Synthesis of nanocrystalline yttria doped ceria powder by urea-formaldehyde polymer gel auto-combustion process

Article abstract of DOI:10.1016/j.materresbull.2006.08.017

Nanocrystalline yttria doped ceria powder has been prepared by auto-combustion of a transparent gel formed by heating an aqueous acidic solution containing methylol urea, urea, cerium(III) nitrate and yttrium(III) nitrate. The TGA and DSC studies showed the combustion reaction of the gel initiated at 225 °C and completed within a short period of time. XRD spectrum of the combustion product reveals the formation of phase pure cubic yttria doped ceria during the combustion process. Loose agglomerate of yttria doped ceria particle obtained by the combustion reaction could be easily deagglomerated by planetary ball milling and the powder obtained contains particles in the size range of 0.05-3.3 μm with D₅₀ value of 0.13 μm. The powder particles are aggregate of nanocrystallites with a wide size range of 14-105 nm. Pellets prepared by pressing the yttria doped ceria powder sintered to 95.2% TD at 1400 °C.

Full text of DOI:10.1016/j.materresbull.2006.08.017

Materials Research Bulletin 42 (2007) 609–617
www.elsevier.com/locate/matresbu
Synthesis of nanocrystalline yttria doped ceria powder by
urea–formaldehyde polymer gel auto-combustion process
*
M. Biswas, K. Prabhakaran , N.M. Gokhale, S.C. Sharma
Ceramic Division, Naval Materials Research Laboratory, Defence Research and Development Organization,
Shil Badlapur Road, Anandnagar P.O., Addl. Ambernath, Thane 421 506, India
Received 14 April 2006; received in revised form 17 July 2006; accepted 10 August 2006
Available online 18 September 2006
Abstract
Nanocrystalline yttria doped ceria powder has been prepared by auto-combustion of a transparent gel formed by heating an
aqueous acidic solution containing methylol urea, urea, cerium(III) nitrate and yttrium(III) nitrate. The TGA and DSC studies
showed the combustion reaction of the gel initiated at 225 8C and completed within a short period of time. XRD spectrum of the
combustion product reveals the formation of phase pure cubic yttria doped ceria during the combustion process. Loose agglomerate
of yttria doped ceria particle obtained by the combustion reaction could be easily deagglomerated by planetary ball milling and the
powder obtained contains particles in the size range of 0.05–3.3 mm with D50 value of 0.13 mm. The powder particles are aggregate
of nanocrystallites with a wide size range of 14–105 nm. Pellets prepared by pressing the yttria doped ceria powder sintered to
9
5.2% TD at 1400 8C.
2006 Elsevier Ltd. All rights reserved.
#
Keyword : A. Oxides; B. Chemical synthesis; C. Differential scanning calorimetry; D. Microstructure
1
. Introduction
Rare earth doped ceria is one of the well known electrolyte materials for low temperature (500–700 8C) solid oxide
fuel cell (SOFC) application due to their high oxygen ion conductivity at low temperature [1–3]. Though dopants such
Sm, Gd and Y showed highest ionic conductivity values, yttria doped ceria is considered as low cost material for SOFC
electrolyte due to the higher relative abundance of Y [4–6]. Dense doped ceria ceramics used for SOFC electrolyte
application are prepared from nanocrystalline powders [7]. Combustion synthesis is a simple and faster method for
preparation of nanocrystalline ceramic powders [8,9]. Combustion synthesis are of two types namely solution
combustion synthesis and gel combustion synthesis. In solution combustion method, an aqueous solution containing
the precursor metal nitrates and a fuel is heated in a muffle furnace at about 500 8C to initiate the combustion reaction
[
9–18]. High temperature generated during the combustion process helps the formation of required ceramic phase in a
short period of time. Organic molecules such as urea, glycine and oxalyl dihydrazide are some of the fuels used in
solution combustion synthesis of various oxide ceramic powders [8–18]. In gel combustion method, a solution
containing the precursor metal nitrates and fuels such as citric acid, glycine, hydrazine and urea is heated to form a
*
Corresponding author. Tel.: +91 251 2621954; fax: +91 251 2620604.
E-mail address: kp2952002@yahoo.co.uk (K. Prabhakaran).
0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2006.08.017

