Journal of Materials Research and Technology Journal of Materials Research and Technology
J Mater Res Technol 2018;7:350-5 DOI: 10.1016/j.jmrt.2018.04.012
Original Article
Magnetic filter produced by ZnFe2O4 nanoparticles using freeze casting
Letícia dos Santos Aguileraa,, , Rubens Lincoln Santana Blazutti Marçala, José Brant de Camposb, Marcelo Henrique Prado da Silvaa, André Ben-Hur da Silva Figueiredoa
a Military Institute of Engineering – IME, Mechanical and Materials Engineering Department, Sq General Tibúrcio, 80, Praia Vermelha, Rio de Janeiro, RJ, Brazil
b State University of Rio de Janeiro – UERJ, Mechanical Engineering Department, São Francisco Xavier St, 524, Maracanã, Rio de Janeiro, RJ, Brazil
Received 20 November 2017, Accepted 24 April 2018
Abstract

Zinc ferrite magnetic nanoparticles were synthesized by combustion method and the obtained powders were processed by the freeze casting technique, to produce magnetic filters. Green bodies were obtained by freezing a water-based slurry of nanoparticles and polyethylene glycol (PEG) of molecular mass 8000. The suspension was frozen to −140°C under the cooling rate of 10°C/min. The obtained bodies were sintered at 1300°C during 1h and the X-ray diffraction (XRD) analyses showed ferrite as the only crystalline phase. Scanning electron microscopy (SEM) analyses revealed formation of porous channels in freezing direction and Archimedes’ density measurements showed porosity values ranging from 48.0% to 63.0%, depending on PEG content.

Keywords
Magnetic nanoparticles, Freeze casting, Zinc ferrite nanoparticles, Magnetic filters
1Introduction

The various technological applications involving magnetic ferrite nanoparticles have been gaining prominence [1–7], with ongoing studies on the chemical, electrical, photoelectric, thermal and magnetic properties of these nanoparticles [8–15]. There are several syntheses routes, such as sol-gel, high energy milling and co-precipitation [12,16–19]. Among the most common methods, solution combustion synthesis is the simplest, quickest and most economical, thus producing high purity nanostructured ceramic powders without the use of further thermal treatments [20,21].

Composite materials based on polymeric, ceramic or metallic matrices reinforced with ferrite nanoparticles have been reported as biomedical materials, drug delivery systems, magnetic contrast agents, catalysts, pigments, purifiers and removing agents such as ions, impurities and microorganisms present on water [22–26].

Freeze casting technique consists of fabricating porous ceramic bodies from a ceramic slurry, that is frozen. The solvent is removed by sublimation and the obtained green body is sintered [27,28]. Factors such as concentration of solids in the slurry, cooling rate and additives such as binders, influence the developed microstructure [29,30].

It is a promising technique for the production of bioceramics, since the porous struts are compatible with porous spongy bone. However, the strut may also be used in applications such as catalysis, where large surface area and the presence of macro, meso and micropores are required [27,29]. Another positive point is the fact that the manufacturing process is a sustainable route, since it is applicable to a wide variety of ceramics such as silicon carbide, silicon nitride, alumina, copper oxide, iron oxide, hydroxyapatite, titanium oxide, silica and clay-based ceramics [29–35]. In this study, high-porosity magnetic ceramic bodies to be used as magnetic filters were produced by zinc ferrite nanoparticles using the freeze casting technique.

2Experimental procedure2.1Solution combustion synthesis

Zinc ferrite nanoparticles were obtained from the solution combustion method – a pyrolysis process – using zinc nitrate hexahydrate (Aldrich brand, 98.0% purity), iron nitrate trihydrate (Aldrich brand, 98.0% purity) as ceramic precursors and glycine (C2H5NO2) (Aldrich brand, 98.5% purity) as organic fuel. The chosen glycine–nitrate (G/N) ratio was 1.0. The slurry was prepared based on stoichiometric calculation to obtain ZnFe2O4. After mixing the reagents, an homogeneous solution was obtained and heated up to 100°C to allow distilled water evaporation, with formation of a highly viscous gel. At this point, the combustion reaction occurred, resulting in the nanostructured powder. The high heating rates involved in the process result in quick exposure of the powder to high synthesis temperatures nucleate small crystallites, as the growth step is almost suppressed. A good advantage is the absence of subsequent heat treatment [21].

