Patent Publication Number: US-2015059306-A1

Title: Honeycomb structure

Description:
TECHNICAL FIELD 
     The present invention relates to a honeycomb structure used as a filter that purifies a gas. 
     BACKGROUND ART 
     A honeycomb structure is widely used as a filter that purifies an exhausted gas from an internal combustion engine, such as a diesel particulate filter (see Patent Literature 1, for example). Since soot removed from an exhaust gas builds up in the honeycomb structure, filter regeneration in which the soot is burned is required every fixed period. To burn soot, a large amount of hot combusted exhaust gas may be supplied to ignite the soot and burn it completely. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-Open Publication No. 2009-202143 
     SUMMARY OF INVENTION 
     Technical Problem 
     When a honeycomb structure is heated beyond an acceptable level by burning the soot in the filter regeneration, excessive thermal stress may occur and break the honeycomb structure. To avoid such breakage, a technology for moderating the burning of the soot in the filter regeneration is required. 
     The present invention has been made in view of the problem described above, and an object of the present invention is to provide a honeycomb structure that allows soot to be mildly burned in filter regeneration. 
     Solution to Problem 
     To achieve the object described above, the present invention relates to a honeycomb structure comprising a first end surface and a second end surface facing each other and a partition wall that forms a plurality of first channels and a plurality of second channels extending in a facing direction between the first end surface and the second end surface. The plurality of first channels are open at the first end surface and closed at the second end surface, and the second channels are closed at the first end surface and open at the second end surface. The plurality of first channels are so provided that the first channels surround the second channels and so disposed that the first channels are adjacent to the second channels, at the first end surface. The partition wall includes standard walls that separate the first channels and the second channels from each other and a common wall that separates two of the first channels adjacent to each other at the first end surface. The standard walls are thicker than the common walls. 
     According to the honeycomb structure described above, since the standard walls are formed to be thicker than the common walls, a gas having flowed through the openings at the first end surface into the first channels does not tend to pass through the standard walls as compared with a case where the standard walls are as thick as the common walls. Therefore, according to the honeycomb structure described above, when a hot gas flows into the first channels to burn soot in the filter regeneration, the burned gas (carbon dioxide gas, for example) produced by the burning does not tend to pass through the standard walls and exits into the second channels, whereby the burned gas that stays in the first channels suppresses further oxygen supply, and the burning of the soot is suppressed or the soot is burned mildly. As a result, a situation in which the honeycomb structure is heated beyond an acceptable level and damaged by excessive thermal stress does not occur, whereby the reliability of the honeycomb structure can be improved. 
     In the honeycomb structure according to the present invention, at least five of the first channels may be so provided that the at least five of the first channels surround the second channels and so disposed that the at least five of the first channels are adjacent to the second channels, at the first end surface. 
     According to the honeycomb structure described above, a high-opening-ratio, efficient channel arrangement is achieved. 
     In the honeycomb structure according to the present invention, all the standard walls between one of the second channels and the first channels that surround the second channel may be thicker than all the common walls between the first channels that surround the second channel. 
     According to the honeycomb structure described above, since all the standard walls around one of the second channel are thicker than all the common walls between the first channels that surround the second channel, the burned gas does not tend to exit from the first channels to the second channel, whereby soot can be burned more mildly. 
     In the honeycomb structure according to the present invention, areas formed by the common walls in the first channels may be smaller than areas formed by the standard walls in the first channels, in the partition wall that forms the first channels. 
     According to the honeycomb structure described above, since the areas of the common walls, which form the first channels, are smaller than the areas of the standard walls, which form the first channels, the soot layer that builds up on the common walls is thicker than the soot layer that builds up on the standard walls. As a result, when soot is burned in the first channels in the filter regeneration, burning of the soot layers having built up to a large thickness on the common walls are not proceeded so fast, whereby the soot is not burned up in a short period and hence the soot can be burned more mildly. 
     Advantageous Effects of Invention 
     Any of the honeycomb structures according to the present invention allows soot to be mildly burned in filter regeneration. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a honeycomb structure according to a first embodiment. 
         FIG. 2  is a cross-sectional view taken along the line II-II in  FIG. 1 . 
         FIG. 3  is an enlarged view for describing the arrangement of channels at a first end surface. 
