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   4647篇
  免费   126篇
  国内免费   18篇
化学   2967篇
晶体学   25篇
力学   62篇
数学   780篇
物理学   957篇
  2023年   22篇
  2021年   61篇
  2020年   75篇
  2019年   96篇
  2018年   83篇
  2017年   56篇
  2016年   116篇
  2015年   85篇
  2014年   113篇
  2013年   228篇
  2012年   220篇
  2011年   246篇
  2010年   134篇
  2009年   157篇
  2008年   218篇
  2007年   186篇
  2006年   176篇
  2005年   218篇
  2004年   148篇
  2003年   143篇
  2002年   129篇
  2001年   55篇
  2000年   50篇
  1999年   43篇
  1998年   43篇
  1997年   41篇
  1996年   58篇
  1995年   64篇
  1994年   62篇
  1993年   57篇
  1992年   53篇
  1991年   45篇
  1990年   42篇
  1989年   40篇
  1988年   43篇
  1987年   35篇
  1986年   28篇
  1985年   33篇
  1984年   48篇
  1983年   26篇
  1982年   35篇
  1981年   50篇
  1980年   43篇
  1979年   52篇
  1978年   36篇
  1977年   43篇
  1976年   42篇
  1975年   43篇
  1974年   34篇
  1973年   33篇
排序方式: 共有4791条查询结果,搜索用时 15 毫秒
1.

Fabricating mechanically strong hydrogels that can withstand the conditions in internal tissues is a challenging task. We have designed hydrogels based on multicomponent systems by combining chitosan, starch/cellulose, PVA, and PEDOT:PSS via one-pot synthesis. The starch-based hydrogels were homogeneous, while the cellulose-based hydrogels showed the presence of cellulose micro- and nanofibers. The cellulose-based hydrogels demonstrated a swelling ratio between 121 and 156%, while the starch-based hydrogels showed higher values, from 234 to 280%. Tensile tests indicated that the presence of starch in the hydrogels provided high flexibility (strain at break?>?300%), while combination with cellulose led to the formation of stiffer hydrogels (elastic moduli 3.9–6.6 MPa). The ultimate tensile strength for both types of hydrogels was similar (2.8–3.9 MPa). The adhesion and growth of human osteoblast-like SAOS-2 cells was higher on hydrogels with cellulose than on hydrogels with starch, and was higher on hydrogels with PEDOT:PSS than on hydrogels without this polymer. The metabolic activity of cells cultivated for 3 days in the hydrogel infusions indicated that no acutely toxic compounds were released. This is promising for further possible applications of these hydrogels in tissue engineering or in wound dressings.

Graphical abstract
  相似文献   
2.
The Ramanujan Journal - We study fixed points in integer partitions viewed, respectively, as weakly increasing or weakly decreasing structures. A fixed point is a point with value i in position i....  相似文献   
3.
We investigate the relationship between structure (crystal and molecular) and tert-butyl and methyl group dynamics in 2-(tert-butyl)-9-(4-(tert-butyl)phenyl)anthracene. Powder and single-crystal X-ray diffraction, taken together, show that different polycrystalline samples recrystallized from different solvents have different amounts of at least four polymorphs (crystallites having different crystal structures), of which we have identified three by single crystal X-ray diffraction. The molecules in the asymmetric units of the different crystal structures differ by the dihedral angle the tert-butylphenyl group makes with the anthracene moiety. Ab initio electronic structure calculations on the isolated molecule show that very little intramolecular energy is required to change this angle over a range of about 60° which is probably the origin of the concomitant polymorphism (crystals of more than one polymorph in a polycrystalline sample). Solid state 1H nuclear magnetic resonance (NMR) spin-lattice relaxation experiments support the powder and single-crystal X-ray results and provide average NMR activation energies (closely related to rotational barriers) for the rotation of the tert-butyl groups and their constituent methyl groups. These barriers have both an intramolecular and an intermolecular component. The latter is sensitive to the crystal structure. The intramolecular components of the rotational barriers of the two tert-butyl groups in the isolated molecule are investigated with ab initio electronic structure calculations.  相似文献   
4.
