首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   240篇
  免费   5篇
  国内免费   4篇
化学   134篇
晶体学   1篇
力学   7篇
数学   65篇
物理学   42篇
  2023年   1篇
  2022年   1篇
  2021年   3篇
  2020年   3篇
  2019年   5篇
  2018年   1篇
  2017年   1篇
  2016年   1篇
  2015年   1篇
  2014年   6篇
  2013年   20篇
  2012年   12篇
  2011年   14篇
  2010年   14篇
  2009年   10篇
  2008年   16篇
  2007年   14篇
  2006年   9篇
  2005年   13篇
  2004年   11篇
  2003年   10篇
  2002年   14篇
  2001年   2篇
  2000年   3篇
  1999年   11篇
  1998年   5篇
  1997年   3篇
  1996年   3篇
  1995年   2篇
  1994年   4篇
  1993年   1篇
  1992年   3篇
  1991年   2篇
  1990年   4篇
  1989年   3篇
  1988年   1篇
  1986年   1篇
  1985年   3篇
  1984年   2篇
  1983年   1篇
  1982年   2篇
  1980年   1篇
  1978年   2篇
  1977年   2篇
  1976年   2篇
  1975年   1篇
  1972年   1篇
  1971年   1篇
  1970年   1篇
  1968年   2篇
排序方式: 共有249条查询结果,搜索用时 17 毫秒
61.
Hsu MH  Chen RT  Sheu WS  Shieh M 《Inorganic chemistry》2006,45(17):6740-6747
The novel hydroxo-bridged dimeric lead-chromium carbonyl complex [Et4N]2[{PbCr2(CO)10}2(mu-OH)2] ([Et4N]2[1]) was synthesized from the reaction of PbCl2 and Cr(CO)6 followed by metathesis with [Et4N]Br in a KOH/MeOH solution. The X-ray crystallographic structure shows that dianion 1 consists of two Pb{Cr(CO)5}2 units bridged by two hydroxo fragments in which the Pb atoms are further coordinated with two Cr(CO)5 groups, resulting in a distorted tetrahedral geometry. A CO2 molecule can insert itself into dianion 1 to form two new carbonate complexes, [Et(4)N]2[{PbCr2(CO)10}(CO3)] ([Et4N]2[2]) and [Et4N]2[{PbCr2(CO)10}2(CO3)] ([Et4N]2[3]), depending on the reaction conditions. In addition, complex 2 can be transformed into 3 in CH2Cl2 solution at an elevated temperature. While the carbonate group in dianion 2 is bonded to one Pb atom, which is coordinated with two Cr(CO)5 fragments, the carbonate group in 3 bridges the two Pb centers in a mu-1kappa2OO':2kappa2OO' fashion in which each Pb atom is further bonded to two Cr(CO)5 moieties. Complexes 2 and 3 can be converted back the hydroxo complex 1 under appropriate conditions. All three unprecedented lead-chromium compounds, 1-3, were fully characterized by spectroscopic methods and single-crystal X-ray diffraction analyses. The nature and formation of complexes 1-3 were also examined by molecular orbital calculations using the B3LYP method of the density functional theory.  相似文献   
62.
Dye-doped organic light-emitting diode of ITO/alpha-NPB (70 nm)/Bebq(2)-1 (7 nm)/BCP (5 nm)/Bebq(2) (33 nm)/LiF (1 nm)/Al (150 nm) shows red electroluminescence with the efficiency of 2.9 cd/A at 100 cd/m(2) and maximum brightness of 62000 cd/m(2). The physical organic aspects of the current-induced fluorescent quenching effect are discussed. [structure: see text]  相似文献   
63.
In this paper, the authors study partial inverse nodal problems for differential pencils on a star-shaped graph. We firstly show that the potential on each edge can be uniquely determined by twin-dense nodal subsets on some interior intervals under certain conditions. Without any nodal information on some potential on the fixed edge, we may add some spectral information to guarantee these uniqueness theorems. We still consider the case of arbitrary intervals having the internal vertex. In particular, we pose and solve a new partial inverse nodal problem for differential pencils on the star-shaped graph from the Weyl m-function and the theory concerning densities of zeros of entire functions.  相似文献   
64.
In this paper, Hamilton’s principle is employed to derive Lagrange’s equations of an liquid crystal display (LCD) glass-handling robot driven by a permanent magnet synchronous motor (PMSM). The robot has three arms driven by two timing belts. The dynamic formulations can be expressed by one and four independent variables, which are named as the rigid and flexible models, respectively. In order to verify the dynamic formulation is correct, we reduce the flexible model to the rigid one under some assumptions. In this paper, we adopt the real-coded genetic algorithm (RGA) to identify all the parameters of the robot and PMSM simultaneously. It is found that the RGA can identify system parameters which are difficult to be measured in practical problems, for examples, the inductance, stator resistance, motor torque constant, damping coefficient of the motor and timing belts. In numerical simulations, vibrations due to flexibility of the timing belts are investigated for the angular displacements, speeds, accelerations of arms, and the horizontal and vertical displacements of the robot. The angular displacements of the robot arm and the translational positions of the robot end are obtained in the numerical simulations and experimental results. From their comparisons, it is demonstrated that identification results of the dynamic model with four independent variables present the better matching with experimental results of the system.  相似文献   
65.
