Source: http://www.google.com/patents/US4851150?dq=7,237,634
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Patent US4851150 - Drying control chemical additives for rapid production of large sol-gel ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsLarge sol-gel derived silicon-containing monoliths are prepared with the use of drying control chemical additives selected from the group consisting of glycerol, formamide, oxalic acid and acids of the formula Cn H2(n-1) O2 N to control the ultrastructure of the gel solid and pore phases. Gelation, aging,...http://www.google.com/patents/US4851150?utm_source=gb-gplus-sharePatent US4851150 - Drying control chemical additives for rapid production of large sol-gel derived silicon, boron and sodium containing monolithsAdvanced Patent SearchPublication numberUS4851150 APublication typeGrantApplication numberUS 06/583,741Publication dateJul 25, 1989Filing dateFeb 27, 1984Priority dateFeb 27, 1984Fee statusLapsedPublication number06583741, 583741, US 4851150 A, US 4851150A, US-A-4851150, US4851150 A, US4851150AInventorsLarry L. Hench, Gerard F. OrcelOriginal AssigneeUniversity Of FloridaExport CitationBiBTeX, EndNote, RefManPatent Citations (4), Non-Patent Citations (2), Referenced by (71), Classifications (11), Legal Events (10) External Links: USPTO, USPTO Assignment, EspacenetDrying control chemical additives for rapid production of large sol-gel derived silicon, boron and sodium containing monoliths
US 4851150 AAbstract
Large sol-gel derived silicon-containing monoliths are prepared with the use of drying control chemical additives selected from the group consisting of glycerol, formamide, oxalic acid and acids of the formula Cn H2(n-1) O2 N to control the ultrastructure of the gel solid and pore phases. Gelation, aging, drying, and densification of the sol-gel derived monoliths may be performed rapidly in tens of hours instead of tens of days without cracking, final densification at temperature of 800� C. to 1200� C. or less being possible. The silicon-containing monolith comprises a ternary SiO2 --B2 O3 --Na2 O system.
1. A method for producing a crack-free sol-gel derived silicon-containing gel monolith comprising(1) forming a silicon-containing sol comprising SiO2 --B2 O3 --Na2 O in a proportion of 42 mole % SiO2, 30 mole % B2 O3 and 28 mole % Na2 O and a drying control chemical additive, said drying control agent enabling the formation of a crack-free gel monolith under drying conditions of elevated temperatures of between 70� and 150� C. for a period of between 18 hours and 96 hours; (2) casting said sol; (3) gelling said cast sol; and (4) drying said gelled cast sol to form a crack-free gelled monolith. 2. The method of claim 1 wherein the drying control chemical additive is selected from the group consisting of glycerol, formamide, oxalic acid and acids of the formula Cn H2(n-1) O2n and mixtures thereof.
It is an object of the present invention to produce large sol-gel derived silicon-containing monoliths.
These and other objects as will herinafter become more apparent result from a process wherein a silicon-containing sol is formed in the presence of a drying control chemical additive selected from the group consisting of glycerol, formamide, oxalic acid and other acids having the formula Cn H2(n-1) O2n, casting the drying control chemical additive-containing sol, gelling said cast sol, drying said gelled cast sol, and densifying said dried gel.
The instant invention makes it possible to control the ultrastructure of the gel solid and pore phases by chemical means so that gelation, aging, drying, and densification of the sol-gel derived monoliths may be performed rapidly in tens of hours instead of tens of days without cracking. By the practice of this invention, it is possible to increase the surface area of a gel and decrease pore size by a factor as much as 200% for a given R mole ratio (H2 O/TMS). Solids with an enormous surface area, in the range of 600 m2 /g to 1100 m2 /g, are obtained without cracking. Further, the enormous surface area makes it possible to convert the dried gels to fully dense objects at temperatures very much lower those required for traditional ceramics and glass processing. For example, pure SiO2 generally requires melt processing temperatures in the range of 2000� C., where as fully dense SiO2 has been prepared using this invention at temperatures below 900� C. Further, the range of densification temperatures of SiO2 can be controlled depending on the R ratio used in making the gel, the residual water content, and the particle size in the sol. Further, fully dense Na2 O--SiO2 glasses have been prepared using this invention at temperatures as low as 570� C.
