Identification of local compactions in ceramics

Fractures of non-fired samples and samples heat-treated in the temperature range 900–1200°C made of a mixture of tinted granules molded at various pressure levels and non-tinted powder are investigated. It is demonstrated that a crack propagates along the boundaries of the granules simulating local compaction areas, and after firing at 1050 – 1100°C, the fracture becomes more level. At the same time, the system (the sample) passes to an unstable state which has the properties of a bifurcation. An increase in the heating rate from 2.5 to 6.5°C/min produces a shift in the transition from an uneven to a level fracture depending on the initial granule density (granule molding pressure). The identification and elimination of the principal bifurcations in a technological process opens up the possibility of developing ceramics with reproducible structure and properties.     

Combustion of carbon in the firing of ceramic heat insulation based on fuel-shale wastes

The firing of ceramic heat insulation based on fuel-shale wastes without traditional materials is investigated. On heating insulation in the range 400–600°C, most of the volatiles are removed. Above 800°C, pyrolytic forms of carbon (coke and semicoke) are produced. In the range 1050–1100°C, the reduction of iron is markedly accelerated. At 1100°C, there are practically no organic compounds in the fired ceramic.     

Development of technology for the production of microspherical alumina support for the alkane dehydrogenation catalyst: III. The effect of the phase composition of microspherical supports on their thermal stability

This publication continues a series of our reports on the optimization of preparation conditions for obtaining a thermally stable support for the alkane dehydrogenation catalyst. The phase composition effect on the stability, particle size distribution, structure, texture, and mechanical properties of supports heated to 1100°C is reported. Microspherical alumina supports obtained by successive thermal and hydrothermal treatments of gibbsite are compared to commercial supports obtained by the thermochemical activation (TCA) of gibbsite. The dimensions of the support granules decrease upon heating because of shrinkage, which is governed by the phase composition of the granules and by the packing of their constituent boehmite and alumina crystallites. Three temperature intervals can be distinguished in the shrinkage of the granules. In region I (<600°C), there is intensive shrinkage via the diffusion glide of crystallites, the mechanical strength of the granules remaining invariable. In region II (600–900°C), the polymorphic transformations of alumina accompanied by sintering via surface diffusion do not affect the dimensions and strength of the granules. In region III (>900–1000°C), shrinkage takes place via coalescent sintering. For commercial manufacturing of microspherical alkane dehydrogenation catalysts and for ensuring their stability at 550–900°C, it is recommended to use alumina supports containing the minimum possible amount of χ-Al₂O₃. As the single-phase boehmite support obtained by our technology is heated to 1100°C, its granules shrink by no more than 14.4% and show an attrition resistance of 89% or above. The support based on the gibbsite TCA products, which contains 14–23 wt % χ-Al₂O₃, is characterized by 3–5% greater granule shrinkage and 6–12% lower mechanical strength.     

The wetting-to-nonwetting transition of CVD BN coatings deposited at different temperatures

Boron nitride (BN) coatings (thickness 20–40 μm) were prepared on graphite substrates by chemical vapor deposition, with precursors of BCl₃ and NH₃ (ratio of 1:4) and pressure of 500 ± 50 Pa. The influence of the deposition temperature (650°C–1250°C) on the wettability of BN coatings with deionized water was studied. The wetting angle rapidly increases at 1100°C–1250°C, and the wetting-to-nonwetting transition occurs. The crystal structure and surface morphology of the BN coatings were characterized by a stylus instrument, scanning electron microscopy, and transmission electron microscopy. Research shows that the contact angle or nonwettability increases with a higher degree of crystallinity and a lower surface roughness, which were both under the control of the deposition temperature since the pressure and gas flows were kept constant in this study. At a deposition temperature of 650°C–950°C, the increase in the degree of crystallinity dominates; at 950°C–1100°C, the increase in surface roughness takes over. At 1100°C–1250°C, the degree of crystallinity continues to increase, while the surface roughness decreases due to the advantage of nucleation and the breakage of large surface clusters into smaller clusters. This results in increases (650°C–950°C), then decreases (950°C–1100°C) and again fast increases (1100°C–1250°C) in the wetting angle between the BN coating and deionized water and finally in the wetting-to-nonwetting transition (1100°C–1250°C).     

