EP2108061A2

EP2108061A2 - Crystal growth in a solution in stationary conditions

Info

Publication number: EP2108061A2
Application number: EP07871782A
Authority: EP
Inventors: Alain Ibanez; Julien Zaccaro
Original assignee: Centre National de la Recherche Scientifique CNRS
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 2006-12-06
Filing date: 2007-12-03
Publication date: 2009-10-14
Also published as: FR2909687B1; US20110142741A1; WO2008081103A2; WO2008081103A3; US8771379B2; FR2909687A1; JP2010511589A

Abstract

The present invention relates to a method for crystal growth in a solution adapted for the fast, controlled and efficient preparation of large-dimension crystals (1) from a supersaturated solution into a compound. The crystal growth is carried out in stationary conditions. In order to do so, the growth is carried out in a crystallisation chamber (10) maintained at a constant temperature Tc and in fluid communication with a saturation chamber (20) at a temperature Ts that is also constant but different from Tc, the compound solubility at the Ts temperature being higher than compound solubility at the temperature Tc. A continuous solution circulation is established between the crystallisation (10) and saturation (20) chambers, thus maintaining a constant supersaturation ratio in the crystallisation chamber (10). The circulating solution is further submitted to a treatment for removing and inhibiting (40) the formation of aggregates, thus inhibiting the nucleation of parasitic crystallites.

Description

Crystalline growth in solution under stationary conditions

The present invention relates to a new crystal growth crystallogenesis process in solution, in particular adapted to an implementation on an industrial scale to prepare in a controlled and efficient way large single crystals, of the order of a few centimeters or more.

At the present time, various techniques for the preparation of commercially exploitable single crystals are known.

A particularly interesting technique in this context is to grow the monocrystal from a molten bath of the desired compound to crystallize, in particular by implementing the usual Czochralski method Such a method has, among others, the advantage to lead to growth rates of particularly high crystals, typically of the order of several centimeters per day. However, crystalline growth from molten baths is not possible for certain compounds, especially those with non-congruent melting (that is to say which have in the liquid state a composition different from that of the state. solid for example due to chemical decomposition phenomena or peritectics), as well as for those having a phase transition between the molten state and the liquid state in a temperature zone between their melting temperature and the ambient temperature. Thus, the melt bath technique is in fact unsuitable for the crystallization of numerous compounds such as, among other numerous examples, the α form of quartz (σ-SiO 2), certain phosphates (such as KH ₂ PO ₄ , KTiOPO ₄ , by example), some borates (such as YAI ₃ (BO ₃ ) ₄ ), or even some metal halides.

To achieve the crystallization of compounds not adapted to the melt bath technique, such as those mentioned above, it has been proposed to implement growth techniques in solution, in which the compound to be crystallized is initially dissolved in a solvent, and where growth is achieved by placing the solution under supersaturation conditions, that is, under conditions where the solute is at a concentration greater than its solubility. In this type of process, at a given instant, the state of supersaturation of the solution in which the crystallization is carried out can be quantified by its relative supersaturation, denoted by s, which is calculated by the following ratio:

. . . (C - C ^Q )

C ₀ where C denotes the concentration of compound dissolved in the solution; and

Co refers to the solubility of the compound dissolved at the temperature at which crystallization is performed.

This state of supersaturation, required for crystallization in solution, is usually obtained according to two types of methods, namely: by solvent evaporation: according to this first variant, the amount of solvent of the solution during the crystallization is decreased , generally by allowing this solvent to evaporate gradually, resulting in an increase in the ratio of the compound / solvent in the medium, which initially makes it possible to increase the concentration to obtain the supersaturation required to start the crystallization, and then to maintain the state of supersaturation as the compound in solution is consumed to form the crystal.

