The temperature dependence of the viscosity and stress‐relaxation kinetics of sol–gel‐derived SiOC glasses that contain up to 14 at.% carbon have been characterized in the temperature range of ...1000°–1400°C. The viscosity, as determined from relaxation experiments, is in good agreement with the creep viscosity and is typically two orders of magnitude higher than the viscosity of vitreous silica. However, materials suffer from partial crystallization at >1150°C, and the precipitation of β‐SiC nanocrystals induces a flow‐hardening behavior and results in a dynamic increase in viscosity, especially at >1200°C and for glasses with a high carbon content.
•SiOC ceramic aerogels are synthesized by pyrolysis of low-density preceramic aerogels.•SiOC ceramic aerogels are studied as anode material for faster charging and discharging.•The porous nature ...allows fast ionic transport.•At a rate of 10C (3600mAg−1) the specific capacities as high as 200mAhg−1 are recovered.
Porous carbon-rich SiOC ceramic aerogels have been synthesized from a linear polysiloxane cross-linked with divinylbenzene (DVB) via hydrosilylation reaction in presence of a Pt catalyst and acetone as a solvent. The obtained wet gels are aged in solvent followed by drying under supercritical conditions using liquid carbon dioxide. The resulting pre-ceramic aerogels are subjected to pyrolysis at 1000°C under controlled argon atmosphere to form the desired SiOC aerogel. The synthesized SiOC ceramics contain 43wt% of free carbon, which is segregated within amorphous SiOC matrix. The high BET surface area up to 230m2g−1 of preceramic aerogels is only slightly diminished to 180m2g−1 after pyrolysis at 1000°C. The electrochemical characterization reveals a high specific capacity of more than 600mAhg−1 at a charging rate of C (360mAg−1) along with a good cycling stability. At a rate of 10C (3600mAg−1) the specific capacities as high as 200mAhg−1 are recovered. The excellent properties of the materials are discussed with respect to their structural features. The porous nature of the carbon rich ceramics allows for fast ionic transport and helps to accommodate the structural changes which in turn allow a stable performance during repeated lithiation/delithiation.
Scheme illustrating the lower density of deep energy states in NTs grown in 12wt.% H2O. As a result, such NTs exhibit efficient electron transport properties. In contrast electron transport is ...difficult in the case of 2wt.% H2O NTs as the density of deep states, acting as recombination center, is larger.
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The microstructural and photoelectrochemical properties of anodized TiO2 Nanotubes (NTs) grown in ethylene glycol containing 0.1M NH4F and a variable amount of H2O, either 2wt.% (low water content) or 12wt.% (high water content) have been investigated. The study has been conducted at a fixed anodizing potential, as it is known that also the applied potential can affect the electron transport properties of the NTs. The amount of water also has an impact on the microstructure of the NTs; those prepared in 12wt.% H2O have a rough wall, whereas those made at 2wt.% are mostly smooth. In addition the water content affects the dissolution of the forming oxide and consequently has an impact also on the diameter and the length of the NTs. For a fixed potential (30V), when working at 12wt.% H2O content, it is necessary to extend the anodizing time to 60min to obtain ∼1.4μm long NTs. A similar length is achieved at 2wt.% H2O content after 20min.
The photocurrent density measurements reveal an improved photoactivity for NTs grown in high water content electrolytes (1.1mA/cm2 in 12wt.% H2O vs 0.75mA/cm2 in 2wt.% H2O at 1V vs RHE). Similarly the Incident Photon to Current Efficiency (IPCE) of NTs obtained in 12wt.% H2O is consistently higher than for 2wt.% H2O over the range 350-300nm (with values > 40% at 320-300nm). Electrochemical Impedance Spectroscopy (EIS) analysis shows similar electron transport and charge transfer properties for NTs grown in low and high water contents. EIS is mainly sensitive to the bottom of the NTs, whereas the NTs wall is not active at potentials more positive than the flat band potential. EIS also reveals that the poorer photoelectrochemical properties of NTs grown in 2wt.% H2O are due to a larger density of deep intra band-gap energy states in comparison with 12wt.% H2O.
Hybrid gels, made from HSi(OEt)
3 and CH
3SiH(OEt)
2, were used to produce SiOC glasses and to study their stability at high temperature. The variation of the hydrolysis ratio allowed the formation ...of either dense or highly porous xerogels. This difference in microstructure was maintained at 1000°C in the glassy state and influenced the stability of these materials at higher temperature. Indeed, it plays a major role on their carbothermal degradation by promoting or hindering the removal of the gaseous products in the case of porous or dense materials respectively. In the latter case, SiOC glass appears to be a suitable candidate for thermostructural applications.
We report the first synthesis of silicon nanocrystals embedded in a silicon nitride matrix through a direct pyrolysis of a preceramic polymer (perhydropolysilazane). Structural analysis carried out ...by XRD, XPS, Raman and TEM reveals the formation of silicon quantum dots and correlates the microstructures with the annealing temperature. The photoluminescence of the nanocomposites was investigated by both linear and nonlinear measurements. Furthermore we demonstrate an enhanced chemical resistance of the nitride matrix, compared to the typical oxide one, in both strongly acidic and basic environments. The proposed synthesis via polymer pyrolysis is a striking innovation potentially allowing a mass-scale production nitride embedded Si nanocrystals.
