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   1742篇
  免费   74篇
  国内免费   170篇
电工技术   14篇
综合类   104篇
化学工业   608篇
金属工艺   158篇
机械仪表   40篇
建筑科学   46篇
矿业工程   18篇
能源动力   35篇
轻工业   277篇
水利工程   5篇
石油天然气   149篇
武器工业   7篇
无线电   43篇
一般工业技术   182篇
冶金工业   78篇
原子能技术   190篇
自动化技术   32篇
  2023年   30篇
  2022年   57篇
  2021年   89篇
  2020年   80篇
  2019年   60篇
  2018年   40篇
  2017年   56篇
  2016年   92篇
  2015年   73篇
  2014年   133篇
  2013年   138篇
  2012年   214篇
  2011年   178篇
  2010年   137篇
  2009年   106篇
  2008年   98篇
  2007年   88篇
  2006年   68篇
  2005年   57篇
  2004年   41篇
  2003年   22篇
  2002年   17篇
  2001年   21篇
  2000年   18篇
  1999年   8篇
  1998年   4篇
  1997年   5篇
  1996年   12篇
  1995年   7篇
  1994年   3篇
  1993年   5篇
  1992年   4篇
  1991年   4篇
  1990年   2篇
  1989年   2篇
  1987年   2篇
  1985年   4篇
  1984年   3篇
  1982年   3篇
  1981年   2篇
  1980年   2篇
  1959年   1篇
排序方式: 共有1986条查询结果,搜索用时 15 毫秒
1.
在航空γ剂量率测量过程中由于海拔高度跨度大,γ射线空气衰减系数会受到海拔高度、温度、气压与空气密度的影响,故需要进行相应修正。利用蒙特卡罗(MCNP)法模拟γ射线在不同空气密度下的线衰减系数。利用经验公式计算不同海拔高度下的空气密度,利用MCNP法建立γ谱仪模型,计算不同海拔高度、不同密度条件下的γ光子注量,根据指数衰减原理计算不同空气密度下的线衰减系数。结果表明,由经验公式求出的空气密度模拟得出的空气质量衰减系数与美国NIST推荐值最大相对误差为-17.3%;在γ射线参考辐射场中用衰减板进行实验验证,60Co源的最大相对误差为6.0%,137Cs源的最大相对误差为5.3%。本工作为后续低空近地辐射剂量的研究工作奠定了基础。  相似文献   
2.
It is extremely desirable to develop high hydrogen evolution activity and stable visible-light-driven photocatalysts. The sluggish oxidation process and holes accumulation are the main obstacles to high catalysis activity and photo-stability. An efficient γ-NiOOH/ZnCdS photocatalyst was prepared by in-situ hydrothermal method. The γ-NiOOH nanosheets distribute on ZnCdS nanospheres surface and accelerate holes transfer. The hydrogen evolution rate is up to 48.60 mmol g?1 h?1 under visible-light illumination (λ = 400–780 nm), about 10.8 times of pure ZnCdS (4.50 mmol g?1 h?1) and 1.8 times of general β-NiOOH modified ZnCdS (27.40 mmol g?1 h?1). And apparent quantum yield of γ-NiOOH/ZCS-100 is up to 18.23% (400 nm). The carrier lifetime extends from 5.50 ns (ZnCdS) to 6.10 ns (γ-NiOOH/ZCS), examined by steady photoluminescence and time-resolved photoluminescence. Moreover, the γ-NiOOH/ZCS photocatalyst has exhibited excellent photo-stability even after one-year of storage. The γ-NiOOH nanosheets can be an excellent co-catalyst on accelerating both holes transfer and oxidation process for high photo-stability and photo-activity.  相似文献   
3.
In view of the practical application of γ-AlON as a promising transparent structural ceramic, in-depth insight into its mechanical and thermal properties is essential. The solid-state MAS NMR technique was combined with XRD Rietveld refinement to confirm the crystal structure of Al(8+x)/3O4-xNx (x = 0.299–0.575). These structural parameters were further applied to predict hardness and elastic properties based on theoretical exploration, which are in good agreement with the experimental values. A slight enhancement of mechanical properties with increasing nitrogen concentration is attributed to the stronger chemical bond in octahedra. The experimental thermal conductivity of γ-AlON transparent ceramics was improved slightly with the rise of x in the temperature range from 298 K to 1074 K. The intrinsic lattice thermal conductivity was determined by eliminating the extrinsic phonon scattering as well as the thermal radiation. The reason for the discrepancy between experimental and intrinsic thermal conductivity was revealed. The present methods provided powerful and accessible guidelines in optimizing the mechanical and thermal properties of oxynitride materials.  相似文献   
4.
