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   7774篇
  免费   14篇
  国内免费   1篇
电工技术   9篇
综合类   3篇
化学工业   164篇
金属工艺   25篇
机械仪表   31篇
建筑科学   41篇
矿业工程   9篇
能源动力   24篇
轻工业   108篇
水利工程   4篇
石油天然气   9篇
无线电   213篇
一般工业技术   172篇
冶金工业   6817篇
原子能技术   7篇
自动化技术   153篇
  2021年   7篇
  2020年   6篇
  2019年   10篇
  2018年   20篇
  2017年   16篇
  2016年   8篇
  2015年   11篇
  2014年   20篇
  2013年   47篇
  2012年   42篇
  2011年   42篇
  2010年   35篇
  2009年   43篇
  2008年   36篇
  2007年   33篇
  2006年   44篇
  2005年   40篇
  2004年   35篇
  2003年   45篇
  2002年   33篇
  2001年   30篇
  2000年   50篇
  1999年   252篇
  1998年   2129篇
  1997年   1186篇
  1996年   843篇
  1995年   462篇
  1994年   369篇
  1993年   412篇
  1992年   74篇
  1991年   105篇
  1990年   79篇
  1989年   85篇
  1988年   78篇
  1987年   87篇
  1986年   85篇
  1985年   75篇
  1984年   17篇
  1983年   17篇
  1982年   36篇
  1981年   50篇
  1980年   74篇
  1979年   15篇
  1978年   26篇
  1977年   149篇
  1976年   342篇
  1975年   17篇
  1974年   16篇
  1973年   7篇
  1971年   6篇
排序方式: 共有7789条查询结果,搜索用时 109 毫秒
21.
Notches were machined in specimens of INCONEL 718 by a broaching process, where differing broaching runs led to differing extents of subsurface deformation and surface roughness. Fatigue tests were carried out at 600 °C with a trapezoidal loading waveform at 0.25 Hz. The broaching process that led to the more severe subsurface deformation (but lower surface roughness) showed the worst fatigue performance. Analysis of total strain amplitude in the notch root with the aid of an elastoplastic finite-element (FE) model showed that the work hardening related to the subsurface deformation caused by the different broaching can account for the difference in fatigue lives. Differences in initiation and growth behavior were seen for the two broached finishes as well as for broached and subsequently polished samples. These differences are discussed in terms of a change in crack growth initiation and growth mechanisms due to the presence of the work-hardened layer.  相似文献   
22.
Seven ferrocene-bound polyester resins were prepared from fumaric acid and diethylene glycol in the presence of different ferrocene derivatives. To accomplish the incorporation of the organometallic moiety into the polyester resin chain, the cyclopentadienyl rings of ferrocene were substituted in the 1,1′ positions with reactive functional groups. The ferrocene derivatives used were: 1,1′-bis(1-hydroxyethyl) ferrocene 1 , 1,1′-ferrocene dicarboxylic acid 2 , and 1,1′-ferrocene diacetic acid 3 . The incorporation of an organometallic complex significantly increased the rate of the polyesterification process. In order of increasing rate enhancement, the effect was 2 < 3 < 1 . The effectiveness of ferrocene derivatives chemically incorporated into the polyester backbone during copolymerization with styrene was studied by monitoring the gel time, cure rate, and the resin properties. The efficiency of the copolymerization reaction of ferrocene-bound polymers with styrene decreased in the order 1 ≥ 3 > 2 . Mechanisms to account for these results are presented. Some of the strength related properties were also studied.  相似文献   
23.
The Magtalo prototype environment harnesses dialogue protocols to support flexible, intuitive interaction with data in complex, contentious domains and provides mechanisms for eliciting structured knowledge. Magtalo represents the first example of an implemented online system that uses a closely specified argument-based dialogue protocol combined with a rich language for representing monologic argument to provide a tool for intuitive user exploration of a disagreement space. This approach also enables the expansion of argument resources through knowledge elicitation, which is structured by the argument dialogue protocol. The continuing aim of our research is to use advances in argumentation theory to push the practice of argumentation technology in providing tools and interfaces that have wide appeal.  相似文献   
24.
Here we are presenting the case of a 70-years-old woman who has hepatic cirrhosis anti-HCV and insulin-dependent diabetes mellitus, without relevant epidemiologic ascendants or previous transfusions and HBV, HIV negatives. On admission to our hospital she showed signs of autoimmune hemolytic anaemia (AHA) which was confirmed by positive direct Coombs test and an improvement in blood test after corticoid treatment. Having discarded other possible causes such as drugs infectious diseases or essential mixed cryoglobulinemia (CME), we put forward the possible association between AHA and infection by HCV, where AHA was an extrahepatic immunological manifestation of HCV. This fact has never been brought to light in previous medical literature.  相似文献   
25.
26.
BACKGROUND: Hypertension and hypercholesterolemia are frequently associated with this leading to considerable cardiovascular risk. METHODS: An open parallel randomized study was performed in which the effects of doxazosin, an alpha-adrenergic blocker and enalapril, an inhibitor of the angiotensin converting enzyme were compared in 70 patients with essential high blood pressure and plasma cholesterol levels greater than 240 mg/dl. Following 2-4 weeks of placebo administration the patients were randomly treated with one of the two drugs. When required doses were increased and hydrochlorothiazide added until blood pressure lower than 160/95 mmHg was achieved. After this period the patients were observed for a minimum of 8 weeks. The mean length of the study was of 22 weeks. RESULTS: Both drugs significantly reduced blood pressure without modifying cardiac frequency. Doxazosin tended to favorably modify the lipid profile of the plasma while enalapril significantly reduced the levels of cholesterol, lipids and high density lipoproteins (HDL). Upon termination of the study the total HDL/cholesterol index increased 8.6% in those treated with doxazosin and decreased 5.5% in those receiving enalapril (p < 0.05). CONCLUSIONS: Although doxazosin and enalapril are potent antihypertensive drugs, the effects on plasma lipid obtained with doxazosin indicate that a reduction in cardiovascular risk was achieved with this drug in the patients included in this study.  相似文献   
27.
