GB1572915A

GB1572915A - Crystallisation device

Info

Publication number: GB1572915A
Application number: GB424377A
Authority: GB
Original assignee: Produits Chimiques Ugine Kuhlmann; Ugine Kuhlmann SA
Current assignee: Produits Chimiques Ugine Kuhlmann; Ugine Kuhlmann SA
Priority date: 1977-01-11
Filing date: 1977-02-02
Publication date: 1980-08-06
Also published as: FR2376697B2; JPS5919914B2; CH617725A5; FR2376697A2; DE2704913A1; DE2704913C2; IT1117104B; JPS5387984A

Description

(54) CRYSTALLISATION DEVICE (71) We, PRODUITS CHIMIQUES UGINE KUHLMANN, a French Body Corporate, of 25 Boulevard de l'Amiral Bruix, Paris 16, France, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a device for the continuous production of preformed plate-shaped crystals by the hanging drop method.

Our British Patent No. 1546843 (hereinafter referred to as the Main Patent) relates to a process for the continuous preparation of a monocrystal of predetermined shape, comprising: (a) placing the material serving to produce the monocrystal in a crucible whose bottom part is provided with a die having one or more capillary apertures each of the capillaries having a height greater than or equal to the height of retention of the molten material in the said capillary at the operative temperature and pressure; (b) bringing the said material to a temperature higher than its melting point with the resultant formation of a hanging drop at the lower end of the capillary; (c) bringing a suitably oriented, preformed crystal nucleus into contact with the hanging drop;; (d) moving the nucleus downwards while feeding the aforesaid material into the crucible at such a rate that the amount of material being introduced per unit of time will at any moment be substantially equal to the amount of material being drawn out in the form of a monocrystal; and (e) removing at selected intervals of time the monocrystal formed.

The above-mentioned patent also relates to an apparatus for the continuous production of monocrystals of predetermined shape, comprising: (a) a crucible the bottom part of which is provided with a die having one or more capillary apertures the axis of which is parallel to that of the crucible, each of the capillaries having a height equal to or greater than the height of retention in the said capillary of the molten material serving to produce the monocrystal, at the temperature and pressure of formation of the monocrystal, (b) a heating device surrounding the said crucible, (c) a system, placed above the said crucible, for feeding thereto raw material for the monocrystal, (d) a vessel provided with a cooling device, surrounding said crucible and the said feed system and having a bottom aperture permitting the passage of the monocrystal to be formed, apertures permitting the circulation of a gas constituting the working atmosphere, and apertures permitting the passage of the heating system, (e) a nucleus carrier system for supporting a preformed monocrystal nucleus, (f) a system permitting the displacement of the said nucleus carrier in the upward and downward directions and its rotation about its own axis, and (g) a regulation system for relating the speed at which the monocrystal formed is drawn to the speed at which the material serving to produce the monocrystal is fed to the crucible.

With a capillary die of rectangular section, plate-shaped monocrystals are obtained.

In the apparatus of the Main Patent the bottom edges defining the lower aperture of the capillary passage are at the same horizontal level, as illustrated in the accompanying Figure 1, which shows a crucible 1, having a capillary passage 2 with two bottom edges 2' and 2". Molten material 3 serving to produce the monocrystal, forms a hanging drop 3' at the lower end of the capillary before a preformed crystal nucleus is brought into contact with the lower aperture of the capillary passage.

In the device of the main patent, the liquid zone, that is to say the liquid space between the bottom of the capillary and the top of the crystal formed from the said nucleus, is of the order of 0.1 to 0.5 mm.

Particularly for the purpose of facilitating the control and regulation of the flow of material fed, and also to facilitate control of the quality of the liquid, it may be desirable to have available a device in which the said liquid zone is larger.

The apparatus of the present invention is the same as an apparatus of the Main Patent having a capillary passage of rectangular cross-section, with the exception that in the present apparatus, the bottom of the crucible is shaped so that one of the edges defining the lower aperture of the capillary passage is not at the same horizontal level as the corresponding edge on the opposite side of the aperture.

