Materials Research Bulletin 44 (2009) 753–758

Electrical properties of MgAl2–2xZrxMxO4 (M = Co, Ni and x = 0.00–0.20)synthesized by coprecipitation technique using urea

Muhammad Javed Iqbal *, Beenish Kishwar

Surface and Solid State Chemistry Laboratory, Department of Chemistry, Quaid-i-Azam University, 3rd Avenue, Capital Terretory, Islamabad-45320, Pakistan

A R T I C L E I N F O

Article history:

Received 6 December 2007

Received in revised form 4 August 2008

Accepted 24 September 2008

Available online 10 October 2008

Keywords:

A. Oxides

B. Chemical synthesis

C. Thermogravimetric analysis

D. Dielectric properties

D. Electrical properties

A B S T R A C T

This paper presents the results of a study concerning the structural and electrical properties of MgAl2–

2xZrxMxO4 (x = 0.00–0.20 and M = Co2+ and Ni2+) prepared by a coprecipitation technique using urea as a

precipitating agent. The X-ray diffraction data for the pure and its doped samples are consistent with the

single-phase spinel and their crystallite sizes are in the range 7–20 � 4 nm. The DC resistivity increases

from 3.09 � 109 V cm to 6.73 � 109 and 8.06 � 109 V cm whereas dielectric constant decreases from 5.80 to

5.11 and 4.95 on doping with Zr–Co and Zr–Ni, respectively. The electrical resistivity variations with increase

in the dopant contents indicate two types of conduction mechanisms in operation. Several parameters such

as, hopping energy (W), metal–semiconductor transition temperature (TMS) and Debye temperature (uD) have

also been determined. The increase in DC resistivity and decrease in dielectric constant suggest that the

synthesized materials can be considered for application as an insulating and structural material in fusion

reactors.

� 2008 Elsevier Ltd. All rights reserved.

Contents lists available at ScienceDirect

Materials Research Bulletin

journa l homepage: www.e lsev ier .com/ locate /matresbu

1. Introduction

In recent years, there has been an increasing interest in thesynthesis and characterization of magnesium aluminate(MgAl2O4) spinel due to its physical, chemical, optical andelectrical properties. This material is of significant technologicalinterest for refractory and structural applications at elevatedtemperature due to its high melting point (2135 8C) and excellentchemical resistance [1]. It is also used as a humidity sensor incertain electronic applications [2–4] and as an insulating materialfor fusion reactor cores [5]. The mechanism of electricalconductivity in spinels depends on the arrangement of ions indifferent sites. The electrical and structural properties of nano-materials are unusually different from those of their singlecrystalline, coarse-grained polycrystalline and thin film counter-parts due to the high surface-to-volume ratio of the grains,quantum confinement of charge carriers, enhanced contributionfrom grains and grain boundary regions, band structure modifica-tion and possibility of holes and defects in grains [6].

The chemical and structural properties of MgAl2O4 aredependent on the method of preparation [7,8]. In recent years, anumber of studies have been carried out to synthesize homo-geneous nanostructures of MgAl2O4 spinel powders with highchemical purity and a narrow particle size distribution. A variety of

* Corresponding author. Tel.: +92 51 90642143; fax: +92 51 90642144.

E-mail address: mjiqauchem@yahoo.com (M.J. Iqbal).

0025-5408/$ – see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2008.09.036

preparation techniques have been used to prepare MgAl2O4 spinel.Of all these techniques, the coprecipitation technique [9] is moreconvenient for the synthesis of small nanosized particles becauseof its simplicity, time effectiveness and better control over thecrystallite size of the product. Studies concerning the effect ofsubstitution of Co2+, Fe3+, Ni2+, V4+, Cr3+, Mn3+ Ca2+, Ba2+ and Sr2+ onthe properties of aluminum based spinel have been carried out byseveral workers [10,11].

