Marco A. Schiavon

Investigation on kinetics of thermal decomposition in polysiloxane networks used as precursors of silicon oxycarbide glasses

In this study, polysiloxane networks prepared by hydrosilylation or hydrolysis/condensation reactions were considered to be potential precursors for Si–C–Osystems. Different precursors had different pyrolytic properties, which was essentially due to their molecular architecture. The kinetics parameters, such as the activation energy, E (kJ/mol) involved in the polymer-to-ceramic conversion, were investigated by thermogravimetry using a multiple heating rate kinetic method. The relationships between the molecular architecture and the precursor composition were compared to that of a linear poly(dimethylsiloxane) precursor. Solid-state 29 Si nuclear magnetic resonance, infrared spectroscopies, density measurements, and X-ray diffraction measurements were made on the final samples. These products were typically amorphous, with a molecular structure formed by a random distribution of different silicon sites and variable amounts of free carbon residue. 2002 Elsevier Science B.V. All rights reserved.

Catalytic activity of nitro- and carboxy-substituted iron porphyrins in hydrocarbon oxidation: Homogeneous solution and supported systems

Abstract A series of iron(III)porphyrins containing NO2-substituents in the meso-ortho-phenyl or COOH-substituents in the meso-para-phenyl rings (FeP) have been used to catalyse hydrocarbon oxidation by iodosylbenzene. The FeP series were efficient and selective catalysts for alkene epoxidation and alkane hydroxylation. The most promising iron porphyrin, 5,10,15-tri(2-nitrophenyl)20-mono-(4-carboxyphenyl)porphyrin iron(III) chloride, Fe(TNMCPP)Cl, was covalently bound to aminopropylated silica (APS) through covalent binding between COOH groups in the FeP (after activation by reaction with SOCl2) and NH2 groups on the funcionalised silica resulting in the anchored catalyst SiNH(TNMCPP)FeCl. This system proved to be a highly efficient catalyst for alkene epoxidation. The same iron porphyrin was also supported on APS through electrostatic binding, resulting the heterogeneous catalyst SiNH3+(TNMCPP)FeCl. The iron centres are in different surroundings on the two supports as demonstrated by EPR, UV/VIS and oxidation reactions results. The SiNH3+(TNMCPP)FeCl is not a good catalyst for hydrocarbon oxidation, which can be attributed to the higher polarity of this support and the bis-axial coordination by the free NH2 groups of the support with the iron centre. Both effects are unfavourable for the interaction between the non-polar substrates and the catalyst.

Effect of surface ligands on the optical properties of aqueous soluble CdTe quantum dots

We investigate systematically the influence of the nature of thiol-type capping ligands on the optical and structural properties of highly luminescent CdTe quantum dots synthesized in aqueous media, comparing mercaptopropionic acid (MPA), thioglycolic acid (TGA), 1-thioglycerol (TGH), and glutathione (GSH). The growth rate, size distribution, and quantum yield strongly depend on the type of surface ligand used. While TGH binds too strongly to the nanocrystal surface inhibiting growth, the use of GSH results in the fastest growth kinetics. TGA and MPA show intermediate growth kinetics, but MPA yields a much lower initial size distribution than TGA. The obtained fluorescence quantum yields range from 38% to 73%. XPS studies unambiguously put into evidence the formation of a CdS shell on the CdTe core due to the thermal decomposition of the capping ligands. This shell is thicker when GSH is used as ligand, as compared with TGA ligands.

