| 1. |
- Rodriguez-Rojas, F., et al.
(författare)
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Anomalous oxidation behaviour of pressureless liquid-phase-sintered SiC
- 2011
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Ingår i: Journal of the European Ceramic Society. - : Elsevier BV. - 0955-2219 .- 1873-619X. ; 31:13, s. 2393-2400
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation behaviour of pressureless liquid-phase-sintered (PLPS) SiC, an important non-oxide engineering ceramic, was investigated, and was found to be universally anomalous. Thermogravimetry oxidation tests performed in oxygen in the temperature range 1000-1225 degrees C on three PLPS SiC ceramics fabricated with different combinations of Al(2)O(3) RE(2)O(3) (RE=Gd, Sc, or Sm) as sintering aids indicated that the oxidation is in all cases passive and protective, but unexpectedly anomalous in the sense that the oxidation resistance does not scale inversely with temperature. In particular, in all cases it was observed that there is less oxidation above 1100 degrees C than below, in clear contradiction to the expectation for a diffusional process. Exhaustive characterization of the oxide scales by scanning electron microscopy, X-ray energy dispersive spectrometry, and X-ray diffractometry, together with detailed modeling of the oxidation curves, showed that the origin of this universal anomalous oxidation behaviour lies in the marked crystallization within the oxide scale of rare-earth silicates that act as effective barriers against the inward diffusion of oxygen thus improving notably the oxidation resistance. A strategy is proposed to provide PLPS SiC, and probably other SiO(2)-scale-forming ceramics that are sintered using rare-earth oxides, with the superior oxidation resistance at moderate temperatures (i.e., <1100 degrees C) that they do not currently have.
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| 2. |
- Rodriguez-Rojas, F., et al.
(författare)
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Oxidation behaviour of pressureless liquid-phase-sintered α-SiC with additions of 5Al2O3 + 3RE2O3 (RE = La, Nd, Y, Er, Tm, or Yb)
- 2010
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Ingår i: Journal of the European Ceramic Society. - : Elsevier BV. - 0955-2219 .- 1873-619X. ; 30:15, s. 3209-3217
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation behaviour of pressureless liquid-phase-sintered (PLPS) α-SiC was investigated as a function of the sintering additives of 5Al2O3 + 3RE2O3 (RE = La, Nd, Y, Er, Tm, or Yb) by thermogravimetry experiments in oxygen at 1075–1400 °C for up to 22 h. It was found that the oxidation is in all cases passive and protective, with kinetics governed by the arctan-rate law. This is because the PLPS SiC ceramics develop oxide scales having no cracks or open porosity and accordingly prevent the parent material from direct contact with oxygen. In addition, these oxide scales crystallize gradually during the exposure to the oxidizing atmosphere with the attendant reduction in the amorphous cross-section available for oxygen diffusion. It was also found that the rate-limiting mechanism of the oxidation is outward diffusion of RE3+ cations from the intergranular phase into the oxide scale, and that the activation energy of the oxidation increases with increasing size of the RE3+ cation. It was also observed that the oxidation of PLPS SiC increases with increasing size of the RE3+ cation, an effect that is especially marked for cation sizes above 0.9 Å because the oxidation rate becomes several orders of magnitude faster. This trend is attributable to the oxide scales being more crystalline, and containing crystals that are more refractory and amorphous residual phases that are more viscous as the size of the RE3+ cation decreases. Finally, implications for the design of PLPS SiC ceramics with superior oxidation resistance are discussed.
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| 3. |
- Zamora, V., et al.
(författare)
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Crystal-size dependence of the spark-plasma-sintering kinetics of ZrB2 ultra-high-temperature ceramics
- 2012
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Ingår i: Journal of the European Ceramic Society. - : Elsevier BV. - 0955-2219 .- 1873-619X. ; 32:2, s. 271-276
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Tidskriftsartikel (refereegranskat)abstract
- The crystal-size dependence of the spark-plasma-sintering (SPS) kinetics of ZrB2 ultra-high-temperature ceramics (UHTCs) was investigated. It was found that refining the starting powder enhances the SPS kinetics, reducing the onset temperatures of sintering and of the intermediate and final sintering regimes, as well as promoting a greater maximum shrinkage rate at lower temperatures. This enhancement was only relevant with reduction in crystal size to the nanoscale. Finally, the implications for low-temperature sintering of ZrB2 UHTCs are discussed.
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| 4. |
- Zamora, V., et al.
(författare)
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On the crystallite size refinement of ZrB(2) by high-energy ball-milling in the presence of SiC
- 2011
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Ingår i: Journal of the European Ceramic Society. - : Elsevier BV. - 0955-2219 .- 1873-619X. ; 31:13, s. 2407-2414
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Tidskriftsartikel (refereegranskat)abstract
- The effect of SiC addition (5, 17.5, and 30 vol.%) on the high-energy ball-milling (HEBM) behaviour of ZrB(2) is investigated. It was found that the presence of SiC during HEBM did not alter ZrB(2) refinement mechanism of repeated brittle fracture followed by cold-welding, thereby leading to the formation of agglomerates consisting of primary nano-particles. SiC did, however, slow down the kinetics of crystallite size refinement and promoted the formation of finer agglomerates. Both of these phenomena became more pronounced with increasing SiC content in the ZrB(2) + SiC powder mixtures, and they were attributed to the energy dissipation effect of the nanocrystalline SiC particles during HEBM of the ZrB(2) + SiC powder mixture. This study offers the first evidence that the addition of harder materials to softer materials can slow down the refinement of crystallite sizes, and thus provides a new mechanism to control crystallite sizes during HEBM. The simultaneous attainment of nano-particles of ZrB2 and SiC, reduced agglomerate sizes, and homogeneous SiC dispersion at the nanometre scale may have important implications for the ultra-high-temperature ceramic community, as it simplifies the processing route and is likely to facilitate the sintering of ZrB(2) SiC composites.
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