Abstract:
The objective of the present invention is to reduce the loss in functionality due to the burn-out of micro-coils in a heating device, which uses micro-coils formed of carbon or molecules that include carbon as a principal component for heating a target object located in a space where high-temperature gases flow. The present invention relates to a heating device comprising: a heat producing layer having a micro-coil formed of carbon or molecules that include carbon as a principal component, where the heat producing layer is installed together with the target object located in a target space where high-temperature gases flow; and an electromagnetic (EM)-wave-emitting device that emits EM radiation into the target space. The target object is heated by producing heat in the micro-coil by EM radiation from the EM wave emitting device emitted to the target space. The heating device further comprises a covering layer, which coats the entirety of the heat producing layer.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a heating device that heats a target object by producing heat in a material that absorbs electromagnetic (EM) radiation. 
       BACKGROUND 
       [0002]    Technologies for heating a target object by producing heat in an EM wave absorber using EM radiation are known. 
         [0003]    For example, JP 2009-036199A1 discloses a technology that heats a filter for capturing particulate matter in exhaust gases by producing heat in a material by absorbing microwave radiation. The microwave absorbing material is attached to the particulate filter. A carbon micro-coil formed of coil-shaped carbon fiber is described as an example of a material that absorbs microwave radiation. 
       PRIOR ART DOCUMENT 
     Patent Document 
       [0004]    Patent Document 1: JP 2009-036199A1 
       SUMMARY OF INVENTION 
     Problems to be Solved 
       [0005]    A micro-coil is installed together with the target object to form a heating device that uses the micro-coil, which is formed of carbon or molecules that include carbon as a principal component, to heat a target object located in a space where a high-temperature gas flows. Therefore, when an object, such as a particulate filter exposed to the high-temperature gas becomes heated, the micro-coil is also exposed to the high-temperature gas. The micro-coil, which is formed of carbon or molecules that include carbon as a principal component, may ignite because of the high temperatures of the exhaust gases. This may cause the micro-coil to burn out, so that the target object may no longer be heated. 
         [0006]    The present invention is in view of this respect. The objective of the present invention is to reduce the degradation resulting from burn-out of the micro-coil in a heating device, which uses a micro-coil formed of carbon or molecules that include carbon as a principle component, for heating a target object located in a space where high-temperature gases flow. 
       Means for Solving the Problem 
       [0007]    The first invention comprises a heat producing layer having a micro-coil formed of carbon or molecules that include carbon as a principle component, where the heat producing layer is installed together with the target object located in a space where high-temperature gases flow, and an EM-wave-emitting device that emits EM radiation in the target space. The target object is heated by producing heat in the micro-coil by EM radiation emitted from the EM-wave-emitting device to the target space. The heating device further comprises a covering layer, which covers the entire region of the heat producing layer. 
         [0008]    In the first invention, significant quantities of oxygen are prevented from reaching the heat producing layer, since it is equipped with a covering layer, which coats the entirety of the heat-generating material. Furthermore, the increase in the temperature of the heat producing layer is controlled because the heat producing layer is not directly exposed to the high-temperature gases. 
         [0009]    The second invention relates to the first invention, wherein the target object is a catalyst that purifies the high-temperature gases, and wherein a catalyst is installed in the covering layer. 
         [0010]    The third invention relates to the second invention wherein the micro-coil is not in contact with the catalyst on the boundary surface of the covering layer and the heat producing layer. 
         [0011]    In the third invention, the micro-coil is not in contact with the catalyst. Therefore, oxidization of the micro-coil by the catalyst is inhibited. 
         [0012]    The fourth invention relates to one of the first to third inventions, wherein the emission of the EM radiation by the EM-wave-emitting device is controlled such that the temperature of the micro-coil does not reach the ignition temperature of the micro-coil. 
         [0013]    In the fourth invention, the temperature of the micro-coil is prevented from reaching the ignition temperature. 
         [0014]    The fifth invention comprises a heat producing layer having a micro-coil formed of carbon or molecules that include carbon as a principle component, where the heat producing layer is installed together with the target object located in a target space where the high-temperature gases flow, and an EM-wave-emitting device that emits EM radiation to the target space. The target object is heated by producing heat in the micro-coil by EM radiation from the EM wave emitting device emitted to the target space. The micro-coil is made of silicon carbide as a principle component. 
