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The substrate/washcoat systems which preserve both the mechanical and thermal attributes of cordierite substrates are most desirable for prolonged durability of automotive catalysts. This paper provides a micromechanics viewpoint of substrate/washcoat composite whose properties are predictable, measurable and relevant to catalyst durability. The micromechanics model helps quantify substrate/washcoat interaction which controls the long-term catalyst performance. Three different examples of substrate/washcoat systems are used here to illustrate the optimization process during the development of new substrates or washcoat technologies to meet the more stringent emission and durability requirements of advanced catalysts for the 1990s.

Significant advances in composition and the manufacturing process have led to thin wall cordierite ceramic substrates with low thermal mass, high surface area, and large open frontal area-properties that are critical for fast light-off, high conversion efficiency and low back pressure. Indeed, such substrates are ideal catalyst supports for meeting the ever-stringent emissions regulations, ala SULEV and ULEV, as demonstrated by recent performance data1. This paper focuses on the physical durability of 400/4 and 600/4 cordierite ceramic substrates. In particular, it presents strength, fatigue, and modulus data which influence the mechanical durability. In addition, it presents thermal expansion data which impact the thermal durability. Both of these durabilities are examined as a function of operating temperature.

The stringent emissions standards in the late 1990's like NLEV, ULEV and SULEV have led to major modifications in the composition and design of ceramic substrates. These changes have been necessitated to reduce cold start emissions, meet OBD-II requirements, and to ensure 100,000 mile durability requirement in a cost-effective manner. This paper presents the key advances in ceramic substrates which include lower thermal expansion, lighter weight, higher surface area and improved manufacturing process all of which help meet performance requirements. In addition to above benefits, the compressive and tensile strengths of lightweight substrates, as well as their thermal shock resistance, are found to be adequate following the application of high surface area alumina washcoat. The strength properties are crucial for ensuring safe handling of the substrate during coating and canning and for its long term mechanical durability in service.

As emissions regulations become more stringent, catalyst supports with higher cell density, smaller wall thickness, higher surface area and lower thermal mass become more desirable for faster light off and higher conversion efficiency. Simultaneously, however, washcoat formulation and loadings have to be adjusted to yield higher and more stable B.E.T. area at operating temperatures representative of close-coupled application. The thermal mass contribution of advanced washcoat system to catalyst supports with 600/4 and 900/2 cell structures may approach or even exceed that of uncoated substrates. Under such high washcoat loadings, the composite properties of advanced catalysts may be affected adversely in terms of their physical durability, notably in close-coupled application. This paper focuses on potential solutions to light-off performance and FTP efficiency, via optimization of substrate/washcoat interaction, geometric design and the mounting system.

The dual requirement of high conversion efficiency and 50K mile durability for cordierite ceramic converters is achievable through optimization of washcoat and catalyst formulation. This paper presents new data for high temperature physical properties, light-off performance, conversion efficiency and pressure drop through an oval cordierite ceramic converter with triangular cell structure and two different washcoat formulations; namely standard vs high-tech. Both of the washcoat systems have a beneficial effect on strength properties with nominal impact on thermal shock resistance. Both the standard and high-tech catalysts provide identical light-off performance for CO, HC and NOx conversion. The high-tech washcoat and catalyst system, in particular, provides consistently superior conversion efficiency for CO, HC and NOx. The pressure drop across the catalyst depends on hydraulic diameter and is only 8% higher for high-tech washcoat than for standard washcoat.

The dynamic fatigue data for two different cordierite ceramic wall-flow diesel filter compositions, EX-54 and EX-66, are obtained at 200° and 400°C using the 4-point bend test. These compositions offer larger mean pore size and experience lower pressure drop than the EX-47 composition, and hence are more desirable for certain applications. Their fatigue behavior in the operating temperature range is found to be equivalent or superior to that of EX-47 composition which helps promote filter durability. The fatigue data are used to arrive at a safe allowable stress, which would ensure the required 290K vehicle mile durability. The paper also discusses the impact of mean pore size on high temperature strength and fatigue properties and their effect on filter durability.

This paper reviews the development and successful application of ceramic catalytic converters for controlling automotive exhaust emissions. It presents the scientific rationale for designing the high surface area substrate to meet both performance and durability requirements. This is followed by a step-by-step design process for each of the converter components. The initial design stage focuses on understanding automaker's requirements and optimizing component design commensurate with them. The intermediate stage involves laboratory testing of converter components in simulated environment and ensuring component compatibility from durability point of view. The final design stage addresses the critical tests on converter assembly to ensure performance and field durability. In addition, it examines the necessary trade-offs and associated design modifications and evaluates their impact on warranty cost for system failure.

