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Microstructural Analysis of Optical Ceramic Materials

Introduction

“Photonic applications using the II-VI semiconductor zinc oxide (ZnO) are becoming increasingly prevalent, and research into even more uses is exploding, with hundreds of labs looking into the material’s unique properties” [1]. One reason to use ZnO is as a potential replacement for gallium nitride and other semiconductors since it is environmentally benign. The advantages are that it is biologically compatible, making it well suited for potential medical applications. It is transparent at wavelengths in the visible part of the electromagnetic spectrum and opaque at ultraviolet wavelengths. Plus, it exhibits piezoelectric and pyroelectric behavior. ZnO has been produced in various forms tailored for specific optoelectronic applications. A form produced by a vapor growth process has been characterized using Electron Backscatter Diffraction (EBSD) in the Scanning Electron Microscope (SEM).

Single crystals of materials like magnesium fluoride (MgF2), alumina (Al2O3), and magnesium aluminate (MgAl2O4) are capable of transmitting both infrared (IR) and visible light ( Figure 1). With recent technological advances in the formation of nanostructured materials, it is now possible to fabricate these materials in polycrystalline form and still retain their transmittance properties. The fabrication of polycrystalline ceramic materials reduces the cost significantly compared to the fabrication of single-crystal materials. These polycrystalline ceramics can be fabricated in highly dense compacts, mitigating pore formation. Pores are deleterious to light transmission. As these are ceramic materials, they have high strength and hardness plus good damage and thermal-shock resistance. Thus, these materials are well suited for application as protective domes and windows. To produce materials with good IR transmission, it is important to understand those factors which affect the light-scattering properties of polycrystalline materials. One of these factors is grain size, or more precisely, grain boundaries. Studies [2, 3] show that decreasing the grain size improves the transparency in MgF2 and alumina. Another factor that affects the transmittance is the preferred crystallographic orientation of the constituent crystals or texture [4]. EBSD is an ideal tool for discerning such microstructural characteristics. In addition, the chemical composition of these materials affects their performance. Energy Dispersive Spectroscopy (EDS) is well suited to measure the chemical composition and its spatial distribution within the microstructure. Examples of EBSD and EDS analysis of these materials are described in the following sections.

Two polycrystalline MgAl2O4 samples showed varying levels of transparency.
Figure 1. Two polycrystalline MgAl2O4 samples showed varying levels of transparency.

 

Zinc Oxide (5)

Figure 2 shows well-faceted ZnO microfibers with periodic junctions. The microfibers were prepared by an evaporation and deposition process. Fibers can be produced with spacings between the junctions ranging from 5 – 30 µm. The spacing can be regulated by controlling the growth conditions. Several techniques were employed to characterize these materials, including X-ray Diffraction (XRD), EDS analysis, SEM, EBSD, and photoluminescence (PL) microscopy. In particular, EBSD was used to characterize the anisotropic growth mechanism of the fibers.

ZnO microfibers with periodic junctions with 6.2 µm spacing. An anisotropic microfiber growth model was postulated, as shown in Figure 3. EBSD measurements confirmed this on several fibers. An example is shown in Figure 4. A small series of orientation measurements were made using EBSD made along the length of the fiber. From the individual orientation measurements analysis, it was first found that the fibers were indeed single crystals.
Figure 2. ZnO microfibers with periodic junctions with 6.2 μm spacing.

 

An anisotropic microfiber growth model was postulated, as shown in Figure 3. EBSD measurements confirmed this on several fibers. An example is shown in Figure 4. A small series of orientation measurements were made using EBSD made along the length of the fiber. From the individual orientation measurements analysis, it was first found that the fibers were indeed single crystals.

Microfiber anisotropic growth model. Through pole figure analysis of the data, it was confirmed that the growth direction of the fibers is <2110>, the “side” surfaces are {0001} planes, and the top and bottom surfaces are {0110} planes. The fiber base was formed by fast growth along the <2110> direction, followed by slow growth along the c-axis [0001] creating the regular prisms.
Figure 3. Microfiber anisotropic growth model.

 

Through pole figure analysis of the data, it was confirmed that the growth direction of the fibers is <2110>, the “side” surfaces are {0001} planes, and the top and bottom surfaces are {0110} planes. The fiber base was formed by fast growth along the <2110> direction, followed by slow growth along the c-axis [0001] creating the regular prisms.

