庞建峰
, 黄文娟
PANG Jianfeng, HUANG Wenjuan
中图分类号: TB34
文章编号: 1005-3093(2016)04-0314-07
通讯作者:
收稿日期: 2014-11-20
网络出版日期: 2016-04-25
版权声明: 2016 《材料研究学报》编辑部 《材料研究学报》编辑部
基金资助:
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摘要
采用溶胶-凝胶自蔓延燃烧法制备出漂珠/钡铁氧体低密度磁性复合材料, 用扫描电镜、热重-差热分析仪、X射线衍射仪、振动样品磁强计和矢量网络分析仪表征了样品的形貌、结构、电磁性能和吸波性能。结果表明: 在漂珠表面形成的钡铁氧体包覆层厚度为5-15 nm, 颗粒粒径均小于60 nm。复合材料由六角晶钡铁氧体、α-氧化铁及少量莫来石和石英组成, 其磁性能随钡铁氧体与漂珠质量比的增大而增强。复合材料在 2-18 GHz 频段具有较好的介电损耗和磁损耗性能, 当吸波材料厚度为 1.5 mm、在 14.2 GHz处反射损耗峰值为-29.2 dB, 反射损耗小于-10 dB的带宽为4.5 GHz。
关键词：
Abstract
The magnetic composites with low density of cenospheres-barium ferrite were prepared by a sol-gel self-propagating combustion technology. The morphology, structure, electromagnetic properties and microwave absorption properties of the composite powders were characterized by the scanning electron microscope, thermogravimetry-differential scanning calorimetry, X-ray diffraction, vibrating sample magnetometer and vector network analyzer. The results show that cenospheres were covered with barium ferrite coating of ca 5 nm to 15 nm in thickness. The size of barium ferrite coated particles is less than 60 nm. In addition, it is found that the composites are composed of barium ferrite, hematite, minor mullite and quartz, and the magnetic properties of the composite material could be enhanced with the increasing mass ratio of barium ferrite to cenospheres. Furthermore, the samples exhibit better dielectric loss and magnetic loss properties within a frequency range from 2 GHz to 18 GHz. The maximum reflection loss of the composite material of 1.5 mm in thickness reaches -29.2 dB at 14.2 GHz and the bandwidth for reflection loss of -10 dB is 4.5 GHz.
Keywords：
