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As explained in my 2023 feature on ceramic matrix composites (CMC), the market for these high-temperature materials is growing. While technical ceramics are resistant to high temperatures (up to 2000°C+) and are chemically nonreactive, they are very brittle. By adding fiber reinforcement, CMC offer toughness and thermal shock resistance with significantly lower density and improved strength-to-weight ratios. Applications include higher temperature industrial applications for increased durability and efficiency, aerospace vehicles that must withstand reentry and/or hypersonic velocities, and the next-generation of nuclear power and concentrated solar power (CSP).

However, traditional CMC — including carbon fiber-reinforced carbon (C/C) and silicon carbide fiber-reinforced silicon carbide (SiC/SiC) — have been very expensive due to long manufacturing times. An alternative is being developed at  (Menlo Park, Calif., U.S.) by senior researcher Junhua Austin Wei, targeting cost-competitive CMC by using a high char yield preceramic resin with functionalized ceramic particles to eliminate the infiltration step which has historically increased process time and cost.

Infiltration increases CMC cost

C/C manufacturing normally begins with carbon fiber-reinforced polymer (CFRP) that is shaped, infused with a preceramic resin and then cured to form a green body. This is then pyrolyzed to convert the polymer matrix into a ceramic matrix. However, shrinkage during pyrolysis creates cracks in the matrix which form into voids. To improve the mechanical properties, these voids are infiltrated with ceramic material to densify the matrix. This infiltration is the most costly step.

Several infiltration methods have been developed, including chemical vapor infiltration (CVI), liquid silicon infiltration (LSI), melt infiltration (MI) and polymer infiltration and pyrolysis (PIP). But all of these have historically required weeks or months to complete and/or special equipment that drives up the manufacturing cost. A preceramic resin with sufficiently high char yield to enable skipping the ceramic infiltration step could make CMC more affordable and broaden applications.

The traditional preceramic resin for C/C is phenolic, which has ~60wt% char yield. For PIP CMC manufacturing, at least five cycles of phenolic resin infiltration into the pyrolyzed CFRP are required to densify the matrix to have <15 vol% voids. Wei and his team have developed an alternative approach called scalable infiltration-free (SIF) CMC. But it actually started with functionalized polymers for non-ceramic composites.

Evolution of SIF CMC

Wei’s background was in using nanoparticles to functionalize polymers. At Palo Alto Research Center (PARC, Palo Alto, Calif., U.S.) Wei’s team used this concept to develop a thermoset resin with increased mechanical properties in order to reduce carbon fiber reinforcement in certain composites. “Our system could increase CFRP tensile strength by 10% because the matrix had a tensile strength of 200 MPa without losing much elongation,” says Wei. (For comparison, unreinforced aerospace-grade epoxies report tensile strength from 90 to 179 MPa.)

Then Wei and his team began reading about how a major issue in C/C materials was how to densify the CMC. “They need CFRP precursor materials that have a high char yield, but current materials have only a 60% char yield,” notes Wei. “And some groups had already tried adding ceramic particles to these pre-ceramic systems, but it was very hard to impregnate all the way through the fiber tows.”

“We thought our highly particle-filled resin could help solve this and make the pre-CMC composite very strong,” he explains. “We targeted C/C to start. Traditionally, its density was only 1.5 grams per cubic centimeter [g/cm³] as the densified carbon used as the matrix of the C/C and the polymer we had developed was 1.3 g/cm³. So, they were similar, and we saw that if we could formulate a resin and then add SiC particles, we could perhaps achieve a char yield of 80%. Then after pyrolyzing, the whole system should have less than 10% voids, which is better than the PIP process typically used to make C/C.”

Wei’s team started to produce some preliminary results in 2017, began making CMC in 2019 and then were sponsored by the Dept. of Energy’s (DOE) Solar Energy Technology Office (SETO) in 2022 to help work on a solar receiver project in 2022. PARC was then acquired by SRI International in 2023, where Wei continues to mature the SIF CMC technology.

