Method for manufacturing a sapphire rod

Abstract

To provide a method of directly manufacturing rod-like sapphire single crystal.SOLUTION: A crucible 100 has a fixed first part 1 with an inside opening 13 and a movable second part 2 forming a bottom part of the crucible and made of a sapphire piece forming seed crystal for sapphire growth. The second part can move in a translationally movable state in the inside opening 13 of the first part. The crucible 100 is arranged in an enclosure under a vacuum or controlled atmosphere, and the enclosure is heated to heat the crucible up to an operation temperature. Raw material is supplied to the crucible through a supply system 3 and molten raw material is formed in the crucible. The molten raw material is gradually solidified by moving the bottom part of the crucible to gradually form a sapphire rod-like body.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for directly producing rod-shaped single crystal sapphire. The present invention also relates to external or functional parts for the watch (e.g., wristwatch, pocket watch) or jewelry industry obtained by cutting such single crystal sapphire. [Background technology]

[0002] There are several ways to obtain artificial single crystals, such as from supersaturated solutions of compounds, from molten compounds, or by chemical vapor transport.

[0003] For example, the Verneuil process is well known. To synthesize corundum, which has the formula Al2O3 and is the compound of ruby ​​and sapphire, the temperature must be raised to a very high level (fusion at 2050°C), reached using an oxyhydrogen torch (H2 + O2 → H2O), with a flame temperature of approximately 2700°C. Alumina, possibly doped, is introduced in the form of a fine powder using a vibrator, which drops a small amount of fine alumina powder directly into the torch flame. The molten alumina droplet thus formed falls on top of a seed crystal and crystallizes according to the crystallographic structure of the seed. The growing single crystal is gradually lowered so that crystallization occurs at a constant temperature. At the end of the synthesis, a bottle-shaped single crystal is obtained.

[0004] However, sapphire single crystals obtained by the Verneuil process have high dislocation densities and uncontrollable local misalignment, and other defects present in the Verneuil single crystal, such as bubbles, raindrops, and other inclusions, tend to be visible to the naked eye in, for example, the finished watch crystal.

[0005] Furthermore, it is relatively difficult to effectively control the shape of a Bernoulli crystal, with only ±2 mm of control over the diameter possible.

[0006] The Czochralski process is a crystallization technique that uses a molten bath in a crucible. In theory, the Czochralski process is well suited to producing cylindrical sapphires, whose size is limited only by the size of the crucible. Because the pulling rate is precisely controlled, the average diameter can be brought very close to the desired value using a sensor that measures the weight of the crystal, which is used to calculate the deviation from this diameter and adjust the crystallization temperature accordingly.

[0007] However, controlling the average diameter may not prevent the actual shape of the sapphire crystal from deviating significantly from the ideal cylindrical shape. Because the growing crystal does not come into contact with the crucible, there are no mechanical constraints limiting its shape. Facets, which are flat, low-energy surfaces, may develop along the pulling direction. In some cases, such as growth along the A-axis, facets perpendicular to the C-axis appear, which can cause very pronounced shape anisotropy.

[0008] The Czochralski process also requires a very tall sapphire bath because all the raw materials used to form the crystal must be contained in the crucible. This makes degassing difficult; because the crystal grows above the bath, bubbles and microbubbles can get inside the crystal.

[0009] In the Edge-Defined Film-Fed Growth (EFG) technique, sapphire crystallization occurs on top of a die immersed in a bath of molten sapphire. The die imparts its contour to the resulting crystal. To approximate the final dimensions, plates, tubes, and small rods can be obtained. However, the thickness of the contour is also limited, typically to a maximum of 20 mm, because the molten sapphire must rise to the top of the die through very thin capillary channels (maximum 0.8–1 mm in one dimension). Furthermore, because crystallization occurs above the bath, bubbles and dissolved gases formed in the bath can rise into the crystal. In particular, microbubbles systematically appear near the surface, typically at depths up to 0.5 mm, limiting the useful portion of the sapphire produced and necessitating processing all surfaces to this depth.

