全文获取类型
收费全文 | 7406篇 |
免费 | 178篇 |
国内免费 | 47篇 |
专业分类
化学 | 4824篇 |
晶体学 | 57篇 |
力学 | 180篇 |
综合类 | 1篇 |
数学 | 1014篇 |
物理学 | 1555篇 |
出版年
2022年 | 42篇 |
2021年 | 58篇 |
2020年 | 78篇 |
2019年 | 82篇 |
2018年 | 55篇 |
2017年 | 71篇 |
2016年 | 131篇 |
2015年 | 150篇 |
2014年 | 157篇 |
2013年 | 390篇 |
2012年 | 336篇 |
2011年 | 484篇 |
2010年 | 215篇 |
2009年 | 184篇 |
2008年 | 443篇 |
2007年 | 423篇 |
2006年 | 407篇 |
2005年 | 411篇 |
2004年 | 328篇 |
2003年 | 277篇 |
2002年 | 259篇 |
2001年 | 133篇 |
2000年 | 132篇 |
1999年 | 67篇 |
1998年 | 76篇 |
1997年 | 78篇 |
1996年 | 124篇 |
1995年 | 103篇 |
1994年 | 103篇 |
1993年 | 102篇 |
1992年 | 84篇 |
1991年 | 91篇 |
1990年 | 74篇 |
1989年 | 49篇 |
1988年 | 60篇 |
1987年 | 38篇 |
1986年 | 51篇 |
1985年 | 94篇 |
1984年 | 78篇 |
1983年 | 65篇 |
1982年 | 93篇 |
1981年 | 93篇 |
1980年 | 83篇 |
1979年 | 87篇 |
1978年 | 83篇 |
1977年 | 96篇 |
1976年 | 78篇 |
1975年 | 79篇 |
1974年 | 58篇 |
1973年 | 63篇 |
排序方式: 共有7631条查询结果,搜索用时 0 毫秒
41.
Appleby I Boulton LT Cobley CJ Hill C Hughes ML de Koning PD Lennon IC Praquin C Ramsden JA Samuel HJ Willis N 《Organic letters》2005,7(10):1931-1934
Two methods to produce (2S)-5-amino-2-(1-n-propyl-1H-imidazol-4-ylmethyl)-pentanoic acid were investigated. Diastereoisomeric salt resolution, using the quinidine salt, gave the desired intermediate in 98% ee and 33% yield. Asymmetric hydrogenation of various substrates gave high conversions, with up to 83% ee. Integration of these two approaches via asymmetric hydrogenation of a quinidine salt substrate followed by crystallization provided the desired intermediate in 94% ee and 76% yield. 相似文献
42.
Kurt V. Schenker Wolfgang Von Philipsborn C. Anderson Evans Werner Skuballa Georg-Alexander Hoyer 《Helvetica chimica acta》1986,69(7):1718-1727
The configurations of the 5,6-double bond in the carbacyclins iloprost ( 3 ; (E)) and isoiloprost ( 4 ; (Z)) are based on a complete assignment of the 13C and 1H resonances determined by 1D and 2D 13C-NMR and 1H-NMR methods. 相似文献
43.
Using flow microcalorimetry, the ion association reaction M2+(aq)+Fe(CN)
6
4–
(aq)=MFe(CN)
6
2–
(aq) (M=Ca, Mg) has been studied at 25°C over the ionic strength range 0.02 to 0.08 mol-dm–3. Analyses of the data to obtain Ho, the enthalpy change at infinite dilution, are described. The value obtained for Ho is sensitive to the kind of functions used to correct for non-ideal behavior. 相似文献
44.
