Chemical Inhomogeneity Research Articles

Zn0.1Ca0.1Sr0.4Ba0.4ZrO3: A non-equimolar multicomponent perovskite ceramic with low thermal conductivity

A non-equimolar multicomponent perovskite of Zn0.1Ca0.1Sr0.4Ba0.4ZrO3 is successfully synthesized. Its single-phase stabilization is achieved by the designed non-equimolar composition that contributes to the nearly zero mixing enthalpy, the high configuration entropy and the moderate atomic difference. Its chemical bonds inhomogeneity is revealed, which contributes to the unique mechanical and thermal properties of Zn0.1Ca0.1Sr0.4Ba0.4ZrO3. Its Young’s modulus is 185.7 GPa and tends to be the average of its components’. The thermal conductivity of Zn0.1Ca0.1Sr0.4Ba0.4ZrO3 ranges from 1.45 to 1.87 W/(m·K), being lower than those of its components (CaZrO3 and BaZrO3) and the typical low thermal conductivity material Y2O3 partially stabilized ZrO2. The low thermal conductivities suggest its potential as thermal barrier materials and this work is supposed to arouse future works on the non-equimolar multicomponent ceramics with targeted performances.

Fe(Se,Te) from melting routes: the influence of thermal processing on microstructure and superconducting properties

Superconducting properties of the Fe(Se,Te) system are directly related to its microstructure. However, in the framework of a not well assessed phase diagram, a large spread of chemico-physical and functional properties is reported for compounds characterized by similar overall chemical compositions. This is in particular observed in materials produced by melting routes, where chemical inhomogeneities and multiple phases are commonly obtained. Both composition and morphology of these inhomogeneities seem to play a role in the behaviour of the superconducting phases, pointing out the need to assess the dependence of phase composition and therefore of the material performance on the sample thermal history.In this work, the effect of different post-synthesis annealing treatments is studied. A melting route has been adopted to synthesize polycrystalline samples, subsequently subjected to thermal treatments at different temperatures. Morphology and microstructure were characterized by means of x-ray diffraction and microscopy techniques. The superconducting properties have been evaluated by means of electrical and magnetic measurements. It is observed how microstructure, chemical composition and morphology of the obtained products strictly depend on the annealing temperature. Moreover, a crucial role of the cooling step is evidenced: in fact, a rapid cooling procedure from high temperature leads to the formation of high temperature (Tconset > 19 K) superconducting phases, but the overall bulk superconducting performance is negatively affected. Other annealing treatments can promote instead an enhancement in the critical current at high fields with respect to pristine sample, suggesting that a careful engineering of a post-synthesis thermal step can represent a useful tool to optimize the material properties.

A Blueprint for the Milky Way’s Stellar Populations: The Power of Large Photometric and Astrometric Surveys

Abstract Recent advances from astronomical surveys have revealed spatial, chemical, and kinematical inhomogeneities in the inner region of the stellar halo of the Milky Way. In particular, large spectroscopic surveys, combined with Gaia astrometric data, have provided powerful tools for analyzing the detailed abundances and accurate kinematics for individual stars. Despite these noteworthy efforts, however, spectroscopic samples are typically limited by the numbers of stars considered; their analysis and interpretation are also hampered by the complex selection functions that are often employed. Here we present a powerful alternative approach—a synoptic view of the spatial, chemical, and kinematical distributions of stars in the Milky Way based on large photometric survey databases, enabled by a well-calibrated technique for obtaining individual stellar metal abundances from broadband photometry. We combine metallicities with accurate proper motions from the Gaia mission along the prime meridian of the Galaxy, and find that various stellar components are clearly separated from each other in the metallicity versus rotation-velocity space. The observed metallicity distribution of the inner-halo stars deviates from the traditional single-peaked distribution, and exhibits complex substructures comprising varying contributions from individual stellar populations, sometimes with striking double peaks at low metallicities. The substructures revealed from our less-biased, comprehensive maps demonstrate the clear advantages of this approach, which can be built upon by future multiband photometric surveys, and used as a blueprint for identifying the stars of greatest interest for upcoming spectroscopic studies.

