P.H. Mutin

Non-hydrolytic sol–gel routes to silica

Abstract Silica was prepared by solvent-free non-hydrolytic condensation reactions from silicon tetrachloride with different oxygen donors: benzyl alcohol and tetrabenzyloxysilane (no catalysis), tetraethoxysilane, tetraisopropoxysilane, diethylether and diisopropylether (catalyzed by FeCl3, TiCl4, AlCl3 or ZrCl4). Highly condensed gels with a low hydroxyl content were obtained in high yields. A solution 29Si NMR study showed that in the presence of FeCl3 the condensation of SiCl4 with Si(OiPr)4 was accompanied by extensive redistribution of Si–Cl and Si–OiPr bonds, leading to increasingly complex mixtures. Non-hydrolytic silica xerogels were characterized by chemical analysis, IR and 29Si NMR spectroscopy and thermogravimetric analysis. Porosity and specific surface area were determined using nitrogen adsorption–desorption. Xerogels displayed a wide variety of textures depending on the nature of the oxygen donor and of the catalyst.

Preparation of alumina gels by a non-hydrolytic sol-gel processing method

Abstract Monolithic alumina gels were prepared by a novel non-hydrolytic sol-gel route based on the condensation reaction between aluminum alkoxides Al(OR)3 and aluminum halides AlX3, through the formation of alkyl halide RX(X = Br, Cl; R = iso-propyl, sec- or ter-butyl), around 100°C. Samples were then calcined and were found to be amorphous up to about 750°C; they kept a high specific surface area to about 900°C. This delayed crystallization is correlated with the large number of five-coordinate aluminum sites measured in dried gels by 27Al NMR spectroscopy.

29Si nuclear magnetic resonance study of the structure of silicon oxycarbide glasses derived from organosilicon precursors

Several silicon oxycarbide glasses with O/Si ratios 1.2 and 1.8 were prepared by pyrolysis below 1000 °C of organosilicon precursors. Their structure was investigated using29Si magic-angle spinning nuclear magnetic resonance. The spectra obtained are consistent with a purely random distribution of Si-O and Si-C bonds, which depends on the O/Si ratio of the glass only, regardless of the structure of the precursors. This result is interpreted by the occurrence, during the pyrolysis of entropically controlled redistribution reactions involving the exchange of Si-O and Si-C bonds.

Preparation and Structure of Silicon Oxycarbide Glasses Derived from Polysiloxane Precursors

Silicon oxycarbide phases with different O/Si ratios were prepared by pyrolysis of highly cross-linked polysiloxane precursors. Their structure was investigated using quantitative29Si solid-state NMR and X-ray photoelectron spectroscopy (XPS). The prominent part played by the redistribution reactions on the ceramic yield, the composition of the oxycarbide phases and their structure was illustrated. In a large range of compositions the oxycarbide phases formed by pyrolysis at moderate temperature (<1000°C) may be described as a purely random arrangement of CSi4 tetrahedra and Si−O−Si bridges.

Novel non-hydrolytic sol-gel route to metal oxides

Monolithic alumina and aluminosilicate gels have been prepared using a novel sol-gel process based on the non-hydrolytic condensation reaction between a metal halide and a metal alkoxide. XRD indicated that the alumina gel remained amorphous at 750°C; solid state 27Al NMR indicated the presence in the dried gel and in the amorphous calcined sample of a large amount of pentacoordinated aluminum atoms. A study of the sol formation using liquid state 27Al NMR suggested that the gel structure was reminiscent of the oligomeric structure of the chloroisopropoxide precursors. Differential thermal analysis and XRD indicated that the aluminosilicate gels were converted to mullite below 1000°C, suggesting a high degree of homogeneity in these precursors.

29Si MAS-NMR Study of Silica Gels and Xerogels: Influence of the Catalyst

Four silica gels were prepared by hydrolysis of tetraethoxysilane (TEOS) in ethanol, using different catalysts: HCl, NaOH, NH3, and NBu4F. Nitrogen adsorption-desorption isotherms indicated that the HCl-catalyzed xerogel was purely microporous, whereas the other samples exhibited a very broad distribution of mesopores and a variable amount of micropores. 29Si MAS NMR spectroscopy of the wet gels (before drying) pointed to a low degree of condensation for the HCl-catalyzed gel, and to the presence of unhydrolyzed TEOS monomer in the NaOH-catalyzed gel. Comparison with the 29Si MAS NMR spectra of the xerogels indicated a significant increase of the degree of condensation during the drying procedure (3 hrs at 120°C under vacuum) for the HCl-catalyzed gel.

THERMAL DECOMPOSITION OF HYDROSILSESQUIOXANE GELS UNDER ARGON

Abstract The thermal decomposition of HSiO 3/2 gels in argon gives rise first to a minor evolution of SiH 4 (below 500°C), then to a release of dihydrogen gas until complete dehydrogenation (up to 1000°C). From NMR, IR and ESR spectroscopy the second step appears to be associated with the formation of SiSi bonds in units Si(Si z O 4−z ); XANES indicates the significant presence of silica- and silicon-like sites as early as 500°C. The phase separation and the crystallisation of silica and silicon occur at higher temperatures. A mechanism based on silylene intermediates is proposed for the first steps of the process.

{sup 29}Si NMR and XPS investigation of the structure of silicon oxycarbide glasses derived from polysiloxane precursors

Two silicon oxycarbide glasses with different compositions (O/Si ratio 1.2 and 1.8) were prepared by pyrolysis at moderate temperature (900 °C) of polysiloxane precursors. Their structure was investigated using quantitative 29 Si solid-state NMR and X-ray photoelectron spectroscopy (XPS). The environment of the silicon atoms in the oxycarbide phase corresponded to a purely random distribution of Si-O and Si-C bonds depending on the O/Si ratio of the glass only and not on the structure of the precursors. At the light of the NMR results, the Si2p XPS spectra of the glasses may be interpreted using the contribution of the five possible S iO x C 4-x tetrahedra. The Cls spectra of these glasses indicated the presence of oxycarbide carbon in C Si 4 tetrahedra, similar to carbide carbon, and graphitic-like excess carbon.

Introduction of Organic Moieties into Transition-Metal Oxide Matrices via Phosphonate Groups

Transition metal oxide / phenylphosphonate hybrids with M/P ratios ranging from 1 to 5, (M= Ti, Zr) and metal phosphonates (M/P = 0.5) have been prepared by a sol-gel process involving in a first step the non-hydrolytic condensation between metal alkoxide and phosphonic acid leading to M-O-P bonds, followed by the hydrolysis-condensation of the remaining M-OR groups. The composition, texture and structure of the materials were investigated using EDX, TGA, XRD, IR and 31 P NMR.

Octadecylphosphonic Acid Self-Assembled Monolayers as Lubricant Coatings Stable in Alkaline Media

The formation of octadecylphosphonic acid self-assembled monolayers (SAMs) on titanium deposited by cathodic sputtering on planar substrates has been studied by water contact angle measurements and Atomic Force Microscopy (AFM). Dense monolayers were obtained after one week of reaction. Tribological experiments indicated that these monolayers had good lubricating properties, which were maintained after immersion in a 1 M NaOH solution at 65 °C for up to 100 min. The stability in alkaline media of octadecylphosphonic acid SAMs on titanium was ascribed to the high chemical stability of Ti-O-P interfacial bonds.