Sodium iodide crystal growth device and crystal growth method

Abstract

The invention discloses a sodium iodide crystal growth device and a crystal growth method, and relates to the technical field of crystal growth, in particular to the sodium iodide crystal growth device which comprises a vacuum device, a crystal furnace, a heating device, a heat preservation device and a crucible, the heating device comprises a main heater, an upper heater and a lower heater, and a single temperature zone is changed into a multi-temperature zone; in the crystal growth process, the heater is subjected to segmented control according to the melting point and growth characteristics of the crystal, so that impurities in the crystal are reduced, defects such as polycrystal are also reduced, and the crystal quality is effectively improved.

Description

Technical Field

[0001] This invention relates to the technical field of crystal growth, specifically to a sodium iodide crystal growth apparatus and crystal growth method. Background Technology

[0002] Sodium thallium-doped iodide (NaI(Tl)) crystal is an excellent scintillation crystal material. For applications such as scintillation crystals in high-energy physics detection, sodium iodide crystals not only require high quality, completeness without cracks, but also the absence of impurities and inclusions. Existing methods for growing sodium iodide crystals typically use the crucible-lowering method under atmospheric atmosphere. However, because sodium iodide readily reacts with oxygen, current growth processes cannot address the issue of internal impurities. Therefore, the prepared sodium iodide crystals usually contain numerous inclusions, fine scattering particles, and other impurities, severely affecting the yield and luminescence performance, increasing production costs, and making it impossible to fabricate large-size (4L) sodium iodide devices. Summary of the Invention

[0003] To address the technical problems of high impurity content, low yield, and poor luminescence performance in sodium iodide crystals prepared in existing technologies, this invention provides the following technical solution:

[0004] An apparatus for growing sodium iodide crystals, comprising:

[0005] A crystal furnace, comprising a furnace body and a lifting shaft, wherein the furnace body is made of double-layered stainless steel, and a coil cooling structure is provided between the inner and outer layers; the furnace body chassis is provided with a vacuum connection pipe; the furnace body contains a heat preservation device, a crucible, and a heater; the heat preservation device includes a heat preservation cylinder and an upper heat preservation cover and a lower heat preservation layer respectively disposed above and below the heat preservation cylinder; the crucible is disposed inside the heat preservation cylinder, and the crucible includes a crucible body and a crucible cover disposed above the crucible body; the interior of the crucible body is hollow and serves as a material hopper.

[0006] The heater is located outside the crucible, and the lifting shaft is located at the bottom of the furnace body and connected to the crucible body.

[0007] A vacuum device, wherein the vacuum device is connected to a vacuum connection pipe on the furnace chassis.

[0008] An air-filling device is installed at the top of the furnace body.

[0009] Preferably, the heater is an annular heater, and an electrode is provided below the heater. The heater includes a main heater, an upper heater, and a lower heater. The main heater is located outside the crucible body. The lower heater is located between the crucible body and the main heater and overlaps the height of the main heater by 50 mm. The upper heater body is located outside the main heater, and the top of the upper heater is bent and located between the main heater and the crucible body, and overlaps the height of the main heater by 50 mm.

[0010] Preferably, the main heater, upper heater, and lower heater are independently temperature controlled.

[0011] Preferably, the distance between the outer wall of the heater and the inner wall of the insulation cylinder is 10-15 mm, and the distance between the inner diameter of the heater and the outer diameter of the crucible is 10-15 mm.

[0012] Preferably, a waste gas treatment device is provided between the vacuum device and the vacuum connection pipe.

[0013] Preferably, the crucible lid is threadedly connected to the crucible cone.

[0014] Preferably, the lifting stroke of the lifting shaft is 300~600mm.

[0015] A method for growing sodium iodide crystals is also provided, comprising the following steps:

[0016] Step 1: Feeding: After thoroughly mixing sodium iodide raw material powder and 0.01~0.05%wt polytetrafluoroethylene powder, load the mixture into the crucible. Add 0.1~0.5wt% thallium iodide by weight of sodium iodide to the bottom of the crucible, cover the crucible, and close the furnace.

[0017] Step 2: Drying the material, evacuate the crystal furnace to 5*10. -3 Pa, the temperature inside the crystal furnace is increased to 150℃ at a rate of 10℃ / h, and kept constant for 20 hours until the vacuum degree stabilizes at 4*10-4Pa or higher, and then the raw materials and impurities in the growth furnace are discharged.

