An in situ synchrotron X-ray diffraction investigation of lepidocrocite and ferrihydrite-seeded Al(OH)3 crystallisation from supersaturated sodium aluminate liquor

Lepidocrocite and ferrihydrite-seeded Al(OH)₃ crystallisation from supersaturated sodium aluminate liquor at 70 °C was investigated using in situ synchrotron X-ray diffraction. The presence of iron oxides and oxyhydroxides in the Bayer process has implications for the nucleation and growth of scale on process equipment, and a greater understanding of the effect they have on Al(OH)₃ crystallisation may allow for development of methods for Al(OH)₃ scale prevention. The early stages of both crystallisation reactions were characterised by nucleation of gibbsite on the seed material. This was followed by a rapid increase in gibbsite concentration, which coincided with the appearance of the bayerite and nordstrandite polymorphs of Al(OH)₃. The lepidocrocite-seeded reaction then proceeded via a mechanism similar to that which has been observed previously for goethite, hematite and magnetite-seeded Al(OH)₃ crystallisation. Different behaviour was observed in the ferrihydrite-seeded experiment, with nucleation as well as growth occurring during the period of rapid increase in gibbsite concentration, followed by a period of diffusion controlled growth.     

Quartz crystal microbalance as a tool for scale deposition monitoring in Bayer process conditions

This paper describes the study of bayerite or gibbsite scale formation and growth from supersaturated sodium aluminate solutions using a quartz crystal microbalance. Analysis of frequency vs time curves and scanning electron microscopy images allowed us to propose a surface nucleation mechanism that leads to cementation of particles produced by catastrophic secondary nucleation. We also highlighted the influence of the filtration of the supersaturated sodium aluminate solution step on the scale nucleation kinetics. It was observed that use of filter with porosity below 2.5 μm delays the formation of scale.     

The role of fuzzy interfaces in the nucleation, growth and agglomeration of aluminum hydroxide in concentrated caustic solutions

Fundamental crystal growth theory relies on classical concepts of monomeric addition at step sites on crystal surfaces. The nucleation and growth of crystalline aluminium hydroxide from concentrated caustic solutions does not follow classical crystal growth mechanistic pathways. Numerous techniques including static and dynamic light scattering, small angle X-ray and neutron scattering, cryovitirification transmission electron microscopy, rheology and atomic force microscopy have been employed in the study of aluminium hydroxide crystallisation from concentrated caustic solutions. The observations from these techniques have been interpreted on the basis polymer crystal growth theory, thermodynamic phase inversions analysis and entropically driven insolubility.

The experimental observations can be interpreted on the basis that aluminium hydroxide nuclei and crystals are surrounded by a diffuse interface which grades in density from the crystalline aluminium hydroxide particle core to the surrounding solution. A mechanism for the nucleation and growth mechanisms of aluminium hydroxide has been proposed: initial solution formation of a loose polymeric network; clustering of this network followed by gradual densification to form amorphous nuclei; further densification of the core of the nuclei to form crystallites and gradual densification but not crystallisation of the still amorphous diffuse interface.

The presence of this diffuse interface enables the slow agglomeration behaviour of aluminium hydroxides particles in concentrated caustic liquors to be explained. In liquors of very high ionic strength (in this case up to 6 M NaOH) particulate agglomeration would be expected to be rapid due to the small double layer thickness as predicted by DLVO theory. During rapid growth the diffuse interface inhibits the sufficiently close approach of the dense part of the particles to the point where attractive inter-particulate van der Waals forces would dominate and agglomeration would take place. As supersaturation is depleted and the growth rate of the diffuse interface decreases but densification is still occurring the particles can approach more closely and agglomeration will occur. Thus it is probable that the observed agglomeration behaviour is supersaturation and growth rate related.     

Ni-containing spinel aluminates glass-ceramic materials obtained from cordieritic bulk glasses

Monolithic glasses with compositions 2MO · 2Al₂O₃ · 5SiO₂, being M=Ni and equimolar mixtures of Ni and Mg, were prepared at 1650 °C by melting mixtures of raw materials. The crystallization of monoliths was produced by heat-treatment at several temperatures up to 1200 °C. The crystallization sequence was followed by differential thermal analysis (DTA), X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) and diffuse reflectance spectroscopies. Surprisingly, the only crystalline phase formed after heating up to 1100 °C was a nickel-containing aluminate spinel for both compositions. The microstructural characterization indicated the volume crystallization of well formed octahedral crystals of spinel with smaller size than 500 nm. Finally, it has been proved that nickel-containing aluminosilicate glasses could be converted into μ- or -cordierite- and spinel-based glass-ceramics by thermal treatment of powdered glasses and monolithic bodies, respectively.     

