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Electrical and thermal behavior of PS/ferrite composite A.H. Ashour, O.M. Hemeda, Z.K. Heiba, S.M. AlZahrani
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S0304-8853(14)00520-4 http://dx.doi.org/10.1016/j.jmmm.2014.06.005 MAGMA59125
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Journal of Magnetism and Magnetic Materials
Received date: 7 October 2013 Revised date: 19 May 2014 Accepted date: 1 June 2014 Cite this article as: A.H. Ashour, O.M. Hemeda, Z.K. Heiba, S.M. Al-Zahrani, Electrical and thermal behavior of PS/ferrite composite, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2014.06.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrical and Thermal Behavior of PS/Ferrite Composite
A.H.Ashourb*, O. M. Hemedac ,Z. K. Heibaa,dand S.M.Al-Zahrania a
Solid State , Physics Dept . , Faculty of Sci. Taif University. Radiation Phys. Dept.,National Center for RadiationResearch and Technology, Egypt c Solid State lab. , Physics Dept . , Faculty of Science. Tanta University , Egypt. d Solid State , Physics Dept . , Faculty of Sci. Ain Shams University Egypt, b
*Corresponding author: A.H.Ashour, E‐mail: [email protected]
Abstract This work aims to study the effect of gamma radiation on the structure , thermal and electrical properties of PS/ferrite composite . The Ni0.6Cd0.4Fe2-xSmxO4 was prepared using a conventional sintering ceramic process. Ferrite powder and Styrene was mixed and achieve polymerization process by gamma irradiation at 50 kGy. The composite samples have single spinel phase structure. Stability of the crystalline structure and no phase transition due to irradiation are found. The bulk density decreases whereas X-ray density increase with increasing Sm contents for both ferrite and PS/ferrite. The tetrahedral radii rA remains constant with Sm content but octahedral radii rB increases for both ferrite and PS/ferrite composite. The grain size shows increasing trend for PS/ferrite composite . The PS nearly coat the grains and so their boundaries become faint and not sharp. The gamma radiation transfer Fe3+ to Fe2+ due to its ionizing effect .The Fe 2+ occupy octahedral site and the stretching vibration of its bond with oxygen (Fe 2+- O2- ) gives absorption at about 392 cm-1, near octahedral absorption at 462 cm-1 .The PS/ Ni0.6Cd0.4SmxFe2-xO4 composite becomes thermally more stable than pure polystyrene. The activation energy of conduction Eσ has a small values and in the range of hopping conduction mechanism. Highlights Composite of was successfully obtained as follows: Ferrite powder and Styrene was mixed and achieve polymerization process by gamma irradiation at 50 kGy. Stability of the crystalline structure and no phase transition due to irradiation are found. The grain size shows increasing trend for PS/ferrite composite . The PS/ Ni0.6Cd0.4SmxFe2-xO4 composite becomes thermally more stable than pure polystyrene. The activation energy of conduction Eσ has a small values and in the range of hopping conduction mechanism.
Keywords Gamma radiation; Polymers; Ferrites; Thermal ; electrical
1
1.Introduction It has been approximately 50 years since researchers first began exposing polymeric materials to ionizing radiation, and reporting the occurrence of cross-linking and other useful effects. Today, a substantial commercial industry is in place based on processing of polymers with radiation. Innovation in this field has by no means ended; important new products made possible through radiation technology continue to enter the marketplace, and exciting new innovations in the application of radiation to macromolecular materials are under exploration at research institutions around the world. An objective of this work is to provide workers , and composite scientists in general, with an overview on the status of progress toward commercial products and technologies in the broad area of radiation processing. More recently, radiation curing of polymer/ceramic composites has begun to come into use. The material prepared by this process has significantly better stability, high-performance and high-temperature applications than polymer only. This technology has been commercialized. More recently, this approach has been extended to the production of Polystyrene/ferrite composite. This material is being explored for use in aggressive environments such as nuclear power plants, high tech electronic devices, integrated capacitors and aerospace applications. The present work is to study gamma radiation effect during composite preparation on the physical, structural, morphology, thermal and electrical properties. The Ni0.6Cd0.4Fe2-xSmxO4 was prepared using a conventional sintering ceramic process. Ferrite product and Styrene was mixed and achieve polymerization process by gamma irradiation at 50 kGy. E. Rezlescu et al. studied the lattice constant, density, electrical resistivity, activation energy, carrier concentration and mobility for rare-earthsubstituted nickel- zinc ferrites with formula Ni0.7Zn0.3Fe1.98R0.02O4 where R = Yb, Er, Dy, Tb, Gd, Sm and Ce. They have found that all the rare-earth ions favor the occurrence of a second phase resulting in an increase of the electrical resistivity, bulk density and hardness. The log ρ = f (1/T) curves show two regions of different activation energy. The difference between the two activation energies was explained by an important activation of the carrier mobility at higher temperatures and by conduction mechanism sensitive to the microstructure [1]. S. Sindhu et al. studied the Synthesis and characterization of ferritepolymer nanocomposite spheres from nickel-ferrite or nickel-zinc-ferrite and hydrophilic polymers such as polyhydroxylated methylmethacrylate (PHPMMA) or polyvinyl alcohol (PVA) is reported. The ferrite filler was 2
synthesized via the nitrate-citrate auto combustion method. The composite samples were prepared by direct mixing of the ferrite and the polymer, followed by sonication. X-ray powder diffraction patterns indicate crystalline nature of the filler particles. Electron microscopic investigation revealed a spherical morphology and an average particle size of 86 - 55nm for the composite. Additional characterization of the composites was done using EDXS and FTIR spectroscopy. The percentage of magnetization induced in composites with PHPMMA was higher than that obtained from PVA based composites[2] . According to Kazuaki Shimba et al, Polymer composites of magnetic particles are widely used as microwave absorbers. An effective method for obtaining thinner microwave absorbers for device design is increasing the volume fraction of magnetic nanoparticles by enhancing the permeability of composites. In this study, composites were prepared using Ni-Zn ferrite nanoparticles surface-modified with 4-META (4-methacryloylioxyethyl trimellitate anhydride) and cross-linked with PEG-4SH (pentaerythritol tetrapolyethylene glycol ether with four thiol-modified terminals). These composites have a high volume fraction of nanoparticles (up to 72 vol%) and permeability (µ״max = 5.9). In addition, the prepared composites showed good microwave absorption properties (R.L<- 20 dB) with a smaller matching thickness than conventional microwave absorber using spinel-type ferrite [3] . The Ni0.6Cd0.4Fe2-xSmxO4 was prepared using a conventional sintering ceramic process. Ferrite product and Styrene was mixed and achieve polymerization process by gamma irradiation at 50 kGy. The aim of the present work is to study the structural , electrical and thermal properties of Ni0.6Cd0.4Fe2-xSmxO4 ferrite / polystyrene composite.
