*3.1. Result of the Ball Milling: Size Parameters*

Figure 1 shows the as-received particle diameter distributions of the ground samples determined by the DLS method. All of them have a broad distribution in the range of a few hundred nanometers while the smallest particle diameter can be achieved by the ball-milling in stainless steel vial. Ball-milling in alumina or tungsten carbide vial resulted in similar particle diameter distributions which did not change considerably if a milling time longer than 5 h was chosen.

**Figure 1.** Particle diameter distributions of the ground samples in different vials determined by the dynamic light scattering (DLS) method.

Since the samples darkened as a result of the grinding process, it was checked whether the greyish color was related to sample contamination originating from the ball or the vial. EDS results showed that no characteristic impurity element at *x* > 0.1 at% concentration was present in the LN powders ground in either the tungsten carbide or the stainless steel vials, while aluminum could be detected when alumina vial was used in the milling process. This also means that the large fraction corresponding to smaller particle sizes observed in sample SS-5 milled in the stainless steel vial (see Figure 1) cannot be ascribed to the presence of an impurity but has to be considered as LN, too.

Neither the occurrence of alumina contamination nor the absence of any stainless steel residue can be explained on the basis of the hardness properties of the materials involved in the ball milling process. Taking into account the order of the Vickers hardness values of the relevant materials (stainless steel: 200–240, LN: ~630 [24], tungsten carbide: 1200–1700, alumina: 1400–1900), one might expect that stainless steel would cause the largest impurity level. However, alumina was the only vial material resulting in sample contamination, although it was the hardest material used. Al contamination was also confirmed by the XRD measurements as indicated by the presence of α-Al2O3 in the XRD pattern. This phenomenon was also proved by Heitjans and co-workers [22]. Although the alumina impurity level was sufficiently high for both XRD and EDS detection (at least 2 vol% which was the detection limit of the XRD method), it did not appear as a discernible fragment in the size distribution curves of the samples ALO-5 and ALO-20 (see Figure 1).

Although the alumina contamination cannot be explained on the basis of the hardness of the unmodified materials, it is understood by taking into account the mechanochemical reactions taking place in the system. As it will be shown in the forthcoming sections, lithium oxide is released from LN during the milling process that can react with the amphoteric alumina, resulting in the modification of its structure and facilitating its degradation. Various mixed oxide compounds of aluminum and lithium are known (such as LiAlO2, LiAl5O8, Li3AlO4; see Ref. [25]). In particular, the synthesis of LiAlO2 is well described by using either sol-based [26] or solid phase reaction-based [27] routes. Although the quantitative yield of the solid-phase reaction is given for temperatures higher than 370 ◦C, a surface-limited reaction can be assumed for the milling conditions applied in the present work. The hardness data of LiAlO2 is not known to the authors, but the Vickers hardness of other alkali aluminates (Na or K) is reported to be quite small (< 100, see [28]). Hence, the sites where the released Li2O reacts with the Al2O3 ball/vials can be assumed to serve as degradation initiation spots during the milling process.

While the particle size of a few hundred nm observed by light scattering did not diminish further for milling times longer than 5 h, a Williamson-Hall type evaluation of the X-ray diffractograms resulted in mean grain sizes decreasing from at about 63 nm to 37 nm for milling times increasing from 5 to 20 h (see Table 2). This reflects the fact that the diffraction-based size analysis provides the diameter of the coherently scattering grains which differs from the much larger particle diameter visible by light scattering or imaging methods. This is direct evidence that the particles formed during high-energy milling consist of a multitude of small grains. The grain diameters found in the present work are in good agreement with those reported before [13,15,20–23] for ball-milled LN crystals.


**Table 2.** Particle and grain sizes of samples ground in different vials.
