Thermoelectric power properties of Zn substituted Cu

Download PDF
0 downloads 0 Views 218KB Size Report
Comment
Thermoelectric power properties of Zn substituted Cu–Ga spinel ferrites ... used for many years as high-frequency devices such as radio ... by sandwiching the sample between two copper electrodes, where ... The percentage porosity ... charge carriers at 300 K also, Curie temperature TC and thermoelectric power transition.
Materials Letters 63 (2009) 1010–1012

Contents lists available at ScienceDirect

Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Thermoelectric power properties of Zn substituted Cu–Ga spinel ferrites M.K. Fayek, S.S. Ata-Allah, Kh. Roumaih ⁎, S. Ismail Reactor Physics Department, Nuclear Research Center, Atomic Energy Authority, Abu Zabal, P. No. 13759, Cairo, Egypt

a r t i c l e

i n f o

Article history: Received 16 December 2008 Accepted 25 January 2009 Available online 31 January 2009 Keyword: Thermoelectric power Cu–Zn–Ga ferrites Dc conductivity

a b s t r a c t Composition and temperature dependence of dc conductivity and Seebeck coefficient for Cu1−xZnxGa0.5Fe1.5O4 (0.0≤ × ≤ 0.5) are discussed. Thermoelectric power studies of this ferrite series are investigated from room temperature to well beyond the Curie temperature by the differential method. The Seebeck coefficient φ is found to be positive for compositions with x ≤ 0.2 indicating that these ferrites behave as p-type semiconductors, while compositions with x ≥ 0.3 show n-type semiconductors with φ negative. Results of the dc conductivity display semiconducting behavior of these materials. Transition temperatures obtained from both studies are in good agreement and was found to be decreased linearly with Zn content. Some physical properties of the samples such as density and porosity are also discussed. The obtained results are discussed in the light of the interactions over the metal sites in the spinel unit cell. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Study of transition metal oxides, especially spinel ferrites is of great importance from both fundamental and applied research points of view [1–3]. The synthesis and sintering of ferrites have become an important part of advanced ceramic research [4–6] due to the variation of their different physical properties with the method of preparation. Zinc substituted copper ferrites have been commercially used for many years as high-frequency devices such as radio frequency coils, transformers cores, rod antennas and magnetic cores of read–write heads for high speed digital tapes [7,8]. The magnetic, dielectric and elastic properties of mixed Cu–Zn ferrites have been studied in detail [9–12]. Crystal structure, magnetic and dielectric properties of Cu–Zn–Ga ferrites was studied earlier [13–16]. However, the dc conductivity and thermoelectric power of these materials is in need to be clarified. In this concern the present work is devoted to investigate these properties as a function of composition and temperature of these ferrites. 2. Experimental detail The thermoelectric power or Seebeck coefficient φ was measured by sandwiching the sample between two copper electrodes, where differential method is used to make a temperature gradient by raising up the temperature of one surface of the sample with a small heating coil. The optimum temperature difference ΔT between the two surfaces of the sample was kept at 10 °C. The potential difference ΔV ⁎ Corresponding author. E-mail address: [email protected] (K. Roumaih). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.01.071

between the two surfaces is recorded using digital voltmeter (Model FLUKE 189). Effect of the reference leads was eliminated by subtracting the absolute potential difference of copper electrodes from the measured thermoelectric voltage at each temperature. As the charge diffuses from hot surfaces of the specimen to the cold one so, the sign of thermoelectric power is taken as the sign of the cold electrode. The Seebeck coefficient (φ) was calculated using the relation; u = limΔTY0 ΔV = ΔT≈ΔV = ΔT;

