Sintering Processing of Complex Magnetic Ceramic Oxides: A Comparison Between Sintering of Bottom-Up Approach Synthesis and Mechanochemical Process of Top-Down Approach Synthesis

Sintering is a common synthesis method for the fabrication of ceramics. The widespread use of sintering for the production of complex ceramic oxide especially ferrites has led to a variety of investigations on the subject. Top-down approach synthesis like mechanochemical process has recently been suggested as a promising synthesis method for replacing bottom-up approach synthesis methods like sintering, questioning its necessity for thermal treatment at high temperature. Understanding of sintering mechanism is crucial in order to optimize and enhance the advantages of sintering, which cannot be replaced by other techniques. In general, ferrites with particular set of behaviors require a particular set of microstructural properties influenced by the sintering steps. The main objective of this chapter is to understand how the increase of sintering temperature affects the microstructural evolution, in order to develop a fundamental science understanding for the mechanism of sintering. In the second part of this chapter, presentation of experimental results on sintering of mechanically activated Ni0.5Zn0.5Fe2O4 nanoparticles and its effect on microstructural, magnetic, and optical properties was reported. Lastly, a comparative study between sintering (bottom-up approach) and mechanochemical (top-down approach) process is presented

Structural transformations of mechanically induced top-down approach BaFe12O19 nanoparticles synthesized from high crystallinity bulk materials

In this work, a top-down approach was applied to high crystallinity BaFe12O19 bulks, breaking them into smaller nanoparticles by mechanochemical route. The effects of milling time, reaction mechanisms and structural information were investigated. Interestingly, three distinct stages of the mechanochemical mechanism were observed. The XRD results indicated that the BaFe12O19 phase existed even though the mechanical energy had induced the formation of an amorphous phase in the material. The average crystallite size decreased during the first stage and the intermediate stage, and increased during the final stage of the mechanical alloying. A Rietveld refinement analysis suggested the deformation of a mechanically-triggered polyhedral in the magnetoplumbite structure. FESEM micrographs indicated that fragmentation predominated during the first and intermediate stages, until a steady equilibrium state was achieved at in the final stage, where a narrow particle size distribution was observed. HRTEM micrographs suggested the formation of a non-uniform nanostructure shell surrounding the ordered core materials at the edge-interface region. The thickness of the amorphous surface layer extended up to 12 nm during the first and intermediate stages, and diminished to approximately 3 nm after 20 h milling. VSM results showed a mixture of ferromagnetic, superparamagnetic, and paramagnetic behaviours. However, different magnetic behaviours predominated at different milling time, which strongly related to the defects, distorted polyhedra, and non-equilibrium amorphous layers of the material

Development of magnetic B-H hysteresis loops through stages of microstructure evolution of bulk BaFe12O19

A series of polycrystalline BaFe12O19 bulk samples produced have been used to investigate the parallel evolving relationship between the microstructural and magnetic properties of the magnetic material. The raw material starting powders were prepared via the mechanical alloying method and subsequently moulded and pressed by using a hydraulic pressing machine. The nanosized samples were sintered from 700 to 1300 ∘C with 100 ∘C increments in static air conditions. The sintered samples were characterized and analysed with increasing sintering temperature to study their crystallinity, microstructural and magnetic properties. The result shows the magnetic B-H hysteresis loop varies with grain size, which was attributed to the increasing crystallinity and changing magnetic parameters. The microstructural properties like grain size were measured by using FeSEM, the phase analysis of the material was analysed with X-ray diffractometry (XRD) and density was determined by a densimeter, while the hysteresis loop was measured by a B-H tracer and Curie temperature was measured with an impedance analyser. The characterized samples were found to be divided into three groups which consisted of grain size varied from 0.30 to 0.39 μm (paramagnetic state), 0.46 to 0.94 μm (intermediate ferromagnetic state) and ≥1.78 μm (strong ferromagnetic state). For the latter two groups, slanting sigmoid hysteresis loops were observed. Well-defined sigmoid-shaped B-H hysteresis loops were obtained when high crystallinity was attained, allowing strong exchange interaction between neighbouring atomic magnetic moments. This indicated that the ferromagnetic state had been achieved. The observed three B-H hysteresis groups exhibited different magnetic properties, and these behaviours are useful for industrial applications

