Kinetics of NiFe2O4 Precipitation in NiO

The kinetics of the precipitation of NiFe₂O₄ from an Fedoped NiO matrix has been studied. Polycrystalline samples were cooled at a constant rate through the solvus temperature, and the various stages of precipitation were monitored by periodically interrupting the cooling to observe the evolving size, distribution, and morphology of the precipitates. The precipitation occurred primarily by homogeneous nucleation and growth. The coherent precipitates have a dendritic morphology with a large interparticle spacing characteristic of fast precipitate growth. Reasonable rates for the nucleation of NiFe₂O₄ were observed to occur between 850° and 800°C, resulting in a 350° to 400°C undercooling. Preferential nucleation occurred at grain boundaries, but these particles were only twice as long as those nucleated homogeneously. The size and shape of the precipitates are independent of the distance from the surface, indicating that precipitation occurs by a local arrangement of the Ni and Fe cations and the preexisting cation vacancies.     

Oxygen Mobility in Polycrystalline NiCr2O4 and α-Fe2O3

Oxygen diffusion coefficients have been determined for polycrystalline samples of NiCr₂O₄ and α-Fe₂O₃ by exchange measurements with oxygen gas containing the stable isotope¹⁸O, using mass spectrometer analysis. Oxygen diffusion in NiCr₂O₄ can be represented by the equation D = 0.017 exp (-65,400/RT); oxygen diffusion in α-Fe₂O₃ can be represented by the equation D = 1 × 10¹¹ exp (-146,000/RT). The large difference between D₀ and activation energy for these materials suggests that different diffusion mechanisms are involved.     

Structural Relaxation around Substitutional Cr3+ Ions in Sapphire

The local structural arrangement of ions around a substitutional Cr³⁺ ion in sapphire (ruby) has been studied experimentally using extended X-ray absorption fine structure (EXAFS) in the vicinity of the Cr absorption edge. The findings are compared with an ionic model (Mott-Littleton) computation using two sets of pairwise potentials. Both the EXAFS results and the computations reveal that when Cr substitutes for an Al ion in sapphire, the surrounding ions relax to an arrangement similar to that for Cr in α-Cr₂O₃. Also, most of the structural relaxation is accommodated by the first shell of oxygen and aluminum ions around the substituted Cr³⁺ ion. The computations also indicate that with applied pressure (tensile and compressive) the ionic positions change self-similarly and in proportion to the macroscopic strain.     

Optical Absorption Spectra of Cr3+ in MgO·Al2O3-MgO·3.5Al2O3 Spinels

Spinel crystals in the system MgO· n Al₂O³:Cr³⁺ (1.04≤ n ≤3.51) were synthesized by flame fusion; their absorption spectra were measured in transmission from 400 to 50,000 cm⁻¹. The O-H band spectra suggest the presence of protons in octahedral site vacancies which appear for n >1. The spectra for Cr³⁺ indicate clearly that these ions occupy only octahedral sites. However, the effective trigonal field, which varies with n , causes the major absorption bands to split in the form of a lower-energy shoulder on a higher-energy component. The red-green transition in these spinels was rationalized and observed to arise from a somewhat different mechanism than that in other oxides in which the site radii and Cr³⁺ concentration vary considerably.     

On the Formation of Fe2+ in the System MgO-Fe2O3-MgFe2O4 at High Temperatures

The formation of ferrous iron in the system MgO-Fe₂O₃MgFe₂O₄ is of interest in connection with its deleterious effect on the microwave performance of magnesium ferrite, MgFe₂O₄. The partial reduction of ferric iron in the system at relatively low temperatures is discussed in terms of the preferential diffusion of iron, and its partial stabilization as ferrous iron from lattice energy considerations.     

