Structural and magnetic properties of pure and doped nanocrystalline cadmium ferrite

Journal of Alloys and Compounds 475 (2009) 832–839

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Structural and magnetic properties of pure and doped nanocrystalline cadmium ferrite N.M. Deraz a,∗ , M.M. Hessien b a b

Physical Chemistry Department, Laboratory of Surface Chemistry and Catalysis, National Research Center, Dokki, Cairo, Egypt Electronic Materials Laboratory, Advanced Materials Department, Central Metallurgical Research and Development Institute, Helwan, Cairo, Egypt

a r t i c l e

i n f o

Article history: Received 17 July 2008 Accepted 12 August 2008 Available online 5 November 2008 Keywords: MgO Al2 O3 -doping CdFe2 O4 calcination temperature Activation energy

a b s t r a c t The solid-state reaction between pure and alumina or magnesia doped Cd/Fe mixed oxides have been investigated using DSC, XRD, IR, TEM and VSM techniques. Equimolar proportions of cadmium nitrate and ␣-Fe2 O3 were employed and also Al and Mg were added as aluminum and magnesium nitrates. Pure and doped mixed solid were subjected to thermal treatment at 500–1200 ◦ C. The results obtained showed that the degree of propagation of the reaction between Fe2 O3 and CdO yielding CdFe2 O4 increased as a function of calcination temperature up to 1000 ◦ C. However, heating of pure mixed solids at 1000 ◦ C for 3 h was sufficient to effect complete conversion of the reacting solids to Cd-ferrite. The crystallite size of produced CdFe2 O4 was reduced nanocrystalline range due to Al2 O3 or MgO-doping. The microstructure and magnetic properties of the investigated solids were strongly dependent on the calcination temperature and doping process. The maximum saturation magnetization (7.79 emu/g) was achieved by doping with 2 mol% MgO at 1100 ◦ C. The formation activation energy of CdFe2 O4 was evaluated be 72, 64 and 27 kJ mol−1 for pure mixed solids and those doped with 6 mol% Al2 O3 and 6 mol% MgO, respectively. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The development of nanocrystalline materials is a subject of considerable interest, both for the scientific value of understanding the unique properties of materials that have relevance to condensed matter studies and for the technological significance of enhancing the performance of existing materials. The ability to produce nanocrystalline ferrites is expected to assist in controlling the ultimate properties of ferrites, an advantage over those fabricated by powder metallurgy process. The magnetic properties in particular are strongly dependent on crystallite size, crystallite size distribution and morphology of crystallites [1–4]. Ferrites have found wide use in many industrial applications and can be prepared by ceramic process involving high temperature solid-state reactions between the constituent oxides and/or carbonates [5–7]. They find extensive applications as catalysts and permanent magnets in microwave devices, television, radio, audio–video and digital recording and bubble devices of computer. They have been regarded as better magnetic materials than pure metals because of their high resistivities, higher efficiency and lower cost. Most of the divalent metal oxides (MOs) interact with

∗ Corresponding author. Fax: +20 233370931. E-mail address: [email protected] (N.M. Deraz). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.08.034

Fe2 O3 yielding the corresponding ferrite with the formula MFe2 O4 [5]. Based on the distribution of cations, among the tetrahedral (A) and octahedral (B) sites of the coordinated oxygen, spinel ferrites can be either normal spinel (M2+ )A [Fe3+ Fe3+ ]B O4 or inverse spinel with half of the trivalent ions in the A-sites and the other half together with divalent ions in the B-sites [8]. It has been found that the ferrites of similar composition differ in their physical and chemical properties depending on distribution of cations, having different oxidation states, among the available tetrahedral and octahedral sites. The cation distribution depends on the preparation technique as well as the processing parameter therein, such as the thermal history of constituent oxides and doping process [9]. The calcination temperature and particle size of the synthesized ferrites could be reduced by doping with small amounts of foreign ions [5]. It has been reported that electronic properties of the Fe3+ in ferrites can be strongly modified with the introduction of a metal with a smaller atomic radius [10,11] as well as by the introduction of a metal with higher electron negativity than iron like rhodium [12]. Cadmium ferrite is a normal spinel with Cd2+ ions in the A-sites and both Fe3+ ions occupying the B-sites. But the anomalous antiferromagnetism of Cd-ferrite could be attributed to small amount of Fe3+ ions occupy A-sites and form magnetic clusters with nearest Fe3+ neighbors at B-sites through coupling by the A–B interaction which is much stronger than the B–B interaction [13–15].

