全文获取类型
收费全文 | 7837篇 |
免费 | 150篇 |
国内免费 | 88篇 |
专业分类
化学 | 4520篇 |
晶体学 | 80篇 |
力学 | 157篇 |
数学 | 1711篇 |
物理学 | 1090篇 |
无线电 | 517篇 |
出版年
2020年 | 74篇 |
2019年 | 63篇 |
2018年 | 88篇 |
2017年 | 52篇 |
2016年 | 143篇 |
2015年 | 113篇 |
2014年 | 131篇 |
2013年 | 414篇 |
2012年 | 325篇 |
2011年 | 377篇 |
2010年 | 236篇 |
2009年 | 212篇 |
2008年 | 341篇 |
2007年 | 343篇 |
2006年 | 414篇 |
2005年 | 381篇 |
2004年 | 339篇 |
2003年 | 292篇 |
2002年 | 296篇 |
2001年 | 97篇 |
2000年 | 87篇 |
1999年 | 90篇 |
1998年 | 109篇 |
1997年 | 107篇 |
1996年 | 148篇 |
1995年 | 95篇 |
1994年 | 88篇 |
1993年 | 85篇 |
1992年 | 96篇 |
1991年 | 89篇 |
1990年 | 71篇 |
1989年 | 68篇 |
1988年 | 61篇 |
1987年 | 74篇 |
1986年 | 69篇 |
1985年 | 109篇 |
1984年 | 129篇 |
1983年 | 96篇 |
1982年 | 99篇 |
1981年 | 124篇 |
1980年 | 135篇 |
1979年 | 102篇 |
1978年 | 124篇 |
1977年 | 92篇 |
1976年 | 100篇 |
1975年 | 109篇 |
1974年 | 103篇 |
1973年 | 102篇 |
1972年 | 55篇 |
1970年 | 50篇 |
排序方式: 共有8075条查询结果,搜索用时 9 毫秒
81.
Hill CD 《Physical review letters》2007,98(18):180501
There are many cases where the interaction between two qubits is not precisely known, but single-qubit operations are available. In this Letter we show how, regardless of an incomplete knowledge of the strength or form of the interaction between two qubits, it is often possible to construct a controlled-NOT gate which has arbitrarily high fidelity. In particular, we show that oscillations in the strength of the exchange interaction in solid state Si and Ge structures are correctable. 相似文献
82.
Motivated by our desire to understand the biophysical mechanisms underlying the swimming of sperm in the non-Newtonian fluids of the female mammalian reproductive tract, we examine the swimming of filaments in the nonlinear viscoelastic upper convected Maxwell model. We obtain the swimming velocity and hydrodynamic force exerted on an infinitely long cylinder with prescribed beating pattern. We use these results to examine the swimming of a simplified sliding-filament model for a sperm flagellum. Viscoelasticity tends to decrease swimming speed, and changes in the beating patterns due to viscoelasticity can reverse swimming direction. 相似文献
83.
Coupled-cluster investigations (CCSD/cc-pVDZ and CCSD/cc-pVQZ//CCSD/cc-pVDZ) of singlet cyclobutadiene and fifteen-substituted analogs were conducted. A local minimum with a square frame does not exist on their potential surfaces. The well-known rectangular D2h minimum, the square D4h transition state, and two additional stationary points were found on cyclobutadiene’s potential surface. This included a transition state with a rhombic carbon ring and C2h symmetry, separating two equivalent puckered C2v local minima. The predicted barriers were 19.7 and 19.8 kcal/mol at the CCSD/cc-pVDZ and CCSD/cc-pVQZ//CCSD/cc-pVDZ levels, respectively. The relative strain energies of rectangular D2h cyclobutadiene and all fifteen-substituted analogs were obtained from isodesmic reactions. Progressive substitution with methyl or BH2 groups continuously lowers ring strain while increasing substitution with fluorines or trifluoromethyl groups steadily increases ring strain. C4(BH2)4 is 16.6 and 13.3 kcal/mol less strained than cyclobutadiene while C4F4 is 17.7 and 21.5 kcal/mol more strained at the levels above. Cyclobutadiene is more strained than both cyclopropene and cyclobutene by 12.2 and 37.0 kcal/mol, respectively. Electron density contours indicate that fluorine substitution raised the electron density especially in the short C=C ring bonds above/below the ring plane (π-electrons) but not in the ring plane (σ-electrons). BH2-substitutions lower the ring π-electron density with little effect in the ring plane. Methyl substituents have little effect on electron densities. All rings retain a strong bond alternation tendency (rectangular) whether substituted with electron-donating or -attracting groups. One-bond coupling constants and the percent p-character in ring C-to-C and C-to-substituent bonds are described. 相似文献
84.