Journal of Thermal Analysis and Calorimetry - We introduce an explicit theoretical formula for the heat capacity peaks that occur in materials during phase changes. Such materials are of importance...  相似文献   
5.
6.
Ruthenium is a platinoid that exhibits a range of unique chemical properties in solution, which are exploited in a variety of applications, including luminescent probes, anticancer therapies, and artificial photosynthesis. This paper focuses on a recently demonstrated ability of this metal in its +3 oxidation state to form highly stable complexes with tris (hydroxymethyl)aminomethane (H2NC(CH2OH)3, Tris‐base or T) and imidazole (Im) ligands, where a single RuIII cation is coordinated by two molecules of each T and Im. High‐resolution electrospray ionization mass spectrometry (ESI MS) is used to characterize RuIII complexes formed by placing a RuII complex [(NH3)5RuIICl]Cl in a Tris buffer under aerobic conditions. The most abundant ionic species in ESI MS represent mononuclear complexes containing an oxidized form of the metal, ie, [XnRuIIIT2 – 2H]+, where X could be an additional T (n = 1) or NH3 (n = 0‐2). Di‐ and tri‐metal complexes also give rise to a series of abundant ions, with the highest mass ion representing a metal complex with an empirical formula Ru3C24O21N6H66 (interpreted as cyclo(T2RuO)3, a cyclic oxo‐bridged structure, where the coordination sphere of each metal is completed by two T ligands). The empirical formulae of the binuclear species are consistent with the structures representing acyclic fragments of cyclo(T2RuO)3 with addition of various combinations of ammonia and dioxygen as ligands. Addition of histidine in large molar excess to this solution results in complete disassembly of poly‐nuclear complexes and gives rise to a variety of ionic species in the ESI mass spectrum with a general formula [RuIIIHiskTm (NH3)n ? 2H]+, where k = 0 to 2, m = 0 to 3, and n = 0 to 4. Ammonia adducts are present for all observed combinations of k and m, except k = m = 2, suggesting that [His2RuIIIT2 ? 2H]+ represents a complex with a fully completed coordination sphere. The observed cornucopia of RuIII complexes formed in the presence of histidine is in stark contrast to the previously reported selective reactivity of imidazole, which interacts with the metal by preserving the RuT2 core and giving rise to a single abundant ruthenium complex (represented by [Im2RuIIIT2 ? 2H]+ in ESI mass spectra). Surprisingly, the behavior of a hexa‐histidine peptide (HHHHHH) is similar to that of a single imidazole, rather than a single histidine amino acid: The RuT2 core is preserved, with the following ionic species observed in ESI mass spectra: [HHHHHH·(RuIIIT2)m ? (3m‐1)H]+ (m = 1‐3). The remarkable selectivity of the imidazole interaction with the RuIIIT2 core is rationalized using energetic considerations at the quantum mechanical level of theory.  相似文献   
7.
