US2414680A

US2414680A - Process of crystal formation

Info

Publication number: US2414680A
Application number: US547263A
Authority: US
Inventors: Cutler D West; Frederick J Binda
Original assignee: Polaroid Corp
Current assignee: Polaroid Corp
Priority date: 1944-07-29
Filing date: 1944-07-29
Publication date: 1947-01-21
Anticipated expiration: 1964-01-21

Description

Ac:. D. wss'r :TAL

PROCESS OF CRYSTAL FORMATION Jan. 2l, 1947.

med my 29, 1944 FIG. l

FIG. 2

Mrs

l Patented Jan. 21,"1947 UNITED Ns'mrrs PATENT orifice PROCESS F CRYSTAL FORMATION Cutler D. West and Frederick J. Binda, Cambridge', Mass.; assignors to PolaroidCorporation, Cambridge, 'Massa a corporation of Delaware Application Jury 29,1944.,"seria1'No.547,263j

This invention relates to crystallography and more particularly to a new process for producing single crystals of predetermined size and orientation.

a solution or melt or by careful control of the conditions of growth, notably the thermal gradients, or both. The processes of the prior art. are all seriously limited from the standpointV both of the size of the nished crystal and the time 18 Claims. (C1. 23-302) 2' necessary for the process. In accordance with the present invention, it has been discovered that greatly improved results are obtained both from the standpoint of the size of the iinished crystal One of the objects of the invention is to pro- 5 and from the standpoint of time by growing single vide a process for growing relatively large single crystals from the melt under the influence oi crystals of uniaxial material and particularly an orienting forcecomprising a totally different crystals of predetermined dimensions and oriencrystalline material and, particularly under the tation. inuence of mica.

Another object is to provide such crystals by l0 An example 0f a unieXial Crystalline material a process of growth from the melt under the in which is particularly adapted to production by uence of an orienting force. means of the process of the present invention Other objects of the invention are to provide is sodium nitrate. Fig. 1 illustrates apparatus a process as outlined above wherein the orienting which has been found useful in Carrying 011.11 One force is a crystal of a diierent material from the l5 fOlm 0f the DrOCess of thenvention 4to produce crystal being grown, to provide such a process a lbasal section of Sodium nitrate: ANCOnlJaner wherein the orienting agent is a crystal ofrmica, Such as'gthe hollow block IE! in Fig. l may coinand to provide such a process which is particuprise any-suitable heat insulating material such, larly applicable to the growth of crystals of soiOr example, asthat sold vunder, the trade name dium nitrate and other alkali metal compounds, Marinite and deineS a Cavity l2 Of any desired such as the alkali halides,` and mixed crystals of shape. Within cal/ity l2 there is ShOWn a 011D alkali metal salts. I4 substantially lled with molten sodium nitrate A particular object of the invention is to prol5. Cup I4 may comprise any of a variety 0f vide a process for growing basal sections of unimaterials, but Preferred results haVe been 0baxial crystals, such as sodium nitrate, of large tained with aluminum fOl elJDrOXiInaiely 00015-, size (area) in an exceedingly short time. 0.003 inch in thickness. Other materials may be Further objects and advantages will in part used provided, however, that if the material be appear and in part be pointed out in the course one to which sodium nitrate will adhere strongly of the following detailed description of one or on solidifying, the material should either 'have more embodiments of the invention, which are substantially the same (2o-efficient of thermal exgiven as non-limiting examples, in connection pansion as solid sodium nitrate, or should be with the accompanying drawing, in which readily deformable by the sodium nitrate as the .Figure 1 is a diagrammatic View illustrating latter cools. On `the other hand, other rigid one embodiment of the process of the invention materials n'iay be Sed vlllOVlded the adhesion andcomprising a vertical section through a mold thereto of the sodium nitrate is not sufiicient'to utilized in said process; crack the crystal o-n cooling. In Fig. 1 it will be l Figure 2 is a diagrammatic sectional view of a noted-that an annular space I6 is provided becrystal grown in accordance with the process tween the outside of cup i4 and blOCk Walls l3- illustrated in Fig. 1; This arrangement is not essential. but is pref Figure 3 is a view similar to Fig. 1 illustrating 40 ferred from the standpoint of convenience of another modified form of the process of the inhandling. Element 20 irl Fig. 1 represents a vention; and cleavage section of mica preferably atleastsuf- Figure 4 is a diagrammatic sectional view illus- Cienly smaller in area thanpllp i4 t0 fili easily trating still another moded form of the process therein. l of the invention. 45 In' the practice of the invention with the The art of growing large single crystals is relaequipment shown in Fig. 1`, cup I4 may be filled tively old and includes work with growth from with ground sodium nitrate and both the hollow solution and growth from the melt. In general, block and cup then placed in a suitable oven such influencing of the shape of the crystal being as an electric muilie oven until the charge is grown has been aCCOmDliShed'ehel' lOy Seeding 50 completely melted. Alternatively, the sodiumni trate may bel melted separately and then poured into cup lll. Ineither case, it is preferred to heat both th melt andhouow` block m substantially above the melting point of sodium ni. trate.4 'For' example, the interiorof the oven may be raised to a temperature of the order of 700 F. in which case the temperature within the melt will be at least of the order of 675 F. Thereafter, the hollow block may for convenience be at least temporarily removed fro-m the oven and mica 20 floated on top of melt I5. This operation will be found relatively simple, since mica is not readily wetted by the melt and will therefore float thereon in spite of its greater density. Care should be taken, however, to avoid wetting the top of the mica by the melt, and to prevent also trapping air bubbles under the mica. It does not appear that there is any particular advantage in using freshly cleaved' mica nor in taking any special precautions'with respect to cleaning its surface coming in contact with melt I5. On the other hand, it does appear that preferred results are obtained when the mica is as flat as possible.