N.M. Deraz, M.M. Hessien / Journal of Alloys and Compounds 475 (2009) 832–839

833

The aim of this work was to study the genesis of cadmium ferrite prepared by a standard ceramic route with the subsequent thermal treatment and doping with either Al2 O3 or MgO at 500–1200 ◦ C and effect of these treatments on the magnetic properties of produced Cd-ferrite. The techniques employed were DSC, XRD, IR, TEM and VSM. 2. Experimental 2.1. Materials Pure mixed oxides were prepared by impregnating equimolar proportions of cadmium nitrate and ␣-Fe2 O3, then dried and subjected to thermal treatment in air at 500–1200 ◦ C for 3 h. Four doped mixed solid specimens were obtained by treating known amounts of Cd-nitrate and ␣-Fe2 O3 with calculated amounts of aluminum and magnesium nitrates dissolved in the least amount of distilled water, dried at 100 ◦ C and then calcined in air at 500, 700, 900, 1000, 1100 and 1200 ◦ C for 3 h. The concentration of aluminum and magnesium expressed as mol% Al2 O3 and MgO were 2 and 6. The chemicals employed in the present work were of analytical grade supplied by Fluka Company. 2.2. Techniques Differential scanning calorimetry (DSC) analysis of the different samples was carried out using a SETARAM thermal analysis apparatus flow rate of argon was 30 ml/min. The rate of heating was kept at 10 ◦ C/min and the mass of sample was 40 mg. An X-ray measurements of various mixed solids heated in air at 500–1200 ◦ C were carried out using a BRUKER D8 advance diffractometer (Germany). The patterns were run with Cu K␣ radiation at 40 kV and 40 mA with scanning speed in 2 of 2◦ min−1 . The crystallite size of Cd-ferrite present in the investigated solids calcined at different temperatures was based on X-ray diffraction line broadening and calculated using Scherrer equation [16]: d =

B ˇ cos 

(1)

where d is the average crystallite size of the phase under investigation, B is the Scherrer constant (0.89),  is the wavelength of X-ray beam used, ˇ is the full-width half-maximum (FWHM) of diffraction, and  is Bragg’s angle. An infrared transmission spectrum of different solids calcined at various temperatures was determined using PerkinElmer Spectrophotometery (type 1430). The IR spectra determined from 900 to 400 cm−1 . Two milligrams of each solid sample were mixed with 200 mg of vacuum dried IR grade KBr. The mixture was dispersed by grinding for 3 min in a vibratory ball mill and placed in a steel die 13 mm in diameter and subjected to a pressure of 12 tons. The sample disks were placed in the holder of the double grating IR spectrometer. Transmission electron micrographs (TEM) were recorded on a Zeiss model EM10 TEM instrument. The samples were dispersed in ethanol and then treated ultrasonically in order disperse individual particles over a gold grids. The magnetic properties of the investigated solids were measured at room temperature using a vibrating sample magnetometer (VSM; 9600-1 LDJ, USA) in a maximum applied field of 15 kOe. From the obtained hysteresis loops, the saturation magnetization (Ms ), remanence magnetization (Mr ) and coercivity (Hc ) were determined.

Fig. 1. DSC curves of ferric oxide impregnated with cadmium nitrate and those treated with calculated amounts of magnesium or aluminum nitrates. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

doped samples. The peak at 468–1036 ◦ C might characterize a possible solid-state interaction between CdO and Fe2 O3 producing Cd–Fe–O compound. The identification of this compound will be discussed latter in the next section of the present work via XRD investigation of different investigated solids. The peak located at 1077–1100 ◦ C may correspond to the partial decomposition of the newly formed Cd–Fe–O compound with subsequent formation of ␥-Fe2 O3 phase. Moreover, an additional peak at 486–591 ◦ C could be observed for the ample treated with 12 mol% aluminum nitrate (6 mol% Al2 O3 ). This peak is broad, weak and might be attributed to solid-state reaction between CdO and Al2 O3 yielding CdAl12 O19 . Since the weight of pure and doped mixed solids was constant in each DSC run, the area of various endothermic peaks for different solids could be looked as measure of the amount undergoing a chemical change. Inspection of the DSC curves of the various investigated solids are given in Fig. 1 reported that the addition of dopants led to an increase in the area of all investigated peaks. This indicates the treatment with either alumina or magnesia brought about an increase of solid-state reaction between Cu/Fe mixed oxide solids and/or phase transformation process of one the resulted products. These findings will be confirmed later by XRD and IR measurements.

3. Results 3.2. XRD analysis 3.1. Thermal analysis DSC curves of pure and heavily doped investigated system were determined and illustrated in Fig. 1. The DSC curves exhibit seven sets of endothermic peaks having minima at 45–73, 109–125, 182–209, 402–414, 432–454, 468–1036 and 1077–1100 ◦ C. The first three sets of peaks are moderate, whereas the fourth and fifth sets consisted of very strong peaks. The sixth set of peaks is very broad and the last set of peaks is weak. The peaks located at 45–73 and 109–125 ◦ C are indicative for departure of physisorbed water of different precursors. The peak at 182–209 ◦ C might indicate thermal decomposition of aluminum and magnesium nitrates to Al2 O3 and MgO, respectively, and one of the steps of thermal decomposing of cadmium nitrate to cadmium oxide. The strong endothermic peaks at 402–414 and 432–454 ◦ C correspond to a complete thermal decomposition of cadmium nitrate to CdO in the pure and

X-ray diffraction patterns of pure and doped Cd/Fe mixed oxide solids calcined in air at 500, 700, 900, 1000, 1100 and 1200 ◦ C were determined. The X-ray diffractograms of pure and variously doped solids calcined at 500 and 700 ◦ C, not given, show that: (i) Pure and doped solids consisted of all diffraction peaks of CdO and Fe2 O3 (Hematite). The augmentation in the calcination temperature of various mixed solids from 500 to 700 ◦ C led to a decrease in the degree of ordering and crystallite size of both CdO and Fe2 O3 phases. This decrease may be due to the solid-state reaction between CdO and Fe2 O3 solids yielding nanocrystalline CdFe2 O4 solids. (ii) The doping process with either Al2 O3 or MgO followed by heating at 500 or 700 ◦ C brought about an increase in the peak height and crystallite size of CdO and Fe2 O3 phases. However, the extent of an increase in the peak height and crystallite size of CdO phase due to doping process decreases by increasing the alumina

834

N.M. Deraz, M.M. Hessien / Journal of Alloys and Compounds 475 (2009) 832–839 Table 1 The effects of calcination temperature and doping on the height of some diffraction lines at “d” spacing of 1.41 Å (53% CdO) and 1.66 Å (69% CdO and 55% CdFe2 O4 ).

