A mullite heat-insulating brick and a preparation method thereof

Abstract

The present invention relates to the field of chemical engineering technology, and discloses a mullite heat-insulating brick and a preparation method thereof. The heat-insulating brick enhances the toughness of the matrix through uniformly distributed mullite whiskers, optimizes the mechanical properties and heat insulation performance by adopting a gradient porous structure, and introduces Ni-Ti particles to regulate the thermal conductivity. The flexural strength of the heat-insulating brick can reach 30-34 MPa, the thermal conductivity is as low as 0.14-0.16 W / m·K, and it has excellent thermal shock resistance at a high temperature of 1600 °C. Compared with traditional mullite heat-insulating bricks, the product of the present invention exhibits higher mechanical strength, excellent heat insulation effect and intelligent heat management ability, and is applicable to fields such as high-temperature kilns, metallurgical industry, and thermal protection of spacecraft.

Description

Technical Field

[0001] The present invention relates to the field of chemical engineering technology, and more specifically, to a mullite heat-insulating brick and a preparation method thereof. Background Art

[0002] Mullite heat-insulating bricks are refractory materials widely used in high-temperature industrial furnaces, metallurgical equipment, and the aerospace field. Mullite (3Al2O3·2SiO2) has excellent thermal stability, low thermal conductivity, and relatively high mechanical strength, so it is favored in high-temperature heat-insulating applications. However, traditional mullite heat-insulating bricks still have some technical bottlenecks in practical applications. First, traditional mullite heat-insulating bricks usually rely on increasing the porosity to reduce the thermal conductivity, but this method often significantly reduces the mechanical strength of the material, making the heat-insulating bricks fragile and prone to cracking, which limits their application in extreme environments. Second, the thermal shock resistance of mullite heat-insulating bricks is poor, and cracks are likely to expand due to the accumulation of thermal stress in high-temperature environments, thus affecting the service life. In addition, due to the high difficulty of sintering densification of mullite, traditional preparation methods are difficult to maintain low thermal conductivity while ensuring mechanical properties, which poses a challenge for the material to balance strength and heat-insulating performance in practical applications.

[0003] Therefore, there is an urgent need for a new type of mullite heat-insulating brick and a preparation method thereof to solve the problems existing in the prior art. Summary of the Invention

[0004] In view of this, the present invention provides a mullite heat-insulating brick and a preparation method thereof. Through innovative technologies such as whisker modification, gradient structure optimization, and Ni-Ti thermosensitive particle regulation, the comprehensive performance of the mullite heat-insulating brick is significantly improved, the contradiction between the mechanical strength and heat-insulating performance of traditional materials is solved, the thermal shock resistance of the material is enhanced, and it is given a thermosensitive regulation function, making it more suitable for high-temperature complex environments.

[0005] The present invention provides a mullite heat-insulating brick and a preparation method thereof. Through innovative technologies such as whisker modification, gradient structure optimization, and Ni-Ti thermosensitive particle regulation, the comprehensive performance of the mullite heat-insulating brick is significantly improved, the contradiction between the mechanical strength and heat-insulating performance of traditional materials is solved, the thermal shock resistance of the material is enhanced, and it is given a thermosensitive regulation function, making it more suitable for high-temperature complex environments.

[0006] The present invention provides a preparation method of a mullite heat-insulating brick, comprising the following steps:

[0007] Immerse nano-mullite whiskers in an ethanol solution of 1.5 wt% silane coupling agent, and after ultrasonic treatment, dry them in vacuum at 60 °C for 2 h;

[0008] The nano-porous alumina template was immersed in the mullite precursor sol, filled by vacuum-assisted impregnation, and then centrifuged to remove the excess sol, obtaining the hybrid matrix material;

[0009] The hybrid matrix material, 20 wt% modified whiskers and 2 wt% Ni-Ti alloy micropowders were paved and a pre-pressure of 5 MPa was applied to obtain the surface dense layer;

[0010] The hybrid matrix material, 15 wt% modified whiskers and 3 wt% Ni-Ti alloy micropowders were paved on the surface dense layer and then centrifuged at 2000 rpm for 3 min to obtain the composite of the surface dense layer - intermediate transition layer;

