Preparation method of high-temperature-resistant graphite pipe for spectral analysis

Abstract

The invention provides a preparation method of a high-temperature-resistant graphite pipe for spectral analysis, and belongs to the technical field of spectral analysis, and the preparation method comprises the following steps: S1, preparing a high-purity graphite pipe matrix and carrying out surface activation treatment; s2, preparing composite sol, wherein the composite sol is formed by mixing silica sol, molybdate and phytic acid; s3, infiltrating the composite sol into the graphite matrix by adopting a vacuum-pressurization alternate impregnation process; s4, carrying out stepped heating heat treatment in an inert environment to form a SiC-MoSi composite reinforced phase; and S5, depositing a pyrolytic carbon layer on the inner wall of the graphite pipe. The silica sol, the molybdate and the phytic acid have a synergistic effect to form a SiC-MoSi composite phase, the phytic acid can prevent the molybdate from gathering and can promote the growth of SiC whiskers in heat treatment, so that the anti-oxidation temperature of the graphite pipe is increased, the temperature resistance and the heat stability of the graphite pipe are improved, and the anti-oxidation performance of the graphite pipe is improved. And finally, the use frequency of the graphite pipe is greatly increased.

Description

Technical Field

[0001] The invention belongs to the technical field of spectral analysis, and in particular relates to a method for preparing a high-temperature resistant graphite tube for spectral analysis. Background Art

[0002] The graphite tube, the core component of an atomic absorption spectrometer, operates on the principle of atomic absorption spectroscopy. When light of a specific wavelength emitted by a light source (such as a hollow cathode lamp) passes through the atomic vapor in the graphite tube, the electrons in the atoms absorb the energy of the photons, transitioning from their ground state to an excited state. According to the Lambert-Beer law, the absorbance is proportional to the atomic concentration, allowing the concentration of elements in the sample to be determined by measuring the absorbance.

[0003] Graphite tubes used for spectral analysis have extremely high material performance requirements and are made of high-purity graphite. One of the core characteristics of graphite tubes is their high-temperature resistance. High-purity graphite can withstand extreme temperatures of up to 3000°C in an inert gas or vacuum environment. In practical applications, the high-temperature resistance of graphite tubes is affected by the following factors: Material purity: High-purity graphite (purity ≥99.9%) has higher thermal stability and oxidation resistance, and can maintain structural integrity at high temperatures. Environmental atmosphere: Graphite exhibits excellent high-temperature resistance in an inert gas (such as argon, nitrogen) or vacuum environment and can be used for a long time at 2500-3000°C.

[0004] However, the most common problems with existing graphite tubes are: 1. High-temperature oxidation: When temperatures exceed 600°C, the graphite tube reacts violently with oxygen, producing CO or CO2, leading to rapid tube wear. 2. Thermal shock damage: The instantaneous high temperature during the atomization phase (typically 2000-3000°C) and the rapid temperature fluctuations during the cooling phase cause microcracks in the graphite tube, leading to structural failure. Both of these phenomena shorten the lifespan of the graphite tube. Summary of the Invention

[0005] Based on this, the present invention provides a method for preparing a high-temperature resistant graphite tube for spectral analysis to solve the technical problem in the prior art that the service life of the graphite tube is shortened due to high-temperature oxidation and thermal shock damage.

[0006] The technical solution of the present invention to solve the above technical problems is as follows:

[0007] A method for preparing a high-temperature resistant graphite tube for spectral analysis, comprising:

[0008] S1: Preparation of high-purity graphite tube substrate and surface activation treatment;

[0009] S2: preparing a composite sol, wherein the composite sol comprises a mixture of silica sol, molybdate and phytic acid;

[0010] S3: infiltrating the composite sol into the graphite matrix using a vacuum-pressure alternating impregnation process;

[0011] S4: performing a stepwise temperature increase heat treatment in an inert environment to form a SiC-MoSi2 composite reinforcement phase;

[0012] S5: Depositing a pyrolytic carbon layer on the inner wall of the graphite tube.

[0013] Preferably, the composite sol comprises, by weight:

[0014] 40-60 parts of the silica sol,

[0015] 3-10 parts of the molybdate,

[0016] 2-8 parts of phytic acid,

[0017] 30-50 parts of alcohol solvent.

[0018] Preferably, the SiO2 content in the silica sol is ≥30wt%; the molybdate is ammonium molybdate ((NH4)2MoO4) or sodium molybdate (Na2MoO4), wherein the content of ammonium molybdate ((NH4)2MoO4) or sodium molybdate (Na2MoO4) is ≥99wt%; and the purity of the phytic acid is ≥50%.

[0019] Preferably, the composite sol further comprises 0.1-3 parts of graphene or graphene oxide.

[0020] Preferably, the vacuum-pressure alternating impregnation process comprises:

[0021] Vacuum stage: vacuum degree -0.08 to -0.1 MPa, maintained for 20-40 minutes;

[0022] Pressurization stage: pressure 0.5-1.2MPa, maintained for 1-3 hours;

[0023] The number of dipping cycles is 2-4 times.