Fig. 2. X-ray diffractograms of pure and doped Cd/Fe mixed oxide solids calcined at 900 and 1000 ◦ C. Lines (2) CdO, (3) CdFe2 O4 and (5) CdAl12 O19 phases.

content from 2 to 6 mol%. This decrease could be attributed to formation of Cd–Al–O compound which not detected in XRD patterns due to its amorphous nature or its presence in minute amount beyond the detection limit of the X-ray diffractometer. Figs. 2 and 3 show the X-ray diffractograms of pure and doped solids calcined at 900, 1000, 1100 and 1200 ◦ C. Examination of these figures revealed the following: (i) Pure and doped solids calcined at 900 ◦ C consisted of CdO and CdFe2 O4 phases. In other words, the rise in the calcination temperature of pure mixed solids from 700 to 900 ◦ C brought about disappearance all diffraction lines of Fe2 O3 and a progressive decrease in the peak height related to CdO with subsequent formation of CdFe2 O4 . (ii) Increasing the calcination temperature of pure mixed oxide solids up to 1000 ◦ C resulted in complete conversion of CdO and Fe2 O3 to yield CdFe2 O4 as a single phase (Fig. 2). (iii) The thermal treatment of pure mixed oxides at 1100 ◦ C led to formation of CdFe2 O4 (major phase) together with a trace amount of ␥-Fe2 O3 (Maghemite). However, pure mixed

Fig. 3. X-ray diffractograms of pure and doped Cd/Fe mixed oxide solids calcined at 1100 and 1200 ◦ C. Lines (1) ␣-Fe2 O3 , (2) CdO, (3) CuFe2 O4 , (4) ␥-Fe2 O3 and (5) CdAl12 O19 phases.

Peak height (a.u.)

Ratio between b/a (R)

Solids

Calcination temperature (◦ C)

1.41 Å (a)

1.66 Å (b)

Pure CdO + Fe2 O3 +2 mol% MgO +6 mol% MgO +2 mol% Al2 O3 +6 mol% Al2 O3

500

35 57 59 55 48

47 87 97 110 78

1.34 1.53 1.64 2.0 1.62

Pure CdO + Fe2 O3 +2 mol% MgO +6 mol% MgO +2 mol% Al2 O3 +6 mol% Al2 O3

700

26 33 31 45 36

38 56 65 100 60

1.46 1.70 2.10 2.22 1.66

Pure CdO + Fe2 O3 +6 mol% MgO +6 mol% Al2 O3

900

25 40 42

50 18 20

2.00 2.20 2.10

Pure CdO + Fe2 O3 +2 mol% MgO +2 mol% Al2 O3

1000

0 0 0

100 55 50

∞ ∞ ∞

oxide sample heated at 1200 ◦ C consisted of CdO, ␣-Fe2 O3 and CdFe2 O4 phases. This indicates partially decomposition and lower thermal stability of produced CdFe2 O4 solid at temperatures above 1000 ◦ C. (v) The doping with either 6 mol% Al2 O3 or 6 mol% MgO followed by calcination at 900–1100 ◦ C resulted in a decrease in the peak height of CdFe2 O4 phase. However, 6 mol% Al2 O3 -doping process followed by heating at 1000 and 1100 ◦ C led to an appearance of the diffraction lines of CdO together with new diffraction lines related CdAl12 O19 phase. (iv) With increasing the calcination temperature of mixed solids, in presence of the dopant, a progressive shift of the diffraction peaks to higher Bragg’s angle was observed. This shift indicates that the dopant enters into the lattices of the reacted oxides and /or produced CdFe2 O4 crystallites. In other words, the observed increase in the peak height at 2.31 Å (CdO) and/or a decrease in that at 2.62 Å (CdFe2 O4 ) due to doping with Al2 O3 or MgO at 900–1100 ◦ C might indicate the substitution of small amounts of Fe and/or Cd species by dopants forming CdAlx Fe2−x O4 and/or Cd1−x Mgx Fe2 O4 lattices, respectively [17]. The promotion effects of calcination temperature and/or the doping with different amounts of Al2 O3 or MgO on CdFeO4 formation are better investigated by measuring the height of certain diffraction lines characteristic for CdO (1.41 Å, 53%) and relative to one the common lines of both CdO (1.66 Å, 69%) and CdFe2 O4 (1.66 Å, 55 %). This was done and the results obtained are given in Table 1. Investigation of this table revealed that: (i) The ratio, R, between the peak height of the lines at “d” spacing of 1.66 and 1.41 Å for pure mixed solids calcined at 500 ◦ C is 1.34 which greater than of pure CdO (1.30). This indicates the formation of CdFe2 O4 phase at 500 ◦ C. (ii) An increase in the calcination temperature of pure and doped solids in the range of 500–1000 ◦ C resulted in a progressive increase in the extent of produced CdFe2 O4 phase. (iii) The doping process with either Al2 O3 or MgO followed by calcination at 500, 700, 900 and 1000 ◦ C brought about an increase in the value of R. these treatments enhanced the formation of CdFe2 O4 to an extent proportional to both the calcination temperature of different solids and the amount of dopant added. Similar results have been reported in case of Al2 O3 or MgO-doping of Cu/Fe mixed oxide solids [17]. An X-ray data enabled us to investigate the role of both the calcination temperature and doping of the system studied in modifying the crystallite size (d), lattice constant (a), unit cell volume (V), X-ray density (Dx ), the distance between the magnetic ions (hopping length) in octahedral and tetrahedral sites and the activation

