全文获取类型
收费全文 | 10537篇 |
免费 | 728篇 |
国内免费 | 599篇 |
专业分类
化学 | 8070篇 |
晶体学 | 34篇 |
力学 | 232篇 |
综合类 | 61篇 |
数学 | 1794篇 |
物理学 | 1673篇 |
出版年
2023年 | 78篇 |
2022年 | 133篇 |
2021年 | 176篇 |
2020年 | 200篇 |
2019年 | 270篇 |
2018年 | 210篇 |
2017年 | 232篇 |
2016年 | 277篇 |
2015年 | 241篇 |
2014年 | 432篇 |
2013年 | 671篇 |
2012年 | 456篇 |
2011年 | 660篇 |
2010年 | 578篇 |
2009年 | 739篇 |
2008年 | 665篇 |
2007年 | 615篇 |
2006年 | 564篇 |
2005年 | 418篇 |
2004年 | 354篇 |
2003年 | 335篇 |
2002年 | 270篇 |
2001年 | 156篇 |
2000年 | 155篇 |
1999年 | 134篇 |
1998年 | 113篇 |
1997年 | 91篇 |
1996年 | 93篇 |
1995年 | 67篇 |
1994年 | 66篇 |
1993年 | 56篇 |
1992年 | 58篇 |
1991年 | 46篇 |
1990年 | 28篇 |
1989年 | 26篇 |
1988年 | 22篇 |
1986年 | 17篇 |
1985年 | 417篇 |
1984年 | 283篇 |
1983年 | 238篇 |
1982年 | 213篇 |
1981年 | 152篇 |
1980年 | 117篇 |
1979年 | 131篇 |
1978年 | 75篇 |
1977年 | 86篇 |
1976年 | 87篇 |
1975年 | 89篇 |
1974年 | 115篇 |
1973年 | 130篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
The objective of the work described in this paper was to produce dispersions of small spherical carbon particles, having particle diameters in the region of 0.1 μm. To this end, the dehydrochlorination of poly(vinylidene chloride) (PVDC) latex particles was attempted. The PVDC latex was prepared by a dispersion polymerization route. Both chemical and thermal dehydrochlorination routes were attempted. Chemical dehydrochlorination, using a variety of base/solvent systems, led to nonporous, spherical black particles of the required size, but which contained only 60% carbon; most of the remainder was oxygen, introduced by nucleophilic substitution reactions. Thermal dehydrochlorination, at 700°C under a nitrogen atmosphere, using a fluidized bed arrangement, on the other hand, led to black particles, having 90% carbon and which retained their sphericity, but which were highly porous. Initial chemical dehydrochlorination, prior to thermal treatment, did not seem to reduce the porosity of the final carbons. Dispersions of the carbon particles in a variety of solvents were readily achieved. 相似文献
72.
Zhengping Hao Lidun An Junling Zhou Hongli Wang 《Reaction Kinetics and Catalysis Letters》1996,59(2):295-300
Supported gold catalyst for eliminating hydrogen from CO2 feed gas in the production of urea is found to be superior to other industrial catalysts (e.g. Pt/Al2O3 and PdPt/Al2O3) in catalytic activity and resistance to sulfur poisoning. 相似文献
73.
N. V. Moskalev M. I. Tartynova I. Yu. Bagryanskaya Yu. V. Gatilov 《Russian Chemical Bulletin》1996,45(2):462-464
Nitrobenzene,p-bromonitrobenzene,N,N-diethyl-p-nitroaniline, and 9-methyl-3nitrocarbazole undergo condensation with two moles of acetophenone in a DMSO-KOH system to form the corresponding (Z)-1,4-diphenyl-2-arylamino-2-butene-1,4-diones.Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 480–482, February, 1996 相似文献
74.
The first evidence that α-acylnitrone is an intermediate in the base-promoted reaction of 4-nitro-N,N-diethylaniline with aceptophenome to give an enaminoketone was obtained.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1645–1647, August, 1998. 相似文献
75.
用AM1方法(采用非限制的Hartree-FockUHF计算)研究乙烯与环己-1,3-二烯的热Diels-Alder反应。结果表明反应是放热的且存在两条竞争的路径;协同反应的活化能以及双自由反应速度控制步骤的活化能分别为112.667kJ/mol和78.406kJ/mol。 相似文献
76.
77.
The magnesium-butadiene 1 : 1 (1), 1 : 2 (II) and 1 : 3 addition compounds obtained by the direct metalation of butadiene with metallic magnesium were isolated. These compounds have a polymeric structure. Structural analysis of these compounds by NMR and IR methods was carried out. The process for the formation of II via I and III via II was investigated. The results of protolysis and alkylation of the above magnesium compounds corresponded well to those of crotylmagnesium compounds. 相似文献
78.
Recently the hydrogen-bond activated reactions have attracted much attention.1 Takemoto2 reported a highly enantioselective Michael addition of manolate to nitroolefins catalyzed by a bifunctional organocatalyst with tertiary amine and thiourea moiety. As we known,stereoselective conjugate additions of thiols are interesting due to the standpoint of biological and synthetic importance, however, only very limited good results have been obtained except for the works of Shibasaki3, Kanemasa4 and Deng5 et al.In this letter, we report an efficient catalytic asymmetric Michael reactions of thiols to a,a-unsaturated carbonyl compounds promoted by bifunctional organocatalysts. A series of organocatalysts with chiral amine and thiourea structures were designed and synthesized and have been successfully applied in the conjugated additions of thiols to a,a-unsaturated imides and enones.The reactions got quantitative yields and the ee values were up to 84%. It is noteworthy that the a-asymmetric protonation (up to 43% ee) also could be achieved.The Michael addition between aromatic thiols and a,a-unsaturated carbonyl compounds isdescribed as follows:Works to further increase the enantioselectivity is under investigation in our laboratory. 相似文献
79.
meso-四(4-磺酸苯基)卟啉与Co(Ⅱ)的显色反应的研究 总被引:3,自引:0,他引:3
本文研究了弱酸性介质中钴(Ⅱ)与TPPS4的显色反应条件。实验发现胺类化合物质子化后,不参与成络反应,而均能使TPPS4试剂的吸光度大大降低,从而提高了测定的灵敏度,Co(Ⅱ)-TPPS4络合比为1:1,ε_(426)=3.54×10 ̄5,钴含量在0~5.0μg/25mL范围内呈线性,并用于烟草、茶叶中痕量钴的测定,得到了满意的结果。 相似文献
80.