Patent Publication Number: US-2022227672-A1

Title: Ceramic body and method for producing same, heater element, heater unit, heater system as well as purification system

Description:
FIELD OF THE INVENTION 
     The present invention relates to a ceramic body and a method for producing the same, a heater element, a heater unit, a heater system, as well as a purification system. 
     BACKGROUND OF THE INVENTION 
     Conventionally, a ceramic body obtained by adding various additive elements to a composition represented by BaTiO 3  is proposed as a material exhibiting a PTC (Positive Temperature Coefficient) property. The PTC property is a property in which a resistant value is rapidly increased at elevated temperature that is higher than or equal to a Curie point. The ceramic body having the PTC property have been used for PTC heaters, PTC switches, overcurrent protection devices, temperature detectors, and the like, and various properties have been improved depending on the applications. 
     Patent Literature 1 discloses that the use of Y, Ho, Er, and Yb in BaTiO 3  and the multiple addition of La, Dy, Eu, and Gd with Y, Ho, Er, and Yb in appropriate amounts provide a porcelain composition for PTC thermistors, which has a small change over time, and practical values for both resistivity at room temperature and resistant temperature coefficient. 
     Patent Literature 2 discloses semiconductor ceramics mainly based on a Ba m TiO 3 -based composition having a perovskite-type structure represented by the general formula: A m BO 3 , wherein W as a semiconducting agent is substituted in a range of 0.05 mol % to 0.3 mol % of 100 mol % of Ti; the symbol m, which is a ratio of A sites occupied mainly by Ba to B sites occupied mainly by Ti, satisfies 0.99 m 1.002; and the semiconductor ceramics contains Ca in a range of 15 mol % or less when the total number of moles of elements constituting the A sites is 100 mol %: and when a temperature at which a resistance value becomes twice a resistance value at 25 ° C. is defined as a doubling point, the doubling point is 100 ° C. or more, and a measured sintered density is 70% or more and 90% or less of theoretical sintered density. It also mentions that the semiconductor ceramics has a stable PTC property, a higher doubling point, and a wider operating temperature range. 
     Patent Literature 3 discloses that a barium titanate PTC thermistor which can easily form a semiconductor in either air or nitrogen atmosphere sintering, and which has lower specific resistivity at normal temperature and a Curie point shifted to a higher temperature side than 120° C., can be obtained by substituting a part of Ba with Bi and alkali metal A (Na or K) in a predetermined range, rather than Pb which has a higher environmental impact, and by maintaining a molar ratio of Ba sites to Ti sites and an amount of Ca added in predetermined ranges. It also discloses that a change over time can be reduced even if the PTC thermistor can be used as a heater element. 
     Patent Literature 4 disclosed that a stacked PTC thermistor having decreased specific resistance at normal temperature and increased voltage resistance can be obtained by using highly crystalline tetragonal barium titanate powder as a raw material in place of cubic barium titanate powder, and by controlling a slope of a change in resistance within grains to an inverse number of a Kelvin temperature to be 135 or more and 340 or more in 123 Kelvin to 163 Kelvin, to have an average porcelain grain diameter of 0.8 μm or less after sintering. 
     Patent Literature 5 discloses a stacked PTC thermistor device that uses a ceramic substrate, wherein the ceramic substrate is mainly based on barium titanate doped with a rare earth element(s), and has an average porcelain grain size of 0.3 [μm] or more and less than 0.5 [μm], and wherein a lower limit of a relative density of the ceramic substrate is 70 [%], and an upper limit of the relative density of the ceramic substrate is −6.43 d+97.83 [%] in which d is the porcelain grain size. It also discloses that the stacked PTC thermistor device can achieve both the low specific resistance at normal temperature and high voltage resistance. 
     Patent Literature 6 discloses a non-lead semiconductor ceramic that is substantially lead-free, wherein the ceramic is mainly based on a BaTiO 3 -based composition having a perovskite-type structure represented by the general formula: A m BO 3 , and a part of Ba making up A sites with an alkali metal element(s), Bi, Ca, Sr, and a rare earth element(s), and wherein Ca and Sr contents when the total number of moles of the elements making up the A sites is one mole, satisfy 0.05≤x≤0.20, 0.02≤y≤0.12, and 2x+5y≤0.7, in which x is a molar ratio of Ca and y is a molar ratio of Sr. It also discloses that the semiconductor ceramic has excellent reliability because surface discoloration does not occur even if a current is conducted to the ceramic for a long period of time, and a change in resistance value is suppressed while maintaining a desired Curie point. 
     CITATION LIST 
     Patent Literatures 
     [Patent Literature 1] Japanese Patent Application Publication No. 2013-79160 A 
     [Patent Literature 2] Japanese Patent No. 5510455 B 
     [Patent Literature 3] Japanese Patent No. 5930118B 
     [Patent Literature 4] Japanese Patent Application Publication No. 2017-27980 A 
     [Patent Literature 5] Japanese Patent No. 5970717 B 
     [Patent Literature 6] Japanese Patent No. 5327553 B 
     SUMMARY OF THE INVENTION 
     The present invention relates to a ceramic body being configured of mainly BaTiO 3 -based crystalline particles in which a part of Ba is substituted with at least one rare earth element, wherein the ceramic body comprises Ba 6 Ti 17 O 40  crystalline particles of from 1.0 to 10.0% by mass. 
     The present invention also relates to a method for producing a ceramic body, comprising: a forming step of forming a green body containing a ceramic raw material comprising BaCO 3  powder, TiO 2  powder, and powder of rare earth nitrate and/or hydroxide to produce a ceramic formed body having a relative density of 60% or more; and a firing step of maintaining the ceramic formed body at a temperature of from 1150 to 1250° C., and then increasing the temperature to a maximum temperature of from 1360 to 1430° C. at a heating rate of from 20 to 500° C./hour and maintaining it for 0.5 to 5 hours. 
     The present invention also relates to a heater element comprising the ceramic body as described above. 
     The invention also relates to a heater unit comprising two or more of the heater elements as described above. 
     The present invention also relates to a heater system, comprising:
         the heater unit;   an inflow pipe for communicating an outside air introduction portion or a vehicle interior with an inflow port of the heater unit;   a battery for applying voltage to the heater unit; and   an outflow pipe for communicating an outflow port of the heater unit with the vehicle interior.       

