Alumina-based composites reinforced with single-walled carbon nanotubes

Monolithic Al[2]O[3] ceramics and Al[2]O[3] - 3 vol% single-walled carbon nanotubes (SWCNTs) composites were prepared by spark plasma sintering. The influence of SWCNTs and sintering temperature on sintering behavior and mechanical properties were investigated. Nanotubes were relatively homogeneously distributed in the composite powder, although some agglomerates/bundles existed. It was found that SWCNTs addition retards slightly the sinterability of alumina by nanotubes hindering of particle rearrangement. The average microhardness of the composites was lower than that of Al[2]O[3] ceramics, but there were also high microhardness values of composites (20.41 GPa). The reduction in microhardness is explained in this paper. The average values of the fracture toughness (4.94 MPa·m{1/2}) from the composite sintered at 1500 °C were approximately 6 % and 12 % higher than those from the Al[2]O[3] ceramics sintered at 1500 °C and 1600 °C, respectively. In addition, the analysis of the phase composition and the parameters of the crystal structure of the samples were made

Spark plasma sintering of ceramic matrix composite based on alumina, reinforced by carbon nanotubes

Alumina composites reinforced with 3 vol.% multi-walled carbon nanotubes (MWCNTs) were prepared by spark plasma sintering (SPS). The influence of sintering temperature (1400-1600 °C) on the composites microstructure and mechanical properties was investigated. Microstructure observations of the composite shows that some CNTs site along alumina grains boundary, while others embed into the alumina grains and shows that CNTs bonded strongly with the alumina matrix contributing to fracture toughness and microhardness increase. MWCNTs reinforcing mechanisms including CNT pull-out and crack deflection were directly observed by scanning electron microscope (SEM). For Al[2]O[3]/CNT composite sintered at 1600 °C, fracture toughness and microhardness are 4.93 MPa•m{1/2} and 23.26 GPa respectively

Improving the Mechanical Properties of SiC-ceramics by means of Vacuum Electron-ion-plasma Alloying with Titanium

The investigation results of elemental and phase composition, state of defective substructure and microhardness of the surface layer of "film (Ti)/substrate (SiC-ceramics)" system (Ti film 0.5 [mu]m thick was deposited on the surface of SiC-ceramics) subjected to treatment with an intense pulsed low-energy electron beam (15 J/cm{2}, 200 [mu]s, 0.3 s{-1}, 20 pulses) are presented. It is shown that irradiation of the "film (Ti)/substrate (SiC-ceramics)" system with an electron beam is accompanied by the formation of multielement multiphase (SiC; TiC; Ti5Si[3]) surface layer having submicro- and nanocrystalline structure. Microhardness of the irradiated surface layer reaches a value of 74 GPa, that is twice the value of microhardness of SiC-ceramics (36 GPa)

Improving the Mechanical Properties of SiC-ceramics by means of Vacuum Electron-ion-plasma Alloying with Titanium

The investigation results of elemental and phase composition, state of defective substructure and microhardness of the surface layer of "film (Ti)/substrate (SiC-ceramics)" system (Ti film 0.5 [mu]m thick was deposited on the surface of SiC-ceramics) subjected to treatment with an intense pulsed low-energy electron beam (15 J/cm{2}, 200 [mu]s, 0.3 s{-1}, 20 pulses) are presented. It is shown that irradiation of the "film (Ti)/substrate (SiC-ceramics)" system with an electron beam is accompanied by the formation of multielement multiphase (SiC; TiC; Ti5Si[3]) surface layer having submicro- and nanocrystalline structure. Microhardness of the irradiated surface layer reaches a value of 74 GPa, that is twice the value of microhardness of SiC-ceramics (36 GPa)

Composition of Sedimentary Organic Matter across the Laptev Sea Shelf: Evidences from Rock-Eval Parameters and Molecular Indicators

Global warming in high latitudes causes destabilization of vulnerable permafrost deposits followed by massive thaw-release of organic carbon. Permafrost-derived carbon may be buried in the nearshore sediments, transported towards the deeper basins or degraded into the greenhouse gases, potentially initiating a positive feedback to climate change. In the present study, we aim to identify the sources, distribution and degradation state of organic matter (OM) stored in the surface sediments of the Laptev Sea (LS), which receives a large input of terrestrial carbon from both Lena River discharge and intense coastal erosion. We applied a suite of geochemical indicators including the Rock Eval parameters, traditionally used for the matured OM characterization, and terrestrial lipid biomarkers. In addition, we analyzed a comprehensive grain size data in order to assess hydrodynamic sedimentation regime across the LS shelf. Rock-Eval (RE) data characterize LS sedimentary OM with generally low hydrogen index (100–200 mg HC/g TOC) and oxygen index (200 and 300 CO2/g TOC) both increasing off to the continental slope. According to Tpeak values, there is a clear regional distinction between two groups (369–401 °C for the inner and mid shelf; 451–464 °C for the outer shelf). We suggest that permafrost-derived OM is traced across the shallow and mid depths with high Tpeak and slightly elevated HI values if compared to other Arctic continental margins. Molecular-based degradation indicators show a trend to more degraded terrestrial OC with increasing distance from the coast corroborating with RE results. However, we observed much less variation of the degradation markers down to the deeper sampling horizons, which supports the notion that the most active OM degradation in LS land-shelf system takes part during the cross-shelf transport, not while getting buried deeper

