A preparation method of alumina foam ceramics

Abstract

The present invention belongs to the technical field of the preparation of thermal insulation materials, and particularly relates to a method for preparing alumina foam ceramics by a foaming method. The preparation method comprises the following steps: using ammonium chloride, urea, sugar, a sintering aid and alumina powder as raw materials, grinding and mixing them, and directly foaming in an inert atmosphere to obtain a porous carbon / alumina composite material; removing carbon from the obtained composite material to obtain a precursor of alumina foam ceramics; and then high-temperature sintering the obtained precursor to obtain porous alumina foam ceramics. The present invention adopts a simple and non-toxic process route, has a short production cycle, and the prepared alumina foam ceramics have the characteristics of high porosity, low bulk density and high-efficiency heat insulation.

Description

Technical Field

[0001] The present invention belongs to the field of preparation of thermal insulation materials, and particularly relates to a method for preparing alumina foam ceramics. Background Art

[0002] Alumina foam ceramics are applied in various technical fields due to their high-temperature stability, low bulk density, high porosity, low thermal conductivity and good mechanical strength, including technical applications such as molten metal filtration, high-temperature thermal insulation, heat exchange, filtration of corrosive hot gases, catalyst carriers, etc.

[0003] Currently, the commonly used methods for foam ceramics include the pore-forming agent addition method, organic foam impregnation method, foaming method, etc. Patent US4448896A discloses a method for preparing porous sintered alumina ceramics by adding carbon black powder as a pore-forming agent. The pore size distribution of the samples obtained by this method is dispersed, and the foam ceramics obtained by the pore-forming agent addition method usually have a relatively low porosity (less than 50%); Patent CN201310539992.0 uses a polyurethane organic foam impregnation method to prepare foam ceramics. The pore size and porosity of the foam ceramics obtained by this method depend on the organic foam itself, and there is an organic system during the sintering process, inevitably generating toxic gases; The foaming method can simply and effectively prepare porous alumina ceramics with high porosity. Currently, the typical direct foaming method is to introduce gas into a suspension or liquid medium to generate porous foam and then sinter after solidification. For example, K. Prabhakaran et al. in a non-patent literature (J. Am. Ceram. Soc. 2005, 88, 2600 - 2603) used a sucrose resin slurry filled with alumina powder for foaming to obtain alumina foam ceramics with a porosity of 93.5 - 96.7%. The foaming method has the characteristics of simple process and low cost, but the thermodynamic instability of the wet foam system will cause small bubbles to merge into large bubbles to reduce the free energy of the system, resulting in uneven pore size, making it difficult to achieve the expected effect in actual use. At the same time, the preparation processes such as pulping, demolding, and drying are time-consuming. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the present invention provides a method for preparing highly porous alumina foam ceramics by foaming. This method has a simple and non-toxic process. The obtained product has relatively uniform pore size, high porosity and low bulk density. At the same time, its low thermal conductivity makes it have the application prospect of becoming an efficient thermal insulation material.

[0005] The technical solution adopted by the present invention includes the following steps:

[0006] (1) Mix ammonium chloride, urea, sugar, a sintering aid, and alumina powder and then grind to obtain a powder mixture;

[0007] (2) Place the above powder mixture in a reaction furnace, introduce an inert atmosphere as a protective gas, and then heat it from room temperature to 160 °C at a heating rate of 25 °C / min, from 160 °C to 400 °C at a heating rate of 4 °C / min, and from 400 °C to 600 °C at a heating rate of 20 °C / min. After cooling, a porous carbon / aluminum oxide composite material is obtained;

[0008] (3) Pass water vapor through the above composite material at high temperature to remove carbon or remove carbon in an oxygen-containing atmosphere to obtain a precursor of alumina foam ceramic;

[0009] (4) Sinter the above-obtained precursor at high temperature in an oxygen-containing atmosphere to obtain an alumina foam ceramic with a high porosity.

[0010] In the present invention, the sugar in step (1) is either sucrose or glucose or a combination of the two, and the sintering aid is either magnesium oxide, silicon oxide, calcium oxide, zirconium oxide or a combination of multiple ones.

[0011] In the present invention, the mass ratio of ammonium chloride, urea, sugar, sintering aid, and aluminum oxide in step (1) is (0.4 - 1):(0.1 - 0.3):(0.6 - 1.5):(0.002 - 0.05):1.

[0012] In the present invention, the particle size of the aluminum oxide powder in step (1) is 0.1 - 10 μm.

