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51.
Application of Ultrafast High-temperature Sintering (UHS) technique to rapidly densify barium titanate ceramics has been explored for the first time. Bulk ceramic with ~94% density was obtained by UHS at ~1340 °C for 60 seconds. The densification process was accompanied with progressive sample discolouration from light to dark grey. Further analysis indicates that oxygen vacancy and its associated Ti-rich phase Ba4Ti12O27 are present in the ceramics. Their roles in ultrafast densification and sample discoloration are discussed. Due to the presence of oxygen vacancies, the UHSed ceramics generally exhibit a colossal dielectric constant of ~ 15–30k at 1 kHz, with dielectric loss of ~0.07–0.10, while the ceramics without oxygen vacancy retain a dielectric constant of ~3000–6000 and dielectric loss of ~ 0.06 at 1 kHz which are comparable to that of the conventionally sintered ceramics. Furthermore, the challenges in applying UHS to sinter thick BT ceramics are discussed, aided by thermal simulations.  相似文献   
52.
High-entropy (HE) ceramics nanoparticles have received much attention due to their interesting properties. However, very limited studies have been conducted on their sintering. Here, we report the sintering behavior of HE A2B2O7 type rare earth oxide nanoparticles obtained by polyol process. HE cerate (HECe) (La0.2Y0.2Nd0.2Sm0.2Gd0.2)2Ce2O7-δ is chosen as an exemplary case, which is considered as a good candidate for thermal insulation. HECe nanoparticles with size of 2.6–7.1 nm can be synthesized through polyol process followed by annealing in air at 300–700 °C. HECe nanoparticle compact can be densified by directly sintering at 1500 °C. The sintering temperature could be further decreased using a two-step sintering process, i.e., 1500 °C 5 min-1300 °C 5 h. Our results show that fine particle and abundant oxygen vacancies probably dominate the densification process. By controlling the sintering regime, we can tune the microstructure of HECe ceramics and thermal conductive properties accordingly.  相似文献   
53.
Single phase multiferroic BiFeO3 (BFO) ceramics were synthesized with controllable amount of Fe2+ and oxygen vacancies in a very wide concentration range. Double ferroelectric hysteresis loops are observed in fresh BFO ceramics at room temperature without any aging processes. Detailed study shows that this is due to the defect dipoles which have preferential orientations antiparallel to the direction of spontaneous polarization, providing a driving force for domain back-switching. The dielectric constant of BFO could be enormously enhanced by 100 times by increasing the Fe2+ content, employing a polaronic relaxation process.  相似文献   
54.
非均匀软岩蠕变机理分析   总被引:5,自引:0,他引:5  
为了对工程围岩的蠕变现象进行解释,探明软岩的蠕变本质,根据岩石压缩过程中变形、弱化本构关系以及变形与承载能力的关系,采用岩石最弱断面并联微元体模型,对非均匀软岩的衰减蠕变和非衰减蠕变机理进行了分析,研究表明,非均匀性岩石内部各点承载能力不同使得变形后岩石内部各点屈服弱化不同步,又由于岩石晶体内部空位或杂质的扩散,二者共同作用导致岩石蠕变.通过对截面积相同,高径比分别为1.5和2.5的页岩岩样进行蠕变试验,验证了岩石的非均匀性是造成岩石蠕变的一个不可忽视的重要因素.  相似文献   
55.
《Ceramics International》2020,46(10):16472-16479
(Pb1-xLax) (Zr0.92Ti0.08)1-x/4O3 (PLZT x/92/8, x = 3, 5 and 7 at%) ceramics with compositions near the antiferroelectric (AFE)-ferroelectric (FE) phase boundary were fabricated by a solid-state reaction method. The effect of lanthanum content on the conduction behaviors and relaxation processes has been investigated. It was verified that the main phase with orthorhombic structure was formed in all compositions. The increase of lanthanum substitution resulted in an enhancement of diffuse phase transition. Impedance analysis suggested that the ac conductivity decreased with increasing lanthanum content. Moreover, thermally stimulated depolarization current study was utilized to establish the correlation between defect structures and relaxation processes. It showed three peaks with distinct characteristics, which originated from dipole orientation, oxygen vacancy migration and phase transition respectively. The oxygen vacancy-related defects induced by lanthanum doping were mainly responsible for the variation of conduction behaviors and relaxation processes.  相似文献   
56.
