Patent Grant
7112769
The present invention relates to a susceptor for a microwave hybrid heating system, a microwave hybrid heating system including such a susceptor, and a method for sintering ceramic materials in such a hybrid microwave heating system.
Microwave energy offers a fast and effective sintering process that can reduce processing time by over 50% and which offers energy savings as a result. These decreased processing times and energy savings associated with microwave sintering, however, can only be applied to materials that can be readily processed by microwaves. The applicability of direct microwave sintering to specific materials is based on the characteristics of the material, that is, whether the dielectric constant and the dielectric loss of the material are such that the material will respond to microwave energy at a specific microwave frequency.
The specific frequency at which a given material will most effectively couple directly with microwave energy is dictated by the complex permittivity characteristics of that material. That is, when a material having a suitable dielectric constant and dielectric loss factor is irradiated with microwaves at a specific frequency, the material will absorb, store, and transform the microwave energy into thermal energy. This behavioral phenomenon in materials is often referred to as susceptibility. The susceptibility of a given material generally increases with temperature, as the dielectric loss factor of the material increases. Susceptibility in some materials diminishes, however, at a certain temperature where the dielectric loss of the material becomes sufficiently high enough such that the same material becomes reflective to microwave energy, even at an elevated temperature.
Near room temperature susceptibility is a desired property for materials to be sintered using microwave energy. Many ceramic materials, however, such as SiO2, Al2O3 and ZrO2, have a low room temperature dielectric loss factor and are virtually transparent to microwaves at room temperature, that is, these materials do not substantially reflect or absorb microwaves. As such, these materials do not directly couple with microwaves at room temperature. Indeed, sintering ceramic materials using direct microwave systems has been problematic if not impossible since most ceramic materials are not readily susceptible to microwaves emitted at a frequency of 2.45 GHz, which is a commercially desirable microwave frequency for materials processing.
That is, the Federal Communication Commission (FCC) has allocated specific uses for all frequencies ranging from 300 MHz to 300 GHz, including applications such as communications, avionics, and naval and other military applications, including radar, satellite, and missile guidance applications. Additionally, all non-military communications, including wireless and cellular communication systems, satellite television, household appliances, and scientific frequencies have been specifically allocated, as well. Large-scale use of any frequency outside of the specific use allocation range detrimentally interferes with the intended applications allocated to the specific frequency range. Accordingly, only those frequencies that have been specifically designated for scientific, industrial, and household use would be suited for material processing with microwaves. As such, viable microwave processes for those applications are limited to the frequencies allocated by the FCC.
In general, microwave technologies have been restricted to frequencies of 2.45, 5.8, 10, 18, 28, 84 and 110 GHz operating systems. Generally speaking, however, higher operating frequencies require a more expensive operating system. For example, in microwave processes involving lower frequencies or lower power requirements, such as power requirements less than 20 KW, magnetron technology is most often used to generate the microwaves. As the power requirements increase, however, more suitable microwave generation sources become klystrons, gyrotrons and gyro-klystrons etc., the system costs of which can easily exceed $500,000.
As a source for microwave generation, magnetron technology is generally well understood and has been well developed. That is, since the advent of the household microwave oven, the focus on cost reductions through “economies of scale” has allowed the market to develop to such a degree that more than 60 million household microwave ovens are produced per year, each of which operates at a frequency of 2.45 GHz using a magnetron source. Thus, microwave processing systems with 2.45 GHz magnetron microwave sources are by far the most economical and readily attainable type of microwave sintering system.
As mentioned above, however, most materials, and particularly, most ceramic materials, are not readily susceptible to microwaves emitted at a frequency of 2.45 GHz at room temperature. Increasing the microwave processing frequency involves a correlating increase in operational expense, and does not necessarily guarantee an energy efficient room temperature response from low dielectric loss (low susceptibility) ceramic materials. Therefore, a material having a high room temperature susceptibility is required to be used in concert with the low susceptibility material to be sintered in order to even make microwave sintering low susceptibility materials at a frequency of 2.45 GHz a possibility. Hybrid microwave sintering involves such a combination.
In hybrid microwave sintering, a high susceptibility material (primary material) is provided that readily couples to and absorbs the microwave energy and transforms it into infrared energy, which is emitted from the primary material to heat a low susceptibility (secondary) material to be sintered. That is, the primary material, also known as a susceptor, responds to microwave energy at room temperature to become an infrared radiant heater. As the temperature of the secondary material increases as a result of the heat emitted from the primary material, the susceptibility of the secondary material increases until the material can directly absorb and couple with the microwave energy. That is, the secondary material responds to the radiant energy of the primary susceptor material until the temperature at which the secondary material can couple directly to the microwave radiation is reached.
