全文获取类型
收费全文 | 15483篇 |
免费 | 663篇 |
国内免费 | 6篇 |
专业分类
化学 | 10544篇 |
晶体学 | 82篇 |
力学 | 245篇 |
数学 | 2251篇 |
物理学 | 2222篇 |
无线电 | 808篇 |
出版年
2023年 | 146篇 |
2022年 | 85篇 |
2021年 | 154篇 |
2020年 | 240篇 |
2019年 | 252篇 |
2018年 | 215篇 |
2017年 | 208篇 |
2016年 | 520篇 |
2015年 | 525篇 |
2014年 | 592篇 |
2013年 | 1073篇 |
2012年 | 1234篇 |
2011年 | 1149篇 |
2010年 | 678篇 |
2009年 | 526篇 |
2008年 | 1036篇 |
2007年 | 1002篇 |
2006年 | 968篇 |
2005年 | 904篇 |
2004年 | 731篇 |
2003年 | 615篇 |
2002年 | 552篇 |
2001年 | 226篇 |
2000年 | 219篇 |
1999年 | 140篇 |
1998年 | 106篇 |
1997年 | 106篇 |
1996年 | 132篇 |
1995年 | 103篇 |
1994年 | 81篇 |
1993年 | 97篇 |
1992年 | 78篇 |
1991年 | 73篇 |
1990年 | 65篇 |
1989年 | 52篇 |
1988年 | 54篇 |
1987年 | 54篇 |
1986年 | 67篇 |
1985年 | 95篇 |
1984年 | 84篇 |
1983年 | 51篇 |
1982年 | 68篇 |
1981年 | 77篇 |
1980年 | 75篇 |
1979年 | 60篇 |
1978年 | 59篇 |
1977年 | 54篇 |
1976年 | 53篇 |
1975年 | 49篇 |
1974年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 13 毫秒
991.
Coral J Karp SG Porto de Souza Vandenberghe L Parada JL Pandey A Soccol CR 《Applied biochemistry and biotechnology》2008,151(2-3):333-341
Propionic acid (PA) is widely used as additive in animal feed and also in the manufacturing of cellulose-based plastics, herbicides, and perfumes. Salts of propionic acid are used as preservative in food. PA is mainly produced by chemical synthesis. Nowadays, PA production by fermentation of low-cost industrial wastes or renewable sources has been an interesting alternative. In the present investigation, PA production by Propionibacterium acidipropionici ATCC 4965 was studied using a basal medium with sugarcane molasses (BMSM), glycerol or lactate (BML) in small batch fermentation at 30 and 36 degrees C. Bacterial growth was carried out under low dissolved oxygen concentration and without pH control. Results indicated that P. acidipropionici produced more biomass in BMSM than in other media at 30 degrees C (7.55 g l(-1)) as well as at 36 degrees C (3.71 g l(-1)). PA and biomass production were higher at 30 degrees C than at 36 degrees C in all cases studied. The best productivity was obtained by using BML (0.113 g l(-1) h(-1)), although the yielding of this metabolite was higher when using glycerol as carbon source (0.724 g g(-1)) because there was no detection of acetic acid. By the way, when using the other two carbon sources, acetic acid emerged as an undesirable by-product for further PA purification. 相似文献
992.
Nuclear magnetic resonance (NMR) spectroscopy arguably provides the greatest insight into the overall chemical composition of dissolved organic matter (DOM). However, in a standard 5 mm NMR probe, a sample of sea water at natural abundance only contains ca. 500-600 ng of organic matter, distributed among the heterogeneous components of DOM. Additionally, the intensity of the water signal, which may be many orders of magnitude greater than the signals from DOM, makes the detection and analysis of DOM at natural abundance extremely demanding. Here, we demonstrate, that although challenging, the application of an improved water suppression technique allows NMR spectra of DOM to be obtained directly (i.e without pre-concentration) for major bodies of water, including rivers, lakes and the ocean. The technique described here provides a compositional overview of an intact sample, permitting researchers to investigate and assess the impact of concentration, isolation and extraction procedures that are employed routinely. Also the technique permits NMR to be performed on 'precious' samples for which traditional isolations are not possible, for example, water from ice cores and pore water, which are key in hydrology and for paleoclimatic reconstruction. 相似文献
993.
Giménez-Romero D Agrisuelas J García-Jareño JJ Gregori J Gabrielli C Perrot H Vicente F 《Journal of the American Chemical Society》2007,129(22):7121-7126
This paper demonstrates the importance of the structural changeover in controlling the physical-chemical properties of hexacyanometalate-like materials (Prussian Blue). A meticulous in situ study of compositional variations using electroacoustic impedance techniques associated to electrogravimetric techniques in hexacyanoferrates containing K+ alkali metals reveals the existence of a nanostructural changeover coupled to a change of the magnetic properties of these electromagnetic materials. In the same way, the electroacoustic impedance techniques can be useful both in the understanding and in the in situ monitoring of the structural changeovers and the magnetic behavior of all kinds of materials. 相似文献
994.
995.
