首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1265篇
  免费   24篇
  国内免费   10篇
化学   954篇
晶体学   21篇
力学   15篇
综合类   1篇
数学   108篇
物理学   200篇
  2021年   7篇
  2019年   16篇
  2018年   12篇
  2017年   13篇
  2016年   9篇
  2015年   17篇
  2014年   25篇
  2013年   108篇
  2012年   34篇
  2011年   67篇
  2010年   31篇
  2009年   44篇
  2008年   70篇
  2007年   69篇
  2006年   66篇
  2005年   66篇
  2004年   71篇
  2003年   60篇
  2002年   51篇
  2001年   23篇
  2000年   14篇
  1999年   11篇
  1998年   23篇
  1997年   17篇
  1996年   15篇
  1995年   18篇
  1994年   13篇
  1993年   11篇
  1992年   8篇
  1991年   12篇
  1990年   14篇
  1989年   11篇
  1988年   19篇
  1987年   10篇
  1986年   7篇
  1985年   22篇
  1984年   28篇
  1983年   13篇
  1982年   21篇
  1981年   16篇
  1980年   12篇
  1979年   15篇
  1978年   16篇
  1977年   12篇
  1976年   10篇
  1974年   5篇
  1972年   8篇
  1967年   6篇
  1959年   5篇
  1956年   6篇
排序方式: 共有1299条查询结果,搜索用时 15 毫秒
981.
2-Diethylaminoethyl methacrylate (DEAEMA)–N-vinyl-2-pyrrolidone (VPy) copolymers of various compositions have been synthesized. The resultant copolymers were examined for their ability to bind methyl orange and its homologs, in particular butyl orange, at 5, 15, 25, and 35°C in aqueous solutions. The amount of binding of butyl orange is much higher with the copolymers than with polyvinylpyrrolidone or with 2-hydroxyethyl methacrylate–N-vinyl-2-pyrrolidone copolymers. Introduction of only 3% of the hydrophobic DEAEMA residue increases markedly the binding affinity toward the cosolute. Maximal binding is obtained at 15°C in the temperature range measured. This peculiar temperature dependence of the extent of binding is explicable on the basis of hydrophobic effects involved in this binding. The peculiar temperature dependence disappeared in aqueous solution of NaSCN which acts as a water-structure breaker: the extent of binding changes regularly with temperature. This is interpretable only in terms of reduction of hydrophobic contribution to the binding. With propyl orange, which is a less hydrophobic cosolute than butyl orange, the peculiarity of the binding was not detected.  相似文献   
982.
A novel type of optically active N-[4-N′-(α-methylbenzyl)aminocarbonylphenyl]maleimide [(R)-MBCP] was synthesized from maleic anhydride, p-aminobenzoic acid, and (R)-methylbenzylamine. Radical homopolymerization of (R)-MBCP was performed in tetrahydrofuran (THF) at 50 and 70°C for 24 h to give optically active polymers having [α]25D = -141° and -129°, respectively. Anionic polymerization of (R)-MBCP with n-butyllithium in THF and N,N-dimethylformamide gave an optically active polymer having ?78 to ?81° of [α]25D. Radical copolymerizations of (R)-MBCP (M1) were performed with styrene (ST, M2) and methyl methacrylate (MMA, M2) in THF at 50°C. The monomer reactivity ratios (r1, r2) and the Alfrey-Price Q-e values were determined as follows: r1 = 0.009, r2 = 0.091, Q1 = 1.30, e1 = 1.87 in the (R)-MBCP-ST; r1 = 0.27, r2 = 1.21, Q1 = 0.93, e1 = 1.46 in the (R)-MBCP-MMA system. Chiroptical properties of the polymers were also investigated. © 1992 John Wiley & Sons, Inc.  相似文献   
983.
