US3009788A

US3009788A - Method of producing synthetic mica

Info

Publication number: US3009788A
Application number: US767824A
Authority: US
Inventors: Daimon Nobutoshi
Original assignee: Individual
Current assignee: Individual
Priority date: 1957-10-25
Filing date: 1958-10-17
Publication date: 1961-11-21
Anticipated expiration: 1978-11-21

Description

Nov. 21, 1961 NOBUTOSHI DAIMON 3,009,788

METHOD OF PRODUCING SYNTHETIC MICA Filed Oct. 1'7, 1958 b axi Fig. 5

fur En [42F IVobu/a .s/n' 00/07 on 3,009,788 METHQD Q1 PRQDUCING YNTHETIC MICA Nobntoshi Daimon, 140 Harnsato-cho, Chikusa-ku, Nagoya, Japan Filed Oct. 17, 1958, Ser. No. 767,824 Claims priority, application Japan Oct. 25, 1957 8 Claims. (Cl. 233tl4) The present invention relates broadly to the art of synthetic mica production, and is more particularly concerned with a new and improved method of making fiuorophlogopite by growing or enlarging mother or seed mica.

Two of the most frequently employed methods of making synthetic mica are to either treat potassium fluosilicate with alumina under pressure and heat, or by melting basic oxides, fluorides and feldspar together. It has been found, however, that when the mica melt cools after being heated by electric current or gas, the melt crystallizes into aggregates of various sizes. This is believed to be due to inadequate control of the cooling rate, disturbances in the temperature gradient, and undercooling of the melt. It is necessary to then separate the crystals from the aggregates by crushing and splitting, however, as a result a major portion of the mica splits into small fragments and the yield of crystals sufficiently large for commercial use is indeed small.

It is accordingly a primary aim of the present invention to provide a method of producing synthetic mica which avoids the noted disadvantages of the prior art processes.

Another object of this invention lies in the provision of a method of making mica crystals suitable for practical commercial uses, which involves essentially utilizing relatively small synthetic mica crystals as the mother or starter for the raw mica components, and which when melted and cooled in a crucible under controlled conditions, grow or enlarge into substantial size single or compound mica crystals.

Still another object of this invention is to provide a method of making a relatively large single mica crystal in a crucible by growing seed crystals whose a, b and c-axes coincide with those of each of the other seed crystals, or stated in the alternative, a method of making relatively large mica crystals whose crystal axes coincide with those of the seed crystals, utilizing seed crystals whose c-axis coincides with that of each of the other seed crystals, although not necessarily coinciding with the a and b-axes thereof.

A further object of the present invention lies in the formation of a relatively large mica crystal or compounded crystals whose size and shapes are determined by the contour of the melt crucible.

Other objects and advantages of the present invention will become more apparent during the course of the following description, particularly when taken in connection with the accompanying drawings.

In the drawings, wherein like numerals are employed to designate like parts throughout the same:

FIGURE 1 is a fragmentary perspective view of a mica sheet crystal employable in the method of this invention;

FIGURE 2 is a vertical sectional view through a crucible adapted to practice the present method;

FiGURE 3 is a vertical sectional view taken through a somewhat different form of melt crucible; and

FIGURE 4 is a graphical illustration plotting temperatures along particular portions of the crucible against the height thereof.

In accordance with the principles of this invention, crystals are grown or formed from seed crystals without the formation of any new nuclei, with the exception of amass Patented Nov. 21, 1961 melt crucible, preferably together with raw materials. These materials are packed upon the top of the mica crystals, which are assembled in the same c-axis direction by standing edgewise with the c-axis of each crystal perpendicular to the temperature gradient. The contents of the crucible are heated with a predetermined temperature gradient in order to completely melt the raw batch while at the same time melting only the upper portion of the seed crystals. When the melt in the crucible is cooled, the unmelted portion of the seed crystals starts to grow without the formation of spontaneously made nuclei. By the described method, a single crystal or layer of relatively large crystals of definite orientation are produced.

