Inventor(s): ROY
RUSTUM [US]; MOELLER WILLIAM [US] + (ROY RUSTUM, ; MOELLER
WILLIAM)
Applicant(s): CLIFTON MINING COMPANY [US] +
(CLIFTON MINING COMPANY, ; AMERICAN BIOTECH LABS, LLC)
Classification: - international:
C22B11/00 - European:
B02C19/18; C22B1/00; C22B11/00; C22B11/04; C22B3/06; C22B4/00
Also published as:
US7850759 // WO2008147420
Inventor(s): ROY RUSTUM [US]; CHENG JIPING
[US]; AGRAWAL DINESH K [US] + (ROY, RUSTUM, ; CHENG, JIPING, ;
AGRAWAL, DINESH, K)
Applicant(s): PENN STATE RES FOUND [US]; ROY
RUSTUM [US]; CHENG JIPING [US]; AGRAWAL DINESH K [US] + (THE PENN
STATE RESEARCH FOUNDATION, ; ROY, RUSTUM, ; CHENG, JIPING, ;
AGRAWAL, DINESH, K)
Classification: - international: C04B35/64;
H05B6/80; (IPC1-7): H01L21/326; H05B6/64; H05B6/74 - European:
C04B35/64; H05B6/80
Also published as:
US6365885 (B1) // AU8027800 (A)
Abstract -- A process for
heating a material (30) includes providing a microwave radiation
source (14) and a processing chamber (12). The process includes
generating a region of pure magnetic field from the microwave
radiation in the processing chamber (12). A region of pure
electric field from the microwave radiation is also generated. The
material (30) is positioned in the region of pure magnetic field
while no portion of the material is positioned in the region of
pure electric field and the material is heated in the region of
pure magnetic field. The heating may be conducted to sinter the
material (30) which may include a metal.
Technical Field
The present invention relates to methods for processing materials
in the presence of microwave energy.
Background
Application of microwave energy to process various kinds of
materials in an efficient, economic, and effective matter, is
emerging as an innovative technology.
Microwave heating of materials is fundamentally different from
conventional radiation-conduction-convection heating. In the
microwave process, the heat is generated internally within the
material instead of originating from external heating sources.
Microwave heating is a sensitive function of the material being
processed.
Microwaves are electromagnetic radiation with wavelengths ranging
from 1 mm to 1 m in free space and frequency between approximately
300 GHz to 300
MHz, respectively. Today, microwaves at the 2.45 GHz frequency are
used almost universally for industrial and scientific
applications. The microwaves can be transmitted, absorbed, or
reflected, depending on the material type with which they
interact.
In conventional sintering processes, extremely high temperatures,
long processing times and, in some cases, hot pressing or hot
isostatic processing must be applied in the fabrication of
products to achieve the highest density and minimum porosity.
Conventional powder processing involves the compaction of a powder
into the desired shape following by sintering. Typically, powders
in the range of 1 to 120 micrometers are employed. The powder is
placed in a mold and compacted by applying pressure to the mold.
The powder compact is porous. Its density depends upon the
compaction pressure and the resistance of the particles to
deformation.
The powder compact is then heated to promote bonding of the powder
particles. The sintering temperature is such as to cause atomic
diffusion and neck formation between the powder particles. The
diffusion process can yield a substantially dense body upon
completion of the sintering cycle. Such a process is used in
industry for a variety of products and applications.
Microwave sintering processes have unique advantages over
conventional sintering processes. The fundamental difference is in
the heating mechanism. In conventional heating, heat is generated
by heating elements (resistive heating) and then transferred to
samples via radiation, conduction and convection. In microwave
heating, sample materials themselves absorb microwave power and
then transform microwave energy to heat within the sample volume.
The use of microwave processing typically reduces sintering time
by a factor of 10 or more. This minimizes grain growth. The fine
initial microstructure can be retained without using grain growth
inhibitors and hence achieve high mechanical strength. The heating
rates for a typical microwave process are high and the overall
cycle times are reduced by similar amounts as with the process
sintering time, for example from hours/days to minutes. Microwave
processing of materials has the potential to yield products having
improved mechanical properties with additional benefits of short
processing times and low energy usage.
Typical microwave processing procedures utilize a mixed electric
field and magnetic field condition, with the sample placed into a
chamber where it is exposed to both the electric field and the
magnetic field generated by the microwaves.
Cherrardi et al., in Electroceramics IV. Vol. II, (eds. Wasner,
R., Hoffmann, S.,Bonnenberg, D., & Hoffmann, C.) RWTN, Aachen,
1219-1224 (1994), published a paper indicating that both the
magnetic field and electric field may contribute to the sintering
of certain materials.
