Wataru Nakao

A novel macroinimer of polyethylene oxide: synthesis of hyper branched networks by photoinduced H-abstraction process

A novel poly(ethylene oxide) macroinimer (PEO-macronimer) possesing methacryloyl and tertiary amino end groups was prepared by ring-opening polymerization of ethylene oxide initiated by potassium 2-methylaminoethoxide and termination of living ends of PEO with methacryloyl chloride. NMR analysis showed that the macronimer contains equal amount of amino and methacrolyl groups indicating efficient initiation and termination processes. The dimethylamine end group in conjunction with benzophenone under UV irradiation produced ternary amine methylene radicals via H-abstraction mechanism which initiates the polymerization. Photopolymerization in solutions at high macroinimer concentration or in films resulted in the formation of insoluble networks. The crosslinked polymers exhibit high swelling capacity in organic solvents and water due to the hyperbranched nature.

Enhancement of the self-healing ability in oxidation induced self-healing ceramic by modifying the healing agent

The available temperature range of the self-healing induced by high temperature oxidation of SiC can be controlled by the particle size of the contained SiC particles. In this study, three types of alumina?SiC composites were prepared. The SiC particle sizes of the composites were 270, ?30?nm, and less than 10 nm. The self-healing abilities were estimated by the strength recovery behavior at several temperatures.The use of nanometer-sized dispersed SiC particles as healing agent decreases the activation energy of the SiC oxidation obtained from the differential thermal analysis with several heating rates. This implies that smaller SiC particles can give rise to the oxidation at lower temperature. Moreover, the lowest temperature at which the cracked strength was completely recovered for 10?h was strongly affected by the SiC particle size. As the SiC particle size varied from 270 to ?30?nm, the lowest temperature varied from 1300 to 950??C. However, alumina composite containing SiC particles whose particle size is less than 10?nm cannot recover completely the cracked strength under every condition, because the space between crack walls cannot be filled with the formed oxide due to the small volume of SiC on the crack walls. Therefore, it was found that there is an optimal SiC particle size for endowing self-healing ability.

Thermodynamic Stability of γ-Aluminum Oxynitride

For the purpose of determining the thermodynamic stability of γ-aluminum oxynitride spinel, alon, equilibrium N 2 -CO gas compositions coexisting with alon-Al 2 O 3 -graphite have been measured by means of gas chromatography and quadrupole mass spectrometry in the temperature range from 1908 to 2023 K. From the results obtained, the standard Gibbs energy change of the carbothermal nitridation reaction of Al 2 O 3 to form alon Al 2 O 3 (s) + 3C(s) + N 2 (g) = Al (64+x)/3 V (8 x)/3 O 32 x N x (Al 2 O sat.) + 3CO(g) has been determined to be Δ r G o /kJ = 726.2 - 0.3611 T (±0.7) (1908-2023 K) where alon is expressed as Al (64-x)/3 V (8-x)/3 O 32-x N x (V is vacancy). Using the obtained Gibbs energy change, the eutectoid decomposition temperature of alon into α-Al 2 O 3 and AIN has been evaluated to be 1903 ± 4 K, and the chemical potential diagram of the Al-O-N-C system has been constructed at 2000 K.

Effect of Difference in Crack-healing Ability on Fatigue Behavior of Alumina/Silicon Carbide Composites

Crack healing has great effect on monotonic strength of ceramics. Also the crack-healing effect on fatigue behavior is an interesting issue. Three kinds of Al2O3/30 vol.% SiC composites that have differences in crack-healing ability and fracture toughness were hot pressed. Static fatigue behaviors of these crack-healed specimens were investigated. From the experimental results, the static fatigue limits of three kinds of Al 2O3/30 vol.% SiC composites were 800, 700, and 1000 MPa, respectively. Furthermore, the fractures seldom initiated from the healed crack. Therefore, it was found that using the crack-healing forbade stress corrosion cracking from the original surface cracks, thereby leading to large increment in fatigue strength. The results, however, demonstrated that fatigue cracks were generated by applying high tensile stress. The general mechanism of fatigue cracks was discussed by using surface morphology.

Methodology for evaluating self-healing agent of structural ceramics:

Simple methodology for evaluating availability of self-healing agent has been established from the investigation on the relationship between oxidation behavior of self-healing agent and self-healing phenomena. The consistency of the established methodology was discussed by comparison in the lower bound of the available temperature range (TH-low) of mullite/TiSi2 composite. From thermo-gravity and differential temperature analysis, the available temperature range for 10-h healing ( T H - low , 10 h est ) was estimated to be 563°C. On the other hand, the value of the lower bound of the available temperature range for 10-h healing ( T H - low , 10 h exp ) was experimentally determined to be 600°C from strength recovery tests. These values showed a good consistency. Also, the data on 1-h healing of mullite/TiSi2 and the reference data on alumina/SiC self-healing ceramics exhibited good consistency. Therefore, the proposed methodology is sufficient for evaluating the advanced healing agent.

