全文获取类型
收费全文 | 3839篇 |
免费 | 97篇 |
国内免费 | 12篇 |
专业分类
化学 | 2895篇 |
晶体学 | 12篇 |
力学 | 66篇 |
数学 | 530篇 |
物理学 | 445篇 |
出版年
2023年 | 23篇 |
2022年 | 30篇 |
2021年 | 28篇 |
2020年 | 46篇 |
2019年 | 38篇 |
2018年 | 24篇 |
2017年 | 22篇 |
2016年 | 102篇 |
2015年 | 78篇 |
2014年 | 73篇 |
2013年 | 146篇 |
2012年 | 169篇 |
2011年 | 257篇 |
2010年 | 127篇 |
2009年 | 129篇 |
2008年 | 238篇 |
2007年 | 194篇 |
2006年 | 201篇 |
2005年 | 178篇 |
2004年 | 180篇 |
2003年 | 125篇 |
2002年 | 118篇 |
2001年 | 69篇 |
2000年 | 69篇 |
1999年 | 48篇 |
1998年 | 45篇 |
1997年 | 65篇 |
1996年 | 55篇 |
1995年 | 58篇 |
1994年 | 72篇 |
1993年 | 66篇 |
1992年 | 38篇 |
1991年 | 52篇 |
1990年 | 43篇 |
1989年 | 46篇 |
1988年 | 54篇 |
1987年 | 39篇 |
1986年 | 40篇 |
1985年 | 53篇 |
1984年 | 38篇 |
1983年 | 40篇 |
1982年 | 41篇 |
1981年 | 49篇 |
1980年 | 31篇 |
1979年 | 28篇 |
1978年 | 21篇 |
1977年 | 24篇 |
1976年 | 32篇 |
1975年 | 28篇 |
1973年 | 22篇 |
排序方式: 共有3948条查询结果,搜索用时 109 毫秒
991.
Zhiyue J. Wang Roland Fischer Zili Chu Donald H. Mahoney Jr. Brigitta U. Mueller Raja Muthupillai Ellen B. James Rajesh Krishnamurthy Taylor Chung Eric Padua Elliott Vichinsky Paul Harmatz 《Magnetic resonance imaging》2010
A magnetic resonance imaging cardiac magnetic susceptometry (MRI-CS) technique for assessing cardiac tissue iron concentration based on phase mapping was developed. Normal control subjects (n=9) and thalassemia patients (n=13) receiving long-term blood transfusion therapy underwent MRI-CS and MRI measurements of the cardiac relaxation rate R2*. Using MRI-CS, subepicardium and subendocardium iron concentrations were quantified exploiting the hemosiderin/ferritin iron specific magnetic susceptibility. The average of subepicardium and subendocardium iron concentrations and R2* of the septum were found to be strongly correlated (r=0.96, P<.0001), and linear regression analysis yielded CIC (μg Fe/gwet tissue)=(6.4±0.4)·R2* septum (s−1) − (120±40). The results demonstrated that septal R2* indeed measures cardiac iron level. 相似文献
992.
Rexford D. Newbould Stefan T. Skare Marcus T. Alley Garry E. Gold Roland Bammer 《Magnetic resonance imaging》2010
By combining a balanced steady-state free precession (bSSFP) readout with an initial inversion pulse, all three contrast parameters, T1, T2 and proton density (M0), may be rapidly calculated from the signal progression in time. However, here it is shown that this technique is quite sensitive to variation in the applied transmit RF (B1) field, leading to pronounced errors in calculated values. Two-dimensional (2D) acquisitions are taxed to accurately quantify the relaxation, as the short RF pulses required by SSFP's rapid TR contain a broad spectrum of excitation angles. A 3D excitation using a large diameter excitation coil was able to correctly quantify the parameters. While the extreme B1 sensitivity was previously problematic and has precluded use of IR-bSSFP for relaxometry, in this work these obstacles were significantly reduced, allowing the rapid quantification of T1, T2 and M0. The results may further be used to simulate image contrast from common sequences, such as a T1-weighted or fluid-attenuated inversion recovery (FLAIR) examination. 相似文献
993.
994.
