Thermoelectric Properties of Zn-Substituted Magnetite

Thermoelectric properties of Zn-substituted magnetite were investigated experimentally. Since Zn is incorporated in the A-site of magnetite for 0 ≤ x ≤ 0.2 in Zn _x Fe_3−x O_4−δ , electrical resistivity remained constant in this region and the thermoelectric power factor (PF) increased with Zn content. At x = 0.2, it attained 1.66 μW/K² cm at 700°C. Above x = 0.2, where Zn began to enter the B-site, the PF decreased with x.     

Uniform Hexagonal Plates of Vaterite CaCO3 Mesocrystals Formed by Biomimetic Mineralization

Vaterite mesocrystals with hexagonal morphology and uniform size have been successfully synthesized in the presence of a N‐trimethylammonium derivative of hydroxyethyl cellulose via aggregation‐mediated crystallization using a simple gas‐diffusion method. The uniform hexagonal plates display sharp facets and edges, even though they are formed by the aggregation of nanocrystals. The results demonstrate that each vaterite plate can be explained as consisting of aggregates of nanoparticles that share the same three‐dimensional orientation. A mechanism for the formation of hexagonal vaterite mesocrystals made of primary nanoparticles and hexagonal units is also presented. An understanding of the mesoscale transformation process will be helpful in controlling the aggregation‐driven formation of complex higher‐order structured materials and will provide new insights into biomineralization mechanisms. For example, the spines of sea urchins can be discussed within the framework of the mesocrystal concept. This study could provide an additional tool for designing advanced materials and could be used for the synthesis of more complex crystalline three‐dimensional structures.     

Highly Flexible and Twistable Freestanding Single Crystalline Magnetite Film with Robust Magnetism

Magnetic materials and devices that can be folded and twisted without sacrificing their functional properties are highly desirable for flexible electronic applications in wearable products and implantable systems. In this work, a high‐quality single crystalline freestanding Fe₃O₄ thin film with strong magnetism has been synthesized by pulsed laser deposition using a water‐dissolvable Sr₃Al₂O₆ sacrificial layer, and the resulting freestanding film, with magnetism confirmed at multiple length scales, is highly flexible with a bending radius as small as 7.18 µm and twist angle as large as 122°, in sharp contrast with bulk magnetite that is quite brittle. When transferred to a polydimethylsiloxane support layer, the Fe₃O₄ film can be bent with large deformation without affecting its magnetization, demonstrating its robust magnetism. The work thus offers a viable solution for flexible magnetic materials that can be utilized in a range of applications.     

Magnetite Crystal Orientation in Magnetosome Chains

One‐dimensional magnetic nanostructures have magnetic properties superior to non‐organized materials due to strong uniaxial shape anisotropy. Magnetosome chains in magnetotactic bacteria represent a biological paradigm of such magnet, where magnetite crystals synthesized in organelles called magnetosomes are arranged into linear chains. Two‐dimensional synchrotron X‐ray diffraction (XRD) is applied to cells of magnetotactic bacteria that are pre‐aligned with a magnetic field to determine the crystallographic orientation of magnetosomes relative to the chain axis. The obtained pole figure patterns reveal a [111] fiber texture along the chain direction for magnetospirilla strains MSR‐1 and AMB‐1, whereas a [100] fiber texture is measured for Desulfovibrio magneticus strain RS‐1. The [100] axis appears energetically unfavorable because it represents a magnetic hard axis in magnetite, but can be turned into an effective easy axis by particle elongation along [100] for aspect ratios higher than 1.25, consistent with aspect ratios in RS‐1 magnetosomes determined earlier. The pronounced fiber textures can be explained either by a strain‐specific biological control on crystal orientation at the chain level or by physical alignment effects due to intra‐chain magnetic interactions. In this case, biological control of the axis of elongation would be sufficient to influence the crystallographic texture of the magnetosome chain.     

