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1.
Sigrun N. Karlsdottir John W. Halloran Carl E. Henderson 《Journal of the American Ceramic Society》2007,90(9):2863-2867
During the high-temperature oxidation of ZrB2 –SiC composites, liquid boron oxide (B2 O3 ) is formed at the zirconium diboride–zirconium oxide interface and transported through the overlying layer of silica liquid by convection, forming distinct convection cells arranged like the petals of a flower. The convection cells are localized by a viscous fingering phenomenon, as the fluid B2 O3 rich liquid solution rises through the viscous silica layer. The upwelling B2 O3 rich liquid contains dissolved zirconium dioxide, which deposits in the center of the flower-like structure as the B2 O3 evaporates. The driving force for the B2 O3 liquid flow is the volume increase upon oxidation of ZrB2 . Convective transport of B2 O3 liquids suggests a novel mechanism for the high-temperature oxidation of these materials. 相似文献
2.
Oxidation Behavior of Titanium Boride at Elevated Temperatures 总被引:3,自引:0,他引:3
Young-Hag Koh Seung-Yong Lee Hyoun-Ee Kim 《Journal of the American Ceramic Society》2001,84(1):239-241
The oxidation behavior of dense TiB2 specimens was investigated. Hot-pressed TiB2 with 2.5 wt% Si3 N4 as a sintering aid was exposed to air at temperatures between 800° and 1200°C for up to 10 h. The TiB2 exhibited two distinct oxidation behaviors depending on the temperature. At temperatures below 1000°C, parabolic weight gains were observed as a result of the formation of TiO2 ( s ) and B2 O3 ( l ) on the surface. The oxidation layer comprised two layers: an inner layer of crystalline TiO2 and an outer layer mainly composed of B2 O3 . When the oxidation temperatures were higher than 1000°C, gaseous B2 O3 was formed along with crystalline TiO2 by the oxidation process. In this case, the surface was covered with large TiO2 grains imbedded in a highly textured small TiO2 matrix. 相似文献
3.
Boron oxide glasses with compositions corresponding to B2 O3 , B2 O3 .0.42H2 O, B2 O3 .0.50H2 O, and B2 O3 .0.63H2 ) have been prepared and their structures have been studied. Diffractometric X-ray scattering measurements have been made, and from these, radial distribution functions have been computed. Comparison of the radial distribution functions indicates that the fundamental triangular coordination characteristic of vitreous B2 O3 is maintained in the water-containing glasses. Consideration of the results of complementary infrared and nuclear magnetic resonance studies along with preliminary results of physical property measurements indicates that the glasses of high water content are heterogeneous. It is probable that disordered regions of high hydrogen bond density, approaching HBO2 in composition, are embedded in a matrix approximating vitreous B2 O3 . 相似文献
4.
Takashi Wakasugi Fumitaka Tsukihashi Nobuo Sano 《Journal of the American Ceramic Society》1991,74(7):1650-1653
The BN solubilities for B2 O3 , B2 O3 ─SiO2 , and B2 O3 ─CaO systems have been measured mainly at 1823 K using a graphite crucible. The capability of the systems for nitrogen dissolution is compared with that of silicate systems in terms of nitride capacity. The dependence of nitrogen solubility in molten CaO containing 15 mol% of B2 O3 on oxygen and nitrogen partial pressures is also investigated. It has been found that there are two mechanisms for nitrogen dissolution, namely as chemically bonded nitrogen and as physically dissolved nitrogen gas. 相似文献
5.
Jun-Youp Choi Suk-Joong L. Kang O. Fukunaga Jong-Ku Park K. Y. Eun 《Journal of the American Ceramic Society》1993,76(10):2525-2528
Four kinds of BN powders—amorphous BN with B2 O3 , partially crystallized BN without B2 O3 , well-crystallized hBN with B2 O3 , and well-crystallized hBN without B2 O3 —were prepared to determine the effect of B2 O3 on the crystallization of amorphous BN and the effect of BN crystallinity on the formation of cBN under high pressure (4–5 GPa) and at high temperature (1350–1450°C). The amorphous BN with B2 O3 easily crystallized and transformed to cBN in the presence of A1N catalyst, while the partially crystallized BN without B2 O3 did not. The well-crystallized hBN transformed very slowly to cBN without B2 O3 , in contrast to fast transformation with B2 O3 . It is thus found that the transformation from hBN to cBN in the presence of AIN catalyst is determined by the degree of BN crystallinity as well as the presence of B2 O3 . Cubic BN can be synthesized only from crystallized hBN under the experimental conditions used. The formation of cBN from amorphous BN is possible through its prior crystallization, which can occur in the presence of B2 O3 . 相似文献
6.