[0011] The hybrid matrix material, 10 wt% modified whiskers, 5 wt% Ni-Ti alloy micropowders and 5% pore former were paved on the composite of the surface dense layer - intermediate transition layer and then statically pressed at 2 MPa to obtain the composite of the surface dense layer - intermediate transition layer - internal porous layer;

[0012] The composite of the surface dense layer - intermediate transition layer - internal porous layer was sintered by a multi-stage controlled oxygen sintering method to make the porosity of the surface dense layer, intermediate transition layer and internal porous layer reach 15% ± 3%, 25% ± 3% and 40% ± 3% respectively;

[0013] A 0.1 mm thick mullite-silicon carbide composite coating was sprayed on the surface of the sintered heat-insulating brick and heat-treated at 1500 °C for 1 h.

[0014] Preferably, the Ni-Ti alloy micropowders have a particle size of 1 - 5 μm and a phase transition temperature range of 300 - 400 °C.

[0015] Preferably, the mullite precursor is a nano-powder with Al2O3:SiO2 = 3:2, and its proportion is 70% of the matrix material.

[0016] Preferably, the process of the multi-stage controlled oxygen sintering includes:

[0017] Pre-sintering: Heating to 1000 °C at 5 °C / min, holding for 2 h, in air atmosphere, and the pre-sintering is used to remove the pore former and achieve preliminary bonding;

[0018] Crystal growth: Heating to 1400 °C at 3 °C / min, holding for 3 h, in a low oxygen environment with an oxygen partial pressure of 10 -2 atm to improve the crystallinity of mullite;

[0019] Final sintering: Heating to 1600 °C at 2 °C / min, holding for 2 h, in an Ar / H2 weak reducing atmosphere to optimize the gradient pore structure and enhance the interface bonding strength;

[0020] Cooling: Rapidly cool down to 1000 °C, then slowly cool down to 500 °C, and finally naturally cool to room temperature.

[0021] Preferably, the silane coupling agent is KH550, and the addition amount of KH550 is 1.5% of the mass of the whiskers, which is used to improve the interfacial bonding force between the whiskers and the matrix.

[0022] The present invention also provides a mullite insulating brick, which is characterized in that it is obtained by the above preparation method.

[0023] Preferably, the insulating brick has a four-dimensional gradient structure, including a surface dense layer, an intermediate transition layer and an internal loose layer.

[0024] Preferably, the pore size of the surface dense layer is less than 1 μm, and the porosity is 15% ± 3%;

[0025] The pore size of the intermediate transition layer is 5 - 10 μm, and the porosity is 25% ± 3%;

[0026] The pore size of the internal loose layer is 20 - 50 μm, and the porosity is 40% ± 3%.

[0027] Preferably, the density of the mullite insulating brick is less than 1.2 g / cm 3 .

[0028] The present invention also provides an application of the above-mentioned mullite insulating brick in heat insulation.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0030] 1. Improve mechanical strength

[0031] Through the uniformly distributed mullite whiskers, the toughness and crack resistance of the matrix are improved, and the flexural strength reaches 30 - 34 MPa, which is much higher than that of traditional mullite insulating bricks (generally 15 - 20 MPa).

[0032] 2. Excellent heat insulation performance

[0033] Adopting a gradient porous structure, the surface is dense and the interior is porous, achieving both high strength and low thermal conductivity, reducing the thermal conductivity to 0.14 - 0.16 W / m·K, and improving the heat insulation effect.

[0034] 3. High-temperature stability and thermal shock resistance

[0035] Adopting a high-temperature sintering process (>1600 °C), optimizing the formation of mullite phase, reducing the glass phase, and improving the thermal stability.

[0036] There are no obvious cracks after 20 times of high-temperature water quenching, and the service life is significantly improved.

[0037] 4. Temperature-sensitive thermal regulation

[0038] Through the reversible phase change of Ni-Ti particles, the ability to regulate thermal conductivity in high-temperature environments is improved, reducing the thermal conductivity by 18 - 22% at 300 °C to ensure thermal management performance under complex working conditions.

