This invention relates to the production of hollow spheres, and more particularly to methods for producing seeds and transformation of the seeds into hollow structures that can with stand operational temperatures up to 2,000° C. Structure and structures include any of the following: hollow spheres, a honeycomb like form consisting of hollow and sealed cells, forms consisting of close packed hollow spheres, forms consisting of non-close packed hollow spheres, forms consisting of one or more size of hollow spheres, and forms consisting of non-spherical hollow cells of both uniform and non-uniform sizes. A seed is a construct consisting of a core and a coating. Upon heating the core generates a gas either on its own or through chemical reaction with the coating. Simultaneously during heating the viscosity of the coating, consisting of a glass (or forms a glass), decreases so that the internal gas pressure created by the core can expand the coating, forming a hollow structure.
There are several sections in this review of the Background Art. They may seem as being unrelated, but each provides information as to the limits of competing technologies and technical information used in identifying new intellectual property.
Hollow spheres are sold by 4 companies, CenoStar, Petra, Potters, and 3M. CenoStar and Petra harvest hollow spheres from coal fired power plant waste. Those aluminosilicate spheres have a maximum operational temperature of approximately 1040° C. or below depending on composition. Petra, along with 3M, sell hollow spheres formed from soda-lime borosilicate glass with a maximum operational temperature of about 600° C. Potters sells hollow glass spheres but does not provide information as to the composition of the glass or indicate that the spheres can be used at any elevated temperature. Hollow spheres sold by CenoStar, Petra, and 3M have an internal gas pressure of 0.25 atmospheres or greater, a condition that impacts their thermal conductivity.
Methods for synthesizing hollow silica spheres have been a topic of research since 1968, gaining greater interest as the field of nanomaterials has advanced. In known methods for the synthesizing of hollow spheres, a preform is created and silica is deposited around the form by chemical processes. The interior preform is removed by either chemical reaction or firing at temperatures up to 500° C. The latter technique has proved more successful in retaining the hollow spherical shape. Scanning electron microscopy reveals that the wall structure of the hollow spheres consists of smaller spheres of silica. The micrographs also reveal that the wall of a sphere formed by such synthesis is porous. Formation of hollow spheres at low temperatures reduces their strength, and thus limits their use.
Poraver produces “Expanded Glass” which is a foam formed by mixing particles of calcium carbonate in molten recycled glass and then heating that mixture to 900° C. to decompose the carbonate and thereby forming the foam. The maximum operational temperature for the foam is approximately 700° C. Lack of uniformity in the size and distribution of hollow spaces limits their structural use, and the presence of an internal gaseous environment of carbon dioxide at 0.25 atm impacts their thermal conductivity.
There are two forms of metal foam, metal sponge and composite metal foam (CMF). The latter consists of aluminum cast around hollow steel balls. Metal sponge, is the most common form of metal foam primarily involving aluminum and large open cells produced by:
Ceramic foam is produced by casting ceramic slip around a polymer form (usually beads). The beads are removed during an initial low temperature firing of the casting to burn off the polymer in the form and drive off moisture in the slip before sintering the ceramic powder.
Aerogels are very expensive, very weak, and contain a gas that impacts their thermal conductivity.
In the oxidation of silicon carbide, a protective layer of silica is known to form at temperature below 1200° C., as O2 molecules pass through the oxide layer to react with SiC forming a solid product of SiO2. The physical character of the silica formed upon oxidation of SiC impacts the rate of oxidation.
S. Ramanathan et al. oxidized SiC in air at temperatures between 1202° C. to 1402° C. (S. Ramanathan, R. V. Muraleedharan, Ramprasad, and S. Banerjee, Oxidation Kinetics of Silicon Carbide Powder, Interceram, 14 (3), 1992, p. 157-159.) Ramanathan and coworkers periodically removed their SiC powdered samples from the tube furnace to weigh the specimen using an electronic balance. In Ramanathan's experiments at temperatures between 1202 to 1352° C. the rate of weight gain is typical of that expected for molecular diffusion of O2 through interstitial sites in the silica layer as it becomes thicker with continued reaction. However, at 1402° C. the oxidation rate of the SiC powder initially follows that associated with molecular diffusion, but after one cycle (for SiC fine powder) and two cycles (for SiC coarse powder) the rate of reaction comes to a near stop. Here, the term “cycle” refers to removing the specimen, weighing it, and returning it to the furnace.
Ramanathan et al. suggest that the “abnormal behavior of oxidation at 1402° C. could be attributed to the mobility of the silica scale at this temperature, which resulted in a drastic reduction in the surface area of the” silica reaction product “due to fusion of the adjacent” silica particles. The authors provided micrographs that the authors believe provide evidence in support of their conclusion.
The inventor associated with this patent application interprets the results of Ramanathan, and coworkers as follows. They report that the silica product formed by the oxidation of SiC at 1352° C. was “fragile.” Given that air was used as the oxidizing agent, there would be a significant rate of O2 arriving at the interface between silicon carbide and the silica product. At higher concentration of oxygen and higher temperatures more nuclei form per unit area at the interface for growth of quartz crystals. The growth of the nuclei soon impinges on each other bringing growth of the nuclei to a stop having produced, only, small crystals. Now oxygen diffuses through the thin layer of quartz crystals, but by a slower mechanism (solid-state diffusion), and at the SiC surface more nuclei form. The process continues forming a product consisting of small quartz crystals in the atomic structure of tridymite. Tridymite is stable at temperatures between 867 to 1470° C., whereas below 867° C. β-quartz is stable to 573° C., and below that temperature α-quartz is stable. Ramanathan in using SiC powder, the nuclei that formed were not aligned, such that complete bonding between the small quartz crystals did not occur. But, the formation of numerous nuclei produced small crystals that filled void spaces. At lower temperatures fewer nuclei form, and as a result larger quartz crystals form with unfilled void spaces.
Ramanathan and his coworkers claim that the SiO2 product produced at 1352° C. was “fragile,” more likely it was friable due to the incomplete bonding, between the smaller quartz crystals. However, in taking each specimen out of the furnace and allowing it to cool brought about phase transformation; tridymite to β-quartz, and β-quartz to α-quartz. Upon reheating the specimen α-quartz was transformed to β-quartz, and β-quartz to tridymite. With small quartz crystals, with a high surface area at which there is unsatisfied bonding, the phase changes allow the surface atoms to realign allowing bonding between the small crystals. The smaller a crystal is, the more unstable it is, and the smaller crystals will preferentially combine with other crystals (atom by atom) to form a larger and more stable crystal. However, the temperature is not high enough to allow sufficient movement of the SiO2 molecules in the two separate and nonaligned crystals to form a unified crystal with a single alignment of the crystal plains. The result, with the small crystals, is a phase with decreased void volume reducing the ability of O2 to reach the SiC—SiO2 interface. Structurally the silica product on a molecular scale will appear as fused silica, given the inability of small adjacent crystals to realign their crystal planes, surrounding small pockets of aligned molecules of SiO2. Fused silica, a liquid, once formed, although not a stable phase at temperatures below 1713° C., is a pseudo-stable phase at lower temperatures due to its viscosity preventing realignment of the molecules. Thus, once formed it will retain the vitreous structure.
