共查询到20条相似文献,搜索用时 468 毫秒
1.
Shiro Torizuka Kaoru Sato Hiroaki Nishio Teruo Kishi 《Journal of the American Ceramic Society》1995,78(6):1606-1610
Compacts of TiB2 with densities approaching 100% are difficult to obtain using pressureless sintering. The addition of SiC was very effective in improving the sinterability of TiB2 . The oxygen content of the raw TiB2 powder used in this research was 1.5 wt%. X-ray photoelectron spectroscopy showed that the powder surface consisted mainly of TiO2 and B2 O3 . Using vacuum sintering at 1700°C under 13–0.013 Pa, TiB2 samples containing 2.5 wt% SiC achieved 96% of their theoretical density, and a density of 99% was achieved by HIPing. TEM observations revealed that SiC reacts to form an amorphous phase. TEM-EELS analysis indicated that the amorphous phase includes Si, O, and Ti, and X-ray diffraction showed the reaction to be TiO2 + SiC → SiO2 + TiC. Therefore, the improved sinterability of TiB2 resulted from the SiO2 liquid phase that was formed during sintering when the raw TiB2 powder had 1.5 wt% oxygen. 相似文献
2.
The presence of TiC or TiN paritcles in an Al2 O3 matrix affects the thermal stability of the composites in oxidizing environments. In isothermic oxidation tests at 700°, 800°, 900°, 1000°, and 1100°C for up to 20 h, two different oxidation regimes have been observed at T < 900°C and at 900°C ≤ T ≤ 1100°C. At low temperatures ( T < 900°C), the oxidation follows a phase-boundary reaction; the reaction product initially consists of aggregates of submicrometer needlelike TiO2 rutile crystals that subsequently grow and coalesce. When a continuous TiO2 rutile layer is formed ( T ≥ 900°C), the oxidation kinetics change to parabolic, and the diffusion of O2 through a thick TiO2 layer is proposed as the governing step. 相似文献
3.
Young-Hag Koh Hae-Won Kim Hyoun-Ee Kim John W. Halloran 《Journal of the American Ceramic Society》2002,85(12):3123-3125
The oxidation behavior and its effect on the mechanical properties of fibrous monolith Si3 N4 /BN after exposure to air at temperatures ranging from 1000° to 1400°C for up to 20 h were investigated. After exposure at 1000°C, only the BN cell boundary was oxidized, forming a B2 O3 liquid phase. With increasing exposure temperature, the Si3 N4 cells began to oxidize, forming crystalline Y2 Si2 O7 , SiO2 , and silicate glass. However, in this case, a weight loss was observed due to extensive vaporization of the B2 O3 liquid. After exposure at 1400°C, large Y2 Si2 O7 crystals with a glassy phase formed near the BN cell boundaries. The oxidation behavior significantly affected the mechanical properties of the fibrous monolith. The flexural strength and work-of-fracture decreased with increasing exposure temperature, while the noncatastrophic failure was maintained. 相似文献
4.
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
5.
Sea-Fue Wang Chiang-Chuang Chung Chai-Hui Wang Jinn P. Chu 《Journal of the American Ceramic Society》2002,85(6):1619-1621
Preparation of dense and phase-pure Ba2 Ti9 O20 is generally difficult using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba2 Ti9 O20 equilibrium phase boundary in the BaO–TiO2 system at high temperatures. In this study, the effects of B2 O3 on the densification, microstructural evolution, and phase stability of Ba2 Ti9 O20 were investigated. It was found that the densification of Ba2 Ti9 O20 sintered with B2 O3 was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid-state sintering became dominant because of the evaporation of B2 O3 . With the addition of 5 wt% B2 O3 , the ceramic yielded a pure Ba2 Ti9 O20 phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO2 or ZrO2 . The facilities of B2 O3 addition to the stability of Ba2 Ti9 O20 are apparently due to the eutectic liquid phase which accelerates the migration of reactant species. 相似文献
6.
