【作者】 聂军武；

【导师】 徐国跃；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2009， 博士

【摘要】 磁电耦合效应是由外加电场或外加磁场感应产生的电场感生磁矩效应或磁场感生电矩效应,具有这种磁电耦合效应的铁电磁功能材料被简称为磁电材料。由于磁电材料是在外加磁场强度H的作用下产生电极化强度P或在外加电场作用下产生诱导磁化,因此可以用转换系数α=dP/dH或dH/dP表征磁电效应的大小,由磁场能感生电场能的转换系数αE即磁电耦合系数。由钴铁氧-钛酸钡两相复合而成的磁电材料（简称磁电复合材料）以其具有较大的磁电耦合系数和无铅、容易合成等特点而受到广泛的研究和关注。本文以制备出高性能的CoFe₂O₄-BaTiO₃磁电复合材料为目标,采用化学方法一次性合成CoFe₂O₄和BaTiO₃两相共存的纳米复合粉体,然后采用不同陶瓷烧结技术对粉体进行烧结,分别实现该体系的铁磁相—铁电相的复合。针对以纳米复合粉体为原料制备磁电复合材料的特点,构建了粉体中两相形成共格界面、依附生长的八面体模型,运用此模型解释了由纳米粉体烧制得到高磁电耦合性能的原因;首次设计了块体二次烧结实验方案,通过改变块体微界面研究其对磁电耦合效应的影响;针对二次烧结块体样的磁电耦合系数下降等问题,提出了适合于混相磁电复合材料的电畴与磁畴作用距离模型,并以此对制备磁电陶瓷中的各种问题作出合理的解释。具体内容包括:1.通过改进了的EDTA-柠檬酸溶液络合法合成CoFe₂O₄-BaTiO₃两相不同配比（1 : 4, 1 : 2, 1 : 1）的纳米复合粉体,研究不同温度下处理的粉体其形貌、粒径和两相界面共生机制等问题,应用改进的普通烧结方法,在1180°C/2h条件下对900°C粉体进行烧结,制得了磁电耦合系数最高值为680μV/Oe·cm的磁电复合材料,分析表明是该中铁磁相含量低、晶粒界面结合不够紧密等因素影响了磁电耦合效应;采用陶瓷热等静压技术烧结了CoFe₂O₄摩尔含量为20%和45%的磁电样品,在1050°C/110 MPa/60 min条件下烧制的0.45CoFe₂O₄-0.55BaTiO₃磁电陶瓷样品具有最大的磁电耦合系数值,达30.09 mV/Oe·cm,研究表明细化晶粒、提高铁磁相含量、增大两相接触面积和提升烧结密度有利于高磁电耦合效应的产生。2.首次采用以单相NaCl为熔融盐介质,以NP-10 （nonylphenyl ether）为表面活性剂的熔融盐法一次性合成出了CoFe₂O₄-BaTiO₃纳米复合粉体,经TEM、XRD等分析手段检测表明于800°C反应合成的粉体中无其它杂相,粉体粒径在100 nm以下,而提高或者降低反应温度都将影响粉体粒径和粉体纯度;用普通陶瓷烧结方法在1180°C/120 min的条件下烧制了CoFe₂O₄摩尔含量约为20%、35%和50%的磁电复合材料样品,后两种样品都获得很好的物理性能（密度达到93.2%TD和94.3%TD）和铁电、铁磁综合性能（如压电系数分别达到14 pC·N-1和17 pC·N-1）,并在50%CoFe₂O₄的样品中获得最高的磁电耦合系数,其值为17.04 mV/Oe·cm;以XRD、SEM等分析手段检测了块体材料的组成和微结构,分析表明两相之间结合紧密且无杂相存在于晶界,晶粒大小控制在0.5μm左右;以熔融盐法合成出了更细小的纳米粉粒,为烧结后的块体样提供了更大的相接触面积,块体烧结致密度也增大,同时获得优异的铁电、铁磁综合性能。3.利用粉体XRD表征、透射电镜衍射花样（SEAD）和计算出的两相晶格参数之间的关系,分析得到两相晶胞在平行于（110）和（220）晶面的平面内可以以晶格匹配的方式共格生长,由此构建了粉体合成时两相共格生长的八面体结构模式;当粉体中两相以这种共格的方式长大时,相与相之间能紧密结合,减少位错等缺陷,因此以全纳米粉体为原料制备的磁电复合材料可以获得更高的磁电耦合效应;以提高块体样品致密度、提升磁电耦合效应为目标,设计了块体二次热压烧结实验方案,实验后发现,溶液法合成的块体样最大磁电耦合系数是680μV/Oe·cm,二次烧结后其系数值降为42.7μV/Oe·cm,对其多项性能进行对比分析,结果表明两相间共格界面遭受最大程度改变是导致耦合系数下降的最直接原因;以熔融盐法制备的块体样（50% CoFe₂O₄和35% CoFe₂O₄）磁电耦合系数可以达到17.04 mV/Oe·cm和6.83 mV/Oe·cm,经过二次烧结后样品的耦合系数降为62.1μV/Oe·cm和65.16μV/Oe·cm,研究分析也表明影响它们的最关键因素是共格界面的破坏。4.从磁电耦合作用最基本的工作原理出发,分析和总结了CoFe₂O₄-BaTiO₃磁电复合材料中磁畴和电畴的形成机制,以及两种畴在各自受外力作用时形变、极化等现象产生的过程和机理;首次提出了畴间作用距离影响磁电耦合效应的模型,不但充分地验证了混相磁电复合材料中耦合效应产生的条件,而且对相界面的变化影响磁电耦合系数大小也作出了圆满的解释。更多还原

【Abstract】 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.更多还原