共查询到20条相似文献,搜索用时 31 毫秒
1.
Maria P. Seabra Maxim Avdeev Victor M. Ferreira Rob C. Pullar Neil McN. Alford Ian M. Reaney 《Journal of the American Ceramic Society》2004,87(4):584-590
The microwave dielectric properties and microstructures of compounds in the solid solution series x BaTiO3 –(1− x )La(Mg1/2 Ti1/2 )O3 (BTLMT) have been investigated. The structural phase transitions that occur as a function of x have been studied and are related to changes in the dielectric properties. For compounds where x ≤ 0.1, X-ray diffraction (XRD) showed evidence of 1:1 ordering between Mg and Ti cations. For x ≤ 0.3, XRD and electron diffraction revealed that compounds were tilted in both antiphase and in-phase. However, for 0.3 < x < 0.7, only antiphase tilting was present. The temperature coefficient of resonant frequency (τf ) vs the relative permittivity (ɛr ) was linear until x = 0.5 at which point in the solid solution the transition to a nontilted structure resulted in nonlinear behavior. τf values close to zero (−2 ppm/°C) were achieved at x = 0.5 (ɛr ∼ 60), which had a quality factor ( Q · f o ) of 9600 GHz. 相似文献
2.
The effects of Si and ZrO2 dopants on the crystallization and phase transformation process in Li2 O · Al2 O3 · 6SiO2 glasses were investigated using differential thermal analysis, X-ray powder diffractometry (XRD), and high-resolution transmission electron microscopy (TEM) interactively. Phase separation was observed in the studied glasses prior to substantial crystallization. Elemental Si (1 mol%) significantly aided in glass devitrification. Dropletlike phase-separated regions in the as-quenched or heat-treated glass devitrified at ∼760°C, which in turn provided sites for the heterogeneous nucleation and growth of β-quartz(ss) (solid solution), which transformed to β-spodumene(ss) at higher temperature. Low-temperature surface crystallization in these glasses occurred as low as 760°C. ZrO2 has limited solubility in this glass system. Small ZrO2 crystallites (·5 nm) in the as-quenched glass acted as sites for the heterogeneous nucleation and subsequent growth of large (<5 μm) β-quartz(ss) crystals in glasses containing 1.0 mol% or more ZrO2 . The transformation from β-quartz(ss) to β-spodumene(ss) was increasingly inhibited with ZrO2 additions. The nucleating efficiency of Si was significantly greater than that of ZrO2 in this glass system. 相似文献
3.
Feridoon Azough Wenjin Wang Robert Freer 《Journal of the American Ceramic Society》2009,92(9):2093-2098
LaAlO3 -stabilized La2/3 TiO3 (LT) ceramics were prepared by the conventional mixed oxide route. Small amounts of manganese oxide were added to eliminate Ti4+ reduction. The powders were calcined at 1150°C and sintered at 1400°–1500°C for 4 h and cooled at rates of 900°–15°C/h. The products were high density and single phase, with an average grain size of 6 μm. The LaAlO3 -stabilized LT ceramics exhibited a relative permittivity (ɛr ) of 64, a positive temperature coefficient of resonant frequency (τf ) of 84, and dielectric Q value × resonant frequency ( Q × f ) values of 16 400 GHz. The crystal structure and microstructures have been investigated using high-resolution transmission electron microscopy (HRTEM) in conjunction with X-ray diffraction (XRD). One candidate crystal structure, a ≈2 a p (where a p is the lattice parameter of the high-temperature form of the cubic perovskite), b ≈2 a p , and c ≈2 a p with a space group Cmmm (65), has been confirmed by XRD, electron diffraction, and lattice imaging techniques. Microtwins, with twin boundaries parallel to the {100} planes, were observed in the microstructures. 相似文献
4.
