Note: Descriptions are shown in the official language in which they were submitted.
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
METHOD AND APPARATUS FOR CONTINUOUS SPIN MELT
CASTING OF MATERIALS
This application is a continuation in part of co-pending U.S. application
number 09/237267, filed on January 25, 1999, entitled "APPARATUS FOR THE
PRODUCTION OF METAL RIBBONS AND METHOD THEREFORE".
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for melt spin
casting of materials, and more specifically, a method and apparatus for
continuous, economical melt spin casting of homogenous materials.
BACKGROUND OF THE INVENTION
A number of techniques are known for the production of homogeneous
materials. In the field of producing powdered materials, for example those
materials used as electrode material for rechargeable electrochemical cells,
property requirements include size, homogenous chemistry, and homogenous
crystalline structure. In the field of battery production, a hydrogen storage
alloy
2o is commonly formed as a bulk ingot from a melt. One method of producing a
hydrogen storage alloy is disclosed in commonly assigned U.S. Pat. No.
4,948,423 to Fetcenko, Sumner, and LaRocca for ALLOY PREPARATION OF
HYDROGEN STORAGE MATERIALS, incorporated herein by reference.
However, an inherent concern with manufacturing materials as an ingot where
homogenous material properties are required is the rate of change of cooling
through the ingot. As the ingot solidifies, the cooling rate of the interior
material
is much less than that of the exterior, resulting in a variation of the
material
crystalline structure.
Hydrogen storage negative electrodes utilizing the aforementioned alloys
are of relatively high hardness. Indeed, these alloys can typically exhibit
Rockwell "C" ("Rc") hardness of 45 to 60 or more. Moreover, in order to attain
the high surtace areas per unit volume and per unit mass necessary for high
capacity electrochemical performance, the alloy must be in the form of fine
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
2
particles. In a preferred exemplification, the hydrogen storage alloy powder
must
pass through a 200 U.S. mesh screen, thus being smaller than 75 microns in
size (200 U.S. mesh screen has interstices of about 75 microns). Therefore,
the
resulting hydrogen storage alloy material is comminuted, e.g., crushed,
ground,
milled or the like, before the hydrogen storage material is fabricated into
electrode form.
Comminution of bulk ingots of hydrogen storage alloy material is made
more difficult because the materials described hereinabove are quite hard, and
therefore do not easily fracture into particles of uniform size and shape. In
commonly assigned U.S. Pat. No. 4,893,756 to Fetcenko, Kaatz, Sumner, and
LaRocca for HYDRIDE REACTOR APPARATUS FOR HYDROGEN
COMMINUTION OF METAL HYDRIDE HYDROGEN STORAGE MATERIAL, the
disclosure of which is incorporated herein by reference, a hydride-dehydride
cycle comminution process was disclosed for initial size reduction of bulk
ingots
of hydrogen storage alloy material to flakes of about 80-100 mesh size. While
this process is effective for the initial size reduction of hydrogen storage
alloy, it
is inadequate for the task of further comminuting particulate hydrogen storage
alloy powder to the required particle size of 75 microns or less (i.e. 200
mesh or
less). Furthermore, the process begins an ingot which by default, is subject
to
the inherent thermal limitations of materials produced in ingot form as
described
above. Still another concern with initiating a powder formation process with
material in the form of an ingot is the size of the ingot with respect to the
final
size of the powder material. Initiating a powder formation process with a
large
piece of material in order to form a powder is hardly efficient.
Any method which can accomplish the objective of providing economical
size reduction of the metal hydride material is a potential candidate for
commercial processing. However, there are numerous characteristics of the
material which require special handling, instrumentation and other
precautions.
These characteristics include: (1 ) inherent alloy powder hardness, i.e.,
approximately Rockwell "C" ("Rc") 60 hardness. This means that conventional
size reduction processes of shear, abrasion and some types of impact
mechanisms as ball mills, hammer mills, shredders, fluid energy, and disk
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
3
attrition, are not very effective for material in the form of an ingot; (2)
sensitivity
to oxidation, such that comminution must be done under an inert environment to
provide a safe environment and maintain acceptable electrochemical
performance; (3) requirement of a specific crystalline structure necessary for
electrochemical activity; i.e., the microstructure of the material cannot be
adversely altered during grinding or atomization to produce powders directly
from a melt; and (4) requirement of a broad particle size distribution with a
maximum size of 75 microns (200 mesh) which provides optimum packing
density and electrochemical accessibility.
