US2447362A - Production of crystals - Google Patents

Description

L. PESSEL Filed Nov. 1, 1945 PRODUCTION OF CRYSTALS Aug. 17, 194s.

Patented 17, 1948 amata raonUc'noN. or cars'rns Leopold remi, Philadelphia. rs., mlm to Radio Corporation of America, a corporation of Delaware Application November 1, 1945,-Serlal No. 626,135

23 Claims. (CL 171-327) 'Ihis invention relates to the art' of producing crystals and 'more specifically to producing synthetically, crystals which exhibit a piezo-electric effect.

One obj ect-of this invention to produce monol crystal bodies. inv general, synthetically by re,-

crystallization of finely subdivided crystalllzable substances. f

Another object is to produce synthetic oscillator plates of quartz or other piezoelectric substances.

Another object is to change the frequency of existing oscillator plates by synthetic crystal growth.

A further object of the invention is to produce synthetic crystals in such a shape and form that a minimum -of additional mechanical operations has to be performed to produce a. finished oscillator plate.

Another object is to produce a large number of oscillator plates simultaneously, with little frequency deviation between them.

occurring therein crystals of suitable quality. Only a very small percentage of the mined mineral is sent to the laboratories where the crystals are cut and shaped. Yet, suchi-s the uncertainty of prevailing tests which can be used in the field,

' that of the supposedly perfect crystals finallyv sent to the laboratory more than half must be I ,discarded when the oscillator plates are being cut.

' Another source of piezoelectric crystals has been the well known Rochelle salt or sodium potassium tartrate. Here the prevailing methods have been to grow crystals from the melt or from solution. Rochelle salt crystals, however, have proven unsuitable for use at high frequencies` and have mechanical limitations such as softness and highy solubility.

Certain methods similar to those used in making Rochelle salt crystals have been tried in making suitable quartz crystals but the results have been disappointing since the product is inferior in both size and quality to the crystals which occur naturally.

` out the world for natural quartz deposits having 2 The present invention represents a distinct change from any of the above described methods and can be applied not only to quartz but to tourmaline, Rochelle salt, and, in fact,to the forH mation of any crystalline material.

In general, the finely subdivided substance, for example, powdered quartz or silica, is mixed with a, small proportion of a solvent. A crystallization catalyst may be added, if speed of reaction is desired. In the case of quartz this catalyst may -be a substance which gives an alkaline reaction such as the hydroxides of the alkali and alkaline earth metals as well as some of the stronger ory ganlc bases. Other types of catalyst which have been found useful are the inorganic fluoride salts and hydrouoric. acid. Although these catalysts are preferred in the case of quartz the invention contemplates the use, in general, of any substance which catalyzes the crystallization of the sub? stance the crystal of which is being formed. The mixture is compressed into a substantially plateshaped body and maintained at elevated pressure and preferably at elevated temperature. The compressed body i-s subjected to high-frequency oscillations by means of suitably spaced electrodes. One frequency component is preferably the natural mechanical vibration frequency of the compressed body. A supersonic frequency component may -be added. It is also possible to use other high .frequencies of alternation which will produce the desired vibrations in the compressed yamorphous material. Novel features are shown for causing such crystallization to take place and to proceed in such an oriented manner thatthe ultimate result is a body` showing the characteristics of a 4monocrystal.'

The invention will now be described more specifically as it relates to quartz bearing in mind,

howevernthat the same general principles apply in the case of other substances.

It is Well known that quartz or silica isvery slightly soluble in water. This solubility increases when the material is powdered and this increase depend-s 'upon the degree of subdivision of the particles. In addition, the solubility is increased by raising the .temperature and pressure. Further increases in solubility are caused by the presence of other substances. Some of those substances which have been found particularly useful in practicing the invention are the alkalies in which themetal is of the class of alkali or alkaline earth metals, as well as strong organic Other classes of substances found usefulbases. are the fluoride salts and hydroiluoric acid.

lThe material used should be extremelysubdivided silica','such as powdered quartz.'ch alcedony.'" The para..v ticles are `preferably of 10 micron diameter 0I although coarser particles may be hydrated silica, oramorphous silica.

pressure used may be made use of.

' I'he powdered silica is mixed with a vsma amount of water which amount is not ilxed but Vdepends -upon particle size and shape as well-as upon other operating conditions. To this mixture is preferably added a small amount of one of the catalysts heretofore enumerated in order The prepared mixture is then treated by the method presently to be described using suitable apparatus which is diagrammatically represented in the various figures, of which:

Fig. 1 represents diagrammatically a longitudinal section view of apparatus suitable for apply ing a single high frequency component. z

Fig. 2 represents a modification in which an additional supersonic component is added at right angles to the high frequencycomponent.

