全文获取类型
收费全文 | 14355篇 |
免费 | 2203篇 |
国内免费 | 1650篇 |
专业分类
化学 | 8084篇 |
晶体学 | 71篇 |
力学 | 668篇 |
综合类 | 80篇 |
数学 | 1097篇 |
物理学 | 4305篇 |
无线电 | 3903篇 |
出版年
2024年 | 70篇 |
2023年 | 431篇 |
2022年 | 444篇 |
2021年 | 619篇 |
2020年 | 610篇 |
2019年 | 519篇 |
2018年 | 437篇 |
2017年 | 393篇 |
2016年 | 622篇 |
2015年 | 604篇 |
2014年 | 709篇 |
2013年 | 1018篇 |
2012年 | 1182篇 |
2011年 | 1207篇 |
2010年 | 807篇 |
2009年 | 859篇 |
2008年 | 927篇 |
2007年 | 827篇 |
2006年 | 781篇 |
2005年 | 671篇 |
2004年 | 463篇 |
2003年 | 438篇 |
2002年 | 387篇 |
2001年 | 317篇 |
2000年 | 344篇 |
1999年 | 374篇 |
1998年 | 320篇 |
1997年 | 270篇 |
1996年 | 274篇 |
1995年 | 235篇 |
1994年 | 203篇 |
1993年 | 167篇 |
1992年 | 155篇 |
1991年 | 117篇 |
1990年 | 107篇 |
1989年 | 69篇 |
1988年 | 57篇 |
1987年 | 51篇 |
1986年 | 33篇 |
1985年 | 33篇 |
1984年 | 14篇 |
1983年 | 10篇 |
1982年 | 16篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1957年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
采用温度作为铝电解电容器寿命加速因子,通常根据“10℃法则”或“7℃法则”计算。作者通过对63V-47μF铝电解电容器在四种不同温度下的负荷试验(+85℃、+95℃、+105℃、+115℃)后对数百只样品、数万个测量数据进行了处理分析。认为单纯地使用“10℃法则”或“7℃法则”不能准确地描述温度加速因子。作者结合铝电解电容器的结构,分析了铝电解电容器的失效因素,给出了负荷试验中温度与铝电解电容器寿命的关系。 相似文献
42.
Yang Ye Xiang Qi Bing Xu Ying Lin Huan Xiang Liang Zou Xiang-Yang Ye Tian Xie 《Chemical science》2022,13(23):6959
Herein, we present a facile and efficient allylation method via Ni-catalyzed cross-electrophile coupling of readily available allylic acetates with a variety of substituted alkenyl bromides using zinc as the terminal reductant. This Ni-catalyzed modular approach displays excellent functional group tolerance and a broad substrate scope, which the creation of a series of 1,4-dienes including several structurally complex natural products and pharmaceutical motifs. Moreover, the coupling strategy has the potential to realize enantiomeric control. The practicality of this transformation is demonstrated through the potent modification of the naturally antitumor active molecule β-elemene.Herein, we present a facile and efficient allylation method via Ni-catalyzed cross-electrophile coupling of readily available allylic acetates with a variety of substituted alkenyl bromides using zinc as the terminal reductant. 相似文献
43.
Shuang Liu Xingzhu Chen Mingyue Yu Jianing Li Jinyao Liu Zunxuan Xie Fengxiang Gao Yuyan Liu 《Molecules (Basel, Switzerland)》2022,27(12)
Breakthroughs in the field of nanotechnology, especially in nanochemistry and nanofabrication technologies, have been attracting much attention, and various nanomaterials have recently been developed for biomedical applications. Among these nanomaterials, nanoscale titanium dioxide (nano-TiO2) has been widely valued in stomatology due to the fact of its excellent biocompatibility, antibacterial activity, and photocatalytic activity as well as its potential use for applications such as dental implant surface modification, tissue engineering and regenerative medicine, drug delivery carrier, dental material additives, and oral tumor diagnosis and treatment. However, the biosafety of nano-TiO2 is controversial and has become a key constraint in the development of nano-TiO2 applications in stomatology. Therefore, in this review, we summarize recent research regarding the applications of nano-TiO2 in stomatology, with an emphasis on its performance characteristics in different fields, and evaluations of the biological security of nano-TiO2 applications. In addition, we discuss the challenges, prospects, and future research directions regarding applications of nano-TiO2 in stomatology that are significant and worthy of further exploration. 相似文献
44.
Tingjunhong Ni Zichao Ding Fei Xie Yumeng Hao Junhe Bao Jingxiang Zhang Shichong Yu Yuanying Jiang Dazhi Zhang 《Molecules (Basel, Switzerland)》2022,27(11)
A series of triazole derivatives containing phenylethynyl pyrazole moiety as side chain were designed, synthesized, and most of them exhibited good in vitro antifungal activities. Especially, compounds 5k and 6c showed excellent in vitro activities against C. albicans (MIC = 0.125, 0.0625 μg/mL), C. neoformans (MIC = 0.125, 0.0625 μg/mL), and A. fumigatus (MIC = 8.0, 4.0 μg/mL). Compound 6c also exerted superior activity to compound 5k and fluconazole in inhibiting hyphae growth of C. albicans and inhibiting drug-resistant strains of C. albicans, and it could reduce fungal burdens in mice kidney at a dosage of 1.0 mg/kg. An in vivo efficacy evaluation indicated that 6c could effectively protect mice models from C. albicans infection at doses of 0.5, 1.0, and 2.0 mg/kg. These results suggested that compound 6c deserves further investigation. 相似文献
45.
