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   44909篇
  免费   7575篇
  国内免费   6256篇
化学   33244篇
晶体学   613篇
力学   2697篇
综合类   485篇
数学   5206篇
物理学   16495篇
  2024年   51篇
  2023年   813篇
  2022年   1019篇
  2021年   1449篇
  2020年   1802篇
  2019年   1830篇
  2018年   1523篇
  2017年   1487篇
  2016年   2126篇
  2015年   2181篇
  2014年   2651篇
  2013年   3423篇
  2012年   4078篇
  2011年   4391篇
  2010年   3133篇
  2009年   2973篇
  2008年   3114篇
  2007年   2873篇
  2006年   2759篇
  2005年   2237篇
  2004年   1795篇
  2003年   1375篇
  2002年   1295篇
  2001年   1097篇
  2000年   963篇
  1999年   878篇
  1998年   697篇
  1997年   592篇
  1996年   569篇
  1995年   558篇
  1994年   523篇
  1993年   390篇
  1992年   369篇
  1991年   330篇
  1990年   296篇
  1989年   232篇
  1988年   161篇
  1987年   140篇
  1986年   133篇
  1985年   122篇
  1984年   60篇
  1983年   61篇
  1982年   55篇
  1981年   28篇
  1980年   21篇
  1979年   16篇
  1977年   7篇
  1975年   6篇
  1971年   7篇
  1957年   12篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Multicompound determination for the quality control of traditional Chinese medicine (TCM) may often be inadequate, since these compounds may not be associated with, or fully represent, the clinical effects of TCM. Moreover, the individual contributions of each constituent to the pharmacological effect are often not considered. In China, Porana sinensis is widely used as a substitute for Erycibe sources to treat joint pain and rheumatoid arthritis. The existing quality control methods for P. sinensis neither consider the individual contributions of various compounds nor control the actual quality associated with different clinical efficacies. In the present study, a novel efficacy-oriented approach, named the effect-constituent index (ECI), was established for P. sinensis. Analyses of the spectrum–effect relationship and components in rat plasma were conducted to systematically and scientifically select quality markers. Quantitative analysis of multicomponents via a single marker method was introduced to enhance the practical application value of the established ECI. The established ECI shows a good ability to distinguish and predict the bioeffect-based quality of P. sinensis. The present study also provides a reference for the establishment and application of ECI as a quality control method for TCMs.  相似文献   
82.
Ocotillol, pseudo-ginsenoside RT5 (RT5), and pseudo-ginsenoside F11 (PF11) are ocotillol-type saponins that have the same aglycone structure but with different numbers of glucose at the C-6 position. In this study, the metabolites of ocotillol, RT5, and PF11 in rat plasma, stomach, intestine, urine, and feces after oral administration were investigated by ultra-performance liquid chromatography coupled with time-of-flight mass spectrometry. The results showed that RT5 was easily biotransformed into metabolites in vivo, whereas PF11 and RT5 were difficult to be biotransformed. Hydrogenation, dehydrogenation, dehydration, deglycosylation, deoxygenation, hydration, phosphorylation, deoxidation, glucuronidation, and reactions combining amino acid were speculated to be involved in the biotransformation of ocotillol, RT5, and PF11. Based on the structural analysis of metabolites, it was deduced that hydrogenation, dehydration, deoxidation, and reactions combining amino acid occurred on the aglycone structure, whereas deglycosylation, hydration, and phosphorylation occurred on the glycosyl chain. Further, metabolites in plasma, urine, feces, and tissues were different: First, glucuronidation products were found in urine, stomach, intestine, and feces, but not in plasma. Second, the ocotillol prototype was not identified in urine samples. Third, the RT5 prototype was found in stomach, intestine, feces, and urine, but not in plasma.  相似文献   
83.
Our previous work demonstrated that total withanolides of Datura metel L. leaves (TWD) exhibited excellent therapeutic effects on psoriasis. However, current knowledge of its mechanisms is incomplete. In this study, integrated spleen and thymus untargeted metabolomics were used to analyze the changes in endogenous metabolites underlying the immunosuppressive activity of TWD on psoriasis animal models induced by imiquimod. The results suggested that TWD treatment markedly attenuated imiquimod-induced psoriasis and showed significant immunosuppressive activity as evidenced by decreased elevation index of spleen and thymus. Meanwhile, TWD significantly reversed the elevation of immunoregulatory factors, including IL-10, IL-17, IL-22 and IL-23. Multivariate trajectory analysis revealed that TWD treatment could restore the psoriasis-disturbed spleen and thymus metabolite profiles towards the normal metabolic status. A total of 25 and 27 metabolites associated with the immunomodulatory effects for which levels changed markedly upon treatment have been identified in spleen and thymus, respectively. These differential metabolites were mainly involved in amino acid metabolism, nucleotide metabolism, fatty acid metabolism and lipid metabolism. Our investigation provided a holistic view of TWD for intervention in psoriasis through immunoregulation and provided further scientific information in vivo about a clinical value of TWD for psoriasis.  相似文献   
84.