The Electronic Conductivity of Single Crystalline Ga‐Stabilized Cubic Li7La3Zr2O12: A Technologically Relevant Parameter for All‐Solid‐State Batteries

AbstractThe next‐generation of all‐solid‐state lithium batteries need ceramic electrolytes with very high ionic conductivities. At the same time a negligible electronic conductivity σeon is required to eliminate self‐discharge in such systems. A non‐negligible electronic conductivity may also promote the unintentional formation of Li dendrites, being currently one of the key issues hindering the development of long‐lasting all‐solid‐state batteries. This interplay is suggested recently for garnet‐type Li7La3Zr2O12 (LLZO). It is, however, well known that the overall macroscopic electronic conductivity may be governed by a range of extrinsic factors such as impurities, chemical inhomogeneities, grain boundaries, morphology, and size effects. Here, advantage of Czochralski‐grown single crystals, which offer the unique opportunity to evaluate intrinsic properties of a chemically homogeneous matrix, is taken to measure the electronic conductivity σeon. Via long‐time, high‐precision potentiostatic polarization experiments an upper limit of σeon in the order of 5 × 10−10 S cm−1 (293 K) is estimated. This value is by six orders of magnitude lower than the corresponding total conductivity σtotal = 10−3 S cm−1 of Ga‐LLZO. Thus, it is concluded that the high values of σeon recently reported for similar systems do not necessarily mirror intragrain bulk properties of chemically homogenous systems but may originate from chemically inhomogeneous interfacial areas.

Universality of Temperature Dependence of Twinning Stress in Ni-Mn-Ga 10M Martensite and Effect of Crystal Quality

We investigate experimentally the universality of the temperature dependences of twinning stress of type Ⅰ and type Ⅱ twins in Ni50Mn28Ga22 five-layered modulated martensite. Single crystals from different producers are compared to distinguish the universal behaviour and the effects of crystal quality. The twinning stress is both measured directly and calculated from magnetisation loops. In the vicinity of martensite transformation, the twinning stress is about the same for both types of twins. For type Ⅰ twins, it increases linearly with decreasing temperature with the slope of -0.045 MPa/K. For type Ⅱ twins, the twinning stress is almost temperature independent, but it increases rapidly near the intermartensite transformation temperature. In the crystal, which does not undergo an intermartensitic transformation and exhibits type Ⅱ twins, the magnetically induced reorientation of martensite is observed down to 2 K. Chemical inhomogeneity and mosaicity of the crystal results in mixed type twin boundaries with temperature dependences deviating from the universal behaviour of type Ⅰ and type Ⅱ twins.

Suppression of nano-hydride growth on Nb(100) due to nitrogen doping.

Niobium superconducting radio frequency (SRF) cavities enable the operation of modern superconducting accelerator facilities. These cavities do not approach the theoretical performance limits of Nb due to the deleterious effects of surface defects and chemical inhomogeneities such as Nb hydrides. Nitrogen doping is known to consistently increase the cavity performance and inhibit Nb hydride growth, but a comprehensive understanding of Nb hydride growth and suppression is not yet realized. Scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations presented herein elucidate the real-time, nanoscale structural and electronic evolution of undoped, hydrogen doped, and hydrogen and nitrogen doped Nb(100) due to the growth and suppression of Nb nano-hydrides. DFT calculations in agreement with the experimental data found unique near-surface phases stabilized upon dopant incorporation. The experimental STM and STS results and DFT calculations reported herein provide the first in situ and real-time nanoscale visualization and characterization of the effects of nitrogen doping on Nb hydride suppression and growth. Such information allows for further optimization of nitrogen doping procedures and advances in the performance of SRF materials for next-generation SRF-based accelerators and free electron lasers.

A plague of magnetic spots among the hot stars of globular clusters

Six decades and counting, the formation of hot ~20,000-30,000 K Extreme Horizontal Branch (EHB) stars in Galactic Globular Clusters remains one of the most elusive quests in stellar evolutionary theory. Here we report on two discoveries shattering their currently alleged stable luminosity. The first EHB variability is periodic and cannot be ascribed to binary evolution nor pulsation. Instead, we here attribute it to the presence of magnetic spots: superficial chemical inhomogeneities whose projected rotation induces the variability. The second EHB variability is aperiodic and manifests itself on time-scales of years. In two cases, the six-year light curves display superflare events a mammoth several million times more energetic than solar analogs. We advocate a scenario where the two spectacular EHB variability phenomena are different manifestations of diffuse, dynamo-generated, weak magnetic fields. Ubiquitous magnetic fields, therefore, force an admittance into the intricate matrix governing the formation of all EHBs, and traverse to their Galactic field counterparts. The bigger picture is one where our conclusions bridge similar variability/magnetism phenomena in all radiative-enveloped stars: young main-sequence stars, old EHBs and defunct white dwarfs.