Life on Earth can be traced back to as far as 3.8 billion years (Ga) ago. The catastrophic meteoritic bombardment ended between 4.2 and 3.9 Ga ago. Therefore, if life emerged, and we know it did, it must have emerged from nothingness in less than 400 million years. The most recent scenarios of Earth accretion predict some very unstable physico-chemical conditions at the surface of Earth, which, in such a short time period, would impede the emergence of life from a proto-biotic soup. A possible alternative would be that life originated in the depth of the proto-ocean of the Hadean Earth, under high hydrostatic pressure. The large body of water would filter harmful radiation and buffer physico-chemical variations, and therefore would provide a more stable radiation-free environment for pre-biotic chemistry. After a short introduction to Earth history, the current tutorial review presents biological and physico-chemical arguments in support of high-pressure origin for life on Earth. 相似文献
995.
Roithová J Zabka J Herman Z Thissen R Schröder D Schwarz H 《The journal of physical chemistry. A》2006,110(20):6447-6453
Structural aspects as well as the stability and reactivity of the CHBr(2+) dication are studied both experimentally and theoretically. Translational energy distributions of the CHBr(+) products from charge transfer between CHBr(2+) and Kr indicate that the dication exists in two isomeric forms, H-C-Br(2+) and C-Br-H(2+). In the reaction of CHBr(2+) with H(2), the dominant channel corresponds to proton transfer leading to CBr(+) + H(3)(+). Other reaction channels involve the formation of the intermediates CH(3)Br(2+) and CH(2)BrH(2+), respectively. Both of the latter dications can either lose a proton to form CH(2)Br(+) or undergo a spin-isomerization followed by cleavage of the C-Br bond. The proposed mechanisms are supported by DFT calculations and deuterium labeling experiments. 相似文献
996.
The recently discovered scaling law for the relaxation times, tau(T,upsilon) = I(Tupsilon(gamma)), where T is temperature and upsilon the specific volume, is derived by a revision of the entropy model of the glass transition dynamics originally proposed by Avramov [J. Non-Cryst. Solids 262, 258 (2000)]. In this modification the entropy is calculated by an alternative route. The resulting expression for the variation of the relaxation time with T and upsilon is shown to accurately fit experimental data for several glass-forming liquids and polymers over an extended range encompassing the dynamic crossover. From this analysis, which is valid for any model in which the relaxation time is a function of the entropy, we find that the scaling exponent gamma can be identified with the Gruneisen constant. 相似文献
997.
Viscosities eta and their temperature T and volume V dependences are reported for seven molecular liquids and polymers. In combination with literature viscosity data for five other liquids, we show that the superpositioning of relaxation times for various glass-forming materials when expressed as a function of TV(gamma), where the exponent gamma is a material constant, can be extended to the viscosity. The latter is usually measured to higher temperatures than the corresponding relaxation times, demonstrating the validity of the thermodynamic scaling throughout the supercooled and higher T regimes. The value of gamma for a given liquid principally reflects the magnitude of the intermolecular forces (e.g., steepness of the repulsive potential); thus, we find decreasing gamma in going from van der Waals fluids to ionic liquids. For some strongly H-bonded materials, such as low molecular weight polypropylene glycol and water, the superpositioning fails, due to the nontrivial change of chemical structure (degree of H bonding) with thermodynamic conditions. 相似文献
998.
Schröter MK Khodeir L van den Berg MW Hikov T Cokoja M Miao S Grünert W Muhler M Fischer RA 《Chemical communications (Cambridge, England)》2006,(23):2498-2500
Free-standing, ZnO surface decorated Cu nanoparticles of 1-3 nm size were obtained by sequential co-pyrolysis of [Cu(OCHMeCH2NMe2)2] and ZnEt2 in squalane in the absence of additional surfactants and proved to be highly active quasi homogeneous catalysts for methanol synthesis from CO and H2. 相似文献
999.
1000.
Siemer AB Arnold AA Ritter C Westfeld T Ernst M Riek R Meier BH 《Journal of the American Chemical Society》2006,128(40):13224-13228
We report the observation of undetected (until now) residues of the prion protein fragment HET-s(218-289) which give rise to well-resolved (13)C, (15)N, and (1)H NMR resonances under high-resolution magic-angle spinning (HRMAS) conditions. The observed signals belong to large polymeric units as shown by measuring the lateral diffusion constants. The amino acids identified in the spectra are compatible with their localization in the segments of the protein which could not be detected in earlier solid-state NMR experiments. The observed chemical shifts indicate that these residues are in a random-coil conformation. Complementary experiments which detect only dynamic or static residues, respectively, strongly suggest that they belong to different parts of the same molecule. 相似文献