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   8429篇
  免费   1038篇
  国内免费   329篇
电工技术   73篇
综合类   647篇
化学工业   4660篇
金属工艺   323篇
机械仪表   91篇
建筑科学   384篇
矿业工程   363篇
能源动力   82篇
轻工业   1122篇
水利工程   68篇
石油天然气   397篇
武器工业   35篇
无线电   137篇
一般工业技术   569篇
冶金工业   730篇
原子能技术   73篇
自动化技术   42篇
  2024年   28篇
  2023年   105篇
  2022年   237篇
  2021年   265篇
  2020年   264篇
  2019年   239篇
  2018年   214篇
  2017年   278篇
  2016年   303篇
  2015年   312篇
  2014年   518篇
  2013年   536篇
  2012年   654篇
  2011年   627篇
  2010年   456篇
  2009年   460篇
  2008年   357篇
  2007年   508篇
  2006年   490篇
  2005年   446篇
  2004年   397篇
  2003年   331篇
  2002年   319篇
  2001年   268篇
  2000年   228篇
  1999年   192篇
  1998年   165篇
  1997年   103篇
  1996年   99篇
  1995年   74篇
  1994年   68篇
  1993年   42篇
  1992年   33篇
  1991年   38篇
  1990年   26篇
  1989年   16篇
  1988年   14篇
  1987年   18篇
  1986年   13篇
  1985年   6篇
  1984年   14篇
  1983年   7篇
  1982年   6篇
  1981年   6篇
  1980年   5篇
  1979年   2篇
  1976年   2篇
  1962年   1篇
  1959年   1篇
  1951年   2篇
排序方式: 共有9796条查询结果,搜索用时 15 毫秒
1.
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   
2.
A strategy that constructs the morphotropic phase boundary and manipulates the domain structure has been used to design the component of 0.96[Bi0.5(Na0.84K0.16)0.5Ti(1-x)NbxO3]-0.04SrTiO3 (BNKT-4ST-100xNb) to enhance the strain properties for actuator application. Non-equivalent Nb5+ donor doping modulates the phase transition from the mixture of rhombohedral and tetragonal phases to the pseudocubic phase and results in the coexistence of multiple phases. Moreover, the high-resolution TEM confirms the existence of polar nano regions that contribute to the macroscopic relaxor behaviour. The size of the domains is reduced with increasing Nb5+, resulting in an enhanced relaxor behaviour. The ferroelectric-relaxor transition temperature decreases from 85 to below 30 °C, implying a non-ergodic to ergodic relaxor transition. An improved strain of 0.56% and a giant normalized strain of 1120 pm/V were achieved for BNKT-4ST-1.5Nb, which were attributed to the unique domain structure in which nanodomains are embedded in an undistorted cubic matrix. Ferroelectric, antiferroelectric, and relaxor phases coexist. As the electric field is large enough, a reversible phase transition occurs. Furthermore, good temperature stability was obtained due to the stability of the nanodomains, and no degradation in strains was observed even after 104 cycles, which may originate from the reversible phase transition and dynamic domain wall. The results show that this design strategy offers a reference way to improve the strain behaviour and that BNKT-4ST-100xNb ceramics could be a potential material for high-displacement actuator applications.  相似文献   
3.
采用水溶液聚合法制备了低分子量聚丙烯酸钾(PAAK),并作为新型消焰剂加入单基发射药中。通过火焰原子吸收光谱法测试了PAAK中钾的含量;用乌氏黏度计测定了特性黏度;采用DSC法研究不同pH值的PAAK与硝化棉(NC)的相容性;利用充氮氧弹法对添加PAAK、硝酸钾KNO3、硫酸钾K2SO4的单基发射药的燃烧残渣进行了对比研究。结果表明,合成的PAAK中,钾的质量分数为15.21%,相对分子量在3 000左右,有利于和NC均匀混合,且在中性或微碱性(pH=7.0~7.5)的情况与NC相容性良好。与传统的KNO3、K2SO4消焰剂相比,PAAK能够和NC均匀混合,制备均质透明的单基发射药;PAAK发射药的燃烧残渣最少,占发射药质量的0.18%。  相似文献   
4.
