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Production of dispersed nanometer sized YAG powders from alkoxide, nitrate and chloride precursors and spark plasma sintering to transparency
Journal of Alloys and Compounds 493 (2010) 391–395
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Production of dispersed nanometer sized YAG powders from alkoxide, nitrate and chloride precursors and spark plasma sintering to transparency M. Suárez a,∗ , A. Fernández b , J.L. Menéndez a , R. Torrecillas a a Departamento de Materiales Nanoestructurados, Centro de Investigación en Nanomateriales y Nanotecnología (CINN), Principado de Asturias – Consejo superior de Investigaciones Científicas (CSIC) – Universidad de Oviedo (UO), Parque Tecnológico de Asturias, 33428 Llanera, (Asturias), Spain b Fundación ITMA, Parque Tecnológico de Asturias, 33428, Llanera, Spain
a r t i c l e
i n f o
Article history: Received 17 September 2009 Received in revised form 17 December 2009 Accepted 18 December 2009 Available online 28 December 2009 Keywords: Ceramic Optical materials Sol–gel process Sintering Optical properties
a b s t r a c t Yttrium aluminum garnet (YAG) was synthesized from different starting materials, i.e., alkoxide, nitrate and chloride precursors. The conversion steps from the precursors to crystalline YAG were studied by Raman spectroscopy. Dispersed YAG powders were formed at a relatively low temperature, around 800 ◦ C by the chlorides route, whereas alkoxide precursors needed firing over 900 ◦ C and nitrates even over 1100 ◦ C. Lyophilized YAG gel was sintered to transparency by the spark plasma sintering method at 1500 ◦ C with in-line transmittances close to 60% at 680 nm and over 80% in the infrared range. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Single crystal yttrium aluminum garnet (YAG) with the chemical composition Y3 Al5 O12 has been widely recognized as a laser gain host material for the last four decades. Traditionally, YAG has been prepared as a single crystal by the Czochralski method but this production method presents two main drawbacks: on one hand, it is very expensive, and, on the other hand, it is difficult to produce large pieces. Over these years, more attention has been paid to the fabrication of polycrystalline YAG ceramics [1–3] since they have shown optical and high temperature mechanical properties comparable to those of the single crystals, but with a much simpler processing and consequently, a lower price, ease of manufacture and mass-production. The interest in polycrystalline YAG as an optical material arises not only from the possibility of developing active media for lasers when doped with rare earth ions, but it may also be used as a window material both for UV and IR optics. It is particularly useful for high temperature and high-energy applications and it shows no trace absorption in the 2–3 m region where glasses tend to be highly absorbent due to the strong H2 O band. A technique commonly used to prepare YAG involves the mixing of Al2 O3 and Y2 O3 powders, followed by calcinations at high
∗ Corresponding author. Tel.: +34 985 98 00 58; fax: +34 985 26 55 74. E-mail address: [email protected] (M. Suárez). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.12.108
temperature. This technique generally requires extensive heat treatment at high temperatures, above 1600 ◦ C [4,5], and a prolonged burning time. On the other hand, several wet-chemical processes have been developed and successfully used in recent years for low-temperature production of pure phase powders. These methods include solvothermal synthesis [6–9], the sol–gel route [10–12], nitrate–citrate combustion [13] and coprecipitation methods [14–19]. These chemical processes achieve intimate mixing of the reactant cations at the atomic level, leading to an increase in the reaction rate and a decrease in the synthesis temperature. Different features of the YAG products such as crystallization, final grain size, morphology, dispersion of the powders and purity of the product, in terms of phases present will depend on the precursors and synthesis routes employed. Raman spectroscopy is a valuable tool to study the structural transformations during the subsequent thermal treatment of an amorphous material on a molecular scale as it is sensitive to molecular vibrations. This technique offers several advantages as a good sensitivity in low crystalline materials means that special conditions for preparation of the samples are not required and the analysis time is very short. It is the aim of this work to explore the possibility of preparing pure crystalline YAG nanopowders by different routes and precursors. To do this, two different wet-chemical processes were used: sol–gel and reverse-strike coprecipitation were compared and three kinds of precursors were used: alkoxides, nitrates and chlorides. The phase transformations of the powders were
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M. Suárez et al. / Journal of Alloys and Compounds 493 (2010) 391–395
