全文获取类型
收费全文 | 31435篇 |
免费 | 3812篇 |
国内免费 | 3200篇 |
专业分类
电工技术 | 2559篇 |
综合类 | 4377篇 |
化学工业 | 2238篇 |
金属工艺 | 2215篇 |
机械仪表 | 2101篇 |
建筑科学 | 2639篇 |
矿业工程 | 706篇 |
能源动力 | 543篇 |
轻工业 | 1454篇 |
水利工程 | 806篇 |
石油天然气 | 708篇 |
武器工业 | 398篇 |
无线电 | 3651篇 |
一般工业技术 | 5037篇 |
冶金工业 | 1023篇 |
原子能技术 | 279篇 |
自动化技术 | 7713篇 |
出版年
2024年 | 144篇 |
2023年 | 432篇 |
2022年 | 690篇 |
2021年 | 853篇 |
2020年 | 1035篇 |
2019年 | 1031篇 |
2018年 | 946篇 |
2017年 | 1299篇 |
2016年 | 1351篇 |
2015年 | 1324篇 |
2014年 | 1782篇 |
2013年 | 2372篇 |
2012年 | 2255篇 |
2011年 | 2390篇 |
2010年 | 1830篇 |
2009年 | 1999篇 |
2008年 | 1893篇 |
2007年 | 2092篇 |
2006年 | 1770篇 |
2005年 | 1616篇 |
2004年 | 1372篇 |
2003年 | 1162篇 |
2002年 | 993篇 |
2001年 | 874篇 |
2000年 | 800篇 |
1999年 | 667篇 |
1998年 | 503篇 |
1997年 | 502篇 |
1996年 | 393篇 |
1995年 | 354篇 |
1994年 | 300篇 |
1993年 | 244篇 |
1992年 | 229篇 |
1991年 | 188篇 |
1990年 | 159篇 |
1989年 | 133篇 |
1988年 | 85篇 |
1987年 | 65篇 |
1986年 | 35篇 |
1985年 | 36篇 |
1984年 | 44篇 |
1983年 | 45篇 |
1982年 | 34篇 |
1981年 | 32篇 |
1980年 | 26篇 |
1979年 | 15篇 |
1978年 | 10篇 |
1977年 | 15篇 |
1976年 | 5篇 |
1959年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
1.
Yasoub Eghbali Mahmoud Ferdosizade Naeiny 《International Journal of Communication Systems》2020,33(14)
In this paper, an off‐grid direction of arrival (DoA) estimation method is proposed for wideband signals. This method is based on the sparse representation (SR) of the array covariance matrix. Similar to the time domain DoA estimation methods, the correlation function of the sources was assumed to be the same and known. A new measurement vector is obtained using the lower‐left triangular elements of the covariance matrix. The DoAs are estimated by quantizing the entire range of continuous angle space into discrete grid points. However, the exact DoAs may be located between two grid points; therefore, this estimation has errors. The accuracy of DoA estimation is improved by the minimization of the difference between the new measurement vector and its estimated values. Simulation results revealed that the proposed method can enhance the DoA estimation accuracy of wideband signals. 相似文献
2.
Optical studies of residual strain in cadmium telluride (CdTe) films grown using molecular beam epitaxy on gallium arsenide (GaAs) substrate have been performed using photoreflectance techniques. Measurements have been conducted to determine the fundamental transition energy, heavy-hole and light-hole transition energy critical-point parameters in a range of temperatures between 12 and 300 K. There are problems inherent in the fabrication of optoelectronic devices using high-quality CdTe films, due to strain effects resulting from both the lattice mismatch (CdTe: 14.6%) and the thermal expansion coefficient difference. The CdTe film exhibits compressive stress causing valence-band splitting for light and heavy holes. We have used different models to fit the obtained experimental data and, although the critical thickness for the CdTe has been surpassed, the strain due to the lattice mismatch is still significant. However, the strain due to the thermal expansion is dominant. We have found that the fundamental transition energy, E0, is affected by the compressive strain and the characteristic values are smaller than those reported. In addition, the total strain is compressive for the full measured range, since the strain due to the lattice mismatch is one order of magnitude higher than that calculated from the thermal expansion. 相似文献
3.
