on the synthesis of Lanthanum magnesium

Ceramics International 44 (2018) 4734–4739

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

The effect of magnesium compounds (MgO and MgAl2O4) on the synthesis of Lanthanum magnesium hexaaluminate (LaMgAl11O19) by solid-state reaction method

T

⁎

M.M. Khorramirad , M.R. Rahimipour, S.M.M. Hadavi, K. Shirvani Jozdani Ceramic Department of Materials and Energy Research Center, Alborz, Iran

A R T I C L E I N F O

A B S T R A C T

Keywords: Lanthanum magnesium hexaaluminate Magnesium oxide Magnesium aluminate spinel Solid-state reaction

In the present study, the lanthanum magnesium hexaaluminate (LaMgAl11O19)(LaMA) powder was synthesized by the solid–state reaction method using two types of magnesium compounds, including magnesium oxide (MgO) and magnesium aluminate (MgAl2O4) spinel (MAS). The effect of substitution of magnesium oxide with MAS on the synthesis temperature, intermediate compounds and morphology of synthesized powders were investigated. The microstructural results showed that the intermediate compounds of lanthanum aluminate (LaAlO3), aluminum oxide and MAS were formed in the presence of magnesium oxide, whereas in the latter case, the LaAlO3 intermediate phase was not observed and La4Al2MgO10 was formed at about 810 °C. Also in both cases, a single LaMA phase with the platelet-like morphology was formed. The thickness of the LaMA platelets decreased from 300 nm to 125 nm and the synthesis temperature increased from 1330 °C to 1355 °C, by replacing MgO with MAS.

1. Introduction Hexaaluminates compounds are widely used in laser and luminescence industries, catalytic substrates, high-temperature catalysts in catalytic combustion chambers, as well as for stabilization and immobilization of nuclear wastes [1]. Lanthanum magnesium hexaaluminate (LaMgAl11O19) (LaMA) has recently been developed as a candidate for thermal barrier coating (TBC) due to its excellent thermal stability at high temperatures, well sintering resistance, high fracture toughness, long thermal cycling lifetime, and low Young modulus [2,3]. These properties drive from the irregular platelet-like structure, that create a product with high porosity and low thermal conductivity [4]. Hexaaluminate compounds are basically in the form of ABAl11O19 composition with a magnetoplumbite (MP) structure. They have hexagonal structure with P63/mmc space group (Fig. 1) and thermo–mechanical and structural stability at high temperatures [5–7]. In various studies, the effect of different elements, A and B cations doping on alumina powder are investigated [8]. The initial composition of LaAl11O18, contains aluminum cations (≈ 8%) and oxygen anion vacancies (≈ 5%), which allow the diffusion of atoms in the structure. By doping alumina powder with MgO, the vacancies are reduced. This means that the vacancies of LaMA crystalline structure are occupied with magnesium and oxygen, therefore leads to high stability at high

⁎

temperatures [9]. In general, the La2O3-xMgO-yAl2O3 formula is desired compound that 0.2 < x < 0.33 and 10 < y < 13. The ideal compound is LaMgAl11O19 and according to the ternary phase- diagram of this compound, it includes 7.1%, 14.3% and 78.6% mol fractions of La2O3, MgO and Al2O3 respectively [9]. The crystalline structure of LaMA is composed of blocks with a spinel structure separated by mirror planes containing lanthanum element. Secondary ion mass spectroscopy (SIMS) results show that the diffusion resistance of oxide ions in c crystallographic axis is much greater than a and b axes. Therefore crystal growth in c direction is limited and the platelet-like crystals are formed. The aspect ratio increases with the growth of the crystal, which leads to an increase in the crystalline surface energy. According to the thermodynamic laws, crystals try to keep their surface energy at the minimum. Thus, crystal growth is minimal even at high temperatures [10]. In other studies, LnMgAl11O19 (Ln˭La, Nd, Gd) powder was synthesized by two methods, solid–state reaction [11,12] and sol–gel, with different chemical composition of raw material, time and temperature [12,13]. It is reported that the synthesized powder by sol–gel method have larger sizes (5–20 µm) compared to the solid–state reaction method (1–3 µm) and the extent of agglomeration is higher in sol–gel method; also, the temperature and time synthesis in the solid state

Corresponding author. E-mail address: [email protected] (M.M. Khorramirad).

https://doi.org/10.1016/j.ceramint.2017.12.056 Received 12 November 2017; Received in revised form 3 December 2017; Accepted 7 December 2017 Available online 08 December 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.


