ISSN 00201685, Inorganic Materials, 2014, Vol. 50, No. 10, pp. 1030–1034. © Pleiades Publishing, Ltd., 2014. Original Russian Text © S.S. Balabanov, E.M. Gavrishchuk, V.V. Drobotenko, A.D. Plekhovich, E.E. Rostokina, 2014, published in Neorganicheskie Materialy, 2014, Vol. 50, No. 10, pp. 1114–1118.
Effect of the Composition of Starting Yttrium Aluminum Hydroxide Sols on the Properties of Yttrium Aluminum Garnet Powders S. S. Balabanova, E. M. Gavrishchuka, b, V. V. Drobotenkoa, A. D. Plekhovicha, b, and E. E. Rostokinaa, b a
Devyatykh Institute of Chemistry of HighPurity Substances, Russian Academy of Sciences, ul. Tropinina 49, Nizhni Novgorod, 603950 Russia b Lobachevsky State University (National State University), pr. Gagarina 23, Nizhni Novgorod, 603950 Russia email:
[email protected] Received November 12, 2013
Abstract—A technique has been developed for the synthesis of yttrium aluminum garnet sols aggregation stable in water as a dispersion medium. Depending on the type of precursor used, the temperature of yttrium aluminum garnet formation varies from 900°C to 1100°C. We have obtained yttrium aluminum garnet nano powders with an average particle size from 40 to 300 nm, depending on the aluminum yttrium hydroxide hydrosol synthesis procedure and annealing temperature. DOI: 10.1134/S0020168514100033
INTRODUCTION
EXPERIMENTAL
Yttrium aluminum garnet (YAG) based materials find wide application in hightemperature devices as optical and luminescent elements and as gain ele ments of high and ultrahighpower solidstate lasers [1]. The use of nanopowders typically allows one to improve the quality of such devices. In the fabrication of laser ceramics, the preparation of nanopowders is a necessary step. One attractive method for the synthesis of pure nanopowders highly uniform in chemical and phase compositions is sol–gel processing. Owing to the molecularscale mixing of starting materials, this method enables the synthesis of chemically homoge neous multicomponent oxides at relatively low tem peratures, and varying the colloidal and chemical properties of the sols, one can control the particle size, morphology, and degree of agglomeration of the pow ders. Even though this method offers a number of advantages, very few theoretical and experimental studies have been concerned with the synthesis of yttrium aluminum garnet hydrosols with controlled composition and high stability.
The starting chemicals used were yttrium oxide, Y2O3 (99.99% purity); aluminum isopropoxide, Al(iOC3H7)3 (99.99% purity); nitric acid, HNO3 (99.99% purity); and acetic acid, CH3COOH (99.9% purity).
The objectives of this work were to develop a tech nique for the synthesis of aggregationstable yttrium aluminum hydroxide hydrosols and yttrium aluminum garnet nanopowders and investigate the properties of the powders in relation to the composition of the pre cursor used.
Yttria nanopowder was prepared by selfpropagat ing hightemperature synthesis (SHS) from yttrium acetate nitrates as described in Ref. [2]. Yttrium hydroxyacetate (Y(OH)2(OOCCH3)) sols were pre pared as described in Ref. [3]. Aqueous solutions of aluminum hydroxynitrate aqua complexes with the general formula Aln(NO3)3(OH)3n – 3 ⋅ 6H2O, where n = 1–6, were synthesized as follows: ground crystalline aluminum isopropoxide was hydrolyzed by atmo spheric moisture for two weeks to give amorphous Al(OH)3 powder, which was dissolved in water with the addition of nitric acid in the ratio Al : HNO3 = 1 : 0.5–3 at a temperature of 90°С, with periodic stirring in an ultrasonic bath for 0.5–1 h. Boehmite (AlOOH) sol was prepared by peptizing boehmite powder with 0.64 M nitric acid with stirring in an ultrasonic bath, followed by heating in a water bath at a temperature of 70–80°С until the formation of a visually transparent sol. The AlO(OH) : HNO3 ratio was 1 : 0.07. Boehmite powder was synthesized by hydrolyzing aluminum iso propoxide in hot water at a temperature of 80°С and (iPrO)3Al : H2O = 1 : 100, as described by Yoldas [4]. Yttrium aluminum hydroxide hydrosols were pre pared by mixing appropriate starting reagents in the
