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   2671篇
  免费   133篇
  国内免费   20篇
电工技术   42篇
综合类   18篇
化学工业   712篇
金属工艺   61篇
机械仪表   52篇
建筑科学   94篇
矿业工程   11篇
能源动力   83篇
轻工业   330篇
水利工程   30篇
石油天然气   25篇
武器工业   2篇
无线电   236篇
一般工业技术   343篇
冶金工业   506篇
原子能技术   14篇
自动化技术   265篇
  2023年   17篇
  2022年   51篇
  2021年   109篇
  2020年   57篇
  2019年   47篇
  2018年   70篇
  2017年   65篇
  2016年   64篇
  2015年   63篇
  2014年   82篇
  2013年   151篇
  2012年   113篇
  2011年   142篇
  2010年   115篇
  2009年   123篇
  2008年   108篇
  2007年   116篇
  2006年   92篇
  2005年   75篇
  2004年   74篇
  2003年   85篇
  2002年   80篇
  2001年   59篇
  2000年   45篇
  1999年   36篇
  1998年   177篇
  1997年   109篇
  1996年   101篇
  1995年   60篇
  1994年   47篇
  1993年   49篇
  1992年   20篇
  1991年   19篇
  1990年   16篇
  1989年   16篇
  1988年   9篇
  1987年   14篇
  1986年   15篇
  1985年   14篇
  1984年   6篇
  1983年   10篇
  1982年   6篇
  1981年   8篇
  1980年   9篇
  1979年   12篇
  1978年   7篇
  1977年   12篇
  1976年   23篇
  1971年   7篇
  1943年   2篇
排序方式: 共有2824条查询结果,搜索用时 203 毫秒
31.
BACKGROUND: The relationship between nonfatal acute myocardial infarction (AMI) and self-reported body weight and body mass index (BMI; Quetelet index, kg/m2) has been investigated. METHODS: A case-control study was conducted between 1983 and 1992 in northern Italy on 432 women with nonfatal AMI and 867 controls in hospital for acute, noncardiovascular, nonneoplastic, nondigestive, non-hormone-related conditions. Odds ratios (OR), with their 95% confidence intervals (CI), were computed by unconditional multiple logistic regression analysis, including terms for age, education, and smoking, plus history of selected diseases. RESULTS: Women with body weight and BMI in the highest quartile had an increased risk of AMI after allowance for age, education, and smoking status (OR 1.5, 95% CI 1.0 to 2.2, and OR 1.7, 95% CI 1.2 to 2.4, respectively). Compared with leaner women, the risk was higher among women with BMI above the median, in association with a history of diabetes (OR 5.2) or hyperlipidemia (OR 6.0). Hypertensive women had similar OR in the two strata of BMI (OR 5.1 and 4.8). The association of BMI with risk of AMI was apparently stronger among women younger than 50 years and among less educated women, but was similar among smokers and never smokers. CONCLUSIONS: The results of this study confirm that AMI among women is related to excess BMI, with a population attributable risk of 17%. The excess risk was substantial among overweight women with history of diabetes or hyperlipidemia, stressing the importance of controlling body weight among these women.  相似文献   
32.
This study reports the establishment of alpha-amylase-producing human parotid pleomorphic adenoma cell lines (2HP and 2HP1) which have been maintained in culture for over 1 yr. The procedures required preparation of cellular clumps from tumor tissue and plating them on plasma clot or precoated dishes. During the initial phase of growth they required modified MCDB-153 medium without serum. When cells showed signs of degeneration they were changed to MCDB-153 medium containing first 2% and then 10% heat inactivated fetal bovine serum. Although cells grew well in MCDB-153 containing 10% serum, the epithelial cell morphology was not distinct. Therefore, the growth and morphology of cells grown in MCDB-10% serum were compared with those in RPMI growth medium containing 10% fetal bovine serum and F12 containing 10% agammaglobulin newborn bovine serum. Although the growth of cells was a little slower in F12 medium than those in MCDB and RPMI, the epithelial cell morphology was maintained better than in other growth media. The cells of 2HP and 2HP1 produce low levels of alpha-amylase and relatively high levels of alpha-amylase mRNAs of 1176 and 702 bp and contain neurofilament-160, a neuronal-specific marker. The cells of 2HP1 are tumorigenic when tested in athymic mice, but the cells of 2HP are not. The establishment of amylase-producing human parotid adenoma cell lines of different characteristics in culture provides a new opportunity to study the mechanisms of differentiation and transformation, and regulation of alpha-amylase in these cells.  相似文献   
33.
