Journal of the European Ceramic Society 41 (2021) 5634–5643 Contents lists available at ScienceDirect Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc Sintering and grain growth behaviour of magnesium aluminate spinel: Effect of lithium hydroxide addition Ali Talimian a,*, H.F. El-Maghraby b,c, Monika Michalkov´ a´ b, Duˇsan Galusek a,b a Centre for Functional and Surface Functionalised Glass, Alexander Dubcek University of Trencin, Trencin, Slovakia b Joint Glass Centre of the IIC SAS, TnUAD and FChPT STU, Trencin, Slovakia c Refractories, Ceramics, and Building Materials Department, National Research Centre, 33 El-Bohous St., 12622, Cairo, Egypt ARTICLE INFO ABSTRACT Keywords: Lithium hydroxide, LiOH, in the amounts ranging from 0.1 to 1.2 wt% has been used as a sintering aid to improve Magnesium aluminate spinel the densificationof MgAl2O4. The addition of 0.3 wt% LiOH promotes densificationand limits grain growth. The Sintering activation energy of sintering, calculated using master sintering curve approach, decreases from 790 ± 20 kJ. Master Sintering Curve 1 1 mol to 510 ± 20 kJ.mol with the addition of 0.3 wt% of LiOH. In addition, MgAl2O4 was also mixed with 10 High-temperature XRD wt% of LiOH to amplify the formation of reaction products. High-temperature XRD results showed that sec­ ◦ ondary phases (MgO and LiAlO2) are produced above 1040 C. The secondary phases start to disappear at T > ◦ 1200 C, and MgAl2O4 is produced. While adding small amounts of LiOH, up to ca. 0.3 wt%, is beneficial for + densification and suppressing grain growth, there exists a critical concentration of Li that is accounted for by the preferential incorporation of lithium ions into MgAl2O4 crystal lattice. 1. Introduction sintering: the incorporation of lithium ions into MgAl2O4 changes cat­ ions’ stoichiometry, introducing oxygen vacancies and improving Polycrystalline magnesium aluminate spinel is an interesting candi­ diffusion [21]. However, other fluorides,such as MgF2 and MnF2, have date for various engineering applications; it exhibits a favourable com­ also been reported to accomplish the task of LiF during sintering [17, bination of chemical and physical properties, such as high melting point, 22]. There are, therefore, some reservations about whether lithium has chemical inertness, low coefficient of thermal expansion, high thermal any significant influence on sintering. Although liquid LiF, acting as a shock resistance and excellent mechanical properties, i.e. hardness, and lubricant, facilitates particles’ rearrangement during sintering, it has fracture toughness [1–3]. Moreover, owing to its isotropic reflection detrimental effects on the properties of finalmaterial; grain growth and index and wide bandgap, polycrystalline magnesium aluminate spinel is cracking at grain boundaries are inevitable consequences of using LiF a cost-effective alternative to sapphire single crystals for optical appli­ that result in the decrease of the mechanical properties or impair the cations [4–6]. Producing dense magnesium aluminate spinel is a pre­ transparency [23–27]. requisite for obtaining excellent properties; however, sintering of While the effects of using LiF as sintering aid on the sintering MgAl2O4 is difficult due to the slow diffusion of constituent elements behaviour of MgAl2O4 have been studied extensively, there are few re­ and oxygen in particular [2,7–9]. Therefore, careful sintering processes ports on the influenceof other sources of lithium ions incorporation on at high temperatures with the application of pressure are required to the densification of magnesium aluminate spinel. Mordekovitz et al. densify MgAl2O4 [6,10–12]; even then, highly dense bodies are usually have studied the sintering of lithium doped magnesium aluminate spinel produced with the help of sintering additives, such as CaO, B2O3, AlF3, [28]. Although the instability of Li-doped MgAl2O4 during sintering transition element fluorides, or LiF [13–20]. Among these additives, results in the formation of secondary phases, such as MgO and γ-LiAlO2, only LiF has been consistently used to fabricate highly dense MgAl2O4. that can suppress grain growth by Zener pinning [28], no meaningful LiF promotes sintering during early-stage densification by producing change in the densificationof samples was observed. Therefore, the role transient liquid. Also, there had been a general agreement on how of lithium ions as solid-state sintering aids remains equivocal. Moreover, lithium facilitates sintering, particularly during the final stages of the possible reactions between the lithium source and MgAl2O4 during * Corresponding author. E-mail address: [email protected] (A. Talimian). https://doi.org/10.1016/j.jeurceramsoc.2021.05.003 Received 24 October 2020; Received in revised form 26 April 2021; Accepted 1 May 2021 Available online 6 May 2021 0955-2219/© 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). A. Talimian