全文获取类型
收费全文 | 4801篇 |
免费 | 260篇 |
国内免费 | 20篇 |
专业分类
化学 | 3385篇 |
晶体学 | 21篇 |
力学 | 104篇 |
数学 | 639篇 |
物理学 | 932篇 |
出版年
2023年 | 48篇 |
2022年 | 65篇 |
2021年 | 97篇 |
2020年 | 137篇 |
2019年 | 133篇 |
2018年 | 90篇 |
2017年 | 74篇 |
2016年 | 218篇 |
2015年 | 161篇 |
2014年 | 194篇 |
2013年 | 265篇 |
2012年 | 390篇 |
2011年 | 492篇 |
2010年 | 231篇 |
2009年 | 152篇 |
2008年 | 317篇 |
2007年 | 333篇 |
2006年 | 302篇 |
2005年 | 283篇 |
2004年 | 199篇 |
2003年 | 133篇 |
2002年 | 112篇 |
2001年 | 49篇 |
2000年 | 37篇 |
1999年 | 34篇 |
1998年 | 22篇 |
1997年 | 19篇 |
1996年 | 42篇 |
1995年 | 38篇 |
1994年 | 34篇 |
1993年 | 38篇 |
1992年 | 31篇 |
1991年 | 11篇 |
1990年 | 12篇 |
1989年 | 13篇 |
1988年 | 8篇 |
1986年 | 13篇 |
1985年 | 14篇 |
1984年 | 14篇 |
1983年 | 12篇 |
1982年 | 20篇 |
1981年 | 10篇 |
1980年 | 9篇 |
1979年 | 21篇 |
1978年 | 11篇 |
1977年 | 10篇 |
1976年 | 13篇 |
1974年 | 8篇 |
1973年 | 35篇 |
1970年 | 7篇 |
排序方式: 共有5081条查询结果,搜索用时 31 毫秒
11.
A Geometric Characterization of Automatic Monoids 总被引:1,自引:0,他引:1
12.
Benjamin S. Hsiao Rong-Ming Ho Stephen Z. D. Cheng 《Journal of Polymer Science.Polymer Physics》1995,33(17):2439-2447
Unique crystallization and melting behavior in poly(aryl ether ketone ketone) containing alternated terephthalic and isophthalic moieties were studied by time-resolved synchrotron x-ray methods. Recently, this material has been shown to exhibit three polymorphs (forms I, II, and III). In this work, we further investigated their distinctive thermal properties and found that form I is the dominating and the most thermally stable phase while form II is favored by fast nucleation conditions and is the least stable phase. On the other hand, form III represents a minor intermediate phase that usually coexists with form I and can be transferred from form II and to form I. Structural and morphological changes in form I have been followed by simultaneous wide-angle x-ray diffraction (WAXD)/small-angle x-ray scattering (SAXS) measurements during cold- or melt-crystallization and subsequent melting. In all cases, a larger dimensional change was found in the crystallographic a-axis than the b-axis during heating and cooling. This may be due to the greater lateral stress variation with respect to temperature along the a direction of the primary lamellae which is induced by either the formation of secondary lamellae or the preferential chain-folding direction in poly(aryl ether ketone ketone)s. During the phase transitions of form II ← III in the cold-crystallized specimen and form III ← I in the melt-crystallized samples, lamellar variables (long period, lamellar thickness, and invariant) obtained from SAXS remain almost constant. This indicates that the density distribution in the long spacing is independent of the melting in form II or III. For melt-crystallization, the corresponding changes in unit-cell dimensions and lamellar morphology during the annealing-induced low endotherm are most consistent with the argument that these changes are due to the melting of thin lamellar population. © 1995 John Wiley & Sons, Inc. 相似文献
13.
14.
15.
Benjamin Falk Santiago M. Vallinas James V. Crivello 《Journal of polymer science. Part A, Polymer chemistry》2003,41(4):579-596
This article describes the development of optical pyrometry (OP) as a new analytical technique for the continuous monitoring of the progress of both free‐radical and cationic photopolymerizations. The method is rapid, reproducible, and very easy to implement. A temperature profile of a photopolymerization can be obtained. Preliminary studies have shown that the temperatures of some polymerizing monomers can easily reach temperatures in excess of 250 °C. The effects of the mass and reactivity of the monomer, light intensity, structures, and concentrations of the photoinitiators and monomers as well as the presence or absence of oxygen on various free‐radical and cationic photopolymerizations were examined with this method. Coupling of real‐time infrared spectroscopy with OP provides a convenient method for simultaneously monitoring both the chemical conversion and the temperature of a photopolymerization. This combined technique affords new insights into the effects of temperature‐induced autoacceleration on the course of photopolymerizations. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 579–596, 2003 相似文献
16.
