Analysis of Key Technologies for the Preparation of Transparent Ceramics

Can ceramics be transparent? Yes, you heard it right! We materials scientists are so ingenious. Traditional ceramic materials are all opaque, like the porcelain bowls we use at home. However, transparent ceramics have very promising and attractive application prospects in "high-end" fields such as national defense industry (such as transparent armor ceramics, transparent scintillation ceramics for medical instruments). Therefore, our research is always in line with the needs of the motherland and the people. Thus, many ceramic material experts and scholars have been thinking about how to make ceramics transparent.

January 28th

Figure 1 Transparent ceramic samples and their microstructure (Figure taken from the webpage of Zhou Youfu's research group at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences)

Up to now, our country has achieved a series of results in the research of transparency. The domestic "specialists" in transparent ceramics include: Shanghai Institute of Ceramics, Chinese Academy of Sciences (with the Key Laboratory of Transparent Optical Functional Inorganic Materials of the Chinese Academy of Sciences), Wuhan University of Technology and other institutions. Alright, let's stop the "foreplay" here! Don't worry, dear readers, the editor will now get to the point.

From traditional opaque to semi-transparent and then to nearly completely transparent ceramics, a large number of fundamental scientific issues are involved, especially the microstructure of materials. To prepare transparent ceramics, first of all, we need to study why ordinary ceramic materials are opaque. This can serve as a very good "negative example" for us to prepare transparent ceramics.

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Figure 2 The propagation process of light in polycrystalline ceramics

Ceramics are formed by sintering powders and are polycrystals composed of grains, grain boundaries and pores. Under the illumination of light, light absorption, reflection, refraction and scattering occur at the grain boundaries and pores. Therefore, from an optical perspective, polycrystalline ceramics are generally opaque. So, how can we prepare highly transparent ceramics? By comparing the characteristics of the "negative examples", the key points for the preparation of transparent ceramics can be concluded as follows:

I. Powder Raw Materials

The purity of raw materials is one of the main factors among many that affect transparency. Therefore, to prepare high-quality transparent ceramics, one must be a "master of powder making". The purity of the powder should be high; if the powder contains impurities, they will become the scattering centers of light, greatly reducing the transparency of the prepared ceramics.

The particle size of the powder should be small. When the particle size of the raw material is very small, it is in a highly dispersed state. During sintering, fine particles can shorten the diffusion path of pores. The finer the particles, the better the effect. It reduces the scattering and reflection of light at the pores, thereby improving the transparency of the ceramic. Nanomaterials have also greatly promoted the development of transparent ceramics.

To obtain transparent ceramics, sometimes additives are needed to inhibit grain growth. The amount of additives is usually very small, so it is required that the additives be evenly distributed in the material. In addition, the additives should be completely soluble in the main crystal phase and not form a second phase substance; otherwise, the added additives may "backfire". Therefore, the use of additives can be said to be an art.

II. Firing System

A well-designed firing system is aimed at eliminating the pores inside ceramics as much as possible (the "damn" pores have the greatest impact on the light transmission performance of transparent ceramics), and improving the density. The sintering system mainly includes sintering temperature, holding time, cooling rate, etc.

When sintering transparent ceramics, the maximum sintering temperature should be determined based on the properties of the sintering material and the properties and size of the green body. During the sintering of transparent ceramics, the heating rate must be controlled to ensure uniform heating of the entire green body, control the crystal growth rate and grain size, and achieve the purpose of eliminating pores. The selection of holding time can be based on the size of the grains and the presence or absence of pores, and the determination of the cooling system should be based on the condition that the ceramic has no deformation and no internal stress. Ordinary sintering techniques cannot meet such requirements, so various advanced sintering equipment and technologies are needed, which will be discussed later.

This is not enough! Finally, it needs to be fired in a vacuum, hydrogen atmosphere or other atmospheres, so that the pores in the ceramic sintered body can be replaced and diffuse quickly, thereby achieving the purpose of eliminating pores. Using this sintering method can make the ceramic transparent.

III. Grain Boundary Structure and Surface Roughness

Grain boundary structure and surface roughness are also factors that need to be considered in the preparation of transparent ceramics.

When incident light enters a grain, it will encounter a grain boundary, resulting in refraction and reflection. If the refractive index of the grain boundary is the same as that of the grain, no refraction or reflection will occur according to relevant formulas. If there is a certain difference in refractive index between the grain boundary and the grain, it will affect the transmittance. Anisotropic crystals have a birefringence effect, and reflection loss will occur at the grain boundary, reducing the transmittance. For cubic crystal structure ceramics, due to their isotropy, light entering the ceramic interior will not produce a birefringence effect. Therefore, many transparent ceramics are cubic crystal systems. With the development of technology, researchers are developing non-cubic crystal system transparent ceramics.

