
Citation: | SUN Yi, XIANG Shikai, GENG Huayun, GAN Yuanchao, WU Fengchao, WANG Yufeng, CHEN Han, LI Jun, GAO Junjie, YANG Jing, DAI Chengda. Automated Calibrated Modeling Method of Multiphase Equations of States: Applied to Tin[J]. Chinese Journal of High Pressure Physics, 2023, 37(2): 021301. doi: 10.11858/gywlxb.20220709 |
纳米磁性材料作为新兴的材料, 在多个领域得到广泛的应用。尤其纳米铁颗粒在电子、航空、吸波以及污水处理等方面的作用逐渐引起人们的重视[1-5]。但由于其在自然条件下易氧化团聚, 因此需要在其晶核外层包覆惰性介质, 目前常用的包覆层介质为C、SiO2等[6-9]。Jeong等[10]采用溶胶-凝胶法制备得到了尺寸为100~150 nm的SiO2包覆铁颗粒, 其外壳尺寸为40~60 nm。Santini等[11]采用层状双金属氢氧化物(LDH)离子交换法成功制备出颗粒粒径约为1.4 nm的Al2O3-ZnO包覆的铁纳米颗粒。Srikanth等[12]采用微波等离子体聚合技术, 以五羰基铁和苯乙烯单体为前驱体, 在等离子体强烈的热作用下生成纳米铁颗粒和苯乙烯自由基, 自由基在颗粒表面聚合, 最终形成粒径为15~20 nm的包覆型纳米铁颗粒。以此形成的壳核结构不仅保护了内部的晶核, 同时也使纳米颗粒具备其他特性[13]。
向纳米颗粒中掺杂其他金属或氧化物可改变其性质[14]。李晓杰等[15]利用爆轰法合成了碳包覆铁镍合金纳米颗粒, 通过改变铁镍源的比例发现, 随着铁镍比的减小, 纳米颗粒的饱和磁化强度增大而其矫顽力却减小。李振湖等[16]利用油酸盐高温热解法制备出钴掺杂的磁性氧化铁纳米颗粒, 发现纳米颗粒的磁性与钴铁比以及反应时间等有关。
目前关于氧化亚铁纳米颗粒的研究较少, 主要是由于铁在氧化过程中很难控制氧化的程度, 很容易被氧化成氧化铁而非氧化亚铁。本实验利用硝酸铁(含水)和硅酸钠(含水)制成的复合炸药前驱体, 在以氩气为保护气的密闭容器中爆轰, 利用爆轰过程产生的高温高压条件合成具有氧化亚铁掺杂的二氧化硅包覆的纳米铁颗粒, 并对纳米颗粒的形貌特征、结构组成及磁性行为进行分析。
实验中以硝酸铁(含水)和硅酸钠(含水)为铁源和硅源, 以无水乙醇和黑索金为碳源。在进行原料配方时, 首先利用控制变量法假定纳米颗粒的壳核尺寸以及爆轰反应所经历的过程, 即预先假定爆轰反应方程式。在实验中假设所制得的纳米颗粒的晶核直径为50 nm, 包覆层的厚度为5 nm, 以及参与爆轰的黑索金质量为70 g, 最终得到的含铁晶核的质量为4 g, 根据B-W法其对应的爆轰反应方程式为
0.010Na2SiO3·9H2O+0.071Fe(NO3)3·9H2O+0.143CH3CH2OH+0.315C3H6O6N6→2.104H2O+1.231CO+0.010SiO2+0.010Na2O+0.071Fe+1.053N2
通过计算得到上述原料的质量比依次为0.267:0.025:0.061:0.647, 然后在烧杯中混合均匀制成塑性混合炸药。在密闭容器中抽真空并通入氩气至常压作为保护气, 利用起爆器引爆雷管使复合炸药爆轰, 待爆轰灰沉淀完全后, 用脱脂棉收集容器内壁的爆轰灰, 然后在实验室中用无水乙醇对脱脂棉进行洗涤并将洗涤液过滤, 最后将滤纸上黑色固体颗粒在烘干箱中干燥得到灰黑色灰粉, 即目标纳米颗粒。
利用X射线衍射(X-Ray Diffraction, XRD)、透射电镜(Transmission Electron Microscope, TEM)及振动样品磁强计(Vibrating Sample Magnetometer, VSM)对目标纳米颗粒的组成、形貌特征及磁性进行分析。
