Journal of Nanomaterials

Volume 2016 (2016), Article ID 1592161, 3 pages

http://dx.doi.org/10.1155/2016/1592161

## Comment on “Correlation between Porosity and Electrical-Mechanical Properties of Carbon Nanotube Buckypaper with Various Porosities”

^{1}Department of Textile Technology, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India^{2}Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Bela ter 1, 6720 Szeged, Hungary^{3}MTA-SZTE “Lendület” Porous Nanocomposites Research Group, Rerrich Bela ter 1, 6720 Szeged, Hungary^{4}University of Borås, 501 90 Borås, Sweden

Received 17 June 2016; Revised 15 September 2016; Accepted 26 September 2016

Academic Editor: Albert Nasibulin

Copyright © 2016 Amit Rawal et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

With great interest we read a recently published article entitled “Correlation between Porosity and Electrical-Mechanical Properties of Carbon Nanotube Buckypaper with Various Porosities” authored by Ling Liu and colleagues in Journal of Nanomaterials [1]. There are certain concerns and issues that question the methodology, reliability of results, and related analysis. One of the key concerns is that the authors have prepared three sets of samples of randomly aligned multiwalled carbon nanotubes (MWCNTs) in the form of buckypapers (BPs) with IDs 1^{#} BPs, 2^{#} BPs and 3^{#} BPs, having exceptionally low levels of porosity, that is, 11.3%, 21.1%, and 39.3%, respectively. The porosity () and fibre volume fraction () of any porous material are related to each other, as shown in the following equation [2]:Based upon the above equation, of sample IDs 1^{#} BPs, 2^{#} BPs and 3^{#} BPs are 88.7%, 78.9%, and 60.7%, respectively. Such values of can be obtained neither through any experimental route for fabricating randomly aligned carbon nanotubes (CNTs) nor from theoretical perspective. To prove this point, we hypothetically analysed the maximum fibre volume fraction () of BPs through the work of Pan et al. [3] dealing with the fibrous network that also includes the case of randomly oriented fibres. Considering each CNT as a fibre in the network [4, 5], the fibrous network consists of three basic segments, namely, the mean length (separation distance) between the centres of two adjacent fibres (), the mean length of contacts (), and the mean free fibre length () (see [3] for details). Hence, the proportions of free length of fibre () and that of contact length () are given below:Theoretically, ; in case this inequality is violated; the free fibre length between the contacts will not exist, which eventually leads to . Based upon these considerations, Pan et al. [3] have formulated the general relationship between and the orientation distribution of fibres, as shown below:where is an orientation parameter defining the orientation characteristics of fibres in the assembly, is the angle between the two axes of fibres having defined types of orientation distributions and , and is the aspect ratio.

Considering the BP as a two-dimensional (2D) random network of CNTs and hence the value as [3] whereas the expression of is given below [3],Thus, a relationship can be obtained between and of CNTs for a 2D random network, as illustrated in Figure 1. Now considering the extreme values of MWCNT diameter and length leading to minimum aspect ratio presented in [1], it can be seen that the maximum diameter and minimum length of CNT are 15 nm and 5 *μ*m, respectively, that results in minimum aspect ratio of 333, which inevitably leads to of 15%. Therefore, porosity of BPs based upon (1) cannot be less than 85% whereas the prepared samples of [1] have a porosity ranging from 11.3 to 39.3%, which does not seem rationally correct.