An electron-escaping cooling thermal protection material and a preparation method and application thereof

Abstract

The present application relates to an electron emission cooling thermal protection material and a preparation method and application thereof, and belongs to the technical field of composite materials, and solves the problem that the existing electron emission cooling thermal protection material cannot consider low emission work and high conductivity. The present application provides a preparation method of an electron emission cooling thermal protection material, ceramic matrix powder and conductive reinforcing phase material are homogenized and mixed, then densification forming and high reduction heat treatment are performed to obtain a composite material with a continuous conductive network. The electron emission cooling thermal protection material prepared by the preparation method has excellent comprehensive performance, the emission work is not higher than 3.5 eV, the conductivity can be up to 4.72*10 ‑2 S / m, and is not prone to performance degradation in a high-temperature oxidation environment. After heating and oxidation treatment of the material in air at 1600 DEG C, the emission work decreases by not more than 15% and the conductivity decreases by not more than 10% after 5 minutes of oxidation treatment, and the electron emission cooling can be realized efficiently and sustainably in a severe environment.

Description

Technical Field

[0001] This invention relates to the field of composite materials technology, and in particular to an electron evaporation cooling thermal protection material, its preparation method, and its application. Background Technology

[0002] Electron emission cooling thermal protection system is a novel thermal protection technology based on the thermally induced electron emission cooling phenomenon. This technology utilizes the release of electron kinetic energy during the thermal electron emission process and the work done by the electron to overcome the potential barrier to achieve energy dissipation, and has significant advantages such as high theoretical cooling efficiency, simple structure, and no need for additional cooling medium.

[0003] However, the actual cooling efficiency of electron emission cooling thermal protection systems is highly dependent on the work function of the material. Materials with low work functions can significantly improve cooling efficiency, but in practical applications, thermally induced electron emission increases electron cloud density near the wall surface, reduces space potential, and consequently triggers space charge-limited emission, inhibiting continuous electron emission and weakening the cooling effect. Therefore, a bias voltage is typically applied to the material wall surface to guide the emitted electrons back into the material to maintain the sustainability of the cooling process. Thus, electron emission cooling materials need high electrical conductivity to apply the bias voltage and reduce Joule heat accumulation during electron migration, avoiding additional heat load. However, most thermoelectric materials currently struggle to simultaneously possess both low work function and high conductivity, resulting in technical bottlenecks in the efficiency and sustainability of electron emission cooling systems, hindering their further application and development. Furthermore, existing electron emission cooling thermal protection materials also suffer from poor high-temperature resistance. Summary of the Invention

[0004] Based on the above analysis, the present invention aims to provide a high-temperature resistant, low work function, and high conductivity electron emission cooling thermal protection material, its preparation method, and its application, in order to solve at least one of the problems of existing electron emission cooling thermal protection materials being unable to simultaneously achieve low work function and high conductivity, and having poor high-temperature resistance.

[0005] This invention discloses a method for preparing an electron evaporation cooling thermal protection material, the method comprising the following steps:

[0006] S1: Prepare ceramic matrix powder with low work function;

[0007] S2: Prepare a high-temperature resistant, highly reducing, conductive reinforcing phase material, and homogenize and mix the ceramic matrix powder with the conductive reinforcing phase material to form a composite material precursor;

[0008] S3: A compact to be sintered is obtained by densification molding, and the compact is subjected to high-reducibility heat treatment to obtain a composite material with a continuous conductive network, namely the electron escape cooling heat protection material.

[0009] Specifically, the ceramic matrix powder is CaO·Al2O3 ceramic powder, wherein the molar ratio of CaO to Al2O3 is 2:1 to 8.

[0010] The preparation process of the CaO·Al2O3 ceramic powder is as follows:

[0011] The calcium oxide precursor and γ-alumina were homogenized and mixed to obtain the CaO·Al2O3 ceramic precursor. The precursor powder was then subjected to high-temperature sintering to obtain CaO·Al2O3 ceramic.

[0012] The CaO·Al2O3 ceramic was crushed and then ultra-finely ground by ball milling to obtain CaO·Al2O3 ceramic powder with a mesh size of 60-100.

[0013] The homogenization mixing process is either ball milling or mechanical stirring. When ball milling is used, the rotation speed is 300-700 rpm and the ball-to-material ratio is 10-50:1.

[0014] The conditions for the high-temperature sintering treatment are as follows: the temperature is increased to 1100-1600℃ at a heating rate of 5-15℃ / min, held for 10-24 hours, and then decreased to 800℃ at a cooling rate of 1-5℃ / min before natural cooling; air is continuously introduced at a flow rate of 60-100mL / min during the high-temperature sintering treatment.

[0015] Specifically, the calcium oxide precursor includes one or more of silicon nitride, calcium oxide, calcium carbonate, and calcium nitrate; the molar ratio of the calcium oxide precursor to γ-alumina is 1-12:1-7.

[0016] Specifically, the highly reducible conductive reinforcing phase material mentioned in step S2 is a carbon-based conductive material;

[0017] The carbon-based conductive material is one or more of the following: high-purity graphite, multi-walled carbon nanotubes, graphene oxide, graphene, vapor-grown carbon fibers, carbon black, amorphous carbon, or carbon aerogel.

[0018] The homogenization mixing in step S2 is performed using ball milling.

[0019] When the carbon-based conductive material is a sheet material, dry ball milling is used with a rotation speed of 300-700 rpm and a ball-to-material ratio of 10-20:1.

[0020] When the carbon-based conductive material is a rod-shaped material, it is initially dispersed by ultrasonication for ≥20 min using a nano-aqueous dispersant, followed by wet ball milling at a speed of 600-800 rpm and a ball-to-material ratio of 20-40:1, with an additional 3-10% by volume of 45-60 mesh zirconium oxide beads.

[0021] When the carbon-based conductive material is a spherical material, dry ball milling is used, with a rotation speed of 400-800 rpm and a ball-to-material ratio of 20-60:1.

[0022] Specifically, in step S2, the mass ratio of ceramic matrix powder to conductive reinforcing phase material is 80-97:3-20; the ratio of the specific surface area of ​​the conductive reinforcing phase material to that of the ceramic matrix powder is 1.5-250.

