首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   23671篇
  免费   3938篇
  国内免费   3700篇
化学   17494篇
晶体学   387篇
力学   1486篇
综合类   298篇
数学   2693篇
物理学   8951篇
  2024年   48篇
  2023年   397篇
  2022年   581篇
  2021年   795篇
  2020年   926篇
  2019年   894篇
  2018年   762篇
  2017年   809篇
  2016年   1075篇
  2015年   1073篇
  2014年   1377篇
  2013年   1828篇
  2012年   2031篇
  2011年   2105篇
  2010年   1570篇
  2009年   1598篇
  2008年   1714篇
  2007年   1554篇
  2006年   1459篇
  2005年   1250篇
  2004年   960篇
  2003年   757篇
  2002年   693篇
  2001年   630篇
  2000年   507篇
  1999年   503篇
  1998年   497篇
  1997年   386篇
  1996年   360篇
  1995年   309篇
  1994年   315篇
  1993年   219篇
  1992年   198篇
  1991年   176篇
  1990年   180篇
  1989年   116篇
  1988年   98篇
  1987年   98篇
  1986年   99篇
  1985年   82篇
  1984年   51篇
  1983年   39篇
  1982年   38篇
  1981年   29篇
  1980年   21篇
  1979年   18篇
  1978年   14篇
  1976年   9篇
  1975年   9篇
  1973年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Mass concentration and isotopic values δ13C and 14C are presented for the water-insoluble refractory carbon(WIRC) component of total suspended particulates(TSP),collected weekly during 2003,as well as from October 2005 to May 2006 at the WMO-GAW Mt.Waliguan(WLG) site.The overall average WIRC mass concentration was(1183±120)ng/m3(n = 79),while seasonal averages were 2081 ±1707(spring),454±205(summer),650±411(autumn),and 1019±703(winter) ng/m3.Seasonal variations in WIRC mass concentrations were consistent with black carbon measurements from an aethalometer,although WIRC concentrations were typically higher,especially in winter and spring.The δ13C PDB value(-25.3 ± 0.8)%.determined for WIRC suggests that its sources are C3 biomass or fossil fuel combustion.No seasonal change in δ13C PDB was evident.The average percent Modern Carbon(pMC) for 14C in WIRC for winter and spring was(67.2 ± 7.7)%(n = 29).Lower pMC values were associated with air masses transported from the area east of WLG,while higher pMC values were associated with air masses from the Tibetan Plateau,southwest of WLG.Elevated pMC values with abnormally high mass concentrations of TSP and WIRC were measured during a dust storm event.  相似文献   
12.
利用分子筛择形特点,对煤直接液化油中的混合酚实施高效分离。本研究选取间甲酚和对甲酚作为分离煤直接液化油馏分段混合酚的模型化合物,采用化学液相沉积法对HZSM-5吸附剂的孔口结构进行改变,分析分子筛硅铝比及颗粒粒径对模型化合物间甲酚和对甲酚吸附分离性能的影响,以获得高性能固相吸附剂,并将其应用于180-190℃馏分段混合酚分离。结果表明,当分子筛硅铝比为25、粒径为3-5 μm时,分子筛的孔口结构调节效果最优;当正硅酸乙酯的最小用量为0.2 mL/g时,固相吸附剂的吸附量为0.03 g/g,对甲酚选择性高于95%。由于外表面沉积物对吸附剂的孔口结构变化,导致对甲酚选择性的提高。进一步采用HZSM-5(1)吸附剂对真实煤直接液化油混合酚的分离中发现,苯酚和对甲酚的选择性均达到100%。  相似文献   
13.
14.
Given an n ‐vertex pseudorandom graph G and an n ‐vertex graph H with maximum degree at most two, we wish to find a copy of H in G , that is, an embedding φ : V ( H ) V ( G ) so that φ ( u ) φ ( v ) E ( G ) for all u v E ( H ) . Particular instances of this problem include finding a triangle‐factor and finding a Hamilton cycle in G . Here, we provide a deterministic polynomial time algorithm that finds a given H in any suitably pseudorandom graph G . The pseudorandom graphs we consider are ( p , λ ) ‐bijumbled graphs of minimum degree which is a constant proportion of the average degree, that is, Ω ( p n ) . A ( p , λ ) ‐bijumbled graph is characterised through the discrepancy property: | e ( A , B ) ? p | A | | B | | < λ | A | | B | for any two sets of vertices A and B . Our condition λ = O ( p 2 n / log n ) on bijumbledness is within a log factor from being tight and provides a positive answer to a recent question of Nenadov. We combine novel variants of the absorption‐reservoir method, a powerful tool from extremal graph theory and random graphs. Our approach builds on our previous work, incorporating the work of Nenadov, together with additional ideas and simplifications.  相似文献   
15.