现在, 吸波材料已成为世界各国民用防电磁辐射和军用隐身技术领域的研究热点。钡铁氧体(BaFe12O19)具有优异的硬磁性能、单轴磁晶各向异性且化学性质稳定、价格低廉, 成为目前研究较多的吸波材料之一[1-5]。但是钡铁氧体的密度较大(5.29 g/cm3), 制成的单一成分吸收剂难以满足新型吸波材料的要求。从粉煤灰中分选出的漂珠(FACs)是一种性能稳定的球状颗粒, 且其质轻、抗压强度高、耐磨性强及分散流动性好, 在建材、冶金、化学、环保、航空航天等领域有良好的应用前景。以漂珠为基材, 在其表面包覆钡铁氧体颗粒, 有望制备出兼具漂珠和钡铁氧体特性的“薄、轻、宽、强”新型吸波材料。但是目前的研究多集中在微珠/铁氧体复合材料工艺参数优化及磁性能方面, 对其吸波性能鲜有报道。同时, 对漂珠表面未作任何处理直接进行包覆, 得到的材料磁性能和吸波性能均不甚理想。本文以漂珠为基材, 用溶胶-凝胶自蔓延燃烧法制备漂珠/钡铁氧体, 研究钡铁氧体与漂珠质量比对磁性能及吸波性能的影响。
实验用试剂: 漂珠, 硝酸银、酒石酸钾钠、硝酸钡、硝酸铁、柠檬酸、硝酸、氨水、3-氨基丙基三乙氧基硅烷（KH550）以及乙醇等均为分析纯。
将漂珠置于体积分数为10%的HNO3溶液中清洗。在乙醇和超纯水混合溶剂中加入KH550, 搅拌后加入酸洗漂珠, 在75℃下磁力搅拌2 h; 过滤出漂珠, 乙醇清洗并烘干, 得到预处理漂珠。
将上述漂珠加入银氨溶液中, 搅拌10 min后再加入C4H4KNaO64H2O溶液, 反应4 h后过滤出漂珠, 用超纯水清洗并烘干, 得到活化漂珠。
根据 BaFe12O19化学成份配比, 将定量Ba(NO3)2和Fe(NO3)39H2O溶于超纯水中形成Ba2+和Fe3+混合液, 再加入C6H8O7H2O溶液, 控制Ba2+∶Fe3+∶柠檬酸摩尔比为1∶11.6∶13[6], 搅拌使其混合均匀。向混合液中滴加氨水, 调节pH值稳定在7.0左右, 形成前驱物溶液。将活化漂珠按一定质量比加入到上述前驱物溶液中, 超声后加热形成干凝胶。再将干凝胶点燃生成前驱物粉体, 将其研磨后在850℃煅烧2 h即得漂珠/钡铁氧体复合材料。为了考察钡铁氧体与漂珠质量比对材料晶体结构、磁性能及吸波性能影响, 取两者的比值分别为2∶8、3∶7和4∶6, 对应样品的编号为S1、S2和S3。在相同反应条件下制备纯BaFe12O19, 编号为S0。
用排水法测试样品密度; 用S-3400NII型扫描电镜(SEM)观察样品微观形貌; 用EX-250型能谱仪(EDX) 分析样品成分; 用STA449C型热重-差示扫描量热仪(TG-DSC)分析样品结构、热性能及物理性质变化之间的关系; 用X’TRA型X射线衍射仪(XRD)分析样品物相组成; 用7304型振动样品磁强计(VSM)测试样品磁性能参数; 用E8363C型矢量网络分析仪(VNA)测试样品电磁参数。测试试样: 将待测粉体与环氧树脂E-44混匀(质量比为4∶1), 压制成外径7.0 mm, 内径 3.0 mm, 厚度1.0-3.0 mm之间的同轴环形样品。
图1给出了漂珠表面包覆钡铁氧体的SEM和EDX图谱。可以看出, 包覆后漂珠表面比较粗糙, 包覆层均匀且呈网络状分布。也能观察到一些微小孔洞分布在包覆层中, 这可能是粉体在自蔓延燃烧过程中柠檬酸盐及水合物释放出COx、NOx及H2O等气体对包覆层造成冲击力所致。从图1(c)清晰可见, 漂珠表面由许多尺寸较小的片状颗粒组成, 还有部分颗粒团聚。其可能的原因是, 用溶胶-凝胶自蔓延燃烧法制备的前驱物粉体处于微米级, 比表面能大, 具有较强的反应活性。经850℃高温热处理后更加剧了颗粒团聚。从EDX能谱图可明显看出包覆层中含有Ba、Fe和O元素, 并且Fe和O元素谱线强度比漂珠基体高出很多。
图1 漂珠/钡铁氧体复合粉体的SEM图谱和EDX图谱
Fig.1 SEM images of the FACs / barium ferrite at low magnification (a), (b); at high magnification (c) and EDX spectrum (d)
钡铁氧体颗粒均匀包覆在漂珠表面后, 可用下式计算钡铁氧体壳层理论厚度
