Glassy Layer Research Articles

Microstructure and oxidation behaviors of dense mullite‑silicon carbide‑silicon coating for graphite fabricated by dipping-pyrolysis and reactive infiltration

A new-designed mullite-SiC-Si coating with dense structure for protecting graphite materials was prepared by dipping-pyrolysis combined with reactive infiltration. The phase synthesis, microstructure and anti-oxidation property of the fabricated mullite-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 1600 °C for 752 h and 550 h, respectively, the mass gains were 0.1% and 0.99%, respectively. The excellent anti-oxidation performance was ascribed to the dense structure of coating and the formed glassy oxide layer on the coating surface. The oxidation process of mullite-SiC-Si coating was controlled by the diffusion of oxygen, and the as-prepared coating after oxidation was composed of oxide layer and dense mullite-SiC-Si layer. In contrast to SiC-Si coating, the high temperature stability of mullite-SiC-Si coating was greatly improved.

Microstructural evolution and oxidation behavior of TiB2[sbnd]SiC[sbnd]B4C composite fabricated by reactive spark plasma sintering

Microstructural evolution and oxidation behavior of TiB2SiCB4C composite fabricated from TiSiCB4C system with different weight ratios via reactive SPS were investigated in detail. Based on the characterizations of SEM, XRD, TEM and HRTEM, it was found that the solid diffusion induces reactions between Ti and B4C to form TiB2(TiB), C and Ti(B,C,Si). Afterwards, the results of oxidation tests at 800 °C, 900 °C and 1000 °C demonstrated that the sample with highest TiB2 contents performed the best oxidation resistance due to the lowest porosity. Furthermore, the smoothly glassy oxidation layer above substrate composed of TiO2 and B2O3 became thicker with temperature increasing, meanwhile, the vaporized B2O3 recrystallized into plate-like, rose-like even large-monolithic grains.

High temperature oxidation resistance of La2O3-modified ZrB2-SiC coating for SiC-coated carbon/carbon composites

To improve the long-term anti-oxidation performance of carbon/carbon (C/C) composites at high temperature, La2O3-modified ZrB2-SiC (ZSL) coating was prepared on SiC-coated C/C composites by pack cementation. The results indicated that significant improvement was achieved on the oxidation resistance of ZSL coated specimens compared with ZrB2-SiC (ZS) coating for SiC-coated C/C composites at 1500 °C and 1600 °C. The mass loss of the ZSL coated specimen was merely 0.6% after a static oxidation test at 1500 °C for 550 h, while that of the ZS coated specimen was 1.89% at the same condition. And the ZSL coating can also protect C/C composites from oxidation for 107 h at 1600 °C with the mass loss of 0.92%. Moreover, the mass gain of ZSL coated specimen was 0.35% when it subjected to the thermal shock between 1500 °C and room temperature for 50 times. The excellent protection ability against oxidation and thermal shock could be attributed to the synergetic effects of generated La-Si-O compound glassy layer and thermally stable phase ZrSiO4.

The effects of carbon/carbon composites blasting treatment and modifying SiC coatings with SiC/ZrB2 on their oxidation and cyclic ablation performances

To enhance the oxidation and ablation performances of SiC coating, C/C composites with a porous surface layer were designed via blasting treatment and then followed by ZrB2 additive in the process of pressure-less reactive sintering. Cyclic ablation test using oxyacetylene torch from 1750 °C to room temperature showed that the coating possessed good ablation performance accompanying with the formation of Zr-O-Si glassy protective layer. The good sintering performance combined with the increased coating/substrate interfaces contributed to reduce the cyclic thermal stress and enhance the stability of the Zr-O-Si layer to the denudation of oxyacetylene torch, thus promoting the ablation performance.

Effect of Mineralogical Structure of Flux Film on Slab Quality for Medium Carbon Steel

In order to avoid the slab quality problems in the process of continuous casting, mineralogical structure of flux film for medium carbon steel was studied systematically and the reasonable mineralogical structure of flux film was confirmed by combining with the lubrication and heat transfer characteristics of the mould flux. The results showed that layered structures of flux film were mostly crystallization phase, and the dominating phase was akermanite, with a small number of cuspidine and wollastonite. The akermanites were distributed on the shell side, wollastonites were spread in the middle of the flux film, and cuspidines were scattered in the glassy layer which was adjacent to the mould side. According to the analysis, the crystallization rate of flux film should be increased and the contents of wollastonite and cuspidine should be reduced to avoid longitudinal cracks and sticking breakout in continuous casting.

