Composites: Part B 51 (2013) 106–111

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Composites: Part B

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Influence of thermal treatment on the structure, humidity sensitivity,electrical and magnetic properties of barium–tungsten ferrite

1359-8368/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2013.03.005

⇑ Corresponding author. Tel.: +40 232 201173.E-mail address: IulianPetrila@gmail.com (I. Petrila).

Florin Tudorache a, Iulian Petrila a,⇑, Paul Dorin Popa b, Sorin Tascu a

a Research Center on Advanced Materials and Technologies, Faculty of Physics, Alexandru Ioan Cuza University of Iasi, Blvd. Carol I, No. 11, 700506 Iasi, Romaniab National Institute of Research & Development for Technical Physics-IFT, Blvd. D. Mangeron, No. 47, 700050 Iasi, Romania

a r t i c l e i n f o

Article history:Received 29 November 2012Received in revised form 5 March 2013Accepted 6 March 2013Available online 19 March 2013

Keywords:A. Metal–matrix compositesB. Electrical propertiesB. Magnetic propertiesE. Heat treatmentHumidity sensitivity

a b s t r a c t

The purpose of this study was to analyze the methods and effects of introducing tungsten ions in a hex-agonal barium ferrite structure by partial substitute of barium with tungsten. For this investigation weprepared a tungsten substitution on the barium hexaferrite Ba0.5W0.5Fe12O19 using sol–gel self-combus-tion technology. The powder was treated for 30 min, without atmosphere precautions, at 850 �C, 900 �C,950 �C, 1000 �C and 1050 �C. Scanning electron microscopy has revealed the crystallite size and shape,and the X-ray diffraction was provided information related to the phase compositions. The investigationwas focused on the variation of permittivity and electrical resistivity, in relation to the treatment temper-ature, frequency and humidity. We have also investigated the influence of thermal treatment on to Curietemperature and frequency characteristics of the relative permeability. Because barium–tungsten ferriteshows a porous structure, the measurements are strongly influenced by humidity conditions, and in thisrespect we analyzed the variations with humidity of permittivity and electrical resistivity.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The multiple experimental [1–3], theoretical [4–6] and numer-ical [7] investigations on to magnetic materials, performed duringto the last period, show that the magnetic materials represents oneof the most versatile materials.

The ferrite materials, thanks to multiple control options on theirelectrical and magnetic properties, shows increased applicativeinterest from the information storage to the sensors [8,9]. By com-bining the temperature treatment with intrusions of different ele-ments in the ferrite structure, can be obtained a hybrid ferriteswith useful properties [10–12].

Various recent studies [13,14] emphasize that the properties ofbarium hexaferrite show high saturation and magneto-crystallineanisotropy, which recommends that potential applications in areassuch as: the microwave sensors, filters, and magnetic recordingmedia. The various substitutions on the barium hexaferrite mayor may not lead to an improvement in electrical properties fromapplications perspective [15–19].

Generally, the tungsten is difficult to include in a ferritic struc-ture especially because of its multiple oxidation states, and ananalysis of how the tungsten is incorporated into a complex ferritestructure can be very useful from both: scientifically and techno-logically perspective. In the present paper, we report the results

of a systematic study of barium–tungsten ferrite Ba0.5W0.5Fe12O19

prepared by sol–gel self-combustion method, in which the electri-cal, magnetic and humidity sensitivity properties are investigatedand the connection with their composition, sintering temperatureand structural characteristics are also explained. In the next sec-tions are presented the preparation method, microstructural anal-ysis, electrical and magnetic properties of the barium–tungstenhexaferrite and the influence of humidity sensitivity and the tem-perature treatment on to electrical properties.

2. Preparation

Since the ferrites preparation method is one of the key factors inobtaining ferrites with various properties, in recent years weredeveloped and reported many preparation methods such as: co-precipitation [20], sol–gel combustion [21], molten salt route[22] and microemulsion [23].

In our study, the barium–tungsten hexaferrite powder was pre-pared by using the sol–gel self-combustion technology [24–26],because for Ba0.5W0.5Fe12O19 ferrite formation is sufficient ashort-term treatment. According to the stoichiometric formula,were weighed and used the raw materials: barium nitrateBa(NO3)2 (99% purity-Merck, Darmstadt, Germany); iron nitrateFe(NO3)3�9H2O (99.99% purity-Sigma–Aldrich, Taufkirchen, Ger-many); ammonia paratungstate (NH4)10H2(W2O7)6�4H2O (99% pur-ity Merck) and Polyvinyl Alcohol [CH2CHOH]n.

Table 1Structural characteristics of sintered Ba0.5W0.5Fe12O19 hexaferrites.

Temperature Tsint

(�C)Contraction DV/V0

(%)Surface S(mm2)

Porosity p(%)

850 45.8 109.36 32900 47.3 107.51 24950 48.8 102.07 151000 49.4 93.14 131050 50.1 91.27 12

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The homogeneous acidic solution was neutralized with 10%ammonia concentration to pH = 7. The resulting mixture was driedat a temperature of 100 �C in order to ignition combustion reaction.After the combustion, were obtained approximately 12 g of hexa-ferrite powder. The barium–tungsten hexaferrite powder (Ba0.5-

W0.5Fe12O19) was pre-calcined for 30 min at 500 �C. By using ahydraulic press of 10 tons, the samples were uniaxially pressedinto several disk-shaped samples with a diameter of 13 mm and2 mm thickness.