Synthesis of a polycyclic silazane network and its evolution to silicon carbonitride glass

Abstract Polysilazanes have been described as excellent polymeric precursors to amorphous silicon carbonitride (SiCN) glasses. In this work, the synthesis, thermal cross-linking and pyrolysis of a polycyclic silazane network precursor for a SiCN glass, were studied. This polymeric precursor was prepared by free radical polymerization of 1,3,5-trimethyl-1′,3′,5′-trivinylcyclotrisilazane, using dicumyl peroxide as radical initiator. The polymer-to-ceramic conversion was investigated by 29 Si and 13 C magic angle spinning nuclear magnetic resonance (MAS NMR) and infrared spectroscopies, X-ray diffraction and by simultaneous thermogravimetric and differential thermal analyses. Based on the results the polymeric precursor can be characterized as a polycyclic silazane network, containing aliphatic carbon segments produced by the vinyl polymerization. 29 Si and 13 C NMR data of this polymeric precursor heated at different temperatures indicated the occurrence of side-reactions in addition to the vinyl polymerization. The SiCN glass was obtained as an amorphous solid up to 1400 ° C . Above this temperature, the crystallization process of the ceramic product was promoted by rearrangement of the different Si sites in the amorphous network. Heat treatment at 1600 ° C , under N2 atmosphere, produced a ceramic containing 85% silicon carbide (SiC) and 15% silicon nitride, determined by 29 Si MAS NMR in relation to the total silicon sites, and under argon atmosphere, SiC.

Microstructural characterisation of monolithic ceramic matrix composites from polysiloxane and SiC powder

Abstract The microstructure of ceramic matrix composites (CMCs) prepared by moulding α-SiC powder in the presence of preceramic polysiloxane as a binder was investigated. During pyrolysis, the powder/polymer system was converted into a ceramic body, in which the shrinkage and the porosity contents could be tailored. Different amounts of polymeric binder and also different firing temperatures were found to have strong influence on the microstructural parameters, such as average pore size, pore size distribution and overall porosity, as determined by both mercury porosimetry and nitrogen adsorption techniques. These parameters were related to shrinkage rate. The morphological characterisation of the composites obtained at 1000 °C, performed by field emission scanning electron microscopy, showed a uniform distribution of the α-SiC, in the SiCxO4−x phase, as well as SiC whisker particles in the pores of the CMCs. Additional characterisation of the ceramic phase, obtained from preceramic polysiloxane, was performed by 29Si and 13C nuclear magnetic resonance and infrared spectroscopies and also by X-ray diffraction. The results suggested a random distribution of silicon sites, SiCxO4−x with 0≤x≤4, in the ceramic bulk, at 1000 °C. This phase evolved to β-SiC at higher temperatures.

Poly(borosiloxanes) as precursors for carbon fiber ceramic matrix composites

Ceramic matrix composites (CMCs), constituted of a silicon boron oxycarbide (SiBCO) matrix and unidirectional carbon fiber rods as a reinforcement phase, were prepared by pyrolysis of carbon fiber rods wrapped in polysiloxane (PS) or poly(borosiloxane) (PBS) matrices. The preparation of the polymeric precursors involved hydrolysis/condensation reactions of alkoxysilanes in the presence and absence of boric acid, with B/Si atomic ratios of 0.2 and 0.5. Infrared spectra of PBS showed evidence of Si-O-B bonds at 880 cm-1, due to the incorporation of the crosslinker trigonal units of BO3 in the polymeric network. X ray diffraction analyses exhibited an amorphous character of the resulting polymer-derived ceramics obtained by pyrolysis up to 1000 °C under inert atmosphere. The C/SiBCO composites showed better thermal stability than the C/SiOC materials. In addition, good adhesion between the carbon fiber and the ceramic phase was observed by SEM microscopy

Synthesis and characterization of a novel series of meso (nitrophenyl) and meso (carboxyphenyl) substituted porphyrins

The anionic 5,10,15-tris(4-carboxyphenyl), 20-mono(2-nitrophenyl) porphyrin (1), 5,10(or 15)-bis(4-carboxyphenyl), 15(or 10),20-bis(2-nitrophenyl)porphyrin (2) and 5-mono(4-carboxyphenyl), 10,15,20-tris(2-nitrophenyl)porphyrin (3) were sinthesized directly by reaction of pyrrole with substituted benzaldehydes in nitrobenzene/propionic acid media. The benzaldehydes molar ratio was controlled to optimize the synthesis and purification of the desired porphyrins. This new series of porphyrins was characterised by TLC, mass spectrometry (FAB MS), 1H NMR, UV/Vis, IR and electrochemistry. 5,10,15,20-Tetrakis(4-carboxyphenyl)porphyrin (4) and 5,10,15,20-Tetrakis(2-nitrophenyl)porphyrin (5) were also characterised for comparative purposes, completing the series The electrochemical reduction was investigated for the free base and corresponding iron(III) porphyrins on glassy carbon and mercury electrodes. The reduction potentials showed the expected dependence on the number of electron-withdrawing nitro groups present on the porphyrin ring providing additional evidences for the characterisation of the synthesised compounds.

