Sapphire rod manufacturing method

Abstract

The invention relates to a method for directly manufacturing rod-like single crystal sapphire, comprising the following steps:-providing a crucible (100) comprising a first fixed part (1) having an internal opening (13) and a second movable part (2) forming the back of the crucible and consisting of a sapphire sheet forming a seed for sapphire growth; -positioning the first part (1) and a second part (2) relative to each other, the second part being translationally movable in the internal opening (13) of the first part; -placing the crucible (100) in a housing (4) under a vacuum or controlled atmosphere and heating the housing (4) to bring the crucible to an operating temperature; feeding a raw material (M) into the crucible to form a molten raw material (F) in the crucible; translating the back (2) of the crucible at a controlled speed so as to gradually solidify the molten raw material and gradually form a sapphire rod (C); -interrupting the feed of the starting material (M) and completely crystallizing the remaining molten material (F) in the crucible; cooling the crucible to ambient temperature; and recovering the sapphire rod bonded to the resulting seed crystal.

Description

Technical Field

[0001] The invention relates to a method for directly manufacturing a rod-shaped single-crystal sapphire. The invention also relates to external and functional components cut from such a single crystal sapphire for use in the watchmaking and jewellery industries. Background Art

[0002] There are several ways to obtain artificial single crystals:

[0003] - from a supersaturated solution of the compound;

[0004] - from molten compounds;

[0005] -Via chemical vapor delivery.

[0006] The Verneuil process is a well-known example. In order to synthesize corundum, which has the chemical formula Al2O3 and constitutes ruby ​​and sapphire, the temperature must be raised to a very high level (melting temperature is 2050°C), which is achieved by an oxyhydrogen torch H2+1 / 2O2→H2O, with a flame temperature of about 2700°C. The aluminum oxide that can be doped is inserted in the form of a fine powder through a vibrator, which drips a small amount of aluminum oxide directly into the flame of the torch. The molten aluminum oxide thus formed drips on top of the seed crystal and crystallizes according to the crystal arrangement of the seed crystal. The growing single crystal is gradually lowered so that crystallization is carried out at a constant temperature. At the end of the synthesis, a bottle-shaped single crystal is obtained.

[0007] However, sapphire single crystals obtained by the Verneuil method exhibit high dislocation density and uncontrollable local dislocations. Other defects are also present in Verneuil single crystals, such as bubbles, raindrops and other inclusions, which may be visible to the naked eye, such as in a finished watch crystal.

[0008] Furthermore, effective control over the shape of the Verneuil crystal is relatively poor, with diameter variations being + / - 2 mm in the best case.

[0009] The Czochralski method is a technique that uses a molten bath in a crucible for crystallization. In theory, it is well suited to producing cylindrical sapphires whose size is limited only by the size of the crucible. Because the pulling speed is precisely controlled, the crystals can be weighed by a sensor so that the average diameter is very close to the desired value, and this weight measurement is used to calculate the deviation from this diameter and adjust the crystallization temperature accordingly.

[0010] However, in some cases, controlling the average diameter does not prevent the actual shape of the sapphire crystal from deviating significantly from the ideal cylinder. Since the growing crystal does not have any contact with the crucible, there are no mechanical constraints to limit its shape. The appearance of facets (planes with lower energy) can be related to the pulling direction. In some cases, such as growth along the A axis, facets perpendicular to C appear, which can lead to very pronounced shape anisotropy.

[0011] It should also be noted that the Czochralski technique requires a very high sapphire bath because all the raw materials used to form the crystal must be contained in the crucible, which makes degassing more difficult. Bubbles and microbubbles may enter the crystal as it is grown above the bath.

[0012] In the EFG (Edge-defined Film-fed Growth) technique, sapphire crystallization takes place on top of a mold, which is immersed in a bath of molten sapphire. The mold imparts a profile to the resulting crystal. To approach the final dimensions, plates, tubes and small rods can be obtained. However, since the molten sapphire must rise to the top of the mold through very thin capillary channels (maximum one dimension of 0.8-1 mm), the thickness dimensions of the profile will also be limited, typically to a maximum of 20 mm. In addition, since the crystallization takes place above the bath, bubbles formed therein and dissolved gases will be able to rise into the crystal. In particular, microbubbles will systematically appear near the surface, typically to a depth of 0.5 mm, which limits the useful part of the sapphire produced and requires machining all surfaces to this depth.