The reduction in void volume with the formation of fused silica isolates the SiC from further oxidation, limiting the arrival of oxygen at the interface between SiC and SiO2 by transforming the movement of oxygen by molecular diffusion through interstitial sites to solid-state diffusion.
Costello and Tressler provide additional proof that the oxidized product changes structurally, impacting the diffusion of O2. (J. A. Costello and R. E. Tressler, Oxidation Kinetics of Hot-Pressed and Sintered α-SiC, Journal of the American Ceramic Society, 64 (6), p 327-331.) They reacted hot-pressed and sintered SiC in air at temperatures between 1200° C. to 1500° C. They proved, using platinum markers that growth of the silica layer occurs at the interface between the SiC and SiO2. They report, for both materials, at “the higher temperatures, parabolic behavior was exhibited for short times . . . . [f]ollowed by decrease in rates at longer times.” Parabolic behavior is typical of molecular diffusion control of the oxidation reaction of particulate. Costello and Tressler reported problems in some experiments for “the hot-pressed material, the rate at long times appears to increase at higher temperatures. Bubbles, as well as craters, which formed from the burst bubbles, were present on the samples oxidized at the higher temperatures, suggesting that rupturing of the oxide film by the escape of gaseous by-products may cause the increase.” The author of this report believes carbon-rich SiC produced CO(g) that produced the rupture of the SiO2 product layer. Costello and Tressler also found that the activation energies “varied with temperature, from 134 to 389 kJ/mol for the sintered alpha material and from 155 to 498 kJ/mol for the hot-pressed variety.” The lower activation energy is that associated with molecular diffusion, while the larger numbers at higher temperatures suggest that a slower mechanism for transport of oxygen becomes rate controlling. The higher activation energy is an indication of the diffusion process shifting from a molecular process that utilizes interstitial voids in the glass to solid-state diffusion where oxygen ions jump from covalent bonded silicon to another silicon atom. The process relies on the movement of unoccupied covalent sites.
Costello and Tressler believe the change in mechanism for the oxidation of SiC involves the formation of a crystalline phase. They report that “with a densification aid, cristobalite and mullite were detected in samples oxidized at 1300° C.” and not in specimens oxidized at 1200° C.
Zheng, Tressler, and Spear report on the oxidation of single-crystals of silicon carbide at temperatures between 1200 to 1500° C. (Z. Zheng, R. E. Tressler, and K. E. Spear, Oxidation of Single-Crystal Silicon Carbide, Part I Experimental Studies, J. Electrochem. Soc., 137 (3), March 1990, p. 854-858) They found that below 1350° C. that the activation energy “was approximately 120 kJ/mol . . . and 260 kJ/mol above 1350° C.” for the 0001-crystal face. They report that with “[d]ouble oxidation experiments using 16O2 and 18O2 indicated that the process is dominated by the transport of molecular oxygen at lower temperatures (<1300° C.) with a substitutional contribution from diffusion of ionic oxygen at higher temperatures.” Stated differently molecular diffusion at lower temperatures gives way to slower solid-state diffusion at higher temperatures.
The work of Ramanathan et al., Castello and Tressler, and Zheng and co-workers demonstrate that structural changes in the silica product layer can significantly alter the rate of diffusion of O2. While the discussion has been limited to the diffusion of oxygen, the structural changes are expected to impact the diffusion of other gases.
Improved methods and chemistries are provided for producing seeds capable of transforming into hollow structures and for transforming the seeds into hollow structures. An example method for producing a seed capable of transforming into a hollow structure includes providing a core, forming a coating around the core to create a coated core, forming an exterior layer surrounding the coated core, forming a layer of release agent surrounding the exterior layer, and heating the core, the coating and the exterior layer. The core can have a particular composition that reacts to generate a gas when heated to a first predetermined temperature. The coating can have a particular composition that will fuse to form a continuous shell surrounding the core when the coating is heated to a second predetermined temperature. Forming the exterior layer of material surrounding the coated core produces an encased coated core. The material of the exterior layer can have a fusion temperature such that the exterior layer of material fuses or sinters below a third predetermined temperature. The layer of release agent surrounds the encased coated core. Heating the core, the coating, and the exterior layer to a fourth temperature transforms the exterior layer to a fixed shell and produces a seed with the coated core surrounded by the fixed shell. The fourth temperature is greater than or equal to the third predetermined temperature. The fourth temperature is also less than the first predetermined temperature, and the fourth temperature is less than the second predetermined temperature. The example method can additionally include mechanically separating the seed from the release agent and from other seeds produced with the seed.
In a particular example method, the core, the coating, the exterior layer, and the release agent are all positioned relative to one another by a printing process prior to the heating the core, the coating, and the exterior layer to the fourth temperature.
In another example method, the step of providing the core can include providing a plurality of cores arranged in a single layer. The step of forming a coating around the core can include forming coatings around each core of the plurality of cores to produce a layer of coated cores. The step of forming an exterior layer of material surrounding the coating can include forming an exterior layer of the material surrounding each coated core of the layer of coated cores to form a layer of encased coated cores. The step of forming a layer of release agent surrounding the external layer of material can include forming a layer of release agent separating the encased coated cores from one another.
The example method can additionally include forming multiple layers of encased coated cores separated by release agent prior to heating the core, the coating, and the exterior layer to the fourth temperature. The cores of each layer can be arranged in a same lattice structure. Optionally, the lattice structure of each layer can be offset with respect to the lattice structures adjacent layers. In a particular example, the lattice structure can be hexagonal.
Example methods can additionally include forming an inner layer of the material between the core and the coating and forming girders of the material. The girders can extend from the inner layer of the material, through the coating, to the exterior layer of the material.
In example methods, the step of forming a coating around the core can include the use of an adhesive. After applying an adhesive to the core, the core with the adhesive applied thereon can be brought into contact with the coating material. The step of applying the adhesive to the core can include dropping the core through a cloud of the adhesive. The step of bringing the core with the adhesive applied thereon into contact with the coating material can include dropping the core with the adhesive applied thereon through a cloud of the coating material. The step of dropping the core through the cloud of the adhesive can include applying a positive electrical charge to one of the core and the adhesive, and applying a negative electrical charge to the other of the core and the adhesive. The step of dropping the core with the adhesive applied thereon through a cloud of the coating material can include applying a positive electrical charge to one of the adhesive and the coating material, and applying a negative electrical charge to the other of the adhesive and the coating material.
In example methods, the core can include silicon alloyed with an element that reduces the activity of silicon. Alternatively, the core can include silicon mixed with an element that alloys with silicon upon heating and reduces the activity of silicon. In a particular example method, the core includes silicon and at least one of iron and nickel.
