sign in sign up
<<
Home , Supercooling

United States Patent (19) 11] 3,985,531 Grossman 45) Oct. 12, 1976

(54) SPONTANEOUSLY-FORMED FLUORMICA Primary Examiner-Winston A. Douglas -CERAMICS Assistant Examiner-Mark Bell Attorney, Agent, or Firm-Clinton S. Janes, Jr.; 75) Inventor: David G. Grossman, Corning, N.Y. Clarence R. Patty, Jr. 73 Assignee: Corning Glass Works, Corning, N.Y. 57 ABSTRACT 22 Filed: Mar. 19, 1975 This invention relates to the manufacture of articles exhibiting the physical properties and internal micro (21) Appl. No.: 559,725 structure of glass-ceramics but which can be formed spontaneously from a glass-forming melt, i.e., the heat 52 U.S. Cl...... 65/33; 106/39.6 treatment of a glass body to cause the 51 Int, C.’...... C03B 32/00; C03C 3/22 in situ thereof conventional in the production of glass 58) Field of Search...... 106/39.6, 39.7, 39.8; ceramic articles is not required. More particularly, this 65/33 invention is concerned with the manufacture of glass ceramic articles having compositions within the NaO 56) References Cited MgO-SiO,-F field, wherein a fluormica constitutes the UNITED STATES PATENTS primary , which can be formed spontane 3,804,608 4/1974 Gaskell et al...... 106139.7 ously from a molten glass batch. 3,839,056 10/1974 Grossman...... 106.139.6 3 Claims, 3 Drawing Figures

