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Synthesis modified structural and dielectric properties of semiconducting zinc ferrospinels
Author’s Accepted Manuscript Synthesis modified structural and dielectric properties of semiconducting zinc ferrospinels N. Kumari, M.S. Dahiya
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S1386-9477(16)30241-7 http://dx.doi.org/10.1016/j.physe.2016.10.007 PHYSE12596
To appear in: Physica E: Low-dimensional Systems and Nanostructures Received date: 17 April 2016 Revised date: 27 September 2016 Accepted date: 9 October 2016 Cite this article as: N. Kumari and M.S. Dahiya, Synthesis modified structural and dielectric properties of semiconducting zinc ferrospinels, Physica E: Lowdimensional Systems and Nanostructures, http://dx.doi.org/10.1016/j.physe.2016.10.007 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.
Synthesis modified structural and dielectric properties of semiconducting zinc ferrospinels N. Kumari, M. S. Dahiya* Physics Department, DCR University of Science and Technology, Murthal -131039 *[email protected] Abstract The influence of preparation techniques on structural and dielectric properties of ZnCrxFe1-xO4 (x=0, 0.1 abbreviated as Z and ZC) ferrite nano-particles synthesized using chemical coprecipitation (CCP), sol-gel (SG) and solid state reaction (SS) techniques is discussed. XRD profiles are used to confirm the single phase spinel ferrite formation. TEM images indicate the change in size and shape of particles on changing either the composition or the synthesis methodology. The TEM micrograph of samples obtained through CCP shows uniform particle size formation compared to those obtained through SG and SS. Sample prepared through CCP possess porosity >70% making these materials suitable for sensing applications. The dielectric loss, dielectric constant and ac conductivity are analyzed as a function of frequency, temperature and composition using impedance spectroscopy. A universal dielectric response and standard dielectric behavior has been reveled by temperature and frequency variations of each dielectric parameter. Dielectric constant is found to possess highest value for sample synthesized through SG which marks the possibility of using the SG derived ferrospinels as microwave device components. Keywords: Magnetic materials; chemical synthesis; impedance spectroscopy; dielectric properties 1. Introduction Nanocrystalline ferrite materials have achieved a primary position of economic and engineering importance within the family of magnetic materials because of their excellent physical and dielectric properties as compared to their bulk counterpart. Physical and dielectric properties of ferrite materials are highly influenced by size, shape of grains, grain boundaries, porosity, composition and preparation techniques [13]. The possibility of preparing ferrite nano-particles 1
has opened a new and interesting research field with revolutionary applications [4]. A remarkable characteristic of spinel ferrites is that their composition and properties can be strongly modified depending upon the applications keeping the basic crystalline structure unaltered. Different techniques like sol-gel, co-precipitation, solid-state, hydrothermal and microwave combustion methods [57] are used to prepare ferrite nanoparticles. A well known problem in synthesis of Znferrite prepared through solid state reaction method is loss of zinc content by evaporation due to high calcination temperature and long time during preparation which results in non-stoichiometric ferrite system [8, 9]. Hence, the challenge is to obtain high quality reproducible ferrites by selecting appropriate method for their preparation. In the present report, we have synthesized nanocrystalline Zn-ferrites and Cr3+substituted Znferrites via three different synthesis methods viz. chemical co-precipitation, sol-gel and solid state reaction. Effect of preparation technique on structural, micro structural and dielectric parameters has been studied and reported. 2. Experimental procedure 2.1 Preparation of materials 2.1.1 Synthesis of ZnFe2O4 and ZnFe1.9Cr0.1O4 by sol-gel technique For synthesis of ZnFe2O4 and ZnFe1.9Cr0.1O4 via sol-gel technique analytical grade ZnCl2 (Zinc Chloride), Cr (NO3)3.9H2O (Chromium Nitrate) and Fe(NO3)3.9H2O (Ferric Nitrate) were used as initial chemicals. Stoichiometric amounts of required metal salts were mixed separately to a gelating agent Ethylene Glycol in molar ratio 1:3 and then were mixed together using constant stirring and heating at 80oC. During heating solution takes the form of a viscous gel and finally became puffy porous mass. These puffy powders were then heated at 300oC to remove excess ethylene glycol and to obtain the final ferrite powder [10]. 2.1.2 Synthesis of ZnFe2O4 and ZnFe1.9Cr0.1O4 by Solid State technique ZnFe2O4 and ZnFe1.9Cr0.1O4 ferrites were synthesized via solid state technique by taking Fe2O3, CrO, ZnO as starting chemicals. Firstly, oxides were mixed together in stoichiometric ratio using agate mortar for 1hr. The mixture was then sintered at 500oC for 5hrs in muffle furnace in air
2
medium followed by cooling at room temperature. The powder so obtained was re-grinded to promote homogeneity and then re-sintered at 1000oC for 3hrs to get the final ferrite material. 2.1.3 Synthesis of ZnFe2O4 and ZnFe1.9Cr0.1O4 by Chemical co-precipitation ZnFe2O4 and ZnFe1.9Cr0.1O4 ferrites were synthesized via chemical co-precipitation technique by taking ZnCl2 (Zinc Chloride), Cr (NO3)3.9H2O (Chromium Nitrate) and Fe (NO3)3.9H2O (Ferric Nitrate) as starting chemicals. Metal salts were taken in stoichiometric ratio and their homogeneous solutions were prepared in distilled water using magnetic stirring. Oleic acid was added to avoid agglomeration of particles and to protect the particles from atmospheric oxygen. Ammonia (NH3) solution was added drop wise under constant stirring for the precipitation of ferrites. Detailed chemical co-precipitation synthesis of ferrite nano-particles has been reported in our earlier publication [11]. 2.2 XRD, D.C. and dielectric measurements As obtained powder samples were first annealed at 350 oC and then characterized for phase identification using an X-Ray Diffactometer (Panalytical X’Pert PRO) in 2 range of 20-70o ( at 2o/minute) with Cu-kα radiations (λ=1.5406Å). A part of each fine powdered sample (~0.25 g) was used to prepare cylindrical pellet (dia 13mm) by putting it in the die under a constant pressure (40 Mpa) for 5 minutes. Thereafter colloidal silver paste (SPI Spectrochem) was applied on both ends of prepared pellets (thickness ~1.4 mm) for electrode formation. D.C. conductivity measurements of these pellets were carried out using Keithley 2401 source-cum-electrometer and a programmable pot muffle furnace in the temperature range from 50400oC in order of decreasing temperature so as to remove the humidity effects in samples. Dielectric measurements of freshly prepared pellets were carried out on an impedance analyzer (HIOKI IM 35700 in the frequency range of 1 kHz to 5 MHz and temperature range of 27oC to 250oC. 2.3 Calculations The crystallite size (D) of ferrite nanoparticles prepared via different techniques was calculated from most intense peak (311) of XRD data using Debye Scherrer’s equation [12]. (1) 3