2.2Preparation of the colloidal slurry

The obtained ferrite nanoparticles were dispersed in distilled water, at room temperature and the pH value was adjusted to 10.0 [14] in order to reach the aqueous slurry stability. The solid rate in the slurry was fixed at 10.0% in weight, and the water-soluble polymeric binder PEG-8000 (Aldrich brand) was added to the slurry in 2.0% and 3.0% by weight, relative to the ceramic weight.

2.3Suspension freezing and drying of samples

The suspension was poured into a polyvinyl chloride (PVC) container with 20.0mm in diameter and 30.0mm in length. The PVC container was placed on a copper disk with 50.0mm in diameter and 5.0mm thickness, with the top exposed to atmospheric conditions. This set was placed in contact with a cold copper finger. Freezing was performed in liquid nitrogen, with a cooling rate of 10°C/min, between 10°C and −145°C. After reaching the temperature of −145°C, the samples were kept 0.5h at this temperature before being retrieved and transfered to a freezer until they were sent to the freeze-drying process by use of a freeze drier. Lyophilization is the step that completes freeze casting, sublimating solid-state water. The used freeze drier was a Labconco model Freezone 2.5, in the following conditions:

  • -

    0.035mBar of pressure in the chamber;

  • -

    30h of time for drying.

2.4Sintering

The green bodies were sintered to improve strength and integrity, achieving suitable mechanical resistance. To do so, samples were placed in a platinum crucible and sintering route was designed to carefully eliminate the binder (PEG-8000) without damaging the green body structure, and achieve proper densification of pore walls, according to Fig. 1:

  • -

    heating rate of 1°C/min to 158°C;

  • -

    dwell time of 1h at 158°C;

  • -

    heating rate of 1°C/min to 375°C;

  • -

    dwell time of 1h at 375°C;

  • -

    heating rate of 3°C/min to 1300°C;

  • -

    dwell time of 1h at 1300°C.

Fig. 1.
(0.07MB).

Heat treatment steps for polymer binder removal and sintering of samples.

2.5Characterization

Sintered samples were characterized by X-ray diffraction (XRD) in an X’PERT PRO PANalytical diffractometer, with monochromatic radiation (Cu Kα, λ=1.5406Å), step size of 0.05°s−1, time per step 150s and 2θ between 10° and 90°. The pores structure morphology was assessed by a scanning electron microscope (SEM) (FEI model Quanta 250) with field emission gun (FEG), under high vacuum. The nanometric powder density was measured by a pycnometer (AccuPyc II) using helium gas, with measurements performed at room temperature. Total porosity measurements were assessed by NBR ISO 5017 standard.

3Results and discussion3.1X-ray diffraction (XRD)

Fig. 2(a) and (b) shows XRD patterns of samples with 2.0wt% and 3.0wt% of PEG-8000, respectively. The XRD patterns were refined using Rietveld's method, by the use of TOPAS-Academic version 4.1. For phases analysis the index card ICSD-91827 was used. Both samples showed ferrite phase as the only present phase, with goodness of fitness (GOF) adjustment of 1.24 and 1.21, as shown in Table 1. There was no phase transformation during sintering and the polymer binder was successfully eliminated. Table 1 shows the parameters obtained by the diffractograms adjustment.

Fig. 2.
(0.14MB).

XRD patterns of ZnFe2O4 samples with 2.0wt% PEG-8000 (a) and 3.0wt% PEG-8000 (b).

Table 1.

Parameters obtained by the Rietveld's method.

  2.0wt%  3.0wt% 
GOF  1.24  1.21 
Formed phase  ZnFe2O4  ZnFe2O4 
Crystal structure  Cubic  Cubic 
Density [g/cm35.32  5.32 
Crystalitte size [nm]  420.1  229.4 
Lattice parameter [Å]  8.44  8.44 

By refining the parameters of sintered samples, it was possible to verify that the sintering process was effective, with crystallite growth and consequent grain growth, starting from nanometric powder.

3.2Scanning electron microscopy (SEM)

The synthesized bodies were observed in cross-section regions of samples, along a direction parallel to freezing front. In Fig. 3(a–f), red arrows represent the freezing direction front of ice crystals.