Full factorial design analysis of carbon nanotube polymer-cement composites

The work described in this paper is related to the effect of adding carbon nanotubes (CNT) on the mechanical properties of polymer-cement composites. A full factorial design has been performed on 160 samples to identify the contribution provided by the following factors: polymeric phase addition, CNT weight addition and water/cement ratio. The response parameters of the full factorial design were the bulk density, apparent porosity, compressive strength and elastic modulus of the polymer-cement-based nanocomposites. All the factors considered in this analysis affected significantly the bulk density and apparent porosity of the composites. The compressive strength and elastic modulus were affected primarily by the cross-interactions between polymeric phase and CNT additions, and the water/cement ratio with polymeric phase factors.

Optimizing photovoltaic performance in CuInS2 and CdS quantum dot-sensitized solar cells by using an agar-based gel polymer electrolyte

Quantum dot-sensitized solar cells (QDSSCs) offer new opportunities to address the clean energy challenge, being one of the top candidates for third generation photovoltaics. Like dye-sensitized solar cells (DSSCs), QDSSCs normally use liquid electrolytes that suffer from issues such as evaporation or leakage. In this study a gel polysulfide electrolyte was prepared containing a natural polymer, agar, and was used as a quasi-solid-state electrolyte in solar cells to replace the conventional liquid electrolytes. This gel electrolyte shows almost the same conductivity as the liquid one. The solar cells were fabricated using CuInS2 quantum dots (QDs), previously synthesized, deposited on TiO2 photoanodes by electrophoretic deposition (EPD). CdS was deposited on TiO2 by successive ionic layer adsorption and reaction (SILAR). Reduced graphene oxide (RGO)–Cu2S, brass, and thin film CuxS were used as counter electrodes. Compared to a liquid polysulfide water based electrolyte, solar cells based on CuInS2 and CdS using gel polymer electrolyte (GPE) exhibit greater incident photon to current conversion efficiency (IPCE = 51.7% at 520 nm and 72.7% at 440 nm), photocurrent density (Jsc = 10.75 and 13.51 mA cm−2), and power conversion efficiency (η = 2.97 and 2.98%) while exhibiting significantly enhanced stability. The solar cells employing the agar-based gel polymeric electrolyte are about a factor of 0.20 more stable than using a liquid electrolyte. The higher photovoltaic performance is due to the good conductivity and high wettability as well as the superior permeation capability of the gel electrolyte into the mesoporous matrix of a TiO2 film.

Influence of Inert and Oxidizing Atmospheres on the Physical and Optical Properties of Luminescent Carbon Dots Prepared through Pyrolysis of a Model Molecule

This work used L-tartaric acid as a model molecule to evaluate how the use of inert and oxidizing atmospheres during pyrolysis affected the physical and optical properties of the resulting carbon dots (CDs). Pyrolysis revealed to be a simple procedure that afforded CDs in a single step, dismissed the addition of organic solvents, and involved only one extraction stage that employed water. By X-ray diffraction a dependency between the structure of the CDs and the atmosphere (oxidizing or inert) used during the pyrolysis was found. Potentiometric titration demonstrated that the CDs were largely soluble in water; it also aided characterization of the various groups that contained sp(3) -hybridized carbon atoms on the surface of the dots. Raman spectroscopy suggested that different amounts of sp(2)- and sp(3)-hybridized carbon atoms emerged on the CDs depending on the pyrolysis atmosphere. In conclusion, the pyrolysis atmosphere influenced the physical properties, such as the composition and the final structure.