[0023] Specifically, the densification molding process in step S3 is either a molding process or a cold isostatic pressing process;

[0024] The specific process and parameters are as follows: the pressure is increased to 40-300 MPa at a rate of 3-10 MPa / s, held for 100 seconds, and then reduced to atmospheric pressure at a rate of 1-5 MPa / s.

[0025] Specifically, the high-reducibility heat treatment process in step S3 adopts one of the following methods: carbon monoxide reduction, graphite embedding reduction, active metal embedding reduction, or hydrogen reduction.

[0026] When using the graphite embedding reduction method, a semi-sealed graphite crucible and high-purity graphite powder are used to embed and compact the green body to be sintered. The particle size of the high-purity graphite powder is 2.6nm~45μm, and the mass ratio of graphite powder to the green body to be sintered is 1~5:1~5. The high-purity graphite powder preferably has a flake graphite structure, which can more uniformly coat the ceramic powder and avoid agglomeration and insufficient local reduction during high-temperature heat treatment.

[0027] The specific process and parameters for high-reducibility heat treatment are as follows:

[0028] The temperature is increased to 1100–1450℃ at a heating rate of 5–15℃ / min and held for 6–24 hours. Then the temperature is decreased to 800℃ at a cooling rate of 1–5℃ / min and then allowed to cool naturally. The flow rate of the protective atmosphere argon during the heat treatment is 60–100 mL / min.

[0029] The present invention also discloses a heat protection material for cooling electron emission with high temperature resistance, low work function and high conductivity, wherein the protective material is prepared by the above preparation method;

[0030] The electrical conductivity of the protective material is ≥1.33×10⁻⁶. -2 S·m -1 The work function is ≤3.5eV. When the material is heated and oxidized in air at 1600℃, the work function decreases by no more than 15% and the conductivity decreases by no more than 10% after 5 minutes of oxidation treatment.

[0031] Furthermore, the content of the highly reducing conductive reinforcing phase material in the protective material is 3-20 wt.%, and the content of the ceramic matrix is ​​80-97 wt.%.

[0032] The conductivity-enhancing phase material has a three-dimensional interconnected network structure, forming a continuous conductive network inside the protective material;

[0033] The ceramic matrix is ​​an electron-electronized CaO·Al2O3 ceramic with an oxygen vacancy structure.

[0034] This invention also discloses the application of an electron escape cooling thermal protection material, which is used as a thermal protection material in high-speed aircraft thermal protection systems, plasma equipment, high-temperature electron emission devices, and other industrial scenarios with high-efficiency heat dissipation.

[0035] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0036] 1. The electron emission cooling thermal protection material provided by this invention has both low work function and high conductivity, with a conductivity ≥ 1.33 × 10⁻⁶. -2 S / m, work function ≤3.5eV. These characteristics effectively avoid Joule heating problems caused by potential current loops, enabling the material to exhibit excellent electron escape cooling performance, thereby significantly reducing the incoming heat on the material wall and reducing heat conduction into the material interior.

[0037] This invention uses a ceramic matrix as the main matrix material. Due to its low work function, the ceramic matrix can release a large number of electrons at high temperatures. The energy required to overcome the potential barrier and the kinetic energy of the emitted electrons effectively carry away heat during the electron escape process. Furthermore, by employing a reasonable composition design and process parameter control (selecting different mixing process parameters according to different conductive reinforcement phase sizes and shapes), the conductive reinforcement phase forms a three-dimensional interconnected network (continuous conductive network) within the ceramic matrix. This significantly reduces resistivity and increases electron mobility, reduces Joule heat accumulation, and enhances electron escape efficiency. This, in turn, ensures smooth electron return, maintains space charge balance, avoids space charge-limited emission, and thus achieves a continuous and efficient cooling effect.

[0038] It is worth noting that the three-dimensional interconnected network cannot be obtained simply by combining the ceramic matrix and the conductive reinforcing phase material (mainly referring to carbon-based conductive materials). Combining conductive carbon materials with ceramic materials to form a continuous conductive network faces several challenges, requiring the simultaneous coordination of multiple parameters: including component content (carbon material needs to account for 3–20 wt%), morphology and size (sheet thickness <1 μm, rod length 1–2 μm, spherical particle size <100 nm), and specific surface area difference (specific surface area of ​​conductive carbon materials 5–250 m²). 2 / g, ceramic powder with a specific surface area of ​​approximately 1-3m²2 Only by combining the surface area ratio of the two (1.5 to 250) with appropriate homogenization and mixing processes and pressing processes can a three-dimensional interconnected network (continuous conductive network) with an ideal structure be obtained.

[0039] Simply mixing the materials is insufficient to achieve a uniform and stable microstructure, which may lead to defects such as interruptions in the conductive network and a conductivity below 10⁻³ S / m, far from meeting the requirements for high-conductivity materials. Furthermore, insufficient or uneven distribution of the conductive phase material results in inadequate overall thermal stability and excessively high local work function, failing to meet the high-temperature stability requirements above 1600℃. Overall conductivity and low material work function are thus difficult to guarantee.

[0040] Particularly preferred, this invention utilizes rod-shaped carbon-based conductive materials and optimizes the ball milling speed, ball-to-material ratio, and dispersion process, combined with chemical dispersants and zirconia beads to achieve uniform encapsulation of conductive carbon materials and ceramic powder. The proportion of carbon material (3–20 wt.%) is adjusted according to the specific surface area and morphological properties of the material to form a network microstructure with carbon material as the framework. After adjustment, the material conductivity can reach 1.33 × 10⁻⁶. -2 With an S / m or higher, the work function can reach 2.85eV, and the temperature resistance is improved to over 1600℃, significantly enhancing conductivity and high-temperature stability.

[0041] Furthermore, the conductive reinforcing phase material exhibits high reducibility, and the resulting three-dimensional interconnect network has two functions:

[0042] (1) Forming a continuous conductive network;

[0043] (2) It is uniformly dispersed in the composite material / ceramic matrix as a reducing agent.

