A cycle of works on manufacturing and studying laser and magnetooptical ceramics with a focus on their thermo-optical characteristics performed by the research team is analyzed. Original results that ...have not been published before such as measurements of the Verdet constant in the Zr:TAG, Re:MgAl2O4, and ZnAl2O4 ceramics are also presented.
MgAl2O4 samples were microwave sintered to near‐full density in rapid processes with heating rates on the order of 100°C/min and zero isothermal hold. The experiments were carried out using a ...gyrotron system for microwave processing of materials operating at a frequency of 24 GHz with a maximum power of 6 kW. In the regimes with a preset heating rate sustained by the automatically regulated microwave power, the maximum achieved density was about 95% of the theoretical value in pristine MgAl2O4 samples (maximum sintering temperature 1650°C) and about 97% in 1 wt.% Y2O3‐doped samples (1700°C). In the regimes with a fixed microwave power (about 3.5 kW), translucent spinel samples with a relative density above 99% were obtained at 1700°C. The duration of the high‐temperature stage of sintering was 1.5‐10 minutes. The suggested mechanism responsible for the enhanced densification involves development of a thermal instability and formation of transient liquid phases at grain boundaries. The estimated specific absorbed power in the samples during the high‐temperature stage of ultra‐rapid microwave sintering was 27‐80 W/cm3, similar to the values observed in dc field‐assisted flash sintering experiments.
A glycine-nitrate self-propagating high-temperature synthesis (SHS) was developed to produce composite MgO-Gd
2
O
3
nanopowders. The X-ray powder diffraction (XRD) analysis confirmed the SHS-product ...consists of cubic MgO and Gd
2
O
3
phases with nanometer crystallite size and retains this structure after annealing at temperatures up to 1200 °C. Near full dense high IR-transparent composite ceramics were fabricated by spark plasma sintering (SPS) at 1140 °C and 60 MPa. The in-line transmittance of 1 mm thick MgO-Gd
2
O
3
ceramics exceeded 70% in the range of 4–5 mm and reached a maximum of 77% at a wavelength of 5.3 mm. The measured microhardness HV0.5 of the MgO-Gd
2
O
3
ceramics is 9.5±0.4 GPa, while the fracture toughness (
K
IC
) amounted to 2.0±0.5 MPa·m
1/2
. These characteristics demonstrate that obtained composite MgO-Gd
2
O
3
ceramic is a promising material for protective infra-red (IR) windows.
This study discusses the usage of Venus gravity assist in order to choose and reaching any point on Venusian surface. The launch of a spacecraft to Venus during the launch windows of 2029 to 2031 is ...considered for this purpose. The constraints for the method are the re-entry angle and the maximum possible overload. The primary basis of the proposed strategy is to use the gravitational field of Venus to transfer the spacecraft to an orbit resonant to the Venusian one – with the aim of expanding accessible landing areas. Results of the current research show that this strategy provides an essential increase in accessible landing areas and, moreover, may provide an access to any point on the surface of Venus with a small increase in
required for launch from the Earth and in the flight duration. The comparison with the landing without using gravity assist near planet is also given.
Abstract
MgAl
2
O
4
samples were microwave sintered to near‐full density in rapid processes with heating rates on the order of 100°C/min and zero isothermal hold. The experiments were carried out ...using a gyrotron system for microwave processing of materials operating at a frequency of 24 GHz with a maximum power of 6
kW
. In the regimes with a preset heating rate sustained by the automatically regulated microwave power, the maximum achieved density was about 95% of the theoretical value in pristine MgAl
2
O
4
samples (maximum sintering temperature 1650°C) and about 97% in 1 wt.% Y
2
O
3
‐doped samples (1700°C). In the regimes with a fixed microwave power (about 3.5 kW), translucent spinel samples with a relative density above 99% were obtained at 1700°C. The duration of the high‐temperature stage of sintering was 1.5‐10 minutes. The suggested mechanism responsible for the enhanced densification involves development of a thermal instability and formation of transient liquid phases at grain boundaries. The estimated specific absorbed power in the samples during the high‐temperature stage of ultra‐rapid microwave sintering was 27‐80 W/cm
3
, similar to the values observed in dc field‐assisted flash sintering experiments.
