ARL-TN-0720 ● DEC 2015

US Army Research Laboratory

Synthesis of Al2O3-Coated Fe3O4 Nanoparticles 

for Thermomagnetic Processing

by Victoria L Blair and Joseph Marsico

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ARL-TN-0720 ● DEC 2015

US Army Research Laboratory

Synthesis of Al2O3-Coated Fe3O4 Nanoparticles 

for Thermomagnetic Processing

by Victoria L Blair 

Weapons and Materials Research Directorate, ARL

by Joseph Marsico

Rochester Institute of Technology, Rochester, NY

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December 2015

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Technical Note

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1 June–30 September 2015

4. TITLE AND SUBTITLE

Synthesis of Al2O3-Coated Fe3O4 Nanoparticles for Thermomagnetic Processing

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Victoria L Blair and Joseph Marsico

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DSI14-WM-014

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US Army Research Laboratory

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Aberdeen Proving Ground, MD 21005-5069

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ARL-TN-0720

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14. ABSTRACT

Current research at the US Army Research Laboratory (ARL) has been focused on understanding the underlying physics of 

external fields on materials synthesis and processing, also known as Energy Coupled to Matter. This technical note will 

describe a combination of synthesis processes by which a new composite material will be produced for future work in 

thermomagnetic processing at ARL. This novel process will be used to fabricate ceramic parts with tailored physical 

properties to enable new applications that benefit the Army. Potential applications could include new devices or equipment 

that enhance the performance of Soldier and vehicle protection systems.

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thermomagnetic processing, nanoparticles, synthesis, core/shell particles, composites

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Victoria L Blair

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Contents

Acknowledgments iv

1. Introduction 1

2. Synthesis Method 1

3. Conclusion and Future Work 3

4. References 4

List of Symbols, Abbreviations, and Acronyms 6

Distribution List 7
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iv

Acknowledgments

Special thanks to the US Army Research Laboratory’s Ceramics and Transparent 

Materials Branch researchers for all their technical guidance.
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1. Introduction

Recent work in materials processing and manufacturing research has been focused 

on using nontraditional parameters to influence the properties of the final parts. 

This includes the application of high-strength magnetic fields on materials during 

synthesis, processing, and/or densification. In the case of ceramics, there is 

significant work being completed on the forming process of alumina under a 

magnetic field.1–6 Most recently by Sakka et al.,

5,6 researchers were able to develop 

a highly textured microstructure from both slip casting and gel casting under a 

magnetic field. The bodies maintained a relatively high degree of texture after 

sintering despite particle rearrangement during the sintering process.

Oak Ridge National Laboratory has conducted research on the application of highstrength magnetic fields during high-temperature processing for alloy formation. It 

has been shown through computer simulations that the addition of an approximately 

9-T external magnetic field can influence phase development in steel alloys.7–9

Furthermore, additional studies have demonstrated texture development in weakly 

magnetic metals during sintering under an applied magnetic field.1–3,9 Current 

research within the US Army Research Laboratory has been focused on Energy 

Coupled to Matter for exploring fundamental physics during processing in external 

fields to obtain novel materials with tailored properties. One future goal is the 

development of a core-shell structures in which the magnetic core is responsive 

under an applied magnetic field and the nonresponsive portion of the shell is the 

majority phase. A process will be described for producing magnetic magnetite 

(Fe3O4) nanoparticles with a thick shell of alumina to be used during 

thermomagnetic processing. The process will be inspired by similar procedures 

described in the literature.10

2. Synthesis Method

Clean laboratory glassware was required to avoid impurities in the end product. 

Deionized (DI) water (H2O) was degassed using house nitrogen (N), allowing for 

the N to bubble through about 100 mL of H2O for 10 min before starting synthesis. 

A 2:1 ratio of iron Fe3+ ions to Fe2+ ions from Fe (III) sulfate and Fe (II) chloride 

was required for the synthesis process. Fe3O4 is a ferrite that has both cation 

valences in the oxide, as demonstrated in the following chemical reaction: 

Fe2+ + 2Fe3+ + 8OH- = Fe3O4 + 4H2O. (1)
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To acquire the correct ratio, it was necessary to know the exact amount of H2O

hydrating the sulfate and the chloride. These data were obtained from 

thermogravimetric analysis up to a temperature of at least 1,000 °C to enable the 

formation of iron oxide. The results were used to back-calculate the exact 

stoichiometry of the precursor materials.