610
M. Biswas et al. / Materials Research Bulletin 42 (2007) 609–617
polymeric gel and the gel undergoes auto-combustion to form either the required ceramic phase or its precursor [19–
3]. The precursor powder is further calcined to form the required crystalline ceramic phase. Generally, in combustion
3
synthesis large volume of gases are generated during the combustion reaction in a short interval of time which makes
the product foamy and soft agglomerate of fine particles [8,9]. Therefore the product obtained in combustion synthesis
can be easily brought down to fine particles by minimum mechanical milling. Combustion synthesis of yttria doped
ceria powder from the corresponding nitrates using citric acid and glycine as fuel has been reported in the literature
[
34,35].
Recently we have shown that urea–formaldehyde (UF) forms a transparent gel in presence of metal ions such as
3
+
2+
2+
La , Sr and Mn . The dried gel undergoes self sustained combustion when initiated with a burning matchstick and
little kerosene to form nanocrystalline lanthanum strontium manganite [36]. In the present work, the urea–
formaldehyde polymer gel combustion synthesis method has been extended for preparation of yttria doped ceria
3
+
3+
powder. Synthesis of transparent urea–formaldehyde polymer gel containing Y and Ce ions and its combustion
behaviour has been studied. The combustion product has been characterized by XRD, SEM and TEM, particle size and
surface area analysis.
2
. Experimental
Ce(III)nitrate (CDH, New Delhi, India) and urea (CDH, New Delhi, India) used were of analytical reagent grade. A
7% (w/v) solution of formaldehyde (A.R. grade) used was procured from S.D. fine chemicals (Mumbai, India).
3
Analytical reagent grade nitric acid (Merck, India) and ammonia solution (Merck, India) were used for pH adjustment.
Y(III) nitrate was prepared by dissolving yttria powder (99.9%, Indian Rare Earth Ltd., Mumbai, India) in dilute nitric
acid.
Flow chart of the urea–formaldehyde gel auto-combustion process is shown in Fig. 1. A methylol urea solution was
prepared by dissolving urea in the formaldehyde solution preadjusted to a pH of 8.5 using ammonia solution and aging
for 24 h [37–39]. The mole ratio of urea and formaldehyde used was 1:4. Stoichiometric amount of the Ce(III) nitrate
and Y(III) nitrate required for the formation of 8 mol% yttria doped ceria were dissolved in the methylol urea solution.
An additional lot of urea was further dissolved in the solution to make the urea: formaldehyde mole ratio 1:2. The mole
ratio of metal ion to urea is also 1:2. The solution was heated on a boiling water bath to form a transparent resin. The
resin was poured in to a glass bowl and dried in an oven at 120 8C. The TGA (HI Res TGA 2950, TA Instruments,
USA) and DSC (MDSC Q 100, TA Instruments, USA) analysis of the dried gel was carried out in air at a heating rate of
1
0 8C/min.
Fig. 1. Flowchart of the urea–formaldehyde gel auto-combustion process.

