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Corning DuraTrap® GC gasoline particulate filter (foreground) and a Corning FLORA® low-mass substrate for catalytic converters (background).

Smog begone! How development of ceramic automotive catalytic

substrates and filters Credit: Corning Incorporated. eep in the heart of your car’s helped reduce Dexhaust system—withstanding temperatures of more than 1,800 degrees Fahrenheit and staying tough over hundreds of air pollution thousands of miles of bumpy roads—a highly engineered ceramic is an important part of helping to prevent harmful emissions from escaping into the air you breathe. By keeping the air cleaner, these same ceramic products help save as many as 160,000 lives each year and help to prevent just By Douglas M. Beall and Willard A. Cutler as many cases of heart disease and asthma.1 From 1975 through today, and heavy-duty emis- sions dropped by an astounding 99%. Ceramic-based mobile Ceramic-based mobile emissions control products have emissions control products prevented more than 4 billion tons of

prevented billions of tons of hydrocarbons, , hydrocarbons, 4 billion tons of nitrogen oxides (NOx), and 40 bil- lion tons of monoxide from entering the atmosphere. carbon monoxide, and particulates from entering the atmo- The clean-air movement brought economic benefits as well. sphere—and researchers continue to innovate to make these Emissions-related represent about $37 billion in products even better. annual economic activity, with a significant portion of the indus- try involving ceramic-based components.2 Capsule summary A DIRTY PROBLEM CERAMIC SOLUTION AHEAD By the mid-1900s, health hazards associated In the 1970s, Corning scientists invented a Most emissions in a typical drive cycle are with poor air quality had come into stark view. substrate for catalytic convertors based on produced in the first minutes of operation, Governments set new federal air pollution cordierite and also a ceramic-based wall-flow so the remaining frontier of a zero-emission regulations that presented both a technologi- particulate filter for diesel engines. Nowadays, internal combustion vehicle is tackling this first cal challenge and business opportunity for ceramic substrates and filters are the keystone minute of operation. manufacturers. of mobile pollution control ecosystems.

