Hydroxide Sol Research Articles

Coagulation mechanisms - nano- and microprocesses

Possible coagulation mechanisms were studied in relatively high alkalinity model systems and surface waters. On the basis of available information, original laboratory experiments and simple calculation were performed in order to show that the adsorption of Al3+ and Fe3+ ions is not the dominant process in decreasing the stability of suspended particles. The ions of the feeding coagulants hydrolyse within short time and form positively charged water soluble aluminium- or ferric hydroxides. Adsorption of these water soluble hydroxides onto the surface of colloids and quasi-colloid particles are restricted because of the quick completion of the hydrolysis process in relatively high alkalinity (>1.2 mmol/L) water. The result of complete hydrolysis of Al3+ or Fe3+ ions are slightly positively charged poorly water soluble aluminium or ferric hydroxide sols. The positively charged hydroxides and the associated water molecules are connected to each other by hydrogen bonds, providing a stabile structure. The hydrogen bonds provide the aggregation of slightly positively charged sol aggregation into flocs. Considering the repulsing forces among the sols, high numbers of individual sol particles (having nm sizes) are able adsorb onto the surface of suspended particles, changing their electrical charge and decreasing the stability of the colloids and quasi-colloid particles. This process is dominant in the destabilisation of suspended particles.

A method for preparing silica-containing iron(III) oxide adsorbents for arsenic removal

A method for preparing iron(III)-based binary oxide adsorbents in a granulated form for arsenic removal was studied. The key step in the method was the simultaneous generation of hydrous ferric oxide (FeOOH) sol and silica sol in situ in one reactor. This eventually led to the formation of Fe–Si complexes. The addition of silica enhanced the granulated adsorbent strength but reduced the arsenic adsorption capacity. An optimum Si/Fe molar ratio in the balance of adsorbent strength and arsenic adsorption capacity was found to be approximately 0.33. The effects of aging time, drying temperature and process pH on adsorbents were also evaluated in the study. X-ray diffraction analysis confirmed that the iron(III) oxide in the Fe–Si binary oxide adsorbents was amorphous, largely due to the retardation of the iron oxide crystallization by the presence of silicate species. The surface area of the Fe–Si adsorbents and the particle size of Fe–Si complexed suspensions were determined as well. The batch strength testing procedure introduced in this study can provide a simple and quick evaluation of granulate strength in a wet status. Generally, this developed method can prepare granulated Fe–Si binary oxide adsorbents for column adsorption of arsenic from water.

Mechanochemical synthesis of lanthanum chromate by grinding of constituent oxides : F. Saito et al. (Tohoku University, Sendai, Japan.) Powder Technol., Vol. 122, No 2–3, 2002, 145–149

Alkaline earth-doped LaCrO3 is a satisfactorily conducting ceramic material for the interconnector of Solid Oxide Fuel Cells (SOFC). For the synthesis of highly sinterable LaCrO3 powders spraydrying of adequate precursors like sols based on alkoxides has been used. Lanthanum chromium hydroxide sol was prepared by hydrolysis of synthesized methoxide complexes. The chemical reactions during hydrolysis and peptization of the precursors are described. The crystalline phase formation during heat treatment of spray-dried amorphous gel consisting of hydroxides and contaminant chloride ligands was characterized by thermal analysis methods (TG, DTG, DTA) and X-ray diffraction analysis. The progressive oxidation of Cr(III) by thermal dehydroxylation transforms the powder into fine crystallized La(III) Cr(V)hydroxo-oxychloride at 450 °C. A thermal treatment permits complete chlorine removal forming sinterable crystalline LaCrO3 perovskite powder at temperatures above 720 °C. The same sol-gel process also can be applied to chlorine-free nitrate-containing alkoxide sols. The properties of sinterable lanthanum chromite powders obtained from these sols are compared with powders made by direct spray-drying of lanthanum-chromium nitrate solutions.

