全文获取类型
收费全文 | 13221篇 |
免费 | 363篇 |
国内免费 | 34篇 |
专业分类
化学 | 8104篇 |
晶体学 | 236篇 |
力学 | 381篇 |
数学 | 1025篇 |
物理学 | 3872篇 |
出版年
2023年 | 132篇 |
2022年 | 285篇 |
2021年 | 235篇 |
2020年 | 257篇 |
2019年 | 345篇 |
2018年 | 265篇 |
2017年 | 289篇 |
2016年 | 428篇 |
2015年 | 288篇 |
2014年 | 515篇 |
2013年 | 1038篇 |
2012年 | 720篇 |
2011年 | 917篇 |
2010年 | 548篇 |
2009年 | 508篇 |
2008年 | 623篇 |
2007年 | 623篇 |
2006年 | 457篇 |
2005年 | 391篇 |
2004年 | 333篇 |
2003年 | 231篇 |
2002年 | 255篇 |
2001年 | 164篇 |
2000年 | 163篇 |
1999年 | 104篇 |
1998年 | 78篇 |
1997年 | 77篇 |
1996年 | 122篇 |
1995年 | 133篇 |
1994年 | 130篇 |
1993年 | 128篇 |
1992年 | 153篇 |
1991年 | 96篇 |
1990年 | 94篇 |
1989年 | 111篇 |
1988年 | 120篇 |
1987年 | 120篇 |
1986年 | 100篇 |
1985年 | 140篇 |
1984年 | 148篇 |
1983年 | 120篇 |
1982年 | 117篇 |
1981年 | 116篇 |
1980年 | 135篇 |
1979年 | 114篇 |
1978年 | 132篇 |
1977年 | 99篇 |
1976年 | 83篇 |
1975年 | 73篇 |
1974年 | 95篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
The treatment of 3,4-dihydropyrimidin-2-ones with n-BuLi at −78 °C, followed by quenching with various electrophiles furnished N3-substituted derivatives, regioselectively. Further, N1,N3-diacyl derivatives were found to transfer N1-acyl groups to nucleophilic sites. 相似文献
82.
We examine a model system to study the effect of pressure on the surface tension of a vapor-liquid interface. The system is a two-component mixture of spheres interacting with the square-well (A-A) and hard-sphere (B-B) potentials and with unlike (A-B) interactions ranging (for different cases) from hard sphere to strongly attractive square well. The bulk-phase and interfacial properties are measured by molecular dynamics simulation for coexisting vapor-liquid phases for various mixture compositions, pressures, and temperatures. The variation of the surface tension with pressure compares well to values given by surface-excess formulas derived from thermodynamic considerations. We find that surface tension increases with pressure only for the case of an inert solute (hard-sphere A-B interactions) and that the presence of A-B attractions strongly promotes a decrease of surface tension with pressure. An examination of density and composition profiles is made to explain these effects in terms of surface-adsorption arguments. 相似文献
83.
N. L. Chutke M. N. Ambulkar V. K. N. Singh A. N. Garg 《Fresenius' Journal of Analytical Chemistry》1994,350(12):723-726
A multielemental instrumental neutron activation analysis (INAA) method has been employed for the determination of 30 elements in seven garden soil samples from Nagpur City and two intercomparison soil standards from IAEA, Vienna and Institute of Radioecology and Applied Nuclear Techniques (IRANT), Slovak Republic. Environmental implications of elemental concentrations in different soils have been discussed. 相似文献
84.
The o-, m-, and p-phenylene bis(1,3-dioxolanium) dications (4-6) and 2,4,6-triphenylene tris(1,3-dioxolanium) trication (7) have been prepared by the ionization of the corresponding 2-methoxyethyl benzoates in FSO(3)H or CF(3)SO(3)H at 40 and 60 degrees C, respectively. The charge delocalization in these carbocations was probed by (13)C NMR chemical shifts and substantiated by GIAO/DFT calculations. Relatively less charge is delocalized into the aromatic ring of the carbotrication 7. The rotational barrier around the C(+)-Ar bond for carbodications 4 and 5 was also estimated to be 8-10 kcal/mol. 相似文献
85.
Summary The macrocycles 2,10-dimethyl-3,4,8,9,15-pentaazabicyclo-[9.3.1]-pentadeca-1 (15),2,9,11,13-penta-ene-5,7-dione (L) and 2,11-dimethyl-3,4,8,9,10,16-pentaazabicyclo[10.3.1]-hexadeca-1 (16),2,10,12,14-penta-ene-5,8-dione (L) were prepared and characterized by elemental, i.r. and mass spectral data. The macrocycles react with various metal(II) chlorides to yield complexes of the types [MLCl2H2O] (M = Mn, Co, Ni, Cu or Zn), [MLCl2H2O] (M = Mn, Ni, Cu or Zn) and [Co3L2Cl4]Cl2. The complexes were characterized by physico-chemical and spectroscopic methods. 相似文献
86.
