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   7411篇
  免费   52篇
  国内免费   1篇
电工技术   84篇
综合类   2篇
化学工业   414篇
金属工艺   20篇
机械仪表   37篇
建筑科学   45篇
矿业工程   2篇
能源动力   25篇
轻工业   109篇
水利工程   9篇
石油天然气   6篇
无线电   327篇
一般工业技术   391篇
冶金工业   5400篇
原子能技术   29篇
自动化技术   564篇
  2021年   26篇
  2020年   18篇
  2019年   26篇
  2018年   26篇
  2017年   31篇
  2016年   20篇
  2015年   26篇
  2014年   23篇
  2013年   134篇
  2012年   64篇
  2011年   78篇
  2010年   85篇
  2009年   85篇
  2008年   94篇
  2007年   71篇
  2006年   80篇
  2005年   70篇
  2004年   48篇
  2003年   81篇
  2002年   76篇
  2001年   67篇
  2000年   55篇
  1999年   232篇
  1998年   1668篇
  1997年   932篇
  1996年   588篇
  1995年   392篇
  1994年   321篇
  1993年   377篇
  1992年   99篇
  1991年   101篇
  1990年   117篇
  1989年   92篇
  1988年   89篇
  1987年   89篇
  1986年   87篇
  1985年   89篇
  1984年   40篇
  1983年   56篇
  1982年   56篇
  1981年   63篇
  1980年   55篇
  1979年   28篇
  1978年   42篇
  1977年   121篇
  1976年   215篇
  1975年   27篇
  1974年   30篇
  1972年   20篇
  1970年   17篇
排序方式: 共有7464条查询结果,搜索用时 12 毫秒
101.
102.
103.
104.
We have made reasonably comprehensive measurements of action potential activity in the Aplysia californica abdominal ganglion to determine the amount of feedback the central nervous system (CNS) receives from a movement which it initiates. Voltage-sensitive dye measurements of action potential activity of cells in the ganglion were made during the gill-withdrawal reflex elicited by siphon stimulation. We compared recordings in two situations which differed dramatically in the amount the gill moved. In the control sea water, the gill withdrawal was normal; in low-Ca2+, high-Mg2+ sea water, the gill movement was blocked. Both the timing and the number of spikes of the individual neurons were similar in the two situations. Histograms of the summed spike activity versus time and histograms of the number of active neurons versus time in the two conditions were also similar. Finally, two numerical measures of trial-to-trial differences, a paired t-test and a measure we named fractional similarity, did not indicate larger differences between two trials in the different sea waters than two trials in the same sea water. Feedback from sensory neurons activated by the gill movement itself does not make a large contribution to the spike activity in the abdominal ganglion. Apparently the Aplysia CNS issues the command for the withdrawal and does not make adjustments for the magnitude of the actual withdrawal. It may not even receive the information necessary for such adjustments to be made. A second motivation for these experiments was to test whether removing the feedback might simplify the neuronal activity that occurs during the gill-withdrawal reflex. This did not occur.  相似文献   
105.
106.
American Academy of Pediatrics (AAP) fellows responded to a questionnaire measuring knowledge of 1991 Centers for Disease Control and Prevention (CDC) guidelines. Generalists' scores on a knowledge scale were positively correlated with reporting CDC and AAP documents as knowledge sources, and negatively correlated with private practice, medical school as a knowledge source, and age. However, private practitioners who read AAP documents scored well.  相似文献   
107.
108.
109.
Real-time ultrasonic measurements of 10th-rib backfat (BF10) and loin muscle area (LMA) were made by a single technician at four mean BW (67.4, 80.3, 93.4, and 104.9 kg) on live hogs to assess the accuracy of predicting carcass measurements before and at slaughter weight. Records were evaluated on 655 purebred barrows and 472 purebred gilts in two tests. Residual correlations-accounting for test, sex, and breed effects, among and between scans and carcass measurements--were moderate to high for BF10 (r=.69 to .82) and LMA (r=.57 to .68), with the largest correlations at 104.9 kg of live weight. Ultrasonic BF10 and LMA were within +/-4 mm and +/-6.45 cm2, respectively, of the corresponding carcass measurement 75.9 and 89.8% of the time. Sex differences for LMA bias were significant (P < .001); ultrasonic LMA was overestimated in barrows by .75 cm2 and underestimated in gilts by .91 cm2. Breed differences were significant (P < .001) for BF10 and LMA bias. Standard errors of prediction (SEP) for BF10 and LMA across the two tests were 3.46 mm and 4.04 cm2, respectively. The SEP for BF10 were 3.60 mm for barrows and 3.19 mm for gilts. The SEP for LMA were 3.77 cm2 for barrows and 4.22 cm2 for gilts. The SEP for BF10 within breeds ranged from 3.25 to 3.72 mm, and for LMA, ranged from 2.98 cm2 to 4.90 cm2. Ultrasound measurements overestimated the carcass measurement by .57 mm for carcasses measuring < 24.1 mm and underestimated by 2.81 mm carcasses with BF10 > 30.3 mm. Ultrasonic LMA overestimated the carcass by 2.35 cm2 in carcasses measuring < 32.5 cm2 and underestimated by 2.29 cm2 in carcasses measuring greater than 37.9 cm2. Results indicate that the magnitude of the carcass measurement affects bias and accuracy of prediction for real-time ultrasonic measurements of BF10 and LMA. The SEP statistic is more consistent in evaluating accuracy of ultrasonic measurement than bias, absolute deviations, and percentage of absolute deviation.  相似文献   
110.
The mechanism of decline in viremia following acute infection with HIV is unknown. To characterize this process virologically, the expression of viral RNAs was analyzed in samples of peripheral blood mononuclear cells (PBMCs) from a patient who experienced a 100-fold decline in plasma viremia over a 13-day period prior to the initiation of antiretroviral therapy. Cell-associated viral RNA declined in association with the decline in plasma virus. During the initial 7 days of observation, plasma viremia declined more than 10-fold with no change in the ratio of unspliced to multiply spliced mRNAs. The efficiency of viral gene expression did not decline during the study period and varied from 380 to 2800 unspliced RNA copies per productively infected cell. Together, these data indicate no change in the relative proportion of cells in late- and early-stage gene expression during the initial decline and provide evidence against shortening of the viral replication cycle by immune surveillance. However, the prevalence of productively infected cells declined markedly during the 13 days of observation, from 1 in 250 to 1 in 25,000 PBMCs. These data are compatible with depletion of available target cells during the initial decline in viremia. As the level of plasma virus stabilized after 8 days of observation, the ratio of unspliced to multiply spliced mRNAs rose; this rise was due to a relatively greater decline in multiply spliced mRNA. These data suggest the possible onset of a blockade to new infection events (for example, by neutralizing antibody or chemokines), causing an increase in the relative proportion of cells in late-stage gene expression. They may also be explained, however, by the persistence of cell-associated virions together with the near disappearance of productively infected cells from the circulation.  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