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   4620篇
  免费   137篇
  国内免费   7篇
化学   3341篇
晶体学   60篇
力学   50篇
综合类   2篇
数学   165篇
物理学   1146篇
  2023年   27篇
  2022年   30篇
  2021年   36篇
  2020年   63篇
  2019年   67篇
  2018年   52篇
  2016年   79篇
  2015年   100篇
  2014年   83篇
  2013年   216篇
  2012年   214篇
  2011年   224篇
  2010年   143篇
  2009年   159篇
  2008年   218篇
  2007年   258篇
  2006年   230篇
  2005年   247篇
  2004年   191篇
  2003年   174篇
  2002年   172篇
  2001年   116篇
  2000年   117篇
  1999年   73篇
  1998年   58篇
  1997年   43篇
  1996年   79篇
  1995年   64篇
  1994年   49篇
  1993年   68篇
  1992年   67篇
  1991年   66篇
  1990年   54篇
  1989年   56篇
  1988年   50篇
  1987年   58篇
  1986年   50篇
  1985年   90篇
  1984年   72篇
  1983年   41篇
  1982年   54篇
  1981年   47篇
  1980年   54篇
  1979年   55篇
  1978年   45篇
  1977年   32篇
  1976年   37篇
  1975年   23篇
  1974年   27篇
  1973年   25篇
排序方式: 共有4764条查询结果,搜索用时 625 毫秒
51.
T. Sato  R. Noyori 《Tetrahedron letters》1980,21(26):2535-2538
A stereocontrolled synthesis of 4′-hydroxymethylated ribo-C-nucleosides and a new psico-C-nucleoside has been accomplished on the basis of the [3 + 4] reductive cyclocoupling reaction of α,α,α′,α′-tetrabromoacetone and furfuryl acetate.  相似文献   
52.
We present a simple and rapid polymerase chain reaction (PCR)-based technique, termed consumed allele-specific primer analysis (CASPA), as a new strategy for single nucleotide polymorphism (SNP) analysis. The method involves the use of labeled allele-specific primers, differing in length, with several noncomplementary nucleotides added in the 5'-terminal region. After PCR amplification, the amounts of the remaining primers not incorporated into the PCR products are determined. Thus, nucleotide substitutions are identified by measuring the consumption of primers. In this study, the CASPA method was successfully applied to ABO genotyping. In the present method, the allele-specific primer only anneals with the target polymorphic site on the DNA, so it is not necessary to analyze the PCR products. Therefore, this method is only little affected by modification of the PCR products. The CASPA method is expected to be a useful tool for typing of SNPs.  相似文献   
53.
The structure of iso-grayanotoxin II, a new diterpenoid from Leucothoe grayana MAX., has been determined as 3 beta,5 beta,6 beta,14 beta,16 alpha-pentahydroxygrayanotox-9(10)-ene by spectroscopic and X-ray crystallographic analysis. The lethal dosage level of iso-grayanotoxin II in mice was lower than that of grayanotoxin III.  相似文献   
54.
55.
56.
The crystal structures of oxo-centered trineclear cobalt-iron chloroacetate complex [CoIIFe 2 III O(CH2ClCO2)6(H2O)3]·3H2O (1) was compared with that of previously reported trinuclear iron complex [FeIIFe 2 III O(CH2ClCO2)6(H2O)3]·3H2O (2) which has an isomorphous structure to 1. Compound 1 crystallizes in space group P21/n with Z=4 in a unit cell of a=14.826 (4) Å, b=4.536 (8) Å, c=14.000 (4) Å, =100.32 (2)0 and V=2968 (11) Å3. The structure was refined to R=0.75 and Rw=0.82. The coordination geometries of the three iron atoms are observed equivalent in 1 indicating a static disorder of the position among cobalt and iron atoms. Two distinct FeIII doublets observed in Mössbauer spectra of 1 become an indistinguishable broad doublet by dehydration of crystal water. On the other hand, no significant line-broadening is observed after the dehydration in complex 2. The results indicate that the dehydration in 2 induces a local environmental change reordering of an electronic configuration around iron atoms, whereas the remaining disordering is reflected in Mössbauer spectrum after the dehydration in 1.  相似文献   
57.
The thermal stress around the sodium level of the LMFBR reactor vessel, caused by the axial temperature distribution during heat-up and cool-down transient conditions, is one of the most important problems for a reactor vessel designer, especially in a conceptual design stage. Key parameters relating to plant operating transient conditions and reactor vessel main configuration should be settled carefully, thermal stresses not being allowed to exceed design stress limits. In order to examine the sodium level thermal stress easily and economically, a simple computer program for parameter survey has been developed. This program uses a one-dimensional Fourier series solution in transient temperature distribution analysis, and an analytical stress solution based on shell theory in stress value estimation. This paper presents a simplified and economical calculation method for the axial temperature distribution and stress value in the LMFBR operating transient conditions, and analysis examples obtained with this computer program.
Zusammenfassung Der durch die axiale Temperaturverteilung während der Übergangsphase von Aufheizen und Abkühlen verursachte thermische Streß rund um den Natriumfüllstand des LMFBR-Reaktorkessels ist für den Reaktorkesselkonstrukteur, besonders im Entwurfstadiuni eines der wichtigsten Probleme. Die mit den transienten Operationsbedingungen der Anlage und der Gestaltung des Reaktorkessels in Beziehung stehenden Grundparameter müssen sorgfältig ermittelt werden, um zu verhindern, daß der thermische Streß die geplanten Streßgrenzen nicht überschreitet. Um den thermischen Streß um den Natriumfüllstand, leicht und ökonomisch zu bestimmen, wurde ein Komputerprogramm für die Parameterübersicht aufgestellt. In diesem Programm wird von einer eindimensionalen Fourrierreihe zur Analyse der transienten Temperaturverteilung und einer auf der Schalentheorie von Streßwertbestimmungen basierende analytische Lösung Gebrauch gemacht. Im vorliegenden Artikel werden eine vereinfachte und wirtschaftliche Berechnungsmethode für die axiale Temperaturverteilung und Streßwerte bei transienten Arbeitsbedingungen mitgeteilt und mit diesem Komputerprogramm ausgeführte Analysenbeispiele angegeben.

, - () , , , . , , , . , , - , . , , .


Paper presented at the World Conference on Thermal Analysis, Madeira (Portugal) 1986.  相似文献   
58.
An asymmetric synthetic route to 2-substituted-3-aminocarbonyl propionic acid, which is the significant component of low-molecular-weight renin inhibitors, is described. The key step of this synthesis is diastereoselective alkylation by using chiral oxazolidinone and benzyl bromoacetate.  相似文献   
59.
A new series of 11-substituted 6,11-dihydrodibenz[b,e]oxepin derivatives was synthesized and evaluated for antiallergic activity. Convenient methods for the preparation of sulfides from alcohols were developed. Structure-activity relationships are described. Compound 7, 11-[2-(dimethylamin)ethyl]thio-6,11-dihydrodibenz[b,e] oxepin-2-carboxylic acid hydrochloride, was the most potent in the rat passive cutaneous anaphylaxis test (ED50 = 0.92 mg/kg p.o.). It had a potent inhibitory effect on anaphylactic bronchoconstriction in guinea pigs (ED50 = 0.029 mg/kg p.o.) and H1 receptor antagonistic effect (Ki = 14 nM) with few central nervous system side effects. Additionally, an antagonistic effect against prostaglandin D2-induced contraction of isolated guinea pig trachea (pA2 = 5.73) was an attractive mechanism of action of the new antiallergic agent. Compound 7 was selected for further evaluation as KW-4994.  相似文献   
60.
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