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991.
Kai Pahnke Josef Brandt Dr. Ganna Gryn'ova Dr. Ching Y. Lin Dr. Ozcan Altintas Dr. Friedrich G. Schmidt Dr. Albena Lederer Prof. Michelle L. Coote Prof. Christopher Barner‐Kowollik 《Angewandte Chemie (International ed. in English)》2016,55(4):1514-1518
We show that, all other conditions being equal, bond cleavage in the middle of molecules is entropically much more favored than bond cleavage at the end. Multiple experimental and theoretical approaches have been used to study the selectivity for bond cleavage or dissociation in the middle versus the end of both covalent and supramolecular adducts and the extensive implications for other fields of chemistry including, e.g., chain transfer, polymer degradation, and control agent addition are discussed. The observed effects, which are a consequence of the underlying entropic factors, were predicted on the basis of simple theoretical models and demonstrated via high‐temperature (HT) NMR spectroscopy of self‐assembled supramolecular diblock systems as well as temperature‐dependent size‐exclusion chromatography (TD SEC) of covalently bonded Diels–Alder step‐growth polymers. 相似文献
992.
Kinetically Controlled Sequential Growth of Surface‐Grafted Chiral Supramolecular Copolymers 下载免费PDF全文
Hendrik Frisch Eva‐Corinna Fritz Friedrich Stricker Lars Schmüser Daniel Spitzer Dr. Tobias Weidner Prof. Dr. Bart Jan Ravoo Prof. Dr. Pol Besenius 《Angewandte Chemie (International ed. in English)》2016,55(25):7242-7246
We report a facile strategy to grow supramolecular copolymers on Au surfaces by successively exposing a surface‐anchored monomer to solutions of oppositely charged peptide comonomers. Charge regulation on the active chain end of the polymer sufficiently slows down the kinetics of the self‐assembly process to produce kinetically trapped copolymers at near‐neutral pH. We thereby achieve architectural control at three levels: The β‐sheet sequences direct the polymerization away from the surface, the height of the supramolecular copolymer brushes is well‐controlled by the stepwise nature of the alternating copolymer growth, and 2D spatial resolution is realized by using micropatterned initiating monomers. The programmable nature of the resulting architectures renders this concept attractive for the development of customized biomaterials or chiral interfaces for optoelectronics and sensor applications. 相似文献
993.
Synthesis of Cadmium Nitride Cd3N2 by Thermolysis of Cadmium Azide Cd(N3)2 and Crystal Structure Determination from X‐ray Powder Diffraction Data Cadmium nitride Cd3N2 was obtained by thermolysis of cadmium azide Cd(N3)2 at 2·10?6 mbar and 210 °C. It was obtained as a black, crystalline powder which becomes brown in air due to formation of cadmium amide Cd(NH2)2. The crystal structure of Cd3N2 was determined from X‐ray powder diffraction data and refined by the Rietveld method ( , a = 10.829(9) Å, V = 1270(3) Å3, Z = 16, R(F2) = 0.1196). Cd3N2 crystallizes in the anti‐bixbyite structure type and is isotypic to Ca3N2. 相似文献
994.