investigated by evaluating their Raman spectra after different thermal treatments. Finally, the best synthesis route was chosen in order to obtain transparent YAG materials by spark plasma sintering (SPS). 2. Experimental procedures Amorphous oxide mixtures with the stoichiometric composition 3Y2 O3 :5Al2 O3 were prepared by two different methods: sol–gel and reverse-strike coprecipitation. In the sol–gel method, aluminum tri-sec-butoxide (Aldrich, 97%) and yttrium methoxyethoxide (ABCR GmbH) were chemically polymerized. Yttrium methoxyethoxide dissolved in ethanol was added dropwise to an aluminum tri-secbutoxide solution in ethanol and it was slightly heated to obtain a clear solution. Then, 0.5 mol/mol alkoxide of 2,4-pentanedione (Fluka, 99.5%) complexing agent, was added to the previous solution. Finally, the appropriate amount of water for hydrolysis was added to the mixture to obtain the aluminum and yttrium hydroxides. The mixture was heated to 60–70 ◦ C in order to evaporate the dissolvent leading to the formation of the gel. Finally, the gel was dried at 120 ◦ C. In the reverse-strike coprecipitation, aluminum and yttrium nitrates (Al(NO3 )3 ·9H2 O (Fluka, 98%) and Y(NO3 )3 ·6H2 O (Aldrich, 99.9%)) or aluminum and yttrium chlorides (AlCl3 ·9H2 O (Fluka, 99%) and YCl3 ·6H2 O (Aldrich, 99.99%)) with a 5:3 molar ratio were mixed with ammonium hydroxide solution (Fluka, 28% in water). Keeping a constant pH value during the process was critical to control the chemical homogeneity within the particles. The stoichiometric coprecipitation of aluminum and yttrium ions was achieved at pH 9. After aging for 24 h, a gelatinous precipitate was obtained. Then, solvent was removed by centrifugation and subsequently the amorphous gel was dried at 60–70 ◦ C. All powders prepared by using different precursors were ground with mortar and pestle, sieved below 60 m and then they were burnt at different temperatures in order to achieve a crystalline material. Thermogravimetric and differential temperature analysis for three products (TGA/DSC Star System, Mettler Toledo) were carried out under air atmosphere on the dried gels at a heating rate of 5 ◦ C/min up to 1500 ◦ C. The phase transitions of the heat-treated samples as a function of the temperature were followed by Raman spectroscopy (Jobin Yvon, HORIBA). X-ray diffraction (XRD) data were obtained with a powder diffractometer (D8 Advance, BRUKER). The particle size and the morphology of the polycrystalline YAG nanopowders synthesized with the three precursors were investigated by TEM (2000 FX, JEOL). Sintering was performed in a spark plasma sintering apparatus (HPD 25/1, FCT) under low vacuum (10−1 mbar). Crystalline powders were placed into a graphite die with an inner diameter of 20 mm and sintered at 1500 ◦ C for 3 min under an applied pressure of 50 MPa and a heating rate of 100 ◦ C/min. The density of the samples was measured by the Archimedes method using distilled water as the immersion liquid and a theoretical density of 4.55 g/cm3 . The microstructure was studied by scanning electron microscopy (SEM, DSM 950, Zeiss). The SEM samples were prepared by mechanical polishing with a diamond spray down to 1 m, followed by a thermal etching. The samples were gold covered prior to observation. The average final grain size was measured using the intercept analysis of Smith and Guttman [20], using 1.56 as the stereological correction factor. The transmission spectrum was recorded on visible (AvaSpec-2048, Avantes) and infrared (IR-560, Nicolet Magna) equipment.
Fig. 1. TG and DTA analysis of the synthesized gels from the alkoxide (a), nitrate (b) and chloride (c) precursors.
3. Results and discussion Fig. 1 shows the differential thermal analysis (DTA) curves corresponding to the gels obtained by the alkoxide (Fig. 1a), nitrate (Fig. 1b) and chloride (Fig. 1c) precursors. Two weight losses in the temperature ranges of 30–200, 200–580 ◦ C are present in the DTA corresponding to the alkoxides route (Fig. 1a). The first one, below 200 ◦ C, is due to the removal of adsorbed and chemically bonded solvent. The second and most significant decomposition step can be attributed to the elimination of alkoxy groups attached to various metal centers of the molecular precursor which is supported by an exothermic peak present in this region as observed in Ref. [2] where aluminum and yttrium tert-butoxide were used. Two exothermic peaks appear at 925 and 1200 ◦ C. The peak at 925 ◦ C indicates the onset of powder crystallization. The second exothermic peak at 1200 ◦ C corresponds to the formation of YAG, whereas the first one probably corresponds to the formation of a phase in the Y2 O3 –Al2 O3 phase diagram, other than YAG. The presence of a shoulder between 800 and 1000 ◦ C is due to the formation of oxo-alkoxide clusters of general formula M5 O(OR)13 . In the case of nitrate precursors, the TG curve (Fig. 1b), shows a weight loss associated with an endothermic peak between 40 and 290 ◦ C corresponding to the dehydration of adsorbed water in the powders and the dehydroxylation processes. This is fol-
lowed by an exothermic peak around 290–515 ◦ C attributed to the decomposition of the precursors in the oxide compounds. Also, two exothermic peaks at 930 and 995 ◦ C can be observed: the sharp exothermic peak at 930 ◦ C was caused by the crystallization of YAP. The less pronounced exothermic peak around 995 ◦ C corresponds to YAP reacting with a polymorph of Al2 O3 to form YAG [21]. Fig. 1c shows the TG-DTA curves for the powders synthesized from chloride precursors. The endothermic peak in the 40–405 ◦ C range present in the thermogram is associated with the dehydration of aluminum mono- and trihydrates [22] and of yttrium hydroxides [23] and the adsorbed moisture. The second weight loss between 405 and 600 ◦ C is due to the evaporation of NH4 Cl which appears during the synthesis as a by-product, and finally, the exothermic peaks around 855 ◦ C can be attributed to the powder crystallization. The thermogravimetric results of the gels synthesized by different precursors show: (1) the product yield obtained in the nitrate and chloride routes is higher than in the alkoxide route where a weight loss of 70% took place and (2) pure YAG was obtained in the chlorides route with no intermediate phases observed, in contrast to the alkoxides and nitrates routes. The XRD patterns of the powders burnt at 800 ◦ C are shown in Fig. 2. Whereas broad bands, rather than peaks, associated with the presence of amorphous powders, are present in the alkoxide and
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Fig. 2. XRD patterns of the powders prepared by the alkoxide, nitrate and chloride precursors burnt at 800 ◦ C for 2 h.
