FR2559165A1 - Process for growing crystals of a compound with reverse solubility and device for its use - Google Patents

Abstract

Preparation of crystals with reverse solubility. The process for growing crystals of a compound with reverse solubility from nuclei in a solution enriched in the compound consists in causing a diffusion from a cold zone of the solution containing the parent substance towards a hot zone where it crystallises, by applying to the solution a horizontal temperature gradient of approximately between 5 and 20 DEG C. The device comprises a substantially cylindrical hydrothermal enclosure with a horizontal axis, containing a solution of the compound comprising a cold part containing the parent substance and a hot part containing the crystallisation nuclei, which are connected by a strangulation neck and means for independently heating the hot part and the cold part. Application to the preparation of crystals usable in frequency filters.

Description

The present invention relates to the preparation of berlinite crystals or analogous compounds, and more particularly to a method of growing crystals of the quartz type, as well as a device for carrying out such a method.

Frequency filtering devices used in various fields such as telecommunications, generally include quartz crystal resonators or, much more rarely, lithium tantalate. Berlinite is intermediate between these two materials, both in terms of bandwidth and electromagnetic coupling coefficient; furthermore its frequency drift as a function of temperature is an order of magnitude smaller than that of quartz.

The crystals used in volume or surface wave resonators must meet rigorous conditions 3 of purity, of volume (several cm), and of optical and mechanical qualities. Quartz a generally gives satisfactory results, but it has also appeared advantageous to use, because of its properties, berlinite crystals (aluminum phosphate AlPO4) obtained by hydrothermal growth, given the retrograde solubility of berlinite in orthophosphoric acid. Unlike quartz, the growth of which can only be obtained from natural germs, the supply of which is limited, berlinite crystals can be prepared from synthetic germs, as berlinite does not exist in the state natural in the form of crystals of sufficient size.

There are two known methods based on the principle of retrograde solubility of berlinite crystals. The so-called "slow temperature rise" method consists in slowly heating a solution of berlinite in phosphoric acid.

However, it is not applicable industrially because it does not readily provide crystals of sufficient dimensions, but it allows initially obtaining small germs of good quality, usable for subsequent growth.

According to the second known method, known as the "vertical temperature gradient" 11, a mother body (AlPO4) is used placed in a pierced basket fixed in the upper part of an autoclave containing the solution of berlinite in phosphoric acid. The rich solution in berlinite (cold part) is entrained by convection currents towards the hot part where it recrystallizes, the temperature gradient being from 2 to 60C approximately. This method implies that the mother body is in the form of crystals of sufficient size to avoid their fall which can induce parasitic crystallization on germs. This defect cannot however be completely avoided because of the progressive dissolution of the mother body during the operation.

This known method also has the disadvantage of a low temperature gradient, less than 60C, and requires an additional step of synthesis of small crystals which can be used as mother body, which makes industrial production delicate and storytelling.

The present invention relates to a method of crystal growth applicable not only to berlinite, but also to compounds with retrograde solubility, such as those of the quartz a type, of formula XY04 where X is aluminum or gallium, and Y is phosphorus or arsenic.

The invention also relates to a method for growing crystals of compounds such as those of formula XYO4 above, by diffusion currents, applicable industrially under good conditions of profitability.

The invention also relates to a device with horizontal hydrothermal enclosure, for implementing the above method.

The crystal growth method according to the invention is applicable to compounds with retrograde solubility and in particular to those of the quartz a type of formula XY04 where X is aluminum or gallium and Y is phosphorus or arsenic, from germs in a solution enriched with compound XY04. According to the method of the invention, the solution is caused to diffuse from the cold zone containing the mother body of formula XY04, to the hot zone where it recrystallizes.

According to the invention, it is advantageous to apply to the solution a horizontal temperature gradient of approximately 5 to 200C, between the cold zone and the hot zone, which are connected by a bottleneck intended to control the diffusion of the solution and to retain the mother body being in the cold zone. The mother body can be in the form of small single crystals or microcrystalline powder of the compound of formula XY04.

