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81.
Abstract The knowledge of water exchange reaction mechanism in aqueous solutions of Gd3+ polyaminocarboxylates is important for the understanding of the relatively slow water exchange rates measured for these complexes. Variable ressure measurements show a change of mechanism from associatively activated on [Gd(H2O)8]3+ and [Gd(PDTA)(H2O2)2]? to probably limiting dissociative on the MRI contrast agents [Gd(DOTA)(H2O)]?, [Gd(DTPA)(H2O)]2? and [Gd(DTPA-BMA)(H2O)]. 相似文献
82.
Powell AB Suzuki Y Ueda M Bielawski CW Cowley AH 《Journal of the American Chemical Society》2011,133(14):5218-5220
We report the synthesis and catalytic activity of a polymeric imidazolium salt. In contrast to the well-known resin-supported N-heterocyclic carbenes (NHCs), the material described herein affords a polymer with NHCs orthogonally positioned along a main chain upon generation in situ. The unique structural characteristics of the corresponding poly(NHC)s enabled the materials to display catalytic activities that were similar or superior to those displayed by monomeric analogues. Moreover, the new catalyst was successfully recovered and reused with minimal loss of performance over several cycles. 相似文献
83.
84.
Feltham HL Klöwer F Cameron SA Larsen DS Lan Y Tropiano M Faulkner S Powell AK Brooker S 《Dalton transactions (Cambridge, England : 2003)》2011,40(43):11425-11432
A family of thirteen tetranuclear heterometallic zinc(II)-lanthanide(III) complexes of the hexa-imine macrocycle (L(Pr))(6-), with general formula Zn(II)(3)Ln(III)(L(Pr))(NO(3))(3)·xsolvents (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm or Yb), were prepared in a one-pot synthesis using a 3:1:3:3 reaction of zinc(II) acetate, the appropriate lanthanide(III) nitrate, the dialdehyde 1,4-diformyl-2,3-dihydroxybenzene (H(2)L(1)) and 1,3-diaminopropane. A hexanuclear homometallic zinc(II) macrocyclic complex [Zn(6)(L(Pr))(OAc)(5)(OH)(H(2)O)]·3H(2)O was obtained using a 2:0:1:1 ratio of the same reagents. A control experiment using a 1:0:1:1 ratio failed to generate the lanthanide-free [Zn(3)(L(Pr))] macrocyclic complex. The reaction of H(2)L(1) and zinc(II) acetate in a 1:1 ratio yielded the pentanuclear homometallic complex of the dialdehyde H(2)L(1), [Zn(5)(L(1))(5)(H(2)O)(6)]·3H(2)O. An X-ray crystal structure determination revealed [Zn(3)(II)Pr(III)(L(Pr))(NO(3))(2)(DMF)(3)](NO(3))·0.9DMF has the large ten-coordinate lanthanide(III) ion bound in the central O(6) site with two bidentate nitrate anions completing the O(10) coordination sphere. The three square pyramidal zinc(II) ions are in the outer N(2)O(2) sites with a fifth donor from DMF. Measurement of the magnetic properties of [Zn(II)(3)Dy(III)(L(Pr))(NO(3))(3)(MeOH)(3)]·4H(2)O with a weak external dc field showed that it has a frequency-dependent out-of-phase component of ac susceptibility, indicative of slow relaxation of the magnetization (SMM behaviour). Likewise, the Er and Yb analogues are field-induced SMMs; the latter is only the second example of a Yb-based SMM. The neodymium, ytterbium and erbium complexes are luminescent in the solid phase, but only the ytterbium and neodymium complexes show strong lanthanide-centred luminescence in DMF solution. 相似文献
85.
