Surface Crystallization and Water Diffusion of Silica Glass Fibers: Causes of Mechanical Strength Degradation

Pristine silica glass fiber is well‐known to become mechanically weaker when heat‐treated in air but the cause of such weakening is not presently known. The time dependence of mechanical degradation of various silica glass fibers containing varying impurity contents were studied in the range from 500°C to 1000°C. Two possible sources of strength degradation were considered: surface crystallization and water diffusion. Surface crystallization kinetics of silica glass fibers were investigated in a wide temperature range, including nanoscale surface nucleation at low temperatures via scanning electron microscopy. From the comparison of the strength degradation, surface crystallization, and water diffusion data in literature, it was concluded that surface crystallization may be responsible for the mechanical weakening observed in silica glass fiber surface during heat‐treatment at temperatures above ~800°C, whereas water diffusion into the glass surface may be responsible for the strength degradation at lower temperatures.     

Graphitization at low temperatures (600–1200 °C) in the presence of iron implications in planetology

Our objective is to understand how graphite can be formed at “low” temperatures (<1200 °C) in contrast to the high temperature of the industrial processes (∼3000 °C), and from precursors which are non-graphitizable by a thermal treatment alone. Blends of iron and saccharose char were heated between 650 and 1600 °C. The carbons obtained were characterized by SEM, TEM and Raman microspectrometry. Our work confirms that graphite can be formed from non-graphitizable carbons during a heat-treatment in the presence of iron. Carbon and iron migrations, below the eutectic temperature (1150 °C), appear to be a key factor for carbon transformation. Iron migration and graphitization could be favored by nucleation of Fe nanoparticles and surface melting, detected as soon as 900 °C. This allows formation of turbostratic macroporous carbons. Above the eutectic, all iron is liquid and graphitization occurs; it is complete at 1600 °C. Heat-treatment duration, observed over 4 orders of magnitude, favors the structural improvement. Concerning applications in planetology these experimental samples are pertinent experimental analogues of natural carbons from differentiated parent-bodies (with an iron core), and explain how graphite can be formed at temperatures below 1200 °C in these environments.     

A novel B2O3/B4C-modified composite adhesive with wide operative temperature range for alumina fiber fabric bonding

A heat resistance adhesive with wide operative temperature range for bonding alumina fiber fabric was developed by organic-inorganic modification. The polyvinyl alcohol modified by B₂O₃ generated a complex cross-linked network connected by B–O–C bonds, which can enhance the bonding strength of adhesive after heat-treatment from RT to 400 °C. Moreover, the addition of B₄C can enhance the bonding strength of adhesive after heat-treatment from 400 °C to 800 °C due to the formation of molten B₂O₃ and borosilicate glass. Significantly, the appropriate addition of B₄C can make the adhesive form a denser structure without transforming the fracture mode of the bonding joints, which is conducive to enhance the strength performance of bonding joints after sintered at 800 °C. On the contrary, the excessive addition of B₄C will transform the fracture mode of the bonding joints into brittle fracture, which will degrade the strength performance of bonding joints.     

Compressive strength of fly‐ash‐based geopolymer concrete at elevated temperatures

This paper presents the compressive strength of fly‐ash‐based geopolymer concretes at elevated temperatures of 200, 400, 600 and 800 °C. The source material used in the geopolymer concrete in this study is low‐calcium fly ash according to ASTM C618 class F classification and is activated by sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) solutions. The effects of molarities of NaOH, coarse aggregate sizes, duration of steam curing and extra added water on the compressive strength of geopolymer concrete at elevated temperatures are also presented. The results show that the fly‐ash‐based geopolymer concretes exhibited steady loss of its original compressive strength at all elevated temperatures up to 400 °C regardless of molarities and coarse aggregate sizes. At 600 °C, all geopolymer concretes exhibited increase of compressive strength relative to 400 °C. However, it is lower than that measured at ambient temperature. Similar behaviour is also observed at 800 °C, where the compressive strength of all geopolymer concretes are lower than that at ambient temperature, with only exception of geopolymer concrete containing 10 m NaOH. The compressive strength in the latter increased at 600 and 800 °C. The geopolymer concretes containing higher molarity of NaOH solution (e.g. 13 and 16 m ) exhibit greater loss of compressive strength at 800 °C than that of 10 m NaOH. The geopolymer concrete containing smaller size coarse aggregate retains most of the original compressive strength of geopolymer concrete at elevated temperatures. The addition of extra water adversely affects the compressive strength of geopolymer concretes at all elevated temperatures. However, the extended steam curing improves the compressive strength at elevated temperatures. The Eurocode EN1994:2005 to predict the compressive strength of fly‐ash‐based geopolymer concretes at elevated temperatures agrees well with the measured values up to 400 °C. Copyright © 2014 John Wiley & Sons, Ltd.     

