Sapphire Quality Matters: Part 1

Sapphire is an extremely versatile material with a growing list of applications in a wide range of industries.  Sapphire suits optical applications because of its scratch resistance and its transmission characteristics.  You’ll find sapphire components such as lenses and windows in medical equipment, lasers, satellites, aircraft, flame detectors, smart phones, cameras and watches.  Recent advances in sapphire crystal growth technology and fabrication have improved the performance, purity, and availability of sapphire for all types of applications.

Recently, Novus Light Today published an article by Dr. Jonathan Levine, Director of Technical Business Development at Rubicon Technology, about sapphire quality.  His article gives a thorough review of the measures of sapphire quality for optical applications.  Levine writes that the quality of a sapphire is determined by how closely the grown crystal matches the ideal structure with respect to the arrangement of atoms within the lattice, dislocations, defects, and stress.  Root causes for these problems often originate from insufficient purity of the starting material and the growth process itself.

The effects of these variables in the final product are commonly quantified by three metrics: chemical analysis, X-ray rocking curves, and optical transmission.  Additionally, the observance of bubbles in the crystal provides a baseline from which crystal quality is determined because bubbles serve as scattering centers for any light transmitted through a sapphire optic, thus reducing its performance.

This week, we look at the first two metrics, chemical analysis and X-ray rocking curves.

Powdered aluminum oxide

Powdered aluminum oxide

 

 

 

 

 

 

Purity of the crystal is highly important.  According to Levine, the presence of certain elements can vary drastically between suppliers, and sapphire manufacturers must exercise proper quality control.  For example, titanium (Ti) and chromium (Cr) impurities can result in pink crystals.  In nature, these impurities lead to rubies and other variations of sapphire depending on the impurity.  Levine says trace amounts of these elements must be kept below 1 ppm.  Levine includes a graphic about other elements that can cause issues including silicon (Si), potassium (K), chlorine (Cl), iron (Fe), lithium (Li), and sodium (Na).  The data was collected using glow discharge mass spectroscopy (GDMS).

Typically, a company can buy two types of raw material for crystal growth that can have impurities.  Levine says it can be purified alumina powder and/or Verneuil sapphire.  Rubicon has developed a new in-house purification process that converts the raw powder into densified pellets for crystal growth without an increase in cycle time or decrease in crystal yield. This process enables Rubicon to eliminate impurities in the alumina power that they use to make crystal.

Levine includes another useful metric for analyzing sapphire, rocking curve data obtained via X-ray diffraction.  A rocking curve helps measure various stresses in a crystal.  Levine says the width of the resulting peak is highly sensitive to strain and defects within the crystal.  A narrow peak, indicated by its full width at half maximum (FWHM) measured in arcseconds, signifies a high quality crystal free of low-angle grain boundaries and lattice strain.  A standard narrow rocking curve for Rubicon’s ES2 sapphire windows is shown below.

Sample rocking curve data from Rubicon ES2 sapphire.

Sample rocking curve data from Rubicon ES2 sapphire.

 

 

 

 

 

 

 

 

What can introduce a poor rocking curve?  Levine says that high thermal gradients, fast growth rates, and impurities contributed by the surrounding insulation can introduce defects and stress into the crystal that subsequently yield poor results in rocking curve data.  He adds that accurately controlling the temperature gradient and maintaining a stable growth interface throughout the entire process can help make higher quality sapphire.

For Further Reading

Novus Light Today, Optical-Grade Sapphire, Where Quality Matters, http://www.novuslight.com/optical-grade-sapphire-where-quality-matters_N1596.html#sthash.giGipxT1.dpuf

Rubicon Develops New Sapphire Growth Platform to Make Large Optical Windows

Sapphire Windows in Lockheed Martin f-35 Lightning Joint Strike Fighter

Sapphire Windows in Lockheed Martin f-35 Lightning Joint Strike Fighter

There are a lot of ways to make large commercial sapphire crystals.  But none of them really work for making large sapphire windows using sapphire’s a-plane. We mean really wide so that they can be used in a military plane as a targeting system window.  So, the Air Force Research Laboratory (AFRL) asked Rubicon Technology to develop a new growth platform for very large polished sapphire infrared (IR) windows.  The project is supported by a grant from the AFRL, with a total value of $4.7 million over three years.

Sapphire is perfect for military and other high-performance applications due to its hardness and strength, transparency in the visible and IR spectrum, thermal conductivity, thermal shock resistance, abrasion resistance, high melting point and chemical inertness.  The project will begin with intermediate sizes, then will step it up to windows as large as 36 x 18 x 0.8 inches.

Why do we need this new method?  Existing methods used to produce sapphire windows don’t meet the demands of military applications for thickness and size.  Rubicon’s new technological platform will grow panels approximately two inches thick, which can be cut into multiple windows of varying thickness.  The new method also allows growth of very large windows up to 36 x 18 inches. This is an improvement over smaller faceted windows that require piecing together for large applications.

The sapphire windows for the AFRL will be applied to fighter jet targeting systems. For example, the Lockheed-Martin F-35 Lighting Joint Strike Fighter’s fuselage has a durable sapphire window in the fuselage for use by the Electro-Optical Targeting System (EOTS).  The sapphire is preferred due to its durability, hardness and strength and the way it allows transmission light in the IR spectrum.

Links:

Lockheed-Martin F-35 Lightning II Joint Strike Fighter, http://airsoc.com/articles/view/id/4fe4b7f8c6f8fa2443000009/lockheed-martin-f-35-lightning-ii-joint-strike-fighter

Clearlysapphire.com. http://www.clearlysapphire.com/Optical__Lasers_Windows_.html

Rubicon Technology, http://rubicon-es2.com/index.php?page_id=23