High char yield resin, functionalized particles

Finding the best resin to make C/C is difficult, says Wei. “Most C/C materials start with phenolic, but its char yield is typically only 60%. We spent one year to find a higher char yield yet low-viscosity resin.”

As Wei and his team explain in a 2024 SAMPE paper, benzoxazine and its derivatives have been explored for years due to their potential to boost char yield.¹ The most promising is PHB-APA, an acetylene- and aldehyde-functionalized dihydrobenzoxazine that features high thermal stability and good mechanical properties in CFRP. However, even with ~75wt% char yield, it still is not possible to fabricate dense CMC by directly pyrolyzing the CFRP made with PHB-APA. Based on rough calculations involving shrinkage, char yield and density changes, Wei’s team determined higher than 80wt% char yield is needed to skip the infiltration process entirely.

“At the same time, we worked to modify SiC nanoparticles (sized 200-350 nanometers) to be compatible with this resin,” says Wei. “To do this, we use the same monomer to functionalize the SiC particles to increase their compatibility. The resulting Bz-SiC nanoparticles can be easily dispersed into the resin without increasing viscosity too much. We also needed to make sure the resin with particles could infiltrate all the way into the fiber tows and remain in place. This is what would allow the C/C to resist all the stress and strain of thermal shock at high temperatures, which is the main benefit of CMC.” He concedes that the particle-filled PHB-APA material is not the easiest to process but it does succeed in all of these functions.

The idea of using SiC particles is not new. Wei describes a 1990 technical paper where NASA used a high char yield resin that impregnated the carbon fiber tows while 30-40% of the resin was particles. “These particles helped to hold the composite together and stop the stress from generating cracks during the shrinkage that occurs in pyrolysis,” he explains. But their approach used a slurry process that required a solvent. “The sintering process was then very difficult and did not generate great results. Learning from them, we did not want to rely only on the SiC particles. Instead, we rely also on the high-char matrix and the particles then help stabilize the shape and prevent shrinkage and large cracking. It’s a hybrid approach.”

Microcracking only between nanoparticles

“In most cases, when cracking occurs, it results from shrinkage because there is a difference in the coefficient of thermal expansion [CTE] between the fibers and the matrix,” says Wei. “And this generates stress as the matrix becomes a glassy carbon, which can create large cracks if the material shrinks too much. But the particles we add don’t shrink; shrinkage during pyrolysis only occurs between the particles. We do see some microcracking within the particles, but we don't see the really large cracks that extend from the top down in the material, which result in the voids that degrade CMC properties. And if you don’t have those large cracks and voids, you no longer need the infiltration step to fill them.”

Demonstrating infiltration-free C/C

Wei’s team successfully produced a flowable preceramic formulation with 40%wt Bz-SiC nanoparticles in a PHB-APA resin that achieved ≈84% char yield. The next step was to demonstrate that dense C/C could be made using these materials without infiltration. The result would not be a traditional C/C-SiC material, but instead a C/C system where SiC nanoparticles comprise >30% of the volume.

The team used Toray (Tokyo, Japan) M40JB carbon fiber in a 4-harness satin fabric from Composite Envisions (Wasau, Wisconsin, U.S.). “This high modulus fiber gives us a good balance between mechanical properties, high thermal conductivity and price, being less expensive than graphite or pitch fiber,” says Wei. For now, it is supplied in a 225 gsm fabric which makes it easy for the resin with particles to penetrate into the middle of the fibers. “Eventually we could use a higher size tow,” he adds.

The fabric is used to make samples with plain PHB-ABA as well as the formulation with 40wt% Bz-SiC particles. Both resin formulations were dissolved into acetone and used to wet laminate the fabric. The acetone was removed under room temperature vacuum to form a prepreg, which was then cured overnight in a stainless steel mold at 120°C under vacuum only and also under vacuum with 2.5 MPa of compression. All samples were post-cured at 240°C for 2 hours.

