Method for preparing thermocouple protection tube by depositing and pyrolyzing boron nitride through CVD (Chemical Vapor Deposition) method

Abstract

The invention discloses a method for preparing a thermocouple protection tube by depositing and pyrolyzing boron nitride through a CVD method. The method comprises the steps of preparation of a hot-pressing boron nitride tube blank, plasma treatment of the outer wall of the hot-pressing boron nitride tube blank, chemical deposition of the outer wall of the hot-pressing boron nitride tube blank and the like. The boron nitride-based thermocouple protection tube is prepared by depositing pyrolytic boron nitride on the outer wall of the hot-pressed boron nitride through a CVD method for the first time, the defects that pure hot-pressed boron nitride is poor in obdurability, difficult to prepare a slender tube, difficult to be compact and the like are overcome, and the defects that time is long and cost is high when pyrolytic boron nitride is used for preparing a thick-wall tube are also overcome; and ammonium chloride plasma surface treatment is combined with plasma air intake, so that the obtained thermocouple protection tube can be quickly inserted into a high-temperature metal solution for temperature measurement in an air environment of 1000 DEG C or below, can be used for a long time at a high temperature of 2200 DEG C in a vacuum or inert gas environment, and has high heat conductivity and high lubrication of hot-pressed boron nitride and high impact resistance of pyrolytic boron nitride.

Description

Technical Field

[0001] The invention relates to the technical field of new materials for thermocouple protection tubes, and in particular to a method for preparing a thermocouple protection tube by depositing pyrolytic boron nitride using a CVD method. Background Art

[0002] Thermocouples have become the most widely used temperature-sensing components in industrial production and scientific research. The properties of the thermocouple protective tube material affect various performance indicators, such as the long-term stability and service life of the thermocouple. This is especially true for thermocouples operating at high temperatures, which place stricter requirements on the tube material.

[0003] Existing thermocouple protective covers are mainly divided into three categories: metal, non-metal, and metal and non-metal composite materials. They need to meet basic performance requirements such as good airtightness (i.e., small porosity), sufficient mechanical strength, stable physical and chemical properties, and good thermal conductivity. In particular, thermocouples have extremely high requirements in the temperature measurement process of high-temperature melts and high-temperature flowing powders.

[0004] When measuring high-temperature melts with thermocouples, the thermocouple must be inserted into the melt. This means the thermocouple must withstand both the thermal shock of the melt and the physical and chemical corrosion of the medium, making it extremely susceptible to damage. This presents one of the most challenging problems in temperature measurement and a key area of ​​research for scientists and engineers.

[0005] For example, in the process of measuring the temperature of molten salts, low-melting-point alloys, molten glass, ceramic precursor melts, or some special industrial melts (such as molten sulfur and molten metal oxides) below 1000°C, a large number of new casing materials have been developed both domestically and internationally, such as ZrB2, Al2O3-Cr, Al2O3-Fe, ZrO2-Mo, MgO-Mo-Cr, and special metal ceramic tubes lined with corundum tubes, with certain results. However, these materials have not yet been widely promoted and applied due to their poor thermal shock resistance, less than ideal melt corrosion resistance, limited lifespan, and high price.

[0006] Based on the above needs and research difficulties, the inventors' team attempted to use boron nitride as the primary material for thermocouple protection sleeves. Boron nitride is categorized into hot-pressed and pyrolytic boron nitride based on its processing technology. Based on its crystal form, it can be divided into cubic boron nitride (c-BN) and wurtzite boron nitride (w-BN) with sp3 hybridized BN bonds, and hexagonal boron nitride (h-BN) and rhombohedral boron nitride (r-BN) with sp2 hybridized BN bonds. h-BN and c-BN are thermodynamically stable structures, while r-BN and w-BN are metastable structures.