     Further, the invention present invention relates to a purification system, comprising:
         a heater element or a heater unit comprising two or more of the heater elements, wherein the heater element comprises: the ceramic body; an adsorbent provided on surfaces of the partition wall of the ceramic body; and a pair of electrodes provided on the first end face and the second end face of the ceramic body;   a battery for applying a voltage to the pair of electrodes of the heater element; and   an inflow pipe for communicating the vehicle interior with the inflow port of the heater element or the heater unit; and   an outflow pipe for communicating the outflow port of the heater element or the heater unit with the vehicle interior and a vehicle exterior; and   a switching valve that can switch the flow of air through the outflow pipe to the vehicle interior or the vehicle exterior, the switching valve being arranged in the outflow pipe.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view of a ceramic body according to an embodiment of the present invention; 
         FIG. 2  is a schematic cross-sectional view orthogonal to a central axis of a honeycomb joined body having five honeycomb segments; 
         FIG. 3  is a schematic perspective view of a heater element according to an embodiment of the present invention; 
         FIG. 4  is a schematic cross-sectional view of the heater element in  FIG. 2  orthogonal to an extending direction of cells of a honeycomb structure; 
         FIG. 5  is a schematic end view of another heater element according to an embodiment of the present invention; 
         FIG. 6  is a schematic cross-sectional view taken along the line a-a′ in the heater element of  FIG. 5 ; 
         FIG. 7  is a schematic front view of a heater unit according to an embodiment of the present invention as viewed from a first end face side of a honeycomb structure; 
         FIG. 8  is a schematic front view of another heater unit according to an embodiment of the present invention as viewed from a first end face side of a honeycomb structure; 
         FIG. 9  is a schematic front view of another heater unit according to an embodiment of the present invention as viewed from a first end face side of a honeycomb structure; 
         FIG. 10  is a schematic view showing an arrangement example of a heater system according to an embodiment of the present invention; 
         FIG. 11  is a schematic enlarged cross-sectional view orthogonal to an extending direction of cells of a honeycomb structure provided with an adsorbent; and 
         FIG. 12  is a schematic view showing an arrangement example of a purification system according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As described above, ceramic bodies having the PTC property have been improved from various viewpoints, but there is still room for development. 
     For example, the application of the honeycomb-shaped ceramic body to a heater element for heating has been studied. If the heater element is to function at a low voltage, lower electrical resistance at room temperature is required. Further, in the heater element, a pair of electrodes are generally provided on end faces (faces perpendicular to an extending direction of cells) of the honeycomb-shaped ceramic body, but the pair of electrodes face a flow path for a gas, which may cause corrosion of the pair of electrodes. Therefore, it has been considered to provide a pair of electrodes on the side of the honeycomb-shaped ceramic body (parallel to the extending direction of the cells). 
     However, in a case where a pair of electrodes are provided on the side of the ceramic body, a distance between the electrodes is larger than in a case where a pair of electrodes are provided on the end faces of the ceramic body. Therefore, it requires lower electrical resistance of the ceramic body at room temperature. This requirement is particularly high in the case of honeycomb-shaped ceramic bodies where the thickness of the partition wall is lower. 
     The present invention has been made in view of the above circumstances. An object of the present invention is to provide a ceramic body having low electrical resistance at room temperature and having a PTC property, and a method for producing the same. Also, an object of the present invention is to provide a heater element, a heater unit, a heater system and a purification system, which have the above ceramic body. 
     As a result of intensive studies for ceramic bodies being configured of mainly BaTiO 3 -based crystalline particles, the present inventors have found that the presence of Ba 6 Ti 17 O 40  crystalline particles is closely related to the electrical resistance at room temperature, and have completed the present invention. 
     According to the present invention, it is possible to provide a ceramic body having low electrical resistance at room temperature and having a PTC property, and a method for producing the same. Also, according to the present invention, it is possible to provide a heater element, a heater unit, a heater system and a purification system, which have the above ceramic body. 
     Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings. It is to understand that the present invention is not limited to the following embodiments, and those which have appropriately added changes, improvements and the like to the following embodiments based on knowledge of a person skilled in the art without departing from the spirit of the present invention fall within the scope of the present invention. 
     &lt;Ceramic Body&gt; 
     (1-1) Crystal Structure and Composition 
     The ceramic body according to an embodiment of the present invention is configured of mainly BaTiO 3 -based crystalline particles in which a part of Ba is substituted with at least one rare earth element. The use of BaTiO 3 -based crystalline particles as the main component can result in a ceramic body that can generate heat by current conduction and has a PTC property. Further, by substituting a part of Ba of the BaTiO 3 -based crystalline particles with the rare earth element, the electrical resistance at room temperature (25° C.) can be reduced. 
     As used herein, “mainly based on” means that a percentage of those particles relative to the total component is 50% by mass or more. 
     The compositional formula of BaTiO 3 -based crystalline particles, in which a part of Ba is substituted with the rare earth element, can be expressed as (Ba 1-x A x )TiO 3 . In the compositional formula, the symbol A represents at least one rare earth element, and 0.001≤x≤0.010. 
     The symbol A is not particularly limited as long as it is the rare earth element, but it may preferably be one or more selected from the group consisting of La, Ce, Pr, Nd, Eu, Gd, Dy, Ho, Er and Yb, and more preferably La. The x value is preferably 0.001 or more, and more preferably 0.0015 or more, and even more preferably 0.002 or more, in terms of suppressing excessively high electrical resistance at room temperature. On the other hand, x is preferably 0.001 or less, and more preferably 0.009 or less, and even more preferably 0.002 or less, in terms of preventing the electrical resistance at room temperature from becoming too high due to insufficient sintering. 
     The BaTiO 3 -based crystalline particles in which a part of Ba is substituted with the rare earth element preferably have a (Ba+rare earth element)/Ti ratio of from 1.005 to 1.050. By controlling the (Ba+rare earth element)/Ti ratio to such a range, the electrical resistance at room temperature can be stably reduced. The element ratio of Ba, the rare earth element, and Ti can be determined by, for example, X-ray fluorescence analysis and ICP-MS (inductively coupled plasma mass spectrometry). 
     The BaTiO 3 -based crystalline particles in which a part of Ba is substituted with the rare earth element preferably have a lattice volume of from 64.000 to 64.3750 Å 3 , and more preferably from 64.4000 to 64.3650 Å 3 . By controlling the lattice volume in such a range, the electrical resistance at room temperature can be stably reduced. 
     The lattice volume of the BaTiO 3 -based crystalline particles can be measured using an X-ray diffractometer. More particularly, the lattice volume can be measured from a lattice constant obtained by analyzing the X-ray diffraction data by the Rietveld method. 
     The BaTiO 3 -based crystalline particles in which a part of Ba is substituted with the rare earth element preferably have an average crystal grain size of from 5 to 200 μm, and more preferably from 5 to 180 μm, and even more preferably from 5 to 160 μm. By controlling the average crystal grain size to such a range, the electrical resistance at room temperature can be stably reduced. 
     The average crystal grain size of the BaTiO 3 -based crystalline particles can be measured as follows. A square sample having 5 mm×5 mm×5 mm is cut out from the ceramic body and encapsulated with a resin. The encapsulated sample is mirror-polished by mechanical polishing and observed by SEM. The SEM observation is carried out using, for example, a model S-3400N from Hitachi High-Tech Corporation, at an acceleration voltage of 15 kV and at magnifications of 3000. In the SEM observation image (30 μm in length×45 μm in width), four straight lines each having a thickness of 0.3 μm were drawn at intervals of 10 μm across the entire vertical direction of the field of view, and the number of BaTiO 3 -based crystalline particles through which these lines pass even at a part of them is counted. An average of the SEM images at four or more positions where the length of the straight line is divided by the number of BaTiO 3 -based crystalline particles is defined as the average crystal grain size. 
     The content of the BaTiO 3 -based crystalline particles in which a part of Ba is substituted with the rare earth element in the ceramic body is not particularly limited as long as it is determined to be the main component, but it may preferably be 90.0% by mass or more, and more preferably 92.0% by mass or more, and even more preferably 94.0% by mass or more. The upper limit of the content of the BaTiO 3 -based crystalline particles is not particularly limited, but it may generally be 99.0% by mass, and preferably 98.0% by mass. 
     The content of the BaTiO 3 -based crystalline particles can be measured by, for example, fluorescent X-ray analysis or EDAX (energy dispersive X-ray) analysis. Other crystalline particles can be measured in the same manner as this method. 
     The ceramic body according to an embodiment of the present invention contains Ba 6 Ti 17 O 40  crystalline particles. The presence of Ba 6 Ti 17 O 40  crystalline particles in the ceramic body can reduce the electrical resistance at room temperature. Although not wishing to be bound by any theory, it is believed that Ba 6 Ti 17 O 40  crystalline particles are liquefied during a firing process to promote rearrangement, grain growth and densification of BaTiO 3 -based crystalline particles, thus reducing the electrical resistance at room temperature. 
     The content of the Ba 6 Ti 17 O 40  crystalline particles in the ceramic body may be from 1.0 to 10.0% by mass, and preferably from 1.2 to 8.0% by mass, and even more preferably from 1.5 to 6.0% by mass. The content of the Ba 6 Ti 17 O 40  crystalline particles of 1.0% by mass or more can provide an effect of the presence of the Ba 6 Ti 17 O 40  crystalline particles (i.e., an effect of reducing the electric resistance at room temperature). Further, the content of the Ba 6 Ti 17 O 40  crystalline particles of 10.0% by mass or less can ensure the PTC property. 
     The ceramic body according to an embodiment of the present invention can further contain BaCO 3  crystalline particles. The BaCO 3  crystalline particles are those derived from BaCO 3  powder, which is a raw material for the ceramic body. 
     The BaCO 3  crystalline particles may not be contained in the ceramic body because they have substantially no effect on the electrical resistance of the ceramic body at room temperature. However, if the content of BaCO 3  crystalline particles in the ceramic body is too high, it may affect the electrical resistance at room temperature, and the number of other crystalline particles may decrease, so that desired properties may not be obtained. Therefore, the content of the BaCO 3  crystalline particles in the ceramic body is preferably 2.0% by mass or less, and more preferably 1.8% by mass or less, and further preferably 1.5% by mass or less. The lower limit of the content of BaCO 3  crystalline particles is not particularly limited, but it may generally be 0.1% by mass, and preferably 0.2% by mass. 
     The ceramic body according to an embodiment of the present invention may further contain a component(s) conventionally added to PTC materials, in addition to the above crystalline particles. Such a component includes additives such as shifters, property improving agents, metal oxides and conductor powder, as well as unavoidable impurities. 
     In terms of reduction of the environmental load, it is desirable that the ceramic body according to an embodiment of the present invention is substantially free of lead (Pb). More particularly, the ceramic body according to the embodiment of the present invention preferably comprises Pb of 0.01% by mass or less, and more preferably 0.001% by mass or less, and still more preferably 0% by mass. The lower Pb content can allow heated air to be safely applied to organisms such as humans by contacting the ceramic body, for example, when the ceramic body is used for the heater element. In the ceramic body according to the embodiment of the present invention, the Pb content is preferably less than 0.03% by mass, and more preferably less than 0.01% by mass, and further preferably 0% by mass, as converted to PbO. The lead content can be determined by, for example, fluorescent X-ray analysis, ICP-MS (inductively coupled plasma mass spectrometry), or the like. 
     It is preferable that the ceramic body according to an embodiment of the present invention is substantially free of an alkali metal which may affect the electric resistance at room temperature. More particularly, the ceramic body according to the embodiment of the present invention preferably comprises an alkali metal of 0.01% by mass or less, and more preferably 0.001% by mass or less, and still more preferably 0% by mass. By controlling the content of the alkali metal to such a range, the electrical resistance at room temperature can be stably reduced. The alkali metal content can be determined by, for example, fluorescent X-ray analysis, ICP-MS (inductively coupled plasma mass spectrometry), or the like. 
     (1-2) Open Porosity 
     The open porosity of the ceramic body according to an embodiment of the present invention is a factor that will affect the electrical resistance at room temperature. Therefore, the open porosity of the ceramic body according to the embodiment of the present invention is preferably controlled to 5.0% or less, and more preferably 4.9% or less. By controlling the open porosity to such a range, the ceramic body can be densified, so that the electrical resistance at room temperature can be stably reduced. The lower limit of the open porosity is not particularly limited, but it may generally be 0.1%, and preferably 0.5%. 
     The open porosity of the ceramic body can be measured by the Archimedes method using pure water as a medium. The open porosity can be controlled by adjusting conditions such as an amount of a pore former or a sintering aid used in production of the ceramic body, and a firing atmosphere. 
     (1-3) Bulk Density 
     The bulk density of the ceramic body according to an embodiment of the present invention is a factor that will affect the electrical resistance at room temperature. Therefore, the bulk density of the ceramic body according to the embodiment of the present invention is preferably controlled to 5.35 g/cm 3  or more. By controlling the bulk density to such a range, the electrical resistance at room temperature can be stably reduced. The upper limit of the bulk density is not particularly limited, but it may generally be 7.00 g/cm 3 , and preferably 6.00 g/cm 3 . 
     The bulk density of the ceramic body can be measured by the Archimedes method using pure water as a medium, as in the open porosity. The bulk density can be controlled by adjusting conditions such as an amount of a dispersion medium, a binder, a plasticizer, a dispersant and the like used in production of the ceramic body, and the firing atmosphere. 
     (1-4) Volume Resistivity 
     The ceramic body according to an embodiment of the present invention preferably has a volume resistivity measured at 25° C. of 150 Ω·cm or less, and more preferably 100 Ω·cm or less, and still more preferably 50 Ω·cm, and particularly preferably 30 Ω·cm. The volume resistivity in such a range can be determined to lower electrical resistance at room temperature. The lower electric resistance at room temperature can ensure heat generation performance required for heating, and can suppress an increase in power consumption. The lower limit of the volume resistivity is not particularly limited, but it may generally be 0.1 Ω·cm, and preferably 1.0 Ω·cm. 
     The volume resistivity of the ceramic body can be measured as follows. Two or more samples each having a dimension of 30 mm×30 mm×15 mm are randomly cut and collected from the ceramic body. The electrical resistance at the measurement temperature is then measured by the two-terminal method, and the volume resistivity is calculated from shapes of the samples. An average value of the volume resistivities of all the samples is defined as a measured value at a measured temperature. 
     (1-5) Application 
     The ceramic body according to an embodiment of the present invention can be used, for example, in a PTC heater, a PTC switch, an overcurrent protection element, and a temperature detector, although not limited thereto. Among these, the ceramic body according to the embodiment of the present invention can be suitably used as a heater element for heating, particularly as a heater element for heating a vehicle interior. The vehicle includes, but not limited to, automobiles and trains. Non-limiting examples of the automobile include a gasoline vehicle, a diesel vehicle, a fuel cell vehicle, an electric vehicle, and a plug-in hybrid vehicle. The heater element according to the embodiment of the present invention can be particularly suitably used for a vehicle having no internal combustion engine such as electric vehicles and trains. 
     (1-6) Shape 
     The shape of the ceramic body according to an embodiment of the present invention may be appropriately selected depending on the applications, and is not particularly limited. For example, when considering the use of the ceramic body as a heater element, it can have a wall-flow type or flow-through type honeycomb shape, but it may preferably have the flow-through type honeycomb shape. 
     Here,  FIG. 1  shows a schematic perspective view of a ceramic body having a flow-through type honeycomb shape (hereinafter, referred to as a “honeycomb structure”) according to an embodiment of the present invention. A honeycomb structure  10  according to the embodiment of the present invention has a honeycomb structure including: an outer peripheral wall  11 ; and a partition wall  12  which are arranged on an inner side of the outer peripheral wall  11 , and which define a plurality of cells  14  that form flow paths from a first end face  13   a  to a second end face  13   b.    
     The shape of each end face (first end face  13   a  and second end face  13   b ) of the honeycomb structure  10  is not particularly limited, but it may be any shape such as a polygonal (quadrangular (rectangular, square), pentagonal, hexagonal, heptagonal, octagonal, etc.) shape, a circular shape, an oval shape and an L shape. If each end face is polygonal, the corners may be chamfered. It is preferable that the shape of each end face and the shape of the cross section orthogonal to the extending direction of the cells  14  are the same as each other. 
     The shape of each cell  14  in the cross section orthogonal to the extending direction of the cells  14  is not limited, but it may preferably be a quadrangle (rectangle, square), a hexagon, an octagon, or a combination thereof. Among these, the shape of each cell  14  is preferably square and/or hexagonal. By forming the cells  14  each having such a shape, it is possible to reduce the pressure loss when a gas passes through the honeycomb structure  10 . The honeycomb structure  10  of  FIG. 1  shows a case where the shape of each cell  14  in the cross section orthogonal to the extending direction of the cells  14  is square. 
     The partition wall  12  preferably has an average thickness of from 50 to 130 μm, and more preferably 55 to 120 μm, and even more preferably 60 to 110 μm, although not limited thereto. The average thickness of the partition wall  12  of 50 μm or more can ensure the strength of the honeycomb structure  10  while lowering the electrical resistance at room temperature. Further, the average thickness of the partition wall  12  of 130 μm or less can lead to a compact honeycomb structure  10 . 
     As used herein, the thickness of each partition wall  12  refers to a length in which a line segment crosses the partition wall  12  when centers of gravity of adjacent cells  14  are connected by the line segment in the cross section orthogonal to the extending direction of the cells  14 . The average thickness of the partition wall  12  refers to an average value of the thicknesses of all portions of the partition wall  12 . 
     A cell density is not particularly limited, but it may preferably be from 15 to 140 cells/cm 2 , and more preferably from 46 to 94 cells/cm 2 . The cell density of 15 cells/cm 2  or more can provide the honeycomb structure  10  suitable for heating while reducing the electrical resistance at room temperature. Further, the cell density of 140 cells/cm 2  or less can allow ventilation resistance to be suppressed and an output of a blower to be suppressed. 
     As used herein, the cell density can be obtained by dividing the number of cells by an area of each end face of the honeycomb structure  10 . 
     The honeycomb structure  10  shown in  FIG. 1  can be used as a heater element and can generate heat by current conduction. Therefore, a gas such as outside air or vehicle interior air can be heated by heat transfer from the heated partition wall  12  from a time when the gas flows in the first end face  13   a  until the gas passes through the plurality of cells  14  and flows out from the second end face  13   b.    
     The ceramic body according to an embodiment of the present invention may be a honeycomb joined body having honeycomb segments and joining layers for joining the honeycomb segments. The use the honeycomb joined body can increase the total cross-sectional area of the cells  14 , which is important for ensuring a flow rate of a gas, while suppressing generation of cracks. 
     Here, as an example,  FIG. 2  shows a schematic cross-sectional view orthogonal to the extending direction of the cells of a honeycomb joined body having five honeycomb segments. 
     As shown in  FIG. 2 , the honeycomb joined body  17  has five honeycomb segments  18  and joining layers  19  for joining the honeycomb segments  18 . Each honeycomb segment  18  has the outer peripheral wall  11  and the partition wall  12  which are arranged on the inner side of the outer peripheral wall  11  and which define a plurality of cells  14  that form flow paths from the first end face  13   a  to the second end face  13   b.    
     Each joining layer  19  can be formed by using a joining material. The joining material is not particularly limited, but a ceramic material obtained by adding a solvent such as water to form a paste can be used. The joining material may contain ceramics having a PTC property, or may contain the same ceramics as the outer peripheral wall  11  and the partition wall  12 . In addition to the role of joining the honeycomb segments  18  to each other, the joining material can also be used as an outer peripheral coating material after joining the honeycomb segments  18 . 
     (1-7) Production Method 
     A method for producing the ceramic body according to the embodiment of the present invention includes a forming step and a firing step. Hereinafter, a case of producing a ceramic body (honeycomb structure  10 ) having a honeycomb shape will be described as an example. 
     In the forming step, a green body containing a ceramic raw material including BaCO 3  powder, TiO 2  powder, and rare earth nitrate and/or hydroxide powder is formed to prepare a ceramic formed body (hereinafter, which may be referred to as a “honeycomb formed body”) having a relative density of 60% or more. In particular, the use of the rare earth hydroxide powder as the ceramic raw material can suppress aggregation of BaCO 3  powder in the green body, which can facilitate uniform liquid phase formation and grain growth in the firing step. As a result, it is easy to obtain the honeycomb structure  10  having lower electrical resistance at room temperature. The rare earth may be one or more selected from the group consisting of La, Ce, Pr, Nd, Eu, Gd, Dy, Ho, Er and Yb, and preferably La. 
     The ceramic raw material can be obtained by dry-mixing the powders so as to have a desired composition. 
     The green body can be obtained by adding a dispersion medium, a binder, a plasticizer and a dispersant to the ceramic raw material and kneading them. The green body may optionally contain additives such as shifters, metal oxides, property improving agents, and conductor powder. 
     The blending amount of the components other than the ceramic raw material is not particularly limited as long as the relative density of the ceramic formed body is 60%. 
     As used herein, the “relative density of the ceramic formed body” means a ratio of the density of the ceramic formed body to the true density of the entire ceramic raw material. More particularly, the relative density can be determined by the following equation: 
       relative density of ceramic formed body (%)=density of ceramic formed body (g/cm 3 )/true density of entire ceramic raw material (g/cm 3 )×100.
 