In the present study, a chemical coprecipitation techniqueinvolving urea (NH2CONH2) as a precipitating agent has been used.Different amines including urea are known to form complexmolecules with metal ions [12] that on addition to the metal saltsolution encircle the metal ions and form tetragonal complex. Thiscomplex, upon decomposition of urea, leads to uniform precipita-tion of the metal hydroxide to produce small and uniform sizeparticles in the samples. The main purpose of this work is toenhance the electrical resistivity and to lower the dielectricconstant of nanoparticles of MgAl2O4 by substitution of Zr–Co andZr–Ni. Inclusion of Zr increases the physical strength and providesthermal stability [13,14] whereas Co and Ni can enhance theresistivity of the material [15].

2. Experimental

2.1. Materials synthesis

Analytical grade chemicals Mg (NO3)3�6H2O (Merck, 99%),Al(NO3)3 (Fluka, 99%), Co (CH3COO)2�H2O (Merck, 99%), NiCl2�6H2O

Fig. 1. XRD patterns of samples: (a) MgAl2O4, (b) MgAl1.84Zr0.08Co0.08O4, and (c)

MgAl1.84Zr0.08Ni0.08O4.

M.J. Iqbal, B. Kishwar / Materials Research Bulletin 44 (2009) 753–758754

(Aldrich, 99%), ZrOCl2�4H2O (BDH, 96%), NH2CONH2 (Merck, 99%)and AgNO3 (Agar, 99%) are used in the synthesis of samples. Theundoped magnesium aluminate and its Zr–Co and Zr–Ni sub-stituted samples of nominal compositions MgAl2–2xCoxZrxO4 andMgAl2–2xNixZrxO4 (where x = 0.00–0.20) are synthesized by achemical coprecipitation method maintaining the Mg/Al molarratio of 0.5. The 2 M solution of NH2CONH2 is used as aprecipitating agent in the reaction. The precipitation reactionoccurs by carefully controlling the decomposition of urea–watersolution at a temperature 363 K for 3 min on a magnetic stirrerwith constant stirring to produce a steady stream of ammoniawhich is released as the main product [16] according to thefollowing scheme:

NH2CONH2þH2O ! 2NH3þCO2

The solution pH of 7 is maintained during precipitation bycarefully controlling the urea decomposition temperature. Theprecipitates are washed several times with the deionized water,dried at 393 K for 6 h and then calcined at 1073 K at a rate of 5 K/min in a temperature programmed tube furnace. The reactionscheme for the coprecipitation reaction is shown below:

MgðNO3Þ2 ðAqÞ þ 2AlðNO3Þ3 ðAqÞ ! Mg � Al2O4þnH2O þ yNOx

It is inferred that the layered double hydroxide (LDH) is formed inthe solution where an isomorphic replacement of Mg2+ ions withtrivalent Al3+ cations occurs. This generates a positive charge onthe layers that necessitates the presence of interlayer chargebalancing anions. The remaining free space is occupied by thewater of crystallization in the interlayer [17].

Similarly Zr4+–Co2+ and Zr4+–Ni2+ doped derivatives of MgAl2O4

were also prepared by adding the appropriate stoichiometricquantities of the dopant salt and following the same procedure asstated above.

2.2. Characterization

The crystal structure is determined by X-ray diffractometer(Jeol JDX-60PX) which uses a Cu Ka radiation source at 45 kV and40 mA. The XRD peaks of the powdered samples are recordedbetween 158 and 858with a step 0.048 and counting time of 1 s/stepand are used to identify the crystal lattice. The DC electricalresistivity measurements are carried out on the sample pellets of13-mm diameter and 3-mm thickness formed under a pressure of90 kN. The resistivity is measured by a two-point probe method ina temperature range of 298–673 K. This technique involvesbringing two probes in contact with the sample. One of the probesis used for sourcing DC voltage from a constant voltage sourcewhich measures voltage with an accuracy of �0.0001 V and theother probe is used for measuring the resulting change in currentacross the surface of samples using the source meter (Keithley, Model2400) with an accuracy of �0.0001 mA. A temperature sensingmultimeter (Uni, UT 55) is connected to the thermocouple (PT100)enabling temperature measurements with an accuracy of �0.01 K.The resistivity r is calculated by the following relation: r = R(A/L)where R is resistance of sample, A is area of the sample pellet and L isits height.