Anal Tarafder Kalyandurg Annapurna Reenamoni Saikia Chaliha Vidya Sagar Tiwari Pradeep Kumar Gupta Basudeb Karmakar 《Journal of the American Ceramic Society》2009,92(9):1934-1939
We report here the processing and properties of transparent glass and glass–ceramic nanocomposites in the Li2 O–Ta2 O5 –SiO2 –Al2 O3 system in the presence of Eu2 O3 as luminescent probe. The formation of the LiTaO3 crystal phase, the crystallite size, and the morphology with the progression of heat treatment have been examined by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transformed infrared reflectance spectroscopy measurements. The crystallite sizes obtained from XRD and TEM are found to increase with heat-treatment time and vary in the range of 2–20 nm. The measured photoluminescence spectra exhibit emission transitions of 5 D0,1 →7 F j ( j =0, 1, 2, 3, and 4) of Eu3+ ions. From the nature of the emission transitions, the site symmetry in the vicinity of Eu3+ ions has been found to be near C3v in the glass–ceramic nanocomposites. An inverse correlation has been observed between the asymmetric ratio ( I ED / I MD ) of Eu3+ ions and the dielectric constant (ɛr ), with an increase in the heat-treatment time of glass, which is caused by the dipole–dipole interaction. 相似文献
5.
Ian MacLaren Roland Schierholz Paul A. Trusty Clive B. Ponton 《Journal of the American Ceramic Society》2007,90(10):3307-3310
Hot-pressed yttrium disilicate ceramics have been characterized using analytical transmission electron microscopy (TEM). The microstructure consists of large grains of the γ phase of stoichiometric γ-Y2 Si2 O7 containing rounded glassy Y-doped SiO2 inclusions; excess glassy SiO2 -rich material is also found at the grain boundaries. Two main reasons are found for the inhomogeneity: a slight SiO2 excess is inferred from the composition measurements, and the LiF flux used in hot pressing would also promote glass formation. Improved high-temperature mechanical properties would only be possible if residual glass formation was minimized, strategies for doing so are discussed, and the importance of analytical TEM for studying such submicron scale inhomogeneity is underlined. 相似文献
6.
Xue-Ning Huang Chu Kun Kuo Patrick S. Nicholson 《Journal of the American Ceramic Society》1995,78(4):892-896
An epitaxial β-alumina crystal growth method was used to modify α-AI2 O3 platelet surfaces before inclusion as a reinforcing phase in partially stabilized zirconia (3Y-TZP). The as-grown surface phase was Na-β"-AI2 O3 . This was converted to Ca-β"-AI2 O3 by ion exchange, as the latter is more temperature-stable at composite sintering temperatures. The conditions of formation, thermal stability, and chemical compatibility of these interfacial phases were examined. α-AI2 O3 platelets with Ca-β"-AI2 O3 film were incorporated into 3Y-TZP. The β"-AI2 O3 /ZrO2 interface was found to promote platelet debonding and pullout, thus enhancing the α-AI2 O3 platelet/crack interactions during the fracture process. 相似文献
7.
PAUL F. BECKER 《Journal of the American Ceramic Society》1981,64(1):37-39
The initial strength of (σi ) and thermal shock resistances (Δ Tc and σr /σi ), as determined by quench tests, of Al2 O3 -ZrO2 composites are increased by increasing amounts of tetragonal ZrO2 second phase for contents of up to ∼15 vol%. For composites with ≤9 vol% ZrO2 the increases in σr and Δ Tc reflect the increase in γIC with addition of ZrO2 However, for ZrO2 contents >9 vol%, the thermal shock resistances (Δ Tc and σr /σi ) and σi are also affected by machining-induced microcracking in the surface of the samples. For ZrO2 contents >14 vol%, bulk microcracking can become extensive and result in a degradation of σi and Δ Tc . 相似文献
8.
