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   1637篇
  免费   1篇
电工技术   11篇
化学工业   20篇
金属工艺   4篇
机械仪表   5篇
建筑科学   6篇
矿业工程   1篇
能源动力   12篇
轻工业   11篇
石油天然气   1篇
无线电   101篇
一般工业技术   106篇
冶金工业   1301篇
原子能技术   3篇
自动化技术   56篇
  2022年   3篇
  2016年   6篇
  2014年   3篇
  2013年   6篇
  2012年   4篇
  2011年   15篇
  2010年   5篇
  2009年   8篇
  2008年   11篇
  2007年   18篇
  2006年   22篇
  2005年   14篇
  2004年   14篇
  2003年   16篇
  2002年   25篇
  2001年   10篇
  2000年   14篇
  1999年   41篇
  1998年   347篇
  1997年   212篇
  1996年   142篇
  1995年   74篇
  1994年   77篇
  1993年   86篇
  1992年   25篇
  1991年   23篇
  1990年   16篇
  1989年   32篇
  1988年   31篇
  1987年   24篇
  1986年   28篇
  1985年   30篇
  1984年   6篇
  1983年   7篇
  1982年   8篇
  1981年   10篇
  1980年   19篇
  1979年   5篇
  1978年   7篇
  1977年   45篇
  1976年   97篇
  1975年   8篇
  1974年   4篇
  1973年   7篇
  1971年   3篇
  1969年   5篇
  1968年   3篇
  1964年   2篇
  1959年   2篇
  1955年   3篇
排序方式: 共有1638条查询结果,搜索用时 15 毫秒
61.
PURPOSE: Children who have brain tumors are at risk for a variety of treatment-related sequelae, including neuropsychological and cognitive impairment, neurologic deficits, and neuroendocrinologic disturbances. We sought to determine the value of proton MR spectroscopy in assessing brain tissue remote from the tumor site to ascertain the effects of chemotherapy and radiation treatment in these patients. METHODS: Single-voxel proton MR spectra from 70 patients (111 spectra) and 11 healthy volunteers (11 spectra) were analyzed. NAA/Cr, NAA/Cho, and Cho/Cr ratios based on peak areas were obtained from nonneoplastic regions of the frontal lobe. The relationship between MR spectroscopic ratios and treatment was determined. RESULTS: NAA-containing ratios were decreased in patients as compared with control subjects. The presence of gadolinium-based contrast material did not cause significant changes in the ratios as compared with precontrast data. When chemotherapy was a component of a child's treatment protocol, we found a significant decline in NAA/Cr ratios. Patients who underwent both chemotherapy and radiation therapy showed a trend toward lower NAA-containing ratios if the chemotherapy was administered before the radiation therapy. Patients receiving whole-brain radiation had a trend toward lower NAA-containing ratios than did those who had only focal tumor treatment. CONCLUSION: In children with brain tumors, MR spectroscopy of brain tissue remote from the tumor reveals treatment-related biochemical changes.  相似文献   
62.
Enterohemorrhagic Escherichia coli (EHEC) strains require intimin to induce attaching and effacing (A/E) lesions in newborn piglets. Infection of newborn calves with intimin-positive or intimin-negative EHEC O157:H7 demonstrated that intimin is needed for colonization, A/E lesions, and disease in cattle. These results suggest that experiments to determine if intimin-based vaccines reduce O157:H7 levels in cattle are warranted.  相似文献   
63.
64.
