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.