全文获取类型
收费全文 | 3886篇 |
免费 | 168篇 |
国内免费 | 47篇 |
专业分类
化学 | 2686篇 |
晶体学 | 31篇 |
力学 | 152篇 |
数学 | 354篇 |
物理学 | 878篇 |
出版年
2023年 | 14篇 |
2022年 | 14篇 |
2021年 | 63篇 |
2020年 | 72篇 |
2019年 | 58篇 |
2018年 | 46篇 |
2017年 | 43篇 |
2016年 | 100篇 |
2015年 | 84篇 |
2014年 | 140篇 |
2013年 | 232篇 |
2012年 | 245篇 |
2011年 | 287篇 |
2010年 | 180篇 |
2009年 | 202篇 |
2008年 | 240篇 |
2007年 | 199篇 |
2006年 | 191篇 |
2005年 | 177篇 |
2004年 | 171篇 |
2003年 | 156篇 |
2002年 | 164篇 |
2001年 | 105篇 |
2000年 | 112篇 |
1999年 | 60篇 |
1998年 | 55篇 |
1997年 | 48篇 |
1996年 | 62篇 |
1995年 | 50篇 |
1994年 | 45篇 |
1993年 | 44篇 |
1992年 | 29篇 |
1991年 | 27篇 |
1990年 | 24篇 |
1989年 | 32篇 |
1988年 | 16篇 |
1987年 | 25篇 |
1986年 | 26篇 |
1985年 | 33篇 |
1984年 | 28篇 |
1983年 | 29篇 |
1982年 | 19篇 |
1981年 | 21篇 |
1980年 | 12篇 |
1979年 | 13篇 |
1978年 | 12篇 |
1977年 | 16篇 |
1976年 | 20篇 |
1975年 | 17篇 |
1974年 | 10篇 |
排序方式: 共有4101条查询结果,搜索用时 187 毫秒
901.
Two weeks after the Chernobyl Nuclear Power Plant accident, a substantial increase in radioactivity above normal background levels was observed in various samples taken in Taiwan, R. O. C., which is 7600 km from Chernobyl. The131I concentrations in grass, rainwater, and milk were monitored continuously in succeeding weeks and correlations with weather conditions are discussed. Levels of radiation fallout over Taiwan due to the Chernobyl accident are much lower than the response levels recommended by local authorities and pose no danger to the public. 相似文献
902.
A E Almeida A G Ferreira M S Crespi Z A Andrade M C Chung 《Journal of Thermal Analysis and Calorimetry》2006,83(2):277-281
Oxamniquine polymeric prodrug with potential antischistosomal
activity was prepared using dextran T-70 as a carrier, which was analysed
by 1HNMR, 13C NMR and
IR spectroscopy. The formation of the oxamniquine salt was confirmed by thermogravimetric
analysis (TG) and differential scanning calorimetry (DSC) which showed a different
thermal behaviour when compared to the physical mixture. 相似文献
903.
Taek Dong Chung 《Journal of inclusion phenomena and macrocyclic chemistry》1998,32(2-3):179-193
The electrochemistry of calixarene as a redox-dependent ionophore and its structural dependence are described. One or more redox-centers such as quinone, ferrocene, cobaltocenium and ruthenium bipyridine moieties have been introduced into the calixarene frame of the lower or upper rim. Although the electrochemical behavior depends mainly on the inherent redox property of these electrochemically active groups, the structural effect and solvent also play important roles, especially, in the presence of charged guests. When cationic species such as metal ions and ammonium ion are added to a quinone-functionalized calixarene solution, electron transfer to quinone is enhanced by the electrostatic effect or the formation of hydrogen bonds. In addition to redox-active hosts for voltammetric use, a number of calixarenes with novel structures have been developed as ionophores for potentiometric analysis and found to be successful for some target ions. In terms of Na+, Cs+ and Ca2+ selective ionophores for ion-selective electrodes, calixarenes are found to be excellent compared to crown ether derivatives or cryptands. Calixarenes can be also utilized to construct chemically modified electrodes, which are sensitive to gas species and biologically important compounds. The sophisticated design and synthesis of calixarenes are essential to specific potential applications to diverse fields. 相似文献
904.
Calorimetric measurements of the enthalpy of reaction of WO3(c) with excess OH?(aq) have been made at 85°C. Similar measurements have been made with MoO3(c) at both 85 and 25°C, to permit estimation of ΔH°=?13.4 kcal mol?1 for the reaction WO3(c)+2OH?(aq)=WO2?4(aq)+H2O(liq) at 25°C. Combination of this ΔH° with ΔH°f for WO3(c) leads to ΔH°f=?256.5 kcal mol?1 for WO2?4(aq). We also obtain ΔH°f=?269.5 kcal mol?1 for H2WO4(c). Both of these values are discussed in relation to several earlier investigations. 相似文献
905.