Sintering of silica gel derived from the alkoxysilane solution containing N,N-dimethylformamide

The sintering of three kinds of dried gels with different microstructures prepared from the tetramethoxysilane (TMOS) solutions containing N,N-dimethylformamide (DMF) with varying [H₂O]/[TMOS] mole ratios and different catalysts was investigated by measuring the pore size distribution, the Vickers hardness, the linear shrinkage and the apparent density of the gels heated to various temperatures. The marked shrinkage of the pores started at temperatures above 900 °C. The temperature required for the conversion of a gel to a pore-free silica glass depended on the average size of the pores contained in the dried gels; the average pore sizes of about 50 Å, 75 Å and 100 Å corresponded to 1040 °C, 1060°C and 1080°C, respectively. The Vickers hardness, the linear shrinkage and the density of the gel showed a corresponding change with a change in heating temperature.     

Novel synthesis of gibbsite by laser-stimulated nucleation in supersaturated sodium aluminate solutions

Laser-induced localized density fluctuations are shown to stimulate primary nucleation of nano-sized aluminum trihydroxide seed crystals in flowing supersaturated industrial sodium aluminate solutions (Bayer liquor). The post-nucleation mass deposition rate is greatly increased over rates attainable through more traditional seeding methods. Near-ideal single-crystal gibbsite is formed under diffusion-limited growth conditions. Laser-stimulated nucleation represents an in situ method for the active local modulation of the supersaturation state, so that the stimulated nucleation occurs under macroscopic solution conditions that are unfavorable for spontaneous nucleation.     

The influence of additives on the crystal habit of gibbsite

Crystallization of gibbsite (Al(OH)₃) is an important stage of the Bayer process, production of alumina from bauxite ores. In both pure or industrial supersaturated sodium aluminate solutions, gibbsite crystals are always agglomerated. In the present paper, we present results of a study concerning the influence of different polycarboxylic acids as crystal habit modifier for gibbsite. In pure solution, agglomerated hexagonal plates are observed. Whereas acicular and tabular morphologies are found in the presence of different additives. These results are discussed referring to the crystallographic structure of gibbsite. It is found that only oxygen atoms are present on gibbsite surface. This observation leads us to propose an additive way of acting by formation of a molecular complex between the growth unit and the carboxylic groups of the additive.     

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.     

Preparation of zirconia aerogel by heating of alcohol–aqueous salt solution

Using zirconyl nitrate dihydrate as starting material, zirconia aerogel with high specific surface area was prepared by heating zirconyl nitrate dihydrate solution with an alcohol–aqueous mixture and supercritical drying method (SCFD). It was characterized by XRD, TEM and BET methods. The specific area of primary ZrO₂ aerogel could reach 675.6 m²/g which was much higher than that of documents so far. While both the specific surface area and pore volume decreased markedly on increasing the calciniation temperature. Contrast with the powder synthesized by ordinary trend, the content of t-ZrO₂ in the aerogel increased when the calcination temperature improved from 500 to 700 °C, the percentage of tetragonal phase was 86% at 700 °C. And it was very interesting that the particle size of the aerogel was bigger than 30 nm after being roasted in 1000 °C, the tetragonal phase still existed.     

The crystallisation of alkaline-earth metal metasilicates from metal chloride melts: Preliminary experimental studies

The crystallisation of alcaline-earth metal metalsilicates was studied by slow cooling of saturated solutions in the metal chloride melts at T₀ = 830° to 1300°C, down to ambient temperature; cooling rates were varied from 20° to 200°C hr⁻¹. Calcium metasilicate crystallised as β-CaSiO₃ prisms, strontium metasilicate as α-SrSiO₃ platelets and barium metasilicate as α-BaSiO₃ platelets. Final crystal lengths increased with reduction in cooling rate and with increase in initial crystallisation temperature.     

The crystallisation of the yttrium–sialon glass: Y15.2Si14.7Al8.7O54.1N7.4

The development of microstructure during crystallisation of a glass with composition Y_15.2Si_14.7Al_8.7O_54.1N_7.4 has been studied by analytical and high resolution transmission electron microscopy. Crystal nucleation at temperatures in the range 965–1050°C occurs by the heterogeneous nucleation of lenticular-shaped yttrium, silicon and aluminium containing crystals on silicon-rich clusters that formed during glass preparation. The lenticular crystals have a wide range of composition after heat treatment at 1050°C; the yttrium cation percentage varies around that of the expected B-phase composition Y₂SiAlO₅N but the aluminium content is lower and the silicon content generally significantly higher than that. The crystals display the hexagonal crystal structure of B-phase, although the results from EDX analysis imply that the atomic arrangement of the lattice is not the previously proposed B-phase structure. Crystal growth during prolonged heat treatment at 1050°C occurs to a significant extent by coalescence.     

