Journal of the Australian Ceramics Society Volume 50[2], 2014, 17 – 24 17

The Formation of Yttrium Aluminium Monoclinic (Y4Al2O9) by Sol-Gel Synthesis at Low Heating Temperature Ikhwan M. K. 1, Azis R.S.1,2*, Hashim M.2, Holland D.3, Howes A.P.3, Zakaria A.1,2 and Hassan J.1,2 1 Department of Physics, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia. 2 Institute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia. 3 Department of Physics, University of Warwick, CV4 7AL, UK. E-mail: rabaah@upm.edu.my  Available Online at: www.austceram.com/ACS-Journal Abstract Y4Al2O9 has been synthesized by means of the citrate-nitrate sol-gel combustion method using yttrium (III) nitrate and aluminium (III) nitrate. DTA/TG analysis, X-ray diffraction (XRD), FT-IR and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) measurements were used to characterize the phase decomposition, weight loss of the sample, the crystal structure and phase formation of the Y4Al2O9 material. XRD shows the Y4Al2O9 starts to crystallize at low temperature, 700 oC, with an average particle size around 49 nm. Keywords: YAM (Y4Al2O9), sol-gel, DTA/TG, XRD, FT-IR, 27Al MAS NMR INTRODUCTION Yttrium aluminium monoclinic (Y4Al2O9, YAM), which has the Y/Al ratio of 2/1 [1], is one of the compounds present within the pseudo-binary system between aluminum oxide (Al2O3) and yttrium oxide (Y2O3) [2]. In the Y2O3-Al2O3 system, there are another two well-known compounds, which are yttrium aluminium garnet Y3Al5O9 (YAG) and the perovskite YAlO3 (YAP) with Y/Al ratios equal to 0.6/1 and 1/1 respectively [1]. It has been reported that the space group of the crystal structure of monoclinic Y4Al2O9 is P21/c [3, 4]. The Al atoms are coordinated to four oxygen atoms and the Y atoms are coordinated to either six or seven oxygen atoms [4], with site symmetry C1[5]. The structure of Y4Al2O9 contain two different AlO4 environments, four yttrium (two YO6 and two YO7), and nine distinct oxygen sites that can be described as four OY3Al, two OY2Al, two OY4 and one OY2Al2 [6]. This Y2O3-Al2O3 system has many applications such as providing scintillation hosts, in semiconductor processing technology and as lasing materials [7-9]. Y2O3-Al2O3 compounds doped with rare earth ions are promising phosphor materials and their photoluminescence properties in the UV-VIS range have been widely studied [10-12]. A recent report has suggested that Y4Al2O9 has potential to be used as a thermal barrier ceramic because of its very low

thermal conductivity [13]. To produce Y4Al2O9 powders using conventional solid-state oxides reaction requires extensive mechanical mixing and a high heating temperature to form a highly pure and homogeneous sample. Medraj et. al [14] reported that the formation of pure Y4Al2O9 by the solid-state method only can be achieved for a sample mixture of 33 mole % Al2O3 and 67% Y2O3 calcined at >1800 oC. A study of the photoluminescence properties of Y4Al2O9:Eu3+ of low temperature (sol-gel) synthesized Y4Al2O9:Eu3+ [15] reported the formation of Y4Al2O9:Eu3+ nanocrystals at 800 oC which is much lower than the solid-state sintering process. Thus, chemical synthesis processing such as sol-gel [15-19], co-precipitation [20] and microwave-solvothermal synthesis [21] can produce Y4Al2O9 at lower temperatures. In this paper, the citrate-nitrate sol-gel combustion method was used to synthesize Y4Al2O9. The method, a combination of a chemical sol-gel process followed by a combustion process at 200 to 250 oC, is based on the gelling and subsequent combustion of an aqueous solution containing salts of the desired metals and some organic fuel. This helps giving a voluminous product with large surface area [22]. This sol-gel method uses inexpensive precursors and can produce large quantities of powder. Structure development during processing has been characterised by different

