Thermal stability and crystallisation behaviour of amorphous alumina-yttria synthesised by co-precipitation and combustion synthesis

Ceramics International 45 (2019) 2368–2380

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Thermal stability and crystallisation behaviour of amorphous alumina-yttria synthesised by co-precipitation and combustion synthesis

T

Indumathi Nainara, , S.S. Bhattacharyaa, Ashutosh S. Gandhib ⁎

a b

Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400076, India

ARTICLE INFO

ABSTRACT

Keywords: Amorphous materials Alumina-yttria Co-precipitation synthesis Solution combustion synthesis

Amorphous alumina-yttria powders with 20, 25, 30 and 37.5 mol% yttria were synthesised by a co-precipitation method using the respective nitrates as precursors for identifying a composition with the best amorphous phase stability. Thermal analysis showed that calcination to yield amorphous oxides completed at around 800 °C. Crystallisation behaviour investigated by x-ray diffraction (XRD) showed that Al2O3–25 mol% Y2O3 (25Y) had the highest crystallisation temperature of 900 °C. Crystallisation involved the appearance and disappearance of metastable hexagonal yttrium aluminate (H-YAlO3) below 1000 °C. The phase evolution sequences for all the compositions were studied in detail. All the phases were nanocrystalline, establishing the efficacy of amorphous phase crystallisation as a viable route for the synthesis of nanocrystalline ceramics. Solution combustion synthesis (SCS) was selected to synthesis 25Y with enhanced amorphous phase stability using different proportions of citric acid and ethylene glycol. Thermal analysis revealed that the powder was free of organic residue after heat treating at 650 °C. The powder produced with 10 vol% of fuel (citric acid + ethylene glycol) yielded the highest crystallisation temperature of 925 °C and the appearance and disappearance of H-YAlO3 between 950 and 1100 °C. Fourier transform infrared (FT-IR) spectroscopy revealed that the co-precipitation synthesis might have residual NH4+ and NO3- radicals whereas SCS did not contain any residual carbonate above 700 °C.

1. Introduction Alumina-based amorphous oxides are of scientific interest owing to their potential for interesting properties, such as high refractive index, chemical inertness and hardness [1,2]. Alumina-zirconia amorphous materials undergo large-scale plastic deformation in compression at temperatures below crystallisation temperatures [3]. Localised deformation in the form of shear bands has been observed in these materials, and the deformation behaviour is similar to that of metallic glasses when deformed below the glass transition temperature. Largescale plasticity has been observed in alumina-yttria amorphous oxides. Alumina-yttria glasses also exhibit the exciting phenomenon of polyamorphism – phase separation into two glassy phases of the same composition but different densities [4,5]. Alumina with lanthanoid oxide additions has also been prepared as glassy materials [6]. An amorphous phase, being far from equilibrium, always has a driving force for crystallisation. The crystallisation kinetics can be such that the crystalline phase(s) are of nanometric dimensions. If the amorphous phase is prepared in the bulk form, then its crystallisation can lead to the formation of bulk nanocrystalline ceramics. This is an

⁎

attractive alternative to sintering of ceramic nanoparticles, because grain growth during sintering makes it difficult to retain nanometric grain sizes while achieving complete densification. This approach of producing nearly fully dense alumina-zirconia amorphous oxide and then crystallising it into bulk nanocrystalline oxide ceramic has been demonstrated by Gandhi et al. [7] and Rosenflanz et al. [6]. In spite of the interesting and potentially useful characteristics such as high hardness and refractive index, alumina-yttria amorphous/glassy materials did not garner a lot attention owing to the difficulties in processing these materials into bulk dense forms. The amorphous material is, of necessity, produced in particulate form and must be sintered without crystallisation. The particulate forms can be produced either by rapid solidification or chemical precursor synthesis. Temperature more than ~ 2000 °C is normally required to melt alumina with zirconia, yttria or lanthanoid oxide additions [6]. Moreover, crystallisation of the melt can only be avoided by employing rapid solidification techniques for obtaining glassy/amorphous oxides. The rapid solidification inherently produces the material in the form of powders or flakes, necessitating a consolidation step to obtain bulk materials [8]. While some success has been achieved in producing dense alumina-zirconia

Corresponding author. E-mail address: [email protected] (I. Nainar).

https://doi.org/10.1016/j.ceramint.2018.10.154 Received 20 August 2018; Received in revised form 16 October 2018; Accepted 18 October 2018 Available online 21 October 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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[7], alumina-yttria [9–11] and alumina-lanthanoid [6] oxides systems by pressure assisted sintering, the processing challenges have not been fully met. The main reason behind the processing difficulty is the low crystallisation temperatures of the amorphous oxides. The thermal activation provided for bringing about sintering also triggers crystallisation. Therefore, the apparent strategy is to increase the crystallisation temperature of the amorphous oxide. Amorphous/glassy alumina based oxides can also be produced by chemical synthesis methods, such as reverse co-precipitation (aluminayttria) [12] and precursor spray pyrolysis (alumina-zirconia) [7]. Maintaining the chemical homogeneity throughout the low temperature synthesis is of importance in the synthesis of amorphous materials. Homogeneous powders of Al2O3-Y2O3 have been synthesised using spray pyrolysis and reverse co-precipitation methods, with amorphous or fine crystalline phases. Ullal et al. [13] observed that amorphous alumina with 4–15 mol% yttria had crystallisation temperature higher than stiochiometric YAG. Among alumina-yttria compositions studied by Gandhi et al. [12], the stoichiometric YAG composition showed the lowest crystallisation temperature. As the composition deviated from the stoichiometric YAG, the crystallisation temperature increased. Hence, the maximum crystallisation temperature is likely between 15 and 37.5 mol% yttria content. With this view, the dependence of the crystallisation temperature of amorphous alumina-yttria oxides was investigated to determine the effect of different composition and chemical synthesis technique (reverse co-precipitation and solution combustion). As remarked above, the amorphous oxides serve as precursors to nanocrystalline ceramics. Hence, this study also investigated the phases formed during crystallisation and subsequent thermal treatment.

