全文获取类型
收费全文 | 916篇 |
免费 | 28篇 |
国内免费 | 78篇 |
专业分类
化学 | 314篇 |
晶体学 | 3篇 |
力学 | 39篇 |
综合类 | 2篇 |
数学 | 321篇 |
物理学 | 343篇 |
出版年
2024年 | 2篇 |
2023年 | 18篇 |
2022年 | 14篇 |
2021年 | 6篇 |
2020年 | 12篇 |
2019年 | 12篇 |
2018年 | 10篇 |
2017年 | 9篇 |
2016年 | 21篇 |
2015年 | 10篇 |
2014年 | 39篇 |
2013年 | 73篇 |
2012年 | 39篇 |
2011年 | 59篇 |
2010年 | 45篇 |
2009年 | 53篇 |
2008年 | 51篇 |
2007年 | 71篇 |
2006年 | 44篇 |
2005年 | 27篇 |
2004年 | 39篇 |
2003年 | 38篇 |
2002年 | 30篇 |
2001年 | 33篇 |
2000年 | 25篇 |
1999年 | 37篇 |
1998年 | 38篇 |
1997年 | 23篇 |
1996年 | 27篇 |
1995年 | 14篇 |
1994年 | 18篇 |
1993年 | 11篇 |
1992年 | 11篇 |
1991年 | 10篇 |
1990年 | 6篇 |
1989年 | 8篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1976年 | 1篇 |
排序方式: 共有1022条查询结果,搜索用时 187 毫秒
991.
基于伪魏格纳分布分解的合成孔径雷达图像目标与阴影分割 总被引:1,自引:1,他引:0
在二维魏格纳分布的框架内,针对魏格纳变换的交叉项问题和计算量大的问题,提出了合成孔径雷达图像局部伪魏格纳变换的目标和目标阴影的分割方法.首先,将合成孔径雷达图像进行二维伪魏格纳变换,得到各像素点的二维能量谱图|然后提取各像素点的二维能量谱图对应位置值形成多个不同频段的与原图像同大小的能量谱图|最后,对不同频段的能量谱图采用不同的处理方法后,将各能量谱图相加处理后形成区域标识图像,最终得到原图像的目标和目标阴影分割图像.本文利用该方法对MSTAR切片图像进行了分割试验,并对分割图像与频谱最大值距离或方位分割算法和基于双参量CFAR与隐马尔科夫联合分割算法进行了分割图像对比度对比.实验结果表明,采用本文算法的合成孔径雷达分割图像,对比度明显提高,且保留了目标图像细节. 相似文献
992.
In this paper, we consider an inventory–routing problem (IRP) in a large petroleum and petrochemical enterprise group. Compared to many other IRPs, the problem in this paper includes some special aspects due to the operational constraints, such as hours-of-service regulations of the company and the industry. Also, in some cases, it is more important to avoid stock out for any station, rather than purely focusing on transportation cost minimization. The objective is to minimize the maximum of the route travel time, which is not addressed in the literature so far. We present a tabu search algorithm to tackle the problem, which builds in an efficient and effective procedure to improve the search quality in each iteration. Moreover, lower bounds of reasonable sized problems, which are intractable in the formulated mathematical model by existing optimization software, are obtained via Lagrangian relaxation technique. Computational results indicate that the lower bounds are tight and the tabu search is capable of providing near optimal, close-to-lower-bound solutions in a computational time effective manner. 相似文献
993.
We propose and demonstrate strain and temperature discrimination technique using a single fiber Bragg grating (FBG) written in the core of an erbium doped fiber. We observed that amplified spontaneous emission power varying linearly from the erbium doped fiber with temperature which determines temperature changes and strain is estimated by subtracting the wavelength shift due to temperature change, from the measured shift corresponding to the dip in the transmission spectrum of the FBG. A simple and compact FBG sensor is presented with improved rms errors of 21.2 μ? and 1 °C over ranges of 0–800 μ? and 40–95 °C, respectively. The sensor is shown to have strain and temperature sensitivity of 0.8 pm/μ? and 12 pm/°C. 相似文献
994.
