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Description  |
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2. Field of the Invention
The present invention is directed to a method and apparatus for growing
high quality crystals, and particularly to a method and apparatus for
growing high quality crystals by programmed pressure reduction of a
supercritical or subcritical solution.
3. Brief Description of the Prior Art
Crystals of solid substances are conventionally grown either from a liquid
or vapor phase of the substance, or from a solution of the substance in a
second substance. The second substance, the solvent, usually is a liquid
at ambient temperature. All methods of crystal growth from a solution
involve shifting of the solid versus solute-solvent equilibrium at least
in the vicinity of the crystal-solution interface so that the solution
becomes supersaturated with the substance to be crystallized.
By far, the most common method of shifting the equilibrium is gradually
changing the temperature of the entire crystal solution system.
Furthermore, because the solubility of most substances in various solvents
usually increases with increasing temperature, the majority of
crystallization processes of the prior art involve gradual cooling of the
crystal-solution system. The method of thermally shifting the equilibrium
between dissolved solid and solution to promote crystallization is
hereinafter briefly referred to as "temperature driven crystallization."
As is appreciated by those skilled in the art, temperature driven
crystallization requires the transfer, usually withdrawal, of large
amounts of heat from the solution.
A majority of the temperature driven crystallization processes are
performed at ambient or near ambient pressure. Nevertheless, in some
instances, such as for example in the industrial process for growing
quartz (SiO.sub.2) crystals, the temperature driven crystallization
process is performed at a high pressure because the solubility of
SiO.sub.2 in the solvent is increased at high pressure.
Although the prior art temperature driven crystallization techniques are
widely used even when relatively large and morphologically perfect or near
perfect crystals are desired, they suffer from several disadvantages. One
disadvantage is that precise timecontrolled temperature regulation of a
solution containing a growing crystal is very difficult because
temperature adjustments of the environment have a relatively long lag-time
before they take effect in the solution-crystal system.
Another disadvantage of the temperature driven crystal growing technique is
that crystal growth by the temperature driven technique in the earth's
gravitational field is invariably accompanied by convection currents. The
convection currents cause morphological irregularities and fluid
inclusions in the crystals. Although several methods have been suggested
for minimizing the adverse effect of the convection currents on the
quality of the resulting.crystals, the prior art is still far from
achieving a fully satisfactory solution to this problem.
The above-noted disadvantages of the temperature driven crystal growing
techniques are buttressed by relatively recent advances in solid state
physics, semiconductor and related technology which have continuously
increased the quantitative and qualitative requirements for large,
morphologically near perfect crystals of various substances. For example,
quartz (SiO.sub.2) crystals are presently in highest demand for industrial
and scientific purposes. Other inorganic crystals of high quality, such as
zinc oxide (ZnO), aluminum phosphate (AlPO.sub.4), beryl and tourmalene
are also in high demand.
In order to overcome the adverse effect of convection currents in the
thermal growth of large crystals, growing of crystals in a zero gravity,
i.e., spaceflight, environment was also suggested. Obviously, due to its
tremendous expense, this solution offers the hope of only a very limited
immediate industrial application.
Another method for reducing convection currents in temperature driven
crystal growing processes in the earth's gravitational field was suggested
at a presentation by P. J. Shlichta to an American Association for Crystal
Growth Meeting in San Diego, Calif. on July 25, 1981, and also in a report
titled Crystal Growth in Spaceflight Environment, Jet Propulsion
Laboratory, California Institute of Technology. This method however is
practically limited to crystal configurations of only a few millimeter
thickness. This size limitation for the purpose of reducing convection
currents, is inherent in any temperature driven crystallization process.
Another technique for shifting the solid versus solute-solvent equilibrium
in a solution and more specifically growing of large, high morphological
quality crystals has been described in an article by P. J. Shlichta and R.
