Abstract:
The present invention is concerned with the preparation of glass microfoams and gas-ceramics having compositions selected from the systems of SiO 2  --Al 2  O 3  --B 2  O 3  --RO--R 2  O, SiO 2  --Al 2  O 3  (B 2  O 3 )--P 2  O 5  --Li 2  O--[ZrO 2  (TiO 2 )], and SiO 2  --P 2  O 5  --B 2  O 3  --[RO], wherein RO is selected from the group of MgO, CaO, SrO, BaO, and ZnO, and R 2  O is selected from the group of alkali metal oxides. The foams comprise hydrogen-containing cells produced through the inclusion in the batch of a hydrogen-generating agent selected from the group of ammonium salts and/or a combination of amines and/or carbohydrates and/or hydrocarbons with phosphates.

Description:
This application is a continuation-in-part application of Ser. No. 737,205, filed May 23, 1985, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     Gas evolution from glasses has been studied both from th standpoints of refining of glass and the creation of glass foams. As an example of a glass foam, a commercial material marketed under the trademark FOAMGLAS by Pittsburgh-Corning, Pittsburgh, Pa. is produced in large volume by melting a typical soda-lime glass under highly oxidizing conditions (utilizing Na 2  SO 4  in the batch), comminuting the glass to a fine particle size, and firing the glass particles in combination with powdered carbon. A coarse foamed glass of low density (&lt;0.2 g/cm 3 ) is formed containing carbon dioxide bubbles of several millimeters&#39; diameter. The glass is gray or black in color with a porous and dull surface. 
     Porous glasses, glass-ceramics, and sintered ceramics have frequently been described in the patent literature. Examples of such disclosures include: 
     U.S. Pat. No. 1,108,007 is directed to the melting of basalt in graphite crucibles. The molten basalt reacts with the graphite and bubbles of a gas (not identified) are entrained in the melt. Upon cooling, the melt crystallizes to a porous body. 
     U.S. Pat. No. 2,978,340 is concerned with the preparation of hollow glass spheres from discrete, solid particles consisting of an alkali metal silicate, e.g., sodium silicate, a metal oxide which forms an insoluble glass when melted with the silicate, e.g., B 2  O 3 , and a bloating agent. An extensive list of gasifying agents is furnished, none of which is singled out as exhibiting any unusual behavior. 
     U.S. Pat. No. 3,189,512 is drawn to foamable ceramic cements wherein a combination of SiC and SO 3  comprised the foaming agent. The cements were composed of PbO, a metal fluoride, SiC, SO 3 , and a lithium aluminosilicate material (conveniently petalite). 
     U.S. Pat. No. 3,261,696 reports a method for forming insulating foamed materials comprising the steps of: (a) combining ZrO 2 , Al 2  O 3 , and powdered aluminum; (b) adding H 3  PO 4  to the mixture to cause a reaction to occur which liberates water vapor and hydrogen to foam the mass; and (c) curing the mass at 150°-800° F. 
     U.S. Pat. No. 3,634,111 discusses foamable ceramic cements. The cements consisted of a glass having a composition within the Li 2  O--Al 2  O 3  --SiO 2  --TiO 2  system containing SiC as the cellulating agent, and being essentially free from PbO, SO 3 , and fluoride. 
     U.S. Pat. No. 3,811,852 discloses the preparation of porous glass-ceramic masses comprising the steps of frothing the initial glass melt with gas liberated through fuel combustion in the melt, forming glass ribbon from the melt, and thereafter heat treating the glass ribbon in a two-step process to convert the glass into a glass-ceramic. 
     U.S. Pat. No. 4,011,093 describes a foamable ceramic cement consisting essentially of a glass frit having a composition within the Li 2  O--Al 2  O 3  --CeO 2  --SiO 2  system with, optionally, ZnO into which SiC is incorporated as a foaming agent. 
     U.S. Pat. No. 4,084,980 is drawn to the production of a foamed body comprising the steps of: (a) mixing the following four components, viz., an aqueous solution of an acid or a water soluble acidic phosphate, a cement material or an anhydrous alkali metal silicate, a metal blowing agent, and a foaming stabilizer, to obtain a pasty mass; (b) shaping the pasty mass into a desired geometry; and (c) allowing the shaped mass to stand to effect foaming. 
     U.S. Pat. No. 4,116,703 is directed to the preparation of a foamable cement which comprises mixing together crystalline hydraulic cement, a hydraulic cement in the form of a silicate glass powder, and quaternary ammonium silicate, and then allowing the mixture to react and set at a temperature below 150° C. 
     U.S. Pat. No. 4,133,691 is concerned with the development of an inorganic foam which comprises the steps of: (a) mixing particulate aluminum with an aqueous solution of an alkali metal base to cause the formation of hydrogen gas; (b) folding that mixture into an aqueous alkali metal silicate solution in a manner to retain concentrated areas of the mixture in the silicate solution; and (c) thoroughly mixing the materials to form a solid foam. 
     U.S. Pat. No. 4,404,291 reports a method for forming a molded sintered porous body comprising the following steps: (a) mixing a powdered organic combustible material with powdered glass, devitrifying solder glass, or glass-ceramic; (b) heating the mixture to a temperature sufficient to burn off the organic material to form open pores in the resultant mass; and then (c) heating the mass to a temperature sufficient to sinter the powders together into an integral body. 
     As can be observed from the above, various mechanisms have been employed to prepare foamed glass and glass-ceramic bodies. Nevertheless, the production of foamed glasses and glass-ceramics exhibiting the highly desirable combination of fine bubble size, low density, and a non-porous surface has not been satisfactorily achieved. Hence, the primary objective of the present invention is to provide such products. cl SUMMARY OF THE INVENTION 
