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1.
The synthesis of 17 O-enriched α-tellurite (α-TeO2 ) is described. TeO2 is obtained stochiometrically by the reaction of tellurium isopropoxide with 17 O-enriched H2 O. The reaction is mild and simple and opens the field of tellurite-based ceramics to 17 O nuclear magnetic resonance spectroscopy. We also report the first 17 O nuclear magnetic resonance spectra of α-TeO2 and Na2 TeO3 . Their isotropic chemical shifts, δiso , are 180 ppm and 158 ppm, respectively, and their quadrupolar coupling parameters are CQ =7.48 MHz and η=0.43 for α-TeO2 , and CQ =6.63 MHz and η=0.33. By using extended Hückel tight binding calculations, we interpret the chemical shifts of tetravalent inorganic oxide network formers (SiO2 , GeO2 , and TeO2 ) in terms of the local paramagnetic shielding effect. We also show that this is the predominant effect in determining the chemical shifts of non-bridging oxide sites in this type of network. 相似文献
2.
Yu Jia Yuji Hotta Kimiyasu Sato Koji Watari 《Journal of the American Ceramic Society》2006,89(3):1103-1106
ZrO2 –Al2 O3 nanocomposite particles were synthesized by coating nano-ZrO2 particles on the surface of Al2 O3 particles via the layer-by-layer (LBL) method. Polyacrylic acid (PAA) adsorption successfully modified the Al2 O3 surface charge. Multilayer coating was successfully implemented, which was characterized by ξ potential, particle size. X-ray diffraction patterns showed that the content of ZrO2 in the final powders could be well controlled by the LBL method. The powders coated with three layers of nano-ZrO2 particles, which contained about 12 wt% ZrO2 , were compacted by dry press and cold isostatically pressed methods. After sintering the compact at 1450°C for 2 h under atmosphere, a sintered body with a low pore microstructure was obtained. Scanning electron microscopy micrographs of the sintered body indicated that ZrO2 was well dispersed in the Al2 O3 matrix. 相似文献
3.
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
4.
Liang Fang Heping Su Qian Yu Hui Zhang Bolin Wu 《Journal of the American Ceramic Society》2008,91(8):2769-2771
A high dielectric constant and low-loss ceramic with composition Sr4 LaTiNb3 O15 has been prepared by the conventional solid-state ceramic route. This compound adopts an A5 B4 O15 cation-deficient hexagonal perovskite structure and crystallizes in the trigonal system with unit cell parameters a =5.6307(2), c =11.3692(3) Å, V =312.16(2) Å3 , and Z =1. The dielectric properties of dense ceramics sintered in air at 1460°C have been characterized at microwave frequencies. The results show that the material affords a relatively high dielectric constant ɛr ∼43, a high quality factor Q × f ∼44 718 GHz, and a low temperature coefficient of resonant frequency TCf ∼13 ppm/°C. 相似文献
5.
(Li1/2 Nd1/2 )2+ substitution into the A site and (Mg1/3 Ta2/3 )4+ substitution into the B site of CaTiO3 ceramic were investigated, respectively. The modified CaTiO3 dielectric ceramics prepared by conventional solid-state method exhibit single perovskite structure and improved dielectric properties. Optimal microwave dielectric properties of ɛr =112.6, Q × f =4480 GHz, τf =8.2 ppm/°C in [Ca0.4 (Li1/2 Nd1/2 )0.6 ] TiO3 and ɛr =60.2, Q × f =36900 GHz, τf =−10.1 ppm/°C in Ca[Ti0.4 (Mg1/3 Ta2/3 )0.6 ] O3 are obtained, which indicates their potential for microwave application. The effects of change of crystal structure on dielectric properties are also discussed. 相似文献
6.
Dispersion of Alumina and Silicon Carbide Powders in Alumina Sol 总被引:1,自引:0,他引:1
Dispersion of Al2 O3 and SiC particles in an alumina sol has been investigated through determination of particle-size distribution, zeta potential, and agglomerate morphology. The particle size of Al2 O3 and SiC (as determined by the particle-size analyzer) is strongly affected by the presence of alumina sol in the solution. The average agglomerate size is decreased by at least 50%. The zeta potential of Al2 O3 in 1 M alumina sol increases slightly, whereas that of SiC reverses its sign over a wide range of pH values. It is proposed that these effects are caused by AlO4 Al12 (OH)24 (H2 O)7+ 12 sol clusters (1-2 nm in size) that are absorbed on the surface of the large (1-5 µm) ceramic particles. The electrostatic and steric effects of the cluster absorption help to control the dispersion and stabilize the suspension of ceramic particles in the alumina sol during wet processing. It is expected that the alumina-sol clusters can be used as an efficient, clean dispersant for single-phase and composite ceramic powders. 相似文献
7.