nitrate routes, small peaks appear in the chloride powders diffractogram showing the beginning of the crystallization. Due to the difficulties in monitoring changes in low crystalline systems, as those shown in this work, by XRD, Raman spectroscopy will be used from now on. Fig. 3 shows the Raman spectra of the synthesized powders fired at different temperatures using a heating rate of 5 ◦ C/min. After calcination at 800 ◦ C (Fig. 3a), powders coming from the alkoxides route exhibit fluorescence due to the oxo-alkoxide clusters forming a stiff structure, whereas the fluorescence produced by the nitrate precursors is rather weak and there are none for the chloride precursors. Powders obtained from the alkoxide and nitrate precursors exhibit broad Raman bands, indicating the presence of amorphous material. For the chloride powders, peaks of crystalline YAG between 200 and 900 cm−1 appear in the spectra, indicating that crystallization is starting. It is very important to remark on the suitability of Raman spectroscopy to monitor changes in these low crystalline systems. There are noticeable differences between Figs. 2 and 3a, in particular for the chloride precursors. Just a broad band around 30◦ is observed by XRD, whereas intense peaks corresponding to the formation of YAG are observed up to 700 cm−1 . The differences arise from the degree of crystallinity of the materials. In the early stages of crystallization, when YAG starts to be formed, just small crystallites are expected to appear and they are poorly detected by XRD, where a large crystalline coherence, over tens of nanometers, is required in order to obtain a well-defined peak. However, Raman scattering relies on molecular vibrations and is not so dependent on the crystal size. The Raman spectra of powders after heating up to 1100 ◦ C for 2 h are shown in Fig. 3b. Powders synthesized by the chloride precursors show well-defined peaks corresponding to crystalline YAG. The phase transformation seems to be finished at 1100 ◦ C, as all the peaks have a small width and only peaks corresponding to pure YAG are observed. On the other hand, some well-defined peaks between 300 and 700 cm−1 appear in the spectra belonging to the alkoxide powders, but the broad band centered around 600 cm−1 indicates that a part of the material still remains amorphous. Weak peaks (350, 900 cm−1 . . .) showing the beginning of the crystallization are observed in the spectrum from the nitrate powders. However, these peaks are not well-defined and the broad peaks in the spectrum indicate the presence of an amorphous material. Fig. 3c shows the spectra of the powders after burning at 1200 ◦ C for 2 h. Whereas powders corresponding to chloride precursors show perfectly crystalline pure YAG peaks, powders synthesized by the alkoxide and nitrate routes are not pure and show phases
Fig. 3. Raman spectra of the powders heat treated at 800 ◦ C (a), 1100 ◦ C (b) and 1200 ◦ C (c) for 2 h prepared by the alkoxide, nitrate and chloride precursors.
other than YAG (indicated with asterisks in the figure). Possible impurities in the prepared oxides, such as the nitrates, may act as suppressors of the transformation process and have an effect on the degree of crystallinity and homogeneity of starting materials and on the kinetics of phase transformation, according to Ref. [24]. TEM images (Fig. 4) show the morphology of the powders synthesized by the alkoxide (a), nitrate (b) and chloride (c) routes burnt at 1200 ◦ C for 2 h. The differences in the morphology of the powders synthesized by the different routes depend on the precursor and on its chemistry, leading to the formation of either colloidal particles or polymeric gels during the chemical reaction. Powders synthesized by the alkoxide route (Fig. 4a) present irregular forms, but mainly elongated, with an inhomogeneous particle size distribution and with an average size of about 100–300 nm formed by small structural units below 100 nm. Powders synthesized from the nitrate precursor (Fig. 4b) are more agglomerated with an inhomogeneous size distribution. In this case, necks between the
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M. Suárez et al. / Journal of Alloys and Compounds 493 (2010) 391–395
Fig. 5. Transmittance spectra normalized to 0.8 mm thickness for experimental measurement (solid line) and simulation (dotted line).
