Ferromagnetic resonance in nanomagnetic metal core and noble metal shell systems

The change in the line widths in the ferromagnetic resonance (FMR) spectra of Co and Ni nanoparticles upon shell formation with noble metals like gold or silver are described. The Ni(core)Ag(shell), Co(core)Ag(shell), and CO(core)Au(shell) nanoparticles were prepared by a simple transmetallation reaction between the Co and Ni nanoparticles and the Ag+ or AuCl4- ions. It is revealed that the FMR line width decreases upon Ag shell formation whereas it increases upon core-shell composite formation with Au. Several probable explanations such as the differences in size distributions before and after the reaction or the changes occurring in shape anisotropy of the particles due to the shell formation or the different extents of electronic interaction between the core and shell materials have been offered for this observation.     

Preparation of Fe (core)/SiO2 (shell) composite particles with improved oxidation-resistance

The preparation of silica coated iron particles with improved oxidation-resistance was described in the paper. XRD, TEM and XPS were used to characterize the product, which indicated that a thin film of silica was coated on the surface of Fe particles through a silica–oxide–iron bond. The content of silica to iron in the sample was quantitatively determined by ICP. Magnetic measurements results of the particles almost kept constant before and after coating process. DTA and TG results showed that the coating shell of silica could effectively protect the Fe cores from oxidation.     

Synthesis and characterization of silica core/nano-ferrite shell particles

Core/shell particles were synthesized by assembling oppositely charged ferrite (Fe₃O₄ or NiFe₂O₄) nanoparticles on the surface of monodispersed silica core particles (having size ～0.4 μm) prepared by hydrolysis and condensation of tetraethylortosilicate. Optimal conditions for synthesis of silica core/nano-Fe₃O₄ shell particles were found at pH ～ 5.4. The obtained particles have superparamagnetic behavior above a blocking temperature of ≈25 K, which make them very attractive for a broad range of biomedical and bioengineering applications. Incorporation of nickel into ferrite structure could not be achieved at lower pH value, so functionalization of core particles was required. Incorporation of nickel into ferrite structure was successful at pH above 7, however at higher pH the formation rate of nickel–ferrite particles becomes very fast and the self-aggregation dominates the competing formation of the nickel–ferrite shell. Because of that the self-aggregation was prevented by surface modification of nickel–ferrite nanoparticles with citric acid before their deposition on the functionalized silica core and homogenous and continuous NiFe₂O₄ shell was finally obtained.     

Hollow MCM-41 microspheres derived from P(St-MMA)/MCM-41 core/shell composite particles

In soap-free latex media, poly(styrene-methyl methacrylate)/MCM-41 core/shell composite microspheres have been fabricated by adding silicate source in batches. In this process, silicate species and the surfactant micelles were self-assembled into 2-dimensional hexagonal arrangement on the surface of P(St-MMA) microspheres. Hollow MCM-41 microspheres were obtained via removing polymer core by solvent. XRD, TEM, IR and N₂ adsorption-desorption analysis were applied to characterize products. The results showed that average diameter and wall thickness of hollow MCM-41 microspheres is about 240 nm and 20 nm, respectively. Results of N₂ adsorption-desorption indicate that hollow MCM-41 microspheres possess a highly ordered mesoporous structure and a narrow pore distribution with a mean value of 2.34 nm.     

Effect of core–shell composite particles on the sintering behavior and properties of nano-Al2O3/polystyrene composite prepared by SLS

Nano-Al₂O₃ particles coated with polystyrene (PS) by emulsion polymerization were used as fillers to reinforce PS based composites prepared by selective laser sintering (SLS). The influences of the treated and untreated nanoparticles on the sintering behavior and mechanical properties of the laser sintered specimens were investigated. It was found that there were many uneven holes in the untreated composites. However, for the treated composites, due to the nanoparticle surfaces treated by emulsion polymerization, the absorbance of laser was improved and the nanoparticles dispersed well in the polymer matrix; a full dense structure was obtained and the properties were enhanced, such as the notched impact strength increased 50%, the maximum value was 12.1 kJ/m²; the tensile strength increased up to 300%, the maximum value was 31.2 MPa, comparing to the unfilled PS. FE-SEM studied the tensile fractured surfaces of the sintered specimens. It was noted that the fractured surfaces of composites with treated nanoparticles were rougher than those of unfilled PS and those of the untreated composite. Drawing from the results, it can be confirmed that a full dense structure can be obtained and the polystyrene matrix was strengthened and toughened when the nanoparticles were coated with PS by emulsion polymerization. This work forms a theoretical and technique basis for the production of selective laser sintered nano-Al₂O₃/PS functional products.     

