全文获取类型
收费全文 | 17296篇 |
免费 | 881篇 |
国内免费 | 751篇 |
专业分类
化学 | 3348篇 |
晶体学 | 32篇 |
力学 | 359篇 |
综合类 | 145篇 |
数学 | 2401篇 |
物理学 | 2523篇 |
综合类 | 10120篇 |
出版年
2024年 | 60篇 |
2023年 | 173篇 |
2022年 | 340篇 |
2021年 | 414篇 |
2020年 | 305篇 |
2019年 | 323篇 |
2018年 | 239篇 |
2017年 | 340篇 |
2016年 | 371篇 |
2015年 | 371篇 |
2014年 | 703篇 |
2013年 | 703篇 |
2012年 | 818篇 |
2011年 | 1001篇 |
2010年 | 853篇 |
2009年 | 1026篇 |
2008年 | 970篇 |
2007年 | 1083篇 |
2006年 | 1049篇 |
2005年 | 872篇 |
2004年 | 701篇 |
2003年 | 608篇 |
2002年 | 539篇 |
2001年 | 462篇 |
2000年 | 393篇 |
1999年 | 305篇 |
1998年 | 248篇 |
1997年 | 242篇 |
1996年 | 214篇 |
1995年 | 178篇 |
1994年 | 158篇 |
1993年 | 152篇 |
1992年 | 95篇 |
1991年 | 115篇 |
1990年 | 93篇 |
1989年 | 94篇 |
1988年 | 81篇 |
1985年 | 421篇 |
1984年 | 286篇 |
1983年 | 237篇 |
1982年 | 208篇 |
1981年 | 151篇 |
1980年 | 119篇 |
1979年 | 130篇 |
1978年 | 76篇 |
1977年 | 86篇 |
1976年 | 87篇 |
1975年 | 88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 171 毫秒
121.
The construction of an expert-like system for machine scheduling called SCHEDULE is presented. Essential parts of SCHEDULE were developed by students in a laboratory course Operations Research on Microcomputers at the University of Karlsruhe, Germany. SCHEDULE consists of the components data base, knowledge base, inference engine, explanation facility, dialog component, and knowledge acquisition component. The knowledge base contains an algorithm base for solving different types of scheduling problems. To establish the rules of the knowledge base the well-known three-field classification of deterministic machine scheduling problems and the concept of the reduction digraph are exploited. Experiences gained during building and demonstrating SCHEDULE are reported. 相似文献
122.
在H-ZSM-5沸石上甲醇转化为汽油的初始产物分布姜玄珍(浙江大学化学系,杭州310027)R.F.Howe(新南威尔士大学物理化学系,澳大利亚)关键词H-ZSM-5沸石,甲醇转化汽油,产物分布以氢型ZSM-5沸石作催化剂转化甲醇为汽油(MTG),在... 相似文献
123.
详细论述了管理评审的基本概念、如何开展管理评审工作以及中国实验室国家认可委员会的有关政策。 相似文献
124.
O. N. Babkina O. M. Chukanova E. E. Faingol"d N. M. Bravaya 《Russian Chemical Bulletin》2004,53(4):785-790
Quantitative characteristics of changes in the energy of charge transfer from -bonded ligands to metal are presented for the series of structurally similar ansa-metallocene complexes of IVB Group elements. The changes are caused by the solvation effect, replacement of -bonded chlorine ligands by methyl groups and of bridging ethylene groups by dimethylsilylene moieties, introduction of the methyl substituent into position 2 and phenyl substituent into position 4 of the indenyl system, variation of the transition metal in metallocene, and formation of complexes with polymethylalumoxane (MAO) at different AlMAO/Zr ratios. These effects are found to be additive. 相似文献
125.
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
126.
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
127.
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
128.
129.
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
130.
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