Method and apparatus for processing dielectric materials using microwave energy
Methods and systems are provided for heating a dielectric preform material. An exemplary method includes inserting the preform material into a microwave cavity along a longitudinal axis of the microwave cavity and supplying the microwave cavity with microwave power having a frequency that corresponds to an axial wavelength along the longitudinal axis of the microwave cavity. The axial wavelength is greater than a length of the preform material along the longitudinal axis. The method includes heating the preform material within the microwave cavity by the microwave power and determining temperatures of the preform material at one or more locations on a surface of the preform material. The method further includes adjusting, based on the temperatures of the preform material, the microwave frequency to achieve substantially uniform heating at least on a sidewall of the preform material along the longitudinal axis.
1. A system for heating a dielectric preform material, the system comprising:
a microwave cavity configured to contain the preform material along a longitudinal axis of the microwave cavity;
a microwave power source, in communication with the microwave cavity, for supplying microwave power to the microwave cavity to heat the preform material;
at least one controller configured to perform, during the heating of the preform material, steps including:
determine temperatures of the preform material at one or more locations on a surface of the preform material; and
adjust a frequency of the microwave power in response to a change in the temperatures of the preform material and based on an energy absorption efficiency of the microwave cavity, wherein adjusting the frequency maintains an axial wavelength of the microwave power greater than a length of the preform along the longitudinal axis, thereby maintaining uniform electric field distribution and uniform heating in the microwave cavity along the length of the preform; and
a dielectric spacer positioned adjacent to the preform material in the microwave cavity to enhance the uniform electric field distribution, if the preform material has a non-uniform thickness or shape along the longitudinal axis.
2. The system of claim 1 , wherein the frequency of the microwave power is between 0.5 GHz and 10 GHz.
3. The system of claim 1 , wherein the frequency of the microwave power is between 2.40 GHz and 2.50 GHz.
4. The system of claim 1 , wherein the microwave cavity is cylindrical.
5. The system of claim 4 , wherein the at least one controller is configured to maintain the frequency of the microwave power at a cutoff frequency of a TM01 mode of the microwave cavity during heating of the preform material.
6. The system of claim 4 , wherein the diameter of the microwave cavity is between 1 and 100 cm.
7. The system of claim 4 , wherein the diameter of the microwave cavity varies along the longitudinal axis of the cavity in response to a variation of the geometry or property of the preform material to maintain the uniform electric field distribution.
8. The system of claim 1 , wherein the microwave cavity is rectangular if the preform material is approximately rectangular in shape.
9. The system of claim 1 , further comprising an annular slit defining an inlet of the microwave power into the microwave cavity, the annular slit dimensioned to enable coupling of the microwave power to the microwave cavity to within a predetermined range of coupling efficiency.
10. The system of claim 9 , wherein the microwave power source and the annular slit are coupled with a coaxial cable.
11. The system of claim 1 , further comprising:
one or more temperature sensors disposed around the microwave cavity to monitor the temperatures of the preform material during heating,
wherein the at least one controller, in communication with the one or more temperature sensors and the microwave power source, is configured to adjust the frequency of the microwave power in response to the monitored temperatures to uniformly heat at least a sidewall of the preform material in the longitudinal axis by maintaining the uniform electric field distribution.
12. The system of claim 11 , wherein the one or more temperature sensors comprise a plurality of temperature sensors spaced at equidistance along the preform material in the longitudinal axis.
13. The system of claim 9 , further comprising:
one or more electric field sensors disposed within the microwave cavity to monitor electric field within the cavity;
wherein the at least one controller, in communication with the one or more electric field sensors and the microwave power source, is configured to adjust the frequency of the microwave power in response to the monitored electric field to maintain the uniform electric field distribution.
14. The system of claim 13 , wherein the one or more electric field sensors comprise a plurality of electric field sensors spaced at equidistance along the preform material in the longitudinal axis.
15. The system of claim 1 , wherein the at least one controller is further configured to set the microwave power to control a heating rate of the preform material and limit the electric field in the microwave cavity to below an air breakdown limit.
16. The system of claim 1 , wherein the dielectric spacer is positioned adjacent to a closed end of the preform material to increase electric field near the closed end.
17. The system of claim 1 , further comprising:
at least one opening for inserting and removing the preform material; and
a microwave choke positioned outside of the opening to reduce leakage of the microwave power from the microwave cavity.
18. The system of claim 1 , wherein the microwave power source is a solid-state microwave generator.
19. The system of claim 1 , wherein the at least one controller is further configured to adjust the frequency of the microwave power in response to a difference between the temperatures of the preform material and a power absorption profile.