20-12-2012, 05:37 PM
Thermogravimetric and magneticproperties of Ni1-XZnxFe2O4 nanoparticles synthesized by coprecipitation
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ABSTRACT
Ni1-xZnxFe2O4 (x = 0 to 1) nanoparticles of size less than 9 nm were prepared by a chemical coprecipitation method which could be used for ferrofluid preparation. XRD, VSM and DTA-TG (STA) were used to study the effect of variation in Zn substitution and its influence on particle size, magnetic properties such as MS, HC and Curie temperature, as well as on the water content. ICP was used to estimate Ni, Zn and Fe concentrations. The average crystallite size (DaveXR) of the particles was found to decrease from 8.95 to 6.92 nm with increasing zinc substitution. The lattice constant (ao) increased with increasing zinc substitution. The specific saturation magnetization (MS) of the particles was measured at room temperature. Magnetic parameters such as MS, Hc, and Mr were found to decrease with increasing zinc substitution. Estimation of the water content, which varies the Zn concentration, plays a vital role for the correct determination of cation contents. The Curie temperature was found to decrease with increasing zinc substitution.
Introduction
Magnetic nanoparticles are of great technological importance because of their use in magnetic fluids, information storage systems, medical diagnostics etc. Various preparation techniques have been used for the synthesis of fine particles of ferrites, which exhibit novel properties when compared to their properties in the bulk. Non-conventional methods such as coprecipitation, thermal decomposition, sol-gel and hydrothermal methods have been widely used. Ultrafine ferrite particles can be prepared by the chemical coprecipitation method. Auzans et al.1,2 have studied the preparation and properties of Mn-Zn ferrite nanoparticles, which were used in ionic and surfacted ferrofluids with different degrees of Zn substitution prepared by the coprecipitation method. Chandana Rath et al.3 have reported the dependence on cation distribution of crystallite size, lattice parameter and magnetic properties in nanosized Mn-Zn ferrite for different degrees of Zn substitution prepared by hydrothermal precipitation method. The use of Mn-Zn ferrite for the preparation of temperature sensitive magnetic fluids by the coprecipitation method has already been studied4-6. Ni0.2Zn0.8Fe2O4 fine particles have been prepared by chemical coprecipitation method followed by sintering7. Control of crystallite size in the nanometer range by variation of synthesis condition is always a difficult task and becomes mandatory in the case of ferrofluid preparation using the coprecipitation method. In order to prepare ferrofluids having such fine particles, a specific size restriction is imposed considering the stability criteria. Ni1-xZnxFe2O4 substituted ferrites with x varying from 0 to 1.0 prepared by the coprecipitation method have not yet been fully studied like Mn-Zn substituted ferrites. In this paper we report preparation of Me1-xZnxFe2O4 fine particles, where Me = Ni2+ with x varying from 0 to 1.0 with crystallite size less than 13 nm by the chemical coprecipitation method and the consequent change in magnetic properties, thermomagnetic coefficient, lattice parameter, particle size and associated water content due to zinc substitution.
2. Synthesis and Characterization of Ni1-xZnxFe2O4 Nanoparticles
The magnetization of substituted ferrite nanoparticles synthesized by coprecipitation depends mostly on parameters such as reaction temperature, pH of the suspension, initial molar concentration etc. 4. Ultrafine particles of Ni1-xZnxFe2O4 with x varying from 0 to 1.0 were prepared by coprecipitating aqueous solutions of NiCl2, ZnCl2 and FeCl3 mixtures in an alkaline medium. The mixed solution of NiCl2, ZnCl2 and FeCl3 in their respective stoichiometry (100 mL of 0.5 M CoCl2, 100 mL of 0.5 M ZnCl2 and 100 mL of 2 M FeCl3 in the case of Ni0.5Zn0.5Fe2O4 and similarly for the other values of x) was prepared and kept at 333 K (60 ºC). This mixture was added to a boiling solution of NaOH (0.63 M dissolved in 1200 mL of distilled water) within 10 s under constant stirring. Nanoferrites are formed by conversion of metal salts into hydroxides, which take place immediately, followed by transformation of hydroxides into ferrites. The solutions were maintained at 358 K (85 ºC) for 1 hr. This time was sufficient for the transformation of hydroxides into spinel ferrite (dehydration and atomic rearrangement involved in the conversion of intermediate hydroxide phase into ferrite)4. A sufficient amount of fine particles was collected at this stage using magnetic separation. These particles were washed several times with distilled water followed by acetone and dried at room temperature.
The X-ray diffraction (XRD) patterns of the samples were recorded on a Philips® PANALYTICAL X' PERT PRO X-ray powder diffractometer using Cu K∝(λ=1.54060 Å) radiation. Slow scans of the selected diffraction peaks were carried out in the step mode (step size 0.05º, measurement time 5 s, measurement temperature 323 K (25 ºC), standard: Si powder). The crystallite size of the nanocrystalline samples was measured from the X-ray line broadening using the Debye- Scherrer formula after accounting for instrumental broadening,