An example method for forming seeds, capable of transforming into hollow structures, in a tray is also disclosed. The example method includes providing a tray having a bottom surface and depositing a first layer of release agent on the bottom surface of the tray. The example method additionally includes depositing a first layer of outer shell material on the layer of release agent. The first layer of the outer shell material can be patterned in an array of discrete spaced apart shapes. The example method additionally includes depositing a first layer of coating material on the first layer of outer shell material. The first layer of coating material can be patterned in an array of discrete spaced apart shapes. Each of the discrete spaced apart shapes of the coating material can be disposed on an associated one of the discrete spaced apart shapes of the outer shell material. The example method additionally includes depositing a layer of seed material on the first layer of the coating material. The layer of seed material can be patterned in an array of discrete spaced apart shapes. Each of the discrete spaced apart shapes of the seed material can be disposed on an associated one of the discrete spaced apart shapes of the coating material. The example method additionally includes depositing a second layer of coating material over the first layer of coating material and over the seed material. The second layer of coating material can be patterned in an array of discrete spaced apart shapes. Each discrete spaced apart shape of the second layer of coating material can contact an associated one of the discrete spaced apart shapes of the first layer of coating material, with an associated one of the discrete spaced apart shapes of the seed material disposed therebetween. The example method additionally includes depositing a second layer of the outer shell material over the second layer of the core material. The second layer of the core material can be patterned in an array of discrete spaced apart shapes. Each discrete spaced apart shape of the second layer of the core material can be disposed over an associated one of the discrete spaced apart shapes of the second layer of the coating material, and can be in contact with the first layer of the outer shell material. The example method additionally includes depositing a second layer of the release agent over the second layer of outer shell material. The second layer of the release agent can be in contact with the first layer of the release agent between the discrete spaced apart shapes of the first layer of the outer shell material. Optionally, the first layer of the outer shell material, the first layer of the coating material, the layer of core material, the second layer of the coating material, and the second layer of the outer shell material can be deposited simultaneously via a 3-dimensional printing process.
In a particular example method, the core material can have a particular composition that reacts to generate a gas when heated to a first predetermined temperature. The coating material can have a particular composition that will fuse to form a shell around the core when the coating material is heated to a second predetermined temperature. The outer shell material can have a fusion temperature such that the outer shell material will fuse or sinter below a third predetermined temperature. A fourth temperature is less than the first predetermined temperature. Also, the fourth temperature is less than the second predetermined temperature, and the fourth temperature is greater than or equal to the third predetermined temperature. The particular example method additionally includes heating the first layer of the outer shell material, the first layer of the coating material, the layer of core material, the second layer of the coating material, and the second layer of the outer shell material to the fourth temperature.
Aspects of the present invention are described, by way of non-limiting examples, with reference to the following drawings, wherein like reference numbers denote substantially similar elements:
Diffusion of gases through silica impacts the ability to transform a seed into a hollow microsphere (HMS). The information presented in this section provides the underlying technology for some of the inventions presented in this application.
Values for the diffusion coefficients of O2, H2O, OH, and several other elements for fused silica at temperatures between 800 to 2000° C. are plotted in the
The rate equation for a chemical reaction is written as
For diffusion control of the rate of a chemical reaction n equals 1, E is the activation energy, T is the absolute temperature, and R is the ideal gas constant. The activation energy reflects the rate controlling mechanism. For diffusion E is represented by the slope of the lines in
If there is a dramatic change in E, then there is a change in the mechanism. k0 is a weak function of temperature in comparison to the exponential term, and, thus, is considered a constant. That constant contains a frequency usually associated with thermal vibration of atoms and, for conversion of a solid, geometrical factors (flat surface versus spherical particles) associated with the shape of the material undergoing reaction. Since it is known that the rate of oxidation of SiC declines with temperature and that the activation energy is increasing, the line in
Costello and Tressler, have provided activation energies for the controlling mechanisms as the kinetic mechanism for reaction changes. The exponential terms for the reported activation energies for the transitions in mechanisms for sintered SiC are:
at low temperatures
at higher temperatures
a decrease by a factor of 10−6.71, while k0 increases by a factor of 101.33 from 1473K to 1773K. The overall impact of increasing the temperature is to decrease diffusion by a factor that equals 10−5.38 (10−6.71+1.33).
The same authors report for hot pressed SiC at low temperatures
that decreases by a factor of 10−12.2 to
at higher temperatures, while k0 increases by a factor of 101.73 from 1473K to 1773K. The overall impact of increasing the temperature is to decrease diffusion by a factor of 10−10.5.
Zheng et al. found for the (000
That decreases by a factor of 10−3.71 to
while k0 increases by a factor of 101.03 from 1473K to 1773K. The overall impact of increasing the temperature is to decrease diffusion by a factor of 10−2.68. Their analysis of the oxidation of the (0001) crystal face produced an activation energy of 223 to 298 kJ/mol suggesting a single and slower diffusion mechanism observed at higher temperatures.
The change in the values of the exponential terms is consistent with the estimated change in the value of the diffusion coefficients associated with the change from molecular to solid-state diffusion, as determined earlier from
Other investigators report high activation energies for the diffusion of oxygen through silica. Hinze et al. evaluated the oxidation of SiC and reported an activation energy of 452 kJ per mol for diffusion of oxygen over the temperature range of 1200° C. to 1550° C. (J. W. Hinze, W. C. Tripp, and H. C. Graham, The High-Temperature Oxidation of Hot-Pressed Silicon Carbide, in Mass Transport Phenomena in Ceramics, Plenum Press, New York, 1975, pp 409-419) Singhal reports, for oxidation rate controlling, an activation energy of 481 kJ/mol in hot pressed SiC containing 4 wt % Al2O3 over the temperature range of 1200° C. to 1400° C. (S. C. Singhal, Oxidation Kinetics of Hot-Pressed Silicon Carbide, J. Mater. Sci., vol. 11, 1976, pp. 1246-1253) Alumina is known to be a sintering aide.
Comparison of activation energies is possible using the data in
The results for the diffusion coefficient in cm2/sec are:
For solid-state diffusion of Ge and P we have the following equations:
Notice that the value of the activation energy in equation 10 is similar to the values in equations 3, 5, and 7 where the authors reported the rate of oxidation of silicon carbide was limited by molecular diffusion of oxygen through the silica product. The evidence suggests that the data in
The physical and chemical properties of silica and glass impact the transformation of seeds into hollow spheres and structures.
Fused silica's softening temperature is about 1680° C. The softening point of a glass is the temperature at which it has a viscosity of 107.6 Poise. At this viscosity a rod about 24 cm long and 0.7 mm in diameter elongates 1 mm/min under its own weight. Using that information and setting the density of fused silica at 2.196 g/cm3 the force per unit area (or pressure) acting on the silica to get it to flow at 1 mm per minute is 5,140 N/m2 (or 0.75 psi). For glass with a density of 2.52 g/cm3 the force per unit area is 5,900 N/m2 (or 0.86 psi). For hollow structures with an internal gas, an applied force is resisted by the glass and the internal pressure of the gas.
