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排序方式: 共有1026条查询结果,搜索用时 234 毫秒
1.
《Ceramics International》2020,46(14):22452-22459
Relaxor ferroelectrics have attracted much attention as electric energy storage materials for intermittent energy storage because of their high saturated polarization, near-zero remnant polarizations, and considerable dielectric breakdown strength (BDS). Despite the numerous efforts, the dielectric energy storage performance of relaxor ferroelectric ceramics is incomplete or unsatisfactory. The enhancement of recoverable energy storage density Wrec usually accompanies with the sacrifice of discharge-to-charge energy efficiency η; therefore, it is an important issue to achieve high recoverable Wrec and large efficiency η simultaneously. In this work, the (1-x)BaTiO3-xBi(Zn1/2Zr1/2)O3 (abbreviated as BT-100xBZZ, 0 ≤ x ≤ 0.20) ferroelectric ceramics were prepared using the conventional solid-state reaction method. The phase structure, microstructural morphology, dielectric and ferroelectric properties, relaxation behaviors, and energy storage properties of BT-BZZ ceramics were investigated in detail. X-ray powder diffraction, dielectric spectra, and ferroelectric properties confirm the transformation of tetragonal phase for normal ferroelectrics (BT) to pseudo-cubic phase for relaxor ferroelectrics (BT-8BZZ). A high recoverable energy storage density Wrec of 2.47 J/cm3 and a large energy efficiency η of 94.4% are simultaneously achieved in the composition of BT-12BZZ, which presents typical weakly coupled relaxor ferroelectric characteristics, with an activation energy Ea of 0.21 eV and a freezing temperature Tf of 139.7 K. Such excellent energy storage performance suggests that relaxor ferroelectric BT-12BZZ ceramics are promising dielectric energy storage materials for high-power pulsed capacitors. 相似文献
2.
《International Journal of Hydrogen Energy》2020,45(27):13975-13984
An intervening barrier for photocatalytic water decomposition and pollutant degradation is the frustratingly quick recombination of e− - h+ pairs. Delicate design of heterojunction photocatalysts by coupling the semiconductors at nanoscale with well-matched geometrical and electronic alignments is an effective strategy to ameliorate the charge separation. Here a facile and environment-friendly l-cysteine-assisted hydrothermal process under weakly alkaline conditions is demonstrated for the first time to fabricate ZnIn2S4/In(OH)3 hollow microspheres with intimate contact, which are verified by XRD, SEM, (HR)TEM, XPS, N2 adsorption-desorption, UV–Vis DRS and photoluminescence spectra. ZnIn2S4/In(OH)3 heterostructure (L-cys/Zn2+ = 4, molar ratio) with a band-gap of 2.50 eV, demonstrates the best photocatalytic performance for water reduction and MB degradation under visible light, outperforming its counterparts (In(OH)3 and ZnIn2S4). The excellent activity of ZnIn2S4/In(OH)3 heterostructure arises from the intercrossed band-edge positions as well as the unique hollow structure with large surface area and wide pore-size distribution, which are beneficial for the efficient charge migration from bulk to surface as well as at the interface between ZnIn2S4 and In(OH)3. This work provides an efficient and eco-friendly strategy for one-pot synthesis of heterostructured composites with intimate contact for photocatalytic application. 相似文献
3.
《International Journal of Heat and Mass Transfer》2006,49(1-2):359-365
Physically substantiated boundary conditions for problems of heat transfer in infiltrated granular beds based on the two-temperature model which allow for the absence of interphase heat transfer on boundaries are formulated. It is shown that classical Dankwerts conditions would be applicable for gas. The problem of porous cooling at the boundary conditions of the 2nd and 3rd kind on the outer boundary is solved in a new formulation. 相似文献
4.
《International Journal of Fatigue》2007,29(9-11):1846-1854
This work presents a survey of the isothermal and anisothermal fatigue behavior of aluminum casting alloys obtained from different processes. Experimental results have shown that porosity, especially large and irregular pores, provides the main factor in decreasing fatigue properties of the tested alloys. In materials with similar porosity levels, other microstructural factors became relevant such as, matrix morphology and the amount of alloying elements. Fractographic analyses showed that fatigue cracks preferentially start to propagate in microcracks or interdendritic shrinkage usually located next to the surface. In most cases, propagation takes place in the eutectic phase, although in the thixoformed material, a transition from transgranular to intergranular mode was observed in the crack propagation mode. 相似文献
5.
《Journal of power sources》2006,159(2):1042-1047
The direct methanol fuel cell (DMFC) is regarded as a promising candidate in portable electronic power applications. Bipolar plate stacks were systematically studied by controlling the operating conditions, and by adjusting the stack structure design parameters, to develop more commercial DMFCs. The findings indicate that the peak power of the stack is influenced more strongly by the flow rate of air than by that of the methanol solution. Notably, the stack performance remains constant even as the channel depth is decreased from 1.0 to 0.6 mm, without loss of the performance in each cell. Furthermore, the specific power density of the stack was increased greatly from ∼60 to ∼100 W l−1 for stacks of 10 and 18 cells, respectively. The current status of the work indicates that the power output of an 18-cell short stack reaches 33 W in air at 70 °C. The outer dimensions of this 18-cell short stack are only 80 mm × 80 mm × 51 mm, which are suitable for practical applications in 10–20 W DMFC portable systems. 相似文献
6.