Where λ is the wavelength of Cu-kα radiation (λ=1.5406Å) and β is FWHM in radians. Lattice constant (a), X-ray density (x) and porosity (P) has been calculated using following standard relations described earlier [11]. The dielectric constant (), dielectric loss (tan) and ac conductivity (ac) were calculated by following the relations described earlier [11]. The values of Rg (grain resistance), Cg (grain capacitance), g (relaxation time) were calculated using equation (8) and (9). Cg (grain capacitance) was calculated using frequency peaks of semicircle arcs since at maximum Z= Z. (2) (3) 3. Results & discussion 3.1 X-ray diffraction and TEM X-ray diffraction analysis of samples ZCCP, ZCCCP after calcinations at 350oC (Fig 1a) clearly indicate single phase formation of prepared ferrites. Fig 1b and Fig 1c show the XRD pattern of as prepared samples ZSG, ZCSG and ZSS, ZCSS respectively. Intensity of peaks in XRD pattern of ferrite samples varies as ZCSSZCSGZCCCP. The difference in intensity is attributed to difference in calcination temperature of samples during preparation. Requirement of higher calcination temperatures in solid state technique over sol-gel and chemical co-precipitation technique can be explained as follows. During calcination reaction between different precursors takes place to obtain the final product. In chemical and sol-gel synthesis precursors are in solid forms which are dissolved in organic solvent and then they are mixed in liquid form together which leads to mixing at molecular level [13]. In solid state process precursors are mixed mechanically using agate and mortar resulting in inhomogeneous mixing. For complete reaction and homogeneous mixing it is required to diffuse different precursors through long distances among themselves and hence high calcination temperature is required in solid state reaction process [14]. The values of crystallite size of samples with uncertainty prepared through different techniques calculated using eq. (1) are reported in Table 1. It is observed that crystallite size varies as ZSSZSGZCCP. Larger value of crystallite size obtained in solid state reaction process is attributed to high calcination temperature during 4
preparation. Further crystallite size ZSGZCCP may be attributed to the fact that in chemical co-precipitation method precursor powder (obtained after drying) possesses finer particle than in sol-gel technique. As finer particles possess lager surface area and hence react at lower temperatures [15]. It is also observed that porosity follows the order ZCCPZSGZSS respectively which is also attributed to calcination temperature during preparation because high calcination temperature removes the pore contents and hence density decreases. Porosity of samples prepared via chemical co-precipitation method is greater than 70% so these materials are good for sensing applications [16]. Fig. 2 shows the TEM images of ZCCP, ZSG and ZSS. The values of average particle size of samples prepared via different techniques calculated using TEM image are reported in Table 1. These values are in good agreement with those calculated using XRD data. It is observed that uniform particle sized distribution is present in sample prepared through chemical coprecipitation method (ZCCP) as compared to sample prepared via sol-gel (ZSG) and solid state reaction (ZSS) technique. There is also change in shape of particles obtained through different techniques. Particles of sample prepared via solid state reaction technique are hexagonal in shape while in chemical co-precipitation and sol-gel technique particles are spherical and cubic respectively. 3.2 Dielectric properties 3.2.1 Dielectric constant Variation of real () and imaginary () part of dielectric constant (for ZCCP, ZSG and ZSS) with frequency at 250oC is shown in Figs. 3 and 4 respectively. It is observed that all these compositions exhibit normal dielectric behavior. High value of dielectric constant at low frequencies is possibly because of occurrence of space charge polarization on account of contact between two differently conducting surface areas [17]. An initial increase in frequency results in reduced mobility of electrons in between Fe2+ and Fe3+ ions which eliminates the surface charge polarization [18, 19] and at sufficiently higher frequencies, electronic movements are unable to synchronize with changing ac field which reduces the dielectric constant. Sample ZSG possesses very high value of dielectric constant as compared to ZCCP and ZSS. This marks the possibility of using these materials as microwave components. Effect of Cr3+ substitution on dielectric constant of ZCCP, ZSG and ZSS is shown in Table 2. It is observed that with Cr3+ 5
substitution dielectric constant decreases due to stable oxidation state of Cr3+ ions as chromium ions do not participate in conduction but hinders the electron hopping between ferrous and ferric ions. Similar type of variation with preparation techniques was observed in Cr3+ substituted samples. Fig. 5 shows the temperature dependence of (at 1MHz) for ZC prepared by CCP, SG and SS techniques. An increase in with increasing temperature as evident from Fig. 5 may be primarily due to an increase in electron hopping between Fe2+ and Fe3+ ions [20]. 3.2.2 Dielectric loss Fig. 6 shows the variation of dielectric loss with frequency for ZCCP, ZSG and ZSS samples at 250oC. The high value of tan at low frequencies is attributed to high resistivity of grain boundaries which are more effective at lower frequencies. It is because high resistivity of grain boundaries requires more energy for electron exchange between ferrous (Fe2+) and ferric (Fe3+) ions thereby maximizing energy loss. Contrary to this, at higher frequencies lesser energy is required for electron exchange which minimizes the energy loss [21]. Trends in dielectric loss are similar that in dielectric constant which can be explained in the same manner as that of dielectric constant. Effect of Cr3+ substitution on dielectric loss of ZCCP, ZSG and ZSS is shown in Table 2. There is decrease in dielectric loss with Cr3+ substitution which can be explained on same basis as that of dielectric constant. Fig. 7 shows the temperature dependence of tan at 1MHz for ZnCrxFe2-xO4 (x = 0.1) sample prepared by all the three techniques. Analysis shows that tan increases with increase in temperature which is primarily due to enhanced hopping of charge carriers. 3.2.3 ac conductivity (ac) The variation of ac with frequency at 250oC for zinc ferrites prepared by CCP, SG and SS is shown in Fig. 8. One can observe from Fig. 8 that the total conductivity increases with increase in frequency which an evidence of universal dielectric behavior. This is possibly because of increased hopping of charge carriers between Fe2+ and Fe3+ ions which support the conduction process. Sample ZCCP possesses least conductivity among all samples. This difference in conductivity can be explained on the basis of calcination temperature during preparation. The volatilization of zinc at higher temperatures leads to formation of Fe2+ ions which increase the electron hopping between Fe2+ Fe3+ and hence increase the conductivity [22, 23]. In this 6