         FIG. 4  shows part of a first end surface of a honeycomb structure according to a second embodiment. 
         FIG. 5  is an enlarged view for describing the arrangement of channels of the honeycomb structure according to the second embodiment. 
         FIG. 6  shows part of a first end surface of a honeycomb structure according to a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the present invention will be described below in detail with reference to the drawings. 
     First Embodiment 
     A honeycomb structure  100  according to a first embodiment is a cylindrical structure used as a filter that purifies an exhaust gas from an internal combustion engine, such as a diesel engine and a gasoline engine, as shown in  FIGS. 1 and 2 . The cylindrical honeycomb structure  100  has a first end surface  100   a  and a second end surface  100   b,  which face each other, and a partition wall  112 , which forms a plurality of channels  110 . 
     The channels  110  are closed by closing members  111  at one of the first end surface  100   a  and the second end surface  100   b,  as shown in  FIG. 2 . Specifically, the plurality of channels  110  are grouped into first channels  110   a,  which are open at the first end surface  100   a  and closed at the second end surface  100   b,  and second channels  110   b,  which are closed at the first end surface  100   a  and open at the second end surface  100   b.    
     The plurality of first channels  110   a  and the plurality of second channels  110   b  are channels extending in the direction in which the first end surface  100   a  and the second end surface  100   b  face each other and have a regular hexagonal cross-sectional shape (cross-sectional shape perpendicular to the direction in which channels  110   a  and  110   b  extend). 
     The first channels  110   a  and the second channels  110   b  are so arranged that the first channels  110   a  surround the second channels  110   b  at the first end surface  100   a.  Specifically, six first channels  110   a  adjacent to each other are so arranged that they surround one second channel  110   b  at the first end surface  100   a.  Each of the six first channels  110   a  are so arranged that they are adjacent to the one second channel  110   b.    
     The thus configured honeycomb structure  100  is disposed, for example, in an exhaust gas channel of an internal combustion engine with the first end surface  100   a  located on the gas upstream side (side facing internal combustion engine) and the second end surface  100   b  located on the gas downstream side (exhaust side). The arrows G represent a primary flow of the exhaust gas passing through the honeycomb structure  100 , which functions as a filter. 
     The exhaust gas from the internal combustion engine flows into the first channels  110   a  through the openings at the first end surface  100   a,  as indicated by the arrows G. The gas having flowed into the channels  110   a  passes through the partition wall  112  into the second channels  110   b  because the channels  110   a  are closed at the second end surface  100   b.  At this point, soot in the exhaust gas is trapped by the partition wall  112 . The gas from which soot has been removed passes through the second channels  110   b  and flows outside through the openings of the second channels  110   b  at the second end surface  100   b.    
     The honeycomb structure  100 , which functions as a filer, is composed, for example, of a porous ceramic material (20 μm in average pore diameter or smaller, for example). Examples of the ceramic material used for the honeycomb structure  100  include oxides such as alumina, silica, mullite, cordierite, glass, and aluminum titanate; silicon carbide; silicon nitride; and metals. The aluminum titanate can further contain magnesium and/or silicon. 
     The honeycomb structure  100  is produced by burning a green molded body (pre-burned molded body) which becomes the ceramic materials described above after extrusion molding thereof, and then performing a predetermined closing treatment. The green molded body contains, for example, inorganic compound source powder that is a raw material of the ceramic material, organic binder such as methyl cellulose, and an additive added as required. 
     In the case of a green molded body made of an aluminum titanate, the inorganic compound source powder contains aluminum source powder such as α-alumina powder and titanium source powder such as anatase-type or rutile-type titania powder and can further contain magnesium source powder such as magnesia powder, and magnesia spinel powder, and/or silicon source power such as silicon oxide powder and glass flit, as required. 
     Examples of the organic binder include celluloses such as methyl cellulose, carboxymethyl cellulose, hydroxyalkyl methyl cellulose, and sodium carboxymethyl cellulose; alcohols such as polyvinyl alcohol; and lignin sulfonate. 
     Examples of the additive include a pore former, a lubricant, a plasticizer, a disperser, and a solvent. 
     Examples of the pore former include carbon materials such as graphite; resins such as polyethylene, polypropylene, polymethylmethacrylate; plant materials such as starch, the shell of a nut, the shell of a walnut and corn; ice; and dry ice. 
