Synthesis of alumina powders by the glycine-nitrate combustion process

Synthesis of alumina powders by the glycine–nitrate

combustion process

J.C. Toniolo*, M.D. Lima, A.S. Takimi, C.P. Bergmann

Department of Materials Engineering, Federal University of Rio Grande do Sul,

99 Osvaldo Aranha Av. 705, 90035190 Porto Alegre, RS, Brazil

Received 3 November 2003; received in revised form 7 May 2004; accepted 23 July 2004

Abstract

The combustion synthesis technique using glycine as fuel and aluminum nitrate as an oxidizer is able to produce

alumina powders. Thermodynamic modeling of the combustion reaction shows that as the fuel-to-oxidant ratio

increases, the amount of gases produced and adiabatic flame temperatures also increases. X-ray diffractions showed

the amorphous structure for as-synthesized powder and presence of well-crystallized a-Al2O3 after calcination at

1100 8C during soaking time of 1 h. Alumina’s largest measured specific surface area was 15 m2/g with BET

method and 0.51 glycine-to-nitrate ratio.

# 2004 Elsevier Ltd. All rights reserved.

Keywords: A. Ceramics; B. Chemical synthesis; C. X-ray diffraction; D. Thermodynamic properties

1. Introduction

Alumina powders with controlled size distribution find a wide variety of applications for advanced

engineering materials today. By and large, they are produced commercially by the Bayer process, which

has some limitation to obtain fine particles and purity.

In recent years increasing attention has been focused on the development of alumina nano-sized

powders. They have high potential for use as coatings [1], abrasives [2], catalyst supports [3], thermal

insulators [4], pollution prevention [5], sintering aid for ceramics [6], biocompatible material for medical

and dental composites [7,8], and nanocomposite for structural [9,10] and electrical [11,12] applications.

www.elsevier.com/locate/matresbu

Materials Research Bulletin 40 (2005) 561–571

* Corresponding author. Tel.: +55 51 32336916; fax: +55 51 33163405.

E-mail address: [email protected] (J.C. Toniolo).

0025-5408/$ – see front matter # 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2004.07.019

In fact, these materials offer novel characteristics in comparison with conventional alumina ceramics

because of their novel properties since as high hardness, toughness, compression strength, transparency,

wear resistance.

In order to achieve these requirements many researches have dedicated themselves to finding

alternatives such as chemical processes. There are several wet chemical techniques like freeze-drying,

spray-drying, co-precipitation, sol–gel and combustion synthesis, which have been employed to obtain

these ceramic powders.

Combustion synthesis is particularly an easy, safe and rapid production process wherein the main

advantages are energy and time savings. This quick, straightforward process can be used to produce

homogeneous, high-purity, crystalline oxide ceramic powders [13]. This method is versatile to synthesize

a broad range of particle sizes, including alumina nano-sized powders as related by Mimani and Patil

[14]. Interestingly, combustion of metal nitrate–glycine–ammonium nitrate redox mixtures [14] or metal

acetate–aluminum nitrate urea mixtures exhibited non-flaming linear combustion to yield nano-sized

oxide products [15].

The basis of the combustion synthesis technique comes from the thermochemical concepts used in the

field of propellants and explosives, and its extrapolation to the combustion synthesis of ceramic oxides

and thermodynamic interpretation is extensively discussed elsewhere [16]. The success of the process is

due to an intimate blending among the constituents using a suitable fuel or complexing agent (e.g., citric

acid, urea, glycine, etc.) in an aqueous medium and an exothermic redox reaction between the fuel and an

oxidizer (i.e., nitrates) [17].

Actually, the mechanism of the combustion reaction is quite complex. The parameters that influence

the reaction include: type of fuel, fuel to oxidizer ratio, use of excess oxidizer, ignition temperature, and

water content of the precursor mixture. In general, a good fuel should react non-violently, produce non-

toxic gases, and act as a complexant for metal cations [18].

One of the cheapest amino acids, glycine (NH2CH2COOH), is known to act as a complexing agent for

a number of metal ions as it contains a carboxylic acid group at one end and an amino group at the other

end [19]. Amino acids become zwitter ions on dissolving in water with both a positive and negative

charges. Such types of zwiterionic character of a glycine molecule can effectively complex metal ions of

varying ionic sizes, which helps in preventing their selective precipitation to maintain compositional

homogeneity among the constituents. On the other hand, glycine can also serve as a fuel during a

combustion reaction, being oxidized by nitrate ions [20].

