全文获取类型
收费全文 | 3477篇 |
免费 | 195篇 |
国内免费 | 63篇 |
专业分类
化学 | 2471篇 |
晶体学 | 16篇 |
力学 | 166篇 |
数学 | 438篇 |
物理学 | 644篇 |
出版年
2024年 | 6篇 |
2023年 | 22篇 |
2022年 | 88篇 |
2021年 | 109篇 |
2020年 | 176篇 |
2019年 | 146篇 |
2018年 | 175篇 |
2017年 | 138篇 |
2016年 | 239篇 |
2015年 | 172篇 |
2014年 | 231篇 |
2013年 | 362篇 |
2012年 | 287篇 |
2011年 | 291篇 |
2010年 | 223篇 |
2009年 | 182篇 |
2008年 | 169篇 |
2007年 | 145篇 |
2006年 | 116篇 |
2005年 | 75篇 |
2004年 | 80篇 |
2003年 | 61篇 |
2002年 | 54篇 |
2001年 | 32篇 |
2000年 | 14篇 |
1999年 | 8篇 |
1998年 | 9篇 |
1997年 | 9篇 |
1996年 | 13篇 |
1995年 | 7篇 |
1994年 | 7篇 |
1993年 | 3篇 |
1992年 | 6篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1976年 | 5篇 |
1975年 | 2篇 |
1974年 | 5篇 |
1973年 | 2篇 |
排序方式: 共有3735条查询结果,搜索用时 15 毫秒
971.
Kathleen L. Caldwell Amir Makhmudov Robert L. Jones Joseph G. Hollowell 《Accreditation and quality assurance》2005,10(7):356-361
In 2001 the Centers for Disease Control and Prevention (CDC) established a program, Ensuring the Quality of Urinary Iodine
Procedures (EQUIP); to assist laboratories around the world and assess the accuracy of their urinary iodine (UI). CDC designed
EQUIP to issue unknown specimens to participating laboratories three times per year. Each laboratory was asked to analyze
unknown samples in duplicate on three different days. During the first five rounds of EQUIP, 41 laboratories participated,
measuring unknown samples and reporting their results to CDC. CDC used these results to prepare a statistical report for the
laboratories. Feedback to the laboratories provided external confirmation regarding performance. As a group, laboratory performance
improved; several laboratories made considerable improvement. Several laboratories that showed no improvement have ordered
new equipment or are arranging for additional training. EQUIP is a key tool used to support laboratory quality assurance in
an effort to eliminate iodine deficiency disorders (IDD) in the world.
相似文献
Kathleen L. CaldwellEmail: |
972.
The spot-test for m-dinitroaromatics and their derivatives, based on the Janovsky reaction, is re-investigated. The sensitivity of the spot-test has been much enhanced by changing the solvent and using the novel “pellet” spot-test. With the “pellet” spot-test and dimethylsulphoxide, 4–50 ng of m-dinitroaromatic compounds and their derivatives can be detected. The change from acetone (used in the original spot-test) to dimethylsulphoxide changes the reaction product in most cases. 相似文献
973.
Vogel MJ Miraghaie R Lopez JM Hirsa AH 《Langmuir : the ACS journal of surfaces and colloids》2004,20(14):5651-5654
Insoluble monolayers on water have been patterned at the macroscopic scale (i.e., at the centimeter scale of the flow apparatus) as well as the mesoscopic scale (i.e., down to the micron scale resolvable via optical microscopy). The macroscopic patterning at the air/water interface results from a hydrodynamic instability leading to a steadily precessing flow pattern. The velocity field is measured, and the associated shear stress at the interface is shown to be locally amplified by the flow pattern. The resulting hydrodynamic effects on two different monolayer systems are explored: (1) the pattern in a model monolayer consisting of micron-size, surface-bound particles is visualized to show that the particles are concentrated into isolated regions of converging flow with high shear, and (2) Brewster angle microscopy of a Langmuir monolayer (vitamin K1) shows not only that the monolayer is patterned at the macroscopic scale but also that the localized high-shear flow further patterns the monolayer at the mesoscale. 相似文献
974.
A flow-injection manifold incorporating immobilized firefly luciferase is described. The detector design and reaction conditions are discussed and results are presented for the determination of adenosine-5′-triphosphate over the range 1 × 10?12?1 × 10?5 M. Modifications for creatine phoshokinase (10–400 U l?1) and creatine phosphate (10?t–10?1 M), both determined indirectly via ATP, are also described. 相似文献
975.
