Paper Review

    Research and Development of Transparent Ceramics for Optical Applications

    Yuji Kintaka

    Original Paper: Research and Development of Transparent Ceramics for Optical Applications
    References

    1. Y. Kintaka, S. Kuretake, N. Tanaka, K. Kageyama, and H. Takagi, “Crystal Structures and Optical Properties of Transparent Ceramics Based on Complex Perovskite Ba(M4+,B12+,B25+)O3(M4+ = Ti, Sn, Zr, Hf; B12+ = Mg, Zn; B25+ = Ta, Nb),” J. Am. Ceram. Soc., 93, 1114-9 (2010).
    2. Y. Kintaka, S. Kuretake, T. Hayashi, N. Tanaka, A. Ando, and H. Takagi, “Crystal Structures and Optical Properties of Transparent Ceramics Based on LaAlO3-Sr(Al,Ta)O3 Solid Solution,” J. Am. Ceram. Soc., 94, 4399-403 (2011).
    3. Y. Kintaka, T. Hayashi, A. Honda, M. Yoshimura, S. Kuretake, N. Tanaka, A. Ando, and H. Takagi, “Abnormal Partial Dispersion in Pyrochlore Lanthanum Zirconate Transparent Ceramics,” J. Am. Ceram. Soc., 95, 2899-905 (2012).

    Announcement venue: The Fulrath Okazaki Memorial Association, Presentation by 2013 Fulrath Award Recipients

    In the process of studying materials and process technologies for the purpose of developing poreless dielectric substrates for millimeter waves, at Murata Manufacturing, we were able to obtain transparent ceramics based on Ba(Mg,Ta)O3 perovskite material. When the refractive index of this material was measured, we found that this had a higher value than that of typical optical glass. In terms of applications as a lens, the focal distance can be shortened by a high refractive index for enabling a compact design for the optical system. On the other hand, wavelength dependency (dispersion) of the refractive index is an important characteristic value for realizing improvement of the image quality by correction of the chromatic aberration.*1 Up to now, there have been no attempts to use transparent ceramics as lenses, and there have been no design guidelines for the refractive index and dispersion in ceramics. Therefore, we examined design guidelines for the refractive index and dispersion based on Ba(Mg,Ta)O3 for the purpose of producing new materials that have a wide range of refractive indices and dispersion.

    Ba(Mg,Ta)O3-Based Transparent Ceramics

    Ba(Mg,Ta)O3 is a material where the B-site in the perovskite structure is occupied by divalent Mg ions and pentavalent Ta ions. Because the difference in valence is large at the B-site in this material, Mg and Ta become an ordered array, which results in a crystalline system having a hexagonal structure. Hexagonal crystals have different refractive indices depending on the crystal orientation, and so light scattering tends to occur at the grain boundaries, and this presents an obstacle in obtaining transparent bodies in polycrystalline ceramics. It has been reported that the difference in valence can be reduced by substituting tetravalent cations in the B-site in Ba(Mg,Ta)O3 and that the cation array at the B-site is randomized so that the crystalline system changes to a cubic crystal (Fig. 1). In a cubic crystal, the refractive index is not dependent on the crystal orientation, and this structure is preferred for obtaining transparent ceramics. Ba(Sn,Zr,Mg,Ta)O3 is a material that has actually become transparent by converting a dielectric substrate for millimeter waves to a poreless structure, and it is a material where tetravalent Sn ions and Zr ions were substituted in the B-site. Therefore, we used the tetravalent ions Ti, Sn, Zr, and Hf, which are near the ionic radii of Mg and Ta, as the B-site substitution elements of Ba(Mg,Ta)O3, and we examined the relationship between these substitution quantities and their optical properties. Examining the relationship between the substitution quantity and the crystalline structure revealed that, for each substitution element, the crystalline structure changes to a cubic crystal at substitution of 16mol% or more at the B-site. Also, the transmittance of the sintered body that was obtained allows changes to the crystalline structure for enabling the transmittance to be improved by substituting 16mol% or more of tetravalent ions. Figure 2 shows the refractive indices and dispersion of a Ba(M,Mg,Ta)O3-based transparent ceramic where the B-site was replaced by tetravalent ions. Here, the dispersion is indicated by the Abbe number vd. The Abbe number is a value calculated from the refractive index values in the three wavelengths in the visible light range. A larger Abbe number indicates that the refractive index has less dependency on the wavelength. Figure 2 shows how refractive index varies depending on the substitution element type and that the refractive index increases considerably when Ti is substituted. This figure further shows that the Abbe number also decreases significantly when Ti is substituted.