Folic acid-functionalized magnetic ZnFe2O4 hollow microsphere core/mesoporous silica shell composite particles: Synthesis and application in drug release

A drug delivery system was designed by deliberately combining the useful functions into one entity, which was composed of magnetic ZnFe₂O₄ hollow microsphere as the core, and mesoporous silica with folic acid molecules as the outer shell. Amine groups coated magnetic ZnFe₂O₄ hollow microsphere core/mesoporous silica shell (MZHM-MSS-NH₂) composite particles were first synthesized by a one-pot direct co-condensation method. Subsequently a novel kind of folic acid-functionalized magnetic ZnFe₂O₄ hollow microsphere core/mesoporous silica shell (MZHM-MSS-NHFA) composite particles were synthesized by conjugating folic acid as targeted molecule to MZHM-MSS-NH₂. Ibuprofen, a well-known antiphlogistic drug, was used as a model drug to assess the loading and releasing behavior of the composite microspheres. The results show that the MZHM-MSS-NHFA system has the higher capacity of drug storage and good sustained drug-release property.     

Core/shell structured sSiO2/mSiO2 composite particles: The effect of the core size on oxide chemical mechanical polishing

In a typical chemical mechanical polishing (CMP) process, the type, morphology, structure, mechanical, and surface characteristics of abrasive particles play an important role in influencing the material removal process. The novel abrasive particles with special mechanical and/or tribochemical properties have been introduced into CMP processes for the improvement of surface quality and finishing efficiency. In this work, the composite particles containing solid silica (sSiO₂) cores and mesoporous silica (mSiO₂) shells were prepared via a developed Stöber method using cetyltrimethylammonium bromide as a structure-templating surfactant. The as-synthesized core/shell structured sSiO₂/mSiO₂ composite particles were characterized by powder X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and nitrogen sorption–desorption measurements. The effect of the sSiO₂ core size of the composite particles on oxide CMP performance was evaluated in terms of surface roughness and material removal rate (MRR). The root-mean-square surface roughness (0.15–0.31 nm) of the polished substrates slightly increased with increasing of the sSiO₂ core size (168–353 nm) of the composites with a comparable mSiO₂ shell thickness (16–18 nm). The sSiO₂/mSiO₂ composite particles with a relatively smaller or larger core presented a relatively high MRR for silicon oxide films. These oxide CMP results could be rationalized according to the contact area mechanism and indentation-based mechanism, incorporating the total contact area and chemical reactivity between particles and wafers, and the indentation depth of an abrasive particle onto the substrate surface.     

Free vibration response of composite sandwich cylindrical shell with flexible core

This study deals with the free vibration analysis of composite sandwich cylindrical shell with a flexible core using a higher order sandwich panel theory. The formulation uses the classical shell theory for the face sheets and an elasticity theory for the core and includes derivation of the governing equations along with the appropriate boundary conditions. The model consists of a systematic approach for the analysis of sandwich shells with a flexible core, having high-order effects caused by the nonlinearity of the in-plane and the vertical displacements of the core. The behavior is presented in terms of internal resultants and displacements in the faces, peeling and shear stresses in the face–core interface and stress and displacement field in the core. The accuracy of the solution is examined by comparing the results obtained with the analytical and numerical results published in the literatures. The parametric study is also included to investigate the effect of geometrical properties such as radius of curvature, length and sector angle of the shell.     

