全文获取类型
收费全文 | 39855篇 |
免费 | 7459篇 |
国内免费 | 7106篇 |
专业分类
化学 | 29484篇 |
晶体学 | 727篇 |
力学 | 2625篇 |
综合类 | 599篇 |
数学 | 4619篇 |
物理学 | 16366篇 |
出版年
2024年 | 116篇 |
2023年 | 693篇 |
2022年 | 1457篇 |
2021年 | 1546篇 |
2020年 | 1593篇 |
2019年 | 1670篇 |
2018年 | 1457篇 |
2017年 | 1550篇 |
2016年 | 1902篇 |
2015年 | 2189篇 |
2014年 | 2572篇 |
2013年 | 3305篇 |
2012年 | 3762篇 |
2011年 | 3832篇 |
2010年 | 2887篇 |
2009年 | 2989篇 |
2008年 | 3247篇 |
2007年 | 2764篇 |
2006年 | 2587篇 |
2005年 | 2097篇 |
2004年 | 1538篇 |
2003年 | 1288篇 |
2002年 | 1259篇 |
2001年 | 1062篇 |
2000年 | 891篇 |
1999年 | 719篇 |
1998年 | 492篇 |
1997年 | 434篇 |
1996年 | 393篇 |
1995年 | 317篇 |
1994年 | 321篇 |
1993年 | 249篇 |
1992年 | 201篇 |
1991年 | 159篇 |
1990年 | 154篇 |
1989年 | 110篇 |
1988年 | 97篇 |
1987年 | 70篇 |
1986年 | 81篇 |
1985年 | 55篇 |
1984年 | 41篇 |
1983年 | 38篇 |
1982年 | 20篇 |
1981年 | 26篇 |
1980年 | 14篇 |
1979年 | 24篇 |
1978年 | 8篇 |
1977年 | 13篇 |
1957年 | 8篇 |
1925年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Samples of poly[1-(3-sulfopropyl)-2-vinyl-pyridinium-betaine] (PSPV) have been synthesized to high conversion by free radical polymerization in aqueous solution of the zwitterionic monomer SPV with several concentrations of the crosslinker N,N′-methylene-bis-acrylamide (MBA). The densities of the resultant xerogels increased regularly with the content of MBA. Hydrogels obtained by swelling them in water and aqueous KSCN solution were examined by gravimetric and dimensional analysis. The water contents increased with decreasing content of MBA, the value of 92.7 wt% at the lowest MBA content being higher than that for other zwitterionic hydrogels. Enhanced swelling occurred in 1 M aq. KSCN at each MBA content, the total swelling being 98.1 wt% at the lowest crosslinker content. Swelling increased with increasing temperature. An approximate procedure to formulating swelling equilibrium in term of the volume fraction of water in hydrogel, in conjunction with the van’t Hoff equation, yields a small positive value for the enthalpy of swelling. This is compared with values derived similarly for other hydrogels. 相似文献
62.
The spin states of iron (II) in FeL2(NCS)2 produced from the thermal decomposition of FeL8(NCS)2 (L=2-py-CR1=NR2, R1 or R2=H, Me(CH3), Ph) under nitrogen atmosphere are investigated by means of Mössbauer spectroscopy and magnetic measurements. It was found that the spin states of iron (II) in substitutited bis (2-pyridylimine) iron (II) thiocyanate are included: intermediate-spin (3T1, S=1), mixed-spin of 5T2(S=2)+3T1 and spin incomplete transition of 5T2?3T1. 相似文献
63.
The intrinsic characteristics of radical pairs produced in squalane and in cetane receiving high gamma-dose are extensively studied with the EPR technique at temperatures from 77°K up to 150°K. The spectra of the paired radicals occur at g=4 with a very low transition probability in contrast to that of isolated radicals which appear at g=2 A well-resolved hyperfine spectrum corresponding to the species (CH3CH2.CH2CH3) is observed in cetane. The isothermal decay rates of radical pairs in cetane below 100°K are significantly slow; however, the decay kinetics at 150°K is first order with rate constant=1.86 min?1. A relatively slower decay rate is obtained for isolated radicals suggesting that the decay mechanism of paired radicals is through geminate recombination. The relative inter-radical distance in radical pairs is known from a decay curve as a function of temperature. The yields of radical pairs are low in both matrices, only few percents of those of isolated radicals. The formation mechanisms of paired radicals with direct radiolytic bond scission process are discussed in connection with the experimental observations. 相似文献
64.
