Synthesis and Characterization of Magnetically Retrievable Fe3O4/Polyvinylpyrrolidone/Polystyrene Nanocomposite Catalyst for Efficient Catalytic Oxidation Degradation of Dyes Pollutants

Recently, the application of metal oxides such as Fe₃O₄ nanoparticles have wide interest for environmental remediation and treatment of wastewater especially contaminated with azo dyes owing to its high degradation efficacy and low toxicity. The recovery of magnetic catalysts without losing their efficiency is an essential feature in the catalytic applications. The aim of this article is to investigate and synthesis of magnetically retrievable Fe₃O₄/polyvinylpyrrolidone/polystyrene (Fe₃O₄/PVP/PS) nanocomposite for the catalytic degradation of azo dye acid red 18 (AR18). Fe₃O₄/PVP/PS nanocomposite was prepared in two steps. Firstly, PVP/PS microsphere was synthesized by γ-irradiation polymerization of styrene in presence of PVP solution. Secondly, deposition of Fe₃O₄ nanoparticles on PVP/PS microsphere was achieved by the alkaline co-precipitation of Fe³⁺/Fe²⁺ ions. The chemical structural and morphological properties of PVP/PS microsphere and Fe₃O₄/PVP/PS nanocomposite were examined by XRD, TEM, DLS, FTIR, EDX and VSM techniques. TEM results showed homogeneous morphology, spherical shaped and well-dispersed Fe₃O₄ nanoparticles with average particle size of 26 nm around PVP/PS microspheres. The VSM measurements of Fe₃O₄/PVP/PS nanocomposite exhibit excellent magnetic response of saturation magnetization 26.38 emu/g which is suitable in magnetic separation. The effect of the synthesized Fe₃O₄/PVP/PS nanocomposite on the catalytic degradation of AR18 in presence of hydrogen peroxide (H₂O₂) as a heterogeneous Fenton-like catalyst was examined. The catalyst Fe₃O₄/PVP/PS/H₂O₂ played basic role in promoting the oxidation degradation efficiency of AR18 of initial concentration 50 mg/L to 94.4% in 45 min with excellent recyclability till the sixth cycles under the best conditions of pH 3, 2% v/v H₂O₂ and 0.3 g catalyst amount. Furthermore, the Fe₃O₄/PVP/PS/H₂O₂ hybrid catalyst system supports high capability for oxidation degradation of mixture of different dyes. The Fe₃O₄/PVP/PS nanocomposite catalyst had high magnetic and recyclability characters which are acceptable for the treatment of wastewater contaminated by various dyes pollutants.

     

Cr(III)‐containing Fe3O4/mercaptopropanoic acid‐poly(2‐hydroxyethyl acrylate) nanocomposite: Highly active magnetic catalyst for direct hydroxylation of benzene

In this study, polymer‐grafted magnetic nanoparticles containing chromium(III) ions incorporated onto Fe₃O₄/mercaptopropanoic acid‐poly(2‐hydroxyethyl acrylate) was prepared via a simple and in situ method. The obtained magnetic nanocomposite exhibited high catalytic activity and excellent selectivity in direct hydroxylation of benzene in the presence of hydrogen peroxide under solvent‐free condition. The magnetic catalyst could be also separated by an external magnet and reused seven times without any significant loss of activity/selectivity. Due to the Lewis acidity of the Fe³⁺ groups in the structure of magnetic nanoparticles, the high efficiency of this catalyst is possibly due to the synergetic effect of Cr³⁺ and Fe³⁺ groups in the structure of magnetic nanocomposite. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40383.     