The backscattering electron images are obtained using a Jeol(JSM-5910) scanning electron microscope (SEM). Energy disper-sive X-ray fluorescence (EDXRF) studies of the samples are carriedout by a Horiba (MESA-500) system. Thermo-gravimetric analysisis carried out by a PerkinElmer (TGA-7) system at a heating rate of5 K/min. The dielectric measurements are carried out at roomtemperature in a frequency range of 100 Hz–1 MHz. The samplepellets of diameter 13 mm and thickness of 1 mm are used as acapacitor. The surface of the capacitors is coated by a layer of silver

paste (Agar Scientific) and air dried for 20 min. The capacitance anddielectric loss measurements of the pellet are measured by a LCRmeter bridge (Wayne Kerr LCR4275) and using Eq. (4).

3. Results and discussion

3.1. X-ray diffraction analysis

The powder XRD patterns of MgAl2O4, MgAl1.84Zr0.08Co0.08O4

and MgAl1.84Zr0.08Ni0.08O4 samples are shown in Fig. 1. The highintensity peaks are recorded for the undoped magnesiumaluminate and its derivatives doped with Zr4+–Co2+ and Zr4+–Ni2+. The hkl values of (1 1 1), (2 2 0), (3 1 1), (4 0 0), (5 1 1) and(4 4 0) are a perfect match with the standard spinel MgAl2O4

compound (JCPDS Card No. 21–1152). Analysis of the XRD patternsand the absence of any extra peaks in these patterns confirm thatall the samples have a single-phase cubic structure [18]. The latticeconstant a is calculated from the powder XRD data using therelation, a = [d2(h2 + k2 + l2)]1/2; where d is value of d-spacing oflines in XRD pattern and hkl are the corresponding indices for thecrystal lattice. Crystallite size (D) is calculated by the Debye–Scherrer equation:

D ¼ KlbcosuB

(1)

where b is the broadening of diffraction line measured at halfwidth of its maximum intensity, l is the X-ray wavelength and isequal to 1.542 A, uB is the Bragg’s angle and K is a constant whichfor cubic system is equal to 0.9. The crystallite sizes of the samplesare in the range of 7–20 nm.

Table 1 shows that the cell volume (V = a3) increases with theaddition of Zr–Co content, x, because the ionic radii of both Co2+

(0.82 A) and Zr4+ (0.80 A) are larger than that of Al3+ (0.50 A).However, Fang et al. [19] have suggested that this change in thelattice parameter is due to entrance in the cubic structure of thealuminates of relatively bigger ions. Whereas, in the case of Zr–Nidoped samples, the lattice parameter decreases as shown inTable 2 possibly due to a random distribution of Ni2+ ions at boththe octahedral and tetrahedral sites. Because some of the smallerNi2+ ions (0.62 A) may have replaced the parent Mg2+ ions (0.65 A)by moving them from tetrahedral to the larger octahedral sites assuggested by Halevy et al. [20]. The ED-XRF analysis of an undopedsample (MgAl2O4) is carried out to confirm the nominal composi-tion. The experimental Al/Mg ratio of 2.11 is in close agreementwith the stoichiometric composition of (Al/Mg = 2.0).

The X-ray density (dX-ray) of the samples is calculated by therelation; dX-ray = zM/VcellNA, where z is the number of molecules per

Table 1Calculated parameters of MgAl2–2xZrxCoxO4 spinel (x = 0.00–0.20).