Subtle neuropsychological deficits have been described in patients affected by amyotrophic lateral sclerosis (ALS) without dementia. Overall, selective impairment in memory function has been ...reported, but the source of memory impairment in ALS has yet to be defined. We performed neuropsychological screening in 20 ALS patients. Semantic encoding and post-encoding cue effects on the retrieval of word lists were investigated in the ALS patients and normal controls. Severity of memory impairment was correlated to cerebral blood perfusion detected by single photon emission computed tomography (SPECT). ALS patients showed moderate impairments in frontal and memory tests. Short-term memory was normal, while serial position retrieval of word lists with normal recency effect but poor primacy effect showed long-term memory deficit. ALS patients performed better in cued encoding than in cued post-encoding recall condition. In the cued post-encoding condition, the primacy effect in word list recall improved significantly in controls, but not in ALS patients, as compared with both the free recall and cued encoding conditions. SPECT hypoperfusion was observed in frontal and temporal areas in ALS patients. ALS patients showed a long-term memory deficit which did not improve in cued post-encoding condition as it does for controls. We hypothesize abnormal retrieval processes related to frontal lobe dysfunction which entails difficulties in generating stable long-memory traces at encoding.
Two polymer-derived SiOC ceramics with different amount of carbon were synthesized either as dense or porous SiOC powders. The dense materials were produced up to a maximum temperature of 1400°C and ...show a phase separated nanostructure consisting of SiO2-rich clusters, nanocrystalline SiC and nanocrystalline carbon phase. The corresponding porous materials were obtained by etching the silica phase of the dense SiOC with 20% HF solution. The electrochemical properties of the dense and porous SiOC ceramics in terms of lithium insertion/extraction were studied. Accordingly, the SiOC materials show a first lithium insertion capacity between 380 and 648mAhg−1 followed by significantly lower extraction capacities between 102 and 272mAhg−1. We consider the free carbon phase present in the ceramic as the major lithium intercalating agent. The porous samples show a stable electrochemical behavior up to 30 cycles while for the dense materials the efficiency drops to almost zero after 10 cycles.
Gel‐derived oxycarbide glasses have atomic network structures similar to that of vitreous silica glass but with carbon‐rich regions consisting of CSi4 tetrahedra and C–Si–O bonds finely dispersed in ...the glass. Therefore, oxycarbide glasses exhibit the so‐called “anomalous” hardness behavior, similar to silica‐rich glasses, with a substantial densification–strain component beneath the indenter. However, the role of carbon is twofold: on the one hand, the covalently bonded carbon atoms slightly affect the behavior, similar to the way network modifiers affect the behavior of silicate glasses, and favor a “normal” indentation behavior; and on the other hand, the free carbon, forming turbostratic graphite domains, provides easy crack initiation sites and low‐energy fracture paths. Almost concentric shear steps and microcracks, which follow the turbostratic graphite domains, are observed after indentation. The ultimate coalescence of the microcracks produces Hertzian‐type cone cracks.
Background: Epidemiological and clinical studies show higher prevalence of amyotrophic lateral sclerosis (ALS) in males than in females and more severe lesions in androgen receptor (AR)‐expressing ...tissues. The AR gene contains a polymorphic CAG trinucleotide repeat, whose expansion over a certain threshold is toxic to motor neurons, causing spinal and bulbar muscular atrophy (SBMA).
Purpose and methods: We tested the hypothesis that the AR CAG repeat linked to SBMA is a risk factor for ALS. We analyzed AR CAG expansions in 336 patients with ALS and 100 controls.
Results: We found a negative association of AR CAG expansions with ALS susceptibility, clinical presentation, and survival.
Conclusions: Our findings do not support a role of the AR CAG repeat length in ALS.
The pathogenesis of amyotrophic lateral sclerosis (ALS) is poorly understood. Increased levels of free radicals derived from nitric oxide (NO), the product of nitric oxide synthase (NOS), may damage ...mitochondrial function leading to motor neurone death. Previous studies demonstrated a specific impairment of mitochondrial function in skeletal muscle of ALS patients. In order to verify a pathogenetic relationship between neuronal NOS (nNOS) and mitochondrial function, we studied nNOS expression by Western blot and mitochondrial enzyme activity by spectrophotometric assays in muscle biopsies of 16 sporadic ALS patients and 16 controls subjects. We observed a reduced activity of respiratory chain complexes with mitochondrial encoded subunits and a lower nNOS amount in ALS muscles. There was a direct correlation between levels of nNOS and values of mitochondrial enzymes function. In ALS muscles we found normal levels of manganese superoxide dismutase (SOD2) that is assumed as related to mitochondrial DNA abnormalities. Our data suggest a beneficial role for NO to mitochondrial function and lead to the hypothesis of a common oxidative damage in motor neurones and skeletal muscle in sporadic ALS patients.
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