Preparation of hexagonal micro-sized α-Al₂O₃ platelets from a milled Al(OH)₃ precursor with NH₄F and NH₄Cl additives

1 Introduction

Plate-like α-Al₂O₃ powders have been widely applied in the fields of high-strength ceramics, abrasives and other refractory materials owing to its excellent properties including good chemical stability, high elastic modulus as well as high tensile and compression strength [1], [2], [3], [4], [5], [6], [7].

Until now, many methods have been developed to synthesize α-Al₂O₃ platelets [8], [9], [10], such as the melt salt method [11], hydrothermal synthesis [12], laser scanning [13], electrostatic spray-assisted chemical vapor deposition [14] and high-temperature calcination [15]. As the crystal growth of α-Al₂O₃ is mainly determined by the relative growth rates of various crystal faces, not only the internal structural factors but also the external conditions [16], [17], [18], such as the type of precursor, the reaction temperature and the addition of crystal seeds (including α-Al₂O₃, α-Fe₂O₃, α-Cr₂O₃, etc.), have been considered to control the morphology of α-Al₂O₃ powders.

In particular, it has been demonstrated that the type and microstructure (morphology, aggregate size and shape) of the precursor are crucial to control the morphology of α-Al₂O₃ powders [19], [20]. As the larger-sized precursor aggregates could be broken into highly active fragments under high-energy ball milling, the neck growth of α-Al₂O₃ during calcination could be efficiently suppressed, and almost-spherical α-Al₂O₃ powders of ~1.0-μm size were synthesized by the calcination of the ground precursor at 1450°C [21]. Moreover, although many kinds of additives [15], [22], [23], [24], such as AlF₃, ZnF₂, NH₄F, etc., have been introduced to improve the α-phase transformation and the morphology of α-Al₂O₃, the effect of fluoride additives on the morphology modification of α-Al₂O₃ particles is different, depending on the precursor [20].

Therefore, considering the above aspects, in current technology, commercial Al(OH)₃ with spherical agglomerates and about 100-μm size is ball milled as the precursor for the preparation of α-Al₂O₃. Composite additives (NH₄F and NH₄Cl) are introduced to modify the α-Al₂O₃ microstructure. In the present work, hexagonal α-Al₂O₃ micro-platelets with a uniform distribution are prepared by high-temperature calcination, and the growth process is discussed.

2 Experimental procedure

The preparation of α-Al₂O₃ powders from a commercial Al(OH)₃ precursor (Shandong Branch of CHACLO, China) was carried out in the following way. First, ball milling treatment of commercial Al(OH)₃ precursor was carried out for 9 h in a batch-type planetary mill (ND7-2L, Nanjing LAB Industrial Co., Ltd., China) with zirconia balls using ethanol as the dispersion medium. The ratio of grinding balls to material was 15:1 in weight, and a rotational speed of 240 r min⁻¹ was employed in the experiments. The milled Al(OH)₃ samples were oven dried at T=80°C for 24 h. Then, 5 wt.% NH₄F (Sinopharm Chemical Reagent Beijing Co., Ltd., China) and 5 wt.% NH₄Cl (Sinopharm Chemical Reagent Beijing Co., Ltd., China) additives were introduced into the milled Al(OH)₃ precursor, and the mixture was ground in an agate mortar for 1 h. Finally, the commercial Al(OH)₃ precursor, the Al(OH)₃ precursor milled for 9 h and the milled Al(OH)₃ precursor with 5 wt.% NH₄F and 5 wt.% NH₄Cl as additives were calcined separately at T=1300°C for 3 h for comparison.

