全文获取类型
收费全文 | 654篇 |
免费 | 26篇 |
国内免费 | 7篇 |
专业分类
化学 | 398篇 |
晶体学 | 4篇 |
力学 | 39篇 |
数学 | 107篇 |
物理学 | 139篇 |
出版年
2023年 | 2篇 |
2022年 | 30篇 |
2021年 | 22篇 |
2020年 | 23篇 |
2019年 | 20篇 |
2018年 | 25篇 |
2017年 | 23篇 |
2016年 | 27篇 |
2015年 | 24篇 |
2014年 | 24篇 |
2013年 | 62篇 |
2012年 | 37篇 |
2011年 | 57篇 |
2010年 | 47篇 |
2009年 | 32篇 |
2008年 | 51篇 |
2007年 | 18篇 |
2006年 | 15篇 |
2005年 | 18篇 |
2004年 | 11篇 |
2003年 | 9篇 |
2002年 | 15篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 4篇 |
1998年 | 10篇 |
1997年 | 5篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1989年 | 3篇 |
1987年 | 2篇 |
1985年 | 2篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 8篇 |
1978年 | 2篇 |
1977年 | 5篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1968年 | 1篇 |
1967年 | 3篇 |
1966年 | 1篇 |
排序方式: 共有687条查询结果,搜索用时 15 毫秒
51.
Action of Hydrazines on 2‐(2‐Oxindolin‐3‐ylidene)malononitrile, (E,Z)‐Ethyl 2‐cyano‐2‐(2‐oxindolin‐3‐ylidene)acetate and Isatin‐β‐thiosemicarbazone as a Source of Spiro Indoline‐pyrazole Systems 下载免费PDF全文
Ahmed S. A. Youssef Magdy M. Hemdan Samir A. Emara Rabaa M. Kamel 《Journal of heterocyclic chemistry》2015,52(5):1331-1336
2‐(2‐Oxindolin‐3‐ylidene)malononitrile ( 1a ) or (E,Z)‐ethyl 2‐cyano‐2‐(2‐oxindolin‐3‐ylidene)acetate ( 1b ) or isatin‐β‐thiosemicarbazone ( 1c ) undergoes reactions with prototype hydrazine hydrate itself and some of its simple congeners to give hydrazone derivatives bearing indoline‐2‐one moiety ( 2 ). The hydrazone derivatives ( 2 ) when heated with acetyl acetone or ethyl acetoacetate in dry pyridine afforded the spiro indoline derivatives ( 3a , 3b ). Also, cinnoline derivative ( 9 ) is obtained by action of hydrazine hydrate on the N‐acetyl derivative of ( 6a ). The structures of the newly synthesized compounds were evaluated by IR, 1H‐NMR spectroscopy, mass spectra and elemental analyses. 相似文献
52.
David A. Keire Hongping Ye Michael L. Trehy Wei Ye Richard E. Kolinski Benjamin J. Westenberger Lucinda F. Buhse Moheb Nasr Ali Al-Hakim 《Analytical and bioanalytical chemistry》2011,399(2):581-591
During the 2007–2008 heparin crisis, it was found that the United States Pharmacopeia (USP) testing monograph for unfractionated
heparin sodium (UFH) did not detect the presence of the contaminant, oversulfated chondroitin sulfate (OSCS) in heparin. In
response to this concern, new tests and specifications were developed by the Food and Drug Administration (FDA) and USP and
put in place to not only detect the contaminant OSCS but also to improve assurance of quality and purity of the drug product.
Additional tests were also developed to monitor the heparin supply chain for other possible economically motivated additives
or impurities. In 2009, a new USP monograph was put in place that includes 500 MHz 1H NMR, SAX-HPLC, %galactosamine in total hexosamine, and anticoagulation time assays with purified factor IIa or factor Xa.
These tests represent orthogonal approaches for UFH identification, measurement of bioactivity, and for detection of process
impurities or contaminants in UFH. The FDA has applied these analytical approaches to the study of UFH active pharmaceutical
ingredients in the marketplace. Here, we describe results from a comprehensive survey of UFH collected from seven different
sources after the 2009 monograph revision and compare these data with results obtained on other heparin samples collected
during the 2007–2008 crisis. 相似文献
53.
Assessment of turbulence modeling for gas flow in two-dimensional convergent–divergent rocket nozzle
In the present study, the turbulent gas flow dynamics in a two-dimensional convergent–divergent rocket nozzle is numerically predicted and the associated physical phenomena are investigated for various operating conditions. The nozzle is assumed to have impermeable and adiabatic walls with a flow straightener in the upstream side and is connected to a plenum surrounding the nozzle geometry and extended in the downstream direction. In this integrated component model, the inlet flow is assumed a two-dimensional, steady, compressible, turbulent and subsonic. The physics based mathematical model of the considered flow consists of conservation of mass, momentum and energy equations subject to appropriate boundary conditions as defined by the physical problem stated above. The system of the governing equations with turbulent effects is solved numerically using different turbulence models to demonstrate their numerical accuracy in predicting the characteristics of turbulent gas flow in such complex geometry. The performance of the different turbulence models adopted has been assessed by comparing the obtained results of the static wall pressure and the shock position with the available experimental and numerical data. The dimensionless shear stress at the nozzle wall and the separation point are also computed and the flow field is illustrated. The various implemented turbulence models have shown different behavior of the turbulent characteristics. However, the shear-stress transport (SST) k–ω model exhibits the best overall agreement with the experimental measurements. In general, the proposed numerical procedure applied in the present paper shows good capability in predicting the physical phenomena and the flow characteristics encountered in such kinds of complex turbulent flow. 相似文献
54.
