Fabrication And Properties Of Transparent Magnesium Aluminate Spinel Ceramics

Sol- gel method has received much attention due to its excellent properties, such as high productivit y, product purity and low cost. However, the report on spinel powder synthetized by sol- gel process was rare. Porosity, defects and impurity have great adverse effects on the light transmittance of the transparent ceramics. Transparent Mg Al2O4 ceramics possess good properties including high transmittance and strength, however, the low sintering activity, high sintering temperature and low powder purity of the spinel have limited their development. Consequently, it is of great importance to prepare transparent Mg Al2O4 ceramics by sol-gel and SPS process.N itrate and citric acid were used as raw materials to prepare nano- Mg Al2O4 powders with high purity by sol- gel method. The results showed that precursor with high component homogeneity and low crystalline temperature was obtained when citric acid and mental cation reacted at 300 o C in 1: 1 molar ratio. The organic burned out at 800 o C and the crystalline temperature of precursor was 600 o C. The size and agglomeration degree of the powders increased with the calcination temperature. The powders possessed high sintering activity, the BET surface was up to 39.2 m2 /g and the size of the particle reached 17.1nm, when calcined at 800 o C for 2h.The sintering process had great influence on the properties and densification behavior of the transparent ceramics. The spine l ceramics fabricated by SPS under 1400 o C /40MPa/20 o C /min condition possessed the highest density and transmittance. The strength, toughness and transmittance of the spinel increased as the heating rate decreased, which indicated that low heating rate may contributed to the high density and large grain size. The graphite contamina tion increased with the ris ing temperature and high porosity may be induced by lower sintering temperature. The strength of the spinel was up to 215 MPa.The initia l sintering temperature was decreased and the density of the samples was increased by using Li F as sintering aids for Mg Al2O4 ceramics. Li F produced gas phase, which can remove the graphite contamination during sintering process, consequently, transmittance of the spinel was improved. The bending strength, hardness and modulus were increased with the sintering aids content. The spinel with 1wt.% Li F possessed highest fracture toughness and transmittance, which were up to 65% at 2000 nm wavelength and 2.92 MPa?m1/2 respectively. But secondary phase may be brought out when sintering aids continue to increase, which may decreased the transmittance of the sample.The spinel ceramics using 1wt.% Li F as sintering aids with homogenous phase were fabricated by SPS at 1400 o C for 10 min with different heating rates ranging from 20 to 50 o C /min. The transmittance was mainly affected by the porosity of the samples. The porosity of the samples increased with the heating rate, resulting in lower transmittance. Thus, in this condition, relatively low heating rate was optimal for fabricating spinel ceramics with high transmittance.