Structure for electrically heatable catalytic core

There is provided an improved electrically heatable core for an electrically heatable catalytic converter. The core is characterized by corrugated thin metal strips and flat thin metal strips in alternating relation with each other and secured to an electrically conductive central core or tube. Alternating with the corrugated thin metal strips and the flat thin metal strips are areas of brazing metal in an axially staggered pattern. These strips are spirally wound and fused to braze them together in a unified monolith. A retainer shell is supplied to hold the monolith together. The core is placed in a housing fitted with insulated means for conducting electric current to the monolith to effect heating. The housing is then inserted in an exhaust line where it is effective to control start-up pollution, and where the monolith is constrained against telescoping.

This invention relates to an improved structure for an electrically 
heatable catalytic core and, more particularly, to an electrically 
heatable catalytic core which resists telescoping in the course of a 
severe testing regime as described below. 
BACKGROUND OF THE INVENTION AND PRIOR ART 
The purpose of a catalytic converter is to convert pollutant materials in 
engine or turbine exhaust, e.g., carbon monoxide, unburned hydrocarbons, 
nitrogen oxides etc., to carbon dioxide, nitrogen and water. Conventional 
catalytic converters utilize a ceramic honeycomb monolith having square or 
triangular straight-through openings or cells with catalyst deposted on 
the walls of the cells, catalyst coated refractory metal oxide beads, 
e.g., alumina beads, or a corrugated thin metal foil monolith, e.g., 
ferritic stainless steel foil, having catalyst carried on or supported by 
the surface The catalyst is normally a noble metal, e.g., platinum, 
palladium, rhodium, ruthenium, or a mixture of two or more of such metals. 
The catalyst catalyzes a chemical reaction, mainly oxidation, whereby the 
pollutant is converted to a harmless by-product which then passes through 
the exhaust system to the atmosphere. However, conversion is not efficient 
initially when the exhaust gases are relatively cold. To be effective at a 
high conversion rate, the catalyst and the surface of the converter with 
which the exhaust gases come in contact must be at a minimum temperature, 
e.g., 390.degree. F. for carbon monoxide, 570.degree. F. for voltile 
organic compounds (VOC) and 1000.degree. F. for methane or natural gas. 
Otherwise, conversion to harmless by-products is poor and cold start 
pollution of the atmosphere is high. Once the exhaust system has come to 
its operating temperature, the catalytic converter is optimally effective. 
Hence, it is necessary to contact relatively cold exhaust gases with hot 
catalyst to effect satisfactory conversion at engine start-up. Both 
compression ignited (diesel) and spark ignited engines have this need. 
To achieve initial heating of the catalyst prior to engine start-up, there 
is provided an electrically heatable catalytic converter formed of a 
corrugated thin metal foil monolith which is connected to a voltage 
source, e.g., a 12 volt or 14 volt automotive battery, and power supplied, 
preferably before and during start-up, to elevate and maintain the 
temperature of the catalyst to at least about 650.degree. F. 
Copending application Ser. No. 587,219 filed 24 Sep. 1990 (and its parent 
case Ser. No. 524,284 filed 16 Apr. 1990, now abandoned) discloses one 
form of electrically heatable catalytic converter which has been found to 
be subject to telescoping of the core, and provides one means for 
offsetting the tendency to telescoping of the core in operation and 
ultimate destruction thereof in the face of "Hot Shake" automotive proof 
tests . The present invention provides a different means for offsetting 
the tendency to telescope. Because much of the disclosure of Ser. No. 
587,219 is relevant to the present application, the disclosure of Ser. No. 
587,219 is incorporated herein by reference thereto. Instead of utilizing 
a ceramic core in juxtaposition with the electrically heatable catalytic 
core to inhibit telescoping, the present invention utilizes inter-leaf 
brazing whereby the leaves are held against telescoping or axial 
displacement. Ser. Nos. 524,284 and 587,219 are commonly owned with the 
present application. 
Copending application Ser. No. 606,130 filed 31 Oct. 1990, now U.S. Pat. 
No. 5,070,694 by William A. Whittenberger and entitled Structure for 
Electrically Heatable Catalytic Converter, and commonly owned with the 
present application, discloses a means for preventing telescoping of a 
spiral or S-wound corrugated thin metal foil monolith by brazing between 
the corrugated leaves according to a pattern (staggered relation). In that 
application, all of the leaves forming the monolith are corrugated. The 
adjacent leaves or strips are in a nonnesting relationship by virtue of 
(1) pattern corrugations, such as herringbone corrugation, or (2) 
straight-through corrugations using corrugated leaves with straight cells 
of differing pitch. (See U.S. Pat. No. 4,810,588 dated 7 Mar. 1989 to 
Bullock) 
The present application is quite similar to the aforesaid U.S. Pat. No. 
