Investigation of growth kinetics and morphology of sodium fluorosilicate ice‐analogue crystals in solutions and gels

The effect of growth method (solution or gel growth) and growth conditions on the morphology of ice analogue crystals (sodium fluorosilicate) has been studied. Many habits typical for atmospheric ice crystals (hexagonal columns, plates, different types of stars or dendrites) were obtained during experiments. The dependence of growth rate of basal and prism faces of columnar crystals on supersaturation was measured for crystals growing by evaporation of solvent. The experiments establish evaporation rate and growth rate ranges at which appearance of certain types of crystals is most probable. The experiments have also shown that good quality sodium fluorosilicate crystals can be obtained in TMS gel. These crystals were significantly bigger than those obtained in solution. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Determination of the size and phase composition of silver nanoparticles in a gel film of bacterial cellulose by small-angle X-ray scattering,electron diffraction,and electron microscopy

The nanoscale structural features in a composite (gel film of Acetobacter Xylinum cellulose with adsorbed silver nanoparticles, stabilized by N-polyvinylpyrrolidone) have been investigated by small-angle X-ray scattering. The size distributions of inhomogeneities in the porous structure of the cellulose matrix and the size distributions of silver nanoparticles in the composite have been determined. It is shown that the sizes of synthesized nanoparticles correlate with the sizes of inhomogeneities in the gel film. Particles of larger size (with radii up to 100 nm) have also been found. Electron microscopy of thin cross sections of a dried composite layer showed that large particles are located on the cellulose layer surface. Electron diffraction revealed a crystal structure of silver nanoparticles in the composite.     

The Effect of Spin Center Parameters on the Photoactivity of Nanocrystalline Titanium Dioxide in the Visible Spectral Range

Crystallography Reports - Nanocrystalline TiO2 samples, obtained by sol–gel and supercritical fluid (SCF) methods, have been studied by optical spectroscopy and electron spin resonance. It is...     

Growth of different habits of lead bromide crystals in silica gel by the new gel technique

Lead bromide crystals of high optical perfection and of different habits have been grown in silica gel by new gel technique starting from colloidal lead tartrate and KBr solution. pH is found to be not a factor for habit modification. Certain habits of the grown crystals exhibit interesting surface features. The growth conditions have been optimized and the various aspects of the growth process have been widely investigated.     

Mössbauer characterization of an amorphous steel with optimal Mo content

Three amorphous steel alloys, Fe_71.2?xC_7.0Si_3.3B_5.5P_8.7Cr_2.3Al_2.0Mo_x (x = 0, 4.5 and 6.5 at.%), have been produced by the melt-spinning technique and characterized by X-ray diffraction and transmission Mössbauer spectrometry (TMS). TMS allows us to study the local environments of the Fe atoms in the glassy state, showing the changes in the amorphous structure due to the addition of Mo. A reduction of the mean hyperfine field is observed as the amount of Mo increases. With intermediate Mo content, this reduction is associated to the substitution of Fe by Mo in a disordered magnetic Fe-rich structure, whereas for high Mo content, this structure is destroyed leading to an increase of paramagnetic environments. Finally, the relationship between the glass forming ability (GFA) of these alloys and their local structure determined by TMS will be discussed.     

Luminescent porphyrinosilica obtained by the sol–gel method

Hybrid materials consisting of a substituted porphyrinic macrocycle covalently bonded to a porous silica matrix have been prepared. The organic macrocycle 5,10,15,20-tetra-(p-hydroxy)phenylporphyrin, H₂T(p-OH)PP, has been covalently bonded to silica gel porous networks, obtained by sol–gel reactions, by means of 3-isocyanatopropyl-triethoxysilane (IPTES), whose molecules act as bridges between the silica precursor tetraethoxysilane (TEOS) and the porphyrinic H₂T(p-OH)PP macrocycles. Electronic absorption spectra show Soret and Q bands characteristic of trapped porphyrins inside the silica networks. Infrared spectroscopy monitoring has been employed to demonstrate the formation of covalent bonds between some substituents of the porphyrin molecules and the isocyanato group of IPTES.     