其中t为包覆层理论厚度, r1为漂珠平均半径, m1和m2分别为漂珠和钡铁氧体理论质量, ρ1和ρ2为漂珠和钡铁氧体密度, 分别为0.62 g/cm3和5.29 g/cm3, 得到钡铁氧体理论包覆层厚度(表1)。用排水法测试样品密度(表1), 可见复合粉体密度远低于钡铁氧体理论值, 表明引入漂珠大大降低了材料密度。
表1 样品S1、S2和S3的理论包覆层厚度及密度
Table 1 Coating thickness and density results of samples S1, S2 and S3
| Samples | S1 | S2 | S3 |
|---|---|---|---|
| Coating thickness / nm | 5.56 | 8.39 | 13.6 |
| Density / gcm-3 | 1.94 | 2.53 | 3.11 |
图2给出了样品S3干凝胶热重-差热分析曲线。可见DSC曲线在130℃和150℃两处有吸热峰, 而TG曲线对应位置没有观察到失重现象, 表明这两处吸收峰应该为干凝胶熔融过程。在200-240℃区间DSC曲线在210℃出现尖锐放热峰, 对应TG曲线上质量损失约为78%, 表明积累足够能量后干凝胶从200℃发生分解并开始燃烧, 且在210℃时最为剧烈。燃烧反应发生的温度范围在40℃左右, 说明自蔓延燃烧过程可在短时间内快速完成。温度高于400℃后TG曲线几乎没有明显变化, 表明粉体的质量基本上稳定了。此外, DSC曲线在640℃附近有一个小吸热峰, TG曲线对应位置也有少许质量损失, 表明该温度附近有新物质形成或存在晶相转变。
图2 样品S3干凝胶的热分析曲线
Fig.2 Dry gel thermal analysis patterns of sample S3 (a) TG and (b) DSC
样品S3自蔓延燃烧产物及该产物在650℃、750℃和850℃高温热处理2 h后样品的XRD谱线, 如图3所示。可以看出, 自蔓延燃烧产物的组成主要为BaCO3和γ-Fe2O3, 表明柠檬酸络合物在燃烧过程中发生了热分解反应, 生成BaCO3和γ-Fe2O3等中间物质。该粉体在650℃热处理后其产物主要为BaCO3和γ-Fe2O3、α-Fe2O3及微量BaFe2O4, 表明BaCO3和γ-Fe2O3发生反应并产生BaFe2O4。温度为750℃时γ-Fe2O3和BaFe2O4反应生成六角晶BaFe12O19, 且α-Fe2O3衍射峰强度增强。当温度升至850℃时产物相中除了大量BaFe12O19外, 仍包含微量α-Fe2O3。上述结果表明, 自蔓延燃烧产物中的γ-Fe2O3, 一部分与BaFe2O4反应生成 BaFe12O19, 其余部分则转变为α-Fe2O3。根据文献[7, 8], γ-Fe2O3属立方结构, 与BaFe2O4结构相同, 在两者共存体系中容易反应生成BaFe12O19; α-Fe2O3则属三角晶系晶体结构, 是非磁性稳定化合物, 不能与BaCO3反应生成BaFe2O4, 因此也不能生成BaFe12O19。而γ-Fe2O3又是α-Fe2O3的亚稳相, 在一定条件下可转化为α-Fe2O3。当自蔓延燃烧粉体在不同温度热处理时, 在燃烧过程中产生的γ-Fe2O3与BaCO3反应生成BaFe2O4, 同时也有部分逐渐转化为稳定的α-Fe2O3相, 因此在XRD谱线中有BaFe12O19和α-Fe2O3两种晶体的峰。根据文献[9]γ-Fe2O3相转变温度为700℃, 结合图2 DSC曲线, 可认为640℃附近出现的吸热峰是γ-Fe2O3转变为α-Fe2O3过程中吸热所致。
图3 样品S3在不同热处理温度的XRD图谱 (a) 自蔓延燃烧粉体及其在不同热处理温度处理2 h (b) 650℃, (c) 750℃, (d) 850℃
Fig.3 XRD patterns of sample S3 (a) auto-combustion powders and combustion powder with different calcining temperatures for 2 h (b) 650℃, (c) 750 and (d) 850℃
