全文获取类型
收费全文 | 523518篇 |
免费 | 4998篇 |
国内免费 | 1075篇 |
专业分类
电工技术 | 8981篇 |
综合类 | 319篇 |
化学工业 | 81270篇 |
金属工艺 | 28059篇 |
机械仪表 | 18713篇 |
建筑科学 | 11167篇 |
矿业工程 | 5104篇 |
能源动力 | 10446篇 |
轻工业 | 34556篇 |
水利工程 | 7050篇 |
石油天然气 | 16785篇 |
武器工业 | 36篇 |
无线电 | 51335篇 |
一般工业技术 | 116941篇 |
冶金工业 | 78737篇 |
原子能技术 | 15797篇 |
自动化技术 | 44295篇 |
出版年
2021年 | 4350篇 |
2019年 | 4146篇 |
2018年 | 12839篇 |
2017年 | 13148篇 |
2016年 | 11679篇 |
2015年 | 5216篇 |
2014年 | 8524篇 |
2013年 | 20181篇 |
2012年 | 14242篇 |
2011年 | 21058篇 |
2010年 | 17943篇 |
2009年 | 18571篇 |
2008年 | 19303篇 |
2007年 | 20090篇 |
2006年 | 12812篇 |
2005年 | 13463篇 |
2004年 | 12356篇 |
2003年 | 11888篇 |
2002年 | 11145篇 |
2001年 | 10442篇 |
2000年 | 10193篇 |
1999年 | 9596篇 |
1998年 | 20553篇 |
1997年 | 15684篇 |
1996年 | 12064篇 |
1995年 | 9438篇 |
1994年 | 8644篇 |
1993年 | 8689篇 |
1992年 | 6950篇 |
1991年 | 7007篇 |
1990年 | 6940篇 |
1989年 | 6816篇 |
1988年 | 6623篇 |
1987年 | 6021篇 |
1986年 | 5937篇 |
1985年 | 6637篇 |
1984年 | 6399篇 |
1983年 | 5980篇 |
1982年 | 5644篇 |
1981年 | 5807篇 |
1980年 | 5685篇 |
1979年 | 5798篇 |
1978年 | 6044篇 |
1977年 | 6490篇 |
1976年 | 7927篇 |
1975年 | 5458篇 |
1974年 | 5389篇 |
1973年 | 5481篇 |
1972年 | 4832篇 |
1971年 | 4447篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Y. L. Hao R. Yang M. Niinomi D. Kuroda Y. L. Zhou K. Fukunaga A. Suzuki 《Metallurgical and Materials Transactions A》2003,34(4):1007-1012
Alloys for implant devices require improved strength but a reduced Young’s modulus, in order to become mechanically more compatible
with adjacent bone tissues. In this study, a new metastable β-type titanium alloy, Ti-29Nb-13Ta-4.6Zr (wt pct), was subjected to aging treatment to produce different microstructures,
and the resulting mechanical properties, including the Young’s modulus, were measured. The Young’s modulus of this alloy is
found to be sensitive to microstructures generated by various heat treatments. For microstructures varying from (α + β) to (α + β + ω) and (β + ω), the Young’s modulus increases with an accompanying increase in tensile strength and hardness, but decreases in ductility.
The (β + ω) microstructure has a low strength, high modulus, and poor ductility and cannot be used for biomedical applications. For
an (α + β) microstructure, the volume fraction of the phases is shown to be the main factor that determines the mechanical properties. 相似文献
962.
Yu. G. Volodin 《Thermal Engineering》2007,54(5):399-402
The results of experimental study of unsteady effects on local heat transfer coefficients caused by abruptly increasing of the gas flow temperature in the inlet section of a cylindrical channel are presented. 相似文献
963.
The processes of optical spin orientation and spin relaxation of electrons are treated theoretically for semiconductor quantum wells, in which the spin splitting of the energy spectrum is comparable with the characteristic energy of charge carriers. The density matrix of photoexcited electrons at the instant of optical excitation is obtained in explicit form. A system of kinetic equations describing the behavior of the spin density matrix at an arbitrary relation between the average energy of charge carriers and the spin splitting is derived. It is demonstrated that, upon photoexcitation, a noticeable degree of orientation can be attained only in the pulse mode of operation, when the photoexcitation pulse duration is comparable with the period of spin precession in the field of spin splitting. It is shown that the total spin of the ensemble of electrons exhibits oscillations damping with time; the shape and damping time of the oscillations are sensitive to the parameters of photoexcitation and the spin splitting. 相似文献
964.
