Glassy Layer Research Articles

Oxidation behavior and ablation mechanism of Cf/ZrB2-SiC composite fabricated by vibration-assisted slurry impregnation combined with low-temperature hot pressing

The Cf/ZrB2-SiC composite was fabricated by vibration-assisted slurry impregnation and low-temperature hot pressing. The Cf/ZrB2-SiC composite exhibited an excellent thermal shock resistance and oxidation resistance with a dense SiO2-rich glassy layer exposing at 1500 °C/1 h condition for static oxidation. Moreover, the high temperature ablation resistance was investigated with an oxyacetylene ablative testing at temperature 1800 °C for 1000s. The mass loss rate and line loss rate were 7.32×10−4 mg/mm2·s and -1.70×10−4 mm/s, respectively. The enriched silica glass layer embedded with a small amount of zirconia particles was contributed mainly to the excellent ablation resistance of Cf/ZrB2-SiC composite.

The influence of microstructure on perpendicular magnetic anisotropy in Co/Dy periodic multilayer systems

The article is dedicated to reveling the reason of unusual magnetic properties, found in one of Co/Dy periodic multilayer systems with different Co/Dy layer thickness by X-Ray reflectivity, scanning/transmission electron microscopy together with energy dispersive X-Ray microanalysis. Magnetic measurements did not demonstrate any peculiarities in the system with thinner Co/Dy layer thickness. However, the system with thicker layers demonstrated an anomalous behavior of the perpendicular magnetic anisotropy. Based on X-Ray reflectivity measurements, it was found that the polarizability value in the system with thicker Co/Dy layer was very high and related to glassy intermetallic DyCo2 compound formation. Electron microscopy and microanalysis data also indicated the presence of glassy intermetallic DyCo2 layers, which could be responsible for the peculiar magnetic behavior. The relation between magnetic properties and microstructure was revealed.

Tailored Oxidation Barrier Coatings for Mo-Hf-B and Mo-Zr-B Alloys.

The cyclic oxidation response of Mo-14Hf-23B and Mo-14.8Zr-26B (compositions in at. %) was investigated in air at 800 °C, which is a critical temperature for Mo-based alloys because of the pesting phenomenon. Rapid oxidation was observed for the unprotected samples, and an oxidation protection coating was developed based on a preceramic polymer with silicon and boron as particulate fillers. Cyclic oxidation tests of the coated samples showed excellent oxidation protection: no Mo, Hf or Zr oxides were found after testing and a small mass gain in the initial stage of oxidation indicated the formation of a glassy protection layer on the alloys surfaces after exposure to air at 800 °C.

Chemical Durability of Thermal Insulating Materials in Hall-Héroult Electrolysis Cells

The most common thermal insulating materials used in the cathode lining in aluminum electrolysis cells are Moler (diatomaceous earth), calcium silicate, or vermiculite based materials. The thermal insulation layer is critical for the overall thermal stability of the cell and is vulnerable to volatile species, such as sodium vapor, that may penetrate through the carbon cathode and refractory layer. Here, we present an investigation of the chemical degradation of typical thermal insulating materials by exposure to sodium vapor in a laboratory test. Changes in microstructure and chemical and mineralogical composition of the exposed materials were characterized by electronic microscopy and powder X-ray diffraction. The materials possess different reaction patterns, ranging from deformation by creep to formation of a glassy layer reducing further sodium penetration. The results from the laboratory test were compared with chemical reactions with sodium predicted by computational thermodynamics and discussed with respect to relevant ternary phase diagrams.

Synthesis, Characterization, and Thermal Behavior of Nanoparticles of Mg(OH)2 to Be Used as Flame Retardants

To study the effects of the precursor materials on the structure, the morphology, and the thermal stability of Mg(OH)2 particles, five samples were prepared by using the same method. The produced powder was characterized by using FTIR, XRD, SEM, and TEM, and the thermal stability was studied by using the thermogravimetric analysis. This study aims to use the advantage prepared material as a flame-retardant for the polymeric materials. The characterization of the obtained samples shows that Mg(OH)2 is formed in hexagonal phase and arranged in different shapes. The analysis of the data obtained from the TGA shows that Mg(OH)2 in general decomposed in three steps, the first one due to the water content and the other volatile materials, the second step represents the decomposition of Mg(OH)2 to produce MgO, and the third step represents the conversion of MgO to glassy layer. The samples prepared in presence of the surfactants gave a higher value for the formation of the glassy layer so that it is recommended to use this sample as flame retardant for the polymeric materials.