Exploring chemical homogeneity in dwarf galaxies: a VLT-MUSE study of JKB 18

ABSTRACT Deciphering the distribution of metals throughout galaxies is fundamental in our understanding of galaxy evolution. Nearby, low-metallicity, star-forming dwarf galaxies, in particular, can offer detailed insight into the metal-dependent processes that may have occurred within galaxies in the early Universe. Here, we present VLT/MUSE observations of one such system, JKB 18, a blue diffuse dwarf galaxy with a metallicity of only 12 + log(O/H)=7.6 ± 0.2 (∼0.08 Z⊙). Using high spatial resolution integral-field spectroscopy of the entire system, we calculate chemical abundances for individual H ii regions using the direct method and derive oxygen abundance maps using strong-line metallicity diagnostics. With large-scale dispersions in O/H, N/H, and N/O of ∼0.5–0.6 dex and regions harbouring chemical abundances outside this 1σ distribution, we deem JKB 18 to be chemically inhomogeneous. We explore this finding in the context of other chemically inhomogeneous dwarf galaxies and conclude that neither the accretion of metal-poor gas, short mixing time-scales or self-enrichment from Wolf–Rayet stars are accountable. Using a galaxy-scale, multiphase, hydrodynamical simulation of a low-mass dwarf galaxy, we find that chemical inhomogeneities of this level may be attributable to the removal of gas via supernovae and the specific timing of the observations with respect to star formation activity. This study not only draws attention to the fact that dwarf galaxies can be chemically inhomogeneous, but also that the methods used in the assessment of this characteristic can be subject to bias.

Optimization of the Sintering Parameters for Materials Manufactured by Powder Injection Molding

The properties of sintered and heat-treated steels produced from the Catamold 8740 material by powder injection molding were examined. Parts made of the Catamold 8740 low-alloy mild carbon steel are used in the military, automotive, and other industries where high reliability and resistance to dynamic loads are required. The Catamold 8740 chemical homogeneity was studied as a function of powder injection molding parameters: heating rate, sintering temperature, holding time, and subsequent heat treatment. Microscopic analysis showed that Catamold 8740 was a mechanical mixture of powders in various sizes (carbonyl iron, nickel, Fe–Mo, Fe–Cr, Fe–Si-Mo, Fe–Si). When the heating rate of the Catamold 8740 powder mixture increased from 2 to 5 °C/min, the microstructural heterogeneity caused by uneven compaction decreased and the alloy formation process activated at 900–1340°C. The impact strength of Charpy V-notch test samples changed from 10.39 to 11.52 J/cm2 with higher sintering temperature and heating rate increasing from 2 °C/min (1270°C) to 5 °C/min (1340°C). When holding time increased from 30 to 90 min at a sintering temperature of 1340°C, the material became denser, the pores rounded, and the ferrite matrix homogeneous. However, the impact strength decreased by a factor of 1.4 because of coarser grain sizes and brittle fracture. Heat treatment processes reduced the impact strength after sintering. The results obtained were used to optimize the powder injection molding parameters. The influence of chemical and dimensional inhomogeneity of the starting Catamold 8740 powders and sintering kinetics on the structure and mechanical properties of parts operating under impact loads was established.

New Universality Class Associated with Jahn–Teller Distortion and Double Exchange

The scaling of the magnetic heat capacity in the two manganites La0.85Ag0.15MnO3 and Sm0.55Sr0.45MnO has given the critical exponents α = –0.23 and ν = 0.7433 of the heat capacity and correlation radius of the magnetic order parameter, respectively, which do not belong to any known universality class. These results cannot be attributed to chemical inhomogeneities and/or structural imperfections because the samples are of a high quality. Thus, unusual critical exponents can be associated not only with the chemical disorder and/or structural defects but also with the collective behavior of the lattice. An analogy has been revealed between the effects of the magnetic field and doping on ternary oxides of transition metals: the magnetic field affecting lattice distortions through the orientation of t2g orbitals acts as chemical doping. It seems that scaling relations are more stable than critical exponents in them. The synchronism of lattice distortions and ferromagnetism leads to a novel criticality, but their desynchronization induced by magnetostructural disorder results in the violation of scaling relations between isothermal and isomagnetic exponents. Although double-exchange systems demonstrate novel criticality, they satisfy scaling relations until the magnetic behavior is synchronized with the coherent lattice behavior in the form of cooperative Jahn–Teller distortions. Breaking of double exchange bonds leads to the formation of metamagnetic clusters with magnetic dipole–dipole interaction between them, which desynchronizes lattice distortions and ferromagnetism, resulting in the violation of scaling relations. The proposed new universality class includes diverse materials such as manganites, cobaltites, crystalline Fe–Pt and amorphous Fe–Mn alloys, and high-Tc superconductors. Unusual criticality in double-exchange systems is due to an unusual semiclassical nature of double-exchange ferromagnetism caused by real exchange, i.e., electron current through Mn3+–O–Mn4+ chains with the conservation of the spin rather than by virtual exchange as in a usual ferromagnet. Double-exchange ferromagnetism arises only because to freely itinerate, electrons orient the magnetic moments of Mn cations in a single direction.

