We are fresh off our stimulating attendance of the 43rd International Conference and Exposition on Advanced Ceramics and Composites, or ICACC19. Over five exciting days, we took in many exciting talks and showcases by research and industry leaders. We also did our part in demoing and testing cutting-edge ceramic products, and we cannot wait to share the scoop with you.

In the following blog post, we will:

• introduce you to the fascinating world of ceramic science;
• explain exactly what makes advanced ceramics such as translucent alumina and transparent alumina a potential game-changer;
• prove that vacuum furnaces and vacuum sintering are essential to manufacturing high-quality ceramic product.

We begin our trip into the realm of advanced ceramics with a quick rundown on its calling-card event, the ICACC.

ICACC19: research and industry meet to drive ceramics innovation

Innovation is the chief source of added value and progress in our societies today, and it largely depends on monetary and brain capital from universities and industry leaders. It is all too often, however, that the two branches of innovation do not get to speak to one another and, thus, miss out on mutually beneficial synergies and knowledge transfer.

This is where the ICACC19 comes in.
It creates a space for scientists, research and development specialists, engineers, and industry representatives to come together and take a deep dive into the latest advances in ceramics research.

The 43rd edition of ICACC focussed on advanced ceramics, bioceramics, and ceramic coatings, among many other thrilling developments in the field. In the framework of seventeen symposia, five focussed sessions, and a special forum for young investigators, ICACC19 was an exciting event on a topic of continued importance.

But what exactly are advanced ceramics and why should you care?
Continue reading and prepare to have your professional and knowledge horizons expanded!

Advanced ceramics: strong, versatile, affordable

Much of recent developments in ceramics research focus on translucent alumina, or see-through ceramic composites. They have a range of favourable mechanical properties, including:

• exceptional hardness
• resistance to heat and corrosion
• various degrees of transparency
• high added value in a range of applications

Thus, transparent ceramics find applications in almost any environment where highly resistant and reliable transparent media are required. From advanced airborne optics to strong yet practically invisible orthodontic brackets, translucent alumina are part of our daily lives already.

The possibility to form complex and intricate shapes is what makes translucent alumina so desirable. Achieving equally great shape precision and transparency at both small and large dimensions used to be quite expensive (think: single crystal growth).

Thanks to recent developments in ceramic science, the costly fabrication procedures are a thing of the past, and we can produce a wide range of robust transparent ceramic items. But how do we achieve translucency in a material that is usually opaque?
It's all about reducing structural defects.

Translucent alumina and the fight against imperfection

When you think of ceramic objects, 'transparent' is usually not a quality that comes to your mind. This is because conventionally sintered (compression-baked) ceramics contain constellations of minor structural defects. While these imperfections do not significantly compromise the ceramic objects' integrity, they do make them opaque.

Achieving ceramic translucence, therefore, entails a battle against structural heterogeneities. While a perfect structure is hardly attainable, keeping problematic spots small and dispersed is key.

• For applications in the visible spectrum, ceramic heterogeneities cannot exceed 50 nm. In the ideal case, they will stay below 10 nm.
• For applications in the infrared spectrum, on the other hand, the tolerance for heterogeneity is much higher and can extend to 1µm.

It follows, then, that transparent ceramics, much like people, are not perfect, but the truly good ones have flaws that are small and far between. How much you put up with depends on what your goals are. Unlike with people, however, you are in full control of how much imperfection you will allow and how clearly you will be able to see through the final product.

Transparent alumina are all about the sinter

The translucency of advanced ceramic composites can only be achieved through a carefully controlled sintering process that keeps in mind two important material properties:

• The size and concentration of the pores
• The fineness of the grain

Since translucent alumina is an anisotropic material, it possesses a grain that gives it its characteristic strength. However, to keep its transparent properties, that grain has to be kept very tiny, and the amount and density of pores in the material has to stay low as well.

Controlling for both of these structural variables is a formidable task. As we mentioned earlier, most sintering processes do not manage to keep grain size and pore concentration at bay, resulting in ceramics that are not so strong and completely opaque.

So, what is the solution to this million-dollar question?
Hint: we have it right in our company's name!

Vacuum furnaces: the best way to streamline translucent alumina production

Above, we explained why a regular sinter yields opaque and brittler ceramic components. They are certainly good enough for everyday household use, for example, but most industrial applications require a much more sophisticated approach.
Enter the all-powerful vacuum furnaces!

Vacuum sintering is the key to controlling the three main parameters that influence the quality of transparent ceramics:

• Sintering temperature;
• Maintenance time at high temperatures;
• Heat-treating atmosphere.

Vacuum sintering has a particularly strong positive effect on the heat-treatment atmosphere. It reduces oxygen levels drastically and eliminates much of pore formation and other impurities within the material.

But how much vacuum is enough to yield these positive effects?

More vacuum, more translucence

An experiment revealed that low vacuum (around 10^-3 mbar) has a notable positive effect on stemming pore formation; it is enough to push air and other absorbents out. However, translucent alumina sintered in low vacuum consistently fall short of the desired optical quality standards.

High vacuum sintering at 10^-6 mbar, on the other hand, gets the job done. It keeps pores out and grain sizes sufficiently small. It beats atmospheric sintering by a long shot, especially when it comes to keeping impurities out.

Keeping Impurities at bay promotes a finer grain

Atmospheric sintering is known for the higher grade of impurities that make it into the final product. Si, Ca, K, and Mg are just some elements that can be incorporated from the walls of the furnace into the alumina during the various preparatory stages or during sintering. They, along with water vapor from the atmosphere, have a deleterious effect on grain size.

Vacuum sintering minimizes these foreign substances thanks to the continuous pumping-out process that occurs in vacuum furnaces. By removing loose atoms and keeping oxygen pressure low, vacuum sintering establishes firm control over grain growth.

At 1350°C and 10^-6 mbar pressure, vacuum-sintered alumina can reach optimal purity, minimal pore density, and minimal grain size - all the qualities that make it strong and translucent for a variety of industrial applications.

The following graph shows how vacuum sintering implies higher transmittances over a wider spectral window with respect to atmosphere treated components.

Red curve: alumina transmittance spectrum after atmosphere sintering.
Blue curve: alumina transmittance spectrum after high vacuum sintering.

Conclusion

In our little crash course on advanced ceramics, we poured the inspiration we got from the great ICACC19 conference and expo into a rundown on what makes translucent alumina such a great material. We described the process of creating this clear and robust substance and the pivotal role vacuum furnaces play in it.

Thanks to this know-how, translucent alumina emerge as an affordable and reliable alternative to the expensive and cumbersome production of intricate parts, both large and small.

Have you already thought about integrating transparent ceramics into your products?
They are definitely worth your while!

Have you missed ICACC19?
Take a look at our presentation slides here below (after the free ebook chapter).

Download the FREE eBook