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   2650篇
  免费   14篇
  国内免费   1篇
电工技术   13篇
综合类   1篇
化学工业   98篇
金属工艺   25篇
机械仪表   17篇
建筑科学   33篇
能源动力   17篇
轻工业   35篇
水利工程   2篇
石油天然气   1篇
无线电   45篇
一般工业技术   192篇
冶金工业   2121篇
原子能技术   8篇
自动化技术   57篇
  2021年   5篇
  2018年   8篇
  2017年   7篇
  2016年   6篇
  2015年   8篇
  2014年   10篇
  2013年   35篇
  2012年   12篇
  2011年   18篇
  2010年   18篇
  2009年   18篇
  2008年   18篇
  2007年   20篇
  2006年   12篇
  2005年   8篇
  2004年   19篇
  2003年   19篇
  2002年   17篇
  2001年   23篇
  2000年   14篇
  1999年   71篇
  1998年   709篇
  1997年   366篇
  1996年   246篇
  1995年   129篇
  1994年   138篇
  1993年   130篇
  1992年   29篇
  1991年   30篇
  1990年   38篇
  1989年   34篇
  1988年   39篇
  1987年   30篇
  1986年   29篇
  1985年   29篇
  1984年   12篇
  1983年   18篇
  1982年   19篇
  1981年   19篇
  1980年   25篇
  1979年   7篇
  1978年   10篇
  1977年   46篇
  1976年   104篇
  1975年   5篇
  1974年   4篇
  1973年   6篇
  1972年   5篇
  1971年   7篇
  1955年   4篇
排序方式: 共有2665条查询结果,搜索用时 15 毫秒
91.
In the present study, we evaluated changes in brain-derived neurotrophic factor (BDNF) immunoreactivity in the rat lumbar (L) 5 dorsal root ganglion (DRG) and areas where afferents from the DRG terminate, the L5 spinal cord and gracile nuclei, following unilateral sciatic nerve transection or crush. From 3 days to 4 weeks following cut or crush injury, the percentage of medium and large BDNF-immunoreactive neurons in the ipsilateral DRG increased significantly compared with those on the contralateral side. Following cut injury, there was no significant change in the percentage of small BDNF-immunoreactive neurons in the ipsilateral DRG; however, the intensity of immunoreactivity of these cells decreased. Following crush injury, however, both the percentage and intensity of small BDNF-immunoreactive neurons in the ipsilateral DRG significantly increased. Following cut injury, the expression of BDNF-immunoreactive axonal fibers decreased markedly in the ipsilateral superficial laminae of the L5 spinal cord and increased significantly in the ipsilateral deeper laminae of the spinal cord and gracile nuclei. Crush injury induced a marked increase in the expression of BDNF-immunoreactive axonal fibers in the superficial laminae of the spinal cord and gracile nuclei. These differences in BDNF response in the DRG and spinal cord after cut or crush injuries may reflect differences in trophic support to the injured DRG neurons and altered neuronal activity in the spinal cord and gracile nuclei following different types of peripheral nerve injury.  相似文献   
92.
AIMS: To assess the effects of a smoking cessation program for recovering alcoholics on use of alcohol, tobacco and illicit drugs after discharge from residential treatment. DESIGN AND SETTING: A randomized community intervention trial design was employed in which 12 residential drug treatment centers in Iowa, Kansas and Nebraska were matched and then randomly assigned to the intervention or control condition. PARTICIPANTS: Approximately 50 adult residents (inpatients) from each site were followed for 12 months after treatment discharge. INTERVENTION: Participating residents in the six intervention centers received a 4-part, individually tailored, smoking cessation program while those in the six control sites received usual care. FINDINGS: Both moderate and heavy drinking rates were reduced in the intervention group. Intervention site participants were significantly more likely than controls to report alcohol abstinence at both the 6-month (OR = 1.59, 95%CI: 1.09-2.35) and 12-month assessment (OR = 1.84, 95%CI: 1.28-2.92). Illicit drug use rates were comparable. Effect of the intervention on tobacco quit rates was not statistically significant. CONCLUSIONS: Counseling alcoholics in treatment to quit smoking does not jeopardize the alcohol recovery process. However, low-intensity tobacco interventions are unlikely to yield high tobacco quit rates.  相似文献   
93.
Gonadal cell types that derive from the coelomic epithelium (sex cords) or mesenchymal cells of the embryonic gonads include granulosa cells, theca cells, fibroblasts, Leydig cells, and Sertoli cells. Ovarian tumors of these cell types are called sex cord-stromal tumors. This group of tumors represents approximately 8% of ovarian neoplasms and affects all age groups. The more common types are granulosa cell tumors (GCTs), fibrothecomas, and Sertoli-Leydig cell tumors. Sex cord-stromal tumors are of interest partly because of their hormonal effects, which are rare for other ovarian neoplasms. These effects include estrogenic effects (pseudoprecocious puberty, endometrial bleeding, endometrial hyperplasia and carcinoma) and virilization. The variety of gross appearances of these tumors, ranging from large multicystic masses to small solid masses, would appear to preclude a specific radiologic diagnosis. However, in many patients, both clinical and radiologic clues can suggest the diagnosis, including predominantly fibrous content at ultrasound or magnetic resonance imaging (fibrothecoma), large hemorrhagic multicystic mass in a child with pseudoprecocious puberty (juvenile GCT), and associated syndromes such as Peutz-Jeghers syndrome (sex cord tumor with annular tubules) or Ollier disease and Maffucci syndrome (juvenile GCT).  相似文献   
94.