2. METHODOLOGY 2.1. Preparation of the Samples: 2.1.1. Ferrite preparation The Ni0.6Cd0.4Fe2-xSmxO4 was prepared using a conventional sintering ceramic process. The Ni0.6Cd0.4Fe2-xSmxO4 was prepared starting with grinding NiO, CdO, SmO and Fe2O3 with purity 99.99% and mixed in molar ratio for 7 hours and presintered at 900 C° for 4 hours and left to be cooled gradually. Finally, all samples were ground and pressed at room temperature into tablets under a pressure of 107 kg/m2 of diameter 2 cm and 0.4 cm thickness. Finally the samples were sintered at 1100 C° for 3 hours. The furnace was left to cool gradually by 2.5 C° /min to room temperature. 2.1.2. Ferrite/polystyrene composite preparation Ferrite powder and Styrene monomer was mixed for about 10 minutes at a rotor speed of 60 rpm. The mixed samples were compressed into discs of 3
desired thickness by hydraulic press. Then , A Cobalt-60 Indian cell GC 4000 A , was used to achieve Polymerization process by irradiation of the samples at 50 kGy with a dose rate of 10 kGy / 100 min [4].
2.2. Identification of the Prepared Samples: The samples were examined by Energy-Dispersive X-ray Microanalysis (EDX) and Scanning Electron Microscope (SEM) using a JEOL model (JSM-6390) , and X-ray diffraction (XRD) using a Philips model (PW-1729) diffractometer . In X-ray diffractometer, the powder specimens were exposed to Cu-k α radiation (λ = 1.5405 Ǻ). Infrared (IR) spectra for the prepared samples were carried out at room temperature by using a PERKIN-ELMER1430 recording infrared spectra in the range 200 to 4000 cm-1 . Thermogravimetry (TGA) and Differential Thermal Analysis (DTA) curves were obtained using a thermo balance from Netzsch STA 449F3. The temperature scale of the instrument was calibrated with high purity calcium oxalate. Sample masses of about 15 mg were set in platinum open crucible and then heated from ambient temperature to 1200 oC at a heating rate of 20 o C min-1 under dynamic nitrogen atmosphere (50 mL min-1).The DC resistance was measured using an Electrometer (Keithely 610 C).
3. Results and Discussion 3.1. X-ray analysis of pure ferrite and PS/ ferrite composite : The XRD patterns are shown in Fig . (1) which indicate that the material has a well defined single spinel phase belonging to fcc lattice. Inspecting this figure, one can notice that the diffraction peaks were slightly shifted to different positions as the Sm content increases .
4
Fig.( 1). X- ray diffraction patterns of Ni0.6 Cd0.4 Smx Fe2-x O4 at (x = 0.01, 0.02, 0.03, and 0.04)
Also, the XRD patterns for the synthesized samples Ni0.6Cd0.4SmxFe2-xO4. (x = 0.01, 0.02,0.03 and 0.4) after irradiation at 50 kGy during the polymerization process of styrene to from PS/ferrite composite are shown in Fig.( 2). All patterns show single spinel phase for all investigated samples which indicates the stability of the crystalline structure and no phase transition are found due to irradiation or due to the construction of Ps / ferrite composite samples.
Fig.( 2) X- ray diffraction of PS/Ni0.6Cd0.4SmxFe2-xO4 composite at (x = 0.01, 0.02, 0.03, and 0.04).
The lattice constant is plotted against the Sm content as shown in Fig. (3) . The Sm3+ ions have relatively large ionic radius equal to 0.97Ao which substitute for Fe3+ ions with radius (0.67 Aο) at octahedral sites during sintering process. The dependence of the lattice constant on Sm content obeys Vegard,s law (i.e. the plot of (a) vs Sm content is linear). The theoretical lattice parameter can be calculated using the radius of tetrahedral (rA),the radius octahedral( rB) and the radius of oxygen ion ro. The rA and rB can be calculated from the following equations : rA = 0.4r Cd +0.6r Fe rB = [0.6r Ni +(x) r Sm +(2-x-0.6) r Fe] /2 . In order to calculate rA and rB, it is necessary to know the cation distribution which can be represented by the proposed formula: 5
(Cd0.4 Fe0.6)[Ni0.6SmxFe 2-x –0.6]O4 As known, Ni and Sm ions prefer positions at octahedral sites while Cd prefers tetrahedral sites. The cation distributions were estimated from the agreement between the theoretical and the experimental lattice parameters. These cation distributions are given in Table( 1). Table( 1) Proposed cation distribution of the system Ni0.6Cd0.4SmxFe2-xO4. Sm content(x) 0.01 0.02 0.03 0.04
Cation distribution (Cd0.4Fe0.6)[Ni0.6Sm0.01Fe1.39] (Cd0.4Fe0.6)[Ni0.6Sm0.02Fe1.38] (Cd0.4Fe0.6)[Ni0.6Sm0.03Fe1.37] (Cd0.4Fe0.6)[Ni0.6Sm0.04Fe1.36]
The theoretical lattice parameter (ath), experimental lattice parameter values (aex) were plotted as a function of Sm content and are shown in Figs.(3a ,b) for pure ferrite and PS/ferrite composite . (a) ( b)
Fig. (3 a,b) The effect of Sm content on (ath. and aexp.) for ferrite and composite samples .
The lattice parameter increases for composite as a result of the formation of ferrous ions (Fe2+) which have a larger radius (0.74) 0A than that of ferric ions (Fe3+) (0.64) 0A. After composite formation, the dependence of lattice parameter on Sm content became nonlinear due to the distorted lattice formed by gamma irradiation. The theoretical lattice parameter ath was calculated using the values of tetrahedral and octahedral radii rA , rB and is given by the following equation
ath=
√
[( rA + r0 ) √ 3 ( rB + r0 )]
(1)
where r0 is the radius of the oxygen ion (1.32) 0A. 6
a
b
Fig.4( a,b) The effect of Sm content on (rA and rB) for pure ferrite and composite.
The ath for pure ferrite has values near aex at low concentration of Sm , but are slightly different at higher concentration of Sm as shown in Fig. (3a) . This means that there is a deviation from the ideal formula of cation distribution at high Sm content . Fig.( 4a,b) shows that rA remains constant with Sm addition but rB increases. The substitution of Sm ions with radius of (0.97Ao) for Fe3+ with radius (0.67Ao) at octahedral site leads to the increase of rB.The presence of Cd2+ ions and Fe3+ ions at tetrahedral site doesn't change rA values .The behavior of rA and rB is similar to that before irradiation. The radius of tetrahedral sites rA for pure ferrite and PS/ferrite composite have the same values, whereas rB for composite is higher than that in ferrite due to the formation of ferrous ions at the octahedral sites. Fig. (5a ) shows that the average crystallite size tm, increases by increasing Sm content . The average crystallite size was calculated using the Scherrer equation (3). (2) where :k = 51.57 is the Scherrer constant ,λ = 1.5404 A D is the wavelength of Cu Kα radiation . h1/2 : Full-width at half- maximum (FWHM) and θ: The peak position. The results show that the crystallite size of pure ferrite ranges from 125to 135 nm.
