Asymmetric thermal field for eliminating impurities in single crystal manufacturing equipment

Abstract

Disclosed herein is a furnace and method for growing high-temperature oxide crystals. The furnace comprises a cylindrical furnace wall, an induction coil disposed within the cylindrical furnace wall, a quartz tube disposed within the induction coil, a refractory lining disposed within the quartz tube, and a crucible (containing a molten material) disposed within the refractory lining. A lid is placed over the crucible. By asymmetrically arranging at least one of the crucible, the refractory lining, the quartz tube, the induction coil, and the lid within the cylindrical furnace wall, a temperature gradient is formed in the molten material, moving the cold point in the molten material from a first position to a second position. A rod having a seed crystal at its end is lowered into the crucible, and the boule is pulled out of the molten material from the first position via the seed crystal.

Description

Background of the Invention

[0001] [Field of the Invention] The present invention generally relates to crystal growth for nuclear medicine imaging, and more particularly to a method for reducing impurities in high temperature oxide crystals grown for use in scintillation detectors for positron emission tomography (PET).

[0002] [Description of the Related Art] When manufacturing boules of lutetium orthosilicate (LSO), lutetium yttrium orthosilicate (LYSO), or gadolinium aluminum gallium garnet (GAGG) type scintillators, powders are added to a crucible in the appropriate stoichiometric ratio and heated to form a melt. Thereafter, a seed crystal is lowered into contact with the melt and then gradually withdrawn from the melt to form a boule. It is desirable for any dopants in the melt to be uniformly distributed during the withdrawal of the boule, and thus it is desirable for the dopant distribution in the boule to be uniform. It is also desirable for impurities not to be incorporated into the boule.

[0003] However, this is usually not the case. Due to the geometric symmetry of the furnace and the resulting thermal field, a symmetric flow field of the melt creates a cold spot (cold spot) at the center of the crucible, forming a location where impurities accumulate. Since the seed crystal contacts the melt at the center (i.e., the cold spot), there is a tendency for impurities to be incorporated into the boule. Therefore, it is desirable to form a flow field within the crucible that moves impurities away from the location where the seed crystal contacts the melt. [Summary of the Invention]

[0004] Disclosed herein is a method for growing high-temperature oxide crystals. The method comprises assembling a furnace including a cylindrical furnace wall, the furnace comprising an induction coil placed within the cylindrical furnace wall, a quartz tube placed within the induction coil, a refractory lining placed within the quartz tube, a crucible (containing a molten material) placed within the refractory lining, and a lid covering the crucible. By asymmetrically arranging any of the crucible, refractory lining, quartz tube, induction coil, and lid with respect to the cylindrical furnace wall, a temperature gradient is created in the molten material, causing the cold point of the molten material to move from a first position to a second position. The boule is then pulled up, avoiding the second position where the cold point is located.

[0005] Also disclosed herein is a furnace for growing high-temperature oxide crystals. The furnace comprises a cylindrical furnace wall, an induction coil disposed within the cylindrical furnace wall, a quartz tube disposed within the induction coil, a refractory lining disposed within the quartz tube, a crucible (containing a molten material) disposed within the refractory lining, and a lid disposed over the crucible. By asymmetrically arranging at least one of the crucible, refractory lining, quartz tube, induction coil, and lid within the cylindrical furnace wall, a temperature gradient is created within the molten material, moving the cold point of the molten material from a first position to a second position. A rod (pulling rod) having a seed crystal at its end is then lowered into the molten material, and the boule is pulled up from the molten material from the first position via the seed crystal.

