Mullite heat-insulating brick and preparation method thereof

Abstract

The invention relates to the technical field of chemical engineering, and discloses a mullite heat-insulating brick and a preparation method thereof.According to the heat-insulating brick, the toughness of a matrix is enhanced through evenly-distributed mullite whiskers, the mechanical property and the heat insulation property of the heat-insulating brick are optimized through a gradient porous structure, and Ni-Ti particles are introduced to regulate and control the heat conductivity. The bending strength of the heat insulation brick can reach 30-34 MPa, the heat conductivity is as low as 0.14-0.16 W / m.K, and the heat insulation brick has excellent thermal shock resistance at the high temperature of 1600 DEG C. Compared with the traditional mullite heat-insulating brick, the product disclosed by the invention shows higher mechanical strength, excellent heat-insulating effect and intelligent heat management capability, and is suitable for the fields of high-temperature kilns, metallurgical industry, spacecraft heat protection and the like.

Description

Technical Field

[0001] The invention relates to the technical field of chemical industry, in particular to a mullite insulation brick and a preparation method thereof. Background Art

[0002] Mullite insulation brick is a refractory material widely used in high-temperature industrial furnaces, metallurgical equipment and aerospace fields. Mullite (3Al2O3·2SiO2) has excellent thermal stability, low thermal conductivity and high mechanical strength, so it is favored in high-temperature insulation applications. However, there are still some technical bottlenecks in the practical application of traditional mullite insulation bricks. First, traditional mullite insulation bricks usually rely on increasing porosity to reduce thermal conductivity, but this method often significantly reduces the mechanical strength of the material, making the insulation bricks fragile and easy to crack, limiting its application in extreme environments. Secondly, mullite insulation bricks have poor thermal shock resistance and are prone to crack propagation due to thermal stress accumulation in high-temperature environments, thereby affecting their service life. In addition, due to the high difficulty of sintering and densification of mullite, traditional preparation methods are difficult to maintain low thermal conductivity while ensuring mechanical properties, which makes the material face the challenge of balancing strength and thermal insulation performance in practical applications.

[0003] Therefore, there is an urgent need for a novel mullite insulation 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 proposes a mullite insulation brick and a preparation method thereof. Through innovative technologies such as whisker modification, gradient structure optimization and Ni-Ti temperature-sensitive particle regulation, the comprehensive performance of the mullite insulation brick is significantly improved, the contradiction between the mechanical strength and thermal insulation performance of traditional materials is solved, the thermal shock resistance of the material is improved, and it is endowed with a temperature-sensitive regulation function, making it more suitable for high-temperature and complex environments.

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

[0006] The present invention provides a method for preparing a mullite insulation brick, comprising the following steps: The nano-mullite whiskers were immersed in an ethanol solution of 1.5 wt% silane coupling agent, and then dried in vacuum at 60 °C for 2 h after ultrasonic treatment. The nanoporous alumina template is immersed in a mullite precursor sol, filled by vacuum-assisted impregnation, and then centrifuged to remove excess sol to obtain a mixed matrix material; The mixed matrix material, 20wt% modified whiskers and 2wt% Ni-Ti alloy powder were paved and subjected to a pre-pressure of 5MPa to obtain a dense surface layer; The mixed matrix material, 15wt% of modified whiskers and 3wt% of Ni-Ti alloy powder are paved on the surface dense layer and then centrifuged at 2000rpm for 3min to obtain a surface dense layer-intermediate transition layer composite; The mixed matrix material, 10wt% modified whiskers, 5wt% Ni-Ti alloy powder and 5% pore-forming agent are paved on the composite of the surface dense layer-middle transition layer and statically pressed at 2MPa to obtain a composite of the surface dense layer-middle transition layer-inner loose layer; The composite of the surface dense layer-intermediate transition layer-internal loose layer is sintered by multi-stage oxygen-controlled sintering to make the porosity of the surface dense layer, the intermediate transition layer and the internal loose layer reach 15%±3%, 25%±3% and 40%±3% respectively; A 0.1 mm thick mullite-silicon carbide composite coating was sprayed on the surface of the sintered insulation brick and heat treated at 1500°C for 1 hour.

