The Influences of Fluorides on the Transformation of Α-Alumina Crystals

Ceramics-Silikáty 61 (4), 357-366 (2017) www.ceramics-silikaty.cz doi: 10.13168/cs.2017.0035 THE INFLUENCES OF FLUORIDES ON THE TRANSFORMATION OF α-ALUMINA CRYSTALS #QINGBO TIAN*,**, XIAOJIE YANG***, YUE WANG**, ZHIJIE LV** *Co-innovation Center for Green Building of Shandong Province,, Shandong Jianzhu University, Jinan 250101, P. R. China; **School of Material Science and Engineering, Shandong Jianzhu University, Jinan 250101, P. R. China; ***Shandong Institute for Product Quality Inspection, Jinan 250100, P. R. China #E-mail: [email protected] Submitted June 20, 2017; accepted July 29, 2017 Keywords: α-Al2O3 , Morphology, Platelet-like Al2O3, Fluorides Influence of addition of aluminum fluoride (AlF3) and ammonium fluoride (NH4F) on transformation of α-Al2O3 from boehmite was investigated. Transformation of α-Al2O3 occurred at about 1050°C and 950°C, when boehmite contained AlF3 and NH4F additives respectively, which was 150°C and 300°C lower than that without any additives. The formed α-Al2O3 consisted of clear, hexagonal plate-like particles about 2-3 µm in diameter and 200 nm in thickness. However, the formed hexagonal platelet particles gradually became blunt at edges and decreased in sizes at the higher calcinating temperatures. The transfer of α-Al2O3 morphology was due to the diminution and disappearance of fluorine. The fluoride coated on the surfaces were firstly decomposed and then fluorine entered the lattices of crystals was volatilized. The fluoride with tiny particles attached on the surface had a same chemical bonding of F–Al–O with that in the lattices of crystals. INTRODUCTION ration of ammonium nitrate induced the transformation of intermediate phases to the stable α-Al2O3 structure at The high pure alumina ceramics have attracted a lower temperature. They concluded that an amount more attentions from the researchers and engineers of high energy defects were introduced in the transition in the ceramic field over the last decades due to their alumina matrix by the decomposition of ammonium unique combination of properties such as high hardness, nitrate and the evolution of oxides of nitrogen and nitric high strength, good electrical resistivity and optical per- acid, which would promote the reaction of transition formance [1-7]. Over the years a number of methods alumina to α-Al2O3 [9, 13]. The main drawback of this have been developed for the fabrication of high dense method was that a large number of ammonium nitrate alumina ceramic. However, to produce high-density were contained and an amounts of nitride inevitably alumina ceramic, the characteristics of the α-alumina volatilized during the calcination processes, causing powders used are always a substantial factor, in which, the environmental problem. Chlorides were expected to the dispersion of α-alumina powders with relatively finer decrease the converting temperature of α-Al2O3, for the grains is generally required. In general, the formation of addition of chloride promoted crystallization of MgO. α-alumina from boehmite occurs at temperature as high However, the discouraging results were often obtained as 1200°C after a series of polymorphic transformation, [11, 17]. Nowadays, fluorides were commonly used as including γ-alumina, δ-alumina, θ-alumina and α-alumi- mineral additives to promote α-Al2O3 crystal growth na [8-11]. A high transformation temperature inevitably [17-20]. It was determined that a small number of results in the coarsening and heavy aggregation of fluorides additives could efficiently induced the trans- α-alumina particles [9, 12]. Therefore, it is imperative formation of transition alumina phases [18, 19, 21]. to reduce the α-alumina transformation temperature in A decline of the converting temperature between 130°C order to achieve the uniform α-alumina particles without to 400°C was reported according to the variety of hard agglomerates. fluorides by the forming bridges between fluoride ions There were some methods invented to decrease the and aluminum ions on alumina surfaces [19]. It see- transformation temperature to overcome the possibility med that some fluorides, such as AlF3, ZnF2, CaF2, of powder agglomeration problems by adding surfactants NH4F and MgF2, had a positive effect on decreasing the [8, 13], the seeds [14-16], or mineralizers [9, 13, 17-23]. transformation temperature of α-Al2O3 [22]. However, Among them, the addition of mineralizer was proved to most of them would introduce other metal elements be an effective method. It was reported that the incorpo- except aluminum, which could apparently not apply to Ceramics – Silikáty 61 (4) 357-366 (2017) 357 Tian Q., Yang X., Wang Y., Lv Z. prepare high purity α-Al2O3. NH4F and AlF3 would be RESULTS AND DISCUSSION decomposed into fluoride or nitride, and volatilized with the rise of transformation