A preparation method of lightweight and low thermal conductivity Yb2SiO5 aerogel

Abstract

The present invention provides a method for preparing a lightweight, low-thermal-conductivity Yb2SiO5 aerogel, which belongs to the field of porous material preparation technology. First, a composite sol of a Yb2SiO5 precursor is obtained by a sol-gel method using citric acid as a rare earth chelating agent and propylene oxide as a proton scavenger. After the gel is allowed to stand, it is added to an aging liquid for aging. Finally, a supercritical drying combined with a heat treatment process is used to prepare a Yb2SiO5 aerogel ceramic material with nano-scale pore size, high porosity, low density and low thermal conductivity, which can withstand extreme temperatures of 1500°C. The Yb2SiO5 ceramic aerogel involved in the present invention has excellent high-temperature thermal stability and chemical stability, and has an X2 phase of Yb2SiO5. The porosity of the prepared material is 86.57% to 87.86%, and the density is 0.8257 to 0.9134 g / cm 3 , the thermal conductivity is 0.125~0.138W / (m·K), which is much lower than the minimum thermal conductivity of its dense material (0.74W / (m·K)).

Description

Technical Field

[0001] The invention relates to a method for preparing lightweight and low thermal conductivity Yb2SiO5 aerogel, belonging to the field of porous material preparation technology. Background Art

[0002] In recent years, ytterbium monosilicate (Yb2SiO5) is a monoclinic ceramic material belonging to the C2 / c space group. Its unit cell parameters are a = 1.5835nm, b = 0.637nm, c = 1.1208nm, and the crystal plane angle β = 128.299°. It has a single X2-RE2SiO5 monoclinic structure. Yb2SiO5 is composed of Si-O tetrahedra and Yb-O polyhedra. Recent studies have shown that Yb2SiO5 has the characteristics of high thermal stability, low thermal conductivity (0.74W / (m·K)), anisotropy, low Young's modulus, good chemical and mechanical compatibility, and excellent corrosion resistance. It has been widely studied in the field of thermal barrier coatings (EBC) [HuiminXiang, ZhihaiFeng, YanchunZhou, Mechanical and thermal properties of Yb2SiO5: First-principlescalculations and chemical bond theory investigations[J], Journal of MaterialsResearch volume, 2014, 29, 1609–1619]. Due to the excellent comprehensive properties of Yb2SiO5, single-phase Yb2SiO5 powder has been synthesized by various methods, such as hydrothermal synthesis, sol-gel method, solid-state reaction, and molten salt synthesis. Khan et al. synthesized Yb2SiO5 powder by solid-state reaction method and measured the thermal expansion coefficient (CTE). From 100℃ to 1300℃, the CTE was 7.1×10 -6 K -1. In addition, these preparation methods all have their own characteristics, but their shortcomings are complex preparation processes and harsh reaction conditions [Khan ZS, Zou B, Huang W, et al. Synthesis and characterization of Yb and Er based monosilicate powders and durability of plasma sprayed Yb2SiO5 coatings on C / C–SiC composites[J]. Materials Science&Engineering B, 2012, 177(2): 184-189.]. Cao Liyun et al. prepared Yb2SiO5 powder by combining the sol-gel method with the lava method. This method reduces the sintering temperature, but the preparation process is complex and requires subsequent centrifugal drying and separation. Cao Liyun. A method for preparing Yb2SiO5 powder: CN201610065520.X[P]. 2018-07-24.

[0003] Traditional oxide thermal insulation materials can no longer meet the harsh aerodynamic heating environment. Widely used traditional oxide aerogels such as SiO2, Al2O3-SiO2, and yttrium-stabilized zirconia (YSZ) cannot break through the bottleneck of 1300°C. However, ytterbium silicate (Yb2SiO5) aerogel has excellent properties such as high thermal stability, low thermal conductivity, low thermal expansion coefficient, oxidation resistance and corrosion resistance. Compared with widely used traditional oxide aerogels, it can be used in higher temperature environments (>1300°C). In addition, compared with non-oxide aerogels such as silicon carbide and silicon nitride, ytterbium silicate aerogel can be used in higher temperatures and more harsh oxidizing environments. Therefore, it can be used as an excellent new type of ceramic aerogel thermal insulation material.

