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   6529篇
  免费   65篇
  国内免费   8篇
电工技术   274篇
综合类   4篇
化学工业   1120篇
金属工艺   195篇
机械仪表   114篇
建筑科学   102篇
矿业工程   1篇
能源动力   187篇
轻工业   461篇
水利工程   22篇
石油天然气   6篇
无线电   825篇
一般工业技术   1036篇
冶金工业   1741篇
原子能技术   175篇
自动化技术   339篇
  2024年   15篇
  2023年   23篇
  2022年   55篇
  2021年   97篇
  2020年   52篇
  2019年   59篇
  2018年   85篇
  2017年   58篇
  2016年   99篇
  2015年   63篇
  2014年   110篇
  2013年   214篇
  2012年   163篇
  2011年   214篇
  2010年   167篇
  2009年   179篇
  2008年   212篇
  2007年   207篇
  2006年   198篇
  2005年   172篇
  2004年   175篇
  2003年   178篇
  2002年   160篇
  2001年   154篇
  2000年   160篇
  1999年   193篇
  1998年   641篇
  1997年   410篇
  1996年   268篇
  1995年   210篇
  1994年   204篇
  1993年   188篇
  1992年   103篇
  1991年   93篇
  1990年   97篇
  1989年   96篇
  1988年   84篇
  1987年   76篇
  1986年   72篇
  1985年   79篇
  1984年   57篇
  1983年   55篇
  1982年   57篇
  1981年   57篇
  1980年   35篇
  1979年   43篇
  1978年   29篇
  1977年   45篇
  1976年   73篇
  1975年   14篇
排序方式: 共有6602条查询结果,搜索用时 0 毫秒
991.
The biochemical activities that underlie the genetically defined activator and repressor functions of the VIVIPAROUS1 (VP1) protein have resisted in vitro analysis. Here, we show that a glutathione S-transferase (GST) fusion protein, including only the highly conserved B3 domain of VP1, has a highly cooperative, sequence-specific DNA binding activity. GST fusion proteins that include larger regions of the VP1 protein have very low activity, indicating that removal of the flanking protein sequences is necessary to elicit DNA binding in vitro. DNA competition and DNase I footprinting analyses show that B3 binds specifically to the Sph element involved in VP1 activation of the C1 gene, whereas binding to the G-box-type VP1-responsive element is of low affinity and is nonspecific. Footprint analysis of the C1 promoter revealed that sequences flanking the core TCCATGCAT motif of Sph also contribute to the recognition of the Sph element in its native context. The salient features of the in vitro GST-B3 DNA interaction are in good agreement with the protein and DNA sequence requirements defined by the functional analyses of VP1 and VP1-responsive elements in maize cells.  相似文献   
992.
BACKGROUND AND AIMS: The aim of the present study was to compare power Doppler imaging with traditional color Doppler imaging and with contrast enhanced computer tomography in the evaluation of intratumoral vascularity of hepatocellular carcinomas at diagnosis and in response to percutaneous ethanol injection. PATIENTS AND METHODS: Sixteen patients with hepatocellular carcinoma underwent colour Doppler, power Doppler and computed tomography at diagnosis. Seventeen patients were studied by the three techniques one month after percutaneous ethanol injection treatment. RESULTS: At baseline evaluation, power Doppler and color Doppler were always in agreement and, with the exception of one case, were also in agreement with the computerized tomography scan. On the contrary, power Doppler and computerized tomography are more sensitive than color Doppler in the evaluation of residual vascularized tumoral tissue after percutaneous ethanol injection. In 3 patients, residual vascularity was demonstrated only by computerized tomography while color and power Doppler were negative. In another 3 cases, a positive power Doppler signal, with a typical arterial Doppler spectrum, was observed while color Doppler and computerized tomography were negative. In these patients, cancer relapse was clinically evident after a few months and treatment was repeated to obtain complete necrosis. CONCLUSIONS: We conclude that only the integration of the results of all these techniques can reliably evaluate tumoral vascularity after percutaneous ethanol injection.  相似文献   
993.