N.M. Deraz, M.M. Hessien / Journal of Alloys and Compounds 475 (2009) 832–839

energies of both formation (Ef ) and sintering (Es ) of produced CdFe2 O4 crystallite in various solids. The computed values of different structural parameters are given in Table 2. It is clear from this table that: (i) The crystallite size of CdFe2 O4 phase present in the investigated mixed solids increased by increasing the calcination temperature of pure and doped solids from 900 to 1000 ◦ C and then decreased above this temperature. (ii) The doping process at 900, 1000 and 1100 ◦ C brought about a decrease in the “d” values of Cd-ferrite. This decrease might be attributed to a substitution of cadmium or iron species by dopant forming Cd1−x Mgx Fe2 O4 or CdAlx Fe2−x O4 compounds. This substitution was based on the smaller ionic radii of Al3+ (60 Å) and Mg2+ (65 Å) than that of Fe3+ (67 Å) and Cd2+ (97 Å) [18]. The substitution process should be normally accompanied by contraction in the crystallite size, lattice constant and unit cell volume of CdFe2 O4 phase (Table 2). These findings run parallel to an increase in the peak height of CdO (2.34 Å) at 900 and 1000 ◦ C, and also formation of CdAl12 O19 phase at 1000 and 1100 ◦ C. The activation energy of formation (Ef ) of Cd-ferrite was determined at temperatures between 900 and 1100 ◦ C for pure and variously doped mixed solids. This had been tentatively achieved from the height of characterized diffraction line 2.62 Å which provide a measure of the amount of CdFe2 O4 phase present in a given solid sample at a definite temperature (T). By plotting the peak height of this line versus 1/T, a straight line is obtained, the slope of which determines Ef value by the direct application of the Arrhenius equation [5]. The computed Ef values of CdFe2 O4 phase were 72, 64 and 27 kJ mol−1 for pure mixed oxides and those with either 6 mol% Al2 O3 or 6 mol% MgO, respectively. This considerable decrease in the value of Ef due to doping process clearly reflects the role of Al2 O3 and MgO treatments in effectively increasing the formation of nanocrystalline cadmium ferrite. The calculated Ef values might suggest that MgO-doping exhibited more enhanced effect towards the formation of CdFe2 O4 than the Al2 O3 treatment. This observation might show that MgO dissolves in the lattices of CdO and/or Fe2 O3 solids easier than Al2 O3 solid. This conclusion might find evidence from a limited solubility of Al2 O3 in the lattice of CdO due to a possible formation CdAl12 O19 phase which has been defected, by XRD technique, at 1000 and 1100 ◦ C. The variation of the X-ray density (Dx ) with the calcination temperature and different dopants is shown in Table 2. The thermal treatment and doping process brought about change in the value Dx of Cd-ferrite. Increasing the calcination temperature of the investigated solids led to an increase in The Dx values due to an increase the oxygen ion diffusion which accelerates the densification process. Moreover, the reduction in the density of samples doped with 6 mol% Al2 O3 or 6 mol% MgO followed by heating at 900–1100 ◦ C may be due to the reduction of oxygen vacancies which play a pre-

Table 2 Structural characteristics of pure and doped CdFe2 O4 solids calcined at different temperatures. Solids

Calcination temperature (◦ C)

d (nm)

a (Å)

V (Å−3 )

Dx (g/cm3 )

Pure CuO + Fe2 O3 +6 mol% MgO +6 mol% Al2 O3

900

104 60 55

8.6869 8.6744 8.6790

655 653 653

5.8380 5.7559 5.7526

Pure CuO + Fe2 O3 +6 mol% MgO +6 mol% Al2 O3

1000

124 81 76

8.6882 8.6612 8.6780

656 650 653

5.8360 5.7824 5.7521

Pure CuO + Fe2 O3 +6 mol% MgO +6 mol% Al2 O3

1100

89 80 85

8.6574 8.6474 8.6548

649 647 648

5.898 5.8101 5.7984

Pure CuO + Fe2 O3

1200

82

8.6244

641

5.9660

835

Table 3 The effects of calcination temperature and doping on the distance between magnetic ions (Dm ) in tetrahedral (A) and octahedral (B) sites of pure and doped spinel Cdferrite solids. Solids

Calcination temperature (◦ C)

Dm in A-site (Å)

Dm in B-site (Å)