         [0015]    In the fifth invention, the micro-coil, made of silicon carbide as a principle component which is heat-resistant and chemically stable, is used to heat the target object. This micro-coil does not burn out easily, even if it is installed together with the target object located in a space where high-temperature gases flow. 
       Advantage of the Present Invention 
       [0016]    In the present invention, significant quantities of oxygen are prevented from reaching the heat producing layer because the covering layer coats the entirety of the heat producing layer. Furthermore, the increase in temperature in the heat producing layer is controlled because the heat producing layer is not exposed directly to the high-temperature gases. Thus, the micro-coil in the heat producing layer is not in contact with oxygen and is prevented from reaching the ignition temperature. Therefore, the micro-coil in the heat producing layer is prevented from burning out, and degradation of the heating device can therefore be reduced. 
         [0017]    In the third invention, the micro-coil is not in contact with the catalyst, and oxidization of the micro-coil by the catalyst is prevented. As a result, micro-coil is not damaged by the contact with the catalyst, and the longevity of the heating device can thereby be extended. 
         [0018]    In the fourth invention, emission of the EM radiation is controlled such that the temperature of the micro-coil does not reach the ignition temperature of the micro-coil. Therefore, the micro-coil in the heat producing layer is prevented from burning out, not only when the target object is exposed to the high-temperature gases, but also when the heat is produced in the micro-coil by the EM radiation. This reduces the degradation of the heating device due to the burnout of the micro-coil. 
         [0019]    In the fifth invention, the micro-coil is formed primarily of silicon carbide, which is heat resistant and chemically stable, and is used for heating the target object. Therefore, the micro-coil is prevented from burning out, even if the micro-coil is installed together with the target object located in a space where the high-temperature gases flow, and degradation of the heating device is thereby inhibited. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  illustrates an outline of the structure of an exhaust-gas-purifying device according to one embodiment. 
           [0021]      FIG. 2  shows a cross sectional horizontal view of the catalyst carrier according to one embodiment. 
           [0022]      FIG. 3  shows a cross sectional vertical view of the catalyst carrier according to one embodiment. 
           [0023]      FIG. 4  shows a cross sectional vertical view of portion of the catalyst carrier according to a second modification. 
           [0024]      FIG. 5  shows a cross sectional vertical view of portion of the catalyst carrier according to a third modification. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    The embodiments of the present invention are detailed with reference to the accompanying drawings. The embodiments below are the preferred embodiments of the invention, but are not intended to limit the scope of present invention and application or usage thereof. 
         [0026]    Heating device  10  of this embodiment is a device that heats catalyst  32  (the target object) of exhaust-gas-purifier  30  that purifies the exhaust gas emitted from the engine of an automobile. 
         [0027]    In this embodiment, catalyst  32  is an active metal such as platinum, palladium, or rhodium, which are the common principal components of a three-way catalyst system. The three-way catalyst system purifies hydrocarbons (HCs), carbon monoxide (CO), and nitrogen oxides (NO x ) contained in the exhaust gases of an automobile that uses gasoline as a fuel. The three-way catalyst oxidizes HCs to form water and carbon dioxide (CO 2 ), oxidizes CO to form CO 2 , and reduces NO x  to form nitrogen (N 2 ). 
         [0028]    The three-way catalyst system is not effective at reducing NO x  at low temperatures, and so is ineffective in this respect immediately following the startup of the engine in cold conditions. To function appropriately following cold-starting of the engine, the three-way catalyst system should be heated so that catalyst  32  can activate. In this embodiment, heating device  10  heats catalyst  32  to activate the catalyst. 
         [0029]    As shown in  FIG. 1 , exhaust-gas purifier  30  has catalyst carrier  11  provided with catalyst  32 ; casing  31 , which accommodates catalyst carrier  11 ; and heating device  10 , which heats catalyst  32 . 
         [0030]    As shown in  FIGS. 2 and 3 , the outer diameter of catalyst carrier  11  is almost identical to the inner diameter of casing  31 . Catalyst carrier  11  is fixed inside casing  31  using a fixing material (not shown in the figure). Catalyst carrier  11  has outer cylinder  12 , honeycomb structure  13 , and catalyst-supporting layer  35 . 
         [0031]    Outer cylinder  12  is cylindrical and formed of an insulating material, which allow transmission of the microwave radiation. In this embodiment, outer cylinder  12  is formed of a ceramic material. Outer cylinder  12  accommodates honeycomb structure  13  which is described next. 
