Pyrolytic boron nitride insulation protection tube for high-temperature thermocouple and manufacturing method thereof

Abstract

The invention relates to the technical field of insulating protection tubes for high-temperature thermocouples, in particular to a pyrolytic boron nitride insulating protection tube for a high-temperature thermocouple and a manufacturing method of the pyrolytic boron nitride insulating protection tube. The insulation protection tube comprises more than two conical connecting tubes, the conical connecting tubes are arranged to be identical in taper and cone diameter, the conical connecting tubes are coaxially connected in a conical mode end to end according to the sequence that the conical heads are small and the conical tails are large and are tightly combined into a whole, the conical head of the conical connecting tube at the front end seals the opening of the conical tail, and the conical heads and the conical tails of the other conical connecting tubes are arranged to be openings. Insulation protection pipes with different lengths can be obtained through combination by adjusting the number, taper, length, wall thickness, cone diameter and other parameters of the conical connecting pipes, stable connection between the connecting pipes can be achieved based on conical surface sealing in a conical connection mode, and rapid disassembly, assembly and replacement are also facilitated. The method has the advantages of simple and feasible implementation steps, short process flow, low production cost and continuous production.

Description

Technical Field

[0001] This invention relates to the technical field of insulating protective tubes for high-temperature thermocouples, specifically to a pyrolytic boron nitride insulating protective tube for high-temperature thermocouples and its manufacturing method, which is particularly suitable for long-scale temperature measurement environments in ultra-high temperature environments of 2000-2800℃. Background Technology

[0002] With the rapid development of science and technology and national defense industry in my country, various scientific and technological departments and cutting-edge industrial fields require the measurement of higher temperatures (2000-2800℃), which places higher demands on corresponding high-temperature resistant insulating materials. Pyrolytic boron nitride (PBN), prepared by chemical vapor deposition, has emerged as one of the best choices for insulating protective tubes of thermocouples due to its excellent properties such as high heat resistance, high thermal conductivity, good high-temperature insulation, and strong chemical stability.

[0003] Chinese invention patent CN1031901C, entitled "A Manufacturing Method for a High-Temperature Thermocouple Insulating Tube," discloses three types of thermocouple insulating protection tubes based on pyrolytic boron nitride. The first type is a pyrolytic boron nitride sheath with a thickness of approximately 1–1.2 mm; the second type is a carbon / carbon composite material sheath with a pyrolytic boron nitride coating; and the third type is a high-strength graphite sheath with a pyrolytic boron nitride coating. These designs have significantly promoted the development and application of, especially, ultra-high temperature tungsten-rhenium metal thermocouples. Considering factors such as purity, overall insulation, airtightness, resistance to electrical interference, and chemical inertness to the metal thermocouple wire, the first type is undoubtedly the most superior and highly favored. In some important temperature measurement scenarios, it can completely replace protective tubes made of high-temperature alloys, alumina, beryllium oxide, hafnium oxide, and other materials. Patent CN219956730U discloses a thermocouple with high-temperature protection, focusing primarily on improving the thermocouple's heat dissipation performance and the high-temperature resistance of the protective tube. Heat dissipation is achieved through structures such as heat dissipation tube covers and heat-conducting columns, but its high-temperature insulation performance is somewhat lacking. Furthermore, its overall structure is relatively complex, increasing manufacturing costs and process difficulty. Patent CN215524850U discloses a coated graphite thermocouple protective tube, using multiple graphite tubes connected by a stop joint and combined with a pyrolytic boron nitride coating to extend the length of the thermocouple protective tube. However, the stability and airtightness of its connection method are relatively poor. Additionally, the size and length range of the graphite tubes are relatively small, making it difficult to meet the needs of some large-scale engineering temperature measurement scenarios requiring longer protective tubes. Patent CN1924532A discloses a thermocouple composite protective sleeve and its manufacturing method. The thermocouple composite protective sleeve mainly involves coating the base tube to improve its corrosion resistance, thermal shock resistance, and mechanical strength. However, since the base tube is made of metal, its overall high-temperature resistance is limited, making it difficult to meet the requirements for higher temperature measurements. Furthermore, its coating structure is relatively complex, including a metal bonding underlayer, an intermediate transition layer, and a surface working layer, resulting in a relatively cumbersome manufacturing process and high cost.