Kyoung R. Han Jin-Wook Jang Seo-Yong Cho Dae-Yong Jeong Kug-Sun Hong 《Journal of the American Ceramic Society》1998,81(5):1209-1214
Low-temperature-sinterable (Zr0.8 Sn0.2 )TiO4 powders were prepared using 3 mol% Zn(NO3 )2 additive and then compared with powders prepared using 3 mol% ZnO additive and no additives. Sintering at 1200°C for 2 h produced a dielectric ceramic with ρ= 98.6% of theoretical density (TD), ɛr = 38.4, Q × f (GHz) = 42000, and τ f =−1 ppm/°C. Sintering at 1250°C resulted in an excellent dielectric with ρ= 99% of TD, epsilonr = 40.9, Q × f (GHz) = 49000, and τ f =−2 ppm/°C. Scanning electron microscopy showed a microstructure with grains measuring 0.5 to 1 μm, and transmission electron microscopy revealed secondary phase in the grain boundaries. 相似文献
9.
Badri Rangarajan Tom Shrout Michael Lanagan 《Journal of the American Ceramic Society》2009,92(11):2642-2647
Nucleation and crystallization kinetics of fresnoite (Ba2 TiSi2 O8 ) crystals in BaO–TiO2 –SiO2 glasses have been explored for dielectric applications. The volume fractions crystallized at different temperatures and times were tracked by XRD analysis. The activation energy of crystallization was estimated from DTA results to be about 528 kJ/mol, which is consistent with the value obtained by XRD results. The Avrami parameter values calculated at different temperatures from DTA results were found to be between 3.2 and 3.9, indicating that the growth is three dimensional and the mechanism of growth is interface-controlled. Additionally, because of compositional similarities, the dielectric contrast between the glass (ɛr ∼15) and the resulting glass–ceramic (ɛr ∼18) was minimal. 相似文献
10.
Sergey Kucheiko Ji-Won Choi Hyun-Jai Kim Hyung-Jin Jung 《Journal of the American Ceramic Society》1996,79(10):2739-2743
The microwave dielectric properties of CaTi1-χ (Al1/2 Ta1/2 )cH O3 solid solutions (0.3 ≤χ≤ 0.5) have been investigated. The ceramic samples had perovskite structures similar to CaTiO3 . The partial substitution of Ti4+ by a coupled Al3+ /Tas+ permitted improvement of the quality factor Q . The dielectric constant (τr ) and temperature coefficient of resonant frequency (τr ) decrease rapidly with an increase of χ. A new high-quality microwave dielectric material was found at χ= 0.46 with σr = 46.5, Q f = 27300 GHz, and πf = 0 ppm/°C. The relationship between microstructures and dielectric properties is presented. 相似文献
11.
The BiVO4 additive was found effective for low-temperature firing of ZnNb2 O6 polycrystalline ceramics below 950°C in air without a serious degradation in their microwave dielectric properties. Dense BiVO4 -doped ZnNb2 O6 samples of a relative sintered density over 95% could be prepared even at 925°C. An optimally processed specimen exhibited excellent microwave dielectric properties of Q · f = 55000 GHz, ɛr = 26, and τf =−57 ppm/°C. With increasing BiVO4 addition up to 20 mol% relative to ZnNb2 O6 , while the quality factor Q · f was gradually decreased, the relative dielectric constant, ɛr , was linearly increased and the temperature coefficient of resonant frequency, τf , was slightly increased. The variations in Q · f and ɛr are surely attributable to the residual BiVO4 in the ZnNb2 O6 matrix. An unexpected slight increase in τf is probably due to the formation of the Bi(V,Nb)O4 -type solid solution. 相似文献
12.
A Zn2 Te3 O8 ceramic was investigated as a promising dielectric material for low-temperature co-fired ceramics (LTCC) applications. The Zn2 Te3 O8 ceramic was synthesized using the solid-state reaction method by sintering in the temperature range 540°–600°C. The structure and microstructure of the compounds were investigated using X-ray diffraction (XRD) and scanning electron microscopy methods. The dielectric properties of the ceramics were studied in the frequency range 4–6 GHz. The Zn2 Te3 O8 ceramic has a dielectric constant (ɛr ) of 16.2, a quality factor ( Q u × f ) of 66 000 at 4.97 GHz, and a temperature coefficient of resonant frequency (τf ) of −60 ppm/°C, respectively. Addition of 4 wt% TiO2 improved the τf to −8.7 ppm/°C with an ɛr of 19.3 and a Q u × f of 27 000 at 5.14 GHz when sintered at 650°C. The chemical reactivity of the Zn2 Te3 O8 ceramic with Ag and Al metal electrodes was also investigated. 相似文献
13.
