Realization of controllable etching for ZnO film by NH4Cl aqueous solution and its influence on optical and electrical properties

ZnO films were deposited on c-plane Al₂O₃ substrates by pulsed laser deposition. The etching treatments for as-grown ZnO films were performed in NH₄Cl aqueous solution as a function of NH₄Cl concentration and etching time. It was found that NH₄Cl solution is an appropriate candidate for ZnO wet etching because of its controllable and moderate etching rate. The influence of etching treatment on the morphology, optical and electrical properties of the ZnO films has been investigated systematically by means of X-ray diffraction, atomic force microscope, photoluminescence and Hall effect. The results indicated that the surface morphology and optical properties of the films were highly influenced by etching treatment.     

创设"问题情境"课堂教学案例——"闭合电路欧姆定律"教学设计

为什么教师在课堂上多次强调的问题，学生在学习中却屡屡犯错？学生为什么会提出一些非常简单的问题，甚至存在明显错误的思路和方法？难道仅仅是记忆力不好，学习不专注等因素造成的吗？其实教师有一定的责任．以往一节课基本上是教师一人唱“独角戏”，这种“牵着鼻子走”的教学模式，学生学到的只是这个问题的“是什么”、“为什么”和“怎么做”．一些需要学生一步一步深入思考的问题，却都是教师代替学生“解决”了．     

Speech enhancement based on multitaper spectrum and psychoacoustical weighting rule

Multitaper spectrum has lower variance than the traditional periodogram. The noise spectrum and the noise to noisy signal spectrum ratio (NNSR) were estimated from the multitaper spectrum of the noisy signal; the pre-enhanced speech for calculating the noise masking threshold was obtained by the spectral amplitude subtraction method, whose gain is a function of NNSR; the final enhanced speech was obtained by suppressing the Fourier spectrum of the noisy speech with the psychoacoustical weighting rule incorporating the noise masking threshold. Because of the low variance feature of the multitaper spectrum, a modified offset formula was proposed to calculate the noise masking threshold, thus the reconstructed speech with this modification has an improvement in MBSD (Modified Bark Spectral Distortion). When a maximum limitation less than one to the psychoacoustical weighting rule is further proposed, the higher the input SNR (> 0 dB) is, the more improvement the segmental SNR and the overall SNR have. The informal listening tests show that there is little speech distortion for the enhanced speech processed by the proposed method, the background noise is reduced much and free of musical noise.     

Copolymerization of carbon dioxide and propylene oxide under inorganic oxide supported rare earth ternary catalyst

Recently, rare earth ternary coordination catalyst represented as Y(CCl₃OO)₃‐Glycerin‐ZnEt₂ has been used for producing poly(propylene carbonate) (PPC, an alternating copolymer of carbon dioxide and propylene oxide) in industry scale, but its catalytic activity needs further improvement. One reason for the relatively low catalytic activity lied in that only 11.7% of active center was efficient due to possible embedding of active center in the heterogeneous catalyst. In this report, supporting strategy was developed, where Y(CCl₃OO)₃‐Glycerin‐ZnEt₂ was supported on various inorganic oxides. Two supporting methods were carried out. One way was to mix Y(CCl₃OO)₃‐Glycerin with inorganic oxide first and then ZnEt₂ was dropped to form the supported catalyst, and the other was to make Y(CCl₃OO)₃‐Glycerin‐ZnEt₂ at first and then mixing with inorganic oxides. The former showed decreasing catalytic activity compared with corresponding unsupported rare earth ternary catalyst, while an improvement of 16–36% in catalytic activity was realized in the latter. PPC with an average number molecular weight (M_n) of over 100 kg/mol and carbonate unit (CU) content of higher than 96% was prepared by both supported catalysts. The catalytic activity of the supported catalyst depended significantly on the supports, which increased in the following order: α‐Al₂O₃ < MgO < ZnO ≈ SiO₂ <γ‐Al₂O₃. γ‐Al₂O₃ was the best support for rare earth ternary catalyst, which showed a remarkable 36% increase in catalytic activity, corresponding to the utilization of 17% of active center. Although MgO supported catalyst gave only an 8% increase in catalytic activity, the M_n and CU content of PPC were raised to about 143 kg/mol and 99%, whereas the PPC from common rare earth ternary catalyst was about 108 kg/mol and 97%, respectively. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Shape optimization for Stokes flow

This paper is concerned with a shape sensitivity analysis of a viscous incompressible fluid driven by Stokes equations. The structures of continuous shape gradients with respect to the shape of the variable domain for some given cost functionals are established by introducing the Piola transformation and then deriving the state derivative and its associated adjoint state. Finally we give the finite element approximation of the problem and a gradient type algorithm is effectively used for our problem.     