Structure and magnetic properties of regenerated cellulose/Fe3O4 nanocomposite films

Cellulose nanocomposites containing high contents of Fe₃O₄ nanoparticles were successfully prepared with regenerated cellulose films as a matrix and mixture solutions of Fe²⁺/Fe³⁺ as precursors. The structure and properties of the magnetic nanocomposite films were investigated with X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and vibrating sample magnetometry. Fe₃O₄ nanoparticles as prepared were irregular spheres and were homogeneously dispersed in the cellulose matrix. With an increase in the concentration of precursors from 0.2 to 1.0 mol/L, the content of Fe₃O₄ nanoparticles in the dried nanocomposites increased from 12 to 39 wt %, and the particle diameter increased from 32 to 64 nm. The cellulose nanocomposite films demonstrated superparamagnetic behavior, and their saturation magnetizations were in the range 4.2–21.2 emu/g, which were related to the increase in Fe₃O₄ nanoparticle content. With increasing nanophase content, the nanocomposite films displayed significantly anisotropic magnetic properties in the parallel and perpendicular directions. This study provided a green and facile method for the preparation of biobased nanocomposite films with high nanophase content and excellent magnetic properties. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Palladium nanoparticles supported on Fe3O4/amino acid nanocomposite: Highly active magnetic catalyst for solvent-free aerobic oxidation of alcohols

In this paper, Fe₃O₄ nanoparticles were coated by a number of amino acids, e.g. cysteine, serine, glycine and β-alanine, via a simple method. Because of the surface modification of the magnetic nanoparticles with amino acid, the obtained magnetic nanocomposite is able to trap palladium nanoparticles through a strong interaction between the metal nanoparticles and the functional groups of amino acids. Among the synthesized nanocomposites, Fe₃O₄/cysteine-Pd exhibited the highest catalytic performance and excellent selectivity in the solvent-free aerobic oxidation of various alcohols, along with high level of reusability.     

Synthesis of nanofibrous chitosan/Fe3O4/TiO2@TiO2 microspheres with enhanced biocompatibility and antibacterial property

Multifunctional materials have received considerable attention as they could integrate different functional components in one-single platform. In this study, novel chitosan/Fe₃O₄/TiO₂@TiO₂ nanowire (NW) microspheres having extracellular matrix-like fibrous surface and photothermal antibacterial property were synthesized through in situ hydrothermal growth of TiO₂ NWs on chitosan/Fe₃O₄/TiO₂ microspheres. It is found that the microspheres were spherical in morphology with a diameter of 100–300 µm and exhibited a hierarchical and nanofibrous feature. Their surface was mainly constructed by numerous TiO₂ NWs with a diameter of 20– 30nm. In vitro biological evaluation indicates that the chitosan/Fe₃O₄/TiO₂@TiO₂ NW microspheres significantly enhanced attachment and proliferation of human umbilical vein endothelial cells compared with chitosan/Fe₃O₄/TiO₂ nanocomposite microspheres due to the presence of nanofibrous surface. Moreover, the microspheres showed photothermal antibacterial property to inhibit the growth of bacteria due to the presence of Fe₃O₄ component.     

Preparation of high-magnetization Fe3O4–NH2–Pd (0) catalyst for Heck reaction

A magnetically separable Fe₃O₄–NH₂–Pd (0) catalyst was easily synthesized by immobilizing Pd nanoparticles on the surface of magnetic Fe₃O₄–NH₂ microspheres. It was found that the combination of Fe₃O₄ and triethylene tetramine (TETA) could give rise to structurally stable catalytic sites. Furthermore, the high-magnetization Fe₃O₄–NH₂–Pd(0) catalyst can be recovered by magnet and reused for six runs for Heck reaction without significant loss in catalytic activity.     

Facile fabrication of nanocomposite microspheres with polymer cores and magnetic shells by Pickering suspension polymerization

Pickering suspension polymerization was used to prepare magnetic polymer microspheres that have polymer cores enveloped by shells of magnetic nanoparticles. Styrene was emulsified in an aqueous dispersion of Fe₃O₄ nanoparticles using a high shear. The resultant Pickering oil-in-water (o/w) emulsion stabilized solely by magnetic nanoparticles was easily polymerized at 70 °C without stirring. Fe₃O₄ nanoparticles act as effective stabilizers during polymerization and as building blocks for creating the organic–inorganic hybrid nanocomposite after polymerization. The fabricated magnetic nanocomposites were characterized by FTIR, XRD, TGA, DSC, GPC, XPS and SEM. The structures of the polymer core and the nanoparticle shell were analyzed. We investigated the effects on the products of the weight of Fe₃O₄ nanoparticles used to stabilize the original Pickering emulsions. Pickering suspension polymerization provides a new route for the synthesis of a variety of hybrid nanocomposite microspheres with supracolloidal structures.     