contrast lowest conductivity of ZCCP sample may be due to lower or no zinc evaporation due to low calcination temperature. Effect of substitution of Cr3+ on ac conductivity is listed in Table 2. 3.2.4 Impedance spectroscopy To resolve the impedance due to grains and grain boundaries in dielectric materials, one have to employ impedance spectroscopy [24]. Impedance spectroscopy is carried out by analyzing ColeCole (real (Z) vs. imaginary (Z) part of impedance) plots. Figs. 9a, 9b and 9c show Cole-Cole plots of samples ZCCP, ZSG and ZSS respectively at temperatures of 200 and 250C. A close observation of these plots shows a decrease in the diameter of semicircles with increase in temperature which marks a decrease in electrical resistance with increasing temperature revealing th semi-conducting behavior. Further analysis of Cole-Cole plots of ZSG and ZSS shows that after extrapolating the Cole-Cole plots, all these semicircles merge and terminate at Z (real) axis at higher frequency side yielding bulk resistance. The values of Rg (grain resistance), Cg (grain capacitance), g (relaxation time) are calculated and shown in Table 3. It is observed that g decreases with increasing temperature and value of Rg for sample prepared via chemical co-precipitation method (ZCCP) goes out of measurement range indicating very high value of resistance. Grain resistance (Rg) calculated from Cole-Cole plots follows the trend ZCCPZSSZSG which also supports the order of variation in ac conductivity. Similar types of variation were observed for Cr3+ substituted sample prepared by all the three techniques. 3.3 dc conductivity Fig. 10 depicts the variation of dc conductivity with temperature. It is observed that dc conductivity increases with increase in temperature indicating semiconducting behavior of all the prepared compositions. This decrease in resistivity with increasing temperature can be explained on the basis that with increase in temperature crystal expands i.e. grain size increases as a result of which grain boundaries decreases which act as a barrier to electron flow and hence resistivity decreases. It may also be due to increase in mobility of charge carriers with increasing temperature which results in increase in conductivity. Activation energy was calculated using Arrhenius relation of eq. (4) [25].
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(4) Where is conductivity at absolute temperature T, 0 is conductivity at 0K, kB is Boltzmann constant, Eρ is activation energy. Effect of Cr3+ substitution on activation energy is reported in Table 1 which shows that there is increase in activation energy with chromium substitution means more energy is required for charge carriers to jump from one cationic site to other site which decreases the dc conductivity. DC conductivity for all the three samples follows the trend ZCCPZSGZSS. Sample ZCCP prepared via chemical co-precipitation possesses high value of dc conductivity which may be due to small size of particles in sample. 4. Conclusions XRD pattern of nanocrystalline ferrite samples prepared by chemical co-precipitation, sol-gel and solid state reaction techniques confirmed single phase cubic spinel structure formation. Porosity of ferrite sample synthesized by chemical co-precipitation method was found greater than 70%, so these materials are good for sensing applications. HRTEM pattern revealed that ferrite sample synthesized by chemical co-precipitation technique possesses uniform particle size distribution as compared to samples synthesized by sol-gel and solid state reaction techniques. Samples prepared through all the three techniques showed normal dielectric behavior. Dielectric constant and dielectric loss were found to increase with increase in temperature. Ferrite sample prepared through chemical co-precipitation method were found to have low value of dielectric loss and ac conductivity, making these materials suitable for use in microwave devices. Acknowledgements Authors are thankful to Coordinator, Central Instrumentation Laboratory, Deenbandhu Chhotu Ram University of Science and Technology, Murthal for providing Impedance Analyzer facility. References: [1] Kumar, S.; Farea, A. M. M.; Batoo, K. M.; Lee, C. G.; Koo, B. H.; Yousef, A. Phys. B. 2008, 403, 3604-3607. [2] Pahuja, P.; Prakash, C.; Tandon, R. P. Ceram. Int. 2014, 40, 5731-5743. [3] Hashim, M.; Alimuddin; Kumar, S.; Ali, S.; Koo, B. H.; Chung, H.; Kumar, R. J. Alloys Compd. 2012, 511, 107-114. 8
[4] Iftikhar, A.; Islam, M. U.; Awan, M. S.; Ahmad, M.; Naseem, S; Iqbal, M. A. J. Alloys Compd. 2014, 601, 116-119. [5] Zapata, A.; Herrera, G. Ceram. Int. 2013, 39, 7853-7860. [6] Wahba, A. M.; Mohamed, M. B. Ceram. Int. 2014, 40, 6127-6135. [7] Gul, I. H.; Abbasi, A. Z.; Amin, F.; Anis-ur-Rehman, M.; Maqsood, A. Ceram. Int. 2007, 311, 494-499 [8] Shrotri, J. J.; Kulkarni, S. D.; Deshpande, C. E.; Mitra, A.; Sainkar, S. R.; Kumar, A. P. S.; Date, S. K. Mater. Chem. Phys. 1999, 59 1-5. [9] Zahi, S.; Daud, A. R.; Hashim, M. Mater. Chem. Phys. 2007, 106, 452-456. [10] Jacob, B. P.; Kumar, A.; Pant, R. P.; Singh, S.; Mohammed, E. M. Bull. Mater. Sci. 2011, 34, 1345-1350. [11] Kumari, N.; Kumar, V.; Singh, S. K. Ceram. Int. 2014, 40, 12199-12205 [12] Cullity, B. D. Elements of X-ray Diffraction; Addision-Wesely, USA, 1978. [13] Fu, L. J.; Liu, H.; Li, C.; Wu, Y. P.; Rahm, E.; Holze, R.; Wu, H. Q. Prog. Mater. Sci. 2005, 50, 881-928. [14] Baranauskas, A.; Jasaitis, D.; Kareiva, A. Vibr. Spectroscopy 2002, 28, 263-275. [15] Palkar, V. R.; Guptasarma, P.; Multani, M. S.; Vijayaraghavan, R. Mater. Lett. 1991, 11, 199-206. [16] Sutka, A.; Mezinsskis, G.; Lusis, A.; Stingaciu, M. Sensors and Actuators B: Chemical 2012, 171-172, 354-360. [17] Wagner, K. W. Ann. Phys. 1973, 40, 817-819. [18] Patange, S. M.; Shirsath, S. E.; Lohar, K. S.; Jadhav, S. S.; Kulkarni, N.; Jadhav, K. M. Phys. B 2011, 406 663. [19] Tan, M.; Köseoğlu, Y.; Alan, F.; Sentürk, E. J. Alloys Compd. 2011, 509 9399-9405. [20] Kumari, N.; Kumar, V.; Singh, S. K. J. Alloys. Compd. 2014, 622 628-634 [21] Jnaneshwara, D. M.; Avadhani, D. N.; Prasad, B. D.; Nagabhushana, B. M.; Nagabhushana, H.; Sharma, S. C.; Prashantha, S. C.; Shivakumara, C. J. Alloys Compd. 2014, 587, 50-58. [22] Verma, A.; Chatterjee, R. J. Magn. Magn. Mater. 2006, 306, 313-320. [23] El-Sayed, A. M. Mater. Chem. Phys. 2003, 82 583-587. [24] Irvine, J. T. S.; Sinclair, D. C.; West, A. R. Adv. Mater. 1990, 2, 132-138.
