共查询到20条相似文献,搜索用时 31 毫秒
1.
Hsin-chun Lu Lawrence E. Burkhart Glenn L. Schrader 《Journal of the American Ceramic Society》1991,74(5):968-972
Barium titanate precursors with Ba/Ti ratio 2:9 and 1:5 were prepared by first hydrolyzing titanium alkoxide and then mixing the resulting titania sol with a barium alkoxide-methanol solution. After drying, the xerogels of the precursors of barium titanates were sintered at temperatures from 700°C (4 h) to 1200°C (110 h or longer). Characterization of the product was performed using X-ray diffraction and laser Raman spectroscopy. At 700°C, BaTi5 O11 was formed from the 1:5 precursor and a two-phase mixture of BaTi2 O5 and BaTi5 O11 was formed from the 2:9 precursor. After prolonged heating at 1200°C, the latter mixture converted to a single-phase material, Ba2 Ti9 O20 . 相似文献
2.
Phase relations in the system BaO-TiO2 from 67 to 100 mol% TiO2 were investigated at 1200° to 1450°C in O2 . Data were obtained by microstructural, X-ray, and thermal analyses. The existence of the stable compounds Ba6 Ti17 O40 , Ba4 Ti13 O30 , BaTi4 O9 , and Ba2 Ti9 O20 was confirmed. The compound BaTi2 O5 is unstable and either forms as a reaction intermediate below the solidus or crystallizes from the melt. The compounds Ba6 Ti17 O40 and Ba4 Ti13 O30 decompose in peritectic reactions, and BaTiO3 and Ba6 Ti17 O40 react to form a eutectic. Special conditions are required for the formation of Ba2 Ti9 O20 , which decomposes in a peritectoid reaction at 1420°C. The new phase diagram is presented. 相似文献
3.
The phase development sequence based on a composition equivalent to Ba2 Ti9 O20 during heating is found to be in the following order: BaTi5 O11 > BaTi4 O9 > Ba2 Ti9 O20 . The lowest rate of formation of Ba2 Ti9 O20 is caused by its high surface energy and interface energy, which result in a low nucleation rate. The existence of BaTi5 O11 in calcined powder helps to form Ba2 Ti9 O20 in sintered compacts. The effect of BaTi5 O11 on Ba2 Ti9 O20 formation can be explained by their similar oxygen packing and by reduced volume change during transformation. The amount of BaTi5 O11 formed during heating depends greatly on the compositional homogeneity of powders. The addition of SnO2 aids the formation of Ba2 Ti9 O20 by reduced strain energy at transformation and reduced surface energy. 相似文献
4.
A double–inverse microemulsion (IME) process is used for synthesizing nano-sized Ba2 Ti9 O20 powders. The crystallization of powders thus obtained and the microwave dielectric properties of the sintered materials were examined. The IME-derived powders are of nano-size (∼21.5 nm) and possess high activity. The BaTi5 O11 , intermediate phase resulted when the IME-derived powders were calcined at 800°C (4 h) in air. However, high-density Ba2 Ti9 O20 materials with a pure triclinic phase (Hollandite like) can still be obtained by sintering such a BaTi5 O11 dominated powders at 1250°C/4 h. The phase transformation kinetics for the IME-derived powders were markedly enhanced when air was replaced by O2 during the calcinations and sintering processes. Both the calcination and densification temperatures were reduced by around 50°C compared with the process undertaken in air. The microwave dielectric properties of sintered materials increase with the density of the samples, resulting in a large dielectric constant ( K ≅39) and high-quality factor ( Q × f ≅28 000 GHz) for samples possessing a density higher than 95% theoretical density, regardless of the sintering atmosphere. Overfiring dissociates Ba2 Ti9 O20 materials and results in a poor-quality factor. 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.
