For many superconducting applications, coatings play a vital role. Electrically insulating coatings have been commonly used on superconducting wires. Newer insulating coatings, such as the ultra-thin polyimide coatings developed by scientists at the RIKEN Center for Life Science Technologies and Chiba University,¹ can be as thin as 4 mm. This is more than 10 times thinner than the conventional insulation used for high-temperature superconducting wires. Wires with the new coating have a cross-section containing just 10% insulation, a fivefold reduction of conventional superconducting wires, and they exhibit�� double the magnetic strength. Reducing the ratio of insulation to conductor in the wire allows for more compact superconducting coils for use in next-generation medical and scientific devices.

Wires with the new coating have a cross-section containing just 10% insulation, a fivefold reduction of conventional superconducting wires, and they exhibit double the magnetic strength.

Special ceramic insulating coatings for Bi-2212 (Bi₂Sr₂CaCu₂O_8−x) high-temperature superconducting round wire² were developed by the National MagLab to enable the wire to be used in ultra-high field nuclear magnetic resonance magnets. Bi-2212 is important because it can be used in magnets that produce very strong magnetic fields. The coating was designed to withstand a temperature of 900°C, the temperature at which Bi-2212 undergoes heat treatment. This temperature would cause conventional polymer-based insulations to melt.

In a recent development, researchers at Florida State University discovered that the use of semiconducting or resistive coatings³ instead of insulating coatings results in the elimination of the need for expensive and complicated quench protection systems. This results in less expensive electromagnets with faster ramping times and reduced eddy current losses during field changes.

Superconducting magnets also have the potential to make fusion power possible.�� Researchers at MIT and the private company Commonwealth Fusion Systems are using a new material—a steel tape coated with the superconducting compound yttrium-barium-copper oxide (YBCO)—to create the world’s most powerful superconducting magnets.⁴ Their goal—and one that scientists have been working on since the 1940s—is to develop a fusion reactor. One of the most promising designs being considered is a tokamak, a chamber that uses powerful magnets to create a magnetic field to confine hot plasma produced by nuclear fusion. MIT is hoping to build a compact tokamak, called SPARC, capable of generating 100 megawatts of heat. This could serve as significant step towards achieving a full-scale fusion reactor.

In some cases, the coatings themselves can act as superconductors. Thin, high-temperature superconducting films are used in a wide range of electronic devices used in physical experiments, such as fault current limiters, SQUIDs (superconducting quantum interference devices), and magnetic traps for cold atoms. Researchers at Saarland University and the Leibniz Institute for New Materials produced an ultrathin, flexible film⁵ from superconducting Bi-2212 nanowires that at approximately –200°C can conduct electricity without loss, levitate magnets, and screen magnetic fields. The lightweight nature and flexibility of the ultrathin films are attractive, because most conventional superconducting films are rigid, brittle, and dense. The new flexible film has properties similar to plastic wrap and can be produced in nearly any size. It has potential uses in medical and space applications, among others.

Microns-thin Nb₃Sn films⁶ applied on the inner surfaces of superconducting radiofrequency (SRF) cavities exhibited improved cryogenic efficiency compared to conventional niobium materials, increasing their value in research and for potential industrial applications. SRF cavities boost the energy of charged particle beams passing through them, making them useful in particle accelerators.⁷ Nb₃Sn cavities may be particularly useful in small-scale SRF accelerators for medical, border security, and flue gas and wastewater treatment applications.

The process of electroplating in the CIRES instrument lab.

A recent find by scientists at the National Institute of Standards and Technology and the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder (CIRES)⁸ could help make higher-temperature superconductors possible. They discovered that an ultrathin layer of rhenium sandwiched between layers of gold, each measuring 1/1000th the diameter of a human strand of hair, can superconduct at a critical temperature above 6 Kelvin. The material is attractive because it is electroplated and could be ideal for use in circuit boards for ultrafast, next-generation computing applications not only because it is superconducting at higher, easier-to-achieve critical temperatures, but also is easy to work with mechanically, is nontoxic, and it melts at high temperatures. According to CIRES, this discovery is already attracting attention from the giants in international computing.