EBSD measurements and pole figure analysis. Photoluminescence studies showed that the optical character of the fibers was related to the structure, as can be observed in Figure 5. The photoluminescence behavior means that these modulated fibers could serve as microscale waveguides and make the creation of microscale light-emitting arrays, as well as bar codes used in biotechnology and electronics possible. EBSD has been used to characterize ZnO in other forms and applications. For example, on polycrystalline ZnO varistors [6] and powder compact specimens [7], single-crystal nanoscrews [8], and tetrapods where each leg of the tetrapod is a single crystal [9].
Figure 4. EBSD measurements and pole figure analysis.

 

Photoluminescence studies showed that the optical character of the fibers was related to the structure, as can be observed in Figure 5. The photoluminescence behavior means that these modulated fibers could serve as microscale waveguides and make the creation of microscale light-emitting arrays, as well as bar codes used in biotechnology and electronics possible. EBSD has been used to characterize ZnO in other forms and applications. For example, on polycrystalline ZnO varistors [6] and powder compact specimens [7], single-crystal nanoscrews [8], and tetrapods where each leg of the tetrapod is a single crystal [9].

Photoluminescence micrograph for the type of fibers shown in Figure 2.
Figure 5. Photoluminescence micrograph for the type of fibers shown in Figure 2.

 

Magnesium Fluoride

Magnesium fluoride (MgF2) has a tetragonal crystal structure. Wen and Shetty [2] have shown that the grain size affects optical transmittance in polycrystalline MgF2. Figure 6 shows orientation maps of MgF2 in the as hot-pressed form and then annealed at different annealing temperatures. The orientation data was obtained using EBSD. Such maps are often termed Orientation Imaging Microscopy (OIM) maps. EBSD is especially well suited for characterizing grain size in these materials. The ability of these materials to transmit light makes it challenging to get accurate grain size measurements using conventional light microscopy. In addition, light microscopy does not have the spatial resolution to resolve the microstructure at the smaller grain sizes. It is clear from the OIM maps shown here that the higher annealing temperatures promote increased grain growth.

Color-coded orientation maps of polycrystalline MgF2 after a) hot-pressing and after b-e) annealing for one hour at the temperatures indicated. (By permission of the authors [2] and sponsoring agency – Naval Air Warfare Center AD.) Optical transmittance measurements were made at several different wavelengths from the polycrystalline MgF2 samples shown in Figure 6. These results are plotted as a function of grain size in Figure 7.
Figure 6. Color-coded orientation maps of polycrystalline MgF2 after a) hot-pressing and after b-e) annealing for one hour at the temperatures indicated. (By permission of the authors [2] and sponsoring agency – Naval Air Warfare Center AD.)

 

Optical transmittance measurements were made at several different wavelengths from the polycrystalline MgF2 samples shown in Figure 6. These results are plotted as a function of grain size in Figure 7.

Optical transmittance of polycrystalline MgF2 as a function of grain size at five different wavelengths. The solid lines are from an analytical approximation [3].
Figure 7. Optical transmittance of polycrystalline MgF2 as a function of grain size at five different wavelengths. The solid lines are from an analytical approximation [3].

 

Magnesium Aluminate Spinel

Traditionally, the word spinels referred to red gemstones, but in modern scientific literature, it refers more specifically to crystals of magnesium aluminum oxide (MgAl2O4) or sometimes a class of minerals with a specific chemical formulation and cubic crystal structure. MgAl2O4 crystals are transparent over a wide range of wavelengths. EDS in the Transmission Electron Microscope (TEM) has been utilized to study the effect of variations in chemical composition across grain boundaries in a fine-grained spinel [10]. Figure 8 shows the microstructure of a MgAl2O4 after hot-pressing and subsequent annealing. This figure was constructed from EBSD scan data on the sample. The colors of the grains in the map correspond to their crystallographic orientation relative to the sample normal. The hue is created by mapping the quality of the corresponding diffraction pattern at each point in the OIM scan to a grayscale.

Orientation map overlaid on an intensity map based on EBSD pattern quality on a hot-pressed MgAl2O4 sample. Analysis of the EBSD data obtained for this sample showed very little preferred orientation of the constituent crystals and that the misorientation at the grain boundaries was random. Ting and Lu [11] have studied the role of grain boundaries and sub-grain boundaries on the evolution of microstructure in these materials using selected area diffraction in the TEM. EBSD is an ideal tool for expanding the study of grain boundaries in these materials as it can analyze many boundaries relatively easily, allowing for statistical analysis of grain boundary misorientation [12].
Figure 8. Orientation map overlaid on an intensity map based on EBSD pattern quality on a hot-pressed MgAl2O4 sample.

