The Formation of Magnesium Oxychloride Phases in the Systems MgO-MgCl2-H2O and NaOH-MgCl2-H2O

The reactions leading to the formation of crystalline Mg₃(OH)₅Cl·4H₂O (phase 5), Mg₂(OH)₃,Cl·4H₂O (phase 3), and Mg(OH)₂ are compared for the systems MgO-MgCl₂-H₂O and NaOH-MgCl₂-H₂O. The crystalline phases were determined by X-ray diffraction analysis. The concentration of the total magnesium and chloride in the solution and the pH of the solution determine the reaction product(s) in both systems. The influence of MgO reactivity and the molar ratio of reactants on the formation and stability of reaction products is discussed and the mechanism of the formation of phases 3 and 5 is explained. In the system MgO-MgCl₂-H₂O, MgO serves only to increase the concentration of total magnesium and the pH of the MgCl₂ solution.     

The System MgO─P2O5─H2O at 25°C

The phase diagram for the ternary system MgO─P₂O₅─H₂O at 25°C has been constructed. The magnesium phosphates represented are Mg(H₂PO₄)₂· n H₂O ( n = 4, 2, 0), MgHPO₄·3H₂O, and Mg₃(PO₄)₂· m H₂O ( m = 8, 22). Because of the large differences in the solubilities of these compounds, the technique which involves plotting the mole fractions of MgO and P₂O₅ as their 10th roots has been employed. With the exception of MgHPO₄·3H₂O, the magnesium phosphates are incongruently soluble. Because incongruency is associated with a peritectic-like reaction, the phase Mg₂(PO₄)₃· 8H₂O persists metastably for an extended period.     

Reaction Products in the System MgCl2-NaOH-H2O

The precipitation process of solid phases Mg₃(OH)₅CI-4H₂O (phase 5), Mg₂(OH)₃CI-4H₂O (phase 3), and Mg(OH)₂ was followed by the addition of NaOH water solution in MgCl₂ water solutions of different concentrations (0.001 to 4.8 mol dm⁻³) and characterized by chemical, potentiometric, coulometric, and X-ray diffraction analyses. The concentration range in which the precipitation of solid phases occurs was determined. The phase distributions relative to the pH of solution and concentrations of magnesium and chloride were defined by the equilibrium diagram. The approximate solubility products of stable solid phases formed at different ionic strengths and at 293 K were determined.     

Fabrication of Translucent Magnesium Aluminum Spinel Ceramics

A precursor for magnesium aluminum spinel powder, composed of crystalline ammonium dawsonite hydrate (NH₄Al(OH)₂CO₃·H₂O) and hydrotalcite (Mg₆Al₂(CO₃)(OH)₁₆·4H₂O) phases, was synthesized via precipitation, using ammonium bicarbonate as the precipitant. The precursor was characterized by differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. Reactive spinel powder, which could be densified to translucency under vacuum at 1750°C in 2 h without additives, was obtained by calcining the precursor at 1100°C for 2 h.     

The System Magnesia-Silica-Water Below 300°C.: I, Low-Temperature Phases from 100° to 300°C. and Their Properties

Reactions in the ternary system MgO-SiO₂-H₂O were studied over the temperature range 100° to 300° C. and were found to produce only two phases. Under conditions of 100° to 200° C. and atmospheric pressure up to 20,000 lb. per sq. in., and regardless of the initial MgO/SiO₂ ratio, the predominant magnesium silicate product was found to have a MgO/SiO₂ ratio of 1.5. In the range 200° to 300°C. at 210 to 20,000 lb. per sq. in. two stable phases, 3MgO.2SiO₂.2H₂O (I) and 3MgO.4SiO₂.H₂O (II), were observed. In this case the phase that was favored was determined by the molar ratio of MgO/SiO₂ of the reaction mixture at the start of the run. The physical and chemical properties of phases (I) and (II) resembled those of the natural minerals serpentine and talc respectively. Two different morphologies were observed in electron micrographs of phase (I). From 100° to 160°C. it occurred as crumpled foils, and at 170°C. fibrous crystallites, which resembled the natural asbestos mineral chrysotile, appeared at the expense of some of the foils.     

Hydrolysis of Pure and Sodium Substituted Calcium Aluminates and Cement Clinker Components Investigated by in Situ Synchrotron X-ray Powder Diffraction

The hydrolysis of pure and sodium-substituted calcium aluminates and cement clinker phases was investigated in situ in the temperature range 25°–170°C, using the angle dispersive powder synchrotron powder X-ray diffraction technique. The final hydrolysis product in all cases was Ca₃Al₂(OH)₁₂. The intermediate phase Ca₄Al₂O₇·19H₂O was formed from the pure calcium aluminates, and the intermediate phases Ca₄Al₂O₇· x H₂O, x = 11, 13, or 19, were formed from the cement clinker phases.     