Referring now to the drawings, there is shown in FIG URE 1 a mica sheet 6 belonging to the monoclinic system to which there has been applied the a, b and c-axes thereof. It is to be noted that the c-axis is perpendicular to the horizontal plane of the sheet or crystal 6, and that the a and b-axes as well as the b and c-axes intersect at right angles to one another. Further, the a and c-axes lie at an angle of approximately 90 one to the other. The angle 5, which is of the order of 95 to 100, shows clearly the intersection of the a and c-axes. Applied also to the mica sheet 6 are the symbols 001, 010 and 221 designating the planes of the crystal as used in crystallographic studies.

Crucibles of various forms and shapes may be employed to practice the method of this invention, and one crucible of illustrative shape is designated in FIGURE 2 by the numeral 5. The crucible as shown is essentially rectangular and located interiorly thereof in upright relation upon the bottom wall of the crucible are a plurality of mica crystals 6. As indicated by the legend applied to. FIGURE 2, the multiple crystals 6 are assembled with the c-axis of each crystal horizontal with the crucible bottom wall, or stated otherwise, the c-axis of each crystal is perpendicular to the side walls of the crucible. Located upon the crystals 6 in tightly packed or pressed relation thereon is a predetermined quantity of raw material or mica scrap 7 of a composition to be later specifically described. As will also be noted in detail, the method of this invention is practiced by only heating a portion of the seed crystals 6, and the numeral 8 in FIG- URE 2 represents the melted section of the starter or mother mica crystals.

The raw materials 7 used in accordance with this invention are aggregates of small crystals or fragments of the seed crystals which are originally inserted into the small crystals of synthetic mica or fiuorophlogopite identified by the formula F KMg AlSi O On the other hand, there may be used aggregates of small crystals or fragments of small crystals of synthetic micas which can be obtained by the replacement of potassium in fluorophlogopite by cesium, sodium, barium, strontium, calcium or the like, by replacing the magnesium by cobalt, nickel, zinc, iron, manganese or the like, by substituting in place of aluminum and silicon such elements as cobalt, zinc, iron, manganese, germanium, gallium, boron, beryllium or the like, or as an alternative substitution for the aluminum and silicon, there may be used mixtures of Slog, A1203, KgsiFe, K2CO3, and other components corresponding to the chemical compositions of the fluor-micas.

It may be found upon occasion when melting the batch mixture that evaporation will occur of a part of the volatile components or a dissolution of a part of the crucible material, such as alumina, causing a change in the melt composition which shifts said composition from the expected ideal chemical composition of fluor-mica. Excess components are accordingly produced which disturb the regular growth of crystal and produce crystals of inferior quality. This results in glass or foreign crystals being deposited between the mica crystal plates, and a marked deterioration in the flexibility of split mica crystals occurs. Further, the excess components also disturb the formation of single crystals, as well as retarding the growth rate.

In accordance with the present invention, and for the purpose of preventing the formation of deteriorating excess components, the chemical composition of the raw batch mixture may be modified so that the resulting melt corresponds essentially with the ideal chemical composition of fluor-mica. To accomplish this purpose, there may be added a small quantity of volatile components to compensate for the evaporation during melting, and there may further be included a small amount of those components which might come from the crucible material to thereby compensate for dissolution during melting.

More specifically, in order to promote the formation of crystals of superior quality, there may be added to the batch mixture in amounts less than 8% by weight fluorides such as PbF ZnF CdF NH F, NH HF K SiF KF and the like. It will be seen upon the practice of the present method that the addition agent evaporates and prevents the decomposition of volatile components of the batch during the melting thereof.

The formation of crystals of good quality may also be promoted by adding in an amount less than 3% by weight ions such as cobalt, nickel, manganese and the like, which have tetrahedral coordination at high temperatures and octahedral coordination at lower temperatures. Optimum crystal formation is also obtained by addition agents which include ions such as barium, strontium and the like, whose ionic radii are almost equal to the ionic radius of potassium and can thereby replace potassium in fluorophlogopite. As indicated, however, particular conditions may demonstrate that addition agents of the character mentioned will not be required.

The shape of the seed or mother or starter crystals may be widely varied without departure from the principles of this invention. Specifically, large crystals may be placed in the same crystallographic orientation, that is, having the same orientation of the a, b, and c axes. The seed crystals may also be small crystals assembled in the same crystallographic orientation, or either large or small crystals placed in the same small c-axis direction. Small crystals may also be assembled in the same c-axis direction and pressed with or wtihout adhesives at room temperature or at various high temperatures up to the melting point of the crystals, or the seed crystal may comprise a relatively large single crystal.