ZrxCox

content x

a (�0.02 A) Vcell

(�4 A3)

D (�4 nm) dX-ray

(�0.16 g cm�3)

r (�1.79 V cm 109) TMS

(�5 K)

uD (�10 K) e0 at 1

MHz (�0.26)

tan d at 1

MHz (�0.0017)

W (�0.06 eV)

0.00 8.10 531 19 3.56 3.09 383 913 5.80 0.0051 0.14

0.04 8.11 533 14 3.64 4.45 398 979 5.62 0.0033 0.27

0.08 8.12 535 9 3.72 6.73 403 1015 5.11 0.0009 0.32

0.12 8.11 533 8 3.85 5.23 408 964 5.20 0.0010 0.27

0.16 8.13 537 13 3.94 4.64 413 665 5.24 0.0012 0.19

0.20 8.15 541 15 3.97 3.14 453 1000 5.45 0.0011 0.21

M.J. Iqbal, B. Kishwar / Materials Research Bulletin 44 (2009) 753–758 755

formula unit (z = 8 for spinel system), M is the molar mass whereasVcell and NA have their usual meanings. The calculated values of thestructural parameters are shown in Tables 1 and 2. Density of thedoped magnesium aluminate samples is higher than that of theundoped sample since the atomic masses of Zr (91.22), Co (58.93)and Ni (58.69) are higher than that of Al (26.98).

SEM for pure MgAl2O4 and its doped derivatives are shown inFig. 2. Slightly rough surface morphology of the samples indicatesthat an agglomeration of finer crystallites has resulted intoformation of the particles at the surface. The microstructureshows a homogeneous distribution of the particles in the samples.The particle size calculated by SEM for pure MgAl2O4 and itsderivatives is found in the range of 70–83 nm. It is clear from Fig. 2that the substituted samples are more homogeneous and theparticle size is also smaller (Fig. 2b and c) than that of the undopedsample (Fig. 2a). The decrease of the particle size of the samples isowing to the presence of zirconium which is reported to increasethe mechanical strength and thermal stability of the synthesizedsamples [13,14].

3.2. Thermal analysis

DTA and TG curves of the undoped magnesium aluminatesample are shown in Fig. 3. Three endothermic peaks are clearlyobserved in the DTA curves of the samples; peaks centered at about403 K and 495 K correspond to the evaporation of water, NH3 andCO2 adsorbed or trapped on the surface, respectively, and the peakat 618 K corresponds to decomposition of the metallic hydroxidesto oxides. The weight loss in TGA corresponding to theendothermic peaks also appeared in DTA profiles. No significantweight loss above 623 K is observed in the TGA profile indicatingthat stable metal oxide must have been fomed.

3.3. Electrical resistivity measurements

The DC electrical resistivity of the samples is measured in thetemperature range of 298–673 K and the calculated values arelisted in Tables 1 and 2. An initial decrease in the resistivity ofMgAl2O4, MgAl1.68Zr0.16Co0.16O4 and MgAl1.68Zr0.16Ni0.16O4 sam-ples with temperature is observed (Fig. 4) which is due to theabsorbed moisture [21] and other impurities, like CO2 gas, trapped

Table 2Calculated parameters of MgAl2–2xZrxNixO4 spinel (x = 0.00–0.20).

ZrxNix

content

a (�0.03 A) Vcell (�5 A3) D (�4 nm) dX-ray

(�0.17 g cm�3)

r (�1.85 V cm

0.00 8.10 531 19 3.56 3.09

0.04 8.02 516 8 3.75 4.61

0.08 8.09 529 16 3.76 6.45

0.12 8.06 524 12 3.87 8.06

0.16 8.08 526 10 3.95 5.95

0.20 8.07 522 15 4.07 3.25

in the samples. However, when all the trapped impurities areremoved at approximately above 350 K the resistivity increasesexponentially with rise in the temperature (positive Dr/DT) andreaches a maximum value at the temperature called, metal–semiconductor transition temperature TMS (Fig. 4). The behaviorbelow the TMS represents a metallic character and indicates thatsufficient energy is available to thermally activate the chargecarriers. However, since the necessary activation energy has beenattained above TMS, the resistivity decreases on further increasingthe temperature (negative Dr/DT). This is the normal semicon-ducting behavior and is observed throughout the remainingtemperature range investigated here. The value of TMS increaseswith the addition of the dopants as shown in Tables 1 and 2. TheZr–Co doped samples have higher TMS values at higher dopantconcentration (x = 0.12–0.20) as compared to the Zr–Ni sub-stituted samples. This may be due to the reason that highernumber of unpaired electrons (3) is involved in Co2+ than in case ofNi2+ (2). The increase in the value of TMS may also be due to the spincanting; by increasing the dopant concentration and temperature,the spin angle of electrons is changed to cause variation ofresistivity [22].