[0018] Step 3: Melting the material. The main heater and the upper heater are heated to 300°C at a rate of 20°C / hour, and the lower heater is heated to 250°C at a rate of 20°C / hour. The temperature is maintained for more than 30 hours to remove the impurities remaining in the heat preservation cylinder. Then the main heater is heated to 630°C at a rate of 10°C / hour, the upper heater is heated to 670°C at a rate of 10°C / hour, and the lower heater is heated to 610°C at a rate of 10°C / hour. The raw material is melted, and the doped thallium iodide is fully melted into the melt.

[0019] Step 4: Crystal growth. The initial temperature gradient at the bottom of the crucible is 10~15℃ / cm. Crystal growth begins. The crucible is lowered at a constant speed of 1~5mm / hr, with a drop distance of more than 450mm.

[0020] Step 5: Annealing. The upper heater, lower heater and main heater are cooled to 400℃, 450℃ and 480℃ respectively at the same time. Then the crucible is raised to the initial position 20~100mm above the ground at a rate of 0.5mm / h and held for 60 hours to start the original furnace annealing.

[0021] Step 6: Cooling down, the main heater cools down to 450℃ at 5℃ / h, the upper heater cools down to below 30℃ at 15℃ / h, and the lower heater cools down to below 30℃ at 15℃ / h.

[0022] Step 7: Remove the crystal. First, turn off the vacuum device. After 2 hours, turn off the mechanical pump, open the vent valve to raise the gas pressure inside the furnace to balance the internal and external pressures, open the furnace chamber, and remove the sodium iodide crystal blank.

[0023] In a further preferred embodiment, in step 4, the crucible descent rate is inversely proportional to the crystal cross-sectional area. The descent rate is calculated as follows: for a sodium iodide crystal growth crucible with a diameter of φ100, the descent rate is 5 mm / h. For every 10% increase in crystal diameter, the crucible descent rate decreases by 5%.

[0024] More preferably, in step 5, the cooling rate of the upper heater, lower heater and main heater is less than 2℃ / h, and the cooling time is more than 100h.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] (1) This application provides a sodium iodide crystal growth apparatus, which includes a vacuum device, a crystal furnace, a heating device, a heat preservation device and a crucible, etc. In this application, the main heater, the upper heater and the lower heater are independently temperature controlled in multiple temperature zones. Through the improvement of equipment and process, impurities inside the crystal are reduced, as well as defects such as polycrystalline, thereby effectively improving the crystal quality.

[0027] (2) The sodium iodide crystal growth device provided in this application has a waste gas treatment device between the vacuum device and the vacuum connection pipe. The volatilized sodium iodide or other compounds are deposited and collected by multiple circulating water condensation to quickly cool down the device, which can effectively prevent the crystal from being contaminated.

[0028] (3) Sodium iodide crystals are prepared by using the growth device and growth method of sodium iodide crystals provided in this application. The multi-heater structure can ensure that sodium iodide crystals have sufficient space and a suitable temperature gradient for growth. Furthermore, impurities in the growth system can be fully removed through process control to prevent impurity nucleation and polycrystalline and microscopic defects, thereby ensuring a high yield and a low number of macroscopic defects. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of a sodium iodide crystal growth apparatus.

[0030] Figure label:

[0031] 1-Furnace body, 2-Lifting shaft, 3-Insulation cylinder, 4-Upper insulation cover, 5-Lower insulation layer, 6-Cruise body, 7-Cruise cover, 8-Main heater, 9-Upper heater, 10-Lower heater, 11-Electrode Detailed Implementation

[0032] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0033] like Figure 1 As shown, the present invention discloses a sodium iodide crystal growth apparatus, including a crystal furnace, a vacuum device and a heating device.

[0034] The crystal furnace has an inner diameter of 600-1000mm and a height of 800-1400mm. It includes a furnace body 1 and a lifting shaft 2. The furnace body 1 is made of double-layered stainless steel, with a coil cooling structure between the inner and outer layers. The inner layer is made of 316 stainless steel with a wall thickness of 20-30mm, and the outer layer is made of 304 stainless steel with a wall thickness of 10-20mm. This thickness ensures sufficient stability, minimizes the impact of vibration during sodium iodide crystal growth, and guarantees a sufficiently high vacuum. The inner thickness of the furnace body 1's base is 20mm, and the outer thickness is 10mm. A movable lifting shaft 2 is located at the center, with a lifting stroke of 300-600mm. The furnace body 1's base is equipped with vacuum connection pipes.