Toughened carbon/epoxy composites made by using core/shell particles

Toughened epoxy resin composites have been prepared by resin-transfer moulding by using a range of toughening agents. Two types of epoxy-functional preformed toughening particles were investigated and have a three-layer morphology in which the inner core is crosslinked poly(methyl methacrylate), the intermediate layer is crosslinked poly(butyl acrylate) rubber and the outer layer is a poly[(methyl methacrylate)-co-(ethyl acrylate)-co-(glycidyl methacrylate)]. The presence of glycidyl groups in the outer layer facilitates chemical reaction with the matrix epoxy resin during curing. Comparisons were made with acrylic toughening particles that have a similar structure, but which do not have the epoxy functionality in the outer shell, and with a conventional carboxy-terminated butadiene acrylonitrile (CTBN) liquid rubber toughening agent. The composites were characterised by using tensile, compression and impact testing. The fracture surfaces and sections through the moulded composites were examined by means of optical and scanning electron microscopy. Short-beam shear tests and fragmentation tests were used to investigate the interfacial properties of the composites. In general, use of the epoxy-functionalised toughening particles gave rise to superior properties compared with both the non-functionalised acrylic toughening particles and CTBN.     

Synthesis of zirconium tungstate–zirconia core–shell composite particles

In this work, ZrW₂O₈–ZrO₂ core–shell composite particles were synthesized. ZrW₂O₈ that was used in the core is a material with negative coefficient of thermal expansion, and it was synthesized from a high-pH precursor based on use of tungstic acid and zirconium acetate. Shell layer was composed of ZrO₂ nanocrystallites and precipitated from an aqueous solution by urea hydrolysis. While volume of the shell was effectively controlled by the initial zirconium ion concentration in the solutions, the rate of precipitation was a function of the ratio of initial concentrations of urea to zirconium ions. It is hypothesized that isolation of the ZrW₂O₈ within a layer of ZrO₂, will be a key element in solving problems associated with reactivity of ZrW₂O₈ towards other components in sintering of ceramic–ceramic composites with tuned or zero thermal expansion coefficient.     

Porous ternary complex metal oxide nanoparticles converted from core/shell nanoparticles

We demonstrate an easy and scalable low-temperature process to convert porous ternary complex metal oxide nanoparticles from solution-synthesized core/shell metal oxide nanoparticles by thermal annealing. The final products demonstrate superior electrochemical properties with a large capacity and high stability during fast charging/discharging cycles for potential applications as advanced lithium-ion battery (LIB) electrode materials. In addition, a new breakdown mechanism was observed on these novel electrode materials.

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Syntactic foam core metal matrix sandwich composite under bending conditions

The present work aims at characterizing a metal matrix syntactic foam core sandwich composite under three-point bending conditions. The sandwich comprises alumina hollow particle reinforced A356 alloy syntactic foam with carbon fabric skins. Crack initiation in the tensile side of the specimen causing failure of the skin, followed by rapid failure of the core in the direction applied load, is observed as the failure mechanism. Crack propagation through the alumina particles is observed in the failed specimens instead of interfacial failure. The average maximum strength, flexural strain and stiffness were measured as 91.2 ± 5.6 MPa, 0.49 ± 0.06% and 20.6 ± 0.7 GPa respectively. The collapse load is theoretically predicted using mechanics of sandwich beams. Experimental values show good agreement with theoretical predictions.     

A polymer-network gel route to oxide composite nanoparticles with core/shell structure

A modified polymer-network gel route has been developed to prepare oxide composite nanoparticles with core/shell structure. The core particles, which are prepared firstly using an acrylamide gel method, are dispersed in the solution of the shell material to form a uniform suspension, and then the suspension system is gelled by the polymerization of acrylamide. The gel is calcined to remove the organic phase, consequently leading to the coating of the shell material on the core particles. The versatility of this sol-gel method in preparing core/shell nanocomposites is evidenced by the example of MgO-coated La_0.67Ca_0.33MnO₃ system. The preparation process has been discussed in detail.     