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[25] Soft Ferrites, Properties and applications, 2nd ed.; Ed. Snelling E. C. Butterworth and Co. Ltd: London, 1988.
Figure Captions: Fig. 1 (a) X-ray diffraction pattern of Z-CCP and ZC-CCP calcined at 350C. (b) X-ray diffraction pattern of Z-SG and ZC-SG. (c) X-ray diffraction pattern of Z-SS and ZC-SS. Fig. 2 HRTEM micrographs of Z-CCP, Z-SG and Z-SS. Fig. 3 Variation of real part of dielectric constant () of Z-CCP, Z-SG and Z-SS samples with frequency at 250C. Fig. 4 Variation of imaginary part of dielectric constant (’) of Z-CCP, Z-SG and Z-SS samples with frequency at 250C. Fig. 5 Variation of ε with temperature for ZnCrxFe2-xO4 (x=0.1) sample prepared by CCP, SG and SS techniques at 1MHz Fig. 6 Variation of dielectric loss (tan) of Z-CCP, Z-SG and Z-SS samples with frequency at 250C. Fig. 7 Variation of dielectric loss (tan) with temperature for ZnCrxFe2-xO4 (x=0.1) sample prepared by CCP, SG and SS techniques at 1MHz
Fig. 8 Variation of ac conductivity (ac) of Z-CCP, Z-SG and Z-SS samples with frequency at 250C. Fig. 9 (a) Cole-Cole plot for Z-CCP. (b) Cole-Cole plot for Z-SG. (c) Cole-Cole plot for Z-SS. at different temperatures i.e. 200C and 250C. Fig. 10 Variation of DC conductivity with temperature for Z-CCP, Z-SG and Z-SS samples (Solid lines are least curve fit for portion used to calculate activation energy).
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Table 1: Crystallite size (D), average particle size calculated from TEM images (DTEM), lattice constant (a), X-ray density (ρx), porosity (P) and activation energy (E) for ZnCrxFe2-xO4 (x=0, 0.1) prepared by CCP, SG and SS techniques. Parameter
CCP
SG
SS
x=0.0
x=0.1
x=0.0
x=0.1
x=0.0
x=0.1
D (nm)
12(0.9)
28(1.7)
18(1.2)
16(1.1)
59(3.0)
37(1.8)
DTEM (nm)
(15-25)
(30-45)
(20-100)
(17-90)
(70-150)
(45-120)
a (Å)
8.457
8.451
8.357
8.331
8.377
8.365
ρx (gm/cc)
5.51
5.29
5.48
5.53
5.50
5.54
P
0.70
0.71
0.58
0.60
0.45
0.47
E (eV)
0.13
0.21
0.25
0.39
0.32
0.41
Crystallite size uncertainty is written in parentheses
Table 2: ac conductivity (ac), dielectric loss (tan), dielectric constant () at 2500C for ZnFe2O4, ZnCr0.1Fe1.9O4 sample prepared by CCP, SG and SS techniques.
Sample
ac ( ohm-1m-1)
tan
ID 1kHz
100kHz 1MHz 1kHz
100kHz
1MHz
1.3x10-5 3.8x10-5
0.8
0.22
0.07
25.9
10.7
8.6
ZC-CCP 8.8x10-6
3.8x10-5 1.5x10-4
2.8
0.43
0.23
57.1
15.7
11.1
Z-SG
2.9x10-4
3.9x10-4 7.7x10-4
22
5.01
0.76
202
33.1
17.3
ZC-SG
1.6x10-4
3.3x10-4 3.0x10-3
20
1.67
0.66
152
107.6
82.8
Z-SS
1.9x10-5
7.0x10-5 1.4x10-4
3.3
0.43
0.11
88.5
28.0
21.8
ZC-SS
8.1x10-6
1.6x10-4 7.4x10-4
2.5
0.67
0.35
68.5
45.4
38.2
Z-CCP
1kHz
100kHz
1.4x10-6
1MHz
11
Table 3: Relaxation time (g), grain resistance (Rg) and grain capacitance (Cg). Sample
Z-CCP
Z-SG
Z-SS
250 0C
200 0C
250 0C
200 0C
250 0C
200 0C
Rg (k)
-
-
19.41
48.67
753.51
3011.19
Cg (pF)
-
-
14.08
14.40
13.50
14.50
g (sec)
-
-
2.735x10-7
7.046x10-7
1.01x10-5
4.38x10-5
Fig. 1(a)
12
Fig. 1(b)
Fig 1(c)
13
Fig. 2
14
Fig. 3 15
Fig. 4
16
Fig. 5
17
Fig. 6
18
Fig. 7
19
Fig. 8
20
Fig. 9(a)
21
Fig. 9 (b)
22
Fig. 9(c)
23
Fig. 10
24
HIGHLIGHTS
Ferrites are successfully prepared by solid state, sol-gel and co-precipitation. Single phase cubic spinel structure formation is confirmed by XRD. Ferrites prepared by co-precipitation are porous, hence suitable for sensing. TEM confirms uniform size formation of particles prepared by co-precipitation. Samples prepared by co-precipitation are suitable for microwave devices.
