全文获取类型
收费全文 | 89459篇 |
免费 | 10970篇 |
国内免费 | 11773篇 |
专业分类
化学 | 33056篇 |
晶体学 | 1013篇 |
力学 | 3006篇 |
综合类 | 721篇 |
数学 | 5705篇 |
物理学 | 19419篇 |
综合类 | 49282篇 |
出版年
2024年 | 327篇 |
2023年 | 1179篇 |
2022年 | 2456篇 |
2021年 | 2646篇 |
2020年 | 2592篇 |
2019年 | 2377篇 |
2018年 | 2282篇 |
2017年 | 2560篇 |
2016年 | 2995篇 |
2015年 | 3951篇 |
2014年 | 5161篇 |
2013年 | 6278篇 |
2012年 | 7009篇 |
2011年 | 7397篇 |
2010年 | 6292篇 |
2009年 | 6664篇 |
2008年 | 7205篇 |
2007年 | 6679篇 |
2006年 | 6035篇 |
2005年 | 5026篇 |
2004年 | 3829篇 |
2003年 | 3041篇 |
2002年 | 3219篇 |
2001年 | 3044篇 |
2000年 | 2578篇 |
1999年 | 1871篇 |
1998年 | 1111篇 |
1997年 | 951篇 |
1996年 | 880篇 |
1995年 | 728篇 |
1994年 | 692篇 |
1993年 | 547篇 |
1992年 | 471篇 |
1991年 | 406篇 |
1990年 | 346篇 |
1989年 | 305篇 |
1988年 | 228篇 |
1987年 | 170篇 |
1986年 | 133篇 |
1985年 | 90篇 |
1984年 | 62篇 |
1983年 | 68篇 |
1982年 | 45篇 |
1981年 | 33篇 |
1980年 | 34篇 |
1979年 | 32篇 |
1978年 | 15篇 |
1977年 | 17篇 |
1936年 | 9篇 |
1925年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The cluster density function of independent percolation in ad-dimensional lattice is considered. For eachn, it is shown that(p) has finitenth leftderivative at critical probabilityp
c ifd is sufficiently large. This result agrees with the Bethe lattice approximation, where thenth one-sided derivative of(p) is bounded atp
c for alln. 相似文献
992.
993.
994.
The equation for the Helmholtz free energy for systems of small anisotropic molecules and ions is deduced by substituting the complete expression for various potential energies (including repulsive, dispersive, electrostatic, and induced energies) into the perturbation expansion. The equation is applied to pure water. The relative dielectric constant is set at unity. Based on the equal chemical potentials of equilibrated vapor and liquid phases, the molecular parameters of water are regressed from the densities of saturated vapor in the temperature range of 0 to 370°C. The ARD of regression is 1.16%. These parameters are used to predict the heat of vaporization and densities of saturated vapor and liquid phases of water in the same temperature range. The ARDs of prediction are 4.5% and 9.8%, respectively. The equation is used to correlate the osmotic coefficients of twelve 1:1 electrolyte solutions. The relative dielectric constant is set at unity. The parameters (Soft-sphere diameter and dispersive constant) of seven ions (Na+, K+, Rb+, Cs+, Cl–, Br–, and I–) are obtained. The total average absolute deviation between calculated and experimental values of the osmotic coefficient is 0.041. The parameters of ions can keep constant in different systems. 相似文献
995.
Herein, we report a novel amino acid based reaction system for CO2 capture and utilization (CCU) to produce formates in the presence of the naturally occurring amino acid l-lysine. Utilizing a specific ruthenium-based catalyst system, hydrogenation of absorbed carbon dioxide occurs with high activity and excellent productivity. Noteworthy, following the CCU concept, CO2 can be captured from ambient air in the form of carbamates and converted directly to formates in one-pot (TON > 50 000). This protocol opens new potential for transforming captured CO2 from ambient air to C1-related products.A novel amino acid based reaction system for CO2 capture and utilization (CCU) to produce formates is presented applying a ruthenium-based catalyst. Noteworthy, CO2 can be captured from ambient air and converted to formates in one-pot (TON > 50 000). 相似文献
996.
Chao Chen Yu-Lin Yang Kun-Lin Huang Zhen-Hua Sun Wei Wang Zhuo Yi Yun-Ling Liu Wen-Qin Pang 《Polyhedron》2004,23(18):3033-3042
With a hydrothermal technique, a layered titanium phosphate with the formula Ti2(H2PO4)(HPO4)(PO4)2 · 0.5C6N2H16 (denoted TP-J2) has been prepared by treating the Ti/H3PO4/H2O/1-methylpiperazine system directly. The as-synthesized products were characterized by powder X-ray diffraction, CP-MAS solid-state 31P NMR spectroscopy, thermogravimetric and differential thermal analyses (TG-DTA). The structure was solved by single-crystal X-ray diffraction analysis and it presents an extended γ-phase intercalated with organic amine. Crystal data: triclinic, , a = 8.106 (2) Å, b = 8.197 (2) Å, c = 11.658 (2) Å. = 78.32 (3)°, β = 80.85 (3)°, γ = 77.90 (3)°, Z = 2. Additionally, the intercalation behavior of TP-J2 with n-alkyl monoamine (n = 2, 3, 4, 6, 8, 10 and 12) was investigated. Owing to the strong brønsted base, N,N′-dimethylpiperazine, resides in the interlayer, it presented unusual features of TP-J2 in contrast with that of γ-Tip. 相似文献
997.