There are, however, drawbacks associated with microwave hybrid heating systems. One problem is that the masses of the susceptible materials are included as an integral part of the materials sintering process, in that the susceptor mass required to radiate a sufficient amount of infrared energy to induce microwave coupling in the material to be sintered becomes an energy consumption consideration. That is, for a specific mass of any given susceptor material, a certain amount of energy input is required in order for the susceptor material to begin radiating heat and in order to increase and maintain the desired level of heat output therefrom. Typically, a large load or a high mass secondary material requires a correspondingly larger mass for the susceptor. In that manner, the susceptor material can act as a thermal well that diminishes the energy efficiency of the overall system.
While the physical space that the susceptor material occupies can be reduced, for example, by reducing the profile of the susceptor or by designing the susceptor material to act as a setter material for the load, a certain amount of energy input is still required in order for the susceptor material to begin radiating heat and to increase and maintain the desired level of heat output. Further, in the case of most solid-state susceptor materials, reducing the mass of the susceptor material may undesirably inhibit the ability of the susceptor to emit enough radiant heat to bring the mass of the secondary material to the coupling-trigger temperature.
Thus, it would be desirable to provide a commercially viable microwave sintering system that addresses the problems currently associated with microwave sintering systems. That is, it would be desirable to provide a hybrid microwave sintering system that can effectively sinter a large material load using an economic, commercially available microwave furnace with a standard 2.45 GHz frequency magnetron source. In conjunction therewith, it would also be desirable to provide a relatively low mass susceptor that can provide a sufficient amount of radiated infrared heat to adequately heat a large load with a low overall microwave energy input and high energy efficiency. It would also be desirable to provide a method for microwave sintering low loss materials, such as ceramic materials, using an energy efficient hybrid microwave heating system.
It is an object of the present invention to overcome the drawbacks described above. It is also an object of the present invention to provide a hybrid microwave sintering system that can effectively sinter a large material load using an economic, commercially available microwave furnace with a standard 2.45 GHz frequency magnetron source. In conjunction therewith, it is an object of the present invention to provide a relatively low mass susceptor that provides a sufficient amount of radiated infrared heat to adequately heat a large load with a low overall microwave energy input and high energy efficiency. It is also an object of the present invention to provide a method for microwave sintering low loss materials, such as ceramic materials, using an energy efficient hybrid microwave heating system.
According to one embodiment of the present invention, a susceptor for a microwave hybrid heating system is provided. The susceptor includes a hollow member surrounding a substance contained within the hollow member that substantially immediately couples to microwave energy at room temperature and emits radiant energy substantially immediately. Preferably, the hollow member comprises a ceramic envelope made of a heat resistant material that does not substantially absorb or reflect microwave energy at room temperature.
The ceramic envelope preferably comprises at least one material selected form the group consisting of quartz, translucent polycrystalline alumina, single crystal magnesium oxide, single crystal sapphire, cubic zirconia and yttrium oxide.
The substance contained within the ceramic envelope preferably substantially immediately forms a plasma when the susceptor is irradiated with microwave energy, and preferably comprises a gas, more preferably a noble gas, having a sufficient volume and a sufficient pressure to ensure safe and sufficient radiant energy emission when the susceptor is irradiated with microwave energy.
A main feature of the invention is containing a plasma within a ceramic envelope to provide a susceptor for a MHH system such that the load to be sintered is provided outside the plasma field. That is, by using a quick-response susceptor comprising a gas-filled ceramic envelope according to the present invention, several major benefits are achieved, as discussed below.
First, the overall mass of the susceptor required for a specified level of radiant heat output is reduced because the mass of the substance contained within the microwave transparent vessel (hollow member) is significantly less than that of a solid state susceptor material. Additionally, the substance interacts with the microwave energy and produces a heat-emitting plasma substantially immediately. In that manner, the energy transfer between the microwave energy and the substance is virtually direct. Further, plasma generates heat at a much higher rate of speed when compared to solid state radiant heat transfer. Moreover, since the energy transfer between the microwaves and the substance is substantially direct and virtually instantaneous, very little energy is lost compared to the energy loss associated with first heating a solid state susceptor material to a radiant temperature and the continued energy input required to maintain the radiant emissions of the solid state susceptor during sintering.