According to Chick [19] glycine–nitrate combustion produces N2, H2O and CO2 as the gaseous

products. The initial composition of the solution containing aluminium nitrate and glycine was derived

from the total oxidizing and reducing valences of the oxidizer and fuel using the concepts of propellant

chemistry [20,21]. Carbon, hydrogen and aluminium were considered as reducing elements with the

corresponding valences of +4, +1 and +3, respectively. Oxygen was considered as an oxidizing element

with the valence of�2, nitrogen was considered to be 0. The total calculated valences of metal nitrates by

arithmetic summation of oxidizing and reducing valences was �15. The calculated valence of glycine

was +9. The stoichiometric composition of the redox mixture demanded that 1(�15) + n(+9) = 0, or n =

1.67 mol. Thus, the reactants were combined in the molar proportion of 1:1.67.

The powder’s characteristics such as crystallite size, surface area, extent, and nature (hard or soft) of

agglomeration are primarily governed by enthalpy or flame temperature generated during combustion,

which is itself dependent on the nature of the fuel and fuel-to-oxidant ratio [22]. Rapid evolution of large

volume of the gaseous products during combustion dissipates the heat of the process and limits

J.C. Toniolo et al. / Materials Research Bulletin 40 (2005) 561–571562

temperature increase, thus reducing the possibility of premature local partial sintering among the primary

particles. The gas evolution also helps in limiting the interparticle contact resulting in a more easily

friable product [20].

Recent research on combustion synthesis has been conducted in order to better understand the role of

the fuel in controlling particle size and microstructure of the product’s combustion. Only a few scientists

[20] have tried to investigate this effect for different fuel-to-oxidant ratios.

In this article, we report the synthesis of a-alumina powders by different fuel-to-oxidant molar ratios

using glycine as fuel and nitrate as an oxidizer. The powders obtained through combustion synthesis have

been characterized by scanning electron microscopy, X-ray diffraction line broadening and surface area

analyses. Enthalpies and adiabatic flame temperatures were calculated theoretically for the combustion

reactions involving different fuel-to-oxidant ratios. The nature of combustion and variation in the powder

characteristics obtained for different fuel-to-oxidant ratios is explained on the basis of adiabatic flame

temperatures.

2. Experimental procedure

Aluminum nitrate Al(NO3)3�9H2O (Vetec Quımica, Brazil) and glycine (Bio-Rad, USA) with 98 and

98.5% purities, respectively (vendor specification) were used as the starting material. Both were capable

of being mixed with minimum amount of deionized water. The solution was heated continuously without

any previous thermal dehydration. Afterwards the solution became transparent viscous gel which auto-

ignited automatically, giving a voluminous and foam product of combustion. The general flowchart for

the process is shown in Fig. 1.

Under continuous intense heating, the precursor mixture auto-ignited at approximately 140–150 8Cand underwent combustion spontaneously forming a powder which did not contain crystalline phases. In

J.C. Toniolo et al. / Materials Research Bulletin 40 (2005) 561–571 563

Fig. 1. Flowchart for the preparation of a-Al2O3 powder.

fact, on all fuel-to-oxidant ratios evaluated, upon auto-ignition, it resulted in a brownish voluminous

product identified by XRD as an amorphous structure, which indicates the incomplete combustion

probably due to characteristics of fuel employed.

Subsequently, these powders were calcined at 800, 900, 1000, 1050 and 1100 8C, at a heating rate of

approximately 9 K/min, during 1 h of soaking time.

Simple equipment was necessary to use as Bunsen-type burner and aluminum milk container. Each

solution was carried out at hood in order to avoid gas escaping. The temperatures of the combustion were

measured using Cyclops optical pyrometer (model 300 AF) and an adjustable type K thermocouple.

X-ray diffraction was executed on combustion-synthesized powders for phase characterization, at a

rate of 18/min, using Cu Ka radiation on a Philips X-ray diffractometer, (model X’Pert MPD). Silicon

was also employed as an external standard for correction due to instrumental broadening. Crystallite sizes

were obtained by X-ray diffraction line broadening through Williamson–Hall plot, assuming Cauchy–

Cauchy profile for size and strain contribution, respectively.