After mica 29 has been put in position, it is preferable to return the hollow block to the oven for a short heating period, for example, 15 minutes, in order to counteract the effects of the cooling while the mica was added and to establish as nearly as possible a uniform temperature -throughout the melt. Thereafter the hollow block and melt may be permitted to cool and the rate of cooling may be relatively rapid, preferred results having been obtained with a cooling cycle of approximately five hours to roomtemperature.

Care should be taken to insure that meltv I5 cool from the top in order that crystallization may be initiated at the surface ofk mica 20 and. in order to prevent the initiation ofr crystallization elsewhere in the melt than at the mica surface.

Such control may be obtained in a variety of ways. For example, the hollow block may be allowed'to remain in an oven in order to minimize convection currents around the cups, and it is also desirable to provide the hollow block with some sort of cover adapted to shield the mica andthe melt from air currents. For example, a large sheet of mica may be used as a cover as indicated by the dotted lines at '22. Satisfactory results have also been obtainedl by removing the hollow block from the oven, providing it with a cover' 22'comprising a thin layer of heat reecting material, such as aluminum foi1 of theorder of 0.001 inch'in thickness, and then permitting the hollow block to cool to room temperature. In general, itappears that the most satisfactory results areobtainedfrom the standpoint of absolute uniformity in the crystal if crystallization can take place in asuccession of planes substantially parallel to the lower face of mica 20. The above described heating` of the hollow'blcck is also of substantial importance in preventing heat loss from the melt except from the top.

The melting point of sodium nitrate is approximately 587 F., and inthe preferred practice of the invention melt I5 will have completely solidifled within approximately 20 minutes after the initiation of crystallization vat the surfaceof mica 2B. Thereafter, the rate of cooling is sub*- stantially less important although certain precautions should be observed. The rate shouldnot be so rapid as to exceed the resistance of the crystalrto thermal stress and produce cleavage. Moreover, sodium nitrate is relatively plastic at elevated temperatures below its melting point, and care should be taken to prevent stresses tending to deform the crystal. The above-mentioned limitations on the choice of material for cup I4 `are important in this connection, although it should be noted that the iJnpQrIialflQE Qf vsaid 4 choice of material decreases inversely with the area of the crystal to be grown. In other words, when the crystal is sufficiently large in area, the relatively small outer portion which may be distorted by adhesion to the walls of the cup may readily be removed by edge grinding.