Authigenic minerals in the bottom sediments from seeps of the Laptev Sea

Метановые сипы являются широко распространенным явлением, наблюдаемым на шельфах и континентальных склонах внутренних и окраинных морей по всему миру, в том числе и в море Лаптевых. Ключевыми биогеохимическими процессами, протекающими в донных осадках этих районов, являются анаэробное окисление метана в сочетании с бактериальной сульфатредукцией. Оба этих процесса контролируют образование специфической аутигенной минерализации. Целью настоящей работы являлось изучение аутигенных минералов донных осадков с аномально высокими концентрациями метана, отобранных на двух сиповых участках в северо-восточной части моря Лаптевых, для определения признаков их идентификации в древних осадочных породах. Приведены результаты литологических и минералогических исследований донных осадков. Было установлено, что основными аутигенными минералами в исследованных образцах донных осадков, отобранных с двух сиповых участков в северо-восточной части моря Лаптевых, являются магнезиальный кальцит, гипс и пирит. Разная специфика аутигенной минерализации, предположительно, указывает на различия в режимах миграции метан-содержащих флюидов на этих участках. Временное снижении интенсивности просачивания метана в пределах «восточного сипа» способствовало насыщению поровой воды ионами SO[4]{2-} и Ca{2+} и, как следствие, осаждению гипса. Близповерхностное положение сульфат-метановой транзитной зоны в «западном сипе», обусловленное высокими потоками метана, благоприятствовало осаждению магнезиального кальцита в верхних горизонтах донных осадков. Присутствие пирита в осадках как восточного, так западного сиповых участков является свидетельством активности процесса бактериальной сульфат-редукции при анаэробном окислении метана.Methane seeps is a widespread phenomenon observed on the shelves and continental slopes of inland and border seas around the world, including the Laptev Sea. Key biogeochemical processes occurring in the bottom sediments of these areas are the anaerobic oxidation of methane in combination with bacterial sulfate reduction. Both of these processes control the formation of specific autigenic mineralization. The aim of this work was to study authigenic minerals of bottom sediments with abnormally high concentrations of methane taken from two seeps in the north-eastern part of the Laptev Sea to determine the signs of their identification in ancient sedimentary rocks. The paper presents the results of lithological and mineralogical studies of bottom sediments. It was found that magnesium calcite, gypsum and pyrite are the main authigenic minerals in the studied samples of bottom sediments taken from two seeps in the north-eastern part of the Laptev Sea. The different specifics of authigenic mineralization indicate differences in conditions of migration of methane-containing fluids in these areas, presumably. Temporary decrease in the rate of methane seepage within the «eastern seep» contributed to the saturation of pore water with SO[4]{2-} and Ca{2+} and, as a consequence, gypsum deposition. The near-surface position of the sulfate-methane transition zone in the «western seep» due to high methane flows favored the precipitation of magnesian calcite in the upper horizons of bottom sediments. The presence of pyrite in sediments of eastern and western seep is evidence of the activity of the bacterial sulfate reduction during anaerobic methane oxidation

Identifying sources of organic carbon in surface sediments of Laptev Sea shelf using a Rock-Eval approach

Актуальность исследования обусловлена необходимостью всестороннего изучения процессов, ответственных за изменения биогеохимического режима арктического региона. Увеличение темпов деградации прибрежной и подводной мерзлоты на Восточно-Сибирском шельфе приводит к вовлечению в современный биогеохимический цикл большого объема ремобилизованного органического углерода. Изучение особенностей его транспорта и преобразования в системе суша-шельф играет важную роль для оценки функционирования крайне хрупкой арктической экосистемы. Цель: изучение геохимических характеристик органического вещества, прослеживаемых по профилю от береговой зоны к континентальному склону моря Лаптевых с применением метода Rock-Eval и оценка их взаимосвязи с литологическими свойствами вмещающих осадков. Объектом исследования явились пробы донных осадков, взятые с поверхностного горизонта (0-2 см). Отбор проб проводился в морских арктических экспедициях 2018-2019 гг. на НИС «Академик Мстислав Келдыш». Результаты. На основе результатов пиролитического анализа дана геохимическая характеристика органического вещества, содержащегося в донных осадках моря Лаптевых. Органическое вещество, экспортируемое с речным стоком и продуктами береговой эрозии, характеризуется относительно низким кислородным (OI) и водородным (HI) индексами в прибрежной зоне и на глубинах до нескольких десятков метров. В районе среднего шельфа существенное влияние на состав органического вещества, по всей видимости, оказывает снос осадочного вещества с Новосибирских островов, где активно действуют термоабразионные процессы (снижение значений HI и увеличение OI). Высказано предположение, что для продуктов, выносимых речным стоком, и продуктов эрозии берегов характерны различные пиролитические параметры, определяемые методом Rock-Eval (в частности, значения HI, OI и Tpeak).An increasing rate of degradation of coastal and subsea permafrost leads to remobilization of huge amounts of organic carbon. To know how this remobilized carbon behaves while being transported through the land-shelf system is crucially important for understanding an extremely fragile Arctic ecosystem. This study is aimed at tracing the geochemical signals of organic matter along the profile from the coastal zone to the continental slope of the Laptev Sea, using the Rock-Eval approach. We investigated surface sediment samples obtained during the Arctic marine expeditions of 2018-2019 on the R/V "Akademik Mstislav Keldysh". The most active oxidation of organic matter, exported with river runoff and products of coastal erosion, occurs in the coastal zone at a depth of several tens of meters. A significant effect on the organic matter composition is exerted by the sediment export from Novosibirsk Islands eroding coastlines. We assume that various products carried by river runoff and coastal erosion are characterized by various signatures detected by the Rock-Eval method (e.g., the OI and Tpeak values). It is also shown that the mineral matrix does not seem to provide a first-order control on preventing organic matter degradation during transport from the coastal zone to deep-sea basins