Steven M. McLeod Charles J. Hages Nathaniel J. Carter Rakesh Agrawal 《Progress in Photovoltaics: Research and Applications》2015,23(11):1550-1556
We report a total‐area power conversion efficiency of 15% for a copper indium gallium disulfoselenide (CIGSSe) solar cell fabricated from a copper indium gallium disulfide (CIGS) nanoparticle ink based process. Careful optimization of the fabrication process has resulted in a significant improvement in efficiency compared to our previously reported efficiency of 12%. This efficiency ranks among the highest reported in the literature for solution processed CIGSSe. Despite having an absorber thickness of approximately 700–800 nm, which is less than half the thickness of high efficiency devices grown by both coevaporation and solution processes in the literature, our devices show good short‐circuit current (32.1 mA/cm2). Surprisingly, the sintered film shows lateral composition fluctuations, which have not been reported for other high efficiency devices and may be responsible for the lower open circuit voltage (636 mV) observed here. This suggests an avenue for further improvement through optimization of the nanoparticle selenization process to better control composition in the sintered film. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
85.
We develop some recursive constructions for rotational Steiner triple systems with which the spectrum of a k-rotational Steiner triple system of order v is completely determined for each positive integer k. © 1996 John Wiley & Sons, Inc. 相似文献
86.
87.
88.
William N. Setzer Shahrara Afshar Norman L. Burns Lucille A. Ferrante Amy M. Hester Edward J. Meehan Gregory J. Grant Saju M. Isaac Christopher P. Laudeman Charles M. Lewis Donald G. VanDerveer 《Heteroatom Chemistry》1990,1(5):375-387
The mesocyclic trithioethers, 1,4,7-trithiacyclodecane, 1,4,7-trithiacycloundecane, 1,4,8-trithiacycloundecane, and 1,5,9-trithiacyclododecane; the mesocyclic trithioether ketones, 1,4,7-trithiacyclodecan-9-one; 1,4,8-trithiacycloundecan-6-one, and 1,5,9-trithiacyclododecan-3-one; and the mesocyclic trithioether alcohols, 1,4,7-trithiacyclodecan-9-ol, 1,4,8-trithiacycloundecan-6-ol, and 1,5,9-trithiacyclododecan-3-ol, have been synthesized using the cesium dithiolate technique. In some cases, the corresponding macrocyclic hexathioether was isolated from the reaction mixture in addition to the mesocyclic trithioether; 1,4,7,11,14,17-hexathiacycloeicosane, 1,4,7,11,14,17-hexathiacycloeicosan-9,19-dione, 1,4,7,12,15,18-hexathiacyclodocosane, and 1,5,9,13,17,21-hexathiacyclotetracosane. Single-crystal X-ray structures have been determined for 1,5,9-trithiacyclododecan-3-ol and 1,4,7,12,15,18-hexathiacyclodocosane. For 1,5,9-trithiacyclododecane-3-ol, the compound crystallizes in the monoclinic space group, C2/c, with a = 10.5926( 9 ) Å, b = 15.582(2) Å, c = 13.6015(8) Å, β = 98.186(6)0, Z = 8, and R = 0.038. The macrocycle, 1,4,7,12,15,18-hexathiacyclodocosane, crystallizes in the orthorhombic space group, Pbca, with a = 21.406(5) Å, b = 9.810(2) Å, c = 10.225(2) Å, Z = 4, and R = 0.020. 相似文献
89.
Transport in Porous Media - Acid mine drainage is generated when sulfide minerals are exposed to air and water through the porous subsurface, and it is significantly accelerated by the action of... 相似文献
90.
Determination of zwitterionic and cationic surfactants by high-performance liquid chromatography with chemiluminescenscent nitrogen detection 总被引:1,自引:0,他引:1
The use of chemiluminescent nitrogen specific detection (CLND) combined with an HPLC separation allows for the identification and quantification of cationic and zwitterionic surfactants. The CLND provides equimolar responses, based on the amount of nitrogen within any compound. This allows for the detection of any nitrogen containing surfactant. Reversed-phase separation methods using cyano columns are developed for cationic and zwitterionic (sulfobetaine) surfactant mixtures. The limits of detection for these surfactants are in the single micromolar range (1 ng N). A linear response was obtained (R2=0.9981) between 50 microM and 5 mM. The methodology was then applied to the determination of an industrial zwitterionic surfactant, Rewoteric AM CAS U [coco(amidopropyl)hydroxyldimethylsulfobetaine]. 相似文献