Preparation of low-density xerogels from mixtures of TEOS with substituted alkoxysilanes. II. Viscosity study of the sol–gel transition

Mixtures of TEOS with substituted methoxysilanes generate low-density xerogels due to a nucleation mechanism involving the substituted alkoxysilane. The sol–gel transition of these mixtures was followed by rheological characterisation. The transition from sol to gel takes place in a few minutes at ambient temperature. For the series exhibiting nucleation by the additive, the gel time goes through a slight minimum when the ratio of additive/main reagent increases. The elastic modulus increases with increasing ratio of additive/main reagent as the particle size decreases because of the nucleation mechanism by the additive. Samples with smaller particles exhibit the highest modulus for equal silica concentrations.     

Surface morphology of iron specimens boronized with BCl3—H—mixtures (I) influence of process parameters

Specimens made of armco iron were boronized with 1.1 Vol.% BCl₃ in H₂ at 1023 K, 1123 K, and 1223 K during different periods. Afterwards, their surfaces were examined by scanning electron microscopy. As a result of variation of the process parameters the surface morphology of the boride layers formed was changed. The etch attack of the boronizing gas mixture on the substrate may be reduced by appropriate parameters.     

Preparation of zirconia mullite flakes via a plasma rapid solidification process using starting materials derived from a sol–gel technique

Flakes of zirconia–mullite with different zirconia contents varying from 3 to 24 wt% were produced from sol–gel derived raw materials via a plasma melting method followed by a rapid solidification process using a rotating copper roll. The morphology, phase constitution and microstructure development of the as-prepared flakes and of the flakes after various heat treatments were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). It was found that the starting materials could be transformed from the initial irregular-shaped powders into flakes which consisted of mullite, zirconia phase, a relatively large amount of glassy phase and pores. Using TEM, it was shown that the crystalline phases consisted of zirconia polymorphs and mullite. The glassy phases consisted of Al₂O₃–SiO₂ solid solution supersaturated with zirconia. Firing at 1500 °C or 1700 °C resulted in full crystallisation of the flakes and a fairly homogeneous distribution of zirconia particles in terms of size and shape dispersed in the mullite matrix, which contained both intra-granular and inter-granular precipitates. The microstructural characteristics of the flakes may provide promising physical properties for applications in high temperature thermal insulation materials.     

1H, 29Si and 27Al NMR study of the destabilization process of a paracrystalline opal from Mexico

This is the first NMR study of the destabilization through whitening of paracrystalline opal-CT (SiO₂, nH₂O). The aim of this study is to understand the process leading to the degradation, particularly any changes in the structure and bonding of this material. ²⁹Si and ²⁷Al NMR signals are not significantly modified. In contrast, using for the first time ¹H as a probe in opal, we show that a comprehensive understanding can be obtained on both the local structure of the opal-CT and the destabilization process. Thus, we propose the latter is related to some changes in ¹H speciation (loss of strongly ‘bound’ water and appearance of new species) which induce structural reorganizations on the surface and in the rim of the silica nanograins constituting opal-CT.     

Small-angle X-ray scattering study of the influence of solvent replacement (from H<Subscript>2</Subscript>O to D<Subscript>2</Subscript>O) on the initial crystallization stage of tetragonal lysozyme

The composition of lysozyme solutions in D₂O under conditions favorable for the formation of tetragonal crystals has been investigated at different protein concentrations by small-angle X-ray scattering using the synchrotron radiation. In addition to lysozyme monomers, dimeric and octameric species are found in the crystallization solutions; the octamer content increases with an increase in the protein concentration. A comparison of the data with those obtained under similar conditions but with H₂O used as a solvent has shown that the replacement of light water with heavy one leads to increase of octamer volume fraction in solution.     

Growth of filamentary KDP crystals from solution with the addition of Al(NO<Subscript>3</Subscript>)<Subscript>3</Subscript> · 9H<Subscript>2</Subscript>O

To elucidate the mechanism of nucleation and growth of filamentary crystals on the bipyramid faces of KH₂PO₄ (KDP) crystals from solution with the addition of Al(NO₃)₃ · 9H₂O, the growth rates and transverse dimensions of the crystals were measured at various supersaturations, temperatures, and impurity concentrations. The dependences obtained can be interpreted with due regard for the competition between the intrinsic and impurity particles in the presence of Cabrera-Vermilyea stoppers.     

Modular design of locally ordered tetrahedral structures of Si,SiO<Subscript>2</Subscript>, and H<Subscript>2</Subscript>O: I. investigation of the possible cooperative local crystallization in amorphous matrices of these compounds

The hypothesis is put forward that the structure of amorphous silicon and other amorphous compounds with a tetrahedral coordination of atoms consists of local regions characterized by an ordered but not crystalline structure and that, owing to the regularity inherent in this structure, solid-phase crystallization of the amorphous compounds under consideration can occur through the cooperative mechanism. The modular design is used to construct a possible structure of similar local regions and to describe the mechanism of its cooperative transformation into a diamond-like structure with the possible formation of numerous microtwins. The difficulties encountered in the course of the cooperative transformation are considered. Algorithms are proposed for constructing one more connected structure of local regions from the same modules (closed hollow loops of bonds), which undergoes a cooperative transformation into crystalline silicon without the aforementioned difficulties.     

Crystal structure of the gold(I) complex [Au(tfp)Cl] with tfp = (C<Subscript>4</Subscript>H<Subscript>3</Subscript>O)<Subscript>3</Subscript>P

The Au(I) complex [Au(tfp)Cl] with tfp = (C₄H₃O)₃P has been prepared and characterised. It crystallises in the monoclinic P2₁/n space group with z = 4, a = 9.837(3), b = 12.684(4), c = 11.103(5)Å, = 91.58(2)° and V = 1384.8(2)ų. The structure of the complex consists of molecules connected in head-to-tail dimers by a metal-halogen contact as generally found in analogus complexes of lighter coinage metals. The typical feature of halogen gold compounds, the Au–Au interaction, seems prevented by the bulk of the tfp ligand.     

Crystal Structures of Energetic Compounds. I. Safe Small-Scale Preparation and X-Ray Structure Determination of the High-Energy Cobalt(III) Complex [Co(NH<Subscript>3</Subscript>)<Subscript>5</Subscript>N<Subscript>3</Subscript>](N<Subscript>3</Subscript>)<Subscript>2</Subscript>

Abstract  Well-formed dark red single-crystals of the potentially explosive cobalt(III) azido complex [Co(NH₃)₅N₃](N₃)₂ (nitrogen content: 72.59%) have been grown using a safe small-scale preparation from [Co(NH₃)₅N₃]Cl₂ and sodium azide. The complex crystallizes in the orthorhombic system, space group Pnma with unit cell parameters a = 12.921(3) ?, b = 10.346(2) ?, c = 8.0315(16) ?, V = 1,073.7(4) ?³ and Z = 4. Five NH₃ ligands and the azide anion surround the Co³⁺ ion in a nearly perfect octahedral geometry. Two azide anions are non-coordinating, and no water of crystallization is present in the crystal structure. Graphical Abstract  Well-formed dark red single-crystals of the potentially explosive cobalt(III) azido complex [Co(NH₃)₅N₃](N₃)₂ (nitrogen content: 72.59%) have been grown using a safe small-scale preparation from [Co(NH₃)₅N₃]Cl₂ and sodium azide. Five NH₃ ligands and the azide anion surround the Co³⁺ ion in a nearly perfect octahedral geometry. Two azide anions are non-coordinating, and no water of crystallization is present in the crystal structure.