全文获取类型
收费全文 | 11019篇 |
免费 | 64篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 13篇 |
化学工业 | 197篇 |
金属工艺 | 14篇 |
机械仪表 | 16篇 |
建筑科学 | 17篇 |
能源动力 | 20篇 |
轻工业 | 202篇 |
水利工程 | 4篇 |
石油天然气 | 9篇 |
无线电 | 73篇 |
一般工业技术 | 115篇 |
冶金工业 | 10320篇 |
原子能技术 | 4篇 |
自动化技术 | 83篇 |
出版年
2023年 | 8篇 |
2022年 | 8篇 |
2021年 | 12篇 |
2020年 | 16篇 |
2019年 | 24篇 |
2018年 | 19篇 |
2017年 | 24篇 |
2016年 | 33篇 |
2015年 | 21篇 |
2014年 | 40篇 |
2013年 | 57篇 |
2012年 | 49篇 |
2011年 | 45篇 |
2010年 | 37篇 |
2009年 | 36篇 |
2008年 | 35篇 |
2007年 | 41篇 |
2006年 | 41篇 |
2005年 | 23篇 |
2004年 | 30篇 |
2003年 | 29篇 |
2002年 | 20篇 |
2001年 | 23篇 |
2000年 | 16篇 |
1999年 | 301篇 |
1998年 | 3368篇 |
1997年 | 1961篇 |
1996年 | 1187篇 |
1995年 | 669篇 |
1994年 | 536篇 |
1993年 | 637篇 |
1992年 | 73篇 |
1991年 | 131篇 |
1990年 | 110篇 |
1989年 | 129篇 |
1988年 | 138篇 |
1987年 | 100篇 |
1986年 | 88篇 |
1985年 | 62篇 |
1983年 | 13篇 |
1982年 | 50篇 |
1981年 | 49篇 |
1980年 | 68篇 |
1979年 | 4篇 |
1978年 | 25篇 |
1977年 | 193篇 |
1976年 | 472篇 |
1975年 | 13篇 |
1955年 | 8篇 |
1954年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
961.
962.
JM Mascaro 《Canadian Metallurgical Quarterly》1995,22(11):823-828
The porphyrias are uncommon diseases caused by enzymatic deficiencies in the heme pathway. In the 25 year period 1969-1994, the Department of Dermatology of the Hospital Clinic of Barcelona has been able to study 793 cases of porphyria (724 cases of PCT, 27 of EPP, 26 of PV, 5 of CEP, 5 of HEP, 5 of AIP, 1 of HCP). Homozygous expression of an enzymatic deficiency in the heme pathway produces severe disease. Commonly, clinical manifestations appear in the homozygous state (the autosomal recessive porphyrias). However, homozygous forms of autosomal dominant porphyrias may occur exceptionally. Moreover, there are cutaneous porphyrias whose clinical manifestations do not permit dermatologists to classify them clearly into one of the well-defined syndromes. These uncommon and atypical forms are difficult to recognize without biochemical and enzyme studies. The porphyrias have a wide clinico-biochemical spectrum, including a large proportion of well defined diseases. Nevertheless, atypical forms occur and may be difficult to evaluate. It is important to note the genetic heterogeneicity of porphyrias, which accounts for the varying phenotypic expression. 相似文献
963.
964.
965.
966.
Many studies have demonstrated that the human visual system is sensitive to very small differences in relative binocular disparity. It is not known over what monocular regions information is spatially integrated to mediate performance in such tasks. In this study we present psychophysical observations that define the smallest spatial scale involved in disparity processing, and we indicate the nature of the computations performed by the units mediating that disparity discrimination. We show that human observers can identify the sign of disparity of a single target dot when it is embedded in a row of identical dots, with these noise dots presented either in the fixation plane or with a proportion binocularly uncorrelated. In conjunction with the psychophysical data, we explore how a class of simple correlator models of stereopsis must be constrained in order to account for human performance for the same fine-scale tasks. Such models can perform the task only when the correlation is carried out over a very small region of the image, for a very small range of disparities. Our results demonstrate that there is a fine-scale input to the stereo system, mediated by foveal mechanisms that spatially integrate visual signals over a region as small as 4-6 arcmin in diameter. 相似文献
967.