[0024] Preferably, the stepwise temperature-increasing heat treatment comprises:

[0025] The first stage: heating to 400-600℃ at 2-10℃ / min and keeping warm for 0.5-2 hours;

[0026] The second stage: heating to 1200-1400℃ at 1-5℃ / min and keeping warm for 1-3 hours;

[0027] The third stage: heating to 1800-2200℃ at 5-15℃ / min and keeping warm for 0.5-2 hours.

[0028] Preferably, the pyrolytic carbon layer is deposited by chemical vapor deposition, the carbon source is methane, acetylene or propane, the deposition temperature is 1600-2000° C., the time is 20-60 minutes, and the thickness of the obtained pyrolytic carbon layer is 10-100 μm.

[0029] Preferably, the graphite tube substrate is isostatic graphite or pyrolytic graphite with a density of 1.7-2.2 g / cm 3 , carbon content ≥99.9wt%, and the substrate surface is plasma treated or acid-activated.

[0030] Preferably, after the stepwise temperature-raising heat treatment, the outer surface of the graphite tube is coated with RLHY-305 graphite antioxidant with a coating thickness of 5-20 μm and cured at 300-500° C.

[0031] Compared with the prior art, the present invention has at least the following advantages:

[0032] 1. The silica sol, molybdate, and phytic acid interact synergistically to form a SiC-MoSi2 composite phase. The phytic acid prevents molybdate aggregation and promotes SiC whisker growth during heat treatment, raising the graphite tube's oxidation resistance temperature. Simultaneously, the phytic acid catalyzes the reconstruction of the graphite matrix edge, forming a unique "toothed interlaced interface." This improves the graphite tube's structural strength, reducing its oxidation weight loss rate from >10% to <3%, thereby enhancing its high-temperature resistance. Repeated heating and rapid temperature changes reduce microcracks on the graphite tube's surface, enhancing its thermal stability. This enhanced temperature resistance and thermal stability ultimately significantly increase the tube's usability.

[0033] 2. The vacuum-pressure alternating impregnation process accelerates the penetration of the composite sol into the graphite matrix, solving the problem of high-density graphite matrix being difficult to penetrate, achieving a combination of internal matrix reinforcement and surface modification, and realizing overall protection "from the inside out".

[0034] 3. The deposited pyrolytic carbon layer reduces the reaction or penetration of the measured elements with graphite at high temperatures, reduces cross contamination, and reduces the memory effect of the graphite tube by 70%, while traditional pyrolytic graphite tubes can only reduce it by 30-40%. It is particularly suitable for the analysis of high melting point elements. DETAILED DESCRIPTION

[0035] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments of the present invention can be combined with each other. The following will further describe the technical solution of the present invention in conjunction with the embodiments of the present invention, and the present invention is not limited to the following specific implementation methods.

[0036] It should be understood that the same or similar reference numerals in the embodiments correspond to the same or similar components. In the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "front", "back", "left", "right", "top", "bottom" and the like indicating an orientation or positional relationship, they are based on the orientation or positional relationship shown in the scheme. This is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the terms describing the positional relationship in the scheme are only used for illustrative purposes and cannot be understood as a limitation of this patent. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to the specific circumstances.

[0037] A method for preparing a high-temperature resistant graphite tube for spectral analysis, comprising:

[0038] S1: preparing a high-purity graphite tube substrate and performing surface activation treatment; the high-purity graphite tube substrate is pressed into a transverse graphite tube by a high-temperature pressing device, which has the effects of uniform temperature distribution, a larger constant temperature area, and a small temperature gradient; and the surface activation treatment of the graphite tube substrate can remove surface defects of the graphite tube substrate and improve the purity of the graphite tube substrate.

[0039] S2: Prepare a composite sol, which is a mixture of silica sol, molybdate and phytic acid; the silica sol is a colloidal solution, which is a dispersion of nano-scale silica particles in water or solvent, and contains a large amount of silicon dioxide SiO2; the molybdate is an inorganic chemical substance, which can be reduced and reacts with graphite to form molybdenum silicide MoSi2 under high temperature environment, and the molybdenum silicide forms a high-temperature resistant coating on the graphite tube. For example, the invention patent No. CN116023157A discloses the high-temperature coating characteristics formed by molybdenum silicide; the phytic acid is also known as inositol hexaphosphate C6H 18 O 24 P6, Traditionally, phytic acid has only been used as a metal ion chelating agent.

[0040] S3: The composite sol is infiltrated into the graphite matrix by a vacuum-pressure alternating impregnation process; that is, the graphite matrix is ​​immersed in the composite sol, and the composite sol is infiltrated into the graphite matrix by alternating vacuum-pressure, thereby solving the problem that the high-density graphite matrix is ​​difficult to penetrate.

[0041] S4: Perform step-by-step temperature increase heat treatment in an inert environment. Argon gas is introduced into the inert environment to isolate oxygen, prevent oxygen from reacting with the graphite tube and graphite in a high-temperature environment, and reduce the generation of CO or CO2. Step-by-step temperature increase means increasing the preset temperature of the reactor within a preset time, stopping / keeping the temperature for a preset time, and then increasing the preset temperature again within a preset time to achieve the effect of step-by-step temperature increase to form a SiC-MoSi2 composite reinforcement phase. The SiC-MoSi2 composite reinforcement phase system is composed of a matrix phase (MoSi2) and a dispersed reinforcement phase (SiC), with molybdenum disilicide (MoSi2) as the matrix and silicon carbide (SiC) as the reinforcement phase.