Continuous production of phosphor YAG:Tb nanoparticles by hydrothermal synthesis in supercritical water

Phosphor YAG:Tb ((Y 2.7Tb 0.3)Al 5O 12) nano particles were synthesized by a hydrothermal method at supercritical conditions (400 °C and 30 MPa) using a flow reactor. Hydroxide sol solutions formed by stoichiometric aluminum nitrate, yttrium nitrate, terbium nitrate and potassium hydroxide solutions. The relationship between particle size and experimental variables including pH, concentration of coexistent ions and hydroxide sol were investigated. Particles were characterized by XRD, TEM and photo-luminescence measurements. Particle size of YAG:Tb became finer as pH was increased or potassium nitrate concentration of the starting metal salt solution was increased. By removing the coexisting ions (NO 3 −, K +) from the metal salt solution, single phase YAG:Tb particles with 20 nm particle size were obtained. The emission spectra of YAG:Tb particles of 14 nm shows a blue shift.

Optical properties and charge transport in nanocrystalline TiO 2–In 2O 3 composite films

This work involves a comparative study of nanocrystalline titanium and indium oxides as well as TiO 2–In 2O 3 composites prepared by a sol–gel method from concentrated hydrous titanium dioxide and indium hydroxide sols. Highly transparent uniform films were fabricated from the sols by a spin-coating technique. TiO 2–In 2O 3 nanocomposites possess pronounced extreme dependencies of both the apparent bandgap and the refractive index on the film composition. No chemical interaction between the oxides was observed during annealing to temperatures between 200 and 700 °C. This behavior was attributed to the effect of surface states formed at the nanocrystallite interfaces. Other possible causes of the pronounced dependence of the optical and electronic properties on the TiO 2-to-In 2O 3 ratio are also discussed. Abrupt changes in resistivity, photoresistivity and photocurrent decay time of the composite films were observed for composites where the TiO 2 content ranged from 25 to 50 wt.% and are associated with the formation of a three-dimensional infinite cluster of interconnected In 2O 3 nanoparticles.

Preparation of Globular Zirconium(IV) Hydroxide by Sol-Gel Process

The sol-gel process of the preparation of globular zirconium(IV) hydroxide is described, which involves formation of zirconium(IV) hydroxide sol by electrolysis of zirconium(IV) chloride solution in a one-compartment cell to a Cl/Zr atomic ratio of less than 0.6, gelation of sol droplets in aqueous ammonia, treatment of gel spheres to remove electrolytes, and drying.

Structural, Optical, and Photoelectrochemical Properties of Nanocrystalline TiO2−In2O3 Composite Solids and Films Prepared by Sol−Gel Method

Nanocrystalline titanium and indium oxides and TiO2−In2O3 composites have been prepared by sol−gel technique using concentrated hydrous titanium dioxide and indium hydroxide sols. Structure, optical, and photoelectrochemical properties of the composites with different molar ratios of TiO2 to In2O3 have been studied. The binary oxide films and xerogels annealed at temperatures from 200 to 700 °C have a nanocrystalline structure without the chemical interaction between the oxides. However, there is a strong mutual influence of the components of the binary oxide systems, during their annealing which manifests itself in thermogravimetry and differential scanning calorimetry measurements and in a marked effect on the oxide crystallite size. Highly transparent TiO2−In2O3 films fabricated by spin-coating technique exhibit extreme dependencies of the various optical and photoelectrochemical properties on the film composition. The band gap edge of the composite films is blue-shifted, as compared with that of singl...

The synthesis and characterization of the hexagonal Z ferrite, Sr 3Co 2Fe 24O 41, from a sol-gel precursor

The Z ferrite Sr 3Co 2Fe 24O 41, produced from a stoichiometric aqueous inorganic sol-gel precursor, is reported and its magnetic hysteresis loop characterized for the first time. The precursor was an iron(III)hydroxide sol stabilised with NO 3 - counterions and doped with stoichiometric nitrate salts, which had a particle size similar to halide stabilised Ba 3Co 2Fe 24O 41 Z ferrite precursor sols investigated previously. When fired the amorphous gel formed a mixture of α-Fe 2O 3, BaM and CoFe 2O 4 from 600°C, but these products then converted directly to the pure Z phase at 1200°C, without first forming the Y ferrite (Ba 2Co 2Fe 12O 22) phase always observed prior to Z ferrite crystallization in Ba 3Co 2Fe 24O 41 systems. The resulting material was a very soft ferrite, with a very low coercivity of 5.6 kA m -1 and a magnetisation of 48.5 emu g -1 at an applied field of 5 T.