M.M. Khorramirad et al.

Fig. 1. The crystalline structure of LaMgAl11O19 [22].

reaction (1500 °C, 4 h) are less than sol–gel method (1600 °C and 8 h) [14]. In this research, the effects of magnesium oxide and MAS have been investigated on the synthesis temperature, intermediate compounds and morphology of LaMA synthesized powders by solid–state reaction method. 2. Experimental procedures La2O3 (MERCK 12220), MgO (ALDRICH 34279-3), γ-Al2O3 (MERCK 101095) and magnesium aluminate (MgAl2O4) spinel (MAS) were used as raw materials. The MAS powder was synthesized by the aqueous solgel method using MgCl2.6H2O (MERCKA378633), AlCl3.6H2O (MERCK 101084) and NH4OH as raw materials, then the calcinations of precipitated mixture of double hydroxide (2Mg(OH)2.Al(OH)3) has been performed at 1000 °C [15]. In order to synthesis the LaMA powder, raw materials were grouped in two categories according to stoichiometric ratios of desired compound, including magnesium oxide and MAS. Then for producing a powder pasty, an amount of distilled water was added into the powder mixture. It was mixed using a planetary ball mill with weight ratio of ball to powder of 10:1 and rotation speed of 250 rpm for 24 h then they were dried in an oven. Finally, the powder mixture was calcined in an air electric furnace at 1400 °C for 6 h to form hexaaluminate powder. X-ray diffraction (XRD) analysis investigated by SIMENS D500 X-ray diffractometer using CuKα (λ = 1.54 A°) radiation, step rate 0.02° and scan ranges of 10–80°. TESCAN scanning electron microscopy (SEM, model VEGA) equipped with an Energy dispersive spectroscopy (EDS) were used in order to evaluation of the microstructural and morphological properties of the synthesized powders. To determine the temperatures of different transformations occurred during the synthesis, simultaneous thermal analyses (STA) including thermo gravimetric (TG) and differential thermal analysis (DTA) was done by NETZSCH STA 409 PC/PG system in air atmosphere with a rising temperature rate of 5 °C/min.

Fig. 2. SEM micrographs of the raw materials morphology.

3. Results and discussion

4Al2O3 + LaAlO3 + MgAl2O4 → LaMgAl11O19

The intermediate compounds such as; α-Al2O3, LaAlO3 and MgAl2O4 formed from the reaction among La2O3, MgO and γ-Al2O3 with morphologies that are shown in Fig. 2. The Eq. (1) was suggested for the final reaction in LaMA synthesis by using magnesium oxide as a raw material. Fig. 3 shows the X-ray diffraction patterns of the raw materials and synthesized LaMA powder.

The spinel phase (MAS) can be produced by solid-state reaction between MgO, Al2O3 and diffusion of Al3+ toward MgO and Mg2+ toward Al2O3. This reaction hardly occurs at temperatures below 1200 °C and it requires several days to be completed at 1500–1700 °C [16,17]. The results of this investigation indicate the formation of MAS at temperature below 1100 °C which is due to the γ-Al2O3 phase. The γAl2O3 phase has a high specific surface area and unique surface 4735

(1)



Fig. 5. Thermal analysis of the powders mixture from γ-alumina and lanthanum oxide in the presence of MgAl2O4.

Fig. 3. The XRD patterns of raw materials and LaMA powder in the presence of MgO.

