Patent Publication Number: US-2023136926-A1

Title: Porous structure such as for filters, and making the same

Description:
BACKGROUND 
     Aspects of the present disclosure generally relate to structures formed from glass bubbles that have been breached, and which may be used with filters. 
     Tiny glass bubbles, also called microballoons or hollow glass “microspheres,” are commercially available, such as from Dennert Forayer GmbH, 3M, Zhongke Yali Technology, Ltd, Fibre Glast Developments Corp., Potters Industries LLC, and others. Such glass bubbles may be used as filler in composite materials, such as concrete. 
     Glass bubbles can be characterized by “diameter,” where diameter refers to the diameter if volume of the glass bubble was arranged in a perfect spherical geometry. In practice, however, glass bubbles may only be generally spherical, such as having a potato-shape, for example. Size of glass bubbles may be selected and characterized based on the diameter, where “D50” particle size corresponds to a 50% pass point of glass bubbles having a diameter of the D50 value, where half in a group are larger and half are smaller in diameter than the D50 value. Likewise, “d50” corresponds to a 50% pore size of a porous structure, and d10 corresponds to a 10% pass point, as measured by ASTM standard with mercury intrusion. Accordingly, the ratio of (d50−d10)/d50 provides insight into pore size. 
     Glass bubbles are generally fragile and conventional practices teach methods to prevent breakage of the glass bubbles so as to maintain internal closed cavities of the glass bubbles, preserving the correspondingly low weight-to-volume relationships that glass bubbles may provide. When integrity of the glass bubbles is maintained, the glass bubbles may be integrated in composite materials for buoyant, load-bearing structures, such as surf boards or supports for offshore drilling equipment. 
     SUMMARY 
     Despite their conventional uses, Applicants discovered glass bubbles may be arranged and processed to make particularly efficient porous structures with open porosity, such as for filters. Structures with open porosity may be formed from tightly packing glass bubbles together, bonding the glass bubbles to one another and also breaching (e.g., breaking, popping, fracturing, opening, exposing hollow cores thereof) the glass bubbles. Voids of the individual glass bubbles open into one another to form porous cavities that extend and interconnect through the overall structure and may open to surfaces thereof. Such structures may be particularly useful with filters, or may be used for other purposes, such as providing a glass skeleton infiltrated with polymer, for example. 
     In some embodiments, a method of making a porous structure, which is configured for use in a filter, includes steps of breaching a plurality of glass bubbles (e.g., at least 100, at least 1000, at least 10,000 glass bubbles) and bonding the plurality of glass bubbles to one another. In aggregate, the bonded, breached glass bubbles form the porous structure, where voids within individual breached glass bubbles open into one another to form cavities that extend through the porous structure and to surfaces thereof. In some such embodiments, the breaching includes expanding gasses within the glass bubbles to rupture the glass bubbles. In other such embodiments, the breaching includes devitrification of glass of the glass bubbles, where softening and movement of amorphous glass relative to the devitrified glass ruptures the glass bubbles. In other such embodiments, the method includes a step of heating the plurality of glass bubbles to at least a softening temperature of amorphous glass of the glass bubbles. The heating may be such that adjoining glass bubbles sinter to one another. The breaching may occur concurrently during the heating. In some embodiments, the method includes a step of cooling the plurality of glass bubbles with adjoining glass bubbles physically bonded directly to one another. Timing and temperatures of the heating and/or cooling may devitrify at least some of the glass of the glass bubbles so that crystals form. Applicants believe that devitrification may aid in rupture of the glass bubbles, such as by limiting shrinkage of glass bubbles under negative core pressures. 
     In some such embodiments, prior to the heating step, the method of making a porous structure includes a step of extruding green material that includes the glass bubbles and an organic binder. Most of the glass bubbles survive the extruding without fracturing. In some such embodiments, heating burns out or chemically changes most of the organic binder in terms of weight from the porous structure. During the heating, the glass bubbles are heated at a temperature increased from ambient temperature to a first temperature with a first dwell time, then the temperature is increased from the first temperature to a second temperature with a second dwell time. The first temperature may be in a range from 300° C. to 400° C. and the first dwell time may be in a range from 1 to 10 hours. In some such embodiments, the second temperature is from 600° C. to 1200° C. and the second dwell time is from 1 to 10 hours. In other embodiments, the temperature is increased from the second temperature to a third temperature with a third dwell time. In at least some of those embodiments, the second temperature is above 400° C. and below a softening point of amorphous glass of the glass bubbles, and the second dwell time is from 1 to 10 hours. The third temperature may be above the softening point of the amorphous glass of the glass bubbles, and the third dwell time may be from 1 to 10 hours. 