Cup I4 may be removed from the solidied crystal at any convenient time during the cooling process. It appears to make no appreciable difference in the results whether it be removed after the crystal cools to room temperature or while it is still heated to temperatures as high as of the order of 400 F. If, for example, aluminum foil is used in constructing cup I4, it will readily peel from `the solidified crystal. Relatively heavy spun aluminum cups have also been used successfully, and in that case it is preferred to cut the cup away from the crystal at relatively high temperatures, Mica readily separates from the solidified crystal at room temperature, although it is preferred to aid the separation with a.y drop of water in order to minimize the danger of stresses producing either cleavage or regions of glide twinning.

When the above steps have been carried out, the product will have the appearance indicatedv in Fig. 2 Yand will comprise two principal portions. Portion will be e, single crystal with its axis perpendicular to the surface which was in contact with the mica as indicated by arrow 32. Portion 33 will comprise an undergrowth of randomly oriented sodium nitrate. The relative thicknesses of

portions

30 and 33 will, in general, reflect the rate of cooling during the crystallizing period. If the thermal gradients in melt I5 are carefully controlled to produce crystallization only at the micasurface, it is possible to avoid the formation of portion 33. In general, however, a certain amount of non-oriented crystal growth will result, but this is immaterial since there is a clearly defined parting plane $4 between the two crystal portions at which they may be easily separated without damage to crystal 30. Thereafter, the latter may be readily ground and polished to any desired thickness and size.

The above-described embodiment of the invention has been successfully practiced to produce single crystals of sodium nitrate of a wide variety of shapes and sizes, including circular, triangular, and rectangular, and including sizes as large as square inches and one-half inch in thickness. The crystalline structure of mica has been the subject of considerable Vinvestigation by crystallographers and is discussed in detail in Atomic .Structure of Minerals by W. L. Bragg7 Cornell University Press, 1937, The Structure of Crystals by R. W. A. Wyckoff, 2nd ed., The Chemical Catalogue Company, The Crystalline State vol.

I, by W. L. Bragg, G. Bell Si Sons, Ltd., 1939, and Introduction to Crystal Chemistry by R. C. Evans, Cambridge University Press, 1939. It isigenerally agreed that the mica crystal is built up of a numberv of sheets comprising hexagonal networks vof linked tetrahedra. These tetrahedra .c are formed of a single silicon atom joined to four oxygen atoms. The three oxygen atoms forming thebase of the tetrahedra are each shared by two tetrahedra. The oxygen atoms forming the vertices of the` tetrahedra are not shared with other tetrahedra. Two 0f these sheets are placed together with the vertices of the tetrahedra. pointingV inwards. The vertices of one sheet are cross-linked with the vertices of the other sheet by Al atoms in muscovite or by Mg or Fel atoms in phlogopite. Hydroxyl groups are also incorpoi plane triangular net of potassium atoms.

rated and linked' to' Al, Mg or Fe alone.` There is thus formed a iirmly bound double sheet with the bases of the tetrahedra on each outer side. The bases of the tetrahedra are symmetrically opposed as if they were mirror images across a center plane. The hexagonal rings are likewise symmetrically opposed and two oi these hexagonal rings, one in each composite sheet, thus outline a large cavity in which a large potassium atom is situated. The potassium atom may be bound to all twelve oxygens in its vicinity. The potassium atoms which join two composite sheets are at the center of each of the smallest hexagons and form, with all the other potassium atoms, a The triangles in this net are equilateral and have a side of 5.17 Angstroms.

The potassium atoms of the triangular net are loosely bound to the oxygen atoms comprising the bases of the linked tetrahedra and it seems that this bond depends to a certain extent upon the substitution of Al for Si in the tetrahedra, In any event the number of bonds per unit area between the oxygen atoms and the potassium atoms is considerably less than the number of bonds per unit area through any other plane of the mica. The cleavage of mica is consequently along the plane in which the potassium atoms lie.

Sodium nitrate which has the same structure as calcite is a rhombohedral crystal having triangular nets of sodium atoms lying in planes perpendicular to the c axis of the crystal. A plane perpendicular to the c axis in this crystal is known as a basal plane. Therefore, the basal plane of sodium nitrate taken at the level of the I sodium atoms would constitute a triangular net of sodium atoms. This triangular net is composed of equilateral triangles. The smallest triangle of the triangular net of a sodium nitrate crystal has a side of 5.06 Angstroms.

