Nanostructured yttrium aluminum garnet powders synthesized by co-precipitation method using tetraethylenepentamine

JOURNAL OF RARE EARTHS, Vol. 27, No. 6, Dec. 2009, p. 967

Nanostructured yttrium aluminum garnet powders synthesized by co-precipitation method using tetraethylenepentamine LI Xianxue (ᴢ‫ܜ‬ᄺ)1,2, WANG Wenju (⥟᭛㦞)3 (1. Department of Environment & Life Science, Putian University, Putian 351100, China; 2. Beijing Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China; 3. Financial Division, Putian University, Putian 351100, China) Received 3 March 2009; revised 14 July 2009

Abstract: Tetraethylenepentamine (C8H23N5, TEPA) has been used as a novel precipitant to synthesize yttrium aluminum garnet (Y3Al5O12, YAG) precursor from a mixed solution of aluminum and yttrium nitrates via a normal-strike co-precipitation method without controlling the pH value during precipitation process. The original precursor was analyzed by thermogravimetry/differential scanning calorimetry (TG/DSC). The evolution of phase composition and micro-structure of the as-synthesized YAG powders were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and transmission electron microscopy (TEM). Compositionally pure YAG nanostructured powders were directly obtained by calcination of the precursor at 900 °C without the formation of any intermediate phases. The average particle size determined from TEM micrograph for the powder obtained at 1000 °C was approximately 50 nm. Keywords: yttrium aluminum garnet; powder technology; co-precipitation; ceramics; rare earths

Yttrium aluminum garnet (Y3Al5O12, YAG) is an ideal host material for phosphors and solid-state lasers when doped with transition or lanthanide element[1,2]. Besides, because of its promising chemical stability and high-temperature mechanical properties[3], YAG is also an important ceramic material. Neodymium-doped YAG (Nd:YAG) transparent laser ceramics have attracted much attention because of their several advantages, such as low cost, short preparation time, high doping concentration and large size, etc. Moreover, some properties of polycrystalline Nd:YAG ceramic laser materials have proved to be comparable or superior to those of single crystal[4,5]. Given in such broad application potential of the YAG materials, the simple and practicable synthesis methods yielding phase-pure nanostructured YAG are highly acceptable. Typically, YAG powders are prepared by a solid-state reaction from their respective oxide powders. This method usually requires high calcining temperature of over 1600 °C and long aging time[6,7]. Compared with solid-state reaction, wet-chemical synthesis methods not only have the advantages of low synthesis temperature and short calcining time, but also can achieve homogeneous mixture of metal ions at the atomic level. Consequently, several wet-chemical methods have been extensively investigated in recent years for preparing pure phase YAG powders. These methods include

co-precipitation[8–10], homogeneous precipitation[11], sol-gel processing[12], hydrothermal treatment[13], spray pyrolysis[14], combustion methods[15,16], etc. Among them, co-precipitation is a relatively simple and cost-effective way for powder synthesis, and many precipitants, such as ammonia[8,17,18] and ammonium hydrogen carbonate[9,19], have been employed to produce YAG powder. However, pH values of the co-precipitation process with these precipitants have to be kept at some constant value ranging from 7.8–9[8,17–19] due to amphoteric properties of Al, which makes the preparation procedures more complex and unmanageable. In the present work, a novel precipitant, tetraethylenepentamine (TEPA), was adopted to synthesize nanostructured stoichiometric YAG powders via a co-precipitation method, and pH value of the reaction solution needs not be especially controlled. This method reported here simplified the manipulation procedures and was more feasible. The results on the synthesis and characterization of nano-sized YAG powder by the co-precipitation method carried out in this laboratory were elaborated in this paper.

1 Experimental 1.1 Synthesis

Corresponding author: LI Xianxue (E-mail: [email protected]; Tel.: +86-594-2652865) DOI: 10.1016/S1002-0721(08)60371-3

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Y(NO3)3·6H2O (>99.9% purity), Al(NO3)3·9H2O (>99.9% purity) and TEPA (analytical grade) were used as raw materials for the synthesis of YAG powder. Concentrated solutions were obtained by dissolving Y(NO3)3·6H2O and Al(NO3)3·9H2O in distilled water, respectively. Then, Y(NO3)3 and Al(NO3)3 mixed solutions were prepared from the above concentrated salt solutions with appropriate volume to maintain the Y:Al molar ratio at 3:5. Subsequently, required amount of 0.5 mol/L TEPA solution (TEPA to metal ion ratio of 5) was added dropwise into the mixed solution (normal-strike method) while being stirred properly at 50 °C. After aging for 30 min, the resultant precipitate was filtered, washed with distilled water and ethanol, and dried at 60 °C for 1 d. The dried cake, which was the so- called original precursor, was put into an alumina crucible, and then calcined in a muffle furnace from 800 to 1000 °C for 2 h in air. 1.2 Characterization The crystalline development of the powders heat-treated at different temperatures was identified by X-ray diffraction (XRD) on a MAC Science MXP21VAHF diffractometer. Thermogravimetry/differential scanning calorimetry (TG/DSC) of the original precursor was carried out on a SETARAM LabsysTM TG-DSC16 thermal analyser. Infrared (IR) spectra were recorded on a Bruker (TENSOR-27) FT-IR spectrometer by a KBr disk method. The particle size and morphology of the heat-treated powders were examined using transmission electron microscopy (TEM) (Model 200, JEOL, Tokyo, Japan).