金属纳米颗粒的相结构可由XRD图谱得出, 图 1是爆轰产物的XRD图谱。从图 1中可以看出, 在2θ=10.0°左右出现二氧化硅的衍射峰, 对应于衍射卡(JCP DS NO.45-0112), 在图 1中用“*”标识。在2θ=26.2°附近未出现石墨的衍射峰, 同时图谱中曲线的背底线幅较小, 说明在爆轰反应中碳基本完全被氧化。在2θ=36.1°, 41.9°, 60.8°, 72.5°和76.5°附近出现5条明显的衍射峰, 对照衍射卡片, 与氧化亚铁的衍射峰(JCP DS NO.06-0615)相对应, 在图 1中用“•”标识。在2θ=42.3°和49.5°左右出现两条不十分明显的衍射峰, 与铁的衍射峰(JCP DS NO.34-0529)相一致, 在图 1中用“♦”标识, 铁峰不明显可能与爆轰反应过程中铁与氧结合形成氧化亚铁从而使铁的晶形不完整有关。在爆轰产物中存在氧化亚铁而非氧化铁, 主要由于爆轰反应过程中提供的高温高压反应条件, 并且有碳源作为还原剂将氧化铁还原成氧化亚铁以及铁。利用Scherrer公式计算氧化亚铁掺杂的铁晶核平均直径, 即
|
(1) |
式中:D为平均晶粒直径; k为形状因子, k=1.05;λ为铜靶波长, λ=0.154 060 nm; β为半高宽; θ为衍射峰对应的半衍射角度。计算得到氧化亚铁掺杂的铁晶核平均直径D=53.20 nm。
利用TEM扫描纳米颗粒可以观察其微观形貌结构, 图 2是爆轰产物的TEM图。从图 2(a)中可以看出, 纳米颗粒的尺寸多数分布在30~60 nm; 从图 2(b)中可以看到, 纳米颗粒金属核呈规则的正球体, 不同于任蓉等[5]用水解法制得的椭球形结构, 金属核的外部被一层浅色的二氧化硅所包覆, 厚度大约为5 nm, 由于磁性材料在电镜下“跑动”使得晶核不是十分清晰。在实验设计配方之初, 利用编写的程序计算时, 假定纳米晶核的尺寸为50 nm, 其包覆层的厚度为5 nm。通过TEM图可以看出, 所得结果与事先假定基本一致, 说明在实验时可以事先假定需要合成的纳米颗粒的尺寸, 然后计算所需的原料比例, 这对爆轰合成包覆型纳米材料具有指导意义。由图 2(c)可看出明显的晶格且壳/核界限清晰, 说明晶体结构完好。
对于磁性材料, 可以用磁滞回线的形状和大小显示其性能的优劣。并且只有铁磁性和铁氧体磁性材料拥有磁滞现象, 从而具有研究和应用价值。在实验中, 用VSM测定爆轰产物在室温条件下的磁滞回线, 并对其磁性进行分析。图 3为爆轰产物的磁滞回线, 可以看出, 氧化亚铁掺杂的二氧化硅包覆铁纳米颗粒表现出独特的磁学性能。对VSM数据进行分析得出, 纳米颗粒的饱和磁化强度Ms=49.86 A·m2·kg-1, 剩余磁化强度Mr=8.63 A·m2·kg-1, 矫顽力A·m-1, 纳米颗粒的剩磁比Mr/Ms=0.173。在磁记录媒介的应用方面[17-18]:Ms越大表示能记录的资料越多, 存取密度也越大; 较高的Mr能提高存储信号的灵敏度; Hc的大小与磁记录材料的保存行息息相关, Hc越大则越不易受到外界磁场的影响。因此高矫顽力、高剩磁比在常温下表现出弱的铁磁性。这种弱铁磁性的纳米材料在储磁[19]方面具有一定优势。
(1) 采用金属硝酸盐为金属核, 硅酸盐为其壳, 乙醇及黑索金作为碳源以还原金属盐, 混合形成塑性炸药; 在以氩气为保护气的密闭容器中爆轰, 成功合成了具有壳核结构的氧化亚铁掺杂的二氧化硅包覆铁纳米颗粒。
(2) 采用TEM、XRD、VSM等手段对爆轰产物进行表征, 发现爆轰产物呈规则的正球形且分散均匀, 其中氧化亚铁掺杂的晶核尺寸分布在50 nm左右, 二氧化硅包覆层的厚度在5 nm左右。同时在XRD图谱中未发现石墨等碳的衍射峰, 说明碳在爆轰反应过程中几乎完全被氧化。利用B-W法书写爆轰反应方程式时未考虑铁未能被完全还原, 说明此方法还有欠缺。从爆轰产物的磁滞回线中可以看出, 其具有高剩磁比、高矫顽力, 表现出弱的铁磁性, 是良好的储磁材料。
(3) 从TEM图可以看出, 爆轰法合成的纳米材料呈规则的球形。主要原因是在颗粒聚集形成过程中爆轰反应释放出大量的热, 在密闭容器中形成高温高压环境, 使颗粒外围受力均匀, 并且合成时间短, 几乎在瞬间完成。因此爆轰合成法具有其他方法所不具备的优势。
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