24 www.ceramics.org | American Ceramic Society Bulletin, Vol. 99, No. 3 in ceramics will continue began its first attempt to legislate a solu- Control Act of 1965, and the Air Quality to be needed for at least the next couple tion with the Air Pollution Control Act Act of 1967 all lacked the teeth to make a of decades. Consumers throughout the of 1955, which focused on research and difference in the deadly problem. world, not only in areas with untreated information, leaving the states to devise But in 1970, sweeping new federal mobile pollution sources, are demanding ways to deal with polluters. A few states, regulations required clean-air compliance better air quality. Areas of focus that con- notably California, had a plan to combat from every segment of industry. The new tinue to drive ceramic innovations include the issue. Environmental Protection Agency (EPA) (1) Ensuring that real-world emissions The U.S. government made several was formed with the mandate to enforce are as low as emissions on the certifica- more attempts to regulate sources of air the regulations. tion test; pollution. However, the Clean Air Act of The Clean Air Act of 1970 estab- (2) Gaseous emission limits reduc- 1963, the Motor Vehicle Air Pollution lished air-quality standards that strictly ing, particularly NOx emissions, while supporting the reduction of greenhouse Clean Air Act of 1970 extreme benefits thanks to this legislation. gases; and For example, a 2011 EPA study2 estimated When it comes to clean air in the United (3) Reducing the last remaining bursts the central benefits exceed costs by a factor States, one of the most important pieces of of gaseous and particulate emissions of more than 30 to one, largely due to reduc- legislation that made it possible is the Clean tions in premature mortality associated with from internal combustions engines, the Air Act (CAA) of 1970. reductions in ambient particulate matter. emissions that occur during the first Prior to CAA 1970, there were a few federal minute or two of vehicle operation—an However, the gains in clean air made thanks acts involving air pollution.1 The first was the event that can happen several times dur- to CAA 1970 and its amendments may be Air Pollution Control Act of 1955, which pro- challenged in the future. The Trump admin- ing a drive with hybrid . vided funds for federal research on air pollu- istration has pursued the rollback of almost tion. This was followed by CAA 1963, which 100 environmental rules, 58 of which are now A growing dirty problem established a federal program within the U.S. completed. Of these completed rollbacks, 16 By the late-1940s, industrial activity in Public Health Service and authorized research involve air pollution and emissions rules.3 the United States had grown for nearly a into techniques for monitoring and control- ling air pollution. In 1967, the Air Quality Act Recent studies also show air quality improve- century. Little attention was paid to the ment is leveling off. A 2018 report4 by the consequences of smoke and pollutants was enacted to expand federal government activities, including conducting the first-ever U.S. PIRG Fund and Environment from coal-burning factories. Suburbs extensive ambient monitoring studies and America Research & Policy Center found were growing. More and more fami- stationary source inspections. 2018 had more days of pollution than each of lies could afford cars. With city traffic the previous five years, a finding supported crowding new highways, unprecedented However, CAA 1970 marked a major shift in by studies by the American Lung Association5 the federal government’s role in air pol- and Carnegie Mellon University.6 levels of harmful chemicals flowed into lution control by substantially expanding the air, especially in major population the government’s enforcement authority.1 References: centers—both in the U.S. and in other It authorized development of comprehen- 1United States Environmental Protection Agency. “Evolution developed countries. sive federal and state regulations to limit of the Clean Air Act.” Accessed 28 Feb. 2020. https:// www.epa.gov/clean-air-act-overview/evolution-clean- The health hazards associated with emissions from both stationary (industrial) air-act poor air quality came into stark global sources and mobile sources. In particular, it 2United States Environmental Protection Agency. “Benefits view in London in late 1952, when air- established four major regulatory programs and Costs of the Clean Air Act 1990-2020, the Second affecting stationary sources: Prospective Study.” Accessed 28 Feb. 2020. https://www. borne pollutants, mostly arising from the epa.gov/clean-air-act-overview/benefits-and-costs-clean- use of coal, mixed with fog to form a thick • National Ambient Air Quality Standards air-act-1990-2020-second-prospective-study blanket of smog over the city. Historical (NAAQS) 3N. Popovich, L. Albeck-Ripka, and K. Pierre-Louis (21 Dec. accounts vary on the details, but most • State Implementation Plans (SIPs) 2019). “95 environmental rules being rolled back under Trump,” The New York Times. https://www.nytimes.com/ agree between 4,000 and 6,000 people • New Source Performance Standards interactive/2019/climate/trump-environment-rollbacks. died over the course of five days from the (NSPS) html choking effects of the Great Smog.3,4 • National Emission Standards for Hazard- 4E. Ridlington, G. Weissman, and M. Folger (Winter 2020). The following year, New York City ous Air Pollutants (NESHAPs) “Trouble in the air: Millions of Americans breathed polluted air in 2018.” U.S. PIRG Education Fund. Accessed 28 Feb. was covered with a toxic mix of sulfur In 1977 and 1990, two sets of major amend- 2020. https://uspirgedfund.org/sites/pirg/files/reports/ dioxide and carbon monoxide that blan- ments were added to CAA 1970.1 The 1977 EnvironmentAmerica_TroubleintheAir_scrn.pdf keted the city. During one week, accord- amendments focused on ensuring attainment 5American Lung Association. “The state of the air 2019.” and maintenance of NAAQS by establishing Accessed 28 Feb. 2020. https://www.lung.org/our-initia- ing to reports, as many as 260 deaths tives/healthy-air/sota/key-findings/ major permit review requirements; the 1990 were attributed to the smog. More 6 amendments substantially increased the fed- K. and N. Z. Muller (October 2019). “Recent deadly smog crises would hit both New increases in air pollution: Evidence and implications for York and London again over the follow- eral government’s authority and responsibility. mortality.” NBER, Working Paper No. 26381. https://www. nber.org/papers/w26381 n ing decades.5 To date, several in-depth benefit-cost analy- With the public health effects of air ses of the CAA Amendments performed by pollution now beyond dispute, the U.S. the Environmental Protection Agency all show

American Ceramic Society Bulletin, Vol. 99, No.3 | www.ceramics.org 25 Smog begone! How development of ceramic automotive catalytic substrates and . . .

a) b) c) d)

Schematic of laboratory scale production

Figure 1. Numerous pathways to fabricating porous ceramic substrates were tried before settling on extruded honeycombs.