     The density of the ceramic formed body can be measured by the Archimedes method using pure water as a medium. Further, the true density of the entire ceramic raw material can be obtained by dividing the total mass of the respective raw materials (g) by the total volume of the actual volumes of the respective raw materials (cm 3 ). 
     Examples of the dispersion medium include water or a mixed solvent of water and an organic solvent such as alcohol, and more preferably water. 
     Examples of the binder include organic binders such as methyl cellulose, hydroxypropoxyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, and polyvinyl alcohol. In particular, it is preferable to use methyl cellulose in combination with hydroxypropoxyl cellulose. The binder may be used alone, or in combination of two or more, but it is preferable that the binder does not contain an alkali metal element. 
     Examples of the plasticizer include polyoxyalkylene alkyl ethers, polycarboxylic acid-based polymers, and alkyl phosphate esters. 
     The dispersant that can be used herein includes surfactants such as polyoxyalkylene alkyl ether, ethylene glycol, dextrin, fatty acid soaps, and polyalcohol. The dispersant may be used alone or in combination of two or more. 
     The ceramic formed body can be produced by extrude the green body. In the extrusion, a die having a desired overall shape, cell shape, partition wall thickness, cell density and the like can be used. 
     The relative density of the ceramic formed body obtained by extrusion is 60% or more, and preferably 61% or more. By controlling the relative density of the ceramic formed body to such a range, the ceramic body can be densified and the electrical resistance at room temperature can be reduced. The upper limit of the relative density of the ceramic formed body is not particularly limited, but it may generally be 80%, and preferably 75%. 
     The ceramic formed body can be dried before the firing step. Non-limiting examples of the drying method include conventionally known drying methods such as hot air drying, microwave drying, dielectric drying, drying under reduced pressure, drying in vacuum, and freeze drying. Among these, a drying method that combines the hot air drying with the microwave drying or dielectric drying is preferable in that the entire formed body can be rapidly and uniformly dried. 
     The firing step includes maintaining the ceramic formed body at a temperature of from 1150 to 1250° C., and then increasing the temperature to a maximum temperature of from 1360 to 1430° C. at a heating rate of 20 to 500° C./hour, and maintaining the temperature for 0.5 to 5 hours. 
     The maintaining of the honeycomb formed body at the maximum temperature of from 1360 to 1430° C. for 0.5 to 5 hours can provide a ceramic body (honeycomb structure  10 ) mainly based on BaTiO 3 -based crystal particles in which a part of Ba is substituted with the rare earth element. 
     Further, the maintaining at the temperature of from 1150 to 1250° C. can allow the Ba 2 TiO 4  crystal particles generated in the firing step to be easily removed, so that the honeycomb structure  10  can be densified. 
     Further, the heating rate of 20 to 500° C./hour from the temperature of 1150 to 1250° C. to the maximum temperature of 1360 to 1430° C. can allow 1.0 to 10.0% by mass of Ba 6 Ti 17 O 40  crystal particles to be formed in the honeycomb structure  10 . 
     The retention time at 1150 to 1250° C. is not particularly limited, but it may preferably be from 0.5 to 5 hours. Such a retention time can lead stable and easy removal of Ba 2 TiO 4  crystal particles generated in the firing step. 
     The firing step preferably includes maintaining at 900 to 950° C. for 0.5 to 5 hours. The maintaining at 900 to 950° C. for 0.5 to 5 hours can lead to sufficient decomposition of BaCO 3 , so that the honeycomb structure  10  having a predetermined composition can be easily obtained. 
     Prior to the firing step, a degreasing step for removing the binder may be performed. The degreasing step may preferably be performed in an air atmosphere in order to decompose the organic components completely. 
     Also, the atmosphere of the firing step may preferably be the air atmosphere in terms of controlling electrical characteristics and production cost. 
     A firing furnace used in the firing step and the degreasing step is not particularly limited, but it may be an electric furnace, a gas furnace, or the like. 
     &lt;Heater Element&gt; 
     The heater element according to an embodiment of the present invention includes the above ceramic body (for example, honeycomb structure  10 ). 
       FIG. 3  is a schematic perspective view of the heater element according to the embodiment of the present invention. Further,  FIG. 4  is a schematic cross-sectional view of the heater element of  FIG. 3 , which is orthogonal to the extending direction of the cells of the honeycomb structure. 
     The heater element  100  according to the embodiment of the present invention includes the honeycomb structure  10  and a pair of electrodes  20  arranged on a surface of the outer peripheral wall  11  of the honeycomb structure  10 . 
     The honeycomb structure  10  used for the heater element  100  preferably has a shape having a long axis and a short axis in a cross section orthogonal to the extending direction of the cells  14 . Further, it is preferable that the pair of electrodes  20  are formed in a band shape extending in parallel with the extending direction of the cells  14 , and arranged on the surface of the outer peripheral wall  11  so as to face each other across the long axis passing through the center of gravity of the honeycomb structure  10  in the cross section orthogonal to the extending direction of the cells  14 . Further, it is preferable that the heater element  100  further includes a plate-shaped external connecting member  30  which is arranged on an end portion side of each electrode  20  and is in contact with each electrode  20  in a plane. By thus arranging the electrodes  20  and the external connecting members  30 , the electrodes  20  and the external connecting members  30  are brought into surface contact with each other, which will easily increase an amount of power supplied from the outside, so that the heat generation performance can be improved. 
       FIG. 5  is a schematic end view of another heater element according to an embodiment of the present invention (i.e., a schematic front view of another heater element according to an embodiment of the present invention as viewed from the first end face side of the honeycomb structure). Further,  FIG. 6  is a schematic cross-sectional view of the heater element of  FIG. 5  along the line a-a′ (i.e., a schematic cross-sectional view of the heater element of  FIG. 5  parallel to the extending direction of the cells of the honeycomb structure). 
     A heater element  200  according to an embodiment of the present invention includes the honeycomb structure  10  and the pair of electrodes  20  arranged on the surfaces of the outer peripheral wall  11  and the partition wall  12  on the first end face  13   a  and the second end face  13   b  of the honeycomb structure  10 . 
     The honeycomb structure  10  used for the heater element  200  preferably has a shorter length in the extending direction of the cells  14 . Such a structure can allow the honeycomb structure  10  having lower electric resistance at room temperature to be applied even to the heater element  200  having a structure in which the pair of electrodes  20  are arranged on the first end face  13   a  and the second end face  13   b.  Further, it is preferable that the heater element  200  further includes a plate-shaped external connecting member  30  on at least a part of the pair of electrodes  20 , which is in contact with each electrode  20  in a plane. The arrangement of such an external connecting member  30  can lead to efficient conduction of current over the pair of electrodes  20  as a whole, thereby improving the heat generation performance. 
     Hereinafter, each component of the heater elements  100 ,  200  will be described in detail. 
     (2-1) Pair of Electrodes  20  in Heater Element  100   
     The pair of electrodes  20  can be provided on the surface of the outer peripheral wall  11  of the honeycomb structure  10 . The pair of electrodes  20  are formed in a band shape extending in parallel with the extending direction of the cells  14  of the honeycomb structure  10 . Further, the pair of electrodes  20  are arranged on the surface of the outer peripheral wall  11  so as to face each other across the long axis passing through the center of gravity of the honeycomb structure  10  in the cross section orthogonal to the extending direction of the cells  14  of the honeycomb structure  10 . By applying a voltage through the pair of electrodes  20  thus arranged, electricity can be conducted to generate heat in the honeycomb structure  10  by Joule heat. 
     The electrode  20  that can be used herein includes, but not particularly limited to, a metal or alloy containing at least one selected from Zn, Cu, Ag, Al, Ni and Si. It is also possible to use an ohmic electrode layer capable of ohmic contact with the outer peripheral wall  11  and/or the partition wall  12 , which has a PTC property. The ohmic electrode layer contains, for example, at least one selected from Al, Au, Ag and In as a base metal, and contains at least one selected from Ni, Si, Ge, Sn, Se and Te for n-type semiconductors as a dopant. Further, the electrode  20  may have one layer or two or more layers. When the electrode  20  has two or more layers, materials of the respective layers may be of the same type or different types. 
     The thickness of each electrode  20  is not particularly limited, but it may be appropriately set depending on the method of forming the electrodes  20 . Examples of the method for forming the electrodes  20  include metal deposition methods such as sputtering, vapor deposition, electrolytic deposition, and chemical deposition. Further, the electrode  20  can be formed by applying an electrode paste and then baking the electrode paste. Further, the electrode  20  can also be formed by thermal spraying. 
     It is preferable that the thickness of each electrode  20  is from about 5 to 30 pm for baking of the electrode paste, from about 100 to 1000 nm for dry plating such as sputtering and vapor deposition, from about 10 to 100 μm for thermal spraying, or from about 5 to 30 μm for wet plating such as electrolytic precipitation and chemical precipitation. 
     (2-2) Pair of Electrodes  20  in Heater Element  200   
     The pair of electrodes  20  can be provided on the surfaces of the outer peripheral wall  11  and the partition wall  12  on the first end face  13   a  and the second end face  13   b  of the honeycomb structure  10 . 
     The pair of electrodes  20  are preferably provided on the first end face  13   a  and the second end face  13   b  without blocking the cells  14 , and more preferably provided on the entire first end face  13   a  and the entire second end face  13   b  without blocking the cells  14 . 
     Since the other features of the electrodes  20  are the same as those set forth in (2-1), descriptions thereof will be omitted. 
     (2-3) External Connecting member  30  in Heater Element  100   
     The external connecting member  30  has a plate shape and can be provided on the end portion side of each electrode  20  so as to be in contact with each electrode  20  in a plane. It is preferable that the external connecting member  30  extends parallel to the long axis passing through the center of gravity of the honeycomb structure  10  in the cross section orthogonal to the extending direction of the cells  14  of the honeycomb structure  10 . By providing such a plate-shaped external connecting member  30 , an amount of power supplied from the outside to the electrodes  20  can be easily increased, so that the heat generation performance can be improved. 
     As used herein, “the end portion side of each electrode  20 ” means a region up to 30% of the entire length of each electrode  20  from the end of each electrode  20  in the longitudinal direction passing through the center of gravity of the honeycomb structure  10  in the cross section orthogonal to the extending direction of the cells  14  of the honeycomb structure  10 . 
     The external connecting member  30  may be arranged on the end portion side of each electrode  20 , and may not necessarily be in contact with the end of each electrode  20 . For example, the external connecting member  30  may be provided with bent portions, and the bent portions may be connected to the respective electrodes  20 . 
     The external connecting member  30  preferably has the substantially same width as that of the end portion of the electrode  20  on the side where the external connecting member  30  is arranged. Such a configuration can increase a contact area of the electrode  20  with the external connecting member  30 , so that the effect of improving the heat generation performance is enhanced. 
