US2546305A

US2546305A - Method of and means for growing crystals

Info

Publication number: US2546305A
Application number: US728310A
Authority: US
Inventors: Jaffe Hans; Edward M Brazis; Kjellgren Bengt
Original assignee: Brush Development Co
Current assignee: Brush Development Co
Priority date: 1947-02-13
Filing date: 1947-02-13
Publication date: 1951-03-27
Anticipated expiration: 1968-03-27

Description

March 27, 1951 H. JAFFE ET AL 2,546,305

' METHOD OF AND MEANS FOR GROWING CRYSTALS Filed Feb. 13, 1947 2 Sheets-Sheet l FIG. 6 FIG. 7

INVENTORS HANS JAFFE BY EDWARD M. BRAZIS I BENGT KJELLGREN I ATTORNEY Patented Mar. 27, 1951 :UNI 'IZED STATES iPAT-EN T OFFICE METHQDOFQAND EANsFoR GROWING "'FGRYSTALS Hans J affe; Cleveland Hei Cleveland: ands-Bengt Heights, hio;.l-assigno.r

ghts; Edward M. Brazis,

. Kjellgren, University to The. Brush Development'Company, Cleveland; Ohio, a corporation of Ohio Aliplication'february 13;;1947;.S.eria1 No. f 7 28,310

,. .23 Claims.

. This invention relates to crystal growing; and,

7 more particularlyeto methods .of and means for growing crystals of substances. which by natural habit tend to grow in solutionsthereof-elongated Pparallel to -foneaxis, :such, "for .example, aszthe.

f primary phosphates; of potassiumand ammonium .v-eand compounds isomorphous therewith.

The-(present: application :is r a vcontinuat-ion-inpart of, applicationaserial.No; 491,252, filed June 1 1'7, 194,3; now? abandoned.

Previousito thisinvention, ithas been-customvary toggrow crysta1s,.:of certain types; such as Rochelle: salt, eitherlfrom small seed-crystals, or :"from artificial seeds-: such as slabs ."or bars. cut

from a mother crystal, the edges of-w hichartificial zseeds are parallel to :the crystallographic 'axes thereof. .T-heilatter practice is exemplified bythe I i United States patents to vKjellgren Re'.:19,697 and -:Re'. 19,698.

,= A group" of crystals- .of elongated habit-and The. term P-type crystal, is to be understood as :emhracingprimary, ammonium phosphate (N H4H2PO4) primary: potassium phosphatac'pri- .:mary=rubidium phosphate, the primary arsenates ofammonium potassium :andrubidium-isomer 1 :phous mixturesofanysof these named compounds,'

- land all otherapiezoeleetrically active crystalline material isomorphous therewith. 'In-Wyckoff; .t- -structure of CrystaI KZnd-ed; N. Y., 1931). this crystal typeis .designated-,-as KHzP-Oi-type. :In. the .Strukturbericht.. (Supplement: to Zeitschrift l sr phy).this.typ is: designated as HP-22. :"Ihe habit of this P-type crystal is a-combina tiorr-of. theseconda-ry' prism and: the secondary 45 bi-pyramid. 'It is; characteristic 1 of s the: P-type crystals, as grown from solutions of their respective .compounds,- that: they are elongated parallel tc theaaxis ofaprism, 'whichis the opticaxis of :theseicrystals and designated herein as the Z.axis,-.-

"All thecr-ystals:enumerated above as members 10f the P-type group-belong to the crystallographic a ymmetry-class designated commonly by the sym- .-.-bol Vd. 1 This classisalso known as the di-tetraga lonal t-alternatingrrcrystal class or as the tetragonal asphenoidal class, the ,latter name bein -1 the one I usedninDana-Ford, Textbook of Mineralogy,- 14th .ed.,- N. Y., 1932. This-crystal class is-characterized bythe'presence of three two-fold axes ;.of. symmetry perpendicular to each other and-twotplanes :of symmetry at right angles, to each othenand intersecting--inone=of the two-fold: axes. f -The -planes cut the, other two two-foldpaxesunder :angles of; This combination: of symmetry elements makesthat axis which is parallel-rtoi the -two planes of symmetry a= four-fold alternating symmetryaxis which is also the opticaxisofg the crystal.

