Recently, fabrication and usage of transparent polycrystalline sintered ceramics are required because easily composition control and upsizing. Optical ceramics, which possess harsh-environment resistance such as heat resistance and chemical resistance, can be used as materials for lasers, phosphor matrix and scintillators, etc., and can be applied for sensors and medical field. 光We aim to fabricate advanced ceramics that not only possess optical functions but also have other functions such as electrical conductivity, mechanical properties, etc. that take advantage of the feature of ceramics.
APPROACH
In order to make transparency in polycrystalline ceramics, it is necessary to achieve densification by ultimately eliminating defects, and it is necessary to understand each of the ceramics processing such as powder synthesis, forming and sintering. It is important to design the process for precisely controlling the microstructure in sintered ceramics. We also focus on the processing using the external fields such as magnetic field and electric field, and in order to clarify their effectiveness, we also use in-situ observations in a magnetic field to elucidate the phenomena that occur during the processing. Furthermore, we are improving the method of the ionic conductivity measurement in addition to measuring optical properties.
FIG 1 Transparent alumina fabricated by Spark Plasma Sintering. (a) is prepared from the powder directly, (b) is prepared by colloidal processing in a magnetic field for controlling the c-axis orientation.
FIG 2 in-situ observation of particle deposition process under high magnetic fields in HGMS (high gradient magnetic separation) (an example of the in-situ observation of behavior of materials under high magnetic fields)
Fabrication of bulk ceramics with controlled microstructure
Overview
Controlling microstructural parameters such as grain size, grain boundaries, second phases, and crystallographic orientations is crucial for enhancing the properties of bulk ceramics. My current research primarily focuses on crystallographic orientation. Although using a magnetic field in diamagnetic and paramagnetic materials presents challenges, colloidal processing enables control over orientation in materials like alumina, zinc oxide, and silicon carbide with the help of a magnetic field. This approach eliminates the need for a magnetic field during sintering, allowing for the application of various sintering techniques. Additionally, this method can be employed to control the alignment of ceramic fillers in resin.
Characteristics
Method for Controlling Crystallographic Orientation in Paramagnetic and Diamagnetic Bulk Ceramics Using a Magnetic Field
Compatible with a wide range of sintering methods, such as spark plasma sintering, allowing for densification with orientation.
Enables control over various functional properties dependent on crystalline orientation, including thermal conductivity, ionic conductivity, thermoelectric properties, and transparency.
Facilitates the control of ceramic filler orientation and short fiber alignment in various matrices, such as resin.
Allows for the alignment of tubular pores in porous ceramics by controlling the orientation of pore-forming agents like nylon.
Major reserch
Superconducting magnets of the 10 Tesla class, which do not require liquid helium supply, can be utilized to align specific crystal axes. Particles with crystalline anisotropy, such as hexagonal and tetragonal crystals, rotate within the slurry during molding. For particles where the c-axis is the easy axis of magnetization, alignment occurs with the c-axis parallel to the applied magnetic field. Conversely, if the c-axis is the hard axis, particles rotate to position the c-axis perpendicular to the magnetic field. This process requires only the application of a magnetic field during molding, allowing various conventional sintering techniques to be applied for subsequent densification. In Al2(WO4)3, a multivalent cation conductor, it's possible to fabricate both b-axis and c-axis oriented structures using static and rotating magnetic fields, respectively. It has been found that b-axis alignment results in higher conductivity compared to c-axis alignment, with randomly oriented Al2(WO4)3 exhibiting conductivity between these alignments. Transparent alumina production is achievable even in randomly oriented bodies using colloidal processing and spark plasma sintering, allowing control over the compact's packing structure (as illustrated in the blue figure below). Due to alumina's anisotropic corundum structure, improving in-line transmittance has been challenging due to birefringence at grain boundaries. We have successfully suppressed birefringence by aligning the c-axis using a magnetic field, demonstrating the production of polycrystalline alumina with excellent in-line transmittance.
summary
Advancements in Material Properties through Crystal Orientation
I have demonstrated significant improvements in ionic conductivity in electrolytes for secondary batteries and fuel cells, enhancement of properties through crystal orientation in anisotropic ceramics such as thermoelectric materials, and improved linear permeability via densification. Looking ahead, my focus will be on exploring conditions to reduce the magnetic field strength necessary for these processes and developing a method that transitions seamlessly from batch to continuous operation. It is anticipated that refinements to the process will facilitate its application to an even broader range of materials.
Development of highly functional electrochemical impedance analysis program
Overview
I have developed a highly functional program that enables various analyses of electrochemical impedance in a single program. It implements original analysis functions such as our original simple artificial intelligence automatic analysis and extended relaxation time distribution analysis. This program is released under a commercial license.
Characteristics
Dynamic modeling through the use of GUI
Implementation of automatic analysis function by simple artificial intelligence
Implementation of original extended relaxation time distribution method applicable to spectra by batteries and capacitors
Implementation of many other original functions
Development of new functions in progress
Major reserch
Overview of the developed electrochemical impedance analysis program. In addition to conventional equivalent circuit analysis, comparison of multiple relaxation time distribution analysis results, etc. are shown. Impedance spectra of non-ideal shapes can also be analyzed intuitively and easily.