It can be observed that values of up to 82% in the infrared region and 56% at 680 nm can be achieved by sintering of dispersed powders synthesized using the chloride route. In order to analyze the in-line transmittance spectra, a model based on that given by Apetz and Van Bruggen [25] was developed [26]. This model enabled the extracting of information concerning the total porosity and its distribution in the sintered YAG material. The average grain size and its distribution were obtained from the SEM characterization and, therefore, are not fitting parameters in this case. As can be seen in Fig. 5, the simulation results fit the experimental measurement of the transmittance well, showing a full density YAG material with a pore size of around 40 nm. The most significant differences can be observed between 1000 and 2000 nm and they can be attributed to the residual graphite in the samples sintered by SPS. It is important to emphasise that in the visible region (400–900 nm), where the changes are more important, the simulation is very close to the experimental line. 4. Conclusions
Fig. 4. TEM morphology of the crystalline YAG powder synthesized by the alkoxide (a), nitrate (b) and chloride (c) precursors.
grains are observed, indicating the onset of sintering at this temperature. Powders obtained by the chloride route (Fig. 4c) show a well-dispersed microstructure with an average grain size in the nanometer range, 100 nm approximately. Therefore, deagglomerated homogeneous nanometric crystalline YAG powders were obtained by reverse-strike precipitation using chlorides as precursors followed by gel lyophilization. Lyophilized YAG nanopowders synthesized by the chloride route were sintered at 1500 ◦ C for 3 min. During the entire sintering cycle, a pressure of 50 MPa was applied and the heating rate was 100 ◦ C/min. Fig. 5 shows the in-line transmittance of the sample normalized to a thickness of 0.8 mm according to Eq. (1).
T
T = (1 − Rs )
measured
1 − Rs
t/d (1)
d: actual thickness of the sample; t: normalizing thickness (0.8 mm in the present work); T: in-line transmittance normalized to thickness t; Rs : reflectance loss, that equals 2R /(1 + R ) where R = (n − 1/n + 1)2 , with n the refractive index of the material, alumina in the present work.
YAG powders were synthesized using three kinds of precursors by different wet-chemical routes. Powders with different average sizes and morphologies can be obtained by choosing the precursor. Whereas chloride precursors lead to powders with a homogeneous morphology and an average grain size in the nm range, alkoxide and nitrate routes produce powders with a large degree of agglomeration. The chloride route has been shown to be the most suitable one to achieve pure and polycrystalline YAG at a lower temperature and with a grain size in the nanometer range: the chloride route enables the obtaining of pure YAG at 1100 ◦ C, whereas fully crystalline powders are not obtained in the alkoxide and nitrate routes up to temperatures as high as 1200 ◦ C. This has an important impact on the sintering conditions, as the powders obtained in the nitrates route already start sintering before they are fully crystallized. The suitability of Raman spectroscopy to monitor the crystallization of the powders has also been demonstrated. Finally, lyophilized powders synthesized by the chloride route were spark plasma sintered into 0.8 mm thick transparent YAG materials with in-line transmittances close to 60% at 680 nm and over 80% in the infrared range. Acknowledgments The authors want to acknowledge the Spanish Ministry of Education and Science and UE for funding through projects
M. Suárez et al. / Journal of Alloys and Compounds 493 (2010) 391–395
MAT2006-01783 and NMP3-CT-2005-515784. One of us, M. Suarez, wants to acknowledge I3P program for PhD grant. References [1] L. Wen, X. Sun, Z. Xiu, S. Chen, C.T. Tsai, J. Eur. Ceram. Soc. 24 (2004) 2681– 2688. [2] M. Veith, S. Mathur, A. Kareiva, M. Jilavi, M. Zimmer, V. Huch, J. Mater. Chem. 9 (1999) 3069–3079. [3] J.G. Li, T. Ikegami, J.H. Lee, T. Mori, J. Am. Ceram. Soc. 83 (2000) 961–963. [4] Q. Zhang, F. Saito, Powder Technol. 129 (2003) 86–91. [5] Ch.Q. Li, H.B. Zuo, M.F. Zhang, J.B. Han, S.H. Meng, Trans. Nonferrous Metal Soc. China 17 (2007) 148–153. [6] X. Zhang, H. Liu, W. He, J. Wang, X. Li, R.I. Boughton, J. Alloys Compd. 372 (2004) 300–303. [7] X. Zhang, H. Liu, W. He, J. Wang, X. Li, R.I. Boughton, J. Cryst. Growth 275 (2005) e1913–e1917. [8] M. Inoue, H. Otsu, H. Kominami, T. Inui, J. Am. Ceram. Soc. 74 (1991) 1254–1452. [9] Z. Wu, X. Zhang, W. He, Y. Du, N. Jia, P. Liu, F. Bu, J. Alloys Compd. 472 (2009) 576–580. [10] G. Gowda, J. Mater. Sci. Lett. 5 (1986) 1029–1032.
