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   10293篇
  免费   831篇
  国内免费   342篇
系统科学   1333篇
丛书文集   168篇
教育与普及   62篇
理论与方法论   304篇
现状及发展   890篇
研究方法   11篇
综合类   8698篇
  2024年   38篇
  2023年   74篇
  2022年   162篇
  2021年   171篇
  2020年   113篇
  2019年   80篇
  2018年   818篇
  2017年   845篇
  2016年   530篇
  2015年   212篇
  2014年   258篇
  2013年   280篇
  2012年   540篇
  2011年   1234篇
  2010年   1094篇
  2009年   726篇
  2008年   796篇
  2007年   1029篇
  2006年   219篇
  2005年   258篇
  2004年   320篇
  2003年   317篇
  2002年   262篇
  2001年   190篇
  2000年   168篇
  1999年   128篇
  1998年   89篇
  1997年   83篇
  1996年   81篇
  1995年   70篇
  1994年   61篇
  1993年   48篇
  1992年   38篇
  1991年   38篇
  1990年   29篇
  1989年   34篇
  1988年   16篇
  1987年   10篇
  1986年   2篇
  1985年   1篇
  1968年   2篇
  1967年   1篇
  1955年   1篇
排序方式: 共有10000条查询结果,搜索用时 875 毫秒
211.
Biotherapeutics have revolutionized modern medicine by providing medicines that would not have been possible with small molecules. With respect to cancer therapies, this represents the current sector of the pharmaceutical industry having the largest therapeutic impact, as exemplified by the development of recombinant antibodies and cell-based therapies. In cancer, one of the most common regulatory alterations is the perturbation of translational control. Among these, changes in eukaryotic initiation factor 4F (eIF4F) are associated with tumor initiation, progression, and drug resistance in a number of settings. This, coupled with the fact that systemic suppression of eIF4F appears well tolerated, indicates that therapeutic agents targeting eIF4F hold much therapeutic potential. Here, we discuss opportunities offered by biologicals for this purpose.  相似文献   
212.
Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease. Neuronal vacuolization and glial activation are pathologic hallmarks in the superoxide dismutase 1 (SOD1) mouse model of ALS. Previously, we found the neuropeptide calcitonin gene-related peptide (CGRP) associated with vacuolization and astrogliosis in the spinal cord of these mice. We now show that CGRP abundance positively correlated with the severity of astrogliosis, but not vacuolization, in several motor and non-motor areas throughout the brain. SOD1 mice harboring a genetic depletion of the βCGRP isoform showed reduced CGRP immunoreactivity associated with vacuolization, while motor functions, body weight, survival, and astrogliosis were not altered. When CGRP signaling was completely disrupted through genetic depletion of the CGRP receptor component, receptor activity-modifying protein 1 (RAMP1), hind limb muscle denervation, and loss of muscle performance were accelerated, while body weight and survival were not affected. Dampened neuroinflammation, i.e., reduced levels of astrogliosis in the brain stem already in the pre-symptomatic disease stage, and reduced microgliosis and lymphocyte infiltrations during the late disease phase were additional neuropathology features in these mice. On the molecular level, mRNA expression levels of brain-derived neurotrophic factor (BDNF) and those of the anti-inflammatory cytokine interleukin 6 (IL-6) were elevated, while those of several pro-inflammatory cytokines found reduced in the brain stem of RAMP1-deficient SOD1 mice at disease end stage. Our results thus identify an important, possibly dual role of CGRP in ALS pathogenesis.  相似文献   
213.
Using 4128 single jumps detected from high frequency data of 220 individual stocks in SZ300P index, this paper investigates the liquidity dynamics around price jumps in Chinese market. Some interesting empirical results are obtained and the corresponding explanations are given. The frequency of positive jumps is quite higher than that of negative jumps. The trading volumes and average trade sizes are all in a high level around positive jumps. The relatively low liquidities around negative jumps show that negative jumps may be generated and enlarged by poor liquidity provision. The price reversal after price jumps is significant, and price reversal lasts longer after positive jumps. Moreover, the size and direction of jumps are significantly correlated with the returns and trades in the post-jump trading time. These findings are believed to be associated with the high proportion of retail investors and their herding behavior for price trend chasing.  相似文献   
214.
