全文获取类型
收费全文 | 3897篇 |
免费 | 129篇 |
国内免费 | 13篇 |
专业分类
化学 | 2696篇 |
晶体学 | 31篇 |
力学 | 100篇 |
数学 | 584篇 |
物理学 | 628篇 |
出版年
2023年 | 38篇 |
2022年 | 57篇 |
2021年 | 94篇 |
2020年 | 92篇 |
2019年 | 88篇 |
2018年 | 53篇 |
2017年 | 37篇 |
2016年 | 124篇 |
2015年 | 100篇 |
2014年 | 114篇 |
2013年 | 218篇 |
2012年 | 246篇 |
2011年 | 297篇 |
2010年 | 148篇 |
2009年 | 131篇 |
2008年 | 194篇 |
2007年 | 193篇 |
2006年 | 209篇 |
2005年 | 175篇 |
2004年 | 122篇 |
2003年 | 128篇 |
2002年 | 113篇 |
2001年 | 46篇 |
2000年 | 66篇 |
1999年 | 34篇 |
1998年 | 26篇 |
1997年 | 35篇 |
1996年 | 32篇 |
1995年 | 31篇 |
1994年 | 31篇 |
1993年 | 18篇 |
1992年 | 30篇 |
1991年 | 29篇 |
1990年 | 29篇 |
1989年 | 25篇 |
1988年 | 23篇 |
1985年 | 19篇 |
1984年 | 27篇 |
1983年 | 33篇 |
1982年 | 45篇 |
1981年 | 39篇 |
1980年 | 36篇 |
1979年 | 36篇 |
1978年 | 25篇 |
1977年 | 27篇 |
1976年 | 23篇 |
1975年 | 26篇 |
1974年 | 23篇 |
1973年 | 29篇 |
1972年 | 19篇 |
排序方式: 共有4039条查询结果,搜索用时 125 毫秒
21.
Ross W. Mair David G. Cory Sharon Peled Ching-Hua Tseng Samuel Patz Ronald L. Walsworth 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,135(2):478-486
Pulsed-field-gradient NMR techniques are demonstrated for measurements of time-dependent gas diffusion. The standard PGSE technique and variants, applied to a free gas mixture of thermally polarized xenon and O2, are found to provide a reproducible measure of the xenon diffusion coefficient (5.71 × 10−6m2s−1for 1 atm of pure xenon), in excellent agreement with previous, non-NMR measurements. The utility of pulsed-field-gradient NMR techniques is demonstrated by the first measurement of time-dependent (i.e., restricted) gas diffusion inside a porous medium (a random pack of glass beads), with results that agree well with theory. Two modified NMR pulse sequences derived from the PGSE technique (named the Pulsed Gradient Echo, or PGE, and the Pulsed Gradient Multiple Spin Echo, or PGMSE) are also applied to measurements of time dependent diffusion of laser polarized xenon gas, with results in good agreement with previous measurements on thermally polarized gas. The PGMSE technique is found to be superior to the PGE method, and to standard PGSE techniques and variants, for efficiently measuring laser polarized noble gas diffusion over a wide range of diffusion times. 相似文献
22.
The recently proposed Harbola–Sahni local potential in its exchange-only version [Phys. Rev. Lett. 62 , 689 (1989)] is found to give the static dipole and quadrupole polarizabilities for the Neisoelectronic series in excellent agreement with the corresponding Hartree–Fock estimates. © 1992 John Wiley & Sons, Inc. 相似文献
23.
By means of exponential laws we show a Bianchi Type I model of the universe where we define overall deceleration and Hubble's parameters that have constant values. We employ a convenient relation for defining the radius of the universe. Pressure and density have constant values, too. 相似文献
24.
Gregory H. Robinson Fritz Moise William T. Pennington Samuel A. Sangokoya 《Journal of chemical crystallography》1988,18(4):387-393
The crystalline condensation product [Al(CH3)2]2[C10H22N4][Al(CH3)3]2 is prepared from reaction of the multidentate open-chain amine 1,4-bis(3-aminopropyl)piperazine with trimethylaluminum in methylene chloride. The compound crystallized in the monoclinic space groupC2/c with unit cell parametersa=25.166(12),b=7.268(4),c=17.316(4) Å,=115.3(4)°, andD
c
=1.06 g cm–3 forZ=4. Least-squares refinement based on 1054 observed reflections with intensitiesI 3(I) in the range 2<2<45° led to a finalR factor of 0.053 (R
w
=0.064). The title compound resides on a crystallographic center of symmetry with independent Al-N distances of 1.889(5), 2.058(5), and 2.030(5) Å. 相似文献
25.
26.
27.
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
28.
Samuel Natelson 《Microchemical Journal》1991,44(3)
When glutamic acid is amidinated, guanidinoglutaric acid is formed. This is readily converted by boiling in water to its lactam [2-imino-4-oxo-5-(3′-propane acid) imidazolidine; GGAL]. In the present study GGAL is dehydrated to form a second lactam, anhydro-GGAL (AGGAL), with trifluoroacetic anhydride. The molecular weight of this compound was ascertained gravimetrically by precipitation with flavianic acid and by mass spectrography to be 153.14. UV and infrared spectra and hydrogen and 13C NMR studies determined its structure as an imidazolidine ring fused to a pyrrol ring, 2-imino-4-oxo-imidazolidine-1,5-pyrrolidone-8. On paper electrophoresis, GGAL traveled to the anode, while AGGAL traveled to the cathode. On TLC plates (butanol 60, H2O 25, acetic 15), the Rf value of GGAL was 0.42 and that of AGGAL was 0.38, confirming that AGGAL was more basic than GGAL. The preparation of 1,3-di-(phenylcarbamoyl)-GGAL is also described. 相似文献
29.
30.
R. R. Mendel Kimball A. Milton James H. Reid Mark A. Samuel 《Zeitschrift fur Physik C Particles and Fields》1986,32(4):517-520
Several methods to determine (or set an upper bound on) the mass of the tau neutrino are described. The decay spectrum of \(\tau \to e\bar \nu _e \nu _\tau \) near the high center-of-mass electron-energy end is particularly sensitive to the tau-neutrino mass. It is shown that an upper bound of 20 MeV or lower may be feasible. In addition, improved limits on the mass and mixing angle of a heavy component of the tau-neutrino may be readily obtained. We also discuss the purely leptonic decays of theD andF mesons, and show how measurements of the ratiosB(D→τντ/B(D→μνμ,B(F→τντ/B(D→μνμ can be sensitive to tau-neutrino masses of 30 MeV or greater. 相似文献