全文获取类型
收费全文 | 3553篇 |
免费 | 81篇 |
国内免费 | 26篇 |
专业分类
化学 | 2430篇 |
晶体学 | 43篇 |
力学 | 111篇 |
数学 | 548篇 |
物理学 | 528篇 |
出版年
2022年 | 24篇 |
2021年 | 25篇 |
2020年 | 38篇 |
2019年 | 32篇 |
2018年 | 25篇 |
2017年 | 21篇 |
2016年 | 62篇 |
2015年 | 64篇 |
2014年 | 53篇 |
2013年 | 136篇 |
2012年 | 182篇 |
2011年 | 236篇 |
2010年 | 107篇 |
2009年 | 92篇 |
2008年 | 188篇 |
2007年 | 196篇 |
2006年 | 202篇 |
2005年 | 176篇 |
2004年 | 174篇 |
2003年 | 138篇 |
2002年 | 155篇 |
2001年 | 73篇 |
2000年 | 56篇 |
1999年 | 45篇 |
1998年 | 38篇 |
1997年 | 32篇 |
1996年 | 51篇 |
1995年 | 35篇 |
1994年 | 47篇 |
1993年 | 34篇 |
1992年 | 44篇 |
1991年 | 41篇 |
1990年 | 34篇 |
1989年 | 45篇 |
1988年 | 34篇 |
1987年 | 22篇 |
1986年 | 35篇 |
1985年 | 37篇 |
1984年 | 54篇 |
1983年 | 40篇 |
1982年 | 59篇 |
1981年 | 51篇 |
1980年 | 51篇 |
1979年 | 32篇 |
1978年 | 36篇 |
1977年 | 33篇 |
1976年 | 34篇 |
1975年 | 46篇 |
1974年 | 30篇 |
1973年 | 27篇 |
排序方式: 共有3660条查询结果,搜索用时 10 毫秒
1.
An optimal control model of exhaustible resources is used to clarify the long run relationship between mineral rent and depletion cost at the industry level. A standard first order condition of the time rate of change of rents is reformulated to reveal that rent data may be used to help forecast the rise in extraction costs resulting from resource depletion. This application of the theory of exhaustible resources is illustrated using historical mineral industry rent and extraction cost data. A forecast of U.S. coal extraction costs, following the method proposed in this paper, suggests that future rates of extraction cost increases will be similar to rates experienced in the past. 相似文献
2.
3.
Tricyclic skeletons have been generated from acyclic enyne precursors by using an intramolecular Pauson-Khand reaction in combination with aldol, Michael and alkylation reactions. 相似文献
4.
Graham Smith 《The Journal of the Operational Research Society》1989,40(9):761-770
Courses which teach discrete-event simulation are based on many different simulation languages. The requirements for a language to support teaching simulation are discussed. In particular, it is recommended that such languages separate into distinct modules those aspects of simulation which are taught as separate topics. Implementation of the separation is discussed. The SEESIM language, developed as a teaching aid, is described, and examples of its use are given. Straightforward use of SEESIM can be learned quickly, yet the language provides facilities for a staged introduction to advanced concepts of simulation. 相似文献
5.
Graham D. Williams 《Results in Mathematics》2005,47(1-2):155-161
For each of a standard set of normal forms for (n × n) complex matrices under the relation of congruence, explicit matrices are exhibited which transform, via congruence, the normal form to its transpose. 相似文献
6.
Philip A. Wheeler Michael Brungs Graham R. Atkins 《Journal of Sol-Gel Science and Technology》2003,27(3):321-326
Organically modified silicates (ormosils) have been viewed as possible materials for optical signal processing devices for communications networks. One significant problem obstructing their use in this application is the absorption of infrared light by hydroxy species (present for example in silanol groups (SiOH) and water) around the communications wavelengths (1310 and 1550 nm). We have investigated several ormosil compositions, synthesised via an aqueous sol-gel method, to determine if residual hydroxy groups can be removed more effectively via reducing the connectivity of the material by increasing the number of organic groups (such as Si—CH3). Furthermore, we have investigated the effect of different inert atmospheres on the drying process. It was found that reducing the connectivity does promote the removal of hydroxy absorption, mainly via the condensation of residual silanol groups, but also by removal of residual water, depending on the other precursors in the system. It was also found that the drying atmosphere used (nitrogen or helium) had no effect on the drying process. 相似文献
7.
8.
Optimized prefactored compact schemes 总被引:1,自引:0,他引:1
The numerical simulation of aeroacoustic phenomena requires high-order accurate numerical schemes with low dispersion and dissipation errors. In this paper we describe a strategy for developing high-order accurate prefactored compact schemes, requiring very small stencil support. These schemes require fewer boundary stencils and offer simpler boundary condition implementation than existing compact schemes. The prefactorization strategy splits the central implicit schemes into forward and backward biased operators. Using Fourier analysis, we show it is possible to select the coefficients of the biased operators such that their dispersion characteristics match those of the original central compact scheme and their numerical wavenumbers have equal and opposite imaginary components. This ensures that when the forward and backward stencils are added, the original central compact scheme is recovered. To extend the resolution characteristic of the schemes, an optimization strategy is employed in which formal order of accuracy is sacrificed in preference to enhanced resolution characteristics across the range of wavenumbers realizable on a given mesh. The resulting optimized schemes yield improved dispersion characteristics compared to the standard sixth- and eighth-order compact schemes making them more suitable for high-resolution numerical simulations in gas dynamics and computational aeroacoustics. The efficiency, accuracy and convergence characteristics of the new optimized prefactored compact schemes are demonstrated by their application to several test problems. 相似文献
9.
Graham R. Knox Peter L. Pauson Debra Willison 《Journal of organometallic chemistry》1993,450(1-2):177-184
Phenylazoferrocene undergoes nickelation and palladation exclusively on the phenyl group. Lithium phenylazocyclopentadienide has been converted into derivatives of Mo, Mn, Fe, Ru and Rh. 相似文献
10.
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