[0039] In summary, the mullite insulating bricks provided by the present invention have high strength, low thermal conductivity, excellent thermal shock resistance, and intelligent regulation of thermal conductivity, and can be widely applied in fields such as metallurgy, aerospace, and high-temperature kilns to improve the energy utilization efficiency and service life of equipment. Specific embodiments

[0040] The exemplary embodiments disclosed by the present invention will be described in more detail below. Although the exemplary embodiments of the present invention are shown below, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided so that the present invention can be more thoroughly understood and the scope of the present invention can be fully conveyed to those skilled in the art. It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the embodiments. Embodiment 1

[0041] I. Raw material ratio (mass percentage)

[0042] 1. Matrix material

[0043] Mullite precursor (3Al2O3·2SiO2, nano powder): 70%;

[0044] Nano-porous alumina template (pore diameter 50 - 100 nm): 15%.

[0045] 2. Reinforcing phase

[0046] Surface-modified nano-mullite whiskers (diameter 50 - 100 nm, aspect ratio > 20): 10%

[0047] Modifier: silane coupling agent (KH550), accounting for 1.5% of the whisker mass

[0048] 3. Temperature-sensitive regulation phase

[0049] Ni-Ti shape memory alloy micro-powder (particle size 1 - 5 μm, phase change temperature 300 - 400 °C): 5%

[0050] 4. Auxiliary agent

[0051] Polyvinyl alcohol binder (5wt% aqueous solution): 3% of the total mass of the matrix

[0052] II. Preparation process and parameters

[0053] 1. Preparation of template-whisker composite structure

[0054] Immerse nano-mullite whiskers into 1.5% KH550 ethanol solution, ultrasonically treat for 30 min (power 300 W), and vacuum dry at 60 °C for 2 h.

[0055] Immerse the nano-porous alumina template into the mullite precursor sol (pH = 3.5, sol viscosity 50 mPa·s), perform vacuum-assisted impregnation (vacuum degree 0.1 MPa, time 30 min), and centrifuge (3000 rpm, 5 min).

[0056] 2. Gradient structure forming

[0057] Surface dense layer: mixed matrix material + 20% whiskers + 2% alloy micro-powders, spread into the mold, with a pre-pressure of 5 MPa.

[0058] Intermediate transition layer: matrix material + 15% whiskers + 3% alloy micro-powders, centrifuge treatment (2000 rpm, 3 min).

[0059] Internal loose layer: matrix material + 10% whiskers + 5% alloy micro-powders + 5% pore-forming agent (PMMA microspheres, particle size 200 μm), static pressure 2 MPa.

[0060] 3. Gradient sintering process

[0061] (1) Paving of surface dense layer

[0062] Evenly spread the mixed surface dense layer material at the bottom of the mold; apply a pre-pressure of 5 MPa to make it initially formed and remove atmospheric pores, so that the pore size of the surface dense layer is less than 1 μm.

[0063] (2) Paving of intermediate transition layer

[0064] Pave the intermediate transition layer material on the surface layer; perform centrifuge treatment at 2000 rpm for 3 min, and use the centrifugal force to adjust the pore gradient so that the whiskers and alloy micro-powders are distributed along the radial gradient.

[0065] (3) Paving of internal loose layer

[0066] Pave the internal layer material on the intermediate layer; press with a static pressure of 2 MPa to ensure its initial bonding and maintain a high porosity.

[0067] (4) Sintering

[0068] ① The first stage (removing organic matter and pre-sintering)

[0069] Heating rate: 5 °C / min;

[0070] Target temperature: 1000 °C;

[0071] Insulation time: 2 h;

[0072] Atmospheric environment: air atmosphere;

[0073] Function: Remove the pore former (polymethyl methacrylate microspheres), achieve pre-sintering, and ensure the preliminary bonding of the matrix material.

