Effect Of Modified Alumina Powders On Its Sinterability And Thermal Conductivity

Alumina,one of the traditional ceramic materials,holds excellent mechanical properties and corrosion resistance.Simultaneously,it contains not only good wear resistance,but also the abundant reserves.Alumina also has low price.Therefore,those advantages make alumina become the most widely studied non-metallic material in current era.In the field of materials engineering,the high-qualited alumina ceramics are mainly used in crucial devices in high-end industries.For instance,laser crystals,high purity containers,lithium-ion battery separators,and thermally conductive substrates.However,there are still many problems in the preparation of high-performance alumina ceramic materials.The drawbacks of alumina,such as single product shape,high sintering temperature,low density and thermal conductivity,still needs to be overcome.At the end of experiments and researchers,scientists find that improving the performance of alumina powders may have a positive effect on the sintering and thermal performance of its ceramics products.Alumina powder with high purity,small and narrow size distribution,and regular morphology is one of the urgent raw materials for the fabrication of high-performance ceramic materials.In this paper,experiments modified the morphology and particle size of powders,using high-purity alumina（99.999%）powders made by metal alkoxide method.The hexagonal flakeαalumina powder with regular morphology was successfully prepared at a low temperature of 700℃via fully mixing between the mineralizer（Al F₃）and raw material.The experiment obtains the best parameters of the process via orthogonal method,and the growth mechanism of hexagonal flaky crystals was analyzed by the model of negative ion coordination polyhedron and surface energy calculation.It reveals that the reaction of the structured water in the boehmite with the mineralizer changes the solid-phase mass transfer to the gas phase,which accelerates the mass transfer rate,and an agglomerated structure is formed between the boehmite particles and Al F₃.This structure provides the conditions of the nucleation,forming hexagonal flaky particles after a polyhedron morphology that has never been discovered in any papers.And the influence of mechanical sanding process on the particle size of the powders is investigated.The result showed that the small molecule electrolyte dispersant ammonium citrate（AC）can effectively reduce the powder agglomeration in the slurry,and improve the sanding efficiency.After adding 2%ammonium citrate and sanding for 100 minutes,the D50of the alumina slurry with 19%solid content can reach 394.81 nm,and the particle size is reduced by nearly 86%.The experiments are performed to investigate the influence of powder morphology and particle size on its sintering and thermal conductivity.In the experiments,different kinds of powders are prepared into ceramic at a certain density under the same conditions（In the high-density ceramics,since the growth space of the particles is limited,their morphology will no longer be representative）.The result shows that the powders after sanding exhibit better sintering performance at low temperature（1300℃）,with lower shrinkage rate and porosity compared with other subjects.On the other side,because of the special morphology of the hexagonal flaky alumina powder,a huge amount of pore which cause the lowest density are formed during the sintering process.Since the density and porosity of every sample cannot be controlled at the same level,it is difficult to analyze the influence factors of thermal conductivity.Hence,the results of the experiments should be discussed from the perspectives of macroscopic and microcosmic.From the aspect of macroscopic,due to the difference in density,the thermal conductivity of each sample changes significantly.The higher the temperature,the lower the thermal conductivity;Microscopically,according to the calculation results of the mean free distance of phonons,thermal conductivity of the alumina ceramic under this condition depends on the porosity instead of the grain size.Consequently,under the same conditions,the flaky powders have a larger porosity after sintering,resulting in a lower thermal conductivity;but the powders after sanding have the lower shrinkage and porosity after sintering,and will display better thermal conductivity.