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   6234篇
  免费   5篇
  国内免费   1篇
电工技术   15篇
综合类   1篇
化学工业   67篇
金属工艺   26篇
机械仪表   16篇
建筑科学   16篇
矿业工程   2篇
能源动力   4篇
轻工业   26篇
水利工程   3篇
石油天然气   6篇
无线电   80篇
一般工业技术   84篇
冶金工业   5733篇
原子能技术   13篇
自动化技术   148篇
  2021年   6篇
  2018年   9篇
  2014年   7篇
  2013年   29篇
  2012年   8篇
  2011年   15篇
  2010年   23篇
  2009年   24篇
  2008年   15篇
  2007年   24篇
  2006年   11篇
  2005年   14篇
  2004年   23篇
  2003年   17篇
  2002年   9篇
  2001年   9篇
  2000年   11篇
  1999年   175篇
  1998年   1719篇
  1997年   938篇
  1996年   680篇
  1995年   346篇
  1994年   341篇
  1993年   360篇
  1992年   48篇
  1991年   62篇
  1990年   91篇
  1989年   82篇
  1988年   76篇
  1987年   96篇
  1986年   68篇
  1985年   67篇
  1984年   25篇
  1983年   23篇
  1982年   49篇
  1981年   52篇
  1980年   49篇
  1979年   8篇
  1978年   17篇
  1977年   158篇
  1976年   353篇
  1975年   20篇
  1974年   15篇
  1973年   10篇
  1972年   11篇
  1971年   6篇
  1970年   6篇
  1966年   5篇
  1965年   3篇
  1955年   5篇
排序方式: 共有6240条查询结果,搜索用时 13 毫秒
11.
12.
Flavobacterium aurantiacum NRRL B-184 possesses the ability to degrade aflatoxin B1 in solution and in several food items. Aflatoxin B1 is a potent carcinogen that causes significant economic losses to the agricultural and food industry. The role of trace metal ions (Cu2+, Mn2+, Zn2+, and Co2+) were studied in an effort to understand the enzymatic system involved in aflatoxin B1 degradation by F aurantiacum. The effect of divalent chelators (EDTA and 1,10-phenanthroline [OPT]) in the presence of the trace metal ions was studied as well. Aflatoxin B1 (10 microg/ml) was added to 72-h cultures of F aurantiacum that had been washed and resuspended in phosphate buffer (pH 7.0). HPLC was used to determine aflatoxin B1 concentration in these cultures. Incubating cells at 30 degrees C with 1 and 10 mM Cu2+, Mn2+, and Zn2+ significantly decreased aflatoxin B degradation after 4 and 24 h (P < 0.05). Decreased degradation was also observed with 1 and 10 mM Cu2+ and Zn2+ after 48 h and with 0.1 mM Cu2+ after 24 and 48 h. Co2+ did not have a significant effect on aflatoxin B1 degradation. EDTA and OPT did not counter the inhibition in the presence of Cu2+. The addition of 1 mM EDTA countered the inhibition by 1 mM Mn2+ after 4 and 24 h, but 1 mM OPT did not counter the inhibition by 10 mM Mn2+ after 4 and 24 h. OPT countered the inhibition by 1 mM Zn2+ after 4 and 48 h. These trace elements inhibit aflatoxin B1 degradation by F aurantiacum. In addition, their presence necessitates higher concentrations (>1 mM) of EDTA and OPT for the removal of their inhibitory effect.  相似文献   
13.
14.
The Galileo spacecraft has been orbiting Jupiter since 7 December 1995, and encounters one of the four galilean satellites-Io, Europa, Ganymede and Callisto-on each orbit. Initial results from the spacecraft's magnetometer have indicated that neither Europa nor Callisto have an appreciable internal magnetic field, in contrast to Ganymede and possibly Io. Here we report perturbations of the external magnetic fields (associated with Jupiter's inner magnetosphere) in the vicinity of both Europa and Callisto. We interpret these perturbations as arising from induced magnetic fields, generated by the moons in response to the periodically varying plasma environment. Electromagnetic induction requires eddy currents to flow within the moons, and our calculations show that the most probable explanation is that there are layers of significant electrical conductivity just beneath the surfaces of both moons. We argue that these conducting layers may best be explained by the presence of salty liquid-water oceans, for which there is already indirect geological evidence in the case of Europa.  相似文献   
15.
“Pure” poly(vinyl chloride) resin and four compositions containing poly(vinyl chloride) were subjected to oxidative thermal degradation in air at &400°C both in a quiescent and a flow system. The volatiles formed were identified and quantitatively determined on a gram-per-gram basis. Hydrogen chloride was the main product found. The nature and relative concentration of the produced organic chlorinated species appeared to be dependent not only on the poly(vinyl chloride) constituent but also on the other ingredients. All the compositions contained phthalate ester plasticizers. In the dynamic system, these distilled largely unchanged, whereas under static conditions transformation into phthalic anhydride occurred.  相似文献   
16.
