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   30428篇
  免费   2536篇
  国内免费   1390篇
电工技术   883篇
技术理论   1篇
综合类   990篇
化学工业   10212篇
金属工艺   2939篇
机械仪表   492篇
建筑科学   339篇
矿业工程   959篇
能源动力   3723篇
轻工业   691篇
水利工程   33篇
石油天然气   725篇
武器工业   61篇
无线电   2258篇
一般工业技术   6357篇
冶金工业   2929篇
原子能技术   337篇
自动化技术   425篇
  2024年   88篇
  2023年   655篇
  2022年   998篇
  2021年   1293篇
  2020年   1170篇
  2019年   1148篇
  2018年   1090篇
  2017年   1146篇
  2016年   1022篇
  2015年   1013篇
  2014年   1581篇
  2013年   1744篇
  2012年   1859篇
  2011年   2535篇
  2010年   1888篇
  2009年   1789篇
  2008年   1537篇
  2007年   1751篇
  2006年   1534篇
  2005年   1243篇
  2004年   1085篇
  2003年   988篇
  2002年   833篇
  2001年   699篇
  2000年   680篇
  1999年   511篇
  1998年   431篇
  1997年   345篇
  1996年   305篇
  1995年   236篇
  1994年   218篇
  1993年   153篇
  1992年   171篇
  1991年   125篇
  1990年   121篇
  1989年   92篇
  1988年   58篇
  1987年   36篇
  1986年   15篇
  1985年   32篇
  1984年   25篇
  1983年   16篇
  1982年   25篇
  1981年   19篇
  1980年   14篇
  1979年   10篇
  1978年   4篇
  1977年   4篇
  1959年   5篇
  1951年   7篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
141.
用共沉淀法制备了TiO2-ZrO2复合氧化物载体,BET,DTA,XRD表征结果表明,该复合载体具有较大的比表面和较好的热稳定性,对于环己酮肟气相Beckmann重排反应制己内酰胺,用复合载体制备的B2O3/TiO2-ZrO2催化剂,比分别以TiO2,ZrO2为载体所制备的B2O3/TiO2和B2O3/ZrO2催化剂具有更高的活性和选择性,实验表明,催化剂表面中等强度的酸位是环己酮肟气相Beckmann重排反应的有效活性中心。  相似文献   
142.
中国软磁铁氧体用氧化铁   总被引:1,自引:0,他引:1  
介绍了中国软磁铁氧体用氧化铁的发展历史和现状,预测了未来的市场需求,并探讨了氧化铁的应用技术问题。  相似文献   
143.
Hall measurements have been used to compare the properties of 4H-SiC inversion-mode MOSFETs with “wet” and “dry” gate oxides. While the field-effect mobilities were approximately 3–5 cm2/Vs, the Hall mobilities in 4H-SiC MOSFETs in the wet and dry oxide samples were approximately 70–80 cm2/Vs. The dry-oxidized metal oxide semiconductor field effect transistors (MOSFETs) had a higher transconductance, improved threshold voltage, improved subthreshold slope, and a higher inversion carrier concentration compared to the wet-oxidized MOSFETs. The difference in characteristics between the wet- and the dry-oxidized MOSFETs is attributed to the larger fixed oxide charge in the dry oxide sample and a higher interface trap density in the wet oxide sample.  相似文献   
144.
谈我国冶金中小企业的融资方式及融资创新   总被引:3,自引:0,他引:3  
徐振华 《有色矿冶》2002,18(2):61-63
受各种条件的制约,长期以来我国冶金中小企业主要依靠自身积累、银行贷款和社会集资等形式融通资金。这样不可避免地要限制资金的使用规模,因此传统的融资方式不能适应企业快速发展的需要。随着我国经济体制改革的深化和资本市场的不断完善,信用担保、上市融资、股权融资、风险投资基金、职工持股等融资手段已经成为我国冶金中小企业新的融资方式。  相似文献   
145.