         [0032]    Honeycomb structure  13  is cylindrical in shape and formed of an insulating material that allows transmission of the microwave radiation. In this embodiment, honeycomb structure  13  is formed of a ceramic material. Honeycomb structure  13  has cylinder  13   a  and lattice  13   b , which is formed of a sectional lattice shape molded together with cylinder  13   a . Honeycomb structure  13  is structured such that the exhaust gases can flow through the spaces between lattices  13   b , as shown by the arrows in  FIG. 3 . 
         [0033]    Catalyst-supporting layers  35  are laminated on cylinder  13   a  and lattice  13   b  of honeycomb structure  13 , respectively, as shown in  FIGS. 2 and 3 . Catalysts  32  are supported on catalyst-supporting layer  35 . A portion of catalyst-supporting layer  35 , except for catalyst  32 , consists a portion of heating device  10 . Catalyst  32  is supported specifically on covering layer  15 , supporting layers  35 , which will be described later. 
         [0034]    Casing  31  is cylindrical and formed of metal, and accommodates catalyst carrier  11 . Casing  31  forms a portion of an exhaust tube of an automobile engine, and exhaust gases flow through in the direction shown by the arrows in  FIG. 1 . The inner part of casing  31  consists exhaust-gas passage  33  (i.e., the target space), through which the exhaust gases flow. 
         [0035]    Opening  34  for inserting antenna  17  (described later) is formed near the lower center side of casing  31 . Microwave radiation is emitted from antenna  17  in exhaust-gas passage  33  inside casing  31 . 
         [0036]    Heating device  10  is a device for heating catalyst  32  (the target object), supported on catalyst-supporting layer  35  of exhaust-gas purifier  30  described above. Heating device  10  has heat producing layer  14 , cover layer  15 , and EM-wave-emitting device  40 . 
         [0037]    Heat producing layer  14  consists of a layer at the lower side of catalyst-supporting layer  35 . Heat producing layer  14  is accommodated inside cylinder  13   a  and lattice  13   b  of honeycomb structure  13 , as shown in  FIGS. 2 and 3 . In heat producing layer  14 , a large quantity of micro-coils  21  are mixed with ceramic binder  14   a , as shown in  FIG. 4 . Heat producing layer  14  is formed by applying a slurry solution, in which ceramic binder  14   a  and micro-coil  21  are mixed, to the surface of honeycomb structure  13 , and is then baked together with honeycomb structure  13 . 
         [0038]    Micro-coil  21  is a carbon micro-coil (CMC), formed of carbon as the principle component. The CMC is a micro-structured carbon fiber formed of a coil that is rolled with a pitch of approximately 0.01-1 μm. A micro-coil made of silicon carbide as the principal component may be used as heat producing layer  14 . 
         [0039]    The CMCs produce heat by absorbing EM radiation. Using this characteristic, heat can be produced from micro-coil  21  by allowing microwave radiation from EM-wave-generating device  16  to be absorbed. Heat producing layer  14  and cover layer  15  installed on heat producing layer  14  (described later) are thereby heated. As a result, catalyst  32  supporting covering layer  15  is also heated. 
         [0040]    Covering layer  15  consist a layer at the upper side of catalyst-supporting layer  35 . Covering layer  15  is a non-ventilated heat-resistant layer formed of a ceramic binder material. Covering layer  15  covers the entirety of heat producing layer  14  to prevent oxygen from reaching heat producing layer  14 . This also prevents the temperature of heat producing layer  14  from increasing when high-temperature gases flow through the spaces in honeycomb structure  13 . 
         [0041]    Covering layer  15  is formed by applying a ceramic binder material that supports catalyst  32  and heat producing layer  14 , followed by a baking process. Thus, catalyst  32 , which is the target object, is installed on the surface of covering layer  15 . 
         [0042]    The CMC may ignite spontaneously in air at temperatures in the range of 500-600° C. or more. The temperature of the exhaust gases from an automobile engine may reach temperatures in excess of 700-800° C. during full load. The temperature may reach 1000 degrees Celsius or more in an uphill or during acceleration. 
         [0043]    Therefore, when the CMC is exposed directly to the exhaust gas that is more than the spontaneous ignition temperature, the CMC may ignite when it exceeds the ignition temperature. Thus, covering layer  15  coats the entirety of heat producing layer  14  where micro-coil  21  is installed in order to prevent oxygen from reaching heat producing layer  14 , and to avoid heat producing layer  14  from being high temperatures. 
