Preparation And Characterization Of CaMnO₃

Perovskite materials, CaMnO₃, are fascinating both because they display a widevariety of fundamental properties, from magnetism to ferroelectricity, from colossalmagnetoresistance to half-metallicity, and because they are used in a number ofimportant technological applications, such as transducers and memories. They alsoshow electronic and structural peculiarities including orbital and charge ordering,formation of local moments, and Jahn-Teller distortions. Such a richness, combinedwith their relatively simple structure, makes them ideal materials for investigating thegeneral principles that govern these properties. Recently, CaMnO₃has been focusedon the TE properties including high thermopower α and relatively low total thermalconductivity κ above room temperature.It is well known that the different preparing methods for the same material canreduce the difference of its properties. In this work, CaMnO₃perovskite samples weresynthesized by three different methods. The structure and morphology of sampleswere characterized by X-ray diffraction （XRD）, Fourier transform infrared （FT-IR）,Scanning electron microscopic （SEM） and transmission electron microscopic （TEM）.Furthermore, the magnetic, electrical and thermal properties of CaMnO₃were alsocharacterized.The single phase CaMnO₃has been successfully obtained by conventionalsolid-state reaction and Sol-gel method.The relative density of CaMnO₃prepared bythe ceramic method was only87%. In addition, the electrical resistivity and thethermal conductivity measured at room temperature （RT） are2.1×10^-1 cm and3.5×10³Wm^-1K^-1, respectively. The purity, compactness and the crystallinity ofCaMnO₃prepared by Sol-gel method are improved by the increase of temperature andthe holding time. The resistivity of CaMnO₃at RT decreases with the increase ofsintering time. The thermal conductivity measured at RT was4.5×10³Wm^-1K^-1whichwas higher than the CaMnO₃prepared by the ceramic method. This result indicatesthat CaMnO₃prepared by Sol-gel method may have more excellent physicalproperties. Furthermore, the thermal conductivities of CaMnO₃decrease with theincrease of temperature. The electrical resistivities of CaMnO₃prepared by sol-gel method arecharacterized in situ at high pressure （atm-25GPa）and decrease with the increase ofpressure. This result indicates that the structural and electronic phase transitions werenot occurred within these pressures.The pure phase of CaMnO₃has been successfully prepared by high pressure andhigh temperature （HPHT） method with Sol-gel precursors for the first time. The grainmorphology of CaMnO₃is different from that prepared by pressureless sintering withthe same precursors. With the increase of sintering time, the crystalline size and therelative density of CaMnO₃increase and the maximum relative density values is95.44%. While the resistivity decrease （the minimum value of3.8·cm） and thethermal conductivity measured at RT increases and up to a maximum value of6.06×10³Wm^-1K^-1. The enhanced thermal conductivity for CaMnO₃may be attributedby the larger of the particle size and the increase of density with the increase ofsintering time. Similar to CaMnO₃prepared by Sol-gel method, the thermalconductivities decrease with the increase of temperature. Based on the above results,CaMnO₃prepared by the HPHT method may have excellent physical properties thanthe samples prepared by the Sol-gel method and Solid-state reaction.The hysteresis curves of CaMnO₃obtained at10K was shown that a coercivity,2502Oe, and the remanent magnetism,0.98emu/g, are corresponding to CaMnO₃prepared at HPHT. While a coercivity,9672Oe, and the remanent magnetism,1.47emu/g, are corresponding to CaMnO₃prepared by Sol-gel method.These resultsindicate that the magnetic properties of CaMnO₃are restrained by the high-pressureeffect.In addition, CaMn₂O₄with spinel structure has been successfully prepared byHPHT method. The magnetic susceptibility χ is measured for CaMn₂O₄at an appliedfield of2T Oe and the TN=224K is obtained. This value is in good agreement withthose reported in literature. This result indicates that the magnetic property ofCaMn₂O₄has not been changed by the high-pressure effect.