[structure: see text] A synthetic strategy promising the establishment of a new star-shaped and linear polycyclic aromatic hydrocarbons (PAHs) family with distinct molecular topologies has been developed. The Sonogashira reaction between the iodide derivatives 2a-e and phenylacetylene catalyzed with Pd(0) affords 3a-e in high yields. The Diels-Alder and decarbonylation reactions between 3a-e and tetraphenylcyclophentadiene following the oxidation by FeCl(3) produce the star-shaped and linear PAHs 5a-e containing a five-membered ring. The structural analysis and the optical properties of all new compounds are performed by a combination of MALDI-TOF mass spectrometry, UV-vis, and fluorescence spectrometry. The electronic and photophysical properties are studied by orthogonal comparisons of the absorption and fluorescence spectra in THF solutions, which not only give insight into the interactions among aromatic submoieties in each molecule and the effects of meta-conjugation and para-conjugation on electronic delocalization, but also indicate effective conjugation length variations from oligophenylacetylenes 3a-e to oligophenylene dendrimers 4a-e and PAHs 5a-e. The star-shaped 5c exhibits the highest aggregation in excited states compared with the other four hexa-peri-hexabenzocoronene (HBC) derivatives. 相似文献
998.
The attempted Baylis-Hillman reactions of sulfonyl aldimines or aryl aldehydes with 3-methylpenta-3,4-dien-2-one or 3-benzylpenta-3,4-dien-2-one gave the corresponding Baylis-Hillman adducts in moderate yields in DMSO under the catalysis of DBU or PMe3, respectively. Moderate diastereoselectivities were observed in the reaction of 3-benzylpenta-3,4-dien-2-one with N-arylmethylidene-1-naphthalenesulfonamides catalyzed by chiral catalyst cinchona alkaloid derivative TQO {4-(3-ethyl-4-oxa-1-azatricyclo[4.4.0.0^3,8]dec-5-yl)quinolin-6-ol}. 相似文献
999.
Determination of amino acids in a complex matrix without derivatization is advantageous, however, difficulties are found in both the detection and the separation of those compounds. In this study, a rapid and reliable LC-MS-MS method for the quantitation of underivatized amino acids in exocellular media was established. Injections were made directly after centrifugation of the samples, without further preparation. The separation of seven underivatized amino acids was achieved on a reversed-phase C18 column with pentadecafluorooctanoic acid as a volatile ion-pair reagent, and the specific detection of most amino acids was achieved by MS-MS of the specific transitions [M + H]+-->[M + H - 46]+. The calibration curves of all analytes were linear over the range of 1.0-1000 microg ml(-1) and the detection limits ranged from 0.1 to 5 ng ml(-1), with an injection volume of 20 microl. The inter-day and intra-day precisions ranged from 2.6 to 5.7% and 4.8 to 8.2%, respectively; the mean recoveries of the seven analytes were 81-104%, 91-107% and 93-101% respectively at the spiked level of 10, 40 and 200 microg ml(-1). A large number of fermentation samples were analysed using this method. The technique is simple, rapid, selective and sensitive, and shows potential for the high-throughput quantitation of amino acids from other biological matrices. 相似文献
1000.
H.-J. Machulla A. Blocher M. Kuntzsch M. Piert R. Wei J.R. Grierson 《Journal of Radioanalytical and Nuclear Chemistry》2000,243(3):843-846
[18F]FLT (3-deoxy-3-[18F]fluorothymidine) turned out to be a tracer particularly suitable for PET imaging of tumor proliferation because of lacking degradation in vivo. To facilitate clinical studies with [18F]FLT, we investigated two new easily accessible precursors, 2,3-anhydrothymidine (AThy) and 5-O-(4,4-dimethoxytriphenylmethyl)-2,3-anhydrothymidine (DMTThy), using a common approach for introducing the label with nucleophilic [18F]fluoride. Radiochemical yields were determined in dependence on substrate concentration, reaction time and temperature. In the case of AThy (10 mg), best FLT yields were 5.3%±1.2 (130 °C, 30 min). Labeling of DMTThy (10 mg) gave 14.3%±3.3 at 160 °C within 10 minutes. Starting with an aqueous solution of 20 GBq [18F]fluoride the new method allows to produce 1.3 GBq [18F]FLT within 90 minutes ready for intravenous injection. The new labeling procedures allow [18F]FLT synthesis without lengthy preparation of the precursor and with high reproducibility mandatory for clinical application. 相似文献