Studies on 4CaOAl2.O3.13H2O and the Related Natural Mineral Hydrocalumite

Single-crystal X-ray and electron-diffraction studies show the existence in one polymorph of 4CaO.Al₂O₃. 13H₂O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca₂(OH)₇- 3H₂O. These structural elements are stacked in a complex way and there are probably two or more poly-types as in SiC or ZnS. Hydrocalumite is closely related to 4CaO.A1₂O₃.13H₂O, from which it is derived by substitution of CO₃^2-for 20H^-+ 3H₂O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al₂O₃.Ca Y ₂- xH₂O and 3CaO Al₂O₃ Ca Y xH₂O. On dehydration, 4CaO.Al₂O₃.13H₂O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH)₂ and 4CaO.3Al₂O₃.3H₂O are formed and, by 1000°C., CaO and 12CaO.7Al₂O₈. The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al₂O₃.3H₂O is formed.     

High-Temperature Stability of Alumina in Argon and Argon/Water-Vapor Environments

The high-temperature stability of alumina (Al₂O₃) in argon and argon/water-vapor (Ar/H₂O) environments has been investigated. Samples were exposed at temperatures of 1300°C–1700°C for 10 h. The microstructure, flexural strength, and volume all showed significant changes in the Ar/H₂O environment at 1700°C. Samples also became whiter, because of the oxidation of graphite impurities that had diffused from the hot-processing dies. In the Ar/H₂O environment at 1700°C, grain-boundary etching occurred and was much more severe than in the pure-argon environment, which was very likely caused by the enhanced formation of gaseous Al(OH)₃ and Al(OH)₂ along grain boundaries. In addition, in the Ar/H₂O environment, substantial grain growth occurred in the surface vicinity. This grain growth, together with grain-boundary etching, led to a decrease in flexural strength.     

Phase Transformation of Hydrous-Zirconia Fine Particles Containing Cerium Hydroxide

Monoclinic hydrous-zirconia fine particles that contained cerium(IV) hydroxide (Ce(OH)₄) were heated from 200°C to 600°C, to investigate the phase transformation to CeO₂-doped tetragonal ZrO₂. Both ZrOCl₂·8H₂O and CeCl₃·7H₂O were dissolved in aqueous solutions and then boiled to prepare the hydrous-zirconia particles. The Ce(OH)₄-containing hydrous-zirconia particles were prepared by adding aqueous ammonia into the boiled solutions. The monoclinic-to-tetragonal ( m right arrow t ) phase transformation of the Ce(OH)₄-containing hydrous zirconias was observed at 300°C using X-ray diffraction (XRD). XRD and Brunauer-Emmett-Teller (BET) specific surface area measurements revealed that the Ce(OH)₄-containing hydrous zirconias had a tendency to transform from the monoclinic phase to the tetragonal phase at lower temperatures as the primary particle size of the hydrous zirconia decreased and the Ce(OH)₄ content increased. These tendencies for the m right arrow t phase transformation agree with the conclusions that have been derived from thermodynamic and kinetic considerations.     

Thermal Analysis of Some Barium and Strontium Titanyl Oxalates

The thermal decompositions of BaTiO(c₂O₄)₂.- 4H₂O, BaTiO(OH)₂C₂O₄.2H₂O, SrTiO(C₂O₄)₂.- 4H₂O, and SrTiO(OH)₂C₂O₄.H₂O were investigated using TGA, DTA, and effluent gas analysis. The stoichiometry of the decompositions is discussed and it is proposed that a reduced state of titanium is formed as an intermediate.     

The System CaO-Al2O3-H2O

Phase equilibria have been determined in the system CaO-Al₂O₃-H₂O in the temperature range 100° to 1000°C. under water pressures of up to 3000 atmospheres. Only three hydrated phases are formed stably in the system: Ca(OH)₂, 3CaO·Al₂O₃·6H₂O, and 4CaO·3Al₂O₃-3H₂O. Pressure-temperature curves delineating the equilibrium decomposition of each of these phases have been determined, and some ther-mochemical data have been deduced therefrom. It has been established that both the compounds CaO·Al₂O₃ and 3CaO·Al₂O₃ have a minimum temperature of stability which is above 1000°C. The relevance of the new data to some aspects of cement chemistry is discussed.     