Ikhwan et al. 18

analytical methods, showing formation of pure Y4Al2O9 phase can be achieved at 700 oC, which is much lower than that required for the solid state reaction. EXPERIMENTAL The Y4Al2O9 materials were prepared by the sol-gel combustion process. Yttrium (III) nitrate – Y(NO3)3.6H2O (99.99%, Alfa Aesar) and aluminium (III) nitrate – Al(NO3)3.9H2O (99.9, Acro organic), in molar ratio of 2:1, were used as the precursors. Propan-2-ol and citric acid were used as a solvent and chelating agent respectively. The ratio between citrate and nitrate ions was chosen to be 0.75. The process was started by mixing the aqueous nitrate salt solution with the propan-2-ol. The transparent nitrate solution, the ‘sol’, was obtained after 20 minutes stirring at ~ 40 oC. The citric acid was then added to the ‘sol’ to form a gel which was then combusted at 200 to 250 oC to form a brown coloured ash. The ashes were then treated at 700-800 oC for 10 h. X-ray diffraction (XRD) of the powder obtained was performed on a Philips X’pert Diffractometer model 7602 EA Almelo. XRD scans were made from 10° to 80° 2θ, using CuKα radiation (1.542 Å) operating at 45 kV and 40 mA. The PANalytical X’Pert Highscore Plus software (version 2.2d (2.2.4)) was used to identify the crystallographic phases and to determine the relative phase compositions. DTA was performed on a Mettler Toledo TG/DSC with Al2O3 as a reference sample and heating rate of 2.5 oC/min from 30 oC to 1000 oC. FT-IR measurement was taken by an in-situ FT-IR catalyst Treatment and characterization system (Thermo Nicolet), in the 4000-400 cm-1 spectral range using the KBr pellet method. Pellets were made by mixing 200 mg of KBr powder with around 10-15 mg of sample. 27Al magic angle spinning (MAS) NMR experiments were performed using a Varian spectrometer at 14.1 T, corresponding to a Larmor frequency, νo, for 27Al of 156.9 MHz. A 3.2 mm rotor was used with MAS frequency 15 kHz. A single pulse program was used with a pulse width of 1.0 μs to give a small tip angle of 15°; a pre-acquisition delay of 6.0 μs; a recycle delay of 2 s; and a typical acquisition time of 30 mins for each sample. These spectra were referenced against commercial YAG as a secondary reference,

with the signal corresponding to the octahedral aluminium site [AlO6] set to 0.7 ppm [22]. The DmFIT2011 program [23] was used to simulate the spectra. RESULTS AND DISCUSSION The thermogravimetry (TG) and differential thermal analysis (DTA) spectra of the ash obtained after the combustion process are given in Figure 1. The TG curves represent a multi-stage decomposition, but in this Figure, stable intermediates are not formed. The weight loss within the sample occurs in the range of 30-500 oC and tends to constant weight at about 900 oC. A total mass loss of about 80 % up to 1000 oC shows that the combustion reaction of the precursor at 150 oC is not complete. The sharp exothermal peak at 427 oC in the DTA curve and accompanied by substantial weight loss in the TG curve is due to evolution of the gases CO, CO2, NO and NO2 resulting from decomposition of excess citric acid and nitrate [17]. So, a precalcination stage at 500 oC is desirable, such that the undecomposed organic compounds from the combustion-formed precursor powder, which could influence the formation of crystalline Y4Al2O9 phase, can be effectively removed. Figure 2 presents the X-ray diffraction patterns of Y4Al2O9 samples, heated at various temperatures between 700-800 oC for 10 h, as compared to the reference patterns. The temperature difference between each sample is 20 oC. This small increment of the heating temperature was applied that any change in phase formation could be monitored easily. Figure 2a shows that the Y4Al2O9 diffraction peaks start to appear at 700 oC i.e. Y4Al2O9 crystals are forming within the amorphous phase. By increasing the heating temperature from 720 oC to 800 oC, the YAM peaks increase in intensity and become much sharper. All of the XRD peaks are matched with the YAM reference pattern and can be indexed with JCPDS 34-368 belonging to the P21/a space group as shown in Figure 2b. There is no trace of secondary crystalline phase and the amorphous phase content decreases with increasing temperature and has gone by 760 oC

Journal of the Australian Ceramics Society Volume 50[2], 2014, 17 – 24 19

Fig. 1: TG/DTA curve of ash measured at a heating rate of 2.5 oC/min

Fig. 2a: X-ray diffraction patterns of the samples calcined between 700-800 oC as compared to the reference patterns

of the constituent oxides.

Ikhwan et al. 20

Fig. 2b: Indexed pattern of Y4Al2O9 heated at 740 oC

Fig. 3: XRD peak FWHM and crystallite size of Y4Al2O9 after heating at different temperatures.

The mean crystallite size (dp) of the powder sample also be calculated using Scherrer’s formula as shown in eq. (1) below: dp = 0.94λ / B1/2 cosθ (1) where dp is the diameter of the crystallite ; λ is the x-ray wavelength; B1/2 is the line broadening obtained

from B1/2 = (B2 – b2)0.5 where B is the FWHM (Full width at half maximum) of the broadened diffraction line on the 2θ scale (radians) and b is the FWHM of a reference sample with large, well-ordered crystallites. The average crystallite size calculated from XRD line broadening for the sample of Y4Al2O9 was shown in Figure 3.

