Nitrogen-rich Atmosphere Research Articles

In-situ synthesis of Yttria-based precipitates and their effects on Fe12Cr6Al in laser powder bed fusion

To advance the suitability of FeCrAl alloys for nuclear applications, radiation damage resistance, creep resistance, corrosion resistance, and mechanical properties enhancement are essential. A promising approach is the introduction of a high density of nitride and oxide nanoprecipitates within the alloy matrix. This study aimed to explore the impact of in-situ formed nitride and oxide precipitates on the properties of Fe12Cr6Al alloy by using a nitrogen-rich atmosphere and incorporating nano-sized Y2O3 (0.45 wt %) during laser powder bed fusion (LPBF). As a result of adding Y2O3 nanoparticles to the Fe12Cr6Al alloy, the grain size decreased from 70 μm to 40 μm, and the Al–O, Al–N, and Al–N–O precipitates were replaced by Y–O, Y–Al–O, and Y–N precipitates. Additionally, the nitrogen contents increased from 220 ppm to 470 ppm. The sample with Y2O3 exhibited higher mechanical properties, with yield strength, tensile strength, and hardness (HV) increasing by 80 MPa, 112 MPa, and 23 HV, respectively. This study deviates from previous research on Y2O3-containing FeCrAl alloys by investigating the impact of in-situ synthesized Y–N nitrides on Fe12Cr6Al. The objective is to elucidate the combined effects of these precipitates on the properties of Fe12Cr6Al alloy in comparison to literature-reported oxide dispersion-strengthened (ODS) FeCrAl alloys, which were fabricated in an argon (Ar) atmosphere.

Continuum absorption in pure N[formula omitted] gas and in its mixture with Ar

The N2−N2and N2−Arcontinuum absorption spectra are calculated using the classical trajectory-based simulation (CTS). The spectra obtained are validated by new measurements in the subTHz spectral range along with the previously reported data in the far infrared. A novelty of our approach consists in the use of the CTS method to simulate both the fundamental nitrogen absorption band and the rototranslational band in N2−Ar, i.e., we succeeded to step beyond the conventionally used approximation of the only rigid monomers. This extension of the theory made it possible, in particular, to demonstrate the validity of the rigid monomer assumption for the CTS simulation of the rototranslational N2−Arband. The broadband spectra within 77–354 GHz were measured using the resonator spectrometer at temperatures of 278–333 K and pressures of 900–1600 Torr. A minor underestimation of the calculated absorption by 3.7% and 5% is shown for the N2−N2and N2−Arsystem, respectively. On the basis of the obtained data, a new analytical model is developed for the N2−N2absorption in the subTHz range, which can be used in radiation propagation codes for the Earth, Titan, or other nitrogen-rich atmospheres. The advantage of the model proposed here over those previously published is discussed.

Adiabatic reactions on excited electronic states of N[formula omitted]O: First computations on the O([formula omitted]P)+N[formula omitted](X)[formula omitted]N([formula omitted]D)+NO(X) and O([formula omitted]P)+N[formula omitted](A)[formula omitted] N([formula omitted]S)+NO(X) reactions

Electronically excited nitrogen atoms and N2 molecules play a major role in nitrogen-rich atmospheres, specially under extreme conditions. Here we computationally explore several excited states of the N2O system and identify two new reactions involving the excited species N2(A) and N(2D) that can occur without electronic transitions. Their rate coefficients are estimated for the first time, and their implication to atmospheric models are discussed. We infer that the O(3P)+N2(A)→N(2D)+NO(X) channel should account for the total experimental NO production at room temperature.

Silicon nitride thin-films deposited by radiofrequency reactive sputtering: Refractive index optimization with substrate cooling in a nitrogen-rich atmosphere

Silicon nitride (SiN) is widely used as a core material in optical waveguides due to its optical properties. The deposition of SiN thin-films by radiofrequency (RF) reactive sputtering is commonly used in low-temperature processes, where the thin-films optical properties can be optimized by controlling the deposition parameters (sputtering power, gases ratio, etc.). This work presents the deposition of several SiN thin-films by RF reactive sputtering with different sputtering powers (ranging from 180 W to 300 W), with a nitrogen-argon ratio of 16:4, and performing substrate cooling in a nitrogen-rich atmosphere immediately after deposition, consisting in keeping the substrate under 16 sccm of nitrogen until it reaches 25 °C. The refractive indices of the SiN thin-films were assessed through ellipsometry, obtaining a maximum refractive index of 1.906 at 400 nm. SiN thin-films were also analyzed by energy-dispersive spectroscopy (EDS) and atomic force microscopy (AFM).

