全文获取类型
收费全文 | 389518篇 |
免费 | 4584篇 |
国内免费 | 1231篇 |
专业分类
电工技术 | 7135篇 |
综合类 | 2657篇 |
化学工业 | 53462篇 |
金属工艺 | 19577篇 |
机械仪表 | 16197篇 |
建筑科学 | 9033篇 |
矿业工程 | 2268篇 |
能源动力 | 7937篇 |
轻工业 | 26490篇 |
水利工程 | 4421篇 |
石油天然气 | 5476篇 |
武器工业 | 68篇 |
无线电 | 47973篇 |
一般工业技术 | 79161篇 |
冶金工业 | 53130篇 |
原子能技术 | 6525篇 |
自动化技术 | 53823篇 |
出版年
2021年 | 2034篇 |
2019年 | 2040篇 |
2018年 | 22506篇 |
2017年 | 22158篇 |
2016年 | 15939篇 |
2015年 | 3265篇 |
2014年 | 4314篇 |
2013年 | 11493篇 |
2012年 | 11186篇 |
2011年 | 23553篇 |
2010年 | 20417篇 |
2009年 | 18310篇 |
2008年 | 19651篇 |
2007年 | 22086篇 |
2006年 | 7959篇 |
2005年 | 11263篇 |
2004年 | 9441篇 |
2003年 | 9033篇 |
2002年 | 7373篇 |
2001年 | 6604篇 |
2000年 | 6421篇 |
1999年 | 6319篇 |
1998年 | 14290篇 |
1997年 | 10294篇 |
1996年 | 8158篇 |
1995年 | 6426篇 |
1994年 | 5740篇 |
1993年 | 5613篇 |
1992年 | 4574篇 |
1991年 | 4253篇 |
1990年 | 4048篇 |
1989年 | 3780篇 |
1988年 | 3631篇 |
1987年 | 3171篇 |
1986年 | 3065篇 |
1985年 | 3611篇 |
1984年 | 3383篇 |
1983年 | 3030篇 |
1982年 | 2852篇 |
1981年 | 2951篇 |
1980年 | 2760篇 |
1979年 | 2649篇 |
1978年 | 2499篇 |
1977年 | 2945篇 |
1976年 | 3554篇 |
1975年 | 2316篇 |
1974年 | 2313篇 |
1973年 | 2318篇 |
1972年 | 1849篇 |
1971年 | 1745篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Simulation-based algorithms for maximizing the average reward of a parameterized Markov chain often rely upon the existence of a state which is recurrent for all choices of parameter values. The question of which recurrent state should serve to mark the end of a regenerative cycle is a very important practical consideration in applications. Even when all of the states of the process are recurrent, some states tend to be visited more often than others, and lengthy renewal cycles tend to result in high variance estimates of the gradient. To address this difficulty, we analyze a mechanism for adjusting this special state dynamically (i/sup */-adaptation) as applied to the "batch" simulation-based optimization algorithm of a previous paper. We show that the desirable convergence properties of the original "batch" algorithm are retained with i/sup */-adaptation, namely the almost sure convergence of the parameter vector to a critical point. 相似文献
993.
The storage time before salvaging of decommissioned nuclear-powered submarines and the floating storage time of reactor units from salvaged submarines can total decades. During this time, radioactive contamination of seawater occurs as a result of the corrosion of the pressure hull in the region of the reactor system and rupturing of the pressure hull - as a result of corrosion of the components of the reactor system. A computational analysis shows that under standard storage conditions the specific activity of water is 105–106 times lower than the intervention level. In the case when the hull and the reactor system are destroyed even with remaining fuel, the specific activity of the sea water near the point of sinking also does not exceed the intervention level because of the slow emission of fission products and transuranium nuclides.Translated from Atomnaya Énergiya, Vol. 97, No. 3, pp. 210–218, September, 2004. 相似文献
994.
V. D. Loktionov 《Atomic Energy》2004,96(3):184-190
Two- and three-parameter quadratic fitting curves, characterizing the basic thermal and deformational parameters of the state of the structural components of a RBMK technological channel heated as a result of an accident, are constructed on the basis of numerical simulation. The parameters characterizing the accidental heating are assumed to be the heat flux density on the inner surface of the channel pipe, the pressure in the channel, and the thermal resistance on contact surfaces. The output functions are the temperature of the channel pipe at the moment it sets on the graphite, the rate of heating, the average rate of radial expansion of the pipe before and after it sets on the graphite, and the temperature of the surface of the opening in the block at the moment the pipe sets on the graphite. The fitting relations constructed can be used to obtain real-time estimates of the basic thermal and deformational characteristics of the heating process due to the accident and to develop new integral criteria for the integrity of a damaged RBMK channel. 相似文献
995.
K.D. Do Author Vitae Z.P. Jiang Author Vitae Author Vitae H. Nijmeijer Author Vitae 《Automatica》2004,40(1):117-124
This paper presents a method to design an output-feedback controller that simultaneously solves global asymptotic stabilization and tracking of an underactuated omni-directional intelligent navigator—a spherical underwater vehicle moving in a horizontal plane (i.e. at a constant depth). The vehicle does not have a sway actuator and has only position and orientation measurements available. The control development is based on Lyapunov's direct method, backstepping technique and use of interconnected structure of the vehicle dynamics. Numerical simulations demonstrate the results. 相似文献
996.