Reactivity of lime and related oxides. XIX. Production of strontium oxide

Strontium oxide has been prepared by calcining the hydroxide and the carbonate at several temperatures up to 1200°. Strontium hydroxide sinters considerably before decomposing above 500°. There is appreciable carbonation of the oxide products by atmospheric carbon dioxide at temperatures up to 800°. Strontium carbonate decomposes at a reasonable yield at temperatures above 1000°. The oxide products can be activated by milling, but they sinter when subsequently calcined, especially at temperatures between 800° and 1000°. Crystallite and aggregate sizes of the strontium oxide produced from the carbonate at 1000°–1200° are similar to those of lime prepared under corresponding conditions.     

Wide Color Variation in SiNO Thin Films Dispersed with Precipitated TiN Nano Particles

Amorphous thin films of Ti_1?ySi_y(N,O) with y ≥ 0.38 were prepared by reactive sputter deposition in a nitrogen atmosphere. Thermal annealing of the films in an ammonia flow above 800°C yielded Si(N,O) amorphous thin films dispersed with precipitated TiN nanosized particles. The film color changed with Si content y and the annealing conditions, from carrot orange to cream yellow in the as‐deposited films due to their oxynitride nature, and from dark green to canary yellow and from iron blue to horizon blue at respective annealing temperatures of 800°C and 900°C due to metallic nature of the TiN nanosized particles precipitated in the annealing.     

Effect of gas environment on properties of particles prepared by spray pyrolysis of metal nitrates

Particles in different states of oxidation were prepared by spray pyrolyses of aqueous solutions of silver and copper nitrates under different gas environments: air, nitrogen or mixture gas of 10 vol%H₂-N₂, respectively. Silver nitrate was converted to phase-pure silver at temperatures below 500 °C whose densification and crystallization were completed around 500 °C, irrespective of the gas environment. On the other hand, phase-pure copper(II) oxide was formed from copper nitrate up to 1,000 °C with air, but below 800 °C with nitrogen, above which copper(I) oxide was produced. Phase-pure copper particles were obtained with the mixture gas at temperatures above 400 °C. Copper(I) oxide was sintered and crystallized more easily than copper(II) oxide. The rates of the metallization, sintering and crystallization of copper were between those of silver and nickel.     

Tensile strength and morphological investigation of SiC-coated carbon fibers

SiC-coated film onto carbon fibers as a barrier of oxidation resistance and reaction between carbon fibers and metals was investigated. The chemical vapor deposition of silicon carbide onto carbon fibers was performed at various temperatures ranging from 700 to 1000°C using triisopropylsilane vapor carried by hydrogen gas. The strength of the SiC-coated carbon fibers was decreased due to deterioration of fibers and chemical attack of hydrogen on the surface of carbon fibers during the coating process. The oxidation and the thermal resistance of the SiC-coated carbon fibers compared to the uncoated carbon fibers were improved at temperature range of 600–800°C and 1000–1200°C, respectively. Morphological change by air oxidation at temperature range of 500–800‡C was also investigated for the SiC-coated and the uncoated carbon fibers, respectively. The SiC-coated film between carbon fiber and aluminum was sufficient as a barrier of reaction on carbon fiber reinforced aluminum at temperature of above 1000°C.     

Variable-range hopping conduction and negative magnetoresistance of disordered carbons at low temperature

Temperature dependence of the electrical conductivity in the range from room temperature down to 1.31 K was studied for three kinds of cokes heat treated at 900°C, bamboo char heat treated at 900°C and glassy carbon heat treated at 1000°C. Localized states as evidenced by variable-range hopping conduction (conductivity proportional to exp T^?1/4) was observed for these carbons at temperatures below 4 K. Room temperature Hall coefficient was studied for these carbons. Magnetoresistance was measured for the samples in the temperature range between 1.31 and 4.2 K. Positive magnetoresistance related to variable-range hopping conduction was observed for a bamboo char to increase with increasing magnetic field and with decreasing temperature. The mechanism of conduction can be interpreted by use of a model of density of states suggested by Davis and Mott for amorphous semiconductor. For cokes and glassy carbon negative magnetoresistance superposes with the positive magnetoresistance. Dependence of conductivity and of magnetoresistance on heat-treatment was investigated in the temperature range from 1.31 to 4.2 K for cokes heat treated at 1100, 1300 and 1500°C. Conductivity was found to be constant against temperature for these samples. With increasing heat-treatment temperature the contribution of negative magnetoresistance was observed for samples heat treated at 1500°C.     