Sridhar Komarneni Colin A. Fyfe Gordon J. Kennedy Harold Strobl 《Journal of the American Ceramic Society》1986,69(3):45-C-
Magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy (27 Al) detected the coordination of Al in clays containing as little as 0.26% Al2 O3 . The 29 Si MASNMR of fluorphlogopite showed three distinct Si chemical environments which suggested short-range ordering. Synthetic laponite and mica-montmorillonite showed broad 29 Si resonances indicative of short-range disorder. A saponite showed four 29 Si resonances. Considerable insight into the short-range ordering of clays can be gained by 27 Al and 29 Si MASNMR. 相似文献
8.
Subhash H. Risbud R. James Kirkpatrick Antoni P. Taglialavore B. Montez 《Journal of the American Ceramic Society》1987,70(1):10-C-
The structures of roller-quenched SiO2 -Al2 O3 glasses containing 10 to 50 wt% Al2 O3 were investigated by TEM and high-resolution 27 Al and 29 Si magic angle sample spinning nuclear magnetic resonance (MASS-NMR) spectroscopy. The chemical shifts observed in the NMR spectra of these phase-separated (on the TEM scale) glasses provide evidence, for the existence of all three (4-, 5-, and 6-jold) Al-coordination units in these glasses and for the presence of A1 in both the Si-rich and Si-poor phases. 相似文献
9.
Chunfeng Hu Fangzhi Li Lingfeng He Mingyue Liu Jie Zhang Jiemin Wang Yiwang Bao Jingyang Wang Yanchun Zhou 《Journal of the American Ceramic Society》2008,91(7):2258-2263
In this work, a bulk Nb4 AlC3 ceramic was prepared by an in situ reaction/hot pressing method using Nb, Al, and C as the starting materials. The reaction path, microstructure, physical, and mechanical properties of Nb4 AlC3 were systematically investigated. The thermal expansion coefficient was determined as 7.2 × 10−6 K−1 in the temperature range of 200°–1100°C. The thermal conductivity of Nb4 AlC3 increased from 13.5 W·(m·K)−1 at room temperature to 21.2 W·(m·K)−1 at 1227°C, and the electrical conductivity decreased from 3.35 × 106 to 1.13 × 106 Ω−1 ·m−1 in a temperature range of 5–300 K. Nb4 AlC3 possessed a low hardness of 2.6 GPa, high flexural strength of 346 MPa, and high fracture toughness of 7.1 MPa·m1/2 . Most significantly, Nb4 AlC3 could retain high modulus and strength up to very high temperatures. The Young's modulus at 1580°C was 241 GPa (79% of that at room temperature), and the flexural strength could retain the ambient strength value without any degradation up to the maximum measured temperature of 1400°C. 相似文献
10.
There are few effective methods for characterizing the molecular scale structural environments of Ca2+ in hydrated cements, which has limited our ability to understand the structure of, for example, Ca–silicate hydrate (C–S–H). 43 Ca nuclear magnetic resonance (NMR) spectroscopy has long been considered too insensitive to provide useful data in this regard, but 43 Ca magic angle spinning (MAS) NMR spectra reported here for synthetic tobermorite and jennite with naturally abundant levels of 43 Ca demonstrate that this is a viable approach. We show that spectra with useful signal/noise ratios can be obtained in a reasonable acquisition period (∼2 days) using an H0 field strength of 21.1 T, 5 mm rotors spinning at a frequency of 5 kHz, and a double frequency sweep preparatory pulse sequence. Tobermorite and jennite produce relatively broad resonances due to their complex structures and structural disorder, however, the chemical shift differences between six-coordinate 43 Ca in jennite and seven-coordinate 43 Ca in 11 Å tobermorite are large enough that the signals are entirely resolved at this field. These data suggest that signal from ideal tobermorite-like and jennite-like sites in cement C–S–H can most likely be distinguished by 43 Ca NMR and that this method will be a powerful approach for studying cement-based ceramic materials in the coming decade. 相似文献
11.
Using a novel combustion method, Eu-doped Eu:yttrium aluminum garnet (YAG) and Eu:YSAG powders, and transparent Eu:YSAG ceramics were fabricated. The optical properties of these transparent ceramics have been measured, and a reduced peak splitting of Eu3+ for 5 D0 →7 F1 and 5 D0 →7 F2 was observed when 10 at.% Al3+ was substituted by Sc3+ . The enhanced symmetry of the Eu sites in YAG lattice, which resulted from the expanded YSAG lattice by Sc3+ doping, is the main reason for the reduced peak splitting. 相似文献
12.