[0007] Hot-pressed boron nitride (primarily h-BN) offers high thermal conductivity, high lubricity, chemical resistance, and high-temperature resistance, all at a relatively low cost, making it a highly suitable material for thermocouple protection tubes. However, it is relatively brittle, vulnerable to impact and friction, and exhibits some hygroscopic expansion. Pyrolytic boron nitride offers extremely high wear and impact resistance, but its relatively high cost makes it difficult to widely use as a primary material. Therefore, finding a cost-effective way to incorporate boron nitride into thermocouple protection tubes remains a significant challenge. Summary of the Invention

[0008] The purpose of the present invention is to overcome the shortcomings of the prior art and attempt to use the CVD method to deposit pyrolytic boron nitride on the outer wall of a hot-pressed boron nitride tube to make a thermocouple protection tube to adapt to the temperature measurement process of thermocouples in high-temperature melts or high-temperature flowing powders. The method of preparing a thermocouple protection tube by depositing pyrolytic boron nitride by CVD is proposed.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions: A method for preparing a thermocouple protection tube by depositing pyrolytic boron nitride using a CVD method comprises the following steps: S1. Preparation of hot pressed boron nitride tube: S101, pre-pressing molding: The boron nitride powder and additives are mixed and filled into a rubber mold, and pre-pressed at room temperature under an isostatic pressure of 100-150 MPa to form a green blank. The purpose of pre-pressing is to increase the initial density of the powder to facilitate subsequent hot pressing and sintering. S102, hot pressing sintering: Using a hot pressing sintering furnace, the sample is heated and pressurized at high temperature and high pressure under the protection of an inert gas (such as argon), and sintered in stages at 500-1900°C to prevent boron nitride from oxidizing at high temperatures. The mold is then removed from the hot pressing sintering furnace, cooled, and demolded to obtain a hot pressed boron nitride tube blank. S2. Plasma treatment of the outer wall of hot-pressed boron nitride tube: Ammonium chloride powder with a purity exceeding 99% is crushed into 1000-2000 mesh and applied to the outer wall of the hot-pressed boron nitride tube blank with a brush. The outer wall of the hot-pressed boron nitride tube blank is evenly sprayed with nitrogen plasma generated by an arc plasma generator. The ammonium chloride penetrates the outer wall of the hot-pressed boron nitride tube blank through the plasma reaction and cleans the easily detached parts, making the surface more dense and facilitating the deposition of pyrolytic boron nitride. The present invention only processes the outer wall of the tube blank, maintaining the low hardness and high lubricity of the inner wall, thereby preventing the precious metal electrode material of the thermocouple from collision damage during the loading process. This is also something that all existing protective tube materials except graphite cannot achieve, but graphite is too weak to withstand thermal shock and various impact damages.

[0010] S3. Chemical deposition on the outer wall of hot pressed boron nitride tube: Dual-source plasma chemical vapor deposition (DB-PECVD) method, the DB-PECVD deposition chamber consists of a plasma zone and a deposition zone. The plasma zone uses a BCl3-NH3 mixed gas; the generated plasma then bombards the substrate at high speed to deposit a cBN film. The temperature of the deposition zone is controlled at 30-1500℃, and the temperature is increased at a rate of 10℃ / min to obtain a thermocouple protection tube with pyrolytic boron nitride deposited on the outer wall of the hot-pressed boron nitride tube. The reaction process is analyzed by analyzing the gas components of the exhaust gas discharged from the deposition zone to obtain the temperature change curve of each gas component in the deposition zone (only the concentration of more than 10 is tested). -7 mol / L gas components), thereby monitoring the deposition reaction in real time; The reaction formula is: BCl3+NH3→H2+HC1+N2+c-BN; NH4C1 (plasma) → NH3 + HC1.

[0011] Preferably, in S101, the purity of the boron nitride powder exceeds 99%, the additive is a mixture of magnesium oxide and aluminum oxide in a weight ratio of 1:3, and the weight ratio of the additive to the boron nitride powder is 1-1.2:100.

[0012] Preferably, in S102, the mold of the hot pressing sintering furnace includes a graphite core rod and an arcuate graphite tube sleeve, the graphite core rod is placed on the inner wall of the blank, and 3-4 arcuate graphite tube sleeves surround the outer wall of the blank, and static pressing and sintering are performed to obtain a hot pressed boron nitride tube blank.

[0013] Preferably, the specific parameters of the staged sintering in S102 are as follows: Low temperature treatment: carried out at 500-800℃, with a holding time of 0.5-2h; Medium temperature pressure holding: carried out at 1000-1500℃, pressure of 15-20 MPa, and pressure holding time of 0.5-2h; High temperature pressure holding: carried out at 1600-1900℃, pressure of 16-25 MPa, and pressure holding time of 1-3h.

[0014] Preferably, the application amount of ammonium chloride powder in S2 is 1.2 g / cm 2 .

[0015] Preferably, the injection parameter of the nitrogen plasma in S2 is a plasma temperature of 60-70° C. at a high-purity nitrogen flow rate of 15 L / min.

