摘要
本论文根据层状双金属氢氧化物(LDHs)在高温焙烧后可转化为尖晶石的实验和理论依据,以及针对传统干法制备尖晶石铁氧体磁性材料的缺点,提出了由层状前体LDHs制备铁氧体磁性材料的若干方法:依据LDHs经高温焙烧后生成尖晶石和二价金属氧化物的混合物,将Zn2+引入LDHs层板上,通过调控层板组成,使其焙烧后为尖晶石和ZnO的混合物,利用ZnO的两性性质将其去除,实现了由层状前体LDHs制备多孔性铁氧体磁性材料的构想;依据LDHs层板金属离子的可调控性和层间阴离子的可交换性,将FeII离子引入层板,以及将[Fe(CN)6]3-络合离子引入层间,其焙烧后所得产物的磁学性能优良,实现了由层状前体LDHs制备高性能铁氧体磁性材料的构想。
一、层状前体法制备多孔性铁氧体磁性材料的研究
1. 层状前体ZnFeIII-SO4-LDHs和NiZnFeIII-SO4-LDHs的合成
采用共沉淀法制备了ZnFeIII-SO4-LDHs,研究了pH值、晶化温度、晶化时间及投料摩尔比对其晶体结构的影响,并对其结构特征进行了分析。在此基础上,将Ni2+离子引入层板,制备了NiZnFeIII-SO4-LDHs,对其晶体结构和热稳定性进行了研究。
2. 多孔性铁氧体ZnFe2O4 和NixZn1-xFe2O4的性能研究
(1)ZnFeIII-SO4-LDHs和NiZnFeIII-SO4-LDHs焙烧产物的多孔性
ZnFeIII-SO4-LDHs焙烧产物中的ZnO被去除后所得产物的孔体积及比表面积明显增大,并且在微孔和中孔范围内孔径分布均匀。不同Ni/Zn/FeIII摩尔比NiZnFeIII-SO4-LDHs在500℃焙烧产物去除ZnO后,产物的孔体积和比表面积明显增加,孔分布均匀。微孔孔体积随着前体中Zn含量的增加略微增大,当Ni/Zn/FeIII摩尔比为1/15/4时,微孔孔体积反而下降;而中孔孔体积变化规律则相反,即随着前体中Zn含量的增加略微减小,当Ni/Zn/FeIII摩尔比为1/15/4时,
微孔孔体积反而增加。当Ni/Zn/FeIII摩尔比1/15/4NiZnFeIII-SO4-LDHs的焙烧温度从500℃升至600℃时,焙烧产物用浓NaOH溶液处理后产物的孔体积和比表面积明显降低,当焙烧温度升至700℃时,去除ZnO后产物的比表面积略有降低,但孔体积却明显增加,其中存在孔径为31nm的孔。
(2)ZnFeIII-SO4-LDHs和NiZnFeIII-SO4-LDHs焙烧产物的磁学性能
由层状前体ZnFeIII-SO4-LDH制备得到的ZnFe2O4为正尖晶石,在300K时表现为顺磁性;在77K时,其磁学性能随着层状前体焙烧温度的改变而改变,在500℃下焙烧所得ZnFe2O4的磁滞回线特征明显且饱和磁化强度较高。由层状前体NiZnFeIII-SO4-LDHs制备得到的NixZn1-xFe2O4,当焙烧温度相同时,其比饱和磁化强度随层状前体中Zn含量的增加呈降低趋势。当焙烧温度为500℃时,产物呈顺磁特性,焙烧温度升至600℃以上时,产物呈亚铁磁性,且饱和磁化强度随着焙烧温度的升高明显增加。
二、层状前体法制备高性能铁氧体磁性材料的研究
1. 层板添加FeII离子层状前体法制备高性能铁氧体磁性材料的研究
合成了不同层板组成 (Mg, Ni, Co)ZnFeIIFeIII-SO4-LDHs,对其组成、晶体结构和热稳定性进行了分析,在此基础上,研究了其焙烧产物的组成和晶体结构。与传统制备尖晶石的干法和湿法相比,由层状前体(Mg, Ni, Co)ZnFeIIFeIII-SO4-LDHs焙烧后所得的尖晶石铁氧体具有优良的磁学性能。
2. 超分子插层组装法制备高性能铁氧体磁性材料的研究
采用离子交换法,以MgFeIII-Cl-LDHs为交换前体合成了单一晶相的MgFeIII-[Fe(CN)6]3--LDHs,对其晶体结构和热稳定性进行了较为详细的研究,结果表明层间阴离子[Fe(CN)6]3-以C3轴垂直于层板的取向排布于LDHs层间,并给出了其结构示意图。对MgFeIII-[Fe(CN)6]3--LDHs在不同温度下焙烧所得产物的组成及晶体结构进行了分析。与传统干法相比,由MgFeIII-[Fe(CN)6]3--LDHs层状前体制备MgFe2O4尖晶石的磁学性能优良。
In this thesis, methods for preparation of spinel ferrite from hydrotalcite-like precursors is presented, which bases that high-temperature calcination of LDHs leads to MIIMIII2O4 spinal, and the conventional method of preparation of MIIMIII2O4 spinal have some disadvantages. According to the fact that high-temperature calcination of LDHs leads to a mixture of MIIMIII2O4 spinal and MIIO, we synthesize LDHs containing Zn2+ ions in the layer. The calcination of the LDHs affords a mixture of MIIMIII2O4 spinal and ZnO by control of content of metal ions in the layer. Treatment of the calcined products with aqueous NaOH leads to dissolution of the ZnO and the formation of a porous MIIMIII2O4 ferrite phase with high surface area and pore volume. According to the fact that the metal ions in the layer can be controlled and the anions in the gallery space of LDHs can be exchanged, we synthesize LDHs containing Fe2+ ions in the layer, and LDHs intercalated by [Fe(CN)6]3- anions. The calcination of the LDHs leads to the formation of MIIMIII2O4 ferrite with excellent magnetic properities.
1. The investigation of highly porous ferrite prepared from layered double hydroxide precursor