Hypopharyngeal cancer is one of the head and neck cancers with poor prognosis. This is due to the unpredictable extension of the local disease and the high ratio of distant metastasis. To verify the morphological behavior of hypopharyngeal cancer, whole-mount horizontal serial sections were developed using the larynges obtained from total pharyngolaryngoesophagotomy. The technique of processing high-resolution three-dimensional (3D) images of the larynges using Nikon Cosmozone 2SA software is presented. Two representative surgical specimens were used and 3D images reconstructed. In these two particular cases, the cancer infiltrated superiorly through submucosal lymphatic ducts and formed separate daughter nests under the intact mucosal epithelium far above the main tumor. 3D images enhanced the morphological features of these cases. These cases also suggested the difficulty of defining the upper resecting limit in the operation of hypopharyngeal cancer. 3D reconstruction is and will be a crucial modality for studying the morphological behavior of hypopharyngeal cancer.  相似文献   
994.
Hierarchical organization of cognitive memory   总被引:1,自引:0,他引:1  
This paper addresses the question of the organization of memory processes within the medial temporal lobe. Evidence obtained in patients with late-onset amnesia resulting from medial temporal pathology has given rise to two opposing interpretations of the effects of such damage on long-term cognitive memory. One view is that cognitive memory, including memory for both facts and events, is served in a unitary manner by the hippocampus and its surrounding cortices; the other is that the basic function affected in amnesia is event memory, the memory for factual material often showing substantial preservation. Recent findings in patients with amnesia resulting from relatively selective hippocampal damage sustained early in life suggest a possible reconciliation of the two views. The new findings suggest that the hippocampus may be especially important for event as opposed to fact memory, with the surrounding cortical areas contributing to both. Evidence from neuroanatomical and neurobehavioural studies in monkeys is presented in support of this proposal.  相似文献   
995.
BACKGROUND: Cardiopulmonary bypass reduces platelet number and function, increases postoperative bleeding time, and is the major, unsolved cause of nonsurgical bleeding after open heart operations. Temporary inhibition of platelet function during cardiopulmonary bypass (platelet anesthesia) protects platelets and reduces postoperative bleeding time and bleeding. METHODS: Integrilin, a short-acting, reversible platelet glycoprotein IIb/IIIa inhibitor was studied in 28 baboons that had 60 minutes of normothermic cardiopulmonary bypass using peripheral cannulas. A control group, two groups that received different doses of Integrilin, and a group that received a combination of Integrilin and low-dose Iloprost were studied. Blood samples for platelet count, aggregation to adenosine diphosphate, beta-thromboglobulin, prothrombin fragment F1.2, thrombin-antithrombin complex, and fibrinopeptide A were obtained at seven time points. Template bleeding times were measured before and at five intervals after cardiopulmonary bypass. RESULTS: Both doses of Integrilin and the combination of Integrilin and Iloprost significantly protected platelet number, inhibited the response to adenosine diphosphate, and reduced postoperative bleeding times, but they did not reduce beta-thromboglobulin release except in the high-dose Integrilin group. Thrombin formation and activity were qualitatively, but not significantly, reduced in all treatment groups. Bleeding times were not significantly different from baseline at the time protamine was given in the combination group and 60 minutes after protamine administration in all treatment groups. CONCLUSIONS: Integrilin alone or in combination with Iloprost significantly reduces platelet activation during cardiopulmonary bypass and produces normal or near-normal bleeding times at the time protamine is given.  相似文献   
996.