         [0044]    EM-wave-emitting device  40  is a device that emits microwave radiation, which will be absorbed by micro-coil  21  of heat producing layer  14  in order to heat catalyst  32 . EM-wave-emitting device  40  includes EM-radiation-generating device  16 , antenna  17 , power-supply unit  18 , and controller  19 . 
         [0045]    EM-wave-generating device  16  generates microwave power using a semiconductor oscillator (not shown in the figure). EM-wave-generating device  16  generates microwaves when electrical power is supplied from power-supply unit  18  via power supply line  18   a . The microwave power is transmitted to antenna  17  via microwave transmission line  16   a.    
         [0046]    Antenna  17  is for emitting microwave radiation using the signal from EM-wave-generation device  16  to exhaust-gas passage  33  inside of casing  31 . Antenna  17  is inserted into opening  34  of casing  31 , and antenna  17  is exposed to exhaust gases in passage  33  in casing  31 . 
         [0047]    Controller  19  is composed of an electronic control device that includes a central processing unit (CPU), memory, and an input/output (I/O) device. Controller  19  controls heating device  10 , and is described below. 
       Operation of the Heating Device 
       [0048]    The operation of heating device  10 , including the operation of controller  19 , is described below. 
         [0049]    Controller  19  outputs an EM-wave-driving signal to power-supply unit  18  immediately following start-up of the automobile engine. Power-supply unit  18  supplies power to EM-wave-generating device  16  when the EM-wave-driving signal is received. The EM radiation generated by EM-wave-generating device  16  is then transmitted into exhaust-gas passage  33  inside casing  31  to antenna  17 . 
         [0050]    Micro-coil  21  of heat producing layer  14  produces heat and reaches an elevated temperature when micro-coil  21  absorbs the microwave radiation that is emitted into exhaust-gas passage  33  from antenna  17 . 
         [0051]    Heat producing layer  14  and coating layer  15  are heated rapidly by micro-coil  21  which is in high temperature. Catalyst  32  that is supported by coating layer  15  is thereby heated. As a result, catalyst  32  reaches the activation temperature quickly. 
         [0052]    In this embodiment, heating device  10  is provided such that catalyst  32  is heated to 300-400° C. for activation. HCs, CO, and NO x  contained in the exhaust gases are resolved at the surface of catalyst  32 , which has reached the activation temperature. The purified exhaust gases are emitted to atmosphere through the exhaust passage (not shown in the figure), which is located at the downstream side. 
       Advantages of this Embodiment 
       [0053]    In this embodiment, very little oxygen reaches heat producing layer  14  because the entirety of heat producing layer  14  is coated by covering layer  15 . Furthermore, since heat producing layer  14  is not exposed directly to the high-temperature exhaust gases, the temperature of heat producing layer  14  is controlled when the high-temperature exhaust gases flow through the structure. Micro-coil  21  of heat producing layer  14  does not make contact with significant quantities of oxygen and so is prevented from igniting. This prevents micro-coils  21  of heat producing layer  14  from burning out, and thereby reduces degradation of heating device  10 . 
         [0054]    Since catalyst  32  and micro-coils  21  are not in contact, oxidization of micro-coil  21  by catalyst  32  is prevented. Thus, the longevity of heating device  10  can be extended because micro-coil  21  is not damaged by contact with catalyst  32 . 
       Modification 1 
       [0055]    In the first modification, micro-coil  21  of heat producing layer  14  is formed primarily of silicon carbide, which is heat-resistant and chemically stable. This reduces the degradation of heating device  10  because micro-coil  21  is prevented from burning out. 
       Modification 2 
       [0056]    In the second modification, an insulation layer formed of insulator  25  is installed between catalyst  32  and micro-coil  21  to prevent chemical reactions between catalyst  32  and micro-coil  21 . As shown in  FIG. 5 , insulator  25  is deposited on heat producing layer  14 . Catalyst  32  and micro-coil  21  are thereby preventing from being in contact. 
         [0057]    In this modification, oxidation of micro-coil  21  by catalyst  32  is prevented because catalyst  32  and micro-coil  21  are not in contact due to the presence of insulator  25 . Therefore, the micro-coil is not damaged due to contact with catalyst  32 , and the longevity of heating device  10  may be extended. 