SEM micrographs were recorded from a Jeol (model JSM-5800) instrument after coating the samples

with gold. Surface area analysis of the alumina powders was performed by a standard BET technique

with N2 adsorption using Autosorb Quantachrome Instrument (model Nova 1200).

3. Results and discussion

The reactivity of the combustion reaction is dependent upon the ligand groups of the molecule and the

compositional ratio of fuel to nitrate. Therefore, the reactivity and the product phase from aluminum

nitrates with glycine were investigated before the synthesis of Al2O3 was undertaken.

There is evidence that Al ion may be complexed with amine group of glycine as it occurs in the

aluminum nitrate–urea process [23]. In this case, the Al ion is strongly complexed due to presence of two

amines on urea fuel. On the other hand, the aluminum nitrate–glycine process promotes lower adiabatic

flame temperatures, which could be seen by the smoldering phenomenon in comparison with the pair

aluminum nitrate–urea [16] that shows incandescent flame growing after auto-ignition. The physical

characteristics of the combustion reaction for different glycine-to-nitrate ratio are illustrated on Table 1.

The self-sustaining nature of the combustion can be clarified as follows. The combination of chemical

reagents contains a strong oxidizer, aluminum nitrate, and glycine, a readily combustible fuel. This

precursor mixture, after thermal dehydration, causes nitrate decompositon giving oxides of nitrogen


Table 1

Physical characteristics of the combustion reaction for different glycine-to-nitrate ratios

Glycine-to-nitrate

molar ratio

Reaction start

temperaturea (8C)

Maximum

temperatureb

(8C)

Flame

characteristic

Color of

fume

Color of

foam as-

synthesized

0.37 152 412 470 Smoldering Brown White-brown

0.43 148 445 500 Smoldering Brown

0.51 146 452 580 Smoldering Brown "0.56 150 422 620 Smoldering Brown

0.69 148 450 665 Smoldering Brown Browna Type K thermocouple measurement.b Type K and optical pyrometer respective measurements.

(NOx). The gaseous NOx reacts with the fuel generating heat and more gases. The homogeneous gas-

phase exothermic oxidation–reduction increases the temperature of the intact viscous mixture at once

adjoining the combustion zone, causing it to react. The reaction process comes out rapidly and sustains

until the entire intact zone is consumed.

The color of the powder as-synthesized was also found to change with the fuel-to-oxidant ratio used on

this process. This is attributed to the carbonaceous residue which remains from glycine due to insufficient

oxidizer specimen quantity.

The values obtained with thermocouple and optical pyrometer showed to be analogous for the ignition

temperature but are significantly different for maximum temperatures involved. These results are an

approach due to probable presence of inaccuracy measurement. Thermocouple has inertia and it is

measured on the punctual form. On the same way, as the thermocouple, it is hard to measure precisely the

temperature of the combustion wave front which appears during combustion using an optical pyrometer.

The emissivity value assumed as 0.9 typical for alumina is another feature, which determines uncertain

consequences of optical pyrometer measurement.

The water remaining in the precursor solution at the temperature of autoignition may also have a strong

effect on the flame temperature in compliance with Purohit et al. [22]. Resulting flame temperatures are

lowered because this residual liquid water must be converted to the vapor and be heated as if it were a

product of combustion. Civera et al. [30] believe that precursors which ignite at relatively low

temperatures are most prone to this effect.

No appreciable change was verified on quantity relationship among alumina powders formed by these

five fuel-to-oxidant ratios and precursor raw materials employed. An efficiency average of 75% of yield

was achieved.

3.1. Thermodynamic modeling

Redox reactions are usually exothermic in nature and often lead to explosion if not controlled. The

combustion of aluminum nitrate–glycine mixture appears to undergo a self-propagating and non-

explosive exothermic reaction.

