Evidence for high saturation of porous amorphous carbon films by noble gases

Results of the electron diffraction study of Kr and Xe deposits on amorphous porous carbon substrates are reported. We have observed mixtures of crystallites typical of condensates formed at such substrates at low deposition temperatures. However, in the warming process at temperatures about three-to-five degrees below the sublimation point characteristic of flat substrates, the diffraction patterns demonstrate that large crystallites gradually disappear and a highly disordered matter forms. Such transformed samples are kept inside substrates several dozens degrees above the sublimation points which are typical of these substances at flat (e.g. metallic) substrates.We ascribe these features to specificity of composites formed from noble gases strongly bonded inside porous carbon matrices due to capillary filling at temperatures close to the sublimation points

On the role of distortion in the hcp vs fcc competition in rare-gas solids

As a prototype of initial or intermediate structure somewhere in between the hcp and fcc lattices we consider a distorted bcc crystal. We calculate the temperature and pressure dependences of the lattice parameters for heavier rare gas solids Ar, Kr, Xe in the quasiharmonic approximation applying the Aziz potentials and confirm that in line with the previously found prevalence of hcp over fcc the hcp structure is still dominant in the bulk over the wide P–T ranges analyzed. The situation is different for confined clusters up to 105 atoms where owing to the specific surface energetics and terminations the structures with five-fold symmetry comprising fcc fragments are dominant. As a next step we consider a free relaxation of differently distorted bcc clusters, and show that two types (monoclinic and orthorhombic) of the initial distortion and its degree is a driving force for the hcp vs fcc final realizations. Possible energetic links between the initial and final structures are shown and analyzed

Neon in carbon nanopores: wetting, growth mechanisms and cluster structures

Low-temperature high-energy (50 keV) electron diffraction study of size-dependent structures and growth mechanisms of neon samples in multiporous «amorphous» carbon films is presented. Electron diffractograms were analyzed on the basis of the assumption that there exists the cluster size distribution in deposits formed in substrate and multi-shell structures such as icosahedra, decahedra, fcc and hcp were probed for different sizes up to approximately 3·10⁴ atoms. The analysis was based on the comparison of precise experimental and calculated diffracted intensities with the help of the R (reliability) — factor minimization procedure. Highly reproducible discrete distribution functions of sizes and structures were found. The time-dependent evolution of diffractograms at earlier stages of growth was revealed. Initially distinct diffraction peaks gradually «disappeared» although the total electron beam absorption evidenced that deposited neon was preserved in the porous substrate. We ascribed this effect to diffusion-like gas penetration from larger to smaller pores which resulted in a highly dispersed or even disordered substance. Evidently, clusters initially grown during deposition were later soaked by a sponge-like substrate due to capillary forces

Selective uptake and desorption of carbon dioxide in carbon honeycombs of different sizes

The EK research was performed in the Center of Excellence of the Institute of Solid State Physics, University of Latvia, supported through European Unions Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under Grant Agreement No. 739508, Project CAMART2.Carbon honeycombs (CHs) are new carbon cellular structures, very promising in many respects, in particular, for high-capacity storage of various materials, especially in gaseous and liquid forms. In this study, we report a strong uptake of carbon dioxide kept inside carbon honeycomb matrices up to temperatures about three times higher as compared with CO2 desorption at ≈ 90 K from flat solid surfaces in vacuum where we conduct our high-energy electron diffraction experiments. Desorption of CO2 from CH matrices upon heating exhibits non-monotone behavior, which is ascribed to carbon dioxide release from CH channels of different sizes. It is shown that modeling of CO2 uptake, storage, and redistribution in the thin CH channels of certain types and orientations upon heating can explain experimental observations.--//-- This is an Open Access article N. V. Krainyukova, D. G. Diachenko, E. A. Kotomin; Selective uptake and desorption of carbon dioxide in carbon honeycombs of different sizes. Low Temp. Phys. 1 January 2024; 50 (1): 97–102. https://doi.org/10.1063/10.0023898 published under the CC BY licence.The EK research was performed in the Center of Excellence of the Institute of Solid State Physics, University of Latvia, supported through European Unions Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under Grant Agreement No. 739508, Project CAMART2

Tunnelling defect nanoclusters in hcp 4He crystals: alternative to supersolidity

A simple model based on the concept of resonant tunnelling clusters of lattice defects is used to explain the low temperature anomalies of hcp 4He crystals (mass decoupling from a torsional oscillator, shear modulus anomaly, dissipation peaks, heat capacity peak). Mass decoupling is a result of an internal Josephson effect: mass supercurrent inside phase coherent tunnelling clusters. Quantitative results are in reasonable agreement with experiments.Comment: 13 pages, 5 figure

Absorption–desorption of carbon dioxide in carbon honeycombs at elevated temperatures

The recently synthesized honeycomb carbon allotrope has numerous potential applications, in particular for storage of gases inside carbon matrices. In this work this carbon form was experimentally studied in its denser form in order to estimate the upper temperature limit for keeping a gas inside the cellular structure. Along with the previously reported random honeycombs of a zigzag type we have also revealed the densest armchair structure. The mechanism of absorption–desorption of carbon dioxide studied by means of high energy electron diffraction at low temperatures showed the two — stage character of the observed desorption at elevated temperatures. This effect is associated to the weaker or stronger bonding of molecules with pore walls depending on the specific configuration of channels with different sizes. We have found that complete desorption of CO₂ does not occur even at the temperatures about three times higher as compared with the sublimation point of carbon dioxide in our vacuum conditions.Нещодавно синтезований вуглецевий стільниковий алотроп має численні потенційні застосування, зокрема для зберігання газів всередині вуглецевих матриць. Таку вуглецеву форму було експериментально досліджено в її більш щільній формі, щоб оцінити верхню границю температури, при якій газ зберігається всередині пористої структури. Поряд з раніше запропонованими випадковими стільниками зиґзаґоподібного типу виявлено найбільш щільну структуру типу armchair. Механізм поглинання–десорбція діоксиду вуглецю, що досліджено за допомогою дифракції електронів високої енергії при низьких температурах, показав двохстадійний характер десорбції, який спостерігається при підвищенні температури. Цей ефект пов’язаний з більш слабким або більш сильним зв’язуванням молекул зі стінками пор в залежності від конкретної конфігурації каналів різного розміру. Виявлено, що повна десорбція СО₂ не відбувається навіть при температурах приблизно в три рази вищих у порівнянні з точкою сублімації вуглекислого газу в наших вакуумних умовах.Недавно синтезированный углеродный сотовый аллотроп имеет множество потенциальных применений, в частности для хранения газов внутри углеродных матриц. Такая углеродная форма была экспериментально исследована в ее более плотной форме, чтобы оценить верхний предел температуры, при которой газ сохраняется внутри ячеистой структуры. Наряду с ранее предложенными случайными сотами зигзагообразного типа обнаружена самая плотная структура типа armchair. Механизм поглощение–десорбция диоксида углерода, изучаемый с помощью дифракции электронов высокой энергии при низких температурах, показал двухстадийный характер наблюдаемой десорбции при повышении температуры. Этот эффект связан с более слабым или более сильным связыванием молекул со стенками пор в зависимости от конкретной конфигурации каналов разного размера. Обнаружено, что полная десорбция СО₂ не происходит даже при температурах примерно в три раза более высоких по сравнению с точкой сублимации углекислого газа в наших вакуумных условиях