combustion synthesis and characterization of nicuzn ferrite powders

Combustion synthesis and characterization of

NiCuZn ferrite powders

Yao Li a,*, Jiupeng Zhao b, Jiecai Han a, Xiaodong He a

a Center for Composite Materials, Harbin Institute of Technology, Harbin 150001, PR Chinab Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China

Received 26 August 2004; received in revised form 19 February 2005; accepted 28 February 2005

Abstract

In this paper, the feasibility of synthesizing NiCuZn ferrite powders by combustion synthesis (CS) reaction is

demonstrated through igniting the mixtures of iron, iron oxide, copper oxide, zinc oxide and copper carbonate

under different oxygen pressure values. The ferrite powders produced directly from the CS reaction and after

annealing at 800 8C for 2 h are characterized by XRD, SEM, XPS and VSM. The results show that the spinel phase

in the combustion products increases with the decrease of the diluent content and the increase of the oxygen

pressure. Heating the as-synthesized ferrite at 800 8C for 2 h affords pure crystalline NiCuZn ferrite, which

possesses better magnetic properties. XPS studies confirm that copper ions in the as-synthesized ferrite are present

in the different ionic states of the A- and B-sites, while copper ion is divalent in the B-sites only for the annealed

products.

# 2005 Elsevier Ltd. All rights reserved.

Keywords: A. Ceramics; A. Magnetic materials; C. X-ray diffraction; D. Magnetic properties

1. 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 [1–3]. The conventional methods for the preparation of NiCuZn ferrite powders involve the

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Materials Research Bulletin 40 (2005) 981–989

* Corresponding author. Tel.: +86 451 86402345; fax: +86 451 86402477.

E-mail address: [email protected] (Y. Li).

0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2005.02.018

solid state reaction of finely ground powders which are heated at high temperatures for up to several hours

and wet chemical methods such as sol–gel and coprecipitation reaction [4–6]. These methods have

multiple step pathways that are time consuming and expensive.

In recent years, combustion synthesis (CS), also called self-propagating high temperature synthesis

(SHS), is initially developed in Russia by Merzhanov and has been successfully used to speed up the

synthesis of complex oxide materials such as ferrite and high temperature superconductors [7,8]. This

method is characterized by its simpler process, a significant saving in time and energy consumption over

the traditional methods. It also has been demonstrated that combustion synthesis of ferrites can produce

metastable phases and such combustion synthesized ferrites possess high sintering activity [9]. Although

CS method has been studied in different ferrite systems, such as, NixZn1�xFe2O4, BaFe12O19 [8,10],

systematic studies about NiCuZn ferrite prepared by this method have not yet been reported in the

literature to our knowledge.

The aim of this work is the first of a serial work to determine the feasibility of utilizing CS method to

produce NiCuZn ferrite powders and to characterize NiCuZn ferrite powders produced directly from the

CS reaction and after annealing by XRD, SEM, XPS and VSM.

2. Experimental procedure

2.1. Preparation of Ni0.25Cu0.25Zn0.50Fe2O4 powders

The raw materials used in the preparation of Ni0.25Cu0.25Zn0.50Fe1.96O3.94 were iron (with the average

particle size of 45 mm), iron oxide (with the average particle size of 1 mm), copper oxide (with the

average particle size of 1 mm), copper carbonate (with the average particle size of 1 mm), zinc oxide

(with the average particle size of 1 mm), and gaseous oxygen. The purity of the raw materials is more than

99%. Copper carbonate was used as the diluent.

Synthesis of Ni0.25Cu0.25Zn0.50Fe1.96O3.94 ferrite proceeds according to the following equations:

CuCO3 ! CuO þ CO2

0:25NiO þ 0:25CuO þ 0:5ZnO þ 2kFe þ ð0:98 � kÞFe2O3 þ 1:5kO2

¼ Ni0:25Cu0:25Zn0:50Fe1:96O3:94

where k is the coefficient, which controls the exothermicity of the mixture. The larger the k value,

the higher the molar ratio Fe/Fe2O3 in the reactants should be. In the experiments, k value is fixed

at 0.5.

The starting materials were weighed according to the required stoichiometric proportion, mixed in

ethanol followed by ball milling for 8 h and then dried in air. The mixture of powders was packed in a

quartz container. A tungsten wire was used to initiate the reaction at various oxygen pressures of 0.1–

0.4 MPa and the experiments were carried out in a water-cooled tube. Within a few seconds the

combustion reaction was completed with the resultant loose products filling in the container, which were

then milled and NiCuZn powders were obtained. The structure of the CS reactor has been reported in a

previous paper [8].

To study the influence of subsequent heat-treatment on phase composition, the as-synthesized samples

were annealed at 700 8C, 800 8C and 850 8C for 2 h, respectively.

