Journal of the American Ceramic Society最新文献

The abnormal expansion associated with the simultaneous synthesis and sintering of Zn₂SnO₄ from ZnO and SnO₂ has been investigated. This expansion was the effect of the synthesis reaction, as it was correlated with the degree of conversion. Two stages have been identified in the reaction. The first one has been modeled with the Avrami equation and the second one with the Dunwald equation, which corresponds to nucleation-and-growth and diffusion-controlled mechanisms, respectively. A hypothesis about the mechanism of the reaction and the origin of the expansion has been proposed. The abnormal expansion was reflected in the microstructure, porosity, and pore size distribution of the test specimens. Results showed that adjusting the thermal cycle, this phenomenon allows the manufacture of porous ceramics with the microstructure of a microfiltration membrane without the addition of pore generators.

Tm³⁺ and Yb³⁺ co-doped yttrium pyrogermanate (Y₂Ge₂O₇) phosphors were synthesized via the conventional solid-state reaction method. The structural properties, optical properties for upconversion luminescence, and optical thermometric properties were studied. Phase analysis using powder x-ray diffraction confirmed the formation of a single-phase tetragonal structure without any secondary phases. The scanning electron microscopy revealed irregular, agglomerated grains of micrometer size range. The diffuse reflectance spectroscopy showed characteristic absorption bands for absorption from the ground state ³H₆ of Tm³⁺ ions to excited states ³F_2,3 (684 nm) and ³H₄ (797 nm), as well as the ²F_7/2 → ²F_5/2 transition of Yb³⁺ ions (∼980 nm), confirming successful incorporation of dopant ions into the host lattice. Photoluminescence spectra under 355 nm excitation exhibited strong blue emission at ∼453 nm (¹D₂ → ³F₄), alongside weaker blue, red, and near infrared emissions at 475, 650, and 792 nm, respectively. The upconversion luminescence emissions under 980 nm excitation produced characteristic blue (∼475 nm; ¹G₄ → ³H₆) and near infrared (∼797 nm; ³H₄ → ³H₆) bands. Notably, the blue emission showed a stronger power-dependent enhancement compared to the near infrared emission, attributed to multiphoton energy transfer upconversion involving Yb³⁺ sensitizers. The maximum intensity was observed in the 2% Tm³⁺ doped sample co-doped with 5% Yb³⁺, confirming efficient Yb³⁺→Tm³⁺ energy transfer. Further, temperature-dependent upconversion luminescence studies demonstrated thermally modulated emission intensity, with calculated activation energies of 0.35 eV (blue emission) and 0.43 eV (near infrared emission). Optical thermometry based on the ratio of emission intensities for 797 and 475 nm exhibited excellent sensitivity, with absolute sensitivity (S_a) of 156.42 × 10⁻² K⁻¹ and relative sensitivity (S_r) of 13.4% K⁻¹ at 673 K. These results highlight Y₂Ge₂O₇:Tm³⁺/Yb³⁺ as a promising phosphor for blue upconversion-based photonic and high-sensitivity ratiometric thermometry applications.

Relaxor-based ferroelectric single crystals, such as Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT), are critical for advanced ultrasonic transducers due to their exceptional piezoelectric and electromechanical properties. To further enhance performance, domain engineering has emerged as a cost-effective and versatile strategy for tailoring material functionality. In this work, an ultrahigh-temperature field cooling poling (UFCP) method is applied to [001]-oriented PMN–PT single crystals. The approach combines DC–AC poling above the Curie temperature with a subsequent field cooling process, enabling controlled domain configuration and enhanced thickness-mode vibration performance. Compared with conventional direct current poling (DCP), UFCP crystals exhibit a 51% increase in piezoelectric coefficient $�d_{33}$ and a 23% improvement in the mechanical quality factor $�Q_m$. Thickness-mode ultrasonic transducers fabricated from these crystals further demonstrate a 7.2% enhancement in −6 dB bandwidth and a 2.3 dB reduction in insertion loss. These results establish UFCP as an effective domain engineering strategy that optimizes phase coexistence to achieve simultaneous improvements in material properties and device performance.

Field assisted sintering techniques (FASTs) are currently being presented as an attractive alternative to the energy and time intensive fabrication routes employed for UO₂ and MOx nuclear fuel manufacture. Current-rate controlled flash sintering (CR-FS) is a novel variant of typical flash sintering (FS) methods that permits heightened sample temperature control during field application, yet the technique has not been extensively trialled on oxide fuel materials, and the impact on resultant pellet properties is not fully understood. In this study, CR-FS has been employed to consolidate cylindrical UO₂ towards the requirements of existing fuel specifications, refining an optimised CR-FS process alongside. An average pellet density $�\sim$96.0 $�\pm$ 0.3% TD was observed, alongside an average grain size of 3.98 $�\pm$ 0.82 $��\mu �m�$ and a final O/M ratio of 2.033 $�\pm$ 0.002. Pellet heterogeneity in the radial direction has been observed and related to intrinsic field and thermal effects.