V. Kevorkijan 《Metallurgical and Materials Transactions A》2004,35(2):707-715
Semi-industrial samples of Mg AZ80/SiC/50p composites (machined bars with o.d.=80 mm and length=150 mm, having a maximum 2 vol pct of retained porosity) were successfully
fabricated by pressureless, low-pressure (under overpressure of about 0.3 MPa) and moderate-pressure (under overpressure of
about 0.8 MPa) infiltration. Pressureless infiltration was completed in 24 hours under the flowing atmosphere of nitrogen,
whereas the moderate and low-pressure infiltrations were performed under a static atmosphere of pressurized nitrogen for 7
and 0.5 hours, respectively. The composite samples obtained by all applied infiltration paths posessed a homogeneous microstructure
and superior mechanical properties as compared to the nonreinforced matrix. Due to the improved productivity of composite
fabrication in comparison with the other two infiltration paths investigated in this work, the moderate-pressure infiltration
process was recognized as the most competitive. 相似文献
997.
A. Phillion H. W. Zurob C. R. Hutchinson H. Guo D. V. Malakhov J. Nakano G. R. Purdy 《Metallurgical and Materials Transactions A》2004,35(4):1237-1242
We have evaluated controlled decarburization as a method for probing the effect of alloying elements on ferrite growth from
austenite. The technique permits the exploration of longer-time ferrite layer growth; it minimizes the effects of interface
structure on ferrite growth; and it permits the isolation of the effects of temperature and alloying element concentration
on ferrite/austenite interface motion. The study of the decarburization of initially homogeneous Fe-C-Ni alloys was complemented
by experiments using specimens with a controlled nickel concentration gradient. Although the decarburization method yields
consistent results at longer times, it is found to be less appropriate for the study of initial ferrite growth. Nucleation
in the gas/solid interface region, coupled with uncertainties about the precise time of decarburization, leads to large relative
errors at the earliest times. For these reasons, the method is considered a valuable complement to studies based on precipitation
boundary conditions.
This article is based on a presentation given in the symposium “The Effects of Alloying Elements on the Gamma to Alpha Transformation
in Steels,” October 6, 2002, at the TMS Fall Meeting in Columbus, Ohio, under the auspices of the McMaster Centre for Steel
Research and the TMS-ASM Phase Transformations Committee. 相似文献
998.
Chul-Hwan Choi Sung-Koo Jo Seon-Hyo Kim Kwang-Ro Lee Jeong-Tae Kim 《Metallurgical and Materials Transactions B》2004,35(1):115-120
To address the role of CaF2 in the CaO-CaF2-SiO2(-MgO) slag system employed for the production of low-pressure rotor steels, the thermodynamic aspects of the slag were investigated
by equilibrating it with liquid iron at 1873 K in CaO or MgO crucibles. Presaturation of slag with an oxide block piece of
CaO or MgO in a Pt crucible and application of a carbon paste to the outside of an oxide crucible were designed to prevent
crucible failure during the slag-metal experiments. The liquidus isotherm and phase boundary of the preceding slag system
were investigated using the slag-metal equilibria. Also, the effect of CaF2 on the sulfide capacity and the activity coefficient of Fe
t
O were of particular interest in controlling the sulfur level and cleanliness of low-pressure rotor steels. 相似文献
999.
Mixtures of Cr and Mo elemental powders, with the nominal compositions Cr25Mo75, Cr50Mo50, and Cr75Mo25, are processed by high-energy ball milling at ambient temperature. Milling is observed to force the mixing of the immiscible
bcc elements Cr and Mo into solid solutions. The lattice parameter of these solid solutions, measured by X-ray diffraction
(XRD), displays the expected positive deviation from Vegard’s law. These deviations are compared to the ones predicted by
Eshelby’s inclusion model for dilute alloys. The conventional Williamson-Hall approach is shown to fail to determine the grain
size in as-milled samples, probably due to the high density of dislocations. Annealing at 700 °C for 10 hours under argon
leads to a large reduction in structural defect density, without inducing any significant decomposition. The mixing measured
in Cr-Mo is discussed in the broader context of the mechanical mixing forced by ball milling in moderately immiscible systems. 相似文献
1000.
Yaojun Lin Yizhang Zhou Enrique J. Lavernia Kevin M. McHugh 《Metallurgical and Materials Transactions A》2004,35(11):3633-3635
From the geometrical standpoint, this article presents a qualitative theoretical analysis and prediction of the transient
to steady-state transition during the spray-rolling process, a novel manufacturing technique for aluminum strips. The analytical
results indicate that, when the deposited materials at the specific points on one roll surface overlap their counterparts
on the other roll surface, spray rolling transits from the transient state to the steady state. The specific points are the
limiting deposition positions of the atomized droplets on the roll surface initially. 相似文献