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[11] R. Manalert, R.M. Rahaman, J. Mater. Sci. Lett. 31 (1996) 3453–3458. [12] L. Yang, T. Lu, H.X. Nian Wei, J. Alloys Compd. 4849 (2009) 449–545. [13] J.J. Zhang, J.W. Ning, X.J. Liu, Y.B. Pan, L.P. Huang, Mater. Lett. 57 (2003) 3077–3081. [14] X. Li, H. Liu, J. Wang, H. Cui, X. Zhang, F. Han, Mater. Sci. Eng., A 379 (2004) 347–350. [15] A.K. Pradhan, K. Zhang, G.B. Loutts, Mater. Res. Bull. 39 (2004) 1291–1298. [16] Z.H. Chen, Y. Yang, Z.G. Hu, J.T. Li, S.L. He, J. Alloys Compd. 433 (2007) 328–331. [17] B. Hoghooghi, L. Healey, S. Powell, J. McKittrick, Mater. Chem. Phys. 38 (1994) 175–180. [18] K. Zhang, H.-Z. Liu, Y.-T. Wu, W.-B. Hu, J. Alloys Compd. 453 (2008) 265–270. [19] Y. Zhang, H. Yu, Ceram. Int. 35 (2009) 2077–2081. [20] C.S. Smith, L. Guttman, Trans. AIME 197 (1953) 81–87. [21] J.G. Li, T. Ikegami, J.H. Lee, T. Mori, Y. Yajima, J. Eur. Ceram. Soc. 20 (2000) 2395–2405. [22] A.B. Carel, D.K. Cabbiness, Am. Ceram. Soc. Bull. 64 (1985) 716–719. [23] E.A. Ivanova, V.G. Konakov, E.N. Solovyeva, Rev. Adv. Mater. Sci. 4 (2003) 41–47. [24] C.N.R. Rao, J. Gopalakrishnan, New Directions in Solid State Chemistry, Cambridge University Press, Cambridge, 1997, p.480. [25] R. Apetz, M.P.B. Van Bruggen, J. Am. Ceram. Soc. 86 (2003) 480. [26] M. Suárez, A. Fernández, J.L. Menéndez, R. Torrecillas, Scripta Mater. 61 (2009) 931.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Production of dispersed nanometer sized YAG powders from alkoxide, nitrate and chloride precursors and spark plasma sintering to transparency M. Suárez a,∗ , A. Fernández b , J.L. Menéndez a , R. Torrecillas a a Departamento de Materiales Nanoestructurados, Centro de Investigación en Nanomateriales y Nanotecnología (CINN), Principado de Asturias – Consejo superior de Investigaciones Científicas (CSIC) – Universidad de Oviedo (UO), Parque Tecnológico de Asturias, 33428 Llanera, (Asturias), Spain b Fundación ITMA, Parque Tecnológico de Asturias, 33428, Llanera, Spain
a r t i c l e
i n f o
Article history: Received 17 September 2009 Received in revised form 17 December 2009 Accepted 18 December 2009 Available online 28 December 2009 Keywords: Ceramic Optical materials Sol–gel process Sintering Optical properties
a b s t r a c t Yttrium aluminum garnet (YAG) was synthesized from different starting materials, i.e., alkoxide, nitrate and chloride precursors. The conversion steps from the precursors to crystalline YAG were studied by Raman spectroscopy. Dispersed YAG powders were formed at a relatively low temperature, around 800 ◦ C by the chlorides route, whereas alkoxide precursors needed firing over 900 ◦ C and nitrates even over 1100 ◦ C. Lyophilized YAG gel was sintered to transparency by the spark plasma sintering method at 1500 ◦ C with in-line transmittances close to 60% at 680 nm and over 80% in the infrared range. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Single crystal yttrium aluminum garnet (YAG) with the chemical composition Y3 Al5 O12 has been widely recognized as a laser gain host material for the last four decades. Traditionally, YAG has been prepared as a single crystal by the Czochralski method but this production method presents two main drawbacks: on one hand, it is very expensive, and, on the other hand, it is difficult to produce large pieces. Over these years, more attention has been paid to the fabrication of polycrystalline YAG ceramics [1–3] since they have shown optical and high temperature mechanical properties comparable to those of the single crystals, but with a much simpler processing and consequently, a lower price, ease of manufacture and mass-production. The interest in polycrystalline YAG as an optical material arises not only from the possibility of developing active media for lasers when doped with rare earth ions, but it may also be used as a window material both for UV and IR optics. It is particularly useful for high temperature and high-energy applications and it shows no trace absorption in the 2–3 m region where glasses tend to be highly absorbent due to the strong H2 O band. A technique commonly used to prepare YAG involves the mixing of Al2 O3 and Y2 O3 powders, followed by calcinations at high
∗ Corresponding author. Tel.: +34 985 98 00 58; fax: +34 985 26 55 74. E-mail address: [email protected] (M. Suárez). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.12.108