References

1. “Ultra-thin Insulation Coatings Makes Superconducting Wires Thinner, More Efficient,” Riken, accessed at: https://www.riken.jp/en/news_pubs/news/2013/20130826_1/.
2. National High Magnetic Field Laboratory, “Ceramic Insulation
   for High-Temperature Superconducting Wire,” accessed
   at: https://nationalmaglab.org/magnet-development/
   magnet-science-technology/publications-mst/highlights-mst/ceramic-insulation-high-temperature-superconducting-wire.
3. “Novel Coatings for Superconducting Magnet Superconducting Wires and Tapes,” FSU Office of Research, accessed at: https://www.research.fsu.edu/research-offices/oc/technologies/novel-coatings-for-
   superconducting-magnet-superconducting-wires-and-tapes/
4. Starr, Michelle, “MIT Is Building the World’s Most Powerful Superconducting Magnets for an Amazing Cause,” Science Alerts (March 12, 2018); accessed at: https://www.sciencealert.com/mit-superconducting-magnets-nuclear-fusion-power-tokamak
5. “Physicists Develop Ultrathin Superconducting Film,” Science Daily (April 6, 2017); accessed at: https://www.sciencedaily.com/releases/2017/04/170406121529.htm
6. Posen, S. and Hall, D.L., “Nb₃Sn Superconducting Radiofrequency Cavities: Fabrication, Results, Properties, and Prospects,” Superconductor Science and Technology, 30, No. 3 (January 23, 2017); accessed at: https://iopscience.iop.org/article/10.1088/1361-6668/30/3/033004
7. “Superconducting Films Particle Acceleration,” Energy.gov (June 13, 2019); accessed at: https://www.energy.gov/science/articles/superconducting-films-particle-acceleration.
8. Cooperative Institute for Research in Environmental Sciences at University of Colorado Boulder, “A Surprising New Superconductor,” CIRES (May 1, 2018): accessed at: https://cires.colorado.edu/news/surprising-new-superconductor.

CoatingsTech | Vol. 17, No. 2 | February 2020

In the simplest terms, sustainability could be defined as less waste and more efficiency. This could mean fewer processing steps, the use of fewer chemicals or other building blocks, or the ability to develop products that last hundreds of years without the need for constant maintenance.

In the coatings (paint) world, sustainability is usually looked at through the lens of fewer chemicals, biomaterial substitutions, shortened or fewer manufacturing steps, or outright chemical bans. Significant efforts are currently underway to reduce petroleum-based materials in the first place along with trying to make the new products last at least as long as their predecessors.

There is another way to look at materials sustainability. What if the pretreatment chemicals and coatings could be eliminated altogether? What if the oxide was also the color coat? What if the longevity of stainless steel could be exploited together with its oxidation potential? What if color could be imparted onto stainless steel strip using only one production operation for both steelmaking and coloring? What if these innovative ideas were actually more cost effective?

First Steps

In 1971, Allegheny Ludlum Steel Corporation took a step in that direction. While certainly not under the guise of sustainability, the company developed a process that produced black stainless steel for use in formed architectural products.¹ In this case, a stainless steel strip was produced by oxidizing the stainless surface to form a porous oxide coating and impregnating it with an alkali metal silicate, and then drying and fusing the silicate by heating between 1400 and 1600 °F. Temper rolling the finished product—a typical steel finishing process—allowed for flattening the strip to facilitate fabrication.

This process was an efficiency improvement over the original pre-1968 process where many more production steps were used. First, the strip was treated to obtain a black oxide and then coated with a water-soluble alkali metal silicate or a water-based thickened or gelled solution of sodium or potassium dichromate. The next step involved removing the water and subsequently heating the coated surfaces to a temperature high enough to form a uniform, black, porous oxide 1,000–500,000 Å thick. Excess coating and loose oxide were then scrubbed off with water.

In addition to reducing the number of processing steps, the 1971 process eliminated many quality problems. There was better thickness uniformity, less visibility of any spray or rolling pattern, good aesthetics, and better condensing humidity resistance, as depicted by the absence of white residue all over the strip. Steam testing, abrasion resistance, chemical resistance, mandrel bend, and impact testing were also improved to the point that this black stainless could be formed into tubes or building panels with decent aesthetics. With better oxide thickness control, color was also more uniform.

Fast Forward to 2013

Over the 40 years since the first steps in manufacturing blackened stainless, many material inventions and improvements have occurred. Stainless steel itself has become a more uniform product, with improved surface finishes that can now be used in a variety of aesthetic-critical applications. Base metal chemistry modifications and more controlled annealing atmospheres and temperatures have led to increased formability as well.

Alongside steel production maturation, the advent of affordable, usable nanotechnology opened a huge window for surface modification. Nanoparticle surface treatments were developed to reduce damaging oxidation and corrosion of stainless steel and other alloy components in oxidating and corrosive conditions.²

Improving the longevity of stainless steel at elevated temperatures was the genesis of this development. Power generation plants often have problems with damaging oxidation from accelerated high-temperature fire-side corrosion due to molten alkali salts. Other problems include accelerated medium-temperature fire-side corrosion due to low oxygen activity environments and the presence of sulfur and steam side oxidation of tubing, piping, and valves in fossil fuel-fired boilers.

