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排序方式: 共有125条查询结果,搜索用时 15 毫秒
1.
N.A. Zhuk M.G. Krzhizhanovskaya N.A. Sekushin V.V. Kharton B.A. Makeev V.A. Belyy R.I. Korolev 《Ceramics International》2021,47(14):19424-19433
The solid solutions based on the pyrochlore-type system Bi2MgNb2-xTaxO9 were formed in the compositional range х = 0–2.0 (Bi1·6Mg0·8Nb1.6-tTatO7.2, t = 0–1.6). The Rietveld method was used to refine the structure for Bi2MgNb2-xTaxO9 (x = 0, 1.0, 2.0). The increasing tantalum content led to the slight decrease in the cubic unit cell parameters from 10.56934 (4) Å for x = 0 and 10.54607 (3) Å for x = 2 (sp.gr. Fd-3m:2). At the same time, tantalum additions suppressed grain growth in the pyrochlore ceramics during sintering and made it possible to obtain materials with an average grain size of 1–2 μm (Bi1·6Mg0·8Ta1·6O7.2). The increase in the Ta5+ concentration led to the decrease in the dielectric permeability from 104 (Bi1·6Mg0·8Nb1·6O7.2) to 20 (Bi1·6Mg0·8Ta1·6O7.2) at room temperature, while the dielectric loss tangent remained lower than 0.002, which is due to the small grain size and the high porosity of the samples. An increase in temperature has practically no effect on the values of the dielectric permittivity in the entire frequency range. The samples have weak through conductivity. The activation energies of electrical conductivity varied in the range of 0.84–1.00 eV, and the less tantalum, the lower the activation energy. The electrical properties of the samples at 200 Hz to 1 MHz are described by the simplest parallel scheme. 相似文献
2.
Nanosized lead zirconate titanate (PZT) powder with Zr:Ti ratio in the morphotropic phase boundary region was synthesized by homogeneous precipitation of metal ions. The powder precipitated at 90°C and at pH 6.7 resulted single-phase perovskite lead zirconate titanate powder when calcined at 550°C and above for 4 hours in air. The solution pH and the precipitation temperature strongly affect the composition of the calcined powder. The results obtained by structural characterization of homogeneously precipitated powder were compared with that obtained from the conventional precipitation method using ammonia in terms of crystallization, homogeneity, and microstructure. The homogeneously precipitated powder showed smaller particle size, minimum agglomeration and uniform shape on calcination and annealing. Powdered samples that precipitated by homogeneous precipitation crystallized directly to perovskite PZT, without any intermediate pyrochlore phase formation. In contrast, the NH3 precipitated powder converted to perovskite PZT via metastable pyrochlore and it showed phase segregation upon annealing at higher temperatures. The reaction kinetics has been studied by X-ray diffraction, differential thermal analysis, and differential scanning calorimetry. 相似文献
3.
本文介绍溶胶-凝胶法制备均匀PbZr-Ti-B-Si凝胶玻璃,并通过适当热处理在凝胶玻璃中原位生长Pb(Zr;Ti)O3微晶的新工艺·利用IR谱考察了凝胶玻璃中的元素键合结构随温度的变化.结合热分析和XRD;SEM技术,研究了Pb(Zr,Ti)O3微晶在凝胶玻璃中的生长过程及该材料的结构特点.发现Pb2(Zr,Ti)2O6+x立方焦绿石介稳过渡相的纳米微晶首先出现于该体系中,并在更高的温度下先后完全转变成三方和四方Pb(Zr,Ti)O3钙钛矿相.电子显微观察结果表明,该工艺制备的Ph(Zr;Ti)O3玻璃陶瓷具有均匀的细晶结构. 相似文献
4.
Shuhui Zhang Mingjun Rao Rendong Xiao Jinxiang You Guanghui Li Tao Jiang 《矿业科学技术学报(英文版)》2022,32(3):615-626
In this work, a novel process consisting of calcining-slaking followed by gravity separation for the enrichment of niobium(Nb) and titanium(Ti) from carbonatite pyrochlore ore was proposed, validated and compared with the current mainstream flotation method. During calcining of the pyrochlore ore, within which the carbonates were transformed into lime. Subsequently, when the calcined ore was slaked, lime was transformed into hydroxide with fine particles which were amenable to gravity separation... 相似文献
5.
《International Journal of Hydrogen Energy》2019,44(8):4167-4176
Dry reforming of methane was carried out using isotopic 13CO2 and C18O2 gases over a Ni-based pyrochlore catalyst that was synthesized using the modified Pechini method. In this method, 1 wt% Ni was doped into the La2Zr2O7 pyrochlore structure. The catalyst was characterized by H2-TPR, TPO, and XRD and tested for its methane reforming activity under CO2 dry-reforming reaction conditions.The results of repeated TPR/TPO cycles up to 950 °C showed that the consecutive TPR profiles were nearly identical, indicating that the catalyst was stable at high temperatures, and that the nickel oxidation/reduction processes were reversible. The dry-reforming experimental results using labelled 13CO2 gas showed the syngas production for this material proceeded through the activation of CH4 with O that came from breaking one of the CO bonds of CO2 with the latter reaction (CO2 activation) likely occurring at oxygen vacancies at or near the Ni particle-pyrochlore interface. It was also found that only a small portion of the CO originated from CH4. A variation of the same experimental test, but using 12C18O2, revealed only 12C18O was formed and no 12C16O was detected, ruling out the possibility of reaction with the lattice oxygen in the catalyst structure with this material. Over this catalyst, the activated CH4 appeared to dissociate to elemental carbon on the catalyst surface, which was determined to be the source of carbon from a post reaction TPO of the catalyst that was exposed to the 13CO212CH4 mixture. No carbon deposition appeared to originate from 13CO2. 相似文献
6.