In the semiconducting region, i.e. above the TMS, resistivity doesnot fit to the Arrhenius equation and indicates a nonlinear behaviorcharacteristic of the small polaron hopping conduction. Thetemperature at which the linearity (in the high temperaturerange) changes is considered to be uD/2, where uD is thecharacteristic Debye temperature. Debye temperature (uD) forthe doped samples is higher as compared to the undoped MgAl2O4

sample as is evident from Tables 1 and 2. The linear semiconduct-ing part, as shown in Fig. 5 for T > uD/2, is satisfactorily explainedby Mott’s small polaron hopping model [23]

sdc ¼ so exp�W

kbT

� �(2)

where so is the temperature independent constant, W and kb arerespectively, the activation energy and the Boltzman constant. Theactivation energy of each sample is determined from the slope ofthe linear plot of the conductivity versus T�1 as shown in Fig. 5. Inthree dimensional cases, the conductivity follows the variablerange hopping (VRH) mechanism and is predicted from the

109) TMS (�5 K) uD (�10 K) e0 at 1

MHz (�0.32)

tan d at 1

MHz (�0.0114)

W (�0.06 eV)

383 913 5.80 0.0051 0.14

453 920 5.57 0.0028 0.25

413 962 5.01 0.0008 0.32

393 1013 4.95 0.0006 0.37

408 995 5.12 0.0010 0.25

418 980 5.33 0.0011 0.21

Fig. 2. SEM photographs of samples: (a) MgAl2O4, (b) MgAl1.84Co0.08Zr0.08O4, and (c)

MgAl1.84Ni0.08Zr0.08O4.

Fig. 3. (a) DTA curves for MgAl2O4, MgAl1.84Zr0.08Co0.08O4, and

MgAl1.84Zr0.08Ni0.08O4; and (b) TGA curves for MgAl2O4, MgAl1.84Zr0.08Co0.08O4,

and MgAl1.84Zr0.08Ni0.08O4.

Fig. 4. Plot of resistivity (r) versus temperature (T) for MgAl2O4,

MgAl1.68Zr0.16Co0.16O4, and MgAl1.68Zr0.16Ni0.16O4.

M.J. Iqbal, B. Kishwar / Materials Research Bulletin 44 (2009) 753–758756

following relation:

s ¼ soexp � To

T

� �1=4" #

(3)

Plots of log s versus T�1/4 are shown in Fig. 6. Straight linesshow that the conductivity data between TMS and uD/2 follows thevariable range hopping mechanism.

Fig. 7 shows the variation of the room temperature DCresistivity with the dopant content, x. The doped Co and Ni ionsare known to occupy the octahedral site while Zr ions havepreference for the tetrahedral sites [24]. In cases of the Zr–Co andZr–Ni doped samples, the DC resistivity reaches a maximum valueat x = 0.08 and x = 0.12, respectively. The reason for this trend is

that Co (6.24 mV cm) and Ni (6.84 mV cm) cations are moreresistive than Al (2.65 mV cm), the resistivity would be enhanced.However, the resistivity would drop on further substitution due topronounced hopping at the octahedral sites. It is also observed thatthe room temperature DC resistivity of Zr–Ni substituted samplesis higher than that of the Zr–Co substituted samples. The higherresistivity value in the former samples is because Ni is moreresistive than Co as stated above. Ni2+ and Co2+ ions are convertedinto Ni3+ and Co3+, respectively, during annealing to form positiveholes. The conductivity is now due also to the hopping of theseholes in the substituted samples, however, ionization ofNi2+! Ni3+ is more difficult than Co2+! Co3+ because the thirdionization energy of cobalt (3232 kJ mol�1) is smaller than that ofthe Ni (3395 kJ mol�1).

