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1.
The decomposition of Al2(1- x ) Mg x Ti(1+ x ) O5 ceramics in air has been studied between 900° and 1175°C for 0 lessthan equal to x lessthan equal to 0.6. The decomposition temperature versus composition x predicted using a thermodynamic model based on the regular solution approach is in satisfactory agreement with the experimental results. The decomposition kinetics has been studied at 1100°C for x = 0, 0.1, and 0.2 and follows a nucleation and growth mechanism. Random nucleation of the reaction products is hindered by the high elastic stresses that result from the molar volume change related to decomposition because of the small chemical driving force available. Decomposition occurs only at a limited number of sites, probably associated with the presence of impurities and/or glassy phase. The decomposition products grow as nodules formed by an Al2 O3 (+ MgAl2 O4 for x > 0) core and a TiO2 shell. The growth is parabolic for x = 0 and linear for x = 0.1 and 0.2. The rate-controlling step in the decomposition mechanism of pure Al2 TiO5 ( x = 0) is the transport of Al3+ ions through the TiO2 -rutile phase. 相似文献
2.
The kinetics and mechanism of eutectoid decomposition of CuFe5 O8 were studied by X-ray diffractometer, electron probe microanalyzer, and reflecting-light microscope techniques. The isothermal decomposition curves were sigmoidal and the maximum rate of decomposition occurred at ∼600°C. Sigmoidal-type curves indicated that the decomposition proceeded by a process of nucleation and growth. Kinetic data were best expressed by Avrami's equation with n≃2 and n≃4 below and above 850°C, respectively. A "site saturation" mechanism is suggested. Bainite-type and pearlite-type microstructures were observed below and above 800°C, respectively. Grain boundaries were active sites in the nucleation process. 相似文献
3.
The kinetics of the thermal decomposition of silicon nitride, Si3 N4 ( s )→3Si( l )+2N2 ( g ), were studied from 1490° to 1750°C using a static system. The reaction was followed by measuring the rate of increase of N2 pressure in the system as a function of time. In the initial stages, the decomposition followed first-order kinetics, with the rate constant expressed as: In the later stages, the experimental data followed the Jander solid-state kinetic law, with the Jander rate constant given by: The effects of excess Si and of Ar and N2 atmospheres on the reaction kinetics are also reported. 相似文献
4.
The thermal stability of Al3 BC3 powder was analyzed. Nearly X-ray-pure Al3 BC3 powder was obtained through the calcination of the aluminum, B4 C, and carbon mixture at 1800°C in Ar. In contrast to the former investigations, which reported the melting of so called "Al8 B4 C7 " at 1800°C, Al3 BC3 did not melt up to 2100°C. Instead, it decomposed by the vaporization of aluminum. The decomposition occurred distinctly at 1400° and 1900°C in flowing Ar and a sealed carbon crucible, respectively. The results indicated that the decomposition temperature depended on the partial pressure of aluminum vapour in the atmosphere. 相似文献
5.
KAMRAN AYKAN 《Journal of the American Ceramic Society》1968,51(10):577-581
The TiO2 -rich side of the system PbO-TiO2 was investigated by X-ray diffraction techniques, and a new compound PbTi3 O7 was found. It has a bimolecular monoclinic unit Cell with a = 10.732 A, b = 3.812 A, c = 6.578 A, and β= 98° 05'. The PbTi3 O7 decomposes into PbTiOs and rutile at temperatures above 850°C. The overall decomposition proceeds according to first-order kinetics with an apparent activation energy of about 170 kcal mole-1 . 相似文献
6.
The solid-state reaction of SrNb2 O6 and BaNb2 O6 to form Sr x Ba1− x Nb2 O6 (SBN) at different temperatures and heating rates was investigated. The reaction kinetics were analyzed by X-ray diffractometry for quenched samples, and the internal-standard method was applied to quantify the extent of the reaction. A nonisothermal kinetic empirical model was proposed to evaluate the activation energy and rate constant of SBN with different Sr:Ba ratios. It was found that the kinetic form would change above and below a transition at a reaction fraction of ∼60%, which might be due to the change of the frequency factor. It was also verified that the model that was presented was more favorable to describe the nonisothermal reaction kinetics of SBN. 相似文献
7.
