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   4056篇
  免费   133篇
  国内免费   10篇
化学   2186篇
晶体学   23篇
力学   127篇
数学   447篇
物理学   1416篇
  2022年   36篇
  2021年   57篇
  2020年   75篇
  2019年   75篇
  2018年   86篇
  2017年   58篇
  2016年   104篇
  2015年   78篇
  2014年   103篇
  2013年   252篇
  2012年   223篇
  2011年   261篇
  2010年   158篇
  2009年   147篇
  2008年   205篇
  2007年   203篇
  2006年   216篇
  2005年   183篇
  2004年   127篇
  2003年   113篇
  2002年   88篇
  2001年   72篇
  2000年   70篇
  1999年   43篇
  1998年   39篇
  1997年   38篇
  1996年   42篇
  1995年   48篇
  1994年   43篇
  1993年   54篇
  1992年   43篇
  1991年   33篇
  1990年   49篇
  1989年   40篇
  1988年   37篇
  1987年   38篇
  1986年   29篇
  1985年   44篇
  1984年   38篇
  1983年   39篇
  1982年   35篇
  1981年   34篇
  1980年   38篇
  1979年   32篇
  1978年   37篇
  1977年   33篇
  1976年   37篇
  1975年   30篇
  1974年   34篇
  1973年   36篇
排序方式: 共有4199条查询结果,搜索用时 131 毫秒
1.
Roy  Sumit 《Archiv der Mathematik》2022,118(5):471-484
Archiv der Mathematik - Let G be a simple simply-connected connected linear algebraic group over $${\mathbb {C}}$$ . We prove a 2-birational Torelli theorem for the moduli space of semistable...  相似文献   
2.
Molecular Diversity - Tuberculosis (TB) is an infectious disease and the leading cause of death globally. The rapidly emerging cases of drug resistance among pathogenic mycobacteria have been a...  相似文献   
3.
4-Hydroxy isoleucine is one of the potent hypoglycemic active constituents of fenugreek seeds. A method capable of reducing biological interferences is required for bioavailability studies. An isocratic separation of 4-hydroxy isoleucine from endogenous interferences was achieved in ZIC-cHILIC column using 0.1% formic acid in water and acetonitrile (20:80, % v/v) pumped at 0.5 ml/min. Quantification was performed in multiple reaction monitoring mode using the transitions of m/z 148.1→102.1 and m/z 276.1→142.2 for 4-hydroxy isoleucine and homatropine (as internal standard), respectively. After full method validation, 4-hydroxy isoleucine levels in human plasma and commercial fenugreek formulations were determined. This method showed good linearity in the range of 50–2000 ng/mL. Intra- and interday accuracies were in the range of 90.64–109.0% and precision was <4.82% CV. The mean (SD) plasma concentration of 4-hydroxy isoleucine in healthy individuals at 2 h after oral administration of fenugreek tablet was found to be 1590 (260) ng/mL. Half of marketed formulations were found to contain <0.05% of 4-hydroxy isoleucine content. We developed a rapid hydrophilic interaction liquid chromatography–tandem mass spectrometry method for analysis of 4-hydroxy isoleucine in human plasma. This method can be applied directly to conduct the clinical pharmacokinetics studies of 4-hydroxy isoleucine in human population.  相似文献   
4.
Bacterial trans-acyltransferase polyketide synthases (trans-AT PKSs) are multimodular megaenzymes that biosynthesize many bioactive natural products. They contain a remarkable range of domains and module types that introduce different substituents into growing polyketide chains. As one such modification, we recently reported Baeyer–Villiger-type oxygen insertion into nascent polyketide backbones, thereby generating malonyl thioester intermediates. In this work, genome mining focusing on architecturally diverse oxidation modules in trans-AT PKSs led us to the culturable plant symbiont Gynuella sunshinyii, which harbors two distinct modules in one orphan PKS. The PKS product was revealed to be lobatamide A, a potent cytotoxin previously only known from a marine tunicate. Biochemical studies show that one module generates glycolyl thioester intermediates, while the other is proposed to be involved in oxime formation. The data suggest varied roles of oxygenation modules in the biosynthesis of polyketide scaffolds and support the importance of trans-AT PKSs in the specialized metabolism of symbiotic bacteria.  相似文献   
5.