.HansJa-fie, one-of the, present applicants, {establishedthat for most applications-of B-type mate- :rial,- such asin piezoelectric transducersand light modulators. it-is-thighly desirable-to makeuse of sections, designated Z-cutj the major 'faces of which lie iii-planessubstantially perpendicular to the optictaxis 'of the. mother ,crystalgaand' often such Z-cut -sections are requiredhavingmajor faces of. very considerahleosizes. -Thismal es it particularly:desirable to provide crystal bodies .having a volume of .clear materialwith very considerable extension at .right .angles :to. the optic axis -.as well as parallel. thereto..aPrioratoathe present invention this .could'loe, accomplished by subjecting crystal-seeds of 4 theP-typelmaterial to growth sumciently prolonged to: produce crystals I having dimensions at'rightanglesttoetha-Z- axis as great, as the corresponding dimensions of. the

' Z-cut piezoelectric sections to- -be produced; --but this called for growingpycles greatly-prolonged in duration because most-of the materialotthese ielongated crystalssis grown bydepositionontheir pyramidal end faces, the lateral growth at :right angles to the Z axis being at azrata-orrtheayerage,

: only-. about one-tenthuasugreatas ,Z-the growth 1 parallel'to.thataxis. Alsossuch a practice would involve the production andhandlin of crystals of great length projectedin ,oppositetdirections fromsmall 'seedwbodies. -;Boththegprolonged growing cycle and the greatly elongated crystals,

for. reasons readily-understood byethose experienced in .such matters, are unfavorable @to the 1 attainment of economic crystal production.

' Agaiminethods of crystal growing using simple artificial seed bodies-which have been successful in thepcase of Roche11e,-:sa1t;-which; grows-with about equal facility and speed in dir,e.ctions:paral- :lel to all three of its crystallographic axes,;=a'-r e not I suitable for-the unflawedtgrowth; of elon ated type crystalline material to producezpiezoelectric e1ements having large dimensions at-rightangles torthe optic axis.

An alternate method disclosed by Busch (Helvetica Physica Acta, vol. 11, pages 269, 273, 1938) used the expedient of reducing the natural acidity observed for pure solutions of the primary phosphates of potassium and ammonium and isomorphous crystals, by addition of substantial amounts of the corresponding secondary salts. While this method may be advantageous for the growing of seed bodies of substantial lateral dimensions, it is not suitable for the growing of crystal material to be used for piezoelectric or electrooptic purposes, because the material deposited from these less acid solutions onto the prism faces of the P-type crystal has a pronounced tendency to be flawed and, in the use of the especially important primary ammonium phosphate, shows electrical conductivity of much higher order than material deposited upon the pyramid faces from the solution having the characteristic acidity of the pure primary phosphates.

Accordingly it has been a principal object of the present invention to provide a method of growing crystal bodies of material having the elongated habit, such as the P-type materials, which should be both practically independent of the low lateral growth rate of such materials and adapted to take full advantage of their rapid growth in the direction of the longitudinal symmetry axis and which, by virtue of these characteristics, should be adapted with good economy of time, labor and equipment to produce crystal bodies of the said materials having any one of a wide range of predetermined dimensions at right angles to their longitudinal symmetr axis and any one of a wide range of predetermined dimensions parallel to said axis, with the latter dimension substantially independent of the former.

A further object of the invention is to provide a method such as specified in the last preceding paragraph which can be carried out with growing solutions of natural acidity favoring the production of sound crystal growth.

Another object of the invention is to produce novel seed bodies of P-type or other elongated crystal material and methods of producing such seed bodies, useful in the practice of the above noted method of growing crystals.

