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1.
The diopside ceramics with a formula of Ca(Mg1−xAlx)(Si1−x/2Alx/2)2O6 (x=0.01–0.3) were synthesized via a traditional solid-state reaction method, and their solid solubility, sintering behavior and microwave dielectric properties were investigated. The results revealed that the solubility limit of Al2O3 in Ca(Mg1−xAlx)(Si1−x/2Alx/2)2O6, which is defined as x, was between 0.15 and 0.2, and a second phase of CaAl2SiO6 presented when the x value reached 0.2. Appropriate Al3+ substitution for Mg2+ and Si4+ could promote the sintering process and lower the densification temperature, and a broadened densification temperature range of 1250–1300 °C was obtained for the compositions of x=0.08–0.15. With the increase of the x value, the dielectric constant (εr) increased roughly linearly, and the temperature coefficient of frequency (τf) showed a rising trend. The Q×f values increased from 57,322 GHz to 59,772 GHz as the x value increased from 0.01 to 0.08, and then they were saturated in the range of x=0.08–0.2. Further increase of the x value (x≥0.25) deteriorated the microwave dielectric properties. Good microwave dielectric properties of εr=7.89, Q×f=59,772 GHz and τf=−42.12 ppm/°C were obtained for the ceramics with the composition of x=0.08 sintered at 1275 °C.  相似文献   

2.
The microwave dielectric properties of (BaxMg1−x)(A0.05Ti0.95)TiO3 (A=Zr, Sn) ceramics were investigated with regard to substitution of Ba for Mg of A-site. The microwave dielectric properties were correlated with the Ba content. With an increase in Ba content from 0.01 to 0.1, the dielectric constant and the τf value increased, but the Q×f value decreased. The sintered (BaxMg1−x)(Zr0.05Ti0.95)TiO3 (called BxMZT) ceramics had a permittivity in the range of 19.1−20.6, quality factor from 180,000 to 25,000 GHz, and variation in temperature coefficient of resonant frequency from −35 to −39 ppm/°C with increasing composition x. For sintered (BaxMg1−x)(Sn0.05Ti0.95)TiO3 (called BxMST) ceramics, the dielectric constant increased from 19 to 20.5, Q×f value increased from 120,000 to 37,000 (GHz), and the τf value increased from −50 to −3.3 ppm/°C as the x increased from 0.01 to 0.1. When A=Sn and x=0.1, (Ba0.1Mg0.9)(Sn0.05Ti0.95)TiO3 ceramics exhibited dielectric constant of 20.5, Q×f value of 37,000 (GHz), and a near-zero τf value of −3.3 ppm/°C sintered at 1210 °C for 4 h.  相似文献   

3.
Microwave dielectric properties of (1 − x)BaZn2Ti4O11-xBaNd2Ti4O12 (x = 0-1.0) ceramics were investigated by the solid-state reaction with the purpose of finding a microwave ceramics with high dielectric constant (?r), high quality factor (Q × f) and low temperature coefficient of resonant frequency (τf). A two phase system BaZn2Ti4O11-BaNd2Ti4O12 was formed and SEM photographs show equiaxed BaZn2Ti4O11 grains and columnar BaNd2Ti4O12 grains. The microwave dielectric properties were strongly determined by the chemical composition. As increasing x from 0 to 1.0, the phase composition varied from pure BaZn2Ti4O11, to the two phase system BaZn2Ti4O11-BaNd2Ti4O12 and then to pure BaNd2Ti4O12. Therefore, the ?r raised from 29.1 to 82.0 and the Q × f values decreased from 54,630 GHz to 8110 GHz, and the τf values increased from −29 ppm/°C to 94 ppm/°C. 0.8BaZn2Ti4O11-0.2BaNd2Ti4O12 ceramics sintered at 1250 °C for 2.5 h had ?r = 39.1, Q × f = 37,850 GHz and τf = −9 ppm/ °C.  相似文献   

4.
A series of ZnxMg1 − xGa2O4:Co2+ spinels (x = 0, 0.25, 0.5, 0.75, and 1.0) was successfully produced through low-temperature burning method by using Mg(NO3)2·4H2O, Zn(NO3)2·6H2O, Ga(NO3)3·6H2O, CO(NH2)2, NH4NO3, and Co(NO3)2·6H2O as raw materials. The product was characterized by X-ray diffraction, transmission electron microscopy, and photoluminescence spectroscopy. The product was not merely a simple mixture of MgGa2O4 and ZnGa2O4; rather, it formed a solid solution. The lattice constant of ZnxMg1 − xGa2O4:Co2+ (0 ≤ x ≤ 1.0) crystals has a good linear relationship with the doping density, x. The synthesized products have high crystallinities with neat arrays. Based on an analysis of the form and position of the emission spectrum, the strong emission peak around the visible region (670 nm) can be attributed to the energy level transition [4T1(4P) → 4A2(4F)] of Co2+ in the tetrahedron. The weak emission peak in the near-infrared region can be attributed to the energy level transition [4T1(4P) → 4T2(4F)] of Co2+ in the tetrahedron.  相似文献   

