全文获取类型
收费全文 | 79755篇 |
免费 | 392篇 |
国内免费 | 390篇 |
专业分类
化学 | 25437篇 |
晶体学 | 798篇 |
力学 | 6775篇 |
数学 | 32266篇 |
物理学 | 15261篇 |
出版年
2018年 | 10446篇 |
2017年 | 10268篇 |
2016年 | 6080篇 |
2015年 | 888篇 |
2014年 | 324篇 |
2013年 | 406篇 |
2012年 | 3871篇 |
2011年 | 10591篇 |
2010年 | 5688篇 |
2009年 | 6079篇 |
2008年 | 6670篇 |
2007年 | 8836篇 |
2006年 | 323篇 |
2005年 | 1368篇 |
2004年 | 1603篇 |
2003年 | 2033篇 |
2002年 | 1061篇 |
2001年 | 280篇 |
2000年 | 304篇 |
1999年 | 163篇 |
1998年 | 211篇 |
1997年 | 161篇 |
1996年 | 230篇 |
1995年 | 134篇 |
1994年 | 91篇 |
1993年 | 120篇 |
1992年 | 70篇 |
1991年 | 87篇 |
1990年 | 70篇 |
1989年 | 77篇 |
1988年 | 81篇 |
1987年 | 68篇 |
1986年 | 71篇 |
1985年 | 76篇 |
1984年 | 66篇 |
1983年 | 52篇 |
1982年 | 69篇 |
1981年 | 66篇 |
1980年 | 61篇 |
1979年 | 78篇 |
1978年 | 54篇 |
1977年 | 47篇 |
1976年 | 41篇 |
1975年 | 42篇 |
1973年 | 44篇 |
1914年 | 45篇 |
1913年 | 43篇 |
1912年 | 40篇 |
1909年 | 43篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Ab initio molecular orbital theory and density functional theory have been used to study nine isomers of N7 ionic clusters with low spin at the HF/6-31G*, MP2/6-31G*, B3LYP/6-31G*, and B3LYP/6-311(+)G* levels of theory. All stationary
points are examined with harmonic vibrational frequency analyses. Four N7
+ isomers and five N7
− isomers are determined to be local minima or very close to the minima on their potential-energy hypersurfaces, respectively.
For N7
+ and N7
−, the energetically low lying isomers are open-chain structures (C
2
v
and C
2
v
or C2). The results are very similar to those of other known odd-number nitrogen ions, such as N5
+, N9
+, and N9
−, for which the open-chain structures are also the global minima. This research suggests that the N7 ionic clusters are likely to be stable and to be potential high-energy-density materials if they could be synthesized.
Received: 16 July 2001 / Accepted: 8 October 2001 / Published online: 21 January 2002 相似文献
992.
The ionic liquid 1-N-butyl-3-methylimidazolium chloride ([C4mim]+Cl−) was investigated as reaction media for the homogeneous acylation of cellulose with 2-furoyl chloride in the presence of
pyridine. The preparation of cellulose furoate depending on the reaction conditions, the cellulose type and the pyridine content
was studied. Cellulose furoates with a degree of substitution in the range from 0.46 to 3.0 were accessible, i.e., under mild
conditions, with a low excess of reagent and in a short reaction time. The products were characterized by elemental analysis,
perpropionylation, 1H- and 13C NMR spectroscopy and FTIR spectroscopy.
Thomas Heinze is the member of the European Polysaccharide Network of Excellence (EPNOE), www.epnoe.eu 相似文献
993.
In this study, poly(N-methylolacrylamide)/polymethylacrylamide (PNMA/PMAA) hybrids were produced successfully by frontal free-radical polymerization
at ambient pressure. In a typical run, the appropriate amounts of reactants (N-methylolacrylamide, NMA; methylacrylamide, MAA) and initiator (ammonium persulfate) were dissolved in dimethyl sulfoxide
at ambient temperature. Frontal polymerization (FP) was initiated by heating the wall of the tube with a soldering iron, and
the resultant hot fronts were allowed to self-propagate throughout the reaction vessel. Once initiated, no further energy
was required for polymerization to occur. The dependences of the front velocity and front temperature on the initiator concentration,
reactant dilution, and NMA/MAA components were thoroughly investigated. The front temperatures were between 69 and 116 °C,
depending on the persulfate concentration. We have also investigated the FP of PNMA/PMAA hybrids with N-methyl-2-pyrrolidone as solvent. Results show that FP can be exploited as a means for the preparation of PNMA/PMAA hybrids
with the potential advantage of higher throughput compared to the traditional mode. 相似文献
994.
A new compound (5R, 10R)-3,8-dihydroxy-5,10-diethoxy-5,10-dihydrochromeno[5,4,3-cde]chromene monohydrate was obtained from 3,4-dihydroxybenzaldehyde in aerobic basic aqueous ethanol solution in the presence
of manganese chloride and triethylamine and crystallized in orthorhombic P212121 space group (denoted as 1). When 1 was recrystallized from aqueous methanol, it was transformed to another crystal (2) with the same composition but in P21/n space group. The drastic difference in the extensive hydrogen bond network makes 1 a 3D and 2 a 2D infinite supramolecular structure, respectively. 相似文献
995.
