The challenges of growing the world’s largest sapphire slab

Can optical quality sapphire be grown in a different way to create a large, thick window to protect equipment on the belly of an aircraft?

This is the question that members of the Rubicon Technology design team were asked when the United States Air Force Research Lab (AFRL) called upon them about three years ago to develop a new growth platform that could produce very large polished sapphire infrared (IR) windows.

In the past, these dimensions were met by piecing together several smaller sapphire windows, with less-than-desired performance.

Despite high interest from the defense and aerospace industries, no method for producing sapphire for optical-grade windows of such great size and thickness existed at the time. Sapphire had been limited to smaller sizes and shapes using traditional growth methods. Rubicon studied the problem and agreed that it could be done. In 2012, the Air Force Research Laboratory granted Rubicon a three-year research contract to develop this product.

With several “firsts” already under its belt, such as having been the first to commercialize 6-inch and 8-inch wafers for the LED industry and the first to produce large-diameter patterned sapphire substrates, Rubicon has now successfully become the first to grow a 36 x 18 x 2-inch sapphire slab.

Drawing upon its strong history of scaling up sapphire growth processes, Rubicon successfully produced large sapphire blanks by highly modifying the Horizontal Directional Solidification (HDS) process and developing LANCE. Unlike the ES2 method, which produces high-quality sapphire crystals in the shape of roughly cylindrical boules, the HDS method produces sapphire plates that are advantageous for shaping into large windows.

Scaling up to larger crystal sizes while maintaining crystal quality was, of course, the real test and involved development of a new type of furnace.

Devising how to maintain crystal quality at such a large size proved to be experimental and time-intensive for the designers. By combining multiple platforms, materials and heater types, Rubicon ultimately demonstrated the ability to pull a horizontal boat through a heater surrounded by thermal insulation to achieve the desired thick slab while maintaining its high quality.

Over the past two years, the design team has continued to tweak and perfect the LANCE process, keeping the ultimate goal in mind of producing a sapphire slab with the cut measurements of 36 x 18 x 2 inches. The project is on track to produce optical quality sapphire at this immense size.

Looking to the future, this monolithic sheet is a hugely innovative advance that will provide a large, seamless optical field for military sensing applications and beyond.

For more information on the LANCE project, visit http://www.rubicontechnology.com/research-development/project-lance-very-large-sapphire-windows.

Case Number 88ABW-2015-1571

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

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