全文获取类型
收费全文 | 9651篇 |
免费 | 260篇 |
国内免费 | 64篇 |
专业分类
化学 | 1183篇 |
晶体学 | 64篇 |
力学 | 618篇 |
综合类 | 26篇 |
数学 | 4218篇 |
物理学 | 3866篇 |
出版年
2024年 | 9篇 |
2023年 | 154篇 |
2022年 | 130篇 |
2021年 | 284篇 |
2020年 | 351篇 |
2019年 | 179篇 |
2018年 | 188篇 |
2017年 | 458篇 |
2016年 | 396篇 |
2015年 | 413篇 |
2014年 | 620篇 |
2013年 | 559篇 |
2012年 | 601篇 |
2011年 | 566篇 |
2010年 | 480篇 |
2009年 | 550篇 |
2008年 | 342篇 |
2007年 | 416篇 |
2006年 | 458篇 |
2005年 | 323篇 |
2004年 | 203篇 |
2003年 | 202篇 |
2002年 | 231篇 |
2001年 | 277篇 |
2000年 | 142篇 |
1999年 | 258篇 |
1998年 | 158篇 |
1997年 | 137篇 |
1996年 | 128篇 |
1995年 | 80篇 |
1994年 | 57篇 |
1993年 | 69篇 |
1992年 | 52篇 |
1991年 | 45篇 |
1990年 | 36篇 |
1989年 | 48篇 |
1988年 | 187篇 |
1987年 | 11篇 |
1986年 | 17篇 |
1985年 | 34篇 |
1984年 | 38篇 |
1983年 | 14篇 |
1982年 | 16篇 |
1981年 | 12篇 |
1980年 | 12篇 |
1979年 | 7篇 |
1978年 | 10篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1975年 | 4篇 |
排序方式: 共有9975条查询结果,搜索用时 15 毫秒
131.
The problem of minimizing a nondifferential functionx f(x) (subject, possibly, to nondifferential constraints) is considered. Conventional algorithms are employed for minimizing a differential approximationf
off (subject to differentiable approximations ofg). The parameter is adaptively reduced in such a way as to ensure convergence to points satisfying necessary conditions of optimality for the original problem.This research was supported by the UK Science and Engineering Research Council, the National Science Foundation under Grant No. ECS-8121149, and the Joint Services Electronics Program, Contract No. F49620-79-C-0178. 相似文献
132.
Refractive index of silicon nitride film deposited in a plasma enhanced chemical deposition system is modeled by using neural network. The deposition process was characterized with a full factorial experiment. Additional 12 experiments were conducted to test model appropriateness. Predicted model behaviors were in good agreement with actual measurements. Deposition mechanisms were qualitatively examined especially with respect to the pressure. Possible interactions between the pressure and other factors (SiH4, NH3, radio frequency (RF) power, and substrate temperature) were examined on the basis of SiH/NH bond ratio. The refractive index increased with increasing either SiH4 flow rate or RF power. In contrast, the refractive index decreased with increasing NH3 flow rate. Little interactions between the pressure and RF power were observed. Pressure effect on the refractive index was quite different depending on the level of SiH4 flow rate or substrate temperature. In general, increasing the pressure increased the refractive index. Meanwhile, the refractive index was insensitive to parameter variations at relatively high pressures. The most complex interaction occurred as the pressure interacted with the temperature. Useful clues to control the refractive index were revealed from the predictive model. 相似文献
133.
