全文获取类型
收费全文 | 261636篇 |
免费 | 3737篇 |
国内免费 | 827篇 |
专业分类
电工技术 | 5164篇 |
综合类 | 339篇 |
化学工业 | 37584篇 |
金属工艺 | 10288篇 |
机械仪表 | 8505篇 |
建筑科学 | 6155篇 |
矿业工程 | 1081篇 |
能源动力 | 6460篇 |
轻工业 | 23196篇 |
水利工程 | 2571篇 |
石油天然气 | 4076篇 |
武器工业 | 69篇 |
无线电 | 34924篇 |
一般工业技术 | 50320篇 |
冶金工业 | 46845篇 |
原子能技术 | 5428篇 |
自动化技术 | 23195篇 |
出版年
2021年 | 2066篇 |
2019年 | 2087篇 |
2018年 | 3523篇 |
2017年 | 3574篇 |
2016年 | 3786篇 |
2015年 | 2365篇 |
2014年 | 4118篇 |
2013年 | 11393篇 |
2012年 | 6658篇 |
2011年 | 9105篇 |
2010年 | 7123篇 |
2009年 | 7927篇 |
2008年 | 8766篇 |
2007年 | 8786篇 |
2006年 | 7883篇 |
2005年 | 7184篇 |
2004年 | 6936篇 |
2003年 | 6780篇 |
2002年 | 6421篇 |
2001年 | 6496篇 |
2000年 | 6200篇 |
1999年 | 6297篇 |
1998年 | 14335篇 |
1997年 | 10353篇 |
1996年 | 8164篇 |
1995年 | 6448篇 |
1994年 | 5724篇 |
1993年 | 5608篇 |
1992年 | 4563篇 |
1991年 | 4220篇 |
1990年 | 4055篇 |
1989年 | 3791篇 |
1988年 | 3626篇 |
1987年 | 3177篇 |
1986年 | 3071篇 |
1985年 | 3620篇 |
1984年 | 3387篇 |
1983年 | 3035篇 |
1982年 | 2854篇 |
1981年 | 2952篇 |
1980年 | 2762篇 |
1979年 | 2648篇 |
1978年 | 2501篇 |
1977年 | 2946篇 |
1976年 | 3561篇 |
1975年 | 2319篇 |
1974年 | 2314篇 |
1973年 | 2319篇 |
1972年 | 1849篇 |
1971年 | 1746篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
172.
173.
Pectic enzymes are widely used in the food industry for fruit juice extraction as well as in the clarification of cloudy juices. Our laboratory has been using pectic enzymes produced from a strain of Saccharomyces cerevisiae (ATCC 52712) in different applications including fruit juice extraction. The enzyme was produced in the laboratory by culturing the yeast in papaya juice supplemented with 1% pectin for 6 days. Known amounts of enzyme preparation (0–40 mg protein) were added to a measured weight of papaya mash for varying reaction periods (30–90 min) and the amount of free‐run juice obtained in each treatment compared with a control sample. Treatment of 200 g of papaya mash with different dosages of the pectic enzyme extract resulted in rapid increases in flow rate of free‐run juice. Mash treated with 32 mg of total protein extract with a 30‐min reaction time was the optimum for a maximum rate of juice flow (25 mL/min) when initial rates were measured. There was no significant difference (P > 0.05) between the red‐ and yellow‐fleshed varieties of the papaya used. When juice flow was monitored over 6 min, the treated samples gave a flow rate that was more than twice those of the untreated samples. This biotechnological approach could be adopted to enhance papaya‐juice production by local fruit juice processors when parameters for scale‐up processes are established. 相似文献
174.
175.
Real-time multimedia applications have to use forward error correction (FEC) anderror concealment techniques to cope with
losses in today’s best-effort Internet. The efficiency of these solutions is known however to depend on the correlation between
losses in the media stream. In this paper we investigate how the packet size distribution affects the packet loss process,
that is, the distribution of the number of lost packets in a block, the related FEC performance and the average loss run length.