Transfer of Amide and 2-Methoxyethoxy Groups and Sodium Encapsulation in the Reaction of TaCl3[N(TMS)2]2 with Sodium Bis(2-methoxyethoxy)aluminum Hydride: X-ray Structure of [NaAl{N(TMS)2}(OCH2CH2OMe)3]2

Abstract  The reaction between the tantalum compound TaCl₃[N(TMS)₂]₂ and the hydridic reducing agent sodium bis(2-methoxyethoxy)aluminum hydride (Vitride) has been investigated in toluene solution at room temperature and found to afford the dimeric aluminate complex [NaAl{N(TMS)₂}(OCH₂CH₂OMe)₃]₂ as the sole isolable product. The molecular structure of the product establishes the existence of a four-coordinate aluminum atom and the formal transfer of the 2-methoxyethoxy and bis(trimethylsilyl)amide groups to the aluminate product. The aggregation of two NaAl{N(TMS)₂}(OCH₂CH₂OMe)₃ units serves to bind the two sodium cations in a crown-ether fashion through six ancillary oxygen atoms. Index Abstract  The reaction between TaCl₃[N(TMS)₂]₂ and the reducing agent Vitride has been examined and found to furnish the dimeric aluminate complex [NaAl{N(TMS)₂}(OCH₂CH₂OMe)₃]₂ in low yields. X-ray diffraction analysis confirms the attendant transfer of the 2-methoxyethoxy and bis(trimethylsilyl)amide groups to the aluminate product, which exists as a dimer that encapsulates two sodium counter ions via the 2-methoxyethoxy appendages.

Growth kinetics,habit modifications and surface designs of lead iodide grown by the new gel technique

Lead iodide crystals have been grown in silica gel by the new gel technique by the inter action between colloidal lead tartrate and KI solution. The crystals are found to exhibit interesting surface designs. Growth kinetics and habit modifications of the crystal have been investigated. Two dimensional surface nucleation and screw dislocation are found to partake in the growth process of the crystals.     

Low temperature synthesis of monolithic silica glass from the system Si(OC2H5)4---H2O---HCl---HOCH2CH2OH by the sol-gel method

To avoid breaking of the gel during drying any organic solvent such as ethanol has not been used. When the amount of HCl is increased and the mixture is stirred vigorously, a transparent silica sol can be prepared immediately with only Si(OC₂H₅)₄ and H₂O. By adjusting the amount of HCl and the temperature, the sol may gel at any time, even in 5 min at room temperature. Such a gel may be dried without cracking under completely open conditions. The addition of glycol as a complex agent can apparently increase the pore size of the gel and decrease its capillary force and surface tension. So, large dried gels can be made in a rather short time. For example, a dried gel of diameter 68 mm and thickness 13 mm can be made in only 4 days. Because of the large pores, the dried gels can be sintered to monolithic silica glasses of practical size with a rather fast heating rate. The mechanisms in the whole process have been explored.     

Effect of synthesis conditions on the fractal structure of yttrium-stabilized zirconium dioxide

The effect of synthesis conditions (sequential precipitation, coprecipitation, and sol–gel method) on the fractal structure of yttrium-stabilized zirconium dioxide has been investigated. It has been shown that the difference between methods manifests itself clearly in xerogel nanostructures, viz. by the number of fractal aggregation levels (three levels in hydroxide precipitates and two in sol–gel) and the absence of mass-fractal aggregation in sol–gel. It is been determined that the mass-fractal aggregation of precursors contributes to good filtration of hydroxide precipitates and allows preparation of oxides with soft, readily destructible aggregates, and that surface-fractal aggregation makes for good pressability of oxides.     

Dislocation etching of tin iodide single crystals

Single crystals of tin-iodide (SnI₂) have been grown using the controlled reaction between SnCl₂ and KI by diffusion process in gel medium. As grown (010) surfaces of the crystals have been optically studied. Characteristic etch pits have been observed on them. This suggests that SnI₂ crystals might go into dissolution in the acid-set gel. By successively etching (010) surfaces in a mixture of ammonia, acetic acid, and CdCl₂ solution, it is established that the pits indicate the site of dislocations in the crystals. This is further confirmed by comparing the etch patterns before and after chemically polishing (010) surfaces. The average dislocation density in the crystals have been evaluated and found to be 3.2 × 10³ cm⁻² and the implications are discussed.     

Study of the gel films of Acetobacter Xylinum cellulose and its modified samples by 1H NMR cryoporometry and small-angle X-ray scattering

Gel films of Acetobacter Xylinum cellulose and its modified samples have been investigated by 1H nuclear magnetic resonance (NMR) cryoporometry and small-angle X-ray scattering. The joint use of these two methods made it possible to characterize the sizes of aqueous pores in gel films and estimate the sizes of structural inhomogeneities before and after the sorption of polyvinylpyrrolidone and Se₀ nanoparticles (stabilized by polyvinylpyrrolidone) into the films. According to small-angle X-ray scattering data, the sizes of inhomogeneities in a gel film change only slightly upon the sorption of polyvinylpyrrolidone and nanoparticles. The impregnated material is sorbed into water-filled cavities that are present in the gel film. ¹H NMR cryoporometry allowed us to reveal the details of changes in the sizes of small aqueous pores during modifications.     

