Muralidhar Chourashiya

The Ph.D. thesis, titled "Studies on synthesis and characterizations of gadolinium doped ceria solid electrolyte," investigates the development of advanced materials for clean energy conversion, specifically focusing on Solid Oxide Fuel Cells (SOFCs). The central goal of the work is to address the high operating temperatures of traditional SOFCs—typically around 1000°C—which cause material degradation, thermal mismatch between components, and high fabrication costs.

To overcome these obstacles, the research explores lowering the operating temperature to an intermediate range of 500–800°C. This is achieved by utilizing Gadolinium Doped Ceria (GDC) as a solid electrolyte, a material that exhibits significantly higher ionic conductivity than standard yttria-stabilized zirconia (YSZ) at lower temperatures.

Fundamental Principles and Defect Chemistry

The study is grounded in the defect chemistry of the fluorite crystal structure. Ceria CeO₂ naturally possesses an open fluorite structure that is stable from room temperature to its melting point. By substituting Ce⁴⁺ host ions with Gd³⁺ dopant ions, the material minimizes internal lattice strain while introducing oxygen vacancies to maintain charge neutrality.

These vacancies act as the primary vehicles for electrical conduction, allowing oxygen ions to migrate through the lattice via a thermally activated hopping mechanism. The thesis details how the ionic conductivity peaks at approximately 10% gadolinium doping, identifying GDC10 as the ideal candidate for intermediate-temperature applications.

Experimental Synthesis and Characterization

The experimental portion of the thesis is divided into two major phases:

1. Bulk Analysis: Initially, GDC was synthesized in bulk form using a cost-effective solid-state reaction method. This allowed for the systematic optimization of processing parameters—such as sintering time and temperature—to understand their influence on the structural and electrical properties of the final ceramic.
2. Thin Film Fabrication: To further reduce ohmic losses and enhance performance, the research shifted to creating thin-film electrolytes with a thickness of 10–20µm. These films were produced using the spray pyrolysis technique (SPT), a simple and scalable chemical deposition method.

The thin films were first optimized on glass substrates to understand growth mechanisms before being deposited onto porous NiO-GDC ceramic anode substrates to form a functional electrode-electrolyte interface. The thesis concludes by testing these optimized GDC structures through Open Circuit Voltage (OCV) measurements, confirming their stability and efficiency in simulated fuel cell environments.

The M.Sc. dissertation, titled "Low Temperature Deposition of Tin Oxide Thin Films by SILAR Method and to Study the Effect of Indium Doping on its Electrical and Optical Properties," explores cost-effective methodologies for fabricating transparent conducting oxide (TCO) films. The central motivation of the research is to develop an inexpensive and low-temperature technique for producing thin films that have significant industrial applications, particularly as gas sensors, solar cell electrodes, and transparent heat mirrors.

To achieve this, the project utilizes the Successive Ionic Layer Adsorption and Reaction (SILAR), also referred to as the Modified Chemical Bath Deposition (M-CBD) method, to grow tin oxide SnO₂ thin films on glass substrates at room temperature. This process acts as the core synthesis technique, relying on the sequential adsorption and reaction of ions at the solid-liquid interface. The films were synthesized using a precursor solution of stannous chloride complexed with EDTA, followed by rinsing in de-ionized water containing hydrogen peroxide to provide the necessary oxygen ions and prevent homogeneous precipitation. Once the baseline parameters for pure SnO₂ deposition were established, the method was advanced by introducing indium sulfate to the precursor, allowing for the fabrication of indium-doped tin oxide films.

The scientific foundation of the work is rooted in solid-state physics and the surface phenomena of two-dimensional structures. The dissertation details the chemical mechanics of the deposition, explaining how SnO₂ cations adsorb onto the substrate and react with SnO₂ anions to form mono-layers of tin oxide. By applying semiconductor physics, the research evaluates how doping with indium modulates the material's properties. Pure SnO₂ is an n-type semiconductor with high resistivity due to its wide band gap, but introducing precise amounts of indium reduces this resistivity by altering the free carrier concentration and structural defects.

Throughout the study, rigorous analytical technologies were employed to ensure the structural integrity and performance of the synthesized materials. Gravimetric weight difference analysis was used to measure film thickness, determining that a maximum thickness of 0.38 microns was achieved after 70 immersion cycles. X-ray Diffraction (XRD) confirmed the formation of a pure, tetragonal SnO₂ crystal structure. Surface morphology and grain size—averaging around 100 nm—were examined using Scanning Electron Microscopy (SEM), ensuring uniform nano-scale deposition. Furthermore, optical absorption data revealed a direct transition optical band gap of 2.8 eV, while two-probe resistivity measurements quantified the electrical improvements brought on by the doping process.

Ultimately, the research successfully bridges fundamental materials science with practical industrial needs by demonstrating that increasing the indium doping percentage up to 40% significantly lowers the electrical resistivity while simultaneously increasing the optical transmittance of the films in the visible spectrum. This work presents a scalable, economically viable pathway for manufacturing high-quality TCO films for advanced optoelectronic and gas-sensing applications.

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