Carbon monoxide is a non‐condensable gas in the Martian atmosphere produced by the photolysis of CO2. Its abundance responds to the condensation and sublimation of CO2 from the polar caps, resulting ...in seasonal variations of the CO mixing ratio. ACS onboard the ExoMars Trace Gas Orbiter have measured CO in infrared bands by solar occultation. Here we provide the first long‐term monitoring of the CO vertical distribution at the altitude range from 0 to 80 km for 1.5 Martian years from Ls = 163° of MY34 to the end of MY35. We obtained a mean CO mixing ratio of ∼960 ppmv at latitudes from 45°S to 45°N and altitudes below 40 km, mostly consistent with previous observations. We found a strong enrichment of CO near the surface at Ls = 100–200° in high southern latitudes with a layer of 3,000–4,000 ppmv, corresponding to local depletion of CO2. At equinoxes we found an increase of the CO mixing ratio above 50 km to 3,000–4,000 ppmv at high latitudes of both hemispheres explained by the downwelling flux of the Hadley circulation on Mars, which drags the CO enriched air. General circulation models tend to overestimate the intensity of this process, bringing too much CO. The observed minimum of CO in the high and mid‐latitudes southern summer atmosphere amounts to 700–750 ppmv, agreeing with nadir measurements. During the global dust storm of MY34, when the H2O abundance peaks, we see less CO than during the calm MY35, suggesting an impact of HOx chemistry on the CO abundance.
Plain Language Summary
Carbon monoxide (CO) is a product of the photodissociation of the main component of the Martian atmosphere, CO2 (96%). During the polar night on Mars, the temperature drops so low that ∼30% of the global atmospheric carbon dioxide freezes out, condensed into snowflakes. They settle to the ground to form a seasonal polar cap, which sublimates in spring. Unlike CO2, CO is not a condensable species, so its relative abundance increases when CO2 and the atmospheric pressure drops, forming a prominent seasonal cycle. This behavior of CO was well documented by spacecraft measurements of column abundances, while its vertical distribution remained poorly known until recently. Still, it helps to separate the impacts of atmospheric dynamics and chemistry, addressing the problem of the Martian atmosphere's stability. We present the first climatology of the CO vertical distribution lasting more than one Martian year based on solar occultations by the ExoMars Trace Gas Orbiter. We report a previously unobserved CO layer near the surface in high southern latitudes in late winter and spring and document the difference between two Martian years caused by the global dust storm. These observations give new challenges to General Circulation Models for understanding the modern atmosphere of Mars.
Key Points
The first climatology of CO vertical profiles is collected for the period Ls = 163° of Martian Year 34 to the end of Martian Year 35
The mean CO volume mixing ratio is ∼960 ppmv at 0–35 km and 45°S–45°N
Strong enrichment of CO, >3,000 ppmv, is detected at 10–20 km at Ls = 100–200 in high southern latitudes and above 50 km at equinoxes near Poles
Temperature and density in the upper Martian atmosphere, above ∼100 km, are key diagnostic parameters to study processes of the species' escape, investigate the impact of solar activity, model the ...atmospheric circulation, and plan spacecraft descent or aerobraking maneuvers. In this paper, we report vertical profiling of carbon dioxide (CO2) density and temperature from the Atmospheric Chemistry Suite (ACS) solar occultations onboard the ExoMars Trace Gas Orbiter. A strong CO2 absorption band near 2.7 μm observed by the middle infrared spectrometric channel (ACS MIR) allows the retrieval of the atmospheric thermal structure in an unprecedentedly large altitude range, from 20 to 180 km. We present the latitudinal and seasonal climatology of the thermal structure for 1.5 Martian years (MYs), from the middle of MY 34 to the end of MY 35. The results show the variability of distinct atmospheric layers, such as a mesopause (derived from 70 to 145 km) and homopause, changing from 90 to 100 km at aphelion to 120–130 km at perihelion. Some short‐term homopause fluctuations are also observed depending on the dust activity.
Plain Language Summary
We report vertical distributions of the density and temperature in the Martian atmosphere in the altitude range from 20 to 180 km. This broad interval of heights embraces regions of the troposphere (<50 km), the mesosphere (50–100 km), and the thermosphere (>100 km). Knowledge of thermal structure in the middle and upper atmosphere (above 50 km) is unique for the Martian climate modeling, studying the atmospheric escape and the impact of solar activity, as well as planning spacecraft maneuvers. Our data are based on remote measurements of the carbon dioxide (CO2) absorption in the atmosphere at very high altitude resolution of one kilometer. Sensing this major component, 95% of the total density, in the infrared wavelength range allows us to derive the atmospheric temperature as well. We observe the climatology of different atmospheric layers depending on latitude and seasons for 1.5 Martian years, from May 2018 to January 2021. For example, we reveal extremely high variability of the coldest layer, mesopause, from 70 km in the southern winter to 150 km in the southern summer.
Key Points
Seasonal variability of the Martian atmospheric thermal structure at altitudes 20–180 km is reported from CO2 infrared spectroscopy
The mesopause altitude rises from 70 to 90 km in the high‐winter latitudes to 130–150 km in the summer season for both hemispheres
The homopause altitude varies from 90 km at aphelion to 130 km at perihelion in the Martian years 34 and 35, and it depends on dust activity
Based on the analysis of scientific literature and theoretical studies by constructing polythermal incisions of multicomponent systems, offered two of ligatures for the smelting of new alloys based ...on silver. The article presents the results of pilot studies on acquisition and processing of new alloys. The evaluation of the properties and structure of the obtained cast semi-finished products of the new silver jewelry alloys 925.
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