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   852篇
  免费   83篇
  国内免费   75篇
化学   395篇
晶体学   3篇
力学   343篇
综合类   15篇
数学   128篇
物理学   126篇
  2024年   3篇
  2023年   2篇
  2022年   16篇
  2021年   20篇
  2020年   25篇
  2019年   27篇
  2018年   22篇
  2017年   27篇
  2016年   27篇
  2015年   30篇
  2014年   40篇
  2013年   44篇
  2012年   58篇
  2011年   71篇
  2010年   53篇
  2009年   67篇
  2008年   65篇
  2007年   50篇
  2006年   33篇
  2005年   44篇
  2004年   38篇
  2003年   34篇
  2002年   28篇
  2001年   12篇
  2000年   29篇
  1999年   15篇
  1998年   24篇
  1997年   12篇
  1996年   19篇
  1995年   10篇
  1994年   13篇
  1993年   5篇
  1992年   10篇
  1991年   9篇
  1990年   2篇
  1989年   2篇
  1988年   4篇
  1987年   3篇
  1986年   1篇
  1985年   1篇
  1984年   2篇
  1983年   1篇
  1982年   6篇
  1980年   1篇
  1979年   4篇
  1973年   1篇
排序方式: 共有1010条查询结果,搜索用时 15 毫秒
971.
M. J. Jiménez  P. Real 《代数通讯》2013,41(9):2731-2743
In this article, in the setting of connected DG-modules, we prove that, for any A -algebra (M, {m i } i≥1), there is a chain contraction from a DG-algebra A M onto the DG-module M such that the A -algebra structure induced by perturbation theory on M is precisely the original one. In fact, the mentioned DG-algebra can be considered a rectification of the A -algebra in the sense of (Boardman and Vogt, 1973 Boardman , J. M. , Vogt , R. M. ( 1973 ). Homotopy Invariant Algebraic Structures on Topological Spaces . Lecture Notes in Mathematics . Springer-Verlag , pp. 347 . [Google Scholar]). Appropiate dual results are given for A -coalgebras.  相似文献   
972.
A basic geometric question is to determine when a given framework G(p)G(p) is globally rigid in Euclidean space RdRd, where G is a finite graph and p is a configuration of points corresponding to the vertices of G  . G(p)G(p) is globally rigid in  RdRd if for any other configuration q for G   such that the edge lengths of G(q)G(q) are the same as the corresponding edge lengths of G(p)G(p), then p is congruent to q. A framework G(p)G(p) is redundantly rigid, if it is rigid and it remains rigid after the removal of any edge of G.  相似文献   
973.
Grapevine is susceptible to fungal diseases generally controlled by numerous chemical fungicides. Elicitors of plant defence are a way of reducing the use of these chemicals, but still provide inconsistent efficiency. Easy-to-analyse markers of grapevine responses to elicitors are needed to determine the best conditions for their efficiency and position them in protection strategies. We previously reported that the elicitor sulphated laminarin induced the emission of volatile organic compounds (VOCs) by grapevine leaves. The present study was conducted to characterise and compare VOC emissions in response to other elicitors. Bastid® was first used to test the conditions of VOC collection and analysis. Using SBSE-GC-MS, we detected several VOCs, including the sesquiterpene α-farnesene, in a time-dependent manner. This was correlated with the induction of farnesene synthase gene expression, in parallel with stilbene synthesis (another defence response), and associated to resistance against downy mildew. The other elicitors (Redeli®, Romeo®, Bion®, chitosan, and an oligogalacturonide) induced VOC emission, but with qualitative and quantitative differences. VOC emission thus constitutes a response of grapevine to elicitors of various chemical structures. Therefore, VOC analysis is relevant for studying the impact of environmental factors on grapevine defence responses and optimising the performance of elicitors in vineyards.  相似文献   
974.
The reaction of the V-shaped linker molecule 5-hydroxyisophthalic acid (H2L0), with Al or Ga nitrate under almost identical reaction conditions leads to the nitration of the linker and subsequent formation of metal–organic frameworks (MOFs) with CAU-10 or MIL-53 type structure of composition [Al(OH)(L)], denoted as Al-CAU-10-L0, 2, 4, 6 or [Ga(OH)(L)], denoted as Ga-MIL-53-L2. The Al-MOF contains the original linker L0 as well as three different nitration products (L2, L4 and L4/6), whereas the Ga-MOF mainly incorporates the linker L2. The compositions were deduced by 1H NMR spectroscopy and confirmed by Rietveld refinement. In situ and ex situ studies were carried out to follow the nitration and crystallization, as well as the composition of the MOFs. The crystal structures were refined against powder X-ray diffraction (PXRD) data. As anticipated, the use of the V-shaped linker results in the formation of the CAU-10 type structure in the Al-MOF. Unexpectedly, the Ga-MOF crystallizes in a MIL-53 type structure, which is usually observed with linear or slightly bent linker molecules. To study the structure directing effect of the in situ nitrated linker, pure 2-nitrobenzene-1,3-dicarboxylic acid (m-H2BDC-NO2) was employed which exclusively led to the formation of [Ga(OH)(C8H3NO6)] (Ga-MIL-53-m-BDC-NO2), which is isoreticular to Ga-MIL-53-L2. Density Functional Theory (DFT) calculations confirmed the higher stability of Ga-MIL-53-L2 compared to Ga-CAU-10-L2 and grand canonical Monte Carlo simulations (GCMC) are in agreement with the observed water adsorption isotherms of Ga-MIL-53-L2.  相似文献   
975.