Amino acid-based chiral ligands have been developed for use in Cu-catalyzed enantioselective allylic alkylations and conjugate additions that allow access to optically enriched compounds that are otherwise difficult to prepare. These chiral ligands are easily modified and have been identified through mechanism-based library screening. The data presented point to the significance of the availability of a collection of catalysts, since subtle variations in substrate or nucleophile structure often call for a different optimal chiral ligand. Can a catalyst be truly "rationally designed" or do we design our search pathway that eventually leads us to such a catalyst? What is meant by a "general catalyst"? Do we need a class of effective catalysts instead? These and related questions are addressed in the context of the above studies. 相似文献
976.
Ahmad VU Hussain J Hussain H Jassbi AR Ullah F Lodhi MA Yasin A Choudhary MI 《Chemical & pharmaceutical bulletin》2003,51(6):719-723
Three new diterpene esters with a myrsinol-type skeleton have been isolated from Euphorbia decipiens BOISS. & BUHSE. The structure elucidation of the isolated compounds was based primarily on two-dimensional (2D)-NMR techniques including correlation spectroscopy (COSY), heteronuclear multiple quantum coherence (HMQC), heteronuclear multiple bond correlation (HMBC) and nuclear Overhauser effect spectroscopy (NOESY) experiments. Compounds 1 and 3 are active against prolyl endopeptidase and compound 2 showed inhibitory activity against urease enzyme. 相似文献
977.
Yasseri AA Syomin D Malinovskii VL Loewe RS Lindsey JS Zaera F Bocian DF 《Journal of the American Chemical Society》2004,126(38):11944-11953
A series of multithiol-functionalized zinc porphyrins has been prepared and characterized as self-assembled monolayers (SAMs) on Au. The molecules, designated ZnPS(n) (n = 1-4), contain from one to four [(S-acetylthio)methyl]phenylethynylphenyl groups appended to the meso-position of the porphyrin; the other meso-substituents are phenyl groups. For the dithiol-functionalized molecules, both the cis- and the trans-appended structures were examined. The ZnPS(n) SAMs were investigated using X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and various electrochemical methods. The studies reveal the following characteristics of the ZnPS(n) SAMs. (1) The ZnPS(n) molecules bind to the Au surface via a single thiol regardless of the number of thiol appendages that are available per molecular unit. (2) The porphyrins in the ZnPS(3) and ZnPS(4) SAMs bind to the surface in a more upright orientation than the porphyrins in the ZnPS(1), cis-ZnPS(2), and trans-ZnPS(2) SAMs. The porphyrins in the ZnPS(3) and ZnPS(4) SAMs are also more densely packed than those in the cis-ZnPS(2) and trans-ZnPS(2) SAMs. The packing density of the ZnPS(3) and ZnPS(4) SAMs is similar to that of the ZnPS(1) SAMs, despite the larger size of the molecules in the former SAMs. (3) The thermodynamics and kinetics of electron transfer are generally similar for all of the ZnPS(n) SAMs. The general similarities in the electron-transfer characteristics for all of the SAMs are attributed to the similar binding motif. 相似文献
978.
S. A. Nabi E. S. M. Abu-Nameh N. Rahman M. I. H. Helaleh 《Fresenius' Journal of Analytical Chemistry》1998,360(1):140-141
A simple, fast and sensitive titrimetric method has been developed for the determination of tetracycline hydrochloride in
human urine using m-dinitrobenzene as color agent and Dowex 1 × 8 as detection medium.
Received: 26 February 1997 / Revised: 23 April 1997 / Accepted: 29. April 1997 相似文献
979.
Javad Safaei-Ghomi Abdolhamid Bamoniri Alireza Hatami Hossein Batooli 《Chemistry of Natural Compounds》2007,43(1):37-39
The essential oil of aerial parts of Stachys acerosa, which belongs to the Lamiaceae family and grows in central Iran, was obtained by a hydrodistileation method and analyzed
by GC and GC-MS apparatus. Fourteen compounds representing 98.8% of the oil were identified. Among them N-methylisatin (30%),
α-pinene (25%), sabinene (12.3%), and 2-hydroxyacetophenone (11.2%) were the major constituents of the oil, which was obtained
in 0.1% yield.
Published in Khimiya Prirodnykh Soedinenii, No. 1, pp. 32–34, January–February, 2007. 相似文献
980.
Regioselective and high-yielding bromination of aromatic compounds using hexamethylenetetramine-bromine 总被引:1,自引:0,他引:1
A regioselective and highly efficient method for bromination of aromatic compounds in the presence of a stoichiometric amount of hexamethylenetetramine-bromine (HMTAB) as an efficient reagent in dichloromethane is reported. The selectivity depends on the temperature and nature of the substituent on the substrate. The reactivity of this reagent was increased by supporting it to silica gel for bromination of less activated compounds. 相似文献