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221.
Molecular structure, ionic mobility and phase transitions in six- and seven-coordinated ammonium oxofluoroniobates (NH4)2NbOF5 and (NH4)3NbOF6 were studied by 19F, 1H NMR and DFT calculations. Equatorial fluorine atoms (Feq) in [NbOF5]2− and [NbOF6]3− are characterized by high 19F NMR chemical shifts while axial fluorine atoms (Fax) have those essentially lower. The high-temperature ionic mobility in (NH4)2NbOF5 does not stimulate the ligand exchange Feq ↔ Fax, whereas it is observed in (NH4)3NbOF6 as pseudorotation typical for seven-coordinated polyhedra. The transformation of pentagonal bipyramidal structure (BP) of [NbOF6]3− into capped trigonal prismatic (CTP) one takes place during the phase transition (PT) at 260 K. The PT of order-disorder type in (NH4)2NbOF5 is accompanied by transition of anionic sublattice to a rigid state. The 19F and 1H NMR data corroborate the independent motions of NH4 groups and anionic polyhedra in (NH4)2NbOF5 while they are coordinated in (NH4)3NbOF6. 相似文献
222.
以3-脲丙基三甲氧基硅烷为偶联剂,制备了一种新型极性脲丙基-C30(TPU-C30)反相色谱固定相。采用扫描电子显微镜、元素分析、红外光谱和热分析等对该固定相进行表征。结果表明,TPU-C30固定相已成功制备,连续制备3次固定相,其元素含量的相对偏差均小于5%,说明该合成工艺重复性良好。以不同极性、位置异构、碱性化合物为溶质探针,以传统的C18色谱柱与C30色谱柱为参比,对制备的固定相的色谱性能进行了研究。研究结果表明,TPU-C30固定相具有不同于传统C18柱、C30柱的选择性和更优的择形性,明显改善了碱性物质的峰形,其具有广阔的应用空间。 相似文献
223.
Haitao Guo Zhiying Wang Liying Xu Hongfen Zhang Ruimiao Chang Anjia Chen 《Electrophoresis》2019,40(4):547-554
A simple, comprehensive, and highly selective MEKC method has been developed for simultaneous analysis of seven bioactive components (triptolide, wilfortrine, wilfordine, wilforgine, wilforine, triptophenolide, and triptonide) in the root extracts of Tripterygium wilfordii Hook. F. (TWHF) and Tripterygium preparations (TPs). Optimal BGE consisted of 10 mM sodium tetraborate, 30 mM SDS, and 30% v/v methanol. The separation voltage was 20 kV and the temperature was 25°C. A DAD was used and the detection wavelength was at 218 nm. Under the optimum conditions, the baseline separation of seven components was achieved in less than 26 min. Excellent precision, good stability, and accuracy were obtained. For all analytes, linear calibrations were established within 10–100 μg/mL. The LOD and LOQ were within 1.2–4.2 μg/mL and 4.0–14 μg/mL, respectively. The developed method was suitable for the determination of key components in TWHF and TPs. 相似文献
224.
本文合成了正电子发射断层显像剂[18F]FET的两个新型前体:N-叔丁氧羰基-O-(2-三氟甲磺酰氧乙基)-L-酪氨酸甲酯9a和N-叔丁氧羰基-O-(2-三氟甲磺酰氧乙基)-L-酪氨酸叔丁酯9b. 化合物9a或9b以L-酪氨酸为原料, 先与甲醇发生酯化反应或与乙酸叔丁酯进行酯交换, 再用叔丁氧羰基保护氨基, 接着在苯环的酚羟基上引入羟乙基, 最后与三氟甲磺酸酐反应形成目标化合物, 这四步反应总收率分别是30%或15%. 相似文献
225.
226.
分别采用分子簇模型、连续介质模型和离散-连续组合模型研究了XH-NH3(X=F,Cl,Br)分子内质子传递的溶剂效应.结果表明,对于弱酸性化合物FH-NH3,其溶剂效应主要为短程作用,3个H2O分子即可使其发生质子传递,而简单的连续介质模型得到的仍为分子化合物形式,需进一步包含溶剂效应短程作用.对于强酸性化合物ClH-NH3和BrH-NH3,较弱的溶剂效应即可促使其发生质子传递,分子簇模型和连续介质模型均可合理描述,且与离散-连续组合模型的结果相近.离散-连续组合模型既在从头算水平考虑了溶质分子和第一溶剂化层中溶剂分子间的短程作用,又包含了溶剂效应的长程静电作用,能更准确地描述溶剂化作用,且对弱酸性化合物和强酸性化合物体系均适用. 相似文献
227.
The First Intramolecular Charge Transfer Transition Based on 2-Ureido-4[1H]-pyrimidinone Binding Module 总被引:1,自引:0,他引:1
The first intramolecular charge transfer transition based on 2-ureido-4[1H]-pyrimidinone binding module was reported. 相似文献
228.
Thermally induced phase separation technique was utilized to fabricate biodegradable poly(l ‐lactic acid) (PLLA) macrocellular foams which were capable of being applied in tissue engineering. The block copolymer Pluronic F127 composed of (polyethyleneoxide)‐(polypropyleneoxide)‐(polyethyleneoxide) [(PEO)‐(PPO)‐(PEO)] was used as a porogen. Water/dioxane mixtures with different volume ratios were used as solvents. The addition of Pluronic F127 could induce an appearance of large pores (50–200 μm) besides small pores (10–20 μm) or a change from a solid–liquid phase separation to a liquid–liquid phase separation. The role of Pluronic F127 depends on the water/dioxane ratios in the PLLA/dioxane/water system. The X‐ray diffraction patterns and porosity measurement results showed that Pluronic F127 was crystallized and existed on the pore wall. The effect of Pluronic F127 on changing pore structure is attributed to the occurrence of the interaction of the lipophilic PPO blocks in Pluronic F127 with PLLA clews, consequently, this results in PLLA aggregation and early phase separation on cooling. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
229.
Using geometry optimization and DFT method at the B3LYP/6-31G* level of theory for C30H20, an equilibrium geometry is identified that has the form of polyhedral hydrocarbon with five carbon–carbon single bonds linking a dodecahedrane cage and a pentaprismane cage. Thus, this molecule is a tri-cage molecule with two pentaprismane cages and one dodecahedrane cage. Vibrational frequencies and the infrared spectrum are computed at the same level of theory. The heat of formation for C30H20 has been estimated in this paper. The heat of formation of C30H20 as well as the vibrational analysis indicates that this molecule enjoys sufficient stability to allow for its experimental preparation. 相似文献
230.
19F and 1H NMR spectra of halocarbons 总被引:1,自引:0,他引:1
Foris A 《Magnetic resonance in chemistry : MRC》2004,42(6):534-555
19F NMR chemical shifts and coupling constants are reported for 215 compounds. For 77 of these compounds, 1H NMR spectral data are also given. Long-range couplings, including 8J(F,F) and 5J(F,H), are reported. The complexity of halocarbon spectra owing to the presence of rotational isomers, asymmetric centers, long-range couplings, and chlorine isotope effects are illustrated, and the methods used for analyzing such complex spectra are briefly discussed. 相似文献