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1.
Single crystals of phenacite (Be2 SiO4 ), bromellite (BeO), and tridymite (SiO2 ) were grown from an Li2 MoO4 -MoO3 flux. Phenacite, with rhombohedral symmetry, grew in three distinct shapes with aspect ratios (length/width) as follows: needles (>3), rods (>1.1 to 1.5), and rhombohedral-faced crystals (=1). The latter grew as single crystals; the others were twinned on the . For most experiments the temperature was held constant at 1165°C and the Li2 MoO4 /MoO3 ratio at 1/16. The growth mechanism for crystallization was the evaporation of MoO3 . The system produced one to three phases, depending on the BeO/SiO2 ratio. Bromellite grew until a BeO/SiO2 ratio of 0.8 was attained. It grew as a hemipyramidal crystal having a short prism with a curved top or as a hexagonal plate. The pyramid- and prism-shaped crystals were twinned, although a few hexagonal plates were single. Tridymite grew in small hexagonal plates when the BeO/SiO2 ratio was less than 1.5. The effect of temperature, nucleation, and flux composition on crystal shape, twinning, and occurrence is discussed. 相似文献
2.
John R. Moyer 《Journal of the American Ceramic Society》1996,79(11):2965-2968
Alumina reacts with 1 atm of SiF4 below 660°± 7°C to form A1F3 and SiO2 . At higher temperatures the product is a mixture of fluorotopaz and AIF3 . Mixtures of fluorotopaz and AIF3 decompose in 1 atm of SiF4 at 973°± 8°C and form tabular α-alumina. The equilibrium vapor pressure of SiF4 above mixtures of fluorotopaz and AlF3 is log p (atm) = 9.198 – 11460/ T (K). Fluorotopaz itself decomposes at 1056°± 5°C in 1 atm of SiF4 to give acicular mullite, 2Al2 O3 . 1.07SiO2 . Alumina and mullite are stable in the presence of 1 atm of SiF4 above 973° and 1056°C, respectively. The phase diagram of the system SiO2 -Al2 O3 -SiF4 shows only gas-solid equilibria. 相似文献
3.
IRVIN C. HUSEBY† HANS L. LUKAS GÜNTER PETZOW 《Journal of the American Ceramic Society》1975,58(9-10):377-380
The 1780°C isothermal section of the reciprocal quasiternary system Si3 N4 -SiO2 -BeO-Be3 N2 was investigated by the X-ray analysis of hot-pressed samples. The equilibrium relations shown involve previously known compounds and 8 newly found compounds: Be6 Si3 N8 , Be11 Si5 N14 , Be5 Si2 N6 , Be9 Si3 N10 , Be8 SiO4 N4 , Be6 O3 N2 , Be8 O5 N2 , and Be9 O6 N2 . Large solid solubility occurs in β-Si3 N4 , BeSiN2 , Be9 Si3 N10 , Be4 SiN4 , and β-Be3 N2 . Solid solubility in β-Si3 N4 extends toward Be2 SiO4 and decreases with increasing temperature from 19 mol% at 1770°C to 11.5 mol% Be2 SiO4 at 1880°C. A 4-phase isotherm, liquid +β-Si3 N4 ( ss )Si2 ON2 + BeO, exists at 1770°C. 相似文献
4.
John R. Moyer 《Journal of the American Ceramic Society》1995,78(12):3253-3258
At 1 atm of SiF4 , mullite and SiF4 react below 660° 7°C to form A1F3 and SiO2 . From 660° to 1056°± 5°C, the product is fluorotopaz. Mullite is stable in the presence of 1 atm of SiF4 above 1056°C. The transition temperatures at other pressures of SiF4 can be calculated from log p (atm) = 11.587 – 10811/ T (K) and log p (atm) = 9.9609 – 13238/ T (K). The phase diagram shows only gas–solid equilibria, but there is evidence for a metastable melt from which acicular mullite and fluorotopaz grow. 相似文献
5.
The equilibrium phase diagram for the system Li2 O-BeO-SiO2 contains only one ternary compound, Li2 BeSi04 . Liquidus relations for compositions containing 33 mol% SiO2 were determined; 10 liquidus invariant points were located and 7 subsolidus compatibility triangles. The most refractory compositions lie on the join BeO-Li2 BeSiO4 , with a solidus temperature of 1320°C. Metastable phases observed were a high-quartz phase, Li2x (Si1 -x Bex )O2 , x 0.33; phase X which is probably a metastable orthosilicate between Li2 BeSiO4 and Be2 SiO4 ; and phase Y which lies on the join Li2 BeSiO4 -SiO2 . The crystal chemistry and glass network-forming properties of BeO are discussed. 相似文献
6.