Near-haploidy is a rare cytogenetic finding in childhood acute lymphoblastic leukaemia (ALL) and is associated with a poor prognosis. A second hyperdiploid line, occurring presumably by endoreduplication of the near-haploid stemline, is often observed. We present a case of common ALL in relapse characterized morphologically by a dual population of small and large lymphoblasts. Cytogenetic analysis supplemented with fluorescence in-situ hybridization (FISH) studies localized near-haploidy and hyperdiploidy to the small and large blast population respectively. DNA ploidy determination confirmed two abnormal clones with near-haploidy as the predominant one. A novel t(9;12)(q11;q13) was present in the near-haploid clone and was duplicated in the hyperdiploid clone. This finding identified cells bearing near-haploidy to be the clonogenic population following malignant transformation and confirmed endoreduplication as the mechanism for the presence of associated hyperdiploidy.  相似文献   
28.
The aim of this study was to document plasma retinol status and nocturnal vision in ten eutrophic adolescents with cystic fibrosis (CF) receiving daily retinol supplementation. Plasma retinol, alpha and beta carotenes and retinol binding protein were measured in ten clinically stable CF patients (mean age: 14.3 years; Shwachman score: 80-100). Nocturnal vision evaluation was performed with a Beyne optometer. Plasma retinol (mean 0.42 +/- 0.16 mg/l), alpha carotene and beta carotene levels were below the lower limit of normal in all but one patient. Five out of ten patients with normal standard opthalmological examination presented a poor (n = 3 patients) or a pathological (n = 2) dark adaptation test. These two patients showed a dramatic increase in nocturnal vision after 1 year of adapted retinol supplementation. CONCLUSION: Low vitamin A levels occur frequently in clinically stable, eutrophic and retinol supplemented CF adolescents. Since vitamin A deficiency is associated with poor nocturnal vision and since this pattern can be reversed by adapted retinol supplementation, we recommend monitoring plasma vitamin A levels in CF patients and evaluation of dark adaptation in retinol deficient patients.  相似文献   
29.
The purpose of this study was to examine the factors which affect the level of fatigue among patients participating in clinical trials in which this symptom had been assessed with the EORTC QLQ-C30. Data were assembled from 2390 patients in ten clinical trials in which the QLQ-C30 had been used to assess baseline and on-study quality of life. The relationship between the level of fatigue reported by the patients on the fatigue scale of this questionnaire and patient and disease characteristics was assessed in univariate and multivariate cross-sectional analyses. In addition, changes in fatigue scores were compared in a longitudinal analysis among patients on two arms of an anti-emetic trial whose emesis control was markedly different. Baseline fatigue levels differed substantially among patients taking part in the different trials. Factors associated with greater fatigue severity on univariate analysis included: female gender, presence of metastatic disease, and poorer performance status. In addition, on multivariate analyses the oldest patients were found to have less fatigue, as were patients with breast cancer, while patients with ovarian and lung cancer experienced greater fatigue. Patients on the arm of the anti-emetic trial in which emesis was better controlled showed significantly less increase in fatigue after receiving chemotherapy. The fatigue scale of the QLQ-C30 appears to provide a useful approach to assessing this important symptom. The relationships found between fatigue and patient and disease characteristics need further exploration as does the degree to which the QLQ-C30 fully captures this dimension of quality of life.  相似文献   
30.
Male strain A/J mice were exposed for 6 h a day, 5 days a week to environmental tobacco smoke (ETS) generated from Kentucky 1R4F reference cigarettes. Chamber concentrations were 87 mg/m3 of total suspended particulate matter (TSP), 246 p.p.m. of CO and 16 mg/m3 of nicotine. After 5 months, 33% of the ETS exposed and 11% of the control animals had one or several lung tumors; the difference was statistically not significant. A second group of animals exposed for 5 months to ETS was allowed to recover for another 4 months in filtered air. When they were killed, 85% of the ETS animals had lung tumors (average number per lung: 1.4 +/- 0.2), whereas in the control group 38% had lung tumors (average number of lung tumors in all animals 0.5 +/- 0.2). The differences in tumor incidence and multiplicity were statistically significant. More than 80% of all tumors were adenomas, the rest adenocarcinomas. When animals were pretreated with a carcinogen, lung tumor multiplicity was lower in the ETS exposed animals after 5 months compared with controls injected with a carcinogen and kept in air. However, after an additional 4 month recovery period in air, lung tumor multiplicities were the same in ETS plus carcinogen exposed mice as in carcinogen-treated air-exposed controls. Histopathologic and morphometric analysis of the lung tissue failed to reveal any differences between ETS exposed and control animals. However, immediately after ETS exposure, immunohistochemistry revealed increased staining for CYP1A1 in airway epithelia and lung parenchyma; following recovery in air, the staining disappeared again. Analysis of cell kinetics showed an initial burst of increased DNA synthesis in the epithelial cells of the airways and a smaller early positive response in the parenchyma. Feeding of butylated hydroxytoluene during ETS exposure did not modulate lung tumor development. It was concluded that ETS is a pulmonary carcinogen in strain A/J mice.  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