[0004] The current problems are as follows: On the one hand, due to limitations such as the size of the reaction zone in the chemical vapor deposition furnace and the deposition reaction mechanism, the actual length of the pyrolytic boron nitride insulating protection tubes produced is usually no more than 400 mm. The longer the length, the more difficult the manufacturing process and the higher the cost. However, the length of engineering temperature measuring thermocouples is increasingly exceeding 400 mm, and even reaching 2000 mm. This makes it impossible for the pyrolytic boron nitride insulating protection tubes manufactured by existing technologies to be widely used in the field of long-size thermocouple protection tubes. On the other hand, if high-temperature adhesive bonding, welding and other methods are used to shorten the length, not only is the connection process complex and prone to weak connection areas, resulting in high equipment investment costs, but it is also difficult to guarantee a stable connection over a long period of time and high airtight protection for the thermocouple. The excellent high-temperature resistance, insulation and high airtightness properties of pyrolytic boron nitride are difficult to fully utilize. Summary of the Invention

[0005] To address the problems existing in the background art, the purpose of this invention is to provide a pyrolytic boron nitride insulating protective tube for high-temperature thermocouples and its manufacturing method, which has excellent properties such as thermal shock resistance, corrosion resistance, high-temperature electrical insulation, and airtightness, and is therefore suitable for the field of long-size high-temperature thermocouple protective tubes.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A pyrolytic boron nitride insulating protection tube for high-temperature thermocouples includes two or more tapered connecting tubes. The tapered connecting tubes are all configured with the same taper and taper diameter. The tapered connecting tubes are connected coaxially and tightly combined into one unit in the order of small taper head and large taper tail. The taper head of the front tapered connecting tube is closed and the taper tail is open. The taper head and taper tail of the other tapered connecting tubes are all configured with open taper heads and taper tails.

[0008] The aforementioned pyrolytic boron nitride insulating protective tube for high-temperature thermocouples has a tapered joint section formed at the insertion point between adjacent tapered connecting tubes, and a tapered surface seal is formed by tight contact interference fit of small-angle tapered surfaces.

[0009] The aforementioned pyrolytic boron nitride insulating protective tube for high-temperature thermocouples has a tapered connecting tube made of pyrolytic boron nitride prepared by chemical vapor deposition, with a density of 1.9–2.2 g / cm³. 3 .

[0010] The aforementioned pyrolytic boron nitride insulating protective tube for high-temperature thermocouples comprises 2 to 50 tapered connecting tubes, with a taper of 0° to 5°, a length of ≤400mm, and a wall thickness of 0.5 to 5mm.

[0011] The pyrolytic boron nitride insulating protective tube for high-temperature thermocouples, preferably, has a taper of 1° to 3° and a length of 50 to 300 mm.

[0012] The aforementioned pyrolytic boron nitride insulating protective tube for high-temperature thermocouples has a tapered connecting tube with an outer diameter of 5-50 mm, an inner diameter of 4-45 mm, an outer diameter of 7-65 mm, an inner diameter of 6-60 mm, and a tapered section length of 5-200 mm.

[0013] The aforementioned pyrolytic boron nitride insulating protective tube for high-temperature thermocouples has a tapered connecting tube with a circular, triangular, or rectangular cross-section.

[0014] A method for preparing a pyrolytic boron nitride insulating protective tube for high-temperature thermocouples includes the following steps:

[0015] Step 1: Preparation of tapered connecting tube

[0016] Based on the required size and length of the thermocouple protection tube, the quantity, inner and outer diameters, wall thickness, and tapered parameters of the required protection tubes are determined, and a tapered graphite mold is designed and processed. The graphite mold is placed in a chemical vapor deposition furnace, and pyrolytic boron nitride is deposited on the surface of the graphite mold by adjusting the chemical vapor deposition process parameters. The deposition process parameters are: deposition temperature of 1200-2100℃, furnace pressure of 100-101325Pa, nitrogen as dilution gas and carrier gas, boron trichloride as boron source, ammonia as nitrogen source, molar ratio of boron trichloride to ammonia of 1:1-1:10, and deposition time of 3-20 hours. After cooling to room temperature in the furnace, the pyrolytic boron nitride tube shell is obtained by depositing on the outer surface of the graphite mold. The machined pyrolytic boron nitride tapered connecting tubes with open ends and one closed end and one open end are oxidized and decarbonized at 600-800℃ for 2-10 hours, and then air-cooled to room temperature.

