全文获取类型
收费全文 | 18370篇 |
免费 | 1510篇 |
国内免费 | 2912篇 |
专业分类
化学 | 17248篇 |
晶体学 | 593篇 |
力学 | 94篇 |
综合类 | 88篇 |
数学 | 2114篇 |
物理学 | 2655篇 |
出版年
2024年 | 44篇 |
2023年 | 288篇 |
2022年 | 423篇 |
2021年 | 462篇 |
2020年 | 624篇 |
2019年 | 612篇 |
2018年 | 504篇 |
2017年 | 676篇 |
2016年 | 640篇 |
2015年 | 553篇 |
2014年 | 873篇 |
2013年 | 1992篇 |
2012年 | 983篇 |
2011年 | 1196篇 |
2010年 | 940篇 |
2009年 | 1075篇 |
2008年 | 1187篇 |
2007年 | 1222篇 |
2006年 | 1108篇 |
2005年 | 1049篇 |
2004年 | 979篇 |
2003年 | 841篇 |
2002年 | 689篇 |
2001年 | 523篇 |
2000年 | 456篇 |
1999年 | 433篇 |
1998年 | 325篇 |
1997年 | 239篇 |
1996年 | 245篇 |
1995年 | 233篇 |
1994年 | 244篇 |
1993年 | 189篇 |
1992年 | 157篇 |
1991年 | 123篇 |
1990年 | 79篇 |
1989年 | 56篇 |
1988年 | 58篇 |
1987年 | 44篇 |
1986年 | 42篇 |
1985年 | 53篇 |
1984年 | 43篇 |
1983年 | 27篇 |
1982年 | 43篇 |
1981年 | 40篇 |
1980年 | 22篇 |
1979年 | 34篇 |
1978年 | 24篇 |
1976年 | 15篇 |
1974年 | 17篇 |
1971年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Noureddine Raouafi Khaled Boujlel Mohamed Lamine Benkhoud 《Phosphorus, sulfur, and silicon and the related elements》2013,188(12):2471-2482
N-(-1H-Benzimidazol-2-yl) imidates 1a–c react with chlorophosphoramide to give the N-[-1-N,N,N′,N′-tetramethylphosphoramidoyl-1H-benzimidazol-2-yl]-imidates 2a–c or with dichlorophosphoramide to yield the bis[(N-1-benzimidazol-2-yl)-imidate] phosphoramide derivatives 3a–b. The reaction of compounds 2a–c toward primary amines is studied. The obtained amidine derivatives 4a–b were unambiguously characterized by different spectroscopic techniques (IR, 1H, 13C, and 31P NMR, and in some cases MS). 相似文献
992.
993.
The oxidative coupling of methane over La 2 O 3 /CaO type-catalyst in a fixed-bed reactor is studied under a wide range of operating conditions(973相似文献
994.
The influent and effluent characteristics of river Ganga in the Hardwar-Narora sector in Uttar Pradesh, India was studied using environmental oxygen-18. δ18O of river water and also of groundwater close to the river course was monitored at 8 stations along the 220 km stretch for a period of ten months. δ18O of the river during the monsoon months (June to September) was about – 10 at all the stations. However, during the dry months from October to April, the δ18O of the river was enriched progressively from Hardwar to Narora. The enrichment indicated groundwater contribution to the river during the non-monsoon months. The contribution was comuted using the principle of isotope balance. 相似文献
995.
996.
Dr. Yongqi Yao Dr. Qifu Lin Dr. Wen Yang Prof. Dr. Weitao Yang Prof. Dr. Fenglong Gu Prof. Dr. Wei Guo Prof. Dr. Dingqiao Yang 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(25):5607-5610
A cobalt(II)-catalyzed [4+2] annulation of picolinamides with alkynes via C−H bond activation has been developed. The operationally simple annulation reaction allows for the synthesis of acyl-substituted 1H-benzoquinoline bearing multiple aromatic rings (up to 96 % yield) without co-oxidant or other oxidation factors under mild conditions. Several control experiments were carried out. This practical [4+2] annulation provides an efficient route to access highly functionalized compounds. 相似文献
997.