[0035] The furnace body 1 is equipped with a heat preservation device, a crucible, and a heater. The heat preservation device uses asphalt-based rigid heat preservation felt with a graphite coating on the surface to prevent volatile impurities or oxygen from corroding the heat preservation cylinder 3. The heat preservation device includes the heat preservation cylinder 3 and an upper heat preservation cover 4 and a lower heat preservation layer 5 respectively set above and below the heat preservation cylinder 3. The heat preservation cylinder 3 is purified and has an ash content of less than 10 ppm and impurities such as Al, Fe, and Si of less than 1 ppm. The thickness of the heat preservation cylinder 3 is 50~80 mm according to the melting point and crystallization characteristics of sodium iodide crystals and the required size of the grown crystals.

[0036] The crucible is housed inside the insulation cylinder 3, with an inner diameter of 180 mm and a uniform height of 600 mm, and its external shape is cylindrical. Specifically, the crucible includes a crucible body 5 and a crucible lid 7 positioned above the crucible body 5. The interior of the crucible body 5 is hollow and serves as a material hopper. Both the crucible body 5 and the crucible lid 7 are made of high-purity isostatic graphite, with the content of transition metals and rare earth elements in the graphite material required to be below 5 ppm, and the graphite particle size less than 5 μm. The crucible body 5 has an inner diameter of 100-300 mm, a thickness of 15 mm, and a total height of 500-700 mm. The bottom angle of the crucible is 90°, 120°, or 150°. Preferably, in this application, the bottom cone angle of the crucible body 5 is 120°, which ensures that the diameter of the grown sodium iodide crystal blank is not less than 160 mm and the height is not less than 400 mm. The crucible lid 7 is threadedly connected to the crucible cone to ensure that a large amount of sodium iodide raw material does not volatilize outside the crucible. The lifting shaft 2 is located at the bottom of the furnace body 1 and connected to the crucible body 5, and is used to drive the crucible to move up and down.

[0037] The heater, made of high-purity graphite, is located outside the crucible. Preferably, the heater is an annular heater, with an electrode 11 located below it. The heater includes a main heater 8 (500 mm high, 200 mm inner diameter), an upper heater 9 (200 mm high, 230 mm inner diameter), and a lower heater 10 (150 mm high, 230 mm inner diameter). The main heater is located outside the crucible body 5. The lower heater 10 is located between the crucible body 5 and the main heater 8, and overlaps the height of the main heater 8 by 50 mm. The upper heater 9 is located outside the main heater 8, and its top is bent and located between the main heater 8 and the crucible body 5, overlapping the height of the main heater 8 by 50 mm. This method can ensure that the adjustment gradient is between 5 and 15 °C / cm, i.e., the total effective heating height is 700 mm.

[0038] Preferably, the main heater 8, upper heater 9, and lower heater 10 are independently temperature-controlled. The main heater 8 melts the raw materials and cools them for crystallization, while the upper heater 9 and lower heater 10 regulate the temperature gradient at the crystallization site. The heater thickness is 10-15 mm, with a thickness tolerance of ±1 mm based on the resistivity of the graphite material. The distance between the outer wall of the heater and the inner wall of the insulation cylinder 3 is 10-15 mm, the heater height is 100 mm-600 mm, and the distance between the inner diameter of the heater and the outer diameter of the crucible is 10-15 mm.

[0039] The vacuum device is a JK200 vacuum pump unit, connected to the vacuum connection pipe of the furnace body 1 chassis. Preferably, a waste gas treatment device is installed between the vacuum device and the vacuum connection pipe. The vacuum device used in this application uses a combination of a rotary vane mechanical pump, an oil diffusion pump, and a molecular pump to create a vacuum. This combination pump has an ultimate vacuum better than 5×10-4 Pa and a fast pumping rate, which is a necessary condition to ensure that sodium iodide crystals are not oxidized and are not prone to impurity formation during the vacuum growth process. The waste gas treatment device with coil water cooling and an activated carbon filter is connected between the vacuum pump pipe and the furnace body 1 to prevent volatile impurities from entering the pump body and affecting the vacuum efficiency.

[0040] The gas filling device is a KF vacuum venting valve. The gas filling device is located at the top of the furnace body 1 and is used to fill the furnace with gas when the crystal is taken out. Because the crystal is in a vacuum during the growth process, the equipment cannot be opened under vacuum. Therefore, it is necessary to fill the furnace with gas to atmospheric pressure after the crystal growth is completed before the equipment can be opened.