[0013] In the Micro-Pulling-Down technique, the crystallization of sapphire occurs below the molten sapphire bath, which limits the above-mentioned problems involving the incorporation or nucleation of bubbles and microbubbles. However, the crystallization zone is always fed by capillary action, i.e., a fine hole or slit is drilled in the back of the crucible containing the sapphire bath, and the sapphire seed must be brought under this hole or slit in order to form a stable liquid meniscus between the crucible and the seed. As the surface area of ​​the meniscus increases, the conditions for meniscus stability decrease, which unfortunately greatly limits the size that can be achieved in micro-pulling.

[0014] Both horizontal and vertical Bridgman crystallization techniques and their variants (HDC - horizontal directional crystallization or Bagdasarov, HEM - heat exchanger method, VGF - vertical gradient freezing, etc.) are characterized by the growth of the crystals directly in contact with the crucible, which precisely determines the shape of the resulting sapphire, making it as close as possible to the shape of the final product. However, this advantage has a disadvantage in terms of cost, since the crucible usually has to be destroyed to recover the crystals and it must be renewed for each crystallization cycle. In addition, all the raw materials used to form the crystals must be contained in the crucible and completely melted, so the height of the bath can be large, which prevents effective degassing and affects the optical quality of the crystals. Summary of the invention

[0015] The object of the present invention is to overcome the above and other problems by providing a method for directly manufacturing rod-shaped sapphire single crystals.

[0016] This method makes it possible to crystallize sapphire directly in the form of rods with optimized cross-section and length that are easily adjustable and precisely controlled, without having to resort to capillary action to limit one of the three dimensions, and as close as possible to the desired final dimensions, in particular those of watch components, and which can also be easily sawn into thin slices by wire sawing, all of which makes it possible to significantly reduce material losses during mechanical processing.

[0017] It is also an object of the present invention to allow the method tool to be reused multiple times to reduce costs.

[0018] The invention also aims to optimize the optical quality of the crystals produced by controlling the atmosphere in which the method is carried out and limiting the amount of the molten bath in order to promote its degassing.

[0019] Finally, the invention aims to optimize the optical quality of the produced crystals by growing the crystals below a molten bath in order to limit the incorporation of bubbles, microbubbles and residual gases in the bath into the crystals.

[0020] To this end, the present invention relates to a method for manufacturing a rod-shaped single-crystal sapphire, the method comprising the following steps:

[0021] - providing a crucible comprising a first fixed part having an internal opening, and a second movable part forming the back of the crucible and consisting of a sapphire piece forming a seed for sapphire growth;

[0022] - positioning the first component and the second component relative to each other at ambient temperature, the second component being movable in translation in the internal opening of the first component;

[0023] - placing the crucible in a vacuum or controlled atmosphere enclosure and heating the enclosure to bring the crucible to operating temperature;

[0024] - feeding raw material into the crucible, preferably via a tank and feed system operating continuously or discontinuously, to form molten raw material above the movable sapphire part in the crucible;

[0025] - translating the back of the crucible toward the lower less heated area at a controlled speed by a translation mechanism, so that the molten raw material gradually solidifies and gradually forms a sapphire rod having the same cross section as the back of the crucible;

[0026] - interrupt the feed of raw materials and allow the remaining molten material in the crucible to completely crystallize;

[0027] - Cool the crucible to ambient temperature;

[0028] - Recovering the sapphire rod obtained, a portion of which, after being sawn, is able to form a new back and a new seed.

[0029] According to the specific implementation of the method of the present invention:

[0030] - the inner opening of the first part is cylindrical and has a cross-section substantially corresponding to the desired cross-section of the sapphire rod;

[0031] - the sapphire back is cylindrical and has a cross-section corresponding to the desired cross-section of the sapphire rod;

[0032] - the first part of the crucible is made of a high melting point metal, such as molybdenum, tungsten or an alloy of these two metals;

[0033] - The operating temperature of the housing is 2000℃ to 2100℃;

[0034] - the first and second components are dimensioned to achieve a minimum gap at operating temperature;

[0035] - The raw material has a pure or doped Al2O3 chemical composition;

[0036] - the raw material is selected from ground and crushed sapphire, sapphire or alumina beads, or densely compacted granular alumina powder;

[0037] - The controlled atmosphere consists of a neutral gas;

[0038] - The neutral gas is argon;

[0039] - Movement is controlled by a motorized translation system;

[0040] - the raw material feed tank has a capacity at least equal to the weight of the crystallizable sapphire rods;

[0041] - the crystal orientation of the sapphire back or seed crystal can be selected indiscriminately from all crystal orientations of sapphire;

[0042] - Once the sapphire rods are obtained, external or functional components are cut from them for use in the watchmaking and jewellery industries;

[0043] - The sapphire rods are cut by wire sawing to produce external or functional components for the watchmaking and jewellery industries;

[0044] -The same vacuum or controlled atmosphere housing is used for multiple crucibles, heating systems, raw material feed and translation mechanisms.