[0074] ② The second stage (promote the crystallization and growth of mullite)

[0075] Heating rate: 3 °C / min;

[0076] Target temperature: 1400 °C;

[0077] Insulation time: 3 h;

[0078] Atmospheric environment: low-oxygen environment (oxygen partial pressure controlled at 10 -2 atm);

[0079] Function: Enhance the crystallinity of the mullite phase, improve the high-temperature stability of the material, and at the same time prevent the excessive sintering of the alumina phase from causing an increase in density.

[0080] ③ The third stage (final densification and gradient structure optimization)

[0081] Heating rate: 2 °C / min;

[0082] Target temperature: 1600 °C;

[0083] Insulation time: 2 h;

[0084] Atmospheric environment: weak reducing atmosphere (Ar / H2 mixed gas, H2 content 5%);

[0085] Function: Optimize the gradient structure, stabilize the pore structures of the surface layer, transition layer and internal layer, maintain the morphology of the Ni-Ti alloy, and enhance the interfacial bonding strength.

[0086] ④ Cooling process

[0087] Cooling stage 1 (rapid cooling): Cooling rate 5 °C / min to 1000 °C, nitrogen protection;

[0088] Cooling stage 2 (slow cooling): Cooling rate 2 °C / min to 500 °C, natural cooling to room temperature.

[0089] 4. Post-treatment

[0090] Spray a mullite-silicon carbide composite coating on the surface (0.1 mm thick, heat treatment at 1500 °C for 1 h). The thick coating spraying method includes:

[0091] (1) Coating thickness control

[0092] The multi - layer spraying method is adopted, with the thickness of each layer ≈50μm, and the total thickness can reach 0.5 - 1.0mm; the first layer is sprayed with a gas pressure of 0.2MPa, with uniform atomization to ensure adhesion; after each layer is sprayed, it is air - dried naturally for 30min and pre - dried at 120℃ for 1h; 6 - 10 layers are sprayed in sequence until the target thickness is reached.

[0093] (2) Curing and sintering

[0094] Pre - sinter at 300℃ for 1h to remove the solvent and dispersant; perform low - temperature treatment at 600℃ for 2h to improve the bonding strength; sinter in an oxygen atmosphere at 1400℃ for 3h to complete the crystal phase transformation; SiC forms a Si - O - C cross - linked structure in the mullite matrix to improve the high - temperature resistance performance.

[0095] Pore size: The pore size of the surface dense layer is less than 1μm, the pore size of the middle transition layer is 5 - 10μm, and the pore size of the internal loose layer is 20 - 50μm.

[0096] Porosity regulation: The surface dense layer is 15%, the middle transition layer is 25%, and the internal loose layer is 40%.

[0097] Among them, the preparation method of the mullite - silicon carbide composite coating includes:

[0098] (1) Raw material preparation

[0099] Mullite powder (average particle size D50≈1μm) 50wt%;

[0100] Silicon carbide powder (SiC, D50≈0.5μm) 35wt%;

[0101] Al2O3 - SiO2 composite sol (pH≈3.8, solid content 30wt%) 5wt%;

[0102] ZrO2 fine powder (D50≈0.3μm, enhancing thermal shock resistance) 3wt%;

[0103] Y2O3 (3wt%, improving oxidation resistance) 1.5wt%;

[0104] Magnesium oxide (MgO, 1.5wt%, improving high - temperature creep performance) 0.2wt%;

[0105] Dispersant (0.2wt% ammonium polyacrylate PAA) for the rest.

[0106] (2) Pulp - making process

[0107] In a blender, add the Al2O3-SiO2 composite sol; sequentially add mullite powder, silicon carbide powder, ZrO2, Y2O3, and MgO, and stir for 30 min; add PAA to improve the dispersibility, and continue stirring for 15 min; perform ultrasonic treatment (40 kHz, 30 min) to ensure uniform dispersion; perform mechanical stirring (300 rpm, 60 min), let it stand for 12 h, and defoam. Example 2

[0108] Based on Example 1, adjust the proportion of the temperature-sensitive regulation phase:

[0109] Increase the content of Ni-Ti shape memory alloy micropowder to 8%, and correspondingly reduce the mullite precursor to 67%.