Alterations in triglyceride and cholesterol metabolism often accompany inflammatory diseases and infections. We studied the effects of endotoxin (lipopolysaccharide [LPS]) and cytokines on hepatic sphingolipid synthesis, activity of serine palmitoyltransferase (SPT), the first and rate-limiting enzyme in sphingolipid synthesis, and lipoprotein sphingolipid content in Syrian hamsters. Administration of LPS induced a 2-fold increase in hepatic SPT activity. The increase in activity first occurred at 16 hours, peaked at 24 hours, and was sustained for at least 48 hours. Low doses of LPS produced maximal increases in SPT activity, with half-maximal effect seen at approximately 0.3 microg LPS/100 g body weight. LPS increased hepatic SPT mRNA levels 2-fold, suggesting that the increase in SPT activity was due to an increase in SPT mRNA. LPS treatment also produced 75% and 2.5-fold increases in hepatic sphingomyelin and ceramide synthesis, respectively. Many of the metabolic effects of LPS are mediated by cytokines. Interleukin 1 (IL-1), but not tumor necrosis factor, increased both SPT activity and mRNA levels in the liver of intact animals, whereas both IL-1 and tumor necrosis factor increased SPT mRNA levels in HepG2 cells. IL- produced a 3-fold increase in SPT mRNA in HepG2 cells, and the half-maximal dose was 2 ng/mL. IL-1 also increased the secretion of sphingolipids into the medium. Analysis of serum lipoprotein fractions demonstrated that very low density lipoprotein, intermediate density lipoprotein, and low density lipoprotein isolated from animals treated with LPS contained significantly higher amounts of ceramide, glucosylceramide, and sphingomyelin. Taken together, these results indicate that LPS and cytokines stimulate hepatic sphingolipid synthesis, which results in an altered structure of circulating lipoproteins and may promote atherogenesis.  相似文献   
95.
We report a murine leukemia cell variant (L1210/DDP), selected for cisplatin (DDP) resistance, to be cross-resistant to methotrexate (MTX). Cross-resistance of L1210 cells to DDP and MTX has been observed by others, and has also been recorded in P388 murine leukemia and SSC-25 human squamous carcinoma cells. We demonstrated that MTX resistance is not due to dihydrofolate reductase (DHFR) gene amplification, increased DHFR enzyme activity or decreased MTX binding to the target enzyme. Of the mechanisms commonly proposed for MTX resistance, only differences in transport were observed when comparing sensitive (L1210/0) and resistant (L1210/DDP) cells. Our results suggest that MTX resistance in L1210/DDP cells is due to altered methotrexate uptake.  相似文献   
96.
A human cancer cell line was found to be heterogeneous for expression of the epidermal growth factor receptor (EGFR). Clones and variants of this cell line were separated on the basis of EGFR expression level, and those expressing high EGFR had different growth characteristics, in vitro and in vivo, than variants expressing low levels of EGFR. Karyotype analysis revealed that the heterogeneity was the result of mixing of two lines, the 2774 ovarian cancer cell line, and the SW620 colon cancer cell line. Our results reinforce the necessity for accurate identification of cell lines. Also, that measurement of gene expression on a single cell level, for example by flow cytometric analysis, can be more informative than measurements of cell lysates, since the initial indication of heterogeneity would not have been detected by northern or western blotting. The different cell types retained characteristic growth patterns when injected i.p. in nude mice, i.e. peritoneal carcinomatosis and ascites formation by the 2774 ovarian cancer cells, and liver metastasis and growth of discrete abdominal tumors by the SW620 colon cancer.  相似文献   
97.
The seven trans-membrane chemokine receptor CCR-5 is a coreceptor for macrophage tropic HIV-1 strains. CCR-5 responds to a number of chemokines, including macrophage inflammatory protein (MIP)-1 alpha. We describe the use of MIP-1 alpha in a biotin tyramine-mediated proximity selection to guide the selection of CCR-5-specific phage antibodies from a large phage display human library. Proximity based selections resulted in a population of antibodies recognizing CCR-5 on primary CD4+ lymphocytes, none of which blocked MIP-1 alpha binding to cells. The selected population of phage antibodies were subsequently used as guide molecules for a second phase of selection that was carried out in the absence of MIP-1 alpha. This generated a panel of CCR-5-binding antibodies, of which around 20% inhibited MIP-1 alpha binding to CD4+. The single chain Fvs (scFv) generated by this step-back selection procedure also inhibited MIP-1 alpha-mediated calcium signaling.  相似文献   
98.
99.
100.
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