Saikat Maitra Narayan Bandyopadhyay Jayita Pal 《Journal of the American Ceramic Society》2008,91(1):337-341
The nonisothermal kinetics of thermal decomposition of SrCO3 and BaCO3 was studied by thermo-gravimetry following non-Arrhenius integral approach of Harcourt and Esson. Variation of activation energy of these two carbonates with fraction decomposition was compared and the average activation energy in the decomposition range was computed. 相似文献
8.
The heterogeneous phase distribution found in Ba2 Ti9 O20 ceramic resonators results from a temperature-dependent phase boundary and slow reaction kinetics. When sintered at 1350°C or higher in oxygen the Ba2 Ti9 O20 phase becomes slightly reduced and barium-rich. Thus a stoichiometric composition forms rutile and "Ba2 Ti9 O20 'phase. On slow cooling the excess barium diffuses to the oxygen-rich surface where it reacts to form an envelope of rutile-free material surrounding a core containing a small amount of rutile. 相似文献
9.
Shinji Nambu Akinori Sato Djuniadi A. Sagala 《Journal of the American Ceramic Society》1992,75(7):1906-1913
Computer simulation was carried out for the kinetics of spinodal decomposition in the tetragonal TiO2 –SnO2 system on the basis of a nonlinear diffusion equation. A time evolution of the microstructure and the effect of coherent strain on the separated two phases were investigated by Langer's approximate method and the finite difference method. It was shown that the composition fluctuations develop in the first stage of the spinodal decomposition, and the formation of interface and the grain growth appear in the second stage. The local stress field and the local strain field with the coherence of the lattice were calculated. Subsequently the appearance of the interface dislocations in the (100) and (010) planes was demonstrated to occur in the third stage. Physical interpretation was given to the experimental observations for the tetragonal TiO2 –SnO2 system on the basis of those calculations. 相似文献
10.
Seong J. Kim Zhien C. Chen Anil V. Virkar 《Journal of the American Ceramic Society》1988,71(10):C428-C432
Solid solutions of various compositions in the system LiAl5 O8 –LiFe5 O8 were prepared by calcining mixtures of Li2 CO3 , Al2 O3 , and Fe2 O3 . Lattice parameters of the solid solutions were determined by X-ray diffraction (XRD). Samples of equimolar composition were also prepared with titanium and magnesium as dopants. The doped and undoped samples were annealed at 1050°C for up to 192 h in air. Annealing at this temperature, which is inside the miscibility gap for an equimolar composition, led to the decomposition of the originally single-phase solid solution. The extent of phase transformation was followed by XRD. The fraction transformed was estimated using integrated peak intensities. The kinetic data were analyzed in light of the conventional kinetic equation and the previously derived equation which takes into account the diffusive and the interface transfer processes. It was observed that titanium enhances and magnesium suppresses the kinetics of phase separation. These results are rationalized on the premise that the predominant point defects are Schottky defects in this spinel system. 相似文献
11.
Dense samples in the TiO2 -SnO2 system were fabricated by pressureless sintering in air using commercial powders as well as powders obtained by coprecipitation. Phase equilibria in the system were examined using X-ray diffraction on samples that were annealed for long periods of time. Samples of near equi-molar composition made with controlled amounts of Al and Ta (separately) as dopants were annealed inside the coherent spinodal. X-ray diffraction and electron microscopy were the principal characterization tools used to follow the kinetics of phase transformation. The composition modulations occurred along the [001] direction consistent with prior work. During the early stages of the decomposition process, interlamellar surfaces were coherent. At later stages, strain mismatch was accommodated by interface dislocations. The kinetics of decomposition were strongly influenced by the type and the amount of dopant. Specifically, trivalent aluminum enhanced the decomposition while pentavalent tantalum suppressed it. The kinetics in a sample doped with 0.5 mol% Al2 O3 were more than 3 orders of magnitude more rapid than a sample doped with 1.0 mol% Ta2 O5 . These observations are rationalized on the premise that cation interstitial mobility is greater than cation vacancy mobility. 相似文献
12.