[0041] The present invention also provides a method for growing sodium iodide crystals, comprising the following steps:

[0042] Step 1: Feeding: Thoroughly mix sodium iodide raw material (purity above 99.9%) powder and 0.01~0.05%wt polytetrafluoroethylene powder and load it into the crucible. Since sodium iodide crystals have a low melting point, low-melting-point polytetrafluoroethylene powder is used as an oxygen removal material to remove water vapor and oxygen. Polytetrafluoroethylene powder can decompose and volatilize at low temperature and will not remain in the growth system. Add 0.1~0.5wt% of sodium iodide thallium iodide to the bottom of the crucible, cover the crucible with the lid, and close the furnace.

[0043] Step 2: Drying the material, evacuate the crystal furnace to 5*10. -3 Pa, the temperature inside the crystal furnace is raised to 150℃ at a rate of 10℃ / h and kept constant for 20 hours. First, impurities such as water vapor and oxygen in the raw materials and growth furnace are discharged. After the vacuum degree is stabilized at better than 3*10-4Pa, the mechanical pump-molecular pump pipeline is switched. After the vacuum degree is stabilized at more than 4*10-4Pa, the impurities in the raw materials and growth furnace are discharged.

[0044] Step 3: Melting the material. The main heater 8 and the upper heater 9 are heated to 300°C at a rate of 20°C / hour, and the lower heater 10 is heated to 250°C at a rate of 20°C / hour. The temperature is maintained for more than 30 hours to remove the impurities remaining in the heat preservation cylinder 3. Then the main heater 8 is heated to 630°C at a rate of 10°C / hour, the upper heater 9 is heated to 670°C at a rate of 10°C / hour, and the lower heater 10 is heated to 610°C at a rate of 10°C / hour. At this time, the raw material temperature reaches the overmelting state, which is 10~20°C higher than the melting point. The raw material melts, and the doped thallium iodide is fully melted into the melt.

[0045] Step 4: Crystal growth. The initial temperature gradient at the bottom of the crucible is 10~15℃ / cm. Crystal growth begins. The crucible is lowered at a constant speed of 1~5mm / hr, with a lowering distance of more than 450mm. The lowering rate of the crucible is inversely proportional to the cross-sectional area of ​​the crystal. The lowering rate is calculated as follows: for a sodium iodide crystal with a diameter of φ100, the lowering rate of the crucible is 5mm / h. For every 10% increase in crystal diameter, the lowering rate of the crucible decreases by 5%. The crystallization process ends when the crucible is lowered to a distance of more than 450mm.

[0046] Step 5: Annealing. The upper heater 9, lower heater 10 and main heater 8 are cooled to 400℃, 450℃ and 480℃ respectively at the same time (cooling rate less than 2℃ / h and cooling time more than 100h). Then the crucible is raised to 20~100mm above the initial position at a rate of 0.5mm / h and held for 60 hours to start the original furnace annealing.

[0047] Step 6: Cooling down, the main heater 8 cools down to 450℃ at 5℃ / h, the upper heater 9 cools down to below 30℃ at 15℃ / h, and the lower heater 10 cools down to below 30℃ at 15℃ / h.

[0048] Step 7: Remove the crystal. First, turn off the vacuum device. After 2 hours, turn off the mechanical pump, open the vent valve to raise the gas pressure inside the furnace to balance the internal and external pressures, open the furnace chamber, and remove the sodium iodide crystal blank.

[0049] Sodium iodide crystal blanks, after being sliced ​​(forged) and packaged, can be used as optical components for applications such as high-energy physics detection.

Claims

1. A sodium iodide crystal growth apparatus, characterized in that: include A crystal furnace, comprising a furnace body and a lifting shaft, wherein the furnace body is made of double-layered stainless steel, and a coil cooling structure is provided between the inner and outer layers; the furnace body chassis is provided with a vacuum connection pipe; the furnace body contains a heat preservation device, a crucible, and a heater; the heat preservation device includes a heat preservation cylinder and an upper heat preservation cover and a lower heat preservation layer respectively disposed above and below the heat preservation cylinder; the crucible is disposed inside the heat preservation cylinder, and the crucible includes a crucible body and a crucible cover disposed above the crucible body; the interior of the crucible body is hollow and serves as a material hopper. The heater is located outside the crucible, and the lifting shaft is located at the bottom of the furnace body and connected to the crucible body. A vacuum device, wherein the vacuum device is connected to a vacuum connection pipe on the furnace chassis. An air-filling device is installed at the top of the furnace body.

