|Title of Invention||
A DEVICE FOR THE SURFACE POLISHING OF GRAPHITE PLATES
|Abstract||A device for the surface polishing of a graphite plate comprises a chamber. The top of the chamber consists of a perforated plate over which the graphite plate is laid and fixed in place. There exists means for creating a vacuum in the chamber to cause the graphite plate to remain flat against the perforated plate under negative pressure and means to polish the surface of the graphite plate.|
|Full Text||This invention relates to a device for the surface polishing of graphite plates
A polymer electrolyte fuel cell is an electrochemical fuel cell, which employs an electrolyte in the form of Membrane, and comprises of an anode, a cathode, and a separator plate. The impervious, highly planar, impregnated graphite plates, also known as flow field plates, are used as gas separators and reactant distributor-cum-current collectors in the fuel cell stack. A Flow field plate for use in a fuel cell using a gaseous fuel and oxidant, comprising a substantially planar surface and a flow field formed in the substantially planar surface is provided. The plate further includes a network of flow passages for receiving the reactant gases discharging from the flow field. The flow field includes a plurality of substantially symmetric flow sectors having separate inlets and outlets communicating with the networks of supply and exhaust flow passage, respectively. Accordingly, these flow field plates with Membrane and Electrode Assemblies are stacked to deliver the required voltage and current. This substantially requires a high planarity in the graphite plates. Commercially available indigenous graphite plates are fine grained and have a planarity of 0.5 mm variation in thickness. This variation generally admits non planarity during stack assembly, which leads to the mixing of oxidant and fuel, in particular, hydrogen and resulting in the uncontrollable combustion, increases the contact resistance, and also damage to the graphite plates, which in turn to the Polymer Exchange Membrane Fuel Cell (PEMFC) stack. Thus, surface grinding of the graphite plates with less than 0.1 mm variation in thickness is necessary for use of these plates in PEM fuel cell. Apart from that, procuring an indigenous low planar graphite plates and surface grinding considerably reduces the capital cost of the fuel cell stack in which graphite plates form more than 50 % of the cost. We propose a device herein, for making the surface of the graphite plates with high planarity, of variation in thickness being less than 0.1 mm, by fabricating a fixture jig with proper dimension, which can be attached to a milling machine, may be used in the manufacture of Polymer Electrolyte Membrane (PEM) fuel cell flow field plates.
The present invention relates generally to a device for surface grinding of flow field graphite bipolar plates, which are used in the PEM fuel cell, more particularly, reactant distributor cum current collector, with perforated base for vacuum, and can be mounted on to a milling machine.
Surface grinding of nonplanar graphite materials with the help of a jig in a milling machine is always desirable in order to make them more planar and also to increase the perfection in alignment during stacking, so that the stack can take up the compression load during the tightening of the plates in between the end plates with the help of bolts and nuts. Indigenous graphite plates have non planarity in the form of variation in thickness, which detracts from use in some applications such as fuel cells those in which they are used as separators. This non planarity generally resulting in the mixing of reactants, increases the contact resistance and also damage to the stack. The variation in thickness also prevents the stack to take the compression load, which leads to the drop in electrical contact between the electrodes, membranes and the graphite plates.
The device proposed herein will now be described with reference to the accompanying
drawings, which illustrate, by way of example, and not by way of limitation, one of the
various possible embodiments of this invention,
Fig. 1 illustrating the embodiment in plan view and
Fig.2 illustrating the view in Fig.l at the section line A .
The chamber C has a top consisting of a perforated plate P with perforations, B.
Over the perforated plate, P the graphite plates is laid and fixed in place.
Means, such as, a vacuum pump, are provided at V, for creating a vacuum in the chamber to
cause the graphite plate to remain flat against the perforated plate, under negative pressure.
Means, such as a milling cutter are provided for polishing the surface of the said graphite
The vacuum of lO-2Torr is maintained for about 10 minutes, which is about the time taken
to complete the polishing operation up to a desired level of planarity. Thereafter, graphite
dust particles are collected in a dust bag with the help of a suction pump. A vacuum cleaner
may also be used.
The graphite plate is fixed in place on the perforated plate, P by a fixture.
Surface polishing of solid porous graphite plates with a jig is frequently desired in order to
make the graphite plates with good planarity. This is accomplished by fixing the plates in a
jig made of mild steel, and a cutting tool, preferably of circular diameter 50 mm. The jig can
be mounted on the rails of the milling machine, while the cutter, preferably of carbide tip
to the milling head.