Other objects more or less incidental or ancillary to those stated above and the manner in which the various objects are attained will be set forth in the following description of specific embodiments of the invention, with reference to the accompanying drawings, and the distinctive features of the invention will be defined in appended claims.

In the drawings,

Fig. 1 is a diagram, in perspective, illustrating the derivation from a P-type mother crystal, such for example as a crystal of primary ammonium phosphate, of several diiferent forms of seed bodies useful in carrying out the invention. Fig. 2 is a view in side elevation of a composite seed bod in accordance with the invention.

Figs. 3 to '7, inclusive, are views, in side elevation or in perspective, of several of the seed bodies shown in Figs. 1 and 2, indicating the manner in which they may be provided with pedestals for supporting them during the growing process.

Fig. 8 is a diagrammatic vertical section through a crystallization tank, combining in the one View a plurality of seed bodies of different types, showing how they may be supported on pedestals from the bottom of the tank and indicating by the double headed arrow the preferred flow of the crystal growth solution with respect to the optic axes of the bodies.

Fig. 9 is a perspective view of a crystal of primary ammonium phosphate grown in accordance with the present invention, the seed body used being shown embedded in the crystal.

Fig. 10 is an elevation of a seed similar to that shown in Fig. 5 but mounted on its pedestal in another manner.

Fig. 11 is a perspective view showing a mosaic of small seed portions forming a larger composite seed body.

Figs. 12 and 13 are side elevations of two of the different seed bodies shown in Fig. 1 show ing modified ways of supporting the bodies in the crystal growing process.

Figs. 14 and 15 are plan views of crystals grown on seed bodies such as are shown by Figs. 12 and 13, respectively.

Fig. 16 is a plan view of a crystallization tank showing seed bodies of the type shown in Fig. 12 suitably arranged therein for crystal growing.

The present invention is made possible by the fact, established by the applicants, that seed bodies of the elongated type crystal material, such as the P-type material, having great extent at right angles to the longitudinal symmetry axis of the material, are capable of supporting sound crystal growth in the direction of the said axis and at substantiall the growth rate characteristic of the specified material, if the seed body has one or more sound surfaces parallel to (or coincident with) a natural end surface of the crystal material.

It has been found, furthermore, that in the case of P-type materials crystal growth of the desired character is facilitated if the seed body has more than one face substantially parallel to (or coincident with) end pyramidal faces of the mother crystal. It has also been established that in the growth of crystals in accordance with the invention, with one or more of the seed body faces parallel to pyramidal faces, a flow of the crystal growing solution relative to the seed body in a direction generally parallel to the longitudinal symmetr axis thereof is favorable to the desired growth.

There are a number of ways in which seeds may be fabricated from an elongated type crystal according to this invention, several of which, in the case of P-type material, are exemplified by Fig. 1. For example, the pyramidal ends 2| of a crystal. which otherwise would be wasted, may be cemented together, as shown in Fig. 2 on a reduced scale, to form a composite seed body 2 la and a mounting pedestal 22 may be cemented to the seed, as shown in Fig. 3.

It is not obligatory that the two pyramidal ends be taken from the same crystal but it is advisable that the base of each be substantially perpendicular to its longitudinal symmetry axis and that the two bases be joined so as to suitably align the two X axes in one part so that they are parallel to the X axes in the other part, whereby symmetrical growth will be materially assisted.

A close scrutiny of the symmetry of the crystal materials, here described as P-type, reveals that the alignment of the axes as described in the last preceding paragraph does not yet assure a complete congruence of the distribution in space of both materials. As has been explained in an earlier paragraph, the symmetry of the P-type crystals is that of the class Vd, also known as t tra cnal sphenoidal. A consequence of this latter symmetry is that the eight edges of the tetragonal bi-pyramid of the P-type crystals are not physically equal but are divided into two groups of four. One group may be called the positive group of the pyramidal edges, the other the negative group of pyramidal edges. These groups differ, for instance, in their piezoelectric behavior, if a pressure be applied upon two edges which are space-opposite. The one of the two corners will acquire a positive charge and the other a negative charge. In each one of the two pyramids positive and negative edges alternate. The orientation of the two end pyramids in the natural crystal is so that, under the stated pressure, the two pyramidal edges appearing on opposite ends of any one of the prism edges are of opposite Sign.

