全文获取类型
收费全文 | 234624篇 |
免费 | 17537篇 |
国内免费 | 9973篇 |
专业分类
电工技术 | 12726篇 |
技术理论 | 10篇 |
综合类 | 15417篇 |
化学工业 | 37763篇 |
金属工艺 | 13690篇 |
机械仪表 | 14365篇 |
建筑科学 | 14555篇 |
矿业工程 | 5509篇 |
能源动力 | 5457篇 |
轻工业 | 15416篇 |
水利工程 | 4549篇 |
石油天然气 | 12186篇 |
武器工业 | 1587篇 |
无线电 | 25584篇 |
一般工业技术 | 33739篇 |
冶金工业 | 13981篇 |
原子能技术 | 5738篇 |
自动化技术 | 29862篇 |
出版年
2024年 | 917篇 |
2023年 | 3024篇 |
2022年 | 5959篇 |
2021年 | 8147篇 |
2020年 | 6184篇 |
2019年 | 5350篇 |
2018年 | 6557篇 |
2017年 | 7260篇 |
2016年 | 6784篇 |
2015年 | 7990篇 |
2014年 | 10463篇 |
2013年 | 12770篇 |
2012年 | 14158篇 |
2011年 | 15214篇 |
2010年 | 13431篇 |
2009年 | 13197篇 |
2008年 | 13077篇 |
2007年 | 12386篇 |
2006年 | 11726篇 |
2005年 | 9948篇 |
2004年 | 7839篇 |
2003年 | 7423篇 |
2002年 | 7975篇 |
2001年 | 7163篇 |
2000年 | 5669篇 |
1999年 | 4689篇 |
1998年 | 3528篇 |
1997年 | 2957篇 |
1996年 | 2646篇 |
1995年 | 2327篇 |
1994年 | 1898篇 |
1993年 | 1556篇 |
1992年 | 1469篇 |
1991年 | 1271篇 |
1990年 | 1255篇 |
1989年 | 1112篇 |
1988年 | 995篇 |
1987年 | 918篇 |
1986年 | 794篇 |
1985年 | 764篇 |
1984年 | 739篇 |
1981年 | 686篇 |
1979年 | 750篇 |
1978年 | 785篇 |
1977年 | 751篇 |
1976年 | 770篇 |
1975年 | 719篇 |
1974年 | 725篇 |
1973年 | 726篇 |
1972年 | 707篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
试论循环水装置的经济性及其成本分析 总被引:2,自引:1,他引:1
通过对循环水装置的投资和成本分析,阐述了建设该装置的经济性和社会意义。 相似文献
52.
53.
E. A. Gurieva P. P. Konstantinov L. V. Prokof D. A. Pshenaĭ-Severin M. I. Fedorov Yu. I. Ravich 《Semiconductors》2006,40(7):763-767
The coefficients of thermopower and electrical and thermal conductivity in the PbTe0.8Se0.1 S 0.1 solid solution with electron concentration (4.6–54) × 1018 cm?3 are studied in the range of 85–300 K (and in some cases up to 700 K). The temperature dependences of electrical and thermal conductivity indicate that the low-temperature electron and phonon scattering initiated by the off-center impurity of sulfur exists. The temperature dependences of the electronic and lattice components of thermal conductivity are calculated in the approximation of a parabolic spectrum and electron scattering by acoustic phonons and neutral substitutional impurities. The lattice thermal conductivity is found to have a feature in the form of a shallow minimum in the range of 85–250 K. A similar feature, while not so clearly pronounced, is found to exist also in Pb1?x SnxTe1?x Sex alloys (x≥0.15) with an off-center tin impurity. An analysis of the possible origins of this effect suggests that, at low temperatures, the Lorentz numbers L of the materials under study are smaller than the L0 numbers employed which correspond to the above scattering mechanisms. The cause of the decrease in L is related to electron scattering at two-level systems, a mechanism whose effect grows with increasing electron energy. An analysis of experimental data obtained at high temperatures, as well as on undoped samples with the lowest possible carrier concentrations, yields the values of L for samples with different electron densities. The minimum value L/L0 = 0.75 is obtained for a lightly doped sample at ~130 K. 相似文献
54.
Ramanujam J. Jinpyo Hong Kandemir M. Narayan A. Agarwal A. 《Signal Processing, IEEE Transactions on》2006,54(1):286-294
Most embedded systems have limited amount of memory. In contrast, the memory requirements of the digital signal processing (DSP) and video processing codes (in nested loops, in particular) running on embedded systems is significant. This paper addresses the problem of estimating and reducing the amount of memory needed for transfers of data in embedded systems. First, the problem of estimating the region associated with a statement or the set of elements referenced by a statement during the execution of nested loops is analyzed. For a fixed execution ordering, a quantitative analysis of the number of elements referenced is presented; exact expressions for uniformly generated references and a close upper and lower bound for nonuniformly generated references are derived. Second, in addition to presenting an algorithm that computes the total memory required, this paper also discusses the effect of transformations (that change the execution ordering) on the lifetimes of array variables, i.e., the time between the first and last accesses to a given array location. The term maximum window size is introduced, and quantitative expressions are derived to compute the maximum window size. A detailed analysis of the effect of unimodular transformations on data locality, including the calculation of the maximum window size, is presented. 相似文献
55.
This letter focuses on the performance analysis of the decorrelating receiver in multipath Rician faded CDMA channels. M-ary QAM scheme is employed to improve the spectral efficiency. Approximate expressions are first derived for the two performance indexes: the average symbol error rate (SER) and the average bit error rate (BER) when the decorrelating-first receiver perfectly knows the channel information of the user of interest. To achieve desirable closed-form expressions of the SER and the BER, we exploit results in large system analysis and make assumptions of a high signal-to-interference ratio (SIR) and/or a small Rician K-factor. To measure the receiver performance in the practical scenario, we further derive expressions to approximate the average SER and BER of the decorrelating-first scheme with channel uncertainty. Simulation results demonstrate that the analytical results can also be employed to evaluate the performance of the combining-first receiver. 相似文献
56.
N.S. Chen S.F. Yu 《Photonics Technology Letters, IEEE》2004,16(7):1610-1612
The effect of external optical feedback on the transient response of antiresonant reflecting optical waveguide (ARROW) vertical-cavity surface-emitting lasers (VCSELs) is studied. It can be shown that the proper design of ARROW can suppress the excitation of high-order transverse leaky mode as well as increase the critical feedback strength so that stable high-power single-mode operation of VCSELs can be obtained even under the influence of strong external optical feedback. 相似文献
57.
58.
59.
60.
根据石油开采业对石油机械产品的需求 ,利用机械理论基础知识 ,通过实践设计 ,提出了对几何尺寸长的金属管类进行调质处理后的均匀冷却方法 ,以满足其性能要求。 相似文献