Oxide CMC exhaust ground test demonstrator consists of a 1.60-m diameter nozzle and 1.14-m diameter × 2.34-m conical centerbody with titanium end cap inspection portal. Credit: Steyer; IJACT.

Ceramic matrix composite (CMC) materials can benefit aerospace in propulsion and exhaust, thermal protection, and hot primary structure applications, according to Todd E. Steyer of The Boeing Company (Huntington Beach, Calif.).

Reviewing aerospace-related presentations from last July’s 4^th International Congress on Ceramics in a recent paper in the ACerS International Journal of Applied Ceramic Technology, Steyer outlined several emerging aerospace opportunities for CMCs, including propulsion and exhaust, thermal protection, and hot primary structure applications.

In the propulsion area, gas turbines have long been dominated by the use of nickel-based superalloys and titanium alloys. According to Steyer, engine manufacturers are now taking a closer look at CMCs for use in engine hot sections. Silicon carbide-based composites can handle temperatures to 1200°C while reducing weight and cooling requirements, resulting in reduced fuel burn and improved performance.

According to an article in MIT Technology Review, new engines being developed by CFM, a partnership between GE and France’s Snecma, feature CMC components that will reduce fuel consumption by about 15 percent—enough to save nearly $1 million per year per airplane, assuming a fuel cost of $2.50 per gallon.

CFM’s LEAP engine uses SiC-reinforced CMC parts that don’t require cooling, enabling air that would normally be diverted to keep superalloy components from melting to be used to generate thrust. It also uses parts produced using a 3-D printing process, according to the MIT article.

The company already has orders for 4,500 of the new engines. In addition to saving money, the engines will help users comply with current and anticipated emissions regulations.

In engine exhaust systems, work is underway to produce an alumina-fiber reinforced aluminosilicate matrix composite centerbody and exhaust nozzle for commercial aircraft. Currently in ground testing, the ceramic nozzle will reduce weight and engine noise and increase component lifetime, Steyer wrote.

Ceramic materials have long been used in aerospace thermal protection applications—for 30 years, ceramic tiles with glass-based coatings provided thermal protection for the US’s now-retired space shuttle fleet. Initially composed of silica fibers with a nominal density of 0.14 g/cm³ and a glaze aimed at controlling emissivity and limiting catalysis for oxygen and nitrogen recombination from the plasma on reentry, the tiles provided effective insulation but required heavy maintenance between flights. Engineers improved durability over the shuttle’s service life using new tile substrates and coatings.

For new thermal protection applications, Steyer reported on CMCs developed and tested by NASA researchers for use at temperatures to 1700°C. Toughened Uni-piece Fibrous Reinforced Oxidation-Resistant Composite (TUFROC) materials build on the success of insulating fibrous tiles with high-emissivity/low-recombination-efficiency coatings using a refractory ceramic carbon-insulated layer for dimensional stability.

Supersonic and hypersonic flight vehicles present unique challenges for primary hot structural materials, and ultrahigh-temperature ceramics (UHTCs) have been emerging as a promising class of materials for leading edges for hypersonic vehicles. The refractory nature of this class of carbides, borides, and nitrides makes them good candidates for the highest heat flux areas as well as areas with high integrated heat load as a function of time, Steyer wrote.

Particulate, whisker, and chopped or continuous fiber reinforcements are resulting in improved mechanical properties, but the materials’ relatively high density and difficulty in large-scale processing are potential drawbacks. Steyer reported one recent example in which CMCs consisting of 0.5- to 1-mm long chopped Hi-Nicalon SiC fibers in a ZrB₂ matrix hot-pressed at 1700°C showed significantly improved chevron-beam fracture toughness at compositions containing up to 20 vol.% fiber.

Increased use of CMCs in aerospace will require microstructure optimization, a path to entry into service, and improved affordability. Steyer believes fundamental and applied research in damage accumulation mechanisms/models, life prediction methodologies and modeling, nondestructive inspection techniques, and robust field and depot-level repair methods will result in more CMCs in aerospace applications.

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Micrograph of one strand of a new spray-on super-nanotube composite developed by the National Institute of Standards and Technology (NIST) and Kansas State University. A ceramic shell surrounds the multiwall nanotube core. The composite is a promising coating for laser power detectors. (Color added for clarity.) Credit: Kansas State University.

How does one measure the optical power output of lasers that are able to—and even designed to—destroy materials? Some lasers with optical output that high are built to be weapons; others are used for friendlier purposes like defusing unexploded landmines.