rapid process for producing ultrafine powders. Three different methods were used with the aim of obtaining an amorphous phase with improved thermal stability. Method 1 (M1): Equal volumes of aqueous metal nitrate solution and aqueous fuel solution (5 vol% of fuel – citric acid + ethylene glycol). Method 2 (M2): Equal volumes of aqueous metal nitrate solution and aqueous citric acid solution (5 vol% of fuel). Method 3 (M3): Aqueous fuel solution (10 vol% of fuel – citric acid + ethylene glycol) was taken 5 times the volume of aqueous metal nitrate solution. Equivalence ratio (φe) of total oxidising (O) and reducing (F) valency was calculated for the stoichiometric composition of metal nitrates (oxidisers) and fuel for the combustion process. Equivalence ratio was calculated to ensure the evolution of energy released during combustion is maximum (O/F = 1) [14]. In the combustion process, aqueous citric acid and ethylene glycol solutions were added to the 0.3 M metal nitrate solution. The aqueous solution dried to form a gel by heating on a hot plate with continuous stirring. The gel was then kept in a furnace preheated at 600 °C for 20 min which leads to a rapid exothermic reaction with the evolution of CO2, H2O and N2 volatile matters. The resultant solid product was ground to produce a fine powder. The as-synthesised powders from co-precipitation and combustion routes were calcined at various temperatures between 700 and 1500 °C for 5 min at a heating rate of 10 °C/min followed by air cooling to investigate the thermal stability of the amorphous phase as well as the crystalline phase evolution.

2. Experimental work 2.1. Powder synthesis by co-precipitation method

2.3. Powder characterisation

The following abbreviations are used throughout the paper: Al2O3–20 mol% Y2O3 (20Y), Al2O3–25 mol% Y2O3 (25Y), Al2O3–30 mol % Y2O3 (30Y), Al2O3–37.5 mol% Y2O3 (37.5Y). Aluminium nitrate nonahydrate (Al(NO3)3·9H2O - CDH laboratory, New delhi, 98% purity) and yttrium nitrate hexahydrate (Y(NO3)3·6H2O - Sigma-Aldrich, USA, 99.8% purity) were used as starting materials. Alumina-yttria powders of desired compositions were prepared by the reverse co-precipitation route. An aqueous chemical precursor solution was prepared by dissolving required amounts of both aluminium nitrate and yttrium nitrate in de-ionised water to obtain a concentration of 0.3 M. Precipitation was prepared by adding the nitrate precursor solution to an ammonia precipitant solution (reverse-strike technique). This technique was selected for obtaining cation homogeneity in the hydroxide precipitate with both the cations. The mixed nitrate solution was added drop-wise to ammonia solution under continuous stirring with the pH being maintained at 9.2–9.3. The resultant gelatinous white precipitate was filtered and washed with de-ionised water. The precipitate was then dried at 120 °C for 14 h and then ground with an agate mortar and pestle to produce a fine powder. At this stage, all the powders were predominantly mixed hydroxides. Calcination was performed between 700 and 1500 °C as described in the following section.

Calcination and crystallisation behaviour of the as-dried reverse coprecipitation and the combustion synthesis powders were examined by simultaneous thermogravimetry and differential scanning calorimetry (TGDSC, TA instruments, SDT Q600) at a heating rate of 10 °C/min from room temperature to 1200 °C in nitrogen atmosphere. The phase evolution and the crystalline structure of all the calcined powders were analysed by X-ray diffraction (XRD, X′Pert Pro, PANalytical) with Cu-Kα radiation in the diffraction angle range of 10–90° and a step size of 0.02°. Table 1 represents the key used for all the phases and their respective JCPDS file number. The crystallite size was calculated using Scherrer formula Eq. (1):

D=

0.94 cos

(1)

where D is the crystallite size in nm, λ is the wavelength, β is the full width at half maximum (FWHM) in radians and θ is the Bragg's angle. Standard silicon was used as a reference for calculating the instrumental contribution to the peak FWHM. Selected area electron diffraction (SAED) study and imaging were performed on the powders by transmission electron microscopy (TEM, Tecnai 20G2) operating at 200 kV. Infrared spectra of the calcined powders were recorded by the Fourier transform infrared spectrometer (FT-IR, Perkin-Elmer Spectrum One) in the wavenumber range of 4000–450 cm−1 using the standard KBr technique to identify the nature of bonding and structural features of the heat-treated powders.

2.2. Powder synthesis by solution combustion method Aluminium nitrate nonahydrate and yttrium nitrate hexahydrate (as detailed above) were used as starting materials. Citric acid monohydrate (C6H8O7·H2O - Merck, Mumbai, 99% purity) and ethylene glycol (C2H6O2 - Merck, Mumbai, 99% purity) were used as fuel. Based on the results of co-precipitation, 25Y was selected for solution combustion synthesis. Solution Combustion Synthesis (SCS) was selected since it is believed to avoid the formation of hydroxides as intermediate compounds before the formation of oxides, in contrast to the reverse coprecipitation technique. Moreover, SCS is known as an inexpensive and

Table 1 Compounds and their respective JCPDS file number.

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Compound

Key

JCPDS file number

γ- Al2O3 (FCC) YAlO3 (hexagonal) Y3Al5O12 (YAG-BCC) α- Al2O3 (Rhombohedral)

γ H G α

10-0425 74-1334 33-0040 46-1212

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Fig. 1. TGA/DSC curves for alumina – yttria powder produced by co-precipitation method (a) TGA curve (b) DSC curve (c) TGA/DSC curve for alumina – 25yttria powder produced by combustion method (M3).

3. Results and discussion 3.1. Co-precipitation synthesis The TG-DSC plots for the as-dried co-precipitated alumina-yttria powders are shown in Fig. 1a-b. The TG plot (see Fig. 1a) shows that the weight loss upon calcination was 44%, 46%, 51% and 41% for 20Y, 25Y, 30Y, and 37.5Y, respectively. The thermal decomposition of all the powders occurred during heating from 30 to 600 °C due to evaporation of water, residual nitrous species and dehydration of mixed Al-Y hydroxide. The completion of thermal decomposition into oxides was observed for all the powders at around 800 °C. The DSC result (see Fig. 1b) reveals that the endothermic peaks at 110 °C and 250–500 °C were due to thermal decomposition and release of residual CO2, NO2, NH4 and other volatile matter. These endotherms enlighten the weight loss shown in the TG plot. In the case of 20Y, the exothermic peak at 910 °C (Fig. 1b) reveals the initiation of the first crystalline γ-Al2O3 phase as shown in XRD pattern (Fig. 2). The exothermic peaks observed for 25Y, 30Y and 37.5Y near 930 °C, 932 °C and 920 °C, respectively, were due to crystallisation of hexagonal YAlO3 (H) with traces of yttrium aluminium garnet (Y3Al5O12–G). The exothermic peaks at 1028 °C for 20Y and 1040 °C for 25Y were due to the formation of G with a small volume fraction of γ-Al2O3. Exothermic peaks at 1031 °C and 1025 °C observed for 30Y and 37.5Y, respectively, were due to the complete transition of phase from H to G which is in line with the XRD results (Fig. 2). In the co-precipitation process, the stoichiometric amounts of

Fig. 2. XRD pattern for alumina – yttria powder prepared by co-precipitation method.

aluminium nitrate and yttrium nitrate were mixed with deionised water to obtain an aqueous solution in which the nitrates would dissociate into metal ions and NO3 [15,16]. Eq. (2) explains the co-precipitation reaction between the aqueous solution of metal ions and NH4OH(aq)

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solution (As the nitrates were dissolved in water, their water of crystallisation is not considered in the reaction).