     Other exemplary embodiments include a method of making a porous structure that includes steps of extruding green material, which includes glass bubbles and an organic binder, where most of the glass bubbles survive the extruding without fracture, but then breaching most of the glass bubbles after the extruding, and bonding the plurality of glass bubbles to one another. In aggregate, the bonded, breached glass bubbles form the porous structure. In some embodiments, during the extruding, the glass bubbles have a D50 size of at least 1 micrometer but less than 100 micrometers, such as a D50 size of at least 5 micrometers and no more than 50 micrometers. In some embodiments, after the breaching, the porous structure comprising the breached glass bubbles have a pore size distribution (d50-d10)/d50 of less than 0.8. In some embodiments, during the extruding, the glass bubbles have an isostatic crush strength of 1000 psi or higher. In some embodiments, during the extruding, most of the glass bubbles have a density of at least 0.1 g/cm 3  but less than 1.5 g/cm 3 . In some embodiments, during the extruding, glass of the glass bubbles is soda lime, borosilicate, and/or aluminum silicate. 
     Still other exemplary embodiments include a method of making a porous structure including steps of extruding green material, which includes glass bubbles and an organic binder, where most of the glass bubbles survive the extruding without fracture; heating the glass bubbles to at least a softening temperature of amorphous glass of the glass bubbles; breaching most of the glass bubbles after the extruding, wherein the breaching includes expanding gasses within the glass bubbles to rupture the glass bubbles; bonding the glass bubbles to one another, wherein the heating is such that adjoining glass bubbles sinter to one another. In aggregate, the bonded, breached glass bubbles form the porous structure, where voids within individual breached glass bubbles open into one another to form cavities that extend through the porous structure and to surfaces thereof. In some such embodiments, the extruded green material floats in water, while the porous structure, which includes the bonded, breached glass bubbles, sinks. 
     In some embodiments a porous structure includes a plurality of glass bubbles. The glass bubbles are sintered to one another such that adjoining glass bubbles are physically bonded directly to one another. Most of the glass bubbles are breached and voids defined within individual breached glass bubbles open into one another to form cavities that extend through the porous structure and to surfaces thereof. The porous structure has at least 50% porosity in terms of volume. In some such embodiments, at least some of the glass of the glass bubbles is devitrified such that the glass includes crystals. In some embodiments, the porous structure is mostly glass (including devitrified glass), in terms of weight, such as at least 90% of glass, and/or where less than 75% of the porous structure is amorphous phase in terms of weight. In some embodiments, the porous structure has at least 65% and no more than 85% porosity in terms of volume. 
     In other embodiments a porous structure is mostly (e.g., at least 90%), in terms of weight, a plurality of ruptured glass bubbles sintered to one another such that adjoining glass bubbles are physically bonded directly to one another. Voids defined within individual ruptured glass bubbles open into one another to form cavities that extend through the porous structure and to surfaces thereof. In some such embodiments, the porous structure has a cellular honeycomb geometry with a web thickness of no more than 9 mils, preferably no more than 8 mils, more preferably no more than 6 mils, and a cell density of at most 300 cells per square inch, preferably at most 200 cells per square inch. Other embodiments include a filter that includes such a porous structure and further includes a coating supported by the porous structure, where the coating may be configured to block and/or attract target particulates (i.e. particulate filter), and a housing at least in part surrounding the porous structure and coating. 
     Still other embodiments include extrusion batch material for making porous structures, which includes a plurality of glass bubbles, where the glass bubbles have a D50 size of at least 1 micrometer and no more than 100 micrometers, and where the glass bubbles have an isostatic crush strength of 1000 psi or higher. The extrusion batch further includes a binder and has a specific gravity with respect to water of less than 1.0. In some embodiments, the extrusion batch further includes a pore former, such as an organic pore former, such as a starch. 