Accordingly in fabricating a composite seed" from two end-pyramids of P-type crystalline material it is important that twoprecautions be taken, namely;

With respect to the second precaution, the positive and negative edges may be recognized by subjecting the oi-pyramid to pressure between two space-opposite edges; and, observing the polarities of the charges developed on those edges. It is much simpler, however, to submit the pyramid faces to the action of a solvent, such as water, for a short periodof time. When that is done, numerous small pits are etched on each face. superficially, the pits appear to. be triangular incontour, although, in reality, they are inverted three-sided pyramids. If the etched Surfaces are viewed by reflected light, they take on the appearance of arrows pointing toward two of the pyramid edges diagonally opposite to each other, and pointingaway from the other two pyramid edges, as shown greatly enlarged in Fig. 9.

The etched pits produced by Wateron the pyramid faces of primary potassium phosphate are. similar to those produced on primary ammonium phosphate, and may be, exhibited to determine thepositive or negative character of the pyramid edges. In case: of primary potassium phosphate, the same purpose can be achieved by observation of etched figures on the, prism faces; These are elongated parallelograms with their longer side at an angle of about 70 to the longitudinal symmetry axis. The inclination is in such a direction that the parallelograms on, the prism faces pointv toward the same pyramid edge as the triangular etch points on: the pyramid faces.

Accordingly, two etched pyramids chosenat random may be matched with each other, as indicated by the small arrows in Fig. 3, with the assurance that the proper orientation is achieved to form a seed body from which a homogeneous crystal may be, subsequently grown.

A single end pyramid may also be used to grow a clearhalf-crystal of P-type substance, in which case it is considered preferable to protect the base thereof against the deposition of crystalline material thereon during the growing process. For example, referring to Fig. 4, the base of a pyramid may be cementedto: the surface of a support such as a plate 23: of Bakelite or the like and a single half crystal may begrown laterally outward therefrom. It is better, however, also to cement a similar pyramidal end to the opposite surface of the plate whereby the crystal may be grown symmetrically thus rendering it less difhcult to support during growth. Obviously, in the case of the composite seed bodies of. Figs 2 and.3 each, pyramidal end section protects the base of the other section. from deposit of crystalline material thereon.

The intermediate support 23,, or bafiie, appears to assist somewhat crystal growth in the plane perpendicular to the longitudinal symmetry axis of the seed body, inasmuch as the deposited crystalline material. creeps along its surfaces quite rapidly before merging at the boundaries thereof into a. single mass.

Seedbodies may also be obtained from the crystal by cutting from it inclined sections 24 as shown in Fig. l and Fig. 5, the major faces of which are substantially parallel to one of the end-pyramid faces. Such seed bodies may be cemented to supporting blocks 22 as shown on reduced scale in Figs. 5 and 8, but a mounting such as that shown in Figs. 10 and 12 is to be preferred. In the latter mountings the seed body is arranged to rest upon one of its minor surfaces which are at, right angles to its major, inclined surfaces. Thus the overhanging and unbalanced mounting of Fig. 5 isavoided. In Fig. 1c the seed is cemented on a supporting block 22, while in Fig. 12 holes drilled in the section 24 receive tightly fitting pins; 25 the projecting ends of which engage holes in a supporting block 255 secured in a slot in the bottom of: the crystallization tank. 7

Sections suitable for use may also be so cut from the mother; crystal that their major faces are, perpendicular to the Z axis. Such a section is designated by the numeral 2? in Fig. 1. Z-cut sections are very desirable for use as transducer elements but for reasons that. will later appear, they do not promote crystal growth, when used as seed bodies, so well as sections of other types herein; disclosed; they have, however, the advan tage of being produced very easily and with consumption of only a small amount of crystal material. This advantage is, of course, shared to some extent by the inclined type'24. Z-cut sections may be provided with a. simple pedestal 22 as shown in Fig. 6, or they may be mounted on pins 25 and block 26 as shown in Fig. 13-.