2. The sodium iodide crystal growth apparatus according to claim 1, characterized in that: The heater is an annular heater, and an electrode is provided below the heater. The heater includes a main heater, an upper heater, and a lower heater. The main heater is located outside the crucible body. The lower heater is located between the crucible body and the main heater, and overlaps the height of the main heater by 50mm. The upper heater body is located outside the main heater, and the top of the upper heater is bent and located between the main heater and the crucible body, and overlaps the height of the main heater by 50mm.

3. The sodium iodide crystal growth apparatus according to claim 2, characterized in that: The main heater, upper heater, and lower heater are independently temperature controlled.

4. The sodium iodide crystal growth apparatus according to claim 3, characterized in that: The distance between the outer wall of the heater and the inner wall of the insulation cylinder is 10~15mm, and the distance between the inner diameter of the heater and the outer diameter of the crucible is 10~15mm.

5. The sodium iodide crystal growth apparatus according to claim 1, characterized in that: A waste gas treatment device is installed between the vacuum device and the vacuum connection pipe.

6. The sodium iodide crystal growth apparatus according to claim 1, characterized in that: The crucible lid is threadedly connected to the crucible cone.

7. The sodium iodide crystal growth apparatus according to claim 1, characterized in that: The lifting stroke of the lifting shaft is 300~600mm.

8. A method for growing sodium iodide crystals, characterized in that: Includes the following steps: Step 1: Feeding: After thoroughly mixing sodium iodide raw material powder and 0.01~0.05%wt polytetrafluoroethylene powder, load the mixture into the crucible. Add 0.1~0.5wt% thallium iodide by weight of sodium iodide to the bottom of the crucible, cover the crucible, and close the furnace. Step 2: Drying the material, evacuate the crystal furnace to 5*10. -3 Pa, the temperature inside the crystal furnace is increased to 150℃ at a rate of 10℃ / h, and kept constant for 20 hours until the vacuum degree stabilizes at 4*10-4Pa or higher, and then the raw materials and impurities in the growth furnace are discharged. Step 3: Melting the material. The main heater and the upper heater are heated to 300°C at a rate of 20°C / hour, and the lower heater is heated to 250°C at a rate of 20°C / hour. The temperature is maintained for more than 30 hours to remove the impurities remaining in the heat preservation cylinder. Then the main heater is heated to 630°C at a rate of 10°C / hour, the upper heater is heated to 670°C at a rate of 10°C / hour, and the lower heater is heated to 610°C at a rate of 10°C / hour. The raw material is melted, and the doped thallium iodide is fully melted into the melt. Step 4: Crystal growth. The initial temperature gradient at the bottom of the crucible is 10~15℃ / cm. Crystal growth begins. The crucible is lowered at a constant speed of 1~5mm / hr, with a drop distance of more than 450mm. Step 5: Annealing. The upper heater, lower heater and main heater are cooled to 400℃, 450℃ and 480℃ respectively at the same time. Then the crucible is raised to the initial position 20~100mm above the ground at a rate of 0.5mm / h and held for 60 hours to start the original furnace annealing. Step 6: Cooling down, the main heater cools down to 450℃ at 5℃ / h, the upper heater cools down to below 30℃ at 15℃ / h, and the lower heater cools down to below 30℃ at 15℃ / h. Step 7: Remove the crystal. First, turn off the vacuum device. After 2 hours, turn off the mechanical pump, open the vent valve to raise the gas pressure inside the furnace to balance the internal and external pressures, open the furnace chamber, and remove the sodium iodide crystal blank.

9. The method for growing sodium iodide crystals according to claim 8, characterized in that: In step 4, the crucible descent rate is inversely proportional to the crystal cross-sectional area. The descent rate is calculated as follows: for a sodium iodide crystal growth crucible with a diameter of φ100, the descent rate is 5 mm / h. For every 10% increase in crystal diameter, the crucible descent rate decreases by 5%.

10. The method for growing sodium iodide crystals according to claim 8, characterized in that: In step 5, the cooling rate of the upper heater, lower heater and main heater is less than 2℃ / h, and the cooling time is more than 100h.

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