In recent years, rare earth oxides have received extensive attention from researchers due to their advantages such as high melting point, high chemical activity, good sintering promotion and strong corrosion resistance [7-10], among which Y2O3 is used as a sintering assistant for mullite [11], spinel [12], AlON [13] and other materials due to its high cost performance and high activity. Yuan et al. [14] studied the effects of two rare earth oxides, CeO2 and Y2O3, on the properties and structure of Mg-Al castable, and found that CeO2 and Y2O3 had different degrees of influence on the in-situ reaction of spinel and the stability of the material, and Y2O3 was more likely to form solid solution with spinel. CeO2 is more likely to enter the calcium hexaluminate (CA6) lattice. To this end, in this paper, with fused white corundum, fused magnesia, activated alumina as the main raw materials, with Y2O3 as the additive, respectively at 1300, 1400, 1500, 1600 ℃ temperature for 3h to prepare aluminum magnesium dry ramming material. The effects of adding Y2O3 at different calcination temperatures on the phase composition, microstructure, sintering properties and mechanical properties of Al-magnesia dry ramming compound were studied in order to provide reference for optimizing the properties of Al-magnesia dry ramming compound.

1.1 Preparation of raw materials and samples

The main raw materials used in the experiment are: fused white corundum (particle size d≤5mm), 97 fused magnesia, activated alumina (CL370, particle size d≤ 3μm), Y2O3 powder (purity 99%, particle size d≤5μm), plus 1.5% of the binding agent. The addition amounts of Y2O3 are 0, 0.5%, 1.0%, 1.5% and 2.0% respectively, which are marked as Y0, Y1, Y2, Y3 and Y4 in sequence. The composition of the test sample is shown in Table 1. First, white corundum, magnesia, active aluminum oxide powder and Y2O3 fine powder are pre-mixed in a polyurethane plastic ball mill tank for 3h to make them evenly mixed. Then, aggregate, binder and pre-mixed fine powder are successively mixed in an Alish power mixer, and the mixing time is controlled at about 5min. Then the mixed material was pressed into 50mm×50mm cylindrical and 25mm×25mm×140mm long strip test samples on the TYE-500B manual pressure test machine under 70MPa pressure, and the formed samples were dried in a constant temperature drying oven at 110℃ for 24h. The dried samples were then divided into calcined at 1300, 1400, 1500 and 1600 ℃ for 3h, and then each item was tested. In addition, the matrix part is extracted separately and pressed into a sample of 20mm ×20mm, and the heat treatment is carried out under the same conditions as above, and then the phase composition and microstructure are analyzed.

Table 1 Composition of the sample (wB / %)

1.2 Performance Detection

The heating line conversion rate, apparent porosity, bulk density and room temperature compressive strength of the test samples were measured according to GB/T5988-2007, GB / 2997-2000 and GB/T5027-2008, respectively. According to GB/T3002-2004, the hot bending strength of samples calcined at 1600 ℃ for 3h and kept at 1400 ℃ for 0.5 h was determined by three-point bending method. The phase composition of the sample was analyzed by X-ray diffractometer (PhilipsX 'PertPro), and the lattice constants of the phase were calculated by MDIJade6.0 software. The microstructures of the samples were observed by scanning electron microscopy (PhilipsXL-30-TMP), and the microelements were analyzed by PHDEMX spectrometer.

2 Results and discussion

2.1 Effects of addition of Y2O3 on the phase composition of dry ramming materials FIG. 1 shows the XRD pattern of samples with different contents of Y2O3 after being held at 1600℃ for 3h. As can be seen from Figure 1, the main crystal phase of each sample is MG-Al spinel (MgAl2O4) after being held at 1600 ℃ for 3h. With the introduction of Y2O3, Y3Al5O12 (YAG) begins to be formed in the sample. Moreover, the intensity of YAG characteristic peaks gradually increases with the increase of Y2O3 content (see the enlarged figure in Figure 1). Spinel from Figure 1. As can be seen from the magnification diagram of the strong peak, with the gradual increase of the addition of Y2O3, the diffraction peak of spinel moves toward the square direction of low angular degree. This is because the rare earth oxide Y2O3 has a high activity and is easy to solid dissolve into the lattice of spinel at high temperature, resulting in lattice stress

Figure 1 XRD pattern of samples with different contents of Y2O3 held at 1600 ℃ for 3h

FIG. 2 shows the XRD pattern of the sample with 1.0% Y2O3 held at different temperatures for 3h. As can be seen from Figure 2, when the temperature is 1400 ℃, there are obvious diffraction peaks of spinel, YAlO3 (YAP) and more Al2O3 in the sample. When the temperature is 1500℃, YAP is replaced by Y3Al5O12 (YAG), but weak Al2O3 diffraction peak can still be observed in the sample. When the temperature is further increased to 1600 ℃, the Al2O3 diffraction peak disappears and is completely transformed into MgAl2O4 and YAG phases

Table 2 shows the lattice constants of the sample spinel (MgAl2O4) and Y3Al5O12 (YAG) after heat treatment under different conditions. It can be seen from Table 2 that after the test sample was calcined at 1600 ℃, the lattice constant of MgAl2O4 gradually increased with the increase of Y2O3 input, while the lattice constant of Y3Al5O12 did not change significantly. This is because Y3+ is more likely to solute into the MgAl2O4 lattice to replace Al3+ at high temperature, and the half-diameter of Y3+ is larger than the radius of the lattice of MgAl2O4 and Y3Al5O12 after heat treatment under different conditions of Al3+, resulting in changes in their lattice parameters. It can also be seen from Table 2 that for sample Y2 with Y2O3 content of 1.0%, the lattice constant of MgAl2O4 gradually decreases with the increase of calcining temperature, which is caused by the decrease of MgO/Al2O3 mass ratio in MgAl2O4 [13].

2.2 Effects of addition of Y2O3 on the microstructure of dry ramming materials FIG. 3 shows the SEM photos and EDS spectra of the samples with different Y2O3 contents after calcination at 1600℃ for 3h. As can be seen from Figure 3, only spinel particles exist in the sample without Y2O3, and the structure is loose. After Y2O3 is added, the sintering performance and density of the sample are significantly improved. When the amount of Y2O3 is 0.5%, besides spinel, bright white round particles can also be observed in the sample. Distributed among the gray spinel crystals, the EDS spectra shown in Figure 3 (f) show that the bright white round granular material is Y3Al5O12 (YAG). It can also be seen from Figure 3 that when the addition of Y2O3 is 1.0%, the density of the sample is further improved, and the spinel structure gradually develops well, showing a regular octahedral shape.