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technique[18,19] (adding salt solution dropwise to the precipitant solution) in co-precipitating multiple components. The possible interpretation may be proposed from the chemical property of TEPA. Belonging to polybasic amine, TEPA hydrolyzes and generates OH– anion in aqueous solution until the hydrolysis achieves the equilibrium. When TEPA is dripped into the mixed solution containing metal ions, hydroxides of Y3+ and Al3+ occur and OH– concentration in aqueous solution decreases in the meantime, which leads to the shift rightward of the hydrolysis balance. Consequently, OH– anions are continuously released at the same time, and the pH in local areas is approximately considered at a relatively invariant value, which guarantees Y3+ and Al3+ undergoing precipitation with a ratio of 3:5, therefore no other intermediate phase, except YAG, occurs in the above XRD spectra. Further heating of the powder to 1000 ºC shows no change in phase composition other than the intensity and sharpness of the diffraction peaks, which indicates the improved crystallinity at a higher calcination temperature. 2.2 Thermal analysis Fig. 2 shows TG/DSC curves of the original precursor. Two major endothermic peaks located at approximately 91

2 Results and discussion 2.1 X-ray diffraction The XRD spectra of the original precursor and powders calcined at various temperatures for 2 h is shown in Fig. 1. The original precursor remains amorphous. As the temperature increases to 800 ºC, the powder exhibits diffraction peaks which are in agreement with that of YAG crystal structure with cubic structure (ICSD Card No. 20090). Nevertheless, the diffuse and week peaks indicate the existence of a considerable amount of amorphous phases. With the heating temperature continuously elevating, the powder presents improved crystallinity due to further decomposition and crystallite growth. Calcining to 900 ºC results in a complete conversion to YAG without any intermediate impurity phases detected, which indicates that phase-pure YAG is achieved and it is not necessary to dominate the pH value during the normal-strike co-precipitation process. However, most chemical precipitations are performed by reverse-strike

Fig. 1 XRD pattern of the YAG powders calcined at various temperatures

Fig. 2 TG/DSC curves of the original precursor

LI Xianxue et al., Nanostructured yttrium aluminum garnet powders synthesized by co-precipitation method using…

and 176 ºC with a weight loss of about 30% are caused by the evaporation of absorbed water and the release of molecular water. The broad endothermic peak centered at approximately 600 ºC (in the range of 300–900 ºC) corresponds to the decomposition of the complex hydrate precipitate. Besides, there still exist two exothermic peaks: the former at 922 °C (very sharp) might result from the progress of crystallization from amorphous YAG due to the decomposition of the hydrate, whereas the latter at 990 ºC (very shallow) is probably caused by the crystal growth of YAG. The assumption is supported by the above XRD results, in which no change in the phase composition is found other than the intensity and sharpness of the diffraction peaks while increasing the heating temperature from 900 ºC to 1000 ºC. On the other hand, the weight loss before and after these two exotherms shows little change on the TG curve, which also indicates that the chemical composition of the powder at these stages is unaltered. Therefore the peaks at 990 ºC are attributed to the crystal growth. 2.3 FT-IR Fig. 3 shows the IR spectra of the precursor and the powders heated at various temperatures. The broad absorption band peaking at 3450 cm–1 is associated with the stretching vibrations of the hydroxyl groups (O–H). Characteristic of H–O–H bending mode of molecular water corresponds to the small band at ~1630 cm–1. Two major peaks at ~1385 cm–1 and 1518 cm–1 may result from diagnostic of NO3–. As the calcination temperature increases to 900 ºC, the double absorption peaks corresponding to NO3– disappear, implying the decomposition of NO3–. In addition, the metal-oxygen vibration characteristics of Al–O, Y–O, and Y–O–Al stretches confirming the formation of a pure YAG phase are clearly observed in the range of ~400–800 cm–1 at 800 ºC or above, which is evidenced by the XRD results in Fig. 1. 2.4 Morphology characterization

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YAG powders calcined at 900 and 1000 ºC for 2 h. It can be seen that the observed particle size becomes larger as the calcining temperature increases from 900 ºC to 1000 ºC. The average particle size determined from the TEM micrograph for the powder obtained at 1000 ºC is about 50 nm, which is consistent with the crystallite size (46.6 nm) calculated from the X-ray line broadening analysis with Scherrer equation for the same powder.

