Industrial applications for ultrahigh-temperature ceramics

Due to the current race in advancing hypersonic technologies,^1,2 a class of materials known as ultrahigh-temperature ceramics (UHTCs) has gained research momentum.

UHTCs are a class of materials comprised of borides, carbides, and nitrides of transition metals that, as the naming suggests, have high melting points, generally above 2,000°C. The broad definition of UHTCs encompasses more than 300 material compositions. However, materials with melting points above 3,000°C are limited to borides, carbides, and nitrides of boron, hafnium, tantalum, niobium, zirconium, titanium, and thorium oxide (Figure 1).^3–9

In addition to high melting points, UHTCs are also heat resistant with high phase stability, meaning they do not easily react with other materials or their environment. These properties make UHTCs ideal for high-temperature applications, such as hypersonics.

Though hypersonic technology is the current poster child for UHTC applications, these materials already have established uses in numerous industrial sectors, including electronics, manufacturing, and energy. In this article, we will look at several of these market-ready compositions, namely BN, HfB₂, HfC, HfN, NbC, TaB₂, TaC, ThO₂, TiC, ZrB₂, and ZrC, and consider how they are sourced, produced, and used.

Material sourcing

We begin our discussion of industrial applications for UHTCs by considering the sourcing of the raw materials (Figure 2).¹⁰ UHTC materials are globally sourced with the exception of thorium oxide, which is sourced from only China and the U.K. The rare earth UHTCs only make up a few thousand metric tons of available stock. Hafnium, being a byproduct of zirconium processing, is actually the rarest based on available data, with only 70 metric tons produced per year. All UHTCs undergo similar processing methods to get them from a raw ore stage to the final product, which involve the reaction of the oxide form into the desired boride, carbide, or nitride component. These methods will be discussed in the next section.

Boron

Boron is the necessary element to produce UHTC borides and boron nitride. Boron, in the form of boron oxide (B₂O₃), is obtained from borate minerals of colemanite, kernite, tincal, and ulexite. Most ore is sourced from Turkey, which provides nearly 50% of the market supply. The United States supplies 25% of the ores, Chile supplies 8%, and half a dozen other countries supply the remainder to produce an approximate world total of 4.1 million metric tons per year.¹¹ From the total supply of boron, 80% of boron is consumed for borate glasses and ceramics in the United States.

Tantalum

Tantalum is sourced from the minerals columbite, microlite, tantalite, and wodginite. The prime producers are from African nations, which make up 60% of the sourcing. Brazil produces 15% of the ores and various other countries contribute a small percentage each to produce a world total of about 2,400 metric tons per year. Due to the limited availability and rarity of tantalum, recovery of tantalum from recycling electronic scraps and super alloys is a practical and economical solution. Of all the tantalum, more than 50% of tantalum is used for the manufacturing of tantalum-based UHTCs.

Zirconium and hafnium

Zirconium is obtained from zircon, baddeleyite, and mineral sands, while hafnium is a byproduct from the processing of zirconium metal. Thus, availability of hafnium correlates to the processing of zirconium. Moreover, zirconium mineral sands are excavated in the same regions as titanium mineral sands, which contributes to the availability of zirconium. More than 50% of zirconium-containing ores originate from Australia and South Africa, with smaller contributions coming from China at 8% and the United States and Indonesia supplying 6% each, amounting to 1.6 million metric tons per year.

Zirconium is processed mostly into the oxide form of ZrO₂, where it is commonly used for ceramics and silicides. Because hafnium is a derivative of zirconium processing, prices and availability for hafnium are controlled by the industrial demand of zirconium-based products, with an approximate production of only 70 metric tons per year. For many cases, zirconium can be interchangeable with hafnium in alloying and some electronic components, which can reduce the burden and use of hafnium.

Titanium

Ilmenite and rutile are the main mineral sources to obtain titanium. Illmenite is mainly sourced from China, Mozambique, and South Africa, while more than 50% of rutile is sourced from Australia, Sierra Leone, the United States, and South Africa. Titanium is one of the most versatile transition metals used in the world, finding application as either a standalone element or composite in paints, polymers, aerospace, medical, and automotive applications. Because of this versatility, the use of titanium in the form of titanium carbide makes up a very small fraction of the total titanium consumption.

