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   6550篇
  免费   60篇
  国内免费   1篇
电工技术   14篇
综合类   5篇
化学工业   424篇
金属工艺   50篇
机械仪表   36篇
建筑科学   75篇
矿业工程   5篇
能源动力   49篇
轻工业   142篇
水利工程   19篇
石油天然气   9篇
无线电   206篇
一般工业技术   263篇
冶金工业   5109篇
原子能技术   9篇
自动化技术   196篇
  2022年   21篇
  2021年   18篇
  2020年   16篇
  2019年   17篇
  2018年   29篇
  2017年   25篇
  2016年   30篇
  2015年   24篇
  2014年   44篇
  2013年   84篇
  2012年   46篇
  2011年   69篇
  2010年   34篇
  2009年   53篇
  2008年   53篇
  2007年   68篇
  2006年   40篇
  2005年   40篇
  2004年   33篇
  2003年   38篇
  2002年   38篇
  2001年   37篇
  2000年   27篇
  1999年   170篇
  1998年   1594篇
  1997年   986篇
  1996年   574篇
  1995年   337篇
  1994年   297篇
  1993年   326篇
  1992年   66篇
  1991年   66篇
  1990年   98篇
  1989年   83篇
  1988年   102篇
  1987年   113篇
  1986年   69篇
  1985年   60篇
  1984年   31篇
  1983年   32篇
  1982年   47篇
  1981年   56篇
  1980年   72篇
  1979年   21篇
  1978年   25篇
  1977年   113篇
  1976年   243篇
  1975年   17篇
  1973年   16篇
  1969年   15篇
排序方式: 共有6611条查询结果,搜索用时 9 毫秒
71.
BACKGROUND: The association of serum lipids with coronary heart disease has been studied extensively in middle-aged men and, to a lesser extent, in similar women. Less well defined are lipid variables predictive of CHD in individuals of age > or = 60 years. METHODS AND RESULTS: The Systolic Hypertension in the Elderly Program recruited 4736 persons (mean age, 72 years; 14% were black; and 43% were men). Mean systolic and diastolic blood pressures were 170 and 77 mm Hg, respectively. Baseline mean total cholesterol was 6.11 mmol/L (236 mg/dL); HDL cholesterol, 1.39 mmol/L (54 mg/dL); and non-HDL cholesterol, 4.72 mmol/L (182 mg/dL). Triglyceride levels were 1.62 mmol/L (144 mg/dL) for fasting participants and 1.78 mmol/L for the total group. LDL cholesterol, estimated in fasting samples with triglycerides of < 4.52 mmol/L, averaged 3.98 mmol/L (154 mg/dL). Mean follow-up was 4.5 years. In multivariate Cox regression analyses, baseline total, non-HDL, and LDL cholesterol levels and the ratios of total, non-HDL, and LDL to HDL cholesterol were significantly related to CHD incidence. HDL cholesterol and triglycerides were not significant in these analyses. In fasting participants with triglyceride levels of < 4.52 mmol/L, a 1.03 mmol/L (40 mg/dL) higher baseline total, non-HDL, or LDL cholesterol was associated with a 30% to 35% higher CHD event rate. CONCLUSIONS: The results of this study support the concept that serum lipids are CHD risk factors in older Americans.  相似文献   
72.
73.
74.
Phenotypic analysis of antigen-specific T lymphocytes   总被引:4,自引:0,他引:4  
Identification and characterization of antigen-specific T lymphocytes during the course of an immune response is tedious and indirect. To address this problem, the peptide-major histocompatability complex (MHC) ligand for a given population of T cells was multimerized to make soluble peptide-MHC tetramers. Tetramers of human lymphocyte antigen A2 that were complexed with two different human immunodeficiency virus (HIV)-derived peptides or with a peptide derived from influenza A matrix protein bound to peptide-specific cytotoxic T cells in vitro and to T cells from the blood of HIV-infected individuals. In general, tetramer binding correlated well with cytotoxicity assays. This approach should be useful in the analysis of T cells specific for infectious agents, tumors, and autoantigens.  相似文献   
75.
