全文获取类型
收费全文 | 1721篇 |
免费 | 219篇 |
国内免费 | 217篇 |
专业分类
化学 | 912篇 |
晶体学 | 18篇 |
力学 | 78篇 |
综合类 | 4篇 |
数学 | 85篇 |
物理学 | 469篇 |
综合类 | 591篇 |
出版年
2024年 | 4篇 |
2023年 | 37篇 |
2022年 | 61篇 |
2021年 | 73篇 |
2020年 | 54篇 |
2019年 | 58篇 |
2018年 | 40篇 |
2017年 | 49篇 |
2016年 | 61篇 |
2015年 | 76篇 |
2014年 | 93篇 |
2013年 | 101篇 |
2012年 | 138篇 |
2011年 | 150篇 |
2010年 | 88篇 |
2009年 | 85篇 |
2008年 | 80篇 |
2007年 | 74篇 |
2006年 | 86篇 |
2005年 | 74篇 |
2004年 | 62篇 |
2003年 | 46篇 |
2002年 | 42篇 |
2001年 | 33篇 |
2000年 | 37篇 |
1999年 | 58篇 |
1998年 | 51篇 |
1997年 | 55篇 |
1996年 | 41篇 |
1995年 | 51篇 |
1994年 | 33篇 |
1993年 | 32篇 |
1992年 | 29篇 |
1991年 | 21篇 |
1990年 | 17篇 |
1989年 | 16篇 |
1988年 | 15篇 |
1987年 | 15篇 |
1986年 | 12篇 |
1985年 | 5篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1957年 | 1篇 |
排序方式: 共有2157条查询结果,搜索用时 15 毫秒
991.
研制了一种用于硝酸根检测的小型电化学测量系统。该系统利用由微机电系统(MEMS)技术制备在玻璃基底上的铜簇修饰工作电极(WE)和铂对电极(CE),实现对水中硝酸根离子的浓度检测。设计了一个低噪声、高精度的恒电位仪模块来实现工作电极表面铜层的自更新、线性扫描电压的产生以及μA~mA级响应电流的检测。应用STM32F407微控制器控制测量过程并将检测结果显示在电容触摸屏上。实验结果表明,在0~71.4μmol·L-1浓度范围内,系统对硝酸根的检测具有较好的线性度(0.996)。与电化学工作站的检测结果进行对比,该小型系统表现出良好的性能。 相似文献
992.
Zhaohui Chen Shilie Pan Qun Jing Yun Yang Xiaoyu Dong Qiang Bian Jian Han Lingyun Dong Zhihua Yang 《无机化学与普通化学杂志》2014,640(1):140-146
The organic‐inorganic hybrid nonlinear optical (NLO) material NH4B(d‐ (+)‐C4H4O5)2 · H2O (NBC) was synthesized in a borate‐carboxylic acid system. Its structure was determined by single crystal X‐ray diffraction. It crystallizes in the orthorhombic system, space group Pna21 (No. 33), with cell parameters a = 11.484(6) Å, b = 5.354(3) Å, c = 21.079(12) Å, V = 1296.0(12), Z = 4. It exhibits a three‐dimensional pseudo tunnel structure consisting of fundamental building block [B(d‐ (+)‐C4H4O5)2]– anions. The small cavities are occupied by the H2O molecules and NH4+ cations, which stabilize the whole structure by O–H ··· O and N–H ··· O hydrogen bonds. The powder X‐ray diffraction (PXRD) of the crystal was also recorded. Elemental analyses, FT‐IR and FT‐Raman spectra analyses, thermal analysis, and diffuse‐reflectance spectra for the compound are also presented, as are band structures and density of states calculation. Nonlinear optical measurements indicate that the material has second harmonic generation (SHG) properties and is phase‐matchable. 相似文献
993.
Li-Juan Zhang Lin-Yan Feng He Bian Ling Pei Da-Zhi Li Hua-Jin Zhai 《International journal of quantum chemistry》2020,120(13):e26229
Boroxol (B3O3) rings and relevant hexagonal B3S3 structural blocks are ubiquitous in boron oxide/sulfide glasses, crystals, and high-temperature liquids. However, the isolation of an ultimate heterocyclic B3O3 or B3S3 cluster in the free-standing form, with as few as six atoms, has been unsuccessful so far. We report on computational design of the simplest case of such a system: the highly symmetric D3h B3S3+ (1A1′) cluster. It is the well-defined global minimum on the potential energy surface, following global searches and electronic structure calculations at the B3LYP and single-point CCSD(T) levels. Chemical bonding analysis reveals an ideal system with skeleton Lewis B S σ single bonds and unique double 6π/2σ aromaticity, which underlies its stability. The cluster turns out to be an inorganic analog of the 3,5-dehydrophenyl cation, a typical double π/σ aromatic system. It offers an example for chemical analogy between boron-based heterocyclic clusters and aromatic hydrocarbons. Double π/σ aromaticity is also a new concept in heterocyclic boron clusters. Previous systems such as borazine, boroxine, and boronyl boroxine only deal with π aromaticity as in benzene. 相似文献
994.
