THE FORMATION OF A SPACE CHARGE REGION IN NANOSCALE MAGNESIUM ALUMINATE SPINEL

Mahdi Halabi 1 Amit Kohn 2 Shmuel Hayun 1
1Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel
2Department of Materials Science and Engineering, Tel Aviv University, Tel Aviv, Israel

Charge distribution in magnesium aluminate spinel (MAS) results in the formation of a space-charge region, which then has a critical role in assigning functional properties. Proposed explanations of this phenomenon are reported though quantitative experimental evidence in the case of nano-scale granular MAS is indirect. In this talk, direct measurements of the space charge region in nanoscale MAS are presented. Consequently, the effect of composition, grain size and applied electric field on the space-charge potential (SCP) is examined.

The electrostatic potential distribution in nonstoichiometric grains was measured by electron holography and compared to the distribution of cations and defects as measured by electron energy loss spectroscopy.

We demonstrated quantitatively [1] that regardless of grain size, excess Mg+2 or Al+3 cations resides in the vicinity of grain-boundaries of Mg- and Al-rich MAS, respectively. Furthermore, applying a moderate electric field during the annealing process modifies lattice ordering (Fig. 1a). We are investigating the effect of an electric field on the SCP in MgO∙1.27Al2O3 when the grain size, sized ~10nm, is comparable to the Debye length (Fig.1b and c).

Grains subjected to thermal annealing of 870°C show charge accumulation resulting in a potential above 4V. However, when such grains are subjected additionally to dc electric fields, up to 1000 Vcm-1, no charge accumulation is detected.

The origin of this surface charging and its discharge in the presence of an electric field will be discussed.

Figure 1. Inversion parameter of MgO•1.27Al2O3 subjected to dc electric fields, up to 1000 Vcm-1, and an annealing temperature of 870°C (a), BF-TEM (phase-contrast) image of as-synthesized grains (b) Phase shift map of the electron wave as measured by electron holography for as synthesized grains (c). Line profiles between grain surfaces of inversion parameter (blue curves), measured phase shift (red) and, phase shift calculated from thickness contribution (black) after annealing at 870°C without (d) and with applying an electric field: 300 Vcm-1(e) and 1000 Vcm-1(f)

References:

1. Halabi, M. et al., J. Am. Ceram. Soc. 2017;100:800– 811.









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