首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   77571篇
  免费   754篇
  国内免费   691篇
电工技术   1456篇
综合类   47篇
化学工业   11749篇
金属工艺   3471篇
机械仪表   2687篇
建筑科学   1779篇
矿业工程   528篇
能源动力   1900篇
轻工业   6635篇
水利工程   863篇
石油天然气   2310篇
武器工业   41篇
无线电   7515篇
一般工业技术   16762篇
冶金工业   13111篇
原子能技术   2648篇
自动化技术   5514篇
  2022年   593篇
  2021年   937篇
  2020年   721篇
  2019年   802篇
  2018年   1580篇
  2017年   1584篇
  2016年   1781篇
  2015年   979篇
  2014年   1587篇
  2013年   3704篇
  2012年   2426篇
  2011年   2910篇
  2010年   2462篇
  2009年   2748篇
  2008年   2711篇
  2007年   2673篇
  2006年   2283篇
  2005年   1999篇
  2004年   1912篇
  2003年   1856篇
  2002年   1774篇
  2001年   1740篇
  2000年   1726篇
  1999年   1628篇
  1998年   3814篇
  1997年   2716篇
  1996年   2007篇
  1995年   1571篇
  1994年   1372篇
  1993年   1403篇
  1992年   1042篇
  1991年   1097篇
  1990年   1114篇
  1989年   1091篇
  1988年   977篇
  1987年   919篇
  1986年   919篇
  1985年   965篇
  1984年   809篇
  1983年   800篇
  1982年   700篇
  1981年   731篇
  1980年   692篇
  1979年   678篇
  1978年   647篇
  1977年   815篇
  1976年   976篇
  1975年   613篇
  1974年   568篇
  1973年   569篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
Previously unknown compounds AI I(BUO5)2·nH2O (AI I = Mg, Ca, Sr, Ba; n = 0-7) were synthesized. Their structure and thermal decomposition were studied by X-ray diffraction, IR spectroscopy, and thermal analysis.  相似文献   
22.
On the synergy between electrical and photonic switching   总被引:1,自引:0,他引:1  
This article focuses on the values of electrical switching vs. photonic switching in the context of telecom transport networks. In particular, we show that the requirement of providing agility at the optical layer in the face of traffic forecast uncertainties is served better through photonic switching. On the other hand, some of the network-level functions, such as fast protection, subwavelength aggregation, and flexible client connectivity, require electrical switching. We further argue that additional values are achieved with hybrid photonic and electrical switching, which do not exist when either of these options is used in isolation.  相似文献   
23.
The mass and charge identification of secondary particles with Z < 4 by a large CsI(T1) scintillation detector is performed using pulse shape analysis and time-of-flight methods. The dependence of the light output on E, A and Z is studied in the energy range of 1–20 MeV/A and special attention is paid to the integration time of the photomultiplier anode signal. It is found that the behaviour of the calibration curves strongly depends on the choice of the integration time interval.  相似文献   
24.
Serum vanadium, aluminum, silicon and beta 2-microglobulin levels as well as the red cell count, hemoglobin and systolic blood pressure were simultaneously measured in 80 chronic hemodialysis patients. The serum vanadium level was positively correlated with the serum levels of aluminum, silicon and beta 2-microglobulin as well as the systolic blood pressure, and was inversely correlated with the red cell count and hemoglobin. The mean serum vanadium level was 18.4 +/- 7.6 ng/ml before hemodialysis and decreased to 13.0 +/- 5.30 ng/ml at the completion of dialysis. The dialysate vanadium level increased from 0.4 +/- 0.2 (inflow) to 1.0 +/- 0.4 ng/ml (outflow). It was concluded that vanadium was transferred from blood to dialysate when purified water was used in the preparation of the dialysate.  相似文献   
25.
The immunological agglutination reactions of physically absorbed F(ab')2 molecules onto anionic and cationic latex particles have been investigated by means of optical absorbance measurements. These measurements have been conducted under different conditions to determine the most influential factors. Surface F(ab')2 and BSA densities, particle concentration in the reaction medium and polyethylene glycol concentration are some of these factors. Sensitized cationic and anionic latexes differ considerably with respect to their colloidal stability and reactivity. As a general rule, the sensitized cationic latex has a relatively higher colloidal stability and hence, it provides reagents with a better optical response. Less than 0.025 microgram/ml of C-reactive protein has been detected using this particle enhanced optical immunoassay.  相似文献   
26.
Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 11, pp. 17–18, 23, November, 1993.  相似文献   
27.
