Journal

J. Am. Ceram. Soc., 88 [7] 2003–2004 (2005)

DOI: 10.1111/j.1551-2916.2005.00382.x

Synthesis of Nanocrystalline Alumina Using Egg White

Santanu Dharaw

Materials Science Centre, IIT, Kharagpur 721302, India

A new simplified route to prepare nanocrystalline alumina pow-ders is proposed. The method involved the use of freshly ex-tracted egg white (ovalbumin) and aluminum nitrate in anaqueous medium. Changes in ionic strength upon addition ofaluminum nitrate resulted in gelation of ovalbumin. The addedmetal ions were dispersed in the gelled ovalbumin matrix. Heattreatment of the gelled mass resulted in nanosized c alumina atas low a temperature as 3301C. TEM analysis showed that theparticle size ranged from 15 to 80 nm.

I. Introduction

NANOCRYSTALLINE ceramics can be prepared through a num-ber of solution-based techniques1 such as the sol–gel,2–4

hydrothermal,5 co-precipitation,5,6 metal ion-polymer precur-sor,7 etc. Sol–gel, a commonly used technique, involves forma-tion of an amorphous gel from a solution of precursor mixture,which contains metal ions. The sol to gel transformation isbrought about by pH adjustment, increasing the ionic strength,and or changing the temperature of the precursor solution. Thegel is heat treated in steps that include dehydration of the gelfollowed by calcination at elevated temperature to obtain fineceramic powders.

Results similar to those in sol–gel processing can be obtainedusing different metal ion–polymer-based precursor solutions.The methods involve mixing of the precursor solution with awater-soluble polymer for obtaining the matrix for dispersion ofthe cations. Typically, aqueous solutions of the metal nitratesalts are used as the precursor. Evaporation of water by slowheating of the polymer solution results in a black fluffy porousmass, which is ground and heat treated at different temperaturesto obtain different phases of the corresponding metal oxides.Different water-soluble polymers like polyvinyl alcohol (PVA),7

sucrose,7,8 diethanolamine (DEA),7 and triethanolamine (TEA)7

have been used for the above processing.The present study describes a simple and innovative approach

for the synthesis of nanocrystalline alumina powders involvingthe use of egg white (ovalbumin). The use of egg white for theabove purpose was motivated by its ready solubility in waterand its ability to associate with metal ions in solution. Egg whiteproteins have also recently been used as a binder cum gel form-ing material in shape forming of bulk and porous ceramics.9–11

The powder prepared by the present technique was character-ized by XRD and TEM.

II. Experimental Procedure

A mix of freshly extracted egg white and water (pH 9.5) washomogenized by using a magnetic stirrer with additions of n-octanol to inhibit foaming of the mixture. Aluminum nitrate

(Qualigens, Mumbai, India) solution of known concentrationwas added drop wise to the egg white–water mixture. In thisstudy, 50 mL of 1.4MAl(NO3)3 was added drop wise to 325 mLmixture of freshly extracted egg white and water (3:2 ratio).

During addition of aluminum nitrate, the whole solution wasstirred continuously with a magnetic stirrer to form a trans-parent solution. After stirring, the solution was allowed to gel9,12

by keeping it at rest for 2 h. Throughout the whole process de-scribed above, no pH adjustment was made. The pH of the ov-albumin-water mix changed from the initial value of 9.5 to 2.3after addition and mixing of aluminum nitrate solution. Thegelled mass was dried by slow heating and continuous stirring for10 h in the temperature range of 801–1501C. The dried yellowishmass was crushed into powder using mortar and pestle. Thecrushed and vacuum dried powder was characterized by simul-taneous thermo gravimetric and differential thermal analysis(TG-DTA) (Perkin-Elmer, Boston, MA, Pyris Diamond TG/DTA) in air.

The crushed powder was calcined at temperatures close towhich decomposition and/or phase changes were expected tooccur as suggested by the TG-DTA analysis. Calcination wascarried out at 3301, 4501, 5601, and 6301C, respectively for 2 hand the resultant phase changes were examined by X-ray dif-fraction (PW1710, Philips, Amsterdam, Holland using Cu targetat 0.051C/s). The Debye–Scherrer formula was used for deter-mination of the crystallite size of the calcined product.

Microstructure of the final calcined powders was studied bytransmission electron microscopy (TE-300, Philips). A suspensionof powders in chloroform was prepared using an ultrasonic bath.The suspension was taken in a syringe and a drop was carefullyplaced on a carbon-coated grid for examination under TEM.