ð1Þ

Where: (ΔT) is small compared to the absolute temperature. The Seebeck coefficient is taken for samples over a wide range of temperature starting from room temperature to 500 K. The dc conductivity measurements are carried out in the temperature range 200–420 K using a conventional four-probe technique [17]. 3. Results and discussion Single-phase spinel ferrites with the compositional formula Cu1−xZnxGa0.5Fe1.5O4, (0.0 ≤ x ≤ 0.5) were conducted to thermoelectric power and dc electrical measurements. The details of preparation and crystal structure were reported earlier [16] where, tetragonal structure is obtained for CuGa0.5Fe1.5O4 and Cu0.9Zn0.1 Ga0.5Fe1.5O4 samples with c/a N 1. As Zn2+substitutes Cu2 + in this system a crystallographic transformation from tetragonalto-cubic structure is occurred at (x ≥ 0.2). The authors attributed the crystallographic transformation to the decrease of the concentration of the JT ions [Cu2+(3d9)] in the host structure at the octahedral sites because of Zn substitution and leads to a high-symmetry phase (the cubic phase). The bulk density D of the specimens has been determined from the geometry and mass of the samples, while the

M.K. Fayek et al. / Materials Letters 63 (2009) 1010–1012 Table 1 Seebeck coefficient (φ), dc conductivity σ , carrier concentration n and mobility µ of charge carriers at 300 K also, Curie temperature TC and thermoelectric power transition temperature TS(K), density and porosity for Cu1−xZnxGa0.5Fe1.5O4 X φ σ(E-5)(Ω- 1 cm− 1) activation energy (eV) n (E+23 cm− 3) µ (E-10 cm2 V− 1 sec− 1) TC (K) TS(K) X-ray density Dx (gm/ cm3) Bulk density D (gm/cm3) Porosity

0.0 15.1 7.75 0.51 3.38 14 410 408 7.8654

0.1 12.7 3.65 1.57 3.33 6.85 350 348 7.8631

4.9945 0.5078

4.9260 0.4992

0.2 5.2 3.15 2.72 3.08 6.39 340 338 5.5540

0.3 − 0.88 3.06 2.92 2.58 7.40 315 – 5.5513

0.4 − 2.86 8.96 2.77 2.9 19.2 286 – 5.5550

0.5 −2.77 2.22 2.09 2.8 5.00 260 – 5.5492

4.7295 5.4191 6.4305 5.0351 0.6715 0.796 0.9776 0.7271

X-ray density (Dx) was computed from the values of the lattice parameter using the formula; Dx = ZM = Na

3

where Z is the number of molecules in unit cell of spinel ferrite (Z = 8), M the molecular weight of the ferrite samples, N the Avogadro's number and a3 is the unit cell volume (lattice parameter) of the ferrite. Both densities Dx and D as a function of Zn concentration are given in Table 1.

Fig. 1. The dc electrical conductivity as ln σ verses 1000/T (K).

1011

From these values, it can be seen that X-ray density decreases with the increase of Zn content x, as it inversely proportional to the lattice constant, which increase with increasing Zn concentration [16]. Also the bulk density nearly reflected the same behavior. The percentage porosity (P) of the ferrite sample was calculated using the formula; Porosity P ðkÞ = ½ð1 − bulk densityÞ = x−ray density × 100 The increased of porosity with increasing Zn2+ions content could be attributed to the rapid densification of ferrite samples and to the difference in specific gravity of the ferrite components [18]. Fig. 1 shows the plot of dc electrical conductivity σ against temperature of the present ferrite series. The relation confirms the semiconducting behavior of these materials, where σ increases with increasing temperature. Also, there is a deviation of the slope in the relation at the transition temperature TC, which is found to decrease linearly with Zn content. This agrees with previous studies for super exchange interaction for various oxides [19]. The variation of the thermoelectric power φ with temperature is shown in Fig. 2. It is clear that, for all compositions, the value of thermoelectric power φ decreases with increasing temperature to a certain temperature (designated as TS (K)) beyond which the value of φ shows abrupt change with increasing temperature. Similar behavior was previously reported for the compounds Cu–Ni [20], Mn–Zn–Gd [21], and Co–Zn [22] ferrites.

Fig. 2. Variation of thermoelectric power with temperature for Cu1−xZnxGa0.5Fe1.5O4.