Synthesis of Mesoporous Needle-Shaped Ytterbium Oxide Crystals by Solvothermal Treatment of Ytterbium Chloride

The reaction of rare-earth (RE; Y, Er, and Yb) chloride hydrates in 1,4-butanediol at 300°C for 2 h gave mixtures of RE(OH)₂Cl and RE₂O₃· x H₂O, and the products were composed of irregularly shaped particles. A prolonged reaction (10 h) yielded a mixture of RE(OH)₂Cl and RE₂O₃· x H₂O for Er or Y, but phase-pure RE₂O₃· x H₂O was obtained for Yb. The product for Yb comprised needle-shaped single crystals of Yb₂O₃· x H₂O with a width of 0.2–0.6 μm and a length of 5–15 μm. The Yb₂O₃· x H₂O phase decomposed to Yb₂O₃ at 350°–500°C, preserving the needle-shaped morphology; this was maintained even after calcination at 1100°C. Single crystals of Yb₂O₃ obtained by the calcination of Yb₂O₃· x H₂O at 500°C had very small voids and the voids were enlarged to 35 Å in diameter by calcination at 800°C.     

Synthesis and Characterization of MgAl2O4 Spinel Precursor from a Heterogeneous Alkoxide Solution Containing Fine MgO Powder

A precursor was synthesized from a heterogeneous alkoxide solution that contained fine MgO powder, which allowed the preparation of MgAl₂O₄ spinel powder with high sinterability characteristics. The precursor consisted of a mixture of boehmite (AlO(OH)) and a mixed hydroxide (Mg₄Al₂(OH)₁₄· 3H₂O). The spinel phase formed through two steps: (i) decomposition of the mixed hydroxide at low temperature and (ii) solid-state reaction between MgO and γ-Al₂O₃ at higher temperatures. Dense polycrystalline spinel could be obtained from the calcined powders at sintering temperatures as low as 1400°C.     

Dehydration Behavior of Aluminum Sulfate Hydrates

An exothermic transition is observed near 400°CC on thermal dehydration of highly crystalline AI₂(SO₄)3.16H₂O, Al₂(S0₄)₃ 14H₂O, and Al₂(S0₄)₃ 9H₂O when the early stages of heating are carried out in vacuum. Amorphous or partially crystalline hydrates do not show the exotherm. No systematic relation is apparent between the decomposition behavior and the pore volume distribution of the various anhydrous A1₂(SO₄)₃ products.     

Chemistry and Morphology of Hydrogarnets Formed in Cement-Based CASH Hydroceramics Cured at 200° to 350°C

We have studied the chemistry and the morphology of hydrogarnet crystals produced in cement-based hydroceramic materials at elevated temperatures (200°–350°C) with silica and alumina additions. Such materials lie within the hydrothermal CaO–Al₂O₃–SiO₂–H₂O (CASH) system. Hydrogarnet Ca₃Al₂(SiO₄)_{3− y} (OH)_{4 y} is the dominant aluminum bearing phase formed and its composition is influenced mainly by the curing temperature and to a lesser degree by the addition of silica. The composition parameter y was estimated by Rietveld refinement of X-ray diffraction (XRD) data. Electron probe microanalysis (EPMA) shows that the hydrogarnets incorporate minor elements such as Fe, Mg, and S. EPMA data confirmed the hydrogarnet composition estimated from XRD. Both octahedral and icositetrahedral forms are observed. The icositetrahedral form is associated with higher minor element content.     

Kinetics of Thermal Dehydroxylation and Carbonation of Magnesium Hydroxide

The kinetics of simultaneous dehydroxylation and carbonation of precipitated Mg(OH)₂ were studied using isothermal and nonisothermal thermogravimetric analyses. Specimens were analyzed using X-ray diffraction, transmission electron microscopy, and through measurements of the volume of carbon dioxide evolved in a subsequent reaction with hydrochloric acid. From 275° to 475°C, the kinetics of isothermal dehydroxylation in helium were best fit to a contracting-sphere model, yielding an activation energy of 146 kJ/mol, which was greater than values reported in the literature for isothermal dehydroxylation under vacuum (53–126 kJ/mol). The carbonation kinetics were complicated by the fact that dehydroxylation occurred simultaneously. The overall kinetics also could be fit to a contracting-sphere model, yielding a net activation energy of 304 kJ/mol. The most rapid carbonation kinetics occurred near 375°C. At this temperature, Mg(OH)₂ underwent rapid dehydroxylation and subsequent phase transformation, whereas thermodynamics favored the formation of carbonate. During carbonation, MgCO₃ precipitated on the surface of disrupted Mg(OH)₂ crystals acting as a kinetic barrier to both the outward diffusion of H₂O and the inward diffusion of CO₂.     

