CN114524674B - Heat-proof, heat-insulation and load-bearing integrated light carbon-ceramic composite material and preparation method thereof - Google Patents

Abstract

The invention discloses a heat-proof, heat-insulation and bearing integrated light carbon-ceramic composite material and a preparation method thereof, and belongs to the technical field of ultrahigh-temperature thermal protection materials. The composite material is prepared by compounding a ceramic matrix with oxidation resistance and ablation resistance with a light carbon-based composite material. The material consists of a fiber reinforcement body, carbon aerogel and a ceramic double-base body, wherein the ceramic base body is uniformly dispersed in a carbon aerogel three-dimensional nano network structure, and the multifunctional requirement under a long-term high-temperature aerobic environment is met by means of heat insulation-bearing of the carbon aerogel and anti-oxidation ablation of the ceramic base body. By changing the types, the contents and the introduction sequence of the ceramic components, the wide-temperature-range antioxidant ablation of the light carbon-based composite material can be realized. Compared with a light carbon-based composite material, the mechanical and oxidation resistance of the material is obviously improved, the compression strength is improved to 90.9MPa, and the weight loss rate of the material is reduced to 9.19 percent after static oxidation for 15min at 1300 ℃.

Description

A light-weight carbon-ceramic composite material integrated with heat protection-heat insulation-carrying capacity and its preparation method

technical field

The invention relates to the technical field of ultra-high temperature thermal protection materials, in particular to a lightweight carbon-ceramic composite material integrating heat protection-heat insulation-carrying capacity and a preparation method thereof.

Background technique

Carbon airgel is a new type of nano-porous carbon material. Three-dimensional nano-carbon particles are stacked inside it to form a rich pore structure, which makes it have excellent properties such as light-weight porous airgel and high-temperature stability of carbon materials. In particular, due to its unique mesoporous structure and nanoparticle network structure to suppress phonon scattering, photon shielding and gas molecule collisions, it can greatly reduce solid-state, gaseous and radiative thermal conductivity, and its thermal insulation performance is obviously superior. Compared with traditional carbon fiber felt and carbon foam, it is a rare rigid insulation material that can be used for a long time above 1600 °C. However, traditional carbon aerogels have a glassy carbon structure, which is brittle and difficult to achieve large-scale preparation. Using high-strength carbon fibers as reinforcements can significantly improve its mechanical properties and large-scale forming capabilities through interface microstructure regulation and other means, so as to realize the integrated function of ultra-high temperature heat insulation and load bearing. The obtained lightweight carbon-based composite The material has great application prospects in thermal protection fields such as aerospace vehicles and their power systems.

However, carbon materials are prone to failure due to oxidation at high temperatures, and it is difficult to meet the performance requirements of thermal protection materials in the aerobic environment of the new generation of aircraft and their power systems. Anti-oxidation ceramic components are used to dope the nano-carbon airgel matrix Modification can effectively improve the anti-oxidation and anti-ablation capabilities of carbon aerogels, and meet the heat protection-insulation-carrying requirements in an aerobic environment. Therefore, the present invention proposes a matrix doping technology suitable for lightweight carbon-based composites (carbon airgel composites, carbon foam composites), by introducing one or more ceramic substrates with anti-oxidation functions into lightweight carbon-based composites. Carbon aerogel and ceramic double matrix are formed in the carbon-based composite material, so as to obtain a lightweight carbon-ceramic composite material with integrated functions of heat protection, heat insulation and load bearing.

Contents of the invention

The purpose of the present invention is to provide a light-weight carbon-ceramic composite material integrated with heat protection-heat insulation-carrying capacity and its preparation method, so as to meet the thermal protection requirements in an ultra-high temperature aerobic environment.

