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1.
The existence of compounds between Si3 N4 -CeO2 and Si3 N4 -Ce2 O3 was investigated for firing temperatures of 1600° to 1700°C. The two new monoclinic compounds found were Ce2 O3 ·2Si3 N4 with lattice parameters a = 16.288, b = 4.848, and c =7.853 Å and β=91.54° and Ce4 Si2 O7 N2 with lattice parameters a = 10.360, b = 10.865, and c =3.974 Å and β=90.33°. Cerium orthosilicate (Ce 4.67 (SiO4 )3 O) is present during firing as a glassy intermediate phase which promotes sintering and densification and then reacts with silicon nitride to form cerium silicon oxynitrde (CeSiO2 N). 相似文献
2.
Kiyoshi Itatani Toshio Nishioka Satoru Seike F. Scott Howell Akira Kishioka Makio Kinoshita 《Journal of the American Ceramic Society》1994,77(3):801-805
Mixed solutions of Ca(NO3 )2 and (NH4 )2 HPO4 with Ca/P = 1.50 were spray-pyrolyzed at 600°C to produce β-calcium orthophosphate (β-Ca3 (PO4 )2 ) powder; the spray-pyrolyzed powder was ground and then calcined at 600°C for 1 h. The best crystalline β-Ca3 (PO4 )2 powder was obtained from the solution with 1.80 mol.L–1 Ca(NO3 )2 , 1.20 mol.L–1 (NH4 )2 HPO4 . The resulting powder was composed of primary particles with sizes of <0.5 μm. Dense β-Ca3 (PO4 )2 ceramics with a relative density of 96.1% could be fabricated by firing this compressed powder at 1070°C for 5 h. 相似文献
3.
Eaden Saw Ken H. Sandhage Patrick K. Gallagher Alan S. Litsky 《Journal of the American Ceramic Society》2000,83(4):998-1000
A novel VIMOX (volume identical metal oxidation) route to near-net-shaped calcium hydroxyapatite, Ca10 (PO4 )6 (OH)2 , is demonstrated: the oxidation of machinable Ca—Ca2 P2 O7 precursors. Mechanically alloyed mixtures of Ca and β-Ca2 P2 O7 were compacted into disk- and bar-shaped preforms. The latter preforms could be machined into cylinders using a metalworking lathe (200 rpm, hardened steel tooling). After oxidation at 600°C in O2 , and then postoxidation annealing in H2 O/O2 mixtures at 850°C and 1150°C, phase-pure hydroxyapatite was obtained. Because of offsetting volume changes from calcium oxidation and hydroxyapatite formation, porous hydroxyapatite bodies were produced that retained the shapes and dimensions (within 1%) of the machined precursors. 相似文献
4.
J. P. COUTURES 《Journal of the American Ceramic Society》1985,68(3):105-107
A tentative phase diagram for the system Al2 03 -Nd2 O3 is presented. Three compounds were obtained: a β -A12 O3 -type compound, the perovskite NdAlO3 , and Nd4 Al2 O9 . The perovskite melts congruently (mp 2090°C), and the two other compounds exhibit incongruent melting behavior: β -Nd/Al2 O3 , mp 1900°C; Nd4 Al2 O9 , mp 1905°C. Two eutectics exist with the following compositions and melting points: 80 mol% Al2 O3 , 1750°C; 23 mol% Al2 O3 ,1800°C. Nd4 Al2 O9 decomposes in the solid state at 1780°C. 相似文献
5.
F. G. BUTTLER L. S. DENT GLASSER H. F. W. TAYLOR 《Journal of the American Ceramic Society》1959,42(3):121-126
Single-crystal X-ray and electron-diffraction studies show the existence in one polymorph of 4CaO.Al2 O3 . 13H2 O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca2 (OH)7 - 3H2 O. 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.A12 O3 .13H2 O, from which it is derived by substitution of CO3 2- for 20H- + 3H2 O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al2 O3 .Ca Y 2 - xH2 O and 3CaO Al2 O3 Ca Y xH2 O. On dehydration, 4CaO.Al2 O3 .13H2 O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH)2 and 4CaO.3Al2 O3 .3H2 O are formed and, by 1000°C., CaO and 12CaO.7Al2 O8 . The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al2 O3 .3H2 O is formed. 相似文献
6.