Since the side of the potassium triangular net of the mica cleavage plane is 5.17 Angstroms in the case of muscovite and 5.32 Angstroms in the case of phlogopite, it is very close to the length of the side of the sodium triangle of the sodium net of NaNOa. It can be seen, then, that these two triangular plane atomic networks have the same geometry. They are both built up of numerous equilateral triangles having sides of substantially the same length.

When the mica plane is cleaved, half of the potassium atoms remain with one sheet, and half go with the other sheet. In a statistical manner, there thus remains on the cleavage plane of mica only half of the potassium atoms of the potassium plane, and these are scattered at random. l

The following is believed to happen when NaNOs is crystallized on a cleavage plane of mica. VI-Ialf of the potassium atoms are already missing from the cleavage plane and the sodium atoms are drawn to and occupy the places formerly occupied by the potassium atoms. The remaining potassium atoms are replaced by the sodium atoms and there is thus produced and held by the mica surface a monatomic network of sodium atoms which corresponds exactly to the monatomic network oi potassium atoms in the `cleavage plane prior to cleavage. This monatomic network of sodium atoms will have the same geometry and substantially the same size as the normal monatomic network in a basal plane of .NaNOs taken ,through the sodium level. Therefore, this monatomic network of sodium atoms will act as the foundation for the NaNOs crystal and the NO3 ions will form a network layer on top of the sodium network. Next will come another network layer of sodium 'atoms and then another network of NO3 ions, etc.

The exchange of sodium atoms for potassium atoms is believed to take place in the manner described above, since it is known that the potassium atoms are readily exchanged for other cations in solution and consequently it is believed that lthis exchange will take place with the sodium cation in the NaNOs melt.

rlfhe above embodiment of the process of the invention is subject to relatively wide variation without producing appreciable changes in the product. In this connection, attention should particularly be called to the fact that the necessity of strict control of all conditions depends to a considerable extent on the quality desired in the product. In other words, if it is desired. to produce a single crystal having the highest possible degree of uniformity from the standpoint of orientation and optic axis, then the greatest care should be exercised to minimize conditions tending to produce strain either during crystallization or during the cooling period, and particularly during the earlier cooling period when the crystal is more plastic. On the other hand, if perfect orientation is not essential in the product, the conditions of cooling are much less critical and, in fact, large single crystals have been successfully grown under conditions of forced cooling such that the entire time necessary to reduce the temperature of the melt and crystal to room temperature is only of the order of 15 minutes. In general it may be stated that the necessity ior care in avoiding cleavage from too rapid cooling increases with the thickness of the crystal, and preferred results have been obtained by following the cooling procedure outlined above and controlling the rate to a cycle of approximately three hours from `approxin'iately 700 F. to approximately 500 F. Thereafter, the blocks may be permitted to stand at room temperature until they have completely cooled.

It is in no way essential to follow the practice above described of iloating the mica on the melt and causing crystallization to proceed from the top downwards in the melt. Excellent results have been obtained either by sinking the mica to the bottom of the melt and causing crystallization to proceed in an upward directiony or by using a relatively deep cup and inserting the mica with its cleavage face perpendicular to the surface of the melt and either at the side or the middle of the cup, in which case it is preferable to keep the melt iluid and draw the mica very slowly therefrom. Apparatus for carrying out the rst of these variations is illustrated in Fig. 3.

Cup ll and hollow block a2 in Fig. 3 correspond to cup` I4 and hollow block Il] in Fig. 1, but instead of positioning cup 43 within mold cavity 44, it is mounted upon a plate or block 45 of heat conducting material such, for example, as brass or aluminum; It will also be noted that mica 4E in Fig. 3 lies on the bottom of cup 40 with melt B8 nlling most of the remainder of the cup.

