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   55705篇
  免费   1302篇
  国内免费   186篇
电工技术   638篇
综合类   636篇
化学工业   6491篇
金属工艺   733篇
机械仪表   1108篇
建筑科学   1122篇
矿业工程   407篇
能源动力   742篇
轻工业   3408篇
水利工程   697篇
石油天然气   90篇
武器工业   6篇
无线电   2203篇
一般工业技术   4433篇
冶金工业   27494篇
原子能技术   248篇
自动化技术   6737篇
  2024年   81篇
  2023年   266篇
  2022年   389篇
  2021年   462篇
  2020年   239篇
  2019年   301篇
  2018年   750篇
  2017年   992篇
  2016年   1382篇
  2015年   990篇
  2014年   714篇
  2013年   1023篇
  2012年   2544篇
  2011年   2987篇
  2010年   1052篇
  2009年   1194篇
  2008年   961篇
  2007年   924篇
  2006年   819篇
  2005年   3495篇
  2004年   2708篇
  2003年   2192篇
  2002年   969篇
  2001年   830篇
  2000年   369篇
  1999年   899篇
  1998年   8111篇
  1997年   4890篇
  1996年   3298篇
  1995年   1871篇
  1994年   1410篇
  1993年   1474篇
  1992年   303篇
  1991年   396篇
  1990年   365篇
  1989年   355篇
  1988年   367篇
  1987年   293篇
  1986年   273篇
  1985年   230篇
  1984年   88篇
  1983年   108篇
  1982年   161篇
  1981年   225篇
  1980年   264篇
  1979年   72篇
  1978年   119篇
  1977年   753篇
  1976年   1630篇
  1975年   113篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
This study analyses the relationship between the use of rotating instruments, the production of a smear layer and the presence of alterations to enamel microstructures. The rotating instruments used were carbide (8-12 blade) and diamond tipped (30-15 m) cutters. Cavities were made in extracted teeth. Subsequently, half the sample was analysed using the rugosimeter before and after the application of ortophosphoric acid at 35% for 15 and the other half suing a Scansion Electronic microscope (SEM). The results obtained showed on the one hand that carbide cutters leave a smoother surfacer than diamond tipped cutters, and on the other that the smear layer is eliminated better by carbide cutters compared to diamond tipped cutters. Moreover, there are no major traumatic-type alterations at the level of the enamel affecting the microstructure after the use of carbide cutters.  相似文献   
62.
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   
63.
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   
64.
Factor V Leiden mutation was initially detected in thrombophilic patients and relatives by PCR RFLP (Restriction Fragment Length Polymorphism) according to Bertina (1). This technique presents some drawbacks and the current trend is to simplify the diagnosis. We describe a technique of Allele Specific Amplification (ASA) which is optimized in terms of reliability: an additional mismatch in antepenultimate position enables to obtain the same specificity as PCR RFLP. Furthermore, coamplification of internal control warrants an optimal sensitivity. All the PCR have been simplified: the DNA extraction improvement allows to analyse the genotype with only a few microliters of whole blood whatever the anticoagulant and the procedure of preservation (freezing, dried blood spots, storage at +4 degrees C for several days). This technique saves time. Moreover, full automation of the ASA technique may be shortened thanks to the lack of extraction and the positive/negative reading of the PCR signal.  相似文献   
65.
Ileal pouch-anal anastomosis is a surgical procedure used for the treatment of people with chronic ulcerative colitis and familial adenomatous polyposis. The surgery is intended to preserve anal sphincter function, but it carries a risk for certain complications, including pouchitis and anastomotic stricture. The purpose of this article is to review the clinical manifestations, causes, and treatment of anastomotic stricture and pouchitis after ileal pouch-anal anastomosis.  相似文献   
66.
67.
OBJECTIVE: Experiences obtained with nonoperative treatment (NOT), i.e. total prohibition of per oral food intake for a minimum of 7 days, administration of combinations of broad-spectrum antibiotics, and parenteral hyperalimentation, are described in the management of esophageal perforations. SUMMARY BACKGROUND DATA: The place, value, and indication of NOT in the management of esophageal perforation has not yet been unequivocally defined. As a result, contradictory data have been published regarding the outcome of NOT. METHODS: During the past 15 years (1979 to 1994), 20 of 86 patients (23.3%) with esophageal perforation have been treated nonoperatively from the outset. In this group, perforations were located to the upper, middle, and lower third of the esophagus in 50%, 30%, and 20%, respectively. In the operative management group (OT)--in which conservative (drainage, endeprothesis), reconstructive (suture, reinforced suture), and radical (resection) surgical methods were applied--lesions were preponderantly located in the lower one third of the esophagus (56.1%--37/66). As to the interval between the perforation and the onset of treatment, 14 patients had been diagnosed within 24 hours, whereas in 6 cases treatment had been begun beyond 24 hours. RESULTS: NOT could be successfully carried out in 16 patients; the decision to use NOT had to be revised in 4 other cases (Table 1). Two patients were lost; the mortality rate was 10% (2 of 20). The rate of complications was lower in the NOT group (20%, or 4 of 20) than in the OT group (50%, or 33 of 66). CONCLUSIONS: NOT can be suggested for the treatment of intramural perforations. In the case of transmural perforation, this approach should be taken into consideration if the esophageal lesion is circumscribed, is not in neoplastic tissue, is not in the abdominal cavity, and is not accompanied by simultaneous obstructive esophageal disease; in addition, symptoms and signs of septicemia should be absent.  相似文献   
68.
69.
BACKGROUND: Disturbed fibrinolytic function may influence the progression of coronary atherosclerosis and contribute to thrombotic cardiovascular (CV) events. METHODS AND RESULTS: In the Angina Prognosis Study in Stockholm (APSIS), patients with stable angina pectoris were studied prospectively during double-blind treatment with metoprolol or verapamil. Various measures of fibrinolytic function were studied in 631 (of 809) patients. During a median follow-up time of 3.2 years (2132 patient-years), 32 patients suffered a CV death, 21 had a nonfatal myocardial infarction (MI), and 77 underwent revascularization. Plasma levels of tissue plasminogen activator (TPA) activity and antigen (ag), plasminogen activator inhibitor (PAI-1) activity at test, and TPA responses to exercise were determined at baseline and after 1 month's treatment and were related to subsequent fatal and nonfatal CV events. Univariate Cox regression analysis revealed that elevated levels of TPA-ag at rest (P < .05), high PAI-1 activity (P < .05), and low TPA-ag responses to exercise (P < .05) were associated with increased risk of subsequent CV death. After adjustment for baseline risk factors, TPA-ag independently predicted CV death or MI. In addition, PAI-1 activity independently predicted CV death or MI in male patients. Verapamil treatment was associated with a 10% decrease of TPA-ag levels and metoprolol treatment with a 2% increase (P < .001 for treatment difference). CONCLUSIONS: Plasma TPA-ag levels at rest, and among male patients PAI-1 activity as well, independently predict subsequent CV death or MI in patients with stable angina pectoris.  相似文献   
70.
A cryptic multidrug resistance (MDR) system in Stenotrophomonas maltophilia, the expression of which is selectable by tetracycline, is described. Tetracycline resistance was the consequence of active efflux of the antibiotic, and it was associated with resistance to quinolones and chloramphenicol, but not to aminoglycosides or beta-lactam antibiotics. MDR is linked to the expression of an outer membrane protein (OMP54) both in a model system and in multidrug-resistant clinical isolates.  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