[0007] Preferably, the particle size of the Ni-Ti alloy powder is 1-5 μm, and the phase change temperature range is 300-400°C.

[0008] Preferably, the mullite precursor is a nano powder with a ratio of Al2O3:SiO2=3:2, which accounts for 70% of the matrix material.

[0009] Preferably, the multi-stage oxygen-controlled sintering process includes: Pre-sintering: heating to 1000°C at 5°C / min, keeping warm for 2h, air atmosphere. The pre-sintering is used to remove the pore-forming agent and achieve preliminary bonding. Crystal growth: 3℃ / min heating to 1400℃, keep warm for 3h, oxygen partial pressure 10 -2 Atm low oxygen environment improves the crystallinity of mullite; Final sintering: heating to 1600℃ at 2℃ / min, keeping warm for 2h, Ar / H2 weak reducing atmosphere, optimizing gradient pore structure and improving interface bonding strength; Cooling: Rapidly cool to 1000℃, then slowly cool to 500℃, and finally cool naturally to room temperature.

[0010] Preferably, the silane coupling agent is KH550, and the added amount of KH550 is 1.5% of the mass of the whisker, so as to improve the interface bonding strength between the whisker and the substrate.

[0011] The present invention also provides a mullite thermal insulation brick, which is characterized in that it is prepared by the above-mentioned preparation method.

[0012] Preferably, the thermal insulation brick has a four-dimensional gradient structure, including a surface dense layer, a middle transition layer and an internal loose layer.

[0013] Preferably, the pore size of the surface dense layer is less than 1 μm, and the porosity is 15%±3%; The pore size of the intermediate transition layer is 5-10 μm, and the porosity is 25% ± 3%; The pore size of the internal loose layer is 20-50 μm, and the porosity is 40% ± 3%.

[0014] Preferably, the density of the mullite insulation brick is less than 1.2 g / cm 3 .

[0015] The invention also proposes an application of the mullite thermal insulation brick in thermal insulation.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. Improve mechanical strength The evenly distributed mullite whiskers improve the toughness and crack resistance of the matrix, making the flexural strength reach 30-34MPa, which is much higher than the traditional mullite insulation bricks (generally 15-20MPa).

[0017] 2. Excellent thermal insulation performance The gradient porous structure is adopted to make the surface dense and the interior porous, achieving both high strength and low thermal conductivity, reducing the thermal conductivity to 0.14-0.16W / m·K and improving the thermal insulation effect.

[0018] 3. High temperature stability and thermal shock resistance The high temperature sintering process (>1600℃) is used to optimize the formation of mullite phase, reduce the glass phase and improve thermal stability.

[0019] After 20 times of high-temperature water quenching, there are no obvious cracks and the service life is significantly improved.

[0020] 4. Thermosensitive thermal control Through the reversible phase change of Ni-Ti particles, the thermal conductivity regulation ability in high temperature environment is improved, and the thermal conductivity at 300°C is reduced by 18-22%, ensuring the thermal management performance under complex working conditions.

[0021] In summary, the mullite insulation brick provided by the present invention has high strength, low thermal conductivity, excellent thermal shock resistance and intelligent regulation of thermal conductivity, and can be widely used in metallurgy, aerospace, high-temperature kilns and other fields to improve the energy utilization efficiency and service life of equipment. DETAILED DESCRIPTION

[0022] The exemplary embodiments disclosed in the present invention will be described in more detail below. Although the exemplary embodiments disclosed in 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 in order to enable a more thorough understanding of the present invention and to fully convey the scope of the present invention to those skilled in the art. It should be noted that, in the absence of conflict, the embodiments in the present invention and the features described in the embodiments can be combined with each other. The present invention will be described in detail below in conjunction with the embodiments. Example 1

[0023] 1. Raw material ratio (mass percentage) 1. Base material Mullite precursor (3Al2O3·2SiO2, nanopowder): 70%; Nanoporous alumina template (pore size 50-100nm): 15%.