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Fig. 5a. the average crystallite size tm as a function of Sm content for ferrite.
Fig. 5b. the average crystallite size tm as a function of Sm content for composite.
As a result of irradiation, the XRD patterns showed a slight shift of the reflected peaks and increasing in the crystallite size values, which is attributed to the produced defects and compressive strain by γ- rays on the crystal lattice. The distortion in the cubic spinel structure after γ-rays irradiation is similar to that observed in a previous study on irradiation of Mg-Mn-Zn ferrite [5]. It is noticed that the gamma rays increase the crystallite size, which is in agreement with the previously reported data relevant to γ-rays irradiation effect on Mg–Mn nanoferrite [6]. The behavior of the crystallite size after irradiation is changed and decrease with increasing Sm content due to the influence of PS which accumulates at the grain boundaries as shown in Fig. (5b).
3.2. Morphological Study of Ferrite and PS/Ferrite composite : The scanning electron microscope (SEM) micrographs of the pure ferrite and PS/ferrite composites are shown in Fig. (6). Fig.( 6a,b) show the presence of a monophasic homogeneous micro-structure with an average particle size 2µm for x=0.01 and 0.04 Sm content. Whereas, the PS/ferrite composite in Fig. ( 6c,d ) show a microstructure with dark ferrite particle 8
and small white area at the particle boundary for polystyrene. It is noticed that the particle size of pure ferrite decreases by increasing Sm content . Generally the rare earth ions are accumulated at the particle boundaries , it is difficult for octahedral B - sites and tetrahedral A - sites to accommodate Sm ions due to its large radius with respect to A and B radii. This process retard the particle growth during sintering process .For composite sample 0.04 Sm content, the PS nearly coat the particle and so their boundaries become faint and not sharp as the other samples.The particle size for composite sample is about 4 µm as shown in Fig (6c) . The SEM results showed that the dislocations were created in the material after irradiation which results in increase of particle size and formation of some amorphous structure as shown in Fig. (6c,d) .
a- Sm 0.01
b- Sm 0.04
c- PS/ Sm 0.01
d- PS/ Sm 0.04
Fig. (6). SEM micrographs of powder ferrite (a,b) and irradiated PS/ferrite (c,d) Samples.
3.3. Infrared Absorption Spectra for pure Ferrite Samples and PS/Ferrite Composite : The room temperature IR spectra of the above mentioned samples Ni0.6 Cd0.4SmxFe2-xO4, (x = 0.01, 0.02, 0.03 and 0.04) are shown in Fig. (7) . 9
No absorption was observed above frequency 1000 cm-1. Waldron and Hafner [7 ]have attributed the band υ1 around 600 cm-1 to stretching vibration of tetrahedral groups Fe3+- O2- and thatν 2 around 400 cm-1 to octahedral groups complex Fe3+- O2- . The first absorption band in the present spectrum is at 570 cm-1 which shifts to higher frequency by Sm addition. The second absorption band υ2 appears at 446 cm-1 for x = 0.0. The difference in vibration positions of υ1 and υ2 for the different composition must be related to the difference in Fe3+- O2- interdistance in each A and B site. In general the rare-earth ions are too large to occupy A and B sites. The presence of small band ν 3 at around 400 cm-1 is a good evidence of the entry of Sm3+ ions instead of Fe3+ at B site and these bands υ3 may be due to Sm3+ - O2- stretching vibration . The absorption bands and its corresponding frequencies and intensities are given in Table (2), have nearly Lorentzian shape for all samples. The shape and the width of these bands depend on the cation distribution. The calculated values of force constant for pure ferrite and for PS/ferrite composite are given in Table(2) and is calculated using the equation: F=4π2 c2υ2µ2
(3)
Where c: is the velocity of light ,υ: is the frequency, and µ: is the reduced mass. F1 and F2 are the force constants for tetrahedral and octahedral sites respectively. The force constant for composite has the same behavior to that for pure ferrite . The results of the IR spectra of the given samples indicate that the Sm addition was within the solubility limit of the rare earth ion in the spinel ferrite.
Fig. ( 7). The IR spectra of Ni0.6Cd0.4SmxFe2-xO4 ferrite. 10
Table 2 IR Data analysis Sm(x) Content% υ1(tet.) 0.01 0.02 0.03 0.04
582 581 585 580
I ab υ2 (oct.)
I ab
υ3
3.1 3.3 3.2 3.1
4.3 4.6 4.8 4.0
466 461 471 455
397 396 400 397
Iab Ferrite Dyne/cm2 Composite Dyne/cm2 FA FB FA FB 1.4 1560 2900 1550 2450 1.5 1560 2900 1550 2450 1.5 1560 2900 1550 2450 1.1 1560 2850 1550 2450
Fig.(8) shows the IR spectra of PS/Ferrite composite. The IR spectra of PS/ Ferrites are almost identical to that of pure PS . The absorption peaks at 579 and 462 cm−1 were the characteristic absorption of Fe3+ - O bond at both tetrahedral and octahedral sites respectively, which confirmed the presence of ferrite particles.. It is observed in IR spectra the splitting of absorption peak at octahedral site for PS / Ferrite composite which may be due to the effect of gamma irradiation used to polymerize the styrene. The gamma radiation transfer Fe3+ to Fe2+ due to its ionizing effect . The Fe 2+ occupy octahedral site and the stretching vibration of its bond with oxygen (Fe 2+- O2- ) gives absorption at about 392 cm-1 near octahedral absorption at 462 cm-1 .The other absorption bands from 1250 to 3300 cm-1 are correspond to polymer chain molecules vibrations.
Fig. (8). The IR spectra of PS/Ferrite composite.
3.4.Thermal Analysis of Ferrite and PS/Ferrite Composite : Figures (9 a,b,c and d) and (10 a,b,c and d ) show the TGA and DTA curves of ferrite samples and PS/ferrite composite samples respectively. The overall shape of the curves shows a slight increase in temperature gradient ∆ T followed by endoderms at 340 0C. The remaining curves then exhibit increases in ∆T until the degradation is complete at 600C for PS. There is also 11
a small endothermic peak at 1100 οC corresponds to the sintering temperature of ferrites whereas the solid state reaction takes place. The thermal decomposition behavior of polystyrene/ Ferrites composites were discussed and compared with that of pure polystyrene..In the TGA curves we can see that the main step of polystyrene degradation is from 300 to 450 οC, attributed to the degradation of main chain of polystyrene [8]. Another degradation at about 340 οC with the evolution of aromatics from the degradation of the styrene. This degradation temperature has shifted to a higher temperature range than that of polystyrene as a result of the presence of ferrite .We can also see from the TGA curves that at 1100 οC, when the residue of polystyrene is almost none, there is another degradation step for ferrite due to the solid state reaction of ferrite.
A B
C
D
Fig. (9). TGA and DTA curves for thermal studied of ferrite samples.
