全文获取类型
收费全文 | 37360篇 |
免费 | 18158篇 |
国内免费 | 57篇 |
专业分类
化学 | 52861篇 |
晶体学 | 23篇 |
力学 | 1082篇 |
数学 | 1131篇 |
物理学 | 478篇 |
出版年
2024年 | 422篇 |
2023年 | 4244篇 |
2022年 | 1397篇 |
2021年 | 2404篇 |
2020年 | 4668篇 |
2019年 | 2225篇 |
2018年 | 2364篇 |
2017年 | 600篇 |
2016年 | 5531篇 |
2015年 | 5495篇 |
2014年 | 4920篇 |
2013年 | 5038篇 |
2012年 | 3035篇 |
2011年 | 967篇 |
2010年 | 3305篇 |
2009年 | 3240篇 |
2008年 | 996篇 |
2007年 | 696篇 |
2006年 | 120篇 |
2005年 | 80篇 |
2004年 | 69篇 |
2003年 | 69篇 |
1997年 | 50篇 |
1996年 | 59篇 |
1995年 | 111篇 |
1994年 | 77篇 |
1993年 | 192篇 |
1992年 | 72篇 |
1991年 | 57篇 |
1988年 | 76篇 |
1987年 | 68篇 |
1986年 | 48篇 |
1985年 | 54篇 |
1984年 | 68篇 |
1983年 | 69篇 |
1982年 | 76篇 |
1981年 | 89篇 |
1980年 | 107篇 |
1979年 | 96篇 |
1978年 | 97篇 |
1977年 | 166篇 |
1976年 | 183篇 |
1975年 | 185篇 |
1974年 | 195篇 |
1973年 | 112篇 |
1972年 | 153篇 |
1971年 | 123篇 |
1970年 | 208篇 |
1969年 | 127篇 |
1968年 | 138篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
Dr. H. Schmieg Prof. Dr. P. Vielsack 《Archive of Applied Mechanics (Ingenieur Archiv)》1987,57(6):446-458
Übersicht Betrachtet wird ein zwangserregtes Zweikörpersystem mit wechselnden Bindungen infolge trockener Reibung. Stationäre Bewegungen werden als Grenzfall instationärer Einschwingvorgänge berechnet. Abhängig von den Systemparametern ergeben sich drei typische Bewegungsformen. Ihnen entsprechen dauernde Haftzustände, wechselnde Haft-Gleitzustände oder dauernde Gleitzustände an der Berührfläche beider Körper.
Intermittant constraints in a two-body-system with dry friction
Summary An externally excited two-body-system with intermittant constraints due to dry friction is considered. Stationary motions are calculated as limit cases of instationary transients. Depending on the parameters of the system, three typical modes are of interest. These correspond to permanent sticking, slipstick behaviour, or to permanent slipping in the contact surface of the bodies, respectively.相似文献
132.
An Nd:YAG laser-pumped periodically poled RbTiOAsO4 (PPRTA) optical parametric oscillator (OPO) near degeneracy is reported. At a pump power of 3.7 W, a net conversion efficiency of 43% was obtained. These crystals were found to be phase-matchable for the higher order quasi-phase-matched second-harmonic generation (SHG) in the visible range. Detailed analyses of the phase-matching properties for these processes were made by using the published Sellmeier and thermo-optic dispersion formulas. 相似文献
133.
Yun-Zhi Tang Shao-Song Qian Xi-Sen Wang Hong Zhao Xue-Feng Huang Yong-Hua Li Xing-Cai Jiao Ren-Gen Xiong Prof. Dr. 《无机化学与普通化学杂志》2004,630(11):1623-1626
The in situ hydrolysis of phthalyl-γ-glutaminyl-histidine with Cd(ClO4)2·6H2O in the presence of water and methanol affords a novel two-dimensional cadmium coordination polymer, a new phase, anhydrous cadmium(II) phthalate, ( 1 ). The structure was determined by single crystal X-ray diffraction. Crystal data: P21/c, a = 13.8168(14), b = 7.0351(7), c = 8.2054(8) Å, β = 105.540(2)°, V = 768.43(13) Å3, Z = 4, R1 = 0.0380, wR2 = 0.1111. 相似文献
134.
Nobuhiro Honda 《Mathematische Nachrichten》2003,256(1):48-57
In a previous paper of the same title the author gave a generalization of the constrution of Donaldson–Friedman, to prove the existence of twistor spaces of n CP 2 with a special kind of divisors. In the present paper, we consider its equivariant version. When n = 3, this gives another proof of the existence of degenerate double solid with C *–action, and we show that the branch quartic surface is birational to an elliptic ruled surface. In case n ≥ 4, this yields new Moishezon twistor spaces with C *–action, which is shown to be the most degenerate ones among twistor spaces studied by Campana and Kreußler. 相似文献
135.
136.
Akira Yanagisawa Prof. Dr. Takahiro Suzuki Takanori Koide Shogo Okitsu Takayoshi Arai Prof. Dr. 《化学:亚洲杂志》2008,3(10):1793-1800
A Barbier‐type regioselective propargylation of aldehydes and ketones with (3‐bromobut‐1‐ynyl)trimethylsilane has been achieved using reactive barium as a low‐valent metal in THF. Especially in the case of ketones, the corresponding homopropargylic alcohols form almost exclusively. In the reaction of α,β‐unsaturated carbonyl compounds, only 1,2‐adducts have been observed. This method is also applicable to propargylation of imines, and the corresponding homopropargylic amines are obtained regiospecifically in good yields with diastereomeric ratios of up to 87:13. 相似文献
137.
138.
In this article, we present results of an empirical study with 500 German students of grades 7 and 8. The study focussed on students' mathematics achievement and their interest in mathematics as well as on the relation between these two constructs. In particular, the results show that the development of an individual student's achievement between grade 7 and grade 8 depends on the achievement level of the specific classroom and therefore on the specific mathematics instruction Interest in mathematics could be regarded a predictor for mathematics achievement Moreover, our findings suggest that the students show hardly any fear of mathematics independent of their achievement level. 相似文献
139.
Several models have been developed in order to categorize the numerous expressions that people use in order to describe their emotional experiences. The focus of the present study is on one of these theoretical classifications proposed by Pekrun (1992) specifically concerning emotions which are directly related to learning and achievement in mathematics. In his model, emotions are classified according to their valence (positive vs. negative) and their level of activation. In testing the assumptions of this model, we investigated students' enjoyment, anxiety, anger and boredom experienced before, during, and after the completion of a math test. Correspondence analyses which were used to generate a graphical illustration of structural interrelationships between these emotions provide empirical support for the theoretical classification. 相似文献
140.
Selecting, modifying or creating appropriate problems for mathematics class has become an activity of increaing importance in the professional development of German mathematics teachers. But rather than asking in general: “What is a good problem?” there should be a stronger emphasis on considering the specific goal of a problem, e.g.: “What are the ingredients that make a problem appropriate for initiating a learning process” or “What are the characteristics that make a problem appropriate for its use in a central test?” We propose a guiding scheme for teachers that turns out to be especially helpful, since the newly introduced orientation on outcome standards a) leads to a critical predominance of test items and b) expects teachers to design adequate problems for specific learning processes (e.g. problem solving, reasoning and modelling activities). 相似文献