In practicing the form of the invention illustrated in Fig. mica d6 may be placed within cup 40 either before or after charge 4B is melted; For example, mica 46 may be placed in the cup and the latter then substantially .filled with ground sodium nitrate. The cup and charge, preferably already mounted on plate 45, may then be placed within an oven and heated until the charge isV completely melted. Thereafter hollow block 42 may be mounted over cup 4i) as shown in Fig.,3 and the Whole may be permitted to cool either by shutting' oft the heat inthe o'venwher'ein the charge' was melted, or by simplyV permitting the cupand plate 45- to stand at room temperature until it has cooled. The use of plate 45 as the conducting material tends to induce the ilow of heat from the melt at the bottom of cup 49 and this procedure is aided by the insulating properties of hollow block 42. When the entire melt has solidified, it will have substantially the same appearance as has already been described in connection with Fig. 2. An advantage of this form of the invention moreover lies in the fact that any bubbles which may form within the melt rise to the top instead of collecting at the mica surface as is sometimes the case when the process is practiced in the manner illustrated in Fig. 1. For many purposes therefore, this form of the process gives preferred results. Y

The process of the present invention is not limited to sodium nitrate nor to the use of mica as .the orienting force, although preferred results have been obtained with` mica. Other crystals which full-lll the above conditions may be used provided that they also are not structurally altered at the fusion temperature of the new crystal and provided they may be detached therefrom without damage to the new crystal. This last essential is particularly aided by iiexibility as in the case of mica. It is also preferred, but not essential, that the orienting substance and new crystal have substantially matching coeflicients of thermal expansion in the plane of contact therebetween.

Among the other crystals which have been successfully gro-wn from mica in sections perpendicular to a three-fold axis by means of the process of the invention are cubic alkali halides, such as potassium iodide, potassium bromide, rubidium iodide, sodium iodide, and mixed crystals such as mixed alkali halides and mixtures of sodium nitrate with, for example, silver nitrate. The basic requirements determining whether a new crystal may be grown by means of the process of the invention are that as the atoms are drawn to and held by the mica or other surface, the atoms will form a network which will constitute the rst layer of the crystal to be formed. This monatomic network does not have to be exactly the same size as the size of the network in a plane of the final crystal, but it should be of substantially the same size.

It can be seen that a number of the alkali halides will satisfy the above-stated basic re-. quire'ments. This is due to the crystalline structure of the alkali halides which are cubic. As can be seen by studying the crystalline structure of a cubic alkali halide, a, plane perpendicular to one of the body diagonals will have therein a triangular network of alkali metal atoms. These triangles are equilateral triangles and the sides of the triangles have lengths quite near the lengths of the sides of the triangles in the potassium triangular net of mica cleavage plane. For instance, potassium bromide and potassium iodide have lengths of 4.81 and 5.16 Angstroms at their respective melting points. It will be understood, however, that the process essentially comprises two stages, the first of which is the formation of the crystal from the melt, and the second of which is the cooling of the new crystal to room temperature. It is possible to form new crystals by the process of the invention at the crystallizing temperature without being able to complete successfully the subsequent cooling stage. This is particularly true in the case of crystals such as potassium nitrate which have a polymorphic transition below the melting point and which 8 cannot be successfully cooled without. transforming to a polycrystallineaggregate.

It is desirable to use a still further `modification of the invention in practicing its processy in connection with alkali halides, owing to the fact that the latter generally have coefficients of thermal expansion substantially different from that of the materials otherwise found most suitable for use in the cups. The preferred cup material when growing alkali halide crystals is platinum, but the adhesion thereto of the solidifying crystal melt is nevertheless so great as to tend to cause shattering of the crystal while cooling to room temperature after formation. In order to overcome this tendency, it has been found desirable to remove the mica with thecrystal adhering thereto before the entire melt has solidiiied. A convenient way ofV accomplishing this step is illustrated in Fig. Il.