[0016] Preferably, the total pressure of the BCl3-NH3 mixed gas in the S3 is 1000 Pa, of which N2 accounts for 90%, and the precursor inlet ratio is calculated as BC13:NH3:H2=1:3:6, that is, N2 is used as the carrier gas, BCl3-NH3 is evaporated outside the reaction chamber, and directly enters the plasma zone to form plasma to bombard the tube blank.

[0017] Preferably, the real-time monitoring process of the deposition reaction of each gas component versus temperature curve is as follows: In the early stage, when deposition begins, the BCl3 concentration in the tail gas decreases; In the middle stage, when the BCl3 concentration in the tail gas is lower than the detection limit, it indicates that the deposition rate reaches the highest; In the middle stage, when the HCl concentration in the tail gas is the lowest, it indicates that the decomposition of ammonium chloride is nearly complete; At the later stage, when BCl3 begins to appear in the tail gas and its concentration increases with the temperature, it indicates that the deposition rate decreases; When the concentrations of BCl3, H2, HCl and N2 in the exhaust gas are basically constant, it indicates that the deposition is completed.

[0018] For example, in a preferred embodiment of the present invention, in the early stage, due to the adsorption of a large amount of ammonium chloride on the outer wall, plasma bombardment continuously produces NH3 and HCl, and the reaction temperature does not meet the requirements, only a small amount of plasma reaction forms c-BN, and gradually deposits after 270°C. After 330°C, the remaining ammonium chloride on the outer wall surface is completely decomposed into NH3 and HCl, further inhibiting the reaction and appropriately reducing the deposition rate. At 450°C, the BCl3 concentration in the gas is lower than the detection limit and the HCl concentration is close to the minimum, indicating that the deposition rate reaches the highest and the decomposition of ammonium chloride on the surface is nearly complete. At 600°C, BCl3 begins to appear in the gas and its concentration increases with increasing temperature, indicating that the deposition rate decreases. At 1100°C, the concentrations of BCl3, H2, HCl and N2 are basically constant, indicating that the deposition is complete. The overall deposition time is 107 min, the deposition thickness is 0.56 μm, and the calculated deposition rate reaches 0.314 μm / h.

[0019] The present invention also proposes the application of the thermocouple protection tube prepared by the above method in the temperature measurement process in high-temperature melt or high-temperature flowing powder. For example, the melt temperature of the downstream special boron glass factory is basically 900±50℃. The existing commonly used protection tubes generally need to be replaced after 100h (domestic) or 200h (foreign) use, otherwise it will cause irreversible damage to the precious metal of the internal thermocouple. The service life of the present invention can reach more than 300h. The reason may be that c-BN will not be converted to h-BN until 1550℃, thereby preserving long-term friction resistance, high density, thermal shock resistance and impact resistance. Even when it is converted to h-BN at a higher temperature, its surface deposition layer is still denser than the internal tube blank, and still has a certain service life limit (the test shows that the service life at 1050℃ is 40-50h); and after process improvement, the cost is close to the product process of special metal ceramic tube lined with corundum tube produced by the Chinese Academy of Sciences.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention, for the first time, utilizes a CVD method to deposit pyrolytic boron nitride on the outer wall of hot-pressed boron nitride to produce a boron nitride-based thermocouple protection tube. This tube combines the high thermal conductivity and lubricity of hot-pressed boron nitride (h-BN) with the high impact resistance of pyrolytic boron nitride (c-BN), overcoming the shortcomings of simple hot-pressed boron nitride, such as poor toughness, as well as the high hardness and high cost of pyrolytic boron nitride (c-BN) on thermocouple electrodes. The resulting thermocouple protection tube can be used for long periods of time in high-temperature environments.

[0021] 2. The present invention uses ammonium chloride plasma surface treatment before CVD, and combines it with plasma intake during the CVD process to significantly improve the adhesion between the pyrolytic boron nitride wear-resistant layer and the hot-pressed boron nitride tube blank, as well as the impact resistance of the outer wall surface. Ammonium chloride plasma treatment can clean the easily detached parts of the outer wall surface, making the surface more dense, thereby enhancing the deposition quality of the pyrolytic boron nitride; plasma bombardment during the CVD process can appropriately increase the small amount of deposition at low temperatures in the early stage, while laser ammonium chloride decomposition suppresses the mid-term deposition rate, thereby maintaining a moderately uniform rate during the deposition process, further improving the surface bonding and density, and reducing the risk of shedding in high-temperature environments.

