全文获取类型
收费全文 | 4768篇 |
免费 | 146篇 |
国内免费 | 1002篇 |
专业分类
化学 | 5206篇 |
晶体学 | 33篇 |
力学 | 54篇 |
综合类 | 59篇 |
数学 | 21篇 |
物理学 | 303篇 |
无线电 | 240篇 |
出版年
2024年 | 4篇 |
2023年 | 20篇 |
2022年 | 73篇 |
2021年 | 88篇 |
2020年 | 104篇 |
2019年 | 109篇 |
2018年 | 94篇 |
2017年 | 151篇 |
2016年 | 155篇 |
2015年 | 116篇 |
2014年 | 174篇 |
2013年 | 425篇 |
2012年 | 248篇 |
2011年 | 273篇 |
2010年 | 225篇 |
2009年 | 254篇 |
2008年 | 253篇 |
2007年 | 303篇 |
2006年 | 294篇 |
2005年 | 302篇 |
2004年 | 283篇 |
2003年 | 280篇 |
2002年 | 212篇 |
2001年 | 187篇 |
2000年 | 158篇 |
1999年 | 175篇 |
1998年 | 140篇 |
1997年 | 132篇 |
1996年 | 129篇 |
1995年 | 106篇 |
1994年 | 81篇 |
1993年 | 72篇 |
1992年 | 62篇 |
1991年 | 56篇 |
1990年 | 40篇 |
1989年 | 41篇 |
1988年 | 32篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 9篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有5916条查询结果,搜索用时 15 毫秒
81.
A novolac phenol-formaldehyde resin was investigated via the thermally stimulated depolarization current, using integral and partial measurements in the temperature range from 137 to 270 K. Tow broadened peaks, assigned as Β1, and Β2, appeared at about 160 and 190 K. The influence of water and ¯Mn was investigated. The activation energy Ea vs. T relationship was analysed, and a search was made for compensation phenomena. The distribution of the relaxing dipolesN vs. Ea was approximated. Three different relaxation ranges were distinguished. All the motions were attributed to the rotation of the phenyl rings. The differences found are due to the heterogeneity in the resin. Contamination with water increases the polarizability, decreases the structure differences and relieves the motions in the resin. A resin with a smaller ¯Mn exhibits a higher polarizability and a decreases in structure variety. 相似文献
82.
1 Introduction
The Suzuki cross-coupling reaction is a powerful and versatile method for the generation of unsymme-trical biaryls from arylboronic acids and aryl halides in a single step~([1-3]).However,the reaction is usually performed in the presence of Pd catalyst along with phosphine ligand,which sometimes creates practical problems because organophosphines tend to be expensive,poisonous,and air sensitive.Recently,phosphine-free ligands,such as nitrogen or sulfur-containing ligands~([4-6]) and N-heterocyclic car-benes~([7-12]) have been applied in Suzuki reaction effectively. 相似文献
83.
Viscoelastic ature is one of the key features of polymeric composites. A series of cyanate ester (CE)‐based composites with different aluminum nitride (AlN) contents for high performance electronic packaging, coded as AlN/CE, were developed; the viscoelastic nature of AlN/CE composites was intensively investigated by employing dynamic mechanical analysis (DMA). Results show that the AlN content has a great effect on dynamic mechanical properties of AlN/CE composites. The storage modulus in the glassy region increases linearly with the addition of AlN as well as the increase of AlN content. Meanwhile, all composites also exhibit notably higher loss modulus than cured CE resin due to the appearance of new energy dissipation forms. In addition, the incorporation of AlN has a significant effect on damping factor peak. All reasons leading to these phenomena are analyzed from the view of structure–property relationship. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
84.
Cross-linked chitosan was chemically modified with di-2-propanolamine via an arm of chloromethyloxirane (CCTS-DPA resin). The adsorption behavior of the resin towards 62 elements was examined using a mini-column pretreatment method, and the collected elements were eluted with 1 mol L− 1 nitric acid before measurement by inductively coupled plasma-mass spectrometry (ICP-MS).The CCTS-DPA resin can adsorb several metal cations and several oxoanionic elements at appropriate pH. However, di-2-propanolamine (DPA) attached to cross-linked chitosan (CCTS) showed excellent ability and selectivity for the adsorption of germanium at pH 6 to 9. The adsorption capacity of the resin for germanium (IV) was found to be 106 mg g− 1 resin, whereas the adsorption rate constant was 9.82 × 10− 2 min− 1. Through the column treatment, alkali and alkaline earth matrices in river water and seawater matrices could be completely removed. The resin can also successfully remove chloride and selenium that can interfere with the direct determination of germanium by ICP-MS. The applicability of the CCTS-DPA resin was further demonstrated for the collection/preconcentration of germanium in environmental water samples and its determination by ICP-MS. The concentrations of germanium in tap water, river water and seawater samples were found in the range of 0.011 to 0.022 μg L− 1. 相似文献
85.
