Abstract: Yttrium oxide （Y₂O₃） transparent ceramics have been widely studied for infrared windows，corrosion-resistant components，solid-state lasers，and scintillator hosts because of their broad optical transparency range，excellent chemical stability，and low thermal expansion coefficient. However，their widespread application is hindered by the high melting point of Y₂O₃，which necessitates energy-intensive sintering processes at temperatures often exceeding 1 800 ℃ to achieve full densification. Conventional approaches to lower the sintering temperature typically rely on either complex，time-consuming synthesis of ultra-high-purity，nano-sized powders or on expensive advanced sintering techniques like hot isostatic pressing （HIP） or spark plasma sintering （SPS），limiting their scalability and cost-effectiveness. In this study，we report a simple，scalable，and low-cost strategy to dramatically enhance the sinterability of commercial Y₂O₃ powder through an oxalic acid liquid-phase pretreatment，enabling its low-temperature densification via vacuum pressureless sintering，which is a method that is both industrially favored and economically viable.The method involves treating ball-milled commercial Y₂O₃ powder with a 1.2 mol/L oxalic acid solution，followed by filtration，washing，and a final calcination at 700 ℃. This treatment is shown to have a profound，multi-faceted activation effect. It not only efficiently removes surface-adsorbed impurities and carbonates，as confirmed by FT-IR analysis，but also refines the average particle size from 1.0~1.3 μm to approximately 300 nm. Crucially，the chelation between Y³⁺ and C₂O₄^2- leads to the in-situ formation of a yttrium oxalate precursor，which upon decomposition generates a substantial population of 30~50 nm Y₂O₃ nanoparticles. These nanoscale particles act as highly reactive sintering seeds，significantly boosting the overall sintering activity of the powder.The impact of this pretreatment on the sintering behavior and optical quality of Y₂O₃ ceramics was systematically investigated. For 2.0%（atomic fraction） ZrO₂-doped Y₂O₃，the oxalic acid treatment reduced the densification temperature by approximately 100 ℃. Near-full densification with a uniform，fine-grained microstructure was achieved at just 1 700 ℃ for 5 h under vacuum，a temperature at which untreated samples remained porous and opaque. This low-temperature densification is attributed to the combined effects of improved powder dispersion，and the presence of the nano-sized Y₂O₃ particles that enhance grain boundary diffusion. The superior sintering activity and microstructural uniformity directly translated into outstanding optical performance. Sintered at 1 750 ℃，the treated Y₂O₃ ceramics exhibit a maximum in-line transmittance of 75.8% at 600 nm，representing a significant advancement over untreated samples which showed no transmittance at this wavelength under identical conditions.In conclusion，this work introduces a highly effective powder activation strategy that bridges the gap between low-cost commercial precursors and the demanding requirements of high-performance transparent ceramics. By demonstrating that a simple oxalic acid wash can match the sintering efficacy of complex powder synthesis routes，this research provides a new paradigm for the low-temperature，cost-effective fabrication of high-melting-point oxide ceramics.

Key words: Y₂O₃ transparent ceramics; oxalic acid pretreatment; vacuum pressureless sintering; optical transmittance; microstructure; densification behavior