Nonwoven metal fabric and method of making same

Details Nonwoven metal fabric and method of making same Nonwoven metal fabric and method of making same

2001-10-26

2003-07-01

Smith, Duane (Department: 1724)

Gas separation

Specific media material

Fibrous or strand form

C055S385300, C055S522000, C055SDIG005, C264SDIG004, C123S19800E, C210S500250, C428S608000, C428S613000, C442S117000, C442S376000, C442S377000, C442S378000, C442S379000

Reexamination Certificate

active

06585794

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a nonwoven metal fabric for use in various kinds of filters for purification for exhaust gas from automobiles and semiconductor manufacturing gas and catalyst carrier of fuel cell and so forth.
The nonwoven metal fabric is often used for the materials for filters because of its superior strength and heat-resistance and low pressure-loss. For example, it is used for a diesel particulate filter (DPF) that reduces particulates such as soot in the exhaust gas which creates social problems of air pollution by the exhaust gases of automobiles.
The nonwoven metal fabric is also used for the filters to remove particles in manufacturing gases in the semiconductor manufacturing processes.
The exhaust gas from a diesel engine and gases emitted in the semiconductor manufacturing processes or a part of the exhaust gases from fuel cells contain corrosive gases. It is therefor an object to improve the corrosion resistance of the nonwoven metal fabric as the nonwoven metal fabric corrodes in the corrosive gas.
A large diameter of metal fiber of the nonwoven metal fabric has been considered as a means of improving the corrosion resistance of the nonwoven metal fabric. The drawback, however, is the reduced performance of pressure loss of the filter and the catalyst carrier if the metal fiber diameter is enlarged.
On the contrary, the object of the present invention is to provide the nonwoven metal fabric that has the improved corrosion resistance without compromising the performance of pressure loss and its preparing method.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, the corrosion resistance of a nonwoven metal fabric formed with mainly metal fibers can be improved without compromising its property in terms of pressure loss by making the fiber diameter Da at the surface part of the gas inlet side and the fiber diameter Db at the central part of the thickness direction to satisfy the equation Da≧1.5×Db, wherein the fiber diameter Da is not less than 15 &mgr;m and not more than 60 &mgr;m. Here, the metal fiber diameter is measured with a scanning electron microscope (SEM).
In the case where the fiber diameter Da at the surface part of the gas inlet side is smaller than 1.5 times the fiber diameter Db at the central part of the thickness direction, the fiber diameters must be made large in the whole nonwoven metal fabric to improve the corrosion resistance of the fabric. However, if the fiber diameter in the whole nonwoven metal fabric is enlarged, the performance of the cloth in terms of the pressure loss declines. Therefore, in order to avoid such degradation of pressure loss paformance, it is necessary for the fiber diameter of the nonwoven metal fabric to satisfy the relationship Da≧1.5×Db.
Also, when the fiber diameter Da at the surface part of the gas inlet side is made smaller than 15 &mgr;m, the smaller the diameter of metallic fibers, the more corrosion occurs at the surface part. This is because the inertia force of the gas flow becomes the highest at the surface of the nonwoven metal fabric when the corrosive gas passes the nonwoven metal fabric.
It is more preferable to have a large diameter fiber Da at the surface part of the metal unwoven fiber to improve the corrosion resistance of the nonwoven metal fabric. The larger the fiber diameter Da becomes, however, the greater the pressure loss when gas and so on pass the nonwoven metal fabric. Accordingly it is necessary that the fiber diameter at the surface part of the nonwoven metal fabric be not more than 60 &mgr;m.
Also it is preferable that the fiber diameter Db at the central part of the nonwoven metal fabric be not more than 35 &mgr;m. If the fiber diameter Db at the central part of the nonwoven metal fabric is more than 35 &mgr;m, it becomes disadvantageous in that the pressure loss increases as in the case with the fiber diameter Da at the surface part.
Also it is preferable that the fiber diameter Dc at the surface part of the gas outlet side satisfies the relation of Dc≧1.5×Db and is not more than 60 &mgr;m. This is because it is possible to manufacture and use the nonwoven metal fabric without distinguishing the front and back and reduce costs to distinguish them. The fiber diameter Dc at the surface part of the gas outlet side, however, is not limited to this condition because of a small effect of the corrosion. On the contrary, if the fiber diameter Dc at the surface part of the gas outlet side is made small, the pressure loss is restrained.
FIG. 1
shows a conceptual figure of the nonwoven metal fabric according to one aspect of the present invention. The diameter of the metal fiber
2
a
and the diameter of the metal fiber
2
b
of the nonwoven metal fabric
1
change along the gas flow direction, which is the thickness direction, when the filter surface
3
is perpendicular to the gas flow direction A. The fiber diameter
2
a
at the surface part of the gas inlet side is made to be different from the fiber diameter
2
b
at the central part of thickness direction by adjusting the maximum current density of the electroplating and metal composition of the plating bath, which is described in the following. The fiber diameter
2
c
at the surface part of the gas outlet side is nearly the same as the fiber diameter
2
a
at the surface part of the gas inlet side in FIG.
1
. It is possible, however, to make the fiber diameter
2
c
smaller than the fiber diameter
2
a
by making the maximum current density at the surface part of the gas outlet side smaller than at the surface part of the gas inlet side.
FIG. 2
shows a conceptual figure of an example of the nonwoven metal fabric of the present invention. The diameter of the metal fiber
2
a
and the diameter of the metal fiber
2
b
of the nonwoven metal fabric
1
change along the gas flow direction, which is the thickness direction, when the filter surface
4
is perpendicular to the gas flow direction A. The nonwoven metal fabric in
FIG. 2
differs from
FIG. 1
in that a sheet of the nonwoven metal fabric
1
is made by piling up the nonwoven metal fabrics
1
a
and
1
b
which have different metal fiber diameters from each other in FIG.
2
. The difference between the fiber diameter
2
a
at the surface part at the gas inlet side and the fiber diameter
2
b
at the central part of the thickness direction can be easily adjusted by such structure. Also, the number of nonwoven metal fabrics can be equal to or more than three sheets if the relation of a fiber diameter Da at the surface part of the gas inlet side and a fiber diameter Db at the central part of the thickness direction satisfies the equation Da≧1.5×Db and the fiber diameter Da at the surface part of the gas inlet side is not less than 15 &mgr;m and not more than 60 &mgr;m, despite the structure in
FIG. 2
consisting of 2 sheets of the nonwoven metal fabric.
It is desirable that the cross section of the metal fiber is not angular, and is close to circle or ellipse. This is because the corrosion originate from an angular part of the metal fiber if the cross section of the metal fiber is angular. Assessment of an angular cross section is made by a scanning electron microscope (SEM) having an expansion magnification of from about 200 times to about 1000 times.
For low production cost of the nonwoven metal fabric it is desirable that the main composition of the metal fiber is Fe or Ni, or both Fe and Ni and the metal fiber is a solid solution alloy or another alloy of these elements. The effect of corrosion resistance is not diminished if the total amount of metallic additive elements added to increase the corrosion resistance such as Al, Ag, Si Ti, V, Cr, Co, Zr, Nb, Mo, or rare-earth metallic elements is not more than 40 weight percent.
The nonwoven metal fabric is prepared by a method in which a conductive nonwoven fabric is electroplated with metal on the surface of the nonwoven metal fabric, and is fired in a reducing atmosphere, and the conductive nonwoven fabric is removed.
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