[0044] For example, when using graphite reduction for high-reducibility heat treatment, the outer surface of the composite material is fully reduced by graphite embedding, and the ceramic matrix inside the composite material is fully and uniformly reduced by a three-dimensional interconnected network of highly reduceable conductive reinforcing materials. The effect of this reduction is to completely remove free oxygen ions in the ceramic matrix, forming oxygen holes, and a large number of electrons are introduced into its dodecahedral cage structure, thereby significantly increasing the electron concentration of the material, and thus uniformly and significantly improving the conductivity of the entire ceramic matrix, ultimately improving the conductivity of the entire composite material.

[0045] 2. The electron evaporation cooling thermal protection material provided by this invention maintains structural integrity and conductivity under high temperature and oxidizing atmospheres, exhibits excellent oxidation resistance and thermal cycling stability, and is suitable for thermal protection scenarios with long-term service.

[0046] By introducing a conductive carbon material phase (melting point exceeding 3500℃) into the ceramic powder, a stable three-dimensional conductive phase is formed. This carbon material provides structural stability to the matrix ceramic material at high temperatures, significantly increasing the composite material's high-temperature stability above 1600℃. Furthermore, the matrix ceramic material is an oxide ceramic material, inherently possessing good oxidation resistance. The conductive carbon material constitutes only a very small portion, and the vast majority of the conductive material is embedded within the matrix material, receiving excellent protection from the oxide matrix. Therefore, the material exhibits high heat resistance and high temperature resistance. The morphology before and after high-temperature heat treatment is as follows: Figure 1 As shown, the morphology and structure are complete and the density is high. After heat treatment at 1600℃ for 300 seconds, the work function is increased by no more than 15%.

[0047] 3. The preparation method provided by the present invention is simple and does not require complicated synthesis steps. Moreover, through cold isostatic pressing technology, the entire molding process is not limited by size and can flexibly realize the free preparation of irregularly shaped components, thus having high process adaptability.

[0048] It is worth emphasizing that this invention ensures the uniformity of the two-phase material distribution during the high-temperature reduction process by homogenizing and mixing a highly reducing continuous conductive phase with CaO·Al2O3 ceramic (mainly referring to the full contact between the ceramic matrix and the three-dimensional interconnected network). The reduction process is not constrained by size, effectively overcoming the problem of insufficient reduction caused by insufficient contact with surface reducing substances in the prior art.

[0049] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0050] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0051] Figure 1 This is a photograph of the composite material prepared in Example 2 after high-temperature heat treatment.

[0052] Figure 2 The left image (a) shows the physical image of the composite material prepared in Example 1, and the right image (b) shows the physical image of the composite material prepared in Example 2.

[0053] Figure 3 The left image (a) is a scanning electron microscope image of the composite material prepared in Example 1, and the right image (b) is a scanning electron microscope image of the composite material prepared in Example 2.

[0054] Figure 4 The left image (a) is a transmission electron microscope image of the composite material prepared in Example 1, and the right image (b) is a transmission electron microscope image of the composite material prepared in Example 2.

[0055] Figure 5 The left image (a) shows the electron paramagnetic resonance spectrum of the composite material prepared in Example 1, and the right image (b) shows the electron paramagnetic resonance spectrum of the composite material prepared in Example 2.

[0056] Figure 6 This is a photograph of the composite material prepared in Comparative Example 4 after high-temperature heat treatment. Detailed Implementation

[0057] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0058] Given that existing electronic cooling and thermal protection materials cannot simultaneously achieve both high conductivity and electron work function, and have poor high-temperature resistance, this invention attempts to improve conductivity by compositing conductive reinforcing phase materials onto a ceramic matrix. Through in-depth research and extensive experiments, the researchers have identified the following technical challenges:

[0059] (1) Low addition of conductive reinforcing phase material will result in poor conductivity improvement, while excessive addition will result in a significant increase in electron work function.

[0060] (2) The poor basic conductivity of the ceramic matrix leads to poor uniformity of conductivity of the entire composite material. Even if homogenization has been basically achieved, the conductivity of the surface and interior of the composite material is still uneven, and there are still potential current loops, which leads to Joule heating.

[0061] (3) Traditional ceramic matrices will experience performance degradation under high-temperature oxidation conditions, which limits their application in extreme environments.

[0062] Based on the above technical difficulties, the researchers of this invention creatively proposed the following approach to prepare a novel composite material, overcoming the shortcomings such as the inability to simultaneously achieve high conductivity and electron work function, and poor high-temperature resistance:

[0063] (1) By selecting a highly reducible conductive reinforcing phase material and selecting different homogenization process parameters according to the morphological characteristics of different materials, a uniform three-dimensional interconnected network can be formed inside the composite material with a lower addition amount.

[0064] (2) The three-dimensional interconnected network improves the electrical conductivity of the composite material relatively uniformly on the one hand, and acts as a reducing material inside the composite material (uniformly distributed and fully in contact with the ceramic matrix) during the high-reducibility heat treatment process.

[0065] (3) CaO·Al2O3 ceramic is preferred as the ceramic matrix (good heat resistance and strong oxidation resistance). During the high reduction heat treatment process, it is ionized, that is, free oxygen ions are transformed into oxygen holes, and a large number of electrons are introduced into its dodecahedral cage structure, thereby significantly increasing the electron concentration of the material and significantly improving the conductivity of the ceramic matrix, making the conductivity of the entire composite material more uniform.

[0066] In summary, this thermal protection material possesses a metal oxide ceramic matrix with high high-temperature stability and oxidation resistance, and a continuous conductive reinforcing framework (three-dimensional interconnected network) with low work function and high electrical conductivity. The coupled design of the continuous conductive reinforcing phase and the high-temperature stable CaO·Al2O3 ceramic achieves the complementary advantages of the two types of materials, resulting in a highly sustainable and efficient electron escape process.

