Emissivity

Measurements made by a variety of aerospace companies confirm Laser Gold’s theoretically low emissivity.  ETI has been able to modify, electrochemically, surfaces that are too delicate or too convoluted to permit physical polishing. The surface of machined materials, Be or Al, for example, can achieve a consistent emissivity between .03 to .05.

Indeed, the ability to achieve a repeatable emissivity value, whatever the substrate, has made Laser Gold the coating specified on an impressive array of military and space programs.

In 2021, Epner Technology designed a study to determine if the surface roughness of a material, in this case Titanium, has an effect on its Emissivity. We concluded that the roughness of the substrate did not fundamentally alter the Emissivity of the plated Laser Gold. All samples tested at greater than 97%. This results of our study and the accompanying matrices can be found at the links below.

Surface Optics Report

Testing Matrix

Sample Roughness Matrix

Emissivity values of Laser Gold courtesy of James Heaney, formally with NASA-Goddard Space Flight Center

The emittance of the gold plated Al samples were measured using a Gier-Dunkel Model DB-100 Infrared Emissometer/Reflectometer.  This instrument is located at NASA Goddard.

The instrument measures near-normal incidence, integrated room temperature, and relative emittance.  That is, the sample is kept at room temperature (~20C) and placed over an opening through which it alternately (~13 Hz) views two black body cavities at near-normal incidence.  These are heated slightly above room temperature, and differ from one another in temperature by about 15 degrees C.  Standard high reflectance and low reflectance samples are used to calibrate the instrument’s read out scale.  The reflectance (emittance) of an unknown sample is measured relative to these known standard reference samples.

The instrument produces a single number; integrated reflectance that is weighted for a room temperature black body.

(1 – reflectance_ = near-normal emittance at room temperature.

For a pure metal, such as gold, hemispherical emittance can be obtained from a theoretical ratio (ref. M. Jackob, “Heat Transfer”):

Hemispherical emittance = normal emittance x 1.3

The Gier-Dunkle DB100 instrument measurements have a precision better than 0.005.  That is, a group of samples whose emittance  varies by this amount or more can be easily ranked by emittance.  The absolute knowledge of the standard samples is always questionable, however, instrument precision is the key factor in distinguishing between ‘before’ and ‘after’ values.  Epner’s gold sample had a measured emittance (.02) that was equal to our gold reference standard to within +/-0.005.

 

 

Laser Gold:  Emissivity Project

University of Hawaii, Institute of Astronomy

An emissivity report was requested by the University of Hawaii, Institute of Astronomy prior to awarding Epner Technology, Inc. a contract for Laser Gold® coating the radiation shield for the Cryo-Camera on the Subaru and Gemini telescopes.  The report was produced at the Energy Materials Testing Laboratory in Biddeford, Maine.

The laboratory measured the hemispherical total emission which is derived from thermal emission of the sample integrated over all wavelengths and over all directions.  This is very different from the approximation for emissivity often used of (1 – reflectance).  This approximation is valid only if the reflectance measurement is made close to the wavelength where most of the energy is being emitted.  More than likely this will not be the case.

The emittance of the Epner sample is about 5% at room temperature.  This would normally be considered high since the reflectance of gold at 2 microns is about 98%, thus suggesting the emissivity should be 2%.  However this refers to a mirror surface.  We consider 5% to be realistic for gold-coated polished aluminum.

Project Comments

“We recently finished a cooled down test of our instrument with all of the gold-coated radiation shields installed.  The test was very successful in that we achieved the temperature that we had expected.  This was a critical test of our cooling system and we were very gratified to find it to perform as expected.  The success of this test is a tribute to the fine gold coating produced by your company.  It also confirms that the emittance (or emissivity) as measured was about right.  The temperature achieved was close to that predicted based upon the emittance measurements.  Finally, I should say that when all of the gold-coated shields are installed the instrument looks so beautiful that I have found myself staring at it in amazement.  Thanks for doing a great job.”

Alan Tokunaga, University of Hawaii, Institute of Astronomy

To view the full report: Click Here

Emissivity Qualification Comparison performed by a major aerospace company involving three plating suppliers and six different substrates
Low Emissivity Gold Plating Vendor and Targeting Hemispherical Emissivity of < 0.04

 

Substrate
Epner Emissivity (Hem)
Company B Emissivity (Hem)
Company C Emissivity (Hem)
 
1100 Al- Side A
 0.023 0.027 0.036
1100 Al- Side B
 0.017 0.030 0.036
1100 Al- Side A
 0.031
1100 Al- Side B
 0.034
_________
 __________ ________ ________
Average
 0.02 0.0305 0.036
6061-T6 Al-Side A
 0.021 0.018 0.021
6061-T6 Al-Side B
 0.014 0.018 0.021
6061-T6 Al-Side A
 0.027
6061-T6 Al-Side B
 0.021
_________
 __________ ________ ________
Average
 0.0175 0.021 0.021
6063-T6 Al-Side A
 0.029 0.011 0.029
6063-T6 Al-Side B
 0.02 0.011 0.031
6063-T6 Al-Side A
 0.014
6063-T6 Al-Side B
 0.014
_________
 __________ ________ _________
Average
 0.0245 0.0125 0.03
Beryllium – Side A
 0.023 0.041 0.043
Beryllium – Side B
 0.016 0.044 0.031
Beryllium – Side A
 0.022 0.039 0.037
Beryllium – Side B
 0.017 0.044 0.037
_________
 __________ ________ _________
Average
 0.0195 0.042 0.037
Titanium 6Al4V – Side A
 0.049 n/a 0.032
Titanium 6Al4V – Side B
 0.020 n/a 0.032
Titanium 6Al4V – Side A
 n/a n/a n/a
Titanium 6Al4V – Side B
 n/a n/a n/a
_________
 __________ ________ ________
Average
 0.0345 n/a 0.032
Titanium 1581
 n/a 0.031 n/a
Titanium 1581
 n/a 0.030 n/a
Titanium 1581
 n/a 0.025 n/a
Titanium 1581
 n/a 0.021 n/a
_________
 __________ ________ ________
Average
 n/a 0.02675 n/a
Vendor Scores – Sum of Average Emissivitys (lowest is best)

0.1160

0.1328

0.1560
Company B
Best Titanium Vendor. Best 6063 Aluminum, Excellent 1100 and 6061 Aluminum. Failed Beryllium
Company C
Overall, highest emissivities (higher risk to processes). Passing for Aluminum and Titanium. Failed Beryllium
EPNER
Overall, lowest emissivities (lowest risk to processes), Best 1100 and 6061 Aluminum Vendor, Excellent Beryllium. Failed Titanium, may be due to substrate.
Failing Criteria
A single Measurement exceeding 0.04