(a) Alternate layers of flat and crimped paper infused with - Credit: Beall and Cutler ceramic (CERCOR material); (b) layers of cordierite glass-ceramic sheets with small “nubbins” to increase the surface area and permit exhaust flow; (c) open, layered structure similar to ribbon candy formed by buckling a hot glass stream and later ceraming the part; and (d) glass tubing fused together, then ceramed, looking like a packet of hollow cigarettes. limited levels of six pollutants that he liked Corning’s ingenuity. He urged tinuously operating melting used threatened public health: sulfur dioxide, the company to investigate the substrate to mass-produce light bulbs as well as nitrogen dioxide, particulate matter, opportunity for catalytic convertors. one of the company’s newest products: carbon monoxide, ozone, and lead.6 (See GM and other automakers had settled PYREX kitchen ware. “Clean Air Act of 1970”) on internal combustion engines and Corning approached the emissions- For automakers, the government’s the , first patented control project by researching a wide vari- marching orders were clear: design vehi- by French-born mechanical ety of designs and . Under con- cles that could run on unleaded gasoline Eugene Houdry in 1955, as the solution sideration, for example, were (Figure 1) and incorporate a new device—the cata- for reducing harmful engine emissions. • Alternate layers of flat and crimped lytic converter—to reduce carbon mon- The converter required unleaded gaso- paper infused with CERCOR glass- and hydrocarbons by 90% from line and a durable substrate with low ceramic material; car exhaust. By 1975, all American-made resistance to flow that also provided a • Layers of cordierite glass-ceramic vehicles were required to meet the new lot of surface area, allowing the exhaust sheets with small “nubbins” to increase emissions requirements—no exceptions. to pass over the platinum group catalysts the surface area and permit exhaust flow; European countries implemented similar supported on high-surface-area gamma- • An open, layered structure, not regulations in 1992. alumina. unlike ribbon candy, formed by buckling Many capable people were trying a hot glass stream and later ceraming A technical challenge, to solve the problem, including those (i.e., heat treating to induce crystalliza- a business opportunity at GM, 3M, W.R. Grace, Engelhard, tion of the glass) the part; and Around the same time in 1970, Johnson Matthey, and more. But as of • Glass tubing fused together, then Corning was bringing its materials sci- 1970, no clear winning substrate tech- ceramed, looking like a packet of hollow ence expertise to General Motors with nology emerged. Corning chief technol- cigarettes. the idea of a lightweight, chemically ogy officer Bill Armistead responded to Competitors were testing potential strengthened glass windshield made on MacAvoy’s challenge by launching an solutions just as wide-ranging. its new fusion-draw process. GM had lit- internal emissions control (EMCON) • American Lava Corporation (a tle interest in Corning’s windshield glass, project and directed significant funding subsidiary of 3M) developed a way to as the recently developed Pilkington’s into its R&D efforts. alternate layers of flat and corrugated float process was more economical. Advanced glass was Corning’s best- ceramic-impregnated paper, which While at GM, Corning president Tom known specialty, but the company’s was then slowly fired. The design used MacAvoy showed GM president Ed Cole skill in ceramics dated back to the late zircon-mullite and cordierite-mullite a sample of a unique glass-ceramic mate- 19th century, when designed compositions. rial called CERCOR®. Light and highly and created durable ceramic crucibles • W.R. Grace devised a ceramic resistant to heat, Corning envisioned for melting glass. In 1920, the company powder-filled plasticized polyolefin sheet marketing CERCOR material as a heat formed a ceramics research group and with ribs. The sheet was rolled and heat- exchanger for engines. Cole soon began making ceramic sealed, providing parallel airflow paths. told MacAvoy that the industry was with extreme chemical durability. The were burned off to form moving away from turbine engines, but The bricks were ideal for lining the con- the final product.

26 www.ceramics.org | American Ceramic Society Bulletin, Vol. 99, No. 3 Figure 2. Results from the first of cellular ceramic (left), and drawings from the patented idea filed in November 1971. First tested in July and first scaled in October 1971. Credit: Beall and Cutler • General Motors’ in-house team heating and cooling experienced within History in Paris in 1813. used a packed bed of catalyst-coated a car’s powertrain system. Lachman The synthetic version of cordierite ceramic rather than a struc- worked with Ron Lewis to improve the that Corning scientists created included tured ceramic. material. The resulting synthetic cordier- no iron and contained ,

