全文获取类型
收费全文 | 9504篇 |
免费 | 381篇 |
国内免费 | 76篇 |
专业分类
化学 | 6663篇 |
晶体学 | 53篇 |
力学 | 244篇 |
数学 | 1770篇 |
物理学 | 1231篇 |
出版年
2023年 | 61篇 |
2022年 | 88篇 |
2021年 | 274篇 |
2020年 | 186篇 |
2019年 | 211篇 |
2018年 | 180篇 |
2017年 | 159篇 |
2016年 | 349篇 |
2015年 | 284篇 |
2014年 | 345篇 |
2013年 | 559篇 |
2012年 | 667篇 |
2011年 | 774篇 |
2010年 | 475篇 |
2009年 | 455篇 |
2008年 | 604篇 |
2007年 | 598篇 |
2006年 | 556篇 |
2005年 | 537篇 |
2004年 | 434篇 |
2003年 | 302篇 |
2002年 | 301篇 |
2001年 | 134篇 |
2000年 | 101篇 |
1999年 | 99篇 |
1998年 | 76篇 |
1997年 | 85篇 |
1996年 | 117篇 |
1995年 | 83篇 |
1994年 | 70篇 |
1993年 | 66篇 |
1992年 | 68篇 |
1991年 | 70篇 |
1990年 | 40篇 |
1989年 | 50篇 |
1988年 | 36篇 |
1987年 | 38篇 |
1986年 | 47篇 |
1985年 | 58篇 |
1984年 | 52篇 |
1983年 | 35篇 |
1982年 | 53篇 |
1981年 | 27篇 |
1980年 | 24篇 |
1979年 | 19篇 |
1978年 | 17篇 |
1977年 | 11篇 |
1976年 | 11篇 |
1975年 | 10篇 |
1974年 | 10篇 |
排序方式: 共有9961条查询结果,搜索用时 296 毫秒
991.
Traditionally, mining engineers plan an open pit mine considering pre-established conditions of operation of the plant(s) derived from a previous plant optimization. By contrast, mineral processing engineers optimize the processing plants by considering a regular feed from the mine, with respect to quantity and quality of the materials. The methods implemented to optimize mine and metallurgical plans simultaneously are known in the mining industry as global or simultaneous optimizers. The development of these methods has been of major concern for the mining industry over the last decade. Some algorithms are available in commercial mining software packages however, these algorithms ignore the inherent geological uncertainty associated with the deposit being considered, which leads to shortfalls in production, quality, and expected cashflows. This paper presents a heuristic method to generate life-of-mine production schedules that consider operating alternatives for processing plants and incorporate geological uncertainty. The method uses iterative improvement by swapping periods and destinations of the mining blocks to generate the final solution. The implementation of the method at a copper deposit shows its ability to control mine and processing capacities while increasing the expected net present value by 30% when compared with a solution generated using a standard industry practice. 相似文献
992.
Gerardo Cáceres Montenegro Carolina G. Gutierrez Santiago E. Vaillard Roque J. Minari Jorge R. Vega Luis M. Gugliotta 《大分子反应工程》2017,11(3)
The synthesis of poly(acrylic acid) (PAA) of low molar mass under safe conditions is difficult due to the high polymerization rate of acrylic acid (AA) and the fast heat generation. The aqueous‐solution “semibatch” polymerization of non‐ionized AA in almost starved conditions involves high initiator loads when low molar masses are required. This article proposes the simultaneous feeding of AA and nonconventional chain transfer agents (CTA) as a strategy aimed at controlling both the molar masses and the generated heat rate. Three CTAs are investigated: 2‐mercaptoethanol, thioglycolic acid, and isopropyl alcohol. Even when PAA of relatively low molar mass can be produced by adequately selecting the flow rates and concentrations of both AA and CTA, it is found that the nature of CTA can have a significant effect on the polymerizations kinetics. The mechanisms responsible for these effects are discussed with the help of a representative mathematical model.
993.
For the nonautonomous dynamics defined by a sequence of bounded linear operators acting on an arbitrary Hilbert space, we obtain a characterization of the notion of a nonuniform exponential dichotomy in terms of quadratic Lyapunov sequences. We emphasize that, in sharp contrast with previous results, we consider the general case of possibly noninvertible linear operators, thus requiring only the invertibility along the unstable direction. As an application, we give a simple proof of the robustness of a nonuniform exponential dichotomy under sufficiently small linear perturbations. 相似文献
994.