国产电子级BaCO_3及TiO_2理化特性的比较研究

对比了国内外电子级ＢａＣＯ３和ＴｉＯ２的理化指标，探讨了国内材料的不足之处及其对ＰＴＣ热敏电阻器生产的影响。对国产ＢａＣＯ３的平均粒径以及ＴｉＯ２的团聚情况进行了分析，提出了两者平均粒径相匹配的要求     

BaCO_3和TiO_2的物化特性对PTC热敏电阻器性能的影响

探讨了ＢａＣＯ３与ＴｉＯ２的物化特性，诸如纯度、杂质含量、晶型、热特性、颗粒形状、粒度分布及平均粒径等对ＰＴＣ热敏电阻器性能的影响。并据此提出了ＢａＣＯ３与ＴｉＯ２的技术标准。     

电动离心喷雾干燥机在高纯碳酸钡生产中的应用及改进

为解决高纯碳酸钡生产干燥过程中金属氧化物的污染、入料量不易控制、成品水分超标等生产难题,通过对所用电动离心喷雾干燥机辅助设备、控制系统的改进及干燥状态参数的确定,使高纯碳酸钡产品质量明显提高,其中:w(Fe)≤5?06,w(H2O)≤0.1%,完全满足PTC用户的要求。生产实践证明:改进后的电动离心喷雾干燥机使用效果良好。     

Colloidal Stabilization of Calcium Carbonate Prenucleation Clusters with Silica

Calcium carbonate precipitation proceeds via a complex multistage scenario involving neutral ion clusters as precursors and amorphous phases as intermediates, which finally transform to crystals. Although the existence of stable clusters in solution prior to nucleation has been demonstrated, the molecular mechanisms by which they precipitate are still obscure. Here, direct insight into the processes that drive the transformation of individual clusters into amorphous nanoparticles is provided by progressive colloidal stabilization of different transient states in silica‐containing environments. Nucleation of calcium carbonate in the presence of silica can only take place via cluster aggregation at low pH values. At higher pH, prenucleation clusters become colloidally stabilized and cannot aggregate. Nucleation through structural reorganization within the clusters is not observed under these conditions, indicating that this pathway is blocked by kinetic and/or thermodynamic means. The degree of stabilization against nucleation is found to be sufficient to allow for a dramatic enrichment of solutions with prenucleation clusters and enable their isolation into the dry state. This approach renders direct analyses of the clusters by conventional techniques possible and is thus likely to facilitate deeper insight into the chemistry and structure of these elusive species in the future.     

Synergy of Torsion Strained and Ligand Effect for Relay Acceleration of Industrial High-pH Hydrogen Evolution

Noble metal Pt-based catalysts have slow water dissociation kinetics at high pH conditions, making it difficult for water molecules to be electrochemically activated. Utilizing ligand effect and strain effect to tailor catalytic active sites is a common method, while the understanding of mechanism of their interaction remains obscure due to the complexity of the process. This study proposes a pulse-induced torsional strained PtRu mesocrystals (PtRu MCs) with 20 times higher mass activity than commercial Pt/C. The combination of experimental results and theoretical calculations reveals that the ligand effect induced by Ru doping accelerates the kinetics of the water dissociation reaction, while the pulse-induced torsion strained dominates the thermodynamic optimization of the hydrogen adsorption reaction. The structure-activity relationship defined by the synergistic effect under the complementary advantages of the strain and doping provides guidance for the design of future basic hydrogen evolution catalysts. The catalyst can run stably at 1 A cm⁻² for 500 h, showing potential for industrial application.     

(Ba,Co,Nb)掺杂SnO2压敏材料电学非线性的研究

通过对样品的伏安特性，晶界势垒的测量和分析，研究了BaCO3对新型(Co，Nb)掺杂SnO2压敏材料微观结构和电学性质的影响。晶界势垒高度测量表明，SnO2晶粒尺寸的迅速减小是压敏电压急剧增高的原因。对Ba含量增加引起SnO2晶粒减小的根源进行了解释。掺杂x(BaCO3)=0．4％的SnO2压敏电阻击穿电压为最小(140V／mm)；掺杂x(BaCO3)-0．8％的SnO2压敏电阻具有最高非线性系数(α=19．6)，最高的势垒电压(ψB=1．28eV)。     

Structural Basis for Metastability in Amorphous Calcium Barium Carbonate (ACBC)

Metastable amorphous precursors are emerging as valuable intermediates for the synthesis of materials with compositions and structures far from equilibrium. Recently, it was found that amorphous calcium barium carbonate (ACBC) can be converted into highly barium‐substituted “balcite,” a metastable high temperature modification of calcite with exceptional hardness. A systematic analysis ACBC (Ca_1‐xBa_xCO₃·1.2H₂O) in the range from x = 0–0.5 is presented. Combining techniques that independently probe the local environment from the perspective of calcium, barium, and carbonate ions, with total X‐ray scattering and a new molecular dynamics/density functional theory simulations approach, provides a holistic picture of ACBC structure as a function of composition. With increasing barium content, ACBC becomes more ordered at short and medium range, and increasingly similar to crystalline balcite, without developing long‐range order. This is not accompanied by a change in the water content and does not carry a significant energy penalty, but is associated with differences in cation coordination resulting from changing carbonate anion orientation. Therefore, the local order imprinted in ACBC may increasingly lower the kinetic barrier to subsequent transformations as it becomes more pronounced. This pathway offers clues to the design of metastable materials by tuning coordination numbers in the amorphous solid state.     