The demetalation process of 10 multi-iron Wells-Dawson polyoxometalates is studied by cyclic voltammetry and controlled potential coulometry. Eight sandwich-type complexes (alphaalphaalphaalpha-Na(16)[(NaOH(2))(2)(Fe(III))(2)(X(2)W(15)O(56))(2)], alphaalphabetaalpha-Na(14)[(NaOH(2))(Fe(III)OH(2))(Fe(III))(2)(X(2)W(15)O(56))(2)], alphabetabetaalpha-Na(12)[(Fe(III)OH(2))(2)(Fe(III))(2)(X(2)W(15)O(56))(2)], and alphabetabetaalpha-Na(14)[(Mn(II)OH(2))(2)(Fe(III))(2)(X(2)W(15)O(56))(2)] (where X = P(V) or As(V))) and two monomeric complexes (alpha-Na(11)[(P(2)(Fe(III)Cl)(2)(Fe(III)OH(2))W(15)O(59))] and alpha-Na(11)[(As(2)(Fe(III)Cl)(2)Fe(III)OH(2))W(15)O(59))]) were selected for this study. All 10 complexes show Fe(III) waves which are well-separated from the redox activity of the W(VI) centers. At room temperature and under mild conditions, iron release from the complexes is observed upon reduction of the Fe(III) centers. This release is controlled by the ionic strength of the medium, the nature and concentration of the anions present in the supporting electrolyte, and by the pH of the solution. This behavior parallels those described for most siderophores which depend on the same parameters. 相似文献
45.
Ian K. Boddy J. Boniface Richard C. Cambie Peter A. Craw David S. Larsen Hamish McDonald Peter S. Rutledge Paul D. Woodgate 《Tetrahedron letters》1982,23(42):4407-4408
Gentle heating of allyloxyanthraquinones with sodium dithionite in dimethylformanide - water effects a rapid and controlled rearrangement to give high yields of 2-allyanthraquinones. 相似文献
46.
Daniel E. Lynch Gillian E. Spicer Ian Mcclenaghan 《Journal of heterocyclic chemistry》2005,42(7):1363-1368
Three α‐phenylmalonamides have been prepared by the selective nucleophilic cleavage of 5,7‐dimethyl‐2‐phenyl‐1‐oxo‐1H‐pyrazolo[1,2‐a]pyrazol‐4‐ylium‐3‐olate in solventless microwave syntheses. The three weak nucleophiles employed were aniline, p‐chloroaniline and m‐toluidine. The α‐phenylmalonamides of these three aniline derivatives could not be prepared using the previously reported solvent syntheses via 3‐oxopyrazolo[1,2‐a]pyrazol‐8‐ylium‐1‐olates. All products were characterised using, infrared spectroscopy, 1H nmr and electrospray mass spectrometry. The single crystal X‐ray structures of the starting pyrazolo‐[1,2‐a]pyrazole and α‐phenylmalon‐m‐toluidide are also reported. 相似文献
47.
48.
49.
Optical studies of pulsed-laser fragmentation of biliary calculi 总被引:2,自引:0,他引:2
P. Teng N. S. Nishioka R. Rox Anderson T. F. Deutsch 《Applied physics. B, Lasers and optics》1987,42(2):73-78
The fragmentation of gallstones and kidney stones using pulsed visible laser radiation has recently been demonstrated; however, the fragmentation mechanism is not well understood. The temporal and spectral characteristics of the bright flash of light accompanying fragmentation of gallstones were studied using 0.8 and 360-s-long, 690-nm-wavelength, dye-laser pulses. Time-resolved visible emission spectra show a broad continuum upon which line spectra are superimposed. The continuum emission is due to free-free and free-bound electron transitions indicative of a plasma and the line spectra are due to neutral and ionized calcium. Initiation of this plasma is fluence rather than intensity dependent. A model is proposed in which laser energy is coupled to the plasma, which then impulsively expands, generating intense acoustic transients which fracture the stone. 相似文献
50.
We consider the statistical mechanics of the traveling salesman problem (TSP) and develop some representations to study it. In one representation the mean field theory has a simple form and brings out some of the essential features of the problem. It shows that the system has spontaneous symmetry breaking at any nonzero temperature. In general the phase progressively changes as one decreases the temperature. At low temperatures the mean field theory solution is very sensitive to any small perturbations, due to the divergence of some local susceptibilities. This critical region extends down to zero temperature. We perform the quenched average for a nonmetric TSP in the second representation and the resulting problem is more complicated than the infinite-range spin-glass problem, suggesting that the free energy landscape may be more complex. The role played by frustration in this problem appears explicitly through the localization property of a random matrix, which resembles the tight binding matrix of an electron in a random lattice. 相似文献