Electric heating – Microwave heating – Fluid heater
Reexamination Certificate
1999-08-24
2001-07-31
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
Fluid heater
C219S745000, C219S726000, C219S751000, C422S021000
Reexamination Certificate
active
06268596
ABSTRACT:
This application is related to U.S. Application No. 09/382,414, entitled “Apparatus And Method For Microwave Processing Of Materials”, filed on the same day, and herein incorporated by reference.
This invention was made with Government support under Contract No. DE-AC
05-96
OR22464 awarded by the U.S. Department of Energy to Lockheed Martin Energy Research, Inc., and the Government has certain rights in this invention.
This invention relates to the field of microwave radiation. More specifically, this invention relates to a microwave furnace having the capability of selectively enhancing the microwave power applied to a liquid sample by the use of tooling within the liquid vessel in the microwave cavity.
In the field of microwave radiation, it is well known that microwave furnaces may be constructed with either a fixed or a variable operating frequency. It has long been known that the interactions of various materials with microwaves are frequency dependent. It has further been observed that sweeping the microwave frequency can be an effective means of creating a relatively uniform power distribution within a multimode applicator cavity (“2 to 18 GHz Broadband Microwave Heating Systems” by R. J. Lauf et al., Microwave Journal, November 1993.) Where uniformity is the main goal, it is therefore desirable to have a microwave furnace that can be operated over a broad frequency range.
Most microwave sources have a very narrow bandwidth because they employ a resonant cavity. Microwave ovens constructed for home use are provided with a magnetron which operates at 2.45 GHz, which is a frequency that has been allocated by the FCC for domestic heating applications. Due to the coupling ability of a 2.45 GHz microwave to water, these ovens are used for cooking foods, drying, and other purposes wherein the principal material to be acted upon is water. However, it is well known that some microwave absorption will normally occur over a range of frequencies when heating a bulk liquid such as organic species and solvents for applications such as microwave assisted chemical synthesis.
The use of frequency sweeping over a wide range as a means of mode stirring has important implications for the use of microwave power to sterilize medical equipment or contaminated wastes. In such uses it is crucial to eliminate “dead” areas in the cavity wherein sufficient power may not be received in order for complete sterilization. Electronic frequency sweeping may be performed at a high rate of speed, thereby creating a much more uniform time-averaged power density throughout the furnace cavity. The desired frequency sweeping may be accomplished through the use of a variety of microwave electron devices. A helix traveling wave tube (TWT), for example, allows the sweeping to cover a broad bandwidth (e.g., 2 to 8 GHz) compared to devices such as the voltage tunable magnetron (2.45±0.05 GHz). Other devices such as klystrons and gyrotrons have other characteristic bandwidths that may be appropriate for selected applications.
Further, fixed-frequency microwave ovens typically found in the home are known to have cold spots and hot spots. Such phenomena are attributed to the ratio of the wavelength to the size of the microwave cavity. With a relatively low frequency microwave introduced into a small cavity, standing waves occur and thus the microwave power does not uniformly fill all of the space within the cavity, and the unaffected regions are not heated. In the extreme case, the oven cavity becomes practically a “single-mode” cavity.
Attempts have been made at mode stirring, or randomly deflecting the microwave “beam”, in order to break up the standing modes and thereby fill the cavity with the microwave radiation. One such attempt is the addition of rotating fan blades at the beam entrance of the cavity.
Another method used to overcome the adverse effects of standing waves is to intentionally create a standing wave within a single-mode cavity such that the workpiece may be placed at the location determined to have the highest power (the hot spot). Thus, only that portion of the cavity in which the standing wave is most concentrated with be used.
Other devices have been produced to change the parameters of the heating process of selected materials. Typical of the art is those devices disclosed in the following U.S. Pat. Nos.
U.S. Pat. No.
Inventor(s)
Issue Date
3,611,135
D. L. Margerum
October 5, 1971
4,144,468
G. Mourier
March 13, 1979
4,196,332
A. MacKay B, et al.
April 1, 1980
4,340,796
M. Yamaguchi, et al.
July 20, 1982
4,415,789
T. Nobue, et al.
November 15, 1983
4,504,718
H. Okatsuka, et al.
March 12, 1985
4,593,167
O. K. Nilssen
June 3, 1986
4,777,336
J. Asmussen
October 11, 1988
4,825,028
P. H. Smith
April 25, 1988
4,843,202
P. H. Smith, et al.
June 27, 1989
4,866,344
R. I. Ross, et al.
September 13, 1989
4,939,331
B. Berggren, et al.
July 3, 1990
5,321,222
D. W Bible et al.
June 14, 1994
5,318,754
M. J. Collins et al.
June 7, 1994
5,520,886
J. P. Bennett et al.
May 28, 1996
The subject matter disclosed by MacKay ('332) is further discussed in an article authored by MacKay B, et al., entitled “Frequency Agile Sources for Microwave Ovens”,
Journal of Microwave Power
, 14 (1), 1979. A microwave furnace having a wide frequency range has been disclosed in U.S. Pat. No. 5,321,222, herein incorporated by reference.
The field-perturbing tool of the present invention should not be confused with various contrivances used generally to modify the thermal environment of the workpiece rather than to perturb the local electric field in a known and controllable way. A typical example thereof is the introduction of relatively lossy materials such as silicon carbide whose role is to absorb microwave power and convert that power to radiant heat thereby providing supplemental or “hybrid” heating to the workpiece [see, for example, U.S. Pat. No. 5,318,754 entitled “Microwave Ashing Apparatuses and Components” by M. J. Collins et al. assigned to CEM Corporation]. That type of contrivance is referred to by various terms, such as the “picket fence” of Janney et al. [see H. D. Kimrey et al. “Microwave Sintering of Zirconia-Toughened Alumina Composites”, Mat. Res. Soc. Symp. Proc. Vol. 189, pp. 243-55, 1991] and the “casketing” of Holcombe et al. [“Importance of “Casketing” for Microwave Sintering of Materials”, Journal of Materials Science Letters 9 (1990), 425-428]. Other contrivances include thermal insulation around the workpiece as well as thermally conductive inserts such as boron nitride to spread the heat within these insulated “caskets” [see, for example, T. N. Tiegs et al. “Comparison of the Properties of Sintered and Sintered Reaction-Bonded Silicon Nitride Fabricated by Microwave and Conventional Heating”, Mat. Res. Soc. Symp. Proc. pp. 501-6, 1994]. Yet other contrivances of that nature include packaging for microwave heatable food products such as popcorn and the like. A field-perturbing tool may, of course, provide some supplemental heating because of its own dielectric loss, but such heating, if any, is an incidental benefit of the field-perturbing tool and not its primary purpose.
A wide variety of materials and designs have been developed over the years for containing materials, particularly liquids, during microwave heating operations. One familiar example is microwave cookware for use in the home. Microwave-transparent vessels for heating liquids, as for example, in analytical chemistry procedures, have been developed that in many cases will resist damage from microwave heating and withstand a certain amount of internal pressure created as the contained liquid is heated [see, for example, U.S. Pat. No. 5,520,886 “Explosion Resistant Reinforced Container Assemblies for Materials to be Microwave Heated” by J. P. Bennett et al. assigned to CEM Corporation]. Many of the aforementioned vessels will be suitable for carrying out the present invention.
It is therefore an object of this invention to provide a microwave heating apparatus in which a volume o
Fathi Zakaryae
Lauf Robert J.
Tucker Denise A.
Leung Philip H.
UT-Battelle LLC
Wilson Kirk A.
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