The Crystallisation of Aluminium Trihydroxide (Gibbsite) from Sodium Hydroxide Solution Onto Fine Seeds: Physico-chemical Studies on the Internal Structure and Agglomeration of Different Crystal Fractions

A batch of Bayer gibbsite crystals (particle size range 3 to 70 μm) was prepared by crystal growth onto fine seeds (1 to 3 μm) from sodium hydroxoaluminate/sodium hydroxid-solution. The internal structures and seed agglomeration of different fractions were examined by optical microscopy, infra-red spectrophotometry, thermal analysis and chemical dissolution analysis. These physico-chemical studies confirmed that crystal growth occurs with some seed agglomeration in the early stages, followed by growth onto the ‘agglomerates’. The agglomerated seeds contents (X) were as follows: for fraction DiD (l₀ = 6.3 μm), X = 0.44; for fraction Gi C (l₀ = 20 μm), X = 0.41; for fraction Gi B (l₀ = 38 μm), X = 0.31 and for fraction Gi A (l₀ = 55 μm), X = 0.24.     

Influence of solution constituents, solution conditioning and seeding on the crystalline phase of aluminium hydroxide using in situ X-ray diffraction

Filtering, drying and aging of crystalline solids can affect crystalline phase(s) and therefore ex situ X-ray diffraction is not necessarily indicative of the in situ crystalline components. This paper examines some of the factors affecting the crystallised phase of aluminium hydroxides in highly caustic solutions. Caustic aluminate solutions were prepared using several different methodologies. Where solutions were prepared from aluminium metal, gibbsite and bayerite were observed to be the crystallisation products. Where gibbsite was used as the aluminium source in the liquors, the crystallised phase was found to be a function of solution conditioning. Solutions prepared at 100°C and at atmospheric pressure crystallised to form only gibbsite. These same solutions subsequently heated to 160°C for 16 h crystallised as gibbsite and bayerite. It has been concluded from these results that the caustic aluminate solutions made using gibbsite were not dissolved on the molecular scale (although optically clear) if the solutions were heated to only 100°C. Further heating at elevated temperatures completed the dissolution process so that the species in solution were similar to those found in solutions made from aluminium metal. Solutions seeded with either bayerite or gibbsite crystallised to form only gibbsite. Only crystallisation in the bulk solution and not on the seed surfaces was recorded. In some cases seeding altered the expected crystallisation products, i.e. from gibbsite and bayerite to only gibbsite.     

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.     

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.     

Growth and microhardness studies of melt-grown lead (II) chloride single crystals

In the present work, lead (II) chloride material is purified by directional solidification and single crystals of PbCl₂ are grown by vertical Bridgman technique employing double zone furnace. The growth conditions and the problems overcome are discussed. The grown crystals of pure and K₊-doped crystals are cleaved and subjected to microindentation tests. The validity of Kick's relation is checked, the value of K₁ the standard hardness number and n the work-hardening coefficient are discussed. The microhardness behaviour of pure and K⁺-doped lead(II) chloride have been compared.     

Doping behavior of silicon in vapor-grown III–V epitaxial films

An expanded silicon-doping model, applicable for unintentionally doped vapor-grown III–V epitaxial materials, is presented based on additional experimental results of doping-level influences of AsH₃, PH₃, and HCl. This model retains the role of HCl as the silicon generation agent, as suggested in a recently reported silicon-doping model for GaAs. It also includes a surface kinetic factor for the silicon-incorporation efficiency in III–V materials as influenced by the relative vapor-phase concentration of As₂, As₄ and P₂, P₄ species. The present studies indicate a different degree of influence of the HCl and AsH₃ partial pressures on the suppression of the GaAs background doping level. The additive effect of both partial pressures is very similar to that found in the chloride system. Qualitatively, the same sensitivity of GaP doping levels to variations in HCl and PH₃ has been observed; however, the magnitudes of the doping-level changes are different. In all instances, the impurity levels in the epitaxial films are reduced by increases in the partial pressures of the various reactants, with the AsH₃ and PH₃ having a greater influence than the HCl.     