U.S. Patent Oct 12, 1976 Sheet 2 of 2 3,985,531

East 3,985,531 1 2 undesirable, because the resultant microstructure con SPONTANEOUSLY-FORMEDFLUORMECA. sists of nonuniformly sized, relatively coarse GLASS-CERAMICS which are often oriented in a plane normal to the sur U.S. applications Ser. Nos. 559,731 and 559,730, face. Such a microstructure customarily provides me filed concurrently herewith by H. L. Rittler, describe chanically weak bodies. the production of spontaneously-formed glass-ceramic Fundamentally, this "normal's devitrification phe articles wherein BaO and/or SrO-FeO-SiO solu nomenon differs from the production of glass-ceramic tion and carnegieite and/or nepheline solid , articles in that it involves, crystallization taking place at respectively, constitutes the primary crystal phase. U.S. or near the . The fusion casting of application Ser. No. 559,786, filed concurrently here 10 refractory ceramic materials is another example of with by G. H. Beall, discloses the formation of spon crystallization occuring at or near the liquidus. Inocula taneously-formed glass-ceramic articles wherein alpha tion procedures have been developed in an attempt to comprises the predominant crystal phase. U.S. nucleate a fine-grain structure to thereby ameliorate application Ser. No. 559,788, filed concurrently, here the inherent weakness of coarse-grain microstructures. with by G. H. Beall, P. E. Blaszyk, and W. T. Brydges, 15 In contrast, the production of glass-ceramic articles III, discusses the production of spontaneously-formed through the controlled crystallization of glass bodies glass-ceramic articles wherein beta-spodumene solid contemplates much below the liquidus. (a solution constitutes the principal crystal phase. United larger degree of supercooling), such that the crystalli States application Ser. No. 559,787, filed concurrently zation process takes place at a much higher herewith by the present applicant discloses the manu 20 level where time can be used to influence the nucle facture of spontaneously-formed glass-ceramic articles ation and rates. wherein a fluormica comprises the primary crystal The present invention relates to spontaneously phase. Finally, U.S. application Ser. Nos. 559,787, formed glass-ceramic articles, that is, articles exhibiting 559,789, and 559,726, also filed concurrently herewith physical behavior and microstructures akin to those of by the present applicant, describe the formation of 25 conventional glass-ceramic bodies, but which can be spontaneously-formed glass-ceramic articles wherein produced through the simple cooling of a molten mass beta-spodumene solid solution, mullite, and celsian, with no subsequent heat treatment of a glass body respectively, constitutes the predominant crystal phase. being required. Thus, certain compositions within the The art of glass-ceramics was founded in U.S. Pat. NaO-MgO-SiO-F field, when cooled from a melt, will No. 2,920,971. As described therein, the classic glass 30. provide a uniform homogeneous of crystals ceramic article is produced through the controlled heat within a glassy matrix without the need for any further treatment of a precursor glass body. Hence, the manu heat treatment. In these compositions, the crystals have facture of a glass-ceramic article customarily involves been found to comprise the predominant proportion of three general steps. A glass-forming batch, to which a the total volume of the body, i.e., greater than about nucleating agent is commonly added, is melted. The 35 50% by volume, and are commonly less than about 5 melt is then simultaneously cooled to a temperature at microns in diameter. least within and, commonly, below the transformation It is believed that FIG. 1 can be useful in arriving at range thereof to yield an essentially crystal-free glass an understanding of the difference in crystallization and an article of a desired geometry shaped therefrom. mechanism involved between the instant spontaneous Thereafter, the glass shape is heated to a temperature 40 ly-formed glass-ceramic bodies and the conventional above the transformation range thereof to cause the glass-ceramics. The key is believed to lie in the overlap growth of crystals therein. In most instances, this crys of the rate curves for and crystallization tallization treatment will comprise two steps. In the pictured there. Hence, below the equilibrium first, the glass shape will be heated to a temperature temperature of a viscous , T, there