by temperature change: according to this other variant, the temperature of the solution is modified so as to induce a decrease in the solubility of the compound to be crystallized in the crystallization solution and to obtain the desired supersaturation. For most compounds, solubility increases with temperature. As a result, in most cases, the desired solubility is reduced by lowering the temperature of the crystallization solution. There are nevertheless some particular compounds, such as limestone for example, whose solubility is retrograde, that is to say, it decreases when the temperature increases. For this type of compounds, conversely, the decrease in the desired solubility is achieved by increasing the temperature. In temperature change processes, additional control of the reaction conditions is required to maintain the crystallization process. Indeed, the formation of the crystal obtained by the temperature change consumes the compound in solution, which decreases the concentration of said compound in the crystallization solution, whereby it tends to deviate from the required conditions of saturation. To counterbalance this phenomenon of desaturation of the solution and maintain a supersaturation state, it is possible in particular to carry out a continuous temperature modification (that is to say most often a continuous lowering of the temperature, or more specifically a continuous increase, in the particular case of compounds with retrograde solubility).

The aforementioned crystallization methods, by solvent evaporation or temperature change, are most often poorly adapted to an implementation on an industrial scale for the preparation of large size crystals.

In this regard, it should first be noted that the currently known solution crystallogenesis methods do not allow a control of the thermodynamic and kinetic conditions of crystal formation:

in the case of crystalline growth by evaporation of the solvent, the conditions of crystallization (volume of the reaction medium, concentration) change constantly during the reaction. In particular, the concentration of impurities in solution tends to increase during growth, which induces an increase over time in the incorporation of these impurities into the crystals in formation. The same is often difficult to control, so in practice, crystal growth techniques by solvent evaporation are only used at the laboratory scale, and they have not actually found application at an industrial level.

similarly, in the methods implementing a temperature change, this modification of the temperature induces, again, a modification of the thermodynamic and kinetic conditions during the crystallogenesis, and maintenance of supersaturation throughout growth is difficult to control and remains in fact empirical.

The evolution of the conditions of crystallogenesis obtained in the context of current crystalline growth techniques in solution has a direct impact on the quality of crystals produced.

This evolution of the crystalline growth conditions proves particularly problematic when it is desired to grow crystals which are not based on defined compounds, but based on intermediate compositions, such as, for example, solid solutions or compounds. doped (in particular intermediate compositions of solid solutions of substitution where several types of atoms of different chemical nature occupy a same site of the crystalline structure and / or compounds doped with interstitial elements, for example crystals doped with ions of d 'insertion). Indeed, in this case, the evolution of thermodynamic and kinetic conditions is usually accompanied by significant changes in crystal formation mechanisms. These modifications may in particular cause a variation in the rate of incorporation of the substitutive and / or interstitial species and thus lead to inhomogeneities in the composition of the crystal formed. These instabilities can also lead to the formation of defects in the crystalline structure (stacking faults, dislocations, etc.) and in extreme cases to the insertion of solvent into the crystal. More generally, these different phenomena are likely to lead to a degradation of the crystalline quality. Typically, the gradual evolution of the rate of incorporation of substitutive and / or doping elements and / or impurities can in particular induce an evolution of the crystal lattice parameters as the growth progresses, which leads most often to to tensions within the crystal, source of many crystalline defects such as dislocations or even more macroscopic fractures. These phenomena are all the more sensitive as the size of the formed crystal is important. Another more general problem with presently known crystalline solution growth methods is that they generally lead to limited crystalline growth rates, which are significantly lower than those obtained using techniques using molten baths. In fact, in crystallogenesis techniques in solution, the crystal growth rate is directly proportional to the relative supersaturation of the solution in which the crystallization is carried out.

However, in most cases, in particular to avoid the spontaneous nucleation of crystallites, the usual solution growths are typically carried out with low levels of supersaturation, typically around 1 to 5%, which corresponds to crystalline growth rates. in solution which are of the order of a few millimeters a day. With this type of reduced crystalline growth rate, the preparation of centimeter-size single crystals can take several weeks, or even several months. The preparation time is even of the order of one year or more for certain compounds.