1.1 The synthesis of ZnFeIII-SO4-LDHs and NiZnFeIII-SO4-LDHs precursor
ZnFeIII-SO4-LDHs have been prepared from zinc and iron(II) precursors by coprecipitation method. The effect of pH, aging temperature, aging time and Zn/FeII molar ratio for the structure of the LDHs has been investigated. And NiZnFeIII-SO4-LDHs have been synthesized and their structure and thermal stability have been investigated.
1.2 The investigation of properties of porous ferrite ZnFe2O4 and NixZn1-xFe2O4
1.2.1 The investigation of porosity of materials produced by calcination of ZnFeIII-SO4-LDHs and NiZnFeIII-SO4-LDHs
Treatment of the calcined products of ZnFeIII-SO4-LDHs with aqueous NaOH leads to dissolution of the ZnO and the formation of a pure zinc ferrite phase with high surface area and pore volume, and uniform pore size distributions in micropore and mesopore regions. The pore volume and BET surface area of the materials producted by calcination of NiZnFeIII-SO4-LDHs with different Ni/Zn/FeIII molar ratios at 500 oC after treatment with apueous NaOH obviously increase and the materials have uniform pore distributions in micropore and mesopore regions. The pore volume in micropore region is increased slightly by increasing Zn content in the layer precursor. In the case of Ni/Zn/FeIII molar ratio of 1/15/4, the pore volume in micropore region is decreased. Nevertheless, compared with the micropore, change of pore volume in mesopore is opposite. The pore volume is decreased slightly by increasing Zn content in the layer precursor. In the case of Ni/Zn/FeIII molar ratio of 1/15/4, the pore volume in is increased. When the temperature of calcination of NiZnFeIII-SO4-LDHs with Ni/Zn/FeIII molar ratio of 1/15/4 increases from 500 oC to 600 oC, the pore volume and BET surface area of the calcined products after treatment with apueous NaOH markedly decrease. When the calcination temperature is up to 700 oC, the BET surface area decrease but the pore volume increase clearly because of existence of large pore of 31 nm.
1.2.2 The investigation of magnetic properties of materials produced by calcination of ZnFeIII-SO4-LDHs and NiZnFeIII-SO4-LDHs
ZnFe2O4 ferrite prepared by calcination of ZnFeIII-SO4-LDHs is a normal spinel. The ferrite present paramagnetism at 300 K whereas its magnetic properties are affected by changing calcination temperature of the ZnFeIII-SO4-LDHs precursor and the saturation magnetization of the calcined product of the LDH at 500 ℃ is higher than that at upwards 500 ℃.
The saturation magnetizations of NixZn1-xFe2O4 ferrite produced by calcination of NiZnFeIII-SO4-LDHs were reduced by increasing Zn content in the LDHs precursor.
When the calcination temperature is 500 ℃, the NixZn1-xFe2O4 ferrite present paramagnetism. When the calcination temperature is up to 600 ℃, the NixZn1-xFe2O4 ferrite present ferromagnetism and its sat
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