Early attempts to provide a method for size reduction of hydrogen storage
alloy materials from ingots proved inadequate due to the extreme hardness of
the hydrogen storage alloy materials. Conventional size reduction processes
employing devices such as jaw crushers, mechanical attritors, ball mills, and
fluid
energy mills consistently fail to economically reduce the size of such
hydrogen
storage materials. Grinding and crushing processes have also proven
inadequate for initial reduction of ingots of hydrogen storage alloy material
to
intermediate sized (i.e. 10-100 mesh) particulate.
There are numerous methods for preparing metal powders. Since the
alloys under consideration are at one stage molten, one might consider
ultrasonic agitation or centrifugal atomization of the liquid stream to
prepare
powders directly. The cost and the product yield are the two main concerns
with
using this approach. The particle shape is also not optimal. Finally, because
it is
difficult to provide a completely inert atmosphere; surface layers, which are
undesirable from an electrochemical perspective, may be formed on the
particulate.
Attempts to embrittle the hydrogen storage alloy material by methods such
as immersion in liquid nitrogen, so as to facilitate size reduction are
inadequate
because: (1 ) the materials are not sufficiently embrittled; (2) the methods
typically introduce embrittlement agents in the alloys which have an
undesirable
effect upon the electrochemical properties of the hydrogen storage alloy
material; and (3) as the materials become more brittle, it becomes
increasingly
difficult to obtain uniform particle size distribution. Other methods for
embrittling
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
4
metals are disclosed, for example, in Canadian patent No. 533,208 to Brown.
Brown employs cathodic charging as a size reduction technique.
Furthermore, material size reduction by an hydrogenation processes is
not desirable because of the inherent dangers associated with hydrogen gas.
Therefore it is desirous to employ a technique that minimizes subsequent size
reduction processes while providing homogeneous material properties.
One alternative for preparing materials for powder formation is rapid
solidification. Rapid solidification refers to a technique for rapidly
quenching
material from a liquid, or molten, state into a solid state at a rate
sufficient to
freeze the position of atoms. One rapid solidification technique is a spin
cast
technique where the molten material into is formed into ribbons. Spin casting
is
a method of dispersing molten material on a rotating wheel, also commonly
known as melt spin casting. The rotating wheel, made of a highly conductive
metal, typically copper, is positioned proximal to a reservoir of molten
material.
The reservoir typically has an orifice or nozzle to direct molten material
onto the
rotating wheel. The molten material is rapidly solidified because of the mass
of
the wheel and the significant difference in temperature between the wheel and
the molten material. The wheel need not be cooled in order to provide a
sufficiently cold moving surface relative to the molten material, however, the
wheel may be cooled to achieve higher quench rate if so desired.
A wheel is typically between 6" and 10" (15.24 and 25.40 centimeters) in
diameter and is rotated at a rotational velocity of between 1,000 and 5,000
rpm
thereby to obtain a linear velocity, at the point of contact at the material
with the
cylindrical periphery of the wheel, of 32.81 to 65.62 feet per second (1,000 -
2,000 centimeters per second).
Materials produced by melt spin casting techniques of the prior art exhibit
material property variations due to a number of factors. Variations such as
flow
rate, stream diameter and material temperature, and compositional variations
including chemistry and impurities within the melt stream contribute to
inhomogeneity of the solidified material. Changes in the diameter and flow
rate
of the molten material stream result in different cooling rates of the
material. As
explained above, different cooling rates will effect the material's
crystalline
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
structure. Inhomogeneity is the main drawback of the melt spin processes of
the
prior art. Increased homogeneity requirements of materials make an improved
technique especially important.