To try to counter this difficulty, it has been developed a technique of crystal growth by lowering the temperature in which it has been proposed to filter and heat treat the solution to avoid spontaneous nucleation, which allows an increase in speed. of decreasing the temperature in order to increase the supersaturation, and consequently the growth rate of the crystal. In this context, it has been described in particular processes for the preparation of giant crystals of potassium diphosphate KH ₂ PO ₄ (or KDP), where it has been sought to increase the state of supersaturation up to values of the order 30% in order to increase growth rates to 3 to 4 cm / day. Such methods are certainly effective with crystals of relatively small size (of the order of a few centimeters), but the rate of growth remains limited, in practice, during the formation of crystals of larger dimensions (for example the order of ten centimeters or more). In fact, the rates of temperature decrease required to obtain significant crystalline growth rates induce a thermal gradient between the core of the macrocrystals in formation and the periphery of the crystal in contact with the colder solution. This gradient generates within the crystal constraints that most often induce fracture phenomena when too high growth rates are used. Thus, although supersaturations of 30% and growth rates of 3 cm / day have been obtained, the growth of large single crystals with this system based on the lowering of temperature can be done only at 1 cm / day at most.

An object of the present invention is to provide a crystallogenesis process implementing crystalline growth in solution which is suitable for implementation at the industrial level, in particular avoiding the aforementioned drawbacks of currently known solution crystallization methods.

For this purpose, according to a first aspect, the subject of the present invention is a process for preparing a crystal based on a compound (C), by crystalline growth in solution, from a solution (S) supersaturated with said compound (C), wherein: - said crystal growth is carried out in a crystallization chamber maintained at a constant crystallization temperature T ₀ ;

- said crystallization chamber is in fluid communication with a saturation chamber, which comprises an excess of compound (C) to the state of a solid feeder body and is brought to a saturation temperature T ₅ constant, different from T ₀ this temperature T ₃ being chosen so that the solubility of the compound (C) in the solution (S) at the temperature T ₅ is greater than the solubility of the compound (C) in the solution (S) at the temperature Tc; and

a continuous circulation of the solution (S), preferably at a substantially constant rate, is established between the crystallization and saturation chambers, whereby a constant supersaturation level is maintained within the crystallization chamber; and

the solution (S) in circulation between the crystallization and saturation chambers is subjected to an elimination treatment of the aggregates likely to form and inhibit the formation of such aggregates, to avoid or inhibit parasitic crystallization nucleation phenomena.

According to a first variant of the process, the compound (C) used is a compound having a solubility in solution (S) which increases with temperature. In this case, the temperature Ts kept constant in the saturation chamber is higher than the temperature Tc kept constant in the crystallization chamber.

According to a more particular variant, the compound (C) is, conversely, a compound with retrograde solubility, namely having a solubility in the solution (S) which decreases with temperature. According to this second variant, the temperature

Ts kept constant in the saturation chamber is lower than the temperature Tc kept constant in the crystallization chamber.

In the process of the invention, the crystallization temperature T _c of the crystallization chamber and the saturation temperature T ₃ of the saturation chamber are both kept constant during the crystallogenesis process. For the purposes of the present description, the term "constant temperature" means a temperature which varies as little as possible on either side of a reference value T ⁰ , this temperature generally varying at most by +/- 0, 1 ° C, more preferably at most +/- 0.05 ° C over time, that is to say that this temperature preferably remains in the range of (T ⁰ - 0.1 ⁰ C) to (T ⁰ + 0.1 ⁰ C), more preferably still (T ⁰ - 0.05 ⁰ C) at (T ⁰ + 0.05 ⁰ C), more preferably (T ⁰ - 0.01 ⁰ C) at (T ⁰ + 0.01 ⁰ C), the acceptable variation of the temperature being a function of the nature of the compound employed, the solvent and also the temperature T ⁰ . It is particularly important to maintain constant the crystallization temperature T _c , the value of which is advantageously set at +/- 0.05 ^° C., preferably at +/- 0.01 ^° C., and more preferably at +/- 0.01 ^° C. +/- 0.005 ⁰ C near. The regulation of the supersaturation temperature T _s may be a little less fine, but remains most often regulated to within +/- 0.1 ^° C, preferably to +/- 0.05 ⁰ C, more preferably + / - 0,01 ⁰ C near. In the process of the invention, the specific maintenance of the crystallization temperature T _c and of the saturation temperature T _s has constant values, associated with the continuous circulation of the solution (S) between the crystallization and saturation chambers. , induces very particular conditions of crystallogenesis. In particular, unlike the crystallogenesis in solution currently proposed, the method of the invention specifically implements crystalline growth under stationary thermodynamic conditions.