[0017] Step 2: Conical joint assembly

[0018] A pyrolytic boron nitride conical connecting tube with one closed end and one open end is placed at the front end, and the other pyrolytic boron nitride conical tubes with open ends are inserted into it in sequence and tightly. Finally, they are combined to obtain a pyrolytic boron nitride insulating protection tube for high-temperature thermocouples.

[0019] The method for preparing a pyrolytic boron nitride insulating protective tube for high-temperature thermocouples, wherein the thickness of the pyrolytic boron nitride tube shell is 1-6 mm.

[0020] The method for preparing a pyrolytic boron nitride insulating protective tube for high-temperature thermocouples involves, during machining, firstly grinding the outer surface of the pyrolytic boron nitride tube shell to achieve a set taper; after demolding, grinding the inner surface of the pyrolytic boron nitride ceramic shell to achieve a set taper; finally, except for one tube whose end cap is left uncut, the end caps of the remaining tubes are cut off, resulting in a pyrolytic boron nitride tapered tube with both ends open and one end closed and the other open.

[0021] The design concept of this invention is:

[0022] Existing high-temperature thermocouple insulating protective tubes are difficult to meet the performance requirements of long length, high airtightness, and high insulation. Furthermore, existing long-length protective tubes suffer from high manufacturing difficulty, high cost, and poor connection reliability. This invention employs multiple tapered connecting tubes with the same taper and diameter, assembled into a single unit through coaxial tapered joints. The tapered surface seal ensures a stable and airtight connection. Simultaneously, the number, taper, length, and wall thickness of the tapered connecting tubes can be flexibly adjusted according to actual needs to obtain the required length of insulating protective tube.

[0023] Each connecting pipe section of this invention is made of pure pyrolytic boron nitride (density 1.9–2.2 g / cm³). 3Prepared by chemical vapor deposition (CVD), it ensures high temperature resistance, insulation, and airtightness. Multiple tapered connecting tubes (2 to 50) with the same taper (0° to 5°) are coaxially tapered together in the order of "smaller taper head and larger taper tail". The self-tightening seal of the tapered surface eliminates the need for additional connecting materials. The length of a single tapered connecting tube is ≤400mm. The taper accuracy is ensured by machining. After combination, any length can be achieved (up to 2000mm or more). By coaxially tapering multiple tapered connecting tubes with the same taper, a long-length protective tube is formed, solving the length limitation problem. Furthermore, the tapered surface seal achieves a stable connection and high airtightness.

[0024] The advantages and beneficial effects of this invention are:

[0025] (1) This invention provides a pyrolytic boron nitride insulating protection tube for high-temperature thermocouples, comprising multiple tapered connecting tubes, which are connected coaxially and tightly in the order of small head and large tail to form a whole. By adjusting the number, taper, length, wall thickness, and taper diameter of the tapered connecting tubes, insulating protection tubes of different lengths can be obtained. Through a unique tapered connection method, a stable connection between the connecting tubes can be achieved based on the tapered surface seal, which is also convenient for quick disassembly and replacement.

[0026] (2) The present invention provides a method for manufacturing a pyrolytic boron nitride insulating protection tube for high temperature thermocouples. The implementation steps are simple and easy, the process flow is short, the production cost is low, and continuous production is possible.

[0027] (3) The present invention provides excellent properties such as good thermal shock resistance, corrosion resistance, high temperature electrical insulation and airtightness, effectively protects thermocouples and improves the accuracy and reliability of temperature measurement.

