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SilcoTek® CVD coatings, such as Dursan® and SilcoNert®, can be used for sulfur analysis in metal capillaries and analytical columns and process systems. This article will discuss how to use them to improve the inertness of metal filters and glass frits to chemicals and corrosion without clogging the filter. 

Sintered metal frit is produced by heating compressed metal powder to fuse powder particles together, thereby providing a complex solid with embedded channels and pores. The porous structure of the sieve plates means that they can be used to hold particles in one place in an industrial process or as a sieve to filter out certain particles.

Although glass frits are mechanically strong, their metallic structure usually means that they can interact with the mixture they filter, making them unsuitable for industrial use. This interaction includes the absorption of reactive particles into the internal structure of the frit or adhesion of particles to the surface of the frit.

SilcoTek provides coatings that are inert, corrosion-resistant and anti-fouling, and can act as a barrier to protect metal frits from reactive chemicals.

The sieve plates are regularly coated to avoid interaction with the chemicals they filter. Commonly used coatings include PVD, epoxy, and paint; however, there are challenges in ensuring that the entire inner surface of the glass frit is evenly coated without clogging the pores-this prevents the glass frit from acting as a filter.

This article will discuss SilcoTek's CVD coating method and how it avoids these problems.

A previous study conducted by SilcoTek explored the coating of 2 µm sintered stainless steel frit with SilcoNert® 2000. The study showed that the internal structure of the glass frit was completely coated with a layer with a thickness of 50 to 80 nm.

Dursan can also be used as a coating for sintered materials, and is used in analytical systems due to its chemical and corrosion inertness, and in coal gasification systems due to its anti-coking properties.

Both SilcoNert and Dursan use different precursor gases, so the question remains – does Dursan coat the internal structure of the sintered material like SilcoNert?

The most common question when using CVD coating is whether it will penetrate all glass frit-this is because most coating techniques are "line of sight", meaning that they only cover the outer surface.

As shown in Figure 1, SilcoTek's CVD coating is out of sight and can coat the internal network of glass frit (including blind holes), while other PVD coatings can only coat the direct target area of ​​the coating deposition source.

Figure 1: CVD-coated parts will have a conformal coating around the entire part, while parts coated by "line of sight" methods (such as PVD) will only coat the area facing the coating source. This will leave uncoated areas and exposed to the environment around the part.

Most PVD coatings provided by SilcoTek are several hundred nanometers thick. For researchers who want to use PVD ​​coated glass frit, this may cause some anxiety, because: if the glass frit has 2 µm wide pores, the PVD ​​coating may occupy a large part of the space . This may limit the size of the hole, creating back pressure when the frit is running.

However, this specification of a few hundred nanometers refers to coating a flat surface. For the coating inside the glass frit (with a large surface area: volume ratio), the coating thickness on the internal structure is much smaller. For example, when using SilcoNert 2000 to coat glass frit with a pore size of 2 µm, the coating thickness on the inner surface is 50 – 80 nm (measured with Filmetrics F40).

It should be noted that in order to achieve the same flow rate through the coated frit, a slightly larger pressure may be required, but a significant increase is not required. You can use flow or bubble point tests to assess increased pressure requirements.

When coating amorphous silicon with a thickness of less than micrometers, a small change in thickness will cause a significant change in the color of the coating.

If the flat surface is exposed to high-temperature coating process gas, physical phenomena (including temperature changes, airflow dynamics, and small changes in surface morphology) can cause changes in coating growth. For this reason, typical coatings can give the surface a rainbow appearance.

Figure 2 is a cross-sectional photo of the glass frit after decomposition in the middle, showing three different colors-blue (80 nanometers thick), pink (65 nanometers) and gold (50 nanometers). It is confirmed by Raman analysis that it is the same coating material in the entire glass frit (amorphous silicon, the material used in Dursan and SilcoNert).

Figure 2 shows a cross-sectional photo of the glass frit after it has been split into two halves. There are three different colors in the glass frit: gold measuring ~50 nm, pink measuring ~65 nm, and blue measuring ~80 nm. Raman spectroscopy is performed to ensure that the material deposited on the innermost part of the frit is the same as the material on the outside.

It has been confirmed that the coating is an amorphous silicon coating, which is the reason for our SilcoNert 2000 and Dursan coatings.

Figure 2: Cross section of glass frit coated with SilcoNert 2000. Although there is a thickness gradient from the center to the edge of the frit, the difference in coating thickness from the center to the edge is only about 30 nm.

Dursan is used to coat 2 µm stainless steel frit using SilcoTek's method. After coating, the glass frit is folded in half to expose the internal structure, which is then imaged using electron microscopy (Figure 3).

As mentioned earlier, this complex network will be difficult to use line-of-sight technology (such as PVD, epoxy, paint) for effective coating. SiloTek's method is not line-of-sight and can reach the internal network of the sintered frit.

Figure 3: Electron micrograph after the sintered glass frit is coated and divided into two halves to expose the inside of the part for analysis.

SilcoTek customers often care about ensuring that all glass frits are coated, as uncoated areas may cause system defects or malfunctions. Dursan contains silicon and oxygen, and the glass frit contains iron, which means that EDS can be used to map where the coating occurs.

Figure 4 shows the silicon, oxygen, and iron diagrams using EDS, which has been superimposed on Figure 3. The figure shows good penetration of the entire glass frit. Figure 4 does show some areas without silicon or oxygen signals, but these areas also show no iron signals, indicating that this is the result of shadows (due to rough surface) rather than coating gaps.

Figures 4 and 5 are high-magnification electron micrographs and EDS images of the center of Dursan coated glass frit. The high magnification allows the coating thickness to be determined.

The result is similar to the SilcoNert 2000 coating, which provides a thickness ranging from 80 nm on the outer surface to 60 nm at the center. The lower thickness in the center is due to the highly complex internal structure of the glass frit.

Figure 4: An electron micrograph of the internal network of a 2 µm sintered glass frit at a higher magnification.

Figure 5: EDS image of silicon in sintered glass frit under higher magnification.

As demonstrated, the new CVD coating technology used by SilcoTek can be used to consistently coat the internal structure of sintered materials. SilcoTek's coatings (Dursan and SilcoNert) are more suitable for this application, while PVD and other line-of-sight coatings are poor at accessing the complex internal structure of the glass frit.

Dursan and SilcoNert 2000 have been proven effective by applying both coatings to a 2 µm stainless steel frit. The coating coats all glass frit uniformly and conformally, including its internal structure, providing a barrier of sufficient thickness that does not block pores (which can cause pressure problems), but protects the material from the surrounding environment.

The coating process used provides uniformity of the coating, with a difference of only 30 nm between the thinnest and thickest coating areas. Researchers who use glass frits that experience service life or activity issues should consider using SilcoTek's coatings to protect their glass frits and obtain the best performance from it.

This information is derived from materials provided by SilcoTek and has been reviewed and adapted.

For more information on this source, please visit SilcoTek.

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