The large frontal area cordierite ceramic flow-through converter for diesel emissions must meet the 290K vehicle mile durability requirement, almost a six fold increase over that of automotive converters. This paper compares the size, the geometry and the operating conditions of automotive vs. diesel converters and suggests ways to design the converter system to meet the challenging durability requirements without compromising its performance with respect to back pressure and conversion efficiency. It is shown that the mechanical durability of the system, which is critical for meeting the 290K vehicle mile durability, can best be met by ensuring good compatibility between the substrate and washcoat and by designing a rugged packaging system with positive mounting pressure under all driving conditions.

The high temperature strength and deformation behavior of ceramic and two different metal substrates were measured in the 25°-1200°C temperature range in uniaxial and biaxial bending using rectangular bars and circular discs, respectively, prepared from the substrates. The data show that both of the metal substrates exhibit permanent deformation and lose their load carrying capability by an order of magnitude above 800°C. The ceramic substrate, on the other hand, preserves its strength and behaves elastically over the entire temperature range exhibiting neither permanent deformation nor cell distortion. These data suggest that the upper use temperature for metal substrates could be significantly lower than that for ceramic substrates to meet 50-100K vehicle mile durability

This paper addresses the packaging design for monolithic cordierite ceramic converters to meet the new, stringent durability requirements of the 1990's, while minimizing warranty cost for the automaker. These objectives are best met by using a systems approach during the early phases of packaging design, i.e. by examining design interactions between the ceramic monolith, alumina coating, ceramic mat or wiremesh mounting material with seals, stainless steel can, heatshields, and associated peripheral components. Failure of any one of these components can prove detrimental to converter durability. In this paper we take advantage of overall understanding of the observed failure modes and individual component behavior, and present new data for optimizing the total converter durability through initial design. In particular, the impact of symmetric gas entry, monolith contour, clamshell anisotropy, mount density, stiffener ribs, and heatshield insulation on total durability is highlighted.

The emissions regulations for the decade of 1990s are not only more stringent but are also required of vehicles other than passenger cars, for example both diesel and gasoline trucks as well as motorcycles. These latter applications involve different operating conditions in terms of space velocities, temperature profiles, and vibrational loads than those typical of passenger cars [1]*. In addition, the performance and durability requirements for these applications call for lower back pressure and longer service life. Furthermore, the space availability and the operating temperature range differ vastly so as to require special packaging designs to meet the durability requirements. This paper provides new data for ceramic insulating mats, both intumescent and non-intumescent [2,3], and ceramic substrates with thin and thick walls and square and triangular cell geometries [4], which are under development for non-passenger car applications indicated above.

The typical ceramic catalyst support for automotive application has a total volume of 1640 cm3. Approximately 10% of this volume is subjected to tensile thermal stresses due to a radial temperature gradient in service [1]*. These stresses are kept below 50% of the substrate strength to minimize fatigue degradation and to ensure long-term durability [2]. However, the tensile strength measurements are carried out in 4-point bending using 2.5 cm wide x 1.2 cm thick x 10 cm long modulus of rupture bars in which the specimen volume subjected to tensile stress is merely 3.2 cm3 or 0.2% of the total substrate volume [3]. Thus, a large specimen population is often necessary (50 specimens or more) to obtain the strength distribution representative of full substrate. This is particularly true for large frontal area substrates for diesel catalyst supports with an order of magnitude larger stressed volume. In this paper, the modulus of rupture data are obtained as function of specimen size.

The stringent durability requirements approaching 100,000 vehicle miles for automotive substrates and 290,000 vehicle miles for large frontal area diesel substrates for 1994+ model year vehicles call for advanced packaging designs with thick ceramic mats and high mount densities. The latter result in high mounting pressure on the substrate and enhance its mechanical integrity against engine vibrations, road shocks and back pressure forces. A novel measurement technique which applies a uniform biaxial compressive load on the lateral surface of ceramic substrates, thereby simulating canning loads, is described. The biaxial compressive strength data obtained in this manner help determine the maximum mounting pressure and mat density for a durable packaging design. The biaxial compressive strength data for both round and non round substrates with small and large frontal area are presented.

