全文获取类型
收费全文 | 83885篇 |
免费 | 988篇 |
国内免费 | 409篇 |
专业分类
电工技术 | 773篇 |
综合类 | 2316篇 |
化学工业 | 11421篇 |
金属工艺 | 4782篇 |
机械仪表 | 3020篇 |
建筑科学 | 2191篇 |
矿业工程 | 567篇 |
能源动力 | 1109篇 |
轻工业 | 3585篇 |
水利工程 | 1272篇 |
石油天然气 | 345篇 |
无线电 | 9253篇 |
一般工业技术 | 16303篇 |
冶金工业 | 2659篇 |
原子能技术 | 254篇 |
自动化技术 | 25432篇 |
出版年
2019年 | 8篇 |
2018年 | 14453篇 |
2017年 | 13380篇 |
2016年 | 9957篇 |
2015年 | 606篇 |
2014年 | 228篇 |
2013年 | 219篇 |
2012年 | 3146篇 |
2011年 | 9405篇 |
2010年 | 8286篇 |
2009年 | 5544篇 |
2008年 | 6778篇 |
2007年 | 7793篇 |
2006年 | 125篇 |
2005年 | 1221篇 |
2004年 | 1139篇 |
2003年 | 1190篇 |
2002年 | 545篇 |
2001年 | 96篇 |
2000年 | 183篇 |
1999年 | 64篇 |
1998年 | 66篇 |
1997年 | 33篇 |
1996年 | 51篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 11篇 |
1992年 | 14篇 |
1991年 | 30篇 |
1988年 | 15篇 |
1981年 | 6篇 |
1969年 | 25篇 |
1968年 | 44篇 |
1967年 | 34篇 |
1966年 | 42篇 |
1965年 | 44篇 |
1964年 | 11篇 |
1963年 | 28篇 |
1962年 | 22篇 |
1961年 | 18篇 |
1960年 | 30篇 |
1959年 | 35篇 |
1958年 | 37篇 |
1957年 | 36篇 |
1956年 | 34篇 |
1955年 | 63篇 |
1954年 | 68篇 |
1952年 | 6篇 |
1950年 | 6篇 |
1949年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
141.
A new technique for the preparation of CuI nanoparticles from CuSO4 and KI ethanol solutions has been developed. Preparation conditions were optimized through a series of experiments. Under
these conditions, the yield of CuI reached 95.39%. The product was characterized and the reaction kinetics was studied. The
results show that the product takes a roughly spherical shape with an average particle size of less then 50 nm. The activation
energy of the formation of CuI is found to be E
a = 0.58 × 102 kJ/mol, and the preexponential factor in the Arrhenius equation is k
0 = 7.43 × 1016 mol/(l s).
The text was submitted by the authors in English. 相似文献
142.
143.
Solidification time and heat treatment are known to have a large effect on the microstructure of cast aluminum alloys. This
study was conducted to quantify how the fatigue properties of a 319-type aluminum alloy are affected by solidification time
and heat treatment. Both porosity-containing (non-hot isostatically pressed (HIP)) and porosity-free (HIP) samples in the
T6 (“peak aged”) or T7 (“overaged”) heattreated conditions were tested. As the solidification time increased, the average
initiating pore diameter increased and stress-controlled fatigue life decreased. Heat treatment was observed to have a large
effect on fatigue properties of the HIP samples. However, in the non-HIP fatigue samples, heat treatment did not significantly
change the fatigue life or fatigue strength of the cast 319-type alloy. The absence of an influence of heat treatment on fatigue
response is attributed to the predominance of the microporosity in fatigue crack initiation in cast aluminum. 相似文献
144.
Nicolas Clauvelin Basile Audoly Sébastien Neukirch 《Scientific Modeling and Simulation》2007,14(1):95-101
The DNA molecule is modeled as an elastic rod with bending and twisting rigidities, subjected to external tension and twist applied at one end, the other end being clamped. We study the plectonemic equilibrium of such a rod, taking into account the impenetrability constraint. Numerical solutions of this boundary value problem have previously shown that purely elastic models can reproduce the supercoiling response of the DNA molecule. Using a variational approach, we derive analytical formulae for the elastic response of the filament, and extend former numerical results. 相似文献
145.
146.
Mariana Ušáková Jozef Lukáč Rastislav Dosoudil Vladimír Jančárik Anna Grusková Elemír Ušák Jozef Sláma Jan Šubrt 《Journal of Materials Science: Materials in Electronics》2007,18(12):1183-1189
The paper is devoted to the preparation of NiZn ferrite with small substitutions of copper by means of ceramic technology.
The influence of small Cu substitution on the microstructural and magnetic properties of NiZn ferrites have been analysed
by means of various experimental methods and interpreted from the point of view of preparation technology optimisation and
possible applications of such materials. A strong correlation between the substituent content and resulting properties has
been observed, thus allowing preparation of material with the properties tailored for any particular application. 相似文献
147.
148.
We show new upper bounds for problems in the W-hierarchy of fixed-parameter complexity. A crucial ingredient of our proofs
is an extension to the W-RAM model, which permits more-powerful operations but remains equivalent to the original. We use
the extended model to give new upper bounds for Subsetsum, Maximum Irredundant Set, and various problems concerning intersection
of finite-state machines. 相似文献
149.
Peter Bro Miltersen 《Theory of Computing Systems》2007,41(1):119-125
We prove that the one-dimensional sandpile prediction problem is in LOGDCFL, a subset of AC1. The previously best known upper bound on the ACi-scale was AC2. Furthermore, we prove that the one-dimensional sandpile prediction problem is hard for TC0 and thus not in AC1-ε for any constant ε > 0. 相似文献
150.
Daniel Meister 《Theory of Computing Systems》2007,41(2):257-289
A finite recurrent system over sets of natural numbers of dimension n is a pair composed of n n-ary functions over sets of
natural numbers and an n-tuple of singleton sets of natural numbers. Every function is applied to the entries of the tuple
and computes a set of natural numbers, that may also be empty. The results are composed into another tuple, and the process
is started anew. Thus, a finite recurrent system defines an infinite sequence of n-tuples containing sets of natural numbers.
The last entry of a generated n-tuple is called the output of a step, and the union of the output sets of all steps is the
set defined by the finite recurrent system. Membership problems ask whether a given number is in a specified output set or
in some output set. We study membership problems for special finite recurrent systems, whose functions are built from the
set operations union, intersection and complementation and the arithmetical operations addition and multiplication. Sum and
product of two sets of natural numbers are defined elementwise. We restrict the set of operations from which functions are
built and determine the impact on the complexity of the membership problems. We focus on PSPACE-decidable membership problems
and show completeness results for the complexity classes NL, NP and PSPACE. 相似文献