全文获取类型
收费全文 | 418328篇 |
免费 | 6129篇 |
国内免费 | 1395篇 |
专业分类
电工技术 | 7751篇 |
综合类 | 366篇 |
化学工业 | 63252篇 |
金属工艺 | 14445篇 |
机械仪表 | 11979篇 |
建筑科学 | 11919篇 |
矿业工程 | 881篇 |
能源动力 | 11300篇 |
轻工业 | 44286篇 |
水利工程 | 3397篇 |
石油天然气 | 1852篇 |
武器工业 | 13篇 |
无线电 | 55711篇 |
一般工业技术 | 77751篇 |
冶金工业 | 75780篇 |
原子能技术 | 5581篇 |
自动化技术 | 39588篇 |
出版年
2021年 | 2818篇 |
2019年 | 2582篇 |
2018年 | 3998篇 |
2017年 | 4048篇 |
2016年 | 4321篇 |
2015年 | 3547篇 |
2014年 | 5941篇 |
2013年 | 19713篇 |
2012年 | 10526篇 |
2011年 | 15138篇 |
2010年 | 11598篇 |
2009年 | 13191篇 |
2008年 | 14250篇 |
2007年 | 14598篇 |
2006年 | 13032篇 |
2005年 | 12178篇 |
2004年 | 11762篇 |
2003年 | 11452篇 |
2002年 | 11073篇 |
2001年 | 11220篇 |
2000年 | 10353篇 |
1999年 | 10841篇 |
1998年 | 24072篇 |
1997年 | 17657篇 |
1996年 | 13961篇 |
1995年 | 11041篇 |
1994年 | 9823篇 |
1993年 | 9400篇 |
1992年 | 7164篇 |
1991年 | 6806篇 |
1990年 | 6421篇 |
1989年 | 6138篇 |
1988年 | 5893篇 |
1987年 | 4917篇 |
1986年 | 4856篇 |
1985年 | 6015篇 |
1984年 | 5636篇 |
1983年 | 4859篇 |
1982年 | 4530篇 |
1981年 | 4463篇 |
1980年 | 4228篇 |
1979年 | 4095篇 |
1978年 | 3788篇 |
1977年 | 4564篇 |
1976年 | 6112篇 |
1975年 | 3121篇 |
1974年 | 3018篇 |
1973年 | 2896篇 |
1972年 | 2322篇 |
1971年 | 2005篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Recently we have been making extensive use of TEM observations of grain boundaries in a research program concerned with the deformation behavior of tungsten bicrystals. A major problem in this research involved the preparation of thin foils which contained the grain boundary in the electron tranparent region, since standard cell type electrojetpolishing techniques do not allow adequate control of the placement of the thin area. Since the grain boundary comprises a very small percentage of a foil, good thin area placement control is critical to obtaining useful samples consistently. Masking techniques which use electrically insulating lacquers are not precise enough to guarantee the position of the thin area. However, we have found that one can use the dishing characteristic of the jetpolisher (i.e., the fact that the center of the foil thins faster than the perimeter) in conjunction with the masking technique to predict the position of the thin area within a preferentially thinned region created by the masking procedure outlined below. This is the essence of the procedure outlined below. 相似文献
993.
In unit selection-based concatenative speech synthesis, join cost (also known as concatenation cost), which measures how well two units can be joined together, is one of the main criteria for selecting appropriate units from the inventory. Usually, some form of local parameter smoothing is also needed to disguise the remaining discontinuities. This paper presents a subjective evaluation of three join cost functions and three smoothing methods. We also describe the design and performance of a listening test. The three join cost functions were taken from our previous study, where we proposed join cost functions derived from spectral distances, which have good correlations with perceptual scores obtained for a range of concatenation discontinuities. This evaluation allows us to further validate their ability to predict concatenation discontinuities. The units for synthesis stimuli are obtained from a state-of-the-art unit selection text-to-speech system: rVoice from Rhetorical Systems Ltd. In this paper, we report listeners' preferences for each join cost in combination with each smoothing method. 相似文献
994.
The applicability of fluidised-bed reactor (FBR) based sulphate reducing bioprocess was investigated for the treatment of iron containing (40-90 mg/L) acidic wastewater at 65 degrees C. The FBR was inoculated with sulphate-reducing bacteria (SRB) originally enriched from a hot mining environment. Ethanol or acetate was supplemented as carbon and electron source for the SRB. A rapid startup with 99.9, 46 and 29% ethanol, sulphate and acetate removals, in respective order, was observed even after 6 days. Iron was almost completely removed with a rate of 90 mg/L.d. The feed pH was decreased gradually from its initial value of 6 to around 3.7 during 100 days of operation. The wastewater pH of 4.3-4.4 was neutralised by the alkalinity produced in acetate oxidation and the average effluent pH was 7.8 +/- 0.8. Although ethanol removal was complete, acetate accumulated. Later the FBR was fed with acetate only. Although acetate was present in the reactor for 295 days, its oxidation rates did not improve, which may be due to low growth rate and poor attachment ability of acetate oxidising SRB. Hence, the oxidation of acetate is the rate limiting step in the sulphidogenic ethanol oxidation by the thermophilic SRB. 相似文献
995.