[0006] These and other features, aspects, and advantages of the present invention will be better understood by referring to the following description and the appended claims, as well as the appended drawings. [Brief explanation of the drawing]

[0007] [Figure 1] Figure 1 is a side view of the furnace in which the boule is grown, illustrating an exemplary embodiment. [Figure 2] Figure 2 shows a side view of the crucible during heating of the molten material inside the crucible in a symmetrical configuration. [Figure 3] Figure 3 shows a plan view of the furnace in the symmetrical configuration shown in Figure 1. [Figure 4] Figure 4 shows a plan view of the furnace, illustrating the first asymmetrical configuration of the furnace. [Figure 5] Figure 5 shows a plan view of the furnace, illustrating the second asymmetrical configuration of the furnace. [Figure 6] Figure 6 shows a plan view of the furnace, illustrating the third asymmetrical configuration of the furnace. [Figure 7] Figure 7 shows a plan view of the furnace in the fourth asymmetrical configuration. [Figure 8] Figure 8 shows a plan view of the furnace in the fifth asymmetrical configuration. [Figure 9] Figure 9 shows a plan view of the furnace in the sixth asymmetrical configuration. [Figure 10] Figure 10 shows a side view of the reactor in the seventh asymmetrical configuration. [Figure 11] Figure 11 shows a diagram of a crucible in an asymmetric configuration.

[0008] It should be understood that various embodiments are not limited to the arrangements and apparatus shown in the drawings. Detailed Description of the Invention

[0009] The present invention will be more readily understood by referring to the following detailed description of preferred embodiments and the examples contained herein. Numerical values ​​herein are considered to be modified by the word “approximately,” whether expressly indicated or not. The word “approximately” generally refers to a range of numerical values ​​(i.e., numerical values ​​having the same function or result) that a person skilled in the art would consider equivalent to the cited value. Often, the word “approximately” may include numerical values ​​rounded to the nearest significant figure.

[0010] Figure 1 is a side view of a furnace 100 in which a boule is grown in an exemplary embodiment. The furnace 100 includes a cylindrical furnace wall 102 surrounding a space 104. A copper coil 106 is wound around the outer surface of the cylindrical furnace wall 102. Water or a coolant flows through the copper coil 106 to remove heat from the furnace 100 or maintain the temperature of the space 104 within the furnace 100. A quartz tube 108 is concentrically arranged within the cylindrical furnace wall 102. An induction coil 110 is positioned in the annular section between the quartz tube 108 and the cylindrical furnace wall 102. A crucible 114 is positioned within the quartz tube 108 at the bottom surface 116 of the furnace 100. The crucible 114 includes a cylindrical wall extending upward from the bottom surface. The crucible 114 can be made of iridium. The crucible 114 extends from the bottom surface 116 to partway into the furnace 100. A brick or ceramic cylinder 115 extends from the top of the crucible 114 to the top of the furnace 100 or to the lid 130. A refractory lining 118 is placed in the annular section 128 between the quartz tube 108 and the crucible 114 / ceramic cylinder 115.

[0011] To produce a boule, the material for forming the molten material 120 is placed in a crucible 114, which is concentrically positioned in a furnace 100. An induction coil 110 is activated to heat the material and form the molten material 120. A seed crystal 122 is lowered through a ceramic cylinder 115 at the end of a rod (pulling rod) 124 and comes into contact with the top of the molten material 120 at the seed position (i.e., the position where the boule is pulled up from the molten material). The seed crystal 122 is then pulled upward from the molten material 120, and the molten material 120 cools and crystallizes around the seed crystal 122. Thus, by pulling the seed crystal 122 upward, a boule or cylindrical crystal is formed.

[0012] Figure 2 shows a side view 200 of a crucible when a molten material is heated in a symmetric configuration. When the molten material 120 is heated above a predetermined temperature, convection cells 202 are formed within the molten material 120. Heat in the walls 204 of the crucible 114 causes the molten material 120 to flow upward along the walls 204. Upon reaching the top of the crucible 114, the molten material 120 moves radially inward toward the central axis 206, and is cooled in the process. Thus, a cold point 215 is formed at the top of the molten material, toward the central axis 206. Impurities tend to accumulate at the cold point 215. Subsequently, the molten material 120 descends toward the bottom of the crucible 114 along the central axis 206. The method disclosed herein creates a temperature gradient within the molten material, moving the cold point 215 from a first position (e.g., the furnace central axis 310 (see Figure 3)) to a second position (e.g., a position away from the central axis 206).

