全文获取类型
收费全文 | 496篇 |
免费 | 9篇 |
国内免费 | 53篇 |
专业分类
化学 | 398篇 |
晶体学 | 8篇 |
力学 | 2篇 |
物理学 | 150篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2022年 | 4篇 |
2021年 | 4篇 |
2020年 | 10篇 |
2019年 | 3篇 |
2018年 | 11篇 |
2017年 | 9篇 |
2016年 | 5篇 |
2015年 | 11篇 |
2014年 | 10篇 |
2013年 | 19篇 |
2012年 | 26篇 |
2011年 | 35篇 |
2010年 | 29篇 |
2009年 | 23篇 |
2008年 | 31篇 |
2007年 | 38篇 |
2006年 | 33篇 |
2005年 | 28篇 |
2004年 | 22篇 |
2003年 | 19篇 |
2002年 | 19篇 |
2001年 | 20篇 |
2000年 | 11篇 |
1999年 | 20篇 |
1998年 | 17篇 |
1997年 | 8篇 |
1996年 | 17篇 |
1995年 | 5篇 |
1994年 | 9篇 |
1993年 | 15篇 |
1992年 | 10篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 5篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1975年 | 1篇 |
1973年 | 3篇 |
1972年 | 2篇 |
排序方式: 共有558条查询结果,搜索用时 15 毫秒
71.
《Analytical letters》2012,45(7):1237-1243
Abstract This research deals with the quantitative formation of carbon dioxide in electrolytic oxidation reactions. The electrolytic reactions were run with barium peroxide to generate the superoxide anion at the anode. With the organic compounds used in these electrolytic studies we needed to develop a method where we could determine the amount of carbon dioxide liberated from these compounds with the superoxide anion. This method degasses an acidified solution with dry nitrogen, which carries the carbon dioxide to a standard solution of sodium hydroxide. Titration of the sodium hydroxide solution with standardized hydrochloric acid revealed the amount of carbon dioxide formed in the reaction after precipitation of the carbonate ions with barium chloride. Blank runs with the apparatus using anhydrous sodium carbonate produced 99% plus results of recovered carbon dioxide from the sodium carbonate. 相似文献
72.
73.
74.
以过氧化钡(Ba O2)晶体为母体钡源,利用溶液中碱性条件下过氧化氢电离控制Ba O2分解速度,结合过量SO42-制备了具有Ba O2宏观形貌的Ba SO4多晶体,该多晶体由条形结构单元组成。进一步通过不同聚合物添加剂调节次级结构单元的形貌,在溶液中引入聚苯乙烯磺酸钠制备了由球形次级结构单元组成的Ba SO4多晶体;而在不同浓度的聚丙烯酸添加剂作用下,可制得由纳米颗粒和纳米棒状结构单元组成的Ba SO4多晶体。制备的多晶体材料为具有母体材料统一宏观形貌的大颗粒,其次级结构可通过聚合物添加剂调控,为合成具有特定形貌的功能晶体材料提供了一个新的途径。 相似文献
75.
Borate is considered one of the most important additives for improving the fire-resistance of combustible polymers because of its smoke suppression, low toxicity, and good thermal stability. However, the size of prepared borate is usually in the micrometer range, which makes it difficult to disperse in a polymer matrix, thus hindering its use as fire-retardant material. The preparation and application of borate nanomaterial as flame retardant is considered an effective method. However, the preparation of barium borate nanomaterials as flame retardant has not been reported. In this paper, nanosheets and nanoribbons with different sizes for a new barium borate BaO·4B2O3·5H2O are prepared by hydrothermal method, and characterized by X-ray diffraction (XRD), Fourier transform infrared spectrum (FT-IR), thermogravimetric analysis-differential scanning calorimetry (TG-DSC), and scanning electron microscope (SEM). The flame-retardant properties of polypropylene (PP)/BaO·4B2O3·5H2O composites are investigated by thermogravimetric analysis (TG), differential scanning calorimetry (DSC) thermal analysis methods and limited oxygen index (LOI) method. Considering the near TG mass losses and the near LOI values for PP with 10% prepared BaO·4B2O3·5H2O nanosheet and nanoribbon, their flame-retardant properties need to be further evaluated by non-isothermal decomposition kinetic method. The apparent activation energy for this decomposition reaction was obtained from the slope by plotting ln(β/Tp2) against 1/Tp according to Kissinger's model. With the reduction of TG mass loss, increased heat absorption in DSC under N2 atmosphere, increased apparent activation energy Ea for the thermal decomposition of PP/BaO·4B2O3·5H2O composite as well as increased LOI value, the flame-retardant performance of prepared BaO·4B2O3·5H2O samples with PP gradually improved from bulk to nanoribbon to nanosheet. This can be attributed to the decrease in the size of BaO·4B2O3·5H2O samples because the smaller sample size leads to improved dispersion and increased contact area with the polymer. The flame-retardant mechanism is discussed by analyzing the after-flame chars of the PP/BaO·4B2O3·5H2O composite in SEM images, which show that the char layer is more compact and continuous for the PP/BaO·4B2O3·5H2O nanosheet composite. The influence of loading BaO·4B2O3·5H2O nanomaterials on the mechanical properties of PP is also tested using a universal material testing machine, in which the PP/BaO·4B2O3·5H2O nanosheet composite has higher tensile strength. The PP/BaO·4B2O3·5H2O nanosheet composite has the best flame-retardant and mechanical properties, which is promising to be developed for the application as flame-retardant material. 相似文献
76.