Several properties of mineral admixtured lightweight mortars at elevated temperatures

The improvement of thermal and mechanical properties of mortars including expanded perlite aggregate (EPA) containing either clinoptilolite, a type of natural zeolite (NZ), waste glass powder (GP) or blast furnace slag (BFS) cured at elevated temperature was analyzed using thermal conductivity, compressive strength, flexure strength and dry unit weight. EPA mortar specimens were prepared by replacing a varying part of the portland cement with the above minerals. All mortar samples were prepared and cured at 23±1°C lime saturated water for 28 days. The maximum thermal conductivity of 1.3511W/mK was determined with the control samples containing plain cement. GP has shown 1 and 4% decrease for both 10, 20% GP and 25% EPA, respectively. Both BFS and NZ have a decreasing effect on thermal conductivity. The experiments were carried out, in which the samples were subjected to temperature of 300, 500 and 800°C for 2 h, then cooled in air. The results indicated that all the mortars exposed to temperature of 500 and 800°C shown a significant decrease in thermal conductivity, compressive strength and flexure strength. However, compared with the mortars including 25% EPA, adding the other admixtures at all level replacement decreased thermal conductivity, compressive strength, flexure strength and dry unit weight as a function of replacement percent. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of fine ceramic aggregates on the residual properties of concrete subjected to elevated temperature

The residual properties of concrete subjected to elevated temperature are of importance to assess the stability of the structure. This paper investigates the performance of concrete containing white ware ceramic sand exposed to elevated temperature. Concrete mixes containing 0%, 50%, and 100% ceramic sand were prepared. The specimen were exposed to elevated temperatures of 200°C, 500°C, and 800°C for a duration of 60 minutes. Their residual mechanical properties (compressive strength, split tensile strength), ultra sonic pulse velocity, and mass change for different cooling regimes were investigated and compared among specimen. The results showed that incorporation of ceramic sand in concrete mixes improved the resistance against elevated temperature of hardened concrete.     

Comparative study of aromatization selectivity during N‐heptane reforming on sintered Pt/Al2O3 and Pt‐Re/Al2O3 catalysts

BACKGROUND: The metal dispersed over a support can be present as small crystallites with sizes less than 5 nm. The smaller crystallites favour aromatization while larger crystallites favour cracking/hydrogenolysis. Sintering results in the agglomerization of smaller metal crystallites. Correlation of size with aromatization selectivity was investigated. RESULTS: The primary products of n‐heptane reforming on fresh Pt were methane, toluene, and benzene, while on fresh Pt‐Re, the only product was methane. Both catalysts exhibited enhanced aromatization selectivity at different oxygen sintering temperatures. The reaction products ranged from only toluene at 500 °C sintering temperature to methane at a sintering temperature of 650 °C with no reaction at 800 °C for the Pt/Al₂O₃ catalyst. On Pt‐Re/Al₂O₃ catalyst, methane was the sole product at a sintering temperature of 500 °C while only toluene was produced at a sintering temperature of 800 °C. CONCLUSION: This is the first time that sintering has been used to facilitate aromatization of supported Pt and Pt‐Re catalysts. A superior selectivity behaviour associated with bi‐metallic Pt catalysts is established. It was found that no reaction occurred on Pt catalyst after sintering at 800 °C whereas sintering Pt‐Re at 800 °C promoted aromatization solely to toluene. Copyright © 2008 Society of Chemical Industry     

Effect of nano silica and fine silica sand on compressive strength of sodium and potassium activators synthesised fly ash geopolymer at elevated temperatures

Environment friendly geopolymer is a new binder which gained increased popularity due to its better mechanical properties, durability, chemical resistance, and fire resistance. This paper presents the effect of nano silica and fine silica sand on residual compressive strength of sodium and potassium based activators synthesised fly ash geopolymer at elevated temperatures. Six different series of both sodium and potassium activators synthesised geopolymer were cast using partial replacement of fly ash with 1%, 2%, and 4% nano silica and 5%, 10%, and 20% fine silica sand. The samples were heated at 200°C, 400°C, 600°C, and 800°C at a heating rate 5°C per minute, and the residual compressive strength, volumetric shrinkage, mass loss, and cracking behaviour of each series of samples are also measured in this paper. Results show that, among 3 different NS contents, the 2% nano silica by wt. exhibited the highest residual compressive strength at all temperatures in both sodium and potassium‐based activators synthetised geopolymer. The measured mass loss and volumetric shrinkage are also lowest in both geopolymers containing 2% nano silica among all nano silica contents. Results also show that although the unexposed compressive strength of potassium‐based geopolymer containing nano silica is lower than its sodium‐based counterpart, the rate of increase of residual compressive strength exposed to elevated temperatures up to 400°C of potassium‐based geopolymer containing nano silica is much higher. It is also observed that the measured residual compressive strengths of potassium based geopolymer containing nano silica exposed at all temperatures up to 800°C are higher than unexposed compressive strength, which was not the case in its sodium‐based counterpart. However, in the case of geopolymer containing fine silica sand, an opposite phenomenon is observed, and 10% fine silica sand is found to be the optimum content with some deviations. Quantitative X‐ray diffraction analysis also shows higher amorphous content in both geopolymers containing nano silica at elevated temperatures than those containing fine silica sand.     