A hybrid sorbent material for removal of hydrogen sulfide from air was developed. The material is based on activated carbon and iron compounds obtained from waste iron(II) sulfate(VI) heptahydrate. The iron salt is deposited on the carbonaceous support and subjected to oxidation (Fe2+ to Fe3+) using atmospheric oxygen under alkaline conditions. An effect of H2O2 addition to the process on the composition of the resultant material was also examined. X-ray diffraction (XRD) analyses confirmed easy conversion of waste FeSO4·7H2O to iron oxides Fe3O4 and FeOOH. The activated carbon supporting iron oxides revealed a higher efficiency in H2S elimination from air compared to the commercial activated carbon, without any modification.  相似文献   
5.
ABSTRACT

In this study, effect of calcium and gypsum on scheelite and fluorite was investigated using sodium oleate as collector. Micro-flotation and contact angle results showed that the adsorption of calcium could inhibit the hydrophobicity of scheelite and fluorite. Moreover, sulfate could enhance the inhibition. FT-IR results showed that calcium could be priori precipitated into calcium oleate and adsorb on mineral surface. The adsorption of calcium could increase the scheelite potential to IEP, while it showed limited effect on fluorite potential. However, the interaction of calcium on scheelite and fluorite in gypsum solution was more complex than that in calcium solution.  相似文献   
6.
This paper reports an assessment of the performance of concrete based on a calcium sulfoaluminate–anhydrite–fly ash cement combination. Concretes were prepared at three different w/c ratios and the properties were compared to those of Portland cement and blast-furnace cement concretes. The assessment involved determination of mechanical and durability properties. The results suggest that an advantageous synergistic effect between and ettringite and fly ash (Ioannou et al., 2014) was reflected in the concrete’s low water absorption rates, high sulfate resistance, and low chloride diffusion coefficients. However, carbonation depths, considering the dense ettringite-rich microstructure developed, were higher than those observed in Portland cement concretes at a given w/c ratio. It was concluded that the amount of alkali hydroxides present in the pore solution is as important factor as the w/c ratio when performance of this type of concrete is addressed.  相似文献   
7.
介绍了硫磷混合酸溶样重铬酸钾容量法测定矿石中铁含量的方法和步骤。重点讨论了一线生产中的质量控制。  相似文献   
8.
9.
The effectiveness of antimicrobial mixtures against Listeria innocua 7, used as a L. monocytogenes surrogate, was investigated in broth and a food system. Synergistic effects were found for nisin (Nis), potassium sorbate (PS), calcium propionate (CP) and sodium lactate (SL), Nis + PS being the most effective binary mixture that exhibited listericidal activity in broth. To assess the effect of adding lactocin AL705 (AL705) to Nis + organic acid salt combinations, tridimensional isobolograms were generated. Sub-MIC combinations of the antimicrobials exerted bactericidal activity against L. innocua 7 after AL705 addition to the binary mixtures. However, when applied on Sardo cheese contaminated with L. innocua 7 (initial inoculum 4.45 ± 0.06 CFU g−1), only Nis + PS + AL705 produced count reductions respect to the control, reaching 3.04 ± 0.35 CFU g−1 counts after 15 days at 15 °C. Ternary combinations containing AL705 showed potential to reduce antimicrobial usages for L. innocua 7 inhibition.  相似文献   
10.
采用新工艺回收7-氨基-3-脱乙酰氧基头孢烷酸(7-ADCA)生产中产生的苯乙酸,并对苯乙酸回收过程中产生的废硫酸进行处理。采用硫酸(9 8%)洗涤二氯甲烷,硫酸与二氯甲烷适宜的体积比为1∶50,洗涤好的二氯甲烷中含苯乙酸4%。蒸馏出二氯甲烷,趁热向熔融苯乙酸中加入母液,降温析出苯乙酸。晾干的苯乙酸为白色鳞片状,含量大于99%,苯乙酸母液补充部分清水套用到下一批析出苯乙酸。废浓硫酸用芬顿试剂氧化,控制80℃氧化4h,双氧水和七水硫酸亚铁重量比为6∶1,废浓硫酸COD下降85%以上。氧化结束后向废硫酸中加入铁粉和双氧水,制得聚合硫酸铁。  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