[0028] (4) The manufacturing method of the present invention is simple and easy to implement, with a short process flow and low cost. It can also be continuously produced, which improves production efficiency and economic benefits.

[0029] (5) The connection method of the present invention is stable, which facilitates quick disassembly and replacement, reducing maintenance costs and time.

[0030] (6) Compared with the Chinese invention patent with authorization announcement number CN1031901C, this invention uses multiple tapered connecting pipes connected coaxially end-to-end in the order of small cone head and large cone tail. Different lengths of insulating protective pipes can be obtained as needed, breaking through length limitations and enabling its widespread application in the field of long-size thermocouple protective pipes, better meeting practical engineering needs. This invention achieves a stable connection between connecting pipes through tapered surface sealing, resulting in high connection strength and good sealing performance. It also facilitates quick disassembly and replacement, making maintenance and use convenient. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of a pyrolytic boron nitride insulating protective tube for high-temperature thermocouples.

[0032] Figure 2 This is a schematic diagram of the tapered connecting pipe at the front end.

[0033] Figure 3 This is a schematic diagram of the remaining tapered connecting pipes.

[0034] Figure 4 This is a cross-sectional view of the tapered section of the tapered connecting pipe.

[0035] Reference numerals: 1. Front tapered connecting pipe, 2. Other tapered connecting pipes, 3. Tapered section of tapered connecting pipe. Detailed Implementation

[0036] The following description, in conjunction with the accompanying drawings and specific embodiments, will provide further details.

[0037] See Figures 1-4 This invention provides a pyrolytic boron nitride insulating protective tube for high-temperature thermocouples, comprising multiple tapered connecting tubes. All tapered connecting tubes are configured with the same taper and diameter. The tapered connecting tubes are coaxially tapered and tightly assembled into a single unit, with the tapered head of the front tapered connecting tube 1 sealing the opening of the tapered tail. The tapered heads and tails of the remaining tapered connecting tubes 2 are all open. The insertion points between adjacent tapered connecting tubes form tapered connecting tube sections 3. These sections 3 achieve a high-temperature seal and stable connection without adhesive through a precisely matched tapered surface geometry design. A tight interference fit between the small-angle tapered surfaces forms a tapered seal, ensuring that axial force is converted into radial compressive stress after insertion.

[0038] The tapered connecting tube is made of pyrolytic boron nitride prepared by chemical vapor deposition, with a density of 1.9–2.2 g / cm³. 3 The number of tapered connecting pipes is 2 to 50; the taper of the tapered connecting pipe (the angle between the axis of the tapered connecting pipe and the generatrix of the tapered surface) is between 0° and 5° (preferably 1° to 3°); the length of the tapered connecting pipe is ≤400mm (preferably 50 to 300mm); the wall thickness of the tapered connecting pipe is 0.5 to 5mm; the outer diameter of the tapered head of the tapered connecting pipe is 5 to 50mm, the inner diameter of the tapered head is 4 to 45mm, the outer diameter of the tapered tail is 7 to 65mm, the inner diameter of the tapered tail is 6 to 60mm, and the length of the tapered section of the tapered connecting pipe is 5 to 200mm (preferably 20 to 100mm); the cross-section of the tapered connecting pipe is circular, but can also be triangular, rectangular, etc.

[0039] This invention allows for the combination of different lengths of insulating protective tubes by adjusting parameters such as the number, taper, length, wall thickness, and taper diameter of the tapered connecting tubes. Furthermore, a stable connection between the connecting tubes can be achieved through a simple tapered connection method based on the tapered surface seal, which also facilitates quick disassembly and replacement.

[0040] The present invention will be further described in detail below through embodiments.

[0041] Example 1

[0042] In this embodiment, a method for manufacturing a pyrolytic boron nitride insulating protective tube for high-temperature thermocouples includes the following steps:

[0043] Step 1: Preparation of tapered connecting tube

[0044] Based on the required size and length of the thermocouple protection tube, the quantity, inner and outer diameters, wall thickness, taper, and other parameters of the required protection tubes are set, and a conical graphite mold is designed and processed. Three conical connecting tubes are produced, each 300mm long, with a taper of 1°, an outer diameter of 10mm at the cone tip, and a wall thickness of 1mm.

