全文获取类型
收费全文 | 6673篇 |
免费 | 545篇 |
国内免费 | 357篇 |
专业分类
化学 | 4173篇 |
晶体学 | 62篇 |
力学 | 309篇 |
综合类 | 40篇 |
数学 | 842篇 |
物理学 | 2149篇 |
出版年
2023年 | 76篇 |
2022年 | 153篇 |
2021年 | 211篇 |
2020年 | 258篇 |
2019年 | 211篇 |
2018年 | 179篇 |
2017年 | 165篇 |
2016年 | 269篇 |
2015年 | 279篇 |
2014年 | 263篇 |
2013年 | 446篇 |
2012年 | 460篇 |
2011年 | 525篇 |
2010年 | 344篇 |
2009年 | 328篇 |
2008年 | 394篇 |
2007年 | 384篇 |
2006年 | 339篇 |
2005年 | 289篇 |
2004年 | 237篇 |
2003年 | 207篇 |
2002年 | 177篇 |
2001年 | 146篇 |
2000年 | 91篇 |
1999年 | 105篇 |
1998年 | 65篇 |
1997年 | 58篇 |
1996年 | 70篇 |
1995年 | 61篇 |
1994年 | 73篇 |
1993年 | 65篇 |
1992年 | 57篇 |
1991年 | 41篇 |
1990年 | 47篇 |
1989年 | 32篇 |
1988年 | 25篇 |
1987年 | 26篇 |
1986年 | 21篇 |
1985年 | 33篇 |
1984年 | 43篇 |
1983年 | 21篇 |
1982年 | 36篇 |
1981年 | 20篇 |
1980年 | 24篇 |
1979年 | 42篇 |
1978年 | 27篇 |
1977年 | 19篇 |
1976年 | 23篇 |
1975年 | 17篇 |
1974年 | 22篇 |
排序方式: 共有7575条查询结果,搜索用时 296 毫秒
141.
Fluorocarbons containing higher halogens have the potency of breaking certain hydrogen bonds in solutions. The relative strength of this potency varies in the series F < Cl < Br < I and the presence of hydrogen atoms in the fluorocarbon increases it. There is a striking parallelism between the hydrogen bond breaking and the anaesthetic potency of these molecules. It is suggested that the breaking of hydrogen bonds is an important step in the mechanism of anaesthesia. 相似文献
142.
Nonisothermal kinetics of the solid‐solid phase transition in (n‐C10H21NH3)2ZnCl4(C10Zn), (n‐C16H33NH3)2ZnCl4(C16Zn) and their binary system were determined by Kissinger and Ozawa methods from DSC measurements. The activation energy Ea of the binary system shows a waving dependence on WC10Zn%, which is caused by not only an intermediate (C10H21NH3)(n‐C16H33NH3)ZnCl4 but also three solid solution ranges (α, β, γ) in the phase diagram of C10Zn‐C16Zn. The variations of the layer d‐spacing are also convenient for the above result. 相似文献
143.
Carlo Carlini Marco Di Girolamo Alessandro Macinai Mario Marchionna Marilena Noviello Anna Maria Raspolli Galletti Glauco Sbrana 《Journal of molecular catalysis. A, Chemical》2003,200(1-2):137-146
The synthesis of isobutanol via the Guerbet condensation between methanol and ethanol was studied by using sodium methoxide (MeONa) as soluble basic component and copper-based catalysts as heterogeneous dehydrogenating/hydrogenating metal species. The effect of the nature of the catalyst and the relative amount of its individual components with respect to the reacting alcohols as well as of temperature on productivity and selectivity of the process was investigated. The collected data indicated that the copper chromite/MeONa was more active than Cu-Raney/MeONa system. The reaction was shown to proceed with the formation only of n-propanol and isobutanol. Ethanol conversion up to 61% with selectivity to isobutanol up to 98.4% was obtained. The same catalytic systems were also employed in the reaction of the methanol/ethanol/n-propanol ternary mixture. Again copper chromite/MeONa resulted more active than the Cu-Raney/MeONa system. Finally, experiments were carried out on methanol/n-propanol mixtures in the presence of the copper chromite/MeONa catalytic system by recycling both the recovered solid copper component and the liquid reaction mixture for evidencing eventual copper leaching by MeONa. On the basis of the obtained results it was concluded that in the Guerbet reaction copper chromite works as heterogeneous catalyst. 相似文献
144.
Di Bussolo V Caselli M Romano MR Pineschi M Crotti P 《The Journal of organic chemistry》2004,69(21):7383-7386
The reaction of alpha vinyl oxirane 5, prepared through a new route to the d-gulal system, with O-nucleophiles (alcohols and di-O-isopropylidene-alpha-d-monosaccharides) and C-nucleophiles (lithium alkyls) affords, in a completely stereoselective way, the corresponding 2-unsaturated alpha O- and C-glycosides having the same configuration as the starting epoxide. 相似文献
145.
氢化物发生—分光光度法测定染发剂中的微量铅 总被引:1,自引:0,他引:1
本文提出了用铁氰化钾将铅氧化至高价,再用氢化物发生-分光光度法测定铅的一种新方法。该法灵敏度高,简便快速,用于分析染发剂中的铅,取得了较好的效果。 相似文献
146.
