全文获取类型
收费全文 | 2246篇 |
免费 | 97篇 |
国内免费 | 15篇 |
专业分类
电工技术 | 27篇 |
综合类 | 6篇 |
化学工业 | 564篇 |
金属工艺 | 74篇 |
机械仪表 | 64篇 |
建筑科学 | 36篇 |
矿业工程 | 2篇 |
能源动力 | 130篇 |
轻工业 | 85篇 |
水利工程 | 13篇 |
石油天然气 | 7篇 |
无线电 | 311篇 |
一般工业技术 | 541篇 |
冶金工业 | 162篇 |
原子能技术 | 15篇 |
自动化技术 | 321篇 |
出版年
2023年 | 43篇 |
2022年 | 80篇 |
2021年 | 85篇 |
2020年 | 69篇 |
2019年 | 69篇 |
2018年 | 77篇 |
2017年 | 78篇 |
2016年 | 85篇 |
2015年 | 52篇 |
2014年 | 83篇 |
2013年 | 173篇 |
2012年 | 105篇 |
2011年 | 123篇 |
2010年 | 101篇 |
2009年 | 98篇 |
2008年 | 89篇 |
2007年 | 81篇 |
2006年 | 78篇 |
2005年 | 48篇 |
2004年 | 47篇 |
2003年 | 37篇 |
2002年 | 40篇 |
2001年 | 38篇 |
2000年 | 24篇 |
1999年 | 26篇 |
1998年 | 42篇 |
1997年 | 44篇 |
1996年 | 32篇 |
1995年 | 25篇 |
1994年 | 33篇 |
1993年 | 29篇 |
1992年 | 16篇 |
1991年 | 20篇 |
1990年 | 10篇 |
1989年 | 8篇 |
1988年 | 10篇 |
1987年 | 11篇 |
1986年 | 19篇 |
1985年 | 19篇 |
1984年 | 25篇 |
1983年 | 16篇 |
1982年 | 16篇 |
1981年 | 24篇 |
1980年 | 20篇 |
1979年 | 19篇 |
1978年 | 17篇 |
1977年 | 17篇 |
1976年 | 14篇 |
1975年 | 8篇 |
1972年 | 5篇 |
排序方式: 共有2358条查询结果,搜索用时 15 毫秒
981.
High temperature tensile creep behaviour of a directionally-solidified Ni3Al-based alloy is presented. The study involved selection of nine alloy systems based on Ni3Al. The alloys contained varying amounts of Cr and Ta, fixed amounts of 1·5 at.% Hf and 0·5 at.% Zr and doped with 0·2 at.%
each of C and B. The alloys were vacuum arc-melted into buttons and homogenized at 1050°C for 68 h. The test pieces of the
alloys were hot compression tested at 600, 700, 800 and 900°C. The yield strength data of some of the alloys were superior
to conventionally cast Mar-M 200, a cast nickel-base superalloy widely used in gas turbine structural applications. The best
alloy system was chosen based on consistent performance in the hot compression studies. The alloy so chosen was directionally
solidified and vacuum-homogenization-treated for 20 h at various selected temperatures. Optimum creep properties were observed
at 1120°C, 20 h treatment. The minimum creep rate data of the DS alloy showed relatively higher values even at lower temperatures
and stress levels as compared to Mar-M 200. Hence, the alloy is less promising in replacing nickel-based superalloys used
as structural materials in gas turbine applications. 相似文献
982.
Bayesian analysis combines the prior belief about the lifetime parameters with the experimental data. The present study develops Bayes estimates for the pointwise and interval availability of a system with constant failure and repair rates. Type I censored failure and repair information has been used in the estimation. 相似文献
983.
This paper reviews the evolution of off-line quality engineering methods with respect to one or more quality criteria, and
presents some recent results. The fundamental premises that justify the use of robust product/process design are established
with an illustrative example. The use of designed experiments to model quality criteria and their optimization is briefly
reviewed. The fact that most design-for-quality problems involve multiple quality criteria motivates the development of multiobjective
optimization techniques for robust parameter design. Two situations are considered: one in which response surface models for
the quality characteristics can be obtained using regression and considered over a continuous factor space, and one in which
the problem scenario and the experiment permit only discrete parameter settings for the design factors. In the former scenario,
a multiobjective optimization technique based on the reference-point method is presented; this technique also incorporates
an inference mechanism to deal with uncertainty in the response surface models caused by finite, noisy data. In the discrete-factors
scenario, an efficient method to reduce computational complexity for a class of models is presented. 相似文献
984.
An algorithm for computing recursively the exact system reliability of k-out-of-n systems is proposed. It is simple, easy to implement, fast, and memory efficient. It gives a reliability expression with minimal number of terms, C(k, n) and involves only a few multiplications. The reduction in number of terms and multiplications is over 50 percent compared to some methods. The recursive nature of the algorithm enables one to design easily the number of units in the system to meet a reliability target. An alternative representation of the algorithm which is easy to remember and good for manual computation is given. However, it involves a few more multiplications compared to the original one but fewer than those required with existing methods. 相似文献
985.