European remote sensing (ERS) satellites synthetic aperture radar (SAR) wind retrievals using CMOD-IFR2 are, for the first time, retrieved in the marginal ice zone (MIZ) and in Arctic coastal areas and compared with in situ observations from reseach vessels (RVs) and output from a high-resolution atmospheric model. The root mean squares (rms) of the comparisons were 1.6 m s/sup -1/ and 2 m s/sup -1/, respectively. The spatial variation of the SAR wind fields established a decrease in wind speed close to the ice edge for the late summer situations where the wind was along the ice edge with the ice to the left. This decrease is believed to be due to changes in atmospheric stability, possibly through development of an internal boundary layer caused by the cold ice cover and melt water. Lower wind speed near the ice edge is confirmed by the atmospheric model and the in situ observations. Furthermore, good results are obtained from SAR wind retrieval in leads when compared with model output during a cold-air outbreak. Routine measurements in the MIZ are useful for estimating the wind stress, and therefore SAR may play an important role in this region. Finally, the identification of a jet out from Hinlopen Strait in the Svalbard region and low wind wakes along the coast in the SAR-retrieved wind field is confirmed by in situ observations as the RV moves through the region. The jet is also confirmed by the atmospheric model, which is able to reproduce the situation.  相似文献   
28.
Results are reported of a detailed optical and electron metallographic study of the effects of variations in quenching rate from the β-phase and aging temperature in the + ZrCr2 phase region, on the recrystallization and grain growth characteristics of Zr-1.14 wt% Cr-0.08 wt% Fe alloy used for nuclear fuel sheathing. It is shown that certain quenching rates, which produce a mixed martensitic-Widmanstätten structure, should be avoided during the fabrication of fuel sheathing, since recrystallization and grain growth occurs at a much lower temperature than a material quenched at such a rate that the structure is wholly Widmanstätten or wholly martensitic. Preaging at 350 or 450°C prior to the high-temperature (700–750°C) aging reduces the incidence of recrystallization and grain growth.  相似文献   
29.
The energy expenditure and heart rate of five Shetland ponies were measured during transport and compared with the values recorded while they were at rest and walking. Secondary aims of the study were to investigate whether there was any correlation between heart rate and energy expenditure and whether different orientations affected the values to different degrees. The measurements were recorded at one-minute intervals while the ponies were at rest, walking and being transported in four different orientations, on journeys lasting 30 minutes. There were significant increases (P < 0.001) in both heart rate and energy expenditure during transport which were similar to those associated with walking, and there was a strong correlation between the two measurements. There was no conclusive evidence that orientation affected the measurements, but it appeared that rear facing transport might have had the least effect.  相似文献   
30.
The aim of this study was to investigate the effect of the absence of elongate spermatids (ES) from the rat seminiferous epithelium on the quantitative secretion and synthesis of the three major Sertoli cell secretory proteins--SGP-1, SGP-2 and CP-2. Seminiferous tubules (ST) were isolated (a) from normal 28-day-old rats, in which the most mature germ cell type is the round spermatid, (b) from normal adult rats at stages IX-XIV of the spermatogenic cycle, i.e. after spermiation, or at stages I-V and VI-VIII, when ES are still attached to the Sertoli cell, and (c) at stages VI-VIII from normal adult rats and from rats treated with methoxyacetic acid (MAA) in order to specifically deplete ES at these stages. Two-dimensional SDS PAGE combined with computerized image analysis was used to analyse 35S-methionine-labelled intracellular and secreted proteins. In the case of SGP-1 and SGP-2, almost all of the protein synthesized by ST was secreted. The total amount of both SGP-1 and CP-2 secreted by unstaged ST from immature rats was significantly lower than that secreted by unstaged ST from adult rats. The total amount of SGP-1 and CP-2 secreted by adult ST at stages IX-XIV of the spermatogenic cycle also declined dramatically compared to ST at earlier stages. The proportion of the total CP-2 synthesized by ST which was secreted also declined in all situations in which ES were absent from the seminiferous epithelium. The synthesis of only SGP-2 was changed by ES depletion from ST at stages VI-VIII, which was almost doubled compared to synthesis of this protein by ST from control rats. Our results suggest strongly that the secretion of SGP-1 and SGP-2 is via the constitutive pathway, and that regulation of these two proteins by ES is at the level of protein synthesis. In contrast, the regulation of CP-2 by ES is predominantly at the level of secretion, suggesting that this protein is secreted via a regulated pathway. Our findings add to the evidence showing that ES play a major role in the regulation of Sertoli cell function.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号