Counter-current chromatographic separation of proteins was performed using a rotary-seal-free nonsynchronous coil planet centrifuge (CPC) fabricated in our laboratory. This apparatus has a unique feature that allows a freely adjustable rotational rate of the coiled separation column at a given revolution speed. The separation was performed using a set of stable proteins including cytochrome c, myoglobin and lysozyme with two different types of aqueous-aqueous polymer phase systems, i.e., PEG (polyethylene glycol) 1000-dibasic potassium phosphate, and PEG 8000-dextran T500 in 5 mM potassium phosphate buffer. Using a set of multilayer coiled columns prepared from 0.8 mm I.D. PTFE tubing with different volumes (11, 24, 39 ml), the effect of the column capacity on the partition efficiency was investigated under a given set of experimental conditions. Among these experiments, the best separation of proteins was attained using the 39 ml capacity column with a 12.5% (w/w) PEG 1000-12.5% (w/w) dibasic potassium phosphate system at 10 rpm of coil rotation under 800 rpm. With lower phase mobile at 0.2 ml/min in the head-to-tail elution, the resolution between cytochrome c and myoglobin was 1.6 and that between myoglobin and lysozyme, 1.9. With upper phase mobile in the head-to-tail elution, the resolution between lysozyme and myoglobin peaks was 1.5. In these two separations, the stationary phase retention was 35.0 and 33.3%, respectively. Further studies were carried out using a pair of eccentric coil assemblies with 0.8 mm I.D. PTFE tubing at a total capacity of 20 ml. A comparable resolution was obtained using both lower and upper phases as a mobile phase in a head-to-tail elution. The results of our studies demonstrate that the nonsynchronous CPC is useful for protein separation with aqueous-aqueous polymer phase systems.  相似文献   
984.
985.
Let be a monomial ideal of . Bayer-Peeva-Sturmfels studied a subcomplex of the Taylor resolution, defined by a simplicial complex . They proved that if is generic (i.e., no variable appears with the same non-zero exponent in two distinct monomials which are minimal generators), then is the minimal free resolution of , where is the Scarf complex of . In this paper, we prove the following: for a generic (in the above sense) monomial ideal and each integer , there is an embedded prime of . Thus a generic monomial ideal with no embedded primes is Cohen-Macaulay (in this case, is shellable). We also study a non-generic monomial ideal whose minimal free resolution is for some . In particular, we prove that if all associated primes of have the same height, then is Cohen-Macaulay and is pure and strongly connected.

  相似文献   

986.
Rotation of Mo-Fibers in Directionally Solidified Ni-Al-Mo Eutectic Alloys   总被引:2,自引:0,他引:2  
The interface structure of -Mo fib ers in directionally solidified Ni-Al-Mo eutectic alloys has been investigated by transmission electron microscopy and computer-aided graphical techniques. The Mo fibers have square or rectangular cross sections with well defined flat facets and fundamentally have the Bain orientation relationship with respect to the matrix lattice. The crystal lattices of the Mo fibers, however, are often rotated with respect to the matrix lattice about their longitudinal axes. Correspondingly, the facet planes deviate from the exact rational (1 1 0)//(1 0 0) relationship. A computer aided graphical plotting of the atom arrangement at the interfaces suggests that good atomic matching is kept even after the rotation of fibers. High resolution electron microscopy revealed the incorporation of single atomic structural ledges to keep the coherency of the lattices of the matrix and rotated fibers. The relation between the lattice rotation and the facet plane rotation is well explained by the moiré and O-lattice models.  相似文献   
987.