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molar ratio Y : Al = 3 : 5 using the following proce dures: 1. To aqueous solutions of an aluminum hydroxy nitrate with the composition Al5(NO3)3(OH)12 was added the SHS yttria powder with stirring in an ultra sonic bath. Subsequent heating of the suspension at a temperature of 50°С with periodic stirring in an ultra sonic bath for 3.5–4 h led to the formation of trans parent, coagulationstable yttrium aluminum hydroxide sols with the tentative composition Y3Al5(NO3)3(OH)21 and a concentration of 40 g/L (hereafter, in terms of yttrium aluminum garnet). The Y3Al5(NO3)3(OH)21 sol transformed into gel at a con centration of 50–60 g/L. 2. Aluminum hydroxynitrate sol with the composi tion Al5(NO3)3(OH)12 was mixed with yttrium hydroxyacetate (Y(OH)2(OOCCH3)) sol. Subsequent processing of the mixture in an ultrasonic bath yielded transparent, aggregationstable yttrium aluminum hydroxide sols with the composition Al5(NO3)3(OH)12 ⋅ 3Y(OH)2(OOCCH3) and a concentration of 60 g/L. The aluminum yttrium hydroxyacetatonitrate sol transformed into gel at a concentration of 175 g/L. 3. A mixture of boehmite sol and yttrium hydroxy acetate sol was processed in an ultrasonic bath until the formation of transparent, coagulationstable alu minum yttrium hydroxyacetate sols with the composi tion 5AlOOH ⋅ 3Y(OH)2(OOCCH3) with a concen tration of 80 g/L. Such sols are stable over time and transform into gel at a concentration of 110 g/L. 4. To boehmite sol was added the SHS yttria pow der with stirring in an ultrasonic bath, followed by heating of the suspension in a water bath. The reaction yielded white viscous aluminum yttrium hydroxide gel with the composition 5AlOOH ⋅ 3/2Y2O3 with a con centration of 120 g/L. The sols (procedures 1–3) and gel (procedure 4) obtained were dried in an SNOL 20/300 LFN drying oven at temperatures between 90 and 150°C for two days, until the formation of xerogels. Next, the xero gels were ground in a planetary mill for 10 min and passed through a 94 µm laboratory sieve. The aluminum yttrium hydroxide powders were annealed in an SNOL 6.7/1300 laboratory furnace in the temperature range 700–1100°С with 30min iso thermal holds. The effect of heattreatment condi tions of aluminum yttrium hydroxides on the phase composition of yttrium aluminum garnet was studied using Xray diffraction (XRD) on a Bruker D8 Dis cover diffractometer with СuКα radiation (λ = 1.5418 Å, 2θ = 10°–80°). The heat effects of yttrium aluminum garnet for mation were assessed using a Netzsch STA 409 PC/PG simultaneous thermal analyzer (Ger many). The samples were enclosed in platinum cruci bles, and the measurements were made in flowing argon at a heating rate of 2.5 K/min. Thermogravi metry (TG)/differential scanning calorimetry (DSC) INORGANIC MATERIALS
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data were obtained in the temperature range from 750 to 1200°С. The specific surface area of the powders was deter mined by BET measurements on a SORBIM ana lyzer. RESULTS AND DISCUSSION Figure 1 shows DSC curves for the phase transfor mations of different xerogels. Analysis of the DSC curves of samples 1–4, prepared from sols of different compositions, allowed us to reveal general trends in yttrium aluminum garnet formation. In all of the samples, chemical reactions below a temperature of 750°С are accompanied by a weight loss. In the temperature range 750–1200°С, only phase transformations are observed. The DSC curve of sample 1 shows two welldefined exothermic peaks: a strong, sharp peak and a weak, broad peak, at temperatures of 922 and 979°С, respec tively. A joint analysis of the DSC curve and XRD data for the powders in the temperature range 900–1000°С leads us to conclude that the former peak is due to YAlO3 crystallization, and the latter corresponds to YAlO3 conversion into Y3Al5O12. The strong exothermic peak in the DSC curve of sample 2 at a temperature of 912°С is due to the crys tallization of amorphous yttrium aluminum garnet. The broader exothermic peak at 1121°С is probably related to recrystallization and sintering processes, driven by the excess surface energy due to the small particle size of the powders. The broad exothermic peak at a temperature of 927°С in the DSC curve of sample 3 arises from the crystallization of an intermediate phase, YAlO3 or Y4Al2O9, and subsequent transformation into yttrium aluminum garnet at 1027°С. The exothermic peaks at 1105 and 1165°С are most likely due recrystallization and sintering processes. The DSC curve of sample 4 shows a welldefined exothermic peak at a temperature of 1100°С, which corresponds to YAlO3 crystallization from Y4Al2O9. In the temperature range in question, no peaks of any other crystalline phases were detected. YAG formation in this system reaches completion at a temperature above 1300°С. The formation of yttrium aluminum garnet in sample 4 through a number of intermediate compounds and at a relatively high temperature (Table 1) indicates that the process occurs through solidstate reactions [5]. The crystallization of YAlO3 in orthor hombic symmetry also suggests that the reaction mechanism in this sample differs from those in sam ples 1–3, where YAlO3 crystallizes in hexagonal sym metry, which is typical of chemical synthesis methods (coprecipitation method, thermal decomposition of nitrates, sol–gel process, and hydrothermal synthesis) [6].
