S. RAVASZOVÁ et al.: SYNTHESIS AND PHYSICOCHEMICAL PROPERTIES OF AN ADVANCED BaO-MgO-Al2O3-SiO2 SYSTEM ... 873–877 SYNTHESIS AND PHYSICOCHEMICAL PROPERTIES OF AN ADVANCED BaO-MgO-Al 2 O 3 -SiO 2 SYSTEM AS AN ATTRACTIVE PROTECTIVE-COATING MATERIAL SINTEZA IN FIZIKALNO-KEMIJSKE LASTNOSTI NAPREDNEGA BaO-MgO-Al 2 O 3 -SiO 2 SISTEMA KOT MATERIALA ZA NAPREDNE ZAŠ^ITNE PREVLEKE Simona Ravaszová 1 *, Karel Dvoøák 1 , Petr Skalka 2 , Mariano Casas Luna 2 , David Jech 2 , Ladislav ^elko 2 1 Brno University of Technology, Faculty of Civil Engineering, Institute of Technology of Building Materials and Components, Veveri 331/95, 602 00 Brno, Czech Republic 2 Brno University of Technology, Central European Institute of Technology, Purkynova 123, 612 00 Brno, Czech Republic Prejem rokopisa – received: 2019-08-01; sprejem za objavo – accepted for publication: 2020-08-13 doi:10.17222/mit.2019.181 One way to increase the turbine efficiency and reduce pollutant emissions would be to work in harsh environments with high temperatures and corrosion environments. Therefore, it is essential to develop new materials that are able to withstand environ- mental and mechanical conditions. Such coatings are usually a combination of ceramic layers that can better counteract aggres- sive environments and protect the substrate. Over many years, many ceramic systems have been developed and studied as ther- mal and ecological barriers. However, the costs and stability of such materials still require research and improvement to decrease the price of production costs, while maintaining the best protective properties. Barium osumilite (BaMg2Al6Si9O30) has many attractive physical and chemical properties that make it most suitable for high-temperature utilities. The main aim of this article is to present a detailed description of a laboratory preparation of Ba-osumilite, including the multistep solid-state reaction after the mechanical activation by high-energy milling and physicochemical characterization of the resulting material, focusing on the microstructure and thermal properties. The thermal-expansion coefficient and microstructure, monitored in view of the changes to the crystallinity, are observed during the heating up to 1200 °C in an Anton Paar HTK 2000 high-temperature cham- ber. Keywords: Ba-osumilite, solid-state synthesis, high-energy milling, heat treatment, thermal-expansion coefficient Eden od na~inov pove~anja u~inkovitosti delovanja turbin in zmanj{anja emisij onesna`evalcev okolja je njihovo obratovanje v te`kih pogojih; to je pri visokih temperaturah in v korozijskem okolju. Zato je klju~no razvijanje novih materialov, ki so sposobni kljubovati tak{nemu okolju ob isto~asnih visokih mehanskih obremenitvah. To so obi~ajno kerami~ne prevleke izdelane v ve~ plasteh, ki lahko kljubujejo agresivnemu okolju in za{~itijo podlago (turbinske lopatice). V preteklih letih so raziskovalci {tudirali in razvili vrsto kerami~nih sistemov, uporabnih kot termi~ne in ekolo{ke bariere. Vendar pa sta tako cena kot stabilnost teh materialov dokaj neugodna in zato je {e vedno potreba po raziskavah in razvoju novih ter izbolj{anju obstoje~ih materialov s stali{~a zmanj{anja njihovih stro{kov izdelave in doseganja najbolj{e za{~ite. Barijev osumilit (BaMg2Al6Si9O30) ima mnoge privla~ne fizikalne in kemijske lastnosti, ki ga uvr{~ajo med najbolj primerne materiale po sestavi za visoko temperaturno uporabo. V tem ~lanku avtorji predstavljajo natan~en opis laboratorijske priprave Ba-osumilita z ve~stopenjsko reaktivno sintezo v trdnem stanju po mehanski aktivaciji z visoko energijskim mletjem. Sledila je fizikalno-kemijska karakterizacija izdelanega materiala s poudarkom na mikrostrukturi in termi~nih lastnostih. Koeficient toplotnega {irjenja in mikrostrukture, ki so jih opazovali s stali{~a sprememb kristalini~nosti, so analizirali med segrevanjem do 1200 °C v Anton Paar HTK 2000 visoko temperaturni komori. Klju~ne besede: Ba-osumilit, reaktivna sinteza v trdnem stanju, visoko energijsko mletje, toplotna obdelava, koeficient toplotnega {irjenja 1 INTRODUCTION During the last decades, barium-magnesium-alumi- niumsilicate (BMAS) has been one of the most promis- ing glass-ceramic systems due to the mechanical and thermal properties that make it suitable for metal-matrix composites such as fibber reinforcements and high-tem- perature structural applications such as refractory and en- vironmental barriers due to its low thermal-expansion coefficient. 1 The thermal stability of BMAS depends on the crys- talline phases that are created during the devitrification of this glass ceramic. Thus, BMAS can include a single phase or a mixture of diverse metastable crystalline phases during the devitrification process, i.e., celsian (BaO·Al 2 O 3 ·2SiO 2 , monoclinic), hexacelsian (BaO· Al 2 O 3 ·2SiO 2 , hexagonal) and Ba-osumilite (BaO·2MgO· 3Al 2 O 3 ·9SiO 2 , hexagonal). All these phases are usually present in a eutectic mixture with / -cordierite (2MgO·2Al 2 O 3 ·5SiO 2 , hexagonal/orthorhombic), de- pending on the chemical route, source and post-process- ing of the BMAS material. 