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1.
We investigated the sintering behavior of Cr2O3–Al2O3 ceramic materials. In our observation of the isothermal shrinkage behavior of Cr2O3–Al2O3 ceramic, the activation energy of sintering reaction was measured to be 102 kJ/mol, that is, the near value of the activation energy of diffusion of Al ions in Al2O3 single crystal. Therefore the diffusion of cations is believed to control the sintering behavior of this material. With the addition of TiO2, (the compound chosen to accelerate the diffusion of cations) to Cr2O3–Al2O3, the sintering behavior was accelerated.  相似文献   

2.
《Ceramics International》2020,46(9):13414-13423
The element/phase loss is undesirable but existing during selective laser melting (SLM) of materials with volatile element/phase, which not only changes the material composition but also affects the molten pool flow. In the previous researches, the effect of remelting on the element/phase loss was neglected during the SLM process, instead, laser energy density was thought to be uppermost. In fact, the SLM process fabricates the parts in a manner of line by line and layer by layer, i.e., additive character, and the remelting in the overlap zone occurs during the SLM process. In this paper, three different Al2O3 loss prediction models of SLM Al2O3–Al composite by considering the additive character of SLM and the distribution of the Al2O3 associated with the different molten pool driving forces were developed. By comparing with the experimental results and predicted results, it is found that the Al2O3 is distributed on both sides of the molten pool under the combined action of the Marangoni flow and the evaporation recoil pressure. This kind of Al2O3 distribution enhances the effect of the remelting on the Al2O3 loss, i.e., the remelting brings a logarithmic increase in the Al2O3 loss rate. This determines the final Al2O3 loss rate of the SLMed 3D samples. During this study, although the Al2O3 loss rate of the single-track is only 33%, the loss rate of SLMed 3D samples increases significantly to 97% when the hatching space of 60 μm and scanning speed of 200 mm/s are utilized, i.e., almost no Al2O3 in the 3D sample. Thus, it is more important to reduce the remelting, i.e., overlap rate for reducing the element/phase loss. This study is a benefit for understanding and reducing the element/phase loss in SLM.  相似文献   

3.
Microsilica addition in Al2O3–MgO and Al2O3–spinel castables helps to improve their flowability and partially accommodate their residual expansion after firing. Nevertheless, there is a lack of conclusive statements in the literature regarding the effects of microsilica on one of the main requisites for steel ladle refractories: corrosion resistance. In the present work, the performance of alumina–magnesia and alumina–spinel with or without microsilica when in contact with a steel ladle slag was evaluated based on three aspects: the material's physical properties, its chemical composition and the microstructural features before the slag attack. According to the attained results, microsilica induced liquid formation and pore growth during sintering, favoring the physical slag infiltration. Moreover, due to this liquid, CA6 was formed in the matrix, mainly for the Al2O3–spinel composition, which also favored the castable dissolution into the molten slag.  相似文献   

4.
《Ceramics International》2020,46(5):6205-6211
This study utilized the single hot thermocouple technique to examine the dissolution behavior of lumped magnesium oxide (MgO) in CaO–Al2O3–SiO2 ternary slags. The aluminum oxide (Al2O3) content in the slag (C/S = 1) varied from 10% to 30%; the MgO sphere with a diameter of 1 mm was placed in molten slags at 1,550 °C. Results showed that the dissolution rate decreased as the Al2O3 content increased up to 20%. Over 20% Al2O3, MgAl2O4 was formed at the interface of MgO and it did not fully melt at 30% Al2O3. The dissolution behavior and the formation of MgAl2O4 were analyzed by a phase diagram provided by Factsage 7.0 software. In the case of less than 20% Al2O3 content, apparent sphere radii were measured; the shrinking core model was then applied to understand the dissolution mechanism. The dissolution rate of both slags was controlled by boundary layer diffusion. The dissolution rate at 20% Al2O3 slag appeared to fit the behavior to the boundary layer diffusion, although it deviated during the middle stage of the dissolution because of MgAl2O4 formation. The 10% Al2O3 slag fitted well to the boundary layer diffusion curve; the obtained diffusion coefficient was 0.94 × 10−9 m2/s.  相似文献   

