How to Increase Surface Emissivity & When It Is Essential?
We have always been fascinated by how engineering systems handle all sorts of critical environmental challenges.
Whether it is NASA dealing with high temperatures (up to 1000 °C) caused due to friction between a space vehicle surface and atmosphere at a hypervelocity, or challenges in achieving accuracy of spectrometer, telescope or microscope, it is essential to know how thermal radiation effects operation and efficiency of a system.
Radiative heat transfer is relevant in a wide variety of industries ranging from sensing and imaging, metallurgy, manufacturing processes, semiconductor, solar, medical, automotive, and many others. Moreover, it plays a significant role in vacuum and space industry.
That is where high emissivity coating helps.
Almost all the space organizations are using specially developed coatings in their launch vehicles and satellites. Example, to protect a spacecraft during atmospheric re-entry, high emissivity coatings play a crucial role in heat shield designing.
Now, for your space or other industrial applications, if you are looking for coating with properties such as stability, low outgassing and low reflectivity at high temperatures, there are a few specially developed coatings available (like coating solutions by Acktar) that enable you to design thermal control into your systems efficiently.
Acktar coatings are suitable for metals and alloys, ceramics, glass, silicon as well as polymers. So if you want to achieve stray light absorption suppression or passive thermal management, black coating is the best possible option.
It also helps in thermal cycling, offering stability and compatibility with other neighboring materials.
You can also use high emissivity coating to keep a room-temperature electronics box from overheating to cool down detectors and optical elements to cryogenic temperatures. It also makes it easier to measure temperatures of low emissivity materials, like gold, silver, and aluminum with a radiation thermometer.
Example: Using PVD or vacuum deposition for Applying special high emissivity coating to an aluminum sheet will increase its emissivity by (based on the source of standard emissivity calculate the difference of the emissivity of a polished aluminum, Anodized aluminum Sheet, Chromic Acid Proc and a sheet of aluminum coated with Acktar’s foil or coating in %).
How to achieve high emissivity?
There are four standard methods to increase emissivity:
- Vacuum Coating
- PVD Coating
- High Emissivity Paint
- High Emissivity Foil
Let’s analyze each one of them.
1. Vacuum coating
Vacuum coating can deposit layers of an extensive range of material.
Deposition can be as thin as one atom to as thick as a few millimeters of multiple layers of either elemental or compound material.
This method also offers non conducting material coatings such as plastic, fiber, and wood.
Desired properties can be achieved by altering the composition of the coating material and the structure of the layers.
If you are looking for a very low outgassing and low reflectivity in your applications, we have recorded a few videos to help our clients. You can watch them here.
Applications of vacuum coating include tools for machining applications, decorative coating for plumbing and other hardware, lamp reflectors and trim applications in automotive, tribological coating, corrosion protection, glass and solar absorbing coating in energy conservation & generation.
2. PVD coating
Physical vapor deposition (PVD) coating is carried out in a high vacuum chamber between 150 and 500 Degrees C at 10-2 to 10-4 millibar.
You can coat on a wide range of substrate and surfaces with almost all types of organic and inorganic materials.
In this method, the material goes from a condensed phase to a vapor phase and then back to a thin film condensed phase.
So if your requirement includes sustainability at high temperatures, strength, durability, highly resistant to scratches and scrapes, PVD coating would be a good fit.
It also offers a brilliant decorative finish with color that does not fade quickly, wear and oxidation resistance and hence suitable for door handles and plumbing fixtures, and many other household items.
PVD coating is also widely used for optical applications, solar panel, high-performance aerospace and automotive component, surgical and medical implants that have the highest degrees of purity requirements.
3. High Emissivity Paint
It’s thermal conductivity ranges from 0.52 W/mK (at 100°C) to 2.05 W/mK (at 700°C), coating thickness around 150 μm and spectrum-dependent transmissivity of 1-2 %.
Some of the unique properties of high emissivity painting include high-temperature tolerance, extremely UV resistant, low weight, and durability. That makes it a preferable coating method for aerospace parts, military, and testing applications.
High emissivity paints are also widely used for parts of automotive and electrical equipment, metal parts of industrial machines, heat resistant building materials.
These paints exhibit properties such as non-toxic, cold to the touch, smooth and exhibit excellent adhesion and flexibility, even on plastics and rubber.
You can also measure non-contact surface temperature.
These paints usually have high outgassing properties, thus not used in areas close to optical parts. Even the best coatings have some non zero outgassing.
4. High emissivity foil
If you have a substrate that you cannot ship to a vendor for coating and want to get it done in-house (DIY), then high emissivity foil is the go-to place.
High emissivity foils exhibit the lowest reflectance, excellent band performance – UV through MWIR, lightweight aluminum construction, high abrasion resistance. Based on your requirement specifications, the price of high emissivity foil ranges from $40 to $4000.
You can easily get a thin, flexible foil that is ready-to-use with low outgassing adhesive for your metal or plastic parts.
Acktar’s high emissivity foils are easy to handle and install, and it can be attached to any shape – flat, concave, convex. They are primarily used for laser beam dump, projectors & displays, baffles & apertures spectrometers, as well as for thermal management.
Today it is easier than ever to alter the emissivity of a surface. It offers improved performance, enhanced capabilities of parts of your equipment or system.
You can use any of the methods based on your requirement specifications.