Technically, several of the designs ite not only had very high temperature aluminum, and (Mg2Al4Si5O18), worked. But the question of manufactur- capabilities (Tm >1,400°C) but also great creating a new material for emissions- ing—specifically, how to produce millions thermal shock resistance. control products. of substrates per year, at a low cost—was The first successful 4.66-inch (118 mm) Irwin Lachman was working with cor- most challenging. diameter extrusion of cordierite was in dierite for other applications at the time 1971 (this diameter is still a common of Bagley’s invention of the die. Lachman A breakthrough solution diameter for automotive exhaust compo- considered cordierite an attractive choice The tide turned in July of 1971 when nents). Corning branded the new ceramic of material for several reasons. two young Corning scientists—Ed Bush substrate as Celcor® and received its first First, the application required a very and Rod Bagley—were in a meeting with a order—from Ford Motor Company—by the high level of thermal shock resistance, colleague at the company’s research cam- end of 1971. and cordierite was known to have a very pus, Sullivan Park. Bagley described an It worked so well that Corning—even low CTE. Second, cordierite had good idea he had mulled over—using extrusion as it was financially challenged in the high-temperature stability and therefore to make a cellular substrate structure. increasingly global color TV glass market— could survive even the highest tempera- He sketched his idea on blackboard. shelved other potential substrate solutions tures that would be encountered in the The original design had offset slots in and quickly invested $25 million into application, and it also had good chemi- both sides, but it was not clear to the a new environmental factory in Erwin, cal stability in the environment that others how it would work. To clarify the N.Y. This Corning factory depreciated would be encountered in the vehicle concept, he ran to the mason’s shop and over five years, as car companies claimed exhaust. Third, cordierite could be syn- grabbed a soft refractory and used they would improve their engines so thesized from relatively inexpensive and a saw to make a 3D model, fur- that catalytic convertors would not be commonly available batch materials such ther demonstrating the concept. needed in the future. Instead, the market as talc (Mg3Si4O10(OH)2), clay

The first practical embodiment was a continued to grow, and the factory is still (Al2Si2O5(OH)4), and gibbsite (Al(OH)3) custom-made prototype die about operational today. or corundum (Al2O3). 25 mm in diameter, which produced Lachman combined these raw materi- parts with 50 cells per square inch and What’s special about cordierite? als along with a methyl cellulose binder cellular walls roughly 0.5 mm thick Naturally occurring cordierite is a and water to produce a mass with (Figure 2). Bagley extruded alumina mineral compound containing magne- a putty-like consistency, which could be through the brass die to test the idea. sium, iron, aluminum, and silicon. It is pushed easily through Bagley’s extrusion Meanwhile, scientist Irwin Lachman found, among other places, near veins of die to produce the honeycomb structure. was developing a cordierite-mullite tin in the mines of Southern England. The extruded honeycomb parts were composition with a remarkably low coef- It draws its name from French geologist then dried to remove the water and fired ficient of (CTE)—a Louis Cordier, who included the min- to a high temperature, allowing the that could remain stable and eral in a much-celebrated geological gal- materials to react together to form the functional despite the repeated extreme lery at the National Museum of Natural cordierite phase.

American Ceramic Society Bulletin, Vol. 99, No. 3 | www.ceramics.org 27 Smog begone! How development of ceramic automotive catalytic substrates and . . .

