High Aspect Ratio Platelets Research Articles

Growth conditions and mechanisms for IrOx nano-platelet formation by reactive sputtering

Iridium oxide (IrOx) forms a wide array of tailorable nanostructures, including plates, rods, and cones. Despite a strong body of literature on nanostructured IrOx, prior work did not explore the specific growth conditions and mechanisms for platelet formation. Here we report on how IrOx nanoplatelets take their form during DC reactive sputtering deposition, highlight the conditions and growth mechanisms that lead to unique nanostructures, and demonstrate changes in morphology and crystal orientation via x-ray diffraction (XRD), atomic force microscopy, scanning electron microscopy and x-ray photoelectron spectroscopy. The XRD spectra indicate that IrOx undergoes a substantial transition with increasing oxygen flow rate during deposition, with the initial face-centered cubic (111), (200) and (220) peaks vanishing, while the rutile (101) and (110) peaks emerge, with several intermediate peaks indicating transition species. Ultimately (101) emerges as the preferred orientation of the IrOx nanostructures, as evidenced by the time-series XRD spectra. Corresponding stress analysis indicates that in the absence of oxygen flow, the IrOx films are highly compressive, and that the film stress becomes significantly less compressive with increasing oxygen flow. This unlocks the use of IrOx as a highly tailorable nanostructured material, ranging from dense film to high aspect ratio platelets, by simply adjusting oxygen flow rate and/or sputtering time.

Pro-inflammatory response and genotoxicity caused by clay and graphene nanomaterials in A549 and THP-1 cells

Nanoclays and graphene oxide nanomaterials represent a class of materials sharing similar shapes constituted of high aspect ratio platelets. The increased production of these materials for various industrial applications increases the risk of occupational exposure, consequently with elevated risk of adverse reactions and development of pulmonary diseases, including lung cancer. In this study, pro-inflammatory responses and genotoxicity were assessed in alveolar epithelial cells (A549) and activated THP-1 macrophages (THP-1a) after exposure to three nanoclays; a pristine (Bentonite) and two surface modified (benzalkonium chloride-coated Nanofil9, and dialkyldimethyl-ammonium-coated NanofilSE3000); graphene oxide (GO) and reduced graphene oxide (r-GO) nanomaterials. The pro-inflammatory response in terms of IL-8 expression was strongest in cells exposed to Bentonite, whereas surface modification resulted in decreased toxicity in both cell lines when exposed to Nanofil9 and NanofilSE3000. GO and r-GO induced a pro-inflammatory response in A549 cells, while no effect was detected with the two nanomaterials on THP-1a cells. The pro-inflammatory response was strongly correlated with in vivo inflammation in mice after intra-tracheal instillation when doses were normalized against surface area. Genotoxicity was assessed as DNA strand breaks, using the alkaline comet assay. In A549 cells, an increase in DNA strand breaks was detected only in cells exposed to Bentonite, whereas Bentonite, NanofilSE3000 and GO caused an increased level of genotoxicity in THP-1a cells. Genotoxicity in THP-1a cells was concordant with the DNA damage in bronchoalveolar lavage fluid cells following 1 and 3 days after intra-tracheal instillation in mice. In conclusion, this study shows that surface modification of pristine nanoclays reduces the inflammatory and genotoxic response in A549 and THP-1a cells, and these in vitro models show comparable toxicity to what seen in previous mouse studies with the same materials.

Nanoscale Thin Film Corrosion Barriers Enabled By Multilayer Polymer Clay Nanocomposites

