全文获取类型
收费全文 | 326174篇 |
免费 | 4463篇 |
国内免费 | 1091篇 |
专业分类
电工技术 | 6127篇 |
综合类 | 280篇 |
化学工业 | 46429篇 |
金属工艺 | 14659篇 |
机械仪表 | 13184篇 |
建筑科学 | 8137篇 |
矿业工程 | 1323篇 |
能源动力 | 7915篇 |
轻工业 | 29662篇 |
水利工程 | 3231篇 |
石油天然气 | 3384篇 |
武器工业 | 5篇 |
无线电 | 40390篇 |
一般工业技术 | 65759篇 |
冶金工业 | 52303篇 |
原子能技术 | 4480篇 |
自动化技术 | 34460篇 |
出版年
2021年 | 1798篇 |
2020年 | 1347篇 |
2019年 | 1667篇 |
2018年 | 11282篇 |
2017年 | 11953篇 |
2016年 | 7596篇 |
2015年 | 2710篇 |
2014年 | 3740篇 |
2013年 | 12007篇 |
2012年 | 9416篇 |
2011年 | 18165篇 |
2010年 | 15639篇 |
2009年 | 15387篇 |
2008年 | 15557篇 |
2007年 | 17596篇 |
2006年 | 8351篇 |
2005年 | 11218篇 |
2004年 | 9216篇 |
2003年 | 8690篇 |
2002年 | 7231篇 |
2001年 | 7084篇 |
2000年 | 6450篇 |
1999年 | 6837篇 |
1998年 | 16381篇 |
1997年 | 11369篇 |
1996年 | 8595篇 |
1995年 | 6552篇 |
1994年 | 5735篇 |
1993年 | 5590篇 |
1992年 | 4011篇 |
1991年 | 3796篇 |
1990年 | 3548篇 |
1989年 | 3369篇 |
1988年 | 3280篇 |
1987年 | 2589篇 |
1986年 | 2487篇 |
1985年 | 3156篇 |
1984年 | 2797篇 |
1983年 | 2559篇 |
1982年 | 2344篇 |
1981年 | 2390篇 |
1980年 | 2201篇 |
1979年 | 2048篇 |
1978年 | 1879篇 |
1977年 | 2176篇 |
1976年 | 2724篇 |
1975年 | 1584篇 |
1974年 | 1489篇 |
1973年 | 1558篇 |
1972年 | 1144篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
962.
M. M. Stadnyk 《Materials Science》2007,43(6):764-768
We deduce the theoretical formula taking into account the influence of biaxial loading of a plate weakened by a crack on the
stress intensity factor K
I. This enables us to compute the characteristic of crack resistance K
c according to the known boundary forces.
__________
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 6, pp. 14–16, November–December, 2007. 相似文献
963.
This paper considers the axisymmetric steady flow driven by exact counter rotation of two co-axial disks of finite radius.
At the edges of the rotating disks one of three conditions is (typically) imposed: (i) zero velocity, corresponding to a stationary,
impermeable, cylindrical shroud (ii) zero normal velocity and zero tangential fluid traction, corresponding to a (confined)
free surface and (iii) an edge constraint that is consistent with a similarity solution of von Kármán form. The similarity
solution is valid in an infinite geometry and possesses a pitchfork bifurcation that breaks the midplane symmetry at a critical
Reynolds number. In this paper, similar bifurcations of the global (finite-domain) flow are sought and comparisons are made
between the resulting bifurcation structure and that found for the similarity solution. The aim is to assess the validity
of the nonlinear similarity solutions in finite domains and to explore the sensitivity of the solution structure to edge conditions
that are implicitly neglected when assuming a self-similar flow. It is found that, whilst the symmetric similarity solution
can be quantitatively useful for a range of boundary conditions, the bifurcated structure of the finite-domain flow is rather
different for each boundary condition and bears little resemblance to the self-similar flow. 相似文献
964.
The paper describes an experimental investigation of the diamond-like carbon deposition process from methane precursor with an innovative high-density inductively coupled plasma source. It appears that a high plasma density allows a fast growth of diamond-like carbon coatings exhibiting a high hardness. In contrast to what is usually found in the literature, it seems that the ion species participate significantly to the growth of the carbon film when exposed to a high ion flux. 相似文献
965.
