全文获取类型
收费全文 | 7619篇 |
免费 | 970篇 |
国内免费 | 821篇 |
专业分类
化学 | 4360篇 |
晶体学 | 76篇 |
力学 | 267篇 |
综合类 | 77篇 |
数学 | 897篇 |
物理学 | 1522篇 |
无线电 | 2211篇 |
出版年
2024年 | 37篇 |
2023年 | 187篇 |
2022年 | 249篇 |
2021年 | 286篇 |
2020年 | 317篇 |
2019年 | 322篇 |
2018年 | 279篇 |
2017年 | 312篇 |
2016年 | 342篇 |
2015年 | 408篇 |
2014年 | 471篇 |
2013年 | 673篇 |
2012年 | 654篇 |
2011年 | 633篇 |
2010年 | 500篇 |
2009年 | 526篇 |
2008年 | 566篇 |
2007年 | 477篇 |
2006年 | 421篇 |
2005年 | 315篇 |
2004年 | 236篇 |
2003年 | 186篇 |
2002年 | 163篇 |
2001年 | 107篇 |
2000年 | 103篇 |
1999年 | 91篇 |
1998年 | 65篇 |
1997年 | 52篇 |
1996年 | 45篇 |
1995年 | 22篇 |
1994年 | 31篇 |
1993年 | 29篇 |
1992年 | 17篇 |
1991年 | 14篇 |
1990年 | 17篇 |
1989年 | 17篇 |
1988年 | 21篇 |
1987年 | 14篇 |
1986年 | 9篇 |
1985年 | 17篇 |
1984年 | 15篇 |
1983年 | 14篇 |
1982年 | 15篇 |
1981年 | 11篇 |
1980年 | 17篇 |
1979年 | 11篇 |
1978年 | 19篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1975年 | 9篇 |
排序方式: 共有9410条查询结果,搜索用时 15 毫秒
81.
82.
A reversed-phase high-performance liquid chromatography/electrospray ionisation mass spectrometry (HPLC/ESI-MS) method has been developed to conclusively differentiate the epimers betamethasone and dexamethasone and various esterification products (betamethasone and dexamethasone 21-acetate, betamethasone and dexamethasone 21-phosphate, betamethasone 17-valerate, betamethasone 21-valerate and betamethasone 17,21-dipropionate) in counterfeit drugs. Good separation with baseline resolution of all epimers or isomers was obtained on a Zorbax Eclipse XDB or Luna C8 column, using a step gradient with mobile phases of 0.05 M ammonium acetate and acetonitrile. Betamethasones can also be distinguished by the relative abundance of their m/z 279 ion in the positive electrospray tandem mass spectra. The LC/MS or LC/MS/MS method developed was successfully applied to the analysis of drug product samples, i.e. creams and tablets. 相似文献
83.
84.
对近期发展的固体圆二色(CD)光谱测试方法进行了概述、评价和比较, 着重探讨了“浓度效应”的存在使固体CD光谱失真的原因. 通过对本课题组和其他作者已报道的四种化合物的固体CD谱再测试的反思, 强调了依手性化合物的手性光谱学性质不同, 根据浓度梯度实验选择其合适测试浓度的必要性. 对固有手性的阻转异构化合物(S)-1,1'-联二萘酚(S-BINOL)进行了成膜法固体CD谱浓度梯度测试, 发现所得固体薄膜CD谱中也存在着“浓度效应” 相似文献
85.
86.
为实现PTS系列频率综合器的微机控制,设计了一种简单的控制电路。在微机的控制下,通过该电路与微机的并行通讯口,用C语言编写的控制程序可以很方便的控制频率综合器的输出频率。控制程序采用菜单操作,电路所需的+5V电源由微机提供,整个控制系统简单实用,值得推广。 相似文献
87.
[chemical reaction: see text]. An efficient Yb(OTf)3-promoted palladium-catalyzed oxidative cyclization of gamma-heteroalkenyl beta-keto amides has been developed. Under simple aerobic condition, a variety of six-, seven-, and eight-membered-ring N- and O-heterocycles were obtained regioselectively in excellent yield. 相似文献
88.
Uniform, well-dispersed silver particles of various morphologies have been prepared by reducing highly acidic silver nitrate solutions with ascorbic acid in the presence of a sodium naphthalene sulfonate-formaldehyde copolymer as dispersing agent. By varying the temperature of the reaction, the free acid content, the addition rate of the reductant, and the aging time, both isometric and anisotropic silver particles could be obtained. It was found that the latter were formed by aggregation of nanosize subunits, which were identified by electron microscopy and X-ray diffractometry. 相似文献
89.
Dan Donescu Sever Serban Kristiana Gosa Cristian Petcu 《Central European Journal of Chemistry》2005,3(1):10-27
The possibility to prepare hybrids made by poly(vinyl acetate) (PVAc), poly(methyl methacrylate) (PMMA) and/or poly(ethyl
acrylate) (PEtA) with TiO2 was studied. The processes of polymer formation-radical polymerization and sol-gel process for inorganic network —were achieved
simultaneously. Due to a high reactivity of titanium isopropoxide (TIP) in the sol-gel process, a complexant comonomer, allyl
acetoacetate (AlAcAc), was used. Covalent bonds between polymer and inorganic chains were obtained by addition of trialkoxysilane
derivates with vinyl (VTES) or methacryloyl (MPTS) groups. The presence of TIP inhibits the radical polymerization of vinyl
acetate (VAc). The PVAc-TiO2 hybrids were produced by the sol-gel process of TIP in the presence of pre-obtained PVAc. Except for VTES and MPTS, trialkoxysilane
derivates with methyl (MeTES), octyl (OTES) and phenyl (PTES) groups were used. The thermal stability of hybrids is strongly
affected by TiO2 presence and by the type of trialkoxysilane derivates. The thermal stability of PVAc hybrids decreases in the presence of
TiO2 inorganic network. The glass transition temperature of polymers increases in the presence of the inorganic network. 相似文献
90.
The fragmentation patterns of 3-methyl-2-butyl trifluoroacetate and 2-methyl-2-butyl trifluoroacetate were investigated by GC/MS/MS, with electron impact and collision-induced dissociation, on regular and isotope-labeled (deuterium and 18O) esters. The atoms found in various fragments could be traced back to specific positions in the parent molecules. In this way, molecular rearrangements potentially occurring during the formation of esters by trifluoroacetolysis of 3-methyl-2-butyl p-toluenesulfonate or trifluoroacetic acid addition to various 2-methylbutenes could be shown. Rearrangements also occurred during the fragmentation, particularly during the expulsion of the small fragments CO, C2H4 and F2CO. For the decompositions of oxygen-containing ions these rearrangements were highly specific. By contrast, alkyl cations lead to fragments that are fully scrambled (statistical label distribution). Alkene radical cations ([CnH2n]+ ˙) fragment to daughter ions that are extensively, but less than statistically scrambled. Hydrogen scrambling may also occur in fluoroalkyl cation fragments. 相似文献