Patent Publication Number: US-2011073592-A1

Title: Applicator for microwave enhanced chemistry

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Pat. No. 7,405,382, filed Apr. 8, 2002, entitled “System for microwave enhanced chemistry,” which is hereby incorporated by reference herein in its entirety, including but not limited to those portions that specifically appear hereinafter, the incorporation by reference being made with the following exception: In the event that any portion of the above-referenced prior application is inconsistent with this application, this application supercedes said above-referenced prior application. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     FIELD OF THE INVENTION 
     The present invention relates to microwave systems, and more particularly microwave systems for accelerating reactions in small reagent volumes. 
     BACKGROUND OF THE INVENTION 
     The invention includes a variable output power microwave generator and a broadly tuned microwave applicator that is connected to the generator though a length of coaxial cable. 
     Many chemical reactions are greatly accelerated by heating with a microwave field as compared to an open flame, oven or oil bath. Systems for microwave chemistry that are available from several manufacturers have the limitations of physical size, limited to reagent volumes greater than a few milliliters, require retuning of the applicator when the reagent mix is changed, and limit the users ability to monitor the progress of the accelerated reaction. 
     U.S. Pat. No. 4,681,740, Apparatus for the Chemical Reaction by Wet Process of Various Products illustrates a length of waveguide that is terminated with end panels made from a conductive material with the magnetron oscillator placed near at one end of the cavity and a provisions to place to insert samples near the other end of the cavity. The waveguide is provided with tuning screws to adjust the tuning of the cavity to the requirements of the sample. 
     Systems such as these are generally suitable for performing reaction on relatively large volumes of chemicals. However the combination of the magnetron with its required cooling system, the sample holder, and the overall packaging make them excessively large for many laboratory applications. 
     U.S. Pat. No. 5,308,944 illustrates a cavity applicator that can be coupled to a microwave generator either by waveguide or coaxial cable. The geometry of this cavity is varied by sliding walls proportionately to adjust for the sample container and sample volume. 
     The paper, “Evaluation of a Microwave Cavity for the Synthesis of PET Radio Pharmaceuticals”; C. S. Dence et al; Journal of Labeling Compounds for Radiopharmaceuticals, 1995, #37, page 115. shows a microwave power oscillator with fixed output power connected to a tunable circular cavity. 
     DISCLOSURE OF THE INVENTION 
     One aspect of the present invention is predicated on the observation that the energy imparted to a sample in a microwave system, and for heating that sample, can be affected by the presence of an inductive tuning post placed within a waveguide of that microwave system. A second aspect is predicated on the observation that microwave systems made for chemical reagent heating generating irregular average power from one sample heating to the next receive little demand from consumers of microwave systems. 
     It is an object of the present invention to provide a microwave system capable of complete distillation of a small reagent sample. It is a second object of the invention to provide a variable power microwave generator with only one transformer for both high voltage and filament pre-heat. 
     According to the present invention, a microwave generator includes a variable voltage provided from a first transformer to a second transformer having a secondary winding split into a filament winding cathode low AC signal and a secondary winding providing a high voltage which is doubled and rectified in a half-wave voltage doubler for providing a pulsed DC signal through a ballast resistor to a magnetron anode which is responsive to an electromagnetic field. In further accord with the present invention, the variable voltage is provided from a bucking transformer secondary winding In still further accord with the present invention, the bucking transformer output variable voltage is provided in response to signals from a thyristor controller, which is itself responsive to commands from a microcontroller. In still further accord with the present invention, the electromagnetic field strength and direction is selectable by outputs from the microcontroller. In still further accord with the present invention, the low AC signal from the filament winding is for bringing the filament cathode to a selectable temperature prior to beginning timing cycle when the filament begins to boil off electrons in a magnetic field created by an electromagnet for producing microwaves from an antenna connected to a cathode plate of the magnetron. The ballast resistor maintains a nearly constant current by compensating for fluctuations in alternating-current powerline voltage by increasing resistance when current increases. The ballast resistor is a means for stabilizing the pulse to pulse magnetron operation at low filament voltage and high standing wave ratio. 
     The “bucking” transformer works by taking the secondary winding and putting it in series but out of phase with the primary of the main transformer. What happens is that the net voltage applied to the high voltage main transformer is reduced, and therefore the output voltage of the high voltage main transformer and hence the magnetron anode is reduced. 