     The basis of the instant invention is the finding that foamed, closed-pore glass and glass-ceramic articles, wherein very fine bubbles composed predominantly of hydrogen are present, can be formed over a range of compositions in the following fundamental systems; viz., SiO 2  --Al 2  O 3  --B 2  O 3  --RO--R 2  O, P 2  O 5  --SiO 2  --B 2  O 3  --[RO], and SiO 2  --Al 2  O 3  (B 2  O 3 )--P 2  O 5  --Li 2  O--[ZrO 2  (TiO 2 )], wherein RO is selected from the group of MgO, CaO, SrO, BaO, and ZnO, and R 2  O is selected from the group of alkali metal oxides, conveniently Li 2  O, Na 2  O, and/or K 2  O. Ammonium salts constitute the preferred source of hydrogen, although similar effects can be obtained in certain compositions through a combination of carbohydrates, hydrocarbons, and amines with phosphates. 
     As used herein, the term gas-ceramic indicates a body formed by a process wherein foaming concurrently accompanies crystallization; glass microfoam designates a body formed by a process wherein foaming is generated without crystallization. Gas-ceramics can be produced either through foaming by controlled nucleation of bubbles upon heat treatment of a precursor glass body or by spontaneous nucleation upon cooling of a molten glass to a solid body. 
     In a general composition survey of the three operable systems, three limitations appear to be unqualifiedly mandatory; viz., at least 8% by weight SiO 2 , at least 30% by weight B 2  O 3  +Al 2  O 3  +P 2  O 5 , and at least 10% by weight B 2  O 3  +P 2  O 5 . Although both B 2  O 3  and P 2  O 5  are desirable in combination in all composition systems, neither alone is absolutely necessary. Furthermore, all the compositions appear to be &#34;acid&#34;; i.e., SiO 2  +B 2  O 3  +Al 2  O 3  +P 2  O 5  &gt;&gt;RO+R 2  O, and B 2  O 3  +Al 2  O 3  +P 2  O 5  &gt;RO+R 2  O. Fluorides appear to be undesirable, as are most easily reducible metal oxides, although sometimes minor amounts of TiO 2  and rather considerable levels of ZnO can be tolerated. 
     The most effective batch ingredients for the introduction of hydrogen-forming species include NH 4  H 2  PO 4 , (NH 4 ) 2  HPO 4 , NH 4  Cl, NH 4  B 4  O 7 , and starch and/or sugar with Al(PO 3 ) 3 . Concentrations of those materials ranging from 0.5 to over 50% of the total batch have been found effective, depending upon the base glass composition. 
     Foaming of the samples was accomplished by heating at about 700°-1000° C., depending upon the base compositions thereof, for times ranging from about 10 minutes to several hours. 
     The hydrogen-containing cells or bubbles are believed to be the result of either the brakdown of ammonium species in the glass, followed by dissolution of hydrogen molecules in the glass at high temperatures and subsequent release at low temperatures, or by the reduction of stable OH -  ions in the glass network through reaction with a reduced phosphorus species such as P +3  or P in the glass. 
     SiO 2  --Al 2  O 3  --B 2  O 3  --RO--R 2  O System 
     Operable compositions in this system, expressed in terms of weight percent on the oxide basis, consist essentially of 25-65% SiO 2 , 15-35% Al 2  O 3 , 12-35% B 2  O 3 , and 1.5-20% RO+R 2  O, consisting of 0-15% RO (alkaline earth oxides and/or ZnO) and 0-20% R 2  O (alkali metal oxides). The level of Al 2  O 3  in terms of mole percent is maintained higher than the sum of RO+R 2  O. The inclusion of 0.5-10% P 2  O 5  is desirable but not mandatory. 
     A heat treatment in the vicinity of 800° C. is generally effective in developing gaseous hydrogen in this system, although temperatures over the range of 700°-950° C. can be operable. Mullite (3Al 2  O 3 .2SiO 2 ) and sometimes anorthite solid solution [(Ca,Sr).Al 2  O 3 .2SiO 2  ] and AlPO 4  have been identified through x-ray diffraction analysis. The preferred alkaline earth oxide is CaO, since its presence favors the generation of small bubbles. CaO-containing, aluminosilicate glasses have been recognized as exhibiting high resistance to gas permeation and low ionic mobility, this latter phenomenon resulting in glasses of high electrical resistivity. Because it has been postulated that the rate of hydrogen diffusion controls the rate of bubble growth and the size of the bubbles, glasses demonstrating low permeability to gases have been deemed to be preferred. 
     The products of this composition system combine a dense glassy skin of good chemical durability with densities typically in the range of about 0.9-2.0 g/cm 3 . The dimensions of the bubbles generally vary from about 50 microns to 1 mm. With the inclusion of low concentrations of Pd, however, which acts as a nucleating agent in amounts of 0.001-0.01% by weight, bubbles having diameters down to 20 microns have been observed. 
     SiO 2  --Al 2  O 3  (B 2  O 3 )--P 2  O 5  --Li 2  O--[ZrO 2  (TiO 2 )] System 
     The compositions of this system normally crystallize to a substantial degree, the major crystal phase being identified as a β-quartz solid solution having the general formula 
     
         nSiO.sub.2.xLiAlO.sub.2.yAlPO.sub.4 
    
     wherein n&gt;x+y. Although the inclusion of B 2  O 3  appears to increase the proportion of glassy phase after crystallization, it has been conjectured that some BPO 4  enters into the quartz structure in place of silica. 
     The principal components of the system comprise SiO 2 , Al 2  O 3 , P 2  O 5 , Li 2  O, and ZrO 2  in the amounts recorded below, expressed in terms of weight percent on the oxide basis, with TiO 2  being capable of replacing at least part of the ZrO 2  as a nucleating agent. Total replacement of ZrO 2  with TiO 2 , however, appears to yield products cointaining coarse blisters. 
     