Sintering kinetics of the system Si3 N4-Y2O3 -Al2 O3 were determined from measurements of the linear shrinkage of pressed disks sintered isothermally at 1500° to 1700°C. Amorphous and crystalline Si3 N4 were studied with additions of 4 to 17 wt% Y2 O3 and 4 wt% A12 O3 . Sintering occurs by a liquid-phase mechanism in which the kinetics exhibit the three stages predicted by Kingery's model. However, the rates during the second stage of the process are higher for all compositions than predicted by the model. X-ray data show the presence of several transient phases which, with sufficient heating, disappear leaving mixtures of β ' -Si3 N4 and glass or β '-Si3 N4 , α '-Si3 N4 , and glass. The compositions and amounts of the residual glassy phases are estimated. 相似文献
13.
Guotian Ye George Oprea Tom Troczynski 《Journal of the American Ceramic Society》2005,88(11):3241-3244
MgAl2 O4 spinel precursor was prepared by a novel method combining a sol–gel process with the "traditional" precipitation process. The thermal decomposition and phase development of the precursor were analyzed, and the degree of agglomeration of the calcined powder was assessed by determining its particle size and crystal size. The optimum calcination temperature was determined based on the variation of specific surface areas, crystal size, and particle size. Completely crystallized ultrafine spinel powder ( d 50 =600 nm, specific surface area=105 m2 /g) was obtained after calcination at 900°C. 相似文献
14.
WILLIAM C. HAGEL P. J. JORGENSEN D. S. TOMALIN 《Journal of the American Ceramic Society》1966,49(1):23-26
Since the difference between oxygen-ion and cation diffusion coefficients is greater for α-Cr2 O3 than for α-Fe2 O3 or α-Al2 O3 , a study of initial-sintering kinetics was undertaken to show unequivocally which species is rate controlling. Fine powders of α-Cr2 O3 , obtained by thermal decomposition of reagent-grade (NH4 )2 Cr2 O7 , were lightly compacted and their isothermal rates of shrinkage were determined between 1050° and 1300°C. Resultant data follow volume-diffusion sintering models, and calculated diffusion coefficients agree with, those measured for oxygen ions in α-Cr2 O3 . There is little evidence that oxygen diffusion along grain boundaries becomes so enhanced that chromium ions are left in control of the process. 相似文献
15.
Phase relations in the system Na2 O· Al2 O3 -CaO· Al2 O3 -Al2 O3 at 1200°C in air were determined using the quenching method and high-temperature X-ray diffraction. The compound 2Na2 O · 3CaO · 5Al2 O3 , known from the literature, was reformulated as Na2 O · CaO · 2Al2 O3 . A new compound with the probable composition Na2 O · 3CaO · 8Al2 O3 was found. Cell parameters of both compounds were determined. The compound Na2 O · CaO-2Al2 O3 is tetragonal with a = 1.04348(24) and c = 0.72539(31) nm; it forms solid solutions with Na2 O · Al2 O3 up to 38 mol% Na2 O at 1200°C. The compound Na2 O · 3CaO · 8Al2 O3 is hexagonal with) a = 0.98436(4) and c = 0.69415(4) nm. The compound CaO · 6Al2 O3 is not initially formed from oxide components at 1200°C but behaves as an equilibrium phase when it is formed separately at higher temperatures. The very slow transformation kinetics between β and β "-Al2 O3 make it very difficult to determine equilibrium phase relations in the high-Al2 O3 part of the diagram. Conclusions as to lifetime processes in high-pressure sodium discharge lamps can be drawn from the phase diagram. 相似文献
16.