[0024] 2. Enhanced phase Surface modified nano mullite whiskers (diameter 50-100nm, aspect ratio > 20): 10% Modifier: Silane coupling agent (KH550), accounting for 1.5% of the whisker mass 3. Temperature sensitive control phase Ni-Ti shape memory alloy powder (particle size 1-5μm, phase change temperature 300-400℃): 5% 4. Auxiliary agents Polyvinyl alcohol binder (5wt% aqueous solution): 3% of the total mass of the matrix 2. Preparation process and parameters 1. Preparation of template-whisker composite structure The nano-mullite whiskers were immersed in 1.5% KH550 ethanol solution, ultrasonically treated for 30 min (power 300 W), and vacuum dried at 60 °C for 2 h.

[0025] The nanoporous alumina template was immersed in the mullite precursor sol (pH = 3.5, sol viscosity 50 mPa·s), vacuum-assisted impregnation (vacuum degree 0.1 MPa, time 30 min), and centrifuged (3000 rpm, 5 min).

[0026] 2. Gradient structure forming Surface dense layer: mixed matrix material + 20% whiskers + 2% alloy powder, spread into the mold, 5MPa pre-pressure.

[0027] Intermediate transition layer: base material + 15% whiskers + 3% alloy powder, centrifugal treatment (2000rpm, 3min).

[0028] Internal loose layer: matrix material + 10% whiskers + 5% alloy powder + 5% pore-forming agent (PMMA microspheres, particle size 200μm), static pressure 2MPa.

[0029] 3. Gradient sintering process (1) Paving of the surface dense layer The mixed surface dense layer material is evenly spread into the bottom of the mold; a pre-pressure of 5 MPa is applied to initially shape it and eliminate the atmospheric pores, so that the pore size of the surface dense layer is less than 1 μm.

[0030] (2) Paving of intermediate transition layer The intermediate transition layer material is laid on the surface layer; the centrifugal treatment is performed at 2000 rpm for 3 minutes, and the pore gradient is adjusted by using the centrifugal force so that the whiskers and alloy powder are distributed along the radial gradient.

[0031] (3) Internal loose layer paving The inner layer material is laid on top of the middle layer; 2MPa static pressure is used to ensure its initial bonding and maintain high porosity.

[0032] (4) Sintering ①The first stage (eliminating organic matter and pre-sintering) Heating rate: 5℃ / min; Target temperature: 1000℃; Insulation time: 2h; Atmosphere: air atmosphere; Function: Remove the pore former (PMMA microspheres), achieve pre-sintering, and ensure initial bonding of the matrix material.

[0033] ②The second stage (promoting mullite crystal growth) Heating rate: 3℃ / min; Target temperature: 1400℃; Insulation time: 3h; Atmosphere: Hypoxic environment (oxygen partial pressure controlled at 10 -2 atm); Function: Enhance the crystallinity of mullite phase, improve the high temperature stability of the material, and prevent the density from increasing due to excessive sintering of alumina phase.

[0034] ③The third stage (final densification and gradient structure optimization) Heating rate: 2℃ / min; Target temperature: 1600℃; Insulation time: 2h; Atmosphere: weak reducing atmosphere (Ar / H2 mixed gas, H2 content 5%); Function: Optimize the gradient structure, stabilize the pore structure of the surface layer, transition layer and internal layer, maintain the shape of the Ni-Ti alloy, and improve the interface bonding strength.

[0035] ④Cooling process Cooling stage 1 (rapid cooling): cooling rate 5℃ / min to 1000℃, nitrogen protection; Cooling stage 2 (slow cooling): cooling rate 2℃ / min to 500℃, and naturally cooling to room temperature.

[0036] 4. Post-processing The surface is sprayed with mullite-silicon carbide composite coating (0.1mm thick, heat treated at 1500℃ for 1h). Thick coating spraying methods include: (1) Coating thickness control A multi-layer spraying method is used, with each layer thickness ≈ 50μm and a total thickness of 0.5-1.0mm. The first layer is sprayed with 0.2MPa air pressure, with uniform atomization to ensure adhesion. After each layer is sprayed, it is naturally dried for 30 minutes and pre-dried at 120℃ for 1 hour. 6-10 layers are sprayed in sequence until the target thickness is reached.