Amorphous phase), and is also pictorially represented in Fig. 3(a). As an example, XRD data for 25Y is shown in Fig. 3(b).

m Al(NO3)3 (aq) + n Y(NO3)3 (aq) + 3(m+n)NH4OH (OH)3(m+n) (ppt) + 3(m+n)NH4NO3 (aq)

20Y: Mixed(Al Y)hydroxide

(aq)→(AlmYn)

(2)

700°C

Am(Al2 O3

1000°C

The co-precipitation process requires proper control over pH, since the precipitation of Al(OH)3 and Y(OH)3 occur in the range of pH 5–9 and pH 7–9, respectively [16]. In the present work, pH of 9.2–9.3 was maintained to control the nucleation and growth of a homogenous precipitate (AlmYn)(OH)3(m+n). For all the compositions, the mixed metal hydroxide gets dehydrated to amorphous form (Al-Y-O) during the calcination process. The Al-Y hydroxide loses its physically adsorbed water when heating in the air and leads to the formation of amorphous Al-Y-O oxide. As Y3+ is a larger cation than Al3+, it acts as a network modifier in an irregularly cross-linked amorphous network which is formed by Al3+ and O2-. With subsequent thermal exposure, the amorphous oxide transforms into crystalline phases. The first crystallisation event detected by XRD is shown in Fig. 2 for all the reverse co-precipitated powders. The primary crystalline phase was γAl2O3 at 900 °C for 20Y, whereas simultaneous formation of H-YAlO3 and YAG was observed at 925 °C for 25Y, 30Y and 37.5Y. The complete phase evolution sequence is given in the Eqs. (3)–(6) (Am represents

Y2O3) 1100°C

Am + G+

900°C

G+

Am+ 1300°C

G+ (3)

25Y: Mixed(Al

Y)hydroxide

700°C

1000°C

30Y: Mixed(Al

Y)hydroxide

700°C

1100°C

37.5Y: Mixed(Al

Y)hydroxide

Am(Al2 O3 G+

G+

G

1400°C

925°C

H+G (4)

G+

Am(Al2 O3

700°C

1100°C

1400°C

Y2O3)

Y2O3)

925°C

H+G (5)

G+

Am(Al2 O3

Y2O3)

925°C

H+G (6)

In the case of 20Y, crystallisation occurred with the evolution of γAl2O3 from the amorphous phase. The XRD pattern of powder heat treated at 950 °C (Fig. 2) indicates the presence of amorphous phase along with γ-Al2O3. It has been demonstrated by Ullal et al. [13] that up to ~4 mol% yttria may be metastably dissolved in γ-Al2O3 when it

Fig. 3. (a) Phase evolution sequence for alumina – yttria powder prepared by co-precipitation method (b) XRD pattern for alumina – 25 yttria powder obtained by calcining at different temperatures for 5 min. (c) Phase evolution sequence for alumina – 25 yttria powder produced by combustion synthesis using 3 different methods (d) XRD pattern for combusted alumina – 25 yttria powder (M3 method) obtained by calcining at different temperatures for 5 min. 2371

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crystallises from the amorphous phase. Hence, in 20Y, excess yttria would be rejected into the remaining amorphous phase during the formation of γ-Al2O3. The crystallisation of G from the residual amorphous phase initiated at 1000 °C, and γ to α transformation occurred at 1300 °C. In accordance with the equilibrium phase diagram, the twophase mixture of α + G was stable up to 1500 °C. There was no appearance of Y-rich phases such as H, orthorhombic YAlO3 (O) and Y in the phase evolution of 20Y. In the compositions of 25Y, 30Y and 37.5Y the amorphous phase was stable up to 900 °C. The metastable H and G formed at 925 °C. At higher temperatures, H transformed into G. A small amount of γ formed in 25Y. G+H transformed into G + γ and G in 30Y and 37.5Y, respectively. The two-phase G + γ microstructure observed at and above 1100 °C in 30Y was due to slower cation diffusion [13]. Hence, the phase evolution for 25Y, 30Y and 37.5Y involved 3 distinct paths: 1) There was always a competition between the formation of H and G crystalline phases from an amorphous phase. If G crystallites had already formed at a lower temperature (850 °C) then the nucleation of metastable H was not favoured due to the growth of pre-existing G crystals [17]. This was due to different nucleation and growth kinetics of these two phases. 2) The crystallisation of γ started after the disappearance of metastable H. 3) γ converted into α at high temperature (except stoichiometric composition 37.5Y). The preference for the formation of H or G from the parent amorphous phase depended on the free energy and the complexity of the structure. From the free energy-composition curve constructed for Al2O3-Y2O3 [13], the free energy G (YAG) = −69 kJ/mol and G (H) = −50 kJ/mol. Garnet structure (G) has the most complex unit cell volume of 1.732 nm3 which is 14.9 times of H unit cell. Hence H can form over a range of temperature as it is easier to nucleate [13]. In the case of stoichiometric composition 37.5Y, only G was observed at high temperature. From the above investigations, 25Y had the highest crystallisation temperature of 900 °C with the appearance and disappearance of H phase below 1000 °C when compared to 30Y and 37.5Y. Intermediate phases such as YAM (monoclinic: Y4Al2O9) and YAP (perovskite: YAlO3) were not observed in any of the compositions. Frequencies of functional groups present in the co-precipitated alumina-yttria powders were studied using FT-IR analysis. Aluminayttria powders with the composition range from 20Y to 37.5Y and heat treated at different temperatures for 5 min were chosen for FT-IR studies. FT-IR spectroscopy reveals crystalline transition and decomposition pathways involved in the powder. As an example, the FT-IR spectra for 25Y at each heat treatment step are shown in Fig. 4a. The detailed