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ТG, %
DSC, mV/mg EXO 0.1
102
(b)
ТG, % 98
0
97
–0.1
96
–0.2
95
–0.3
94
–0.4
93
DSC, mV/mg EXO 0.05 0
101
–0.05 1121°С
979°С
100
–0.10
99
–0.15 –0.20 912°С
922°С 98 870
900
930 960 990 Temperature, °C (c)
ТG, % 93
1020
800
DSC, mV/mg 0 EXO –0.1
92 927°С
–0.2
900 1000 1100 Temperature, °C (d)
1200
DSC, mV/mg EXO –0.2 –0.3
94.5
–0.4
–0.3
91
–0.4 90
–0.6
1105°С
89
1165°С
–0.7 –0.8
950
1000
1050
–0.5
94.0
–0.6
–0.5
1027°С
88 900
ТG, % 95.0
–0.25
1100
1150
Temperature, °C
93.5
–0.7
–0.8 1100°С 93.0 1000 1020 1040 1060 1080 1100 1120 1140 1160 Temperature, °C
DSC curves for the phase transformations of different xerogels: (a) composition 1, (b) composition 2, (c) composition 3, (d) com position 4.
Table 1 presents Xray diffraction data for the yttrium aluminum garnet powders. The present results demonstrate that the hydrosol preparation procedure has a significant effect on the structural transforma tions of the precursor during heat treatment. Even though procedures 1–3 lead to the formation of stable yttrium aluminum garnet sols, the mechanism of YAG formation depends on the type of precursor used. The use of Al(OH)3 as the aluminum source (procedures 1 and 2) leads to YAG crystallization at a lower temper ature, which can be interpreted as evidence for more homogeneous mixing of the starting reagents in the system (molecularscale mixing of the metals) and crystallization directly from the amorphous phase. In the case of sols of boehmite and yttrium hydroxyace tate (procedure 3) with particle sizes in the range 25– 80 nm, no molecularscale mixing of the metals occurs. Because of this, during heat treatment of such xerogels garnet formation takes place at higher tem peratures in comparison with the first two procedures
and involves the formation of intermediate crystalline phases. Because of the alkaline reaction of yttria, mix ing its with boehmite sol (procedure 4) leads to coag ulation of the latter. The result is gel consisting of yttria agglomerates coated with boehmite gel. Heat treat ment of this mixture at temperatures above 1300°С leads to the synthesis of yttrium aluminum garnet and also involves the formation of several intermediate phases. This is also evidenced by the fact that, at low heat treatment temperatures (700–800°С), Xray dif fraction identified crystalline Y2O3, in contrast to the first three procedures. The use of a mixture of commercially available yttria and alumina sols and colloidal stabilizers [7, 8] and yttria powder dissolution in alumina sol [1], or the use of Al2(OH)nCl6 – n sol and yttrium acetate as pre cursors [9] lead to the formation of yttrium aluminum garnet at a temperature 100 to 500°C higher than the above values (for procedures 1–3). INORGANIC MATERIALS
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Table 1. Phase composition of yttrium aluminum garnet powders as determined by XRD (the powders were prepared from sols 1–3 and gel 4) (cr. = crystalline phase, am. = amorphous phase) Phase composition Composition 700°C 1. Al5(NO3)3(OH)12 ⋅ 3/2 Y2O3
800°C
6% cr. Y3Al5O12, 6% cr. Y3Al5O12, 64% cr. 94% am. 94% am. Y3Al5O12, 36% cr. YAlO3 100% am. 100% am. Cr. Y3Al5O12
2. Al5(NO3)3(OH)12 ⋅ 3Y(OH)2(OOCCH3) 3. 5AlOOH ⋅ 3Y(OH)2(OOCCH3) 100% am.
100% am.