A chemo-mechanical grain boundary model and its application to understand the damage of Li-ion battery materials

Although the unique mechanical and transport features of grain boundaries (GBs) in polycrystalline ion conductors have been recognized, the understanding of the chemo-mechanical interplay and its impact is insufficient. We present a coupled GB model which includes both the damage-dependent across-grain transport and a mechanical cohesive zone law. 3D simulations on LiNixMnyCozO2 demonstrate that the chemical process and the mechanical degradation go hand-in-hand: the enhanced intergranular chemical inhomogeneity challenges the GB mechanical strength, while the GB damage influences or even blocks the across-grain transport. Results explain well the experimentally observed features including chemical hot spots and surface layer delamination.

A comprehensive study of the glass/translucent anti-glass/transparent ceramic structural ordering in the Bi2O3[sbnd]Nb2O5–TeO2 system

The full and congruent crystallization of the Bi0.8Nb0.8Te2.4O8 (BNT) glass leads to perfectly transparent ceramic in both the visible and near infrared ranges. While heating the parent glass, an intermediate structural regime order – called anti-glass – that shows chemical inhomogeneity at the nanometer scale and with a long-range cationic ordering while retaining a strong local anionic disorder is observed. Here, we report on the crystallization sequence starting from the Bi0.8Nb0.8Te2.4O8 glass system and show that an intermediate Bi0.8Nb0.8Te2.4O8 anti-glass phase (polymorph 1) is first formed, before transforming into a more ordered and ceramic material (polymorph 2). In particular, we provide a comprehensive structural study of the crossover between glass and its crystalline counterpart through intermediate steps. Similarities and differences between the structures, microstructures of Bi0.8Nb0.8Te2.4O8 glass, anti-glass and ceramic materials of identical composition are investigated in details at different length scales by X-ray and neutron powder diffraction, SEM-EBSD, TEM, NMR and Raman spectroscopy. The optical properties of the anti-glass and ceramic are discussed in correlation with the structure and microstructure observations. This study paves the way for understanding the complex mechanisms at the origin of the full and congruent crystallization from the parent glass.

Mass transfer of low-mass binaries and chemical anomalies among unevolved stars in globular clusters

ABSTRACT While it is well known that mass transfer in binaries can pollute the surfaces of the accretors, it is still unclear whether this mechanism can reproduce the observed chemical inhomogeneities in globular clusters. We study the surface abundances of the accretors in low-mass binaries, as a first step towards understanding whether mass transfer in low-mass binaries is one of the potential origins of the aforementioned abundance anomalies in globular clusters. We use the mesa (Modules for Experiments in Stellar Astrophysics) code to calculate binary evolutionary models with different initial donor masses between 0.9 and 1.9 $\rm {M}_\odot$ for an initial metallicity of Z = 0.0034. The results show that in some low-mass binary systems, the accretors exhibit peculiar chemical patterns when they are still unevolved stars, e.g. C and O depletion; Na and N enhancement; and constant Mg, Al, and C+N+O. The abundance patterns of the accretors are significantly different from their initial abundances (or that of normal single stars), and can match the observed populations. These abundance patterns strongly depend not only on the initial parameters of binaries (donor mass, mass ratio, and orbital period), but also on the assumptions regarding mass-transfer efficiency and angular momentum loss. These results support the hypothesis that mass transfer in low-mass binaries is, at least, partly responsible for the unevolved anomalous stars in globular clusters. More work on binary evolutionary models and binary population synthesis is required to fully evaluate the contribution of this scenario.