《Polyhedron》2005,24(16-17):2562-2567
Molecular magnets have been recently proposed as possible building blocks for a solid-state quantum computer. In order to substantiate and develop such a proposal, one needs to identify those molecules that are best suited for the qubit encoding and manipulation. Here, we focus on a heterometallic molecular ring, namely Cr7Ni, where the substitution of one Cr3+(S = 3/2) with Ni2+(S = 1) provides an extra spin to the otherwise compensated molecule. We show that its ground state consists in an S = 1/2 doublet, energetically well separated (Δ0/kB ∼ 13 K at zero magnetic field) from the first excited multiplet. This relatively large value of Δ0, together with the reduced mixing of the subspaces corresponding to different values of the total spin S, enables a safe encoding of the |0〉 and |1〉 states with the ground-state doublet, and allows to coherently rotate the effective S = 1/2 spin, while keeping the population loss to the excited states negligible. A further, intriguing challenge is represented by the implementation of the conditional dynamics (two-qubit gates). We present here preliminary characterization of molecular “Cr7Ni-dimers”, i.e., derivatives in which two Cr7Ni rings are linked with each other by means of delocalized aromatic amines. The resulting intercluster couplings are estimated to be ⩽1 K and are expected to be permanent, i.e., not tuneable during gating, as required by the standard approach to quantum computation. We discuss a computational scheme that allows in principle to overcome this limitation. The most relevant decoherence mechanisms for Cr7Ni and possible ways to reduce their effects are discussed as well. 相似文献
134.
Ab initio (SCF and CI) calculations have been performed for the linear approach (C∝v) of C+ to H2. For the 2 Σ+ surface the saddle point and barrier height were determined. The interaction of the 2Σ+ and 2Π surfaces was investigated, leading to the conclusion that in near-C∝v symmetry a low-energy path exists by which CH+1 Σ+ can be formed, with negligible barrier in excess of endothermicity. 相似文献
135.
Collisionless plasma expansion into a vacuum is investigated by computational treatment of a system of Vlasov equations for both ions and electrons coupled by a selfconsistent field. High-velocity low-density mass expansion is observed as a result of an electrostatic acceleration of ions. 相似文献
136.
137.
《Radiation Physics and Chemistry》2006,75(1):87-97
The effect of γ-irradiation on mechanical, thermal, physicochemical and structural properties of polypropylene (PP) syringes was studied. Irradiation doses of 30, 60 and 120 kGy were used with non-irradiated PP syringes serving as control samples. Irradiation caused a significant deterioration in mechanical properties of samples. The compression strength of whole syringe body decreased with increasing irradiation dose. Similarly % extension at break decreased with increasing irradiation dose. Of the physicochemical properties tested, both degree of yellowness and extractable radiolysis products increased with increasing irradiation dose. Melting temperature as well as specific melting enthalpy decreased with increasing irradiation dose. Minor differences in FTIR spectra were observed, mainly in the region of 1720 cm−1, corresponding to the absorption of carbonyl compounds indicating the formation of increased amounts of oxidation products at high irradiation doses. Gas chromatography- mass spectroscopy analysis indicated the formation of a number of radiolysis compounds as a result of irradiation. The number and concentration of these compounds increase progressively with increasing dose until 60 kGy. At the same time a number of compounds initially present in non-irradiated syringes were destroyed by irradiation. 相似文献
138.
The relativistic electronic structures of the Ag2 and Au2 molecules have been calculated using the recently developed self-consistent-field Xα Dirac-scattered-wave programs. Calculations have been carried out for transition energies and ionization potentials for selected molecular orbitals. The results indicate a large degree of s-d hybridization in the ground state of the Au2 molecule. The calculations are in good agreement with other theoretical work and with existing experimental results. 相似文献
139.
《Current Opinion in Colloid & Interface Science》2007,12(6):272-284
We summarize experimental observations of fluorescence intermittency of single semiconductor nanocrystals and single molecules. We review the main models proposed earlier to explain the broad power-law distributions of on- and off-blinking times. We argue that a self-trapping model with a distribution of trapping distances can account for most, if not all, observations to date. We propose possible scenarios for photo-ionization, the switching to states with long on-times and the influence of disorder and surfaces on the trapping dynamics. 相似文献
140.
The shift operator matrix (SOM) method is discussed. We show that in the thermodynamic limit, the largest eigenvalue of the SOM determines the grand canonical partition function for situations when simple, nearest-neighbor and next-nearest neighbor interacting particles are distributed on anM ×N lattice space. In addition, we present a method for calculating the appropriate shift operator matrices. 相似文献