The thermal behaviour of [Ba(C2H6O2)4][Sn(C2H4O2)3] used as a BaSnO3 precursor, and its phase evolution during thermal decomposition are described. The initially formed transient barium tin oxycarbonate phase disintegrates into BaCO3 and SnO2, reacting subsequently to BaSnO3. The existence of the intermediate oxycarbonate phase is evidenced by Fourier transformed infrared (FT-IR), X-ray powder diffraction (XRD) and electron energy loss spectroscopy (EELS (ELNES)) investigations. 相似文献
77.
聚苯胺钡铁氧体纳米复合材料的制备、表征及性能 总被引:7,自引:1,他引:7
采用原位掺杂聚合法, 将聚苯胺(PANI)对粒径在60~80 nm的M型钡铁氧体颗粒(BaFe12O19)进行了包覆, 得到了具有棒状结构的复合材料. 通过X射线衍射(XRD)、傅里叶变换红外光谱(FTIR)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)等测试手段对材料的形貌和结构进行了表征. 结果表明, PANI链段与BaFe12O19颗粒之间存在作用力. 使用振动磁强计和四探针法测定了复合材料的磁性能与电性能后发现, 饱和磁化强度与矫顽力均随聚苯胺含量的增加呈规律性下降趋势, 而电导率呈上升趋势. 复合材料的吸收特性测试结果表明, 该材料反射率小于-20 dB时, 带宽可以达到15.07 GHz. 同时详细地讨论了纳米复合材料的聚合机理及相互作用. 相似文献
78.
S.I. Lopatin S.M. Shugurov Z.G. Turnina 《The Journal of chemical thermodynamics》2006,38(12):1706-1710
In the present study, the stability of gaseous barium silicates was confirmed by the high temperature mass spectrometry. On the basis of equilibrium constants measured for gas-phase reactions, the standard formation enthalpies were determined for gaseous barium silicates as (−510 ± 15) kJ · mol−1 and (−884 ± 18) kJ · mol−1 at 298 K; standard atomization enthalpies as (1637 ± 17) kJ · mol−1 and (2318 ± 20) kJ · mol−1 at 298 K for BaSiO2 and BaSiO3, respectively. Based on the results obtained the critical analysis of the literature data was carried out. 相似文献
79.
Barium calcium magnesium silicate (BaCa2MgSi2O8), a compound whose space group was obtained via X-ray diffraction data, was re-investigated using neutron diffraction techniques. A combined powder X-ray and neutron Rietveld method revealed that BaCa2MgSi2O8 crystallizes in the trigonal space group P3? (Z=1, a=5.42708(5) Å, c=6.79455(7) Å, V=173.310(4) Å3; Rp/Rwp=5.52%/7.63%), instead of the previously believed space group P3?m1. The difference in the two structures arises from the displacement of the O2 atom. Blue emission from Ba0.98Eu0.02Ca2MgSi2O8 under 325-nm excitation is ascribed to the 4f65d1→4f7 transitions of Eu2+ ions at Ba sites and Ca sites. Site assignment of Eu2+ ions in the titled compound was performed by analysis of emission spectra at temperatures in the range of 4.2-300 K. 相似文献
80.
On the Crystal Chemistry of a New Barium Rare-Earth Oxozincate: Ba2Er2Zn8O13 High temperature reactions led to single crystals of Ba2Er2Zn8O13. It crystallizes with orthorhombic symmetry, space group C122v? Cmc21, a = 6.276, b = 10.871, c = 10.195 Å, Z = 2. The hitherto unknown crystal structure shows Zn2+ with tetrahedral, Er3+ octahedral and Ba2+ cuboctahedral coordination by O2?. It will be shown that parts of the [Zn8O13] network are fragments of the ZnO structure showing O2? within a tetrahedral zinc coordination. A deficit of two O2? ions per unit cell is focused on two point positions. 相似文献