A comparison of natural and experimental long-term corrosion of uranium-colored glass

The alteration features of historical U-colored glasses exposed to natural weathering for over 150 years were compared with the experimental alteration of similar glass with ～0.3 wt% of uranium using a long-term (up to 426 days) kinetic laboratory batch leaching test in deionized water. Two types of natural corrosion crusts were identified by a combination of SEM/EDS, HRTEM/SAED, EPMA and XRD: (i) formation of a leached layer (up to ～600 μm thick) depleted in alkalis and enriched in Si with stable concentration of U and Al and (ii) formation of lamellae depleted in alkalis, Si and U and enriched in Al. The presence of newly formed gibbsite (Al(OH)₃) and kaolinite (Al₂Si₂O₅(OH)₄) were confirmed in the second type of corrosion crust by HRTEM. Dissolution of the glass components including uranium was determined during the laboratory leaching test. Several μm thick alkali-depleted alteration zones with stable U content relatively enriched in Si and Al were formed on the glass surface. The PHREEQC-2 modeling also predicted the precipitation of secondary gibbsite and kaolinite in the late stages of the leaching. These phases may form especially when sufficient amounts of Al are available from the environment (e.g., soil). Furthermore, they provide surfaces for sorption and may, in some cases, affect the mobility of U ions released from the glass in dependence on pH and U speciation.     

Solvothermal synthesis and structural characterization of unfilled and Yb‐filled cobalt antimony skutterudite

Unfilled and ytterbium (Yb)‐filled cobalt antimony (CoSb₃) nanoparticles were synthesized via solvothermal route using sodium borohydride (NaBH₄) as a reducing agent. The effect of various amounts of sodium borohydride on the formation of as‐synthesized CoSb₃ nanoparticles with pure phase was investigated. It is found that a sufficient amount of NaBH₄ was required in order to form pure phase CoSb₃. In addition, the effect of annealing time and temperature on the phase transformation of the as‐prepared non‐pure phase CoSb₃ sample was also investigated. It is found that annealing at 500 °C for 5 h would eliminate those non‐CoSb₃ phases and result in pure cubic skutterudite phase CoSb₃. Structural characterization of the as‐prepared unfilled and Yb‐filled nanoparticles was carried out with transmission electron microscopy (TEM) which revealed the formation of highly crystalline cubic phase of skutterudite Yb‐filled CoSb₃. Laser induced breakdown spectroscopy (LIBS) confirmed the presence of ytterbium in the Yb‐filled CoSb₃ samples.     

Effect of CuCl2 · 2 H2O additive in ethanol on dissolution and selective etching behaviour of CsI crystals

{100} faces of Kyropoulos grown CsI crystals had undergone a selective etching, and CsI spheres of ∼ 10 mm in diameter were subjected to dissolution at room temperature for 10 hrs. The results are: pure ethanol does not bring about any anisotropy in dissolution, addition of CuCl₂ · 2 H₂O leads to an anisotropy in dissolution of CsI, change in CuCl₂ · 2 H₂O concentration changes etch pit morphology, whereas the dissolution form does not depend on additive concentration. With increasing CuCl₂ · 2 H₂O etching rates at first increase, later decrease, dissolution rates linearly increase.     

Modeling the atomic structure of an amorphous Ni71Nb29 alloy produced by mechanical alloying using reverse Monte Carlo simulations

The local atomic structure of an amorphous Ni₇₁Nb₂₉ alloy produced by mechanical alloying technique was determined using only one X-ray total structure factor S(K) as input data for reverse Monte Carlo (RMC) simulations. The results showed that the amorphous alloy has a local atomic structure similar to that predicted by the additive hard sphere (AHS) model for a Ni and Nb mixture with same composition of the alloy, and quite different of that found in the rhombohedral NiNb crystal. The obtained coordination numbers for the first neighbors showed that the amorphous alloy has a preference to form homopolar pairs.     

Effect of Impurities on the Etching of Sodium Chloride Crystals

The results of a study of the effect of HgCl₂, ZnCl₂, PbCl₂ and CaCl₂ on the surface micro-morphology and kinetics of etching of {100} planes of NaCl crystals in methanol and ethanol are described and discussed. It was found that addition of an impurity to the solvent leads to the formation of contrasting dislocation etch pits, and that the overall dissolution rate in a solvent decreases with an increase in additive concentration. In the case of HgCl₂ impurity added to methanol terraced etch pits are observed, but their terracing behaviour diminishes with the increasing impurity concentration.     