Growth of octacalcium phosphate and dicalcium phosphate dihydrate crystals in silica gels

The crystallization of octacalcium phosphate (OCP) and dicalcium phosphate dihydrate (DCPD) have been studied at ambient temperature in silica gel using double diffusion system. The first stage of evolution of the OCP spherulites as well as their growth have been observed by optical microscopy. The growth rate of the OCP spherulites under stable external conditions was constant. The formation of DCPD crystals has been observed in the last stages of experiments.     

Crystalline quality of lysozyme crystals grown in agarose and silica gels studied by X-ray diffraction techniques

The crystalline quality of hen egg white lysozyme (HEWL) crystals grown in agarose gels and in silica gels has been characterized by measuring resolution and mosaic spread. These crystals have been compared to solution grown ones. A quasi-plane-wave X-ray topography study has also been done on some crystals. The study concerns triclinic, monoclinic, orthorhombic and tetragonal forms of HEWL. One observes that the resolution is not really changed by gelling the growth medium, even for rather high gel contents (agarose 0.5% wt/wt). On the contrary, mosaicity, characterized through reflection profile recordings allows to differentiate crystals grown by different techniques: agarose gel grown crystals are, on average, better than solution grown ones but the best crystals are obtained in silica gel. X-ray topography confirms this result.     

Growth of Potassium Sulphate Crystals by Silica Gel Technique

Single and transparent crystals of potassium sulphate up to 30 × 3 × 3 mm in size have been grown by silica gel technique, using various methods (two-layers three-layers and hybrid gel technique) which are described. The effects of the three differents alcohols (used as top solutions) upon the quality and nucleation of crystals are also reported. The “growth and equilibrium” morphologies and microstructures of the habit faces are discussed.     

Dielectric properties of ferroelectric lead hydrogen phosphate single crystals

The ferroelectric lead hydrogen phosphate (LHP) single crystals have been grown by employing the gel technique. The controlled reaction between lead nitrate and orthophosphoric acid solutions by diffusion process in gel medium has been used. The dielectric constant (ϵ′) and the dielectric loss (ϵ″) have been measured for the first time as a function of frequency in the range 10² to 10⁴ Hz. They have also been determined as a function of temperatures in the range 300 to 680 K. The polarisation effect contributing to the dielectric constant has been studied. It has been observed that the mechanism of the dielectric behaviour is different in the lower and higher temperatur and frequency ranges. The activation energy of the oscillators has been calculated and found to be 1.12 eV. Attempt has been made to draw some qualitative conclusions, taking in view, the existing theories of various kinds of polarization and implications have been discussed.     

Thermosoftening phenyl siloxane glasses prepared via sol concentration

Thermosoftening phenyl polysiloxane glasses have been prepared by concentration of the corresponding sol above the melting temperature of the resultant materials, which we call the ‘sol concentration method’ (SCM). Because the glass melt is directly obtained without a gelation process, a remarkable shortening of the process time and suppression of crack formation in the sol–gel transition region have been realized. That is the whole process time of the SCM (about 24 h) is much shorter than that of the conventional sol–gel process (about 10 days). Thermosoftening phenyl polysiloxane glasses with high transparency in the UV region were obtained by the SCM.     

Growth and Characterisation of Pure and Nickel-doped Strontium Tartrate Tetrahydrate Single Crystals

Strontium tartrate tetrahydrate single crystals doped with nickel have been grown in silica gel. The grown crystals have been characterized by X-ray, atomic absorption spectroscopy (AAS), thermogravimetric analysis (TGA), and etching technique.     

Fine-patterning on glass substrates by the sol-gel method

A novel process has been developed to form fine patterns on glass substrates. The process consists of the following steps: (1) coating of gel film on a substrate, (2) patterning on the gel film, and (3) heat-treatment. The key point in this process is the control of hardening of the gel films by the addition of organic polymers such as polyethylene glycol (PEG). The addition of PEG increased the viscosity of the coating solutions and delayed the gelation time, but had little effect on the optical properties of the resultant heat-treated glass films. The newly developed process is expected to be applicable to the formation of the optical devices such as diffraction gratings and pre-groovesof optical memory disks.     

Crystal Growth in Gel: Nuclei and Precipitation Distribution in Gel

The investigation of the gel structure and properties influence upon the crystallization processes have been carried out and are reported in this paper. The gel acidity and nucleation centers distribution are chosen as the parameters describing this process. The analytical approximations for the nucleation and precipitation density along the gel column are proposed. It has been shown that in the view of the parameters chosen it is impossible separately to determine the gel structure and proton concentration effects on crystallization processes.