    The refractive index in the visible light range is determined by the electron polarization. In the development of optical glass, the actual measurement values for the refractive index and the empirical equation known as the Gladstone-Dale relation (equation (1)) are said to show a strong correlation.

    equation (1): n−1=ρΣfiki

    Fig. 1 Crystalline Structure of Ba(Mg,Ta)O3 and Crystalline Structure of Ba(M,Mg,Ta)O3 When Tetravalent Ion M Is Substituted in the B-Site

    Fig. 1 Crystalline Structure of Ba(Mg,Ta)O3 and Crystalline Structure of Ba(M,Mg,Ta)O3 When Tetravalent Ion M Is Substituted in the B-Site

    Fig. 2 Refractive Index (Left Figure) and Abbe Number (Right Figure) of Ba(M,Mg,Ta)O3-Based Transparent Ceramics

    Fig. 2 Refractive Index (Left Figure) and Abbe Number (Right Figure) of Ba(M,Mg,Ta)O3-Based Transparent Ceramics

    Here, ρ is the material density, fi is the weight fraction of the material components, and ki is the specific refractive energy of the components (empirical parameter indicating the size of the electron polarization). The actual measured values of Ba(M,Mg,Ta)O3-based transparent ceramics match the values calculated using this empirical equation within an accuracy of ±3%, and this shows that the relation is valid even for transparent ceramics. Based on this empirical equation, when using an oxide where the product of the molecular weight m and the specific refractive energy k is large, the refractive index can be expected to be larger. Table 1 shows the molecular weights and specific refractive energies of various oxides related to these material types. The B-site of Ba(Mg,Ta)O3 consists of MgO and TaO2.5, and calculating the product mk of the B-site as a weighted average from this ratio gives us 24.786. On the other hand, the product mk of TiO2 is larger than this value, and the product mk of SnO2 is smaller than this value. Because the tendency of the actual measured values of the refractive index and the size relationship of the product mk show a general correlation, the product mk can be considered to be an effective indicator for control of the refractive index.

    Table 1: Molecular Weight m and Specific Refractive Energy k for Various Oxides, and Their Product mk

    Table 1: Molecular Weight m and Specific Refractive Energy k for Various Oxides, and Their Product mk

    In the absorption of light, electrons absorb light energy, and this excites the high energy orbitals. In crystal materials such as ceramics, optical absorption occurs from the visible light to the ultraviolet light range due to the band gap. Because the refractive index increases suddenly near wavelengths where light is absorbed, to reduce changes in the refractive index at visible light ranges, light absorption due to the band gap should occur at wavelength ranges that are far from visible light. In other words, the band gap and dispersion in the visible light range are correlated, and materials with a larger band gap are expected to have a smaller dispersion, while materials with a smaller band gap are expected to have a larger dispersion. As shown in Fig. 2, compositions where Ti is substituted in the B-site have a smaller Abbe number (larger dispersion). Based on the measurement results of the wavelength dependency for the transmittance, substituting Ti shows that the absorption edge is shifted to the long wavelength side. This signifies that the band gap has become smaller and that the relationship between the band gap and dispersion matched the above expectations.

    Substituting Ti in the B-site of Ba(Mg,Ta)O3 enabled the obtaining of a material with a high refractive index and low Abbe number, but this material was brown in color. This brown coloring is thought to be due to color centers*2 originating in lattice imperfections that are formed during the sintering process, and we found that adjusting the Ca substitution of the A-site and the A/B-site ratio enabled reduction of the color centers. The left side of Fig. 3 is a Ba(Ti,Mg,Ta)O3 sintered body where Ca substitution has not been performed, and the right side of this figure is a (Ba,Ca)(Ti,Mg,Ta)O3 sintered body where the Ca substitution and A/B-site ratio have been adjusted. This shows that adjustment of the composition enables reduction of the brown coloring. The (Ba,Ca)(Ti,Mg,Ta)O3 transparent ceramics shown on the right side of Fig. 3 had a refractive index nd=2.14 and an Abbe number vd=24.0. The Ba(Sn,Zr,Mg,Ta)O3 transparent ceramic obtained by converting a dielectric substrate for millimeter waves to transparent form had nd=2.08 and vd=30.3. This shows that we were able to obtain transparent ceramics with a higher refractive index and lower Abbe number.