et al. Journal of the European Ceramic Society 41 (2021) 5634–5643 the initial stages of sintering have never been examined. LiOH) spread over a Pt strip acting both as heater and as sample holder, In the present work, the sintering behaviour of a commercial mag­ using the hight temperature diffraction chamber Anton Paar HTK 16. nesium aluminate spinel powder doped with LiOH has been studied. The The measurements were performed by heating the samples at the con­ ◦ ◦ effects of the addition of LiOH, up to 1.2 wt%, on the densificationand stant heating rate of 5 C up to 1200 C and recording the diffraction ◦ ◦ grain growth of MgAl2O4 were investigated. The reactions between Li2O pattern every 10 C, over the 2θ range between 20 and 38 . Additional and MgAl2O4 during the sintering and the stability of transient phases HT-XRD isothermal experiments were also carried out at the tempera­ ◦ have been studied by adding a relatively large amount of LiOH, 10 wt%, tures 1050, 1070, and 1120 C. The measured data were fitted and to make any reaction detectable. analysed by Rietveld refinement methods using the MAUD software package [31]; the total occupancies of tetrahedral and octahedral sites 2. Experimental procedures were constrained to stoichiometry values describing the Mg-Al distri­ + bution and assuming both sites are able to host Li . The background was Commercial magnesium aluminate spinel powder, S30CR (Bai­ modelled using a fourth-order polynomial, and the crystallographic kowski, Paris, France), and lithium hydroxide monohydrate, ACS grade variables were the lattice constants and occupancies of the tetrahedral > 99.0 (Sigma-Aldrich, MO, USA) were used as raw materials in this and octahedral sites. study. The main impurities of the spinel powder, reported by the sup­ plier, are (in wt ppm): Na: 70, K: 60, Ca: 60, Si:30, Fe:15, and S:600. The 2.1. Master Sintering Curve spinel powder was dispersed in isopropanol using an ultrasonic mixer (Sonopuls HD 3400, BANDELIN, Berlin, Germany). Then, an aqueous The densification kinetics of samples was studied by constructing solution of LiOH (25 mg/mL) was added to the suspension in order to Master Sintering Curves (MSC) and following the method developed by prepare a mixture with up to 1.2 wt % of LiOH, with respect to spinel. Su and Johnson [32]. The instantaneous densification rate of samples Afterwards, the mixture was transferred to a rotary evaporator, during sintering, (dρ/dt), can be described using the combined-stage ◦ concentrated under vacuum, and then dried at 120 C overnight. The sintering model proposed by Hansen et al. [33]: ( ) obtained powder was granulated by passing through a sieve of 0.5 mm 1 dρ 3γΩ δD Γ D Γ = b b + V V mesh. 4 3 (1) ρ dt kBT G G Cylindrical pellets with a diameter of 8 mm were produced using uniaxial pressing at 80 MPa. A portion of samples was sintered in a where γ and Ω are the surface energy and atomic volume, respectively; D thermo-mechanical analyser (TMA 402 Hyperion, Netzsch, Germany); being the diffusion coefficient. Γ is a scaling parameter related to the the temperature was increased at constant heating rates of 5, 10 and 20 ◦ ◦ driving force, mean diffusion distance and the microstructural features C min 1 up to 1550 C and a dwell time of 1 min. Some samples were ◦ of sintered samples. The indices are pointing out the diffusion mecha­ prepared by sintering at 1400 and 1500 C with a constant heating rate nism: b, grain boundary and V, lattice diffusion. K is Boltzmann con­ ◦ – .B. of 10 C and various dwelling time between 1 240 min in an electric stant, and T represents absolute temperature. G represents the grain size. furnace under ambient condition; the samples were removed from the It is considered that the diffusion is a thermally activated process furnace and quenched to room temperature to freeze the microstructure. following the Arrhenius relation: The density of samples was measured using Archimedes’ principle, with the theoretical density of MgAl O assumed to be 3.58 g cm 3. Q 2 4 D = D0exp( ) (2) The chemical reactions between the spinel powder and LiOH were RT investigated using a Simultaneous Thermal Analyser (STA 449 F1 where D , Q and R are a pre-exponential factor of diffusion, the apparent Jupiter®, Netzsch, Germany) in DTA-TG configurationupon heating to 0 ◦ ◦ diffusion activation energy and the gas constant, respectively. The sin­ 1350 C using a constant heating rate of 10 C.min 1. The measurements tering mechanism is controlled by the diffusion of the slowest species in were carried out by heating a powder mixture comprising MgAl2O4 and the fastest diffusion path [2]; hence, one can assume that the densifi­ 10 wt % LiOH; a relatively large amount of LiOH was added to the spinel cation occurs through only one mechanism [32,34], and Eq. (1) can be powder to make any reaction detectable, adopting the approach applied simplified, rearranged and integrated: by Rozenburg et al.