Chin Jong Chan Waltraud M. Kriven J. Francis Young 《Journal of the American Ceramic Society》1992,75(6):1621-1627
It has been shown that the monoclinic β -phase of dicalcium silicate (Ca2 SiO4 ) can be stabilized against transformation to the orthorhombic γ -phase by physical rather than chemical factors. Stabilization was studied in different types of microstructures fabricated under various processing conditions such as different powder or grain sizes, chemical additives, cooling kinetics, or high-temperature annealing treatments. The observations can be explained in terms of a critical particle size effect controlling nucleation of the transformation. Rapid quenching through the high-temperature hexagonal ( α ) to orthorhombic ( a' H ) transformation at 1425°C, which is accompanied by a −4.7% volume decrease, causes periodic fracture of β -twins due to accumulated strains. Chemical doping with K2 O or Al2 O3 promotes the formation of amorphous phases which mold themselves around β -Ca2 SiO4 grains. Annealing treatments cause crystallization of the glass and subsequent transformation to the γ -phase. 相似文献
7.
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
8.
Sanjeevi Kannan Friedlinde Goetz-Neunhoeffer Jürgen Neubauer Jose M.F. Ferreira 《Journal of the American Ceramic Society》2009,92(7):1592-1595
Zinc substituted β-tricalcium phosphate [β-Ca3 (PO4 )2 ] was formed by substituting a zinc precursor in calcium-deficient apatite through aqueous precipitation technique. Heat treatment at 1000°C led to the formation of well crystalline β-Ca3 (PO4 ). Refinement technique was used to determine the influence of incorporated zinc in the β-Ca3 (PO4 ) structure. The structural data for all the four different zinc substituted β-Ca3 (PO4 ) ranging from 0–9 mol% of zinc investigated in the present study confirmed the rhombohedral structure of β-Ca3 (PO4 ) in the hexagonal setting (space group R 3 c ). The incorporation of lower sized Zn2+ (0.745 Å for sixfold coordination with O) at the higher sized Ca2+ (1.00 Å for sixfold coordination with O) site in the β-Ca3 (PO4 ) structure led to the contraction of unit cell parameters. The added zinc prefers to occupy the Ca(5) site of β-Ca3 (PO4 ) structure. 相似文献
9.
TiO2 above 4 mol% is an effective nucleating agent for CaO–P2 O5 glass which also contains substantial SiO2 and Al2 O3 additions. Glass ceramics can be made from this glass using a single slow heating ramp with no need for a nucleating heat treatment step. Powder of this composition crystallizes rapidly to β-Ca2 P2 O7 , whereas bulk glass crystallizes from diphasic nuclei consisting of a central cubic Ca-P-Ti-Si-Al oxide phase surrounded by impure AlPO4 dendrites. Metastable calcium phosphate grows on the AlPO4 dendrites and later transforms to β-Ca2 P2 O7 . 相似文献
10.
Jana Bezjak Aleksander Renik Botjan Janar Philippe Boullay Ivana Radosavljevi Evans Danilo Suvorov 《Journal of the American Ceramic Society》2009,92(8):1806-1812
Polymorphic phase transitions in Ba4 Nb2 O9 were studied by thermal analyses, high-temperature transmission electron microscopy and X-ray powder diffractometry. Two stable polymorphs were isolated, low-temperature α-modification and high-temperature γ-modification, with the endothermic phase transition at 1176°C. The α→γ transformation is accompanied by the formation of a 120° domain structure, which is a consequence of hexagonal→orthorhombic unit cell reconstruction. Reheating the presintered γ-Ba4 Nb2 O9 results in the formation of a metastable γ'-modification (formerly known as β-polymorph) in the temperature range between 360° and 585°C, before the γ→α transformation at 800°C. Above ∼490°C Ba4 Nb2 O9 becomes moderately sensitive to a loss of BaO. In air the surface of Ba4 Nb2 O9 grains decomposes to nanocrystalline Ba5 Nb4 O15 and BaO, which instantly reacts with atmospheric CO2 to form BaCO3 . Surface reaction delays γ→α transformation up to 866°C in air. In vacuum the loss of BaO is even more enhanced and consequently the formation of minor Ba3 Nb2 O8 phase is observed above 1150°C. 相似文献
11.
Hideo Hosono Zhibo Zhang Yoshihiro Abe 《Journal of the American Ceramic Society》1989,72(9):1587-1590
A porous glass-ceramic in the CaO–TiO2 —P2 O5 system has been prepared by crystallization and subsequent chemical leaching of the corresponding glass. By applying a two-step heat treatment to 45CaO · 25TiO2 · 30P2 O5 glasses containing a few mol% of Na2 O, volume crystallization results in the formation of dense glass-ceramics composed of CaTi4 (PO4 )6 and β-Ca3 (PO4 )2 phases. By leaching the resultant glass ceramics with HCI, β-Ca3 (PO4 )2 is selectively dissolved out, leaving a crystalline CaTi4 (PO4 )6 skeleton. The surface area and mean pore radius of the porous glass-ceramics were approximately 40 m2 /g and 13 nm, respectively. 相似文献
12.