5,070,694, except that the monolith is made up of both corrugated and flat 
thin stainless steel strips. Nesting is not a problem with a structure in 
which corrugated foil strips are alternated with flat strips. However, 
telescoping of the core under the severe conditions of the "Hot Shake" 
test described below is still a problem. 
Reference may also be had to U.S. Pat. No. 4,381,590 dated 3 May 1983 to 
Nonnenmann et al which discloses a spirally wound monolith made up of 
corrugated and flat continuous strips which are brazed together. The 
present invention utilizes a plurality of strips, corrugated alternating 
with flat, all emanating from a central core and having a length so as to 
get appropriate wattage (on a 12 volt battery voltage source) between the 
central core and the outer shell. Generally, this power level ranges from 
about 1500 watts to about 12000 watts. Reference may be had to Ser. No. 
587,219, supra, Tables I and II. 
The electrically heatable catalytic cores hereof are normally spirally 
wound, or S-wound, corrugated and flat thin metal foil strips in 
alternating laminar relation. The corrugated and flat thin metal foil 
strips are not initially washcoated with a refractory metal oxide coating, 
e.g., an alumina coating, and a catalyst. The latter treatment steps come 
later according to the present invention for reasons which will appear. 
The corrugated and flat thin metal cores are subjected to a severe test 
which they must pass in order to be acceptable for automotive use. This 
test (so called "Hot Shake Test") involves oscillating (100-200 Hertz and 
28 to 60 G inertial loading) the device in a vertical attitude at high 
temperature (between 700.degree. and 950.degree. C., 1292.degree. F. and 
1742.degree. F., respectively) with exhaust gas from a running internal 
combustion engine being passed through the device. If the electrically 
heatable catalytic device telescopes in the direction of gas flow or 
breaks up after a predetermined time, e.g., 5 to 200 hours, the device is 
said to fail the test. Usually, the test device will fail in 5 hours if it 
is going to fail. Five hours is equivalent to 1.8 million cycles at 100 
Hertz. 
Accordingly, it is a principal object of &he present invention to provide a 
device which will pass the foregoing test and thereby prove reliable in 
extreme field service. 
Reference may be had to U.S. Pat. No. 3,768,982 to Kitzner dated 30 Oct. 
1973. In this patent, heat from a centrally located electric heater is 
transferred by conduction through a monolithic catalyst support to heat 
the catalyst to optimum operating temperature. Reference may also be had 
to U.S. Pat. No. 3,770,389 to Kitzner dated 30 Oct. 1990 which discloses a 
central electrically heated core within a ceramic monolith, heat being 
transmitted by conduction to the catalyst contained in the openings of the 
ceramic monolith. The heating core is formed of metal sheets, one 
corrugated, the other flat, coated with alumina and also bearing a 
catalyst. The metallic core is heated electrically by virtue of its own 
electrical resistance. Heating of the ceramic core by conduction takes too 
long to solve the problem of atmospheric pollution at start-up. These 
cores are, moreover, subject to telescoping under the conditions of the 
foregoing severe test. 
Reference may also be had to U.S. Pat. No. 4,711,009 to Cornelison et al 
dated 8 Dec. 1987 for details of a process for corrugating thin metal foil 
strips. The coating of the surface with a refractory metal oxide is not 
done in the present case or the application of the catalyst as described 
in that patent. These steps cannot be performed in the present case as the 
resulting surfaces cannot be brazed. Also, for brazing purposes, it is 
desirable not to stress relieve aluminum-containing stainless steels as 
there is a tendency to form oxides on the surface to which the brazing 
does not adhere well. However, the corrugating of a thin metal foil strip 
with a herringbone, or chevron pattern as taught therein is applicable to 
the present application and to that extent, the disclosure of the 
aforesaid U.S. Pat. No. 4,711,009 is incorporated herein by reference. The 
refractory metal oxide coating and the catalyst are applied by dipping in 
the present process after formation of the core. The composition of the 
washcoat and the catalyst treating solutions as taught in U.S. Pat. No. 
4,711,009 are pertinent to the present process. 
Reference may also be had to International PCT publication numbers WO 
89/10471 and WO 10470 each filed 2 Nov. 1989. S-wound cores composed of 
corrugated and flat strips in alternating relation are disclosed in these 
publications. However, there is no teaching of brazing between the 
corrugated thin metal foil layers, and telescoping of the core under the 
conditions of the severe test described above occurs. 