5.
The literature suggests that MgAl2O4 can accelerate SiC oxidation in Al2O3–MgAl2O4–SiC–C refractory castables. Thus, in this work thermodynamic calculations have been carried out using FactSage® software in order to explore, search for and understand the role of MgAl2O4 on the SiC oxidation. According to the thermodynamic predictions, at 1500 °C and under a reducing atmosphere, there is no evidence that spinel might directly affect SiC oxidation. The increase of SiC content in an Al2O3–SiC–C (AL) castable composition was mainly related to the reaction between mullite and carbon. On the other hand, the SiC generation in the Al2O3–MgAl2O4–SiC–C (SP) composition was a result of the reaction involving liquid SiO2 and carbon from the refractory. Therefore, the lower SiC content in the SP castable resulted from the refractory's phase transformations. It was also suggested that the samples thermally treated 15 times at 1500 °C did not reach the equilibrium condition, which explains the differences between experimental and thermodynamic results.  相似文献   

6.
AC and DC resistivity of Cr–Al2O3 and ZrxAly–Al2O3 composites with varying metal content were measured. A strong percolation behavior was observed in the Cr–Al2O3 system, where the AC resistivity varied nine orders of magnitude close to the percolation threshold of 28 vol.%. AC measurements were less dependant on the contact resistance than DC measurements. The best reproducibility was obtained at a frequency of 100 kHz. AC resistivity values of insulating composites differed from DC values and may also be frequency-dependant. DC measurements up to 600 °C indicate that the intermetallic phases ZrAl3 and ZrAl are PTC conductors. The electrical properties of ZrxAly–Al2O3 samples with a metal content of 29 vol.% were anisotropic, with a much higher resistivity in the pressing direction.  相似文献   

7.
8.
The glass structure, wetting behavior and crystallization of BaO–Al2O3–B2O3–SiO2 system glass containing 2–10 mol% Al2O3 were investigated. The introduction of Al2O3 caused the conversion of [BO3] units and [BO4] units to each other and it played as glass network former when the content was up to 10 mol%, accompanied by [BO4]  [BO3]. The stability of the glass improved first and then decreased as Al2O3 increased from 2 to 10 mol%, the glass with 5 mol% Al2O3 being the most stable one. The wetting behavior of the glasses indicates that excess Al2O3 leads to high sealing temperature. The glass containing 5 mol% Al2O3 characterized by a lower sealing temperature is suitable for SOFC sealing. Al2O3 improves the crystallization temperature of the glass. The crystal phases in the reheated glasses are mainly composed of Ba2Si3O8, BaSiO3, BaB2O4 and BaAl2Si2O8. Al2O3 helps the crystallization of BaSiO3 and BaAl2Si2O8.  相似文献   

9.
Compared with monolithic fine grained Al2O3, Al2O3 nanocomposites reinforced with SiC nanoparticles display especially high modulus of rupture as well as reduced creep strain. Taking into account the fracture mode change, the morphology of ground surfaces showing plastic grooving, the low sensitivity to wear and the low dependence of erosion rate with grain size, it can be reasonably assumed that the strength improvement is associated with an increase of the interface cohesion (due to bridging by SiC particles) rather than with a grain size refinement involving substructure formation (as initially suggested by Niihara). In the present work, creep tests have been performed and the results agree with such a reinforcement of the mechanical properties by SiC particle bridging Al2O3–Al2O3 grain boundaries. Indeed, particles pinning the grain boundaries hinder grain boundary sliding resulting in a large improvement in creep resistance. In addition, SiC particles, while counteracting sliding, give rise to a recoverable viscoelastic contribution to creep. Because of the increased interface strength, the samples undergoing creep support stress levels, greater than the threshold value required to activate dislocation motion. The high stress exponent value as well as the presence of a high dislocation density in the strained materials suggests that a lattice mechanism controls the deformation process. Finally, a model is proposed which fits well with the experimental creep results.  相似文献   

10.
《Ceramics International》2023,49(15):25261-25268
A new type of glass-ceramic BaO–CaO–Al2O3–SiO2 (BCAS) was developed to join Al2O3 ceramics, adding TiO2 to the glass-ceramics can promote the crystallization behavior of the glass-ceramics. Through the observation of the joints, rutile TiO2 whiskers can grow on the surface of Al2O3 ceramics, and the grown TiO2 whiskers are one-dimensional needle-like whiskers growing in different directions in the joints, providing mechanical support for the joints. The aspect ratio of TiO2 whiskers was changed by controlling the addition of TiO2, and the crystallization behavior and microstructure of the joints were studied. The experimental results show that when the amount of TiO2 added is 10% (wt%), the density of TiO2 whiskers in the joint is the largest, the strengthening effect on the joint is the best, and the shear strength can reach 94.33 MPa.  相似文献   

