Raghavendra Karkala Gururaj

Supercritical fluid processing has developed into a versatile and environmentally responsible approach for the controlled fabrication of nanomaterials, distinguished by low density, viscosity, high diffusivity, and negligible surface tension. These intrinsic features enhance nucleation, efficient mass transport, and accurate control over particle dimensions, morphology, and compositional uniformity. This review comprehensively examines the fundamental principles of supercritical fluids, encompassing phase behavior, thermophysical characteristics, and the mechanisms governing nanoparticle formation, along with important engineering aspects related to both batch and continuous‐flow reactor configurations. The preparation of metal, metal oxide, and composite nanostructures through supercritical techniques is critically evaluated, emphasizing their benefits compared with traditional solid‐state and solution‐based synthesis routes. Special attention is given to the evolution toward high‐entropy oxides, where supercritical conditions enable homogeneous multication distribution, enhanced phase development, and defect modulation under kinetically advantageous environments. By establishing clear relationships between processing conditions, structural characteristics, and electrochemical performance, this review highlights the promise of SCF‐enabled methodologies as scalable and sustainable pathways for designing advanced multicomponent oxide materials suited for energy storage and catalytic applications.

Developing materials with high third‐order nonlinear susceptibility and low optical limiting thresholds remains a challenge in photonic device engineering. Bimetallic nanoparticles offer a compelling route to enhanced nonlinear optical (NLO) performance by coupling plasmonic field amplification with interfacial electronic structure. Herein, we report the synthesis of AgCo bimetallic nanoparticles with a surface‐confined Co ₃ O ₄ layer and a systematic investigation of their third‐order NLO response under both continuous‐wave (CW) and nanosecond pulsed excitation. The system exhibits strong reverse saturable absorption, with β _eff reaching 7.5 × 10 ⁻³ cm/W at 638 nm under CW excitation, β = 0.054 m/GW, and a third‐order susceptibility χ ⁽³⁾ = 4.12 × 10 ⁻¹¹ esu at 532 nm under nanosecond excitation, which is higher than that of many reported molecular and nanostructured systems. Crucially, the intensity‐dependent nonlinear absorption coefficient confirms a sequential excited‐state absorption mechanism rather than two‐photon absorption, driven by hot‐carrier injection across the Ag‐Co ₃ O ₄ Schottky junction into mixed‐valence intermediate states. This distinguishes the system from conventional monometallic optical limiters.

High‐entropy oxides (HEOs) are a promising class of novel ceramic materials characterized by their complex chemistry and multifunctionality. By leveraging high configurational entropy, HEOs stabilize single‐phase structures with significant lattice distortion and tuneable electronic properties through the incorporation of multiple metal cations. This versatility makes HEOs particularly relevant for photocatalytic applications, such as dye degradation in wastewater treatment. This review explores the advancements in HEO catalysts for photocatalysis, detailing their evolution from alloys to ceramics and the thermodynamic principles that influence phase stabilization. Key distinctions between HEOs and traditional oxides are highlighted, along with the significant capabilities of HEO photocatalysts to address challenges posed by conventional systems. The article explores the crystal structure, synthesis methods, and material design for enhanced charge separation and light absorption. It compares the performance of various HEO photocatalysts and discusses their advantages over single or binary‐oxide alternatives. Finally, the review identifies critical knowledge gaps, such as the quantitative effects of entropy, and suggests future directions involving operando characterization, first‐principles modeling, and machine learning for the rapid development of next‐generation HEO materials.

The structural response of ceramics to extreme deformation is of significant scientific and technological relevance since such conditions are commonly encountered during both processing and service. In this study, monoclinic zirconia was subjected to high-energy ball milling for extended durations of 80 h and 120 h, followed by annealing at 1000 °C. X-ray diffraction revealed a progressive increase in the tetragonal phase content with milling duration, while subsequent annealing promoted its consolidation alongside the principal monoclinic phase, resulting in a stable biphasic structure. The phase evolution is also evaluated through a Raman spectroscopy analysis and correlated with the morphology, mechanical properties, and surface area analyses. Scanning electron microscopy confirmed the preservation of nanoscale features in the milled and annealed specimens, in contrast to the unmilled sample, which exhibited pronounced grain coarsening. The combined presence of nanostructural stability and biphasic phase constitution underscores the efficacy of high-energy ball milling, in conjunction with thermal treatment, as an effective strategy to tailor the microstructure and phase stability of zirconia ceramics for advanced engineering applications.

Zirconium dioxide (ZrO ₂ ) is widely recognized for its outstanding mechanical, thermal, and optical properties, making it a material of choice for advanced applications in catalysis, coatings, and other structural applications. In this study, monoclinic ZrO ₂ is ball milled for various milling durations up to 40 h, and changes in the microstructure and phase are identified. A dominant monoclinic phase is observed along with a minor tetragonal phase when milled beyond 10 h, with corresponding crystallite size reduction and enhanced surface area with increased milling duration. The examined photoluminescence spectra revealed a noticeable reduction in the peak intensity, which is attributed to a drop in the charge recombination. The deconvolution of photoluminescence spectra indicates the suitability of 20 h sample for enhanced catalysis. In line with this, the catalytic reduction-assisted degradation of rhodamine B (RhB) demonstrated a substantial increase in the catalytic activity for the 20 h sample, with about 97.11% degradation in 40 min, verifying the suitability of ball milled ZrO ₂ as an efficient catalyst for wastewater treatment.

Rare Earth Zirconates with pyrochlore/flourite structure have attracted interests owing to their immense potential in modern technological applications. Among these, Lanthanum Zirconates are a promising top coat thermal barrier material and also a candidate nuclear waste host matrix for radionuclide immobilization. Despite its candidacy for these high-tech applications, the fundamental questions related to process—structure—property correlation still lacks a thorough understanding and this article is an attempt to bridge this gap. The current study investigates the microstructural characteristics of Lanthanum Zirconate powders synthesized using co-precipitation method under various pH, annealing temperature and hold durations. An evolution from disordered to ordered pyrochlore structure is observed at a relatively lower annealing temperature of 950 °C, as investigated through x-ray diffraction and Raman spectroscopy, confirmed via synchrotron investigations. Further examinations of the samples synthesized at varied pH conditions indicated a relatively high surface area and stable microstructure with a lower propensity for grain growth for the samples synthesized at pH 11. The comprehensive material microstructural information as a function of process conditions studied here is thought to be crucial for microstructure tailoring in both pristine and doped Lanthanum Zirconates.

A potential but an underexplored application of FIB_SEM is its ability to image the subsurface microstructure and capability for an associated chemical analysis. In this article, agglomerated microstructures of two model oxide dispersion strengthened alloy systems, consisting of ZrO₂ and Y₂O₃ dispersions, are evaluated to understand its elevated temperature behaviors. The systems under evaluation are relevant as candidate nuclear structural materials for next-generation fast breeder reactors, and FIB_SEM technique is effectively used along with TEM and EDS for a comprehensive understanding of the material microstructure, and the results are discussed. Distinct microstructures are observed for the two systems with a difference in crystallite size distribution and presence of micron-sized dispersoids in Y₂O₃. The varied behavior of dispersoids is discussed in terms of its pre-annealing microstructures, and the superiority of ZrO₂ over Y₂O₃ as a dispersoid for ODS alloys is emphasized.