In order to actually use it as optical window materials for sensors, the materials should simultaneously possess excellent performance such as mechanical and thermal properties in addition to optical properties. However, since it is not easy to realize multiple functionalities in simple material system, the development of new optical composite materials (CMC: Ceramic Matrix Composite) that combines multiple materials with different functionality has been required.
For example, the right figure shows the photo of MgAl2O4/Al2O3 laminated optical ceramic composite, which attained high mechanical property with simultaneously maintaining broadband transparency; mechanical property (high hardness) is 2.0-2.5 times higher than that of monolithic spinel ceramics. The high performance visible-infrared transparent MgAl2O4 spinel was realized by laminating its surface by high-hardness α-Al2O3 alumina phase through optimizing the fabrication processing.
Polycrystalline Optical Material Group
Group leader
Group members
AIM and GOAL
- Optical materials for sensor windows and light sources are one of key components for realizing smart society. Our group is aiming to develop new polycrystalline optical ceramics that possess excellent mechanical and thermal properties in addition to visible to near-infrared broadband transmission.
- In order to attain a major breakthrough in the optical ceramics, our group are now focusing on complex composition systems as a new challenging field of the optical ceramics.
APPROACH
- New optical materials：
By expanding the search area from simple composition systems to unexplored complex composition systems, we are expecting to develop new optical ceramics that possess excellent mechanical and thermal properties in addition to broadband transmission. - Bulk processing：
We are aiming to develop densification techniques that enable to synthesize polycrystalline optical ceramics with complex composition systems through utilizing external field effects such as current/electric field and high pressure. - Applications：
New polycrystalline optical ceramics applicable to broadband sensor windows and light sources.
FIG 1 Application examples of optical ceramics in sensing windows and light sources for realizing smart society.
FIG 2 Examples of (b) high strength alumina/spinel laminated composite and (c) cross-sectional interface microstructure fabricated through (a) a pulsed current sintering technique, and (d) (Y,Gd)2O3:Eu3+ transparent polycrystalline ceramics and (e) red luminescence under UV excitation.
MORITA, Koji / Group Leader
Developments of new polycrystalline optical materials and its fabrication processing
Overview
Optical ceramics are one of the key components necessary in sensor window and light source materials, which can provide a highly efficient, safe and secure society. In order to realize the breakthrough in next-generation optical ceramics, it might be essential to challenge in the development of new research fields.
Therefore, as a challenging research that will attain the breakthrough in the optical ceramics, this work will promote the research of complex optical materials such as "middle and high entropy" material systems in addition to the conventional simple materials. This work is aiming to the development of complex optical materials, from the synthesis of fine powders to the development of processing for attaining the polycrystalline optical materials.
Characteristics
- Broadband transparent ceramics in visible/near-infrared range
- Search of complex polycrystalline optical ceramics
- Development of new processing for attaining dense bulk ceramics
- High performance optical ceramics that can simultaneously attain optical, thermal and mechanical properties
Major reserch 1
Major reserch 2
It has been expected to develop new research fields and material systems that enable to realize breakthrough in the pre-existed optical ceramics. In more recently, high entropy materials have been attracted many attentions as a new material system that enable to attain new function properties. High entropy is a general term of materials that are formed by mixing relatively large proportions more than five elements. The high entropy materials have been examined mainly on metallic and structural materials, and then, several new functional properties that exceed the pr-existed materials have been reported. Currently, high-entropy optical ceramics have also been reported.
Since the high-entropy has infinite combinations, it can be expected to develop high-performance new optical ceramics that fit well with the needs of industrial requirements. This work is also aiming to develop new fabrication processing to realize multi-functional high-entropy optical ceramics.
Summary
Polycrystalline optical ceramics that can fabricate from powder sources are expected at attain low cost, highly productive and scaling-up. In addition, it can also be formed final products directly due to near net shaping and expected to have excellent mechanical properties.
Furthermore, it is expected industrial applications because of its flexibility to adjust the composition, to combine with different phases and to refine the microstructure for simultaneously improving its optical/mechanical properties.
TODOROKI, Shin-ichi
Effects of external energy exchange in fiber fuse propagation through single-mode silica glass fibers
Overview
Silica glass optical fibers are vulnerable to high-power light transmission. Accidental external heating makes the glass less transparent to induce light-heat conversion at more than 1000℃. This sometimes causes a catastrophic breakdown along the fiber length.
The damage sites are formed by captured high-temperature gas at the fiber core, called fiber fuse, running toward the light source and leaving periodic voids in the single-mode core.
This periodicity is found to be modulated when external conditions are modified.
This behavior is reasonably explained with the energy flow in the captured high-temperature gas and glass melt surrounding it.
Characteristics
- Discovery of the relation between void interval and energy flow
- Insight into how it appears based on the viscosity of molten silica glass
- Experimental skills to manipulate fiber fuse for data acquisition
- Analytical skills to extract valuable data from massive photographs
- Video editing skills to make the data easily understandable (see the QR-codes in the following slides)
Major reserch
It is hard to examine fiber fuse behavior running at about 1 m/s. However, valuable information is left behind in the form of periodic voids (See Slide 1).