In order to achieve the above object, the technical scheme adopted in the present invention is as follows:

A method for preparing a heat-proof-heat-insulation-carrying integrated lightweight carbon-ceramic composite material, comprising the following steps:

(1) The fiber-reinforced lightweight carbon-based composite material is used as the base material and processed into the desired shape, after the surface is blown clean, it is cleaned ultrasonically with alcohol, and then placed in an oven at 90-120°C for 24-48 hours;

(2) Prepare material A, mix material A with a solvent in a certain proportion, and mechanically stir for 1 to 4 hours to obtain an impregnating solution; the material A is boric acid or phosphoric acid; or, the material A is ceramics of anti-oxidation components powder; or, the material A is an organic or inorganic precursor of an antioxidant component; the antioxidant component is one or more of SiBCN, SiCO, SiC, ZrC, ZrB ₂ , HfC and HfB ₂ ; The solvent is one or more of xylene, ethanol and distilled water;

(3) Immerse the lightweight carbon-based composite material sample in the impregnation solution prepared in step (2), and use methods such as ultrasonic vibration, vacuum or normal pressure impregnation to immerse and evenly distribute the solution in the lightweight carbon-based composite material sample, keeping After a certain period of time, take out the sample and dry it;

(4) The light carbon-based composite material sample impregnated in step (3) is placed in a drying oven for curing and drying under normal pressure, and then the composite material is subjected to high-temperature heat treatment to obtain a light carbon-ceramic composite material containing antioxidant components Material; curing and drying process parameters are 80~170℃, heat preservation 2~4h;

(5) Repeat the process from step (2) to step (4) for 0-5 times.

In the above step (1), the density of the lightweight carbon-based composite material ranges from 0.2 to 0.7 g/cm ³ , and the lightweight carbon-based composite material is a fiber-reinforced carbon airgel composite material or a carbon foam composite material.

In the above step (2), when the material A is boric acid or phosphoric acid, the preparation method of the impregnating solution is as follows: pour the boric acid or phosphoric acid powder into deionized water at 90°C, stir mechanically until most of the powder is dissolved, and then put it into an ultrasonic oscillator Dissolve with ultrasonic vibration at 110°C to obtain boric acid or phosphoric acid impregnation solution; wherein: the weight ratio of boric acid powder or phosphoric acid powder to water is 1:(3~10).

In the above step (2), when the material A is a ceramic powder of anti-oxidation components, the preparation method of the impregnating solution is as follows: pour the ceramic powder into deionized water or ethanol, stir magnetically for 2-4 hours, and then prepare a ceramic Powder impregnation solution; wherein: the weight ratio of the amount of ceramic powder to the solvent is (5-30):100.

In the above step (2), when the material A is an organic or inorganic precursor of an antioxidant component, the preparation method of the impregnation solution is: mix the precursor of the antioxidant component with xylene in a certain proportion, and magnetically stir Prepare a precursor impregnation solution after 2-4 hours; wherein the weight ratio of the precursor of the anti-oxidation component to the xylene solvent is (5-30):100; the precursor of the SiBCN is polyborosilazane PSNB, SiCO The precursor is polysiloxane PSO, the precursor of SiC is polycarbosilane PCS, the precursor of ZrC is the organic zirconium precursor PZC, the precursor of _ZrB is the organic zirconium precursor PZB, and the precursors of HfC and _HfB are based on HfCl ₄ is an organic precursor prepared from a hafnium source.

In the above step (3), when the impregnation solution is prepared by boric acid or phosphoric acid, ultrasonic vibration is used for impregnation, and the impregnation time is 0.5 to 2 hours; when the impregnation solution is prepared by using ceramic powder or precursor of anti-oxidation components, Vacuum-atmospheric pressure impregnation is used, the specific method is: put the sample into a beaker and place it in a vacuum impregnation tank, evacuate the impregnation tank into a vacuum (vacuum degree ≤ -0.1MPa), and use the pressure difference to introduce the precursor impregnation solution into the container. In the beaker of the sample, maintain the vacuum degree for 0.5-2 hours, and then maintain the normal pressure for 0.5-2 hours.

In the above step (4), when boric acid or phosphoric acid impregnation solution is used, the heat treatment process of the sample is carried out in multiple steps: under an inert atmosphere, the temperature is raised from normal temperature to 170-250°C at a heating rate of 5°C/min for 0.5-1h, and then Continue to raise the temperature to 300-400°C and keep it warm for 0.5-1h, then raise the temperature to 500-700°C and keep it warm for 0.5-1h.

In the above step (4), when the impregnating solution is prepared from ceramic powder or precursor of anti-oxidation components, the heat treatment process of the sample is as follows: the temperature is raised to 800-800°C at a rate of 5°C/min under an inert atmosphere in a cracking furnace. 1500°C, keep warm for 0.5-2 hours, and cool down naturally under a protective atmosphere.

