Oxides reactions with a High-chrome sesquioxide refractory

In slagging coal-gasifier systems, the combination of oxides present as impurities in coal and combustion temperatures that can exceed 1650°C restrict the use of liner materials in the coal combustion chambers to refractories. In this study, the slag-refractory interactions of a new high chrome sesquioxide refractory was characterized. High-temperature cup tests showed that the molten oxides infused into the refractory and that the sesquioxide refractory reacts with the oxides in a manner similar to spinel phase refractories. Studies of the coal slag's individual oxide components showed CaO reacts with the chrome refractory to form a low melting Ca(CrO₂)₂. FeO reacts with the sesquioxide to form a interface layer of (Cr,Fe)₃O₄ spinel phase. Results of this study now make it possible to design studies for improving corrosion resistance to increase refractory life.     

Modeling of failure in a high temperature black liquor gasifier refractory lining

Recently, in the pulp and paper industry, black liquor gasification (BLG) has been of interest as a potential replacement for the Tomlinson recovery boiler. Some commercial-scale BLG units have been developed. However, the full potential of high temperature gasifiers has not been realized because of the refractory failure. Computer simulation of existing materials will accelerate materials research in developing new refractory materials. A model based on continuum damage mechanics is presented to analyze the failure behavior of gasifier refractory lining under high temperature and chemical reactive environments by using the finite element method. This work provides a new methodology for the failure analysis of refractory material and would provide a powerful tool in the development of the high temperature gasifier refractories.     

Synthesis of Al_2O_3-SiC Composite and Its Effect on the Properties of Low-carbon MgO-C Refractories

Al2O3-SiC composite was synthesized with pyrophyllite and natural graphite as raw materials by carbothermal reduction reaction under argon atmosphere. The effect of synthesis temperature on phase composition and microstructure was investigated. Low-carbon MgO-C refractories were prepared by using the synthesized Al2O3-SiC composite as additive. The effect of its addition on the slag penetration and corrosion resistance as well as oxidation resistance of the refractories was investigated, and the slag resist...     

Oxidation behaviour of a pressureless sintered AlN-SiC composite

The present study aims to investigate the oxidation behaviour of an AlN-SiC composite, pressureless sintered with the addition of Y₂O₃. Two main aspects are considered: (1) the evaluation of the oxidation kinetics in the temperature range 1300–1450°C for short term tests (30 h) and (2) the degradation of the flexural strength after oxidation at temperatures from 1000 to 1400°C for 100 h, in relationship with the microstructure of the exposed surfaces.The material starts to oxidize notably at temperatures higher than 1300°C. The oxidation kinetics is parabolic in the temperature range 1350–1450°C, the oxidation products are dependent on temperature and exposure time and are mainly constituted by crystalline mullite and alumina.The surface modification induced by long term oxidation does not affect mechanical strength until 1200°C, while after oxidation at 1400°C, the residual strength is about 25% of the starting one. These results are discussed in terms of the microstructure modifications induced by oxidation.     

Interfacial fatigue crack propagation in Ni-base superalloy protective coatings

It is important to control the interface strength of coatings and composite materials; a feature that has been noted for many years. However, how can interface strength be evaluated and controlled? In order to explore this aspect, subcritical fatigue crack growth behaviour was investigated along the interface of a CoNiCrAlY coated Ni‐Base superalloy. According to the traditional fatigue test methodology, fatigue crack propagation tests were carried out, using double cantilever beam specimens. The resistance to the fatigue crack propagation was successfully evaluated by a fracture mechanics approach. Particular attention was given to the effects of surface finish of the substrate as a coating parameter, the test temperature, and long‐term thermal ageing after the coating.     

Creep in polyethylene and hydroxyapatite reinforced polyethylene composites

Isochronous stress-strain relationships and long term creep performance for unfilled and hydroxyapatite filled polyethylene composites have been studied. The tests were carried out in a buffered (pH=7.5) Ringer's solution at 37°C. It was observed that the inclusion of hydroxyapatite does not remove the non-linear viscoelasticity of polyethylene. The creep resistance is found to increase with increase in volume fraction of hydroxyapatite. The creep failure of composites at long times can occur due to debonding of the interface.     

