全文获取类型
收费全文 | 69548篇 |
免费 | 23735篇 |
国内免费 | 3429篇 |
专业分类
化学 | 76178篇 |
晶体学 | 375篇 |
力学 | 3746篇 |
综合类 | 322篇 |
数学 | 5878篇 |
物理学 | 10213篇 |
出版年
2024年 | 505篇 |
2023年 | 4953篇 |
2022年 | 2358篇 |
2021年 | 3563篇 |
2020年 | 5933篇 |
2019年 | 3454篇 |
2018年 | 3227篇 |
2017年 | 1400篇 |
2016年 | 7002篇 |
2015年 | 7080篇 |
2014年 | 6720篇 |
2013年 | 7254篇 |
2012年 | 5452篇 |
2011年 | 3249篇 |
2010年 | 5184篇 |
2009年 | 5173篇 |
2008年 | 2764篇 |
2007年 | 2240篇 |
2006年 | 1481篇 |
2005年 | 1228篇 |
2004年 | 1005篇 |
2003年 | 777篇 |
2002年 | 788篇 |
2001年 | 672篇 |
2000年 | 529篇 |
1999年 | 476篇 |
1998年 | 390篇 |
1997年 | 454篇 |
1996年 | 420篇 |
1995年 | 448篇 |
1994年 | 356篇 |
1993年 | 426篇 |
1992年 | 241篇 |
1991年 | 237篇 |
1990年 | 187篇 |
1988年 | 214篇 |
1981年 | 186篇 |
1980年 | 220篇 |
1979年 | 212篇 |
1978年 | 220篇 |
1977年 | 336篇 |
1976年 | 400篇 |
1975年 | 486篇 |
1974年 | 503篇 |
1973年 | 325篇 |
1972年 | 455篇 |
1971年 | 419篇 |
1970年 | 625篇 |
1969年 | 459篇 |
1968年 | 502篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
991.
Yanlin Feng Jiali Lin Gang He Li Liang Qijun Liu Jun Yan Qian Yao 《Molecules (Basel, Switzerland)》2022,27(15)
Pomegranate peel extract (PPE), which is abundant in polyphenols, holds immerse prospects for the treatment of airway infection. In this study, water and ethanol of 30%, 50%, and 80% were used to prepare PPE. A total of 18 phenols belonging to 8 categories of polyphenols were identified in PPE by HPLC-MS/MS. The PPE from the four extraction solvents possessed different antioxidant, antibacterial, and anti-inflammatory activities. Principal component analysis revealed that though total flavonoids (TFs), total polyphenols (TPs), and total tannins (TTs) were responsible for the reducing power of PPE, only TFs contributed to the effect of PPE in inhibiting lipid membrane peroxidation. TPs, TTs, and punicalagin were positively correlated with the antibacterial strength against S. aureus while TTs alone contributed to the inhibition of methicillin-resistant S. aureus, implying the crucial role of TT in suppressing bacteria. Meanwhile, TTs was associated with the prevention of IL-6 release. The PPE with higher contents of TPs, TTs, and punicalagin had a weaker capacity to decrease nitric oxide secretion. PPE of 30% ethanol gained the highest integrated score due to its stronger antioxidant, antibacterial, and anti-inflammatory activities. It is a suitable candidate for the therapy of respiratory tract infection. 相似文献
992.
993.