Y. Li et al. / Materials Research Bulletin 40 (2005) 981–989982

2.2. Measurements

The major parameters of the CS process (combustion temperature and combustion wave velocity)

were measured with Pt/Rh thermocouples pressed into the mixture. Phase composition and microscopic

morphology of the as-synthesized and annealed samples were investigated by X-ray diffraction analysis

(XRD) and scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) results were

obtained with a MICROLAB MKII spectrometer (UK). Magnetic properties of the samples were

conducted on a vibrating sample magnetometer (VSM, M-9500, USA).

3. Results and discussion

3.1. Effects of oxygen pressure and diluent content on the CS reaction

The effect of oxygen pressure and dilution on combustion temperature is shown in Fig. 1(a). There is a

clear trend showing an increase in the temperature with increasing oxygen pressure and a decrease with

increasing dilution for any given pressure. The effect of oxygen pressure and dilution on wave velocity

can be seen in Fig. 1(b). As the pressure is increased, the wave velocity increases, and the sample with

0 mol% dilution experiences the largest increase whereas the 30 mol% diluted sample has only a small

increase. The wave velocity decreases with the increase of the diluent content.

In order to show the effect of oxygen pressure on phase composition of the products, CS reactions were

conducted at various oxygen pressures (in the range of 0.1–0.4 MPa), using the reactants with the diluent

content of 20 mol%. Fig. 2 shows the XRD patterns of the combustion products synthesized at different

oxygen pressures. The XRD curve of 0.1 MPa suggests that the sample is not well crystallized, being

composed of the spinel crystalline phase and the secondary phases, NiO, ZnO, CuO and Fe2O3. As the

oxygen pressure is increased, the peaks corresponding to NiO, ZnO and CuO disappear, while the peak

intensities related to Fe2O3 phase gradually decrease and those related to NiCuZn ferrite spinel phase

increase.

To determine the CuCO3 diluent content for the CS reaction, the effect of the dilution content on phase

composition of the combustion products was investigated. Fig. 3 shows the XRD patterns of the

Y. Li et al. / Materials Research Bulletin 40 (2005) 981–989 983

Fig. 1. Effect of oxygen pressure on combustion temperature and wave velocity for samples using different diluent contents.

combustion products synthesized at 0.3 MPa, using the reactants with various diluent contents in the

range of 0–30 mol%. It can be seen that the peak intensity of the ferrite spinel in the combustion products

increases as the diluent content in the reactants decreases. When the diluent content is 30 mol%

(Fig. 3(a)), besides the main lines of the ferrite matrix, additional lines belonging to the ZnO and Fe2O3

phases are seen in the XRD patterns. While decreasing the amount of diluent in the reaction mixture to

0 mol% can produce pure NiCuZn phase (Fig. 3(d)). However, such products possess low sintering

activity due to self-sintering at high combustion temperature.


Fig. 2. XRD patterns of the combustion products synthesized at different oxygen pressures of (a) 0.1 MPa, (b) 0.2 MPa, (c)

0.3 MPa and (d) 0.4 MPa.

Fig. 3. XRD patterns of the combustion products synthesized at 0.3 MPa, using the reactants with various diluent contents of (a)

30 mol%, (b) 20 mol%, (c) 10 mol% and (d) 0 mol%.

Fig. 4 shows the morphology of NiCuZn powders formed using 0 mol% and 20 mol% CuCO3 diluent.

When the diluent content is 0 mol%, the products are heavily sinter-agglomerated and their size is about

5 mm (Fig. 4(a)). When the diluent content is increased to 20 mol%, NiCuZn is formed as discrete

particles that has size in the range of 0.5–3 mm.

In order to show the ferritization degree of the combustion products after heat-treatment, the as-

synthesized samples obtained at 0.3 MPa with 20 mol% diluent were annealed at 750 8C, 800 8C and

850 8C, respectively. Fig. 5 shows the XRD patterns of the combustion products after heat-treatment. The

phase compositions can be indexed as NiCuZn as major phase and the only secondary phase present is

Fe2O3 for the sample after annealing at 750 8C (Fig. 5 (c)). After annealing at 800 8C and 850 8C, the


Fig. 4. SEM morphology of NiCuZn powders formed with (a) 0 mol% and (b) 20 mol% CuCO3 diluent.

Fig. 5. XRD patterns of the combustion products after annealing at (a) 750 8C, (b) 800 8C and (c) 850 8C for 2 h.

peaks corresponding to Fe2O3 completely disappear and the samples have a single spinel structure, as

exhibited in Fig. 5(a) and (b). With the increase of the annealing temperature, the increase in sharpness of

XRD lines indicates the growth of crystallite size. The XRD patterns indicate that the spinel phase, which

usually forms at high temperatures in conventional method, has been completely formed at 800 8C during

combustion synthesis and subsequent heat-treatment.