Samples were then pyrolized. “The current process we use is either at 900°C or 1500°C for 3 hours in an argon atmosphere without vacuum, which is pretty standard,” says Wei. “We’re doing no infiltration and then directly using the part. But we probably will do some machining on the surface, because we'll have some microcracking there. And then we can do a coating on top. This is to increase our ability to absorb solar energy for the concentrated solar receiver application.”

For comparison, a sample made by the plain PHB-APA is fabricated. The pyrolyzed green body is very porous. Even after two cycles of PIP process, infiltration and curing of the plain resin and then pyrolysis, the final sample is still very porous compared to the sample made by the formulation with Bz-SiC.

Mercury intrusion porosimetry (MIP) results  at right compare porosity for the CFRP and CMC produced using PHB-APA. The CMC made with Bz-SiC particles can produce a CMC with ≈5% porosity without using infiltration. In comparison, the CMC from plain PHB-APA had a much higher porosity (~25 vol%) even with two additional cycles of infiltration with additional plain PHB-APA, and cannot be filled by further PIP cycles, says Wei.

The micrographs at right show that the C/C-SiC produced using the SIF CMC process was sufficiently dense to require no further processing. “This reduces the manufacturing time to 3-5 days,” says Wei.

Eliminating infiltration, cost

Skipping the infiltration step is a big benefit. “It takes months to produce a dense C/C structure with PIP because your green body has 25% voids, where ours can have less than 10%,” says Wei. “The five to seven pyrolysis and infiltration cycles required for PIP take weeks, while our SIF CMC process takes days. And that process time also increases labor cost for the people to conduct the process as well as energy consumption.”

SRI has calculated that for a production capacity of 1 ton per year, the manufacturing cost of SIF CMC plates is ~$300/kilogram. Most of that is from the raw materials, due to low commercial availability for certain key constituents. In comparison, the CMC plates made with two PIP cycles using PHB-APA without Bz-SiC particles cost ~$500/kilogram. Though raw material costs for this CMC are still significant, the largest cost is labor, due to the long and multistep PIP process. Surprisingly, PIP CMC made using phenolic resin was calculated to be the costliest (~$600/kilogram), due to the high labor cost from four PIP cycles.

Another benefit of the SIF CMC process is shape fidelity. Wei points out that shrinkage measured after pyrolysis was <1% in-plane and ~4% out of plane. “This favors the manufacturing of CMC with complex shapes,” he adds. “For example, if you have 30% voids in a T-shaped part after pyrolysis, you aren’t sure the shape has been maintained. One of the C/C manufacturers we spoke with said that ≈50% of their labor comes from machining to ensure part dimensions and tolerances. Our current shrinkage for the structure is roughly 5%. The geometric fidelity due to this low shrinkage, along with the simplified SIF CMC process and equipment requirements and shorter process time make it scalable for larger structures and production volumes.”

Wei notes that the SIF CMC work completed so far has been for the SETO-sponsored project to develop a corrosion-resistant solar receiver that can operate at temperatures above 700°C. “It does not require mechanical performance as high as aerospace,” he explains. “But the market for decarbonization solutions is extremely cost sensitive. We have no market if our cost is higher than current nickel alloys, but we do need to offer a better performance. Thus, we are targeting 80% of the performance of C/C and C/C-SiC materials at 50% of the cost.”

Molten salt receiver project

A key component in a concentrated solar power (CSP) plant, a solar receiver converts sunlight into thermal energy. It is located at the top of a tall tower and is heated by sunlight that is concentrated by a large number of flat mirrors called heliostats. A common receiver design is a tube panel, comprising multiple parallel tubes. Molten salt is an excellent storage medium, flowing through the tubes and acting as a heat exchanger, transferring the solar energy to the salt fluid. Although it offers excellent thermal stability at high temperatures for high efficiency (≈90%), and is also non-flammable and non-toxic, molten salt is extremely corrosive, especially at temperatures above 700°C — the operating conditions targeted for next-generation (Gen3) CSP chloride salt receivers. Typically, salt receivers are made from specialized Inconel or Incoloy steel alloys that can resist high temperatures and corrosion, but which are also expensive and difficult to machine and fabricate. 