Zhang  Jia-Rui  Zhang  Jia-Qi  Zheng  Zhao-Lin  Lin  Da  Shen  Yu-Jia 《Nonlinear dynamics》2022,108(2):1005-1026
Nonlinear Dynamics - Parametrically excited oscillators are used in several domains, in particular to improve the dynamical behaviour of systems like in the case of the parametric amplification or...  相似文献   
16.
Dioscin (DIS), one of the most abundant bioactive steroidal saponins in Dioscorea sp., is used as a complementary medicine to treat coronary disease and angina pectoris in China. Although the pharmacological activities and pharmacokinetics of DIS have been well demonstrated, information regarding the final metabolic fates is very limited. This study investigated the in vivo metabolic profiles of DIS after oral administration by ultra‐performance liquid chromatography quadrupole time‐of‐flight mass spectrometry method. The structures of the metabolites were identified and tentatively characterized by means of comparing the molecular mass, retention time and fragmentation pattern of the analytes with those of the parent compound. A total of eight metabolites, including seven phase I and one phase II metabolites, were detected and tentatively identified for the first time. Oxidation, deglycosylation and glucuronidation were found to be the major metabolic processes of the compound in rats. In addition, a possible metabolic pathway on the biotransformation of DIS in vivo was proposed. This study provides valuable and new information on the metabolism of DIS, which will be helpful for further understanding its mechanism of action. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   
17.
Fully utilizing solar energy for catalysis requires the integration of conversion mechanisms and therefore delicate design of catalyst structures and active species. Herein, a MOF crystal engineering method was developed to controllably synthesize a copper–ceria catalyst with well-dispersed photoactive Cu-[O]-Ce species. Using the preferential oxidation of CO as a model reaction, the catalyst showed remarkably efficient and stable photoactivated catalysis, which found practical application in feed gas treatment for fuel cell gas supply. The coexistence of photochemistry and thermochemistry effects contributes to the high efficiency. Our results demonstrate a catalyst design approach with atomic or molecular precision and a combinatorial photoactivation strategy for solar energy conversion.  相似文献   
18.
A new asymmetric Salamo‐based ligand H2L was synthesized using 3‐tertbutyl‐salicylaldehyde and 6‐methoxy‐2‐[O‐(1‐ethyloxyamide)]‐oxime‐1‐phenol. By adjusting the ratio of the ligand H2L and Cu (II), Co (II), and Ni (II) ions, mononuclear, dinuclear, and trinuclear transition metal (II) complexes, [Cu(L)], [{Co(L)}2], and [{Ni(L)(CH3COO)(CH3CH2OH)}2Ni] with the ligand H2L possessing completely different coordination modes were obtained, respectively. The optical spectra of ligand H2L and its Cu (II), Co (II) and Ni (II) complexes were investigated. The Cu (II) complex is a mononuclear structure, and the Cu (II) atom is tetracoordinated to form a planar quadrilateral structure. The Co (II) complex is dinuclear, and the two Co (II) atoms are pentacoordinated and have coordination geometries of distorted triangular bipyramid. The Ni (II) complex is a trinuclear structure, and the terminal and central Ni (II) atoms are all hexacoordinated, forming distorted octahedral geometries. Furthermore, optical properties including UV–Vis, IR, and fluorescence of the Cu (II), Co (II), and Ni (II) complexes were investigated. Finally, the antibacterial activities of the Cu (II), Co (II), and Ni (II) complexes were explored. According to the experimental results, the inhibitory effect was found to be enhanced with increasing concentrations of the Cu (II), Co (II), and Ni (II) complexes.  相似文献   
19.
An unprecedented Mn(I)-catalyzed selective hydroarylation and hydroalkenylation of unsaturated amides with commercially available organic boronic acids is reported. Alkenyl boronic acids have been successfully employed for the first time in Mn(I)-catalyzed carbon–carbon bond formation. A wide array of β-alkenylated amide products can be obtained in moderate to good yields, which offers practical access to five- and six-membered lactams. This protocol has predictable regio- and chemoselectivity, excellent functional group compatibility and ease of operation in air, representing a significant step-forward towards manganese-catalyzed C−C coupling.  相似文献   
20.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号