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Parvathy N. N. Pajonk G. M. Rao A. Venkateswara 《Journal of Materials Synthesis and Processing》1999,7(4):221-228
Quantized CdS crystallite-doped tetraethylorthosilicate (TEOS) silica xerogels are prepared by the sol-gel method. In this method, cadmium acetate [Cd(CH3COO)22H2O]-doped TEOS alcogel is formed by the hydrolysis and polycondensation of ethanolic TEOS in the presence of hydrochloric acid (HCl) and ammonium hydroxide (NH4OH) catalysts and Cd(CH3COO)2.2H2O. The CdS crystallites are formed in the alcogel by the reaction of Cd(CH3COO)2.2H2O present in the gel and methanolic sodium sulfide (Na2S), which is added over the alcogel. The effect of CdS/TEOS, EtOH/TEOS, S/Cd molar ratios, and temperature on the optical properties and CdS crystallite sizes in the xerogels are studied. A blue shift is observed in optical absorption spectra by decreasing the CdS/TEOS molar ratio from 2 × 10–2 to 1 × 10–4. It is observed that the crystallite size is increased from 1.6 to 3.4 nm by increasing the EtOH/TEOS molar ratio from 2 to 20, respectively, for a constant CdS/TEOS molar ratio of 5 × 10–4. Emission spectra of xerogels are measured and found that the emission peak maxima shifted toward lower energies (higher wavelengths) by increasing the CdS/TEOS molar ratio in the xerogels. It is known from the X-ray diffraction (XRD) measurements of CdS-doped xerogels that the CdS crystallite structure in the xerogels is hexagonal wurtzite. The crystallite sizes were calculated from the XRD patterns and tight bonding calculations. There is a significant change in the color and size of CdS crystallite in the xerogels with a variation in temperature from 200 to 400°C. 相似文献
53.
BaMnZnCoTi—W型铁氧体微波吸收特性的研究 总被引:6,自引:1,他引:5
对用Co2 + 、Ti4 + 置换Fe3 + 的BaMnZn -W型铁氧体的制备和微波吸收性能进行了分析研究 ,发现涂层厚为 1.0 6mm在 7.0~ 12 .0GHz频率范围内有 3个损耗吸收峰 ,峰值最高达 3 4dB以上 ;同时测量了复数磁导率 μ~( =μ′ -jμ″) ,复介电常数ε~( =ε′ -jε″) ,求出了磁损耗角正切tgδm 和介电损耗角正切tgδe,初步分析了 3个损耗吸收峰可能产生的机制。比较了在相同工艺条件下 ,随Co2 + 、Ti4 + 含量增加 ,铁氧体的吸收频率特性和涂层厚度的关系。 相似文献
54.
The alkylation of isobutane with 2-butene on hexagonal (protonated form = H EMT, investigated, for the first time on this reaction) and cubic faujasite (protonated form = H FAU) with Si/Al ratios of 3.5 was studied at 80°C and compared with the results obtained for a cerium exchanged Y zeolite (Ce Y). The alkylate composition consisted of 90% and about 80% C8 paraffins for H EMT and H FAU/Ce Y, respectively. Within the C8 fractions the three trimethylpentanes (TMP) 2,2,4-, 2,3,3- and 2,3,4-TMP were the dominating product compounds in all tests, whereas a higher content of dimethylhexanes (DMH) was observed for the H FAU and Ce Y. However, among the four trimethylpentanes 2,3,3,-TMP and 2,3,4-TMP were always the main compounds formed. Oligomerization was favoured only by the H FAU and Ce Y, with a strong increase of C8 olefins after about one third of the observed reaction time. On the contrary, H EMT exhibited much less formation of C8 olefins (and on a constant level) even after 300 min reaction time. The deactivated samples showed only formation of non-aromatic coke. The differences in the product pattern were attributed to the slightly larger size of the second type of supercage in EMT and to the stronger Bronsted acid sites in the hexagonal faujasite.Dedicated to Professor Dr. Martin Klessinger on the occasion of his 60th birthday. 相似文献
55.
纳米晶金刚石的爆炸合成及超微金刚石的应用 总被引:1,自引:0,他引:1
爆炸合成的金刚万中存在一种具有纳米级亚晶的金刚石织构,它是由石墨经固态无扩散相变而成的,具有明显择优取向的纤维状组织。除常见的立方金刚石外,还存在一种六方金刚石,二者常共生于一个晶粒中。六方金刚石也是超硬材料之一,经碎化后的纳米级金刚石超微粉,是磁头、电接触头等耐磨性能要求高的材料中的良好的强化相。 相似文献
56.
A 4‐Fold‐Symmetry Hexagonal Ruthenium for Magnetic Heterostructures Exhibiting Enhanced Perpendicular Magnetic Anisotropy and Tunnel Magnetoresistance 下载免费PDF全文
57.
Epitaxial Growth of Molecular Crystals on van der Waals Substrates for High‐Performance Organic Electronics 下载免费PDF全文
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