Mica disk 5l! in Fig. i is illustrated as supported by three or more fine wires 52, which may comprise copper, gold or any other suitable metal capable of withstanding the temperature'of the process and may be xed in any suitable way to the edges of the mica. In carrying out this em` bodiment of the process of the invention, a cup 54 may be first substantially iilled with the desired alkali halide melt 55, and mica 50 may then be lowered until its surface just touches the surface of the melt. This step may be more easily controlled if the cup is of at least appreciably greater diameter than the mica. The melt is then permitted to cool until a crystal of suflicient thickness has grown downward from the mica in the melt, care being taken during this phase of the process to insure cooling of the'melt from the top as outlined above in connection with sodium nitrate. The time necessary for this phase of the process will usually be determined by the operator himself but will in general'be of the order of l0 minutes, and the mica and attached crystal should be withdrawn from the melt before the latter has completely solidified and may then be transferred to an oven for slow cooling if desired. On the other hand, it has been found that the resistance to thermal shock of alkali halide crystals, such particularly as potassium iodide and potassium bromide, is so great that said crystals may be quenched in oil without affecting their optical properties. It should be noted, however, thatin View of the substantial difference between the coefficients of thermal eX- pansion of alkali halides and mica, it is preferred to separate the newly-formed crystal from the mica at high temperatures of the order of 500 F.

The embodiment of the invention just described has certain advantages. Itis particularly useful in working with alkali halide melts since the latter wet mica readily and it will not float thereon as satisfactorily as in the case of sodium nitrate. The arrangement of wires 52, illustrated in Fig. 4, is of considerable advantage in aiding to keep the mica centered symmetrically with respect to the cup. Another advantage of this form of the invention is that choice of material for use in cup 54 becomes less important and highly satisfactory results have been obtained with materials such as porcelain, which would not otherwise lend themselves to the practice of the invention.

Since certain changes in carrying out the above process may be made without departing from its scope, it is intended that all matter contained in the above description or shown inthe accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

What is claimed is:

1. `In a `process of producing a predeterminedly oriented section of a single crystal, the steps comprising forming a melt comprising said crystalline material, bringing into contact with said melt a cleavage surface of mica, and initiating crystallization of said melt on said cleavage surface by cooling said melt adjacent said surface at a faster rate than the remainder of said melt.

2. In a process of producing a predeterminedly oriented section of a single crystal, the steps comprising forming a melt comprising said crystalline material, bringing into contact with said melt a cleavage surface of mica, initiating crystallization of said melt on said cleavage surface by cooling said melt adjacent said surface at a faster rate than the remainder ofsaid melt, continuing crystallization of said melt on said cleavage surface by maintaining a thermal gradient in said melt from said surface and in a direction substantially perpendicular thereto, and cooling said newly formed crystal to room temperature.

3. In a process of producing a basal section of sodium nitrate, the steps comprising forming amelt comprising sodium nitrate, `bringing into contact with said melt a cleavage surface of mica, initiating crystallization of said melt on said cleavage surface by cooling said melt adjacent said surface at a faster rate than the remainder of said melt, and continuing crystallization of said melt on said cleavage surface by maintaining a thermal gradient in said melt from said surface and in a direction substantially perpendicular thereto.

4. In a process of producing a basal section of sodium nitrate, the steps comprising forming a melt comprising sodium nitrate in an open vessel comprising material having such thickness and physical characteristics as to be readily deformable by sodium nitrate cooling therein from the melt to room temperature, bringing intocontact with said melt a cleavage surface of mica, initiating crystallization of said melton said mica cleavage surface by cooling said melt adjacent said surface at a faster rate than the remainder of said melt, and continuing crystallization of saidl melt on said mica cleavage surface by maintaining a thermal gradient in said melt from said surface and in a direction substantially perpendicular thereto.

5. In a process of producing a basal section of sodium nitrate, the steps comprising forming a melt comprising sodium nitrate in an open vessel comprising material to which. the adhesion of sodium nitrate in crystallizing from `the melt is relatively low, initiating crystallization of said melt on said mica cleavage surface by cooling said melt adjacent said surface at a faster rate than the remainder of `said melt, and continuing crystallization of said melt on a mica cleavage surface by maintaining a thermal gradient in said melt from said surface and in a direction substantially perpendicular thereto.

6. In a process of producing a basal section of sodium nitrate, the stepsucomprising forming a melt comprising sodium nitrate, floating on said melt a cleavage section of mica, andinitiating crystallization of said melt on said mica by coollng the surface of said melt at a faster rate than the remainder thereof.