Benjamin M. NugentAmie L. Williams E.N. PrabhakaranJeffrey N. Johnston 《Tetrahedron》2003,59(45):8877-8888
The complete scope of free radical-mediated vinyl amination is described, using 5-exo-trig cyclizations of vinyl radicals to the nitrogen of azomethines. The focus is primarily on N,N-dialkyl enamines since their nucleophilicity renders them the most challenging enamines to synthesize using redox conditions. These studies establish several encouraging precedents for the broader application of this strategy. 相似文献
17.
The formation of the 28-membered ring cyclo[(6-amino-6-deoxy-d-galactonic acid)4] by cyclisation of a protected open chain fully hydroxylated nylon 6 linear tetramer in modest yield provides the first example of a new class of carbopeptoid-cyclodextrin. 相似文献
18.
Benjamin W. Parcher 《Tetrahedron letters》2004,45(12):2677-2679
The scope of a cyclization reaction between amino-acetonitriles and isocyanates was investigated. The study revealed that this reaction tolerates various functional groups leading to highly functionalized 5-amino-imidazole-2-ones, which are useful intermediates for purine synthesis. 相似文献
19.
Rajesh H. Somani Igors Sics Benjamin S. Hsiao 《Journal of polymer science. Part A, Polymer chemistry》2006,44(24):3553-3570
Combined in situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) studies using couette flow geometry were carried out to probe thermal stabilty of shear-induced oriented precursor structure in isotactic polypropylene (iPP) at around its normal melting point (162 °C). Although SAXS results corroborated the emerging consensus about the formation of “long-living” metastable mesomorphic precursor structures in sheared iPP melts, these are the first quantitative measures of the limiting temperature at which no oriented structures survive. At the applied shear, rate = 60 s−1 and duration ts = 5 s, the oriented iPP structures survived a temperature of 185 °C for 1 h after shear, while no stable structures were detected at and above 195 °C. Following Keller's concepts of chain orientation in flow, it is proposed that the chains with highly oriented high molecular weight fraction are primarily responsible for their stability at high temperatures. Furthermore, the effects of flow condition, specifically the shear temperature, on the distributions of oriented and unoriented crystals were determined from rheo-WAXD results. As expected, at a constant flow intensity (i.e., rate = 30 s−1 and duration, ts = 5 s), the oriented crystal fraction decreased with the increase in temperature above 155 °C, below which the oriented fraction decreased with the decrease in temperature. As a result, a crystallinty “phase” diagram, i.e., temperature versus crystal fraction ratio, exhibited a peculiar “hourglass” shape, similar to that found in many two-phase polymer–polymer blends. This can be explained by the competition between the oriented and unoriented crystals in the available crystallizable species. Below the shear temperature (155 °C), the unoriented crystals crystallized so rapidly that they overwhelmed the crystallization of the oriented crystals, thus depleting a major portion of the crystallizable species and increasing their contribution in the final total crystalline phase. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3553–3570, 2006 相似文献
20.
Benjamin A. Lotto 《Integral Equations and Operator Theory》1990,13(2):216-230
For a given functionb in the unit ball ofH
and an arbitraryH
functionm, the question of whenm is a multiplier of the de Branges space
(that is, when
is invariant under multiplication bym) is examined. Some necessary and sufficient conditions thatm be a multiplier of
are found and it is shown that there are no nonconstant inner multipliers of
whenb is a nonconstant extreme point of the unit ball ofH
. A new proof is given of the known fact that
is invariant under multiplication byz whenb is not an extreme point of the unit ball ofH
. Finally, we give a new proof of the known fact that an inner functionm is a multiplier of
forb(z)=(1+z)/2 if and only ifm belongs to the range of
.Some of the work in this paper originally appeared in the author's doctoral disseratation written at the University of California at Berkeley under the supervision of Donald Sarason. 相似文献