11.
Compounds in the system Y2O3-Al2O3 are promising materials for optical, electronic and structural applications. In this study, a melt extraction process with a new approach to making ceramic fibers was used to produce amorphous fibers in the Y2O3–Al2O3 system within the 20–30-micron size range. Smooth and uniform cross section fibers with relatively high tensile strength were obtained depending on the wheel velocity. X-ray diffraction of as-extracted fibers revealed the non-crystalline nature of the yttria-alumina compositions. The crystallization and glass transition temperatures of non-crystalline fibers were determined using differential thermal analysis (DTA). Crystalline phases were identified by X-ray diffraction in the fibers after heat treatment.  相似文献   

12.
The beneficial effects of adding nanostructured expandable graphite (EG) hybridized yttrium aluminium garnet (EG\YAG) powder as a composite reinforcement in improving the oxidation resistance, hot-strength, and microstructure development in Al2O3–MgO–C refractories were studied. The refractory components reinforced with EG\YAG exhibited more than 60% of oxidation resistance enhancement and as high as 200% increase in hot-strength performance over the standard refractories, formulated without EG\YAG. Correlating the damage parameter (DE) calculations based on ultrasonic measurements with residual strength data (Rc, Rb) showed that there was a progressive increase in Rc and Rb values with consistent reduction in the oxidative damage of EG\YAG reinforced refractories. Analysis indicated that these beneficial features were majorly ascribed to the in-situ development of bimodal microstructure with EG\YAG sintered framework throughout the refractory interior in these new class of reinforced systems. Additionally, the mechanism of toughening and implications of these results to materials design are discussed.  相似文献   

13.
To support commercialization of the MgO–Al2O3–B2O–SiO2-based low-dielectric glass fibers, crystallization characteristics of the relevant glasses was investigated under various heat-treatment conditions. The study focused on the effects of iron on the related thermal properties and crystallization kinetics. Both air-cooled and nucleation-treated samples were characterized by using the differential thermal analysis/differential scanning calorimeter method between room temperature and 1200°C. A collected set of properties covers glass transition temperature (Tg), maximum crystallization temperature (Tp), specific heat (ΔCp), enthalpy of crystallization (ΔHcryst), and thermal stability (ΔT=TpTg). Using the Kinssiger method, the activation energy of crystallization was determined. Crystalline phases in the samples having various thermal histories were determined using powder X-ray diffraction (XRD) and/or in situ high-temperature XRD method. Selective scanning electron microscope/energy-dispersive spectroscopy analysis provided evidence that crystal density in the glass is affected by the iron concentration. Glass network structures, for air-cooled and heat-treated samples, were examined using a midinfrared spectroscopic method. Combining all of the results from our study, iron in glass is believed to function as a nucleation agent enhancing crystal population density in the melt without altering a primary phase field. By comparing the XRD data of the glasses in two forms (bulk versus powder), the following conclusions can be reached. The low-dielectric glass melt in commercial operation should be resistant to crystallization above 1100°C. Microscopic amorphous phase separation, possibly a borate-enriched phase separating from the silicate-enriched continuous phase can occur only if the melt is held at temperatures below 1100°C, that is, below the glass immiscibility temperature. The study concludes that neither crystallization nor amorphous phase separation will be expected for drawing fibers between 1200°C and 1300°C in a commercial operation.  相似文献   

14.
《Ceramics International》2023,49(13):22022-22029
The in-situ controllable synthesis of AlN–SiC solid solution reinforcement in large-sized Al–Si3N4–Al2O3 composite refractory by two-steps nitriding sintering was examined. In the first step, a dynamic Al@AlN structure was constructed in the composite by pre-nitriding at 580 °C. During the subsequent sintering process, it cracked above ∼900 °C, and micronized Al cluster (mixture of droplets and vapor) was extracted out gradually. As a result, multiple AlN mesophases were formed through different reaction paths, including i) initial AlN shell formed by solid Al with N2, ii) reaction of Al cluster with N2, and iii) reaction of Al cluster with Si3N4 from 900 °C to 1500 °C. The Si3N4 precursor serves as both a solid nitrogen source and an active Si source, and the controllable reaction between Al and Si3N4 leading to uniformly distributed AlN and Si mesophases. AlN–SiC solid solution is significantly formed when liquid Si appears. The shell, granule and whisker SiC–AlN solid solution were observed mainly depending on the dynamic AlN mesophase. The SiC–AlN solid solution reinforced Al2O3 materials is a novel promising refractory for large-scale blast furnace lining.  相似文献   