Yu-Chen Lee Shaw-Bing Wen Liang Wenglin Chih-Peng Lin 《Journal of the American Ceramic Society》2007,90(6):1723-1727
An experimental study has been conducted to evaluate the formation of nano α-Al2 O3 under various conditions, such as different calcining temperatures and emulsion ratios of aqueous aluminum nitrate solutions and oleic acid with a high-speed stirring mixer. Four batches of the precursor powders were calcined at three different temperatures of 1000°, 1050°, and 1100°C for 2 h and a terminal product of nano α-Al2 O3 powders was obtained. The products have been identified by X-ray diffraction (XRD), specific surface area measurement scanning electron microscope, and transmission electron microscope (TEM). The XRD results show that the phase of powders is determined to be α-Al2 O3 , indicating that the overall process has been effective. The optimum calcination temperature of the precursor powder for crystallization of nano α-Al2 O3 was found to be 1000°C for 2 h. The TEM image indicates that the particle grains have a sub-spherical shape with a mean size of 50–100 nm. 相似文献
14.
Dong-Wan Kim Kyung Hyun Ko Do-Kyun Kwon Kug Sun Hong 《Journal of the American Ceramic Society》2002,85(5):1169-1172
(1− x )ZnNb2 O6 · x TiO2 ceramics were prepared using both anatase and rutile forms of TiO2 . At a composition of x = 0.58, a mixture region of ixiolite (ZnTiNb2 O8 ) and rutile was observed and the temperature coefficient of resonant frequency (τf ) was ∼0 ppm/°C. We found that although ɛr and τf were comparable, the quality factor ( Q × f , Q ≈ 1/ tan δ, f = resonant frequency) of 0.42ZnNb2 O6 ·0.58TiO2 prepared from anatase and rutile was 6000 and 29 000, respectively. The origin of the difference in Q × f of both samples was investigated by measuring electrical conductivity and by analysis of the anatase–rutile phase transition. The anatase-derived sample had higher conductivity, which was related to the reduction of Ti4+ . It is suggested that the increase of dielectric loss originates from an increase in Ti3+ and oxygen vacancies due to an anatase–rutile phase transition. 相似文献
15.
Wolfgang Winter Axel Berger Gerd Muller Wolfgang Pannhorst 《Journal of the American Ceramic Society》1993,76(7):1837-1843
MAS-osumilite is a metastable double-ring silicate which is attractive for use in glass-ceramics because of its low thermal expansion coefficient. The phase can be crystallized from glass powders of MAS-osumilite composition containing small amounts of BaO. The nucleation and crystallization process has been investigated by means of TEM and SEM. Crystallization starts with the precipitation of β-quartzss of composition MgAl2 O4 · n SiO2 . BaO concentrates in a residual glass phase between the β-quartzss crystals. In a second stage small amounts of Ba-osumilite (BaMg2 Al6 -Si9 O30 ) nucleate and crystallize from this glass phase, In a third stage BaO-free MAS-osumilite crystals are formed by epitactic growth on the Ba-osumilite surfaces and grow into adjacent β-quartzss grains. Two factors are essential for the suppression of undesired further phases to develop a monophase (besides some Ba-osumilite) MAS-osumilite glassceramic: a residual glass phase with a composition close to Ba-osumilite, and β-quartzss crystals having a composition close to that of MAS-osumite. 相似文献
16.
L. K. V. LOU T. E. MITCHELL A. H. HEUER 《Journal of the American Ceramic Society》1978,61(9-10):392-396
Impurity phases in commercial hot-pressed Si3 N4 were investigated using transmission electron microscopy. In addition to the dominant, β-Si3 N4 phase, small amounts of Si2 N2 O, SiC, and WC were found. Significantly, a continuous grain-boundary phase was observed in the ∼ 25 high-angle boundaries examined. This film is ∼ 10 Å thick between, β-Si3 N4 grains and ∼ 30 Å thick between Si2 N2 O and β-Si3 N4 grains. 相似文献
17.
