全文获取类型
收费全文 | 59649篇 |
免费 | 4170篇 |
国内免费 | 1343篇 |
专业分类
电工技术 | 520篇 |
综合类 | 2855篇 |
化学工业 | 27707篇 |
金属工艺 | 1430篇 |
机械仪表 | 578篇 |
建筑科学 | 790篇 |
矿业工程 | 1146篇 |
能源动力 | 1221篇 |
轻工业 | 19274篇 |
水利工程 | 93篇 |
石油天然气 | 3380篇 |
武器工业 | 72篇 |
无线电 | 402篇 |
一般工业技术 | 2790篇 |
冶金工业 | 2091篇 |
原子能技术 | 478篇 |
自动化技术 | 335篇 |
出版年
2024年 | 282篇 |
2023年 | 783篇 |
2022年 | 1688篇 |
2021年 | 2083篇 |
2020年 | 1832篇 |
2019年 | 1785篇 |
2018年 | 1574篇 |
2017年 | 2017篇 |
2016年 | 2092篇 |
2015年 | 2058篇 |
2014年 | 3087篇 |
2013年 | 3644篇 |
2012年 | 4170篇 |
2011年 | 4279篇 |
2010年 | 3074篇 |
2009年 | 3136篇 |
2008年 | 2703篇 |
2007年 | 3530篇 |
2006年 | 3261篇 |
2005年 | 2754篇 |
2004年 | 2339篇 |
2003年 | 2027篇 |
2002年 | 1792篇 |
2001年 | 1538篇 |
2000年 | 1399篇 |
1999年 | 1197篇 |
1998年 | 964篇 |
1997年 | 729篇 |
1996年 | 635篇 |
1995年 | 518篇 |
1994年 | 462篇 |
1993年 | 383篇 |
1992年 | 284篇 |
1991年 | 226篇 |
1990年 | 178篇 |
1989年 | 110篇 |
1988年 | 76篇 |
1987年 | 93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 0 毫秒
11.
12.
13.
14.
15.
16.
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
17.
18.
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
19.
The steady-state performance of a two-stage recycle fermenter with two different types (I and II) of configurations for staging was investigated numerically for the continuous production of lactic acid. In Type I the bleed broth withdrawn from the first stage is supplied to the second fermenter, and in Type II both filtrate and bleed broth from the first stage are introduced to the second stage. Using four different kinetic models taken from the literature, the effects of operating parameters on the overall lactic acid productivities and the overall substrate conversions are examined. At moderate conversions, productivities for Type I operation are found to be higher than those for Type II and for the single-stage system. In the case of high conversions close to complete consumption, Type II operation is more efficient than the Type I and single-stage systems. For 99% conversion and 40 kg m-3 substrate feed concentration, productivities for Type II are predicted to be 14-74% higher compared to those for the single-stage system at the same bleed ratios. 相似文献
20.
Tomi Gominek Andrej Lubej Ciril Pohar 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》2005,80(8):939-947
Precipitation of calcium sulfate dihydrate, gypsum, from (i) a pure sulfuric acid/lime suspension and (ii) a waste sulfuric acid/lime suspension in a continuous pilot plant in the temperature range from 40 °C to 80 °C was studied. It was observed that in the case of waste sulfuric acid with a high content of Mg2+ and Fe2+ ions, several hours after the beginning of the precipitation, partial dissolution of the product and modification of the crystals from needle‐ and plate‐like to agglomerated structures occurred. It is suggested that the secondary changes occur due to the increased concentration of Mg2+ and Fe2+ ions in the reactor. Below 60 °C, and above 70 °C plate‐like and needle‐like single crystals respectively were formed. The mean size (d50) of the crystals was found to increase with increasing temperature and decrease with the initial concentration of lime. Gypsum produced between 70 °C and 80 °C is suitable for further processing for construction plaster. In a precipitation process with pure sulfuric acid only single perfect needle‐like crystals occurred. Copyright © 2005 Society of Chemical Industry 相似文献