[0010] In the micro-pulldown technique, sapphire crystallization is performed below the molten sapphire bath, which reduces the problems associated with entrapment and nucleation of bubbles and microbubbles. However, the crystallization zone is always fed by capillary action, i.e., through a narrow hole or slit in the bottom of the crucible containing the sapphire bath, and a sapphire seed crystal must be placed below this hole or slit to form a stable liquid meniscus between the crucible and the seed crystal. The condition for meniscus stability decreases with increasing meniscus surface area, which significantly limits the dimensions accessible by micro-pulldown.

[0011] Horizontal and vertical Bridgman crystallization techniques and their variants (such as HDC (Horizontal Directional Crystallization) or Bagdasarov Method), HEM (Heat Exchanger Method), and VGF (Vertical Gradient Freeze) all feature the advantage that the crystal grows in direct contact with the crucible, which allows the resulting sapphire to have a precisely defined shape and that this shape is as close as possible to the final product. However, this advantage comes at a cost cost, since the crucible typically needs to be destroyed to recover the crystal and a new crucible must be used for each crystallization cycle. Furthermore, since all the raw materials for forming the crystal must be contained within the crucible and completely melted, the bath height increases considerably, which can hinder efficient degassing from the viewpoint of the crystal's optical quality. Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention aims to overcome these and other problems by providing a method for directly producing rod-shaped sapphire single crystals.

[0013] This method allows for the direct crystallization of sapphire rods with easily adaptable and precisely controllable cross sections and lengths, without relying on capillary effects that limit one of the three dimensions, and allows for easy thin cutting with a wire saw, as close as possible to the required final dimensions, especially for timepiece components. All of this significantly reduces material loss during processing.

[0014] It is a further object of the present invention to enable the tools used in the method to be reused multiple times in order to reduce costs.

[0015] The present invention further aims to optimize the optical quality of the crystals produced by controlling the atmosphere in which the method is carried out and limiting the volume of the molten bath to promote degassing.

[0016] Finally, the present invention aims to optimize the optical quality of the produced crystals by growing the crystals below the molten bath, thereby reducing the incorporation of bubbles, microbubbles and residual gases in the molten bath into the crystals. [Means for solving the problem]

[0017] To this end, the present invention relates to a method for producing rod-shaped single crystal sapphire, the method comprising: providing a crucible having a fixed first portion with an internal opening and a movable second portion comprising a piece of sapphire that forms the bottom of the crucible and forms a seed crystal for sapphire growth; disposing the first portion and the second portion relative to one another at an ambient temperature, the second portion being translatably movable within the inner opening of the first portion; placing the crucible in an enclosure under vacuum or controlled atmosphere and heating the enclosure to heat the crucible to an operating temperature; supplying raw material to the crucible, preferably via a tank and supply system operating continuously or discontinuously, to form molten raw material within the crucible on the movable second portion; using a moving means to move the bottom of the crucible at a controlled speed towards a lower, cooler zone to gradually solidify the molten material and gradually form a sapphire rod having the same cross section as the bottom of the crucible; interrupting the supply of raw material and allowing the molten material remaining in the crucible to completely crystallize; cooling the crucible to ambient temperature; and recovering the resulting sapphire rods; After the sapphire rod is cut, it can form both a new base and a new seed crystal.

[0018] Some particular implementations of the method according to the invention have the following characteristics: the inner opening of the first portion is cylindrical and has a cross section that substantially corresponds to the desired cross section of the sapphire rod; The sapphire base is cylindrical and has a cross section corresponding to the desired cross section of the sapphire rod. the first part of the crucible is made of a refractory metal such as molybdenum, tungsten or an alloy of these two metals; The operating temperature of the enclosure is in the range of 2000°C to 2100°C. - the first and second portions are dimensioned to achieve a minimum clearance at operating temperature; The raw material has the chemical composition of pure or doped Al2O3. the raw material is selected from crushed and crushed sapphire, sapphire or alumina beads, or densified and compressed alumina powder in pellet form; The controlled atmosphere is composed of a neutral gas. the neutral gas is argon. The movement is controlled by a motorized translation system. the capacity of said supply system for supplying raw material is equal to or greater than the weight of the crystallizable sapphire rods; The crystallographic orientation of the base or seed crystal made of sapphire can be chosen independently of all crystallographic orientations of sapphire. After obtaining the sapphire rod, it is cut to produce external or functional parts for the watch or jewellery industry. The sapphire rod is cut by a wire saw to produce external or functional parts for the watch or jewellery industry. - Use the same vacuum or controlled atmosphere enclosure for multiple crucibles, heating systems, and material feed and transfer means.