The ability of cells to migrate to the destined tissues or lesions is crucial for physiological processes from tissue morphogenesis, homeostasis and immune responses, and also for stem cell-based regenerative medicines. Cytosolic Ca2+ is a primary second messenger in the control and regulation of a wide range of cell functions including cell migration. Extracellular ATP, together with the cognate receptors on the cell surface, ligand-gated ion channel P2X receptors and a subset of G-protein-coupled P2Y receptors, represents common autocrine and/or paracrine Ca2+ signalling mechanisms. The P2X receptor ion channels mediate extracellular Ca2+ influx, whereas stimulation of the P2Y receptors triggers intracellular Ca2+ release from the endoplasmic reticulum (ER), and activation of both type of receptors thus can elevate the cytosolic Ca2+ concentration ([Ca2+]c), albeit with different kinetics and capacity. Reduction in the ER Ca2+ level following the P2Y receptor activation can further induce store-operated Ca2+ entry as a distinct Ca2+ influx pathway that contributes in ATP-induced increase in the [Ca2+]c. Mesenchymal stem cells (MSC) are a group of multipotent stem cells that grow from adult tissues and hold promising applications in tissue engineering and cell-based therapies treating a great and diverse number of diseases. There is increasing evidence to show constitutive or evoked ATP release from stem cells themselves or mature cells in the close vicinity. In this review, we discuss the mechanisms for ATP release and clearance, the receptors and ion channels participating in ATP-induced Ca2+ signalling and the roles of such signalling mechanisms in mediating ATP-induced regulation of MSC migration.  相似文献   
215.
TNF-related apoptosis-inducing ligand (TRAIL) is a prominent cytokine capable of inducing apoptosis. It can bind to five different cognate receptors, through which diverse intracellular pathways can be activated. TRAIL’s ability to preferentially kill transformed cells makes it a promising potential weapon for targeted tumor therapy. However, recognition of several resistance mechanisms to TRAIL-induced apoptosis has indicated that a thorough understanding of the details of TRAIL biology is still essential before this weapon can be confidently unleashed. Critical to this aim is revealing the functions and regulation mechanisms of TRAIL’s potent death receptor DR5. Although expression and signaling mechanisms of DR5 have been extensively studied, other aspects, such as its subcellular localization, non-signaling functions, and regulation of its membrane transport, have only recently attracted attention. Here, we discuss different aspects of TRAIL/DR5 biology, with a particular emphasis on the factors that seem to influence the cell surface expression pattern of DR5, along with factors that lead to its nuclear localization. Disturbance of this balance apparently affects the sensitivity of cancer cells to TRAIL-mediated apoptosis, thus constituting an eligible target for potential new therapeutic agents.  相似文献   
216.
217.
泵是流体输送系统的核心动力源,开展泵优化设计、实现节能降耗一直是流体机械领域的研究热点。文章回顾了基于一元、二元、三元流动理论进行水力设计的各种方法,总结了各个水力设计方法的优缺点;概括了近年来在叶轮、导叶、蜗壳水力设计方面的研究热点和主流方法,为今后泵设计方法的发展奠定了基础。以数值模拟结合三元流动理论和优化算法的设计方法已成为当前开展泵水力设计的主流手段,可以预期,随着计算机计算能力的进一步提高和优化算法的快速发展,泵的水力设计水平将会有大幅度提升并会促进泵行业的整体技术进步。  相似文献   
218.
The author starts from the observation that citizenship and voluntarism are contested terms with diverse meanings. They have also been appropriated by politicians of various persuasions and imbued with meanings associated with ‘feel good’ factors that emphasize serving in a community. Therefore, voluntarism has the potential to continue the exclusion of minority groups, marginalized individuals and collective groupings at the expense of their citizenship rights, particularly those identified by Hannah Arendt as the ‘right to have rights’ that have been endorsed through public policy but today are being undermined by the ‘age of austerity’ in publicly funded welfare states. Against the background of the political context of UK, and the public rhetoric on the ‘Big Society’, the author examines whether citizenship discourses allied with voluntarism support a meaningful endorsement of altruistic solidarity or whether they endorse exploitative relationships under the guise of meeting the public needs.  相似文献   
219.
This paper studies the first passage time problem for a reflected two-sided jump-diffusion risk model with the jumps having a hyper-Erlang distribution. The authors give the explicit closed-form expression for the joint Laplace transform of the first passage time and the overshoot for the reflected process. Finally, the formula is applied to the ruin problem under the barrier dividend strategy and the pricing of the Russian option.  相似文献   
220.
Huge quantities of zinc leaching residues (ZLRs) generated from zinc production are dumped continuously around the world and pose a potential environmental threat because of their considerable amounts of entrained heavy metals (mainly lead). Most ZLRs have not been properly treated and the valuable metals in them have not yet been effectively recovered. Herein, the deep cleaning of a ZLR and recovery of valuable metals via a hydrometallurgical route were investigated. The cleaning process consists of two essential stages:acid leaching followed by calcium chloride leaching. The optimum conditions for extracting zinc, copper, and indium by acid leaching were a sulfuric acid concentration of 200 g·L-1, a liquid/solid ratio of 4:1 (mL/g), a leaching time of 2 h, and a temperature of 90℃. For lead and silver extractions, the optimum conditions were a calcium chloride concentration of 400 g·L-1, a pH value of 1.0, a leaching time of 1 h, and a temperature of 30℃. After calcium chloride leaching, silver and lead were extracted out and the lead was finally recovered as electrolytic lead by electrowinning. The anglesite phase, which poses the greatest potential environmental hazard, was removed from the ZLR after deep cleaning, thus reducing the cost of environmental management of ZLRs. The treatment of chlorine and spent electrolyte generated in the process was discussed.  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