analysis of all the powders is given in Table 2. For all the heat treated powders between 700 and 1200 °C, the broad absorption band observed near 3443 and 1639 cm−1 was associated with O-H stretching and O-H bending vibrations respectively, which can be the adsorbed water during the exposure of powders to ambient air [18,19]. This remained unchanged with increase in calcination temperature up to 1200 °C. The bands at around 1515 and 1395 cm−1 were associated with the stretching vibrations of residual NH4+ (ammonia) and NO3− (nitrate) groups, respectively. The bands became very weak and nearly disappeared with an increase in calcination temperature (below 900 °C) due to the decomposition of residual NH4+ and NO3-, prior to crystallisation. This was an important observation, indicating that complete conversion to amorphous oxide occurred before crystallisation. However, in these powders prepared by the co-precipitation route, residual NH4+ and NO3− persisted even after calcination at 800 °C, which might have influenced the amorphous phase characteristics and subsequent crystallisation behaviour. The FT-IR data were consistent with the TGA plots (Fig. 1a). The broad and smooth absorption band from 550 to 900 cm−1 indicated the development of amorphous metal oxides up to 900 °C for 25Y-37.5Y was due to the disordered distribution of free volume and variation of bond length in an amorphous material [20]. The FT-IR data on crystalline powders are presented in this section. In the case of 20Y, γ started to evolve at 900 °C as the first crystalline phase. However, majority of the phase was still amorphous. This is seen from the presence of a broad amorphous peak in the XRD pattern (Fig. 2) and further confirmed from the broad and smooth absorption band at 550–900 cm−1 in the FT-IR spectrum. In all the compositions from 20Y-37.5Y, above 900 °C, FT-IR showed several intense peaks in the range of 1000–450 cm−1 indicating metal-oxygen (M-O) vibrations. The compositions 20Y (1000–1200 °C), 25Y (925–1200 °C), 30Y (1000–1100 °C) and 37.5Y (1000 °C) exhibited the characteristic vibration bands of both AlO4 tetrahedral and AlO6 octahedral units. The compositions 30Y (1200 °C) and 37.5Y (1100–1200 °C) revealed the presence of only AlO6 octahedral unit. The bands associated with Al-O, Y-O and Y-O-Al stretching were consistent with the formation of the G phase, and the peak intensities increased with an increase in the heat treatment temperature. The absence of AlO4 vibration bands in 30Y and 37.5Y may be due to the relatively low overall fraction of tetrahedral Al3+, although G has tetrahedral Al3+. These results were in good agreement with the crystallisation peaks observed in DSC and XRD analysis. The summary of crystallite size evolution in the alumina-yttria systems synthesised by reverse co-precipitation is shown in Table 3.

Fig. 4. FT-IR sequence for alumina – 25yttria powder produced by a) co-precipitation method (b) combustion method (M3). The powder was obtained by calcining at different temperatures for 5 min. 2372

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1100

1000

900

800

700

Temperature (°C)

α~3443 cm−1 β~1639 cm−1 ε~455, 470 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ ~621 cm−1 α~3443 cm−1 β~1639 cm−1 ε~455, 470 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ~621 cm−1

α~3443 cm β~1639 cm−1 γ ~1515 cm−1 δ~1395 cm−1 Am~550–900 cm−1 α~3443 cm−1 β~1639 cm−1 γ~1515 cm−1 δ~1395 cm−1 Am~550–900 cm−1 α~3443 cm−1 β~1639 cm−1 550–900 cm−1

α~3443 cm−1 β~1639 cm−1 ε~470 cm−1 η~803 cm−1 λ ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ~621 cm−1 α~3443 cm−1 β~1639 cm−1 ε~470 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ~621 cm−1

α~3443 cm β~1639 cm−1 γ ~1515 cm−1 δ~1395 cm−1 Am~550–900 cm−1 α~3443 cm−1 β~1639 cm−1 γ~1515 cm−1 δ~1395 cm−1 Am~550–900 cm−1 α~3443 cm−1 β~1639 cm−1 Am~550–900 cm−1

−1

−1

37.5Y −1

α~3443 cm−1 β~1639 cm−1 ε~455 cm−1 η~803 cm−1 λ~720, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 α~3443 cm−1 β~1639 cm−1 η~803 cm−1 λ~720, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1

α~3443 cm−1 β~1639 cm−1 ε~470 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ~621 cm−1

α~3443 cm β~1639 cm−1 γ~1515 cm−1 δ~1395 cm−1 Am~550–900 cm−1 α~3443 cm−1 β~1639 cm−1 γ~1515 cm−1 δ~1395 cm−1 Am~550–900 cm−1 α~3443 cm−1 β~1639 cm−1 Am~550–900 cm−1

α~3443 cm−1 β~1639 cm−1 ε~455, 470 cm−1 η~803 cm−1 λ~720, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1

α~3443 cm β~1639 cm−1 γ~1515 cm−1 δ~1395 cm−1 Am~550–900 cm−1 α~3443 cm−1 β ~1639 cm−1 γ ~1515 cm−1 δ~1395 cm−1 Am~550–900 cm−1 α~3443 cm−1 β~1639 cm−1 Am ~550–900 cm−1

30Y

α~3443 cm−1 ʊ~2350 cm−1 β~1639 cm−1 ε~455 cm−1 η~803 cm−1 λ~720 cm−1 φ~680 cm−1 α~3443 cm−1 ʊ~2350 cm−1 β~1639 cm−1 ε~455 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 α~3443 cm−1 ʊ~2350 cm−1 β~1639 cm−1 ε~455, 470 cm−1 η~803 cm−1 λ~720,531 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ~621 cm−1

α~3443 cm−1 ʊ~2350 cm−1 β~1639 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 α~3443 cm−1 ʊ~2350 cm−1 β~1639 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 α~3443 cm−1 ʊ~2350 cm−1 β~1639 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ~621 cm−1

α~3443 cm−1 ʊ~2350 cm−1 β~1639 cm−1 Am~550–900 cm−1

α~3443 cm−1 ʊ~2350 cm−1 β~1639 cm−1 550–900 cm−1

−1

α~3443 cm ʊ~2350 cm−1 ¥~1565, 1425 cm−1 Am~550–900 cm−1

M2

α~3443 cm ʊ~2350 cm−1 ¥~1565, 1425 cm−1 550–900 cm−1

−1

M1

25Y

20Y

−1

Combustion synthesis methods for 25Y

Mole percentage of yttria for co-precipitation

Table 2 FT-IR sequence for alumina- yttria powder produced by co-precipitation and combustion methods.