Pure CdO + Fe2 O3 +6 mol% MgO +6 mol% Al2 O3

900

3.0710 3.0668 3.0685

3.7615 3.7561 3.7581

Pure CdO + Fe2 O3 + 6 mol% MgO + 6 mol% Al2 O3

1000

3.0717 3.0622 3.0681

3.7621 3.7504 3.7577

Pure CdO + Fe2 O3 +6 mol% MgO +6 mol% Al2 O3

1100

3.0608 3.0573 3.0599

3.7488 3.7444 3.7476

Pure CdO + Fe2 O3

1200

3.0492

3.7345

dominant role in accelerating densification [19], i.e. the decrease in oxygen ion diffusion would retard the densification. In addition, the atomic weight of Al and Mg are less than that of Cd and Fe which is one of the reasons which decrease the density. These findings could be investigated by determination the sintering activation energy (Es ) of CdFe2 O4 phase present in pure and doped mixed oxides calcined at 900–1100 ◦ C. This has been achieved from the result of crystalline size “d” of CdFe2 O4 phase (3 1 1) measured for different mixed oxides calcined at various temperatures by plotting ln d versus 1/T adopting direct application of the Arrhenius equation [5]. The calculated Es values of Cd- ferrite were 28, 48 and 42 kJ mol−1 in pure mixed oxides and those doped with either 6 mol% Al2 O3 or 6 mol% MgO, respectively. These results clearly indicate that the doping process hindered the sinterability of the treated solids. The effects of thermal treatment and doping process on the distance between magnetic ions (hopping length) in octahedral and tetrahedral sites for pure and doped solids calcined at 900–1100 ◦ C are listed in Table 3. It is clear from this table that the distance between the magnetic ions decrease as the calcination temperature of pure and doped specimens increases. However, the doping with either 6 mol % Al2 O3 or 6 mol % MgO followed by calcination at 900, 1000 and 1100 ◦ C led to a decrease in the distance between the magnetic ions. This may be explained on the basis of the smaller radii of Al3+ and Mg2+ than that of Fe3+ , which makes the magnetic ions become closer to each other and the hopping length decreases [19]. 3.3. IR spectroscopic analysis The study of IR spectrum is an important tool to get information about the positions of the ions in the crystal through the crystal’s vibration modes [20]. It is know that cubic spinels have three IR bands [21]. It has been reported that these bands are due to tetrahedral and octahedral complexes [20]. Fig. 4 shows the IR spectra of pure and doped samples with either 6 mol% Al2 O3 or 6 mol% MgO followed by heating at 700 and 900 ◦ C. It is obvious from this figure that: (i) The spectra of pure and doped mixed solids consisted of two main absorption bands at 534–559 (1 ) and 430–455 cm−1 (2 ). These bands are attributed to the vibration on iron ions in both tetrahedral (A) and octahedral (B) sites of the spinel structure, respectively [22]. The third frequency band at 405–410 cm−1 is associated with divalent octahedral metal ions and oxygen complex. Finally, the existence of a weak shoulder (1 ) at 634–650 cm−1 around the 1 band could be attributed to presence of the Cd2+ ions in tetrahedral site. The average value of the threshold energy is 0.084–0.96 eV. (ii) The doping process brought about a remarkable shift in the absorp-

836

N.M. Deraz, M.M. Hessien / Journal of Alloys and Compounds 475 (2009) 832–839

size and shape particles. Increasing calcination temperature of pure mixed oxides from 900 to 1100 or 1200 ◦ C brought about smaller particles (Fig. 5B and C). They were quasi-spherical particles with a quite uniform size distribution between 31 and 124 nm. This can be attributed to the higher diffusion rate at higher temperatures, giving rise to the conventional solid stat reaction between CdO and Fe2 O3 yielding nanocrystalline CdFe2 O4 particles. The doping process with either 6 mol% Al2 O3 or 6 mol% MgO followed by heating at 1100 ◦ C led to smaller spherical particles with diameters between 12 and 81 nm (Fig. 5D and E). They exhibited a slight trend to an agglomeration with MgO-treatment. The selected area electrodiffraction (SAED) pattern is shown in the upper right inset of Fig. 5, corresponding to that of a spinel phase. The polycrystalline nature of the spinel phase is confirmed by the presence of sharp rings in the SAED patterns. 3.5. Magnetic measurements

Fig. 4. IR spectra of pure and doped Cd/Fe mixed solids calcined at 700 and 900 ◦ C.

tion bands towards higher wave number. This could be attributed to the substitution of Fe3+ ions by Al3+ or Mg2+ ions which have smaller ionic radii than that of Fe3+ ions [18]. (iii) The introduction of Al3+ into the lattice of CdFe2 O4 results in a decrease in the number of Fe3+ ions in octahedral and tetrahedral sites, which leads to a decrease in the intensity of the absorption bands. Whereas, the substitution of Mg2+ in Cd-ferrite lattice led to an increase the intensity of the absorption bands due to displacement of smaller Fe3+ ion from B-site to A-site [15]. It can be suggested that the spinel structure in the prepared materials having the formulas Cd1−x Mgx Fe2 O4 and CdAlx Fe2−x O4 and can be expressed as (Cd1−x Mgx−y Fe1−x−y )A [Mgy Fe1+x+y ]B O4

(2)