Melt spin casting is an attractive method for producing ribbons of material
5 because of the lack of complexity involved. This technique may be employed
to
form ribbons of material such as metals, metal alloys, or thermoplastics. One
method of producing materials by melt spin casting is disclosed in commonly
assigned U.S. Pat. No. 4,637,967 to Keem et al for ELECTRODES MADE WITH
DISORDERED ACTIVE MATERIAL AND METHODS OF MAKING THE SAME,
the disclosure of which is incorporated herein by reference. The '967 patent
discloses a heated crucible that is equipped with means for pressurizing the
crucible to extrude molten material through a nozzle onto the surface of a
chill
wheel. Although this method is very good for forming homogeneous materials,
the process efficiency is reduced because the crucible must be pressurized to
extrude the material. The flow rate is also inconsistent as the material level
in
the crucible changes, the flow rate of the material changes. This process is
not
continuous, therefore an hiatus in the production of ribbon material is
inevitable,
resulting in a reduced capacity.
Another method and apparatus for spin melt casting of materials is
commonly assigned U.S. Pat. No. 4,339,255 to Ovshinsky et al for METHOD
AND APPARATUS FOR MAKING A MODIFIED AMORPHOUS GLASS
MATERIAL, the disclosure of which is incorporated by reference. Although the
teachings of the '255 patent disclose the advantages of spin melt casting over
thin film processes for making amorphous materials, the process has inherent
limitations. A piston is provided to cause material within a crucible to be
ejected
onto a rotating wheel. However, once all of the material is driven from the
crucible, the process must be halted and the crucible refilled. This method is
also susceptible to temperature and chemistry variations because he process
must be stopped and started.
In order to improve process efficiency, a hydrostatic system may be
employed to provide the necessary force to extrude the molten material from
the
crucible. Such a device is disclosed in U.S. Pat. No. 4,485,839 to Ward for
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
6
RAPIDLY CAST ALLOY STRIP HAVING DISSIMILAR PORTIONS. The '839
patent discloses a planar flow casting technique for drawing thin ribbons.
Although this technique includes a hydrostatic system for delivering the
molten
material, there are shortcomings associated with this invention. The device
disclosed is not capable of providing a true, continuous melt casting
operation.
Furthermore, the disclosed operation relies on heating a crucible to prevent
the
nozzles from clogging, which is ineffective since it is commonly known that
stags
and other impurities are present within the crucible. Also, because of the
tolerances associated with this device, less than 0.120" (3.05 mm), the
surtace of
1o the chill wheel must be constantly maintained.
Accordingly, there exists a need for an efficient, continuous spin melt
casting process for producing homogeneous materials.
SUMMARY OF THE INVENTION
The present invention disclosed herein is an apparatus for continuous
melt spin casting of materials. The apparatus comprises a supply crucible and
a
casting crucible disposed a chamber. The supply crucible provides molten
material to the casting crucible for ejecting at least two streams of molten
material upon a chill wheel for solidification. Each stream is ejected through
one
of at least two orifices in the casting crucible. Monitoring means determine
the
level of molten material in the casting crucible.
The apparatus of the present invention includes a selectively actuated
flow control valve for controlling the flow of molten material from the supply
crucible to the casting crucible to maintain the level of molten material in
the
casting crucible within a range. Valve control means actuate the flow control
valve as a function of material level in the casting crucible as sensed by the
monitoring means. The present invention includes means for filtering the
molten
material prior to casting and thermal control means for controlling the
temperature of material within said casting crucible. In one embodiment, the
means for filtering is one or more removable filter elements disposed in a
movable support assembly.
Each orifice is at an equal distance from a point of contact on the chill
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
7
wheel. Equal length streams of material at a uniform temperature and flow rate
are rapidly solidified form homogenous ribbons by the chill wheel.
The apparatus further comprises a melting crucible to providing the supply
crucible with molten material. The melting crucible may be disposed within the
chamber or be located external to the chamber. Conveyor means to supply the
melting crucible with ingot material is also disclosed. Means for removing
accumulated material from an external surtace of said orifices is also
disclosed,
the means may be the chill wheel or an attachment such as a diamond wheel.