Once the DI H2O was degassed, it was poured into a sample reaction vessel with 

the precursor materials. This was an exothermic reaction completed in a fume hood, 

adding the H2O slowly. While the mixture was boiling and cooling down, a solution 

of sodium hydroxide (NaOH) was prepared in DI H2O and degassed with house N

(as described earlier) for a minimum of 60 min. Once the NaOH solution was

finished degassing, an Fe/H2O mixture was slowly added to the NaOH solution 

while continuing to bubble house N through the system. Keeping oxygen (O) out 

of the system was necessary to prevent premature oxidation of the Fe cations. After 

this step, the Fe3O4 particles precipitated out of solution, leaving behind H2O, NaCl,

and Na2SO4.

Next, the solution was heated at approximately 85 °C and stirred while continuing 

to bubble N through the suspension. After 3 h, the heat was turned off, allowing the 

solution to cool while keeping the N bubbling. Once the suspension was cooled, it 

could be filtered. While filtering, the mother suspension continued to bubble N. The 

Fe3O4 was washed twice in DI H2O and once in isopropyl alcohol (IPA). Following 

the IPA rinse, the Fe3O4 was heated in a beaker up to 100 °C with N blowing on it 

so the IPA could evaporate. Once the IPA was evaporated, drying was continued in 

an oven. The surface area was measured (by N absorption in BET [BrunauerEmmet-Teller]) on the resulting Fe3O4 nanoparticles and found to be 133 m2

/g. 

An in situ nanoprecipitation process was used to coat the Fe3O4 particles with 

alumina.

11 Prior to precipitation, 2 solutions were prepared, including an acidic 

solution (solution A) and a basic solution (solution B). The acidic solution consisted

of aluminum (Al) nitrate, magnesium (Mg) nitrate, and rare earth (RE) nitrate in 

stoichiometric amounts to acquire a composition of RE0.002Al1.998O3 with 250 ppm 

of magnesium oxide. Adding Mg to alumina had the dual purpose of creating 

structural distortion to assist in dissolving the erbium into the Al octahedral site and 

behaving as a grain growth inhibitor. DI H2O was added to the mixed nitrates to 

acquire a 7.5-M solution of Al(NO3)3 (RE and Mg nitrates were not included in the 

calculation). The basic solution consisted of 11% by weight ammonium 

bicarbonate and 3% by weight ammonium hydroxide in DI H2O. Both solutions 

were stirred until all crystals were dissolved, adding heat when necessary. 
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Once solutions A and B were prepared, a third solution, referred to as the buffer, 

was mixed to host the reactions. The amount of buffer solution (2% by weight

ammonium bicarbonate in DI H2O) was dependent on the batch size. The pH of the 

buffer was adjusted to approximately 7 by adding a small amount of nitric acid. To 

coat the Fe3O4 particles, approximately 1.5-wt% Fe3O4 (based on the calculated 

mass of alumina) was added to the buffer solution and allowed to stir vigorously, 

keeping the particles suspended. Finally, solutions A and B were added drop-wise 

to the buffer/Fe3O4 suspension such that the pH remained at approximately 7 during 

the entire precipitation step. When solution A was exhausted, the resulting 

suspension was allowed to age while stirring vigorously overnight.

The following day, the suspension was filtered from the NaCl solution. The 

resulting powder was washed twice with DI H2O and once with IPA. After washing, 

the powder was placed into an oven to dry. The dry powder was gently crushed and 

calcined at 1,300 °C for 30 min in air. The final surface area of the calcined 

composite powder was 7.28 m2

/g, which was a significant change from the 

as-synthesized Fe3O4 nanoparticles. It is possible that this was the result of the 

coating of alumina on the surface reducing the total surface area.