We introduce a notion of q ‐pseudoconvex domain of new type for a bounded domain of ?n and prove that for given a ‐closed (p, r)‐form, rq, that is smooth up to the boundary, there exists a (p, r – 1)‐form smooth up to the boundary which is a solution of ‐equation on a bounded q ‐pseudoconvex domain. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
976.
We investigate the use of different Machine Learning methods to construct models for aqueous solubility. Models are based on about 4000 compounds, including an in-house set of 632 drug discovery molecules of Bayer Schering Pharma. For each method, we also consider an appropriate method to obtain error bars, in order to estimate the domain of applicability (DOA) for each model. Here, we investigate error bars from a Bayesian model (Gaussian Process (GP)), an ensemble based approach (Random Forest), and approaches based on the Mahalanobis distance to training data (for Support Vector Machine and Ridge Regression models). We evaluate all approaches in terms of their prediction accuracy (in cross-validation, and on an external validation set of 536 molecules) and in how far the individual error bars can faithfully represent the actual prediction error.  相似文献   
977.
某PBX炸药的动态力学性能研究   总被引:1,自引:0,他引:1  
 PBX炸药作为现代武器的主装药,它的力学行为决定着武器的生存能力。为了研究PBX炸药的动态力学特性,采用分离式霍普金森压杆(Split Hopkinson Pressure Bar,SHPB)作为加载手段,结合半导体应变片测试技术和压电晶体监测技术,保证了实验数据的有效性。利用SHPB加载波整形技术,实现了材料两端应力平衡和常应变率加载,得到了不同应变率(90~410 s-1)下材料的应力-应变曲线。根据材料的模量、破坏强度和破坏应变随应变率的变化规律,采用粘性修正的Sargin模型,得到了该PBX炸药在单轴压缩下的唯象本构模型,模拟结果与实验曲线符合较好。  相似文献   
978.
A sensitive and reproducible stir bar sorptive extraction and HPLC-UV detection method was used for the therapeutic drug monitoring of chlorpromazine and trifluoperazine in human serum. The separation was achieved using a C(18) column. The mobile phase consisted of methanol/sodium acetate buffer (pH 4.1; 0.1 M) (95:5, v/v) including 0.5% triethylamine. This miniaturized method can result in faster analysis, lower solvent consumption and less workload per sample while maintaining or even improving sensitivity. In the second part, stir bar sorptive extraction/HPLC-UV method was optimized by a chemometrics approach. An experimental design was therefore used to evaluate the statistically influential and/or interacting factors, among those described in the literature, and to find the best extraction and desorption conditions. Optimal sample volume of 1 mL, extraction time of 24 min at 31°C with pH 8.1 were obtained in a screening 2(5) half fractional factorial design followed by a Box-Behnken design. For the desorption conditions, a Box-Behnken design showed that the best conditions were 150 μL mobile phase for 20 min at 50°C. The optimized method was repeatable (CV<10%, linear (LOQ-500 ng/mL)), with the LOQs equal to 0.7 and 1.5 ng/mL for chlorpromazine and trifluoperazine, respectively.  相似文献   
979.
A review with 75 references is presented that deals with the reported methods for analysis of some important central nervous system (CNS) drugs in biological fluids utilizing stir bar sorptive extraction (SBSE) technique covering the years from 2000 to 2008. The theoretical aspects of SBSE, as well as an significant number of applications have been published, showing the advantages of this technique over the classical extraction techniques (liquid–liquid extraction (LLE) and solid-phase extraction (SFE). In this review, recent SBSE developments and a focus on the development of new instrumental approaches and sorbent phases are presented.  相似文献   
980.
吴昊  姜锡权 《爆炸与冲击》2011,31(4):392-396
利用加长型分离式霍普金森压杆(入射杆长6000mm、子弹长800mm)研究聚氨脂泡沫材料在较 低应变率大应变条件一维应力状态下的动态力学性能,获得了约550s的长加载脉冲,得到了该材料在应变 率520s-1、应变0.15条件下的应力应变曲线,对较低应变率条件下,应变率与动态应力平衡之间的关联进行 了讨论。  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