Group Leader Koji Watari, Senior Researcher Masaki Yasuoka, and Expert Researcher Takashi Shirai of the Advanced Sintering Technology Group at the Advanced Manufacturing Research Institute (Director: Hideto Mitome) of National Institute of Advanced Industrial Science and Technology (President: Hiroyuki Yoshikawa) (hereinafter referred to as AIST) have developed a new process that permits the manufacture of ceramic products with excellent shape retention without the use of an organic binder; the process involves facilitating the hydration reaction of ceramic particles by irradiating them with microwaves.

In the normal process for forming ceramics, organic compounds are added as binders (binding agents) to maintain the shape of the compact. When the compact is heated during a subsequent calcination process, the binders vaporize and decompose, producing emissions of toxic gases and greenhouse effect gases, such as carbon dioxide. These emissions are a serious problem, so reducing the amount of binder that is used is a highly desirable objective.

Our newly developed technique involves facilitating the hydration reaction between the moisture that is present in a wet compact and the surface of the particles by heating the compact by microwave irradiation for about 15 minutes. This generates hydrates or hydroxide groups on the surfaces of the particles, causing them to bind together tightly (figure) without the need for an organic binder.

AIST is attempting to simplify and shorten the processes that are involved in manufacturing ceramics, from forming to sintering, to reduce the environmental burden produced by the industry. As part of this research, we are developing a technology that utilizes the unique reaction field that is present in the electromagnetic environment created by microwave radiation.

In conventional processes for the production of ceramics, particularly in the wet forming method, large amounts of organic binder (10-15 wt% of the raw materials of ceramics) are used to maintain the shapes of compacts during manufacture.

These organic substances in these compacts have to be completely burned away during later stages of ceramic manufacture. This results in an environmental burden because, besides consuming large amount of energy, burning of organic substances generates carbon dioxide and toxic gases that exacerbate global warming and air pollution. With a backdrop of this situation, there is an urgent need to develop a ceramics manufacturing process that realizes tighter bonds between particles to give shape retention without the need for an organic binder.

In the conventional wet forming method, it is necessary to add large amounts of organic binders to ensure a high degree of formability and of shape retention after shape forming, but the use of organic substances requires operations such as degreasing in the steps following calcination. The development of a binder free forming technology would make it possible to eliminate the degreasing step, simplifying the manufacturing process, and would open the road to a manufacturing process that requires less energy and fewer resources and generates a smaller environmental burden.

In this research, we found that irradiating a ceramics sample with microwaves facilitates the hydration reaction between the moisture that is inside the compact and the surfaces of the particles, and controls the process of creating hydrates on the particle surfaces. By using this technology, we succeeded in creating hydrates or hydroxides on the contact surfaces, causing the particles to bind tightly together. The photograph shows alumina compacts created by this process that were left in water for 30 days at normal temperatures before calcination. Whereas a sample created by the conventional binder free method broke up in water as a result of the weak binding force between the particles, the sample created by the newly developed method retained its shape for as long as 30 days, even in water.

The new technology makes it possible to manufacture mechanically strong compacts with excellent shape retention without the use of an organic binder and to produce high quality sintered ceramic compact with few of the defects (voids or cracks) that are attributable to organic substances. The process decreases the energy requirements for ceramic manufacturing and cuts the environmental burden arising from the generation of carbon dioxide and toxic gases. At the same time, it has the advantage of decreasing the incorporation of impurities, because it uses the hydrates or hydroxides on the surfaces of the ceramic materials as the binding agents.

 

We will conduct further research to make the technology more versatile and easier to handle, in addition to pursuing cost reduction. We will also make further efforts to apply it as part of a unified process from forming to calcination.