[0013] In the present invention, the inert atmosphere in step (2) is either nitrogen, ammonia, helium or a combination of multiple ones, and nitrogen is preferably used in terms of cost.

[0014] In the present invention, the temperature for carbon removal by water vapor in step (3) is 750 - 1150 °C, and the heat preservation time is 10 - 60 min. The water vapor is realized by one or more methods of injection, bubbling, and vaporization.

[0015] In the present invention, the sintering temperature in step (4) is 1300 - 1600 °C, and the sintering time is 30 - 240 min.

[0016] Compared with the prior art, the preparation method of the present invention has the following prominent advantages:

[0017] 1) Adopt a simple and non-toxic process route, with low requirements for equipment, short foaming time, and can realize the mass preparation of alumina foam ceramics.

[0018] 2) The molten sugar forms a polymer chain polymerization and plays a good bonding role. In an inert atmosphere, the inhibition of oxygen on the polymerization of sugar is avoided. At the same time, the molten urea is beneficial to the uniform distribution of the mixture. Foaming agents such as ammonium chloride decompose and release gases to form bubbles, thus simply, stably, and effectively constructing a three-dimensional cross-linked network structure.

[0019] 3) By using blowing agents with different melting points and decomposition temperatures, in the foaming process, through different heating rate programs and a large number of nucleation points brought by multiple materials, the possibility of small bubbles merging into larger bubbles is reduced, resulting in a foam with a relatively dense, uniform pore volume, pore diameter, and pore distribution.

[0020] 4) The obtained alumina foam ceramic has a uniform and complete structure. Changes in process parameters within the scope defined by the claims of this application have little impact on the performance of the product, and it has excellent stability.

[0021] 5) The alumina foam ceramic provided by the present invention is characterized by high porosity, low bulk density, and low thermal conductivity, and has excellent performance in the field of heat insulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is an optical photograph after sintering of the porous carbon / alumina composite material and alumina foam ceramic obtained in Example 1 of the present invention.

[0023] Figure 2 It is an X-ray diffraction spectrum diagram of the ground alumina foam ceramic obtained in Example 1 of the present invention.

[0024] Figure 3 It is a scanning electron microscope photograph of the fracture surface of the alumina foam ceramic obtained in Example 1 of the present invention.

[0025] Figure 4 It is the temperature rise curve of the alumina foam ceramic obtained in Example 1 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] The present invention will be further introduced below with reference to the drawings and through specific embodiments, but the embodiments are only for illustration and do not constitute a limitation to the present invention.

[0027] Example 1:

[0028] (1) 0.75 g of ammonium chloride and 0.15 g of urea were mixed and ground, then 1 g of sucrose, 0.03 g in total of alumina powder with a particle size of 0.8 g to 0.2 μm, silica powder and magnesium oxide powder with similar particle sizes were added as a sintering aid and mixed and ground to obtain a white mixed powder; (2) The mixed powder was placed in a tubular furnace. After evacuation, it was first heated from room temperature to 160 °C at a heating rate of 25 °C / min under a nitrogen atmosphere of 60 sccm. In the temperature range of 160 °C to 400 °C, it was heated at a rate of 4 °C / min. Subsequently, it was heated to 600 °C at a heating rate of 20 °C / min to further completely decompose the foaming agent, and then cooled to 500 °C at a cooling rate of 2 °C / min, and then naturally cooled to obtain a porous carbon / alumina composite; (3) The above composite was placed in a tubular furnace and heated to 1050 °C under a nitrogen atmosphere of 60 sccm. Subsequently, 6 mL of high-temperature water vapor was injected uniformly within a holding time of 10 min. After the holding ended, it was naturally cooled to room temperature to obtain a white alumina precursor; (4) The alumina precursor obtained in the above step was dried and placed in a tubular furnace with dry air. When the temperature was above 850 °C, it was heated to 1400 °C at a heating rate of 2 °C / min and held for 150 min to obtain alumina foam ceramics.

[0029] The porous carbon / alumina composite obtained in Example 1 above macroscopically shows porous alumina coated with black carbon ( Figure 1 the left figure in the middle), and after carbon sintering, a white lightweight porous foamy structure ( Figure 1 the right figure in the middle) is obtained. The X-ray diffraction pattern ( Figure 2 ) of the ground powder shows that the characteristic peaks of the sample almost completely correspond to the characteristic peaks of the α-alumina standard card (PDF#10-0173), indicating the high crystallinity and purity of alumina; the scanning electron microscope photograph ( Figure 3 ) at the fracture surface of the sample shows the three-dimensional skeleton structure of the alumina foam ceramics, with many macropores having a pore diameter of 100 - 150 μm.