[0037] (2) Curing and sintering Pre-sintering at 300℃ for 1h to remove solvent and dispersant; low-temperature treatment at 600℃ for 2h to improve bonding strength; sintering at 1400℃ in oxygen atmosphere for 3h to complete crystal phase transformation; SiC forms Si-OC cross-linking structure in mullite matrix to improve high temperature resistance.

[0038] 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.

[0039] Porosity control: the surface dense layer is 15%, the middle transition layer is 25%, and the internal loose layer is 40%.

[0040] Among them, the preparation method of mullite-silicon carbide composite coating includes: (1) Raw material preparation Mullite powder (average particle size D50≈1μm) 50wt%; Silicon carbide powder (SiC, D50≈0.5μm) 35wt%; Al2O3-SiO2 composite sol (pH ≈ 3.8, solid content 30wt%) 5wt%; ZrO2 powder (D50≈0.3μm, enhanced thermal shock resistance) 3wt%; Y2O3 (3wt%, improves oxidation resistance) 1.5wt%; Magnesium oxide (MgO, 1.5wt%, to improve high temperature creep properties) 0.2wt%; Dispersant (0.2wt% polyammonium acrylate PAA), balance.

[0041] (2) Pulping process In a blender, add Al2O3-SiO2 composite sol; add mullite powder, silicon carbide powder, ZrO2, Y2O3, and MgO in sequence and stir for 30 minutes; add PAA to improve dispersibility and continue stirring for 15 minutes; ultrasonic treatment (40kHz, 30 minutes) to ensure uniform dispersion; mechanical stirring (300rpm, 60 minutes), let stand for 12 hours, and degas. Example 2

[0042] The ratio of the temperature-sensitive regulating phase is adjusted based on Example 1: The content of Ni-Ti shape memory alloy powder is increased to 8%, and the content of mullite precursor is correspondingly reduced to 67%.

[0043] The remaining process steps are the same. Example 3

[0044] The sintering process was adjusted based on Example 1: A three-stage sintering process is adopted: 1200℃ (2h) → 1400℃ (3h) → 1650℃ (1h) to improve the density of the material.

[0045] The remaining process steps are the same.

[0046] Comparative Example 1 Compared with Example 1, no whisker surface modification was performed.

[0047] The main changes are: no whisker surface modification is performed, and nano-mullite whiskers are directly added to the matrix material.

[0048] Process change: Omit the KH550 modification step of nano-mullite whiskers and directly add the whiskers into the matrix mixture.

[0049] The other steps remain the same.

[0050] Comparative Example 2 does not use a gradient structure The main changes are: the gradient structure is cancelled, and all layers use the same ratio of base material (10% whiskers and 5% Ni-Ti alloy).

[0051] Process change: The centrifugal pressing step is eliminated, and all materials are evenly mixed and then molded at one time (4MPa pressure).

[0052] The other steps remain the same.

[0053] Comparative Example 3 Sintering temperature is reduced Main changes: Lowering the maximum sintering temperature (from 1600°C to 1350°C).

[0054] Process change: Change the sintering plan to 1000℃ (2h) → 1200℃ (3h) → 1350℃ (2h).

[0055] The other steps remain the same.

[0056] Test Example 1 The mullite insulation bricks prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to quality and performance tests to evaluate their mechanical properties, thermal properties and high temperature resistance, and to verify the effects of gradient structure, whisker modification and sintering process on the performance of the final product.

[0057] 1. Testing process 1. Sample preparation Sample size: 50mm×50mm×10mm Number of samples: Prepare 5 groups of samples for each material and take the average value.

[0058] 2. Physical performance testing (1) Bulk density detection Standard: Tested according to ASTM C20.

[0059] Method: Measure the sample mass and volume, and calculate the density (g / cm 3 ).

[0060] (2) Porosity test Standard: Tested according to ASTMC373.

[0061] Method: The open porosity and total porosity (%) were calculated by water immersion method.

[0062] 3. Mechanical properties testing (3) Bending strength test Standard: Three-point bending test according to ASTM C1161.

[0063] Equipment: Electronic universal testing machine.

