【作者】 刘俊杰；

【导师】 李峰；

【作者基本信息】 北京化工大学 ， 应用化学， 2003， 硕士

【摘要】 尖晶石铁氧体是一种典型的软磁材料,由于其独特的结构及性能,在许多方面得到了广泛的应用,其可作为颜料、脱氢催化剂、传感元件、光电材料和磁敏光记录材料等。目前制备尖晶石的方法主要有传统干法和湿化学法(共沉淀法、溶胶凝胶法和微乳剂胶团法)。传统干法制备是通过焙烧各种金属的氧化物、氢氧化物或其它沉淀混合物后得到的,但由于反应物的活性、混合均匀度和细度不高,因此生产工艺存在反应物活性较差和反应不易完全的缺陷,最终影响到铁氧体的磁性能。针对这种状况,人们积极地进行了制备工艺的改进,如文献C. Jovalekic, M.Zdujic, A. Radakovic, and M.Mitic. Mater.Lett. 24,365(1995)和文献 J.Ding, H.Yang, W.F.Miao,P.G.McCormick, and R.Street. J.Alloys Compd. 221,959(1995) 通过高能碾磨制备出了阳离子分散性不同于传统法的纳米尖晶石铁氧体,但是由于仍然采用混合物作为焙烧前体原料,焙烧前体的混合均匀度和细度无法从微观上进行很大程度调整,也很难再进一步提高铁氧体的磁性能;而在湿化学制备过程中,由于引入的沉淀剂和表面活性剂不易被除去,从而使得产物的纯度及性能受到一定的影响。层状双金属氢氧化物是一种层状结构的功能材料,典型的是含碳酸根的镁铝水滑石[Mg6Al2(OH)16CO3·4H2O](LDH) 。当LDH位于层板上的Mg2+,Al3+及层间CO32-被其他相应的二三价离子及阴离子取代或交换后,可以得到组成不同的类水滑石化合物(LDHs)。在高温焙烧条件下,LDHs会向尖晶石铁氧体转化;且在LDHs晶体结构中,由于受晶格能最低效应及其晶格定位效应的影响,金属离子在层板上以一定方式均匀分布,即在层板上每一个微小的结构单元中,其化学组成不变。正是由于其结构上的这种特点,使其焙烧后能够得到成分均匀、结构均匀的尖晶石铁氧体,从而使得此磁性产物中的磁畴结构单一,可大幅度提高其磁学性能,这是传统干法制备尖晶石所无法比拟的。因此通过设计,可以向其层板引入潜在的磁性物种,<WP=5>制备得到层板组成不同的LDHs,然后以其为前体经高温焙烧后得到磁学性能不同的尖晶石铁氧体。但是由于尖晶石型铁氧体中二、三价离子的化学计量比为1/2,远小于LDHs中二、三价离子的化学计量比,直接焙烧产物中会有非磁性的M(Ⅱ)的氧化物生成,从而最终影响产物的磁学性能。为此,可利用水滑石的结构特征,先将Fe2+引入水滑石层板,制备得到层板含Fe(Ⅱ)和Fe(Ⅲ) 的LDHs,再利用Fe(Ⅱ)易被氧化的特点,通过高温焙烧最终降低焙烧产物中的MⅡ/MⅢ摩尔比,得到晶相单一的尖晶石铁氧体。为此本文采用共沉淀法,在层板中引入磁性物种Ni、Zn、Co、Fe(Ⅱ)和Fe(Ⅲ),合成得到MgFe(Ⅱ)Fe(Ⅲ)-LDHs、CoFe(Ⅱ)Fe(Ⅲ)-LDHs、NiFe(Ⅱ)Fe(Ⅲ)-LDHs和NiZnFe(Ⅱ)Fe(Ⅲ)-LDHs化合物,并通过对M2+/(Fe2++Fe3+)摩尔比与尖晶石中二三价离子的化学计量比相同的层状前体进行高温焙烧的方法,来实现由层状前体制备晶相单一的尖晶石铁氧体的构想。更多还原

