磁场下镍离子的化学还原反应与产物的磁畴结构研究
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摘要
在化学反应中通过引入磁场,可改变产物的形貌和性质;磁性主要来源于电子自旋磁矩,对磁畴进行分析有助于从微观角度了解磁性材料的磁性能。因此本论文致力于探讨磁场下镍离子化学还原反应的特征,并研究磁场对产物磁畴结构及磁性能的影响。详细内容可归纳如下:
1.为了研究磁场对化学反应的影响,施加磁场的反应体系应该尽量满足以下条件:以顺磁性离子Ni~(2+)和Cu~(2+)作为研究对象;考虑到镍的居里温度为358℃,所以反应温度不应太高,而且温度过高时,体系热运动占主导,这时候磁场的影响就较为微弱;反应应该尽可能的缓慢进行,离子缓慢成核与生长才有可能观察到磁场对粒子的诱导作用,如果反应进行的很快,那么磁场还没有起到作用时就立即成核并生长了。因此,低温混合热溶剂法是一个较理想的反应体系,而且方便施加外磁场。于是,我们在一个较温和的低温混合热溶剂体系下,选择被PVP络合的镍离子作为研究对象,研究了各种反应条件对其形貌的影响,探索了适合观察磁场效应的化学反应体系。以蒸馏水—乙醇为溶剂,氯化镍为镍源,PVP为表面活性剂,用水合肼还原得到了单质镍。研究发现,通过调节混合溶剂中蒸馏水/乙醇的体积比(w)、表面活性剂PVP的用量、反应温度及镍离子浓度可以有效控制产物的形貌和尺寸,从而获得均匀多晶镍纳米球。该体系中制得的镍纳米球具有良好的铁磁性质,样品的饱和磁化强度(Ms)和矫顽力(Hc)分别为53 emu/g和82 Oe。然而,研究发现磁场对铜离子的还原反应影响不明显。这可能是由于产物铜不是铁磁性物质造成的。
2.基于制备镍纳米球的工作基础,我们选择在一个合适的化学反应体系中施加外磁场,最终获得了一维镍纳米线。然后以其中涉及的化学反应为例,探讨了磁场下镍离子的化学还原特征,及镍纳米线的成核与生长机理。我们认为一维镍纳米线的形成,是反应速率与磁场作用相互协作的结果。当反应物Ni~(2+)浓度较低时,反应速率相对较慢,[Ni(N_2H_4)_x]~(2+)优先选择迁移到磁力线附近并且沿着磁力线方向排列,导致电极电势增大,于是[Ni(N_2H_4)_x]~(2+)的化学还原反应也是沿着磁力线发生,结果导致了镍纳米线的形成。而当Ni~(2+)浓度较高时,反应速率相对较快,[Ni(N_2H_4)_x]~(2+)可能在还没来得及迁移到磁力线之前,还原反应就已经很快的在整个体系中发生了,导致出现了产物中纳米球和一维镍线共存的现象。我们对无磁场下制得的样品,在相同的磁场强度下也进行了磁场后处理,然而却没发现有镍纳米线生成。可见,在反应中施加的外磁场可以影响粒子的成核与生长,而不是诱导粒子的简单组装。磁测量结果表明,与无磁场下的产物相比,在磁场下生成的样品具备较好的磁学性质,另外,我们对在无磁场及有磁场下得到的产物分别做了它们的微波吸收性质的测试。实验结果表明,一维镍线在8.5-12.5 GHz下的微波吸收性质优于镍纳米球的,该结果进一步证明了镍线不是纳米粒子的简单组装,其磁畴结构可能已被外加磁场改变。
3.利用磁力显微镜(MFM)分别研究了在无磁场及有磁场下的产物镍的磁畴结构特征,并探讨外加磁场对产物磁畴结构及其磁性能的影响。MFM观测结果表明,直径250 nm的镍纳米线拥有单磁畴结构;而直径2μm的镍线的磁力图呈现的是新颖的核—壳圆柱形磁畴。我们认为这种新颖的多畴结构可能是由于在化学反应中所施加的外磁场造成的。于是,我们利用示意模型来解释这两种不同的磁畴构型。为了验证我们所提出的磁畴模型,在相同的反应体系下,在无外加磁场时制得了镍膜ZF,在0.25 T磁场下得到了镍膜AF样品。ZF样品的磁力图中含有的是暗色斑点畴,对应着该样品形貌图中的纳米颗粒;AF样品的磁力图中发现大量的暗色圆形畴和一些典型的同心圆环磁畴结构,分别对应着直径250 nm的镍纳米线中的单磁畴结构,和直径2μm的镍线中的核—壳圆柱形磁畴结构。这两个镍膜的磁测量结果差别很大,我们认为这是由于化学反应中施加的外磁场对产物磁畴结构的改变造成的。磁测量的结果与MFM观测的结果能很好的对应,进一步证实了我们所提出的磁畴模型的合理性,可见利用外磁场可以实现对产物磁畴结构的有效调控。
The morphology and properties of materials can be controlled by use of magnetic field in chemical reactions.Magnetism mainly comes from electron spin magnetic moments,so studying magnetic domains contributes to understand magnetic properties of magnetic materials in the view of microcosmic.Therefore,the objective of this dissertation is to explore magnetic field effects on chemical reduction of Ni~(2+), magnetic domain structures and magnetic properties of nickel.More details are summarized as follows:
1.In order to study the influence of magnetic field on chemical reactions,the reaction system imposed by the magnetic field should meet the following conditions. Firstly,the research objects should be paramagnetic ions,such as Ni~(2+) or Cu~(2+); Secondly,the reaction temperature can not be too high,for the Curie temperature of nickel is 358℃.Also,if the temperature is too high,the thermal motion would be dominant in the system,so the influence of magnetic field would become weak.Lastly, the reaction should be carried out as slow as possible,and then the slow nucleation and growth of crystal would be helpful to observe the induction of magnetic field on the particles.Therefore,low-temperature solvothermal system is an optimal reaction system,and facilitates the imposition of external magnetic field.So,in a moderate low-temperature solvothermal system,Ni-PVP complex was