[0045] The invention also relates to external and functional components for the watchmaking and jewellery industries, in particular bridges, plates, crystals, cases and dials or bracelet links, cut from a single crystal of sapphire obtained according to the method of the invention.

[0046] Thanks to these characteristics, the invention provides a method for manufacturing watch glasses under easier machining conditions and with minimal losses. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Other features and advantages of the invention will become more apparent from the following detailed description of an example embodiment of the method according to the invention, which example is given by way of non-limiting illustration only in conjunction with the accompanying drawings, in which:

[0048] - Figure 1 The second step of the method according to the invention is shown;

[0049] - Figure 2 The third step of the method according to the invention is shown;

[0050] - Figure 3 The relative expansion coefficients of molybdenum, tungsten and sapphire are shown as a function of temperature (according to C. Miyogawa et al., J. Cryst. Growth 372 (2013) pp. 95-99);

[0051] - Figure 4 The fourth and fifth steps of the method according to the invention are shown;

[0052] - Figure 5 Schematic diagram showing a furnace for growing multiple sapphire rods in parallel. DETAILED DESCRIPTION

[0053] The present invention relates to a method for directly manufacturing (or crystallizing) rod-shaped single-crystal sapphire.

[0054] The first step of the method is to provide a crucible 100 for receiving a raw material "M" in which the raw material "M" is melted by providing heat.

[0055] The raw material "M" used to make sapphire has a chemical composition of A12O3 and is pure or doped. The raw material "M" can be selected from ground and crushed sapphire, sapphire or alumina beads, or densely compacted granular alumina powder.

[0056] According to the invention, the crucible 100 comprises a first fixed part 1 having an internal opening 13, and a second movable part 2 forming the back of the crucible and consisting of a sapphire piece forming a seed for sapphire growth.

[0057] The thickness of the crucible back 2 may be 1 cm to 10 cm.

[0058] In an alternative embodiment, an intermediate metal part may also be provided, which forms the circular metal back, to which the seed crystal is fixed.

[0059] As shown, the inner opening 13 of the first part 1 is cylindrical, extends over the height of the first part, and has a cross-section that substantially corresponds to the cross-section of the sapphire rod to be produced. Similarly, the sapphire back 2 is cylindrical and has a cross-section that corresponds to the desired cross-section of the sapphire rod. The shape and dimensions of the cylinder will accurately define the shape and dimensions of the sapphire rod to be crystallized.

[0060] It goes without saying that the inner opening 13 can have a variety of cross-sectional shapes relative to the longitudinal axis of the crucible, and the shape of the cross section depends on the cross section of the sapphire crystal to be produced.

[0061] The inner cross section may thus be circular, oval or polygonal. A polygonal cross section may, for example, take the form of a square, rectangle, pentagon, hexagon or octagon.

[0062] The first part 1 of the crucible 100 is preferably made of a high melting point metal, such as molybdenum, tungsten or an alloy of these two metals.

[0063] The next step of the method is to position the first component 1 and the second component 2 relative to each other at ambient temperature, wherein the second component 2 is movable in translation in the internal opening 13 of the first component.

[0064] The first metal part 1 of the crucible and the sapphire back 2 of the crucible (or the seed crystal) must be positioned relative to each other while the system is still at ambient temperature. To this end, there must be enough space between the two parts of the same shape, such as Figure 1 shown.

[0065] Next, the crucible 100 is placed in a housing 4 under vacuum or controlled atmosphere and heated to the operating temperature by a heating system 5. The operating temperature in the crucible is 2000°C to 2100°C, and the temperature at the top of the back 2 is preferably at least 2050°C, which is the temperature at which sapphire melts.