Formation of LiNbO3 films by hydrolysis of metal alkoxides

Homogeneous, crack-free, thin films of crystalline LiNbO₃ were synthesized above 250°C on Si(100) substrates by the dip-coating method using a double alkoxide solution. The coating solution, which was prepared by the controlled partial hydrolysis of the double alkoxide, gave stoichiometric LiNbO₃ crystalline films at temperatures as low as 250°C. The concentration of the alkoxide solution influenced both of thickness and quality of films. Crystallinity of thin film top-coated directly on the substrate affected the crystallization state of films coated on the film remarkably. Films crystallized on -Al₂O₃(0001) showed preferred orientation along the c-axis, while the preferred orientation could not be observed on Si(100) substrates.     

Effect of heat treatment on phase transformation of aluminum nitride ultrafine powder prepared by chemical vapor deposition

Ultrafine aluminum nitride (AlN) powders were obtained by chemical vapor deposition via AlCl₃–NH₃–N₂ system operated at various temperatures and at a same 200 cm³/min flow rate of NH₃ and N₂, respectively. It has been shown that when the reaction temperature of AlCl₃ and NH₃ was above 600°C, then crystalline AlN powder can be formed; whereas, amorphous AlN was obtained with NH₄Cl if reacted in a lower-temperature zone of the reaction chamber. The amorphous AlN powder was heat treated at 1400°C under NH₃ and N₂ atmosphere for 2 h, then the crystalline phases of the obtained powder belong to a single phase of AlN; a mixture of AlN and Al₂O₃ and only AlON, respectively. On the other hand, if crystalline AlN powder is heat treated at 1400°C under N₂ atmosphere for 2 h, the crystalline phases were composed of the major phase of AlN and a minor phase of Al₂O₃. The morphology, particle size and agglomerate size of the AlN powder were strongly dependent on the heat-treatment temperature. The particle size of AlN powder increases from 28.1 to 45.0 nm, as the heat treatment temperature increases from 800 to 1400°C.     

电熔镁铝尖晶石和轻烧镁粉对合成镁铁铝系复合尖晶石的影响

本文研究电熔镁铝尖晶石和轻烧镁粉对合成镁铁铝复合尖晶石的影响.电熔镁铝尖晶石和轻烧镁粉分别与氢氧化铁、活性氧化铝混合均匀制得φ20 mm×10 mm的试样,在空气气氛烧结条件下,于1550℃×6 h高温烧成.采用XRD、SEM和EDS等手段对烧后试样的物相组成和显微结构进行表征.结果表明:加入电熔镁铝尖晶石的试样中存在刚玉相、赤铁矿、铁铝尖晶石相和镁铁铝复合尖晶石相四种矿物相,镁铁铝复合尖晶石的化学式为Mg796Al15.31Fe0.68O32;加入轻烧镁粉的试样中存在刚玉相、铁铝尖晶石相和镁铁铝复合尖晶石相三种矿物相,镁铁铝复合尖晶石的化学式为Al15.71Mg3.35Fe4.94O32;加入电熔镁铝尖晶石的试样中铁铝尖晶石的晶粒尺寸和镁铁铝复合尖晶石相差无几,加入轻烧镁粉的试样中铁铝尖晶石的晶粒尺寸明显大于镁铁铝复合尖晶石.     

Analysis of precursor residues in lithium aluminosilicate gels using XPS and RGA

Lithium aluminosilicate gels of composition 15 mol% lithia-2 mol% alumina-83 mol% silica were prepared by adding nitrates to tetraethyl-orthosilicate (TEOS) and going through the sol-gel process. Samples were prepared in thin film and bulk form. Dried and outgassed samples were studied with X-ray photoelectron spectroscopy (XPS) d residual gas analysis (RGA). XPS spectra show similar species in both thin film and bulk samples, but different relative quantities of each species. Some oxidation of organic groups by nitrate is evident in bulk samples which were heated to 70°C during the drying process. In both thin film and bulk samples, the O 1s spectra indicate oxygens associated with a silicate network and higher binding energy species such as - -O-. The C 1s spectra of bulk gels heated to 140° and 350°C in vacuum show some organics are vaporized at low temperature, while the oxidized-organic residues are decomposed to CO₂ at high temperature. These species are also observed with residual gas analysis at the corresponding temperatures.     

