Exchange reactions of organic halides and organo-silicon compounds with boron tribromide and boron triiodide

Reactions of the boron halides with organic halides and organo-silicon compounds have been investigated. The results show exchange of halogens between the BX3 (X = Br# 1) and the organic halidef exchange of the halogen of the C-X bond being proved. The rates of halogen exchange vary. Reaction of the heavier halides with organo-silicon compounds indicated that the silicon-carbon bonds ruptured in the absence of electronegative atom attached to the silicon. The presence of an electronegative atom (halogen or oxygen) attached to the silicon causes the bond between the silicon and the electronegative atom to be preferentially broken. Products of exchange reactions of the boron halides and the organic halides or the organo-silicon compounds were studied by use of 1H NMR and GC/MS. From these results some possible mechanisms for the exchange reactions are postulated, but further work is indicated to prove the real courses of the reactions

Si3N4 single-crystal nanowires grown from silicon micro- and nanoparticles near the threshold of passive oxidation

A simple and most promising oxide-assisted catalyst-free method is used to prepare silicon nitride nanowires that give rise to high yield in a short time. After a brief analysis of the state of the art, we reveal the crucial role played by the oxygen partial pressure: when oxygen partial pressure is slightly below the threshold of passive oxidation, a high yield inhibiting the formation of any silica layer covering the nanowires occurs and thanks to the synthesis temperature one can control nanowire dimensions

Si3N4 single-crystal nanowires grown from silicon micro- and nanoparticles near the threshold of passive oxidation

A simple and most promising oxide-assisted catalyst-free method is used to prepare silicon nitride nanowires that give rise to high yield in a short time. After a brief analysis of the state of the art, we reveal the crucial role played by the oxygen partial pressure: when oxygen partial pressure is slightly below the threshold of passive oxidation, a high yield inhibiting the formation of any silica layer covering the nanowires occurs and thanks to the synthesis temperature one can control nanowire dimensions

Composite Material Sensitive to Volatile Organic Compounds

This work evaluates fluorinated thin films and their composites for sensor development. Composites were produced using 5 µm starch particles and plasma films obtained from organic fluorinated and silicon compounds reactants. Silicon wafers and aluminum trenches were used as substrates. Film thickness, refractive index and chemical structure were also determined. Scanning electron microscopy shows conformal deposition on aluminum trenches. Films deposited on silicon were exposed to vapor of volatile organic compounds and CV curves were obtained. A qualitative model (FemLab 3.2® program) was proposed for the electronic behavior. These environmentally correct films can be used in electronic devices and preferentially reacted to polar compounds. Nonetheless, due to the difficulty in signal recovery, these films are more effective in one-way sensors, in sub-ppm range.

Mechanical Characterization of Multicrystalline Silicon Substrates for Solar Cell Applications

The possibility of using more economical silicon feedstock, i.e. as support for epitaxial solar cells, is of interest when the cost reduction and the properties are attractive. We have investigated the mechanical behaviour of two blocks of upgraded metallurgical silicon, which is known to present high content of impurities even after being purified by the directional solidification process. These impurities are mainly metals like Al and silicon compounds. Thus, it is important to characterize their effect in order to improve cell performance and to ensure the survival of the wafers throughout the solar value chain. Microstructure and mechanical properties were studied by means of ring on ring and three point bending tests. Additionally, elastic modulus and fracture toughness were measured. These results showed that it is possible to obtain marked improvements in toughness when impurities act as microscopic internal crack arrestors. However, the same impurities can be initiators of damage due to residual thermal stresses introduced during the crystallization process.