15.
Nine samples of Al2O3–30 wt.% TiC composites were prepared by hot-pressing the Al2O3 powder mixed with TiC particles. The average sizes of the TiC particles used for preparing the nine samples were different with each other. Knoop hardness measurements were conducted on these nine samples, respectively, in the indentation load range from 1.47 to 35.77 N. For each sample, the measured Knoop hardness decreases with the increasing indentation load. The classical Meyer's power law and an empirical equation proposed originally by Bückle were verified to be sufficiently suitable for describing the observed load-dependence of the measured hardness. Analysis based on Meyer's law can not provide any useful information about the cause of the observed ISE while true hardness values, which are load-independent, can be deduced from the Bückle's equation. It was found that the deduced true hardness increases with the average size of TiC particles existing in the sample.  相似文献   

16.
The present investigation focuses on the effect of Cr2AlC MAX phase addition on erosion and oxidation-induced crack healing behavior of Ni–Mo–Al alloy. For this, Ni–Mo–Al and 20 wt% Cr2AlC-blended Ni–Mo–Al powders were coated by Air Plasma Spray (APS). For oxidation-induced crack healing studies, the samples were heat treated at 500, 800, and 1100°C in the air for 5 hours. The heat-treated samples were analyzed by X-Ray Diffraction (XRD) analysis, Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS) for the phases, morphology, and composition. Erosion behavior studies were carried out at 30, 250, 500, 800, and 1000°C temperatures. The average hardness was obtained to be 400 ± 10 HV for Ni–Mo–Al coating and 580 ± 10 HV for 20 wt% Cr2AlC-blended Ni–Mo–Al coating. The addition of Cr2AlC MAX into Ni–Mo–Al matrix reduces the overall erosion rate and improved the crack healing ability. This was attributed to the presence of in-situ-formed Cr7C3 and Al2O3 phases.  相似文献   

17.
《应用陶瓷进展》2013,112(6):256-259
Abstract

Amorphous fibres of the Al2O3–Y2O3 system were prepared by a melt extraction technique, and subjected to crystallisation. The quality of the melt extracted fibres is controlled by the wheel edge and rotational speed, with both having a significant effect on fibre diameter and avoidance of irregularities and instabilities along the fibre length. Tensile strength in the glassy state varied from 0·6 to 1·0 GPa. Crystallisation activation energies calculated from scan-rate dependence of DTA peaks are 741 and 1374 kJ mol-1 for E1 (Al2O3–yttrium aluminium garnet (YAG) eutectic), 390 kJ mol-1 for YAG, and 438 kJ mol-1 for E2 (YAG–yttrium aluminium perovskite (YAP) eutectic) by the Kissinger method; and 698 and 1346 kJ mol-1 for E1, 352 kJ mol-1 for YAG, and 399 kJ mol-1 for E2 by the Augis–Bennett method.  相似文献   

18.
19.
《Ceramics International》2016,42(9):10908-10912
Pure chemosynthetic Al2O3–2SiO2 powders fabricated by a sol–gel method exhibit high phosphoric acid-activated properties and high compressive strengths. The phosphoric acid-activated properties could be characterized by compressive strength. The phase structure evolution of synthetic powders and the resulting geopolymers were investigated by DTA-TG, XRD, FTIR and MAS NMR analysis. These results show that the phosphoric acid-activation region of the synthetic powders was in the range of 200–800 °C, which was much lower than the temperature at which kaolinite was converted into metakaolinite. 31P MAS NMR analysis revealed that [PO4] tetrahedra were part of the geopolymer structure.  相似文献   

20.
Al2O3–C refractories are potential candidates for use in gasifiers, and they are Cr2O3-free. However, the oxidation of the carbon species and ceramic phases within the high-temperature water vapor environment may deteriorate the integrity of the working lining. Ti3AlC2 has been verified as an effective antioxidant for Al2O3–C refractories in air. In this study, the structural transformation of Ti3AlC2 during heat treatment and the water vapor resistance of Ti3AlC2-containing Al2O3–C refractories are investigated. The results show that the oxidation of Ti3AlC2 and Si in the matrix contributes to the in situ formation of a multilayer core–shell structure of TiC–AlTi2O5–Al6Si2O13. These structural evolutions improve densification and stimulate pore size refinement, which enhances the mechanical properties and thermal stress resistance of the specimens. In particular, the refined pore size contributes to the significantly improved water vapor resistance at high temperatures.  相似文献   

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