Ag-sheathed (Bi,Pb)2 Sr2 Ca2 Cu3 O, (2223) tapes were made by the oxide-powder-in-tube method. Tapes were heat-treated isothermally at several different temperatures in 7.5% O2 /Ar, then quenched into oil to retain the phase assemblages at the reaction temperatures. 2223 formed between ∼810° and ∼837°C. The Avrami equation was applied to describe the kinetics of 2223 formation from a mixture of Bi2 Sr2 CaCu2 O x and nonsuperconducting phases, mainly Ca2 PbO4 and CuO. The calculated Avrami exponent, n ∼ 1, indicated that the kinetics in this system could be described as a diffusion-controlled, two-dimensional nucleation and growth process. The apparent activation energy for forming 2223 was ∼2900 kJ/mol from ∼817° to ∼825°C and ∼890 kj/mol from ∼825° to ∼837°C. A temperature-time-transformation diagram was constructed based on the kinetic data; it describes the transformational behavior of this particular system. 相似文献
17.
A novel and cost-effective electrostatic spray-assisted vapor deposition (ESAVD) was used to deposit Y3 Al5 O12 (YAG) coatings. Polycrystalline single-phase Y3 Al5 O12 coatings were synthesized using the ESAVD method in an open atmosphere at 650°C, and then annealed at 700°–900°C for 1 h. The ESAVD process involves the decomposition and chemical reactions of charged aerosol in vapor phase. The low-temperature coating deposition characteristics of the ESAVD process using a suitable sol precursor decreases the reaction and crystallization temperatures for forming Y3 Al5 O12 coatings. The microstructure of the Y3 Al5 O12 coating prepared using the ESAVD method is columnar and such strain-resistance microstructure could be useful for thermal barrier coating applications. 相似文献
18.
RODNEY D. BAGLEY IVAN B. CUTLER D. LYNN JOHNSON† 《Journal of the American Ceramic Society》1970,53(3):136-141
Alpha alumina with additions of TiO2 sintered more rapidly than "pure" alumina. The rate of initial sintering increased approximately exponentially with titania concentration up to a percentage beyond which the rate of sintering remained approximately constant or decreased slightly with additional titania. The concentration which produces the maximum rate of sintering is thought to be the solubility limit of TiO2 in Al2 O3 . For alumina particles larger than about 2 μm, the kinetic process was mainly grain-boundary diffusion. With smaller particles, volume diffusion increased. The "solubility limit" increased with decreasing particle size, indicating an excess surface concentration of TiO2 . The data may be interpreted in terms of a region of enhanced diffusion at the grain boundary that increases with TiO2 concentration. With small alumina particles, this region is large enough to become a significant portion of the volume of the particle, and the small particles appear to sinter by volume diffusion kinetics, but the diffusion coefficient corresponds to an enhanced diffusion coefficient. 相似文献
19.
The system TiO2 -Bi2 Ti4 O11 was examined by Raman spectroscopy and X-ray diffraction to determine whether TiO2 is soluble in Bi2 Ti4 O11 . The Raman spectral data obtained from preparations made at ∼ 1050°C and cooled to room temperature led us to conclude that TiO2 is not soluble in the "high-temperature" form of Bi2 Ti4 O11 . It was also found that extensive grinding of the phase identified as the "high-temperature" form converts it to the "low-temperature" form, stable below 250°C. 相似文献
20.
Priyadarshi G. Desai Zhengkui Xu Jennifer A. Lewis 《Journal of the American Ceramic Society》1995,78(11):2881-2888
Novel microcomposite powders, consisting of inert cores (αAL-Al2 O3 ) surrounded by reactive cement-based coatings (CaAl2 O4 ), were synthesized by a modified Pechini process. The evolution of the crystalline CaAl2 O4 phase during calcination was studied using multiple analytical techniques, including DRIFTS,13 C and 27 AlMAS FT-NMR, and XRD, for both pure CaAl2 O4 and CaAl2 O4 -coated Al2 O3 precursor powders. In both powders, decomposition proceeded via hydrocarbon chain scission and removal of ester groups at low temperatures ( T < 450°C), followed by the formation of inorganic carbonates at higher temperatures ( T > 450°C). These decomposition processes were accelerated by the underlying Al2 O3 cores. Transmission electron microscopy (TEM) of the fully calcined powders showed that the inert αAL-Al2 O3 particles were surrounded by relatively uniform CaAl2 O4 coatings ranging in thickness from approximately 10 to 100 nm. 相似文献