Fig. 3.
(1.15MB).

SEM micrographs of ZnFe2O4 sintered with 2.0wt% PEG-8000 (a–c), and with 3.0wt% of PEG-8000 (d–f).

By SEM analyses, it was possible to confirm the formation of porous channels in freeze direction for both 2.0wt% and 3.0wt% binder samples, although it is more evident for 3.0wt% samples. The formation of lamellar structure, characteristic of ice crystals growth, is also evident, with a columnar freezing front [27,36], essential for application in filtration [37]. The presence of homogeneous spherical pores and cross-linked pores [38], with primary alignments of freezing front and, secondary alignments between the porous channels, derived from the presence of the high molecular weight PEG [39], are shown in Fig. 3(b).

3.3Measures of density, porosity, densification and sintering shrinkage

Density of nanometric powders, measured by a pycnometer at room temperature, was 6.6415g/cm3 with a standard deviation of 0.0164g/cm3. Apparent density of sintered specimens was assessed by the Archimedes’ method, following the NBR ISO 5017 standard [40], and the values obtained are presented in Table 2, as well as the values for apparent porosity. These results suggest small fraction of interconnected pores [41], with water absorption of 17.1% and 33.0% for 2.0wt% and 3.0wt% of PEG-8000, respectively, whereas the apparent porosity found is below the expected values for 10.0wt% of solid, when it is compared to other ceramics studies [27]. No cracks were observed in sintered samples, but there was a volumetric shrinkage of 73.0% in the 2.0wt% PEG-8000 sample and 56.0% in the 3.0wt% PEG-8000 during sintering. The measurement of volumetric retraction was made by comparing samples diameter and height before and after sintering.

Table 2.

Measured values of density, porosity, densification and sintering shrinkage of samples.

  2.0wt%  3.0wt% 
Apparent density [g/cm32.83  1.93 
Solid apparent density [g/cm35.48  5.35 
Apparent porosity [%]  48.34  63.83 
Abosrtion of water [%]  17.10  33.05 
Densification [%]  56.24  38.44 
Sintering shrinkage [%]  73.64  56.89 

Both high sintering shrinkage and low porosity can be explained by the sintering temperature and route: high temperatures tend to increase densification because sintering is a thermally activated process, which lead to a decrease in porosity, closing pores, changing the structure formed during freezing, and the crystallite growth, as observed by XRD. Low radii ions, such as zinc, with an atomic radius of 0.74Å, while iron's is 0.77Å, are easy to diffuse, increasing densification.

It is also evident the role of additive in the slurry stability, growth of ice crystals tips [37], mechanical strength of green bodies before sintering [39] and formation of porous morphology. PEG-8000 increased the viscosity of suspension by reducing porosity, with linear and volumetric shrinkage of ceramic bodies. This effect is similar to that observed in processes using gelatin as solvent [38,39]. The increase from 2.0wt% to 3.0wt% of PEG provided higher porosity in the ceramic bodies, delaying the process of grains coalescence, because the high molecular weight PEG tends to form thin porous channels [37].

4Conclusions

Two magnetic filters were produced, made of zinc ferrite nanoparticles, by the freeze casting technique, with 2.0wt% and 3.0wt% of PEG – 8000 as binder. The sintered bodies showed porosities of 48.0% and 63.0%, respectively. Due to the high molecular weight of PEG used, the studied samples did not show expected porosity values for percentage of solids added. However, low molecular weight PEG is being considered in future woks in order to obtain higher porosity values.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

The authors are grateful to the Mechanical and Materials Engineering Department of the Military Engineering Institute, X-ray Crystallography and Diffraction Laboratory of the Brazilian Center for Physical Research and the Center for Mineral Technology for their assistance in conducting the tests and analyzes and CNPq by financial support under grant number 141012/2017-0.

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Paper was part of technical contributions presented in the events part of the ABM Week 2017, October 2nd to 6th, 2017, São Paulo, SP, Brazil.

Corresponding author. (Letícia dos Santos Aguilera le_aguilera13@hotmail.com)
Copyright © 2018. Brazilian Metallurgical, Materials and Mining Association
J Mater Res Technol 2018;7:350-5 DOI: 10.1016/j.jmrt.2018.04.012
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