全文获取类型
收费全文 | 4806篇 |
免费 | 327篇 |
国内免费 | 405篇 |
专业分类
化学 | 1333篇 |
晶体学 | 4篇 |
力学 | 241篇 |
综合类 | 61篇 |
数学 | 2868篇 |
物理学 | 1031篇 |
出版年
2024年 | 12篇 |
2023年 | 100篇 |
2022年 | 126篇 |
2021年 | 112篇 |
2020年 | 144篇 |
2019年 | 155篇 |
2018年 | 163篇 |
2017年 | 186篇 |
2016年 | 169篇 |
2015年 | 121篇 |
2014年 | 282篇 |
2013年 | 380篇 |
2012年 | 214篇 |
2011年 | 303篇 |
2010年 | 257篇 |
2009年 | 266篇 |
2008年 | 311篇 |
2007年 | 308篇 |
2006年 | 253篇 |
2005年 | 224篇 |
2004年 | 208篇 |
2003年 | 180篇 |
2002年 | 181篇 |
2001年 | 132篇 |
2000年 | 109篇 |
1999年 | 120篇 |
1998年 | 120篇 |
1997年 | 81篇 |
1996年 | 65篇 |
1995年 | 42篇 |
1994年 | 31篇 |
1993年 | 25篇 |
1992年 | 17篇 |
1991年 | 8篇 |
1990年 | 27篇 |
1989年 | 15篇 |
1988年 | 9篇 |
1987年 | 4篇 |
1986年 | 11篇 |
1985年 | 10篇 |
1984年 | 10篇 |
1983年 | 3篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1976年 | 3篇 |
1973年 | 3篇 |
1936年 | 2篇 |
排序方式: 共有5538条查询结果,搜索用时 15 毫秒
71.
Shun-Chen Niu 《Queueing Systems》1988,3(2):157-178
We give in this paper a detailed sample-average analysis of GI/G/1 queues with the preemptive-resume LIFO (last-in-first-out) queue discipline: we study the long-run state behavior of the system by averaging over arrival epochs, departure epochs, as well as time, and obtain relations that express the resulting averages in terms of basic characteristics within busy cycles. These relations, together with the fact that the preemptive-resume LIFO queue discipline is work-conserving, imply new representations for both actual and virtual delays in standard GI/G/1 queues with the FIFO (first-in-first-out) queue discipline. The arguments by which our results are obtained unveil the underlying structural explanations for many classical and somewhat mysterious results relating to queue lengths and/or delays in standard GI/G/1 queues, including the well-known Bene's formula for the delay distribution in M/G/l. We also discuss how to extend our results to settings more general than GI/G/1. 相似文献
72.
Two greener procedures for flow-injection spectrophotometric determination of nitrite in natural waters were developed and critically compared. Replacement of toxic reagents, waste minimization and treatment were exploited to attend the standards of clean chemistry. The flow system was designed with solenoid micro-pumps in order to minimize reagent consumption and waste generation. The first procedure is based on the Griess diazo-coupling reaction with sulfanilamide and N-(1-naphthyl)ethylenediamine (NED) yielding an azo dye, followed by photodegradation of the low amount of waste generated based on the photo-Fenton reaction. The second procedure is based on the formation of iodine from nitrite and iodide in acid medium in order to avoid the use of toxic reagents. For Griess method, linear response was achieved up to 1.0 mg L− 1, described by the equation A = − 0.007 + 0.460C (mg L− 1), r = 0.999. The detection limit was estimated as 8 μg L− 1 at the 99.7% confidence level and the coefficient of variation was 0.8% (n = 20). The sampling rate was estimated as 108 determinations per hour. The consumption of the most toxic reagent (NED) is reduced 55-fold and 20-fold in comparison to batch method and flow injection with continuous reagent addition, respectively. A colorless residue was obtained by in-line photodegradation with reduction of 87% of the total organic carbon content. The results obtained for natural water samples were in agreement with those achieved by the reference method at the 95% confidence level. For the nitrite–iodide method, linear response was observed up to 2.0 mg L− 1, described by the equation A = − 0.024 + 0.148C (mg L− 1), r = 0.999. The detection limit was estimated as 25 μg L− 1 at the 99.7% confidence level and the coefficient of variation was 0.6% (n = 20). The sampling rate was estimated as 44 determinations per hour. Despite avoiding the use of toxic reagents, the nitrite–iodide method presented worst performance in terms of selectivity and sensitivity. 相似文献
73.
