共查询到20条相似文献,搜索用时 31 毫秒
1.
《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
2.
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
3.
《Journal of the European Ceramic Society》2000,20(2):195-199
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
4.
Barbara Lothenbach Laure Pelletier-Chaignat Frank Winnefeld 《Cement and Concrete Research》2012,42(12):1621-1634
The solubility of AH3, CAH10, C2AH7.5, and C3AH6 was determined experimentally at 7 to 40 °C and up to 570 days. During the reaction of CA, at 20 °C and above initially C2AH7.5 formed which was unstable in the long-term. The solubility products calculated indicate that the solubilities of CAH10, C2AH7.5 and C4AH19 increase with temperature while the solubility of C3AH6 decreases. Thus at temperatures above 20 °C, C3AH6 is stable, while at lower temperature also CAH10 and C4AH19 are stable, depending on the C/A ratio.At early hydration times, CAH10 can be stable initially at 30 °C and above, as the formation of amorphous AH3 stabilises CAH10 with respect to C3AH6 + 2AH3. With time, as the solubility AH3 decreases due to the formation of microcrystalline AH3, CAH10 becomes unstable at 20 °C and above. 相似文献
5.
6.
《Journal of the European Ceramic Society》2002,22(13):2149-2153
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.
《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
8.
《Journal of the European Ceramic Society》2004,24(7):2103-2111
Neodymium aluminosilicate (Nd2O3–Al2O3–SiO2; NdAS) glasses have been investigated for the effect of concentration of TiO2 on the crystallization mechanism and for the effect of surface condition on crystal growth. NdAS glasses with 0–10 wt.% TiO2 were heat-treated for nucleation and crystal growth and were examined for phase separation and morphology of surface crystals as well as for crystal growth rate. All the glasses exhibit surface crystallization, however, the glass having 8 wt.% TiO2 also exhibits internal crystallization after a two-stage heat treatment. Surface crystallization was dependent on the condition of the glass surface and the amount of TiO2. The crystal growth on the cut surface was faster than on the fractured surface and the growth rate in surface increased with increasing TiO2. The phase separation found in NdAS glasses containing above 8 wt.% TiO2, was confirmed to be an important factor controlling the internal crystallization process in Nd2O3–Al2O3–SiO2–TiO2 glasses. 相似文献
9.
《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
10.
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=Tp—Tg). 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
11.
《Journal of the European Ceramic Society》2000,20(8):1091-1098
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.
13.
《Ceramics International》2016,42(12):14006-14010
Formation of WSi2–Al2O3 and W5Si3–Al2O3 composites was studied by thermite-based combustion synthesis. The addition of two thermite combinations composed of WO3+2Al and 0.6WO3+0.6SiO2+2Al into the W-Si reaction systems facilitated the combustion wave propagating in a self-sustaining manner and contributed to the in situ formation of tungsten silicides along with Al2O3. Experimental results showed that the former thermite mixture is more exothermic than the latter, and a decrease in the combustion temperature and flame-front velocity with increasing silicide phase formed in the composite. Depending on the reaction stoichiometry, the combustion wave velocity varied from 9.5 to 3.7 mm/s and temperature from 1650 to 1280 °C. A complete phase conversion and a broad range of the molar ratio of WSi2/Al2O3 from 0.8 to 4.0 were achieved for the production of the WSi2–Al2O3 composites. Due to the lower formation exothermicity, the W5Si3–Al2O3 composites were produced with a narrower range of W5Si3/Al2O3 from 0.4 to 2.0, beyond which combustion failed to proceed. Moreover, there exist WSi2 and unreacted W in the as-synthesized W5Si3–Al2O3 composites. 相似文献
14.
