全文获取类型
收费全文 | 511924篇 |
免费 | 5016篇 |
国内免费 | 1118篇 |
专业分类
电工技术 | 9128篇 |
综合类 | 351篇 |
化学工业 | 75849篇 |
金属工艺 | 27263篇 |
机械仪表 | 18614篇 |
建筑科学 | 10663篇 |
矿业工程 | 4644篇 |
能源动力 | 10886篇 |
轻工业 | 30305篇 |
水利工程 | 6516篇 |
石油天然气 | 15256篇 |
武器工业 | 40篇 |
无线电 | 54973篇 |
一般工业技术 | 115517篇 |
冶金工业 | 80106篇 |
原子能技术 | 15039篇 |
自动化技术 | 42908篇 |
出版年
2021年 | 4514篇 |
2019年 | 4284篇 |
2018年 | 12832篇 |
2017年 | 13252篇 |
2016年 | 11704篇 |
2015年 | 5186篇 |
2014年 | 8644篇 |
2013年 | 20127篇 |
2012年 | 14106篇 |
2011年 | 21107篇 |
2010年 | 17921篇 |
2009年 | 18595篇 |
2008年 | 19336篇 |
2007年 | 20276篇 |
2006年 | 12882篇 |
2005年 | 13170篇 |
2004年 | 12218篇 |
2003年 | 12210篇 |
2002年 | 10968篇 |
2001年 | 10837篇 |
2000年 | 10360篇 |
1999年 | 10099篇 |
1998年 | 22910篇 |
1997年 | 16592篇 |
1996年 | 12738篇 |
1995年 | 9806篇 |
1994年 | 8926篇 |
1993年 | 9024篇 |
1992年 | 6949篇 |
1991年 | 6907篇 |
1990年 | 6854篇 |
1989年 | 6534篇 |
1988年 | 6363篇 |
1987年 | 5841篇 |
1986年 | 5702篇 |
1985年 | 6106篇 |
1984年 | 5762篇 |
1983年 | 5543篇 |
1982年 | 5213篇 |
1981年 | 5193篇 |
1980年 | 5141篇 |
1979年 | 5119篇 |
1978年 | 5139篇 |
1977年 | 5637篇 |
1976年 | 7030篇 |
1975年 | 4591篇 |
1974年 | 4637篇 |
1973年 | 4640篇 |
1972年 | 4080篇 |
1971年 | 3625篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
972.
The mean convergence of various versions of a genetic algorithm are considered. A number of convergence statements are formulated
and relevant estimates are obtained. A hypothesis concerning the form of these estimates under variation of the structure
of a genetic algorithm is put forward.
Roman Riviyanovich Sharapov. Born 1981. Graduated from the Faculty of Mechanics and Mathematics, Ural State University, in 2003. Presently, a postgraduate
at the Department of Mathematical Economics at the same university. Scientific interests: genetic algorithms, neural networks,
and financial mathematics.
Aleksandr Vyacheslavovich Lapshin. Born 1980. Graduated from the Faculty of Mechanics and Mathematics, Ural State University, in 2003. Presently, a postgraduate
at the Department of Mathematical Economics at the same university. Scientific interests: financial mathematics, genetic algorithms,
and neural networks. 相似文献
973.
过去几年里,一些非洲国家开始起步创建或加强核技术和平利用管理的法律、行政、和技术机制,并在于国际标准加强辐射防护措施的有效性和持续性。人们越来越多的认识到适当的国家基础结构是实施安全标准、实现和维护理想的防护和安全水平的先决条件,特别是在公共健康和工业部门。其他一睦具有全球或地区意义的问题,如辐射源的管理,包括有害废物的处理和放射应急的响应力等都有助于更好地认识到缺乏足够的国家辐射防护管理机构带来危害,而且往往这种认识也已经不适应辐射源管理审管框架建立过程的进展。本文总结了所有非洲国家辐射防护基础结构的现状。基于当前存在的弱点和面临的挑战,综述了IAEA对于援助需求的响应,讨论了IAEA在这一领域的计划。 相似文献
974.