         ______________________________________SiO.sub.2    40-50          ZrO.sub.2                           0-10Al.sub.2 O.sub.3    15-35          B.sub.2 O.sub.3                           0-15P.sub.2 O.sub.5    10-25          TiO.sub.2                           0-5Li.sub.2 O    1-7______________________________________ 
    
     Ammonium acid phosphate or aluminum metaphosphate combined with starch comprises the preferred hydrogen-gnerating batch materials. The principal advantages of the gas-ceramics derived from this composition system are two; viz., the potential for materials exhibiting very low coefficients of thermal expansion arising from the presence of the β-quartz solid solution phase, and substantial mechanical strength due to the high crystallinity of the bodies. Heat treatments between about 775°-1000° C. are generally satisfactory to secure the desired high crystallinity. The bubble sizes typically vary over the range of about 0.5-5 mm. 
     SiO 2  --B 2  O 3  --P 2  O 5  --[RO] System 
     This pseudobinary system, in which SiO 2  and BPO 4  are the major glass forming constituents, yields the most uniform glass microfoams with bubbles of the smallest dimensions. Very smooth and uniform solid glass skins encase the foamed articles, the thickness of such skins being variable according to the body composition. Hence, cell diameters range about 1-100 microns, with preferred foams having 5-20 micron cells. The densities of the products average about 1.0 g/cm 3 . 
     Operable compositions, expressed in terms of weight percent on the oxide basis, consist essentially of 10-65% SiO 2 , 5-20% B 2  O 3 , and 15-60% P 2  O 5 , with a narrower composition area being defined in terms of 40-60% SiO 2 , 7-20% B 2  O 3 , and 20-35% P 2  O 5 . In order to insure the production of uniform foams with glassy skins, an alkaline earth oxide and/or ZnO (RO) may desirably be added. Thus, the lowest density foams have contained about 0-20% MgO, 0-20% CaO, and 0-15% ZnO. It has been hypothesized that those additions prevent restriction of cell expansion resulting from the crystallization of BPO 4 , and enable the generation of a very fine uniform cell size and the formation of a smooth, glassy, monolithic skin on the foam bodies. Ammonium acid phosphate is the preferred batch ingredient for both hydrogen generation and as the source of P 2  O 5 . It has been observed that compositions containing &gt;13% MgO and/or ZnO tend to foam spontaneously upon cooling, either as poured into a glass body or after the glass body has been placed into an annealer. It has also been observed, however, that MgO and CaO additions greater than about 5% and ZnO additions greater than about 10% may result in the body exhibiting a measure of hygroscopic behavior or poor chemical durability. Therefore, from a practical point of view, the total of MgO+CaO+ZnO will be limited to 0.5-10%, consisting of 0-5% MgO, 0-5% CaO, and 0-10% ZnO. 
     The preferred compositions in this system crystallize only partially (customarily &lt;50% by volume) during foaming, thereby permitting foam cell expansion. Where crystallization does occur (750°-950° C.), the principal phases are BPO 4 , exhibiting a β-cristobalite structure, and/or magnesium pyrophosphate, Mg 2  P 2  O 7 . 
     The chemical durability of BPO 4  -containing glass systems is typically quite poor, but can be improved dramatically by: (a) increasing SiO 2  above 40% by weight; (b) reducing B 2  O 3  below 10% by weight; (c) reducing modifier additions to less than 5% by weight; and/or (d) adding Al 2  O 3  up to 10% by weight. 
     Structure and Properties of the Inventive Products 
     Several analyses of the bubbles occurring in the three composition systems were conducted utilizing mass spectrometry. Hydrogen always constituted the predominant species. Nitrogen was usually present to some extent, occasionally appearing as air contamination (with the normal ratio of argon), but often as primary nitrogen, i.e., without argon, presumably resulting from NH 3  breakdown. 
     The effects of varying the thermal treatment on the generation of hydrogen bubbles are generally dissimilar to those observed in conventional glass-ceramics. For example, whereas it is normally beneficial to optimize nucleation at high viscosities and low crystal growth rates with a &#34;nucleation hold&#34;, the rate of bubble nucleation at high viscosities does not appear optimum. Thus, better nucleation is often achieved at higher temperatures where the glass readily deforms; viz., at least 200° C. above the glass transition temperature. 
     Furthermore, there seems to be no clear relationship between the crystallization event and the formation of hydrogen bubbles. To illustrate, in the mullite system (SiO 2  --Al 2  O 3  --B 2  O 3  --RO--R 2  O) foaming appears to accompany crystallization in most compositions, but generally precedes crystallization in the other two systems. Even in the high alumina compositions, bubble nucleation appears geometrically unrelated to that of mullite crystallization, with bubble nuclei occurring widely separated by about 100 microns and mullite crystals separated by less than 0.1 micron. In fact, palladium has a far greater effect upon hydrogen nucleation rates than does mullite. Hence, less than 0.01% by weight Pd can increase hydrogen nucleation rates by over an order of magnitude in the high alumina glasses. 
     The hydrogen bubbles are generally spherical in shape and produce a closed pore foam. As the volume percent of gas increase, the bubbles begin to impinge, thereby producing flat glassy regions separating bubbles. Customarily, the volume percent of gas does not exceed about 75%. Smaller bubbles are frequently nucleated in the glassy region between large bubbles, thereby indicating that nucleation continues during bubble growth. 
     The size of the bubbles is dependent upon the rate of nucleation, that rate appearing to be most rapid in the alkaline earth borosilicophosphate system and slowest in the β-quartz gas-ceramic system. The average bubble diameter in low density foams ranges from about 10 microns in Mg-Zn borosilicophosphates to several millimeters in the β-quartz system. Bubbles of smaller size (˜1 micron) can be observed during the early stage of bubble generation, but the volume percent of gas in the bodies is quite small at that time. 
     A unique and useful characteristic of the inventive foams is the glossy dense skin which is maintained during bubble generation. It appears that the bubbles do not nucleate or grow within the surface of the bodies. This phenomenon is believed to be due to hydrogen diffusing out from the surface which causes a depleted layer where bubbles cannot form. The thickness of this hydrogen-depleted layer is variable and can range up to 1 mm. 
     This dense skin imparts several advantages to these foams. First, it provides relatively higher strength to the body because no bubbles penetrate the surface to create large flaws. Second, it permits the surface to be cleaned easily and creates a barrier to penetration by foreign particles. Third, the aesthetic appearance of the inventive materials is far superior to those of standard commercial foam glasses. Fourth, increased strength through thermal tempering may be imparted. 