Microstructure Formations Resulting from Nanosecond and Picosecond Laser Irradiation of a Ti-Based Alloy under Controlled Atmospheric Conditions and Optimization of the Irradiation Process.

This paper presents a study and comparison of surface effects induced by picosecond and nanosecond laser modification of a Ti6Al4V alloy surface under different ambient conditions: air and argon- and nitrogen-rich atmospheres. Detailed surface characterization was performed for all experimental conditions. Damage threshold fluences for picosecond and nanosecond laser irradiation in all three ambient conditions were determined. The observed surface features were a resolidified pool of molten material, craters, hydrodynamic effects and parallel periodic surface structures. Laser-induced periodic surface structures are formed by multi-mode-beam nanosecond laser action and picosecond laser action. Crown-like structures at crater rims are specific features for picosecond Nd:YAG laser action in argon-rich ambient conditions. Elemental analysis of the surfaces indicated nitride compound formation only in the nitrogen-rich ambient conditions. The constituents of the formed plasma were also investigated. Exploring the impact of process control parameters on output responses has been undertaken within the context of laser modification under different environmental conditions. Parametric optimization of the nanosecond laser modification was carried out by implementing an advanced method based on Taguchi's parametric design and multivariate statistical techniques, and optimal settings are proposed for each atmosphere.

Fabrication of Cu2-xO thin films directly bondable to copper on aluminum nitride substrates using the sol-gel method

Controlling the oxygen content becomes a formidable challenge when using the traditional method of copper pre-oxidation to attach copper metal to AlN ceramic substrates. The issue results in copious amounts of copper oxide (CuO) formation. Excessive CuO causes voids to form at the interfacial junction, harming the long-term reliability of the copper-bonded substrate. To address these issues, this study employed the sol-gel technique, in which a copper-containing solution was applied to the AlN substrate via spin coating, followed by thermal treatment in a nitrogen-rich atmosphere. The goal was to create thin films of Cu2-xO to improve the adhesion between copper and AlN substrates. It was observed that the surface roughness of the resulting Cu2-xO thin film after 5 spin-coating cycles was the lowest (3.48 nm). Furthermore, at a fixed thickness of the Cu2-xO oxide layer (approximately 36 nm), insufficient oxygen content in the thin film heated at 250 °C prevented the formation of sufficient Cu–O eutectic liquid at high temperatures, resulting in insufficient copper-bonded substrate bonding strength (0.47 MPa). When the thin film was heated to 400 °C, the proportion of the CuO phase in the film increased, effectively increasing the bonding strength of the copper-bonded substrate (0.84 MPa).

Manufacturing of Corrosion-Resistant Surface Layers by Coating Non-Alloy Steels with a Polymer-Powder Slurry and Sintering.

This paper describes the combination of surface engineering and powder metallurgy to create a coating with improved corrosion resistance and wear properties. A new method has been developed to manufacture corrosion-resistant surface layers on steel substrate with additional carbide reinforcement by employing a polymer-powder slurry forming and sintering. The proposed technology is an innovative alternative to anti-corrosion coatings applied by galvanic, welding or thermal spraying techniques. Two different stainless-steel powders were used in the research. Austenitic 316 L and 430 L ferritic steel powders were selected for comparison. In addition, to improve resistance to abrasive wear, coatings containing an additional mixture of tetra carbides (WC, TaC, TiC, NbC) were applied. The study investigates the effects of using multicomponent polymeric binders, sintering temperature, and atmosphere in the sintering process, as well as the presence of reinforcing precipitation, microstructure and selected surface layer properties. Various techniques such as SEM, EDS, hardness and tensile tests and corrosion resistance analysis are employed to evaluate the characteristics of the developed materials. It has been proven that residual carbon content and nitrogen atmosphere cause the release of hard precipitations and thus affect the higher mechanical properties of the obtained coatings. The tensile test shows that both steels have higher strength after sintering in a nitrogen-rich atmosphere. Nitrogen contributes over 50% more to the tensile strength than an argon-containing atmosphere.

