Effect of agglomerates in ZrO2 powder compacts on microstructural development
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Author : LZH
Update time : 2023-08-17 17:43:06
What is ZrO2? ZrO2-nanoparticles are used as model substances to study agglomeration processes in colloidal suspensions. The sol–gel method and coprecipitation from solutions form, together with hydrolysis and colloidal processes, the chemical method category. The chemical methods, especially the sol–gel method, are characterized by flexibility, non-aggressive solutions, low cost, and relatively simple reaction conditions. The sol–gel method is based on a molecular synthesis of nanoparticles. During nanopowder formation, close control over the nucleation and growth of the particles is required because the particles easily adhere and form agglomerates. The methods reported in the literature for synthesizing zirconia particles (include the following: from solutions of inorganic salt or alkoxide complexes and hydrothermal method [6]) have been widely employed. To control the formation of sub-micrometer particles, including YSZ powders, chemical methods, such as sol–gel processing, have been applied and investigated extensively.
Effect of agglomerates in ZrO2 powder compacts on microstructural development Ultrafine zirconia powders were prepared by a coprecipitation and spray-drying method. Agglomerates may be fragmented or present in green bodies after compaction. The effect of agglomerates on sintering and microstructural development was studied, and it was found that the agglomerate content in compacts was a major factor affecting the microstructure development and the sintered densities. The interaction between agglomerates themselves and between agglomerates and the primary particle matrix is discussed. It is argued that the hard agglomerates in the powder from the water-washed coprecipitate are formed by oxo bridging between non-bridging hydroxyl groups present in the zirconium hydroxide structures due to the effect of hydrogen bonding in the aqueous system. The substitution of organic -OR groups for the non-bridging hydroxyl groups removes this hydrogen-bonding effect between the zirconium hydroxide units and thus eliminates the cause of accumulation. A new production method for high-purity ZrO2 powder has been developed. Non-stabilized ZrO2 powder high purity can be produced by heating a mixture of zircon (ZrO2･SiO2) and carbon powders under reduced pressure. Stabilized ZrO2 powders with high purity can also be obtained by adding a stabilizer such as CaO or Y2O3 to the zircon/carbon mixture. The generation of SiO(g) from a zircon/carbon mixture is greatly accelerated by heating under reduced pressure compared with under atmospheric pressure. The purity of ZrO2 powder produced by this method is about 99.8%, and the average grain size is smaller than 5μm. Fine Y2O3-partially-stabilized ZrO2 powder can also be obtained by wet grinding as the raw material for a ZrO2-sintered body with high strength and toughness. In addition, as a by-product of this process, ultra-fine SiO powder can be obtained by vapor-phase SiO(g) condensation.
In-situ evaluation of particle size distribution of ZrO2-nanoparticles obtained by sol–gel Sol–gel processing involves hydrolysis and polycondensation of a precursor and the subsequent formation of a gel. Heat treatment of the formed gel results in a crystalline network structure and, depending on the gelling solution, may be formed as fibers, monoliths, thin and thick film coatings, and powders. The ability to control the parameters of the final product, such as purity and microstructure, and to consolidate the particles at relatively low temperatures, are some of the advantages of the sol–gel method for materials produced via conventional synthesis routes. The literature suggests that gelation of hydrated zirconium oxide takes place in two steps: step one — formation of sol-primary particles, their growth, hydrolysis, and polymerization; and step two — coagulation of sol and formation of a polymeric gel. The largest particle size was micrometers for other reaction media. The comparison of the diversity synthesis routes investigated did not enable us to observe one trend combined with increased particle size when compared to reaction times in the synthesis of zirconia. This work shows how the reaction medium influences obtaining a colloidal dispersion using nanometric sizes obtained from the sol–gel process. At 27 °C, the particles tend to agglomerate, increasing in size. After this time, the agglomerated particles tend to decrease their size. When samples were refluxed under reflux at 70 °C, a maximum particle size of 100 nm was observed after two h. In 3 h, the average size reached 280 nm in the presence of NaCl at 27 °C, and a maximum particle size of 70 nm after two h was observed. It is well-known that colloidal dispersions have a high area or volume ratio regarding the electrical loads on their surfaces when exposed to polar solvents. The electrical potential between the interface of the particle surface decreases more rapidly as the ionic strength increases since the electric double layer is compressed towards the surface by the concentration of ions of the solution. One possible explanation for the suspension's stability is the formation of particle clusters due to hydrophilic and electrolytic forces. If the kinetic energy exceeds the repulsive potential of the electric charge, the particles aggregate. There are two limiting regimes for the aggregation of colloidal particles. The first is related to diffusion-limited colloid aggregation (DLCA), where a stable aggregate is formed every time two particles collide. In this case, the aggregation rate is fully determined by the number of collisions per time. In the second regime, the repulsive barrier due to surface charges or steric effects hinders the aggregation, and many collisions are required to form a stable coagulate. This is the so-called reaction-limited colloid aggregation (RLCA).
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