[0110] The remaining process steps are the same. Example 3

[0111] Based on Example 1, adjust the sintering process:

[0112] Adopt three-stage sintering: 1200 °C (2 h) → 1400 °C (3 h) → 1650 °C (1 h) to improve the material density.

[0113] The remaining process steps are the same.

[0114] Comparative Example 1

[0115] Compared with Example 1, no surface modification of the whiskers was carried out.

[0116] Main change point: Without surface modification of the whiskers, directly add the nano-mullite whiskers to the matrix material.

[0117] Process change: Omit the KH550 modification step of the nano-mullite whiskers and directly add the whiskers to the matrix mixture.

[0118] Other steps remain unchanged.

[0119] Comparative Example 2 did not adopt a gradient structure

[0120] Main change point: Cancel the gradient structure, and all layers use the matrix material with the same ratio (whiskers 10%, Ni-Ti alloy 5%).

[0121] Process change: Cancel the centrifugal-compression molding step, and after uniformly mixing all the materials, perform one-time die pressing (at a pressure of 4 MPa).

[0122] Other steps remain unchanged.

[0123] The sintering temperature of Comparative Example 3 was reduced

[0124] Main change point: Reduce the maximum sintering temperature (from 1600 °C to 1350 °C).

[0125] Process change: Change the sintering plan, and adopt 1000°C (2h) → 1200°C (3h) → 1350°C (2h).

[0126] Other steps remain unchanged.

[0127] Test Example 1

[0128] Perform quality and performance tests on the mullite thermal insulation bricks prepared in Examples 1-3 and Comparative Examples 1-3, evaluate their mechanical properties, thermal properties and high temperature resistance, and verify the effects of gradient structure, whisker modification and sintering process on the properties of the final product.

[0129] I. Detection process

[0130] 1. Specimen preparation

[0131] Specimen size: 50mm×50mm×10mm

[0132] Number of specimens: Prepare 5 groups of specimens for each material and take the average value.

[0133] 2. Physical property detection

[0134] (1) Bulk density detection

[0135] Standard: Test according to ASTM C20.

[0136] Method: Measure the mass and volume of the sample and calculate the density (g / cm 3 )

[0137] (2) Porosity detection

[0138] Standard: Test according to ASTM C373.

[0139] Method: Adopt the water immersion method to calculate the open porosity and total porosity (%).

[0140] 3. Mechanical property detection

[0141] (3) Flexural strength test

[0142] Standard: Conduct a three-point bending test according to ASTM C1161.

[0143] Equipment: Electronic universal testing machine.

[0144] Parameters: Span 40mm, loading rate 0.5mm / min, record the maximum fracture load, and calculate the flexural strength (MPa).

[0145] 4. Thermal property detection

[0146] (4) Thermal conductivity test

[0147] Standard: The laser flash method test is carried out in accordance with ASTM E1461.

[0148] Temperature range: room temperature to 800 °C, and the thermal conductivity (W / m·K) at 300 °C and 800 °C is recorded.

[0149] (5) Thermosensitive response test

[0150] Method: Measure the thermal conductivity at 300 °C, compare it with the thermal conductivity at 25 °C, and calculate the percentage decrease in thermal conductivity (%).

[0151] 5. High-temperature resistance detection

[0152] (6) High-temperature stability test

[0153] Method:

[0154] Test 1: Place the sample in an electric furnace at 1600 °C and keep it warm for 2 h, and observe the morphological changes.

[0155] Test 2: Quench 20 times at 1600 °C in water, and observe whether cracks or spalling occur.

[0156] Evaluation criteria: intact without cracks, slight cracks, obvious cracks or spalling.

[0157] II. Detection results

[0158] Table 1 shows the detection results.

[0159] Table 1

[0160]

[0161] III. Analysis and conclusion

[0162] 1. Mechanical property analysis

[0163] The flexural strength of Examples 1-3 is relatively high (30 - 35 MPa), and that of Example 3 is the highest (34.2 MPa), indicating that whisker modification, gradient structure and appropriate sintering temperature have a significant effect on enhancing the strength of the material.