Nitrogen partitioning between silicate phases and aqueous fluid depends on concentration

The distribution of nitrogen in geologic systems is modulated by its partitioning between silicate (mineral and melt) and fluid phases. Under geologically applicable oxygen fugacity, pressure, and temperatures, nitrogen can be multiply-speciated, with N2 coexisting with reduced nitride (N−3) species. Non-polar, neutral species, including N2, tend to concentrate in fluids, while charged nitride species have a greater propensity to concentrate in silicate phases. The stoichiometry of converting N2 to single N atom nitride species implies that nitrogen speciation may depend on its concentration, and this leads to the hypothesis that the partitioning of nitrogen between silicate and fluid phases also depends on concentration, potentially biasing prior experimental work in doped systems and influencing the behavior of nitrogen in geologic systems. To test this hypothesis, we have completed a series high pressure (∼1.75 GPa, 800 °C) experiments that react minerals, melts, and fluids with variable nitrogen concentrations (3.1–17.1 wt% N). Our results imply order-of-magnitude-scale increases in mineral/melt and melt/fluid partitioning as nitrogen concentrations decrease within natural ranges. For example, decreasing the N concentration from 2500 to 2 ppm increases predicted Dmelt/fluidN values by over an order of magnitude at constant PT conditions. This means that loss of nitrogen from a degassing magma or dehydrating slab is a self-limiting process that becomes increasingly inefficient as nitrogen concentration falls. Despite this, nitrogen remains highly concentrated in the atmosphere, which receives N from fluids exsolved from slabs and magmas. To maintain a nitrogen-rich atmosphere we therefore suggest that warm and oxidizing conditions have prevailed over subduction zones because warm slabs dehydrate under lower pressures where nitrogen is more easily partitioned into fluids, and oxidizing conditions also promote nitrogen partitioning into fluids. Concentration-dependent partitioning of nitrogen will also serve to moderate any initial variations of N/K in slab materials upon dehydration, and this may help to explain the relatively uniform N/K ratio of MORB mantle. We supplement our nitrogen concentration experiments with a temperature series (1.5–2 GPa, 750–950 °C). Our temperature series data reveal that at high temperature nitrogen favors melts over fluids, while temperature has no resolvable effect of biotite-fluid partitioning.

1,3-Butadiene on Titan: Crystal Structure, Thermal Expansivity, and Raman Signatures

Titan is one of the most Earth-like objects in the Solar System, hosting a nitrogen-rich atmosphere and an impressive inventory of organics ranging from simple to complex. 1,3-Butadiene has been suggested to be one of the photochemical products that could be present on Titan’s surface. As the simplest conjugated hydrocarbon, it is expected to partake in many chemical processes that influence Titan’s geology and evolution. However, the crystal structure and thermal expansivity of 1,3-butadiene, two of its most fundamental properties, have remained largely unknown for over eight decades, especially at cryogenic conditions often encountered on planetary bodies. In this work, a series of powder X-ray diffraction and micro-Raman experiments have been conducted in order to elucidate the crystal structure and thermal behavior of solid 1,3-butadiene between 85 and 165 K. Specifically, a single crystalline phase (space group P21/c) is observed in the temperature range of study in which the butadiene molecules form layers of zigzag chains that are stabilized by an intricate network of intermolecular C–H···π interactions. This monoclinic structure is shown to exhibit significant anisotropic thermal expansion along the three directions with the majority occurring along the crystallographic b-axis. The calculated bulk density of 1,3-butadiene (0.877 g/cm3 at 150 K) is considerably lower than previously realized. These results help enhance our understanding of the putative molecular solids that make up Titan’s surface and could hold important implications for physical as well as mechanical processes occurring on this icy satellite.

Oxynitride-surface engineering of rhodium-decorated gallium nitride for efficient thermocatalytic hydrogenation of carbon dioxide to carbon monoxide