[0045] A graphite mold was placed in a chemical vapor deposition (CVD) furnace, and pyrolytic boron nitride was deposited on the surface of the mold by adjusting the CVD process parameters. The deposition parameters were: deposition temperature of 1800℃, furnace pressure of 500Pa, nitrogen as dilution gas and carrier gas, boron trichloride (BCl3) as boron source, and ammonia (NH3) as nitrogen source, with a BCl3:NH3 molar ratio of 1:4, and deposition time of 8 hours. After cooling to room temperature in the furnace, the pyrolytic boron nitride tube shell was deposited on the outer surface of the graphite mold, with a thickness of 1.2mm. Machining was then performed. First, the outer surface of the pyrolytic boron nitride tube shell was polished to achieve the set taper. After demolding, the inner surface of the pyrolytic boron nitride ceramic shell was polished to achieve the set taper. Finally, except for one tube end cap, the end caps of the remaining tubes were removed, resulting in a pyrolytic boron nitride tapered tube with both ends open and one end closed and one end open. The machined boron nitride tapered connecting pipe with both ends open and one end closed and the other open was oxidized and decarbonized at 700°C for 5 hours, then air-cooled to room temperature and cleaned.

[0046] Step 2: Conical joint assembly

[0047] A pyrolytic boron nitride conical connecting tube with one closed end and one open end is placed at the front end, and the other pyrolytic boron nitride conical tubes with open ends are inserted tightly into it in sequence. Finally, a long-length, highly airtight pyrolytic boron nitride insulating protection tube for high-temperature thermocouples is obtained.

[0048] In this embodiment, the performance indicators of the pyrolytic boron nitride insulating protection tube for high-temperature thermocouples are as follows: After the connecting tube is tapered and assembled, it can be made into a thermocouple insulating protection tube with a length of 530mm and an outer diameter of 10mm. The 185mm long tapered seal gives the protection tube excellent high-temperature insulation performance and airtightness, which can meet the long-size temperature measurement requirements of high-temperature thermocouples.

[0049] Example 2

[0050] In this embodiment, a method for manufacturing a pyrolytic boron nitride insulating protective tube for high-temperature thermocouples includes the following steps:

[0051] Step 1: Preparation of tapered connecting tube

[0052] Based on the required size and length of the thermocouple protection tubes, the quantity, inner and outer diameters, wall thickness, taper, and other parameters of the required protection tubes are set, and a conical graphite mold is designed and processed. Six conical connecting tubes are produced, each 200mm long, with a taper of 2°, an outer diameter of 20mm at the cone tip, and a wall thickness of 1.5mm.

[0053] A graphite mold was placed in a chemical vapor deposition (CVD) furnace, and pyrolytic boron nitride was deposited on the surface of the mold by adjusting the CVD process parameters. The deposition parameters were: deposition temperature of 1900℃, furnace pressure of 1000Pa, nitrogen as dilution gas and carrier gas, boron trichloride (BCl3) as boron source, and ammonia (NH3) as nitrogen source, with a BCl3:NH3 molar ratio of 1:6, and deposition time of 12 hours. After cooling to room temperature in the furnace, the pyrolytic boron nitride tube shell was deposited on the outer surface of the graphite mold, with a thickness of 1.8mm. Machining was then performed. First, the outer surface of the pyrolytic boron nitride tube shell was polished to achieve the set taper. After demolding, the inner surface of the pyrolytic boron nitride ceramic shell was polished to achieve the set taper. Finally, except for one tube end cap, the end caps of the remaining tubes were removed, resulting in a pyrolytic boron nitride tapered tube with both ends open and one end closed and one end open. The machined boron nitride tapered connecting pipe with both ends open and one end closed and the other open was oxidized and decarbonized by air burning at 750°C for 6 hours, then air-cooled to room temperature and cleaned.

[0054] Step 2: Conical joint assembly

[0055] A pyrolytic boron nitride conical connecting tube with one closed end and one open end is placed at the front end, and the other pyrolytic boron nitride conical tubes with open ends are inserted tightly into it in sequence. Finally, a long-length, highly airtight pyrolytic boron nitride insulating protection tube for high-temperature thermocouples is obtained.