国产电子级碳酸钡理化性能的比较研究

采用粒度分析、数码显微分析以及AAS分析等手段,比较了国产电子级碳酸钡从发展初期至近年,七家具有代表性的生产厂家样品理化性能的演变情况。结果表明:碳酸钡样品的形貌、分散性以及酸根含量的变化情况不容忽视。样品理化性能的波动,特别是比表面积以及硝酸根含量的波动较大,必须引起粉体生产厂家以及用户的高度重视。建议将比表面积以及硝酸根含量增加为产品的理化指标,并对电子级碳酸钡产品的质量加强控制。     

B位Sn4+取代铌锌酸钡陶瓷及其介电性能

向铌锌酸钡中添加适量的锡酸钡,使Sn4+取代B位的Zn以改善其介电性能。锡酸钡为立方钙钛矿结构,正值温度系数τC约为+190×10–6℃–1。锡离子的引入会增大钙钛矿晶胞中B位阳离子的平均半径,致使氧八面体阴阳离子间的空隙减小,降低晶格的非简谐相互作用,造成衰减因子γ减小,tgδ降低。当锡酸钡添加量(摩尔分数)为0.226或0.32时,其τC接近于0,τf位于(–5～0)×10–6℃–1。     

Ultrabright Fluorescent Silica Mesoporous Silica Nanoparticles: Control of Particle Size and Dye Loading

The synthesis of ultrabright fluorescent mesoporous silica nanoparticles (UFSNPs) of various sizes loaded with different amounts of fluorescent dye (Rhodamine 6G) is reported here. The dye is physically entrapped inside the nanochannels of the silica matrix created during templated sol–gel self assembly. Due to the specific nanoenvironment, the fluorescence of the encapsulated dye molecules remains unquenched up to very high concentrations, which results in relatively high fluorescence. The particle size (ranging from 20–50 nm) and dye loading (0.8–9.3 mg dye per g particles) are controlled by the timing of the synthesis and the concentration of several organotriethoxysilanes, which are coprecursors of silica. The quantum yields of the encapsulated dye range from 0.65 to 1.0. The relative brightness of a single particle is equivalent to the fluorescence of 30–770 free nondimerized R6G dye molecules in water, or to that of 1.5–39 CdSe/ZnS quantum dots. Despite the presence of some hydrophobic groups on the particles' surfaces, colloidal suspensions of the particles are relatively stable (as monitored for 120 days).     

钛酸钡纳米粉体的共沉淀法合成与研究

采用TiCl4和BaCl2·2H2O原料,以正丁醇为分散剂,NH4HCO3和NH3·H2O作为沉淀剂合成钛酸钡前驱体,在900 ℃煅烧制备分散性良好的钛酸钡纳米粉体.利用XRD、透射电镜(TEM)和 SEM等手段分析了反应温度、TiCl4浓度、分散剂掺杂量等反应参数对粉体的晶相组成、晶粒度、形貌等的影响,并且测试了相应陶瓷烧结体的介电常数.结果表明,反应温度为900 ℃,TiCl4浓度为0.6 mol/L,分散剂用量为3‰条件下,保温2 h可制备高分散性的纳米级粉体.用以上方法制备的粉体烧结而成的陶瓷片介电常数约为3 400.     

Biosynthesis of Zinc Substituted Magnetite Nanoparticles with Enhanced Magnetic Properties

The magnetic moments of magnetite nanoparticles are dramatically enhanced through the addition of zinc in a microbiologically driven synthesis procedure. The particles are produced through the reduction of Fe(III)‐compounds containing Zn(II) by the iron reducing bacterium Geobacter sulfurreducens. Results indicate a significant increase in the saturation magnetization by over 50% compared to magnetite at both room and low temperatures for relatively minor quantities of zinc substitution. A maximum saturation magnetization of nearly 100 emu g^?1 of sample is measured at room temperature. Analysis of the cation site ordering reveals a complex dependence on the Zn content, with the combined effect of Zn substitution of Fe³⁺ ions on tetrahedral sites, together with Fe²⁺ cation oxidation, leading to the observed magnetization enhancement for low Zn doping levels. The improved magnetic properties give superior performance in MRI applications with an MRI contrast enhancement among the largest values reported, being more than 5 times larger than a commercial contrast agent (Feridex) measured under identical conditions. The synthesis technique applied here involves an environmentally benign route and offers the potential to tune the magnetic properties of magnetic nanoparticles, with increased overall magnetization desirable for many different commercial applications.     