EFFECT OF REDUCING GASES ON THE TRANSVERSE STRENGTH OF FIRECLAY BRICKS1

It was reported that fireclay brick, when heated in the presence of carbon monoxide, were disintegrated by the progressive deposition of finely divided carbon at the “iron spots” in the brick. The conditions necessary for the occurrence of this phenomenon were not definitely known, although the known reversibility of the catalytic reaction around 650°C and the outcome of small scale experiments indicated that disintegration would not occur above this temperature. To obtain more definite information on this score, the effect of city gas at 550°C and 1100°C on the transverse strength of three brands of fireclay brick was determined. No significant changes in strength occurred at 1100°C. At 550°C two of the brands suffered very significant decreases in strength, but the other brand was unaffected, although it had the highest iron content.     

Study on the oxygen diffusion in the oxide layers of SiBCN ceramics by SIMS

SiBCN, SiC and SiC-BN ceramics/composites were prepared by mechanical alloyed combined hot-pressing sintering at 1900 °C, and the oxidation kinetics of SiBCN, SiC and SiC-BN were calculated based on the thickness of oxide layers at 1100～1500 °C. The oxide layer can be divided into outer and inner parts under 1300 °C. At 1100 °C, the oxygen molecules diffused in SiC through the gaps in lattice, while diffused in SiBCN by substituting the O in SiO₂. Moreover, BN(C) phase in SiBCN can slow down the generation rate of gases such as CO, N₂, NO₂ and B₂O₃.     

Formation of porous α-alumina from ammonium aluminum carbonate hydroxide whiskers

Ammonium aluminum carbonate hydroxide (AACH) whiskers prepared by hydrothermal technique were employed as precursor material for development of porous alumina. After compaction of AACH whiskers at 8 bars, calcination was performed at 650?°C followed by sintering at different temperatures. The sintered samples were characterized by XRD, FTIR, SEM and mercury intrusion porosimetry. Mechanical strength was determined by compression testing. At sintering temperatures of 1200?°C to 1400?°C, the % age porosity was around 80%. At 1500?°C, the percentage porosity decreased to 71%. The as-prepared AACH consisted of bundles of whiskers with diameters as thick as 0.7?µm, while an individual whisker had a diameter of about 100?nm with an aspect ratio of about 33. A two-phase mixture consisting of θ- and α-alumina was obtained at 1100?°C, while at 1200?°C and above, single phase α-alumina was formed. θ-alumina retained the bundle-like morphology. However, transformation to α-alumina was accompanied by formation of bead-like morphology. These beads were joined together through necks/stems within the whiskers as well as across the parallel-lying whiskers. These necks grew at 1300?°C to form aggregates with smooth surfaces. At 1400?°C, these aggregates started joining with each other by neck formation and at 1500?°C, a three-dimensional network was formed. For sintering temperatures of up to 1400?°C, pores with sizes around 260?nm were very stable. At 1500?°C, significant pore growth took place along with an overall densification. Therefore, number of pores with sizes of around 260?nm decreased and those with sizes around 10?µm, 1?µm and 5?nm increased. The compression strength of samples sintered at 1100?°C to 1300?°C was in the range of 3.4–4.3?MPa. At 1400?°C, the strength increased to 5.2?MPa, while at 1500?°C, it jumped to 10.8?MPa due to the formation of three-dimensional network.     

On a solar reflective ceramic based glaze for asphalt shingle

Solar reflective materials are one of the most effective solutions to counteract Urban Heat Island effect. Among them, asphalt shingles are one of the most widely used products. To improve solar reflectance of these surfaces usually both polymeric paint on the final product or ceramic glazes applied directly on the granules surface through rotary kiln are used. In this study the Design of Experiment approach is applied to an industrial formulation for ceramic glaze for asphalt shingles in order to find the optimal combination between pigment (Rutile and Talc), liquid phase (Sodium Silicate and Water) and heat treatment (700 °C −1100 °C). On the most significant samples, moreover, XRD and ESEM characterization has been performed in order to better understand the behaviour of the studied system. Interesting values in Solar reflectance were obtained, reaching ρ_sol=0.882 creating a good solar reflective product ready to be applied, through rotary kiln, on mineral granules for asphalt shingles.     