According to another embodiment of the present invention, a method for sintering a ceramic member using microwave hybrid heating system is provided. The method includes the steps of:
The substance contained within the hollow member substantially immediately couples to the microwave energy in step (h) such that the susceptor emits radiant energy substantially immediately and the temperature of the ceramic member within the thermal containment chamber is raised via the radiant energy emitted from the susceptor to the microwave coupling-trigger temperature of the ceramic member, at which time the ceramic member directly couples to the microwave energy such that the ceramic member is sintered by the microwave energy in cooperation with the radiant energy emitted from the susceptor.
Preferably, a plurality of the susceptors are provided in step (d) and positioned adjacent and proximate peripheral portions of the inner peripheral surface of the thermal containment unit in step (e) so as to substantially peripherally surround the ceramic member when the ceramic member is positioned in step (f).
According to yet another embodiment of the present invention, a microwave hybrid heating system is provided. The system includes a microwave furnace including an applicator in communication with at least one microwave source, the applicator having a microwave chamber lined with a material that reflects microwave energy, and a thermal containment unit provided within the microwave chamber of the applicator. The thermal containment unit comprises a material that does not substantially absorb or reflect microwave energy at room temperature or at any temperature less than a maximum sintering temperature of the ceramic member to be sintered, and the thermal containment unit has an inner surface and an outer surface defining a thermal containment chamber. The system also includes at least one susceptor provided within the thermal containment chamber of the thermal containment unit. The susceptor comprises a hollow member and a substance contained within the hollow member that substantially immediately couples to microwave energy at room temperature and emits radiant energy substantially immediately. Preferably, the hollow member comprises a heat resistant ceramic envelope made of a material that does not substantially absorb or reflect microwave energy at room temperature. When the microwave chamber is irradiated with microwave energy from the microwave source, the substance contained within the hollow member of the susceptor substantially immediately couples to the microwave energy such that the susceptor emits radiant energy substantially immediately and the temperature of a ceramic member to be sintered positioned within the thermal containment chamber is raised via the radiant energy emitted from the susceptor to a microwave coupling-trigger temperature of the ceramic member, at which time the ceramic member directly couples to the microwave energy and begins sintering by the microwave energy in cooperation with the radiant energy emitted from the susceptor.
Preferably, the thermal containment unit comprises at least one material selected from the group consisting of silica, boron nitride and alumina. More preferably, the thermal containment unit comprises fibrous alumina or foam silica.
It is also preferred to use a plurality of susceptors and a substantially cylindrical thermal containment unit. The plurality of susceptors are preferably arranged at equiangular positions with respect to a central axis of the substantially cylindrical thermal containment unit.
According to another embodiment of the present invention, a continuous microwave hybrid heating system is provided. The continuous microwave hybrid heating system includes a microwave furnace including at least one applicator in communication with at least one microwave source, and the applicator has a microwave chamber lined with a material that reflects microwave energy. At least one thermal containment unit is provided within the microwave chamber of the applicator. The thermal containment unit comprises a material that does not substantially absorb or reflect microwave energy at room temperature or at any temperature less than a maximum sintering temperature of a ceramic member to be sintered, and the thermal containment unit has an inner surface and an outer surface defining a thermal containment chamber. At least one susceptor is provided within the thermal containment chamber of the thermal containment unit. The susceptor comprises a hollow member surrounding a substance that substantially immediately couples to microwave energy at room temperature and emits radiant energy substantially immediately. The continuous microwave hybrid heating system also includes transport means for continually transporting a plurality of thermal containment units through the microwave chamber. The microwave chamber is irradiated with microwave energy from the microwave source, the substance contained within the hollow member of the susceptor substantially immediately couples to the microwave energy at room temperature and substantially immediately emits radiant energy so that the temperature of the thermal containment chamber is raised via the radiant energy emitted from the susceptor and such that the ceramic member to be sintered is heated to a microwave coupling-trigger temperature thereof, at which time the ceramic member directly couples to the microwave energy and begins sintering by the microwave energy in cooperation with the radiant energy emitted from the susceptor as the thermal containment unit member is transported though the microwave chamber via the transport means. According to one embodiment, the applicator comprises a plurality of applicators arranged in a predetermined configuration to define a single continuous microwave chamber.