Fig. 3 IR spectra of the original precursor and the powders heated at different temperatures

3 Conclusions Single-phase nanostructured YAG powders were synthesized from a mixed solution of aluminum and yttrium nitrates via a normal-strike co-precipitation method followed by calcinations using a novel precipitant TEPA. The pH value of precipitation process was not especially dominated, facilitating the control of the Y:Al stoichiometric ratio and simplifying the manipulation procedures. Characterization results indicated that the co-precipitation synthesized precursor was converted directly to pure YAG after being calcined at 900 °C for 2 h. The resultant polycrystalline YAG powders obtained at 1000 °C showed an average particle size of approximately 50 nm.

Fig. 4 shows TEM morphologies of the as-synthesized

Fig. 4 TEM morphologies of YAG powders calcined at 900 °C (a) and 1000 °C (b)

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References: [1] Wang S J, Xu Y B, Lu P X, Xu C F, Cao W. Synthesis of yttrium aluminum garnet (YAG) from an ethylenediaminetetraacetic acid precursor. Materials Science Engineering B, 2006, 127: 203. [2] Fu Y P. Preparation of Y3Al5O12: Ce powders by microwaveinduced combustion process and their luminescent properties. Journal of Alloys and Compounds, 2006, 414: 181. [3] Takaichi K, Yagi H, Lu J R, Shirakawa A, Ueda K, Yanagitani T, Kaminskii A A. Yb3+-doped Y3Al5O12 ceramics-a new solid-state laser material. Physica Status Solidi (A), 2003, 200: 5. [4] Lu J R, Song J, Prabhu M, Xu J Q, Ueda K, Yagi H, Yanagitani T, Kudryashov A. High-power Nd:Y3Al5O12 ceramic laser. Japanese Journal of Applied Physics, 2000, 39: 1048. [5] Lu J R, Prahu M, Song J, Li C, Xu J Q, Ueda K, Kaminskii A A, Yagi H, Yanagitani T. Optical properties and highly efficient laser oscillation of Nd:YAG ceramics. Applied Physics B, 2000, 71: 469. [6] Li J G, Ikegami T, Lee J H, Mori T. Well-sinterable Y3Al5O12 powder from carbonate precursor. Journal of Materials Research, 2000, 15: 1514. [7] Messier D, Gazza G. Synthesis of MgAl2O4 and Y3Al5O12 by thermal decomposition of hydrated nitrate mixtures. American Ceramic Society Bulletin, 1972, 51: 692. [8] Su J, Zhang Q L, Gu C J, Sun D L, Wang Z B, Qiu H L, Wang A H, Yin S T. Preparation and characterization of Y3Al5O12 (YAG) nano-powder by co-precipitation method. Materials Research Bulletin, 2005, 40: 1279. [9] Xu G G, Zhang X D, He W, Liu H, Li H, Boughton R I. Preparation of highly dispersed YAG nano-sized powder by co-precipitation method. Materials Letters, 2006, 60: 962. [10] Tong S H, Lu T C, Guo W. Synthesis of YAG powder by al-

JOURNAL OF RARE EARTHS, Vol. 27, No. 6, Dec. 2009 cohol-water co-precipitation method. Materials Letters, 2007, 61: 4287. [11] Ramanathan S, Roy S K, Bhat Y J. Transparent YAG from powder prepared by homogeneous precipitation reactionAl(NO3)3+Y(NO3)3+(NH4)2SO4+CO(NH2)2. Journal of Materials Science Letters, 2001, 20: 2119. [12] Lu Q M, Dong W S, Wang H J, Wang X K. A novel way to synthesize yttrium aluminum garnet from metal-inorganic precursors. Journal of the American Ceramic Society, 2002, 85: 490. [13] Zhang X D, Liu H, He W, Wang J Y, Li X, Boughton R I. Novel synthesis of YAG by solvothermal method. Journal of Crystal Growth, 2005, 275: 1913. [14] Jung K Y, Lee D Y, Kang Y C. Morphology control and luminescent property of Y3Al5O12: Tb particles prepared by spray pyrolysis. Materials Research Bulletin, 2005, 40: 2212. [15] Qiu F G, Pu X P, Li J, Liu X J, Pan Y B, Guo J K. Thermal behavior of the YAG precursor prepared by sol-gel combustion process. Ceramics International, 2005, 31: 663. [16] Li J, Pan Y B, Qiu F G, Wu Y S, Liu W B, Guo J K. Nanostructured Nd:YAG powders via gel combustion: the influence of citrate-to-nitrate ratio. Ceramics International, 2008, 34: 141. [17] Wang H Z, Gao L, Niihara K. Synthesis of nanoscaled yttrium aluminum garnet powder by the co-precipitation method. Materials Science Engineering A, 2000, 288: 1. [18] Palmero P, Esnouf C, Montanaro L, Fantozzi G. Influence of the co-precipitation temperature on phase evolution in yttrium-aluminium oxide materials. Journal of the European Ceramic Society, 2005, 25: 1565. [19] Yuan F L, Ryu H. Ce-doped YAG phosphor powders prepared by co-precipitation and heterogeneous precipitation. Materials Science Engineering B, 2004, 107: 14.