Choi SH Lee S Soto HE Lodge TP Bates FS 《Journal of the American Chemical Society》2011,133(6):1722-1725
Assessing the state of mixing on the molecular scale in soft solids is challenging. Concentrated solutions of micelles formed by self-assembly of polystyrene-block-poly(ethylene-alt-propylene) (PS-PEP) diblock copolymers in squalane (C(30)H(62)) adopt a body-centered cubic (bcc) lattice, with glassy PS cores. Utilizing small-angle neutron scattering (SANS) and isotopic labeling ((1)H and (2)H (D) polystyrene blocks) in a contrast-matching solvent (a mixture of squalane and perdeuterated squalane), we demonstrate quantitatively the remarkable fact that a commercial mixer can create completely random mixtures of micelles with either normal, PS(H), or deuterium-labeled, PS(D), cores on a well-defined bcc lattice. The resulting SANS intensity is quantitatively modeled by the form factor of a single spherical core. These results demonstrate both the possibility of achieving complete nanoscale mixing in a soft solid and the use of SANS to quantify the randomness. 相似文献
55.
el-Nahass MM Kamel MA el-Deeb AF Atta AA Huthaily SY 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,79(3):443-450
The optimized geometry and vibrational frequencies of P-N,N-dimethylaminobenzylidenemalononitrile (DBM) were obtained by ab initio HF and DFT/B3LYP levels with complete relaxation in the potential energy surface using 6-31++G(d,p) and 6-311++G(d,p) basis sets. The Fourier-transform infrared (FT-IR) spectrum of DBM has been recorded in the region 4000-400 cm(-1). The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FT-IR spectrum. The calculated frequencies are in good agreement with the experimental frequencies. 相似文献
56.
Bouderdara N Elomri A Djarri L Medjroubi K Seguin E Vérité P 《Natural product communications》2011,6(1):115-117
The essential oil of aerial parts of Cachrys libanotis L. (Apiaceae) from east Algeria was extracted by hydrodistillation and analyzed by GC-FID and GC-MS. Thirty-one compounds were identified, the main components being germacrene-D (18.0%), gamma-terpinene (6.4%), p-cymene (5.5%), caryophyllene oxide (5.1%), and limonene (5.1%). 相似文献
57.
Hosni K Msaâda K Ben Taârit M Chahed T Marzouk B 《Natural product communications》2011,6(11):1731-1734
The chemical composition of the essential oils obtained by hydrodistillation from the aerial parts of the Tunisian Hypericum perforatum and H. ericoides ssp. roberti was elucidated by a combination of GC and GC-MS analyses. The main constituents of the oil of H. perforatum were alpha-pinene (11.8%), alpha-ylangene (10.4%), germacrene-D (9.5%), n-octane (6.5%) and alpha-selinene (5.9%). The oil of H. ericoides ssp. roberti exhibited a higher amount of aliphatic and branched hydrocarbons and the main constituents were n-octane (29.1%), alpha-pinene (10.9%), pulegone (7.7%) and acetophenone (7%). Both qualitative and quantitative differences were observed between the studied oils. This chemical variability seems likely to result from the genetic variability, since samples of both species were collected at the same location and processed under the same conditions. 相似文献
58.
ABSTRACTHydrogen storage reactions on Ni ? C59X(X = B, N) heterofullerene are investigated by using the state-of-the-art density functional theory calculations. The Ni atom prefers to bind at the bridge site between two hexagonal rings, and can bind up to five hydrogen molecules with average adsorption energies of (?0.94, ?0.48, ?0.33, ?0.25 and ?0.20 eV) per hydrogen molecule for Ni ? C59B, while (?1.20, ?0.60, ?0.41, ?0.28 and ?0.23 eV) per hydrogen molecule for Ni ? C59N. With no metal clustering, the system gravimetric capacities are expected to be as large as 10.87 and 10.85 wt % for 5H2NiC59B?and 5H2NiC59N, respectively. While the desorption activation barriers of the complexes 1H2 + C59X?(X = B, N)?are outside the Department of Energy domain (?0.2 to ?0.6 eV), the desorption activation barriers of the complexes nH2 + C59X(X = B, N)(n = 2 ? 5) are inside this domain. The hydrogen storage of the irreversible 1H2 + NiC59X?(X = B, N) and reversible 2H2 + NiC59X?(X = B, N) interactions is characterised in terms of density of states and projected densities of states, pairwise and non-pairwise additivity, infrared, Raman, electrophilicity and molecular electrostatic potentials. 相似文献
59.
The compound Mn2N0.86 which belongs to the crystallographic space group P6322 undergoes at 308K a paramagnetic-antiferromagnetic transition accompanied by a crystallographic distorsion. The magnetic structure is collinear with a magnetic moment of 1.7 μB per atom Mn. The Shubnikov group is Cp22'2'1 (PA21212). 相似文献
60.
Mohamed Rifaat Hamza Elmoghayar Abdel-Ghani Ali El-Agamey Mohamed Yousri Abdel-Samad Nasr Mohamed Mohamed Mohamed Sallam 《Journal of heterocyclic chemistry》1984,21(6):1885-1887
The reaction of 2-cyanoethanoic acid hydrazide and arylidenemalononitrile was studied as a new route for the synthesis of N-amino-2-pyridones. Pyrano[2,3-c]pyrazole and thiazolo[2,3-a]pyridine could be prepared from the reaction of arylideneazolones with the same reagent. 相似文献