     It will be appreciated that the extent of bubble development is affected by ambient pressure. For example, when a material that would normally generate a gas-ceramic containing very small cells and exhibiting a density greater than 1 g/cm 3  upon heating under one atmosphere pressure was heated in a vacuum furnace, a coarse foam of very low density, i.e., less than 0.5 cm 3  was produced. Conversely, when a small glass slab was heated between glass-ceramic plates with excess pressure being applied by stacking refractory bricks upon the upper glass-ceramic plate, the resulting foam had a higher than normal density and the bubbles were elongated parallel to the plates, i.e., in the minimum stress direction. Furthermore, the specimen, itself, was substantially elongated in the same direction and the surface thereof took on the characteristics of the surfaces of the plates. Quite unexpectedly, no adhesion persisted between the foam body and the plates after cooling. The above activity clearly demonstrates that the inventive gas-ceramics and glass microfoams may be reshaped, reformed, and embossed during thermal treatment without loss of the dense surface layer. 
     The phenomenon of photosensitive behavior was observed in foams of the borophosphosilicate composition system. After hydrogen generation, the foam bodies typically exhibit a white coloration, but, after being exposed to the radiation from laboratory fluorescent lamps for a few hours, the surfaces of the bodies take on a distinct pink, orange, or brownish hue. Exposure to direct sunlight produces a more neutral gray coloration in the surface. The glass bodies before hydrogen generation manifest no photosensitive effects. 
     The development of the pink-orange hue has also been observed when the glasses are undertreated, i.e., heated at temperatures below those at which the best foaming occurs. Also, it has been observed that the color can be bleached out of the surface by heating the bodies above about 500° C. for a few minutes. 
     The color of the original glasses in the three composition systems ranges from colorless through gray or brown to black. In the mullite system the color depends upon the presence of P 2  O 5  and ammonium salts. Where NH 4  Cl constitutes the sole ammonium salt in the glass batch, the original glasses are commonly light brown or salmon colored. When ammonium phosphate is present, the glasses are dark brown or black. Upon generation of hydrogen bubbles, those glasses generally become white and opaque. The opacity is deemed to be due to light scattering by the bubbles, but the change in color from dark to white is not fully understood but is believed to represent the oxidation of reduced phosphorus ions as hydrogen is generated. 
     In the Li 2  O-containing, higher phosphate, β-quartz gas-ceramics, the original glasses are yellow to brown, or, if TiO 2  is present, dark blue to black. The foamed bodies are commonly gray-white or blue, again with a general lightening of color. 
     In contrast, the borophosphosilicate original glasses are clear or pale blue or violet, but can darken to a brown or pink-orange color upon hydrogen generation. When fully foamed at the upper end of the heat treating temperatures, however, the foamed bodies typically tend to become white, as previously described. 
     Because of the glassy and non-porous skin characteristic of hydrogen glass microfoams, the inventive products are much stronger than the standard commercial foam glasses. For example, abraded modulus of rupture values have been measured on typical mullite gas-ceramics with a density of about 1.3 g/cm 3  averaging about 4500 psi. Borophosphosilicate glass microfoams of the smallest cell size (10 micron cell diameter and about 1 g/cm 3  density), exhibit moduli of rupture averaging about 2500 psi. 
     A wide variety of thermal expansion characteristics can be found in the inventive gas-ceramic systems. To illustrate, gas-ceramics containing mullite and AlPO 4  generally exhibit coefficients of thermal expansion (0°-300° C.) between about 40-90×10 -7  /°C.; β-quartz solid solution gas-ceramics can demonstrate coefficients as low as 10×10 -7  /°C.; and the smallest bubble foams in the alkaline earth borophosphosilicate system display coefficients over the 40-50×10 -7  /°C. interval, a good match for silicon metal. 
     Because of the inclusion of P 2  O 5  and/or B 2  O 3  in a number of the compositions, the refractoriness thereof is not very high. Thus, the top use temperature of the products with no thermal distortion will range about 450°-900° C. 
     Exceptional dielectric properites have been measured on the inventive products prepared from non-alkali metal compositions in both the SiO 2  --Al 2  O 3  --B 2  O 3  --CaO and P 2  O 5  --SiO 2  --B 2  O 3  --MgO systems. Very low dielectric constants, low loss tangents, and high electrical resistivities are quite prevalent. For example, a dielectric constant of 2.6 at 25° C. and 100 KHz with a corresponding loss tangent of 0.01 was measured on a mullite gas-ceramic, and a dielectric constant ranging from 2.21-2.27 and a loss tangent ranging from 0.000-0.002 over an interval of temperatures (25°-200° C.) and frequencies (100-10 5  Hz) were measured on a borophosphosilicate glass microfoam. Inasmuch as those products exhibit thermal expansions closely tracking alumina and silicon, their potential for electronic packaging is clear. 
     In like manner to the circumstances present in many conventional glass and glass-ceramics containing substantial quantities of B 2  O 3  and/or P 2  O 5 , the silicophosphate foams containing B 2  O 3  demonstrate poor chemical durability. However, additions of Al 2  O 3  or an increase in SiO 2  appear to significantly improve the resistance to chemical attack, as does the crystallization of BPO 4  and/or Mg 2  P 2  O 7  therein which leaves a siliceous continuous glassy phase. 
     In the mullite-containing gas-ceramics with little or no P 2  O 5 , and Al 2  O 3  present in amounts greater than 20% by weight, the chemical durability is quite good. The resistance to chemical attack exhibited by the β-quartz solid solution-containing materials can also be quite good. 
     Related Application 
     U.S. application Ser. No. 737,204, filed concurrently with the predecessor of the instant application, now U.S. Pat. No. 4,576,920, discloses the preparation of conventional glass-ceramic bodies having compositions within the system B 2  O 3  --P 2  O 5  --SiO 2 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Table I records a number of batched glass compositions, expressed in terms of parts by weight on the oxide basis, illustrating the compositional parameters of the inventive products encompassed within the SiO 2  --Al 2  O 3  --B 2  O 3  --RO--R 2  O system. Because the sum of the individual constituents totals or closely approximates 100, for all practical purposes the figure tabulated for each component can be considered to indicate percent by weight. Other than the hydrogen-forming ingredients, which are tabulated in excess of the base composition, the actual batch constituents may comprise any materials, either oxides or other compounds, which, when melted together, will be converted into the desired oxides in the proper proportions. It will be appreciated that volatile species like NH 3  do not fully remain in the glass. 
     The batches were compounded, ballmilled to aid in securing a homogeneous melt, and charged into silica crucibles. The crucibles were covered, introduced into a furnace operating at about 1400°-1600° C., and the batches melted for about 2-4 hours. The melts were cast into glass slabs having dimensions of about 8&#34;×4&#34;×0.375&#34;, and those slabs were annealed overnight at about 500°-700° C. 
     