With regard now to the raw or batch material 7, the batch may be made of fluor-mica crystals, or may be a raw material mixture of precisely or about the chemical composition of fluor-mica. Further, a crystallized aggregate or aggregates obtained from melt precisely or about the same as the chemical composition of fluor-mica may be employed, or quenched glass of the melt may be utilized. In either case, the batch is packed upon the standing edges of the mother or starter crystals in the melt crucible, in the manner shown in FIGURE 2.

The vessel or crucible containing the seed crystals 6 and raw batch 7 is heated in either an electric or gas furnace, or may be heated internally by passing an electric current through the batch. The temperature gradient is set exactly or substantially perpendicular to the c-axis of the seed crystals in order to first melt the raw batch and thereafter only that part of the seed crystals 6 which is in contact with the batch 7. The melted part of the seed crystals is identified in FIGURE 2 by the numeral 8. The crystals whose c-axis coincides with that of the unmelted seed crystals are then grown by either cooling the entire vessel '5 relatively slowly with a fixed temperature gradient, by moving the vessel in the direction of a lower temperature, or by moving the furnace so that the vessel is located in the cooler part of said furnace.

Substantial investigations have been undertaken of the method herein disclosed, and specific illustrative procedures which may be followed are set forth in the examples now to follow.

Example 1 Several single crystals of fluorophlogopite were placed as seed crystals on the bottom of a rectangular crucible measuring 1 cm. in width, 1 cm. in length and 5 cm. in depth. The crystals were arranged in the same crystal lographic orientation, that is, in the same orientation of a, b and c-axes with their c-axes being essentially parallel to the plane of the bottom of the vessel. A powdered mass of fiuorophlogopite was then packed upon the top of the upper ends of the seed crystals, and the furnace heated to 1400 C. at the level 5 mm. above the upper end of the seed crystal layer, providing a temperature gradient of 30 C./cm. around the upper end of said seed crystal layer. The crucible was then moved downwardly in the vertical furnace at a rate of descent of 0.7 mm./hr., which corresponds to a cooling rate of 2 C./hr. with the temperature gradient noted. After approximately 50 hours of cooling, the crucible was taken from the furnace, and it was. observed that the entire batch had grown into a single crystal.

Example 2 Relatively small mica crystals were assembled in the same orientation of their c-axes, and after mixing with a relatively small amount of the adhesive agent PbF and hot-pressed with a temperature of kg./cm. at about 1300 C. The block as thus formed was placed as seed crystals on the bottom of a rectangular crucible corresponding to the vessel 5 of FIGURE 2, with the c-axis of the block essentially parallel to the plane of the bottom of the crucible. Fluorophlogopite powder was then packed in the crucible in contact with the block and the level about 5 mm. above the upper end of the block was heated up to about 1400* C. and maintained at this temperature with gradient of 60 C./m. After the entire batch and the upper part of the block were melted, the crucible was slowly lowered at a rate of 0.4 mm./hr. to be thereby cooled very slowly at a rate of 2.4 C./hr. After the entire melt had solidified, the crucible was removed from the furnace. It was seen that the entire charge of batch and the melted portion of the seed crystal block had grown into relatively large crystals, whose c-axes coincided with the c-aXis of the block.

Example 3 A wedge-shaped crucible having a wedge angle of 20, and measuring 1 cm. across the top and 5 cm. in height was employed. The configuration of such a crucible is shown in FIGURE 3, the crucible being identified at 11. Several single crystals having the same orientation as that described in Example 1 above were assembled in wedge form, and placed in the lower end of a crucible with the same orientation as in the first example. The crystals of wedge shape are designated in FIGURE 3 by the numeral 14, and as will be now noted, a raw batch 13 is preferably packed thereupon.