For the purposes of this discussion, the sol-gel technique will be viewed as comprising six bacic steps, (1) mixing the sol, (2) casting the sol, (3) gelling the sol, (4) aging the gel, (5) drying the gel, and (6) densification of the gel. By introducing the drying control chemical additives (DCCA) during the first step of the process wherein the sol is initially formed, it is possible to control each of the five subsequent steps.
Typical precursors for the SiO2 monomer units and the Si--O--Si bonds in the structure are silicon tetraalkoxides having the general formula Si(OR)4 wherein R is an alkoxide group. These compounds, known generally as tetraalkoxide silanes, include but are not limited to, tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetrabutoxysilane. Tetramethoxysilane Si(OCH3)4 is the preferred tetraalkoxide silane. Additionally, other monomers precursors may be included as well. For example, B(OCH3)3 may be used as a source of B2 O3, NaOCH3 may be used as a source for Na2 O, and LiOCH3 and LiNO3 are suitable as lithia precursors for binary and ternary systems.
Where pure silicon dioxide is the desired final product, tetramethoxysilane (TMS) is preferred as the precursor for the SiO2 monomer units and Si--O--Si bonds in the structure. The mixing of water with TMS forms a silica sol through the following hydrolysis and polycondensation reactions:
Si(OCH3)4 +4(H2 O)&#8594;Si(OH)4 +4CH3 OH (1)
Typically, in the first mixing step to form the sol, the DCCA is added to 5 to 40 mols of distilled water. In the preferred embodiment, wherein pure silica is produced as the end product, TMS is the precursor monomer and oxalic acid is the DCCA. Oxalic acid, in a mole ratio of 0.01 to 0.40 mole per mole of TMS is added to 5 to 40 moles of distilled water. The critical range of oxalic acid to TMS for ultrastructure drying control is from 0.1% by weight to 10% by weight. Smaller concentrations are not effective and larger percentages are uneconomical and affect the surface of the object in a negative fashion. 1 to 5 moles of TMS is added to the solution while mixing continuously for a time between 5 to 120 minutes while increasing the temperature of 70� to 100� C. Higher mixing temperatures produce a short gelation time. The presence of the DCCA permits the use of a substantially higher mixing temperature since the DCCA slows down the gelation time by approximately 2 to 6 times.
Gelation occurs in the mold with the solid object resulting taking the shape, configuration, and surface finish of the mold. Gelation times with the oxalic acid DCCA are typically 12-24 hours at 25� C., 2 to 6 hours at 70� C., and 5 minutes to 2 hours at 100� C. The solidified gel is then placed into an aging oven and at a temperature ranging from 60� to 150� C. for a fixed time ranging between 1 to 15 hours. Alternatively, the temperature of the mold or gel may be varied from 60� to 150� C. through the use of heater coils or microwave without handling of the mold or gel. During the aging process, densification of the gel occurs without drying. Aging is accomplished in a closed container and takes place under an atmosphere estalished by the vapor pressure of the pore liquid. The rate of shrinkage is greatest for the higher aging temperatures. During shrinkage liquid is being expelled or expressed from the gel into the container. During aging, the strength of the fibrillar network of the gel increases manyfold. Additionally, the DCCA gels demonstrate a much smaller and more uniform distribution of pores than those absent the DCCA material. Since larger pores serve as crack initiators during drying, their absence contributes to less cracking of the monolith. Finally, the presence of the DCCA seems to prevent the growth of individual silica particles at the expense of the fibrillar network.
The thus aged gel is now subjected to drying. Drying is carefully controlled to proceed at an evaporation rate that does not exceed the shrinkage rate of the gel. Typically, any excess liquid present after the aging process above is removed. Then the pore liquid is removed by evaporation over a temperature range between 70� to 150� C. for a fixed time varying from 18 to 96 hours. During this stage of the process, the DCCA permits controlled evaporation of the pore liquid by controling the vapor pressure thereof. As the gel is dried, those gels with an optimal ultrastructure and superior resistance to drying stresses are characterized by a change in visible optical scattering while they are dried. The optical sequence is: transparent, very slight bluish tint, white opaque, transparent, very slightly blue and finally transparent. These changes in optical properties as well as vapor pressure and weight of the gel may be used to monitor the drying process and thereby used in a feedback loop to optimize the rate of drying. Ultimately, the final stage of drying is monitored by measuring vapor pressure and weight loss. When the theoretical molecular weight of silica is reached, drying may be terminated. Surface areas of the fully dried silica monoliths prepared in this manner range from 600 m2 /g to 1100 m2 /g.