A stoichiometric mixture of fuel and oxidant is one in which the quantity of oxidant present is

theoretically correct for complete oxidation [20]. The combustion reactions can be represented,

respectively, as follows:

Stoichiometry

1:0AlðNO3Þ3 � 9H2Oþ 1:67NH2CH2COOH! 0:5Al2O3 þ 2:33N2 þ 3:33CO2

þ 13:17H2O (1)

Fuel-rich (+ 24.55%)

1:0AlðNO3Þ3 � 9H2Oþ 2:08NH2CH2COOHþ 0:94O2! 0:5Al2O3 þ 2:54N2 þ 4:17CO2

þ 14:21H2O (2)

Fuel-lean (�8.98%)


þ 12:79H2Oþ 0:34O2 (3)


Fuel-lean (� 23.35%)


þ 12:21H2Oþ 0:87O2 (4)

Fuel-lean (�33.53%)


þ 11:78H2Oþ 1:25O2 (5)

If the quantity of oxygen in the combustible mixture is in excess of that required for complete

combustion of the fuel, then a portion of the oxygen does not react and appears in the exhaust [20].

Available thermodynamic data in literature [24,25] for various reactants and products are presented in

Table 2. It is well known that the enthalpy of combustion can be expressed as:

DH0 ¼X

nDH0p

� ��

XnDH0

r

� �

and

DH0 ¼Z T

T0

XnCp

� �dT ; T ¼ T0 þ

DH0r � DH0

p

Cp

where n is the number of the mol, DH0r and DH0

p the enthalpies of formation of the reactants and

products, respectively, T the adiabatic flame temperature, T0 the 298 K and Cp is the heat capacity of

products at constant pressure. Using the thermodynamic data for various reactants and products listed in

Table 2, the enthalpy of combustion and the theoretical adiabatic flame temperatures as a function of

glycine-to-nitrate molar ratio can be calculated. Nevertheless, the measured flame temperatures are

typically much lower than calculated values as a result of radioactive losses, incomplete combustion,

and heating of air.

The adiabatic flame temperature T of the reaction is influenced by the type of fuel, fuel to oxidizer

ratio, and the amount of water remaining in the precursor solution at the ignition temperature [26]. The


Table 2

Relevant thermodynamics data

Compound DHf (kcal mol�1) Cp (Cal mol�1 K�1)a

Al(NO3)3�9H2O (c) �857.59 –

NH2CH2COOH (c) �79.71 –

NH4NO3 �87.40 –

Al2O3 (c) �399.09 28.062 + 0.01038T

CO2 (g) �94.051 10.34 + 0.00274T

N2 (g) 0 6.50 + 0.0010T

O2 (g) 0 5.92 + 0.00367T

H2O (g) �57.796 7.20 + 0.0036T

NO2 (g) �33.2 –

(c): Crystalline; (g): gas.; T : absolute temperature.a Calculated from the discrete values.

https://www.researchgate.net/publication/25312841_Lange's_Handbook_of_Chemistry_11th_Ed?el=1_x_8&enrichId=rgreq-2791b9a2-e1f3-4d69-9a46-4e6af9f02b11&enrichSource=Y292ZXJQYWdlOzIyMjIzNTU0MTtBUzoxMDQ5NDQxNzA0Mzg2NjBAMTQwMjAzMjA0OTUyMg==

flame temperature can be increased with the addition of excess oxidizer such as ammonium nitrate [18],

or by increasing the fuel/oxidizer molar ratio. Segadaes and co-workers [16] calculated theoretical

adiabatic flame temperatures in the case of urea–nitrate combustion synthesis of ZnO. Purohit et al. [20]

also calculated theoretical adiabatic flame temperatures involving glycine–nitrate combustion synthesis

of CeO2.

The variation of enthalpy and adiabatic flame temperature with the glycine-to-nitrate molar ratio could

be seen in Figs. 2 and 3. As expected, they increase substantially with the amount of the fuel used during

combustion.

3.2. Phase formation and morphology

Pramanik and co-workers [27] carried out thermogravimetric and differential thermal analysis (TG–

DTA) in air at a heating rate of 5 K/min. The DTA curve indicated that Al2O3 precursor had been

decomposed exothermically, with sharp peaks at 534 and 362 8C. As seen, from Fig. 3, all calculated

adiabatic flame temperatures for the combustion synthesis reactions are much higher than the decom-

position temperature of Al(NO3)3�9H2O. The XRD of the as-synthesized powder also confirmed the

amorphous structure, which denotes the absence of crystalline phase from precursor solution for all

compositions. The formation of crystalline alumina in situ did not occur for all experiments because the

temperature generated was not enough to promote some alumina crystallization as expected.