temperature, could not Phase transformation introduce metal impurities contamination to the powders. Figures 1 and 2 show XRD patterns of boehmite Meanwhile, the addition of fluorides to a transi- samples without any additives calcinated at various tem- tion alumina or transition alumina precursor enhanced peratures and times. As shown in Figure 1a, after the the formation of α-Al2O3 crystal with the preferred powder is calcinated at 1100°C and holding for 1 h, morphology. A plate-like α-Al2O3 crystal was prepared the XRD patterns mainly illustrated are diffraction by the addition of aluminum fluoride powder to γ-alu- peaks of the transient alumina phases, such as θ-Al2O3, mina or η-Al2O3 [23, 24]. In another work the hydro- δ-Al2O3 and γ-Al2O3. After calcinated at 1150°C for fluoric acid was added to boehmite, alumina platelets 1 h, the diffraction peaks of the transient alumina phases with an average size of about 25 mm were achieved [25]. and the peaks of α-Al2O3 appear (seeing Figure 1b). With The α-Al2O3 particles having a very high aspect ratio the increase of calcinating temperature, the intensity of were used as fillers or pigments in rubbers or plastics diffraction peaks of α-Al2O3 is evidently strengthened. to exhibit good dispersibility [26], or as reinforcements As the calcinating temperature reaches 1200°C holding in ceramics to improve their mechanical properties, for 1 h, all the diffraction peaks have vanished except especially the fracture toughness [27, 28]. In this paper, two kinds of fluoride, aluminum fluoride (AlF3) and ammonium fluoride (NH4F) were used to reduce the phase transformation temperature of α–Al2O3 θ–Al2O3 α-Al2O3 in calcination process of boehmite precursor. δ–Al2O3 The influences of the fluorides on the phase transfor- γ–Al O mation processes and morphology were characterized. 2 3 And the mechanism action of them was also analyzed. d) EXPERIMENTAL c) A high-purity boehmite powder with a purity of at b) least 99.995 % was supplied by the Advanced Ceramic Center, Dalian Jiaotong University, China. The chemical a) agents of AlF3 and NH4F (chemical pure grade, > 99.5 %) as additives were purchased from Guoyao Group Che- 20 30 40 50 60 70 mical Reagent Co. Ltd., China. The fixed amounts of 2θ (°) the additives were wet-mixed with boehmite powder in Figure 1. XRD patterns of boehmite powders calcinated at: the deionized water and thoroughly dispersed by stirring a) 1100°C for 1 h, b) 1150°C for 1 h, c) 1200°C for 1 h, and for 5 h. The amount of additive was 2.8 % (mass ratio d) 1300°C for 0.5 h. of fluorine to boehmite). Then the mixture slurry was dried, calcinated between 850 - 1300°C for 1 - 10 h. As a comparison, boehmite powder without any additives α–Al2O3 was also prepared by the same processes. The size and θ–Al2O3 morphology of the powders were observed using scan- δ–Al2O3 ning electron microscope (SEM: Quanta FEG 250, γ–Al2O3 USA). The phases evolvements of boehmite with temperatures were identified by x-ray diffraction (XRD: Riguku D/Max-2400, Japan). The content of fluorine c) in the samples was evaluated by X-ray fluorescence analysis (XRF: ZSX Primus II, Japan). The fluorine bonding in the samples was analyzed by X-ray photoelec- tron spectroscopy (XPS: Escalab 250, UK). The system b) pressure during analyses was 1.33 × 10-7 Pa. The radiation was a monochromatized Al source (1486.6 eV). All a) spectra were referenced to C1s located at binding energy 20 30 40 50 60 70 (BE) of 284.8 eV. Surveys were recorded at 150 eV pass 2θ (°) 1s energy and high resolution F spectrum at 20 eV pass Figure 2. XRD patterns of AlOOH powders calcinated at: energy, then fitted with XPSPEAK 4.1 software. a) 1100°C for 600 min, b) 1150°C for 300 min, and c) 1400°C for 20 min. 358 Ceramics – Silikáty 61 (4) 357-366 (2017) The influences of fluorides on the transformation of α-alumina crystals the peaks of α-Al2O3, indicating that α-Al2O3 with of α-Al2O3 is a kind of reconstruction of crystal lattice complete crystallization is synthesized (Figure 1c). and a nucleation growth process requiring large amounts Further increasing the calcinating temperature to above of energy to overcome the barrier energy for nucleation 1200°C, no diffraction peaks of new phases are emerged [8, 29], so in the case of no external interference, the (Figure 1d). phase transformation temperature is usually over 1200°C The calcinating times also have significant influen- [30]. Although different mechanisms for addition of ces on the transformation of α-Al2O3 from the transient fluorides on decreasing the formation temperature of phases. Calcinated at 1150°C for 1 h, as it is seen in α-Al2O3 were presented, the most accepted view was Figure 1b, most of the diffraction peaks are identified that the deformed crystal lattice of the transient alumina as the transient alumina phases.

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