[0030] The porosity of the sample was calculated to be 98.1% by the static weighing method using Archimedes' principle. Based on the transient plane heat source method, the temperature rise curve ( Figure 4 ) of the alumina foam ceramic sample was obtained by a thermal conductivity tester. According to the temperature rise curve, the heat transfer was fitted, and the thermal conductivity of the sample at room temperature was obtained as 0.0457 W / (mK), showing high thermal insulation performance.

[0031] Example 2:

[0032] Change the step of preparing the alumina precursor in step (3) of Example 1 to heat preservation in air at 850 °C for 30 min, and keep other operations the same as those in Example 1. The obtained alumina foam ceramic has a porosity of 96.6% and a thermal conductivity of 0.0565 W / (mK) at room temperature.

[0033] Example 3:

[0034] Change the alumina in step (1) of Example 1 to powder with a particle size of 1 g to 2 μm, change the particle size of the sintering aid to ~2 μm, change the sintering temperature in step (4) to 1450 °C, and change the heat preservation time to 120 min. Keep other operations the same as those in Example 1. The obtained alumina foam ceramic has a porosity of 98.3% and a thermal conductivity of 0.0378 W / (mK) at room temperature.

[0035] Example 4:

[0036] Change the alumina in step (1) of Example 1 to 1.2 g of powder with a particle size of 5 - 6 μm, select powder with a similar particle size as the sintering aid, change the sintering temperature in step (4) to 1500 °C, and keep other operations the same as those in Example 3. The obtained alumina foam ceramic has a porosity of 95.0% and a thermal conductivity of 0.0662 W / (mK) at room temperature.

[0037] Example 5:

[0038] Change ammonium chloride in step (1) of Example 3 to 0.6 g, change urea to 0.12 g, change sucrose to 0.8 g, and change the water vapor injection amount in step (3) to 5 mL. Keep other operations the same as those in Example 3. The obtained alumina foam ceramic has a porosity of 97.2% and a thermal conductivity of 0.0557 W / (mK) at room temperature.

[0039] Example 6:

[0040] Change sucrose in step (1) of Example 3 to glucose, and keep other operations the same as those in Example 3. The obtained alumina foam ceramic has a porosity of 97.8% and a thermal conductivity of 0.0493 W / (mK) at room temperature.

Claims

1. A preparation method of alumina foam ceramics, characterized in that, The method comprises the following steps: (1) Mix ammonium chloride, urea, sugar, a combustion aid, and alumina powder and then grind them to obtain a powder mixture; (2) Place the above powder mixture in a reaction furnace, introduce an inert atmosphere as a protective gas, and then heat from room temperature to 160°C at a heating rate of 25°C / min, heat from 160°C to 400°C at a heating rate of 4°C / min, and heat from 400°C to 600°C at a heating rate of 20°C / min. After cooling, a porous carbon / alumina composite material is obtained; (3) Subsequently, pass water vapor through the above composite material at high temperature or directly remove carbon in an oxygen-containing atmosphere to obtain a precursor of alumina foam ceramic; (4) Sinter the precursor of alumina foam ceramic obtained above at high temperature in an oxygen-containing atmosphere to obtain an alumina foam ceramic with a high porosity.

2. The preparation method of an alumina foam ceramic according to claim 1, characterized in that, In the step (1), the sugar is either sucrose or glucose or a combination of the two, and the combustion aid is any one or a combination of magnesium oxide, silicon oxide, calcium oxide, titanium oxide, and zirconium oxide.

3. The preparation method of an alumina foam ceramic according to claim 1, characterized in that, In the step (1), the mass ratio of ammonium chloride, urea, sugar, combustion aid, and alumina powder is (0.4 - 1):(0.1 - 0.3):(0.6 - 1.5):(0.002 - 0.05):

1.

4. The preparation method of an alumina foam ceramic according to claim 1, wherein In the step (1), the particle size of the alumina powder is 0.1 - 10 μm.

5. The preparation method of an alumina foam ceramic according to claim 1, characterized in that, In the step (2), the inert atmosphere is any one or a combination of nitrogen, argon, and helium.

6. The preparation method of an alumina foam ceramic according to claim 1, wherein, In the step (3), the temperature for carbon removal by water vapor is 750 - 1150°C, and the heat preservation time is 10 - 60 min.

7. The preparation method of an alumina foam ceramic according to claim 1, wherein, In the step (4), the sintering temperature is 1300 - 1600°C, and the sintering time is 30 - 240 min.

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