Ya. A. Ilyushin V. E. Kunitsyn 《Journal of Communications Technology and Electronics》2008,53(5):545-549
The possibility for determining the structure of the surface layer of a cometary nucleus from the data of bistatic radar sensing is investigated. A rigorous solution formulated for this problem on the basis of vector equations of the electromagnetic field is presented. 相似文献
965.
Porous carbon nanofibers are synthesized by CVD method from acetylene with use of iron-containing catalysts. Activation of the nanofibers in melted potassium hydroxide results in increasing surface area from initial 300–400 m2 g−1 to 1700 m2 g−1. As follows from XRD data, activated nanofibers do not contain regular packages of graphene layers, but retain high electric conductivity. Deposition of copper improves electrochemical hydrogen storing characteristics of carbon nanofibers. Carbon nanomaterials obtained can be used as hydrogen storing materials in batteries instead of hydride forming metals. 相似文献
966.
Six synthesized magnesium aluminate spinel-based refractory compositions used in steel and cement applications, were prepared using a two stage sintering process at 1760 °C, starting with approximately 1:1 wt% ratio of pure magnesia and alumina with additions of zirconium silicate (0.5, 1.0 and 2.0 wt%) and chromite (2.0, 3.0 and 5.0 wt%). These compositions were investigated for effects on densification, chemical and mineralogical phases formed. 相似文献
967.
A membrane reactor containing different types of ZSM-5/porous SS membranes was used to perform the xylene isomerization reaction. The parent Na-ZSM-5 layer was synthesized by secondary growth on top of porous stainless steel tubes. The xylene isomerization reaction was carried out at different temperatures in the membrane reactor and in a fixed-bed reactor of identical geometry for comparison. Two different kinds of membranes were prepared by ion exchange: a Pt/H-ZSM-5 catalytic membrane and two Ba-ZSM-5 composites with different Ba2+ concentration. The p-xylene production using 100% exchanged Ba-membrane was about 28% higher than the fixed-bed reactor at 370 °C, when m-xylene was fed. 相似文献
968.
The photoelectric parameters of silicon solar cells degraded under the action of 60Co gamma-radiation can be partly restored using an ultrasonic treatment (UST). The growth of the maximum output power of solar cells after the UST is related to a redistribution of the radiation defects and an increase in the homogeneity of a semiconductor crystal structure. 相似文献
969.
The effects of a biquadratic exchange and an external magnetic field on the phase states of a twodimensional non-Heisenberg ferromagnet have been studied with allowance for magnetoelastic and magnetodipolar interactions. 相似文献
970.
Carlos Fernandes Agostinho C. Rosa 《Soft Computing - A Fusion of Foundations, Methodologies and Applications》2008,12(10):955-979
Mate selection plays a crucial role in both natural and artificial systems. While traditional Evolutionary Algorithms (EA)
usually engage in random mating strategies, that is, mating chance is independent of genotypic or phenotypic distance between
individuals, in natural systems non-random mating is common, which means that somehow this mechanism has been favored during
the evolutionary process. In non-random mating, the individuals mate according to their parenthood or likeness. Previous studies
indicate that negative assortative mating (AM)—also known as dissortative mating—, which is a specific type of non-random mating, may improve EAs performance by maintaining the genetic diversity of the
population at a higher level during the search process. In this paper we present the Variable Dissortative Mating Genetic Algorithm (VDMGA). The algorithm holds a mechanism that varies the GA’s mating restrictions during the run by means of simple rule
based on the number of chromosomes created in each generation and indirectly influenced by the genetic diversity of the population.
We compare VDMGA not only with traditional Genetic Algorithms (GA) but also with two preceding non-random mating EAs: the
CHC algorithm and the negative Assortative Mating Genetic Algorithm (nAMGA). We intend to study the effects of the different methods in the performance of GAs and verify the reliability of
the proposed algorithm when facing an heterogeneous set of landscapes. In addition, we include the positive Assortative Mating Genetic Algorithm (pAMGA) in the experiments in order test both negative and positive AM mechanisms, and try to understand if and when negative
AM (or DM) speeds up the search process or enables the GAs to escape local optima traps. For these purposes, an extensive
set of optimization test problems was chosen to cover a variety of search landscapes with different characteristics. Our results
confirm that negative AM is effective in leading EAs out of local optima traps, and show that the proposed VDMGA is at least
as efficient as nAMGA when applied to the range of our problems, being more efficient in very hard functions were traditional
GAs usually fail to escape local optima. Also, scalability tests have been made that show VDMGA ability to decrease optimal
population size, thus reducing the amount of evaluations needed to attain global optima. We like to stress that only two parameters
need to be hand-tuned in VDMGA, thus reducing the tuning effort present in traditional GAs and nAMGA. 相似文献