Electroceramics from Source Materials via Molecular Intermediates: PbTiO3 from TiO2 via [Ti(catecholate)3]2-

The precursor [NH₄]₂[Ti(catecholate)₃] · 2H₂O is known to react with Ba(OH)₂· 8H₂O in an acid/base process that generates Ba[Ti(catecholate)₃] · 3H₂O, a compound which undergoes low-temperatue calcination to produce BaTiO₃ powder. Attempts to develop similar routes to PbTiO₃ have been frustrated, since lead(II) hydroxide does not exist. The amphoteric yellow PbO and the basic oxide, Pb₆O(OH)₆⁴⁺, are both insufficiently basic to react with [NH₄]₂[Ti(catecholate)₃] · 2H₂O. Based on the large sizes of both the [Ti(catecholate)₃]^2- anion and the Pb²⁺ cation, a precipitation method has been developed in which lead nitrate and [NH₄]₂[Ti(catecholate)₃] · 2H₂O are added together in an aqueous medium causing precipitation and leaving only NH₄NO₃ in solution. The lead-titanium-catecholate complex that forms in this manner undergoes low-temperature pyrolysis to produce PbTiO₃. SEM indicates a submicrometer ultimate crystallite size.     

Electroceramics from Source Materials via Molecular Intermediates: BaTiO3 from TiO2 via (Ti(catecholate)3)2-

Rutile or anatase may be depolymerized and complexed by sequential treatment with (i) H₂SO₄/(NH₄)₂SO₄, (ii) H₂O, and (iii) catechol/NH₄OH to produce the intermediate (NH₄)₂(Ti(catecholate)₃) · 2H₂O. Treatment with Ba(OH)₂· 8H₂O leads to an acid-base reaction generating Ba(Ti(catecholate)₃) · 3H₂O, in which the Ba:Ti ratio is held at 1:1 at the molecular level. Calcination produces BaTiO₃ powder.     

Thermal Decomposition of Some Substituted Barium Titanyl Oxalates and Its Effect on the Semiconducting Properties of the Doped Materials

Thermal analysis has been performed on BaTiO(C₂O₄)₂.4H₂O, Ba_0.6Sr_0.4TiO(C₂O₄)₂.4H₂O, Sr(TiO(C₂O₄)₂.4H₂O, Ba_0.9Pb_0.1TiO(C₂O₄)₂.4H₂O, and BaTi_0.9Zr_0.1O(C₂O₄)₂.4H₂O. It was observed that the strontium compound decomposes differently than the others. Previous investigators have proposed conflicting mechanisms for the pyrolysis of the barium salt and these results are discussed in comparison with this work. The electrical resistivity and temperature coefficient of fired lanthanum-doped materials were found to vary with the calcination temperature. Maximum conductivity was observed in samples calcined at 900°C whereas maximum positive temperature coefficient was observed for materials calcined at 1050°C. Particle sizes of the calcined material were compared with grain sizes in the fired pieces and correlated with the electrical properties. A cursory examination was made on the effects of fabrication pressure, 1.25 to 15 tsi, on the electrical conductivity. Both the conductivity and positive temperature coefficient were found to increase with decreasing fabrication pressure.     

Nanosized Zinc-Oxide Particles Derived from Mechanical Activation of Zn5(NO3)2(OH)8·2H2O in Sodium Chloride

Nanosized ZnO particles are successfully synthesized via mechanical activation of a zinc nitrate hydroxide hydrate (Zn₅(NO₃)₂(OH)₈·2H₂O) precursor in NaCl matrix for 15 h. The ZnO particles obtained are in the nanosize range of ∼20 nm, with a well-established hexagonal morphology. They compare favorably with those derived from conventional calcination of the precursor. The decomposition of Zn₅(NO₃)₂(OH)₈·2H₂O precursor and formation of nanocrystalline ZnO cannot be completed by mechanical activation in the absence of NaCl, which acts as both an effective dispersing matrix and drying agent although it remains chemically inert during mechanical activation. The powder derived from calcination at 400°C does not possess powder characteristics comparable to that of the powder derived from the mechanical activation in NaCl, because of the extensive particle coarsening and aggregation at the calcination temperature.