Niobium pentoxide (T form, orthorhombic system) was utilized to promote devitrification in Li2 O · Al2 O3 · 6SiO2 glasses. Two or more mole percentage of this nucleating dopant enhanced crystallization in these glasses. Glasses containing 4.0 and 8.0 mol% T-Nb2 O5 exhibited a high tendency to form dispersed TT-Nb2 O5 (monoclinic system) precipitates during the glass quenching process. The crystallization process in glasses containing 2.0 or 4.0 mol% T-Nb2 O5 occurred as microphase separation, followed by the formation of dispersed TT-Nb2 O5 crystalline precipitates (760°C), followed by β-quartz solid-solution ( ss ) formation (850° to 900°C) heterogeneously nucleated from the precipitates. β-quartz( ss ) transformed to β-spodumene( ss ), along with a polymorphic transition from the TT-Nb2 O5 to M-Nb2 O5 (tetragonal system) crystalline phase. 相似文献
13.
Silicon nitride (Si3 N4 ) ceramics, prepared with Y2 O3 and Al2 O3 sintering additives, have been densified in air at temperatures of up to 1750°C using a conventional MoSi2 element furnace. At the highest sintering temperatures, densities in excess of 98% of theoretical have been achieved for materials prepared with a combined sintering addition of 12 wt% Y2 O3 and 3 wt% Al2 O3 . Densification is accompanied by a small weight gain (typically <1–2 wt%), because of limited passive oxidation of the sample. Complete α- to β-Si3 N4 transformation can be achieved at temperatures above 1650°C, although a low volume fraction of Si2 N2 O is also observed to form below 1750°C. Partial crystallization of the residual grain-boundary glassy phase was also apparent, with β-Y2 Si2 O7 being noted in the majority of samples. The microstructures of the sintered materials exhibited typical β-Si3 N4 elongated grain morphologies, indicating potential for low-cost processing of in situ toughened Si3 N4 -based ceramics. 相似文献
14.
Ji-Guang Li Takayasu Ikegami Toshiyuki Mori Yoshiyuki Yajima 《Journal of the American Ceramic Society》2003,86(9):1493-1499
Two wet-chemical routes have been used to synthesize Sc2 O3 nanopowders from nitrate solutions employing ammonia water (AW) and ammonium hydrogen carbonate (AHC) as the precipitants. The precursors and the resultant oxides are characterized by elemental analysis, X-ray diffractometry, differential thermal analysis/thermogravimetry, high-resolution scanning electron microscopy, and Brunauer-Emmett-Teller analysis. Crystalline γ-ScOOH· n H2 O ( n ≈ 0.5) is the only phase obtained by the AW method. This phase dehydrates to Sc2 O3 at ∼400°C, yielding hard aggregated nanocrystalline Sc2 O3 powders. Three types of precursors have been synthesized by the AHC method, depending on the AHC/Sc3+ molar ratio ( R ): amorphous basic carbonate [Sc(OH)CO3 ·H2 O] at R ≤ 3, crystalline double carbonate [(NH4 )Sc(CO3 )2 ·H2 O] at R ≥ 4, and a mixture of the two phases at 3 < R < 4. Among these precursors, only the basic carbonate shows spherical particle morphology, ultrafine particle size (∼50 nm), and weak agglomeration. Sc2 O3 nanopowders (∼28 nm) with high surface area (∼49 m2 /g) have been prepared by calcining the basic carbonate at 700°C for 2 h. 相似文献
15.
Phase equilibria data, obtained both by differential thermal analysis and by quenching, are presented for the system Na2 O-Nb2 O5 . Five compounds corresponding to the formulas 3Na2 O.1Nb2 06 , lNa2 O. 1Nb2 O5 , lNa2 O 4Nb2 O6 , lNaz O.7Nb2 O5 , and lNa2 O. 10Nb2 O6 have been found. The compound 3Naz 0.lNb2 O5 melts congruently at 992°C. The compounds 1Na2 O. 4Nb2 O6 , lNa2 O.7Nb2 O, and 1Na2 O. 1Onb2 O5 melt incongruently at 1265°, 1275°, and 1290°C., respectively. The well-known perovskite structure phase NaNbO3 was found to melt congruently at 1412°C. The transition temperatures in NaNbO5 were checked by thermal analysis and only the major structural changes at 368° and 640°C. could be detected. A new disordered form of NaNbO3 could be preserved to room temperature by very rapid quenching. 相似文献
16.
