Preparation and characterization of NiCuZn ferrite nanocrystalline powders by auto-combustion of nitrate–citrate gels

doi:10.1016/S0921-5107(99)00152-X

Materials Science and Engineering: B

Volume 64, Issue 1, 15 September 1999, Pages 68-72

https://doi.org/10.1016/S0921-5107(99)00152-X Get rights and content

Abstract

A novel nitrate–citrate gel process was used to synthesize NiCuZn ferrite powders. The thermal decomposition and combustion process of nitrate–citrate gels were investigated by using DTA-TG, IR spectra and XRD techniques. The results revealed that the nitrate–citrate gels exhibit self-propagating behavior after ignition in air at room temperature. NiCuZn ferrite powders with a particle size of 20–50 nm were directly formed after combustion. The ratio of metal nitrates to citric acid in the starting solution affects the combustion process, and then determines the particle size of the as-synthesized powder. The magnetic structure of powders was characterized using Mössbauer spectroscopy.

Introduction

NiCuZn ferrites have been inventively studied in recent years for multilayer chip inductors (MLCIs) applications because of their good electro-magnetic properties at high frequency and low sintering temperature [1], [2], [3]. In MLCI applications, ferrites must be densified at 900°C or below, in order to co-fire with a silver internal electrode to form a monolithic structure. In general, the low-melting compounds, such as Bi₂O₃ and PbO, are doped to promote the low-temperature sintering of NiCuZn ferrites [4], [5]. Unfortunately, these additives are detrimental to the electro-magnetic properties of MLCIs by promoting Ag diffusion into ferrites. Therefore, low-temperature sintered NiCuZn ferrites with no low melting additives are especially required for high-performance MLCIs with high quality factor and reliability.

The wet-chemical synthesis of high reactive powders has been proved to be one of the most effective routes to decrease the sintering temperature of ferrites. A variety of chemical synthesis methods such as co-precipitation, hydrothermal synthesis and sol–gel process have been developed [6], [7], [8]. Of these methods, co-precipitation and sol–gel are most popular. However, any co-precipitation process is, in general, highly PH sensitive and not preferred for complex systems like NiCuZn ferrite. On the other hand, the sol–gel technique is more sophisticated requiring stringent drying conditions and expensive alkoxide precursors. In addition, any wet-chemical synthesis method, in general, needs calcination at high temperature to obtain the final product of powder with expected crystal structure. The aggregates can be formed during the calcination process and then decrease the sinterability of powders.

Recently, a sol–gel auto-combustion method was used by several researchers to synthesize spinel and perovskite powders [9], [10], [11], [12], [13], [14], [15], [16]. This is a novel way and unique combination of the chemical sol–gel process and the combustion process. The process has the advantages of inexpensive precursors, a simple preparation method, and a resulting nano-sized powder. In the present study, the auto-combustion method of the nitrate–citrate gel was used to synthesize NiCuZn ferrite-reactive powders.

Section snippets

Experimental procedure

Analytical-grade nickel nitrate, zinc nitrate, copper nitrate, iron nitrate, and citric acid were used as raw materials to prepare Ni_0.25Cu_0.25Zn_0.50Fe₂O₄ ferrite. The ferrite powder was synthesized in the following way. The appropriate amount of metal nitrates and citric acid, which could form 50 g of NiCuZn ferrite powder, was first dissolved in 600 ml of deionized water. A small amount of ammonia was added into the solution to adjust the PH value to about 7. During the procedure, the

Thermal analysis

The experimental observation showed that the nitrate–citrate gels with all three molar ratios of metal nitrates to citric acid exhibited self-propagating combustion behavior. When the dried gels were ignited at any point, the combustion rapidly propagated forward until all gels were completely burnt out to form loose powders. It was also observed that the combustion rate is associated with the ratio of nitrates to citric acid. The combustion rate is most rapid for the gel with 1:1 ratio, and

Summary

The mixture solutions with the appropriate ratios of metal nitrates to citric acid can transform into dried nitrate–citrate gels. These gels exhibit an auto-combustion behavior once ignited in air at room temperature. After combustion, the gel powders are directly transformed into nanocrystalline ferrite powders. The combustion process is an oxidation–reduction reaction, in which the NO₃⁻ ion is oxidant and the carboxyl group is reductant. The combustion rate can be controlled by changing the

Acknowledgements

This work was supported by the High Technology Research and Development Project of the People’s Republic of China.

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Preparation and characterization of NiCuZn ferrite nanocrystalline powders by auto-combustion of nitrate–citrate gels

Abstract

Introduction

Section snippets

Experimental procedure

Thermal analysis

Summary

Acknowledgements

J. Magn. Magn. Mater.

J. Magn. Magn. Mater.

J. Magn. Magn. Mater.

Mater. Lett.

Solid State Commun.

J. Am. Ceram. Soc.

IEEE Trans. Magn.

J. Am. Ceram. Soc.