The weakly absorbed 1.2-μ combination band of water was accurately studied as a function of temperature, in both its profile and first derivative, to obtain information on the bulk properties of liquid water. Arrhenius plots of integrated component intensity ratios showed well aligned experimental points, except for a neatly defined break occurring in the 30 to 40°C temperature interval for pure water A similar break, shifted by some 18°C towards lower temperature, was found in the case of 1M NaClO4 aqueous solutions. The breaks are tentatively assumed to result from abrupt though subtle changes involving low-frequency modes. This tentative, assumption appears independent of specific models and assignments and, although in agreement with available neutron-scattering and NMR data, needs more direct verification. 相似文献
147.
一种分离抗凝血酶Ⅲ的高效亲和色谱填料的制备及表征 总被引:6,自引:0,他引:6
以聚甲基丙烯酸环氧丙酯微球 ( PGMA)为基质 ,利用还原氨化方法将肝素键合在其表面上 ,得到一种以肝素为配基的高效亲和色谱填料 ,该填料对抗凝血酶 的亲和容量为 1 .2 mg/g,对抗凝血酶 和人血清白蛋白的回收率分别为 96.9%和 94 .4 % .利用前沿色谱法测定了肝素与抗凝血酶 之间的表观解离常数为 1 .96× 1 0 - 5mol/L.利用该填料可直接从人血浆中分离出抗凝血酶 相似文献
148.
R. CurciF. Di Furia 《Tetrahedron》1972,28(14):3905-3913
The influence of OH− concentration, in the solvent medium dioxane-water 40:60 at 25·0, on the oxidation rates of diphenylphosphine oxide (DPPO) by t-BuOOH, H2O2 and p-nitroperoxybenzoic acid, and of bis-p-tolylphosphine oxide by t-BuOOH has been investigated. For the oxidation of the phosphorus substrates by the hydroperoxides above, the rate law found (R = k2″[Ar2PHO] [ROO−]) differs from the rate law confirmed to hold for the oxidation of DPPO by peroxyacids in alkaline media, i.e.: R = (k2″ + k3[OH−])[Ar2PHO] [RCO3−]. This is interpreted on the basis of one common reaction mechanism involving formation of an intermediate of similar structure, wherein a change in the rate controlling step is likely to occur on passing from p-O2N·C6H4CO3− to t-BuOO−. In the same solvent medium, pKa′ values for t-BuOOH and H2O2 have been estimated. 相似文献
149.
Albano VG Di Serio M Monari M Orabona I Panunzi A Ruffo F 《Inorganic chemistry》2002,41(10):2672-2677
The silver-assisted ligand metathesis reaction involving a platinum(II) complex of formula [PtClMe(N,N-chelate)] with acetonitrile has been investigated. By using a suitably hindered N,N-chelate, an otherwise hardly detectable trinuclear species has been isolated and characterized through X-ray diffractometry. The trinuclear cation consists of two nearly orthogonal [PtCl(Me)(N,N-chelate)] square-planar units entrapping an Ag(+) cation through the chloride ligands that, acting as bidentate, form a linear AgCl(2) unit with two nonequivalent Ag-Cl bonds. The residual acidity of the silver cation is satisfied by one secondary Ag-Pt interaction [Ag-Pt(1) = 2.82 A] in which the platinum atom acts as a donor. Kinetic studies have demonstrated that the silver assistance operates both through a simple associative step and through a pathway in which the above trinuclear complex is an active intermediate. In a noncoordinating solvent the latter species evolves with AgCl loss and formation of a dinuclear Pt,Pt complex showing a rare single chloride bridge. 相似文献
150.
de Souza BM Marques MR Tomazela DM Eberlin MN Mendes MA Palma MS 《Rapid communications in mass spectrometry : RCM》2004,18(10):1095-1102
The social wasp P. paulista is relatively common in southeast Brazil causing many medically important stinging incidents. The seriousness of these incidents is dependent on the amount of venom inoculated by the wasps into the victims, and the characteristic envenomation symptoms are strongly dependent on the types of peptides present in the venom. In order to identify some of these naturally occurring peptides available in very low amounts, an analytical protocol was developed that uses a combination of reversed-phase and normal-phase high-performance liquid chromatography (HPLC) of wasp venom for peptide purification, with matrix-assisted laser desorption/ionization time-of-flight post-source decay mass spectrometry (MALDI-Tof-PSD-MS) and low-energy collision-induced dissociation (CID) in a quadrupole time-of-flight tandem mass spectrometry (QTof-MS/MS) instrument for peptide sequencing at the sub-picomole level. The distinction between Leu and Ile was achieved both by observing d-type fragment ions obtained under CID conditions and by comparison of retention times of the natural peptides and their synthetic counterparts (with different combinations of I and/or L at N- and C-terminal positions). To distinguish the isobaric residues K and Q, acetylation of peptides was followed by Q-Tof-MS analysis. The primary sequences obtained were INWLKLGKMVIDAL-NH(2) (MW 1611.98 Da) and IDWLKLGKMVMDVL-NH(2) (MW 1658.98 Da). Micro-scale bioassay protocols characterized both peptides as presenting potent hemolytic action, mast cell degranulation, and chemotaxis of polymorphonucleated leukocyte (PMNL) cells. The primary sequences and the bioassay results suggest that these toxins constitute members of a new sub-class of mastoparan toxins, directly involved in the occurrence of inflammatory processes after wasp stinging. 相似文献