Synthesis of nanocrystilline ferrites by sol–gel combustion process: the influence of pH value of solution

Abstract

The dried nitrate–citrate gels exhibit auto-catalytic combustion behavior, which can be used to synthesize the nanocrystalline ferrite powders. In this study, NiCuZn ferrite nanocrystalline powders with composition of (Ni_0.25Cu_0.25Zn_0.5)O(Fe₂O₃)_0.98 were synthesized by a sol–gel auto-catalytic combustion process. The influences of pH value of the precursor solution on the gel morphology, combustion behavior and crystallite size of synthesized powders were investigated with the help of scanning electron microscopy observation, thermal analysis, infrared (IR) spectra and X-ray diffraction technique. The experiments showed that the pH value of the mixed precursor solutions has a significant influence on the morphology of dried gels. The high porous precursors with network structure can be formed at high pH values after the solutions were dried at 135°C. With increasing pH value, the combustion rate is increased significantly. The as-burnt powders become uniform in size and the crystallites size is increased from 26 to 48 nm with pH value increasing from 2 to 7. The combustion reaction mechanisms were put forward with the help of IR spectra and differential thermal analysis techniques.

Introduction

With the development of surface mounting technology for miniaturization of electronic devices, multilayer ceramic components play an important role because of their compact size, excellent electronic and magnetic characteristics and low manufacturing cost. The multilayer chip inductor (MLCI) is one of the most widely used and most important passive components in the circuitry of the latest communicative electronic products [1]. These chip inductors are fabricated by laminating ferrite layers and internal conductors alternatively and then co-firing to form the monolithic structure. Silver, Ag, is usually used as the internal electrode material of MLCIs due to its low resistivity and lower cost. Therefore, low-temperature sintered ferrites are required for MLCIs [2]. These materials are generally prepared through a solid-state route by firing mixtures of NiCuZn ferrite and low-melting additions [3], [4]. However, low-temperature synthesis of ferrites leading to highly homogeneous powders with fine particle size and better sinterability has attracted considerable attention in recent years.

It is well known that low-temperature sintering of ferrites can be achieved by using active ultrafine powders synthesized via wet-chemical method. Several chemical processing techniques, such as hydrothermal, sol–gel, co-precipitation and combustion synthesis have been investigated to prepare ultrafine ferrite powders [5], [6], [7], [8], [9], [10]. Among these techniques, combustion synthesis has been proved to be a simple and economic way to prepare nanoscale powders [11], [12], [13]. In this technique, a thermally induced anionic redox reaction takes place. The energy from the exothermic reaction between oxidant and reductant can be high enough to form a desirable phase within a very short time.

In order to improve the electro-magnetic properties of low-temperature sintered MLCIs, we have successfully used the auto-combustion characteristics of citrate–nitrate gels for the preparation of nanocrystalline powders of several spinel compounds like NiCuZn ferrite [14], LiZn ferrite [15], MgCuZn ferrite [16], and Z-type hexaferrite [17] for MLCI application and Ni–Zn manganite for NTC applications [18]. In these studies, we observed that some factors including pH value of the mixing solution, concentration of the chelation (citric acid) and environmental temperature can affect sol–gel transition and auto-combustion process as well as the particle size and morphology of as-prepared powders. In the present investigation, the influence of pH value of mixed solution on the combustion behavior of the precursor gel was studied.