Mechanical Activation of Heterogeneous Sol–Gel Precursors for Synthesis of MgAl2O4 Spinel

MgAl₂O₄ spinel precursor was prepared using a heterogeneous sol–gel process. The effect of high-energy milling on the precursor decomposition and spinel formation was investigated. The milling decreased the Al(OH)₃ dehydroxylation temperature from 190° to about 130°C. The activation energy for spinel formation decreased from 688 kJ/mol for the as-prepared precursors to 468 kJ/mol for the precursors milled for 5 h. Milling of the precursor lowered the incipient temperature of spinel formation from 900° to 800°C, and the temperature of complete MgAl₂O₄ spinel formation from >1280° to ∼900°C.     

Communications of the American Ceramic Society Estimation of Thermochemical Data for Calcium Silicate Hydrate (C-S-H)

Calcium silicate hydrate (C-S-H) can be viewed as a solid solution, 0.833Ca(OH)₂.SiO₂.0.917H₂O-xCa(OH)₂, at equilibrium at 30°C. On this basis, the change in Gibbsfree energy (ΔG_r) in the solid-solution reaction was calculated from solubility duta for C-S-H in water. The change in ΔG_r with real ratio decreased notably for the higher calcium contents (CaO/Si0₂1.7; ×0.867). Thermochemical values for C-S-H (CaO/SiO₂=1.7) were estimated to be ΔH°=-2890 kJ/mol, ΔG°=-2630 kJ/mol, and S°=200 J1/mol.K at 298 K .     

Single-Calcination Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 and Pb(Mg1/3Nb2/3)O3 Ceramics Using a Coating Method

A coating approach for synthesizing 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃ (0.9PMN–0.1PT) and PMN using a single calcination step was demonstrated. The pyrochlore phase was prevented by coating Mg(OH)₂ on Nb₂O₅ particles. Coating of Mg(OH)₂ on Nb₂O₅ was done by precipitating Mg(OH)₂ in an aqueous Nb₂O₅ suspension at pH 10. The coating was confirmed using optical micrographs and zeta-potential measurements. A single calcination treatment of the Mg(OH)₂-coated Nb₂O₅ particles mixed with appropriate amounts of PbO and PbTiO₃ powders at 900°C for 2 h produced pyrochlore-free perovskite 0.9PMN–0.1PT and PMN powders. The elimination of the pyrochlore phase was attributed to the separation of PbO and Nb₂O₅ by the Mg(OH)₂ coating. The Mg(OH)₂ coating on the Nb₂O₅ improved the mixing of Mg(OH)₂ and Nb₂O₅ and decreased the temperature for complete columbite conversion to ∼850°C. The pyrochlore-free perovskite 0.9PMN–0.1PT powders were sintered to 97% density at 1150°C. The sintered 0.9PMN–0.1PT ceramics exhibited a dielectric constant maximum of ∼24 660 at 45°C at a frequency of 1 kHz.     

Assessment of the Thermodynamic Values for PuO1.5 and High-Temperature Determination of the Values for PuC1.5

Thermodynamic values for PUO_1.5 were assessed using an improved method for estimating fef ° 1.5 and new data for S°₂₉₈ 1.5. Based on the assessment, a value of ΔH°₂₉₈, 1.5=–828 kJ/mol is recommended. Measurements of (CO) pressure over the nominal equilibrium 1.5+ 1.5+ C were performed between 1348 and 1923 K, yielding pressures between 0.644 and 11600 Pa. Second- and third-law analyses were used to obtain a value for ΔH°₂₉₈ 1.5=–93.3°3.3 kJ/mol.     

Oxygen Self-Diffusion in Magnesium- or Titanium-Doped Alumina Single Crystals

Oxygen Setf-diffusion coefficients have been measured in single crystals of N₂O₃ doped with Mg or Ti under an oxygen partial pressure of 20 kPa in the temperature range 1400° to 1700°C. Diffusion coefficients in Mg-doped crystals obey the equation D_o (cm²/s) = 1 × 10¹¹∼ (1×10¹³) exp[−915 ± 50(kJ/mol)/ RT ]. The diffusivity of oxygen in Ti-doped A1₂O₃ is lower than Mg-doped A1₂O₃. A vacancy mechanism explains these results.     