Mechanical characterization of multicrystalline silicon wafers

The possibility of using more economical silicon feedstock, i.e. as support for epitaxial solar cells, is of interest when the cost reduction and the properties are attractive. We have investigated the mechanical behavior of two blocks of upgraded metallurgical silicon, which is known to present high content of impurities even after being purified by the directional solidification process. The impurities are mainly metals like Al and silicon compounds. Thus, it is important to characterize their effect in order to improve cell performance and to ensure the survival of the wafers throughout the solar value chain. Microstructure and mechanical properties were studied by means of ring on ring and three point bending tests. Additionally, Young’s modulus, hardness and fracture toughness were measured. These results showed that it is possible to obtain marked improvements in toughness when impurities act as microscopic internal crack arrestors. However, the same impurities can be initiators of damage due to residual thermal stresses introduced during the crystallization process.

Synthesis and characterization of cyclotriphosphazenes containing silicon as single solid-state precursors for the formation of silicon/phosphorus nanostructured materials

The synthesis and characterization of new organosilicon derivatives of N3P3Cl6, N3P3[NH(CH2)3Si(OEt)3]6 (1), N3P3[NH(CH2)3Si(OEt)3]3[NCH3(CH2)3CN]3 (2), and N3P3[NH(CH2)3Si(OEt)3]3[HOC6H4(CH2)CN]3 (3) are reported. Pyrolysis of 1, 2, and 3 in air and at several temperatures results in nanostructured materials whose composition and morphology depend on the temperature of pyrolysis and the substituents of the phosphazenes ring. The products stem from the reaction of SiO2 with P2O5, leading to either crystalline Si5(PO4)6O, SiP2O7 or an amorphous phase as the glass Si5(PO4)6O/3SiO2·2P2O5, depending on the temperature and nature of the trimer precursors. From 1 at 800 °C, core−shell microspheres of SiO2 coated with Si5(PO4)6O are obtained, while in other cases, mesoporous or dense structures are observed. Atomic force microscopy examination after deposition of the materials on monocrystalline silicon wafers evidences morphology strongly dependent on the precursors. Isolated islands of size ∼9 nm are observed from 1, whereas dense nanostructures with a mean height of 13 nm are formed from 3. Brunauer−Emmett−Teller measurements show mesoporous materials with low surface areas. The proposed growth mechanism involves the formation of cross-linking structures and of vacancies by carbonization of the organic matter, where the silicon compounds nucleate. Thus, for the first time, unique silicon nanostructured materials are obtained from cyclic phosphazenes containing silicon.

Investigating routes toward atomic layer deposition of silicon carbide: Ab initio screening of potential silicon and carbon precursors

Silicon carbide (SiC) is a promising material for electronics due to its hardness, and ability to carry high currents and high operating temperature. SiC films are currently deposited using chemical vapor deposition (CVD) at high temperatures 1500–1600 °C. However, there is a need to deposit SiC-based films on the surface of high aspect ratio features at low temperatures. One of the most precise thin film deposition techniques on high-aspect-ratio surfaces that operates at low temperatures is atomic layer deposition (ALD). However, there are currently no known methods for ALD of SiC. Herein, the authors present a first-principles thermodynamic analysis so as to screen different precursor combinations for SiC thin films. The authors do this by calculating the Gibbs energy ΔGΔG of the reaction using density functional theory and including the effects of pressure and temperature. This theoretical model was validated for existing chemical reactions in CVD of SiC at 1000 °C. The precursors disilane (Si2H6), silane (SiH4), or monochlorosilane (SiH3Cl) with ethyne (C2H2), carbontetrachloride (CCl4), or trichloromethane (CHCl3) were predicted to be the most promising for ALD of SiC at 400 °C.

Accurate prediction of the Si/SiO(2) interface band offset using the self-consistent ab initio DFT/LDA-1/2 method

We use the density functional theory/local-density approximation (DFT/LDA)-1/2 method [L. G. Ferreira , Phys. Rev. B 78, 125116 (2008)], which attempts to fix the electron self-energy deficiency of DFT/LDA by half-ionizing the whole Bloch band of the crystal, to calculate the band offsets of two Si/SiO(2) interface models. Our results are similar to those obtained with a ""state-of-the-art"" GW approach [R. Shaltaf , Phys. Rev. Lett. 100, 186401 (2008)], with the advantage of being as computationally inexpensive as the usual DFT/LDA. Our band gap and band offset predictions are in excellent agreement with experiments.