[0067] This invention discloses a method for preparing an electron evaporation cooling thermal protection material, the method comprising the following steps:

[0068] S1: Prepare ceramic matrix powder with low work function;

[0069] S2: Prepare a high-temperature resistant, highly reducing, conductive reinforcing phase material, and homogenize and mix the ceramic matrix powder with the conductive reinforcing phase material to form a composite material precursor;

[0070] S3: A compact to be sintered is obtained by densification molding, and the compact is subjected to high-reducibility heat treatment to obtain a composite material with a continuous conductive network, namely the electron escape cooling heat protection material.

[0071] The ceramic matrix powder is CaO·Al2O3 ceramic powder, wherein the molar ratio of CaO to Al2O3 is 2:1 to 8, for example, 2:1, 2:2, 2:3, 2:4, 2:5, 2:6, 2:7, 2:8. The ceramic matrix exhibits strong high-temperature stability, good oxidation resistance, and low electron work function.

[0072] The preparation process of the CaO·Al2O3 ceramic powder is as follows:

[0073] The calcium oxide precursor and γ-alumina were homogenized and mixed to obtain CaO·Al2O3 ceramic precursor. The precursor powder was subjected to high-temperature sintering to obtain CaO·Al2O3 ceramic with strong high-temperature stability.

[0074] The CaO·Al2O3 ceramic is crushed and then subjected to ultrafine grinding (particle size below 0.25 mm) by ball milling to obtain CaO·Al2O3 ceramic powder of 60-100 mesh, preferably 100 mesh. At this time, the specific surface area of ​​the ceramic material is 1-3 m². 2 / g, which allows for sufficient contact between conductive carbon materials (conductive reinforcing phase).

[0075] The homogenization mixing process is either ball milling or mechanical stirring.

[0076] When using ball milling, the rotation speed is 300-700 rpm and the ball-to-material ratio is 10-50:1.

[0077] The conditions for the high-temperature sintering treatment are as follows: the temperature is increased to 1100-1600℃ at a heating rate of 5-15℃ / min to ensure the formation of the CaO·Al2O3 ceramic crystal phase and to avoid abnormal grain growth caused by over-firing.

[0078] The ceramic is kept at a temperature of 10–24 h, then cooled to 800 °C at a rate of 1–5 °C / min and then allowed to cool naturally to optimize the densification and work function of the ceramic. The cooling rate can be 1, 2, 3, 4, or 5 °C / min.

[0079] During the high-temperature sintering process, air is continuously introduced at a flow rate of 60–100 mL / min. For example, 60, 70, 80, 90, or 100 mL / min.

[0080] Preferably, after preparing CaO·Al2O3 ceramics, the crystal phase structure is analyzed by X-ray diffraction (XRD) to confirm the formation of CaO·Al2O3 ceramics.

[0081] Specifically, the calcium oxide precursor includes, but is not limited to, one or more of calcium oxide, calcium carbonate, and calcium nitrate; the molar ratio of the calcium oxide precursor to γ-alumina is 1–12:1–7. The selection of the calcium oxide precursor is not limited to, but includes, one or more of calcium oxide, calcium carbonate, and calcium nitrate. Calcium oxide, as a key reactant, can be directly mixed with alumina, which is beneficial for achieving high-purity calcium-aluminum ceramic phase materials. Calcium oxide generated through the decomposition or conversion of the precursor at high temperatures has high chemical stability and reactivity, avoiding the influence of environmental factors such as moisture. The selection of these precursors aims to optimize the pyrolysis behavior, reaction uniformity, and compatibility with γ-alumina of the raw materials to ensure the structural integrity of the composite material.

[0082] Furthermore, the molar ratio of calcium oxide precursor to γ-alumina was set at 1–12:1–7, as shown below. This was based on the stoichiometric requirements of the calcium-aluminum compounds generated in the reaction and the control of material properties. Lower molar ratios are suitable for generating a single calcium-aluminum phase, enhancing the material's high-temperature stability (>1600℃); higher molar ratios (close to 12:7) promote the formation of multiphase structures, optimizing electrical conductivity and specific surface area. This range was experimentally verified to balance the crystal phase composition, porosity, and interfacial bonding effect with conductive carbon materials of the reaction products.

[0083] Specifically, the conductive reinforcing phase material has a conductivity higher than 100 S / m at room temperature.

[0084] Specifically, in step S2, the mass ratio of ceramic matrix powder to conductive reinforcing phase material is 80-97:3-20; the ratio of the specific surface area of ​​the conductive reinforcing phase material to that of the ceramic matrix powder is 1.5-250.

[0085] Specifically, the highly reducible conductive reinforcing phase material mentioned in step S2 is a high-temperature resistant carbon-based conductive material;

[0086] The carbon-based conductive material is one or more of the following: high-purity graphite, multi-walled carbon nanotubes, graphene oxide, graphene, vapor-grown carbon fibers, carbon black amorphous carbon, or carbon aerogel.

[0087] It is worth noting that, due to the different structural characteristics of various carbon-based conductive materials, different addition amounts and homogenization mixing process parameters should be matched to ensure that they can subsequently form a three-dimensional interconnect network. The mixing process should be adjusted according to the specific surface area and morphological characteristics of the materials; the preferred specific surface area of ​​the ceramic powder is 1–3 m². 2 / g, the specific surface area of ​​sheet-like carbon materials (such as high-purity graphite, graphene, and graphene oxide) is 5-10m². 2 / g, the specific surface area of ​​rod-shaped carbon materials (such as multi-walled carbon nanotubes and vapor-grown carbon fibers) is 30-50m². 2 / g, the specific surface area of ​​spherical carbon materials (such as carbon black, amorphous carbon, carbon aerogel) is 150-250m². 2 / g. The ratio of the specific surface area of ​​conductive carbon materials to ceramic materials varies depending on the material morphology. For sheet-like materials, it is approximately 1.5 to 10; for rod-like materials, it is approximately 10 to 50; and for spherical materials, it is approximately 50 to 250.