In the following description, reference will be made to "ferritic" 
stainless steel. A suitable formulation for this alloy is described in 
U.S. Pat. No. 4,414,023 dated 8 Nov. 1983 to Aggen et al. A specific 
ferritic stainless steel useful herein contains 20% chromium, 5% aluminum, 
and from 0.002%to 0.05% of at least one rare earth metal selected from 
cerium, lanthanum, neodymium, yttrium and praseodymium, or a mixture of 
two or more thereof, balance iron and steel making impurities. 
In the following description, reference will also be made to fibrous 
ceramic mat or insulation. Reference may be had to U.S. Pat. No. 3,795,524 
dated 5 Mar. 1974 to Sowman for formulations and manufacture of ceramic 
fibers and mats useful herein. One such ceramic fiber material is 
currently commercially available from 3-M under the registered trademark 
"INTERAM." 
BRIEF STATEMENT OF THE INVENTION 
Briefly stated, the present invention is an improved structure for an 
electrically heatable catalytic converter. The structure is characterized 
by a plurality of corrugated thin metal leaves or strips each in 
alternating relation with flat metal leaves or strips extending from an 
electrically conducting central core, these leaves being in alternating 
relation with thin narrow brazing metal strips. The thin, narrow brazing 
metal s&rips are staggered from side to side of the thin metal strips or 
leaves, lying adjacent first one marginal edge of a thin metal strip, and 
then adjacent the opposite marginal edge of the next succeeding thin metal 
strip. The assembly of from 12 to 40 thin metal strips (alternating 
corrugated and flat) and the interleafed thin narrow brazing strips is 
then spirally wound, or S-wound, into a tight core, and a retaining shell 
placed around the tightly wound core with an intervening wrapping of 
brazing metal and the whole unit heated to a temperature sufficient to 
fuse the brazing metal, e.g. 2100.degree. to 2300.degree. F. 
Simultaneously, the interleafed brazing strips are fused, and the assembly 
becomes a rigid unit. Then the entire unit is dipped into a washcoat of a 
refractory metal oxide dispersed in water, and the coating baked to 
tightly adhere the metal oxide, usually gamma alumina, with or without a 
small amount of ceria, to the metal surface. Then a noble metal catalyst 
is deposited on the metal oxide surface from an aqueous solution of the 
noble metal. The temperature is then raised to decompose the noble metal 
containing compound and deposit the catalyst on the surface. 
Alternatively, the catalyst may be dissolved in the aqueous medium of the 
washcoat before dipping. 
In use, a voltage source, usually a 12 or 24 volt automotive battery, is 
connected across the central conductive bar and the outer retaining shell 
to provide power to heat the monolith to a minimum of about 650.degree. F. 
in about 5 to about 30 seconds. Power in the range of 1500 to 12000 watts 
may be supplied through a MOSFET power supply system such as described in 
copending applications Ser. No. 587,219, supra, and Ser. No. 524,284 filed 
16 Apr. 1990. 
The resulting catalytically active electrically heatable unit now will 
resist telescoping during the severe test outlined above. Moreover, the 
current, instead of flowing through the brazing locations from one leaf to 
the next, will flow mainly along the length of the corrugated thin metal 
strip and the flat thin metal strip as it provides the shortest path from 
negative to positive. This provides for uniform heating per unit volume 
and avoids hot spots that would have an adverse effect on the catalyst.

DETAILED DESCRIPTION OF THE DRAWINGS 
As indicated above, the present invention is in a structure for an 
electrically heatable catalytic core and a converter containing the same. 
This core is able to withstand the severe "Hot Shake" test described above 
without telescoping. 
Referring more particularly to FIGS. 1 and 2, there is here shown in 
cross-section an electrically heatable catalytic core, generally indicated 
at 10. The core 10 is composed of a series of corrugated thin metal strips 
12 secured to a central post 14 by an suitable means, the corrugated 
strips 12 being in alternating relation with flat thin metal strips 13, 
also secured to the central post 14. The preferred securing means is spot 
welding. Alternatively, and as shown in the aforesaid Ser. No. 524,284, 
the central post 14 may be made up of a group of individual wires or rods 
with a thin herringbone corrugated metal strip overfolded over each wire. 