The coprecipitation of zinc aluminium hydroxides (refractory-, absorbent-, and catalyst precursors) from aqueous solution with ammonium hydroxide: Precipitate compositions and coprecipitation mechanisms

Zinc aluminium hydroxide hydrates were coprecipitated from different mixed cation solutions at Zn/Al₂ ratios from 1/2 to 4/1. The coprecipitations were monitored by potentiometric titrations and the final coprecipitate compositions were examined by chemical analysis and atomic absorption spectrophotometry, X-ray diffraction and preliminary thermal analysis. The product from Zn/Al₂ = 1/2 solution was amorphous: at Zn/Al₂ = 1/1.5, the main phase (after drying at 95 °C) was a zinc hydroxoaluminate Zn[Al(OH)₄]₂ together with some gibbsite: at Zn/Al₂ = 1, the main phase was probably a solid solution (of Zn[Al(OH)₄]₂ with Zn₂[Al₂(OH)₁₀]) together with Zn₂[Al₂(OH)₁₀]: at Zn/Al₂ = 2, the main phase was a mixture of Zn₂[Al₂(OH)₁₀] with (ZnOH)₄ [Al₂(OH)₁₀] and some gibbsite: at Zn/Al₂ = 4, the main phase was (ZnOH)₄ [Al₂(OH)₁₀] with some zinc hydroxide.     

CdCr2Se4−CdCl2 phase diagram for the crystallization of semiconducting spinel CdCr2Se4 from high temperature solution

The part of the CdCr₂Se₄-CdCl₂ phase diagram within the range of 75–100 mole% of CdCl₂ has been examined. The composition CdCr₂Se₄ · CdCl₂ was found to be stable below 542°C. The crystallisation processes of CdCr₂Se₄ spinel for two mixture compositions: 88% CdCl₂-12% CdCr₂Se₄ and 80% CdCl₂-20% CdCr₂Se₄ have been carried out and the parameters of CdCr₂Se₄ crystal growth from the CdCl₂ flux were determined.     

Solvothermal growth of ZnO

The growth of ZnO single crystals and crystalline films by solvothermal techniques is reviewed. Largest ZnO crystals of 3 inch in diameter are grown by a high-pressure medium-temperature hydrothermal process employing alkaline-metal mineralizer for solubility enhancement. Structural, thermal, optical and electrical properties, impurities and annealing effects as well as machining are discussed. Poly- and single-crystalline ZnO films are fabricated from aqueous and non-aqueous solutions on a variety of substrates like glass, (100) silicon, -Al₂O₃, Mg₂AlO₄, ScAlMgO₄, ZnO and even some plastics at temperatures as low as 50 °C and ambient air conditions. Film thickness from a few nanometers up to some tens of micrometers is achieved. Lateral epitaxial overgrowth of thick ZnO films on Mg₂AlO₄ from aqueous solution at 90 °C was recently developed. The best crystallinity with a full-width half-maximum from the (0002) reflection of 26 arcsec has been obtained by liquid phase epitaxy employing alkaline-metal chlorides as solvent. Doping behavior (Cu, Ga, In, Ge) and the formation of solid solutions with MgO and CdO are reported. Photoluminescence and radioluminescence are discussed.     

Preparation and characterisation of alkoxide-derived celsian glass-ceramic

An all-alkoxide route to stoichiometric celsian glass has been developed. Sintering to 97% of full glass density, prior to the onset of appreciable crystallisation, was achieved after 30 min at 900 °C. However, crystallisation of metastable hexacelsian, rather than the preferred monoclinic celsian, from stoichiometric glass was difficult to avoid.     

The stability and surface reactivity of gallium phosphide nanocrystals

The stability and surface reactivity of Gal? nanocrystals were characterized by TG-DTA, XRD and XPS measurements. The samples were heated in O₂ and N₂ atmospheres respectively. The experimental curve which describes the mass change as a function of temperature shows that the mass decreases under 330°C but increases in two stages between 33°C and 550°C. The curves obtained in O₂ and N₂ are obviously different. The results of XPS indicate that the density of oxygen atoms bonded chemically to the gallium atoms on the surface increases when the sample is heated in O₂ up to 200°C, whereas the density of nitrogen atoms increased when heated to 330°C in N₂. It can be concluded that the N₂ could be activated on the surface of GaP nanocrystals at a rather low temperature, and highly reactive nitrogen atoms formed. Our result obtained makes it possible to synthesize a series of nitrogen-containing compounds in N₂ gas under moderate conditions.