(Cd Fe1−0.5x )A [Alx Fe1−0.5x ]B O4

(3)

The cation distribution is based on the following: (i) Cd ions prefer to occupy the tetrahedral sites [13]. (ii) Concerning Al3+ and Mg2+ ions distribution, there are two points of view. The first one is that the Mg2+ ions in spinel ferrite are distributed on both tetrahedral and octahedral sites, such that the majority of Mg2+ ions occupy the tetrahedral position [9]. The other one is that Al3+ ions occupy completely B-sites [8]. 3.4. TEM analysis The pure mixed oxide particles calcined at 900 ◦ C were agglomerates, which consisted of particles larger than 100 nm in diameter (Fig. 5A). The rise in calcination temperature caused changes both a

The changes of magnetic moment as a function of magnetic field (magnetization hysteresis loops) were measured for pure and doped solids calcined at 500–1100 ◦ C and are given in Figs. 6–8 and Table 4. It is clear from this table that: (i) The heat treatment and doping process brought about some modifications in the magnetization and coercivity of the investigated solids. The saturation magnetization, Ms , of pure and doped solids increased by increasing the calcination temperature. On the other hand, the augmentation in the calcination temperature of pure and doped oxides resulted in a decrease in the coercive force, Hc , from 3005 to 10.41 Oe. (ii) The presence of MgO as dopant agent led to some changes in the magnetization of the investigated solids calcined at 900–1100 ◦ C. The doping with 2 mol% MgO followed by heating at 900–1100 ◦ C showed an increase in the Ms value. Increasing the amount of MgO to 6 mol% affected a decrease in the magnetization. In other words, higher Ms value of about 6.72 and 7.79 emu/g were obtained for pure and 2 mol% MgO-doped solids calcined at 1100 ◦ C. (iii) Al2 O3 treatment at 900–1100 ◦ C brought about a progressive decrease in the values of Ms to an extent proportional to the amount of dopant added. The induced decrease in The Ms value of Al2 O3 -doped samples at 900–1100 ◦ C is to be expected as a result of formation of non-magnetic species (CdAl12 O19 ). Indeed, XRD measurements revealed that Al2 O3 -doping at 1000 and 1100 ◦ C led to formation of CdAl12 O19 phase. The observed increase in Ms of pure and MgO doped solids calcined at 900–1100 ◦ C may be discussed in the following: (i) The solid-state reaction between Fe and Cd oxides, leading to formation of Cd-ferrite solid which behaves as magnetic material. (ii) The formation of ␥-Fe2 O3 (Meghamite) phase endowed with a high saturation magnetization. In fact, the pure and doped solids calcined at 1100 ◦ C included ␥-Fe2 O3 crystallite. (iii) Decreasing the crystallite size of Cd-ferrite and the distance between magnetic ions due to the doping process followed by heating 900–1100 ◦ C leads to stronger magnetic solid. 4. Discussion Cadmium ferrite particles are commonly produced by a ceramic process involving high temperature solid-state reactions between the cadmium and ferric oxides [13]. The kinetic description of these reactions in the ferrite formation is a complex issue and the literature provides several models that may describe the rate at which these type of reactions proceed [5,23]. Most of these models imply that a thin film of MFe2 O4 may be formed at relatively low temperatures and covers the surfaces of the reacting Fe2 O3 grains, hindering the thermal diffusion of divalent cations through it yielding their ferrites. The produced ferrite film acts as an energy barrier

N.M. Deraz, M.M. Hessien / Journal of Alloys and Compounds 475 (2009) 832–839

837

Fig. 5. TEM images of pure and doped Cd/Fe mixed oxides calcined at 900, 1100 and 1200 ◦ C. (A) Pure mixed solids at 900 ◦ C, (B) pure mixed solids at 1100 ◦ C, (C) pure mixed solids at 1200 ◦ C, (D) +6 mol% MgO at 1100 ◦ C and (E) +6 mol% Al2 O3 at 1100 ◦ C.

against the further formation of MFe2 O4 phase several successful attempts have been done to stimulate the ferrite formation via doping with certain foreign oxides [5,17]. In fact that the heat treatment of cadmium an iron oxides at 1000 ◦ C for 3 h was sufficient to effect the complete conversion of the reacting oxides into Cd-ferrite particles. However, the doping process and thermal

treatment of the investigated mixed oxides resulted in an increase in the values of R (I1.66 /I1.41 ) indicating an increase in Cd-ferrite formation. The enhancement formation of CdFe2 O4 phase due to the doping with either Al2 O3 or MgO could result from an induced increase in the concentration of the reacting species and/or increasing their