     In still further accord with the present invention, a microwave sample cavity includes a waveguide, and a hole for receiving a sample to be microwave-heated, and an inductive tuning post all on a straight line, wherein the hole is adjacent the waveguide and the post is distant from the waveguide. 
     In still further accord with the present invention, the cavity receives the microwaves from the microwave generator for heating a sample to a selectable temperature for a selectable time. 
     An advantage of this arrangement is more complete distillation of the sample. The invention includes a variable power microwave generator that is capable of withstanding a high SWR, a coaxial transmission line, and a fixed tuned cavity of intentionally lowered Q and increased bandwidth, the lowered Q being obtained by the choice of materials from which the cavity is made, by leaving a rough surface finish, by the insertion of inductive tuning posts into the cavity at positions adjacent to the inserted sample, and further by the introduction of a dielectric lens into the cavity to increase the focusing on the sample. 
     Further advantages include, in a microwave system, having a magnetron and an electromagnet but reduced filament voltage and reduced high voltage which is also smoothed because the ballast resistor gives the tube some slack to work against when it misfires. A second advantage of the present invention is that the presence of the ballast resistor allows an increased filament tube voltage. 
     The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a microwave system. 
         FIG. 2  is a schematic of a microwave signal generator. 
         FIG. 3  is a top and side cross sectional view of a microwave cavity applicator with one inductive tuning post. 
         FIG. 4  is a top and side cross sectional view of a microwave cavity applicator with one inductive tuning post and a dielectric insert. 
         FIG. 5  is a graph of return loss versus sample volume. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     In  FIG. 1 , a microwave power generator  2  has a display of microwave system parameters  4  concerning microwave heating of a sample  6  in a microwave applicator cavity  8 , reagent container  10 , via interconnecting coaxial cable  12 . The heating can be effected and monitored the reaction process at computer  14 , through serial cable  16  between the computer  14  and the generator  2 . The antenna  18 , a probe or loop connected to the anode and extending into one of the tuned cavities, is coupled to the waveguide  8  into which it transmits the RF energy. Previously, the waveguide used was a rectangular channel made of sheet metal having reflective walls to allow transmission of microwaves from a magnetron (not shown) within a generator to microwave cavity; minimum frequency which can be propagated is related to rectangular cross section; energy should be reflected off walls and travel through waveguide into load and not reflected back into magnetron, damaging it. According to the present invention, a flexible coaxial cable  12  is used instead to connect the antenna  18  to a circular cavity  8  as shown in  FIG. 3 . 
     In  FIG. 2 , a microwave power generator  2  includes a microcontroller  20  that has four principal ports: a) a low power output, b) a high power output, c) a cathode current monitor and d) a fine electromagnetic power control for transceiving on lines  23 ,  23 ,  25 ,  27 . The low and high power signals provide coarse voltage control of bucking transformer  22 . They do this because each of the low and high signals gates thyristors  24 ,  26 , respectively. Bucking transformer  22  is responsive to an input AC voltage. The “bucking” transformer  22  works by taking a secondary winding of a transformer and putting it in series but out of phase with a primary winding of that transformer  22 . What happens is that the net voltage applied to a second transformer  36  is reduced as compared with what it would be if a non-bucking transformer were used in place of bucking transformer  22 . Therefore, the output voltage of the second transformer which is applied to a magnetron anode is reduced. The thyristors  24 ,  26  control the conversion of that input AC voltage  19  provided bucking transformer  22  into another voltage provides across lines  28 ,  30  to a second transformer  36 . Because the voltage across lines  28 ,  30  is variable in response to dicated commands from the microcontroller  20 , that voltage across lines  28 ,  30  is called a variable voltage. 
     That gating of the thyristors  24 ,  26  provides coarse control over a pulsed DC voltage seen by the anode  32  of magnetron  34 . The voltage across lines  28 ,  30  is provided to a high voltage system. The high voltage system is comprised of the secondary winding of the second transformer  36 , a half-wave voltage doubler, and a ballast resistor. The purpose of the high-voltage system is to generate microwave energy by stepping up AC voltage from the secondary winding of transformer  60  to a higher voltage, thereby changing a high AC voltage to an even higher DC voltage, and then converts the DC power to RF energy. Microcontroller  8  also provides a low power range active signal on a line  21  to a thryistor  24  for gating AC voltage through bucking transformer  22 . Microcontroller  20  provides a high power active signal on a line  25  to the gate of a thyristor  26  for gating more AC line voltage through bucking transformer  22 . 