     Still other embodiments include a porous structure that may be a filter substrate or body (e.g., honeycomb) or may serve other purposes, such has providing support for liquid electrolyte or a skeleton to be infiltrated with a polymer. The structure includes a plurality of glass bubbles having a D50 size of less than 100 micrometers. The glass bubbles are sintered to one another such that adjoining glass bubbles are physically bonded directly to one another. Most of the glass bubbles are breached, and voids defined within individual breached glass bubbles open into one another to form cavities that extend through the structure and to surfaces thereof. In some embodiments, at least some of the glass of the glass bubbles is devitrified such that the glass includes crystals. 
     Additional features and advantages are set forth in the Detailed Description that follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings. It is to be understood that both the foregoing general description and the following Detailed Description are merely exemplary and are intended to provide an overview or framework to understand the nature and character of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The accompanying Figures are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiments, and together with the Detailed Description explain principles and operations of the various embodiments. As such, the disclosure will become more fully understood from the following Detailed Description, taken in conjunction with the accompanying Figures, in which: 
         FIG.  1    is a perspective view of a filter in cross section, as may embody technology disclosed herein. 
         FIG.  2    is a perspective view of a porous structure, such as honeycomb body, as may embody technology disclosed herein. 
         FIG.  3    is a perspective view of another structure as may embody technology disclosed herein. 
         FIG.  4    is a micrograph of a green structure that includes glass bubbles according to an exemplary embodiment. 
         FIG.  5    is a micrograph of a green structure that includes glass bubbles according to another exemplary embodiment. 
         FIG.  6    is a micrograph of a structure similar to that of  FIG.  4   , but with the glass bubbles ruptured and devitrified, according to an exemplary embodiment. 
         FIG.  7    is a micrograph of a structure similar to that of  FIG.  5   , but with the glass bubbles ruptured and devitrified, according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Before turning to the following Detailed Description and Figures, which illustrate exemplary embodiments in detail, it should be understood that the present inventive technology is not limited to the details or methodology set forth in the Detailed Description or illustrated in the Figures. For example, as will be understood by those of ordinary skill in the art, features and attributes associated with embodiments shown in one of the Figures or described in the text relating to one of the embodiments may well be applied to other embodiments shown in another of the Figures or described elsewhere in the text. 
     Referring to  FIG.  1   , an assembly, shown as a filter  110 , includes a porous structure  112  (e.g., substrate, body) in the form of a honeycomb and a housing  114  (e.g., frame), such as metal can. The housing  114  is at least in part surrounding the porous structure  112 . More specifically,  FIG.  1    shows the housing  114  in cross-section, with a portion removed to show the porous structure  112  within the housing  114 . In some embodiments, the porous structure  112  may be coated, such as with a coating configured to attract or otherwise influence target particulates (e.g., exhaust emissions, air particulates, volatile organic compounds). For example, a substrate in a catalytic converter assembly may receive washcoats including aluminum oxide, titanium and silicon dioxides, and may further be coated with catalysts that include precious metals, such as platinum. 
     Referring to  FIG.  2   , a porous structure  210 , similar to the porous structure  112  of  FIG.  1   , may be used in a filter  110  or otherwise. The porous structure  210  is a “honeycomb” in that the porous structure  210  includes elongate channels  212  that extend generally through at least a portion of the porous structure  210 , such as extending linearly from an outer surface  214  (e.g., face) of the porous structure  210  to or near an opposing outer surface of the porous structure  210 . In some embodiments, the filter  110  may include plugs where air flows through walls and webs of the filter, but particulates are caught. In other embodiments, some or all of the elongate channels  212  are unplugged, allowing fluids (e.g., exhaust, water, etc.) to flow through the elongate channels  212 . In still other embodiments, the porous structure may be porous but not include elongate channels  212  (see generally  FIG.  3   ). 