Temperature and aging dependence of strain‐induced crystallization and cavitation in highly crosslinked and filled natural rubber

ABSTRACTWe have investigated the structural changes occurring in highly crosslinked and carbon‐black filled natural rubber under uniaxial extension by small‐ and wide‐angle X‐ray scattering using synchrotron radiation. The experiments focused on strain‐induced crystallization (SIC) and nanocavitation and were carried out on a model series of materials as a function of temperature and aging conditions. We find that for all materials both SIC and cavitation decrease markedly with temperature and aging. However, the presence of carbon black filler shifts the ceiling temperature where SIC is observed to at least 120°C, presumably by a nucleating effect, maintaining the high strength of the elastomers. Interestingly, although in pure elastomers, the cavitation strength decreases with temperature, we find that in these filled elastomers the critical stress for the onset of cavitation increases significantly with temperature strongly suggesting that cavitation is due to the local confinement between fillers and supporting the idea of a glassy layer near the filler. Aging for 10 days at 110°C in oxygen‐free conditions decreases both SIC and cavitation and reduces the strength of the elastomer at high temperature. This is attributed to the formation of sulfur side chains hindering the crystallization. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 780–793

Bismuth titanate-based UV filters embedded mesoporous silica nanoparticles: Role of bismuth concentration in the self-sealing process

The development of new safe inorganic UV filters to effectively protect the skin from ultraviolet (UV) radiation effects is an emerging issue. Bismuth titanate-based UV filters embedded into mesoporous silica nanoparticles (MSN) represent a new class of inorganic sunscreens, with excellent UVA and UVB shielding properties. In addition, the presence of bismuth ions promotes a self-sealing process, allowing (i) the entrapment of the active phases in the deepest core of the system and (ii) the formation of an external glassy silica layer with a consequent suppression of the photocatalytic activity.In this work, aimed at studying in detail the self-sealing mechanism and accessing the role of bismuth ions in the formation of the system, a series of samples impregnated with a different amount of bismuth were investigated. The self-sealing process already occurs at the lowest content of bismuth and the mechanism is demonstrated to be triggered by the ability of Bi to work as a low-melting point agent for silica. Finally, a sunscreen formulation containing the new UV filter was prepared and the Sun Protection Factor (SPF), the pH and the viscosity were measured, demonstrating the potential of the proposed material for large-scale applications.

Cover Image, Volume 102, Issue 6

Cover Photograph: Glasses in the Al2O3‐SiO2 system are notoriously difficult to form without phase‐separation or crystallization, as shown for a composition quenched from the melt along Path A. By instead subjecting alkali aluminosilicate glasses to a poling process under electric‐fields at low temperature (Path B), surface depletion layers of the same composition are formed that circumvent these issues, and moreover show unique structural characteristics within the glassy layer. DOI: 10.1111/jace.16405. image

Molecular Effects of Glycerol on Lipid Monolayers at the Gas-Liquid Interface: Impact on Microbubble Physical and Mechanical Properties.

The production and stability of microbubbles (MBs) is enhanced by increasing the viscosity of both the formation and storage solution, respectively. Glycerol is a good candidate for biomedical applications of MBs, since it is biocompatible, although the exact molecular mechanisms of its action is not fully understood. Here, we investigate the influence glycerol has on lipid-shelled MB properties, using a range of techniques. Population lifetime and single bubble stability were studied using optical microscopy. Bubble stiffness measured by AFM compression is compared with lipid monolayer behavior in a Langmuir-Blodgett trough. We deduce that increasing glycerol concentrations enhances stability of MB populations through a 3-fold mechanism. First, binding of glycerol to lipid headgroups in the interfacial monolayer up to 10% glycerol increases MB stiffness but has limited impact on shell resistance to gas permeation and corresponding MB lifetime. Second, increased solution viscosity above 10% glycerol slows down the kinetics of gas transfer, markedly increasing MB stability. Third, above 10%, glycerol induces water structuring around the lipid monolayer, forming a glassy layer which also increases MB stiffness and resistance to gas loss. At 30% glycerol, the glassy layer is ablated, lowering the MB stiffness, but MB stability is further augmented. Although the molecular interactions of glycerol with the lipid monolayer modulate the MB lipid shell properties, MB lifetime continually increases from 0 to 30% glycerol, indicating that its viscosity is the dominant effect on MB solution stability. This three-fold action and biocompatibility makes glycerol ideal for therapeutic MB formation and storage and gives new insight into the action of glycerol on lipid monolayers at the gas-liquid interface.