[0022] 3. The hot-pressed boron nitride tube of the present invention has a low thermal expansion coefficient and excellent thermal shock resistance. The surface density is increased by depositing pyrolytic boron nitride to overcome the defects of hygroscopicity and low impact strength, so that the protective tube can still maintain good mechanical properties and impact resistance in a high temperature environment (such as 900±50℃).

[0023] 4. The present invention improves the balance of deposition rate through material optimization, combining plasma treatment and CVD deposition process, and uses real-time monitoring of deposition rate by exhaust gas concentration. The resulting new boron nitride-based thermocouple protection tube still maintains excellent thermal conductivity and impact resistance in high-temperature environments, and its performance and service life are superior to other ceramic materials. DETAILED DESCRIPTION

[0024] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0025] 1. Preparation of hot pressed boron nitride tube: 1) Pre-pressing: The boron nitride powder and the additive are mixed and filled into a graphite mold, wherein the purity of the boron nitride powder exceeds 99%, the additive is a mixture of magnesium oxide and aluminum oxide in a weight ratio of 1:3, and the weight ratio of the additive to the boron nitride powder is 1-1.2:100. The mold is pre-pressed at an isostatic pressure of 100-150 MPa and room temperature to form a green blank. The purpose of the pre-pressing is to increase the initial density of the powder to facilitate subsequent hot pressing and sintering. 2) Hot pressing sintering: A hot pressing sintering furnace is used to heat and pressurize the sample at high temperature and high pressure under the protection of an inert gas (such as argon). The mold of the hot pressing sintering furnace includes a graphite core rod and a curved graphite tube sleeve. The graphite core rod is placed on the inner wall of the blank, and 3-4 curved graphite tube sleeves surround the outer wall of the blank. Segmented sintering is performed at 500-1900℃. The specific parameters of the segmented sintering are as follows: Low temperature treatment: carried out at 500-800℃, with a holding time of 0.5-2h; Medium temperature pressure holding: carried out at 1000-1500℃, pressure of 15-20 MPa, and pressure holding time of 0.5-2h; High temperature pressure holding: carried out at 1600-1900℃, pressure of 16-25 MPa, and pressure holding time of 1-3h; The hot-pressed boron nitride tube blank is obtained by static pressing sintering to prevent the boron nitride from oxidizing at high temperature. The mold is then taken out of the hot-pressing sintering furnace and demoulded after cooling to obtain the hot-pressed boron nitride tube blank.

[0026] The specific preparation example formula is shown in Table 1 below, and low-temperature (25°C) strength and high-temperature (1200°C) strength tests were conducted. Since the present invention is original, reference was made to the ISO 6872 standard, which is applicable to the bending strength test of ceramic materials. The three-point bending method or the four-point bending method is used to measure the material's ability to resist damage under bending load.

[0027] Table 1. Formulation and strength of hot-pressed boron nitride tube blanks:

[0028] It can be seen from Table 1 that within the appropriate range, the greater the additive content, the higher the low-temperature bending strength of the tube billet, and the lower the additive content, the higher the high-temperature bending strength of the tube billet; when the additive content is too low, the strength is low, and when the additive content is too high, the low-temperature bending strength is high and the high-temperature bending strength is low; and the decrease in the aluminum content in the additive leads to a decrease in strength.

[0029] 2. Preparation of thermocouple protection tube: Example 1:

[0030] A method for preparing a thermocouple protection tube by depositing pyrolytic boron nitride using a CVD method comprises the following steps: S1. Plasma treatment of the outer wall of hot pressed boron nitride tube: Ammonium chloride powder with a purity exceeding 99% was crushed into 1000-2000 mesh and applied to the outer wall of the hot-pressed boron nitride tube obtained in Preparation Example 1 by a brush. The outer wall of the hot-pressed boron nitride tube was uniformly sprayed with nitrogen plasma generated by an arc plasma generator. The ammonium chloride penetrated the outer wall of the hot-pressed boron nitride tube through the plasma reaction and cleaned the easily detached parts, making the surface more dense and facilitating the deposition of pyrolytic boron nitride. The application amount of ammonium chloride powder was 1.2 g / cm 2 ; The injection parameters of nitrogen plasma are high-purity nitrogen flow rate of 15 L / min and the plasma temperature is 60-70℃.