2,3 Materiali in tehnologije / Materials and technology 54 (2020) 6, 873–877 873 UDK 620.1:67.017:542.06 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(6)873(2020) *Corresponding author's e-mail: ravaszova.s@fce.vutbr.cz (Simona Ravaszova) The crystallization behaviour of a BMAS system has been studied for the production of different phases via various synthesis routes like sol-gel, melting of initial oxides, solid-state reaction and others. 1,3–5 Despite all the effort, a multiple phase system is usually obtained and the resulting phases can change after a post-heating treat- ment. Therefore, the formation of stoichiometric phases is not an easy task, especially in the case of stoichio- metric Ba-osumilite because it is thermally instable and tends to transform into a mixture of the rest of the BMAS crystalline phases. 2,6 This thermal instability pro- vides a better comprehension of the thermal behaviour of the BMAS glass-ceramics and stimulates the interest in the research of its stabilization process, justified by the fact that Ba-osumilite exhibits a high melting point (1370 °C) and a low thermal-expansion coefficient ( = 2.8×10 –6 K –1 in the 20–500 °C range). 6 The present article is devoted to the high-temperature solid-state synthesis of Ba-osumilite after the mechanical activation caused by high-energy milling and to the char- acteristics of the microstructure in terms of the crystallite size and thermal expansion after heating from room tem- perature up to 1200 °C. 2 EXPERIMENTAL PART For the synthesis of Ba-osumilite, a mechanical acti- vation process with a high-temperature solid-state reac- tion was used. Barium oxide (BaO, p.a. purity, Merck, Germany), silica oxide (SiO 2 , p.a. purity, Lach-Ner, Czech Republic), aluminium oxide (Al 2 O 3 , p.a. purity, Lach-Ner, Czech Republic) and magnesium oxide (MgO, p.a. purity, Lach-Ner, Czech Republic) as the basic pre- cursors were dosed according to the stoichiometric ratio for the formation of Ba-osumilite, 2MgO·BaO.3Al 2 O 3 · 9SiO 2 . The semi-wet approach for the raw-material mix- ture preparation was used. At first, barium oxide and sil- ica oxide in a molar ratio of 1:9 mol were weighed on a laboratory scale with an accuracy of ±0.01 g and pre- milled/pre-homogenized in a planetary mill (PULVERI- SETTE 6, Fritsch, Germany). The volume of the used steel-milling bowl was 0.5 dm 3 and it was filled with 25 steel grinding balls with 20 mm in diameter. Pre-milling of the raw-material mixture was carried out for 60 s at a speed of 350 min –1 . The raw-material mixture was used to create a water-based slurry (1:1 wt./wt.) for the me- chanical activation by high-energy milling (Simoloyer CM01, Zoz GmbH, Germany). Stainless-steel balls with 4.76 mm in diameter were used. The milling time was 30 min at a speed of 800 min –1 . The mechanically activated raw-material mixture was dried in a laboratory dryer (BINDER C170, Czech Republic) at 110 °C for 24 h. Nodules of 10–15 mm in diameter were formed naturally during the drying. The nodules in an amount of 694.14 g were directly dosed to platinum crucibles to react at a temperature of 1150 °C for 8 h inside a high-temperature furnace with superkanthal heating elements (2017S, CLASIC, Czech Republic). The heating rate was 8 °C/min. The product (BaSi 9 O 19 ) in an amount of 694.14 g was slowly cooled down to room temperature for 24 h. The product in the form of a block was crushed into powder in a vibratory-disc mill (RS 200, Retsch, Germany) at 900 min –1 for 20 s. After the first solid-state reaction, 3 mol of aluminum oxide was added to 1 mol of the BaSi 9 O 19 product and again mechanically activated so that the slurry was sub- jected to the same process and parameters as in the pre- vious step. The second solid-state reaction was carried out at 1200 °C for 8 h inside the high-temperature fur- nace again. The obtained product was crushed into pow- der in the vibratory-disc mill at 700 min –1 for 20 s and milled in the planetary mill at 350 min –1 for 120 s. After this milling process, 2 mol of magnesium oxide was added to 1 mol of the BaAl 6 Si 9 O 28 product and me- chanically activated, following the same parameters in the high-energy mill (Simoloyer CM01, Zoz GmbH, Germany). The third solid-state reaction was carried out at 1300 °C for 8 h inside the high-temperature furnace and then slowly cooled down to room temperature. The final powder was adjusted with milling in the vibratory and planetary mill for the analysis with an XRD appara- tus. The