Interdiffusion in SiC–AlN and AlN–Al2OC Systems

AlN, Al₂OC, and the 2 H form of SiC are isostructural. Both SiC–AlN and AlN–Al₂OC form homogeneous solid solutions above 2000° and 1950°C, respectively. The kinetics of phase separation in the two systems, however, are quite different. Interdiffusion in both SiC–AlN and AlN-Al₂OC systems was examined in the solid-solution regime in an attempt to elucidate differences in the kinetics of phase separation that occur in the two systems when annealed at lower temperatures. Diffusion couples of (SiC)_0.3(AlN)_0.7/(SiC)_0.7(AlN)_0.3 and (AlN)_0.7(Al₂OC)_0.3/(AlN)_0.3(Al₂OC)_0.7 were fabricated by hot pressing and were annealed at high temperatures by encapsulating them in sealed SiC crucibles to suppress loss due to evaporation. Interdiffusion coefficients in (SiC)_0.3-(AlN)_0.7/(SiC)_0.7(AlN)_0.3 diffusion couples were measured at 2373, 2473, and 2573 K, and the corresponding activation energy was determined to be 632 kJ/mol. (AlN)_0.7(Al₂OC)_0.3/ (AlN)_0.3(Al₂OC)_0.7 samples were annealed at 2273 K. The interdiffusion coefficient measured in the AlN–Al₂OC system was much larger than that in the SiC–AlN system.     

Interpretation of the Parabolic and Nonparabolic Oxidation Behavior of Silicon Oxynitride

The oxidation process of Si₂N₂O, prepared by a hot isostatic pressing technique, has been studied by the thermogravimetric method. The oxidation has been performed in oxygen for 20 h in the temperature range 1300° to 1600°C, producing oxide scales of amorphous SiO₂ and α-cristobalite. The weight gain for T 1350°C does not begin to follow a parabolic rate law, until a certain time, t ₀. The A ₀ parameter in the parabolic rate law, (Δ w / A ₀)²= K _p t + B , represents the cross section area, A , through which the oxygen diffuses; in the derivation of this law A is assumed to be constant during the experiment. If crystallization occurs during the oxidation process, A will decrease with time. A function, A ( t ), describing the time dependence, has been developed and incorporated into the parabolic rate law, yielding a new rate law, which reads Δ W/A ₀= a arctan √ bt + c √ t . This new rate law is valid in the time interval t < t ₀, whereas, for t > t ₀, the oxidation process follows the equation (Δ w/A ₀)²= K °_p t + B ₀. The relation of the latter equation to the common parabolic rate law is described. All of the oxidation curves are described by these equations. The activation energy of the oxygen diffusion (and of the oxidation ( K _p)) is found to be 245 ± 25 kJ/mol, which is consistent with literature values reported for oxygen diffusion.     

Sol-Gel Synthesis of Amorphous Calcium Phosphate and Sintering into Microporous Hydroxyapatite Bioceramics

A new route for preparing hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) bioceramic has been described. An amorphous, nanosized, and carbonate-containing calcium phosphate powder that had a Ca:P ratio of 1.67 was synthesized from calcium diethoxide and phosphoric acid in ethanol via a sol-gel method. The powder was pressed at 98 MPa into green specimens and then heated to a temperature range of 500°-1300°C. At 600°C, the powder crystallized to a carbonated hydroxyapatite and a trace of ß-tricalcium phosphate before converting to hydroxyapatite at 900°C. The thermal crystallization was associated with grain growth, shrinkage, and active surface diffusion. The activation energy of grain growth was 37 ± 2 kJ/mol. After sintering at 1100°C, the decomposition of carbonated hydroxyapatite generated a microporous ceramic with an average pore size of 0.2 µm and an open porosity of 15.5%. This microporous bioceramic can be used as a bone filler.     