838

N.M. Deraz, M.M. Hessien / Journal of Alloys and Compounds 475 (2009) 832–839

Fig. 6. Magnetic hystersis curves measured at a room temperature for pure mixed solids calcined at 500–1100 ◦ C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Fig. 8. Magnetic hystersis curves measured at a room temperature for pure and doped mixed solids calcined at 1100 ◦ C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

mobility. Similar results have been reported that ZnO or Li2 Odoping much enhances the solid-state reaction between CuO and Fe2 O3 producing Cu-ferrite [23]. Aluminum or magnesium oxides can be dissolved in the lattices of Fe2 O3 and CdO because of the smaller of ionic radii of Al3+ , Mg2+ than that of Cd2+ and Fe3+ ions [18]. This process can be simplified by the use of Kröger’s notations [24] in the following manner:

tively; C.V. and A.V. the created cationic and anionic vacancies, respectively; Al and Mg the trivalent aluminum ions and divalent magnesium ions retained in the interstitial positions of Fe2 O3 and CdO lattices (Eqs. (8) and (9)). The equations from (4)–(7) indicate the dissolution of Al3+ and Mg2+ cations in the lattices of Fe2 O3 and CdO via substitution mechanism. All reactions followed by creation of cationic and anionic vacancies (Eqs. (5), (6), (8) and (9)) are expected to enhance the solid-state reaction between the free oxides yielding nanocrystalline Cd-ferrite. The stimulation effect of the ferrite formation results mainly from an effective increase the mobility of Fe3+ and Cd2+ cations through the initial produced ferrite film. Moreover, most of MgO and Al2 O3 dopants might dissolve in the lattices of Fe2 O3 and CdO solid via substitution mechanism (Eqs. (4)–(7)). The fact that MgO and Al2 O3 doping followed by calcination at 500–1000 ◦ C enhanced the formation of Cd-ferrite to an extent proportional to their amounts added suggested that the most of the dopants added dissolved in the lattices of Fe2 O3 and CdO with subsequent creation of cationic and anionic vacancies. In addition, the calculated values of formation activation energy (Ef )

Al2 O3 + 2Fe3+ → 2Al(Fe3+ ) Al2 O3 + 2Cd

2+

2MgO + 2Fe MgO + Cd

2+

Al2 O3 + 2Fe MgO + Cd

→ 2Al(Cd

3+

2+

→ 2Mg(Fe

→ Mg(Cd

3+

2+

2+

(4)

) + 2Cd

3+

2+

) + 2Fe

+ C.V.

3+

+ A.V.

)

(6) (7)

→ 2Al + 2Fe

→ Mg + Cd

(5)

3+

2+

+ 2 C.V.

+ C.V.

(8) (9)

Al(Fe3+ ) and Al(Cd2+ ) are trivalent aluminum ions located in the positions of Fe3+ and Cd2+ of Fe2 O3 and CdO lattices, respec-

Table 4 The effects of calcination temperature and doping on the magnetic properties (Ms , Mr and Hc ) of pure and doped CdFe2 O4 solid.

Fig. 7. Magnetic hystersis curves measured at a room temperature for pure and doped mixed solids calcined at 1000 ◦ C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Solids

Calcination temperature (◦ C)

Ms (emu/g)

Mr (emu/g)

Hc (Oe)

Pure CdO + Fe2 O3

500 700

0.2884 0.4714

0.07878 0.09035

3005 422.3

Pure CdO + Fe2 O3 +2 mol% MgO +6 mol% MgO +2 mol% Al2 O3 +6 mol% Al2 O3

900

0.4740 0.8988 0.8477 0.3273 0.4102

0.0865 0.08047 0.02616 0.07962 0.9897

728.9 940.8 233.8 1282 1003

Pure CdO + Fe2 O3 +2 mol% MgO +6 mol% MgO +2 mol% Al2 O3 +6 mol% Al2 O3

1000

1.7100 1.5050 1.8171 1.8179 0.8778

0.09519 0.0517 0.0655 0.1291 0.0617

Pure CdO + Fe2 O3 +2 mol% MgO +6 mol% MgO +2 mol% Al2 O3 +6 mol% Al2 O3

1100

6.7160 7.7950 4.4150 5.2340 0.8320

0.5198 0.4866 0.1678 0.3702 0.0038

56.95 77.59 64.84 29.52 517.3 10.41 70.72 1.508 26.82 32.91

N.M. Deraz, M.M. Hessien / Journal of Alloys and Compounds 475 (2009) 832–839

of Cd-ferrite at temperatures between 900 and 1100 ◦ C for pure and doped solids were found to a decrease by doping with either Al2 O3 or MgO indicating the role of these dopants in effectively increasing of the mobility of the reacting cations. MgO-doping exhibited more enhancement effect towards the formation of Cd-ferrite than Al2 O3 dopant. This indicates the amount of MgO dissolved in lattices of the reacting oxides is greater than that of Al2 O3. This conclusion has been verified experimentally from XRD investigations of various solids that showed the possible formation of CdAl12 O19 phase. Also, IR data revealed that MgOdoping exhibited an increase in the intensity of adsorption bands greater than that due to the Al2 O3 treatment. This indicates the dissolution of MgO in the lattices of CdO and Fe2 O3 solids easier than Al2 O3 dopant. Additionally, the doping process with either Al2 O3 or MgO followed by calcination at 500–1100 ◦ C enhanced the formation of nanosized and unsintered Cd-ferrite. These findings are supported by the decrease in both the crystallite size and the sintering activation energy of Cd-ferrite due to doping with 6 mol% Al2 O3 or 6 mol% MgO at 900–1100 ◦ C. Finally, the thermal treatment and doping process brought about change in the magnetization of Cd/Fe mixed oxides calcined at 900–1100 ◦ C due to formation of well crystalline nanomagnetic particles of Cd-ferrite. Higher saturation magnetization value of 7.99 emu/g was obtained for 2 mol% MgO-doped solids calcined at 1100 ◦ C. This value was higher than the bulk value (1.4 emu/g) reported by Desai et al. [13]. This could be attributed to the change in the crystallite size of the produced ferrite and formation of nanosized ␥-Fe2 O3 crystallite. According to Kamiyama et al. [15] the small deviation of cation distribution of Fe3+ ions from B-site to A-site results into localized magnetic clusters. The size and shape of these magnetic clusters are influenced by strong A–B interaction between Fe ions on the A-sites and that on B-sites. A random arrangement of Fe ions on both sites will yield many clusters with different volumes and magnetic order. 5. Conclusions The observations from DSC, XRD, IR, TEM and VSM studies are summarized as follows: (1) Heating of Cd/Fe mixed oxides at 1000 ◦ C for 3 h led to complete conversion of the reacting oxides into nanocrystalline Cd-ferrite. (2) The doping of CdO/Fe2 O3 system with MgO or Al2 O3 followed by calcination at 900–1100 ◦ C led to dissolution of some dopants in the CdO and Fe2 O3 lattices via different mechanisms. Some of these processes created cationic and anionic vacancies leading to an increase in the mobility of reacting species. This effect enhanced the ferrite formation.