     As used herein, the phrase “substantially the same width as that of the end portion of the electrode  20 ” means that the width is within ±20% of the width of the end portion of the electrode  20 . 
     It is preferable that each of the external connecting members  30  is arranged on one end portion side of the electrode  20  parallel to the extending direction of the cells  14  of the honeycomb structure  10 . One end portion side on which the external connecting member  30  is arranged may be the same or different side in the cross section orthogonal to the extending direction of the cells  14  of the honeycomb structure  10 . One end portion side is more preferably the same side. It is preferable that each of the external connecting members  30  extends in the same direction from the end portion side thereof to the outside. Such a configuration can lead to a compact heater element  100  when the honeycomb structure  10  is applied to the heater element  100 . 
     The external connecting member  30  may be made of, for example, a metal, although not limited thereto. The metal that can be used herein includes a single metal, an alloy, and the like. In terms of corrosion resistance, electrical resistance, and linear expansion rate, for example, the metal may preferably be an alloy containing at least one selected from the group consisting of Cr, Fe, Co, Ni, Cu, and Ti, and more preferably stainless steel, Fe-Ni alloys, and phosphorus bronze. 
     The shape and size of the external connecting member  30  are not particularly limited, but they may be appropriately adjusted according to the structure of the heater unit to be produced. 
     A method of connecting each external connecting member  30  to each electrode  20  is not particularly limited as long as they are electrically connected. They may be connected by, for example, diffusion joining, a mechanical pressurizing mechanism, welding, or the like. 
     (2-4) External Connecting Member  30  in Heater Element  200   
     The external connecting member  30  has a plate shape and can be provided so as to be in contact with each electrode  20  in a plane. 
     The external connecting members  30  are preferably arranged on the electrodes  20  provided on the outer peripheral wall  11  of the first end face  13   a  and the second end face  13   b.  Such a configuration can lead to efficient conduction of current over the entire electrode  20 . 
     Since the other features of the external connecting members  30  are the same as those set forth in (2-3), descriptions thereof will be omitted. 
     (2-5) Use Method 
     Each of the heater elements  100 ,  200  according to the embodiment of the present invention can be suitably used as a heater element for heating a vehicle interior of a vehicle. 
     Each of the heater elements  100 ,  200  according to the embodiment of the present invention can generate heat by applying a voltage from an external power source to the honeycomb structure  10  via the external connecting members  30  and the electrodes  20 . The applied voltage is preferably from 12 to 800 V. Specifically, in the heater element  100 , the applied voltage is preferably from 100 to 800 V. Further, in the heater element  200 , the applied voltage is preferably from 12 to 60 V. The adjustment of the applied voltage to such range can suppress power consumption while performing rapid heating. Moreover, since the voltage is lower, the safety is higher. Furthermore, since the safety specifications are not severe, the equipment around the heater can be manufactured at lower cost. 
     When the honeycomb structure  10  generates heat due to the application of the voltage, the gas can be heated by allowing the gas to flow through the cells  14 . 
     The temperature of the gas flowing into the cells  14  can be, for example, −60° C. to 20° C., and typically −10° C. to 20° C. 
     Since the heater element  100  according to the embodiment of the present invention uses the honeycomb structure  10  having the PTC property and low electrical resistance at room temperature, it can be driven at a lower voltage. 
     Further, the heater element  100  according to the embodiment of the present invention has a simpler structure than that of an existing heater element in which a PTC element and an aluminum fin are integrated via an insulating ceramic plate, and can prevent the heater unit from becoming larger. Further, in the existing heater element, the PTC element is not in direct contact with the gas, resulting in an insufficient heating rate (heating time) of the gas, whereas in the heater element  100  according to the embodiment of the present invention, the honeycomb structure  10  in which the outer peripheral wall  11  and the partition wall  12  are made of a material having the PTC property is in direct contact with the gas, resulting in an increased heating rate of the gas. 
     Further, the heater element  100  according to the embodiment of the present invention is provided with the electrodes  20  and the external connecting members  30  as described above, so that it is easy to increase an amount of power supplied from the outside to the electrode  20 , thus enabling the heat generation performance to be improved. 
     &lt;Heater Unit&gt; 
     The heater unit according to an embodiment of the present invention can be suitably used as a heater unit for heating a vehicle interior of a vehicle. In particular, since the heater unit according to the embodiment of the present invention uses a ceramic body (honeycomb structure  10 ) having a PTC property, which has low electrical resistance at room temperature, for the heater element  100 ,  200 , it can be driven at a lower voltage. In particular, the arrangement of a plurality of heater elements  100 ,  200  in parallel can result in a practical heater unit that can be used at a lower voltage. Further, since the heater unit according to the embodiment of the present invention uses the heater elements  100 ,  200  having higher heat generation performance, the heat generation performance of the heater unit can be improved. Furthermore, since the heater elements  100 ,  200  can be made compact, it is possible to prevent the heater unit from becoming larger. 
       FIG. 7  is a schematic front view of a heater unit according to an embodiment of the present invention as viewed from the first end face side of the honeycomb structure. 
     As shown in  FIG. 7 , a heater unit  600  according to an embodiment of the present invention includes two or more heater elements  100 . Further, in the heater unit  600 , the heater elements  100  are stacked so that the surfaces of the outer peripheral walls  11  of the honeycomb structures  10  including the long sides of the first end faces  13   a  and the second end faces  13   b  face each other. Such a configuration can produce a compact heater unit  600 . 
     The heater unit  600  according to the embodiment of the present invention may further include a housing (housing member)  610 . 
     The housing  610  may be made of any material, including, but not limited to, for example, metals and resins. Among them, the material of the housing  610  is preferably the resin. The housing  610  made of the resin can suppress electric shock without grounding. 
     The shape and size of the housing  610  are not particularly limited, but they may be the same as those of the existing heater unit. 
     The heater unit  600  according to the embodiment of the present invention may further include insulating materials  620  each arranged between the heater elements  100  which are stacked. Such a configuration can suppress an electrical short circuit between the plurality of heater elements  100 . 
     The insulating materials  620  that can be used herein include plate materials, mats, clothes, and the like, which are formed of an insulating material such as alumina or ceramics. 
     The heater unit  600  according to the embodiment of the present invention can have a wiring structure capable of controlling the heater elements  100 . More particularly, the heater unit  600  according to the embodiment of the present invention may further include wirings  630  connected to the external connecting members  30  of the heater element  100 . 
     The wiring structure is not particularly limited, but as shown in  FIG. 7 , each of the heater elements  100  may be independently controllable. More particularly, the wiring  630  can be connected to each of the external connecting members  30  of the heater element  100 . The wirings  630  are connected to an external power source (not shown). Such a wiring structure can allow the heater elements  100  to be each independently controlled, thereby enabling fine temperature tuning. 
     As shown in  FIG. 8 , the wiring structure may be a parallel wiring structure in which two or more heater elements  100  can be collectively controlled. More particularly, the parallel wiring  640   a  may be connected to one of the external connecting members  30  of each heater element  100 , and one parallel wiring  640   b  may be connected to the other external connecting member  30 . Such a wiring structure can suppress the power consumption of the heater unit  700 . 
     Further, as shown in  FIG. 9 , it may be a parallel wiring structure where two or more heater elements  100  can be collectively controlled using the electrodes  20  between the stacked heater elements  100  as one electrode  20  common to the stacked heater elements  100 . More particularly, each external connecting member  30  may be arranged at the end of each electrode  20 , the parallel wiring  640   a  may be connected to one of the external connecting members  30  of each heater element  100 , and one parallel wiring  640   b  may be connected to the other external connecting member  30 . Such a structure can eliminate necessity to arrange the insulating material  620  between the stacked heater elements  100 , so that the heater unit  800  can be made compact and the power consumption can be suppressed. 
     &lt;Heater System&gt; 
     The heater system according to an embodiment of the present invention can be suitably used as a heater system for heating a vehicle interior of a vehicle. Especially, in the heater system according to the embodiment of the present invention, the heater unit  600  that can be driven at a lower voltage is used, so that power consumption can be suppressed. Further, in the heater system according to the embodiment of the present invention, the heater unit  600  having high heat generation performance is used, so that the heat generation performance of the heater system can be improved. Furthermore, the heater unit  600  can be made compact, so that it is possible to prevent the heater system from becoming larger. 
       FIG. 10  is a schematic view showing an arrangement example of a heater system according to an embodiment of the present invention. 
     As shown in  FIG. 10 , a heater system  900  according to the embodiment of the present invention includes: the heater unit  600  according to the embodiment of the present invention; inflow pipes  920   a,    920   b  for communicating an outside air introduction portion or a vehicle interior  910  with an inflow port  650  of the heater unit  600 ; a battery  940  for applying a voltage to the heater unit  600 ; and an outflow pipe  930  for communicating an outflow port  660  of the heater unit  600  with the vehicle interior  910 . It is also possible to use the heater unit  700 ,  800  according to the embodiment of the present invention in place of the heater unit  600 . 
     The heater unit  600  can be configured to energize and generate heat by connecting to the battery  940  with an electric wire  950  and turning on a power switch in the middle of the wiring, for example. 
     Disposed on the upstream side of the heater unit  600  can be a vapor compression heat pump  960 . In the heater system  900 , the vapor compression heat pump  960  is configured as a main heating device, and the heater unit  600  is configured as an auxiliary heater. The vapor compression heat pump  960  can be provided with a heat exchanger including: an evaporator  961  that functions to absorb heat from the outside during cooling to evaporate a refrigerant; and a condenser  962  that functions to liquefy a refrigerant gas to release heat to the outside during heating. The vapor compression heat pump  960  is not particularly limited, and a vapor compression heat pump known in the art can be used. 
     On the upstream side and/or the downstream side of the heater unit  600 , a blower  970  can be installed. In terms of ensuring safety by arranging high-voltage parts as far as possible from the vehicle interior  910 , the blower  970  is preferably installed on the upstream side of the heater unit  600 . As the blower  970  is driven, air flows into the heater unit  600  from the inside or outside of the vehicle interior  910  through the inflow pipes  920   a,    920   b.  The air is heated while passing through the heating unit  600  that is generating heat. The heated air flows out from the heater unit  600  and is delivered into the vehicle interior  910  through the outflow pipe  930 . The outlet of the outflow pipe  930  may be arranged near the feet of an occupant so that the heating effect is particularly high even in the vehicle interior  910 , or a pipe outlet may be arranged in a seat to warm the seat from the inside, or may be arranged near a window to have an effect of suppressing fogging of the window. 