[0013] Figure 3 shows a plan view 300 of the symmetrical configuration of the furnace in Figure 1. The crucible 114 / ceramic cylinder 116, refractory lining 118, quartz tube 108, induction coil 110, and cylindrical furnace wall 102 are concentric with each other. In other words, the crucible central axis 302 of the crucible 114, the lining central axis 304 of the refractory lining 118, the quartz central axis 306 of the quartz tube 108, the coil central axis 308 of the induction coil 110, and the furnace central axis 310 of the cylindrical furnace wall 102 share the same axis. As a result of this concentric arrangement, the cold point appears along the shared central axis, and consequently, impurities are incorporated into the furnace.

[0014] Figure 4 is a plan view 400 showing the first asymmetrical configuration of the furnace. The lining central axis 304, the quartz central axis 306, and the coil central axis 308 are all concentric with the furnace central axis 310, while the crucible central axis 302 is eccentric from the furnace central axis 310. Due to the eccentricity of the crucible 114, the cold point 215 moves away from the crucible central axis 302 (and to a position shared by the lining central axis 304, the quartz central axis 306, the coil central axis 308, and the furnace central axis 310).

[0015] The rod 124 and seed crystal 122 can be lowered into the molten material and pulled up along the central axis 302 of the crucible. This allows the boule to form away from the accumulation of impurities at the cold point 215.

[0016] Figure 5 is a plan view 500 showing a second asymmetrical configuration of the furnace. The crucible central axis 302, the quartz central axis 306, the coil central axis 308, and the furnace central axis 310 are concentric with each other. The refractory lining 118 is thicker on the first side 502 than on the opposite second side 504. As a result, a temperature gradient is formed throughout the molten material, and the cold point 215 is formed off-center from the furnace axis 310.

[0017] Figure 6 is a plan view 600 showing a third asymmetrical configuration of the furnace. The crucible central axis 302, the lining central axis 304, and the coil central axis 308 are concentric with the furnace central axis 310. The quartz central axis 306 is eccentric from the furnace central axis 310. As a result of the eccentric arrangement of the quartz central axis 306, the cold point 215 is formed at a position away from the furnace central axis 310. The rod 124 and seed crystal 122 can be placed on the furnace central axis 310.

[0018] Figure 7 shows a plan view 700 of the furnace in the fourth asymmetrical configuration. The induction coil is positioned eccentrically with respect to the rest of the furnace. The crucible central axis 302, the lining central axis 304, and the quartz central axis 306 are concentric with the furnace central axis 310. The coil central axis 308 is eccentric from the furnace central axis 310. This eccentricity creates a temperature gradient, resulting in a cold point 215 being formed at a location away from the furnace central axis 310.

[0019] Figure 8 shows a plan view 800 of the furnace in the fifth asymmetrical configuration. The refractory lining 118 has holes 802 formed on one side of the crucible 114. The holes 802 create a temperature gradient within the molten material 120, shifting the cold point 215 to one side of the crucible 114, specifically to the location of the holes 802. Additional holes may also be formed within the refractory lining 118. The shape, size, and / or location of the additional holes are selected or designed to position the cold point 215 at a desired location.

[0020] Figure 9 shows a plan view 900 of the furnace in the sixth asymmetric configuration. The cooling finger 902 is arranged eccentrically with respect to one side of the crucible 114 at the bottom of the quartz tube 108. The cooling finger 902 moves the cold spot 215 away from the central axis and towards the cooling finger. The cooling finger 902 can be supplied with a coolant so as to maintain different temperatures from the surroundings inside the furnace.

[0021] Figure 10 shows a side view 1000 of the furnace in the seventh asymmetric configuration. The lifting rod is arranged eccentrically with respect to the center of the crucible, and thus is away from the cold spot. Therefore, the boule is lifted from a position away from the cold spot 215.

[0022] Figure 11 shows a view 1100 of the crucible 114 in the asymmetric configuration. The crucible 114 can be arranged symmetrically inside the furnace (i.e., the crucible central axis 302 coincides with the furnace central axis 310). On the other hand, the lid 130 can be arranged asymmetrically with respect to the crucible 114 (i.e., the central axis 1102 of the lid is eccentric from the crucible central axis 302).