Optical and mechanical properties of transparent alumina fabricated by high-pressure spark plasma sintering

Transparent alumina was fabricated from untreated commercial powder by high-pressure spark plasma sintering (HPSPS) at temperatures of 1000, 1050 and 1100 °C under pressures of 250-800 MPa. It was established that transparency strongly depends on the HPSPS parameters. At all temperatures, there was a certain point when increasing the pressure led to decreasing transparency. At 1100 °C, relatively high pressure led to excessive grain growth, as well as the formation of creep-induced porosity at the center of the samples. Hardness values decreased with pressure due to grain growth, correlated with the Hall-Petch relationship. The optimal combination of optical and mechanical properties (68% in-line transmittance at a wavelength of 640 nm and a hardness value of about 2300 HV2) was achieved after sintering at 1050 °C under 600 MPa.     

Microstructural and mechanical characterization of a mullite fiber

The effect of thermal exposure on a mullite fiber was analyzed. This type of mullite fiber, consisting of γ-Al₂O₃ and amorphous SiO₂, was developed for high-temperature applications. Heat treatments at temperatures ranging from 900 °C to 1500 °C for 1h were performed in air. Investigations showed that the tensile strength of the initial fiber was about 1.60 GPa. And the elastic modulus was about 133.51 GPa. The bundles’ strength decreased at 900 °C slightly after thermal treatment, then increased and got a maximum at 1100 °C with 1.65 GPa. At above 1100 °C, the strength degraded sharply due to the mullite phase transformation and the exaggerated grain growth. At 1300 °C, the phase reaction almost finished with a tensile strength of 0.86 GPa. And the strength retention was only 47.50%. When the heat-treated temperature got to 1500 °C, the density of surface defects in the fiber surged, making it too fragile and weak to go through the tensile tests.     

Reactions of fuel-nitrogen compounds under conditions of inert pyrolysis

As part of a study of the chemical mechanisms involved in the conversion of fuel-nitrogen compounds to nitric oxide during combustion, fossil fuels and model nitrogen compounds were pyrolysed in helium in a small quartz flow reactor. Hydrogen cyanide was the major nitrogen-containing product obtained in all cases indicating that hydrogen cyanide is formed during the initial pre-flame stages of combustion and is the principal intermediate in the formation of fuel nitric oxide. At a nominal residence time of one second, 50% decomposition of pyrrole, quinoline, benzonitrile and pyridine occurs at 905, 910, 922 and 954 °C, respectively. The fraction of the nitrogen in pyridine that is converted to hydrogen cyanide increases from 40% at 960 °C to 100% at 1100 °C. Benzonitrile produces similar amounts of hydrogen cyanide (49 and 82%). The hydrogen cyanide yields from coals and residual fuel oils increase from the range of 15–25% at 950 °C to 23–42% at 1100 °C. It is not possible to determine from these single-stage experiments if most of the hydrogen cyanide forms in the primary pyrolysis process or in secondary reactions.     

Fatigue life prediction of thermal barrier coatings using a simplified crack growth model

Models that can predict the life of thermal barrier coatings (TBCs) during thermal cycling fatigue (TCF) tests are highly desirable. The present work focuses on developing and validating a simplified model based on the relation between the energy release rate and the TCF cycles to failure. The model accounts for stresses due to thermal mismatch, influence of sintering, and the growth of TGO (alumina and other non-protective oxides). The experimental investigation of TBCs included; 1) TCF tests at maximum temperatures of 1050 °C, 1100 °C, 1150 °C and a minimum temperature of 100 °C with 1 h and 5 h (1100 °C) hold times. 2) Isothermal oxidation tests at 900, 1000 and 1100 °C for times up to 8000 h. The model was calibrated and validated with the experimental results. It has been shown that the model is able to predict the TCF life and effect of hold time with good accuracy.     