Effect of chromium additive on sintering and oxidation behavior of HfB2-SiC ceramics

The effect of chromium admixture on the processes in the HfB2-SiC ceramic powder system during its pressureless sintering at 1600 °C was studied. It was shown that an increase in chromium content from 0% to 15.5% in the HfB2-SiC ceramic powder mixture leads to a continuous increase in its relative density up to 90%. A transient liquid phase Cr-Si-C-B is formed at 1600 °C, and it promotes intense sintering of HfB2 and SiC powders. The oxidation resistance of HfB2-SiC-Cr ceramics was studied in static air at 1000–1500 °C. It was shown that the oxidation resistance is greatly improved due to a decrease in the porosity of the sintered ceramic system because of chromium additive. The presence of chromium oxide in the formed surface glassy layer can also lead to the increase in the oxidation resistance. These results suggest that chromium can be considered as a promising sintering additive for HfB2-SiC and similar systems.

Numerical simulation and experimental investigation on fracture mechanism of granite by laser irradiation

Numerical simulation and experimental tests are conducted to determine the stress distribution and fragment mechanism of granite during laser perforation. The mechanisms of material removal through laser perforation are spallation, melting and ablation. High intensity laser energy concentrates locally on the granite surface area and causes the local temperature to increase instantaneously. The closer the center of laser beam is, the higher the temperature is. With the laser power increasing the size of laser perforation hole is expanded. In addition, the high heating and cooling rates induced by laser irradiation brings out the generation of high tensile stress (481–536 MPa), which is far higher than the tensile strength (11 MPa) of granite sample, and fracture or spallation occurs. Fracture analysis showed the inner wall of the glassy layer after laser perforation is smooth with lots of pores. Moreover, there is a hackle region with regular strips of micron/nano scale at the edge to the glassy layer, and the secondary cracks propagate perpendicular to the regular strips. Compared with the fracture morphology of the glassy layer, the fracture surface of the interfacial transition zone is coarse, and obvious secondary cracks also generate. Obviously, the fracture mechanism of granite faced to laser irradiation is the typical brittle fracture.

First‐principles study in an inter‐granular glassy film model of silicon nitride

AbstractBased on a previously constructed intergranular glassy film (IGF) model for bulk silicon nitride, a large periodic model of 3864 atoms containing 2 grains of different orientations from the main bulk β‐Si3N4 and 2 IGFs was fully relaxed using the ab initio density‐functional theory package VASP. The relaxed structure was then used to calculate the electronic structure, density of states, interatomic bonding, partial charge distribution, and electron localization using the OLCAO method. Analysis of the data focuses on the interfacial regions between bulk β‐Si3N4 and the Si‐O‐N glass layer with different orientations. The total bond order density (TBOD) is evaluated in different interfacial and bulk regions. We show minor differences in the internal cohesion between crystalline grains of different facets. However, the overall charges in the bulk crystal grains and in the glassy regions are electropositive which are balanced by the negatively charged interfacial region between the two. The presence of a less rigid glassy layer is the reason for structural flexibility in ceramics without a huge penalty in the steric energy. The optimization of the interfacial structure and bonding via the creation of defective sites is the atomic origin for the existence of the IGF in silicon nitride. The insights obtained from this detailed quantum mechanical analysis of a realistic IGF model are discussed including implications on the strength, fracture toughness, and processing methods. We also discuss the potential applications of our method to other complex materials systems.

Oxidation protective ZrB2-MoSi2-SiC-Si coating for graphite materials prepared by slurry dipping and vapor silicon infiltration

In order to protect graphite materials from oxidation, a dense single layer ZrB2-MoSi2-SiC-Si coating was for the first time prepared on the surface of graphite substrate by slurry dipping and vapor silicon infiltration process. The phase compositions and microstructure of the prepared coating were analyzed by XRD and SEM. The isothermal thermal cycle oxidation tests were conducted at 1600 °C in air. The results showed that the prepared coating exhibited excellent oxidation protection ability, after oxidation for 150 h, the weight loss was only 0.21%, which was considered to be the depletion of the outer coating and the appearance of defects in the coating. The excellent protection ability was attributed to the dense structure of the coating and the formation of thick oxide layer with immiscible ZrO2 on the outer surface of coating. Two layers could be observed in the coating after oxidation, namely, glassy oxide layer and ZrB2-MoSi2-SiC-Si layer. After thermal shock between 1200 °C and room temperature for 100 cycles, the weight gain was 0.11%. The good thermal shock resistance could be ascribed to the gradient thermal expansion coefficients of different components in the coating and the C/SiC interface layer between the substrate and coating.