[0031] S2. Chemical deposition on the outer wall of hot pressed boron nitride tube: Dual-source plasma chemical vapor deposition (DB-PECVD): The deposition chamber of DB-PECVD consists of a plasma zone and a deposition zone. The plasma zone uses a BCl3-NH3 mixed gas. The total pressure of the BCl3-NH3 mixed gas is 1000 Pa, of which N2 accounts for 90%. The precursor inlet ratio is calculated as BCl3:NH3:H2=1:3:6. That is, N2 is used as the carrier gas to evaporate BCl3-NH3 outside the reaction chamber. N2 is used as the carrier gas to evaporate BCl3-NH3 outside the reaction chamber and directly enters the plasma zone to form plasma to bombard the tube blank. Then the generated plasma bombards the substrate at high speed to deposit the cBN film. The temperature of the deposition zone is controlled at 30-1500℃, and the temperature is increased at a rate of 10℃ / min to obtain a thermocouple protection tube with pyrolytic boron nitride deposited on the outer wall of the hot-pressed boron nitride tube. The reaction process is analyzed by analyzing the gas components of the exhaust gas discharged from the deposition zone to obtain the curve of the gas components in the deposition zone with temperature change (only the concentration of more than 10 is tested). -7 mol / L gas components), thereby monitoring the deposition reaction in real time; The reaction formula is: BCl3+NH3→H2+HC1+N2+c-BN; NH4C1 (plasma) → NH3 + HC1; In the early stage, due to the large amount of ammonium chloride adsorbed on the outer wall, the plasma continuously bombarded to produce NH3 and HCl, and the reaction temperature did not reach the requirement, so only a small amount of plasma reaction formed c-BN. Gradual deposition began after 270°C, and after 330°C, the remaining ammonium chloride on the outer wall surface was completely decomposed into NH3 and HCl, further inhibiting the reaction and appropriately reducing the deposition rate. At 450°C, the BCl3 concentration in the gas was below the detection limit and the HCl concentration was close to the minimum, indicating that the deposition rate reached the highest and the decomposition of surface ammonium chloride was almost complete; at 600°C, BCl3 began to appear in the gas and its concentration increased with increasing temperature, indicating that the deposition rate decreased; at 1100°C, the concentrations of BCl3, H2, HCl and N2 were basically constant, indicating that the deposition was complete. The overall deposition time was 107 min, the deposition thickness was 0.56 μm, and the calculated deposition rate reached 0.314 μm / h. Example 2:

[0032] It is basically the same as Example 1, except that the hot-pressed boron nitride tube obtained in Preparation Example 2 is used. Example 3:

[0033] It is basically the same as Example 1, except that the hot-pressed boron nitride tube obtained in Preparation Example 3 is used.

[0034] Comparative Example 1: The method is basically the same as Example 2, except that the hot-pressed boron nitride tube blank is not subjected to plasma treatment on the outer wall. That is, the hot-pressed boron nitride tube blank obtained in Preparation Example 2 is directly subjected to dual-source plasma chemical vapor deposition (DB-PECVD): the deposition chamber of DB-PECVD consists of a plasma zone and a deposition zone, and the plasma zone uses a BCl3-NH3 mixed gas; then, the generated plasma bombards the substrate at high speed to deposit a cBN film, and the temperature of the deposition zone is controlled to be 30-1500°C, and the temperature is increased at a heating rate of 10°C / min to obtain a thermocouple protection tube with pyrolytic boron nitride deposited on the outer wall surface of the hot-pressed boron nitride tube blank. The reaction process is carried out by analyzing the gas components of the exhaust gas discharged from the deposition zone to obtain a temperature change curve of each gas component in the deposition zone (only the concentration of more than 10 is tested). -7 mol / L gas components), thereby monitoring the deposition reaction in real time; The deposition conditions are as follows: in the early stage, due to the reaction temperature not meeting the requirements, only a small amount of plasma reaction formed c-BN. After 270°C, large-scale deposition began. At 400°C, the BCl3 concentration in the gas was lower than the detection limit and the HCl concentration was close to the minimum, indicating that the deposition rate reached the highest. At 550°C, BCl3 began to appear in the gas and its concentration increased with increasing temperature, indicating that the deposition rate decreased. At 900°C, the concentrations of BCl3, H2, HCl and N2 were basically constant, indicating that the deposition was completed. The overall deposition time was 107 minutes, the deposition thickness was 0.52 μm, and the calculated deposition rate reached 0.359 μm / h.