Transportation demand of shipping container fluctuates due to the seasonality of international trade, thus, every 3–6 months, the liner company has to alter its current liner shipping service network, redeploy ships and design cargo routes with the objective of minimizing the total cost. To solve the problem, the paper presents a mixed integer linear program model. The proposed model incorporates several relevant constraints, such as weekly frequency, the transshipment of cargo between two or more service routes, and transport time. Extensive numerical experiments based on realistic date of Asia–Europe–Oceania shipping operations show that the proposed model can solve real-case problems efficiently by CPLEX. The results demonstrate that the model can reduce ship’s capacity consumption and raise ships’ capacity utilization. 相似文献
4.
ABSTRACTIn this paper, we review some algebraic control system. Precisely, linear and bilinear systems on Euclidean spaces and invariant and linear systems on Lie groups. The fourth classes of systems have a common issue: to any class, there exists an associated subgroup. From this object, we survey the controllability property. Especially, from those coming from our contribution to the theory. 相似文献
5.
In this letter, we address the problem of Direction of Arrival (DOA) estimation with nonuniform linear array in the context of sparse Bayesian learning (SBL) framework. The nonuniform array output is deemed as an incomplete-data observation, and a hypothetical uniform linear array output is treated as an unavailable complete-data observation. Then the Expectation-Maximization (EM) criterion is directly utilized to iteratively maximize the expected value of the complete-data log likelihood under the posterior distribution of the latent variable. The novelties of the proposed method lie in its capability of interpolating the actual received data to a virtual uniform linear array, therefore extending the achievable array aperture. Simulation results manifests the superiority of the proposed method over off-the-shelf algorithms, specially on circumstances such as low SNR, insufficient snapshots, and spatially adjacent sources. 相似文献
7.
8.
基于承包商预期收益不变的不平衡报价模型 总被引:1,自引:0,他引:1
在招标实践中,通常采用最低价中标原则,使得投标人不得不压低其投标价格。基于承包商预期收益不变的不平衡报价模型,是在“工程量清单报价”的条件下,考虑到施工中工程量的变化及施工顺序的影响,体现了资金的时间价值,在保持承包商预期收益不变的前提下,通过调整各分项工程的单价,来降低工程报价,以增加中标机会而建立的。所建立的模型简单,易于操作,并配以具体的工程实例,通过计算机求得其最优解,可供承包商投标报价时参考。 相似文献
9.
Rheological properties of MR fluids under large step strain shear are presented in this paper. The experiments were carried out using a rheometer with parallel-plate geometry. Under the large step strain shear, MR fluids behave as nonlinear viscoelastic properties, where the stress relaxation modulus, G(t, γ), shows a decreasing trend with step strain. The experimental results indicate that G(t, γ) obeys time-strain separability. Thus, a mathematical form based on finite exponential serials is proposed to predict MR behavior. In this model, G(t, γ) is represented as the product of a linear stress relaxation, G(t), and the damping function, h(γ), i.e. G(t, γ)=G(t) h(γ). G(t) is simply represented as a three-parameter exponential serial and h(γ) has a sigmoidal form with two parameters. The parameters are identified by adopting an efficient optimization method proposed by Stango et al. The comparison between the experimental results and the model-predicted values indicates that this mathematical model can accurately predict MR behavior. 相似文献
10.
讨论了输出终端带不等式约束情形的多输入多输出系统,说明了这些约束可以通过修改输入柔化系数而使其得以满足,在柔化系数的选择范围确定过程中,为了避免矩阵求逆,采用利用已知数据辨识参数的方法来确定柔化系数,从而使得问题简化。降低了计算量且加快了收敛速度。 相似文献