Jürgen Köhler Prof. Dr. Holger A. Friedrich Changhoon Lee Myung‐Hwan Whangbo Prof. Dr. 《无机化学与普通化学杂志》2007,633(9):1464-1471
IrIn7GeO8 = [IrIn6](GeO4)(InO4) and Compounds of the Solid Solution Series [IrIn6](Ge1+xIn1?4x/3O8) (0 ≤ x ≤ 0.75): First Oxides containing [IrIn6] Octahedra The low valent indiumoxides IrIn7GeO8 = [IrIn6](GeO4)(InO4) and [IrIn6](Ge1+xIn1?4x/3O8) (0 x ≤ 0.75) are formed by heating intimate mixtures of Ir, In, In2O3 and GeO2 in corundum crucibles under an atmosphere of argon (1420 K, 70 h). The compounds are black and semiconducting. X‐ray powder diffraction patterns can be indexed on the basis of a face centered cubic unit cell with lattice parameters ranging from a = 1012.3(1) pm (x = 0) to a = 1007.3(1) pm (x = 0.75). Characteristic building units in [IrIn6](Ge1+xIn1?4x/3O8) are isolated [IrIn6]9+ octahedra with short Ir‐In distances of 253.5 pm, which are linked via [GeO4]4? and [InO4]5? tetrahedra to a three dimensional framework. Starting from IrIn7GeO8 = [IrIn6](GeO4)(InO4), the isoelectronic substitution of 4 In3+ ions by 3 Ge4+ ions and one Ge‐vacancy leads to the formation of a solid solution series [IrIn6](GeO4)1+x(O4)x/3(InO4)1?4x/3, which shows a slight decrease in the cubic lattice parameter with increasing x. According to Rietveld refinements the structure of [IrIn6](GeO4)(InO4) exhibits a statistical distribution of the tetrahedrally coordinated Ge and In atoms ( , R(prof.) = 4.4 %, R(int.) = 2.5 %). The crystal and electronic structures of [IrIn6](GeO4)(InO4) are discussed on the basis of first principles electronic structure calculations. 相似文献
995.
We determined the isotropic, isothermal compressibility of ubiquitin by pressure tuning spectral holes burnt into the red edge of the absorption spectrum of the single tyrosine residue. The pressure shift is perfectly linear with burn frequency. From these data, a compressibility of 0.086 GPa(-1) in the local environment of the tyrosine residue could be determined. This value fits nicely into the range known for proteins. Although the elastic behavior at low temperatures does not show any unusual features, the pressure tuning behavior at room temperature is quite surprising: the pressure-induced spectral shift is close to zero, even up to very high pressure levels of 0.88 GPa, well beyond the denaturation point. The reason for this behavior is attributed to equally strong blue as well as red spectral pressure shifts resulting in an average pressure-induced solvent shift that is close to zero. 相似文献
996.
Rajeev Ranjan Anatoliy Senyshyn Hans Boysen Friedrich Frey 《Journal of solid state chemistry》2007,180(3):995-1001
Neutron powder diffraction studies of Na1/2La1/2TiO3 and Na1/2Tb1/2TiO3, and a synchrotron X-ray powder diffraction study of Na1/2Eu1/2TiO3 have been carried out. Rietveld refinement of the structures confirmed that Na1/2Eu1/2TiO3 and Na1/2Tb1/2TiO3 crystallizes in GdFeO3 type of structure (space group Pbnm). The ambiguity in the literature regarding the crystal structure of Na1/2La1/2TiO3 has been resolved and the correct space group is found to be . A detailed list of refined structural parameters of these compounds is presented for the first time. 相似文献
997.
Manuel Reiter Arthur Markus Anton Jian Chang Friedrich Kremer Miriam M. Unterlass Jiayin Yuan 《Journal of polymer science. Part A, Polymer chemistry》2021,59(14):1518-1527
Herein, we report a simple and versatile synthetic approach towards siloxane-based poly(ionic liquid)s (PILs) with unusually low glass transition temperatures (Tg) down to −73°C, and thus “liquid-like” behavior at room temperature. We designed a polydimethylsiloxane-derived copolymer carrying dialkylimidazolium moieties, and by careful selection of the side-chain length and the type of anions we were able to manipulate its Tg over a wide range and reach high ionic conductivities (σDC) up to 4.8 × 10−5 S/cm at 300 K. The ionized species make up only a minor fraction (<25 mol%) of the overall repeating units and are supposedly randomly distributed: Yet our results indicate dramatic effects on the thermal properties due to repulsive interactions between ionic and non-ionic segments. 相似文献
998.