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www.elsevier.com/locate/physe
PII: DOI: Reference:
S1386-9477(16)30241-7 http://dx.doi.org/10.1016/j.physe.2016.10.007 PHYSE12596
To appear in: Physica E: Low-dimensional Systems and Nanostructures Received date: 17 April 2016 Revised date: 27 September 2016 Accepted date: 9 October 2016 Cite this article as: N. Kumari and M.S. Dahiya, Synthesis modified structural and dielectric properties of semiconducting zinc ferrospinels, Physica E: Lowdimensional Systems and Nanostructures, http://dx.doi.org/10.1016/j.physe.2016.10.007 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.
Synthesis modified structural and dielectric properties of semiconducting zinc ferrospinels N. Kumari, M. S. Dahiya* Physics Department, DCR University of Science and Technology, Murthal -131039 *[email protected] Abstract The influence of preparation techniques on structural and dielectric properties of ZnCrxFe1-xO4 (x=0, 0.1 abbreviated as Z and ZC) ferrite nano-particles synthesized using chemical coprecipitation (CCP), sol-gel (SG) and solid state reaction (SS) techniques is discussed. XRD profiles are used to confirm the single phase spinel ferrite formation. TEM images indicate the change in size and shape of particles on changing either the composition or the synthesis methodology. The TEM micrograph of samples obtained through CCP shows uniform particle size formation compared to those obtained through SG and SS. Sample prepared through CCP possess porosity >70% making these materials suitable for sensing applications. The dielectric loss, dielectric constant and ac conductivity are analyzed as a function of frequency, temperature and composition using impedance spectroscopy. A universal dielectric response and standard dielectric behavior has been reveled by temperature and frequency variations of each dielectric parameter. Dielectric constant is found to possess highest value for sample synthesized through SG which marks the possibility of using the SG derived ferrospinels as microwave device components. Keywords: Magnetic materials; chemical synthesis; impedance spectroscopy; dielectric properties 1. Introduction Nanocrystalline ferrite materials have achieved a primary position of economic and engineering importance within the family of magnetic materials because of their excellent physical and dielectric properties as compared to their bulk counterpart. Physical and dielectric properties of ferrite materials are highly influenced by size, shape of grains, grain boundaries, porosity, composition and preparation techniques [13]. The possibility of preparing ferrite nano-particles 1
has opened a new and interesting research field with revolutionary applications [4]. A remarkable characteristic of spinel ferrites is that their composition and properties can be strongly modified depending upon the applications keeping the basic crystalline structure unaltered. Different techniques like sol-gel, co-precipitation, solid-state, hydrothermal and microwave combustion methods [57] are used to prepare ferrite nanoparticles. A well known problem in synthesis of Znferrite prepared through solid state reaction method is loss of zinc content by evaporation due to high calcination temperature and long time during preparation which results in non-stoichiometric ferrite system [8, 9]. Hence, the challenge is to obtain high quality reproducible ferrites by selecting appropriate method for their preparation. In the present report, we have synthesized nanocrystalline Zn-ferrites and Cr3+substituted Znferrites via three different synthesis methods viz. chemical co-precipitation, sol-gel and solid state reaction. Effect of preparation technique on structural, micro structural and dielectric parameters has been studied and reported. 2. Experimental procedure 2.1 Preparation of materials 2.1.1 Synthesis of ZnFe2O4 and ZnFe1.9Cr0.1O4 by sol-gel technique For synthesis of ZnFe2O4 and ZnFe1.9Cr0.1O4 via sol-gel technique analytical grade ZnCl2 (Zinc Chloride), Cr (NO3)3.9H2O (Chromium Nitrate) and Fe(NO3)3.9H2O (Ferric Nitrate) were used as initial chemicals. Stoichiometric amounts of required metal salts were mixed separately to a gelating agent Ethylene Glycol in molar ratio 1:3 and then were mixed together using constant stirring and heating at 80oC. During heating solution takes the form of a viscous gel and finally became puffy porous mass. These puffy powders were then heated at 300oC to remove excess ethylene glycol and to obtain the final ferrite powder [10]. 2.1.2 Synthesis of ZnFe2O4 and ZnFe1.9Cr0.1O4 by Solid State technique ZnFe2O4 and ZnFe1.9Cr0.1O4 ferrites were synthesized via solid state technique by taking Fe2O3, CrO, ZnO as starting chemicals. Firstly, oxides were mixed together in stoichiometric ratio using agate mortar for 1hr. The mixture was then sintered at 500oC for 5hrs in muffle furnace in air
2
medium followed by cooling at room temperature. The powder so obtained was re-grinded to promote homogeneity and then re-sintered at 1000oC for 3hrs to get the final ferrite material. 2.1.3 Synthesis of ZnFe2O4 and ZnFe1.9Cr0.1O4 by Chemical co-precipitation ZnFe2O4 and ZnFe1.9Cr0.1O4 ferrites were synthesized via chemical co-precipitation technique by taking ZnCl2 (Zinc Chloride), Cr (NO3)3.9H2O (Chromium Nitrate) and Fe (NO3)3.9H2O (Ferric Nitrate) as starting chemicals. Metal salts were taken in stoichiometric ratio and their homogeneous solutions were prepared in distilled water using magnetic stirring. Oleic acid was added to avoid agglomeration of particles and to protect the particles from atmospheric oxygen. Ammonia (NH3) solution was added drop wise under constant stirring for the precipitation of ferrites. Detailed chemical co-precipitation synthesis of ferrite nano-particles has been reported in our earlier publication [11]. 2.2 XRD, D.C. and dielectric measurements As obtained powder samples were first annealed at 350 oC and then characterized for phase identification using an X-Ray Diffactometer (Panalytical X’Pert PRO) in 2 range of 20-70o ( at 2o/minute) with Cu-kα radiations (λ=1.5406Å). A part of each fine powdered sample (~0.25 g) was used to prepare cylindrical pellet (dia 13mm) by putting it in the die under a constant pressure (40 Mpa) for 5 minutes. Thereafter colloidal silver paste (SPI Spectrochem) was applied on both ends of prepared pellets (thickness ~1.4 mm) for electrode formation. D.C. conductivity measurements of these pellets were carried out using Keithley 2401 source-cum-electrometer and a programmable pot muffle furnace in the temperature range from 50400oC in order of decreasing temperature so as to remove the humidity effects in samples. Dielectric measurements of freshly prepared pellets were carried out on an impedance analyzer (HIOKI IM 35700 in the frequency range of 1 kHz to 5 MHz and temperature range of 27oC to 250oC. 2.3 Calculations The crystallite size (D) of ferrite nanoparticles prepared via different techniques was calculated from most intense peak (311) of XRD data using Debye Scherrer’s equation [12]. (1) 3