Jia-Le Xie Zheng Xu Han-Qi Zhou Yi-Xue Nie Jian Cao Guan-Wu Yin Jean-Philippe Bouillon Li-Wen Xu 《中国科学:化学(英文版)》2021,(5):761-769
Hydrosilylation is one of the most important reactions in synthetic chemistry and ranks as a fundamental method to access organosilicon compounds in industrial and academic processes. However, the enantioselective construction of chiral-at-silicon compounds via catalytic asymmetric hydrosilylation remained limited and difficult. Here we report a highly enantioselective hydrosilylation of ynones, a type of carbonyl-activated alkynes, using a palladium catalyst with a chiral binaphthyl phosphoramidite ligand. The stereospecific hydrosilylation of ynones affords a series of silicon-stereogenic silylenones with up to 94% yield, 20:1 regioselectivity and 98:2 enantioselectivity. The density functional theory(DFT) calculations were conducted to elucidate the reaction mechanism and origin of high degree of stereoselectivity, in which the powerful potential of aromatic interaction in this reaction is highlighted by the multiple C–H-π interaction and aromatic cavity-oriented enantioselectivitydetermining step during desymmetric functionalization of Si–H bond. 相似文献
46.
Mingzhu Du Shuanghong Yang Tong Jiang Tingting Liang Ying Li Shuzhen Cai Qingping Wu Jumei Zhang Wei Chen Xinqiang Xie 《Molecules (Basel, Switzerland)》2022,27(14)
Expression and purification of β-galactosidases derived from Bifidobacterium provide a new resource for efficient lactose hydrolysis and lactose intolerance alleviation. Here, we cloned and expressed two β-galactosidases derived from Bifidobacterium. The optimal pH for BLGLB1 was 5.5, and the optimal temperature was 45 °C, at which the enzyme activity of BLGLB1 was higher than that of commercial enzyme E (300 ± 3.6 U/mg) under its optimal conditions, reaching 2200 ± 15 U/mg. The optimal pH and temperature for BPGLB1 were 6.0 and 45 °C, respectively, and the enzyme activity (0.58 ± 0.03 U/mg) under optimum conditions was significantly lower than that of BLGLB1. The structures of the two β-galactosidase were similar, with all known key sites conserved. When o-nitrophenyl-β-D-galactoside (oNPG) was used as an enzyme reaction substrate, the maximum reaction velocity (Vmax) for BLGLB1 and BPGLB1 was 3700 ± 100 U/mg and 1.1 ± 0.1 U/mg, respectively. The kinetic constant (Km) of BLGLB1 and BPGLB1 was 1.9 ± 0.1 and 1.3 ± 0.3 mmol/L, respectively. The respective catalytic constant (kcat) of BLGLB1 and BPGLB1 was 1700 ± 40 s−1 and 0.5 ± 0.02 s−1, respectively; the respective kcat/Km value of BLGLB1 and BPGLB1 was 870 L/(mmol∙s) and 0.36 L/(mmol∙s), respectively. The Km, kcat and Vmax values of BLGLB1 were superior to those of earlier reported β-galactosidase derived from Bifidobacterium. Overall, BLGLB1 has potential application in the food industry. 相似文献
47.
48.
Feng Wang Jiale Ma Fangyan Xie Linkai Li Jian Chen Jun Fan Ni Zhao 《Advanced functional materials》2016,26(20):3417-3423
The applications of organotin halide perovskites are limited because of their chemical instability under ambient conditions. Upon air exposure, Sn2+ can be rapidly oxidized to Sn4+, causing a large variation in the electronic properties. Here, the role of organic cations in degradation is investigated by comparing methylammonium tin iodide (MASnI3) and formamidinium tin iodide (FASnI3). Through chemical analyses and theoretical calculations, it is found that the organic cation strongly influences the oxidation of Sn2+ and the binding of H2O molecules to the perovskite lattice. On the one hand, Sn2+ can be easily oxidized to Sn4+ in MASnI3, and replacing MA with FA reduces the extent of Sn oxidation; on the other hand, FA forms a stronger hydrogen bond with H2O than does MA, leading to partial expansion of the perovskite network. The two processes compete in determining the material's conductivity. It is noted that the oxidation is a difficult process to prevent, while the water effect can be largely suppressed by reducing the moisture level. As a result, FASnI3‐based conductors and photovoltaic cells exhibit much better reproducibility as compared to MASnI3‐based devices. This study sheds light on the development of stable Pb‐free perovskite optoelectronic devices through new material design. 相似文献
49.
可见光LED的进展——超高亮度LED及应用(二)张万生梁春广(电子工业部第十三研究所,石家庄,050051)4超高亮度发光管的发展[9~14]4.1InGaAlPDHLED发光强度达到坎德拉级发光管的高亮度化一直是半导体材料和器件的前沿课题之一,超高... 相似文献
50.
薄板模具钢脉冲Nd:YAG激光熔凝区显微硬度特征的影响因素 总被引:1,自引:0,他引:1
本文在厚度为 0 .5— 2 .0mm的 5Cr4Mo3SiMnVAl(0 12Al)模具钢和Cr12MoV模具钢薄板上 ,采用脉冲Nd :YAG激光进行了激光熔凝实验 ,研究了工艺参数 (脉冲宽度和脉冲频率 )、材质和材料厚度对激光熔凝后熔凝层显微硬度特征的影响。结果表明 :随着脉冲宽度的增加或脉冲频率的减少 ,激光熔凝区的显微硬度有减少的趋势 ;Cr12MoV模具钢的激光熔凝区的显微硬度比 0 12Al模具钢的低 ;随着材料厚度的增加 ,激光熔凝区的显微硬度表现为先增加后减少的趋势。激光熔凝工艺参数、材料的热扩散情况和材料的热物性参数的不同是造成上述现象的主要原因。 相似文献