    Fig. 3 Appearance of Ba(Ti,Mg,Ta)O3 Ceramics (Left Side) and (Ba,Ca)(Ti,Mg,Ta)O3 Ceramics (Right Side)

    Fig. 3 Appearance of Ba(Ti,Mg,Ta)O3 Ceramics (Left Side) and (Ba,Ca)(Ti,Mg,Ta)O3 Ceramics (Right Side)

    LaAlO3-Sr(Al,Ta)O3-Based Transparent Ceramics

    Image pickup-type lenses in digital cameras, video cameras, and other devices use multiple lens materials with different refractive indices and dispersion for correcting for color bleeding and image blurring. Thus, while materials with a low Abbe number such as (Ba,Ca)(Ti,Mg,Ta)O3 transparent ceramics are needed, materials with a high Abbe number are also needed. For this reason, we decided to develop materials with a high refractive index and high Abbe number, and we focused on LaAlO3 as a material base. The band gap of LaAlO3 is 5.6eV, and because this is a value larger than the band gap (4.4eV) calculated from the absorption edge of Ba(Sn,Zr,Mg,Ta)O3, we expect a high Abbe number for this material. Also, the refractive index of LaAlO3 based on the Gladstone-Dale relation is 2.06, and so a high refractive index can also be expected. However, because the crystalline structure of the LaAlO3 itself is a rhombohedron, a high transmittance cannot be expected in its unmodified form as a polycrystalline body. LaAlO3 and Sr(Al,Ta)O3-based solid solutions are well-known as LaAlO3-based cubic crystal materials. The composition is 0.25LaAlO3-0.75Sr(Al,Ta)O3, and it is used as a single-crystal substrate material for forming thin films, but no reports on polycrystalline ceramics based on these solid solutions were found. This led us to fabricate ceramics in a wide range of composition ratios based on these solid solutions for examining their crystalline structures and optical characteristics. The measurement results for the transmittance of LaAlO3-Sr(Al,Ta)O3-based solid solution ceramics with various composition ratios are shown in Fig. 4. X-ray diffraction was used to find that the crystals have a cubic structure for a Sr(Al,Ta)O3-based solid solution of 40mol% or more, and it is also clear from the transmittance results that these crystals show a high transmittance in a cubic crystal composition (x=0.4 or higher in Fig. 4). The measurement values for the refractive index and Abbe number are nd=2.06 and vd=42.8, respectively, for LaAlO3(x=0.0) and are nd=2.01 and vd=34.3 for Sr(Al,Ta)O3 (x=1.0), which shows that as the solid solution quantity (x value) of Sr(Al,Ta)O3 increases, nd and vd both decrease monotonically. The refractive index and Abbe number of compositions with a high transmittance in a (1-x)LaAlO3-xSr(Al,Ta)O3 solid solution were, for instance, nd=2.04 and vd=37.8 at x=0.5.

    In this way, we were able to obtain transparent ceramics with a high refractive index and high Abbe number in LaAlO3-Sr(Al,Ta)O3-based solid solutions. Figure 5 shows a graph plotted with the refractive indices and Abbe numbers of various types of transparent ceramic materials developed by Murata Manufacturing compared to the values of optical glass materials. Optical glass materials include various types of compositions, but their refractive indices and Abbe numbers are clustered in a band shape as shown in Fig. 5. This shows that the transparent ceramics of Murata Manufacturing have characteristic values that are distinct from this group of optical glasses.

    Fig. 4 Relationship Between Composition Ratio and Transmittance of (1-x)LaAlO3-xSr(Al,Ta)O3-Based Solid Solution Ceramics This shows the data when the thickness of the measurement sample was changed.

    Fig. 4 Relationship Between Composition Ratio and Transmittance of (1-x)LaAlO3-xSr(Al,Ta)O3-Based Solid Solution Ceramics This shows the data when the thickness of the measurement sample was changed.

