收费全文 | 7658篇 |
免费 | 1223篇 |
国内免费 | 653篇 |
化学 | 5258篇 |
晶体学 | 86篇 |
力学 | 477篇 |
综合类 | 70篇 |
数学 | 775篇 |
物理学 | 2868篇 |
2023年 | 172篇 |
2022年 | 167篇 |
2021年 | 228篇 |
2020年 | 372篇 |
2019年 | 349篇 |
2018年 | 284篇 |
2017年 | 248篇 |
2016年 | 380篇 |
2015年 | 367篇 |
2014年 | 499篇 |
2013年 | 579篇 |
2012年 | 643篇 |
2011年 | 686篇 |
2010年 | 465篇 |
2009年 | 372篇 |
2008年 | 497篇 |
2007年 | 421篇 |
2006年 | 384篇 |
2005年 | 316篇 |
2004年 | 217篇 |
2003年 | 205篇 |
2002年 | 149篇 |
2001年 | 113篇 |
2000年 | 133篇 |
1999年 | 139篇 |
1998年 | 120篇 |
1997年 | 126篇 |
1996年 | 142篇 |
1995年 | 104篇 |
1994年 | 101篇 |
1993年 | 83篇 |
1992年 | 71篇 |
1991年 | 62篇 |
1990年 | 66篇 |
1989年 | 43篇 |
1988年 | 39篇 |
1987年 | 20篇 |
1986年 | 27篇 |
1985年 | 18篇 |
1984年 | 17篇 |
1983年 | 12篇 |
1982年 | 14篇 |
1981年 | 13篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1970年 | 12篇 |
1966年 | 3篇 |
1937年 | 5篇 |
We investigate the nature of the relationship between corporate social responsibility (CSR) and corporate financial performance (CFP) by examining how it changes across a third dimension that accounts for firm-specific factors. We propose a semi-latent specification of an endogenous control variable, which can, for the first time, explicitly identify, for each individual firm, the threshold level where the marginal impact of CSR on CFP turns positive. We provide empirical evidence that this threshold depends on the additional dimension and consequently, the previously reported U-shape seems to be an aggregation of relationships of differential magnitude and direction. This disaggregation fits the data better and therefore, we maintain that the addition of a higher dimension, along with the identification of the threshold level, can explain the conflicting results in the literature.
相似文献Stereocomplex (SC) crystallization has been an effective way to improve the physical performances of stereoregular polymers. However, the competition between homo and SC crystallizations can lead to more complicated crystallization kinetics and polymorphic crystalline structure in stereocomplexable polymers, which influences the physical properties of obtained materials. Herein, we select the medium-molecular-weight (MMW) poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) asymmetric blends with different PDLA fractions (fD=0.01–0.5) as the model system and investigate the effects of fD and crystallization temperature (Tc) on the crystallization kinetics and polymorphic crystalline structure. We observe the fractionated (i.e., multistep) crystallization kinetics and the formation of peculiar β-form homocrystals (HCs) in the asymmetric blends under quiescent conditions, which are strongly influenced by both fD and Tc. Precisely, crystallization of β-form HCs is favorable in the MMW PLLA/PDLA blends with high fD (≥0.2) at a low Tc (80–100 °C). It is proposed that the formation of metastable β-form HCs is attributed to the conformational matching between β-form HCs and SCs, and the stronger constrain effects of precedingly-formed SCs in the early stage of crystallization. Such effects can also cause the multistep crystallization kinetics of MMW PLLA/PDLA asymmetric blends in the heating process.
相似文献Side-chain engineering has been demonstrated as an effective method for fine-tuning the optical, electrical, and morphological properties of organic semiconductors toward efficient organic solar cells (OSCs). In this work, three isomeric non-fullerene small molecule acceptors (SMAs), named BTP-4F-T2C8, BTP-4F-T2EH and BTP-4F-T3EH, with linear and branched alkyl chains substituted on the α or β positions of thiophene as the side chains, were synthesized and systematically investigated. The results demonstrate that the size and substitution position of alkyl side chains can greatly affect the electronic properties, molecular packing as well as crystallinity of the SMAs. After blending with donor polymer D18-Cl, the prominent device performance of 18.25% was achieved by the BTP-4F-T3EH-based solar cells, which is higher than those of the BTP-4F-T2EH-based (17.41%) and BTP-4F-T2C8-based (15.92%) ones. The enhanced performance of the BTP-4F-T3EH-based devices is attributed to its stronger crystallinity, higher electron mobility, suppressed biomolecular recombination, and the appropriate intermolecular interaction with the donor polymer. This work reveals that the side chain isomerization strategy can be a practical way in tuning the molecular packing and blend morphology for improving the performance of organic solar cells.
相似文献