E. Knox in the Journal of Crystal Growth 3, 4 (1968), pp. 808-813, North
Holland Publishing Co., Amsterdam. In accordance with this technique,
localized supersaturation of an on-the-average saturated solution, and
thereby crystal formation are achieved in certain parts of a solution when
the solution is subjected to a centrifugal force field of 10.sup.4
-10.sup.6 g magnitude.
Other prior art methods of general interest to the present invention, which
achieve supersaturation of a solution for the purpose of promoting crystal
growth, involve gradual removal of the solvent by evaporation, and
interdiffusing two solutions having respective solutes which chemically
react with one another to form a substance to be crystallized.
The relatively recent scientific and industrial requirements for
morphologically perfect or near perfect crystals of certain substances,
such as quartz, have also prompted increased scientific efforts to develop
techniques for studying the mechanism of crystallization, and more
particularly, the interface of a crystal with a slightly super- or
slightly under-saturated solution. Several authors suggested a "thermal
wave technique" or experiment for this purpose. Briefly, in a thermal wave
experiment, a crystal is located in substantially one end of an elongated
container and the crystal is in contact with a solution of the substance
comprising the crystal. The entire system is kept, on the average, at a
predetermined temperature. Under conditions of equilibrium, the solution
is, of course, saturated with the substance comprising the crystal. In
order to cause the crystal-liquid interface to periodically grow and
decrease macroscopically, temperature of the remote, liquid-containing end
of the container is periodically oscillated between temperature values
which respectively exceed and fall below the predetermined temperature.
Each change in the temperature of the remote end of the container, travels
across the solution in the container in the form of a "thermal wave" and
correspondingly affects the crystal-liquid interface. The interface may be
observed visually and by appropriate instrumental techniques.
An important purpose of the thermal wave experiments, other than scientific
study, is to establish optimal conditions and parameters for growing of
large, high morphological quality crystals for industrial purposes.
However, as is explained below, because of its many disadvantages, the
above-noted purpose is not realized well by the thermal wave technique.
More specifically, a substantial time-lag is involved in the travel of the
thermal wave through the solution, and, therefore, convection currents
arise inevitably in the solution. Therefore, the exact conditions at the
crystal-liquid interface are not well known; they must be either measured,
or inferred from calculations. Measurements of the conditions of the
crystal-liquid interface, however, may be accomplished only with dubious
precision. Calculations of the conditions on the other hand, require
complex mathematical treatment and many assumptions, so that the
calculated conditions are of dubious validity. The difficulty in obtaining
and analyzing data relating to crystal growth by the thermal wave
technique becomes readily apparent for example, by study of the article
"Mathematical Analysis of the Thermal Wave Technique for Square-Law
Kinetics," by R. F. Sekerka in The Journal of Chemical Physics, Volume 46,
pp. 2341-2351 (1966).
Dependence of solubility of a solid substance in a liquid solvent as a
function of pressure of the system was recognized a long time ago. Thus,
it was known in the prior art that whenever dissolution of a solid in a
liquid decreases the overall volume of the entire solid-liquid system, an
increase in the pressure of the system causes an increase in the
solubility of the solid. In other words, the Le Chatelier Braun principle
is applicable to the influence of pressure to the solubility of a solid
substance in a liquid.
Furthermore, it was recognized at least theoretically, in the prior art
that supercritical fluids are capable of dissolving solid substances to a
significant extent. An article written by H. S. Booths and R. M. Bidwell
in Chemical Reviews, Volume 44, (1949) pp. 477-513 titled "Solubility
Measurement in the Critical Region" contains a review of prior art
experiments regarding solubility of solids in fluids near or above the
critical temperature of the fluids, and at high pressures. The above-noted
article shows, for example, that quartz (SiO.sub.2) is approximately five
times more soluble in steam at 650.degree. F. at approximately 425 PSI
than at 150 PSI. In a manuscript of a paper by V. J. Krukonis, A. R.