[0042] S5: Deposit a pyrolytic carbon layer on the inner wall of the graphite tube. Methods for depositing a pyrolytic carbon layer include chemical vapor deposition and pyrolysis. Chemical vapor deposition is a method for depositing a thin film on a substrate through the gas phase. The carbon-containing precursor gas is dissociated at high temperature, causing carbon atoms to be deposited on the substrate, that is, on the inner wall of the graphite tube. The pyrolysis method is to prepare pyrolytic carbon by high-temperature pyrolysis of hydrocarbon gases (such as propane, methane, etc.). It is carried out in a high-temperature furnace, and carbon atoms are deposited on the inner wall of the graphite tube through processes such as pyrolysis and polycondensation.

[0043] In a possible embodiment, the composite sol comprises, by weight:

[0044] The silica sol is 40-60 parts. The silica sol is a dispersion of nano-sized silicon dioxide particles with a high specific surface area and active hydroxyl groups. It can form a dense silicon-oxygen network structure (-Si-O-Si-), enhancing the mechanical strength and high temperature resistance of the coating (resistant to 1500-1600°C). The amount of 40-60 parts ensures sufficient film-forming material and coverage while avoiding brittleness or cracking caused by excessive use.

[0045] The molybdate is 3-10 parts. Molybdate (such as sodium molybdate) forms a molybdenum oxide passivation film by oxidizing the metal surface to inhibit electrochemical corrosion. It is particularly effective when replacing toxic chromate. The concentration of 3-10 parts balances cost and performance. Excessive use may affect coating uniformity due to solubility limitations or crystallization precipitation.

[0046] The phytic acid molecule contains 12 phosphate groups and can react with metal ions (such as Fe 3+ 、Al 3+ ) forms a stable chelate, generates a dense monomolecular film on the metal surface, blocks the corrosion path, and 2-8 parts are sufficient to cover the metal surface and form a continuous film. Excessive use may cause the film to be too thick and brittle.

[0047] 30-50 parts of alcohol solvent. Alcohols (such as ethanol and ethylene glycol) are used as solvents to promote the uniform dispersion of silica sol, molybdate, and phytic acid and prevent particle agglomeration. 30-50 parts ensures the full dissolution of each component and process operability. Excessive amounts may delay curing or reduce coating hardness.

[0048] In a preferred embodiment, the composite sol comprises, by weight: 40 parts of the silica sol, 3 parts of the molybdate, 2 parts of the phytic acid, and 30 parts of the alcohol solvent.

[0049] In a preferred embodiment, the composite sol comprises, by weight: 50 parts of the silica sol, 6 parts of the molybdate, 5 parts of the phytic acid, and 40 parts of the alcohol solvent.

[0050] In a preferred embodiment, the composite sol comprises, by weight: 60 parts of the silica sol, 10 parts of the molybdate, 8 parts of the phytic acid, and 50 parts of the alcohol solvent.

[0051] In a possible embodiment, the SiO2 content in the silica sol is ≥30wt%; the molybdate is ammonium molybdate ((NH4)2MoO4) or sodium molybdate (Na2MoO4), wherein the content of ammonium molybdate ((NH4)2MoO4) or sodium molybdate (Na2MoO4) is ≥99wt%; and the purity of the phytic acid is ≥50%.

[0052] In a preferred embodiment, the SiO2 content in the silica sol is 30wt%; the molybdate is ammonium molybdate ((NH4)2MoO4) or sodium molybdate (Na2MoO4), wherein the content of ammonium molybdate ((NH4)2MoO4) or sodium molybdate (Na2MoO4) is 99.5wt%, that is, the purity is 99.5%; the purity of the phytic acid is 50%.

[0053] It is important to further clarify that, while conventional wisdom holds that heat treatment of silica sol only produces SiO₂, the present invention demonstrates a synergistic effect between the silica sol, molybdate, and phytic acid. The silica sol provides a skeletal structure, while the molybdate fills the pores, synergistically enhancing corrosion resistance. The chelation effect of the phytic acid complements the oxidative passivation of the molybdate, enhancing comprehensive metal protection. The presence of molybdate and phytic acid promotes the in-situ formation of a unique structure of SiC whiskers and MoSi₂ nanoparticles at temperatures between 1600 and 1800°C. Electron microscopy analysis reveals that this structure effectively pins the graphite matrix grain boundaries, inhibiting crack propagation at high temperatures. Furthermore, while phytic acid has traditionally been used only as a metal ion chelator, the present invention demonstrates for the first time that it can catalyze the reconstruction of the graphite matrix edges at high temperatures (>1200°C), forming a unique "toothed interdigitated interface" that increases the coating-substrate bond strength by more than three times.

[0054] In a possible embodiment, the composite sol further includes 0.1-3 parts of graphene or graphene oxide. The quantum effect of graphene or graphene oxide and the interface bonding force of SiC are synergistically optimized, which can improve the strength of the graphite tube and the thermal conductivity of the graphite tube.