     According to an exemplary embodiment, the elongate channels  212  may have relatively high aspect ratios, such as length-to-width or length-to-diameter, where length L is oriented along the flow path of the elongate channels  212 , between openings on the outer surface  214  provided by the elongate channels  212  on opposing outer surfaces  214  of the porous structure  210 , as shown in  FIG.  2   . According to an exemplary embodiment, elongate channels  212  are elongate such that the aspect ratio, defined as the length of an elongate channel  212  in relation to widest cross-sectional dimension of the respective elongate channel  212  orthogonal to the length L, of at least some of (e.g., most, &gt;90%, all) the channels is at least ten, at least twenty, at least fifty, at least one-hundred, and/or no more than 50,000. 
     While  FIGS.  1 - 2    show porous structures  112 ,  210  having a generally cylindrical geometry, other geometries are contemplated, such as cube, box, sheet, and more complex geometries. For example, referring now to  FIG.  3   , structure  310  is generally a rectilinear sheet, which may be used as a filter substrate or for other purposes. In some embodiments, the structure  310  of  FIG.  3    has a generally uniform density and heterogeneous pore distribution, essentially a sheet of glass foam, without elongate channels. The foam may be highly porous, coated, and/or partially- or fully-filled with liquid material (e.g., electrolyte), solid material (e.g., dielectric), or otherwise. 
     According to an exemplary embodiment, the structure  310  is highly porous, and the pores (e.g., cavities, voids, space between structure) are open to one another such that fluids may pass through the pores, into and through the structure  310 . However, the structure  310  may be only semi-permeable, in some such embodiments, allowing only some fluids and/or smaller particulates to pass through the structure  310 , but trapping or blocking others. 
     According to an exemplary embodiment, structures, such as the porous structures  112 ,  210  and structure  310  of  FIGS.  1 - 3   , may include and/or be at least partially formed from a plurality of glass bubbles (e.g., hollow microspheres, see glass bubbles  512 ,  512 ′ of  FIGS.  5  and  7   ), where “plurality” may include more than 100, such as more than 1000. In some such embodiments, the glass bubbles have a D50 size of at least 1 micrometer (μm) and no more than 1000 μm, such as at least 5 μm, at least 25 μm, and/or no more than 500 μm, such as no more than 250 μm, such as no more than 100 μm, as per ASTM standards, such as D4284-12. In some such embodiments, the porous structures including the glass bubbles have a pore size distribution (d50−d10)/d50 of less than 0.8, such as less than 0.4, such as less than 0.2, such as less than 0.1, such as less than 0.06. In some such embodiments, most of the glass bubbles (prior to breach, as discussed below) have a density of at least 0.1 g/cm 3 , such as at least 0.3 g/cm 3 , and/or less than 1.5 g/cm 3 , such as less than 0.7 g/cm 3 , where density accounts for mass per volume, including interior bubble volume. 
     According to an exemplary embodiment, the porous structures  112 ,  210 , and the structure  310 , in terms of weight, are mostly glass or crystallized glass (glass-ceramic, ceramic), such as at least 70% of the weight, such as at least 80%, and such as at least 90%. Such large portions of the structures  112 ,  210 ,  310  formed from glass or crystallized glass of glass bubbles may be surprising or counterintuitive for those in industry because they may expect such structures to be particularly fragile and/or not hold together at all. However, in some contemplated uses, porous space of the porous structures  112 ,  210 , and structure  310  may later be at least partially filled by other materials (e.g., fluids), while the porous structures  112 ,  210  and structure  310  largely hold together due to methods of making such structures as taught herein. 
       FIGS.  4 - 5    include “green” (e.g., pre-fired, pre-sintered) structures  410 ,  510 . More specifically, green structure  410  of  FIG.  4    may be an exterior wall of a porous structure, such as a honeycomb porous structure as shown in  FIG.  1   . While green structure  510  of  FIG.  5    may be an interior wall or web of a porous structure, such as a honeycomb body as shown in  FIG.  1   . 
     The green structures may be formed from extruded batch material. According to an exemplary embodiment, the green structures  410 ,  510  include glass bubbles  412 ,  512  held in binder  414 ,  514  (e.g., organic binder, mostly-organic binder). In some embodiments, the batch material may include glass bubbles of particle size 3 to 100 micrometers, such as having a particle distribution of (d90−d10)/d90 less than 2, such as less than 1.5, or less than 1. According to an exemplary embodiment, the batch density (e.g., “wet batch” density) is less than 1.5 g/cm 3 , such as less than 1.0 g/cm 3 , such as less than 0.5 g/cm 3 , such as less than 0.3 g/cm 3 . According to an exemplary embodiment, the green material and batch material float (i.e. specific gravity less than 1, compared to water) of  FIGS.  4 - 5   , while finished porous structures, after firing and/or breaching of the glass bubbles, may sink (see  FIGS.  6 - 7   ). 