A V-cycle multigrid method for a viscoelastic fluid flow satisfying an Oldroyd-B-type constitutive equation

A V-cycle multigrid method is developed for a time-dependent viscoelastic fluid flow satisfying an Oldroyd-B-type constitutive equation in two-dimensional domains. Also existence, uniqueness, and error estimates of an approximate solution are discussed. The approximate stress, velocity, and pressure are, respectively, σ _k -discontinuous, u _k -continuous, and p _k -continuous.     

Effect of cellulose–lignin interactions on char structural changes during fast pyrolysis at 100–350 °C

This study investigates the cellulose–lignin interactions during fast pyrolysis at 100–350 °C for better understanding fundamental pyrolysis mechanism of lignocellulosic biomass. The results show that co-pyrolysis of cellulose and lignin (with a mass ratio of 1:1) at temperatures < 300 °C leads to a char yield lower than the calculated char yield based on the addition of individual cellulose and lignin pyrolysis. The difference between the experimental and calculated char yields increases with temperature, from ～2% 150 °C to ～6% at 250 °C. Such differences in char yields provide direct evidences on the existence of cellulose–lignin interactions during co-pyrolysis of cellulose and lignin. At temperatures below 300 °C, the reductions in both lignin functional groups and sugar structures within the char indicate that co-pyrolysis of cellulose and lignin enhances the release of volatiles from both cellulose and lignin. Such an observation could be attributed to two possible reasons: (1) the stabilization of lignin-derived reactive species by cellulose-derived reaction intermediates as hydrogen donors, and (2) the thermal ejection of cellulose-derived species due to micro-explosion of liquid intermediates from lignin. In contrast, at temperatures ≥ 300 °C, co-pyrolysis of cellulose and lignin increases char yields, i.e., with the difference between the experimental and calculated char yields increasing from ～1% at 300 °C to ～8% at 350 °C. The results indicate that the cellulose-derived volatiles are difficult to diffuse through the lignin-derived liquid intermediates into the vapor phase, leading to increased char formation from co-pyrolysis of cellulose and lignin as temperature increases. Such an observation is further supported by the increased retention of cellulose functional groups in the char from co-pyrolysis of cellulose and lignin.     

Thermally Evaporated Ag–Au Bimetallic Catalysts for Efficient Electrochemical CO2 Reduction

CO₂ reduction reaction (CO₂RR) has indispensable significance for carbon recycling and renewable energy production. As typical electrochemical catalysts, Au and Ag show relatively high reaction activity and selectivity in CO₂RR. In this study, a series of Ag–Au bimetallic catalysts are designed and synthesized through the thermal evaporation method for efficient yet massive production of electrochemical catalysts. The Ag–Au catalysts show significantly enhanced activity and selectivity in CO₂RR, which is mainly attributed to the increased grain boundaries with well-dispersed single Ag atoms. After the optimization, Au20Ag10 exhibits the best performance with a CO Faraday efficiency of 89% at −0.9 V (vs the reversible hydrogen electrode) with good stability.     

Progress in thermal-mechantical effects induced by laser

对激光辐照诱导的热与力学问题研究进展进行了综述,包括材料在高温、高升温速率下的本构关系,典型薄板和柱壳等结构在激光辐照下的热力破坏效应,多层材料体系的激光破坏行为等几个方面,并着重介绍了包含相变与烧蚀过程的激光破坏分析模型与机制研究,激光辐照效应的流-热-固耦合数值模拟方法,以及短脉冲激光引起的冲击与破坏机理等方面的研究新进展.