全文获取类型
收费全文 | 9108篇 |
免费 | 138篇 |
国内免费 | 37篇 |
专业分类
化学 | 5609篇 |
晶体学 | 124篇 |
力学 | 283篇 |
数学 | 1029篇 |
物理学 | 2238篇 |
出版年
2021年 | 50篇 |
2020年 | 79篇 |
2019年 | 84篇 |
2017年 | 50篇 |
2016年 | 98篇 |
2015年 | 97篇 |
2014年 | 130篇 |
2013年 | 407篇 |
2012年 | 323篇 |
2011年 | 385篇 |
2010年 | 204篇 |
2009年 | 222篇 |
2008年 | 382篇 |
2007年 | 398篇 |
2006年 | 403篇 |
2005年 | 364篇 |
2004年 | 325篇 |
2003年 | 292篇 |
2002年 | 283篇 |
2001年 | 193篇 |
2000年 | 208篇 |
1999年 | 106篇 |
1998年 | 99篇 |
1997年 | 102篇 |
1996年 | 139篇 |
1995年 | 116篇 |
1994年 | 167篇 |
1993年 | 179篇 |
1992年 | 166篇 |
1991年 | 120篇 |
1990年 | 119篇 |
1989年 | 129篇 |
1988年 | 99篇 |
1987年 | 121篇 |
1986年 | 111篇 |
1985年 | 189篇 |
1984年 | 155篇 |
1983年 | 101篇 |
1982年 | 127篇 |
1981年 | 121篇 |
1980年 | 137篇 |
1979年 | 133篇 |
1978年 | 131篇 |
1977年 | 115篇 |
1976年 | 130篇 |
1975年 | 115篇 |
1974年 | 132篇 |
1973年 | 128篇 |
1972年 | 55篇 |
1971年 | 64篇 |
排序方式: 共有9283条查询结果,搜索用时 8 毫秒
131.
132.
Madison A. Kelley Sean R. Smith Jason M. Porter 《Proceedings of the Combustion Institute》2019,37(3):3061-3069
A novel single particle experiment was developed to allow for detailed char gasification measurements in pure steam at temperatures from 1000 °C to 1400 °C and pressures from 1 bar to 15 bar. A coal-derived activated carbon was thoroughly characterized with respect to its composition, physical structure, and reactivity revealing properties consistent with chars reported in the literature. The single particle approach allowed for the boundary and initial conditions to be well known and for the mass of the particle to be accurately measured before and after testing to provide high-quality conversion data. The resulting conversion data were analyzed using the random pore model and the shrinking core model, of which the random pore model provided the best fit. Apparent activation energies were calculated using the random pore model which provided values ranging from 57.1 kJ/mol to 129 kJ/mol which are nominally half of the magnitude of the values reported in the literature under kinetically controlled conditions, thus demonstrating that regime II conditions were present. Additionally, the activation energies decreased with increasing temperature further demonstrating the presence of regime II conditions. The calculated reaction order ranged from 0 to 0.5 and decreased with increasing pressure agreeing well with literature values and trends. 相似文献
133.
134.
Nikolaos V. Tzouras Dr. Fady Nahra Dr. Laura Falivene Prof. Dr. Luigi Cavallo Marina Saab Prof. Dr. Kristof Van Hecke Dr. Alba Collado Dr. Christopher J. Collett Prof. Dr. Andrew D. Smith Prof. Dr. Catherine S. J. Cazin Prof. Dr. Steven P. Nolan 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(20):4515-4519
We have been puzzled by the involvement of weak organic and inorganic bases in the synthesis of metal–N-heterocyclic carbene (NHC) complexes. Such bases are insufficiently strong to permit the presumed required deprotonation of the azolium salt (the carbene precursor) prior to metal binding. Experimental and computational studies provide support for a base-assisted concerted process that does not require free NHC formation. The synthetic protocol was found applicable to a number of transition-metal- and main-group-centered NHC compounds and could become the synthetic route of choice to form M–NHC bonds. 相似文献
135.