We present mathematical models for the loss process of the MMPP+M/D/1/K and the MMPP+M/M/1/K queues; we validate the models
via simulations, and compare the results to simulation results with an MPEG-4 coded video trace. We conclude that the deterministic
packet size distribution (PSD) not only results in lower stationary loss probability than the exponential one, but also gives
a less correlated loss process, both at a particular average link load and at a particular stationary loss probability as
seen by the media stream.Our results show that for applications that can only measure the packet loss probability, the effects
of the PSD on FEC performance are higher in access networks, where a single multimedia stream might affect the multiplexing
behavior. Our results show that the effects of the PSD on FEC performance are higher in access networks, where a single multimedia
stream might affect the multiplexing behavior and thus can improve the queuing performance by decreasing the variance of its
PSD. 相似文献
176.
In multi-user communications, the access point (AP) has several alternatives for distributing the scarce resources among users. Since there exists a trade-off between the global performance and the individual needs, an analytical framework to study fairness is derived, which completes the scope given by the existing fairness indexes in the literature. The framework proposed in this paper is a way to interpret fairness that has been inspired by portfolio selection; basically, it analyzes the mean vs. standard deviation trade-off. In this work, the target application is a multi-antenna AP transmitting simultaneously to several single-antenna terminals, although this framework is valid to analyze other procedures in multi-user communications. 相似文献
177.
S.J. Choi K. Djordjev Zhen Peng Qi Yang Sang Jun Choi P.D. Dapkus 《Photonics Technology Letters, IEEE》2004,16(10):2266-2268
All-buried InP-InGaAsP ring resonators laterally coupled to bus waveguides are demonstrated. The buried configurations offer a lower built-in refractive index step along the resonator periphery, which affords enhanced optical coupling coefficients between the waveguides and reduced scattering losses caused by the resonator sidewall imperfections. Very low optical intensity attenuations of 0.4 cm/sup -1/ and coupling-limited quality factors of greater than 10/sup 5/ are observed from 200-/spl mu/m-radii ring resonators. The measured spectral linewidth is as narrow as 0.0145 nm. 相似文献
178.
179.
Fatigue crack propagation of multiple coplanar cracks with the coupled extended finite element/fast marching method 总被引:1,自引:0,他引:1
A numerical technique for modeling fatigue crack propagation of multiple coplanar cracks is presented. The proposed method couples the extended finite element method (X-FEM) [Int. J. Numer. Meth. Engng. 48 (11) (2000) 1549] to the fast marching method (FMM) [Level Set Methods & Fast Marching Methods: Evolving Interfaces in Computational Geometry, Fluid Mechanics, Computer Vision, and Materials Science, Cambridge University Press, Cambridge, UK, 1999]. The entire crack geometry, including one or more cracks, is represented by a single signed distance (level set) function. Merging of distinct cracks is handled naturally by the FMM with no collision detection or mesh reconstruction required. The FMM in conjunction with the Paris crack growth law is used to advance the crack front. In the X-FEM, a discontinuous function and the two-dimensional asymptotic crack-tip displacement fields are added to the finite element approximation to account for the crack using the notion of partition of unity [Comput. Meth. Appl. Mech. Engng. 139 (1996) 289]. This enables the domain to be modeled by a single fixed finite element mesh with no explicit meshing of the crack surfaces. In an earlier study [Engng. Fract. Mech. 70 (1) (2003) 29], the methodology, algorithm, and implementation for three-dimensional crack propagation of single cracks was introduced. In this paper, simulations for multiple planar cracks are presented, with crack merging and fatigue growth carried out without any user-intervention or remeshing. 相似文献
180.
Linear transformation shift registers 总被引:6,自引:0,他引:6
Dewar M. Panario D. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(8):2047-2052
In order to exploit word-oriented operations for linear-feedback shift registers (LFSRs), Tsaban and Vishne [2002] introduced the notion of linear transformation shift registers (TSRs). An implementation of their primitive TSR generating algorithm shows that the LFSR are paired for all transformations. We prove that the characteristic polynomials of a pair of LFSRs are either both irreducible or both reducible for all transformations. This allows some time improvement when finding primitive TSRs. The authors give a full enumeration of all primitive TSRs with transformations of order 8 and LFSRs of order 3, 4, 5, and 6. 相似文献