Abstract:
There is described a power splitter for directing electromagnetic power comprising: an input port for receiving the electromagnetic power; at least one dielectric element placed inside the power splitter; at least two output ports for outputting the power according to a splitting ratio, the at least two output ports placed on a surface opposite to the input port; and at least one dielectric moving device for positioning the at least one dielectric element between the at least two output ports to dynamically direct the power into the at least two output ports according to the power splitting ratio.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims benefit under 35 U.S.C. 120 and is a continuation of U.S. patent application Ser. No. 11/638,567, filed Dec. 14, 2006, the contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1) Field of the Invention 
         [0003]    The invention relates to microwave-assisted heating, and more particularly, to systems for microwave processing of a plurality of laboratory samples. 
         [0004]    2) Description of the Prior Art 
         [0005]    Most chemical reactions either require or benefit from the application of heat. Developments have provided for the use of microwave heating instead of typical Bunsen burners or “hot plates”. The use of microwave energy is known to be quite appropriate for many chemical reactions. Microwave heating represents the use of radiation energy at wavelengths residing in the electromagnetic spectrum, or between the far infrared and the radio frequency (from about one millimeter (mm) to about 30 centimeters (cm) wavelengths, or with corresponding frequencies in the range of about 1 to 300 gigahertz (GHz)). The exact upper and lower limits defining “microwave” radiations are somewhat arbitrary. 
         [0006]    Microwave radiation is widely used in several fields like spectroscopy, communication, navigation, medicine, and heating. Substances that respond quite well by increasing their temperature levels when under microwave radiation usually have a high dielectric absorption. The use of microwave heating in laboratories is known to people skilled in the art and is often referred to as “microwave assisted” chemistry. A number of laboratory microwave heating devices are thus commercially available. These microwave heating devices typically use a magnetron as the microwave source, a waveguide (usually hollow circular or rectangular metal tube of uniform cross section) to guide the microwaves, and a resonator (sometimes also referred to as the “cavity”) into which the microwaves are directed to heat a sample. The microwave source can also be a Klystron, traveling wave tubes, oscillators, and certain semiconductor devices. Most devices use magnetrons, however, as these are simple and economical. One disadvantage of magnetrons is that the control of radiation power directed towards a specific sample inserted inside a resonator is somewhat complex. One known method of controlling the radiation of the magnetron is to run it at its designated constant power while turning it on and off on a cyclical basis in order to have a certain temperature control of the sample(s) located inside separate containers or loads made of a microwave transparent material such as some types of glass, plastic or ceramic. Usually, for convenience, only one load is monitored within the group of loads each containing a sample, the remaining loads estimated to behave somewhat similarly. This leads to large amounts of uncertainty as to the evolution of reactions inside other loads, since even when a “stirring” device can produce quite uniform radiation inside the cavity of a microwave heater, several other factors, such as the presence of samples and sample containers in the microwave oven, can also change the interference pattern within the cavity and thus affect the energy distribution inside the cavity. 
         [0007]    Accordingly, when multiple samples are to be treated under one microwave source, the treatment should be uniform and controllable. Hence, there is a need to provide for the ability to vary the radiation power levels sent to each sample using a limited number of microwave sources in order to maintain low costs and high efficiency. There is also a need to be able to precisely know and control the temperature or amount of radiation power sent to each individual sample. 
       SUMMARY OF THE INVENTION 
       [0008]    There is described herein a system wherein a single microwave source is cascaded with microwave splitters and applicators such that a precise control of radiation power is offered to each sample placed within a vessel, alternatively referred to as a load. Stepper motors and feedback mechanisms are used to control each microwave splitter according to a desired end result. While the cascading provides the ability to use only one microwave source for a group of multiple loads, the control of the microwave splitters offers the ability to precisely direct a certain amount of radiation power to the subsequent level of microwave splitters, until the cascade reaches an end characterized by an applicator dedicated to an individual load. The amount of power reaching the end of the cascade is therefore precisely known and controllable. 
         [0009]    According to one aspect of the present invention, there is provided an apparatus for microwave heating comprising: a microwave source for generating electromagnetic radiation; a first microwave radiation splitter connected to the microwave source via an input port and having at least two output ports for outputting the electromagnetic radiation received at the input port; at least one dielectric element placed inside the first microwave radiation splitter between the at least two output ports and adapted to dynamically direct the electromagnetic radiation received at the input port to the at least two output ports according to a power splitting ratio; and a load connected to each of the at least two output ports for receiving the electromagnetic radiation. 