 

Analysis of the EBSD data obtained for this sample showed very little preferred orientation of the constituent crystals and that the misorientation at the grain boundaries was random. Ting and Lu [11] have studied the role of grain boundaries and sub-grain boundaries on the evolution of microstructure in these materials using selected area diffraction in the TEM. EBSD is an ideal tool for expanding the study of grain boundaries in these materials as it can analyze many boundaries relatively easily, allowing for statistical analysis of grain boundary misorientation [12].

Figure 9 shows two EDS spectra, one corresponding to a hot-pressed sample and the other to the hot-pressed and annealed sample shown in Figure 7. The spectrum shows that the ratio of Mg/O and Al/O decreases after annealing.

EDS spectra from a hot-pressed MgAl2O4 sample and a hot-pressed and annealed sample.
Figure 9. EDS spectra from a hot-pressed MgAl2O4 sample and a hot-pressed and annealed sample.

 

Alumina

It is interesting to note that texture plays a role in the transmittance performance of ceramic materials [4]. Generally, hot-pressed powder compacts do not exhibit much texture as the forming process tends to be isotropic. However, some texture can be observed in these materials. For example, Figure 10 shows the microstructure of a hot-pressed transparent alumina sample accompanied by the corresponding texture in the form of an inverse pole figure. An inverse pole figure shows the preference of specific crystal axes to align with a specific sample direction, in this case, the sample normal. The material has an average grain size of approximately 300 nm. The scan area contains 1650 individual grains. This number of grains should provide a reasonable assessment of the texture [13]. The texture is relatively weak; the highest intensity is nearly two times random for the c-axes to be aligned with the sample normal.

Orientation map and corresponding texture for a hot-pressed Al2O3 sample.
Figure 10. Orientation map and corresponding texture for a hot-pressed Al2O3 sample.

 

Conclusion

There are several new and exciting areas of research in the arena of optical materials, both in developing novel materials and forms of existing material systems. Understanding the development of microstructure in these materials and their role in optoelectronic performance is important for improving their fabrication, performance, and expanding their application into new technology fields. The capability of EBSD and EDS to characterize different aspects of microstructure makes them important tools for gaining the necessary insight into linking microstructure with properties.

Bibliography

  1. L Savage (2010). Making electro-optical sense with Zinc Oxide” Photonics Spectra 44(2): 47-49
  2. T.-C. Wen and D. K. Shetty (2009). Birefringence and grain-size effects on optical transmittance of polycrystalline magnesium fluoride.” Proceedings of the SPIE - Window and Dome Technologies and Materials XI, 7302: 73020Z-1- 73020Z-6
  3. R. Apetz and M. P. B. van Bruggen (2003). “Transparent Alumina: A Light-Scattering Model.” Journal of the American Ceramic Society, 86: 480-486.
  4. C. S. Chang and M.-H. Hon (2003). “Texture effect of hot-pressed magnesium fluoride on optical transmittance” Materials Chemistry and Physics 81: 27-32
  5. L. Huang, S. Wright, S. Yang, D. Shen, B. Gu and Y. Du (2004). ZnO well-faceted fibers with periodic junctions”” Journal of Physical Chemistry B 108: 19,901-19,903
  6. C. Leach (2005). ““Grain boundary structures in zinc oxide varistors”.” Acta Materialia 53: 237-245
  7. Y. J. Kim, D.-W. Kim, S. J. Ahn, H. S. Kim and S. Ahn (2003). ““Study on the non-linear property of abnormally grown grain ZnO”.” Materials Chemistry and Physics 82: 410-413
  8. L. Liao, J. C. Li, D. H. Liu, C. Liu, D. F. Wang, W.Z. Song and Q. Fu (2005) “Self-assembly of aligned ZnO nanoscrews: growth, configuration and field emission.” Applied Physics Letters, 86: 083106-1-083106-2.
  9. B.-B. Wang, J.-J. Xie, Q, Yuan and Y.-P. Zhao (2008) “Growth mechanism and joint structure of ZnO tetrapods.” Journal of Physics D, 41: 102005-1-102005-6
  10. N. Nuns, F. Béclin and J. Crampon (2005). “Space charge characterization by EDS microanalysis in spinel MgAl2O4.” Journal of the European Ceramic Society, 25: 2809-2811
  11. C.-J. Ting and H.-Y Lu (1999) “Hot-Pressing of Magnesium Aluminate Spinel – II. Microstructure Development.” Acta Materialia, 47: 831-840
  12. V. Randle (2004). ““Application of electron backscatter diffraction to grain boundary characterisation”.” International Materials Reviews 49: 1-11
  13. S. I. Wright, M. M. Nowell and J. F. Bingert (2007). ““A Comparison of Textures Measured Using X-Ray and Electron Backscatter Diffraction”.” Metallurgical and Materials Transactions A 38: 1845-1855