图4给出了样品S1、S2、S3和S0在850℃热处理2 h的XRD图谱。可以看到, 随钡铁氧体与漂珠比的增大复合粉体中钡铁氧体衍射峰强度逐渐增加且峰形愈加尖锐, 表明漂珠表面形成的晶体晶型更加完整。根据Scherrer公式
可计算出钡铁氧体晶粒大小, 其中D为钡铁氧体晶粒大小(nm); K为晶体形状因素, 其值为0.89; λ为衍射采用的放射源波长, 对于Cu 靶其λ为0.15406 nm; θ为最强衍射峰衍射角(rad); β为最强衍射峰半峰宽(rad)。计算结果表明, 样品S1、S2、S3和S0钡铁氧体晶粒尺寸分别为37.7 nm、40.7 nm、54.1 nm和43.8 nm。有研究者[8]计算出六角晶钡铁氧体单畴颗粒的临界尺寸为 460 nm, 本实验中钡铁氧体颗粒尺寸均小于60 nm为单磁畴结构。
图4 样品S1、S2、S3和S0 XRD图谱
Fig.4 XRD patterns of the samples (a) S1, (b) S2, (c) S3 and (d) S0
图5给出了S1、S2、S3和S0的磁滞回线, 由此得到饱和磁化强度(Ms)、剩余磁化强度(Mr)和矫顽力(Hc)等参数(表2)。由磁滞回线可看出, 样品均具有硬磁特性, 且磁滞回线内部面积随钡铁氧体与漂珠比增加而增大, 表明材料储存磁能的能力也随之增强。由表2可知, 复合材料的Ms、Mr及Hc均随钡铁氧体与漂珠比增加而增大。XRD分析结果表明, 漂珠/钡铁氧体主要由六角晶钡铁氧体、α-氧化铁及少量莫来石、石英组成; 材料组分构成中钡铁氧体是性能优异的硬磁性材料, 漂珠是非磁性物质, α-氧化铁则是弱铁磁性物质, 显然材料磁性能主要来源于钡铁氧体。因此随钡铁氧体与漂珠比的增大, 复合粉体Ms随粉体单位质量内钡铁氧体含量的增加而增大。此外, 磁性材料在磁化过程中同时有磁畴转动和畴壁移动两种物理变化, 当磁性颗粒尺寸小于临界尺寸时就只存在磁畴结构, 其磁化机制仅为磁畴转动。前文结果已表明, 钡铁氧体颗粒为单磁畴结构, 其磁化程度由颗粒各向异性和颗粒间的磁交互作用来决定。随钡铁氧体量在复合粉体中的增加磁性颗粒间的相互作用加强, 增强了颗粒之间的各向异性作用, 从而使复合粉体的Hc也呈现增大的态势。
图5 样品S1、S2、S3和S0磁滞回线
Fig.5 Magnetic hysteresis loops of the samples (a) S1, (b) S2, (c) S3 and (d) S0
表2 样品S1、S2、S3和S0的磁性能参数
Table 2 Magnetic parameters of the samples (a) S1, (b) S2, (c) S3 and (d) S0
| Samples | Saturation magnetization Ms / emug-1 | Remanence Mr / emug-1 | Coercivity Hc / Oe |
|---|---|---|---|
| S1 | 17.5 | 11.4 | 4541 |
| S2 | 26.7 | 16.9 | 4937 |
| S3 | 43.6 | 30.5 | 5239 |
| S0 | 55.8 | 36.4 | 5536 |
图6(a)和6(b)分别给出了漂珠及S1、S2、S3和S0的复介电常数实部
图6 漂珠及样品S1、S2、S3和S0电磁参数
Fig.6 Electromagnetic parameters of FACs and the samples S1, S2, S3 and S0 (a) ε', (b) ε", (c) μ', (d) μ"
漂珠及S1、S2、S3和S0的复磁导率实部
图7给出了S1、S2、S3和S0在不同厚度的反射损耗曲线, 表3列出了最低反射损耗(RL)、匹配厚度(dm)、匹配频率(fm)及-10 dB吸收带宽等参数。可以看出, 每条反射损耗曲线至少有两个吸收峰, 原因是漂珠/钡铁氧体兼具介电损耗和磁损耗共同特性。S1、S2和S3的最低反射损耗值随材料中钡铁氧体量的增加而降低; 三个样品的匹配厚度均为1.5 mm, 匹配频率移向低频, 均具有较强的吸波性能。比较而言, S3吸波效果更优, 不仅反射损耗峰值最低(-29.2 dB), 且厚度薄(1.5 mm)、-10 dB吸收频段最宽(4.5 GHz)。
