| 1. |
- Ahlen, N., et al.
(författare)
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On the carbothermal vapour-liquid-solid (VLS) mechanism for TaC, TiC, and TaxTi1-xC whisker growth
- 2000
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 20:14-15, s. 2607-2618
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Tidskriftsartikel (refereegranskat)abstract
- The growth of TaC, TiC and TaxTi1-xC whiskers has been studied in some detail. The whiskers were synthesised via a vapour-liquid-solid (VLS) growth mechanism in the temperature range 1220-1400 degreesC. The starting materials were Ta2O5, TiO2, C, NaCl, and a catalyst metal (Ni, Co, Fe, and Cu were tested). The main reaction during synthesis was a carbothermal reduction of Ta2O5 and TiO2, and NaCl was added to form the oxochlorides and chlorides of Ta and Ti that account for the transport to the catalyst metal. The syntheses were made in a protecting Ar atmosphere. From experiments interrupted after different times at the synthesis temperature it is clear that sodium tantalates form as intermediate products, whereas sodium titanates cannot be identified. Only metals that are able to dissolve the elements building up the whiskers work as catalysts. Whisker growth starts either from a catalyst droplet in contact with carbon or from an oxide particle in contact with both catalyst metal and carbon. For TaC and TaxTi1-xC the only growth direction observed is [100], while TiC may grow either along [100] or along [111].
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| 2. |
- Alkebro, J, et al.
(författare)
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Mechanical alloying of alumina-yttria powder mixtures
- 2000
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 20:12, s. 2169-2174
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Tidskriftsartikel (refereegranskat)abstract
- Mechanical alloying has been used to prepare powder mixtures of alumina and yttria as a means to create composites with a dominant matrix phase together with small particles of a dispersed second phase. The yttria-alumina system, containing five possible phases, has the potential for creating eight combinations of matrix and dispersed phases. Here compositions designed to give YAlO3(YA) dispersed in Y3Al5O12(Y3A5 i.e. YAG) or Y4Al2O9(Y2A) were studied. After milling with steel tools for times up to 8 h, the powders were subjected to thermal cycles up to 1500 °C during which the phase evolution was monitored using X-ray diffractometry (including high-temperature XRD) and differential thermal analysis. During milling the original crystal structures were quickly broken down, in some cases partially replaced by an intermediate structure after milling. Upon subsequent heating the milled mixtures crystallized to give the expected phases, YA in Y3A5 and YA in Y2A respectively, but the reaction route was seen to be different depending on the amount of amorphization of the yttria. Contamination by iron was seen to affect the phase distribution and the lattice parameters.
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| 3. |
- Antti, Marta-Lena, et al.
(författare)
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Thermal degradation of an oxide fibre (Nextel 720)/aluminosilicate composite
- 2004
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 24:3, s. 565-578
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Tidskriftsartikel (refereegranskat)abstract
- The effect of thermal exposure on the microstructure and tensile stress-strain behaviour has been investigated for composites of woven continuous oxide fibres (Nextel 720) in a porous aluminosilicate matrix. The tensile tests were carried out on straight-sided, centre hole notched plates with 0/90° and ±45° orientations. The as-received material was slightly notch sensitive in that the net section fracture stress decreased somewhat with increasing hole diameter but much less than predicted for an ideally elastic, fully notch-sensitive material. After exposure at 1100°C and for long time at 1000°C in air the composite was embrittled. In the 0/90 composite this resulted in a reduced fracture strength, a reduced strain to failure as well as a reduced fracture toughness and damage zone size. After exposure for 100 h at 1100 °C (the most extreme exposure applied) the material also became significantly more notch sensitive and had failure characteristics similar to those of a monolithic ceramic. The ±45 composite was also embrittled which resulted in a reduced strain to failure but an increase in fracture strength. Density measurements and observations on the microstructure and fracture surfaces indicated that the embrittlement was due mainly to localised densification of the matrix and an increase in fibre/matrix bonding.
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| 4. |
- Ashkin, A., et al.
(författare)
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At-temperature observation of phase development in yttrium α-Sialon
- 1995
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Ingår i: Journal of the European Ceramic Society. - : Elsevier BV. - 0955-2219 .- 1873-619X. ; 15:11, s. 1101-1109
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Tidskriftsartikel (refereegranskat)abstract
- A powder mixture of α-Si3N4, AlN and Y2O3 corresponding to an yttrium α-sialon composition, Y0.4Si10.2Al1.8O0.6N15.4, was sintered both in a high temperature X-ray diffraction unit and in a regular sintering furnace. X-ray analysis was performed and it was shown that high temperature X-ray diffraction can be used to monitor the kinetics of the α-sialon phase formation during sintering at temperatures between 1450 and 1580 °C as the reactions take place. A variety of yttrium-rich intermediate and secondary phases are formed during sintering; those formed in the HT-XRD unit were not the same as those formed in the regular sintering furnace, but they do not significantly influence the overall reaction sequence or the amount of α-sialon formed. Quantitative analysis of the acquired data was used to evaluate the kinetics of the α-sialon phase formation; the estimated activation energy of the initial stage α-sialon formation was 330kJ/mol. After this initial stage of relatively rapid α-sialon formation the process became inhibited
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| 5. |
- Ashkin, Daniel
(författare)
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Nitridation Behaviour of Silicon with Clay and Oxide Additions: Rate and Phase Development
- 1997
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 17:13, s. 1613-1624
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Tidskriftsartikel (refereegranskat)abstract
- Samples pressed from fine powders of Si, Si + SiO2, Si + kaolinite and Si + SiO2 + Al2O3 were nitrided by both the nitrogen demand method and a constant rate of heating using a graphite resistance furnace at temperatures of 1100 to 1450 °C. The nitridation rate for different compositions and input parameters were determined by continuously monitoring the time, temperature and pressure within the furnace. By comparing the different compositions, the effect of impurities, silica and particle size on nitriding rate, phase composition and percent theoretical weight gain were determined. Compositions containing high purity additives were found to nitride at higher temperatures and to be more susceptible to weight loss during nitridation. The prime cause for low weight gains was determined to be volatilization of silicon out of the samples. Si2N2O/O′ Sialon was formed in all compositions containing the as-used additives and acted to retain the oxygen within the sample.