Three-dimensional Fourier transform (3-DFT) magnetic resonance imaging (MRI) offers advantages in terms of signal-to-noise (S/N) per unit of time for the case where a large number of slices is desired. This advantage is enhanced when the relaxation time, T(1 ), is short. Because time limitations in 3-DFT imaging force the use of short time intervals, TR, between excitations of a slice, lesion contrast is often undesirable at mid-and high-field strength even when the S/N is good. At low fields, where T(1) values are short, high S/N and contrast can both be achieved with 3-DFT MR images. The conceptual and analytical aspects of low-field 3-DFT MRI are presented and demonstrated at 640 G.  相似文献   
17.
Previous research in this laboratory has shown that preweaning and postweaning juvenile meadow voles, Microtus pennsylvanicus, can acquire a spatial task, the Morris water-maze task. The present study examined the influence of age of juvenile acquisition ("before weaning" (BW; Day 10 and 15 after birth) and "after weaning" (AW; Day 20 and 25 after birth)) of a spatial task on subsequent re-acquisition of the same hidden-platform spatial water-maze task. This study also compared sex differences and litter sex-ratio effects on reacquisition performance. Fifteen litters of adults were re-tested in the same water maze 6 weeks after being initially tested as juveniles. All analyses were conducted using a covariate that removed the group differences in the original task performance. Adult voles from female-biased litters, that had previously learned the task at an older juvenile age (AW), reacquired the same task faster than adults that had previously learned the task at a younger juvenile age (BW). In the adult BW group there was also a significant litter sex-ratio effect such that voles born into a female-biased litter re-acquired the task more slowly than did voles born into a male-biased litter. There were no significant sex or litter sex-ratio effects on spatial learning in the AW group. These results show that adult meadow voles can require a spatial task more quickly if they initially learned the task at an older juvenile age, suggestive of a period of infantile amnesia. In addition, these results indicate that the litter sex-ratio can affect adult spatial performance, suggesting that the relative amount of androgens in utero may influence the development of sexually-dimorphic spatial ability in adulthood.  相似文献   
18.
The purpose of the present study was to evaluate the effect of barrier membrane exposure on the success of guided tissue regeneration in Class II furcation defects. Twenty-six subjects with mandibular Class II furcation defects received initial periodontal therapy followed by guided tissue regeneration surgery. The membrane was placed and the flaps were repositioned so that the membrane was totally submerged. Membranes were removed 4 to 6 weeks later, at which time the extent of their exposure was recorded. An overall improvement in all clinical parameters was observed for all subjects 1 year after surgery. Half of the patients had experienced no membrane exposure, while the other 13 subjects had experienced mild to pronounced exposure; both groups showed similar improvement in all clinical and surgical parameters. In light of the comparable results obtained in exposed sites, and the anatomic difficulties sometimes encountered in covering a membrane completely, in some of these cases the membrane may be left only partially submerged. This approach will allow for tighter occlusal "seal" of the tooth-membrane interface and preservation of the keratinized gingiva.  相似文献   
19.
The use of the Internet for conducting research has found that the level of anonymity intrinsic to web-based surveys is useful in deterring various participant biases. Though, concerns about Internet privacy could interfere with the effects of perceived anonymity and influence how a person responds. In this study, participants were asked to take an Internet survey and then were randomly assigned to conditions with varying degrees of implied privacy in which they answered questions about their desire for casual sex (sociosexuality) and their lifetime sexual partners. Only in the condition without any threat to the participant’s anonymity was there no difference between men and women’s reports of their sexual behavior. However, in every condition men reported having a significantly higher sociosexuality than women. Findings are consistent with both the biologic and gender-biased explanations in differences of self-report for sexual information. Implications for social constraints and Internet based research are discussed.  相似文献   
20.
Remote sensing of suspended sediments and shallow coastal waters   总被引:8,自引:0,他引:8  
Ocean color sensors were designed mainly for remote sensing of chlorophyll concentrations over the clear open oceanic areas (Case 1 water) using channels between 0.4-0.86 /spl mu/m. The Moderate Resolution Imaging Spectroradiometer (MODIS) launched on the National Aeronautics and Space Administration Terra and Aqua spacecrafts is equipped with narrow channels located within a wider wavelength range between 0.4-2.5 /spl mu/m for a variety of remote sensing applications. The wide spectral range can provide improved capabilities for remote sensing of the more complex and turbid coastal waters (Case 2 water) and for improved atmospheric corrections for ocean scenes. We describe an empirical algorithm that uses this wide spectral range to identify areas with suspended sediments in turbid waters and shallow waters with bottom reflections. The algorithm takes advantage of the strong water absorption at wavelengths longer than 1 /spl mu/m that does not allow illumination of sediments in the water or a shallow ocean floor. MODIS data acquired over the east coast of China, west coast of Africa, Arabian Sea, Mississippi Delta, and west coast of Florida are used.  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