全文获取类型
收费全文 | 6985篇 |
免费 | 1515篇 |
国内免费 | 1764篇 |
专业分类
化学 | 3171篇 |
晶体学 | 186篇 |
力学 | 337篇 |
综合类 | 190篇 |
数学 | 592篇 |
物理学 | 2172篇 |
无线电 | 3616篇 |
出版年
2024年 | 41篇 |
2023年 | 93篇 |
2022年 | 271篇 |
2021年 | 217篇 |
2020年 | 209篇 |
2019年 | 216篇 |
2018年 | 212篇 |
2017年 | 290篇 |
2016年 | 219篇 |
2015年 | 365篇 |
2014年 | 482篇 |
2013年 | 541篇 |
2012年 | 553篇 |
2011年 | 577篇 |
2010年 | 577篇 |
2009年 | 629篇 |
2008年 | 642篇 |
2007年 | 575篇 |
2006年 | 620篇 |
2005年 | 517篇 |
2004年 | 441篇 |
2003年 | 285篇 |
2002年 | 276篇 |
2001年 | 294篇 |
2000年 | 312篇 |
1999年 | 160篇 |
1998年 | 63篇 |
1997年 | 51篇 |
1996年 | 57篇 |
1995年 | 50篇 |
1994年 | 57篇 |
1993年 | 46篇 |
1992年 | 57篇 |
1991年 | 31篇 |
1990年 | 40篇 |
1989年 | 28篇 |
1988年 | 41篇 |
1987年 | 29篇 |
1986年 | 17篇 |
1985年 | 20篇 |
1984年 | 10篇 |
1983年 | 13篇 |
1982年 | 9篇 |
1981年 | 12篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1973年 | 1篇 |
1966年 | 1篇 |
1965年 | 2篇 |
1959年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
161.
1 INTRODUCTION In recent years, chiral β-amino alcohols have attracted much attention due to their special bio- logical functions and catalytic activities. Chiral β- amino alcohol moieties are usually found not only in natural products (e.g., cinchona alkaloids, ephe- drine, toxal, etc.) with special biological activities[1, 2], but also critical structural segments of some syn- thesized biologically active compounds, such as adrenergic agonists or antagolist[3, 4] and inhibitors for HI… 相似文献
162.
163.
在甲烷氧化偶联制C2烃的研究中,人们发现碱土金属与稀土金属组成的复合氧化物催化剂具有高的催化活性[1,2].稀土化合物作为有希望的工业催化剂之一,已受到广泛的关注.一般认为,它们含有的氧空位对活化甲烷的有效氧物种有利.Debey等在1%Sr/La2O... 相似文献
164.
有机阳离子[C18mim]+在蒙脱土层间的物理化学吸附与聚集状态* 总被引:2,自引:0,他引:2
咪唑基离子液体(N鄄十八烷基鄄N忆鄄甲基溴化咪唑盐[C18mim]Br)中的[C18mim]+可以通过离子交换反应进入蒙脱土层间.本文用TGA、FTIR和XRD 研究了[C18mim]+在蒙脱土层间的物理和化学吸附及其聚集状态. 研究结果表明, 蒙脱土存在着饱和的化学吸附量, 其总吸附量随物理吸附量的增加而增加;随着[C18mim]+在蒙脱土层间吸附量的增加, 其聚集态结构的有序性增加, 并伴有分子链的构象变化,导致层间距的增加. 相似文献
165.
考察了阳离子、非离子和阴离子表面活性剂存在下水杨酸-2’-乙基己基酯(EHS)的吸收光谱和激发态分子内质子转移(ESIPT)荧光光谱. 结果表明, EHS可增溶在胶束中, 2’-乙基己基碳链朝向胶束内核, 而水杨酸基朝向胶束-水界面; 胶束环境有利于EHS分子对紫外光的吸收和分子内氢键的形成, 从而使ESIPT 荧光显著增强, 激发态分子以发射可见光和非辐射去活化方式衰减; 并根据EHS和表面活性剂分子的结构和大小, 解释了EHS分子在胶束中的结合位点, 荧光猝灭和酯水解的光谱测量进一步为此结合位点提供了佐证. 相似文献
166.