The individual wires with strips welded thereto are gathered together in a 
bundle and welded at the ends to form the central post 14. In the present 
and preferred case, one end of each strip is tack welded to a stainless 
steel or nickel center post or tube 14 which is closed at at least one 
end. The corrugated and the flat thin metal strips are from about 1.5" to 
about 3.6" wide and from 8 to 24 inches long, e.g. 12" long. The length of 
the strips is calculated from the resistance of the thin metal strip per 
unit length and the desired power level between the central core 14 and 
the shell 18. Corrugation is done by passing between rolls having the 
corrugation pattern formed therein. Reference may be had to U.S. Pat. No. 
4,711,009, supra, for details of one mode of forming corrugated thin metal 
strips useful herein. The corrugated thin metal strips are desirably 
stainless steel, and preferably a ferritic stainless steel about 0.0015 to 
0.0025 inch thick, e.g., 0.0016" thick. A particularly useful ferritic 
stainless steel contains 20% chromium, 6% aluminum, 0.002%to 0.05% of at 
least one rare earth metal selected from cerium, lanthanum, neodymium, 
Yttrium and praseodymium, or a mixture thereof, and the balance iron plus 
steel-making impurities. Reference may be had to U.S. Pat. No. 4,414,023 
to Aggen et al dated 8 Nov. 1988 for suitable ferritic stainless steel 
alloys useful herein. 
The corrugated thin stainless steel strip is corrugated continuously, 
preferably in a straight through sinusoidal or triangular (with apices 
rounded off) pattern and later cut to the desired length. The straight 
through corrugation pattern provides for reduced back pressure through the 
electrically heatable catalytic converter unit. Prior to coating the thin 
metal strips, both flat and corrugated, with a refractory metal oxide and 
catalyst, the strips are, as indicated, secured to a central post 14. 
Brazing strips 16 which are 0.05" to 1" wide and of a length equal to the 
length of the thin metal strips, are alternately located (FIGS. 2 and 3) 
and positioned in staggered relation (FIG. 3). Thereafter, the central 
post 14 is twisted and the thin metal foil and brazing foil strips 
spirally wound into a tight cylinder as shown in FIG. 1 and encased in a 
binding tube 18 of stainless steel or nickel, or other suitable metal. In 
order to provide better electrical contact between the binding tube 18 and 
the spirally wound core 20, a layer of brazing foil 22 is desirably 
interposed between the core 20 and the binding tube 18. The tube 14 is 
desirably circular and closed at at least one end to force the exhaust gas 
through the foil portion of the converter. The brazing foil layer 22 has 
an axial length equal to the axial length of the thin metal strips 12 and 
13. The binding tube 18 may conveniently be formed of two metallic half 
shells, e.g., stainless steel, nickel, or the like, to be welded together 
on assembly. 
The brazing foil is rolled or strip cast to about 0.001" to about 0.003" 
thick. Alternatively, brazing paste, brazing wire or preforms may be used. 
The braze material is desirably a nickel/chromium/boron/silicon brazing 
alloy analyzing 75% to 83% nickel with a liquidus temperature of 
2100.degree. to 2300.degree. F. such as commercially available from Allied 
Metglas Products of Parsippany, N.J. See specifically alloys 50/50A and 
80/80A as currently available. 
Where a platinum group metal is to be used as the catalytic agent, 
phosphorus is avoided in the alloy. Other nickel-containing brazing alloys 
containing from 7% to 14% chromium, 3% to 4.5% iron, 3.5% to 4.5% silicon, 
2% to 3% boron, balance nickel and having liquidus temperatures of above 
about 2100.degree. F. may also be used. After fusing, the fused brazing 
material holds the stainless steel corrugated and flat strips together in 
substantially the known corrugated/flat spiral or S-wound configuration as 
shown in the aforesaid WO 89/10471, supra. 
After the core of FIGS. 1 and 1a is fully assembled, it is heated to a 
temperature sufficient to fuse the brazing strips 16 and 22 to the 
contiguous members This has the effects of securing the corrugated and 
flat strips together without significantly adding to the overall diameter 
of the core, and preventing telescoping of the core in the direction of 
gas flow during the severe high frequency and high temperature "Hot Shake" 
test described above. The overall open area of the catalytic core ranges 
from about 80% to about 90% and is not materially changed by the use of 
the thin brazing foil as this fuses and for all practical purposes merely 
bonds the thin stainless steel strips together. 