Where λ is the wavelength of Cu-kα radiation (λ=1.5406Å) and β is FWHM in radians. Lattice constant (a), X-ray density (x) and porosity (P) has been calculated using following standard relations described earlier [11]. The dielectric constant (), dielectric loss (tan) and ac conductivity (ac) were calculated by following the relations described earlier [11]. The values of Rg (grain resistance), Cg (grain capacitance), g (relaxation time) were calculated using equation (8) and (9). Cg (grain capacitance) was calculated using frequency peaks of semicircle arcs since at maximum Z= Z. (2) (3) 3. Results & discussion 3.1 X-ray diffraction and TEM X-ray diffraction analysis of samples ZCCP, ZCCCP after calcinations at 350oC (Fig 1a) clearly indicate single phase formation of prepared ferrites. Fig 1b and Fig 1c show the XRD pattern of as prepared samples ZSG, ZCSG and ZSS, ZCSS respectively. Intensity of peaks in XRD pattern of ferrite samples varies as ZCSSZCSGZCCCP. The difference in intensity is attributed to difference in calcination temperature of samples during preparation. Requirement of higher calcination temperatures in solid state technique over sol-gel and chemical co-precipitation technique can be explained as follows. During calcination reaction between different precursors takes place to obtain the final product. In chemical and sol-gel synthesis precursors are in solid forms which are dissolved in organic solvent and then they are mixed in liquid form together which leads to mixing at molecular level [13]. In solid state process precursors are mixed mechanically using agate and mortar resulting in inhomogeneous mixing. For complete reaction and homogeneous mixing it is required to diffuse different precursors through long distances among themselves and hence high calcination temperature is required in solid state reaction process [14]. The values of crystallite size of samples with uncertainty prepared through different techniques calculated using eq. (1) are reported in Table 1. It is observed that crystallite size varies as ZSSZSGZCCP. Larger value of crystallite size obtained in solid state reaction process is attributed to high calcination temperature during 4
preparation. Further crystallite size ZSGZCCP may be attributed to the fact that in chemical co-precipitation method precursor powder (obtained after drying) possesses finer particle than in sol-gel technique. As finer particles possess lager surface area and hence react at lower temperatures [15]. It is also observed that porosity follows the order ZCCPZSGZSS respectively which is also attributed to calcination temperature during preparation because high calcination temperature removes the pore contents and hence density decreases. Porosity of samples prepared via chemical co-precipitation method is greater than 70% so these materials are good for sensing applications [16]. Fig. 2 shows the TEM images of ZCCP, ZSG and ZSS. The values of average particle size of samples prepared via different techniques calculated using TEM image are reported in Table 1. These values are in good agreement with those calculated using XRD data. It is observed that uniform particle sized distribution is present in sample prepared through chemical coprecipitation method (ZCCP) as compared to sample prepared via sol-gel (ZSG) and solid state reaction (ZSS) technique. There is also change in shape of particles obtained through different techniques. Particles of sample prepared via solid state reaction technique are hexagonal in shape while in chemical co-precipitation and sol-gel technique particles are spherical and cubic respectively. 3.2 Dielectric properties 3.2.1 Dielectric constant Variation of real () and imaginary () part of dielectric constant (for ZCCP, ZSG and ZSS) with frequency at 250oC is shown in Figs. 3 and 4 respectively. It is observed that all these compositions exhibit normal dielectric behavior. High value of dielectric constant at low frequencies is possibly because of occurrence of space charge polarization on account of contact between two differently conducting surface areas [17]. An initial increase in frequency results in reduced mobility of electrons in between Fe2+ and Fe3+ ions which eliminates the surface charge polarization [18, 19] and at sufficiently higher frequencies, electronic movements are unable to synchronize with changing ac field which reduces the dielectric constant. Sample ZSG possesses very high value of dielectric constant as compared to ZCCP and ZSS. This marks the possibility of using these materials as microwave components. Effect of Cr3+ substitution on dielectric constant of ZCCP, ZSG and ZSS is shown in Table 2. It is observed that with Cr3+ 5
substitution dielectric constant decreases due to stable oxidation state of Cr3+ ions as chromium ions do not participate in conduction but hinders the electron hopping between ferrous and ferric ions. Similar type of variation with preparation techniques was observed in Cr3+ substituted samples. Fig. 5 shows the temperature dependence of (at 1MHz) for ZC prepared by CCP, SG and SS techniques. An increase in with increasing temperature as evident from Fig. 5 may be primarily due to an increase in electron hopping between Fe2+ and Fe3+ ions [20]. 3.2.2 Dielectric loss Fig. 6 shows the variation of dielectric loss with frequency for ZCCP, ZSG and ZSS samples at 250oC. The high value of tan at low frequencies is attributed to high resistivity of grain boundaries which are more effective at lower frequencies. It is because high resistivity of grain boundaries requires more energy for electron exchange between ferrous (Fe2+) and ferric (Fe3+) ions thereby maximizing energy loss. Contrary to this, at higher frequencies lesser energy is required for electron exchange which minimizes the energy loss [21]. Trends in dielectric loss are similar that in dielectric constant which can be explained in the same manner as that of dielectric constant. Effect of Cr3+ substitution on dielectric loss of ZCCP, ZSG and ZSS is shown in Table 2. There is decrease in dielectric loss with Cr3+ substitution which can be explained on same basis as that of dielectric constant. Fig. 7 shows the temperature dependence of tan at 1MHz for ZnCrxFe2-xO4 (x = 0.1) sample prepared by all the three techniques. Analysis shows that tan increases with increase in temperature which is primarily due to enhanced hopping of charge carriers. 3.2.3 ac conductivity (ac) The variation of ac with frequency at 250oC for zinc ferrites prepared by CCP, SG and SS is shown in Fig. 8. One can observe from Fig. 8 that the total conductivity increases with increase in frequency which an evidence of universal dielectric behavior. This is possibly because of increased hopping of charge carriers between Fe2+ and Fe3+ ions which support the conduction process. Sample ZCCP possesses least conductivity among all samples. This difference in conductivity can be explained on the basis of calcination temperature during preparation. The volatilization of zinc at higher temperatures leads to formation of Fe2+ ions which increase the electron hopping between Fe2+ Fe3+ and hence increase the conductivity [22, 23]. In this 6
