全文获取类型
收费全文 | 1167篇 |
免费 | 72篇 |
专业分类
电工技术 | 18篇 |
化学工业 | 309篇 |
金属工艺 | 7篇 |
机械仪表 | 17篇 |
建筑科学 | 45篇 |
能源动力 | 41篇 |
轻工业 | 139篇 |
水利工程 | 18篇 |
无线电 | 113篇 |
一般工业技术 | 184篇 |
冶金工业 | 180篇 |
原子能技术 | 16篇 |
自动化技术 | 152篇 |
出版年
2024年 | 3篇 |
2023年 | 8篇 |
2022年 | 66篇 |
2021年 | 62篇 |
2020年 | 30篇 |
2019年 | 26篇 |
2018年 | 36篇 |
2017年 | 31篇 |
2016年 | 32篇 |
2015年 | 35篇 |
2014年 | 58篇 |
2013年 | 69篇 |
2012年 | 65篇 |
2011年 | 52篇 |
2010年 | 49篇 |
2009年 | 52篇 |
2008年 | 52篇 |
2007年 | 35篇 |
2006年 | 37篇 |
2005年 | 29篇 |
2004年 | 33篇 |
2003年 | 33篇 |
2002年 | 27篇 |
2001年 | 19篇 |
2000年 | 10篇 |
1999年 | 28篇 |
1998年 | 57篇 |
1997年 | 36篇 |
1996年 | 31篇 |
1995年 | 17篇 |
1994年 | 17篇 |
1993年 | 11篇 |
1992年 | 12篇 |
1991年 | 4篇 |
1990年 | 8篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 8篇 |
1974年 | 2篇 |
排序方式: 共有1239条查询结果,搜索用时 15 毫秒
71.
DJ McKenzie G Piraccini A Felskie P Romano P Bronzi CL Bolis 《Canadian Metallurgical Quarterly》1999,72(1):116-125
Nile tilapia (Oreochromis niloticus) were infused with ammonium salts, acid, and base to investigate the effects of changes in arterial plasma total ammonia content (Tamm) and pH (pHa) on plasma urea-nitrogen (urea-N) levels and urea-N excretory fluxes (Jurea-N). The tilapia did not possess a functional hepatic ornithine urea-cycle (no significant carbamyl phosphate synthetase III activity). Infused substances were dissolved in a saline vehicle and injected twice (5 mL kg-1), the first infusion to "prime" the animal and promote a more marked response to the second infusion, given 2.5 h later. The results reported are those of the second infusion. Infusion of 200 mM NH4Cl increased Tamm, reduced pHa, and increased plasma urea-N and Jurea-N. Two hundred mM NH4HCO3 increased Tamm and arterial plasma total CO2 content (TaCO2), reduced pHa, and increased Jurea-N. Fifty mM HCl reduced pHa but had no effects on urea dynamics. Fifty mM NaOH increased pHa, plasma urea-N levels, and Jurea-N. Two hundred mM NaHCO3 increased pHa, TaCO2, plasma urea-N levels, and Jurea-N. Infusion of the saline vehicle was without effect. The results indicate that ammonia loading and plasma alkalosis both stimulate urea excretion in uricolytic fish. The responses to hyperammonemia or alkalosis were not modified when combined with elevated plasma bicarbonate levels. 相似文献
72.
PURPOSE: Ischemic neurodegeneration contributes to many retinal diseases. An isolated retina model has been used to examine the neuronal cell death induced by deprivation of oxygen and glucose (simulated ischemia) as a model for ischemic disease. METHODS: Neurodegeneration in the isolated chick embryo retina was induced by simulated ischemia and assessed using biochemical (lactate dehydrogenase release) and morphologic (light microscopy) techniques. RESULTS: Simulated ischemia led to lactate dehydrogenase release gradually in a period of 6 to 24 hours. Light microscopic observations demonstrated morphologic cell degeneration well before lactate dehydrogenase release occurred. N-Methyl-D-aspartate (NMDA) and non-NMDA receptor blockers individually provided partial protection, and the combination was fully protective. No protection was provided if the antagonists were added after simulated ischemia. When NMDA receptors were blocked by MK-801, cyclothiazide, an inhibitor of desensitization at non-NMDA receptors, enhanced lactate dehydrogenase released after 1 or 2 hours of simulated ischemia. Low concentrations of glucose effectively prevented lactate dehydrogenase release, despite anoxic conditions. CONCLUSIONS: The isolated retina provided a convenient system to characterize quantitatively ischemic cell death. Retinal ischemic neurodegeneration is an excitotoxic process that involves overactivation of NMDA and non-NMDA glutamate receptors. Blockade of both of these receptor subtypes was necessary for complete neuroprotection. Receptor desensitization played a protective role. If even low concentrations of glucose were delivered to an ischemic retina in vitro, substantial neuroprotection could be achieved. This may have implications for the management of acute retinal ischemic episodes. 相似文献
73.