[0035] Comparative Example 2: The method is basically the same as Example 2, except that the dual-source plasma chemical vapor deposition method is not used, that is, a single reaction chamber is used for the hot-pressed boron nitride tube blank with plasma treatment on the outer wall of Example 2, and the dual-source plasma chemical vapor deposition method is not used. The total inlet pressure is 1000 Pa, the precursor inlet ratio is calculated as BC13:NH3:H2=1:3:6, and N2 accounts for 90%; the temperature of the deposition zone is controlled to be 30-1500°C, and the temperature reaction is carried out at a heating rate of 10°C / min to deposit the cBN film. The temperature change curves of each gas component in the deposition zone are detected (only the concentration of more than 10 is tested). -7 mol / L of gas components); The deposition situation is as follows: in the early stage, due to the adsorption of more ammonium chloride on the outer wall, but no NH3 and HC1 were produced, and the reaction temperature did not meet the requirements, no c-BN deposition occurred. After 270°C, deposition began gradually, and after 330°C, the ammonium chloride on the outer wall surface decomposed into NH3 and HC1, inhibiting the reaction and appropriately reducing the deposition rate. At 550°C, the BCl3 concentration in the gas was lower than the detection limit and the HC1 concentration was close to the lowest, indicating that the deposition rate reached the highest and the decomposition of ammonium chloride on the surface was almost complete; at 700°C, BCl3 began to appear in the gas and its concentration increased with increasing temperature, indicating that the deposition rate decreased; at 1300°C, the concentrations of BCl3, H2, HC1 and N2 were basically constant, indicating that the deposition was complete. The overall deposition time was 127 min, the deposition thickness was 0.63 μm, and the calculated deposition rate reached 0.298 μm / h. It can be seen from this that plasma bombardment can appropriately increase the small amount of deposition at the early low temperature (<270℃), while causing a large amount of ammonium chloride to be buried inside the middle-term (270-700℃) deposits. The rapid decomposition of ammonium chloride leads to a significant decrease in the density of the deposits, which in turn leads to a significant decrease in the bonding strength of pyrolytic boron nitride on the tube wall, and there is a risk of falling off during use.

[0036] Comparative Example 3: The method is basically the same as Example 2, except that the hot-pressed boron nitride tube blank is not subjected to outer wall plasma treatment, and the dual-source plasma chemical vapor deposition method is not used. That is, the hot-pressed boron nitride tube blank obtained in Preparation Example 2 is directly subjected to single-chamber deposition, and the dual-source plasma chemical vapor deposition method is not used. The total inlet pressure is 1000 Pa, the precursor inlet ratio is calculated to be BC13:NH3:H2=1:3:6, and N2 accounts for 90%; the temperature of the deposition zone is controlled to be 30-1500°C, the temperature is increased at a heating rate of 10°C / min, the cBN film is deposited, and the temperature change curve of each gas component in the deposition zone is detected; The deposition results are as follows: Initially, no c-BN deposition occurred due to the reaction temperature not meeting the required conditions. After 270°C, significant deposition began. At 450°C, the BCl3 concentration in the gas fell below the detection limit, and the HCl concentration reached its highest point, indicating the highest deposition rate. At 650°C, BCl3 began to appear in the gas, and its concentration increased with increasing temperature, indicating a decrease in the deposition rate. By 1500°C, the concentrations of BCl3, H2, HCl, and N2 remained essentially constant, indicating completion of deposition. The total deposition time was 147 minutes, the deposited thickness was 0.53 μm, and the calculated deposition rate reached 0.216 μm / h. This indicates that the absence of ammonium chloride adsorption increases the deposition rate in the intermediate stage, while the absence of plasma bombardment significantly reduces the deposition rate in the later stages. However, the bonding strength of pyrolytic boron nitride to the tube wall remains low, indicating the presence of a distinct phase interface. This indirectly demonstrates that ammonium chloride adsorption after plasma treatment can reduce the phase interface, likely due to the dual effects of tube surface cleanliness and surface activity, resulting in a significant risk of shedding during use.

[0037] Comparative Example 4: The hot-pressed boron nitride tube blank of Preparation Example 2 was directly used as the thermocouple protection tube.