CHIN-JONG CHAN WALTRAUD M. KRIVEN J. FRANCIS YOUNG 《Journal of the American Ceramic Society》1988,71(9):713-719
Dicalcium silicates having CaO/SiO2 molar ratios of 1.8 to 2.2 were sintered at 1450°C for 90 min with or without small quantities of dopants (K2 O or Al2 O3 ) and were air quenched. The microstructures of the fired samples were characterized using electron microscopy (SEM and TEM) and associated microanalytical techniques. There was no evidence for the existence of Ca1.8 SiO3.8 or Ca2.2 SiO4.2 . Amorphous grain-boundary phases were observed between grains and as inclusions within the grains; the amounts decreased as CaO/SiO2 ratios increased. The compositions of the amorphous phases were always rich in dopants and had a CaO/SiO2 ratio close to that of wollastonite. High levels of Al2 O3 were observed to enter the β-Ca2 SiO4 grains under lime-rich conditions (CaO/SiO2 = 2.2) up to a saturation level of about 3.0 wt%. Some additional crystalline phases were observed to form depending on stoichiometry and dopant level. 相似文献
17.
Yong W. Bae Woo Y. Lee David P. Stinton 《Journal of the American Ceramic Society》1995,78(5):1297-1300
Crystalline β-Ta2 O5 coatings were deposited on hot-isostatically-pressed Si3 N4 by reacting TaCl5 with H2 and CO2 in the temperature range of 1000°–1300°C and at a pressure of 660 Pa. The Ta2 O5 coatings generally consisted of wellcoalesced 2–3 μm grains, resulting in the formation of a nonporous coating morphology. However, the presence of microcracks on the as-deposited surface was consistently observed. The surface morphology, texture, and growth rate of the coatings were examined as a function of deposition parameters. 相似文献
18.
Koichiro Fukuda Iwao Maki Suketoshi Ito Hideto Yoshida 《Journal of the American Ceramic Society》1995,78(12):3387-3389
A pseudobinary phase equilibrium diagram has been established for the P2 O5 -bearing Ca2 SiO4 -CaFe4 O7 system to confirm the occurrence of remelting reaction in α-Ca2 SiO4 solid solutions (C2 S(ss)). The reaction started at 1290°C immediately after the α-to-α'H transition and finished at 1145°C. The reaction products were made up of about 1 wt% of liquid and 99 wt% of solid α'H -C2 S(ss). The liquid exsolved at 1290°C was rich in Fe2 O3 , consisting of about 30 wt% of Ca2 SiO4 and 70 wt% of CaFe4 O7 . The liquid coexisting with α-C2 S(ss) precipitated new α'-phase crystals in association with the remelting reaction. 相似文献
19.
SHIGERU ITO NOBUO KUBO SHINYA NARIKI NOBORU YONEDA 《Journal of the American Ceramic Society》1987,70(12):874-879
The potassium ions in potassium β-ferrite ((1 + x)K2 O ·11Fe2 O3 ) crystals were exchanged with Na+ , Rb+ , Cs+ , Ag+ , NH4 + , and H3 O+ in molten nitrates or in concentrated H2 SO4 . On the other hand, spinel and hexagonal ferrites were formed by soaking the crystals in the melt of divalent salts. The crystals of K+ , Rb+ , and Cs+ β-ferrites decomposed to form α-Fe2 O3 at high temperatures of 800° to 1100°C. In addition, H3 O+ , NH4 + , and Ag+ β-ferrites decomposed to form α-Fe2 O3 at relatively low temperatures of 350° to 650°C, in accordance with the stabilities of the inserted ions. The electrical properties of some β-ferrites were measured. 相似文献
20.
A study of the system Al2 O3 –Ga2 O3 –H2 O has resulted in the determination of equilibrium diagrams for the systems Al2 O3 –Ga2 O3 and Al2 O3 .-H2 O–Ga2 O3 .H2 O. Extensive solid solution characterizes the α -Al2 O3 and β -Ga2 O3 structures at high temperatures, but it is shown that below 810°C. a compound, GaAlO3 , and a new series of (Al, Ga)2 O3 structures are stable. Among the hydrates, a complete series of diaspore solid solutions extends from Al2 O3 .H2 O to Ga2 O3 .H2 O. Boehmite solid solutions extend to approximately the composition 70Al2 O3 .H2 O, 30Ga2 O3 .H2 O. 相似文献