Kinetics of Barium Titanate Synthesis

Reaction curves were obtained at various temperatures and concentrations for the formation of BaTiO₃ from particulate titania in Ba(OH)₂ solution. Kinetic analyses were performed by constructing mathematical models which took into account the particle size distribution of the reactant titania for both the topochemically-rate-controlled and the diffusion-rate-controlled reactions. At [Ba(OH)₂] > ca. 0.1 M the rate-controlling step is the Ba reaction with TiO₂ at the interface. The measured activation energy is 105.5 kJ/mol. The rates are independent of Ba(OH)₂ concentration, indicating that the TiO₂ interface is saturated. At [Ba(OH)₂] < ca. 0.1 M the rate-determining step shifts to diffusion through the product BaTiO₃ layer, the rates are concentration dependent, and the BaTiO₃ particle sizes are inversely proportional to the Ba(OH)₂ concentrations used.     

Effect of Seeding and Water Vapor on the Nucleation and Growth of α-Al2O3 from γ-Al2O3

Isothermal transformation kinetics and coarsening rates were studied in unseeded and alpha-Al₂O₃-seeded γ-Al₂O₃ powders heated in dry air and water vapor. Unseeded samples heated in dry air transformed to alpha-Al₂O₃ with an activation energy of 567 kJ/mol. Seeding with alpha-Al₂O₃ increased the transformation rates and reduced incubation times by providing low-energy sites for nucleation/growth of the alpha-Al₂O₃ transformation. The activation energy for the transformation was reduced to 350 kJ/mol in seeded samples heated in dry air. Seeded samples completely transformed to alpha-Al₂O₃ after 1 h at 1050°C when heated in dry air compared to 1 h at 925°C when heated in saturated water vapor. The combined effects of a lower nucleation barrier due to seeding and the increased diffusion due to water vapor reduced the activation energy for the transformation by 390 kJ/mol and the transformation temperature by ∼225°C compared to the unseeded samples heated in dry air. The accelerated kinetics is believed to be due to increased surface diffusion.     

Enthalpy of Formation of Zircon

Using high-temperature solution calorimetry in molten 2PbO. B₂O₃, the enthalpy of reaction of the formation of zircon, ZrSiO₄, from its constituent oxides has been determined: Δ_r H ₉₇₇(ZrSiO₄) =−27.9 (± 1.9) kJ/mol. With previously reported data for the heat contents of ZrO₂, SiO₂, and ZrSiO₄ and standard-state enthalpies of formation of ZrO₂ and SiO₂, we obtain Δ_f H °₂₉₈· (ZrSiO₄) =−2034.2 (±3.1) kJ/mol and Δ_f G °₂₉₈ (ZrSiO₄) =−1919.8 kJ/ mol. The free energy value is in excellent agreement with a range previously estimated from solid-state reaction equilibria. At higher temperature also the data are in close agreement with existing data, though the data sets diverge somewhat with increasing T . The limitations of the data for predicting the breakdown temperature of zircon into its constituent oxides are discussed.     

Low-Temperature Synthesis of Calcium Hexa-Aluminate

Calcium hexa-aluminate (CaO·6Al₂O₃) has been prepared from calcium nitrate and aluminum sulfate solutions in the temperature range of 1000°–1400°C. A 0.3 mol/L solution of aluminum sulfate was prepared, and calcium nitrate was dissolved in it in a ratio that produced 6 mol of Al₂(SO₄)₃·16H₂O for each mole of Ca(NO₃)₂·4H₂O. It was dried over a hot magnetic stirrer at ∼70°C and fired at 1000°–1400°C for 30–360 min. The phases formed were determined by XRD. It was observed that CaO·Al₂O₃ and CaO·2Al₂O₃ were also formed as reaction intermediates in the reaction mix of CaO·6Al₂O₃. The kinetics of the formation of CaO·6Al₂O₃ have been studied using the phase-boundary-controlled equation 1 − (1 − x )^1/3= K log t and the Arrhenius plot. The activation energy for the low-temperature synthesis of CaO·6Al₂O₃ was 40 kJ/mol.     

Reactions and Equi ibria in Magnesium Oxych oride Cements

Phase equi ibria in the system MgO-MgC ₂-H₂O at 2°±3°C were determined and the reactions by which the equi ibrium phases, 5Mg(OH)₂ MgC ₂ 8H₂O and 3Mg(OH)₂ MgC ₂8H₂O, deve op were studied by X-ray diffractometry. As reactive MgO is disso ved by magnesium ch oride so utions, a thixotropic suspension is converted to a ge, which then crysta izes to form the ternary oxych oride phases. Insufficient y active MgO disso ves more s ow y so that, in an open system, bu k composition can shift by evaporation of water, resu ting in crysta ization of a nonequi ibrium phase assemb age, with residua magnesium ch oride so ution and unreacted MgO. Imp ications of these nonequi ibrium reactions for the performance of magnesium oxych oride cements are discussed.     