Designing a power detector that can capture and measure very high laser power without vaporizing away is one application of a new coating developed by researchers at Kansas State University and NIST. The team, led by Gurpreet Singh at KSU published results on a new carbon nanotube-ceramic composite coating in ACS Applied Materials and Interfaces, in their recent article, “Very high laser-damage threshold of polymer-derived Si(B)CN-carbon nanotube composite coatings.”

According to a NIST press release, NIST has been coating optical detectors with carbon nanotubes because their intense black color maximizes light absorption. This new coating comprises multiwall carbon nanotubes (MWCNT) encased in an amorphous SiBCN shell, as shown in the image above. Adding boron increases the refractoriness of the coating.

The KSU team developed the composite with an assist from the NIST researchers who suggested using toluene for both the preceramic polymer solvent and for the MWCNT dispersant. Singh explained in an email, “Toluene-CNT dispersions were more stable and homogeneous [than dispersions based on] chloroform, acetone, or water.” The NWCNTs are dispersed in toluene into which preceramic polymer is added. When the solution is heated to 1,100°C, an amorphous SiBCN shell forms over the MWCNTs. The composite is ground into a fine powder, dispersed in toluene, and sprayed onto copper substrates.

The optical power meter works by absorbing the high-intensity laser light on its inside surface, which is typically a copper cone calorimeter coated with a black absorbing material (for example, the new MWCNT-ceramic coating). It absorbs the incident light and converts it to heat. The heat transfers to water flowing behind the copper heat sink. By precisely measuring the water flow and temperature increase, the energy absorbed can be calculated. (See schematic of device.)

To test the efficacy of the composite coating, the team subjected it to 10.6-micrometer wavelength irradiation from a 2.5 kW CO₂ laser. The composite coating outperformed other tested materials-MWCNT, single wall CNT, and carbon paint- by an order of magnitude or more. According to the paper’s abstract, the damage threshold for the composite coating was 15 kWcm^-2 with an optical absorbance of 97 percent. Essentially, the coating absorbed all of the light.

In contrast, the MWCNT-only coating exhibited damage at 1.4 kWcm^-2 with 76 percent absorbance. SWCNT broke down at 0.8 kWcm^-2 and only 65 percent absorbance, and damage started in the carbon paint coating at 0.1 kWcm^-2 and 87 percent absorbance.

According to the press release, the MWCNT component absorbs the irradiation and transmits the heat, while the ceramic shell provides oxidation and damage resistance. Apparently, though, under the right conditions, the outer shell oxidizes partially to form an external silica layer, which can be used to tune the coating depending on the application.

Singh said there are other possible applications for the MWCNT-ceramic coating, such as lithium-ion cycling. They are also looking into applications such as nanostructured coatings for protection in extreme environments like rocket nozzles.

This last application reminded me of an interview I did several years ago with NASA Space Shuttle astronaut, Danny Olivas. Olivas is a metallurgist and was very involved in the materials aspects of the failure analysis after Columbia disintegrated in 2003. In the aftermath, he also led the effort to develop an in-flight repair kit to mitigate damage to the heat shield tiles. (It was determined that a breach of the heat shield contributed to the Columbia tragedy.) The team developed a similar material: a preceramic polymer that fired to silicon carbide. The idea was that the polymer would be “painted” onto the damaged area and would fire, literally, using the reentry atmosphere itself as the “furnace.”

To the best of my knowledge, the system was never used (thankfully). The Shuttle program ended in 2011, so we will never know whether it would have worked.

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Official video (in German) of STO In Aevero aerogel insulation boards. Credit: STO.

One of the promises of your basic silica-based aerogel is that it would make a fantastic component in insulation systems—but there have always been a lot of manufacturing and processing “ifs” involved. Nevertheless, several companies are starting to make headway with emerging commercial products.

Before I get into the details, I always try to point out that in the big picture the importance—in terms of energy consumption—of improved building insulation varies among regions of the globe. While it is a second-tier concern in North America, the energy-consumption pattern in many European nations is dominated by heating. Germany is one of the best examples, where well over 25 percent of the nation’s energy consumption goes into residential and commercial space heating. Much of the problem is related to the age of the building stock. Besides the heat leakage problems that come from very old buildings, remediation is also a challenge because of sheer space limitations.