The apparatus of the invention therefore comprises: (a) a crucible, and in the bottom of the crucible a capillary passage of rectangular cross-section and of sufficient length to allow, when the apparatus is in use, a drop of molten material from which the monocrystal is to be produced to remain suspended from the lower aperture of the capillary passage, one of the edges defining the lower aperture of the capillary passage being at a different horizontal level from that of the corresponding edge on the opposite side of the aperture, (b) a heating device surrounding the said crucible; (c) a system, placed above the said crucible, for feeding raw material for producing the mono-crystal to the crucible; (d) a vessel, provided with a cooling device, surrounding the said crucible and the said feed system and having a bottom aperture for withdrawal of the monocrystal produced;; (e) a nucleus carrier system for supporting a preformed monocrystal nucleus; (f) means for moving the nucleus carrier upwards or downwards and for rotating the carrier about a vertical axis; and (g) means for regulating the speed at which the carrier is moved.

According to a further aspect of the present invention, there is provided a process for the continuous preparation of monocrystalline plates, the process comprising: (a) placing the material serving to produce the monocrystal in the crucible of an apparatus in accordance with the present invention (b) bringing the said material to a temperature higher than its melting point with the resultant formation of a hanging drop at the lower end of the capillary:: (c) bringing a suitably oriented, preformed crystal nucleus into contact with the hanging drop; (d) moving the nucleus downwards while feeding the aforesaid material into the crucible at such a rate that the amount of material being introduced per unit of time will at any moment be substantially equal to the amount of material being withdrawn out in the form of a monocrystal; (e) removing at selected intervals of time the monocrystal formed.

The bottom edges defining the lower aperture of the capillary passage are for example offset by a height h of from 1 to 3 mm.

Accompanying Figure 2 shows a crucible 1 having a capillary passage 2 whose bottom edges 2' and 2" are offset by a height h.

The bottom edges of the capillary passage may have different thicknesses. In particular, the longer edge may have the greater thickness, as illustrated in accompanying Figure 3, in order to prevent excessively rapid cooling of the liquid of the drop.

As in the Main Patent, the bottom ends of the capillary may be bevelled as illustrated in accompanying Figures 4 and 5.

The crucible of the apparatus is made of a material which at the working temperature is inert with respect to the material used to produce the monocrystal. The crucible may for example be of platinum for monocrystals of NaCI, of sintered silicon carbide for silicon monocrystals, of molybdenum for monocrystals of Awl203, or likewise of iridium.

As in the Main Patent, the crucible is of very small dimensions, of the order of 20 to SOcc.

Any suitable heating system may be used, for example a resistance heater or else a high frequency induction heater equipped with windings adapted to the shape and material of the crucible and working at from 20 to 500 kHz in order to ensure coupling with the materials of the crucible and developing up to 50 kW.

The drop formed at the end of the capillary passage is a dissymmetrical hanging drop, as shown diagrammatically in accompanying Figure 6. The process described in the Main Patent remains essentially the same when applied to the device of the present invention.

It should be noted that although the hanging drop is dissymmetrical, the capillary phenomenon continues to serve the purpose of regulation and there is no trickling of liquid along the longer lip.

Among the numerous advantages of the present apparatus mention may be made of the following: I. The better visual examination of the quality of the liquid, which is made possible by the offsetting (h) between the two bottom edges 2' and 2" of the capillary aperture 2, making it possible in particular to detect any presence of impurities which would result in inclusions-for example fragments of the crucible originating from a slight attack-and also the presence of microbubbles of gas or of volatile materials. In other words the apparatus of the present application provides a means of inspecting the quality of the liquid and determining the moment when crystallisation may commence.

II. The possibility of measuring all along the liquid the temperature on a horizontal band above the crystallisation front. This easier examination of the temperature gradient horizontally above the crystallisation front makes it possible to control the heating of the plate so as to obtain greater homogeneity, and leads to a linear crystallisation front (that is to say a plane liquid-solid interface).

III. Easier inspection by optical means makes it possible for the supply of material to be controlled if necessary.

IV. Better control of the flow of the liquid makes it possible to work at higher speeds of flow than in the main patent, for example 50 higher.

V. Exact control of the temperature of the liquid made possible by the offsetting of the bottom edges of the capillary passage, thus making it possible to work exactly at the melting point of the material without any risk of overheating.

The device of the present application makes it possible to produce monocrystals of NaCI, Si, and Awl703 (sapphire, doped or undoped). However, it also applies to all monocrystals which have the following characteristics: a clearly defined melting point; will wet a die material without attacking it chemically; will withstand the atmospheres and pressures necessary for reaching the desired crystalline phase; will pass from the melting point to ambient temperature without passing through crystalline phases entailing the risk of fractures.