Catalytic activity of coal minerals in hydrogasification of coal

Hydrogasification of six bituminous coals was studied in a fixed-ped flow reactor at pressures up to 2 MPa and temperatures from 790 to 960 °C. Ranges of distinct methane formation are found with all coals between 500 and 600 °C, 750 to 800 °C and >850 °C. The reactions in the first two ranges are determined by the molecular structure of coal and are not affected by catalytic activities of constituents of coal minerals. In the third range, >850 °C, iron as a constituent of mineral matter of coal can accelerate methane formation significantly if the pressure is sufficiently high. Thermodynamic calculations indicate, and were verified by thermogravimetric studies, that iron disulphides in original coals can be desulphurized during gasification. Alkali and alkali earth oxides and carbonates can act as sulphur scavengers via an exchange reaction and thus accelerate the desulphurization of iron sulphides.     

Microstructural evolution and oxidation resistance of polyacrylonitrile-based carbon fibers doped with boron by the decomposition of B4C

Boron (B) was doped into polyacrylonitrile-based carbon fibers (CFs) in various contents by exposing the CFs in a vapor of B by the decomposition of B₄C. The structural evolution of B-doped CFs on different B concentrations and heat-treatment temperatures was studied using scanning electron microscope, transmission electron microscope, X-ray diffraction and Raman spectroscopy. It was found that introducing B changed the morphology of the CFs. At high temperatures, the strong erosion of B vapor, not only changed the primary structure of CFs, but also produced more flaws. Structural analysis indicates that B is mostly substitutionally bonded within the graphene sheet regardless of the experimental conditions. As increasing from 2200 to 2600 °C in the graphite crucible containing 10% B₄C, the concentration of substitutional B increased progressively from 0.81 to 2.57 at.%. Thermogravimetric analysis revealed that B-doping greatly improved oxidation resistance of the CFs with the oxidation onset temperature increasing from 640 °C (0 at.% B doped, 2.1% weight loss) to about 800 °C (3.65 at.% B doped, 0.3% weight loss). The correlation between structural evolution and oxidation resistance of B-doped CFs provides a new route for fabricating the CFs with oxidation resistance up to 800 °C.     

The influence of carbon source on the wall structure of ordered mesoporous carbons

A series of ordered mesoporous carbons (OMCs) have been synthesized by filling the pores of siliceous SBA-15 hard template with various carbon precursors including sucrose, furfuryl alcohol, naphthalene and anthracene, followed by carbonization and silica dissolution. The carbon replicas have been characterized by powder XRD, TEM and N₂ adsorption techniques. Their electrochemical performance used as electric double-layer capacitors (EDLCs) were also conducted with cyclic voltammetry and charge-discharge cycling tests. The results show that highly ordered 2D hexagonal mesostructures were replicated by using all these four carbon sources under the optimal operation conditions. Physical properties such as mesoscopic ordering, surface areas, pore volumes, graphitic degrees, and functional groups are related to the precursors, but pore sizes are shown minor relationship with them. The sources, which display high yields to carbons, for example, furfuryl alcohol and anthracene are favorable to construct highly ordered mesostructures even at high temperatures (1300 °C). OMCs prepared from non-graphitizable sources such as sucrose and furfuryl alcohol display amorphous pore walls, and large surface areas and pore volumes. The functional groups in the precursors like sucrose and furfuryl alcohol can be preserved on carbon surfaces after the carbonization at low temperatures but would be removed at high temperatures. The graphitizable precursors with nearly parallel blocks and weak cross-linkage between them like anthracene are suitable for deriving the OMCs with graphitic walls. Therefore, the OMCs originated from sucrose and furfuryl alcohol behave the highest capacitances at a carbonization of 700 °C among the four carbons due to the high surface areas and plenty of functional groups, and a declination at high temperatures possibly attribute to the depletion of functional groups. Anthracene derived OMCs has the lowest capacitance carbonized at 700 °C, and a steady enhancement when heated at high temperatures, which is attributed to the graphitization. The OMCs derived from naphthalene have the stable properties such as relatively high surface areas, few electroactive groups and limited graphitizable properties, and in turn medium but almost constant capacitances.     