OBJECTIVE: To test the hypothesis that reductions in mean arterial pressure (MAP) induced by food-intake restriction in aortic coarctation hypertension are the result of a reduction of the sympathetic support of the MAP. We also wanted to determine whether the baroreflex control of the heart rate, and alpha- and beta-adrenergic responsivenesses were influenced by chronic food-intake restriction. METHODS: Four days after aortic coarctation, female Sprague-Dawley rats were assigned to a group that had access ad libitum to food (CON; n = 19) or to a food-intake-restricted group (FRG; n = 17) that was allowed 60% of the CON group's food intake per rat. After 3 weeks, carotid and jugular catheters were implanted for measurement of the MAP and infusion of drugs into conscious rats. The sympathetic contribution to the blood pressure was assessed by measuring the depressor response to ganglionic blockade by hexamethonium plus atropine (30.0 and 0.1 mg/kg intravenously). The baroreflex control of the heart rate was assessed by administering alternating bolus doses of phenylephrine and nitroprusside. The alpha-adrenergic sensitivity was assessed by measuring the response of the MAP to phenylephrine in areflexive rats (after ganglionic blockade), and the beta-adrenergic sensitivity was assessed by measuring the responses of the MAP and heart rate to isoproterenol administration both in reflexive and in areflexive rats. RESULTS: Four days after catheterization, both the MAP (CON 150 +/- 5 mmHg, FRG 116 +/- 4 mmHg) and the heart rate (CON 414 +/- 8 beats/min, FRG 365 +/- 11 beats/min) were significantly lower in rats of the FRG. That the sympathetic support of the MAP had diminished in FRG rats was evidenced by an attenuated depressor response to ganglionic blockade (40 +/- 3 versus 65 +/- 3 mmHg). FRG rats exhibited significantly greater reflex bradycardia in response to phenylephrine (slope -1.44+/- 0.07 versus -0.54 +/- 0.05 beats/min per mmHg), whereas their reflex tachycardia was not altered (slope -1.58 +/- 0.08 versus -1.53 +/- 0.13 beats/min per mmHg). FRG rats also displayed blunted responses of the heart rate and MAP to isoproterenol administration. CONCLUSION: Food-intake restriction attenuates the rise in MAP which occurs after aortic coarctation significantly. The antihypertensive effect of food-intake restriction may be mediated via a reduction in sympathetic tone. 相似文献
968.
JM Tourani C Jaillon-Abraham V Lucas Y Chrétien D Mayeur M Di Palma C Boaziz P Grise C Varette JM Pavlovitch D Larregain E Ecstein M Untereiner JM Andrieu 《Canadian Metallurgical Quarterly》1997,84(4):351-356
Angina, myocardial infarction, and sudden cardiac death are associated with thrombus formation in coronary arteries. It is generally believed that these conditions benefit from long-term exercise. However, the evidence for such amelioration or prevention is inconclusive and so is the mechanism through which long-term exercise exerts its beneficial effect on various ischaemic conditions. Because of the thrombotic events, the effects of exercise on platelet reactivity, blood coagulation and fibrinolysis have been extensively studied, but the findings are not consistent (1-9). This is mainly due to methodological problems. Analysis of platelet response to a variety of agonists and the dozen of coagulation and fibrinolysis variables makes the assessment of the overall platelet function, coagulation and fibrinolysis status extremely difficult. 相似文献
969.
MP Golombek RA Cook T Economou WM Folkner AF Haldemann PH Kallemeyn JM Knudsen RM Manning HJ Moore TJ Parker R Rieder JT Schofield PH Smith RM Vaughan 《Canadian Metallurgical Quarterly》1997,278(5344):1743-1748
Chemical analyses returned by Mars Pathfinder indicate that some rocks may be high in silica, implying differentiated parent materials. Rounded pebbles and cobbles and a possible conglomerate suggest fluvial processes that imply liquid water in equilibrium with the atmosphere and thus a warmer and wetter past. The moment of inertia indicates a central metallic core of 1300 to 2000 kilometers in radius. Composite airborne dust particles appear magnetized by freeze-dried maghemite stain or cement that may have been leached from crustal materials by an active hydrologic cycle. Remote-sensing data at a scale of generally greater than approximately 1 kilometer and an Earth analog correctly predicted a rocky plain safe for landing and roving with a variety of rocks deposited by catastrophic floods that are relatively dust-free. 相似文献
970.
F García-Río JM Pino-García S Serrano MA Racionero JG Terreros-Caro R Alvarez-Sala C Villasante J Villamor 《Canadian Metallurgical Quarterly》1997,10(10):2371-2375
The multibreath helium equilibration method is the technique recommended for routine measurement of static lung volumes in normal subjects. However, pregnancy could be an exception to this general rule, due to airway closure during the second half of gestation. The aim of this study was to compare the measurements of lung volumes by plethysmography and helium dilution during pregnancy. Twenty three healthy women were studied at 12, 24 and 36 weeks of pregnancy, and 4 months postpartum. Total lung capacity (TLC), functional residual capacity (FRC) and residual volume (RV) were measured by multibreath helium equilibration (TLCHe, FRCHe and RVHe) and by plethysmography (TLCbox, FRCbox and RVbox). Only at 36 weeks were there differences between the two methods. RVbox was significantly larger than RVHe (1.01+/-0.18 vs 0.77+/-0.21 L; p<0.001). FRCbox was larger than FRCHe (1.95+/-0.32 vs 1.60+/-0.32 L; p<0.001) and TLCbox was larger than TLCHe (4.83+/-0.52 vs 4.45+/-0.51 L; p<0.05). The 95% limits of agreement for differences between lung volumes measured by the two techniques (helium dilution - plethysmography) at 36 weeks were: -0.42 to -0.06 L for RV; -0.54 to -0.17 L for FRC; and -0.66 to -0.11 L for TLC. We conclude that using the multibreath helium equilibration method to measure lung volumes in at-term pregnant women results in underestimation of functional residual capacity and total lung capacity. 相似文献