In Situ Monitoring of Generation and Precipitation of Ferric Hydroxide Sol with a Piezoelectric Quartz Crystal Impedance Analyzer

The piezoelectric quartz crystal (PQC) impedance technique was applied to monitor in situ generation and precipitation of the ferric hydroxide sol in aqueous solutions at 90°C. Equivalent circuit parameters and resonant frequencies as well as the half-peak width of the electroacoustic conductance spectrum δƒG1/2 for the PQC resonance were obtained and analyzed. Three stages, sol generation and simultaneous adsorption, adsorption equilibrium, and precipitation of ferric hydroxide sol could be identified in the process of adding the ferric nitrate solution into the hot water. A scheme of two consecutive reactions occurring at the electrode/solution interface was used to analyze the adsorption kinetics of ferric hydroxide sol onto the Au electrode. In addition, the electrolyte-induced precipitation of the colloid was monitored and discussed. Temperature effect on the PQC resonance behavior in liquid was also investigated. Since the PQC impedance technique provides multidimensional piezoelectric information in situ, it is highly recommended for studying the process of sol–gel generation and precipitation.

Post-Deposition Treatment of Zirconia Thermal Barrier Coatings Using Sol-Gel Alumina

This article addresses the problem of gas permeability of thermal sprayed yttria-stabilized zirconia thermal barrier coatings (TBC)s. The objective of this study was to decrease the open porosity of TBCs through deposition of dense alumina ceramic on the surface of the pores. A simple infiltration technique was used, beginning with aluminum isopropoxide as sol precursor, subsequently hydrated to aluminum hydroxide sol, which decomposed at relatively low temperatures to extra-fine, readily sinterable aluminum oxide. In some experiments, the sol-gel (SG) precursor was combined with fine grains of calcined alumina, constituting high solid-yield composite sol-gel (CSG) deposits within the pores of TBCs. Sinterability in the model systems, including aluminum hydroxide sol-calcined alumina and aluminum hydroxide sol-calcined alumina-zirconia, has been studied. A number of TBC specimens were impregnated with suspensions of alumina sols and CSG. It is shown that these ceramics effectively penetrated into the pores and cracks of TBCs and reduced the coating permeability to gases. The overall reduction of porosity was however small (from ∼12 to ∼11%), preserving the strain and thermal shock tolerance of the coatings. Burner rig tests showed an increase in sealed coating lifetime under thermomechanical fatigue conditions.

Cryosol Synthesis of Al2-xCrxO3 Solid Solutions

We report for the first time synthesis of Al2-xCrxO3 solid solutions by a newly developed cryosol method. The essence of the method is the treatment of an aqueous solution containing chromium and aluminum nitrates by an anion-exchange resin in the OH- form producing hydroxide sols. The freeze-drying of these sols results in hydroxide precursors, and their further annealing yields finally solid solutions. It is shown that the application of the crysol method enables the synthesis of single-phase ruby powders containing 7 mol % Cr at relatively low annealing temperatures (T = 950 °C) and times (2.5 h). Also, we investigated the influence of the organization of the initial colloid system on the properties of final Al2-xCrxO3 solid solutions. The data are used for the refinement of the sub-solidus part of the Al2O3−Cr2O3 phase diagram.

The influence of molybdenum on the properties of SnO2 ceramic sensors

Abstract The properties of SnO2–MoO3 ceramic sensors for CH4 and CO detection and structural features of sensitive layers, containing different concentrations of Sn and Mo, have been studied. The sensitive layers were obtained by means of the sol–gel method—from tin hydroxide sols, stabilised with polyvinyl alcohol (PVA), with Mo6+ added. The structure of the ceramic layer corresponds to Mo6+ in SnO2 substitution solid solution (up to 25 at.% Mo) at all Mo6+ concentrations in sols. Mo5+ stabilises in SnO2 interstitial positions. At definite ratios of components and operating currents, SnO2–MoO3 sensors can selectively detect CO in CO+CH4 mixtures.

