全文获取类型
收费全文 | 1418篇 |
免费 | 29篇 |
国内免费 | 8篇 |
专业分类
化学 | 1002篇 |
晶体学 | 9篇 |
力学 | 24篇 |
数学 | 208篇 |
物理学 | 212篇 |
出版年
2021年 | 14篇 |
2020年 | 14篇 |
2019年 | 20篇 |
2018年 | 14篇 |
2016年 | 29篇 |
2015年 | 13篇 |
2014年 | 22篇 |
2013年 | 34篇 |
2012年 | 42篇 |
2011年 | 61篇 |
2010年 | 30篇 |
2009年 | 27篇 |
2008年 | 56篇 |
2007年 | 49篇 |
2006年 | 64篇 |
2005年 | 56篇 |
2004年 | 58篇 |
2003年 | 39篇 |
2002年 | 41篇 |
2001年 | 19篇 |
2000年 | 48篇 |
1999年 | 29篇 |
1992年 | 11篇 |
1988年 | 8篇 |
1987年 | 11篇 |
1984年 | 10篇 |
1983年 | 14篇 |
1982年 | 18篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 13篇 |
1978年 | 9篇 |
1977年 | 11篇 |
1975年 | 8篇 |
1974年 | 9篇 |
1973年 | 13篇 |
1944年 | 9篇 |
1915年 | 8篇 |
1911年 | 8篇 |
1910年 | 12篇 |
1909年 | 10篇 |
1907年 | 8篇 |
1901年 | 8篇 |
1900年 | 12篇 |
1898年 | 11篇 |
1892年 | 11篇 |
1891年 | 12篇 |
1890年 | 9篇 |
1882年 | 10篇 |
1870年 | 8篇 |
排序方式: 共有1455条查询结果,搜索用时 248 毫秒
1.
Haixing Guan Qianwen Zhang Prof. Dr. Patrick J. Walsh Prof. Dr. Jianyou Mao 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(13):5210-5215
A unique nickel/organic photoredox co-catalyzed asymmetric reductive cross-coupling between α-chloro esters and aryl iodides is developed. This cross-electrophile coupling reaction employs an organic reductant (Hantzsch ester), whereas most reductive cross-coupling reactions use stoichiometric metals. A diverse array of valuable α-aryl esters is formed under these conditions with high enantioselectivities (up to 94 %) and good yields (up to 88 %). α-Aryl esters represent an important family of nonsteroidal anti-inflammatory drugs. This novel synergistic strategy expands the scope of Ni-catalyzed reductive asymmetric cross-coupling reactions. 相似文献
2.
The method described in this paper allows an investigator to determine the intrinsic stress of a polymer layer in a way that
does not result in damage to devices or test structures. The method requires that a small area of the polymer be released
from the substrate to form a diaphragm. The diaphragm is stimulated with acoustic white noise and the diaphragm movement is
monitored with a laser vibrometer. The first few resonance frequencies of the diaphragm are obtained using a laser vibrometer
and then those frequencies are used to calculate the membrane intrinsic bi-axial tension. 相似文献
3.
Ohne Zusammenfassung 相似文献
4.
E. Utescher Ed. Ritsert Th. Salzer und C. Schwarz 《Fresenius' Journal of Analytical Chemistry》1888,27(1):665-668
Ohne Zusammenfassung 相似文献
5.
6.
A planar map is a 2-cell embedding of a connected planar graph, loops and parallel edges allowed, on the sphere. A plane map is a planar map with a distinguished outside (“infinite”) face. An unrooted map is an equivalence class of maps under orientation-preserving homeomorphism, and a rooted map is a map with a distinguished oriented edge. Previously we obtained formulae for the number of unrooted planar n-edge maps of various classes, including all maps, non-separable maps, eulerian maps and loopless maps. In this article, using the same technique we obtain closed formulae for counting unrooted plane maps of all these classes and their duals. The corresponding formulae for rooted maps are known to be all sum-free; the formulae that we obtain for unrooted maps contain only a sum over the divisors of n. We count also unrooted two-vertex plane maps. 相似文献
7.