Takahiro Takada Sea Fue Wang Shoko Yoshikawa Sei-Joo Jang Robert E. Newnham 《Journal of the American Ceramic Society》1994,77(7):1909-1916
The effect of glass addition on the properties of BaO–TiO2 -WO3 microwave dielectric material N-35, which has Q = 5900 and K = 35 at 7.2 GHz for samples sintered at 1360°C, was investigated. Several glasses including B2 O3 , SiO2 , 5ZnO–2B2 O3 , and nine other commercial glasses were selected for this study. Among these glasses, one with a 5 wt% addition of B2 O3 to N-35, when sintered at 1200°C, had the best dielectric properties: Q = 8300 and K = 34 at 8.5 GHz. Both Q and K increased with firing temperature as well as with density. The Q of N-35, when sintered with a ZnO–B2 O3 glass system, showed a sudden drop in the sintering temperature to about 1000°C. The results of XRD, thermal analysis, and scanning electron microscopy indicated that the chemical reaction between the dielectric ceramics and glass had a greater effect on Q than on the density. The effects of the glass content and the mixing process on the densification and microwave dielectric properties are also presented. Ball milling improved the densification and dielectric properties of the N-35 sintered with ZnO–B2 O3 . 相似文献
7.
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
8.
Phase relations at liquidus temperatures in the system La2 O3 -TiO2 were studied in air. The existence of two previously unreported compounds, La2 O3 . TiO2 and 2La2 O3 -TiO2 , is postulated on the basis of X-ray and microscopic examination of crystalline samples, and in the case of La2 O3 .-TiO2 , on a maximum in the liquidus curve at that composition. Quenched liquids of the primary-phase field of rutile were found to be semiconducting. This property was related to oxygen loss from both liquid and crystalline phases and is discussed in the light of weight loss experiments, microscopic examination of quenched samples, and information obtained from the literature. Dielectric constant and loss factor of the compounds La2 O3 -TiO2 , La2 O3 -2TiO2 , and 2La2 O3 -9TiO2 are reported at 1 Mc over the temperature range 25° to 500°C. 相似文献
9.
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
10.
Takahiro Takada Sea Fue Wang Shoko Yoshikawa Sei-Joo Jang Robert E. Newnham 《Journal of the American Ceramic Society》1994,77(9):2485-2488
The effects of glass additions on the properties of (Zr,Sn)TiO4 as a microwave dielectric material were investigated. The (Zr,Sn)TiO4 ceramics with no glass addition sintered at 1360°C gave Q = 4900 and K = 37 at 7.9 GHz . Several glasses, including SiO2 , B2 O3 , 5ZnO–2B2 O3 , and nine commercial glasses, were tested during this study. Among these glasses, (Zr,Sn)TiO4 sintered with ZnO-B2 O3 –SiO2 (Corning 7574) showed more than 20% higher density than that of pure (Zr,Sn)TiO4 sintered at the same temperature. A 5-wt% addition of SiO2 , to (Zr,Sn)TiO4 , when sintered at 1200°C, gave the best Q : Q = 2700 at 9 GHz. Results of XRD analysis and scanning electron microscopy and the effect of glass content are also presented. 相似文献
11.
Low-Temperature Fabrication of Cordierite Ceramics from Kaolinite and Magnesium Hydroxide Mixtures with Boron Oxide Additions 总被引:3,自引:0,他引:3
Katsuhiro Sumi Yuichi Kobayashi Etsuro Kato 《Journal of the American Ceramic Society》1999,82(3):783-785
Thermal reactions of mixtures of ultrafine particles of magnesium hydroxide (Mg(OH)2 ) and kaolinite in a composition of MgO:Al2 O3 :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 (B2 O3 ) (in the form of magnesium borate, 2MgOB2 O3 ), 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 B2 O3 promoted the densification at 850°-900°C and accelerated the crystallization of alpha-cordierite. The specimen with 3 mass% of B2 O3 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. 相似文献
12.
Myeong-Heon Um Chul-Tae Lee Hidehiro Kumazawa 《Journal of the American Ceramic Society》2001,84(5):1181-1183
TiO2 fibers were formed by thermal treatment of layered H2 Ti4 O9 (hydrous titanium dioxide) and KHTi4 O9 synthesized by ion-exchange reactions. The calcination of the former at 900° and 1050°C for 3 h yielded TiO2 fibers with anatase and rutile phases, whose length and diameter were 15–20 and 2–5 μm and 10–15 and 3–5 μm, respectively. The thermal treatment of the latter at temperatures of 250° to 500°C yielded pure K2 Ti8 O17 , which tended to decompose to K2 Ti6 O13 and TiO2 at temperatures >600°C. At 1050°C, K2 Ti6 O13 phase was formed with rutile TiO2 fibers, whose length and diameter were 10–20 and 1–3 μm, respectively. 相似文献
13.