' Fig. 3 represents a modification in lwhich the v high frequency electrodes are placed at an' angle to theplane of the faces of the chamber. -v

Fig. 4 represents a modification in which the high frequency electrodes are placed parallelto the main faces of the oscillator plate buthaving [the electrodes of the supersonic component at -some angle other than a right angle inrelat'ion.

' to the'highfrequency electrodes. v l v Fig. 5 shows a modiilcationin `.whichgthefce'nters of thehigh frequency electrodes are "laterally-5 displaced.

' Fig..6 shows a variation of the abovednwhich theelectrodes of the supersonwboinponent are 1 also atan angle other than afrightfangleto the "planesofthe high frequencyfelectrodes'-j. Fig. .7.1 shows a modification' irl-which quartz.

oscillator plates vare used in place of metalfelectrodes. f

.Where it is not necessary or desirable to speed up the solution process, ordinary room temperature may beused. As in the case of the catalyst,v

increased temperature is used only to speed u'p the process and-may be omitted if desired.

Elevated pressure is then applied to top plate fl through shaft i5 in order to compact the particles and accelerate the recrystallization rej action. 'I'his pressure is preferably not less than the vapor pressure of water at the temperature where `=the lfundamental thickness-mode in Fig. 8 represents a modification bylinea'ns' of which a quartz crystal of given frequency'may a thicker crystal ofA lower free be changed intol quency. Y l

Fig. 9 represents a modication in which more selected but may be considerably higher.

An electric ileld alternating at high frequency is maintained between electrodes i and 2 which are insulated from each other. This high frequency field may be supplied by any conventional type of high frequency' oscillator such as shown diagrammatically atA. The electrodes are connected to the generator through conductors I 9.

Where only a single alternating field component is usedit is preferable that said frequency correspond to vthe natural'vibration frequency of the Acrystal being formed. However, some other frequency suitable'for setting up the necessary mechanical strains may be used. For quartz this frequency mayvary all the way from kilocycles' per second (50,000 cycles/second) to 10 megacyclesper second (10,000,000 cycles/second) and "evenhigher.v

'Ifhe frequency with which a quartz plate oscil- 'latesis. `represented by the equation r ft" megacycles/second, t is -thickness in mils of an inchand K i's'ja constant, 99 for the V2 cut 'quartz crystal'and 66 for the VI cut (see U. S.

patent; .to Gerber, 2,304,760). Since the frequency is 'inversely proportional to the thickness,

the greater-the thickness the lower will be the .frequency of the oscillator plateproduced. The

frequency range Amentioned represents about the practical? limits of commercially usable quartz vvoscillator plates at the present time `although it is possible togetsome results-.with both higher than twoelectrodes are used and in which the fields are parallel.

Fig. l0 represents a modification in' whichmore than two electrodes are used and in whi'ch the elds are not parallel. f Fig. l1 represents a view oi' right angles to the view shown in Fig. 1.

the' appara-.tus I. shown diagrammatically in Fig. 1 but vtaken kat' 'and catalyst added is packed inthe compartment; Y

as-shown in Fig. l. The mass is raised tolan "5 elevated temperature which may be anywhere i'rom slightly above room temperature, as 50 C up to the inversion temperature of quartz (5'I 3 C.). A preferred temperatureis the criti-y '1 cal point oi' water (374 CJ. 'I'he temperature is controlled by resistance heaters I4 connected 'to' the A. C.mains by conductors 20.' Any suitable A. C. current controlling means of conventional and lower -frequencies.

Although not wishing tolbe limited to the theory ,of what supposedly occurs during the above-described process it is thought to take place as follows: l

When the mixture of finely divided silica,

water and catalyst is subjected to elevated temperature and pressure, some of the silica goes into solution. Since this reaction takes place in a-closed system'some of the silica first dissolves then redeposits inaccordance with the principles of fphase. equilibrium resulting in -a .gradual growth of the crystalline particles. This growth should theoretically proceed until the whole mass `r`has been transformed into one single crystal. 1 However, when*` nov -means are provided for orlenting tlie c'rystallites,` growth takes place in a haphazardfmanner as toV crystal orientation.

In addition-ae the crystallits grew there is a corresponding decrease in the active reaction area. The result of these two factors, if allowed to proceed'without the conditions prescribedl by `the present invention, would be the'formation ot a crystalline aggregate showing extensive twinning rather than formation of a monocrystal.