Branfman, M. G. Broome, and A. T. Sneden titled "Supercritical Fluid
Extraction of Plant Materials Containing Chemotherapeutic Drugs", the
authors describe selective extraction of certain organic substances by
solvents in supercritical condition. The solvents disclosed were carbon
dioxide (CO.sub.2), and ethylene (CH.sub.2 .dbd.CH.sub.2). The paper which
was orally presented at the 87th National Meeting of the American
Institute of Chemical Engineers in 1979 in Boston, Mass. includes a graph
showing the pressure dependent solubilities of p-iodochlorobenzene in
ethylene at 25.degree. C., and of naphtalene in carbon dioxide at
45.degree. C.
The above-noted paper by V. J. Krukonis et al. also discloses an apparatus
wherein the extraction of naphtalene, or plant material with supercritical
carbon dioxide was performed. Briefly, the apparatus includes a pressure
vessel adapted for containing the solute (naphtalene) and the solvent
carbon dioxide at 50.degree. C. and 300 atmosphere pressure, and a second
pressure vessel wherein the carbon dioxide containing dissolved solids is
expanded to about 100 atm. causing precipitation of the solids.
The above-noted article by H. S. Booth and R. M. Bidwell in Chemical
Reviews contains a suggestion to the effect that information gleaned from
studying the behavior of solute-solvent systems near the critical region
may become useful as a basis for growing large crystals of various
substances for optical and other scientific purposes. The context of this
article indicates, however, that the suggestion relates to the possibility
of taking advantage of increased solubilities of a solute at high
pressures in a temperature driven crystallization process.
The present applicant is unaware of prior art experimental or practical
investigations or results wherein the strong pressure dependence of
certain solids in fluids is utilized under isothermal or adiabatic
conditions for slow growing of relatively large, high morphological
quality crystals. The present applicant is also unaware of prior art
wherein the pressure dependence of the solubility of a solid in a suitable
fluid is utilized for studying the crystal-solvent interface.
SUMAMRY OF THE INVENTION
It is an object of the present invention to provide a process and apparatus
for growing relatively large, high morphological quality crystals of a
desired substance from a solution of the substance.
It is another object of the present invention to provide a process and
apparatus for experimentally studying a changing crystal-solution
interface in a crystal-solution-system wherein conditions of the system
are precisely and instantaneously controllable.
These and other objects and advantages are attained by dissolving a
substance to be crystallized in a suitable fluid solvent under high
pressure, and by subsequently, slowly, and gradually decreasing the
pressure while maintaining minimal heat flow conditions so as to cause
crystallization of the substance. The minimal heat flow conditions are
attained when the solution and the crystallizing solid is kept under
isothermal or adiabatic conditions. In a technique for studying a
crystal-solution interface principally for the purpose of optimizing
parameters of crystal growth in industrial processes, controlled
conditions at a changing crystal-solution interface are obtained by
precisely controlled, programmed oscillation of the pressure of a crystal
solution system at a substantially constant temperature.
The features of the present invention can be best understood together with
further objects and advantages by reference to the following description,
taken in connection with the accompanying drawings wherein like numerals
designate like parts.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the increase of solubility of quartz (SiO.sub.2)
in steam at various temperatures as a function of pressure;
FIG. 2 is a graph showing the increase of solubility of p-iodochlorobenzene
in ethylene and of naphtalene in carbondioxide (CO.sub.2) as a function of
pressure;
FIG. 3 is a schematic view showing a preferred embodiment of the apparatus
of the present invention, performing a step in the process of the present
invention wherein a solid substance is dissolved in a fluid solvent under
high pressure P.sub.1 in a first pressure vessel;
FIG. 4 is a schematic view showing the apparatus of FIG. 3, performing a
step in the process wherein a fluid solution saturated with the solid
substance at pressure P.sub.1 is tranferred through a suitable filter
system into a second pressure vessel;
FIG. 5 is a schematic view showing the apparatus of FIG. 3, performing a
step in the process wherein pressure in the second pressure vessel is
slowly and gradually decreased from P.sub.1 to P.sub.2 and the solid
substance crystallizes from the fluid solution;
FIG. 6 is a schematic view showing the apparatus of FIG. 3, performing a
step in the process wherein fluid solution is retransferred from the
second pressure vessel into the first pressure vessel to dissolve newly
added solid substance at pressure P.sub.1 ;
FIG. 7 is a schematic view showing a step in a "pressure wave-type"
crystal-liquid interface observation experiment in accordance with the
present invention;
FIG. 8 is a schematic view showing another step in the pressure wave-type
crystal-liquid interface observation experiment;
FIG. 9 is a schematic view showing still another step in the pressure
wave-type crystal-liquid interface observation experiment, and
FIG. 10 is a graph showing the variance of pressure of a fluid solution as
a function of time in the pressure wave-type crystal-liquid interface
observation experiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The following specification taken in conjunction with the drawings sets
forth the preferred embodiment of the present invention in such a manner
that any person skilled in the arts of growing crystals and of building
pressure vessels can use the invention. The embodiments of the invention
disclosed herein are the best modes contemplated by the inventor for
carrying out his invention, although it should be understood that various
modifications can be accomplished within the parameters of the present
invention.
