Research and Development of Transparent Ceramics for Optical Applications

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 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)O3Fig. 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

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.

Paper Review