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   516篇
  免费   21篇
  国内免费   1篇
化学   317篇
晶体学   9篇
力学   19篇
数学   111篇
物理学   82篇
  2023年   5篇
  2022年   5篇
  2021年   7篇
  2020年   12篇
  2019年   15篇
  2018年   13篇
  2017年   10篇
  2016年   36篇
  2015年   20篇
  2014年   28篇
  2013年   89篇
  2012年   28篇
  2011年   41篇
  2010年   35篇
  2009年   21篇
  2008年   21篇
  2007年   20篇
  2006年   14篇
  2005年   9篇
  2004年   3篇
  2003年   3篇
  2002年   8篇
  2001年   5篇
  2000年   5篇
  1999年   5篇
  1998年   6篇
  1997年   8篇
  1996年   7篇
  1995年   5篇
  1994年   4篇
  1993年   6篇
  1992年   4篇
  1991年   6篇
  1990年   4篇
  1989年   7篇
  1987年   2篇
  1986年   1篇
  1985年   2篇
  1984年   1篇
  1983年   3篇
  1982年   1篇
  1981年   3篇
  1980年   1篇
  1976年   1篇
  1975年   2篇
  1948年   1篇
  1932年   1篇
  1908年   1篇
  1905年   2篇
  1887年   1篇
排序方式: 共有538条查询结果,搜索用时 46 毫秒
41.
Intra-organizational network research had its first heyday during the empirical revolution in social sciences before World War II when it discovered the informal group within the formal organization. These studies comment on the classic sociological idea of bureaucracy being the optimal organization. Later relational interest within organizational studies gave way to comparative studies on the quantifiable formal features of organizations. There has been a resurgence in intra-organizational networks studies recently as the conviction grows that they are critical to organizational and individual performance. Along with methodological improvements, the theoretical emphasis has shifted from networks as a constraining force to a conceptualization that sees them as providing opportunities and finally, as social capital. Because of this shift it has become necessary not only to explain the differences between networks but also their outcomes, that is, their performance. It also implies that internal and external networks should no longer be treated separately.Research on differences between intra-organizational networks centers on the influence of the formal organization, organizational demography, technology and environment. Studies on outcomes deal with diffusion and adaptation of innovation; the utilization of human capital; recruitment, absenteeism and turnover; work stress and job satisfaction; equity; power; information efficiency; collective decision making; mobilization for and outcomes of conflicts; social control; profit and survival of firms and individual performance.Of all the difficulties that are associated with intra-organizational network research, problems of access to organizations and incomparability of research findings seem to be the most serious. Nevertheless, future research should concentrate on mechanisms that make networks productive, while taking into account the difficulties of measuring performance within organizations, such as the performance paradox and the halo-effect.  相似文献   
42.
Laser spectroscopic techniques at low temperature, such as fluorescence line-narrowing and hole burning, enable an increase of spectral resolution by a factor of 103–105 compared to conventional spectroscopy at room temperature. With these methods, it is possible to retrieve a fingerprint of the species involved and to measure the rates of dynamic processes that normally remain hidden in the broad absorption bands. A few applications carried out in our laboratory will be discussed: (1) the determination of energy transfer rates in the peripheral LH2 complex of purple bacteria; (2) the study of spectral diffusion and its implications in three types of systems: (a) the B820 and B777 subunits of the LH1 complex of purple bacteria, (b) the photosystem II reaction center (PSII RC) and CP47 antenna complex of green plants, and (c) an organic glass doped with bacteriochlorophyll a; (3) the unraveling of 0-0 transitions and the pathways of photoconversion between a number of conformations of the green fluorescent protein mutant S65T; (4) the measuring of electron-phonon coupling strengths in PSII RC and the red fluorescent protein DsRed; and (5) the determination and comparison of the homogeneous linewidths and optical dephasing in photosynthetic chromoprotein complexes and autofluorescent proteins.  相似文献   
43.
The problem of embedding of linear spaces in finite projective planes has been examined by several authors ([1], [2], [3], [4], [5], [6]). In particular, it has been proved in [1] that a linear space which is the complement of a projective or affine subplane of order m is embeddable in a unique way in a projective plane of order n. In this article, we give a generalization of this result by embedding linear spaces in a finite projective plane of order n, which are complements of certain regularA-affine linear spaces with respect to a finite projective plane.  相似文献   
44.