Fig. 5. Linear fit of conductivity (s) versus 1/T data to the polaron model in a

temperature range above uD/2. (a) MgAl2O4, (b) MgAl1.68Zr0.16Co0.16O4, and (c)

MgAl1.68Zr0.16Ni0.16O4.

Fig. 6. Linear fit of conductivity (s) versus T�1/4 data to the VRH model in

temperature ranging between TMS and uD/2. (a) MgAl2O4, (b) MgAl1.68Zr0.16Co0.16O4,

and (c) MgAl1.68Zr0.16Ni0.16O4.

Fig. 7. Plot of resistivity of MgAl2–2xMxZrxO4 spinel samples at 298 K versus the

dopants (M = Co2+and Ni2+) contents (x = 0.00–0.20).

M.J. Iqbal, B. Kishwar / Materials Research Bulletin 44 (2009) 753–758 757

3.4. Dielectric properties

Dielectric constant (e0) of the synthesized samples is calculatedby the following equation:

e0 ¼ Cd

eA(4)

where C is the capacitance in Farad, d the thickness of the pellet, A

the cross-sectional area of the flat surface of the pellet and e0the permittivity constant of the free space. The variation ofthe dielectric constant and the dielectric loss (tan d) ofsamples measured at room temperature by varying thefrequency of applied field are shown in Fig. 8(a and b). Fromthese figures, it can be seen that both e0 and tan d initiallydecrease rapidly with increase in frequency and as the frequencyis further increased both attain constant values. This dielectricbehavior of samples can be explained by the mechanism ofpolarization [25] according to which the electrons exchangebetween Al3+ and Al2+ gives local displacement of electrons in thedirections of an applied electric field, which induces polarizationin aluminates. Tables 1 and 2 show that small values at highfrequency are due to high concentration of the holes and thedefects associated with the grain boundaries. Materials with high

resistivity would therefore exhibit low dielectric losses and viceversa [26].

The compositional dependence of dielectric constant atfrequency of 1 MHz is illustrated in Fig. 9. The dielectric constantand dielectric loss are in excellent agreement with the DCresistivity data as shown in Tables 1 and 2.

Fig. 8. (a) Comparisons of the dielectric constant, e0 , of MgAl2O4, MgAl2–

2xCo0.2Zr0.2O4, MgAl2–2xNi0.2Zr0.2O4 (b) Comparisons of the dielectric loss, tan d,

of MgAl2O4, MgAl2–2xCo0.2Zr0.2O4 and MgAl2–2xNi0.2Zr0.2O4 as a function of

frequency.

Fig. 9. Plot of dielectric constant (e0) of MgAl2–2xMxZrxO4 samples at 298 K versus

the dopant (M = Co2+ and Ni2+) contents (x = 0.00–0.20).

M.J. Iqbal, B. Kishwar / Materials Research Bulletin 44 (2009) 753–758758

4. Conclusions

Single-phase MgAl2O4 and its derivatives MgAl2–2xMxZrxO4

doped with M (Co2+ and Ni2+) have been synthesized

by a chemical coprecipitation method using urea as aprecipitating agent. The crystallite sizes of the samples aredetermined by Scherrer formula to be in the range of 7–20 nm.This synthesis technique is simpler and more efficientcompared to some of the other techniques. The small polaronand VRH models are applied to the resistivity data above TMS inthe semiconducting region. The small polaron model is found tobe applicable in the temperature range above Debye tempera-ture uD/2 and the VRH model below uD/2. The metal–semiconductor transition temperature, TMS, of the dopedsamples is higher than the undoped sample. Resistivity is foundto increase by doping of MgAl2O4 with Zr–Co and Zr–Ni. Owingto the increased resistivity, the doped materials can beconsidered for application as insulating and structuralmaterials in fusion reactors. Whereas the dielectric parametersof the Zr–Co and Zr–Ni doped samples are lower as compared tothe undoped MgAl2O4 that makes them suitable forapplications in the fields of microwave and millimeter-wavecommunication.

