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spp1997新虫 (初入文坛)
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In glass technology crystallization must usually be avoided; however, partially crystallized glasses or ‘glass-ceramics’, containing fine crystallized particles (nanoparticles), have been developed for specific applications. These nanoparticles confer particular characteristics to glasses, such as strength and impact resistance, lower thermal expansion coefficient, partial or complete opacity, etc [1]. A special type of nanoparticles, ferro- or ferrimagnetically ordered crystalline nanoparticles imbedded in different diamagnetic matrices such as fluids or polymers, gives rise to a new kind of magnetism called superparamagnetism and results in a number of properties which are interesting from a technical viewpoint, for example see [2]. Partially crystallized glasses containing ferromagnetic nanoparticles constitute a novel and much more complex class of superparamagnetic materials. Their properties are mainly determined by morphology (size and shape distributions) and magnetic constants of such particles. Electron magnetic resonance, sensitive to both the magnetic constants and the structure of the environment of a magnetic ion, is well adapted to the study of superparamagnetic nanoparticles [3]. In this case it may be called superparamagnetic resonance (SPR) to distinguish it from the resonance of individual electron spins usually referred to as electron paramagnetic resonance (EPR). Up until now, only a few attempts have been made to carry out quantitative, computer-simulation based analysis of SPR spectra of magnetic particles dispersed in polycrystalline or glassy systems [3–5]. In a previous work [3] we have dealt with transformations arising in the electron magnetic resonance spectra of borate glasses containing small amounts of iron oxide after heat treatment above the glass transition temperature. Namely, the resonance characteristic of diluted Fe3+ ions in the glass, observed at the effective g-value gef = 4.3 [6], progressively disappears and a new resonance, consisting of superposed broader and narrower components, grows at gef ≈ 2.0. This compound resonance signal has been ascribed to a superparamagnetic assembly of crystalline ferromagnetic single-domain particles arising in the diamagnetic glassy matrix under heat treatment. Good computer fits to this resonance have been obtained assuming a monodisperse distribution of the particle sizes. This paper presents a more detailed analysis of room temperature X-band SPR spectra obtained after repeated annealing steps of iron-doped borate glass. First, data are reported on the evolution of the number of spins with annealing, as obtained from resonance intensity measurements. Then, geometrical characteristics of the magnetic particles obtained from a series of computer simulations of the spectra are related to the anneal temperature. Finally, the mechanism of growth of crystallized magnetic particles is discussed. |
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jiangguofeng
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【答案】应助回帖
| 在玻璃工艺当中通常必须避免结晶；然而,现已为专门的应用研制出了含有细小结晶粒子（纳米粒子）的部分结晶的玻璃或"玻璃陶瓷"。这些纳米粒子赋予玻璃一些特殊性能,例如，强度和耐冲击性,降低热膨胀系数,部分或全部不透明性,等等[1]。铁磁或亚铁磁的有序结晶纳米粒子是一种特殊类型的纳米粒子,被包埋在流体或高聚体一类的不同抗磁基质中，产生一种称为超顺磁性的新的磁性并带来了从技术视角来看值得关注的多种性质，具体详见[2]。含有铁磁纳米粒子的部分结晶玻璃构成了一种新颖的更加复杂的超顺磁性材料类型。 它们性质主要是通过此类粒子的形态（大小与形状分配）和磁常数来确定。电子磁共振对磁常数和磁离子环境结构都敏感，非常适于超顺磁性纳米粒子的研究[3]。在这种情况下，为了与通常称为电子顺磁共振(EPR)的单个电子自旋相区别，这可以被叫做超顺磁共振(SPR)。迄今为止，只有少数尝试去进行基于分散于多晶或玻璃状系统中的磁粉的SPR谱图的定量分析和计算机模拟[3-5]。在以前的研究工作中[3]，我们对经高于玻璃态转化温度热处理后的含少量氧化铁的硼酸盐玻璃的电子磁共振谱图中产生的改变进行了处理。 也就是,于有效G值gef=4.3观察到的掺入这种玻璃中的Fe3+离子的共振特性[6]逐渐地消失，于gef≈2.0处逐渐产生一种由重叠的更宽和更窄组分组成的新的共振。 这种化合物共振信号已分配给热处理进行时抗磁玻璃基质中产生的结晶铁磁单畴颗粒的一种超顺磁组装。已经得到针对这种共振的良好计算机配置用以担任粒子粒径的单分散分配。本文给出掺铁硼酸盐玻璃重复退火步骤后得到的室温X波段SPR谱图更为详细的分析。首先,报告了由共振强度测定得到的有关随着退火自旋数目演变的情况。然后,由谱图的一系列计算机模拟得到的磁粉的几何特性与退火温度有关。最后,讨论了结晶磁粉的生长机理。 |
2楼2015-06-18 11:56:15