     In some embodiments, the green structures  410 ,  510  may further include a slip agent and/or lubricant, such as oil. Sodium stearate or another sintering aid may be added to the batch. In some embodiments, the binder may include methylcellulose. In some embodiments, the batch may further include a pore former, such as an organic pore former, such as a starch (e.g., corn starch, pea starch). According to an exemplary embodiment, glass bubbles  412 ,  512  may be a “stand-alone” composition in terms of the inorganic constituents (&gt;90% wt of inorganics in the batch, &gt;95% wt, skeleton). In other embodiments, the batch may further include a second inorganic material with a softening temperature greater than the glass bubbles, such as clay, talc, silica, alumina, minerals, synthetic oxides, other types of glass or ceramic particles and/or bubbles. 
     In some embodiments, particularly resilient glass bubbles  412 ,  512  are used, such as those having a mean isostatic crush strength of at least 1000 psi, such as at least 2000 psi, such as at least 3000 psi (see Measuring Isostatic Pressing Strength of Hollow Glass Microspheres by Mercury-injection Apparatus by Yun and Shou, Key Engineering Materials, vol. 544, pp. 460-5 (2013)). Also, rates and pressures through the corresponding extruder may vary depending upon the size of the glass bubbles, their material, and the extruding device. In some embodiments, extrusion pressures are in the range of less than 2500 psi, such as less than 2000 psi, and/or at least 500 psi. 
     According to an exemplary embodiment, the glass bubbles  412 ,  512  in binder  414 ,  514  have been extruded (e.g., twin-screw) at a rate and pressure to preserve integrity of most (e.g., more than 50%, more than 75%, more than 90%) of the glass bubbles  412 ,  512 . As shown in  FIGS.  4 - 5   , most of the glass bubbles  412 ,  512  appear fully intact. With that said, in other contemplated embodiments, extrusion rate and pressure may preserve integrity of many of the glass bubbles, but not most, such as less than 50%, but at least 25%, or at least 20%. Preserving integrity of the glass bubbles  412 ,  512  allows the glass bubbles to occupy relatively large volumes of space within the green structures  410 ,  510  with voids between the glass bubbles  412 ,  512  and within the glass bubbles  412 ,  512 . 
     Extruding the green structures  410 ,  510  may be particularly efficient for forming through-channels (e.g., elongate channels  212  as shown in  FIG.  2   ) in porous structures such as the honeycomb of  FIG.  1   , or other regular features in the respective green structures  410 ,  510 . However, in other contemplated embodiments, such structures, including glass bubbles in binder may be molded, tape-cast, or otherwise shaped or processed, which may better or alternatively preserve integrity of the glass bubbles  412 ,  512 . In still other contemplated embodiments, structures with shapes far different from those of porous structures  112 ,  210 , such as the structure  310 , may be extruded or otherwise formed. 
     The glass bubbles  412 ,  512  may include glass (e.g., consist of, consist mostly of by volume, comprise), such as soda lime glass, borosilicate, or other glasses. The glass of the glass bubbles  412 ,  512  may be fully amorphous, crystalline, polycrystalline, etc., such as two-phase glass-ceramic. In some embodiments, the glass of the glass bubbles  412 ,  512  may be amorphous prior to heating, and subsequently may devitrify and/or crystallize. For clarity, “glass” as used herein includes amorphous glass, devitrified glass with crystals, such as glass-ceramic and crystalline phase. In at least some contemplated embodiments, the glass bubbles  412 ,  512  may include and/or be formed form other materials, such as synthetic minerals, polymers, ceramics, fly ash/cenospheres, metals, etc. 
     According to an exemplary embodiment, glass bubbles with high crystallinity at softening temperatures of the glass bubbles, such as glass bubbles including more than 45% SiO 2  and/or CaSiO 3 , etc. by weight, facilitate transformation processes from internal porosity to open connected porosity, as discussed below. Exemplary glass composition constituents include more than 74% SiO 2 , more than 6.5% CaO, less than 7% and at least some B 2 O 3 , less than 1% and at least some Al 2 O 3 , at least some Fe 2 O 3 , less than 2.5% and at least some Na 2 O, and/or at least some K 2 O. In some embodiments, glass of the glass bubbles is, is mostly, or includes soda lime, borosilicate, and/or aluminum silicate glass. Some exemplary compositions and corresponding glass bubble attributes are provided in Tables 1 and 2 below: 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Exemplary glass bubble compositions 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Sample 
                 A1 
                 A2 
                 A3 
                 A4 
                 A5 
                 B1 
                 B2 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 SiO 2   
                 79.4 
                 79.3 
                 79.3 
                 79.3 
                 79.5 
                 73.6 
                 71.1 
               