Lachman found that, after firing, the of the expansion coefficients in the Ed Bush discovered the CTE hyster- raw materials converted to over 95% three directions is 1.8x10–6°C–1 (from esis sometimes observed in this material cordierite phase. The fired ceramic hon- 25°C–800°C), which is a very low value was due to microcracking, which could eycomb was porous, containing about relative to most ceramic materials. be intentionally induced to become 30 vol% of in the walls. The However, measurements of the thermal engineered expansion joints (serving a presence of the porosity proved to be expansion of the cordierite honeycombs similar function to expansion joints on advantageous compared to a dense ceram- consistently showed expansion coeffi- a bridge), further lowering the CTE of ic because the pores in the ceramic walls cients of half that value or even lower. the structure. Control over the size and allowed the washcoat containing the cata- Researchers also found their choice density of stable engineered expansion lyst to be slip casted onto the walls. The of raw materials and firing cycle could joints in the matrix is important. The porosity also fortuitously decreased the change the amount of suppression of combined impacts were found to be heat capacity of the honeycomb relative the thermal expansion. Analysis of the capable of reducing the thermal expan- to a dense ceramic, allowing it to heat up of the ceramic walls of sion coefficient of the cordierite honey- faster in use to the temperature where the the substrates revealed the reasons for comb by up to an order of magnitude or catalyst became active. the suppression of the bulk thermal more (compared to the average crystallo- Furthermore, the surface pores served expansion. Ronald Lewis discovered the graphic value), which is important to cre- as anchor points for the high surface cordierite had a preferred ating the severe thermal shock resistance area washcoat and catalyst, increasing the orientation, with a majority of the crys- required for this application. capability of the catalyst in the tallites oriented with the negative expan- harsh environment of thermal cycling sion c-axis lying within the plane of the Ceramic flow-through substrates and mechanical vibrations that would be ceramic walls. Therefore, the thermal The first commercial ceramic substrates encountered when in the vehicle. expansion coefficient within the plane of were low cell density (about 200 cells/in2) One of the most interesting findings the ceramic walls was depressed, relative with thicker walls (about 12 mil or 0.012” was that dilatometric measurements of to the average lattice expansion value. or 0.3 mm) with a substrate volume about the synthetic cordierite honeycomb struc- Lewis and Lachman determined the four times that of engine displacement ture showed the bulk CTE was lower than preferred orientation resulted from the (i.e., cylinder volume of the engine). As expected based on what they knew about position of the platy raw materi- material and processing pro- the structure of mineral cordierite als during the extrusion process as the gressed, higher cell densities, thinner walls, (also known as iolite) from X-ray diffrac- materials passed through the thin slots and higher became possible. tion data describing the lattice expansion. in the extrusion die. This discovery For historical reasons, substrates are The thermal expansion of cordierite led to the granting of a U.S. patent to commonly referred to by their cell den- is anisotropic with a negative thermal Lachman and Lewis for an anisotropic sity/wall thickness moniker. For example, expansion in the c-axis of the crystal cordierite monolith with designed pre- a 400/4 substrate is one in which the and positive expansion in the a and b ferred orientation and low bulk thermal “400” defines the cell density in cells/in2 axes of the cyclosilicate. The average expansion coefficient.7 (or cpsi) and the “4” defines the nominal wall thickness in 0.001” increments (or mils). In the average U.S. gasoline engine sedan, there are two or three substrates at work to meet the rigorous U.S. gaseous emissions standards. Right off the engine, close-coupled substrate(s) with high cell density (750/2 or 900/2) provide a lot of geometric surface area to allow the catalyst to do the initial gaseous conversions. In the underfloor position

Credit: Beall and Cutler there is generally a lower Figure 3. (a) Schematic of orthorhombic cordierite showing anisotropy in thermal expansion. cell density substrate like (b) SEM micrograph of the web surface of a cordierite honeycomb showing orientation of individual 400/4, to help clean up cordierite crystallites and microcracks that act as engineered expansion joints. Dark areas are pores. final emissions. Table 1 Width of view is approximately 80 µm. (c) Expansion joint in a bridge, which allows for thermal expansion of the bridge components, but the length of the bridge itself remains constant. describes common macro

28 www.ceramics.org | American Ceramic Society Bulletin, Vol. 99, No. 3 and micro properties of ceramic sub- cept to filter soot strates and filters. particles from diesel exhaust for diesel Development of particulate filters engines running in The flow-through honeycomb substrate confined spaces, was the ideal platform for supporting like vehicles. catalysts that eliminate harmful gaseous air Rod thought his pollutants but did little to remove harmful ceramic-based wall- particulates from exhaust. Particulates in flow concept might exhaust are often the result of incompletely work better for this combusted fuel and influenced by varied application. He had factors including, but not limited to, ambi- some prototypes ent temperature, altitude, fuel quality, vehi- made and testing