The synthesis of a fully aromatic oligoamide by low temperature interfacial polycondensation between terephthaloyl chloride and 1,4-phenylenediamine is reported. The reaction product was characterized with respect to its physical properties and chemical structure by infrared, X-rays, electron spectroscopy for chemical analysis (ESCA), and mass spectroscopic techniques. Elemental and thermal analysis, as well as viscosimetric and titrimetric methods were applied to evaluate the average molecular weight of the oligomer, whose chain length consists mainly of 3 to 4 monomeric units. 相似文献
74.
75.
The effects of hypothalamic disconnection on body temperature and hypothalamo-pituitary-adrenal activity following acute and repeated exposures to heat were studied. Intact male rats, or animals with complete posterior or anterior hypothalamic disconnection, were exposed to a temperature of 36 degrees C and a relative humidity of 35-45%. In the complete posterior and anterior hypothalamic disconnected rats the basal Tre was higher than that of the intact rats; the rise in Tre following heat exposure was lower in the operated rats than in the intact animals. All the experimental animals, except for those with anterior hypothalamic disconnection, showed a significant inhibition of corticosterone release on exposure to heat for 30 min, but no inhibition was observed in any of the disconnected rats when they were exposed to heat for 120 min. These results suggest that the main stimulus for ACTH release, during the first 30 min of heat exposure, is mediated by a neural input through the posteroir hypothalamus and this is followed by a nerural and/or humoral mechanism which enables the animals to increase their corticosterone secretion. 相似文献
76.
Experiments were carried out on the skeletal muscle fibres of the cricket Acheta domesticus using conventional microelectrode methods. Both potassium and ammonium ions depolarized the fibres but with a considerable divergance from the slope of 58 mV. Calcium ions hyperpolarized the fibres. High concentrations of magnesium ions or dramatic increase in the osmotic pressure of the bathing medium exerted no influence on the resting potential. It seems likely that skeletal muscle fibres of Acheta domesticus at rest are multionic electrodes, but the role played by various ions needs further studies. 相似文献
77.
Persico R Soldovieri F 《Journal of the Optical Society of America. A, Optics, image science, and vision》2004,21(1):35-45
We deal with the inverse-scattering problem for a dielectric slab embedded in a three-layered medium starting from multifrequency scattered field data under the framework of the Born approximation. This allows us to state the problem as a linear inverse one, and the singular-value decomposition (SVD) of the relevant operator makes it possible to investigate and to solve it. In particular, the SVD tool allows an analysis of the reconstruction capabilities of the algorithm in terms of spatial variability of the unknowns that can be retrieved. The new contribution consists in an analysis of the role of the discontinuity of the dielectric properties between the second and the third medium. This analysis is performed with regard both to the class of retrievable dielectric profiles and to the model error deriving from the Born approximation and shows, finally, that this discontinuity can be troublesome. 相似文献
78.
Effects of uncertainty on background permittivity in one-dimensional linear inverse scattering 总被引:1,自引:0,他引:1
Persico R Soldovieri F 《Journal of the Optical Society of America. A, Optics, image science, and vision》2004,21(12):2334-2343
We discuss the role of uncertainties in the knowledge of the background in inverse scattering for a buried object under the distorted Born approximation. In particular, we focus on the role played by inaccuracy in the knowledge of the dielectric permittivity of the host medium, with reference to both a lossless half-space and a lossless three-layered medium. This investigation allows us to show how reconstruction of an inhomogeneity in a three-layered medium is more critical than in the case of a half-space (two-layered) geometry. 相似文献
79.
This paper presents costs per US motor vehicle crash victim differentiated into many more diagnostic categories than prior estimates. These unit costs, which include the first keyed to the 1990 edition of Abbreviated Injury Scale (AIS) threat-to-life severity scores, are reported by body part, whether a fracture/dislocation was involved, and the maximum AIS score among the victim's injuries. This level of detail allows for a more accurate estimation of the social costs of motor vehicle crashes. It also allows for reliable analyses of interventions targeting narrow ranges of injuries. The paper updates the medical care data underlying the US crash costs from 1979 to 1986 to the mid 1990s and improves on prior productivity cost estimates. In addition to presenting the latest generation of crash victim costs, this paper analyzes the effects of applying injury costs classified by AIS code from the 1985 edition to injury incidence data coded with the 1990 edition of AIS. This long-standing practice results in inaccurate cost-benefit analyses that typically overestimate benefits. This problem is more acute when old published costs adjusted for inflation are used rather than the recent costs. 相似文献
80.
Carbon-13 chemical shifts are reported for tetralin, hydrophenanthrenes and hydropyrenes and their alkyl substituted derivatives. Mono- and di- plus tri-aromatic fractions of hydrogenated phenanthrene and pyrene were also examined by carbon-13 n.m.r. and, wherever possible, the components in them identified. 相似文献