全文获取类型
收费全文 | 3785篇 |
免费 | 51篇 |
国内免费 | 27篇 |
专业分类
化学 | 2592篇 |
晶体学 | 59篇 |
力学 | 113篇 |
数学 | 720篇 |
物理学 | 379篇 |
出版年
2020年 | 28篇 |
2016年 | 41篇 |
2015年 | 36篇 |
2014年 | 42篇 |
2013年 | 166篇 |
2012年 | 128篇 |
2011年 | 139篇 |
2010年 | 81篇 |
2009年 | 74篇 |
2008年 | 146篇 |
2007年 | 149篇 |
2006年 | 176篇 |
2005年 | 164篇 |
2004年 | 132篇 |
2003年 | 144篇 |
2002年 | 108篇 |
2001年 | 52篇 |
2000年 | 52篇 |
1999年 | 42篇 |
1998年 | 30篇 |
1997年 | 57篇 |
1996年 | 59篇 |
1995年 | 60篇 |
1994年 | 61篇 |
1993年 | 66篇 |
1992年 | 44篇 |
1991年 | 66篇 |
1990年 | 51篇 |
1989年 | 48篇 |
1988年 | 45篇 |
1987年 | 38篇 |
1986年 | 40篇 |
1985年 | 72篇 |
1984年 | 78篇 |
1983年 | 50篇 |
1982年 | 80篇 |
1981年 | 73篇 |
1980年 | 65篇 |
1979年 | 78篇 |
1978年 | 70篇 |
1977年 | 73篇 |
1976年 | 60篇 |
1975年 | 56篇 |
1974年 | 69篇 |
1973年 | 80篇 |
1972年 | 39篇 |
1971年 | 39篇 |
1970年 | 35篇 |
1969年 | 28篇 |
1968年 | 30篇 |
排序方式: 共有3863条查询结果,搜索用时 15 毫秒
131.
The interaction of pairs of bubbles with equal diameters grown on adjacent capillaries in aqueous magnesium sulfate solutions is observed for varying electrolyte concentrations and bubble diameters. As in previous investigations, a sharp transition from coalescence to bubble detachment without coalescence is observed with increasing electrolyte concentration. The critical electrolyte concentration for this transition is found to increase with decreasing bubble diameter for bubble diameters of 1.4 to 4.2 mm. 相似文献
132.
The water exchange of [V(H2O)6]Cl2 in aqueous solution has been studied as a function of temperature and pressure (up to 250 MPa), by measuring the 17O-FT-NMR. line-widths of the free water resonance at 8.13 MHz. The kinetic parameters obtained are K = 87±4 s?1, ΔH* = +61.8 ± 0.7 kJ mo1?1 and ΔS* = ?0.4±1.9 J mol?1 K?1. A pressure-independent volume of activation ΔV* = ?4.1±0.1 cm3 mol?1 is obtained, suggesting an associative interchange (Ia) mechanism for this early divalent metal ion. 相似文献
133.
William J. Patterson Donald E. Morris 《Journal of polymer science. Part A, Polymer chemistry》1972,10(1):169-178
The synthesis and polymerization of several silphenylene siloxane polymer precursors containing a perfluoroalkylsegment in the backbone was carried out. The molecular weight characteristics of polymers from 1,3-bis[p(-hydroxydimethylsilyl)phenyl]hexafluoropropane and 1,3-bis[p(-dimethylaminodimethylsilyl)phenyl]hexafluoropropane were studied as a function of polymerization conditions. Polymers containing the dodecafluorohexane chain segment were also prepared. Polymers having inherent viscosities of 0.55 to 0.9 were obtained. The polymers crosslinked at room temperature to thermoset elastomers which were characterized by improved thermal and oxidative stability over dimethylsilicones. Room temperature swelling of the experimental polymers hydrocarbon solvents was also much lower than that of dimethylsilicones. The polymers containing the (CF2)3 and (CF2)6 linkages had glass transition points of ?12°C and ?34°C, respectively. 相似文献
134.