From the perspective of surface roughness, when light is incident on a rough surface, it will undergo diffuse reflection. The greater the surface roughness of sintered ceramics, the lower its transmittance. These problems can be solved by grinding and polishing the ceramic surface.

After introducing the key points of transparent ceramic sintering... let's take a look at the sintering technology.

Transparent ceramic sintering technology

Hot-pressing sintering

Hot-press sintering involves applying pressure while heating the powder, so the sintering process mainly depends on plastic flow rather than diffusion. Compared with normal pressure sintering, hot-press sintering of the same material requires a much lower sintering temperature and results in a lower porosity in the sintered body. Additionally, sintering at a lower temperature inhibits grain growth, leading to a dense sintered body with higher strength. However, hot-press sintering has disadvantages such as long heating and cooling times, the need for post-processing, low production efficiency, and the inability to produce products with complex shapes.

Figure 3 Schematic Diagram of High-Temperature and High-Pressure Sintering Equipment

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2. Hot Isostatic Pressing Sintering

It is a production technology that integrates high temperature and high pressure. The heating temperature is usually 1000 to 2000℃. By using high-pressure inert gas or nitrogen in a sealed container as the pressure transmission medium, the working pressure can reach 200MPa. Under the combined effect of high temperature and high pressure, the processed parts are evenly compressed in all directions. Therefore, the processed products have high density, good uniformity and excellent performance. At the same time, this technology has the characteristics of short production cycle, few processes, low energy consumption and small material loss.

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Figure 4 Effects of Different Pre-sintering Temperatures on the Preparation of Transparent Ceramics

3. Vacuum sintering

The gases such as water vapor, hydrogen and oxygen contained in the pores of oxide ceramic green bodies can escape from the pores through dissolution and diffusion along the grain boundaries or through the grains during sintering. However, gases like carbon monoxide, carbon dioxide, especially nitrogen, with low solubility, are not easy to escape from the pores, resulting in pores in the products and a decrease in density. If the green bodies are sintered under vacuum conditions, all gases will escape from the pores before the green bodies are completely sintered, making the products free of pores and thus increasing the density of the products.

4. Spark Plasma Sintering

Spark plasma sintering (SPS) is a sintering technology developed in the 1990s. The SPS device is very similar to a hot-pressing sintering furnace, except that in this process, a controllable pulsed current is applied to a pressure-bearing and conductive mold. The pulsed current passes through the mold and the sample itself, and a portion of it penetrates the gap between the sample and the mold. The portion of the current passing through the sample and the gap activates the grain surfaces, breaks through the residual gas in the pores, causes local discharge, and even generates plasma, promoting local bonding between grains. The portion of the current passing through the mold heats the mold, providing an external heat source for the sample. Therefore, in the SPS process, the sample is heated both internally and externally, and the heating can be very rapid.

Moreover, since only the mold and the sample get heated after they are connected, and they are rapidly cooled once disconnected, the cooling rate can reach over 300°C per minute. The advantage of the SPS sintering technology lies in its rapid heating characteristic, which is conducive to controlling abnormal grain growth. Meanwhile, the pressure applied by the mold promotes the densification of ceramics. However, its drawback is that due to the fast heating rate, the holding time is relatively short, making it difficult to completely eliminate pores.

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Figure 5 Schematic diagram of spark plasma sintering equipment

5. Microwave sintering

A new rapid sintering technology that achieves densification by heating ceramic and its composite materials as a whole to the sintering temperature through dielectric loss of materials in microwave electromagnetic fields. Microwave sintering is fast and takes a short time, thus avoiding abnormal grain growth of ceramics during the sintering process, and ultimately high-strength and high-density transparent ceramics can be obtained. The key in the microwave sintering process is to ensure the temperature uniformity of the sintering sample and prevent local thermal fracture.

Summary

The main focus is on controlling the microstructure of the sintered body, especially the porosity.

The cost of preparing transparent ceramics is among the highest of all ceramic materials, if not the highest. The average person can't afford it. The equipment mentioned above is all very expensive. During the process of preparing transparent ceramics, one has to be extremely careful. Even the slightest flaw can lead to the failure of the experiment. But taking on high challenges is the nature of us materials scientists. When we see the transparent ceramics we made presented before us, the sense of pride and achievement is something ordinary people can't enjoy. Finally, I'd like to add that the porosity of transparent ceramics generally needs to reach 95% or even higher. So, come on, fellow researchers in transparent ceramics!