[0088] To ensure that the conductivity meets the standard, the mass ratio of conductive carbon material needs to be selected based on the material's structural characteristics and specific surface area ratio:

[0089] When the specific surface area ratio is 1.5 to 10, the mass fraction of the conductive material needs to be at least 8 wt.% to achieve a conductivity ≥ 4.72 × 10⁻² S / m;

[0090] When the specific surface area ratio is 10 to 50, the mass fraction of the conductive material needs to be ≥3wt.% to achieve a conductivity ≥1.33×10-2S / m;

[0091] When the specific surface area ratio is 50–250, the mass fraction of the conductive material must be at least 6 wt.% to ensure a conductivity ≥ 3.54 × 10⁻⁶. -2 S / m. While excessively high carbon content can significantly improve conductivity, it also leads to a significant increase in the work function (which can increase to over 5 eV), affecting thermoelectric emission performance and long-term stability.

[0092] Specifically, when high-purity graphite powder is used, the homogenization process in S2 is as follows:

[0093] Dry ball milling is employed, with the rotation speed controlled between 300 and 700 rpm and the ball-to-powder ratio set at 10 to 20:1. Excessive ball-to-powder ratio should be avoided to prevent powder accumulation and compaction. The addition of high-purity graphite powder should be at least 8 wt.% to ensure an electrical conductivity ≥ 4.72 × 10⁻⁶. -2 S / m.

[0094] Specifically, when multi-walled carbon nanotubes are used, the homogenization process in S2 is as follows: first, ultrasonic treatment with a nano-aqueous dispersant for at least 20 minutes (e.g., 20, 25, or 30 minutes) is performed to achieve initial dispersion. Then, wet ball milling is carried out at a speed of 600–800 rpm with a ball-to-material ratio of 20–40:1. Additionally, 3–10% by volume of 45–60 mesh zirconia beads are added to enhance the ball-to-material contact area and fully disperse the easily agglomerated rod-shaped structure. The mass fraction of multi-walled carbon nanotubes added must be at least 3 wt.% to ensure an electrical conductivity ≥ 1.33 × 10⁻² S / m.

[0095] Specifically, when carbon black is selected, the homogenization process in S2 is as follows: dry ball milling is adopted, with a speed range of 400-800 rpm, the ball-to-material ratio is adjusted to 20-60:1, and the mass fraction of carbon black added is at least 6 wt.% to ensure that the electrical conductivity is ≥3.54×10⁻⁶. -2 S / m.

[0096] Preferably, the carbon-based conductive material is a multi-walled carbon nanotube (MWCNT), which is composed of multiple concentric cylindrical graphene layers and has a specific surface area of ​​approximately 30–60 m². 2 / g. Its tubular structure originates from sp. 2 The six-membered ring network formed by hybrid carbon atoms exhibits excellent electron transport capabilities (conductivity can reach 10). 4Multi-walled carbon nanotubes (S / m) can form a continuous conductive network in a ceramic matrix due to their high aspect ratio and specific surface area, significantly improving the electrical conductivity of the composite material. Simultaneously, their multilayer structure enhances thermal stability, and their tight bonding with the ceramic matrix interface allows for temperature resistance exceeding 1600℃, superior to sheet-like or spherical carbon materials, making them a preferred material for achieving high conductivity and high-temperature stability.

[0097] Specifically, the densification molding process in step S3 is either a molding process or a cold isostatic pressing process;

[0098] The specific process and parameters are as follows: the pressure is increased to 40-300 MPa at a rate of 3-10 MPa / s, held for 100 seconds, and then reduced to atmospheric pressure at a rate of 1-5 MPa / s.

[0099] Specifically, the high-reducibility heat treatment process in step S3 adopts one of the following methods: carbon monoxide reduction, graphite embedding reduction, active metal embedding reduction, or hydrogen reduction; preferably, it adopts graphite embedding reduction, wherein the graphite particle size in the graphite embedding reduction method is preferably 1 nm to 100 μm, and the mass ratio of graphite to green body is preferably 1 to 10: 1 to 5.

[0100] More preferably, when using the graphite embedding reduction method, a semi-sealed graphite crucible and high-purity graphite powder are used to embed and compact the green body to be sintered. The particle size of the high-purity graphite powder is 2.6 nm to 45 μm, and the mass ratio of graphite powder to the green body to be sintered is 1–5:1–5. The parameter selection is based on the matching and bonding requirements of the specific surface area of ​​carbon materials and ceramic materials. The graphite powder particle size range (2.6 nm to 45 μm) corresponds to a specific surface area of ​​approximately 5–10 m². 2 / g, with ceramic matrix (specific surface area 1-3m²) 2 The ratio of the specific surface area of ​​graphite powder to its g / g is 1.5–10 to ensure that the graphite powder can uniformly coat the ceramic particles, forming an effective contact interface, promoting the reduction reaction, and improving the conductivity of the conductive network. A mass ratio of 1–5:1–5 ensures that the graphite powder is sufficient to embed the green body (when the mass ratio is ≥1:1, the coating rate is >90%). High-purity graphite powder with a flake structure is preferred for embedding, as it has more sufficient contact with the surface of the green body and a better reduction effect.

[0101] It is worth noting that the interior of the green body is actually "filled" with a reducing agent, namely a highly reducing conductive reinforcing phase material (mainly carbon-based material) that forms a three-dimensional interconnected network. This material was subsequently reduced / electronized inside the green body.

[0102] The specific process and parameters for high-reducibility heat treatment are as follows:

[0103] The temperature is increased to 1100–1450℃ at a heating rate of 5–15℃ / min and held for 6–24 hours. Then the temperature is decreased to 800℃ at a cooling rate of 1–5℃ / min and then allowed to cool naturally. The flow rate of the protective atmosphere argon during the heat treatment is 60–100 mL / min.

[0104] It is worth noting that the selection of heat treatment parameters is based on the reduction process characteristics of the ceramic matrix under the action of reducing substances. A heating rate of 5–15 °C / min ensures gradual heating to 1100–1450 °C, avoiding rapid heating that could cause cracking of the green body. Simultaneously, to meet the thermodynamic requirements of C12A7 reduction, the carbon material can only effectively remove oxygen atoms to form oxygen vacancies, thereby generating C12A7:e, when the reduction process reaches a specific temperature. - The heat preservation time of 6–24 hours is based on the requirements of kinetic control; electron migration and diffusion require sufficient time (diffusion coefficient D≈10). -12 m 2 / s) to achieve uniform reduction within the material (electron concentration up to 10). 18-21 cm -3 The cooling rate is 1–5 °C / min up to 800 °C, combined with natural cooling, to prevent thermal stress caused by rapid cooling and ensure crystal structure stability. An argon flow rate of 60–100 mL / min provides a protective atmosphere to maintain a reducing environment (oxygen partial pressure <10). -3 Pa) to avoid oxidation and ensure uniform gas flow.