闫腾飞  刘俊秋 《化学学报》2020,78(8):713-718
利用共价自组装的方法,将刚性组装基元与柔性链在一定条件下进行横向交联,可以方便地制备出单层高分子纳米胶囊.相比于传统的非共价超分子囊泡,这种新型的共价纳米胶囊具有结构稳定、尺度可控且分散性优异等诸多优点.因此,如何利用化学合成的手段来制备新型的纳米胶囊,并进一步实现对其结构调控和性能的探究具有十分重要的意义.针对这些问题,分别发展了功能化的柱[5]芳烃、四苯乙烯、卟啉、三嗪、苯硼酸酐等不同类型构筑基元,并使之与两端具有活性位点的柔性烷基链在适当的溶剂中进行聚合反应,最终获得了一系列的共价纳米胶囊.通过对其结构的修饰和调控,发现这些功能化的高分子纳米胶囊在光捕获、人工酶、抗菌材料以及药物载体等领域具有诸多潜在应用价值.未来,新型共价高分子纳米胶囊的开发、功能化以及应用有望得到进一步的拓展.  相似文献   
85.
Zhao  Jun  Chen  Xiaoyu  Zhou  Yuhan  Tian  Hongjing  Guo  Qingjie  Hu  Xiude 《Research on Chemical Intermediates》2020,46(3):1805-1822
Research on Chemical Intermediates - Nano-TiO2 photocatalysts doped with La–N(La–N–TiO2) were prepared by solgel method. The La–N–TiO2 was loaded on the cellulose/SiO2...  相似文献   
86.
Xi  Junting  Zhang  Yiping  Chen  Xing  Hu  Ying 《Research on Chemical Intermediates》2020,46(4):2205-2214
Research on Chemical Intermediates - Defective TiO2 nanocrystals (D-TiO2) were synthesized by a simple, mild and green sol–gel hydrothermal method. The as-prepared sample possesses a high...  相似文献   
87.
Recently, nonmetal doping has exhibited its great potential for boosting the hydrogen evolution reaction (HER) of transition-metal (TM)-based electrocatalysts. To this end, this work overviews the recent achievements made on the design and development of the nonmetal-doped TM-based electrocatalysts and their performance for the HER. It is also shown that by rationally doping nonmetal elements, the electronic structures of TM-based electrocatalysts can be effectively tuned and in turn the Gibbs free energy of the TM for adsorption of H* intermediates (ΔGH*) optimized, consequently enhancing the intrinsic activity of TM-based electrocatalysts. Notably, we highlight that concurrently doping two nonmetal elements can continuously and precisely regulate the electronic structures of the TM, thereby maximizing the activity for HER. Moreover, nonmetal doping also accounts for enhancing the physical properties of the TM (i.e. surface area). Therefore, nonmetal doping is a robust strategy for simultaneous regulation of the chemical and physical features of the TM.  相似文献   
88.
High-throughput metabolic analysis is of significance in diagnostics, while tedious sample pretreatment has largely hindered its clinic application. Herein, we designed FeOOH@ZIF-8 composites with enhanced ionization efficiency and size-exclusion effect for laser desorption/ionization mass spectrometry (LDI-MS)-based metabolic diagnosis of gynecological cancers. The FeOOH@ZIF-8-assisted LDI-MS achieved rapid, sensitive, and selective metabolic fingerprints of the native serum without any enrichment or purification. Further analysis of extracted serum metabolic fingerprints successfully discriminated patients with gynecological cancers (GCs) from healthy controls and also differentiated three major subtypes of GCs. Given the low cost, high-throughput, and easy operation, our approach brings a new dimension to disease analysis and classification.  相似文献   
89.
提出了一类新的相对性区域创新指数,并采用世界专利申请数据对其进行了具体计算.基于区域创新同经济发展水平之间的超线性关系,该指数消除了经济发展水平对创新能力的影响,可以实现对不同发展水平的经济体之间进行有效的创新能力横纵对比.该创新指数尽管极其简单,却揭示出一系列迥异于传统认知的现象,例如中国大陆地区的技术创新能力在1980年代就已经位居世界前列.采用该指数,不但可以在较高水平上解释世界各国的经济增长,还发现它同经济增长率之间的相关性存在一个20年的经济周期.这些结果显示,该指数作为一个单一性指标,以极小的数据依赖就实现了较高程度的解释性,不但重新定位了世界各经济体的创新能力,对深入理解创新同经济发展之间的关系提供了新的角度,而且暗示着这类相对性经济指标的发展潜力与应用空间.  相似文献   
90.
曾宇  户文成 《应用声学》2020,39(3):409-416
针对公共场所异常声的感知和识别问题,提出一种基于贝叶斯优化卷积神经网络的识别方法。提取声信号的Gammatone倒谱系数、倍频程功率谱、短时能量和谱质心,组合成声信号的特征图。构建卷积神经网络作为分类器,利用递增的卷积核设置和池化操作处理不同尺度的特征。基于贝叶斯优化算法优化卷积神经网络的模型参数,对包括火苗噼啪声、婴儿啼哭声、烟花燃放声、玻璃破碎声和警报声的5种公共场所异常声进行识别。该方法的识别结果与基于不同的特征提取和分类器方案得到的识别结果进行比较,结果表明该方法的识别效果优于其他特征提取和分类器方案的识别效果。最后分析了该方法在不同信噪比噪声干扰下的识别结果,验证了该方法的有效性。  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