More precisely, because of maintaining the saturation temperature T _s at a constant value and the continuous circulation of the solution (S) between the crystallization and saturation chambers, the concentration of compound in the saturation chamber (C) remains constant, namely equal to the solubility of the compound (C) at the temperature T ₃ . To optimize this effect of maintaining the concentration, a minimum flow rate of the solution (S) between the crystallization chambers must generally be applied. It is the competence of the specialist in the field to adapt the flow rate of the solution (S) for this purpose, on a case by case basis.

Given the effect of stabilizing the concentration obtained according to the process of the invention, the solution (S) which leaves the saturation chamber and which is then injected into the crystallization chamber has a constant constant constant concentration the solubility of the compound (C) at the temperature T ₈ .

Therefore, since, in the crystallization chamber, the crystallization temperature T ₀ is maintained at a temperature where the compound (C) has a solubility lower than its solubility at the saturation temperature T _5, the solution (S) is injected into the crystallization chamber with a concentration of compound (C) greater than the solubility of the compound (C) at the temperature T ₈ , that is to say that this solution is constantly in a state of supersaturation within of the crystallization chamber. Thus, in the process of the invention, a state of supersaturation is continuously maintained within the crystallization chamber). Insofar as the crystallization temperature T ₀ is kept substantially constant in the crystallization chamber, the relative supersaturation of the solution (S) is perfectly defined and remains constant during the crystallization. Thus, the process developed by the inventors makes it possible, according to a very simple technique, to obtain a constant relative supersaturation in a crystallization process in solution.

More generally, according to the method of the invention, all crystallogenesis conditions are maintained substantially constant throughout the crystal growth. The process of the invention makes it possible in particular to maintain both constant temperature, concentration and supersaturation conditions.

To further stabilize the crystal growth conditions, it is preferred that the continuous circulation of the solution (s) between the crystallization and saturation chambers takes place at a substantially constant rate. This continuous circulation is generally carried out in a circular manner, the solution (S) starting from the saturation chamber, in the saturated state of compound (C), towards the crystallization chamber along a first channel, then leaving the crystallization chamber to the saturation chamber to "recharge" in compound (C) along a second channel.

The stationary thermodynamic conditions of crystalline growth obtained in the context of the process of the invention make it possible in particular to avoid the growth incidents observed with currently known crystalline solution growth processes which implement non-stationary thermodynamic conditions.

Compared with the non-stationary conditions of currently known methods, the conditions for implementing the method of the invention have many other advantages. Work under stationary conditions according to the invention makes it possible in particular to stabilize the crystalline growth conditions, which leads to a significant improvement in the crystalline quality and to the production of crystals of controlled composition, which proves particularly advantageous in the case of intermediate composition crystals, in particular solid solutions or crystals containing one or more doping element (s), the process then making it possible to obtain crystals of homogeneous composition. The process of the invention makes it possible to regulate and very simply define the value of the relative supersaturation of the solution (S) in the crystallization chamber, and hence the rate of crystallization. According to the invention, this regulation operates simply by varying the values of the temperatures Tc and Ts, the supersaturation increasing with the difference in solubility of the compound (C) at the temperatures Ts and Tc. The technique developed by the inventors proves in particular to be much easier to implement than the techniques where the supersaturation is induced by lowering the temperature, where the lowering speed must be continuously adjusted. In the process of the invention, on the contrary, in order to induce the required supersaturation, it suffices to keep the temperatures T _s and T ₀ constant and distinct in the saturation and crystallization chambers (with T _S > T _C for the compounds of which the solubility increases with the temperature and T _s <T _c in the case of compounds with retrograde solubility), which is feasible very easily and at low cost, in particular by using thermostatic baths.

In addition, due to the specific implementation of stabilized growth conditions, the method of the invention is not limited as to the degree of supersaturation employed. In particular, it makes it possible to work with high levels of supersaturation inducing a very high crystalline growth rate, without inducing the thermal gradient between the core and the outside of the crystal obtained when implementing solution growth techniques by rapid decrease in temperature. In contrast, in the process of the invention, whatever the growth rate of the crystal, it remains perfectly thermostated at the temperature T _c within the crystallization chamber, from the beginning to the end of its growth.