[0064] Parameters: span 40 mm, loading rate 0.5 mm / min, record the maximum breaking load, and calculate the flexural strength (MPa).

[0065] 4. Thermal performance testing (4) Thermal conductivity test Standard: Laser flash test according to ASTM E1461.

[0066] Temperature range: room temperature to 800°C, record thermal conductivity (W / m·K) at 300°C and 800°C.

[0067] (5) Temperature sensitive response test Method: Measure the thermal conductivity at 300°C, compare it with the thermal conductivity at 25°C, and calculate the thermal conductivity decrease ratio (%).

[0068] 5. High temperature resistance test (6) High temperature stability test method: Test 1: The sample was placed in an electric furnace at 1600°C for 2 h and the morphological changes were observed.

[0069] Test 2: Water quench at 1600℃ for 20 times and observe whether cracks or peeling occur.

[0070] Evaluation criteria: complete without cracks, slight cracks, obvious cracks or peeling.

[0071] 2. Test results Table 1 shows the test results.

[0072] Table 1

[0073] 3. Analysis and Conclusion 1. Mechanical properties analysis The flexural strength of Examples 1-3 is relatively high (30-35 MPa), among which Example 3 has 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.

[0074] The flexural strength of comparative example 1 (unmodified whiskers) is only 18.6 MPa, indicating that the whisker modification can improve the interface bonding between the substrate and the whiskers and increase the strength.

[0075] The flexural strength of comparative example 2 (without gradient structure) is 20.5 MPa, which is 35% lower than that of the embodiment, indicating that the gradient structure plays an important role in flexural reinforcement.

[0076] The bending strength of Comparative Example 3 (low-temperature sintering) is the worst, only 15.2 MPa, which indicates that insufficient sintering leads to a decrease in material density and a sharp drop in strength.

[0077] 2. Thermal conductivity and temperature sensitive response analysis The thermal conductivity of Examples 1-3 decreased significantly (18-22%) at 300°C, indicating that the temperature-sensitive regulation effect of the Ni-Ti shape memory alloy is obvious.

[0078] The temperature sensitivity responses of Comparative Examples 1-3 decreased (5-9%), among which Comparative Example 3 had the lowest response, only 5%, indicating that the uniformity of Ni-Ti particle distribution, interface bonding and sintering temperature affect the ability to regulate thermal conductivity.

[0079] Comparative Examples 1 and 2 have higher thermal conductivity (0.20-0.25 W / m·K), indicating that whisker modification and gradient structure are helpful to optimize thermal conductivity.

[0080] 3. High temperature stability analysis After quenching in water at 1600° C. for 20 times, Examples 1-3 showed no cracks, demonstrating excellent thermal shock resistance.

[0081] Comparative Examples 1 and 2 showed slight cracks, indicating that whisker modification and gradient structure had a positive effect on thermal stability.

[0082] Comparative Example 3 produced obvious cracks after quenching at 1600° C. for 20 times, indicating that low-temperature sintering resulted in insufficient material density and poor thermal stability.

[0083] 4. Principle Analysis 1. Microscopic effect of whisker modification (Comparative Example 1 / Comparative Example 1) (1) Experimental observation Under a scanning electron microscope (SEM), Examples 1-3 showed uniformly distributed mullite whiskers, which were staggered and 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.

[0084] (2) Micro-mechanism analysis Strengthening effect: The dispersed distribution of mullite whiskers forms a "bridging effect", which can prevent crack propagation and improve bending strength.

[0085] Toughening mechanism: Under the action of external force, whiskers can consume energy through pulling out and bending, thereby improving fracture toughness.

[0086] Thermal stability: Uniform distribution of whiskers can form a stable microscopic support framework to reduce high temperature deformation.

[0087] (3) Comparison of results In Example 1-3, the mullite whiskers are evenly distributed and the flexural strength is significantly improved (>30 MPa).

[0088] In Comparative Example 1, since the whiskers have not been surface modified, the bonding strength with the matrix is ​​poor, resulting in cracks that easily extend along the whisker aggregation area and a decrease in strength (18.6 MPa).