properties. A sharp decrease in the specific surface area of synthesized powder can be observed with increasing the temperature and occurring allotropic transformations [16,18]. Some elements such as Ce, Ba, La, Sm, Sr in hexaaluminate compounds, delays the reduction of specific surface area [16]. Therefore, the formation of MAS at the lower temperatures in the solid-state reaction can be attributed to the instability of γ-Al2O3 in comparison with the α-Al2O3. Also the high specific surface area, spinel structure of the transition phase (γ-Al2O3) [19] and the existing of lanthanum element in the compounds, increase the rate of reaction and reduce the synthesis time. Finally, led to formation of LaMA at 1330 °C. In order to prevent the formation of MAS as an intermediate compound and to recognize the effect of MAS on the synthesis temperature, intermediate compounds and morphology of synthesized powder, the MAS was used as a raw material in solid–state reaction method. As a result of that, MgAl2O4 stoichiometric powder was produced with high purity and fine particle size. Fig. 4 shows the X-ray diffraction pattern of synthesized MAS powder. The STA curves of MAS containing precursor are shown in Fig. 5, where an endothermic peak at 100 °C and three exothermic peaks at 325, 810 and 1355 °C can be seen. The endothermic peak at about 100 °C is attributed to the evaporation of physically absorbed water of precursor. The exothermic peak at about 325 °C is caused by the lanthanum hydroxide (La(OH)3) decomposition in the raw materials. In Xray diffraction pattern (Fig. 6), it can be seen that the La2O3 powder (MERCK 1220) due to exposure to the environment, has been transformed to lanthanum hydroxide with hexagonal structure and P63/m space group. Also all the XRD peaks are matched with JCPDS (Joint Committee on Powder Diffraction Standard) card no. 00-036-1481 [20]. As shown in Fig. 7, thermal analysis results of lanthanum hydroxide show two endothermic peaks, in the temperature ranges of 330–370 °C and 480–540 °C. According to the studies by other researchers [20,21], it can be stated that in the first step, La(OH)3 converts to LaOOH and in the second step, it transforms to La2O3, which can be described by Eqs.

Fig. 6. The XRD pattern of the La2O3 powder.

Fig. 7. DTA analysis of the La2O3 powder.

(2) and (3). La(OH)3→LaOOH + H2O ΔH = 87 KJ/mole

(2)

2LaOOH→La2O3 + H2O ΔH = 53 KJ/mole

(3)

The reaction enthalpies of the dehydration process could be determined by DTA measurement, which they are 87 and 53 KJ/mole, respectively. As compared with the data of Neumann et al. [20], it is confirmed that these two reactions occur in the synthesis process of LaMA powder. Fig. 8 demonstrates X-ray diffraction pattern of powder mixture containing MAS, which calcined at 850 °C. It can be seen that the La4Al2MgO10 phase (JCPDS card no. 00-043-0922) with orthorhombic structure and α-Al2O3 phase (JCPDS card no. 88-0826) are formed. As a result, the exothermic peak at 810 °C in DTA curve (Fig. 5) is occurred because of formation of La4Al2MgO10 and α-Al2O3 compounds. The X-ray diffraction peaks of the calcined powders at 1400 °C (Fig. 9) are matched with the JCPDS cards no. 00-026-0873 and 00078-1845 of LaMgAl11O19. It can be concluded that the exothermic peak in DTA curve at temperature about 1355 °C is due to the chemical

Fig. 4. The XRD pattern of MAS.

4736



Fig. 8. The XRD pattern of the powder calcined at 850 °C.

Fig. 11. Comparison of XRD patterns from the synthesized LaMA powder in the presence of MgAl2O4 and MgO.

The results confirm that the following chemical reactions (Eq. (4), Eq. (5)) occur during the synthesis of LaMA compound in the presence of MAS as a raw material. 2La2O3 + MgAl2O4→La4Al2MgO10

(4)

La2O3 + 27Al2O3 + 5MgAl2O4 + La4Al2MgO10→6LaMgAl11O19

(5)

Eq. (4) evidences that the alumina did not participate in the first step of synthesis process but it participated in the final step (Eq. (5)). The Eqs. (4) and (5) exhibit that the lanthanum aluminate cannot be observed in the reactions, due to the higher entropies of MgAl2O4 and La4Al2MgO10 compounds. The SEM micrographs of synthesized powders are shown in Fig. 12. As it can be observed, in both cases, the synthesized powder has platelet like morphology with high aspect ratio and nanometer thickness, which confirming the formation of LaMA phase. The planer growth of these particles is occurred because of the lowest growth in the [0001] direction of LaMA crystal [5]. The lanthanum cation (La+3) in the mirror plane of the hexaaluminate compound, suppressing the diffusion of Al+3 and O−2 and as a result, crystalline growth along c axis is prevented. The thickness of the platelets has a reverse relationship with the ionic radius of existing rare-earth elements in the hexaaluminate compounds. Therefore, the LaMA powder have the lowest thickness in LnMgAl11O19 (Ln= La, Nd, Sm, Gd) compounds [13]. By replacing magnesium oxide with MAS in the raw materials, the particle size of LaMA powder is sharply reduced and the agglomeration is increased. Also the thickness of platelet like particles is reduced from 300 to 125 nm. In fact, MAS is a composition of three main elements of LaMA; Mg, Al, O. So, it has the highest thermodynamic entropy than MgO compound. Therefore, MAS acts as a nucleation and transformation initiating site, then the size of particles is decreased. The DTA curves of milled powder mixture from γ-alumina/

