FAN Wugang ¹, ZHOU Xiang ², ZHAO Guannan ², LI Ling ², CAO Xiong ², ZHANG Zhaoquan ¹

(1. Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China;

2. Shanghai Nuclear Engineering Research and Design Institute Co., Ltd., Shanghai 200233, China)

Extended Abstract:[Background and purpose] 8 mol% Y₂O₃ doped ZrO₂ (8YSZ) has cubic crystal structure and is also known as fully stabilized zirconia (FSZ). 8YSZ is widely used in applications, such as solid oxide fuel cells (SOFC), thermal barrier coatings (TBCs) and gas sensors, due to its superior oxygen ionic conductivity, low thermal conductivity and excellent chemical stability. In these applications, 8YSZ typically exists as thin films or porous structures. With the ongoing trend toward reactor miniaturization and modularization, there is an increasing demand for integrated pressurizers within the reactor pressure vessel to reduce overall reactor size. Conventional pressurizers rely on thermal insulation designs that include porous materials such as glass fiber and mineral wool, cavities to reduce thermal convection and metal foils to minimize radiative heat transfer. It was recently demonstrated that dense 8YSZ ceramics offer excellent resistance to subcritical water corrosion, potentially preventing failure of thermal insulation caused by coolant infiltration into cavities and pores during accidental events. Furthermore, 8YSZ ceramics can reduce strain on the metallic shell, enhance insulation stability and improve the safety margins of mechanical and structural designs due to their high compression resistance and modulus of elasticity. However, the high-temperature stability, thermal aging and some mechanical properties of 8YSZ ceramics at pressurizer operating temperatures (~350 ℃) have not been well understood.[Methods] In this study, commercial 8YSZ powder (D₅₀=200 nm) prepared by using the co-precipitation method was calcined in at 1200 ℃ for 2 h and then sand-grinding was carried out for refinement after adding a certain amount of deionized water. The milled slurry was mixed with polyvinyl alcohol (PVA) binder, which accounted for 0.8% of the 8YSZ mass ratio. Then, centrifugal spray drying was carried out after mechanical stirring and mixing. After the granulated powder was sieved through 40 mesh sieves, it was dry pressed at 50 MPa and then cold isostatic pressed at 200 MPa. The green bodies were sintered at 1500 ℃ for 2 h, with heating and cooling rates of 2 ℃·min⁻¹. Density of the samples was measured according to Archimedes' principle. Phase compositions were analyzed by using X-ray diffractometer (D8, Bruker) with Cu target. Surface and cross-section microstructures were analyzed using a scanning electron microscope (SEM, EM6900, KYKY). Thermal diffusion coefficient and specific heat were measured by using pulsed laser thermal conductivity meter (DLF-2800, TA) and high temperature calorimetry (LFA467, Netzsch), respectively. The steady state thermal conductivity was tested by using protective heat flow meter (DTC 300, TA). The coefficient of thermal expansion (75VS, LISEIS) was characterized by push-rod technique. Mass change and caloric effect of the green body and 8YSZ ceramics in air were tested by using TG-DTA (STA449C, Netzsch). Three-point flexural strength was tested using a universal material testing machine (Instron-5566, Canton) and average value was taken with 20 samples. Weibull's modulus was calculated using the three-point bending strength results. Elasticity modulus was tested by using the bending method, with a loading rate of 0.1 mm∙min⁻¹. Compressive strength was tested by using a universal materials testing machine at room temperature (Instron-5566, Canton) and 350 ℃ (CMT-5305, SHSAS). A Vickers hardness tester (Wilson-Wolpert Tukon 2100B) was used to test hardness (HV) of the samples and calculate the fracture toughness (K_IC) with an indenter load of 1 kg.[Results] XRD results confirmed the cubic phase of the sintered 8YSZ ceramics, with only a minor monoclinic phase observed in the raw powder. The TG-DTA curve indicated an exothermic peak at 381 ℃, with a total weight loss of about 3.6%. SEM images revealed that the sintered 8YSZ ceramics exhibited grain sizes of 3–6 μm, with residual pores (~300 nm) at grain boundaries. Thermal conductivity had a temperature-independent value of about 1.9 W·m^⁻1·K^⁻1, from room temperature to 500 ℃, while the heat flow meter method provided a slightly lower value. The coefficient of thermal expansion at 360 ℃ was 9.3×10⁻6 K^⁻1, which is significantly lower than that of 316L stainless steel, making it a suitable candidate for thermal insulation applications in pressurizers. Thermal aging test results at 360 ℃, 450 ℃, and 550 ℃ for up to 1000 h confirmed phase stability of the cubic 8YSZ ceramics. Thermal shock test results, involving rapid cooling from 800 °C to room temperature in deionized water, showed minimal structural damage, indicating good thermal shock resistance. Mechanical test results revealed a compressive strength of 1.2 GPa at 350 ℃, twice that at room temperature, along with a Poisson's ratio of 0.285 and an elastic modulus of 164.9 GPa. Vickers hardness and fracture toughness were consistent with values reported in the literature. The bending strength reached 212 MPa and the Weibull modulus was calculated to be 16.03, indicating high reliability and quality.[Conclusions] To meet the demands for thermal insulation materials with inherent safety for in-vessel pressurizer of pressurized water reactors, 8YSZ ceramics with a relative density greater than 96% are fabricated through calcining commercial powder and sand milling treatment. Thermal physical properties of the 8YSZ ceramics are systematically examined, including thermal conductivity, thermal expansion coefficient, resistance to thermal aging, high-temperature stability and resistance to thermal shock, as well as key mechanical properties, such as compressive strength, flexural strength, hardness, fracture toughness, elastic modulus and Poisson's ratio. These experimental results provide a solid base for the application of 8YSZ ceramics as thermal insulation in in-vessel pressurizer.

Key words: 8YSZ ceramic; thermophysical properties; mechanical properties; pressurizer; thermal insulation materials