Fig. 2. Variation of enthalpy as a function of glycine-to-nitrate molar ratio.

Fig. 3. Variation of adiabatic flame temperature as a function of glycine-to-nitrate molar ratio.

After the calcination procedure the XRD identification was performed, revealing presence of well-

crystallized a-alumina for all glycine-to-nitrate ratios at 1100 8C as shown in Fig. 4. The g-phase has

been detected since 800 8C (Table 3).

The minimum amount of fuel used in the case of the fuel-lean results in a small enthalpy and hence the

local temperature of the particles remains low, which may prevent the formation of a dense structure.

Associated gas evolution results in highly porous structure, i.e., as the amount of gas increases

agglomerates are more likely to break up and more porosity will be observed. Table 3 shows an

increase in the total number of mol of gases evolving during the reaction when the glycine-to-nitrate

molar ratio increases. There is possibly a competition on the reaction between the adiabatic flame

temperature and gases evolving.

Our results tend to indicate that the former plays a predominant role for stoichiometric, fuel-lean and

fuel-rich compositions, though it may be not verified for the high value obtained with fuel-lean (0.51


Fig. 4. X-ray diffractions of a-alumina ceramic powder (0.37 ratio): (a) as-synthesized, (b) 800 8C, (c) 900 8C, (d) 1000 8C, (e)

1050 8C and (f) 1100 8C.

Table 3

Effect of glycine-to-nitrate molar ratio on adiabatic flame temperature and total number of mol of gases evolved

Glycine-to-nitrate molar ratio Adiabatic flame temperature (8C) Total number of mol of gases

0.37 732.65 17.31

0.43 992.44 17.78

0.51 1320.13 18.42

0.56 1518.34 18.83

0.69 1871.68 21.86

ratio). It seems that there might possibly be a narrow range of composition close to this ratio, which is

more susceptible to the presence of gases.

Table 4 shows variation in the specific surface area of the alumina as a function of the glycine-to-

nitrate molar ratio. The composition fuel-lean (0.51 ratio) has the highest surface area, while the other

glycine-to-nitrate ratios exhibited low value in comparison to each other. Actually, there is a meaningful

tumble of the values present towards both sides (full-lean 0.37 ratio and fuel-rich 0.69 ratio). In fact, the

15 m2/g optimal surface area found is larger than the 8 m2/g one found in the aluminium nitrate-urea

process [23].

Crystallite sizes, as calculated using Williansom–Hall plot [28], are presented in Fig. 5 above. The

crystallite sizes increase with glycine-to-nitrate ratio increases. It is attributed to an increase in flame

temperature, which assists crystal growth. There is a correlation between the increase in crystallite size

and the reduction of surface area as a function of fuel content in the case of glycine–nitrate combustion.

Many authors have already observed this relationship [22,29].

The SEM morphology of the agglomerates of the alumina is shown in Fig. 6. It exhibited foamy

agglomerated particles with a wide distribution (low magnification) and presence of larger voids in their

structure (high-magnification). Formation of these features is attributed to the evolution of a larger

amount of gas during combustion.


Table 4

Effect of surface area and crystallite size at 1100 8C for different glycine-to-nitrate ratios

Glycine-to-nitrate molar ratio Surface area (m2/g) Crystallite size (nm)

0.37 8.7 90.3

0.43 11.3 84.7

0.51 15.1 90.3

0.56 10.9 96.8

0.69 8.3 123.2

Fig. 5. Surface area and crystallite size at 1100 8C.

4. Conclusion

Glycine–nitrate combustion synthesis has an outstanding potential for producing pure alumina

powders. Optimal surface area could be obtained with 0.51 glycine-to-nitrate molar ratio. Thermo-

dynamic modeling of the combustion reaction shows that when fuel-to-oxidant ratio increases the

amount of gas produced and adiabatic flame temperature also increases.

Acknowledgements

Thanks are due to Undergraduate student Renato Bonadiman for combustion synthesis measurements,

Eng. Monica J. de Andrade for providing support on Scanning Electron Microscopy and Dr. Joao Marcos

Hohemberger for the aid in X-ray techniques.

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Synthesis of alumina powders by the glycine-nitrate combustion process