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   12378篇
  免费   1381篇
  国内免费   928篇
电工技术   239篇
技术理论   1篇
综合类   1104篇
化学工业   670篇
金属工艺   555篇
机械仪表   592篇
建筑科学   828篇
矿业工程   265篇
能源动力   157篇
轻工业   863篇
水利工程   109篇
石油天然气   133篇
武器工业   47篇
无线电   1106篇
一般工业技术   1838篇
冶金工业   973篇
原子能技术   243篇
自动化技术   4964篇
  2024年   25篇
  2023年   139篇
  2022年   284篇
  2021年   325篇
  2020年   340篇
  2019年   318篇
  2018年   297篇
  2017年   360篇
  2016年   371篇
  2015年   365篇
  2014年   658篇
  2013年   780篇
  2012年   773篇
  2011年   943篇
  2010年   829篇
  2009年   844篇
  2008年   922篇
  2007年   880篇
  2006年   772篇
  2005年   720篇
  2004年   672篇
  2003年   549篇
  2002年   463篇
  2001年   367篇
  2000年   293篇
  1999年   236篇
  1998年   160篇
  1997年   154篇
  1996年   146篇
  1995年   141篇
  1994年   97篇
  1993年   91篇
  1992年   55篇
  1991年   65篇
  1990年   35篇
  1989年   26篇
  1988年   27篇
  1987年   28篇
  1986年   19篇
  1985年   24篇
  1984年   20篇
  1983年   10篇
  1982年   8篇
  1981年   9篇
  1980年   6篇
  1979年   8篇
  1976年   9篇
  1975年   7篇
  1973年   3篇
  1972年   3篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
本文报道了用氨化与不氨化P_(507)萃取分组Nd-Sm的对比效果。并对氧化萃取分组Nd-Sm的既定条件如相混合时间,萃取酸度、氨化率、相比等进行了探讨。在上述条件试验的基础上,完成了Nd-Sm分组分馏萃取串级实验。证明用氯化P_(507)萃取及分离稀土元素,能够提高萃取稀土的效果,提高萃取剂的萃取容量。是一种有发展前途的新的萃取工艺方法。  相似文献   
72.
湖南下寒武统石煤中的镍钼铂族元素的地球化学特征   总被引:8,自引:0,他引:8  
李有禹 《煤炭学报》1996,21(3):261-264
从岩石学、矿床学、微量元素地球化学、稳定同位素方面对湖南下寒武统石煤中镍、钼、铂族元素为主的伴生元素的来源和富集成因进行了研究研究认为,含矿岩系中发育的硅岩是一种喷流岩;有海底喷流作用直接堆积的金属硫化物筒状富矿体;矿石中各种胶状结构发育;矿床具两套成矿系统;微量元素Se,As,Ba高地壳丰度数倍至数万倍;矿层中硫同位素值为-1300%~-1761%等证实了湖南下寒武统石煤中以镍、钼、铂族元素为主的伴生元素,其来源和富集并非陆源搬运沉积,而是与海底喷流作用有关  相似文献   
73.
采用共沉淀法合成稀土(Eu,Dy)-苯甲酸钡二元配合物及稀土(Eu,Dy)-苯甲酸钡盐-邻啡罗啉三元配合物,元素分析和TG-DTA曲线确定了系列配合物的组成分别为DyL3,EuBaL5.6H2O,DyBaL5.6H2O和EuBaL5.Phen.2H2O。采用KBr压片法测定了配合物的红外光谱,结果表明苯甲酸中的氧原子和Phen的氮原子参与了配位。波长295nm紫外光激发荧光光谱分析表明,配合物中的配体可将吸收的能量传递给稀土离子,使其发出特征荧光,第二配体的引入,有利于配体对能量的吸收和传递,增强了荧光强度。  相似文献   
74.
煤矸石中金属元素对人体健康影响的研究   总被引:1,自引:0,他引:1  
矿业活动是环境中金属元素的主要来源,而煤矸石是我国矿业固体废料中产生量、累计积存量和占地面积最大的固体废物,它含有铁、钠、锌、铜、锰等22种有害微量元素。分析煤矸石中这些金属元素在环境中的迁移转化途径及其对生物体的毒性效应,对人体健康影响的研究具有重要意义。本文详细阐述了金属元素对生物体(植物和人)的生物效应,重点分析了煤矸石中几种常见的金属元素在环境中的迁移途径及它们对人体健康的影响和毒性效应,并提出了防治煤矸石环境污染的相应对策。  相似文献   
75.
通过对可视电话认证测试中静电放电和辐射骚扰试验问题对策的分析,探讨在测试过程中如何分析电磁兼容问题的三要素并依据这三个要素进行改进,最终通过认证测试,并总结了电信终端设备电磁兼容认证测试中有关问题的分析方法.  相似文献   
76.
In 1991, the U.S. Geological Survey (USGS) began the first cycle of its National Water Quality Assessment (NAWQA) Program. The Program encompassed 51 river basins that collectively accounted for more than 70% of the total water use (excluding power generation), and 50% of the drinking water supply in the U.S. The basins represented a variety of hydrologic settings, rock types (geology), land-use categories, and population densities. One aspect of the first cycle included bed sediment sampling; sites were chosen to represent baseline and important land-use categories (e.g., agriculture, urban) in each basin. In total, over 1200 bed sediment samples were collected. All samples were size-limited (< 63 µm) to facilitate spatial and/or temporal comparisons, and subsequently analyzed for a variety of chemical constituents including major (e.g., Fe, Al,) and trace elements (e.g., Cu, Zn, Cd), nutrients (e.g., P), and carbon. The analyses yielded total (≥ 95% of the concentrations present), rather than total-recoverable chemical data.Land-use percentages, upstream underlying geology, and population density were determined for each site and evaluated to asses their relative influence on sediment chemistry. Baseline concentrations for the entire U.S. also were generated from a subset of all the samples, and are based on material collected from low population (≤ 27 p km− 2) density, low percent urban (≤ 5%), agricultural or undeveloped areas. The NAWQA baseline values are similar to those found in other national and global datasets. Further, it appears that upstream/underlying rock type has only a limited effect (mostly major elements) on sediment chemistry. The only land-use category that appears to substantially affect sediment chemistry is percent urban, and this result is mirrored by population density; in fact, the latter appears more consistent than the former.  相似文献   