The supported Ni-based catalyst is widely used in the methanation process. Nevertheless, the major disadvantages of this catalyst are a poor behavior in the water-gas-shift (WGS) reaction and the deactivation at higher temperatures. A new kind of catalyst, nickel-containing oxides catalyst (NiMgAl), obtained from thermal treatment of hydrotalcite-like compounds (HTlcs) was prepared using the co-precipitation method. The performance of this catalyst was systematically investigated and compared with that of the Ni/Al2O3 catalyst. It was found that the NiMgAl catalyst shows an enhanced methanation activity compared to that of the Ni/Al2O3 catalyst and the former catalyst shows a better performance for the methanation especially at temperature over 550°C. Three NiMgAl catalysts with different nickel content were prepared and tested in the methanation operated at a GHSV of 15000 h?1 and n(H2)/n(CO) of 1.5. The results indicate that with the NiMg8 catalyst a higher activity and stability could be achieved than with the NiMg5 and NiMg6 samples, the effect mainly attributed to a higher extent of Ni dispersion was confirmed by XRD results. 相似文献
995.
Potentially scalable total synthesis of reblastatin was achieved based on Panek's previous study. Novel and convenient synthetic routes were developed for the known C8–C20 and C1–C7 coupling partners. The challenging C8–C20 fragment was prepared from TBS protected (S)-5-(hydroxymethyl)dihydrofuran-2(3H)-one (6) in nine steps (20% overall yield), and the C1–C7 fragment was synthesized from commercially available 3,4,6-tri-O-acetyl-d-glucal (9) in eight steps (35% overall yield). On a larger scale, Panek's eight-step assembly of the target molecule from the two partners was also slightly modified, giving 45 mg reblastatin (19% overall yield) in the first batch synthesis. Notable feature of our study is the settlement of the C14 chirality through a diastereoselective α-alkylation of 6 followed by a three-step full reduction of the lactone carboxyl, making vastly available 6 a universally applicable C11–C14 synthon for benzenoid/benzoquinone ansamycins. 相似文献
996.
Xiao-Lan Hu Xi-Qing Bian Xin Wu Jian-Yong Li Hui-Ming Hua Yue-Hu Pei Ai-Hong Han Jiao Bai 《Tetrahedron letters》2014
Penioxalamine A (1), a novel prenylated spiro-oxindole alkaloid having a unique seven-membered nitrogen heterocycle system, was isolated from the fungus Penicillium oxalicum TW01-1. The structure of 1 was elucidated on the basis of the spectral data, single-crystal X-ray diffraction, and CD analysis. Compound 1 showed moderate cytotoxicity against HL-60 cell line. The possible biosynthetic pathway of 1 was also proposed. 相似文献
997.
Jie Ma Huiming Lin Rong Xing Xiaofeng Li Chunhui Bian Di Xiang Wei Guo Fengyu Qu 《Journal of Sol-Gel Science and Technology》2014,69(2):364-369
A novel mesoporous silica tubes (MMT) which possessed pH-sensitive controlled release ability had been fabricated and synthesized by using carbon nanotubes (CNTs) as template. The sample replicated the morphologies of the CNTs successfully. The Brunauer–Emmett–Teller surface area of the materials can reach 1,017 m2 g?1 with the pore size of 3.8 nm. As a model drug, metformin HCl was applied to study the drug loading and control release ability of the materials. MMT possesses higher drug loading ratio (36 %) than that of MCM-41 (27.5 %). The release kinetics were studied in simulated gastric fluid (pH = 1.2) and in simulated proximal intestine fluid (pH = 7. 4), respectively. The result shows that the delivery systems exhibit well pH-sensitive control release ability and the as-synthesized materials have potential application in biomedical field. 相似文献
998.
Composite PET Membrane with Nanostructured Ag/AgTCNQ Schottky Junctions: Electrochemical Nanofabrication and Charge‐Transfer Properties
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Li Huang Prof. Yong Chen Shujuan Bian Dr. Yi‐Fan Huang Prof. Zhong‐Qun Tian Prof. Dongping Zhan 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(3):724-728
Large‐area nanostructured Ag/Ag‐tetracyanoquinodimethane (TCNQ) Schottky junctions are fabricated electrochemically on a mesoporous polyethylene terephthalate (PET) membrane‐supported water/1, 2‐dichloroethane (DCE) interface. When the interface is polarized, Ag+ ions transfer across the PET membrane from the aqueous phase and are reduced to form metallic Ag on the PET membrane, which reacts further with tetracyanoquinodimethane (TCNQ) in the DCE phase to form nanostructured Ag/AgTCNQ Schottky junctions. Once the mesoporous membrane is blocked by metallic Ag, a bipolar mechanism is proposed to explain the successive growth of AgTCNQ nanorods and Ag film on each side of the PET membrane. Due to the well‐formed nanostructure of Ag/AgTCNQ Schottky junctions, the direct electrochemical behavior is observed, which is essential to explain the physicochemical mechanism of its electric performance. Moreover, the composite PET membrane with nanostructured Ag/AgTCNQ Schottky junctions is tailorable and can be assembled directly into electric devices without any pretreatment. 相似文献
999.
Mathematics and Financial Economics - We discuss the turnpike property for optimal investment and consumption problems. We find there exists a threshold value that determines the turnpike property... 相似文献
1000.
The allylation reactions of aromatic aldehydes with allyl bromide were carried out in 89–98% yield with Sb-H2O-KF-CH3OH under ultrasound irradiation at rt for 2.5 h. The reactions in the same system gave allylic alcohols in 30–69% yield with stirring for 24 h. The main advantages of the present procedure are shorter reaction time, better yield, and environmental friendliness. 相似文献