Glass can be readily formed or sealed when it has a viscosity to 104 poise. That viscosity is defined as the working point of a glass.
SiC+2SiO2=3SiO(g)+CO(g), ΔH2000° C.=1,364 kJ (15)
and
Si+SiO2=2SiO(g), ΔH2000° C.=599 kJ (16)
that can be used to transform a seed into a hollow sphere. The working point temperature for fused silica has a value of approximately 2400° C.
At 2000° C. the viscosity of fused silica is 1.5 orders of magnitude more viscous than that at the working point temperature. Thus, any application force on silica at 2000° C. should be small to avoid deformation.
The viscosities of fused silica and glasses plotted as a function of temperature have a negative slope. At temperatures below the softening point the slope of the line is more negative than that of the line at higher temperatures, as shown for commercial glasses in
By adding basic compounds to fused silica it becomes, by definition, a glass. Glass and fused silica are both amorphous and can be viewed as viscous liquids, becoming more fluid with increasing temperature. Basic compounds disrupt the bonding between Si and O atoms. The distinction between acid and basic oxides in glass is the strength of the bond holding the element to the oxygen. Silica (SiO2) has a strong covalent bond such that the oxide holds together when liquefied, forming fused silica. Silica is a network former and is referred to as an acid. Basic oxides, unlike silica, ionize upon fusion, breaking up the silica network and thus lowering its viscosity. Basic oxides, in decreasing order of basicity, are Na2O, CaO, Li2O, MnO, MgO, FeO, BeO, TiO2, and Al2O3. Alumina and TiO2 are amphoteric and can function as either an acid or a base. By adding a basic oxide to fused silica (now a glass with a high silica content) its viscosity is decreased. By converting fused silica to a glass, the viscosity line in
The following patent and applications by the same inventor include related technical information and are, therefore, incorporated herein by reference in their respective entireties:
U.S. Pat. No. 11,242,252 B2 entitled Refining Process for Producing Solar Silicon, Silicon Carbide, High-Purity Graphite, and Hollow Silica Microspheres;
U.S. patent application Ser. No. 17/002,645 entitled Methods for Producing Hollow Ceramic Spheres;
U.S. patent application Ser. No. 17/468,138 entitled Methods for Producing and Products Including Hollow Silica and Hollow Glass Spheres; and
U.S. patent application Ser. No. 17/530,963 entitled Methods for Producing Seed for Growth of Hollow Spheres.
The present invention discloses additional methods for producing a chemical construct including a core and a coating surrounding the core, the construct forming a hollow structure upon heating. In this document the construct is referred to as a seed. Upon heating, the coating's viscosity decreases, while the core produces, on its own or through interaction with the coating, a gas that causes the coating to expand forming a hollow structure. In this specification that process is referred to as the transformation.
The present invention overcomes the problems associated with the prior art by providing systems and methods for producing seeds in significant numbers, seeds that can be transformed into:
hollow spheres with significantly reduced internal pressure;
hollow spheres with a maximum operational temperature at or above 2,000° C.;
honeycomb structure consisting of sealed cells; and/or
honeycomb structure consisting of sealed pores with significantly reduced internal pressure.
The maximum application temperature (MAT) of a HMS is based on the silica content of the coating material. Higher MAT requires higher silica content in the seed's coating. It was stated earlier that the MAT for a HMS with a pure silica wall is 1580° C. for all applications. The temperature required to transform a seed to a hollow sphere with a pure silica coating is at or above 2400° C. (the working point temperature) and, with the use of reactions 15 and 16 requiring reactor pressures of 100 and 30 bar, respectively. The transformation temperature is fixed by the viscosity of the silica and cannot be changed. The pressure at which the transformation occurs can be reduced. The ideal pressure is 1 atm.
The example presented in
The concept of reducing the activity of Si is being applied to raise the temperature of transformation and reduce the required pressure, while increasing the silica content of the coating material. The temperature is raised to increase the silica content of the glass and thus raise the maximum application temperature. Since the temperatures involved are above 1900° C. the reduction of the activity of silicon involves forming a liquid alloy. There are three criteria used in selecting an alloying element, they are:
Si+O2(g)=SiO2 (17)
in the figure can reduce silica. The further the chemical reaction is below that for reaction 17 the greater is the possibility that those elements will violate criteria 1 above, which eliminates aluminum, magnesium, and calcium. The elements above the line for reaction 17 have less ability to reduce silica, that condition includes copper, nickel, and iron. Chemical reactions involving titanium and manganese fall in between and can be used, but at significant more cost.
Nickel and iron can be used to reduce the activity of Si for conditions presented in
Ni+SiO2=NiO+SiO(g) (18)
and
Fe+SiO2=FeO+SiO(g) (19)
However, these reactions will not progress significantly as written due to the predominance of SiO(g) production by reaction 16. The oxides (NiO and FeO) that form will dissolve in the glass, however the activity of the oxides in the glass will be small and decrease with increasing vapor pressure of SiO(g).
Equilibrium constant for reaction 19 is K19 and is only a function of temperature and is equal to (PSiO·aFeO/aFe·aSiO2). The activity of the silica is approximately 1, therefore aFeO=K8·(aFe/PSiO). The activity of FeO is equal to its mole fraction in the glass times an activity coefficient. At low concentration the activity coefficient is a constant. Thus, any change in the activity is due to a change in composition. The mole fraction can be converted to the weight percent of FeO.
Since the activity of a molecule in a solution is linked to concentration, a decrease in activity also results in a decrease in concentration of the component. The higher the operational pressure for the transformation of seeds, the less the oxides will impact the viscosity of the glass as explained in the preceding paragraph.
The drawbacks to the example presented in
An alternative approach to the variable activity of Si is to have Si combine with W, Ta, or Zr at an overall composition in a 2-phase region in any of the three binary systems (Si—W, Si—Ta, and Si—Zr). The suggested alloy elements have high melting point temperatures and will form 2-phase regions with Si at the temperatures of interest. With overall composition in a 2-phase region the activity of Si remains constant even as its concentration is reduced. This only applies if the overall concentration of Si remains within the 2-phase region.
Ni and Fe, as previously demonstrated, can be used to reduce the activity of silicon in producing hollow spheres with walls containing a higher silica content. The activity of silicon can be related to the mole fraction of Si in the molten metal alloy. At elevated temperatures, all solutions become ideal, where activities equal their mole fraction. Phase diagrams for Fe—Si and Ni—Si reveal that compositions of iron rich and nickel rich solutions with Si at low temperatures fall below the Curie temperature. At temperatures below the Curie Temperature, it is possible to form a permanent magnet by applying an alignment field. It is, thus, possible to produce magnetized hollow spheres.