High-resolution transmission electron microscopy investigation of diffusion in metallic glass multilayer films

Lack of plasticity is one of the main disadvantages of metallic glasses. One of the solutions to this problem can be composite materials. Diffusion bonding is promising for composite fabrication. In the present work the diffusion process in glassy multilayer films was investigated. A combination of advanced transmission electron microscopy (TEM) methods and precision sputtering techniques allows visualization and study of diffusion in amorphous metallic layers with high resolution. Multilayered films were obtained by radio frequency sputter deposition of Zr-Cu and Zr-Pd. The multilayers were annealed under a high vacuum (10−5 Pa) for 1 and 5 h at 400 °C, that is, well below the crystallization temperatures but very close to the glass-transition temperatures of both types of the glassy layer. The structural evolution in the deposited films was investigated by high-resolution transmission electron microscopy. It was observed that, despite the big differences in the atomic mass and size, Pd and Cu have similar diffusion coefficients. Surprisingly, 1 h of annealing results in formation of metastable copper nanocrystals in the Zr-Cu layers which, however, disappear after 5 h of annealing. This effect may be connected with nanovoid formation under a complex stress state evolving upon annealing, and is related to the exceptionally slow relaxation of the glassy layers sealed with a Ta overlayer.

Impact of Carbamide Peroxide Whitening Agent on Dentinal Collagen

Carbamide peroxide (CP) is widely used as a tooth-whitening agent in self-administered tooth-bleaching products. In this study, the effects of 5% and 10% CP on dentinal collagen structure and chemical properties were evaluated in vitro. Thirty-five intact teeth were exposed to 2 whitening protocols (2 or 4 h daily) with either 5% or 10% CP gel for 1 wk. Shade changes before and after the whitening protocol were captured colorimetrically using a spectroshade. Collagen scaffold models and demineralized dentine disc samples were prepared and exposed to CP droplets (5% or 10%). Structural changes were investigated using electron microscopy. Finally, mineralized dentine disc samples were prepared postbleaching to assess chemical changes resulting from CP exposure in dentinal collagen using Raman spectroscopy. Results showed a difference in tooth shade when exposed to 5% and 10% CP whitening protocols, with a significantly (P ≤ 0.01) greater change reported for the 10% CP/4-h group. Imaging of the collagen scaffold model following exposure to CP showed a gelatinization process indicating that the free radical by-products from CP are able to disrupt the quaternary structure of noncrosslinked collagen. The most significant damage on the collagen scaffold was seen for the 10% CP exposure for 4 h. Imaging of the demineralized discs displayed the same glassy amorphous layer appearance as found in the collagen scaffold. Raman spectra of the mineralized dentine discs showed a significant decrease (P ≤ 0.01) in the integrated area of amide I and amide III values in the 4 test groups following CP application. Amide I was more affected as both the exposure time and concentration of CP increased. Despite the claimed safety of whitening agents, this in vitro study concludes that even low concentrations of CP result in a deleterious change in dentinal collagen.

Structure of Solidified Films of CaO-SiO2-Na2O Based Low-Fluorine Mold Flux

As an essential synthetic material used in the continuous casting of steels, mold fluxes improve the surface quality of steel slabs. In this study, a CaO-SiO2-Na2O-based low-fluorine mold flux was solidified by an improved water-cooled copper probe with different temperatures of molten flux and different probe immersion times. The heat flux through solid films and the film structures were calculated and inspected, respectively. Internal cracks (formed in the glassy layer of films during solidification) were observed. The formation and evolution of those cracks contributed to the unstable heat flux density. The roughness of the surface in contact with the water-cooled copper probe formed as films were still glassy and the roughness had no causal relationship with crystallization or devitrification. Combeite with columnar and faceted dendritic shapes were the main crystal in the film.

Polymer dynamics under confinement.

