Thermoelectric Power of Cu-Zn Ferrites

doi:10.4236/msa.2011.211210

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Materials Science s a nd Applications, 2011, 2, 1572-1577

doi:10.4236/msa.2011.211210 Published Online November 2011 (http://www.SciRP.org/journal/msa)

Thermoelectric Power of Cu-Zn Ferrites

Hussain Abd-Elkariem Dawoud

Physics Department, Islamic University of Gaza, Gaza Strip, Palestine.

Email: [email protected]

Received May 24th, 2011; revised August 24th, 2011; accepted October 4th, 2011.

ABSTRACT

A series of Cu-Zn mixed ferrites with composition formula Cu1–xZnxFe2O4 is prepared by the double sintering ceramic

technique. Thermoelectric power studies are performed over a temperature range of 300 to 800 k by a deferential

method. The results showed a negative value for the Seebeck coefficient S for all samples, and all compositions exhib-

ited an n-type semiconductors behavior in the measured range of temperature. The values of charge carrier concentra-

tion n and the Fermi energy were determined. The values of n were found to decrease as temperature increased, while

Fermi energy directed to more negative values when Zn content is increased. On the basis of these results a mechanism

for the conduction in Cu-Zn ferrites is suggested and the properties of the mention compounds were determined.

Keywords: Ferrites, Thermoelectric Power, Fermi Energy, Seebeck Coefficient

1. Introduction

Spinel ferrites are commercially important materials be-

cause of their excellent magnetic and electrical properties.

Interesting physical and chemical properties of the mag-

netically diluted ferrites arise from the ability of these

compounds to distribute the cations amongst the avail-

able tetrahedral A- and octahedral B-sites. Ferrites are

able to fulfill a wide range of applications from micro

wave to radio frequencies, are of great importance from

both fundamental and applied research points of view

[1,2]. Because Zinc is non-magnetic divalent ions that

occupy essentially tetrahedral A sites, when substituted

in ferrites [3]. Gonchar and Andreev [4] have reported

that Ni-Cu-Zn ferrites with less content of Zn could ob-

tain high Curie temperature, but the initial permeability

of Ni-Cu-Zn ferrites reached. The substitution of Cu

brings about a structural phase transition accompanied by

the reduction in the crystal symmetry due to co-operative

John-Teller effect [5,6], which ultimately results in some

interesting electrical and magnetic properties. Various in-

vestigations [7,8] studied the electrical and thermal po-

wer of the spinel ferrites and have found that they have

semi-conducting properties of n or p-type. In this article,

the electric and thermoelectric power of Cu-Zn ferrites

have been studied experimentally as a function of com-

position and temperature in order to understand the con-

duction mechanism in these samples.

2. Experimental Technique

The polycrystalline Zinc-substituted Copper ferrites hav-

ing compositional formula Cu1–xZnxFe2O4 where x step-

ped 0 ≤ 0.2 ≥ 1, were prepared by the double sintering

ceramic technique [9], from pure oxides powders Fe2O3,

CuO and ZnO. Oxides were mixed in stoichiometric pro-

portions and prefired at 750˚C for 5 h. The prefired

powders were grounded and compressed at constant pre-

ssure (3 × 108 Pa) in the form of disk shape of 13 mm

diameter. The samples were finally sintered at 1150˚C

for 5 hours and then slowly cooled to room temperature

with cooling rate of 2˚C/min.

The electrical measurements were performed by two-

probe method; silver past was applied on both sides of

the samples to make good ohmic contact.

Thermoelectric power studies were carried out as a

function of composition and temperature by differential

method [10]. The sample holder for measuring the See-

beck coefficient consists of two non-magnetic copper elec-

trodes between which the sample is fixed. An auxiliary

heating coil is fixed to upper electrode for additional

heating to maintain a temperature difference of about 15

degree between the two faces of the sample. A tempera-

ture of both surfaces of the sample was measured by the

same type of two thermocouples. The Seebeck coeffi-

cient S was calculated using the relation

SET





where ∆E is the thermoelectric potential difference pro-

Thermoelectric Power of Cu-Zn Ferrites1573

duced across the sample due to the temperature differ-

ence ∆T.

3. Results and Discussion

The variation values of Seebeck coefficient with tem-

perature for each sample of Cupper zinc ferrite having a

compositional formula Cu1–xZnxFe2O4 where x stopped 0

≤ 0.2 ≥ 1, have bean calculated from the experimental

values of thermoelectric motive force, and depicted against

the temperature as shown in Figures 1(a)-(f). It is noticed

that the sign of the Seebeck coefficient for all samples is

negative, indicting that the Cu-Zn ferrite behave as n-

type semiconductor. The conduction mechanism in these

ferrites is due to electrons [11], and many authors ob-

tained similar results [12,13].