12
A
B
C
D
Fig. 10. TGA and DTA curves for thermal studied of PS/ ferrite composite samples.
3.5.Electrical Conductivity of pure Ferrite and PS/ferrite composite : The electrical properties of the ferrite system Ni0.6Cd0.4SmxFe2-xO4 where (x = 0.01, 0.02,0.03, 0.04) were studied. The dc resistivity is given by the (4) formula: ρ = ρD e E / kT σ
Where Eσ is the activation energy of the conduction process obtained from the slope of Ln ρ versus 1000/T as shown in Fig. (11). The temperature variation of conductivity exhibits a break at Curie temperature and two regions are noticed. Such a break is associated with the change in the slope which is attributed to the change of magnetic order from ferrimagnetic to paramagnetic phase. Others confirmed that the change in the slope can be linked with magnetic ordering or with conductivity mechanism. One can see that the electrical resistivity decreases up to x = 0.3 and then increases . When Sm content increases, no new phases is formed and Sm entered the lattice at octahedral sites. After a certain limit x=0.3 , Sm acts as a trap center for hopping electrons between Fe2+ and Fe3+ and increases the interionic distance between Fe2+ and Fe3+, which lead to increase of resistivity. 13
Table ( 3) shows that as the Sm content increases, the Curie point changes from (426-378) K. A similar behavior in Tc caused by Ti3+ [9], for Co2+ ions substituted in place Ni2+ [10] and by Al3+ ions in Ni-Al ferrite [11].The substitution of Fe3+ magnetic ions by Sm3+ nonmagnetic ions at tetrahedral sites affects the magnetic order and helps the material to transfer to paramagnetic state at different temperature. The reason of the variation of Tc may be due to the variation of A-B exchange interaction which change the magnetic order state of the material and hence change Tc . The measured Tc values (426 K) for our samples are comparable with the results obtained by the previous work [12] for NiGd xFe2-xO4 at which Tc = 430K.
Fig. 11. The relation between electrical dc resistivity (Ln ρ) of Ni0.6Cd0.4SmxFe2-xO4 ferrite and inverse of absolute temperature. Table (3) Phase transition temperature TC and activation energy of the system Ni0.6Cd0.4SmxFe2-xO4 ferrite and PS/ferrite composite. X Sm
0.01 0.02 0.03 0.04
Tc
161 203 210 185
Ef
0.38 0.42 0.24 0.15
E σ (eV) Ferrite Ep Ef-E
0.48 0.50 0.16 0.77
p
0.10 0.07 0.08 0.62
14
E σ (eV) T g( 0C ) PS/ferrite composite
0.345 0.590 0.354 0.640 82
83 114 108
The Sm ions at octahedral sites causes a distortion in the unit cell which affects the distance between the neighboring ions and the conduction process is expected to be modulated. In our study the activation energy of conduction Eσ has a small values suggesting the hopping conduction mechanism . The activation energy increases on passing from ferrimagnetic (Ef) to the The increase of Ep above Tc is a paramagnetic (Ep) phase i.e. E p >E f . proof of the influence of magnetic ordering upon the conductivity process in ferrites.
Fig.( 12 a ,b ,c ,d). The relation between electrical dc resistivity (Ln ρ ) of PS/ferrite and inverse of absolute temperature.
Fig. (12 )shows the values of d.c. resistivity for PS / ferrite composite with different Sm content . In this figure ln resistivity is plotted against inverse of temperature. A transition in conductivity is observed at a critical temperature Tg for all composite samples. From the previous data for pure PS ,the critical temperature is found to be 147 °C. It can be seen from Fig. (12) that the transition temperature shifts to lower values with increasing Sm content. An increase of the order of 109 magnitude in resistivity of PS / ferrite composite is observed compared with the pure ferrite . The low value of resistivity for PS composite at 147 οC corresponds to its Polymer matrix 15
collapsed at 110-140°C as given from TGA analysis. On doping with Sm this temperature shifts to lower values . Considering the energy transfer mechanism, oxygen atoms from the ferrite to the bond with the PS, thereby giving them a greater degree of freedom for rotation about C-C axis, resulting in a lowering of Tg. Fig. (12) shows that the electrical resistivity has three regions with increasing temperature , the first below Tg the resistivity decrease with temperature which is the normal behavior of ferrite .In this range the mobility of charge carriers is thermally activated and is responsible for the decrease of resistivity. The second region above Tg where the resistivity increase with temperature. The interpretation of this is that the polymeric chains above Tg traps the charge carriers which transited by hopping process in ferrite leading to the increase of resistivity. The third region belongs to ferrite conduction mechanism at high temperature ( paramagnetic region). The activation energy of Ps/ferrite composite increase compared with pure ferrite due to the increase of composite resistance ,as given in Table (3).
4. Conclusions Analysis of the changes in the physical, structural, morphology, thermal and electrical properties of the PS/ Ni0.6Cd0.4SmxFe2-xO4 composite submitted to polymerization by using gamma irradiation at 50 kGy shows that : 1- The bulk density decreases and X-ray density increase with increasing Sm contents for ferrite and PS/ferrite composite. 2- The tetrahedral radii rA remains constant with Sm concentration but octahedral radii rB increases for ferrite and PS/ferrite composite. 3- The average crystallite size tm increase with increasing of Sm content for ferrite and decrease with increasing of Sm content for composite but the crystallite size generally increase for composite at the same Sm content. 4- The gamma radiation transfer Fe3+ to Fe2+ due to its ionizing effect , the Fe 2+ occupy octahedral site and the stretching vibration of its bond with oxygen (Fe 2+- O2- ) gives absorption at about 392 cm-1 near octahedral absorption at 462 cm-1 .
16
5- The PS/ Ni0.6Cd0.4SmxFe2-xO4 composite become thermally more stable than pure polystyrene. 6- The activation energy of conduction Eσ has a small values in the range of hopping conduction mechanism . 7- Resistivity of
PS/ferrite
composite increase by about 109 order of
magnitude than the ferrite. 8- The electrical resistivity for composite has three regions with increasing temperature, below Tg, above Tg and at high temperatures .The first and third region belongs to ferrite conduction mechanism .
References [1] E. Rezlescu, N. Rezlescu, P.D. Popa, L. Rezlescu, and C. Pasnicu, Phys Stat.Sol. (a) 162 (1997)673. [2] S.Sindhu et al . Journal of Magnetism and Magnetic Materials 296 (2006) 104. [3] Kazuaki Shimba, MaterialsTrasactions,Vol.52,No.4,PP.7410-745 (2011). [4] Khan and Darshane, J. Mater. Sci. 24 (1988)2615. [5] G.K. Joshi, A.Y. Khot and C.R. Swant, Solid State Comm. 65(12),(1988) 1593. [6] A.M. Clogston and Bell Syst. Tech. J.34 (1955)739. [7] R.D. Waldron and Z. Hafner, Phys. Rev. 99 (1955) 1727. [8] J.H. de Boer and E.J. Verwey , Proc. Soc., 119A (1975)59. [9] Mazher Addin Rana, "Cation distribution and magnetic properties of ferrites", Ph. D. Thesis, Department of Physics, Bahaa Addin Zakarya University , Pakistan (1988). [10] M.A. El. Hiti, M.K. El- Nimr, M.A. Ahmad, Phase Transition 54 (1995)137. [11] M.A. Ahmad, M.K. El- Nimr, A. Tawfik, A.M. El- Hassab,Phys. Stat. Sol. A 123 (1991)501. [12] O.M. Hemeda, M.Z. Said, M.M. Barakat, Journal of Magnetism and Magnetic Materials 224 (2001)132.