Beryllium nitride (Be3 N2 ) vaporizes congruently in the range 1640° to 1960°K by the reaction Be, N2 ( c ) = 3Be( g ) + N2 ( g ). The equilibrium nitrogen partial pressure, in atmospheres, at the composition for congruent sublimation is given by the expression log P N2 = [(–1.952 ± 0.038) × 104 ] T −1 + (6.509 ± 0.207). The measured enthalpy of decomposition (370 ± 5 kcal at 298° K) yields an enthalpy of formation for Be3 N2 ( c ) of –136 ± 6 kcal/mole. The upper limit to the evaporation coefficient at 1600° to 2000°K can be set as 10–4 by comparison of equilibrium data to Langmuir data obtained with a sample of 18% porosity. The apparent enthalpy of activation for the reaction is 409 ± 7 kcal/mole at 1800°K for the porous Langmuir specimen. An expression is developed to predict the temperature dependence of the reduced apparent pressures in Knudsen studies of substances with low evaporation coefficients in terms of the enthalpy of activation. The variation in temperature dependence of the Langmuir measurements and Knudsen measurements with three different-sized orifices is consistent with predictions from this expression. 相似文献
7.
New data obtained on the join Ca2 SiO4 -CaMgSiO4 established a limit of crystalline solubility of Mg in α-Ca2 SiO4 corresponding to the composition Ca1.90 Mg0.10 SiO4 at 1575°C. The α-α'Ca2 SiO4 inversion temperature is lowered from 1447° to 1400°C by Mg substitution in the lattice. α'-Ca2 SiO4 takes Mg into its lattice up to the composition Ca1.94 Mg0.06 SiO4 at 1400°C and to Ca1.96 Mg0.04 SiO4 at 900°C. A new phase (T) reported previously by Gutt, with the approximate composition Ca1.70 Mg0.30 SiO4 , was stable between 979° and 1381°C, and should be stable at liquidus temperatures in multicomponent systems involving CaO–MgO–SiO2 . 相似文献
8.
Additional information on phase equilibria in the subsolidus region of the system LiF-BeF2 was obtained and a study was made of the system PbF2 –BeF2 . In the system LiF-BeF2 the compound LiBeF3 occurs; it decomposes below the solidus at 300° C. to yield Li2 BeF4 and the quartz form of BeF2 . An additional subsolidus compound having the probable composition LiBe2 F5 forms below 275°C. No evidence was found for the existence of the alleged compound LiBe2 F5 . In the system PbF2 -BeF2 two compounds occur: 3PbF2 .BeF2 and PbF2 -BeF2 ; the former melts congruently at 482°± 5°C. and the latter at 585°± 5°C. Extensive solid solution exists between PbF2 .BeF2 and BeF2 . The cristobalite form of BeF2 crystallizes from glasses of high BeF2 content at temperatures of 450°C. or lower but is converted to the quartz form by heating at higher temperatures or in the presence of liquid. 相似文献
9.
Compound formation in the system PbO-SiO2 was studied; the results are contrasted with those previously reported. Fifteen binary phases, 4 of which had not been reported, were prepared in the present study. The new phases include Pb5 Si8 O21 , an orthorhombic polymorph of PbSiO3 , Pb3 SiO5 , and a high-SiO2 phase containing ∼ 60 mol% SiO2 . It was shown that Pb3 SiO5 is thermodynamically stable relative to Pb2 SiO4 and 4PbO.SiO2 below 640±5°C. A revised phase equilibrium diagram is presented. 相似文献
10.
Data obtained by quenching, thermal, and high-temperature X-ray techniques are presented for the three binary systems CaF2 –BeF2 , MgF2 –BeF2 , and LiF–MgF2 . The systems CaF2 –BeF2 and MgF2 –BeF2 are presented as weakened models of the systems ZrO2 –SiO2 and TiO2 –SiO2 , respectively. The compound CaBeF4 is a model of ZrSiO4 (zircon). New data obtained for the system LiF–MgF2 explain many discrepancies among the results of previous authors. Solid solution is almost complete between LiF and MgF2 at elevated temperatures, but a small gap occurs at the eutectic (735°C.) with extensive exsolution at lower temperatures. 相似文献
11.
