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   7449篇
  免费   1020篇
  国内免费   599篇
化学   1961篇
晶体学   22篇
力学   2631篇
综合类   187篇
数学   2156篇
物理学   2111篇
  2024年   30篇
  2023年   88篇
  2022年   219篇
  2021年   220篇
  2020年   291篇
  2019年   222篇
  2018年   249篇
  2017年   295篇
  2016年   323篇
  2015年   278篇
  2014年   303篇
  2013年   576篇
  2012年   391篇
  2011年   398篇
  2010年   307篇
  2009年   342篇
  2008年   366篇
  2007年   369篇
  2006年   412篇
  2005年   368篇
  2004年   332篇
  2003年   346篇
  2002年   261篇
  2001年   239篇
  2000年   255篇
  1999年   217篇
  1998年   189篇
  1997年   156篇
  1996年   158篇
  1995年   126篇
  1994年   108篇
  1993年   99篇
  1992年   86篇
  1991年   72篇
  1990年   67篇
  1989年   40篇
  1988年   54篇
  1987年   37篇
  1986年   22篇
  1985年   28篇
  1984年   27篇
  1983年   9篇
  1982年   28篇
  1981年   13篇
  1980年   6篇
  1978年   8篇
  1977年   7篇
  1976年   4篇
  1975年   4篇
  1957年   13篇
排序方式: 共有9068条查询结果,搜索用时 10 毫秒
21.
We present a complete characterization of Gabor frame operators on finite dimensional Hilbert space $L^2(\Gamma)$, where $\Gamma = \mathbb{Z}_{m_1} \times \mathbb{Z}_{m_2} \times \dots \times \mathbb{Z}_{m_p}$ and $m_1,m_2,\dots,m_p$ are positive integers. The notion of generalized $B$-modulation and generalized $B$-translation is introduced and some significant properties of the generalized pseudo $B$-Gabor like frames are discussed.  相似文献   
22.
随着光伏行业的快速发展, 对硅单晶的品质和长晶装备的稳定性的要求也不断提高。直拉法是生产硅单晶的主要方法,通过提高单晶炉副室的高度以扩大单晶硅的生产规模。由于副室高度的大幅增加,且单晶炉提拉头质心相对于旋转轴心有一定距离,对单晶炉整体稳定性有较大影响,从而降低了单晶硅的生产质量。针对此问题,对单晶炉建立可靠的力学分析模型,采用数值仿真方法,对单晶炉整体进行动力学响应分析,计算得到副室高度增加后的单晶炉工作时中钨丝绳下端晶棒的运动规律以及最大摆动幅度,为改进设计提供依据。数值仿真分析表明提高单晶炉副室高度后,提拉头较大的质心偏心是单晶炉提拉系统发生摆动的主要原因。在此基础上提出在提拉头上添加质心调节装置,通过控制系统调节可保证提拉头质心位置在旋转轴线上以降低提拉系统的摆动。  相似文献   
23.
As important biomolecules in Camellia sinensis L., amino acids (AAs) are considered to contribute to the overall green tea sensory quality and undergo dynamic changes during growth. However, limited by analytical capacity, detailed AAs composition in different growth stages remains unclear. To address this question, we analyzed the dynamic changes of 23 AAs during leaf growth in Xinyang Mao Jian (XYMJ) green tea. Using amino acid analyzer, we demonstrated that most AAs are abundant on Pure Brightness Day and Grain Rain Day. After Grain Rain, 23 AAs decreased significantly. Further analysis shows that theanine has a high level on the day before Spring Equinox and Grain Rain, accounting for 44–61% of the total free AAs content in tea leaves. Glu, Pro, and Asp are the second most abundant AAs. Additionally, spinasterol and 22,23-dihydrospinasterol are first purified and identified in ethanol extract of XYMJ by silica gel column chromatography method. This study reveals the relationship between plucking days and the dynamic changes of AAs during the growth stage and proves the rationality of the traditional plucking days of XYMJ green tea.  相似文献   
24.