图7 样品S1、S2、S3和S0在不同厚度的反射损耗曲线
Fig.7 Frequency dependence of reflection loss for the composites at different thickness (a) S1, (b) S2, (c) S3 and (d) S0
表3 复合粉体的微波吸收性能
Table 3 Microwave absorption properties of the composites
| Samples | fm / GHz | dm / mm | Minimal RL / dB | Bandwidth*/GHz |
|---|---|---|---|---|
| S1 | 16.6 | 1.5 | -24.6 | 2.7 |
| S2 | 13.0 | 1.5 | -26.6 | 1.6 |
| S3 | 14.2 | 1.5 | -29.2 | 4.5 |
| S0 | 11.6 | 1.0 | -32.6 | 2.4 |
| S0 | 11.7 | 1.5 | -21.8 | 3.5 |
制备的漂珠/钡铁氧体复合材料具有良好的吸波性能, 主要原因有: (1)钡铁氧体与漂珠复合在电磁特性上有一定互补性。钡铁氧体为磁性材料, 具有磁损耗和介电损耗两种功能; 漂珠为非磁性材料, 其引入可显著提高复合材料的介电损耗(图6(b))。(2)复合材料的“核壳”结构及漂珠中空结构对吸波性能有贡献。当电磁波入射至材料表面时小部分被反射, 其余大部分以折射方式进入钡铁氧体包覆层-壳层, 其中部分被反射回壳层并在传播过程中逐渐衰减, 还有部分以折射方式进入漂珠-核层。进入漂珠内部的电磁波在空腔内继续发生反射甚至多次反射和折射, 大部分电磁波被吸收和衰减后, 其余电磁波才会通过钡铁氧体壳层进入自由空间。这表明, 电磁波在核壳材料中的传播历程比在单相钡铁氧体中要复杂。此外, 漂珠的中空结构对电磁波具有散射作用, 其核层折光指数远低于壳层折光指数, 有可能对微波电磁场产生“黑洞”效应而提高材料吸波效果。(3)钡铁氧体与漂珠间的界面效应对吸波性能有贡献。有学者认为, 核壳之间的界面容易发生界面极化弛豫现象使材料介电损耗增加, 即在核壳界面微区会发生介电弛豫损耗而使材料吸波能力增强[10-14]。对于本文实验中的漂珠/钡铁氧体复合材料, 第一介电损耗峰值为0.42, 远高于单相钡铁氧体峰值0.23, 其原因可能是核壳界面介质内部粒子在电磁波作用下发生了界面极化, 提高了复合材料的介电损耗。此外, 还有核壳界面的磁交换耦合作用。磁交换耦合的影响范围一般为纳米数量级, 其强弱程度与晶粒耦合度和晶粒尺寸密切相关。晶粒界面间的直接耦合越多, 晶粒越小, 界面的交换作用越强[15-17]。本文实验中复合材料壳层的钡铁氧体粒子尺寸约为45 nm, 晶粒非常细小, 因此在核壳界面及磁性晶粒之间也可能发生磁交换耦合作用。
1. 以漂珠为基材, 用溶胶-凝胶自蔓延燃烧法可制备漂珠/钡铁氧体低密度磁性复合材料。钡铁氧体包覆层厚度为5-15 nm, 颗粒尺寸为30-60 nm。复合材料由六角晶钡铁氧体、α-氧化铁及少量莫来石和石英组成, 其磁性随钡铁氧体与漂珠质量比增加而增强, Ms、Mr和Hc最大值分别为43.6 emu/g、30.5 emu/g和5239 Oe。
2. 复合材料对电磁波的反射损耗随着钡铁氧体与漂珠质量比的增大而增强。当钡铁氧体与漂珠质量比为4∶6、吸波层厚度为1.5 mm、频率为14.2 GHz时吸收峰值为-29.2 dB, 反射损耗小于-10 dB 的带宽为4.5 GHz。漂珠与钡铁氧体在电磁特性上互补、复合材料的“核壳”结构及漂珠中空结构和漂珠与钡铁氧体之间的界面效应均对材料的吸波性能有贡献。
The authors have declared that no competing interests exist.