Control of materials behavior and materials separation using the magnetic force
Overview
Using magnetic fields of around 10 T generated by superconducting magnets, non-contact mechanical effect can be exerted on various materials such as water, plastics, ceramics, and so on. This feature enables us a various novel process such as the control of material structures through the magnetic alignment based on the materials anisotropy, control of the position or the behavior of materials, separation and analysis, magnetic levitation or the control of the gravitational effects, and so on. Types of effects and their intensities depend on properties of materials, however, magnetism is common property that all materials have, therefore magnetic field effects or control can be applied to many substances by adjusting the conditions properly. In this research, magnetic field effects on some processes are investigated though the visualization and simulation to deepen the understandings and to optimize the magnetic field effects.
Characteristics
Utilize optical visualization devices that can be used even under high magnetic fields
The confocal scanning laser microscope usable up to 13 T magnetic field is unique device
Developed the magneto-Archimedes effect, that is the way to enhance the magnetic field effects and apply this technique to various processes
Process design from the perspective of how the magnetic field effect is applied on which elemental processes
Major reserch
Levitation of water attained by the magneto-Archimedes levitation technique which enhance the effect of magnetic force by adjusting the property of surrounding medium.
Formation of triangular lattice of diamagnetic gold spheres of 1 mm in diameter through the induced magnetic dipole interaction between particles due to their magnetization. The lattice constant can be controlled by changing the conditions.
An example of the visualization of particles deposition process on the magnetic filter during the high gradient magnetic separation (HGMS). In HGMS, particles suspended in a fluid can be separated at high speed and efficiency based on their magnetization. HGMS can be applied to the separation of many substances by applying the magnetic seeding technique. Therefore, further understandings of the deposition process during HGMS will contribute to the improvement of the separation performance.
The confocal scanning laser microscope usable under 13 T magnetic fields is unique device. This device will contribute to deepen the understandings of phenomena occurred under high magnetic fields.
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summary
Utilization of high magnetic fields are expected to serve an unique environment for materials processing through the control of dynamical effects.
High magnetic fields are expected to be used as a novel way of control for the materials processing, separation, and analysis without any direct contact with the matter.
Practical applications of the functional ceramics are actively advanced in a lot of scientific fields like optical, electrical, magnetic, filtering and catalytic materials. For activating and/or improving them, there are numerous issues like, miniaturization, enhancing performances and multi-functionalization. Cost reduction is also important. In addition, we should take into account for the influences on the environmental impact, resource issue and energy loads, seriously. For these issues, recent studies focuses on the characteristic functionality appeared in the nano-scaled area, and the materialization is the key for this approach. I investigate chemical synthesis procedure of the advanced ceramics and the characteristics properties of the products in order to tailor a novel advanced ceramics and/or to develop effective processing technique of promising materials.
Characteristics
There are some differences between generally known ceramics fabricated at high temperature and chemically synthesized material, even though their chemical composition are the same. I investigated this difference from viewpoint of physics.
Chemically synthesized ceramics often configure original structure like metastable phase, nano-materialization with quantum property and the hybridization with organic molecules. I focus on these characteristic materials for tailoring novel advanced ceramics.
For developing effective processing techniques, I often design and prepare original synthesis tools. This approach is believed to produce some originality.
Major reserch 1
Development of hydrothermal technique for synthesizing ZnAl2O4 on a metal aluminum.
ZnAl2O4 consist is one of the promising material, which consists of ubiquitous elements, as sensing devices. Hydrothermal treatment for the surface on a metal aluminum is expected as effective synthesis technique with low cost for preparing ZnAl2O4/Al device. However, it was difficult, since hydrothermal condition strongly oxidizes metal aluminum. I succeeded in developing this synthesis route, and demonstrated the performance of synthesized ZnAl2O4/Al device as humidity sensor. This fabrication technique is applicable for synthesizing ZnAl2O4 on the surface of metal aluminum with small size and/or complex shape with low cost. In addition, this fabrication technique is believed to be applicable for synthesizing other aluminum compounds.
Major reserch 2
Discovery of novel hybrid nanosheet with ultimate device structure of 1D ceramics.
Low dimensional material is focused for investigating quantum effect of ceramics. We discovered a novel material showing quasi one dimensional (1D) magnetism. This material configures a hybrid nanosheet structure with quasi 1D ceramics and organic molecule. Interestingly, this novel inorganic-organic hybrid-nanosheet is considered to align the 1D transition metal oxide vertically against the wide nanosheet surface. Then, this quasi 1D magnetism has a critical temperature at 21 K implying a relatively low magnetic fluctuation state as 1D spin system. These unique characteristics are quite interesting. For instance, it can easily detect and bundle the characteristic electrical quantum-properties of 1D ceramics by sandwiching this nanosheet with electrodes.
summary
I investigated on advanced functional ceramics from the viewpoints of chemical synthesis and physics. Chemically synthesized ceramics often shows characteristic property, since low temperature and homogeneous synthesis conditions often yields metastable phase with original (and unknown) structure. This is the key of my work. I want to materialize these characteristic properties. Recently, I developed novel synthesis technique of aluminum compound and discovered novel hybrid nanosheet showing 1D magnetism. I want to improve these results more practically.