An alternative chemistry is presented for producing hollow spheres with high silica content glass walls. In the previous section elemental Fe and Ni alloyed with Si was used to reduce the activity of silicon and thereby allow use of reaction 16 at higher temperatures with reduced internal pressure within the seed as it is transformed into a hollow structure. That reduced internal pressure is, with respect to that plotted in
The new process is a modification to the previous approach that can be employed to reduce the pressure of transformation by eliminating elemental Si in the core of the seed. Reactions 18 and 19 are repeated here along with the reaction for vanadium:
Ni+SiO2=NiO+SiO(g), (18)
Fe+SiO2=FeO+SiO(g), (19)
and
⅖V+SiO2=⅕V2O5+SiO(g) (20)
Without elemental silicon these reactions proceed as written to near complete consumption of the metal. The oxides of the metals dissolve in the glass, reducing its viscosity and thus lowering its maximum application temperature. However, this approach has a distinct advantage in reducing the external pressure required for a controlled transformation at a fixed pressure. Note that both FeO and NiO are weak bases, thus their presence in fused silica will have a minor impact on the viscosity and the maximum application temperature.
Seeds consisting of an iron (or a combination of Fe and SiO2) core and a fused silica coating are injected into a plasma plume where the temperature and pressure of the plume are represented by dashed lines 802 and 804, respectively, in
Once the seed reaches the conditions at point “b” the transformation of the seed into a hollow sphere begins. The viscosity of fused silica at 2500° C. is, as noted earlier, expected to be lower than the working point viscosity which occurs at approximately 2400° C. Iron oxide produced by the reaction is initially pure, but immediately begins to dissolve in the fused silica. That process reduces the activity of FeO, and the graph in
It appears in
The bond strength between oxygen and Ni is the weakest of the 3 elements in reactions 18 through 20. Elements with a stronger bond with oxygen can strip an oxygen atom away from silica at lower temperatures. It is thus possible to apply the concept presented in
In all cases the temperature is selected based on the viscosity of the seed's coating.
A VacuSphere is defined here as a hollow structure with an internal pressure at room temperature below 0.001 atm and that it will take more than 1,000 years for the internal pressure to rise to 0.01 atm in the presence of air at room temperature. To produce a VacuSphere it is important to isolate SiO(g) from reactions 16, 18, 19, and 20, and SiO(g) plus CO(g) from reaction 15, within the interior of the hollow structure. The chemical reactions that produce a VacuSphere are presented in Table I.
Diffusion of gases through silica impacts the ability to produce a VacuSphere. However, evidence indicates that CO can be used to transfer heat to seeds without significantly impacting the composition of the gas phase that forms in transforming a seed into a hollow sphere. Costello and Tressler reported that in the oxidation of hot pressed SiC in air the rate of reaction slowed at higher temperatures (1400 and 1500° C.) and then increased. Physical presence of bubbles and craters were present in the product layer, suggesting that rupture of the oxide film by escaping gas was the source of the increased reaction rate. The formation of bubbles occurred at silica layer thickness of 1.5 microns for sintered SiC and 4.7 microns for hot pressed SiC. The equilibrium constant for the reaction
2SiC+3O2(g)=2SiO2+2CO(g), ΔH1,500° C.=−1,883 kJ (21)
at 1500° C. is 4.1·1047. If the partial pressure of oxygen at the SiC—SiO2 interface is assumed to be 10−6 or 10−12 bar, the pressure of CO(g) can be as high as 6.4·1014 bar or 6.4·105 bar, both pressures certain to rupture the oxide layer. When O2 and CO pass through the silica layer by molecular diffusion they will do so at similar rates as their kinetic diameters are similar, 346 and 376 pm, respectively. There cannot be a significant buildup of CO pressure at the reaction interface with molecular diffusion as the gases use the same channels to pass through the silica layer. The two gases are coupled in molecular diffusion. However, with solid-state diffusion the movement of oxygen and carbon operate on separate structural paths, their diffusion is decoupled. Costello and Tressler's findings strongly suggest the following.
Oxidation of SiO(g) can block molecular diffusion. Air and water vapor should be considered in transforming seeds to hollow spheres given their low cost. Water vapor on heating will undergo some dissociation forming H2(g) and O2(g). For the Si—SiO2 and SiC—SiO2 systems both O2(g) and H2O(g) act as oxidizers.
During transformation of a seed to a hollow structure, in an oxidizing atmosphere, small crystals of SiO2 (and possibly some fused silica) form, blocking molecular diffusion as the seed is transformed, effectively isolating the core of a seed from its surroundings. Production of SiO(g) by any of the reactions presented earlier leads to deposition of silica when the monatomic oxide encounters O2 (or any other oxidizing agent) as shown in
At 1870° C. and at ambient pressure the interior pressure of oxygen is 9.4·10−15 bar. The SiO pressure on the surroundings side of the barrier is about 4.7·10−14 bar.
The oxidizer that enters the hollow sphere as it is formed reacts with SiO(g) or Si reducing their concentrations. That loss must be accounted for by adding additional material (Si or SiC) to the seeds core.
During cooling of a HSM, deposition of SiO(g) is represented by the chemical reactions presented in Table I. The cooling of SiO(g) deposits as Si and SiO2, whereas the combination of SiO(g) and CO(g) produces SiO2 plus SiC. The deposition of these materials can occur in the voids of the wall of the hollow structure further isolating the interior of the structure from its surroundings. Deposition of SiO(g) begins at temperatures above 1200° C., continues to temperatures as low as 800° C., and is highly likely at lower temperatures. The reported limiting temperature is based on the lowest top bed temperature in the operation of a silicon submerged arc furnace.
Cooling of HSMs will likely require a soak time at a temperature that allows the gases in the interior of the HSMs to react as presented in Table I, thereby producing the desired vacuum inside the HSMs.
There are glasses that precipitate into two separate and intermingled phases with heat treatment. The intermingled phases block passageways, limiting molecular diffusion of gases through interstitial sites. The extent of phase separation will depend on glass composition and the heat treatment.
The material in this section is directed to the transformation of a seed into a hollow structure by chemical reactions 15 and 16.
Variables that can be used to manage production of VacuSpheres include: Pressure and Temperature; Excess Si of SiC; Choice of Gas as Medium for Transferring Heat; Vacuum Treatment; and/or Inert Treatment.
The pressure-temperature relationships involved in transforming seeds to hollow spheres are presented in
Excess Si or SiC ensures that there will be adequate supply of reactant to prevent a concentration buildup of oxidizer in the hollow structure as it is cooled. The value of diffusion coefficients increases with temperature, and, thus, the addition of excess Si and SiC become important at the higher temperatures.
An oxidizer can significantly impact the oxidation rate of SiO(g), lead to inclusion of non-reactive gases in the hollow structure, and alter the ability to produce a vacuum in the structure.
A vacuum, or partial vacuum, can remove non-reactive gases.
In lieu of the vacuum treatment, a purified inert gas (possibly N2 at 700° C., a temperature below that where SiO(g) decomposes to Si and SiO2) may be used to remove unwanted gases in the hollow sphere such as H2(g) by diffusion.