         [0010]    According to another aspect of the present invention, there is provided a method for directing electromagnetic power from an input port to at least two output ports in a power splitter, the method comprising: providing at least one dielectric element inside the power splitter; receiving the power at an input port; positioning the at least one dielectric element between the at least two output ports to dynamically direct the power thereto according to a power splitting ratio; and outputting the power to the at least two output ports in accordance with the power splitting ratio. 
         [0011]    According to yet another aspect of the present invention, there is provided a power splitter for directing electromagnetic power comprising: an input port for receiving the electromagnetic power; at least one dielectric element placed inside the power splitter; at least two output ports for outputting the power according to a splitting ratio, the at least two output ports placed on a surface opposite to the input port; and at least one dielectric moving device for positioning the at least one dielectric element between the at least two output ports to dynamically direct the power into the at least two output ports according to the power splitting ratio. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
           [0013]      FIG. 1 . shows a microwave heating device according to a first embodiment of the invention; 
           [0014]      FIG. 2   a  shows a microwave source with a primary microwave splitter and a stepper motor according to an embodiment of the present invention; 
           [0015]      FIG. 2   b  shows a top view of the cavity of the primary microwave splitter of  FIG. 2   a  in accordance with an embodiment of the invention; 
           [0016]      FIG. 3   a  shows a secondary microwave splitter and stepper motor, according to an embodiment of the present invention; 
           [0017]      FIG. 3   b  shows a top view of the cavity of the secondary microwave splitter of  FIG. 3   a  in accordance with an embodiment of the invention; 
           [0018]      FIG. 4   a  is a schematic illustrating a two-level cascade system in accordance with an embodiment of the invention; 
           [0019]      FIG. 4   b  is a schematic illustrating a one-level cascade system in accordance with an embodiment of the invention; and 
           [0020]      FIG. 5  is a schematic illustrating the position of elements within the cavity of a microwave splitter in accordance with an embodiment of the invention. 
       
    
    
       [0021]    It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Referring to  FIG. 1 , and according to an embodiment of the present invention, a rack  102  containing twelve vessels  101  (herein referred to as loads containing sample mixtures for example) is inserted inside the microwave-assisted processing system made of a metal tunnel-shaped cavity through microwave-safe doors  108  and  109 . Twelve applicators  100  are used to direct the heat to the loads  101  individually. The applicators are to be understood as being energy directing devices that transmit the energy to the loads, like antennas. The applicators are optional to the system but are used in the embodiment described herein. Multiple applicators can be connected together to redirect the energy in a desired direction to a desired destination. In the embodiment shown in the figure, six applicators  100  are located on each side of the microwave-assisted processing system such that each is placed at the corresponding position of a load  101  once the rack  102  is placed inside the system. The loads  101  can be in vessels made of various microwave transparent materials depending on the sample type and mixture. Examples of possible materials include but are not limited to some types of glass, plastic, ceramic, or more specifically, quartz and Perfluoroalkoxy (PFA). The position of the applicators along with the inserted loads  101  is determined during fabrication using a network analyzer for example. Once the rack  102  containing the loads  101  is inserted inside the cavity, the loads  101  are automatically in their correct positions with respect to the applicators  100 . Each applicator  100  receives radiation energy according to a splitting ratio of a variable microwave radiation splitter  103 . A coaxial cable  106  connected to one of the two output ports  300  of the variable microwave radiation splitter  103  (also referred to as a secondary microwave radiation splitter) is used to transmit the radiation energy from the output port of the splitter  103  to the applicator  100 , as determined by the control of a stepper motor  104  located on each variable microwave radiation splitter  103 . 
         [0023]    According to the illustrated embodiment of  FIG. 1 , since there are six loads to be heated on each side of the system, each pair of loads being controlled by a single variable microwave radiation splitter  103  with its stepper motor  104 , there are thus six variable microwave radiation splitters  103  and stepper motors  104 . In a preferred embodiment, only one exhaust fan is installed on the cavity (not shown) in order to release unwanted fumes in case a vessel breaks inside the cavity, but more than one may be present. Other safety features can also be added to prevent vessel rupture and operator harm. Each variable microwave radiation splitter  103  receives radiation energy from one of the two outputs of another variable microwave radiation splitter  201 , itself controlled by another stepper motor  104 . The variable microwave radiation splitter  201  is for splitting the power received from a source of microwave radiation  200 , herein shown as a magnetron. 