cle power-to-weight ratio, drive cycle, and proved his design Credit: Corning Incorporated engine hardware and software. The World worked very well. Figure 4. Pictured is a cross-section of a cell wall of a diesel Health Organization (WHO) cautions that Corning DuraTrap particulate filter, showing an accumulated soot layer at 200x microscopic carbon particles, when inhaled, filters are still pro- on top of the porous ceramic wall. “can penetrate the lung barrier and enter duced with the Frost design, as are almost Diesel engines often produce a lot of the blood system. Chronic exposure to par- all other particulate filters. These filters soot particles, from both a particle num- ticles contributes to the risk of developing have a cellular honeycomb ceramic with ber and a mass perspective. This particle cardiovascular and respiratory diseases, as engineered wall porosity to capture fine output, combined with low engine-out 8 well as of lung cancer.” particles. Individual channels are open temperatures, often results in a soot In the late 1970s, an aluminum manu- and plugged at alternating ends, like a cake on the inlet wall surfaces of the facturer asked Corning if they had a checkerboard. Exhaust gases enter the wall-flow diesel particulate filter (DPF). product that could be used to filter impu- open (inlet) channels, flow down the The porous deposit of nano to submi- rities from molten aluminum. Corning channel, and escape only through the cron soot particles on the filter wall can scientist Rod Frost, who had led the engineered porosity of the cellular walls. increase the native filtration efficiency development of the process for producing The walls offer little flow resistance and (Figure 4). The same mechanism oper- the ceramic honeycomb substrate, con- particles become trapped in the porosity ates in gasoline particulate filter (GPF) ceived of the wall-flow particulate filter as and collect on the filter walls instead of applications, where the number of a possible solution for this application. being released into the atmosphere. The particles in the exhaust can be high but As sometimes happens in R&D, cleaned gas exits the filter through the the particles have less mass. Collecting the concept did not work for this par- adjoining (outlet) channels. Filters can be a soot cake in gasoline applications is ticular application. However, months used in their bare state, or in conjunction more difficult and typically does less to later, diesel engine manufacturers with catalysts to assist in gaseous-emis- aid filtration. came to Corning looking for a con- sions reduction, or to aid soot burning. In both diesel and gasoline applica- Table 1. Flow-through ceramic substrates for automobiles through heavy duty vehicles tions, the captured particles remain Common Possible Comments in the filter until exhaust conditions are appropriate to burn the particles Macro properties to “clean” or “regenerate” the filter. Cell density (cpsi) 200–900 100–3000 >900 cpsi - very high drop (∆p) Regenerating wall-flow filters can take Wall thickness (mil) 2–8 2–17 < 2 mil carries significant cost and isostatic strength place passively (as a by-product of the implications time/temperature/atmosphere) or active- Cell shape square asymmetric Cell shape is often balanced with cost, isostatic strength, hexagonal triangular geometric surface area and ∆p ly (triggered by additional sensors and rectangular round software). For example, in the diesel case, Isostatic strength (bar) 7–10.5 5–15 Usually important for canning strength active regeneration increases the filter inlet temperature in the presence of the Diameter (mm) ~101–330 ~50–610 Larger diameters can be available as assemblies of smaller (4–13”) (2–24”) blocks appropriate -containing species to Length (mm) ~76–300 ~50–600 Length can be dictated by ∆p and dimensional burn the combustible particles, returning (3–12”) (2–24”) control implications the filter to its nearly clean state. Filters CTE (x10-6/°C, RT 0.05–1.0 0–50 Very low coefficient of thermal expansion (CTE), enables generally last the life of the vehicle. to 800°C) thermal shock resistant structures The selection of filter materials depends Wall porosity (%) 25–55 5–70 More porosity leads to lower mass, but also potentially lower on the filtration efficiency requirements strength (microstructure) and the heat capacity Mean pore size (µm) 2–20 0.5–30 Engineered porosity is more important for filters than flow requirements (material choice, porosity, through substrates cell density/wall thickness). For gasoline