The device proposed herein is made of mild steel container with top portion large enough to accommodate the plates to be surface polished. In the preferred embodiment, the fixture area may be 400 mm *240 mm, and also 400 mm * 360 mm, with a thickness of 33 mm and 35 mm respectively. The fixtures can be changed for various sizes of the palates. The perforated base consists of several holes for suction. The suction holes(perforations, B) are preferably 10mm in diameter, and in the preferred embodiment, the number of holes are 49, arranged in many columns, preferably in 9 columns, and are equally distributed throughout the surface of the base. The perforated plate, P dimension is preferably of the size of the flow field plate and is 300*200 mm2, and 300 * 300 mm2. The thickness of the plate may preferably be 10mm. The flow field plate e.g the graphite plate is fixed on the perforated plate, P, and is slightly above the surface of the fixture. This is preferable, in order to prevent the grinding of the fixture at their sides. This fixture jig is ideal, if the surface grinding material is non magnetic and also thickness of the plates is less than 10mm. A hole, preferably of 10mm is drilled on one of the side surface of the surface-polishing jig for vacuum connection. The flow field plates, in the preferred embodiment, are also held in its position by applying vacuum uniformly. Non-uniform distribution of vacuum results in the warping and breaking of the plates. For evacuating the jig, a simple rotary vacuum pump is sufficient. The flow field plates, in the preferred embodiment, the graphite plates, are placed
in the fixture, preferably in the space allotted for the plate, with other support from the perforated base. In the preferred embodiment, the region between the graphite plates and the base are evacuated with the help of suction holes provided at the perforated base and by the vacuum hole. No pre-treatment of the graphite is necessary. The fixture may generally be fitted to the milling machine at one particular position on the rails and the cutter in the milling head. It is necessary that the cutter with a carbide tip is fixed at one end of the graphite plate. With the help of a screw gauge, initially, the point at which the plate is showing minimum thickness is observed. The cutter is fixed at this position, by just touching the surface at this point, and then the cutter is allowed to move from the vertical direction, horizontally over the plate, during which process, the excess non planarity is removed. Since, the cutter dimension is 50mm, the above-mentioned procedure is repeated by moving the jig at a position slightly away from 50mm from the original position. There exists a digital read out to see the position of the cutter in the plates in the X and Y direction. The measure of non-planarity of the plate, preferably the thickness of the plate could be read from the circular dial in the milling machine. By looking at the dial position, the reduction in non-planarity can be assessed. The planarity of the plates can also be verified with the help of a dial gauge, and by horizontally moving the plate, preferably in X and Y directions. The graphite fine powder formed during the surface polishing is collected with the help of hood of dimension 10mm*20mm, using an air suction pump. The hood is placed on top of the fixtures, preferably slightly above the plates, and the graphite fine particles are forced to go to various thick cotton bags, preferably six in numbers, which are connected to the air suction pump.
Graphite plates with 10-20 % porosity were used for polishing to determine the planarity of the plates of dimension 300mm x 200mm and also 300 mm x 300 mm. The graphite plate is arranged in parallel by fixing them in the jig, sitting on the perforated base. The plates are then evacuated under vacuum 10-2 tore and the cutter was allowed to move horizontally over the graphite plates. Preferably six to nine times the cutter was moved over the plates in
1. A device for the surface polishing of a graphite plate comprising a
chamber, the top of the chamber consisting of a perforated plate over
which the graphite plate is laid and fixed in place; means for creating a
vacuum in the chamber to cause the graphite plate to remain flat against
the perforated plate, under negative pressure; and means to polish the
surface of the graphite plate
2. A device as claimed in Claim 1 wherein the means for creating a vacuum
in the chamber comprises a vacuum pump.
3. A device as claimed in Claim 1 or Claim 2 wherein the graphite plate is
fixed in place by a jig and fixture.
4. A device for the surface polishing of a graphite plate substantially as
herein described and illustrated.
|Indian Patent Application Number||396/MAS/2001|
|PG Journal Number||08/2007|
|Date of Filing||15-May-2001|
|Name of Patentee||SPIC SCIENCE FOUNDATION|
|Applicant Address||MOUNT VIEW 111 MOUNT ROAD, GUINDY, CHENNAI 600032.|
|PCT International Classification Number||B24B5/00|
|PCT International Application Number||N/A|
|PCT International Filing date|