Sapphire: The Clear Choice for Military Applications and Beyond

In case you missed it, something BIG is happening in the sapphire industry.

But just what are the practical applications for a massive slab of sapphire?

Military

First and foremost, sapphire is hard, and as a result, extremely durable. On a scale of hardness — the Mohs scale to be exact — sapphire is the second hardest naturally occurring material, next only to diamond.

Sapphire Infographic

This makes sapphire a clear choice over glass for use in harsh environments, such as deserts. Sapphire is harder than quartz (the main component of sand), and does not scratch as easily as glass would, making it ideal for a variety of applications that come in contact with sand. One that comes to mind is the lens of a military-issue scope. Currently, the military is beginning to swap out scopes that have lenses made of glass that scratch easily, in favor of sapphire lenses.

Besides being useful as a lens, sapphire is so hard it can be used as transparent armor — hard enough that it can even shatter a .50 caliber bullet if thick enough. This leaves endless possibilities for its use in tank windows, transport vehicles, etc.

Aerospace

Sapphire can withstand extreme temperatures without losing its integrity, making it ideal for space exploration. Some regions of space get less than I degree C above absolute zero.  On the other hand, during re-entry, NASA says the external surfaces of spacecraft can reach a temperature in excess of 1,500 degrees Celsius. With its ability to withstand extreme high and low temperatures, sapphire is an ideal material for use in the cones of spacecraft.

Along with the ability to withstand extreme temperatures, sapphire is also known for being “radiation hard,” meaning that sapphire does not break down when exposed to radiation. Once astronauts leave the protection of earth’s magnetic field and atmosphere, radiation becomes a concern. Most materials rapidly change chemical properties under radiation exposure, but because sapphire is less susceptible, it is well suited for space exploration.

Rocket

Oil and Gas

Nasty chemicals and harsh sea air take a toll on glass and metals, but sapphire is chemical-resistant. In the oil and gas industry, harsh chemicals are omnipresent, so tools and machinery need to be able to withstand the effects of exposure. Thanks to its corrosion resistance, sapphire is a perfect material for use in the oil and gas industry, specifically in the harsh remote areas where oil and gas exploration takes place.

Sky's the Limit

For sapphire, the sky’s the limit when it comes to future uses. Whether it is in spacecraft or oil and gas exploration, new manufacturing methods are ensuring that sapphire will be an important material for years to come.

Sapphire – Quality Matters, Part 2: Transmission Quality

Recently, Novus Light Today published an article by Dr. Jonathan Levine, Director of Technical Business Development at Rubicon Technology, about sapphire quality.  His article shares a thorough review of the measures of sapphire quality for optical-grade applications.  Last week, we looked at the first two metrics, chemical analysis and X-ray rocking curves.  This week, we’ll look at transmission quality.

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.

Sapphire exhibits excellent transmission in the ultraviolet (UV) to the mid-infrared (IR) range (~200 – 5000 nm).   According to Levine, conditions within the sapphire growth furnace can induce subtle interactions between the molten sapphire and the growth environment.  These interactions can produce bubbles, dislocations and other stresses that could impact optical performance.   Levine says that carefully controlling the growth environment produces sapphire that maintains excellent transmission at 200 nm through the mid-IR wavelengths.  He illustrates the impact of furnace interactions by comparing Rubicon’s ES-2 sapphire with another commercial sapphire maker’s crystal produced using a different growth method in the figure below.  From the image in the post, you can see a sharp absorption peak at 200 nm for sapphire produced by the commercial maker that is absent in sapphire grown by Rubicon.

Optical transmission of sapphire depicting a sharp absorption peak at 200 nm for sapphire produced by a commercial producer that is absent in sapphire grown by Rubicon.  Inset: Optical transmission for Rubicon sapphire from the visible to mid-IR range approaching 90% due to the high quality of the material.

Optical transmission of sapphire depicting a sharp absorption peak at 200 nm for sapphire produced by a commercial producer that is absent in sapphire grown by Rubicon. Inset: Optical transmission for Rubicon sapphire from the visible to mid-IR range approaching 90% due to the high quality of the material.

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

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