One hypothesis for the transcontinental and intra-Great Lakes basin transfer of round gobies (Neogobius melanostomus) has been that round gobies were pumped into the ballast water of ships. During June 2005 in Lake Erie, we obtained evidence of a vertical migration of round goby larvae, when we collected 167 round goby larvae in surface ichthyoplankton net tows at night and zero during day. These results complemented similar findings from the Muskegon River estuary of Lake Michigan during 2003 and 2004, documenting diel vertical migration for the first time in larval round gobies. We suggest vertical migration behavior may have allowed larval round gobies to be transported to and within the Great Lakes via ballast water and dispersed in the Great Lakes via advection of 6.5–8.5-mm long larvae at the surface. Based on our results, if ballast water was only taken on near the surface during daylight hours from May through September when larval round gobies were present, it would have mitigated the spread of round gobies throughout the Great Lakes. 相似文献
996.
In various medium‐to‐large‐scale fire test equipments like the ISO room corner test (RC), and more recently, the single burning item test (SBI) the mass flow rate measurement of the combustion gases plays a key role in the determination of the heat‐release rate and smoke‐production rate. With the knowledge of the velocity profile and the temperature of the flow, the mass flow rate is obtained by measuring the velocity on the axis of the duct. This is done by means of a bi‐directional probe based on the pitot principle. However, due to the variation of the mean temperature and the temperature gradient in any cross section of the duct, introduced by ever changing combustion gas temperatures, the velocity nor the density profile are constant in time. This paper examines the resulting uncertainty on the mass flow rate. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
997.
Chen J.J. Gao G.-B. Chyi J.-I. Morkoc H. 《Electron Devices, IEEE Transactions on》1989,36(10):2165-2172
Avalanche breakdown behavior at the collector junction of the GaAs/AlGaAs HBT (heterojunction bipolar transistor) has been studied. Junction breakdown characteristics displaying hard breakdown, soft breakdown, and negative resistance breakdown behavior were observed and are interpreted by analysis of localized microplasma effects, uniform microplasma-free behavior, and associated current gain measurements. Light emission from the collector-base junction of the GaAs/AlGaAs HBT was observed and used to investigate breakdown uniformity. Using a simple punchthrough breakdown model, the theoretical breakdown curves at different collector doping concentrations and thicknesses were computed and found to be in agreement with maximum breakdown voltages measured from devices displaying the most uniform junction breakdown. The serious current gain degradation of GaAs/AlGaAs HBTs at low current densities was analyzed in connection with the measurement of a large collector-emitter breakdown voltage. The unexpected functional relationship between the collector-emitter breakdown voltage and collector-base breakdown voltage is explained by the absence of a hole-feedback effect for devices not exhibiting transistor action 相似文献
998.
999.
The iron-free heme derivative protoporphyrin-IX, was added to a cooked emulsified beef product to mimic the typical cured-color obtained with nitrite. Although the pigment was stable and could be recovered from the emulsion it imparted a purple hue as noted by visual observation and Hunterlab color values (L,a,b). The possible implications of this pigment as a meat colorant are discussed. 相似文献
1000.
V. Elliott Smith John M. Spurr John C. Filkins Jody J. Jones 《Journal of Great Lakes research》1985,11(3):231-246
Organochlorine analysis was performed on carcasses of 13 diving ducks from a 1981 wintering population that foraged on contaminated sediments in the lower Detroit River. Mean total PCB concentrations were 10 mg/kg for seven lesser scaups (Aythya affinis), 11 mg/kg for three greater scaups (A. marila), and 7.6 mg/kg for three goldeneyes (Bucephala clangula). Highest mean levels of other residues were measured for hexachlorobenzene (1.7 mg/kg) in goldeneyes, and transnonachlor (0.33 mg/kg) and 4,4’-DDE (1.3 mg/kg) in greater scaup. Quantitative analysis of 72 PCB congeners also was applied to water, seston, sediment, benthie oligochaetes, and carp from the same site. Principal congeners in most of the samples included some of the more toxic and persistent PCBs. Results of a multivariate analysis indicated that ratios of more conservative to less conservative PCBs did not vary significantly among ducks (α = .05), but differed from those in carp, oligochaetes, and sediment. For the February through March period of fat mobilization, concentrations of total PCB in lipid were inversely correlated with percent lipid (r = 0.76) in ducks. The percentage of conservative PCBs increased slightly. The distribution and partitioning of organochlorines, including toxic PCB congeners, varied considerably within this water column-sediment-fauna ecosystem. Estimates of toxic exposure based on total PCB values may be unreliable. 相似文献