OBJECTIVE: To identify possible causes for the increased cardiovascular morbidity and mortality seen in patients with primary hyperparathyroidism. DESIGN: Prospective, blind study. SETTING: University hospital, Sweden. SUBJECTS: 44 Patients with primary hyperparathyroidism and 23 (sex and age matched) control patients with atoxic nodular goitres. INTERVENTIONS: Exploration of the neck with removal of pathological parathyroid glands or thyroid resection. Echocardiography before, and one year after, the operation. MAIN OUTCOME MEASURES: Blood pressure and echocardiographic findings. RESULTS: Hyperparathyroid patients had higher blood pressure and greater left atrial diameter than control patients preoperatively. They also had a significantly lower E:A ratio (mitral flow velocity pattern) than the controls (p = 0.02) indicating a disturbance in early diastolic filling of the left ventricle. The E:A ratio correlated negatively with the systolic blood pressure. 19 of the hyperparathyroid patients (43%) had cardiac calcifications as did 14 (61%) of the controls. Most of calcifications were located in the aortic and mitral valves; only a few patients had calcifications in the myocardium. No significant changes had occurred one year after parathyroidectomy, except for a reduction in systolic blood pressure, in the hyperparathyroid patients. CONCLUSION: Echocardiographic investigation of patients with primary hyperparathyroidism shows early signs of left ventricular dysfunction that may be of clinical importance.  相似文献   
76.
This article is a review of how metallic implant corrosion byproducts affect the long-term survival of the dental implant bone bed. Cell structure, bone loss, metal release and corrosion mechanisms are explained in relation to how they influence the metabolic factors that control bone remodeling.  相似文献   
77.
78.
BACKGROUND AND PURPOSE: Epidemiological studies indicate a higher incidence of intracerebral (but not subarachnoid) hemorrhagic stroke among persons with low total serum cholesterol levels. This report further examines the prospective relationship of total serum cholesterol with subsequent intracerebral hemorrhage in a large, well-defined population. METHODS: The cohort included 61756 enrollees in a health plan from the San Francisco-Oakland metropolitan area (46% men, 63% white), aged 40 to 89 years and free of cardiovascular disease at baseline. Sixteen-year incidence of combined nonfatal and fatal intracerebral hemorrhagic stroke (International Classification of Diseases [ICD], 8th revision, code 431, or ICD, 9th revision, codes 431 and 432) was investigated in relation to serum cholesterol measured in multiphasic health checkups made in 1977 through 1985. Intracerebral hemorrhagic events were ascertained using hospital discharge records and as underlying cause of death by the California Mortality Linkage Information System. RESULTS: From 1978 through 1993 (average of 10.7 years), there were 386 events (201 in men, 29% fatal; 185 in women, 42% fatal). By multivariate proportional hazards life-table regression analysis, serum cholesterol level below the sex-specific 10th percentile (< 4.62 mmol/L [178 mg/dL] in men), compared with higher cholesterol level, was associated with a significantly increased risk of intracerebral hemorrhage in men aged 65 years or older (relative risk, 2.7; 95% confidence interval, 1.4 to 5.0). An excess risk was also observed among elderly women at the lowest cholesterol range, but a chance finding could not be ruled out. No relationship was seen among men or women aged 40 to 64, and no statistical interaction of low serum cholesterol with hypertension was found in either sex. CONCLUSIONS: In these data, the association between low serum cholesterol level and intracerebral hemorrhage was confined to elderly men.  相似文献   
79.
BACKGROUND AND OBJECTIVE: Spectroscopic guidance of laser angioplasty has been attempted using a diagnostic He-Cd laser in addition to the therapeutic laser system. This study evaluated a single-laser approach for simultaneous ablation and fluorescence excitation. STUDY DESIGN/MATERIALS AND METHODS: A spectroscopy system was coupled to a clinical XeCl excimer laser. Ablation of 162 human aortic samples in saline and blood with 45 mJ/mm2 per pulse yielded 676 fluorescence spectra validated histologically. The same equipment was used in 16 patients for angioplasty of 18 coronary stenoses applying 500 to 1,725 pulses with 45 to 60 mJ/mm2 under saline flushing. A total of 783 spectra were recorded and validated by intracoronary ultrasound (categories: atheroma, fibrous plaque, calcified lesion). RESULTS: In vitro, 5 types of spectra could be differentiated: (1) atheroma, (2) fibrous plaque, (3) calcified lesion in saline, (4) media, and (5) calcified lesion in blood. Discriminant analysis prospectively classified 576 validation spectra with the following sensitivity and specificity for each type: (1) 83.5 and 97.1%, (2) 85.7 and 96.8% (3) 100 and 98.5%, (4) 98.1 and 99.3%, (5) 98.9 and 100%, respectively. In vivo type 1, 2, 3, and 5 spectra were also observed, but not the media spectrum. The predominant sonographic category also prevailed in spectroscopy. Calcified lesions yielded type 3 and 5 as well as mixed spectra. CONCLUSIONS: Using an excimer laser for angioplasty allows combining ablation and fluorescence excitation without a diagnostic laser. Principal types of atherosclerotic lesions and the media can be differentiated spectroscopically with this approach.  相似文献   
80.
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