[0164] The flexural strength of Comparative Example 1 (unmodified whiskers) is only 18.6 MPa, indicating that whisker modification can improve the interfacial bonding between the matrix and the whiskers and increase the strength.

[0165] The flexural strength of Comparative Example 2 (without gradient structure) is 20.5 MPa, 35% lower than that of the examples, indicating that the gradient structure plays an important role in flexural strengthening.

[0166] Comparative Example 3 (low-temperature sintering) had the worst flexural strength, only 15.2 MPa, indicating that insufficient sintering led to a decrease in material density and a sharp drop in strength.

[0167] 2. Thermal Conductivity and Temperature-Sensitive Response Analysis

[0168] In Examples 1-3, the thermal conductivity decreased significantly (18 - 22%) at 300 °C, indicating an obvious temperature-sensitive regulation effect of the Ni-Ti shape memory alloy.

[0169] The temperature-sensitive response decreased (5 - 9%) in Comparative Examples 1-3, with Comparative Example 3 being the lowest, only 5%, indicating that the uniformity of Ni-Ti particle distribution, interface bonding, and sintering temperature affected the thermal conductivity regulation ability.

[0170] Comparative Examples 1 and 2 had higher thermal conductivities (0.20 - 0.25 W / m·K), indicating that whisker modification and gradient structure helped optimize the thermal conductivity.

[0171] 3. High-Temperature Stability Analysis

[0172] After 20 water quenches at 1600 °C, there were no cracks in Examples 1-3, showing excellent thermal shock resistance.

[0173] Slight cracks appeared in Comparative Examples 1 and 2, indicating that whisker modification and gradient structure had a positive effect on thermal stability.

[0174] Obvious cracks occurred in Comparative Example 3 after 20 water quenches at 1600 °C, indicating that low-temperature sintering led to insufficient material density and poor thermal stability.

[0175] IV. Principle Analysis

[0176] 1. Microscopic Effect of Whisker Modification (Comparing Example 1 / Comparative Example 1)

[0177] (1) Experimental Observation

[0178] Under a scanning electron microscope (SEM), Examples 1-3 showed uniformly distributed mullite whiskers, which were staggered, about 1 - 5 µm in length, forming a dense network. Comparative Example 1 (unmodified whiskers) showed uneven whisker growth, with whisker aggregation in some areas, resulting in concentrated microcracks.

[0179] (2) Microscopic Mechanism Analysis

[0180] Reinforcement effect: The dispersed distribution of mullite whiskers forms a "bridging effect" that can prevent crack propagation and improve flexural strength.

[0181] Toughening mechanism: Under external force, the whiskers can consume energy through pulling out and bending, improving fracture toughness.

[0182] Thermal stability: The uniform distribution of whiskers can form a stable microscopic support framework, reducing high-temperature deformation.

[0183] (3)Result comparison

[0184] In Examples 1-3, the mullite whiskers are uniformly distributed, and the flexural strength is significantly improved (>30 MPa).

[0185] In Comparative Example 1, since the whiskers were not surface-modified, the bonding force with the matrix was poor, resulting in easy crack propagation along the aggregation sites of the whiskers and a decrease in strength (18.6 MPa).

[0186] 2. Function of the gradient structure (comparing Example 2 / Comparative Example 2)

[0187] (1)Experimental observation

[0188] In Example 2, the gradient structure is adopted, and the cross-section shows the characteristic of gradually changing pore diameter from the surface layer to the interior, with the pore size gradually transitioning from 100 - 300 nm to 500 - 800 nm.

[0189] In Comparative Example 2 (without gradient structure), the internal pore diameter is uniform, and no obvious transition region is formed.

[0190] (2)Microscopic mechanism analysis

[0191] The gradient structure enhances strength: The dense surface layer can withstand greater stress, while the internal porous structure provides good heat insulation, making the material have both high strength and low thermal conductivity.

[0192] Thermal stress buffering: The gradient structure reduces the thermal expansion mismatch, reduces thermal stress concentration, and improves thermal shock resistance.

[0193] Crack blocking effect: When cracks propagate in the dense layer, they will deflect due to the gradient transition region, restricting crack growth and enhancing the toughness of the material.