The rapid growth of a through-the-thickness crack in a nominally elastic plate under Mode I conditions is considered, with emphasis on the three-dimensional aspects of the elastodynamic crack growth process. For points near the crack edge, the assumptions on stress state consistent with plane stress analysis are not valid, and the possibility of an inertial effect associated with rapid lateral contraction must be considered. The steady crack growth process is analyzed here by means of the finite element method, and the degree to which the local fields are described by plane stress analysis is established. In addition, synthetic shadow spot patterns are generated from the numerical three-dimensional solutions. The inferred values of stress intensity factor are compared with the values of the corresponding plane stress dynamic stress intensity factor. 相似文献
136.
Summary Polyamines and polyamine conjugates display a diverse range of important biological functions, ranging from antibiotics to immunosuppressants and glutamate receptor antagonists. For these reasons, polyamines provide an excellent template/scaffold for combinatorial chemistry. In this paper we present methods for the solid-phase immobilisation of polyamines for use in synthetic and combinatorial chemistry and describe how they have been employed in the preparation of a number of important polyamine conjugates and polyamine libraries. Thus, we have designed, synthesised and utilised a number of polyamine linkers for both solution and resin screening combinatorial application. 相似文献
137.
Christopher M. Collins Shizhe Li Qing X. Yang Michael B. Smith 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,125(2):233-241
A method for calculatingB1field strength and homogeneity as functions of radiofrequency shield geometry is presented. The method requires use of three-dimensional finite-element analysis, birdcage-coil theory, and antenna-array theory. Calculations were performed for a 12-element birdcage coil (19 cm diameter, 21 cm length) at 125 MHz. CalculatedB1field strengths and homogeneities for the coil in 25 different shields and in no shield are given. For configurations where the shield is longer than the coil, bothB1field strength and homogeneity decrease as shield diameter decreases or as shield length increases. In configurations where the shield is shorter than the coil and has a diameter of 25.6 cm,B1homogeneity is greater than in an unshielded coil.B1field strength was measured experimentally at 125 MHz in a birdcage coil of the same geometry as the model within shields of four different diameters. Calculated results very closely matched experimental measurement. 相似文献
138.
Graph-based causal inference has recently been successfully applied to explore system reliability and to predict failures in order to improve systems. One popular causal analysis following Pearl and Spirtes et al. to study causal relationships embedded in a system is to use a Bayesian network (BN). However, certain causal constructions that are particularly pertinent to the study of reliability are difficult to express fully through a BN. Our recent work demonstrated the flexibility of using a Chain Event Graph (CEG) instead to capture causal reasoning embedded within engineers’ reports. We demonstrated that an event tree rather than a BN could provide an alternative framework that could capture most of the causal concepts needed within this domain. In particular, a causal calculus for a specific type of intervention, called a remedial intervention, was devised on this tree-like graph. In this paper, we extend the use of this framework to show that not only remedial maintenance interventions but also interventions associated with routine maintenance can be well-defined using this alternative class of graphical model. We also show that the complexity in making inference about the potential relationships between causes and failures in a missing data situation in the domain of system reliability can be elegantly addressed using this new methodology. Causal modelling using a CEG is illustrated through examples drawn from the study of reliability of an energy distribution network. 相似文献
139.
Perturbation theory is used to consider expansions for the radial distribution function, g 2(r), of a fluid with a soft core. We consider the Lennard-Jones (12, 6) potential and divide it into repulsive and attractive regions. In the repulsive region we expand the function exp (β u(r))g 2(r) about a hard sphere value. For the first-order contribution of the attractive region we consider a simple approximation to the exact analytical expression. The resulting g 2(r) is accurate at densities below about ρσ 3=0·5. 相似文献
140.
H. V. Smith 《BIT Numerical Mathematics》1984,24(2):253-256
The object of this paper is to derive a method for the numerical approximation of integrals of the form $$\int_{ - 1}^1 {w(x)f(x)(x^2 + \mathop {a^2 }\limits^ - )^{ - 1} } dx$$ where w(x)=(1?x 2)±1/2 or ((1?x)/(1+x))1/2. 相似文献