When the xnicrocrystallineA or amorphous powder is compressed, as in the present invention. to-such an extent that itbehaves like a rigid body, the possibility is given to direct the orientation of the growing particles into uniform axial directions. It is well lfnown that crystal growth shows directional response to mechanical stresses. In the method described, such mechanical stresses are.gcreated by subjecting the compressed mixture to one or more alternating electric fields.

There are various modifications of the method which may be used to advantage some of which may be utilized to produce crystals of special' types.

In the modification shown in Fig. 2, there is used in addition to the, high frequency ileldI a source of supersonic vibrations B connected through conductors 2| to end electrodes I8. The use of these supersonic vibrations aids in the mechanical reorientation of the particles and thus promotes oriented crystallization. The frequency range useful here is 15-50 kilocycles/second.

In order to produce quartz oscillator plates of other than the X-cut type other modifications of the method may be used. As in Fig. 3, the high frequency field `may be applied by electrodes 26, 21 whose surfaces are placed at some angle with respect to the main faces of the insulating plates i2 and 25.

In Fig. 4 the high frequency electrodes I and 2 are placed as in Fig. 1 but the electrodes Il ccnnected to the source B of supersonic vibrations are placed at an angle to thehigh frequency electrodes. y

In order to provide for a high frequency field which will travel at an angle through the powdered mass, the electrodes may be arranged as at 3 and l in Fig. 5. Here the high frequency electrodes are set in the insulating plates I3 .with their centers laterally displaced. V

A further modification is shown in Fig. 6 in which the electrodes connected to the source of supersonic vibrations are also set at an angle to the main faces of the oscillating plate being formed.

It has also been found desirable to sometimes set up two or more high frequency fields in orienting thefcrystallites. Such an arrangement is shown in Fig. 9 in which the fields are parallel. Electrodes 'l and 8, when connected to high-frequency source A2 produce one field while electrodes 9 and I0 connected to high frequency source A1, produce another field parallel to the first. The elds may have the same or different frequencies.

In Fig. 10 the electrodes 9 and i0 produce a eld which is not parallel to that set up between electrodes 'l' and 8'.

The alternating mechanical stresses required for oriented recrystallization are best produced, in piezoelectric substances, by an alternating electric field. In other materials, however, such stresses may be applied to the parent body by an `oscillating element or elements, such as quartz oscillator plates Vibrating at the proper frequencies which may, wholly or partly, replace the metal electrodes previously described. This is illustrated in Fig. 7 where 5 and 6 are hooked up to high frequency oscillator C. Electrodes 21 and 28 are metal plates through which quartz plate 5 is caused to vibrate. Electrodes 29 and asavgsea 6 lll perform a similar function for quarts plate l. The invention may alsobe used in buildingup the dimensions of existing quartz oscillator plates.

As illustrated in Fig. 8, a quartz oscillator plate 2l is packed in a powdered silica, water, catalyst mixture 22. Either a high frequency field only may be applied or a two component field of both high frequency and supersonic vibrations may be used. In this way the original crystal may be made larger.

The invention has been described mainly in connection with the manufacture of synthetic are met.

' These conditions are:

1. The material is placed in'a of subdivision.

\ 2. A liquid is added veryl fine state in which the finely divided material is but slightly or moderately soluble.

3. A crystallization catalyst is desirable although not absolutely necessary.

4. Elevated temperature isused when accelerated reaction is desired.

. 5. Elevated pressure of sullieient degree is used to compress the mixture so that it acts as a rigid body.

6. An alternating field of one or more components is used, at least one of which is preferably the natural mechanical vibration frequency of the parent body.

.While in some cases it is desired to produce a l crystal which has most of the sameproperties as one occurring naturally, it ls possible to produce crystals which are distinctly different. For example, the synthetic crystals may have different lattice constants than those of natural origin. This is possible because of the fact that the synthetic crystals have been produced under mechanical strains not present in the kenvironment when the natural crystals were formed.

In the illustrations the top plate of the compartment is assumed to be movablel to provide for exerting pressure on the mass andin all cases means are assumed for suitably heating the Kas-.-v

sembly where this is desired.

The method ofv heating the assembly is not by any meanslimited to the use of resistance wireheating elements. For example, the vmetal elecbe heated by induction and the heat trodes may thus transferred to the powdered mass. `Also it is feasible to use high frequency dielectric heating in which case in the powdered substance.

I claim as my inventlonz 1. The processV of producing a substantially monocrystalline body comprising mixing a .powder of a crystallizable substance with a liquid capable of bringing about partial solution of said substance, compressing said mixture within a compartment, subjecting the compressed mixture to elevated temperature, and subjecting the compressed, heated mixture to high-frequency vibrations until substantially all of the mixture has crystallized into a solid body.