Microstructure evolution during air bonding of Al2O3 to Al2O3 joints using bismuth–borate–zinc glass
《Journal of the European Ceramic Society》2017,37(13):4015-4023
Alumina ceramics with 95 wt.% purity were sealed together using a bismuth based glass, 40Bi2O3–40B2O3–20ZnO (mol.%). The wettability of the glass on the Al2O3 substrate was investigated. The results showed a contact angle of ≤36.5° was achieved when the temperature was ≥630 °C. Subsequently, sealing cycles were performed at temperatures of 520–700 °C for 30 min. The dependence of microstructure evolution of the joints on temperature was investigated. Bi24B2O39 was detected to be the product in the joints sealed at 530–580 °C, while ZnAl2O4 was identified to be the main product when sealing at temperature of ≥650 °C due to the reaction between the Al2O3 substrate and ZnO from the glass. The influence of dwelling time at 700 °C on microstructure evolution of the joints was also studied. The results showed that the size of ZnAl2O4 increased with increasing holding time. 相似文献
15.
《Journal of the European Ceramic Society》1999,19(13-14):2475-2485
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
16.
《应用陶瓷进展》2013,112(6):256-259
AbstractAmorphous 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
17.
《Ceramics International》2019,45(14):16809-16813
When used as implants, Al2O3 is unable of directly achieving good chemical bonding with soft and hard tissues. To overcome this problem, SiAlON–Al2O3 ceramics were prepared in this study by direct nitridation. Phase composition, porosity, bulk density, and compression strengths were examined, and biological properties were evaluated by cell culture on ceramic surface. Major phase of SiAlON–Al2O3 ceramics was identified as Si4Al2O2N6, formed by reaction of Si, Al and Al2O3 under nitrogen atmosphere at high temperature. As Al2O3 content increased, porosity and compressive strength decreased. Therefore, Si4Al2O2N6 phase could improve sintering, leading to formation of composites with better properties. The porosity and compression strength were found suitable for requirement of biomaterials. Cell culture experiments showed that cells could proliferate and survival well on ceramic surface, indicating good biocompatibility of Si4Al2O2N6 phase in SiAlON–Al2O3 ceramics. Overall, these data look promising and might provide novel strategies for development of future SiAlON–Al2O3 bioceramics. 相似文献
18.
《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
19.
《Ceramics International》2020,46(10):16008-16019
The electrical conductivity of alumina-silicon carbide (Al2O3–SiC) and alumina-multiwalled carbon nanotube (Al2O3-MWCNT) nanocomposites prepared by sonication and ball milling and then consolidated by spark plasma sintering (SPS) is reported. The effects of the nanophase (SiC and MWCNTs) and SPS processing temperature on the densification, microstructure, and functional properties were studied. The microstructure of the fabricated nanocomposites was investigated using field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The phase evolution was determined using X-ray diffraction (XRD). The room-temperature direct current (DC) electrical conductivity of the monolithic alumina and nanocomposites was determined using the four-point probe technique. The EDS mapping results showed a homogenous distribution of the nanophases (SiC and MWCNTs) in the corresponding alumina matrix. The room-temperature DC electrical conductivity of monolithic alumina was measured to be 6.78 × 10−10 S/m, while the maximum electrical conductivities of the alumina-10 wt%SiC and alumina-2wt%MWCNT samples were 2.65 × 10−5 S/m and 101.118 S/m, respectively. The electrical conductivity increased with increasing nanophase concentration and SPS temperature. The mechanism of electrical conduction and the disparity in the electrical performance of the two investigated nanocomposite systems (alumina-SiC and alumina-MWCNT) are clearly described. 相似文献
20.
《Journal of the European Ceramic Society》2003,23(1):1-8
The vaporization behaviour of pure Al2O3, Y2O3 and SiC as well as SiC–Al2O3 and SiC–Al2O3/Y2O3 mixtures has been analysed by thermodynamic calculations in an open system. Pure Al2O3 and Y2O3 evaporate congruently in the 1200–2300 K temperature range. Pure SiC vaporizes in a non-congruent manner leading to graphite formation as by-product. A SiC–Al2O3 mixture evaporates congruently according to the main vaporization reaction, 2 SiC(s) + Al2O3(s) +Al2O(g) ⇆ 2 SiO(g) + 2 CO(g) +4 Al(g), but the overall composition changes: for SiC rich samples, the mixture tends towards pure SiC in time, and for Al2O3 rich samples towards pure Al2O3. A SiC–Al2O3/Y2O3 mixture shows similar behaviour. 相似文献