Sashi Tandon Ranchhor D. Agrawal Madan L. Kapoor 《Journal of the American Ceramic Society》1994,77(4):1032-1036
The viscosity of sodium borate slags at high Na2 O concentrations (37.3 to 49.4 mol%) and high temperatures (1000° to 1300°C) follows an Arrhenius-type relationship. This relationship was also observed for sodium borate slags (mass% Na2 O/mass% B2 O3 = 0.86) containing CaO and CaF2 for the same temperature range. There has been a reduction in viscosity of the sodium borate slags (mass% Na2 O3 mass% B2 O = 0.53 to 0.86) with increase in Na2 O concentration. On adding CaO (10 to 50 mass%) to the sodium borate slag (mass% Na2 O/mass% B2 O3 = 0.86), the viscosity increased considerably, while an addition of CaF2 (S to 15 mass%) to the slag (30.9 mass% Na2 O3 35.8 mass% B2 O3 , 33.3 mass% CaO) decreased the viscosity. The average activation energies of Na2 O─B2 O3 , Na2 O─B2 O3 ─CaO3 and Na2 O─B2 O3 ─CaO─CaF2 slag systems have been estimated as 14.6, 124.7, and 41.4 kJ/mol, respectively, for the given composition ranges and 1000° to 1300°C temperature range. 相似文献
7.
Dong-Wan Kim Jeong-Ryeol Kim Sung-Hun Yoon Kug Sun Hong Chang Kyung Kim 《Journal of the American Ceramic Society》2002,85(11):2759-2762
The effect of B2 O3 on the sintering temperature and microwave dielectric properties of Ba5 Nb4 O15 has been investigated using X-ray powder diffraction, scanning electron microscopy, and a network analyzer. Interactions between Ba5 Nb4 O15 and B2 O3 led to formation of second phases, BaNb2 O6 and BaB2 O4 . The addition of B2 O3 to Ba5 Nb4 O15 resulted in lowering the sintering temperature from 1400° to 925°C. Low-fired Ba5 Nb4 O15 could be interpreted by measuring changes in the quality factor ( Q × f ), the relative dielectric constant (ɛr ), and the temperature coefficient of resonant frequency (τf ) as a function of B2 O3 additions. More importantly, the formation of BaNb2 O6 provided temperature compensation. The microwave dielectric properties of low-fired Ba5 Nb4 O15 had good dielectric properties: Q × f = 18700 GHz, ɛr = 39, and τf = 0 ppm/°C. 相似文献
8.
The effect of Al8 B4 C7 used as an antioxidant in MgO–C refractories and the behavior of Al8 B4 C7 in CO gas were investigated in the present study. Al8 B4 C7 was found to react with CO gas, to form Al2 O3 ( s ), B2 O3 ( l ), and C( s ), at temperatures >1100°C. The Al2 O3 reacts with MgO to form MgAl2 O4 near the surface of the material. At the same time, B2 O3 ( l ) evaporates and reacts with MgO, to form a liquid phase, at >1333°C, the eutectic point between 3MgO·B2 O3 and MgO. The coexistence of the liquid and MgAl2 O4 makes the protective layer more dense, thus inhibiting oxidation of the refractory. At >1333°C, the process apparently is controlled by oxygen diffusion, whereas it is controlled by chemical reaction when the temperature is <1333°C. 相似文献
9.
LEON MAYA 《Journal of the American Ceramic Society》1977,60(7-8):323-328
The system B2 O3 -NaF-NaBF, has been studied by subjecting selected compositions to thermal treatment in the range 400° to 600°C. Weight losses, chemical analyses, ir, Raman, and X-ray diffraction techniques were used to define the composition of the crystalline phases and the structural units being formed in the system. The stoichiometry of the BF3 evolved from NaBF4 -B2 O3 mixtures indicated that a composition corresponding to Na2 B3 F5 O3 was formed in mixtures containing up to 33.3 mol% B2 O3 . At higher boron oxide concentrations, Na2 B3 F5 O3 was consumed, yielding 2NaF.3B2 O3 . The crystalline compounds Na3 B3 F6 O3 , 2NaF.3B2 O3 , and phase B (apparently NaF.B2 O3 ) were formed in the system. The compound Na3 B3 F6 O3 appeared as the stable oxygen-containing species in NaBF4 -NaF mixtures of low oxide content. The main fluorine-containing structural units of the system are BF4 , (–O)3 BF, (–O)2 BF2 , (–O)2 BF, whereas the main structure for binary NaF-B2 O3 mixtures is (–O)3 BF. 相似文献
10.