Ya. O. Shablovskii 《Inorganic Materials》2007,43(12):1345-1349
The pyroelectric properties of lithium sulfate have been studied theoretically on the hypothesis of a pseudosymmetry of the structure of its polar phase. Analytical expressions are proposed for the temperature dependences of its pyroelectric polarization and pyroelectric coefficient at low temperatures and near the polymorphic transformation. The pyroelectric and piezoelectric coefficients of a polar crystal are shown to be in direct proportion. 相似文献
966.
Termites live in a material world: exploration of their ability to differentiate between food sources. 下载免费PDF全文
Ra Inta Joseph C S Lai Eugene W Fu Theodore A Evans 《Journal of the Royal Society Interface》2007,4(15):735-744
Drywood termites are able to assess wood size using vibratory signals, although the exact mechanism behind this assessment ability is not known. Important vibratory characteristics such as the modal frequencies of a wooden block depend on its geometry and boundary conditions; however, they are also dependent on the material characteristics of the block, such as mass, density and internal damping. We report here on choice experiments that tested the ability of the drywood termite Cryptotermes secundus to assess wooden block size using a solid wooden block paired with a composite block, the latter made of either wood and aluminium or wood and rubber. Each composite block was constructed to match mass or low-frequency vibratory modes (i.e. fundamental frequency) of the solid wooden block. The termites always chose the blocks with more wood; they moved to the solid wooden blocks usually within a day and then tunnelled further into the solid wooden block by the end of the experiment. Termites offered composite blocks of wood and rubber matched for mass were the slowest to show a preference for the solid wooden block and this preference was the least definitive of any treatment, which indicated that mass and/or damping may play a role in food assessment. This result clearly shows that the termites were not fooled by composite blocks matched for mass or frequency, which implies that they probably employ more than a single simple measure in their food assessment strategy. This implies a degree of sophistication in their ability to assess their environment hitherto unknown. The potential importance of alternative features in the vibrational signals is discussed. 相似文献
967.
An optimum power metal-oxide-semiconductor field effect transistor (MOSFET) width technique is proposed for enhancing the efficiency characteristics of switching DC-DC converters. By implementing a one-cycle buck DC-DC converter, it is demonstrated that the dynamic power MOSFET width controlling technique has a much improved power reduction whether the load current is light or heavy. The maximum efficiency of the buck converter is ~92% with a 3% efficiency improvement for the heavy load condition. The efficiency is further improved by ~16% for the light load condition as a result of the power reduction from the large power MOSFET transistors. Also proposed is a new error-correction loop circuit to enable a better load regulation than that of previous designs. Compared with the adaptive gate driver voltage technique, the optimum power MOSFET width can achieve a significant improvement in power saving. It is also superior to the low-voltage-swing MOSFET gate drive technique for switching DC-DC converters 相似文献
968.
Split ring-rod combination metamaterial is used as a substrate material in a microstrip guided wave structure to determine what the effect is of a material with potentially excessive dispersion or loss or both. A Green's function method readily incorporates the metamaterial permittivity and permeability tensor characteristics. Ab initio calculations are performed to obtain the dispersion diagrams of several complex propagation constant modes of the structure. Analytical analysis is done for the design and interpretation of the results, which demonstrate remarkable potential for realistic use in high frequency electronics, while showing modes with extremely low loss bands. 相似文献
969.
The performance of netted radar is evaluated in terms of a netted version of the radar equation and the ambiguity function. A software simulation has been developed to allow assessment of netted radar sensitivity and ambiguity properties in both two-and three-dimensional space. Simulation results demonstrate that coherent netted radar sensitivity depends not only on the radar parameters, but also on the numbers, locations and specification of transmit and receive nodes. The netted radar ambiguity function is affected by a combination of many factors. In addition to those that determine the form of the monostatic ambiguity function, the locations of transmit and receive nodes as well as the position and velocity of the target all are shown to play an important role in evaluating system performance. As a result of these dependencies, it is shown that there is severe degradation of range and Doppler resolution when a target crosses sufficiently close to a transmit-receive baseline. 相似文献
970.
D. Boucher W. Geiselmann F. Ulmer 《Applicable Algebra in Engineering, Communication and Computing》2007,18(4):379-389
We generalize the notion of cyclic codes by using generator polynomials in (non commutative) skew polynomial rings. Since
skew polynomial rings are left and right euclidean, the obtained codes share most properties of cyclic codes. Since there
are much more skew-cyclic codes, this new class of codes allows to systematically search for codes with good properties. We
give many examples of codes which improve the previously best known linear codes. 相似文献