       Modification 3 
       [0058]    Both micro-coil  21  and catalyst  32  may be installed on heat producing layer  14 , as shown in  FIG. 6 , and covering layer  15  may be omitted when micro-coil  21  formed primarily of silicon carbide is used instead of a carbon micro-coil. 
         [0059]    Micro-coil  21 , formed of silicon carbide as the principle component, which is heat-resistant and chemically stable, is employed to heat catalyst  32 . Micro-coil  21  is thereby prevented from burning out, even when it is installed together with catalyst  32 , which is located in the space where the high-temperature exhaust gases flow. Degradation of heating device  10  is thereby inhibited. Micro-coil  21  is installed nearer to catalyst  32  compared with the main embodiment. Therefore, the temperature of catalyst  32  may be increased more rapidly. 
       Modification 4 
       [0060]    In the fourth modification, the microwave radiation is controlled by EM-wave-emitting device  40  such that the temperature of micro-coil  21  does not reach the ignition temperature. Control device  19  determines a suitable time to terminate the EM radiation, which is based on the time required for micro-coil  21  to reach a predefined temperature, which is lower than the ignition temperature when EM-wave emitting device  40  continues to emit microwave radiation. The upper limit temperature may be equal to or less than 50° C. 
         [0061]    When sufficient time has elapsed since the start of the emission of microwave radiation, which occurs immediately following the start-up of the engine, control unit  19  outputs an instruction to power-supply unit  18  to terminate the power supply to EM-wave-generating device  16 . Power-supply unit  18  then terminates the power supply to EM-wave-generating device  16 , and the emission of microwave radiation is terminated by EM-wave emitting device  40 . 
         [0062]    In the fourth modification, the emission of microwave radiation is controlled such that the temperature of micro-coil  21  does not reach the ignition temperature. Thus, micro-coil  21  can be prevented from burning out, not only during the period whereby catalyst  32  is exposed to the high-temperature exhaust gases, but also during the period while micro-coil  21  is heated by the microwave radiation. 
         [0063]    Control device  19  may output the termination instruction to power-supply unit  18  based on the reading from a temperature sensor used to determine the temperature of heat producing layer  14 . 
       Other Embodiments 
       [0064]    The following embodiments can be contemplated. 
         [0065]    Heating device  10  is not limited to heating catalyst  32  that purifies the exhaust gases of an automobile. The heating device may be applied to other types of catalyst installed in such a space where gases flow. For example, the heating device may be applied to heat a catalyst that purifies the exhaust gases of a combustion furnace or a burner reactor. 
         [0066]    The shape of casing  31  or the location of opening  34  for receiving the microwave radiation are not limited to those described in the above embodiments. For example, casing  31  may be formed of a non-cylindrical shape. Opening  34  may be located at places other than as described above, such as above casing  31 . 
         [0067]    Catalyst  32  is not limited to those used for three-way catalysts. For example, it may also be applied to selective catalytic reduction (SCR), which requires an elevated temperature for activation. 
         [0068]    Binder  14   a  of heat producing layer  14  does not have to be a ceramic binder, so long as it is heat resistant and can fix micro-coils  21  to honeycomb structure  13 . 
         [0069]    The binder for covering layer  15  does not have to be a ceramic binder. Other binders can be applied, as long as catalyst  32  can be fixed to heat producing layer  14  and can cover the entirety of heat producing layer  14 . 
         [0070]    A magnetron may be used as EM-wave-generating device  16  instead of a semiconductor oscillator, for generating microwave radiation. 
         [0071]    Controller  19  may be used to control the microwave radiation to micro-coil  21  by controlling power-supply unit  18  prior to the startup of the automobile at a low-temperature environment. This allows the exhaust gases to be purified earlier. 
         [0072]    The target objects may be other than catalyst  32 , such as a sensor installed in the exhaust passage. 
       INDUSTRIAL APPLICABILITY 
       [0073]    As discussed above, the present invention is useful for heating a device that heats a target object producing heat using a micro-coil by EM radiation, where a high-temperature gas flows in the target space. 
       EXPLANATION OF REFERENCE NUMERALS 
       [0000]    
       
           10  Heating device 
           14  Heat producing layer 
           15  Covering layer 
           21  Micro-coil 
           25  Insulator 
           32  Catalyst (target object) 
           33  Exhaust-gas passage (target space) 
           40  EM-wave-emitting device