     The inflow pipe  920   a  and the inflow pipe  920   b  merge in the middle. The inflow pipe  920   a  and the inflow pipe  920   b  can be provided with valves  921   a  and  921   b,  respectively, on the upstream side of the confluence. By controlling the opening and closing of the valves  921   a,    921   b,  it is possible to switch between a mode where the outside air is introduced into the heater unit  600  and a mode where the air in the vehicle interior  910  is introduced into the heater unit  600 . For example, the opening of the valve  921   a  and the closing of the valve  921   b  results in the mode where the outside air is introduced into the heater unit  600 . It is also possible to open both the valve  921   a  and the valve  921   b  to introduce the outside air and the air in the vehicle interior  910  into the heater unit  600  at the same time. 
     &lt;Purification System&gt; 
     The purification system according to an embodiment of the present invention can also be suitably used as a purification system for removing harmful components in a vehicle interior of a vehicle. In particular, the purification system according to the embodiment of the present invention uses the heater element including the ceramic body having low electric resistance at room temperature or the heater unit including two or more heater elements, so that the purification performance can be obtained while suppressing power consumption. 
     The heater element used in the purification system according to the embodiment of the present invention includes: the ceramic body (honeycomb structure  10 ) as described above; an adsorbent provided on surfaces of the partition wall  12  of the honeycomb structure  10 ; and a pair of electrodes  20  provided on the first end face  13   a  and the second end face  13   b  of the honeycomb structure  10 . 
     Here,  FIG. 11  shows a schematic enlarged cross-sectional view orthogonal to the extending direction of the cells  14  of the honeycomb structure  10  provided with the adsorbent. 
     As shown in  FIG. 11 , the surfaces of the partition wall  12  of the honeycomb structure  10  are provided with adsorbents  50 . Thus, the providing of the adsorbents  50  can absorb harmful volatile components from the air flowing in the cells  14 . The harmful volatile components include, for example, volatile organic compounds (VOCs) and odorous components. Specific examples of the harmful volatile components include ammonia, acetic acid, isovaleric acid, nonenal, formaldehyde, toluene, xylene, paradichlorobenzene, ethylbenzene, styrene, chlorpyrifos, di-n-butyl phthalate, tetradecane, di-2-ethylhexyl phthalate, diazinon, acetaldehyde, and 2-(1-methylpropyl)phenyl N-methylcarbamate. 
     The adsorbent  50  may be appropriately selected depending on the volatile components to be adsorbed, and is not particularly limited. Examples of the adsorbent  50  include zeolite and the like. Further, if the adsorbent  50  capable of adsorbing CO 2  at room temperature and releasing CO 2  at higher temperature is selected, CO 2  in the vehicle interior can be discharged to the outside of the vehicle. Further, the use of the adsorbent  50  in combination with a noble metal such as Pt or an oxidation catalyst of a metal oxide can lead to easy removal of the harmful volatile components from the air flowing through the cells  14 . 
     The heater element used in the purification system according to the embodiment of the present invention may be provided with the external connecting members  30  on the pair of electrodes  20  arranged on the first end face  13   a  and the second end face  13   b  (for example, on the outer peripheral portions of the electrodes  20  arranged on the outer peripheral wall  11  of the honeycomb structure  10 ). 
     The heater element used in the purification system according to the embodiment of the present invention can be produced according to the above method. For example, an electrode paste can be applied to the first end face  13   a  and the second end face  13   b  of the honeycomb structure  10  and baked to form the electrodes  20 , and the surfaces of the partition wall  12  can be then coated with the adsorbent  50  to produce a heater element. The coating method of the adsorbent  50  is not particularly limited, but for example, the honeycomb structure  10  may be immersed in a slurry containing the adsorbent  50 , an organic binder, and water, and excess slurry on the end faces and the outer periphery of the honeycomb structure  10  is removed by blowing or wiping. Subsequently, the slurry can be dried at a temperature of about 550° C. to provide the adsorbents  50  on the surfaces of the partition wall  12 . This step may be performed once, but this step may be repeated several times to provide a desired amount of the adsorbent  50  on the surfaces of the partition wall  12 . 
     Further, when the external connecting members  30  are provided, the external connecting members  30  may be arranged at predetermined positions of the electrodes  20  and joined thereto. 
       FIG. 12  is a schematic view showing an arrangement example of a purification system according to an embodiment of the present invention. 
     As shown in  FIG. 12 , a purification system  1000  according to an embodiment of the present invention includes: the heater element or heater unit  1100  as described above; a battery (power supply)  1200  for applying a voltage to a pair of electrodes  20  of the heater element or heater unit  1100 ; an inflow pipe  1300  for communicating a vehicle interior with an inflow port  1110  of the heater element or heater unit  1100 ; an outflow pipe  1400  for communicating an outflow port  1120  of the heater element or heater unit  1100  with the vehicle interior and a vehicle exterior; and a switching valve  1500  which is arranged in the outflow pipe  1400  and which can switch the flow of air flowing through the outflow pipe  1400  to the vehicle interior or the vehicle exterior. 
     The heater element or the heater unit  1100  can be configured to energize and generate heat by connecting to the battery  1200  via an electric wire  1210  and turning on a power switch in the middle of the wire, for example. 
     The switching between ON and OFF of the power switch can be performed by a control unit  1600  electrically connected to the power switch. Further, the switching of the switching valve  1500  can also be performed by the control unit  1600  electrically connected to the switching valve. 
     In the purification system  1000  having the above structure, the air from the vehicle interior is fed from the inflow port  1110  to the heater element or the heater unit  1100  through the inflow pipe  1300 . After performing predetermined processing by the heater element or the heater unit  1100 , the air is discharged from the outflow port  1120  and returned to the vehicle interior through the outflow pipe  1400  or discharged to the vehicle exterior. 
     When closing the flow path of the outflow pipe  1400  to the vehicle exterior with the switching valve  1500  so that the air returns to the vehicle interior, the power switch is turned off and the heater element or the heater unit  1100  is maintained at room temperature. Such a control can allow the harmful volatile components contained in the air from the vehicle interior to be removed by adsorbing them to the adsorbents  50  of the heater element or the heater unit  1100 . 
     On the other hand, when the flow path of the outflow pipe  1400  to the vehicle interior is closed by the switching valve  1500  so that the air is discharged to the vehicle exterior, the power switch is turned on to heat the heater element or the heater unit  1100 . Such a control can release the harmful volatile components adsorbed on the adsorbents  50  of the heater element or the heater unit  1100  to regenerate the function of the adsorbents  50 , and to discharge the harmful volatile components to the vehicle exterior. 
     By repeating the switching of the power switch and the switching valve  1500  as described above in a fixed cycle, it is possible to discharge stably the harmful volatile components in the vehicle interior to the vehicle exterior. 
     In the purification system  1000 , the heater element or the heater unit  1100  is preferably arranged at a position close to the vehicle interior, in terms of stably ensuring the above functions. Therefore, from the viewpoint of preventing electric shock, the drive voltage is preferably 60 V or less. Since the honeycomb structure  10  used in the heater element or the heater unit  1100  has lower electrical resistance at room temperature, the honeycomb structure  10  can be heated at the lower drive voltage. The lower limit of the drive voltage is not particularly limited, but it may preferably be 10 V. If the drive voltage is less than 10 V, the current during heating of the honeycomb structure  10  is increased, so that the electric wire  1210  must be thickened. 
     The honeycomb structure  10  used in the heater element or the heater unit  1100  preferably has a higher opening ratio of the cells  14 , a higher cell density and a higher surface area of the partition wall  12 , from the viewpoints that a large amount of adsorbent  50  is provided on the surfaces of the partition wall  12 , and the adsorption function of the absorbent is sufficiently ensured. In a typical embodiment, the honeycomb structure  10  has an opening ratio of the cells  14  of preferably 75% or more, and more preferably 80% or more. The cell density is preferably from 15 to 94 cells/cm 2 , and more preferably from 31 to 70 cells/cm 2 . Further, the diameter of the short axis (thickness of the honeycomb structure  10 ) in the cross section orthogonal to the extending direction of the cells  14  is preferably from 3 to 15 mm, and more preferably 4 to 10 mm. 
     If the diameter of the short axis (thickness of the honeycomb structure  10 ) in the cross section orthogonal to the extending direction of the cells  14  is smaller, the amount of the adsorbent  50  may be insufficient. Therefore, it is preferable to use the heater unit in the purification system  1000  from the viewpoint of sufficiently ensuring the absorption function. As described above, the heater unit can be produced by arranging a plurality of heater elements in parallel. By using the heater unit, the amount of the adsorbent  50  can be increased, and the heating speed and the cooling speed of the honeycomb structure  10  can be increased when the power switch is switched on and off. Therefore, the practicality of the purification system  1000  can be improved. 
     From the viewpoint of efficiently improving the adsorption function of the adsorbent  50 , the thickness of the adsorbent  50  provided on the surfaces of the partition wall  12  should not be too large. This is because if the adsorbent  50  is too thick, it will be difficult to be brought into contact with the air flowing in the cells  14 , and an efficiency of the adsorbing function is decreased. Therefore, the thickness of the adsorbent  50  is preferably from 0.01 to 0.5 mm. 
     EXAMPLES 
     Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples. 
     (1) Production of Ceramic Body 
     Examples 1 to 9, Comparative Examples 1 to 4 
     As ceramic raw materials, BaCO 3  powder, TiO 2  powder and La(NO 3 ) 3 .6H 2 O powder were prepared. These powders were weighed so as to have the composition shown in Table 1 after firing, and dry-mixed to obtain a mixed powder. The dry mixing was carried out for 30 minutes. Subsequently, from 3 to 30 parts by weight of water, a binder, a plasticizer and a dispersant in total were added by a proper amount, based on 100 parts by mass of the obtained mixed powder, such that ceramic formed bodies having the relative density shown in Table  1  were obtained after extrusion, and then kneaded to obtain green bodies. Methyl cellulose was used as the binder. Polyoxyalkylene alkyl ether was used as the plasticizer and the dispersant. 
     Each green body was introduced into an extrusion molding machine and extruded using a predetermined die to obtain a rectangular parallelepiped honeycomb formed body. The density of the honeycomb formed body was measured according to the above method. 
     The obtained honeycomb formed body was subjected to dielectric drying and hot air drying, and both end faces were cut to have predetermined dimensions to obtain a honeycomb dried body. 
     The shape of the dried honeycomb is as follows:
     Overall shape: rectangular parallelepiped having 45 mm×45 mm×200 mm in height (extending direction of the cells);   Cell shape in cross section orthogonal to extending direction of cell: square;   Cell density: 62 cells/cm 2 ; and   Thickness of partition wall: 4 mil (101.6 μm).   

