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   23676篇
  免费   1252篇
  国内免费   10篇
电工技术   112篇
综合类   13篇
化学工业   3876篇
金属工艺   351篇
机械仪表   348篇
建筑科学   536篇
矿业工程   40篇
能源动力   386篇
轻工业   3704篇
水利工程   222篇
石油天然气   81篇
武器工业   2篇
无线电   827篇
一般工业技术   3060篇
冶金工业   8856篇
原子能技术   89篇
自动化技术   2435篇
  2024年   32篇
  2023年   146篇
  2022年   190篇
  2021年   455篇
  2020年   417篇
  2019年   471篇
  2018年   782篇
  2017年   739篇
  2016年   750篇
  2015年   596篇
  2014年   776篇
  2013年   1466篇
  2012年   1174篇
  2011年   1046篇
  2010年   906篇
  2009年   834篇
  2008年   821篇
  2007年   764篇
  2006年   521篇
  2005年   410篇
  2004年   437篇
  2003年   392篇
  2002年   340篇
  2001年   255篇
  2000年   225篇
  1999年   414篇
  1998年   2868篇
  1997年   1803篇
  1996年   1089篇
  1995年   579篇
  1994年   504篇
  1993年   568篇
  1992年   126篇
  1991年   135篇
  1990年   151篇
  1989年   148篇
  1988年   153篇
  1987年   126篇
  1986年   113篇
  1985年   117篇
  1984年   49篇
  1983年   49篇
  1982年   66篇
  1981年   94篇
  1980年   97篇
  1979年   27篇
  1978年   34篇
  1977年   190篇
  1976年   394篇
  1975年   20篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
62.
Calcium-phospholipid-dependent protein kinase (PKC) has long been suggested to play an important role in modulating synaptic efficacy. We have created a strain of mice that lacks the gamma subtype of PKC to evaluate the significance of this brain-specific PKC isozyme in synaptic plasticity. Mutant mice are viable, develop normally, and have synaptic transmission that is indistinguishable from wild-type mice. Long-term potentiation (LTP), however, is greatly diminished in mutant animals, while two other forms of synaptic plasticity, long-term depression and paired-pulse facilitation, are normal. Surprisingly, when tetanus to evoke LTP was preceded by a low frequency stimulation, mutant animals displayed apparently normal LTP. We propose that PKC gamma is not part of the molecular machinery that produces LTP but is a key regulatory component.  相似文献   
63.
Papaya proteinase omega (pp omega) has been purified from dried latex both by immunoaffinity and traditional methods. Kinetic analysis revealed that (1), the pp omega-catalysed hydrolysis of N-benzoyl-L-arginine p-nitroanilide (BApNA) has a lower specificity (kcat/Km) than the same reaction catalysed by papain; (2), the pp omega-catalysed hydrolysis of a tripeptide substrate having phenylalanine at the second position (S2-site) showed a more similar specificity to that catalysed by papain; (3), the significant difference between the two enzymes is that steady state kinetics with both L-BApNA and a tripeptide enables the identification in pp omega of other ionizations affecting binding. The active sites of papain and pp omega can therefore be distinguished by pH-dependence of kcat/Km.  相似文献   
64.
Indices of fluid balance were evaluated during and after aortic surgery in 16 consecutive patients. Thoracic electrical impedance (TI), heart rate (HR), central venous (CVP), pulmonary artery mean (PAMP), pulmonary wedge (PWP) and mean arterial (MAP) pressures as well as fourteen arterial and venous blood gas variables were followed. Consistent with a reduction of TI during the operation, fluid balance was in excess, and it remained elevated on the first postoperative morning. The HR, MAP and PWP remained stable, while CVP and PAMP decreased. Of the determined variables only TI revealed a meaningful correlation to fluid balance (rho = -0.41; p < 0.01). The results indicate that while central venous and pulmonary artery mean pressures gave the impression of a volume deficit, the positive fluid balance was mirrored by thoracic electrical impedance.  相似文献   
65.
We describe a case of a hyperplastic anterior choroidal artery (AChA) with double persistent anastomotic channels. This hyperplastic AChA, which was detected at vertebral angiography, is different from previously reported cases, because it has an unusual, secondary anastomotic channel.  相似文献   
66.