[0004] Currently, most reports focus on the preparation of Yb2SiO5 powders, but there are no reports on the preparation of Yb2SiO5 aerogels. Given that Yb2SiO5 aerogel ceramic materials with excellent high-temperature thermal stability are still lacking, it is very important to provide a simple and efficient method for preparing Yb2SiO5 aerogels. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for preparing lightweight and low thermal conductivity Yb2SiO5 aerogel. Yb2SiO5 aerogel ceramic material with nano-scale pore size, ultra-high porosity, lower density and thermal conductivity is prepared through simple sol-gel, supercritical drying and calcination processes. It can withstand extreme temperatures of 1500°C, and the material can be lightweight, crack-free, and complete in block form, and can achieve near-net-size molding.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: A method for preparing lightweight and low thermal conductivity Yb2SiO5 aerogel, using in-situ synthesis combined with supercritical process, the specific steps of which are as follows:

[0007] (1) Preparation of composite sol: According to the chemical formula Yb2SiO5, ytterbium source and silicon source precursors were weighed and added to a container containing ethanol and water. Citric acid was added and stirred until completely hydrolyzed. Finally, propylene oxide was added and stirred thoroughly to obtain a composite sol.

[0008] (2) Preparation and aging of wet gel: Pour the composite sol into a mold, seal it and let it stand until the gel is completely gelled, then add aging solution and age it until it can be demoulded;

[0009] (3) Preparation of xerogel: supercritical drying the wet gel obtained in step (2) to obtain xerogel;

[0010] (4) Preparation of aerogel: The dry gel prepared in step (3) is placed in a muffle furnace in an oxygen atmosphere for heat treatment to obtain Yb2SiO5 ceramic aerogel.

[0011] Preferably, the ytterbium source in step (1) is one or more of ytterbium chloride hexahydrate (YbCl3·6H2O) or ytterbium nitrate pentahydrate.

[0012] Preferably, the silicon source precursor in step (1) is one or more of tetraethyl orthosilicate or methyl orthosilicate.

[0013] Preferably, the molar ratio of the ytterbium source, silicon source precursor, ethanol, water, citric acid and propylene oxide in the composite sol in step (1) is 1:0.5:(15-40):(6-15):(0.3-1.5):(6-8).

[0014] Preferably, the aging time in step (2) is 48 to 72 hours, and the aging solution is replaced every 6 to 12 hours.

[0015] Preferably, the aging liquid in step (2) is one or more of ethanol, acetone, n-hexane or isopropanol.

[0016] Preferably, the supercritical drying in step (3) is CO2 supercritical drying, the temperature of CO2 supercritical drying is 32-50°C, the pressure is 9-12 MPa, and the drying time is 5-8 h.

[0017] Preferably, the heat treatment temperature in step (4) is 1200-1500° C., the heating rate is 2-20° C. / min, and the heat treatment time is 0.5-3 h.

[0018] Beneficial effects:

[0019] (1) The present invention prepares Yb2SiO5 aerogel through an in-situ sol-gel combined with a supercritical drying process. The material has a high porosity and can be presented in a lightweight, crack-free, complete block shape, which can be used for near-net-size molding.

[0020] (2) The present invention is simple to operate, can realize fluidized production, and is easy to industrialize.

[0021] (3) Compared with the porous materials prepared with Yb2SiO5 cooked powder, Yb2O3 and SiO2 as raw materials reported in the literature, the Yb2SiO5 ceramic aerogel prepared by the in-situ synthesis method in the present invention has higher porosity, better thermal stability and lower thermal conductivity, and can withstand extreme temperatures of 1500°C.

[0022] (4) The most important thing is that the present invention introduces citric acid as a structural support agent. Citric acid can form a stable soluble chelate with rare earth ions. Citric acid removes protons from carboxylic acid and participates in coordination in the form of acid radicals, which inhibits the Yb 3+ The element loss during aging replacement and supercritical drying of ions ensures the formation of pure single-phase ceramic materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a photo of the Yb2SiO5 ceramic aerogel prepared in Example 1;

[0024] Figure 2 This is the XRD pattern of the Yb2SiO5 ceramic aerogel prepared in Example 1;

[0025] Figure 3 (a) and (b) are SEM images of the Yb2SiO5 ceramic aerogel prepared in Example 1; DETAILED DESCRIPTION