7. In a process of producing a basal section of sodium nitrate, the steps comprising forming a melt comprising sodium nitrate in an open vessel, placing said vessel in a container having side and bottom walls comprising heat-insulating material, said container having the top thereof open and being heated to a temperature of the order of the melting point of sodium nitrate, floating on said melt a cleavage section of mica, providing said container with a top cover comprising material permitting `the escape of heat therethrough more readily than the side and bottom walls of said container, and permitting said melt and container to cool until said melt has crystallized.

8. In a process of producing a basal section of 'sodium nitrate, the steps comprising forming a melt comprising sodium nitrate in an open vessel, positioning at the bottom of said melt a crystal of mica having a cleavage surface uppermost, mounting said vessel and melt on a support comprising heat-conducting material, covering said vessel and melt with a container having side and top walls comprising heat-insulating material, said container having the bottom thereof open and being heated to a temperature of the order of the melting point of sodium nitrate, and permitting said melt and container` to cool until said melt has crystallized. Y

9. In a process of producing a predeterminedly oriented section of a single crystal, comprising an alkali metal compound, the steps comprising forming a melt comprising said crystalline material, bringing into contact with said` melt a cleavage surface of mica, and initiating crystallization of said melt on said cleavage surface by cooling said melt adjacent said surface at a faster rate than the remainder of said melt.

10. In a process of producing a predetermined section of a crystal comprising an alkali halide, said crystal section being substantially perpendicular to a three-fold axis in said crystal, `the steps comprising forming a melt comprising said alkali halide, suspending in contact with said melt a cleavage surface of mica, initiating crystallization of said melt on said mica cleavage surface by cooling the surface of said melt at a faster rate than the remainder thereof, and continuing crystallization of said melt on said mica cleavage surface by maintaining a thermal gradient in said melt'from the top to the bottom thereof.

l1. In a process of producing a predetermined section of a crystal comprising an alkali halide, said crystal section being substantially perpendicular to a three-fold axis in said crystal, the stepscomprising forming a melt comprising said alkali halide, suspending in contact with said melt a cleavage surface of mica, continuing crystallization of said melt on said mica cleavage surface. by maintaining a thermal gradient in said melt from the top to the bottom thereof,

removing said mica and newly formed crystal from said melt before said melt has completely crystallized, separating said mica from said newly formed crystal ata temperature substantially 0 above room temperature, and cooling said newly formed crystal to room temperature.

12.In a process of producing a basal section of sodium nitrate, the steps comprising forming a melt comprising sodium nitrate in an open vessel comprising aluminum, bringing into contact with said melt a cleavage surface of mica, initiating crystallization of said melt on said mica cleavage surface by cooling said melt adjacent said surface at a faster rate than the remainder of said melt, and continuing crystallization of said melt on said mica cleavage surface by maintaining a thermal gradient in said melt from said surface and in a direction substantially perpendicular thereto.

13. A process of producing a single crystal, having a predetermnedly oriented axis and a relatively large cross-sectional area from a crystalline material which comprises the steps of lforming a melt of said crystalline material, bringing into contact with said melt a single crystal of a material substantially infusible at the temperature of said melt, substantially insoluble in said melt, separable from a formed crystal and different .than said melt, said second-named crystal having a plane surface thereof in contact with -said melt and having said surface extending over 4at least a greater part of the cross-sectional are/a of the melt, said surface substantially den- -ing the desired cross-sectional area 'of the said crystal to be formed, initiating crystallization of said melt on said crystal surface by cooling said melt adjacent said surface at a faster rate than the remainder of said melt, and separating said first-named crystal after formation from said second-named crystal.

14. A process of producing a single crystal, having a predeterminedly oriented axis and a relatively large cross-sectional area from a crys- 'tallne material which comprises the steps of forming a melt of-said crystalline material, bringing into contact with said melt a single crystal of a material substantially infusible at the temper'ature of said melt, substantially insoluble in said melt, separable from va formed crystal and different 'than said melt, said second-named crystal hav-ing a plane surface thereof in contactwith said melt and having said surface extending over Yat -least a greater part =of the cross-sectional area `of the melt, said surface Ysubstantially defining the desired cross-sectional area of the said crystal to be formed, said crystal surface having a-n atomic structure capable of drawing out of the melt-and holding a monatomic layer of atoms in a predetermined network, constituting the rst layer of thecrystal to be formed, said predetermined network having the same geometry as, and -substantiallythe same size as, a like atomic network-in the final formed crystal, initiating crystallizationof'said melt on said crystal surface by cooling said melt adjacent said surface at a faster 5' rate than the remainder of said melt, and separating-said first-named crystal after formation vfrom said second-named crystal.