Mary E. Neubert DAVID G. ABDALLAH Jr Sandra S. Keast Julie M. Kim Soonnam Lee Ryan M. Stayshich Margaret E. Walsh Rolfe G. Petschek Shin-Tson Wu 《Liquid crystals》2003,30(6):711-731
New diphenyldiacetylenes of the type with A, B = H and/or F; m = 0, 1; n = 1-4; and X = C n H 2n + 1 , F, CF 3 or CN were synthesized and their mesomorphic properties determined by hot stage polarizing microscopy and DSC. When m = 0, all of these compounds showed only a nematic phase except when X = CF 3 when both nematic and smectic A phases were seen. Both clearing and melting temperatures were higher than those reported for substitution with the corresponding alkyl chains but the much larger increase in clearing temperatures produced considerably wider nematic phases. Eutectic mixtures of a few of these olefins yielded nematic materials also having much wider temperature ranges and higher clearing temperatures than the eutectic mixtures of the alkyl compounds, while retaining their high birefringence and low viscosities. Such materials are of interest for beam-steering devices.
Four of the diacetylenes with m = 1 ( A, B = H) were also prepared ( X = C 6 H 13 , F, n = 2, 3). When X was C 6 H 13 ( n = 2), the nematic range was smaller in the 2- than in the 1-olefin but wider than in the alkyl series. When X = F, either no nematic phase or a monotropic one was observed, whereas the 1-olefins gave a much wider nematic phase. Both transition temperatures were lower than those for the corresponding 1-olefin and alkyl analogues. The compound with X = C 6 H 13 and n = 2 had a melting temperature below room temperature. 相似文献
Four of the diacetylenes with m = 1 ( A, B = H) were also prepared ( X = C 6 H 13 , F, n = 2, 3). When X was C 6 H 13 ( n = 2), the nematic range was smaller in the 2- than in the 1-olefin but wider than in the alkyl series. When X = F, either no nematic phase or a monotropic one was observed, whereas the 1-olefins gave a much wider nematic phase. Both transition temperatures were lower than those for the corresponding 1-olefin and alkyl analogues. The compound with X = C 6 H 13 and n = 2 had a melting temperature below room temperature. 相似文献
8.
Mary Beth Walsh Connie M. Moss Benny G. Johnson Dale A. Holder Jeffry D. Madura 《The Chemical Educator》2002,7(6):379-383
The need for improved interactive tutoring capabilities in educational software for chemistry problem solving is an important one clearly articulated by teachers and students. To deliver the next generation of individualized interactive capabilities users demand, it is necessary to go beyond the conventional computer-assisted instruction methodology. The focus of this paper is the assessment with first-semester general chemistry students of a recently developed artificial intelligence (AI) tutor for balancing chemical equations. This is the first such assessment of an AI-based learning tool in chemistry. Students in CHEM 121 in the Fall 2001 semester at Duquesne University (N = 273) participated in the study. Students were divided into a test group that used the AI tutor as part of their study activities and a control group that did not use the tutor. It was found that the tutor improved the performance of the test group students to a statistically significant degree, helping the weakest students the most. This study establishes the feasibility of an AI-based approach to creating advanced new tutoring software for chemistry problem solving. Access to a Web-based demonstration of the equation-balancing tutor may be obtained by emailing the corresponding author. 相似文献
9.
This paper presents results on symmetries of the spectrum of singularities for random cascades found in the statistical theory of turbulence. It is shown that empirical dimension curves possess a natural symmetry whose presence restricts the class of allowable probability distributions of the cascade generator in a simple manner. In particular, necessary and sufficient conditions on the probability distribution of the generator are obtained for symmetry of the singularity spectrum within a large class of cascade models. 相似文献
10.