The device of the present invention permits the production of monocrystal plates which have numerous applications.

For example, ruby monocrystals can be used for the purposes of jewellery, watchmaking, and electronics or in order to obtain a laser effect. Sapphire monocrystals, that is to say of pure alpha- alumina, are used as insulating plates serving as supports for electronic circuits and as windows transparent to radiations from ultraviolet to near infrared. Silicon monocrystals are used in the electronics industry on account of their semi-conductor properties, particularly for the production of transistors and of photovoltaic cells intended for picking up solar energy. Monocrystals of sodium chloride are used in infrared optics, monocrystals of yttrium garnet are used as laser crystals, while those of quartz are used as piezoelectric crystals.

The following Example illustrates the invention.

EXAMPLE Production of a monocrystalline plate of Si The crucible is of sintered silicon carbide, with a total volume of 20 cc and with a capillary passage of rectangular section, size lx 15 mm, one of whose lower edges extends 2 mm below the other. The crucible together with the supply system is contained in a quartz vessel flushed with a current of argon without oxygen. The quartz vessel is provided with a cooling device. The crucible is fed with doped silicon powder of great purity, with a granulometry of 0.1 to I mm, and is brought to a temperature of 15000C+100C (melting point of Si: 14700C) with the aid of a high frequency generator working at 300 kHz and developing a permanent powder of 10 kW. The crucible is fed at a mean rate of 28.5 g of Si powder per hour.The drops formed at the bottom of the die are allowed to flow until inspection of the liquid reveals that it contains no apparent impurities. At this moment a plate of oriented silicon serving as nucleus, size lox 15 mm, is brought into contact with the drop which has formed at the bottom end of the capillary passage, and the drop is bonded to this nucleus. The nucleus then starts to be pulled downwards at the rate of 75 cm per hour, and at the same time the crucible is still fed at a mean rate of 28.5 g of silicon powder per hour. After pulling for 20 minutes a silicon plate of a section of about lx15 mm and a length of about 225 mm, with a relatively plane surface, is obtained.

Under X-ray diffraction examination this plate appears to be monocrystalline with a low grain content and with crystallographic orientation identical to that of the nucleus.

Our co-pending application No. 4242/77 (Serial No. 1572914) relates to a process and an apparatus for producing multi-doped preformed monocrystals.

WHAT WE CLAIM IS: 1. An apparatus for producing monocrystalline plates, comprising: (a) a crucible, and in the bottom of the crucible a capillary passage of rectangular cross-section and of sufficient length to allow, when the apparatus is in use, a drop of molten material from which the monocrystal is to be produced to remain suspended from the lower aperture of the capillary passage, one of the edges defining