Hollow polyaniline/Fe3O4 microsphere composites: Preparation,characterization, and applications in microwave absorption

Hollow polyaniline/Fe₃O₄ microsphere composites with electromagnetic properties were successfully prepared by decorating the surface of hollow polyaniline/sulfonated polystyrene microspheres with various amounts of Fe₃O₄ magnetic nanoparticles using sulfonated polystyrene (SPS) as hard templates and then removing the templates with tetrahydrofuran (THF). The synthesized hollow microsphere composites were characterized by FT-IR, UV/Vis spectrophotometry, SEM, XRD, elemental analysis, TGA, and measurement of their magnetic parameters. Experimental results indicated that the microspheres were well-defined in size (1.50–1.80 μm) and shape, and that they were superparamagnetic with maximum saturation magnetization values of 3.88 emu/g with a 12.37 wt% content of Fe₃O₄ magnetic nanoparticles. Measurements of the electromagnetic parameters of the samples showed that the maximum bandwidth was 8.0 GHz over ?10 dB of reflection loss in the 2–18 GHz range when the Fe₃O₄ content in the hollow polyaniline/Fe₃O₄ microsphere composites was 7.33 wt%.     

Hybrid Au nanoparticles on Fe3O4@polymer as efficient catalyst for reduction of 4-nitrophenol

In this paper, we attempted to synthesize a hybrid nanostructure by the incorporation of Au nanoparticles (NPs) with polymer-coated Fe₃O₄ microspheres. Also, Au NPs on 3-aminopropyl triethylsilane (APTS)-modified Fe₃O₄@SiO₂ and Fe₃O₄@polymer microspheres were synthesized to assess the catalytic activity of Au NPs on Fe₃O₄@polymer microspheres for the reduction of 4-nitrophenol. It was found that Au NPs on Fe₃O₄@polymer catalysts showed higher catalytic activity and recyclability than other APTS-modified catalysts.     

Facile synthesis of magnetic Fe3O4@Chitosan nanocomposite as environmentally green catalyst in multicomponent Knoevenagel-Michael domino reaction

Magnetic Fe₃O₄ has interesting characteristics such as large surface area, low toxicity, ferromagnetic, and biocompatible. The presence of magnetic properties in Fe₃O₄ provides an advantage in its application as a heterogeneous catalyst. This study highlights the synthesis of Fe₃O₄ modified chitosan composite and evaluates the catalytic ability in multicomponent Knoevenagel-Michael domino reaction. The synthesis and characterization of pristine Fe₃O₄ and Fe₃O₄@Chi samples were investigated. The XRD analysis combined with refinement technique indicates that the pristine Fe₃O₄ crystallized in a cubic structure with Fd-3mz symmetry and the presence of chitosan in Fe₃O₄ sample did not change its structure. The FTIR analysis also demonstrated the binding of chitosan to the Fe₃O₄ nanoparticles. TEM image reveals the presence of chitosan in the composite sample formed a core-shell interaction and covered the surface of Fe₃O₄ nanoparticles. The evaluation of Fe₃O₄@Chi catalytic ability in multicomponent Knoevenagel-Michael domino reaction demonstrated reliable catalyst performance with a yield of 92.3% at optimum conditions. Fe₃O₄@Chi could be classified as a green catalyst because it can be used repeatedly with high yield and easy separation.     

Multihollow nanocomposite microspheres with tunable pore structures by templating Pickering double emulsions

Micrometer-sized dual nanocomposite polymer microspheres with tunable pore structures were fabricated using a simple and straightforward method based on Pickering double emulsions. First, a primary water-in-styrene (oil) emulsion (w₁/o) was prepared using the hydrophobic silica nanoparticles as a particulate emulsifier without any molecular surfactants. Then, a water-in-styrene-in-water (w₁/o/w₂) Pickering emulsion was produced by the emulsification of the primary w₁/o emulsion into water using Fe₃O₄ nanoparticles as external emulsifier. The big styrene droplets containing small water droplets were polymerized after the formation of the double emulsions. Nanocomposite polystyrene microspheres with a multihollow structure were obtained and their morphological structures were studied by scanning electron microscopy (SEM). The pore structure of the microspheres could be tuned by the volume ratio of the internal water phase to the medium oil phase (w₁:o) of the primary emulsions. With increasing w₁:o from 1:8 to 4:1, the amount of the pores in one microsphere increased gradually and the pore structures changed from close to interconnected. The resulting multihollow microspheres had a responsive ability to magnetic stimulus due to the existence of Fe₃O₄ nanoparticles. This kind of multihollow hybrid polymer microspheres is expected to have a wide potential application in materials science and biotechnology.     

Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>–β-cyclodextrin–Chitosan Bionanocomposite for Arsenic Removal from Aqueous Solution

The aim of this present work is to investigate the adsorption capacity, kinetics and mechanism of arsenite ion removal onto beta-Cyclodextrin–Chitosan–Fe₃O₄ nanocomposite (β-CD–CS–Fe₃O₄ nanocomposite) from aqueous solutions. Iron oxide nanoparticles (Fe₃O₄) were synthesized using the co-precipitation method and the nanocomposite was successfully prepared via the solution-blending method. The analysis to determine arsenite ion adsorption was carried out using ICP-MS by varying pH, contact time and arsenite concentration parameters. The sorption of arsenite was found to be dependent on pH, time and arsenite initial concentrations. The adsorption equilibrium was reached in the first 20 min with the maximum uptake of 96%. Adsorption data were fitted well to the Langmuir isotherm describing a monolayer adsorption mechanism and pseudo-second-order models for kinetic study. It was established that the β-CD–CS polymer blend grafted with Fe₃O₄ nanoparticles enhanced the adsorption capacity because of the complexation abilities of the multiple OH and NH₂ groups in the polymer backbone with metal ions. Subsequently, the mechanism of adsorption was investigated by studying the physicochemical properties of the adsorbent and the adsorbed species using the FTIR, TGA, DSC, XRD, SEM and TEM techniques. The characterizations before and after incorporations of the β-CD–CS composite with Fe₃O₄ nanoparticles showed well-improved properties for better adsorption of arsenite (As(III)) ions.     

Fe3O4@mesoporouspolyaniline: A Highly Efficient and Magnetically Separable Catalyst for Cross‐Coupling of Aryl Chlorides and Phenols

A high surface, magnetic Fe₃O₄@mesoporouspolyaniline core‐shell nanocomposite was synthesized from magnetic iron oxide (Fe₃O₄) nanoparticles and mesoporouspolyaniline (mPANI). The novel porous magnetic Fe₃O₄ was obtained by solvothermal method under sealed pressure reactor at high temperature to achieve high surface area. The mesoporouspolyaniline shell was synthesized by in situ surface polymerization onto porous magnetic Fe₃O₄ in the presence of polyvinylpyrrolidone (PVP) and sodium dodecylbenzenesulfonate (SDBS), as a linker and structure‐directing agent, through ‘blackberry nanostructures’ assembly. The material composition, stoichiometric ratio and reaction conditions play vital roles in the synthesis of these nanostructures as confirmed by variety of characterization techniques. The role of the mesoporouspolyaniline shell is to stabilize the porous magnetic Fe₃O₄ nanoparticles, and provide direct access to the core Fe₃O₄ nanoparticles. The catalytic activity of magnetic Fe₃O₄@mesoporousPANI nanocomposite was evaluated in the cross‐coupling of aryl chlorides and phenols.     

Synthesis of new electromagnetic nanocomposite based on modified Fe3O4 nanoparticles with enhanced magnetic,conductive, and catalytic properties

A new method for the fabrication of an electromagnetic nanocomposite based on Fe₃O₄ and polyaniline (PANI) is offered. The authors focused on improvement of the physical and electromagnetic properties of the nanocomposite using a new synthetic method. Supermagnetic Fe₃O₄ nanoparticles were synthesized through coprecipitation method. As a chemical modification, the third generation of poly (amidoamine) dendrimer was grafted on the surface of the nanoparticles. PANI was grafted from –NH₂ functional groups of dendrimer via in situ polymerization of aniline. Finally, Au nanoparticles were loaded on the nanocomposite and its catalytic activity for reduction reactions was studied.     