Differential abundances of open clusters and their tidal tails: Chemical tagging and chemical homogeneity

Context. Well studied open clusters (OCs) of the solar neighborhood are frequently used as reference objects to test galactic and stellar theories. For that purpose, their chemical composition needs to be known with a high level of confidence. It is also important to clarify if each OC is chemically homogeneous and if it has a unique chemical signature. Aims. The aims of this work are (1) to determine accurate and precise abundances of 22 chemical species (from Na to Eu) in the Hyades, Praesepe, and Rupecht 147 by using a large number of stars at different evolutionary states, (2) to evaluate the level of chemical homogeneity of these OCs, and (3) to compare their chemical signatures. Methods. We gathered ∼800 high resolution and high signal-to-noise spectra of ∼100 members in the three clusters, which were obtained with the latest memberships based on Gaia DR2 data. We built a pipeline, which computes atmospheric parameters and strictly line-by-line differential abundances among twin stars in our sample. With this method, we were able to reach a very high precision in the abundances (0.01–0.02 dex in most of the elements). Results. We find large differences in the absolute abundances in some elements, which can be attributed to diffusion, non-local thermodynamic equilibrium (non-LTE) effects, or systematics in the analysis. For the three OCs, we find strong correlations in the differential abundances between different pairs of elements. According to our experiment with synthetic data, this can be explained by some level of chemical inhomogeneity. We compare differential abundances of several stars from the Hyades and Praesepe tails: The stars that differ more in chemical abundances also have distinct kinematics, even though they have been identified as members of the tail. Conclusions. It is possible to obtain high precision abundances using a differential analysis even when mixing spectra from different instruments. With this technique, we find that the Hyades and Preasepe have the same chemical signature when G dwarfs and K giants are considered. Despite a certain level of inhomogeneity in each cluster, it is still possible to clearly distinguish the chemical signature of the older cluster Ruprecht 147 when compared to the Hyades and Praesepe.

Facile Synthesis of Hollow Carbon Nanospheres and Their Potential as Stable Anode Materials in Potassium-Ion Batteries.

Hollow carbon nanospheres (HCNs) have found broad applications in a large variety of application fields. Unfortunately, HCNs are known for their tedious operations and are incompetent for scalable synthesis for those widely adopted nanocasting-based routes. Here, we report a facile and highly efficient method for the creation of hollow carbon structures by tuning the growth kinetics of its polymeric precursor. We identified that a controlled polymerization of Cu2+-poly(m-phenylenediamine) (Cu-PmPD) could form nanospheres with modulated inner chemical inhomogeneity, where the core of the particles was low in polymerization degree and water soluble, whereas the outer part was water insoluble. Therefore, a simple water washing of the prepared polymeric particles directly formed hollow nanospheres with a good control on the structural features including their cavity size and shell thickness. HCNs were formed through a following heat treatment and were able to exhibit promising potential as a stable anode material when tested in potassium-ion batteries.

Thermal and mechanical properties of ferroelastic RENbO4 (RE = Nd, Sm, Gd, Dy, Er, Yb) for thermal barrier coatings

A new series of rare-earth niobates (RENbO4) are synthesized and investigated for potential thermal barrier coating (TBC) application. The thermal conductivities of RENbO4 are much lower than yttria stabilized zirconia (YSZ) mainly due to the chemical inhomogeneity. The lattice distortion contributes to the higher coefficients of thermal expansion (CTEs, 12.8 × 10−6 K−1) of RENbO4 below the critical temperature. Especially, RENbO4 exhibits high fracture toughness (1.93–2.77 MPa m0.5) comparable to YSZ, owning to the absorption of crack energy by the ferroelastic domain switching. The excellent thermal and mechanical properties give RENbO4 great potential as new durable TBC materials.

From suppressed void growth to significant void swelling in NiCoFeCr complex concentrated solid-solution alloy