Thus, while the availability of aerogel-containing insulation panels and systems may not be front-page news in the United States, it is a fairly big deal in Europe (where the EU is already funding a major research and commercialization initiative). It’s worth keeping this in mind as you read about the developments below, and illustrated in the video above.

The first is that an internal insulation and finishing system developed by STO AG—”STO in Aevero”—recently received the “Award for Product Innovation” at the BAU 2013 trade fair. At least in terms of product recognition, this is a nice accomplishment because BAU, as far as I know, is the world’s largest expo for architecture, materials, and systems. STO’s system uses aerogel develop by Cabot.

Sixty companies were part of the competition, vying for three prizes and six awards. The STO/Cabot system won the events “Investing in the Future” award, which apt. A Cabot news release describes the product as a “super slim system is comprised of a composite board that combines Cabot’s aerogel particles for superior energy-savings performance with STO’s binder and composite technology. This results in an insulation board that offers greater energy efficiency than traditional materials. Cabot’s aerogel enables an ultra-low thermal conductivity of 0.016 W/mK applied in very thin insulation thicknesses from 10 to 40 millimeters (R3.5 - R14).”

Here is a summary of STO In Aevero’s properties:

• Thermal conductivity: 0.016 W / (mK)
• Compressive strength: ≥ 100 kPa
• Water vapor diffusion resistance factor μ: 10
• Tensile strength perpendicular to faces: ≥ 20 kPa
• Density: ≥ 150 kg / m³
• Panel thicknesses: 10, 15, 20, 30, 40 mm
• Sheet size: 580 x 390 mm
• Material class B2 according to DIN 4102 (B1 in the system)

STO’s contribution is significant in that it had to design and manufacture a composite board that, besides incorporating the aerogel, also addresses permeability and vapor control and delivers a product in a thin form factor. STO also has developed some important installation methodologies, and leveraged its experience with installing high-quality facade, plasters, paints and rain screen-cladding systems.

In the Cabot release, Raj Chary, vice president and general manager for Cabot Aerogel says, ”[STO's] modern, intelligent solutions in reconstruction, renovation and renewal work are helping architects and builders meet the highest regional and industry standards for energy conservation, [and to] help deliver energy efficient renovation solutions for historical buildings as well as new construction.”

One frustrating thing that unfortunately is lacking in these announcements is pricing/installation cost information relative to traditional insulation.

While STO and Cabot seem to be staking out the building sector, Aspen Aerogels continues to refine its products for industrial application. Aspen was one of the first companies that demonstrated a flexible insulation system that could, for example, be used as a wrapable barrier around pipes. Aspen and others saw a business opportunity with insulating pipes that pass through cold regions, such as the petroleum pipelines that cross Alaska.

Apparently the company was also keeping an eye on high-temperature applications, too. Recently Aspen, announced a new high-temperature insulation, Pyrogel XT-E. A news release from the company indicates that the new product is a variation of if the existing XT product, and that it is being aimed at uses refining, petrochemical, power and other facilities. Given the recent boom in the drilling and refining industries, a product aimed at this sector makes a lot of sense.

In the Aspen release, Don Young, president and CEO, says Pyrogel XT-E is “the most effective high-temperature insulation material in the industrial market and improves our customers’ ability to use our product in the most demanding environments.”

A document (pdf) on the Aspen websites says that it is available in rolls of sizes of 850 and 1,500 square feet, is available in 5 and 10-millimeter thicknesses, and has a density of about 12.5 pounds per cubic foot.

Like Aspen’s other products, the Pyrogel XT-E comes in rolls that makes it a “labor saver.” Aspen also says it has been able to significantly reduce the dust that comes from installation. It says its flexible blanket form is up to five-times thinner than competing insulation products and can serve in applications that range from -270 °C to 650°C.

No pricing was mentioned for the Aspen product, either.

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A record-breaking 200-plus attended the 49th Annual Symposium on Refractories. Credit: ACerS.

I just got back from the 49^th Annual Symposium on Refractories that was held Wednesday and Thursday at the combined St. Louis Section and Refractory Ceramics Division meeting. The theme of this year’s meeting was “Refractory Challenges in the Chemical and Petro-Chemical Industries.”

To open the symposium, Ed Linck (Linck Refractory Services) provided an overview of refractories challenges and technologies in the refining industry, particularly as they are used in fluid catalytic cracking units (FCCUs). In the 1950s when he began working in the industry, FCCUs ran 6-9 months before needing repair. Today, FCCUs run five years continuously, and new technologies are driving the life cycle toward seven years.