Weiguo Wang Minkang Feng Xiaomeng Li Feiyu Chen Zhihao Zhang Wenlong Yang Chen Shao Liming Tao Yang Zhang 《Molecules (Basel, Switzerland)》2022,27(15)
Actinomycetes play a vital role as one of the most important natural resources for both pharmaceutical and agricultural applications. The actinomycete strain SPRI-371, isolated from soil collected in Jiangsu province, China, was classified as Streptomyces aureus based on its morphological, physiological, biochemical and molecular biological characteristics. Its bacterial activity metabolites were identified as aureonuclemycin (ANM), belonging to adenosine derivatives with the molecular formula C16H19N5O9 for ANM A and C10H13N5O3 for ANM B. Simultaneously, the industrial fermentation process of a mutated S. aureus strain SPRI-371 was optimized in a 20 m3 fermentation tank, featuring a rotation speed of 170 rpm, a pressure of 0.05 MPa, an inoculum age of 36–40 h and a dissolved oxygen level maintained at 1–30% within 40–80 h and at >60% in the later period, resulting in an ANM yield of >3700 mg/L. In the industrial separation of fermentation broth, the sulfuric acid solution was selected to adjust pH and 4# resin was used for adsorption. Then, it was resolved with 20% ethanol solution and concentrated in a vacuum (60–65 °C), with excellent results. Antibacterial experiments showed that ANM was less active or inactive against Xanthomonas oryzae pv. oryzae, Xanthomonas citri subsp. citri and Xanthomonas oryzae pv. oryzicola and most bacteria, yeast and fungi in vitro. However, in vivo experiments showed that ANM exhibited extremely significant protective and therapeutic activity against diseases caused by X. oryzae pv. oryzae and X. oryzae pv. oryzicola in rice and X. citri in oranges and lemons. In field trials, ANM A 150 gai/ha + ANM B 75 gai/ha exhibited excellent therapeutic activity against rice bacterial leaf blight, citrus canker and rice bacterial leaf streak. Furthermore, as the dosage and production cost of ANM are lower than those of commercial drugs, it has good application prospects. 相似文献
994.
Xiaoyan Guo Lu Yao Xiangyan Hou Xiaofeng Wu Yaowen Zhang Qian Zhu Zhangtao Guo Shuting Li Yilan Jiang Shouhua Feng Keke Huang 《Chemical science》2022,13(32):9440
Exsolution is an ingenious strategy for the in situ construction of metal- or alloy-decorated oxides and, due to its promising energy related catalysis applications, has advanced from use in perovskites to use in spinels. Despite its great importance for designing target composites, the ability to identify whether active metal ions at octahedral or tetrahedral sites will preferentially exsolve in a spinel remains unexplored. Here, an inverse spinel NiFe2O4 (NFO) was employed as a prototype and FeNi/NFO composites were successfully constructed via exsolution. The preferential breaking of octahedral metal–oxygen bonds in the spinel oxide was directly observed using Mössbauer and X-ray absorption spectroscopy. This was further verified from the negative segregation energies calculated based on density-functional theory. One exsolved FeNi/NFO composite exhibits enhanced electrochemical activity with an overpotential of 283 mV at 10 mA cm−2 and a long stability time for the oxygen evolution reaction. This work offers a unique insight into spinel exsolution based on the preferential breaking of chemical bonds and may be an effective guide for the design of new composite catalysts where the desired metal ions are deliberately introduced to octahedral and/or tetrahedral sites.The preferential breaking of octahedral metal–oxygen bonds is exploited to construct an exsolved FeNi/NFO composite for an efficient oxygen evolution reaction. 相似文献
995.
A key element for reducing energy consumption and improving thermal comfort on high-speed rail is controlling air-conditioning temperature. Accurate prediction of air supply temperature is aimed at improving control effects. Existing studies of supply air temperature prediction models are interdisciplinary, involving heat transfer science and computer science, where the problem is defined as time-series prediction. However, the model is widely accepted as a complex model that is nonlinear and dynamic. That makes it difficult for existing statistical and deep learning methods, e.g., autoregressive integrated moving average model (ARIMA), convolutional neural network (CNN), and long short-term memory network (LSTM), to fully capture the interaction between these variables and provide accurate prediction results. Recent studies have shown the potential of the Transformer to increase the prediction capacity. This paper offers an improved temporal fusion transformers (TFT) prediction model for supply air temperature in high-speed train carriages to tackle these challenges, with two improvements: (i) Double-convolutional residual encoder structure based on dilated causal convolution; (ii) Spatio-temporal double-gated structure based on Gated Linear Units. Moreover, this study designs a loss function suitable for general long sequence time-series forecast tasks for temperature forecasting. Empirical simulations using a high-speed rail air-conditioning operation dataset at a specific location in China show that the temperature prediction of the two units using the improved TFT model improves the MAPE by 21.70% and 11.73%, respectively the original model. Furthermore, experiments demonstrate that the model effectively outperforms seven popular methods on time series computing tasks, and the attention of the prediction problem in the time dimension is analyzed. 相似文献
996.