temperature. This technique generally requires extensive heat treatment at high temperatures, above 1600 ◦ C [4,5], and a prolonged burning time. On the other hand, several wet-chemical processes have been developed and successfully used in recent years for low-temperature production of pure phase powders. These methods include solvothermal synthesis [6–9], the sol–gel route [10–12], nitrate–citrate combustion [13] and coprecipitation methods [14–19]. These chemical processes achieve intimate mixing of the reactant cations at the atomic level, leading to an increase in the reaction rate and a decrease in the synthesis temperature. Different features of the YAG products such as crystallization, final grain size, morphology, dispersion of the powders and purity of the product, in terms of phases present will depend on the precursors and synthesis routes employed. Raman spectroscopy is a valuable tool to study the structural transformations during the subsequent thermal treatment of an amorphous material on a molecular scale as it is sensitive to molecular vibrations. This technique offers several advantages as a good sensitivity in low crystalline materials means that special conditions for preparation of the samples are not required and the analysis time is very short. It is the aim of this work to explore the possibility of preparing pure crystalline YAG nanopowders by different routes and precursors. To do this, two different wet-chemical processes were used: sol–gel and reverse-strike coprecipitation were compared and three kinds of precursors were used: alkoxides, nitrates and chlorides. The phase transformations of the powders were
392
M. Suárez et al. / Journal of Alloys and Compounds 493 (2010) 391–395
investigated by evaluating their Raman spectra after different thermal treatments. Finally, the best synthesis route was chosen in order to obtain transparent YAG materials by spark plasma sintering (SPS). 2. Experimental procedures Amorphous oxide mixtures with the stoichiometric composition 3Y2 O3 :5Al2 O3 were prepared by two different methods: sol–gel and reverse-strike coprecipitation. In the sol–gel method, aluminum tri-sec-butoxide (Aldrich, 97%) and yttrium methoxyethoxide (ABCR GmbH) were chemically polymerized. Yttrium methoxyethoxide dissolved in ethanol was added dropwise to an aluminum tri-secbutoxide solution in ethanol and it was slightly heated to obtain a clear solution. Then, 0.5 mol/mol alkoxide of 2,4-pentanedione (Fluka, 99.5%) complexing agent, was added to the previous solution. Finally, the appropriate amount of water for hydrolysis was added to the mixture to obtain the aluminum and yttrium hydroxides. The mixture was heated to 60–70 ◦ C in order to evaporate the dissolvent leading to the formation of the gel. Finally, the gel was dried at 120 ◦ C. In the reverse-strike coprecipitation, aluminum and yttrium nitrates (Al(NO3 )3 ·9H2 O (Fluka, 98%) and Y(NO3 )3 ·6H2 O (Aldrich, 99.9%)) or aluminum and yttrium chlorides (AlCl3 ·9H2 O (Fluka, 99%) and YCl3 ·6H2 O (Aldrich, 99.99%)) with a 5:3 molar ratio were mixed with ammonium hydroxide solution (Fluka, 28% in water). Keeping a constant pH value during the process was critical to control the chemical homogeneity within the particles. The stoichiometric coprecipitation of aluminum and yttrium ions was achieved at pH 9. After aging for 24 h, a gelatinous precipitate was obtained. Then, solvent was removed by centrifugation and subsequently the amorphous gel was dried at 60–70 ◦ C. All powders prepared by using different precursors were ground with mortar and pestle, sieved below 60 m and then they were burnt at different temperatures in order to achieve a crystalline material. Thermogravimetric and differential temperature analysis for three products (TGA/DSC Star System, Mettler Toledo) were carried out under air atmosphere on the dried gels at a heating rate of 5 ◦ C/min up to 1500 ◦ C. The phase transitions of the heat-treated samples as a function of the temperature were followed by Raman spectroscopy (Jobin Yvon, HORIBA). X-ray diffraction (XRD) data were obtained with a powder diffractometer (D8 Advance, BRUKER). The particle size and the morphology of the polycrystalline YAG nanopowders synthesized with the three precursors were investigated by TEM (2000 FX, JEOL). Sintering was performed in a spark plasma sintering apparatus (HPD 25/1, FCT) under low vacuum (10−1 mbar). Crystalline powders were placed into a graphite die with an inner diameter of 20 mm and sintered at 1500 ◦ C for 3 min under an applied pressure of 50 MPa and a heating rate of 100 ◦ C/min. The density of the samples was measured by the Archimedes method using distilled water as the immersion liquid and a theoretical density of 4.55 g/cm3 . The microstructure was studied by scanning electron microscopy (SEM, DSM 950, Zeiss). The SEM samples were prepared by mechanical polishing with a diamond spray down to 1 m, followed by a thermal etching. The samples were gold covered prior to observation. The average final grain size was measured using the intercept analysis of Smith and Guttman [20], using 1.56 as the stereological correction factor. The transmission spectrum was recorded on visible (AvaSpec-2048, Avantes) and infrared (IR-560, Nicolet Magna) equipment.
Fig. 1. TG and DTA analysis of the synthesized gels from the alkoxide (a), nitrate (b) and chloride (c) precursors.