The ultraporous dried silica gels are converted to partially dense or fully dense monolith objects by heating from 150� C. up to 800�-1200� C. over a period of from 1 day to 3 days. Little change in density is observed up to 650� C. By 750� C., the silica object is at 90% density. Full density is reached between 900� C.-1200� C. depending on heating rate and the gel ultrastructure prior to densification.
By the practice of the present invention as described above, the solid silica produced can be varied from an ultraporous substance with a surface area of 1100 m2 /g to a full dense material. Additionally, the fully dried silica objects may be made within a 24 hour processing time. Fully dense silica materials may be made within a 1-4 day total process time at temperatures of approximately 900� to 1200� C. Additionally, any shape or size of object may be made by the DCCA silica invention by simple solution casting at temperatures no higher than 150� C. While oxalic acid represents the preferred DCCA, used in a critical range of 0.1% by weight to 10% by weight based on the weight of the precursor, glycerol in amounts up to 100 weight percent and formamide in amounts up to 100 weight percent are within the contemplation of the present invention as well.
Additionally, as mentioned above, the invention is not limited to the formation of pure silica objects. For example, alkali-silicate objects may be made essentially as described above with only small variations in the various process parameters. The major difference occurs in the densification step, the major difference in the densification being the densification temperature. Preparation of a fully densified 20 mol percent sodium oxide-80 mol percent silicon dioxide glass using a DCCA material requires only 540� to 580� C. densification temperature. Dense monolithic materials over the compositional range of from 10% Na2 O to 40% Na2 O with the remainder being SiO2 can be prepared with heating to 600� C. or less.
However, the composition 20 mole % Na2 O 80 mole % SiO2 (20N) represents a preferred embodiment. Various metalorganic precursors for the soda constituent of the gel are compared and optimized for use with tetramethoxysilane (TMS), solvent, and the DCCA method of controlling drying stresses. Suitable sodium compounds, with critical properties, are shown in Table 1, form a noncrystalline homogeneous 20N soda silicate sol upon reacting with polysilicic acid formed with TMS. The ten compounds represent a broad range of solubilities in either water or organic solvents and also a broad range of bond energies.
An equivalent sequence of four processing stages is used to make the gel monoliths from the various sodium precursors, as shown in Chart 1. However, the nature of the solvent, the pH, temperature of mixing and R ratio (H2 O/TMS) is varied for each precursor. Table 2 summarizes the values of these processing variables for each sodium precursor.
(iv) The choice of quantity of water used for hydrolysis depends on whether the solution is basic or not. For a basic medium the water ratio is chosen so that the gelation of the mixture is completed in about 3 minutes after removal from the 0� C. mixing bath (see Table 2).
(v) After finishing the sol preparation (Stage 1, Chart 1), the sol is cast into a polypropylene container and put into a fixed temperature oven for gelation (Stage 2, Chart 1) and further aging for a fixed time (Stage 3, Chart 1). The gelation and aging temperatures may range from 60� C. to 125� C. and the aging times from a few minutes to 1.5 hr.
(vii) During drying, the oven temperature is typically slowly increased from 60� C. to 125� C. in order to accelerate the evaporation rate. Drying at the higher temperature is desirable since it accelerates new bond formation during the rearrangement of the soda-silicate particles as the pore liquid is removed.
(viii) By following this procedure with NaOH or NaOCH3 precursors, dried monolithic gels with the nearly theoretical molecular weight of 20 mol % Na2 O-80 mol % SiO2 may be obtained with a total processing time of about 24 hours. A number of other sodium precursors produce acceptable monoliths through the gelation and the aging stages but crystallization of the gel occurs during drying. The crystals are due to reaction of the sodium with air, forming sodium salts.