Given a graph G=(V,E) and a positive integer D , we consider the problem of finding a minimum number of new edges E' such that the augmented graph G'=(V,E\cup E') is biconnected and has diameter no greater than D. In this note we show that this problem is NP-hard for all fixed D , by employing a reduction from the DOMINATING SET problem. We prove that the problem remains NP-hard even for forests and
trees, but in this case we present approximation algorithms with worst-case bounds 3 (for even D ) and 6 (for odd D ). A closely related problem of finding a minimum number of edges such that the augmented graph has diameter no greater than
D has been shown to be NP-hard by Schoone et al. [21] when D=3 , and by Li et al. [17] when D=2.
Received April 19, 1999; revised June 5, 2001. 相似文献
975.
Volatile Compounds of Raw Beef from 5 Local Spanish Cattle Breeds Stored Under Modified Atmosphere 总被引:7,自引:0,他引:7
Volatile compounds of raw beef from Asturiana de los Valles, Morucha, Parda Alpina, Pirenaica, and Retinta cattle breeds were studied. Steaks were packaged under 60% O2, 30% CO2, and 10% N2, and 53 volatile compounds were tentatively identified by purge‐and‐trap extraction and gas chromatography with mass spectrometry after 0, 5, 10, and 15 d of chill storage. The degradation of beef quality with increasing storage time was evidenced by the increase in 2,3,3‐trimethylpentane, 2,2,5‐trimethylhexane, 3‐octene, 3‐methyl‐2‐heptene, 2‐octene, and 2‐propanone and by the decrease in dimethyl sulfide. Consequently, some of the volatile compounds, which are believed to be formed by thermal degradation of meat, might be formed during chill storage, and the rate of formation of some was dependent on the specific compound. 相似文献
976.
ABSTRACT: The thermal stability of Bacillus licheniformis α-amylase at low moisture content was studied, based on isothermal experiments performed in a temperature range 113 to 125 °C. The thermal inactivation was monitored by measuring the decrease in thermal denaturation enthalpy and/or the decrease in enzymic activity on pnitrophenyl-α-D-maltoheptaoside, or on starch as a substrate. Based on enthalpy readings, an enzymic system with a z-value of 10.4 °C was observed when using a relative humidity of 81% at 4 °C. A theoretical study showed that this system could be used as a Time Temperature Integrator (TTI) to monitor the safety of sterilization processes of numerous food products. 相似文献
977.
Androstenedione labeled with tritium at positions 1 and 2 was prepared. The molar radioactivity of the sample prepared using homogeneous catalysts was as high as 1.4-1.5 PBq mol-
1. [1,2-3H]Androst-4-ene-3,17-dione can be converted into the [1-3H]steroid with the molar radioactivity of 0.9 PBq mol-
1. 相似文献
978.
979.
Field R. D. Hartwig K. T. Necker C. T. Bingert J. F. Agnew S. R. 《Metallurgical and Materials Transactions A》2002,33(13):965-972
The equal-channel angular extrusion (ECAE) technique has been applied to a powder metallurgy (P/M) source Be alloy. Extrusions
have been successfully completed on Ni-canned billets of Be at approximately 425°C. No cracking was observed in the billets,
and significant grain refinement was achieved. In this article, microstructural features and dislocation structures are discussed
for a singlepass extrusion, including evidence of <c> and <c+a> dislocations. Significant crystallographic texture developed during ECAE, which is discussed in terms of this unique deformation
processing technique and the underlying physical processes which sustain the deformation.
S.R. AGNEW, formerly with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115
This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals
and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following
ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic
Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. 相似文献
980.
Validity on the well-known Onsager reciprocal relations L 12=L 21 is verified in the kinetic theory of gases taking into account mass and heat transfer near the surface. Using an analytical solution to the ellipsoidal statistical equation (providing for the true Prandtl number), it is demonstrated that the Onsager relations are obeyed, at least to within exponential corrections of the type exp(−1/Kn) in the Knudsen number. 相似文献