Reactions and Microstructure Development in Mullite Fibers 总被引:3,自引:0,他引:3
Elizabeth A. Richards Chris J. Goodbrake Harold G. Sowman 《Journal of the American Ceramic Society》1991,74(10):2404-2409
Microstructural and compositional changes during heat treatment of sol–gel-derived mullite fibers with additions of 2 wt% B2 O3 , 2 wt% P2 O5 , 2 wt% Cr2 O3 , and (1 wt% P2 O5 + 1 wt% Cr2 O3 ) were compared with those of undoped mullite fibers. For all compositions the sequence of phase development was the crystallization of a spinel phase (†-Al2 O3 or Al–Si spinel) from amorphous material, followed by the formation of mullite at higher temperatures. Differential thermal analysis showed that additions of B2 O3 and P2 O5 increased the temperature of spinel formation and that B2 O3 significantly decreased the temperature of mullite formation. After 1 h at 1200°C, the size of mullite grains in fibers that contained B2 O3 was less than 1000 Å the grains in fibers of other compositions were 6000 to 12000 Å. After 60 h at 1400°C, fibers modified with B2 O3 had a grain size less than 2000 to 3000 Å the grains in fibers of other compositions were 6000 to 12000 Å. B2 O3 was the most volatile additive. 相似文献
11.
Martin Boike Klaus Hilpert Franz Müller 《Journal of the American Ceramic Society》1993,76(11):2809-2812
Eight glass samples in the B2 O3 -SiO2 system with compositions from 20 to 90 mol% B2 O3 were prepared. The equilibrium vaporization was studied by Knudsen effusion mass spectrometry at temperatures between 1450 and 1500 K. B2 O3 ( g ) was the most abundant boron-containing species in the vapor; no silicon-containing gaseous species were detected. Thermodynamic activities of B2 O3 in the liquid were determined at 1475 K. Thermodynamic activities of SiO2 and integral excess Gibbs energies were estimated from the thermodynamic activities of B2 O3 . The thermodynamic data support the results obtained by other methods indicating the existance of a miscibility gap in the metastable liquid. 相似文献
12.
ERNEST M. LEVIN CARL R. ROBBINS JON L. WARING 《Journal of the American Ceramic Society》1961,44(2):87-91
The phase equilibrium diagram for the system La2 O3 -B2 O3 has been determined experimentally. The compounds La2 O3 -3B2 O3 and La2 O3 -B2 O3 melt congruently at 1141°± 5°C. and 1660°± 15°C, respectively. At 1488°± 5°C, La2 O3 -B2 O3 inverts from the aragonite-type structure to a high-temperature form. Trilanthanum borate, 3La2 O3 B2 O3 , melts incongruently at 1386°± 5°C. to give liquid and La2 O3 . No solid solutions exist in the system. A region of liquid immiscibility exists in the system and extends at 1136°± 5°C. from almost pure B2 O3 to 21.5 mole % La2 O3 . The experimental value for the extent of immiscibility agrees with that calculated from theoretical considerations. A second method for estimating immiscibility in the system is demonstrated, which requires experimentally only the determination of the index of refraction of the modifier-rich liquid. Principles governing immiscibility are discussed. 相似文献
13.
A new crystalline variety of B2 O3 has been prepared at pressures above 22,000 atmospheres and above 400°C. The properties are listed. These moderate pressures also are sufficient to catalyze the crystallization of the more common hexagonal form from B2 O3 glass. Pressures, however, of up to 50,000 atmospheres will not convert GeO2 quartz to GeO2 rutile at room temperature. From the infrared absorption patterns of appropriate phases it can be shown that in coesite the tetrahedral coordination of Si4+ (or Be2+ in the model BeF2 -coesite phase) must be unchanged. In the new form of B2 O3 , however, the coordination number must be substantially different from that in the common hexagonal B2 O3 . 相似文献
14.
Takashi Harada Hiroyuki In Hiromichi Takebe Kenji Morinaga 《Journal of the American Ceramic Society》2004,87(3):408-411
The effect of B2 O3 addition on the thermal stability of BaO–P2 O5 glasses is studied by differential thermal analysis (DTA), X-ray diffraction (XRD) analysis, scanning electron microscopy, and micro-Raman spectroscopy. The difference between glass-transition and onset-crystallization temperatures increases monotonically with increasing B2 O3 concentration. The DTA result reveals that no exothermic peak due to surface crystallization exists in the BaO–P2 O5 glass doped with 3 mol% B2 O3 . A single-mode BaO–P2 O5 -B2 O3 glass fiber could be fabricated by a rod-in-tube technique. The modification of glass structure due to B2 O3 addition is qualitatively discussed. 相似文献
15.
A partial molar volume technique was used to estimate the coordination number of oxygen ions around a boron ion in PbO·2B2 O3 and BaO·2B2 O3 melts. The boron coordination number appeared to be lower in the melt than in the crystal of PbO·2B2 O3 , whereas it was nearly the same in the melt and crystal of BaO·2B2 O3 . 相似文献
16.