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Synthesize of barium ferrite nanowire array by self-fabricated porous silicon template ,
In this work, we synthesize barium ferrite (BaFe 12 O 19 ) nanowire array in porous silicon template. The porous silicon templates are prepared via gold-assisted chemical etching method. The gold (Au) nanoparticles with mean diameter of 30nm and distance of 100nm were ordered on the surface of Si substrate through the Polystyrene (510000)-block-poly (2-vinylpyridine) (31000) (PS 510000 -b-P2VP 31000 ) diblock copolymer. Porous silicon templates with mean diameter of 500nm and distance between the pores of 500nm were fabricated by two etching steps. BaFe 12 O 19 nanowires with mean diameter of 200nm were synthesized into a porous silicon template by a sol鈥揼el method. Magnetic hysteresis loops show an isotropic feature of the BaFe 12 O 19 nanowires array. The coercivity ( H c ) and squareness ratio ( M r / M s ) of nanowire arrays are 2560Oe and 0.6, respectively.
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Surface modification of M-Ba-ferrite powders by polyaniline: Towards improving microwave electromagnetic response ,
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| [3] |
Effect of high energy milling on the synthesis temperature, magnetic and electrical properties of barium hexagonal ferrite ,
Nanoparticles of barium hexagonal ferrite (BaFeO) have been synthesized by initial high energy milling of the precursors and sintering subsequently. X-ray Diffraction pattern reveals the hexagonal phase with a crystallite size of 42 nm. Scanning Electron Micrograph (SEM) shows the size of nanoparticles to lie in the range of single domain particles. X-ray Photoelectron Spectroscopy (XPS) indicates Fe in +3 state and an associated surface peak indicates its presence at different environments. Vibrating Sample Magnetometer measurements (VSM) illustrate ferrimagnetism with high coercivity and non-saturation of hysteresis behavior up to 8500 Oe. The initial high energy milling confines the particle size to the nanoscale which leads to high coercivity. Temperature dependent magnetization shows a sharp peak before transition which is the characteristic of the single domain magnetic behavior. Resistivity plot shows a decreasing trend with temperature. Two thermal activation energy values of 0.81 eV and 0.29 eV indicate the existence of two different conduction processes. The frequency dependent dielectric constant plot shows a semiconducting behavior. The dielectric loss plot shows a drastic change at high temperature due to the anion vacancies in addition to four types of polarization.
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Synthesis of barium ferrite ultrafine powders by a sol-gel combustion method using glycine gels , |
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Synthesis of the plate-shaped barium ferrite by the second chemical co-precipitation method and investigation of the magnetic properties ,二次化学共沉淀法制备片状钡铁氧体的形成历程及磁性能研究 ,
采用二次化学共沉淀法制备出六角或近六角片状BaFe<sub>12</sub>O<sub>19</sub>, 其颗粒径向尺寸和径厚比分别为0.4--2 μm和4--20。通过XRD、FTIR、TG/DTA及SEM/EDS分析技术研究了片状BaFe<sub>12</sub>O<sub>19</sub>的形成历程。结果表明: 前驱体为非晶态BaCO<sub>3</sub>、低结晶态Fe(OH)<sub>3</sub>和晶态α--Fe2O3包覆原料BaFe<sub>12</sub>O<sub>19</sub>复合物; 前驱体在焙烧过程中经过</br>Fe(OH)<sub>3</sub>脱水、BaCO<sub>3</sub>分解反应、中间相α--Fe<sub>2</sub>O<sub>3</sub>和BaO反应得到终产物BaFe<sub>12</sub>O<sub>19</sub>。基于形成历程, 六角片状BaFe<sub>12</sub>O<sub>19</sub>较原料BaFe<sub>12</sub>O<sub>19</sub>表现出显著提高的颗粒径向尺寸和径厚比、较高的纯度和略低的结晶有序程度, 进而表现出明显提高的矫顽力、略低的饱和磁化强度和剩余磁化强度。
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Preparation and magnetic properties of barium hexaferrite nanorods ,
The barium hexaferrite nanorods were successfully prepared by sol-gel technique combined with polymethylmethacrylate as template. The crystal structure, morphology and magnetic properties of BaFe{sub 12}O{sub 19} with different shape were investigated with X-ray diffraction, field emission scanning electron microscope and vibrating sample magnetometry. The results show that diameters and lengths of magnetic nanorods are about 60 nm and 300 nm, respectively. The coercivity of rod-shaped BaFe{sub 12}O{sub 19} is increased to 5350 Oe, in comparison with 4800 Oe with plate-shape. The formation mechanism of BaFe{sub 12}O{sub 19} nanorods and reasons resulting in high coercivity are discussed.