Qiaoshu Chen Dr. Yibo Liu Prof. Jianbo Liu Prof. Juewen Liu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(70):16659-16665
Peroxidase-mimicking nanozymes such as Fe3O4 nanoparticles are promising substitutes for natural enzymes like horseradish peroxidase. However, most such nanozymes work efficiently only in acidic conditions. In this work, the influence of various liposomes on nanozyme activity was studied. By introducing negatively charged liposomes, peroxidase-mimicking nanozymes achieved oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in neutral and even alkaline conditions, although the activity towards anionic 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) was inhibited. The Fe3O4 nanoparticles adsorbed on the liposomes without disrupting membrane integrity as confirmed by fluorescence quenching, dye leakage assays, and cryo-electron microscopy. Stabilization of the blue-colored oxidized products of TMB by electrostatic interactions was believed to be the reason for the enhanced activity. This work has introduced lipids to nanozyme research, and it also has practically important applications for using nanozymes at neutral pH, such as the detection of hydrogen peroxide and glucose. 相似文献
998.
Saulius Kaciulis Alessio Mezzi Roberto Montanari Maria Richetta Ekaterina Pakhomova Alessandra Varone 《Surface and interface analysis : SIA》2020,52(12):1093-1097
The W-1%La2O3 alloy has been irradiated by a single laser pulse (λ = 1064 nm) to simulate transient thermal loads of high energy occurring in a tokamak under operative conditions. A zone with a diameter of ~2 mm, namely, much larger than the focal spot, results to be affected by the pulse, and a crater of about 300 μm is observed in its center. La2O3 particles are not present inside the crater. The change of surface morphology is accompanied by elemental redistribution. Multipoint XPS analysis evidenced that the concentration of La is very low in the crater and increases moving toward the border of the affected zone while that of W shows an opposite trend. The composition changes involve only the outmost 5 nm of the sample: through depth profiling, no differences of chemical composition were detected deeper in the alloy between the center and external border of the affected area. 相似文献
999.
Dr. Laura Ielo Prof. Dr. Vittorio Pace Prof. Dr. Wolfgang Holzer Md. Mahbubur Rahman Dr. Guangrong Meng Dr. Roman Szostak Prof. Dr. Michal Szostak 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(69):16246-16250
The structure and properties of amides are of tremendous interest in organic synthesis and biochemistry. Traditional amides are planar and the carbonyl group non-electrophilic due to nN→π*C=O conjugation. In this study, we report electrophilicity scale by exploiting 17O NMR and 15N NMR chemical shifts of acyclic twisted and destabilized acyclic amides that have recently received major attention as precursors in N-C(O) cross-coupling by selective oxidative addition as well as precursors in electrophilic activation of N-C(O) bonds. Most crucially, we demonstrate that acyclic twisted amides feature electrophilicity of the carbonyl group that ranges between that of acid anhydrides and acid chlorides. Furthermore, a wide range of electrophilic amides is possible with gradually varying carbonyl electrophilicity by steric and electronic tuning of amide bond properties. Overall, the study quantifies for the first time that steric and electronic destabilization of the amide bond in common acyclic amides renders the amide bond as electrophilic as acid anhydrides and chlorides. These findings should have major implications on the fundamental properties of amide bonds. 相似文献
1000.
By the reaction of 4-chlorobenzaldehyde with ethyl acetoacetate, malononitrile, and hydrazine hydrate, 6-amino-4-(4-chlorophenyl)-3-methyl-2,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile was prepared and then reacted with salicylaldehyde and CoCl2·6H2O to produce nano-Co-[4-cholorophenyl-salicylaldimine-methylpyranopyrazole]Cl2 (nano-[Co-4CSMP]Cl2). The prepared nano-Schiff base complex was reported for the first time and fully characterized by Fourier transform-infrared spectroscopy, thermal gravimetric analysis, differential thermal gravimetric analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and Brunner–Emmett–Teller analyses and applied as an efficient catalyst for the synthesis of some 1-amidoalkyl-2-naphthol derivatives. 相似文献