     The honeycomb dried body was cut to have a height of 35 mm, and then degreased in an air atmosphere (450° C. for 4 hours) in a firing furnace, and then fired in an air atmosphere to obtain a ceramic body. The conditions of the firing step were as shown in Table 1. More particularly, the firing step carried out a maintaining step A, a maintaining step B, and a maintaining step C in this order. It should be noted that the maintaining step C is a maintaining step at the maximum temperature. 
     The following evaluations were performed on the obtained ceramic body. 
     Examples 10 and 11 
     Ceramic bodies were prepared in the same method as that of Example 1 or the like as described above, with the exception that La(OH) 3  powder was used in place of La(NO 3 ) 3 .6H 2 O powder, and the following evaluations were performed. 
     (2) Chemical Analysis 
     The chemical composition of each ceramic body was analyzed by ICP emission spectroscopy, and the atomic ratios of elements such as La, Ba, and Ti were determined. Table 1 shows the atomic ratio of La of the BaTiO 3 -based crystalline particles (x value) and (Ba+La)/Ti ratio obtained by the analysis. 
     Moreover, the analysis results confirmed that the ceramic bodies produced in Examples and Comparative Examples did not contain Pb and an alkali metal. 
     (3) Identification of Crystalline Particles and Lattice Volume of BaTiO 3 -based Crystalline Particles 
     Crystal particles of each ceramic body were identified using an X-ray diffractometer. As the X-ray diffractometer, a multifunctional powder X-ray diffractometer (D8Avance from Bruker) was used. The conditions for the X-ray diffraction measurement were: a CuKα radiation source; 10 kV; 20 mA; 2θ=5 to 100°. The obtained X-ray diffraction data was then analyzed by the Rietveld method using an analysis software TOPAS (from BrukerAXS) to identify the crystalline particles. 
     The lattice volume of the BaTiO 3 -based crystal particles was determined by calculation from the lattice constant obtained by the analysis of the X-ray diffraction data. 
     Table 1 shows these results. 
     (4) Content of Each Crystal Particle 
     The content of each crystal particle was measured using an X-ray diffractometer. As the X-ray diffractometer, the same equipment and analysis software as described above were used, and the content of each crystal particle was determined by the Rietveld method. 
     (5) Measurement of Average Crystal Grain Size 
     The average crystal grain size of each ceramic body was measured according to the above method. The SEM observation was carried out using a model S-3400N from by Hitachi High-Tech Corporation at an acceleration voltage of 15 kV and at magnifications of 3000. The results are shown in Table 1. 
     (6) Open Porosity 
     The open porosity of each ceramic body was measured according to the above method. The results are shown in Table 1. 
     (7) Bulk Density 
     The bulk density of each ceramic body was measured according to the above method. The results are shown in Table 1. 
     (8) Volume Resistivity 
     The volume resistivity of each ceramic body at room temperature (25° C.) was measured according to the above method. The measured value of the volume resistivity was an average value of the measured volume resistivities. The results are shown in Table 1. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 Examples 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Relative Density (%) of 
                 61.9 
                 61.9 
                 61.9 
                 63.6 
                 63.9 
               