Enhanced cycling performance of Fe3O4-graphene nanocomposite as an anode material for lithium-ion batteries

Fe₃O₄-graphene nanocomposite was prepared by a gas/liquid interface reaction. The structure and morphology of the Fe₃O₄-graphene nanocomposite were characterized by X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. The electrochemical performances were evaluated in coin-type cells. Electrochemical tests show that the Fe₃O₄-22.7 wt.% graphene nanocomposite exhibits much higher capacity retention with a large reversible specific capacity of 1048 mAh g⁻¹ (99% of the initial reversible specific capacity) at the 90th cycle in comparison with that of the bare Fe₃O₄ nanoparticles (only 226 mAh g⁻¹ at the 34th cycle). The enhanced cycling performance can be attributed to the facts that the graphene sheets distributed between the Fe₃O₄ nanoparticles can prevent the aggregation of the Fe₃O₄ nanoparticles, and the Fe₃O₄-graphene nanocomposite can provide buffering spaces against the volume changes of Fe₃O₄ nanoparticles during electrochemical cycling.     

Chitosan- and dehydroascorbic acid-coated Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles: preparation,characterization and their potential as draw solute in forward osmosis process

Forward osmosis (FO) is a natural osmosis process that has attracted a significant attention due to its many advantages. However, the development of FO process depends on the development of proper draw solutions. In this work, chitosan (CS)-coated Fe₃O₄ nanoparticles and dehydroascorbic acid (DHAA)-coated Fe₃O₄ nanoparticles were successfully synthesized by co-precipitation method and their performance as draw solutes was investigated for application in FO systems. CS and DHAA could improve the surface hydrophilicity of the Fe₃O₄ nanoparticles. The synthesized nanoparticles were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometry (VSM) which the results presented a small size, crystalline morphology and high magnetization value for their structure as well as a good dispersion in water. Cellulose triacetate/cellulose acetate (CTA/CA)-based membranes were also prepared by immersion precipitation and used as FO membranes. The synthesized FO membranes were characterized by FESEM. The performance evaluation of synthesized nanoparticles revealed that the water flux of Fe₃O₄ nanoparticles capped with DHAA was higher than that of the chitosan-coated Fe₃O₄ nanoparticles. At the end of the process, the Fe₃O₄ nanoparticles were easily separated from the diluted draw solution by applying the magnetic field.     

Facile fabrication of highly efficient bifunctional polydopamine–polyethyleneimine copolymerization-modified magnetic material and its effective applications for both adsorption of gold ions and catalytic reduction of 4-nitrophenol

The heavy metal ions and organic pollutants are major harmful substances in wastewater because of their toxicity on human health and environment. Nowadays, the adsorption–catalysis treatment to remove them simultaneously is still a challenge because of their different reaction mechanisms. Herein, a novel bifunctional polymer magnetic material Fe₃O₄@PDA-PEI was successfully developed through a facile co-deposition method by the copolymerization modification of polydopamine (PDA) and polyethyleneimine (PEI) on magnetic ferroferric oxide, which can both be utilized for both heavy metal ion removal from simulated industrial wastewater and further effective catalytic reduction of toxic 4-nitrophenol (4-NP) to useful 4-aminophenol. The maximum adsorption capacity of Fe₃O₄@PDA-PEI for gold ions was 465.12 mg/g at 35°C, and the adsorption thermodynamic parameters ΔG, ΔH, and ΔS of Fe₃O₄@PDA-PEI (m(PDA):m(PEI) = 2:1) were −5.28 (35°C), 25.58, and 111.30 J K⁻¹ mol⁻¹, respectively. In addition, a sustainable strategy of converting gold ions to gold nanoparticles has been demonstrated for additional catalytic functionality of Fe₃O₄@PDA-PEI, and Fe₃O₄@PDA-PEI-Au could effectively catalyze the reduction of 4-NP at ambient temperature within 9 min. Fe₃O₄@PDA-PEI with the advantages of facile fabrication, easy separation, excellent adsorption, and catalysis performance could be used as a promising polymer composite material in complicated wastewater environment. © 2023 Wiley Periodicals LLC.     