In addition, in the method of the invention, the supersaturated solution (S) used is specifically subjected to a treatment making it possible to eliminate the aggregates that may form therefrom and / or to inhibit the formation of such aggregates for avoid or inhibit parasitic crystallization nucleation phenomena. The potential nucleation of parasitic crystallites is a problem that is systematically encountered in any solution crystallization process. This disadvantage is related to the implementation of supersaturated, metastable solutions, where Aggregates are systematically formed from the species in solution. As long as these aggregates have a sufficiently small size (called "subcritical"), they are not stable thermodynamically and therefore do not lead to the formation of crystallites. On the other hand, parasitic crystallites form when the aggregates reach or exceed a critical radius r ^* . This critical radius r ^* is all the weaker as the supersaturation is high in the medium, that is to say one moves away from the equilibrium conditions of the system.

This phenomenon is particularly marked in solution crystallization processes known hitherto, especially in temperature-lowering processes, insofar as they involve working away from thermodynamic equilibrium conditions. In this context, in order to avoid this phenomenon of parasitic nucleation, it is known to continuously treat and filter the growth solution in order to dissociate the aggregates in formation and to inhibit their development in the form of crystallites. For example, it has been proposed to dissociate the aggregates by continuously circulating the growth solution in a treatment cell where the solution is filtered very thoroughly (typically by membranes of porosity of 20 nm) and subjected to overheating. .

In the process of the invention, especially in view of the established stationary conditions, it is particularly easy to apply this type of anti-nucleation treatment of the solution (S) to inhibit the formation of subcritical aggregates and / or dissociate the aggregates formed.

In this context, any type of treatment for removing or dissociating the subcritical aggregates in the supersaturated solution (S) in circulation may be considered within the scope of the invention. Advantageously, this treatment comprises, among other possible means, the submission of the solution (S) to ultrasound between the crystallization chamber and the saturation chamber. In conjunction with this ultrasonic treatment, it is advantageous to implement a heat treatment of the solution between the crystallization chamber and the saturation chamber, namely:

in the case of the use of a compound (C) having a solubility increasing with temperature: a heating of the solution (S) between crystallization chamber and the saturation chamber, advantageously at a temperature of at least 10 ^c C higher than that of the saturation temperature T ₃ to which is carried the crystallization chamber;

in the case of the use of a compound (C) having a retrograde solubility decreasing with temperature: a cooling of the solution (S) between the crystallization chamber and the saturation chamber, advantageously at a temperature of at least 10 ^° C. lower than that of the saturation temperature T ₅ .

Optionally, the solution (S) may optionally be subjected to filtration between the crystallization chamber and the saturation chamber, so as to retain any aggregates in formation, which is however not always required.

Given these different elements, the method of the invention is a very interesting alternative to the currently known processes, insofar as it allows the preparation of very large single crystals with very high speeds by inhibiting the risk of incidents of growth, and more generally allowing access to optimized crystallization properties.

In this context, quite surprisingly, the method proposed by the inventors, although very simple to implement in practice, both to optimize the crystalline properties obtained and the crystal growth rate. One of the appreciable assets of the method of the invention and it allows to increase the speed of production while jointly lowering the risk of growth incident. In addition, the crystal growth rates that can be obtained according to the method of the invention are generally comparable to those recorded with the techniques of molten baths, namely of the order of at least several centimeters per day.

Another advantage of the process of the invention with respect to currently known solution crystallization techniques is that it allows the crystalline growth of compounds having very low solubilities or very slight evolutions of solubility with temperature. The process of the invention is more generally adapted to the crystal growth of a large number of chemical compounds. In fact, in practice, any compound can be used as compound (C) in the process of the invention, provided that it is solubilizable in a solvent. In all cases, the method of the invention is suitable for the preparation of single crystals substantially free of defects, which may have high dimensions, for example of the order of ten centimeters or more. The invention thus makes it possible to access new single crystals of quality at least similar to that of single crystals obtained by the technique of molten baths, but with compounds (C) which can not be crystallized according to this technique. In particular, the invention provides access with fast growth rates to large single crystals, substantially free of defects and based on compounds (C) having no congruent melting point or compounds (C) having a phase transition between the molten state and the solid state.

