全文获取类型
收费全文 | 6517篇 |
免费 | 1237篇 |
国内免费 | 1012篇 |
专业分类
化学 | 4680篇 |
晶体学 | 188篇 |
力学 | 392篇 |
综合类 | 99篇 |
数学 | 697篇 |
物理学 | 2710篇 |
出版年
2024年 | 16篇 |
2023年 | 117篇 |
2022年 | 272篇 |
2021年 | 281篇 |
2020年 | 321篇 |
2019年 | 351篇 |
2018年 | 297篇 |
2017年 | 325篇 |
2016年 | 393篇 |
2015年 | 425篇 |
2014年 | 471篇 |
2013年 | 623篇 |
2012年 | 706篇 |
2011年 | 639篇 |
2010年 | 503篇 |
2009年 | 496篇 |
2008年 | 479篇 |
2007年 | 391篇 |
2006年 | 311篇 |
2005年 | 279篇 |
2004年 | 219篇 |
2003年 | 158篇 |
2002年 | 144篇 |
2001年 | 108篇 |
2000年 | 68篇 |
1999年 | 78篇 |
1998年 | 33篇 |
1997年 | 47篇 |
1996年 | 49篇 |
1995年 | 48篇 |
1994年 | 20篇 |
1993年 | 23篇 |
1992年 | 12篇 |
1991年 | 14篇 |
1990年 | 11篇 |
1989年 | 9篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 7篇 |
1984年 | 2篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1980年 | 4篇 |
1936年 | 2篇 |
排序方式: 共有8766条查询结果,搜索用时 15 毫秒
81.
Information-rich molecules provide opportunities for evolution. Genetically engineered materials are superior in that their properties are coded within genetic sequences and could be fine-tuned. In this review, we elaborate the concept of genetically engineered materials(GEMs) using examples ranging from engineered protein materials to engineered living materials. Proteinbased materials are the materials of choice by nature. Recent progress in protein engineering has led to opportunities to tune... 相似文献
82.
83.
A series of novel N-dichloroacetyl oxazolidine herbicide safeners was synthesized and characterized by IR, 1H NMR, 13C NMR, elemental analysis and X-ray diffraction. The preliminary biological test shows that the compounds could protect maize against injury caused by chlorsulfuron to a certain extent. 相似文献
84.
通过天然γ能谱测量方法测定建材样品中的镭含量,计算理论上衰变产生的氡密度,以及用硫化锌闪烁法测定由样品释放到空气中的氡密度的方法来计算氡的射气系数。通过改变样品的颗粒度大小,以及改变样品的灼烧温度等条件,来研究氡射气系数的变化规律。结果表明,土坯砖射气系数在8.5%~29.4%,红砖射气系数在0.031%~3.5%之间,瓷砖射气系数在0.74%~4.5%之间;建筑材料的氡射气系数随着其自身颗粒度的增大而减小,随加工(灼烧)温度升高而减小。 相似文献
85.
基于光谱曲线形态的高光谱影像检索方法研究 总被引:1,自引:0,他引:1
随着传感器技术、数据通讯技术的飞速发展,利用各种机载和星载传感器,己经获取到各种不同的海量遥感影像数据。巨大的数据量带来了数据存储和管理的问题,如何实现从海量影像数据中检索出我们所需要的信息显得十分迫切。影像检索最早由Chang于1980年提出,是对传统信息检索的扩展。针对海量遥感影像高效检索的需求和高光谱遥感影像波段数目多的特点,分析了影像检索中的影像距离函数和相似性度量问题,基于经典的曲线简化Douglas-Peucke算法(简称DP算法)提取光谱曲线的形态特征,利用“提取特征”的思想,提出了基于DP算法的光谱曲线和影像检索(简称DPSR)方法,将光谱形态特征应用于影像检索当中。DPSR利用光谱曲线上的特征点,减小了计算量,实现了有效地匹配和检索,适合高光谱遥感影像的光谱检索。文章选择了OMISI高光谱数据的四种易混分地类进行了相似性度量的对比实验。通过与常规的分析方法光谱角匹配(SAM)、光谱信息散度(SID)的对比可以看到,DPSR在较少计算量的情况下能保持较高的计算精度,提供了一种新的影像光谱高效检索方法。此外,文章还提出了尚待进一步研究的问题。 相似文献
86.