5.
Ceramic samples based on ZnO-Nb2O5-TiO2 compositions have been prepared using solid state ceramic route. The work was carried out over a wide range of initial ZnNb2O6 and Zn0.17Nb0.33Ti0.5O2 compounds concentration. The crystal structure and microstructure developments were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was shown that the phase compositions of the samples present itself a columbite type and mixture of two phases—solid solutions of columbite and rutile types.The sintering behavior, permittivity, its temperature coefficients and quality factor had been characterized for ceramic samples in depending on compositions. The permittivity of the samples in this system is within the limits from 24 to 80, τ? from 150 to −560 ppm/°C. For the samples with τ? ∼ 0, ?r ∼ 43.8 and Q·f = 35000 GHz at f = 9 GHz. The comparatively low sintering temperature (≤1080 °C) and high dielectric properties in microwave range make these ceramics promising for application in electronics.  相似文献   

6.
Layered metastable lithium manganese oxides, Li2/3[Ni1/3−xMn2/3−yMx+y]O2 (x = y = 1/36 for M = Al, Co, and Fe and x = 2/36, y = 0 for M = Mg) were prepared by the ion exchange of Li for Na in P2-Na2/3[Ni1/3−xMn2/3−yMx+y]O2 precursors. The Al and Co doping produced the T#2 structure with the space group Cmca. On the other hand, the Fe and Mg doped samples had the O6 structure with space group R-3m. Electron diffraction revealed the 1:2 type ordering within the Ni1/3−xMn2/3−yMx+yO2 slab. It was found that the stacking sequence and electrochemical performance of the Li cells containing T#2-Li2/3[Ni1/3Mn2/3]O2 were affected by the doping with small amounts of Al, Co, Fe, and Mg. The discharge capacity of the Al doped sample was around 200 mAh g−1 in the voltage range between 2.0 and 4.7 V at the current density of 14.4 mA g−1 along with a good capacity retention. Moreover, for the Al and Co doped and undoped oxides, the irreversible phase transition of the T#2 into the O2 structure was observed during the initial lithium deintercalation.  相似文献   

7.
TiO2, CaTiO3 and SrTiO3 were added to the 0.79ZnAl2O4-0.21Co2TiO4 (ZACT in abbreviation) system to control its temperature coefficient of resonant frequency (τf). The effects of these additions on sinterability, phase compositions and microwave dielectric properties of the ceramics synthesized by the solid-state reaction were investigated. The results show that TiO2, CaTiO3 and SrTiO3 can all reduce the densification temperature of the ZACT ceramics within the scope from 75 to 150 °C. CoTi2O5 second-phase with negative τf value appears in the TiO2 doped ZACT system, which inhibits TiO2 addition's function for adjusting τf value of ZACT ceramics. While, CaTiO3 and SrTiO3 can both tune effectively τf value to obtain temperature-stable materials.  相似文献   

8.
Several compositions of NdYb1−xGdxZr2O7 (0 ≤ x ≤ 1.0) ceramics were prepared by pressureless-sintering method at 1973 K for 10 h in air. The relative density, microstructure and electrical conductivity of NdYb1−xGdxZr2O7 ceramics were analyzed by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance plots measurements. NdYb1−xGdxZr2O7 (0 ≤ x ≤ 0.3) ceramics have a single phase of defect fluorite-type structure, and NdYb1−xGdxZr2O7 (0.7 ≤ x ≤ 1.0) ceramics exhibit a single phase of pyrochlore-type structure; however, the NdYb0.5Gd0.5Zr2O7 composition shows mixed phases of both defect fluorite-type and pyrochlore-type structures. The measured values of the grain conductivity obey the Arrhenius relation. The grain conductivity of each composition in NdYb1−xGdxZr2O7 ceramics gradually increases with increasing temperature from 673 to 1173 K. NdYb1−xGdxZr2O7 ceramics are oxide-ion conductor in the oxygen partial pressure range of 1.0 × 10−4 to 1.0 atm at all test temperature levels. The highest grain conductivity value obtained in this work is 1.79 × 10−2 S cm−1 at 1173 K for NdYb0.3Gd0.7Zr2O7 composition.  相似文献   