3. Results and discussion Fig. 1 shows the differential thermal analysis (DTA) curves corresponding to the gels obtained by the alkoxide (Fig. 1a), nitrate (Fig. 1b) and chloride (Fig. 1c) precursors. Two weight losses in the temperature ranges of 30–200, 200–580 ◦ C are present in the DTA corresponding to the alkoxides route (Fig. 1a). The first one, below 200 ◦ C, is due to the removal of adsorbed and chemically bonded solvent. The second and most significant decomposition step can be attributed to the elimination of alkoxy groups attached to various metal centers of the molecular precursor which is supported by an exothermic peak present in this region as observed in Ref. [2] where aluminum and yttrium tert-butoxide were used. Two exothermic peaks appear at 925 and 1200 ◦ C. The peak at 925 ◦ C indicates the onset of powder crystallization. The second exothermic peak at 1200 ◦ C corresponds to the formation of YAG, whereas the first one probably corresponds to the formation of a phase in the Y2 O3 –Al2 O3 phase diagram, other than YAG. The presence of a shoulder between 800 and 1000 ◦ C is due to the formation of oxo-alkoxide clusters of general formula M5 O(OR)13 . In the case of nitrate precursors, the TG curve (Fig. 1b), shows a weight loss associated with an endothermic peak between 40 and 290 ◦ C corresponding to the dehydration of adsorbed water in the powders and the dehydroxylation processes. This is fol-
lowed by an exothermic peak around 290–515 ◦ C attributed to the decomposition of the precursors in the oxide compounds. Also, two exothermic peaks at 930 and 995 ◦ C can be observed: the sharp exothermic peak at 930 ◦ C was caused by the crystallization of YAP. The less pronounced exothermic peak around 995 ◦ C corresponds to YAP reacting with a polymorph of Al2 O3 to form YAG [21]. Fig. 1c shows the TG-DTA curves for the powders synthesized from chloride precursors. The endothermic peak in the 40–405 ◦ C range present in the thermogram is associated with the dehydration of aluminum mono- and trihydrates [22] and of yttrium hydroxides [23] and the adsorbed moisture. The second weight loss between 405 and 600 ◦ C is due to the evaporation of NH4 Cl which appears during the synthesis as a by-product, and finally, the exothermic peaks around 855 ◦ C can be attributed to the powder crystallization. The thermogravimetric results of the gels synthesized by different precursors show: (1) the product yield obtained in the nitrate and chloride routes is higher than in the alkoxide route where a weight loss of 70% took place and (2) pure YAG was obtained in the chlorides route with no intermediate phases observed, in contrast to the alkoxides and nitrates routes. The XRD patterns of the powders burnt at 800 ◦ C are shown in Fig. 2. Whereas broad bands, rather than peaks, associated with the presence of amorphous powders, are present in the alkoxide and
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Fig. 2. XRD patterns of the powders prepared by the alkoxide, nitrate and chloride precursors burnt at 800 ◦ C for 2 h.
nitrate routes, small peaks appear in the chloride powders diffractogram showing the beginning of the crystallization. Due to the difficulties in monitoring changes in low crystalline systems, as those shown in this work, by XRD, Raman spectroscopy will be used from now on. Fig. 3 shows the Raman spectra of the synthesized powders fired at different temperatures using a heating rate of 5 ◦ C/min. After calcination at 800 ◦ C (Fig. 3a), powders coming from the alkoxides route exhibit fluorescence due to the oxo-alkoxide clusters forming a stiff structure, whereas the fluorescence produced by the nitrate precursors is rather weak and there are none for the chloride precursors. Powders obtained from the alkoxide and nitrate precursors exhibit broad Raman bands, indicating the presence of amorphous material. For the chloride powders, peaks of crystalline YAG between 200 and 900 cm−1 appear in the spectra, indicating that crystallization is starting. It is very important to remark on the suitability of Raman spectroscopy to monitor changes in these low crystalline systems. There are noticeable differences between Figs. 2 and 3a, in particular for the chloride precursors. Just a broad band around 30◦ is observed by XRD, whereas intense peaks corresponding to the formation of YAG are observed up to 700 cm−1 . The differences arise from the degree of crystallinity of the materials. In the early stages of crystallization, when YAG starts to be formed, just small crystallites are expected to appear and they are poorly detected by XRD, where a large crystalline coherence, over tens of nanometers, is required in order to obtain a well-defined peak. However, Raman scattering relies on molecular vibrations and is not so dependent on the crystal size. The Raman spectra of powders after heating up to 1100 ◦ C for 2 h are shown in Fig. 3b. Powders synthesized by the chloride precursors show well-defined peaks corresponding to crystalline YAG. The phase transformation seems to be finished at 1100 ◦ C, as all the peaks have a small width and only peaks corresponding to pure YAG are observed. On the other hand, some well-defined peaks between 300 and 700 cm−1 appear in the spectra belonging to the alkoxide powders, but the broad band centered around 600 cm−1 indicates that a part of the material still remains amorphous. Weak peaks (350, 900 cm−1 . . .) showing the beginning of the crystallization are observed in the spectrum from the nitrate powders. However, these peaks are not well-defined and the broad peaks in the spectrum indicate the presence of an amorphous material. Fig. 3c shows the spectra of the powders after burning at 1200 ◦ C for 2 h. Whereas powders corresponding to chloride precursors show perfectly crystalline pure YAG peaks, powders synthesized by the alkoxide and nitrate routes are not pure and show phases
Fig. 3. Raman spectra of the powders heat treated at 800 ◦ C (a), 1100 ◦ C (b) and 1200 ◦ C (c) for 2 h prepared by the alkoxide, nitrate and chloride precursors.