selected.The reaction conditions involved were studied systematically to select the optimal reaction parameters to reveal the effects of magnetic fields on chemical reactions.Uniform polycrystalline nickel nanospheres were prepared in a water-ethanol mixed solvent solution using Nickel Chloride(NiCl_2),hydrazine(N_2H_4),and polyvinylpyrrolidone (PVP) as starting materials.The product exhibited good ferromagnetism,and its saturation magnetization(Ms) and coercivity(Hc) are 53 emu/g and 82 Oe, respectively.It can be deduced that 1D nickel wires with controlled shapes can be obtained by introducing a magnetic field in the same system.However,research shows that the influence of magnetic field on the reduction reaction of copper ions was not obvious,as copper is not a ferromagnetic material.
2.Based on the study of several control experiments performed without a magnetic field,1D nickel wires with controlled shapes were obtained by use of a magnetic field in the optimal reaction systems.The involved chemical reaction was taken as an example to study the effects of magnetic fields on the chemical reduction of Ni~(2+),and the nucleation and growth of nickel nanowires.The formation of our 1D nickel wires can be ascribed to the cooperative effect of the reaction rate and magnetic field.The reaction rate in the system is controlled mainly by adjusting the reactant concentration when other reaction conditions are fixed.When the concentration of Ni~(2+) is low,the reaction rate is correspondingly slow,so the complex[Ni(N_2H_4)_x]~(2+) preferentially migrates to the magnetic line of force since paramagnetic metal ions are attracted toward the maximum field.Then the chemical reduction of[Ni(N_2H_4)_x]~(2+) may occur along the magnetic line of force due to the enhanced electrode potential,leading to the formation of uniform 1 D wires.While with the increase of the concentration of Ni~(2+), the reaction rate increases too,so it can be deduced that the chemical reduction of [Ni(N_2H_4)_x]~(2+) may be taken place quickly in the whole system,leading to the formation of isolated particles coexisted with the wires composed of microspheres which formed along the magnetic line of force.Additionally,to explore well the effect of magnetic fields on the nucleation and growth of magnetic particles,a postsynthesis magnetic alignment experiment was carried out on the sample prepared without a field.However,one-dimensional wires were not found,which interprets that magnetic fields applied during the chemical reaction have influenced the nucleation and growth of nickel,instead of inducing a simple assembly of particles.Magnetic measurement results show that the nickel wires prepared with a magnetic field had remarkably improved magnetic properties,compared to that of nickel nanospheres prepared without a field.Moreover,the enhanced microwave absorption property of the nickel wires/PMMA composite at 8.5-12.5 GHz in comparison with that of nickel microspheres/PMMA composite was observed,which indicated that the anisotropic wires were not the simple assembly of particles,and the magnetic domain structures in wires had been changed by the applied field in chemical reaction.