Journal of the Australian Ceramics Society Volume 50[2], 2014, 17 – 24 21

27Al MAS NMR is very helpful in discussing the aluminium-oxygen environment in the sol-gel derived Y4Al2O9. In the Y4Al2O9 lattice, aluminium ions are known to occupy two types of tetrahedral site (AlO4) in the ratio of 1:1 [24]. Florian et.al [24] determined the quadrupolar parameters for single crystal Y4Al2O9 : the quadrupole coupling constant CQ is 10.8 MHz and 10.4 MHz and asymmetry parameter Q = 0.48 and 0.77 for each AlO4 site. Figure 4a shows the 27Al MAS NMR spectra of sol-gel derived Y4Al2O9 heated at 700, 800 and 1000 oC performed at Bo= 14.1 T. All spectra are simulated using the DmFIT programme [23], with AlO6 and AlO4 sites being simulated using Gaussian-Lorentzian and QMAS lineshapes, respectively [22]. For low heating temperatures, a spectrum with broad peaks corresponding to an amorphous phase containing AlO6, AlO5 and AlO4 sites is obtained. The peaks are too broad to simulate with quadrupole lineshapes and therefore Gaussian-Lorentzian lineshapes only have been used to obtain peak positions. For the sample heated at 700 oC (see Figure 4b), these peak positions are at 3.3 1.0 ppm, 39.3 1.0 ppm and 68.0 1.0 ppm and are due to AlO6, AlO5 and AlO4 sites respectively. The AlO5 site is found only in the amorphous phase and not in any of the related crystal phases. This agrees with the XRD which shows the existence of the amorphous phase at this temperature. Figures 4c show the experimental and simulated spectra of Y4Al2O9 heated at 800 oC. The peak positions are at 1.6 1.0 ppm, 73.3 1.0 ppm and 77.2 1.0 ppm and are due to AlO6 and two AlO4 sites respectively. The

quadrupole lineshapes of the contributing species are now quite clear and quadrupole parameters can be determined. Florian et al [24] reported the δiso for the AlO4(I) and AlO4(II) are located at 78.2 1 ppm and 76.2 1 ppm respectively. All respective 27Al MAS NMR parameters for Y4A2O9 heated at 700 and 800 oC are summarized in Table 1.

Fig. 4a: 27Al MAS NMR for samples heated at

700/800/1000 oC at B0=14.1T

Fig. 4b: Experimental and simulated spectra for 27Al MAS NMR for Y4A2O9 heated at 700 oC.

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Fig. 4c: Experimental and simulated spectra for 27Al MAS NMR for Y4A2O9 heated at 800 oC.

Table 1: 27Al MAS NMR parameters for Y4A2O9 samples heated at varied temperatures simulated using DmFit2011.

Sample (oC)

Site Crys %

δiso

( 1.0 ppm) CQ

( 0.1 MHz) ηQ

0.05

% calc. .

700 AlO6 3.3 - - 35.1

AlO5 39.3 - - 17.4 AlO4 68.0 - - 47.5

800

AlO6 1.6 - - 13.3

AlO4 73.3 - - 16.1 AlO4 50 77.2 10.3 0.79 24.5 AlO4 50 79.8 10.8 0.48 42.1

In order to further investigate the existence and the structure of the amorphous phase which could be present in the sample along with the crystalline phase, FT-IR analysis was used to characterize the materials. Figure 5 shows the FT-IR spectra of the Y4Al2O9 phase heated at various temperatures. The intensity of the band located at in the region 2000-1300 cm-1 decrease with increasing temperature, and the intensity of the band located between 1000-500 cm-1 increases with increasing temperature. The band between 2000-1300 cm-1 consists of stretching and bending vibrations of precursor compounds which are

largely decomposed on heating, whereas, the band around 1000-400 cm-1 is the region where metal-oxygen vibrations occur. The peaks around 1634 and 1384 cm-1 are due to stretching vibration of C=C and N-O groups respectively [25]. The broader peak at 1553 cm-1 probably arises from precursor groups bound to metal in the amorphous phase and has been removed by 1000 oC. The peaks at about 900, 760, 712 and 554 cm-1 are the characteristic metal-oxygen (Y-O) and (Al-O) vibrations [15, 25] in the Y4Al2O9 phase.

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Fig. 5: FTIR spectra of Y4Al2O9 heated at various temperatures.

CONCLUSION Y4Al2O9 phase was obtained by sol-gel synthesis using Y(NO3)3.6H2O and Al(NO3)3.9H2O as the precursors. Use of small increments of heating temperature between 700 to 800 oC was able to determine the optimum temperature to produce a single Y4Al2O9 phase as low as 760 oC which is much lower than that normally required (>1000 oC) in a conventional solid state method. The compositional homogeneity at the molecular level in the solution is retained in the gel state, leading to uniform reactivity due to the homogeneous distribution of reactants and the reduction of reaction temperature. The 27Al MAS NMR analysis also shows the presence of an amorphous second phase for samples heated at 700 to 800 oC, agreeing with the results from FTIR and XRD. As the heating temperature is increased, the peak widths decrease, in agreement with XRD analysis, though there is some retention of precursor organics even to 1000 oC. ACKNOWLEDGMENTS The authors are grateful to Research University Grant, RUGS (9199848) Universiti Putra Malaysia (UPM) and Fundamental Research Grant, FRGS (5524218), Ministry of Education (MOE) Malaysia for financial assistance. The authors also would like to thank Graduate Research Fellowship (GRF) and

MyBrain15 for the research funding. D. Hammond is thanked for the DTA/TG measurement. The UK 850 MHz solid-state NMR Facility used in this research was funded by EPSRC and BBSRC, as well as the University of Warwick including via part funding through Birmingham Science City Advanced Materials Projects 1 and 2 supported by Advantage West Midlands (AWM) and the European Regional Development Fund (ERDF).

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