To coat the surface of the thin metal strips, the entire unit is dipped 
into a slurry of a refractory metal oxide, e.g., an alumina slurry, 
preferably one containing a minor amount of ceria, e.g. from 0.5% up to 
about 25% by weight of the solids. The unit (FIG. 4) after wiping off the 
excess slurry from the outer shell is then baked at a temperature 
sufficient to remove the liquid base (water, principally) from the coating 
and set to coating on the surface, e.g. about 1000.degree. F. The unit 
(FIG. 4) is then dip coated with an aqueous solution of a noble metal 
catalyst material. The porous nature of the gamma alumina coating traps 
and absorbs the aqueous catalyst-containing solution. Heating of the 
catalyst treated core to the decomposition temperature of the 
catalyst-containing compound yields the catalyst metal on the substrate. 
The noble metals useful as catalysts are palladium, platinum, rhodium, 
ruthenium and mixtures of two or more of such metals. Reference may be had 
to U.S. Pat. No. 4,711,009 for details of catalytic solutions useful in 
applying the catalyst to &he refractory metal oxide washcoated surface. 
At this point, a terminal post 26 is secured to the outside of the outer 
shell 18 as by welding. One of the poles of a voltage source, e.g., the 
positive pole of an automotive vehicle battery, is later secured to the 
terminal post 26 by a suitable cable. The opposite pole of the voltage 
source, e.g., the negative pole of an automotive vehicle battery is later 
attached to the central post 14 by a suitable cable. To effect rapid 
heating of the electrically heatable catalytic core, considerable power 
must be drawn from the voltage source. For details of a suitable power 
supply system, reference may be had to Ser. No. 587,219, supra. 
When the assembly as shown in FIG. 4 is completed, it is placed in a 
suitable housing such as shown in FIG. 5. FIG. 5 shows a completed 
electrically heatable catalytic converter, generally indicated at 60. 
There is provided a steel, preferably stainless steel, housing 52 having 
adapter end caps 64 and 66 having nipples 58 and 60 which reduce the 
diameter to the diameter of the exhaust pipe, not shown. The unit 10 (See 
FIGS. 1-4) is disposed within the housing 52 which is conveniently formed 
of half shells welded together at the seam, and the end caps 54 and 56 
later welded on. Because the outer metal binder shell 18 will become 
positively charged, it must be insulated from the housing 62, and to this 
end a flexible ceramic insulation mat 62 (such as described in U.S. Pat. 
3,795,524, supra, is interposed between the housing 52 and the metal 
binder shell 18. An aperture 64 is provided in the mat 62 to accommodate 
the positive terminal post 26 and suitable insulation means are provided 
for traversing the housing 52. There is provided an aperture 66 through 
the housing 52 into which is threadedly engaged a bushing 76 which holds 
an insulating sleeve 74 which in turn surrounds the post 26. A metal cap 
78 provides a seat for a positive cable clamp, not shown. Current flows to 
the central post 14 through a transverse bar 80 welded as at 82 to the 
central post 14 at its midpoint and to the end cap 64 at its extremities. 
Alternatively, one end of the transverse bar 80 may be extended through 
end cap 54 and threaded to accept a cable connector from the battery. The 
flow of power to the electrically heatable unit 10 is effected by suitable 
switching means 44. Reference may be had to Ser. No. 587,219, supra, for 
the details of a suitable power switching means. 
The temperature of the core 10 is conveniently sensed by a thermocouple 56 
extending through the housing 52 with a metal bushing 59. A suitable 
insulating sleeve 58 and an inner tube 57 wherein the conducting leads 
from the junction 61 are carried spaced from each other as is known, 
extends into the interior of the housing 52. The leads terminate in a 
junction 61 which is embedded in the core 10. 
There have thus been provided means for securing spirally wound, or 
S-wound, corrugated plus flat, thin me&at foil strips against telescoping 
under severe testing conditions. No intra-core insulation is required in 
the devices hereof. The flow of current through the entire foil strip 
lengths is preserved because there is sufficient inter-lamination 
dielectric strength to prevent lamination-to-lamination shorting of any 
significant amount of current. Interlamination shorting has the effect of 
(1) lowering the characteristic resistance of the assembly (which is 
undesirable) and (2) concentrating the electrical energy more in the 
center of the core rather than evenly distributing it across the exhaust 
gas intercepting face of the core. Current will prefer to flow from one 
end of the thin metal strip at the central post to the other at the inner 
periphery of the housing. The appropriate resistance in ohms is thus 
provided. The refractory metal oxide coating on the surface of the foil 
strips has a high dielectric strength and for all practical purposes 
prevents shorting between contiguous layers of foil except in the areas 
where the brazing metal is located. Hence, the electrical integrity and 
performance of the device as disclosed in the aforesaid Ser. No. 587,219 
is preserved. The devices hereof may be used alone or in tandem with 
conventional ceramic catalytic converters in a given exhaust system.