[0019] The invention further relates to external or functional parts for the watch or jewellery industry, in particular bridges, plates, crystals, watch cases, dials or bracelet links, obtained by cutting the sapphire single crystals obtained according to the method according to the invention.

[0020] Thanks to these features, the invention is able to provide a method that makes it possible to manufacture watch crystals under easier processing conditions and with minimal losses.

[0021] Other characteristics and advantages of the present invention will become more apparent from a reading of the following detailed description of an exemplary embodiment of the method according to the invention, with reference to the drawings, in which this example is given purely for illustrative and not limiting purposes. [Brief explanation of the drawings]

[0022] [Figure 1] 4 shows a second step of the method according to the invention. [Figure 2] 4 shows a third step of the method according to the invention. [Figure 3] Comparative expansion coefficients of molybdenum, tungsten, and sapphire as a function of temperature (C. Miyogawa et al, J. Cryst. Growth 372 (2013), pages 95-99). [Figure 4] 4 shows the fourth and fifth steps of the method according to the invention. [Figure 5] FIG. 1 shows a schematic diagram of a furnace for growing multiple sapphire rods in parallel. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention relates to a method for directly producing (or crystallizing) rod-shaped single crystal sapphire.

[0024] The first step of the method involves providing a crucible 100 intended to receive raw material "M", in which the raw material "M" is melted by the supply of heat.

[0025] The raw material "M" used in the production of sapphire has the chemical composition Al2O3 and can be pure or doped. Raw material "M" can be selected from crushed and crushed sapphire, sapphire or alumina beads, or densified and compressed alumina powder in pellet form.

[0026] According to the present invention, the crucible 100 has a fixed first portion 1 with an inner opening 13 and a movable second portion 2 with a piece of sapphire forming the bottom of the crucible and forming the seed crystal for sapphire growth.

[0027] The thickness of the bottom 2 of the crucible is in the range of 1 cm to 10 cm.

[0028] In an alternative embodiment, there may also be an intermediate metal part, which forms a circular metal base on which the seed crystal is fixed.

[0029] As shown, the inner opening 13 of the first portion 1 is cylindrical, extends the length of the first portion, and has a cross-section that substantially corresponds to the cross-section of the sapphire rod to be produced. Similarly, the sapphire bottom portion 2 is cylindrical and has a cross-section that corresponds to the desired cross-section of the sapphire rod. The shape and size of the cylinder precisely determine the shape and size of the sapphire rod to be crystallized.

[0030] Naturally, the inner opening 13 can have a wide variety of cross-sectional shapes relative to the longitudinal axis of the crucible, the cross-sectional shape depending on the cross-section of the sapphire crystal being produced.

[0031] Thus, the inner cross section can be circular, elliptical, or polygonal. The polygonal cross section can be, for example, square, rectangular, pentagonal, hexagonal, or octagonal in shape.

[0032] The first portion 1 of the crucible 100 is preferably made of a refractory metal such as molybdenum, tungsten, or an alloy of these two metals.

[0033] The next step in the method is to position the first part 1 and the second part 2 relative to each other at ambient temperature, with the second part 2 being movable so that it can translate within the inner opening 13 of the first part.

[0034] When the system is still at ambient temperature, the metallic first part 1 of the crucible and the sapphire bottom part 2 of the crucible (or seed crystal) must be placed against each other. To make this possible, there must be enough space between the two parts, which have the same corresponding shape, as shown in Figure 1.

[0035] In the next step, the crucible 100 is placed in an enclosure 4 under vacuum or controlled atmosphere and heated via a heating system 5 to an operating temperature. The operating temperature within the crucible at the top of the base 2 is in the range of 2000°C to 2100°C, preferably above 2050°C, the temperature at which sapphire melts.