, 15. Aprocess of producing a single crystal, having a predeterminedly oriented axis and a relatively -largeicross-sectional area from a crystalline 'material which comprises the steps of forming a melt of said crystalline material, placing on the surface'V of said melt a single crystalof a material substantially infusible at the temperature of sammelt, substantially insoluble in said melt, separable from a formed crystal and different than said melt, 'said second-named crystal having'a plane lsurface thereof in Contact" with said melt and "having said surface kextending over at least a greater part of the cross-sectional area of the melt, said surface substantially defining the desired cross-sectional area of thc said crystal 'to be formed, -said crystal 'surface having an atomic structure capable 'ofdrawing out of the i melt and holding a monatomic layer of atoms iii a predetermined .network constituting the first layer of the crystal to be formed, said predetermined network having the same geometry as, and substantially the same size as, alikea'tomic network in the final formed crystal, initiating crystallization of said melt on said crystal surface by cooling said melt adjacent saidsurface at a faster rate than the remainderof said melt, and separating said rst-named crystal .after formation from said second-named crystal.

16. A processof producing a single crystaL'having a predeterminedly oriented axis Vand a rela-V tively large cross-sectional area from a crystalline material which comprises the steps offorming a melt of said crystalline material, positioning at the bottom of said melt a single crystal of armaterial ysubstantially infusible at the temperature of said melt, substantially .insoluble in said melt, separable from a formed crystal and different than said melt, saidsecond-named crystal having a plane surface thereof in contact with said melt and hav-ing said surface extending over at least a greater part of the cross-sectional area of the melt, said surface substantially defining the desired cross-sectional area of the said crystal to be formed, said crystal surface having an atomic structure capable of drawing out of the melt and holding a monatomic layer of atoms in a predetermine'd network constituting the rst layer of the Vcrystal to be formed, said predetermined network having the same geometry as, and substanltially the same size as, a like atomic network in the final formed crystal, initiating crystallization of said melt on said crystal surface by cooling said melt adjacent said surface at a faster rate than the remainder .of said melt, and separating said first-named crystal after formation from said second-named crystal.

V'17. A process of producing a single crystal, having a predeterminedly oriented axis and a relatively large cross-sectional area from a 'crystalline material which comprises the steps of forming a melt of said crystalline materiaLbring- 'ing' "into contact with said melt a single crystal of a material substantially infusible at the temperature of said melt, substantially insoluble 'in said melt, separable from a formed crystal vand v"different `than said KLmelt, said second-named crystal having a'plane surface thereof in contact with said melt and Shaving said surface extending over Aatfleasta greater part lof the cross-sectional area 'ofthe melt, sa'id surface substantially defining thedesired cross-sectional area of the said crystal to'bev form-ed, said crystal surface having ian atomic structure capable of ydrawingvout 'of the melt-and holding a monatomic layer of atoms `in a predetermined network constituting the rst layer of the crystal to vbe formed, said predetermined network having the same geometry as, and substantially the same size as, a like atomic network in the nal formed crystal, initiating y crystallization of said melt on said crystal surface by cooling said melt adjacent said surface at aY faster rate than the remainderl of said melt, continuing crystallization of said melt on said crystal surface by maintaining a thermal gradient in said melt from said surface and in a direction Tsubstantially perpendicular thereto, `and separating said first-named crystal after formation from said second-named crystal.

18. `In aiprocess 'of producing abasal section of sodium nitrate, the'steps comprising forming Va melt comprising sodium nitrate ingan open vessel, p'lacingfs'aidlvessel in a containerzcomprising heat the escape of heat therethrough more readily than through the walls of said heat insulating container and permitting said melt and container to cool until said melt has crystallized.

CUTLER D. WEST. FREDERICK J. BINDA.