β-赛隆(Sialon)/刚玉复相耐火材料研究

采用氮化反应烧结工艺制备β－Ｓｉａｌｏｎ／刚玉复相耐火材料．结果表明,在低于１４５０～１６００℃下的流动氮气气氛中直接氨化反应一定比例的ＡＩ、Ｓｉ、Ａｌ_２Ｏ_３微粉,以及刚玉细粉和颗粒,可以制备不同Ｚ值的β－赛隆（Ｓｉ_６-ｚＡｌ_ｚＯ_ｚＮ_８-ｚ）／刚玉复相材料．但是,最终产物中的β－赛隆相的Ｚ值与设计值存在偏差,这可能与反应过程有关。不同Ｚ值的β－赛隆／刚玉复相材料均显示良好的抗渣铁侵蚀性．同时抗碱试验表明;当预设计Ｚ值为１．５～２．５时具有良好的抗碱性能,而当预设计Ｚ值等于４时,抗碱性下降,这可能与复相材料中Ｓｉａｌｏｎ含量及其Ｚ值有关．     

Investigation of the thermomechanical properties of industrial refractories: the French programme PROMETHEREF

This paper summarises the work that has been done in the framework of the French programme PROMETHEREF. This programme was concerned with the thermomechanical properties at high temperature of two industrial refractories: fused-cast materials for glass melting and alumina castables for steel production. At high temperature, both materials exhibit creep, that has been characterised by tension, compression and bending tests. The microstructural mechanisms of deformation have been investigated and allowed the macroscopic viscoplasticity to be understood. Both types of materials exhibit damage processes that have also been characterised mechanically and microstructurally. The nature and the adhesion of the aggregates have been shown to have a great influence on the mechanical behaviour of the castables, as well as the continuous zirconia skeleton observed in high-zirconia fused-cast refractories by X-rays tomography.     

Stress relaxation of a polycarbonate blend after hygrothermal aging

This work has studied the competing effects of physical aging and moisture absorption on the relaxation behaviour of a polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) polymer blend after being subjected to both thermal and hygrothermal aging. Physical aging was simulated by thermal aging the blend at temperatures up to 80 ^°C which is below the glass transition temperatures of ABS and PC in the blend, respectively. Nine different combinations of relative humidity and temperature were investigated for hygrothermal aging. In both cases, two sets of aging times were applied: “short” term (≤16 hour aging) and “long” term (@168 hour aging). The short term tests were used to generate momentary master curves by applying time/aging-time and time/moisture superposition principles. Using the Kohlrausch–Williams–Watts (KWW) expression, the model parameters determined from curve fitting master curves were used to predict longer term stress relaxation behaviour. The predictions, verified against experimental results, showed reasonable agreement for all hygothermal exposure conditions except for full immersion. Moreover, comparisons of KWW parameters for both short and long term tests suggested that physical aging processes dominate absorbed moisture effects in terms of influencing viscoelastic behaviour, especially when the aging temperature approaches 80 ^°C.     

Rapidly sintering nanosized SiC particle reinforced TiC composites by the spark plasma sintering (SPS) technique

Different content of nanosized SiC reinforced TiC matrix composites were fabricated at 1600°C by spark plasma sintering (SPS) without any aids. It was found that the materials could be sintered in a relatively short time (12 min) and low sintering temperature (1600°C) to satisfactory relative density (99%). The phase distribution and microstructure of composites have been investigated by optical microscopy and scanning electron microscope. Fracture toughness and Vickers hardness at room temperature were also measured by indentation tests. The results showed that nanosized SiC particles addition could inhibit the coalescence of TiC grains and increase fracture toughness of composites due to the crack deflections.     