We review recent neutron scattering work and related results from simulation and complementary techniques focusing on the microscopic dynamics of polymers under confinement. Confinement is either realized in model porous materials or in polymer nanocomposites (PNC). The dynamics of such confined polymers is affected on the local segmental level, the level of entanglements as well as on global levels: (i) at the segmental level the interaction with the surface is of key importance. At locally repulsive surfaces compared to the bulk the segmental dynamics is not altered. Attractive surfaces slow down the segmental dynamics in their neighborhood but do not give rise to dead, glassy layers. (ii) Confinement generally has little effect on the inter-chain entanglements: both for weakly as well as for marginally confined polymers the reptation tube size is not changed. Only for strongly confined polymers disentanglement takes place. Similarly, in PNC at higher NP loading disentanglement phenomena are observed; in addition, at very high loading a transition from polymer caused topological constraints to purely geometrical constraints is observed. (iii) On the more global scale NSE experiments revealed important information on the nature of the interphase between adsorbed layer and bulk polymer. (iv) Polymer grafts at NP mutually confine each other, an effect that is most pronounced for one component NP. (v) Global diffusion of entangled polymers both in weakly and strongly attractive PNC is governed by the ratio of bottle-neck to chain size that characterizes the 'entropic barrier' for global diffusion.

A Multilayer SiC/ZrB2/SiC Ablation Resistance Coating for Carbon/Carbon Composites

To improve the ablation resistance of carbon/carbon (C/C) composites, a new SiC/ZrB2/SiC multilayer coating is designed and fabricated by a three‐step method of pack cementation, supersonic plasma spraying, and low pressure chemical vapor deposition. In this study, ablation behavior of SiC/ZrB2/SiC coating is investigated under oxyacetylene torch environment. Oxyacetylene ablation testing results show that the linear ablation rate and mass ablation rate of SiC/ZrB2/SiC coated C/C composites are only −5.0 × 10−4 mm s−1 and −9.6 × 10−4 g s−1, respectively, for 20 s ablation under heat flux of 2400 kW m−2. However, the ablative linear rate of ZrB2/SiC coated C/C composites is 7.5 × 10−4 mm s−1 and the ablative mass rate is zero. The better ablation resistance is mainly attributed to a molten SiO2–ZrO2 hybrid glassy layer formed after ablation on the surface of the composites and acting as thermal barrier layer inhibiting inward diffusion of oxygen.

Bound ("Glassy") Rubber as a Free Radical Cross-linked Rubber Layer on a Carbon Black.

A model of rubber with a cross-linked rubber layer on a carbon black filler has been proposed. The cross-links are the result of free radical reactions generated by carbon atoms with unpaired electrons at the edge of graphitic sheets in a carbon black filler. The experimental study of the cross-linking reactions in polyisoprene was done on a flat carbonized surface after ion beam implantation. The cross-linking process in the polyisoprene macromolecules between two particles was simulated. The model with a cross-linked rubber layer on a carbon filler as a “glassy layer” explains the mechanical properties of the rubber materials.

Ablation properties of HfB2 coatings prepared by supersonic atmospheric plasma spraying for SiC-coated carbon/carbon composites

In order to improve the ablation resistance of carbon/carbon (C/C) composites, hafnium boride (HfB2) coatings were prepared on the surface of SiC-coated C/C composites by supersonic atmospheric plasma spraying (SAPS). The anti-ablation property was investigated in an oxyacetylene torch environment under different heat flux. The phase composition, surface and cross-section microstructure and ablation behavior of the prepared coated C/C composites were characterized by using X-ray diffraction and scanning electron microscopy equipped with energy dispersion spectroscopy. The HfB2 coating exhibited excellent ablation resistance under 2400 kW/m2 after ablation for 30 s and no obvious penetrated crack and critical defect could be observed in the coating. The increase of ablation rates with the formation of rugged surface structure and pores indicated that HfB2 coating suffered more serious mechanical denudation and thermochemical erosion. The formation of protective glassy Hf-O layer with low oxygen permeability, generated due to the oxidation of HfB2 phase, inhibited the inward diffusion of oxygen and improved remarkably the ablation resistance of C/C composites.

In-situ synthesis of MoSi2 coating on Mo substrate under carbon protection and its short-term oxidation behavior

MoSi2 coating was in-situ synthesized on pure Mo substrate by a simple and improved packing method, in which carbon powder was used for providing a protective atmosphere. The microstructure and phase composition of the coated samples were characterized by SEM, EDS and XRD techniques. The mechanism of coating preparation was discussed. Results showed that the addition of carbon could effectively protect Mo from oxidizing during the coating preparation. A compact MoSi2 coating with a loose surface layer was obtained. It was suggested that the trace oxygen availed the formation of the compact internal structure and the loose surface layer. And this particular coating structure was expected to contribute to a better oxidation resistance in high temperature. Additionally, the short-term oxidation behavior of the as-prepared MoSi2 coating was investigated. The as-prepared MoSi2 coating provided an unexceptionable protection for the Mo substrate from oxidation at 1600 °C for 1 h. The presence of a glassy silica layer that formed on the MoSi2 coating during the oxidation exposure was proposed to explain the behavior. It shows the simple and improved packing method with carbon powder as protective medium is promising potential for the preparation of various anti-oxidation coatings.