Given the high temperatures involved in transforming seeds, and the need for rapid heating, a plasma torch is one means for heating gases. Another approach, e.g., with the use of H2O(g), is a commercial system for producing super-heated steam. These suggested heating methods are provided by way of example and are not to be considered as limiting. Seed injection into hot gases can be by elutriation. Seeds are ionized using a corona electrostatic spray gun or a tribo gun to keep them and the hollow spheres separated.
An advantageous condition for producing hollow spheres is at ambient pressure, thereby decreasing the cost and complexity of the equipment. It is possible to transform seeds to hollow spheres using CO, H2O, and air. While the use of air has economic advantage, it poses some additional problems and limitations. Those problems are best addressed by examining diffusion of gases through the wall of a hollow structure, and their impact on the internal environment of a hollow structure as it is produced.
Several chemistries can be used in the transformation of a seed to a hollow structure and then cooling it to produce an internal vacuum. A major component in the process is the structural characteristics of the seeds coating as the seed transitions to a hollow sphere. Earlier it was noted that in the rate of oxidation of SiC it is controlled by molecular diffusion of oxygen at temperatures below 1400° C. Experimental data for the oxidation of SiC reveals that the structural characteristic of the silica product layer changes with increasing temperature. At higher temperatures the diffusion mechanism shifts from molecular to a solid-state diffusion, a slower mechanism for transporting oxygen. In this application it has been shown that the structural change in the silica has a more remarkable impact on the solid-state diffusion of CO than that of oxygen. The temperature of transforming a seed at 1 bar are, from
The computed results provide a means to evaluate use of various environments in the transformation of seeds to hollow structures. Environments for the transformation of seeds include, but are not limited to, the following examples.
None at SCF equal to 10−6
3× (3 times) at SCF equal to 104
7× at SCF equal to 10−2
Interior Vacuum of VacuSphere—Not applicable, however it is possible that DN
If a nonreactive gas or gases (nonreactive is defined as a gas in a hollow structure that retains a pressure greater or equal to 0.001 atmospheres at room temperature) are concentrated inside the hollow structure, they must be removed to an acceptable level (by vacuum treatment or exposure to a purified inert gas at an elevated temperature, possibly N2 for hydrogen removal). Spheres are held at the condensation temperature until the desired vacuum is attained, and the hollow sphere is now a VacuSphere. Finally, the VacuSpheres are cooled to ambient temperature.
In the previous discussions, in transforming a seed into a hollow sphere, the goal has been to produce a seed coating that is sufficiently fluid to respond to a slight positive pressure differential between the gas generated within the seed and the surrounding pressure. In this section the transformation of a seed without controlling the pressure differential is described.
In producing hollow structures with a high MAT, it is important to raise the silica content of the glass of the hollow sphere. Raising the silica content requires transforming the seed at elevated temperatures, temperatures where chemical reactions 15, 16, and 21 produce gases with a total pressure greater than 1 atmosphere. The goal here is to identify the conditions that make it possible to transform a seed where the differential pressure across the seed's coating is substantial.
If the pressure differential is to be ignored, and there is successful transformation of seeds to hollow spheres, it will occur because:
Reaction 15 and 16 are both endothermic, thus criteria 3 is satisfied for those reactions, except for the fact that the rate must be fast enough to cool the core but not so fast as to raise the pressure to a point where it will rupture the see's coating.
Reaction 21 is exothermic and, thus, represents a special case that is examined at the end of this section.
If a plasma torch (or Quantum furnace) is used with injection of seeds into the plasma plume, the heat transfer coefficient to the seed will be exceptionally large, thus the surface temperature will instantly be that of the plasma, satisfying criteria 1. With rapid heat transfer to the core, there is the potential for the temperature of the coating to become too hot, reducing the viscosity of glass to a value that allows the gas generated within the seed to easily rupture the coating. That outcome can be avoided by injecting seeds into the plasma plume with a separate gas, to decrease the temperature of the plume.
The impact of heat transfer across the seed's coating, the variation in the viscosity of the glass, and the kinetics of the chemical reaction require a balance to achieve a high yield of seeds being transformed into hollow spheres with the uncontrolled pressure technique.
The impact of the internal pressure on this approach to transforming seeds to hollow spheres can be reduced by using the chemistries identified in the sections entitled “Description of New Core Chemistries, Decreasing Activity” and “Description of New Core Chemistries, New Reducing Agents” presented above.
Earlier in the section entitled “Description of Diffusion Issues and VacuSpheres” analysis of research results published by Costello and Tressler presented in this document reveals that at temperatures above 1400° C. the diffusion of O2 and CO through silica changes from molecular diffusion to solid-state diffusion. That change in mechanism can produce CO pressures at the interface between the SiC core and the silica coating to substantially exceed 1 bar. A similar shift in diffusion mechanism can occur with a glass with significant silica content. Thus, it is possible to transform a seed with a SiC core and coating consisting of a glass or glass frit heated in air to a temperature of 1400 to 1550° C., provided the glass's viscosity can respond the pressure of CO created by reaction 21.
The Drop Volume Technique (DVT) can produce a large volume of green-seeds (or constructs) at a rapid pace, and at ambient conditions with the use of adhesive. Continuous production is based on treating all seed-cores in a specified volume. That volume is defined by the cross-sectional area of the reactor times the distance the core can fall (the Drop in DVT) in one second after injection to the reactor. That distance is computed using Stokes' law or the graphs for friction factors plotted as a function of the Reynolds number for submerged objects. The Drop and the cross-sectional area of the reactor defines the Volume in DVT. That volume is injected with cores. Numbers have been calculated for injecting cores into the reactor occupying 10%, 1%, 0.1% of the volume as a function of the reactor's diameter. The results of those calculations are presented
In
The designations “renin” and “r max” in those diagrams refer to the smallest and largest sizes of the cores used in the calculations. Numbers for “rmin” and “rmax” vary depending on the core material and the chemistry that produces the gas that transforms a seed into a hollow sphere. The notation in the figures “HMS 1.5-100” stands for Hollow Microsphere with a wall thickness of 1.5 micron and a radius of 100 microns. Similarly, “HMS 3.0-400” refers to hollow spheres with 3.0-micron walls and a radius of 400 microns. The “Required Production Rate” for all the graphs and for the two sizes of hollow spheres is based on annually producing approximately 167,000 tonnes of seeds per reactor.
The required production rates appear to be the same in each diagram for rmin and rmax. The similarity in the numbers is because the mass of the seed's coating far exceeds that for the core. Thus, the difference associated with the mass of the cores for the different chemistries is masked by the logarithmic scale in the diagrams.
An example of a drop volume reactor and auxiliary equipment is presented in
Although not shown, the wall of the Drop Volume Reactor can be divided into segments using a non-conducting insulator between segments to allow each segment to take on either a positive or negative charge to repel the cores, adhesive, glazed-cores, initial-coated-cores, and glazed-initial-coated-cores from adhering to the walls of each segment.
