全文获取类型
收费全文 | 262129篇 |
免费 | 3545篇 |
国内免费 | 747篇 |
专业分类
电工技术 | 4751篇 |
综合类 | 800篇 |
化学工业 | 43355篇 |
金属工艺 | 9925篇 |
机械仪表 | 8158篇 |
建筑科学 | 6377篇 |
矿业工程 | 1574篇 |
能源动力 | 5823篇 |
轻工业 | 27042篇 |
水利工程 | 2716篇 |
石油天然气 | 5270篇 |
武器工业 | 59篇 |
无线电 | 27233篇 |
一般工业技术 | 47630篇 |
冶金工业 | 48465篇 |
原子能技术 | 6107篇 |
自动化技术 | 21136篇 |
出版年
2021年 | 2641篇 |
2019年 | 2413篇 |
2018年 | 3983篇 |
2017年 | 3889篇 |
2016年 | 4196篇 |
2015年 | 2646篇 |
2014年 | 4523篇 |
2013年 | 11459篇 |
2012年 | 7006篇 |
2011年 | 9312篇 |
2010年 | 7360篇 |
2009年 | 8132篇 |
2008年 | 8425篇 |
2007年 | 8556篇 |
2006年 | 7587篇 |
2005年 | 6619篇 |
2004年 | 6449篇 |
2003年 | 6461篇 |
2002年 | 6816篇 |
2001年 | 6529篇 |
2000年 | 5936篇 |
1999年 | 5776篇 |
1998年 | 13661篇 |
1997年 | 9626篇 |
1996年 | 7380篇 |
1995年 | 5689篇 |
1994年 | 5010篇 |
1993年 | 4920篇 |
1992年 | 3840篇 |
1991年 | 3525篇 |
1990年 | 3721篇 |
1989年 | 3584篇 |
1988年 | 3421篇 |
1987年 | 2992篇 |
1986年 | 3032篇 |
1985年 | 3510篇 |
1984年 | 3297篇 |
1983年 | 2978篇 |
1982年 | 2813篇 |
1981年 | 2910篇 |
1980年 | 2801篇 |
1979年 | 2676篇 |
1978年 | 2725篇 |
1977年 | 3036篇 |
1976年 | 3975篇 |
1975年 | 2415篇 |
1974年 | 2272篇 |
1973年 | 2419篇 |
1972年 | 1998篇 |
1971年 | 1882篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Two international trials of potential biomass species families and clones planted in 1985 have been analyzed to determine changing patterns in growth, diseases, and insects. In the Swedish trial, with close spacing and short rotations, Salix viminalis clones gave the best average yield of 17.4 Mg/Ha/yr on the first coppice rotation. In the Minnesota, USA trial, with wider spacings and a longer rotation Populus alba hybrid clones gave the best average yields of 5.5 Mg/Ha/yr. The most important disease and insect problems encountered were Septoria musiva stem cankers on some Populus clones; frost die back in some Salix clones; and the leaf blotch miner, Fenusa dohrnii, on most Alnus families. Recommendations for future international cooperation in field trials are presented. 相似文献
992.
Various specimen sizes of an unreinforced polymer, Hercules 3501 -6 thermosetting epoxy, were subjected to a tensile test. The general specimen geometry was a rectangular dog-bone shape with constant gauge length, but with each specimen size having a different crosssectional area. These cross-sectional areas were obtained by varying the thickness of the epoxy during casting, and the gauge section width during grinding. The resulting failure surfaces of the specimens were observed and photographed using scanning electron microscopy. The results indicate that failure stress, dimensions of the critical flaw which caused failure, and a quantity which is proportional to the fracture toughness, are all correlated with specimen size. 相似文献
993.
Molybdenum black solar selective coatings have been produced on cobalt by immersion in a solution of ammonium paramolybdate
and nickel sulphate. The cobalt was electroplated on nickel-plated copper prior to immersion in the paramolybdate solution.
The maximum solar absorptance of the resulting molybdenum black coating was about 0.91. The minimum emittance was about 0.1
for coatings on cobalt deposited with an addition agent and about 0.23 for coatings on cobalt deposited without addition agent
in the 60 °C plating solution. These differences have been related to the coating morphology determined by scanning electron
microscopy. The emittance of coatings on cobalt deposited without addition agent decreases (or remains unchanged) during short-term
heat treatment while that of coatings deposited on cobalt plated with addition agent increases somewhat. Reduction of the
cobalt plating bath temperature to 45 °C can also lead to good initial coating properties but without the requirement for
an addition agent in the cobalt plating bath. XPS studies show that the oxidation state of molybdenum in the coatings is approximately
+ 5 corresponding to Mo4O11. This reduces to + 4 after argon ion bombardment. Some cobalt may be present in the coatings in the form of CoO. 相似文献
994.