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. by the offsetting (h) between the two bottom edges 2' and 2" of the capillary aperture 2, making it possible in particular to detect any presence of impurities which would result in inclusions-for example fragments of the crucible originating from a slight attack-and also the presence of microbubbles of gas or of volatile materials. In other words the apparatus of the present application provides a means of inspecting the quality of the liquid and determining the moment when crystallisation may commence. II. The possibility of measuring all along the liquid the temperature on a horizontal band above the crystallisation front. This easier examination of the temperature gradient horizontally above the crystallisation front makes it possible to control the heating of the plate so as to obtain greater homogeneity, and leads to a linear crystallisation front (that is to say a plane liquid-solid interface). III. Easier inspection by optical means makes it possible for the supply of material to be controlled if necessary. IV. Better control of the flow of the liquid makes it possible to work at higher speeds of flow than in the main patent, for example 50 higher. V. Exact control of the temperature of the liquid made possible by the offsetting of the bottom edges of the capillary passage, thus making it possible to work exactly at the melting point of the material without any risk of overheating. The device of the present application makes it possible to produce monocrystals of NaCI, Si, and Awl703 (sapphire, doped or undoped). However, it also applies to all monocrystals which have the following characteristics: a clearly defined melting point; will wet a die material without attacking it chemically; will withstand the atmospheres and pressures necessary for reaching the desired crystalline phase; will pass from the melting point to ambient temperature without passing through crystalline phases entailing the risk of fractures. The device of the present invention permits the production of monocrystal plates which have numerous applications. For example, ruby monocrystals can be used for the purposes of jewellery, watchmaking, and electronics or in order to obtain a laser effect. Sapphire monocrystals, that is to say of pure alpha- alumina, are used as insulating plates serving as supports for electronic circuits and as windows transparent to radiations from ultraviolet to near infrared. Silicon monocrystals are used in the electronics industry on account of their semi-conductor properties, particularly for the production of transistors and of photovoltaic cells intended for picking up solar energy. Monocrystals of sodium chloride are used in infrared optics, monocrystals of yttrium garnet are used as laser crystals, while those of quartz are used as piezoelectric crystals. The following Example illustrates the invention. EXAMPLE Production of a monocrystalline plate of Si The crucible is of sintered silicon carbide, with a total volume of 20 cc and with a capillary passage of rectangular section, size lx 15 mm, one of whose lower edges extends 2 mm below the other. The crucible together with the supply system is contained in a quartz vessel flushed with a current of argon without oxygen. The quartz vessel is provided with a cooling device. The crucible is fed with doped silicon powder of great purity, with a granulometry of 0.1 to I mm, and is brought to a temperature of 15000C+100C (melting point of Si: 14700C) with the aid of a high frequency generator working at 300 kHz and developing a permanent powder of 10 kW. The crucible is fed at a mean rate of 28.5 g of Si powder per hour.The drops formed at the bottom of the die are allowed to flow until inspection of the liquid reveals that it contains no apparent impurities. At this moment a plate of oriented silicon serving as nucleus, size lox 15 mm, is brought into contact with the drop which has formed at the bottom end of the capillary passage, and the drop is bonded to this nucleus. The nucleus then starts to be pulled downwards at the rate of 75 cm per hour, and at the same time the crucible is still fed at a mean rate of 28.5 g of silicon powder per hour. After pulling for 20 minutes a silicon plate of a section of about lx15 mm and a length of about 225 mm, with a relatively plane surface, is obtained. Under X-ray diffraction examination this plate appears to be monocrystalline with a low grain content and with crystallographic orientation identical to that of the nucleus. Our co-pending application No. 4242/77 (Serial No. 1572914) relates to a process and an apparatus for producing multi-doped preformed monocrystals. WHAT WE CLAIM IS:

1. An apparatus for producing monocrystalline plates, comprising: (a) a crucible, and in the bottom of the crucible a capillary passage of rectangular cross-section and of sufficient length to allow, when the apparatus is in use, a drop of molten material from which the monocrystal is to be produced to remain suspended from the lower aperture of the capillary passage, one of the edges defining

the lower aperture of the capillary passage being at a different horizontal level from that of the corresponding edge on the opposite side of the aperture, (b) a heating device surrounding the said crucible; (c) a system, placed above the said crucible, for feeding raw material for producing the monocrystal to the crucible; (d) a vessel, provided with a cooling device, surrounding the said crucible and the said feed system and having a bottom aperture for withdrawal of the monocrystal produced; (e) a nucleus carrier system for supporting a preformed monocrystal nucleus; (f) means for moving the nucleus carrier upwards or downwards and for rotating the carrier about a vertical axis; and (g) means for regulating the speed at which the carrier is moved.

2. An apparatus according to Claim 1 in which the said edge is from 1 to 3 mm higher than the said opposite edge.

3. An apparatus according to Claim 1 or Claim 2, in which the said edge and opposite edge are of different thicknesses.

4. An apparatus according to Claim 3, in which the longer edge has the greater thickness.

5. An apparatus according to Claim 1 substantially as hereinbefore described with reference to, and as illustrated in Figures 2 to 6 of the accompanying drawings.

6. A process for the continuous preparation of monocrystalline plates, the process comprising: (a) placing the material serving to produce the monocrystal in the crucible of an apparatus as claimed in any one of the preceding claims; (b) bringing the said material to a temperature higher than its melting point with the resulting formation of a hanging drop at the lower end of the capillary; (c) bringing a suitably oriented, preformed crystal nucleus into contact with the hanging drop; (d) moving the nucleus downwards while feeding the aforesaid material into the crucible at such a rate that the amount of material being introduced per unit of time will at any moment be substantially equal to the amount of material being withdrawn out in the form of a monocrystal; (e) removing at selected intervals of time the monocrystal formed.

7. A process according to Claim 6, substantially as described in the foregoing Example.

8. A monocrystal produced using an apparatus as claimed in any one of Claims 1 to 5.

9. A monocrystal produced by a process as claimed in Claim 6 or 7.