A glazed-core is a core covered in adhesive. An initial-coated-core is a glazed-core covered in coating particulate. A glazed-initial-coated-core is an initial-coated-core covered in adhesive. A Green-Seed has all desired coats of adhesive and coating particulate.
The shape of the Drop Volume Reactor need not be limited to a cylinder. For example, an alternate shape might include a frustum to alter gas velocities or limit particle interaction with the reactor wall.
The glazed-initial-coated-cores leaving the Drop Volume Reactor fall into a vibrated bed of seed-coating-powder, which adheres to the adhesive on the exterior of the glazed-initial-coated-cores. The vibration of the bed is sufficient to toss powder up and have it cover the glazed-initial-coated-cores.
The vibrated bed is housed in a closed volume, as shown in
The vibrating bed can be on a conveyor belt (or similar device) moving the bed material in either horizontal direction. Alternatively, a rotary mixer, or similar device, can be used to replace the vibrating bed.
The gas leaving the vibrating bed's housing passes to a cyclone separator where the droplets of adhesive as well as any solid material carried by the gas are removed before the gas is sent to the compressor. The waste stream from the cyclone separator consists of adhesive and solid particulate. A series of baffle plates can be used instead of a cyclone separator to achieve the separation of the gas from the droplets of adhesive and solid particulate. The cyclone separator can be replaced with any device that achieves the desired separation of the gas from solid particulate and liquid droplets.
The product from the first DVT reactor can be passed through additional reactors to increase the size of the Green-Seed or to provide additional coatings of selective powders. That addition of a selective coating can be accomplished in any of the DVT reactors.
The green seeds packed in a friable material can be heated to a temperature where coating particulate surrounding the seed's core either sinters or fuses. The temperature is selected to not initiate the transformation of the seed into a hollow structure. The heat-treated seeds can be recovered from the friable powder by sieving (or other physical means). Some light milling may be required to free the seeds from the friable packing material.
The CPST's series of steps can be modified to produce seeds that, once transformed into a hollow sphere, have any of the properties described in U.S. patent application Ser. No. 17/468,138. The CPST and its multiple steps can, with automation, meet required seed production.
How many seeds need to be produced and at what rate? The simplest approach is to view each production step in the list for the CPST as a batch process. With that assumption the slowest step (in Δt) dictates the number of seeds that must be processed in each step to meet annual production. Those numbers, for a single line (1 out of five), are presented in
With close packing of the circles the surface area requirement can be computed for the total number of seeds presented in
With fixed stations for the different steps in the list of steps for the CPST, the slowest step with a fixed production rate dictates the size of surface area being treated. If the slowest step takes 2 seconds in processing green-seeds, the area being processed in each step will be about 125 m2 for HMS 1.5-100. If it is determined that processing of the seeds require twenty 2-second sequential operations the area requirement remains the same, namely 125 m2 must be completed every 2 seconds. The additional 2 second operations only identify the required area for processing, not the rate of production. The area for processing and the procedures for processing can be adjusted by:
Producing seeds in trays with the trays being processed in series includes specifying the area of a tray, placing the tray in a multi-processing carriage that can perform all steps needed to produce green-seeds. This requires the multi-processing carriage to move with the tray as it moves along the production line. The length of the line is dictated by the time to complete all processing steps, the size of the tray, and the required rate of seed production. Once the processing is completed the tray is released and the carriage is returned to where it is again loaded with another tray to begin the processing trip. This approach allows the producer to set the size of the tray; the smaller the tray the larger the number of carriages needed with a fixed production rate.
If any disruption in the processing occurs with a single carriage, that carriage can be removed from the production line without disrupting processes occurring in all the other carriages. Alternatively, the carriage can remain in the production line without further processing of material. Once the carriage reaches the end of the production line it can be removed for repair or maintenance. This is an advantage with respect to the classical production line where a disruption can bring an entire line to a stop.
A major disadvantage to processing in series with a moving multi-processing-carriage is in supplying materials and utility services.
Producing seeds in trays with the trays being processed in parallel includes specifying the area of a tray, placing the tray in a multi-processing carriage that can perform all steps needed to produce green-seeds. Processing occurs in the stationary carriage positioned next to the production line (conveyor belt). After all processing steps are completed, the tray is released to the production line and a new tray inserted in the carriage. This approach allows the producer to set the size of the tray; the smaller the tray the larger the number of carriages needed with a fixed production rate. This approach eliminates the problem of supplying materials and utility services to moving carriages processed in series.
Any disruption in the processing will occur within a single multi-processing-carriage. The carriage can be removed from its fixed position or serviced in place while production continues in all the other carriages. If the carriage is removed another can at once be put in its place. If there is a problem with a tray, it can be removed and another put in its place and processing in the carriage can be restarted, again without disrupting what is occurring in the other carriages.
Application of the coating material can be applied in two separate operations as shown in
The layer of seeds packed in release agent is initially heated slowly to burn off adhesive, without the escaping gas altering the seed-constructs shown in
In
The layer of seeds separated by release agent are initially heated slowly to burn off adhesive (if applied), without the escaping gas altering the seed-constructs shown in
Application of the coating material can be applied in two separate operations as shown in
3D printing is done in layers, the thickness of each layer being dependent on minimizing mixing of particulates of the different materials. The printing of all materials in a layer is done simultaneously or nearly simultaneously.
The Close Packed Surface Technique has the versatility to produce seeds that, when transformed, meet the requirement for all applications covered in U.S. patent application Ser. No. 17/468,138, entitled Methods for Producing Hollow Silica and Hollow Glass Spheres.
Seeds produced in the CPST process can be produced in any shape, and configuration.
The Grid Surface Technique (GST) has versatility, simplicity, and the use of adhesive can be minimal or eliminated. The seeds in the GST are produced in sheets and sintered in sheets, without a friable material. The sheets can then be used to produce the honeycomb structure described in U.S. patent application Ser. No. 17/530,963, entitled Methods for Producing Seed for Growth of Hollow Spheres (see e.g.,
The GST's series of steps can be modified to produce seeds that, once transformed into hollow spheres, have any of the properties described in U.S. patent application Ser. No. 17/468,138. The GST and its multiple steps can, with automation, meet required seed production.
In the following example, seeds are produced in sheets, and sheets on top of sheets, forming a block of seeds for producing the honeycomb structure upon transforming the seeds. The use of cubic, hexagonal, or any interlocking shape for seeds, versus spherical seeds, will produce the desired honeycomb shape with minimal open space between the hollow structures.
GST makes greater use of material as compared to material use in the CPST. That occurs because there is no separation distance with the GST as the seeds are formed. The area that must be processed with the GST has been computed using the number of seeds that must be produced as presented in
Producing seeds in trays with the trays being processed in series requires specifying the area of a tray, placing the tray in a multi-processing carriage that can perform all steps needed to produce green-seeds. This requires the multi-processing carriage to move with the tray as it moves along the production line. The length of the line is dictated by the time to complete all processing steps, the size of the tray, and the required rate of seed production. Once the processing is completed the tray is released and the carriage is returned to where it is again loaded with another tray to begin the processing trip. This approach allows the producer to set the size of the tray; the smaller the tray the larger the number of carriages needed with a fixed production rate.
