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1.
《Ceramics International》2022,48(14):20096-20101
A series of Mn2+-doped Mg1-xMnxTa2O6 (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12) ceramics were synthesized by solid-state reaction method. The influence of introducing Mn–O bonds as a partial replacement for Mg–O bonds on the lattice and microwave dielectric properties was systematically investigated. XRD and Rietveld refinement confirm that Mn2+ occupies the 2a Wyckoff position and forms a pure trirutile phase. Moreover, based on the chemical bond theory, the dielectric constant is mainly affected by the ionicity of the Ta–O bond. The lattice and dielectric properties remain relatively stable with Mn2+ doping below 0.1, but excessive Mn2+ doping leads to pronounced distortion of the lattice, which is not beneficial for lattice stability and microwave dielectric properties. Introducing an appropriate amount of Mn–O bonds with high bond dissociation energy facilitates MgO6 octahedron stability, which improves the thermal stability of the lattice. Accordingly, the microwave dielectric properties for 0.06 Mn2+-doped MgTa2O6 ceramics were determined: εr = 28, Q × f = 105,000 GHz (at 7.5 GHz), τf = 19.5 ppm/°C. 相似文献
2.
In the attempt to reduce surface free energy of silica to improve interaction of silica with silver, silica was doped by different amounts of low surface energy lanthanum, La_2O_3, through impregnation. The doped and undoped silica were used as supports for preparation of Na/Ag/Mo/La_2O_3-SiO_2 catalysts. Catalytic performances of the catalysts were evaluated in direct epoxidation of propylene(DPO) using molecular oxygen under atmospheric pressure and without adding hydrogen. Adding 5 wt%La to the Na/Ag/Mo/SiO_2 catalyst improves both the catalysts electivity in DPO and its stability over 20h of time-on-stream.The characterization results show that La_2O_3 species interact strongly with silver particles on the silica surface which result in significant improvement in the dispersion profile of silver and marked decrease in the size of silver nanoparticles(AgNPs). The estimated mean diameter of AgNPs is ca. 4.0 nm in Na/Ag/Mo/5 wt%La_2O_3-SiO_2, which is smaller than that(53.9 nm) found in Na/Ag/SiO_2. The presence of subnanometer AgNPs on Ag/La_2O_3-SiO_2 prior addition of MoO_3 and NaCl to the sample can enhance the mutual electronic synergism between Ag, MoO_3 and Na for selective production of propylene oxide. 相似文献
3.
This paper is the second of a two-paper set on stress-controlled direct shear testing of geosynthetic clay liners (GCLs). Design of the apparatus, preliminary experiments, and shear deformation mechanisms in heat-treated and non-heat treated needle-punched (NP) GCLs were discussed in Part I. The objective of Part II (this paper) was to evaluate the effects of physical factors (i.e., peel strength and initial normal stress, σni), environmental factors (i.e., temperature and hydration solution), and creep on the internal shear behavior of NP GCLs. In addition, failure conditions of GCLs in the stress-controlled direct shear tests were compared to displacement-controlled direct shear tests to verify results. An increase in internal shear strength developed from increased GCL peel strength or increased normal stress. Elevated temperatures were observed to decrease internal shear strength for both non-heat treated and heat-treated NP GCLs. Specimens hydrated with a calcium-rich synthetic mining solution experienced increased internal shear strength due to cation exchange in the bentonite, whereas specimens hydrated with a highly alkaline synthetic mining solution experienced decreased internal shear strength. Creep tests revealed an increase in time-to-failure with decrease in applied shear stress. Finally, stress states at failure from stress-controlled and displacement-controlled shear tests corresponded to a unique failure envelope, which validates the efficacy of using stress-controlled direct shear tests to assess internal shear behavior and shear strength of NP GCLs. 相似文献
4.
