Research On The Preparation And Sintering Activity Of High Purity Ultrafine Alumina Powders

High purity and ultra-fine alumina powders have the characteristics of high wear resistance,high temperature corrosion resistance,high resistivity,high thermal conductivity,and low cost,rich reserves.It is widely used in mechanical industry,refractory,electronic industry,lighting,petrochemical industry,aerospace and other fields.Chemical co-precipitation method has the advantages of simple equipment,low price of raw materials,high yield and high purity of powders.It is an effective method for large-scale production of high-purity ultrafine alumina powders.However,there are many technological conditions in the chemical co-precipitation method,and the influence of technological parameters on the quality of the powders is complex,and the powders are easy to agglomerate.Therefore,in this paper,the precursors were prepared by chemical co-precipitation method with ammonium aluminum sulfate and ammonium bicarbonate as raw materials,and the high-purity ultrafine alumina powders were prepared by calcining the precursors.The orthogonal experimental system was designed to study the comprehensive effects of temperature,dropping speed and pH value on the preparation of precursors and alumina powders in the co-precipitation reaction,and by adding seeds,dispersant,pre-calcination and ball-milling,the calcination process of the precursor and the sintering activity of the calcined alumina powders were studied,which provided the experimental basis for the industrial stable production and application of the high-purity ultrafine alumina powders.Taking the reaction temperature,pH and dropping speed as the factors,the orthogonal experiment of three factors and four levels was designed.The phase,morphology and particle size distribution of precursors and alumina powders were characterized by SEM,XRD and FT-IR.It was found that pH value was the main factor affecting the formation of precursor phase and crystal phase grain size,the dropping speed of ammonium aluminum sulfate solution was the main factor affecting the particle size of aluminum oxide powders and the content of ?-Al2O3 phase,and the temperature is an important factor affecting the grain size of the precursor ammonium aluminum carbonate and alumina powders.After optimizing the process,adding 3 wt.%seeds,controlling the temperature of 5?,dropping speed of 2 mL/min,pH=10,the precursors prepared by reaction can be calcined at 1100? for 2 hours to obtain vermicular alumina powders with good dispersion.The relative density of alumina ceramics sintered at 1400? for 4 h is 93%,and the relative density of alumina ceramics sintered at 1500?for 4 h is 98.1%.Through the study of the calcination process of ammonium aluminum carbonate,it is found that the pre-calcination and ball milling process change the shape of calcined powder particles from vermicular to angular block particles.The phase transformation temperature of ?-Al2O3 phase was increased by pre-calcination above 700? and ball milling.With the increase of pre-calcination temperature,the phase transformation temperature of the powders increased more significantly.The pre-calcination and ball milling process increase the grain size of calcined powders,and with the increase of pre-calcination temperature,the grain size of calcined powders increase more significantly.After being milled for 24 hours and calcined at 1100? for 2 hours,a-alumina powders with average particle size of 183 nm and good dispersity can be obtained.The relative density of the powders sintered at 1400? is 91.9%,and the alumina ceramics with 99.55%relative density can be prepared at 1500?.It is found that the effect of dispersant PEG on sintering activity of alumina powders is related to the molecular weight and total amount of PEG.The addition of PEG2000 is beneficial to inhibit the agglomeration of particles in the solution and improve the sintering activity of calcined alumina powders.The addition of PEG200 has no obvious effect on the sintering activity of alumina powders.The addition of PEG11000 can improve the sintering activity of calcined alumina powders,but the effect is not as good as PEG2000.Too much PEG can reduce the sintering activity of calcined alumina powders.The relative density of ceramics compacted of alumina powders prepared by adding 0.5 wt.%PEG2000 is 98.5%at 1400? and 99.23%at 1500? for 4 hours.