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Polymer‐derived SiOC‐C composites are typically obtained through pyrolysis of a polysiloxane precursor in inert atmosphere. Recent studies have shown that novel SiOC microstructures and compositions can be obtained when the pyrolysis is carried out in a reactive environment, as CO2, which leads to a selective oxidation of the Si─C bonds leaving a microstructure constituted by a nano‐dispersed sp2 carbon phase within an SiO2 matrix. However, little is known about the reaction mechanisms between CO2 and the preceramic polymer to date. In this work, we investigated the pyrolysis of a methyl‐silsesquioxane in reactive (CO2) and inert (Ar or He) atmosphere by combining TG/MS and FT‐IR analysis. The results showed that CO2 starts to react with the preceramic polymer from ≈750°C when the Si─CH3 groups start to form Si─CHx‐Si units. The reaction breaks the Si─C bond increasing the amount of the free carbon phase and releasing water vapor, detected by MS, even at temperatures exceeding 900°C. At higher temperatures (≈950°C), CO2 reacts with the free carbon phase leading to a weight loss and the formation of CO. [ABSTRACT FROM AUTHOR]

This work investigates the potentialities of the thermal analysis (TA) coupled with the mass spectrometry (MS) technique by studying the thermal decomposition of calcium oxalate monohydrate (CaC2O4·H2O). The aim of this work is twofold: to demonstrate, at first, the efficacy of coupling the thermal gravimetric (TG)‐MS experimental approach to check the presence of intermediate reactions beyond the conventional three‐step decomposition: dehydration, decarbonylation, and decarbonation; second, to test the reliability of different modeling approaches in determining the activation energy (E) including, as innovative alternative, the use of the MS signals. The TA is carried out at selected constant heating rates by recording both the thermal gravimetric analysis, differential thermal analysis) profiles, and, simultaneously from the MS data, the signals of the total ion current and the ion currents selected to monitor the release of the H2O, CO, and CO2 species. These experimental results are effective in determining the E through different modeling approaches based on the maximum reaction rate method and isoconversional procedures including both TG and MS signals. Coupling the TA‐MS technique has allowed us to check the concurrent presence of the dismutation reaction of carbon monoxide, occurring during the decarbonylation event, and to determine the corresponding E value (E = 169.7 kJ/mol). These results present a surprising correlation between the global enthalpy of the two reactions involved in this second step and their activation energy values. On the whole, the results satisfy the conventional constraints usually adopted to test the reliability of the E results and are consistent with the majority of the datasets published in the literature on this subject. [ABSTRACT FROM AUTHOR]

The catalytic effect of SnO doping on the hydrogen thermal desorption of MgH was investigated by coupled thermogravimetric-mass spectrometric (TG-MS) analyses, among a set of samples prepared with different SnO/MgH molar ratios. A small release of water and carbon dioxide was detected at temperatures preceding to the reductive thermal decomposition of MgH yielding molecular hydrogen. While TG and DTA data collected at different heating rate provided an apparent activation energy for the entire thermal process, appropriate ion-current signals from the MS-data were used to obtain the activation energies of the single chemical reactions yielding the formation of HO, CO, and H. The most common equations derived from model-free isoconversion methods (Kissinger-Akahira-Sunose; Flynn-Wall-Ozawa; Starink) were used by processing temperatures referred to the different kinds of signals (TG-, DTG-, DTA-, TIC-, molecular-ion IC-curves) for isoconversion points corresponding to the maximum rate of transformation. Temperature and activation energy values were used to compare the kinetic behavior of MgH powder with the best performance of a doped sample, prepared by high energy grinding of SnO/MgH mixture. Doped-MgH samples prepared with SnO mass ratio of 0.2 showed the best kinetic thermal decomposition behavior, with respect to the pristine MgH powder: a decreasing of the main thermal decomposition event of ca. 51-57 °C associated to a coherent decreasing of ca. 20 % (ca. 30 kJ mol) of its activation energy value. [ABSTRACT FROM AUTHOR]

The thermal desorption of hydrogen from commercial MgH powders was investigated by coupled thermogravimetry and mass spectroscopy (TG-MS). The analysis of the evolved gas species gives a detailed picture of the composition of the initial specimen and of its thermal decomposition: just before the H release from MgH, additional HO and CO are detected, sensing the presence of MgCO and Mg(OH) in the initial specimen. Measurements done at different heating rates allowed the determination of apparent activation energies for the whole thermal process. Moreover, by considering appropriate ion current signals of the MS data, the activation energies of the single chemical reactions, leading to the formation of HO, CO, and H, were obtained. Differences coming from the choice of the (model-free isoconversion) method of analysis (Kissinger-Akahira-Sunose; Flynn-Wall-Ozawa; Starink; Friedman equations) are evidenced and discussed. [ABSTRACT FROM AUTHOR]