Polyethersulfone (PES) hollow fiber membranes for kidney dialysis application were prepared by the dry-jet wet-spinning method. A dual-coagulation bath technology was first time employed for fabricating the kidney dialysis membranes with a tight inner skin and loose outer supporting layer structure. A weak coagulant isopropanol (IPA) was served as the first external coagulation bath, while water as the second bath. Experiments demonstrate their advantages of better controlling both inner and outer skin morphology. The as-spun fibers have a higher mean effective pore size (μp), pure water permeation flux (PWP) and molecular weight cut-off (MWCO) with an increase in N-methyl-2-pyrrolidone (NMP) percentage in bore fluid (i.e., internal coagulant). After being treated in 8000 ppm NaOCl solution for 1 day, fibers show larger pore sizes and porosity in both inner and outer surfaces, and thinner inner and outer layers than their as-spun counterparts. Among them, the bleached fibers spun with 50 wt.% NMP in bore fluid have the MWCO (43 kDa) and PWP (40 × 10−5 L m−2 Pa−1 h−1) suitable for kidney dialysis application. Based on SEM observations and solute rejection performance, the further heat treated fibers in an aqueous solution is found to be an effective way to fine tune membranes morphology and MWCO for kidney dialysis application. The solute rejection performance data of the hollow fiber membranes spun with 55 wt.% NMP in bore fluid after heat treated at 90 °C in water for 2 h were found to be very appropriate for the kidney dialysis application. 相似文献
906.
A novel synthetic biodegradable oligomer based upon methacrylated aminocaproyl maleamic acid (MACMA), was synthesized and characterized. Injectable and in situ crosslinkable polymer networks were formulated by copolymerization of MACMA with triethyleneglycol dimethacrylate (TEGDMA). In addition, composites composed of MACMA, TEGDMA and beta-tricalcium phosphate (β-TCP) were prepared. The networks and composites were initiated by photo- and redox-polymerization, respectively. The initial compressive (CS) and diametral tensile strengths (DTS) of these materials were determined and used to evaluate the effects of MACMA/TEGDMA ratios on the degradation behavior of the materials. The neat resin networks exhibited initial CS values ranging from 6.7 to 284.2 MPa and the composites demonstrated initial DTS values ranging from 2.8 to 20.8 MPa and CS values ranging from 19.1 to 119.5 MPa. During the course of degradation the polymer neat resins lost 51%, 69% and 61% of their initial CS after 3 weeks for the MACMA/TEGDMA ratios at 25/75, 50/50 and 75/25, respectively. The resin with the MACMA/TEGDMA ratio of 75/25 completely degraded after 6 months. The composite with the MACMA/TEGDMA ratio of 25/75 exhibited a significant increase in CS after an initial decrease for 7 days and then lost 57% of its initial CS after 3 months. The composite composed of poly(MACMA) homopolymer showed a complete degradation after 21 days. 相似文献
907.
The distillation characteristics of osmium and ruthenium, which have been studied by using both radioactive tracers and inactive compounds, are reported. The tracer solutions were prepared from hexachloro and hexabromo compounds and from the purified sponges of both metals. A quantitative separation of osmium from ruthenium can be made by distillation from peroxide-sulphuric acid solution at 110 degrees for about 40 min. Slow and incomplete distillations are observed when osmium is present as either the chloro or bromo complex. Optimum conditions for the dissolution of acid-insoluble materials containing osmium and ruthenium are discussed. 相似文献
908.
The total syntheses of furan and thiophene PAH's related to acephenanthrylene are reported. Cyclobutanones which were obtained by [2 + 2] addition of ketenes with 2,3-dihydrofuran or 2,3-dihydrothiophene regioselectively could be converted into angular PAH skeleton molecules by rearrangements with polyphosphoric acid. 2,3-Dichloro-5,6-dicyanobenzoquinone was applied for aromatization to achieve the formation of PAH's. 相似文献
909.
Deok-Heon Kweon Ju-Wha Chung Su-Dong Cho Sung-Kyu Kim Yong-Jin Yoon 《Journal of heterocyclic chemistry》1998,35(6):1401-1403
Some 1-alkyl-4-(4-amino-2,6-dichlorophenoxy)-5-halopyridazin-6-ones were synthesized chemoselectively from 1-alkyl-4,5-dihalopyridazin-6-ones and 4-amino-2,6-dichlorophenol via a fluoride ion-assisted reaction. 相似文献
910.
The crystal structure of [Co2(μ-dmg)2(μ-dmg H)(dmg H)(Pø3)] · 1/2 dmg H2 · 1/2 CH3OH has been determined by single crystal X-ray diffraction. The title complex crystallizes in triclinic space group P&1bar; with a=10.859(3), b=13.299(3), c=15.247(3)Å, α=98.52(2), β=99.26(2), γ=97.80(2)° and Z=2. The agreement indices are R(F)=0.062 and Rw(F)=0.085 for 5231 observations and 515 variables. Both cobalt atoms have an oxidation state of 3+ and are both six-coordinated; one Co(III) atom is bonded to a phosphorous atom of triphenylphosphine, four nitrogen atoms of two planar ligands, dmg2? and dmgH?, and one oxygen atom of another dmg2? ligand. The other Co(III) atom is coordinated by four nitrogen atoms of two cis nonplanar ligands, dmg2?, dmgH?, and two oxygen atoms of the other dmg2? and dmgH? ligands. There are three N-O bridged ligands namely two dmg2? and one dmgH? between two Co(III) atoms. A free molecule of dmgH2 and a molecule of CH3OH are also found in the crystal lattice. The structure interestingly demonstrates the possible various forms of dmg, not only on their charge but also in different conformations upon coordination. 相似文献