9.
In this study, the effects of CaTiO3 addition on the sintering characteristics and microwave dielectric properties of BiSbO4 were investigated. Pure BiSbO4 achieved a sintered density of 8.46 g/cm3 at 1100 °C. The value of sintered density decreased with increasing CaTiO3, and sintering at a temperature higher than 1100 °C led to a large weight loss (>2 wt%) caused by the volatile nature of the compound. Samples either sintered above 1100 °C or with a CaTiO3 content exceeding 3 wt% showed poor densification. SEM micrographs revealed microstructures with bimodal grain size distribution. The size of the smaller grains ranged from 0.5 to 1.2 μm and that of the larger grains between 3 and 7 μm. The microwave dielectric properties of the (1−x) BiSbO4−x CaTiO3 ceramics are dependent both on the x value and on the sintering temperature. The 99.0 wt% BiSbO4–1.0 wt% CaTiO3 ceramic sintered at 1100 °C reported overall microwave dielectric properties that can be summarized as εr≈21.8, Q×f≈61,150 GHz, and τf≈−40.1 ppm/°C, all superior to those of the BiSbO4 ceramics sintered with other additives.  相似文献   

10.
(1 − x)ZnAl2O4xTiO2 (x = 0.21) ceramics were synthesized at 1500 °C for 3 h using the solid-state reaction at a heating rate from 1 to 7 °C/min. The effects of heating rate on the microstructure, phase composition and oxidation state of titanium in the ceramics were investigated. The XRD results show that this system is composed of two phases, i.e. ZnAl2O4 spinel and rutile. The “black core” phenomenon resulting from reduction of Ti4+ ion valence appears after the ceramics are sintered at the speed of 1 and 3 °C/min. As the heating rate increases, the density and quality factor (Q·f) increase initially and reach the maximum value when the heating rate is 5 °C/min, and then reduce quickly to the minimum, while the dielectric constant (?r) and temperature coefficient of resonator frequency (τf) nearly do not change. The optimal microwave dielectric properties can be achieved in (1 − x)ZnAl2O4xTiO2 (x = 0.21) ceramics sintered at a heating rate of 5 °C/min with an ?r value of 11.6, a Q·f value of 74,000 GHz (at about 6.5 GHz), and a τf value of −0.4 ppm/°C.  相似文献   

11.
Ni/MgxTi1 − xO catalysts were prepared through a wet impregnation method by dispersing Ni on MgxTi1 − xO composite oxides obtained via a sol–gel technique. The Ni/MgxTi1 − xO catalysts were characterized by various means including ICP–OES, BET, XRD, H2–TPR, SEM, and TG. No free NiO peak was found in all XRD patterns of the Ni/MgxTi1 − xO catalysts. The H2–TPR and chemisorption results indicated that adding Ti to the NiO–MgO system obstructed the formation of solid solution, and thus increased the reducibility of the catalysts. The prepared MgxTi1 − xO composite oxides had the same ability to disperse Ni as TiO2 and MgO. The tri-reforming (simultaneous oxygen reforming, carbon dioxide reforming, and steam reforming) of methane over Ni/MgxTi1 − xO catalysts was carried out in a fixed bed flow reactor. The conversions of CH4 and CO2 can respectively be achieved as high as above 95% and 83% over Ni/Mg0.75Ti0.25O catalyst under the reaction conditions. The activity of Ni/Mg0.75Ti0.25O and Ni/Mg0.5Ti0.5O did not decrease for a reaction period of 50 h, indicating their rather high stability. The experimental results showed that the nature of support, the interaction between metal and support, and the ability to be reduced played an important role in improving the stability of catalysts.  相似文献   

12.
Phase equilibria and glass formation studies of the (1 − x)TeO2-xCdO system (0.05 ≤ x ≤ 0.33 mol) were realized by using differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The samples were prepared by applying a conventional melt-quenching technique at 800 °C. The glass formation range of the system was determined as 0.05 ≤ x < 0.15 and the sample containing 10 mol% CdO showed the highest glass stability. Crystallization behavior of the TeO2-CdO glasses was investigated and formation and/or transformation of different phases were detected for each crystallization reaction. In order to obtain thermal stability of the system, as-cast samples were heat-treated above all crystallization reaction temperatures at 550 °C for 24 h. A binary eutectic: liquid → TeO2 + CdTe2O5 was detected at 638 ± 4 °C. Crystallization behavior of the TeO2-CdO glasses and microstructural characterization of the TeO2-CdTe2O5 system was realized.  相似文献   