MANUFACTURE AND PROPERTIES OF PLASMA SPRAYED Cr2O3

Low porosity (some %) chromia coatings have been plasma sprayed onto several substrates by atmospherical and vacuum plasma spraying. Different phases appear in the sprayed product; metallic Cr and the metastable tetragonal Cr₃O₄ phase have been identified. A post annealing treatment completely removes these phases. This behaviour can be correlated with differences between the coefficients of thermal expansion comparing the as-sprayed with the annealed material. Hot Isostatic Pressing improves the interface bonding between coating and substrate and reduces porosity as can be seen from the results of metallographical investigations. If aluminium is present in the substrate alloy, chemical interactions have been observed. First results from measurements of residual stresses in coatings show the influence of coefficients of expansion and substrate temperature during spraying. Gastightness of the sprayed chromia could not yet be established.     

The thermal diffusivity and conductivity of transformation-toughened solid solutions of alumina and chromia

The thermal diffusivity of a series of solid solutions of alumina and chromia transformation toughened with a dispersed phase of unstabilized zirconia was measured by means of the laser-flash method from room temperature to 1400° C. It was found, in general, that the thermal diffusivity could be decreased significantly by the combined effects of solid solution alloying, microcracking and by the presence of the low conductivity dispersed phase of zirconia. The decrease in thermal diffusivity by microcracking was found to be present in the solid solution with low chromia content which underwent extensive grain growth. The effectiveness of solid solution formation and microcracking on thermal diffusivity was found to be greatest at the lower and intermediate ranges of temperature. The decrease in the thermal diffusivity due to the zirconia inclusions was found to be effective over the total temperature range. A numerical example is presented for the thermal conductivity calculated from the thermal diffusivity multiplied by the volumetric heat capacity.     

Thermal characterization of screen printed conductive pastes for RFID antennas

Thermal resistance is an essential aspect of electronic circuits designing. It leads to unexpected changes in electronic components during their work. In this study, new materials for screen printed RFID tag's antennas were characterized in terms of their resistance to thermal exposure. Polymer materials containing silver flakes, silver nanopowder, carbon nanotubes or conductive polymer PEDOT:PSS were elaborated and used for antenna printing on flexible materials. In order to verify their long term susceptibility to damages caused by the changing environmental conditions, the temperature cycling test was used in three different temperature ranges: +65 °C, −12 °C, −40 °C/+85 °C (3 h in each temp., dwell time 1 h). The highest durability to thermal exposure exhibited the paste with carbon nanotubes dispersed in poly(methyl methacrylate) PMMA and the lowest one – the paste with conductive polymer PEDOT:PSS.     

Effects of high temperature mechanical damage on physical properties of unidirectional Ti/SiC metal matrix composite

Abstract

A miniaturised test system was used to investigate how the thermal and electrical properties of a unidirectionally reinforced titanium alloy (Ti–6Al–4V)/SiC (SM1140+)metal matrix composite change with mechanical damage at elevated temperature. Thermal conductivity and expansion measurements were obtained in the longitudinal and transverse direction both before and after short term strength and creep tests and at intervals during tests to assess changes in interface characteristics as functions of mechanical or thermal damage. The mechanical tests included monotonic stress–strain and ramp creep at temperatures between 500 and 650°C. The changes in thermal properties were compared with model predictions for the dependence of thermal properties on interface characteristics. The agreement was good for thermal expansion changes but not for thermal conductivity. This was ascribed to the nature of the damage at the interface that probably still allowed thermal transport but not mechanical load transfer.     

Experimental investigation of interfacial fracture behavior in foam core sandwich beams with visco-elastic adhesive interface

Interfacial toughness as well as toughness enhancement and long term interfacial fracture behavior in sandwich beams were experimentally examined. The interfacial toughness was enhanced by inserting a chopped glass fiber mat at the interface during the fabrication process. Robust interfacial load capacity and improved toughness were obtained by the method. The long term interfacial fracture behavior presents the characteristics of the three-parameter solid (standard solid) model. The results of the long term fracture tests show a significant increment in crack opening displacement and nominal stress intensity factors. It indicates that during the process of structural design and analysis the interfacial fracture resistance may be overestimated without taking the visco-elastic effect of the materials of the structure into consideration.     

