MacDiarmid - Grain Growth Kinetics of ZnOAl Nanocrystalline Powders

1516 other studies on nanocrystalline zno have

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Unformatted text preview: bserved to be significantly lower than their corresponding bulk materials for many systems. Two studies on yttriastabilized zirconia (YSZ) by Shukla et al. and Kuo et al. yielded activation energies of 13 kJ/mol for 15À20 nm particles18 and 4À5 kJ/mol for 6À10 nm particles,19 in contrast to the bulk value of 580 kJ/mol.18 This dramatic reduction of the activation energy was attributed to the increased concentration of oxygen vacancies as the particle size decreases below 20 nm, as observed in CeO2.20 Lai and Shek reported activation energies for nanocrystalline SnO2 derived from solÀgel synthesis of between 30 and 45 kJ/mol for grain sizes around 10À30 nm.15,16 Other studies on nanocrystalline ZnO have yielded grain growth activation energies of 20 kJ/mol for an initial 26 nm grain size,17 which is similar to the value we obtained for the 0% Al sample of 24 ( 3 kJ/mol. A study of the sintering of larger (100 nm) particles resulted in a higher activation energy of 63 kJ/mol.21 As well as low values for the activation energies, similar values of m to what we reported (0.1À0.2) have also been observed before for grain growth in nanocrystalline SnO2.15,16 These values, which in the generalized parabolic grain growth model (Model 1) correspond to exponents n of 5À10, are outside the range expected for this model. Typical values are 1À3, depending on the dimensionality of growth (1 = 1-dimensional growth, forming needles; 2 = 2-dimensional growth, forming plates; 3 = 3-dimensional growth).22 Higher values of n indicate that another mechanism is responsible for the grain growth kinetics observed. For our results, particularly at higher temperatures, D∞ tends toward infinity (i.e., the limiting grain size is much greater than the measured grain size) and the relaxation model kinetics are equivalent to the generalized parabolic grain growth. The relaxation model itself is derived from glass kinetics, with the activation energy relating to reorganization of the bonds at the surface of the grains. This model is applicable to nanocrystalline materials because they have a significant fraction of their atoms at crystalline interfaces and grain boundaries.15 The reorganization of bonds in the interfacial region dominates the kinetics. It is possible to estimate the volume of this interface component as a ARTICLE fraction of the total crystalline volume. This is given as Ct = 3Δ/D, where Ct is the volume fraction of the interface, Δ is the thickness of the interface region, taken as 1 nm, and D is the grain size.16 For 10 nm grains, this corresponds to a value for Ct of 30%; for 60 nm grains (the largest observed in our study) Ct is then 5%. Upon addition of Al, the activation energy increases by a factor of 2 to 43 ( 4 kJ/mol (see Figure 12). Over the Al concentration range studied, there is no statistically significant variation in the activation energy. In our previous study on ZnO:Al powders, 27Al NMR was used to provide a measure of the fraction of Al being incorporated into various sites: (a) crystalline tetrahedral, (b) amorphous tetrahedral, and (c) amorphous octahedral. The crys...
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This note was uploaded on 06/15/2013 for the course MSE 101 taught by Professor Sen during the Spring '12 term at Indian Institute of Technology, Kharagpur.

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