Whatever the nature of the compound (C) used, the process of the invention preferably has one or more of the additional characteristics described below.

In the process of the invention, the supersaturation level maintained in the crystallization chamber is advantageously the highest possible, which makes it possible to increase the crystal growth rate all the more. This degree of supersaturation can be modulated very simply by varying the saturation and crystallization temperatures Ts and Tc, and in particular by varying the difference ΔT between these two temperatures. The exact value of the degree of supersaturation in the crystallization chamber can vary to a very large extent depending on the nature of the compound (C), but it should be noted that the supersaturation levels that can be achieved according to the invention are higher than those that can be used in current growth processes in solution. Typically, according to the process of the invention, the degree of supersaturation can be easily modulated and kept constant at a value ranging generally from 1% to 30%, for example between 1 and 25%, this level of supersaturation which may especially reach values greater than 10%, for example at least 15%, or even at least 20%, which are much higher than the supersaturation rates that can be envisaged with current solution growth processes.

According to a very specific embodiment of the invention, the process developed by the inventors can in particular be used for the preparation of potassium biphosphate mass monocrystals of the type used in optical applications, in particular for constitution of LASER type devices. In this particular context, the compound (C) is potassium diphosphate KH ₂ PO ₄ (KDP), preferably in at least partially deuterated form (DKDP). The process of the invention makes it possible to easily and rapidly produce optical quality crystals based on these compounds.

It should be emphasized that although part of the following description is made specifically with reference to the preparation of KDP or DKDP crystals, this embodiment is only one mode among others and would not be considered as limiting of the invention.

In the specific context of using KDP or DKDP as compound (C), the temperature T _c at which the crystallization chamber is maintained is advantageously in the range between 5 and 80 ^° C. In a more particular embodiment, the KDP or DKDP crystallization process according to the invention can advantageously be carried out at low temperature, with a temperature T _c at which the crystallization chamber is maintained is less than 40 ^° C., preferably less than 40 ^° C. 30 ^° C., for example between 15 and 25 ° C. Moreover, the crystallization temperature Tc advantageously varies by at most +/- 0.1 ^° C., more preferably at most +/- 0.05 ^° C. Thus, typically, the temperature T _c within the chamber of crystallization can be set at 20 ° C +/- 0.005 ⁰ C.

Moreover, when the compound (C) is KDP or DKDP, the temperature difference (T _s -T _c ) between the saturation chamber and the crystallization chamber is preferably of the order of a few degrees, this difference temperature can vary depending on T ₀ and the desired growth rate.

Typically, this difference in temperature is between 2 and 30 ^° C., this difference preferably being at least 1 O ⁰ C, for example between 15 and 2O ⁰ C.

The DKDP mass monocrystals substantially free of defects that can be obtained according to this specific embodiment of the invention, advantageously having at least one dimension greater than or equal to 10 cm, constitute, according to a particular aspect, another object of the invention. present invention.

These single crystals of DKDP have very good qualities and are in particular essentially free from defects and have a homogeneous deuterium / hydrogen ratio within the material, which makes them very good materials for optical applications.

According to a more specific aspect, the present invention also relates to a device for implementing the method of the invention. This device comprises:

a crystallization chamber, adapted to the growth of a crystal in solution, and provided with an inlet and an outlet, as well as temperature regulation means making it possible to maintain the said crystallization chamber at a crystallization temperature T _c substantially constant, typically at temperatures ranging from -30 ⁰ C to +150 ⁰ C;

a saturation chamber, suitable for bringing a solution into contact with an excess of solid solute, provided with an inlet in fluid communication with the outlet of the crystallization chamber and an outlet in fluid communication with the entry of the crystallization chamber, as well as temperature control means for maintaining said saturation chamber at a crystallization temperature T ₃ substantially constant and distinct from T ₀ ; and

means for the continuous circulation of a solution between the two chambers, from the exit of the saturation chamber to the entrance of the crystallization chamber, and from the exit of the crystallization chamber towards the entrance of the chamber of saturation, these means of circulation being in further provided means for removing aggregates likely to form in the circulating solution and for inhibiting the formation of such aggregates.