An efficient synthetic method has been developed for the synthesis of 2-arylacrylic esters from the corresponding aryl methyl ketones via Wittig reaction and singlet oxygen ene reaction. Wittig reaction to aryl methyl ketones with (methoxymethyl)triphenylphosphonium chloride in basic condition afforded the methyl enol ethers, and then 2-arylacrylic esters were obtained by singlet oxygen ene reaction, followed by tosylation and elimination in one-pot to the methyl enol ethers in good yields. 相似文献
86.
Liang Gao Zhiwei Yao Haifeng Gao Yue Sun Feixue liang 《Phosphorus, sulfur, and silicon and the related elements》2018,193(5):267-272
It was the first time that phenolic resin (PR) was used as a carbon source for the synthesis of nanostructured monometallic Mo and bimetallic CoMo carbides via carbothermal reduction route. The results showed that phase-pure β-Mo2C can be formed under an Ar atmosphere at 900°C or a H2 atmosphere above 700°C. However, almost pure CoMo carbides (Co3Mo3C and Co6Mo6C) can be obtained only under a H2 atmosphere at a low temperature of 630°C for 24 and 48 h, respectively. The role of PR in the preparation process has been investigated and a detailed formation mechanism was proposed based on the experimental results. 相似文献
87.
Electrical conductivity and elongation at break of epoxy filled with electroconductive carbon black, graphite or with silver-coated basalt particles or fibres were investigated in this paper. Percolation concentrations were determined to be 14 vol% for epoxy/carbon black composites, 22 vol% for epoxy/graphite composites, 28-29 vol% for both epoxy/silver-coated basalt particles and fibres. The steepest increase in electrical conductivity and the most pronounced decrease in elongation at break occurs at similar filler concentration range for all investigated systems. A good correlation between phenomenological model, introduced in [J. Mater. Sci. Lett. 18 (1998) 1457] and experimental data for all investigated systems was observed. 相似文献
88.
本实验合成了一系列含p-苯磺酸异丙酯基亲和性变化的二元、三元共聚物,并对其结构、聚合特征f1-F1曲线以及热分解性能进行了研究.对共聚物的DSC测试发现,随着p-苯磺酸异丙酯基含量的增加,树脂中磺酸酯基的分解温度向低温方向移动,范围在120-170℃;而磺酸酯基的分解能量随其在树脂中含量的增加而升高,范围在20-30kJ/mol.动力学研究结果显示,聚合物中磺酸酯基的热分解过程属于自加速类型,反映了热分解产生的磺酸对磺酸异丙酯热分解过程的催化作用.加热分解后树脂水溶性的研究表明,在二元共聚物中磺酸酯基单元含量等于或大于27%,在引入羧酸基的三元共聚物中磺酸酯基单元含量为21%时,热分解后共聚物涂层表现出优良的水溶性,能够在中性水中快速、彻底溶解并脱离支持版基. 相似文献
89.
Esther Domínguez Concepción Laborra Aranzazu Linaza Ana Madoz Issa A. Katime 《Monatshefte für Chemie / Chemical Monthly》1989,120(8-9):743-748
Summary A series of mono and diesters of itaconic acid containing alkoxy and sulfide side chains of variable lengths has been synthesized. Their structures have been established on the basis of their spectral data.
Eine Serie von Mono- und Diestern von Itaconsäure: Synthese und Strukturbestimmung
Zusammenfassung Es wurde eine Serie von Mono- und Diestern von Itaconsäure mit Alkoxy- und Sulfidseitenketten variabler Länge synthetisiert. Die Strukturaufklärung erfolgte mittels spektroskopischer Daten.相似文献
90.
Electrochemical oxidation of malonic esters in acetonitrile in the presence of iodides follows two different pathways depending
on the nature of the cation. In the presence of Lil, alkyl 1,1,2,2,3,3-propanetetracarboxylates were obtained in 85–98% yields.
In the presence of Nal, Kl, or Bu4NI, the formation of 1,1,2,2-ethanetetracarboxylates and their subsequent dehydrogenation to ethenetetracarboxylates was the
main reaction pathway.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 620–623, March, 1997. 相似文献