全文获取类型
收费全文 | 866557篇 |
免费 | 10586篇 |
国内免费 | 2229篇 |
专业分类
电工技术 | 15360篇 |
综合类 | 744篇 |
化学工业 | 132966篇 |
金属工艺 | 36688篇 |
机械仪表 | 26975篇 |
建筑科学 | 20452篇 |
矿业工程 | 5898篇 |
能源动力 | 21669篇 |
轻工业 | 70154篇 |
水利工程 | 9847篇 |
石油天然气 | 20365篇 |
武器工业 | 57篇 |
无线电 | 92115篇 |
一般工业技术 | 175889篇 |
冶金工业 | 158021篇 |
原子能技术 | 20651篇 |
自动化技术 | 71521篇 |
出版年
2021年 | 8009篇 |
2019年 | 7500篇 |
2018年 | 13211篇 |
2017年 | 13173篇 |
2016年 | 13882篇 |
2015年 | 9076篇 |
2014年 | 15008篇 |
2013年 | 38740篇 |
2012年 | 23222篇 |
2011年 | 31313篇 |
2010年 | 25011篇 |
2009年 | 28060篇 |
2008年 | 28700篇 |
2007年 | 27980篇 |
2006年 | 24138篇 |
2005年 | 22102篇 |
2004年 | 21264篇 |
2003年 | 20831篇 |
2002年 | 20074篇 |
2001年 | 19937篇 |
2000年 | 18786篇 |
1999年 | 19151篇 |
1998年 | 46890篇 |
1997年 | 33877篇 |
1996年 | 26018篇 |
1995年 | 19709篇 |
1994年 | 17400篇 |
1993年 | 17247篇 |
1992年 | 12841篇 |
1991年 | 12394篇 |
1990年 | 12218篇 |
1989年 | 11909篇 |
1988年 | 11275篇 |
1987年 | 10166篇 |
1986年 | 9976篇 |
1985年 | 11169篇 |
1984年 | 10567篇 |
1983年 | 9639篇 |
1982年 | 9020篇 |
1981年 | 9294篇 |
1980年 | 8904篇 |
1979年 | 8871篇 |
1978年 | 8871篇 |
1977年 | 10155篇 |
1976年 | 13043篇 |
1975年 | 7872篇 |
1974年 | 7579篇 |
1973年 | 7644篇 |
1972年 | 6578篇 |
1971年 | 6005篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
An inventory analysis of rural pollution loads in China. 总被引:1,自引:0,他引:1
Rural pollution has attracted increasing attention over the past decade for its important consequences on surface and groundwater quality. This is particularly so in China due to the wide and effective control of industrial and municipal wastewater discharges over the past decade. Based on the discussion of rural activities in China, this paper develops a new method of inventory analysis through the definition of elementary unit (EU). This inventory analysis approach is then applied to estimate the total rural pollution loads of COD, TN and TP, as well as their spatial distributions in China. Further analysis is also conducted to investigate the correlations between rural pollution loads and agricultural production outputs, as well as population. It is found that China's agriculture is developing largely at the cost of environment quality, and a high population, as well as the increasing demand for agricultural products, is one of the dominant driving forces. The constructed EKC, which describes the relationship between rural pollution loads and agricultural income, also reveals that current agricultural development in China is mostly far from de-locked from the environmental problem. 相似文献
22.
It was found that vanadyl porphyrin complexes synthesized from petroleum metal porphyrin concentrates stimulated epoxidation during the olefin oxygenation process. The yields of obtained oxiranes turned out to be 38–75%, depending on the olefin structure. An epoxidation mechanism that suggests the formation of a protonated dioxygen adduct as an intermediate during oxygenation of olefins in the presence of vanadyl porphyrin complexes was proposed. An analogy is drawn between the epoxide formation reaction upon the catalytic oxygenation of olefins and the Prilezhaev reaction. 相似文献
23.
Presented here are details of the development of a novel membrane integrated circuit (IC) probe card structure based on microsystems technology. The device design allows probing of both solder bumps and pads. A self-limiting sensor was integrated to prolong device lifetime. Comparison with and discussion of the use of modelling is made. Possible enhancements to the probing structure are discussed to improve alignment and measurements. Also shown is data using our microsystems probe card to access a simple IC device. Our device has a contact resistance of less than 0.5 Ω for a force of 0.004 N. A method to implement our probing structure for commercial application and the potential developments which can be made to improve its ease of use are then discussed. 相似文献
24.
Good requirements practices are neither necessary nor sufficient 总被引:2,自引:2,他引:0
25.
Improved homogenization of Ni in sintered steels through the use of Cr-containing prealloyed powders
The homogenization of Ni in powder metal (PM) steel compacts is usually difficult even after high-temperature sintering at
1250°C. An earlier study by the authors demonstrated that this problem can be alleviated through the addition of 0.5 wt pct
Cr in the form of stainless steel powders. To further improve the microstructure and mechanical properties of Ni-containing
PM steels and to understand the mechanisms, an attempt was made in this study using the Fe-3Cr-0.5Mo prealloyed powder as
the base material. The results showed that the distribution of the Ni additives was significantly improved. As a result, the
tensile strength of the Fe-3Cr-0.5Mo-4Ni-0.5C compact sintered at 1250°C reached 1323 MPa. The elongation was higher than
1 pct. These sinter-hardened properties, which were attained using a slow furnace cooling rate, were comparable to those of
the sinter-hardened alloys reported in the literature using accelerated cooling and were equivalent to those of the best quenched-and-tempered
alloys registered in the Metal Powder Industries Federation (MPIF) standards. These improvements were attributed to the positive
effect of Cr addition on alloy homogenization due to the reduction of the repelling effect between Ni and C, as was demonstrated
through the thermodynamic analysis using the Thermo-Calc program. 相似文献
26.
Note on B-splines, wavelet scaling functions, and Gabor frames 总被引:3,自引:0,他引:3
Grochenig K. Janssen A.J.E.M. Kaiblinger N. Pfander G.E. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(12):3318-3320
Let g be a continuous, compactly supported function on such that the integer translates of g constitute a partition of unity. We show that the Gabor system (g,a,b), with window g and time-shift and frequency-shift parameters a,b>0 has no lower frame bound larger than 0 if b=2,3,... and a>0. In particular, (g,a,b) is not a Gabor frame if g is a continuous, compactly supported wavelet scaling function and if b=2,3,... and a>0. We give an example for our result for the case that g=B/sub 1/, the triangle function supported by [-1,1], by showing pictures of the canonical dual corresponding to (g,a,b) where ab=1/4 and b crosses the lines N=2,3,. 相似文献
27.
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
28.
The basic methods of verifying continuous automatic belt weighers are described. A comparative analysis of these methods on the basis of experimental studies is made and ways of implementing the results in industry are recommended. 相似文献
29.
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
30.
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