Further, in accordance with the invention, the seed-body may take, the form of a prism of the type designated by the numeral 23 and shown in, Figs. 1' and 81,. the prism having its long axis parallel to one of the X axes of the mother crystal and its faces intersecting the Z axis at substantially the natural angle. of the pyramidal faces at the ends of the mother crystal. The natural angle, in primary ammonium phosphate. is 45"; in primary potassium phosphate, it is about 47. Such prism seed bodies may be provided with mounting pedestals as shown in 8..

A triangular prism, as designated by the nu-- meral 29, may also be cut: from the mother crystal to serve as a seed body, the slant faces of which intersect the Z axis at substantially the natural angle, and provided with supporting pedestal 2-2 as shown in- Fig. 7. As will be apparent, this form of seed body can be cut from the mother crystal with little waste of crystal- 7 line material, especially in the case of primary ammonium phosphate.

Fig. 11 illustrates a mosaic or composite seed 30 composed of a plurality of Z-cut sections such as sections 21, arranged side-by-side to provide a seed of greater extent in directions perpendicular to the Z axis, whereby crystals may be grown having a greater width-to-length ratio than a typical natural crystal of the same substance. The advantage to be gained with this composite type of seed is limited by the fact that it is difficult to secure consistenly a unitary merging of the crystal growths on the several mosaic blocks 2?.

The method of crystal growing according to this invention is exemplified by diagrammatic Figs. 8 and 16. In Fig. 8 a tank 3! having a tightly fitting cover 32 is provided, on the bottom of which is disposed a sheet 33 of Bakelite, hard rubber or the like having slots or wells into which the mounting blocks or pedestals of the seed bodies may be cemented, or simply secured by friction. Means (not shown) are provided whereby the tank may be rocked slowly from side to side to cause the saturated solution of the crystalline material to flow gently back and forth in the direction of the Z axes of the seed bodies, as indicated by the double ended arrow.

In Fig. 8 a single row containing closely spaced seed bodies of five different types is shown simply for purposes of explanation; in practice it is preferable to load the tank with seed bodies of a single type and it is to be understood that the invention is not limited to any particular number of rows of bodies nor to any particular number of bodies per row, but, of course, the seed bodies in each row must be planted farther apart than in Fig. 8 to insure suitable space for growth. Such a spacing is illustrated in Fig. 16 showing a crystallization tank 34 planted with several rows of seeds of the type shown in Fig. 12, the pedestal blocks 2'6 in this case being carried by metal channels 34a having their ends adjustably mounted on angle rails 3% attached to the bottom of the tank 34.

The supporting means of the seed bodies are preferably narrower in the direction of the X axes than the seed bodies themselves and of a suitable height (as shown, in the case of block 22, in Figs. 8 and 9) so that the growing solution can flow beneath the seed (and the crystal as it develops) to better insure symmetrical growth of the crystal, permit any spurious crystals formed to fall to the bottom of the tank and avoid attachment to the crystal being grown and to prevent attachment of the main crystal to the bottom of the tank. To this end the pin and block supports shown in Figs. 12 and 13 are superior to the simple block supports shown in the other figures because the pins offer less bstruction to the solution flow. The feature of supporting the seed body above the bottom of the crystallization tank is the invention of C. K. Gravley and is disclosed and claimed in his application Serial No. 728,293, filed February 13, 1947.

