Effect Of Additives On The Sintering Densification Process And Properties Of Magnesium Borate Whisker Porous Ceramics

Sintering is an indispensable step for porous ceramics preparation. For the in-situ synthesis of whisker reinforced porous ceramics composites, both uniform distribution of whiskers and well properties of the ceramics are required. When it comes to study the whisker growth and sintering of ceramic in in-situ synthesis of whisker reinforced porous ceramic composites through solid sintering method, a large number of article show that the addition of transition metal oxides are beneficial to the overall performance of porous ceramics and the growth of whisker. In this study, Ni(NO3)2, Fe(NO3)3 and NiO are choosed for additives to research the effects of additives on the sintering process, microstructure, morphology and performance of in-situ synthesis magnesium borate whisker porous ceramic. Magnesium carbonate and boric acid as the main raw material, and the Mg: B moore ratio is 1: 1.5. Through the modern material analysis testing technologies, which contain thermoanalysis(TG-DTA), phase analysis(XRD), thermomechanical analysis(DIL), morphology analysis(SEM), strength and porosity analysis, the following conclusions are achieved.1. when additives aren't added, the green body thant has pretreated sinter with a heating rate of 10?/min from room temperature to 900?. In the whole process the total shrinkage amount of the sample is 5.98%, shrinkage start and end temperatures are 629? and 738? respectively. And the heating rate has little effect on the densification rate in the process of sintering, but the sample shrinkage amount decrease slightly with the increase of heating rate;2. Research shows that the sample's overall performance obtained the best by 850? sintering 4 h, in which the mainly phases are composed of Mg2B2O5 and MgB4O7, and the diameter and length of the whiskers are 0.3-0.5 ?m and 3-6 ?m respectively, but a small amount of grain or granular grain still exist. And the porosity and three-point bending strength of the porous ceramics are 65% and 4.72MPa;3. 5% of Ni(NO3)2 as an additive can effectively reduce shrinkage start and end temperature, postpone the reaction of B2O3 and MgO, and let the shrinkage of the sample uniform during sintering process. And the total shrinkage amount is 7.86% in the whole sintering stage. When the samples are sintered at 850? for 4 h, the whiskers of sample are 0.2-0.3 ?m in diameter and 4-8 ?m in length, in addition to the porosity and three-point bending intensity of the porous ceramics are 63% and 6.47 MPa;4. The effect of 5% Fe(NO3)3 as the additives on the in situ synthesis of magnesium borate whisker porous ceramics' sintering process and phase transformation are similar with the Ni(NO3)2 as an additive, but it almost doesn't affect the amount of shrinkage in the whole sintering process. When its samples are sintered at 850? for 4 h, the diameter and length of resulting magnesium borate whiskers are 0.3-0.8 ?m and 3-5 ?m, and the porosity of the porous ceramics and three-point bending strength are 60% and 6.0 MPa;5. Add 5% NiO as an additive, there are almost little effect on the start and end shrinkage temperature, and its total body shrinkage amount is 5.45% during the whole sintering process. And when its sample sintering at 850? for 4 h, the resulting whisker growth uniformly, and the length up to 6-10 ?m, but the diameter of whisker is thick, about 0.7-1.5 ?m. Moreover the porosity and three-point bending strength of the porous ceramic are 67% and 4.25 MPa;6. Compared with adding 5% additive, when the amount of the additive is 1% or 10%, the resulting whisker growth nonuniformly, and the ratio of length and diameter is very small, meanwhile the density and flexural strength of sintered samples decrease and the porosity of the porous ceramics increase, but the porosity of the increased amount is not large during sintered at 850? for 4 h;7. Integrating the sintering process, phase, morphology and physical properties of the in situ synthesis of magnesium borate whisker porous ceramics, when add 5% Ni(NO3)2 as an additives and sintered at 850? for 4 h, the resulting magnesium borate whisker porous ceramics have optimal performance.