NaOCH3 +H2 O&#8594;Na+ +OH- +CH3 OH
The 20N gel made from sodium methylate with a solvent/total volume ratio of 85% and with formamide as a DCCA may be dried at 125� C. to nearly the theoretical molecular weight. Considerable shrinkage occurs during this drying process.
However, the formamide DCCA causes problems in the conversion of the dried gel into glass. Since formamide (HCONH2) has an active amine group, it has a strong affinity to absorb moisture from the air. The absorption occurs prefrentially on the surface of the gel and results in uneven stresses between the gel surface and the bulk and thereby produces cracks.
This surface reaction is more serious in a fully dried gel than in a half dried gel. The stresses are sufficiently great in fully dried gels that they crack explosively when exposed to air. Reducing the quantity of formamide is only a partial solution. Only complete removal by thermal or chemical treatment eliminates the hydration cracking. Another alternative is to use a DCCA which does not produce a reaction with moisture. Glycerol (C3 H8 O3) is an alternative. However, the glycerol with its --C--O--H bond will decompose and react with the sodium ions in higher surface areas of the 20N gel bodies to form a sodium carbonate crystalline phase at a higher temperature.
X-ray diffraction of 20N gels made with glycerol as the DCCA show development of sodium carbonate during heating to 300� C. Scanning EM micrographs also show evidence of the carbonate crystals on the gel surface exposed to air, whereas there are no crystals in the freshly fractured bulk of the gel. This behavior is similar to the Na2 O--SiO2 gel system made without the use of DCCA chemicals. Thus, the primary advantage offered by use of the DCCA is control of the large shrinkage stresses during drying. However, the residual DCCA left in the pores can lead to subsequent environmental instability. This is especially true for formamide which produces explosive hydration stresses when exposed to moisture. Glycerol is a very much less reactive DCCA and does not induce moisture sensitivity to the gels. However, glycerol still produces a sodium carbonate reaction during heating due to formation of CO2 when it decomposes. The resulting sodium carbonate compounds make it difficult to control the gel-glass transformation. This is due to bloating that occurs when the surface carbonate decomposes first, the surface layer densifies, followed by decomposition of the carbonate in the interior of the gel. Removal of residual glycerol .sbsp.the only way to prevent this problem.
Glycerol as a DCCA also controls the rate of pore liquid evaporation and results in 100% reliable formation of dried 20N gel monoliths. There is no moisture sensitivity with the 20N gel monoliths made with a glycerol DCCA. The gels can be heated to several hundred �C. without problems. However control of full densification and the gel-glass transformation is difficult due to formation of sodium carbonates which leads to bloating during high temperatures.
Also suitable is a 20 mole % Li2 O-80 mole % SiO2 gel (20L system). Consequently, a number of soluble lithium compounds were investigated as possible lithia precursors to determine their effect on gel processing variables for the 20L system. Throughout the study formamide (NH2 CHO) was used as a DCCA.
(b) The ratio of CH3 OH to NH2 CHO DCCA was 1:1, and the ratio of solvent to TMS or TEOS was kept at 1:1.
(c) The water/TMS ratios (R) was varied for each precursor such that the gelation time at 0� C. was approximately 2 minutes; i.e., enough time to stir and cast before gelation occurred.
(d) The processing temperature of 0� C. slowed the reaction rate allowing higher R ratio to be used.
This first set of experiments showed the lithium-i-propoxide (LiOC3 H7) was the best organic precursor. Lithium methoxide (LiOCH3) formed gels almost as easily. Since LiOC3 H7 gave dark green gels, due to high impurity levels in the chemical, and has a higher molecular weight, the LiOCH3 precursor was selected for further study and process optimization.
Lithium nitrate (LiNO3), gave the best results for the inorganic precursor tested so it also was investigated further.
In this second set of experiments using (LiOCH3) and LiNO3 as lithia precursors, the processing variables R, pH and solvent/TMS ratio were optimized to give uncracked monoliths in the shortest processing time.
