全文获取类型
收费全文 | 8750篇 |
免费 | 1608篇 |
国内免费 | 2414篇 |
专业分类
化学 | 4158篇 |
晶体学 | 36篇 |
力学 | 1342篇 |
综合类 | 49篇 |
数学 | 2389篇 |
物理学 | 4798篇 |
出版年
2023年 | 166篇 |
2022年 | 182篇 |
2021年 | 147篇 |
2020年 | 150篇 |
2019年 | 120篇 |
2018年 | 190篇 |
2017年 | 80篇 |
2016年 | 184篇 |
2015年 | 220篇 |
2014年 | 361篇 |
2013年 | 322篇 |
2012年 | 425篇 |
2011年 | 407篇 |
2010年 | 388篇 |
2009年 | 304篇 |
2008年 | 225篇 |
2007年 | 466篇 |
2006年 | 454篇 |
2005年 | 547篇 |
2004年 | 514篇 |
2003年 | 346篇 |
2002年 | 429篇 |
2001年 | 524篇 |
2000年 | 417篇 |
1999年 | 510篇 |
1998年 | 224篇 |
1997年 | 147篇 |
1996年 | 194篇 |
1995年 | 257篇 |
1994年 | 208篇 |
1993年 | 213篇 |
1992年 | 203篇 |
1991年 | 155篇 |
1990年 | 193篇 |
1989年 | 186篇 |
1988年 | 220篇 |
1987年 | 227篇 |
1986年 | 194篇 |
1985年 | 255篇 |
1984年 | 170篇 |
1983年 | 114篇 |
1982年 | 109篇 |
1981年 | 90篇 |
1975年 | 114篇 |
1959年 | 83篇 |
1958年 | 125篇 |
1957年 | 123篇 |
1956年 | 110篇 |
1955年 | 118篇 |
1954年 | 90篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
152.
153.
Hong Xin SHI * Hui LIN Gérard MANDVILLE Zhejiang Key Lab. of Green-chemical Synthetic Technology Zhejiang University of Technology Hangzhou .Department of Chemistry Institute of Science Zhejiang University Hangzhou Université de Paris-Sud ICMO Orsay Cédex France 《中国化学快报》2004,15(3):288-291
The reaction of aromatic carboxylic acid with oxalyl chloride gives rise to the corresponding acid chloride which without purification is treated with the sodium salt of mercaptopyridine oxide in the presence of 2,2-azo-bisisobutyronitrile (AIBN), radical initiator to give a brominated aromatic compound. After etherification and oxidation, 5-iodovaniline was converted to trisubstituted benzene carboxylic acid which give 1-bromo-3-methoxy-4-propoxy-5-iodobenzene by this new brominating process with a yield of 74%. 相似文献
154.
Molecular mobility in thermotropic polyesters and side-chain polymers with different struc-ture of mesogens and spacers has been studied by dielectrical method in dilutesolutions. The results made it possible to establish the multiplicity of dielectric relaxationtransitions which reflects the small- and large-scale types of molecular motion. It was shownthat dielectric relaxation processes occurring in accordance with local mechanism (relaxationtimes 10~(-9)--10~(-7)s. and the activation energy 10--50kJ/mol) are due to the mobility of kineticchain elements of different length within a monomer units. It was found that the dielectricrelaxation process connected with a large-scale form of molecular motion (relaxation times10~(-5)--10~(-6)s. and the activation energy 100kJ/mol) did not depend on the molecular massbut was infiuenced by factors changing the conformational state of the macromolecule. It isestablished tha the cooperative reorientation mobility of associated mesogenic fragments isthe source of the large-scale process. 相似文献
155.
