全文获取类型
收费全文 | 73223篇 |
免费 | 7749篇 |
国内免费 | 14209篇 |
专业分类
化学 | 65534篇 |
晶体学 | 2650篇 |
力学 | 1337篇 |
综合类 | 593篇 |
数学 | 5085篇 |
物理学 | 19982篇 |
出版年
2024年 | 155篇 |
2023年 | 1209篇 |
2022年 | 2396篇 |
2021年 | 2391篇 |
2020年 | 2628篇 |
2019年 | 2439篇 |
2018年 | 1999篇 |
2017年 | 2671篇 |
2016年 | 2748篇 |
2015年 | 2436篇 |
2014年 | 3225篇 |
2013年 | 6531篇 |
2012年 | 4599篇 |
2011年 | 5070篇 |
2010年 | 4302篇 |
2009年 | 5112篇 |
2008年 | 4956篇 |
2007年 | 5031篇 |
2006年 | 4708篇 |
2005年 | 4191篇 |
2004年 | 3961篇 |
2003年 | 3240篇 |
2002年 | 2803篇 |
2001年 | 2241篇 |
2000年 | 2147篇 |
1999年 | 1691篇 |
1998年 | 1460篇 |
1997年 | 1290篇 |
1996年 | 1094篇 |
1995年 | 1042篇 |
1994年 | 972篇 |
1993年 | 788篇 |
1992年 | 686篇 |
1991年 | 519篇 |
1990年 | 352篇 |
1989年 | 322篇 |
1988年 | 276篇 |
1987年 | 182篇 |
1986年 | 156篇 |
1985年 | 164篇 |
1984年 | 127篇 |
1983年 | 62篇 |
1982年 | 104篇 |
1981年 | 135篇 |
1980年 | 105篇 |
1979年 | 110篇 |
1978年 | 74篇 |
1977年 | 76篇 |
1976年 | 50篇 |
1973年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
Several α-ketoalcohols of synthetic value were resolved using lipase as a catalyst. Lipoprotein lipase (LPL) provided the best rate of hydrolysis and kinetic differentiation. One of these optically pure α-ketoalcohol was converted to (5)-ibuprofen in good optical purity. The stereospecific inversion of (R)-alcohol to (S)-alcohol is described. 相似文献
192.
The products from the condensation of acetone-anils ( 1 ) and TCNE reaction were characterized after separation. The main products, tricyanoethylene derivatives ( 7–9 ) were isolated with the malonodinitrile derivatives ( 10–12 ), respectively. The [2+2] cycloaddition product ( 13 ), namely 5-methoxy-3,4-(2′,2′,3′,3′-tetracyanocyclobutano)-l,2-dihydro-2,2,4-trimethylquinoline also has been isolated. The distribution of these three products is substituent dependent. 相似文献
193.
194.
本文给出了对高功率横流CO_2激光器脉冲预电离过程的理论和实验研究.表明了预电离过程中光电离的重要作用.实验结果表明,脉冲预电离可增大高功率横流CO_2.激光器的pd值(p为放电气压、d为放电间隔),增大放电区注入功率密度.对于提高此类型激光器的放电稳定性和输出激光功率是一种技术简单而有效的手段. 相似文献
195.
该文用NMR方法研究暗霉素T的化学结构.从1D谱和几种2D技术确认了暗霉素T是氨基糖苷类化合物.讨论了化合物的构型,并通过改变溶剂推断了氨基的连接位置. 相似文献
196.
Semi-empirical molecular orbital calculations were carried out for the compounds (C2H5)3As, (C2H5)3Ga and RAsH2 (R = C2H5, i-C3H7, i-C4H9, and t-C4H9) by using the CNDO/2-U program, and their capability of β-elimination reaction is compared on the basis of the torsion energy to the transition state, electrostatic interactions and orbital overlapping between the central atom and the β-hydrogen, and bond order of the metal-carbon, and carbon-hydrogen bond. In the comparison of (C2H5)3As with (C2H5)3Ga, we found that the β-elimination of (C2H5)3As could hardly be expected to take place in the thermal decomposition. The capability of β-elimination would be smaller in C2H5AsH2 than that in (C2H5)3As. Moreover when the ethyl group is replaced by a t-butyl group in RAsH2, the β-elimination reaction appears to become more difficult and a large possibility for a radical process is suggested. 相似文献
197.
The response to Hg(II) of a thin all-solid-state Te-doped silver chalcogenide membrane, described by the general formula Ag2 + δSe1 − xTex, which has been electrochemically prepared following a previously proposed approach, has been investigated. The kinetics of formation of the membrane's secondary dynamic response to Hg(II) has been successfully combined with the precise timing and transient signal, typical for flow-injection (FI) measurements, in developing a sensitive and reliable mercury FI detector. Under optimized stream conditions it exhibits a linear Nernstian response, with a double slope of the calibration graph of 59 mV dec−1, over the mercury(II) concentration range 10−6 − 10−3 M, the typical sample throughput amounting to about 70 samples per hour. The observed chemical amplification of the signal is due to the specificity of the processes dominating the initial step in formation of the steady-state signal of the membrane to mercury. The analytical performance of the Hg(II) FI detector, as regards sensitivity, reproducibility, selectivity and long-term stability has been thoroughly investigated. The exact procedure for membrane electrodeposition is given and the potential of the proposed approach as a cost-effective way for preparing chalcogenides of unique structure and properties has been outlined in the above context. 相似文献
198.
In the present study, proteins and glycoproteins of mouse embryos at 2-cell,morula and blastocyst stages were analyzed.The techniques of ~(35)S-Met incorporation,ConA antiserum-precipitating ConA-binding proteins, mini-2D-electrophoresis, fluorography,video densitometer scanning and the computer-lmage system were used for analyses.Results of the investigations indicated that proteins and glycoproteins were synthesized by the embryos in a gradual increase manner from 2-cell to blastocyst. A relatively large amount of glycoproteins was synthesized during the morula and blastocyst stages.Two specific small glycoproteins respectively with molecular weights about 6500 and 9000 and PIs at 5.0 and 6.5 were apparently synthesized in the blastocyst but not in the 2-cell or the morula. 相似文献
199.
I.IntroductionThepcrformanccsofAcousto-optic(AO)dcviccareoftenlimitcdbythepropertiesofAOmcdiawhenitisusedinAOsigna1processingsystems.Thespectrumanalyzersandcorre1ationreceivers,forinstancc,areexpcctcdtoimprovcthereso1utionanddynamicrangcwhilereducingtheproccssorvolume.Rccent1y,ancwclassofmateria1s,themercurousha1ides,promiscstocontributeimportantsolutionstosuchnccdsandisrefincdtopractica1uti1izations.Themecurousha1ideweconsidcrinthispapcrismecurouschloride(Hg,C1,).Thesing1ecrystalofHg,Cl… 相似文献
200.
The electrochemical oxidation of 1, 3-benzenedithiol was investigated in a 0. 100 mol/L tetrabutylammonium perchlorate/acetonitrile electrolyte. The electrochemical techniques used were potential sweep, bulk electrolysis, rotating disc and the potential step method. The combination of the techniques yielded the number of electrons transferred per molecule, the reaction order, the transfer coefficient, the diffusion coefficient and concentration of dithiol anions, the standard heterogeneous rate constant as well as the formal potential and equilibrium constant of the preceeding dissociation reaction. This paper also illustrates the methods for studying the electrode kinetics of reactions which (a) involve a chemical reaction preceeding the electron-transfer process, (b) have insoluble polymer products, and (c) are totally irreversible. 相似文献