全文获取类型
收费全文 | 53669篇 |
免费 | 8072篇 |
国内免费 | 12002篇 |
专业分类
化学 | 38443篇 |
晶体学 | 1339篇 |
力学 | 3339篇 |
综合类 | 1150篇 |
数学 | 7771篇 |
物理学 | 21701篇 |
出版年
2024年 | 129篇 |
2023年 | 604篇 |
2022年 | 1405篇 |
2021年 | 1444篇 |
2020年 | 1521篇 |
2019年 | 1486篇 |
2018年 | 1340篇 |
2017年 | 1546篇 |
2016年 | 1723篇 |
2015年 | 2109篇 |
2014年 | 2406篇 |
2013年 | 3448篇 |
2012年 | 3764篇 |
2011年 | 5150篇 |
2010年 | 3868篇 |
2009年 | 4001篇 |
2008年 | 3960篇 |
2007年 | 3554篇 |
2006年 | 3378篇 |
2005年 | 3149篇 |
2004年 | 3393篇 |
2003年 | 2428篇 |
2002年 | 2255篇 |
2001年 | 2079篇 |
2000年 | 1762篇 |
1999年 | 1369篇 |
1998年 | 1063篇 |
1997年 | 863篇 |
1996年 | 856篇 |
1995年 | 749篇 |
1994年 | 692篇 |
1993年 | 988篇 |
1992年 | 978篇 |
1991年 | 716篇 |
1990年 | 684篇 |
1989年 | 642篇 |
1988年 | 361篇 |
1987年 | 244篇 |
1986年 | 247篇 |
1985年 | 226篇 |
1984年 | 194篇 |
1983年 | 148篇 |
1982年 | 145篇 |
1981年 | 103篇 |
1980年 | 89篇 |
1979年 | 88篇 |
1978年 | 55篇 |
1976年 | 45篇 |
1974年 | 42篇 |
1973年 | 49篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The applicability of (1R,2R)-1,3-diacetoxy-1-(4-nitrophenyl)-2-propyl isothiocyanate [(R,R)-DANI] as a recently developed chiral derivatizing agent for the enantioseparation of a series of β-blockers is described. The thiourea diastereomers formed were analyzed by reversed-phase high-performance liquid chromatography, mixtures of water and methanol or acetonitrile being used for elution. Conditions of derivatizations (temperature, reagent excess and reaction time) were optimized, and the effects of organic modifiers on the retention and separation were investigated; the diastereomers could readily be baseline separated with methanol-containing mobile phases with resolutions between 1.58 and 2.72. 相似文献
992.
993.
By means of a vacuum time-lag method, gas transport properties of apolyimide based on 2, 2- bis (3, 4- decarboxyphenyl ) hexafluoropropane dianhydride (6FDA )and meta- phenylenediamine (mPDA ) have been measured as a function of upstream pres-sure and temperature. The results show that no gas-induced plasticization occurs for thispolyimide in the upstream pressure range from 1 atm to 20atm. The temperature depen-dence of P and D can be described by the Arrhenius equations. The activation energiesof permeation and diffusion were obtained for the gas/polymer pair studied and correlatedwith the size of penetrant gas. 相似文献
994.
995.
This review aims to provide a summary of the progress in TP small molecule fluorescent probes for enzymes in recent years and displays the main fluorescent mechanisms that have been applied to design probes. 相似文献
996.
997.
Teng-Yuan Dong Chienlin Huang Chiao-Pei Chen Ming-Cheng Lin 《Journal of organometallic chemistry》2007,692(23):5147-5155
We describe the formation of stable dithiol-bifunctionalized Ru(II)-terpyridine monolayer onto gold electrode. The coverage-dependent behavior onto gold electrode has been studied by electrochemical technique. The stability, surface charge coverage, and electron-transfer kinetics were assessed by cyclic voltammetry. Functionalized monolayer-protected Au clusters (MPCs) were also prepared. The spectroscopic characterization data of MPCs using UV-Vis and TEM techniques are discussed. TEM images showed that functionalized spherical nanoclusters of 4.7 ± 0.3 and 4.3 ± 0.2 nm were produced. The particle sizes are uniform with a narrow size distribution. The morphology of Au(1 1 1) metal surface modified with MPCs was imaged using atomic force microscopy (AFM). The nanoparticle layer exhibits a distinct surface morphology, irregularly shaped domains with dimensions from 20 to 60 nm and root mean square heights of 2.401 nm. 相似文献
998.
R.-L. Yun Y.-M. Chang C.-H. Lin K.-H. Hu C.-M. Shu 《Journal of Thermal Analysis and Calorimetry》2006,85(1):107-113
In industrial processes, information
on the safety property of chemicals is essentially crucial for safe handling
during unit operations. Ensuring the safe use of combustible or flammable
substances in processes is unlikely without detailed investigations of their
flammability characteristics and related hazards. We studied 3-methyl pyridine
(3-picoline), e.g., flammability limits (LFL/UFL), maximum explosion pressure
(P
max), maximum
explosion pressure rise (dP/dt)max, minimum oxygen concentration
(MOC), vapor deflagration index (K
g),
and characterized the influence of inert steam (H2O)
on critical parameters for 3-picoline/water mixtures at 270°C, 1 atm,
various oxygen concentrations, and vapor mixing ratios (100/0, 30/70, 10/90
and 5/95 vol.%) with a 20-L-Apparatus in simulated conditions, respectively.
The results showed that the flammability characteristics
of 3-picoline(aq) all increased with the oxygen concentration.
However, as the composition of inert steam increased, the flammability parameters
and the degree of fire and explosion hazards were significantly reduced, instead.
This study elucidated the flammability properties of 3-picoline mixed with
inert steam. The conclusions could be applied to proactively prevent the relevant
processes from incurring fire and explosion accidents. 相似文献
999.
A New Diterpene Acid from the Flowers of Heteropappus altaicus 总被引:2,自引:0,他引:2
QingHuaLIU JunShanYANG YangLU QiTaiZHENG LiMAO 《中国化学快报》2005,16(7):921-924
A new compound, named heteraltaic acid was isolated from the flowers of Heteropappus altaicus (willd) Novopokr. The compound was elucidated as (5R, 6S, 8aS) -5-[2-(3-furyl) ethyl-5, 6, 8a-trimethyl-4a, 5, 6, 7, 8, 8a -hexahydro-l-naphthalenecarboxylic acid] by the combination of 1D and 2D NMR techniques (HSQC, HMBC) and X-ray analysis. 相似文献
1000.
In this work, a new ligand, 2,9-bis[N,N-bis(carboxymethyl)-aminomethyl]-1,10-phenanthroline (BBCAP), was synthesized and
used to establish a novel fluorimetric method for the determination of trace amounts of terbium in a binary system. Its luminescence
mechanism was studied. The excitation and emission wavelengths are 284 nm and 547 nm, respectively. Other rare-earth metal
ions in 100-fold excess caused no interference. The fluorescence intensity was a linear function of the concentration of terbium
in the range of 5.0 × 10−10–1.0 × 10−6 mol/L. The detection limit was 2.0 × 10−11 mol/L (n = 12). The standard addition method was used to determine the terbium in a synthetic rare-earth sample with satisfactory
results. This method represents a direct, rapid, selective and sensitive analytical procedure with a widely linear range.
Received November 29, 2001 Revision February 9, 2002 相似文献