     Examples of the lubricant and the plasticizer include alcohols such as glycerin; higher fatty acids such as caprylic acid, lauric acid, palmitic acid, arachidic acid, oleic acid and stearic acid; metal salt stearates such as aluminum stearate, and polyoxy alkylene alkyl ether (POAAE). 
     Examples of the disperser include inorganic acids such as nitric acid, hydrochloric acid and sulfuric acid; organic acids such as oxalic acid, citric acid, acetic acid, malic acid and lactic acid; alcohols such as methanol, ethanol and propanol; surfactant such as polycarboxylic aluminum. 
     Examples of the solvent include alcohols such as methanol, ethanol, butanol and propanol; glycols such as propylene glycol, polypropylene glycol and ethylene glycol; and water. 
     The material of the closing members  111  may be the same as the material of the green molded body described above or may be different therefrom. The closing members  111  may instead be composed of a material through which the exhaust gas from the internal combustion engine cannot pass. 
     The channels  110  and the partition wall  112  of the honeycomb structure  100  will subsequently be described in detail. 
       FIG. 3  is a figure for describing the arrangement of the channels  110  at the first end surface  100   a.    FIG. 3  shows channels  121  and  122  of the plurality of first channels  110   a  and a channel  123  of the plurality of second channels  110   b  by way of example. 
     The partition wall  112  has standard walls  112   a,  which separate the first channels  121  and  122  from the second channel  123  adjacent to each other, and a common wall  112   b,  which separates the adjacent two first channels  121  and  122  from each other, as shown in  FIG. 3 . The first channels  121  and  122  are formed by the standard walls  112   a  and the common walls  112   b,  and the second channel  123  is formed only by the standard walls  112   a.    
     In the honeycomb structure  100 , the thickness t s  of the standard wall  112   a  is set to be greater than the thickness t c  of the common walls  112   b.  That is, the distance between the first channels  121 ,  122  and the second channel  123  is set to be greater than the distance between the first channels  121  and  122 . A gas in the first channels  121  and  122  therefore does not tend to flow into the second channel  123  as compared with a case where the thickness t s  of the standard walls  112   a  is equal to the thickness t c  of the common walls  112   b.    
     The thickness of all the standard walls  112   a  between the plurality of first channels  110   a,  which surround the second channels  123 , and the second channels  123  is set to be greater than the thickness of all the common walls  112   b  between the first channels  110   a  adjacent to each other at the first end surface  100   a,  as in the case of the first channels  121  and  122 , as shown in  FIG. 1 . The thickness t s  of the standard walls  112   a  and the thickness t c  of the common walls  112   b  preferably satisfy the relationship 0.1&lt;t c /t s &lt;0.9. 
     The exhaust gas having flowed through the openings at the first end surface  100   a  into the first channels  121  and  122  passes through the standard walls  112   a,  flows into the second channel  123 , and then exits outside through the openings at the second end surface  100   b,  as described above. 
     In this process, soot contained in the exhaust gas is trapped by the standard walls  112   a,  and a soot layer Sa shown in  FIG. 3  is formed by the trapped soot. The soot layer Sa is formed on standard wall surfaces  121   a  and  122   a  of the first channels  121  and  122 . The standard wall surface  121   a  is a surface that is part of the first channel  121  and formed by the corresponding standard wall  112   a,  and the standard wall surface  122   a  is a surface that is part of the first channel  122  and formed by the corresponding standard wall  112   a.  Similarly, the second channel  123  has standard wall surfaces  123   a  and  123   b  formed by the standard walls  112   a.    
     The standard wall surface  123   a  of the second channel  123  is a surface that faces the standard wall surface  121   a  of the first channel  121  with the corresponding standard wall  112   a  therebetween. Further, the standard wall surface  123   b  of the second channel  123  is a surface that faces the standard wall surface  122   a  of the first channel  122  with the corresponding standard wall  112   a  therebetween. In other words, the portion of the partition wall  112  that is sandwiched between the standard wall surfaces  121   a,    122   a  of the first channels  121  and  122  and the standard wall surfaces  123   a,    123   b  of the second channel  123  forms the standard walls  112   a.    