[0036] When the crucible 100 is at operating temperature, it must reach a temperature at the surface of the seed crystal slightly above the melting point of sapphire, 2050°C. Therefore, both the metallic first portion 1 and the sapphire second portion expand with increasing temperature. However, the metallic portion 1 expands less than the sapphire bottom portion 2, as can be seen from the expansion coefficient curves of these two materials in Figure 3. Therefore, it is possible to calculate the corresponding dimensions of the two portions so that the clearance between them is essentially zero at operating temperature. This configuration ensures that the assembly is impermeable to the flow of molten sapphire and that the bottom portion 2 (or the sapphire seed crystal) can move downward and slide within the inner opening 13 in the metallic first portion of the crucible, which remains fixed.

[0037] The remainder of the method involves feeding feedstock "M" into the crucible 100 thus formed and converting it at any one time into a relatively small amount of molten feedstock "F." The zone of liquid (or molten) sapphire above the seed crystal is therefore relatively thin, and there is an exchange surface "S" with the atmosphere of the enclosure in which the system is located, this exchange surface "S" having a cross section corresponding to that of a cylinder and being relatively large.

[0038] Thus, the degassing of the liquid sapphire "D" can be maximized. 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 either a high vacuum or an airtight enclosure that has previously been completely evacuated of air with a neutral gas.

[0039] In order to maintain the zone of molten sapphire raw material F at a relatively small and constant amount at all times, it is preferable to use a supply system 3, and the supply in this supply system 3 can be continuous or discontinuous.

[0040] The continuous supply system 3 allows the raw material M to be supplied in a finely divided form, so that the amount supplied at any given time can be precisely adjusted, thereby precisely corresponding to the amount crystallized at the same time.

[0041] The melting of the raw material M is preferably carried out in a zone separate from the zone directly above the seed crystal so as not to interfere with the crystallization. Advantageously, as shown in Figures 1, 2 and 4, the outer surface of the crucible around the metallic first part 1 is provided with a receiving groove 10 forming a melting zone, which receiving groove 10 communicates with an inner opening 13 via a feed channel 11. Advantageously, metal chips, splinters, granules, pellets or capsules 12, such as molybdenum or tungsten, are placed in this melting zone, where the raw material M is melted.

[0042] The molten raw material F then flows into the inner opening 13 of the crucible through the channel 11. This configuration provides an additional degassing effect.

[0043] The remainder of the method is carried out in a continuously pumped vacuum P or, preferably, in an atmosphere of argon or other neutral gas, which is reduced in pressure and continuously pumped P to facilitate degassing of the molten sapphire.

[0044] Additionally, metallic heating elements are preferred because a carbon (C)-free heating and insulating environment is preferably used to avoid the formation of carbon monoxide (CO) gas, which can dissolve in the molten sapphire.

[0045] The next step involves translating the bottom 2 of the crucible at a controlled rate to gradually solidify the molten raw material F and gradually form a rod-shaped sapphire single crystal C. This movement is towards the lower, relatively unheated, and therefore cooler, zone within the enclosure 4 to solidify the molten raw material.

[0046] Crystallization of the sapphire is achieved by translating the bottom 2 of the crucible (i.e., the sapphire seed crystal) downward at a rate controlled by a motorized translation system 6, where the metallic first portion 1 of the crucible is supported by a structure integral with the enclosure 4 and therefore remains stationary during production of the sapphire single crystal.

[0047] Thus, the molten raw material F (or liquid sapphire) in the boundary region with the bottom 2 displaces and solidifies into the lower temperature zone while retaining the orientation and contour of the bottom 2 (i.e., seed crystal) of the crucible. If bubbles or dissolved gases are still present, they will not be incorporated into the crystallized sapphire if the displacement is not too rapid.

[0048] The position of the liquid / solid boundary region remains substantially constant throughout the method. The length of the crystallizable sapphire rod depends on the total movement of the translation system, and therefore the capacity of the source tank must be equal to or greater than the capacity of the weight of the crystallizable rod.

[0049] The method is completed by interrupting the supply of raw material M, allowing the molten sapphire zone to fully crystallize, and then cooling the crucible 100 to ambient temperature.

[0050] After everything has cooled, the sapphire rod C attached to the seed crystal 2 can be recovered and a part of the seed crystal 2 and / or rod C can be cut off to form a new base 2, thus forming a new seed crystal.

[0051] Thus, the method can be repeated by placing a new base or seed in the first part of the crucible made of the same metal. This new seed can be the seed used initially and / or a small portion of the resulting rod recovered by cutting. Thus, seed crystals can be recreated indefinitely without the need for complex reworking operations to obtain a base (or seed) of the correct dimensions.