夏校良 《稀有金属材料与工程》2016,45(8):2003-2008
采用X射线衍射、高分辨透射电镜、选区电子衍射、拉曼光谱以及阻抗谱等手段研究了稀土锆酸盐(A_2Zr_2O_7)材料的晶体结构和电学性能,发现稀土锆酸盐A_2Zr_2O_7的有序度随稀土阳离子半径的减小逐渐降低。Sm_2Zr_2O_7和Eu_2Zr_2O_7陶瓷为有序的烧绿石相,而Gd_2Zr_2O_7和Dy_2Zr_2O_7为无序的缺陷型萤石相。与其它几种稀土锆酸盐的电导率相比,有序度相对较低的烧绿石相Eu_2Zr_2O_7具有最大的晶粒电导率(1.03×10~(-2)S·cm~(-1),1173 K)。 相似文献
7.
Subsolidus pyrochlores with the proposed formula, Bi3+(5/2)xMg2−xNb3−(3/2)xO14−x (0.14≤x≤0.22) were successfully synthesised at the firing temperature of 1025 °C using conventional solid-state reaction. The excess Bi3+ charge was offset by removal of relative proportion of Mg2+ and Nb5+ together with creation of oxygen non-stoichiometry in order to preserve electroneutrality of the system. These samples were crystallised in cubic structure with space group of Fd3m, No. 227 and their refined lattice parameters were in the range of 10.5706 (3)–10.5797 (7) Å. The surface morphologies of the samples as confirmed by scanning electron microscopy analysis were of irregular shaped grains while their crystallite sizes of ~30–85 nm were calculated using the Scherrer equation and the Williamson–Hall method. No thermal event was discernable indicating these pyrochlores were thermally stable within a studied temperature range of ~30–1000 °C. The recorded dielectric constants of Bi3+(5/2)xMg2−xNb3−(3/2)xO14−x (0.14≤x≤0.22) subsolidus pyrochlores were generally above ~160 and their dielectric losses were in the order of 10−4–10−3 at the frequency of 1 MHz and temperature of ~30 °C. Meanwhile, these ceramic samples also exhibited negative temperature coefficient of relative permittivity between −528 and −742 ppm/°C in the temperature range of ~30–300 °C. 相似文献
8.
《Journal of the European Ceramic Society》2014,34(16):4373-4381
From the characterization of a series compositions with general stoichiometry as Ca1−xZr1−xSm2xTi2O7 (0.00 ≤ x ≤ 1.00), the phase evolution between zirconolite (CaZrTi2O7) and pyrochlore type Sm2Ti2O7 has been elucidated. All the compositions were prepared by high temperature solid state reaction and characterized by powder X-ray diffraction (XRD) and electron probe for microanalyses (EPMA). Three major phase fields, namely two layer (2-M) or four layer (4-M) monoclinic zirconolite and cubic pyrochlore structure types were observed in this system. In addition, a feeble amount of perovskite type phase is found to coexist with zirconolite phase. 4-M zirconolite phase is observed as single phase field at the composition with x = 0.30 and 0.35, while cubic pyrochlore phase is observed as single phase at the compositions with x ≥ 0.60. Further, the composition and microstructure of coexisting phases are verified by back scattered electron image and EPMA studies. 相似文献
9.
Juanli Zhao Yuchen Liu Yun Fan Wei Zhang Chengguan Zhang Guang Yang Hongfei Chen Bin Liu 《材料科学技术学报》2021,73(14):23-30
Various excellent properties of rare earth zirconate and stannate pyrochlores are close related with their native point defects.First-principles calculations are performed to systematically investigate the point defect mechanism and the oxygen diffusion behavior of A2B2O7 (A=La,Ce,Pr,Nd,Pm,Sm,Eu,Gd;B=Zr,Sn).The possible defect complexes and their associated reactions under stoichiometric and nonstoichiometric conditions are explored.The O Frenkel pairs are the most stable defect structure in stoichiometric zirconates,whereas the cation antisite defects are the predominant one in stoichiometric stannates.In the case of BO2 excess zirconates and stannates,the BA cation antisite defect with the A vacancy and/or the oxygen interstitial is energetically favorable,whereas the AB antisite defect together with the oxygen vacancy and/or the A interstitial is preferable under the A2O3 excess condition.Meanwhile,the maximum point defect concentrations of zirconates are much higher than those of stannates.Furthermore,the oxygen migration barriers are similar in these compounds,ranging in 0.68 eV-0.80 eV.The predicted point defects and oxygen diffusion mechanisms play the critical role in their engineering applications and are expected to guide the future property improvement of pyrochlores through the control of point defects and/or composition. 相似文献
10.
Ln2B2O7 (Ln=Sm, Eu, Gd and Tb; B=Zr or Ti) with pyrochlore structure was prepared by sol–gel method for the high-temperature catalytic combustion. The crystal structure of Ln2B2O7 was identified by XRD and their surface area was about 4 m2/g after calcinations at 1200 °C. Catalytic activity of methane combustion was observed for Ln2Zr2O7 series and the best catalyst was Sm2Zr2O7. Its relative reaction rate per unit surface area at 600 °C was 2 cm3/m2 min, which was twice higher than that of Mn-substituted Sr hexaaluminate. From surface analysis by XPS, the low binding energy of each Ln element of Ln2Zr2O7 compared to that of Ln2Ti2O7, gave the catalytic activity of methane combustion. 相似文献