               
                 CaO 
                 7.72 
                 7.43 
                 7.43 
                 7.43 
                 7.89 
                 6.19 
                 5.67 
               
               
                 B 2 O 3   
                 5.95 
                 5.95 
                 5.95 
                 5.95 
                 5.09 
                 7.38 
                 8.73 
               
               
                 Al 2 O 3   
                 0.52 
                 0.55 
                 0.55 
                 0.55 
                 0.49 
                 1.01 
                 1.07 
               
               
                 Fe 2 O 3   
                 0.057 
                 0.056 
                 0.056 
                 0.056 
                 0.035 
                 0.089 
                 0.081 
               
               
                 NaO 
                 1.68 
                 1.59 
                 1.59 
                 1.59 
                 1.74 
                 2.58 
                 2.60 
               
               
                 K 2 O 
                 0.14 
                 0.14 
                 0.14 
                 0.14 
                 0.14 
                 0.19 
                 0.21 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Attributes of glass bubbles 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Sample 
                 A1 
                 A2 
                 A3 
                 A4 
                 A5 
                 B1 
                 B2 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Softening temp. (° C.) 
                 ~800 
                 ~800 
                 ~800 
                 ~800 
                 ~800 
                 ~600 
                 ~600 
               
               
                 Density (g/cm 3 ) 
                 0.38 
                 0.60 
                 0.64 
                 0.55 
                 0.48 
                 0.45 
                 0.34 
               
               
                 Porosity (%) 
                 82.9 
                 73.1 
                 71.3 
                 75.3 
                 78.4 
                 79.8 
                 84.8 
               
               
                 Shell Thickness (μm) 
                 1.07 
                 1.45 
                 1.18 
                 1.76 
                 0.91 
                 0.70 
                 1.24 
               
               
                 Particle Size D 10  (μm) 
                 21.1 
                 17.0 
                 15.2 
                 27.9 
                 15.0 
                 10.5 
                 21.9 
               
               
                 Particle Size D 50  (μm) 
                 35.5 
                 28.7 
                 22.2 
                 39.1 
                 23.4 
                 19.2 
                 35.1 
               
               
                 Particle Size D 90  (μm) 
                 58.3 
                 58.9 
                 33.5 
                 58.9 
                 37.0 
                 35.4 
                 55.4 
               
               
                 Distribution (D 90 -D 10 )/D 50   
                 1.05 
                 1.45 
                 0.83 
                 0.79 
                 0.94 
                 1.30 
                 0.95 
               
               
                 Crush strength (psi) 
                 3000 
                 10000 
                   
                   
                 4000 
                 3000 
                 4000 
               
               
                   
               
            
           
         
       
     