[0066] When the crucible 100 is at operating temperature, the temperature of the seed crystal surface must be slightly higher than the sapphire melting temperature of 2050°C. Therefore, both the first metal component 1 and the second sapphire component will expand as the temperature rises. However, the expansion degree of the metal component 1 will be smaller than that of the sapphire back 2. Figure 3 The expansion coefficient curves of the two materials can be seen in Fig. 1. Therefore, the respective dimensions of the two parts can be calculated so as to achieve an almost zero gap between the two parts at the operating temperature. This arrangement ensures that the assembly is not affected by the flow of molten sapphire and the back 2 (or sapphire seed) can be driven downwards, sliding in the internal opening 13 of the first metal part of the crucible, while the first metal part of the crucible remains stationary.

[0067] The remainder of the method consists in feeding the crucible 100 thus formed with raw material "M" in order to transform it into molten raw material "F", each time in relatively small quantities. Thus, the liquid (or molten) sapphire zone above the seed crystal is relatively thin and has an exchange surface "S" with the atmosphere of the enclosure in which the system is placed, which is equal to the cross-sectional area of ​​the cylinder and is relatively large.

[0068] Thus, the degassing "D" of the liquid sapphire can be maximized. In order to be compatible with the use of molybdenum or tungsten metals or alloys of these two metals at high temperatures, the enclosure in which the method is carried out is a gas-tight enclosure in which a high vacuum has been previously applied or the air has been completely evacuated with a neutral gas.

[0069] In order to keep the area of ​​molten sapphire raw material F at a relatively small constant amount at all times, it is preferred to use a feeding system 3, the feeding of which may be continuous or discontinuous.

[0070] The continuous feed system 3 allows the raw material M to be supplied in finely divided form and therefore allows the amount supplied to be adjusted precisely each time so that it corresponds precisely to the amount crystallized simultaneously.

[0071] The melting of the raw material M is preferably carried out in a region separated from the region directly above the seed crystal to avoid disrupting the crystallization. Advantageously, the receiving groove 10 forming the melting zone is arranged on the outer surface around the first metal part 1 of the crucible and is connected to the inner opening 13 through the feed channel 11, such as Figure 1 , 2 4. The melting zone is advantageously lined with metal flakes, chips, granules, pellets or capsules 12, for example molybdenum or tungsten, on which the raw material M is melted.

[0072] The molten raw material F then flows into the inner opening 13 of the crucible via the channel 11. This arrangement provides an additional degassing effect.

[0073] The remainder of the method may still be carried out in vacuum by continuous pumping P, or in an atmosphere of argon or other neutral gas, but is preferably carried out under reduced pressure by continuous pumping P to facilitate degassing of the molten sapphire.

[0074] Furthermore, it is preferred to use a heating and insulating environment that does not contain carbon (C) to avoid the formation of carbon monoxide (CO) gas that can dissolve in the molten sapphire, and therefore it is preferred to use a metal heating element.

[0075] The next step is to translate the crucible back 2 at a controlled speed to gradually solidify the molten raw material F and gradually form a rod-shaped sapphire single crystal C. This movement is performed toward the lower less heated and therefore cooler area in the housing 4 to solidify the molten raw material.

[0076] The crystallization of sapphire is achieved by translating the back 2 of the crucible (i.e., the sapphire seed crystal) downward at a speed controlled by a motorized translation system 6. The first metal part 1 of the crucible is supported by a structure integral with the housing 4 and therefore remains stationary during the manufacture of the sapphire single crystal.

[0077] Therefore, the molten raw material F (or liquid sapphire) at the interface with the back 2 is transferred to a cooler area and solidified while maintaining the direction and contour of the crucible back 2 (i.e., the seed crystal). If there are still any bubbles and dissolved gases, and if the displacement does not occur too quickly, the bubbles and dissolved gases are not incorporated into the crystallized sapphire.

[0078] The position of the liquid / solid interface remains essentially constant throughout the process. The length of the crystallizable sapphire rod depends on the total stroke of the translation system, so the capacity of the raw material tank must be at least equal to the weight of the crystallizable rod.

[0079] The method is completed by interruption of the supply of raw material M and complete crystallization of the molten sapphire region, followed by cooling of the crucible 100 to ambient temperature.

[0080] Once everything has cooled down, the sapphire rod C bonded to the seed crystal 2 can be recovered and the seed crystal 2 and / or a portion of the rod C can be sawn off to form a new back 2, thereby forming a new seed crystal.

[0081] Thus, the method can be repeated by placing a new back or seed crystal into the same first metal part of the crucible. This new seed crystal can simply be the seed crystal initially used and / or a small portion of the resulting rod recovered by sawing. Thus, the seed crystal can be regenerated indefinitely without the need for complex re-machining operations to obtain a back (or seed crystal) of the correct size.