【Abstract】 Spinels are a typical magnetically soft material and play a significant role in the development of numerous aspects of solid-state physics and chemistry. These materials in the form of fine powers are finding applications in dehydrogenation catalysis, sensor, pigment, humidity-sensing and photoelectrical materials.Currently, the main method of synthesis of spinel ferrite is conventional solid-state reaction (further referred to as the nonactivated sample). However, in conventional solid-state reaction, inadequate mixing of the reaction components, low contact surface, and strong diffusion resistance make it difficult to reaction completely. The formations of complex oxides with the spinel structure using the conventional solid-state reaction between simple oxides proceeds especially slowly and requires prolong exposure at considerably elevated temperatures. Directed to this problem, new methods of material synthesis have received increased attention in recent years. Of particular interest are low-temperature technique, such as room-temperature ball-milling, which offer the possibility of forming structures exhibiting new and unusual properties. The mechanochemical route for the preparation of spinel ferrite has been reported starting from MeO and α-Fe2O3 powder in equimolar ratio. But in this method, the reactant is also the nonactivated α-Fe2O3/ MeO mixture, so a single homogeneous spinel is also difficult to achieve.Alternative wet chemical methods have been proposed including coprecipitation from aqueous solution, sol-gel synthesis involving supercritical drying to provide aerogels and use of micellar microemulsions. In these cases, it is difficult to prevent contamination of the product by cations arising from the precipitants or organic residues from the precursor mixtures. In order to avoid compromising the purity and <WP=7>properties of spinel ferrite and related materials, it would be desirable to prepare them from a single solid precursor, which can be prepared in a pure state in which the Me2+ and Fe3+ cations are uniformly distributed on an atomic level. In this Report, we show how synthesis of a Layered Double Hydroxide precursor with the correct stoichiometry allows this objective to be realized.Layered double hydroxide hydrotalcite are a kind of two-dimensional nanometric material, represented typically by the hydrotalcite Mg6Al2(OH)16CO3·4H2O. The structure of it consists of brucite-like, positively charged layers of magnesium and aluminum hydroxide octahedral sharing edges and has interstitial carbonate anions to charge compensate. Water molecules are also between the metal hydroxide layers. Layered double hydroxides (LDHs) can be obtained, when Mg2+, Al3+ and CO32- are substituted by other cations or anions. The LDHs generic formula is [MⅡ1-XMⅢX (OH)2]X+(An-)X/n·mH2O, where MⅡis a divalent cation (Mg2+, Ni2+, Co2+, Zn2+ or Cu2+); MⅢ is a trivalent cation ( Al3+, Cr3+, Fe3+or Sc3+)in the octahedral interstices of the hydroxide layer and An- is the charge-balancing interlayer gallery anion (CO32-, NO3-, Cl-, OH-, SO42- or PO43-).It is well known that spinel can be achieved from LDHs at high temperature (above 750 oC). But in LDHs, the divalent cation is always present in greater amounts than the trivalent cation (the stoichiometric coefficient x above is usually found in the range 0.2 – 0.33, corresponding to MII/MIII ratios of 2 – 4) whereas in a spinel the required ratio is MII/MIII = 0.5. So in the calcined products of LDHs, there are always mixed with the non-magnetism oxide of the divalent metal.Based on this fact, we put forward the substitution by Fe2+ in hydrotalcite layers to synthesize MeFe(Ⅱ)Fe(Ⅲ)-LDHs, which are potential precursors to pure MeFe2O4 spinels since the Fe2+ ions will be oxidized on calcination in air to give additional Fe3+ ions, thus overcoming the deficiency of trivalent ions discussed above. In this report, we have synthesized MgFe(Ⅱ)Fe(Ⅲ)-LDHs, CoFe(Ⅱ)Fe(Ⅲ)-LDHs, NiFe(Ⅱ)Fe(Ⅲ)-LDHs and NiZnFe(Ⅱ)Fe(Ⅲ)-LDHs precursors with the composition req更多还原