The structural analyzes were performed using X-ray diffraction(Shimadzu LabX XRD-6000) and scanning electron microscope(Hitachi S3400-N).

For performing magnetic investigations, the ferrite sampleswere uniaxially pressed into torus-shaped form with internaldiameter of 5 mm and an outer diameter of 13 mm.

In order to investigate the influence of the heat treatment onthe structural, electrical and magnetic properties, the sampleswere annealed, without precautions atmosphere, for 30 min at:850 �C, 900 �C, 950 �C, 1000 �C and 1050 �C.

For the electrical investigations, on both sides of the disk-shaped samples were deposition silver sandwich electrodes (por-ous in the case of humidity analysis in order to allow the accessof water vapors). The electrical measurements were performed atroom temperature by using an Agilent E4980A LCR Meter, in thefrequency range 20 Hz–2 MHz.

In order to analyze the influence of humidity on the electricalproperties of barium hexaferrite, we investigated the variation ofelectrical resistivity and permittivity depending on the relativehumidity.

3. Microstructural characteristics

The microstructure was investigated by X-ray diffraction usinga Shimadzu LabX XRD-6000 diffractometer using Cu Ka radiationwith k = 1.5405 Å and a scanning speed of 4 degrees/min.

Fig. 1. XRD patterns of BaW0.5Fe11.5O19 hexaferrites sintered at differenttemperatures.

From X-ray analysis of XRD patterns, shown in Fig. 1, it is foundthat the untreated hexaferrite sample it occur a peak of WO3 at 27�[24,25,27], marking that the tungsten was integrated into the fer-rite structure.

Also, from the XRD analysis it is observed that, by performingthe sintering treatment at 850 �C for 30 min, there is barium hexa-ferrite formation but also is observed the presence of a small pro-portion of simple phase of hematite (Fe2O3). For the treatments attemperatures above 850 �C it is observed an increase in the amountof hexaferrite barium and a decrease in the amount of hematite be-cause the 64� peak is not found in samples sintered at 1000 �C and1050 �C. The structural characteristics of barium hexaferrite withtungsten substitutions are shown in Table 1.

It is observed that with increasing sintering temperature thereis an increase in the sample volume contraction DV/V0, but equallyit produces a decrease of porosity. The higher value of porosity wefound for the hexaferrite sintered at 850 �C and the lowest for thesample treated at 1050 �C, these results will also be confirmed bythe electrical measurements under humidity influence.

The shape and size of crystallites investigation, which can beobserved in Fig. 2, was performed on fractured samples by usinga scanning electron microscope (SEM) Hitachi S3400-N. It seemsthat by performing a heat treatment at temperatures of 850 �C,900 �C and 950 �C, the Ba0.5W0.5Fe12O19 ferrite leading to a finegranular structure that containing inter-granular pores.

For the samples with sintered temperatures below 1000 �C isobserved clusters of small crystallites with dimensions of the orderof hundred of nanometers. On the SEM micrographs of Ba0.5W0.5-

Fe12O19 samples treated at temperatures of 1000 �C and 1050 �Care observed the formations of hexagonal faceted crystallites withan average size of between 2 and 4 lm [28] which are correlatedwith the difractograms shown in Fig. 1.

4. Electrical properties

The electrical measurements on to Ba0.5W0.5Fe12O19 hexaferritesamples were made by measuring of permittivity and electricalresistivity variation in the frequency range 20 Hz–2 MHz, at roomtemperature by using an RLC Metter. It is known the fact that fre-quency characteristics of complex conductivity, in case of disor-dered materials, can arise from hopping mechanism, interfacialpolarization at contacts, grain boundaries and other in-homogene-ities presents in the sample [4].

In Fig. 3 are shown the frequency dependence of relative per-mittivity of the investigated samples for all sintering temperatures.It is found that increasing the sintering temperature increases theelectrical permittivity of Ba0.5W0.5Fe12O19 hexaferrite. The samplesintered at 850 �C shows the lowest value of electrical permittivitybecause it has a sub-micron fine-grained structure (see Fig. 2) andhas the highest non-homogeneity and porosity (see also Table 1).The samples sintered at temperatures of 1000 �C and 1050 �Cshows higher values of electrical permittivity, at least an order ofmagnitude because it shows larger crystallites with relative thin-ner grain boundaries (see Fig. 2).

Fig. 2. SEM micrographs of Ba0.5W0.5Fe12O19 powders sample: (a) untreated, (b) 850 �C, (c) 900 �C, (d) 950 �C, (e) 1000 �C and (f) 1050 �C.

Fig. 3. Dependences of relative permittivity of investigated hexaferrite onfrequency.

Fig. 4. The variation of electrical resistivity versus frequency for the investigatedhexaferrites.

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Fig. 6. The influence of thermal treatment on the temperature characteristics of themagnetic susceptibility and Curie temperature.