77.
ICP-AES法测定天青石中主次量元素的研究   总被引:1,自引:0,他引:1  
谢华林  李坦平 《石材》2003,(12):37-39
采用偏硼酸锂熔样,电感耦合等离子体原子发射光谱(ICP-AES)法,对天青石样品实现了一次熔样,主次量元素同时分析。对影响其光谱测量的各种因素进行了较为详细的研究,研究了实验的最佳测定条件。结果表明,方法的检出限为0.0011-0.022lμg·ml-1,回收率为92.87%-106.37%,RSD小于3.89%。该法准确、快速、简便,应用于天青石样品中主次量元素的测定,结果满意。  相似文献   
78.
Aström M  Corin N 《Water research》2003,37(2):273-280
The abundance, fractionation and physicochemical forms of rare earth elements (REEs) were determined in five boreal humus-rich streams (dissolved organic carbon, DOC = 14-40 mg/l) affected by acid sulphate soils. The sampling was carried out during high-water flow in autumn when the acid sulphate soils are extensively flushed. The analytical procedures included ion-exchange experiments in field and ICP-MS determination. There was a general decrease in pH (range 4.5-6.2) and increase in the REE concentrations (La range 0.82-23 microg/l) as the proportion of the catchment cover of acid sulphate soils increased, explained by high amounts of REEs in the acidic runoff from such soils. In each stream, four different REE fractions were identified: (1) A cationic fraction, which is dominant in the REE-rich runoff from the acid sulphate soils and which is depleted in HREEs due to hydrochemical and/or geochemical processes, (2) an anionic fraction identified as humus-REE complexes, which in general is more abundant the higher the DOC concentrations and which also becomes increasingly abundant across the lanthanide series, (3) a fraction having a well-developed MREE enrichment, presumably consisting of colloidal REEs, and (4) a minor uncharacterised particle-associated fraction. The REE pool in the streams thus consists of several coexisting and contrasting REE species. The identification and quantification of such species is a prerequisite for the precise and accurate characterisation of the REE hydrochemistry of the streams.  相似文献   
79.
大门具有众多功能实在的景观价值和文化内涵。认识大门与围墙、主体建筑、道路、山水植物及雕塑等要素的景观配置艺术是必要的。  相似文献   
80.

Objective

To investigate blood lead level and its relationship to copper, zinc, calcium, magnesium and iron in the children aged 0 to 14 years old from Beijing, China.

Methods

We classified 3181 children into one of the four groups: Group A (n = 783, < 1 year old); Group B (n = 1538, 1-3 years old); Group C (n = 443, 3-7 years old); and, Group D (n = 417, 7-14 years old). All these metal elements were determined by atomic absorption spectrometry.

Results

The blood lead level was 0.207 ± 0.105 μmol/L. There was a significant gender difference for zinc (P < 0.05) in Group C, and there was also a significant gender difference for copper (P < 0.05) and lead (P < 0.05) in Group D. Controlling for gender and age, we observed that there was a negative correlation of lead with zinc (r = − 0.052, P < 0.01), magnesium (r = − 0.042, P < 0.05) and iron (r = − 0.031, P < 0.05), respectively. Furthermore, in the children aged 1-7 years old, we also found there was a negative linear correlation of lead with zinc, magnesium and iron, respectively (P < 0.01).

Conclusion

Blood lead level in children from Beijing was markedly decreased. And deficiency of zinc, magnesium and iron is related to the elevated blood lead level in the children aged 1-7 years.  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