The interval increases at a heated segment (see Slide 2) because of an additional heat-induced excitation.
Self-pumping occurs when a fuse propagates in a coating-free segment via back-reflection at the fiber surface and makes the gas volume larger (See Slide 3).
The void periodicity is broken when a sudden change of the pumping light intensity (See Slide 4). This is due to the delayed response of molten glass surrounding the captured gas.
Summary
- The void interval of fiber fuse damage is found to increase with increasing temperature, in addition to the formerly known relationship with pumping light intensity.
- The volume of captured gas increases when it propagates in a bare fiber segment which causes self-pumping through back-reflection of original emission.
- The molten glass surrounding fiber fuse is the origin of broken void-periodicity caused by a sudden change of the pumping light intensity.
LI, Jiguang
Inorganic optical materials
Overview
Phosphors and transparent ceramics are optically functional materials that are finding wide applications in LED lighting, display, solid state lasers, and scintillation.
The current advances in the above technological fields are imposing a strong need for new materials and fabrication technologies to achieve cost reduction and improved/novel optical properties.
Characteristics
Through rational design of precursors and synthesis technologies, highly sinterable ceramics powders that can be fully densified at significantly reduced temperatures can be produced Through morphology control of the crystallites (size/shape/exposed facets), improved/novel luminescence properties can be attained Through polyhedron design of the activator ion and composition/structure design of the host lattice, high performance new phosphors can be expected
Major reserch
- Figure shows multicolor emitting (Gd,Ln)3(Al,M1/M2)5O12:R3+phosphors, where Ln is a lanthanide element, M1 and M2 are 2+ and 4+ charged dopant ions, respectively, and RE3+is an activator ion. The compounds were derived by modifying the Gd and Al sites of Gd3Al5O12 garnet (GAG), and such a strategy not only stabilized the metastable lattice of GAG but also produced multicolor luminescence by doping different types of activators.
- Figure shows the (Y,Gd)2O3:Eu3+transparent ceramics, which were fabricated via vacuum sintering at only 1700 °C for 4 h but are as transparent as the corresponding single crystals. The oxide powders used for sintering were obtained by engineering of layered hydroxide nanosheets, including composition design and thickness control, followed by proper calcination. The derived oxide particles showed high dispersion, high specific surface area and unimodal size distribution, revealing the significant advantages of the synthesis technology.
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Summary
The ultimate goal of research is to develop advanced phosphors and transparent ceramics for application in the important fields of lighting/display, solid laser and scintillation (imaging), which is largely based on controllable processing of powders and rational design of chemical composition and crystal structure of the materials.
VASYLKIV, Oleg
Deformation-resistant multipurpose ultra-hight temperature ceramics
Overview
We are currently conducting research in the chemical structural engineering of deformation-resistant UHTC carbides, borides, nitrides, and composites with ultra-hardness and ultra-high strength.
We have combined the merits of powder synthesis and electric current activated sintering technique for the design of techniques applied on powder body with establishing morpho-structural and compositional features, which lead to the fabrication of bulk ceramics with superior characteristics.
Characteristics
- multipurpose deformation-resistant UHTC carbides, borides, nitrides and composites
- sufficient balance between ultra-high hardness, ultra-strength, toughness and modulus
- morpho-structural and compositional features with superior characteristics
- gas turbine operation in a combined cycle power plants
Major reserch
Deformation-resistant UHTC high-entropy ceramics and composites becoming extremely attractive. Light, ultra-hard bulk B4C-based composites with hierarchical superstructure with deformation resistivity from RT to 2000°C (Fig. 1(a)) exhibit change in the deformation mechanism from brittle fracture to plastic deformation, and flexural strength far exceeding 1000MPa at 1800 - 2000°C (Fig. 1(b, c)).
Depending on the loading rate, B4C-based ceramic showed 1000 - 8400MPa strength at 2000°C (Fig. 1(b)). Bulk ultrastrong TiB2-B4C ceramic exhibits a mean flexural strength of 1000MPa up to 1800°C, and further increasing to 1760MPa at 2000°C. Recently produced bulk, ultrahard, tough, deformation-resistant Ta diboride, Ta monoboride, Zr-Ta multiboride, and high-entropy TaB2-ZrB2-TiB2-HfB2.
Summary
The request for new multipurpose deformation-resistant ultra-high temperature ceramics (UHTC), able to act as special engine and vehicle protection, ceramic segmented leading edge components for aerospace, plasma-facing, ceramic parts for solar towers used for gas turbine operation in a combined cycle power plants (grids, superheaters, reheaters, evaporators, steam turbines, condensers, and chimneys) cause the worldwide demand in a new class of ceramic composites of incredible high strength, the sufficient balance between high toughness, hardness, and high-modulus.
Fields of Electronic and Photofunctional Materials Research Center
Ultra-wide Bandgap Semiconductors Group
Optical Single Crystals Group