[0052] An example of an apparatus for directly producing rod-shaped sapphire single crystals is shown in Figure 5. A single vacuum or controlled atmosphere enclosure can contain multiple crucibles and seed crystals, along with a corresponding number of heating, insulation, continuous feed, and translation systems operating in parallel, thereby increasing the production of sapphire rods by this method at low cost.

[0053] The invention furthermore makes it possible to manufacture crystals and case backs for watchmaking from the sapphire rods obtained using the method described above. Naturally, the examples given here are purely illustrative and are not intended to be limiting, and it is also possible to manufacture external or functional parts, such as bridges, plates, watch cases, dials or bracelet links, in particular for the watchmaking or jewellery industry. [Explanation of symbols]

[0054] 1. First Part 2. Second Part 3. Supply System 4. Enclosure 5. Heating System 6 Translational Movement System 10 Receiving groove 11 Supply Channels 13 Inner opening 100 crucibles C Sapphire rod F molten material M Raw material S exchange surface

Claims

1. A method for directly producing rod-shaped single crystal sapphire, comprising the steps of: providing a crucible (100) having a fixed first part (1) with an inner opening (13) and a movable second part (2) consisting of a piece of sapphire forming the bottom of the crucible and forming a seed crystal for sapphire growth; placing the first part (1) and the second part (2) relative to each other at ambient temperature, the second part being translatably movable within the inner opening (13) of the first part; placing the crucible (100) in an enclosure (4) under vacuum or controlled atmosphere and heating the crucible via a heating system (5) located within the enclosure (4) to heat the crucible to an operating temperature; feeding raw material (M) into the crucible via a feeding system (3) to form a molten raw material (F) on the movable second portion within the crucible; Moving the bottom (2) of the crucible via a moving means at a controlled speed to gradually solidify the molten raw material and gradually form a sapphire rod (C) having the same cross section as the bottom (2) of the crucible (100); interrupting the supply of raw material (M) and completely crystallizing the molten material (F) remaining in the crucible; cooling the crucible to ambient temperature; recovering the sapphire rod bonded to the seed crystal; Optionally, cutting a portion of the seed crystal and / or sapphire rod to form both a new base and a new seed crystal. A method comprising:

2. The inner opening (13) of the first portion is cylindrical and has a cross section that substantially corresponds to the desired cross section of the sapphire rod.

2. The method of claim 1 .

3. The sapphire bottom (2) is cylindrical and has a cross section corresponding to the desired cross section of the sapphire rod.

2. The method of claim 1 .

4. The first part (1) of the crucible is made of a refractory metal such as molybdenum, tungsten or an alloy of these two metals.

2. The method of claim 1 .

5. The operating temperature in the crucible (100) is in the range of 2000°C to 2100°C.

2. The method of claim 1 .

6. The first portion (1) and the second portion (2) are sized to achieve a minimum clearance at operating temperature.

2. The method of claim 1 .

7. The raw material (M) is pure or doped Al 2 O 3 having a chemical composition of 2. The method of claim 1 .

8. The raw material (M) is selected from crushed and crushed sapphire, sapphire or alumina beads, or densified and compressed alumina powder in pellet form.

8. The method of claim 7.

9. The controlled atmosphere is composed of a neutral gas.

2. The method of claim 1 .

10. The neutral gas is argon 10. The method of claim 9.

11. The movement means is managed by a motorized translation system (6).

2. The method of claim 1 .

12. The capacity of the supply system (3) for supplying the raw material (M) is equal to or greater than the weight of the sapphire rod-shaped body.

2. The method of claim 1 .

13. The crystallographic orientation of the sapphire base or seed crystal can be selected independently of all crystallographic orientations of sapphire.

2. The method of claim 1 .

14. After obtaining the sapphire rod, it is cut into external or functional parts for the watch or jewelry industry.

2. The method of claim 1 .

15. The external or functional parts for the watch or jewellery industry are obtained by cutting the sapphire rod with a wire saw.

15. The method of claim 14.

16. Use of the same vacuum or controlled atmosphere enclosure for multiple crucibles, heating systems, feed systems, and translation means 2. The method of claim 1 .

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