Summary
One embodiment relates to a process including providing a
microwave radiation source and a processing chamber. The process
includes generating a region of pure magnetic field from the
microwave radiation in the processing chamber. A region of pure
electric field from the microwave radiation is also generated. A
material is positioned in the region of pure magnetic field while
no portion of the material is positioned in the region of pure
electric field, and the material is heated in the region of pure
magnetic field. In one aspect of certain related embodiments, the
heating is conducted to sinter the material. In another aspect of
certain related embodiments, the material includes a metal.
Another embodiment relates to a method including providing a
microwave radiation source and a processing chamber. A first
region of maximum magnetic field from the microwave radiation is
generated in the processing chamber, and a second region of
maximum electric field from the microwave radiation is generated
in the processing chamber. A body is positioned in only one of the
first region and the second region during a first time period. The
body is positioned in the other of the first region and the second
region during a second time period. In an aspect of certain
related embodiments, a first portion of the body is heated during
the first time period and a second portion of the body is heated
during the second time period, wherein during the first time
period the first portion is heated to a higher temperature than
the second portion, and during the second time period the second
portion is heated to a higher temperature than the first portion.
Another embodiment relates to a method for heating including
providing a substrate having a layer of material thereon. One of
the substrate and the layer is positioned in one pure field
selected from the group consisting of a microwave generated pure
magnetic field and a microwave generated pure electric field,
during a first time period. The one of the substrate and the layer
is heated layer to a temperature greater than that of the other.
Brief Description of the Drawings
Certain embodiments of the invention are described with reference
to the accompanying figures which, for illustrative purposes, are
not necessarily drawn to scale.
Fig. 1 is a schematic of a
microwave system used for processing materials in magnetic and
electric fields.
Figs. 2 (a) is a top sectional
view of the cavity of the microwave system including a quartz
tube therein.
Fig. 2 (b) is a view along the
line A-A of Fig. 2 (a).
Fig. 2 (c) is a side sectional
view illustrating a sample positioned in the electric field
region and the location of the temperature monitor.
Fig. 2 (d) illustrates the
microwave field distribution within the TE, 03 single mode
microwave cavity along the length a shown in Fig. 2 (b) and
sample locations in the maximum magnetic H field region and the
maximum electric E field region.
Fig. 3 (a) illustrates a
comparison of the heating rates of powdered metal compact (Fe-2%
Cu-0.8% C) samples in different microwave fields.
Fig. 3 (b) illustrates a
comparison of the heating rates of cobalt powder compact samples
in different microwave fields.
Fig. 3 (c) illustrates a
comparison of the heating rates of copper powder compact samples
in different microwave fields.
Fig. 3 (d) illustrates a
comparison of the heating rates of solid copper rod samples in
different microwave fields.
Fig. 3 (e) illustrates a
comparison of the heating rates of alumina powder compact
samples in different microwave fields.
Fig. 3 (f) illustrates a
comparison of the heating rates of tungsten carbide
powder-compact samples in different microwave fields.
Fig. 3 (g) illustrates a
comparison of the heating rates of alumina-metal composite
powder-compact samples in different microwave fields.
Fig. 3 (h) illustrates a
comparison of the heating rates of tungsten carbide cobalt
powder-compact samples in different microwave fields.
Fig. 4 illustrates the sintering
of a pure Cu powder metal compact including a fully sintered
core, a partially sintered interlayer and a surface layer.
Fig. 5 illustrates an example of
microwave heating of a Fe-Cu-C powder compact metal sample in a
magnetic field region according to an embodiment of the present
invention.
Detailed Description
Fig. 1 illustrates a microwave system 10 including a finely tuned
waveguided cavity 12 with a cross-sectional area of 86mm by 43 mm
which works in TE, 03 single mode which was used to investigate
the microwave heating of several materials in different microwave
fields using a 2.45 GHz, 1.2 kW microwave generator 14 (Toshiba,
Japan). The system also includes a circulator and water dummy load
16, a frequency tuner 18, microwave power monitor 18, and
temperature monitor 20 (infrared pyrometer from Mikron Instrument
Co., Model
M90-BT with a temperature range of-50 C to 1000 C) connected to
the cavity 12.
A gas supply may also be connected for atmospheric control.
Figs. 2 (a)-2 (d) illustrates the cavity 12 in more detail. The
cavity is rectangular in cross-section and has a length a = 12.4
cm. A quartz tube 22 is positioned in the cavity 12 to hold the
sample 30 and to permit easy atmospheric control. As seen in Figs.