3.2. Chemical state of the ions of NiCuZn ferrite

Surface studies of the samples were carried out by XPS to check the chemical state of the as-

synthesized ferrite and the annealed ferrite (800 8C). The Cu 2p, Fe 2p and O 1s photoelectron spectra are

shown in Figs. 6–8. The Cu 2p3/2 spectrum of the as-synthesized ferrite exhibits an intense peak in

binding energy range 932–934 eV (Fig. 6(b)) and a satellite peak at about 942 eV which is solely related

to Cu2+ cations. The large full-width at half-maximum (FWHM) value seems to be strong evidence for

the presence of copper ions in different binding states. As a matter of fact, the Cu 2p3/2 peak contains

three signals whose binding energies have the following values: 931.2 eV, 933 eV and 934.6 eV. From

XPS investigations of different copper species, Lenglet et al. [11] have reported copper binding energies

in relation to its valency and its tetrahedral or octahedral environment: CuA2+ at 936.2 eV, CuB

2+ at

934 eV, CuA+ at 932.8 eV and CuB

+ at 931.4 eV. In comparison with the above result, the peak at

934.6 eV in the spectrum can be interpreted as belonging to Cu2+ on B-sites, the peak at 933 eV to Cu+ on


Fig. 6. XPS spectra of the Cu 2p3/2 region of (a) annealed sample (800 8C) and (b) as-synthesized sample.

A-sites, and at 931.2 eV to Cu+ on B-sites. Therefore, it confirms that the combustion process results in

the formation of NiCuZn ferrites characterized by varying cation oxidation and distribution. In Fig. 6(a),

the Cu 2p3/2 spectrum of the annealed ferrite shows a signal with a small FWHM value (2.8 eV) due to

the absence of CuA+ and CuB

+ in the structure. The curve yields only one signal caused by Cu2+ ions on B-

sites. The XPS results show the subsequent annealing change of Cu from Cu+ to Cu2+.

In Fig. 7, the Fe 2p photoelectron spectra for the as-synthesized ferrite and the annealed ferrite are

shown. The binding energy is in the 711–711.4 eV range with a satellite-primary peak energy separation

of about 8.5 eV, which indicates that iron is in an oxygen environment.

The binding energy of the O 1s peak is about 530 eV in the as-synthesized samples (Fig. 8). This

energy is in agreement with results from the literature. However, the shoulder at higher energy derives

from adsorbed species like C O, OH, etc. [12]. The O 1s photoelectron spectrum of the annealed samples

is almost the same as Fig. 8.


Fig. 7. XPS spectra of the Fe 2p region of (a) annealed sample (800 8C) and (b) as-synthesized sample.

Fig. 8. XPS spectrum of the O 1s region of the as-synthesized samples.

3.3. Magnetic properties

The room temperature magnetic properties of the as-synthesized and annealed (800 8C)

Ni0.25Cu0.25Zn0.50Fe1.96O3.94 ferrite powders are determined. The results of the studies are summarized

in Table 1 and Fig. 9. It can be seen that the annealed products show better magnetic properties than the

as-synthesized NiCuZn powders. The maximum saturation magnetization, Ms, of the as-synthesized

samples (42.68 emu g�1) is lower than that of the annealed samples (67.75 emu g�1), while coercive

force Hc is higher. In agreement with the XPS results, this may be due to the diamagnetic Cu+, which

creates the dilution of the spin magnetic moment of B-site and lowers the Ms.

4. Conclusions

We have prepared NiCuZn ferrite powders by combustion synthesis method using the reactants

containing 0–30 mol% diluent and under oxygen pressure in the range of 0.1–0.4 MPa. XRD and SEM

results show that preferable products can be obtained from the CS reaction performed using 20 mol%

diluent and under an oxygen pressure of 0.3 MPa. XPS studies show that copper ions in the as-

synthesized ferrite are present in the different ionic states in the A- and B-sites. While heating the as-

synthesized ferrite at 800 8C for 2 h affords pure crystalline NiCuZn ferrite. Moreover, the annealed

products show better magnetic properties than the initial CS NiCuZn powders.


Table 1

Magnetic properties of NiCuZn ferrite powders

Samples Hc (A m�1) Ms (emu g�1)

CS 8624.09 42.68

800 8C 5753.42 67.75

Fig. 9. Hysteresis loop of NiCuZn ferrite powders (a) annealed at 800 8C and (b) as-synthesized.

Acknowledgment

The first author is grateful to the Youth Scientific Foundation of Hei Longjiang Province (Grant No.

QC02C39) and the Multidiscipline Scientific Research Foundation of Harbin Institute of Technology

(Grant no. MD. 2002.03) that supported this research.

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