Oxidation behaviour of laminated BN/ZrB2–SiC ceramics

Laminated BN/ZrB2–SiC ceramics were successfully fabricated by tape casting and hot-pressing. The oxidation behaviour of the ceramics at 1300, 1500 and 1600 °C in air was investigated. The results indicated their flexural strength and fracture toughness reached up to 583 ± 27 MPa and 16.1 ± 0.4 MPa m1/2, respectively, after oxidation at 1300 °C for 0.5 h. These values were higher than those obtained at room temperature (381 ± 31 MPa and 11.8 ± 0.5 MPa m1/2). The improved mechanical properties could be attributed to the slight oxidation and healing effect to the defects of the SiO2 glassy layer produced during oxidation. At 1600 °C, the mechanical properties decreased constantly, which was attributed to the substantive oxidation and formation of a thick, porous ZrO2 layer under the thin, dense SiO2 layer.

Oxidation and ablation resistant properties of pack-siliconized Si-C protective coating for carbon/carbon composites

Uniformly SiC coating was established on carbon/carbon (C/C) composites using halide activated pack cementation (HAPC). The siliconizing temperature in HAPC process played an evident effect on the deposition efficiency and oxidation behavior of SiC coating. The SiC coating revealed a typical crystalline structure with limited porosity and combined well with the substrate. Oxidation tests showed that the coating can withstand the 25 h of continuous isothermal oxidation at 1773 K and 10 thermal cycles between 1773 K and room temperature due to the good self-healing ability of formed glassy SiO2 scale. An oxyacetylene flame test demonstrated that the SiC coating can effectively protect C/C composites from ablation for 45 s. The ablation central region presented a discontinuous and damaged morphology due to the volatilization of oxides and thermos-mechanical denudation, while a continuous glassy SiO2 layer as the primary ablation barrier availably prevented the transitional region from being heavily oxidized.

Influence of molecular weight on molecular dynamics and dynamic rheology of polypropylene glycol filled with silica.

Molecular weight strongly influences the molecular dynamics and rheological responses of nanocomposites, which is far from being well understood. Herein molecular dynamics and rheological behaviors of hydrophilic fumed silica filled unentangled polypropylene glycol (PPG) were investigated as a function of weight averaged molecular weight (Mw) of PPG and volume fraction (∅) of silica. It is shown that Mw does not affect the glassy layers surrounding the nanoparticles and the segmental dynamics of the mobile PPG phase. On the other hand, the mobile PPG phase in the highly filled nanocomposites exhibits an abnormal “more fragile” to “stronger” transition with increasing Mw. The reinforcement and thinning behaviors are stronger in lower-Mw nanocomposites with the “more fragile” mobile PPG phase. The results suggest that reinforcement of nanocomposites affects the dynamic fragility of the mobile phase of the matrix.

Solid-State Electrochemical Bidirectional Alkali Vapor Source for Cold Atom Microsystems