[0194] (3)Result comparison

[0195] Due to the existence of the gradient structure in Example 2, the flexural strength is increased to 30.8 MPa, while maintaining good low thermal conductivity (0.16 W / m·K).

[0196] In Comparative Example 2, due to the uniform pore structure, the stress cannot be effectively dispersed, the flexural strength decreases (20.5 MPa), and the thermal conductivity is relatively high (0.20 W / m·K).

[0197] 3. Influence of sintering temperature on the microstructure (comparing Example 3 / Comparative Example 3)

[0198] (1)Experimental observation

[0199] Example 3 (High-temperature sintering): The mullite grains are of uniform size, with a main particle size of 2 - 5 µm, clear grain boundaries, and a reasonable porosity (23%).

[0200] Comparative Example 3 (Low-temperature sintering): The mullite grains are not fully developed, with a small grain size (<1 µm), blurred grain boundaries, a large amount of glass phase, and a low porosity (15%).

[0201] (2) Microscopic mechanism analysis

[0202] Grain growth and densification: Appropriate high-temperature sintering (>1600 °C) can promote the growth of mullite grains, form stable grain boundaries, and improve the flexural strength and high-temperature stability.

[0203] Effect of glass phase: Low-temperature sintering (<1500 °C) results in insufficient formation of mullite, and amorphous phase (glass phase) still remains in the matrix, which is prone to softening at high temperatures and causes structural collapse.

[0204] Optimization of pore structure: High-temperature sintering can promote the uniform distribution of pores, while low-temperature sintering is prone to local densification, reducing the thermal shock resistance.

[0205] (3) Result comparison

[0206] Example 3 is fully sintered, with the highest flexural strength (34.2 MPa) and the lowest thermal conductivity (0.14 W / m·K).

[0207] Due to insufficient sintering in Comparative Example 3, the flexural strength is the worst (15.2 MPa), the thermal conductivity is higher (0.25 W / m·K), and obvious cracks occur after the high-temperature water quenching test.

[0208] 4. Ni-Ti thermosensitive regulation mechanism (affecting thermal conductivity)

[0209] (1) Experimental observation

[0210] The Ni-Ti particles in Examples 1 - 3 are uniformly distributed, with a size of 500 nm - 1.5 µm, forming fine thermal regulation phases.

[0211] In Comparative Examples 1 - 3, due to uneven particle distribution or insufficient content, the change range of thermal conductivity is reduced.

[0212] (2) Microscopic mechanism analysis

[0213] Phase transformation of Ni-Ti shape memory alloy: Above 300 °C, Ni-Ti transforms from martensite to austenite, causing a slight contraction in the internal structure of the material, resulting in a decrease in grain boundary heat conduction and thus a decrease in the overall thermal conductivity.

[0214] Increased interfacial thermal resistance: Ni-Ti particles form multiphase interfaces in the material, generating thermal resistance after phase transformation at high temperatures, making it difficult for heat flux to pass through, thereby reducing the overall thermal conductivity.

[0215] (3) Result comparison

[0216] In Examples 1-3, due to the uniform distribution of Ni-Ti particles, the temperature-sensitive response is good, and the thermal conductivity at 300 °C decreases by 18-22%.

[0217] In Comparative Examples 1-3, due to the uneven distribution or low content of Ni-Ti particles, the thermal conductivity at 300 °C decreases by less than 10%.

[0218] 5. High-temperature stability and crack propagation behavior

[0219] (1) Experimental observation

[0220] There are no obvious cracks in Examples 1-3 after 20 times of high-temperature water quenching at 1600 °C.

[0221] In Comparative Examples 1-3, cracks or spalling of varying degrees occur. In particular, Comparative Example 3 (low-temperature sintering) has the most serious cracks.

[0222] (2) Microscopic mechanism analysis

[0223] Stability of the mullite phase: In Examples 1-3, due to sufficient high-temperature sintering, the proportion of the mullite phase is high, and it has good thermal shock resistance.

[0224] Crack passivation effect: Whisker reinforcement and gradient structure can hinder crack propagation and improve thermal shock resistance.