The structure and properties of polymer-derived Si-( B-) O- C glasses have been shown to be significantly influenced by the boron content and pyrolysis temperature. This work determined the impact of these two parameters on the thermodynamic stability of these glasses. High-temperature oxide melt solution calorimetry was performed on a series of amorphous samples, with varying boron contents (0-7.7 at.%), obtained by pyrolysis of precursors made by a sol-gel technique. Thermodynamic analysis of the calorimetric results demonstrated that at a constant pyrolysis temperature, adding boron makes the materials energetically less stable. While the B-containing glasses pyrolyzed at 1000°C were energetically less stable than the competitive crystalline components, increasing the pyrolysis temperature to 1200°C led to their enthalpic stability. 29 Si and 11B MAS nuclear magnetic resonance ( NMR) spectroscopy measurements on selected samples confirmed a decrease in the concentrations of mixed Si-centered SO i C4− i and B-centered BO j C3− j bonds at the expense of formation of SiO4 and B( OSi)3 species (indicating a tendency toward phase separation) when the boron content and pyrolysis temperature increased. In light of the findings from calorimetry and NMR spectroscopy, we propose a structure-energetic relationship in Si-( B-) O- C glasses. [ABSTRACT FROM AUTHOR]

Ceramic aerogels possess intriguing thermophysical properties which make them excellent candidates for high temperature thermal insulators. However, their properties can degrade at high temperature because of crystallization phenomena or because of densification (causing a sensible reduction of their specific surface area and porosity). The polymer derived ceramic (PDC) route is a relatively new way of developing ceramic aerogels. Several aspects influence the properties of the final product when dealing with preceramic polymers, among them their chemical composition and molecular architecture. In this work, we investigated the possibility of producing aerogels belonging to the SiCN system from polysilazanes mixtures, namely perhydropolysilazane (PHPS) and a methyl/vinyl-containing polysilazane, namely Durazane 1800®, thus changing the C/Si ratio of the amorphous pyrolyzed products. It is shown that the chemical composition of the ceramic aerogel affects the main properties of the porous materials, such as thermal stability and specific surface area (SSA). Results show that the presence of carbon in the aerogels inhibits crystallization of Si 3 N 4 up to 1600 °C in N 2 and allows to maintain a SSA of ~90 m2/g up to this temperature. [ABSTRACT FROM AUTHOR]

In this work the room temperature electrical conductivity of Si(B) OC glasses made via polymer pyrolysis at 1200°C and 1400°C (maximum temperature) and having different amount of boron was measured. When B content is increased from zero (pure Si OC glass) up to B/Si=0.5 the electrical conductivity increases in 2 orders of magnitude from 4.09±0.64×10−5 up to 2.93±1.91×10−3 with a corresponding decrease in the activation energy from about 1.08 to 0.51 eV. This results shows for the first time that the electrical conductivity of Si-based polymer-derived ceramics can be controlled by the amount of the doping element. The structure of the Si(B) OC glasses has been studied with different techniques including FT- IR, XRD and Raman spectroscopy. The Raman study indicates that B partially substitutes C into the sp2 C planes of the free carbon phase forming trigonal BC3 units. Accordingly, the evolution of the electrical properties with the B content has been correlated with the corresponding structural evolution and a hypothesis is presented to rationalize the role of boron on the electrical conductivity of Si OBC glasses. [ABSTRACT FROM AUTHOR]

Silicon oxycarbide (SiOC) aerogels have been synthesized from preceramic polymers via pyrolysis in inert atmosphere at 1200 and 1300 °C. The as synthesized materials have a typical colloidal microstructure with mesoporosity in the range 10–50 nm and no microporosity. HF acid attack of the SiOC aerogels dissolves preferentially the SiO 2 -rich phase and creates micro-and (small)mesopores (<10 nm) in the aerogels microstructure finally leading to a materials with hierarchical porosity. The HF post-pyrolysis treatment is more efficient for the SiOC aerogels pyrolyzed at the maximum temperature, i.e. 1300 °C, leading to a maximum value of specific surface area of 530 m 2 /g and total porosity of 0.649 cc/g. [ABSTRACT FROM AUTHOR]

The feasibility of flash joining conductive ceramics using spark plasma sintering is demonstrated. It is shown that graphite disks can be joined in a few seconds (6–10 s electric discharge time) using a SiOC precursor (methyl silicone resin) as an interlayer. Differently from usual flash experiments, the process does not require any pre-heating, allowing a dramatic reduction of the processing time. XPS analysis of the joint revealed a clear evolution of the chemical environment of silicon with a progressive transition from SiOC to SiO 2 /SiC. Mechanical tests were performed to determine the fracture toughness (1.0 ± 0.2 MPa m0.5) and fracture energy (40.6 ± 9.8 J/m2) of the interface Flash joining can be applied beyond graphite joining, and opens a novel and flexible processing route for many conductive ceramics, as demonstrate by preliminary work on Kanthal® Super MoSi 2 and C f -reinforced SiC. [ABSTRACT FROM AUTHOR]

