全文获取类型
收费全文 | 2542篇 |
免费 | 65篇 |
国内免费 | 8篇 |
专业分类
化学 | 1727篇 |
晶体学 | 23篇 |
力学 | 32篇 |
数学 | 564篇 |
物理学 | 269篇 |
出版年
2022年 | 17篇 |
2020年 | 45篇 |
2019年 | 29篇 |
2018年 | 17篇 |
2017年 | 25篇 |
2016年 | 42篇 |
2015年 | 68篇 |
2014年 | 52篇 |
2013年 | 92篇 |
2012年 | 92篇 |
2011年 | 99篇 |
2010年 | 75篇 |
2009年 | 58篇 |
2008年 | 118篇 |
2007年 | 110篇 |
2006年 | 95篇 |
2005年 | 102篇 |
2004年 | 79篇 |
2003年 | 63篇 |
2002年 | 50篇 |
2001年 | 22篇 |
2000年 | 32篇 |
1999年 | 25篇 |
1998年 | 30篇 |
1997年 | 22篇 |
1996年 | 22篇 |
1995年 | 26篇 |
1994年 | 27篇 |
1993年 | 35篇 |
1992年 | 28篇 |
1991年 | 18篇 |
1989年 | 25篇 |
1988年 | 30篇 |
1987年 | 30篇 |
1985年 | 34篇 |
1984年 | 46篇 |
1983年 | 26篇 |
1982年 | 20篇 |
1981年 | 31篇 |
1980年 | 38篇 |
1979年 | 23篇 |
1978年 | 23篇 |
1977年 | 26篇 |
1976年 | 22篇 |
1975年 | 26篇 |
1974年 | 26篇 |
1966年 | 16篇 |
1965年 | 16篇 |
1964年 | 16篇 |
1956年 | 24篇 |
排序方式: 共有2615条查询结果,搜索用时 15 毫秒
991.
Michal Ilavsk Karel Bouchal Helena Valentovä František Lednick Antonín Sikora Josef Baldrian 《Journal of Macromolecular Science: Physics》2013,52(5):645-666
Synthesis of the liquid crystalline (LC) diol 6,6′-[ethylenebis(l,4-phenylene-oxy)]-dihexanol (I) is described. The structure of polyurethanes prepared from diol I and 4,4′-methylenedi(phenyl isocyanate) (MDI), 4,4′-methylenedi(cyclohexyl isocyanate) (HMDI), or 2(4)-methyl-l,3-phenylene diisocyanate (TDI) at 1:1 molar ratios of isocyanate and hydroxy groups is studied by dynamic mechanical spectroscopy, differential scanning calorimetry (DSC), polarizing microscopy, and x-ray scattering. The polymer prepared from HMDI and the diol (I/HMDI) shows, on cooling, thermal behavior typical of amorphous polymers. A frequency-temperature superposition could be applied to the mechanical data, and the horizontal shift factor satisfied the Williams-Landel-Ferry (WLF) equation. A more-complex thermal behavior was found for I/HMDI polymer during subsequent heating; above 70°C, the formation of an ordered structure takes place, and this structure melts at about 120°C. Complex thermal behavior is exhibited by I/TDI polymer. On cooling its melt, the polymer forms a nematic phase at about 80°C, which freezes into the LC glassy state. On heating, the mesophase melts, and. subsequently, a better-ordered smectic phase is formed at 95°C, which melts at 120°C. This structure buildup is accompanied by a rapid increase in storage modulus G′, and the sample shows thermorheologically complex mechanical behavior. The polymer formed from the diol and MDI (I/MDI) exhibits a most-complex thermal behavior. On cooling and heating, four transitions can be detected in its thermal mechanical behavior, and the structure of the polymer is strongly dependent on its thermal history. 相似文献
992.
993.
The spin dipole–spin dipole and spin–orbit coupling contributions to the zero-field splitting parameters D and E of [CH3–N–CH3]+, [CH3–P–CH3]+, and [CH3–As–CH3]+ have been calculated from CASSCF(14,14)/cc-pVTZ wave functions and the Breit–Pauli Hamiltonian at T1 B3LYP/cc-pVTZ optimized geometries. The spin–orbit coupling contributions represent a minor correction for the dimethylnitrenium cation, which has a triplet ground state. They dominate the spin–spin terms by an order of magnitude in the dimethylphosphenium cation and by more than two orders of magnitude in the dimethylarsenium cation, both of which have a singlet ground state. The properties of all these biradicaloids follow expectations based on the simple algebraic 2-in-2 model of biradical structure. 相似文献
994.
995.
Josef Brody Stanley H. Erlwanger 《International Journal of Mathematical Education in Science & Technology》2013,44(4):645-659
This paper extends the themes on the computational approach discussed in Parts 1 and 2 to the differentiation of functions. One theme is that constructing diagrams for programs using a calculator helps students to visualize processes and thus acquire the concepts involved. The ability to construct diagrams for a given problem is identical to that of programming a calculator to solve the problem. However, in this case constructing the diagram for derivatives from the diagram of the function also represents the ability to teach a calculator how to perform formal differentiation. A second theme is that the approach promotes sequential or procedural thinking which is important in both programming and mathematics. A third theme is that the approach permits the generation of many subproblems, thereby encouraging one to pose questions rather than merely to solve those set by others. Consequently, it becomes clear that the approach has wider applications in the learning of mathematics. 相似文献
996.
