全文获取类型
收费全文 | 240篇 |
免费 | 22篇 |
专业分类
化学 | 175篇 |
晶体学 | 4篇 |
力学 | 7篇 |
数学 | 26篇 |
物理学 | 50篇 |
出版年
2023年 | 4篇 |
2022年 | 2篇 |
2021年 | 5篇 |
2020年 | 8篇 |
2019年 | 6篇 |
2018年 | 9篇 |
2017年 | 4篇 |
2016年 | 10篇 |
2015年 | 5篇 |
2014年 | 16篇 |
2013年 | 22篇 |
2012年 | 20篇 |
2011年 | 18篇 |
2010年 | 25篇 |
2009年 | 15篇 |
2008年 | 13篇 |
2007年 | 18篇 |
2006年 | 5篇 |
2005年 | 7篇 |
2004年 | 9篇 |
2003年 | 4篇 |
2002年 | 3篇 |
1999年 | 5篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 3篇 |
1993年 | 1篇 |
1992年 | 3篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1967年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有262条查询结果,搜索用时 15 毫秒
251.
Özek G Ishmuratova M Tabanca N Radwan MM Göger F Özek T Wedge DE Becnel JJ Cutler SJ Can Başer KH 《Journal of separation science》2012,35(5-6):650-660
Gas chromatographic analysis revealed that the oil of Crinitaria tatarica was rich in sabinene (32.1%), β-pinene (8.8%), and two unknown (M+200) compounds (I) and (II) (21.4% and 3.4%). One-step multiple fractionation of the oil and separation of two unknown constituents were performed using preparative capillary gas chromatography connected to preparative fraction collector system. This combination allowed separation and recover of sufficient quantities of two unknown compounds with high purity from complex oil matrix. Separation conditions (column temperature, cooling temperature, flow rate, injection volume, cut time) were optimized to achieve the best isolation and successful collection. The target compounds were separated from the oil using a HP Innowax (Walt & Jennings Scientific, Wilmington, DE, USA) preparative capillary column in rapid one-step manner with 95.0% purity. Trapping of the isolated compounds in collector system was facilitated by cooling with liquid nitrogen. Structure determination was accomplished by spectral analysis including ultraviolet, nuclear magnetic rezonance, and high-resolution electrospray ionization mass spectrometry. Z- (I) and E-artemidin (II) were isolated for the first time from this species. Crinitaria tatarica oil and Z- (I) and E-artemidin (II) were evaluated for biological activity. 相似文献
252.
Five period InGaN/GaN multi quantum well (MQW) light emitting diode (LED) structures were grown by a metalorganic chemical vapor deposition (MOCVD) system on c‐plane sapphire. The structural characteristics as a strain‐stress analysis of hexagonal epilayers MQWs were determined by using nondestructive high resolution x‐ray diffraction (HRXRD) in detail. The strain/stress analysis in AlN, GaN, and InGaN thin films with a variation of the In molar fraction in the InGaN well layers was conducted based on the precise measurement of the lattice parameters. The a‐ and c‐lattice parameters of the structures were calculated from the peak positions obtained by rocking the theta axis at the vicinity of the symmetric and asymmetric plane reflection angles, followed by the in‐plane and out‐of‐plane strains. The biaxial and hydrostatic components of the strain were extracted from the obtained a‐ and c‐direction strains values. 相似文献
253.
Nickel complex of O, O'-dinonyldithiophosphoric acid was prepared and characterised. Solvent extraction of nickel dinonyldithiophosphate [Ni(DNDTP)2] from acidic aqueous solutions into different organic phases was investigated and concentration of nickel was determined spectrophotometrically. A simple and selective spectrophotometric method was developed and applied to sediment samples. 相似文献
254.
A series of new highly active Ru(II) complexes with two new (N-diphenylphosphino)isopropylanilines, having an isopropyl substituent at carbon 2- (1) or 2,6- (2) and two new bis(diphenylphosphino)isopropylanilines, having an isopropyl substituent at carbon atom 2- (3) or 4- (4), were prepared starting from the dimeric complex [Ru(η6-p-cymene)(μ-Cl)Cl]2. All the compounds have been fully characterized by microanalysis, IR, 31P{1H} NMR, 1H NMR and 13C NMR spectroscopies. Following activation by NaOH, complexes 5–8 were tested in the transfer hydrogenation of acetophenone derivatives with iso-PrOH as the hydrogen source. Catalytic studies showed that the complexes are excellent catalytic precursors for the transfer hydrogenation of acetophenone derivatives. 相似文献
255.