[0055] In a preferred embodiment, the amount of graphene or graphene oxide is 0.5 parts.

[0056] In a preferred embodiment, the amount of graphene or graphene oxide is 2 parts.

[0057] In a possible embodiment, the vacuum-pressure alternating impregnation process includes:

[0058] Vacuum stage: vacuum degree -0.08 to -0.1 MPa, maintained for 20-40 minutes;

[0059] Pressurization stage: pressure 0.5-1.2MPa, maintained for 1-3 hours;

[0060] The number of dipping cycles is 2-4 times.

[0061] The composite sol can penetrate the graphite tube matrix, facilitating the in-situ formation of a unique structure of SiC whiskers and MoSi2 nanoparticles during heating. This solves the problem of high-density graphite matrix penetration, achieving a combination of internal matrix reinforcement and surface modification, providing comprehensive protection from the inside out.

[0062] In a preferred embodiment, during the vacuum stage, the vacuum degree is -0.08 MPa and maintained for 30 minutes; during the pressurization stage, the pressure is 1.2 MPa and maintained for 2 hours; and the number of immersion cycles is 2 times.

[0063] In a preferred embodiment, during the vacuum stage, the vacuum degree is -0.04 MPa and maintained for 30 minutes; during the pressurization stage, the pressure is 0.8 MPa and maintained for 2 hours; and the number of immersion cycles is 2 times.

[0064] In a preferred embodiment, during the vacuum stage, the vacuum degree is -0.02 MPa and maintained for 30 minutes; during the pressurization stage, the pressure is 0.5 MPa and maintained for 2 hours; and the number of immersion cycles is 4 times.

[0065] In a possible embodiment, the stepwise temperature-increasing heat treatment includes:

[0066] The first stage: heating to 400-600℃ at 2-10℃ / min and keeping warm for 0.5-2 hours;

[0067] The second stage: heating to 1200-1400℃ at 1-5℃ / min and keeping warm for 1-3 hours;

[0068] The third stage: heating to 1800-2200℃ at 5-15℃ / min and keeping warm for 0.5-2 hours.

[0069] In a preferred embodiment, the step-by-step temperature increase heat treatment includes: the first stage: heating to 500°C at 2°C / min and keeping warm for 1 hour; the second stage: heating to 1300°C at 2°C / min and keeping warm for 1 hour; the third stage: heating to 2000°C at 5°C / min and keeping warm for 1 hour.

[0070] In a preferred embodiment, the step-by-step temperature increase heat treatment includes: the first stage: heating to 500°C at 5°C / min and keeping warm for 1 hour; the second stage: heating to 1300°C at 3°C / min and keeping warm for 1 hour; the third stage: heating to 2000°C at 8°C / min and keeping warm for 1 hour.

[0071] In a preferred embodiment, the step-by-step temperature increase heat treatment includes: the first stage: heating to 500°C at 10°C / min and keeping warm for 1 hour; the second stage: heating to 1300°C at 5°C / min and keeping warm for 1 hour; the third stage: heating to 2000°C at 15°C / min and keeping warm for 1 hour.

[0072] In a possible embodiment, the pyrolytic carbon layer is deposited by chemical vapor deposition, the carbon source is methane, acetylene or propane, the deposition temperature is 1600-2000°C, the time is 20-60 minutes, and the thickness of the obtained pyrolytic carbon layer is 10-100 μm. At high temperature, the carbon-containing methane, acetylene or propane gas is dissociated, and carbon atoms are deposited on the substrate, that is, on the inner wall of the graphite tube. The chemical vapor deposition method can control the film thickness, uniformity and carbon layer structure. During high-temperature operation, the pyrolytic carbon layer on the inner wall will be selectively consumed to form a dense glassy carbon film. The adaptive protection mechanism enables the graphite tube to maintain stable analytical performance under extreme conditions, reduces the reaction or penetration of the measured element with graphite at high temperature, reduces the phenomenon of cross contamination, and reduces the memory effect of the graphite tube by 70%, while the traditional pyrolytic graphite tube can only reduce it by 30-40%, which is particularly suitable for the analysis of high-melting-point elements.

[0073] In a preferred embodiment, the pyrolytic carbon layer is deposited by chemical vapor deposition, with methane as the carbon source, a deposition temperature of 1600° C., and a deposition time of 45 minutes. The resulting pyrolytic carbon layer has a thickness of 50 μm.

[0074] In a preferred embodiment, the pyrolytic carbon layer is deposited by chemical vapor deposition, the carbon source is acetylene, the deposition temperature is 1800° C., the deposition time is 45 minutes, and the obtained pyrolytic carbon layer has a thickness of 50 μm.

[0075] In a preferred embodiment, the pyrolytic carbon layer is deposited by chemical vapor deposition, the carbon source is propane, the deposition temperature is 2000° C., the time is 45 minutes, and the obtained pyrolytic carbon layer has a thickness of 50 μm.

[0076] In one possible embodiment, the graphite tube substrate is isostatic graphite or pyrolytic graphite with a density of 1.7-2.2 g / cm 3 , carbon content ≥99.9wt%, and the substrate surface is plasma treated or acid-activated.

