Element for filtering and/or purifying hot gases, and a process for manufacturing same

An element is disclosed for filtering and/or purifying hot gases, comprising a fibrous structure with a predetermined degree of porosity and formed of metal alloy fibrils of the MCrAlX type, in which M is a matrix chosen from iron and/or nickel and/or cobalt, and X is an element reinforcing the adhesion of the oxide layer which is formed on the fibrils in contact with the hot gases, chosen from zirconium, yttrium, cerium, lanthanum and misch metal.

The present invention relates to improvements to the structure and the 
process for manufacturing filtering elements and/or catalysis purifying 
elements for purifying hot gases, for example (but not exclusively) the 
exhaust gases of internal combustion engines. 
The active material supports used today in catalysis filtering and/or 
purifying elements for purifying hot gases and in particular in exhaust 
silencers for treating the exhaust gases of motor vehicles are essentially 
formed from ceramic (cordierite). The supports thus formed have the 
defects inherent in these materials, namely great mechanical fragility 
(sensitivity to vibrations and thermal shocks) which form a handicap in 
particular for the automobile application, high calorific capacity and 
mediocre heat conductivity (which increase the time required for obtaining 
optimum operating conditions), and a difficult manufacturing process 
(which considerably increases the cost). 
Solutions have also been put forward using metal structures, particularly 
strips obtained by rolling. Generally, the cost is greater than that of 
ceramics, whose raw material is very cheap. 
In all cases, known structures comprise rectilinear channels through which 
the gas to be purified flows : the flow is laminar and is not favourable 
for providing homogeneous contacting of the whole of the gaseous mass with 
the catalyst lining the walls of the channels. Solutions to this problem, 
it is true, allow a relative turbulence to be created increasing the 
efficiency of treatment of the gases, but the overall efficiency of the 
treatment is far from being completely satisfactory, and the structure of 
the silencer is complicated thereby. 
Furthermore, known metal catalytic silencers are, it is true, adapted for 
treating the gaseous phase of the exhaust gases, particularly in the case 
of explosion engines, but are without appreciable effect for filtering out 
the solid particles in suspension in the gases, in particular for 
retaining the unburnt particles present in diesel engine exhaust gases. 
An object of the invention is therefore to overcome these different 
drawbacks of presently known structures, by providing an improved 
structure, as well as its method of manufacture, which is more efficient 
in purifying the gases, which filters out the solid particles present 
therein, the same structure being able to fulfil a single one or both 
functions, which is simpler and less costly to manufacture and whose cost 
as a whole is lower than that of present structures. 
For this, in a first aspect of the invention, an element is provided for 
filtering and/or purifying hot gases, which is characterized in that it 
comprises a fibrous structure having a predetermined degree of porosity 
and formed of metal alloy fibrils of MCrAlX type, in which M is a matrix 
chosen from iron and/or nickel and/or cobalt, and X is an element 
reinforcing the adhesion of the oxide layer which is formed on the fibrils 
in contact with the hot gases, chosen from zirconium, hafnium, yttrium, 
cerium, lanthanum and misch metal. By "misch metal" is meant a combined 
metal of ceric earths (lanthanum, cerium, praseodymium, neodymium, 
proethium, samarium) containing all the elements of the rare earths 
present in monazite (ceric earth orthophosphate containing yttric earths 
(europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, 
ytterbium, lutetium) in low proportions, as well as thorium and silica), 
or more particularly : 
52% Ce, 18% Nd, 5% Pr, 1% Sm, 24% (La and other rare earths), 
or else, in a lanthanum enriched formula : 
47% Ce, 19% Nd, 6% Pr, 1% Sm, 27% (La and other rare earths). 
A fibrous structure of the claimed type withstands high temperatures and in 
particular the temperatures of about 150.degree. C. to 900.degree. C. met 
with in the exhaust gases of internal combustion engines; furthermore, the 
component materials are little oxidized and/or corroded at the 
temperatures considered, by the treated gases, in particular the corrosive 
exhaust gases of engines. Thus, a suitable lifespan of the fibrous 
filtering mass is ensured. 