N_Ox storage and reduction(NSR) technology has been regarded as one of the most promising strategies for the removal of nitric oxides(NO_x) from lean-burn engines, and the potential of the plasma catalysis method for NO_x reduction has been confirmed in the past few decades. This work reports the NSR of nitric oxide(NO) by combining non-thermal plasma(NTP) and Co/Pt/Ba/γ-Al_2O_3(Co/PBA) catalyst using methane as a reductant. The experimental results reveal that the NO_x conversion of NSR assisted by NTP is notably enhanced compared to the catalytic efficiency obtained from NSR in the range of 150 °C–350 °C, and NO_x conversion of the 8% Co/PBA catalyst reaches 96.8% at 350°C. Oxygen(O_2) has a significant effect on the removal of NO_x, and the NO_x conversion increases firstly and then decreases when the O_2 concentration ranges from 2% to 10%. Water vapor reduces the NO_x storage capacity of Co/PBA catalysts on account of the competition for adsorption sites on the surface of Co/PBA catalysts. There is a negative correlation between sulfur dioxide(SO_2) and NO_x conversion in the NTP system, and the 8% Co/PBA catalyst exhibits higher NO_x conversion compared to other catalysts, which shows that Co has a certain SO_2 resistance.  相似文献   
5.
本文用MCNP软件进行模拟计算,设计了屏蔽一定能量中子和γ射线的屏蔽材料,用此屏蔽材料制作中子、γ外照射防护马甲,防护马甲存在厚又重的问题,人体很难承受它的重量。通过研究助力型外骨骼装置,将助力型外骨骼装置与防护马甲组合形成防护服,用助力型外骨骼装置承受防护马甲的重量,解决了人体需要承重的难题。并用放射源和在现场对研制的防护服进行性能测试,屏蔽率、厚度、均匀性等各项技术参数均达到预期要求。  相似文献   
6.
为了探讨γ-氨基丁酸(γ-aminobutyric acid, GABA)对采后香菇品质的影响,本文研究了不同浓度的GABA处理对香菇外观和营养品质的影响。结果表明,不同浓度的GABA处理能够不同程度的降低香菇的失重率、相对电导率和丙二醛含量,抑制香菇的呼吸作用、较好的保持香菇的L*值和硬度,同时促进抗坏血酸、总酚和蛋白质的积累。从呼吸强度来看,GABA处理仅在贮藏前期(9 d)抑制香菇的呼吸作用,其中5.0 g·L?1 GABA处理的效果最好;从失重率、L*值、硬度等外观品质以及抗坏血酸、总酚、蛋白质等品质指标来看,5.0 g·L?1 GABA处理的效果最好;从相对电导率和丙二醛含量来看,2.5 g·L?1 GABA处理的效果最好,但是与5.0 g·L?1 GABA处理之间无显著差异。综合考虑GABA对香菇各项指标的影响,确定其最佳处理浓度为5.0 g·L?1。以上结果表明GABA可以通过抑制呼吸,减缓细胞膜的透性的增大,减少香菇的失水进而较好的保持香菇的硬度和色度;同时能够通过促进抗坏血酸、总酚和蛋白质的积累来保持香菇的营养品质。  相似文献   
7.
8.
9.
In this study, the accumulation of GABA and its inherent factors across different varieties of mung bean (Vigna radiata L.) in response to heat and relative humidity (HRH) were investigated. Results showed the average GABA content in mung bean varieties was increased 7.52 times following HRH treatment, and the black mung bean variety (A8) exhibited the highest GABA accumulation capability (1.76–84.57 mg per 100 g DW). From the perspective of GABA shunt metabolites, the free glutamic acid content of mung beans significantly decreased (P < 0.05) after HRH treatment and presented a significant correlation (P < 0.05) with GABA content. In polyamine degradation pathway, although the average levels of spermine and spermidine of mung bean varieties significantly decreased (P < 0.05) after HRH treatment, no significant correlation with GABA content was identified. Hence, the GABA accumulation was predominantly attributed to GABA shunt. Besides, free amino acids including glutamic acid, serine, ornithine, arginine and glycine in mung beans showed a significant positive correlation (P < 0.05) with GABA content and increment following HRH treatment, which suggested that mung beans enriched in these free amino acids might accumulate higher amounts of GABA after HRH treatment and be useful for industrial applications.  相似文献   
10.
以一种第四代镍基单晶高温合金为研究对象,采用变截面蠕变试样,在1100℃、43~96 MPa条件下进行200 h蠕变中断实验,利用SEM和TEM观察了微观组织演变规律,利用同步辐射高能XRD和EPMA分析了高温低应力条件下镍基单晶高温合金的蠕变组织演变。结果表明:随着应力的增大,镍基单晶高温合金的γ′相体积分数降低,筏化程度增大且筏排厚度下降,同时,γ相通道宽度逐渐增大,而γ/γ’两相界面位错网间距逐渐减小。固溶强化元素Re、Mo和Cr等在γ相中的富集导致γ/γ’两相错配度绝对值增大。蠕变过程中γ’相体积分数降低和γ’相筏排厚度减小显著降低了合金的强度。另外,位错在γ′相溶解所导致的弯曲相界处的塞积,使位错易于切入γ′相,也是镍基单晶高温合金室温硬度下降的重要原因。  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