III. Results and Discussion

It is well known that egg white or ovalbumin in solution pos-sesses net charge, the magnitude and nature of which is a func-tion of the solution pH. This enables association of the solvatedions in solution with the charged sites of the protein molecules.13

This association of cations and the ovalbumin helps in keepingthe metal ions dispersed within the resulting gel structure. Sincethe isoelectric point of ovalbumin is 4.9, the ovalbumin was ex-pected to have a net negative charge at the solution pH of 9.5 inthe present study. The association of the cations and the proteinmolecules was verified by changes in conductivity measure-ments. It was observed that the conductivity of aqueous oval-bumin solution remained unchanged with even upto 25 mLaddition of (M/10) aluminum nitrate solution to 50 mL of1 vol% ovalbumin solution.

The dried gel containing the dispersed metal ions when heattreated at high temperatures resulted in crystallization of thenanosized oxide particles. The various stages of the process canbe traced by the following methods. X-ray diffraction pattern ofthe yellowish precursor powder showed that it is amorphous innature (Fig. 1). The results from the simultaneous TG-DTA ofdried ovalbumin and the precursor are shown in Fig. 2. The TG-DTA peaks of the precursor are shifted to higher temperatures,relative to ovalbumin, possibly due to the association between

2003

A. Bandyopadhyay—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: santanuhara@rediffmail.com

Manuscript No. 10875. Received February 24, 2004; approved March 16, 2004.

protein molecules, metal cations, initially, and association ofcarbonaceous residue, crystallized metal oxide particles as theprecursor was heated to higher temperature.

Comparison of the DTA curves, for ovalbumin and the pre-cursor, indicates that the exothermic peak at 3301C (Fig. 2) canbe attributed to oxidative breakdown of the ovalbumin compo-nent as well as crystallization of the precursor, which is con-firmed from the XRD of the precursor heat treated at the sametemperature (Fig. 1). Heat treatment of the precursor at highertemperatures results in further development of the crystallinephase (g-alumina) as indicated by the increase in peak intensi-ties. Comparison of the DTA curves also confirmed that theexothermic peaks at temperatures of B5601 and 6301C corre-spond to the oxidative degradation of the remaining part of ov-albumin.

The above inferences drawn from TG-DTA and XRD anal-yses can be correlated to the visual observations of the precursorduring heat treatment. Decomposition of the precursor startedjust above 2001C and was accompanied by formation of nitro-genous gases (brown fumes) as also observed during the calci-

nation process. The precursor transforms to crystalline g-alumi-na but it appears black in color due to the presence of thecarbonaceous materials. The powder gradually becomes increas-ingly white till it turns to fully white after heat treatment at6301C.

The crystallite sizes of g alumina powders, as calculated usingthe Debye–Scherrer relation from the above diffraction patterns,were in the range of 15–25 nm. The transmission electron mi-crograph of the calcined powder shows an agglomerated nano-crystalline powder (Fig. 3(a)). Examination of the powder athigher magnification indicated that the individual powder par-ticles had sizes in the range of 15–80 nm (Fig. 3(b)).

IV. Summary

The gel formed by water-soluble egg white proteins serves as aperfect matrix for entrapment of metal ions that, upon heattreatment, give rise to ultrafine amorphous or crystalline oxideparticles. The use of egg white simplifies the process and pro-vides another alternative process for economical synthesis ofnanocrystalline ceramic particles. In this study, alumina parti-cles with crystallite sizes of 15–80 nm could be produced by heattreatment of the gelled mass at as low as 3301C.

Acknowledgment

The author would like to thank Dr. P. Bhargava (Asst. Prof. in Materials Sci-ence Centre, IIT Kharagpur) for his kind support.

References

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12R. Lumry and H. Eyring, ‘‘Conformation Changes of Proteins,’’ J. Phys.Chem., 58, 110–20 (1954).

13B. L. Oser, inHawk’s Physiological Chemistry, 1054. McGraw Hill Publishers,New York, 1965. &

20 30 40 50 60 70

Arb

itra

ry u

nit

2θ 2θ (degree)

(a)

(c)

(b)

(e)

(d)

Fig. 1. X-ray diffractograms (using CuKa radiation) of (a) the yellow-ish precursor and precursor after heat treatments at (b) 3301C, (c) 4501C,(d) 5601C, and (e) 6301C for 2 h.

−10

30

70

110

50 250 450 650

TG

(%

Wei

ght)

−10

0

10

20

30

DT

A (

µv)

DTA-Ovalbumin

Temperature (°C)

TG-Precursor

DTA-Precursor

TG-Ovalbumin

Fig. 2. Simultaneous TG and DTA curves for ovalbumin and precur-sor indicating the decomposition of the precursor and the crystallizationof alumina.

Fig. 3. Transmission electron microscopy of the calcined powder (a)showing agglomerates and (b) particles at higher magnification of thealumina powder.

2004 Communications of the American Ceramic Society Vol. 88, No. 7