Solubility of NiO in Al2O3

Solubility of NiO in Al₂O₃ was determined by electron probe microanalysisy A diffusion couple method was used by coupling an NiO-doped Al₂O₃ polycrystal to a pure single crystal of Al₂O₃. The solubility of NiO in Al₂O₃ in air was 230 wt ppm (157 at. ppm of cations) and 170 wt ppm (116 at. ppm) at 2073 and 1973 K, respectively. The solubility of NiO in Al₂O₃ obtained in this work was compared with our previous work of the solubility of MgO in Al₂o₃.     

Kinetics of Solid-State Reaction of Bi2O3 and Fe2O3

Solid-state reactions of equimolar mixtures of Bi₂O₃ and Fe₂O₃ from 625° to 830°C and their kinetics were investigated. The reaction rates were determined from the integrated X-ray diffraction intensities of the strongest peaks of the reactants and products. The activation energy for the formation of BiFeO₃ was 96.6±9.0 kcal/mol; that for a second-phase compound, Bi₂Fe₄O₉, which formed above 675°C, was 99.4±9.0 kcal/mol. Specific rate constants for these simultaneous reactions were obtained. The preparation of single-phase BiFeO₃ from the stoichiometric mixture of Bi₂O₃ and Fe₂O₃ is discussed.     

Control of Grain-Boundary Pinning in Al2O3/ZrO2 Composites with Ce3+/Ce4+ Doping

The control of the microstructure of Ce-doped Al₂O₃/ZrO₂ componsites by the valence change of cerium ion has been demonstrated. Two distinctively different types of microstructure, large Al₂O₃ grains with intragranular ZrO₂ particles and small Al₂O₃ grains with intergranular ZrO₂ particles, can be obtained under identical presintering processing conditions. At doping levels greater than ∼ 3 mol% with respect to ZrO₂, Ce³⁺ raises the alumina grain-boundary to zirconia particle mobility ratio. This causes the breakaway of grain boundary from particles and the first type of microstructure. On the other hand, Ce⁴⁺ causes no breakaway and produces a normal intergranular ZrO₂ distribution. The dramatic effect of Ce³⁺ on the relative mobility ratio is found to be associated with fluxing of the glassy boundary phase and is likewise observed for other large trivalent cation dopants. The ZrO₂ second phase acts as a scavenger for these trivalent cations, provided their solubility limit in ZrO₂ is not exceeded.     

Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame

SiO₂, Al₂O₃, and 3Al₂O₃.2SiO₂ powders were synthesized by combustion of SiCl₄ or/and AlCl₃ using a counterflow diffusion flame. The SiO₂ and Al₂O₃ powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al₂O₃.2SiO₂ powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.     

Decomposition Kinetics of CaYb2O4

The kinetics of decomposition of CaYb₂O₄ were studied using quantitative analysis by X-ray diffraction methods and a modified method of "standard binary mixtures." This type of method was required as total composition remained constant with only the phase assemblage changing. The experimental factors considered in obtaining meaningful results from such a procedure are covered. The linearity of the fraction decomposed or concentration unreacted versus time curves at various temperatures revealed that the decomposition proceeded according to "zero-order" kinetics. The activation energy for this process was 100 kcal/mole.     

Synthesis of Er3+ Doped Y2O3 Nanophosphors

The synthesis and characterization of yttrium hydroxyl carbonate (Y(OH)CO₃²⁻) and yttrium nitrate hydroxide hydrate (Y(OH)NO₃H₂O) precursor materials as well as Y₂O₃ nanoparticles are reported. The resultant precursor particle size is about 10–12 nm with a narrow size distribution by the enzymatic decomposition method, whereas the particle size was smaller than those acquired by the homogeneous and alkali precipitation methods. The formation of Y(OH)CO₃²⁻ and Y(OH)NO₃H₂O species was also evident from the fourier-transform infrared spectrometry (FT-IR) analysis. Precipitated Y(OH)CO₃²⁻ precursors have an amorphous nature whereas Y(OH)NO₃H₂O precursors have a crystalline nature, which was manifested from the XRD analysis. Moreover, precipitated Y(OH)NO₃H₂O precursors were found in the agglomerated form and Y(OH)CO₃²⁻ was established in the monodispersed form, as determined from the FE-SEM, TEM and DLS measurements. It was demonstrated that calcination of precursor materials at 900°C eventually removed the inorganic anions from the precursors and consequently produced crystalline Y₂O₃ nanoparticles, which was evident from the XRD and FT-IR analysis. The EDS analysis confirms Er³⁺ doping in the Y₂O₃ nanoparticles. The morphology and the size of the Y₂O₃ nanoparticles are almost unchanged before and after the calcination.     