In the above step (5), when steps (2) to (4) are repeated, the composition of the modified component of the lightweight carbon-based composite material can be adjusted by changing the concentration of the impregnation solution, the type of the impregnation solution, and the order of the impregnation. The impregnation solution concentration, type, sequence and obtained modified composite materials include but not limited to the following 10 types:

(1) Immersion sequence: 25wt.% boric acid solution; obtained material: boron oxide modified lightweight carbon-based composite material;

(2) Impregnation sequence: 25wt.% boric acid solution, 10wt.% PSNB; obtained material: boron oxide-SiBCN modified lightweight carbon-based composite material;

(3) Impregnation sequence: 20wt.% PSNB; obtained material: SiBCN modified lightweight carbon-based composite material;

(4) Immersion sequence: 30wt.% phosphoric acid solution; obtained material: phosphoric acid modified lightweight carbon-based composite material;

(5) Impregnation sequence: 25wt.% PCS; obtained material: SiC modified lightweight carbon-based composite material;

(6) Impregnation sequence: 30wt.% PSO; obtained material: SiCO precursor modified lightweight carbon-based composite material;

(7) Impregnation sequence: 15wt.% PZB; obtained material: ZrB ₂ modified lightweight carbon-based composite material;

(8) Impregnation sequence: 20wt.% PCS, 10wt.% PZC; obtained material: SiC-ZrC modified lightweight carbon-based composite material;

(9) Impregnation sequence: 10wt.% PCS, 20wt.% HfB ₂ organic precursor; obtained material: SiC-HfB ₂ modified lightweight carbon-based composite material;

(10) Impregnation sequence: 10wt.% PCS, 15wt.% PZB, 15wt.% PZC; obtained material: SiC-ZrB ₂ -ZrC modified lightweight carbon-based composite material.

The prepared integrated lightweight carbon-ceramic composite material is composed of fiber reinforcement, carbon airgel and ceramic double matrix. The ceramic matrix is uniformly dispersed in the carbon aerogel three-dimensional nano-network structure. Heat insulation-carrying and anti-oxidation ablation of the ceramic matrix to meet the multi-functional requirements under long-term high-temperature aerobic environment.

Design mechanism of the present invention is as follows:

Using impregnation solutions prepared with different concentrations of antioxidant components (or their precursors), the matrix doping of lightweight carbon-based composites is carried out by vacuum impregnation, dehydration and drying, or ceramic precursor impregnation pyrolysis (PIP); using liquid With the characteristics of good phase fluidity, the prepared solution is introduced into the airgel matrix through the pores, and after curing and heat treatment, a new type of composite material with antioxidant components uniformly dispersed in the three-dimensional nano network structure is obtained, which significantly improves its performance. Oxidation resistance and ablation performance in high temperature aerobic environment. As a low-temperature anti-oxidation component, boron/phosphoric acid can form a glass phase oxide after drying and dehydration, which has a high viscosity and strong adhesion to the substrate, and can form a protective layer on the inner and outer surfaces; The anti-oxidation component is formed by pyrolysis of its precursor solution at high temperature. It has excellent anti-oxidation and ablation properties, and its temperature resistance can reach above 2000 °C. In the present invention, the impregnating solution prepared by selecting a single or mixed anti-oxidation component has good fluidity, has good wettability and permeability to carbon airgel materials, and the anti-oxidation component infiltrated in the nano-carbon network can consume oxygen, And inhibit the diffusion of oxygen, while reducing the contact between the substrate and oxygen, thereby improving the oxidation ablation performance.

The present invention has the following beneficial effects:

1. The present invention adopts ultrasonic vibration and vacuum plus atmospheric pressure impregnation process to introduce boron/phosphoric acid, anti-oxidation ceramic powder and its precursor into the light carbon-based composite material, and obtain carbon airgel through curing and heat treatment processes -Ceramic double-matrix composite material, which significantly improves the overall anti-oxidation and ablation performance of the material, so as to meet the needs of integrated use of heat protection-heat insulation-carrying in high-temperature aerobic environments.

2. The method of the present invention can realize the anti-oxidation and ablation of the light carbon-based composite material in a wide temperature range by changing the type, content and introduction sequence of the ceramic components. Compared with the lightweight carbon-based composite material, the mechanical and oxidation resistance properties of the material of the present invention are significantly improved, the compressive strength is increased from 62.7MPa to 90.9MPa, and the weight loss rate of static oxidation at 1300°C for 15 minutes is reduced from 14.92% to 9.19%.