is a tempera slightly above the transformation range and maintained 45 ture interval in which nuclei do not form a detectable thereat for a sufficient length of time to achieve sub rate. This range of temperatures (T-T) is the metasta stantial nucleation. In the second step, the nucleated ble Zone of supercooling. With conventional glass body will be heated to a higher temperature, frequently ceramic composition systems, no crystals form at or above the softening point of the precursor glass, to just below the metastable zone because of the low rate cause the growth of crystals on these nuclei. The SO of nucleation existing there. Thus, nucleation occurs transformation range has been defined as that tempera within the temperature. interval T-T. ture at which a molten mass is converted to an anor The crystallization process utilized in the production phous solid and has generally been deemed to lie in the of conventional glass-ceramic bodies is illustrated in vicinity of the point of a glass. FIG. 1(A). As is pictured there, crystallization is se As a result of this careful heat treatment of the glass 55 cured by reheating the supercooled liquid (a glass body, a homogeneously crystallized article is produced body) first into the temperature region of maximum wherein the crystals are relatively uniform in size. In nucleation, holding thereat for a sufficient length of general, glass-ceramic articles are highly crystalline time to achieve the substantial development of nuclei, and the crystals, themselves, very finegrained. How and then into the region of maximum crystal growth ever, as was noted above, for a detailed discussion of 60 where it is held to complete the crystallization. the theoretical aspects and practical considerations FIG. (B) depicts the nucleation-crystallization rela intrinsic to glass-ceramic article production, reference tionship obtaining in the case of spontaneously-formed is called to U.S. Pat. No. 2,920,971. glass-ceramics. As, can be observed, the metastable It has frequently been observed in cooling molten Zone of supercooling is much foreshortened, and the batches into glass bodies that crystallization will occur, 65 rates of nucleation and crystal growth much greater. ordinarily originating from the surface and growing Under such circumstances, nucleation and crystalliza inwardly. This phenomenon has been referred to as tion can take place with sufficient rapidity at tempera “normal devitrification and is almost always deemed tures just below the zone of metastable supercooling 3,985,531 3 4 that substantial dwell times within those respective temperatures used. The actual batch ingredients may regions are not demanded. Thus, a simple cooling comprise any materials, either the oxide or other com schedule applied to the melt will be adequate to pro pounds, which, when melted together, will be con duce a body having a uniform crystal dispersion verted to the desired oxide in the proper proportions. therein. . In the compositions recorded in Table I, MgF2 was It is believed likely that the nucleation rate is en employed to provide the fluoride content. hanced in the composition system of the instant inven The batch ingredients were blended together in a tion by a concurrently-occurring phase separation phe ballmill to aid in achieving a homogeneous melt and nomenon. Spontaneous opal resulting from then turned into a platinum crucible. After covering, phase separation are well-known in fluorine-containing O the crucible was placed in an electrically-fired furnace glass systems. and the batch melted at 1450-1500°C. for about four U.S. Pat. No. 3,804,608 discloses a number of com hours. The melt was thereafter poured into a graphite positions which can be formed into glass-ceramic arti or steel mold to yield a slab about 6 x 6 X % inch. After cle without employing the reheating step required in allowing the slab to cool in the ambient environment to the production of conventional glass-ceramic bodies. 15 about 750-850°C., as measured by means of an optical However, no mention is made therein of compositions pyrometer, this cooling requiring about one minute, it within the NaO-MgO-SiO-F field which can be crys was transferred to an annealer operating at about tallized to articles having a microstructure akin to con 650-700°C., depending upon the composition of the ventional glass-ceramic articles and wherein the pre starting batch. Table I 3 4. 