Referring now to the drawing Figures, and particularly to the graphs of
FIGS. 1 and 2, the theoretical basis for the novel process and apparatus
of the present invention, is disclosed. As it is stated in the
introductory section of the present application for patent, the solubility
of substances in a liquid solvent depends on the pressure of the system as
well as on its temperature. In accordance with the well-known Le Chatelier
Braun principle, an increase in the pressure of a fluid-solvent
solid-solute system at a constant temperature causes dissolution of solid
provided the total volume of the systems is decreased by the dissolution
of the solid.
Furthermore, the prior art established that many inorganic and organic
substances have very significant solubilities in supercritical fluids. As
is wellknown by those skilled in physics or chemistry, the critical
temperature of a fluid is that temperature above which the fluid cannot be
compressed to yield a two phase liquid and gas system, no matter how large
a pressure is used for the compression. In other words, the fluid can only
exist in one phase above the critical temperature. In the ensuing
description, the respective states of a fluid solvent are referred to as
subcritical, or supercritical, as applicable, and the term subcritical
fluid should be understood to mean the liquid phase of a subcritical
fluid.
Thus, in accordance with the present invention it was discovered that pure,
very good morphological quality, and relatively large crystals of various
solid substances may be grown by dissolving the solid substance in a
subcritical or supercritical fluid at a high pressure, and subsequently,
slowly, and gradually decreasing the pressure while minimizing flow of
heat between the solid-solution system and its environment. Conditions
wherein heat flow between the solid-solution system and its environment is
minimized are attained when the system is kept under isothermal or
adiabatic conditions during decrease of the pressure.
The process of the present invention is applicable to those solid-solute
fluid-solvent systems wherein increase of pressure results in increasing
solubility of the solid in the fluid solvent. Generally speaking, the
above requirement is met by the predominant majority of solid-fluid
systems. Particularly, all open frame-work crystalline substances, as well
as many other crystalline substances, in combination with most inorganic
solvents, such as H.sub.2 O, NH.sub.3, CO.sub.2, H.sub.2 S and most
organic solvents, meet the above-noted requirement. Very importantly from
a utilitarian viewpoint, the SiO.sub.2 -water system, as well as most
silicates and ceramic materials in water, meet the above-noted
requirement.
With regard to the pressures required for performing the process of the
present invention, it is noted that the solubility versus pressure curve
of most systems of interest, such as for example of the SiO.sub.2 -water
system, increases significantly in pressure ranges exceeding 10
atmosphere. An upper limit of the pressures which may be used in
accordance with the present invention appears to be defined by the
engineering limitations of an apparatus wherein the process is performed,
rather than by the process itself.
The pressure dependence of the solubility of solid substances in fluids
usually increases rapidly with increasing temperature, and usually becomes
very significant at temperatures approximately equalling or exceeding the
critical temperature of the solid-fluid system. Therefore, the process of
the present invention is preferably practiced with supercritical fluids,
or with fluids having a temperature equalling or being only slightly below
the critical temperature.