By referring to the Kjellgren reissued patents already identified herein, or to the United States patent to Moore, No. 1,347,350, directions will be found for growing crystals from seeds by gradually lowering the temperature of a heated saturated solution in which the seeds are immersed. In general, that method of crystallization may be followed with P-type seed bodies. For example, if the seed bodies are from a crystal of primary ammonium phosphate, the initial temperature of the mother liquor may be of the order of 44 degrees centigrade, and the concentration of the liquid such that it becomes saturated at a temperature of 40 degrees centigrade. The solution which is saturated at 40 C. is obtained by dissolving 567 grams of pure salt in 1000 grams of water. The specific gravity measured at the same temperature is 1.204. The growing of primary potassium phosphate crystals may be started by employing the same saturation temperature. For obtaining clear crystals of this substance it has been found advantageous to use a solution of the chemically pure salt in 0.5 normal solution of potassium hydroxide. This solution is found to be saturated in respect to primary potassium phosphate at 40 C. if its specific gravity, at the same temperature, is 1.240.

Crystals of primary ammonium phosphate grown in accordance with the present invention, are shown in Figs. 9, 14 and 15. Especially in the cases of Figs. 9 and 14 there is a marked freedom, both of parasitic crystals on the faces of the embedded seeds and of flaws in the crystal growth. In the case of Fig. 9 the seed body has the end pyramid faces initially fully established and complete natural growth occurs from the outset. In Fig. 14 it is necessary at the outset to develop three additional pyramid surfaces at each side of the seed body but, with the one large pyramidal surface and natural minor surfaces parallel to the Z axis to start with, the additional surfaces are established readily without flawed growth. In the case of Fig. 15, showing crystal growth on a Z-cut seed body 21, like element 2! of Fig. 1, the first part of the growth serves to establish pyramid faces 35. Some flawed growth, indicated at 36, always occurs before solid pyramidal faces 35 are established; but once the solid pyramidal shells are formed, further crystal growth is clear and solid, in ac cordance with this invention. Flawless growth is more readily established by starting with a seed body having one or more exposed faces coincident with or parallel to a natural pyramidal end face of the crystal to be grown, and all things considered the seed elements 24, preferably supported as shown in Fig. 12, are considered best suited for the economic practice of the invention. However, the crystal body produced by crystal growth starting with the Z-cut plate as in Fig. 13 and carried to completion of the pyramidal shells with faces 35, may itself be considered and treated as a seed body and, like the other seed bodies herein, may be made, used and sold as such.

To initiate the practice of the invention a crystal of a particular material of the elongated crystal habit can be grown by the prior conventional method from a natural seed body until a predetermined thickness dimension is attained equal to that necessary to produce Z-cut piezoelectric elements of a size that may be required. In a long growth period, special precautions and care are required to do this and more than one trial may be necessary to produce a sound crystal of sufliciently large thickness dimension; but the trouble and expense of doing so is not significant because, once a body of clear crystal material of the required size is secured, a number of seed bodies of the character herein disclosed may be prepared from it and used in accordance with the present invention to grow additional crystals of the required thickness dimensions. After initiation of the process, obviously a certain percentage of the crystals grown by the improved method may be used to produce seed bodies to maintain the production process.

From the foregoing description it will be seen that the invention is characterized by the use of seed bodies having (a) one or more plane faces disposed at substantially the same angle to the longitudinal symmetry axis of their crystalline material as pyramidal'end faces of the natural crystal and (b) a thicknesseto-length ratio which is much larger than the thickness-to-length ratio of a natural crystal of the same material or, in other words, much larger than the ratio of the natural rate of lateral growth to the natural rate of longitudinal growth of the material in a solution thereof, it being these widely different rates of lateral growth and longitudinal growth that determines the thickness-to-length ratio of the natural crystal. In the expression thicknessto-length ratio the word thickness refers to over-all dimensions measured at right angles to the longitudinal symmetryaxis of the material and the word length refers to over-all dimensions measured parallel to that axis.