Wen-Yi Lin Robert F. Speyer Wesley S. Hackenberger Thomas R. Shrout 《Journal of the American Ceramic Society》1999,82(5):1207-1211
The effects of solid-solution additives, their concentration, and the thermal processing schedule on the microstructure evolution and microwave properties of Ba2 Ti9 O20 were studied. The solubility of tin in Ba2 Ti9 O20 was higher than that of zirconium. Both elements facilitated the formation of phase-pure Ba2 Ti9 O20 resonators. Ba2 Ti9 O20 formed most easily with low dopant concentrations (0.82 mol%) (most impressively for ZrO2 substitutions). Extended heat treatment (16 h versus 6 h at a temperature of 1390°C) resulted in volatilization of the grain-boundary liquid phase, which leads to more-porous resonators that have correspondingly lower permittivities. Increasing the dopant concentration resulted in minor increases in the quality factor; doping with zirconium led to slightly higher values (a maximum of 13900 at a frequency of 3 GHz). Increasing the measurement temperature degraded the quality factor (most precipitously for BaTi4 O9 ). The temperature coefficient decreased as the ZrO2 substitution increased but was largely unaffected by the SnO2 concentration. Excess TiO2 in a batch with no other dopants demonstrated degraded microwave properties. 相似文献
7.
Jin-Ho Choy Yang-Su Han Jeong-Ho Sohn Mitsuru Itoh 《Journal of the American Ceramic Society》1995,78(5):1169-1172
Microwave dielectrics of the TiO2 -rich BaO-TiO2 system (BaTi4 O9 and Ba2 Ti9 O20 ) were prepared by the citrate route. Pure and well-crystallized BaTi4 O9 and Ba2 Ti9 O20 particles of nanometer size (30–50 nm) could be obtained by thermal decomposition of citrate gel precursors. After sintering at 1200°–1350°C (for 2–10 h), dense compounds with >90% of theoretical density could be obtained. Dielectric properties of disk-shaped sintered specimens, in the microwave frequency region, were measured in the TE01δ mode. They were found to have excellent microwave characteristics: for BaTi4 O9 , εr = 36, Q = 4900 at 10.3 GHz, and τf = 16 ppm/°C; and for Ba2 Ti9 O20 , εr = 37, Q = 5300 at 10.7 GHz, and τf =−6.0 ppm/°C. 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.
Processing and Characterization of BaTi4 O9 总被引:1,自引:0,他引:1
S. G. Mhaisalkar W. E. Lee D. W. Readey 《Journal of the American Ceramic Society》1989,72(11):2154-2158
BaTi4 O9 powder prepared by calcining BaCO3 and TiO2 powders was sintered to over 97% of theoretical density. Less than 5% Ba2 Ti9 O20 occurred as a second phase in "pure" BaTi4 O9 , and Al2 O3 impurities from processing formed isolated hollandite (∼BaAl2 Ti6 O16 ) grains, which were identified by fringes in bright-field TEM images. For pure BaTi4 O9 at 1 MHz, a dielectric loss (tan δ) of 5 × 10−4 and dielectric constant of 39 were recorded. Hollandite impurities were found to increase tan δ by 2 orders of magnitude, whereas firing in oxygen decreased tan δ by an order of magnitude. 相似文献
10.
Hong-Wen Wang 《Journal of the American Ceramic Society》1995,78(4):1134-1135
in a recent article of the Journal , Yu et al .1 reported their experimental results on the effect of Al2 O3 and Bi2 O3 on the formation mechanism of Sn-doped Ba2 Ti9 O20 . They claimed that both Al2 O3 and Bi2 O3 can dramatically assist the formation of Sn-doped Ba2 Ti9 O20 but are based on different mechanisms. They concluded that first, Bi2 O3 melts above 830°C and accelerates the migration of the involved reactants to form Ba2 Ti9 O20 ; second, Al2 O3 can reduce the height of the potential energy barrier of the formation of Ba2 Ti9 O20 due to the intergrowth of BaAl2 Ti6 O16 phase. They explained their results from a point of view that the formation of Ba2 Ti9 O20 is controlled by (1) the migration of reactants to the interfaces and (2) the height of the potential-energy barrier of the reaction at the interfaces. However, based on their results, we feel their conclusions are incautious and may be misleading, as will be discussed later. 相似文献
11.