镧掺杂钡铁氧体-聚苯胺复合材料的制备与表征

采用溶胶凝胶法制备了镧掺杂钡铁氧体（BaLaxFe12-xO19,x=0、0.01、0.03、0.05）。通过XRD表征表明它们的结晶性良好,均为单一的磁铅石型钡铁氧体晶相结构,随镧掺杂量的增加,得到的镧掺杂钡铁氧体的粒径随之减小。采用溶液原位聚合法制备镧掺杂钡铁氧体-聚苯胺复合材料,红外光谱分析可知聚苯胺-铁氧体纳米复合物的生成,揭示了聚合物分子链与铁氧体纳米粒子之间存在一定的键合作用,TEM表征显示复合材料粒子呈球形,且粒径为60～80 nm左右。随着铁氧体含量的降低,样品的磁性能随之降低,即铁氧体含量是影响磁性能的主要因素。     

Protein‐Mediated Synthesis of Uniform Superparamagnetic Magnetite Nanocrystals

Magnetite nanocrystals are synthesized in the presence of a recombinant Mms6 protein thought to be involved in the biomineralization of bacterial magnetite magnetosomes, the mammalian iron‐storage protein, ferritin, and two proteins not known to bind iron, lipocalin (Lcn2) and bovine serum albumin (BSA). To mimic the conditions at which magnetite nanocrystals are formed in magnetotactic bacteria, magnetite synthesis is performed in a polymeric gel to slow down the diffusion rates of the reagents. Recombinant Mms6 facilitates formation of ca. 30 nm single‐domain, uniform magnetite nanocrystals in solution, as verified by using transmission electron microscopy analysis and magnetization measurements. The nanocrystals formed in the presence of ferritin, Lcn2, and BSA, do not exhibit the uniform sizes and shapes observed for those produced in the presence of Mms6. Mms6‐derived magnetite nanoparticles show the largest magnetization values above the blocking temperature, as well as the largest magnetic susceptibility compared to those of the nanomaterials synthesized with other proteins. The latter is indicative of a substantial effective magnetic moment per particle, which is consistent with the presence of magnetite with a well‐defined crystalline structure. The combination of electron microscopy analysis and magnetic measurements confirms our hypothesis that Mms6 promotes the shape‐selective formation of uniform superparamagnetic nanocrystals. This provides a unique bioinspired route for synthesis of uniform magnetite nanocrystals.     

Fabrication and Electromechanical Characterization of a Piezoelectric Structural Fiber for Multifunctional Composites

The use of piezoceramic materials for structural sensing and actuation is a fairly well developed practice that has found use in a wide variety of applications. However, just as advanced composites offer numerous benefits over traditional engineering materials for structural design, actuators that utilize the active properties of piezoelectric fibers can improve upon many of the limitations encountered when using monolithic piezoceramic devices. Several new piezoelectric fiber composites have been developed; however, almost all studies have implemented these devices such that they are surface‐bonded patches used for sensing or actuation. This paper will introduce a novel active piezoelectric structural fiber that can be laid up in a composite material to perform sensing and actuation, in addition to providing load bearing functionality. The sensing and actuation aspects of this multifunctional material will allow composites to be designed with numerous embedded functions, including structural health monitoring, power generation, vibration sensing and control, damping, and shape control through anisotropic actuation. This effort has developed a set of manufacturing techniques to fabricate the multifunctional fiber using a SiC fiber core and a BaTiO₃ piezoelectric shell. The electromechanical coupling of the fiber is characterized using an atomic force microscope for various aspect ratios and is compared to predictions made using finite element modeling in ABAQUS. The results show good agreement between the finite element analysis model and indicate that the fibers could have coupling values as high as 68% of the active constituent used.     

石英半球型微谐振器的仿真与加工

该文对石英半球型微谐振器进行了模态仿真,分析了前十阶振型及适合陀螺工作的模态,讨论了谐振器壁厚变化对四波腹模态谐振频率的影响。利用吹玻璃法和湿法腐蚀制作了石英半球型微谐振器,跟踪不同的腐蚀速率得出湿法腐蚀可精确控制壁厚的结论。测试腐蚀后的谐振器表面粗糙度仅为0.581nm,保留了原子级别的光滑度。