Magnetic phase transition of mechanically alloyed single sample Co0.5Ni0.5Fe2O4

The parallel evolutional relationship between microstructural properties and magnetic and electrical properties was elucidated through this study. A Co0.5Ni0.5Fe2O4 rod sample was prepared via high energy ball milling and subsequent moulding into a nano-sized compacted powder. This single sample was sintered through 10 cycles at different sintering temperatures in the range of 500 °C–1400 °C. After each sintering, the sample was characterized for its phase, microstructural, density, magnetic and electrical properties using XRD, SEM, B-H tracer, Curie temperature measurement and two probes method. An integrated study of microstructural properties with elevating sintering temperature would point to the existence of three stages of sintering, which involved atomic, interfaces (lattice and boundaries), and volume diffusions respectively. Three distinct shape-differentiated groups of B–H hysteresis loops were observed. The existence of these groups was associated with microstructural properties such as phase purity, volume fraction of disordered phase or grain boundaries, and grain size. In terms of average grain size, from 48.25 nm to 71.93 nm, a weak paramagnetic behaviour was observed; while from 83.65 nm to 374.79 nm, a relatively square-shaped hysteresis loops with moderate ferromagnetic behaviours were observed. The occurrences of erect and well-defined sigmoid-shape were observable when there were sufficiently high single-phase purity and crystallinity, where the average grain size was in the range of 964.73 nm–11215.91 nm. The critical grain size of 186.75 nm was found by plotting average grain size against coercivity, suggesting the number of single-domain particles was reduced, and the number of multi-domain particles was increased by increasing sintering temperature. The electrical resistivity variations were strongly related to the microstructural properties

DDX3X loss is an adverse prognostic marker in diffuse large B-cell lymphoma and is associated with chemoresistance in aggressive non-Hodgkin lymphoma subtypes.

Funder: addenbrooke's charitable trust, cambridge university hospital

Microstructural, optical and magnetic properties of barium hexaferrite and nickel zinc ferrite synthesized via mechanochemical procedure

Mechanochemical process is a powder processing technique that utilises mechanical energy to grind down bulk materials. Mechanochemical process has received a lot of interest for producing technologically important ferrites because it is a solvent-free technique and hence green process. Throughout the centuries, the applications of mechanochemical process are limited to diminution of particles because the lack of systematic studies on the process mechanisms of mechanochemical process. The immediate objective of this research is devoted to this subject by developing a systematic study on top-down approach mechanochemical process (referring to the production of nanoparticles by mechanochemical process) and mechanochemical activation-based synthesis (referring to mechanochemical process, used to activate the starting powders, before a sintering step to induce the formation of final product). For top-down approach mechanochemical process, starting bulk materials were mechanically treated for different milling time ranging from 1 to 20 hours at room temperature, for the preparation of nanoparticles. Evidence of the presence of single phase ferrites was identified by XRD. Rietveld refinement analysis suggested the deformation of a mechanically triggered polyhedral in the magnetoplumbite structure of BaFe12O19 and spinel structure Ni0.5Zn0.5Fe2O4. Three distinct stages of the mechanochemical mechanism were observed when the milling time was extended. The average crystallite size decreased at different rate during the first stage and the intermediate stage, and increased during the final stage of the mechanochemical process. FESEM micrographs showed the particle size decreased from 432.96 nm to 81.43 nm for BaFe12O19 and 371.68 nm to 158.49 nm for Ni0.5Zn0.5Fe2O4 during the first stage and the intermediate stage. In the final stage, particle size increased to 134.15 nm for BaFe12O19 and 193.60 nm for Ni0.5Zn0.5Fe2O4. HRTEM micrographs suggested the formation of a non-uniform nanostructure shell surrounding the ordered core materials. The thickness of the shell extended up to 12 nm during the first and intermediate stages, and diminished to approximately 3 nm during final stage. VSM results showed a mixture of ferromagnetic, superparamagnetic, and paramagnetic behaviours attributed to the defects, distorted polyhedra, and non-equilibrium amorphous layers induced by the mechanical energy. The observed spectral shift from UV-Vis spectra was ascribed tothe competition between quantum confinement effects and structural disorder bandgap narrowing effect. For mechanochemical activationbased synthesis, mechanochemical process on the starting powders and subsequent sintering was carried out to synthesize BaFe12O19 and Ni0.5Zn0.5Fe2O4 nanoparticles. The XRD results indicated an improvement of crystallinity with increasing sintering temperature. Single phase ferrites were observed at 1100 ℃ for BaFe12O19 and 700 ℃ for Ni0.5Zn0.5Fe2O4. FESEM micrographs showed the particle size increased from 42.24 nm to 913.96 nm for BaFe12O19 and 66.39 nm to 1084.27 nm for Ni0.5Zn0.5Fe2O4 when sintering temperature were elevated from 600 ℃ to 1200 ℃. Morphological studies showed three stages of sintering with distinct microstructure features. By sintering from 600 ℃ to 1200 ℃, a dependence of magnetic properties on sintering temperature was found. Maximum magnetization at 10 kOe improved with elevating sintering temperature. The optical bandgap values decreased with increasing crystallite size, showing the dominancy of quantum confinement effects. It can be concluded top-down approach mechanochemical process is capable of producing single phase nanoparticles; and mechanochemical activation-based synthesis has significantly reduced the sintering temperature required for the formation of final product. The systematic studies on the process mechanisms of top-down approach mechanochemical process and mechanochemical activation-based synthesis developed a fundamental knowledge to tailor nanoparticles with specific properties according to its possible industrial applications