Also, it is clear that'in the use of the improved method to produce a crystal body of predetermined thickness; a suitable choice of the thick- 'ness dimension of the seed body may make the growing process practically independent of the slow rate of lateral growth so that the length of r the crystal grown may be determined, not by the thickness dimension required but by ractical considerations determining convenience and economy of production. .It is demonstrable that to attain the minimum cost of producing Z-cut crystal elements from elongated type crystal material the grown crystal bodies should be of moderate length-decidedly shorter, in fact, than is necessary to produce the larger sizes of Z-cut elements by the conventional natural crystal growing methods. Obviously the method takes full advantage of the rapid endwise growth of the elongated type crystal materials.

In the use of the various forms of seed bodies herein disclosed it has been observed that the crystal bodies grown from them sometimes taper somewhat toward their ends, due perhaps to specific differences in the seed bodies or to the presence of impurities in the growing solution. This presents no difficulty in the practice of the invention since the small transverse growth that will occur in the production of crystal bodies of economiclength will serve to compensate for the taper so that the prism portion of the grown crystal will have suflicient thickness from end to end to produce Z-cut elements of the predetermined size for which the crystal bodies are intended.

It is to be understood that,' while disclosure has been made of preferred forms of seed bodies and of methods of producing them and using them for the economic production of masses of elongated type crystal material, other equivalent forms may be used within the scope of the invention as defined in the claims.

What is claimed is:

1. In. the growth of crystal bodies formed of material of an elongated crystal habit consisting of end pyramids and an interposed prism elongated parallel to its axis and to the optic axis of the material, the method of producing crystal bodies of the said material having large predetermined transverse dimensions and moderate lengths which comprises the steps of providing a seed body formed of the said crystalline material having at least one plane face disposed at an angle to its optic axis substantially equal to the angle between an end pyramid face and the optic axis of a natural crystal of the said material and also having a thickness-to-length ratio that is large in comparison with the ratio of the natural rate of lateral growth to the natural rate of Iongitudinal growth of the said material; immersing the seed body in a solution of the said crystalline material; and causing the material in solution to crystallize out upon the surfaces of the seed body; whereby, with a seed body of suitable transverse dimensions, a crystal body of the predetermined transverse dimensions can be grown to a length suitable for economy at the rapid rate of natural longitudinal growth of the said crystalline material.

2. A method as claimed in claim 1 in which the seed body and the material in solution are P-type material.

3. A methodas claimed claim 1 in which the solution of crystalline material is caused to flow over the seed body in a general direction substan tially parallel to the optic axis of the body to facilitate crystallization upon the body.

4. A method as claimed in claim 1 in which the seed body of crystalline material is plate-like in form with both of its major faces disposed at substantially the same angle to the optic axis of'the material as one of the pyramidal en'd faces of a natural crystal of said material.

5. A method as claimed in claim 1 in which the seed body of crystalline material is plate-like in form with both of its major faces disposed at substantially the same angle to the optic axis of the material as one of the pyramidal end faces of a natural crystal of said material and in. which during crystallization the solution of crystalline material is caused to flow over the seed-body in a general direction substantially parallel to the optic axis of the seed body.

6. The steps in the manufacture of a P-type crystal which include: selecting a plurality of seed pieces of P-type crystalline material; orienting said plurality of seed pieces in the same manner with respect to the crystallographic axes and with respect to the piezoelectric properties of the material; with said seed pieces so oriented connecting them directly together to form a composite seed; immersing said composite seed in a salt solution; and causing the salt in said solution to crystallize out upon said composite seed.

7. As an article of manufacture, a crystalline seed body formed of crystalline material having an elongated habit composed of end pyramids and an interposed prism elongated parallel to its axis and to the optic axis of the material, said seed body having at least one plane face disposed at an angle to its optic axis substantially equal to the angle between an end pyramid face and the optic axis of a natural crystal of the same material and also having a thickness-to-length ratio that is large in comparison with the ratio of the natural rate of lateral growth to the natural rate of longitudinal growth of the said material.