     The thickness t s  of the standard walls  112   a  corresponds to the distance between the standard wall surfaces  121   a,    122   a  of the first channels  121 ,  122  and the standard wall surfaces  123   a,    123   b  of the second channel  123 . 
     Further, part of the soot contained in the exhaust gas adheres to the common wall  112   b  while flowing through the first channels  121  and  122  to form a soot layer Sb. The soot layer Sb is formed on common wall surfaces  121   b  and  122   b  of the first channels  121  and  122 . The common wall surface  121   b  is a surface that is part of the first channel  121  and formed by the common wall  112   b,  and the common wall surface  122   b  is a surface that is part of the first channel  122  and formed by the common wall  112   b.    
     The common wall surface  121   b  and the common wall surface  122   b  face each other with the common wall  112   b  therebetween. In other words, the portion of the partition wall  112  that is sandwiched between the common wall surface  121   b  and the common wall surface  122   b  forms the common wall  112   b.    
     The thickness t c  of the common wall  112   b  corresponds to the distance between the common wall surface  121   b  and the common wall surface  122   b.  Although not shown, soot builds up on the other surfaces that form the first channels  121  and  122 . 
     In the honeycomb structure  100 , filter regeneration that allows restoration of the filter function by burning the soot layers Sa and Sb and other soot layers with a hot gas is performed. The arrows A and B indicate an example of the flow of the hot gas in the filter regeneration. 
     In the filter regeneration, after a hot gas produced by the internal combustion engine flows into the first channels  121  and  122 , part of the hot gas circulates as indicated by the arrows A and moves along the surfaces of the first channels  121  and  122  to facilitate burning of the soot layers Sb. A burned gas produced by the burning of the soot layers Sb passes through the standard walls  112   a  while burning the soot layers Sa and flows into the second channel  123 , as indicated by the arrows B. The burned gas having flowed into the second channel  123  exits outside through the opening at the second end surface  100   b.  In addition, part of the hot gas directly flows to the soot layers Sa along the arrows B, passes through the standard walls  112   a  while burning the soot layers Sa, flows into the second channel  123 , and exits outside. 
     In the honeycomb structure  100  according to the first embodiment described above, since the thickness ts of the standard walls  112   a  is set to be greater than the thickness t c  of the common walls  112   b,  the gas having flowed through the openings at the first end surface  100   a  into the first channels  110   a  does not tend to pass through the standard walls  112   a  as compared with a case where the thickness t s  of the standard walls  112   a  is equal to the thickness t c  of the common walls  112   b,  and the gas does not tend to enter the second channels  110   b,  which are open at the second end surface  100   b.  Therefore, according to the honeycomb structure  100 , when the hot gas flows into the first channels  110   a  to burn soot in the filter regeneration, the burned gas (carbon dioxide gas, for example) produced by the burning does not tend to pass through the standard walls  112   a  or exit into the second channels  110   b,  whereby the burned gas that stays in the first channels  110   a  suppresses further oxygen supply, and the burning of the soot is suppressed or the soot is burned mildly. As a result, a situation in which the honeycomb structure  100  is heated beyond an acceptable level and damaged by excessive thermal stress does not occur, whereby the reliability of the honeycomb structure  100  can be improved. 
     Further, since six first channels  110   a  are so disposed that they surround one second channel  110   b  and are adjacent to the second channel  110   b  at the first end surface  100   a,  the channels  110  having a hexagonal cross-sectional shape can be efficiently arranged with a high opening ratio. 
     Second Embodiment 
     A honeycomb structure  101  according to a second embodiment differs from the honeycomb structure  100  according to the first embodiment in terms of the cross-sectional shape of the first channels.  FIG. 4  shows part of a first end surface  101   a  of the honeycomb structure  101  according to the second embodiment. 
     A plurality of channels  130  of the honeycomb structure  101  according to the second embodiment are grouped into first channels  130   a,  which are open at the first end surface  101   a  and closed at a second end surface (not shown), and second channels  130   b,  which are closed at the first end surface  101   a  and open at the second end surface, as shown in  FIG. 4 . The second channels  130   b  are closed by closing members  134 . 