Pressure has been placed on power plants to increase efficiency, meet stringent environmental regulations, and ensure plant reliability, availability, and maintainability—ideally while minimizing costs.³

Some of the nanoparticles used as surface treatments for corrosion and oxidation resistance include aluminum, silicon, scandium, titanium, yttrium, zirconium, niobium, lanthanum, hafnium, tantalum cerium oxide (nanoceria), and thorium. These nanoparticles do not really form coatings but rather are doping agents that become embedded within the oxide structure.

Over time, yttrium has become a preferred nanoparticle for these types of applications. Yttrium, particularly in the form of yttrium oxide nanoparticles (Y2O3 NPs), is considered superior because of its exceptional properties around high thermal stability, strong chemical stability, mechanical strength, and strong corrosion resistance. Perhaps the greatest attribute under the veil of sustainability is that yttrium has low toxicity. Because of its low toxicity, this nanoparticle is often used in biomedical applications.

Minimox® is one of the yttrium-based, self-protective alloy treatments that minimizes oxidation of alloys at elevated temperature. It changes the structure and/or chemistry of the thermal oxide, which becomes denser and more adherent than thermal oxide without Minimox® doping. Flaking, protrusions, and microvoids are eliminated, resulting in a smooth, dense, adherent surface.⁴

Depending upon the time and temperature of the annealing process, thin film oxides can be grown to thicknesses between 500 and 9,000 Å (1 Å = 0.1 nanometer). Oxidized surfaces doped with yttrium are characterized by excellent cyclic and isothermal oxidation, corrosion resistance in severe environments, and high resistance to severe heat and chemical environments. As a bonus, the thickness of the oxide can be controlled within a high-temperature annealing atmosphere to purposefully impart color onto a metal surface.

Modern Colored Steel

The black stainless steel for architectural applications referenced earlier in this article did not result in large volumes of product released into the field. Despite the excellent properties derived from the surface modification, stainless steel was too expensive for run-of-the-mill building applications. However, the idea of a black surface that did not require painting simmered in the background for many years. The thought of being able to colorize a surface using existing in-line annealing practices was a tantalizing thought. Black stainless manufactured using the existing in-line annealing process, without the addition of either paint or doping agents, was successfully accomplished in 2018.⁵ However, stainless remained an extremely expensive substrate for many applications.

It was determined that other substrates that were more in line with cost constraints of industries, such as appliance and automotive, would also benefit from this oxide modification methodology. Electrogalvanized steel (EG) was textured to the required product finish and the zinc coating was subsequently annealed. The resulting black substrate met all the requirements of appliance finish specifications for refrigerator door applications (the most stringent requirements in the appliance world). This “faux stainless” EG product was compared to black polished stainless and the GE Black PVD standard, and it was found to meet all requirements, including color (Figure 1), gloss, stain/aging, anti-fingerprinting, corrosion resistance, and mechanical properties—all at a much lower cost.

FIGURE 1 Black color comparison between standard product, black stainless steel and “faux stainless steel” textured electrogalvanized.

Finally, in 2023, a method of colorizing stainless steel using yttrium doping and annealing was developed. To prove the concept, stainless steel coils were coated with an aqueous yttrium pretreatment solution and batch annealed for several hours to oxidize the surface. Batch annealing is not particularly uniform, and is not the streamlined process of choice, but enough of the coil exhibited the required properties to move forward and optimize the process.

The basic idea was to dope the surface with an aqueous yttrium nano-solution using an in-line roll coater, followed by continuously annealing the strip while varying time and/or temperature in the annealing furnace to grow the oxide and obtain the desired color.⁶ Even keeping the annealing furnace atmosphere constant, varying line speeds (time in anneal) can allow for the efficient production of different colors. (See Table 1.) These colored stainless coils were fabricated and placed on structures in the Nashville, TN, area (Figure 2) and are still in place today.

FIGURE 2 Colored stainless panels on structures in the Nashville, TN, area.

All the aforementioned work has culminated in less waste and more efficiency. Technological improvements in substrate quality, steel processing, and nanotechnology have bolstered the development of a variety of products previously deemed impossible, too expensive, or of insufficient quality for commercial production. By looking at outside-the-box alternatives, it is possible to obtain color by manipulating the oxide and removing the organic coating completely.

References

1. Helgert, Harold. L; et.al. Method of Making Black Stainless Steel Sheet. U.S. Patent 3,556,871, January 19, 1971.
2. Kerber, Susan J. Nanoparticle Surface Treatment. U.S. Patent 8,568,538 B2.
3. Stott, F. H. Influence of Alloy Additions on Oxidation. Materials Science Tech. 1989, 5 (8), 734.
4. Material Interface Inc. Protecting Furnace Components with Minimox® Alloy Treatment. Minimox® Product Data Bulletin. 2016.
5. Myers, F.A. Black Steel. AK Steel Presentation, 2018.
6. Myers, F. A.; Price, L. R. Method of Colorizing Stainless Steel Using Strip Anneal Processing. U.S. Patent 11,584,997 B2, February 21, 2023.