2 (a) and 2 (b), the samples may be positioned at location (a) at
the center of the chamber or at position (b) at the side wall of
the cavity. Fig. 2 (c) illustrates the position of the temperature
monitor 20 when a sample 30 is positioned in the electric field
region. Fig. 2 (d) shows the electric field E and magnetic field H
distributions along the length a (as seen in Fig. 2 (b) of the
chamber. In the location halfway along the length of the cavity,
the maximum electric field is in the center of the cross-section,
where the magnetic field is a minimum, and the maximum magnetic
field is near the wall, where the electric field is at a minimum.
During experimental processing runs nitrogen gas was passed
through the tube 22 to avoid oxidation of metal samples at
elevated temperatures.
A number of samples were prepared and sintered using the
experimental set up described above. The samples were centered
either at the electric field maximum node or at the magnetic field
maximum node. The electric field maximum region and the magnetic
field maximum region in the experimental set up described above
are separated by about 6 cm, hence, the sample sizes of about
6.25mm (1/4 inch) diameter pellets are small. It should be noted
that depending on the size of the sample, it is possible that the
entire sample may not lie at the absolute maximum of the magnetic
field or electric field. For example, if the maximum electric
field is located at the center point of the cavity, then part of
the sample may be positioned exactly at the center point, and part
of the sample may be positioned slightly adjacent to the center
point. As such, the part that is positioned slightly adjacent may
not be exposed to the maximum electric field and minimum magnetic
field. As a result, when the samples are described herein as being
positioned at a"maximum" field region or a"minimum"field region,
or at a"pure"field region, it is understood that a portion of the
sample may be positioned slightly adjacent to the maximum field
point, minimum field point, or pure field point.
A first sample was a commercial powdered metal having a
composition of Fe-2wt% Cu-0.8wt% C (obtained from Keystone
Powdered-Metal Company, Saint Marys, PA). In the pure or maximum
electric field region, the sample reached a maximum temperature of
180 C after being microwave heated for 8 minutes, as seen in Fig.
3 (a). In the maximum magnetic field region, the sample heated up
quickly and uniformly. The heating rate was higher than 300 C per
minute in the first two minutes, then it slowed down. The final
temperature reading was 780 C after 10 minutes.
A cobalt powder-compact sample displayed similar behavior to the
Fe2wt% Cu-0.8wt% C sample described above. There was little
heating effect in the maximum electric field region, with the
sample reaching a temperature of only about 150-200 C in 10
minutes. There was a high heating rate in the maximum magnetic
field region, with the sample reaching a temperature of about 550
C in about 2 minutes, and further increasing to a temperature of
about 700 C at 10 minutes, as seen in Fig. 3b.
A copper powder-compact sample was also tested and displayed fast
heating when placed in the maximum electric field region and when
placed in the maximum magnetic field region. As seen in Fig, 3c,
the sample temperature rose to about 600700 C and then dropped
down to-500 C and remained within the range of about 500-550 C
during the heating. In the maximum electric field region, the
sample reached maximum temperature in about 1-2 minutes. In the
magnetic field region, the same reached maximum temperature in
about 3-4 minutes.
For comparison to the copper powder-compact sample, a solid copper
bar with the same shape and size was tested to determine its
energy absorption and heating behavior (Fig. 3d). There was little
or no temperature rise in the solid copper bar in either the
maximum electric field region or the maximum magnetic field
region. After being exposed in the microwave field for 10 minutes,
the sample remained at room temperature, as seen in Fig. 3d.
Non-metal samples were also tested. Alumina is a typical ceramic
material with excellent dielectric properties. Alumina usually has
a very low dielectric loss, and it is generally not easy to heat
up by microwaves, particularly at lower temperatures. Since the
dielectric loss of alumina increases with temperature, microwave
heating of alumina becomes more efficient at high temperature.
Alumina powder-compact samples doped with 0. 05wt% MgO (from
Baikowski
International, Charlotte, NC) were tested (Fig. 3e). In the
maximum electric field region, the heating rate speeded up after
the sample reached a temperature of about 400-500 C. In the
maximum magnetic field region, the alumina sample barely heated
up, as seen in Fig. 3e.
Tungsten carbide (from Teledyne) powder-compact samples were also
tested.
The WC samples exhibited different behavior than the alumina
samples. As seen in
Fig. 3f, the heating rate was rapid in the maximum magnetic field
region, reaching a maximum of about 700 C in 3-4 minutes and then
leveling out. In the maximum electric field region, the heating
rate was slow, and after 7 minutes of heating the sample
temperature was only about 180 C and there was some electrical
discharging around the sample.