J. B. HOLT 《Journal of the American Ceramic Society》1974,57(3):126-129
The thermal diffusivities of polycrystalline Be4 B, Be2 B, and BeB6 were measured by the flash method. Generally, the thermal diffusivities at a given temperature decrease with increasing boron content. The thermal diffusivities of Be4 B, Be2 B, and BeB6 varied from 0.13 to 0.072 to 0.031 cm2 /s, respectively, at 200°C and from 0.068 to 0.038 to 0.007 cm2 /s at 1000°C. Heat transport in BeB6 is expected to occur almost entirely by phonon conduction, whereas electronic conduction probably plays a major role in Be4 B and Be2 B. Analytical expressions for the thermal diffusivities (α) of Be4 B and Be2 B at 200° to 1000°C and of BeB6 at 25° to 1500°C are: α(Be4 B)=1/(5.83+9.05×10 3 T ), α(Be2 B)=1/(10.92+1.40×10 2 T ), and α(BeB6 )=5.60×10 4+5.72/ T +77.3/T2 -4.09×104 /T3 cm2 /s. 相似文献
12.
JOYDEB MUKERJI 《Journal of the American Ceramic Society》1965,48(4):210-213
Differential thermal analysis and quenching experiments were used to establish the ternary phase diagram CaO-CaF2 -2CaO.SiO2 . Hermetically-closed platinum capsules were used to prevent fluorine loss in the form of HF, SiF4 , and CaF2 by reaction of the CaF2 with water vapor or SiO2 , or by evaporation. The melting point of pure CaF2 was 1419°± 1°C. There is one binary eutectic in the system CaO-CaF2 and there are two ternary eutectics in the system CaO-CaF2 -2CaO.SiO2 . The results of the present study were combined with literature data to construct the phase diagram CaO-CaF2 -SiO2 . 相似文献
13.
Fine-particle beta sodium ferrite (β-NaFeO2 ), rather than α-Fe2 O3 , may be responsible for superparamagnetic behavior in a glass of composition (in mole fractions) 0.37Na2 O-0.26Fe2 O3 -0.37SiO2 . The 700°C isothermal section of the phase diagram of the Na2 O-Fe2 O3 -SiO2 system is given, showing a three-phase field bounded by Na2 SiO3 -NaFeO2 -Fe2 O3 ; there is no evidence for the existence (at 700°C) of compounds of molar composition 6Na2 O-4Fe2 O3 -5SiO2 or 2Na2 O-Fe2 O3 -SiO2 . The Moessbauer spectrum of β-NaFeO2 has an internal magnetic field of 487 kOe at room temperature. 相似文献
14.
The subsolidus region of the PbO-SiO2 system was studied by DTA and X-ray diffraction. X-ray diffraction analysis showed the presence of five compounds: 4PbO.SiO2 , 3PbO·SiO2 , 2PbO·SiO2 , 3PbO·2SiO2 , and PbO·SiO2 . The compound 4PbO·SiO2 has previously been reported to have three polymorphic forms; there are two polymorphs of 2PbO·SiO2 with the inversion at 460°±15°C. The compounds 3PbO·SiO2 and 3PbO·2SiO2 were unstable above 430°±10° and 585°±15°C, respectively; PbO·SiO2 was unstable below 525°±15°C. DTA patterns were determined for glasses of the composition of each of these compounds. 相似文献
15.
The system Ba2 SiO4 -Ca2 SiO4 was studied by heating mixtures of Ba2 SiO4 and Ca2 SiO4 at 1723 K. Six distinct phases resulted; they were examined by both X-ray diffraction and differential thermal analysis. The phases β -(Ba0.05 Ca1.95 )SiO4 and α-(Ba0.15 Ca1.85 )SiO4 are isostructural with corresponding high-temperature modifications of Ca2 SiO4 . The X phase (Ba0.48 Ca1.52 SiO4 ) is orthorhombic, is a pure phase rather than a solid solution, and is defined for the first time in the present work. The T phase (Ba1.31 Ca0.69 SiO4 ) is hexagonal and interpreted in terms of a unit cell with a doubled c parameter, in contrast with literature data. 相似文献
16.