A Novel Design Approach for Self-Crack-Healing Structural Ceramics with 3D Networks of Healing Activator

Self-crack-healing by oxidation of a pre-incorporated healing agent is an essential property of high-temperature structural ceramics for components with stringent safety requirements, such as turbine blades in aircraft engines. Here, we report a new approach for a self-healing design containing a 3D network of a healing activator, based on insight gained by clarifying the healing mechanism. We demonstrate that addition of a small amount of an activator, typically doped MnO localised on the fracture path, selected by appropriate thermodynamic calculation significantly accelerates healing by >6,000 times and significantly lowers the required reaction temperature. The activator on the fracture path exhibits rapid fracture-gap filling by generation of mobile supercooled melts, thus enabling efficient oxygen delivery to the healing agent. Furthermore, the activator promotes crystallisation of the melts and forms a mechanically strong healing oxide. We also clarified that the healing mechanism could be divided to the initial oxidation and additional two stages. Based on bone healing, we here named these stages as inflammation, repair, and remodelling stages, respectively. Our design strategy can be applied to develop new lightweight, self-healing ceramics suitable for use in high- or low-pressure turbine blades in aircraft engines.

Enhancement of In situ Self-crack-healing Efficient Temperature Region by SiC Nanosizing

Self-crack-healing by oxidation of silicon carbide (SiC) is the most effective method to improve the reliability of ceramics, as this eliminates surface cracks completely. Furthermore, if the service condition corresponds to the state under which self-crack-healing can occur, cracks introduced during service can be completely healed during service. However, the ceramic composites having crack-healing ability exhibit only a small efficient temperature region for crack-healing during service. Therefore, the authors have tried to decrease the temperature at which crack-healing reaction becomes active, as well as to increase the temperature limit for bending strength by size reduction in the contained SiC particles. The present study adapts the following self-propagating high temperature synthesis (SHS) to fabricate alumina composite containing nanosized SiC particles: 3(3Al2O32SiO2) + 8Al + 6C = 13Al2O3 + 6SiC. The formed nanosized SiC particles have a particle size of 10—30 nm and exist in alumina grain. As a result, the temperature at which self-crack-healing becomes active can be decreased 200°C by using SHS. Furthermore, the crack-healed specimen has no large strength decrease up to 1300°C. Therefore, new alumina/ SiC nanocomposite is found to exhibit the in situ self-crack-healing efficient temperature region of 300°C.

Self-crack-healing behavior in ceramic matrix composites

Abstract: Self-crack-healing is one of the most valuable phenomena to overcome the decrease in reliability of ceramics caused by cracking. This is because self-healing is triggered by crack initiation and gives complete strength recovery through the passive oxidation of SiC. Thus, self-healing ceramics are candidates for high-temperature materials for gas turbine components, i.e., turbine blades and stator vanes. Nano-composite and multi-composite concepts for self-healing ceramics are described. The design concept is based on ‘damage management’ rather than ‘damage prevention’. The kinetics of self-crack-healing of ceramic/SiC composites including the effects of temperature and oxygen partial pressure is introduced. The time for crack-healing with low oxygen partial pressure can be estimated. Furthermore, the effects of oxygen partial pressure on self-healing under tensile stress are discussed. The design stress of ceramic turbine blades with self-crack-healing is considered.

Crack-Healing Ability of Structural Ceramics and Methodology to Guarantee the Reliability of Ceramic Components

The heat-resistant limit of structural ceramics is 1273K ~ 1773K, which is greatly superior to that of metallic material. Structural ceramics are a candidate element for high-temperature apparatuses such as gas-turbines and fusion reactors. However, the fracture toughness of ceramics is fairly low compared with metallic material, and the following problems have occurred. (1) Cracks occur by the usual machining process (grinding, polish, etc.), lowering the reliability. In order to prevent this, precise polishing is required in the final stage, which is time-consuming, and there are also problems with fabrication efficiency and fabrication cost (2) Crack sizes of about 10 ~ 30 μm in depth affect the reliability. The nondestructiveinspection technology for detecting cracks of 10 ~ 30 μm is underdeveloped. Therefore, the reliability of major parts is low. (3) There is a possibility that a crack will occur in the components while they are being used at higher temperatures, by whatever cause. When a crack occurs, the reliability is greatly lowered. The options for resolving these problems are as follows. (a) Improve the fracture toughness of the material by such means as microstructure control and fiber reinforcement. (b) Conduct a nondestructive inspection before use, and detect and repair any dangerous cracks found. (c) Conduct a proof test to prevent use of a low reliability member. (d) Induce a selfcrack-healing ability, so that all dangers cracks can be healed. There are world-wide active investigations of options (a) to (c). In this chapter, special attention is paid to method (d), the self–crack-healing ability of structural ceramics. There are three advantages to using a material that can heal surface cracks. (A) If the selfhealing of the surface crack which exists is carried out after an efficient machine operation is performed, then there is a great advantage in fabrication efficiency and fabrication cost. (B) Since all surface cracks are healed, reliability improves greatly

Performance of capsule-type micro actuator using hydrogen storage alloys

Capsule-type micro actuator driven by the volume change of hydrogen storage alloys (HSA-CMA) was proposed to be applied as an actuator mounted on the joints of the super multilink manipulator to capture space debris. This actuator consists of a hollow sphere frame made of aluminum, a membrane of hydrogen storage alloys and a valve, and is compact and lightweight. The aim of this study was to evaluate the availability of HSA-CMA. Finite element analyses were performed to investigate the actuator displacement and the generating force as the static performance, and the obtained static performances were compared with that of other smart actuators. The analysis on the actuator displacement led to the design map of HSA-CMA. The comparison result suggests that HSA-CMA has the actuator displacement and the generating force as well or better than other smart actuators and that it is suitable for use in space.Copyright