[0077] In a preferred embodiment, the graphite tube substrate is isostatically pressed graphite with a density of 1.82 g / cm 3 The carbon content is 99.95wt%, and the substrate surface is activated by acid pickling, ultrasonically cleaned in 5% nitric acid solution for 30 minutes, rinsed with deionized water and then dried at 120℃.

[0078] In a preferred embodiment, after the stepwise temperature-raising heat treatment, the outer surface of the graphite tube is coated with RLHY-305 graphite antioxidant with a coating thickness of 5-20 μm and cured at 300-500°C.

[0079] In a preferred embodiment, the RLHY-305 graphite antioxidant is coated with a thickness of 10 μm and cured at 400° C.

[0080] Comparative Example 1

[0081] Matrix preparation: Graphite powder with a purity of 99.95% was selected and isostatically pressed to form a standard transverse graphite tube with a maximum length of 55 mm, a maximum width of 30 mm, and a maximum thickness of 12 mm. The tube was polished with 200-mesh silicon carbide sandpaper, ultrasonically cleaned in 5% nitric acid solution for 30 minutes, rinsed with deionized water, and dried at 120°C.

[0082] Performance test: 1. Oxidation weight loss: use a piece of graphite tube, heat it in air at 2800℃ for 10 minutes, cool it down, heat it again, repeat the heating 10 times, calculate the weight loss rate, weight loss rate % = (weight before heating - weight after heating) / weight before heating. 2. Microcracks: after the weight loss rate is calculated, observe the outer surface of the graphite tube under a microscope to see if there are microcracks. 3. Service life: use another piece of graphite tube, continuously measure the aluminum sample (atomization temperature 2500℃), and record the total number of times the graphite tube is used before failure (graphite tube failure specifically refers to an abnormal increase in the absorbance value or obvious cracks or deformation on the surface of the graphite tube. Either of the two can be considered as graphite tube failure). See Table 1 below for details.

[0083] Comparative Example 2

[0084] Matrix preparation: Graphite powder with a purity of 99.95% was selected and isostatically pressed to form a standard graphite tube with a maximum length of 55 mm, a maximum width of 30 mm, and a maximum thickness of 12 mm. The surface was polished with 200-mesh silicon carbide sandpaper, ultrasonically cleaned in 5% nitric acid solution for 30 minutes, rinsed with deionized water, and dried at 120°C.

[0085] Preparation of composite sol: 40 parts of silica sol (SiO2 content 30wt%), 3 parts of phytic acid (purity 70%), and 42 parts of ethanol were stirred and mixed in a 40°C water bath for 2 hours to obtain a uniform and stable composite sol No. 1.

[0086] Impregnation treatment: 1. Place the graphite tube in a vacuum tank and evacuate to -0.08 MPa for 30 minutes. 2. Slowly inject the composite sol until the tube is completely immersed, then pressurize to 1.2 MPa and maintain for 2 hours. 3. Perform two immersion cycles. 4. Remove the tube and centrifuge at 3000 rpm to remove excess sol from the surface.

[0087] Heat treatment: 1. Dry at 80℃ for 12 hours; 2. Raise the temperature to 500℃ at 5℃ / min under argon protection and keep warm for 1 hour; 3. Continue to raise the temperature to 1300℃ at 3℃ / min and keep warm for 1 hour; 4. Finally, raise the temperature to 2000℃ at 8℃ / min, keep warm for 1 hour, and cool with the furnace.

[0088] Performance test: 1. Oxidation weight loss: use a piece of graphite tube, heat it in air at 2800℃ for 10 minutes, cool it down, heat it again, repeat the heating 10 times, calculate the weight loss rate, weight loss rate % = (weight before heating - weight after heating) / weight before heating. 2. Microcracks: after the weight loss rate is calculated, observe the outer surface of the graphite tube under a microscope to see if there are microcracks. 3. Service life: use another piece of graphite tube, continuously measure the aluminum sample (atomization temperature 2500℃), and record the total number of times the graphite tube is used before failure (graphite tube failure specifically refers to an abnormal increase in the absorbance value or obvious cracks or deformation on the surface of the graphite tube. Either of the two can be considered as graphite tube failure). See Table 1 below for details.

[0089] Comparative Example 3

[0090] Matrix preparation: Graphite powder with a purity of 99.95% was selected and isostatically pressed to form a standard graphite tube with a maximum length of 55 mm, a maximum width of 30 mm, and a maximum thickness of 12 mm. The surface was polished with 200-mesh silicon carbide sandpaper, ultrasonically cleaned in 5% nitric acid solution for 30 minutes, rinsed with deionized water, and dried at 120°C.

[0091] Preparation of composite sol: 40 parts of silica sol (SiO2 content 30wt%), 5 parts of ammonium molybdate (purity 99.5%), and 42 parts of ethanol were stirred and mixed in a 40°C water bath for 2 hours to obtain a uniform and stable composite sol No. 2.

[0092] Impregnation treatment: 1. Place the graphite tube in a vacuum tank and evacuate to -0.08 MPa for 30 minutes. 2. Slowly inject the composite sol until the tube is completely immersed, then pressurize to 1.2 MPa and maintain for 2 hours. 3. Perform two immersion cycles. 4. Remove the tube and centrifuge at 3000 rpm to remove excess sol from the surface.