CNT-MXene membranes for electrochemical energy storage applications
Overview
Superior prospect of 2D MXenes for electrochemical energy storage applications.
Restacking and agglomeration of MXenes considerably limit their true potential for fast ion transport.
CNTs were dispersed to control the structure and porosity of MXene membranes.
CNT–MXene hybrid membranes show dramatically improved Li-ion transport properties.
Characteristics
A scalable method to fabricate ultralight yet continuous CNT–MXene membranes with uniform/3D CNTs dispersion.
Correlation among CNTs content, surface microstructure, MXenes’ stacking structure and ion transport properties of the films.
Li-ion transport mechanisms in the CNT–MXene membranes.
MXene membranes with tunable 2D and 3D structures with improved ion-transport performances.
Potential application of selected CNT–MXene ultralight membranes as interlayers for Li-O2 batteries.
Major reserch
The compact surface microstructure of MXene membranes is dramatically changed as CNTs occupy MXene/MXene edge interfaces.
The 2D stacking order of MXenes is preserved up to 30 wt% CNTs.
The 2D alignment is completely disrupted at 40 wt% CNTs, and a more pronounced surface opening and internal expansion of ~770% are realized.
Both 30 wt% and 40 wt% membranes show stable cycling performance under a significantly higher current density due to faster transport channels.
Notably, for the 3D 40 wt% membrane, over-potential during repeated Li deposition/dissolution reactions is further reduced by another ~50%.
Ultralight yet continuous hybrid films comprising up to ~0.027 mg/cm2Ti3C2 MXene can be prepared using aqueous colloidal dispersions and vacuum filtration for specific applications
summary
A method to fabricate ultralight yet continuous CNT–MXene membranes for electrochemical energy storage applications.
Inexpensive multi-walled CNTs control the structure of MXene membranes and improve their ion-transport properties.
Non-cubic laser ceramics using grain control technology
Overview
Transparent Ceramics with laser-quality attributes are a class of fine ceramics from which scattering sources, such as residual pores and impurities, have been ultimately eliminated. These ceramics are also used as solid-state laser materials, with some achieving higher laser outputs than conventional single crystals. Recognizing that the advancement of optical technologies is fundamentally linked to the development of novel optical materials, my efforts have focused on demonstrating innovative laser ceramics and advancing bonding techniques for optical elements. This includes the fabrication of transparent non-cubic ceramics, overcoming the constraints of conventional technologies, for a wide range of applications.
Characteristics
Realization of transparent nano-ceramics with an average grain size of approximately 100 nm
Demonstration of laser oscillation in non-cubic apatite ceramics
Demonstration of effective bonding between single crystals and polycrystalline materials
Major reserch 1
The traditional manufacturing of transparent ceramics typically involves high-temperature, prolonged heat treatments to eliminate pores. This process often results in grain size that exceed the wavelength of light, causing grain boundary scattering due to birefringence in optically anisotropic materials. By controlling crystal grains to be significantly smaller than the wavelength of light, we can reduce this scattering effect. To achieve optical quality comparable to single crystals in anisotropic ceramics, our research focuses on precise grain size control through liquid-phase synthesis of fine initial powders and low-temperature, short-duration sintering using pulsed electric current sintering. As shown in the figure, we have successfully created fully dense and transparent ceramics with average grain sizes of approximately 100 nm in hexagonal apatite, and proven laser oscillation in samples doped with rare earth elements. Future prospects include expanding into a wide range of optical fields such as scintillators, phosphors, bio-optical materials, and magneto-optical materials.
Major reserch 2
In the development of high-power lasers or white light sources, minimizing the thermal issues in phosphors is crucial. In laser materials, internal temperature gradients can cause thermal lensing and thermal birefringence, which degrade beam quality. To overcome these thermal issues, we have demonstrated an effective bonding method that combines high thermal conductivity sapphire with laser materials, thus reducing thermal effects. We are currently conducting fundamental evaluations, aiming not only for higher output power in laser devices but also for a variety of material combinations and enhanced functionality. We anticipate applications beyond laser processing light sources, extending to white lighting and other fields.
summary
Developed transparent nano-ceramics with an approximate grain size of 100 nm.
Fabricated non-cubic transparent apatite ceramics and demonstrated laser oscillation.
Invented a new bonding technology for different optical materials, showing improvements in laser performance.
Achieving laser quality beyond existing technologies remains issues, with further improvements in optical quality being the goal.
There is anticipation for expansion and societal implementation in a wide range of optical fields beyond lasers.
Fields of Electronic and Photofunctional Materials Research Center
Fields of Electronic and Photofunctional Materials Research Center
Ultra-wide Bandgap Semiconductors Group
Optical Single Crystals Group