     According to an exemplary embodiment, the green structures  410 ,  510  are heated (e.g., fired in a furnace, laser heated). Heating may burn out, char, chemically transform, or otherwise influence the binder  414 ,  514 . According to an exemplary embodiment, the green structures  410 ,  510  are heated at least to a softening temperature of glass of the glass bubbles  412 ,  512 . But, the glass bubbles  412 ,  512  are not overheated, such as well above a liquidus temperature, where the glass bubbles  412 ,  512  may fully lose cohesion or structure. Depending upon the materials, the heating may be to at least 400° C., at least 600° C., at least 800° C., at least 1200° C., and/or no more than 2000° C., such as no more than 1600° C., such as no more than 1400° C. In contemplated embodiments, the glass bubbles  412 ,  512  may have other softening temperatures. 
     According to an exemplary embodiment, conditions and handling of the green structures  410 ,  510  during the heating is such that adjoining glass bubbles  412 ,  512  physically interact with one another, such as directly bond to one another (e.g., sinter, weld, melt-into), but without fully losing their individual structures. Put another way, in at least some such embodiments, the conditions and handling are such that the glass bubbles  412 ,  512  do not fully liquify and/or completely lose structure, and instead become bonded to one another such that, in the aggregate, the resulting structure is cohesive and rigid. 
     According to a further such exemplary embodiment, conditions and handling of the green structures  410 ,  510  during the heating may be such that many (e.g., most, &gt;90%, &gt;95%, &gt;99%) of the glass bubbles  412 ,  512  breach or break, such as by rupture from internal gas expansion and/or by devitrification or otherwise. In some such embodiments, the glass bubbles  412 ,  512  are heated to a point that the glass bubbles  412 ,  512  lose integrity and glass of the glass bubbles  412 ,  512  shatters or is otherwise breached. In other contemplated embodiments, the glass bubbles may be breached by microwaves, sound, or other phenomena. 
     Breaching the glass bubbles  412 ,  512  may be counterintuitive to those in industry, where glass bubbles may be relied upon to provide buoyancy and/or prevent inflow of materials into voids within the glass bubbles or through the glass bubbles. However, Applicants have found that by breaching the glass bubbles  412 ,  512  of structures, as disclosed herein, voids of the glass bubbles  412 ,  512  may be maintained and/or even enlarged and joined to one another. 
     Following heating, the green structures  410 ,  510  may be cooled, such as to a temperature at least 100° C. less than the temperatures to which the green structures  410 ,  510  were heated, such as to less than 100° C., such as less than 50° C. During the cooling, the adjoining glass bubbles  412 ,  512 , which may be less spherical at this point, are and/or remain physically bonded to one another, such as directly or indirectly bonded, with intermediate bonding agents. 
     In some such embodiments, the cooling includes dwelling at temperatures above room temperature (e.g. at annealing point of the glass of the glass bubbles), but below the heating temperature. Dwelling may occur at incremental steps, in some embodiments, or may be in the form of very gradual temperature declines within certain temperature ranges in other embodiments, both of which may allow for formation of crystals in the materials of the glass bubbles  412 ,  512 , and/or may facilitate relaxing of residual stresses by to annealing. 
     Similarly, Applicants have discovered a unique firing process to breach/open the glass bubbles. During the heating, the glass bubbles may be heated from ambient temperature to a first temperature(s) (e.g., fixed temperature and/or temperatures in a limited range) with a first dwell time, such as where the first temperature(s) is at least 200° C., such as from 300° C. to 400° C., and/or where the first dwell time is at least 1 minute, such as from 1 to 10 hours. In some such embodiments, the temperature is then increased from the first temperature to a second temperature(s) with a second dwell time, such as where the second temperature(s) is greater than 400° C., such as from 600° C. to 1200° C., and where the second dwell time is also at least 1 minute, such as from 1 to 10 hours. In some embodiments, during the heating, the temperature is increased from the second temperature(s) to a third temperature(s) with a third dwell time, such as where the second temperature is above 400° C. and below a softening point of glass of the glass bubbles  412 ,  512  and the third temperature is above the softening point of the glass of the glass bubbles. The third dwell time may be at least 1 minute, such as from 1 to 10 hours. 
     Referring now to  FIGS.  6 - 7   , structures  410 ′,  510 ′ of  FIGS.  6 - 7    are related to the green structures  410 ,  510 . More specifically, structure  410 ′ of  FIG.  6    may be an exterior wall of a porous structure, such as a honeycomb as shown in  FIG.  1    and structure  510 ′ of  FIG.  7    may be an interior wall or web of a porous structure, such as a honeycomb as shown in  FIG.  1   . However, structures  410 ′,  510 ′ are not “green” structures. Instead, shells or husks of glass bubbles  412 ′,  512 ′ are bonded to one another and the glass bubbles  412 ′,  512 ′ are breached, where interior volumes of the glass bubbles  412 ′,  512 ′ are exposed and spaces between the glass bubbles  412 ′,  512 ′ are open and interconnected with one another, forming cavities  416 ′,  516 ′ (tortuous pathways) extending throughout and to surfaces of the structures  410 ′,  510 ′. 