Richard E. Chinn Michael J. Haun Chan Young Kim David B. Price 《Journal of the American Ceramic Society》1998,81(9):2285-2293
The use of transient glass-phase processing to lower the glass-melting temperature and subsequent heat-treatment temperature of stoichiometric SrAl2 Si2 O8 to produce the stable monoclinic form has been described. Two nonstoichiometric, low-melting, alumina-deficient, strontium aluminosilicate compositions were melted, quenched, and milled into glass powders. B2 O3 was dissolved into one of the glass compositions to control the crystallization behavior. The glass powders were then wet-mixed with enough alpha-Al2 O3 powder so that the overall composition was that of stoichiometric SrAl2 Si2 O8 (B2 O3 neglected). The four compositions were dry-pressed into pellets and sintered in three processes. Glass-alumina pellets with dissolved B2 O3 were densified via viscous-phase sintering at 1100°C, followed by complete dissolution of the alumina and crystallization to ~100% monoclinic SrAl2 Si2 O8 . Pellets without dissolved B2 O3 required considerably higher temperatures to form ~100% monoclinic SrAl2 Si2 O8 in a modified process. 相似文献
17.
The energy relations existing between the congruently melting compounds Li2 O -2B2 O3 , Na2 O–2B2 O3 , Na2 O-4B2 O3 , and K2 O-4B2 O3 and their glasses have been determined for the temperature range 25° to 1100°C. High-temperature heat-content, entropy, and heat of solution data are given for both the glasses and the corresponding devitrified materials. A comparison of the heats of fusion of the alkali borates on a gram atom of oxygen basis shows that they follow the order Li > Na > K. The entropy differences between the glass and the corresponding crystalline material have been determined at 25°C. The free-energy change at 25°C. for the reaction crystal → glass has been calculated for the four compounds. 相似文献
18.
Jin-Seong Kim Ngoc-Huan Nguyen Jong-Bong Lim Dong-Soo Paik Sahn Nahm Jong-Hoo Paik Jong-Hee Kim Hwack-Joo Lee 《Journal of the American Ceramic Society》2008,91(2):671-674
B2 O3 was added to nominal composition Zn1.8 SiO3.8 (ZS) ceramics to decrease their sintering temperature for application to low-temperature cofired ceramic (LTCC) devices. B2 O3 reacted with SiO2 to form a liquid phase containing SiO2 and B2 O3 . The composition and melting temperature of the liquid phase depended on the sintering temperature and the B2 O3 content. The specimen containing 20.0 mol% of B2 O3 sintered at 900°C exhibited high microwave dielectric properties of Q × f =53 000 GHz, ɛ r =5.7, and τf =−16 ppm/°C, confirming the promising potential of the B2 O3 -added ZS ceramics as candidate materials for the LTCC devices. 相似文献
19.
Takashi Yamaguchi Hiroko Ayaki Lika Asai 《Journal of the American Ceramic Society》1993,76(4):993-997
Interfacial and powder reactions between CaTiO3 and 90PbO–10B2 O3 and 75PbO–25SiO2 binary glasses were studied. The reaction has been analyzed as the effect of B2 O3 and SiO2 additions on the interaction between CaTiO3 and PbO, and discussed from thermodynamic and kinetic points of view. For a fixed CaTiO3 /PbO ratio2 the product perovskite phase became enriched with lead as the amount of additives increased, which is more pronounced with B2 O3 addition. The reaction of CaTiO3 with the lead–boron glass was controlled by a dissolution-precipitation mechanism, and that with the lead-silica glass by a diffusion mechanism. 相似文献
20.
Chen-Feng Chan Bernard B. Argent William E. Lee 《Journal of the American Ceramic Society》1998,81(12):3177-3188
The microstructures of Al2 O3 –SiO2 –SiC–C refractory matrices with aluminum, silicon, Si3 N4 , BN, B2 O3 , and B4 C additives are characterized before and after a crucible slag test, and the phases present are compared to those expected at thermodynamic equilibrium. The carbon content dominates the resistance to CaO–MgO–Al2 O3 –SiO2 slag penetration, while the viscosity of liquid phases present has a significant influence when the matrix carbon contents are similar. Silicon and Si3 N4 additives reduce slag penetration resistance because of indirect oxidation of carbon to form SiC. B4 C, in particular, and B2 O3 also reduce slag penetration resistance because of formation of a more fluid boron-containing liquid, while aluminum and BN addition have no significant effect. Carbon and BN hardly react with the slag, while SiC partially reacts with it, leading to deposition of carbon as a dense layer. Corundum present in the refractories also readily dissolves in the slag. Microstructurally, slag penetration resistance is associated with the dense carbon layer located at the slag-refractory interface. 相似文献