Linck noted that good materials are important, but installation is critical. “You can have the best material in the world, but if you don’t put it in right, it won’t work right,” he says. Downtime costs a refiner about $1 million per day, according to another speaker, Richard Parkinson of UOP LLC/Honeywell. Even though it takes about 100-150 tons of refractory material to line a FCCU, the refining industry accounts for only about 5-10 percent of the overall refractories market, with the majority of refractories going to the steel, metals, glass, and cements industries.

According to Parkinson, there are about 500 FCCUs worldwide that account for about 40 percent of the global gasoline production. A FCCU can process 20,000-200,000 barrels of oil per day. The catalytic cracking reaction occurs at about 500-550°C, and the catalysts are primarily zeolite and aluminosilicate fine powders. Consequently, many of the talks discussed challenges relating to corrosion, heat, and abrasion.

John Hellman, from Pennsylvania State University, is working on proppant technology, which are at the vortex of current events relating to hydrofracturing. He says that the Marcellus Shale reserve alone has enough natural gas reserves to supply the US energy needs for the next 150 years, which he describes as “the Saudi Arabia of natural gas.”

“This is an industry that is going to be large,” Hellman says, “and we have to do it right.” According to him, an important part of “doing it right” is advancing the proppant technology. The numbers are compelling. Each lateral well (12-20 come off each vertical wellhead) calls for 3,000 tons of proppant. The present market is in the neighborhood of 100 billion tons per year, up from 12 billion tons only seven years ago. At those volumes, proppants need to be available locally and cheap. His group is looking at ways to use discarded materials—such as glass cullet, slags, wrong-size roofing shingle minerals, and even the drill cuttings pulled from the wells—to make high-quality, inexpensive proppants, while solving some other industrial disposal problems. He also offered some intriguing ideas for “smart proppants” based on magnetic particles with piezoelectric outer shells that could provide in situ monitoring of the wells.

The RCD regulars like to characterize themselves as a “family.” To my delight, this family welcomed me into the clan by singing “Happy Birthday” to me yesterday to celebrate my N^th 39^th birthday!

Paul Schlett of Valero Energy Corp. introduces the first speaker, Ed Linck of Linck Refractory Services. Between them they have over 100 years experience meeting the refractory needs of the refining industry. Credit: ACerS.

Wagner Silva of Magnesita Refractories presents the work that was the basis on his Allen Award-winning JACerS paper. Credit: ACerS.

ACerS president, Richard Brow, reports on ACerS activities and new initiatives to promote ceramic engineering education. Credit: ACerS.

J.P. Willi, the 2013 Planje Award recipient, regaled the audience with highlights from the many "new opportunities" that his career in refractories afforded him. Credit: ACerS.

Vendors and attendees had plenty of opportunity to talk shop at the expo and reception. Credit: ACerS.

The RCD leadership. From left: Lora Cooper Rothen, ACerS RCD board liaison; Lou Trostel, counselor; Bill Headrick, immediate past chair, Dave Tucker, chair; Jens Decker, secretary; Ben Markel, vice chair; and Josh Pelletier, member-at-large. Credit: ACerS.

The Planjes—previous Planje Award winners with 2013 winner, J.P. Willi. Credit: ACerS.

Conner Jackson with his grandfather, J.P. Willi. Jackson picked up the shades-'stache combo on a trip to Willi's hometown of Chicago. Is there a refractories career in Jackson's future? Credit: ACerS.

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(a) Photograph of new UFAs with diverse shapes; (b) a 100 cm³ cylinder standing on a flower like “dog’s tail”; (c) a ~1000 cm³ pie pan-shaped cylinder (21 cm in diameter and 3 cm in thickness). Credit: Gao et al.; Advanced Materials.

I suspect this story may seem a little like old news to some readers, but a lot of the pop-sci reporting in the last few days about a new ultralight aerogel (actually, a UFA or ultra-flyweight aerogel, i.e., less than 1 mg cm⁻³) has missed the point by a wide margin. The big takeaway from this story, as documented in a new Advanced Materials paper, is that the graphene–carbon nanotube (CNT) aerogel is relatively easy to make, appears to be easily scalable, and is moldable into any shape. Moreover, the new UFA, besides having ultralow density, also is extremely flexible and elastic—with the elasticity independent of temperature—and is thermally stable, a good electrical conductor, hydrophobic, and capable of absorbing extremely high capacities for organic liquids and phase change materials.

