全文获取类型
收费全文 | 440篇 |
免费 | 20篇 |
学科分类
数理化 | 460篇 |
出版年
2023年 | 6篇 |
2022年 | 6篇 |
2021年 | 22篇 |
2020年 | 12篇 |
2019年 | 18篇 |
2018年 | 11篇 |
2017年 | 7篇 |
2016年 | 16篇 |
2015年 | 13篇 |
2014年 | 19篇 |
2013年 | 25篇 |
2012年 | 38篇 |
2011年 | 26篇 |
2010年 | 13篇 |
2009年 | 7篇 |
2008年 | 24篇 |
2007年 | 16篇 |
2006年 | 17篇 |
2005年 | 13篇 |
2004年 | 20篇 |
2003年 | 11篇 |
2002年 | 15篇 |
2001年 | 12篇 |
2000年 | 8篇 |
1999年 | 9篇 |
1998年 | 7篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 7篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 3篇 |
1985年 | 3篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1978年 | 3篇 |
1976年 | 1篇 |
1975年 | 5篇 |
1973年 | 1篇 |
1970年 | 2篇 |
1963年 | 2篇 |
1962年 | 1篇 |
1957年 | 2篇 |
1938年 | 2篇 |
1933年 | 1篇 |
1930年 | 2篇 |
排序方式: 共有460条查询结果,搜索用时 15 毫秒
1.
Jonathan M. Levin Lawrence L. Wald Marc J. Kaufman Marjorie H. Ross Luis C. Maas Perry F. Renshaw 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,130(2):292-295
Residual effects of an initial bolus of gadolinium contrast agent have been previously demonstrated in sequential dynamic susceptibility contrast MR experiments. While these residual effects quickly reach a saturation steady state, their etiology is uncertain, and they can lead to spurious estimates of hemodynamic parameters in activation experiments. The possible influence ofT1effects is now investigated with experiments in whichT1weighting is varied as well as with serial regionalT1measurements. Little evidence for significant residualT1effects is found, suggesting instead that susceptibility effects underlie these observations. An initial saturation dose of contrast agent minimizes this effect. 相似文献
2.
Tin and titanium ferrocyanides were studied as adsorbents for alkali metal ions, viz., 134Cs and 22Na, which represent radioactive wastes. The ferrocyanides were prepared in granular form. The tin version contained 11.2% water, while the titanium version contained 17.7% water. The exchange capacities for Cs+ and Na+ in the hydrated tin version were about 1.5 and 0.7 meq/g, respectively, while those in the titanium version were 2.2 and 1.2 meq/g, respectively. Drying at 250°C decimated those capacities. The diffusional time constant of Cs+ at 25°C, determined via Fick's second law, was of order of magnitude 1 × 10–3 s–1, though there were minor differences due to particle size and the form of ferrocyanide. Similarly, the effective diffusivity was of order of magnitude 1 × 10–8 cm2/s. The titanium version responded slightly faster than the tin version. Likewise, equilibrium measurements in mixtures with sodium nitrate, potassium nitrate, or uranium oxide, showed that the titanium version exhibited significantly greater selectivity for Cs+ than did the tin version. Unfortunately, tests of complete elution of the Cs+ from the ferrocyanides were mostly disappointing. Work continues on that subject. 相似文献
3.
4.
Pongaglabol [8-hydroxy-5-phenyl-furo[2,3-h]benzo(b)pyran-7-one] was synthesized and tested for antibacterial effects againstShigella dysenteriae, Salmonella typhi, Streptococcus β-haemolyticus andStaphylococcus aureus. The synthesized compounds were characterized using UV, IR and1H NMR spectral data 相似文献
5.
LetX,X
1,X
2,... be i.i.d. random vectors in d. The limit laws that can arise by suitable affine normalizations of the partial sums,S
n=X
1+...+X
n, are calledoperator-stable laws. These laws are a natural extension to
d
of the stable laws on. Thegeneralized domain of attraction of [GDOA()] is comprised of all random vectorsX whose partial sums can be affinely normalized to converge to . If the linear part of the affine transformation is restricted to take the formn
–B for some exponent operatorB naturally associated to thenX is in thegeneralized domain of normal attraction of [GDONA()]. This paper extends the theory of operator-stable laws and their domains of attraction and normal attraction. 相似文献
6.
An attempt was made to prepare 2-benzylquinoxalin-3-one by hydrolyzing the azlactone, 2-phenyl-4-benzylidene-5-oxazolone to β-phenylpyruvic acid and then treating this in situ with o-phenylenediamine (OPDA). The initial hydrolysis apparantly proceeded only as far as opening the azlactone ring forming 2-benzamidocinnamic acid which condensed with OPDA to form a substituted styrylbenzimidazole. 相似文献
7.