3.Magnetic force microscopy(MFM) investigations were carried out to study the domain structures of the nickel samples,and magnetic field effects on magnetic domain structures and magnetic properties of nickel were explored.Nickel wires with a diameter of approximately 250 nm had single-domain structure,while unique core-shell cylindrical domain structures were found in wires with a diameter of 2μm. It is suggested that the unique multidomain may be aroused by the applied field in the chemical reaction.Then a schematic domain structure was used to tentatively explain the unique multidomain structure.To further support the core-shell cylindrical model, nickel film AF was prepared with the same condition as the wires,while film ZF was obtained by the same synthesis procedure as AF except without a field applied.MFM images of the AF sample show many dark circular domains,corresponding to the wires in a diameter of about 250 nm.Additionally,there are also some typical concentric annular zone domains with dark inner core and bright outer shell.While the dark spot domains of ZF are corresponding to the nanoparticles in the sample.The different magnetic properties of AF and ZF resulted from their different domains that controlled by the magnetic field in the chemical reactions.The magnetic measurements show a good agreement with our MFM results,further confirming the magnetic domain model proposed.So magnetic field in chemical reactions can be used to control magnetic domains of materials.
1.为了研究磁场对化学反应的影响,施加磁场的反应体系应该尽量满足以下条件:以顺磁性离子Ni~(2+)和Cu~(2+)作为研究对象;考虑到镍的居里温度为358℃,所以反应温度不应太高,而且温度过高时,体系热运动占主导,这时候磁场的影响就较为微弱;反应应该尽可能的缓慢进行,离子缓慢成核与生长才有可能观察到磁场对粒子的诱导作用,如果反应进行的很快,那么磁场还没有起到作用时就立即成核并生长了。因此,低温混合热溶剂法是一个较理想的反应体系,而且方便施加外磁场。于是,我们在一个较温和的低温混合热溶剂体系下,选择被PVP络合的镍离子作为研究对象,研究了各种反应条件对其形貌的影响,探索了适合观察磁场效应的化学反应体系。以蒸馏水—乙醇为溶剂,氯化镍为镍源,PVP为表面活性剂,用水合肼还原得到了单质镍。研究发现,通过调节混合溶剂中蒸馏水/乙醇的体积比(w)、表面活性剂PVP的用量、反应温度及镍离子浓度可以有效控制产物的形貌和尺寸,从而获得均匀多晶镍纳米球。该体系中制得的镍纳米球具有良好的铁磁性质,样品的饱和磁化强度(Ms)和矫顽力(Hc)分别为53 emu/g和82 Oe。然而,研究发现磁场对铜离子的还原反应影响不明显。这可能是由于产物铜不是铁磁性物质造成的。
2.基于制备镍纳米球的工作基础,我们选择在一个合适的化学反应体系中施加外磁场,最终获得了一维镍纳米线。然后以其中涉及的化学反应为例,探讨了磁场下镍离子的化学还原特征,及镍纳米线的成核与生长机理。我们认为一维镍纳米线的形成,是反应速率与磁场作用相互协作的结果。当反应物Ni~(2+)浓度较低时,反应速率相对较慢,[Ni(N_2H_4)_x]~(2+)优先选择迁移到磁力线附近并且沿着磁力线方向排列,导致电极电势增大,于是[Ni(N_2H_4)_x]~(2+)的化学还原反应也是沿着磁力线发生,结果导致了镍纳米线的形成。而当Ni~(2+)浓度较高时,反应速率相对较快,[Ni(N_2H_4)_x]~(2+)可能在还没来得及迁移到磁力线之前,还原反应就已经很快的在整个体系中发生了,导致出现了产物中纳米球和一维镍线共存的现象。我们对无磁场下制得的样品,在相同的磁场强度下也进行了磁场后处理,然而却没发现有镍纳米线生成。可见,在反应中施加的外磁场可以影响粒子的成核与生长,而不是诱导粒子的简单组装。磁测量结果表明,与无磁场下的产物相比,在磁场下生成的样品具备较好的磁学性质,另外,我们对在无磁场及有磁场下得到的产物分别做了它们的微波吸收性质的测试。实验结果表明,一维镍线在8.5-12.5 GHz下的微波吸收性质优于镍纳米球的,该结果进一步证明了镍线不是纳米粒子的简单组装,其磁畴结构可能已被外加磁场改变。