Jiaqing Yu Hangwei Zhao Jiasheng Wang Fei Xia 《Journal of the American Ceramic Society》1994,77(4):1052-1056
High-performance Ba2 Ti9 O20 ceramics are attracting great attention, but their formation mechanism still is somewhat unclear. The present investigation shows that the formation of Ba2 Ti9 O20 can be promoted strikingly by the participation of Bi2 O3 and Al2 O3 . The effect of Bi2 O3 on the formation of Ba2 Ti9 O20 is attributed to the fact that migration of the involved reactants is accelerated by liquid which forms from the melting of Bi2 O3 above 830°C. This migration, however, is not the only rate-limiting factor. A high potential-energy barrier, resulting from stress that arises along the crystal-structured layers, also heavily restricts the formation of Ba2 Ti9 O20 . The participation of Al2 O3 , on the other hand, can reduce the height of this potential-energy barrier and effectively improve the kinetics of the formation of Ba2 Ti9 O20 by causing the formation of BaAI2 Ti6 O16 crystals; these crystals intergrow with Ba2 Ti9 O20 crystals and result in decreased stress. 相似文献
12.
Stephan Senz reas Graff Werner Blum Dietrich Hesse Hans-Peter Abicht 《Journal of the American Ceramic Society》1998,81(5):1317-1321
Fresnoite grows at 700° and 800°C, and Ba6 Ti7 O40 grows at 1200°C with definite orientations, which are determined by X-ray diffraction pole figure analysis. Partially textured fresnoite is formed at higher temperatures. The SiO2 films react with the BaTiO3 crystals, forming the phases Ba2 TiSi2 O8 (fresnoite) and Ba6 Ti17 O40 . At 700° and 800°C, both phases grow with definite orientations, which are determined by X-ray diffraction pole figure analysis. Partially textured polycrystalline phases are formed at higher temperatures. 相似文献
13.
An investigation of the ternary systems BaO-TiO2 -SnO2 and BaO-TiO2 -ZrO2 led to the discovery of two new compounds belonging to the system BaO-TiO2 . These compounds, Ba2 Ti9 -O20 and Ba2 Ti9 O20 , are stabilized by minute additions of SnO2 or ZrO2 . The known compound BaTi2 O5 can be obtained only from the molten phase and decomposes below 1300°C. into Ba2 Ti5 O12 and BaTiO2 . In these systems no ternary compounds are found. The ternary phase diagrams can be divided into regions with high and low dielectric losses, which are in accordance with the phase relations. Tables with crystallographic data of the new compounds are included. 相似文献
14.
Pure Ba2 Ti9 O20 (BT29) was synthesized by a solid-state reaction in one step with various amounts of ZrO2 powder additive. The transformation kinetics of BT29 were investigated by quantitative X-ray diffractometry (XRD). The results show that stoichiometric powder mixtures transform to the BT29 phase by nucleation and growth mechanism between 1200° and 1300°C with 1.0 mol% ZrO2. The activation energy of the transformation was found to be 620±60 kJ/mol, but decreases to 515±30 kJ/mol when doped with 1.0 mol% ZrO2 . The addition of ZrO2 possibly changes the phase transformation mechanism of BT29 from diffusion controlled to interface controlled. 相似文献
15.
H. M. O'BRYAN W. H. GRODKIEWICZ J. L. BERNSTEIN 《Journal of the American Ceramic Society》1980,63(5-6):309-310
Ba2 Ti9 O20 crystallizes in the monoclinic system with α= l.4818(5) nm, b = 1.4283(6), and c = 0.7109(2) with β = 98.37°±0.07°. The most likely space group is P 21 / m , Z = 4 with a calculated density 4.58 g/cm3 . The powder pattern was indexed. The Ba2 Ti9 O20 crystals form as stellated groups when melts of BaCl2 + 20 to 50% TiO2 cool from 1275°C. 相似文献
16.
Ba6−3 x Nd8+2 x Ti18 O54 ceramic powders were synthesized by the modified Pechini method using ethylenediaminetetraacetic acid (EDTA) as a chelating agent. A purplish red, molecular-level, homogeneously mixed gel was prepared, and transferred into a porous resin intermediate through charring. Single-phase and well-crystallized Ba6−3 x Nd8+2 x Ti18 O54 powders were obtained from pulverized resin at a temperature of 900°C for 3 h, without formation of any intermediate phases. Meanwhile, the molar ratio of EDTA to total metal cation concentration had a significant influence on the crystallization behavior of Ba6−3 x Nd8+2 x Ti18 O54 . The Ba6−3 x Nd8+2 x Ti18 O54 ( x = 2/3) ceramics prepared via EDTA precursor have excellent microwave dielectric characteristics: ɛ= 87, Qf = 8710 GHz. 相似文献
17.
