全文获取类型
收费全文 | 59651篇 |
免费 | 4173篇 |
国内免费 | 1343篇 |
专业分类
电工技术 | 520篇 |
综合类 | 2853篇 |
化学工业 | 27698篇 |
金属工艺 | 1431篇 |
机械仪表 | 578篇 |
建筑科学 | 790篇 |
矿业工程 | 1146篇 |
能源动力 | 1220篇 |
轻工业 | 19280篇 |
水利工程 | 93篇 |
石油天然气 | 3382篇 |
武器工业 | 72篇 |
无线电 | 402篇 |
一般工业技术 | 2794篇 |
冶金工业 | 2096篇 |
原子能技术 | 478篇 |
自动化技术 | 334篇 |
出版年
2024年 | 282篇 |
2023年 | 783篇 |
2022年 | 1688篇 |
2021年 | 2083篇 |
2020年 | 1831篇 |
2019年 | 1786篇 |
2018年 | 1574篇 |
2017年 | 2017篇 |
2016年 | 2094篇 |
2015年 | 2060篇 |
2014年 | 3086篇 |
2013年 | 3642篇 |
2012年 | 4171篇 |
2011年 | 4278篇 |
2010年 | 3076篇 |
2009年 | 3135篇 |
2008年 | 2703篇 |
2007年 | 3532篇 |
2006年 | 3262篇 |
2005年 | 2754篇 |
2004年 | 2339篇 |
2003年 | 2028篇 |
2002年 | 1793篇 |
2001年 | 1539篇 |
2000年 | 1398篇 |
1999年 | 1198篇 |
1998年 | 965篇 |
1997年 | 730篇 |
1996年 | 635篇 |
1995年 | 517篇 |
1994年 | 462篇 |
1993年 | 383篇 |
1992年 | 284篇 |
1991年 | 226篇 |
1990年 | 175篇 |
1989年 | 110篇 |
1988年 | 76篇 |
1987年 | 92篇 |
1986年 | 62篇 |
1985年 | 76篇 |
1984年 | 74篇 |
1983年 | 37篇 |
1982年 | 25篇 |
1981年 | 18篇 |
1980年 | 29篇 |
1979年 | 12篇 |
1978年 | 10篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1974年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
实验研究了以工业硫酸亚铁及废硫酸为原料,分别采用NaClo_3、KClO_3、H_2O_2及(o_2+NaNO_2)为氧化剂成聚硫酸铁的条件。在以(O_2+NaNO_2)为氧化剂的合成过程中,发现添加微量助催化剂(型号为HG-1、HG-2和HG-3),对提高合成速度及反应过程的稳定性具有重大作用。 相似文献
12.
13.
14.
15.
16.
17.
18.
TeMxMo1.7O mixed oxides (M = V and/or Nb; x = 0-1.7) have been prepared by calcination of the corresponding salts at 600 °C in an atmosphere of N2. A new crystalline phase, with a Te/V/Mo atomic ratio of 1/0.2-1.5/1.7, has been isolated and characterised by XRD and IR spectroscopy. This phase is observed in the TeVMo or TeVNbMo mixed oxide but not in the TeNbMo mixed oxide. The new crystalline phase shows an XRD pattern similar to Sb4Mo10O31 and probably corresponds to the M1 phase recently proposed by Aouine et al. (Chem. Commun. 1180, 2001) to be present in the active and selective MoVTeNbO catalysts. Although these catalysts present a very low activity in the propane oxidation, they are active and selective in the oxidation of propene to acrolein and/or acrylic acid. However, the product distribution depends on the catalyst composition. Acrolein or acrylic acid can be selectively obtained from propene on Nb-free or Nb-containing TeVMo catalysts, respectively. The presence of both V and Nb, in addition to Mo and Te, appears to be important in the formation of acrylic acid from propene. 相似文献
19.
20.
A series of new alternating aromatic poly(ester‐imide)s were prepared by the polycondensation of the preformed imide ring‐containing diacids, 2,2′‐bis(4‐trimellitimidophenoxy)biphenyl (2a) and 2,2′‐bis(4‐trimellitimidophenoxy)‐1,1′‐binaphthyl (2b) with various aromatic dihydroxy compounds in the presence of pyridine and lithium chloride. A model compound (3) was also prepared by the reaction of 2b with phenol, its synthesis permitting an optimization of polymerization conditions. Poly(ester‐imides) were fully characterized by FTIR, UV‐vis and NMR spectroscopy. Both biphenylene‐ and binaphthylene‐based poly(ester‐imide)s exhibited excellent solubility in common organic solvents such as tetrahydrofuran, m‐cresol, pyridine and dichloromethane. However, binaphthylene‐based poly(ester‐imide)s were more soluble than those of biphenylene‐based polymers in highly polar organic solvents, including N‐methyl‐2‐pyrrolidone, N,N‐dimethylacetamide, N,N‐dimethylformamide and dimethyl sulfoxide. From differential scanning calorimetry thermograms, the polymers showed glass‐transition temperatures between 261 and 315 °C. Thermal behaviour of the polymers obtained was characterized by thermogravimetric analysis, and the 10 % weight loss temperatures of the poly(ester‐imide)s was in the range 449–491 °C in nitrogen. Furthermore, crystallinity of the polymers was estimated by means of wide‐angle X‐ray diffraction. The resultant poly(ester‐imide)s exhibited nearly an amorphous nature, except poly(ester‐imide)s derived from hydroquinone and 4,4′‐dihydroxybiphenyl. In general, polymers containing binaphthyl units showed higher thermal stability but lower crystallinity than polymers containing biphenyl units. Copyright © 2005 Society of Chemical Industry 相似文献