Oxidation of quadruply bonded metal-metal dimers in the presence of good π-accepting ligands results in the formation of MoV---MoV compounds of the type [MO2(μ-X)2(Y)(Y′)]2+ (X = O or S; Y,Y′ = O,O; S,S; O,S). Reaction of MO2(O2CCH3)4 with oxygen in the presence of Na2mnt (mnt = 1,2-dicyanoethylene-2,2-dithiolate) gives [MO2(μ-S)2(O)(S)(mnt)2]2− (1). The compound crystallizes in the monoclinic space group P21/c, with cell dimensions a = 19.547(4), b = 15.210(4), c = 18.754(6) Å, β = 101.69(2)°, V= 5460(2) Å3, and Z = 4. Similarly, oxidation of o-dichlorobenzene solutions of Mo2Cl4(CH3CN)4 and 4,4′-dimethyl-2,2′-dipyridyl (dmpby) or, more directly, the reaction of Mo2Cl4(dmbpy)2 with oxygen leads to the formation of a red solid, which was characterized by X-ray crystallography to be Mo2(μ-O)2(O)2(Cl)2(dmbpy)2 (2). Red diamond crystals, prepared by slow evaporation of CH3CN solutions of 2, are trigonal and in the space group P3121 with cell dimensions a = 16.135(4), b = 16.135(4), c = 10.709(3) Å, V = 2414.4(13) Å3 and Z = 3. In both structures, the geometry about each of the molybdenum atoms is a distorted square pyramid with terminal oxygen or sulphur atoms at the apices and in a syn conformation. The molybdenum-molybdenum bond distances of 2.858(1) Å and 2.562(2) Å in structures of 1 and 2, respectively, are typical of other MoV---MoV dimers and indicative of a single Mo---Mo bond. 相似文献
135.
The chemical-physical basis of loading and release of K(+) and Na(+) ions in and out of the selectivity filter of the K(+) channel has been investigated using the B3LYP method of density functional theory. We have shown that the difference between binding free energies of K(+) and Na(+) to the cavity end of the filter is smaller than the difference between the K(+) and Na(+) solvation free energies. Thus, the loading of K(+) ions into the cavity end of the selectivity filter from the solution phase is suggested to be selective prior to the subsequent conduction process. It is shown that the extracellular end of the filter is only optimal for K(+) ions, because K(+) ions prefer the coordination environment of eight carbonyl oxygens. Na(+) ions do not fit into the extracellular end of the filter, since they prefer the coordination environment of six carbonyl oxygens. Overall, the results suggest that the rigid C(4) symmetric selectivity filter is specifically designed for conduction of K(+) ions. 相似文献
136.
Summary Conventional numerical methods, when applied to the ordinary differential equations of motion of classical mechanics, conserve the total energy and angular momentum only to the order of the truncation error. Since these constants of the motion play a central role in mechanics, it is a great advantage to be able to conserve them exactly. A new numerical method is developed, which is a generalization to arbitrary order of the discrete mechanics described in earlier work, and which conserves the energy and angular momentum to all orders. This new method can be applied much like a corrector as a modification to conventional numerical approximations, such as those obtained via Taylor series, Runge-Kutta, or predictor-corrector formulae. The theory is extended to a system of particles in Part II of this work. 相似文献
137.
Chen Zhou Matthew R. Hermes Dihua Wu Jie J. Bao Riddhish Pandharkar Daniel S. King Dayou Zhang Thais R. Scott Aleksandr O. Lykhin Laura Gagliardi Donald G. Truhlar 《Chemical science》2022,13(26):7685
Strong electron correlation plays an important role in transition-metal and heavy-metal chemistry, magnetic molecules, bond breaking, biradicals, excited states, and many functional materials, but it provides a significant challenge for modern electronic structure theory. The treatment of strongly correlated systems usually requires a multireference method to adequately describe spin densities and near-degeneracy correlation. However, quantitative computation of dynamic correlation with multireference wave functions is often difficult or impractical. Multiconfiguration pair-density functional theory (MC-PDFT) provides a way to blend multiconfiguration wave function theory and density functional theory to quantitatively treat both near-degeneracy correlation and dynamic correlation in strongly correlated systems; it is more affordable than multireference perturbation theory, multireference configuration interaction, or multireference coupled cluster theory and more accurate for many properties than Kohn–Sham density functional theory. This perspective article provides a brief introduction to strongly correlated systems and previously reviewed progress on MC-PDFT followed by a discussion of several recent developments and applications of MC-PDFT and related methods, including localized-active-space MC-PDFT, generalized active-space MC-PDFT, density-matrix-renormalization-group MC-PDFT, hybrid MC-PDFT, multistate MC-PDFT, spin–orbit coupling, analytic gradients, and dipole moments. We also review the more recently introduced multiconfiguration nonclassical-energy functional theory (MC-NEFT), which is like MC-PDFT but allows for other ingredients in the nonclassical-energy functional. We discuss two new kinds of MC-NEFT methods, namely multiconfiguration density coherence functional theory and machine-learned functionals.This feature article overviews recent work on active spaces, matrix product reference states, treatment of quasidegeneracy, hybrid theory, density-coherence functionals, machine-learned functionals, spin–orbit coupling, gradients, and dipole moments. 相似文献
138.