Ultrathin nanocomposite films fabricated by layer-by-layer (LbL) assembly are an emerging class of coatings with remarkable barrier performance. Here, we examine LBL polymer-clay nanocomposite (PCN) coatings that afford anti-corrosion properties comparable to those exhibited by more traditional films that are orders of magnitude thicker. PCN barrier thin films comprised of up to 40 bilayers of polyethyleneimine (PEI) and exfoliated laponite (LAP), montmorillonite (MMT), or vermiculite (VMT) clay platelets were coated onto steel plates using a scalable LBL dip-coating process. A combination of electrochemical impedance spectroscopy and ex-situ electron and vibrational spectroscopy were used to evaluate coating performance during exposure in 0.6 M NaCl for up to 7 days. A PEI/VMT crosslinked coating of only 250 nm thick slowed the corrosion rate of mild steel by three orders of magnitude (Rct = 106 vs. 103 Ω-cm2) after seven days in 0.6 M NaCl. This performance exceeded the reported performance of other LBL coatings along with much thicker multilayer composite coatings and several conventional pretreatments on mild steel under similar test conditions. The exceptional performance is attributed to the effective barrier properties imparted by a high loading (>80 wt%) of orientated, high aspect ratio (2000:1) platelet layers in a nanobrick wall morphology. Substitution of VMT with lower aspect ratio clay platelets of either MMT (400:1) or LAP (30:1) showed that coated steel corrosion rates and apparent coating permeability scales with platelet size following LAP > MMT >> VMT. Thus, this study has for the first time shown that platelet aspect ratio is a primary means of improving the performance LbL nanocomposite coatings which is in line with the expected behavior of conventional nanocomposites. Given film debonding from the substrate was observed as a common failure mode during exposure for all coating types, replacement of the PEI polymer component with a more debonding-resistant polymer may be an additional pathway to further improve LbL PCN anticorrosion performance.

Phosphonated Poly(ethylene terephthalate) ionomers as compatibilizers in extruded Poly(ethylene terephthalate)/Poly(m-xylylene adipamide) blends and oriented films

Blends of poly(ethylene terephthalate) (PET) and poly(m-xylylene adipamide) (MXD6) were successfully compatibilized by using a phosphonated PET ionomer in the Na+ form (Na+-PPET) as a compatibilizer. The compatibilized 90:10 (by weight) polyester/polyamide blends containing various amounts of the phosphonated polyester were prepared by melt mixing in a twin-screw extruder. Scanning electron microscopy (SEM) revealed that the extruded blends contained spherical MXD6 domains dispersed in the PET matrix. The phase-separated MXD6 domain size decreased with increasing ionic monomer concentration, and the efficient compatibilization was attributed to specific interactions between the ionic phosphonate groups on the Na+-PPET ionomer and the amide linkages of MXD6. Thermal analysis suggested that the small MXD6 domains nucleated the crystallization of PET upon cooling as indicated by a shift of crystallization temperature of PET to higher temperatures. Furthermore, the specific interactions appeared to inhibit crystallization of MXD6. Biaxial orientation of melt processed films of the blends with a draw ratio of 3 × 3 transformed the spherical MXD6 domains into high aspect ratio platelets that were oriented in the plane of the film. Compared to the uncompatibilized base blends, optical property characterization on the oriented films revealed a decreased haziness by 56% and an improved transparency by 35% with 0.22 mol% of phosphonated ionic monomers incorporated in the polyester component. Moreover, a 33% decrease in oxygen permeability was found for the oriented Na+-PPET/PET/MXD6 blend composition containing 0.11 mol% of phosphonated monomers.

Synergistic Effect of Mica, Glass Frit, and Melamine Cyanurate for Improving Fire Resistance of Styrene-Butadiene Rubber Composites Destined for Ceramizable Coatings

Synergistic effects of different fillers are widely utilized in polymer technology. The combination of various types of fillers is used to improve various properties of polymer composites. In this paper, a synergistic effect of flame retardants was tested to improve the performance of ceramizable composites. The composites were based of styrene-butadiene rubber (SBR) used as polymer matrix. Three different types of flame retardants were tested for synergistic effect: Mica (phlogopite) high aspect-ratio platelets, along with low softening point temperature glass frit (featuring ceramization effect), and melamine cyanurate, a commonly used flame retardant promoting carbonaceous char. In order to characterize the properties of the composites, combustibility, thermal stability, viscoelastic properties, micromorphology, and mechanical properties were tested before and after ceramization. The results obtained show that the synergistic effect of ceramization promoting fillers and melamine cyanurate was especially visible with respect to the flame retardant properties resulting in a significant improvement of fire resistance of the composites.