(continued on next page)

α~3443 cm−1 ʊ~2350 cm−1 β~1639 cm−1 ε~455 cm−1 η~803 cm−1 λ~720, 531 cm−1 φ~680 cm−1 ϴ~569 cm−1 α~3443 cm−1 ʊ~2350 cm−1 β~1639 cm−1 ε~470 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ~621 cm−1

α~3443 cm−1 ʊ~2350 cm−1 β~1639 cm−1 Am~550–900 cm−1

α~3443 cm−1 ʊ~2350 cm−1 β~1639 cm−1 Am~550–900 cm−1

α~3443 cm−1 ʊ~2350 cm−1 ¥~1565, 1425 cm−1 Am~550–900 cm−1

M3

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α~3443 cm β~1639 cm−1 ε~455, 470 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ~621 cm−1 α~3443 cm β~1639 cm−1 ε~470 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ~621 cm−1

−1

30Y −1

37.5Y −1

α~3443 cm β~1639 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ~621 cm−1

α~3443 cm β~1639 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1

α~3443 cm ʊ~2350 cm−1 β~1639 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ~621 cm−1

−1

−1

α~3443 cm ʊ ~2350 cm−1 β~1639 cm−1 ε~455, 470 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ~621 cm−1

M2

M1

25Y

20Y

−1

Combustion synthesis methods for 25Y

Mole percentage of yttria for co-precipitation

b is for broad and w is for weak band.

Symbol α β γ δ Am ε η λ φ ϴ μ ¥

1200

Temperature (°C)

Table 2 (continued)

Compounds OH stretch (b) OH bend (w) NH4+ stretch (w) NO3− stretch (w) Amorphous AlO4 tetrahedra (stretch) AlO6 octahedra (stretch) Y-O vibrations Y-O-Al (stretch) Al-O (stretch) Al-O-Al vibrations CO32− stretch (w)

α~3443 cm−1 ʊ~2350 cm−1 β~1639 cm−1 ε~470 cm−1 η~803 cm−1 λ~720, 531, 590 cm−1 φ~680 cm−1 ϴ~569 cm−1 μ~621 cm−1

M3

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Table 3 Summary of crystallite size evolution in alumina –yttria powder produced by co-precipitation. Crystallite size at various temperature (nm) mol%

Phases

20Y

G Gamma Alpha H G Gamma Alpha H G Gamma Alpha H G

25Y

30Y

37.5Y

900 °C

925 °C

950 °C

975 °C

3.16

3.56

4.48

19.13 24.11

20.83 25.18

17.94 24.94

18.50 26.13

25.30 27.48

15.22 21.91

16.84 22.26

24.68 25.93

22.95 26.07

1000 °C

1100 °C

1200 °C

1300 °C

1400 °C

1500 °C

22.54 6.89

26.39 7.97

33.61 10.93

39.83

56.00

72.08

44.32

54.71

62.78

27.37 6.30

30.50 6.67

33.35 8.47

39.74 19.89

45.79

78.59

49.25

72.98

33.01 16.21

36.83 18.77

39.61 19.98

63.21

77.04

61.57

80.63

27.91

30.13

42.16

49.64

73.64

Fig. 5. Crystallite size of alumina – yttria powder produced by co-precipitation as a function of temperature.

Several different pathways for crystallite size evolution, as shown in Fig. 5, were identified:

16.2–20 nm up to 1300 °C for 25Y and 30Y respectively. 3) At high temperature, the transition from γ to α was attributed to a drastic increase in the crystallite size which occurred at 1300 °C for 20Y, 1400 °C for 25Y and 30Y. 4) For 20Y, G phase evolved at 1000 °C with a crystallite size of 22.5 nm and slowly increased up to 1300 °C. No H phase was observed for 20Y. 5) With the increase in yttria content, the crystallite size of the H phase

1) γ phase first nucleated with crystallite sizes of 3.2 nm at 900 °C for 20Y, 6.3 nm at 1000 °C for 25Y and 16.2 nm at 1100 °C for 30Y. There was no existence of γ phase in 37.5Y. 2) The crystallite sizes increased slowly with increase in temperature from 3.2 nm to 10.9 nm up to 1200 °C for 20Y, 6.3–19.9 nm and 2375

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Fig. 6. Crystallite size of alumina – 25yttria powder produced by solution combustion methods M1, M2 and M3 as a function of temperature.

Fig. 7. (a-c) TEM images for co-precipitated alumina – 25yttria powder heat treated at 900 °C. 2376

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Fig. 8. (a-c) TEM images for co-precipitated alumina – 25yttria powder heat treated at 1000 °C.

decreased. H phase was first crystallised at 925 °C, with a size of 19.1, 17.9, 15.2 nm for 25Y, 30Y and 37.5Y, respectively. The crystallite sizes increased slowly up to 975 °C for 25Y and 1000 °C for 30Y and 37.5Y. Similarly, G first crystallised at 925 °C with a size of 24.1, 24.9 and 21.9 nm for 25Y, 30Y and 37.5Y, respectively. The crystallite size for G increased slowly up to 1300 °C. 6) The crystallite size observed in γ to α transition was higher than H to G transition. H to G transition occurred at 1000 °C for 25Y and at 1100 °C for 30Y and 37.5Y. 7) For all the compositions, the crystallite sizes increased drastically above 1300 °C for both α and G phases.