Synthesis of BaCu(B2 O5 ) Ceramics and their Effect on the Sintering Temperature and Microwave Dielectric Properties of Ba(Zn1/3 Nb2/3 )O3 Ceramics 总被引:1,自引:0,他引:1
Min-Han Kim Jong-Bong Lim Jae-Chul Kim Sahn Nahm Jong-Hoo Paik Jong-Hee Kim Kyeong-Soon Park 《Journal of the American Ceramic Society》2006,89(10):3124-3128
BaCu(B2 O5 ) ceramics were synthesized and their microwave dielectric properties were investigated. BaCu(B2 O5 ) phase was formed at 700°C and melted above 850°C. The BaCu(B2 O5 ) ceramic sintered at 810°C had a dielectric constant (ɛr ) of 7.4, a quality factor ( Q × f ) of 50 000 GHz and a temperature coefficient of resonance frequency (τf ) of −32 ppm/°C. As the BaCu(B2 O5 ) ceramic had a low melting temperature and good microwave dielectric properties, it can be used as a low-temperature sintering aid for microwave dielectric materials for low temperature co-fired ceramic application. When BaCu(B2 O5 ) was added to the Ba(Zn1/3 Nb2/3 )O3 (BZN) ceramic, BZN ceramics were well sintered even at 850°C. BaCu(B2 O5 ) existed as a liquid phase during the sintering and assisted the densification of the BZN ceramic. Good microwave dielectric properties of Q × f =16 000 GHz, ɛr =35, and τf =22.1 ppm/°C were obtained for the BZN+6.0 mol% BaCu(B2 O5 ) ceramic sintered at 875°C for 2 h. 相似文献
13.
Quadrupole interactions of 11 B and 27 Al in SiO2 -B2 O3 -Al2 O3 -R2 O glass systems were investigated to determine the structure of these glasses, which should be amenable to chemical strengthening. The ratio of BO4 units to BO3 units approached unity as the R2 O/Al2 O3 ratio for compounds having fixed B2 O3 contents approached unity. Nuclear quadrupole coupling constants ( e2 Qq/h =2.73 to 2.93 MHz) were measured for the NMR spectra of 11 B triangles. The line shapes of 27 Al spectra varied with chemical composition, but a few glasses exhibited 27 Al line shapes similar to those of the AlO4 triclusters in SiO2 -Al2 O3 -Na2 O glasses. Compositional trends in the formation of BO4 and AlO4 were deduced from the NMR spectra. 相似文献
14.
Zhaohui Wang Egil Skybakmoen Tor Grande 《Journal of the American Ceramic Society》2009,92(6):1296-1302
A thorough analysis of a silicon nitride (Si3 N4 )-bonded SiC sidelining material from a Hall-Heroult electrolysis cell is reported. Phase composition before and after chemical degradation of the material is obtained by quantitative analysis using Rietveld refinement of X-ray diffraction data and chemical analysis. The main degradation products as a result of the oxidation of Si3 N4 binder phase are Si2 ON2 in the upper part and Na2 SiO3 in the lower part of the sidelining. The microstructure of α-Si3 N4 (needle) and β-Si3 N4 (shell) as well as the degradation products Si2 ON2 (fiber) and Na2 SiO3 (flake) were revealed by electron microprobe analysis. Chemical reactions and degradation mechanisms are proposed based on the presented findings. The degradation in the lower part is more severe than that in the upper part because Na diffusion from the cathode enhances the oxidation of Si3 N4 . The degradation changes the physical properties of Si3 N4 -bonded SiC such as density and porosity. 相似文献
15.
Kazumi Kato Can Zheng Jeffrey M. Finder Sandwip K. Dey Yasuyoshi Torii 《Journal of the American Ceramic Society》1998,81(7):1869-1875
Precursors for layer-structured perovskite thin films of SrBi2 Ta2 O9 (SBT) and SrBi2 Nb2 O9 (SBN) were prepared by the reactions of a strontium-bismuth double methoxyethoxide and tantalum or niobium methoxyethoxide in methoxyethanol, followed by partial hydrolysis. Several spectroscopic techniques, such as 1 H-, 13 C-, and 93 Nb-NMR (nuclear magnetic resonance), and Fourier-transform infrared spectroscopy were used to analyze the arrangement of the metals and oxygen in the precursor molecules. The precursors contained Sr-O-M (where M is Ta or Nb) bonds (i.e., a strontium is connected to two MO6 octahedra) and Sr-O-Bi bonds with a bismuth atom bonded to the oxygens of the MO6 octahedron. The arrangement of metals and oxygens was considered to be similar to the layer-structured perovskite crystal sublattice. As a result, the sol-gel-derived SBT thin films crystallized, by rapid thermal annealing in an oxygen atmosphere below 550°C, and they exhibited preferred (115) orientation. The crystallinity improved and the crystallite size increased with temperature up to 700°C. In the case of SBN thin films, a low heating rate (2°C/min) was necessary for the control of the crystallographic (115) orientation, whereas a rate of 200°C/s (rapid thermal annealing) produced films that exhibited c -axis orientation. The (115) SBT thin film, heated to 700°C, exhibited improved ferroelectric properties. 相似文献
16.