The monolithic, large frontal area (LFA), extruded ceramic substrates for diesel flow-through catalysts offer unique advantages of design versatility, longterm durability, ease of packaging and low Cost [1, 2]*. This paper examines the effect of cell density and cell size on catalyst light-off performance, back pressure, mechanical and thermal durability, and the steady-state catalytic activity. The factors which affect these performance characteristics are discussed. Certain trade-offs in performance parameters, which are necessary for optimum systems design, are also discussed. Following a brief discussion of design methodology, substrate selection, substrate/washcoat interaction and packaging specifications, the durability data for ceramic flow-through catalysts are summarized. A total of over 18 million vehicle miles have been successfully demonstrated by ceramic LFA catalysts using the systems design approach.

Ceramic preconverters have become a viable strategy to meet the California LEV and ULEV standards. To minimize cold start emissions the preconverter must light-off quickly and be catalytically efficient. In addition, it must also survive the more severe thermomechanical requirements posed by its close proximity to the engine. The viability of the ceramic preconverter system to meet both emissions and durability requirements has also been reported recently(1,2). This paper further investigates the impact preconverter design parameters such as cell density, composition, volume, and catalyst technology have on emissions and pressure drop. In addition, different preconverter/main converter configurations in conjunction with electrically heated catalyst systems are evaluated. The results demonstrate that ceramic preconverters substantially reduce cold start emissions. Their effectiveness depends on preconverter design and volume, catalyst technology, and the system configuration.

High temperature modulus of rupture (MOR) data, published previously, show that the ceramic catalyst supports get stronger with temperature due to the absence of water vapor and closure of microcracks which would otherwise act as stress concentrators [1, 2 and 3]*. The increased MOR value is partially responsible for the excellent durability of ceramic catalyst supports at high temperature. In this paper, we will present the compressive strength data of ceramic substrates at high temperature, namely the crush strength along B-axis and biaxial compressive strength of the whole substrate. Since the honeycomb strength is directly related to that of the individual cell wall, the compressive strength should also increase with temperature similar to the modulus of rupture. Accordingly, the ceramic substrates are capable of supporting higher mounting pressures exerted by the intumescent mat at high temperature [4].

The 290,000 vehicle-mile durability requirement for diesel/natural gas oxidation catalysts calls for robust packaging systems which ensure a positive mounting pressure on the ceramic flow-through converter under all operating conditions. New data for substrate/washcoat interaction, intumescent mat performance in dry and wet states, and high temperature strength and oxidation resistance of stainless steels, and canning techniques insensitive to tolerance stack-up are reviewed which help optimize packaging durability. Factors contributing to robustness of converter components are identified and methods to quantify their impact on design optimization are described. CERAMIC FLOW-THROUGH catalysts for diesel exhaust aftertreatment have met with much success since their introduction in 1993.

An advanced three-way converter system with significant improvements in light-off performance, conversion efficiency, thermal stability and physical durability at high operating temperature is described. The converter system is comprised of a light-weight ceramic substrate with high surface area triangular cell structure, a new catalyst formulation with enhanced thermal stability and good substrate compatibility, and a durable packaging design which together lead to consistent improvements in high temperature performance and durability. Experimental data including FTP performance, canning trials, and high temperature vibration and thermal shock tests for both the advanced and standard three-way converter systems are presented.

Under certain operating conditions when the combined stresses in a ceramic wall-flow diesel filter from mechanical, thermal, and vibrational loads exceed its threshold strength, the fatigue effects become important. This paper reviews the theory of static and dynamic fatigue, and presents fatigue data for Coming's high efficiency filter composition (EX-47, 100/17) in the temperature range 25° - 400°C which is representative of the stressed peripheral region during regeneration. The measurement and analysis of fatigue data, together with the implication on long-term durability of cordierite ceramic filters, is discussed.

An important element of the diesel filter assembly is a resilient ceramic mat placed between the ceramic filter and the stainless steel can. It has four key functions: i) to provide adequate gripping pressure, ii) to permit free axial expansion of can, iii) to act as a seal for gases, and iv) to minimize temperature gradients in the filter, which require certain mat properties, namely low-to-medium compression modulus, low shear modulus, and low friction coefficient between mat and filter. This paper compares the properties and performance of two different mats, Interam® I and III, in “hot shake” and “exhaust gas simulator” tests. The results indicate that Interam® III is a superior material for diesel filter application and that a complete coverage by this mat will prolong the durability of the filter.