American Ceramic Society Bulletin, Vol. 99, No. 3 | www.ceramics.org 29 Smog begone! How development of ceramic automotive catalytic substrates and . . . vehicles and many heavy-duty diesel substrates and, in many cases, wall-flow first minute or two of operation. Once vehicles, where passive regeneration domi- filters are sent to catalyzers to apply a catalysts are hot, they are extremely effec- nates, cordierite is the material of choice washcoat and catalyst. These catalysts tive. Therefore, the remaining frontier due to its low cost, low heat capacity, high- can be platinum-group -based of a zero-emission internal combustion melting-point, low CTE, and low-thermal- catalysts supported on high surface area vehicle is tackling this first minute conductivity with macrostructural and alumina, ceria, zirconia, zeolite, or can of operation, including vehicles with microstructural flexibility. be other catalysts or sorbents. multiple engine starts like start/stop In some diesel applications, particular- After coating, the composite product vehicles and hybrid vehicles, which may ly in the diesel passenger car segment, the is sent to a canner and wrapped in a switch between electric and conventional mass of the soot collected is high, result- ceramic-based mat material (Figure 5). engines several times during the drive ing in the possibility of higher exotherms The ceramic-based component must have cycle. Due to low engine outlet tem- when the filter is regenerated. The heat sufficient isostatic strength to withstand peratures (increased engine efficiency or generated only has two paths to dissipate. the canning process, where the material is frequent starts and stops), active devices It can be carried away in the exhaust gas squeezed (stuffing process) or compressed may also play a role. or is adsorbed by the filter. In regenera- (tourniquet process) into the can. The Current designs and materials for tion conditions with little exhaust flow, ceramic-based mat provides heat insula- ceramic-based substrates, filters, catalysts, high-heat-capacity alternatives to cordier- tion and a holding force to maintain the and mats will continue to play an impor- ite, such as silicon or aluminum ceramic in the can while it accommodates tant role in meeting future requirements. titanate, are used to adsorb the exother- the differential shrinkage between the However, new ceramic processes, materi- mic reaction to prevent filter damage. ceramic (low expansion) and the metallic als, , and designs will be can (high expansion). The canned system needed to enable products that heat-up How these products are used in is then welded into the complete exhaust faster and have higher pollution removal the system system and integrated with sensors, so that efficiencies. Thinner walls, higher poros- Ceramic substrates and filters are the computers on the vehicle assure the emis- ity levels, tighter pore size control, and keystone of mobile pollution control sions are compliant under all conditions. additional durability are all on the list. ecosystems. The vehicle emission system Cellular ceramics proved to be an effec- is composed of ceramic substrates and Innovation continues tive way of packing a lot of geometric sur- filters and ceramic-based catalysts held The growth of pure electric vehicles— face area into a small volume, and the cel- in place by a ceramic-based mat mate- which have no tailpipe emissions issues lular form factor is an important success rial. The ceramic substrates and filters (although issues exist at the electricity factor for mobile emissions remediation provide a thermal-mechanically-stable source and from the braking system)—are (Figure 6). There are likely benefits for base that can withstand extremes in tem- changing the baseline and societal expec- cellular ceramics beyond mobile emissions perature and vibration and last for the tations. However, even with aggressive as similar benefits may extend to catalysis, life of the vehicle. In the case of filters, battery electric vehicle penetration, the sorption, and filtration in other industries they also provide the engineered micro- combination of geographic expansion such as petrochemicals, fine chemical, structure that allows for the particulate of regulations and regulation tightening clean water, and more. For these reasons, filtration function. All flow-through in existing regions is likely to increase ceramic filters and substrates are likely to ceramic substrate and filter volume remain a key factor in the improvement over the next 10 to 15 years. To stay of the environment around the world. competitive, some internal combus- tion engine-based vehicles are going Regulations, impact, and well beyond regulatory require- societal benefits ments—to nearly emissions free—to Over the past nearly 50 years, ceramic meet consumer expectations. substrates and filters brought dramatic Because of advancements in benefits to society. Since the U.S. adopted engine, vehicle, and emissions the Clean Air Act of 1970, the nation’s control, the capacity exists to make economy grew fourfold. At the same time, vehicles achieve “negative emis- ambient air pollution dropped more than sions” after the first couple of min- 70%. The greatest contributor to this utes of vehicle operation in many improvement in air quality—reduced vehicle cities, meaning that the air coming emissions—is largely due to the three-way out of the tailpipe is cleaner than catalytic converter, so called because it miti- that going in. gates NO , CO, and hydrocarbon. With Credit: Corning Incorporated. x Figure 5. Ceramic substrates and filters and Most of the gaseous and par- the addition of the wall-flow filter, in some ceramic-based catalysts held in place within a ticulate emissions in a typical locales the air coming out of the engine can metal can using a ceramic-based mat material. drive cycle are produced in the be cleaner than air going into the engine.

30 www.ceramics.org | American Ceramic Society Bulletin, Vol. 99, No. 3 vehicles. Some California lawmakers

are working to tighten NOx emissions by more than 90% by 2027 to address urban ozone issues. They are also consid- ering doubling or tripling the regulatory full-useful life and emissions warranty for trucks and other heavy-duty equip- ment. Similar initiatives are in the works in Europe and China.