In this paper, we show that one can interrelate pluriharmonic maps with para-pluriharmonic maps by means of the loop group method. As an appendix, we give examples for the interrelation between pluriharmonic maps and para-pluriharmonic maps. Moreover, we investigate the relation among CMC-surfaces by use of such maps. 相似文献
997.
Lunkenbein T Kamperman M Li Z Bojer C Drechsler M Förster S Wiesner U Müller AH Breu J 《Journal of the American Chemical Society》2012,134(30):12685-12692
Nanostructured inverse hexagonal polyoxometalate composite films were cast directly from solution using poly(butadiene-block-2-(dimethylamino)ethyl methacrylate) (PB-b-PDMAEMA) diblock copolymers as structure directing agents for phosphomolybdic acid (H(3)[PMo(12)O(40)], H(3)PMo). H(3)PMo units are selectively incorporated into the PDMAEMA domains due to electrostatic interactions between protonated PDMAEMA and PMo(3-) anions. Long solvophilic PB chains stabilized the PDMAEMA/H(3)PMo aggregates in solution and reliably prevented macrophase separation. The choice of solvent is crucial. It appears that all three components, both blocks of the diblock copolymer as well as H(3)PMo, have to be soluble in the same solvent which turned out to be tetrahydrofuran, THF. Evaporation induced self-assembly resulted in highly ordered inverse hexagonal nanocomposite films as observed from transmission electron microscopy and small-angle X-ray scattering. This one-pot synthesis may represent a generally applicable strategy for integrating polyoxometalates into functional architectures and devices. 相似文献
998.
Šardzík R Green AP Laurent N Both P Fontana C Voglmeir J Weissenborn MJ Haddoub R Grassi P Haslam SM Widmalm G Flitsch SL 《Journal of the American Chemical Society》2012,134(10):4521-4524
O-mannosyl glycans are known to play an important role in regulating the function of α-dystroglycan (α-DG), as defective glycosylation is associated with various phenotypes of congenital muscular dystrophy. Despite the well-established biological significance of these glycans, questions regarding their precise molecular function remain unanswered. Further biological investigation will require synthetic methods for the generation of pure samples of homogeneous glycopeptides with diverse sequences. Here we describe the first total syntheses of glycopeptides containing the tetrasaccharide NeuNAcα2-3Galβ1-4GlcNAcβ1-2Manα, which is reported to be the most abundant O-mannosyl glycan on α-DG. Our approach is based on biomimetic stepwise assembly from the reducing end and also gives access to the naturally occurring mono-, di-, and trisaccharide substructures. In addition to the total synthesis, we have developed a "one-pot" enzymatic cascade leading to the rapid synthesis of the target tetrasaccharide. Finally, solid-phase synthesis of the desired glycopeptides directly on a gold microarray platform is described. 相似文献
999.
J Kohl-Landgraf M Braun C Ozçoban DP Gonçalves A Heckel J Wachtveitl 《Journal of the American Chemical Society》2012,134(34):14070-14077
The reversible switching of a water-soluble spiropyran compound is recorded over 1 ns by means of femtosecond vis-pump/vis- and IR-probe spectroscopy under aqueous conditions. Our investigations reveal that the photochemical conversion from the closed spiropyran to the open merocyanine takes 1.6 ps whereas the reversed photoreaction is accomplished within 25 ps. The combination of time-resolved and steady-state observations allows us to reveal central parts of the reaction pathway leading to either form. The enhanced water solubility, its fast and efficient switching behavior, and its stability against hydrolysis over a time range of several weeks make this compound an attractive and versatile tool for biological applications. 相似文献
1000.
Josef Jablonsky 《Central European Journal of Operations Research》2012,20(3):435-449
Data envelopment analysis models usually split decision making units into two basic groups, efficient and inefficient. Efficiency score of inefficient units allows their ranking but efficient units cannot be ranked directly because of their maximum efficiency. That is why there are formulated several models for ranking of efficient units. The paper presents two original models for ranking of efficient units in data envelopment analysis—they are based on multiple criteria decision making techniques—goal programming and analytic hierarchy process. The first model uses goal programming methodology and minimizes either the sum of undesirable deviations or maximal undesirable deviation from the efficient frontier. The second approach is analytic hierarchy process model for ranking of efficient units. The two presented models are compared with several super-efficiency models and other approaches for ranking decision making units in DEA models including definitions based on distances from optimistic and pessimistic envelopes and cross efficiency evaluation models. The results of the analysis by all presented models are illustrated on a real data set—evaluation of 194 bank branches of one of the Czech commercial banks. 相似文献