The quite particular efficiency of the fibrous structure used as filter 
results from the fact that the porosity may be adapted with precision to 
the filtering requirements, and especially to the fact that this fibrous 
structure, whatever its construction, does not have privileged paths for 
the gases; the small flow section offered to the gases between the 
adjacent fibrils and the incoherent, although homogeneous, distribution of 
the fibrils break up the paths followed by the gases, increase the 
turbulence and generally improve the contact between the gaseous mass and 
the fibrils. 
The fibrils may be left in a random tangle and be compacted so as to obtain 
an homogeneous fibrous mass having said predetermined degree of porosity; 
but recourse may also be had to an organized structure and, for example, 
the fibrils may be woven and the appropriate woven layers may be stacked 
so as to obtain said predetermined degree of porosity. 
Desirably, the fibrous structure is sintered so as to ensure mechanical 
interlocking between the fibrils which ensures better mechanical strength 
and maintains the homogeneity of the fibrous mass in time. 
Advantageously, the fibrous mass is enclosed in a container so as to obtain 
an element which can be easily handled and which is protected against 
attacks by the surrounding pollution (mud, dust), especially when it is a 
question of forming exhaust silencers. The filter is then in the form of a 
cartridge which is easy to position. 
Preferably, in this case, the container is made from metal and, because of 
sintering, the fibrils are not only interlocked with each other but, in 
addition, the peripheral fibrils are attached to the wall of the 
container, which prevents the fibrils from moving then in the container 
and contributes to conserving the initial homogeneity of the obtain 
structure in time. 
With the arrangements which have just been discussed, a filtering element 
is obtained which is particularly well adapted for efficiently retaining 
the particles in suspension in the gases, particularly unburnt, oil, etc. 
particles in suspension in the exhaust gases of engines and in particular 
of diesel engines. 
Furthermore, in a second aspect of the invention, a catalysis purifying 
element is provided for the catalytic purifying of hot gases which, 
besides the fact that it comprises all or parts of the above arrangements 
of the filtering elements, is characterized in that the fibrils are coated 
with a layer of a catalytic compound. 
Preferably, the fibrils are coated with an intermediate layer of a wash 
coat and the catalytic compound is deposited on this intermediate layer. 
The presence of the wash coat considerably increases the effective contact 
area between the catalyst and the gases and the efficiency of the 
purifying element is therefore improved, for the same size.

Thus, with the invention, a catalysis purifying element is obtained which 
is of simple construction, of remarkable efficiency because of the 
turbulence of the gases caused by the absence of privileged paths and the 
large contact area means that substantially the whole of the gaseous mass 
which passes through it is contacted with the catalyst and undergoes 
purifying treatment. Furthermore, because the active part of the purifying 
element is formed by a fibrous mass identical to that which forms the 
above filtering element, the result is that, in a catalysis purifying 
element in accordance with the invention, the filtering and purifying 
functions are provided in a combined way by the same member, whence a gain 
in volume, weight and cost. 
Finally, in a third aspect of the invention, a process is provided for 
manufacturing one or other of the above filtering or purifying elements. 
The process according to the invention for manufacturing an element for 
filtering and/or purifying hot gases is characterized in that it comprises 
the following steps : 
metal alloy fibrils of the MCrAlX type are prepared in which M is a matrix 
chosen from iron and/or nickel and/or cobalt and X is an element 
reinforcing the adhesion of the oxide layer which is formed on the fibrils 
in contact with the hot gases, chosen from zirconium, hafnium, yttrium, 
cerium, lanthanum and misch metal, 
and these fibrils are assembled so as to form a fibrous structure having a 
predetermined degree of porosity. 
In a particularly advantageous way, because of its efficiency, the fibrils 
are obtained by a roller overhardening process, known per se. In this 
connection reference may be made to the patent application FR 2 587 635. 