M. Jayalakshmi P. Radhika K. Phani Raja M. Mohan Rao 《Journal of Solid State Electrochemistry》2007,11(2):165-172
Nanoparticles of α-phase nickel hydroxide were synthesized by a single-step hydrothermal method using urea as the hydrolytic agent. Precipitated powders were of pure turbostratic α-phase as confirmed by x-ray diffraction profile. The ageing of α-Ni(OH)2 in 1.0 M alkali solutions is investigated for pure non-intercalated α-Ni(OH)2 and thiourea intercalated/absorbed α-phase nanomaterials. The α-Ni(OH)2 powder immobilized on the surface of graphite electrodes shows a gradual α→β phase transformation with continuous voltammetric cycling, and the concentration gradient of water that exists in the layered-double-hydroxide-like interlayers of α-phase and the solution was shown to play a crucial role on the high electrochemical activity of this phase nickel hydroxide. To understand the role of water in the ageing process, concomitant entries of non-aqueous solvents like ethanol and acetonitrile along with thiourea were effected. Cyclic voltammetric measurements of thiourea-treated α-Ni(OH)2 samples revealed that hydroxyl ion influx during the anodic oxidation depends on the counter flux of solvent molecules, and if the intercalated the solvent is acetonitrile, then the electrochemical activity of α-Ni(OH)2 reduced drastically; Q
a/Q
c>1 for water as solvent in the interlayers α-Ni(OH)2 and Q
a/Q
c<1 for ethanol and acetonitrile as solvents. The α-phase gets stabilized in the presence of thiourea with water and ethanol as co-intercalates. Transmission electron microscope images of α-Ni(OH)2 and thiourea-treated samples show a change in particle size and morphology. Elemental CHNS analysis confirms the presence of sulphur in the thiourea intercalated samples. 相似文献
996.
The oxidation of selected clinically important neurotransmitter metabolites with acidic potassium permanganate in the presence of polyphosphates evokes chemiluminescence of sufficient intensity to enable the sensitive determination of these species. Limits of detection for 5-hydroxyindole-3-acetic acid (5-HIAA), vanilmandelic acid (VMA; α,4-dihydroxy-3-methoxybenzeneacetic acid), 4-hydroxy-3-methoxyphenylglycol (MHPG), homovanillic acid (HVA, 4-hydroxy-3-methoxyphenylacetic acid) and 3,4-dihydroxyphenylacetic acid (DOPAC) were between 5 × 10−9 and 4 × 10−8 M, using flow-injection analysis methodology. In addition, we demonstrate the rapid determination of homovanillic acid and 5-hydroxyindole-3-acetic acid in human urine - without the need for extraction procedures - using monolithic column chromatography with chemiluminescence detection. 相似文献
997.
Two novel RuII complexes [Ru(phen)2(PNOPH)]2+ and [Ru(dmp)2 (PNOPH)]2+ (phen = 1,10-phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline, PNOPH = 2-(4-nitrophenyl)imidazo-[4,5-f][1,10]phenanthroline) and their deprotoned complexes were synthesized and characterized by ES–MS, 1H - n.m.r, u.v.–vis. and electrochemistry. The crystal structure of the deprotonated complex [Ru(dmp)2 (PNOP)][ClO4] · CH3CN was determined by means of X-ray single crystal diffraction. Nonlinear optical properties of the RuII complexes were investigated by Z-scan techniques in DMF solution, and all of them exhibited both NLO absorption and self-defocusing effect. The corresponding effective NLO susceptibilities |3 | of the complexes are 2.39 × 10-12–5.80 × 10-12 esu. 相似文献
998.
Mason Gene Bailey Billy E. Gillett 《The Journal of the Operational Research Society》1980,31(3):257-262
An algorithm is presented for solving families of integer linear programming problems in which the problems are "related" by having identical objective coefficients and constraint matrix coefficients. The righthand-side constants have the form b + θd where b and d are conformable vectors and θ varies from zero to one.The approach consists primarily of solving the most relaxed problem (θ = 1) using cutting planes and then contracting the region of feasible integer solutions in such a manner that the current optimal integer solution is eliminated.The algorithm was applied to 1800 integer linear programming problems with reasonable success. Integer programming problems which have proved to be unsolvable using cutting planes have been solved by expanding the region of feasible integer solutions (θ = 1) and then contracting to the original region. 相似文献
999.
Previous works on stochastic inventory problems have often assumed that an item's lead time demand follows a "convenient" distribution such as the normal, the γ or the Weibull. First, this paper argues that these convenient distributions may be overly restrictive and unrealistic, and points out the versatility and realism of using four-parameter distributions of the Pearson's and the Schmeiser-Deutsch's systems. Second, using these four-parameter distributions, this paper presents practical `manual" methods for computing the stock-out probability, reorder level and expected lost sales of an inventory item and for solving the lot-size reorder-point model. Some of these methods are actually simpler than the ones developed previously for the more restrictive distributions. 相似文献
1000.
Rolfe Tomlinson 《The Journal of the Operational Research Society》1980,31(6):467-476
This paper is concerned with the need for more interdisciplinary, systems oriented, research directed towards major problems encountered by decision makers in industry and government; a need which is more difficult to meet in the face of traditional methods of organising knowledge and research. It emphasises the need to acknowledge the true complexity of the problems and the interactive nature of any effective research procedure. As an illustration, both of the need and the problems involved in meeting it, the development of a new program of research into problems of Management and Technology at the International Institute of Applied Systems Analysis, which is supported by seventeen nations of all political complexions, is analysed. The implication is that we can do something about the future, but we must be prepared to do. 相似文献