Though once considered flawed and too expensive, CSP is experiencing renewed interest. The market jumped to $53 billion in 2023 and is expected to grow steadily at 17% to reach $212 billion by 2032. Governments around the globe are investing in improving this technology. The U.S. recently broke ground on a new G3P3 plant that promises improved conversion efficiencies and thermal storage; India is seeking to use modular CSP in 50% of its renewable tenders; and Australia is developing new installations and use of ceramic particles to capture and store heat.²

The project to develop a molten salt receiver using SIF CMC began while Wei was at PARC. It calls for Wei’s team to design the SIF CMC and demonstrate CMC fabrication. Project partners include:

• University of Wisconsin-Madison: Corrosion testing and analysis of CMC
• Ceramic Tubular Products (Lynchburg, Va., U.S.): Fabricate, test and analyze CMC tubes and CSP receiver efficiency
• National Renewable Energy Laboratory (NREL, Golden, Col., U.S.): Optical property measurements of ceramic and CMC, as well as data and receiver efficiency analyses.

The project consortia has selected a receiver design, completed a cost analysis and is now in the process of fabricating and optimizing SIF CMC plates to meet the properties required as specified by modeling and engineering analyses. The project also includes testing SIF CMC receiver tubes and completing a commercialization analysis.

“We’re currently making plates using compression molding,” says Wei. “But we are also working on the next step, where our project partner will use wet filament winding to make the CFRP tubes.” Wei’s team will then complete pyrolysis, and the next stage will be connecting the CMC tubes. “We are considering a U-shape CMC for that,” he says. “We will try to make the shape as simple as possible. After that, we must connect our tube system with a metal channel developed by another party.” Wei concedes this joining of SIF CMC and metal parts is a challenge because the CTEs are so different. “We are currently working with several universities to develop solutions,” he adds.

Wei notes that SIF CMC can be tailored to meet different requirements. “Our original formulation had low through-plane thermal conductivity,” he explains. “In order to meet the efficiency requirements of the receiver, we changed the formulation to increase this property. Due to that change, the porosity after pyrolysis increased to ~13vol% while shrinkage decreased to ~2%. We will continue to optimize the formulation and process parameters as needed but are encouraged by our progress to date.”

Joining CMC

Wei’s team is also maturing a method to join CMC parts that is included in the patent awarded for the SIF CMC process. “We can make a paste from the same matrix and particles used to make our C/C-SiC parts,” he explains. “We use that to adhesively bond the parts together and then proceed through pyrolysis. The high char yield and SiC particles act in the same way to prevent microcracking.” Using the same material ensures compatibility and there are no elements that might be susceptible to corrosion if bonding smaller tubes into larger ones, for example. “But we are not infiltrating, so the process is challenging.”

Another alternative is mechanical joining. “Our material has good shape fidelity, so we can perform surface machining and use CMC fasteners to make the assemblies.”

SiC/SiC

In addition to the C/C-SiC material, Wei’s team is also working to develop a SiC/SiC material using more traditional processing. This is part of a different project, funded by the DOE’s (AMMTO), aimed at heat recovery for industrial processes like steel production. “We’re trying to use this material to recover heat from the molten slag,” says Wei. “If we can achieve ≈30% energy recovery from this 1200-1500°C material, then that can be pumped back into the steel manufacturing or used for other energy generation needs. At these temperatures, carbon fiber cannot last long enough, even with protective coatings,” notes Wei. “So, we are trying to use short SiC fibers, which are easier for us to source than continuous SiC fibers. We’re also looking into more cost-effective methods for applying coatings that can further extend the performance of this CMC.”