[0089] 2. Effect of Gradient Structure (Comparative Example 2 / Comparative Example 2) (1) Experimental observation Example 2 adopts a gradient structure, and the cross section shows a characteristic of gradual change in pore size from the surface to the inner layer, and the pore size gradually transitions from 100-300nm to 500-800nm.

[0090] The internal pore size of Comparative Example 2 (non-gradient structure) is uniform, and no obvious transition area is formed.

[0091] (2) Micro-mechanism analysis Gradient structure improves strength: the dense surface area can withstand greater stress, while the internal porous structure provides good thermal insulation, making the material have both high strength and low thermal conductivity.

[0092] Thermal stress buffering: The gradient structure reduces thermal expansion mismatch, reduces thermal stress concentration, and improves thermal shock resistance.

[0093] Crack barrier effect: When cracks propagate in the dense layer, they will be deflected by the gradient transition area, which will limit the crack growth and improve the toughness of the material.

[0094] (3) Results comparison In Example 2, due to the presence of the gradient structure, the flexural strength is increased to 30.8 MPa, while maintaining a good low thermal conductivity (0.16 W / m·K).

[0095] In Comparative Example 2, due to the uniform pore structure, the stress could not be effectively dispersed, the flexural strength decreased (20.5 MPa), and the thermal conductivity was relatively high (0.20 W / m·K).

[0096] 3. Effect of sintering temperature on microstructure (Comparative Example 3 / Comparative Example 3) (1) Experimental observation Example 3 (high temperature sintering) The mullite grain size is uniform, the main particle size is 2-5µm, the grain boundary is clear, and the porosity is reasonable (23%).

[0097] In comparative example 3 (low temperature sintering), the mullite grains are not fully developed, the grain size is small (<1µm), the grain boundaries are blurred, there are more glass phases, and the porosity is low (15%).

[0098] (2) Micro-mechanism analysis Grain growth and densification: Appropriate high-temperature sintering (>1600℃) can promote the growth of mullite grains and form stable grain boundaries, thereby improving bending strength and high-temperature stability.

[0099] Influence of glass phase: Low-temperature sintering (<1500℃) leads to insufficient formation of mullite, and the amorphous phase (glass phase) still remains in the matrix, which is easy to soften at high temperature and cause structural collapse.

[0100] Optimization of pore structure: High-temperature sintering can promote uniform distribution of pores, while low-temperature sintering can easily lead to local densification and reduce thermal shock resistance.

[0101] (3) Comparison of results After being fully sintered, Example 3 has the highest flexural strength (34.2 MPa) and the lowest thermal conductivity (0.14 W / m·K).

[0102] Comparative Example 3 has the worst flexural strength (15.2 MPa) and high thermal conductivity (0.25 W / m·K) due to insufficient sintering, and obvious cracks occur after the high-temperature water quenching test.

[0103] 4. Ni-Ti temperature-sensitive regulation mechanism (affecting thermal conductivity) (1) Experimental observation The Ni-Ti particles of Examples 1-3 are uniformly distributed with a size of 500nm-1.5µm, forming a fine thermally controlled phase.

[0104] In Comparative Examples 1-3, the variation range of thermal conductivity is reduced due to uneven particle distribution or insufficient content.

[0105] (2) Micro-mechanism analysis Ni-Ti shape memory alloy phase transformation: Above 300°C, Ni-Ti transforms from martensite to austenite, causing a slight contraction of the internal structure of the material, resulting in reduced grain boundary heat conduction, thereby reducing the overall thermal conductivity.

[0106] Increased interfacial thermal resistance: Ni-Ti particles form a multiphase interface in the material, which generates thermal resistance after phase change at high temperatures, making it difficult for heat flow to pass through, thereby reducing the overall thermal conductivity.

[0107] (3) Comparison of results In Example 1-3, since the Ni-Ti particles are evenly distributed, the temperature-sensitive response is good, and the thermal conductivity at 300°C decreases by 18-22%.

[0108] In Comparative Examples 1-3, due to the uneven distribution or low content of Ni-Ti particles, the thermal conductivity at 300°C decreased by less than 10%.

[0109] 5. High temperature stability and crack propagation behavior (1) Experimental observation After quenching in water at 1600°C for 20 times, there was no obvious crack in Examples 1-3.

