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   535960篇
  免费   8902篇
  国内免费   1461篇
电工技术   9627篇
综合类   533篇
化学工业   80474篇
金属工艺   20142篇
机械仪表   15407篇
建筑科学   13000篇
矿业工程   2304篇
能源动力   14719篇
轻工业   49302篇
水利工程   5352篇
石油天然气   9376篇
武器工业   39篇
无线电   60483篇
一般工业技术   103609篇
冶金工业   106419篇
原子能技术   10600篇
自动化技术   44937篇
  2021年   5181篇
  2020年   3780篇
  2019年   4870篇
  2018年   8354篇
  2017年   8138篇
  2016年   8468篇
  2015年   5964篇
  2014年   9760篇
  2013年   25101篇
  2012年   15383篇
  2011年   20766篇
  2010年   16344篇
  2009年   18182篇
  2008年   18580篇
  2007年   18235篇
  2006年   15795篇
  2005年   14431篇
  2004年   13851篇
  2003年   13607篇
  2002年   12814篇
  2001年   12952篇
  2000年   11964篇
  1999年   12735篇
  1998年   33718篇
  1997年   23481篇
  1996年   17835篇
  1995年   13240篇
  1994年   11450篇
  1993年   11156篇
  1992年   7967篇
  1991年   7466篇
  1990年   7308篇
  1989年   7062篇
  1988年   6517篇
  1987年   5801篇
  1986年   5698篇
  1985年   6315篇
  1984年   5929篇
  1983年   5198篇
  1982年   4943篇
  1981年   5015篇
  1980年   4744篇
  1979年   4629篇
  1978年   4431篇
  1977年   5406篇
  1976年   7110篇
  1975年   3807篇
  1974年   3561篇
  1973年   3674篇
  1972年   2985篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
The proliferating need for sustainability intervention in food grain transportation planning is anchoring the attention of researchers in the interests of stakeholders and environment at large. Uncertainty associated with food grain supply further intensifies the problem steering the need for designing robust, cost-efficient and sustainable models. In line with this, this paper aims to develop a robust and sustainable intermodal transportation model to facilitate single type of food grain commodity shipments while considering procurement uncertainty, greenhouse gas emissions, and intentional hub disruption. The problem is designed as a mixed integer non-linear robust optimisation model on a hub and spoke network for evaluating near optimal shipment quantity, route selection and hub location decisions. The robust optimisation approach considers minimisation of total relative regret associated with total cost subject to several real-time constraints. A version of Particle Swarm Optimisation with Differential Evolution is proposed to tackle the resulting NP-hard problem. The model is tested with two other state-of the art meta-heuristics for small, medium, and large datasets subject to different procurement scenarios inspired from real time food grain operations in Indian context. Finally, the solution is evaluated with respect to total cost, model and solution robustness for all instances.  相似文献   
62.
Urbano  Cristóbal  Ardanuy  Jordi 《Scientometrics》2020,124(1):575-602
Scientometrics - The interdisciplinary nature of library and information science (LIS) research has been highlighted for some time now. The term “interdisciplinary” is used primarily in...  相似文献   
63.

Surface integrity characterization of manufactured component is very important as it significantly affects the in-service performance of the component. Till now, surface integrity was evaluated using conventional measurement technique like microhardness tester, X-ray diffraction, optical microscopy and surface roughness tester. But, this technique being laboratory based cannot be used for in-service monitoring of the surface integrity. The present study focuses on the characterization of surface integrity upon electric discharge machined sample using non-destructive magnetic Barkhausen noise technique. Electric discharge machining was performed in die-sinking mode on die steel using copper–tungsten electrode (negative polarity). Experiment was performed by selecting different levels of peak current, gap voltage and pulse on time. Surface integrity characteristics like microhardness change, residual stress, microstructural alteration and surface roughness were analysed using microhardness tester, X-ray diffraction, optical microscopy and surface roughness tester, respectively, and were then correlated with magnetic parameter like root mean square value and peak value obtained from Barkhausen noise signal. The results show a good correlation between magnetic parameter (RMS and Peak value) of Barkhausen noise with the microhardness and surface roughness of the machined sample.

  相似文献   
64.
65.
66.
67.
68.
RCAS1 is a protein that participates in regulation of the tumor microenvironment and its immune responses, all in order to evade the immune system. The aim of this study was to analyze RCAS1 expression in urothelial bladder cancer cells (and in fibroblasts and macrophages of the tumor stroma) and its relationship with the histological pattern of malignancy. Eighty-three postcystectomy patients were enrolled. We analyzed the histological maturity (grade), progress (pT stage), tissue invasion type (TIT), nonclassic differentiation number (NDN), and the ability to metastasize (pN). The expression of RCAS1 protein was analyzed by immunohistochemistry. Indicators of histological malignancy were observed solely in association with the RCAS1 expression in cells in the border parts (BPs) of the tumor. Histological malignancy of the tumor, indicated by the pT and pN, and metastasis-free survival time, correlated significantly with RCAS1 expression in tumor neoplastic cells, whereas malignancy determined by grade, TIT, and NDN correlated with RCAS1 expression in fibroblasts and macrophages in the tumor microenvironment. These findings suggest that the increased RCAS1 expression depends on its cellular source and that RCAS1 expression itself is a component of various signaling pathways. The immune escape occurs within the tumor BPs, where the increase in the RCAS1 expression occurs within tumor cells and stromal cells in its microenvironment. We conclude that the histological pattern of tumor malignancy, indicated by grade, TIT, NDN, pT, and pN is a morphological indicator of immune escape.  相似文献   
69.
70.
A layer-by-layer analysis of the structures, the phase compositions, and the defect substructures of differentially hardened rails has been carried by optical and transmission electron microscopy. It is found that the material volume of the rail head fillet is cooled faster than the volume located along the central axis.  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