XRD analysis was performed with a multi- functional diffractometer (XRD, Empyrean, PANalytical B.V., Netherlands) with a Cu anode and K as the radia- tion source, = 1.540598 for K 1 , an accelerating voltage of 45 kV, beam current of 40 mA, diffraction angle 2 in a range from 5° to 90° with a step scan of 0.01°. The ICSD (released in 2012) was used for the qualitative analysis of diffraction patterns. Quantification was per- formed with the Rietveld method. For analysing the crys- tal-lattice evolution in the powder during the heat treat- ment, diffractometer PANalytical Empyrean with a high-temperature chamber with a platinum heating strip (HTK 2000N, Anton Paar GmbH, Austria) was used. The Ba-osumilite sample was heated from 25 °C to 1200 °C with a ramp of 30 °C/min and dwell time of 2 min. XRD measurements were taken every 100 °C. The collected data were evaluated using the HighScore Plus software (3.0e, PANalytical B.V., Netherlands) for the Rietveld refinement without an internal standard. Based on the in-situ XRD analyses, the crystallite size and linear thermal-expansion coefficient of the cell parameters (a, b and c) were calculated. The calculation of the crystallite size was based on the measurement of FWHM (full width of half maximum) and the Scherrer equation 7 with the Warren correction 8 : L KK Bb = ⋅ ⋅= ⋅ ⋅ − cos cos 11 22 (1) L is the crystallite size, K is the Scherrer constant (the value of 0.89 was used), is the K 1 wavelength of the X-ray radiation, is the diffraction angle, B is FWHM (full width of half maximum) and b is FWHM of the S. RAVASZOVÁ et al.: SYNTHESIS AND PHYSICOCHEMICAL PROPERTIES OF AN ADVANCED BaO-MgO-Al2O3-SiO2 SYSTEM ... 874 Materiali in tehnologije / Materials and technology 54 (2020) 6, 873–877 size/strain standard used (lanthanum hexaboride). The other factors affecting the accuracy of this evaluation method, such as the strain, were neglected because the aim of this experiment was to assess the trend of the crystallite-size development during the heating up. The linear thermal-expansion coefficient was based on the thermal expansion where coefficient alpha was calculated. The linear thermal-expansion coefficient of the cell parameters (a, b and c) were obtained from the following equation: L L L T =⋅ 1d d (2) L is the particular-length measurement and dL/dT is the rate of change of that linear dimension per unit change in temperature. 3 RESULTS The XRD patterns of the prepared material are shown in Figure 1. For a better illustration of the crystal-struc- ture development, a 3D graph for the Ba-osumilite main diffraction line determined with Miller indices "hkl" 002 was created and is shown in Figure 2. The major phase identified in the XRD patterns is Ba-osumilite (ICSD 79843). The XRD patterns show the presence of two other phases: dialuminium magnesium oxide (ICSD 24766) and aluminium oxide (ICSD 75560). All the identified phases were quantified. The amount of the residual oxides is approximately 5 %. In terms of purity, for an in-situ HTK experiment, where the mineral structure is studied, 95 % purity of Ba-osumilite is sufficient. Based on these results, the HighScore Plus software using the Rietveld refinement was used to calculate the crystallinity growth and the result is shown in Figure 3. During the heating, the crystallite size increases pro- portionally up to 800 °C where it reaches the value of 187 nm. As the temperature increases, there is a change in the crystallite-growth trend. The crystallite size reaches 200 nm at 900 °C and then decreases. S. RAVASZOVÁ et al.: SYNTHESIS AND PHYSICOCHEMICAL PROPERTIES OF AN ADVANCED BaO-MgO-Al2O3-SiO2 SYSTEM ... Materiali in tehnologije / Materials and technology 54 (2020) 6, 873–877 875 Figure 3: Effect of heat treatment on the crystallite size Figure 1: X-ray diffraction patterns of Ba-osumilite Figure 4: a) Effect of heat treatment on cell parameters a, b, b) effect of heat treatment on cell parameter c Figure 2: 3D graph of the crystal structure development Similar to the crystallite size, the cell parameters characterized by vectors a, b and c increase proportion- ally during the heating. The thermal-expansion coefficient evaluated for indi- vidual vectors is shown in Figure 5. The thermal-expansion coefficient for vector c in- creases proportionally to the maximum value of 3.0.10 –6 K –1 at 1200 °C. The change in the growth trend of the thermal-expansion coefficient for vectors a and b is visible compared with vector c. While the thermal-ex- pansion coefficient for vector c increases proportionally with the increasing temperature, the coefficient for vec- tors a and b is hyperbolic, reaching a maximum of only 2.5.10 –6 K –1 . 