3.利用磁力显微镜(MFM)分别研究了在无磁场及有磁场下的产物镍的磁畴结构特征,并探讨外加磁场对产物磁畴结构及其磁性能的影响。MFM观测结果表明,直径250 nm的镍纳米线拥有单磁畴结构;而直径2μm的镍线的磁力图呈现的是新颖的核—壳圆柱形磁畴。我们认为这种新颖的多畴结构可能是由于在化学反应中所施加的外磁场造成的。于是,我们利用示意模型来解释这两种不同的磁畴构型。为了验证我们所提出的磁畴模型,在相同的反应体系下,在无外加磁场时制得了镍膜ZF,在0.25 T磁场下得到了镍膜AF样品。ZF样品的磁力图中含有的是暗色斑点畴,对应着该样品形貌图中的纳米颗粒;AF样品的磁力图中发现大量的暗色圆形畴和一些典型的同心圆环磁畴结构,分别对应着直径250 nm的镍纳米线中的单磁畴结构,和直径2μm的镍线中的核—壳圆柱形磁畴结构。这两个镍膜的磁测量结果差别很大,我们认为这是由于化学反应中施加的外磁场对产物磁畴结构的改变造成的。磁测量的结果与MFM观测的结果能很好的对应,进一步证实了我们所提出的磁畴模型的合理性,可见利用外磁场可以实现对产物磁畴结构的有效调控。
The morphology and properties of materials can be controlled by use of magnetic field in chemical reactions.Magnetism mainly comes from electron spin magnetic moments,so studying magnetic domains contributes to understand magnetic properties of magnetic materials in the view of microcosmic.Therefore,the objective of this dissertation is to explore magnetic field effects on chemical reduction of Ni~(2+), magnetic domain structures and magnetic properties of nickel.More details are summarized as follows:
1.In order to study the influence of magnetic field on chemical reactions,the reaction system imposed by the magnetic field should meet the following conditions. Firstly,the research objects should be paramagnetic ions,such as Ni~(2+) or Cu~(2+); Secondly,the reaction temperature can not be too high,for the Curie temperature of nickel is 358℃.Also,if the temperature is too high,the thermal motion would be dominant in the system,so the influence of magnetic field would become weak.Lastly, the reaction should be carried out as slow as possible,and then the slow nucleation and growth of crystal would be helpful to observe the induction of magnetic field on the particles.Therefore,low-temperature solvothermal system is an optimal reaction system,and facilitates the imposition of external magnetic field.So,in a moderate low-temperature solvothermal system,Ni-PVP complex was selected.The reaction conditions involved were studied systematically to select the optimal reaction parameters to reveal the effects of magnetic fields on chemical reactions.Uniform polycrystalline nickel nanospheres were prepared in a water-ethanol mixed solvent solution using Nickel Chloride(NiCl_2),hydrazine(N_2H_4),and polyvinylpyrrolidone (PVP) as starting materials.The product exhibited good ferromagnetism,and its saturation magnetization(Ms) and coercivity(Hc) are 53 emu/g and 82 Oe, respectively.It can be deduced that 1D nickel wires with controlled shapes can be obtained by introducing a magnetic field in the same system.However,research shows that the influence of magnetic field on the reduction reaction of copper ions was not obvious,as copper is not a ferromagnetic material.