Anisotropic Mg Electrodeposition and Alloying with Ag-based Anodes from Non-Coordinating Mixed-Metal Borohydride Electrolytes for Mg Hybrid Batteries

A highly anisotropic electrodeposition was observed using the hybrid battery electrolyte Mg(BH4)2 with LiBH4 in diglyme. At low overpotentials high aspect ratio platelet morphologies are observed with a strong fiber texture composed of a {10-10} and a {11-20} component, the first evidence of behavior of this kind in magnesium battery electrolytes. At high overpotentials the deposit aspect ratio is indistinguishable but the texture is shown to be primarily composed of a {11-20} fiber texture. The kinetic parameters relative to the relevant crystallographic faces are extracted from electron microscopy images and compared with the observed bulk rate extracted from the electrochemical data. The use of polycrystalline Ag foil substrates with little preferred orientation at the surface allowed highly polycrystalline nucleation at lower overpotentials than that of platinum, likely due to Ag alloying with Mg. Characterization using focused ion beam (FIB) cross-sections with Auger Electron Spectroscopy (AES) elemental analysis confirm that the deposits are primarily Mg although Mg‐Ag alloys of various compositions were observed. It is proposed that the orientation at slow rates of growth is due to the underlying kinetics of adatom diffusion on Mg and that higher rates diminish the phenomenon due to decreased time for adatom diffusion and instead are governed by the rates of adatom formation or more specifically the adatom vacancy formation on the different low-index planes of Mg.

Polypropylene nanocomposites with oxo‐degradable pro‐oxidant: Mechanical, thermal, rheological, and photo‐degradation performance

The objective of the study was to generate degradable polypropylene nanocomposites by incorporation of pro‐oxidant and different fillers like silica, silicate, and thermally reduced graphene. Graphene‐based composites exhibited higher crystallinity attributed to better dispersion and high aspect ratio platelets. Graphene composites with 2.5% additive content significantly enhanced the peak degradation temperature to 464°C as compared to 448°C for pure polymer. The processing conditions used for the nanocomposite generation were optimum as a uniform distribution of filler particles (or platelets) was observed in the PP matrix. The tensile modulus of the graphene composite with 2.5% additive content was 80% higher than pure PP, as compared to 60 and 30% for silicate and silica composites, respectively. Similarly, the storage modulus of the graphene nanocomposite with 1% additive content had 30% increment at 40°C as compared to pure PP. PP‐additive blends as well as PP nanocomposites with silica and silicate were observed to attain 100% degree of embrittlement within 6 months of UV exposure at 30°C. Graphene composites, though, had delayed photo‐degradation due to UV absorption by the platelets and high aspect ratio platelets acting as oxygen barrier for PP matrix, but the pro‐oxidant was successful in attaining controlled degradation. POLYM. ENG. SCI., 56:1229–1239, 2016. © 2016 Society of Plastics Engineers

High CEC generation and surface modification in mica and vermiculite minerals

Montmorillonite layered silicate has been commonly used to reinforce polymer matrices. Due to its swelling in water, organic modification of the mineral surface is easily achieved which makes the surface compatible with polymers. Other minerals like mica and vermiculite though can also lead to high aspect ratio platelets in nanocomposites, but they do not swell in water owing to much stronger electrostatic forces of attraction holding their platelets together (layer charge density >0.5 eq · mol−1 in comparison with 0.25–0.5 eq · mol−1 for montmorillonite). In current study, milling, delamination and cation exchange processing of mica and vermiculite minerals has been reported to explore their potential as reinforcement materials. Wet grinding and subsequent sieving of the coarse minerals led to fine-sized particles suitable to perform chemical delamination in water. The delamination process resulted in Li-mica and Na-vermiculite with enhanced access to the interlayer cations, thus, higher CEC. Successful surface modification of the delaminated minerals with alkyl ammonium ions could be achieved which resulted in significant enhancements in their basal plane spacing. Peak degradation temperatures of 260°C were measured for C18 and 2C18 modified vermiculite, whereas 300°C and 275°C were observed respectively for C18 and 2C18 modified mica minerals which make them suitable for compounding with polymers at high temperature.

Crystallographic and magnetic identification of secondary phase in orientated Bi5Fe0.5Co0.5Ti3O15 ceramics

The fabrication of highly-oriented polycrystalline ceramics of Bi5Fe0.5Co0.5Ti3O15, prepared via molten salt synthesis and uniaxial pressing of high aspect ratio platelets is reported. Electron backscatter images show a secondary phase within the ceramic which is rich in cobalt and iron. The concentration of the secondary phase obtained from scanning electron microscopy is estimated at less than 2% by volume, below the detection limit of x-ray diffraction (XRD). The samples were characterized by x-ray diffraction, polarization-electric field measurements, superconducting quantum interference device as a function of sample orientation and vibrating sample magnetometry as a function of temperature. It is inferred from the data that the observed ferromagnetic response is dominated by the secondary phase. This work highlights the importance of rigorous materials characterisation in the study of multiferroics as small amounts of secondary phase, below the limit of XRD, can lead to false conclusions.