[0093] Heat treatment: 1. Dry at 80℃ for 12 hours; 2. Raise the temperature to 500℃ at 5℃ / min under argon protection and keep warm for 1 hour; 3. Continue to raise the temperature to 1300℃ at 3℃ / min and keep warm for 1 hour; 4. Finally, raise the temperature to 2000℃ at 8℃ / min, keep warm for 1 hour, and cool with the furnace.

[0094] Performance test: 1. Oxidation weight loss: use a piece of graphite tube, heat it in air at 2800℃ for 10 minutes, cool it down, heat it again, repeat the heating 10 times, calculate the weight loss rate, weight loss rate % = (weight before heating - weight after heating) / weight before heating. 2. Microcracks: after the weight loss rate is calculated, observe the outer surface of the graphite tube under a microscope to see if there are microcracks. 3. Service life: use another piece of graphite tube, continuously measure the aluminum sample (atomization temperature 2500℃), and record the total number of times the graphite tube is used before failure (graphite tube failure specifically refers to an abnormal increase in the absorbance value or obvious cracks or deformation on the surface of the graphite tube. Either of the two can be considered as graphite tube failure). See Table 1 below for details.

[0095] Comparative Example 4

[0096] Matrix preparation: Graphite powder with a purity of 99.95% was selected and isostatically pressed to form a standard graphite tube with a maximum length of 55 mm, a maximum width of 30 mm, and a maximum thickness of 12 mm. The surface was polished with 200-mesh silicon carbide sandpaper, ultrasonically cleaned in 5% nitric acid solution for 30 minutes, rinsed with deionized water, and dried at 120°C.

[0097] Preparation of composite sol: 5 parts of ammonium molybdate (purity 99.5%), 3 parts of phytic acid (purity 70%), and 42 parts of ethanol were stirred and mixed in a 40°C water bath for 2 hours to obtain a uniform and stable composite sol No. 3.

[0098] Impregnation treatment: 1. Place the graphite tube in a vacuum tank and evacuate to -0.08 MPa for 30 minutes. 2. Slowly inject the composite sol until the tube is completely immersed, then pressurize to 1.2 MPa and maintain for 2 hours. 3. Perform two immersion cycles. 4. Remove the tube and centrifuge at 3000 rpm to remove excess sol from the surface.

[0099] Heat treatment: 1. Dry at 80℃ for 12 hours; 2. Raise the temperature to 500℃ at 5℃ / min under argon protection and keep warm for 1 hour; 3. Continue to raise the temperature to 1300℃ at 3℃ / min and keep warm for 1 hour; 4. Finally, raise the temperature to 2000℃ at 8℃ / min, keep warm for 1 hour, and cool with the furnace.

[0100] Performance test: 1. Oxidation weight loss: use a piece of graphite tube, heat it in air at 2800℃ for 10 minutes, cool it down, heat it again, repeat the heating 10 times, calculate the weight loss rate, weight loss rate % = (weight before heating - weight after heating) / weight before heating. 2. Microcracks: after the weight loss rate is calculated, observe the outer surface of the graphite tube under a microscope to see if there are microcracks. 3. Service life: use another piece of graphite tube, continuously measure the aluminum sample (atomization temperature 2500℃), and record the total number of times the graphite tube is used before failure (graphite tube failure specifically refers to an abnormal increase in the absorbance value or obvious cracks or deformation on the surface of the graphite tube. Either of the two can be considered as graphite tube failure). See Table 1 below for details.

[0101] Example 5

[0102] Matrix preparation: Graphite powder with a purity of 99.95% was selected and isostatically pressed to form a standard graphite tube with a maximum length of 55 mm, a maximum width of 30 mm, and a maximum thickness of 12 mm. The surface was polished with 200-mesh silicon carbide sandpaper, ultrasonically cleaned in 5% nitric acid solution for 30 minutes, rinsed with deionized water, and dried at 120°C.

[0103] Preparation of composite sol: 40 parts of silica sol (SiO2 content 30wt%), 5 parts of ammonium molybdate (purity 99.5%), 3 parts of phytic acid (purity 70%), and 42 parts of ethanol. The above components were stirred and mixed in a 40°C water bath for 2 hours to obtain a uniform and stable composite sol No. 4.

[0104] Impregnation treatment: 1. Place the graphite tube in a vacuum tank and evacuate to -0.08 MPa for 30 minutes. 2. Slowly inject the composite sol until the tube is completely immersed, then pressurize to 1.2 MPa and maintain for 2 hours. 3. Perform two immersion cycles. 4. Remove the tube and centrifuge at 3000 rpm to remove excess sol from the surface.

[0105] Heat treatment: 1. Dry at 80℃ for 12 hours; 2. Raise the temperature to 500℃ at 5℃ / min under argon protection and keep warm for 1 hour; 3. Continue to raise the temperature to 1300℃ at 3℃ / min and keep warm for 1 hour; 4. Finally, raise the temperature to 2000℃ at 8℃ / min, keep warm for 1 hour, and cool with the furnace.