     The first channels  130   a  and the second channels  130   b  are so disposed at the first end surface  101   a  that the first channels  130   a  surrounds the second channels  130   b.  Specifically, six first channels  130   a  adjacent to each other are so disposed at the first end surface  101   a  that they surround one second channel  130   b.  The six first channels  130   a  are so disposed that each of them is adjacent to the one second channel  130   b.    
     The second channels  130   b  are channels having a regular hexagonal cross-sectional shape as in the first embodiment. On the other hand, the first channels  130   a  have a ordered hexagonal cross-sectional shape (for example, hexagonal shape having a shorter edge facing a edge of the hexagonal cross-sectional shape of an adjacent second channel  130   b  and a longer edge longer than the shorter edge with the longer edges and the shorter edges facing each other). 
     The first channels  130   a  and the second channels  130   b  are so disposed that the longer edges of the ordered hexagonal cross-sectional shape of the six first channels  130   a  faces each edge of the regular hexagonal cross-sectional shape of the second channel  130   b.    
       FIG. 5  is a figure for describing the arrangement of the channels  130  at the first end surface  101   a.    FIG. 5  shows channels  131  and  132  of the plurality of the first channels  130   a  and a channel  133  of the plurality of the second channels  130   b  by way of example. 
     A partition wall  135  is grouped into standard walls  135   a,  which separate the first channels  131 ,  132  from the second channel  133  adjacent to each other, and a common wall  135   b,  which separates the adjacent two first channels  131  and  132  from each other, as shown in  FIG. 3 . 
     The first channels  131  and  132  are channels formed by the standard walls  135   a  and the common walls  135   b,  and the second channel  133  is a channel formed only by the standard walls  135   a.    
     In the honeycomb structure  101  according to the second embodiment, the thickness t s  of the standard walls  135   a  is set to be greater than the thickness t c  of the common walls  135   b,  as in the first embodiment. That is, the distance between the first channels  131 ,  132  and the second channel  133  is set to be greater than the distance between the first channels  131  and  132 . 
     Further, in the honeycomb structure  101  according to the second embodiment, the area of a common wall surface  131   b  of the first channel  131  is set to be smaller than the area of a standard wall surface  131   a  of the first channel  131 . That is, the area formed by the common wall  135   b  is smaller than the area formed by the standard wall surface  131   a,  in the standard walls  135   a  and the common walls  135   b  that form the first channel  131 . This means that the area of the standard wall surface  131   a,  through which burned soot gas exits, is greater than the area of the common wall surface  131   b,  on which soot builds up. 
     Further, also in the honeycomb structure  101  according to the second embodiment, the thickness t s  of the standard wall  135   a  between the standard wall surface  131   a  of the first channel  131  and a standard wall surface  133   a  of the second channel  133  is uniform and set to be greater than the thickness t c  of the common walls  135   b.    
     In the honeycomb structure  101  according to the second embodiment described above, since the thickness t s  of the standard walls  135   a  is set to be greater than the thickness t c  of the common walls  135   b,  the burned gas in the first channels  131  and  132  does not tend to pass through the standard walls  135   a  in the filter regeneration as compared with a case where the thickness t s  of the standard walls  135   a  is equal to the thickness t c  of the common walls  135   b.  Therefore, in the honeycomb structure  101  according to the second embodiment, further oxygen supply into the first channels  131  and  132  is suppressed from the same reason as in the honeycomb structure  100  according to the first embodiment, whereby the burning of the soot is suppressed or the soot is burned mildly. 
     Further, in the honeycomb structure  101  according to the second embodiment, since the area of the common wall surface  131   b,  which forms the first channel  131 , is smaller than the area of the standard wall surface  131   a,  which forms the first channel  131 , the soot layer that builds up on the common wall surface  131   b  is thicker than the soot layer that builds up on the standard wall surface  131   a.  As a result, when the hot gas flows into the first channel  131  in the filter regeneration and the soot is burned from the surface of the soot layer Sb as indicated by the arrow A, burning of the soot layer Sb having built up to a large thickness is not proceeded so fast, whereby the soot is not burned up in a short period and hence the soot can be burned more mildly. 
     Further, in the honeycomb structure  101  according to the second embodiment, which employs an asymmetric cell structure (asymmetric grid structure) having channels having different cross-sectional shapes, the filter area per unit volume of the filter can be larger than that of a symmetric cell structure. A large filter area reduces loss in the pressure of the exhaust gas, which is advantageous in improvement in fuel consumption of the internal combustion engine. 