According to yet another embodiment of the present invention, a continuous microwave hybrid heating system is provided. The continuous system includes a microwave furnace including a microwave source and a microwave chamber lined with a material that reflects microwave energy, and at least one thermal containment unit provided within the microwave chamber of the furnace. The thermal containment unit comprises a material that does not substantially absorb or reflect microwave energy at room temperature or at any temperature less than a maximum sintering temperature of the ceramic member to be sintered, and the thermal containment unit has an inner surface and an outer surface defining a thermal containment chamber. The continuous system also includes at least one susceptor provided within the thermal containment chamber of the thermal containment unit. The susceptor comprises a hollow member and a substance contained within the hollow member. The substance comprises a material that substantially immediately couples to microwave energy at room temperature and emits radiant energy substantially immediately. The continuous system further includes transport means for continually transporting one or more ceramic members to be sintered through the thermal containment chamber. The microwave chamber is irradiated with microwave energy from the microwave source, the substance contained within the hollow member of the susceptor substantially immediately couples to the microwave energy at room temperature and substantially immediately emits radiant energy so that the temperature of the thermal containment chamber is raised via the radiant energy emitted from the susceptor and such that the ceramic members to be sintered are heated to a microwave coupling-trigger temperature thereof, at which time the ceramic members directly couple to the microwave energy and begin sintering by the microwave energy in cooperation with the radiant energy emitted from the susceptor as the ceramic members are transported though the thermal containment chamber via the transport means. According to one embodiment, the thermal containment unit comprises a plurality of thermal containment units arranged in a predetermined configuration to define a single continuous thermal containment chamber.
For a more complete understanding of the nature and objects of the present invention, reference should be made to the following detailed description of a preferred mode for practicing the present invention, read in connection with the accompanying drawings, in which:
Microwave energy is provided by one or more microwave energy sources (not shown), such as magnetron sources, which can be either directly incorporated into the structure of the applicator 110 or provided in a distant position.
That is, as shown in
The wave guides 121 preferably comprise a material that is reflective to microwave energy to effectively contain and transport the energy from the source to the microwave chamber 115 without any significant energy loss. As mentioned above, however, the microwave source can also be directly incorporated with the applicator structure to further reduce the potential for energy loss on transfer to the microwave chamber 115.
A thermal containment unit, such as the thermal containment unit 10 shown and described in more detail below with respect to
As shown in
The shape of the thermal containment unit 10 is not limited to the embodiment shown in
The thermal containment unit 10 preferably comprises a heat resistant material that is virtually transparent to microwaves, that is, a material that does not substantially absorb or reflect microwaves at any temperature less than (or equal to) the maximum sintering temperature of the system. Suitable materials for the thermal containment unit 10 include, but are not limited to, boron nitride, foam silica and fibrous alumina.
A plurality of susceptors 20 are also provided within the thermal containment chamber 15 of the thermal containment unit 10. Each susceptor 20 comprises a hollow member having an outer surface 23 and an inner surface 24 extending from a first end 21 to a second end 22 thereof to define a susceptor chamber 25.
The hollow member of the susceptor 20 preferably comprises a heat resistant material, such as a ceramic envelope, that is virtually transparent to microwaves at any temperature less than (or equal to) the maximum sintering temperature of the system. Suitable examples of materials for the hollow members include, but are not limited to, sealed tubes made of quartz, translucent polycrystalline alumina, single crystal magnesium oxide, single crystal sapphire, cubic zirconia and yttrium oxide.
A substance 30 that substantially immediately couples to microwave energy at room temperature and substantially immediately emits radiant energy is provided within the susceptor chamber 25. The first and second ends 21, 22 of the hollow member of each susceptor 20 are sealed by any appropriate means such that the substance 30 is completely contained within the susceptor chamber 25. The substance 30 is preferably provided in an appropriate volume and pressure state such that the substance will sufficiently interact with the microwave energy to produce a sufficient amount of heat without causing a catastrophic pressure situation (i.e., an explosion).
For example, the substance 30 is preferably a gas or vapor that substantially immediately forms plasma when irradiated with microwave energy. More preferably, the substance is a noble gas, such as xenon. Other suitable examples of the substance include mercury vapor and sodium vapor. The particular volume and pressure of the system contained with the susceptor chamber 25 is application dependent. That is, the specific volume and pressure of the substance 30 required to produce a sufficient amount of plasma to generate a sufficient amount of radiant energy depends, for example, on the size of the susceptor chamber 25, the mass of the load to be sintered, and the specific couple triggering temperature of the load to be sintered.