It is possible to leave the fibrils in a random distribution and compact 
them so as to obtain a felt having the desired degree of porosity; in this 
case, it may be desirable to crush the fibrils before compacting them so 
as to give them a sufficiently small length so that they are suitably and 
homogeneously tangled. 
But it is also possible to use fibrils in an organized form, for example by 
forming a meshed fabric and stacking several fabric thicknesses so as to 
obtain the desired degree of porosity. 
Preferably, the fibrous structure is subjected to a sintering treatment so 
that the fibrils are interlocked with each other. 
Advantageously, the fibrous structure is introduced into a metal container 
and the assembly is subjected to the sintering treatment so that the 
fibrous mass is attached, by its peripheral fibrils, to the wall of the 
container. 
To obtain a purifying element with catalytic operation for purifying hot 
gases by catalysis, the process may be continued by then coating the 
fibrils with a layer of a catalytic compound, preferably after the fibrils 
have undergone a surface treatment for increasing their binding power. 
Preferably also, before depositing the catalytic compound, the fibrils are 
coated with an intermediate layer of a wash coat. 
The invention will be better understood from the following detailed 
description. 
The invention uses the high resistance to oxidation at high temperatures of 
alloyed metal materials of MCrAlX type, in which M is a matrix chosen from 
iron and/or nickel and/or cobalt and X is an element reinforcing the 
adhesion of the oxide layer which is formed on the fibrils in contact with 
the hot gases and chosen from zirconium, hafnium, yttrium, cerium, 
lanthanum and misch metal, as mentioned above. 
Abradable metal materials MCrAlY are known, used for example for protecting 
turbine blades (F. DUFLOS et al, New materials for honey comb structures 
with improved resistance to high temperature oxidation, high temperature 
alloys for gas turbine and other applications, 1986, Proceedings of the 
Conference, Liege, Belgium, Oct. 6-9, 1986, D. Reidel publishing company) 
for obtaining which a good performing manufacturing method has been 
proposed (patent application FR 2 587 635). 
In brief, this method consists in projecting a jet of liquid metal alloy on 
the periphery of a rotating metal wheel. The liquid metal solidifies 
rapidly in contact with the wheel and gives rise to the formation either 
of a ribbon or of fibrils, depending on the configuration of the 
solidification surface. The transverse dimensions (width and thickness) 
are adjusted mainly by the ejection slit width, the distance between wheel 
and crucible and the speed of rotation of the wheel. The advantages of 
this method is twofold. It makes is possible to obtain strips or fibrils 
in alloy compositions which could not be transformed because of their lack 
of ductility. For example, in the case of MCrAlY alloys, it has made it 
possible to increase the aluminium content, and so their resistance to 
oxidation. This could not be achieved using conventional metallurgical 
processes for these alloys which can be neither rolled nor drawn. There 
exist nevertheless limits which are imposed by the phase diagrams, the 
presence of a fragile compound possibly reducing the ductility of the 
material or making use thereof difficult. 
It has also simplified the manufacturing cycle which, after manufacture of 
the alloy ingot, comprises only an operation of casting on a wheel while 
leading to a material in which the added elements are distributed very 
homogeneously. 
The fibrils produced by this process are in the form of very fine slivers, 
of a semi circular cross section whose two transverse dimensions (width 
and thickness) are adjustable by adjusting the geometry of the wheel and 
the conditions of ejection of the liquid metal. Their length, which is 
between a few centimetres and about one metre, may possibly be reduced by 
passing through a blade crusher. 
Other techniques are known for obtaining individual fibres or coarse 
fibrils of a metal alloy of this type (particularly by cold clawing or 
drawing) which make it possible to manufacture felts, steel wools or steel 
shavings. However, in all cases, the alloy grades used must have 
properties adapted to the processes used ; in particular these alloys must 
be sufficiently ductile. This condition then restricts the nature of the 
fibres to materials moderately charged with hardening elements. In the 
case of MCrAlX alloys, the aluminium content, which conditions the 
resistance to oxidation, must be limited to a sufficiently low level so as 
to avoid excessive hardening, making the transformation difficult, even 
impossible. Even though, for some materials, particularly elaborate 
transformation conditions make it possible to obtain relatively fine 
threads, the production cost remains very high. 