     High voltage transformer  36  at its secondary winding is monitored at one end  37  on line  38  by a cathode current monitor circuit  40 . The cathode current monitor circuit  40  is itself connected to a cathode current monitor port on line  25  of microcontroller  20 . The end  37  of bucking transformer  22  is connected to an anode of a diode  44  in the cathode monitor converter circuit  40  and a resistor  46 . The cathode of the diode  44  is connected to a resistor-capacitor circuit having an electrolytic capacitor  50  in parallel with a resistor  52 . Microcontroller  20  monitors bucking transformer  22  by line  25  to resistor  52  in the cathode current monitor circuit  40 . 
     The second transformer  36  is responsive to voltage provided to its primary winding from the bucking transformer  22 . Most microwave systems have a transformer for providing pulsed DC to a magnetron anode, and an additional filament transformer for providing an AC signal to the filament cathode of a magnetron for preheating that filament cathode. The present invention provides instead a single transformer  36  which achieves both purposes. This is done by a single transformer  36  with a single primary winding but a split secondary winding of transformer  36 —split into two sub-windings. One of these two sub-windings functions as the filament winding  39  for providing AC to the magnetron filament cathode and the other as the high voltage winding  41  (for providing pulsed DC to the magnetron anode). 
     Subwinding  41  of second transformer  36  is connected to a half-wave voltage doubler circuit  60 . The circuit of  FIG. 2  serves a continuous wave magnetron  34  designed to energized by pulsed DC to produce a high frequency microwave energy coupled to a waveguide by means of antenna  18 . The cathode of the magnetron is at high voltage relative to the anode which is at ground potential. The half wave voltage doubler includes a charging capacitor  64  and a return path rectifying diode  66 . The magnetron  34  is energized by only half cycles of voltage, the alternate cycles being ineffective due to the diode action of the rectifying diode  66 , the current passing through the rectifying diode  66  on these alternate half cycles. 
     Half-wave voltage doubler circuit  60  includes a capacitor  64  connected to both a cathode of a diode  66  and a resistor  68 . The half-wave voltage doubler circuit  60  doubles the voltage across bucking transformer  22  which is presented to it on line  68  and presents a pulsed DC signal to the anode of magnetron  34 . This pulsed DC high power signal is not passed directly to the magnetron  34  but rather through a ballast resistor  70 . A ballast resistor  70  is placed in series between the half-wave voltage doubler and the anode  32  of magnetron  34 . The addition of this resistor  70  reduces the cycle to cycle instability otherwise encountered when the magnetron  34  is operated at reduced filament cathode voltage and a high voltage standing wave ratio. A value of 1500 ohms was established for the ballast resistor  70  production purposes. 
     The ballast resistor  70  maintains a nearly constant current by compensating for fluctuations in alternating-current powerline voltage by increasing resistance when current increases. Without the ballast resistor  70  large variations would result in the power contained in the pulsed DC signal. These variations would principally show up in the average power required for heating a sample  6 . The result is that from one sample  6  to the next sample  6  a user of the present invention would notice that different lengths of time for heating would be required. Since most scientific progress depends on standardization of tasks such ask heating, the lack of the ballast resistor  70  leads to a microwave system that has no practical use to the researcher. 
     The filament winding  39  of the secondary of transformer  36  is connected to filament cathode of the magnetron  34  which applies a low voltage to the filament cathode which in turn causes it to heat up (filament voltage is usually 3 to 4 VAC). The temperature rise causes increased molecular activity within the filament cathode to the extent that it begins to “boil” off or emit electrons. Electrons leaving the surface of a heated filament wire like molecules leaving the surface of boiling water in the form of steam. The electrons, however, do not evaporate. They float just off the surface of the cathode, waiting for some momentum provided by the negative high voltage DC, which is produced by means of the high-voltage transformer and the doubler action of the diode and capacitor. A negative 4000-volt potential on the cathode puts a corresponding positive high potential on the anode. The electrons leave the vicinity of the cathode and accelerate straight toward the positive anode  32  and encounter an powerful magnetic field provided by electromagnet  72 , which is itself responsive to a fine power control signal from microcontroller  20  through a power amplifier  74 . The resulting microwave is transferred by an antenna  18  to a coaxial cable  78  for communication to the sample  6  held in cavity  8  shown in  FIGS. 1 ,  3 . While coarse power control is provided by the thyristor controller  15 , fine power control is provided by electromagnet  72 . Electromagnet  72  is available from Richardson. It has two poles—one is a permanent magnet and the other pole is an electromagnetic winding. The combination is electromagnet  72 . The magnetron  34  with electromagnet  72  can be bought from Richardson Electronics in LaFox, Ill. 