1012

M.K. Fayek et al. / Materials Letters 63 (2009) 1010–1012

Fig. 3. The carrier concentration (n) versus temperature T (K) for sample CuFe1.5 Ga0.5O4.

The values of TS for some samples are given in Table (1) which shows good agreement with the TC values obtained from the dc conductivity [Fig. 1]. This indicates that; the change in behavior of the thermoelectric power with temperature could be due to magnetic transition from the ordered (magnetic) state to the disordered (paramagnetic) state. It is established that Curie temperature TC in these materials depends primarily upon the number of Fe3+−O2− −Fe3+ linkages [14]. The suppression of TC in these spinel ferrites displayed in Table (1) results from the nonmagnetic substitution in the unit cell which decreases the number of Fe3+−O2− Fe3+linkages and therefore the transition temperature. Table (1) gives also, the experimental values of electrical conductivity and Seebeck coefficient at room temperature. The carrier concentration (n) can be calculated from the following formula [23], n = N exp ð − u e = kÞ;

ð2Þ

where N is density of states or concentration of electronic levels involved in the conduction process, in case of low mobility semiconductors like ferrites the values of N can be substituted by 1022 cm− 3 [24–26], φ is Seebeck coefficient, e is charge of electron, k is Boltzmann constant. The computed values of carrier concentration (n) are included in Table (1). It can be also seen from the table the value of charge carrier concentration (n) slowly decreases with increasing Zn ions content (≈3⁎1023 cm− 1). Also, the mobility (µ) for these ferrites were calculated from the experimental values of the dc conductivity (σ) and carrier concentration (n), and included in Table (1). It is seen that the mobility decreases with increasing Zn ions content. Similarly, the conductivity has the same trend except for sample with x ≥ 0.3 which could be attributed to different cation distribution of this sample [16]. The temperature variation of charge carrier concentration of these samples is shown in Fig. 3 for sample with x = 0.0 (as an example of the present series). It can be seen from the figure that the charge carrier concentration decreases with increasing temperature. There is also, an abrupt change in the in the relation coincides with the transition temperature Ts (K). It was previously reported [20] that; CuFe2O4 sample has a positive Seebeck coefficient φ with p-type semiconductor at room temperature while, at high temperature φ becomes negative with n-type behavior. In CuFe2O4 with tetragonal unit cell Cu2+ions are occupying the octahedral B-sites as reported earlier [13,17] and the cation distribution in this sample has the form (Fe3+)[Fe3+Cu2+]O2− 4 where the parenthesis refer to the tetrahedral A-site and square bracket refer to the octahedral B-site. Goodenough [27] and the previous works [20,28,29] pointed out that both the cation–anion–cation interactions and the cation–cation interactions can be simultaneously present in the rocksalt-type and spinel structures which contains transition metal ions at B-sites. The obtained results could be explained on the