7.
《Catalysis communications》2007,8(3):241-246
A simple method for evaluating the surface acidity of different cation-exchanged montmorillonite (mont) clay catalysts, Mn+-mont (Mn+=Al3+, Fe3+, Cr3+, Zn2+, Ni2+, Cu2+, and H+), involving treatment with pyridine is described. After treating with pyridine, the samples were heated at 120 °C and the FT-IR spectra were directly recorded in the region 1650 and 1350 cm−1. The data obtained show the presence of both Lewis and Brønsted acid sites. The activities of the catalysts to bring about Brønsted acid catalysed esterification of succinic acid with iso-butanol to yield di-(iso-butyl) succinate have been studied. The Brønsted acidity data obtained for Mn+-mont correlated well with activity in the esterification reaction. The activities of the catalysts were found to decrease in the order of exchange ions Al3+ > Fe3+ > Cr3+ > Zn2+ > Ni2+ > Cu2+ > Na+-mont. They also correlated well with the charge to radius ratio of the cations. The catalysts exchanged with trivalent cations showed stronger absorption bands attributed to Brønsted acidity (1540 cm−1) whereas those exchanged with divalent cations showed an increased Lewis acidity (1450 cm−1) and reduced Brønsted acidity along with charge to radius ratio. Zn2+-, Cu2+- and Ni2+-exchanged clays showed an additional peak around 1605 cm−1 which is attributed to the pyridine adsorption on surface sites through its π electrons. The method suggested here to evaluate the acidity is suitable for active sites which are thermally unstable such as water molecules in the hydration shell of a cation in exchanged clay. 相似文献
8.
《Ceramics International》2019,45(10):13179-13186
Pb-free piezoceramics of K0.5Na0.5Nb1-xSbxO3 + 1 mol% CuO are synthesized via a solid-state reaction. Furthermore, the transformation of hardening to softening behaviors induced by Sb substitution is exhibited and the corresponding microscopic mechanism is proposed. The CuO-doped K0.5Na0.5NbO3 ceramic without adding Sb exhibits extremely hardening characteristics (i.e., ultrahigh Qm of ∼2426, low tanδ of 0.32%, and pinched ferroelectric hysteresis loop) due to the formation of defect combinations ( and ). Whereas, the addition of Sb dramatically reduces the levels of defect combinations, leading to obviously softening properties (d33 > 210 pC N−1, kp > 40%, low Qm, and normal single P-E hysteresis loop). Our results indicate that the decrease of defect combinations with Sb addition should be responsible for the hardening-softening transformation of piezoelectricity and ferroelectricity in CuO-doped K0.5Na0.5Nb1-xSbxO3 piezoceramics. 相似文献
9.
《Fuel》2006,85(10-11):1509-1517
FT-IR/Raman spectroscopies have been used to identify the structural features of Victorian brown coal chars during the gasification in air at 400 °C. The deconvolution of the Raman spectra has allowed us to identify the main structural sites in char where preferential reaction with O2 takes place. The presence of Na and Ca catalysts is shown to alter the reaction pathways between char and O2. In the absence of a catalyst, the O-containing functional groups formed in char during gasification were closely associated with the aromatic structure and thus tended to loosen the aromatic structure. The non-catalysed gasification was slow and took place on some specific (especially sp3-rich or sp2–sp3 mixture) sites distributed throughout the char. In the presence of a catalyst (Na or Ca), the O-containing functional groups were not closely associated with the main aromatic structure throughout the char. The catalytic gasification reactions were localised on the sites associated with the catalysts. The preferential removal of smaller aromatic ring systems and the persistence of cross-linking structures in the presence of a catalyst mean that the large aromatic ring systems were increasingly concentrated with little flexibility, affecting the dispersion of catalyst. 相似文献
10.
Fault diagnosis of power transformer based on support vector machine with genetic algorithm 总被引:1,自引:0,他引:1
Diagnosis of potential faults concealed inside power transformers is the key of ensuring stable electrical power supply to consumers. Support vector machine (SVM) is a new machine learning method based on the statistical learning theory, which is a powerful tool for solving the problem with small sampling, nonlinearity and high dimension. The selection of SVM parameters has an important influence on the classification accuracy of SVM. However, it is very difficult to select appropriate SVM parameters. In this study, support vector machine with genetic algorithm (SVMG) is applied to fault diagnosis of a power transformer, in which genetic algorithm (GA) is used to select appropriate free parameters of SVM. The experimental data from several electric power companies in China are used to illustrate the performance of the proposed SVMG model. The experimental results indicate that the SVMG method can achieve higher diagnostic accuracy than IEC three ratios, normal SVM classifier and artificial neural network. 相似文献