[0038] 3. Application of thermocouple protection tubes in temperature measurement of high temperature melts or high temperature flowing powders: Specifically, it is used for environmental measurement in a special boron glass factory where the melt temperature is 900±50℃. The existing commonly used protective tubes generally need to be replaced after 100h (domestic) or 200h (abroad) of use, otherwise it will cause irreversible damage to the precious metal of the internal thermocouple. The service life of the present invention can reach more than 300h. The reason may be that c-BN will not be converted into h-BN until 1550℃, thereby preserving the long-term properties of friction resistance, high density, thermal shock resistance and impact resistance (only explained by the use time and compression strength). Even when it is converted into h-BN at a higher temperature, its surface deposition layer is still denser than the internal tube blank, and still has a certain service life (the test shows that the service life is 40-50h at 1050℃); and after process improvement, the cost is close to the product process of special metal ceramic tube lined with corundum tube produced by the Chinese Academy of Sciences.

[0039] Thermal conductivity: According to ASTM-D5470, the thermal conductivity of the inner and outer wall surfaces of the protective tube is measured using the steady-state heat flow method (taking the average value of the radial and axial directions); Impact resistance: Refer to GB / T 6408-2018: This standard specifies the compressive strength of cubic boron nitride (CBN) in detail. A graphite core rod is filled into the inner wall of a protective tube. Compressive strength is measured by applying pressure vertically from the outer wall to the inner wall of the tube. The 900°C test involves inserting the protective tube into a melt at 900°C for 2-3 seconds, removing it, and cooling it to measure the strength.

[0040] The details are shown in Table 2 below: Table 2. Properties of Thermocouple Protection Tubes

[0041] As can be seen from Table 2, the combination of ammonium chloride plasma treatment and plasma treatment during the reaction process can greatly improve the adhesion of the pyrolytic boron nitride wear-resistant layer, while improving the impact resistance of the outer wall surface and slightly affecting the thermal conductivity of the outer wall. That is, the present invention improves the surface impact resistance by improving the chemical deposition pretreatment process, especially increases the impact resistance of the protective tube in a high temperature environment.

[0042] In addition, according to some application tests, the present invention adopts the CVD method for the first time to deposit pyrolytic boron nitride on the outer wall of hot-pressed boron nitride to produce a boron nitride-based thermocouple protection tube. The boron nitride-based thermocouple protection tube combines the high thermal conductivity and high lubricity of hot-pressed boron nitride with the high impact resistance of pyrolytic boron nitride. It overcomes the shortcomings of simple hot-pressed boron nitride, such as poor strength and toughness, difficulty in preparing slender tubes, and difficulty in achieving 100% density. It also overcomes the shortcomings of pyrolytic boron nitride, such as the long time and high cost of preparing thick-walled pipe fittings. The obtained thermocouple protection tube can be controlled to have an overall thickness of 1-5 mm, and the outer pyrolytic boron nitride layer has a thickness of 0.3-1 mm. It can be quickly inserted into a high-temperature metal melt for temperature measurement below 1000°C in an air environment. After the melt is withdrawn, no residual metal will adhere to the surface. It can be used for a long time at high temperatures of 2200°C in a vacuum or inert gas environment. The present invention uses ammonium chloride plasma surface treatment combined with plasma air intake to significantly improve the density of the pyrolytic boron nitride wear-resistant layer, its adhesion to the hot-pressed boron nitride tube blank, and its impact resistance. Its performance and service life are superior to other ceramic materials.