Upcycling of carbon dioxide towards fuels and value-added chemicals poses an opportunity to overcome challenges faced by depleting fossil fuels and climate change. Herein, combining highly controllable molecular beam epitaxy growth of gallium nitride (GaN) under a nitrogen-rich atmosphere with subsequent air annealing, a tunable platform of gallium oxynitride (GaN1-xOx) nanowires is built to anchor rhodium (Rh) nanoparticles for carbon dioxide hydrogenation. By correlatively employing various spectroscopic and microscopic characterizations, as well as density functional theory calculations, it is revealed that the engineered oxynitride surface of GaN works in synergy with Rh to achieve a dramatically reduced energy barrier. Meanwhile, the potential-determining step is switched from *COOH formation into *CO desorption. As a result, significantly improved CO activity of 127 mmol‧gcat−1‧h−1 is achieved with high selectivity of >94% at 290 °C under atmospheric pressure, which is three orders of magnitude higher than that of commercial Rh/Al2O3. Furthermore, capitalizing on the high dispersion of the Rh species, the architecture illustrates a decent turnover frequency of 270 mol CO per mol Rh per hour over 9 cycles of operation. This work presents a viable strategy for promoting CO2 refining via surface engineering of an advanced support, in collaboration with a suitable metal cocatalyst.

Investigation of crystallographic changes across the Cr/a-Si interface by X-ray absorption spectroscopy

X-ray Absorption Fine Structure spectroscopy (XAFS) has been employed to investigate the crystallographic changes that evolved across the Cr/a-Si interface as a function of annealing temperature. A stack of 400nm a-Si and50nm metallic Cr film is deposited onto a fused silica substrate by electron beam evaporation. Thesestacks are subjected to annealing temperature from 200°C to 700°C in nitrogen-rich atmosphere to facilitate the metal diffusion and subsequent phase formation across the interface. XAFS investigations were carried out in twoconfigurations to study the change in local chemistry of Cr indepth andtop layers of the stack simultaneously. XAFS analysis confirmed thatthe FSC stack annealed at 200°C has a dominant metallic Cr concentration along with a minority content of Cr2O3 andCr3Si. On increasing the annealing temperature, metallic Cr converted into Cr2O3 andCr3Si up to an intermediate temperature of 500°C and finally at 700°C it transformed to CrSi2. It is also confirmed from total reflection XAFSthatCr diffusion intoa-Si is the preferred phenomenon that takes place acrossthe Cr/a-Si interface. The diffusion of Cr and subsequent thermodynamic conditions required for the formation of different Cr phases are discussed to understand the factors that affect the Schottky properties of the Cr/a-Si interface.

A nitrogen-rich atmosphere on ancient Mars consistent with isotopic evolution models

The ratio of nitrogen isotopes in the Martian atmosphere is a key constraint on the planet's atmospheric evolution. However, enrichment of the heavy isotope expected due to atmospheric loss from sputtering and photochemical processes is greater than measurements. A massive, multi-bar early CO2-dominated atmosphere and recent volcanic outgassing have been proposed to explain this discrepancy, and many previous models have assumed atmospheric nitrogen rapidly reached a steady state where loss to space balanced volcanic outgassing. Here we show using time-dependent models that the abundance and isotopic composition of nitrogen in the Martian atmosphere can be explained by a family of evolutionary scenarios in which the initial partial pressure of nitrogen is sufficiently high that a steady state is not reached and nitrogen levels gradually decline to present-day values over 4 billion years. Our solutions do not require a multi-bar early CO2 atmosphere and are consistent with volcanic outgassing indicated by both geologic mapping and the atmospheric 36Ar/38Ar ratio. Monte Carlo simulations that include these scenarios estimate that the partial pressure of N2 was 60 - 740 mbar (90% confidence, with a median value of 310 mbar) at 3.8 billion years ago when the valley networks formed. We suggest that such a high nitrogen partial pressure could have contributed substantially to warming on early Mars.

Regulating Morphology and Composition of Laser-Induced Periodic Structures on Titanium Films with Femtosecond Laser Wavelength and Ambient Environment.

Recently, highly uniform thermochemical laser-induced periodic surface structures (TLIPSS) have attracted significant research attention due to their practical applicability for upscalable fabrication of periodic surface morphologies important for surface functionalization, diffraction optics, sensors, etc. When processed by femtosecond (fs) laser pulses in oxygen-containing environments, TLIPSS are formed on the material surface as parallel protrusions upon local oxidation in the maxima of the periodic intensity pattern coming from interference of the incident and scattered waves. From an application point of view, it is important to control both the TLIPSS period and nanoscale morphology of the formed protrusions that can be expectedly achieved by scalable shrinkage of the laser-processing wavelength as well as by varying the ambient environment. However, so far, the fabrication of uniform TLIPSS was reported only for near-IR wavelength in air. In this work, TLIPSS formation on the surface of titanium (Ti) films was systematically studied using near-IR (1026 nm), visible (513 nm) and UV (256 nm) wavelengths revealing linear scalability of the protrusion period versus the fs-laser wavelength. By changing the ambient environment from air to vacuum (10−2 atm) and pressurized nitrogen gas (2.5 atm) we demonstrate tunability of the composition and morphology of the Ti TLIPSS protrusions. In particular, Raman spectroscopy revealed formation of TiN together with dominating TiO2 (rutile phase) in the TLIPSS protrusions produced in the nitrogen-rich atmosphere.