Two types of composite samples were also tested, including
aluminapowdered metal (50% A1203 and 50% (Fe-2wt% Cu-0.8wt% C))
and tungsten carbide-cobalt (WC-10% Co).
For the alumina-metal composition, in the maximum magnetic field
region, the sample was rapidly heated to a temperature of about
900 C in about 1-2 minutes and then leveled out. In the maximum
electric field region, the reached a temperature of about 400 C in
about 2 minutes and then leveled out.
For the tungsten carbide-cobalt sample, in the maximum magnetic
field region, the sample was rapidly heated to a temperature of
about 400 C in about 2 minutes and then continued to increase to a
temperature of about 650 C in 10 minutes. In the maximum electric
field region, the sample was barely heated at all, reaching a
temperature of about 100 C in 10 minutes.
It should be noted that there was no insulation placed around the
samples, and as such, thermal loss was likely significant at the
higher temperatures, leading to lower heating rates at the higher
temperature ranges. In addition, for the experimental set-up
described above, it is believed that placing the center of the
sample within about 3 mm of the maximum or minimum field point
will yield similar results. In addition, a fixed microwave power
for testing runs was not used because for some samples, the
temperature increase was too fast to be measured with the
pyrometer, and for certain conditions, discharging and arcing
occurred. As a result, the samples were tested at the powers set
forth in Table 1 below.
Table I. Microwave power applied
to test materials.
Test Material Microwave Power
powdered metal compact (Fe-2% Cu-0.8% C) samples 200
cobalt powder-compact samples 150
copper powder-compact samples 150
solid copper rod samples 150
alumina powder-compact samples 180
tungsten carbide powder-compact samples 150
alumina-powder-compact metal composite samples 120
tungsten carbide-cobalt powder-compact samples 200
From the above results, it is apparent that different materials
have different heating behaviors in the electric and magnetic
field regions, and that exposure to either the electric field
alone or the magnetic field alone can be used for processing a
variety of materials. In general, the higher conductivity samples,
such as the powdered metal sample, can be more rapidly and
efficiently heated in the maximum magnetic field region. The pure
ceramic alumina sample with low conductivity exhibited a more
rapid and efficient heating rate in the maximum electric field
region.
Fig. 4 illustrates microstructures of a pure Cu powder compact
microwave heated in the maximum magnetic field region for 10
minutes, including (A) surface region with no sintering and
considerable porosity; (B) interlayer region with little sintering
and some porosity ; and (C) core region fully sintered with little
or no porosity. Fig. 5 shows a portion of the experimental set-up
during microwave heating of a Fe-Cu-C powdered metal sample in the
maximum magnetic field region.
Embodiments may find application in a variety materials processing
applications. For example, for certain types of electronic devices
it may be desirable to heat only a portion of the device. By
properly positioning the device, a particular portion may be
subjected to the maximum magnetic field or electric field region
in order to heat up the particular portion. One example of an
application would be to heat a metal deposited on a ceramic
substrate. By subjecting the metal to the magnetic field region,
it may be possible to heat the metal while the ceramic is not
heated, due to the different interactions of the metal and the
ceramic with the magnetic field. Such a process may be suitable
for activating catalysts, processing semiconductor devices,
forming coating, etc., where different materials can be heated
differently depending on their interactions with the magnetic
field or electric field generated by the microwave processing
system. Numerous materials may be processed according to
embodiments of the present invention, including, but not limited
to metals, ceramics, semiconductors, superconductors, polymers,
composites and glasses. The term metals includes not only pure
metals but also other materials having metallic properties, such
as alloys.
In semiconductor processing, it is sometimes necessary to heat a
particular layer in order to, for example, activating a dopant,
annealing a metal, causing reflow of an electrode, etc. Microwave
processing by exposing the necessary region to a separate
essentially pure magnetic field and/or electric field enables one
region to be heated while other regions, which may be damaged by
heat, are kept at a lower temperature. In addition, it is possible
to configure a processing system so that a sample can be moved
through the regions of maximum magnetic and/or electric field as
desired. Such a system may be a stand alone processing system or
attached to a larger processing system having multiple processing
chambers, such as a semiconductor processing cluster tool.
It will, of course, be understood that modifications of the
present invention, in its various aspects, will be apparent to
those skilled in the art. The scope of the invention should not be
limited by the particular embodiments described herein.
Other embodiments are possible, their specific features depending
upon the particular application.