It is an important feature of the novel process of the present invention,
however, that during a crystallization step while the pressure of the
supercritical or subcritical solution is gradually decreased to promote
crystal growth, the entire system is kept under conditions wherein
interchange of heat between the system and its surrounding is kept at a
minimum. These conditions are practically attained by performing the
crystallization step under isothermal conditions, wherein the entire
system is kept at as constant temperature as possible. Alternatively,
minimal heat flow conditions may also be attained when the system is kept
under adiabatic conditions, i.e., as well insulated from its environment
as is practically possible.
While at first glance, isothermal and adiabetic conditions for crystal
growth may appear to represent two opposite extremes, it is worthy of
consideration in this regard, that under conditions of the prior art
"temperature driven crystllization techniques," a substantial amount of
heat is deliberately withdrawn from the crystallizing system. In contrast,
with the prior art and in accordance with the present invention, under
isothermal conditions the temperature of the crystallizing system is kept
constant. Therefore, only a small amount of heat is added or withdrawn
(usually withdrawn) from the system to compensate for the heat of
crystallization, and for possible change of heat content of the fluid
solvent as it expands. As is well-known by those skilled in physics and
chemistry, the heat of crystallization is usually small. Furthermore,
expansion of a fluid against no resistance results only in a small change
of the heat content of the fluid, in case of an ideal gas such change is
theoretically zero. It is readily apparent from the above, that the amount
of heat withdrawn or added to the system under the above-noted isothermal
conditions is very small compared to the amount of heat withdrawn from the
system in accordance with the prior art, where the solution containing the
crystals to be grown is deliberately cooled to promote crystal growth.
The above-noted considerations are also applicable when the pressure
reduction step in the crystallization of the present invention is
performed under adiabatic conditions. Under these conditions, any
temperature change in the solid-solution system is due to the heat of
crystallization, and to changes in the heat content of the expanding
fluid. Since the above-noted changes are small, the temperature changes
are small, and the adiabatic conditions actually approximate the
isothermal conditions. Both are in contrast, however, with the prior art,
where the solid-solution system is deliberatiely cooled, and the principal
driving force for crystallization is a decrease in temperature. In
accordance with the present invention, the principal driving force for
crystallization is a change in the pressure of the system.
Referring now to the schematic views of FIGS. 3-6, an apparatus 20
particularly adapted for performing the process of the present invention,
is disclosed. Because crystalline, pure SiO.sub.2 (quartz) of high
morphological quality is in high demand for industrial and scientific
purposes, the apparatus 20 is described as the present invention. It
should be understood, however, that neither the apparatus nor the process
of the present invention is limited to the growing of quartz crystals. For
example, zinc oxide (ZnO), aluminum phosphate (AlPO.sub.4) beryl,
tourmalane and other crystals of industrial or scientific importance may
also be advantageously grown in accordance with the present invention.
As is shown in FIGS. 3-6, the apparatus 20 includes a first pressurizable
vessel 22 comprising a cylinder 24 and a piston 26 movably mounted in the
cylinder 24. The solid material 28, such as Si).sub.2 to be grown into
crystals, is placed into the first pressure vessel 22 and a solvent 30,
such as an alkaline aqueous solution, is added to the first pressure
vessel 22 through a suitable addition valve 32. Thereafter, pressure is
applied to the first pressure vessel 22 through the movable piston 26. The
applied, substantially constant pressure (P.sub.1) is maintained for a
prolonged period of time to affect full or partial dissolution of the
solid 28 which is contained in the first pressure vessel 22.
Although it is not shown on the schematic view of FIG. 1, the temperature
of the fluid and solid contents of the first pressure vessel 22 is kept
substantially constant during the above-noted dissolution step. This may
be accomplished by using state-of-the-art technology, such as a
temperature probe (not shown) mounted in the interior 34 of the first
pressure vessel 22, and heating coils or elements (not shown) incorporated
or mounted around the first pressure vessel 22.