Also disclosed herein is a method for continuous melt spin casting of
materials. The novel method comprises the steps of providing a material supply
crucible and a casting crucible disposed within a chamber, where the material
supply crucible is provided to receive molten material to be supplied to the
casting crucible; monitoring the level of molten material in the casting
crucible;
releasing molten material from the supply crucible through a flow control
valve;
filtering the molten material released from the material supply crucible prior
to
solidification; maintaining the molten material level in the casting crucible,
whereby the molten material flow rate through the orifices is controlled by
the
static pressure of the molten material in the casting crucible to provide a
constant material flow rate; maintaining the temperature of the molten
material in
the casting crucible to provide material for casting at a consistent
temperature;
ejecting at least two streams of molten material from the casting crucible,
each
stream ejected through one of at least two orifices in the casting crucible;
and
rapidly solidifying the molten material by ejecting equal length and diameter
streams of molten material upon the chill wheel.
Further included is the step of providing a melting crucible for providing
the supply crucible with molten material, the melting crucible may be disposed
within the chamber or external to the chamber.
The molten material may be filtered through a filter element that is
removable during continuous casting. A movable support assembly, such as a
turntable is provided wherein one or more filter elements may be disposed in a
movable support assembly. At least two nozzles may be provided, each nozzle in
communication with one of at least two orifices. accumulated material deposits
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
8
are removed from the orifices or nozzles by means including a shearing device,
the chill wheel or a diamond wheel. Conveyor means may be provided to supply
the melting crucible with ingot material.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross sectional view of one embodiment of the present
invention taken through a chamber to reveal the operative elements therein.
Figure 2 is a cross sectional view showing one operating position of one
embodiment of a casting crucible in relation to a chill wheel.
Figure 3 is a sectional view of the casting crucible and chill wheel taken
along section A-A of Figure 2.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed toward an apparatus and method for
forming ribbons of material by melt spin casting. Referring now to figure 1,
the
apparatus 10 of the present invention includes a chamber 20 containing a
supply
crucible 30 and casting crucible 40. The supply crucible 30 is provided to
receive
molten material and has a selectively actuated flow control valve 70 for
releasing
molten material into the casting crucible 40. Referring now also to figure 2,
the
casting crucible 40 has at least two orifices 50, each orifice 50 for ejecting
a
stream of molten material upon a chill wheel 110 having a horizontal axis of
rotation.
In the preferred embodiment, a melting crucible 120 is disposed within the
chamber 20 to provide molten material to the supply crucible 30. However, it
should be noted that melting crucible 120 may be located external to the
chamber 20. Conveyor means (not shown) may be employed to supply the
apparatus 10 with material for casting. A loading vessel 130 in communication
with the chamber 20 provides material to the melting crucible 120 without
exposing the materials within the chamber 20 to contaminants by incorporating
a
flap valve 135. It should be noted that although a flap valve is used to seal
the
chamber 20, any suitable means known in the art may be substituted for a flap
valve. An induction heater 240 is employed to heat material within the melting
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
9
crucible 120. The melting crucible 120 has a flow control valve 250 that is
selectively actuated by valve control means 260 for releasing material to the
supply crucible 40. Upon melting, the material within the melting crucible 120
is
stirred electrodynamically, by agitation, or any other suitable means known in
the
art.
The apparatus 10 may also include at least two nozzles 55, each nozzle
in communication with one of the orifices 50 in the casting crucible 40. The
flow
control valve 70 is selectively actuated by valve control means 80 to provide
the
molten material to the casting crucible 40. The valve control means 80 may be
manually operated or automatically controlled by a controller (not shown). The
valve control means 80 are actuated as a function of the material level in the
casting crucible 40.
Molten material is ejected from the casting crucible 40 onto the chill wheel
110. The molten material level within the casting crucible 40 provides
hydrostatic pressure at the orifices 50 to eject molten material upon the
chill
wheel 110. The material level in the casting crucible 40 is maintained in
order to
maintain a uniform flow rate. The chill wheel 110 is preferably formed of a
material having a high thermal conductivity such as copper. The temperature of
the chill wheel 110 may be controlled by any suitable cooling means (not
shown)
2o known in the art, including a cooling medium such as a water and ethylene
glycol
mixture. In the preferred embodiment, the chill wheel 110 has a passage to
allow the cooling medium to pass through and draw heat away from the casting
wheel 110.
The supply crucible 30 is heated by thermal control means 100; in the
preferred embodiment, the thermal control means is an induction heater 180.