Acknowledgements The authors would like to thank Tim Johnson, Anne Kenlon, Kerstin Stobbe, and Agatha Lutoborski for their help with this article. Credit: Cutler Figure 6. Pressure drop as a function of specific surface area for various form factors About the authors of media, where L is ring length, D is ring diameter, and OFA is open frontal area. Douglas M. Beall is a Development The success of Clean Air Act stan- What’s ahead? Fellow at Corning Incorporated. Willard dards in its early years gave the EPA Despite the short-term fluctuations A. Cutler is division vice president impetus to add even more-stringent of governments, the overall global direc- and commercial technology director of amendments in 1990. These standards, tion is toward tightening standards for Environmental Technologies at Corning according to government studies, are sav- vehicular emissions. This attention to air Incorporated. Contact Cutler at ing an additional 160,000 lives per year. quality intersects the continuing increase [email protected]. The 1990 law also prevents an estimated in vehicle usage. Many forecasts indicate 13 million lost workdays, 1.7 million vehicle growth of up to 50% over the References cases of exacerbated asthma, and roughly next 20 years. Virtually all this growth 1United States Environmental Protection Agency. 1 “Benefits and costs of the Clean Air Act 1990–2020, the 130,000 cases of heart disease each year. will be in heavily populated and develop- second prospective study.” Accessed January 14, 2020. Beyond these benefits, the emissions-con- ing countries. www.epa.gov/clean-air-act-overview/benefits-and-costs- trol systems turned out to be a bargain Following this trend, expect to see clean-air-act-1990-2020-second-prospective-study the eventual expansion of regulations 2Gocha, A., Ross, A., Oney, F., and De Guire, E. “What’s for automakers as well. Catalyst systems in and on that car? The role of ceramics and glass in the represent less than 5% of the sticker in the Asean and Africa regions, as $4 trillion auto industry.” American Ceramic Society Bulletin price of most vehicles, and the societal well as the tightening of regulations in 96(9), 2017: 10–23. benefit—even as regulations became more emerging economies. China, India, and 3National Geographic. “Dec 4, 1952 CE: Great Smog of 1952.” Accessed January 14, 2020. www.nationalgeograph- stringent over the years—is still about $10 South America now are implementing ic.org/thisday/dec4/great-smog-1952/ for every $1 spent. vehicle tailpipe regulations on par with 4Excell, J. (22 Dec 2015) “The lethal effects of London Regulations for heavy-duty vehicles those in Europe but still with looser gas- fog.” BBC. Accessed January 14, 2020. www.bbc.com/ future/article/20151221-the-lethal-effects-of-london-fog and nonroad machineries were also eous emissions than in North America. 5Tracton, S. (20 Dec 2012) “The killer London smog adopted, expanding ceramic substrates Because the technology is already avail- event of December, 1952: a reminder of deadly and filters to larger vehicles and increas- able to make the air even cleaner and smog events in U.S.” The Washington Post. Accessed easily deployed, for example, from the January 14, 2020. www.washingtonpost.com/blogs/ ing component size up to 20 times larger capital-weather-gang/post/the-killer-london-smog-event- than light-duty vehicles, requiring new U.S. to Europe, further regulatory tight- of-december-1952-a-reminder-of-deadly-smog-events-in- developments to make ening in all regions is quite likely. us/2012/12/19/452c66bc-498e-11e2-b6f0-e851e741d196_ blog.html ceramic structures and catalyze the large- Though vastly improved over the past 6United States Environmental Protection Agency. frontal area substrates and filters. 50 years, air quality in the U.S.—spe- “Evolution of the Clean Air Act.” Accessed January 14, The honeycomb ceramic substrates cifically, in large cities and traffic-heavy 2020. www.epa.gov/clean-air-act-overview/evolution-clean- air-act and filters had such a profound impact California—still has room for significant 7Lachman, I. M. inventor; Lewis, R. M. inventor. on air quality that in 2002, the National improvement. Reductions in real world Anisotropic cordierite monolith. U.S. Patent 3,885,977. 1975. Inventors Hall of Fame inducted the three driving emissions for NOx and particulates Corning scientists who developed the are still needed. Some in California are 8World Health Organization (2 May 2018) “Ambient exploring new standards that would enable (outdoor) air pollution.” Accessed January 20, 2020. www. innovation: Dr. Irwin Lachman, Dr. Rod who.int/news-room/fact-sheets/detail/ambient-(outdoor)- Bagley, and Mr. Ron Lewis. They were rec- the use of gasoline particulate filters, air-quality-and-health n ognized for creating the extrusion method matching the low particulate output of for forming the thin-walled structures. And European and Chinese gasoline vehicles. in 2005, those same three scientists won Improvement also continues in long- the National Medal of Technology. term emissions standards for heavy-duty

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