全文获取类型
收费全文 | 18229篇 |
免费 | 799篇 |
国内免费 | 21篇 |
专业分类
电工技术 | 215篇 |
综合类 | 18篇 |
化学工业 | 3802篇 |
金属工艺 | 325篇 |
机械仪表 | 405篇 |
建筑科学 | 1013篇 |
矿业工程 | 49篇 |
能源动力 | 504篇 |
轻工业 | 1698篇 |
水利工程 | 125篇 |
石油天然气 | 56篇 |
武器工业 | 1篇 |
无线电 | 1591篇 |
一般工业技术 | 3115篇 |
冶金工业 | 3089篇 |
原子能技术 | 189篇 |
自动化技术 | 2854篇 |
出版年
2023年 | 157篇 |
2022年 | 296篇 |
2021年 | 462篇 |
2020年 | 343篇 |
2019年 | 368篇 |
2018年 | 424篇 |
2017年 | 391篇 |
2016年 | 491篇 |
2015年 | 427篇 |
2014年 | 524篇 |
2013年 | 1077篇 |
2012年 | 859篇 |
2011年 | 1126篇 |
2010年 | 773篇 |
2009年 | 831篇 |
2008年 | 873篇 |
2007年 | 765篇 |
2006年 | 705篇 |
2005年 | 591篇 |
2004年 | 491篇 |
2003年 | 449篇 |
2002年 | 423篇 |
2001年 | 295篇 |
2000年 | 322篇 |
1999年 | 353篇 |
1998年 | 745篇 |
1997年 | 488篇 |
1996年 | 400篇 |
1995年 | 294篇 |
1994年 | 273篇 |
1993年 | 270篇 |
1992年 | 190篇 |
1991年 | 159篇 |
1990年 | 138篇 |
1989年 | 139篇 |
1988年 | 124篇 |
1987年 | 119篇 |
1986年 | 117篇 |
1985年 | 140篇 |
1984年 | 134篇 |
1983年 | 119篇 |
1982年 | 90篇 |
1981年 | 111篇 |
1980年 | 87篇 |
1979年 | 97篇 |
1978年 | 87篇 |
1977年 | 118篇 |
1976年 | 140篇 |
1975年 | 83篇 |
1974年 | 72篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
The deformation characteristics of artificially cemented calcareous soil subjected to undrained cyclic triaxial loading are investigated at different confining pressure and cyclic stress levels. The influence of cementation on the shear stiffness is investigated by comparing the behavior of cemented and uncemented soils with similar initial conditions. It is observed that the deviator stress and the deviatoric strain at yield reduced with increasing number of cycles for cemented sand due to progressive degradation of bond, which results in significant decrease in stiffness. On the other hand, a strain-hardening effect is observed in uncemented sand and this results in increasing yield stress and strain with progressive number of cycles. A linear relationship between degradation index and number of cycles is observed for cemented sand. This relationship has been synthesized in the form of an empirical equation by modifying a previously proposed equation for cohesive soils. This empirical equation was further used to evaluate the fatigue life of soils by adopting a failure criterion. 相似文献
72.
G. G. Martin 《Journal of Failure Analysis and Prevention》2004,4(1):37-39
Piston failures are not a common occurrence, but they do occur, and failure is usually associated with fatigue crack growth.
Most failures initiate at the gudgeon pin hole or in the skirt of the piston. Occasionally they fail elsewhere. In the example
covered in this paper, failure initiated in the crown and progressed down to the gudgeon pin before final failure occurred.
This paper outlines the cause and mode of failure and shows that small metallurgical discontinuities can contribute to these
failures under the right circumstances. 相似文献
73.
Alloying zinc coatings with elements such as magnesium and titanium increases the corrosion resistance in industrial tests. Based on the fundamental mechanism of cathodic delamination, a simple concept how alloying zinc with specific elements might improve its resistance to cathodic delamination is presented in this paper. As an example, results concerning the delamination of a simple polymer from the zinc‐magnesium intermetallic MgZn2 are presented. From electrode potential measurements obtained with the Scanning Kelvin Probe it can be concluded that cathodic delamination from this alloy is completely inhibited. This behaviour is attributed to the electronic properties of the magnesium‐rich oxidic passive layer, that inhibits the oxygen reduction reaction at the metal oxide/polymer interface. 相似文献
74.