Effects of Epoxidized Natural Rubber on Gas Barrier and Mechanical Properties of NR/BIIR Composites

The mechanical properties and gas permeability of natural rubber/bromobutyl rubber blends were investigated when using epoxidized natural rubber (ENR) as a compatibilizer. The blend composites used in this study were composites of commercial precipitated silica. Several types of layered silicates were investigated here as secondary fillers due to their high-aspect ratio platelet structures, which could help delaying the gas permeation rate. These fillers include talcum, montmorillonite clay, and kaolinite clay. We found that, ENR could promote faster cure and could improve the mechanical properties, compression set, and abrasion resistance of the composites with significant improvement in its gas barrier property. Among different types of fillers, talcum could provide superior gas barrier property to other types of fillers, even though the strength and modulus of talcum composite were still inferior to those of other composites. However, the addition of ENR to talcum composite did not further improve its gas barrier property further, though it could improve the other properties.

Structure and Gas Barrier Properties of Poly(propylene-graft-maleic anhydride)/Phosphate Glass Composites Prepared by Microlayer Coextrusion

Alternating (A−B−A)n polymer films with individual layer thicknesses in the 3−25 μm range were produced using a robust layer multiplying coextrusion process. Such a film with layers of a poly(propylene-graft-maleic anhydride) (PPgMA) alternating with layers of PPgMA containing a phosphate glass (Pglass) upon biaxial orientation exhibited percolation of high barrier filler platelets resulting in reduction in gas permeation by two to 3 orders of magnitude. This unprecedented reduction in oxygen permeability was attributed to the high volume fraction of highly aligned Pglass platelets in the polymer matrix resembling a brick wall microstructure. Biaxially oriented films with 20 vol % Pglass content exhibited a microstructure resembling alternating organic and inorganic layers, a close replica of natural biocomposites. Structural models for permeability indicated that enhanced barrier resulted from increased tortuosity of the diffusion pathway provided by the aligned high aspect ratio platelets. Aspect ratios...

Self-Piling Silicate Rods and Dendrites from High Aspect-Ratio Clay Platelets

We observed the formation of unique rod-like microstructures by self-assembling of random mica platelets with a high aspect-ratio and counterion charged. The platelets are polydisperse with dimensions of 300−1000 nm in width and 1 nm in thickness and are prepared from the exfoliation of layered synthetic-fluorinated-mica. These platelets of polygon shape are characterized by transmission electron microscopy and atomic force microscopy. When finely dispersed in water followed by controlled evaporation, the randomized silicate platelets self-align into a rod-like morphology. The restructuring force is attributed to the intensive attraction of surface charges on high aspect-ratio platelets. The resulting thin platelets tend to self-align in a size-selective manner through face-to-face piling into rod-like microstructures of 3−20 μm in length and 300−1000 nm in diameter (observed by field emission-scanning electron microscopy). The self-assembled rods, under an ultrasonic treatment, may further rearrange into fern-leaf dendrites via the process of fragmentation and restacking. The sequential transformation of platelet microstructures in a hierarchical manner allows a morphological manipulation for constructing patterns of regularity.

The use of plant waxes as templates for micro- and nanopatterning of surfaces

Small wax sculptures on plant surfaces are responsible for a variety of functions, including the maintenance of sability, self-cleaning properties, prevention of insect attachment and reflection of light. Here we report on a method for using recrystallized plant wax crystals on flat technical surfaces for generating micro- and nanopatterned polymers with particular functions. Therefore a more than 4-thousand year old replica technique called the “lost wax technique” has been adapted for reproduction of complex, high aspect ratio surfaces and the large scale reproduction of micro- and nanopatterned surfaces. Highly ordered pyrolytic graphite substrates, covered with perpendicularly oriented platelets formed by the primary alcohol octacosan-1-ol, the main component of wheat ( Triticum aestivum) leaf waxes, were used as templates to make an epoxy resin surfaces with high aspect ratio platelets 400–700 nm wide and 10–50 μm thick. The technique holds great promise on the reproduction of micro- and nanopatterned surfaces.