Niobium

Niobium is sourced from the mineral pyrochlore. Brazil mines 90% of the ore, while Canada mines 8%. Niobium is mostly used as additives for alloying steels, with niobium carbide having a specialized use in nuclear reactors.

Thorium

Thorium is primarily sourced from monazite, but it has limited supply due to radioactivity concerns. More than 80% of thorium-containing ores come from China and the remainder come out of the U.K. Thorium is not the sole material found in monazite, accommodating on average 10% of the total composition, accompanied by other rare earth elements. The ores are treated with either an acid or base to break down the mineral into individual rare earth components and can be followed with several solvent extractions or calcination depending on the exact processing method used. The processing of thorium ores results in the production of ThO₂ and only requires further processing for specialized applications, such as coatings or production of pure thorium metal.

Processing of UHTC powders

There are generally two methods employed for the commercial processing of UHTC powders: solid-state reactions and thin film deposition.

Solid-state reactions

Solid-state reactions are an effective way to produce a large quantity of UHTC powders. This processing approach involves the reaction of transition metal powders to produce UHTC powders in the range of micrometers. Prior to reaction, powders can be mixed in a ball mill to uniformly disperse the reactants. Alternatively, the powders can be processed using high-energy milling, where mechanical forces are higher to greatly reduce particle size and promote cold welding of particles.

Though there are several methods reported in academic journals to describe how a specific composition is made, this article discusses the most common methods used for commercial sale.

The common reactant used to form UHTC borides and boron nitride is B₂O₃. This compound is one step removed from mined boron ores, where boron is extracted in the form of H₃BO₃. After a heat treatment up to 600°C, B₂O₃ forms as water evaporates.¹² It is then reacted with ammonia (NH₃) to form hexagonal boron nitride (h-BN). h-BN can be further processed into the high temperature allotrope of cubic boron nitride (c-BN) by processing at temperatures as high as 1,800°C and pressures in the range of 50,000–90,000 atm.¹³

The processing of UHTC carbides goes through the carbothermal reduction of oxides using the oxide form and reacting with graphite at temperatures ranging betweeen 1,500–1,900°C. HfC, NbC, TaC, TiC, and ZrC are formed through the carbothermal reduction of their oxide species. Zirconium and hafnium are sourced from their raw material as chlorides (ZrCl₄ and HfCl₄) and begin the process through the hydrolysis and calcination of their metal chloride to produce ZrO₂ and HfO₂. To get the metal boride, the oxides go through a borocarbothermal reaction using B₂O₃ and a carbon source.

Unlike HfC and HfB₂, the production of HfN begins with the pure metal form of hafnium. HfN is made from the nitridization of hafnium metal in a nitrogen environment at 1,300°C.

Thin film deposition

High-purity nanopowders can be produced via chemical vapor deposition. In this process, one or more gaseous precursors are introduced into a reaction chamber, where they adsorb onto a surface and form a thin layer of material that reacts to form the desired composition on a substrate. A post heat treatment or anneal at a lower temperature than the solid-state reaction may be applied to assure the reaction has completed.

In some cases, physical vapor deposition may be used, which involves bombarding a sputtering target made of the transition metal with ions to eject material that reacts with the environment and forms the final composition as it is deposited on the substrate. The transition metal is typically a chloride, which reacts with boron trichloride to form the boride, methane to form a carbide, and ammonia to form a nitride.

Exploration of UHTC applications in industry

As noted in the introduction, UHTCs are heat-resistant materials with high phase stability, which makes them ideal for applications that expose materials to high thermal loads. In addition, UHTCs exhibit unique mechanical and electrical properties, as seen in Table 1,^{3, 7–9,14–18} which expands their possible uses.

The current industrial uses of UHTCs include electronic device components, additives for cutting tools and optical devices, and coatings for nuclear energy applications (Figure 3). We will briefly discuss the important material properties required for each application.

Electronic device components

Capacitors store and release electrical energy. The push for miniaturization of electronic components drives demand for capacitor materials with high dielectric constants and high dielectric strengths. HfN, TaN, and ThO₂ have uses as capacitors, with ThO₂ being the least used due to radioactivity concerns. The intrinsic high thermal stability of HfN and TaN allows for the production of smaller capacitors with the same capacitance and reduced energy loss.¹⁹

Transistors act as a switch or amplify a signal. Boron nitride is generally a good choice for transistors because it has a large band gap to prevent current leakage and a low dielectric constant for faster switching in high-frequency applications. In the specific case of field-effect transistors, which use an electric field to control the current flow through a semiconductor channel, a material with higher electrical conductivity is desired. The preferred band gap is determined by the application, with a large band gap being ideal for high-power applications and a small band gap being used for low-power electronics. HfN is suitable for the latter application due to its small band gap, and with a higher electrical conductivity than boron nitride, it consumes less power when the transistor is on.