Enhancing the oxidation resistance of Nb-Si based alloys by yttrium addition

The effect of yttrium on the microstructure and high-temperature oxidation performance of Nb-15Si-24Ti-4Cr-2Al-2Hf (at.%) alloys was investigated. Alloys containing different yttrium contents were designed, aiming at maximizing the beneficial effect of yttrium addition and avoiding the possible adverse effect of over-doped yttrium. Results showed that Y was primarily partitioned in γ-Nb5Si3, relative to Nb solid solution (Nbss), Nb3Si and α-Nb5Si3. Microstructural refinement and oxidation resistance enhancement were achieved by yttrium addition up to 0.3 at.%, accompanied by the transition towards parabolic oxidation kinetics. An adverse effect of over-doped yttrium (0.5 at.% addition) by coarsening the microstructure and deteriorating the oxidation resistance was confirmed. A relatively continuous glassy SiO2 layer was generated in the oxide scale formed on the Y-containing alloys with enhanced oxidation resistance.

Oxidation and ablation protection of multiphase Hf0.5Ta0.5B2-SiC-Si coating for graphite prepared by dipping-pyrolysis and reactive infiltration of gaseous silicon

A novel monolayer Hf0.5Ta0.5B2-SiC-Si coating with defect-free structure for protecting graphite materials was prepared by dipping-pyrolysis combined with reactive infiltration of gaseous silicon. The phase synthesis, microstructure, oxidation and ablation resistance properties of the prepared Hf0.5Ta0.5B2-SiC-Si coating were studied. Results demonstrated that the as-prepared coating exhibited excellent low and high temperature oxidation resistance, after isothermal oxidation at 900 °C and 1500 °C for 1320 h and 2080 h, respectively, the mass gains were 0.14% and 1.74%, respectively. The excellent anti-oxidation performance at 900 °C was ascribed to the defect-free structure of coating, while the formed compound Hf-Ta-Si-O glassy oxide layer on the coating surface was responsible for the excellent oxidation resistance at 1500 °C. Moreover, the Hf0.5Ta0.5B2-SiC-Si coating had good ablation resistance, after ablation for 60 s, the mass and linear ablation rates of the coated sample were 1.05 mg/s and −10.2 μm/s, respectively. The ablation behaviors of the coated sample mainly included thermal-physical and thermal-chemical erosion along with thermos-mechanical denudation.

Synergic Swelling of Interactive Network Support and Block Copolymer Films during Solvent Vapor Annealing.

We report the effect of "interactive" polymer network (PN) supports on the solvent-vapor processing of thin polymer films. Densely cross-linked surface-attached network exhibits under experimental time scale a glassy swelling behavior with the conformational states and solvent-uptake clearly sensitive to the degree of solvent vapor saturation in the atmosphere. Pretreatment of the thermally cured PN films by complete immersion or by swelling in saturated chloroform vapors facilitates relaxation of the residual stresses and induces irreversible changes to the network structure as revealed by the swelling/deswelling tests. The presence of a polymer film on top of the PN support results in a mutual influence of the layers on the respective swelling kinetics, steady-state solvent uptake, and chain dynamics. Using UV-vis ellipsometry, we revealed a significantly faster swelling and higher solvent uptake of glassy PN layer below a polymer film as compared to a single PN layer on silicon substrate. Remarkably, the swelling of the network support continues to increase even when the overall swelling of the bilayer is in a steady-state regime. Block copolymer films on PN supports exhibit a faster ordering dynamics and exceptional stability toward dewetting as compared to similar films on silicon wafers. The mechanical stress produced by continuously swelling PN is suggested to account for the enhanced segmental dynamics even at low solvent concentration in the block copolymer film. Apart from novel insights into dynamics of solvent uptake by heterogeneous polymer films, these results might be useful in developing novel approaches toward fast-processing/annealing of functional polymer films and fibers.