Direct observation by high resolution autoradiography of interfacial diffusion in a Ni,Cr-TaC composite

Interfacial diffusion has been evidenced in a eutectic-like Ni, Cr-TaC composite, using high resolution autoradiographic techniques. The structure of the fibre-matrix interface has been determined by using high voltage electron microscopy. The fibre-matrix interface has been shown to be a diffusion short circuit in the temperature range 700° C to 900° C.     

耐火材料发展概述

本文简要介绍了中国耐火材料现状, 结合高温工业技术发展需求, 阐述了耐火材料的发展态势和发展方向。指出结构功能一体化设计与制备是以长寿化、功能化、轻量化、智能化、绿色化为特征的先进耐火材料发展的核心。结合新型高效隔热耐火材料、钢铁冶金连铸用先进功能耐火材料的研究, 介绍了先进耐火材料的组成-结构-性能-功能一体化设计理念与制备技术, 采用有限元数值模拟、融合先进陶瓷技术及梯度多层复合设计, 实现了关键服役性能的最优化设计与制备。     

The effect of thermal cycling on steam oxidation behaviour of TP347H FG at 650 °C

Abstract

The cyclic oxidation behaviour of fine-grained Type 347 stainless steel (TP347H FG) at 650 °C in air saturated steam and deoxygenated steam environments for 100–1000 h has been investigated. Electron microscopy, Energy Dispersive and Wavelength Dispersive X-ray Spectroscopy (EDS and WDS, respectively) have been used to characterise the samples. Short term oxidation tests have shown only haematite spallation occurs whereas longer term tests have shown magnetite also spalls on cooling to room temperature. In all cases cyclic oxidation showed spallation does not occur after long term tests and is only visible in small amounts after short term tests subsequent to the initial spallation event.     

Mechanisms of chromia scale failure during the course of 15–18Cr ferritic stainless steel oxidation in water vapour

Abstract

Breakaway oxidation of 15–18 % Cr ferritic stainless steels occurring in water vapour is described in the temperature range 800–1000°C. The failure of the protective chromia scale leads to iron oxide(s) nodule formation with accelerated kinetics. Characterisation of the (Fe,Cr)₂O₃ initial oxide scale by Raman spectroscopy and photoelectrochemistry shows chemical evolution with oxidation time, with increasing Cr/Fe ratio before haematite suddenly appears at the steel-oxide interface. The mechanisms for such a phenomenon are discussed, first on a thermodynamic point of view, where it is shown that chromium (VI) volatilisation or chromia destabilisation by stresses are not operating. It is rather concluded that mechanical cracking or internal interface decohesion provide conditions for haematite stabilisation. From a kinetic point of view, rapid haematite growth in water vapour compared to chromia is thought to be the result of surface acidity difference of these two oxides.     

Yttrium implantation effect on 304L stainless steel high temperature oxidation at 1000°C

Scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDXS) and in situ X-ray diffraction techniques were carried out to observe the oxide scale evolutions of yttrium implanted and unimplanted commercial 304L stainless steels during and after their high temperature oxidation at 1000°C for 100 h. Our results clearly demonstrate that yttrium implantation promotes a faster oxide scale growth and the formation of a more uniform chromia layer due to a higher chromium selective oxidation compared to unimplanted 304L stainless steel. Moreover, the presence of yttrium also leads to the formation of an enriched silicon layer at the metal-oxide interface limiting the growth of iron-based oxides which were not detected (even during cooling) in the case of yttrium implanted samples. These results allow to understand the low weight gain of yttrium implanted 304L stainless steel observed by thermogravimetry and underline the beneficial effect of yttrium implantation on the 304L oxidation resistance at high temperature.