Climate change, environmental pollution and associated abiotic stresses are beginning to meaningfully affect agricultural production worldwide. Salt stress is, however, one of the most important threats that significantly impairs plant growth and development. Plants in their early growth stages such as seed germination, seed emergence and early seedling growth are very sensitive to salt stress. Among the range of sustainable techniques adopted to improve seed germination and early plant growth is seed priming; however, with the use of ecofriendly substances, this is one of the most effective and economically viable techniques to improve seed tolerance against such environmental stresses. For instance, priming with appropriate non-synthetic compounds including microbial biostimulants are prominent ways to sustainably address these challenges. Therefore, in this research, by using the “priming technique”, two biostimulants were tested for their potential as sustainable approaches to improve canola and soybean seed germination under salt stress and optimal growth conditions. Canola and soybean seeds were primed with flavonoids extracted from citrus fruits (flavopriming) and cell-free supernatant (CFS; produced by a novel strain of Devosia sp.—SL43), alone and in combination, and exposed to low–higher levels of salt stress and ideal growth conditions. Both biostimulants showed promising effects by significantly improving seed germination of soybean and canola under both ideal and stressful conditions. However, increases in seed germination were greater under salinity stress as flavonoids and CFS with stress amelioration effects showed substantial and statistically significant improvements in seed germination under varying levels of salt stress. In addition, combinations (mixtures) of both biostimulants were tested to determine if their effects might be more additive or multiplicative than the individual applications. However, results suggested incompatibility of both biostimulants as none of the combinations showed better results than that of the individual applications of either flavonoids or CFS. Conceivably, the use of flavonoids and this novel Devosia sp. CFS could be significant plant growth enhancers, perhaps much better than the few other biostimulants and bacterial-based compounds currently in use. 相似文献
139.
The impact of Cu leveling additives on electrodeposited Cu topography and subsequent planarization behaviour was studied on both the pattern and wafer scales. The leveling agent significantly reduces as-deposited Cu topography, especially “mounding”. The reduction in topography results in a higher effective Cu removal rate during subsequent Cu planarization, both at the pattern and wafer scales. On the wafer scale, this effect is more evident for lower overburdens as the topography must be eliminated in a shorter total polish time. For Cu electrodeposited from leveler additive-free chemistries, significant pattern-scale topography persists throughout almost the entire planarization process, whereas for Cu deposited using a leveling agent only very wide features (~ > 100 μm) show any significant topography evolution during Cu polish. It is shown that excess electrodeposited Cu topography can lead to poor in-plane Cu wiring leakage performance. 相似文献
140.
Amina Benylles Donald Cairns Philip J. Cox Graeme Kay 《Acta Crystallographica. Section C, Structural Chemistry》2013,69(6):658-664
Reaction between cysteamine (systematic name: 2‐aminoethanethiol, C2H7NS) and L‐(+)‐tartaric acid [systematic name: (2R,3R)‐2,3‐dihydroxybutanedioic acid, C4H6O6] results in a mixture of cysteamine tartrate(1−) monohydrate, C2H8NS+·C4H5O6−·H2O, (I), and cystamine bis[tartrate(1−)] dihydrate, C4H14N2S22+·2C4H5O6−·2H2O, (III). Cystamine [systematic name: 2,2′‐dithiobis(ethylamine), C4H12N2S2], reacts with L‐(+)‐tartaric acid to produce a mixture of cystamine tartrate(2−), C4H14N2S22+·C4H4O62−, (II), and (III). In each crystal structure, the anions are linked by O—H...O hydrogen bonds that run parallel to the a axis. In addition, hydrogen bonding involving protonated amino groups in all three salts, and water molecules in (I) and (III), leads to extensive three‐dimensional hydrogen‐bonding networks. All three salts crystallize in the orthorhombic space group P212121. 相似文献