J. F. EICHELBERGER C. R. HUDGENS L. V. JONES G. PISH T. B. RHINEHAMMER P. A. TUCKER L. J. WITTENBERG 《Journal of the American Ceramic Society》1963,46(6):279-283
The phase equilibrium diagram for the ternary fused-salt system NaF–BeF2 –UF4 was based on evidence from differential thermal analysis, polarized light microscopy, and X-ray examination of samples obtained from thermal-gradient quenching studies and from high-temperature filtration experiments. All the stable compounds in the binary systems NaF–BeF2 and NaF–UF4 displayed primary-phase fields in the ternary system, including a new subsolidus compound which was observed and tentatively assigned the formula NaF · 4UF4 . No ternary compounds or solid solutions were observed. The following three eutectics were found in the ternary system: (1) 72.5 NaF, 17.0 BeF2 , and 0.5 mole % UF4 at 486°C, (2) 56.0 NaF, 43.5 BeF2 , and 10.5 mole % UF4 at 339°C, and (3) 43.5 NaF, 56.0 BeF2 , and 0.5 mole % UF4 at 345°C. 相似文献
17.
Jin-Seong Kim Ngoc-Huan Nguyen Myung-Eun Song Jong-Bong Lim Dong-Soo Paik Sahn Nahm Jong-Hoo Paik Byung-Hyun Choi Soon-Jae Yu 《International Journal of Applied Ceramic Technology》2009,6(5):581-586
Bi2 O3 was added to a nominal composition of Zn1.8 SiO3.8 (ZS) ceramics to decrease their sintering temperature. When the Bi2 O3 content was <8.0 mol%, a porous microstructure with Bi4 (SiO4 )3 and SiO2 second phases was developed in the specimen sintered at 885°C. However, when the Bi2 O3 content exceeded 8.0 mol%, a liquid phase, which formed during sintering at temperatures below 900°C, assisted the densification of the ZS ceramics. Good microwave dielectric properties of Q × f =12,600 GHz, ɛr =7.6, and τf =−22 ppm/°C were obtained from the specimen with 8.0 mol% Bi2 O3 sintered at 885°C for 2 h. 相似文献
18.
Crystalline solubility relations in the system MgO-Mg2 SiO4 MgAl2 O4 (periclase-forsterite-spinel) were studied using coprecipitated gels as starting materials. The substitution 2Al = Mg + Si was investigated along the join Mg2 SiO4 -Mg-Al2 O4 ,. At 1720°C the maximum crystalline solution in forsterite is about 0.5 mole % MgAI2 O4 , and in spinel it is slightly more than 5.0 mole % Mg2 SiO4 . The solubility of MgO in forsterite was 0.5 mole % at 1860°C, whereas more than 11 mole % Mg2 SiO4 can be dissolved in the periclase structure at this temperature. Ternary crystalline solution exists in the periclase structure to a composition of Mg0.853 Al0.063 Si0.026 O at 1710°C. 相似文献
19.
In the course of work on all compositions which would probably yield silica-structure phases, a detailed study was made of the A3+ B5+ O4 -type compounds with gallium as the trivalent element. Two forms of GaPO4 exist corresponding to quartz and cristobalite; the quartz form has a thermal expansion of the low-quartz type and no stable α-β transition is found in this phase. It inverts stably to the cristobalite form at 933°± 4°C. The cristobalite form melts at 1670°C. and the liquid cannot be made to form a glass. The cristobalite form also has a metastable displacive transition at 616°± 2°C. GaAsO4 exists only in a form corresponding to the lowquartz structure. It disproportionates into Ga2 O3 and As2 O5 at about 1000°C. GaSbO4 could only be prepared in one form corresponding to the rutile structure. The influence of such phases on SiO2 was determined by studying a few mixtures in the system SiO2 -GaPO4 . The SiO2 end member appears to admit about 25% GaPO4 into solid solution in the cristobalite phase at the highest temperature. Even under hydrothennal conditions, however, equilibrium cannot be easily attained at the lower temperatures in a few hundred hours. The results are presented in a not-impossible diagram for the system GaPO4 -SiO2 . 相似文献
20.
The liquidus-solidus relations along the join Ca2 SiO4 -Ca(OH), in the system CaO-SiO2 -H2 O 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 Ca2 SiO2 + liquid (composition 23 wt% Ca2 Si04 ) at 955°C. The eutectic between calcium hydroxide and calciochondrodite lies at 13% Ca2 Si04 and 822°C. Preliminary experiments, also at 1000 atm, in the ternary system CaO-Ca2 Si04 -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 Ca2 SiOl + 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. 相似文献