[0105] Specifically, after high-reducibility heat treatment, the reducing substances (graphite, conductive reinforcing phase materials, etc.) fully react with CaO·Al2O3 ceramics. After the reduction reaction, the electronic process of CaO·Al2O3 ceramic materials is successfully realized.

[0106] This invention also discloses a high-temperature resistant, low work function, and high-conductivity electron emission cooling thermal protection material, which is prepared by the above-described method; the conductivity of the protective material is ≥1.33×10⁻⁶. -2 S·m -1 The work function is ≤3.5eV. When the material is heated and oxidized in air at 1600℃, the work function decreases by no more than 15% and the conductivity decreases by no more than 10% after 5 minutes of oxidation treatment.

[0107] The protective material contains 3-20 wt.% highly reducing conductive reinforcing phase material and 80-97 wt.% ceramic matrix.

[0108] The conductivity-enhancing phase material has a three-dimensional interconnected network structure, forming a continuous conductive network inside the protective material;

[0109] The ceramic matrix is ​​an electron-electronized CaO·Al2O3 ceramic with an oxygen vacancy structure.

[0110] This invention also discloses the application of an electron emission cooling thermal protection material, which is used as a thermal protection material in the thermal protection system of high-speed aircraft, plasma equipment (such as high-temperature electron emission devices), and other industrial scenarios with high-efficiency heat dissipation.

[0111] Specifically, the high-temperature resistant, low work function, and high-conductivity electron emission cooling thermal protection material can be used in the field of thermal protection materials, particularly in critical hot-end areas of aircraft and large-area heat protection areas such as windward surfaces. In addition, the material can also be used in plasma equipment (such as plasma surface materials in nuclear fusion reactors), high-temperature electron emission devices (such as field emission cathodes), and other industrial scenarios requiring efficient heat dissipation (such as high-temperature furnace linings or spacecraft reentry thermal protection layers). Due to its high conductivity and low work function, the material can also be used as an electron emission material or conductive coating in high-temperature environments, applied to field emission displays or high-temperature sensors.

[0112] Example 1

[0113] A low work function and high conductivity electron emission cooling thermal protection material and its preparation method, the method comprising the following steps:

[0114] Step 1. Select calcium carbonate with a particle size ≤30μm and a purity ≥99.0% and γ-alumina powder with a particle size ≥500nm and a purity ≥99.99% as raw materials, and blend them in a spherical ink jar at a molar ratio of 12:7. Use a planetary ball mill to ball-mill the mixture at 600rpm with a ball-to-material ratio of 10:1 for a total of 3 milling cycles. Each milling cycle consists of 10 minutes of ball milling followed by 5 minutes of cooling to prevent overheating and decomposition of the powder. The final product is CaO·Al2O3 ceramic powder to be sintered.

[0115] Step 2. Place the precursor powder in an alumina crucible, introduce dry air at a rate of 80 mL / min, raise the temperature to 1400℃ at a rate of 5℃ / min, hold for 10 h, and then lower the temperature to 800℃ at a rate of 3℃ / min to prevent the ceramic powder from crumbling. Then allow it to cool naturally.

[0116] Step 3. Next, place the high-temperature stable CaO·Al2O3 ceramic in a ball mill jar and crush and refine it using a planetary ball mill at a speed of 700 rpm and a ball-to-material ratio of 30:1.

[0117] Step 4. Premix the high-temperature stable CaO·Al2O3 ceramic powder with high-purity graphite powder with a particle size ≤2.6μm and a purity ≥99.95% at a mass fraction of 8wt.%, and then use a planetary ball mill to ball-mill the mixture at a ball-to-material ratio of 10:1 and a rotation speed of 600rpm.

[0118] Step 5. The mixed powder is first pre-pressed using a molding process at a pressure of 40 MPa, with a pressurization rate of 1 MPa / s, a holding pressure of 40 MPa for 10 min, and a depressurization rate of 1 MPa / s to obtain the blank to be sintered.

[0119] Step 6. Place the pre-pressed, high-temperature stable CaO·Al2O3 ceramic mixed with high-purity graphite powder into a cold isostatic pressing equipment, pressurize it to 300MPa at a pressurization rate of 3MPa / s, hold it for 100s, and then reduce the pressure to normal at a depressurization rate of 3MPa / s to obtain a high-density pre-sintered body.

[0120] Step 7. Place the high-density green body to be sintered in a high-purity graphite crucible, and use high-purity graphite powder with a particle size ≤45μm and a purity of 99.95% to embed the green body to be sintered, and compact it by vibration. The mass ratio of graphite powder to green body to be sintered is 5:1.

[0121] Step 8. Place the graphite crucible in a corundum tube furnace, raise the temperature to 1450℃ at a rate of 5℃ / min, hold for 24 hours, then lower the temperature to 800℃ at a rate of 3℃ / min, followed by natural cooling. During sintering, high-purity argon gas with a purity ≥99.999% is introduced at a flow rate of 60 mL / min, ultimately obtaining the composite material 8-Graphite-12CaO·7Al2O3:e- (abbreviated as: 8-Graphite-C12A7:e-).

[0122] After preparation, the integrity of the CaO·Al2O3 ceramic crystal phase was confirmed by XRD, the distribution of the conductive network was observed by SEM, and the conductivity was tested by the four-probe method to ensure that the material properties met expectations.

[0123] Figure 2 (a) is a physical image of the 8-Graphite-C12A7:e- composite material prepared in Example 1. Its structure has a dense and smooth surface with high integrity and densification.

[0124] Figure 3 Image (a) is a scanning electron microscope image of the 8-Graphite-C12A7:e- composite material prepared in Example 1. Observation revealed that the graphite flakes were slightly dispersed in the ceramic matrix, forming a continuous conductive network.