M. Biswas et al. / Materials Research Bulletin 42 (2007) 609–617
611
The dried gel was ignited inside a glass beaker at room temperature. The ignition of the gel was initiated using a
glowing matchstick. XRD (Philips Analytical, The Netherlands) analysis of the combustion product was carried out
using Cu Ka radiation. The combustion product was deagglomerated by planetary ball milling in isopropanol using
zirconia grinding media for a period of 24 h. A charge to ball ratio of 1:4 by weight was used for deagglomeration.
Aggregate size distribution of the combustion product and particle size distribution of the deagglomerated powder was
measured by laser scattering technique (Malvern Master Sizer 2000, UK). He–Ne laser of wavelength 660 nm was
used for the measurement. Both the combustion product and deagglomerated powder were dispersed in water by
ultrasonic treatment for 5 min before the measurement. Surface area (Sorptomatic 1990, Thermo Finnigan, Italy) of
the powder obtained after deagglomeration was measured by BET method using nitrogen gas adsorption. The
microstructure of the combustion product and the powder were observed using a scanning electron microscope (LEO
1
455, UK). Crystallite size in powder particles was measured from TEM.
Green pellets were prepared by pressing the deagglomerated yttria doped ceria powder at 50 MPa pressure using a
hardened steel die set. The pellets were sintered at various temperatures in the range of 1250–1450 8C for 4 h. Heating
rate used was 3 8C/min up to the sintering temperatures. Density of the sintered samples was measured by Archimedes
principle. Microstructure of the sintered sample was observed on fractured surfaces using a scanning electron
microscope.
3
. Results and discussion
It is well known that urea reacts with formaldehyde in presence of base catalyst (pH ꢀ 8.5) to form methylol urea
derivatives [37–39]. When Ce(III) nitrate and Y(III) nitrate were dissolved in the solution the pH has been shifted
nearly to 1 due to hydrolysis of the nitrates. This solution, when supplemented with additional amount of urea to make
the urea to formaldehyde mole ratio 1:2 and heated on a boiling water bath for 30 min, form a transparent resin. The
resin on cooling set in to a clear transparent gel. On the other hand, a solution of methylol urea and urea without the
metal nitrates at pH of nearly 1 when heated formed a white opaque gel in 2–3 min. The slow polymerization of
methylol urea and urea and formation of transparent UF gel in presence of metal ions has been explained in our earlier
3
+
3+
publication [36]. Accordingly, the Ce and Y ions form labile complexes with methylol urea molecules and
3
+
3+
stabilize them against condensation with urea. These labile complexes slowly revert back to free Ce and Y ions and
methylol urea. These methylol urea molecules undergo condensation polymerization with the urea to form urea–
3
+
3+
formaldehyde (UF) polymer. During the course of the polymerization the Ce and Y ions act in between the
growing UF polymer chains by interacting through OH, NH and CO groups in the polymer structure by co-ordination.
3
+
3+
The interaction between the Ce and Y ions and UF polymer chains prevented the self assembly of UF polymer
chains by inter-chain hydrogen bonding, resulted in a transparent gel. The chemical reactions and structure of the
3
transparent UF gel containing Ce and Y ions is shown in Fig. 2.
+
3+
Camino et al. [40] reported that UF polymer decompose in three stages, 100–200 8C corresponding to the cross-
linking reactions, 200–300 8C corresponding to its decomposition to CO , H O, NH and ammines with the formation
2
2
3
of more thermally stable structures and 400–600 8C corresponding to the decomposition of these stable structures.
Unlike this, the transparent UF polymer containing Ce(III) nitrate and Y(III) nitrate showed a weight loss in two
stages. A weight loss of nearly 12% up to a temperature of 200 8C could be due to cross-linking reactions. Further, it
showed a rapid weight loss of nearly 58% at 225 8C. Fig. 3 shows TGA of the dried transparent UF gel sample
containing Ce(III) nitrate and Y(III) nitrate. Corresponding to this rapid weight loss at 225 8C a strong exothermic
peak was observed in the DSC indicating a combustion reaction. The cross-linked UF polymer gel served as fuel and
Ce(III) nitrate and Y(III) nitrate act as oxidant for the combustion reaction. DSC plot of the dried UF gel containing
Ce(III) nitrate and Y(III) nitrate is shown in Fig. 4.
The dried UF gel containing Ce(III) nitrate and Y(III) nitrate in a glass beaker undergoes auto-combustion when
initiated with a glowing matchstick. The combustion reaction was so vigorous that the dried gel for 100 g yttria doped
ceria batch undergoes complete combustion within few seconds. Fig. 5 is a photograph showing auto-combustion of
UF polymer gel containing Ce(III) nitrate and Y(III) nitrate in a glass beaker. Fig. 6 shows XRD pattern of the
combustion product (uncalcined product). The XRD shows all peaks corresponding to the cubic yttria doped ceria
phase. It is clear that the heat energy generated during the combustion of UF polymer gel could be sufficient for
3
+
3+
formation of cubic crystalline yttria doped ceria phase. Further, homogeneous and even distribution of Ce and Y
ions in the urea–formaldehyde polymer gel facilitated the crystallization of yttria doped ceria during combustion

612
M. Biswas et al. / Materials Research Bulletin 42 (2007) 609–617
3
Fig. 2. Chemistry of transparent urea–formaldehyde polymer gel formation in presence of Ce and Y ions and structure of the gel.
+
3+
Fig. 3. TGA plot of the dried transparent urea–formaldehyde gel containing Ce(III) nitrate and Y(III) nitrate.

M. Biswas et al. / Materials Research Bulletin 42 (2007) 609–617
613
Fig. 4. DSC plot of the dried transparent urea–formaldehyde gel containing Ce(III) nitrate and Y(III) nitrate.
˚
process. The lattice parameter of the yttria doped ceria powder calculated from the XRD data was 5.405 A. This lattice
parameter value is very close to that reported in JCPDS file no. 75-0174. The crystallite size calculated from the XRD
spectrum by Scherer equation is 45.7 nm. The combustion product showed negligible weight loss when heated in a
muffle furnace up to 800 8C indicating complete consumption of both fuel and oxidant during the combustion process.
Fig. 5. Photograph showing auto-combustion of the transparent urea–formaldehyde polymer gel containing Ce(III) nitrate and Y(III) nitrate.

614
M. Biswas et al. / Materials Research Bulletin 42 (2007) 609–617
Fig. 6. XRD spectrum of the combustion product showing formation of cubic yttria doped ceria phase.
Fig. 7. SEM photograph of combustion product.
Fig. 8. Particle size distribution of yttria doped ceria powder obtained by planetary ball milling of the combustion product.