     Referring to  FIGS.  6 - 7   , internal walls formed between pores within the structures  112 ,  210 ,  310  may be particularly thin, such as less than 1 millimeters (mm) in thickness, such as less than 500 micrometers (μm), such as less than 100 μm, such as less than 50 μm, such as less than 10 μm, such as less than 5 μm in some contemplated embodiments, such as where particularly small glass bubbles are used, as discussed below. 
     As may be seen in  FIGS.  6 - 7   , in some embodiments, glass of the glass bubbles  412 ′,  512 ′ devitrifies and forms crystals. In  FIGS.  6 - 7   , the devitrified glass appears light gray. Gradually heating, and dwelling, as disclosed herein, may facilitate crystal growth, which may toughen the resulting structures  410 ′,  510 ′. While green material may include amorphous glass bubbles, the processed structures, after firing, may be a glass-ceramics with crystallinity over 45% by weight (e.g., at least 50% wt; e.g., 64% wt crystallinity). In some embodiments, the porous structure (after firing), in terms of weight, consists mostly of glass (including devitrified glass), such as consisting at least 90% of glass. In some such embodiments, the porous structure consists less than 55% of amorphous phase by weight. As may be seen in  FIG.  6   , white needle-like structures on as-fired surface may be tridymite crystals. 
     Referring to  FIG.  7   , the cavities  416 ′,  516 ′ formed by the breached glass bubbles  412 ′,  512 ′ and/or voids left behind from burned-out binder (see binder  414 ,  514  of  FIGS.  4 - 5   ) may lead to particularly high porosity of resulting structures  410 ′,  510 ′. Applicants believe that the presently disclosed technology provides for high porosity, such as at least 40%, at least 60%, at least 70%, at least 80% by volume (see ASTM D6761-07). According to an exemplary embodiment, the porous structures  112 ,  210 , and structure  310  have at least 65% and/or no more than 85% porosity in terms of volume. 
     While  FIGS.  4 - 7    show microstructure and surface features, according to at least some exemplary embodiments, porous structures  112 ,  210 , and structure  310  may have a total volume, within outer surfaces thereof (see, e.g., outer surface having the openings on outer surface  214  of  FIG.  2   ), of at least 1 cubic centimeter (cm 3 ), such as at least 2 cm 3 , such as at least 10 cm 3 , such as at least 50 cm 3 , and/or no more than 2000 cm 3 , such as no more than 1000 cm 3 ; but in other embodiments, the volume may be much larger, such as for large frontal area filters. 
     In contemplated embodiments, processes and technology disclosed herein are used with honeycomb filters, such as diesel engine particulate filters. Glass bubbles are selected with sufficient crush strength and small enough geometry to facilitate extrusion of honeycomb bodies having at least 50 cells per square inch, such as at least 100 cells per square inch, such as at least 200 cells per square inch, such as at least 300 cells per square inch, and/or web thickness of no more than 10 mils (i.e. thousandths of an inch), such as no more than 8 mils, such as no more than 7 mils, such as no more than 6 mils, such as no more than 5 mils, such as for example cell geometries at, at least as dense as, no denser than, or about 200/8 cells per square inch over web thickness in mils, 400/7, 400/6, 400/5, 400/4, 400/3, 400/2, 300/7, 300/6, 300/5, 300/4, 300/3, 300/2, 200/7, 200/6, 200/5, 200/4, 100/8, 100/7, 100/6, 100/5, 50/8, 50/7, 50/6, etc. 
     At least some such embodiments have a cylindrical geometry, with a diameter of at least 4 inches, such as at least 6 inches, such as at least 8 inches, such as at least 12 inches, such as at least 24 inches, and/or no more than 64 inches, such as no more than 36 inches. Other such embodiments have a generally square, rectangular, or other polygonal geometry in cross-section, with sides of at least 4 inches, such as at least 6 inches, such as at least 8 inches, such as at least 12 inches, such as at least 24 inches, and/or no more than 64 inches, such as no more than 36 inches. Other contemplated embodiments have other sizes or shapes. Such geometries may facilitate low pressure drop, high dust loading, and high filtration efficiency. 
     Construction and arrangements of the porous structures, assemblies, and structures, as shown in the various exemplary embodiments, are illustrative only. Although only a few embodiments have been described in detail in this disclosure, many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes, and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations) without materially departing from the novel teachings and advantages of the subject matter described herein. Some elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. The order or sequence of any process, logical algorithm, or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present inventive technology.