全文获取类型
收费全文 | 243585篇 |
免费 | 2953篇 |
国内免费 | 1242篇 |
专业分类
电工技术 | 4833篇 |
综合类 | 265篇 |
化学工业 | 35441篇 |
金属工艺 | 10864篇 |
机械仪表 | 7067篇 |
建筑科学 | 6240篇 |
矿业工程 | 481篇 |
能源动力 | 6280篇 |
轻工业 | 24167篇 |
水利工程 | 1837篇 |
石油天然气 | 1336篇 |
武器工业 | 3篇 |
无线电 | 31043篇 |
一般工业技术 | 44894篇 |
冶金工业 | 50120篇 |
原子能技术 | 3861篇 |
自动化技术 | 19048篇 |
出版年
2021年 | 1568篇 |
2019年 | 1426篇 |
2018年 | 2327篇 |
2017年 | 2226篇 |
2016年 | 2446篇 |
2015年 | 1910篇 |
2014年 | 3165篇 |
2013年 | 10799篇 |
2012年 | 5451篇 |
2011年 | 7791篇 |
2010年 | 6286篇 |
2009年 | 6965篇 |
2008年 | 7563篇 |
2007年 | 7659篇 |
2006年 | 7034篇 |
2005年 | 6520篇 |
2004年 | 6246篇 |
2003年 | 6265篇 |
2002年 | 6243篇 |
2001年 | 6373篇 |
2000年 | 5803篇 |
1999年 | 6421篇 |
1998年 | 16566篇 |
1997年 | 11601篇 |
1996年 | 8948篇 |
1995年 | 6690篇 |
1994年 | 5909篇 |
1993年 | 5728篇 |
1992年 | 4051篇 |
1991年 | 3994篇 |
1990年 | 3583篇 |
1989年 | 3505篇 |
1988年 | 3440篇 |
1987年 | 2908篇 |
1986年 | 2842篇 |
1985年 | 3415篇 |
1984年 | 3051篇 |
1983年 | 2743篇 |
1982年 | 2547篇 |
1981年 | 2569篇 |
1980年 | 2431篇 |
1979年 | 2271篇 |
1978年 | 2235篇 |
1977年 | 2810篇 |
1976年 | 4173篇 |
1975年 | 1900篇 |
1974年 | 1796篇 |
1973年 | 1785篇 |
1972年 | 1462篇 |
1971年 | 1318篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
42.
The problem of operating freeze drying of pharmaceutical products in vials placed in trays of a freeze dryer to remove free water (in frozen state) at a minimum time was formulated as an optimal control problem. Two different types of freeze dryer designs were considered. In type I freeze dryer design, upper and lower plate temperatures were controlled together, while in type II freeze dryer design, upper and lower plate temperatures were controlled independently. The heat input to the material being dried and the drying chamber pressure were considered as control variables. Constraints were placed on the system state variables by the melting and scorch temperatures during primary drying stage. Necessary conditions of optimality for the primary drying stage of freeze drying process in vials are derived and presented. Furthermore, an approach for constructing the optimal control policies that would minimize the drying time for the primary drying stage was given. In order to analyze optimal control policy for the primary drying stage of the freeze-drying process in vials, a rigorous multi-dimensional unsteady state mathematical model was used. The theoretical approach presented in this work was applied in the freeze drying of skim milk. Significant reductions in the drying times of primary drying stage of freeze drying process in vials were obtained, as compared to the drying times obtained from conventional operational policies. 相似文献
43.
必须利用水库多蓄水,另外再通过回灌把水蓄在含水层中供缺水时使用。城市废水也是一种重要水资源,但.必须加强规划和管理以防止造成对健康的负面影响,用于灌溉时则应防止污染地下水。长期以来地下水不仅受到非点源污染,还因对其开采超过回灌导致含水层减少。在灌区,地下水位必须通过排水或抽水系统加以控制,以防止水涝和土壤盐碱化。含盐的水排出后必须妥善处置。缺水的国家可以通过从其他国家进口大部分粮食和电力来节水,这往往比开发本国的水资源划算。本地水源就可用于更具社会、生态或经济回报的用途.或以备不时之需。 相似文献
44.
T. Choi J.‐H. Jang C.K. Ullal M.C. LeMieux V.V. Tsukruk E.L. Thomas 《Advanced functional materials》2006,16(10)
The probing of the micromechanical properties within a two‐dimensional polymer structure with sixfold symmetry fabricated via interference lithography reveals a nonuniform spatial distribution in the elastic modulus “imprinted” with an interference pattern in work reported by Tsukruk, Thomas, and co‐workers on p. 1324. The image prepared by M. Lemieux and T. Gorishnyy shows how the interference pattern is formed by three laser beams and is transferred to the solid polymer structure. The elastic and plastic properties within a two‐dimensional polymer (SU8) structure with sixfold symmetry fabricated via interference lithography are presented. There is a nonuniform spatial distribution in the elastic modulus, with a higher elastic modulus obtained for nodes (brightest regions in the laser interference pattern) and a lower elastic modulus for beams (darkest regions in the laser interference pattern) of the photopatterned films. We suggest that such a nonuniformity and unusual plastic behavior are related to the variable material properties “imprinted” by the interference pattern. 相似文献
45.