Bodo Felsner Dr. Volodymyr Bon Dr. Jack D. Evans Friedrich Schwotzer Dr. Ronny Grünker Dr. Irena Senkovska Prof. Dr. Stefan Kaskel 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(37):9708-9715
The switching mechanism of the flexible framework Zn4O(benztb)1.5 (benztb=N,N,N’,N’-benzidine tetrabenzoate), also known as DUT-13, was studied by advanced powder X-ray diffraction (PXRD) and gas physisorption techniques. In situ synchrotron PXRD experiments upon physisorption of nitrogen (77 K) and n-butane (273 K) shed light on the hitherto unnoticed guest-induced breathing in the MOF. The mechanism of contraction is based on the conformationally labile benztb ligand and accompanied by a reduction in specific pore volume from 2.03 cm3 g−1 in the open-pore phase to 0.91 cm3 g−1 in the contracted-pore phase. The high temperature limit for adsorption-induced contraction of 170 K, determined by systematic temperature variation of methane adsorption isotherms, indicates that the DUT-13 framework is softer than other mesoporous MOFs like DUT-49 and does not support the formation of overloaded metastable states required for negative gas-adsorption transitions. 相似文献
999.
Alexander Friedrich Jonathan Eyselein Holger Elsen Jens Langer Jürgen Pahl Michael Wiesinger Sjoerd Harder 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(28):7756-7763
Strongly Lewis acidic cationic aluminium complexes, stabilized by β–diketiminate (BDI) ligands and free of Lewis bases, have been prepared as their B(C6F5)4− salts and were investigated for catalytic activity in imine hydrogenation. The backbone (R1) and N (R2) substituents on the R1,R2BDI ligand (R1,R2BDI=HC[C(R1)N(R2)]2) influence sterics and Lewis acidity. Ligand bulk increases along the row Me,DIPPBDI<Me,DIPePBDI≈tBu,DIPPBDI<tBu,DIPePBDI; DIPP=2,6-C(H)Me2-phenyl, DIPeP=2,6-C(H)Et2-phenyl. The Gutmann-Beckett test showed acceptor numbers of: (tBu,DIPPBDI)AlMe+ 85.6, (tBu,DIPePBDI)AlMe+ 85.9, (Me,DIPPBDI)AlMe+ 89.7, (Me,DIPePBDI)AlMe+ 90.8, (Me,DIPPBDI)AlH+ 95.3. Steric and electronic factors need to be balanced for catalytic activity in imine hydrogenation. Open, highly Lewis acidic, cations strongly coordinate imine rendering it inactive as a Frustrated Lewis Pair (FLP). The bulkiest cations do not coordinate imine but its combination is also not an active catalyst. The cation (tBu,DIPPBDI)AlMe+ shows the best catalytic activity for various imines and is also an active catalyst for the Tishchenko reaction of benzaldehyde to benzylbenzoate. DFT calculations on the mechanism of imine hydrogenation catalysed by cationic Al complexes reveal two interconnected catalytic cycles operating in concert. Hydrogen is activated either by FLP reactivity of an Al⋅⋅⋅imine couple or, after formation of significant quantities of amine, by reaction with an Al⋅⋅⋅amine couple. The latter autocatalytic Al⋅⋅⋅amine cycle is energetically favoured. 相似文献
1000.
Friedrich Martin Schneider 《Algebra Universalis》2017,78(3):337-354
Birkhoff’s HSP theorem characterizes the classes of models of algebraic theories as those being closed with respect to homomorphic images, subalgebras, and products. In particular, it implies that an algebra B satisfies all equations that hold in an algebra A of the same signature if and only if B is a homomorphic image of a subalgebra of a (possibly infinite) direct power of A. The former statement is equivalent to the existence of a natural map sending term functions of the algebra A to those of B—the natural clone homomorphism. The study of continuity properties of natural clone homomorphisms has been initiated recently by Bodirsky and Pinsker for locally oligomorphic algebras.Revisiting the argument of Bodirsky and Pinsker, we show that for any algebra B in the variety generated by an algebra A, the induced natural clone homomorphism is uniformly continuous if and only if every finitely generated subalgebra of B is a homomorphic image of a subalgebra of a finite power of A. Based on this observation, we study the question as to when Cauchy continuity of natural clone homomorphisms implies uniform continuity. We introduce the class of almost locally finite algebras, which encompasses all locally oligomorphic as well as all locally finite algebras, and show that, in case A is almost locally finite, then the considered natural homomorphism is uniformly continuous if (and only if) it is Cauchy-continuous. In particular, this provides a locally finite counterpart of the result by Bodirsky and Pinsker. Along the way, we also discuss some peculiarities of oligomorphic permutation groups on uncountable sets. 相似文献