全文获取类型
收费全文 | 11669篇 |
免费 | 1055篇 |
国内免费 | 1164篇 |
专业分类
13888篇 |
出版年
2024年 | 44篇 |
2023年 | 196篇 |
2022年 | 409篇 |
2021年 | 674篇 |
2020年 | 474篇 |
2019年 | 556篇 |
2018年 | 523篇 |
2017年 | 397篇 |
2016年 | 524篇 |
2015年 | 767篇 |
2014年 | 921篇 |
2013年 | 921篇 |
2012年 | 1088篇 |
2011年 | 979篇 |
2010年 | 554篇 |
2009年 | 537篇 |
2008年 | 618篇 |
2007年 | 532篇 |
2006年 | 418篇 |
2005年 | 332篇 |
2004年 | 317篇 |
2003年 | 268篇 |
2002年 | 258篇 |
2001年 | 186篇 |
2000年 | 167篇 |
1999年 | 154篇 |
1998年 | 109篇 |
1997年 | 97篇 |
1996年 | 101篇 |
1995年 | 73篇 |
1994年 | 83篇 |
1993年 | 59篇 |
1992年 | 69篇 |
1991年 | 72篇 |
1990年 | 51篇 |
1989年 | 34篇 |
1988年 | 42篇 |
1987年 | 23篇 |
1986年 | 31篇 |
1985年 | 31篇 |
1984年 | 20篇 |
1983年 | 26篇 |
1982年 | 16篇 |
1980年 | 14篇 |
1979年 | 14篇 |
1977年 | 11篇 |
1975年 | 13篇 |
1974年 | 12篇 |
1973年 | 11篇 |
1970年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Huidong Li Hao Feng Weiguo Sun Qunchao Fan Yaoming Xie R. Bruce King Henry F. Schaefer III 《Journal of molecular modeling》2013,19(7):2723-2737
The bis(heptalene)dimetal complexes (C12H10)2M2 of the first row transition metals from Ti to Ni are predicted by density functional theory to exhibit “submarine” sandwich structures with a pair of metal atoms sandwiched between the two heptalene rings. For the early transition metal derivatives (C12H10)2M2 (M = V, Cr) there are two types of such structures. In one structural type the metals are sandwiched between two heptahapto heptalene rings with metal-metal distances (3.5–3.8 Å) too long for direct metal-metal bonding. The other type of (C12H10)2M2 (M = V, Cr, Mn) structure has a pair of bonded metal atoms sandwiched between a fully bonded heptalene ligand and a heptalene ligand bonded to the metals only through an eight-carbon heptafulvene subunit, leaving an uncomplexed cis-1,3-diene unit. The formal metal-metal bond orders in these latter structures are 3, 2, and 1 for M = V, Cr, and Mn with predicted bond lengths of 2.5, 2.7, and 2.8 Å, respectively. For (C12H10)2Fe2 a singlet structure with each iron atom sandwiched between a hexahapto and a tetrahapto heptalene ring is energetically preferred over an alternate structure with ferrocene-like iron atoms sandwiched between two pentahapto heptalene rings. Partial bonding of each heptalene ring to the metal atoms occurs in the late transition metal derivatives (C12H10)2M2 (M = Co, Ni). This leads to an unsymmetrical structure for the cobalt derivative and a structure for the nickel derivative with each nickel atom sandwiched between a trihapto ligand and a tetrahapto ligand. Figure
The bis(heptalene) dimetal complexes (C12H10)2M2 (M = Ti to Ni) are predicted by density functional theory to have a “submarine” sandwich structure with a pair of metal atoms sandwiched between the two heptalene rings. In the early transition metal derivatives (C12H10)2M2 (M = V, Cr) the metal atoms are sandwiched between two heptahapto heptalene rings. In contrast, for (C12H10)2M2 (M = Mn, Fe, Co, Ni) the heptalene rings are only partially bonded to the metal atoms. 相似文献
122.
Min Cen Jian Fen Fan Dong Yan Liu Xue Zeng Song Jian Liu Wei Qun Zhou He Ming Xiao 《Journal of molecular modeling》2013,19(2):601-611
Molecular dynamic (MD) simulations have been performed to study the behaviors of ten kinds of cyclo-hexa-peptides (CHPs) composed of amino acids with the diverse hydrophilic/hydrophobic side chains at the water/cyclohexane interface. All the CHPs take the “horse-saddle” conformations at the interface and the hydrophilicity/hydrophobicity of the side chains influences the backbones’ structural deformations. The orientations and distributions of the CHPs at the interface and the differences of interaction energies (ΔΔE) between the CHPs and the two liquid phases have been determined. RDF analysis shows that the H-bonds were formed between the OC atoms of the CHPs’ backbones and Hw atoms of water molecules. N atoms of the CHPs’ backbones formed the H-bonds or van der Waals interactions with the water solvent. It was found that there is a parallel relationship between ΔΔE and the lateral diffusion coefficients (D xy ) of the CHPs at the interface. The movements of water molecules close to the interface are confined to some extent, indicating that the dynamics of the CHPs and interfacial water molecules are strongly coupled. Figure
Scheme of the ten kinds of CHPs formed by even alternating D- and L- amino acids with the different hydrophilic/hydrophobic side chains. The letters in the parentheses stand for the abbreviations of the composed amino acids in the CHPs 相似文献
123.