Description of drawings

Fig. 1 is the flowchart of the process of the embodiment of the present invention.

Figure 2 shows the microstructure morphology of light carbon-based composites before and after doping and after oxidation; among them: (a) before doping; (b) after doping SiBCN ceramics; (c) microstructure after oxidation.

Figure 3 is a comparison of oxidation weight loss before and after doping of lightweight carbon-based composites.

Figure 4 is a comparison of the compressive strength before and after doping of lightweight carbon-based composites.

Detailed ways

In order to further understand the present invention, the present invention is described below in conjunction with examples, but the examples are only to further illustrate the features and advantages of the present invention, rather than limiting the claims of the present invention.

Since SiBCN ceramics have excellent high-temperature stability and anti-oxidation performance in a wide temperature range, the following uses SiBCN high-temperature ceramics as an anti-oxidation and ablation component to carry out matrix doping on lightweight carbon-based composite materials as an example to further the present invention. Description, to help understand the present invention better, Fig. 1 is its process flow chart, but the protection scope of the present invention is not limited to embodiment.

Example 1:

This example is the preparation of an integrated lightweight carbon-ceramic composite material, and the specific process is as follows:

(1) The lightweight carbon-based composite material substrate with a density of 0.6g/ ^cm3 was processed into a block sample with a size of 12.7×23.2×32.0mm, and the surface was cleaned by ultrasonic cleaning with alcohol, and then placed in an oven Dry at 120°C for 24 hours.

(2) Dissolving the organic precursor of SiBCN ceramics (PSNB) in xylene solvent, and stirring it thoroughly for 2 hours by magnetic force to obtain a PSNB precursor impregnating solution with a concentration of 20%.

(3) Put the beaker containing the lightweight carbon-based composite material sample into a vacuum impregnation tank and evacuate it into a vacuum (vacuum degree≤-0.1MPa), and introduce the above-mentioned PSNB precursor impregnation solution into the beaker by using the pressure difference, After maintaining the pressure state for 0.5h, immerse under normal pressure for 0.5h, take out the sample and dry it.

(4) After the impregnated sample was placed in an oven at 170°C for 2 hours, it was placed in a cracking furnace and introduced into a protective atmosphere, and the temperature was raised to 900°C at a rate of 5°C/min, and then kept for 1 hour to obtain SiBCN modified light carbon matrix composites.

(5) Repeat steps (2) to (4) once.

After impregnating with 20% PSNB precursor solution, amorphous SiBCN ceramics are evenly distributed in the porous body of lightweight carbon-based composite materials, and the weight gain rate is 11.16%. Oxygen, the weight loss rate of the sample was 9.19% after constant temperature oxidation for 15 minutes. Compared with the unimpregnated material, the oxidation weight loss rate of the modified and doped composite material decreased by about 38%, and the oxidation resistance performance was significantly improved. Figure 2 shows the microscopic appearance of the material before and after doping and after oxidation, Figure 3 shows the oxidation weight loss rate before and after material doping, and Figure 4 shows the compressive strength before and after material doping.

Example 2:

The difference from Example 1 lies in the type of dipping solution and the dipping process. In the present invention, the process parameters that affect the oxidation resistance of the light carbon-based composite material are mainly the concentration, type, and introduction sequence of the impregnating solution. Doping is performed to further explain the present invention, specifically including the following steps:

(1) The lightweight carbon-based composite material substrate with a density of 0.4g/ ^cm3 was processed into a block sample with a size of 13.1×23.0×32.5mm, and the surface was cleaned by ultrasonic cleaning with alcohol, and then placed in an oven Dry at 120°C for 24 hours.

(2) Pour the boric acid powder into water and stir it with a glass rod until most of the powder dissolves, then use an ultrasonic instrument to heat and ultrasonically vibrate and dissolve at 110°C to obtain a 25% boric acid solution; use the same method as (2) in Example 1 to obtain 7% PSNB precursor impregnation solution.

(3) Put the beaker containing the lightweight carbon-based composite material sample into a vacuum impregnation tank and evacuate it into a vacuum (vacuum degree≤-0.1MPa), and introduce the above-mentioned PSNB precursor impregnation solution into the beaker by using the pressure difference, After maintaining the pressure state for 0.5h, immerse under normal pressure for 0.5h, take out the sample and dry it.