Recuperação de metais de catalisadores de hidrorrefino usados via fusão com KHSO4

This work describes a process for metal recovery from spent NiMo and CoMo/Al2O3 commercial hydrorefining catalysts. The samples were treated by fusion with potassium hydrogen sulfate (5 h, 600 ºC) with a KHSO4/catalyst mass ratio of 10:1. After fusion the solid was solubilized in water (100 ºC), leaving silicon compounds as residue. Losses of nickel and cobalt may reach 16 wt% of the amount present in the sample, depending on the silicon content. Soluble metals were isolated by selective precipitation techniques (nickel, cobalt, aluminum) or by solvent extraction with methyl-isobutyl ketone (molybdenum) in a hydrochloric acid medium. All metals were recovered in very good yields except for nickel and cobalt in the presence of considerable amounts of silicon. Soluble wastes consist of potassium/sodium sulfates/chlorides. Solid wastes correspond to about 4 wt% of the catalyst and can be discarded in industrial dumps.

Reactivity of Tungsten-aryloxides with Hydrosilane Cocatalysts in Olefin Metathesis

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Strain behavior of lanthanum modified BiFeO3 thin films prepared via soft chemical method

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

HMDSO plasma polymerization and thin film optical properties

Thin films were deposited from hexamethyldisiloxane (HMDSO) in a glow discharge supplied with radiofrequency (rf) power. Actino-metric optical emission spectroscopy was used to follow trends in the plasma concentrations of the species SiH (414.2 nm), CH (431.4 nm), CO (520.0 nm), and H (656.3 nm) as a function of the applied rf power (range 5 to 35 W). Transmission infrared spectroscopy (IRS) was employed to characterize the molecular structure of the polymer, showing the presence of Si-H, Si-O-Si, Si-O-C and C-H groups. The deposition rate, determined by optical interferometry, ranged from 60 to 130 nm/min. Optical properties were determined from transmission ultra violet-visible spectroscopy (UVS) data. The absorption coefficient α, the refractive index n, and the optical gap E04 of the polymer films were calculated as a function of the applied power. The refractive index at a photon energy of 1 eV varied from 1.45 to 1.55, depending on the rf power used for the deposition. The absorption coefficient showed an absorption edge similar to other non-crystalline materials, amorphous hydrogenated carbon, and semiconductors. For our samples, we define as an optical gap, the photon energy E04 corresponding to the energy at an absorption of 104 cm-1. The values of E04 decreased from 5.3 to 4.6 as the rf power was increased from 5 to 35 W. © 1995.

Monolithic diphasic gels of mullite by sol-gel process under ultrasound stimulation

Diphasic gel in the mullite composition was prepared from a colloidal sol of boehmite mixed with a hydrolyzed tetraethoxisilane (TEOS) solution. The boehmite sol was obtained by peptization of a poorly crystallized or very small mean crystallite size (∼34 Å) precipitate, resulting from the reaction between solutions of aluminum sulfate and sodium hydroxide. Ultrasound was utilized in the processes of the TEOS hydrolysis and the boehmite peptization, and also for complete homogenization of the mixture to gel. The wet gel is almost clear and monolithic. The gel transparency is lost on drying, when syneresis has ended, so that the interlinked pore structure starts to empty and is recovered upon water re-absorption. Cracking closely accompanies this critical drying process. Differential thermal analysis (DTA) and X-ray diffraction (XRD) show that the solid structure of the gel is composed of an amorphous silica phase, as a matrix, and a colloidal sized crystalline phase of boehmite. Upon heat treatment, the boehmite phase within the gel closely follows the same transition sequence as in pure alumina shifted towards higher temperatures. Orthorhombic mullite formation was detected at 1300°C. © 1998 Elsevier Science B.V. All rights reserved.