The crystallization temperature is chosen for example from the solubility curves as a function of the concentration of acid and of compound XY04. In the case of berlinite in phosphoric acid, this temperature is 1700C for an H3PO4 / Al P04 solution with concentrations equal to approximately 6.5M and 1M, respectively. In general, solutions of sufficiently high concentrations are used to provide satisfactory solubility, knowing that the solubility decreases when the concentration decreases.

According to the operating conditions, and in particular the volume of the hot part of the hydrothermal enclosure containing the solution and the seeds of crystallization, it is preferable to heat only the upper part of the enclosure at the level of the hot part, while the heating can be applied over the entire periphery of the enclosure at the level of the cold part. It is thus possible to avoid parasitic recrystallization on the lower wall of the enclosure to the detriment of the slides or germs, by creating a vertical temperature gradient from 1 to 50C, and preferably from 3 to 40C.

The solvent can be chosen from all those in which the compounds exhibit retrograde solubility, and for the compounds of general formula XY04 above, the corresponding acid of formula H 3Y04 is preferably used where Y retains the same meaning.

When the crystals tend to redissolve in the solution during the warming-up operation, it is possible at first to keep the seeds outside of the solution until the equilibrium temperature is reached, then they are immersed in it for a determined period of time to cause the crystals to grow.

The process of the invention applies to compounds of the quartz a type of formula XYO4 above, such as berlinite (aluminum phosphate), gallium arsenate, gallium phosphate or aluminum arsenate . It preferably applies to the growth of berlinite crystals.

The process of the invention has the advantage of being able to be easily implemented industrially, in particular because of the large temperature gradient which can be easily adapted to the growth speed of the crystal plates, which on the contrary turns out to be very difficult with the known method of the vertical temperature gradient due to the limitation of this gradient to 5-60C. Another important advantage is the possibility of using a microcrystalline powder as the mother body.

The process of the invention can be carried out in an appropriate device making it possible to establish a horizontal temperature gradient.

 The device according to the present invention makes it possible to ensure the growth of the crystals under excellent conditions. It essentially consists of a substantially cylindrical hydrothermal enclosure, with a horizontal axis, containing a solution of the compound of formula XY04, comprising a cold part containing the mother body, and a hot part containing the seeds of crystallization, connected by a neck of 'throttle, as well as means for independently heating the hot part and the cold part.

According to a preferred embodiment, the volume of the cold part occupying one end of the enclosure represents less than 30 Ó of the total volume, and the bottleneck connecting the hot and cold parts is formed by an inert screen opposite. compounds and solution, partially sealing the cylinder section.

The hot and cold parts of the cylinder are preferably separated by a thermal insulating screen and are heated separately by conventional means, for example by an electrical device. It may be advantageous to use a metal cylindrical autoclave placed in a sliding enclosure made up of two parts separated from each other.

A nonlimiting exemplary embodiment is described below with reference to the appended drawing which represents a schematic axial section of a cylindrical hydrothermal enclosure as defined in the invention.

The cylindrical heating chambers for the cold part t1) and for the hot part (2) are juxtaposed along the same axis, their free ends being closed by a plug (3), and are separated by an insulating screen (4). In the enclosure thus formed, a cylindrical autoclave body (5) of steel is placed, the opening of which can be closed by a shutter (6) made of steel and a plug (7) made of Teflon.

The autoclave (5) comprises a liner (8) made of noble metal (gold, platinum or tantalum) and is crossed by two pyrometric rods (9) and (10), lined in noble metal for controlling the temperature in the upper zone and in the lower zone respectively; moreover, the pyrometric rod (9) serves as a support for the germs (11). In the cold part, a Teflon compartment (12) is placed separated from the rest of the volume of the enclosure by a Teflon screen (13) having a hole of appropriate size. The compartment (12) contains the mother body (14) in the form of microcrystalline powder.