[0056] In this embodiment, the performance indicators of the pyrolytic boron nitride insulating protection tube for high-temperature thermocouples are as follows: After the connecting tube is tapered and assembled, it can be made into a thermocouple insulating protection tube with a length of 629mm and an outer diameter of 20mm. The 114mm long tapered seal gives the protection tube excellent high-temperature insulation performance and airtightness, which can meet the long-size temperature measurement requirements of high-temperature thermocouples.

[0057] Example 3

[0058] In this embodiment, a method for manufacturing a pyrolytic boron nitride insulating protective tube for high-temperature thermocouples includes the following steps:

[0059] Step 1: Preparation of tapered connecting tube

[0060] Based on the required size and length of the thermocouple protection tubes, the quantity, inner and outer diameters, wall thickness, taper, and other parameters of the required protection tubes are set, and a conical graphite mold is designed and processed. Fifteen conical connecting tubes are produced, each 100mm long, with a taper of 3°, an outer diameter of 15mm at the cone tip, and a wall thickness of 2mm.

[0061] A graphite mold was placed in a chemical vapor deposition (CVD) furnace, and pyrolytic boron nitride was deposited on the surface of the mold by adjusting the CVD process parameters. The deposition parameters were: deposition temperature of 2000℃, furnace pressure of 200Pa, nitrogen as dilution gas and carrier gas, boron trichloride (BCl3) as boron source, and ammonia (NH3) as nitrogen source, with a BCl3:NH3 molar ratio of 1:8, and deposition time of 15 hours. After cooling to room temperature in the furnace, the pyrolytic boron nitride tube shell was deposited on the outer surface of the graphite mold, with a thickness of 2.2mm. Machining was then performed. First, the outer surface of the pyrolytic boron nitride tube shell was polished to achieve the set taper. After demolding, the inner surface of the pyrolytic boron nitride ceramic shell was polished to achieve the set taper. Finally, except for one tube end cap, the end caps of the remaining tubes were removed, resulting in a pyrolytic boron nitride tapered tube with both ends open and one end closed and one end open. The machined boron nitride tapered connecting pipe with both ends open and one end closed and the other open was oxidized and decarbonized by air burning at 800℃ for 8 hours, then air-cooled to room temperature and cleaned.

[0062] Step 2: Conical joint assembly

[0063] A pyrolytic boron nitride conical connecting tube with one closed end and one open end is placed at the front end, and the other pyrolytic boron nitride conical tubes with open ends are inserted tightly into it in sequence. Finally, a long-length, highly airtight pyrolytic boron nitride insulating protection tube for high-temperature thermocouples is obtained.

[0064] In this embodiment, the performance indicators of the pyrolytic boron nitride insulating protection tube for high-temperature thermocouples are as follows: After the connecting tube is tapered and assembled, it can be made into a thermocouple insulating protection tube with a length of 1170mm and an outer diameter of 15mm. The 24mm long tapered seal gives the protection tube excellent high-temperature insulation performance and airtightness.

[0065] The results show that the present invention achieves efficient manufacturing of long-length high-temperature thermocouple insulating protection tubes, and breaks through the limitations on the length of the protection tube imposed by factors such as the size of the reaction zone of the chemical vapor deposition furnace. By adjusting the number, taper, length, wall thickness, and taper diameter of the tapered connecting tubes, insulating protection tubes of different lengths can be obtained, thereby meeting the needs of engineering temperature measuring thermocouples with lengths exceeding 400mm or even up to 2000mm.

Claims

1. A pyrolytic boron nitride insulation protection tube for high temperature thermocouples, characterized in that, The application relates to a high-temperature thermocouple protection tube, which comprises two or more than two taper connecting pipes, the taper connecting pipes are arranged with the same taper and taper diameter, the taper connecting pipes are coaxially connected in sequence from the small taper head to the big taper tail and are tightly combined into an integrated body, the taper head of the front-end taper connecting pipe is closed and the taper tail is opened, and the taper head and the taper tail of the remaining taper connecting pipes are both arranged as openings.