For dissolution of SiO.sub.2, the constant temperature of the interior 34
of the vessel 22 may equal or even exceed 300.degree. C. The substantially
constant pressure (P.sub.1) in the first vessel 22 may be as high as 200
atm. For comparison it is noted, that in accordance with prior art
techniques, quartz crystals are grown under "temperature driven
crystallization" conditions which involve a temperature gradient in a
pressurized vessel and temperatures and pressures comparable to the
above-noted temperature and pressure parameters utilized in the process of
the present invention. Furthermore, when the process of the present
invention is used for growing quartz crystals, the aqueous solvent is
rendered alkaline by addition of sodium hydroxide, sodium carbonate or a
like mineralizing agent. This is also done in the prior art to increase
the solubility of SiO.sub.2 in the aqueous solvent. FIG. 3 shows the
apparatus 20 of the present invention performing the step of dissolution,
as described above.
In alternative embodiments of the process of the present invention, the
solvent may be ammonia, hydrogen sulphide, sulphur dioxide, or other
unorganic or organic solvents.
After the dissolution step has been performed for a sufficiently long
period of time to substantially saturate the solution in the first
pressure vessel 22, the saturated solution 36 is transferred into a second
pressure vessel 38. The step of transferring the saturated solution 36
into the second pressure vessel 38 through a suitable filter system 40 is
shown on FIG. 4.
Valves 42 and 44 are connected to the respective first and second presure
vessels 22 and 38 to permit the transfer of the saturated solution 36
through the filter system 40 at a pressure which is only slightly higher
than P.sub.1. This pressure is indicated on FIG. 4 as P.sub.1 +.DELTA..
During the transfer of the saturated solution 36 from the first pressure
vessel 22 into the second pressure vessel 38, the temperature of the
solution 36 is kept as constant as practically possible, and the pressure
in the second, vessel 38 is kept at P.sub.1.
The second pressure vessel 38 is substantially identical in construction to
the first pressure vessel 22. Thus, it also includes a cylinder 24, and a
movably mounted piston 26 which is capable of regulating pressure in the
cylinder 24. In addition to the valve 44, an additional valve 46 is
attached to the second pressure vessel 38. The additional valve 46 is
adapted for draining fluid from the second pressure vessel 38.
A suitable seed crystal 48 is mounted to the piston 26 to prompt and
promote crystal growth in the second pressure vessel 38. The seed crystal
48 for the growing of quartz is preferably a flat plate shaped crystal as
is shown in the drawing Figures.
FIG. 5 discloses the step of crystallization in the process of the present
invention. During crystallization, pressure in the second pressure vessel
38 is gradually and slowly decreased from P.sub.1 to a substantially lower
value, P.sub.2. For example, during the crystallization of quartz, the
pressure may be decreased from approximately 50 atm to approximately 10
atm. The temperature of the solution 50 within the second pressure vessel
38 is kept as constant as possible during the crystallization step.
Alternatively, as was discussed above, the second pressure level 38 may be
well heat insulated so that the crystallization is adiabatic. As it was
further discussed above, under adiabatic conditions, the temperature of
the solution 36 and of the crystals therein undergoes only an
insignificant change compared to the change in pressure. The
crystallization step may be performed for several weeks or months in order
to obtain very high quality crystals. As is shown on FIG. 5, the crystal
51 grows principally on, and around the seed crystal 48.
Because the seed crystal 48 is mounted to the piston 26, crystal growth in
the second pressure vessel 38 is substantially in a downward direction. As
the prior art already realized this, crystal growth in a downward
direction is already conducive to the formation of high quality crystals
because downward growth is likely to minimize convection currents. In the
herein described system, heat flow within the system is minimal and
thermal convections in the solvent are virtually nonexistent. If
minimization of convection becomes less important for some reason, the
seed crystal 48 may be located elsewhere in the cylinder 24 of the second
pressure vessel 38.
