With the in-depth development of my country's economy and society, the contradiction of energy shortages has become increasingly prominent, and the high-temperature kiln industry urgently needs heat-insulating refractories with better energy-saving effects. Among many heat-insulating refractory materials, mullite combined with alumina hollow sphere products have the characteristics of high service temperature and excellent thermal shock resistance, and are widely used in high-temperature kiln linings. Mullite bonded alumina hollow sphere products (referred to as mullite bonded products) are heat-insulating and refractory using mullite as the matrix bonding phase and alumina hollow spheres as aggregates material. Under normal circumstances, the lower the density of the refractory, the lower its thermal conductivity, the less heat storage, and the better the energy-saving effect. Compared with heavy refractory materials, the existing mullite combined products have a certain energy saving effect, but still have a higher density (usually 1.2 ~ 1.6 g·cm -3), leading to high temperature furnace industry energy consumption is too high.
Foaming method combined with gel injection molding is a new process for preparing porous ceramics in recent years. It has successfully prepared lightweight and high-strength porous ceramics such as alumina and mullite. The principle of the process is to prepare porous ceramic bodies by in-situ polymerization and solidification of the foamed ceramic slurry of organic monomers, and obtain porous ceramics by high-temperature treatment. In this work, we tried to use foaming method combined with gel injection molding process to produce a density lower than 1. 0 g·cm -3 lightweight mullite bonded alumina hollow sphere products, and studied the microstructure, bulk density, porosity, compressive strength and thermal conductivity of mullite bonded products with different foaming times Impact.
1. Test
1. 1 Raw materials
The raw materials used in the test are alumina micropowder (d50 = 3μm), silica micropowder (d50 = 5μm), alumina hollow spheres (particle size ≤3 mm), organic monomer acrylamide (analytical purity), crosslinking agent methyl bisacrylamide (analytical grade), catalyst N, N, N, N'-tetramethyl ethylene diamine (analytical grade), initiator ammonium persulfate aqueous solution, foaming agent dodecyl sulfate triethanolamine (industrial Grade), dispersant polycarboxylate-based polymer FS20 and deionized water.
1. 2 Sample preparation
First, acrylamide and methylene bisacrylamide are mixed with deionized water at a mass ratio of 20：1 to prepare a premix. Then weigh 280 g of silica powder, 720 g of alumina powder and a small amount of dispersant polycarboxylate-based polymer FS20 according to the theoretical proportion of mullite to obtain a mixed powder. Add 300 g of a premixed liquid of acrylamide and methylene bisacrylamide to the mixed powder, stir and mix for 60 min to obtain a slurry. Add 50 g of foaming agent to the slurry and stir quickly for 5, 10, 15 and 20 minutes to foam. After the foaming is completed, add 0 to the foam slurry in sequence. 3% (w) catalyst N,N,N',N'-tetramethylethylenediamine and 1% (w) initiator ammonium persulfate aqueous solution, stir well, then add 1000 g alumina hollow ball and continue stirring After being evenly poured into the mold, it will be cured and cured at rest. After demolding, the green body is dried, and finally in it was calcined at 1650 ℃ for 6 h, and then cooled to room temperature with the furnace.
1. 3 performance characterization
Measure the mass m and volume V of the sample, use the formula ρ = m /V to obtain the volume density of the sample; use the pycnometer method to test the true density ρ1 of the sample, use the formula P = (ρ1 － ρ) /ρ1 to get the sample’s volume density Porosity; Use the WHY-5 pressure testing machine to test the compressive strength at room temperature of the sample. The size of the sample is 25 mm × 25 mm × 25 mm. The average value of 5 samples is used to indicate the compressive strength at room temperature of the sample; YB /T 4130—2005 (flat plate method) Use PBD-12-4P thermal conductivity meter to test the thermal conductivity of samples at 600, 800 and 1000 ℃ respectively; use PHILIPS X'pert X-ray diffractometer to analyze For the phase composition of the sample, use the PHILIPS-XL30 scanning electron microscope to observe the fracture morphology of the sample.
2 Results and discussion
2. 1 Phase composition and microstructure
The XRD patterns of the samples prepared with different foaming times are shown in Figure 1. It can be seen that all the samples are composed of mullite and corundum, which shows that after calcination at 1650 ℃ for 6 h, the silica and alumina in the samples completely reacted to form mullite.