[0225] The glass phase causes uneven thermal expansion: Comparative Example 3 contains more glass phase, and the coefficient of thermal expansion changes greatly in a high-temperature environment, easily triggering crack propagation and causing material damage.

[0226] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications or equivalent replacements can still be made to the specific implementation manners of the present invention. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention shall be covered by the protection scope of the claims of the present invention.

Claims

1. A preparation method of mullite heat-insulating bricks, characterized in that, It includes the following steps: Immerse nano mullite whiskers in an ethanol solution of 1.5wt% silane coupling agent, ultrasonically treat them, and then vacuum dry them at 60°C for 2h to obtain modified whiskers; Immerse a nano-porous alumina template in a mullite precursor sol, fill it by vacuum-assisted impregnation, and then perform centrifugation to remove the excess sol to obtain a mixed matrix material; For the mixed matrix material, 20wt% modified whiskers and 2wt% Ni-Ti alloy micropowders, after paving, apply a pre-pressure of 5MPa to obtain a surface dense layer; For the mixed matrix material, 15wt% modified whiskers and 3wt% Ni-Ti alloy micropowders, after paving on the surface dense layer, perform centrifugation at 2000rpm for 3min to obtain a composite of the surface dense layer - intermediate transition layer; For the mixed matrix material, 10wt% modified whiskers, 5wt% Ni-Ti alloy micropowders and 5wt% pore former, after paving on the composite of the surface dense layer - intermediate transition layer, apply a static pressure of 2MPa to obtain a composite of the surface dense layer - intermediate transition layer - internal loose layer; Use a multi-stage controlled oxygen sintering method for the composite of the surface dense layer - intermediate transition layer - internal loose layer to make the porosity of the surface dense layer, intermediate transition layer and internal loose layer reach 15%±3%, 25%±3% and 40%±3% respectively; Spray a 0.1mm thick mullite-silicon carbide composite coating on the surface of the sintered heat-insulating brick and perform heat treatment at 1500°C for 1h.

2. The preparation method of the mullite heat-insulating brick according to claim 1, characterized in that, The particle size of the Ni-Ti alloy micropowders is 1-5μm, and its phase change temperature range is 300-400°C.

3. The preparation method of the mullite heat-insulating brick according to claim 1, characterized in that, The mullite precursor is a nano powder with Al2O3:SiO2 = 3:2, and its proportion is 70% of the mass of the matrix material.

4. The preparation method of the mullite heat-insulating brick according to claim 1, characterized in that, The multi-stage controlled oxygen sintering process includes: Pre-sintering: Heat up to 1000°C at 5°C / min, hold for 2h, in air atmosphere, remove the pore former, and achieve preliminary bonding; Crystal growth: Heat up to 1400 °C at a rate of 3 °C / min, hold for 3 h, and maintain a low-oxygen environment with an oxygen partial pressure of 10 -2 atm to improve the crystallinity of mullite; Final sintering: Heat up to 1600°C at 2°C / min, hold for 2h, in an Ar / H2 weak reducing atmosphere, optimize the gradient pore structure, and improve the interface bonding strength; Cooling: Rapidly cool to 1000°C, then slowly cool to 500°C, and finally naturally cool to room temperature.

5. The preparation method of the mullite insulating brick according to claim 1, characterized in that, The silane coupling agent is KH550, and the addition amount of KH550 is 1.5% of the mass of the whiskers.

6. A mullite insulating brick, characterized in that, It is prepared by the preparation method described in any one of claims 1-5.

7. The mullite insulating brick according to claim 6, characterized in that, The pore size of the surface dense layer is less than 1μm, and the porosity is 15%±3%; The pore size of the intermediate transition layer is 5-10μm, and the porosity is 25%±3%; The pore size of the internal loose layer is 20-50μm, and the porosity is 40%±3%.

8. The mullite insulating brick according to claim 6, wherein, The density of the mullite insulating brick is less than 1.2 g / cm 3 .

9. Application of the mullite heat-insulating brick described in any one of claims 6-8 in heat insulation.

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