995.
Javier Cano Javier M. Moguerza Francisco J. Prieto 《Journal of Optimization Theory and Applications》2017,174(2):474-499
In this work, we describe the efficient use of improved directions of negative curvature for the solution of bound-constrained nonconvex problems. We follow an interior-point framework, in which the key point is the inclusion of computational low-cost procedures to improve directions of negative curvature obtained from a factorisation of the KKT matrix. From a theoretical point of view, it is well known that these directions ensure convergence to second-order KKT points. As a novelty, we consider the convergence rate of the algorithm with exploitation of negative curvature information. Finally, we test the performance of our proposal on both CUTEr/st and simulated problems, showing empirically that the enhanced directions affect positively the practical performance of the procedure. 相似文献
996.
Jordi Castro Stefano Nasini Francisco Saldanha-da-Gama 《Mathematical Programming》2017,163(1-2):411-444
We propose a cutting-plane approach (namely, Benders decomposition) for a class of capacitated multi-period facility location problems. The novelty of this approach lies on the use of a specialized interior-point method for solving the Benders subproblems. The primal block-angular structure of the resulting linear optimization problems is exploited by the interior-point method, allowing the (either exact or inexact) efficient solution of large instances. The consequences of different modeling conditions and problem specifications on the computational performance are also investigated both theoretically and empirically, providing a deeper understanding of the significant factors influencing the overall efficiency of the cutting-plane method. The methodology proposed allowed the solution of instances of up to 200 potential locations, one million customers and three periods, resulting in mixed integer linear optimization problems of up to 600 binary and 600 millions of continuous variables. Those problems were solved by the specialized approach in less than one hour and a half, outperforming other state-of-the-art methods, which exhausted the (144 GB of) available memory in the largest instances. 相似文献
997.
998.
Luis H. Cisneros Ricardo Cortez Christopher Dombrowski Raymond E. Goldstein John O. Kessler 《Experiments in fluids》2007,43(5):737-753
Nearly close-packed populations of the swimming bacterium Bacillus subtilis form a collective phase, the “Zooming BioNematic” (ZBN). This state exhibits large-scale orientational coherence, analogous
to the molecular alignment of nematic liquid crystals, coupled with remarkable spatial and temporal correlations of velocity
and vorticity, as measured by both novel and standard applications of particle imaging velocimetry. The appearance of turbulent
dynamics in a system which is nominally in the regime of Stokes flow can be understood by accounting for the local energy
input by the swimmers, with a new dimensionless ratio analogous to the Reynolds number. The interaction between organisms
and boundaries, and with one another, is modeled by application of the methods of regularized Stokeslets. 相似文献
999.
Three-dimensional elastostatics of a layer and a layered medium 总被引:4,自引:0,他引:4
This paper is concerned with the determination of the distribution of stresses and displacements in an infinite three-dimensional, linear, elastic, isotropic, homogeneous layer subjected to concentrated body forces acting upon an arbitrary internal point.In §2 and §3 the governing partial differential field equations are reduced to a system or ordinary differential equations by the use of the two-dimensional Fourier transform, taken with respect to the two in-plane geometric variables (§4). Analytical expressions for the stresses and displacements are then obtained for the particular case of concentrated body forces, represented as Dirac delta functions (§5).The results are subsequently utilized to formulate the multilayered medium problem by means of transfer matrices. In §8 the typical problem of a non-adhesive layered medium is undertaken. 相似文献
1000.
Kadir Sentosun Marta N. Sanz Ortiz K. Joost Batenburg Luis M. Liz‐Marzán Sara Bals 《Particle & Particle Systems Characterization》2015,32(12):1063-1067
Characterization of core–shell type nanoparticles in 3D by transmission electron microscopy (TEM) can be very challenging. Especially when both heavy and light elements coexist within the same nanostructure, artifacts in the 3D reconstruction are often present. A representative example would be a particle comprising an anisotropic metallic (Au) nanoparticle coated with a (mesoporous) silica shell. To obtain a reliable 3D characterization of such an object, a dose‐efficient strategy is proposed to simultaneously acquire high‐angle annular dark‐field scanning TEM and annular dark‐field tilt series for tomography. The 3D reconstruction is further improved by applying an advanced masking and interpolation approach to the acquired data. This new methodology enables us to obtain high‐quality reconstructions from which also quantitative information can be extracted. This approach is broadly applicable to investigate hybrid core–shell materials. 相似文献