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Key step in synthesis of ultrafine BaFe12O19 by sol-gel technique , |
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Preparation and surface modification of spherical nano-BaFe12O19 ,李世普球形纳米 BaFe12O19的制备及表面改性 , |
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Preparation and properties of nanocrysta γ-Fe2O3 ,纳米γ-Fe2O3的合成及气敏性能 ,
硝酸铁乙二醇甲醚溶液体系中加入硅酸乙酯,用溶胶-凝胶法制备γ-Fe2O3纳米晶粉体.硅酸乙酯的加入不但加速凝胶化过程, 而且有效抑制氧化铁晶粒的生长,提高γ-Fe2O3向α-Fe2O3转变的相变温度.TG-DTA热分析结果表明这个相变温度为700℃左右.改变硅酸乙酯的加入量可制备出具有不同晶相的粉体;TEM观测表明粉体的粒径约为10nm. γ-Fe2O3纳米晶粉体的气敏特性具有迅速的响应-恢复性能, 掺入少量Zn可改善气敏特性.
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Microwave absorption properties of the carbon-coated nickel nanocapsules ,
The carbon-coated Ni(C) nanocapsules were prepared by a modified arc-discharge method in methane atmosphere. Its electromagnetic parameters were measured at 2-18GHz. It is observed that the natural resonance which appeared at 5.5GHz is dominant among microwave absorption properties of Ni(C) nanocapsules, as the consequence of the increased surface anisotropic energy for nanosized particles. The measured relative complex permittivity indicates that a high resistivity existed in Ni(C) nanocapsules samples. The maximum reflection loss of Ni(C) nanocomposites can reach 32dB at 13GHz with 2mm in thickness. The microwave absorptive mechanisms of Ni(C) nanocapsule absorbent were discussed.
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Microwave absorbing materials using Ag-NiZn ferrite core-shell nanopowders as fillers , |
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Fabrication and microwave absorption of carbon nanotubes/CoFe2O4 spinel nanocomposite ,
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X. L.Liang, Microwave absorption enhancement and complex permittivity and permeability of Fe encapsulated within carbon nanotubes ,
CNT/crystalline Fe nanocomposites (see Figure) have excellent microwave-absorption characteristics. This absorption property is shown to result from the confinement of crystalline Fe in carbon nanoshells, deriving mainly from magnetic rather than electric effects-the complex permittivity and permeability depend both on the shape and phase of the CNT/Fe nanocapsulates.
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Synthesis, magnetic and microwave absorbing properties of core-shell structured MnFe2O4/TiO2 nanocomposites , |
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ZHU mingGang, LI Wei, Exchange-coupling interaction and effective anisotropy of NdFeB nanocomposite permanent magnetic materials ,NdFeB纳米复合永磁材料的交换耦合相互作用和有效各向异性 ,
以Nd<sub>2</sub>Fe<sub>14</sub>B/αFe为例，采用立方体晶粒结构模型，研究了纳米复合永磁材料中不同磁性晶粒间的交换耦合相互作用和有效各向异性.纳米复合永磁材料的有效各向异性Keff等于软、硬磁性相各向异性的统计平均值，每个晶粒的各向异性由晶粒表面交换耦合部分和晶粒内部未交换耦合部分的各向异性共同确定.计算结果表明，软、硬磁性相晶粒尺寸分布显著地影响有效各向异性Keff的值.当软、硬磁性晶粒尺寸D相同时，Keff随晶粒尺寸和硬磁性相体积分数的降低而减小, 当D<20nm 时，K
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Effective anisotropy between magnetically soft and hard grains in nanocomposite magnets ,
Effect of grain size on the effective anisotropy constant between magnetically soft (伪-Fe) and hard (NdFeB) grains in nanocomposite magnets, , has been investigated. The results show that the values of increase with decreasing size of soft grain, D, for the given size of hard grain, D. For the given D, increases with increasing D, and then approaches a steady value. In order to get a high value of , the hard grain size should be larger than 30 nm and the soft grain size should be about 10 nm.
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High-frequency ferromagnetic resonance in Nd2Fe14 B/α-Fe nanocomposite films, Magnetics , |
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