Nanofillers in ZnO based materials: a ‘smart’ technique for developing miniaturized high energy field varistors† S. Anas, * ab K. V. Mahesh, a V. Jobin, a S. Prasanth a and S. Ananthakumar * a The present work encompasses the strategic design, synthesis and development of high energy field ZnO varistors. The key aspect of the study involves the chemical synthesis of ZnO based nanorod nanofillers by a pH selective precipitation and reflux method. High field varistors were developed out of these crystalline nanofillers by employing them as fillers to the micron sized commercial varistor powder. A systematic investigation of the microstructural changes of the nanofiller added varistor samples were carried out under step-sintering and the normal sintering conditions. Comparative studies were also performed with the nano and micro counterparts. The influence of nanofillers in the varistor powder packing, grain structure refinement, grain size reduction and on the I–V properties were analyzed under the step- sintering conditions. Based on the performance analysis, the current study foresees ample opportunities for miniaturizing varistors with at least half the size of currently available varistors. Introduction Ever since Matsuoka put forward the use of ZnO for the varistor and surge arrestor applications, 1 extensive research has been carried out on low, high and variable voltage ZnO varistors. 2–4 For the last four decades, researchers have worked on many key aspects of varistors ranging from its material synthesis to its fabrication and commercialization. 1–5 However, in the current scenario, the major quest in this eld is the preparation of miniaturized light weight varistors with high energy handling capabilities but without exceeding the current production cost. 5 It greatly demands novel approaches and strategies for fabri- cating size and shape controlled varistors with desired micro- structures. It has been demonstrated very recently that carbon nanotubes, graphene sheets, ceramic and polymeric nano- particles etc. can be applied as ‘llers’ for the enhancement of electronic, optical, mechanical, thermal and functional prop- erties of diverse industrial products. 6,7 Nanollers are now being added to paints and functional coatings, porcelain and polymer insulators, rubbers and high temperature refractories for their benecial properties. 8,9 Even though tremendous opportunities are there, this potential nanoller strategy has not been effectively established in the varistor eld. As a nano technological approach, the usage of nanopowders as ‘nano- llers’ can be advantageously employed in ZnO varistor industries to minimize the current problems associated with the industry. The commercial ZnO varistors produced from the solid-state based ceramic processing route have signicant disadvantages, especially for many modern commercial applications. 5,10 The major disadvantages of this route are the lack of additive homogeneity at the microscopic level, uncontrolled growth of the ZnO grain size, a large volume requirement, poor break- down eld and non-linear characteristics. 10 In recent years, researchers were successful in establishing that ZnO based nanomaterials derived from the solution phase methods were useful alternatives to micron sized commercial varistor powder for many advantageous applications. 11 However, current studies claim that the use of nano material for varistor applications has many practical disadvantages, like high production costs asso- ciated with the nano material synthesis, problems associated with the handling of nanopowders during packing and varistor disc preparations and installations of new infrastructures like laser or plasma sintering facilities for obtaining defect free well sintered nano products. 12 By taking account of all the earlier investigations in the varistor eld, the present study introduces ‘nano ller strategy’ which is an alternative approach to the purely nano and micro powders based varistors. The microstructure modications as expected in the nano- ller based composite approach is depicted schematically in Scheme 1. As shown in the scheme, before sintering, the diameter of the industrially processed spherical granule is in the range of <50 mm and the granule contains loosely adhered spherical particles of size 3–5 mm with distributions of nano to micro scale interstitial porosities. 12,13 These large amounts of nano and micropores on the powder granule and the interstitial macro pores produced during compaction can cause sintering a National Institute for Interdisciplinary Science and Technology (NIIST), Council of Scientic and Industrial Research (CSIR), Trivandrum, Kerala, 695 019, India. E-mail: anas.tkmchem@gmail.com; ananthakumar70@gmail.com b T. K. M. College of Arts and Science, Kollam, India † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3tc31049c Cite this: J. Mater. Chem. C, 2013, 1, 6455 Received 2nd June 2013 Accepted 12th August 2013 DOI: 10.1039/c3tc31049c www.rsc.org/MaterialsC This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. C, 2013, 1, 6455–6462 | 6455 Journal of Materials Chemistry C PAPER Published on 12 August 2013. Downloaded by OULUN YLIOPISTO on 16/11/2014 14:28:58. View Article Online View Journal | View Issue