In a recent paper, Tripathi et al. 1 reported on a study of the microstructure and properties of electrochemically deposited Ni-Fe/Si 3 N 4 nanocomposites from a DMF bath. Among the physical properties, also the room-temperature electrical resistivity was investigated for the nanocomposites and for an electrodeposited Ni coating. The authors claim that their measured resistivity values are much lower than the bulk values for Ni-Fe alloys and Ni metal. Although the focus of the paper was

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... ly not the resistivity, we should comment on that part of the paper since we believe that the resistivity values reported by Tripathi et al. 1 are unphysical. The authors write the following about their resistivity data. "The apparent electrical resistivity of the Ni-Fe/Si 3 N 4 nanocomposite coatings was found to be many orders lower than that of the bulk Ni-Fe alloy (of the order of 10 −6 cm). 2 The resistivity for pure nanocrystalline Ni coating prepared at current density 3.0 Adm −2 was measured to be 4.5 × 10 −9 cm. Thus the electrodeposited nano-Ni and Ni-Fe/Si 3 N 4 nanocomposites both have electrical resistivity much lower than those of the bulk Ni and Ni-Fe alloy." The room-temperature resistivity measured by the authors (4.5 × 10 −9 cm) for nanocrystalline Ni is indeed by about three orders of magnitude smaller than the room-temperature literature value for bulk Ni (7.2 × 10 −6 cm). 3 The authors give the following explanation for this finding. "According to the theory of electron scattering in solids, the electrical resistivity of nanocrystalline materials is expected to be higher than that in the corresponding coarse-grained polycrystalline ones due to the increased volume fraction of atoms lying on the grain boundaries. 4 However in the present case the electrical resistivity of Ni-Fe/Si 3 N 4 nanocomposites is observed several folds lower than that of the coarse-grained Ni or Ni-Fe alloy which can be due to either absence or significantly low density of defects and dislocations in the electrodeposited coatings in comparison to the bulk alloy; since electrodeposition from non-aqueous media produces nearly defect-free coatings 5 in comparison to aqueous media. Being almost defect-free nanostructures the electron scattering is highly reduced and hence the electrical resistivity too." It should be pointed out that the resistivity values reported by Tripathi et al. 1 are unphysical. The reason for this is that the bulk values of the resistivity of a metal refer to a well-annealed, highpurity state. For example, in the work by Laubitz et al., 3 the reported room-temperature resistivity value cited above refers to a bulk Ni sample of high purity (5N), annealed for 2h at 1400 K in vacuum and then slowly cooled; the residual resistivity ratio (RRR) was at least 220. This sample can really be considered as defect-free Ni metal. Therefore, at a fixed temperature, one cannot measure a lower resistivity value for a Ni coating whatever way it was produced than that of the reported bulk value measured at the same temperature. The authors claim that the electrodeposited Ni coating they investigated was nanocrystalline. This means that there is a significant fraction of Ni atoms in the grain boundaries, i.e., the grain-boundary contribution to the resistivity will be non-negligible and, thus, the resistivity should be higher than the bulk value. This has indeed been observed in electrodeposited nanocrystalline Ni metal. 4,6-8 * Electrochemical Society Active Member. z E-mail: bakonyi.imre@wigner.mta.hu ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 207.241.231.81 Downloaded on 2018-07-18 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 207.241.231.81 Downloaded on 2018-07-18 to IP