Poly(acrylic acid) (PAA) and its salt poly(acrylate) (PA) have been synthesized through modified free radical polymerization in environmentally begin aqueous medium under ultrasound to make the process robust. The synthesized polymer is well-characterized through conventional techniques. Its salt is employed to produce highly stable and stimuli sensitive colloidal silver (Ag-PA sol) without using any additional reducing chemical reagents like sodium borohydride, ascorbic acid, hydrazine, etc or UV/Gamma radiation. A detailed mechanistic path of the polymerization and reduction of Ag+ on polyacrylate chains has been evaluated. Finally Ag-PA sol is used for pH sensing through naked eye to eliminate the need for sophisticated instrument for data collection. The present work focuses mainly the development of a low cost pH sensing system based on colorimetric ‘smart polymer’ having high practical utility. The unique structural and photo-physical features of nano-scaled materials open new opportunities for the applications of colorimetric pH sensor. In the present work, Ag nano-clusters capped by PA? are employed as an effective colorimetric pH sensor for the first time, requiring no further functionalization.  相似文献   
6.
Acta Mathematica Sinica, English Series - The purpose of the present paper is to deliberate *-conformal Yamabe soliton, whose potential vector field is torse-forming on Kenmotsu manifold. Here, we...  相似文献   
7.
Two tripodal C3-symmetric photoswitchable molecular systems T1 and T2 are reported that have extended conjugation at external and internal positions using an acryl group. The influence of the extended π-bonds in their absorption properties, thermal relaxation of the photoisomers and their propensities in forming supramolecular self-assemblies have been explored through spectroscopy, and microscopic studies. In particular, the investigations on the self-assembly have been carried out using scanning electron microscopy (SEM), transmission electron microscopy (TEM), polarized optical microscopy (POM), X-ray diffraction studies (XRD) and atomic force microscopy (AFM). Remarkably, the position of the acryl group influences the behaviour of the two target molecules in supramolecular assembly, and also in the formation of photoresponsive organic hydrogels or microcrystals.  相似文献   
8.
Roy  A.  Mukherjee  R. 《Fluid Dynamics》2022,56(1):S1-S18

This paper identifies laminar separation bubbles at the root or span-wise midsection of a rectangular wing using direct surface pressure measurements in the wind tunnel and analyses their behavior at different Reynolds numbers and angles of attack. The separation, transition, and reattachment locations are determined as functions of the angles of attack and the Reynolds number. The transition structure and turbulence characteristics in the separated shear layer are studied using laser Doppler velocimetry. Surface pressure data and simultaneously acquired velocity signals are correlated to show the pattern of growing disturbances in the shear layer. Surface oil flow visualizations clarified the wingtip and separation bubble’s interactions near the leading edge of the wing at the higher angles of attack. Turbulence statistics are also calculated from the streamwise velocity distributions, and an apparent deviation is observed for the skewness and flatness values from the normal distributions in the near-wall region. The separation bubble effect on aerodynamic coefficients of a 3D rectangular wing root section is studied and reported.

  相似文献   
9.
Hyperpolarisation methods that premagnetise agents such as pyruvate are currently receiving significant attention because they produce sensitivity gains that allow disease tracking and interrogation of cellular metabolism by magnetic resonance. Here, we communicate how signal amplification by reversible exchange (SABRE) can provide strong 13C pyruvate signal enhancements in seconds through the formation of the novel polarisation transfer catalyst [Ir(H)22‐pyruvate)(DMSO)(IMes)]. By harnessing SABRE, strong signals for [1‐13C]‐ and [2‐13C]pyruvate in addition to a long‐lived singlet state in the [1,2‐13C2] form are readily created; the latter can be observed five minutes after the initial hyperpolarisation step. We also demonstrate how this development may help with future studies of chemical reactivity.  相似文献   
10.
Dinuclear gold(I)-N-heterocyclic carbene complexes were developed for the hydrohydrazidation of terminal alkynes. The gold(I)-N-heterocyclic carbene complexes 2a-2b were synthesized in good yields from silver complexes synthesized in situ, which in turn were obtained from the corresponding imidazolium salts with Ag2O in dichloromethane as a solvent. The new air-stable gold(I)-NHC complexes, 2a - 2b, were characterized using NMR spectroscopy, elemental analysis, infrared, and mass spectroscopy studies. The gold(I) complex 2a was characterized using X-ray crystallography. Bis-N-heterocyclic carbene–based gold(I) complexes 2a - 2b exhibited excellent catalytic activities for hydrohydrazidation of terminal alkynes yielding acylhydrazone derivatives. The working catalytic system can be used in gram-scale synthesis. In addition, the catalytic reaction mechanism of the hydrohydrazidation of terminal alkynes by gold(I)-NHC complex was studied in detail using density functional theory.  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