研究目的
To investigate the phase stability of CuFeO2 delafossites at high temperatures and varying oxygen partial pressures, focusing on their application in thermoelectric generators.
研究成果
The phase stability of CuFeO2 is highly dependent on oxygen partial pressure and temperature, with a phase transition to CuFe2O4 and CuO occurring at elevated oxygen concentrations. Theoretical Ellingham diagrams support experimental results, defining a stability window. CuFeO2 is not stable in ambient air above 700°C, limiting its use in thermoelectric generators to low-oxygen environments or requiring encapsulation. Future work could explore the bipolar phase for n-type thermoelectrics.
研究不足
The study is limited to CuFeO2 delafossites and specific temperature and oxygen partial pressure conditions. The oxygen diffusivity is low, requiring long annealing times. EDX analysis has deviations in oxygen measurement up to 3 mol%. The findings may not generalize to other materials or conditions without further investigation.
1：Experimental Design and Method Selection:
The study combined classical material characterization methods including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) to analyze phase composition. Theoretical calculations using the Ellingham diagram were also employed. Hot-stage XRD and long-term annealing tests were conducted for in-situ and stability analysis.
2：Sample Selection and Data Sources:
Delafossite powders were synthesized using a mixed-oxide technique with high-purity copper(I) oxide and iron(III) oxide. Samples were annealed at 900°C under different oxygen concentrations (0%, 1%, 5%, 10%, 20%, 100% in nitrogen) for 12 hours to reach equilibrium.
3：List of Experimental Equipment and Materials:
Equipment included a wet planetary ball mill (Fritsch), rotary evaporator (Heidolph Instruments), high-temperature furnace (STF/15 450, Carbolite-Gero), X-ray diffraction system (PANalytical), scanning electron microscope (LEO 1450 VP, Zeiss), and hot-stage XRD (D8 ADVANCE, Bruker with HTK 1200-N, Anton Paar). Materials included copper(I) oxide (99.9%, Alfa-Aesar), iron(III) oxide (99%, Alfa-Aesar), cyclohexane solvent, and nitrogen gas with varying oxygen concentrations.
4：9%, Alfa-Aesar), iron(III) oxide (99%, Alfa-Aesar), cyclohexane solvent, and nitrogen gas with varying oxygen concentrations.
Experimental Procedures and Operational Workflow:
4. Experimental Procedures and Operational Workflow: Powders were ball-milled, calcined at 1050°C in gas atmospheres, reground, sieved, and dried. Pellets were cold-pressed and annealed. SEM/EDX and XRD analyses were performed on samples. Hot-stage XRD was conducted from 20 to 900°C under nitrogen, with patterns recorded at discrete temperatures after equilibration. Long-term tests involved annealing for 96 hours in nitrogen.
5：Data Analysis Methods:
XRD patterns were compared to reference spectra (JCPDS). EDX provided quantitative element analysis. Theoretical calculations used Gibbs free energy equations to derive Ellingham diagrams for phase stability.
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