• K J Rao

Articles written in Bulletin of Materials Science

• Microwave synthesis of electrode materials for lithium batteries

A novel microwave method is described for the preparation of electrode materials required for lithium batteries. The method is simple, fast and carried out in most cases with the same starting material as in conventional methods. Good crystallinity has been noted and lower temperatures of reaction has been inferred in cases where low temperature products have been identified

• Preparation of Si3N4–SiC composites by microwave route

Si3N4–SiC composites have been microwave sintered using 𝛽-Si3N4 and 𝛽-SiC as starting materials. Si3N4 rich compositions (95 and 90 vol.% Si3N4) have been sintered above 96% of theoretical density without using any sintering additives in 40 min. A monotonic decrease in relative density is observed with increase in SiC proportion in the composite. Decrease in relative density has manifested in the reduction of fracture toughness and microhardness values of the composite with increase in SiC content although the good sintering of matrix Si3N4 limits the decrease of fracture toughness. Highest value of fracture toughness of 6.1 MPa m1/2 is observed in 10 vol.% SiC composite. Crack propagation appears to be transgranular in the Si3N4 matrix and the toughening of the composites is through crack deflection around hard SiC particles in addition to its debonding from the matrix.

• Conductivity studies in SnO–NaPO3 glasses

Na+ ion conductivity has been studied in SnO.NaPO3 glasses, which have been prepared over a wide range of compositions using a microwave melting technique. D.c. activation barriers seem to reflect the structural changes in system. A.c. conductivity analysis has revealed that while the power law exponent, 𝑠, seem to bear correlation to the structural changes, the exponent 𝛽 of the stretched exponential function describing the dielectric relaxation is largely insensitive to the structure. Possible importance of the correlation of transport property to the variation of available non-bridging oxygen (NBO) atoms in the structure is discussed.

• Li+ ion conductivities in boro-tellurite glasses

Lithium ion conductivity has been investigated in a boro-tellurite glass system, LiCl.LiBO$_{2}\cdot$TeO2.In the absence of LiCl, the conductivity increases with increasing non-bridging oxygen (NBO) concentration. LiCl addition has little influence on total conductivity although the observed barriers are low. Formation of LiCl clusters appears evident. In the a.c. conductivity and dielectric studies, it is observed that the conductivity mechanism remains the same in all compositions and at all temperatures. A suggestion is made that Li+ ion transport may be driven by bridging oxygen $\leftrightarrow$ non-bridging oxygen (BO $\leftrightarrow$ NBO) switching, which is why the two different types of Li+ ions in the clusters and in the neighbourhood of NBOs, do not manifest in the conductivity studies.

• A strategic approach for preparation of oxide nanomaterials

A microwave assisted solvothermal method is described for rapid preparation of nano-oxides. This method is based on exploiting differential dielectric constants to induce preferred heating and decomposition of the oxide precursors in the presence of suitable capping agents. This strategic approach has been used to prepare nanopowders of MgO, NiO, ZnO, Al2O3, Fe2O3 and ZrO2.

• Structural and electrochemical behaviour of sputtered vanadium oxide films: oxygen non-stoichiometry and lithium ion sequestration

Structural and electrochemical aspects of vanadium oxide films recently reported from ICMCB/ENSCPB have been examined using appropriate structural models. It is shown that amorphous films are nonstoichiometric as a result of pre-deposition decomposition of V2O5. It is proposed that the structure of amorphous films corresponds to a nanotextured mosaic of V2O5 and V2O4 regions. Lithium intercalation into these regions is considered to occur sequentially and determined by differences in group electronegativities. Open circuit voltages (OCV) have been calculated for various stoichiometric levels of lithiation using available thermodynamic data with approximate corrections. Sequestration of lithium observed in experiments is shown to be an interfacial phenomenon. X-ray photoelectron spectroscopic observation of the formation of V3+ even when V5+ has not been completely reduced to V4+ is shown to be entirely consistent with the proposed structural model and a consequence of initial oxygen nonstoichiometry. Based on the structural data available on V2O5 and its lithiated products, it is argued that the geometry of VO𝑛 polyhedron changes from square pyramid to trigonal bipyramid to octahedron with increase of lithiation. A molecular orbital based energy band diagram is presented which suggests that lithiated vanadium oxides, Li𝑥 V2O5, become metallic for high values of 𝑥.

• Nanocrystalline TiO2 preparation by microwave route and nature of anatase–rutile phase transition in nano TiO2

Nanopowders of TiO2 has been prepared using a microwave irradiation-assisted route, starting from a metalorganic precursor, bis(ethyl-3-oxo-butanoato)oxotitanium (IV), [TiO(etob)2]2. Polyvinylpyrrolidone (PVP) was used as a capping agent. The as-prepared amorphous powders crystallize into anatase phase, when calcined. At higher calcination temperature, the rutile phase is observed to form in increasing quantities as the calcination temperature is raised. The structural and physicochemical properties were measured using XRD, FT–IR, SEM, TEM and thermal analyses. The mechanisms of formation of nano-TiO2 from the metal–organic precursor and the irreversible phase transformation of nano TiO2 from anatase to rutile structure at higher temperatures have been discussed. It is suggested that a unique step of initiation of transformation takes place in Ti1/2O layers in anatase which propagates. This mechanism rationalizes several key observations associated with the anatase–rutile transformation.

• A new approach for understanding ion transport in glasses; example of complex alkali diborate glasses containing lead, bismuth and tellurium oxides

Mechanism of ion transport in glasses continues to be incompletely understood. Several of the theoretical models in vogue fail to rationalize conductivity behaviour when d.c. and a.c. measurements are considered together. While they seem to involve the presence of at least two components in d.c. activation energy, experiments fail to reveal that feature. Further, only minor importance is given to the influence of structure of the glass on the ionic conductivity behaviour. In this paper, we have examined several general aspects of ion transport taking the example of ionically conducting glasses in pseudo binary, 𝑦Na2B4O7.(1−𝑦) M$_{a}$O$_{b}$ (with 𝑦 = 0.25–0.79 and M$_{a}$O$_{b}$ = PbO, TeO2 and Bi2O3) system of glasses which have also been recently characterized. Ion transport in them has been studied in detail. We have proposed that non-bridging oxygen (NBO) participation is crucial to the understanding of the observed conductivity behaviour. NBO–BO switching is projected as the first important step in ion transport and alkali ion jump is a subsequent event with a characteristically lower barrier which is, therefore, not observed in any study. All important observations in d.c. and a.c. transport in glasses are found consistent with this model.

• # Bulletin of Materials Science

Current Issue
Volume 42 | Issue 5
October 2019

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019