[0026] The present invention is described in detail below with reference to specific embodiments. It must be understood that the present invention adopts an in-situ sol-gel combined with a supercritical drying process, and finally obtains a Yb2SiO5 ceramic aerogel with an X2 phase by calcination. The key point is the introduction of citric acid, which reduces the volume shrinkage of the material caused by a large amount of organic matter during the calcination process and increases the specific surface area of ​​the material. The prepared sample has the characteristics of high-temperature thermal stability, high porosity, low shrinkage, low density and low thermal conductivity, and can be presented in a lightweight, crack-free, complete block shape. The Yb2SiO5 ceramic aerogel sample prepared by the present invention has a porosity of 86.57% to 87.86% and a density of 0.8257 to 0.9134 g / cm 3 , the thermal conductivity is 0.125~0.138W / (m·K), which is much lower than the minimum thermal conductivity of its dense material (0.74W / (m·K)).

[0027] Example 1

[0028] 0.002 mol of ytterbium chloride hexahydrate, 0.0006 mol of citric acid, 0.03 mol of ethanol and 0.012 mol of water were accurately weighed using an electronic balance and added into a container and stirred. Then, 0.001 mol of tetraethyl orthosilicate was added and continued to stir until completely hydrolyzed. Then, 0.012 mol of propylene oxide was added and stirred thoroughly to obtain a mixed sol. The mixed sol was poured into a mold, sealed and allowed to stand for gelation, and then added to ethanol for aging for 48 hours, with the aging solution replaced every 6 hours to obtain an aged wet gel. The sample was taken out and subjected to CO2 supercritical drying at a drying temperature of 32°C, a pressure of 9 MPa, and a drying time of 5 hours to obtain a dry gel. Finally, the dry gel was heat-treated.

[0029] The heat treatment equipment: muffle furnace

[0030] Heat treatment temperature: 1200℃,

[0031] Insulation time: 3h,

[0032] Heating rate: 5℃ / min,

[0033] After the heat treatment is completed, Yb2SiO5 ceramic aerogel is obtained. The prepared material has a porosity of 85.43% and a density of 0.8257 g / cm 3 , the thermal conductivity at room temperature is 0.125W / (m·K).

[0034] Figure 1 This is a photo of the Yb2SiO5 ceramic aerogel prepared in Example 1. The Yb2SiO5 ceramic aerogel obtained in Example 1 was characterized by XRD. Figure 2 As shown, it is shown that the ceramic aerogel obtained is a single X2-Yb2SiO5 phase. Figure 3 (a) and (b) are SEM images of the Yb2SiO5 ceramic aerogel prepared in Example 1 at different magnifications.

[0035] Example 2

[0036] 0.002 mol of ytterbium chloride hexahydrate, 0.0012 mol of citric acid, 0.04 mol of ethanol and 0.016 mol of water were accurately weighed using an electronic balance and added into a container and stirred. Then, 0.001 mol of methyl orthosilicate was added and continued to stir until completely hydrolyzed. Then, 0.014 mol of propylene oxide was added and stirred thoroughly to obtain a mixed sol. The mixed sol was poured into a mold, sealed and allowed to stand for gelation, and then added to acetone for aging for 56 hours, with the aging solution replaced every 12 hours to obtain an aged wet gel. The sample was taken out and subjected to CO2 supercritical drying at a drying temperature of 40°C, a pressure of 10 MPa, and a drying time of 6 hours to obtain a dry gel. Finally, the dry gel was heat-treated.

[0037] The heat treatment equipment: muffle furnace,

[0038] Heat treatment temperature: 1300℃,

[0039] Insulation time: 2h,

[0040] Heating rate: 2℃ / min,

[0041] After the heat treatment is completed, Yb2SiO5 ceramic aerogel is obtained. The prepared material has a porosity of 86.99% and a density of 0.8564g / cm 3 , the room temperature thermal conductivity is 0.128W / (m·K).

[0042] Example 3

[0043] 0.002 mol of ytterbium nitrate pentahydrate, 0.002 mol of citric acid, 0.06 mol of ethanol and 0.02 mol of water are accurately weighed using an electronic balance and added into a container and stirred. Then, 0.001 mol of methyl orthosilicate is added and stirring is continued. After stirring until complete hydrolysis, 0.015 mol of propylene oxide is added and stirred thoroughly to obtain a mixed sol. The mixed sol is poured into a mold, sealed and allowed to stand for gelation, and then added to n-hexane for aging for 64 hours, and the aging liquid is replaced every 12 hours to obtain an aged wet gel. The sample is taken out and subjected to CO2 supercritical drying at a drying temperature of 45°C, a pressure of 11 MPa, and a drying time of 7 hours to obtain a dry gel. Finally, the dry gel is heat-treated.