Jue Donald; Meador Kimford J.; Zamrini Edward Y.; Allen Martha E.; Feldman Daniel S.; Loring David W. 《Canadian Metallurgical Quarterly》1992,6(4):331
Studied memory for visuospatial arrays in 28 male and 34 female healthy dextral adults (aged 20–90 yrs). 10 abstract figures were presented in each hemispace over a series of learning trials, and then delayed free recall of figure placement was obtained. Figures were misplaced significantly downward and leftward in both hemispaces. Both vertical and horizontal absolute error increased significanty with age, but no age effects on directional errors were observed. The increase in absolute error over age is consistent with decreased anterograde memory and visuospatial skills in the elderly. The directional errors are believed to result from an attentional shift toward proximal peripersonal space, in combination with attentional shifts from differential left vs right cerebral activation. Gender effects were also observed. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
995.
William E. Cooper Jr. Christopher S. Deperno Johnny Arnett 《Journal of chemical ecology》1994,20(11):2867-2881
Experimental tests showed that poststrike elevation in tongue-flicking rate (PETF) and strike-induced chemosensory searching (SICS) in the gila monster last longer than reported for any other lizard. Based on analysis of numbers of tongue-flicks emitted in 5-min intervals, significant PETF was detected in all intervals up to and including minutes 41–45. Using 10-min intervals, PETF lasted though minutes 46–55. Two of eight individuals continued tongue-flicking throughout the 60 min after biting prey, whereas all individuals ceased tongue-flicking in a control condition after minute 35. The apparent presence of PETF lasting at least an hour in some individuals suggests that there may be important individual differences in duration of PETF. PETF and/or SICS are present in all families of autarchoglossan lizards studied except Cordylidae, the only family lacking linguallly mediated prey chemical discrimination. However, its duration is known to be greater than 2-min only in Helodermatidae and Varanidae, the living representatives of Varanoidea. That prolonged PETF and SICS are typical of snakes provides another character supporting a possible a varanoid ancestry for Serpentes. Analysis of 1-min intervals showed that PETF occurred in the first minute. A review of the literature suggests that a pause in tongue-flicking and delay of searching movements are absent in lizards and the few nonvenomous colubrid snakes tested. The delayed onset of SICS may be a specific adaptation of some viperid snakes to allow potentially dangerous prey to be rendered harmless by venom following voluntary release after envenomation and preceding further physical contact with the prey. 相似文献
996.
Phase effects arising in a semiconductor laser with diffraction extraction of radiation and a distributed Bragg reflector
on the substrate side are taken into account exactly quantitatively and the possibilities of using these effects in lasers
is analyzed. It is shown that the phase effects studied can be used to increase the laser efficiency substantially.
Pis’ma Zh. Tekh. Fiz. 23, 7–11 (September 26, 1997) 相似文献
997.
Memorializes Richard Solomon, one of the major learning theorists of the 2nd half of the 20th century. Solomon was named the 1st James M. Skinner University Professor of Science in 1975, was elected to the National Academy of Sciences, and was awarded, among other honors, the Warren Medal for research by the Society of Experimental Psychologists, the American Psychological Association Distinguished Scientific Contribution Award, and the American Psychological Foundation's Award for Distinguished Teaching in Experimental Psychology. He conducted research on a broad array of topics such as word frequency and perceptual defense, children's estimation of the size of tokens, hoarding behaviors in rats, and personality ratings and sociometric patterns. His work made a major contribution to the exploration of Pavlovian conditioning. Other topics of research focused on in Solomon's laboratories included avoidance learning, learned helplessness, punishment, and the opponent-process theory of acquired motivation. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
998.
Cardioprotective effects of a standardized extract from leaves with flowers of Crataegus (WS-1442; content of oligomeric procyandins [OPC]: 18.75%) have recently been demonstrated in an ischemia-reperfusion model in rats. Further studies were now conducted to clarify the mechanism of action and to identify active constituents involved in these effects of WS-1442. Exhausting partitioning between ethyl acetate/water and successive ultrafiltration of the aqueous layer led to the quantitative recovery of three fractions, which were tested for their in vitro radical scavenging (RS) and human neutrophil elastase (HNE) inhibitory activity. The lipophilic ethylacetate-soluble fraction A, enriched in flavone derivatives and constituting 14.9% of WS-1442, was as active as WS-1442 in inhibiting HNE. However, its RS activity was only about half that of the primary extract. Although 67.9% of WS-1442 was recovered in a water-soluble low molecular weight fraction B, this fraction displayed only weak RS and HNE inhibiting activity. In contrast, the RS and HNE inhibiting potencies of an essentially flavone-free and OPC-rich fraction C (21.3% of WS-1442) were significantly higher (inhibition of lipid peroxidation: IC50 0.3 microgram/ml; inhibition of HNE: IC50 0.84 microgram/ml) as those of WS-1442. The RS and HNE inhibitory activities of the extract and those of its fractions correlated well with their OPC-content but not with their concentration of flavonols. These results demonstrate that OPCs of Crataegus extracts possess stronger radical scavenging activities than flavone derivatives or other constituents. In addition, the oligomeric components are potent inhibitors of HNE. Oral administration of 20 mg/kg/d of the OPC-rich fraction C to rats afforded similar protection against ischemia-reperfusion induced pathologies as treatment with WS-1442 at a dose of 100 mg/kg/d. These observations indicate that radical scavenging and elastase inhibitory activities could indeed be involved in the observed cardioprotective effects of WS-1442, and demonstrate that OPCs are major orally active constituents of WS-1442. Thus, Crataegus extracts used therapeutically for cardiovascular diseases should be analyzed and standardized for their OPC-content. 相似文献
999.