It is preferred that the susceptors 20 are arranged to substantially surround the load of the material to be sintered within the thermal containment chamber 15 of the thermal containment unit 10, however, the particular configuration is not limited to the structures shown and described herein. For example, a single susceptor may be provided proximate a portion of the peripheral wall 14 of the thermal containment chamber 15. Another example susceptor configuration is shown in
Although it is not shown in the drawings, it is preferred that the susceptors 20 are positioned within the thermal containment chamber 15 in a quasi-free-standing arrangement spaced a distance from, but proximate, the peripheral wall 14. This can be accomplished by using any appropriate means, including, but not limited to, eyelet type stand-offs. The material of the stand-offs is preferably transparent to microwaves and examples of suitable materials include, but are not limited to, quartz, BN, high purity Al2O3, and refractory metals.
It is also possible to affix the susceptors 20 directly to a portion of the peripheral wall 14 of the thermal containment chamber 15 to create a semi-mechanical engagement between the susceptors 20 and the peripheral wall 14 in a propped configuration by altering the surface structure of the peripheral wall 14.
As shown in
As shown in
According to the embodiment shown in
One or more susceptors 220 according to the present invention are provided within the continuous thermal containment chamber 215. The susceptors 220 interact with the microwave energy and emit radiant heat as described above such that the temperature within a corresponding portion of the thermal containment chamber 215 is substantially elevated as each ceramic member 50 to be sintered is transported therethrough. The susceptors 220 can be positioned at varying points along the axial length of the continuous thermal containment chamber 215 so as to provide a pre-heat stage I, a direct and cooperative sintering stage II and a cooling stage III.
In that manner, as the ceramic member 250 to be sintered is transported through the continuous thermal containment chamber 215, the ceramic member 250 is pre-heated in stage I along a portion of the axial length of the continuous thermal containment chamber 215 until the ceramic member is heated to the microwave coupling-trigger temperature of the ceramic material, at which time the ceramic member 250 directly couples to the microwaves and is directly and cooperatively sintered in the direct and cooperative sintering stage II further along a downstream portion of the axial length of the continuous thermal containment chamber 215. A cooling stage III, which either does not include any of the susceptors 220 or includes a fewer number of susceptors 20, may also be provided along a further downstream portion of the length of the continuous thermal containment chamber 215 in this continuous operation.
In a similar continuous MHH system according to another embodiment of the present invention that is not shown in the drawings, the second end 112 of the applicator 110 of
In this embodiment, the number of ports 120 and wave guides 121 can be varied at varying points along the axial length of the applicator arrangement so as to provide a pre-heat zone, a direct a direct and cooperative sintering zone and a cooling stage zone in the continuous microwave chamber 115.
In that manner, as the thermal containment units 10 are transported through the continuous microwave chamber 115, the ceramic members within the thermal containment units 10 are pre-heated as they travel through a first microwave zone along the axial length of the continuous microwave chamber 115. Microwave introduction zones are provided, either as a continuous zone or a plurality of grouped zones, downstream along the axial length of the continuous microwave chamber 115. As the thermal containment units 10 travel along the continuous microwave chamber 115, the temperatures of the ceramic members within the individual thermal containment units 10 increase in response to the radiant thermal energy emitted from the susceptors that are also within the thermal containment units 10. When the ceramic members are heated to their respective microwave coupling-trigger temperatures, the ceramic members begin to directly couple to the microwaves. In this zone, the ceramic members are directly and cooperatively sintered as the thermal containment units 10 move further downstream along the axial length of the continuous microwave chamber 115. A cooling zone, that either does not include any ports 120 and wave guides 121 or includes a fewer number of ports 120 and wave guides 121 (for controlled cooling), can also be provided in further downstream portions of the continuous microwave chamber 115 in the continuous operation according to this embodiment of the present invention. In that manner, when the thermal containment units 10 exit the continuous microwave chamber 115, the ceramic members are fully sintered and cooled to a temperature at which they can be further processed (i.e., removed form the thermal containment units manually).
While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.
This application claims the benefit of U.S. Provisional Application Ser. Nos. 60/514,871 filed Oct. 27, 2003 and Ser. No. 60/531,742 filed Dec. 22, 2003, the entireties of which are incorporated herein by reference.
| Number | Name | Date | Kind |
|---|---|---|---|
| 6512216 | Gedevanishvili et al. | Jan 2003 | B1 |
| 6710315 | Chisholm | Mar 2004 | B1 |
| 6744028 | Chisholm et al. | Jun 2004 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20060016805 A1 | Jan 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60531742 | Dec 2003 | US | |
| 60514871 | Oct 2003 | US |