The molten material within the supply crucible 30 may be mixed by any suitable
means known in the art to maintain homogeneity. The casting crucible 40 is
heated by thermal controls means 105, and in the exemplary embodiment, the
thermal control means is an induction heater 190 as well. Likewise, the
casting
3o crucible 40 may be stirred by any suitable means known in the art.
The atmosphere in the chamber 20 may consist of an insert gas or may be
pumped down to a vacuum to prevent contamination. Once the supply crucible
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
has received the materials for casting, heat is provided by the induction
heater
180 to maintain viscosity. The material within the supply crucible 30 is mixed
to
maintain homogeneity. By lowering the frequency of the induction heater 180,
the material may be electrodynamically mixed. In the exemplary embodiment,
5 the induction heaters 180 and 190 are reduced below 1000 Hz, resulting in
excellent mixing results. It should be noted that other mixing operations may
be
substituted for electrodynamic mixing, such as agitation.
Referring now also to figure 2, the chill wheel 110 is shown in one of many
potential locations. The chill wheel 110 is movable in the X, Y, and Z-axis,
10 providing many advantages to the present invention. By adjusting the
position
of the chill wheel 110 along the Z-axis, the length of the streams is changed,
changing the exposure time to ambient conditions and ultimately, temperature.
Therefore, fine temperature adjustments of the molten material prior to
contacting the chill wheel 110 may be made by adjusting the position of the
chill
wheel 110 along the Z-axis. The form of the ribbons produced by apparatus 10
of the present invention may be altered by moving the chill wheel 110 along
the
X-axis, which will change the angle of incidence of the material streams on
the
chill wheel 110.
Referring now also to figure 3, the chill wheel 110 may be positioned by
any combination of orthogonal coordinate changes and then translated along the
Y-axis to remove material which has accumulated upon the orifices 50 by
shearing the accumulated material with the chill wheel 110. Any other suitable
means for removing accumulated material 160, including shearing or grinding
means may otherwise be adapted to remove accumulated material from the
orifices 50 or nozzles 55 to avoid contact with the chill wheel 110, for
example a
diamond wheel. Furthermore, the orifices 50 or nozzles 55 may be heated to
reduce material accumulation.
A thermal screen 220 may also be disposed below the supply crucible 40
to stabilize the flow rate of the molten material exiting the supply crucible
40.
The thermal screen 220 is heated whereby the material temperature is preserved
as the material exits the supply crucible 40. Means for filtering 90 the
molten
material prior to casting are provided. Referring again to figure 3, the means
90
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
11
may be a filter element 230, such as a ceramic filter capable of high
temperature
filtering of molten materials for the removal of stags, oxides or other
impurities. A
common occurrence experienced when melting is pieces of the crucible break
away due to thermal cycling, and become inclusions in the melt.
In the preferred embodiment, the means for filtering 90 molten material
include a removable filter element 140 disposed in a movable support assembly
150. The filter element 140 may comprise a fine filter component and coarse
filter component to improve filter element 140 viability. The filter element
140 and
support assembly 150 are disposed between the casting crucible 40 and the
supply crucible 30. When a filter element 140 must be replaced, the movable
support assembly 150 is cycled while the supply crucible 30 is not providing
material to the casting crucible 40. By cycling the movable support assembly
150, each contaminated filter element 140 may be replaced with a new filter
element 140. A seal 155 isolates the chamber 20, preventing contamination of
the casting material when exchanging filter elements 140.
The temperature of the material within the casting crucible 40 is
maintained by the induction heater 190 and stirred to maintain homogeneity.
Monitoring means 60, such as a sight glass for measuring height or a balance
for
determining material mass, is provided to evaluate the material level in the
casting crucible 40. The flow rate of the material from the casting crucible
40 is
governed by hydrostatic pressure. The material level in the casting crucible
40
will determine the rate material is ejected from the casting crucible 40. The
material level must be maintained within a range in order to provide a uniform
flow rate of the molten material stream. Once the material stream is ejected
from
the casting crucible 40 upon the rotating chill wheel 110, the material is
rapidly
solidified and is projected from the chill wheel 110 and the resulting ribbons
of
material are captured in a material collector 210.