Several features can be obtained from Figures 1(a)-(f),

-250

-200

-150

-100

-50

11.522.533.5

)

/10

(



S(μV·K

–

)

(10

/T)K

–

)

1 1.5 2 2.5 3 3.5

–50

–100

–150

–200

–250

-350

-300

-250

-200

-150

-100

-5 0

11.522.533.54

)

(



S(μV·K

–

(103/T)K

–

Tt→c

t→c

(a) (b)

-400

-350

-300

-250

-200

-150

-100

-5 0

11.522.533.54

)

(



S(μV·K

–1

)

(10

/T)K

–

)

t→c

-900

-800

-700

-600

-500

-400

-300

-200

-100

11.522.533.54

)

(



S(μV·K

–

)

(10

/T)K

–

)

t→c

-12 0 0

-10 0 0

-8 00

-6 00

-4 00

-2 00

11.522.533.54

13 )/10( 

S(μV·K

–

)

(10

/T)K

–

)

1 1.5 2 2.5 3 3.5 4

–200

–400

–600

–800

–1000

–1200

-1200

-1000

-800

-600

-400

-200

11.522.533.54

)

(



S(μV·K

–

)

(10

/T)K

–

)

1 1.5 2 2.5 3 3.5 4

–200

–400

–600

–800

–1000

–1200

(e) (f)

Figure 1. (a) The variation of Seebeck coefficient (S) with 103/T (for sample of x = 0.0); (b) The variation of Seebeck coeffi-

cient (S) with 103/T (for sample of x = 0.2); (c) The variation of Seebeck coefficient (S) with 103/T (for sample of x = 0.4); (d)

The variation of Seebeck coefficient (S) with 103/T (for sample of x = 0.6); (e) The variation of Seebeck coefficient (S) with

03/T (for sample of x = 0.8); (f) The variation of Seebeck coefficient (S) with 103/T (for sample of x = 1.0). 1

Thermoelectric Power of Cu-Zn Ferrites

1574

compositions of x ≤ 0.6 have two change of slope in S

against temperature, the first change at lower temperature

can referred to structural change from tetragonal to cubic

system. The transition from tetragonal to cubic Tt→c was

found to decrease with increasing Zn content as shown in

Figure 2. In other words the tetragonality is in agreement

with the results of [14].

At higher temperature Seebeck coefficient S decrease

in negativity with increasing temperature, up to a parti-

cular temperature, hereafter referred to as Seebeck coef-

ficient transition temperature Tc. However, by further in-

crease of temperature, the values are found to decrease in

negativity. The minimum values observed in Seebeck co-

efficient may be attributed to a magnetic transition, where

the ferromagnetic material becomes paramagnetic. On

the other hand, in the case of samples of x = ≥ 0.8, the

Seebeck coefficient is found to be increased slightly as

the temperature increased unlike other samples, and there

is no transition Tc was found.

This is due to the fact that these samples having higher

Zinc concentration with weak tetrahedral A-site and oc-

tahedral B-site super exchange interaction. This behavior

is obvious because both samples have higher concentra-

tion of Zinc with paramagnetic structure at room tem-

perature [14]. Similarly, they behave like normal solid

with slightly increase in Seebeck coefficient values with

temperature. However, the change of slope could not be

observed in the case of the two samples i.e. of x = ≥ 0.8

since they are diamagnetic at room temperature [15].

Generally, the change of slope for samples of x ≤ 0.6

is attributed to change in conductivity mechanism. The

conduction at lower temperature (below Curie tempera-

ture) is due to hopping of electrons [12] between Fe3+ and

Fe2+ ions, whereas at higher temperature (above Curie tem-

perature) due to polaron hopping [15].

t→c

Following according to Wu et al. [16] the Seebeck co-

efficient S is expressed in terms of Fe3+ and Fe2+ ions,

considering the small polaron hopping conduction me-

chanism as follows:







SlnFeFeke







 





 

where [Fe3+]B and [Fe2+]B are the concentration of the

Fe3+ and Fe2+ ions in the octahedral sites, respectively

and β = 1. Dawoud et al. [17] reported that the increase

of Zn substitution tends to increase the Fe2+ ions as de-

mon-strated in above equation.

In addition, the increase of Zn content is associated

with a decrease in copper content. In turn the possibility

of Cu+ formation and Cu2+↔Cu1+ hopping process and

the number of holes involved will be reduced. As a result

S becomes more negative with the increase of x (Zinc

content).

