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71.
《Journal of the European Ceramic Society》2020,40(3):672-681
Promising piezoelectric properties have been reported in potassium sodium niobate-based ceramics by introducing Bi0.5(Na0.82K0.18)0.5ZrO3 (BNKZ) into K0.48Na0.52Nb0.95Sb0.05O3 (KNNS) solid solutions in order to control the polymorphic phase transformation temperatures. In the present study, synchrotron x-ray powder diffraction (SXPD) was employed in combination with dielectric and ferroelectric measurements in order to clarify the influence of BNKZ on the phase transition temperatures of (1-x)KNNS-(x)BNKZ ceramics (with x = 0 to 0.05). The results, presented in terms of temperature-dependent SXPD patterns, dielectric permittivity and thermal depolarisation characteristics, confirmed that polymorphic phase transformation temperatures all shifted in a systematic manner with increasing BNKZ content. Broadening of the phase transition regions was also observed with increasing BNKZ content, leading to improvements in thermal stability of the ferroelectric properties. Microstructural examination of the KNNS-BNKZ ceramics revealed the presence of core-shell microstructures; this was correlated with the presence of weak shoulders on the diffraction peaks. 相似文献
72.
Adaptive backstepping repetitive learning control design for nonlinear discrete‐time systems with periodic uncertainties 下载免费PDF全文
Qiao Zhu Jian‐Xin Xu Shiping Yang Guang‐Da Hu 《International Journal of Adaptive Control and Signal Processing》2015,29(4):524-535
This paper addresses a tracking problem for uncertain nonlinear discrete‐time systems in which the uncertainties, including parametric uncertainty and external disturbance, are periodic with known periodicity. Repetitive learning control (RLC) is an effective tool to deal with periodic unknown components. By using the backstepping procedures, an adaptive RLC law with periodic parameter estimation is designed. The overparameterization problem is overcome by postponing the parameter estimation to the last backstepping step, which could not be easily solved in robust adaptive control. It is shown that the proposed adaptive RLC law without overparameterization can guarantee the perfect tracking and boundedness of the states of the whole closed‐loop systems in presence of periodic uncertainties. In addition, the effectiveness of the developed controller is demonstrated by an implementation example on a single‐link flexible‐joint robot. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
73.
在社会转型期,面对日益激烈的市场竞争环境,如何制定民办高效发战战略,引导民办高效的可持续发展,是关系到民办高校兴衰的关键。本文主要从民办高校品牌战略入手,对社会转型及高等教育转型进行论述,并对如何加强民办高校品牌建设进行探讨。 相似文献
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77.
《中国有色金属学会会刊》2022,32(5):1728-1740
The hexagonal to orthorhombic (HO) transformation from β-Ni3Sn2 (hexagonal) phase to α’-Ni3Sn2 (orthorhombic) phase was confirmed in directionally solidified Sn–Ni peritectic alloys. It is shown that the remelting/resolidification process which is caused by both the temperature gradient zone melting (TGZM) and Gibbs?Thomson (G?T) effects can take place on secondary dendrites. Besides, the intersection angle between the primary dendrite stem and secondary branch (θ) is found to increase from π/3 to π/2 as the solidification proceeds. This is the morphological feature of the HO transformation, which can change the diffusion distance of the remelting/ resolidification process. Thus, a diffusion-based analytical model is established to describe this process through the specific surface area (SV) of dendrites. The theoretical prediction demonstrates that the remelting/resolidification process is restricted when the HO transformation occurs during peritectic solidification. In addition, the slope of the prediction curves is changed, indicating the variation of the local remelting/resolidification rates. 相似文献
78.
The investigation of phase transformations in metastable ceramic systems such as zirconia often requires local phase analysis within the areas of interest. Electron backscatter diffraction is a suitable method in combination with focused ion beam sample preparation. The interaction between ion beam and sample has to be carefully considered. In case of metastable Y-PSZ and Mg-PSZ, phase transformations were observed after FIB preparation with 30?kV, 30?nA and 5° incidence angle. Damage was the dominating effect for angles of 72°. The expected local temperature increase due to the ion bombardment with 30?kV and 30?nA is 700?K for ZrO2. Thus, the observed phase transformations can be explained on the basis of the temperature increase in the corresponding Y-PSZ phase diagram. In case of Mg-PSZ, the transition temperature is 1083?°C. The local temperature increase was obviously lower. The excitation energy for the observed phase transformation was smaller than expected from the phase diagrams of the thermodynamic equilibrium. Using 5?kV, 4.8?nA and 5° incidence angle, no phase transformations and no damage were observed. Thus, these conditions are well suited for the FIB preparation of metastable zirconia. 相似文献
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80.
Crystal Structures and Phase Behavior of Sulfadiazine and a Method for the Preparation of Aggregates with Good Performance 下载免费PDF全文
Songgu Wu Shichao Du Mingyang Chen Kangli Li Lina Jia Dejiang Zhang Estevao G. J. Macaringue Baohong Hou Junbo Gong 《化学工程与技术》2018,41(3):532-540
The solvate and the solvent‐free form of sulfadiazine (SD) were investigated. SD was found to exist in one solvent‐free form and the N‐methylpyrrolidone (NMP) solvate form. The NMP solvate was shown to be a channel‐type compound. The intrinsic properties of the solvents were used to evaluate the effects of solubility on the phase transformation of SD and the NMP solvate. The SD phase could transform to the NMP solvate by NMP‐mediated phase transformation, which was governed by crystallization of the NMP solvate. The crystalline NMP solvate could transform to the solvent‐free solid state through solid‐solid transformation upon heating or water penetration‐mediated phase transformation. The rate of this water penetration‐mediated phase transformation of the NMP solvate to SD was unusually fast. It can be used to obtain SD aggregates of well‐defined shape and good powder properties. 相似文献