Kinetic Model for α–Tricalcium Phosphate Hydrolysis

A mechanistic model for the kinetics of hydrolysis of α-tricalcium phosphate (α–Ca₃(PO₄)₂ or α-TCP) to hydroxyapatite (Ca_{10− x} (HPO₄) _x (PO₄)_{6− x} (OH)_{2− x} or HAp) has been developed. The model is based on experimental hydrolysis rate data obtained using isothermal calorimetry. Analysis of the kinetic data according to the general kinetics models in terms of the fractional degree of reaction and time suggests the hydrolysis to be controlled by different rate-limiting mechanisms as reaction proceeds. Initially, the hydrolysis kinetics depend on the surface area of the anhydrous α-TCP. Subsequently, they change to a dependence on the rate of HAp product formation controlled by a nucleation and growth mechanism. The model predicts that HAp nuclei form at essentially one time and growth occurs in two dimensions, leading to a platelike morphology. The change in the reaction mechanism occurs at a fractional degree of hydrolysis, which does not change significantly with temperature in the range of 37°–56°C.     

Rutherford Backscattering Spectroscopy Study of the Kinetics of Oxidation of (Mg, Fe)2SiO4

Single crystals of oilivine, (Mg_0.9Fe_0.1)₂SiO₄, have been oxidized in air at temperatures between 700° and 1100°C for times from 0.5 to 100 h. Both an internal and an external oxidation layer developed. Transmission and analytical electron microscopy observations reveal that the internal oxidation layer is composed of precipitates of magnetite plus amorphous silica, which nucleated heterogeneously on dislocations and grew in an Fedepleted matrix of olivine. Rutherford backscattering spec-trometry (RBS) demonstrates that the thin external oxidation layer is free of Si; that is, it is made up of Mg-Fe oxide phases. Thus, the oxidation process is primarily controlled by diffusion of Fe²⁺ and Mg²⁺ ions toward the surface with Si⁴⁺ and O^2- remaining largely immobile. The kinetics of oxidation, as determined from RBS analyses of the external oxidation layer, are parabolic with an activation energy of 140 kJ/mol. Although this activation energy is lower than that reported for self-diffusion of Mg in Mg₂SiO₄, the diffusivity calculated from the reaction rate constant is in good agreement with published values for lattice diffusion of Mg in the limited temperature range in which data overlap. However, the rate of accumulation of Fe in the external layer is more rapid than expected for lattice diffusion, indicating that the transport of Fe is dominated by short-circuit diffusion along the precipitate complexes which decorate dislocations.     

Low-Temperature Fabrication of Cordierite Ceramics from Kaolinite and Magnesium Hydroxide Mixtures with Boron Oxide Additions

Thermal reactions of mixtures of ultrafine particles of magnesium hydroxide (Mg(OH)₂) and kaolinite in a composition of MgO:Al₂O₃:2SiO₂ were investigated to obtain dense cordierite ceramics at temperatures <1000°C. While heating the mixture of kaolinite and Mg(OH)₂ with the equivalent of 2 mass% of boron oxide (B₂O₃) (in the form of magnesium borate, 2MgOB₂O₃), an amorphous phase formed at a temperature of ∼850°C after thermal decomposition. Firing the mixture at a temperature of 900°C yielded dense ceramics with an apparent porosity of almost zero. The addition of B₂O₃ promoted the densification at 850°-900°C and accelerated the crystallization of alpha-cordierite. The specimen with 3 mass% of B₂O₃ that was fired at a temperature of 950°C showed a linear thermal expansion coefficient of ∼3 × 10⁻⁶ K⁻¹, a bending strength of >200 MPa, and a relative dielectric constant of 5.5 at 1 MHz. These cordierite ceramics may be used as substrate materials for semiconductor interconnection applications.     

