Several Ferroic Materials: Synthesis, Characterization And Their Properties

The information age results in colossal amounts of data to store and process.Traditional planar longitudinal recording has reached its limit, thus arises theperpendicular recording. Fabrication of large density magnetic nanowire array withhigh coercivity and squareness ratio is a premise. We adopt a new high-voltage pulsedelectrodeposition method to manufacture and study two kinds of CoFexarrays.Countless wireless communication network and radar would impel the research onmicrowave absorbents, especially for military stealth purpose. We are witnessing theflourishment of study on ferroic materials again. Low reflection loss requirescombination of magnetic and dielectric properties and the usual method is tocompound two kinds of material. Instead, we tune the dielectric and microwaveabsorption property of single magnetic material by changing the stoichiometry. Also,we found that for special morphology ferroelectric (ferroelastic) material, dielectricresonance occurs with surface acoustic wave propagating, which can be discernedfrom the dispersion spectra of complex permittivity in microwave frequency range.The electromagnetic wave energy can be transferred into mechanical energy anddissipated heat ultimately. The potential application is electromagnetic interferenceshielding and fixed frequency notch filter.1. A high voltage pulsed electrochemical deposition (50Hz,-50V) techniquewas used to synthesize CoFex(x=1.57,2.02) nanowire arrays in anodic aluminumoxide templates. The nanowire arrays (NA) are different in diameter (NA A,40nm,NA B,50nm), center-to-center distance (NA A,60nm; NA B,100nm) and length(NA A,4μm; NA B,8μm). Microstructural characterization shows high crystallinityof the nanowires. The magnetic measurements indicate that the nanowire arrayspossess uniaxial anisotropy with the easy magnetization axis along the nanowire.When magnetic field is applied in this direction, the coercivities of both nanowirearrays are larger than1600Oe. The shape difference between two magnetizationhysteresis loops is due to dipolar magnetostatic interaction, and analytical calculationis performed to interpret the magnetic properties as a function of the wire geometry.The results suggest that during high voltage electrodeposition, reduced atoms arehighly energetic and the crystallographic growth planes can be (110),(111),(211)and(100).2.CoxFe3-xO4(x=0-1) spheres are synthesized via solvothermal reaction usingethylene glycol (EG) as solvent. They are characterized and the results show that the prepared spheres are mainly300-500nm in diameter and constituted by small grains.For the EG solution containing stoichiometric ingredients (atomic ratio ofCo2+:Fe3+=1:2), the obtained spheres are Co0.9Fe2.1O4at200oC (sphere A) andCo0.74Fe2.26O4(sphere B) at300oC, whose crystallite is23nm and30nm in sizerespectively. VSM measurements reveal improved properties with sphere B. Thevariation of complex permittivity and permeability for different composite (75%massratio of spheres) has been studied as a function of frequency. The calculatedreflectivity indicates that the composite containing sphere A displays better microwaveabsorption capability. The minimum reflection loss reaches-41.1dB at12.08GHz andthe matching thickness is2mm. Dielectric loss contributes even more than magneticloss in the frequency range of3-14GHz. The synergistic effect of dual losses makesthe submicrosphere a promising absorbent in X and Ku bands. The compositeconsisting of spheres B is inferior in dielectric property owing to the ferrous ionsmigration from octahedral to tetrahedral sites and big crystallites lacking defects. Aftercalcination treatment of the spheres at700oC, the dielectric loss turns out to be littledue to the disappeared Fe2+Fe3+pairs in adjacent octahedral sites and loss of defects.Variation of cobalt ratio in spheres can change the resonant frequency and crystallinityof the spheres, and ultimately the minimum reflection loss and correspondingfrequency band. The microwave absorption properties of mixed magnetite and cobaltferrite spheres are influenced by the cationic stoichiometry and crystalline integrity.3.Barium titanate (BTO) with different morphology is prepared throughhydrothermal method using titania spheres as precursor, then calcined at differenttemperatures and ultimately coated with cobalt ferrite (BTO/CFO). The dielectricdispersion of the composite containing BTO (75wt.%ratio in paraffin wax) showsevidence of resonant behaviour in the microwave spectrum, rather than the usuallyobserved relaxation mode. The imaginary part of permittivity (”) displays a strongpeak in the10-13GHz frequency region, especially for buckhorn-like BTO(hydrothermally synthesized at110oC and calcined at1100oC). The dielectric responseanomaly of BTO in special morphology is due to the emission of plane acoustic wavescaused by electrostrictive or converse piezoelectric effects. An accepted model isadopted to simulate the resonant frequency. The minimum reflection loss ofcauliflower-like BTO (hydrothermally synthesized at110oC, then calcined at600oCfor2h,75wt.%ratio) in paraffin wax reaches-30.8dB at10.56GHz with a matchingthickness of2mm, lower than all the reported values. When the sintering temperature is changed to1100oC (buckhorn-like BTO), the minimum reflection loss value is-24.4dB at12.56GHz under the thickness of3mm. After combination with CFO, thevalue of the composite reaches-42.7dB when the thickness is5.6mm. The ginger-likeBTO (hydrothermally synthesized at200oC and calcined at different temperatures) isinferior in microwave reflection reduction. The electromagnetic interference shieldingeffectiveness of buckhorn-like BTO composite is calculated to be-12.7dB (94.6%shielding) at resonance frequency11.52GHz with2mm thickness. This work clearlyshows the potential to tune the dielectric property of ferroelectrics through control ofmorphology, facilitating new comprehension of the ferroelectrics in microwaveregime.