Cobalt(Ⅱ) can form complexes with Hdbm in different environments. Hdbm reacted with cobalt nitrate to give complex 1 [Co(dbm)2·2H2O]. When complex 1 reacted with pyridine, α-stilbazole or 4,4′-bipyridine respectively, complex 2 [Co(DBM)2Py2] (Py=pyridine), 3 [Co(DBM)2Sbz2] (Sbz=α-stilbazole) or 4 [Co(DBM)2BPy]n was obtained in turn through metathetical reaction. The coordination modes are octahedral polyhedrons. In the crystal structures, the two dbms take the plane position and two other donor molecules take the axial position. CCDC: 196070 for complex 2; 186859 for complex 3. 相似文献
167.
A determination method has been optimized and validated for the simultaneous analysis of tetracycline (TC), oxytetracycline (OTC), chlortetracycline (CTC) and doxycycline (DC) in honey. Tetracyclines (TCs) were removed from honey samples by chelation with metal ions bound to small Chelating Sepharose Fast Flow columns and eluted with Na2EDTA-Mcllvaine pH 4.0 buffers. Extracts were further cleaned up by Oasis HLB solid-phase extraction (SPE), while other solid-phase extraction cartridges were compared. Chromatographic separation was achieved using a polar end-capped C 18 column with an isocratic mobile phase consisting of oxalic acid, acetonitrile and methanol. LC with ultraviolet absorbance at 355 nm resulted in the quantitation of all four tetracycline residues from honey samples fortified at 15, 50, and 100 ng/g, with liner ranges for tetracyclines of 0.05 to 2 μg/mL. Mean recoveries for tetracyclines were greater than 50% with R.S.D. values less than 10% (n= 18). Detection limits of 5, 5, 10, 10 ng/g for oxytetracycline, tetracycline, chlortetracycline and doxycycline, respectively and quantitation limits of 15 ng/g for all the four tetracyclines were determined. Direct confirmation of the four residues in honey (2-50 ng/g) was realized by liquid chromatography-tandem mass spectrometry (LC/MS/MS). The linear ranges of tetracyclines determined by LC/MS/MS were between 5 to 300 ng/mL, with the linear correlation coefficient r〉 0.995. The limits of detection of 1 to 2 ng/g were obtained for the analysis of the TCs in honey. 相似文献
168.
Synthesis and characterization of a series of Polysulfone (PSF)-Polyester (PEs) block copolymers were studied.The degree of randomness (B) of these block eopolymers was calculated from the intensities of their proton signals in ~1H NMR spectra and lies in the region of 0相似文献
169.
Wei SUN Ya Qin DING Kui JIAO 《中国化学快报》2005,16(11):1515-1518
The interaction of phenosafranine (PSF) with a glycosaminoglycans of heparin (Hep) in aqueous solution has been characterized by UV-Vis absorption spectrophotometry and cyclic voltammetry in pH 1.5 Britton-Robinson (B-R) buffer solution. The addition of Hep caused decrease of the absorbance of PSF at 532 nm and the redox peak current of PSF. The study showed that an supramolecular complex of PSF-Hep was formed because of the electrostatic attraction of negatively charged Hep with the positively charged PSF, which resulted in the decrease of the equilibrium concentration of PSF in solutions, and the decrease of the absorbance or the peak current of PSF. The stoichiometry of the Hep/PSF complex was further calculated by voltammetric data with the result of 1:1 complex. 相似文献
170.
红外光谱反应出的是特征化学基团的振动 ,对于高聚物的具体单体组成分析 ,只能借助于紫外光谱、核磁共振及质谱进行综合判断才能得到圆满的鉴定结果[1,2 ] 。本工作针对在红外光谱仪不能分辨的情况下通过气相色谱 质谱联用技术进行综合分析鉴定 ,结果可为其它高聚物分析研究提供实验依据。1 实验1 1 仪器及试验条件未知高聚物试样为白色颗粒状 (晨光化工二厂 )。SYSTEM2 0 0 0FTIR ,PE公司造IR谱仪 ;CDS2 0 0 0铂金丝裂解器 ,裂解室温度 2 5 0℃ ,裂解温度 6 0 0℃ ,升温速率 1 40℃ ms;HP6 890型 ,Compond… 相似文献