John A. Powell Jong J. Whang Frederick H. Owens Roger K. Graham 《Journal of polymer science. Part A, Polymer chemistry》1967,5(10):2655-2664
Copolymers of methyl methacrylate with methyl and ethyl α-hydroxymethylacrylate and with α-hydroxymethylstyrene have been prepared with free-radical initiators at temperatures below 80°C. At higher reaction temperatures or under extrusion conditions, alcohol was eliminated and the free hydroxyl content was greatly decreased. All evidence indicates the formation of six-membered lactone groups in this post-polymerization reaction: direct evidence for their formation is lacking, however, since neither infrared nor nuclear magnetic resonance spectra could be used to detect lactonization in this system. The loss of activity from 14C ester-labeled methyl methacrylate copolymer on heating could be correlated with the extent of lactonization. The degree of lactonization is relatively less with copolymers containing higher amounts of hydroxymethyl groups. The resulting polymers exhibit higher heat distortion temperatures and decreased impact resistance when compared to poly(methyl methacrylate). Attempts were made to incorporate similar lactone structures by cyclocopolymerization with methyl methacrylate of α-methacryloxymethylstyrene or ethyl α-methacryloxymethylacrylate, but only crosslinked polymers or polymers with pendent unsaturation were found. 相似文献
86.
M. J. D. Powell 《Mathematical Programming》1977,12(1):141-147
Davidon [1] provides a variable metric algorithm for minimization calculations that achieves quadratic termination without any line searches by a method that uses positive definite second derivative approximations. We give a new and elementary proof of these termination properties. It shows the great freedom allowed by the algorithm in the choice of search directions and it explains the effect of a restart procedure. 相似文献
87.
M. J. D. Powell 《Mathematical Programming》1984,29(3):297-303
Many trust region algorithms for unconstrained minimization have excellent global convergence properties if their second derivative
approximations are not too large [2]. We consider how large these approximations have to be, if they prevent convergence when
the objective function is bounded below and continuously differentiable. Thus we obtain a useful convergence result in the
case when there is a bound on the second derivative approximations that depends linearly on the iteration number. 相似文献
88.
John?J.?BenedettoEmail author Wojciech?Czaja Przemystaw?Gadziński Alexander?M.?Powell 《Journal of Geometric Analysis》2003,13(2):239-254
For any positive real numbers A, B, and d satisfying the conditions
, d>2, we construct a Gabor orthonormal basis for L2(ℝ), such that the generating function g∈L2(ℝ) satisfies the condition:∫ℝ|g(x)|2(1+|x|
A
)/log
d
(2+|x|)dx < ∞ and
. 相似文献
89.
John T. Powell Benjamin O. Akhuetie‐Oni Dr. Zhao Zhang Prof. Chenxiang Lin 《Angewandte Chemie (International ed. in English)》2016,55(38):11412-11416
Mechanically interlocked supramolecular assemblies are appealing building blocks for creating functional nanodevices. Herein, we describe the multistep assembly of large DNA origami rotaxanes that are capable of programmable structural switching. We validated the topology and structural integrity of these rotaxanes by analyzing the intermediate and final products of various assembly routes by electrophoresis and electron microscopy. We further analyzed two structure‐switching behaviors of our rotaxanes, which are both mediated by DNA hybridization. In the first mechanism, the translational motion of the macrocycle can be triggered or halted at either terminus. In the second mechanism, the macrocycle can be elongated after completion of the rotaxane assembly, giving rise to a unique structure that is otherwise difficult to access. 相似文献
90.
We report on further developments in the three-dimensional tracking of a particle deep within the tumbling ball charge of
an experimental mill. The experimental X-ray program employing the use of bi-planar X-ray angiography now includes the tracking
of a typical 6 mm bulk charge particle in three dimensions with a spatial resolution that is accurate to within 0.15 mm. The
improved experimental tracking techniques presented were developed for the purpose of generating accurate three-dimensional
particle trajectory data against which to validate a numerical method for the simulation of discrete media, namely the discrete
element method (DEM). These improvements are complimented with techniques for comparing charge profiles between numerical
DEM simulations and three-dimensional experimental trajectory data. 相似文献