5 6 7 10.7 8.5 8.6 8.0 10.6 7.3 15.3 15.4 15.5 13.7 67.2 724 73.2 72.3 71.6 8.2 6.5 4.6 7.2 7.1 s 79 103.4 102.7 08 103.0 103.0 - 3.4 - 2.7 - 18 - 3.0 - 30 100.0 100.0 100.0 100.0 100.0 f} 10 12 13 NaO 12.6 38 6.4 6.3 MgO 3.7 3.8 14.9 13.1 SiO | 69.3 68.7 74.9 76.8 : 7.7 6.5 6.5 6.5 102. 103.3 102.8 102.7 102.7 Os F - 3.2 - 2.7 - 2.7 - 2.7 100. 100. 100. 100.0 100.0 dominant crystal phase is a fluormica. The glasses of the present invention which are capa As the molten batch cooled, the melt seemed to ble of being spontaneously crystallized into glass stiffen in the normal manner until a temperature of ceramic articles consist essentially, by weight on the 40 about 950-1000°C. was reached, as determined by an oxide basis as calculated from the batch, of about optical pyrometer. At or about that temperature range, 5-15% NaO, 10-20% MgO, 65-76% SiO, and 5-10% a hazy opalization was observed in the surface of the F. In the preferred embodiment, the compositions will slab and at the edges thereof which quickly moved consist solely of the four constituents to insure uni toward the center of the slab. A wave of opaque crys formity of crystallinity in the final product. Neverthe 45 tallization followed closely behind. less, minor additions of various compatible metal ox The formation of this opalization is demanded to ides can be tolerated up to a total of about 10% by secure the subsequent growth of the desired fluormica weight. For example, such materials as LiO, K2O, crystallization. An examination of FIG. 1 (B) can be BOs, Al2O3, SrO, and Fe2O3 can enter into the fluor helpful in arriving at an understanding for this. Hence, mica structure without destroying the basic structural 50 as is pictured therein, there must be a very high degree nature thereof. However, POs and, in some instances, of nucleation at temperatures near or at the optimum BOs may be incorporated into the residual glassy ma growth temperature to assure fine-grained crystalliza trix which will lower the temperture capabilities of the tion as the melt cools. This nucleation, customarily glass-ceramic and sharply alter the coefficient of ther taking place at about 100-300°C, above the annealing mal expansion thereof. 55 point of the glass, appears to be supplied contempora Table I records a group of glass compositions, ex neously with the opalization phenomenon. pressed in parts by weight on the oxide basis as calcu Nevertheless, although spontaneous opalization has lated from the batch, which were subjected to the pro been observed in numerous glasses, unless one of the cess parameters of the instant invention. However, amorphous phases involved in the opalization is at least inasmuch as the totals of the individual batches equal 60 partially unstable as a glass such that of or approximate 100, the value recorded for each com some type are precipitated to function as nuclei, there ponent may reasonably be deemed to be reported in will be no subsequent spontaneous crystallization of the terms of percent. Since it is not known with which major glass components. Therefore, whereas the mech cation(s) the fluorine is combined, it is reported as anism working to provide the growth of crystals is not fluoride (F) and the oxygens fluorine correction factor 65 fully understood, it is believed that crystallites formed reported in accordance with conventional glass analysis at temperatures well above the annealing point of the practice. Fluorine volatilization during melting will glass during or immediately after the opalization phe average about 25-50%, depending upon the melting nomenon, which provide available nuclei for the 3,985,531 5 6 growth of crystals as the glass moves into the tempera to be a tetrasilicic variety having the probable formula ture region for maximum crystallization...... NaMg2.5SiOoF...... - TABLE II Example No. Visual Description: Crystal Phases Coef. Exp. (x10 I.C.) Very fine-grained, shiny sodium tetra- 80.0 fracture, translucent silicic mica . . . 2. Fific-graincd, smooth 50.5 fracture, opaque . . . 3 Fine-grained, smooth dull it . - fracturc, opaquc ...... 4 , 49. 5 Light opal glassy Nonc 70. appearance - ... ' ' , 6 Fine-grained, smooth dull sodium tetra-. . . . 65.3 fracturc, opaque silicic mica 7 Medium. opal glassy . . . " 15.6 appearance R . . . 8 Very fine-graincd, silky FF () fracture, opaquc 9 P 60.9 10 Finc-grained, smooth Y 97.2 fracture, opaque Medium opal glassy 97.9 appearancc - 2 Fine-grained, smooth dull 52.5 fracturc, opaquc : 13 Coarse-grained, sugary ' ' ' - fracture