[0125] Figure 4 Image (a) is a transmission electron microscope image of the 8-Graphite-C12A7:e- composite material prepared in Example 1. The image shows that the graphite sheets exhibit a layered, regular two-dimensional structure at the nanoscale, with characteristic spacings marked as 0.233 nm and 0.337 nm, corresponding to the (5-10) and (002) crystal planes of graphite, respectively, indicating that the graphite sheets maintain their highly ordered graphene layer stacking. However, the connections between the graphite sheets cannot be made very tight, which may lead to limited charge transport, affecting the conductivity of the composite material and exhibiting local dispersion and a small number of conductive paths.

[0126] Figure 5 Image (a) shows the electron paramagnetic resonance (EPR) spectra of the 8-Graphite-C12A7:e- composite material. The spectrum exhibits a sharp resonance peak with a g-value of approximately 2.002, consistent with the characteristic signals reported for oxygen vacancies in graphite. This indicates that oxygen vacancies were formed in the graphite during material preparation, thus partially completing the reduction reaction and potentially generating a certain electron concentration. This increased electron concentration may have the potential to enhance the thermoelectric emission properties of the material.

[0127] Overall, the performance of a three-dimensional interconnect network formed using sheet-like carbon-based conductive materials is slightly worse than that of a three-dimensional interconnect network formed using rod-shaped carbon-based conductive materials of the same mass content.

[0128] Example 2

[0129] Example 2 is basically the same as Example 1, except that high-temperature stable CaO·Al2O3 ceramic powder and high-purity multi-walled carbon nanotubes with a diameter of 20-40 nm, a length of 1-2 μm, and a purity of ≥95% are premixed at a mass fraction of 3 wt.%, and then ball-milled using a planetary ball mill to finally obtain 3-CNT-C12A7:e - .

[0130] Figure 2 Image (b) shows the physical specimen of the 3-CNT-C12A7:e- composite material prepared in Example 2, which exhibits a high degree of surface densification.

[0131] Figure 3 As shown in (b), CNTs are densely distributed in the form of clustered fibers, closely contacting or connecting with each other to form a conductive path similar to a "mesh". The elongated shape of CNTs and the physical connection between ceramics reduce resistance, resulting in high overall material conductivity.

[0132] Figure 4Image (b) shows a transmission electron microscope (TEM) image of the 3-CNT-C12A7:e-prepared material prepared in Example 2. The image shows that the CNTs exhibit a slender, tubular morphology at the nanoscale, with a characteristic spacing of 0.36 nm, corresponding to the graphene interlayer spacing (002 crystal plane) of the CNTs, indicating a complete and ordered structure. The CNTs are connected through physical contact or entanglement, forming highly efficient conductive paths. This dense, interconnected distribution reduces resistance and significantly improves the electrical conductivity of the composite material, exhibiting excellent electrical properties.

[0133] Figure 5 Figure (b) shows the electron paramagnetic resonance spectra of the 3-CNT-C12A7:e- composite material and pure CNTs. - A significant resonance signal appeared at g = 2.002, while the signal from pure CNTs was weaker and lacked a distinct characteristic peak. This g value is close to the g-factor of free electrons (g ≈ 2.0023), indicating a high concentration of free electrons in the composite material. This phenomenon originates from C12A7:e - After oxygen vacancies are formed, reduction treatment introduces a large number of electrons into its dodecahedral cage structure, significantly increasing the electron concentration of the material. In contrast, CNTs themselves exhibit a weaker paramagnetic signal, indicating a lower free electron concentration. C12A7:e - The high electron concentration helps to reduce the electron work function of the material and improve the electron emission performance, providing support for its application in low work function electron sources and field emission, and also helps to reduce the resistance of the ceramic matrix.

[0134] The three-dimensional interconnected network formed by the rod-shaped carbon-based conductive material has better continuity and is more uniform, but because the amount added is lower than that in Example 1, the absolute value of the conductivity is lower than that in Example 1.

[0135] Example 3

[0136] Example 3 is basically the same as Example 1, except that CaO·Al2O3 ceramic powder with strong high-temperature stability is premixed with high-purity carbon black with a particle size ≤150μm and a purity ≥99.0% at a mass fraction of 6wt.%, and then ball-milled using a planetary ball mill to finally obtain 6-balckcarbon-C12A7:e - .

[0137] Comparative Example 1

[0138] Comparative Example 1 is basically the same as Example 1, except that high-temperature stable CaO·Al2O3 ceramic powder and high-purity graphite powder with a particle size ≤2.6μm and a purity ≥99.95% are premixed at a mass ratio of 5wt.%, and then ball-milled using a planetary ball mill to finally obtain 55wt.% Graphite-C12A7:e- .

[0139] Comparative Example 2

[0140] Comparative Example 2 is basically the same as Example 2, except that the high-temperature stable CaO·Al2O3 ceramic powder and high-purity multi-walled carbon nanotubes with a diameter of 20-40nm, a length of 1-2μm, and a purity of ≥95% are premixed at a mass ratio of (99:1) and then ball-milled using a planetary ball mill to finally obtain the composite material.

[0141] Comparative Example 3

[0142] Comparative Example 3 is basically the same as Example 3, except that the high-temperature stable CaO·Al2O3 ceramic powder and high-purity carbon black with a particle size ≤150μm and a purity ≥99.0% are premixed at a mass fraction of 3wt.%, and then ball-milled using a planetary ball mill to finally obtain the composite material.

[0143] In Comparative Examples 1-3, the amount of conductive reinforcing phase added was lower than the minimum amount added corresponding to the specific surface area, which prevented the formation of an ideal three-dimensional interconnected network (due to defects such as discontinuity and inhomogeneity). This resulted in a decline in the overall performance of the composite material, including electrical conductivity and cooling performance (due to insufficient electronification of the ceramic matrix).