Cold atom microsystems have the potential to bring the precision measurement capabilities of cold atom clocks, gyroscopes, and accelerometers to portable systems and platforms (1). Trapping cold atoms in a vapor cell requires controlling alkali vapor pressure, which is a function of temperature. Using temperature control to control alkali vapor pressure is common in the lab, but requires too much power for portable microsystems. Techniques such as light-induced atomic desorption, pulsed alkali dispensers, and double MOT chambers have also been used to control alkali vapor density in the lab, but none is rapid, bidirectional, low-power, and miniaturizable as needed for portable cold atom microsystems. Solid ion conductors have been used with widely spaced Pt electrodes as alkali sources, but operate at high voltages (≥50V) and elevated temperatures (≥120°C) (2), thus requiring more power that allowable for portable microsystems applications. We present a low-power bidirectional alkali vapor source based on solid-state electrochemistry. The device comprises a solid electrolyte, beta”-alumina, in between two electrodes. One electrode contacts the alkali vapor inside the vapor cell and consists of a uniform 10nm glassy carbon layer covered with widely spaced Pt lines. The other electrode, which serves as a solid-state alkali reservoir, consists of widely space Pt lines and graphite particles. The thin glassy carbon layer getters rubidium until it becomes saturated. When a positive voltage is applied across the device electrodes, alkali atoms in the glassy carbon layer are oxidized and alkali ions transport into the solid electrolyte. This reduces the rubidium concentration in the glassy carbon and, rubidium vapor gettering resumes, causes rubidium vapor pressure reduction. Alkali ions are reduced at the other electrode and alkali atoms are intercalated in the graphite in the solid state alkali reservoir to maintain ion balance in the solid electrolyte. When a negative voltage is applied the process reverses. Alkali ions are reduced at the glassy carbon electrode, thus quickly saturating the glassy carbon with alkali atoms to stop rubidium gettering and further providing additional rubidium to evaporate and increase the rubidium vapor pressure in the vapor cell. Reversible actuation with this device has been demonstrated with voltages as low as 5 V and peak power as low as 3.4 mW. Furthermore, we will present the use of this device in a feedback loop to control Rb vapor density. The ability to electrically control alkali vapor pressure at low power is an enabling advance for cold atom microsystems. This material is based upon work supported by the Defense Advanced Research Projects Agency (DARPA) and Space and Naval Warfare Systems Center Pacific (SSC Pacific) under Contract No. N66001-15-C-4027. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of DARPA or SSC Pacific. 1. J. Kitching, S. Knappe and E. A. Donley, IEEE Sensors J, 11, 1749 (2011). 2. J. J. Bernstein, A. Whale, J. Brown, C. Johnson, E. Cook, L. Calvez, X. Zhang and S. W. Martin, in Solid-State Sensors, Actuators and Microsystems Workshop, p. 180, Hilton Head Island, South Carolina (2016).

Surface Protection of Low-Melting Glass Based Silicon Substrates for Silicon Transistor Fabrication

Different methods are proposed for protecting the surfaces of silicon substrates in the fabrication of silicon transistors. The results of thermal operations with technological regimes for obtaining a low-melting glassy layer in order to protect the surface of p – n junction substrates from different external actions are presented.

Ca-α-SiAlON:Eu Phosphors: Oxidation States, Energy Transfer, and Emission Enhancement by Incorporation-Aimed Surface Engineering.

Ca-α-SiAlON:Eu is one of the most promising phosphors exhibiting potential applications in high power LEDs owing to its excellent thermal stability and relatively high luminescence efficiency comparable to YAG:Ce commercial phosphors. The oxidation states of Eu ions in Ca-α-SiAlON:Eu are still questionable, and furthermore it exhibits lower luminescence intensity than commercial yellow phosphors. Therefore, in the present work, the valences of Eu ions in Ca-α-SiAlON have been examined, from which a mixture of divalence and trivalence is observed. Further improvement in emission intensity involves finding a way to increase the incorporation of larger Eu ions into the Ca-α-SiAlON. Here, we observed preferred doping of Eu around phosphor grain surface and then propose a surface engineering strategy involving HF pickling of glassy surface layer, formation of Eu-rich precursors, and finally post-annealing, aiming to increase surface incorporation of Eu ions. The surface engineered samples exhibit great enhancement in emission intensity with a maximum increment by 80%.

Effect of Filler Morphology and Distribution State on the Linear and Nonlinear Mechanical Behavior of Nanofilled Elastomers

We extend the model proposed by Merabia et al. [2008] regarding reinforcement mechanisms of filled elastomers. This model is based on the presence of glassy layers around filler particles which may bridge neighboring particles. The model was solved by Merabia et al. for spherical particles only and for a single dispersion state. However, experiments show that mechanical properties depend crucially on the complex shape of fillers as well as on their spatial distribution. We consider extensively both aspects in this article. We show that the distribution state of the fillers is key for controlling the reinforcement at high temperature. For a given distribution state, we show a strong effect of filler morphology on reinforcement. Distances between fillers are smaller with fractal aggregates, which leads to stronger reinforcement. Our model opens the path for the development of systems with tailored properties by tuning the filler distribution state and morphology.