74.
Jia Rui Wang Chu Ting Yang Lei Liu Qing Xiang Guo Department of Chemistry University of Science Technology of China Hefei China 《中国化学快报》2007,18(2):133-136
Aerobic oxidation of electron-rich benzylic and phenyl allylic alcohols was achieved with high yields with only 0.1 mol.% of Pd(OAc)2 catalyst in the absence of any ligand. This procedure was expected to be valuable for realistic industrial-scale applications from both economic as well as environmental points of view. 相似文献
75.
76.
Analytic expressions are given for integrals of the Coulomb Green function with Slater type atomic orbitals. The results involve hypergeometric functions.Supported by the National Institutes of Health, Grant No. GM23223. 相似文献
77.
I. M. Krukovskii M. S. Molchanova A. V. Evtushenko V. A. Shlyapochnikov 《Russian Chemical Bulletin》1998,47(7):1266-1273
The ratio between the numbers of structural formulas of C,H,N,O-containing energetic compounds belonging to the classes of
fuels (low values of the oxygen coefficientA), explosives (mediumA), and oxidants (highA values) was studied by a computer generation procedure. The number of the theoretically possible structural formulas was
found to decrease rapidly on going from fuels to explosives and then to oxidants; this observation agrees with the data on
the numbers of various energetic compounds currently used and proposed. The strategy of the search for new compounds with
the specified properties is described in brief, and its applicability to the search for explosives and oxidants with a small
(up to 12) number of atoms in a molecule is evaluated.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1304–1310, July, 1998. 相似文献
78.
Christopher Imrie Phumelele Kleyi Thomas I.A. Gerber Jennifer Look 《Journal of organometallic chemistry》2007,692(16):3443-3453
Grinding of 1,1′-ferrocenedicarboxaldehyde with a 2.2 molar equivalent of an aromatic amine in a solvent-free environment provided excellent yields of 1,1′-ferrocenyldiimines. After mixing the aldehyde and amines, a gum or melt formed which eventually solidified to the product. An analytically pure sample of the product was obtained by cold recrystallization. Grinding of ferrocenecarboxaldehyde and 4-substituted phenylacetonitriles under solvent-free conditions provided good yields of the corresponding ferrocenylacrylonitriles. The yield in this reaction was very low when the substituent group para to the acetonitrile group was electron-donating. 相似文献
79.
Beate Hager Bettina Schwarzinger Heinz Falk 《Monatshefte für Chemie / Chemical Monthly》2006,137(2):163-168
Summary. Two model compounds for the green fluorescent protein chromophore were prepared. One of them incorporates the natural 4-hydroxybenzylidene
group of the natural tyrosin derived chromophore, the other one bears a methyl group instead of the hydroxy group. Whereas
the photochemically prepared (E)-diastereomer of the first compound very effectively reverted thermally (room temperature) to the thermodynamically stable
(Z)-diastereomer, the (E)-diastereomer of the second derivative proved to be stable even at elevated temperatures for more than a day. This finding
can be rationalized by constructing the appropriate resonance structures showing that only in the first case an effective
delocalization enables partial single bond character of the benzylidene double bond. From the standpoint of chemical etiology,
only Nature’s choice of the tyrosin derived chromophore of the green fluorescent protein provides an efficient radiationless
thermal relaxation channel for the unwanted photo-diastereomerization product formed after excitation besides the dominating
fluorescence channel of its chromophore. 相似文献
80.
A novel methodology in on-line pyrolysis-gas chromatography (Py-GC) for the fast analysis of fatty acids in vegetable oils with minimal sample treatment and the use of non-toxic reagents is described. Pyrolysis at 500 degrees C for 10 s of sub-microgram quantity of vegetable oil dissolved in dimethyl carbonate (DMC) and in the presence of nanopowder titanium silicon oxide resulted in the production of fatty acid methyl esters (FAMEs) as unique products. Pyrolysis performed by means of a resistively heated filament pyrolyser interfaced to a GC-MS apparatus enabled the direct analysis of evolved FAMEs. The DMC/Py-GC-MS analysis was tested on soybean, coconut, linseed, walnut and olive oil and the results compared to the classical BF(3)-methanol as reference methodology. The DMC method exhibited a lower precision and was biased towards lower levels of polyunsaturated fatty acids (PUFA) in comparison to the BF(3)-methanol method, but was more advantageous in terms of reduced sample treatment, waste generation and risk factors of employed chemicals. 相似文献