M. Biswas et al. / Materials Research Bulletin 42 (2007) 609–617
615
Fig. 9. SEM photograph of the yttria doped ceria powder after planetary ball milling.
Fig. 7 shows SEM photograph of the combustion product. The combustion product is loose agglomerate of fine
yttria doped ceria particles. Particle aggregates with size in the range of few microns to few hundred microns were
observed in the SEM photograph. However, the aggregate size measured by laser scattering technique after dispersing
in water by ultrasonic treatment for 5 min showed aggregate size in the range of 2–13 mm with a D50 value of 3.2 mm.
This indicates that the porous and loose agglomerates generated during combustion process could be easily broken
down in to primary particles by mechanical methods. Fig. 8 shows particle size distribution of the yttria doped ceria
powder obtained by planetary ball milling of the combustion product. The yttria doped ceria powder contains particles
in the size range from 0.05 to 3.3 mm. More than 95% of the particles are in the submicron range. D50 value of the
2
powder observed was 0.13 mm. Surface area of the deagglomerated powder measured was18.1 m /g. Fig. 9 shows
SEM photograph of the yttria doped ceria powder. The fine yttria doped ceria particles and their agglomerates are
Fig. 10. TEM photograph of the yttria doped ceria powder.

616
M. Biswas et al. / Materials Research Bulletin 42 (2007) 609–617
Fig. 11. Sintered density vs. sintering temperature plot of the pellets prepared from the yttria doped ceria powder.
Fig. 12. SEM photomicrograph of fractured surface of the yttria doped ceria ceramics sintered at 1400 8C.
clearly seen in the SEM photograph. TEM shows that the yttria doped ceria powder particles are aggregates of
nanocrystallites. Crystallites with wide size range are observed in the TEM. Fig. 10 is the TEM photograph of the yttria
doped ceria powder. Most of the crystallites are in the size range from 20 to 60 nm. However, crystallites with size as
low as 14 nm and as high as 115 nm can also be seen on the TEM photograph.
Density of the yttria doped ceria ceramics increased with sintering temperature up to 1400 8C and thereafter showed
a decreasing trend. Maximum sintered density of 95.2% of theoretical density was achieved at 1400 8C. Blisters were
observed on the samples sintered at 1450 8C. Decrease in density at higher sintering temperature is due to the
blistering. Fig. 11 shows sintering behaviour of the yttria doped ceria powder. Microstructure of fractured surface of
the yttria doped ceria ceramics sintered at 1400 8C is shown in Fig. 12. Average grain size calculated from SEM
photograph using linear intercept method was 1.3 mm.
4
. Conclusions
A transparent gel formed by heating an aqueous acidic solution containing methylol urea, urea, Ce(III) nitrate and
Y(III) nitrate undergoes auto-combustion when initiated with a glowing matchstick. TGA and DSC of the dried gel
revealed the combustion reaction initiated at 225 8C and completed within short period of time. The combustion
product is loose agglomerate of phase pure yttria doped ceria particles and could be deagglomerated to a powder
containing more than 95% submicron particles with D50 value of 0.13 mm. The powder particles are aggregates of
nanocrystallites with wide size range of 14–105 nm. Pellets prepared from the deagglomerated yttria doped ceria
powder sintered to 95.2% TD at 1400 8C.