In Situ Synthesis of Polymer−Clay Nanocomposites from Silicate Gels

Polymer-containing silicate gels were hydrothermally crystallized to form layered magnesium silicate hectorite clays containing polymers that are incorporated in situ. Gels consist of silica sol, magnesium hydroxide sol, lithium fluoride, and the polymer of choice. Dilute solutions of gel in water are refluxed for various lengths of time and then isolated via centrifugation, washed, and air-dried. Polymer loadings up to 86% were attained by adding more polymer to the solutions after 2-day reaction times, reacting for another 24 h, and continuing this process prior to isolation. Polyaniline (PANI)− and polyacrylonitrile (PACN)−clay samples contain up to 57% and 76% polymer, respectively, after just one sequential addition at high polymer loading. Series of PANI−, PACN−, poly(vinylpyrrolidone) (PVP)−, and hydroxypropylmethylcellulose (HPMC)−clays also were prepared by several sequential additions of lower polymer loading to the silicate gel during crystallization. Final polymer loadings were determined by thermal analysis. Basal spacings between clay interlayers were measured by X-ray powder diffraction for all samples. Increases in polymer loadings and basal spacings were observed for all the neutral polymers studied, until or unless delamination occured. Delamination was evident for PACN− and PANI−clay nanocomposites. The highest loadings were observed for the PACN−clays, up to 86%. For the cationic polymer polydimethyldiallylammonium chloride, however, the loading could not be increased beyond about 20%. This is due to electrostatic interactions that balance the negatively charged sites on the silicate lattice with those on the cationic polymer chain. Beyond charge compensation, there is no driving force for further incorporation. Charge compensation in the case of the neutral polymers is attained by interlayer lithium(I) cations.

The manufacture and characterisation of single phase magnetite and haematite aligned fibres from an aqueous sol-gel process

A stable iron(III)hydroxide sol was produced, spun as a gel fibre and collected as an aligned tow blanket. The alignment of the fibre was found to be 88.8% within ± 20° of the axis of alignment, comparable to that of some commercially developed ceramic fibres. Heating in air at a temperature of 250 °C for 1 hour yielded single phase haematite fibres, and these were characterised by X-ray diffraction and electron microscopy. Upon reduction of the gel fibre in 5% H2 in N2 at 350 °C for 1 hour, single phase magnetite was obtained and characterised by X-ray diffraction, infrared and Mossbauer spectroscopy. The morphology of the fibre was studied, and it was found that the transformation to magnetite was accomplished with no compromise of integrity of the fibre or its alignment, although there was an accompanying change in microstructure. The strain to break of the magnetite fibres was measured to be of a least 0.6%, and this compares well with some commercial fibres.

Structure and the point of zero charge of magnesium aluminium hydroxide

Positively charged magnesium aluminium hydroxide sol has been prepared by coprecipitation. The structure and properties were studied using X-ray diffraction (XRD), thermogravimetry (TG), differential thermal analysis (DTA), transmission election microscopy (TEM), FTIR and potentiometric titration (PT). The structure was shown to be hydrotalcite-like; the lattice constants a and c both increased with decreasing mole fraction of Al, x=[Al]/([Al]+[Mg]); the highest value of x was 0.332. Three weight loss peaks were detected by TG–DTA of the sol particles. IR spectroscopy showed that the position of the OH-stretching vibration peak shifted to high frequency and the peaks broadened with decreasing x. PT showed that the sol particles bore positive charges; the point of zero charge (pzc) and the amount of permanent positive charge both increased with increasing x.

Kinetics of monolayer particle deposition

The kinetics of adsorption of a stable monodisperse spherocylindrical iron(III) hydroxide sol to a hydrophilic silica–titania surface have been measured at very low ionic strength over four orders of magnitude of bulk particle concentration, using a planar optical waveguide as the adsorbing substrate and recording the shift of the lightmode spectrum of the waveguide, from which the number of deposited particles can be accurately calculated as adsorption proceeds. Deposition is irreversible and saturates at submonolayer coverage. The kinetics are predominantly governed by electrostatic interactions and the geometry of exclusion of the adsorbed particles. The kinetics of deposition at all bulk concentrations could be fitted with just two common parameters, the particle surface potential ψp, and the hydration energy for adsorption Uh. ψp was ca. 8 mV, about an order of magnitude smaller than that deduced from measurements of the electrophoretic mobility. Uh was ca. 4 kT per particle, i.e. 0.3 mJ m-2, much less than that required for complete dehydration of the particle/surface interfacial region.