A series of alkyl α‐hydroxymethacrylate derivatives with various secondary functionalities (ether, ester, carbonate, and carbamate) and terminal groups (alkyl, cyano, oxetane, cyclic carbonate, phenyl and morpholine) were synthesized to investigate the effect of intermolecular interactions, H‐bonding, π–π interactions, and dipole moment on monomer reactivity. All of the monomers except one ester and one ether derivative are novel. The polymerization rates, determined by using photo‐DSC, showed the average trend (aromatic carbamate > hydroxyl > ester > carbonate ~ aliphatic carbamate ~ ether), with several exceptions due to the differences in terminal groups. There is a correlation between the chemical shift differences of the double bond carbons, the calculated dipole moments, and the reactivities only for nonhydrogen bonded monomers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011  相似文献   
45.
Let (u n ) be a sequence of real numbers. In this paper we introduce some Tauberian conditions in terms of regularly generated sequences for (A, k) summability method.  相似文献   
46.
The title polymeric complex {[Mn(O2C(CF2)8CO2)(phen)2]H2O}n was synthesized through the reaction of 1,10-phenanthroline, perfluorosebacic acid and MnCO3 · H2O. The molecular structure was characterized by X-ray diffraction, elemental analysis, thermal gravimetry, IR and UV–Vis spectroscopy and its catalytic activity has been studied. X-ray structure analysis shows that each Mn(II) ion is octahedrally coordinated by two bidentate phenanthroline ligands and the carboxylate oxygen atoms from two symmetry related perfluorosebacate ligands, which are coordinated in cisoid positions. The structure consists of polymeric chains, with the perfluorosebacato ligand bridging the Mn(II) ions in a monodentate fashion. Crystallographic characterization shows a supramolecular structure involving hydrogen bonds, π–π and π-ring interactions. The catalytic results indicated that the complex has reasonably good activity towards the disproportionation of hydrogen peroxide into water and dioxygen in methanol and it does not exhibit saturation kinetics with the substrate. The initial reaction rates and their temperature and base dependencies were investigated by monitoring the dioxygen evolution. Kinetic studies revealed a first-order dependence on the catalyst concentration. Activation parameters have been calculated at 301 K.  相似文献   
47.
Beam search (BS) is used as a heuristic to solve various combinatorial optimization problems, ranging from scheduling to assembly line balancing. In this paper, we develop a backtracking and an exchange-of-information (EOI) procedure to enhance the traditional beam search method. The backtracking enables us to return to previous solution states in the search process with the expectation of obtaining better solutions. The EOI is used to transfer information accumulated in a beam to other beams to yield improved solutions.  相似文献   
48.
A series of seven-membered ditopic ring-expanded N-heterocyclic carbene (dre-NHC) precursors, bearing sterically demanding and electron-rich aryl groups, were synthesised in moderate yields via the reaction of 1,2,4,5-tetrakis(bromomethyl)benzene with the corresponding N,N′-diarylformamidines in the presence of K2CO3 in acetonitrile under an air atmosphere. All new compounds were characterised by HRMS, NMR spectroscopy, and microanalysis, as well as X-ray crystallography for compound 1c. The development of an efficient catalytic system for the Suzuki-Miyaura coupling reaction of aryl chlorides with various boronic acids was also investigated using the in situ generated dre-NHC ligands.  相似文献   
49.

Thin film CdIn2Te4/CdS solar cells were deposited onto the ITO-coated glass substrate by electron beam evaporation (e-beam) technique, and the the effect of annealing on their structural properties is studied. The annealing was performed under nitrogen atmosphere for 1 h. The manufactured solar cells were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDAX) analysis. Crystallite size (D), inter-planer distance (d) and lattice constant (a) values were calculated for the thin film solar cell from XRD data. Annealed samples display well defined XRD patterns with three diffraction peaks. We observed increased peak intensity in the annealed films. EDAX analysis showed that only CdIn2Te4 is present in absorber layer and CdS is found in the window layer, but no impurity atoms are present the structure. It is observed that surface roughness of the annealed films incresed, according to SEM images. The I–V characteristics show that the current is increased for annealed thin films solar cells.

  相似文献   
50.
Oxalic acid adsorption from aqueous solution is studied in this work. Multi-walled carbon nanotubes (MWCNT) were used as an adsorbent. The investigated adsorption variables are equilibrium time, initial acid concentration, and temperature. The experimental results were presented using equilibrium isotherm and kinetic models. The used equilibrium models are Langmuir, Freundlich, and Temkin adsorption isotherms. And the kinetic models are Elovich, Lagergren pseudo-first-order and pseudo-second-order kinetic models. The thermodynamics studies were carried out at three different temperatures: 278, 298, and 318 K. Langmuir isotherm was the best fitted equilibrium model for the experimental data. The all applied kinetic models fitted the data suitably.  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