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Electrical and thermal behavior of PS/ferrite composite A.H. Ashour, O.M. Hemeda, Z.K. Heiba, S.M. AlZahrani
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PII: DOI: Reference:
S0304-8853(14)00520-4 http://dx.doi.org/10.1016/j.jmmm.2014.06.005 MAGMA59125
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Journal of Magnetism and Magnetic Materials
Received date: 7 October 2013 Revised date: 19 May 2014 Accepted date: 1 June 2014 Cite this article as: A.H. Ashour, O.M. Hemeda, Z.K. Heiba, S.M. Al-Zahrani, Electrical and thermal behavior of PS/ferrite composite, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2014.06.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrical and Thermal Behavior of PS/Ferrite Composite
A.H.Ashourb*, O. M. Hemedac ,Z. K. Heibaa,dand S.M.Al-Zahrania a
Solid State , Physics Dept . , Faculty of Sci. Taif University. Radiation Phys. Dept.,National Center for RadiationResearch and Technology, Egypt c Solid State lab. , Physics Dept . , Faculty of Science. Tanta University , Egypt. d Solid State , Physics Dept . , Faculty of Sci. Ain Shams University Egypt, b
*Corresponding author: A.H.Ashour, E‐mail: [email protected]
Abstract This work aims to study the effect of gamma radiation on the structure , thermal and electrical properties of PS/ferrite composite . The Ni0.6Cd0.4Fe2-xSmxO4 was prepared using a conventional sintering ceramic process. Ferrite powder and Styrene was mixed and achieve polymerization process by gamma irradiation at 50 kGy. The composite samples have single spinel phase structure. Stability of the crystalline structure and no phase transition due to irradiation are found. The bulk density decreases whereas X-ray density increase with increasing Sm contents for both ferrite and PS/ferrite. The tetrahedral radii rA remains constant with Sm content but octahedral radii rB increases for both ferrite and PS/ferrite composite. The grain size shows increasing trend for PS/ferrite composite . The PS nearly coat the grains and so their boundaries become faint and not sharp. The gamma radiation transfer Fe3+ to Fe2+ due to its ionizing effect .The Fe 2+ occupy octahedral site and the stretching vibration of its bond with oxygen (Fe 2+- O2- ) gives absorption at about 392 cm-1, near octahedral absorption at 462 cm-1 .The PS/ Ni0.6Cd0.4SmxFe2-xO4 composite becomes thermally more stable than pure polystyrene. The activation energy of conduction Eσ has a small values and in the range of hopping conduction mechanism. Highlights Composite of was successfully obtained as follows: Ferrite powder and Styrene was mixed and achieve polymerization process by gamma irradiation at 50 kGy. Stability of the crystalline structure and no phase transition due to irradiation are found. The grain size shows increasing trend for PS/ferrite composite . The PS/ Ni0.6Cd0.4SmxFe2-xO4 composite becomes thermally more stable than pure polystyrene. The activation energy of conduction Eσ has a small values and in the range of hopping conduction mechanism.
Keywords Gamma radiation; Polymers; Ferrites; Thermal ; electrical
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1.Introduction It has been approximately 50 years since researchers first began exposing polymeric materials to ionizing radiation, and reporting the occurrence of cross-linking and other useful effects. Today, a substantial commercial industry is in place based on processing of polymers with radiation. Innovation in this field has by no means ended; important new products made possible through radiation technology continue to enter the marketplace, and exciting new innovations in the application of radiation to macromolecular materials are under exploration at research institutions around the world. An objective of this work is to provide workers , and composite scientists in general, with an overview on the status of progress toward commercial products and technologies in the broad area of radiation processing. More recently, radiation curing of polymer/ceramic composites has begun to come into use. The material prepared by this process has significantly better stability, high-performance and high-temperature applications than polymer only. This technology has been commercialized. More recently, this approach has been extended to the production of Polystyrene/ferrite composite. This material is being explored for use in aggressive environments such as nuclear power plants, high tech electronic devices, integrated capacitors and aerospace applications. The present work is to study gamma radiation effect during composite preparation on the physical, structural, morphology, thermal and electrical properties. The Ni0.6Cd0.4Fe2-xSmxO4 was prepared using a conventional sintering ceramic process. Ferrite product and Styrene was mixed and achieve polymerization process by gamma irradiation at 50 kGy. E. Rezlescu et al. studied the lattice constant, density, electrical resistivity, activation energy, carrier concentration and mobility for rare-earthsubstituted nickel- zinc ferrites with formula Ni0.7Zn0.3Fe1.98R0.02O4 where R = Yb, Er, Dy, Tb, Gd, Sm and Ce. They have found that all the rare-earth ions favor the occurrence of a second phase resulting in an increase of the electrical resistivity, bulk density and hardness. The log ρ = f (1/T) curves show two regions of different activation energy. The difference between the two activation energies was explained by an important activation of the carrier mobility at higher temperatures and by conduction mechanism sensitive to the microstructure [1]. S. Sindhu et al. studied the Synthesis and characterization of ferritepolymer nanocomposite spheres from nickel-ferrite or nickel-zinc-ferrite and hydrophilic polymers such as polyhydroxylated methylmethacrylate (PHPMMA) or polyvinyl alcohol (PVA) is reported. The ferrite filler was 2
synthesized via the nitrate-citrate auto combustion method. The composite samples were prepared by direct mixing of the ferrite and the polymer, followed by sonication. X-ray powder diffraction patterns indicate crystalline nature of the filler particles. Electron microscopic investigation revealed a spherical morphology and an average particle size of 86 - 55nm for the composite. Additional characterization of the composites was done using EDXS and FTIR spectroscopy. The percentage of magnetization induced in composites with PHPMMA was higher than that obtained from PVA based composites[2] . According to Kazuaki Shimba et al, Polymer composites of magnetic particles are widely used as microwave absorbers. An effective method for obtaining thinner microwave absorbers for device design is increasing the volume fraction of magnetic nanoparticles by enhancing the permeability of composites. In this study, composites were prepared using Ni-Zn ferrite nanoparticles surface-modified with 4-META (4-methacryloylioxyethyl trimellitate anhydride) and cross-linked with PEG-4SH (pentaerythritol tetrapolyethylene glycol ether with four thiol-modified terminals). These composites have a high volume fraction of nanoparticles (up to 72 vol%) and permeability (µ״max = 5.9). In addition, the prepared composites showed good microwave absorption properties (R.L<- 20 dB) with a smaller matching thickness than conventional microwave absorber using spinel-type ferrite [3] . The Ni0.6Cd0.4Fe2-xSmxO4 was prepared using a conventional sintering ceramic process. Ferrite product and Styrene was mixed and achieve polymerization process by gamma irradiation at 50 kGy. The aim of the present work is to study the structural , electrical and thermal properties of Ni0.6Cd0.4Fe2-xSmxO4 ferrite / polystyrene composite.