Environmentally responsive core/shell particles via electrohydrodynamic co-jetting of fully miscible polymer solutions

Herein it is demonstrated that electrohydrodynamic co-jetting is not limited to Janus-type particles, but can also be used for the preparation of core/shell particles. Using side-by-side flow of miscible polymer solutions, electrohydrodynamic co-jetting offers an elegant and scalable route towards preparation of core/shell particles with otherwise difficult-to-prepare particle architectures, including particles with hydrophilic shell and core. Throughout this study, electrohydrodynamic co-jetting of aqueous solutions consisting of a mixture of PAAm-co-AA and PAA is used, and a range of different types of particles with distinct compartments are observed. Transition from Janus particles to core/shell particles appears to be caused by changes in the relative conductivity of the two jetting solutions. After crosslinking, the core/shell particles are stable in aqueous solution and exhibit reproducible swelling behavior while maintaining the original core/shell geometry. In addition, the pH-responsiveness of the particles is demonstrated by repeatedly switching the environmental pH between 1.3 and 12. Moreover, the core/shell particles show surprising uptake selectivity. For instance, a 450% increase in uptake of 6-carboxyfluorescein over rhodamine B base is found.     

炭化聚甲基丙烯酸甲酯／聚丙烯腈核壳聚合物制备炭纳米空心球

采用炭化聚甲基丙烯酸甲酯(PMMA)/聚丙烯腈(PAN)核壳聚合物的方法制备了炭纳米空心球.以两步无皂乳液聚合法制备了PMMA/PAN核壳粒子:首先以间歇无皂乳聚合法制备出直径约200 nm的PMMA粒子乳液,再以其作为种子乳液,以饥饿滴定法在PMMA外表而聚合一层厚度约30 nm的PAN外壳.将制备的PMMA/PAN乳液冷冻干燥后,分别经过250℃预氧化及1000℃炭化工艺,制备了炭纳米空心球.透射电镜结果显示所有核壳粒子均炭化成空心球并呈现交联状态.     

Surface-biofunctionalized multicore/shell CdTe@SiO(2) composite particles for immunofluorescence assay

Strongly fluorescent multicore/shell structured CdTe@SiO(2) composite particles of ～ 50 nm were synthesized via the reverse microemulsion method by using CdTe quantum dots co-stabilized by thioglycolic acid and thioglycerol. The optical stability of the CdTe@SiO(2) composite particles in a wide pH range, under prolonged UV irradiation in pure water, or in different types of physiological buffers was systematically investigated. Towards immunofluorescence assay, both poly(ethylene glycol) (PEG) and carboxyl residues were simultaneously grafted on the surface of the silanol-terminated CdTe@SiO(2) composite particles upon further reactions with silane reagents bearing a PEG segment and carboxyl group, respectively, in order to suppress the nonspecific interactions of the silica particles with proteins and meanwhile introduce reactive moieties to the fluorescent particles. Agarose gel electrophoresis, dynamic light scattering and conventional optical spectroscopy were combined to investigate the effectiveness of the surface modifications. Via the surface carboxyl residue, various antibodies were covalently conjugated to the fluorescent particles and the resultant fluorescent probes were used in detecting cancer cells through both direct fluorescent antibody and indirect fluorescent antibody assays, respectively.     

Antimicrobial activity of composite nanoparticles consisting of titania photocatalytic shell and nickel ferrite magnetic core

Reverse micelle and hydrolysis have been combined to synthesize composite nanoparticles consisting of anatase–titania photocatalytic shell and nickel ferrite magnetic core. The average particle size of the composite nanoparticles was in the range of 10–15 nm. The photocatalytic shell of titania is responsible for the photocatalytic and anti-microbial activity and nickel ferrite magnetic core is responsible for the magnetic behavior, studied by superconducting quantum interference device. The anatase TiO2 coated NiFe₂O₄ nanoparticles retains the magnetic characteristics of uncoated nanocrystalline nickel ferrites, superparamagnetism (absence of hysteresis, remanence and coercivity at 300 K) and non-saturation of magnetic moments at high field. The magnetic measurements results encourage their application as removable anti-microbial photocatalysts. Bacterial inactivation with UV light in the presence of titania-coated NiFe₂O₄ nanoparticles is faster than the action with UV light alone.