Abstract: Lead metaniobate (PbNb₂O₆, PN) piezoelectric ceramics are promising candidates for deep-well high-temperature ultrasonic transducers owing to their high Curie temperature (T_C = 570 ℃) and exceptionally low mechanical quality factor (Q_m~10). However, the significant decrease in electrical resistivity of conventionally sintered PN ceramics in high-temperature (> 200 ℃) silicone oil severely limits their application in deep petroleum exploration. This study aims to enhance the insulation performance of Pb_0.86Ba_0.14Nb₂O₆ (PBN) piezoelectric ceramics in high-temperature silicone oil by improving their relative density through oxygen-atmosphere hot-pressing sintering. PBN ceramic samples were fabricated via conventional sintering (CS) and hot-pressing sintering (HPS) with systematic investigation of sintering temperature (1230-1310 ℃), atmosphere (air/oxygen), and applied pressure (10-20 MPa). The microstructure and phase structure were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Rietveld refinement. The electrical resistivity was measured in silicone oil from 25 ℃ to 235 ℃, and the high-temperature stability was evaluated after 3-h immersion at 235 ℃. Hot-pressing sintering increased the relative density of PBN ceramics from 89.13% (CS) to 95.18% (HPS). Rietveld refinement revealed that the orthorhombic ferroelectric phase content decreased from 89.70% to 83.85% (in mass), while the trigonal non-ferroelectric phase increased correspondingly. The resistivity of HPS-PBN in silicone oil at 235 ℃ reached 3.1×10⁷ Ω·m, which is one to two orders of magnitude higher than that of CS-PBN. The piezoelectric voltage constant g₃₃ was calculated to be 30.97×10^-3 (V·m)/N (CS-PBN) and 26.99×10^-3 (V·m)/N (HPS-PBN), respectively. After high-temperature immersion testing, HPS-PBN exhibited smaller performance degradation compared to CS-PBN, demonstrating superior thermal stability. Although hot-pressing sintering reduced the piezoelectric coefficient d₃₃ from 85 pC/N to 65 pC/N due to decreased ferroelectric phase content, the elimination of the anomalous dielectric peak at 270 ℃ and the significant improvement in high-temperature insulation performance outweigh this drawback for deep-well logging applications. Oxygen-atmosphere hot-pressing sintering provides a practical and effective solution for the insulation failure of PBN piezoelectric ceramics in high-temperature silicone oil, demonstrating significant engineering application potential for deep-well logging environments above 230 ℃.

Key words: hot-pressing sintering, lead metaniobate, high temperature, relative density, electrical resistivity