Thermodynamic Properties of Fe3O4-FeV2O4 and Fe3O4-FeCr2O4 Spinel Solid Solutions

Solid solutions of Fe304-FeV204 and Fe304-FeCr204 were prepared and equilibrated with Pt under controlled streams of CO/CO, gas mixtures at 1673 K. The concentration of Fe in Pt was used to determine the activity of Fe304 in the solid solutions. The activity of the second component was calculated by Gibbshhem integration. From these data, the Gibbs energy of mixing was derived for both systems. The experimental results and theoretical values which are determined from calculated cation distribution compare favorably in the case of vanadite solid solutions but not in the case of chromite solid solutions. The difference is attributed to a heat term arising from lattice distortion due to cation size difference. The positive heat of mixing will give rise to a miscibility gap in the system Fe304-FeCr204 at lower temperatures.     

Effects of Sm2O3 and Gd2O3+ Nd2O3 on Electromechanical Properties of PbTiO3 Ceramics

The electromechanical properties of PbTiO₃ ceramics, modified by substitution of Sm or Gd + Nd (same average atomic radius as Sm) for Pb, were studied in the range of 6% to 14% substitution. The modified PbTiO₃ ceramics were stable, and the Curie temperature decreased linearly over this composition range. The 10% Sm composition had a large anisotropy in the coupling factor ratio, k _t / k _p , and a similar, but weaker, effect developed for 12% (1/2 Gd + 1/2 Nd). This indicates that other than average ion size may influence the electromechanical coupling factor ratio.     

Initial Sintering Kinetics of MgAl2O4

Results are presented for the initial-stage sintering of spinel in air at 1050° to 1300°C. Spinel powders of high purity were prepared from magnesium-aluminum hydroxide coprecipitates by calcining at 1100°C for 4 h. The sintering mechanism is volume diffusion, and the temperature dependence of the apparent diffusion coefficients can be represented by D = 18.6 exp [(-116,000 cal/mol)/RT] cm²/s.     

Preparation and Upconversion Properties of Yb3+, Ho3+: Lu2O3 Nanocrystalline Powders

Yb³⁺, Ho³⁺: Lu₂O₃ nanocrystalline powders were synthesized by a reverse-strike co-precipitation method using NH₄HCO₃ and NH₃·H₂O as precipitators. X-ray diffraction analysis and field emission scanning electron microscopy observation showed that the phase composition of the powders was cubic and the particle size was 30–50 nm. Under the excitation of a 980 nm continuous wave diode laser, green and red emissions centered around 548 and 667 nm, respectively, were observed and the green emission dominated the upconversion spectrum. Power studies revealed that a two-photon process was involved in the upconversion emissions and the possible upconversion mechanisms were discussed.     

Sintering and Grain-Growth Kinetics of MgAl2O4

The sintering and grain-growth kinetics of finely divided MgAl₂O₄ were determined from 1300° to 1600°C for densities up to 96% of theoretical. These results show that sintering is governed by volume diffusion and that the temperature dependence of the diffusivities is 157 exp [(-118 kcal/mol)/ RT ] cm²/s. Grain growth follows the expression ( G ²- G ₀²)= Kt , where K =51.3 exp [(-110 kcal/mol)/ RT ] cm²/s.