The process recommends roller over-hardening so as to get round this 
difficulty, since no thermomechanical transformation is required for 
obtaining the fibril. 
The individual fibres, or fibrils, possibly crushed (so as to increase more 
easily the apparent density of the felts) in order to obtain lengths of 
the order of a few millimetres, for example 10 to 15 mm, may be assembled 
together in a randomly disordered, but homogeneous, distribution so as to 
form a felt whose porosity may be adjusted by simple compression (porosity 
possibly exceeding 95%). 
It is also possible to use relatively long fibres for forming an ordered 
structure, for example so as to form a fabric. The meshwork of the fabric, 
with possible compacting, here again makes it possible to obtain a desired 
degree of porosity. 
Mechanical consolidation of the porous fibrous structure thus obtained is 
achieved by halogenated phase sintering. This technique is made necessary 
by the presence of aluminium which, during heat treatment, even in a high 
vacuum, leads to the formation on the surface of the fibres of a thin 
alumina layer hindering their bonding by diffusion. On the other hand, the 
halogen formed from a donor cleans the surface of the fibres and allows 
sintering. During this treatment, the chemical activity of the elements in 
the gaseous phase is adjusted by a cement adapted to the composition of 
the fibres so as to avoid any modification of the composition due to an 
exchange between the felt and the treatment enclosure (see particularly : 
P. GALMICHE, Application en constructions aerospatiales et retombees des 
techniques thermochimiques ONERA, L'aeronautique et l'Astronautique, no. 
41 --1973 --3, page 33 to 42 ; R. PICHOIR, Etude comparative de deux 
methodes de projection pour superalliages cas de revetements de type NiAl 
sur alliage IN 100, La Recherche Aerospatiale, no. 1974-5, p. 277 to 289. 
The fibrous mass may be disposed in a powdery cement formed, on the one 
hand, of the same metal elements as the fibrils, or of a part of these 
elements, at closely related concentrations, and, on the other hand, a 
chemically inert ceramic diluent (generally alumina) for avoiding setting 
of the cement during the sintering operation. The treatment, which lasts 
several hours, is carried out at a high temperature in a protective 
hydrogen or argon atmosphere. 
To avoid immersion in the powdery cement, a time consuming operation making 
the procedure discontinuous, a treatment may be carried out in a gaseous 
atmosphere. The fibrous mass is then placed in an enclosure where a gas 
atmosphere prevails containing a halide coming from an appropriate cement. 
The treatment may thus be continuous, which reduces the handling 
operations and the cost of this production step. 
It is desirable for the fibrous mass to be disposed in a container, such as 
a tube, so as to form a protected and more easily handled component. This 
container is open so as to allow the hot gases to pass. Preferably, the 
fibrous mass is placed in a metal tube before the sintering operation, and 
the treatment is carried out on the assembly formed by the tube and the 
fibrous mass which it contains. There is then mechanical bonding by 
sintering between the peripheral fibres and the wall of the tube. The felt 
is then attached to the tube, and the assembly forms a sort of cartridge 
which may be introduced in any filtering and/or purifying system and which 
is related to a catalytic silencer structure. It goes without saying that 
the form of the tube (circular, square section, . . . ), its composition 
in the case of a metal tube (in the limit of the compatibility of the 
material with the process) and its structure (sealed, apertured, formed of 
a metal sheet or a sieve, . . . ) are parameters which are quite 
compatible with the sintering technique used. This process makes it 
possible to develop various geometries, such for example as a structure 
permitting axial introduction of the gases with radial flow towards a 
manifold. 
The filter thus obtained is capable of retaining particles in suspension in 
gases, for example for filtering the exhaust gases of internal combustion 
engines and, in particular, of diesel engines. 