4 DISCUSSION This paper deals with the barium magnesium alu- minium silicate as an attractive coating material. The ar- ticle is divided into two main sections, dealing with the laboratory preparation of Ba-osumilite and an investiga- tion of the physicochemical properties of the obtained material during the heat treatment. The formation of the stoichiometric phase of Ba-osu- milite is quite complicated because it is a thermally instable material, which tends to transform into a mix- ture of the rest of the BMAS crystalline phases. The composition of the raw-material mixture and the heat-treatment parameters are the most influential factors that determine the final-phase composition of the mate- rial. Another important factor is the mechanical activa- tion by high-energy milling, which was used for the preparation of Ba-osumilite. The high-energy ball mill- ing is an effective solid-state method for the production of powders with attractive properties. During high-en- ergy ball milling, friction and high-energy kinetic colli- sions are the main processes, under which the powder particles are repeatedly subjected to deformation. Espe- cially at the beginning of milling, friction and fracturing are the dominant processes that result in a particle-size reduction. 9,10 Mechanochemical activation is engaged in physico- chemical changes in substances in all states of aggrega- tion due to mechanical energy. 11 The advantages of this technology include a reduction in the reaction tempera- ture of the material, an increase in the reactivity of the material, shorter milling times (reduced energy demands) and a reduction in the number of technological steps. 12 Using high-energy milling, Ba-osumilite was success- fully prepared. It can be stated that high-energy milling facilitated a better material formation through the forma- tion of active surfaces that could support the formation of Ba-osumilite within the multistep solid-state reaction. The objective of the first part of the study was fully fulfilled because the Ba-osumillite phase without any other crystalline phases such as celsian or hexacelsian was prepared. To achieve higher purity (> 95 %), further mechanochemical activation and the fourth step of the solid-state reaction are suggested. However, for the main purpose of this experiment, a 95-% purity of the Ba-osumilite without any other BMAS phases was suffi- cient. Based on these results, the second part of the experi- ment was completed. The Scherrer method was used to assess the effect of the heating process on the develop- ment of the crystal structure. It can be stated that the crystallite grew proportionally until the size of approxi- mately 188 nm was achieved. The values around 200 nm were disregarded because the method was no longer ap- plicable at this level. Vectors a, b and c exhibit similar crystallinity and grow proportionally with the increasing heating tempera- ture. According to the available literature, the thermal-ex- pansion coefficient of Ba-osumilite is 2.7.10 –6 K –1 ob- tained in a temperature range of 20–900 °C. The results presented in this paper correspond to this fact where at 900 °C the thermal-expansion coefficient for vector c is 2.68.10 –6 K –1 and for vectors a and b, it is 2.28 .10 –6 K –1 . With the temperature increasing to 1200 °C, there was only a slight increase in the thermal-expansion coeffi- cient. Despite the high temperature of 1200 °C, to which the material was subjected in the high-temperature cham- ber, the coefficient of thermal expansion is very low. This implies that this excellent thermal property makes the material suitable for use in metal-matrix composites as a fibber reinforcement and in high-temperature struc- tural applications as refractory and environmental barri- ers. 5 CONCLUSIONS The aim of this article was to describe a process of a laboratory preparation of Ba-osumilite using a multistep solid-state reaction after the mechanical activation by high-energy milling and characterize the microstructure and thermal properties during the heating up to 1200 °C. Through a simple method based on the mechanical activation by high-energy milling in combination with a multistep solid-state reaction, we successfully prepared Ba-osumilite. S. RAVASZOVÁ et al.: SYNTHESIS AND PHYSICOCHEMICAL PROPERTIES OF AN ADVANCED BaO-MgO-Al2O3-SiO2 SYSTEM ... 876 Materiali in tehnologije / Materials and technology 54 (2020) 6, 873–877 Figure 5: Thermal-expansion curve of Ba-osumilite for vectors a, b and c The crystallite size increased proportionally with the increasing temperature. The thermal-expansion coefficient for the c-lattice parameter exhibited linear behaviour and reached 3.0.10 –6 K –1 at 1200 °C. The glass-ceramic with the composition and thermal properties presented in this paper can be used as the sub- strate material for thermal and environmental barriers. 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