Batch materials were prepared by mixing alumina, magnesia, silica, and potassium silicium fluoride of the GP. grade. The composition of the batch was essentially, by Weight, the 11.5% A1 0 27.0% MgO, 30.7% SiO and 25.1% K SiF After the entire batch and a portion of the seed crystal in direct contact therewith had melted, the crucible was withdrawn from the furnace to crystallize the melted batch and melted seed crystal into a single crystal of the same orientation as the crystal layer which was unmelted. The conditions employed for growing a single crystal included a temperature at the upper end of the seed crystal of about 1400 C., a temperature gradient of 20 C./cm., and a rate of descent of 1 mm./hr., providing a cooling rate of 2 C./hr. After solidification of the entire melt,

the rate of withdrawal was increased from 0.7 rum/hr. to 50 mm./hr.

Example 4 A wedge-shaped crucible of the same character described in the foregoing example was employed, and an essential departure in the procedure followed was to eliminate the use of raw batch. The entire charge used was a wedge-shaped block, assembled in the same mannor as the seed crystal block employed in Example 3. The wedge block was packed in the crucible with its caxis perpendicular to one of the wedge walls of the crucible and a substantial portion of the block with the exception of the apex thereof was melted at a temperature of about 1400f C. The unmelted portion or tip served as seed crystals in cooling. In this procedure a temperature gradient of C./cm. was employed, and a rate of descent of 0.2 mm./hr. utilized to provide a cooling rate of 04 C./hr. The crucible and furnace were then cooled by cutting ofi the heat supply.

It will now be seen that a single crystal or large crystals may be formed having sizes and shapes determined by the contour of the melt crucible, by utilizing synthetic mica crystals as the starter, the crystals being arranged so that their c-axes coincide. While it was brought out that the crucibles are removed slowly from the heat zone, essentially the same results may be obtained with a fixed crucible and a movable furnace. Also, essentially the same results are obtained by cooling the temperature of the crucible placed in the furnace, which has a predetermined temperature gradient. Gas or electrical furnaces may be employed, and horizontal furnaces utilized as a substitute for the vertical type. When using a horizontal furnace, the c-axis of the seed crystals is placed perpendicular to the axis of the furnace.

The synthetic mica herein produced is characterized by a higher temperature stability than natural mica, and thereby finds important applications such as in various electronic tubes the high operating temperatures of which cause fatigue and short life in natural mica. The synthetic mica as made by the processes herein disclosed is of a more pure composition than natural mica since all elements can be more readily controlled. The replacement of the (OH) radical from natural mica with P to produce synthetic mica of course results in the advantage that synthetic mica can withstand temperatures in excess of 1000 C., since the F is not readily vaporized. Accordingly, if the (OH) radical is replaced by a fluoride, natural mica scrap can be employed as the batch material over synthetic seed mica. As earlier disclosed, these new and improved results are obtained by arranging synthetic mica crystals with their c-axes perpendicular to the temperature gradient as shown in FIGURE 4 of the drawings.

It is to be understood that various other modifications may be eifected in the compositions, structures and procedures herein utilized without departing from the novel concepts of the present invention.

I claim as my invention:

1. A method of producing fluor-mica, which comprises arranging at least one fluor-mica seed crystal on edge in a heating vessel with the c-axis of the crystal substantially perpendicular to the temperature gradient which is perpendicular to the bottom of the vessel, 10- cating upon the exposed upper edge of the crystal a raw material having a chemical composition corresponding to that of fluor-mica, applying a controlled heat and melting the raw material and the upper portion only of the crystal to form a crystal the c-axis of which is oriented with the c-axis of the unmelted portion of the seed crystal.

2. A method of producing fluor-mica, which comprises arranging a plurality of fluor-mica seed crystals on edge in a heating vessel with the small c-axes of the crystals substantially perpendicular to the temperature gradient which is perpendicular to the bottom of the vessel, and with the a, b and c-axis of each crystal oriented with 6 the same axes of the other crystals, locating upon the exposed upper edges of the "crystals an aggregate of relatively small fragments of iluo-r-mica, applying a controlled heat and melting the aggregate and the upper portion only of the crystals, and cooling the melted aggregate and melted portion of the crystals to form at least one single crystal the c-axis of which is oriented with the c-axes of the unmelted portion of the seed crystals.