basis of the electrical interactions between cation–cation [Cu–Cu] and cation–anion–cation [Fe3+ – O2– – Fe3+] occurred over the octahedral Bsites in these compounds. Cation–cation interactions via direct overlap of cation (Cu) delectron wave functions leads to a semimetal or metal like behavior [19] and could lead to a p-type semiconductor by generating holes as a charge carrier. The interactions that occur via an anion intermediary [cation–anion–cation] for elements (with 3dm; 5 = b m = b 8 where, m is the number of electrons in d-levels) which is Fe3+ – O2– – Fe3+ [14,19]. The later interactions lead to a semiconducting behavior and n-type semiconductor by generating electrons as a charge carrier. Introduction of Ga3+with (3d10) in place of Fe3+in this unit cell first decreases the Fe3+ – O2– – Fe3+ interaction at the B-site and consequently decrease the charge carriers result from this interaction (electrons). Therefore the predominant interaction is [Cu–Cu] where the charge carriers lead to p-type semiconductor. Substitution of Zn2 + (with closed 3d10 shell) in place of Cu in these compounds replaces Fe at the tetrahedral A-site [13,17]. This occupation of zinc at A-site results in transferring more Fe ions to the B-site. This means that, the number of Fe3+ – O2– – Fe3+ interactions increases at B-sites as Zn content increases (x = 0.3, 0.4 and 0.5) and consequently leads to decrease the [Cu–Cu] interactions. This explains the n-type semiconductor behavior for the compounds with (x ≥ 0.3). 4. Conclusion The conduction mechanism in these materials is mainly due to [Cu–Cu] and [Fe3+ – O2– – Fe3+] interactions occurred over the B-sites. The samples with (x b 0.3) Cu–Cu interaction is the most predominate leading to a p-type semiconductors. However [Fe3+ – O2– – Fe3+] interactions are the most predominate for samples with (x ≥ 0.3) results in n-type semiconductors. A good agreement between the values of TS and TC is obtained. The suppression of TC in these spinel ferrites is attributed to the nonmagnetic substitution in the unit cell which decreases the number of [Fe3+ – O2– – Fe3+] linkages and therefore the transition temperature. References [1] Pardavi-Horvath M. J Magn Magn Mater 2000;215–216:171. [2] Ammar Souad, Helfen Arnaud, Jouini Noureddine, Fiévet Fernand, Rosenman Izio, Villain Françoise, et al. J Mater Chem 2001;11:186. [3] Joseph Sebastian MD, Rudraswamy B, Radhakrishna MC, Ramani. Bull Mater Sci 2003;26:509. [4] Arulmurugan R, Jeyadevan B, Vaidyanathan G, Sendhilnathan S. J Magn Magn Mater 2005;288:470. [5] Virden AE, O'Grady K. J Magn Magn Mater 2005;290–291:868. [6] Verma Seema, Joy PA, Khollam YB, Potdar HS, Deshpande SB. Mater Lett 2004;58:1092. [7] Kulikowski J, Leniewski A. J Magn Magn Mater 1980;19:117. [8] Riches EE. In: Gordon Cook J, editor. Ferrites, A Review of Materials and. Applications. London: Mills and Boon. Limited; 1972. p. 17. [9] Satyamurthy N, Natera MG, Yourself SI, Begum RJ, Srinivastava CM. Phys Rev 1969;181:969. [10] Kulkarni RG, Patil VU. J Mater Sci 1982;17:843. [11] Rezlescu N, Rezlescu E. Phys Status Solidi (a) 1974;23:575. [12] Ravinder D, Vijaya Kumar K, Boyanov BS. Mate Lett 2000;299:5. [13] Ata-Allah SS, Hashhash A. J Magn Magn Mater 2006;307:191. [14] Ata-Allah SS. J Magn Magn Mater 2004;284:227. [15] Ata-Allah SS. J Solid State Chem 2004;177:4443. [16] Fayek MK, Ata-Allah SS, Zayed HA, Kaiser M, Ismail S. J Alloys Compd (in press). [17] Ata-Allah SS, Fayek MK, Yehia M. J Magn Magn Mater 2004;279:411. [18] El-Sayed AM. Ceram Int 2002;28:363. [19] Ata-Allah SS, Fayek MK. J Phys Chem Solids 2000;61:1529. [20] Roumaih Kh. J Alloys Compd V 2008;465:291–5. [21] Ravi Kumar B, Ravinder D. Mater Lett 2002;53:441. [22] Ramana Reddy AV, Ranga Mohan G, Boyanov BS, Ravinder D. Mater Lett 1999;39:153. [23] Morin FJ, Gebella TH. Phys Rev 1955;99:467. [24] Morin FJ, Gebella TH. Phys Rev 1970;93:433. [25] Ravinder D. J Alloys Compd 1999;291:208. [26] Mazen SA, Mansour SF, Elmosalami TA, Zaki HM. J Alloys Compd (in press). [27] Goodenough John B. Phys Rev 1960;6:1442. [28] Ata-Allah SS, Fayek MK. Phys Stat Sol (a) 1999;175:725. [29] Ata-Allah SS. Mater Chem Phys 2004;87:378.