     Because such power supplies are intended for magnetron output powers in excess of 500 watts the output voltage from the supply is reduced by placing the output voltage from a low voltage transformer, bucking transformer  22 , in series with the primary of a high voltage transformer  36 . 
     The power control cycle provides for a six to eight second preheating of the magnetron with the high voltage set at the lower of the two power ranges and with no current in the electromagnet of magnetron  34 . After the pre-heat interval the high voltage and electromagnet current are set according to the output power set by the user and continued at the setting until the end of the power cycle. 
     Practical power outputs in the range of 5 to 300 watts may be set for any desired time interval. It is also possible to provide for negative feedback control of temperature and pressure at the sample container. The input current to the magnetron&#39;s beam supply is monitored by a sampling resistor  52  that is placed into the grounded end of the high voltage transformer secondary. The forward and reflected powers inside the waveguide transition that couples the magnetron  34  to the coaxial cable  2  are sampled by a coupling loop inserted into the waveguide and detected by pair of hot carrier diodes (not shown). 
     Thus the a) cathode current, b) forward power, and c) the standing wave ratio are displayed on a front panel and are available for remote monitoring through an RS232 serial port. 
       FIGS. 3A and 3B  are top and side cross-sectional views along lines  3 - 3 , respectively. They show a circular disc-shaped applicator cavity  8  and a single inductive tuning post  74  for affecting microwaves coming from direction  76  on the other side of hole  78  from the post  74 . The cavity  8  includes a loop  80  for the coupling to a microwave field emitted from the antenna of the magnetron  34 . Cavity  8  includes an opening  78  in the top cover to admit a reagent vial  6 , and a recess  84  in the bottom of the cavity  8  which acts as the seat for a bottom of the reagent vial  6 . Cavity  8  also includes a hole  86  in the bottom of the cavity  8  by which the reagent container  6  may be weighed or monitored for changes in its color or light absorption. The top cover  88  is anchored to the reagent container by the inductive tuning post  74 . The cavity is sized to accommodate about 2 milliliters of liquid in the microwave field. The diameter of the cavity  8  is related to the wavelength of the microwave field, in a known manner, within the disc-shaped cavity  8  which is walled  87 . 
     In  FIGS. 4A  &amp; B, top and cross-sectional views of cavity  8  are shown wherein the cavity includes a dielectric lens  89  to the cavity in  FIG. 3 . Dielectric  89  lens is doughnut-shaped for mounting it concentrically and surrounding with the reagent vial  6 . The reagent vial  6  can be seated within a hole of the doughnut-shaped dielectric  28 . The lens  89  may be made of any material with a low loss tangent at microwave frequencies. Commercial materials such as Teflon or Stycast have a controlled dielectric constant and do not absorb moisture from the atmosphere. The addition of this lens improves the ability of the applicator to couple power to sample volumes of 250 microliters or less. When such a lens is added to the cavity applicator the diameter of the cavity must be reduced according to the dielectric constant of the material, in a known manner. 
     The present invention allows the operator to separately position controls from the sample chamber. Sometimes this is done for convenience. Further, where reagents are radio-labeled, the sample cavity can be placed behind a lead shield, for example, while still connected by means of the cable. This protects the researcher. 
     The cavity design  8  is such that the sample vial  10  rests in a position optimized to the electric field of the cavity  8 . This substantially lowers the power required to heat milliliter-volume reagents. 
     Power may be set from 20 to 200 Watts and time from 1 to 600 seconds. Power requirements are 118 VAC at 8 Amps. The sample cavity is single mode TE. 
       FIG. 5  is a graph of return loss versus sample volume for demonstrating the ability of the applicator to couple power to a wide range of reagent volumes. The reagents used were acetonitrile and distilled water. Tests were performed at 2450 MHz with the reagents in a 5 mL Wheton vial with a conical bottom and an applicator cavity with a fixed inductive post as shown in  FIGS. 3 and 4 . 
     Although the invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that various other changes, omissions and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.