Crystallization of Polymer-Derived Silicon Carbonitride at 1873 K under Nitrogen Overpressure 总被引:1,自引:0,他引:1
Martin Friess Joachim Bill Jerzy Golczewski re Zimmermann Fritz Aldinger Ralf Riedel Rishi Raj 《Journal of the American Ceramic Society》2002,85(10):2587-2589
The chemical stability of an amorphous silicon carbonitride ceramic, having the composition 0.57SiC·0.43Si3 N4 ·0.49C is studied as a function of nitrogen overpressure at 1873 K. The ceramic suffers a weight loss at p N2 < 3.5 bar (1 bar = 100 kPa), does not show a weight change from 3.5 to 11 bar, and gains weight above 11 bar. The structure of the ceramic changes with pressure: it is crystalline from 1 to 6 bar, amorphous at ∼10 bar, and is crystalline above ∼10 bar. The weight-loss transition, at 3.5 bar, is in excellent agreement with the prediction from thermodynamic analysis when the activities of carbon, SiC, and Si3 N4 are set equal to those of the crystalline forms; this implies that the material crystallizes before decomposition. The amorphous to crystalline transition that occurs at ∼10 bar, and which is accompanied by weight gain, is likely to have taken place by a different mechanism. A nucleation and growth reaction with the atmospheric nitrogen is proposed as the likely mechanism. The supersaturation required to nucleate α-Si3 N4 crystals is calculated to be 30 kJ/mol. 相似文献
75.
C Benz WR Martin J Arnold R Jakobs JF Riemann 《Canadian Metallurgical Quarterly》1997,122(13):391-395
OBJECTIVE: To evaluate whether push enteroscopy of the small bowel (PES), undertaken after extensive previous investigations in suspected intestinal bleeding from an uncertain site, chronic diarrhoea or lymphoma of the small intestine, contributes to the diagnosis, and to ascertain the results of PES and its clinical significance. PATIENTS AND METHODS: 56 consecutive patients (29 men, 27 women; mean age 63 years) were investigated prospectively. The main indications for PES were the search for the source of intestinal bleeding in 79% of patients (group A), chronic diarrhoea or tropical sprue in 16% (group B) and search for tumour of lymphoma in 5% (group C). PES was always performed in fasting patients under sedation/analgesia using a video PES, which contrary to catheter enteroscopy provides a channel for intervention. RESULTS: In group A 27% of patients were found to have lesions, in particular angiodysplasias, or (in once case) leiomyoma. Half of these patients were successfully treated endoscopically without later surgical intervention being required (mean follow-up of six months). In the other half operation became necessary, either because the disease itself indicated it or the bleeding persisted, the source being in the more distant small intestine and thus not accessible to endoscopic intervention: only 50% of the length of the small intestine proved to be within reach of the instrument. No abnormalities were discovered in patients of groups B and C. CONCLUSION: In cases of gastrointestinal bleeding from an uncertain source PES should be performed first, because in many cases it may obviate surgical intervention. But PES seems to contribute little of diagnostic value in other indications. 相似文献
76.
77.
78.
Heat Transport to the Wall of Packed Tubes. Radial conduction of heat in packed tubes has a crucial influence on yield and selectivity of many heterogeneously catalyzed reactions. In spite of many years of intensive research in the field, there are still tremendous discrepancies between correlations of different origin. Even the standard model using two constant heat transport parameters, which was introduced in the fifties and has since become most widely accepted, has been controversially discussed and called into question. The unsatisfactory state of the art has been an incentive for several groups of researchers to take up this old topic once again. Three parallel experimental investigations on heat transport with air flowing in packed tubes of similar dimensions, electrically heated, steam-heated, or water-cooled, were completed in 1991. Comparative evaluation of the results of these three investigations, together with other data from the relevant literature, now provides the first clear answers to some of the questions so controversially discussed in the past. 相似文献
79.
Eroding poly(DL-lactide-co-glycolide)(PDLLG) washers were observed chronically in vitro and in vivo following loading in a bone chamber tibial implant (BCI). Images were recorded using intravital microscopy of the implanted rabbit and direct pH measurements were obtained of the tissue in the bone chamber using a combination probe-reference microelectrode. While statistically significant pH differences were found between the control (unloaded) and experimental (PDLLG-bearing) BCIs, they were only of the order of 0.2 pH units. This value proved to be physiologically insignificant as no statistically significant difference in bone defect healing, as indicated by angiogenesis, was detected. It was shown that the measured small pH changes during PDLLG washers erosion would result whether the buffer was phosphate-buffered saline replaced weekly or interstitial fluid subject to vascular exchanges. 相似文献
80.
This work demonstrates that for constant oxide reliability stresses in the Fowler-Nordheim regime a low initial rate of charge trapping/detrapping results in long times to breakdown. It was found for MOS gate oxides that when the initial trapping has been completed at low fields times to breakdown enhance. Depending on the stress sequence measurement results can vary significantly which is of great relevance for correct oxide lifetime predictions. 相似文献