Al-P-Ti-Si-O催化剂上合成邻羟基苯乙醚   总被引:6,自引:1,他引:5  
采用不均匀沉淀法制备了Al-P -Ti-Si-O催化剂 ,并研究了邻苯二酚与乙醇在该催化剂上的气相O -乙基化反应 ,考察了催化剂组成、反应温度、邻苯二酚 /乙醇摩尔比、进料速率对反应性能的影响 ,同时考察了催化剂的稳定性。结果表明 ,催化剂组成对反应性能有较大影响 ,在研究的范围内 ,四组分催化剂Al1 P1 3 0 Ti0 3 0 Si0 1 7显示出了较好的催化性能 ,当催化剂用量 1 4g、n(邻苯二酚 ) /n(乙醇 ) =1/5、进料速率 0 5ml/h ,反应温度 2 6 0℃时 ,邻苯二酚转化率和邻羟基苯乙醚收率分别可达到 91 5 %和 85 3% ,并在 10 0h内催化剂具有很好的稳定性  相似文献   
146.
Cold cracking of structural steel weldments is a well-characterized, well-documented, and well-understood failure mechanism. Extensive effort has been put forth to recognize the welding and materials selection parameters that are conducive to cold cracking; however, these engineering efforts have not fully eliminated the occurrence of such failures. This article describes cold cracking failure specifically related to the construction industry. This particular failure was successfully identified prior to final erection of the structural member, and the weld was successfully reworked. These actions potentially prevented a serious catastrophic event that could have occurred have occurred either later in the construction process or possibly during the use of the building. Individual welding parameters, such as electrode/wire selection, joint design, and pre/postheating, played a role in the failure, and a number of human factors relating to the actual fabrication practices also contributed to the failure process.  相似文献   
147.
利用有限元分析方法数值模拟了界面形状对氧化物高温熔体碟形浮区中的温度分布和重力对流速率的影响,发现界面形状对重力对流速率有较大的影响,对于凸面,相对于炉圈平面5%的偏离使重力对流速率增大25%,偏离越大,重力对流速度越大,对于凹面,5%的偏离使重力对流速率减小15.7%,偏离越大,重力对流速度越小。  相似文献   
148.
The structural and electrochemical properties of LiNiO2 powders were investigated as a function of the oxygen flow rate employed in the preparation of lithium nickel oxide. It was found that oxygen played an important role in the synthesis of highly crystallized LiNiO2(Rm). In the crystallization process of LiNiO2, a deficiency of oxygen in the calcination reactor induced the formation of impurities and cubic rock-salt structure (Fm3m) in LiNiO2 powders. For LiNiO2 prepared at higher oxygen flow rates, the electrode delivered high discharge capacities with relatively good retention rates. But very low electrode capacity was obtained from LiNiO2 prepared at lower oxygen flow rates.  相似文献   
149.
The influence of convective heat transfer on constant current density anodizing of aluminium in sulfuric acid has been examined in a wall-jet electrode reactor. The uniformity of the anodic film thickness is related to the local electrode temperature distribution, which is dependent on the convection. The higher the local temperature, the greater the local oxide thickness. An increased local temperature enhances local field assisted oxide dissolution at the pore bases, and consequently acts to increase the local current density. At relatively high current densities, local features develop on the electrode surface, accompanied by high initial, local temperature rises. The relevance of such local features, limiting useful oxide growth, is considered further.  相似文献   
150.
Tin oxide–silica composite aerogels were successfully prepared with a co-fed precursor sol–gel process. The crystallinity of the tin oxide nanoparticles, embedded in the mesoporous SiO2 network, was improved with increasing the post-reaction thermal treatment temperature. The composite aerogels exhibited a rich photoluminescence (PL) emission contributed by both SnO2 and SiO2. The PL peak of 346 nm was from the near band edge emission of the tin oxide nanoparticles, and the ones located at 310 and 476 nm were attributable to the oxygen deficiencies of the silica network. Three more emission peaks, 387, 432, and 522 nm, were observed, with the 387 nm peak contributed by the oxygen vacancies VO++, the 432 nm peak by the Sn interstitials, and the 522 nm peak by the oxygen vacancies VO+, respectively, of the tin oxide nanoparticles. The intensities of these three defect level emissions were found decreased, as compared to that of the near band edge emission, with increasing the post-reaction thermal treatment temperature as the tin oxide crystallinity improved.  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