Table 4 Thermodynamic data. Compounds

∆Hf (KJ/mol)

Cp (J/mol K)

Al(NO3)3·9H2O Y(NO3)3·6H2O C6H8O7·H2O C2H6O2 Al2O3 Y2O3 CO2 N2 H2O

− 3590.55 − 3043.86 − 1837.6 − 460 − 1675.7 − 1905.3 − 393.5 0.0 − 241.8

– – – – 79.0 102.5 37.1 29.1 33.6

The TEM images and selected area electron diffraction (SAED) pattern (see Fig. 7a-c) of co-precipitated 25Y powder heat treated at

Fig. 9. (a-c) TEM images for combusted alumina – 25yttria (M3 method) powder heat treated at 900 °C. 2377

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Y2O3]+30CO2+70H2O+9N2

Table 5 Heat of combustion, adiabatic temperature and number of moles of gas as a function of fuels. Fuels

∆Hf (KJ/mol)

Tad (K)

No. of moles of gas/mol (Al2O3-Y2O3)

Citric acid and ethylene glycol Citric acid

− 6951.56 − 5011.27

1633 1551

71 72.67

(8)

The thermodynamic data for various reactants and products are given in Table 4. Assuming that the combustion reactions were complete, the enthalpy of combustion and adiabatic temperature for each reaction was calculated using Eqs. (9)–(11). Enthalpy of combustion:

H=

(n Hp)

(n Hr )

(9)

Adiabatic temperature:

900 °C showed that the powder was entirely amorphous phase. The TEM images of co-precipitated 25Y powder heat treated at 1000 °C (Fig. 8a-b), reveal fine crystallites with an average particle size of 25 nm. The lattice spacing measured from TEM image (Fig. 8b) was 0.269 nm which corresponded to the {4 2 0} lattice plane of G phase. Fig. 8c shows the same powder in which G is the predominant phase. The measured spacing was in good agreement with the theoretical crystallographic parameter of the G phase. The efficacy of amorphous phase crystallisation in obtaining nanocrystalline oxides was evident in the crystallite size data in Fig. 5. The metastable phases had crystallite sizes as small as 3.2 nm. The equilibrium G phase formed with sizes as small as 28 nm. The most significant results were for α-Al2O3. It is seen that this phase formed with a size as small as 44.3 nm. Further, even after exposure to 1500 °C, the maximum crystallite size in this study was 80.6 nm.

H=

Tad 298

Tad = 298+

(

nCp) dT

Hr

(10)

Hp Cp

(11)

Here, n is the number of moles, ∆Hr and ∆Hp are the enthalpy of formation of reactant and product, respectively. Tad is the adiabatic temperature and Cp is the heat capacity of product under constant pressure. Using the thermodynamic data given in Table 4, the enthalpy of combustion, adiabatic temperature and number of moles of gases for each reaction were calculated and the same is given in Table 5. From Eqs. (7) and (8) it is clear that, the number of moles of gases liberated is similar for both the cases. However, the theoretical combustion temperature for citric acid as fuel is lower than that of the citric acid and ethylene glycol fuel mixture. Assuming no heat losses and complete conversion of the reactant, the adiabatic temperature can be considered as the highest possible temperature during the reaction. In practice, the combustion temperature is lower than the adiabatic temperature due to heat losses. It also yielded stable amorphous oxide powder up to 925 °C, when the amount of fuel mixture added was very high (e.g. M3). It is believed that avoiding hydroxide formation and keeping the flame temperature sufficiently low using complex fuels and oxidisers, improved thermal stability of the amorphous phase can be achieved. The summary of phase evolution of the 25Y powder prepared by method 1, 2 and 3 of combustion synthesis is given in Fig. 3c. The ascombusted powder, heated at 700 °C, resulted in higher amorphous content in M2 and M3 when compared to M1 was attributed by the better mixing of components. In case of M1, G started to crystallise at 700 °C with some residual amorphous phase. However for M2, the amorphous phase was stable up to 800 °C. The phase evolution for M1 and M2 involved 3 distinct paths:

3.2. Solution combustion synthesis Based on the results obtained from the reverse co-precipitation, the composition 25Y was identified as the enhanced stability of the amorphous phase. In order to explore the possibility of improving its stability further, the 25Y composition was synthesised by the solution combustion synthesis (SCS) technique. The TG-DSC curve for the as-synthesised 25Y through solution combustion using Method 3 (M3) is shown in Fig. 1c. A weight loss of only 16% was observed up to 650 °C. Since, the combustion between oxidizer and fuel was exothermic, almost all the organic contents were converted to CO2, N2 and H2O. Hence, the weight loss can be due to the burnout of fine amorphous carbon [21]. The weight of the powder remained constant after 650 °C indicating that the decomposition of the metal nitrate-fuel system was complete, and the powder was free from organic residues above this temperature. However, the powder was fine and highly porous due to the evolution of large amount of gases as can be observed in the TEM image (Fig. 9a). The endothermic peak at 338 °C (Fig. 1c) was due to the evolution of gaseous products such as NO2, CO2, CO and H2O [22]. The sharp exothermic peak at 943 °C with no weight loss was due to crystallisation of G along with a small proportion of H from the pure amorphous phase and the peak at 1090 °C was due to the appearance of G + γ which is in line with the XRD results (Fig. 3d). In the combustion process, the oxidizer to fuel ratio plays an important role for determining the exothermicity of the combusted product. Based on the concept of propellant chemistry [14,23], the stoichiometric balance of the redox mixture was calculated by considering the total oxidising and reducing valencies of the oxidizer and fuel. For stoichiometric balance, this was used as the numerical coefficient, so that the equivalence ratio (O/F) was maintained at unity. The valency of nitrogen was taken as zero because of its conversion to molecular nitrogen during the combustion process. The theoretical stoichiometric equations assuming complete combustion can be written as: For citric acid and ethylene glycol as fuel:

1) Direct nucleation of G from the amorphous precursor leaving some residual amorphous phase. 2) γ yields from the residual amorphous phase leading to phase separation upon crystallisation. 3) Transformation of γ to α at high temperature. In case of M3, the amorphous phase was stable up to 925 °C as shown in Fig. 3d. At 950 °C, crystallisation of G started to nucleate (as confirmed from DSC – Fig. 1c) along with traces of H. The appearance and disappearance of metastable H was observed between 950 and 1100 °C. Once the metastable H disappeared, the crystallisation of γ evolved at 1100 °C followed by transformation of γ to α at 1400 °C. Both the phases α and G were stable up to 1500 °C. The proportion of G increased with an increase in the temperature above 1000 °C in all methods of solution combustion synthesis. Using lower amount of fuel mixture (M1) led to crystallisation of G at 700 °C. However, a larger amount of fuel mixture (M3) led to the amorphous phase being stable up to 925 °C. Also, with the citric acid being the only fuel (M2), the amorphous phase was stable up to 800 °C. This was due to the combustion characteristics of the fuel mixture with nitrates to provide sufficient energy for complete combustion as shown in Eqs. (7, 8). Hence, the fuel mixture in M3 may be the best suited for synthesis of amorphous phase with pure and homogeneous powders. The