Lamrath Abdul Khalam Sherin Thomas Mailadil T. Sebastian 《International Journal of Applied Ceramic Technology》2007,4(4):359-366
The columbites MgNb2 O6 , MgTa2 O6 , and corundum-type Mg4 Nb2 O9 ceramics were prepared by the conventional solid-state ceramic route. The structure and microstructure of the sintered samples were investigated by X-ray diffraction and scanning electron microscopic techniques. The microwave dielectric properties of the samples were measured by the resonance method in the frequency range 4–6 GHz. The dielectric properties have been tailored by forming a solid solution between MgNb2 O6 and MgTa2 O6 and by the substitution of TiO2 for Nb2 O5 in both MgNb2 O6 and Mg4 Nb2 O9 ceramics. The Mg(Nb0.7 Ta1.3 )O6 has ɛr =29, Q u × f =67 800 GHz, and τf =0.8 ppm/°C and the MgO–(0.4)Nb2 O5 –(1.5)TiO2 composition has ɛr =34.5, Q u × f =81 300 GHz, and τf =−2 ppm/°C. 相似文献
18.
Koji Tsukuma 《Journal of the American Ceramic Society》2000,83(12):3219-3221
Composites of β-Ce2 O3 ·11Al2 O3 and tetragonal ZrO2 were fabricated by a reductive atmosphere sintering of mixed powders of CeO2 , ZrO2 (2 mol% Y2 O3 ), and Al2 O3 . The composites had microstructures composed of elongated grains of β-Ce2 O3 ·11Al2 O3 in a Y-TZP matrix. The β-Ce2 O3 ·11Al2 O3 decomposed to α-Al2 O3 and CeO2 by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce2 O3 ·11Al2 O3 divided into several grains of α-Al2 O3 and ZrO2 doped with Y2 O3 and CeO2 . High-temperature bending strength of the oxygen-annealed α-Al2 O3 composite was comparable to the β-Ce2 O3 ·11Al2 O3 composite before annealing. 相似文献
19.
Three types of dicalcium silicate (Ca2 SiO4 –calcium zirconate (CaZrO3 ) composites were fabricated and their microstructures correlated with their mechanical properties. In the first type, Ca2 SiO4 was added as a minor phase. The second type consisted of a 50 vol% Ca2 SiO4 -50 vol% CaZrO3 mixture, while in the third type, CaZrO3 constituted the minor phase. Pure CaZrO3 was also studied as a control and found to have a toughness which depended on its grain size. In composites with Ca2 SiO4 as the minor phase, a toughness increase was observed and found to be a function of matrix grain size. The composite with the second type of microstructure had the highest toughness of about 4.0 Mpa. m1/2 , which was about double that of the monolithic CaZrO3 . No evidence was found for transformation toughening by the orthorhombic (β) to monoclinic (γ) transformation in Ca2 SiO4 . The main toughening mechanisms identified were crack deflection and crack branching. Microstructural observations indicated the existence of weak grain boundaries in CaZrO3 agglomerates as well as weak interfaces between the two phases. 相似文献
20.
Takashi Mizuguchi Shu-Qi Guo Yutaka Kagawa 《Journal of the American Ceramic Society》2009,92(5):1145-1148
Microstructure of the hot-pressed ZrB2 with MoSi2 additive was investigated by transmission electron microscopy (TEM). The effect of MoSi2 addition on the microstructure of the ceramic was assessed. For the pure ZrB2 , the microstructure consisted of the equiaxed ZrB2 grains and a few elongated ZrB2 grains. For the ZrB2 with MoSi2 additive, the microstructure consisted almost entirely of equiaxed ZrB2 grains. A few dislocations were present in the ZrB2 grains. In addition, high-resolution TEM observations showed that the intergranular amorphous phase was absent at two ZrB2 grain boundaries in the ZrB2 with MoSi2 additive. 相似文献