     Third Embodiment 
     A honeycomb structure  102  according to a third embodiment differs from the honeycomb structure  101  according to the second embodiment in terms of the cross-sectional shape of first channels  140   a.    FIG. 6  shows part of a first end surface  102   a  of the honeycomb structure  102  according to the third embodiment. 
     Specifically, the first channels  140   a  of the honeycomb structure  102  according to the third embodiment have a flat hexagonal cross-sectional shape (for example, hexagonal shape having a longer edge facing a edge of a regular hexagonal cross-sectional shape of an adjacent second channel  140   b  and a shorter edge shorter than the longer edge with two longer edges facing each other and two sets of two shorter edges facing each other), as shown in  FIG. 6 . 
     The first channels  140   a  and the second channels  140   b  are so disposed that one of the longer edges of the flat hexagonal cross-sectional shape of each of six first channels  140   a  faces the corresponding edge of the regular hexagonal cross-sectional shape of the second channel  140   b.    
     In the honeycomb structure  102  according to the third embodiment, standard walls  145   a  are thicker than common walls  145   b,  as in the first embodiment. Viewed from each of the first channels  140   a,  the standard walls  145   a  are a part of the partition wall corresponding to the respective shorter edges of the flat hexagonal cross-sectional shape, and the common wall  145   b  is a part of the partition wall corresponding to the shorter edges of the flat hexagonal cross-sectional shape. 
     In the honeycomb structure  102  according to the third embodiment, the standard walls  145   a  and the common walls  145   b,  which form each of the first channels  140   a,  are so configured that the area formed by each of the common walls  145   b  is smaller than the area formed by a standard wall surface  141   a,  as in the second embodiment, as in the honeycomb structure  102  according to the second embodiment. 
     The honeycomb structure  102  according to the third embodiment described above can also provide the same advantageous effects provided by the honeycomb structure  101  according to the second embodiment. 
     The embodiments according to the present invention have been described above, but the present invention is not limited thereto. For example, the honeycomb structure does not necessarily have a cylindrical shape and may have a columnar shape having, for example, an oval or polygonal cross-sectional shape. Further, each of the channels does not necessarily have a hexagonal cross-sectional shape and may have any other polygonal shape or a circular or elliptical shape. Moreover, the cross-sectional shape of the plurality of first channels may differ from the cross-sectional shape of the plurality of second channels, and each of the plurality of first channels (or plurality of second channels) may contain a channel having a different cross-sectional shape. 
     Further, the arrangement of the channels are not limited to those described above. From a viewpoint of efficiency, at least five first channels may be so provided that they surround one second channel and are adjacent thereto at the first end surface. In the second embodiment, which has been described with reference to the case where the area of the common wall surface that forms each of the first channels is smaller than the area of the standard wall surface that forms the first channel, but the area of the common wall surface that forms each of the first channels may be greater than the area of the standard wall surface that forms the first channel. 
     Further, the thickness of the standard walls may vary depending on the position therealong, and the thickness of the common walls may vary depending on the position therealong. In this case, a minimum thickness of all the standard walls that form one second channel only needs to be greater than a maximum thickness of all the common walls between the first channels that surround the second channel. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to a honeycomb structure that allows soot to be mildly burned in filter regeneration. 
     REFERENCE SIGNS LIST 
       100 ,  101 ,  102  . . . Honeycomb structure,  100   a,    101   a,    102   a  . . . First end surface,  100   b  . . . Second end surface,  110 ,  130 ,  140  . . . Channel,  110   a,    121 ,  122 ,  130   a,    131 ,  132 ,  140   a,    141 ,  142  . . . First channel,  110   b,    123 ,  130   b,    133  . . . Second channel,  111  . . . Closing member,  112 ,  135 ,  145  . . . Partition wall,  112   a,    135   a,    145   a  . . . Standard wall,  112   b,    135   b,    145   b  . . . Common wall,  121   a,    122   a,    123   a,    123   b,    131   a,    132   a  . . . Standard wall surface,  121   b,    122   b,    131   b,    132   b  . . . Common wall surface, Sa, Sb . . . Soot layer