Figure 1. XRD patterns of samples prepared at different foaming times
Figure 2 shows the scanning electron micrographs of the samples prepared at different foaming times. It can be seen from Figure 2 that all samples are composed of porous mullite matrix and alumina hollow spheres. The porous mullite matrix and alumina hollow spheres are tightly combined, and the pore diameter of the matrix is 50-100 μm. As the foaming time increases, the pore structure of the porous matrix does not change significantly.
Figure 2 .SEM photos of samples prepared at different foaming times
2. 2 The influence of foaming time on sample bulk density, porosity and compressive strength
Figure 3 shows the bulk density and porosity of the samples prepared with different foaming times.
Figure 3. Bulk density and porosity of samples prepared at different foaming times
It can be seen from Figure 3 that as the foaming time increases, the volume density of the sample changes from 0. 94 g·cm -3 is reduced to 0.71 g·cm-3, and the porosity from 73. 8% increased to 80. 2% . This shows that the foaming method combined with the gel injection molding process, the density <1.0 g·cm-3can be prepared, and the lightweight mullite with porosity> 70% combined with alumina hollow sphere products. Prolonging the foaming time can effectively increase the porosity of mullite-bound products. This is mainly because long-term stirring can introduce more air into the slurry and combine with the foaming agent to form bubbles, increase the volume of the slurry, and improve the production porosity of the product.
Figure 4 shows the normal temperature compressive strength of samples prepared with different foaming times. It can be seen that as the foaming time increases, due to the increase in the number of pores in the porous matrix, the compressive strength of the sample at room temperature changes from 5. 46 MPa is reduced to 1. 81 MPa.
Table 1 lists the properties of the lightweight mullite combined with alumina hollow sphere products prepared by the foaming method in this study and other lightweight mullite materials. It can be seen that the small pore foam mullite ceramic prepared by the foaming method has the smallest bulk density, but has the largest compressive strength. In this study, the lightweight mullite prepared by the foaming method combined with oxidation.The compressive strength of aluminum hollow sphere products is significantly higher than that of lightweight mullite bricks prepared with polystyrene spheres as pore formers , showing higher mechanical properties. The strength of lightweight mullite combined with alumina hollow sphere products is lower than that of traditional mullite combined with alumina hollow sphere products, mainly due to the decrease in strength due to the porous matrix.
Figure 4. Normal temperature compressive strength of samples prepared with different foaming times
Table 1. Performance comparison of different lightweight mullite materials
2. 3 The influence of foaming time on the thermal conductivity of the sample the relationship between foaming time and the thermal conductivity of the sample is shown in Figure 5. Can
it can be seen that the lightweight mullite combined with alumina hollow sphere products have low thermal conductivity. As the foaming time increases, the thermal conductivity of the sample at 1 000 ℃ decreases from 0. 681 W·m-1·K-1 reduced to 0. 575 W·m-1·K-1. Foaming the thermal conductivity of the samples at a time of 20 minutes is lower than that of other samples, and the thermal conductivity at 600 ℃ is even as low as 0. 467 W·m-1·K-1, with the best thermal insulation performance. The thermal conductivity of traditional mullite combined alumina hollow sphere products at 800 ℃ is 0. 900 W·m-1·K-1, and the thermal conductivity of all samples in this work is lower than 800 ℃. 0.640 W·m-1·K-1, especially the thermal conductivity of the sample with a foaming time of 20 minutes at 800 ℃ is as low as 0. 561 W·m-1·K-1. Compared with traditional mullite bonded products, the thermal conductivity of the lightweight mullite bonded alumina hollow sphere products prepared in this work is significantly reduced, and has a broad field of energy saving and consumption reduction in industrial kilns.
Figure 5. Thermal conductivity of samples prepared with different foaming times at different temperatures
3 Conclusion
(1) As the foaming time increases, the porosity of the sample increases, while the bulk density, compressive strength and thermal conductivity decrease.
(2) The sample matrix is porous structure, the foaming time increases, the pore structure of the porous matrix does not change significantly, and the porous matrix is tightly combined with the hollow alumina spheres.
(3) The thermal conductivity of the sample with a foaming time of 20 minutes at 600 ℃ is 0. 467 W·m -1·K-1, the bulk density is 0. 71 g·cm -3, the compressive strength at room temperature is 1. 81 MPa, which has broad application prospects for energy saving and consumption reduction in industrial kilns.
