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   82524篇
  免费   436篇
  国内免费   381篇
化学   26808篇
晶体学   911篇
力学   6868篇
数学   32301篇
物理学   16453篇
  2022年   117篇
  2021年   110篇
  2020年   116篇
  2019年   126篇
  2018年   10518篇
  2017年   10373篇
  2016年   6227篇
  2015年   966篇
  2014年   467篇
  2013年   699篇
  2012年   4039篇
  2011年   10754篇
  2010年   5812篇
  2009年   6204篇
  2008年   6812篇
  2007年   8926篇
  2006年   345篇
  2005年   1429篇
  2004年   1622篇
  2003年   2054篇
  2002年   1107篇
  2001年   299篇
  2000年   338篇
  1999年   186篇
  1998年   221篇
  1997年   180篇
  1996年   237篇
  1995年   144篇
  1994年   106篇
  1993年   133篇
  1992年   103篇
  1991年   98篇
  1990年   89篇
  1989年   107篇
  1988年   102篇
  1987年   100篇
  1986年   90篇
  1985年   107篇
  1984年   99篇
  1983年   76篇
  1982年   85篇
  1981年   89篇
  1980年   91篇
  1979年   90篇
  1978年   77篇
  1977年   69篇
  1976年   69篇
  1975年   60篇
  1974年   49篇
  1973年   52篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
n-Propyl tin trichloride and its complexes with bases as pyridine, α, β-and γ-picolines, isoquinoline, piperidine, morpholine, aniline and benzylamine have been prepared. The complexes have been characterised by their I.R. and molar conductance measurements. Assignments for some I.R. frequencies have been reported and discussed. Molar conductance values of some of the complexes in nitrobenzene categories them to be uni-univalent electrolytes.  相似文献   
42.
Abstract— The relationship between energy metabolism and UV-induced (251 nm) DNA repair has been investigated in leukocytes obtained from peripheral blood of chronic myeloid leukaemia patients. Energy supply was modulated by using inhibitors of respiratory (antimvcin-A) and glycolytic (2-deoxy-o-glucose and other glucose analogues) pathways for adenosine-triphosphatc (ATP) production. DNA repair was measured by the unscheduled DNA synthesis technique. Parameters of energy metabolism like glucose utilization. lactate production and ATP content of cells. were measured under similar experimental conditions. The observations made indicate the following results: (1) ATP for DNA repair can be supplied by the respiratory and/or by the glycolytic pathway. (2) In the absence of respiration, the rate of glycolysis bears a linear correlation with DNA repair. (3) A minimum threshold rate of ATP-production is necessary for DNA repair.  相似文献   
43.
Reactions of salicylaldehydes with boronate ester derivatives of aniline have been examined. Addition of these Schiff base ligands to palladium acetate or Na2PdCl4 afforded novel boron-containing trans-bis(N-arylsalicylaldiminato) palladium complexes.Condensation of salicylaldehyde (2-HOC6H4C(O)H) with H2NC6H4Bpin (pin=1,2-O2C2Me4) afforded the boron-containing Schiff bases, 2-HOC6H4C(H)=NC6H4Bpin (1–3a). Similar reactivity with 2-hydroxy-5-nitrobenzaldehyde and 2-hydroxy-1-naphthaldehyde gave the corresponding Schiff bases (1-3b) and (1-3c), respectively. Reaction of Schiff bases (2) and (3) with palladium acetate or Na2PdCl4 afforded complexes of the type PdL2 (4,5), where L=deprotonated Schiff base. The molecular structure of the nitro-salicylaldehyde 4-Bpin palladium complex (5b) was characterized by an X-ray diffraction study. All new palladium compounds have been characterized fully and tested for their antifungal activity against Aspergillus niger and Aspergillus flavus.  相似文献   
44.
Raman spectra of NaNO2 have been studied as a function of hydrostatic pressure to 40 kbar at 295 and 348 K. Slight changes in slope of mode frequency versus pressure plots support the view that a structural anomaly exists at 9 ± 1 kbar. The absence of qualitative changes in the Raman spectra allow the space group of NaNO2 IV to be specified as one of P1, P2, B2, Pm or Bm. The Raman spectrum of NaNO3 has been studied to 87 kbar. The changes observed are fully consistent with a second-order transition to a phase with symmetry C63v, as indicated by previous X-ray work, although the transition is sluggish.  相似文献   
45.
An electrospray ionization (ESI) ambient pressure ion-mobility spectrometer (APIMS) interfaced to an orthogonal reflector time-of-flight mass spectrometer (TOFMS) was evaluated for the first time as a detector for the identification of phenylthiohydantoin (PTH)-derivatized amino acids, the final products in the Edman sequencing process of peptides and proteins. The drift and flight times of the twenty common PTH amino acids were characterized by a well-defined 2-D mobility/mass spectral pattern. The combination of mobility/mass modes of analysis gave rise to a unique trend-line formation for the series of PTH amino acids. In addition, each PTH amino acid had a unique reduced mobility constant K(o), thus enabling the differentiation of all the amino acid derivatives including the PTH-leucine and PTH-isoleucine isomers. More importantly it was shown that it was possible to resolve a complete reference mixture of PTH amino acids in a single experimental run in less than 1 min. Detection limits for the PTH amino acids were found to range from 1.04 to 3.52 ng; indicating that the limits of detection were less than 17.0 pmol for all of the PTH amino acids.  相似文献   
46.
The coordination compound [Co(C10H7COO)2(H2O)3]2n · 4nH2O was prepared by the reaction of 1-naphthoic acid and cobalt(II) acetate tetrahydrate in basic solution, and was fully characterized by X-ray diffraction, element analysis, FTIR, TG-DTA and luminescent spectra. In the crystal the six-coordinated Co(II) centers are linked into one-dimensional zigzag chains by water molecules, which are further assembled into a two-dimensional network through weak inter-chain C–H···π interactions. The solid complex exhibits favorable fluorescent properties similar to those of free ligand at room temperature, which can be assigned to the intraligand electronic transfer.  相似文献   
47.
It has been established that on heating, 3-benzoyl-4-hydroxy-1-methyl-4-phenylpiperidine is ring-opened in the presence of arylamines by a type of retroaldol reaction, with subsequent transamination of the intermediate Mannich base and the formation of 3-arylamino-1-oxo-1-phenylpropanes. When using arylhydrazines this γ-piperidol is recyclized with the formation of 1,3-diarylpyrazoles and their 4,5-dihydro derivatives. The mass spectral behavior of a series of 3-arylamino-substituted 1-phenylpropanones has been studied. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1486–1495, October, 2007.  相似文献   
48.
Summary. Upper critical solution temperatures (UCSTs) for liquid–liquid demixing in a set of mixtures of linear alkanes (pentane (N 1=5) to pentacontane (N 1=50)) with an oligostyrene (1241amu, N 2=12) are reported. We find strong correlation between the Hildebrand solubility parameters of the alkanes and the UCST. Correlations are developed which enable predictions concerning the miscibility of mixtures of compounds with longer chains.  相似文献   
49.
50.
The time-consuming process of solving large-scale Mixed Integer Programming problems using the branch-and-bound technique can be speeded up by introducing a degree of parallelism into the basic algorithm. This paper describes the development and implementation of a parallel branch-and-bound algorithm created by adapting a commercial MIP solver. Inherent in the design of this software are certain ad hoc methods, the use of which are necessary in the effective solution of real problems. The extent to which these ad hoc methods can successfully be transferred to a parallel environment, in this case an array of at most nine transputers, is discussed. Computational results on a variety of real integer programming problems are reported.  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