This work considers space-time channel coding for systems with multiple-transmit and a single-receive antenna, over space uncorrelated block-fading (quasi-static) channels. Analysis of the outage probability over such channels reveals the existence of a threshold phenomenon. The outage probability can be made arbitrary small by increasing the number of transmit antennas, only if the E/sub b//N/sub 0/ is above a threshold which depends on the coding rate. Furthermore, it is shown that when the number of transmit antennas is increased, the /spl epsi/-capacity of a block-fading Rayleigh channel tends to the Shannon capacity of an additive white Gaussian noise channel. This paper also presents space-time codes constructed as a serial concatenation of component convolutional codes separated by an interleaver. These schemes provide full transmit diversity and are suitable for iterative decoding. The rate of these schemes is less than 1 bit/s/Hz, but can be made arbitrary close to 1 bit/s/Hz by the use of Wyner-Ash codes as outer components. Comparison of these schemes with structures from literature shows that performance gains can be obtained at the expense of a small decrease in rate. Computer simulation results over block-fading Rayleigh channels show that the frame-error rate of several of these schemes is within 2-3 dB from the theoretical outage probability. 相似文献
46.
Takauchi H. Tamura H. Matsubara S. Kibune M. Doi Y. Chiba T. Anbutsu H. Yamaguchi H. Mori T. Takatsu M. Gotoh K. Sakai T. Yamamura T. 《Solid-State Circuits, IEEE Journal of》2003,38(12):2094-2100
We describe a CMOS multichannel transceiver that transmits and receives 10 Gb/s per channel over balanced copper media. The transceiver consists of two identical 10-Gb/s modules. Each module operates off a single 1.2-V supply and has a single 5-GHz phase-locked loop to supply a reference clock to two transmitter (Tx) channels and two receiver (Rx) channels. To track the input-signal phase, the Rx channel has a clock recovery unit (CRU), which uses a phase-interpolator-based timing generator and digital loop filter. The CRU can adjust the recovered clock phase with a resolution of 1.56 ps. Two sets of two-channel transceiver units were fabricated in 0.11-/spl mu/m CMOS on a single test chip. The transceiver unit size was 1.6 mm /spl times/ 2.6 mm. The Rx sensitivity was 120-mVp-p differential with a 70-ps phase margin for a common-mode voltage ranging from 0.6 to 1.0 V. The evaluated jitter tolerance curve met the OC-192 specification. 相似文献
47.
48.
Space-time block codes (STBCs) allow utilising the diversity provided by multiple-input-multiple-output (MIMO) communication channels, thereby decreasing the outage probability for a given communication rate. The contribution of this letter is the derivation of a closed-form expression of the outage probability of distributed STBCs deployed over Nakagami flat fading channels with different channel gains and fading parameters. 相似文献
49.
Sheldon H. Jacobson Julie L. Virta Jon M. Bowman John E. Kobza John J. Nestor 《IIE Transactions》2003,35(3):259-269
Aviation security protects vital national interests, as well as passengers and aircraft. Key components of an aviation security system include baggage and passenger screening devices and operations. Determining how and where to assign (deploy) such devices can be quite challenging. Moreover, even after such systems are in place, it can be difficult to measure their effectiveness. This paper describes how discrete optimization models can be used to address these questions, based on three performance measures that quantify the effectiveness of airport baggage screening security device systems. These models are used to solve for optimal airport baggage screening security device deployments considering the number of passengers on a set of flights who have not been cleared using a security risk assessment system in use by the Federal Aviation Administration (i.e., passengers whose baggage is subjected to screening), the number of flights in this set, and the size of the aircraft for such flights. Several examples are provided to illustrate these results, including an example that uses data available from the Official Airline Guide. 相似文献
50.
On the capacities of bipartite Hamiltonians and unitary gates 总被引:2,自引:0,他引:2
Bennett C.H. Harrow A.W. Leung D.W. Smolin J.A. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(8):1895-1911
We consider interactions as bidirectional channels. We investigate the capacities for interaction Hamiltonians and nonlocal unitary gates to generate entanglement and transmit classical information. We give analytic expressions for the entanglement generating capacity and entanglement-assisted one-way classical communication capacity of interactions, and show that these quantities are additive, so that the asymptotic capacities equal the corresponding 1-shot capacities. We give general bounds on other capacities, discuss some examples, and conclude with some open questions. 相似文献