Diversity and phylogeny of Sargassum (Fucales, Phaeophyceae) in Singapore

Sargassum species play key ecological roles on coral reefs, yet their diversity remains poorly known. Precise identification of Sargassum species, however, is improving with molecular genetic tools, though these have yet to be applied rigorously in Singapore. Historical records list 41 species, but no more than ten were verified based on herbarium vouchers, and even fewer (five species) were confirmed in the field based on a single nuclear gene marker in a previous study. Here, we revised the diversity of Sargassum in Singapore by examining all the morphologically distinct forms collected from the local coral reef environment. A total of six morphotypes, Sargassum aquifolium (Turner) C.Argardh (1820), S. cf. granuliferum C.Argardh (1820), S. ilicifolium (Turner) C.Argardh (1820), S. swartzii C.Argardh (1820), S. polycystum C.Argardh (1824), and an undescribed taxon ‘Sargassum sp.’ (Mattio and Payri 2009), were delineated based on morphological characteristics. The morphotypes were placed in five molecular clades based on phylogenetic analyses of the nuclear ITS-2, chloroplastic partial RuBisCO operon rbcLS, and mitochondrial cox3. Sargassum cf. granuliferum, though morphologically distinct from all other species, is not phylogenetically distinct from S. polycystum. Our results provide a species list for Singapore that will be valuable for future studies on macroalgal biogeography and species-specific ecological relationships with other reef organisms, particularly corals

Magnetic phase transition of mechanically alloyed single sample Co0.5Ni0.5Fe2O4

The parallel evolutional relationship between microstructural properties and magnetic and electrical properties was elucidated through this study. A Co0.5Ni0.5Fe2O4 rod sample was prepared via high energy ball milling and subsequent moulding into a nano-sized compacted powder. This single sample was sintered through 10 cycles at different sintering temperatures in the range of 500 °C–1400 °C. After each sintering, the sample was characterized for its phase, microstructural, density, magnetic and electrical properties using XRD, SEM, B-H tracer, Curie temperature measurement and two probes method. An integrated study of microstructural properties with elevating sintering temperature would point to the existence of three stages of sintering, which involved atomic, interfaces (lattice and boundaries), and volume diffusions respectively. Three distinct shape-differentiated groups of B–H hysteresis loops were observed. The existence of these groups was associated with microstructural properties such as phase purity, volume fraction of disordered phase or grain boundaries, and grain size. In terms of average grain size, from 48.25 nm to 71.93 nm, a weak paramagnetic behaviour was observed; while from 83.65 nm to 374.79 nm, a relatively square-shaped hysteresis loops with moderate ferromagnetic behaviours were observed. The occurrences of erect and well-defined sigmoid-shape were observable when there were sufficiently high single-phase purity and crystallinity, where the average grain size was in the range of 964.73 nm–11215.91 nm. The critical grain size of 186.75 nm was found by plotting average grain size against coercivity, suggesting the number of single-domain particles was reduced, and the number of multi-domain particles was increased by increasing sintering temperature. The electrical resistivity variations were strongly related to the microstructural properties