Dr. Christophe André Dr. Baptiste Legrand Laure Moulat Emmanuel Wenger Dr. Claude Didierjean Dr. Emmanuel Aubert Dr. Marie Christine Averlant‐Petit Prof. Jean Martinez Dr. Muriel Amblard Dr. Monique Calmes 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(50):16963-16971
The combination of a non‐functionalized constrained bicyclo[2.2.2]octane motif along with urea linkages allowed the formation of a highly rigid 2.512/14 helical system both in solution and the solid state. In this work, we aimed at developing stable and functionalized systems as promising materials for biological applications in investigating the impact of this constrained motif and its configuration on homo and heterochiral mixed‐oligourea helix formation. Di‐, tetra‐, hexa‐, and octa‐oligoureas alternating the highly constrained bicyclic motif of (R) or (S) configuration with acyclic (S)‐β3‐amino acid derivatives were constructed. Circular dichroism (CD), NMR experiments, and the X‐ray crystal structure of the octamer unequivocally proved that the alternating heterochiral R/S sequences form a stable left‐handed 2.5‐helix in contrast to the mixed (S/S)‐oligoureas, which did not adopt any defined secondary structure. We observed that the (?)‐synclinal conformation around the Cα? Cβ bond of the acyclic residues, although sterically less favorable than the (+)‐synclinal conformation, was imposed by the (R)‐bicyclic amino carbamoyl (BAC) residue. This highlighted the strong ability of the BAC residue to drive helical folding in heterochiral compounds. The role of the stereochemistry of the BAC unit was assessed and a model was proposed to explain the misfolding of the S/S sequences. 相似文献
996.
Gaël Rouillé Dr. Cornelia Jäger Dr. Mathias Steglich Friedrich Huisken Prof. Dr. Thomas Henning Prof. Dr. Gabriele Theumer Ingmar Bauer Dr. Hans‐Joachim Knölker Prof. Dr. 《Chemphyschem》2008,9(14):2085-2091
The spectroscopic characterization of corannulene (C20H10) is carried out by several techniques. The high purity of the material synthesized for this study was confirmed by gas chromatography‐mass spectrometry (GC‐MS). During a high‐performance liquid chromatography (HPLC) process, the absorption spectrum of corannulene in the ultraviolet (UV) and visible (vis) ranges is obtained. The infrared (IR) absorption spectrum is measured in CsI pellets, and the Raman scattering spectrum is recorded for pure crystal grains. In addition to room temperature measurements, absorption spectroscopy in an argon matrix at 12 K is also performed in the IR and UV/Vis ranges. The experimental spectra are compared with theoretical Raman and IR spectra and with calculated electronic transitions. All calculations are based on the density functional theory (DFT), either normal or time‐dependent (TDDFT). Our results are discussed in view of their possible application in the search for corannulene in the interstellar medium. 相似文献
997.
Valfredo Azevedo Lemos Moacy Selis Santos Márcio José Silva dos Santos Daniel Rodrigues Vieira Cleber Galvão Novaes 《Mikrochimica acta》2007,157(3-4):215-222
Cloud point extraction employing the new reagent 6-[2′-(6′-methyl-benzothiazolylazo)]-1,2-dihydroxy-3,5-benzenedisulfonic
acid as complexing agent and Triton X-114 as the surfactant is proposed for copper determination. A sample volume of 10 mL
was used. Dilution of the surfactant-rich phase with acidified methanol was performed after phase separation, and the copper
contents were measured by flame atomic absorption spectrometry. Variables affecting the system were optimized using factorial
design and Doehlert matrix. Signals were measured as peak height using an instrument software. Using the experimental conditions
defined in the optimization, the method allowed copper determination with a detection limit of 1.5 μg L−1. The calculated enrichment factor is 14. The effects of foreign ions are reported. The accuracy of the procedure was tested
by analyzing certified reference material. The method was successfully applied to copper determination in natural and drinking
water samples. 相似文献
998.
Purification methods for proteomics samples are of crucial concern for improving the quality of the sample delivered to the
mass spectrometer. They constitute the link between the mass spectrometer and protein processing and peptide isolation steps
that usually require solvents, buffers, or detergents completely incompatible with MS-analysis conditions. This work describes
three new clean-up procedures using synthetic membranes and polymer media and compares them with standard procedures. The
efficiency of each of the purification procedures was studied via application to four standards and two membrane proteins.
This work highlights the importance of versatility in sample preparation, especially for MS-based proteomic investigations.
Figure PMF spectra obtained after MALDI-TOF measurements of bovine mitochondrial complex III (A) and complex IV (B) in-solution digests, with and without purification 相似文献
999.
Trilleras J López LG Pacheco DJ Quiroga J Nogueras M Torre JM Cobo J 《Molecules (Basel, Switzerland)》2010,15(10):7227-7234
A series of pyrimido[4,5-b]quinolines (5-deazaflavines), were synthesized by microwave assisted intramolecular cyclization. The N?-substituted-2,4-diamino-6-chloro-pyrimidine-5-carbaldehydes, were prepared by selective monoamination of 2-amino-4,6-dichloropyrimidine-5-carbaldehyde with aliphatic and aromatic amines. 相似文献
1000.
We examine the generation of terahertz (THz) by optical rectification of a fundamental infrared beam with its first harmonic in ionized air. From polarization measurements we identify an important, yet so far unreported (to our knowledge), cross term (chi(3)(xyxy)) + chi(3)(xyyx)) of the nonlinear susceptibility tensor. Omission of this term leads to an error by a factor of 10(6) of the THz intensity obtained with certain polarization configurations. 相似文献