全文获取类型
收费全文 | 552211篇 |
免费 | 29926篇 |
国内免费 | 16251篇 |
专业分类
电工技术 | 24856篇 |
技术理论 | 45篇 |
综合类 | 25485篇 |
化学工业 | 86030篇 |
金属工艺 | 28073篇 |
机械仪表 | 27817篇 |
建筑科学 | 31582篇 |
矿业工程 | 12419篇 |
能源动力 | 14253篇 |
轻工业 | 38266篇 |
水利工程 | 8414篇 |
石油天然气 | 26229篇 |
武器工业 | 2801篇 |
无线电 | 65513篇 |
一般工业技术 | 75961篇 |
冶金工业 | 63562篇 |
原子能技术 | 7147篇 |
自动化技术 | 59935篇 |
出版年
2024年 | 1491篇 |
2023年 | 5834篇 |
2022年 | 10560篇 |
2021年 | 15048篇 |
2020年 | 11320篇 |
2019年 | 9773篇 |
2018年 | 11900篇 |
2017年 | 13097篇 |
2016年 | 12236篇 |
2015年 | 15306篇 |
2014年 | 19973篇 |
2013年 | 28678篇 |
2012年 | 27025篇 |
2011年 | 31118篇 |
2010年 | 27037篇 |
2009年 | 26700篇 |
2008年 | 26609篇 |
2007年 | 25730篇 |
2006年 | 25537篇 |
2005年 | 22828篇 |
2004年 | 17116篇 |
2003年 | 15694篇 |
2002年 | 14729篇 |
2001年 | 14039篇 |
2000年 | 13589篇 |
1999年 | 14746篇 |
1998年 | 22631篇 |
1997年 | 16345篇 |
1996年 | 13657篇 |
1995年 | 10515篇 |
1994年 | 8856篇 |
1993年 | 7608篇 |
1992年 | 5436篇 |
1991年 | 4815篇 |
1990年 | 4428篇 |
1989年 | 4016篇 |
1988年 | 3592篇 |
1987年 | 2754篇 |
1986年 | 2705篇 |
1985年 | 2854篇 |
1984年 | 2512篇 |
1983年 | 2224篇 |
1982年 | 2029篇 |
1981年 | 2117篇 |
1980年 | 1880篇 |
1979年 | 1701篇 |
1978年 | 1673篇 |
1977年 | 2043篇 |
1976年 | 2729篇 |
1975年 | 1428篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
The toughness of 31Mn2SiRE wear-resistance cast steel were increased by means of RE compound modification and high temperature austenitizing. The results show that the microstructures can be refined, needle and network ferrite are eliminated, the dislocation density and the quantity of dislocated martensite are increased remarkably, and the shape and distribution of inclusions are improved by the addition of RE. Therefore, the mechanical properties of the modified steel can be greatly increased, especially the toughness (αK) by 44%, yield strength (σs) by 10%, and elongation (δ5) by 42%. 相似文献
12.
The In-site of Ba2In2O5 with Brownmillerite structure was partially substituted for Ce4+ ions in order to examine the doping effect on the order-disorder transition. Ba2In2 – x
Ce
x
O5 + x/2 (x = 0.1, 0.2, 0.3, 0.5, 1.0, and 1.5) were prepared by solid state reaction. X-Ray diffraction analyses of these powder samples demonstrated that Ba2In2 – x
Ce
x
O5 + x/2 (x = 0.1 and 0.2) possesses Brownmillerite structure. With increasing content of Ce4+ ion the crystal system of Ba2In2 – x
Ce
x
O5 + x/2 (x = 0.3, 0.5, and 1.0) changed to cubic perovskite structure above the order-disorder transition temperature of Ba2In2O5. Arrhenius plots of the electrical conductivities of Ba2In2 – x
Ce
x
O5 + x/2 (x = 0.2, 0.3, and 1.0) exhibited no discontinuity. These compounds had high transference numbers of oxide ion above 973 k. 相似文献
13.