         [0024]    More particularly, and referring to  FIG. 2   a , the source of microwave radiation  200 , is mounted on a variable microwave radiation splitter  201 . The variable microwave radiation splitter  201  is also dynamically controlled by a stepper motor  104  with a feedback signal coming from temperature monitoring of samples  101 . For example, temperature feedback can be implemented using any temperature sensor, such as IR sensors, located underneath each load. The variable microwave radiation splitter  201  is also referred to as a primary microwave splitter. Referring to  FIG. 2   b , variable microwave radiation splitter  201  performs a first division of the radiation energy of the microwave source  200  received at an input port  205  in accordance with a first splitting ratio. Input port  205  is located on one side of the rectangular waveguide forming the variable microwave radiation splitter  201 . The radiation energy is then outputted into two output ports  300  located on a second opposite side. The control of the splitting ratio is provided by the stepper motor  104  (shown in  FIG. 2   a ), which moves, or rotates, a dielectric element  105  placed inside the rectangular waveguide cavity forming the variable microwave radiation splitter  201 , and via the hole or shaft  206 . More particularly, the dielectric element  105  is placed and moved between the two output ports  300 , and as shown later in  FIG. 5 . 
         [0025]      FIG. 3   a  shows the variable microwave radiation splitter  103 , dynamically controlled by the stepper motor  104 . The variable microwave radiation splitter  103  is referred to as a secondary microwave splitter as it performs a second division of the radiation energy from the microwave source in accordance with a second splitting ratio. Radiation energy already split by a first variable microwave radiation splitter (element  201  in  FIG. 2   a ) is received at an input port  205  ( FIG. 3   b ) located on a first side of the rectangular waveguide forming the variable microwave radiation splitter  103 . This power is then split once again according to the second splitting ratio and is directed into two output ports  300  located on a second side opposite to the first side where the input port is located. The control of this second splitting ratio is provided by the associated stepper motor  104 , which moves or rotates a dielectric element  105  placed inside the rectangular waveguide cavity forming the variable microwave radiation splitter  103  in the same manner as described above, and via the rotation hole or shaft  206  ( FIG. 3   b ). 
         [0026]      FIG. 4   a  illustrates both primary  201  and secondary  103  microwave radiation splitters as they are assembled inside the system according to one embodiment. For each pair of secondary microwave radiation splitters  103 , one magnetron  200  connected to a primary splitter  201  communicates radiation energy to each individual secondary splitter  103  via a coaxial connector  106  connected to its two output ports  300  according to a first splitting ratio. This first splitting ratio is controlled by the stepper motor  104  and a feedback mechanism coming from the monitoring of four loads (A, B, C and D for example) in order to treat each pair of loads  101  (A-B, and C-D) as desired. Each secondary splitter  103  communicates part of the received radiation energy to each dedicated applicator  100  and according to a second splitting ratio. This second splitting ratio is controlled by the stepper motor  104  and a feedback mechanism coming from the monitoring of each individual load in order to treat each load  101  within each pair of loads as desired. Insertion sleeves  402  are also used to connect each input and output port to the coaxial cables  106 . 
         [0027]    A one-level cascade system consists of two loads  101 , one variable microwave radiation splitter  201  and one source of radiation energy  200 , as illustrated in  FIG. 4   b . A two-level cascade system, as in  FIG. 4   a , consists of four loads  101 , two secondary variable microwave radiation splitters  103 , one primary variable microwave radiation splitter  201 , and one source of radiation energy  200 . The system can also be made of a three-level cascade arrangement or more. 
         [0028]    In a two-level cascade arrangement, the difference in temperature between the pair of loads A and B is used to control the splitting ratio of the secondary splitter  103 . Similarly, the difference in temperature between the pair of loads C and D is used to control the splitting ratio of the secondary splitter  103 . Once the temperatures of the two pairs of loads are as desired and within a given tolerance level, the second splitting ratio of the secondary splitter  201  is dynamically controlled in such a way to achieve a balanced temperature for each of the two pairs of loads; i.e. A and B is one set of temperatures to be compared to C and D for the other set of temperatures. The same principle applies for other groups of four loads; E, F, G and H. Software may be programmed to perform the above-described procedure, as is understood by a person skilled in the art. 
