Three-phase ceramic composites constituted from equal volume fractions of a-Al 2 O 3 , MgAl 2 O 4 spinel, and cubic 8 mol% Y 2 O 3 -stabilized ZrO 2 (8YSZ) were flash-sintered under the influence of DC electric fields. The temperature for the onset of rapid densification (flash sintering) was measured using a constant heating rate at fields of 50-500 V/cm. The experiments were carried out by heating the furnace at a constant rate. Flash sintering occurred at a furnace temperature of 1350°C at a field of 100 V/cm, which dropped to 1150°C at a field of 500 V/cm. The sintered densities ranged from 90% to 96%. Higher electric fields inhibited grain growth due to the lowering of the flash temperature and an accelerated sintering rate. During flash sintering, alumina reacted with the spinel phase to form a high-alumina spinel solid solution, identified by electron dispersive spectroscopy and from a decrease in the spinel lattice parameter as measured by X-ray diffraction. It is proposed that the solid solution reaction was promoted by a combination of electrical field and Joule heating. K E Y W O R D S alumina, composites, field-assisted sintering technology (FAST), spinel, yttria stabilized zirconia
In situ X-ray diffraction measurements at the Advanced Photon Source show that α-Al 2 O 3 and MgAl 2 O 4 react nearly instantaneously and completely, and nearly completely to form single-phase high-alumina spinel during voltage-to-current type of flash sintering experiments. The initial sample was constituted from powders of α-Al 2 O 3 , MgAl 2 O 4 spinel, and cubic 8 mol% Y 2 O 3 -stabilized ZrO 2 (8YSZ) mixed in equal volume fractions, the spinel to alumina molar ratio being 1:1.5. Specimen temperature was measured by thermal expansion of the platinum standard. These measurements correlated well with a black-body radiation model, using appropriate values for the emissivity of the constituents. Temperatures of 1600-1736°C were reached during the flash, which promoted the formation of alumina-rich spinel. In a second set of experiments, the flash was induced in a current-rate method where the current flowing through the specimen is controlled and increased at a constant rate. In these experiments, we observed the formation of two different compositions of spinel, MgO•3Al 2 O 3 and MgO•1.5Al 2 O 3 , which evolved into a single composition of MgO•2.5Al 2 O 3 as the current continued to increase. In summary, flash sintering is an expedient way to create single-phase, alumina-rich spinel. K E Y W O R D Salumina, composites, field assisted sintering technology, spinels, zirconia: yttria stabilized
For many complex materials systems, low-energy electron microscopy (LEEM) offers detailed insights into morphology and crystallography by naturally combining real-space and reciprocal-space information. Its unique strength, however, is that all measurements can easily be performed energy-dependently. Consequently, one should treat LEEM measurements as multi-dimensional, spectroscopic datasets rather than as images to fully harvest this potential. Here we describe a measurement and data analysis approach to obtain such quantitative spectroscopic LEEM datasets with high lateral resolution. The employed detector correction and adjustment techniques enable measurement of true reflectivity values over four orders of magnitudes of intensity. Moreover, we show a drift correction algorithm, tailored for LEEM datasets with inverting contrast, that yields sub-pixel accuracy without special computational demands. Finally, we apply dimension reduction techniques to summarize the key spectroscopic features of datasets with hundreds of images into two single images that can easily be presented and interpreted intuitively. We use cluster analysis to automatically identify different materials within the field of view and to calculate average spectra per material. We demonstrate these methods by analyzing bright-field and dark-field datasets of few-layer graphene grown on silicon carbide and provide a high-performance Python implementation. this, a wide range of properties can be studied, for example, layer interaction, electron bands [5], layer stacking [2], catalysis [6], plasmons [7], and surface corrugation [8].
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