An uncontrolled sol has a pH 12. Reducing the pH with concentrated HNO3 increases the gelation time so the processing temperature do not need to be controlled and higher R ratios can still be used. Chart 3 shows the optimized procedure for making the 20L gels with the LiOCH3 precursor. Gelation and aging is similar to the process shown in Chart 2 but the gels are dried undirectionally by pouring out the liquor and leaving the gels to dry in the vial with the lid off.
Chart 3 shows the optimized procedure for 20L gels made with the LiNO3 precursor. This process and precursor also results in uncracked monolithic gels within 2 days or less.
Table 3 shows the results of each Li2 O precursor with both TMS and TEOS as the precursor for silica, along with the R ratio of the sol. The lithium alkoxides all produce monoliths, with LiOCH3, and LiOC3 H7 showing the best resistance to cracking. LiNO3 is also an acceptable inorganic precursor for the 20L gels.
For the binary Na2 O--SiO2 system, a typical composition includes 67 mole % SiO2 -33 mole % Na2 O (33N). For the ternary SiO2 --B2 O3 --Na2 O, system, a typical composition includes 42 mole % SiO3 -30 mole % B2 O3 -28 mole % Na2 O. Formamide is the DCCA studied. The DCCA materially reduces drying time requirements by binding the gel such that the increased drying rate does not destroy the monolithic character of the molded shape. Additionally addition of the DCCA modifies the dependence on the gelation time on the R ratio of the sol.
The source of SiO2 in the sols is preferably TEOS, Si(OC2 H5)4. A preferred source of B2 O3 is TMB, B(OCH3)3, and the preferred Na2 O precursor is SMM, NaOCH3 in CH3 OH.
A schematic representation of the processes used to produce monolithic gels is given in Chart 5. The solvent is either pure methanol CH3 OH, or a mixture of methanol and DCCA, in the proportion of 50 volume % each. The molarity of the acidic solution 0.14M HCL. The molarity of the basic solution varies from 0 to 7.53M NH4 OH. In the preferred process, a concentration of 1.20M is used.
The gels are cast in polyethylene, polypropylene, Teflon, Pyrex glass containers at room temperature, then aged at 60� C. for 24 hours. Teflon containers have the lowest adherence, regardless of the formamide content in the gel, whereas the adherence of the gel to Pyrex is a function of the DCCA content.
TABLE 1__________________________________________________________________________SOME CRITICAL PROPERTIES OF SODIUM PRECURSORS                SOLUBILITY PER 100 cc WATER OR INSODIUM  MELTING         BOILING               OTHERPRECURSOR   POINT �C.         POINT �C.               COLD WATER                        HOT WATER                                SOLVENT__________________________________________________________________________NaCl    801   1413  35.7     39.12   sl. s. alNaClO   in solution only               --       --      --NaClO3   248   d     79.0     230     s. cl. gl,c.NaClO4   d482  d     s        v. s.   s. cl.Na2 CO3   851   d     7.1      45.5    sl. s. al.NaOH    318.4 1390  42       347     sl. s. cl. gl,c.NaOCH3   d-CH3 OH         --    S. d     --      S. MeOHHCOONa  253   d     97.2     160     sl. s. alCH3 CO2 Na   324   --    119      170.15  sl. s. alNaNO3   306   d380  92.1     180     s. al.__________________________________________________________________________ Sl. s. al. = slightly soluble in alcohol s. al. = soluble in alcohol
TABLE 2__________________________________________________________________________THE OBSERVATION OF SOME SODIUM PRECURSORS REACT WITH TMSSODIUM  SOLVENT         MIXING       OBSERVATION   CONCLUSIONPRECURSOR   VOL. %         TEMP �C.               pH  R  GELATION                              AGING DRYING__________________________________________________________________________NaCl    H2 O       85         90    2-8 45 Precipitate                      White ColorNaClO   MeOH       85         25    7-10                   0  Good Blue Transparent Gel                                    Crystallization                                    Inside Gel BodyNaClO3   H2 O       85         90    2-7 45 Good Blue Transparent Gel                                    Color Changed                                    to White                                    CrystallizedNaClO4   H2 O       85         90    2-7 45 Good Blue Transparent Gel                                    White Color                                    CrystallizedNa2 CO3   H2 O       85         90    9-12                   45 Good Blue Transparent Gel                                    PrecipitateNaOH    MeOH       85         25    