Ultrafine palladium nanoparticle-bonded to polyetheylenimine grafted reduced graphene oxide nanosheets: Highly active and recyclable catalyst for degradation of dyes and pigments

Much attention has been increasingly focused on the applications of noble metal nanoparticles (NPs) for the catalytic degradation of various dyes and pigments in industrial wastewater. We have demonstrated that Pd NPs/Fe₃O₄-PEI-RGO nanohybrids exhibit high catalytic activity and excellent durability in reductive degradation of MO, R6G, RB. Specific surface area was successfully prepared by simultaneous reduction of Pd(OAc)₂ chelating to PEI grafted graphene oxide nanosheets modified with Fe₃O₄. The as-prepared Pd NPs/Fe₃O₄-PEI-RGO nanohybrids were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, high-resolution TEM and energy dispersive X-ray spectroscopy, and UV-lambda 800 spectrophotometer, respectively. The catalytic activity of Pd NPs/Fe₃O₄-PEI-RGO nanohybrids to the degradation of MO, R6G, RB with NaBH4 was tracked by UV-visible spectroscopy. It was clearly demonstrated that Pd NPs/Fe₃O₄-PEI-RGO nanohybrids exhibited high catalytic activity toward the degradation of dyes and pigments, which could be relevant to the high surface areas of Pd NPs and synergistic effect on transfer of electrons between reduced graphene oxide (RGO), PEI and Pd NPs. Notably, Pd NPs/Fe₃O₄-PEI-RGO nanohybrids were easily separated and recycled thirteen times without obvious decrease in system. Convincingly, Pd NPs/Fe₃O₄-PEI-RGO nanohybrids would be a promising catalyst for treating industrial wastewater.     

Facile Synthesis and Catalytic Application of Silver-Deposited Magnetic Nanoparticles

In this work, we have investigated a novel one-step fabrication of Ag-deposited Fe₂O₃ nanoparticles and their application for the catalytic reduction of 4-nitrophenol to 4-aminophenol by NaBH₄. To deposit Ag onto them, Fe₂O₃ particles were dispersed in a reaction mixture consisting of ethanolic AgNO₃ and butylamine, and then the reaction mixture was incubated and shaken for 40 min at 45 °C. With this simple and surfactant-free fabrication of Ag-deposited Fe₂O₃ nanoparticles, we can avoid contamination in the final product, which makes them suitable for further catalytic applications. Since the magnetic particles are readily recovered from the solution phase without centrifugation and/or filtering, the Ag-deposited Fe₂O₃ nanoparticles prepared in this work have been exploited as solid phase catalysts for the reduction of 4-nitrophenol in the presence of NaBH₄. At the end of the reaction, the Ag-deposited Fe₂O₃ catalyst particles still remain active. They can thus be separated from the product, 4-aminophenol, simply using a neodium magnet and can be recycled a number of times.     

Porous Fe3O4/C microspheres for efficient broadband electromagnetic wave absorption

Porous Fe₃O₄/C microspheres, which were Fe₃O₄ nanocrystals (～8?nm) embedded in an open nanostructured carbon network, were successfully synthesized via a facile hydrothermal process. The porous Fe₃O₄/C microspheres possessed many distinct attributes that facilitate efficient broadband electromagnetic wave absorption (EMWA). EMWs were attenuated through multiple reflections and absorption in the 3D interconnected porous structure of the microspheres; these processes collectively improved the interaction between the EMWs and the absorber. Additionally, the carbon network and embedded Fe₃O₄ nanoparticles caused significant dielectric losses and magnetic losses, respectively, which also enhanced EMWA. The EMWA characteristics of the microspheres could be precisely tuned via changing the carbon content to achieve optimized impedance matching. Porous Fe₃O₄/C microspheres with a 71.5?wt% carbon content displayed particularly impressive EMWA properties: a maximum reflection loss (RL) value of ??31.75 across broad band frequencies in the range of 7.76–12.88?GHz (RL < ?10?dB) at an absorber thickness of 3.0?mm. These excellent EMWA properties may be attributed to both dielectric loss (carbon) and magnetic loss (Fe₃O₄). Additionally, the 3D interconnected porous structure of the Fe₃O₄/C microspheres is especially favorable for impedance matching.