(4) After the impregnated sample was placed in an oven at 170°C for 2 hours at normal pressure and cured for 2 hours, it was placed in a cracking furnace and introduced into a protective atmosphere, and the temperature was raised to 900°C at a rate of 5°C/min and then kept for 1 hour to obtain SiBCN modified Lightweight carbon-based composite material.

(5) Then the sample was impregnated with boric acid solution, dried at 170°C for 2h after impregnation, and dehydrated at 330°C for 0.5h under a protective atmosphere to obtain B ₂ O ₃ -SiBCN modified lightweight carbon-based composite materials. The sample weight gain rate is 11.38%. B ₂ O ₃ is in the form of molten glass at medium and low temperatures, which can effectively isolate oxygen and protect the carbon matrix. SiBCN has excellent anti-oxidation and ablation performance at high temperatures. Oxidation resistance temperature range of carbon matrix composites.

Claims (8)

1. A method for preparing heat-proof-heat-insulating-carrying integrated lightweight carbon-ceramic composite material, characterized in that: the method comprises the following steps: (1) Take the fiber-reinforced lightweight carbon-based composite material as the base material and process it into the desired shape. After blowing the surface, clean it ultrasonically with alcohol, and then dry it in an oven at 90-120°C for 24-48 hours; (2) Prepare material A, mix material A with a solvent in a certain proportion, and mechanically stir for 1 to 4 hours to obtain an impregnating solution; the material A is boric acid or phosphoric acid; or, the material A is ceramics with anti-oxidation components powder; or, the material A is an organic or inorganic precursor of an antioxidant component; the antioxidant component is one or more of SiBCN, SiCO, SiC, ZrC, ZrB ₂ , HfC and HfB ₂ The solvent is one or more of xylene, ethanol and distilled water; wherein: when the material A is boric acid or phosphoric acid, the impregnating solution preparation method is: pour boric acid or phosphoric acid powder into 90°C deionized water , mechanically stirred until most of the powders are dissolved, then placed in an ultrasonic oscillator for 110°C ultrasonic vibration and dissolution to obtain a boric acid or phosphoric acid impregnating solution; wherein: the weight ratio of boric acid powder or phosphoric acid powder to water is 1:(3~ 10); When the material A is ceramic powder of anti-oxidation components, the preparation method of the impregnation solution is as follows: pour the ceramic powder into deionized water or ethanol, and mechanically stir to obtain a ceramic powder impregnation solution; wherein: the ceramic powder The weight ratio of dosage to solvent is (5~30):100; When the material A is an organic or inorganic precursor of an antioxidant component, the preparation method of the impregnating solution is as follows: mix the precursor of the antioxidant component with xylene in a certain proportion, stir magnetically for 2 to 4 hours, and prepare a precursor Body impregnation solution; wherein the weight ratio of the precursor of the antioxidant component to the xylene solvent is (5~30):100; (3) Immerse the lightweight carbon-based composite material sample in the impregnating solution prepared in step (2), and immerse the solution into the lightweight carbon-based composite material sample by ultrasonic vibration, vacuum or normal pressure impregnation method, keep it for a certain period of time, and take it out Dry the sample; (4) Place the sample of the lightweight carbon-based composite material impregnated in step (3) in a drying oven for curing and drying, and then perform high-temperature heat treatment on the composite material to obtain a lightweight carbon-ceramic composite material containing antioxidant components; (5) Steps (2) to (4) are repeated, and the composition of the modified component of the lightweight carbon-based composite material is adjusted by changing the concentration of the impregnation solution, the type of the impregnation solution, and the order of the impregnation. 2. The preparation method of heat-proof-heat-insulation-carrying integrated lightweight carbon-ceramic composite material according to claim 1, characterized in that: in step (1), the density range of the lightweight carbon-based composite material is 0.2-0.7 g/cm ³ , the lightweight carbon-based composite material is a fiber-reinforced carbon airgel composite material or a carbon foam composite material. 