3. Conclusion and Future Work

Fe3O4 nanoparticles have been synthesized with high surface area using a mixture 

of Fe valence cations to precipitate the particles from solution. The particles were 

confirmed to be magnetite by X-ray diffraction, consistent with the observation of 

particles sticking to magnetic stir bars. The resulting nanoparticles were coated with 

alumina by nanoprecipitation onto the particle surface and calcined. The surface 

area was shown to decrease, likely due to either alumina coating or grain growth of 

the powders during calcination.

Future work will involve additional characterization of the alumina/Fe3O4

composite powder, including detailed X-ray diffraction with a molybdenum X-ray 

source to limit florescence from the Fe in the sample. Microscopy will be conducted 

to determine if the core/shell structure was obtained, and transmission electron 

microscopy may be used to observe the bonding between the Fe3O4 core and the 

alumina shell. Once the characterization has been completed, work will begin on 

the magnetic field processing of this new composite material.
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4. References

1. Sun ZHI, Guo M, Vleugels J, Van der Biest O, Blanpain B. Strong static 

magnetic field processing of metallic materials: a review. Current Opinion in 

Solid State and Materials Science. 2012;16(5):254–267.

2. Sun ZHI, Guo M, Vleugels J, Van der Biest O, Blanpain B. Processing of nonferromagnetic materials in strong static magnetic field. Current Opinion in 

Solid State and Materials Science. 2013;17(4):193–201.

3. Sun ZHI, Guo X, Guo M, Vleugels J, Van der Biest O, Blanpain B. Alignment 

of weakly magnetic metals during solidification in a strong magnetic field. 

Journal of Alloys and Compounds. 2013;551:568–577.

4. Sun ZHI, Zhang X, Guo M, Pandelaers L, Vleugels J, Van der Biest O, Van 

Reusel K, Blanpain B. Strong magnetic field effects on solid-liquid and 

particle-particle interactions during the processing of a conducting liquid 

containing non-conducting particles. Journal of Colloid and Interface Science.

2012;375:203–212.

5. Szudarska A, Sakka Y, Suzuki TS, Mizerski T, Szafran M. Magnetic field 

alignment in highly concentrated suspensions for gelcasting process. Ceramics 

International. 2016:42;294–301.

6. Szudarska A, Mizerski T, Sakka Y, Suzuki TS, Szafran M. Fabrication of 

textured alumina by magnetic alignment via gelcasting based on low-toxic 

system. Journal of the European Ceramic Society. 2014;34(15):3841–3848.

7. Jaramillo RA, Babu SS, Ludtka GM, Kisner RA, Wilgen JB, MackiewiczLudtka G, Nicholson DM, Kelly SM, Murugananth M, Bhadeshia HKDH.

Effect of 30T magnetic field on transformations in a novel bainitic steel.

Scripta Materialia. 2005;52:461–466.

8. Ludtka GM, Jaramillo RA, Kisner RA, Nicholson DM, Wilgen JB, 

Mackiewicz-Ludtka G, Kalu PN. In situ evidence of enhanced transformation 

kinetics in a medium carbon steel due to a high magnetic field. Scripta 

Materialia. 2004;51:171–174.

9. Radhakrishnan B, Nicholson DM, Eisenbach M, Parish C, Ludtka GM, Rios 

O. Alignment of iron nanoparticles in a magnetic field due to shape anisotropy. 

Journal of Magnetism and Magnetic Materials. 2015;394:481–490.

10. Laurent S, Forge D, Port M, Roch A, Robic C, Vander Elst L, Muller RN. 

Magnetic iron oxide nanoparticles: synthesis, stabilization, vectorization, 
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physicochemical characterizations, and biological applications. Chemical 

Reviews. 2008;108(6):2064–2110.

11. Sanamyan T, Pavlacka R, Gilde G, Dubinskii M. Spectroscopic properties of 

Er3+ -doped α-Al2O3. Optical Materials. 2013;35:821–826.
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List of Symbols, Abbreviations, and Acronyms

Al aluminum

DI deionized

Fe3O4 magnetite

H2O water

IPA isopropyl alcohol

Mg magnesium

N nitrogen

NaOH sodium hydroxide

O oxygen

RE rare earth
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