The presence and formation of a large variety of organic molecules in the interstellar medium is evident from both astronomical data of absorption and emission bands at different regions of the spectrum. Specifically, polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the interstellar medium (ISM). The mechanism for their formation in ISM‘s low temperature environment is, as of yet, a mystery nonetheless. Understanding the mechanism of formation of complex molecules such as PAHs and nitrogen-based PAHs (PANH) in the ISM is a long-standing challenge which has been drawing a growing attention for the past several decades. In this review we wish to emphasize two things: Firstly, the essential role quantum chemistry can play in the study of astrochemical reactions. Secondly, we wish to demonstrate that said variety of possibilities for chemical reaction, starting upon ionization of van der Waals clusters. The potential for different chemical reactions to occur within a cluster environment arises from the fact that such processes can take place at low temperatures as the systems pose large amounts of energy upon ionization. Moreover, the spectator molecules in the cluster can provide a dissipation route for energy by detachment from the cluster, thus the system can stabilize efficiently even at low densities. The spectator molecules can also change the potential energy surface, by which it will pose a catalytic effect for certain reactions. We will demonstrate this by presenting Ab Initio Molecular Dynamic results on ionization of small acetylene clusters. 相似文献
5.
This study conducted short-term assessments of perceived air quality (PAQ) for six different realistic concentrations of ozone and limonene, separately or together, in room air. The impact of filtration and the influence of the ozone generation method were also examined. The evaluations were made in four identical 40 m3 low-polluting test offices ventilated at 1.4 h(-1) or in two identical 30 m3 stainless-steel chambers ventilated at 1.9 h(-1). Concentrations of ozone, total volatile organic compounds and size-fractionated particles were continuously monitored in each experiment. The results indicate that, for each of the six conditions, the PAQ was poorer when ozone and limonene were present together compared with when only ozone or only limonene was present. In the test offices a correlation was observed between the number of secondary organic aerosols produced by a given ozone/limonene condition and the sensory pollution load for that condition. The particles themselves do not appear to be the primary causative agents, but instead are co-varying surrogates for sensory offending gas-phase species. PRACTICAL IMPLICATIONS: Although the health consequences of long-term exposures to the products of ozone-initiated indoor chemistry remain to be determined, we judge that the sensory offending nature of selected products provides an additional reason to limit indoor ozone levels. Devices that emit ozone at significant rates should not be used indoors. Ozone-filtration of make-up air should also be beneficial in mechanically ventilated buildings located in regions that repeatedly violate outdoor ozone standards. Additionally, the use of limonene containing products should be curtailed during periods when indoor ozone levels are elevated. 相似文献
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7.
景观设计和艺术的讨论--兼与王向荣老师商榷 总被引:3,自引:0,他引:3
设计先于艺术,设计有单纯的设计,也有融合了艺术的设计,景观设计是一种融合了艺术的设计,景观的生态功能和其他使用功能,只是单纯的设计的目的,只有加上人文情趣,融合了艺术的设计,才是真正的景观设计。虽然艺术及其思想有不同的流派和形式,但美是评判艺术的重要标准。 相似文献
8.
《Urban Water Journal》2013,10(2):156-162
ABSTRACTSingapore has adopted a low impact development equivalent of stormwater management philosophy under a national program called ‘Active, Beautiful and Clean’ (ABC) employing soft-engineering techniques to manage rainfall runoff in the face of climate change and rapid urbanisation. This study makes use of the MIKE URBAN modelling tool to evaluate the performance of ABC units in a new residential precinct based on flow data collected. Calibration results show an overall good fit between the measured data and simulated results based on three goodness-of-fit stats. The runoff factors computed for scenarios with and without the presence of ABC units show an improvement in the range of 21–72%. This study demonstrates the usefulness of using a calibrated model to evaluate the performance of constructed units using measurements. It is recommended that this modelling tool be also used for evaluating water quality performance of the units as well as applying it for use in the planning and implementation process. 相似文献
9.
青年教师是大学师资队伍的生力军。如何提高青年教师的教学能力一直是备受关注的问题。笔者从讲授无机化学课程的经验出发,浅谈提高青年教师教学能力的几点体会。 相似文献
10.