[0144] Comparative Example 4

[0145] Comparative Example 4 is essentially the same as Example 1, except that nano-titanium powder (particle size 60 nm, purity ≥99.8%) is used as the conductive reinforcing phase, with a mass ratio of CaO·Al2O3 ceramic:titanium powder = 19:1. The resulting composite material has an electrical conductivity of 1.46 × 10⁻⁶. 1 S·m -1 With a work function of 2.91 eV, it exhibits excellent potential for electron exhaust cooling. After preparation, the integrity of the CaO·Al2O3 ceramic crystal phase was confirmed by XRD, the distribution of the conductive network was observed by SEM, and the conductivity was tested using the four-probe method to ensure that the material properties met expectations.

[0146] Although titanium powder can form a good conductive network (and also has reducing properties) and significantly improve conductivity, its poor high-temperature resistance leads to a precipitous drop in the composite material's performance after high-temperature treatment (structural damage). This makes it unsuitable for applications under extreme conditions. The actual product after high-temperature heat treatment is shown in the image below. Figure 6 As shown.

[0147] Comparative Example 5

[0148] An electron evaporation cooling thermal protection material was selected as a comparative material; the preparation process of the selected electron evaporation cooling thermal protection material is consistent with Example 1 in patent CN 115504814 B.

[0149] The conductivity test materials were conducted according to national standards GB / T 26074-2010 and SEMI MF43-0705. The work function was determined using ultraviolet photoelectron spectroscopy (UPS) with silver (Ag) as a standard, whose known work function is 4.26 eV. During the test, the Fermi level of silver was first measured to calibrate the instrument's energy scale. Subsequently, the sample under test was tested under the same conditions, and its work function was calculated based on the position of the secondary electron cutoff edge. This method can accurately characterize the electron emission characteristics of material surfaces, providing a reliable basis for evaluating their electron emission performance.

[0150] The high-temperature treatment conditions were as follows: heat treatment was carried out in an air atmosphere at 1600℃ in a rapid pyrolysis furnace for 300s.

[0151] Table 1. Summary of composite material properties for each embodiment and comparative example.

[0152]

[0153] *The specific surface area ratio is the ratio of the specific surface area of ​​the conductive reinforcing phase material to that of the ceramic matrix powder.

[0154] In summary, the protective material provided by this invention has an electrical conductivity ≥ 1.33 × 10⁻⁶. -2 With a work function ≤3.5eV, the work function decreases by no more than 15% and the conductivity decreases by no more than 10% after the material is heated and oxidized in air at 1600℃ for 5 minutes.

[0155] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing an electron evaporation cooling thermal protection material, characterized in that, The preparation method includes the following steps: S1: Prepare ceramic matrix powder with low work function; S2: Prepare a high-temperature resistant, highly reducing, and conductive reinforcing phase material. Homogenize and mix the ceramic matrix powder with the highly reducing, and conductive reinforcing phase material to form a composite material precursor. S3: A compact to be sintered is obtained by densification molding, and the compact is subjected to high-reducibility heat treatment to obtain a composite material with a continuous conductive network, namely the electron escape cooling heat protection material. The ceramic matrix powder is CaO·Al2O3 ceramic powder, wherein the molar ratio of CaO to Al2O3 is 2:1 to 8; The highly reducing conductive reinforcing phase material mentioned in step S2 is a carbon-based conductive material; In step S2, the mass ratio of ceramic matrix powder to highly reducing conductive reinforcing phase material is 80-97:3-20; the ratio of the specific surface area of ​​the highly reducing conductive reinforcing phase material to that of ceramic matrix powder is 1.5-250. Wherein, when the carbon-based conductive material is a sheet material, the morphological size is: sheet thickness < 1 μm; when the carbon-based conductive material is a rod material, the morphological size is: rod length 1~2 μm; when the carbon-based conductive material is a spherical material, the morphological size is: spherical particle size < 100 nm.

2. The preparation method according to claim 1, characterized in that, The ceramic matrix powder is CaO·Al2O3 ceramic powder, wherein the molar ratio of CaO to Al2O3 is 2:2 to 8.

3. The preparation method according to claim 1, characterized in that, The preparation process of the CaO·Al2O3 ceramic powder is as follows: The calcium oxide precursor and γ-alumina were homogenized and mixed to obtain the CaO·Al2O3 ceramic precursor. The precursor powder was then subjected to high-temperature sintering to obtain CaO·Al2O3 ceramic. The CaO·Al2O3 ceramic was crushed and then subjected to ultrafine grinding by ball milling to obtain CaO·Al2O3 ceramic powder with a mesh size of 60-100.

4. The preparation method according to claim 3, characterized in that: The calcium oxide precursor includes one or more of silicon nitride, calcium oxide, calcium carbonate, and calcium nitrate; the molar ratio of the calcium oxide precursor to γ-alumina is 1-12:1-7.

5. The preparation method according to claim 1, characterized in that, The carbon-based conductive material is one or more of the following: high-purity graphite, multi-walled carbon nanotubes, graphene oxide, graphene, vapor-grown carbon fibers, carbon black, amorphous carbon, or carbon aerogel.

6. The preparation method according to claim 1, characterized in that, In step S3, the densification molding process is either molding or cold isostatic pressing. The specific process and parameters are as follows: the pressure is increased to 40-300 MPa at a rate of 3-10 MPa / s, held, and then reduced to atmospheric pressure at a rate of 1-5 MPa / s.

7. The preparation method according to claim 1, characterized in that, The high-reducibility heat treatment process in step S3 adopts one of the following methods: carbon monoxide reduction, graphite embedding reduction, active metal embedding reduction, or hydrogen reduction.

8. A heat-protective material for cooling electron emissions, characterized by high temperature resistance, low work function, and high conductivity, wherein... The protective material is prepared by the preparation method according to any one of claims 1 to 7; the electrical conductivity of the protective material is ≥1.33×10⁻⁶. -2 S·m -1 The work function is ≤3.5 eV. When the material is heated and oxidized in air at 1600℃, the work function decreases by no more than 15% and the conductivity decreases by no more than 10% after 5 minutes of oxidation treatment.

9. The application of an electron evaporation cooling thermal protection material, characterized in that, The application of the protective material of claim 8 in the field of thermal protection.

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