A simple, rapid, cost-effective and accurate high performance thin layer chromatographic method has been developed for quantification
of valerenic acid in Valeriana jatamansi and Valeriana officinalis which is one of the stable compounds of Valeriana officinalis and designated as a key marker compound. Valerenic acid makes substantial contribution to the sedative and spasmolytic activity
of the essential oil and extract of Valeriana officinalis. Separation and quantification was achieved by HPTLC using ternary mobile phase of hexane: ethyl acetate: acetic acid (80:20:0.5
v/v) on precoated silica gel 60F254 aluminium plates and densitometric determination was carried out after derivatization with anisaldehyde-sulphuric acid reagent
at 700 nm, in absorption-reflectance mode. The calibration curves were linear in the range of (500 ng–2.5 μg). This is the
first HPTLC report for the identification and quantification of valerenic acid in Valeriana jatamansi and Valeriana officinalis. 相似文献
87.
Total synthesis of (-)-lentiginosine was achieved from D-mannitol using highly stereoselective reactions. Similarly, (+)-lentiginosine was synthesized from L-tartaric acid. 相似文献
88.
R. Dustan Myrex Gary M. Gray Young-Jin Wright Prakash C. Bharara Keith E. Branham 《European Polymer Journal》2003,39(6):1105-1115
Polymeric phosphonate esters are an interesting class of organophosphorus polymers because both the polymer backbone and phosphorus substituents can be modified. These polymers have been prepared by ring-opening polymerizations of cyclic phosphites, stoichiometric polycondensations of dimethyl phosphonate with diols in conjunction with diazomethane treatment and by transesterification of polyphosphonate oligomers. Our initial attempts to prepare high molecular weight polymeric phosphonate esters by the transesterification methods were unsuccessful. Results indicate that the reactions of dimethyl phosphonate with diols to form polyphosphonate oligomers with only methyl phosphonate end groups are plagued by a serious side reaction that forms phosphonic acid end groups. These end groups do not participate in the transesterification reaction and limit the molecular weights of the polymers that can be obtained. The phosphonic acid end groups can be converted into reactive methyl phosphonate end groups by treatment with diazomethane, however diazomethane is explosive and the polymerization is slow. An alternative route for the production of high molecular weight polymers is the transesterification of the 1,12-bis(methyl phosphonato)dodecane, formed by the reaction of excess dimethyl phosphonate and 1,12-dodecanediol, with a Na2CO3 promoter. This allows polymers with molecular weights of up to 4.5×104 to be prepared, and no phosphonic acid end groups are observed in these polymers. Thermal analyses of the poly(1,12-dodecamethylene phosphonate) have shown that this polymer has reasonable thermal stability (onset of thermal decomposition at 273 °C). This polymer also undergoes a cold crystallization process at 15 °C similar to that which has been observed in some polyesters, polyamides and elastomers. 相似文献
89.
Self-assembled multilayer thin films have been prepared on Au substrate by alternate surface derivatization with L-cysteine hydrochloride and cupric perchlorate. The layer-by-layer structure at each step of multilayer formation was investigated by X-ray photoelectron spectroscopy. The measurements indicate that there are two structure modes in the multilayers. One is that Cu(2+) sandwiches between two amino acid groups. The other one is that Cu(+) is bonded through disulfide and thiolate. This process is also confirmed by cyclic voltammetry of Cu ion at different self-assembled multilayers. Steps further on will lead to repeated multilayer films. 相似文献
90.
Bjelopavlic M Singh PK El-Shall H Moudgil BM 《Journal of colloid and interface science》2000,226(1):159-165
Hydrogen bonding is generally thought to be an ubiquitous adsorption mechanism, which often foils selective adsorption schemes. Through investigation of hydrogen bonding energy and its dependence on surface molecular architecture, it may be possible to develop new methodologies to control the adsorption of surfactants and polymeric flocculants, depressants, and dispersants used in particulate processing industries. A model system using St?ber silica spheres and polyethylene oxide, a polymer known for its ability to form hydrogen bonds, was examined. The effect of two different surface treatments of the silica particles, calcination and rehydroxylation, upon the adsorption of two polymer molecular weights was studied. The adsorption behavior was then linked to the respective surface structures via characterization of the surfaces using FTIR, NMR, and Raman techniques. In this paper role of hydrogen bonding sites and surface architecture on adsorption is discussed. Copyright 2000 Academic Press. 相似文献