Various modifications can be made to the device without departing from the scope of the invention. In particular, for small quantities of product, the cylindrical hydrothermal enclosure can be constituted in a simplified form by a vacuum-sealed ampoule having a constriction towards a quarter or a third of its length. The constriction reduces the cross-section of the bulb by approximately a third.

The following examples, given without limitation, illustrate in more detail the preparation of crystals of compounds of formula XY04 according to the process of the invention.

EXAMPLE 1
Berlinite crystals, AlPO4
A vacuum-sealed ampoule with an outside diameter equal to 28 mm is used, having a throttle with an inside diameter close to 10 mm to one third of the length. In the shortest part, cold part, 139 microcrystalline berlinite are deposited, in the other, hot part, 5 Z blades pierced and fixed by gold wires to a glass portal frame. The enclosure is filled to 80% by an H3PO4 / AlPO4 solution (6.5 and 1.1 M respectively).

The two parts are brought as quickly as possible to the desired equilibrium temperature (hot part = 1700C, cold part = 1650C) with a vertical temperature gradient in the hot part, AT2 = 4 C. The horizontal gradient, 170 / 1650C is modulated over time to reach the value 170 / 1550C after 15 days.

After opening after 3 weeks, growth of 0.4 mm per day and per side of the Z slides used is recorded.

EXAMPLE 2
AlAs04 crystals
An enclosure identical to that of Example 1 is used.

In the cold part, 99 monocrystalline AlAs04 are deposited and in the hot part, 4 Z blades. In this case, the filling, with an H3As04 / AlAs04 solution (6.5 M and 1.2 M respectively) is 60e0.

During the rise in temperature, the enclosure is maintained so as to keep the slides outside of the solution to avoid their redissolution. Once equilibrium has been reached, the temperature gradient AT1 being 230 / 2250C, the enclosure is rotated by 1800C to immerse the slides in the solution. The vertical gradient of the hot part, aT2 is maintained at 30C. The horizontal gradient is modulated to reach 2300-2150C in 10 days.

After opening, after 17 days, a growth of the Z blades of 0.39 mm per day and per side is observed.

Claims (10)

RE \ IENDICATIONS 1. Method for growing crystals of compound with retrograde solubility from germs in a solution enriched with the compound, characterized in that it causes a diffusion of a cold zone of the solution containing the mother body, towards a hot zone where it crystallizes. 2. Method for growing crystals of compound with retrograde solubility of the quartz a type, of formula XY04 where X is aluminum -or gallium and Y is phosphorus or arsenic, from germs in a solution enriched with the compound XY04, characterized in that it causes a diffusion of a cold zone of the solution containing the mother body of formula XY04, towards a hot zone where it crystallizes. 3. Method according to any one of claims 1 and 2, characterized in that the solution is subjected to a horizontal temperature gradient between 5 and 200C approximately. 4. Method according to any one of claims 1 and 2, characterized in that the mother body being in the cold zone of the solution is in the form of crystals or microcrystalline powder. 5. Method according to claim 4, characterized in that the mother body is a microcrystalline berlinite powder. 6. Method according to any one of claims 1 to 3, characterized in that a vertical temperature gradient between 1 and 50C is applied to the hot zone of the solution. 7. Method according to claim 2, characterized in that the solution of the compound of formula XY04 is a solution of the corresponding acid H3Y04 where Y has the same meaning. 8. Method according to any one of claims 1 and 2, characterized in that the seeds are kept out of the solution until a determined temperature, then are immersed therein to cause the growth of the crystals. 9. Device for implementing the crystal growth method according to claim 1, characterized in that it comprises a substantially cylindrical hydrothermal enclosure of horizontal axis, containing a solution of the compound of formula XY04, comprising a cold part containing the mother body and a hot part containing the seeds of crystallization connected by a bottleneck, as well as means for independently heating the hot part and the cold part. 10. Device according to claim 9, characterized in that the volume of the cold part represents less than approximately 30 of the total volume.