2. METHODOLOGY 2.1. Preparation of the Samples: 2.1.1. Ferrite preparation The Ni0.6Cd0.4Fe2-xSmxO4 was prepared using a conventional sintering ceramic process. The Ni0.6Cd0.4Fe2-xSmxO4 was prepared starting with grinding NiO, CdO, SmO and Fe2O3 with purity 99.99% and mixed in molar ratio for 7 hours and presintered at 900 C° for 4 hours and left to be cooled gradually. Finally, all samples were ground and pressed at room temperature into tablets under a pressure of 107 kg/m2 of diameter 2 cm and 0.4 cm thickness. Finally the samples were sintered at 1100 C° for 3 hours. The furnace was left to cool gradually by 2.5 C° /min to room temperature. 2.1.2. Ferrite/polystyrene composite preparation Ferrite powder and Styrene monomer was mixed for about 10 minutes at a rotor speed of 60 rpm. The mixed samples were compressed into discs of 3
desired thickness by hydraulic press. Then , A Cobalt-60 Indian cell GC 4000 A , was used to achieve Polymerization process by irradiation of the samples at 50 kGy with a dose rate of 10 kGy / 100 min [4].
2.2. Identification of the Prepared Samples: The samples were examined by Energy-Dispersive X-ray Microanalysis (EDX) and Scanning Electron Microscope (SEM) using a JEOL model (JSM-6390) , and X-ray diffraction (XRD) using a Philips model (PW-1729) diffractometer . In X-ray diffractometer, the powder specimens were exposed to Cu-k α radiation (λ = 1.5405 Ǻ). Infrared (IR) spectra for the prepared samples were carried out at room temperature by using a PERKIN-ELMER1430 recording infrared spectra in the range 200 to 4000 cm-1 . Thermogravimetry (TGA) and Differential Thermal Analysis (DTA) curves were obtained using a thermo balance from Netzsch STA 449F3. The temperature scale of the instrument was calibrated with high purity calcium oxalate. Sample masses of about 15 mg were set in platinum open crucible and then heated from ambient temperature to 1200 oC at a heating rate of 20 o C min-1 under dynamic nitrogen atmosphere (50 mL min-1).The DC resistance was measured using an Electrometer (Keithely 610 C).
3. Results and Discussion 3.1. X-ray analysis of pure ferrite and PS/ ferrite composite : The XRD patterns are shown in Fig . (1) which indicate that the material has a well defined single spinel phase belonging to fcc lattice. Inspecting this figure, one can notice that the diffraction peaks were slightly shifted to different positions as the Sm content increases .
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Fig.( 1). X- ray diffraction patterns of Ni0.6 Cd0.4 Smx Fe2-x O4 at (x = 0.01, 0.02, 0.03, and 0.04)
Also, the XRD patterns for the synthesized samples Ni0.6Cd0.4SmxFe2-xO4. (x = 0.01, 0.02,0.03 and 0.4) after irradiation at 50 kGy during the polymerization process of styrene to from PS/ferrite composite are shown in Fig.( 2). All patterns show single spinel phase for all investigated samples which indicates the stability of the crystalline structure and no phase transition are found due to irradiation or due to the construction of Ps / ferrite composite samples.
Fig.( 2) X- ray diffraction of PS/Ni0.6Cd0.4SmxFe2-xO4 composite at (x = 0.01, 0.02, 0.03, and 0.04).
The lattice constant is plotted against the Sm content as shown in Fig. (3) . The Sm3+ ions have relatively large ionic radius equal to 0.97Ao which substitute for Fe3+ ions with radius (0.67 Aο) at octahedral sites during sintering process. The dependence of the lattice constant on Sm content obeys Vegard,s law (i.e. the plot of (a) vs Sm content is linear). The theoretical lattice parameter can be calculated using the radius of tetrahedral (rA),the radius octahedral( rB) and the radius of oxygen ion ro. The rA and rB can be calculated from the following equations : rA = 0.4r Cd +0.6r Fe rB = [0.6r Ni +(x) r Sm +(2-x-0.6) r Fe] /2 . In order to calculate rA and rB, it is necessary to know the cation distribution which can be represented by the proposed formula: 5
(Cd0.4 Fe0.6)[Ni0.6SmxFe 2-x –0.6]O4 As known, Ni and Sm ions prefer positions at octahedral sites while Cd prefers tetrahedral sites. The cation distributions were estimated from the agreement between the theoretical and the experimental lattice parameters. These cation distributions are given in Table( 1). Table( 1) Proposed cation distribution of the system Ni0.6Cd0.4SmxFe2-xO4. Sm content(x) 0.01 0.02 0.03 0.04
Cation distribution (Cd0.4Fe0.6)[Ni0.6Sm0.01Fe1.39] (Cd0.4Fe0.6)[Ni0.6Sm0.02Fe1.38] (Cd0.4Fe0.6)[Ni0.6Sm0.03Fe1.37] (Cd0.4Fe0.6)[Ni0.6Sm0.04Fe1.36]
The theoretical lattice parameter (ath), experimental lattice parameter values (aex) were plotted as a function of Sm content and are shown in Figs.(3a ,b) for pure ferrite and PS/ferrite composite . (a) ( b)
Fig. (3 a,b) The effect of Sm content on (ath. and aexp.) for ferrite and composite samples .
The lattice parameter increases for composite as a result of the formation of ferrous ions (Fe2+) which have a larger radius (0.74) 0A than that of ferric ions (Fe3+) (0.64) 0A. After composite formation, the dependence of lattice parameter on Sm content became nonlinear due to the distorted lattice formed by gamma irradiation. The theoretical lattice parameter ath was calculated using the values of tetrahedral and octahedral radii rA , rB and is given by the following equation
ath=
√
[( rA + r0 ) √ 3 ( rB + r0 )]
(1)
where r0 is the radius of the oxygen ion (1.32) 0A. 6
a
b
Fig.4( a,b) The effect of Sm content on (rA and rB) for pure ferrite and composite.