4[Al(NO3)3·9H2O]+4[Y(NO3)3·6H2O]+5[C6H8O7·H2O]+3C2H6O2→ 2[Al2O3-Y2O3]+36CO2+94H2O+12N2 (7) For citric acid as fuel: 3[Al(NO3)3·9H2O]+3[Y(NO3)3·6H2O]+5[C6H8O7·H2O]→1·5[Al2O32378

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Table 6 Summary of crystallite size evolution in alumina –25yttria powder produced by different methods (M1, M2 and M3) of solution combustion synthesis. Crystallite size at various temperature (nm)

M1 M2 M3

Phases

700 °C

800 °C

900 °C

G Gamma Alpha G Gamma Alpha H G Gamma Alpha

16.98

18.45

950 °C

1000 °C

1100 °C

1200 °C

1300 °C

1400 °C

1500 °C

21.96

23.07

27.86 13.29

35.74 14.08

47.82

67.23

88.33

26.38

27.95

30.47 5.01

34.01 5.68

38.35 37.59

55.98 56.68

86.34 64.28

40.95

58.35

73.16

26.21 6.10

32.28 8.22

44.74 22.70

63.35

92.81

51.21

86.65

17.87 20.45

975 °C

19.15 21.47

21.47 23.24

remained amorphous. The broad band at 550–900 cm−1 was due to the presence of amorphous phase. In case of M2 (700–800 °C) and M3 (700–925 °C), FT-IR showed a broad and smooth absorption band at 550–900 cm−1 indicating the presence of the amorphous phase. Several intense peaks observed in the range of 1000–450 cm−1 above 800 °C for M1, M2 and at 950 °C for M3 indicate metal-oxygen (M-O) vibrations. In the case of M1 (900–1200 °C), the characteristic vibration bands revealed the presence of the AlO6 octahedral unit. For M2 (900–1200 °C) and M3 (950–1200 °C), the characteristic vibration band revealed the presence of both AlO4 (tetrahedral) and AlO6 (octahedral) units. The bands associated with Al-O, Y-O and Y-O-Al stretching confirmed the formation of the G phase. As an example, the FT-IR spectra for 25Y (heat treatment at different temperature for 5 min) synthesised using M3 is shown in Fig. 4b. All FT-IR results were consistent with the crystallisation processes observed from the XRD results. Thus, it was seen that while co-precipitation synthesis may have residual NH4+ and NO3− radicals present up to 800 °C, no such residual carbonate above 700 °C was present in case of combustion synthesis. In other words the SCS process resulted in purer powders when treated above 700 °C. The summary of crystallite size evolution in 25Y by SCS using different methods is shown in Table 6. In the case of M1 (citric acid and

composition 25Y prepared by solution combustion synthesis (M3) showed the highest crystallisation temperature of 925 °C when compared with the co-precipitation (900 °C). This was most likely due to the avoidance of large excess of NH 4 and OH−- as well as carbonate radicals in the precursor to the amorphous oxides. This reasoning is supported by the FT-IR results described below. FT-IR analysis for 25Y powder from three methods (M1, M2 and M3) is given in Table 2 which was important for understanding the decomposition mechanism and crystallisation from the amorphous phase. For all the powders, the broad absorption bands near 3443 cm−1 (700–1200 °C) and 1639 cm−1 (800–1200 °C) remained unchanged with increase in calcination temperature since they were assigned to adsorption of water at room temperature from the ambient air. The weak band around 2350 cm−1 in the temperature range from 700° to 1200°C was observed in all the spectra is due to adsorbed carbon dioxide from atmosphere which is the result of inequalities in the path length [18,24]. The bands near 1565 and 1425 cm−1 at 700 °C for all the powders were assigned to the stretching vibrations of CO32− (carbonate) groups. These stretching vibrations confirmed the presence of residual carbonate from the fuel. In case of M1, G started to form at 700–800 °C as the first crystalline phase while the majority still

Fig. 10. (a-c) TEM images for combusted alumina – 25yttria (M3 method) powder heat treated at 1000 °C. H-YAlO3 (Hexagonal) and G-Y3Al5O12 (Garnet). 2379

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ethylene glycol – 5 vol%) and M2 (citric acid – 5 vol%) four steps are involved in the pathways of the crystallite size evolution (see Fig. 6):

materials were studied and analysed. Both, metastable and stable phases retained nanometric grains even after thermal exposure at 1500 °C.

1) G first crystallised with a size of 17 nm at 700 °C and 26.4 nm at 900 °C for M1 and M2 respectively. 2) γ phase nucleated at 1100 °C with the crystallite size of 13.3 nm and 5 nm for M1 and M2, respectively. 3) In both G and γ, crystallite sizes increased slowly up to 1200 °C. 4) At 1300 °C, γ to α transition occurred with a drastic increase in the crystallite size.