Fig. 9. The XRD patterns of the raw materials and LaMA powder in the presence of MgAl2O4.

reaction between the intermediate compounds and the formation of LaMA compound. In other words, the synthesis temperature of LaMA by using MAS as a raw material is 1355 °C. The LaMgAl11O19 and LaAl11O18 compounds with JCPDS cards no. 00-78-1845 and 00-33-0699, respectively, have the same chemical composition and space group. It is difficult to distinguish between these compounds only by XRD results. The LaMA compounds containing aluminum, lanthanum, magnesium and oxygen elements. these compounds can be distinguished by quantitative characterization, using Xray energy dispersive spectrometer (EDS) along with X-ray diffraction pattern [9]. The XRD and EDS results (Fig. 9, Fig. 10) confirm the formation of single-phase LaMA powders. The X-ray diffraction patterns of the final synthesized powders by using of magnesium oxide and spinel as raw materials are compared in Fig. 11. As a result, the same phase structure and composition were synthesized in both cases.

Fig. 10. EDS analysis of the synthesized LaMA powder by solidstate method in the presence of MgAl2O4.

4737



Fig. 14. TG curves of the milled powder mixture from γ-alumina/Lanthanum oxide/ MgAl2O4 or MgO.

adsorbed and structural water of Mg(OH)2 compound. 4. Conclusion In the presence of magnesium oxide and MAS as raw materials, LaMA Phase was synthesized as a single-phase material with platelet like morphology. The synthesis temperature of LaMA in the presence of magnesium oxide and MAS were determined to be 1330 °C and 1355 °C, respectively. The replacement of magnesium oxide by MAS caused a sharp reduction in the size of the synthesized particles, thus their agglomeration increased; also the thickness of platelet like particles reduced from 300 to 125 nm. In this case, the La4Al2MgO10 compound as an intermediate phase with an orthorhombic structure was formed at temperatures below 810 °C before forming the final compound. Also the intermediate phase of lanthanum aluminate was not observed unlike the using of magnesium oxide. The weight loss during the synthesis of LaMA in the presence of magnesium oxide was higher than that using MAS as a raw material. Future research Fig. 12. Morphological comparison of the synthesized LaMA powders in the presence of MgO (up) and MgAl2O4 (down).