839

(3) A limited portion of Al2 O3 or MgO was dissolved in the individual oxide lattices, while the other portion contributes in the formation of a mixed ferrites CdAlx Fe2−x O4 or Cd1−x Mgx Fe2 O4 , respectively. (4) The heat treatment and doping process were strongly influenced on the microstructure of the investigated solids. (5) The IR analysis supports the distribution of Mg ions in both A and B sites and also Al in B-sites. (6) Mg ions substitutions improves the saturation magnetization which reduced by Al ions substitution. The Ms values initially increase with the addition of Mg2+ ions and show a rapid decrease at higher concentration in case of Al3+ addition. (7) The computed values of formation activation energy of CdFe2 O4 were 72, 64 and 27 kJ mol−1 for pure mixed solids and those doped with 6 mol% Al2 O3 and 6 mol% MgO, respectively. This might reflect an effective increase in the mobility of reacting cations (Cd2+ and Fe3+ ) through the reacting oxides and produced ferrite due to creation of cationic and anionic vacancies in the lattices of doped CdO/Fe2 O3 system. References [1] L. Li, H. Liu, Y. Wang, J. Jiang, F. Xu, J. Colloid Interface Sci. 321 (2) (2008) 265. [2] V. Lagashetty, S. Havonoor, S.D. Basavaraja, A. Balaji, Venkataraman, Sci. Technol. Adv. Mater. 8 (6) (2007) 484. [3] N. Das, R. Majumdar, A. Sen, H.S. Maiti, Matt. Lett. 61 (10) (2007) 2100. [4] S. Kundu, H.C. Anand, Verma, Powder Technol. 132 (2–3) (2003) 131. [5] N.M. Deraz, Thermochim. Acta 401 (2003) 175. [6] M.M. Hessien, M.H. Khedr, Mater. Res. Bull. 42 (7) (2007) 1242. [7] G. Mu, X. Pan, H. Shen, M. Gu, Mater. Sci. Eng. A 445–446 (2007) 563. [8] J.A. Toledo, M.A. Valenzuela, P. Bosch, H. Armendariz, A. Montoya, N. Nava, A. Vazquez, Appl. Catal. A 198 (2000) 235. [9] B.P. Ladgaonkar, P.N. vasambekar, A.S. vaingankar, Turk. J. Phys. 25 (2001) 129. [10] M.D. Osborne, M.E. Fleet, G.M. Bancroft, J. Solid State Chem. 53 (1984) 174. [11] J.A. Toledo, P. Bosch, M.A. Valenzuela, A. Montaya, N. Nava, J. Mol. Catal. 125 (1997) 53. [12] G.M. Bancroft, M.D. Osborne, M.E. Fleet, Solid State Commun. 47 (1983) 623. [13] B. Desai, R.V. Metha, R.V. Upadhyay, A. Gupta, A. Praneet, K.V. Rao, Bull. Mater. Sci. 30 (3) (2007) 197. [14] M. Yokoyama, T. Sato, E. Ohta, T. Sato, J. Appl. Phys. 80 (1996) 1015. [15] T. Kamiyama, K. Haneda, T. Sato, S. Ikeda, H. Asano, Solid State Commun. 81 (1992) 563. [16] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesly Publishing Co. Inc., 1976 (Chapter 14). [17] N.M. Deraz, J. Anal. Appl. Pyrol. 82 (2008) 212. [18] N.N. Greenwood, Ionic Crystal Lattice Defects and Non-stoichiometry, Butterworths, London, 1968. [19] S. Mazen, B.A. Sabrah, Thermochim. Acta 105 (1986) 1. [20] K. Mohan, Venudhar, J. Mater. Sci. Lett. 18 (1999) 13. [21] Ravender, J. Mater. Lett. 40 (1999) 205. [22] A. Satter, H.M. El-Sayed, K.M. El-Shokrofy, M.M. El- Tabey, J. Appl. Sci. 5 (1) (2005) 162. [23] N.M. Deraz, Ph.D., University, Zagazig University, Zagazig Egypt, 1999. [24] F.A. Kröger, Chemistry of Imperfect Crystals, North-Holland, Amsterdam, 1964.