Cynthia A. Gosselin, Ph.D.,��is director at The ChemQuest Group,��www.chemquest.com. Email:��cgosselin@chemquest.com.

Electrification has been a hot topic for the last few years. The aging power grid where 30% of electric lines are approaching their typical 50- to 80-year lifecycle, leading to brownouts in the western United States. Artificial intelligence (AI) advancements that require exponentially more power than traditional data centers, crypto mining, and a growing population that is using more and more electronics are causing hook-up bottlenecks, particularly for clean energy, large users, new technologies, and an increasing number of consumers.

Adding to these needs, the North American Electric Reliability Corporation predicts that rising peak demands, projected capacity deficits, fuel vulnerabilities, and insufficient dispatchable resources are risks to the availability of power if growth of transmissible power stalls. The ever-increasing energy needs and the vulnerability of our current resources have Microsoft partnering with Constellation Energy to reopen Unit 1 at the Three Mile Island nuclear plant as the Crane Clean Energy Center to provide enough power to operate their projected AI data warehouses.¹

On October 18, 2023, the U.S. Department of Energy (DOE) announced up to $3.5 billion for 58 projects across 44 states to strengthen grid reliability nationwide.² In order to achieve these goals, it will be necessary to manufacture a significantly larger number of transformers and generators. Each of those pieces of equipment will require electrical steel laminations with highly specialized coatings to meet power requirements, insulation levels, durability, corrosion resistance, and temperature resistance to improve energy efficiency.

This projected growth is so important to the national economy that Cleveland Cliffs, a large producer of electrical steel in the United States, announced plans in 2022 to repurpose a shuttered tin-plate facility to produce state of the art electrical transformers in Weirton, WV.³ Lourenco Goncalves, chairman, president, and CEO of Cleveland Cliffs, has emphasized the critical shortage of electrical transformers and its impact on economic growth throughout the country. He has famously stated there will be no AI without electricity and there will be no electricity without transformers.

The global electrical steel coatings market was valued at $304 million in 2023 and projected to grow at a 5.3% CAGR to $490.3 million from 2024 to 2032. Growth will be driven by an increasing need for energy-efficient transformers and electric motors as industries strive to reduce energy losses and meet various environmental regulations. The chromium-containing segment is currently 42%, with the C-5 coating segment valued at $198.1 million.⁴ North America is a prime market due to industrial automation, growth in the EV market, high consumer spending, industrial technology advancements, and the competitive growth of energy-devouring AI initiatives.

Grain Oriented Electrical Steel (GOES) has the largest market share because it can generate magnetic flux while at rest and not rotating. This is the best electrical steel for large, high-performance generators and energy-efficient and traditional transformers because of superior magnetic properties. GOES exhibits lower core losses, strong magnetic directionality, and superior magnetic permeability (amount of magnetization produced in response to a magnetic field). Grains are parallel to the rolling direction. Absolute magnetic properties are dependent upon the heat treatment imparted during production.

Non-Oriented Electrical Steel (NOES) is used primarily for rotating equipment such as electric motors, generators, and over-frequency and high-frequency converters. Both types of steel chemistries contain varying levels of iron (93–98% by weight), silicon (1.2–3.5% by weight), and chromium (0–1.5% by weight). Trace elements of raw materials and/or alloying elements are also present, including aluminum, boron, calcium, carbon, columbium, copper, manganese, molybdenum, nickel, phosphorus, sulfur, titanium, and vanadium.⁵ Electrical steels are coated according to ASTM A976-18 Standard Classification of Insulating Coatings for Electrical Steels by Composition, Relative Insulating Ability, and Application.

Electrical steels are sold as either semi-processed or fully processed. The main difference is that semi-processed steels receive the final heat treatment subsequent to punching into the required shape while fully processed steels are sold with an insulating coating, full heat treatment, and specific magnetic properties.

Rembrantin, Axalta, and Chemetall are well-known global electrical steel coatings suppliers, and Henkel provides a product within the United States. All have recently increased their offerings to include chromium-free and/or formaldehyde-free alternatives to meet current or potential environmental regulations.^6,7,8

There are nine categories of electrical steel coatings currently documented in ASTM A976. Table 1 lists the chemistries of the various coatings. Table 2 provides a description of the coating characteristics for each classification.

The electrical steel coatings market has been growing over the past few years and is slated to play an ever-increasing leading role in the facilitation of all aspects of electrification equipment for upgrading the power grid, enhancements for AI power generation, and clean energy initiatives in the industrial sector.

Cynthia A. Gosselin, Ph.D.,��is director at The ChemQuest Group,��. Email:��cgosselin@chemquest.com.

References