Void swelling can result in dimensional instability and undermine the safe operation of nuclear reactors. Current strategies to inhibit void swelling mainly focus on enhancing defect absorption and recombination by introducing high-density defect sinks. Complex concentrated solid-solution alloys (CSAs), including high-entropy alloys, can withstand severe radiation damage due to their inherent chemical complexity without interfaces. However, the underlying mechanisms for void suppression in CSAs are far from clear. In this research, we studied the void evolution with respect to irradiation depths, doses, and temperatures in equiatomic NiCoFeCr under 3 MeV Ni ion irradiations. At relatively low doses (16 and 54 displacements per atom, dpa), voids form mainly outside of the ion-damaged region, and void formation in the peak damage region is suppressed, leading to negligible swelling. However, with further increase of dose (86 up to 250 dpa), significant void growth occurs in the peak damage region and extended dislocation lines dominate instead of short dislocation lines and loops formed at lower doses. From 500 to 700 °C, the dislocation density decreases while dislocations grow. Although the overall void swelling increases dramatically from 500 to 580 °C at 54 dpa, void growth in the peak damage region is still suppressed. The transition from suppressed void growth to significant void swelling is attributed to dislocation evolution and local chemical inhomogeneity (enrichment of Fe/Cr in the matrix) at higher doses. Our study shows that controlling element diffusion and defect evolution through tuning chemical complexity can further enhance the swelling resistance of CSAs.

Advanced single voxel 1 H magnetic resonance spectroscopy techniques in humans: Experts' consensus recommendations.

Conventional proton MRS has been successfully utilized to noninvasively assess tissue biochemistry in conditions that result in large changes in metabolite levels. For more challenging applications, namely, in conditions which result in subtle metabolite changes, the limitations of vendor-provided MRS protocols are increasingly recognized, especially when used at high fields (≥3 T) where chemical shift displacement errors, B0 and B1 inhomogeneities and limitations in the transmit B1 field become prominent. To overcome the limitations of conventional MRS protocols at 3 and 7 T, the use of advanced MRS methodology, including pulse sequences and adjustment procedures, is recommended. Specifically, the semiadiabatic LASER sequence is recommended when TE values of 25-30 ms are acceptable, and the semiadiabatic SPECIAL sequence is suggested as an alternative when shorter TE values are critical. The magnetic field B0 homogeneity should be optimized and RF pulses should be calibrated for each voxel. Unsuppressed water signal should be acquired for eddy current correction and preferably also for metabolite quantification. Metabolite and water data should be saved in single shots to facilitate phase and frequency alignment and to exclude motion-corrupted shots. Final averaged spectra should be evaluated for SNR, linewidth, water suppression efficiency and the presence of unwanted coherences. Spectra that do not fit predefined quality criteria should be excluded from further analysis. Commercially available tools to acquire all data in consistent anatomical locations are recommended for voxel prescriptions, in particular in longitudinal studies. To enable the larger MRS community to take advantage of these advanced methods, a list of resources for these advanced protocols on the major clinical platforms is provided. Finally, a set of recommendations are provided for vendors to enable development of advanced MRS on standard platforms, including implementation of advanced localization sequences, tools for quality assurance on the scanner, and tools for prospective volume tracking and dynamic linear shim corrections.

OUP accepted manuscript

One of the key findings of the Rosetta's mission to the Jupiter family comet 67P/Churyumov-Gerasimenko was its peculiar bilobed shape along with the apparent north/south dichotomy in large scale morphology. This has re-ignited scientific discussions on the topic of origin, evolution and age of the nucleus. In this work we set up a general numerical investigation on the role of solar driven activity on the overall shape change. Our goal is to isolate and study the influence of key parameters for solar driven mass loss, and hopefully obtain a classification of the final shapes. We consider five general classes of three-dimensional (3D) objects for various initial conditions of spin-axis and orbital parameters, propagating them on different orbits accounting for solar driven CO ice sublimation. A detailed study of the coupling between sublimation curve and orbital parameters (for CO and H$_{2}$O ices) is also provided. The idealizations used in this study are aimed to remove the ad-hoc assumptions on activity source distribution, composition, and/or chemical inhomogeneities as applied in similar studies focusing on explaining a particular feature or observation. Our numerical experiments show that under no condition a homogeneous nucleus with solar driven outgassing can produce concave morphology on a convex shape. On the other hand, preexisting concavities can hardly be smoothed/removed for the assumed activity. In summary, the coupling between solar distance, eccentricity, spin-axis and its orientation, as well as effects on shadowing and self-heating do combine to induce morphology changes that might not be deducible without numerical simulations.

Диссипация механической энергии в осциллирующем адгезионном контакте между жестким индентором и эластомером

A model of hysteresis in an adhesive contact under oscillating loading is proposed, based on the chemical inhomogeneity of the indenter surface. Results of numerical simulation are compared with experimental data obtained with a setup allowing direct observation of the dynamics of contact area. It is shown that the hysteresis almost disappears if the amplitude of the oscillating load is smaller than a critical value depending on the maximum indentation depth.