The ath for pure ferrite has values near aex at low concentration of Sm , but are slightly different at higher concentration of Sm as shown in Fig. (3a) . This means that there is a deviation from the ideal formula of cation distribution at high Sm content . Fig.( 4a,b) shows that rA remains constant with Sm addition but rB increases. The substitution of Sm ions with radius of (0.97Ao) for Fe3+ with radius (0.67Ao) at octahedral site leads to the increase of rB.The presence of Cd2+ ions and Fe3+ ions at tetrahedral site doesn't change rA values .The behavior of rA and rB is similar to that before irradiation. The radius of tetrahedral sites rA for pure ferrite and PS/ferrite composite have the same values, whereas rB for composite is higher than that in ferrite due to the formation of ferrous ions at the octahedral sites. Fig. (5a ) shows that the average crystallite size tm, increases by increasing Sm content . The average crystallite size was calculated using the Scherrer equation (3). (2) where :k = 51.57 is the Scherrer constant ,λ = 1.5404 A D is the wavelength of Cu Kα radiation . h1/2 : Full-width at half- maximum (FWHM) and θ: The peak position. The results show that the crystallite size of pure ferrite ranges from 125to 135 nm.
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Fig. 5a. the average crystallite size tm as a function of Sm content for ferrite.
Fig. 5b. the average crystallite size tm as a function of Sm content for composite.
As a result of irradiation, the XRD patterns showed a slight shift of the reflected peaks and increasing in the crystallite size values, which is attributed to the produced defects and compressive strain by γ- rays on the crystal lattice. The distortion in the cubic spinel structure after γ-rays irradiation is similar to that observed in a previous study on irradiation of Mg-Mn-Zn ferrite [5]. It is noticed that the gamma rays increase the crystallite size, which is in agreement with the previously reported data relevant to γ-rays irradiation effect on Mg–Mn nanoferrite [6]. The behavior of the crystallite size after irradiation is changed and decrease with increasing Sm content due to the influence of PS which accumulates at the grain boundaries as shown in Fig. (5b).
3.2. Morphological Study of Ferrite and PS/Ferrite composite : The scanning electron microscope (SEM) micrographs of the pure ferrite and PS/ferrite composites are shown in Fig. (6). Fig.( 6a,b) show the presence of a monophasic homogeneous micro-structure with an average particle size 2µm for x=0.01 and 0.04 Sm content. Whereas, the PS/ferrite composite in Fig. ( 6c,d ) show a microstructure with dark ferrite particle 8
and small white area at the particle boundary for polystyrene. It is noticed that the particle size of pure ferrite decreases by increasing Sm content . Generally the rare earth ions are accumulated at the particle boundaries , it is difficult for octahedral B - sites and tetrahedral A - sites to accommodate Sm ions due to its large radius with respect to A and B radii. This process retard the particle growth during sintering process .For composite sample 0.04 Sm content, the PS nearly coat the particle and so their boundaries become faint and not sharp as the other samples.The particle size for composite sample is about 4 µm as shown in Fig (6c) . The SEM results showed that the dislocations were created in the material after irradiation which results in increase of particle size and formation of some amorphous structure as shown in Fig. (6c,d) .
a- Sm 0.01
b- Sm 0.04
c- PS/ Sm 0.01
d- PS/ Sm 0.04
Fig. (6). SEM micrographs of powder ferrite (a,b) and irradiated PS/ferrite (c,d) Samples.
3.3. Infrared Absorption Spectra for pure Ferrite Samples and PS/Ferrite Composite : The room temperature IR spectra of the above mentioned samples Ni0.6 Cd0.4SmxFe2-xO4, (x = 0.01, 0.02, 0.03 and 0.04) are shown in Fig. (7) . 9
No absorption was observed above frequency 1000 cm-1. Waldron and Hafner [7 ]have attributed the band υ1 around 600 cm-1 to stretching vibration of tetrahedral groups Fe3+- O2- and thatν 2 around 400 cm-1 to octahedral groups complex Fe3+- O2- . The first absorption band in the present spectrum is at 570 cm-1 which shifts to higher frequency by Sm addition. The second absorption band υ2 appears at 446 cm-1 for x = 0.0. The difference in vibration positions of υ1 and υ2 for the different composition must be related to the difference in Fe3+- O2- interdistance in each A and B site. In general the rare-earth ions are too large to occupy A and B sites. The presence of small band ν 3 at around 400 cm-1 is a good evidence of the entry of Sm3+ ions instead of Fe3+ at B site and these bands υ3 may be due to Sm3+ - O2- stretching vibration . The absorption bands and its corresponding frequencies and intensities are given in Table (2), have nearly Lorentzian shape for all samples. The shape and the width of these bands depend on the cation distribution. The calculated values of force constant for pure ferrite and for PS/ferrite composite are given in Table(2) and is calculated using the equation: F=4π2 c2υ2µ2
(3)
Where c: is the velocity of light ,υ: is the frequency, and µ: is the reduced mass. F1 and F2 are the force constants for tetrahedral and octahedral sites respectively. The force constant for composite has the same behavior to that for pure ferrite . The results of the IR spectra of the given samples indicate that the Sm addition was within the solubility limit of the rare earth ion in the spinel ferrite.
Fig. ( 7). The IR spectra of Ni0.6Cd0.4SmxFe2-xO4 ferrite. 10
Table 2 IR Data analysis Sm(x) Content% υ1(tet.) 0.01 0.02 0.03 0.04
582 581 585 580
I ab υ2 (oct.)
I ab
υ3
3.1 3.3 3.2 3.1
4.3 4.6 4.8 4.0
466 461 471 455
397 396 400 397
Iab Ferrite Dyne/cm2 Composite Dyne/cm2 FA FB FA FB 1.4 1560 2900 1550 2450 1.5 1560 2900 1550 2450 1.5 1560 2900 1550 2450 1.1 1560 2850 1550 2450
Fig.(8) shows the IR spectra of PS/Ferrite composite. The IR spectra of PS/ Ferrites are almost identical to that of pure PS . The absorption peaks at 579 and 462 cm−1 were the characteristic absorption of Fe3+ - O bond at both tetrahedral and octahedral sites respectively, which confirmed the presence of ferrite particles.. It is observed in IR spectra the splitting of absorption peak at octahedral site for PS / Ferrite composite which may be due to the effect of gamma irradiation used to polymerize the styrene. The gamma radiation transfer Fe3+ to Fe2+ due to its ionizing effect . The Fe 2+ occupy octahedral site and the stretching vibration of its bond with oxygen (Fe 2+- O2- ) gives absorption at about 392 cm-1 near octahedral absorption at 462 cm-1 .The other absorption bands from 1250 to 3300 cm-1 are correspond to polymer chain molecules vibrations.
Fig. (8). The IR spectra of PS/Ferrite composite.