The size and the form of the ribbons can be modified by changing the
rotational speed and diameter of the chill wheel 110. By increasing the speed
of
the chill wheel, thin ribbons are formed and the material dwell time is
reduced.
In the preferred embodiment, the surface of the casting wheel 40 is polished
to
provide sufficient mechanical and thermal contact with the melt streams.
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
12
By providing multiple orifices 50, process throughput is increased. The
orifices 50 are positioned at a uniform distance from a contact point of the
material stream upon the chill wheel 110. Flow rate, temperature, material
purity, and homogeneity must be simultaneously maintained in order to obtain
uniform material properties such as crystallite size and homogeneity of the
solidified material. The present invention discloses an economical solution to
melt spin casting concerns found in the state of the art. The present
invention
has improved throughput without a need for a mechanical device to provide
additional pressure within the casting crucible 40, such as a piston. Also, an
uninterrupted stream of material with excellent homogeneity and purity for
rapid
solidification upon the chill wheel 110 is provided.
EXAMPLE
In the present example, the chamber 20 is hermetically sealed and
operated in a vacuum to prevent oxidizing of the melt, thereby achieving
higher
purity. An ingot of negative electrode material for a rechargeable
electrochemical storage cell is provided to the melting crucible 120 within
chamber 20 by the loading vessel 130 and heated to 1550°C. After
melting, the
melt it is mixed electrodynamically to increase homogeneity.
Once liquefied, flow control means 260 open the flow control valve 250 to
released the molten material into the supply crucible 30. Thermal control
means
100 sustain the material temperature until the material is released into
casting
crucible 40. The molten material within the supply crucible 30 is
electrodynamically mixed. By lowering the frequency of the induction heater to
below 1000 Hz frequency, more efficient mixing is achieved.
The molten material in the supply crucible 30 is released to the casting
crucible 40 by actuating the valve control means 80 to open the flow control
valve 70. The material released by the control valve 70 passes through the
thermal screen 220 where the material temperature is stabilized to avoid
cooling.
3o The material then passes through the filter element 140 and into the
casting
crucible 40. The material temperature in the casting crucible 40 is stabilized
by
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
13
an induction heater 190. When the level of the material in the casting
crucible
40 rises to about 200 mm, the valve control means 80 close the supply crucible
flow control valve 70.
The material level in the casting crucible 40 is evaluated by a sight glass
disposed within chamber 20 to assure a material height of 200 mm is
maintained.
In the present example, ten streams of molten material, each stream formed by
one of ten calibrated orifices 50, are ejected onto the rotating chill wheel
110.
Each orifice 50 being uniform in diameter and equidistant from the chill wheel
110, forms a melt stream that is equal in length and diameter. In this
example,
the orifices are disposed about 150 mm from the contact point on chill wheel
110.
fn the present example, a cooling medium is flowed through a passage in
the chill wheel 110. By cooling the chill wheel 110, ribbons are formed having
a
constant width and thickness. The ribbons produced by this technique exhibit
high homogeneity of properties and uniform crystallite size while increasing
the
productivity of the process.
The temperature of the casting crucible is regulated to about
1500°C and
the level of the melt bath to about 200 mm. The casting crucible 40 flow rate
is
between about 0.15 to 0.32 L/min (1 to 2.5 kg/min). The ten streams have an
equal length, less than about 150 mm in the present example, and a diameter
between about 1.0 to 2.5 mm. The chill wheel 110 rotates with a linear speed
of
between about 5 to 25 m/sec.
The apparatus and method of the present can be used to produce
threads, films, ribbons and any variant thereof. Furthermore, although
metallic
materials and alloys have been specifically referenced, it should become
apparent to those skilled in the art that a variety of material, including non
metallic materials, such as plastics, may be formed by employing the teachings
set forth herein.
While the invention has been described in connection with preferred
embodiments and procedures, it should be understood that it is not intended to
limit the invention to the described embodiments and procedures. On the
CA 02362434 2001-08-09
WO 00/47351 PCT/US00/00685
14
contrary, it is intended to cover all alternatives, modifications and
equivalents
which may be included within the spirit and scope of the claims appended
hereto.