[0023] Although the present invention has been described in detail with respect to specific preferred embodiments, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions included herein.

[0024] The reader's attention should be directed to all documents and writings submitted simultaneously with this specification and published together with this specification, and the content of all such documents and writings is incorporated herein by reference.

[0025] All features disclosed in this specification (including the appended claims, abstract and drawings) can be replaced by alternative features that serve the same, equivalent or similar purposes, unless explicitly stated to the contrary. Therefore, unless explicitly stated to the contrary, each feature disclosed is only an example of a general series of equivalent or similar features.

[0026] Any element in the claims that is not explicitly described as a “means for” or a “step” for performing a particular function should not be construed as a “means” or “step” claim clause as defined in 35 U.S.C. §112, paragraph 6. In particular, the use of the term “step” in the claims herein is not intended to apply the provisions of 35 U.S.C. §112, paragraph 6.

Claims

1. A method for growing high-temperature oxide crystals, Steps to assemble a furnace comprising a cylindrical furnace wall, an induction coil disposed within the cylindrical furnace wall, a quartz tube disposed within the induction coil, a refractory lining disposed within the quartz tube, a crucible (containing molten material) disposed within the refractory lining, and a lid covering the crucible. The steps of creating a temperature gradient in the molten material by asymmetrically arranging any of the crucible, refractory lining, quartz tube, induction coil, and lid with respect to the cylindrical furnace wall, thereby moving the cold point in the molten material from a first position to a second position in the molten material, and The step of lifting the boule from the first position, which is away from the second position where the cold point is located. A method that includes this.

2. A method according to claim 1, further comprising the step of forming a temperature gradient by offsetting the central axis of the crucible from the central axis of the furnace.

3. A method according to claim 1, further comprising the step of forming a temperature gradient such that the fire-resistant lining has a first side that is thicker than a second side on the opposite side.

4. A method according to claim 1, further comprising the step of forming a temperature gradient by offsetting the quartz central axis of a quartz tube from the furnace central axis of a furnace.

5. A method according to claim 1, further comprising the step of forming a temperature gradient by offsetting the coil central axis of an induction coil from the furnace central axis of a furnace.

6. A method according to claim 1, further comprising the step of forming a temperature gradient by asymmetrically positioning a lid with respect to the crucible.

7. A method according to claim 1, further comprising the step of forming a temperature gradient by introducing holes into the refractory lining on one side of the crucible.

8. A method according to claim 1, further comprising the step of forming a temperature gradient by applying a cooling finger to a quartz tube on one side of a crucible.

9. A furnace for growing high-temperature oxide crystals, Cylindrical furnace wall, An induction coil arranged within the cylindrical furnace wall, A quartz tube placed inside the induction coil, The fire-resistant lining placed inside the quartz tube, A crucible placed within the refractory lining, comprising a crucible containing molten material, A lid positioned on the crucible, wherein at least one of the crucible, the refractory lining, the quartz tube, the induction coil, and the lid is arranged asymmetrically within the cylindrical furnace wall to form a temperature gradient in the molten material and move the cold point in the molten material from a first position to a second position, and A rod having a seed crystal at its end, wherein the rod is configured to pull the boule out of the molten material from a first position via the seed crystal, A furnace containing a furnace.

10. A furnace according to claim 9, wherein the central axis of the crucible is eccentric from the central axis of the furnace.

11. A furnace according to claim 9, wherein the refractory lining is thicker on the first side than on the opposite second side.

12. A furnace according to claim 9, wherein the quartz central axis of the quartz tube is eccentric from the furnace central axis of the furnace.

13. A furnace according to claim 9, wherein the coil central axis of the induction coil is eccentric with respect to the furnace central axis of the furnace.

14. A furnace according to claim 9, further wherein the lid is arranged asymmetrically with respect to the crucible.

15. A furnace according to claim 9, further comprising a hole in the refractory lining on one side of the crucible that forms the temperature gradient.

16. A furnace according to claim 9, further comprising a cooling finger that forms the temperature gradient and is applied to the quartz tube on one side of the crucible.

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