2. A pyrolytic boron nitride insulation protection tube for high temperature thermocouple according to claim 1, characterized in that, The taper connecting pipe taper connection section is formed between the adjacent taper connecting pipes, and the taper surface sealing is formed through the small-angle taper surface tight contact interference fit.

3. The pyrolytic boron nitride insulation protection tube for high temperature thermocouple according to claim 1, characterized in that, The taper connecting pipe is made of pyrolytic boron nitride prepared by chemical vapor deposition, and has a density of 1.9-2.2 g / cm 3 .

4. The pyrolytic boron nitride insulation protection tube for high temperature thermocouple according to claim 1, characterized in that, The number of the taper connecting pipes is 2-50, the taper of the taper connecting pipes is 0-5 DEG, the length of the taper connecting pipes is less than or equal to 400 mm, and the wall thickness of the taper connecting pipes is 0.5-5 mm.

5. A pyrolytic boron nitride insulation protection tube for high temperature thermocouples according to claim 4, characterized in that, Preferably, the taper of the taper connecting pipes is 1-3 DEG, and the length of the taper connecting pipes is 50-300 mm.

6. A pyrolytic boron nitride insulation protection tube for high temperature thermocouple according to claim 4, characterized in that, The taper head outer diameter of the taper connecting pipes is 5-50 mm, the taper head inner diameter is 4-45 mm, the taper tail outer diameter is 7-65 mm, the taper tail inner diameter is 6-60 mm, and the length of the taper connecting pipe taper connection section is 5-200 mm.

7. A pyrolytic boron nitride insulation protection tube for high temperature thermocouple according to claim 4, characterized in that, The cross section of the taper connecting pipe is circular, triangular or rectangular.

8. A method for producing a pyrolytic boron nitride insulation protection tube for a high-temperature thermocouple according to one of claims 1 to 7, characterized in that The application further discloses a preparation method of the high-temperature thermocouple protection tube. According to the size and length of the thermocouple protection tube, the number, inner and outer diameters, wall thickness and taper parameters of the required protection tube are set, the taper graphite mould is designed and processed, the graphite mould is placed in a chemical vapor deposition furnace, pyrolytic boron nitride is deposited on the surface of the graphite mould by adjusting the chemical vapor deposition process parameters, the deposition process parameters are as follows: the deposition temperature is 1200-2100 DEG C, the furnace pressure is 100-101325 Pa, nitrogen is used as the dilution gas and carrier gas, boron trichloride is used as the boron source, ammonia is used as the nitrogen source, the molar ratio of the boron trichloride to the ammonia is 1:1-1:10, and the deposition time is 3-20 hours; the pyrolytic boron nitride tube shell is obtained by taking out the graphite mould after cooling to room temperature in the furnace; the two-end-opened and one-end-closed pyrolytic boron nitride taper connecting pipe is subjected to air calcination and carbon removal at 600-800 DEG C for 2-10 hours, and then air cooling to room temperature. The one-end-closed and one-end-opened pyrolytic boron nitride taper connecting pipe is arranged at the front end, the two-end-opened pyrolytic boron nitride taper connecting pipes are sequentially and tightly inserted into the one-end-closed and one-end-opened pyrolytic boron nitride taper connecting pipe, and finally the high-temperature thermocouple pyrolytic boron nitride insulation protection tube is obtained. The thickness of the pyrolytic boron nitride tube shell is 1-6 mm. During the machining, the outer surface of the pyrolytic boron nitride tube shell is first polished to reach the set taper, the inner surface of the pyrolytic boron nitride ceramic shell is polished to reach the set taper after demoulding, and finally the end caps of the pyrolytic boron nitride ceramic shells are all cut off except for one, so that the two-end-opened and one-end-closed pyrolytic boron nitride taper connecting pipe is obtained.

9. The method of claim 8, wherein the method further comprises the steps of: providing a graphite tube; and coating the graphite tube with the pyrolytic boron nitride. ​ 10. The method of claim 8, wherein the method further comprises the step of: ​ ​

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