Table II provides a summary of the physical charac The criticality of composition is evidenced in Exam teristics observed when the slabs were removed from ples 5 and 13. Thus, where fluoride is present in the annealer. Thus, a visual description of the slab amounts less than about 5% (Example 5), the fluor mica crystallization does not appear. Example 13 exteriorcrystal phases and a observed fractured in surface each as isdetermined provided andthrough the 30 points to the coarse-grained body obtained where X-ray diffraction analysis. Also, the coefficient of ther excess SiO, is present...... ' ' ' , mal expansion is reported as measured over the range The process of the instant invention can be summa of room temperature to 500°C. in the conventional rized as consisting of four general steps. First, a glass manner on bar samples cut from the slab. forming batch having a composition within the above The characteristic microstructure of the highly-crys- 35 stated ranges is melted. Second, the molten batch is talline slabs can be viewed in FIG. 2 which is a replica simultaneously cooled to a temperature about electron micrograph of the product of Example 4. The 100-300°C. above the annealing point of the glass to white bar at the base of the photomicrograph repre produce phase separation and nucleation and a glass sents a distance of one micron. The micrograph reveals body shaped therefrom. Third, the glass body is ex a crystal growth pattern superimposed upon a phase 40 posed to a temperature between the annealing point of separated glass. The exact role which the phase separa the glass and the temperature of phase separation and tion plays in influencing the nucleation and crystal nucleation for a period of time sufficient to cause crys growth rates is not fully understood at this time. How tallization of the desired fluormica phase. Fourth, the ever, the platey nature of the crystals is quite apparent, crystallized body is cooled to room temperature. with the mica platelets being grouped in clusters of 5 As has been explained above, the second or phase about 1-2 microns in diameter and having grown separation step is of vital significance to the successful around the phase separated droplets which are believed operation of the invention. Therefore, the rate of cool to precede the mica development. ing to which the molten batch is subjected must not be High temperature viscosity measurements have indi so rapid that adequate time is not provided for the cated a "setting point" in the temperature interval 50 required phase separation and nucleation to occur. between about 950-250°C. It has been theorized that With the glass compositions of the instant invention, this "setting point' probably corresponds to the onset laboratory experimentation has demonstrated that of crystallization which could thus account for a sud cooling rates between about 10-1000°C/minute as sure satisfactory phase separation and nucleation. denThe increase micas inconstitute apparent a viscosity. family of silicate minerals 5.5 These reactions commonly occur at temperatures be having a unique two-dimensional or sheet structure. tween about 850-1000°C. Most naturally-occurring micas are hydroxyl silicates, Since the compositions of this invention crystallize whereas micas produced synthetically have commonly very rapidly after the appearance of the opalization, involved replacing the hydroxyl groups within the crys exposure times of as little as two minutes within the tal structure with fluorine. These synthetic micas have 60 crystallization range may be sufficient to achieve the been termed fluormicas as a result of that substitution. desired high crystallinity, i.e., at least 50% by volume of That terminology is employed here. the body. Customarily, the crystallization will take The crystal phase observed to form in accordance place at temperatures between about 650-850°C. with the method of this invention is a fluormica which However, as was illustrated above in the specific exam could not be positively identified. The X-ray diffraction 65 ples of the invention, ease in production has recom pattern exhibits the characteristics of a 1M polymorph mended that the phase separated and nucleated body and, because there are no available trivalent cations to be placed into an annealer operating at a temperature substitute for silicon, the sodium fluormica is reasoned within or slightly above the crystallization range. Meth 3,985,531 7 8 ods and apparatus conventional in the glassmaking art but immersion in various such as oils and are similarly appropriate here. Annealing schedules as baths can also be operable. , brief as 0.5 hour can be successfully utilized, but the The mechanism by which this improvement in me more usual practice contemplates periods up to two chanical strength is imparted is not fully compre hours or longer. Nevertheless, the internal microstruc hended, although it is believed to involve the small ture and the physical properties of the final product do amount of residual glass which it thought to exist as a not appear to be significantly affected by very long continuous phase throughout the crystallized body. annealing schedules. Thus, such are not generally This assumption is believed to be supported by a study viewed with much favor from a practical point of view. of FIG. 2 wherein the residual glass is seen as depressed Although the preferred practice of the invention is 10 regions due to its greater solubility in the etchant used founded upon crystallizing the article as the phase sep to produce the replica electron micrograph. arated and nucleated glass body is cooled to room claim: temperature, it is possible to cool the melt to room 1. A method for making a highly crystalline glass temperature at such a rapid rate that phase separation ceramic article consisting essentially of fluormica crys and nucleation will take place but the desired crystalli 15 tals dispersed within a glassy matrix, said crystals con zation will not, resulting in a body that is essentially stituting the predominant proportion of said article, glassy. However, the desired crystallization can be at which comprises the steps of: tained by exposing the glassy body to a temperature a melting a batch for a glass consisting essentially, by within the crystallization range in like manner to that described above when the melt is simply cooled to 20 weight on the oxide basis, of about 5-15% Na2O, room temperature. Hence, again, it is the occurrence of 10-20% MgO, 65–75% SiO, and 5-10% F; phase separation and nucleation at temperatures above b. simultaneously cooling said melt at a rate between the crystallization range which is of critical importance about 10-1000°C./minute to a temperature about to the operability of the invention. 850-1000°C. to shape said melt into a glass body Finally, the mechanical strength of the crystallized 25 and secure phase separation and nucleation bodies may be materially improved by utilizing a ther therein; mal tempering process such as is employed with glass c. further cooling said shaped glass body and expos articles. Hence, as is shown in Ser. No. 559,788, supra, ing said glass body to a temperature between about filed concurrently herewith by Beall, Blaszyk, and 650-850° C. for a sufficient length of time to cause Brydges, a comparison of the mechanical strength dem 30 crystallization of the fluormica phase in said glass onstrated by annealed crystallized articles with that body; and then exhibited by crystallized articles rapidly chilled from d. cooling the crystallized body to room temperature. the crystallization range to room temperature can evi 2. A method according to claim 1 wherein said time dence a substantial enhancement in strength in the sufficient to cause crystallization is at least about two latter articles. The quick quenching can be especially 35 minutes. . . . effective when the crystallization is undertaken at the 3. A method according to claim 1 wherein said crys upper extreme of the crystallization range. Air temper tallized article is cooled to room temperature by means ing, viz., exposing the crystallized article to a blast of of a quick chilling technique to thermally temper said cold air to chill it to room temperature, is the preferred article. technique due to ease of practice and relative low cost, 40 + k + k sk

45

50

55

60

65 UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. : 3,985,531 Page l of 2 DATED October 12, 1976 INVENTOR(S) : David G. Grossman It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below: Column l, line 23, "559,787" should be -- 559,727 --. Column l, line 59, "finegrained" should be - fine-grained --. Columns 5 and 6, Table II, Heading "Coef. Exp. (x10 T/oc.)" should be - Coef. Exp. (xio /°C.)--. Figure l as shown on the attached sheet should appear on the cover sheet of printed patent. signed and sealed this Sixth Day of December 1977 SEAL Attest:

RUTH C. MASON LUTRELLE F. PARKER Attesting Officer Acting Commissioner of Patents and Trademarks