4. 5AlOOH ⋅ 3/2Y2O3
Cr. Y2O3
Cr. Y2O3
The average particle size of the garnet was assessed using specific surface area data for the powders (BET analysis) under the assumption that the particles were spherical in shape. The results are summarized in Table 2. For samples 3 (900°С) and 4 (900–1100°C), we indicate the average particle size of aluminum yttrium oxides (because there is no yttrium aluminum garnet at these temperatures). For sample 4, the average par ticle size of yttrium aluminum garnet at 1300°С, eval uated using BET analysis, is 347 nm, and the crystal lite size is 78 nm. As would be expected, with increasing powder annealing temperature the specific surface area gradu ally decreases and the average particle size increases. The largest specific surface area was offered by the yttrium aluminum garnet powder prepared from a mixture of boehmite and yttrium hydroxyacetate sols (composition 3). Table 2. Specific surface area (S) and average particle size (d) of yttrium aluminum garnet powders. The numbers in parentheses indicate the average crystallite size of the yttri um aluminum garnet as determined from the broadening of diffraction lines Com posi tion
S, m2/g
1 2 3 4
4.4 11.0 98.8 45.7
d, nm
S, m2/g
900°C
d, nm
1000°C
S, m2/g
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8% cr. Y4Al2O9, 28% cr. YAlO3, 64% am. Cr. 81% Y2O3, 19% Y4Al2O9
1000°C
1100°C
Cr. Y3Al5O12
Cr. Y3Al5O12
Cr. Y3Al5O12
Cr. Y3Al5O12
84% cr. Y3Al5O12, 16% cr. YAlO3 Cr. 18% Y2O3, 66% Y4Al2O9, 16% YAlO3
Cr. Y3Al5O12 Cr. 23% Y3Al5O12, 4% Y2O3, 28% Y4Al2O9, 45% YAlO3
CONCLUSIONS We have developed a technique for the synthesis of aggregationstable sols with the yttrium aluminum garnet composition and water as a dispersion medium. We have obtained hydrosols with the compositions Al5(NO3)3(OH)12 ⋅ 3/2Y2O3, Al 5(NO3)3(OH)12 ⋅ 3Y(OH)2(OOCCH3), and 5AlOOH ⋅ 3Y(OH)2(OOCCH3) and concentrations from 40 to 80 g/L and a gel with the composition 5AlOOH ⋅ 3/2Y2O3. The procedure used to prepare aluminum yttrium hydroxide hydrosols has been shown to influence the temperature of yttrium aluminum garnet formation and the particle size of the resultant powders. Depend ing on the type of precursor used, the temperature of yttrium aluminum garnet formation varies from 900°С for the Al5(NO3)3(OH)12 ⋅ 3/2Y2O3 and Al5(NO3)3(OH)12 ⋅ 3Y(OH)2(OOCCH3) sols to 1100°С for the 5AlOOH ⋅ 3Y(OH)2(OOCCH3) sol. YAG formation in the 5AlOOH ⋅ 3/2Y2O3 gel proceeds through the formation of a number of crystalline com pounds at a temperature above 1300°С, indicating that yttrium aluminum garnet synthesis occurs through solidstate reactions. We have obtained yttrium aluminum garnet nanop owders with an average particle size from 40 to 300 nm, depending on the type of precursor and annealing temperature.
1100°C
300 (43) 3.7 356 (40) 2.5 120 (41) 6.2 213 (38) 3.3 13 29.9 44 (22) 17.3 29 38.6 34 14.6
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ACKNOWLEDGMENTS We are grateful to P.A. Yunin (Institute for Physics of Microstructures, Russian Academy of Sciences) for the Xray diffraction characterization of the samples.
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5. Lee, S.H. et al., Solidstate reactive sintering of trans parent polycrystalline Nd:YAG ceramics, J. Am. Ceram. Soc., 2006, vol. 89, no. 6, pp. 1945–1950. 6. Sim, S.M. et al., Phase formation in yttrium alumi num garnet powders synthesized by chemical methods, J. Mater. Sci., 2000, vol. 35, no. 3, p. 713–717. 7. Tanner, P.A. et al., Preformed sol–gel synthesis and characterization of lanthanide iondoped yttria–alu mina materials, Phys. Status Solidi A, 2003, vol. 199, no. 3, pp. 403–415. 8. King, B.H. and Halloran, W., Polycrystalline yttrium aluminum garnet fibers from colloidal sols, J. Am. Ceram. Soc., 1995, vol. 78, no. 8, pp. 2141–2148. 9. Li, G. Zhang, Y., et al., Preparation, microstructure and properties of yttrium aluminum garnet fibers pre pared by sol–gel method, Mater. Chem. Phys., 2009, vol. 113, no. 1, pp. 31–35.
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