According to an advantageous embodiment, the means allowing the continuous circulation of the solution between the two chambers are provided with sonification means, generally associated with heat treatment means for heating or cooling the solution, and possibly filtration means, as a rule between the exit of the crystallization chamber and the entrance of the saturation chamber.

The invention will be further illustrated by means of the appended figures in which:

- Figure 1 is a schematic representation of a possible device for implementing the method of the invention;

FIG. 2 represents a concrete example of a device that can be used to effect the crystallization of the invention.

In these figures is represented the growth of a crystal 1 from a compound (C) (which may for example, but not necessarily be DKDP) within a crystallization chamber 10 provided with an inlet 11 and an outlet 12. The temperature T ₀ within this chamber 10 is kept constant by appropriate control means, such as the thermostatic bath 15 shown in FIG. 2.

The crystallization chamber is fed continuously with a solution (S) resulting from a saturation chamber 20 provided with an inlet 21 and an outlet 22. This chamber is also maintained at a constant temperature (T _s ) by suitable control means, such as the thermostatic bath 25 shown in FIG. 2. In the case of the growth of DKDP, the temperature T ₃ is less than T ₀ , for example at least 10 ^° C. In the case of more generally, T ₅ differs from T ₀ by several degrees, or even several tens of degrees. The saturation chamber contains an excess of compound (C) in the state of a solid feeder body 28, which makes it possible to saturate the solution at the level of said saturation chamber (in the chamber the concentration of the compound (C) in the solution (S) is equal to the saturation concentration of the compound (C) at the temperature T ₈ ).

The chambers 10 and 20 are generally provided with stirring and homogenizing means, in particular of the type represented in FIG. 2.

The flow direction of the solution (S) is indicated by arrows in the figures: the solution leaves the outlet 22 of the saturation chamber 20 towards the inlet 11 of the crystallization chamber 10, then leaves this chamber 10 by the exit 12 and goes back to the input 21 of the saturation chamber. This circulation is provided by suitable means represented by 31 and 32 in the figures, which may in particular be peristaltic pumps as shown in FIG. 2. These means are generally used to ensure a constant flow rate of the solution (S). throughout the crystallogenesis.

The saturated solution that leaves the saturation chamber 20 at the temperature T ₅ is sent into the growth chamber 10 maintained at a constant temperature distinct from T _c , chosen to place the solution in a supersaturation condition, thereby allowing the crystal to grow. 1 from the solution (S).

The solution (S) depleted in compound (C) by consumption in the crystalline growth then leaves the crystallization chamber to re-enrich in compound 5c) in the saturation chamber 20.

Advantageously, the circulation circuit of the solution (S) comprises, between the outlet 12 of the crystallization chamber and the inlet 21 of the saturation chamber, a chamber 40 provided with sonification means, where it is furthermore preferable that the solution is heat-treated by heating or cooling, in the direction inducing a saturation of the solution, these means making it possible to inhibit the parasitic nucleation phenomena in the solution by limiting the formation of aggregates and / or by disaggregating the aggregates existing. For safety, the circulation circuit of the solution (S) may further comprise means for filtering the solution. As can be seen from the figures, the process of the invention is very easy to implement and only requires trivial means for precisely regulating temperature and flow rate, which are much easier to use than the control means. required in currently known crystalline growth processes employing a controlled decrease in temperature.

Claims

A process for preparing a crystal based on a compound (C), by crystalline growth in solution, from a solution (S) supersaturated with said compound (C), wherein: said crystal growth is carried out in a crystallization chamber (10) maintained at a constant crystallization temperature T ₀ ;

said crystallization chamber (10) is in fluid communication with a saturation chamber (20), which comprises an excess of compound (C) in the state of a solid feeder body (28), and is brought to a temperature constant saturation T _s , different from T _c , this temperature T ₃ being chosen so that the solubility of the compound (C) in the solution (S) at the temperature T ₃ is greater than the solubility of the compound (C) in the solution (S) at temperature T _c ; a continuous circulation of the solution (S) is established between the crystallization and saturation chambers, whereby a constant supersaturation rate is maintained within the crystallization chamber, and

the solution (S) in circulation between the crystallization and saturation chambers is subjected to an elimination treatment of the aggregates liable to form therein and to the inhibition of the formation of such aggregates, in order to avoid or inhibit the phenomena of nucleation of parasitic crystallites.