1H NMR measurements are reported for the CD2Cl2/CDCl3 solutions of the Co(II) calix[4]arenetetraphosphineoxide complex (I). Temperature dependences of the 1H NMR spectra of I have been analyzed using the line shape analysis, taking into account the temperature variation of paramagnetic chemical shifts, within the frame of the dynamic NMR method. Conformational dynamics of the 2:1 Co(II) calix[4]arene complexes was conditioned by the pinched conepinched cone interconversion of I (with activation Gibbs energy ΔG(298K) = 40 ± 3 kJ/mol. Due to substantial temperature dependence of paramagnetic shifts, complex I can be used as model compound for designing an NMR thermosensor reagent for local temperature monitoring.  相似文献   
25.
提出了采用高阶常微分方程模型代替传统时序分析中所用的ARMA模型来实现一维时间序列的建模和预报.设计的将遗传程序设计与遗传算法相嵌套的动态演化建模算法,用遗传程序设计优化模型结构,以遗传算法优化模型参数,边收集数据边建模边预报,首次成功地实现了时间序列实时预报的程序自动化.两个时间序列的应用实例表明采用此算法可获得较好的实时预报效果.  相似文献   
26.
The effect of H impurity on the misfit dislocation in Ni-based single-crystal superalloy is investigated using the molecular dynamic simulation.It includes the site preferences of H impurity in single crystals Ni and Ni 3 Al,the interaction between H impurity and the misfit dislocation and the effect of H impurity on the moving misfit dislocation.The calculated energies and simulation results show that the misfit dislocation attracts H impurity which is located at the γ/γˊinterface and Ni3 Al and H impurity on the glide plane can obstruct the glide of misfit dislocation,which is beneficial to improving the mechanical properties of Ni based superalloys.  相似文献   
27.
本文介绍了一种测试继电器在动态过程中动态参数测试的方法。采用此方法能直接测量和观察继电器动态过程中的线圈电压、电流 ,同时可测出继电器的动作时间 ,还可了解其接点在接触过程是否有振动现象存在。所以 ,此方法是一种多用途的继电器动态参数测量方法  相似文献   
28.
Some new 1,2,4-triazole derivatives have been synthesized and characterized by TLC, IR, NMR and mass spectra. Densities, viscosities and ultrasonic velocities of these compounds have been measured over the wide composition range at 308.15?K in dimethyl formamide (DMF) and tetrahydro furan (THF). From these data, various acoustical and thermodynamic parameters (C.V. Suryanarayana, J. Kuppuswamy. J. Acoust. Soc. Ind., 4, 75 (1976); H. Erying, M.S. John, Significance of Liquid Structures, Wiley, New York (1969); G.K. Johri, R.C. Misra. Acustica, 56, 66 (1989)) were evaluated. Some of these parameters are isentropic compressibility (κs), intermolecular free length (L f), relaxation strength (r), relative association (R A), Rao's molar constant (R m), van der Waal's constant (b), molar compressibility (W), internal pressure (π), free volume (V f), solvation number (S n) etc. The behavior of solutions of these compounds in DMF and THF are explained from the evaluated parameters.  相似文献   
29.
Emamectin benzoate, a macrocyclic lactone, can be used in low quantities to control arthropod pests, effectively. However, its poor photostability prevents its further use. To delay its photodegradation, novel acrylate‐type polymeric nanoparticles were synthesized and tested as materials for improving pesticide photostability. N‐acylated emamectin benzoate was synthesized via bonding emamectin benzoate to acrylamide. The resulting pesticide, containing the double bond linkage –C=C–N–, was copolymerized with butyl acrylate and methyl methacrylate by the emulsion polymerization method. The refined polymers were characterized by Fourier transform infrared spectroscopy spectroscopy, and result illustrated the pesticide was conjugated to the polymers. Atomic force microscope and dynamic light scattering analyses were also used for determining the average particle diameters of pesticide–polymer conjugates. Photostability tests showed that the nanoparticles obtained exhibited greatly improved photostability. Additionally, the laboratory toxicity tests demonstrated that the insecticidal effects of the novel emamectin benzoate formulation were better than those of the control pesticide formulation (emamectin benzoate EC). Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
30.
The dispersed structures of mixtures of five different gasolines with anhydrous ethanol were investigated by the dynamic light scattering (DLS) method. The aim of the work was to find whether these blends are colloid systems. Influence of different parameters was investigated to verify the results of this research. Ethanol‐gasoline blends were found to be colloid systems with the drop size of 20–150 nm.  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