[0106] Performance test: 1. Oxidation weight loss: use a piece of graphite tube, heat it in air at 2800℃ for 10 minutes, cool it down, heat it again, repeat the heating 10 times, calculate the weight loss rate, weight loss rate % = (weight before heating - weight after heating) / weight before heating. 2. Microcracks: after the weight loss rate is calculated, observe the outer surface of the graphite tube through a microscope to see if there are microcracks. 3. Service life: use another piece of graphite tube, continuously measure the aluminum sample (atomization temperature 2500℃), and record the total number of uses before the graphite tube fails (graphite tube failure specifically refers to an abnormal increase in the absorbance value or obvious cracks or deformation on the surface of the graphite tube. Either of the two can be considered as graphite tube failure. The number is the approximate number of uses). See Table 1 below for details.

[0107] Example 6

[0108] Matrix preparation: Graphite powder with a purity of 99.95% was selected and isostatically pressed to form a standard graphite tube with a maximum length of 55 mm, a maximum width of 30 mm, and a maximum thickness of 12 mm. The surface was polished with 200-mesh silicon carbide sandpaper, ultrasonically cleaned in 5% nitric acid solution for 30 minutes, rinsed with deionized water, and dried at 120°C.

[0109] Preparation of composite sol: 50 parts of silica sol (SiO2 content 30wt%), 6 parts of ammonium molybdate (purity 99.5%), 5 parts of phytic acid (purity 70%), and 42 parts of ethanol. The above components were stirred and mixed in a 40°C water bath for 2 hours to obtain a uniform and stable composite sol No. 5.

[0110] Impregnation treatment: 1. Place the graphite tube in a vacuum tank and evacuate to -0.08 MPa for 30 minutes. 2. Slowly inject the composite sol until the tube is completely immersed, then pressurize to 1.2 MPa and maintain for 2 hours. 3. Perform two immersion cycles. 4. Remove the tube and centrifuge at 3000 rpm to remove excess sol from the surface.

[0111] Heat treatment: 1. Dry at 80℃ for 12 hours; 2. Raise the temperature to 500℃ at 5℃ / min under argon protection and keep warm for 1 hour; 3. Continue to raise the temperature to 1300℃ at 3℃ / min and keep warm for 1 hour; 4. Finally, raise the temperature to 2000℃ at 8℃ / min, keep warm for 1 hour, and cool with the furnace.

[0112] Performance test: 1. Oxidation weight loss: use a piece of graphite tube, heat it in air at 2800℃ for 10 minutes, cool it down, heat it again, repeat the heating 10 times, calculate the weight loss rate, weight loss rate % = (weight before heating - weight after heating) / weight before heating. 2. Microcracks: after the weight loss rate is calculated, observe the outer surface of the graphite tube under a microscope to see if there are microcracks. 3. Service life: use another piece of graphite tube, continuously measure the aluminum sample (atomization temperature 2500℃), and record the total number of times the graphite tube is used before failure (graphite tube failure specifically refers to an abnormal increase in the absorbance value or obvious cracks or deformation on the surface of the graphite tube. Either of the two can be considered as graphite tube failure). See Table 1 below for details.

[0113] Example 7

[0114] Matrix preparation: Graphite powder with a purity of 99.95% was selected and isostatically pressed to form a standard graphite tube with a maximum length of 55 mm, a maximum width of 30 mm, and a maximum thickness of 12 mm. The surface was polished with 200-mesh silicon carbide sandpaper, ultrasonically cleaned in 5% nitric acid solution for 30 minutes, rinsed with deionized water, and dried at 120°C.

[0115] Preparation of composite sol: 60 parts of silica sol (SiO2 content 30wt%), 10 parts of ammonium molybdate (purity 99.5%), 8 parts of phytic acid (purity 70%), and 42 parts of ethanol. The above components were stirred and mixed in a 40°C water bath for 2 hours to obtain a uniform and stable composite sol No. 6.

[0116] Impregnation treatment: 1. Place the graphite tube in a vacuum tank and evacuate to -0.08 MPa for 30 minutes. 2. Slowly inject the composite sol until the tube is completely immersed, then pressurize to 1.2 MPa and maintain for 2 hours. 3. Perform two immersion cycles. 4. Remove the tube and centrifuge at 3000 rpm to remove excess sol from the surface.

[0117] Heat treatment: 1. Dry at 80℃ for 12 hours; 2. Raise the temperature to 500℃ at 5℃ / min under argon protection and keep warm for 1 hour; 3. Continue to raise the temperature to 1300℃ at 3℃ / min and keep warm for 1 hour; 4. Finally, raise the temperature to 2000℃ at 8℃ / min, keep warm for 1 hour, and cool with the furnace.

[0118] Performance test: 1. Oxidation weight loss: use a piece of graphite tube, heat it in air at 2800℃ for 10 minutes, cool it down, heat it again, repeat the heating 10 times, calculate the weight loss rate, weight loss rate % = (weight before heating - weight after heating) / weight before heating. 2. Microcracks: after the weight loss rate is calculated, observe the outer surface of the graphite tube under a microscope to see if there are microcracks. 3. Service life: use another piece of graphite tube, continuously measure the aluminum sample (atomization temperature 2500℃), and record the total number of times the graphite tube is used before failure (graphite tube failure specifically refers to an abnormal increase in the absorbance value or obvious cracks or deformation on the surface of the graphite tube. Either of the two can be considered as graphite tube failure). See Table 1 below for details.