Yuhan Hu Fang Chen Kexin Zhou Zhe Zhang Fei Li Jianfeng Zhang Youzhi Tang Zhen Jin 《Molecules (Basel, Switzerland)》2022,27(15)
The novel pleuromutilin derivative, which showed excellent in vitro antibacterial activity against MRSA, 22-(2-(2-(4-((4-(4-nitrophenyl)piperazin-1-yl)methyl)-1H-1,2,3-triazol-1-yl)acetamido)phenyl)thioacety-l-yl-22-deoxypleuromutilin (Z33), was synthesized and characterized in our previous work. In this study, the preliminary pharmacodynamics and safety of Z33 were further evaluated. In in vitro antibacterial activity assays, Z33 was found to be a potent bactericidal antibiotic against MRSA that induced dose-dependent growth inhibition and long-term post-antibiotic effect (PAE). The drug-resistance test demonstrated that Z33 possessed a narrow mutant selection window and lower propensities to select resistance than that of tiamulin. Cytochrome P450 (CYP450) inhibition assay determined that the inhibitory effect of Z33 was similar to that of tiamulin against the activity of CYP3A4, and was lower than that of tiamulin on the activity of CYP2E1. Toxicity determination showed that both Z33 and tiamulin displayed low cytotoxicity of RAW264.7 cells. Furthermore, Z33 was found to be a high-security compound with a 50% lethal dose (LD50) above 5000 mg/kg in the acute oral toxicity test in mice. In an in vivo antibacterial activity test, Z33 displayed better therapeutic effectiveness than tiamulin in the neutropenic mouse thigh infection model. In summary, Z33 was worthy of further development as a highly effective and safe antibiotic agent against MRSA infection. 相似文献
87.
Yue Liu Congmin Wang Rong Guo Juexiu Li Quan Zhao Weiqiang Wang Fei Qi Haifang Liu Yang Li Huifan Zheng 《Molecules (Basel, Switzerland)》2022,27(15)
Iron–manganese silicate (IMS) was synthesized by chemical coprecipitation and used as a catalyst for ozonating acrylic acid (AA) in semicontinuous flow mode. The Fe-O-Mn bond, Fe-Si, and Mn-Si binary oxide were formed in IMS on the basis of the results of XRD, FTIR, and XPS analysis. The removal efficiency of AA was highest in the IMS catalytic ozonation processes (98.9% in 15 min) compared with ozonation alone (62.7%), iron silicate (IS) catalytic ozonation (95.6%), and manganese silicate catalytic ozonation (94.8%). Meanwhile, the removal efficiencies of total organic carbon (TOC) were also improved in the IMS catalytic ozonation processes. The IMS showed high stability and ozone utilization. Additionally, H2O2 was formed in the process of IMS catalytic ozonation. Electron paramagnetic resonance (EPR) analysis and radical scavenger experiments confirmed that hydroxyl radicals (•OH) were the dominant oxidants. Cl−, HCO3−, PO43−, Ca2+, and Mg2+ in aqueous solution could adversely affect AA degradation. In the IMS catalytic ozonation of AA, the surface hydroxyl groups and Lewis acid sites played an important role. 相似文献
88.
Three new isostructural coordination polymers, namely, [Mg(cpna)(H_2O)_2]_n(1), [Mn(cpna)(H_2O)_2]_n(2) and [Co(cpna)(H_2O)_2]_n(3)(H_2 cpna = 5-(3-carboxylphenyl)nicotic acid) are reported. They were synthesized by hydrothermal reactions of transition metal or alkaline earth metal chloride with 5-(3-carboxylphenyl)nicotic acid,respectively. Complexes 1~3 exhibit 2D layers with a 3,3-connected topology with Schl?fli symbol {4.82}. Such layers including hexagonal rings and a quadrangular ring are further extended into an ordered 3D framework by hydrogen bonds between the cpna~(2-) ligands and water molecules. The rare complex 1 has excellent luminescence and can be used as luminescent materials, while 2 and 3 possess prominent magnetism with potential applications in magnetic materials. 相似文献
89.
阐述了中国聚变工程实验堆(CFETR)中涉氚阀门的技术要求及现状。CFETR涉氚阀门应选择寿命长、响应时间快的全金属阀门,在工作压力0.25MPa下整体漏率小于10-9Pa·m3·s-1。 相似文献
90.