         [0029]    Referring to  FIG. 5 , the dielectric element  105  placed inside variable microwave radiation splitters ( 201  and  103 ) can be designed in the shape as illustrated in the drawings or in any other shape to provide for high splitting efficiency. The dielectric element  105  can be made of an aggregate of several different materials with a high permittivity, such as Teflon or alumina. For example, a material made of 99.9% alumina is found to be very effective. When the dielectric element ( 105 ) is rotated between the two output ports  300  by the stepper motor  104  up to an angle of 170 degrees, the arrangement provides for up to 5 dB of control in the difference between the radiation power sent to each of the two output ports  300 . When the dielectric element  105  is in its original position, i.e. not rotated or in what is referred to as the zero degree position, the dielectric element  105  provides up to a 3 dB difference between the radiation power sent and the two output ports  300 . While the positioning of the dielectric element  105  inside the cavity forming the variable microwave radiation splitters ( 201  or  103 ) may be varied to change the power splitting ratio, the placement of the input port  205  and output ports  300  will further determine the power splitting efficiency. 
         [0030]      FIG. 5  illustrates how all the elements present in the cavity of the microwave splitter are positioned with respect to each other according to an embodiment that provides for a relatively high power splitting efficiency. Various other designs are however possible. For example, the cavity of the microwave splitter ( 103  or  201 ) can either be rectangular, square-like or even cylindrical. In one embodiment, the cavity shape can take, for example, a rectangular size 72.14 millimeters (mm) by 34.01 mm, such that it is functional in the S-band of frequencies. Good adaptation and contrasts were also achieved with a length of 72 mm and 75 mm, which may be varied and further depends on the placement of the ports ( 205 ,  300 ) and the dielectric element  105  as well as the shape of the cavity. Hence, the placement of the input  205  and output ports  300  as well as the dielectric element  105  are determinant and can be varied depending on the various specifications needed for the microwave splitter design. For example, still in the S-band of frequencies, good adaptation can be achieved by placing the input port 26 mm from one end of the cavity and 36 mm from a side of the cavity at a height of 24 mm 
         [0031]    Moreover, in  FIG. 5 , the dielectric element  105  is rectangular in shape (for example, 5 mm by 10 mm by 32 mm) and placed such that its height extends from a first side of the cavity having an input port  205  to a second side opposite to the first side of the cavity and having the output ports  300 . The placement and shape of the dielectric element  105  can be changed. For example, it was found that when the displacement of the dielectric  105  is performed closer to the output ports  300 , the contrast between the output powers is better. Also, displacement performed behind the output ports  300  results in a better adaptation. A circular movement or a rotation of the dielectric element  105  around an axis  501  parallel to its height provides for a combination of both higher contrasts and better adaptation. The circular movement can be achieved though the use of an arm  502  connecting the tip of the dielectric element  105  with a directing device or a motor through a hole or shaft  206  following the axis of rotation  501 . The hole or shaft  206  does not cause any further coupling effects if the hole is maintained small enough in diameter; for example 1.5 mm. 
         [0032]    Both primary and secondary variable microwave radiation splitters ( 201  and  103 ) disclosed herein are not limited to controlling heat directed to each load placed within the system. Any embodiment wherein the splitter is used to control a source of radiation energy towards two or more outputs falls within the scope of this invention. More precisely, the variable microwave radiation splitters ( 201  and  103 ) disclosed herein are used to control how radiation energy or power is directed between two or more output ports  300 . The system and variable microwave radiation splitters ( 201  and  103 ) can also function at other frequencies, and is not restricted to using sources that emit at the typical microwave frequency of 2.54 GHz. The microwave radiation source  200  can be any appropriate source, including magnetrons, klystrons, traveling wave tubes, various electronic oscillators and solid states sources including various transistors and diodes. It should also be understood that the displacement of the dielectric may be translational and/or rotational. The shape of the dielectric and the microwave power splitter have been described for optimum performance but may vary depending on the system&#39;s requirements. 
         [0033]    An embodiment for the power splitter having more than two ports to output the radiation power is, for example, three ports with a single dielectric element positioned in front of a central port, the dielectric element being rotated from a first port to a second port to the third port to split the radiation power three ways according to different proportions. The dielectric element may also be moved in a translational motion instead of a rotational motion, thereby enabling a design with more than two ports and a single dielectric element that can be slid across a surface to correctly divide the radiation power amongst the multiple ports. Another embodiment is to have four ports and two dielectric elements, one dielectric element for each set of two ports. A first set of two ports is positioned at one end of the power splitter with one dielectric therebetween, while a second set of two ports is positioned at another end of the power splitter with the second dielectric therebetween. The person skilled in the art will understand that while the embodiments illustrated in the present figures show two ports and a single dielectric element, many variants exist on this design without deviating from the spirit of the present invention. 
         [0034]    The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.