67.
AIMS: This series analyses the results of conservative surgery for large lower pole breast cancers by lumpectomy associated with a bilateral remodelling mammoplasty, in order to avoid residual deformities. METHODS: This retrospective study concerns 50 patients with a lower pole breast cancer treated between 1986 and 1996 by lumpectomy, mammoplasty and irradiation. The contralateral breast was immediately made symmetrical in all cases. The mean tumour size was 32.5 mm. RESULTS: The mean weight of the lumpectomy specimen was 270 g. Resection margins were tumour-free in 90% of cases. The main complication observed was delayed healing, thus postponing post-operative treatment in 6.5% of cases. The median follow-up was 48 months. The 5-year actuarial ipsilateral local recurrence rate was 7% and 5-year actuarial metastasis-free and overall survival rates were 81 and 97%, respectively. Cosmesis was satisfactory in 85% of patients. We observed better results when radiotherapy was performed after rather than prior to surgery (92 vs. 67%: NS). CONCLUSIONS: Performing a bilateral mammoplasty at the time of initial surgery for large breast cancers situated in the lower quadrants of the breast facilitates larger lumpectomies with good cosmetic results.  相似文献   
68.
On the basis of observations that endemic fluorosis occurs more often in malnourished populations, a series of studies tested the hypothesis that deficient dietary intake of calcium, protein or energy affects fluoride metabolism so that the margin of safe fluoride exposure may be reduced. The objective of the investigation was to determine whether changes in fluoride metabolism in nutritionally deficient rats resulted in manifestation of any extraskeletal toxic fluoride effects not observed in healthy animals. This investigation included two studies, one that monitored the effect of calcium deficiency on the effects of chronic fluoride exposure, and a second study that observed fluoride effects in rats that were deficient either in protein or in energy and total nutrient intake. Control and experimental rats received drinking water containing 0, 0.26 (5), 0.79 (15) or 2.63 (50) mmol fluoride/L (mg/L) for 16 or 48 wk. Control rats were fed optimal diets and experimental rats were fed diets deficient in calcium (Study 1) or protein (Study 2). An additional group of experimental rats (Study 2) was provided with a restricted amount of diet; thus these rats were deficient in energy and total nutrient intake. The intake, excretion and retention of fluoride were monitored; after the rats were killed, tissue fluoride levels and biochemical markers of tissue function were analyzed. Bone marrow cells were harvested from some of the rats, after 48 wk of treatment, for determining the frequency of sister chromatid exchange, a marker of genetic damage. Although there were significant differences among fluoride treatment groups in fluoride excretion and retention that resulted in significantly greater fluoride levels in tissues of the experimental rats, we were unable to detect any harmful, extraskeletal biochemical, physiologic or genetic effects of fluoride in the nutritionally deficient rats.  相似文献   
69.
Several in vivo and in vitro methods for monitoring immunological properties of two allergoids obtained by formaldehyde treatment of ovalbumin (OA) were developed. The calculated molecular weight of allergoids was 80 kD (OA-F1) and 165 kD (OA-F2), respectively. The allergenic activity in vitro of allergoids in mast-cell histamine release assay was 1000 times lower than of OA. Both allergoids showed reduced ability to induce passive cutaneous anaphylaxis in the Sprague-Dawley rats or systemic anaphylaxis in Dunkin-Harley guinea-pigs. The ability of OA and allergoids to bind to the OA-specific IgE antibodies was measured in vivo by the inhibition of passive cutaneous anaphylaxis (PCA-inhibition). Allergoid binding to IgE was 51-66% lower than the native allergen. Moreover, the avidity of OA-specific IgG antibodies, measured by ELISA-inhibition, for allergoids and allergen was of the same order. Allergoids induced a different pattern of humoral immune response from that, induced by the native allergen. Thus, after immunization of BALB/c mouse, both allergoids induced a higher production of IgG and a lower production of IgE than OA, only OA-F2 induced a lower production of IgG1. The differences in the IgA response to the immunogens was not significant. Delayed hypersensitivity studies in the BALB/c mouse showed that allergoids were 5- to 12-times less effective in inducing a cell-mediated immune response than OA. The present study provides a battery of immunological methods for preclinical testing of modified allergens.  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