The internal structure of nanostructured pyrolyzed SiBOC glasses has been analyzed using a combination of transmission electron microscopy techniques including high resolution imaging, EELS spectroscopy and 3D imaging. For the first time, we provide direct information on the 3D architecture within such complex glasses containing SiC nanograins, silica-rich and carbon nanodomains. Their sizes and relative 3D distribution have been precisely quantified. It was shown that the SiC nanocrystals and the silica-rich domains exhibit different types of connectivities with the graphitic carbon network. A direct evidence of the existence of "covalent" bridges between the edges of graphitic nanoplatelets and the SiC nanocrystals is provided. In addition, it was demonstrated that the silica rich domains are not covalently bonded to the graphitic platelets. As a consequence, this amorphous phase can be dissolved into HF and may generate a bimodal porosity, in strong relationship with the initial nanostructure of the SiBOC glass specimen. [ABSTRACT FROM AUTHOR]

This work investigates the lithium doping effect on the structural and thermal behavior of (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2)O high entropy oxide (HEO) obtained by co-precipitation. The powders are characterized by differential thermal and thermogravimetric analyses (DTA-TG), mass spectroscopy, dilatometry, X-ray diffraction (XRD), magic angle spinning nuclear magnetic resonance (MAS NMR), Raman spectroscopy, X-ray photoelectron spectra (XPS) and electron spin resonance (ESR). The results point out that Co3+ ions (within a spinel phase) are reduced to Co2+ before the formation of the high entropy oxide in the absence of Li. Conversely, no reduction is observed in the case of Li-doping, thus indicating the presence of Co3+ within the high entropy rock salt lattice. At high temperature (>1050–1150 °C), the HEO phase loses oxygen changing the charge compensation mechanism for Li incorporation (mostly based on the presence of 3 + cations and oxygen vacancies at low and high temperatures, respectively). Moreover, it is found that lithium lies in two well-distinct chemical environments in HEO, which cannot be completely explained by assuming a random organization of the high entropy phase. This suggests the existence of some short-range order and possible M3+-Li+ pairs. • Li dissolves into the rock salt HEO phase in substitutional positions. • Single-phase HEO is formed without the Co3+ reduction in the case of Li-doping. • Li sits in two well-distinct chemical environments within the HEO structure. • The presence of some kind of short-range order within the Li-doped HEO is hypothesized. [ABSTRACT FROM AUTHOR]

Synthesis and characterization of polymer-derived SiCN aerogels are reported. The wet pre-ceramic gels are synthesized via hydrosilylation reaction of a polysilazane with divinylbenzene as cross-linker using cyclohexane as solvent. Subsequently, the wet polymeric gels are dried using supercritical carbon dioxide for the removal of the solvent. The SiCN ceramic aerogels are obtained through pyrolysis in N 2 at 1000 and 1500 °C. The ceramic aerogels show a low density (<0.1 g/cm 3 ), high porosity (>90%), high specific surface area (>150 m 2 /g) and display a hierarchical pores size distribution which ranges from micropores to mesopores and small macropores. [ABSTRACT FROM AUTHOR]

Abstract: Nanocrystalline TiO2 powders were synthesized by thermohydrolysis of TiCl4 in HCl or NaCl aqueous solutions. Rutile, mixtures of brookite and rutile or mixtures of anatase, brookite and rutile were obtained depending on the acidity of the medium. Crystalline phases and composition of the mixtures were identified by using XRD analysis. Pure brookite nanoparticles, separated from the mixtures of brookite and rutile by simple peptization with water, were stable against transformation to rutile up to 750°C. The prepared TiO2 powders were characterized by thermal analysis, diffuse reflectance spectroscopy and BET surface area determinations. The band gap of bulky brookite was estimated 3.29eV. 4-Nitrophenol photodegradation was used to evaluate the photocatalytic activity of the various samples. The highest activity corresponded to the powders consisting of more than one crystalline phase. [Copyright &y& Elsevier]

Cross‐linking polymethylhydrosiloxane (PMHS) with divinylthiophene (DVT) via hydrosilylation in highly dilute conditions and subsequent supercritical drying in CO2 yield a polymeric aerogel containing aromatic sulfur integrally and uniformly distributed throughout the monolith. Fourier‐transform infrared (FT‐IR) spectroscopy indicates almost complete consumption of vinyl groups and SiH bonds in the product. Both FT‐IR and Raman spectroscopic analyses support loss of conjugation of vinyl groups with the retained double bonds of the thiophene ring. Scanning electron microscopy (SEM) indicates a condensed colloidal structure with characteristic particulate diameters of about 165 nm. SEM coupled with energy dispersive X‐ray spectroscopy elemental mapping shows that sulfur is distributed homogeneously in the polymeric aerogel. Porosimetry of the mesoporous aerogel indicates the effective average pore diameters are about 12 nm. Thermogravimetric analysis (TGA) establishes greater thermal stability of the PMHS‐DVT product than either of the pure unreacted components. TGA coupled with mass spectrometric (TG‐MS) identification of the volatiles released during pyrolysis shows that sulfur is driven from the cross‐linked polymer as thiophene and its derivatives. Recorded mass spectra support the hypothesis that cross‐linking DVT bridges between PMHS chains in the polymeric aerogel, and that this results in a more thermally stable monolith. [ABSTRACT FROM AUTHOR]