Effect of Temperature and Water-Solid Ratio on Growth of Ca(OH)2 Crystals Formed During Hydration of Ca3SiO5

The growth behavior, time of nucleation, and morphology of Ca(OH)₂ crystals formed during the hydration of Ca₃SiO₅, at 15°, 25°, and 35°C at water-solid ratios ( w/s ) from 0.3 to 5.0 were studied by optical microscopy. In samples with w/s >0.5 growth of Ca(OH)₂ in the c -axis direction is initially dominant. Growth in this direction ends after a few hours, but growth perpendicular to the c axis continues for several days and produces a dendritic morphology. Growth behavior is not so well defined for w/s <0.5, in part because of the large number of unhydrated particles engulfed. Increasing temperature resulted in an increase in the number of Ca(OH)₂ nuclei and a decrease in nucleation time and crystal size. Increasing the w/s ratio improved the euhedral character of the Ca(OH)₂ crystals, decreased the number of engulfed Ca₃SiO₅ particles, and increased the nucleation time. Dendritic morphology was most pronounced in the samples for which w/s = 1. Growth rates and the ultimate size of the Ca(OH)₂ crystals varied within a given sample. The effects of temperature and the w/s ratio on the heat evolved during the hydration were studied by isothermal calorimetry. The times of nucleation of crystalline Ca(OH)₂ estimated from calorimetry were similar to those derived from growth curves determined by optical microscopy.     

Melting Phenomena in the System CaO–SiO2– H2O: I, The Join Ca2SiO,-Ca(OH)2

The liquidus-solidus relations along the join Ca₂SiO₄-Ca(OH), in the system CaO-SiO₂-H₂O have been determined at 1000 atm up to 1110°C. This join is binary and contains the calcium silicate hydrate, calciochondrodite, Ca5-(SiO₄(OH)₂. Calciochondrodite melts incongruently to Ca₂SiO₂+ liquid (composition 23 wt% Ca₂Si0₄) at 955°C. The eutectic between calcium hydroxide and calciochondrodite lies at 13% Ca₂Si0₄ and 822°C. Preliminary experiments, also at 1000 atm, in the ternary system CaO-Ca₂Si0₄-Ca(OH), indicate that the eutectic at which the fields of primary Ca(OH)₂, CaO, and Ca₂(Si0₄)₂(OH)₂ meet is close to the CaO-Ca. (OH), side of the triangle at approximately 805° C. The ternary reaction point Ca₂SiO_l+ liquid ⇌Ca₅(SiO₄)₂(OH)₂+ CaO + liquid is believed to lie in the low-CaO (<5%) high-Ca(OH)₂ (>70%) part of the system.     

Crystallization Kinetics of Lithium Orthosilicate Glasses

The formation of crystalline Li₄SiO₄ and Li₂SiO₃ from lithium orthosilicate glasses has been studied by means of in situ high-temperature X-ray diffraction. The first phase that crystallizes from the glass could not be identified, but was followed by the transformation to crystalline orthosilicate and metasilicate. Orthosilicate formation was tracked at temperatures between 600° and 650°C, whereas at higher temperatures, the formation of a small amount of crystalline metasilicate occurs. The kinetics of the initial phase of both crystallization processes are described by the Avrami–Erofeev equation, resulting in activation energies of 90 kJ/mol for the formation of the unidentified phase, and 68 kJ/mol for the formation of Li₄SiO₄. The rate constant for the crystallization of the unidentified phase is 0.014 s⁻¹ at 510°C, equaling that of the orthosilicate formation at 630°C. After isothermal heat treatment for 100–800 s, depending on temperature, 80%–95% of the sample is crystallized and further crystallization is controlled by diffusion in both cases.     

Grain Growth in Sintered ZnO and ZnO-Bi2O3 Ceramics

Grain growth in a high-purity ZnO and for the same ZnO with Bi₂O₃ additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: G ⁿ— G ⁿ₀= K ₀ t exp(— Q/RT ). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn²⁺ lattice diffusion mechanism. Additions of Bi₂O₃ to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi₂O₃ content. The preexponential term K ₀ was also independent of Bi₂O₃ content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi₂O₃ ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi₂O₃-rich liquid phase.