Preparation and characterization of agglomerated porous hollow silica supports for olefin polymerization catalyst

Self-prepared porous hollow silica (PHS) nanoparticles were agglomerated with an oil (kerosene)–ammonia method and used as a novel support of metallocene catalysts for olefin polymerization. Transmission electron microscopy (TEM), scanning electron microcopy (SEM), small-angle X-ray diffraction (SA-XRD) and nitrogen adsorption–desorption were employed to characterize the morphologies and mesostructures of the PHS particles and agglomerated PHS particles. It was found that the agglomerated PHS particles had an average diameter between 15 and 30 μm, a specific surface area of 214 m²/g, a pore diameter of 11–18 nm and a pore volume of 1.41 mL/g. Metallocene catalysts prepared with the agglomerated PHS as support demonstrated a very high activity of 8350 g PE/gCata. for ethylene polymerization due to their particular morphology and mesostructures. Analysis with Differential Scanning Calorimetry (DSC), SEM, TEM and molecular weight analysis indicated that the polyethylene product was composed of tiny spherical particles with good crystalline and narrow molecular weight distribution, and the morphology and structure of the individual PHS support particle remained unchanged in polyethylene following the polymerization.     

Synthesis of MoSe2/Reduced graphene oxide composites with improved tribological properties for oil‐based additives

In this work, we developed a facile and effective hydrothermal method synthesis of MoSe₂ nanoflowers on reduced graphene oxide (RGO) sheets. The as‐prepared MoSe₂/reduced graphene oxide (MoSe₂/RGO) composites are characterized by X‐ray diffractometer (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicated that MoSe₂ nanoflowers were successfully deposited on RGO nanosheets to form a well interconnected hybrid structure. The tribological properties of MoSe₂/RGO composites as lubricating oil additive were investigated by a UMT‐2 ball‐on‐plate friction and wear tester. By the addition of MoSe₂/RGO composites in paraffin oil, the antiwear ability was improved and the friction coefficient was decreased. The based oil with MoSe₂/RGO composites showed better tribological properties than the oil with MoSe₂ and pure oil. The good friction and wear properties of MoSe₂/RGO composites as additives were attributed to the formation of a thin physical tribofilm on the substrate.     

Influence of some foreign metal ions on crystal growth kinetics of brushite (CaHPO4·2H2O)

Brushite, CaHPO₄·2H₂O, has been precipitated at 25 °C in the presence of Mg²⁺, Ba²⁺ or Cu²⁺ at concentrations up to 0.5 mM. When initial pH is sufficiently low to exclude nanocrystalline apatite as the initial solid phase, overall crystal growth rate may be determined from simple mass crystallization by recording pH as function of time. A combination of surface nucleation (birth-and-spread) and spiral (BCF) growth was found. Edge free energy was determined from the former contribution and was found to be a linear function of chemical potential of the additive, indicating constant adsorption over a wide range of additive concentrations. Average distances between adsorbed additive ions as calculated from slopes of plots are compatible with lattice parameters of brushite: 0.54 nm for Mg²⁺, 0.43 nm for Ba²⁺ and 0.86 nm for Cu²⁺. With the latter a sharp decrease in growth rate occurred early in the crystallization process, followed by an equally sharp increase to the previous level. When interpreted in terms of the Cabrera–Vermilyea theory of crystal growth inhibition, the results are consistent with an average distance between Cu ions of 0.88 nm, in perfect agreement with the above value.     

Reverse Monte Carlo simulations of an amorphous Cr25Nb75 alloy produced by mechanical alloying

The local atomic structure of an amorphous Cr₂₅Nb₇₅ alloy produced by mechanical alloying was determined using only one X-ray total structure factor S(K) as input data for reverse Monte Carlo simulations. The results showed that the amorphous alloy has a local atomic structure similar to that predicted by the additive hard sphere model for a Cr and Nb mixture with same composition of the alloy, and quite different of those found in the cubic and hexagonal Cr₂Nb crystals.     