2. The process of producing a substantially monocrystalline body comprising mixing a' powder of a crystallizable substance with a liquid capable of bringing about partial solution of said f' substance to which a catalyst of crystallization is added, compressing said mixture within a compartment, subjecting the compressed mixture to elevated temperature, and subjecting the compressed, heated mixture to high-frequency vibrathe heat is induced directly withv.

the mixture has vmixture to high-frequencyI vibrations untilsubstantially all of the mixture has crystallized into a solid body. v 4. The invention as set forth in claim l, characterized by the high-frequency vibrations being applied through the medium of insulated metal electrodes. v

5. The invention as set forth in claim ,1, characterized by the high-frequency vibrations being applied through the medium of piezoelectric oscil-v lator plates.

6. The invention as set forth in claim 1, characterized by said mixture being compressed into a substantially piate-shapedbody and said highfrequency vibrations being set up by an alternat ing electric leld of a frequency corresponding to the natural mechanical vibration frequency of said body. f

7. The invention as set forth in claim 1, char- 4 acterized by said mixture being compressed into asubstantially plate-shaped body and said highfrequency vibrations being set up by an alternating electric field one frequency component of which isd the natural mechanical vibration frequency of the compressed body and another frequency component of which is in 'the supersonic range.

I acterized 'by more than `il. The invention as set forth in claim 1, .charn "acterized by the mixture being compressed into a substantially plate-shaped body, said highfrequency vibrations being set up by an alternating electric field which is not vertical to the main plate faces of said body.

9. The invention as set forth in claim 1, char'- acterized by the mixture-being compressed into a substantially plate-shaped body, saidbody beingvsubiected to alternatingjelectrlc elds, the fields being in a direction not parallel to each other, one field alternating at the natural mechanical vibration frequency of said body and another field alternating at a frequency in the supersonic range.

10. I'he invention as set forth in claim 1, characterized by the crystallizable substance .being capable of forming piezoelectric crystals.

11. The process of producing a quartz oscillator plate comprising mixing flnely'divlded silica with water. compressing the mixture into a substantially plate-shaped body within a compartment, heating the mixture to a temperature between 50 C. and 573 C. while maintaining a pressure notlower than the vapor pressure of water at said temperature, and maintaining an alternatlng electric field across the body.

12. The process of claim 11, characterized by the water containing a dissolved crystallization catalyst.

13. The process of claim 11, characterized by the water containing a dissolved crystallization catalyst, said catalyst being a compound which-is alkaline in nature.

, frequency corresponding 14. The process of claim 11, characterized by the water containing a dissolved crystallization catalyst, said catalyst being a compound selected from the group consisting of hydroiluoric acid and metallic uorides. 15. The process as set forth in claim 11, characterized by the alternating electric field being of a frequency corresponding to the natural mechanical vibration frequency of the compressed body.

16. The process as set forth in claim 11, characterized by said alternating electric field having more than one component, one of which is the natural mechanical vibration frequency of the compressed body and another component of which is a frequency in the supersonic range.

17. The process as set forth in claim 11, charone type of alternating field being maintained across the compressed body. the direction of which fields is not parallel to each other, 'and at least one of said directions being not vertical to the 'main faces of the plate, the frequency of' one field being the natural mechanical vibration frequency of the compressed body, the frequency .of another field being in the supersonic range. y

18'. The process of superimposing oriented growth upon a monocrys'tal body comprising packing around said body ay mixture consisting of the finely subdivided substance'of said body and a solvent in which said substance is slightly soluble, exposing said Vcombination of body and mixture to elevated temperature and pressure and Amaintaining across said! combination an alternating electric eld.

19. The .invention as set forth in claim 18, characterized by the body being a quartz plate oscillator.

5 20.-'.I'he process ,as set forth in claim 18, characterized by the alternating electric eld being of a frequency corresponding tothe natural vibration frequency'of the compressed combination.

21..;The invention-as set 'forth in claim 18, characterized byfsaid' alternating' field having morethan one component, one of which is of a to the natural mechanical vibration frequency oi said combination and another of which is a frequency in the supersonic range.

22. A-y monocrystalline bodyf produced by oriented v'growtbof n'elydivid'ed crystallizable substance in`v the presence of a lsolvent for "said substanceA'under-the influence of elevated temperature,.pressure, .andan alternating electric field.

23. A monocrystalline body produced by oriented growth of finely divided silica in the presence of a small amount of a solvent for said substance under the inuence ofelevated temperature, pressure and an alternating velectric field.

' LEOPOLD PEssEL.

REFERENCES forrnn UNITED STATES PATENTS Name Date Meissner Nov. 1, 1932 Number

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