The process according to claim 1, wherein the aggregate removal and inhibition of formation of such aggregates in the circulating SJ solution comprises subjecting said solution (S) to ultrasound between the crystallization chamber. and the saturation chamber.

The process according to claim 1 or 2, wherein the compound (C) has a solubility in solution (S) which increases with temperature, and wherein the temperature Ts kept constant in the saturation chamber (20) is greater than at the temperature Tc kept constant in the crystallization chamber (10).

The process according to claim 3, wherein the aggregate removal and inhibition of formation of such aggregates in the circulating solution (S) between the crystallization and saturation chambers comprises the submission of the solution (S) with ultrasound, associated with a heating of the solution (S) between the crystallization chamber and the saturation chamber

The process according to claim 1 or 2, wherein the compound (C) is a compound having a solubility in solution (S) which decreases with temperature, and wherein the temperature Ts kept constant in the saturation chamber (20). ) is lower than the temperature Tc kept constant in the crystallization chamber (10).

The method of claim 5, wherein the aggregate removal and inhibition of formation of such aggregates in the circulating SJ solution comprises subjecting the solution (S) to ultrasound, associated with a cooling the solution (S) between the crystallization chamber and the saturation chamber.

7. A process according to any one of claims 1 to 6, wherein the continuous circulation of the solution (SJ) between the crystallization and saturation chambers takes place at a substantially constant rate.

8. Process according to any one of Claims 1 to 7, in which the level of supersaturation kept constant in the crystallization chamber greater than

10%.

The process according to any one of claims 1 to 8, wherein the compound (C) is KH ₂ PCU potassium diphosphate or potassium diphosphate in at least partially deuterated form.

10. The method of claim 6, conducted at low temperature, wherein the temperature T _c at which the crystallization chamber is maintained is less than 40 ° C, preferably less than 30 ⁰ C, for example between 15 and 25 ° C.

11. The method of claim 9 or 10, wherein the temperature difference (T ₃ - T ₀ ) between the saturation chamber and the crystallization chamber is between 2 and 30 ⁰ C.

A substantially defect-free single crystal obtainable by the process of any one of claims 1 to 8, wherein the compound (C) does not have a congruent melting point or has a phase transition between the melted state and solid state.

13. DKDP mass monocrystal substantially free from defects obtainable according to the method of claims 9 to 11, substantially free of defects and having a homogeneous deuterium / hydrogen ratio within the material.

Apparatus for carrying out the method of one of claims 1 to 9, comprising:

a crystallization chamber adapted to the growth of a crystal in solution, and provided with an inlet 15) for maintaining said crystallization chamber at a substantially constant crystallization temperature T _c ;

a saturation chamber (20), suitable for bringing a solution into contact with an excess of solid solute (28), provided with an inlet (21) in fluid communication with the outlet (12) of the chamber of crystallization and an outlet (22) in fluid communication with the inlet (11) of the crystallization chamber, and temperature control means (25) for maintaining said saturation chamber (20) at a temperature of crystallization T ₃ substantially constant and distinct from T _c ; and

means for continuously circulating a solution between the two chambers (31, 32), from the outlet (22) of the saturation chamber to the inlet (1 1) of the crystallization chamber, and from the outlet (12) of the crystallization chamber to the inlet (21) of the saturation chamber, these circulation means being further provided with means (40) for eliminating the aggregates likely to form in the circulating solution and inhibit the formation of such aggregates.

15. Device according to claim 14, wherein the means for the continuous flow of the solution between the two chambers comprise sonication means (40).

Apparatus according to claim 15, wherein the sonication means (40) are associated heat treatment means for heating or cooling the solution in the direction inducing under-saturation of the solution.