By way of purely illustrative example, two fibril compositions have been 
formed by over-hardening a composition of the Fe Ni Cr Al Y type and a 
composition of the Fe Cr Al Y type (percentage by weight 36.5 Fe --37 Ni 
--20 Cr --6 Al --0.5 Y.). As hardened, the length of the fibrils is about 
one metre. 
For the first composition (Fe Ni Cr Al Y) (weight percentage 36.5 Fe --37 
Ni --20 Cr --6 Al --0.5 Y.), a cylindrical cartridge of 50 mm in diameter 
and 100 mm in length was formed by pressing to the porosity of 95% of 
fibrils distributed isotropically in a stainless steel tube. 
The fibrils of the second composition (Fe Cr Al Y) were reduced to a few 
centimetres in length (2 cm) by means of shears then crushed in a blade 
crusher so as to give them an irregular shape favourable to high 
porosities. These Fe Cr Al Y fibrils were pressed to the porosity of 90% 
in a dense alumina tube of the same dimensions as said stainless steel 
tube. 
The sintering was carried out under hydrogen in a halogenated atmosphere in 
cement at 1050.degree. C. for 16 hours. The nickel or steel box containing 
the cement and the parts to be sintered is previously exhausted, then 
filled with argon, the operation being repeated three times so as to 
eliminate a portion of the oxygen retained. The cement is formed of 
alumina (50%), chromium (42.5%) and aluminium (7.5%) powder in which 1% of 
activator is incorporated (here ammonium chloride). When hot, the 
activator is decomposed and forms a chromium halide and aluminium halide 
atmosphere. Depending on the activity (i.e. the partial balance pressure) 
of these halides at the surface of the parts, a chromium or aluminium 
deposit is formed or not on these surfaces. It may be advantageous not to 
have any transfer : in this case, the atmosphere is protective and the 
sintering takes place without co-deposit. For the cement and the alloys 
used, the sintering was accompanied by a slight aluminium deposit. 
Cooling takes place in a hydrogen atmosphere. The parts are then carefully 
cleaned so as to remove any trace of cement. 
In the first case, the stainless steel tube adheres to the fibrils which it 
contains, by diffusion the cartridge can be used directly. 
In the second case, the alumina tube does not adhere to the fibrils. The 
sintered fibril cylinder is ready to be included in an assembly. 
The filtering efficiency of these cartridges on soot particles from diesel 
engines proved to be very high. Cyclic oxidation tests (cycle of 1 hour) 
showed that the Fe Ni Cr Al Y felts withstand more than 1000 cycles at 
1050.degree. C. 
On the basis of the fibrous structure obtained, following the sintering 
operation, it is possible to obtain a catalysis purifying filter by 
depositing a catalytic compound on the fibrils. 
Although the catalytic compound may be deposited directly on the surface of 
the fibrils, after a possible surface treatment for increasing the binding 
power of the compound, it is however preferable to deposit an intermediate 
layer (wash coat) formed of alumina which is consolidated by heat 
treatment. This intermediate layer with porous surface considerably 
increases the effective contact area with the surrounding atmosphere, 
which area may reach 30 to 40 m.sup.2 /g, instead of about 1 m.sup.2 / g 
for the combined surface of the fibres. The catalytic compound is then 
supplied from a solution of metal salts forming the compound and which are 
distributed on the surface of the intermediate layer and, after 
decomposition heat treatment, form crystallites of a few nanometres. 
Following which, a filter is obtained which is not only capable of 
providing a function of filtering out the particles contained in the 
gases, as mentioned above but, in addition, is capable of fulfilling the 
function of catalytic purification of the gases in a particularly 
efficient way because of the turbulences introduced by the fibrous 
structure in the gas flow and because of the considerable contact area 
between the gases and the catalyst. 
A filter in accordance with the invention is then particularly well adapted 
to be used as a catalytic silencer for the treatment of the exhaust gases 
of internal combustion engines, of whatever type. 
As is evident and as it follows moreover from what has gone before, the 
invention is in no wise limited to those of its modes of application and 
embodiments which have been more particularly considered; it embraces, on 
the contrary, all variants thereof.