10-13                   2  Good Blue Transparent Gel                                    Good Blue Trans-                                    parent Gel Under                                    MeOH Atm.NaOCH3   MeOH       85         0     10-13                   2  Good Blue Transparent Gel                                    Good Blue Trans-                                    parent Gel Under                                    MeOH Atm.NCOONa  H2 O       85         90    2-6 45 Good Blue Transparent Gel                                    Crystallized                                    Inside Gel BodyCH3 COONa   H2 O       85         90    2-7 45 Good Blue Transparent Gel                                    Crystallized                                    Inside Gel BodyNaNO3   H2 O       85         90    2-7 45 Good Blue Transparent Gel                                    Crystallized                                    Inside Gel Body__________________________________________________________________________
TABLE 3______________________________________Lithia Precursor Cracking/No Cracking ResultsLithia       Silica PrecursorPrecursor    TMS           TEOS______________________________________        Cracking      No CrackingLithium Methoxide        Clear Yellow Gel                      Clear Yellow GelLiOCH3  R = 1.75      R = 2        No Cracking   No CrackingLithium-i-Propoxide        Dark Brown Gel                      Dark Brown GelLiOCH3 H7        R = 2         R = 2        No Cracking   No CrackingLithium-t-Butoxide        Dark Yellow Gel                      Dark Yellow GelLiOC4 H9        R = 2         R = 2        Cracking      CrackingLithium Hydroxide        White Gel     White GelLiOH         R = 1.5       R = 1.5        Cracking      CrackingLithium Phenoxide        Dark Green Gel                      Dark Brown GelLiOC6 H5        R = 1.5       R = 2        CrackingLithium Acetate        White Gel     No Gel FormedLiOOCCH3        R = 2        Cracking      CrackingLithium Benzoate        White Gel     White GelLiOOCC6 H5        R = 2         R = 2Lithium OxalateLi2 C2 O4        No Gel Formed No Gel Formed        No Cracking   No CrackingLithium Nitrate        Clear Gel     Clear GelLiNO.sub. 3  R = 2         R = 2______________________________________ ##STR2##
Schematic diagram of sol-gel processes for (a) binary 33N and (b) ternary 42S-28N-30B gel derived monoliths.
Having now generally described the invention, a better understanding may be obtained by reference to the following examples, which are provided herein for purposes of illustration only and not intended to be limiting unless otherwise specified.
0.1 mole (9.0 g) of oxalic acid is added to 10 moles (180 g) of distilled water. To this solution, 2 moles (304 g) of TMS is added while mixing continuously for 120 minutes, while the temperature is increased to 70� C. The intimately mixed sol is cast into a shallow rectangular polyethylene mold. The mold is maintained at ambient temperatures, 25� C., with gelation occurring in approximately 24 hours. This solidified gel is then placed in an aging oven at a temperature of 60� C. for a time of approximately 15 hours. During this aging process, a linear shrinkage of approximately 15% occurs. Excess liquid is removed from the gel mold and the gel subjected to evaporation at a temperature of 75� C. for a period of approximately 32 hours. The fully dried silica monolith prepared in this manner has a surface area of 850 m2 /g. The resulting ultraporous dried silica gel shape is converted to a fully dense monolithic object by heating up to 1050� C. during a two day period.
Example 1 is repeated with the exception that formamide is used as the DCCA, 10 mole % of formamide based on TMS being used. A large fully dense monolithic silica object results, the formamide providing the necessary control of the gel ultrastructure.
Example 1 is repeated with the exception that glycerol is used as the DCCA, 15 mole % of glycerol based on TMS being used. The resulting monolithic gel is final fired at 720� C. to produce a monolithic shaped silica article having a density approximately 90% that of fully densified silica.
2 moles of a silicon-containing precursor which is a binary mixture of NaOCH3 and TMS such that the final product is a 20 mole % Na2 O-80 mole % SiO2 product and 0.1 mole of oxalic acid are mixed together with 10 moles of water. The processing conditions of the mixing, casting, gelling, aging and drying steps of Example 1 are repeated. The final dried gel is fully densified at 600� C. to form a crack-free monolith.
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