The phase compositions and the crystallinity of the samples after milling and calcination were identified by powder X-ray diffraction (XRD; X′ Pert Pro MPD, Philips, Eindhoven, Netherlands) with CuKα radiation (λ =0.154 nm, 40 kV, 40 mA) in the 2θ range 20–80° and with a scan rate of 5° min⁻¹. Fourier transform infrared (FTIR; Tensor II, Bruker) spectra of the α-Al₂O₃ samples were collected between 4000 and 400 cm⁻¹ on a Tensor 37 infrared spectrometer with a wavenumber resolution of 4 cm⁻¹. The microstructure of the milled and unmilled Al(OH)₃ samples was analyzed by cold field emission scanning electron microscopy (SEM; JSM-7500F, JEOL, Tokyo, Japan) at an accelerating voltage of 15 kV, and the morphology evolution of the α-Al₂O₃ samples upon calcination at T=1300°C was observed by field emission SEM (Sigma HD, Zeiss, Oberkochen, Germany) at an accelerating voltage of 10 kV.

3 Results and discussion

The effect of the ball milling treatment on the phase composition and crystallinity of the precursor is shown in Fig. 1. The XRD patterns of the commercial samples and those milled for 9 h indicate that the crystallinity of the precursor could be significantly decreased by the ball milling treatment, because the diffraction peaks (shown in the XRD pattern of the 9-h milled precursor) related to gibbsite (JCPDS File No: 74-1775) are weakened and broadened after 9 h of ball milling. In particular, the peak centered at 68.88º corresponding to the 026 reflection of gibbsite even disappears. These results demonstrate that the 9-h ball milling treatment results in a grain refinement and poor crystallinity of the Al(OH)₃ precursor [25], although the milled and unmilled Al(OH)₃ precursors are identical in phase composition.

Fig. 1:

XRD patterns of the Al(OH)₃ precursors before and after ball milling.

Figure 2 presents the microstructure of the commercial Al(OH)₃ precursor. It is seen that the Al(OH)₃ precursor without ball milling consists mainly of spherical agglomerates in the size range of 100–200 μm (as shown in Fig. 2a), which consist of many smaller-sized grains (as shown in Fig. 2b). In comparison, the morphology of the ball-milled Al(OH)₃ precursor shown in Fig. 3 indicates that these large agglomerates are mostly broken into irregularly shaped particles in the size range of ≤5 μm by 9-h ball milling, accompanied by a formation of a small amount of irregular pieces in the size range of 10–30 μm. The above results reveal that the ball milling treatment leads to a significant refinement in morphology and particle size distribution of the Al(OH)₃ precursor.

Fig. 2:

SEM photographs of the commercial Al(OH)₃ precursor. Picture (b) is the magnification of the selected area in picture (a).

Fig. 3:

SEM photographs of the Al(OH)₃ precursor after 9 h of ball milling. Picture (b) is the magnification of the selected area in picture (a).

Figure 4 shows the XRD patterns of the samples obtained by calcination of the commercial Al(OH)₃ precursor, the 9-h ball-milled Al(OH)₃ precursor and the milled Al(OH)₃ precursor with 5 wt.% NH₄F and 5 wt.% NH₄Cl additives at 1300°C for 3 h. The characteristic peaks at 25.590º, 35.155º, 37.781º, 43.353º, 52.561º, 57.504º, 61.310º, 66.530º, 68.215º and 76.883º corresponding to the 012, 104, 110, 113, 024, 116, 018, 214, 300 and 1010 reflections of α-Al₂O₃, respectively [15], confirm that single-phase α-Al₂O₃ could be synthesized by the calcination at T=1300°C.

Fig. 4:

XRD patterns of samples obtained by calcination at T=1300°C.

To further prove the above result, these calcined samples were analyzed by FTIR. As shown in Fig. 5, intense bands between 1000 and 400 cm⁻¹ assigned to the Al–O–Al stretching emerge in the spectra [26], while the peaks at 688, 641, 595, 493, 469 and 453 cm⁻¹ result from the vibrations of Al–O bonds of the AlO₆ units [21]. The FTIR results confirm that the samples calcined at T=1300°C are composed of α-Al₂O₃.

Fig. 5:

FTIR spectra of samples obtained by calcination at T=1300°C.