Silver nanoparticles (AgNPs) were proposed as a new chemical modifier for the elimination of interferences when determining arsenic and antimony in aqueous NaCl or Na2SO4 solutions and in sea-water by electrothermal atomic absorption spectrometry. For this purpose, the AgNPs were prepared simply by reducing silver nitrate with sodium citrate. The effects of pyrolysis and atomization temperatures and the amounts of interferents and modifiers on the sensitivities of these elements were investigated. In the presence of the proposed modifier, a pyrolysis temperature of at least 1100 °C for arsenic and 900 °C for antimony could be applied without the loss of analytes, and the interferences were greatly reduced to allow for interference-free determination. The detection limits (N = 10, 3σ) for arsenic and antimony were 0.022 ng and 0.046 ng, respectively. AgNPs are cheaper and more available compared to many other modifiers. No background was detected, and the blank values were negligible. 相似文献
256.
We study a non-cooperative game for joint replenishment by n firms that operate under an EOQ-like setting. Each firm decides whether to replenish independently or to participate in joint
replenishment, and how much to contribute to joint ordering costs in case of participation. Joint replenishment cycle time
is set by an intermediary as the lowest cycle time that can be financed with the private contributions of participating firms.
We characterize the behavior and outcomes under undominated Nash equilibria. 相似文献
257.
IntroductionPfaff[1]hasbeendefinedaquaternionproductonleaf (theplanecontainingtheOx_axis) .Byaidoftheproduct,anewproductisdefinedonplaneswhichpassesthroughtheorigindonotcontaintheOx_axisandsomeofthepropertiesofthisproducthavebeeninvestigated .Agrawal[2 ]gavesomealgebricpropertiesofHamiltonoperators.Also ,quaternionswereexpressedintermsof 4× 4matricesbymeansoftheseoperators.Yayli[3]gavehomotheticmotionswithaidoftheHamiltonoperatorsatE4 .Hacisalihoglu[4 ]showedthatallHomotheticmotionsinn_di… 相似文献
258.
Macromolecular chains obey purely Gaussian statistics close to the theta temperature. An asymptotic regime is also reached far above the theta point. Thermal blob theory permits an approximate calculation of thermal expansion factor. In this theory, an adjustable parameter is used as a prefactor (A*N1) in order to calculate the reduced blob parameter (N/Nc). In this work, we proposed a different approach to evaluate (A*N1) by plotting τMω1/2/αη5 against τMω1/2. Chain expansion data of poly(4-chlorostyrene) in various solvents is used to evaluate viscosity expansion factors at various temperatures. It seems that predictions of thermal blob theory provide a better fit to experimental points rather than those evaluations based on Flory-type approaches. © 1996 John Wiley & Sons, Inc. 相似文献
259.
E. Elif Gürel Nilhan Kayaman Bahattin M. Baysal Frank E. Karasz 《Journal of Polymer Science.Polymer Physics》1999,37(16):2253-2260
The present article considers the coil‐to‐globule transition behavior of atactic and syndiotactic poly(methyl methacrylates), (PMMA) in their theta solvent, n‐butyl chloride (nBuCl). Changes in Rh in these polymers with temperature in dilute theta solutions were investigated by dynamic light scattering. The hydrodynamic size of atactic PMMA (a‐PMMA‐1) in nBuCl (Mw: 2.55 × 106 g/mol) decreases to 61% of that in the unperturbed state at 13.0°C. Atactic PMMA (a‐PMMA‐2) with higher molecular weight (Mw: 3.3 × 106 g/mol) shows higher contraction in the same theta solvent (αη = Rh(T)/Rh (θ) = 0.44) at a lower temperature, 7.25°C. Although syndiotactic PMMA (s‐PMMA) has lower molecular weight than that of atactic samples (Mw: 1.2 × 106), a comparable chain collapse was observed (αη = 0.63) at 9.0°C. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2253–2260, 1999 相似文献
260.
Simulated annealing (SA) is a popular global minimizer that can conveniently be applied to complex macromolecular systems. Thus, a molecular dynamics or a Monte Carlo simulation starts at high temperature, which is decreased gradually, and the system is expected to reach the low-energy region on the potential energy surface of the molecule. However, in many cases this process is not efficient. Alternatively, the low-energy region can be reached more effectively by minimizing the energy of selected molecular structures generated along the simulation pathway. The efficiency of SA to locate energy-minimized structures within 5 kcal/mol above the global energy minimum is studied as applied to three peptide models with increasing geometrical restrictions: (1) The linear pentapeptide Leu-enkephalin described by the ECEPP potential, (2) a cyclic hexapeptide described by the GROMOS force field energy EGRO alone, and (3) the same cyclic peptide with EGRO combined with a restraining potential based on 31 proton–proton restraints obtained from nuclear magnetic resonance (NMR) experiments. The efficiency of SA is compared to that of the Monte Carlo minimization (MCM) method of Li and Scheraga, and to our local torsional deformations (LTD) method for the conformational search of cyclic molecules. The results for the linear peptide show that SA provides a relatively weak guidance towards the most stable energy region; as expected, this guidance increases for the cyclic peptide and the cyclic peptide with NMR restraints. However, in general, MCM and LTD are significantly more efficient than SA as generators of low-energy minimized structures. This suggests that LTD might provide a better search tool than SA in structure determination of protein regions for which a relatively small number of restraints are provided by NMR. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1659–1670, 1999 相似文献