13.
SnO2-doped CaSiO3 ceramics were successfully synthesized by a solid-state method. Effects of different SnO2 additions on the sintering behavior, microstructure and dielectric properties of Ca(Sn1−xSix)O3 (x=0.5–1.0) ceramics have been investigated. SnO2 improved the densification process and expanded the sintering temperature range effectively. Moreover, Sn4+ substituting for Si4+ sites leads to the emergence of Ca3SnSi2O9 phase, which has a positive effect on the dielectric properties of CaO–SiO2–SnO2 materials, especially the Qf value. The Ca(Sn0.1Si0.9)O3 ceramics sintered at 1375 °C possessed good microwave dielectric properties: εr =7.92, Qf =58,000 GHz and τf=−42 ppm/°C. The Ca(Sn0.4Si0.6)O3 ceramics sintered at 1450 °C also exhibited good microwave dielectric properties of εr=9.27, Qf=63,000 GHz, and τf=−52 ppm/°C. Thus, they are promising candidate materials for millimeter-wave devices.  相似文献   

14.
Sintering behavior, microstructure and microwave dielectric properties of (1 − x)Li3NbO4-xLiF (0 ≤ x ≤ 0.9) ceramics have been studied in this paper. Continuous solid solution with rock salt structure formed across the entire compositional range. Phase transformed from ordered body-centered cubic phase to short range ordered face-centered cubic phase with the addition of LiF. The x > 0.4 compositions could be densified at 850 °C/2 h. The optimized Q × f value for each composition increased with the increase of LiF content and saturated at about 75,000 GHz when x ≥ 0.15, whereas the optimized dielectric permittivity decreased with the increase of LiF content. All specimens exhibited negative τf value. The chemical compatibilities with copper (Cu) in the case of x = 0.4 composition and silver (Ag) in the case of x = 0.5 composition were also investigated, respectively. No chemical reaction has taken place between the matrix phase and Ag or Cu after sintering at 850 °C/2 h or 950 °C/2 h, respectively.  相似文献   

15.
The Mg-Zn interaction effect of KyMg1 − xZn1 + xO3 heterogeneous type catalyst and its performance on transesterification of palm oil have been studied using the response surface methodology and the factorial design of experiments. The catalyst was synthesized using the co-precipitation method and the activity was assessed by transesterification of palm oil into fatty acid methyl esters. The ratio of the Mg/Zn metal interaction, temperature and time of calcination were found to have positive influence on the conversion of palm oil to fatty acid methyl ester (FAME) with the effect of metal to metal ratio and temperature of calcination being more significant. The catalytic activity was found to decrease at higher calcination temperature and the catalyst type K2Mg0.34Zn1.66O3 with Mg/Zn ratio of 4.81 gave FAME content of 73% at a catalyst loading of 1.404 wt.% of oil with molar ratio of methanol to oil being 6:1 at temperature of 150 °C in 6 h. A regression model was obtained to predict conversions to methyl esters as a function of metal interaction ratio, temperature of calcination and time. The observed activity of the synthesized catalyst was due to its synergetic structure and composition.  相似文献   

16.
The La2−xAxMo2O9−δ (A = Ca2+, Sr2+, Ba2+ and K+) series has been synthesised as nanocrystalline materials via a modification of the freeze-drying method. The resulting materials have been characterised by X-ray diffraction (XRD), thermal analysis (TG/DTA, DSC), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The high-temperature β-polymorph is stabilised for dopant content x > 0.01. The nanocrystalline powders were used to obtain dense ceramic materials with optimised microstructure and relative density >95%. The overall conductivity determined by impedance spectroscopy depends on both the ionic radius and dopant content. The conductivity decreases slightly as the dopant content increases in addition a maximum conductivity value was found for Sr2+ substitution, which show an ionic radii slightly higher than La3+ (e.g. 0.08 S cm−1 for La2Mo2O9 and 0.06 S cm−1 for La1.9Sr0.1Mo2O9−δ at 973 K). The creation of extrinsic vacancies upon substitution results in a wider stability range under reducing conditions and prevents amorphisation, although the stability is not enhanced significantly when compared to samples with higher tungsten content. These materials present high thermal expansion coefficients in the range of (13-16) × 10−6 K−1 between room temperature and 753 K and (18-20) × 10−6 K−1 above 823 K. The ionic transport numbers determined by a modified emf method remain above 0.98 under an oxygen partial pressure gradient of O2/air and decreases substantially under wet 5% H2-Ar/air when approaching to the degradation temperature above 973 K due to an increase of the electronic contribution to the overall conductivity.  相似文献   