Fabrication and oxidation behavior of Al4SiC4 powders

Al₄SiC₄ powders with high purity were synthesized by heating the powder mixture of aluminum (Al), silicon (Si), and carbon (C) at 1800°C in argon. The microstructure is characterized as platelike single grain. Both the nonisothermal and isothermal oxidation behavior of Al₄SiC₄ was investigated at 800°C‐1500°C in air by means of thermogravimetry method. It is demonstrated that Al₄SiC₄ powder possesses good oxidation resistance up to 1200°C and is almost completely oxidized at 1400°C. At 800°C‐1100°C, the oxide scales consist of an Al₂O₃ outer layer and a transition layer. Al₄SiC₄ remains the main phase. At 1200°C, some spallation resulting from the increment of Al₂O₃ and the mismatch of thermal expansion coefficient between different product layers can be observed. Above 1300°C, the oxide layer is composed of two part, i.e., large‐scale Al₂O₃ crystals (outer layer) and mullite with less amount of SiO₂ (inner layer). The oxidation behavior changes due to the different oxide products. For the reaction kinetics, a new kind of real physical picture model is adopted and obtains a good agreement with the experimental data. The apparent activation energy is calculated to be 176.9 kJ/mol (800°C‐1100°C) and 267.1 kJ/mol (1300°C‐1400°C).     

Ti3SiC2 interphase coating in SiCf/SiC composites: Effect of the coating fabrication atmosphere and temperature

The SiC fibers were coated with Ti₃SiC₂ interphase by dip-coating. The Ti₃SiC₂ coated fibers were heat-treated from 900 °C to 1100 °C in vacuum and argon atmospheres to comparatively analyze the effect of temperature and atmosphere on the microstructural evolution and mechanical strength of the fibers. The results show that the surface morphology of Ti₃SiC₂ coating is rough in vacuum and Ti₃SiC₂ is decomposed at 1100 °C. However, in argon atmosphere, the surface morphology is smooth and Ti₃SiC₂ is oxidized at 1000 °C and 1100 °C. At 1100 °C, Ti₃SiC₂ oxidized to form a thin layer of amorphous SiO₂ embedded with TiO₂ grains. Meanwhile, defects and pores appeared in the interphase scale. As a result, the fiber strength treated in the argon was lower than that treated in vacuum. The porous Ti₃SiC₂ interphase fabricated under vacuum was then employed to prepare the SiC_f/SiC mini composite by chemical vapor infiltration (CVI) combined with precursor infiltration pyrolysis (PIP), and can effectively improve the toughness of SiC_f/SiC mini composite. The propagating cracks can be deflected within the porous interphase layer, which promotes fiber pull-outs under the tensile strength.     

Controlling micro-porous size in TiO2 pellets processed by sol-gel and rapid liquid phase sintering

A fast and lower electric energy consumption process to synthesize TiO₂ pellets with interconnected micropores, is proposed. Pellets were prepared by rapid liquid-phase sintering (RLPS) at different temperatures (900, 1000 and 1100 °C) and times (2, 5, 7 and 10 min). The density of these samples increases when temperature rises and decreases for longer sintering times; the highest density, of 2.78 g/cm³ was obtained when sintering at 1100 °C/2min. The addition of PEG and the annealing at 450 °C/2 min produced pores of 38.51 ± 27.51 μm and 48.98 ± 32.34 μm when PEG3350 and PEG8000 respectively, were used. An additional RLPS at 1100 °C/2 min gives rise to TiO₂ pellets in a rutile phase, with pores of 76.82 ± 34.23 μm and 173.04 ± 68.03 μm for PEG3350 and PEG8000, respectively. Interconnectivity of pores is obtained in all samples. The elastic module of these pellets was 39.22 ± 0.16 GPa, for the sample prepared with PEG3350; and 121.30 ± 0.04 GPa for the one made with PEG8000. The achieved pore size and interconnectivity at 1100 °C/2 min are a result of the optimized sintering conditions and the better control of PEG vapor pressure released when the intermediate annealing at 450 °C/2 min is introduced.     

Microstructure and Li‐Ion Conductivity of Hot‐Pressed Cubic Li7La3Zr2O12

The effect of hot‐pressing temperature on the microstructure and Li‐ion transport of Al‐doped, cubic Li₇La₃Zr₂O₁₂ (LLZO) was investigated. At fixed pressure (62 MPa), the relative density was 86%, 97%, and 99% when hot‐pressing at 900°C, 1000°C, and 1100°C, respectively. Electrochemical impedance spectroscopy showed that the percent grain‐boundary resistance decreased with increasing hot‐pressing temperature. Hot pressing at 1100°C resulted in a total conductivity of 0.37 mS/cm at room temperature where the grain boundaries contributed to 8% of the total resistance; one of the lowest grain‐boundary resistances reported. We believe hot pressing is an appealing technique to minimize grain‐boundary resistance and enable correlations between LLZO composition and bulk ionic conductivity.     