Large single-ion magnetic anisotropy is observed in lithium nitride doped with iron. The iron sites are two-coordinate, putting iron doped lithium nitride amongst a growing number of two coordinate transition metal single-ion magnets (SIMs). Uniquely, the relaxation times to magnetisation reversal are over two orders of magnitude longer in iron doped lithium nitride than other 3d-metal SIMs, and comparable with high-performance lanthanide-based SIMs. To understand the origin of these enhanced magnetic properties a detailed characterisation of electronic structure is presented. Access to dopant electronic structure calls for atomic specific techniques, hence a combination of detailed single-crystal X-ray absorption and emission spectroscopies are applied. Together K-edge, L2,3-edge and Kβ X-ray spectroscopies probe local geometry and electronic structure, identifying iron doped lithium nitride to be a prototype, solid-state SIM, clean of stoichiometric vacancies where Fe lattice sites are geometrically equivalent. Extended X-ray absorption fine structure and angular dependent single-crystal X-ray absorption near edge spectroscopy measurements determine FeI dopant ions to be linearly coordinated, occupying a D6h symmetry pocket. The dopant engages in strong 3dπ-bonding, resulting in an exceptionally short Fe–N bond length (1.873(7) Å) and rigorous linearity. It is proposed that this structure protects dopant sites from Renner–Teller vibronic coupling and pseudo Jahn–Teller distortions, enhancing magnetic properties with respect to molecular-based linear complexes. The Fe ligand field is quantified by L2,3-edge XAS from which the energy reduction of 3dz2 due to strong 4s mixing is deduced. Quantification of magnetic anisotropy barriers in low concentration dopant sites is inhibited by many established methods, including far-infrared and neutron scattering. We deduce variable temperature L3-edge XAS can be applied to quantify the J = 7/2 magnetic anisotropy barrier, 34.80 meV (∼280 cm−1), that corresponds with Orbach relaxation via the first excited, MJ = ±5/2 doublet. The results demonstrate that dopant sites within solid-state host lattices could offer a viable alternative to rare-earth bulk magnets and high-performance SIMs, where the host matrix can be tailored to impose high symmetry and control lattice induced relaxation effects.

Taking advantage of synchrotron light source methods, we present the geometric and electronic structure of iron doped in lithium nitride.  相似文献   
8.
Triene 6π electrocyclization, wherein a conjugated triene undergoes a concerted stereospecific cycloisomerization to a cyclohexadiene, is a reaction of great historical and practical significance. In order to circumvent limitations imposed by the normally harsh reaction conditions, chemists have long sought to develop catalytic variants based upon the activating power of metal–alkene coordination. Herein, we demonstrate the first successful implementation of such a strategy by utilizing [(C5H5)Ru(NCMe)3]PF6 as a precatalyst for the disrotatory 6π electrocyclization of highly substituted trienes that are resistant to thermal cyclization. Mechanistic and computational studies implicate hexahapto transition-metal coordination as responsible for lowering the energetic barrier to ring closure. This work establishes a foundation for the development of new catalysts for stereoselective electrocyclizations.  相似文献   
9.
The atomic physics collaboration SPARC is a part of the APPA pillar at the future Facility for Antiproton and Ion Research. It aims at atomic-physics research across virtually the full range of atomic matter. An emphasis of this contribution are the atomic physics experiments addressing the collision dynamics in strong electro-magnetic fields as well as the fundamental interactions between electrons and heavy nuclei at the HESR. Here we give a short overview about the central instruments for SPARC experiments at this storage ring.  相似文献   
10.
We report the structural properties of ultra-small ThO2 and UO2 nanoparticles (NPs), which were synthesized without strong binding surface ligands by employing a covalent organic framework (COF-5) as an inert template. The resultant NPs were used to observe how structural properties are affected by decreasing grain size within bulk actinide oxides, which has implications for understanding the behavior of nuclear fuel materials. Through a comprehensive characterization strategy, we gain insight regarding how structure at the NP surface differs from the interior. Characterization using electron microscopy and small-angle X-ray scattering indicates that growth of the ThO2 and UO2 NPs was confined by the pores of the COF template, resulting in sub-3 nm particles. X-ray absorption fine structure spectroscopy results indicate that the NPs are best described as ThO2 and UO2 materials with unpassivated surfaces. The surface layers of these particles compensate for high surface energy by exhibiting a broader distribution of Th–O and U–O bond distances despite retaining average bond lengths that are characteristic of bulk ThO2 and UO2. The combined synthesis and physical characterization efforts provide a detailed picture of actinide oxide structure at the nanoscale, which remains highly underexplored compared to transition metal counterparts.

ThO2 and UO2 nanoparticles synthesized using a COF-5 template exhibit unpassivated surfaces and provide insight into nanoscale properties of actinides.  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