Various electronic components have copper and silicon interfaces, and copper is known to diffuse quickly due to its susceptibility to electromigration. To prevent the diffused copper from reacting and mixing with other key constituents of the device, a diffusion barrier is used. The diffusion barrier requires resistance to dielectric breakdown; thermodynamic phase stability that prevents reaction with copper, silicon, or other substrate materials; and low kinetic mobility of copper. HfN, TaC, ZrC, and TaN are UHTCs that can serve as diffusion barriers.^19–21 Additionally, NbC and TaC are used as electrical contacts due to their high thermal conductivity and resistance to arc erosion.

Cutting tools and optical devices

UHTCs can be used as additives to increase a cutting tool’s hardness and thus enhance cutting capability and reduce wear. Regarding optical devices, HfB₂ and ZrB₂ are used as substrates for the growth of nitrides in LEDs due to having good electrical conductivity as well as compatible thermal expansion coefficients and lattice parameter spacing compared to gallium nitride-based semiconductors.²² ThO₂ was previously used as an additive in optical lenses for its high refractive index to improve image sharpness, but it is not used anymore due to radioactivity concerns. To replace the use of ThO₂, other compounds are used, including TaN coatings.

Nuclear energy applications

In the generation of nuclear energy, materials are bombarded with neutrons and exposed to high temperatures. Uranium oxide (UO₂) is used as the fuel source, but UHTCs are starting to be used as supplements or as alternative fuels to extend the life of fuel cells.

ThO₂ is suggested as a main alternative, but currently it is used as an additive to UO₂. Benefits of ThO₂ include providing greater thermal–chemical stability, which can enhance life, and being more readily available than UO₂.²³ ZrB₂ and HfB₂ are also in current use as coatings for UO₂ fuel rods.²⁴ Although UHTC carbides have been investigated as an alternative to the borides for coating nuclear fuel, carbon does not absorb neutrons as effectively as boron.

Boron has a high neutron absorption cross-section and helps control the nuclear reaction. The benefits of using boride UHTCs as a boron source is that they offer high-temperature stability along with oxidation resistance and high mechanical strength to support UO₂ rods.

Emerging technologies

As noted in the introduction, research on the use of UHTCs for hypersonic technologies is expanding, and this research largely involves UHTC borides and carbides with melting points above 3,000°C. Based on market availability, rare earth UHTCs may be used in small quantities, but zirconium- and titanium-based UHTCs are likely to be used more commonly due to their greater market supply. However, hafnium-based UHTCs have better high-temperature phase stability than the zirconium counterpart. Based on planned flight paths and vehicle design, hafnium-based UHTCs may see roles in critical components exposed to the highest temperatures.

Future directions for UHTCs

UHTCs are increasingly associated with the advancement of hypersonic flight systems, but they already have established uses in various industrial sectors ranging from electronics and manufacturing to energy and more. Developments in advanced material processing techniques, such as direct current sintering and microwave sintering, have increased the accessibility of these materials.

However, challenges still remain with fabricating these materials to withstand the harsh environmental conditions posed by hypersonic flight. For example, the ability to produce structures with controlled microstructures on a large scale,^3,25,26 the enhancement of mechanical properties and oxidation resistance at high temperatures,^13,27,28 and the development of testing facilities that can emulate aspects of extreme environments.^29,30

Despite these challenges, the continued and expanded use of UHTCs in existing commercially relevant sectors remains promising. With their ability to maintain properties at high temperatures, UHTCs are leading materials for further miniaturization of electronics. The continued use of UHTCs as additives for cutting tools and optical devices is also expected, while ZrB₂ and HfB₂ maintain a firm hold as the preferred coating for nuclear fuel rods.

To date, these applications of UHTCs use only single-compound UHTC compositions. The solubility of UHTC compounds with each other can open the door to solid solution compounds with five or more components, known as high-entropy UHTCs. The ability to tailor UHTC properties through solid solution mixing can provide a path for increased customization of UHTCs in the future.