                                           TABLE I__________________________________________________________________________  1  2  3  4  5  6  7   8   9  10 11 12__________________________________________________________________________SiO.sub.2  60.0     58.3        35.0           37.0              37.0                 40.0                    35.0                        40.0                            37.5                               40.0                                  38.0                                     37.0Al.sub.2 O.sub.3  20.0     20.1        25.0           25.0              25.0                 25.0                    25.0                        30.0                            25.0                               25.0                                  26.0                                     27.0B.sub.2 O.sub.3  17.5     19.1        25.0           25.0              25.0                 30.0                    25.0                        30.0                            30.0                               28.0                                  28.0                                     28.0Li.sub.2 O  -- -- -- -- -- 5.0                    --  5.0 2.0                               2.0                                  -- --Na.sub.2 O  2.5     2.5        -- -- -- -- --  --  -- -- 2.0                                     2.0K.sub.2 O  -- -- 15.0           13.0              13.0                 -- 15.0                        --NH.sub.4 Cl  1.0     1.0        2.5           -- 0.5                 0.5                    0.5 0.5 0.5                               0.5                                  0.5                                     0.5(NH.sub.4).sub.2 HPO.sub.4  -- -- -- 1.0              1.0                 1.0                    1.0 1.0 1.0                               1.0                                  -- --Cr.sub.2 O.sub.3  0.05     -- -- -- -- -- --  --F      -- 1.0        -- -- -- -- --  --Pd     -- -- -- -- -- -- 0.0006                        0.0006                            .0006                               .0006                                  .0006                                     .0006CaO                              5.5                               5.0                                  5.0                                     5.0NH.sub.4 H.sub.2 PO.sub.4        -- -- 2.0                                     4.0BaO                              -- -- -- --P.sub.2 O.sub.5                  -- -- -- --StarchNH.sub.3Cl__________________________________________________________________________  13 14 15  16 17 18 19 20 21 22 23 24__________________________________________________________________________SiO.sub.2  31.0     39.0        37.5            40.0               40.0                  38.0                     40.0                        40.0                           35.0                              38.0                                 40.0                                    33.0Al.sub.2 O.sub.3  26.0     27.0        27.0            25.0               25.0                  25.0                     25.0                        25.0                           20.0                              25.0                                 25.0                                    24.0B.sub.2 O.sub.3  30.0     28.0        28.0            15.0               15.0                  17.0                     15.0                        20.0                           20.0                              17.0                                 15.0                                    16.0Li.sub.2 O  -- 1.0        --  --Na.sub.2 O  -- 1.5        --  --K.sub.2 ONH.sub.4 Cl  0.7     0.5        0.5 0.5(NH.sub.4).sub.2 HPO.sub.4  -- -- --  --Cr.sub.2 O.sub.3Pd     .0006     .0006        .0006            .0006               -- 0.01                     0.003                        0.001                           0.001                              0.001                                 -- 0.0006CaO    5.0     3.5        7.5 10.0               10.0                  10.0                     -- -- -- -- 10.0                                    10.0NH.sub.4 H.sub.2 PO.sub.4  1.5     10.0        8.0 --BaO    8.0     -- --  -- -- -- -- -- -- -- -- 12.0P.sub.2 O.sub.5  -- -- --  10.0               10.0                  10.0                     10.0                        10.0                           10.0                              10.0                                 10.0                                    5.0Starch              1.0                  -- -- -- -- -- -- --NH.sub.3            -- 1.4                     1.5                        0.7                           1.5                              1.5                                 0.5                                    1.0Cl                  -- 0.33                     0.33                        0.33                           0.33                              0.33                                 -- 0.5__________________________________________________________________________ 
    