3.4.Thermal Analysis of Ferrite and PS/Ferrite Composite : Figures (9 a,b,c and d) and (10 a,b,c and d ) show the TGA and DTA curves of ferrite samples and PS/ferrite composite samples respectively. The overall shape of the curves shows a slight increase in temperature gradient ∆ T followed by endoderms at 340 0C. The remaining curves then exhibit increases in ∆T until the degradation is complete at 600C for PS. There is also 11
a small endothermic peak at 1100 οC corresponds to the sintering temperature of ferrites whereas the solid state reaction takes place. The thermal decomposition behavior of polystyrene/ Ferrites composites were discussed and compared with that of pure polystyrene..In the TGA curves we can see that the main step of polystyrene degradation is from 300 to 450 οC, attributed to the degradation of main chain of polystyrene [8]. Another degradation at about 340 οC with the evolution of aromatics from the degradation of the styrene. This degradation temperature has shifted to a higher temperature range than that of polystyrene as a result of the presence of ferrite .We can also see from the TGA curves that at 1100 οC, when the residue of polystyrene is almost none, there is another degradation step for ferrite due to the solid state reaction of ferrite.
A B
C
D
Fig. (9). TGA and DTA curves for thermal studied of ferrite samples.
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A
B
C
D
Fig. 10. TGA and DTA curves for thermal studied of PS/ ferrite composite samples.
3.5.Electrical Conductivity of pure Ferrite and PS/ferrite composite : The electrical properties of the ferrite system Ni0.6Cd0.4SmxFe2-xO4 where (x = 0.01, 0.02,0.03, 0.04) were studied. The dc resistivity is given by the (4) formula: ρ = ρD e E / kT σ
Where Eσ is the activation energy of the conduction process obtained from the slope of Ln ρ versus 1000/T as shown in Fig. (11). The temperature variation of conductivity exhibits a break at Curie temperature and two regions are noticed. Such a break is associated with the change in the slope which is attributed to the change of magnetic order from ferrimagnetic to paramagnetic phase. Others confirmed that the change in the slope can be linked with magnetic ordering or with conductivity mechanism. One can see that the electrical resistivity decreases up to x = 0.3 and then increases . When Sm content increases, no new phases is formed and Sm entered the lattice at octahedral sites. After a certain limit x=0.3 , Sm acts as a trap center for hopping electrons between Fe2+ and Fe3+ and increases the interionic distance between Fe2+ and Fe3+, which lead to increase of resistivity. 13
Table ( 3) shows that as the Sm content increases, the Curie point changes from (426-378) K. A similar behavior in Tc caused by Ti3+ [9], for Co2+ ions substituted in place Ni2+ [10] and by Al3+ ions in Ni-Al ferrite [11].The substitution of Fe3+ magnetic ions by Sm3+ nonmagnetic ions at tetrahedral sites affects the magnetic order and helps the material to transfer to paramagnetic state at different temperature. The reason of the variation of Tc may be due to the variation of A-B exchange interaction which change the magnetic order state of the material and hence change Tc . The measured Tc values (426 K) for our samples are comparable with the results obtained by the previous work [12] for NiGd xFe2-xO4 at which Tc = 430K.
Fig. 11. The relation between electrical dc resistivity (Ln ρ) of Ni0.6Cd0.4SmxFe2-xO4 ferrite and inverse of absolute temperature. Table (3) Phase transition temperature TC and activation energy of the system Ni0.6Cd0.4SmxFe2-xO4 ferrite and PS/ferrite composite. X Sm
0.01 0.02 0.03 0.04
Tc
161 203 210 185
Ef
0.38 0.42 0.24 0.15
E σ (eV) Ferrite Ep Ef-E
0.48 0.50 0.16 0.77
p
0.10 0.07 0.08 0.62
14
E σ (eV) T g( 0C ) PS/ferrite composite
0.345 0.590 0.354 0.640 82
83 114 108
The Sm ions at octahedral sites causes a distortion in the unit cell which affects the distance between the neighboring ions and the conduction process is expected to be modulated. In our study the activation energy of conduction Eσ has a small values suggesting the hopping conduction mechanism . The activation energy increases on passing from ferrimagnetic (Ef) to the The increase of Ep above Tc is a paramagnetic (Ep) phase i.e. E p >E f . proof of the influence of magnetic ordering upon the conductivity process in ferrites.
Fig.( 12 a ,b ,c ,d). The relation between electrical dc resistivity (Ln ρ ) of PS/ferrite and inverse of absolute temperature.
Fig. (12 )shows the values of d.c. resistivity for PS / ferrite composite with different Sm content . In this figure ln resistivity is plotted against inverse of temperature. A transition in conductivity is observed at a critical temperature Tg for all composite samples. From the previous data for pure PS ,the critical temperature is found to be 147 °C. It can be seen from Fig. (12) that the transition temperature shifts to lower values with increasing Sm content. An increase of the order of 109 magnitude in resistivity of PS / ferrite composite is observed compared with the pure ferrite . The low value of resistivity for PS composite at 147 οC corresponds to its Polymer matrix 15
collapsed at 110-140°C as given from TGA analysis. On doping with Sm this temperature shifts to lower values . Considering the energy transfer mechanism, oxygen atoms from the ferrite to the bond with the PS, thereby giving them a greater degree of freedom for rotation about C-C axis, resulting in a lowering of Tg. Fig. (12) shows that the electrical resistivity has three regions with increasing temperature , the first below Tg the resistivity decrease with temperature which is the normal behavior of ferrite .In this range the mobility of charge carriers is thermally activated and is responsible for the decrease of resistivity. The second region above Tg where the resistivity increase with temperature. The interpretation of this is that the polymeric chains above Tg traps the charge carriers which transited by hopping process in ferrite leading to the increase of resistivity. The third region belongs to ferrite conduction mechanism at high temperature ( paramagnetic region). The activation energy of Ps/ferrite composite increase compared with pure ferrite due to the increase of composite resistance ,as given in Table (3).
4. Conclusions Analysis of the changes in the physical, structural, morphology, thermal and electrical properties of the PS/ Ni0.6Cd0.4SmxFe2-xO4 composite submitted to polymerization by using gamma irradiation at 50 kGy shows that : 1- The bulk density decreases and X-ray density increase with increasing Sm contents for ferrite and PS/ferrite composite. 2- The tetrahedral radii rA remains constant with Sm concentration but octahedral radii rB increases for ferrite and PS/ferrite composite. 3- The average crystallite size tm increase with increasing of Sm content for ferrite and decrease with increasing of Sm content for composite but the crystallite size generally increase for composite at the same Sm content. 4- The gamma radiation transfer Fe3+ to Fe2+ due to its ionizing effect , the Fe 2+ occupy octahedral site and the stretching vibration of its bond with oxygen (Fe 2+- O2- ) gives absorption at about 392 cm-1 near octahedral absorption at 462 cm-1 .
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5- The PS/ Ni0.6Cd0.4SmxFe2-xO4 composite become thermally more stable than pure polystyrene. 6- The activation energy of conduction Eσ has a small values in the range of hopping conduction mechanism . 7- Resistivity of
PS/ferrite
composite increase by about 109 order of
magnitude than the ferrite. 8- The electrical resistivity for composite has three regions with increasing temperature, below Tg, above Tg and at high temperatures .The first and third region belongs to ferrite conduction mechanism .
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