Effect of porous properties on self-cooling of fired clay plate by evaporation of absorbed water

Porous ceramic plates were prepared from clay and wood charcoal powder at 900 and 1100?°C and their porous properties, water absorption and the cooling effect of porous plates were investigated to produce eco-friendly porous ceramics for cooling by the evaporation of absorbed water. Porous properties were dependent on the firing temperature, and total pore volume, average pore size and porosity, which were 0.38–0.39 cm³/g, 0.15–0.17 μm and 49–50%, respectively at 900?°C and 0.31–0.33 cm³/g, 2.47–2.59 μm and 43–44%, respectively at 1100?°C. By the addition of wood charcoal powder, the cooling rate of porous plate fired at 1100?°C was 1.7 times faster than that of the plate fired at 900?°C and the cooling temperature difference (?T) was around 2.3?°C at 22.5?°C and 52–54% of relative humidity and around 3.2?°C at 29?°C and 77–80% of relative humidity. The porous ceramic plates developed here are potential materials for cooling buildings.     

Effects of sintering atmosphere on the densification and microstructure of yttrium aluminum garnet fibers prepared by sol-gel process

Yttrium aluminum garnet (YAG) fibers were prepared by a sol-gel method, and then sintered in air or nitrogen atmosphere, respectively. The effects of sintering atmosphere on the densification process and microstructure of YAG fibers were investigated. No obvious difference can be found in the fibers sintered below 800 °C. At 1100 °C, the grain size of YAG fibers sintered in nitrogen is much smaller than in air. This difference is much clearer at the higher temperature of 1200 °C. The fine grains are explained by the existence of residual carbon in YAG fibers, which can be trapped at the grain boundaries to hinder the movement of grain boundary. Meanwhile, the densification degree of fibers sintered in nitrogen is higher than in air at 1200 °C, due to the smaller grain size and higher oxygen vacancy concentration generated in the nitrogen atmosphere, which leads to a higher fiber densification rate.     

Deposition Rate,Texture, and Mechanical Properties of SiC Coatings Produced by Chemical Vapor Deposition at Different Temperatures

Silicon carbide (SiC) coatings were produced on carbon/carbon (C/C) composites substrates using chemical vapor deposition (CVD) at different temperatures (1100°C, 1200°C, and 1300°C). The deposition rate was found to increase with deposition temperature from 1100°C to 1200°C. From 1200°C to 1300°C, the deposition rate decreased. SiC coating produced at 1200°C exhibited a strong (111) texture compared with the coatings produced at other temperatures. Both hardness and Young's modulus were also found to be higher in the coating produced at 1200°C. The variation in mechanical properties with the increase in temperature from 1100°C to 1300°C showed a direct correlation with the change in deposition rate and (111) texture. Microstructure analysis shows that the change in CVD temperature leads to the change in grain size, crystallinity, and density of stacking faults of SiC coatings, which appears to have no significant effect on mechanical properties of SiC compared with the texture observed in SiC coating. For the coating deposited at 1200°C, both the hardness and Young's modulus increased gradually from the substrate/coating interface to the top surface. The nonuniformity of mechanical properties along the cross‐section of the coating is attributed to the nonuniform microstructure.     

The role of SiC as a diffusion barrier in the formation of graphene on Si(111)

In this paper, we investigate the role of SiC as a diffusion barrier for Si in the formation of graphene on Si(111) via direct deposition of solid-state carbon atoms in ultra-high vacuum. Therefore, various thicknesses of the SiC layer preformed on the Si substrates were produced in order to evaluate its influence on the quality of graphene formation at different substrate temperatures from 900 °C to 1100 °C. At a given temperature of 1100 °C, we found that a thicker SiC layer can suppress silicon-out diffusion from the substrate and improve the structural quality of the graphene layer. The samples were analyzed by low energy electron diffraction, Auger electron spectroscopy, X-ray photoemission spectroscopy, Raman spectroscopy, and scanning tunneling microscopy.