     Specimens of suitable geometry for use in conducting tests for various physical and chemical properties were cut from the slabs and those specimens placed inside an electrically-heated furnace. Although more rapid or slower heating rates can be employed, as a matter of convenience the temperature within the furnace was raised as a rate of about 5° C./minute to the foaming temperature recited in Table II, and that temperature maintained for the period of time listed in Table II. Likewise, whereas faster or slower rates of cooling the foamed bodies to room temperature can be operable, as a matter of convenience the electric power to the furnace was merely cut off and the furnace permitted to cool with the bodies retained therewithin. This cooling practice has been termed &#34;cooling at furnace rate&#34; and averages about 2°-3° C./minute. 
     Table II also records a visual description of the original glass, a visual description of the gas-ceramic, various properties exhibited by the gas-ceramics, and crystal phases identified therein. 
     
                       TABLE II______________________________________Ex-               Heatam-  Glass        Treatment Gas-Ceramics Appearance,ple  Appearance   °C.-hrs.                       Properties, and Crystals______________________________________ 1   Transparent, 750-2     Transparent, turquoisesome seeds,  800-4     color, grown seeds,turquoise              mullitecolor 2   Clear, some  750-2     Scattered bubbles,seeds and    800-4     mullitestones 3   Clear, brown 750-2     &lt;1 mm bubbles, ˜10%streaks, few 800-4     by volume bubbles,seeds                  some distortion 4   Clear, brown 750-2     Clear surface layer,streaks, few 800-4     ˜10% by volume fineseeds                  bubbles 5   Dark brown-  750-2     Clear surface layer,to-black,    800-4     ˜10% by volume finestreaky                bubbles 6   Dark brown-  750-2     ˜20% by volume fineto-black,    800-4     bubblesstreaky 7   Gray,         700-12   Glassy skin, ˜15%swollen                by volume finebubble area,           bubblespatchy bottomnucleation 8   Black with   850-4     Blue-gray, bubbles,some area of           ˜50% by volume bodybrown translucent      expansion 9   Clear, pale  800-4     Fine gas-ceramic,gray                   streaky, ˜30-40%                       by volume body                       expansion, 50-100                       micron bubbles,                       mullite10   Gray with    800-4     25% linear, 20%fine bubbles           vertical, and 40%on bottom              by volume body                       expansion, glossy,                       mullite11   Black        800-4     45% hydrogen bubbles,streaks                white, fine-grained12   Black        800-4     50% hydrogen bubbles,streaks                white, fine-grained,                       density 1.28 g/cm.sup.3,                       4400 psi MOR13   Black-       800-4     30% by volume bodyclear,                 expansion, medium-cordy                  grained14   Bulged,      800-4     Fine-grained gas-fine glass-            ceramicceramic inmiddle15   Largely      800-4     120% by volume bodyblack with             expansion, 60%light brown            hydrogen fine bubbles,cord                   mullite16   Clear,       800-4     130% by volume bodypale brown             expansion, floats on                       water, white, fine                       bubbles, AlPO.sub.417   Clear with   950-4     Clear with manydark cord              blisters18   Brown,       850-4     Gray, fine-grainblack                  fracture, floats on                       water19   Black with   850-4     Gray, medium bubbles,areas of               floats on waterbrowntranslucence20   Clear with   850-4     Medium-coarse bubbles,brown streaks          blue-white, density                       0.7 g/cm.sup.321   Gray, light  850-4     Glossy skin, mediumbrown areas            bubbles, density 1.1 g/cm.sup.3of transparency22   Seedy        850-4     Blisters and bubbles,                       floats on water23   Clear, dark  950-4     Clear with mediumstreaks                bubbles, ˜10% by                       volume bubbles24   Black,       850-4     White, fine bubbles,bands of               density 1.2 g/cm.sup.3,translucency           30% expansion thickness______________________________________ 
    
     Table III lists several batched glass compositions, expressed in terms of parts by weight on the oxide basis, illustrative of compositions included within the SiO 2  --Al 2  O 3  (B 2  O 3 )--P 2  O 5  --Li 2  O--[ZrO 2  (TiO 2 )] system. Inasmuch as the sum of the individual constituents of the base glass totals or closely approximates 100, for all practical purposes the value recited for each component may be deemed to reflect weight percent. Other than the hydrogen-forming ingredients, which are tabulated in excess of the base composition, the actual batch constituents may comprise any materials, either oxides or other compounds, which, when melted together, will be converted into the desired oxides in the proper proportions. Again, the volatile compounds NH 3  and starch are vaporized off to a great extent during glass formation. 
     The batches were compounded, ballmilled to assist in obtaining a homogeneous melt, and charged into silica crucibles. The crucibles were covered, introduced into a furnace operating at about 1500° C., and the batches melted for about 4 hours. The melts were cast into glass slabs having dimensions of about 8&#34;×4&#34;×0.375&#34;, and those slabs were annealed overnight at about 500° C. 
     
