Synthesis And Characteristic Investigation Of Magnetoelectric Composites CoFe₂O₄-BaTiO₃

Magnetoelectric(ME) coupling effect is an induction of magnetic moment in an adscititious electric field or an induction of electric moment in an magnetic field, and the functional materials which have this properties can be called ME materials. Since the polarization intensity P of ME material is an induction effect in an adscititious magnetic intensity H, the conversion coefficientαcan be expressed as dP/dH, vice versa. Because those materials revealed high ME coefficient, and they are lead-free, easy to be synthesized, the CoFe₂O₄-BaTiO₃ ME composites have been a focus in ME research recently.In this paper, we focus on the preparation of CoFe₂O₄-BaTiO₃ ME composites with high coefficientα, using chemical methods to synthesize the nano-powder with two phases concurrently, and sintering those powders into ME ceramic samples by using different techniques. There are many advantages in those sintered ME ceramics, such as lower sintering temperature, higher density and finer grains in those samples, when they were sintered by using nano-particle powders, which may make great progress in their properties. A octahedron model of sharing a O atom layer in crystal lattice when CoFe₂O₄ growing up on BaTiO₃ was constructed, and the causation of high coefficient in ME nano-composites was explained clearly by using this model; a series of two-time sintering experiments were carried into execution to change the interface of two phases in ME composites, and the influences of the alterable interface on ME coupling effect were studied; a hypothesis of interactional model between electric domain and magnetic domain was assumed to explain the severe decreasing in those two-time sintering ME samples and other problems in preparation of other ME composites. The followings are the main content of this paper:1.The nano-powders of CoFe₂O₄-BaTiO₃ ME compound with different ratio(1 : 4, 1 : 2, 1 : 1) were synthesized by using the improving one-pot solution of EDTA-CA(citric acid) method, and the shape, size and symbiosis mechanism of those nano-particles was studied when they were treated under different temperatures. Those nano-powders were sintered at different conditions by using the common sintering method, and one of the ME ceramics which was sintered on 1180°C/2 hour has achieved the highest ME coefficient, 680μV/Oe·cm, in all of them. The research on them shows that the low content of CoFe₂O₄ phase and bad conbined interface between the CoFe₂O₄ phase and BaTiO₃ phase were the main factors to the low ME coefficient. Two series of composites with CoFe₂O₄ content of 20% and 45% (in mol) were prepared by using the Hot Iso-static Press sintering technique, then their grain sizes, theory density and interface between two phases were studied systemically. A sample of 0.45CoFe₂O₄-0.55BaTiO₃ composites has the biggest ME coefficient of them, that is 30.09 mV/Oe·cm, when it was sintered at 1050°C/110 MPa/60 min by HIP condition; the study shows it was attributed to the finer grain, higher content of CoFe₂O₄, bigger interface areas and increasing in density that the composite has achieved great improvement in ME coefficient.2.It was the first time that the CoFe₂O₄-BaTiO₃ ME nano-powders was synthesized successfully by using the molten salt method, as well as NaCl being medium and NP-10 (nonylphenyl ether) being active reagent. Those powders, with CoFe₂O₄ content being 20%, 35% and 50%, were detected by TEM and XRD after treating at different conditions, and the result shows that one of those powders has no other phase beside CoFe₂O₄ and BaTiO₃ and their particle size is under 100 nm when it was treated at 800°C. On the other hand, when the treated temperature changed, there were some impurity phases in those powders or their size would be bigger, which would bring some deleterious results to ME ceramics finally. We sintered all of the nano-powder into ceramics at 1180°C/120 min and tested the physical properties of them; when the content of CoFe₂O₄ was 35% and 50% in the composites, they have outstanding ferroelectric and ferromagnetic properties (such as piezoelectric coefficient d33 were 14 pC·N-1 and 17 pC·N-1 , respectively) as well as good physical properties (the density of them were 93.2%TD and 94.3%TD). A biggest value of all ME coefficient of them was 17.04 mV/Oe·cm, which was tested in the sample of 0.5CoFe₂O₄-0.5BaTiO₃ composite, and it can be attributed to the excellent all-around piezoelectric-piezomagnetic properties in this sample. The phases and micro-structure of those ceramics were detected by XRD and SEM, and the result shows there was not any impurity between the interface and the grain size was about 0.5μm, which is benefit to rising the contacted area of two phases. It was because of using nano-powders that the density and resistance of the ceramics were raised remarkably.3.The relations of the XRD peaks, SEAD of TEM and crystal lattice parameters between two phases(CoFe₂O₄, BaTiO₃) in powders were lucubrated, and we found there is a plane parallel to the (110) and (220) crystal lattice plane for CoFe₂O₄ and BaTiO₃, respectively. In this plane, the crystalize of two phases can grow up with sharing a O atom layer where the CoFe₂O₄ and BaTiO₃ were octahedron model on each side, and there was less defect in the contacted interface when the two phases combined as it. Thus the causation of high coefficient in ME composite by using the nano-particle can be clear according to this model. A series of two-time Hot-press sintering experiments were carried out in ME ceramic samples in order to develop the density and enhance the ME coupling between the two phases, but an unexpected result was discovered. A two-time sintered sample, for example, which was synthesized by the one-pot solution method and has the ME value of 680μV/Oe·cm, did not be with a higher coefficient but with a lower value as 42.7μV/Oe·cm; the similar phenomena was found in the samples prepared by molten salt method, that is to say the value of 17.04 mV/Oe·cm and 6.83 mV/Oe·cm for 50% CoFe₂O₄ and 35% CoFe₂O₄ samples have a great decrease to be 62.1μV/Oe·cm , 65.16μV/Oe·cm, respectively. Many factors were studied according to their influences on ME coupling, but the result shows that the disappear of the symbiotic interface between two phases was the most important one of them.4.Based on the essential principle of the ME coupling effect, the form mechanism of the magnetic domain and electric domain in CoFe₂O₄-BaTiO₃ ME composites were studied and summarized, then the process and mechanism of the distortion as well as the polarization in the domains were also analyzed when they were under outside force. The interactional model between different domains was assumed to be a hypothesis, which can explain not only the foundations for ME coupling effect, but also the change of coefficient value in those two-time sintering ME composites.