Effect of gas atmosphere change on radical reaction and indicator gas release during coal oxidation

Coal spontaneous combustion (CSC) caused by coal oxidation is the leading cause of mine fires. The essence of most CSC preventive measures is to change an oxygen-containing atmosphere into a nitrogen-rich atmosphere for suppressing continuous coal oxidation. This study investigates the effect of gas atmosphere change on the micro-group reaction and gas product release during coal oxidation. To achieve this aim, the radical parameters and CO were measured with the aid of an electron spin resonance (ESR) spectrometer and a gas chromatograph. The following conclusions show that, after converting the gas atmosphere from dry air to nitrogen, the linewidth of the ESR spectrum drops suddenly and ultimately stabilizes at around 0.46. Besides, a lower conversion temperature corresponds to a higher turning point temperature and a smaller maximum value of the g factor, especially when the temperature is below 90℃. After the gas atmosphere is converted, the radical concentration drops suddenly and rises again. The second increase of radical concentration is significant when the conversion temperature exceeds 130℃. While the atmosphere changes, CO release falls abruptly, and then it continuously increases. Finally, the mechanism of the effect of gas atmosphere conversion on the radical reaction of coal was analyzed. The results could enrich the coal oxidation research and be applied in CSC management and control.

Facile synthesis of cadmium sulfide and the effect of thermal annealing in N2-rich atmosphere on its structural, morphological, chemical, and optical properties

This works aims to evaluate the effect of the annealing process under a nitrogen-rich atmosphere on the properties of cadmium sulfide (CdS). The CdS studied in this work was produced by a simple one-pot reaction between cadmium chloride and sodium sulfide in an aqueous solution. After the synthesis pathway, the powdered material was annealed in a furnace at different temperatures under an N2-rich atmosphere and the resulting samples were characterized to evaluate the transformations obtained. The X-ray diffraction results show that the annealing temperature has a major impact on the structural composition of the CdS, as new phases (CdO and CdSO4) are obtained at higher temperatures due to the oxidation of the samples. The scanning electron micrographs show that a higher annealing temperature leads to the coalescence of the CdS particles, as the spherical particles obtained without thermal treatment are transformed into a smooth surface at temperatures higher than 600 °C. The optical properties of the composite, such as bandgap energy and visible light absorption, can be modified using different annealing temperatures. The differential scanning calorimetry and thermogravimetry confirm the phase transformations observed, showing that the CdS is oxidized to CdO at temperatures above 700 °C.

Structure and Properties of Co-Cr-Mo Alloy Manufactured by Powder Injection Molding Method.

Cobalt–chromium–molybdenum alloys samples were obtained by the powder injection molding method (PIM). PIM is dedicated to the mass production of components and can manufacture several grades of dental screws, bolts, stabilizers, or implants. As a skeleton component, ethylene–vinyl acetate (EVA copolymer) with a low temperature of processing and softening point was used. The choice of a low-temperature binder made it necessary to use a coarse ceramic powder as a mechanical support of the green sample during sintering. The injection-molded materials were thermally degraded in N2 or Ar-5%H2 and further sintered in N2-5%H2 or Ar-5%H2 at 1300 or 1350 °C for 30 min. The structure of the obtained samples was characterized by X-ray diffraction and electron microscopy. Mechanical properties, including hardness and three-point bending tests, confirmed that a nitrogen-rich atmosphere significantly increases the bending strength compared to the material manufactured in Ar-5%H2. This is due to the precipitation of numerous fine nitrides and intermetallic phases that strengthen the ductile γ-phase matrix.