Partially oxidized one-dimensional (1D) Pt-Pt chain compounds [Pt2(MeCS2)4]4ClO4.5PhCN (1) and [Pt2(EtCS2)4]5(ClO4)2 (2) were synthesized by electrocrystallization of diplatinum(II,II) complexes from different solvents. 1 and 2 consist of 1D Pt-Pt chains of stacked Pt-Pt dimers with short interdimer S...S contacts. Depending on the number of ClO4- per dimer and their positions, 1 forms a regular stack of Pt-Pt dimers, whereas 2 forms pentamer of dimers in the 1D chain. 1 exhibits high electrical conductivity (4.2-8.0 S cm-1) at 300 K and metallic behavior above 125 K. 2 is a semiconductor. 1 exhibits almost temperature independent magnetic susceptibility (ca. 1.1 x 10-4 emu mol-1) which is attributed to Pauli paramagnetism, whereas the spin degree of freedom in 2 has been lost. Although the basic structures are closely related, they exhibited different solid-state properties that depend on the valence state of the platinum atoms and the periodicity within the 1D chain.  相似文献   
988.
We report optical band gap modifications of single-walled carbon nanotubes upon C60 insertions by using photoluminescence and the corresponding excitation spectroscopy. The shifts in optical transition energies strongly depend on the tube diameter (dt) and the "2n + m" family type, which can be explained by the local strain and the hybridization between the nanotube states and the C60 molecular orbitals. The present results provide possible design rules for nanotube-based heterostructures having a specific type of electronic functionality.  相似文献   
989.
The low-frequency mode activity of metalloporphyrins has been studied for iron porphine-halides (Fe(P)(X), X = Cl, Br) and nitrophorin 4 (NP4) using femtosecond coherence spectroscopy (FCS) in combination with polarized resonance Raman spectroscopy and density functional theory (DFT). It is confirmed that the mode symmetry selection rules for FCS are the same as for Raman scattering and that both Franck-Condon and Jahn-Teller mode activities are observed for Fe(P)(X) under Soret resonance conditions. The DFT-calculated low-frequency (20-400 cm (-1)) modes, and their frequency shifts upon halide substitution, are in good agreement with experimental Raman and coherence data, so that mode assignments can be made. The doming mode is located at approximately 80 cm (-1) for Fe(P)(Cl) and at approximately 60 cm (-1) for Fe(P)(Br). NP4 is also studied with coherence techniques, and the NO-bound species of ferric and ferrous NP4 display a mode at approximately 30-40 cm (-1) that is associated with transient heme doming motion following NO photolysis. The coherence spectra of three ferric derivatives of NP4 with different degrees of heme ruffling distortion are also investigated. We find a mode at approximately 60 cm (-1) whose relative intensity in the coherence spectra depends quadratically on the magnitude of the ruffling distortion. To quantitatively account for this correlation, a new "distortion-induced" Raman enhancement mechanism is presented. This mechanism is unique to low-frequency "soft modes" of the molecular framework that can be distorted by environmental forces. These results demonstrate the potential of FCS as a sensitive probe of dynamic and functionally important nonplanar heme vibrational excitations that are induced by the protein environmental forces or by the chemical reactions in the aqueous phase.  相似文献   
990.
We present herein a simple but versatile microfluidic system for the treatment of cells with millisecond chemical stimulus, by rapidly exchanging the carrier-medium of cells twice in a microchannel. A technique we refer to as 'hydrodynamic filtration' has been employed for the exchange of medium, in which the virtual width of flow in the microchannel determines the size of filtered cells/particles. The treatment time of cells could be rigidly adjusted by controlling the inlet flow rates and by changing the volume of the stimulating area in the microchannel. In the experiment, two types of microdevices were designed and fabricated, and at first, the ability for carrier-medium exchange was confirmed. As an application of the presented system, we examined the influence of the treatment time of HeLa cells with Triton X-100, a non-ionic surfactant used to solubilize the cellular membrane, on cell viability, varying the average treatment time from 17 to 210 ms. Both quantitative and qualitative analyses were performed to estimate the damage on cell membrane, demonstrating that the cell viability dramatically decreased when the treatment time was longer than approximately 40 ms. The obtained results demonstrated the ability of the presented system to conduct the rapid stimulation of cells, which would be useful for the analysis of biochemical reactions at the cell surface.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号