Chi Feng Tomohiro Seki Shunichi Sakamoto Toshiyuki Sasaki Satoshi Takamizawa Hajime Ito 《Chemical science》2022,13(33):9544
Shape-memory materials can be mechanically deformed and subsequently reverse the deformation upon changing the temperature. Shape-memory materials have attracted considerable attention for basic research and industrial applications, and polymer and alloy shape-memory materials have been well studied; however, it is formidably challenging to develop functional shape-memory materials, such as materials with multi-stage and anisotropic shape changes and shape changes accompanied by changes in color and light emission. Here, we found a reversible multi-stage shape-changing effect after mechanical deformation in a molecular crystal induced by multi-step thermal phase transitions with reversible shape changes and luminescence-color changes. Using single-crystal structure and thermal analyses as well as mechanical property measurements, we found that the reversible multi-stage shape-changing effect was achieved by a combination of a twinning deformation and multi-step thermal phase transitions. The changes in the crystal shape and luminescence suggest novel strategies for imparting known shape-memory materials with additional functionalities.A reversible multi-stage shape-changing material based on an organic crystalline material is presented. Intriguingly, the interconversion among the three different shapes is accompanied by changes of the luminescence color. 相似文献
997.
Xiao-Xue Jing Da-Qing Li Yong Zhang Xiang-Yu Hou Jie Jiang Xing-Ce Fan Meng-Chen Wang Shao-Peng Feng Yuan-fang Yu Jun-Peng Lu Zhen-Liang Hu Zhen-Hua Ni 《中国物理快报》2021,(7):65-75
Two-dimensional surface-enhanced Raman scattering(SERS) substrates have drawn intense attention due to their excellent spectral reproducibility, high uniformity and perfect anti-interference ability. However, the inferior detection sensitivity and low enhancement have limited the practical application of two-dimensional SERS substrates. To address this issue, we propose that the interaction between the MoTe_2 substrate and the analyte rhodamine 6 G molecules could be remarkably enhanced by the introduced p-doping effect and lattice distortion of MoTe_2 via hydrogen plasma treatment. After the treatment, the SERS is greatly improved, the enhancement factor of probe molecules reaches 1.83 × 10~6 as well as the limit of detection concentration reaches 10-13 M.This method is anticipated to afford new enhancement probability for other 2D materials, even non-metal oxide semiconductor SERS substrates. 相似文献
998.
正Lithium niobate(Li Nb O_3), so-called "Silicon in Photonics," is a multifunctional crystal with a combination of a number of excellent physical properties. In optics and photonics, the Li Nb O_3-based devices, such as modulators, wavelength converters, waveguide amplifiers, and quantum photonic chips, have been realized and widely applied in various areas. In addition to the traditional waveguides, 相似文献
999.
Sulfonamides have a special role on medicine due to their broad biological activities, as bacterial infections, diabetes mellitus, oedema, hypertension prevention and treatment. In addition, sulfonamides are also useful in herbicides and pesticides. Herein, we communicate an efficient strategy for the preparation of sulfonamides via NH4I‐mediated amination of sodium sulfinates. This new method provides a general and environmentally friendly access to sulfonamide compounds and tolerates a wide range of functional groups. 相似文献
1000.
探讨了金与铝改性的SBA-15分子筛催化剂(Au-Al/SBA-15)上活性中心与葡萄糖氧化制葡萄酸反应性能的关系.通过固体核磁共振波谱(ss NMR)、X射线光电子能谱(XPS)、紫外-可见光谱(UV-Vis)和透射电子显微镜(TEM)等手段对不同Au及Al含量的Au-Al/SBA-15催化剂进行了结构表征.发现Al改性能够在SBA-15载体上产生四配位Al物种,Au主要以单质的形式存在; SBA-15上的四配位Al物种具有稳定Au颗粒大小的作用.通过对比发现,催化剂上的Au和骨架四配位Al物种间的相互协作促进了对葡萄糖氧化的活性,而非骨架Al物种会导致副反应的发生,降低葡萄糖酸产物的选择性.通过固体13C NMR揭示了反应体系中的NaOH能够将强吸附的产物葡萄糖酸分子从催化剂表面解离,保持活性位不被覆盖进而促进反应持续进行. 相似文献