Temperature Dependence of the Absorption Spectra and Optical Parameters in TGS and Cu2+-Doped TGS Crystals

The effect of temperature on the optical absorption spectra and optical parameters is investigated for pure TGS and TGS doped with Cu²⁺ ions. Absorption measurements cover the range from room temperature to about 355 K in the energy range 3-5.5 eV. The temperature dependence of the band gap E_g(T) reveals an anomaly at the phase transition temperature for both pure and Cu²⁺-doped TGS crystals. In the region of the absorption edge the absorption coefficient is found to display Urbach-rule behaviour. The characteristic Urbach parameters are determined and their temperature dependence is investigated.     

Phase Transition and Phase Diagram of Anion Radical Salts of [(C6H5)3PCH3]+ [(C6H5)3AsCH3]+ ×(TCNO)− 2 (0 ≤×≤ 1)

Abstract

Much attention has been paid to the solid anion radical salts of 7,7,8,8-tetracyanoquinodimethane (TCNQ), because of their prominent electronic properties.^1–4 In particular, the salts containing mixed cations represented by [(C₆H₅)₃PCH₃]⁺ _1–x [(C₆H₅)₃AsCH₃]⁺ _× (TCNQ)^? ₂, (0 ≤×≤ 1), are known to undergo phase transitions at 1 atm pressure in the solid state.^1–4 The phase transition of pure methyltriphenylphosphonium salt, (x = 0.00), takes place at 315.7 K. Heat-capacity measurements of this phase transition have been made by Kosaki et al. ³ The transition has thus been found to be of the first order. The enthalpy and the total entropy change associated with the phase transition were experimentally determined to be 485.18 cal/mol and 1.7206 cal/deg.mol, respectively. For the solid solutions, it was found that the transition temperature (T_c ) is increased, while the magnitude of the heat of transition (δH) is decreased, progressively with an increase in the composition parameter (x) and that pure methyltriphenylarsonium salt, (x = 1.00), has no such phase transition up to the decomposition temperature of about 480 K at 1 atm pressure.^1–3 Figure 1 shows the experimental relation between T _c and x, together with the relation between δH and x.⁴ In the present paper, we attempted to explain thermodynamically the phase diagram of Figure 1 for the solid solutions of those TCNQ anion radical salts.     

Formation of Pure Haematite by Hydrolysis of Iron (III) Salt Solutions under Hydrothermal Conditions

Comparing investigations were carried out of the hydrolysis of iron(III) sulfate, nitrate, and chloride under hydrothermal conditions with an aim to find a new way for the synthesis of pure α-Fe₂O₃ (haematite). Rich yields of highly pure haematite can be precipitated from iron(III) nitrate solutions. The strong decrease of the impurity level in the hydrothermal formation of haematite permits to precipitate from impure technical solutions α-Fe₂O₃ of adequate purity for many purposes. In contrast, the precipitates from iron(III) sulfate or chloride solutions obtained under comparable conditions are rather uncomplete or the phases formed contain a more or less percentage of the respective anion.     

Synthesis and characterization of PbI2 polycrystals

Single‐phase PbI₂ polycrystalline material for single crystal growth was synthesized by two‐temperature vapor‐transporting method (TVM), directly from highly pure lead and iodine with excess lead without according to the PbI₂ stoichiometry. It is found that there is an immiscible phenomenon of two melts in the synthesis experiment, and the melts solidified until cooled down to room temperature. X‐ray diffraction (XRD) analysis and Energy dispersive X‐ray (EDX) microanalysis indicate that the solidified materials are single‐phase PbI₂ polycrystal and nearly pure lead. Considering the observed immiscible phenomenon and the data given in paper [1], it is able to confirm that there is a new immiscible region L₂+L₃ in Pb‐I phase diagram, which is very important to PbI₂ polycrystal synthesis and single crystal growth. Using single‐phase PbI₂ polycrystalline material synthesized by our method, PbI₂ single crystal with size of ?15mm×30mm was grown by vertical Bridgman method. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

Photorefractive Properties of Stoichiometric Lithium Niobate Single Crystals

The specific features of photorefractive light scattering in nominally pure stoichiometric (Li/Nb = 1) sin- gle crystals grown from a melt with 58.6 mol % Li₂O (LiNbO₃st) and in the stoichiometric single crystals grown from a melt of congruent composition in the presence of K₂O flux (LiNbO₃stK₂O) have been investi- gated. At an excitation power of 30 mW, LiNbO₃stK₂O single crystals are found to exhibit a stronger photo- refractive effect than LiNbO₃st single crystals.