               
                 Ceramic Formed Body 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Firing 
                 Maintaining 
                 950° C. × 
                 950° C. × 
                 950° C. × 
                 950° C. × 
                 950° C. × 
               
               
                 Step 
                 Step A 
                 1 hr 
                 1 hr 
                 1 hr 
                 1 hr 
                 1 hr 
               
               
                   
                 Maintaining 
                 1200° C. × 
                 1200° C. × 
                 1250° C. × 
                 1200° C. × 
                 1200° C. × 
               
               
                   
                 Step B 
                 1 hr 
                 1 hr 
                 1 hr 
                 1 hr 
                 1 hr 
               
               
                   
                 Maintaining 
                 1400° C. × 
                 1360° C. × 
                 1380° C. × 
                 1400° C. × 
                 1400° C. × 
               
               
                   
                 Step C 
                 2 hrs 
                 2 hrs 
                 2 hrs 
                 2 hrs 
                 2 hrs 
               
               
                   
                 Heating Rate (° C./hr) 
                 50 
                 50 
                 50 
                 50 
                 50 
               
               
                   
                 from Maintaining Step B 
               
               
                   
                 to Maintaining Step C 
               
               
                   
                 (Maximum Temperature) 
               
               
                 Ceramic 
                 BaTiO 3 -based Crystalline 
                 96.8 
                 97.0 
                 96.9 
                 97.3 
                 97.2 
               
               
                 Body 
                 Particle (% by mass) 
               
               
                   
                 Ba 6 Ti 17 O 40  Crystalline 
                 1.9 
                 1.8 
                 2.1 
                 3.9 
                 1.6 
               
               
                   
                 Particle (% by mass) 
               
               
                   
                 BaCO 3  Crystalline 
                 1.3 
                 1.2 
                 1.0 
                 1.0 
                 1.2 
               
               
                   
                 Particle (% by mass) 
               
               
                   
                 Ba 2 TiO 4  Crystalline 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                   
                 Particle (% by mass) 
               
               
                   
                 La Atomic Ratio (x 
                 0.001 
                 0.001 
                 0.001 
                 0.002 
                 0.004 
               
               
                   
                 value) of BaTiO 3 -based 
               
               
                   
                 Crystalline Particle 
               
               
                   
                 (Ba + La)/Ti Ratio 
                 1.010 
                 1.010 
                 1.010 
                 1.010 
                 1.010 
               
               
                   
                 of BaTiO3-based 
               
               
                   
                 Crystalline Particle 
               
               
                   
                 Lattice Volume of BaTiO 3 - 
                 64.3945 
                 64.3923 
                 64.3920 
                 64.3887 
                 64.3685 
               
               
                   
                 based Crystalline Particle (Å 3 ) 
               
               
                   