Low-Temperature Sintering and Microwave Dielectric Properties of Zinc Metatitanate-Rutile Mixtures Using Boron 总被引:18,自引:0,他引:18
Hyo Tae Kim Seong Ho Kim Sahn Nahm Jae Dong Byun Yoonho Kim 《Journal of the American Ceramic Society》1999,82(11):3043-3048
Mixtures of zinc metatitanate and rutile (ZnTiO3 + x TiO2 , where x = 0-0.5) have been prepared via the conventional mixed-oxide method. Centrifugal planetary milling with zirconia beads 1 mm in diameter produced very fine powders (mean particle size of 0.2 µm), which allowed the synthesis of ZnTiO3 and sintering at temperatures <945°C, which is the decomposition temperature of ZnTiO3 . Sintering of the mixtures was enhanced further by the addition of B2 O3 . Densities of >94% of the theoretical density have been attained for the specimens that were sintered at 875°C for 4 h with B2 O3 additions of <1 wt%. Microwave dielectric properties of the aforementioned compositions were as follows: dielectric constant of 29-31, normalized quality factor of 56000-69000 GHz, and a temperature coefficient of resonance frequency between -10 and +10 ppm/°C. Sintering was enhanced by the formation of a ZnO-B2 O3 liquid phase, which affected the microwave properties, because of variation in the phase composition. 相似文献
14.
A study of the solid solution of TiO2 , Fe2 O3 , and Cr2 03 in mullite was made by measuring the changes in lattice parameters and unit-cell volume. Synthetic mullite (3O3 -2SiO2 ) was reacted with up to 12 weight % of the oxides at temperatures ranging from 1000° to 17000C. The approximate minimum temperature required for the formation of solid solution was 12000C. for Fe2 03 and 1400°C. for Cr2 O3 and TiO3 . The maximum amount of solid solution found was 2 to 4% TiO2 at 1600°C., 10 to 12% Fe2 Os at 1300°C., and 8 to 10% CrZ O3 at 1600OC. Lattice parameters and unit-cell volumes for each solid solution series increased with increasing amounts of foreign oxide. There was good agreement between the calculated and observed increase in cell dimensions for the iron oxide series. Except in the case of titania, there was good agreement between X-ray data and petrographic observations. 相似文献
15.
In the system TiO2 —Al2 O3 , TiO2 (anatase, tetragonal) solid solutions crystallize at low temperatures (with up to ∼ 22 mol% Al2 O3 ) from amorphous materials prepared by the simultaneous hydrolysis of titanium and aluminum alkoxides. The lattice parameter a is relatively constant regardless of composition, whereas parameter c decreases linearly with increasing Al2 O3 . At higher temperatures, anatase solid solutions transform into TiO2 (rutile) with the formation of α-Al2 O3 . Powder characterization is studied. Pure anatase crystallizes at 220° to 360°C, and the anatase-to-rutile phase transformation occurs at 770° to 850°C. 相似文献
16.
BaTiO3 -based ferroelectric glass-ceramics with the composition 0.65(Ba1− X Sr X )TiO3 ·0.27SiO2 ·0.08Al2 O3 ( X = 0.2–0.6) were fabricated, and their tunable dielectric properties were measured at microwave frequency. Major crystalline phases that precipitated during thermal treatment up to 1000°C were (Ba,Sr)TiO3 , Ba2 TiSi2 O8 , and BaAl2 Si2 O8 . The Curie temperatures of the heated bulk samples were successfully tuned near room temperature at the composition between X = 0.2 and 0.3. A thick-film sample with X = 0.3 showed 27% tunability at 5 GHz under 10 kV/cm bias voltage. 相似文献
17.
