Patent Application
20070292657
A high temperature ceramic structure in the form of a porous ceramic honeycomb 10 provided according to the present invention is schematically illustrated in
Honeycomb structure 10 may be formed of any channel density; for example channel densities in the range of 100-400 channels per square inch of honeycomb cross-section are suitable for the construction of diesel engine exhaust filters, while densities of 300-1000 channels per square inch of honeycomb cross-section maybe selected for flow-through catalyst supports. For the purposes of the present description the term honeycomb is intended to include materials having a general honeycomb structure wherein cross-sectional channel shapes of square, hexagonal, triangular, square, circular, or any other open channel shapes may be provided.
In manufacturing a substrate for use in accordance with the present invention, a ceramic batch is first formed from carbide, oxide or mineral oxide (e.g. clay or talc) powders, the powders being blended with binders such as methylcellulose, lubricants such as sodium stearate, and a vehicle such as water to form plasticized powder mixtures for forming. The plasticized mixtures are then extruded or otherwise formed into green honeycomb bodies that are dried and fired to sinter or reaction-sinter the powders into porous ceramic honeycombs. U.S. Pat. No. 6,864,198 discloses examples of the preparation of cordierite (Mg2Al4Si5O18) honeycombs from batches comprising powder constituents such as talc, alumina, aluminum hydroxide, kaolin clay and silica.
As shown in
For washcoating deposition, the particle size of the phase change materials may suitably be adjusted to be similar to that of the washcoating materials in order to avoid changing the viscosity of the washcoating slurry. The concentration of the phase change materials in the slurry will be adjusted to levels that will allow detection by X-ray diffraction or other methods when the substrate is analyzed following use, but generally not at levels to high as to interfere with the function of the ceramic product or any washcoating and/or catalyst provided thereon.
Most of the phase changes occurring in solid state materials involve changes in physical structure that can be detected by X-ray diffraction or similar techniques. While any suitable phase change material may be used, common and low cost materials such as alumina, titanium dioxide, zirconium dioxide, and niobium pentoxide may be favored due to their low reactivity with typical ceramic substrate materials and other materials commonly used for washcoating.
Alumina is an important material in catalysis because of its porous structure, fine particle size, high surface area, and high catalytic surface activity. As a result, alumina is widely used as a catalyst, an absorbent, and as a support for industrial catalysts. It is also used as a main component in the washcoat of catalyzed DPF to provide high dispersion of precious metals. γ-alumina is metastable and exhibits a phase change to δ-alumina at 900° C. Another phase change to α-alumina occurs at 1100° C. Each of these phases can be detected by X-ray powder diffraction to determine the maximum temperature reached during the life of the substrate. The presence of γ-alumina indicates a maximum temperature of less than 900° C.
Gallium oxide (Ga2O3) is readily available and commonly used in the semiconductor industry. At low temperatures the ε-phase of gallium oxide is stable; the ε-phase is converted to β-Ga2O3 at 870° C. The presence of ε-phase indicated a maximum temperature of less than 870° C., while the presence of β-phase indicates a maximum temperature in excess of 870° C.
Titanium dioxide (TiO2) has extensive industrial applications and can exist in three crystalline forms, i.e., anatase, rutile and brookite. Anatase and rutile are tetragonal forms and brookite is orthorhombic. At about 750° C. the brookite phase is converted to anatase and at about 915° C. anatase is converted to the rutile structure.
Zirconium dioxide (ZrO2) has three well-established polymorphs: monoclinic, tetragonal and cubic. The transition temperature from monoclinic to tetragonal is around 1100° C. Between 1000° C. and 1150° C. a tetragonal phase is present above 1350° C. a cubic phase is formed.
Niobium pentoxide (NbO5), a very stable compound under a redox atmosphere, exists in at least four well-established polymorphic forms, including a pseudo hexagonal TT-phase, an orthorhombic T-phase, a higher temperature B-phase and an H-phase. The phase change temperatures are 410° C. for TT-phase to T-phase conversion, 817° C. for T-phase to B-phase conversion, and 960° C. for the B-phase to H-phase conversion.
By selecting a number materials such as alumina, gallium oxide, titanium dioxide, zirconium dioxide, niobium pentoxide with different phase change temperatures the maximum temperature to which a substrate has been heat may be precisely determined. By sampling the substrate at a number of different positions, the temperature profile across the substrate may be determined.
There are several selection factors that may be considered when selecting suitable phase-change materials for use as chemical temperature sensors. Among those are the degree of material compatibility with catalysts and/or catalyst support (washcoat) materials such as alumina, ceria, and precious metals, and with ceramic substrate materials such as cordierite, aluminum titanate, mullite, and/or silicon carbide. In most cases it is important that no solid state reactions are likely between the selected phase change material and the catalysts, washcoats, and ceramic supports under the range of operating conditions to be encountered.
Also desirable is that the selected phase change material be chemically stable under strongly oxidizing or reducing atmospheres, since oxidants and/or reducing species such as carbon monoxide, hydrocarbons, and some nitrogen oxides can be present in combustion exhaust gases. Typically, the phase change material will demonstrate good thermal and hydrothermal stability as well as good resistance to thermal cycling damage during the operating lifetime of the catalyst support or filter. Also, the phase changes of the selected material should be irreversible or nearly irreversible, and the material should have little or no adverse impact on the catalytic performance of any catalysts present in the system. Finally, the addition of the selected material into any washcoating slurry intended to be applied to the support or filter should not affect the physical or chemical properties of the slurry in a manner unacceptably interfering with washcoat adherence to the filter or support.
The presence of some dopants such as lanthanide components in these phase change materials may be useful where it predictably increases or decreases the phase transition temperatures exhibited by the materials. Other conditions such as heating rate, system pressure, and the existence of other materials can also influence the phase change temperature. Any materials that can meet some or all of the selection criteria discussed above can be used as chemical sensors on substrates. The in-situ temperature sensor technology of the present invention is applicable to a variety of products and applications.
In an offline detection system, a substrate or filter incorporating a phase change material such as described may be removed from the internal combustion engine and the thermal history may be determined with an off-line detection method such as x-ray diffraction. The substrate is typically cut to form a number of samples from any desired location for measurement of targeted characteristic physical properties or chemical states. This process provides information about filter in application as well as the thermal causes of substrate failure.
In an online detection system, the substrate may be heated and ongoing phase detection may be preformed. One advantage to an online detection system is that both reversible and irreversible material property changes may be monitored. The reversibly changing phases can operate as thermal sensors, while irreversible changes can be used as substrate failure indicators. Application methods suitable for the application of phase change materials to supports or filters in accordance with the present invention include washcoating, chemical vapor deposition, and thermal spraying, either individually or in conjunction with process steps such as catalyzation process or as a separate post-process step.
While the invention has been described above with reference to specific embodiments or examples thereof, those embodiments and examples are presented for purposes of illustration only and are not intended to be limiting. Thus a wide variety of alternative materials and methods may be selected for the purpose of carrying out the invention within the scope of the appended claims.
