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   78781篇
  免费   6575篇
  国内免费   8969篇
化学   55992篇
晶体学   2083篇
力学   5972篇
综合类   832篇
数学   10226篇
物理学   19220篇
  2024年   158篇
  2023年   720篇
  2022年   1797篇
  2021年   1825篇
  2020年   2128篇
  2019年   2001篇
  2018年   1762篇
  2017年   2180篇
  2016年   2976篇
  2015年   2575篇
  2014年   3367篇
  2013年   5604篇
  2012年   5540篇
  2011年   4319篇
  2010年   3659篇
  2009年   4668篇
  2008年   4774篇
  2007年   5113篇
  2006年   4564篇
  2005年   4162篇
  2004年   4070篇
  2003年   3427篇
  2002年   3238篇
  2001年   2276篇
  2000年   2295篇
  1999年   1926篇
  1998年   1751篇
  1997年   1486篇
  1996年   1423篇
  1995年   1316篇
  1994年   1199篇
  1993年   950篇
  1992年   931篇
  1991年   660篇
  1990年   573篇
  1989年   403篇
  1988年   401篇
  1987年   266篇
  1986年   234篇
  1985年   266篇
  1984年   217篇
  1983年   142篇
  1982年   174篇
  1981年   128篇
  1980年   115篇
  1979年   143篇
  1978年   86篇
  1977年   70篇
  1976年   64篇
  1973年   57篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
51.
Technical advances in the development of field-deployable capillary and microchip electrophoretic instruments and reports of their deployment between 2013 and 2017 were reviewed. Strategies and considerations in the design of the injection, separation and detection hardware, chemistry and associated infrastructure were discussed from an in-field perspective, with portability, robustness and automation/“ease of use” featuring as key requirements. Integration of functionality is important for adequate in-field performance. Progress was made towards the use of multiple channel devices for increased throughput and/or resolving power, mixing devices for on-line/in-line sample derivatization, battery operation and temperature control. The strengths and weaknesses of the various approaches described in the literature are discussed from the perspective of in-field operation. An overview of the applications of the field electrophoretic instruments is provided, including environmental science and planetary investigation.  相似文献   
52.
文[1]提出了两个DEA的逆问题,并用搜索法来解.而本文根据所证的定理,对每个问题一般只要解二、三个线性规划问题就能得到答案.  相似文献   
53.
1INTRODUCTION Recently,the design and preparation of coordina-tion polymers with new porous frameworks are of great interest due to their potential functionalities such as catalysis,gas absorption,separation and molecu-lar recognition.In designing1-D,2-D and3-D porous coordination polymers,the selection of appropriate ligands is crucial for determining the structure.The mixed-linker systems of both carboxylates and pyri-dyls have proved to be effective for the preparation of novel coordi…  相似文献   
54.
Sensitivity analysis is a mathematical tool, first developed for optimization methods, which aim is to characterize a system response through the variations of its output parameters following modifications imposed on the input parameters of the system. Such an analysis may quickly become laborious when the thermal model under consideration is complex or the number of input parameters is high. In this paper, we develop a mathematical model to analyse the heat exchanges in four different types of solar air collectors. When building this thermal model we show that for each collector, at quasi-steady state, the energy balance equations of the components of the collector cascade into a single first-order non-linear differential equation that is able to predict the thermal behaviour of the collector. Our heat transfer model clearly demonstrates the existence of an important dimensionless parameter, referred to as the thermal performance factor of the collector, that compares the useful thermal energy which can be extracted from the heater to the overall thermal losses of that collector for a given set of input parameters. A sensitivity analysis of our thermal model has been performed for the most significant input parameters such as the incident solar irradiation, the inlet fluid temperature, the air mass flow rate, the depth of the fluid channel, the number and nature of the transparent covers in order to measure the impact of each of these parameters on our model. An important result which can be drawn from this study is that the heat transfer model developed is robust enough to be used for thermal design studies of most known flat plate solar air heaters, but also of flat plate solar water collectors and linear solar concentrators.  相似文献   
55.
The polycrystalline perovskitelike manganese oxides La1-xAxMnO (A = Na, and K, ) have been fabricated by sol-gel technique. For all the compositions explored in this work, the average manganese oxidation state is practically constant, at for A = Na, and for A = K, respectively. A close relationship is confirmed to hold between the Curie temperature (Tc) and the bond distance of Mn-O. Results of magnetic measurements show that these materials can be utilized as suitable candidates for magnetic refrigerants with wide applied temperature span, for their significant entropy change and the easily tuned Curie temperature. Received: 12 September 1997 / Revised: 18 December 1997 / Accepted: 21 January 1998  相似文献   
56.
57.
58.
本文给出了C^n中子空间之间最大和最小主角在矩阵逼近,投影算子的扰动分析,群逆以及Oraxin逆的扰动估计,条件数理论,Bott-Duffin系统扰动分析中一些应用。  相似文献   
59.
L-Quebrachitol (1) isolated from the root of Elaeagnus formosana Nakai (Elaeagnaceae) has not been reported before for any species of the genus of Elaeasnus. Its structure was established from spectral data and was completely characterized by single-crystal X-ray analysis. L-Quebrachitol crystallizes in the monoclinic system, space group P21, with cell parameters are a = 6.702(4), b = 7.207(4), c = 8.758(5) Å, β = 90.24(5)° and Z = 2.  相似文献   
60.
 应用高压原位差热方法,直接测量了压力下锗的固化参数─—固化温度与过冷度。高压差热信号表明,当压力大于3 GPa时,锗在凝固过程中可能发生结构相变。X射线结构分析表明,在最终的样品中除GeⅠ相外,还形成GeⅢ相和GeⅣ相。  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