[0110] Comparative Examples 1-3 all had cracks or peeling to varying degrees, especially Comparative Example 3 (low temperature sintering) had the most severe cracks.

[0111] (2) Micro-mechanism analysis Stability of the mullite phase: Since Examples 1-3 are sufficiently sintered at high temperatures, the mullite phase accounts for a high proportion and has good thermal shock resistance.

[0112] Crack passivation effect: Whisker reinforcement and gradient structure can hinder crack propagation and improve thermal shock resistance.

[0113] Glass phase causes uneven thermal expansion: Comparative Example 3 contains more glass phase, and the thermal expansion coefficient changes greatly under high temperature environment, which can easily cause crack propagation and lead to material damage.

[0114] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing mullite insulation bricks, characterized in that: The following steps are involved: The nano-mullite whiskers were immersed in an ethanol solution of 1.5 wt% silane coupling agent, and then dried in vacuum at 60 °C for 2 h after ultrasonic treatment. The nanoporous alumina template is immersed in a mullite precursor sol, filled by vacuum-assisted impregnation, and then centrifuged to remove excess sol to obtain a mixed matrix material; The mixed matrix material, 20wt% modified whiskers and 2wt% Ni-Ti alloy powder were paved and subjected to a pre-pressure of 5MPa to obtain a dense surface layer; The mixed matrix material, 15wt% of modified whiskers and 3wt% of Ni-Ti alloy powder are paved on the surface dense layer and then centrifuged at 2000rpm for 3min to obtain a surface dense layer-intermediate transition layer composite; The mixed matrix material, 10wt% modified whiskers, 5wt% Ni-Ti alloy powder and 5wt% pore-forming agent are paved on the composite of the surface dense layer-middle transition layer and statically pressed at 2MPa to obtain a composite of the surface dense layer-middle transition layer-inner loose layer; The composite of the surface dense layer-intermediate transition layer-internal loose layer is sintered by multi-stage oxygen-controlled sintering to make the porosity of the surface dense layer, the intermediate transition layer and the internal loose layer reach 15%±3%, 25%±3% and 40%±3% respectively; A 0.1 mm thick mullite-silicon carbide composite coating was sprayed on the surface of the sintered insulation brick and heat treated at 1500°C for 1 hour.

2. The method for preparing the mullite insulation brick according to claim 1, characterized in that: The particle size of the Ni-Ti alloy powder is 1-5 μm, and the phase change temperature range is 300-400°C.

3. The method for preparing the mullite insulation brick according to claim 1, characterized in that: The mullite precursor is a nano powder of Al2O3:SiO2=3:2, which accounts for 70% of the mass of the matrix material.

4. The method for preparing the mullite insulation brick according to claim 1, characterized in that: The multi-stage oxygen-controlled sintering process includes: Pre-sintering: 5℃ / min heating to 1000℃, keep warm for 2h, air atmosphere; Crystal growth: 3℃ / min heating to 1400℃, keep warm for 3h, oxygen partial pressure 10 -2 Atm low oxygen environment improves the crystallinity of mullite; Final sintering: heating to 1600℃ at 2℃ / min, keeping warm for 2h, Ar / H2 weak reducing atmosphere, optimizing gradient pore structure and improving interface bonding strength; Cooling: Rapidly cool to 1000℃, then slowly cool to 500℃, and finally cool naturally to room temperature.

5. The method for preparing the mullite insulation brick according to claim 1, characterized in that: The silane coupling agent is KH550, and the added amount of KH550 is 1.5% of the whisker mass.

6. A mullite insulation brick, characterized in that: The method is prepared by the method according to any one of claims 1 to 5.

7. The mullite insulation brick according to claim 6, characterized in that: The heat-insulating brick has a four-dimensional gradient structure, including a surface dense layer, a middle transition layer and an internal loose layer.

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

9. The mullite insulation brick according to claim 6, characterized in that: The density of the mullite insulation brick is lower than 1.2g / cm 3 .

10. Use of the mullite insulation brick according to any one of claims 6 to 9 in thermal insulation.

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