This study proposes a novel Forward Search and Backward Trace (FSBT) technique based on Rough Set Theory to improve data analysis and extend the scope of observations made from sample data to solve personal investment portfolio problems. Rough Set Theory mathematically classifies data into class sets. The class set with the most objects may generate one decision rule. The rules generated from RST are rough and fragmented, that are very difficult to interpret the information. An empirical case is used to generate more than 85 rules by the RST method in comparison with FSBT method which only generated 14 rules. This result can show our proposed method is better than traditional RST method based on class sets that contain the most objects. Much of human knowledge is described in natural language. It is a very important thing to convert information from computer databases into normal human language. Sample data taken from features with the same backgrounds are used to compile different portfolios that investment companies and investment advisors can employ to satisfy the investor’ needs. The method not only can provide decision-making rules, but also can offer alternative strategies for better data analysis. We believe that the FSBT technique can be fully applied in research on investment marketing.  相似文献   
66.
We prove that under certain conditions, a vectorial Sturm-Liouville differential equation of dimension can only possess finitely many eigenvalues which have multiplicity . For the case , we find a sufficient condition on the potential function , and a bound depending on , such that the eigenvalues of the equation with index exceeding are all simple. These results are applied to find some sufficient conditions which imply that the spectra of two potential equations, or two string equations, have finitely many elements in common, and an estimate of the number of elements in the intersection of two spectra is provided.

  相似文献   

67.
In this paper, a new method for robust H controller of uncertainsystems with time-varying state and control input delays isproposed. A sufficient condition, which is dependent on themaximum value of the time derivative of time-varying delay,is also presented for the existence of H controller. The proposedcontroller not only stabilizes closed-loop uncertain systemsbut also guarantees a prescribed H norm bound of the closed-looptransfer matrix from the disturbance to controlled output. Furthermore,we propose a new robust stability criterion to decide the allowedmaximum bound of plant uncertainty which still can be stabilizedby proposed robust controller. An example given illustratesthe availability of the proposed design method.  相似文献   
68.
Au nanoparticles modified with 21-base thiolated-oligonucleotides have been evaluated as delivery vehicles for the development of a nonviral transfection platform. The electromigration combined with electroporation for DNA delivery in an osteoblast like cell was employed to test on microchips. Electroporation introduces foreign materials into cells by applying impulses of electric field to induce multiple transient pores on the cell membrane through dielectric breakdown of the cell membrane. On the basis of the characteristic surface plasmon of the Au particles, UV-vis absorption was utilized to qualitatively judge the efficiency of delivery. Transmission electron microscopy images and atomic absorption measurements (quantitative analysis) provided evidence of the bare Au and Au/oligonucleotide nanoparticles before and after electroporation and electromigration function.The experiments demonstrated that electrophoretic migration followed by electroporation significantly enhanced the transportation efficiency of the nanoparticle-oligonucleotide complexes as compared with electroporation alone. Most interestingly, Au capped with oligonucleotides led to optimal performance. On the other hand, the bare Au colloidal suspensions resulted in aggregation, which might be an obstacle to the internalization process. In addition, analytical results demonstrated an increase in the local particle concentrations on the cell surface that provided additional support for the mechanism underlying the improved Au nanoparticle transportation into cells in the presence of electromigration function.  相似文献   
69.
Compounds used in high throughput screening (HTS) are typically dissolved in DMSO. These solutions are stored automation-friendly racks of wells or tubes. DMSO is hygroscopic and quickly absorbs water from the atmosphere. When present in DMSO compound solutions, water can accelerate degradation and precipitation. Understanding DMSO hydration in an HTS compound library can improve storage and screening methods by managing the impact of water on compound stability. A non-destructive, acoustic method compatible with HTS has been developed to measure water content in DMSO solutions. Performance of this acoustic method was compared with an optical technique and found to be in good agreement. The accuracy and precision of acoustic measurements was shown to be under 3% over the tested range of DMSO solutions (0% to 35% water by volume) and insensitive to the presence of HTS compounds at typical storage concentrations. Time course studies of hydration for wells in 384-well and 1536-well microplates were performed. Well geometry, fluid volume, well position and atmospheric conditions were all factors in hydration rate. High rates of hydration were seen in lower-volume fills, higher-density multi-well plates and when there was a large differential between the humidity of the lab and the water content of the DMSO. For example, a 1536-well microplate filled with 2microL of 100% DMSO exposed for one hour to a laboratory environment with approximately 40% relative humidity will absorb over 6% water by volume. Understanding DMSO hydration rates as well as the ability to reverse library hydration are important steps towards managing stability and availability of compound libraries.  相似文献   
70.
The occurrence of hydride-transfer reactions during the cationic polymerization of trioxane was demonstrated, and rate constants were obtained. The donor of hydride ions in the transfer reactions was the monomer. The hydride-transfer reaction was a first-order reaction with respect to the concentration of the monomer, and it was governed, just as polymerization and depolymerization were (Shieh, Y. T.; Chen. S. A. J. Polym. Sci. Part A: Polym. Chem. 1999, 37, 483–492) by morphological changes. The hydride-transfer rate constants were 5 orders of magnitude smaller than those for polymerizations and depolymerizations. The rate constants for the reactions, including the polymerizations, depolymerizations, and hydride transfers, were smaller for the active centers on the solid surface than for those in solution, that is, kp was less than kp, kd was less than kd, and kht was less than kht. As a reaction medium, benzene had special effects on the kinetics of the cationic polymerization of trioxane. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4198–4204, 1999  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号