Selective and Controlled Synthesis of α- and β-Cobalt Hydroxides in Highly Developed Hexagonal Platelets

We report on the controlled synthesis of single-crystal platelets of alpha- and beta-Co(OH)2 via homogeneous precipitation using hexamethylenetetramine as a hydrolysis agent. The alpha- and beta-Co(OH)2 hexagonal platelets of several micrometers in width and about 15 nm in thickness were reproducibly yielded in rather dilute CoCl2 solutions in the presence and absence of NaCl at 90 degrees C, respectively. The phase and size control of the products were achieved by varying both CoCl2 and NaCl concentrations. Polarized optical microscope observations revealed clear liquid crystallinity of colloidal suspensions of these high aspect-ratio platelets. The as-prepared alpha-Co(OH)2 containing interlayer chloride ions was intercalated with various inorganic or organic anions, keeping its high crystallinity and hexagonal platelike morphology.

Synthesis of High Aspect Ratio Platelet SrTiO3

A method for synthesis of high aspect ratio platelet seeds by growth of SrTiO3 on Sr3Ti2O7 core particles is reported. The aim of this study was to identify and control the morphology and size of SrTiO3 particles via molten salt synthesis. Platelet and tabular morphologies with rectangular faces were obtained using rutile and anatase, respectively. Platelet SrTiO3 particles with an edge length of 10–40 μm and a thickness of 1–4 μm were obtained. High aspect ratios (edge length to thickness) of 7–10 were measured for platelet particles as opposed to lower aspect ratios of 2–4 for tabular particles. Highly anisotropic platelets are suitable template candidates to achieve textured ceramics.

Zeolite Growth in Nonionic Microemulsions: Synthesis of Hierarchically Structured Zeolite Particles

The synthesis of silicalite-1 in nonionic microemulsions is explored. The work presented here is a systematic study showing how parameters such as synthesis temperature, microemulsion composition, and surfactant identity impact the formation and growth of silicalite-1. The work demonstrates the possibility of using microemulsions to manipulate the shape and size of silicalite-1 materials, growing both spheres and high-aspect ratio platelets. In both cases, these large particles are robust aggregates of small submicron particles that are stable to calcination. The size of the primary particles is adjustable based on the synthesis temperature, whereas the size of the large aggregates is adjustable based on the silicalite-1 content of the mixture. On the basis of the results presented, a mechanism is proposed illustrating the role of both the confined space presented by the microemulsion as well as the importance of the surfactant−silicate interactions leading to the formation of the large aggregates. The re...

On the actual contribution of crack deflection in toughening platelet-reinforced brittle-matrix composites

The effectiveness of crack deflection as a single toughening mechanism is evaluated in the case of brittle-matrix reinforced by high aspect-ratio platelets. Microstructural parameters characterizing size, morphology, orientation and distribution of the second phase including aspect ratio, volume fraction and interparticle distance, were experimentally obtained by means of image analysis techniques on a model composite. Introducing “true” microstructural parameters in the fracture mechanics formulation of the strain energy release rate for a deflected crack, it is demonstrated that no appreciable toughening can be obtained due to the geometrical local misalignment of the crack front even in the case of increasing aspect-ratio or volume fraction of second phase. It is concluded that crack deflection processes may play an important role only as precursors for other operative mechanisms, being, in the general case, the deflection of the crack path only phenomenologically related to toughening.

A quantitative evaluation of microstructure in Si3N4/SiC platelet and participate composites

A microstructural evaluation of Si3N4 containing 15–40 vol% SiC platelets or particles is presented. All the composites were fully densified by hot isostatic pressing without external addition of sintering aids. Size, morphology, surface roughness and crystal structures of the SiC phases before and after sintering were compared in order to discuss the structural stability of the reinforcements up to 2050 °C in Si3N4 matrix. Morphology and phase characteristics of the grain boundary are also discussed. In addition, homogeneity and isotropy of the composite bodies were quantitatively examined by image analysis techniques and it was recognized that, for a similar degree of dispersion, the characteristic of three-dimensional randomness could be preserved only at V f<30% in the composites containing high aspect ratio platelets.