contrast lowest conductivity of ZCCP sample may be due to lower or no zinc evaporation due to low calcination temperature. Effect of substitution of Cr3+ on ac conductivity is listed in Table 2. 3.2.4 Impedance spectroscopy To resolve the impedance due to grains and grain boundaries in dielectric materials, one have to employ impedance spectroscopy [24]. Impedance spectroscopy is carried out by analyzing ColeCole (real (Z) vs. imaginary (Z) part of impedance) plots. Figs. 9a, 9b and 9c show Cole-Cole plots of samples ZCCP, ZSG and ZSS respectively at temperatures of 200 and 250C. A close observation of these plots shows a decrease in the diameter of semicircles with increase in temperature which marks a decrease in electrical resistance with increasing temperature revealing th semi-conducting behavior. Further analysis of Cole-Cole plots of ZSG and ZSS shows that after extrapolating the Cole-Cole plots, all these semicircles merge and terminate at Z (real) axis at higher frequency side yielding bulk resistance. The values of Rg (grain resistance), Cg (grain capacitance), g (relaxation time) are calculated and shown in Table 3. It is observed that g decreases with increasing temperature and value of Rg for sample prepared via chemical co-precipitation method (ZCCP) goes out of measurement range indicating very high value of resistance. Grain resistance (Rg) calculated from Cole-Cole plots follows the trend ZCCPZSSZSG which also supports the order of variation in ac conductivity. Similar types of variation were observed for Cr3+ substituted sample prepared by all the three techniques. 3.3 dc conductivity Fig. 10 depicts the variation of dc conductivity with temperature. It is observed that dc conductivity increases with increase in temperature indicating semiconducting behavior of all the prepared compositions. This decrease in resistivity with increasing temperature can be explained on the basis that with increase in temperature crystal expands i.e. grain size increases as a result of which grain boundaries decreases which act as a barrier to electron flow and hence resistivity decreases. It may also be due to increase in mobility of charge carriers with increasing temperature which results in increase in conductivity. Activation energy was calculated using Arrhenius relation of eq. (4) [25].
7
(4) Where is conductivity at absolute temperature T, 0 is conductivity at 0K, kB is Boltzmann constant, Eρ is activation energy. Effect of Cr3+ substitution on activation energy is reported in Table 1 which shows that there is increase in activation energy with chromium substitution means more energy is required for charge carriers to jump from one cationic site to other site which decreases the dc conductivity. DC conductivity for all the three samples follows the trend ZCCPZSGZSS. Sample ZCCP prepared via chemical co-precipitation possesses high value of dc conductivity which may be due to small size of particles in sample. 4. Conclusions XRD pattern of nanocrystalline ferrite samples prepared by chemical co-precipitation, sol-gel and solid state reaction techniques confirmed single phase cubic spinel structure formation. Porosity of ferrite sample synthesized by chemical co-precipitation method was found greater than 70%, so these materials are good for sensing applications. HRTEM pattern revealed that ferrite sample synthesized by chemical co-precipitation technique possesses uniform particle size distribution as compared to samples synthesized by sol-gel and solid state reaction techniques. Samples prepared through all the three techniques showed normal dielectric behavior. Dielectric constant and dielectric loss were found to increase with increase in temperature. Ferrite sample prepared through chemical co-precipitation method were found to have low value of dielectric loss and ac conductivity, making these materials suitable for use in microwave devices. Acknowledgements Authors are thankful to Coordinator, Central Instrumentation Laboratory, Deenbandhu Chhotu Ram University of Science and Technology, Murthal for providing Impedance Analyzer facility. References: [1] Kumar, S.; Farea, A. M. M.; Batoo, K. M.; Lee, C. G.; Koo, B. H.; Yousef, A. Phys. B. 2008, 403, 3604-3607. [2] Pahuja, P.; Prakash, C.; Tandon, R. P. Ceram. Int. 2014, 40, 5731-5743. [3] Hashim, M.; Alimuddin; Kumar, S.; Ali, S.; Koo, B. H.; Chung, H.; Kumar, R. J. Alloys Compd. 2012, 511, 107-114. 8
[4] Iftikhar, A.; Islam, M. U.; Awan, M. S.; Ahmad, M.; Naseem, S; Iqbal, M. A. J. Alloys Compd. 2014, 601, 116-119. [5] Zapata, A.; Herrera, G. Ceram. Int. 2013, 39, 7853-7860. [6] Wahba, A. M.; Mohamed, M. B. Ceram. Int. 2014, 40, 6127-6135. [7] Gul, I. H.; Abbasi, A. Z.; Amin, F.; Anis-ur-Rehman, M.; Maqsood, A. Ceram. Int. 2007, 311, 494-499 [8] Shrotri, J. J.; Kulkarni, S. D.; Deshpande, C. E.; Mitra, A.; Sainkar, S. R.; Kumar, A. P. S.; Date, S. K. Mater. Chem. Phys. 1999, 59 1-5. [9] Zahi, S.; Daud, A. R.; Hashim, M. Mater. Chem. Phys. 2007, 106, 452-456. [10] Jacob, B. P.; Kumar, A.; Pant, R. P.; Singh, S.; Mohammed, E. M. Bull. Mater. Sci. 2011, 34, 1345-1350. [11] Kumari, N.; Kumar, V.; Singh, S. K. Ceram. Int. 2014, 40, 12199-12205 [12] Cullity, B. D. Elements of X-ray Diffraction; Addision-Wesely, USA, 1978. [13] Fu, L. J.; Liu, H.; Li, C.; Wu, Y. P.; Rahm, E.; Holze, R.; Wu, H. Q. Prog. Mater. Sci. 2005, 50, 881-928. [14] Baranauskas, A.; Jasaitis, D.; Kareiva, A. Vibr. Spectroscopy 2002, 28, 263-275. [15] Palkar, V. R.; Guptasarma, P.; Multani, M. S.; Vijayaraghavan, R. Mater. Lett. 1991, 11, 199-206. [16] Sutka, A.; Mezinsskis, G.; Lusis, A.; Stingaciu, M. Sensors and Actuators B: Chemical 2012, 171-172, 354-360. [17] Wagner, K. W. Ann. Phys. 1973, 40, 817-819. [18] Patange, S. M.; Shirsath, S. E.; Lohar, K. S.; Jadhav, S. S.; Kulkarni, N.; Jadhav, K. M. Phys. B 2011, 406 663. [19] Tan, M.; Köseoğlu, Y.; Alan, F.; Sentürk, E. J. Alloys Compd. 2011, 509 9399-9405. [20] Kumari, N.; Kumar, V.; Singh, S. K. J. Alloys. Compd. 2014, 622 628-634 [21] Jnaneshwara, D. M.; Avadhani, D. N.; Prasad, B. D.; Nagabhushana, B. M.; Nagabhushana, H.; Sharma, S. C.; Prashantha, S. C.; Shivakumara, C. J. Alloys Compd. 2014, 587, 50-58. [22] Verma, A.; Chatterjee, R. J. Magn. Magn. Mater. 2006, 306, 313-320. [23] El-Sayed, A. M. Mater. Chem. Phys. 2003, 82 583-587. [24] Irvine, J. T. S.; Sinclair, D. C.; West, A. R. Adv. Mater. 1990, 2, 132-138.
