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   248575篇
  免费   1910篇
  国内免费   263篇
电工技术   4038篇
综合类   90篇
化学工业   40105篇
金属工艺   15149篇
机械仪表   10061篇
建筑科学   4175篇
矿业工程   3756篇
能源动力   3872篇
轻工业   9696篇
水利工程   4084篇
石油天然气   13006篇
武器工业   28篇
无线电   20184篇
一般工业技术   62780篇
冶金工业   32162篇
原子能技术   11138篇
自动化技术   16424篇
  2021年   2750篇
  2019年   2730篇
  2018年   5432篇
  2017年   5712篇
  2016年   6046篇
  2015年   3053篇
  2014年   5327篇
  2013年   10087篇
  2012年   7309篇
  2011年   8933篇
  2010年   7301篇
  2009年   8112篇
  2008年   8183篇
  2007年   7860篇
  2006年   6365篇
  2005年   5562篇
  2004年   5483篇
  2003年   5304篇
  2002年   5137篇
  2001年   4955篇
  2000年   4944篇
  1999年   4254篇
  1998年   7172篇
  1997年   5832篇
  1996年   4425篇
  1995年   3653篇
  1994年   3422篇
  1993年   3623篇
  1992年   3214篇
  1991年   3359篇
  1990年   3481篇
  1989年   3369篇
  1988年   3267篇
  1987年   3260篇
  1986年   3156篇
  1985年   3314篇
  1984年   3252篇
  1983年   3146篇
  1982年   2993篇
  1981年   3045篇
  1980年   3071篇
  1979年   3257篇
  1978年   3490篇
  1977年   3460篇
  1976年   3911篇
  1975年   3209篇
  1974年   3293篇
  1973年   3341篇
  1972年   2987篇
  1971年   2754篇
排序方式: 共有10000条查询结果,搜索用时 625 毫秒
971.
OBJECTIVE: To assess the results of a policy of tailored conservative surgical management for young women with stage I ovarian carcinomas. DESIGN: Retrospective study. PARTICIPANTS: Ninety-nine women aged 40 years or younger who underwent either primary surgery in our department or were referred after primary surgery performed elsewhere. METHODS: Of the 99 women in our study, 56 underwent fertility-sparing surgery and 43 more radical surgery. Minimal requirements for conservative management were adequate staging and complete information about the therapeutic options. Factors important in the choice of the treatment were, age, wish to preserve fertility, histologic type and grade, and the stage of the tumour. RESULTS: Conservative treatment was conducted in 84% of nulliparous and in 33% of parous women; 62% of grade 1 tumours, 48% of grade 2, and 50% of grade 3 were treated conservatively. With a median follow up of seven years, we observed five recurrences (9%) of carcinoma in women treated conservatively and five (12%) in those treated more radically. Two women (one in each treatment arm) were saved after recurrence. Two recurrences after conservative surgery involved the residual ovary (3.6%). Two women developed borderline tumour in the contralateral ovary and both were treated by surgery. CONCLUSION: After adequate staging and accurate information is given to the patient, conservative treatment may be safe in some women with early ovarian cancer. The risk of recurrence in the contralateral ovary is low. Conservative surgery may be also considered in some Stage I grade 3 tumours and in some women with stage JC tumours.  相似文献   
972.
Knowledge of the directional and hemispherical reflectance properties of natural surfaces, such as soils and vegetation canopies, is essential for classification studies and canopy model inversion. The Multi-angle Imaging SpectroRadiometer (MISR), an instrument to be launched in 1998 onboard the EOS-AM1 platform, will make global observations of the Earth's surface at 1.1-km spatial resolution, with the objective of determining the atmospherically corrected reflectance properties of most of the land surface and the tropical ocean. The algorithms to retrieve surface directional reflectances, albedos, and selected biophysical parameters using MISR data are described. Since part of the MISR data analyses includes an aerosol retrieval, it is assumed that the optical properties of the atmosphere (i.e. aerosol characteristics) have been determined well enough to accurately model the radiative transfer process. The core surface retrieval algorithms are tested on simulated MISR data, computed using realistic surface reflectance and aerosol models, and the sensitivity of the retrieved directional and hemispherical reflectances to aerosol type and column amount is illustrated. Included is a summary list of the MISR surface products  相似文献   
973.
The decrease in the density of dangling silicon-silicon bonds in a-Si:H films as a result of annealing in an atomic-hydrogen atmosphere is determined by their density in the initial (nonannealed) film. The change in the total hydrogen density in a-Si:H films, annealed in an atomic-hydrogen atmosphere, is determined by the type of silicon-hydrogen bonds and the impurity content: The hydrogen content can decrease to 1 at. % in the presence of monohydride bonds (2020 cm−1) and no change is observed in the hydrogen content in the presence of oxygen (≲0.1 at. %). A decrease in the defect density as a result of annealing in an atomic-hydrogen atmosphere is observed for all films. The Staebler-Wronski effect — AM-1 irradiation for 10 h — is observed for all films irrespective of the total hydrogen density, the type of silicon-hydrogen bonds, and the presence of oxygen. Fiz. Tekh. Poluprovodn. 32, 620–626 (May 1998)  相似文献   
974.