                 Average Crystal Grain Size 
                 12 
                 5 
                 6 
                 8 
                 5 
               
               
                   
                 (μm) of BaTiO 3 -based 
               
               
                   
                 Crystalline Particle 
               
               
                   
                 Open Porosity (%) 
                 3.3 
                 2.4 
                 4.0 
                 4.9 
                 3.0 
               
               
                   
                 Bulk Density (g/cm 3 ) 
                 5.35 
                 5.36 
                 5.40 
                 5.49 
                 5.65 
               
               
                   
                 Volume Resistivity 
                 69 
                 101 
                 45 
                 30 
                 150 
               
               
                   
                 at R.T (Ω · cm) 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 Examples 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 6 
                 7 
                 8 
                 9 
                 10 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Relative Density (%) of 
                 64.8 
                 64.8 
                 63.4 
                 63.4 
                 62.8 
               
               
                 Ceramic Formed Body 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Firing 
                 Maintaining 
                 950° C. × 
                 950° C. × 
                 950° C. × 
                 900° C. × 
                 950° C. × 
               
               
                 Step 
                 Step A 
                 1 hr 
                 1 hr 
                 1 hr 
                 1 hr 
                 1 hr 
               
               
                   
                 Maintaining 
                 1200° C. × 
                 1200° C. × 
                 1200° C. × 
                 1150° C. × 
                 1200° C. × 
               
               
                   
                 Step B 
                 1 hr 
                 1 hr 
                 1 hr 
                 1 hr 
                 1 hr 
               
               
                   
                 Maintaining 
                 1400° C. × 
                 1400° C. × 
                 1430° C. × 
                 1430° C. × 
                 1400° C. × 
               
               
                   
                 Step C 
                 2 hrs 
                 2 hrs 
                 2 hrs 
                 2 hrs 
                 2 hrs 
               
               
                   
                 Heating Rate (° C./hr) 
                 50 
                 200 
                 200 
                 300 
                 200 
               
               
                   
                 from Maintaining Step B 
               
               
                   
                 to Maintaining Step C 
               
               
                   
                 (Maximum Temperature) 
               
               
                 Ceramic 
                 BaTiO 3 -based Crystalline 
                 94.6 
                 95.0 
                 94.9 
                 94.6 
                 95.9 
               
               
                 Body 
                 Particle (% by mass) 
               
               
                   
                 Ba 6 Ti 17 O 40  Crystalline 
                 5.1 
                 4.0 
                 4.1 
                 4.2 
                 3.9 
               
               
                   
                 Particle (% by mass) 
               
               
                   
                 BaCO 3  Crystalline 
                 0.3 
                 1.0 
                 1.0 
                 1.2 
                 0.3 
               
               
                   
                 Particle (% by mass) 
               
               
                   
                 Ba 2 TiO 4  Crystalline 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                   
                 Particle (% by mass) 
               
               
                   
                 La Atomic Ratio (x 
                 0.001 
                 0.001 
                 0.001 
                 0.001 
                 0.001 
               
               
                   
                 value) of BaTiO 3 -based 
               
               
                   
                 Crystalline Particle 
               
               
                   
                 (Ba + La)/Ti Ratio 
                 1.030 
                 1.030 
                 1.050 
                 1.050 
                 1.030 
               
               
                   
                 of BaTiO3-based 
               
               
                   
                 Crystalline Particle 
               
               
                   
                 Lattice Volume of BaTiO 3 - 
                 64.3937 
                 64.3910 
                 64.3933 
                 64.3962 
                 64.3947 
               
               
                   
                 based Crystalline Particle (Å 3 ) 
               
               
                   
                 Average Crystal Grain Size 
                 96 
                 20 
                 150 
                 120 
                 60 
               
               
                   
                 (μm) of BaTiO 3 -based 
               
               
                   
                 Crystalline Particle 
               
               
                   
                 Open Porosity (%) 
                 1.1 
                 2.2 
                 4.2 
                 3.6 
                 4.0 
               
               
                   
                 Bulk Density (g/cm 3 ) 
                 5.88 
                 5.66 
                 5.39 
                 5.50 
                 5.44 
               
               
                   
                 Volume Resistivity 
                 24 
                 14 
                 23 
                 25 
                 14 
               
               
                   
                 at R.T (Ω · cm) 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Examples 
                 Comparative Examples 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 11 
                 1 
                 2 
                 3 
                 4 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Relative Density (%) of 
                 62.8 
                 63.6 
                 65.6 
                 64.8 
                 62.8 
               
               
                 Ceramic Formed Body 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Firing 
                 Maintaining 
                 950° C. × 
                 950° C. × 
                 950° C. × 
                 950° C. × 
                 950° C. × 
               
               
                 Step 
                 Step A 
                 1 hr 
                 1 hr 
                 1 hr 
                 1 hr 
                 1 hr 
               
               
                   
                 Maintaining 
                 1200° C. × 
                 1200° C. × 
                 1200° C. × 
                 1200° C. × 
                 1200° C. × 
               
               
                   
                 Step B 
                 1 hr 
                 1 hr 
                 1 hr 
                 1 hr 
                 1 hr 
               
               
                   
                 Maintaining 
                 1400° C. × 
                 1350° C. × 
                 1400° C. × 
                 1440° C. × 
                 1400° C. × 
               
               
                   
                 Step C 
                 2 hrs 
                 2 hrs 
                 2 hrs 
                 2 hrs 
                 2 hrs 
               
               
                   
                 Heating Rate (° C./hr) 
                 50 
                 50 
                 50 
                 50 
                 200 
               
               
                   
                 from Maintaining Step B 
               
               
                   
                 to Maintaining Step C 
               
               
                   
                 (Maximum Temperature) 
               
               
                 Ceramic 
                 BaTiO 3 -based Crystalline 
                 95.5 
                 95.3 
                 99.6 
                 100 
                 89.1 
               
               
                 Body 
                 Particle (% by mass) 
               
               
                   
                 Ba 6 Ti 17 O 40  Crystalline 
                 4.1 
                 0.0 
                 0.0 
                 0.0 
                 0.0 
               
               
                   
                 Particle (% by mass) 
               
               
                   
                 BaCO 3  Crystalline 
                 0.4 
                 4.5 
                 0.4 
                 0.0 
                 0.6 
               
               
                   
                 Particle (% by mass) 
               
               
                   
                 Ba 2 TiO 4  Crystalline 
                 0.0 
                 0.2 
                 0.0 
                 0.0 
                 0.0 
               
               
                   
                 Particle (% by mass) 
               
               
                   
                 La Atomic Ratio (x 
                 0.001 
                 0.001 
                 0.0008 
                 0.001 
                 0.001 
               
               
                   
                 value) of BaTiO 3 -based 
               
               
                   
                 Crystalline Particle 
               
               
                   
                 (Ba + La)/Ti Ratio 
                 1.030 
                 0.940 
                 0.990 
                 0.990 
                 1.070 
               
               
                   
                 of BaTiO3-based 
               
               
                   
                 Crystalline Particle 
               
               
                   
                 Lattice Volume of BaTiO 3 - 
                 64.3939 
                 64.3956 
                 64.3905 
                 64.3928 
                 64.3971 
               
               
                   
                 based Crystalline Particle (Å 3 ) 
               
               
                   
                 Average Crystal Grain Size 
                 50 
                 3 
                 6 
                 6 
                 50 
               
               
                   
                 (μm) of BaTiO 3 -based 
               
               
                   
                 Crystalline Particle 
               
               
                   
                 Open Porosity (%) 
                 2.8 
                 5.0 
                 5.3 
                 4.2 
                 3.8 
               
               
                   
                 Bulk Density (g/cm 3 ) 
                 5.46 
                 5.26 
                 5.48 
                 5.55 
                 5.77 
               
               
                   
                 Volume Resistivity 
                 10 
                 2604 
                 4000 
                 1326 
                 3638 
               
               
                   
                 at R.T (Ω · cm) 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, each of the ceramic bodies according to Examples 1 to 11 had a significantly lower volume resistivity at room temperature than that of each of the ceramic bodies of Comparative Examples 1 to 4. In particular, the ceramic bodies according to Examples 4 and 6 to 11 were able to reduce the volume resistivity at room temperature to 30 Ω·cm or less. 
     As can be seen from the above results, according to the present invention, it is possible to provide a ceramic body having low electrical resistance at room temperature and having a PTC property, and a method for producing the same. Also, according to the present invention, it is possible to provide a heater element, a heater unit, a heater system and a purification system, which have the above ceramic body. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           10  Honeycomb Structure 
           11  outer peripheral wall 
           12  partition wall 
           13   a  first end face 
           13   b  second end face 
           14  cell 
           17  honeycomb joined body 
           18  honeycomb segment 
           19  Joining layer 
           20  electrode 
           30  external connecting member 
           50  adsorbent 
           100 , 200  heater element 
           600 , 700 , 800  heater unit 
           610  housing 
           620  insulating material 
           650 ,  1110  inflow port 
           660 ,  1120  outflow port 
           900  heater system 
           910  vehicle interior 
           920   a,    920   b,    1300  Inflow pipe 
           921   a,    921   b  valve 
           930 ,  1400  outflow pipe 
           940 ,  1200  battery 
           950 ,  1210  electric wire 
           960  vapor compression heat pump 
           961  evaporator 
           962  condenser 
           970  blower 
           1000  purification system 
           1100  heater element or heater unit 
           1500  switching valve 
           1600  control unit