3. A method of producing fluor-m-ica, which comprises assembling a plurality of floor-mica seed crystals with their c-axes aligned and with each seed crystal adhesively secured to another crystal to form a shaped block, locat ing the crystal block on edge in a heating vessel with the c-axcs of the crystals substantially perpendicular to the temperature gradient which is perpendicular to the bottom of the vessel, locating upon the exposed upper edges of the crystals an aggregate of relatively small fragments of fluor-mica, applying a controlled heat and melting the aggregate and the upper portion only of the seed crystals, and cooling the melted aggregate and melted portion of the crystals to form at least one single crystal c-axis of which is oriented with the c-axes of the 1mmelted portion of the seed crystals.

4. A method of producing fluor-mica, which comprises arranging a plurality of fluor-mica crystals on edge in a heating vessel with the c-axes of the crystals substantially perpendicular to the temperature gradient which is perpendicular to the bottom of the vessel, locating upon the exposed upper edges of the crystals a raw batch mixture comprising synthetic mica aggregate and an addition agent selected from the group consisting of NH F, NH F ZnF CdF PbF- and K SiF applying a controlled heat and melting the aggregate and the upper portion only of the seed crystals, and cooling the melted aggregate and melted portion of the crystals to form at least one single crystal the c-axis of which is oriented with the c-axes of the unmelted portion of the seed crystals.

5. A method of producing fiuorophlogopite, which comprises arranging a plurality of fluorophlogopite seed crystals on edge in a heating vessel with the c-axes of the crystals substantially perpendicular to the temperature gradient which is perpendicular to the bottom of the vessel, locating upon the exposed upper edges of the crystals a raw batch mixture comprisingfluorophlogopite aggregate and an addition agent selected from the group consisting of a compound of cobalt, manganese and nickel and whose coordination number changes 4-6 times during crystallization, applying a controlled heat and melting the aggregate and the upper portion only of the seed crystals, and cooling the melted aggregate and melted portion of the crystals to form at least one single crystal the c-axis of which is oriented with the c-axes of the unmelted portion of the seed crystals.

6. A method of producing fluorophlogopite, which comprises arranging a plurality of fluorophlogopite seed crystals on edge in a heating vessel with the c-axes of the crystals substantially perpendicular to the temperature gradient which is perpendicular to the bottom of the vessel, locating upon the exposed upper edges of the crystals a raw material having a chemical composition corresponding to that of fluor-mica and not more than 3% by weight of an addition agent selected from the group consisting of a compound of cobalt, manganese and nickel and whose coordination number changm 4-6 times during crystallization, applying a controlled heat and melting the raw material and the upper portion only of the seed crystals, and cooling the melted raw material and melted portion of the crystals to form at least one single crystal the c-axis of which is oriented with the c-axes of the unmelted portion of the seed crystals.

7. A method of producing fluor-mica, which comprises arranging a plurality of fluor-mica seed crystals on edge in a heating vessel with the c-axes of the crystals substanaggregate and melted portion of the crystals to form at 10 least one single crystal the c-aXis of which is oriented with the c-axes of the unmelted portion of the seed crystals.

8. A method of producing fluor-mica as defined in claim 1, in which the temperature gradient is parallel to the horizontal plane of the heating vessel.

References Cited in the file of this patent UNITED STATES PATENTS 2,516,983 Hatch Aug. 1, 1950 2,645,060 Waggoner M July 14, 1953 2,675,853 Hatch et a1 Apr. 20, 1954 OTHER REFERENCES Hatch: Synthetic Mica Investigation, Bureau of Mines Report of Investigation 5337, June 1957, pages 24-45.

Kendall: Proceedings of International Congress of Pure and Applied Chemistry, pages 167 to 170, 1947.

Claims

1. A METHOD OF PRODUCING FLOUR-MICA, WHICH COMPRISES ARRANGING AT LEAST ONE FLOUR-MICA SEED CRYSTAL ON EDGE IN A HEATING VESSEL WITH THE C-AXIS OF THE CRYSTAL SUBSTANTIALLY PERPENDICULAR TO THE TEMPERATURE GRAADIENT WHICH IS PERPENDICULAR TO THE BOTTOM OF THE VESSEL, LOCATING UPON THE EXPOSED UPPER EDGE OF THE CRYSTAL A RAW