[0119]

[0120]

[0121] Table 1

[0122] As can be seen from Table 1 above and the description of the present application, the silica sol, the molybdate and the phytic acid have a synergistic effect to form a SiC-MoSi2 composite phase. Phytic acid can not only prevent the aggregation of molybdate, but also promote the growth of SiC whiskers during heat treatment, thereby increasing the oxidation resistance temperature of the graphite tube. At the same time, phytic acid can catalyze the reconstruction of the edge of the graphite matrix to form a unique "toothed staggered interface". On the basis of realizing spectral analysis in graphite tubes, the structural strength of the graphite tubes is improved, and the oxidation weight loss rate of the graphite tubes is reduced from >10% to <3%, thereby improving the high-temperature resistance of the graphite tubes. After repeated heating and rapid temperature changes, the microcracks on the surface of the graphite tubes are reduced, thereby enhancing the thermal stability of the graphite tubes. By improving the temperature resistance and thermal stability of the graphite tubes, the number of uses of the graphite tubes is ultimately greatly increased. Furthermore, the vacuum-pressure alternating impregnation process accelerates the penetration of the composite sol into the graphite matrix, solving the problem of the difficulty of penetrating the high-density graphite matrix, achieving a combination of internal matrix reinforcement and surface modification, and realizing overall protection "from the inside out." The deposited pyrolytic carbon layer reduces the reaction or penetration of the measured elements with graphite at high temperatures, reduces cross-contamination, and reduces the memory effect of the graphite tube by 70%, while traditional pyrolytic graphite tubes can only reduce it by 30-40%, making it particularly suitable for the analysis of high-melting-point elements.

[0123] Obviously, the above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention are intended to be included within the scope of protection of the present invention.

Claims

1. A method for preparing a high-temperature resistant graphite tube for spectral analysis, characterized in that: include S1: Preparation of high-purity graphite tube substrate and surface activation treatment; S2: preparing a composite sol, wherein the composite sol comprises a mixture of silica sol, molybdate and phytic acid; S3: infiltrating the composite sol into the graphite matrix using a vacuum-pressure alternating impregnation process; S4: performing a stepwise temperature increase heat treatment in an inert environment to form a SiC-MoSi2 composite reinforcement phase; S5: Depositing a pyrolytic carbon layer on the inner wall of the graphite tube.

2. The method for preparing a high-temperature resistant graphite tube for spectral analysis according to claim 1, wherein: The composite sol comprises, by weight: 40-60 parts of the silica sol, 3-10 parts of the molybdate, 2-8 parts of phytic acid, 30-50 parts of alcohol solvent.

3. The method for preparing a high temperature resistant graphite tube for spectral analysis according to claim 2, wherein: The SiO2 content in the silica sol is ≥30wt%; the molybdate is ammonium molybdate ((NH4)2MoO4) or sodium molybdate (Na2MoO4), wherein the content of ammonium molybdate ((NH4)2MoO4) or sodium molybdate (Na2MoO4) is ≥99wt%; and the purity of the phytic acid is ≥50%.

4. The method for preparing a high-temperature resistant graphite tube for spectral analysis according to claim 3, wherein: The composite sol further includes 0.1-3 parts of graphene or graphene oxide.

5. The method for preparing a high-temperature resistant graphite tube for spectral analysis according to claim 1, wherein the vacuum-pressure alternating impregnation process comprises: Vacuum stage: vacuum degree -0.08 to -0.1 MPa, maintained for 20-40 minutes; Pressurization stage: pressure 0.5-1.2MPa, maintained for 1-3 hours; The number of dipping cycles is 2-4 times.

6. The method for preparing a high-temperature resistant graphite tube for spectral analysis according to claim 1, wherein the stepwise temperature-increasing heat treatment comprises: The first stage: heating to 400-600℃ at 2-10℃ / min and keeping warm for 0.5-2 hours; The second stage: heating to 1200-1400℃ at 1-5℃ / min and keeping warm for 1-3 hours; The third stage: heating to 1800-2200℃ at 5-15℃ / min and keeping warm for 0.5-2 hours.

7. The method for preparing a high-temperature resistant graphite tube for spectral analysis according to claim 1, wherein the pyrolytic carbon layer is deposited by chemical vapor deposition, the carbon source is methane, acetylene or propane, the deposition temperature is 1600-2000°C, the time is 20-60 minutes, and the resulting pyrolytic carbon layer has a thickness of 10-100 μm.

8. The method for preparing a high-temperature resistant graphite tube for spectral analysis according to claim 1, wherein the graphite tube substrate is isostatically pressed graphite or pyrolytic graphite, has a density of 1.7-2.2 g / cm³, a carbon content of ≥99.9 wt%, and the substrate surface is plasma treated or acid-activated.

9. The method for preparing a high-temperature resistant graphite tube for spectral analysis according to claim 1, wherein after the stepwise temperature-increasing heat treatment, a graphite antioxidant is coated on the outer surface of the graphite tube to a thickness of 5-20 μm and then cured at 300-500°C.

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