Zaine Teixeira Oswaldo Luiz Alves Italo Odone Mazali 《Journal of the American Ceramic Society》2007,90(1):256-263
The FTIR, Raman, UV-Vis, 31 P MAS-NMR, DTA, and refractive index measurements have been combined to investigate a series of glasses with the general formula 20Na2 O–5Al2 O3 − x TiO2 –(45− x )Nb2 O5 –30P2 O5 , 15≤ x ≤45. The glass structure, as well as thermal, optical, and chemical durability properties, were then described as functions of the f Nb / f Ti ratio. An increase of the f Nb / f Ti ratio correlates with a decrease in length of the average phosphate chains linked through Nb–O–P and Ti–O–P bonds, with an increase in the glass stability and with increase in the linear refractive indices at 632.8 nm from 1.79 to 1.89. Furthermore, niobium is more effective than titanium in improving chemical durability. 相似文献
17.
Paul J. Schilling Leslie G. Butler Amitava Roy Harvill C. Eaton 《Journal of the American Ceramic Society》1994,77(9):2363-2368
b29 Si and 27 Al MAS-NMR were performed on NaOH-activated blast-furnace slag to better characterize the amorphous and poorly crystalline phases which occur in this system. The unreacted glass has a mainly dimeric silicate structure represented by a broad 29 Si peak (FWHM = 15 ppm) centered at –74.5 ppm [ Q 1 ], with aluminum present exclusively in tetrahedral coordination. Upon reaction with 5M NaOH ( w/s = 0.4), three new 29 Si peaks with widths of ca. 2 ppm are formed at -78.5 Q 1 , –81.4 [ Q 2 (1Al)J, and -84.3 [ Q 2 ]. Relative peak areas indicate a mostly dimeric silicate structure for the tobermorite-like C─S─H layers, with roughly a third of the bridging sites occupied by aluminum, and less than 10% by silicon. In addition to the tetrahedrally coordinated aluminum substituted in the C─S─H structure, 27 Al MAS-NMR reveals the presence of aluminum in octahedral sites, which is attributed to the aluminate phase (C,M)4 AH13 . 29 Si results indicate rapid initial consumption of the glass, with roughly a third of the glass reacting within the first day and another third consumed over the following 27 days. 相似文献
18.
The electrical properties of Sr0.5 Ba0.3 TiO3 in the presence of Nb2 O5 as a donor, 3Li2 O · 2SiO2 as a sintering agent, and Bi2 O3 as a dopant have been studied. When the compositions of the ceramics were 1 mol Sr0.7 Ba0.3 TiO3 + 0.5 mol% Nb2 O5 + 2 mol% 3Li2 O · 2SiO2 + 0.2 mol% Bi2 O3 , the ceramics were sintered at 1100°C and exhibited the following characteristics: apparent dielectric constant ɛ, 25000; loss factor tan δ, 2%; insulating resistivity ρj , 1010 Ω· cm; variation of dielectric constant with temperature Δɛ/ɛ (−25° to +85°C), +10%, −14%. ɛ and tan δ show only small changes with frequency. The study shows this ceramic can be used in multilayer technology. 相似文献
19.
Richard K. Brow R. James Kirkpatrick Gary L. Turner 《Journal of the American Ceramic Society》1993,76(4):919-928
We have used 27 Al and 31 P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to describe quantitatively the relationships between the composition, structure, and properties of glasses in the Na2 O.Al2 O3 .P2 O5 (NAP) system. In general, the glass properties (evaluated in part I) are most sensitive to changes in Al coordination. 27 Al MAS NMR spectra reveal that octahedrally coordinated Al is most abundant in glasses with O/P ratios less than 3.5, the pyrophosphate structural limit. Tetrahedrally coordinated Al is most abundant in glasses with O/P greater than 3.5. Decreasing Al(OP)6 /Al(OP)4 ratio generally correlates with decreasing glass transition temperature and refractive index. The compositional dependence of glass structure and properties can be qualitatively understood using a crystal chemical model based on oxygen charge balancing by the different Al and P moieties. 相似文献
20.
It was shown that rapid melt quenching can extend the glass-forming region of a ZrF4 –BaF2 –YF3 –AlF3 composition considerably. Cooling at a rate of ∼102 K · s−1 prevented glass formation at compositions less than about 25 mol% BaF2 , whereas quenching at a rate of ∼2×106 K · s−1 resulted in glasses with a BaF2 content as low as about 13 mol%. This high quenching rate was achieved by a specially designed twin-roller apparatus. 相似文献