J. K. PLOURDE D. F. LINN H. M. O'BRYAN Jr .JOHN THOMSON Jr . 《Journal of the American Ceramic Society》1975,58(9-10):418-420
Microwave measurements of Ba2 Ti9 O20 show that this ceramic is uniquely suited for dielectric resonators. (Suitable ceramics should have a high dielectric constant K , a low dielectric loss (high Q ), and a low temperature coefficient of resonant frequency, τ.) At 4 GHz, Ba2 Ti9 O20 resonators have Q >8000, K = 39.8, and τ=2 ppm/°C. Measurements of Q and τ were made on unmetallized ceramic resonator disks positioned in a waveguide; K was measured using a dielectric post resonator technique. From 4 to 10 GHz, Q approaches that for a copper waveguide cavity, whereas the temperature coefficient is typically 8 times lower. 相似文献
18.
Pazhoor V. Bijumon Yahia M. M. Antar Alois P. Freundorfer Michael Sayer 《International Journal of Applied Ceramic Technology》2008,5(4):382-393
This paper describes the synthesis of Ba2 Ti9 O20 ceramics at extremely low temperatures (∼150°C) and the subsequent growth of bulk resonators on silicon substrates by hydrothermal processing of their sol–gel composites. X-ray diffraction analysis shows excellent crystallinity, while scanning electron microscopy evidenced densification and development of bridging structures at the grain boundaries and interfaces. Transmission electron micrographs further confirmed the development of sol–gel-derived crystalline interfaces between sol–gel-derived material and powder particles. The dielectric properties of a resonator measured in the 5–6 GHz microwave frequency range were ɛr =38 and Q u × f =12,000 at 5.6 GHz and τf =+6 ppm/°C. The density, dielectric properties, and mechanical strength at the bulk ceramic–thin film interface are enhanced by a hydrothermally induced dissolution–crystallization process, which leads to interparticle bridges. The novel low-temperature ceramic process has high potential for the growth of ceramic resonators on integrated circuits and is demonstrated by the fabrication of an integrated dielectric resonator antenna for system-on-chip applications. 相似文献
19.
The system BaO-TiO2 was investigated using quenching, strip-furnace, and thermal techniques. Five compounds were found to exist in the system: Ba2 TiO4 , BaTiO3 , BaTi2 O5 , BaTi3 O7 , and BaTi4 O9 . Of these, only barium metatitanate (BaTiO3 ) melts congruently (at 1618°C.). The dititanate melts incongruently at 1322° C. to yield BaTiO3 and liquid; the trititanate melts at 1357°C. to yield BaTi4 O9 and liquid; the tetra-titanate melts to TiO2 and liquid at 1428° C. The nature of melting of the orthotitanate could not be determined accurately because of the high temperature involved and the rapid reaction with platinum. The two eutectics in the system occur between Ba2 TiO4 and BaTiO3 at 1563°C. and between BaTi2 O5 and BaTi3 O7 at 1317°C. The temperature of the cubic-hexagonal transition in barium metatitanate was determined as 1460°C. and the transition has been shown to be reversible. The transition temperature is raised sharply by the addition of a small percentage of TiO2 although the extent of solid solution is quite limited. Some applications to the manufacture of titanate bodies and to the growth of single crystals of barium metatitanate are discussed. 相似文献
20.
Susumu Nishigaki Shinsuke Yano Hiroshi Kato Takami Hirai Toshio Nonomura 《Journal of the American Ceramic Society》1988,71(1):11-C-
Microwave ceramic resonators composed of BaO-TiO2 -WO3 were developed. The effect of WO3 addition on the system of BaO·xTiO2 ·(1+x)yWO3 (x=4 and 4.5, y=0 to 0.04) was studied. The ceramics of this system are composed of crystallines including Ba2 Ti9 O20 , BaTi4 O9 , BaWO4 , and TiO2 . At y=0.02, the BaO·4TiO2 ·0.1WO3 ceramic was found to have excellent microwave properties such as ε=35, Q=8400 at 6 GHz, and nearly 0 ppm/°C of τf . 相似文献