M. Sc. Julia Mausz, Application Scientist, Gatan/EDAX
Synthetic gemstone quality rubies are commonly manufactured with the Verneuil process, which got its name from its ”father ” Dr. A.V.L. Verneuil. This process was designed to produce single crystalline synthetic rubies and can now be used to melt a variety of high melting point oxides. The details of this flame fusion process were already published in 1902-1904 [1]. As I have neither a ruby mine nor a flame fusion device handy, I aimed to manufacture rubies using a different approach. However, I was unsure if it was possible to form single crystals or even large grains with this technique.
Like in the Verneuil process, the starting point of my synthetic rubies was Al2O3 and Cr2O3 powder. Those were homogeneously mixed, aiming at 1 – 2 at. % chromium content. Considering the melting point of Al2O3 (2,038 °C) [2] and Cr2O3(2,435 °C) [3], the maximum local temperature required to melt a powder mixture of both is 2,435 °C.
A microwave-induced plasma will supply the heat. With an operational frequency of about 2.450 GHz, kitchen microwaves can create high temperature plasmas, even at atmospheric pressure [4]. While bulk metals undergo little heating from microwaves due to the reflection of the waves, it is possible to heat fine-grained metal particles with dielectric heating. However, there is a more effective phenomenon to heat metal with microwaves. Electric discharge can occur due to changes in the distribution of charges when a conductive material with a sharp edge or tip is exposed to microwaves in that frequency regime. The heat resulting from the discharge dissipates very locally into the conductive material, resulting in temperature hot spots able to melt metals and metal oxides in direct contact with the metal, as shown later [5] [6] [7].
The main gases relevant for the plasma will be nitrogen (approx. 78%) and oxygen (approx. 21%) from the surrounding air. The electron source to ignite the plasma will be fine, sharp aluminum edges. Therefore, the powder mixture was placed in a glass crucible and covered with a network of fine aluminum stripes. The crucible was shallow and closed with a glass lid to prevent the hot gas from rising away from the powder. Then, the microwave was operated at 900 W and could sustain the plasma for 60 s. Then, the fused parts were collected from the powder, cleaned, and mounted onto an aluminum stub for observation in the SEM. The resulting fused particles were in the order of 0.5 – 2 mm and already showed the expected pink to purple color, which can be seen in Figures 1a and 1b. The fluorescence yield of rubies can be seen under black light. Without blacklights available, I needed to rely on the 8 kV argon ion beam from the Gatan PECS™ II, and the resulting fluorescence is shown in Figure 1c.
The Zeiss Sigma 500 VP SEM was set to 12 kV acceleration voltage, 120 μm aperture, and 3 Pa low vacuum to prevent charging. The microstructure was then analyzed on the unpolished surface using the EDAX Velocity Super EBSD detector. After fusion of the powder, the resulting ruby has a smooth surface with the crystal structure extending all the way to the surface. Therefore, the ruby could be indexed without any polishing step. It is fascinating with how much ease and speed an unpolished, charging material could be analyzed.
Hough indexing already achieved high indexing rates, considering the dirt and the shadowing on the sample. To bring back even more shadowed points and to refine the grain boundaries, I reprocessed the dataset using Neighbor Pattern Averaging & Reindexing (NPAR™) [8] and spherical indexing [9]. For spherical indexing, a dynamic simulation of trigonal Al2O3 was used. For each, the image quality (IQ) map [10] and confidence index (CI) map, an overlay of the orientation map is shown in Figure 2.
The dataset clearly shows a polycrystalline structure. Note that although the grains can be easily recognized, the shape and size of the grains are distorted due to the variation in surface topography.
In contrast to the grain shape, misorientation and texture analyses are unaffected. The detected bands in the EBSD patterns are direct projections of the lattice planes. As the active lattice planes are independent of the surface structure, the measured crystal orientation is not affected by the surface orientation.
The orientation map is displayed in Figure 3a after applying the confidence index standardization (CIS) procedure and a CI filter of 0.2. Figure 3b shows the overlay of this orientation map with its corresponding CI map and the grain boundaries with a minimum misorientation angle of 5° marked in black.
Interestingly, the as-fused state of the ruby showed a clear spike in the misorientation angle of 60°, as shown in Figure 4a. The twin boundaries of 60° with a tolerance angle of 2° are marked in black on top of the detail orientation map in Figure 4b. The crystal wire figure is schematically shown on both sides of the twin boundary, showing a 60° rotation along the c-axis.
In Figure 5, the (0001) texture pole figure reveals a weak texture. The orientation maximum is shifted somewhat towards the top-right, corresponding to the surface’s slanting in the same direction. This suggests that there is a weak preferred orientation of the (0001) planes parallel to the surface of the ruby aggregate particle.
It is possible to form synthetic rubies using microwave-induced plasma in a commercial microwave oven. However, the resulting rubies are small, of unpredictable shape, and due to their polycrystalline nature, not of high clarity. While ruby production in the microwave did not qualify to open a gemstone side business, it is a reliable source for making interesting EBSD samples, and we might see some more gemstone blogs in the future.
References
- NASSAU, K. Dr. AVL Verneuil: The man and the method. Journal of Crystal Growth, 1972, 13. Jg., S. 12-18.
- SCHNEIDER, Samuel J.; MCDANIEL, C. L. Effect of environment upon the melting point of Al2O3. Journal of Research of the National Bureau of Standards. Section A, Physics and Chemistry, 1967, 71. Jg., Nr. 4, S. 317.
- GIBOT, Pierre; VIDAL, Loïc. Original synthesis of chromium (III) oxide nanoparticles. Journal of the European Ceramic Society, 2010, 30. Jg., Nr. 4, S. 911-915.
- KOCH, Helmut; WINTER, Michael; BEYER, Julian. Optical Diagnostics on Equilibrium and Non-equilibrium Low Power Plasmas. In: 48th AIAA Plasmadynamics and Lasers Conference. 2017. S. 4158.
- SUN, Jing, et al. Review on microwave–metal discharges and their applications in energy and industrial processes. Applied Energy, 2016, 175. Jg., S. 141-157.
- LIU, Wensheng; MA, Yunzhu; ZHANG, Jiajia. Properties and microstructural evolution of W-Ni-Fe alloy via microwave sintering. International Journal of Refractory Metals and Hard Materials, 2012, 35. Jg., S. 138-142.
- ZHOU, Chengshang, et al. Effect of heating rate on the microwave sintered W–Ni–Fe heavy alloys. Journal of Alloys and Compounds, 2009, 482. Jg., Nr. 1-2, S. L6-L8.
- WRIGHT, Stuart I., et al. Improved EBSD Map Fidelity through Re-indexing of Neighbor Averaged Patterns. Microscopy and Microanalysis, 2015, 21. Jg., Nr. S3, S. 2373-2374.
- LENTHE, W. C., et al. Spherical indexing of overlap EBSD patterns for orientation-related phases–Application to titanium. Acta Materialia, 2020, 188. Jg., S. 579-590.
- WRIGHT, Stuart I.; NOWELL, Matthew M. EBSD image quality mapping. Microscopy and Microanalysis, 2006, 12. Jg., Nr. 1, S. 72-84.