KE Ensrud DM Black L Palermo DC Bauer E Barrett-Connor SA Quandt DE Thompson DB Karpf 《Canadian Metallurgical Quarterly》1997,157(22):2617-2624
BACKGROUND: The efficacy of antiresorptive therapy in preventing fractures in women at highest fracture risk, such as very elderly women or those with severe osteoporosis, is uncertain. PARTICIPANTS AND METHODS: Using data from a double-blind, randomized, placebo-controlled clinical trial that enrolled 2027 postmenopausal women aged 55 to 81 years with low femoral neck bone mineral density (BMD) and existing vertebral fractures, we examined the consistency of the effect of treatment with alendronate sodium in preventing fractures within a priori-specified risk subgroups defined at baseline by age, bone density, number of preexisting vertebral fractures, and history of postmenopausal fracture. The women were randomized to oral administration of alendronate or placebo and followed up for an average of 2.9 years. The initial dose of alendronate sodium was 5 mg/d; the dosage was increased from 5 to 10 mg/d at 24 months. New vertebral fractures, the primary end point of this arm of the trial, were defined by morphometry as a decrease of 20% and at least 4 mm in any vertebral height between baseline and a follow-up radiograph at 36 months. Incident clinical fractures, the secondary end point, included nonspine and clinical (symptomatic) vertebral fractures. All clinical fractures were confirmed with x-ray film reports or, in the case of clinical vertebral fractures, x-ray films. RESULTS: Overall, there was a 47% significant reduction in risk of new vertebral fractures in the alendronate group compared with the placebo group. The reduction in risk of new vertebral fracture was consistent across fracture risk categories including age (relative risk [RR], 0.49 in women < 75 years compared with 0.62 in those > or = 75 years), BMD (RR, 0.54 in women with a femoral neck BMD < 0.59 g/cm2 [median] compared with 0.53 in those with a BMD > or = 0.59 g/cm2), and number of preexisting vertebral fractures (RR, 0.58 in women with 1 vertebral fracture compared with 0.52 in those with > or = 2). The overall significant 28% reduction in risk of incident clinical fractures in the alendronate group compared with the placebo group was also observed within these subgroups. Compared with the number of lower-risk women, a similar or smaller number of high-risk women needed to be treated to prevent 1 fracture. For example, 8 women aged 75 years or older compared with 9 women younger than 75 years, or 4 women with 2 or more existing vertebral fractures compared with 16 women with 1 existing vertebral fracture, needed to be treated with alendronate for 5 years to prevent 1 new vertebral fracture. CONCLUSIONS: Alendronate effectively reduces fracture risk in postmenopausal women with vertebral fractures and low BMD, including those women at highest risk because of advanced age or severe osteoporosis. Since the risk reductions observed with alendronate treatment were consistent within fracture risk categories, more fractures were prevented by treating women at highest risk. 相似文献
1000.
Microscopic examination of the microstructure and deformation of conventional and auxetic foams 总被引:1,自引:0,他引:1
Auxetic materials have a negative Poisson’s ratio, that is, they expand laterally when stretched longitudinally. One way of
obtaining a negative Poisson’s ratio is by using a re-entrant cell structure. Auxetic foam was fabricated from a conventional
polymeric foam. Assuming similar mechanical properties for the solid material comprising the foams, the principle variable
affecting the properties of the foam is the geometry of the cells. This means that the unusual mechanical properties of auxetic
foams are attributed to the deformation characteristics of re-entrant microstructures. In this paper, the results of optical-
and scanning electron-microscopic studies of the geometrical parameters for the different foams examined are presented. Examples
of the microstructural deformation mechanisms observed are also presented. Comparison between the conventional foams and their
auxetic conversions are also made.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献