17.
A series of Ni substituted spinel LiNixMn2−xO4 (0 ≤ x ≤ 0.5) have been synthesized to study the evolution of the local structure and their electrochemical properties. X-ray diffraction showed a few Ni cations moved to the 8a sites in heavily substituted LiNixMn2−xO4 (x ≥ 0.3). X-ray photoelectron spectroscopy confirmed Ni2+ cations were partially oxidized to Ni3+. The local structures of LiNixMn2−xO4 were studied by analyzing the and A1g Raman bands. The most compact [Mn(Ni)O6] octahedron with the highest bond energy of Mn(Ni)O was found for LiNi0.2Mn1.8O4, which showed a Mn(Ni)O average bond length of 1.790 Å, and a force constant of 2.966 N cm−1. Electrolyte decomposition during the electrochemical charging processes increased with Ni substitution. The discharge capacities at the 4.1 and 4.7 V plateaus obeyed the linear relationships with respect to the Ni substitution with the slopes of −1.9 and +1.9, which were smaller than the theoretical values of −2 and +2, respectively. The smaller slopes could be attributed to the electrochemical hysteresis and the presence of Ni3+ in the materials.  相似文献   

18.
The optical properties and microwave dielectric properties of transparent polycrystalline MgAl2O4 ceramics sintered by spark plasma sintering (SPS) through homemade nanosized MgAl2O4 powders at temperatures between 1250 °C and 1375 °C are discussed. The results indicate that, with increasing sintering temperatures, grain growth and densification occurred up to 1275 °C, and above 1350 °C, rapid grain and pore growth occurred. The in-line light transmission increases with the densification and decreases with the grain/pore growth, which can be as high as 70% at the wavelength of 550 nm and 82% at the wavelength of 2000 nm, respectively. As the sintering temperature increases, Q×f and dielectric constant εr values increase to maximum and then decrease respectively, while τf value is almost independent of the sintering temperatures and remains between −77 and −71 ppm/°C. The optimal microwave dielectric properties (εr=8.38, Q×f=54,000 GHz and τf=−74 ppm/°C) are achieved for transparent MgAl2O4 ceramics produced by spark plasma sintering at 1325 °C for 20 min.  相似文献   

19.
J. Jiang 《Electrochimica acta》2005,50(24):4778-4783
Samples of the layered cathode materials, Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 (x = 1/12, 1/4, 5/12, and 1/2), were synthesized at 900 °C. Electrodes of these samples were charged in Li-ion coin cells to remove lithium. The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with EC/DEC solvent or 1 M LiPF6 EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were then welded closed. The reactivity of the samples with electrolyte was probed at two states of charge. First, for samples charged to near 4.45 V and second, for samples charged to 4.8 V, corresponding to removal of all mobile lithium from the samples and also concomitant release of oxygen in a plateau near 4.5 V. Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 samples with x = 1/4, 5/12 and 1/2 charged to 4.45 V do not react appreciably till 190 °C in EC/DEC. Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 samples charged to 4.8 V versus Li, across the oxygen release plateau, start to significantly react with EC/DEC at about 130 °C. However, their high reactivity is similar to that of Li0.5CoO2 (4.2 V) with 1 μm particle size. Therefore, Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 samples showing specific capacity of up to 225 mAh/g may be acceptable for replacing LiCoO2 (145 mAh/g to 4.2 V) from a safety point of view, if their particle size is increased.  相似文献   

20.
The complex perovskite oxide Ba(Zn1/3Nb2/3)O3 (BZN) has been studied for its attractive dielectric properties which place this material interesting for applications as multilayer ceramics capacitors or hyperfrequency resonators. This material is sinterable at low temperature with combined glass phase–lithium salt additions, and exhibits, at 1 MHz very low dielectric losses combined with relatively high dielectric constant and a good stability of this later versus temperature. The 2 wt.% of ZnO–SiO2–B2O3 glass phase and 1 wt.% of LiF-added BZN sample sintered at 900 °C exhibits a relative density higher than 95% and attractive dielectric properties: a dielectric constant ?r of 39, low dielectrics losses (tan(δ) < 10−3) and a temperature coefficient of permittivity τ? of 45 ppm/°C−1. The 2 wt.% ZnO–SiO2–B2O3 glass phase and 1 wt.% of B2O3-added BZN sintered at 930 °C exhibits also attractive dielectric properties (?r = 38, tan(δ) < 10−3) and it is more interesting in terms of temperature coefficient of the permittivity (τ? = −5 ppm/°C). Their good dielectric properties and their compatibility with Ag electrodes, make these ceramics suitable for L.T.C.C applications.  相似文献   

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