1. Introduction

In recent years, a variety of industrial applications have been identified for cavitation processing, including peening, surface and molecular structure modifications, fatigue strength improvement, and surface cleaning. Waterjet cavitation involves imparting mechanical action to the surface of a material as the result of the extremely high pressures generated during the collapse of a microjet close to the surface [1]. In contrast, ultrasonic cavitation is known to generate hot spots that can promote chemical reaction by producing temperatures of several thousand K [2]. The authors have developed the multifunction cavitation, which is caused by ultrasonic irradiation of a waterjet during floating cavitation to generate microjets with hot spots. This technique combines the properties of both waterjet and ultrasonic cavitation. Furthermore, the additional nozzle was put on the waterjet nozzle (ejector nozzle) in order to increase the temperature and pressure of bubble. The generation mechanism of ultra-high temperature and pressure cavitation was clarified theoretically. The multifunction cavitation was applied to the enhancement of the photocatalytic properties of titanium oxide under visible light.

2. Theory of multifunction cavitation and ultra-high temperature and

Figure 1 shows the mechanism of our multifunction cavitation process, in which ultrasonic radiation is imparted to the cavitation cloud generated by a high pressure waterjet. In this method, the microjets generate mechanical action, but so-called "hot spots [12]" at which chemical reactions can occur are also formed. As the ultrasonic cavitation proceeds, isothermal expansion takes place once the pressure is above the Blake threshold value. Following this expansion, Rayleigh shrinkage occurs. Repeated expansion and shrinkage leads to cavitation, which in turns results in high pressure collapse and the generation of high temperature hot spots. In the conventional heat treatment of metals, the application of heat treatment after cold working is quite different from cold working after heat treatment. In contrast, multifunction cavitation has the capacity to allow so-called microlevel or nanolevel forging, in which the

Nanolevel Surface Processing of Fine Particles by Waterjet Cavitation and Multifunction Cavitation to Improve…

http://dx.doi.org/10.5772/intechopen.79530

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Figure 1. The multifunction cavitation mechanism and mechanical-electrochemical processing by multifunction cavita-

pressure cavitation

tion microjet.

2.1. Multifunction cavitation mechanism

material is simultaneously worked and heat treated.

At present, there are many technical challenges associated with the development of new energy sources as alternatives to thermal and nuclear power generation. Since the use of hydrogen as a fuel generates only water, hydrogen is seen as a potential clean energy source that does not produce hazardous exhaust gases such as carbon monoxide (CO), hydrocarbons (HCs), or nitrogen oxides (NOx). Hydrogen generation via the decomposition of water using a photocatalyst has been studied extensively, and photocatalytic hydrogen generation under UV irradiation has been developed, typically using titanium oxide as a substrate. Ideally, this technology would exhibit high efficiency in the visible light portion of the electromagnetic spectrum. The study reported herein represents an attempt to move closer to this goal by developing highly efficient, visible light-responsive photocatalyst materials [3].

Titanium oxide (TiO2) is widely used as a white pigment in paints, UV absorbing materials, and cosmetics. It is both inexpensive and safe, and can even be used as a food additive. In addition, the application of titanium oxide to photoelectrodes and photocatalyst materials with applications in solar energy conversion is also anticipated [4]. The TiO2 photocatalyst reaction proceeds most efficiently under irradiation by UV light (350 nm). However, only a portion of the sun's emission spectrum is at this wavelength, so it is important for effective energy conversion to be able to use visible light, which comprises one half of the solar energy output. There have been many studies attempting to append other compounds to titanium oxide-based catalysts to obtain visible light responsiveness, but none have successfully produced hydrogen using these catalysts. Previously, we developed a new method for the nanoscale processing of TiO2 particles as well as a new technique for adding a Pt co-catalyst to TiO2 particles [5]. Furthermore, TiO2 particles were processed by multifunction cavitation [6–11] in order to modify the surface morphology and electrochemical surface condition. It was determined that multifunction cavitation is effective at improving the photocatalytic properties of TiO2 under visible light irradiation.