Fig. 5. The influence of thermal treatment on the frequency characteristics of therelative permeability.

Fig. 7. The influence of the relative humidity and thermal treatment on the frequency characteristics of the relative permittivity.

F. Tudorache et al. / Composites: Part B 51 (2013) 106–111 109

110 F. Tudorache et al. / Composites: Part B 51 (2013) 106–111

In the Fig. 4 are shown the variations of electrical resistivitywith sintering temperature in the frequency range 20 Hz–2 MHz.It is found that increasing sintering temperature induce decreasingof electrical resistivity due to decreasing the amount of hematite(see Fig. 1).

The variation of permittivity and resistivity with frequency is inagreement with Maxwell–Wagner–Debye relaxation [29]. Thesamples sintered at temperatures of 1000 �C and 1050 �C shows amore rapid decreasing of electrical resistivity with frequency dueto increasing the granulation of samples (see Fig. 2) and decreasingthe grain boundary resistivity by decreasing hematite content (seeFig. 1).

5. Magnetic properties

The magnetic permeability measurements were performed atroom temperature by using an Agilent model E4980A RLC circuit.In the Fig. 5 is presented the variation of magnetic permeabilityof sintered barium–tungsten hexaferrite function of frequency fordifferent sintering temperatures.

It is observed that the increasing treatment temperature leadsto an increase in magnetic relative permeability, the minimum va-lue of 2.5 is seen for the sample treated at 850 �C and a maximum

Fig. 8. The influence of the relative humidity and thermal treatme

of 3.7 belonging to the sample treated at 1050 �C [30]. In order todetermine the Curie temperature of Ba0.5W0.5Fe12O19 hexaferrite,the samples as toroidal cores were placed in a non-magnetic ovenand were measured the inductance variation with the tempera-ture. The thermal characteristics of magnetic susceptibility ex-tracted from the inductance are represented, in normalized form,in Fig. 6.

It is observed that by performing heat treatment at tempera-tures of 850 �C, 900 �C, 950 �C, 1000 �C and 1050 �C there is a slightshift of the Curie point to higher values with increasing of the tem-perature treatment.

6. Electrical properties under humidity influence

The humidity sensitivity of Ba0.5W0.5Fe12O19 hexaferrite wereanalyzed in a room, at constant temperature (25 �C) for variousknown values of relative humidity. Under these conditions, weremeasured the frequency characteristics of permittivity and electri-cal resistivity, in the frequency range 20 Hz–2 MHz and humidityrange 0% RH–85% RH.

As can be seen in Fig. 7, for low values of relative humidity,Ba0.5W0.5Fe12O19 hexaferrite treated at 850 �C, 900 �C and 950 �Cshows the relative permittivity values of the order of 103, and for

nt on the frequency characteristics of the electrical resistivity.

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high values of relative humidity, the relative permittivity valuesare of the order of 104 or higher. The significant sensitivity tohumidity is observed for the samples treated at 850 �C, 900 �Cand 950 �C and is decreasing with the frequency.

From the Fig. 8 it can be observed that, for low values of relativehumidity, Ba0.5W0.5Fe12O19 hexaferrite sintered at 850 �C, 900 �Cand 950 �C shows the variation of electrical resistivity values from104 to 105 X �m, and for high values of relative humidity, the rel-ative permittivity values are of the order of 103 X �m. For thesamples sintered at temperatures of 1000 �C and 1050 �C is ob-served higher electrical resistivity variation with almost 3 ordersof magnitude, in the frequency range 20 Hz–2 MHz, but the varia-tion of electrical resistivity with humidity is important for low fre-quency. For the samples sintered at 850 �C, 900 �C and 950 �C, thevariation of electrical resistivity with humidity is significant in thewhole frequency range. In the essence, the humidity influence onthe frequency characteristics of the permittivity and electricalresistivity is explained by contribution to the Maxwell–Wagner–Debye relaxation of the vapors that fill the material pores.

The large variations of electrical properties (permittivity andelectrical resistivity) of Ba0.5W0.5Fe12O19 under humidity influence,suggest that barium–tungsten ferrite can be used as both capaci-tive and resistive humidity sensors.

7. Conclusions

The XRD and SEM analysis showed that for reducing simplephase of hematite (Fe2O3) and obtaining the micron-sized granulesof Ba0.5W0.5Fe12O19 hexaferrite it need a that temperature treat-ment to be at least 1000 �C.

With increasing treatment temperature are increase both: per-meability and the Curie temperature of barium–tungsten ferrite.

According to electrical measurements, Ba0.5W0.5Fe12O19 hexa-ferrite treated at temperatures higher than 1000 �C shows a signif-icant sensitivity to humidity only at low frequency.

By following the investigations of humidity sensitivity, one mayobserves that Ba0.5W0.5Fe12O19 hexaferrite sintered at 850 �C,900 �C and 950 �C shows sensitivity to the humidity, so it can beused as an application as capacitive or resistive humidity sensorelement.

Acknowledgment

This work was performed within the project ‘‘Research Centeron Advanced Materials and Technologies – RAMTECH’’ 162/15.06.2010 of POS CCE – A2-O2.1.2.

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