Masatoshi Takeda Hiroki KurisuSetsuo Yamamoto Hamazo NakagawaKatsunobu Ishizawa 《Applied Surface Science》2011,258(4):1405-1411
This paper addresses a hydrogen outgassing mechanism in titanium materials with extremely low outgassing property by investigating the distribution of hydrogen atoms concentration in depth below the surface, and the activation energy for desorption of dissolved hydrogen atoms into the boundary region between the surface oxide layer and the bulk titanium and that of adsorbed hydrogen atoms on the surface. The distribution of hydrogen atoms concentration in depth below the surface was analyzed by a time-of-flight secondary ion mass spectrometry (TOF-SIMS). The activation energy for desorption of dissolved hydrogen atoms was estimated by the thermal desorption spectroscopy (TDS) measurement with various heating rates. The activation energy for desorption of adsorbed hydrogen atoms was estimated by the temperature dependence of the outgassing rate in titanium material. In the titanium material, hydrogen atoms show maximum concentration at the boundary between the surface oxide layer and the bulk titanium. Concentration of hydrogen atoms decreases rapidly at the surface oxide layer, while it decreases slowly in the deep region below the surface layer-bulk boundary by the vacuum evacuation without/with the baking process. The activation energy for desorption of 1.02 eV of dissolved hydrogen atoms into the surface layer-bulk boundary is about three times as large as that of 0.38 eV of the adsorbed hydrogen atoms on the surface. These results suggest that the hydrogen outgassing mechanism in the titanium material is composed the follows processes, i.e. the slow hydrogen atoms diffusion at the surface layer-bulk boundary, quick hydrogen atoms diffusion at the surface oxide layer and rapid desorption of adsorbed hydrogen atoms on the surface. This outgassing mechanism gives very low hydrogen concentration near the surface, which results in the extremely low outgassing rate in titanium materials. 相似文献
995.
The gain characteristics of ErxY2 − xSiO5 waveguide amplifiers have been investigated by solving rate equations and propagation equations. The gain at 1.53 μm as a function of waveguide length, Er3+ concentration and pump power is studied pumping at three different wavelengths of 654 nm, 980 nm and 1480 nm, respectively. The optimum Er3+ concentrations of 1 × 1021 cm− 3-2 × 1021 cm− 3 with the high gain are obtained for all three pump wavelengths. Pumping at 654 nm wavelength is shown to be the most efficient one due to weak cooperative upconversion. A maximum 16 dB gain at 1 mm waveguide length under a 30 mW pump with Er3+ concentration of 1 × 1021 cm− 3 is demonstrated pumping at 654 nm wavelength. 相似文献
996.
Using convex-roof extended negativity, we generalize previous qubit results of entanglement distribution and entanglement dynamics into arbitrary-dimensional quantum systems for isotropic states and the depolarizing channel. We further investigate a relation between these two types of entanglement properties. 相似文献
997.
The distribution function of dust particles immersed in a weakly-ionized plasma is calculated with regard to electron and ion absorption by grains and fluctuations of grain-charging currents. The dust-particle energy is shown to undergo abrupt changes due to specific velocity dependence of the grain friction coefficient and stochastic character of grain charging. Their effect can crucially influence the state of the grain. This model can give some indication concerning the possibility of structure formation and melting of a dusty crystal structure. The hysteresis in the transition from the one-peak distribution function to twin-peak distribution function can be observed with changing of plasma parameters. 相似文献
998.
We consider the infinite horizon inventory routing problem in a three-level distribution system with a vendor, a warehouse and multiple geographically dispersed retailers. In this problem, each retailer faces a demand at a deterministic, retailer-specific rate for a single product. The demand of each retailer is replenished either from the vendor through the warehouse or directly from the vendor. Inventories are kept at both the retailers and the warehouse. The objective is to determine a combined transportation (routing) and inventory strategy minimizing a long-run average system-wide cost while meeting the demand of each retailer without shortage. We present a decomposition solution approach based on a fixed partition policy where the retailers are partitioned into disjoint and collectively exhaustive sets and each set of retailers is served on a separate route. Given a fixed partition, the original problem is decomposed into three sub-problems. Efficient algorithms are developed for the sub-problems by exploring important properties of their optimal solutions. A genetic algorithm is proposed to find a near-optimal fixed partition for the problem. Computational results show the performance of the solution approach. 相似文献
999.
分配网络流广泛应用于解决水源、电力的调度及工厂的产品运输、分配、合成等问题.本文提出一个分配网络流的最小费用流算法. 相似文献
1000.
Let D be a region, {rn}n∈N a sequence of rational functions of degree at most n and let each rn have at most m poles in D, for m∈N fixed. We prove that if {rn}n∈N converges geometrically to a function f on some continuum S⊂D and if the number of zeros of rn in any compact subset of D is of growth o(n) as n→∞, then the sequence {rn}n∈N converges m1-almost uniformly to a meromorphic function in D. This result about meromorphic continuation is used to obtain Picard-type theorems for the value distribution of m1-maximally convergent rational functions, especially in Padé approximation and Chebyshev rational approximation. 相似文献