芦荟中多糖的光度测定新方法 总被引:3,自引:0,他引:3
利用正交试验考察了芦荟多糖与刚果红显色反应的最佳条件;研究了麦芽糖糊精等多糖、蔗糖等低聚糖和葡萄糖等单糖对显色反应的影响;建立了简便快速、常见糖类基本无干扰的测定芦荟多糖含量的吸光光度法。对芦荟鲜叶和芦荟制品中的多糖进行分析,加标回收率在92%~96%范围内,相对标准偏差≤5%。 相似文献
65.
Mutation of the active-site residue Cys38 of N-Ada converts it from a sacrificial DNA repair protein to an enzyme that uses methanethiol as an external sacrificial reagent to repair DNA methyl phosphotriesters catalytically. 相似文献
66.
YanXiaCAO MiaoDU QiangZHENG 《中国化学快报》2004,15(5):567-570
The study on rheological properties of a series of mica-filled polypropylene (PP) composites was carried out. The influence of surface-treatment of mica particles on dynamic rheological behavior of the composites were dealt with. The viscosity (η) and dynamic modulus ( G‘ ) of the composite melts were higher than those of PP matrix, especially those for systems treated with silane, which was attributed to the interfacial adhesion enhancement. However, surface-treatment of mica by titanate resulted in lower η and G‘, as compared with the treatment by silane. The reason for this is believed to be the formation of the mono-molecular layer on the mica surface. 相似文献
67.
Two new uncommon nitrogenous sesquiterpenes, 11‐ethoxy‐3‐formamidotheonellin ( 1 ) and 7‐ethoxy‐3‐formamidobisabolane‐8,10‐diene ( 2 ), together with two known related sesquiterpenes, 3‐formamidotheonellin (=theonellin formamide; 3 ) and theonellin isothiocyanate ( 4 ), were isolated from the Hainan sponge Axinyssa aff. variabilis. Their structures were determined on the basis of extensive spectroscopic analysis and by comparison of their NMR data with those of known compounds. 相似文献
68.
The reaction of 1-alkynylcyclobutanols with aryl iodides in the presence of Pd(OAc)2 and Et3N in acetonitrile at 80°C for 24 h gives 2-disubstituted methylenecyclopentan-1-ones in modest to good yields. The tandem insertion-ring expansion process proceeds via the formation of an alkynyl π-complex, followed by migration of a carbon-carbon bond of the tert-alkanol to form the cyclopentanones stereoselectively. 相似文献
69.
Yang Nan Subhash P. Upadhyaya Wei Xu Kathrine E. Hughes William J. Dunn Ludwig Bauer Hemendra N. Bhargava George A. Doss 《Journal of heterocyclic chemistry》1996,33(2):399-407
A number of (E)-7-arylidenenaltrexones were synthesized by azeotropic distillation of water from a benzene solution of naltrexone and an aromatic aldehyde (benzaldehyde, 4-chloro- and 4-fluorobenzaldehyde, 3-and 4-pyridinecarboxaldehyde and 1-methyl-2-imidazolecarboxaldehyde) using piperidine as a catalyst. In addition, (E)-7-benzylidenenaloxone was prepared by the previously published Claisen-Schmidt condensation using sodium hydroxide in methanol. The stereochemistry of these arylidene derivatives 3–9 was determined to be (E) by means of nuclear Overhauser enhancement experiments. The 13C nmr spectra of (E)- 3–9 are recorded in deuteriochloroform and those of the hydrochlorides in deuteriodimethyl sulfoxide. 相似文献
70.
在pH5.0的NaOAc-HOAc缓冲溶液中,过量的硝酸银完全沉淀叶酸,用原子吸收光谱法测定剩余的硝酸银,间接计算出叶酸的含量。方法简单快速,回收率在98%~104%之间,相对标准偏差为2.4%。 相似文献