The electrochemical behaviour, FTIR spectrum and the morphology of polypyrrole-polystyrenesulphonate (PPy-PSS) films obtained potentiostatically at 0.6 V in a 0.05m Py + 1.7 × 10?2m NaPSS medium have been analysed. Different electrochemical parameters have been modified in order to establish the first polaron formation, the reversibility of the redox process and the doping mechanism. The FTIR spectrum confirms the existence of C?O groups in the film structure and two possible explanations are suggested. SEM reveals a higher homogeneity in these films than in PPy films doped with smaller counterions.  相似文献   
34.
35.
对Ad hoc网络各种路由协议进行仿真评估,分析它们在不同约束条件下不同度量的相对性能,是了解与学习路由协议性能的重要手段.目前的度量标准主要集中在吞吐量、延时、抖动、丢包率等QoS因子或路由负载、寻路时间等外部特性上,还没有以Ad hoc网络系统本身的连通度作为度量标准进行仿真,评价各种路由协议在不同试验条件下对于同一网络系统连通度的影响的相关研究.基于能量模型、节点密度、停留时间及移动速率四种约束条件对MANET提出的四种路由协议进行了关于网络连通度的仿真与评估,给出了这方面的初步结论.  相似文献   
36.
The design, stereoselective synthesis and in vivo antiallodynic activity of four novel paroxetine analogs, named 3-hydroxy paroxetines (3HPXs), is reported herein. Among the novel synthesized compounds, three showed an antiallodynic effect, while (R,R)-3HPX was found to be 2.5 times more bioactive than (-)-paroxetine itself in neuropathic rats. Consequently, the current investigation not only discloses a novel promising analgesic drug, but also reveals that functionalization at the C3 position of paroxetine could be as effective as the common functionalization at either C4 or within the sesamol group.  相似文献   
37.
Alzheimer’s disease (AD) is a neurodegenerative disease, characterized histopathologically by intra-neuronal tau-related lesions and by the accumulation of amyloid β-peptide (Aβ) in the brain parenchyma and around cerebral blood vessels. According to the vascular hypothesis of AD, an alteration in the neurovascular unit (NVU) could lead to Aβ vascular accumulation and promote neuronal dysfunction, accelerating neurodegeneration and dementia. To date, the effects of insoluble vascular Aβ deposits on the NVU and the blood–brain barrier (BBB) are unknown. In this study, we analyze different Aβ species and their association with the cells that make up the NVU. We evaluated post-mortem AD brain tissue. Multiple immunofluorescence assays were performed against different species of Aβ and the main elements that constitute the NVU. Our results showed that there are insoluble vascular deposits of both full-length and truncated Aβ species. Besides, insoluble aggregates are associated with a decrease in the phenotype of the cellular components that constitute the NVU and with BBB disruption. This approach could help identify new therapeutic targets against key molecules and receptors in the NVU that can prevent the accumulation of vascular fibrillar Aβ in AD.  相似文献   
38.
Films for agricultural applications, such as greenhouses films or mulching films are generally made of polyolefins such as linear low-density polyethylene (LLDPE) or low-density polyethylene. However, the use of biodegradable and/or compostable polymers is increasing, which enjoy the additional advantage that they can be left on the site since a fine life would be gradually assimilated to the underlying soil. Nevertheless, biodegradable polymeric films often do not have suitable mechanical performances. In this work, biodegradable polymer-based nanocomposite films are prepared by film blowing and compared with traditional LLDPE based nanocomposites. In particular, a biodegradable polymer blend and two different inorganic nanofillers (an organo-modified clay and a calcium carbonate with a hydrophobic coating) are used for the preparation of the nanocomposites. A detailed investigation of obtained materials is performed through rheological, mechanical, and optical characterizations. Adding nanofillers led to an increase of rigidity and tear strength of blown films without negatively affecting their ductility.  相似文献   
39.
自由曲面的测量、建模、加工一体化过程研究   总被引:1,自引:0,他引:1  
本文以一个鼠标模型为例,介绍了自由曲面的测量、建模、加工一体化过程,对一体化过程的各个步骤进行了研究为实现逆向工程集成化奠定了基础。  相似文献   
40.
A molecular distillation plant, built particularly to increase the separation efficiency and to obtain safer working conditions, was tested to remove cholesterol from anhydrous butter and lard. A preliminary experiment was carried out with butter to evaluate the fractionation obtained at temperatures between 190 and 250°C and residual pressures between 10−3 and 10−4 torr. A second experiment was carried out at 185°C and at the maximum operational vacuum, evaluating the fractionation achieved within a time scale between 30 and 180 min. Cholesterol was almost completely removed during the second hour with minimal loss of low-molecular weight triglycerides. An experiment was carried out with lard at 250°C and maximum achievable operational vacuum (10−4 Torr), lasting approximately 6 h, and cholesterol was removed almost completely during the second hour without significant modifications in the triglyceride composition. This situation remained constant throughout the duration of the test.  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