[0082] Figure 5An example of an apparatus for directly manufacturing a rod-shaped sapphire single crystal is shown. A single enclosure under vacuum or controlled atmosphere can include multiple crucibles and seed crystals, as well as as many heating systems, thermal insulation systems, continuous raw material feeding systems, and translation systems operating in parallel to increase the production of sapphire rods at a lower cost by this method.

[0083] The invention also allows the manufacture of watch glasses and back covers from sapphire rods obtained using the method described above. Needless to say, this example is merely illustrative and not limiting, and it is also possible to manufacture external and functional components, in particular for the watchmaking and jewellery industries, such as bridges, cocks, cases and dials or bracelet links.

Claims

1. A method for directly manufacturing rod-shaped single crystal sapphire, the method comprising the following steps: - providing a crucible (100) comprising a first fixed part (1) having an internal opening (13) and a second movable part (2) forming the back of the crucible and consisting of a sapphire sheet forming a seed for sapphire growth; - positioning the first component (1) and the second component (2) relative to each other at ambient temperature, the second component being movable in translation in the internal opening (13) of the first component; - placing the crucible (100) in a housing (4) under vacuum or controlled atmosphere and heating the housing (4) to bring the crucible to operating temperature; - feeding raw material (M) into the crucible via a feeding system (3) to form molten raw material (F) above the movable sapphire part in the crucible; - translating the back (2) of the crucible at a controlled speed by means of a moving mechanism so that the molten raw material gradually solidifies and gradually forms a sapphire rod (C) having the same cross section as the back (2) of the crucible (100); - interrupting the feeding of the raw material (M) and allowing the remaining molten material (F) in the crucible to completely crystallize; - Cool the crucible to ambient temperature; - Recover the sapphire rod bonded to the seed crystal; - Optionally sawing off the seed crystal and / or part of the rod to form a new back and a new seed crystal.

2. A method according to claim 1, wherein the inner opening (13) of the first component is cylindrical and has a cross-section substantially corresponding to the desired cross-section of the sapphire rod.

3. A method according to claim 1 or 2, wherein the sapphire back (2) is cylindrical and has a cross-section corresponding to the desired cross-section of the sapphire rod.

4. The method according to claim 1 or 2, wherein the first part (1) of the crucible is made of a high melting point metal, such as molybdenum, tungsten or an alloy of these two metals.

5. The method according to claim 4, wherein the high melting point metal is molybdenum, tungsten or an alloy of these two metals.

6. The method according to claim 1 or 2, wherein the working temperature in the crucible (100) is 2000°C to 2100°C.

7. The method according to claim 1 or 2, wherein the dimensions of the first component (1) and the second component (2) are designed to achieve a minimum gap at operating temperature.

8. The method according to claim 1 or 2, wherein the raw material (M) has a pure or doped Al2O3 chemical composition.

9. The method according to claim 8, wherein the raw material (M) is selected from ground and crushed sapphire, sapphire or alumina beads, or densely compacted granular alumina powder.

10. The method of claim 1 or 2, wherein the controlled atmosphere consists of a neutral gas.

11. The method for manufacturing single crystal sapphire according to claim 9, wherein the neutral gas is argon gas.

12. The method according to claim 1 or 2, characterized in that: The translation mechanism is managed by a motorized translation system (6).

13. The method according to claim 1 or 2, wherein: The capacity of said system (3) for feeding the raw material (M) is at least equal to the weight of the crystallizable sapphire rod.

14. The method according to claim 1 or 2, wherein the crystal orientation of the sapphire back or seed crystal can be selected indiscriminately from all crystal orientations of sapphire.

15. The method according to claim 1 or 2, characterized in that: Once the sapphire rod is obtained, external or functional components for the watchmaking and jewellery industries are cut from it.

16. The method according to claim 15, characterized in that External or functional components for the watchmaking and jewellery industries are cut from the sapphire rods by means of a wire saw.

17. The method according to claim 1 or 2, wherein: The same housing under vacuum or controlled atmosphere is used for multiple crucibles, heating systems, raw material feed systems and translation mechanisms.

18. External and functional components for the watchmaking and jewellery industries cut from a sapphire single crystal obtained by carrying out the manufacturing method according to any one of claims 1 to 17.

19. The exterior and functional assembly according to claim 18, characterized in that They are the bridges, plates, crystal, case and dial or bracelet links.

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