While the step of crystallization is performed in the second pressure
vessel 38, the first pressure vessel may be idle, or may be replenished
with solid material 28. The first pressure vessel 22 may also be utilized
for dissolution of more solid material 28 while crystallization is
performed in the second pressure vessel 38.
After the step of crystallization is completed in the second pressure
vessel 38, the liquid contents 50 of the same may be retransferred to the
first pressure vessel 22 to act therein as the solvent 30 in a subvessel
sequent dissolution step. The step of retranferring the solvent 30 is
schematically shown on FIG. 6. Thereafter, the grown crystal or crystals
51 are removed from the second pressure vessel 38. Alternatively, the
grown crystals 51 may be left in the second pressure vessel 38 to receive
further growth from a freshly saturated solution 36 in a subsequent
crystallization step.
The above described apparatus performs the steps of dissolution and
pressure reduction driven crystallization in a step-wise manner. However,
an apparatus may also be devised which performs the process of the present
invention in a continuous rather than step-wise cycle.
As it was stated above, the process of the present invention is preferably
practiced with supercritical fluids, and the steps of dissolution and
crystallization may require considerably long time periods. As is
schematically shown on FIGS. 3-6, the pressures in the pressure vessels 22
and 38 are regulated automatically in the predetermined, desired manner by
an electronic memory and control device 52. The electronic memory and
control device 52 is constructed in accordance with the state of the
electronic arts. It may also advantageously regulate the valves 32, 42, 44
and 46, so that during the prolonged dissolution and crystallization
steps, operation of the apparatus 20 is automatic.
Referring now to FIGS. 7-10, utilization of the process of the present
invention for the purpose of observing and studying a growing or
dissolving crystal-solution interface, is disclosed. As it was described
in the introductory section of the present application for patent,
scientific study of such a crystal-solution interface is undertaken in the
prior art in a "thermal wave" experiment or technique. The thermal wave
technique, however, suffers from many disadvantages. The disadvantages are
principally caused by the fact that propagation of a heat wave is not
instantaneous in an elongated tube.
In accordance with the present invention, a solid-crystalline substance 54
is contained in an elongated pressure vessel 56 in contact with a
saturated solution 57 of the substance 54 at a predetermined time average
pressure P.sub.0. In other words, the pressure is kept at P.sub.0 for
sufficiently long time to cause a suitable solvent in contact with the
crystalline substance 54 to become saturated with regard to the substance
54. Interchange of heat of the system with its environment is minimized,
which, as it was discussed above, may be accomplished by insulating the
pressure tube 56 or keeping the temperature of the entire contents of the
pressure tube 56 as constant as practically possible throughout the entire
experiment.
After equilibrium is reached with regard to the saturation of the solvent
with the crystalline substance 54, the pressure in the pressure vessel 56
is periodically changed in the manner shown on the graph of FIG. 10, so
that the time average pressure remains substantially at P.sub.0.
In accordance with the above-stated principles, increasing the pressure
causes dissolution of more crystalline substance 54, and therefore a
dissolving crystal-liquid interface. Conversely, decreasing the pressure
causes a growing crystal-fluid interface. The interface is observed by
visual and instrumental techniques as in the prior art. The principal
advantages of the presently described "pressure wave technique" are that
propagation of pressure in the system is uniform and instantaneous, and
thermal convections are eliminated or very significantly minimized in the
system. Consequently, the conditions of the experiment are much better
controlled than in the analogous "thermal wave" experiment and the
pressure wave technique is well adapted for optimizing the parameters or
industrial crystal growing processes.
What has been principally described above is a novel process and apparatus
for growing relatively large, high morphological quality crystals in
supercritical or near critical fluid solvents by programmed reduction of
the pressure of the fluid solvent. Several modifications of the
above-described process and apparatus may become readily apparent to those
skilled in the art in light of the above disclosure. Consequently, the
scope of the present invention should be interpreted solely from the
following claims.
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