Preparation of nanocrystalline Fe3O4 by γ-ray radiation

Abstract The γ-radiation method has been successfully used to prepare pure nanocrystalline Fe3O4 powders from ferric hydroxide sols. Electron microscopy and X-ray powder diffraction techniques show that the average particle size is 16 nm. The influences of experimental conditions on the preparation of nanocrystalline Fe3O4 are discussed.

Grain size control in nanocrystalline In 2O 3 semiconductor gas sensors

In 2O 3 thin films prepared by sol–gel method make it possible to detect low levels (several hundreds ppb) of nitrogen dioxide in air. The possibility of grain size control in indium oxide-sensing layers has been established by using of two preparation methods—electron beam evaporation (EB) and sol–gel technique (SG). SG-prepared samples show smaller particles sizes (down to 5 nm), higher state of agglomeration, higher sensor resistance in air and higher response to NO 2 in comparison to EB samples. Sol–gel technique leads to the preparation of polycrystalline indium oxide with particle sizes of about 5–6 nm after calcination at 400°C and 20 nm after calcination at 700°C. The initial state of particle agglomeration in initial indium hydroxide sol (IHS), stabilized with nitric acid, influences the structure and surface morphology of the resulting indium oxide. While the In 2O 3 layer prepared by using low agglomerated IHS is smooth and porous, In 2O 3 layers prepared from highly agglomerated IHS consist of two regions—thin layer and crystallite agglomerates in cubic and rectangular parallelepiped form. The last shows the best results in terms of NO 2 sensitivity. Sensor resistance and NO 2 sensitivity increase with the decrease of the grain sizes in In 2O 3.

Realistic coagulation mechanisms in the use of aluminium and iron(III) salts

The processes leading from coagulant feed to the development of flocs suited to solid-liquid phase separation are examined and analyzed in the light of available information and experimental data. It is concluded that at the relatively high buffering capacity (HCO3− concentration above 1.8 meg/L) and close to neutral pH values (7.0<pH<8.4) typically found in the surface waters of Hungary, flocs suited to solid-liquid phase separation develop from the coagulant added within a very short period of time (10-20 seconds). These flocs are not capable of changing the charge of the particles on the colloidal, quasi-colloidal, or destabilizing the dispersion. Between the hydrolysis of the coagulant and the development of large flocs, “short-lived” water soluble aluminium and iron(III) hydroxide complexes, metal hydroxide sols are formed, which also carry a positive charge. These latter two metal hydroxide types (which also exist for a brief period - a few seconds - only) are the ones capable of destabilizing the dispersion. The bond between the suspended solids to be removed and the metal hydroxide sols and water soluble metal hydroxide complexes must be established within this short period. Rapid mixing of the coagulant will ensure rapid hydrolysis of the coagulant, contact between the sols and the suspended solids and will retard the development of large flocs which are inactive in destabilizing the dispersion.

Fabrication of (expandable mica)/nylon 6 composites

(Sodium fluorine mica)/nylon 6 composites were fabricated by the polymerization of 6-aminocaproic acid (ACA) and ε-caprolactam (CPL) mixed with the mica. The basal spacings (d b) of the mica in mica/(ACA + CPL) and mica/nylon 6 were 14–15 Å and 21–22 Å, respectively. The d b value of the latter composite indicates the intercalation of three nylon layers. On the other hand, by adding aluminum hydroxide sol (AHS) to the mica before mixing with the monomers, the d b value in the resulting mica/AHS/(ACA + CPL) increased up to ~30 Å. After the polymerization of this mixture, the mica/alumina/nylon 6 showed a basal spacing of 31 Å or no XRD peak of mica, depending on the composition. This basal spacing indicates that five nylon layers were intercalated in the mica. The viscosity average molecular weight of the nylon 6 in the composite decreased with increasing mica content.