AIST Stories No2
From AIST to the Innovative World4hypothesis.”This is, so to speak, just like a steamed bun impacting and solidifying. When the steamed bun impacts, its skin is ripped apart and its contents, red bean paste, are discharged. Then, this red bean paste forms bonds between adjacent steamed buns. Is this hypothesis really correct? In order to see whether the particle fragmentation phenomenon was occurring as predicted by his hypothesis, Akedo went about forming dense films by spraying mixtures of different particles and observing the fine structures of their cross-sections. The results indicated that particles that had collided with the substrate were fragmented at the nano-level and were deformed.This proved that Akedo’s hypothesis was correct. At this time, a phenomenon of sparks flying during collisions could be observed just like sparks being emitted from flint. An alternative hypothesis was thus “Was the impacted surface temperature rising to the point where sparks were emitted?” However, the cause of the sparks was found to be dependent on the type of gas that accelerated and carried the particles; it was not because of a high temperature but rather, electrons being discharged physically from the surfaces when particles collided and this caused the surrounding gas to emit light.Akedo called this phenomenon the room temperature impact consolidation phenomenon and the method derived from it the Aerosol Deposition (AD) method.Solving the mechanism to target application developmentCompared with films fabricated by traditional methods, the mechanical properties of films fabricated using the AD method are extremely good and the films form much faster. On account of the dense solidification, materials such as alumina*3 can be formed into transparent films, whereas such transparency is not easily achieved with the traditional sintering method. Besides microdevices, application was thought to be possible in a wide variety of fields; however, even when presentations were made at academic meetings, reactions were muted because people found it hard to believe in a method that overturned existing conventional wisdom by solidifying ceramic particles at room temperature. Thereupon, Akedo modified his strategy, prototyping an actuator that used this film and comparing the film’s performance with those fabricated using other methods. Through this, from around 1999, companies started to show interest. However, several companies that wished to carry out joint research raised concerns, saying “Even if film forming is achieving even denser adhesion, the opposite happened: the substrate was abraded.”Subsequently, a new method was developed for measuring the particle impact velocity: after direct measurement of these impact velocities, we found that the velocity that could form films ranged at most between 200 and 300 m/s. According to simulations, at such impact velocities, the surface temperature of the fine particles would not reach that required for sintering (>1000°C). In other words, some change occurring when the particles collided was transforming kinetic energy into binding energy, but it could not be explained with a traditional theory of melting and densification. Akedo believed that it must involve some unknown mechanism. Akedo decided to develop a hypothesis that could explain how particles could form a dense, hard film without undergoing melting.“When collisions occur between ceramic particles and they break into even finer particles, they reach an unstable “active state” with the stable electron bonds between atoms slipping and detaching. When they return to a stable state, minute broken particles immediately adjacent to each other adhere together and densify. This was the essence of my *3 Alumina: Aluminum oxide. Sapphire is a high purity singlecrystalline form.AD coating experimental setup▼Substrate sprayed with fine particles at reduced pressure.