Raman and ultraviolet–visible spectroscopy of titanium chromium nitride thin films

ABSTRACT Titanium chromium nitride (TiCrN) films were prepared under various nitrogen flow rates (7–18 sccm) via co-sputtering. It was argued that the target poisoning effect governs the deposition process in nitrogen-rich atmospheres. The observed X-ray diffraction patterns suggested that the crystal structure of TiCrN coatings is a cubic close-packed (TiCr)N solid solution with (111) crystalline planes. The patterns also implied that the increased nitrogen content is accompanied by a tendency towards crystallite size enlargement. In addition to the main vibrational transitions, the micro-Raman spectroscopy detected the Raman third-order vibrational transition (2A + O) and a peak corresponding to one of the Raman second-order transitions (A + O). It was also revealed that the area under the peaks corresponding to chromium nitride first-order and second-order acoustic vibrational transitions increase with the increase in the nitrogen content of the samples. Ultraviolet–visible spectroscopy showed that the coatings can have a potential application in under-marine UV photodetector designs.

Thermal oxidation of oxynitride films as a strategy to achieve (Sr2Ta2O7)100-x(La2Ti2O7)x based oxide perovskite films with x = 1.65

•This study concerns STLTO compounds of the ferroelectric (Sr2Ta2O7)100-x(La2Ti2O7)x solid solution. The purpose is to produce the STLTO composition x = 1.65 as thin films by thermal oxidation of the corresponding oxynitride composition. Indeed, the combination of an STLTO oxide target with a dioxygen-rich reactive atmosphere during the sputtering deposition leads to Sr-deficient oxide thin films, shifting composition and structure from the perovskite to the tetragonal tungsten bronze type. An alternative synthesis pathway is to first deposit, under nitrogen-rich atmosphere, stoichiometric oxynitride films and produce, by thermal annealing under air, the stoichiometric oxide. For low oxidation temperatures ([550–600 °C]), samples remain intact and display an oxide character but still contain a significant amount of nitrogen, they could be described as intermediate phases containing nitrogen-nitrogen pairs as demonstrated by Raman. Dielectric characteristics of these original film materials are of interest with a tunability value of 26 % at 30 kV/cm (10 kHz).

Nitrogen enriched C:H:N:O thin films for improved antibiotics doping

In this study we investigate the ability of RF magnetron sputtered nylon 6,6 films (C:H:N:O) to store and release two model antibiotics, ampicillin and ciprofloxacin, that differ significantly in their chemical structure. We demonstrate that the addition of nitrogen to the deposition process leads to a substantial enhancement in the abundance of nitrogen-containing functional groups in the resulting coatings as well as to the significant changes in their swelling and dissolution. These variations were found to result in different impregnation and release capabilities of the produced coatings: an almost 5 times higher amount of released antibiotics was observed for the samples prepared in nitrogen-rich atmosphere when compared to films deposited in pure argon. It was also found that the amount of released antibiotics strongly depends on the natural character of the antibiotics used. However, it is demonstrated that minimum inhibitory concentrations against S. epidermidis and E. coli can be exceeded for both antibiotics.

Pluto’s ocean is capped and insulated by gas hydrates

Many icy Solar System bodies possess subsurface oceans. On Pluto, Sputnik Planitia’s location near the equator suggests the presence of a subsurface ocean and a locally thinned ice shell. To maintain an ocean, Pluto needs to retain heat inside. On the other hand, to maintain large variations in its thickness, Pluto’s ice shell needs to be cold. Here we show, by thermal evolution and viscous relaxation calculations, that the presence of a thin layer of clathrate hydrates (gas hydrates) at the base of the ice shell can explain both the long-term survival of the ocean and the maintenance of shell thickness contrasts. Clathrate hydrates act as a thermal insulator, preventing the ocean from completely freezing while keeping the ice shell cold and immobile. The most likely clathrate guest gas is methane, derived from precursor bodies and/or cracking of organic materials in the hot rocky core. Nitrogen molecules initially contained and/or produced later in the core would probably not be trapped as clathrate hydrates, instead supplying the nitrogen-rich surface and atmosphere. The formation of a thin clathrate hydrate layer cap to a subsurface ocean may be an important generic mechanism to maintain long-lived subsurface oceans in relatively large but minimally heated icy satellites and Kuiper belt objects. Pluto’s subsurface ocean and thickness variation in its ice shell may be maintained by a layer of methane clathrates forming an insulating cap to the ocean, according to calculations of thermal evolution and viscous relaxation.