Rheological properties of MR fluids under large step strain shear are presented in this paper. The experiments were carried out using a rheometer with parallel-plate geometry. Under the large step strain shear, MR fluids behave as nonlinear viscoelastic properties, where the stress relaxation modulus, G(t, γ), shows a decreasing trend with step strain. The experimental results indicate that G(t, γ) obeys time-strain separability. Thus, a mathematical form based on finite exponential serials is proposed to predict MR behavior. In this model, G(t, γ) is represented as the product of a linear stress relaxation, G(t), and the damping function, h(γ), i.e. G(t, γ)=G(t) h(γ). G(t) is simply represented as a three-parameter exponential serial and h(γ) has a sigmoidal form with two parameters. The parameters are identified by adopting an efficient optimization method proposed by Stango et al. The comparison between the experimental results and the model-predicted values indicates that this mathematical model can accurately predict MR behavior. 相似文献
14.
Behavior of Magnesium‐Alloys for Automotive Applications under Mechanical and Environmental Loading: Influence of Passivating Films and Mechanisms of Local Breakdown To assure an efficient design of components under cyclic loading, all available data concerning fatigue have to be observed. Therefore the influences of manufacturing on the material condition, the mechanical loads and environmental effects have to be analysed. Magnesium‐alloys are of special interest for lightweight applications because of their excellent strength‐density ratio. The corrosion resistance of magnesium‐alloys depends on the same factors that are critical to other metals. The alloys have a good stability to atmospheric exposure and a good resistance to attack by alkali, chromic and hydrofluoric acids. However, because of the electrochemical activity of magnesium, the relative importance of some factors is greatly amplified. The nature and composition of passive films formed on magnesium‐alloys depend on the prevailing conditions, viz. alloy‐composition, passivation potential, pH, electrolyte composition and temperature. Passive films may be damaged by local breakdown. Because of this, magnesium‐alloys suffer a degradation of their properties when exposed to an aqueous environment. The main topic of the present investigations is the verification of mechanisms of the local breakdown of the protecting film. At least two mechanisms are possible for this localization: mechanical breakdown by slip steps and electrochemical breakdown (for e.g. by the effects of chloride ions). Corrosion and passivation of different high purity alloys have been studied in different solutions (neutral, alkaline with specific anions and cations) using electrochemical techniques. The diecasted alloys were tested as produced and machined. The results clarified that depending on alloy/material and surface condition/corrosion environment different mechanisms for electrochemical breakdown of the protecting films are possible. Hence fatigue life under environmental loading is influenced by surface and testing conditions. 相似文献
15.
城域网的建设是今后光纤网络建设的一个热点,根据城域网的特点和系统技术要求选择光纤光缆是网络设计的重要组成部分。本根据城域网的特点和需求,对市场上现有的光纤光缆做了详细的分析和探讨,提出了城域网规划中光纤光缆选择的原则和具体的建议。 相似文献
16.
大牛地气田为低渗-致密气田,因存在启动压力梯度等因素,修正等时试井曲线按常规方法处理出现异常,无法准确获取气井的产能方程.采用LIT法和异常曲线校正法对气田异常试井资料进行了重新处理,最终获得气井产能方程,为同类气藏产能试井可能出现的异常资料处理解释探索了新的途径. 相似文献
17.
VE432可控震源控制系统常见故障分析 总被引:1,自引:0,他引:1
VE432系统为新一代可控震源控制系统,文中简要地介绍了该系统的功能,根据实际生产中出现的常见故障进行分析并给出解决方法. 相似文献
18.
19.
Korolev I. A. Alekseenko N. N. Porodnov B. T. Sapunov V. A. Savel'ev D. V. 《Measurement Techniques》2003,46(9):865-871
The design of a sylphon bellows sensor and the basic circuits of an LC-generator and of a microprocessor unit are presented. An analytical pressure–frequency conversion function and a special method of adjusting the sensor ensure an error of less than 0.05%. The dynamic range is up to 105. The instruments developed cover the ranges 103, 104, and 105 Pa. 相似文献
20.
Honour Index (HoI), a method to evaluate research performance within different research fields, was derived from the impact
factor (IF). It can be used to rate and compare different categories of journals. HoI was used in this study to determine
the scientific productivity of stem cell research in the Asian Four Dragons (Hong Kong, Singapore, South Korea and Taiwan)
from 1981 to 2001. The methodology applied in this study represents a synthesis of universal indicator studies and bibliometric
analyses of subfields at the micro-level. We discuss several comparisons, and conclude the developmental trend in stem cell
research for two decades.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献