                       TABLE III______________________________________    25   26          27     28______________________________________SiO.sub.2  42.1   42.0        47.0 48.5Al.sub.2 O.sub.3      29.3   25.0        20.0 --B.sub.2 O.sub.3      --     10.0        10.0 31.0Li.sub.2 O  2.0    4.0         4.0  6.0P.sub.2 O.sub.5      21.6   20.0        20.0 14.5MgO         5.0   --          --   --TiO.sub.2  --     --           5.0 --ZrO.sub.2  --      5.0        --    3.0NH.sub.3    5.3    4.9         4.9  3.5Starch     --      1.0        --   --______________________________________ 
    
     Samples of proper configuration for use in conducting tests for various physical and chemical properties were cut from the slabs and those samples, along with the remainder of the slabs, were inserted into an electrically-heated furnace. In like manner to the glasses of Table I, the samples were heated at a rate of about 5° C./minute to the foaming temperature recorded in Table IV, maintained at that temperature for the period of time listed in Table IV, and thereafter cooled at furance rate. 
     Table IV also presents a visual description of the original glass, a visual description of the gas-ceramic, various properties exhibited by the gas-ceramics, and crystal phases identified therein. 
     
                       TABLE IV______________________________________             Heat  Glass      Treatment Gas-Ceramic Appearance,Example  Appearance °C.-hrs.                       Properties &amp; Crystals______________________________________25     Clear, seedy             775-2     Gray, ˜15% by             990-4     volume bubbles,                       fine-grained, β-                       quartz solid                       solution26     Black with 750-4     Blue, ˜75% by  raised               volume bubbles,  blisters             coarse-grained,                       floats on water27     Dark blue  850-4     ˜50% by volume                       coarse bubbles,                       blisters28     Seedy,     850-4     Clear, cherty  colorless            fracture, ˜50% by                       volume bubbles,                       blisters, β-quartz                       solid solution______________________________________ 
    
     Table V records a variety of batched glass compositions, expressed in terms of parts by weight on the oxide basis, indicating compositions encompased within the P 2  O 5  --SiO 2  --B 2  O 3  --[RO] system. Since the total of the individual constituents of the base glass equals or closely approaches 100, for all practical purposes the concentration listed for each component may be considered to comprise weight percent. Other than the hydrogen-forming ingredients, which are tabulated in excess of the base composition, the actual batch constituents may be any material, either the oxide or other compound, which, when melted together, will be converted into the desired oxide in the proper proportions. Yet again, most of the NH 3  is volatilized off during glass formation. 
     The batches were compounded, ballmilled to aid in achieving a homogeneous melt, and charged into silica crucibles. The crucibles were covered, introduced into a furnace operating at about 1600° C., and the batches melted for about 2 hours. The melts were cast into glass slabs having dimensions of about 8&#34;×4&#34;×0.25&#34;, and those slabs were annealed overnight at about 600° C. 
     
                                           TABLE V__________________________________________________________________________   29 30 31 32 33 34 35 36 37 38 39 40__________________________________________________________________________SiO.sub.2   30.0 23.7    33.3       10.0          15.0             25.0                35.6                   21.7                      34.1                         34.8                            33.8                               33.8B.sub.2 O.sub.3   11.2 11.4    19.0       14.4          13.6             12.0                25.8                   12.6                      19.8                         15.2                            14.7                               14.7MgO   13.0 -- -- 16.7          15.8             13.9                -- 14.5                      5.7                         8.8                            8.5                               --CaO   -- 18.3    4.1       -- -- --P.sub.2 O.sub.5   45.8 46.0    43.7       59.0          55.6             49.1                38.6                   51.2                      40.4                         41.2                            40.0                               40.0NH.sub.3   11.2 11.4    10.7       14.4          13.6             12.0                9.4                   12.5                      9.9                         10.1                            9.8                               9.8ZnO                  -- -- -- -- -- 11.5Al.sub.2 O.sub.3     -- -- -- -- 2.9                               --__________________________________________________________________________   41  42 43 44 45 46 47  48 49  50 51__________________________________________________________________________SiO.sub.2   40.0  39.0     41.1        40.8           37.3              50.5                 52.8                     53.4                        56.1                            58.0                               59.4B.sub.2 O.sub.3   13.9  13.6     14.3        9.5           17.3              11.0                 11.5                     15.4                        9.8 6.7                               17.2MgO   2.7 -- -- -- -- -- 4.4 -- --  -- --CaOP.sub.2 O.sub.5   37.9  36.9     38.9        38.6           35.3              29.9                 31.3                     31.4                        26.5                            27.4                               23.4NH.sub.3   9.3 9.0     9.5        9.4           8.6              7.3                 7.7 7.7                        6.5 6.7                               5.7ZnO   5.4 10.6     5.6        11.1           10.1              8.6                 --  -- 7.6 7.9                               --Al.sub.2 O.sub.3__________________________________________________________________________ 
    