8. An article of manufacture as claimed in claim '7 in which the seed body has all of its bounding faces substantially parallel to natural faces of the crystalline material of which the body is formed and at substantially the same angles to the optic axis as the corresponding natural faces.

9. An article of manufacture as claimed in claim 7 in which the seed body is plate-like in form with the mutually parallel major faces of the plate disposed at the angle to its optic axis specified in claim 7 for the plane face.

10. An article of manufacture as claimed in claim '7 in which the seed body is in the form of a prism having its axis substantially perpendicular to the optic axis of its crystalline material.

11. An article of manufacture as claimed in claim '7 in which the seed body is in the form of a triagonal prism having its axis substantially perpendicular to the optic axis of its crystalline material.

12. An article of manufacture as claimed in claim '7 in which the seed body is composed of P-type crystalline material and is in the form of a prism having its axis substantially perpendicular to its end faces and to the optic axis of its crystalline material.

13. An article of manufacture as claimed in claim '7 in which the seed body is composed of P-type crystalline material and is in the form of a triagonal prism having its axis substantially perpendicular to its end faces and to the optic axis of its crystalline material.

14. An article of manufacture as claimed in claim 7 in which the seed body has two oppositely facing sets of pyramidal surfaces.

15. An article of manufacture as claimed in claim '7 in which the seed body is substantially completely bounded by two oppositely facing sets of pyramidal surfaces.

16. In the production of a seed unit from P- type crystalline material characterized by a nat ural crystal form consisting of a combination of the secondary prism and the secondary by pyramid, the steps of severing from a crystal of such material at least a portion, pyramidal in form, of one of the pyramidal parts thereof; similarly preparing from the same or another crystal a second pyramidal portion with its base substantially matching for size the base of the first portion; and thereafter joining together the severed portions in aligned base-to-base relation with their optic axes in substantial parallelism and with the same orientation otherwise as in the natural crystal.

17. A piezoelectric crystal grown from a seed body out from another crystal, said seed body having a face upon which said crystal is grown, said face of said seed body being a face extending substantially parallel to a grown cap face of said grown crystal, and said seed body having a plurality of peripheral edge faces corresponding in number with and being substantially parallel 12 to the corresponding grown prism faces of said crystal.

18. A piezoelectric crystal as set forth in claim 17, further characterized by said grown crystal being of tetragonal form having four prism faces and having at an end thereof four pyramid faces, the cut face of said seed body being substantially parallel to one of said pyramid faces.

19. A piezoelectric crystal as set forth in claim 18, further characterized by said grown crystal and said seed body comprising primary ammonium phosphate.

20. A piezoelectric crystal grown from a seed plate cut from another crystal, said seed plate having a pair of substantially plane parallel major faces upon which said crystal is grown, said major faces of said seed plate being faces extending substantially parallel to a grown cap face of said grown crystal, said seed plate having a plurality of edge faces corresponding in number with and being substantially parallel to the corresponding grown prism faces of said grown crystal.

21. A piezoelectric crystal as set forth in claim 20, further characterized by said crystal and said seed plate comprising primary ammonium phosphate.

22. An artificially grown piezoelectric crystal comprising as a part thereof a diagonal seed plate, the prism faces of said crystal being grown parallel with the corresponding peripheral edge faces of said seed plate, and the two major faces of said seed plate extending substantially paral lel to a pyramidal cap face of said grown crystal, said crystal comprising uniformly oriented crystalline material that is substantially free from flawed regions adjacent said major faces of said seed plate.

23. A piezoelectric crystal as set forth in claim 22, further characterized by said crystal and said seed plate comprising primary ammonium phos phate.

HANS JAFFE. EDWARD M. BRAZIS. BENGT KJELLGREN.

REFERENCES CITED UNITED STATES PATENTS Name Date Haas, Jr July 22, 1947 Number