Ping Wang Jiugang Yuan Xuehong Ren Li Cui Qiang Wang Xuerong Fan 《Engineering in Life Science》2013,13(1):102-108
Covalently bound lipids cover the wool surface and make enzymatic degradation of wool scales very difficult. In this paper, methanolic potassium hydroxide (MPH) pretreatment was used prior to enzymatic treatment of wool with protease, aiming at hydrolyzing the outmost lipids on the wool surface and promoting the subsequent proteolytic reaction. The efficacy of lipid removal from the fiber surface and the properties of the protease‐treated wool were evaluated. The results indicated that mild MPH pretreatment with 0.10 mol/L MPH for 10 min improved the wettability of the wool without adverse impacts on its mechanical properties. The wetting time and area shrinkage of the wool fabric reached 0.5 s and 5.6%, respectively, and the strength loss was within the acceptable range. Pretreatment with high concentrations of MPH for longer times led to significant damage to the wool fibers and caused heavy strength loss, without improving the antifelting properties after protease treatment. Thus, the combination of mild MPH and protease treatments endowed the wool with desirable properties in contrast to the treatment with protease alone. 相似文献
124.
125.
126.
127.
Guoxu Ma Haifeng Wu Jingquan Yuan Lizhen Wu Qingxia Zheng Zhaocui Sun Xiaoru Fan Hua Wei Junshan Yang Xudong Xu 《Phytochemistry letters》2013,6(1):152-155
Five phenylpropanoids including one new compound balanophonin A (1), one new natural compound balanophonin B (2) were isolated from the seeds of Lithocarpus pachylepis for the first time. Their structures were elucidated by various spectroscopic techniques (UV, IR, MS, CD, 1D and 2D NMR). All compounds were evaluated for their anti-inflammatory activities on lipopolysaccharide (LPS)-induced nitric oxide (NO) production in RAW 264.7. 相似文献
128.
Background
Total hip or knee arthroplasty is an elective procedure that is usually accompanied by substantial blood loss, which may lead to acute anemia. As a result, almost half of total joint arthroplasty patients receive allogeneic blood transfusions (ABT). Many studies have shown that post-operative auto-transfusion (PAT) significantly reduces the need for ABT, but other studies have questioned the efficacy of this method.Methods
The protocol for this trial and supporting CONSORT checklist are available as supporting information; see Checklist S1. To evaluate the efficacy of PAT, we conducted a Cochrane systematic review that combined all available data from randomized controlled trials. Data from the six included trials were pooled for analysis. We then calculated relative risks with 95% confidence intervals (CIs) for dichotomous outcomes and mean differences with 95% CIs for continuous outcomes.Findings and Conclusion
To our knowledge, this is the first meta-analysis to compare the clinical results between PAT and a control in joint replacement patients. This meta-analysis has proven that the use of a PAT reinfusion system reduced significantly the demand for ABT, the number of patients who require ABT and the cost of hospitalization after total knee and hip arthroplasty. This study, together with other previously published data, suggests that PAT drains are beneficial. Larger, sufficiently powered studies are necessary to evaluate the presumed reduction in the incidence of infection as well as DVT after joint arthroplasty with the use of PAT. 相似文献129.
130.
Shumei Li Bin Wang Pengfei Xu Qixiang Lin Gaolang Gong Xiaoling Peng Yuanyuan Fan Yong He Ruiwang Huang 《PloS one》2013,8(8)
Diffusion-weighted MRI (DW-MRI), the only non-invasive technique for probing human brain white matter structures in vivo, has been widely used in both fundamental studies and clinical applications. Many studies have utilized diffusion tensor imaging (DTI) and tractography approaches to explore the topological properties of human brain anatomical networks by using the single tensor model, the basic model to quantify DTI indices and tractography. However, the conventional DTI technique does not take into account contamination by the cerebrospinal fluid (CSF), which has been known to affect the estimated DTI measures and tractography in the single tensor model. Previous studies have shown that the Fluid-Attenuated Inversion Recovery (FLAIR) technique can suppress the contribution of the CSF to the DW-MRI signal. We acquired DTI datasets from twenty-two subjects using both FLAIR-DTI and conventional DTI (non-FLAIR-DTI) techniques, constructed brain anatomical networks using deterministic tractography, and compared the topological properties of the anatomical networks derived from the two types of DTI techniques. Although the brain anatomical networks derived from both types of DTI datasets showed small-world properties, we found that the brain anatomical networks derived from the FLAIR-DTI showed significantly increased global and local network efficiency compared with those derived from the conventional DTI. The increases in the network regional topological properties derived from the FLAIR-DTI technique were observed in CSF-filled regions, including the postcentral gyrus, periventricular regions, inferior frontal and temporal gyri, and regions in the visual cortex. Because brain anatomical networks derived from conventional DTI datasets with tractography have been widely used in many studies, our findings may have important implications for studying human brain anatomical networks derived from DW-MRI data and tractography. 相似文献