Now, certainly, there is a significant gee-whiz factor for gaining the “world’s lightest material” title, an achievement based on a density of 0.16 mg cm⁻³, accomplished by the research team at Zhejiang University headed by Gao Chao.

The group’s material is not the first UFA. Within the past 18 months, other researchers have constructed nickel foams with density of 0.9 mg cm⁻³ (via electroless plating and subsequently etching away a polymer template), and another group constructed an aerographite with a density of 0.18 mg cm⁻³ (via a ZnO template-based chemical vapor deposition approach). But, both groups’ dependency on a template also creates enormous limitations to scalability. Sol-gel-derived, low-density aerogels can be made on fairly large scales, but with sol-gel processes it is difficult to control the dimensions of the structures.

Gao’s group, instead, uses a process that involves freeze-drying aqueous solutions of CNTs and giant graphene oxide (GGO) sheets, followed by chemical reduction of graphene oxide into graphene using hydrazine vapor. The researchers named their method a “sol-cryo” approach and note in their paper that it is easy to make large samples:

Because of the simplicity of assembly process in our template-free “sol-cryo” methodology and the large-scale availability of GGO and CNTs, the integrated all-carbon aerogels with desired densities and shapes such as rods, cylinders, papers and cubes were readily accessible. More significantly, UFAs can be easily manufactured in a large-scale. For example, a UFA cylinder up to 1000 cm³ was made with a mold of 1-liter plate.

A story on the university’s website reports quotes Gao saying, ”With no need for templates, its size only depends on that of the container. [A] bigger container can help produce the aerogel in bigger size, even to thousands of cubic centimeters or larger.” The story also reports that Gao believes ”the value of this achievement lies not in the record but in its simple way in developing the material and the superior performance exhibited.”

Briefly speaking, the microstructure of the material is a 3D porous framework constructed with cell walls of randomly oriented, crinkly graphene sheets and CNT “ribs.” The macropores ranged from hundreds of nanometers to tens of micrometers. The authors of the paper say the properties of the UFA derive from the graphene-CNT synergy: “Giant graphene flakes build a framework with macro-pores, making the aerogel ultralight; the coating of CNTs reinforces the relatively flexible graphene substrate and endows their intrinsic elasticity to the coorganized aerogel.”

One of the UFA’s interesting properties is its elasticity that allows it to be repeatedly compressed and returned to its nearly original size. This sponginess comes in handy in combination with another property: It can rapidly absorb up to 900 times its own weight in oil or other organic liquids. For example, one gram can absorb 68.8 grams of organics per second. The elasticity remained the same in tests that ranged from −190 to 300°C. The elasticity also remained after researchers annealed the UFA at 900°C for five hours.

There are probably many ways the elasticity and absorbability can be useful, not the least of which is that, because of its hydrophobicity, it could be a reusable medium to soak up oil spills on lakes and oceans. Gao says in the university story, “Maybe one day when oil spill occurs, we can scatter them on the sea and absorb the oil quickly. Due to its elasticity, both the oil absorbed and the aerogel can be recycled.” (Another Chinese research group working at Tsinghua and Peking Universities published a paper in 2010 about the use of CNTs for oil spills, and I suspect that there is some overlap between the work of the two groups.)

Another property of the UFA is that it has elasticity-dependent electrical conductivity. For example, they report connecting an LED lamp top the UFA bulk, and “its brightness fluctuates upon compressing and releasing the aerogel. This phenomenon promises the application of UFAs as pressure-responsive sensors.”

They also say that by loading the UFA with tiny amounts of certain liquids (say, CCl₄ or 1-hexadecanol), they can make conductive composites with very high electrical conductivity compared to just CNT- or graphene-based composites.

Video of variable conductivity of UFA demonstrating current change to LED when aerogel is compressed and released. Credit: Gao et al.; Advanced Materials.

Gao and the other authors also suggest the UFA could find use as supercapacitors and catalyst beds, but another intriguing application they only hint at is its use as a medium that enhances phase-change energy storage materials. For example, unlike other composites, a UFA-paraffin combination delivers higher phase-change enthalpy (ΔH) than ordinary paraffin.

These applications may only be the beginning. When it comes to learning how to leverage the properties of their fluffy stuff, the researchers say their new UFA is “just like a new-born baby.” A baby that was, perhaps, born with a silver spoon in its mouth.

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