In the future, the synthesized powders would be used in the plasma sprayed thermal barrier coatings. Also the effect of the size of the primary powder on the coating properties, the amorphous phase and the stability of the coating against re-crystallization would be investigated. References [1] X. Chen, et al., Thermal aging behavior of plasma sprayed LaMgAl11O19 thermal barrier coating, J. Eur. Ceram. Soc. 31 (13) (2011) 2285–2294. [2] X. Chen, L. Gu, B. Zou, Y. Wang, X. Cao, New functionally graded thermal barrier coating system based on LaMgAl11O19/YSZ prepared by air plasma spraying, Surf. Coat. Technol. 206 (8–9) (2012) 2265–2274. [3] X. Chen, B. Zou, Y. Wang, H. Ma, X. Cao, Microstructure and thermal cycling behavior of air plasma-sprayed YSZ/LaMgAl11O19 composite coatings, J. Therm. Spray. Technol. 20 (6) (2011) 1328–1338. [4] S.M. Naga, Ceramic Matrix Composite Thermal Barrier Coatings for Turbine Parts, Woodhead Publishing Limited, 2014. [5] R. Vaßen, M.O. Jarligo, T. Steinke, D.E. Mack, D. Stöver, Overview on advanced thermal barrier coatings, Surf. Coat. Technol. 205 (4) (2010) 938–942. [6] X.Q. Cao, R. Vassen, D. Stoever, Ceramic materials for thermal barrier coatings, J. Eur. Ceram. Soc. 24 (1) (2004) 1–10. [7] X. Chen, et al., Thermal cycling failure of new LaMgAl11O19/YSZ double ceramic top coat thermal barrier coating systems, Surf. Coat. Technol. 205 (10) (2011) 3293–3300. [8] R.V., D.S. Gerhard Pracht, Lanthanum-lithium hexaaluminate-a new material for thermal, Adv. Ceram. Coat. Interfaces 27 (3) (2007) 87–99. [9] G. S. Rainer Gadow, Thermal insulating material and method of producing same, US 6,998,064 B2, 2006. [10] M. Johansson, E.M. Papadias, D. Thevenin, P.O. Ersson, A.G. Gabrielsson, R, S.P.G. Björnbom, P.H, Järås, Combustion for gas turbine applications, Catal. – Spec. Period. Rep. 14 (1999) 183–198 (Cambridge). [11] X. Chen, Y. Zhao, L. Gu, B. Zou, Y. Wang, X. Cao, Hot corrosion behaviour of plasma sprayed YSZ/LaMgAl11O19 composite coatings in molten sulfate-vanadate salt, Corros. Sci. 53 (6) (2011) 2335–2343. [12] D. Stöver, G. Pracht, H. Lehmann, M. Dietrich, J.-E. Döring, R. Vaßen, New material

Fig. 13. DTA curves of the milled powder mixture from γ-alumina/Lanthanum oxide/ MgAl2O4 or MgO.

Lanthanum oxide/MgAl2O4 or MgO are illustrated in Fig. 13. The synthesis temperatures of LaMA powders by using of MgO and MgAl2O4 are 1330 °C and 1355 °C, respectively. It was observed that the synthesis temperature of LaMA in the presence of MAS increased, while it was expected a reduction in the synthesis temperature, because of the MAS was not formed as an intermediate compound. Fig. 14 reveals that, the weight loss in the presence of magnesium oxide was higher than that MAS as a raw material. The weight loss rate at temperatures below 500 °C can be attributed to the removal of 4738



[13] [14]

[15] [16] [17]

Microporous Mesoporous Mater. 196 (2014) 191–198. [18] I. Levin, D. Brandon, Metastable alumina polymorphs: crystal structures and transition sequences, J. Am. Ceram. Soc. 81 (8) (2005) 1995–2012. [19] W.E. Lee, Ceramic processing and sintering, Int. Mater. Rev. 41 (1) (1996) 36–37. [20] A. Neumann, D. Walter, The thermal transformation from lanthanum hydroxide to lanthanum hydroxide oxide, Thermochim. Acta 445 (2) (2006) 200–204. [21] M. Ozawa, R. Onoe, H. Kato, Formation and decomposition of some rare earth (RE = La, Ce, Pr) hydroxides and oxides by homogeneous precipitation, J. Alloy. Compd. 408–412 (2006) 556–559. [22] X. Chen, et al., Thermal cycling behaviors of the plasma sprayed thermal barrier coatings of hexaluminates with magnetoplumbite structure, J. Eur. Ceram. Soc. 30 (7) (2010) 1649–1657.

concepts for the next generation of plasma-sprayed thermal barrier coatings, J. Therm. Spray. Technol. 13 (1) (2004) 76–83. J. Zhang, et al., Thermal-shock resistance of LnMgAl11O19 (Ln = La, Nd, Sm, Gd) with magnetoplumbite structure, J. Alloy. Compd. 482 (1–2) (2009) 376–381. R.X. Zhu, Z.G. Liu, J.H. Ouyang, Y. Zhou, Preparation and characterization of LnMgAl11O19 (Ln˭La, Nd, Gd) ceramic powders, Ceram. Int. 39 (8) (2013) 8841–8846. R. Castañeda, E. Chavira, O. Peralta, Product prediction: intermediates formed during rare earth reactions, J. Mex. Chem. Soc. 58 (1) (2014) 82–87. W.P, Q.H. Ruren Xu, Modern inorganic synthetic chemistry, Angew. Chem. Int. Ed. 51 (2) (. 2012) 305a–307. P. Alphonse, B. Faure, Thermal stabilization of alumina modified by lanthanum,

4739