[0044] The heat treatment equipment: muffle furnace

[0045] Heat treatment temperature: 1400℃,

[0046] Insulation time: 1h,

[0047] Heating rate: 10℃ / min,

[0048] After the heat treatment is completed, Yb2SiO5 ceramic aerogel is obtained. The prepared material has a porosity of 87.4% and a density of 0.8846 g / cm 3 , the thermal conductivity at room temperature is 0.134W / (m·K).

[0049] Example 4

[0050] 0.002 mol of ytterbium nitrate pentahydrate, 0.003 mol of citric acid, 0.08 mol of ethanol and 0.03 mol of water are accurately weighed using an electronic balance and added into a container and stirred. Then, 0.001 mol of tetraethyl orthosilicate is added and stirring is continued. After stirring until complete hydrolysis, 0.016 mol of propylene oxide is added and stirred thoroughly to obtain a mixed sol; the mixed sol is poured into a mold, sealed and allowed to stand for gelation, and added to isopropanol for aging for 72 hours, and the aging liquid is replaced every 12 hours to obtain an aged wet gel; the sample is taken out and subjected to CO2 supercritical drying at a drying temperature of 50°C, a pressure of 12 MPa, and a drying time of 8 hours to obtain a dry gel, and finally the dry gel is heat treated.

[0051] The heat treatment equipment: muffle furnace

[0052] Heat treatment temperature: 1500℃,

[0053] Insulation time: 0.5h,

[0054] Heating rate: 20℃ / min,

[0055] After the heat treatment is completed, Yb2SiO5 ceramic aerogel is obtained. The prepared material has a porosity of 87.86% and a density of 0.9134 g / cm 3 , the room temperature thermal conductivity is 0.138W / (m·K).

Claims

1. A method for preparing a lightweight and low thermal conductivity Yb2SiO5 aerogel, the specific steps of which are as follows: (1) Preparation of composite sol: According to the chemical formula Yb2SiO5, an ytterbium source and a silicon source precursor are weighed and added to a container containing ethanol and water, citric acid is added and stirred until completely hydrolyzed, and finally propylene oxide is added and stirred thoroughly to obtain a composite sol; wherein the molar ratio of the ytterbium source, silicon source precursor, ethanol, water, citric acid and propylene oxide in the composite sol is 1:0.5:(15-40):(6-15):(0.3-1.5):(6-8); (2) Preparation and aging of wet gel: Pour the composite sol into a mold, seal it and let it stand until the gel is completely gelled, then add aging solution and age it until it can be demoulded; (3) Preparation of xerogel: supercritical drying the wet gel obtained in step (2) to obtain xerogel; (4) Preparation of aerogel: The dry gel prepared in step (3) is placed in a muffle furnace in an oxygen atmosphere for heat treatment to obtain Yb2SiO5 ceramic aerogel.

2. The preparation method according to claim 1, wherein The ytterbium source in step (1) is one or more of ytterbium chloride hexahydrate (YbCl3·6H2O) or ytterbium nitrate pentahydrate.

3. The preparation method according to claim 1, wherein The silicon source precursor described in step (1) is one or more of tetraethyl orthosilicate or methyl orthosilicate.

4. The preparation method according to claim 1, characterized in that The aging time in step (2) is 48 to 72 hours, and the aging solution is replaced every 6 to 12 hours.

5. The preparation method according to claim 1, characterized in that The aging liquid in step (2) is one or more of ethanol, acetone, n-hexane or isopropanol.

6. The preparation method according to claim 1, characterized in that The supercritical drying described in step (3) is CO2 supercritical drying, the CO2 supercritical drying temperature is 32-50°C, the pressure is 9-12 MPa, and the drying time is 5-8 hours.

7. The preparation method according to claim 1, characterized in that The heat treatment temperature in step (4) is 1200-1500° C., the heating rate is 2-20° C. / min, and the heat treatment time is 0.5-3 h.

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