[0043] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for preparing a thermocouple protection tube by depositing pyrolytic boron nitride by CVD, characterized in that: The following steps are involved: S1. Preparation of hot pressed boron nitride tube: S101, pre-pressing molding: The boron nitride powder and additives are mixed and filled into a rubber mold, and pre-pressed at room temperature under an isostatic pressure of 100-150 MPa to form a green blank. The purpose of pre-pressing is to increase the initial density of the powder to facilitate subsequent hot pressing and sintering. S102, hot pressing sintering: Using a hot pressing sintering furnace, the sample is heated and pressurized at high temperature and high pressure under inert gas protection, and sintered at 1900-2000°C. The mold is then removed from the hot pressing sintering furnace, cooled, and demolded to obtain a hot pressed boron nitride blank. The hot pressed boron nitride tube is then obtained by machining. S2. Plasma treatment of the outer wall of hot-pressed boron nitride tube: Ammonium chloride powder with a purity of more than 99% is crushed into 1000-2000 mesh, and applied to the outer wall of the hot-pressed boron nitride tube blank with a brush. The outer wall of the hot-pressed boron nitride tube blank is evenly sprayed with nitrogen plasma generated by an arc plasma generator; S3. Chemical deposition on the outer wall of hot pressed boron nitride tube: The dual-source plasma chemical vapor deposition method is adopted. The DB-PECVD deposition chamber consists of a plasma zone and a deposition zone. The plasma zone uses a BCl3-NH3 mixed gas. The generated plasma then bombards the substrate at high speed to deposit a PBN film. The temperature of the deposition zone is controlled at 1800-2000℃, and the deposition time is 5-40 hours. A thermocouple protection tube with pyrolytic boron nitride uniformly deposited on the outer wall surface of the hot-pressed boron nitride tube is obtained. The reaction process is carried out by analyzing the gas components of the exhaust discharged from the deposition zone to obtain the temperature change curve of each gas component in the deposition zone, and the deposition reaction is monitored in real time.

2. The method for preparing a thermocouple protection tube by depositing pyrolytic boron nitride using a CVD method according to claim 1, characterized in that: In S101, the purity of the boron nitride powder exceeds 99%, the additive is a mixture of magnesium oxide and aluminum oxide in a weight ratio of 1:3, and the weight ratio of the additive to the boron nitride powder is 1-1.2:

100.

3. The method for preparing a thermocouple protection tube by depositing pyrolytic boron nitride using a CVD method according to claim 1, characterized in that: In S102, the mold of the hot pressing sintering furnace includes a graphite core rod and an arc-shaped graphite tube sleeve. The graphite core rod is placed on the inner wall of the blank, and 3-4 arc-shaped graphite tube sleeves surround the outer wall of the blank. Static pressure sintering is performed to obtain a hot pressed boron nitride tube blank.

4. The method for preparing a thermocouple protection tube by depositing pyrolytic boron nitride using a CVD method according to claim 1, characterized in that: The specific parameters of the staged sintering in S102 are as follows: Low temperature treatment: carried out at 500-800℃, with a holding time of 0.5-2h; Medium temperature pressure holding: carried out at 1000-1500℃, pressure of 15-20 MPa, and pressure holding time of 0.5-2h; High temperature pressure holding: carried out at 1600-1900℃, pressure of 16-25 MPa, and pressure holding time of 1-3h.

5. The method for preparing a thermocouple protection tube by depositing pyrolytic boron nitride using a CVD method according to claim 1, characterized in that: The amount of ammonium chloride powder applied in S2 is 1.2 g / cm 2 .

6. The method for preparing a thermocouple protection tube by depositing pyrolytic boron nitride using a CVD method according to claim 1, characterized in that: The injection parameters of the nitrogen plasma in S2 are as follows: the plasma temperature is 60-70° C. at a high-purity nitrogen flow rate of 15 L / min.

7. The method for preparing a thermocouple protection tube by depositing pyrolytic boron nitride using a CVD method according to claim 1, characterized in that: The total pressure of the BCl3-NH3 mixed gas in the S3 is 1000 Pa, of which N2 accounts for 90%, and the precursor inlet ratio is BC13:NH3:H2=1:3:

6. BCl3-NH3 evaporates outside the reaction chamber and directly enters the plasma zone to form plasma to bombard the tube blank.

8. The method for preparing a thermocouple protection tube by depositing pyrolytic boron nitride using a CVD method according to claim 1, characterized in that: The real-time monitoring process of the deposition reaction of each gas component with temperature change curve in S3 is as follows: In the early stage, when deposition begins, the BCl3 concentration in the tail gas decreases; In the middle stage, when the BCl3 concentration in the tail gas is lower than the detection limit, it indicates that the deposition rate reaches the highest; In the middle stage, when the HCl concentration in the tail gas is the lowest, it indicates that the decomposition of ammonium chloride is nearly complete; At the later stage, when BCl3 begins to appear in the tail gas and its concentration increases with the temperature, it indicates that the deposition rate decreases; When the concentrations of BCl3, H2, HCl and N2 in the exhaust gas are constant, it indicates that the deposition is complete.

9. Use of the thermocouple protection tube prepared by the method according to any one of claims 1 to 8 in the temperature measurement process of a high-temperature melt or a high-temperature flowing powder.

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