References

[1] L.R. Ping, A.-M. Azad, T.W. Dung, Mater. Res. Bull. 23 (2001) 1417.[2] S.G.N.M. Sharafurt, P.I.H. Cooke, R.C. Martin, F. Najamabad, K.R. Schultz, C.P.C.

Wong, Fusion Eng. Des. 23 (1993) 99.[3] G.M.G. Gusmano, E. Travera, A. Bearzotti, Sens. Actuator B 13/14 (1993) 525.[4] Y.A.N. Schimizu, T. Seiyama, Sens. Actuator B 7 (1985) 11.[5] H. Cynn, S.K. Sharama, T.F. Oohey, M. Nicol, Phys. Rev. B 45 (1992) 500.[6] B. Thomas, M.A. Khadar, Pramana, J. Phys. 45 (1995) 431.[7] J. Paramentier, M. Richard-Plouet, S. Vilminot, Mater. Res. Bull. 33 (1998 1717).[8] C. Garapon, H. Manaa, R. Moncoge, J. Chem. Phys. 95 (1991) 5501.[9] G. Gusmano, P. Nunziante, E. Traversa, G. Chiozzini, J. Eur. Ceram. Soc. 7 (1991)

31.[10] K. Izumi, S. Miyazaki, S. Yoshida, T. Mizokawa, E. Hanamura, Phys. Rev. B 76 (2007)

075111.[11] M.J. Iqbal, S. Farooq, Mat. Sci. Eng. B 136 (2007) 140.[12] J. Hagen, Industrial Catalysis, 2nd Ed., Wiley-VCH, Germany, 2006, pp. 21–22.[13] Y.H. Sohn, R.R. Biederman, R.D. Sisson, Thin Solid Films 250 (1994) 1.[14] D. Yang, H. Son, J. Feng, S. Li, J. Ling, China’s Refractories 9 (2000) 25.[15] X.B. Wang, C. Song, D.M. Li, K.W. Geng, F. Zeng, F. Pan, Appl. Surf. Sci. 253 (2006)

1639.[16] A.M.G. Gentemann, J.A. Caton, 2nd Joint Meeting of the United States Sections

of the Combustion Institute, The Oakland Marriott City Center Oakland, CA,2001.

[17] Steven P. Newman, W. Jones, New J. Chem. (1998) 105.[18] B.P. Ladgaonkar, P.N. Vasambekar, A.S. Vaingankar, J. Mater. Sci. Lett. 19 (2000)

1375.[19] Q. Fang, H. Cheng, K. Huang, J. Wang, R. Li, Y. Jiao, J. Magn. Magn. Mater. 294 (2005)

281.[20] I. Halevy, D. Dragoi, E. Ustundag, J. Phys. Condens. Matter. 14 (2002) 10511.[21] K. Ahn, B.W. Wessels, S. Sampath, Sens. Actuator B. Chem. 107 (2005) 342.[22] U.V. Chhaya, R.G. Kulkarni, Mater. Lett. 39 (1999) 91.[23] A. Ghosh, S. Bhattacharya, D.P. Bhattacharya, A. Gosh, J. Phys. Condens. Matter. 19

(2007) 10622.[24] A. Gruskova, J. Lipka, M. Papanova, D. Kevicka, A. Gonzales, G. Mendoza, I. Toth, J.

Slama, Hyperfine Interact. 156/157 (2004) 187.[25] A.N. Patil, M.G. Patil, K.K. Patankar, Bull. Mater. Sci. 5 (2000) 447.[26] A.Y. Lipare, P.N. Vasambekar, A.S. Vaingankar, Mater. Chem. Phys. 81 (2003)

108.