M. Biswas et al. / Materials Research Bulletin 42 (2007) 609–617
617
Acknowledgement
The authors are thankful to Dr. J. Narayana Das, Director, Naval Materials Research Laboratory for his
encouragement and keen interest in this work.
References
[
[
[
[
[
[
[
[
[
1] G.M. Christie, F.P.F. VanBerkel, Solid State Ionics 83 (1996) 17.
2] B.C.H. Steele, Solid State Ionics 129 (2000) 95.
3] R. Doshi, V.L. Richards, J.D. Carter, X. Wang, M. Krumpelt, J. Electrochem. Soc. 146 (1999) 1273.
4] H. Inaba, H. Tagawa, Solid State Ionics 83 (1996) 1.
5] H. Yahiro, Y. Baba, K. Eguchi, H. Arai, J. Electrochem. Soc. 135 (1988) 2077.
6] J. Van Herle, T. Horita, T. Kawada, N. Sakai, H. Yokokawa, M. Dokiya, Solid State Ionics 86–88 (1996) 1255.
7] M. Oljaca, R. Maric, S. Shanmugham, A. Hunt, Am. Ceram. Soc. Bull. 82 (2003) 38.
8] A. Varma, A.S. Ragadev, A.S. Mukasyan, Adv. Chem. Eng. 24 (1998) 79.
9] K.C. Patil, S.T. Aruna, S. Ekambaram, Curr. Opin. Solid State Mater. Sci. 2 (1997) 1501.
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
10] T. Mimani, K.C. Patil, Mater. Phys. Mech. 4 (2001) 134.
11] A.S. Mukasyan, C. Costello, K.P. Sherlock, D. Lafarga, A. Varma, Sep. Purif. Tech. 25 (2001) 117.
12] F.B. Noronha, L.V. Mattos, H.P. de Souza, M.R. Morelli, F.B. Passos, M.C. Greca, Mater. Res. 7 (2004) 319.
13] Y. Xijuan, X. Pingbo, S. Qingde, Phys. Chem. Chem. Phys. 3 (2001) 5266.
14] S.T. Aruna, M.M. Muthuraman, K.C. Patil, J. Mater. Chem. 7 (1997) 2499.
15] T. Mimani, S. Ghosh, Curr. Sci. 78 (2000) 892.
16] P. Bera, M.S. Hegde, K.C. Patil, Curr. Sci. 80 (2001) 1576.
17] K. Deshpande, A. Mukasyan, A. Varma, J. Am. Ceram. Soc. 86 (2003) 1149.
18] L.A. Chick, L.R. Pederson, G.D. Maupain, J.L. Bates, L.E. Thomas, G.J. Exarhod, Mater. Lett. 10 (1990) 6.
19] M.B. Kakade, S. Ramanathan, P.K. De, Brit. Ceram. Trans. 102 (2003) 211.
20] R.E. Juarez, D.G. Lamas, G.E. Lascalea, N.E. Walsoe de Reca, J. Eur. Ceram. Soc. 20 (2000) 133.
21] K.H. Wu, T.H. Ting, M.C. Li, W.D. Ho, J. Magn. Magn. Mater. 298 (2006) 25.
22] R.S. Guo, Q.T. Wei, H.L. Li, F.H. Wang, Mater. Lett. 60 (2006) 261.
23] G.E. Lascalea, D.G. Lamas, E. Djurado, E.D. Cabanillas, N.E. Walsoe, Mater. Res. Bull. 40 (2005) 2029.
24] A. Mali, A. Ataie, Scripta Mater. 53 (2005) 1065.
25] Z. Xiu, M. Lu, G. Zhou, B. Su, H. Zhang, Mater. Res. Bull. 40 (2005) 1617.
26] T. Mahata, G. Das, R.K. Mishra, B.P. Sharma, J. Alloys Compd. 391 (2005) 129.
27] F. Qiu, X. Pu, J. Li, X. Liu, Y. Pan, J. Guo, Ceram. Int. 31 (2005) 663.
28] S. Kikkawas, H. Sasaki, H. Tamura, S. Hosokawa, H. Ogavwa, Mater. Res. Bull. 39 (2004) 1821.
29] J. Zhang, J. Ning, X. Liu, Y. Pan, L. Huang, Mater. Res. Bull. 38 (2003) 1249.
30] M. Wu, A. Chen, R. Xu, Y. Li, Mater. Sci. Eng. B 99 (2003) 336.
31] X. Qi, J. Zhou, Z. Yue, Z. Gui, L. Li, Mater. Chem. Phys. 78 (2003) 25.
32] S.A. Antony, K.S. Nagaraja, G.L.N. Reddy, O.M. Sreedharan, Mater. Lett. 51 (2001) 414.
33] L. Fraigi, D.G. Lamas, N.E. Walsoe de Reca, Nanostruct. Mater. 11 (1999) 311.
34] M.F. Bianchetti, R.E. Juarez, D.G. Lamas, N.E. Walsoe de Reca, J. Mater. Res. 17 (2002) 2185.
35] S.V. Chavan, A.K. Tyagi, J. Mater. Res. 19 (2004) 474.
36] K. Prabhakaran, J. Joseph, N.M. Gokhale, S.C. Sharma, R. Lal, J. Am. Ceram. Soc. 89 (2006) 2335.
37] K.J. Saunnders, Organic Polymer Chemistry, Chapman and Hall, London, 1977, pp. 121, 301–317.
38] K. Prabhakaran, C. Pavithran, Ceram. Int. 26 (2000) 67–71.
39] K. Prabhakaran, N. Ramesh Babu, S. Rajesh Kumar, K.G.K. Warrier, J. Am. Ceram. Soc. 85 (2002) 3126–3128.
40] G. Camino, L. Operty, L. Costa, L. Trosarelli, in: B. Miller (Ed.), Thermal Analysis, vol. II, John Wiley, New York, 1992, pp. 1144–1149.