BACKGROUND & AIMS: Interleukin 12 (IL-12) is a heterodimeric, macrophage-derived cytokine that is elevated in Crohn's disease (CD). Epstein-Barr virus-induced gene 3 (EBI3) is a recently characterized human glycoprotein that is homologous to the 40-kilodalton chain of IL-12 and forms a heterodimer with the 35-kilodalton chain of IL-12. We investigated the expression of EBI3 in colonic mucosa of normal control subjects, patients with ulcerative colitis (UC), and patients with CD. METHODS: Colonic tissue was analyzed for messenger RNA (mRNA) expression by quantitative polymerase chain reaction and for protein expression by immunohistology and Western blotting. RESULTS: EBI3 mRNA was present in intestinal biopsy specimens from healthy subjects and patients with CD but was elevated only in active UC. EBI3 levels in UC specimens correlated with histological scores of activity and T-cell infiltration. EBI3-positive cells that had a shape consistent with that of macrophages were identified in the lamina propria, and protein was detected by Western blotting. CONCLUSIONS: EBI3 is a novel IL-12-related cytokine that is expressed by macrophage-like cells in normal intestine and CD and has enhanced expression in active UC but not in active CD.  相似文献   
997.
Hepatocyte growth factor (HGF) is most likely a physiological hepatotrophic factor that triggers regeneration of the injured liver. Histamine may also be important in the pathophysiology of the injured liver. Previously we showed that histamine production was increased in liver macrophages of mice injected with CCI4, a well-known hepatotoxin. Therefore, it is likely that the biological actions of histamine in repairing processes of the injured liver are mediated by HGF. This study was aimed at examining the effects of histamine on production of HGF using, as a model, the human promyelocytic leukemia cells, HL-60. 12-o-Tetradecanoylphorbol-13-acetate (TPA) markedly stimulated HGF production and release from the cells; the maximal amount of HGF was released at a concentration of 3 ng/ml of TPA. Histamine significantly stimulated the TPA-induced HGF production and release in these cells, depending on incubation time and its dose. These actions of histamine were abrogated by a H2 receptor antagonist, ranitidine.  相似文献   
998.
Recently, the marked decline in renal carnitine reabsorption has been thought to account fotr the systemic carnitine deficiency in juvenile visceral steatosis (JVS) mice. We have conducted a kinetic analysis using embryonic fibroblasts derived from normal, heterozygous, and homozygous jvs mice and found that the high-affinity carnitine transporter (Km = 5.5 microM), which shows Na+ and temperature dependency and stereospecificity, is defective in homozygous jvs mice. Moreover, a gene dose-dependent decrease of carnitine transport activity, which was due to a decrease in the number of the transporter molecules, was found in heterozygous jvs mice. Similar phenomena have been observed in human primary carnitine deficiency. Therefore, JVS mice may be useful for understanding this extremely rare human hereditary disorder.  相似文献   
999.
The purpose of this study was to investigate what viruses infiltrate into our nursing home, assess the related clinical symptoms in elderly subjects, and compare the incidence of infections with those in the general community. Between July 1994 and June 1995, 40 elderly persons dwelling in the nursing home were evaluated by clinical observation, serologic analysis, and viral culture of samples to determine the presence of viruses. Enteroviruses and herpes simplex I virus could be isolated from asymptomatic elderly subjects. Individuals with influenza B virus and RSV infections diagnosed by serology did have clinical symptoms. The prevalence of both influenza B virus and enteroviruses was related to that of such viruses in the general community. Our data suggest that respiratory viruses are readily transmitted from the community to nursing home residents.  相似文献   
1000.
Radiofrequency transitions within K = 2 asymmetry doublets have been observed for the CO2-CO van der Waals complex. A Stark effect measurement on the J = 2, K = 2 transition provides an electric dipole moment of μ = 0.2493(1) D. Combining this result with the permanent moment of CO, μCO = 0.1098 D, gives a change of moment on complex formation of Deltaμ = 0.140 D. The sign of Deltaμ is such that the CO end of the complex is more positive than CO2. The origin of Deltaμ should not be attributed to any single mechanism, and several different contributions to Deltaμ are discussed. Copyright 1998 Academic Press.  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