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| 6. |
- Babushkin, O., et al.
(författare)
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Kinetic Aspects of the Formation of Lead Zirconium Titanate
- 1996
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Ingår i: Journal of the European Ceramic Society. - : Elsevier BV. - 0955-2219 .- 1873-619X. ; 16:12, s. 1293-1998
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Tidskriftsartikel (refereegranskat)abstract
- The kinetics of the second calcination step in the formation of PZT solid solution (with perovskite ABO3 lattice) has been investigated by using two different particle sizes of the B-site precursor (1·91 and 5·08 μm), the finer size being obtained by prolonged milling. In-situ analysis performed by high-temperature X-ray diffractometry in a non-isothermal mode (20-800°C) revealed a reduction of the calcination temperature by 100°C with a decrease in particle size of the precursor. In order to clarify the mechanism of the solid-state reaction to PZT, isothermal heat treatment of the mixtures was performed in the temperature range 540-700°C. The activation energies for the fine and the coarse powders were estimated as 150 and 210 kJ mol 1 respectively, and the reaction was found to follow the Jander model for diffusion-controlled solid-state reaction kinetics
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| 7. |
- Babushkin, O., et al.
(författare)
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PZT phase formation monitored by high-temperature X-ray diffractometry
- 1997
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 17:6, s. 813-818
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Tidskriftsartikel (refereegranskat)abstract
- The crystallisation kinetics of amorphous sol-gel PZT thin films were investigated using high-temperature X-ray diffraction. Crystallisation for different isotherms was monitored as a function of time. Phase transformation data were obtained from integrated X-ray peak intensities which were calibrated based on image analysis of the surface microstructure of the samples at the end of the isothermal treatments. An activation energy of 310 kJ/mol was obtained without assuming a specific kinetic model. From the transformation data, a TTT diagram was constructed for the ranges studied.
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| 8. |
- Babushkin, Oleg, et al.
(författare)
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Reaction sequence in the formation of perovskite Pb(Zro0.48Ti0.52)O3-Pb(Nb2/3Ni1/3)O3 solid solution : Dynamic heat-treatment
- 1998
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 18:7, s. 737-744
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Tidskriftsartikel (refereegranskat)abstract
- The sequence of the solid state reactions in the PbOZrO 2-TiO2-Nb2O5-NiO system has been investigated. The oxide mixing route utilised in sample preparation was selected in order to determine the basic reaction path in the formation of the PZTPNN perovskite phase. It has been established that the main intermediate phases formed prior to PZTPNN are PbTiO3 and pyrochlore Pb-Nb-based phases. The sequence in the pyrochlore formation was from tetragonal Pb3Nb2O8 (500 °C) to rhombohedral Pb2Nb2O7 (600-750 °C) and finally to cubic Pb3Nb4O13 (650-850 °C). The formation of the perovskite phase proceeded from mutual solubility of PbTiO3 and pyrochlore Pb3Nb4O13 phases, accompanied by dissolving of residuals (PbZrO3 and NiO) in the perovskite solid solution formed
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| 9. |
- Casellas, D., et al.
(författare)
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On the transformation toughening of Y-ZrO2 ceramics with mixed Y-TZP/PSZ microstructures
- 2001
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 21:6, s. 765-777
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Tidskriftsartikel (refereegranskat)abstract
- Heat treatment of Y-TZP at high temperatures produces materials with a mixed Y-TZP/PSZ phase assemblage, which exhibit a unique combination of high mechanical strength and fracture toughness, uncommon in zirconia ceramics. The microstructure and crack growth resistance of the Y-TZP/PSZ materials developed by treating at 1650°C in air a fine-grained Y-TZP was studied. XRD as well as Raman spectroscopy results indicate that the obtained microstructure allow the retention of large tetragonal grains (up to ˜4 μm), resulting in both phase transformability enhancement and pronounced R-curve behavior. The large transformation zone, discerned from accurate measurements with Raman microprobe spectroscopy, sustains the above assessment and points out tetragonal to monoclinic phase transformation as the main toughening mechanism in the investigated Y-TZP/PSZ microstructures. This was confirmed by satisfactory agreement between the transformation toughening estimated from numerical analysis and the crack shielding experimentally determined from the R-curve measurements.
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| 10. |
- Deng, Shiqiang, et al.
(författare)
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Creep properties of single crystal oxides evaluated by a Larson-Miller procedure
- 1995
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Ingår i: Journal of the European Ceramic Society. - 0955-2219 .- 1873-619X. ; 15:6, s. 513-520
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Tidskriftsartikel (refereegranskat)abstract
- A collation of published results of compressive and tensile creep studies of single and binary oxide single crystals is presented. For the purposes of comparison the results were normalised by means of a Larson-Miller procedure. This method proved to be effective in providing a ranking of the oxides in terms of their creep resistance. Binary oxides with complex crystal structures exhibited the highest creep strengths; no direct correlation with the melting point could be found.
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