Cyclosporine (CsA) impairs renal function, probably by preglomerular vasoconstriction. Vasodilating substances may therefore be of benefit to ameliorate CsA-induced renal dysfunction. We studied the acute effects on blood pressure and renal function of the dihydropyridine calcium antagonist nifedipine (10 mg orally) in 20 CsA-treated renal transplant patients. In addition, we compared the effects of nifedipine when given immediately before and 4 weeks after elective conversion from CsA to azathioprine. Compared with placebo (n = 14), administration of nifedipine led to a significant decrease in blood pressure and a strong natriuretic and diuretic response. Despite the reduction in blood pressure, glomerular filtration rate improved from 60 +/- 20 (mean +/- SD) to 69 +/- 24 mL/min/1.73 m2 (P < 0.001) and renal plasma flow (RPF) increased from 260 +/- 87 to 338 +/- 120 mL/min/1.73 m2 (P < 0.001). The combination of a decreased blood pressure with an increased RPF was reflected in a sharp decrease in renal vascular resistance (0.34 +/- 0.18 units v 0.23 +/- 0.10 units; P < 0.001). The conversion from CsA to azathioprine by itself led to significant increases in glomerular filtration rate (62 +/- 15 mL/min/1.73 m2 v 76 +/- 18 mL/min/1.73 m2; P < 0.05) and RPF (280 +/- 86 mL/min/1.73 m2 v 334 +/- 66 mL/min/1.73 m2; P < 0.05). During treatment with azathioprine an effect of nifedipine on glomerular filtration rate and RPF was no longer observed, although the natriuretic effect was similar on both occasions. The decrease in renal vascular resistance was larger during treatment with CsA than during treatment with azathioprine (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
65.
To determine the effect of life-long alcohol consumption on the adult and aged rat model, 4-week-old, female Sprague-Dawley rats were divided into three diet groups. Alcohol-treated animals were fed a modified Lieber-DeCarli diet ad libitum containing 35% ethanol-derived calories, whereas the pair-fed animals (weight-matched to ethanol rats) received an isocaloric liquid diet in which maltose-dextrin substituted calories supplied by ethanol. Chow animals were fed a standard rat chow ad libitum. Proximal tibiae (primarily cancellous bone) and femora (primarily cortical bone) were removed for analysis after 3, 6, 9, 12, or 18 months on the diets. Serum was collected for analysis of calcium levels, the calcium regulating hormones; parathyroid hormone, 25-hydroxyvitamin D, calcitonin, corticosterone, estradiol, testosterone, and IGF-1. Creatinine, SGOT/AST, and SGPT/ALT levels were measured to determine kidney and liver integrity. Previous studies, with young animals, showed that chronic alcohol consumption during the age of bone development reduced bone density and bone mass in both cortical and cancellous bone. The present study demonstrates that these reductions last throughout life, whereas morphological values, such as length and diameter, attain control levels. Calcium regulating hormones and sex hormones are essentially normal and do not appear to be the primary causative agent for adult alcohol-induced osteopenia, but it appears to be due to a more direct effect of alcohol on bone cells.  相似文献   
66.
A receiver targeting OC-48 (2.488 Gb/s) serial data link has been designed and integrated in a 0.8-μm CMOS process. An experimental receiving front-end circuit demonstrates the viability of using multiple phased clocks to overcome the intrinsic gate-speed limitations in the demultiplexing (receiving) and multiplexing (transmitting) of serial data. To perform clock recovery, data is 3× oversampled so that transitions can be detected to determine bit boundaries. The design of a transmitter for the high-speed serial data is also described. The complete transceiver occupies a die area of ~3×3 mm2  相似文献   
67.
68.
69.
The mixture of conjugated diene hyperperoxide isomers obtained from autoxidation of methyl linoleate was separated by high performance liquid chromatography (HPLC). Four major isomers were obtained from adsorption chromatography and identified as the 9 and 13 positional isomers having the trans-trans and cis-trans configurations. The latter geometrical isomers have the trans double bond adjacent to the hydroperoxide group. The hydroxy compounds (methyl hydroxylinoleates) obtained from the hydroperoxides by NaBH4 reduction were similarly separated but with improved resolution. This is the first instance of the complete separation of these compounds and provides a rapid method for their analysis. Unlike adsorption chromatography, reversed-phase chromatography separates the mixtures only according to the geometrical isomerism of the double bonds and not according to the position of the hydroxy or hydroperoxide function.  相似文献   
70.
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