4. Charge Carrier Concentration (n)

In the case of low mobility semiconductor like ferrites

having exceedingly levels , the value of N, the density of

state can be taken as 1022 cm–3 [18].



n1expNV NSek









The value of charge carriers concentration/unit volume

have been calculated for all the samples by using the

values of the Seebeck coefficient S, and the relation be-

tween n and temperature is depicted in Figures 3(a)-(c)

for samples of x = 0.0, 0.2 and 0.4 respectively. The val-

ues of n for all samples are also listed in Table 1. The

figures illustrated that, the values of n decreased with the

increasing the temperature.

5. Fermi Energy (EF)

The thermoelectric power data were utilized to determine

the Fermi energy. The Fermi energy of Cu-Zn ferrite was

calculated employing the following expression [10,19].

EeSTAKT

where K, e, S and T are Boltzmann constant, electronic

charge, Seebeck coefficient and absolute temperature res-

pectively. A is a dimensionless constant related to the

kinetic energy of the charge carrier, and has values of 0

or 2. Two series of values of EF were obtained, over the

temperature range from 300 K up to 800 K, for A = 0 and

A = 2. Figure 4(a)-(c) represent the variation of Fermi

energy with temperature. The extrapolation of the linear

part of the two series of EF, for A = 0 and A = 2 inter-

sects at T = 0.

The corresponding value of EF, the point of intercep-

tion at ordinate, gives the Fermi energy at zero tempera-

Figure 2. The variation of Tt→c and TS with composition x.

Thermoelectric Power of Cu-Zn Ferrites1575

0100 200 300 400500 600700 800900

n×10

T(K)

(a)

0100200300 400500 600700 800900

n×10

T(K)

(b)

0100200 300 400500 600 700 800 900

n×10

T(K)

(c)

Figure 3. (a) Temperature dependence of charge carrier n

for sample of x = 0.0; (b) Temperature dependence of charge

carrier n for sample of x = 0.2; (c) Temperature dependence

of charge carrier n for sample of x = 0.4.

ture EF (0); and Figures 4(a)-(c) indicates the variation of

Fermi energy with temperature. Also Table 1 lists the

values of Fermi energy obtained for each composition. It

is noticed that EF (0) is positive for sample of x = 0 and

sample of x = 0.2 then it decreases to the negative value

i.e. at the left hand side of the gape, as zinc ion increased.

Table 1. The values of Seebeck coefficient, Curie tempera-

ture transition temperarure from tetragonal to cubic Tt→c

and Fermi energy for Cu-Zn ferrites.

x S at 374 K TS (K) Tt→c (K) Ef (ev)

0.0–53.8 688 540 2.4 × 10–20

0.2–158 630 512 1.6 × 10–20

0.4–166.7 569 482 0.4 × 10–20

0.6–515 509 439 –0.6 × 10–20

0.8–800 - - –0.8 × 10–20

1.0–636.7 - - –1.0 × 10–20

(a)

(b)

Figure 4. (a) Plot of Ef against temperature for sample of x

= 0.0; (b) Plot of Ef against temperature for sample of x =

0.2; (c) Plot of Ef against temperature for sample of x = 1.0.

Thermoelectric Power of Cu-Zn Ferrites

1576

Mazen et al. [10] reported that for Cu ferrite, EF (0) is

positive at T = 0 K. Patil et al. [11] reported that when

heating CuFe2O4 sample, two temperature domains are

noted in which the concentration of B-site Cu2+ ions is

increased and the interstice cation exchange is given as

23 2

AB B

Cu FeCu Fe

 

 



(1)

This reaction gives n-type condition, while at low tem-

perature the conduction process for CuFe2O4 at lower

temperature is

12 2

AA A

Cu CuCuCu

 

 (2)

This gives only p-type conduction depending on the

relative concentration of Cu1+ of Cu2+ ions on A-sites.

For the present system Fermi energy is positive for Cu

ferrite and reaction (2) is fevered. While as the Zinc con-

tent is increased, Fermi energy goes to negative and reac-

tion (1) is fevered since the increasing of the Zinc con-

tent tend to decrease Cu content at A-site this tend to

increase the negativity of Fermi energy at absolute zero.

6. Conclusions

1) The Cu-Zn ferrite behaves as n-type semiconductor.

2) The transition from tetragonal structure to cubic

was found to decrease with increasing of Zn content.

3) The minimum values observed in Seebeck coeffi-

cient may be attributed to a magnetic transition, where

the ferromagnetic material becomes paramagnetic.

4) The conduction at lower temperature (below Curie

temperature) is due to hopping of electrons between Fe3+

and Fe2+ ions, whereas at higher temperature (above Cu-

rie temperature) due to polaron hopping.

5) For the present system Fermi energy is positive for

Cu ferrite while as the Zinc content is increased, Fermi

energy goes to negative value.

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