Stevens SM Chung AY Chow MC McClung SH Strachan CN Harmon AC Denslow ND Prokai L 《Rapid communications in mass spectrometry : RCM》2005,19(15):2157-2162
A fluorescent affinity tag (FAT) was synthesized and was utilized to selectively modify phosphorylated serine and threonine residues via beta-elimination and Michael addition chemistries in a 'one-step' reaction. This labeling technique was used for covalent modification of both phosphoproteins and phosphopeptides, allowing identification of these molecular species by fluorescence imaging after solution- or gel-based separation methods. In addition to the strong fluorescence of the rhodamine tag, a commercially available antibody can be used to enrich low-abundance post-labeled phosphopeptides present in complex mixtures. Application of this methodology to phosphorylation-site mapping has been evaluated for a phosphoprotein standard, bovine beta-casein. Initial results demonstrated low femtomole detection limits after fluorescence image analysis of FAT-labeled proteins or peptides. 相似文献
8.
Papastavros E Shea PJ Langell MA 《Langmuir : the ACS journal of surfaces and colloids》2004,20(26):11509-11516
Finely ground and pretreated iron substrates known as "zerovalent iron" or "Fe0" are used as reductants in the environmental remediation of halogenated hydrocarbons, and the composition of their surfaces significantly affects their reactivity. Samples of unannealed and annealed (heat-treated under H2/N2) zerovalent iron were analyzed using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Surface concentration of the iron and of the impurities observed by XPS and AES, carbon, chlorine, sulfur, and oxygen, were measured before and after soaking in trichloroethylene (TCE) and in water saturated with TCE (H2O/TCE) to simulate chlorocarbon remediation conditions. Samples pretreated by annealing at high temperature under H2 contained less iron carbide. The carbide contaminant was evident in both iron and carbon XPS spectra, with binding energies of 709.0 and 283.3 eV for the Fe 2p3/2 and C 1s, respectively. The annealed Fe0 surface also contained more sulfur. The carbide concentration was essentially unchanged by TCE and H2O/TCE exposure, whereas the sulfur decreased in proportion to chlorine adsorption following the dechlorination reaction. While oxygen concentration is initially lower on the annealed substrate surface, it rapidly increased during the model TCE remediative treatment process and thus does not represent a significant effect of the annealing process on surface reactivity. 相似文献
9.
Marjorie J. Hackathorn M. J. Brock 《Journal of polymer science. Part A, Polymer chemistry》1975,13(4):945-954
The technique of microozonolysis followed by gas chromatography has been used to determine the amount of 1,2-structures occurring as 1,4–1,2–1,4 sequences in n-butyl-lithium-initiated polybutadienes. The product obtained from the ozonolysis of these sequences, 3-formly-1,6-hexanedial, was directly proportional to the 1,2 (vinyl) content of the polymers as measured by infrared or NMR spectroscopy. An unusual ozonolysis product, 4-octene-1,8-dial, has been found in the ozonolysis products of high-1,4 poly-butadienes. Ozonolysis has also been used to determine alternating structures in propylene-butadiene copolymers. The product obtained from ozonolysis of such structures is 3-methyl-1,6-hexanedial. Ozonolysis of a polyisoprene containing equal amounts of 1,4 and 3,4 structures indicated it to have a nonalternating structure with long blocks of 1,4-isoprene units. Gas chromatographic separation was accomplished by using sucrose acetate isobutyrate with Carbowax 20M as the liquid phase. Retention times were established by using model compounds where possible. 相似文献
10.
E. S. Zakaria 《Journal of Radioanalytical and Nuclear Chemistry》2005,264(3):629-635
Summary This paper reports the radiochemical study of the ion-exchange of Cs+, Na+, Sr2+ and Eu3+ ions with H+ by chromium hexacyanoferrate(II) which was prepared in a granular form using a gel method. The slow steps which determine the rate of exchange of these ions are directly proportional to the particle diameter and this is confirmed from the linearity test of Bt vs. t plots at different particle diameters. Boyed’s equation and Reichenberg’s tables were used for evaluating all the kinetic parameters. The results reveal that the effective particle radii are unchanged for both chromium hexacyanoferrate(II) dried at 60 and 120 °C. The obtained data were analyzed using McKay plots and Arrhenius equation and the kinetic and thermodynamic parameters, e.g., effective diffusion coefficient, activation energies and entropies of activation have been evaluated. The mobility of these ions inside the particles of chromium hexacyanoferrate(II) decrease in the order of Eu3+>Sr2+>Na+⊃Cs+. 相似文献