2.Based on the study of several control experiments performed without a magnetic field,1D nickel wires with controlled shapes were obtained by use of a magnetic field in the optimal reaction systems.The involved chemical reaction was taken as an example to study the effects of magnetic fields on the chemical reduction of Ni~(2+),and the nucleation and growth of nickel nanowires.The formation of our 1D nickel wires can be ascribed to the cooperative effect of the reaction rate and magnetic field.The reaction rate in the system is controlled mainly by adjusting the reactant concentration when other reaction conditions are fixed.When the concentration of Ni~(2+) is low,the reaction rate is correspondingly slow,so the complex[Ni(N_2H_4)_x]~(2+) preferentially migrates to the magnetic line of force since paramagnetic metal ions are attracted toward the maximum field.Then the chemical reduction of[Ni(N_2H_4)_x]~(2+) may occur along the magnetic line of force due to the enhanced electrode potential,leading to the formation of uniform 1 D wires.While with the increase of the concentration of Ni~(2+), the reaction rate increases too,so it can be deduced that the chemical reduction of [Ni(N_2H_4)_x]~(2+) may be taken place quickly in the whole system,leading to the formation of isolated particles coexisted with the wires composed of microspheres which formed along the magnetic line of force.Additionally,to explore well the effect of magnetic fields on the nucleation and growth of magnetic particles,a postsynthesis magnetic alignment experiment was carried out on the sample prepared without a field.However,one-dimensional wires were not found,which interprets that magnetic fields applied during the chemical reaction have influenced the nucleation and growth of nickel,instead of inducing a simple assembly of particles.Magnetic measurement results show that the nickel wires prepared with a magnetic field had remarkably improved magnetic properties,compared to that of nickel nanospheres prepared without a field.Moreover,the enhanced microwave absorption property of the nickel wires/PMMA composite at 8.5-12.5 GHz in comparison with that of nickel microspheres/PMMA composite was observed,which indicated that the anisotropic wires were not the simple assembly of particles,and the magnetic domain structures in wires had been changed by the applied field in chemical reaction.
3.Magnetic force microscopy(MFM) investigations were carried out to study the domain structures of the nickel samples,and magnetic field effects on magnetic domain structures and magnetic properties of nickel were explored.Nickel wires with a diameter of approximately 250 nm had single-domain structure,while unique core-shell cylindrical domain structures were found in wires with a diameter of 2μm. It is suggested that the unique multidomain may be aroused by the applied field in the chemical reaction.Then a schematic domain structure was used to tentatively explain the unique multidomain structure.To further support the core-shell cylindrical model, nickel film AF was prepared with the same condition as the wires,while film ZF was obtained by the same synthesis procedure as AF except without a field applied.MFM images of the AF sample show many dark circular domains,corresponding to the wires in a diameter of about 250 nm.Additionally,there are also some typical concentric annular zone domains with dark inner core and bright outer shell.While the dark spot domains of ZF are corresponding to the nanoparticles in the sample.The different magnetic properties of AF and ZF resulted from their different domains that controlled by the magnetic field in the chemical reactions.The magnetic measurements show a good agreement with our MFM results,further confirming the magnetic domain model proposed.So magnetic field in chemical reactions can be used to control magnetic domains of materials.
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