     Specimens of the proper shape for use in determining various physical and chemical properties were cut from the slabs and those specimens, along with the remainder of the slabs, except for Examples 32-34, were inserted into an electrically-heated furnace. Examples 29-31 and 35-42 were heated at a rate of about 5° C./minute to the foaming temperatures recorded in Table VI, held at that temperature for the period of time reported in Table VI, and then cooled at furnace rate. Examples 43-46 were heated at a rate of about 50° C./hour to the foaming temperature reported in Table VI, held thereat for the times specified in Table VI, and thereafter cooled at furnace rate. Examples 47-51 were heated at a rate of about 5° C./minute to about 800° C. and thereafter the samples were raised at about 50° C./hour to the foaming temperatures listed in Table VI, maintained at that temperature for the times recorded, and then cooled at furnace rate. Examples 32-34 spontaneously developed into gas-ceramics as the melts cooled to slabs. 
     Table VI also includes a visual description of the original glass, a visual description of the heat treated product, various properties exhibited by the heat treated product, and crystal phases identified therein. Several specimens were immersed into boiling water for 1-3 hours and their appearance examined thereafter. A rating of 1-5 was assigned; 1 indicating very little or no change and 5 reflecting severe attack. 
     
                       TABLE VI______________________________________Ex-             Heat      Heat Treatedam-  Glass      Treatment Product Appearance,ple  Appearance °C.-hrs.                     Properties, &amp; Crystals______________________________________29   White,     775-4     White, deformed, densitytranslucent          0.6 g/cm.sup.3, fine-medium                     bubbles30   White      775-4     White, deformed, fineopal, waxy           bubbles, density 0.7fracture             g/cm.sup.331   Clear,     775-4     Medium-to-coarse bubbles,seedy                orange skin, density                     0.8 g/cm.sup.3, floats on                     water32   Spontaneous          Glassy skin, 300%gas-ceramic          by volume body expansion33   Spontaneous          Glassy skin, 250% bygas-ceramic          volume body expansion34   Spontaneous          Glassy skin, 400% bygas-ceramic          volume body expansion35   Clear      900-2     Fine-grained, light                     orange, glossy skin,                     few large blisters,                     BPO.sub.436   Clear      800-2     Medium cells, uneven                     matte skin, 400% by                     volume body expansion,                     Mg.sub.2 P.sub.2 O.sub.737   Clear      900-2     Fine cells, glossy skin,                     350% by volume body                     expansion, amorphous38   Clear      900-2     Very fine bubbles, light                     pink, glossy skin, 300%                     by volume body expansion,                     amorphous, H.sub.2 O test 539   Translucent           900-2     Coarse cells, white,White                matte skin, 400% by volume                     body expansion, H.sub.2 O test 340   Clear      900-2     Very fine cells, pink,                     glossy skin, 300% by volume                     body expansion, hygroscopic,                     amorphous41   Clear      900-2     Very fine ceIls, ivory-                     white, glass skin, 300%                     by volume body expansion,                     amorphous, H.sub.2 O test 242   Clear      900-2     Very fine cells, ivory-                     white, glass skin, 300%                     by volume body expansion,                     amorphous, H.sub.2 O test 243   Clear      900-2     Extremely fine cells,                     orange, glass skin,                     200% by volume body                     expansion, H.sub.2 O test 1+44   Clear      900-2     Extremely fine cells,                     orange, glass skin,                     200% by volume body                     expansion, H.sub.2 O test 1+45   Clear      900-2     Very fine cells, ivory,                     glass skin, 300% by                     volume body expansion,                     amorphous, H.sub.2 O test 546   Clear      900-2     Very fine cells, ivory,                     glass skin, few glassy                     cords, 150% by volume                     body expansion, H.sub.2 O                     test 1+47   Clear      950-2     Very fine-celled, ivory-                     white with smooth, glassy                     skin, 300% by volume body                     expansion, H.sub.2 O test 248   Clear      1000-1    Fine-celled, salmon pink                     foam with glassy skin,                     very few small blisters,                     150% by volume body                     expansion, H.sub.2 O test 149   Clear      950-2     Very fine-celled light                     orange foam with smooth                     glassy skin, 300% by                     volume body expansion,                     H.sub.2 O test 150   Clear      950-2     Very fine-celled light                     orange foam with smooth                     glassy skin, 200% by                     volume body expansion,                     H.sub.2 O test 151   Clear      1000-1    Very fine-celled white                     foam with white opal glass                     skin, 200% by volume body                     expansion, H.sub.2 O test 1______________________________________ 
    
     Table VII illustrates that the density of the inventive products decreases with heat treatment, corresponding to an increase in cell volume and cell diameter. 
     
                       TABLE VII______________________________________Example No. 42Heat Treatment      Density (g/cm.sup.3)                   Cell Diameter (microns)______________________________________None       2.40         --5° C./min. to      1.64          5850° C.-hold2 hrs.5° C./min. to      1.10         20900° C.-hold2 hrs.5° C./min. to      1.04         50950° C.-hold2 hrs.______________________________________ 
    
     Table VIII records the results of mass spectrometry bubble analyses on several foam compositions in terms of mole percent. 
     
                       TABLE VIII______________________________________Example        Hydrogen  Nitrogen______________________________________ 3             98.7%     1.3%12             95.9%     4.1%17             98+%      --26             98.7%     1.3%33             99.3%     0.7%______________________________________