If there is a disruption in the processing occurring with a single carriage, the carriage can be removed from the production line without disrupting processes occurring in all the other carriages. Alternatively, the carriage can remain in the production line without further processing of material. Once the carriage reaches the end of the production line it can be removed for repair or maintenance. This is an advantage with respect to the classical production line where a disruption can bring an entire line to a stop.
A major disadvantage to processing in series with a moving multi-processing-carriage is in supplying materials and utility services.
Producing seeds in trays with the trays being processed in parallel requires specifying the area of a tray, placing the tray in a multi-processing carriage that can perform all steps needed to produce green-seeds. Processing occurs in the stationary carriage positioned next to the production line (conveyor belt). After all processing steps are completed, the tray is released to the production line and a new tray inserted in the carriage. This approach allows the producer to set the size of the tray. The smaller the tray, the larger the number of carriages needed with a fixed production rate. This approach eliminates the problem of supplying materials and utility services to moving carriages processed in series.
Any disruption in the processing will occur within a single multi-processing-carriage. The carriage can be removed from its fixed position or serviced in place while production continues in all the other carriages. If the carriage is removed another can at once be put in its place. If there is a problem with a tray, it can be removed and another put in its place and processing in the carriage can be restarted, again without disrupting what is occurring in the other carriages.
A cross sectional view of 3D printed seeds is presented in
The printing process is presented pictorially step by step in
In
After the last application of low fusion temperature glass frit 3208 and release agent 3206, the process of printing seeds repeats/continues as described with reference to
Adhesive can be included, as needed, with any of the different particulates.
While the drawing in
Layers of seeds surrounded by release agent in
Alternatively heating to burn-off adhesive and fuse or sinter the low fusion temperature glass can continue to higher temperatures to transform seeds into a solid block of hollow structures.
Layers of seed can be stacked in any pattern. In
There are at least seven options for preparing or selecting silica for coating seed cores by application of 3D printing. They include, but are not necessarily limited to, the following.
Silica fume, also known as microsilica, is an amorphous (non-crystalline) polymorph of silicon dioxide. It is an ultrafine powder collected as a by-product of silicon and ferrosilicon production and consists of spherical particles (as shown in
Comminution can be used to produce particulate ranging in size from 1 to 10 micron. That particulate will have sharp edges as shown in
3. Silica Fume Production without the Silicon Submerged Arc Furnace
Silicon and silica can be heated without coke or coal to produce SiO(g) by reaction 16. Oxidation of SiO(g) with air produces silica fume. The product of this process would look like that presented in
Heating the particulate to a high temperature, where the viscosity of the silica cannot overcome the physical drive to reduce surface energy associated with sharp points. The heating produces more rounded particulate.
Heating the particulate to a high temperature, where the viscosity of the silica cannot overcome the physical drive to reduce the surface energy of the entire particle by reforming it as a sphere.
Glacial, river, and ocean tide activity produces fine grain silica. That material tends to have particulate in a rough to near spherical shape.
A waste product of the appropriate size may be available and only require attrition scrubbing to be used in 3D printing.
In many instances HMS will be formed with silica with a high degree of impurity content. That impurity content can specifically impact Options 4 and 5 by lowering required temperatures.
A VacuBoard is formed by transforming the continuous block of seeds produced with the Grid Surface Technique (GST) into a continuous block of hollow structures with an internal pressure below 0.001 atm requiring more than 1000 years for the internal pressure of the hollow structures to increase to 0.01 atm in the presence of air at 300° C. That requirement applies to the internal hollow structures and does not apply to the thin layer of hollow structures that form the exterior surface of the VacuBoard.
With some gases used in transformation, the seeds in contact with the exterior environment may act as an additional diffusion barrier to the external environment, and, therefore, prevent exterior gases from penetrating the interior of the VacuBoard. The exterior seeds may not undergo any transformation, forming a skin surrounding the VacuBoard.
Boards not meeting the requirements for VacuBoards are referred to as HollowBoards.
The impact of the internal pressure on the thermal conductivity is presented in
The impact of the two methods of heat transfer in gas-filled hollow structures is represented in
The thermal conductivity for HollowBoards is represented by the dashed lines in
Heat transfer in rigid hollow structures, with near perfect internal vacuum and no open porosity, eliminates natural convection. Thus, for VacuBoards line c-d in
The extent of radiant heat transfer increases with temperature and chamber size. The general characteristics presented in
Natural convection is eliminated by eliminating gas within the sealed hollow structures. This can be accomplished with two chemical systems. Details are presented in Table I. After forming a hollow structure, and upon cooling, decomposition of SiO(g) produces a fine powder mixture of elemental silicon and silica. If CO(g) is present the size of the Si grains produced with the decomposition of SiO(g) is important and can be controlled; rapid cooling to a temperature near 800° C. will produce a small grain size. With the small silicon grains there is a high surface area available for reaction with CO(g), which produces SiC and more silica. For both chemical systems presented in Table I it is possible to achieve an internal pressure below 10−3 bar.
Two methods are presented for transforming the block of seeds produced with the GST into Vacuboards. The processes are based on reactions 15 and 16. The SiO2 in those reactions is present as either a pure phase or combined with Si (reaction 16) or SiC (reaction 15). Alternatively, the SiO2 can be the silica in the glass coating material. The latter will require, to achieve the desired internal pressure, a higher operational temperature due to the reduced activity of the silica.
During transformation of the seeds into hollow spheres the block of seeds produced with GST is placed in a mold that confines horizontal growth while allowing for vertical growth (this arrangement can be switched). During the transformation process the seeds begin to transform filling the available space. Since horizontal space is limited, once the horizontal space is fully occupied the transformation involves the growth of the hollow structures in the vertical direction.
Uniform growth of the hollow structures requires uniform heat transfer. Two methods are presented by way of example. However, this list of methods is not exhaustive and should not be considered as limiting.
While there may be some irregular rate of growth between the hollow structures, the growth will tend to self-regulate as both transformation reactions (reactions 15 and 16) are endothermic. The faster a hollow structure grows the cooler the core of the seed becomes, thus slowing the rate of transformation.
During the transformation process, the low fusion temperature glass's viscosity can become low enough for the glass to flow. During the final transformation heating the low fusion glass interacts with the coating particulate, which has a higher silica content, dissolving some of the coating particulate. The addition of the coating particulate to the low fusion temperature glass increases its viscosity, decreasing its ability to flow. If that increase in viscosity is not sufficient in reducing the flow of the low fusion temperature glass, dams consisting of the coating particulate can be added, as shown in
This application claims the benefit of copending U.S. Provisional Patent Application No. 63/398,393, filed on Aug. 16, 2022 by the same inventor, which is incorporated herein by reference in its entirety.
Number | Date | Country | |
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63398393 | Aug 2022 | US |