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1.
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
2.
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
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.
BaTi4 O9 and Ba2 Ti9 O20 precursors were prepared via a sol–gel method, using ethylenediaminetetraacetic acid as a chelating agent. The sol–gel precursors were heated at 700°–1200°C in air, and X-ray diffractometry (XRD) was used to determine the phase transformations as a function of temperature. Single-phase BaTi4 O9 could not be obtained, even after heating the precursors at 1200°C for 2 h, whereas single-phase Ba2 Ti9 O20 (as determined via XRD) was obtained at 1200°C for 2 h. Details of the synthesis and characterization of the resultant products have been given. 相似文献
5.
EKKEHART TILLMANNS WOLFGANG HOFMEISTER WERNER H. BAUR 《Journal of the American Ceramic Society》1983,66(4):268-271
A single-crystal X-ray study of dibarium nonatitanate, Ba2 Ti9 O20 , yielded the triclinic space group P 1 with a =0.7471(1), b= 1.4081(2), c= 1.4344(2) nm, α=89.94(2)°, β= 79.43(2)°, γ= 84.45(2)°, V = 1.476 nm3 Z = 4, and Dx = 4.61 Mg/m3 . A refinement of atomic coordinates and isotropic thermal parameters led to a residual of 0.03. The structure consists of hexagonally closest-packed layers of Ba and O atoms in the sequence (hch)3 . All Ti atoms reside in octahedral interstices of this closest packing. The various Ti coordination octahedra share only edges and corners with each other. One-half of the Ba atoms is twelve-coordinated by oxygen atoms, the other half is eleven-coordinated. 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.
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
8.
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
9.
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
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.
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
12.
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
13.
The orientational relationship between intergrown lamellae of tetragonal (t) BaTiO3 and monoclinic (m) Ba6 Ti17 O40 was found to be ( 1 1 1 )t ||(001)m , (001)t ||( 1 0 1 )m , and [110]t ||[0 1 0]m . On the basis of this relationship a model of a topotactic boundary between these two phases was constructed. 相似文献
14.
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
15.
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
16.
H. S. MAITI K. V. G. K. GOKHALE E. C. SUBBARAO 《Journal of the American Ceramic Society》1972,55(6):317-322
The monoclinic-tetragonal transformation in ZrO2 was studied by DTA. The reverse transformation occurs at 850° to 1000°C, depending on the crystallite size, which is a function of the experimental variables. Isothermal kinetics are exhibited by material containing small crystallites; athermal behavior begins to appear as the crystallite size increases. On cooling, strain-free samples undergo a very rapid transformation which is evident as a burst. 相似文献
17.
Steven A. Markgraf Shiv K. Sharma Amar S. Bhalla 《Journal of the American Ceramic Society》1992,75(9):2630-2632
Raman spectra are reported for fresnoite (Ba2 Ti(Si,Ge)2 O8 glasses, and comparison is made between the Raman spectra of the corresponding crystalline powders and glasses of Ba2 TiSi2 O8 and Ba2 TiGe2 O8 . The Ba2 TiGe2 O8 glass spectra show correspondence with the Ba2 TiGe2 O8 crystalline Raman spectra; the v s (Ge–O–Ge) mode occurs at 518 cm−1 in the glass and at 521 cm−1 in the crystalline material. Five-fold coordinated titanium is the majority species present in the Ba2 TiGe2 O8 glass as revealed by a strong band at 824 cm−1 in the I glass spectrum. The Ba2 TiSi2 O8 glass spectra are similar to the Ba2 TiSi2 O8 crystalline spectrum; the strongest band is found at 836 cm−1 in the I glass spectrum. Through comparison with the previous Raman data of other titania silicate glasses, we conclude that the Ba2 TiSi2 O8 glass has a structure similar to the crystalline phase. 相似文献
18.
Fred F. Lange Burt I. Davis Douglas O. Raleigh 《Journal of the American Ceramic Society》1983,66(3):50-C-
The solid sodium electrolyte β"-Al2 O3 (Li-stabilized) was strengthened with additions of tetragonal ZrO2 (15 vol%). The conductivity of this composite material, measured in an Na/Na cell, was 7.7 Ω· at 300°C. Average values of strength and the critical stress intensity factor were 350 MPa and 4.5 MPa·m1/2 , respectively, for the sintered composite material. 相似文献
19.
A TEM investigation was conducted on the structure of a second phase precipitated between the grains of a polycrystalline TiO2 -rich BaTiO3 which was doped with 8 mol% Ca. This phase was identified as Ca-stabilized Ba2 Ti5 O12 with a 10-layer orthorhombic structure and unit-cell parameters a=0.990, b=1.131, and c=2.330 nm. 相似文献
20.
La2 Ti2 O7 powders were prepared using three different techniques. Single-phase material was obtained at 1150°C by calcination of mixed oxides, at 1000°C by molten salt synthesis, and at 850°C by evaporative decomposition of solutions. Particle sizes and morphologies of the powders differed substantially, as did the sintered microstructures and dielectric properties. Very dense (99%), translucent, grain-oriented lanthanum titanate was fabricated by hot-forging at 1300°C under a 200-kg load. Anisotropy was demonstrated by X-ray diffraction, scanning electron microscopy, thermal expansion, and dielectric measurements. 相似文献