SYNTHESIS OF MESOPOROUS POLY(STYRENE-co-MALEIC ANHYDRIDE)/SILICA HYBRID MATERIALS VIA A NONSURFACTANT-TEMPLATED SOL-GEL PROCESS* 总被引:1,自引:0,他引:1
Jie-bin Pang Kun-yuan Qiu Yen Wei Department of Polymer Science Engineering College of Chemistry Molecular Engineering Peking University Beijing China. Department of Chemistry Drexel University Philadelphia Pennsylvania USA. 《高分子科学》2000,(5):469-472
Mesoporous poly (styrene-co-maleic anhydride)/silica hybrid materials have been prepared. The synthesis wasachieved by the HCl-catalyzed sol-gel reactions of tetraethyl orthosilicate (TEOS) and styrene-maleic anhydride copolymerin the presence of 3-aminopropyl triethoxysilane (APTES) as a coupling agent and citric acid as a nonsurfactant template orpore-forming agent, followed by ethanol extraction. Characterization results from nitrogen sorption isotherms and powder X-ray diffraction indicate that polymer-modified mesoporous materials with large specific surface areas (e.g. 900 m~2/g) andpore volumes (e.g. 0.6 cm~3/g) could be prepared. As the citric acid concentration is increased, the specific surface areas, porevolumes and pore diameters of the hybrid materials increase. 相似文献
156.
CMAO Jiang-Gao 《结构化学》1998,(5)
1INTR0DUCTI0NLanthanidecarboxylatecomplexeshavebeenwidelystudiedt1iandm0starefoundtoexhibitavarietyofdimericorinfinitechainstructures.Recentstudiesinourlaboratoryhavedem0nstratedthattheprototypetertiaryaminebetaine(Me,N CHzC0z-)anditsderivativesareusefulinthegenerationoflanthanidecarb0xylate-likecomplexeswithnewstructuralfeaturesduetotheiroverallchargeneutralityandpossibleinclusionofvariouskindsofcounteranionsinthestructuret2~4i.Thelan-thanidecomplexesisolatedsofarhavebeenfoundtobemononu… 相似文献
157.
环境水中甲基对硫磷、对硫磷和辛硫磷农药残留的SPE-HPLC分析 总被引:29,自引:0,他引:29
采用离线固相萃取 (SPE)富集 -高效液相色谱(HPLC)分离和紫外分光光度法检测 ,对环境水中甲基对硫磷、对硫磷和辛硫磷3种有机磷农药进行分析;固相萃取用C18 萃取柱 ,用甲醇洗脱 ,高效液相色谱分离以Shim_PackCLCODS柱(150mm×4.6mmid,5μm)为分离柱 ,流动相为甲醇 -水(体积比70∶30) ,紫外检测波长为280nm;该法稳定可靠 ,回收率高 相似文献
158.
金属的电解制取由铜的含氧矿物(孔雀綠、氧化铜及氧化亚铜等)制取铜的原理,在学校里很容易演示。在一含有10%硫酸溶液的烧杯中,投入研碎的矿粉,加热直至得到淡颜色的硫酸铜溶液。将所得溶液用过濾法使与沉淀分离,然后把溶液电解,铜即沉积于炭的阴电极上。由熔融鹽制取金属的原理,最好用电解氯化锌为例说明。为了熔化氯化锌,可应用容积30毫升的铁坩埚。演示实验时,可盛结晶氯化锌至坩埚的大约一半地方,放在酒精灯或煤气灯上加热,经2-3分鐘,氯化锌即熔融(t=313℃),继续加热,在熔融鹽中浸入两根 相似文献
159.
Feng Liu Si Guo Yuan Xiao Li Wang A.E Polikarpov A.A. Shunkevich 《中国化学快报》2007,18(5):588-590
A fibrous sorbent possessing abundant micropore structure was firstly prepared via post-crosslinking reaction on the PP-ST-DVB original fiber. Its micromorphology and sorptive properties were investigated, and the results demonstrated that the novel fibrous hypererosslinked sorbent has narrow pore-size distribution, small average porous radius (1.90 nm), high specific surface area (362.31 m^2/g), and fine sorptive properties for small organic molecules. 相似文献
160.
在某些能够打斷聚合物自由基的增長并在聚合物分子中引進末端基團的物質存在下,不飽和化合物的聚合反應稱為調節聚合反應。這個反應可以製備比較低分子的化合物,可用下式表示之: 相似文献