The experience of the State Joint-Stock Company for Stabilization of Bases and Structures (Gidrospetsstroi) in eliminating a technogenic chalk karst under complex hydrogeological conditions is reported. The causes of formation of the karst at a depth of 30–40 m and methods of eliminating piping and karstification by injecting cement grouts into the chalk by a special technology are examined. Translated from Osnovaniya, Fundamenty i, Mekhanika Gruntov, No. 3, pp. 21–23, May–June, 1994.  相似文献   
975.
976.
977.
1. The synthesis and release of nitric oxide may play a role in the pathogenesis of peripheral vasodilatation and hyperdynamic circulation observed in liver cirrhosis. In this work, we analysed the synthesis of nitric oxide by the lympho-mononuclear cells of peripheral blood from patients with chronic alcoholic and non-alcoholic liver disease and we identified the isoform of nitric oxide synthase involved in the increased nitric oxide synthesis. 2. Patients were classified following clinical and histological criteria in non-alcoholic cirrhotic, alcoholic cirrhotic and non-cirrhotic chronic liver disease. We studied clinical and analytical characteristics, haemodynamic parameters and endotoxin levels in these patients. 3. Cirrhotic patients showed an increase of cardiac output and a decrease of peripheral vascular resistance. These patients had higher levels of plasma endotoxin than those observed in the control group. N omega-Nitro-L-arginine methyl ester (L-NAME)-inhibitable nitrite production from mononuclear lymphocyte cells was higher in patients than in the control group, the highest levels being in non-alcoholic cirrhotic patients, and the lowest levels in patients with non-cirrhotic alcoholic liver disease. 4. Immunocytochemistry studies revealed a positive immunoreactivity for the inducible isoform of nitric oxide synthase in lympho-mononuclear cells that was more evident in non-alcoholic than in alcoholic cirrhotic patients. By Northern blot, inducible nitric oxide synthase mRNA expression was observed only in lymphomononuclear cells from non-alcoholic cirrhotic patients. 5. Our patients show a correlation between nitric oxide synthesis, endotoxin levels and haemodynamic parameters. 6. These findings indicate that lympho-mononuclear cell stimulation may play a role in elevated nitric oxide production in hepatic cirrhosis. Thus, this increased nitric oxide synthesis could be implicated in the pathogenesis of the haemodynamic disturbances frequently found in cirrhotic patients. This increase seems to be induced, at least in part, by activation of an inducible isoform of nitric oxide synthase.  相似文献   
978.
979.
Sulfonylureas have, in the past, been reported to have adverse cardiovascular effects. Glimepiride is a new sulfonylurea. In spite of stimulating less insulin secretion, it has, depending on the species, equal or higher blood glucose decreasing activity and according to preliminary studies less cardiovascular activity than glibenclamide. Further studies were performed to confirm the lower cardiovascular activity of glimepiride. The IC50 for inhibition of rilmakalim-activated KATP channel currents in isolated ventricular myocytes was 31.6 nM for glimepiride and 6.8 nM for glibenclamide. In endotoxin shock-rats at a dose of 1 x 2 mg/kg i.v., glibenclamide induced a significantly higher blood pressure increase than glimepiride. At two i.v. doses of 20 mg/kg 4 min apart, in normal rats, glibenclamide produced signs of ischemia in the ECG in nearly all animals, glimepiride almost none, in diabetic rats, glibenclamide produced in all animals a lethal cardiogenic shock preceeded by serious ECG changes, glimepiride only in one fifth of the animals. In open-chest dogs, on intracoronary infusion of equieffective blood glucose-lowering doses, glibenclamide, gliclazide and glimepiride all reduced coronary blood flow, increased coronary resistance, depressed the mechanical activity of the heart, enhanced myocardial O2-extraction, reduced the serum potassium level and induced a moderate endocardial ST-segment elevation, but glimepiride to a significantly less extent than glibenclamide and gliclazide. The presented data confirm that glimepiride at equivalent blood glucose decreasing doses has less cardiovascular activity than conventional sulfonylureas.  相似文献   
980.
Basic parameters of a resource-saving technology for manufacturing unfired articles for bottom steel casting that was developed under laboratory conditions are presented. Industrial tests of an experimental batch of unfired bottom tubes produced by the suggested technology have shown high performance properties for them and applicability of them for bottom casting of steel.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 8, pp. 27–29, August, 1996.  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