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[25] Soft Ferrites, Properties and applications, 2nd ed.; Ed. Snelling E. C. Butterworth and Co. Ltd: London, 1988.
Figure Captions: Fig. 1 (a) X-ray diffraction pattern of Z-CCP and ZC-CCP calcined at 350C. (b) X-ray diffraction pattern of Z-SG and ZC-SG. (c) X-ray diffraction pattern of Z-SS and ZC-SS. Fig. 2 HRTEM micrographs of Z-CCP, Z-SG and Z-SS. Fig. 3 Variation of real part of dielectric constant () of Z-CCP, Z-SG and Z-SS samples with frequency at 250C. Fig. 4 Variation of imaginary part of dielectric constant (’) of Z-CCP, Z-SG and Z-SS samples with frequency at 250C. Fig. 5 Variation of ε with temperature for ZnCrxFe2-xO4 (x=0.1) sample prepared by CCP, SG and SS techniques at 1MHz Fig. 6 Variation of dielectric loss (tan) of Z-CCP, Z-SG and Z-SS samples with frequency at 250C. Fig. 7 Variation of dielectric loss (tan) with temperature for ZnCrxFe2-xO4 (x=0.1) sample prepared by CCP, SG and SS techniques at 1MHz
Fig. 8 Variation of ac conductivity (ac) of Z-CCP, Z-SG and Z-SS samples with frequency at 250C. Fig. 9 (a) Cole-Cole plot for Z-CCP. (b) Cole-Cole plot for Z-SG. (c) Cole-Cole plot for Z-SS. at different temperatures i.e. 200C and 250C. Fig. 10 Variation of DC conductivity with temperature for Z-CCP, Z-SG and Z-SS samples (Solid lines are least curve fit for portion used to calculate activation energy).
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Table 1: Crystallite size (D), average particle size calculated from TEM images (DTEM), lattice constant (a), X-ray density (ρx), porosity (P) and activation energy (E) for ZnCrxFe2-xO4 (x=0, 0.1) prepared by CCP, SG and SS techniques. Parameter
CCP
SG
SS
x=0.0
x=0.1
x=0.0
x=0.1
x=0.0
x=0.1
D (nm)
12(0.9)
28(1.7)
18(1.2)
16(1.1)
59(3.0)
37(1.8)
DTEM (nm)
(15-25)
(30-45)
(20-100)
(17-90)
(70-150)
(45-120)
a (Å)
8.457
8.451
8.357
8.331
8.377
8.365
ρx (gm/cc)
5.51
5.29
5.48
5.53
5.50
5.54
P
0.70
0.71
0.58
0.60
0.45
0.47
E (eV)
0.13
0.21
0.25
0.39
0.32
0.41
Crystallite size uncertainty is written in parentheses
Table 2: ac conductivity (ac), dielectric loss (tan), dielectric constant () at 2500C for ZnFe2O4, ZnCr0.1Fe1.9O4 sample prepared by CCP, SG and SS techniques.
Sample
ac ( ohm-1m-1)
tan
ID 1kHz
100kHz 1MHz 1kHz
100kHz
1MHz
1.3x10-5 3.8x10-5
0.8
0.22
0.07
25.9
10.7
8.6
ZC-CCP 8.8x10-6
3.8x10-5 1.5x10-4
2.8
0.43
0.23
57.1
15.7
11.1
Z-SG
2.9x10-4
3.9x10-4 7.7x10-4
22
5.01
0.76
202
33.1
17.3
ZC-SG
1.6x10-4
3.3x10-4 3.0x10-3
20
1.67
0.66
152
107.6
82.8
Z-SS
1.9x10-5
7.0x10-5 1.4x10-4
3.3
0.43
0.11
88.5
28.0
21.8
ZC-SS
8.1x10-6
1.6x10-4 7.4x10-4
2.5
0.67
0.35
68.5
45.4
38.2
Z-CCP
1kHz
100kHz
1.4x10-6
1MHz
11
Table 3: Relaxation time (g), grain resistance (Rg) and grain capacitance (Cg). Sample
Z-CCP
Z-SG
Z-SS
250 0C
200 0C
250 0C
200 0C
250 0C
200 0C
Rg (k)
-
-
19.41
48.67
753.51
3011.19
Cg (pF)
-
-
14.08
14.40
13.50
14.50
g (sec)
-
-
2.735x10-7
7.046x10-7
1.01x10-5
4.38x10-5
Fig. 1(a)
12
Fig. 1(b)
Fig 1(c)
13
Fig. 2
14
Fig. 3 15
Fig. 4
16
Fig. 5
17
Fig. 6
18
Fig. 7
19
Fig. 8
20
Fig. 9(a)
21
Fig. 9 (b)
22
Fig. 9(c)
23
Fig. 10
24
HIGHLIGHTS
Ferrites are successfully prepared by solid state, sol-gel and co-precipitation. Single phase cubic spinel structure formation is confirmed by XRD. Ferrites prepared by co-precipitation are porous, hence suitable for sensing. TEM confirms uniform size formation of particles prepared by co-precipitation. Samples prepared by co-precipitation are suitable for microwave devices.
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