Effect of calcium compounds on lightweight aggregates prepared by firing a mixture of coal fly ash and waste glass

Abstract Mixtures of coal fly ash and waste glass are amended with three calcium compounds, pressed into green pellets, and fired to prepare lightweight aggregates at 1050–1175 °C. The aggregate particle density, being associated with microstructure, depends on both firing temperature and calcium species. Calcite and gypsum are found to be good flux promoters. In contrast, calcium chloride is an ineffective flux promoter due to the poor solubility between calcium chloride and coal fly ash/glass mixture, thus the glassy layer formed is too insufficient to effectively envelope the gases/vapor released from pellet interior. Further, the calcium chloride-containing lightweight aggregates prepared at 1050 °C and 1100 °C sorb much more water than those amended with the other two calcium compounds. Such higher water sorption capacity results from the poor solubility of major phases (present in coal fly ash/glass mixture) in calcium chloride liquid phase during firing, as evidenced by the prominent presence of sinjarite crystalline phase in the fired pellets. In fact, the 1050 °C and 1100 °C calcium chloride-containing lightweight aggregates were observed to slightly dissolve in the 24-h water sorption capacity test, thereby forming more open pores. Firing the calcium compound-amended green pellets leads to disappearance of corundum and wustite phases, while with crystobalite, diopside and wollastonite emerging as new phases.

In situ stable crack growth at the micron scale

Grain boundaries typically dominate fracture toughness, strength and slow crack growth in ceramics. To improve these properties through mechanistically informed grain boundary engineering, precise measurement of the mechanical properties of individual boundaries is essential, although it is rarely achieved due to the complexity of the task. Here we present an approach to characterize fracture energy at the lengthscale of individual grain boundaries and demonstrate this capability with measurement of the surface energy of silicon carbide single crystals. We perform experiments using an in situ scanning electron microscopy-based double cantilever beam test, thus enabling viewing and measurement of stable crack growth directly. These experiments correlate well with our density functional theory calculations of the surface energy of the same silicon carbide plane. Subsequently, we measure the fracture energy for a bi-crystal of silicon carbide, diffusion bonded with a thin glassy layer.

Preparation, ablation behavior and thermal retardant ability of C/C-HfB2-SiC composites

C/C-HfB2-SiC composites were prepared by precursor infiltration and pyrolysis followed by reactive melt infiltration. Thermogravimetric analysis showed that C/C-HfB2-SiC composites possessed better oxidation resistance than that of C/C composites. Ablation behavior and thermal retardant ability of C/C-HfB2-SiC composites were investigated using oxyacetylene torch. Compared with C/C, thermal retardant ability of C/C-HfB2-SiC composites was improved by 80.45%, which was 406°C/mm. After ablation for 90s, the linear and mass ablation rates of C/C-HfB2-SiC composites were reduced by 73.59% and 77.84%, which were 2.06μm·s−1 and 0.129mg·cm−2·s−1, respectively. Two typical morphologies of HfO2 skeleton layer and borosilicate glassy layer were formed after ablation, which were indicated to be beneficial for the improvement of thermal retardant ability and ablation performance.

Fire resistance and mechanical properties of enamelled aluminium foam

Aluminium foams show very interesting properties for a wide range of applications, in particular good mechanical properties and fire resistance. The particular cellular structure reduces the corrosion resistance of these materials in several environments. The application of a vitreous enamel layer is a possible solution to obtain a good protective barrier that can also improve the resistance at high temperatures. In this work these properties are evaluated for different porcelain enamelled aluminium foam samples. Starting from plate samples of aluminium foam produced by Alporas method, different enamel systems are studied. A frit without vanadium suitable for aluminium substrate was used. Microstructure analysis was made to highlight the adhesion between enamel and metallic substrate, as well as the filling of the open cells and the presence of defects in the glassy layers. The response to flame exposure of enamelled samples was tested in comparison to the uncoated foam. Finally, mechanical properties were studied by 4-points bending tests. This study has shown that the presence of the vitreous enamel coating increases the fire resistance of the aluminium foam. In addition, it was found that the flexural rigidity increases strongly and it is connected with the particle dimension in the frit.