《Ceramics International》2020,46(11):18690-18697
Bi0.9Er0.1Fe1−xMnxO3 (BEFMxO, x = 0.00–0.03) films are synthesized by a sol–gel technique. The BEFO film exhibits a conduction mechanism based on electron tunneling. The high applied electric field causes dissociation of the defect complex, and the resulting oxygen vacancies contribute to fake polarization. Consequently, the BEFO film has poor polarization stability at high applied electric fields. Coexistence of two phases (with space groups R3c:H and R3m:R) and reduced concentrations of oxygen vacancies and Fe2+ in BEFMxO are achieved by co-doping with Er and Mn. The presence of bulk-based conduction in the BEFMxO films then leads to ferroelectric domain switching contributing to the real polarization and to excellent ferroelectric stability. In addition, the BEFM0.02O film shows a typical symmetrical butterfly curve, the highest remnant polarization of ~109 μC/cm2, and the highest switching current of ~1.66 mA. It also has the smallest oxygen vacancy concentration and thus the smallest amount of defect complex, which means that there are fewer pinning effects on ferroelectric domains and therefore excellent ferroelectric stability. This excellent ferroelectric stability makes the BEFMxO films obtain good stability and reliability in the application of ferroelectric memory devices.  相似文献   
57.
In the last decades, many reports dealing with technology for the catalytic combustion of methane (CH4) have been published. Recently, attention has increasingly focused on the synthesis and catalytic activity of nickel oxides. In this paper, a NiO/CeO2 catalyst with high catalytic performance in methane combustion was synthesized via a facile impregnation method, and its catalytic activity, stability, and water-resistance during CH4 combustion were investigated. X-ray diffraction, low-temperature N2 adsorption, thermogravimetric analysis, Fourier transform infrared spectroscopy, hydrogen temperature programmed reduction, methane temperature programmed surface reaction, Raman spectroscopy, electron paramagnetic resonance, and transmission electron microscope characterization of the catalyst were conducted to determine the origin of its high catalytic activity and stability in detail. The incorporation of NiO was found to enhance the concentration of oxygen vacancies, as well as the activity and amount of surface oxygen. As a result, the mobility of bulk oxygen in CeO2 was increased. The presence of CeO2 prevented the aggregation of NiO, enhanced reduction by NiO, and provided more oxygen species for the combustion of CH4. The results of a kinetics study indicated that the reaction order was about 1.07 for CH4 and about 0.10 for O2 over the NiO/CeO2 catalyst.  相似文献   
58.
The ultrathin two-dimensional (2D) nanomaterials display unique properties owing to their ultrahigh specific surface area and strong quantum confinement of electrons in two dimensions. In this work, we fabricated a rapid gasochromic and electrical hydrogen sensing system containing 2D WO3 and Pd nanoparticles. 2D WO3 nano-plates (NP) are synthesized using sol–gel method and Pd nanoparticles are coated on WO3 by green photochemical deposition method. The sensor is fabricated by dispersing the 2D WO3/Pd composite on filter paper. In presence of hydrogen gas, 2D WO3/Pd composite produces visible change in color from brown to dark blue. With the fabricated sensor, as low as 0.1% H2 gas in air at room temperature can be easily detected using electrical sensing scheme whereas for higher concentration from 1 to 100%, eye readable gasochromic scheme can be utilized. The use of 2D WO3 decreased the response time in great deal compared to WO3 nanoparticles indicating the advantage of 2D structure in fabricating rapid response H2 sensors. The proposed method is simple and can be easily employed to large scale fabrication system for commercial applications.  相似文献   
59.
为了研究MnO_x-CeO_2复合氧化物催化氧化柴油机NO的性能,采用溶胶凝胶法制备了不同Mn/Ce摩尔比的xMn10Ce/γ-Al_2O_3(x=5,6,7,8)催化剂。利用X射线衍射(X-ray diffraction,XRD)、X射线光电子能谱(X-ray photoelectron spectroscopy,XPS)和H_2程序升温还原(H_2temperature programmed reduction,H_2-TPR)技术分别对其物相结构、元素价态及还原性能进行了表征。结果表明:CeO_2晶粒尺寸随Mn含量增加而逐渐变大,同时添加适量的Mn也有利于提高催化剂中Mn~(4+)的数量,从而促进NO氧化;当x=6时,催化剂的氧化还原性能最好。最后,基于模拟气试验平台考察了不同Mn/Ce比对xMn10Ce/γ-Al_2O_3催化剂氧化NO性能的影响。结果表明:在350℃时,NO浓度达饱和所需时间和反应速率与Mn添加量呈非线性关系;在250~450℃范围内,6Mn10Ce/γ-Al_2O_3的NO氧化性能最好,其NO浓度在350℃时最先达到稳定,仅需4min,但在450℃时则延长至6min,这主要是受热力学平衡及随反应进行活性氧数量及氧空位逐渐饱和共同作用的结果。  相似文献   
60.
氧化铝生产中“硅渣”的组成和结构研究   总被引:11,自引:0,他引:11  
邓红梅  曾文明  陈念贻 《金属学报》1996,32(12):1248-1251
综合运用X射线衍射,红外光谱,差热分析等方法,研究氧化铝生产脱Si反应生成物--水合铝硅酸钠渣的组成和结构,证明其为有硅原子缺位的羟基方钠石,SiO2与Al2O3摩尔比小于2.0主要系方钠石晶格骨架上硅原子缺位所致。  相似文献   
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