References [1] J.R. Berchmans, S. Angappan, A. Visuvasam, K.B.R. Kumar, Preparation and characterization of LaAlO3, Mater. Chem. Phys. 109 (2008) 113–118. [2] Anna Haliakova, Anna Prnova, Robert Klement, Dusan Galusek, Wei-Hsing Tuan, Flame-spraying synthesis of aluminate glasses in the Al2O3-La2O3 system, Ceram. Int. 38 (2012) 5543–5549. [3] A.S. Gandhi, V. Jayaram, Plastically deforming amorphous ZrO2-Al2O3, Acta Mater. 51 (2003) 1641–1649. [4] S. Aasland, P.F. McMillan, Density-driven liquid-liquid phase separation in the system Al2O3-Y2O3, Nature 369 (1994) 633–636. [5] M.C. Wilding, C.J. Benmore, P.F. McMillan, A neutron diffraction study of yttrium and lanthanum aluminate glasses, J. Non-Cryst. Solids 297 (2002) 143–155. [6] A. Rosenflanz, M. Frey, B. Endres, T. Anderson, E. Richards, C. Schardt, Bulk glasses and ultrahard nanoceramics based on alumina and rare-earth oxides, Nature 430 (2004) 761–764. [7] Ashutosh S. Gandhi, Vikram Jayaram, Atul H. Chokshi, Dense amorphous zirconiaalumina by low-temperature consolidation of spray-pyrolyzed powders, J. Am. Ceram. Soc. 82 (1999) 2613–2618. [8] A.S. Gandhi, V. Jayaram, Pressure consolidation of amorphous ZrO2-Al2O3 by plastic deformation of powder particles, Acta Mater. 50 (2002) 2137–2149. [9] Samrat Choudhury, Ashutosh S. Gandhi, Vikram Jayaram, Bulk dense nanocrystalline yttrium aluminum garnet by consolidation of amorphous powders at low temperatures and high pressures, J. Am. Ceram. Soc. 86 (2003) 247–251. [10] Pathikumar Sellappan, Vikram Jayaram, Atul H. Chokshi, Synthesis of bulk dense nanocrystalline yttrium aluminum garnet from amorphous powders, J. Am. Ceram. Soc. 90 (2007) 3638–3641. [11] Nithyanantham Thangamani, Ashutosh S. Gandhi, Vikram Jayaram, Atul H. Chokshi, Low-temperature high-pressure consolidation of amorphous Al2O3-15 mol% Y2O3, J. Am. Ceram. Soc. 88 (2005) 2696–2701. [12] Ashutosh S. Gandhi, Carlos G. Levi, Phase selection in precursor-derived yttrium aluminum garnet and related Al2O3-Y2O3 compositions, J. Mater. Res. 20 (2005) 1017–1025. [13] C.K. Ullal, K.R. Balasubramaniam, A.S. Gandhi, V. Jayaram, Non-equilibrium phase synthesis in Al2O3-Y2O3 by spray pyrolysis of nitrate precursors, Acta Mater. 49 (2001) 2691–2699. [14] S.R. Jain, K.C. Adiga, V.R. Pai Verneker, A new approach to thermochemical calculations of condensed fuel-oxidizer mixtures, Combust. Flame 40 (1981) 71–79. [15] C.J. Brinker, G.W. Scherer, The Physics and Chemistry of Sol-Gel Processing, Academic Press, New York, 1990. [16] Prabhu Ramanujam, Bala Vaidhyanathan, Jon Binner, Aashu Anshuman, Chris Spacie, A comparative study of the synthesis of nanocrystalline yttrium aluminium garnet using sol-gel and co-precipitation methods, Ceram. Int. 40 (2014) 4179–4186. [17] Paola Palmero, Claude Esnouf, Laura Montanaro, Gilbert Fantozzi, Influence of the co-precipitation temperature on phase evolution in yttrium-aluminium oxide materials, J. Eur. Ceram. Soc. 25 (2005) 1565–1573. [18] A. Leleckaite, A. Kareiva, Synthesis of garnet structure compounds using aqueous sol-gel processing, Opt. Mater. 26 (2004) 123–128. [19] P. Vaqueiro, M.A. Lopez-Quintela, Influence of complexing agents and pH on yttrium-iron garnet synthesized by the sol-gel method, Chem. Mater. 9 (1997) 2836–2841. [20] C.H. Shek, J.K.L. Lai, T.S. Gu, G.M. Lin, Transformation evolution and infrared absorption spectra of amorphous and crystalline nano-Al2O3 powders, Nanostruct. Mater. 8 (1997) 605–610. [21] R.V. Mangalaraja, J. Mouzon, P. Hedstrom, Carlos P. Camurri, S. Ananthakumar, M. Oden, Microwave assisted combustion synthesis of nanocrystalline yttria and its powder characteristics, Powder Technol. 191 (2009) 309–314. [22] J. Chandradass, Ki. Hyeon Kim, Mixture of fuels approach for the solution combustion synthesis of LaAlO3 nanopowders, Adv. Powder Technol. 21 (2010) 100–105. [23] N. Arul Das, K.C. Patil, Combustion synthesis and properties of zirconia-alumina powders, Ceram. Int. 20 (1994) 57–66. [24] B. Schrader, Infrared and Raman Spectroscopy- Methods and Applications, VCH, Weinheim, 1995. [25] R.S. Mishra, V. Jayaram, B. Majumdar, C.E. Lesher, A.K. Mukherjee, Preparation of a ZrO2-Al2O3 nanocomposite by high-pressure sintering of spray-pyrolyzed powders, J. Mater. Res. 14 (1999) 834–840. [26] Ting C. Chou, Tai G. Nieh, Nucleation and concurrent anomalous grain growth of αAl2O3 during γ – α phase transformation, J. Am. Ceram. Soc. 74 (1991) 2270–2279.

The crystallite sizes increased drastically above 1300 °C for both G and α phases. The pathway for the crystallite size evolution for M3 (citric acid and ethylene glycol – 10 vol%) involved four steps: 1) H and G nucleated at 950 °C with the crystallite size of 17.9 and 20.4 nm, respectively. 2) The crystallite size increased slowly up to 1000 °C for H and 1300 °C for G. 3) γ phase with a crystallite size of 6.1 nm evolved at 1100 °C. The crystallite size increased drastically at 1400 °C which was attributed by the transformation of γ to α. It is known from literature that the γ to α transition yields large α grain. This is due to factors such as large decrease in molar volume, and templated growth of α [25,26]. 4) The crystallite size increased drastically above 1400 °C for both G and α phases. Naturally, the crystallinity improved with the temperature. Fig. 9a shows the TEM image of combusted 25Y powder heat treated at 900 °C. It clearly shows that the powder had lot of internal porosity due to the evolution of a large amount of gases. Fig. 9b-c shows that the powder was totally amorphous at 900 °C, as confirmed from the diffraction pattern. The TEM image of M3 25Y powder heat treated at 1000 °C (Fig. 10a) revealed the presence of porosity in the powder particles. The dhkl values measured from the lattice fringes (Fig. 10b) was 0.273 nm which corresponded to the {1 0 2} lattice planes of H. Fig. 10c shows the SAED patterns of both H and G phases. The measured dhkl values were in good agreement with the crystallographic parameters of H and G phases. It is noteworthy that even though the SCS process involves high temperature, the amorphous phase was retained up to 925 °C. 4. Conclusions Reverse co-precipitation was used for synthesising amorphous alumina-yttria powders with systematically varying yttria content. The alumina-25 mol% yttria (25Y) composition was identified as the one with the most stable amorphous phase (up to 900 °C). A solution combustion synthesis technique was also used for synthesising 25Y powders which enhanced the amorphous phase stability to 925 °C. This enhancement was attributed to the absence of radicals such as NH 4 and OH− as well as carbonate. The amorphous phase characteristics are likely have been influenced by these factors, leading to improved thermal stability. The role of solution combustion synthesis parameters in amorphous phase synthesis was studied. A combination of citric acid and ethylene glycol as fuel during SCS resulted in an amorphous phase with further enhanced thermal stability. Crystallisation of amorphous phases into nanocrystalline oxide ceramics has been studied in detail. The phase evolution sequences in both, reverse co-precipitated and solution combustion synthesised

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