彭晓
,
王福会
,
D.R.
,
Clarke
金属学报
磁控溅射Co-Cr-Al(Y)纳米涂层在1000, 1100和1200℃氧化一定时间后, 用光激发荧光谱技术表征热生长的Al2O3相. 发现氧化层局部区域存在由非稳态相向稳态相的转变, 即: γ→θ→α; 其转变过程随温度升高显著加快, 并在1200℃下变昨不明显. 在相同温度下, Y明显减缓Al2O3相转变过程.
关键词:
光激发荧光谱术
,
null
,
null
Journal of the American Ceramic Society
Combining Clarke's model with first-principles calculation of average sound velocity, the minimum lattice thermal conductivities (kappa(min)) of Y3Al5O12 (YAG), YAlO3 (YAP) and Y4Al2O9 (YAM) are predicted to be 1.59, 1.61, and 1.10 W.(m.K)(-1), respectively. The weak Y-O polyhedra provide "weak zones" that scattering phonons and lead to the low kappa(min) of ternary Y-Al-O compounds. In addition, the extremely low kappa(min) of YAM is attributed to its higher levels of local disorder of crystal structure and weaker chemical bonding compared with those of YAG and YAP. Inspired by theoretical predictions, dense and phase-pure YAM is synthesized and the experimental thermal conductivity is only 1.56 W.(m.K)(-1) at 1273 K. Finally, YAM is highlighted as a potential thermal barrier material for its low thermal conductivities at temperatures from 473 to 1273 K.
关键词:
barrier coatings;single-crystal;gas;stability
Journal of the American Ceramic Society
Search for new thermal barrier materials with high-temperature stability is very difficult because there are intractable choices among thousands of possible candidates. The present paper shows that identifying and developing new low thermal conductivity materials can be accomplished by first-principles calculations of equilibrium crystal structure and elasticity, and thereafter making theoretical prediction of minimum high-temperature lattice thermal conductivity basing on David Clarke's method for insulating materials. We highlight a new oxynitride ceramic, Y4Si2O7N2, with a low theoretical minimum thermal conductivity of 1.12 similar to W center dot m-1 center dot K-1. The theoretical result is further validated by experimental measurement of thermal conductivity of dense and bulk Y4Si2O7N2. The experimental thermal conductivity is 1.50 similar to W center dot m-1 center dot K-1 at 1300 similar to K. Furthermore, thermal properties of Y4Si2O7N2 are investigated for evaluating its potential performances.
关键词:
theoretical elastic stiffness;barrier coatings;mechanical-properties;gas;gamma-y2si2o7;crystal;y2sio5;system;phase;si3n4-sio2-y2o3
Acta Materialia
In order to achieve better understanding of the structural/property relationships of La(2)T(2)O(7) (T = Ge, Ti, Sn, Zr, Hf) pyrochlore, first-principles calculations were conducted to investigate the bonding characteristics, elastic stiffness, structural stability and minimum thermal conductivity. The results show that the relatively weak La-O bonds play a predominant role in determining the structural stability, mechanical and thermal properties of these compounds. In addition, the elastic and thermal properties are influenced when the T atom changes from Ge to Hf. When the bonding strength is enhanced by applying hydrostatic pressure, apart from c(11), c(12), and B, which normally increase at high pressures, it is found that the shear elastic moduli, c(44) and G, which relate to the shear deformation resistance, abnormally remain almost constant. The underlying mechanism may help to explain the damage tolerance of pyrochlore compounds. After comprehensive consideration of the elastic anisotropy, a modified David Clarke-type equation is used to calculate the minimum thermal conductivity of the studied pyrochlore materials, which display an extraordinary low thermal conductivity. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
关键词:
Non-binary oxides;Mechanical properties;Thermal conductivity;Density;functional theory;Local density approximations (LDA);oxide pyrochlores;a(2)b(2)o(7);simulation;disorder
李明春
,
周盛全
,
辛梅华
,
宋宇宁
,
王朝
功能材料
先用甲醛-甲酸法（eschweiler-clarke反应）合成N,N-二甲基壳聚糖（DMC）,再与溴代烷进行Hoffman烷基化反应制备了N-十二烷基-N,N-二甲基壳聚糖季铵盐（DODMC）和N-十六烷基-N,N-二甲基壳聚糖季铵盐（HDMC）,用FT-IR、1 H NMR、EA、TG等对产物进行表征。抗菌实验结果表明所合成产物具有较好的抗菌活性,对革兰氏阳性菌S.aureus的抗菌活性优于革兰氏阴性菌E.coli,抗菌活性随着烷基链长度的增加而增强;产物在pH值=5.5比在pH值=7.2条件下表现出更好的抗菌活性;在碱性及中性条件下,HDMC的抗菌活性随着季铵化度的提高而提高,而在酸性条件下抗菌活性则随着季铵化度提高而降低。
关键词:
壳聚糖
,
N-烷基化
,
季铵盐
,
抗菌活性