    Fig. 5 Refractive Indices and Abbe Numbers of Various Materials ●: Optical glass material, ○: 0.5LaAlO3-0.5Sr(Al,Ta)O3, △: Ba(Sn,Zr,Mg,Ta)O3, □: (Ba,Ca)(Ti,Mg,Ta)O3

    Fig. 5 Refractive Indices and Abbe Numbers of Various Materials
    ●: Optical glass material, ○: 0.5LaAlO3-0.5Sr(Al,Ta)O3,
    △: Ba(Sn,Zr,Mg,Ta)O3, □: (Ba,Ca)(Ti,Mg,Ta)O3

    La2Zr2O7-Based Transparent Ceramics

    The above-described transparent ceramics all had a perovskite structure, but we also fabricated transparent ceramics based on La2Zr2O7 that had a pyrochlore structure. Because La2Zr2O7 originally has a crystalline structure with a cubic system, it was possible to obtain a transparent sintered body by simply optimizing the fabrication process. The refractive index and Abbe number were nd=2.09 and vd=32.5, respectively, and although these were values near Ba(Sn,Zr,Mg,Ta)O3-based materials, the distinctive feature of these materials is that they have abnormal partial dispersion. Figure 6 shows the relationship between the partial dispersion ratio*3 Pg,F and the Abbe number for various materials. The line joining the characteristics of two typical glass materials is called a standard line, and materials having characteristics that deviate significantly from this standard line are called materials with large abnormal partial dispersion. Also, when a material has a Pg,F value that is lower than the standard line, the material is said to have negative abnormal partial dispersion. From Fig. 6, we can see that La2Zr2O7 is a material with a large negative abnormal partial dispersion. There are no other materials with a negative abnormal partial dispersion that is this large around vd=30. Materials with a large abnormal partial dispersion can be used in optical systems for providing a higher level of correction for chromatic aberration, and these materials have practical applications in high-power zoom lenses, telescopes, microscopes, and other devices. This abnormal partial dispersion of La2Zr2O7 is thought to be due to its electron structure.

    Fig. 6 Partial Dispersion Ratio and Abbe Number of Various Materials LZO: La2Zr2O7, LAO-SAT: 0.5LaAlO3-0.5Sr(Al,Ta)O3, BZMT: Ba(Zr,Mg,Ta)O3

    Fig. 6 Partial Dispersion Ratio and Abbe Number of Various Materials
    LZO: La2Zr2O7, LAO-SAT: 0.5LaAlO3-0.5Sr(Al,Ta)O3,
    BZMT: Ba(Zr,Mg,Ta)O3

    Conclusion

    One feature of transparent ceramics is a high refractive index, and this feature was developed for practical use for lenses of digital cameras in 2004. This was the world’s first instance that ceramics had been developed for practical use as lenses. This paper describes lens applications for transparent ceramics, but these transparent ceramics can all also be used as host materials for light-emitting elements using rare-earth elements and other substances. For example, for Ba(Mg,Ta)O3-based material and LaAlO3-Sr(Al,Ta)O3-based material, the cation array is randomized, and so they are characterized by a broad light-emitting peak due to the substituted rare-earth elements and the capability of light emission over a wide range. As described in this paper, a cubic crystalline structure is desirable for attaining transparency of ceramics, but recently, it has been reported that the use of smaller diameters for crystal grain sizes enables high transmittance even for hexagonal crystal materials. The elimination of limitations on the crystalline structure in this way will enable more freedom in the design of transparent ceramics so that we anticipate the introduction of transparent ceramics with even greater functionality and improved characteristics in coming years.

    Glossary

    *1 Chromatic aberration:

    The refractive index of optical materials depends on the wavelength. As a result, the focal point of light passing through the lens varies depending on the wavelength (color). This difference is called the chromatic aberration.

    *2 Color center:

    Light in the visible light range is sometimes absorbed when an electron is captured in a defect in a crystal. This type of defect is called a color center.

    *3 Partial dispersion ratio:

    The difference in refractive indices at two different wavelengths is called the partial dispersion, and the value when this difference is divided by the primary dispersion (difference between the refractive index at the F-line and the refractive index at the C-line) is called the partial dispersion ratio.
    Pg,F is a value found by taking the difference between the refractive index at the g-line and the refractive index at the F-line and dividing it by the primary dispersion.

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