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   1868817篇
  免费   51149篇
  国内免费   18943篇
电工技术   51901篇
技术理论   56篇
综合类   25610篇
化学工业   326512篇
金属工艺   81670篇
机械仪表   64386篇
建筑科学   72455篇
矿业工程   22418篇
能源动力   59113篇
轻工业   137412篇
水利工程   20881篇
石油天然气   59078篇
武器工业   2742篇
无线电   230095篇
一般工业技术   325752篇
冶金工业   211190篇
原子能技术   37202篇
自动化技术   210436篇
  2022年   18839篇
  2021年   27516篇
  2020年   21204篇
  2019年   22054篇
  2018年   25675篇
  2017年   26289篇
  2016年   30578篇
  2015年   29136篇
  2014年   43778篇
  2013年   105503篇
  2012年   56285篇
  2011年   71665篇
  2010年   61979篇
  2009年   68887篇
  2008年   63734篇
  2007年   60452篇
  2006年   63039篇
  2005年   55885篇
  2004年   51959篇
  2003年   50611篇
  2002年   48512篇
  2001年   45208篇
  2000年   44041篇
  1999年   45285篇
  1998年   62414篇
  1997年   51388篇
  1996年   44904篇
  1995年   37790篇
  1994年   34515篇
  1993年   33090篇
  1992年   28836篇
  1991年   25486篇
  1990年   25388篇
  1989年   24225篇
  1988年   22572篇
  1987年   20531篇
  1986年   19730篇
  1985年   22813篇
  1984年   22699篇
  1983年   20604篇
  1982年   19593篇
  1981年   19712篇
  1980年   18306篇
  1979年   18663篇
  1978年   17917篇
  1977年   18085篇
  1976年   19750篇
  1975年   16073篇
  1974年   15544篇
  1973年   15660篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
A set of novel hydrazone derivatives were synthesized and analyzed for their biological activities. The compounds were tested for their inhibitory effect on the phosphorylating activity of the protein kinase CK2, and their antioxidant activity was also determined in three commonly used assays. The hydrazones were evaluated for their radical scavenging against the DPPH, ABTS and peroxyl radicals. Several compounds have been identified as good antioxidants as well as potent protein kinase CK2 inhibitors. Most hydrazones containing a 4-N(CH3)2 residue or perfluorinated phenyl rings showed high activity in the radical-scavenging assays and possess nanomolar IC50 values in the kinase assays.  相似文献   
52.
目的:设计一款满足安全、卫生、操作便捷、结构性能稳定等要求的糖霜制品3D打印机.方法:打印机主要包括机械结构和控制系统,机械结构方面采用龙门架结构,并对传统的打印料罐进行改良.控制系统采用“工控机+运动控制卡”的模式.结果:该打印机可快速更换料罐(30s内),打印机成型速度快(≥5000mm/min),速度可调;成型精度控制在1mm 以内;成型件的强度、膨胀率和表面粗糙度指标均较好.自带光源的图像识别及处理系统能够识别产品形状、个数及完整性等.结论:改良后的料罐提高了半流体食品打印的卫生性、生产效率,且便于耗材的更换,能加工出复杂糖画模型以满足造型独特的个性化糖艺制品的需求.  相似文献   
53.
Cell surface proteoglycans are known to be important regulators of many aspects of cell behavior. The principal family of transmembrane proteoglycans is the syndecans, of which there are four in mammals. Syndecan-1 is mostly restricted to epithelia, and bears heparan sulfate chains that are capable of interacting with a large array of polypeptides, including extracellular matrix components and potent mediators of proliferation, adhesion and migration. For this reason, it has been studied extensively with respect to carcinomas and tumor progression. Frequently, but not always, syndecan-1 levels decrease as tumor grade, stage and invasiveness and dedifferentiation increase. This parallels experiments that show depletion of syndecan-1 can be accompanied by loss of cadherin-mediated adhesion. However, in some tumors, levels of syndecan-1 increase, but the characterization of its distribution is relevant. There can be loss of membrane staining, but acquisition of cytoplasmic and/or nuclear staining that is abnormal. Moreover, the appearance of syndecan-1 in the tumor stroma, either associated with its cellular component or the collagenous matrix, is nearly always a sign of poor prognosis. Given its relevance to myeloma progression, syndecan-1-directed antibody—toxin conjugates are being tested in clinical and preclinical trials, and may have future relevance to some carcinomas.  相似文献   
54.
Elevated activation of the autophagy pathway is currently thought to be one of the survival mechanisms allowing therapy-resistant cancer cells to escape elimination, including for cytarabine (AraC)-resistant acute myeloid leukemia (AML) patients. Consequently, the use of autophagy inhibitors such as chloroquine (CQ) is being explored for the re-sensitization of AraC-resistant cells. In our study, no difference in the activity of the autophagy pathway was detected when comparing AraC-Res AML cell lines to parental AraC-sensitive AML cell lines. Furthermore, treatment with autophagy inhibitors CQ, 3-Methyladenine (3-MA), and bafilomycin A1 (BafA1) did not re-sensitize AraC-Res AML cell lines to AraC treatment. However, in parental AraC-sensitive AML cells, treatment with AraC did activate autophagy and, correspondingly, combination of AraC with autophagy inhibitors strongly reduced cell viability. Notably, the combination of these drugs also yielded the highest level of cell death in a panel of patient-derived AML samples even though not being additive. Furthermore, there was no difference in the cytotoxic effect of autophagy inhibition during AraC treatment in matched de novo and relapse samples with differential sensitivity to AraC. Thus, inhibition of autophagy may improve AraC efficacy in AML patients, but does not seem warranted for the treatment of AML patients that have relapsed with AraC-resistant disease.  相似文献   
55.
To investigate the evolution of the structural and enhanced magnetic properties of GdMnO3 systems induced by the substitution of Mn with Cr, polycrystalline GdMn1-xCrxO3 samples were synthesized via solid-state reactions. XRD characterization shows that all GdMn1-xCrxO3 compounds with single-phase structures crystallize well and that Cr3+ ions entering the lattice sites of GdMnO3 induce structural distortion. SEM results indicate that the grain size of the synthesized samples (a few microns) decreases as the Cr substitution concentration increases. Positron annihilation lifetime spectroscopy reveals that vacancy-type defects occur in GdMn1-xCrxO3 ceramics and that the vacancy size and concentration clearly change with the Cr content. The temperature and field dependence of the magnetization curves show that Cr substitution significantly influences the magnetic ordering of the gadolinium sublattice, improving the weak ferromagnetic transition temperature and magnetization of GdMn1-xCrxO3. The enhanced magnetization of GdMn1-xCrxO3 is closely related to the vacancy defect concentration.  相似文献   
56.
Wireless Personal Communications - Besides anti-malware usage for the eradication of malicious attacks, researchers have developed epidemic models in order to gain more insights into the spread...  相似文献   
57.
Telecommunication Systems - Energy saving in User Equipment (UE) is one of the important issues for limited sources of power in the device. It is critical for the UE to maximize its energy...  相似文献   
58.
Semiconductors - Abstract—In our work, we carry out a structural-spectroscopic study of AlGaN/GaN epitaxial layers grown by molecular-beam epitaxy with nitrogen-plasma activation on a hybrid...  相似文献   
59.
Journal of Communications Technology and Electronics - A new design of wavelets based on the convolution of a compactly supported function with a rectangular pulse is proposed and theoretically...  相似文献   
60.
Journal of Communications Technology and Electronics - A statistical study of the effectiveness of the non-threshold search procedure for a noise-like phase-shift keyed signal by the delay time is...  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