Powder compacts consisting of Al, Al2 O3 , and ZrO2 were heated by microwave radiation. Tracing the phase evolution during reaction bonding revealed the reaction mechanism. In the case of conventional heating, the compacts expanded slightly at temperatures of <700°C due to Al surface oxidation and expanded sharply at temperatures greater than 700°C as oxidation proceeded from the surface to the interior. Then, the compacts shrank at 1550°C due to sintering. For the case of microwave heating, the compacts expanded at temperatures of <550°C due to the formation of Al3 Zr. This Al3 Zr formation was caused by the preferential heating of ZrO2 relative to Al and Al2 O3 by microwave radiation. Then, Al3 Zr was oxidized to form Al2 O3 and ZrO2 at temperatures of >1000°C. Finally, the compacts shrank at 1550°C due to sintering, similarly to conventional sintering. 相似文献
18.
Jonathan Y. Lok Kathryn V. Logan Jairaj J. Payyapilly 《Journal of the American Ceramic Society》2009,92(1):26-31
The stoichiometric self-propagating high-temperature synthesis (SHS) thermite reaction involving magnesium (Mg), titanium dioxide (TiO2 ), and boron oxide (B2 O3 ) forms MgO and titanium diboride (TiB2 ) as final products. Selective acid leaching is used to remove the MgO leaving TiB2 powder. This study investigates the acid leaching of SHS-produced MgO/TiB2 powders and a stoichiometric mixture of commercially obtained MgO and TiB2 powders. Leaching was conducted at pH levels of 4.0, 2.5, and 1.0 by the introduction of concentrated aliquots of HNO3 . This method maintains a minimum pH target throughout the leaching process, thereby sustaining a dynamic concentration to remove the oxide. The optimal leaching conditions were determined to be at 90°C at a minimum pH target of 2.5 for the SHS-produced product. At these conditions, conversion percentages of 83%–84% of MgO were measured with only trace amounts of TiB2 measured in the solution (<100 μg/L). Conversion percentages for each leaching condition and dissolution mass of solid MgO and TiB2 at each pH are also reported. Results from powder X-ray diffraction confirm the removal of MgO and minimal dissolution of TiB2 , and indicate the formation of unidentified compounds. Inductively coupled plasma mass spectrometry (ICP) was used to analyze the ionic composition and extent of leaching. Scanning electron microscopy was used to observe the particle morphology of the leached powders. 相似文献
19.
A type of new low sintering temperature ceramic, Li2 TiO3 ceramic, has been found. Although it is difficult for the Li2 TiO3 compound to be sintered compactly at temperatures above 1000°C for the volatilization of Li2 O, dense Li2 TiO3 ceramics were obtained by conventional solid-state reaction method at the sintering temperature of 900°C with the addition of ZnO–B2 O3 frit. The sintering behavior and microwave dielectric properties of Li2 TiO3 ceramics with less ZnO–B2 O3 frit (≤3.0 wt%) doping were investigated. The addition of ZnO–B2 O3 frit can lower the sintering temperature of the Li2 TiO3 ceramics, but it does not apparently degrade the microwave dielectric properties of the Li2 TiO3 ceramics. Typically, the good microwave dielectric properties of ɛr =23.06, Q × f =32 275 GHz, τf = 35.79 ppm/°C were obtained for 2.5 wt% ZnO–B2 O3 frit-doped Li2 TiO3 ceramics sintered at 900°C for 2 h. The porosity was 0.08%. The Li2 TiO3 ceramic system may be a promising candidate for low-temperature cofired ceramics applications. 相似文献
20.
Al2 O3 -TiN composites varying from 60 to 66.6 mol% TiN were prepared by an in situ reaction between TiO2 and AlN. N2 or O2 evolution takes place, depending on the composition selected. A pseudobrookite (PB) phase appears in the reaction product, the amount decreasing as the TiO2 :AlN ratio becomes poor in AlN. The in situ reaction product can be pressureless sintered to 94% to 97% theoretical density at 1600°C in N2 . The four-point flexural strength varies from 280 to 430 MPa at room temperature. The fracture toughness is 3 to 4.7 MPa.m1/2 . Oxidation of a 94% dense TiN-Al2 O3 composite in the temperature range 710° to 1050°C was also studied. A layer of TiO2 (rutile) protects the composite at 710°C from further oxidation with a weight gain of 0.08 mg/cm2 in 90 min. In the temperature range 820° to 1050°C, the initial oxidation kinetics are parabolic, with an activation energy of 216.5 kJ/mol. Linear oxidation kinetics with an activation energy of 113.7 kJ/mol pertain at longer times. 相似文献