivotal to the evolution of geology, weather and climate, and the possibility of life. Here we extend recent advances in calculating these near‐surface turbulent fluxes in the Earth's atmospheric boundary layer to any terrestrial planetary body with an atmosphere. These improvements include: (a) incorporating Monin‐Obukhov similarity functions that encompass the entire range of atmospheric stability expected on terrestrial planetary bodies, (b) accounting for the additional shear associated with buoyant plumes under unstable conditions, (c) using surface renewal theory to calculate transfer rates within the interfacial layer adjacent to the surface, and (d) explicitly accounting for key humidity effects that become especially important when a volatile is more buoyant than the ambient gas (e.g., on Mars where H2O is lighter than CO2). We tested and validated our model using in situ data collected on Earth, Mars, and Titan under a wide range of atmospheric stability, pressure, and surface roughness conditions. The model shows up to 71% better agreement with measurements compared to methods commonly used on Mars for evaporation/sublimation. Compared to previous estimates for H2O ice on Mars, our model predicts up to 1.5–190x lower latent heat fluxes under stable atmospheric conditions (depending on the wind speed) and 1.78x higher latent heat fluxes under unstable conditions. Our results provide improved constraints on the stability of ice on Mars and will help determine whether ice can melt under present‐day conditions.

Climate Change and Stroke: A Topical Narrative Review.

The impacts of accumulating atmospheric greenhouse gases on the earth's climate are now well established. As a result, there have been increases in ambient temperatures and resultant higher frequency and duration of temperature extremes and other extreme weather events, which have been linked to a wide range of adverse health outcomes. This topical narrative review provides a summary of published evidence on the links between climate change and stroke. There is consistent evidence of associations between stroke incidence and mortality and increasing ambient temperature and air pollution. Associations have also been shown for changes in barometric pressure, wildfires, and desert dust and sandstorms, but current evidence is limited. Flooding and other extreme weather events appear to primarily cause service disruption, but more direct links to stroke may emerge. Synergies between dietary changes that reduce stroke risk and may also reduce carbon footprint are being explored. We also discuss the impact on vulnerable populations, proposed pathophysiologic mechanisms, mitigation strategies, and current research priorities. In conclusion, climate change increasingly impacts the stroke community, warranting elevated attention.

Longitudinal Variations of Mars Thermosphere CO2, Ar, N2, and O Densities From MAVEN: Dependencies on Species Mass, Solar Flux, and Local Time

AbstractMeasurements of CO2, Ar, N2, and O densities between 150 and 200 km from the Mars Atmosphere and Volatile Evolution Neutral Gas and Ion Mass Spectrometer during February 2015 to February 2022 are analyzed to provide a comprehensive analysis of their longitudinal wavenumber k = 1, 2, and 3 components. Variations in density amplitudes (Ak) with solar flux are marginally detectable during this period. The Ak binned and averaged in latitude, local solar time and Ls are referenced to diurnal‐ and zonal‐mean backgrounds in accord with how tides and stationary planetary waves (SPWs) are defined in theory and modeling. The resulting global Ak distributions are the interference patterns formed by superposition of diurnal tides, SPWs and/or semidiurnal tides; consequently, a simple dependence on species mass consistent with thermal expansion (diffusive equilibrium) that might exist for some individual wave components is obscured. Additionally, vertical winds likely contribute to deviations from diffusive equilibrium. Complementary analyses of the Mars Climate Database indicate that the major contributors to the Ak are DE2, SE1, DE1, and SPW1, 2, 3; support the absence of significant variability due to solar flux; and indicate a more well‐defined sensitivity to species mass. The Ak and their phases (longitudes of maxima) for the whole data set are available as part of Supporting Information S1.

Effect of long-term high-temperature exposure in a controlled atmosphere on mullite-corundum ceramics for advanced applications in nuclear power technology

This work investigates the behavior and properties of corundum and mullite ceramics at high temperatures and during very long exposures in a controlled atmosphere. The motivation of the work lies primarily in advanced applications in nuclear energy technology, specifically in the search for materials for helium-cooled reactors of the 4th generation. The studied ceramic samples Al63, Al70, Al76, Al96, Al100, where the number indicates the mass percentage of Al2O3, were exposed for 1000 h at a temperature of 900 °C in gaseous atmospheres of helium, nitrogen and air. The properties of the exposed and unexposed samples were evaluated by measuring the bending strength at room temperature (CMOR), hot modulus of rupture at high temperature (HMOR), and isothermal creep, BET method, and using electron microscopy. The color changes induced by the reducing atmosphere and temperature were also interpreted using a simple thermochemical model. The CMOR measurement results show that all ceramic samples do not significantly change their properties due to long-term exposure. The detected deviations in the exposed samples from the unexposed ones can be considered random caused by the inherent heterogeneous structure. A similar conclusion also applies in the case of the HMOR measurement results, except for the results for the Al63 sample, where the drop in the strength of the exposed samples by approx. 37 MPa (20 %) may be related with the highest proportion of polyvalent oxide admixtures of all studied ceramics, i.e. 1.01 wt % Fe2O3 and 0.52 wt % TiO2. The change in color indicates that the iron and titanium present are being reduced. However, none of the conducted experiments indicate that changes in the redox state associated with the formation of oxygen vacancies would significantly reduce strength at high temperatures or increase creep in mullite ceramics.

Advances in Atmospheric Radiation: Theories, Models, and Their Applications. Part I: Atmospheric Gas Absorption and Particle Scattering

Advances in Atmospheric Radiation: Theories, Models, and Their Applications. Part I: Atmospheric Gas Absorption and Particle Scattering