Carbonization behavior of polyimide films with various chemical structures

Six kinds of polyimide films with different molecular structures were synthesized and carbonized up to 1100°C. The carbonization behavior of polyimides was followed by measuring the changes in weight, size, and electrical conductivity. Pyrolysis gases evolved on the way to carbonize up to 1000°C were analyzed by gas chromatography. All films showed appreciable shrinkage and became a black color above 500°C. No cracks and pores were observed on the films heated up to 1100°C, even under the scanning electron microscope. A large weight decrease of 35–50% was observed in a narrow temperature range from 500 to 650°C, which seemed to be due to the departure of CO groups as either CO or CO₂. An additional weight decrease occurred gradually above 800°C, due to N₂ departure. A remarkable increase of electrical conductivity along the film surface, more than 2 orders, was observed with the increase in heat-treatment temperature. The polyimide film with a flat molecular structure (PMDA/PPD) gave the highest conductivity: 3.7 × 10²S/cm.     

Effect of cooling methods on residual compressive strength and cracking behavior of fly ash concretes exposed at elevated temperatures

This paper presents the effects of cooling methods on residual compressive strength and cracking behavior of concretes containing four different class F fly ash contents of 10%, 20%, 30% and 40% as partial replacement of cement at various elevated temperatures. The residual compressive strength of the aforementioned fly ash concretes is measured after being exposed to 200, 400, 600 and 800 °C temperatures and two different cooling methods, for example, slow cooling and rapid water cooling. Results show that the residual compressive strengths of all fly ash concretes decrease with increase in temperatures irrespective of cooling regimes, which is similar to that of ordinary concrete. Generally, control ordinary concrete and all fly ash concretes exhibited between 10% and 35% more reduction in residual compressive strength because of rapid cooling than slow cooling except few cases. Cracks are observed over concrete specimens after being exposed to temperatures ranging from 400 to 800 °C. Samples that are slowly cooled developed smaller cracks than those rapidly cooled. At 800 °C, all fly ash concretes that are exposed to rapid cooling showed the most severe cracking. X‐ray diffraction analysis shows reduction of Ca(OH)₂ peak and formation of new calcium silicate peak in concretes containing 20% and 40% fly ash when subjected to 800 °C in both cooling methods. Thermo gravimetric analysis and differential thermal analysis results show increase in thermal stability of concrete with increase in fly ash contents. The existing Eurocode also predicted the compressive strength of fly ash concretes with reasonable accuracy when subjected to the aforementioned elevated temperatures and cooling methods. Copyright © 2014 John Wiley & Sons, Ltd.     

Thermal conductivities and Raman spectra of boron-doped carbon materials

Boron-doped carbons were prepared at temperatures from 1000 to 2800°C. The effect of boron doping on the thermal conductivity of carbons has been studied and discussed with the results from Raman scattering. Boron doping above 2200°C depressed the thermal conductivity of carbons and increased the intensity of the 1360-cm⁻¹ Raman band. It appeared that lowering the thermal conductivity is mainly caused by a distortion of the graphite lattice due to boron doping.     

Mechanical properties of porous carbon material: Woodceramics

The mechanical properties of Woodceramics which were made from medium-density fiberboard have been investigated with special reference to the effect of burning temperature on their bending Young's modulus and bending strength. Woodceramics made from beech wood have also been tested to clarify the compressive strength anisotropy, and the role of phenol resin impregnation in strengthening the beech based charcoal.The bending Young's modulus hardly varies for burning temperatures between 300 and 500°C, but it improves remarkably for burning temperatures between 500 and 800°C. The bending strength degrades with temperature for burning temperatures between 300 and 500°C, but it improves remarkably with increasing temperature of burning between 500 and 800°C. The bending Young's modulus and bending strength gradually degrade with temperature for burning temperatures at and above 2000°C.The compressive strength of beech wood burned at 800°C in the longitudinal direction is greater than that in the radial direction, which in turn is greater than that in the tangential direction; this reflects the anisotropy of wood. Woodceramics made from beech wood have a compressive strength superior to beech charcoal in any of the following three directions: 4.5 times in the longitudinal direction, 3.4 times in the radial direction, and 2.0 times in the tangential direction. Both for beech charcoal and beech Woodceramics, brittle fracture is brought about by the buckling of cell wall in compression along the longitudinal direction but by the bending of cell wall in the compression along radial and tangential direction.