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

New Applications for Sapphire: Aerospace & Defense, Part 1 of 3

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Range of sapphire products available from Rubicon Technology including large optical windows and other shapes for aerospace and defense.

Sapphire’s unique properties make it a perfect material for high-performance applications due to its optical transparency, physical strength, resistance to abrasion and corrosion, temperature durability, chemical inertness, and bio-compatibility. As a result, it is perfectly suited for extreme environments where material durability is just as important as optical clarity.

One extreme use case is in the aerospace and defense industry where there’s a need for rugged windows for targeting pods and missile domes, most notably for the US F-35 fighter jet, that may come in contact with harsh conditions from the harsh, gritty desert with extremely high temperatures to high altitudes with extreme low temperatures.

Market research firm Yole Developpement determined that non-substrate applications for sapphire in the defense, semiconductor and other applications represent 25% of the sapphire industry revenue in a new study.  The market represents a solid growth opportunity for sapphire makers.

While there is opportunity, innovation is needed.  Sapphire traditionally has been limited to smaller shapes and sizes using traditional growth methods.  As sensor technology and applications, in defense and aerospace in particular, have evolved, the size requirements for sapphire windows have grown substantially.  One company that is innovating sapphire crystal growth is Rubicon Technology.

In a recent paper, Rubicon’s Dr. Jonathan Levine, Director of Technical Business Development, detailed how Rubicon successfully produced very large sapphire blanks using a highly modified horizontal directional solidification process. This new method, named the Large‐Area Netshape Crystal Extraction (LANCE) system is currently able to produce crystals of several different orientations. The company plans to produce sapphire windows as large as 36 x 18 x 0.8 inches.

For Further Reading

Clearlysapphire.com Blog, Opportunities for Sapphire: New Applications & Markets Explained, http://blog.clearlysapphire.com/?p=426

Clearlysapphire.com Blog, How Large Can You Go? Sapphire Windows Grow Up and Across, http://blog.clearlysapphire.com/?p=409

Rubicon Technology, Synthesis and characterization of large optical-grade sapphire windows produced from a horizontal growth process, http://www.rubicontechnology.com/sites/default/files/Synthesis%20and%20Characterization%20of%20Large%20Optical%20Grade%20Sapphire%20Windows.pdf

How Large Can You Go? Sapphire Windows Grow Up and Across

 

Dr. Jonathan Levine, Director of Technical Business Development, Rubicon Technology, at SPIE Defense, Security and Sensing 2013

SPIE Defense, Security and Sensing 2013 was held last week in Baltimore, MD.  There was a very interesting paper on large sapphire optical windows.  Rubicon Technology’s Dr.  Jonathan Levine, Director of Technical Business Development, talked about recent advances in making very large sapphire optical windows from his paper, “Synthesis and Characterization of Large Optical-Grade Sapphire Windows Produced from a Horizontal Growth Process,” that he presented at the conference.

As sensor technology and applications, in defense and aerospace in particular, have evolved, the size requirements for sapphire sensor windows have grown substantially. Dr. Levine detailed how Rubicon successfully produced very large sapphire blanks using a highly modified horizontal directional solidification process. This new method, named the Large‐Area Netshape Crystal Extraction (LANCE) system is currently able to produce crystals of several different orientations weighing up to 50 kg with plans to expand the process to larger 100 kg sapphire crystals.

Based on a different technique than Rubicon’s ES-2 method, Levine’s team used the LANCE method because it produces plates or slabs of sapphire instead of cylindrical boules.  Using LANCE is advantageous since the near-net shape dramatically reduces machining and associated fabrication costs.  Further, since the melt is horizontal, capillary forces play a minimal role during growth and the crystal is not limited in size or thickness as compared to edge‐defined film-fed growth (EFG) crystals.

According to Levine, Rubicon has been able to synthesize several prototypes up to 1.75 inches thick, 14 inches wide and 20 inches long. The research sets the standard for high quality monolithic sapphire sheets large enough for use as seamless integrated optical windows in both military and civilian applications.

Sapphire is very good for defense and aerospace applications due to its excellent transmission in the UV through IR spectrum, high impact durability, and corrosion resistance in harsh environments.  Sapphire applications include missile domes, transparent armor systems, FLIR and enhanced vision systems for aircraft, and shaped optics for land, air and sea vehicles.

For Further Reading

Rubicon Technology, Rubicon Technology Develops New Sapphire Crystal Growth Platform to Manufacture Large Optical-Grade Windows for Military and Industrial Applications, http://phx.corporate-ir.net/phoenix.zhtml?c=215281&p=irol-newsArticle&ID=1724966&highlight=

SPIE Defense, Sensing and Security 2013, http://spie.org/x6765.xml?WT.svl=mddce7