The morphological evolution of the α-Al₂O₃ samples can be followed in Figs. 6–8 , which present SEM photographs of the α-Al₂O₃ prepared by the calcination of commercial Al(OH)₃, 9-h ball-milled Al(OH)₃ and 9-h ball-milled Al(OH)₃ with NH₄F and NH₄Cl additives at 1300°C, respectively. As shown in Fig. 6a, the α-Al₂O₃ particles obtained from the commercial Al(OH)₃ precursor fired at 1300°C maintain the initial aggregate shape with a worm-like structure formed by the neck growth of α-Al₂O₃ grains. By comparing Fig. 7 with Fig. 6, it is seen that the aggregate size has been significantly decreased due to the morphology refinement of the milled Al(OH)₃ (see Fig. 2). The above results indicate that the α-Al₂O₃ powders obtained by the calcination of the unmilled and milled Al(OH)₃ precursors at 1300°C have similar worm-like microstructures because the formation of the as-synthesized α-Al₂O₃ mainly follows the solid phase mass transfer mechanism [27].

Fig. 6:

SEM photographs of the α-Al₂O₃ sample obtained by the calcination of commercial Al(OH)₃ at T=1300°C for 3 h. Picture (b) is the magnification of the selected area in picture (a).

Fig. 7:

SEM photographs of the α-Al₂O₃ sample obtained by the calcination of 9-h ball-milled Al(OH)₃ at T=1300°C for 3 h. Picture (b) is the magnification of the selected area in picture (a).

Fig. 8:

SEM photographs of the α-Al₂O₃ sample obtained by the calcination of 9-h ball-milled Al(OH)₃ containing NH₄F and NH₄Cl additives. Picture (b) is the magnification of the selected area in picture (a).

In contrast, α-Al₂O₃ powders obtained from the milled Al(OH)₃ precursor with the 5 wt.% NH₄Cl and 5 wt.% NH₄F additives exhibit a well-defined hexagonal platelet morphology as shown in Fig. 8. The diameters of the α-Al₂O₃ platelets are about 1 μm, and the thicknesses are about 200 nm. It can be seen that the introduction of the NH₄Cl and NH₄F additives results in a significant morphological modification of the α-Al₂O₃ powders from the milled Al(OH)₃ precursor calcined at T=1300°C.

The phenomenon suggests that the formation of hexagonal α-Al₂O₃ platelets is highly related to the addition of NH₄Cl and NH₄F, because the additives cause a different growth rate along the respective crystal axis and consequently influence the microstructure of α-Al₂O₃. Previous studies have reported [9], [28], [29] that the (0001) surface is the most stable surface of α-Al₂O₃ and appears predominantly, thereby promoting the formation of hexagonal platelets. In the present study, the addition of NH₄Cl and NH₄F accelerates gas phase mass transport with formation of gaseous intermediates including mainly AlOF and AlOCl, etc. [23], [30], and hence promotes the growth rate of α-Al₂O₃ along the crystal orientation (101̅0). As a result, the growth velocity of the (101̅0) crystal faces is significantly increased, and the growth velocity of the (0001) plane is greatly reduced in the presence of NH₄Cl and NH₄F. Thus, the α-Al₂O₃ powders prepared by calcination of the milled Al(OH)₃ precursor with NH₄Cl and NH₄F additives appear as well-developed hexagonal platelets.

4 Summary

Well-shaped hexagonal α-Al₂O₃ platelets of ~1.0-μm diameter and ~0.2-μm thickness have been synthesized from a ball-milled Al(OH)₃ precursor with NH₄Cl and NH₄F additives calcined at 1300°C. As spherical Al(OH)₃ agglomerates in the size range of 100–200 μm are mostly broken into irregularly shaped particles in the ≤5 μm size range, the size distribution of α-Al₂O₃ powders is significantly improved through the incorporation of the morphology refinement of the precursor caused by the ball milling treatment. Moreover, the growth velocity of the (0001) plane could be reduced by the introduction of NH₄Cl and NH₄F additives, because the growth of α-Al₂O₃ crystals along (101̅0) is greatly promoted by gaseous AlOF and AlOCl or other intermediates. Therefore, the addition of NH₄Cl and NH₄F appears to be helpful in the generation of α-Al₂O₃ platelets.