A TEM study of microstructural changes during retrogression and reaging in 7075 aluminum 总被引:11,自引:0,他引:11
Nguyen Cong Danh Krishna Rajan W. Wallace 《Metallurgical and Materials Transactions A》1983,14(9):1843-1850
Microstructural changes occurring during retrogression, and during retrogression plus reaging in 7075-T6 aluminum alloy have
been investigated by means of transmission electron microscopy, and related to mechanical properties. TEM results indicate
that the drop in strength during the initial stage of retrogression was due to the partial dissolution of G.P. zones while
the growth of the semi-coherentη ′ was responsible for the rapid recovery of strength. It is suggested that the retrogression and reaging treatment resulted
in the increase in volume fraction of G.P. zones and especially η′ precipitates over both the T6 and retrogressed conditions,
therefore significantly improving the strength of the alloy. 相似文献
986.
Dielectric constant (K) and loss (tan δ) of selenite single crystals have been measured in the frequency region 102 to 107 Hz and in the temperature range 30 to 400° C. Measurements have also been taken after X-ray or γ-ray irradiation for these crystals. At 30° C,K, which has larger values at lower frequencies, decreases with frequency, reaching a constant value of 5.8 beyond 105 Hz; similar behaviour is exhibited by tan δ. Variation ofK and tan δ with temperature at different frequencies shows peaks around 120 and 265° C, the peak positions remaining the same at all frequencies. X-ray irradiation of the sample increasesK and tan δ values at lower frequencies significantly, but decreases them at the peaks. It is observed that the changes ofK and tan δ are greater with γ-ray irradiation than with X-ray irradiation. An attempt is made to understand the results. 相似文献
987.
Bacteria possess multiple mechanisms for the transport of metal ions. While many of these systems may have evolved in the first instance to resist the detrimental effects of toxic environmental heavy metals, they have since become adapted to a variety of important homeostatic functions. The 'P'-type ATPases play a key role in metal ion transport in bacteria. A Cu+-ATPase from the intracellular bacterium Listeria monocytogenes is implicated in pathogenesis, and similar pumps in Mycobacterium tuberculosis and M. leprae may play a comparable role. Intracellular bacteria require transition metal cations for the synthesis of superoxide dismutases and catalases, which constitute an important line of defence against macrophage-killing mechanisms. The macrophage protein Nramp1, which confers resistance to a variety of intracellular pathogens, has also been shown recently to be a divalent amphoteric cation transporter. Mycobacterial homologues have recently been identified by genomic analysis. These findings suggest a model in which competition for divalent cations plays a pivotal role in the interaction between host and parasite. 相似文献
988.
Labourdette Jean-François Bouillet Eric Ramamurthy Ramu Ellinas Georgios Chaudhuri Sid Bala Krishna 《Photonic Network Communications》2002,4(3-4):219-235
Wavelength division multiplexed (WDM)-based mesh network infrastructures that route optical connections using intelligent optical cross-connects (OXCs) are emerging as the technology of choice to implement the next generation core optical networks. In these architectures a single OXC is capable of switching tens of terabits of traffic per second. With such data transfer rates at stake, it becomes increasingly challenging for carriers to (1) efficiently and cost-effectively operate and manage their infrastructure, and (2) cope with network failures while guaranteeing prescribed service level agreements (SLAs) to their customers. Proper routing of primary and backup paths is a critical component of the routing and restoration architecture required to meeting these challenges. In this paper we review some of the various strategies and approaches proposed so far to intelligently route connections while at the same time providing guaranteed protection against various types of network failures. We explore the tradeoffs associated with these approaches, and investigate in particular different, sometimes competing aspects, such as cost/capacity required, level of protection (link vs. node failure), restoration time, and complexity of route computation. 相似文献
989.
On optimal design of multitier wireless cellular systems 总被引:2,自引:0,他引:2
We present a general cell-design methodology for the optimal design of a multitier wireless cellular network. Multitier networks are useful when there are a multitude of traffic types with drastically different parameters and/or different requirements, such as different mobility parameters or quality-of-service requirements. In such situations, it may be cost-effective to build a multitude of cellular infrastructures, each serving a particular traffic type. The network resources (e.g., the radio channels) are then partitioned among the multitude of tiers. In general terms, we are interested in quantifying the cost reduction due to the multitier network design, as opposed to a single-tier network. Our study is motivated by the expected proliferation of personal communication services, which will serve different mobility platforms and support multimedia applications through a newly deployed infrastructure based on the multitier approach 相似文献
990.
The probability of capture under a model (for a land mobile radio direct sequence spread spectrum system) based on the ratio of the largest received power to the sum of interference powers is examined in the limit of a large number of transmitting stations. It is shown in great generality that the limit depends only on the capture ratio threshold and the roll-off exponent of the distribution of power received from a typical station. This exponent is insensitive to many typical channel effects such as Rician or Rayleigh fading and log-normal shadowing. The model is suitable for large systems with noncoherently combined interference 相似文献