During the sintering process, porous ceramics undergo significant shrinkage with volume changes even half. Such volumetric alterations may lead to materials cracking or deformation, severely affecting the yield of the prepared products. Researchers have proposed several methods such as adjusting the sintering temperature, utilizing reactions and expansion phases during the sintering process to address these issues. The most commonly method was to utilize the expansion characteristics of the materials themselves to solve the shrinkage problem. However, this method requires complex raw materials and lacks universality. Recently, our group proposed an in-situ hollow sphere method with the addition of spherical metal powders, successfully preparing porous ceramics without sintering shrinkage. The main theoretical support for this process is the Kirkendall effect that occurs during two-phase diffusion. When metal spherical particles are oxidized, the difference in diffusion rates forms a hollow structure, effectively offsetting the shrinkage caused by sintering. In this work, hollow zinc oxide particles were successfully prepared using the direct oxidation of metallic zinc powders and applied in the preparation of zinc oxide porous ceramics. During the oxidation process of metallic zinc powder, its addition amount, particle size, and their effects on the properties and microstructure of porous ceramics were investigated. The relationship between the pore size of porous ceramics and the particle size of metallic zinc particles was analyzed. This study could broad the application scenarios of this process and hollow zinc oxide particles.

Firstly, zinc oxide powder and metallic zinc powder with different particle size were thoroughly ground and mixed in a grinding bowl according to a certain ratio (the volume percentage of metallic zinc powder to zinc oxide powder). Subsequently, a 5% PVA aqueous solution (50 g/L) was added to the total mass of powder as a binder, then stired further until the raw powders and binder were uniformly mixed. Secondly, the mixture was poured into molds for dry pressing at a pressure of 5 MPa for 2 min. Finally, the samples were demolded and subjected to oxidation and sintering with a rate of 1 ℃·min^-1 and a holding time of 2 h. The linear shrinkage were calculated by measuring the changes in diameter of samples before and after sintering. The volume density and apparent porosity of samples were determined using the Archimedes' displacement method, and relative density calculated as the measured volume density divided by the theoretical density of zinc oxide (5.6 g·cm^-3). Weight changes, as well as endothermic and exothermic effects during heating were measured using a thermal analyzer. The morphology of the samples was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray diffraction (XRD) was employed to analyze the phase composition of the samples. The three-point bending strength of the samples was tested using an electronic universal testing machine, with a span of 30 mm, a crosshead speed of 0.5 mm·min^-1, and sample dimensions of 3 mm × 4 mm, ensuring the length exceeded the span.

From TG-DTA analysis, it can be observed that the oxidation process of metallic zinc powder can be divided into three stages: a low-speed oxidation stage before 400 ℃, a high-speed oxidation stage from 400 ℃ to 900 ℃, and a complete oxidation stage after 900 ℃. However, when varied the particle size of metallic zinc powders, XRD results revealed that some unoxidized metallic zinc remained in the samples containing 1μm metallic zinc powder, being due to the relationship between the particle size of metallic zinc and zinc oxide; Regarding the shrinkage rate, when amount of added metallic zinc powder increased and the particle size enlarged, the reduction in the linear shrinkage of the porous ceramic during sintering process became notably apparent, suggesting that the shrinkage rate of samples were mainly dependent on the size of the metallic particles. When size of particle was 10 μm and addition amount equaled to 50%, the linear shrinkage rate of the sample was only -0.36%, with a porosity of 25%, achieving the near net-size preparation of the sample. As for the microscopic structure, it can be observed that distinct pore structures existed inside the material, and the pores size of was closely related to the particle size of the added metallic zinc powder. In TEM analysis, it can be clearly seen that particle size of metallic zinc affected the temperature at which metal zinc begins to hollow, and the radius of Kirkendall pores formed inside the material after the oxidation of metallic zinc was 0.54 to 0.99 times the initial radius of metallic zinc.

The in-situ hollow sphere method allowed for the preparation of near-net-size zinc oxide porous ceramics, offering a simple and efficient process without the addition for any organic pore-forming agents. When temperatures exceeded it's melting point, metallic zinc powder undergone vigorous oxidation, typically concluding around 900 ℃. The linear shrinkage of the sintered zinc oxide porous ceramics was closely related to the amount and particle size of added metallic zinc; higher amounts and larger particle sizes of added metallic zinc powders resulted in a lower linear shrinkage rate, potentially leading to negative shrinkage (expansion). After computational analysis, it was determined that the pores formed in the porous ceramics were smaller than the particle size of metallic zinc powder, consistent with the Kirkendall effect. The pore diameter was approximately 0.54 to 0.99 times the initial spherical metallic zinc particle size.