other than YAG (indicated with asterisks in the figure). Possible impurities in the prepared oxides, such as the nitrates, may act as suppressors of the transformation process and have an effect on the degree of crystallinity and homogeneity of starting materials and on the kinetics of phase transformation, according to Ref. [24]. TEM images (Fig. 4) show the morphology of the powders synthesized by the alkoxide (a), nitrate (b) and chloride (c) routes burnt at 1200 ◦ C for 2 h. The differences in the morphology of the powders synthesized by the different routes depend on the precursor and on its chemistry, leading to the formation of either colloidal particles or polymeric gels during the chemical reaction. Powders synthesized by the alkoxide route (Fig. 4a) present irregular forms, but mainly elongated, with an inhomogeneous particle size distribution and with an average size of about 100–300 nm formed by small structural units below 100 nm. Powders synthesized from the nitrate precursor (Fig. 4b) are more agglomerated with an inhomogeneous size distribution. In this case, necks between the
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Fig. 5. Transmittance spectra normalized to 0.8 mm thickness for experimental measurement (solid line) and simulation (dotted line).
It can be observed that values of up to 82% in the infrared region and 56% at 680 nm can be achieved by sintering of dispersed powders synthesized using the chloride route. In order to analyze the in-line transmittance spectra, a model based on that given by Apetz and Van Bruggen [25] was developed [26]. This model enabled the extracting of information concerning the total porosity and its distribution in the sintered YAG material. The average grain size and its distribution were obtained from the SEM characterization and, therefore, are not fitting parameters in this case. As can be seen in Fig. 5, the simulation results fit the experimental measurement of the transmittance well, showing a full density YAG material with a pore size of around 40 nm. The most significant differences can be observed between 1000 and 2000 nm and they can be attributed to the residual graphite in the samples sintered by SPS. It is important to emphasise that in the visible region (400–900 nm), where the changes are more important, the simulation is very close to the experimental line. 4. Conclusions
Fig. 4. TEM morphology of the crystalline YAG powder synthesized by the alkoxide (a), nitrate (b) and chloride (c) precursors.
grains are observed, indicating the onset of sintering at this temperature. Powders obtained by the chloride route (Fig. 4c) show a well-dispersed microstructure with an average grain size in the nanometer range, 100 nm approximately. Therefore, deagglomerated homogeneous nanometric crystalline YAG powders were obtained by reverse-strike precipitation using chlorides as precursors followed by gel lyophilization. Lyophilized YAG nanopowders synthesized by the chloride route were sintered at 1500 ◦ C for 3 min. During the entire sintering cycle, a pressure of 50 MPa was applied and the heating rate was 100 ◦ C/min. Fig. 5 shows the in-line transmittance of the sample normalized to a thickness of 0.8 mm according to Eq. (1).
T
T = (1 − Rs )
measured
1 − Rs
t/d (1)
d: actual thickness of the sample; t: normalizing thickness (0.8 mm in the present work); T: in-line transmittance normalized to thickness t; Rs : reflectance loss, that equals 2R /(1 + R ) where R = (n − 1/n + 1)2 , with n the refractive index of the material, alumina in the present work.
YAG powders were synthesized using three kinds of precursors by different wet-chemical routes. Powders with different average sizes and morphologies can be obtained by choosing the precursor. Whereas chloride precursors lead to powders with a homogeneous morphology and an average grain size in the nm range, alkoxide and nitrate routes produce powders with a large degree of agglomeration. The chloride route has been shown to be the most suitable one to achieve pure and polycrystalline YAG at a lower temperature and with a grain size in the nanometer range: the chloride route enables the obtaining of pure YAG at 1100 ◦ C, whereas fully crystalline powders are not obtained in the alkoxide and nitrate routes up to temperatures as high as 1200 ◦ C. This has an important impact on the sintering conditions, as the powders obtained in the nitrates route already start sintering before they are fully crystallized. The suitability of Raman spectroscopy to monitor the crystallization of the powders has also been demonstrated. Finally, lyophilized powders synthesized by the chloride route were spark plasma sintered into 0.8 mm thick transparent YAG materials with in-line transmittances close to 60% at 680 nm and over 80% in the infrared range. Acknowledgments The authors want to acknowledge the Spanish Ministry of Education and Science and UE for funding through projects
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