3. The preparation method of heat-proof-heat-insulation-carrying integrated lightweight carbon-ceramic composite material according to claim 1, characterized in that: the precursor of SiBCN is polyborosilazane PSNB, SiCO precursor It is polysiloxane PSO, the precursor of SiC is polycarbosilane PCS, the precursor of ZrC is organic zirconium precursor PZC, the precursor of ZrB ₂ is organic zirconium precursor PZB, the precursor of HfC and HfB ₂ is HfCl ₄ Organic precursor formulated for hafnium source. 4. The preparation method of heat-proof-heat-insulation-carrying integrated lightweight carbon-ceramic composite material according to claim 1, characterized in that: in step (3), when the impregnating solution is prepared with boric acid or phosphoric acid, Ultrasonic vibration impregnation is used, and the impregnation time is 0.5~2h; when the impregnation solution is prepared from ceramic powder or precursor of antioxidant components, vacuum-atmospheric pressure impregnation is used. The specific method is: put the sample in a beaker and place In the vacuum impregnation tank, evacuate the impregnation tank into a vacuum with a vacuum degree of ≤ -0.1MPa, use the pressure difference to introduce the impregnation solution into the beaker containing the sample, maintain the vacuum degree for 0.5~2h, and then maintain the normal pressure for 0.5~2h. 5. The preparation method of heat-proof-heat-insulation-carrying integrated lightweight carbon-ceramic composite material according to claim 1, characterized in that: in step (4), when boric acid or phosphoric acid impregnation solution is used, the sample is The heat treatment process is carried out in multiple steps: under a protective atmosphere, the temperature is raised from room temperature to 170~250°C at a rate of 5°C/min for 0.5~1h, and then the temperature is continued to 300~400°C for 0.5~1h, and then heated to 500~700°C and keep warm for 0.5~1h. 6. The preparation method of heat-proof-heat-insulation-carrying integrated lightweight carbon-ceramic composite material according to claim 1, characterized in that: in step (4), when the impregnating solution is ceramic powder of anti-oxidation components When the sample is prepared from the body or precursor, the heat treatment process of the sample is as follows: under the protective atmosphere, the temperature is raised to 800~1500°C at a rate of 5°C/min, and the temperature is kept for 0.5~2h. 7. The preparation method of heat-proof-heat-insulation-carrying integrated lightweight carbon-ceramic composite material according to claim 1, characterized in that: in step (5), typical impregnating solution concentration, type, sequence and obtained Modified composite materials include but are not limited to the following 10 types: (1) Impregnation sequence: 25 wt.% boric acid solution; obtained material: boron oxide modified lightweight carbon-based composite material; (2) Impregnation sequence: 25 wt.% boric acid solution, 10 wt.% PSNB; obtained material: boron oxide-SiBCN modified lightweight carbon-based composite material; (3) Impregnation sequence: 20 wt.% PSNB; obtained material: SiBCN modified lightweight carbon-based composite material; (4) Impregnation sequence: 30 wt.% phosphoric acid solution; obtained material: phosphoric acid modified lightweight carbon-based composite material; (5) Impregnation sequence: 25 wt.% PCS; obtained material: SiC modified lightweight carbon-based composite material; (6) Impregnation sequence: 30 wt.% PSO; obtained material: SiCO precursor modified lightweight carbon-based composite material; (7) Impregnation sequence: 15 wt.% PZB; obtained material: ZrB ₂ modified lightweight carbon-based composite material; (8) Impregnation sequence: 20 wt.% PCS, 10 wt.% PZC; obtained material: SiC-ZrC modified lightweight carbon-based composite material; (9) Impregnation sequence: 10 wt.% PCS, 20 wt.% HfB ₂ organic precursor; obtained material: SiC-HfB ₂ modified lightweight carbon-based composite material; (10) Impregnation sequence: 10 wt.% PCS, 15 wt.% PZB, 15 wt.% PZC; obtained material: SiC-ZrB ₂ - ZrC modified lightweight carbon-based composites. 8. A heat-proof-heat-insulation-carrying integrated lightweight carbon-ceramic composite material prepared by any one of claims 1-7, characterized in that: the material is composed of fiber reinforcement, carbon airgel and Composed of ceramic double matrix, the ceramic matrix is evenly dispersed in the carbon airgel three-dimensional nano-network structure, relying on the heat insulation-carrying capacity of the carbon airgel and the anti-oxidation ablation of the ceramic matrix to meet the long-term high-temperature aerobic environment. Functional Requirements.

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