The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Thomas Tsakalakos - One of the best experts on this subject based on the ideXlab platform.
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direct in situ observation of electric field assisted Densification of zno by energy dispersive x ray diffraction
Ceramics International, 2019Co-Authors: Hulya Bicer, Berrra Beyoglu, Ertugrul T Ozdemir, John S Okasinski, Thomas TsakalakosAbstract:Abstract Electric field assisted sintering techniques are used to enhance the Densification process, reducing sintering time and temperature. Here, we report the in situ characterization of non-isothermal Densification behavior of zinc oxide under simultaneous application of thermal and electric field by energy dispersive X-ray diffraction. 99% density was achieved under the electric field of 50 V/mm in the 588–843 °C range in a short time without grain growth. The in-situ energy dispersive X-ray diffraction study revealed the abnormal volumetric unit cell expansion of 0.9% at 843 °C associated with the observed sudden rise of the power absorption by the specimen during Densification. Moreover, the variation in peak broadening as a function of time exhibited singularity in the 588–843 °C range implying the formation and rearrangement of defects correlated with Densification during electric field assisted sintering.
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anomalous lattice expansion in yttria stabilized zirconia under simultaneous applied electric and thermal fields a time resolved in situ energy dispersive x ray diffractometry study with an ultrahigh energy synchrotron probe
Journal of Applied Physics, 2013Co-Authors: E K Akdogan, I şavkliyildiz, Hulya Bicer, William A Paxton, F Toksoy, Zhong Zhong, Thomas TsakalakosAbstract:Nonisothermal Densification in 8% yttria doped zirconia (8YSZ) particulate matter of 250 nm median particle size was studied under 215 V/cm dc electric field and 9 °C/min heating rate, using time-resolved in-situ high temperature energy dispersive x-ray diffractometry with a polychromatic 200 keV synchrotron probe. Densification occurred in the 876–905 °C range, which resulted in 97% of the theoretical density. No local melting at particle-particle contacts was observed in scanning electron micrographs, implying Densification was due to solid state mass transport processes. The maximum current draw at 905 °C was 3 A, corresponding to instantaneous absorbed power density of 570 W/cm3. Densification of 8YSZ was accompanied by anomalous elastic volume expansions of the unit cell by 0.45% and 2.80% at 847 °C and 905 °C, respectively. The anomalous expansion at 905 °C at which maximum Densification was observed is characterized by three stages: (I) linear stage, (II) anomalous stage, and (III) anelastic recove...
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anomalous lattice expansion in yttria stabilized zirconia under simultaneous applied electric and thermal fields a time resolved in situ energy dispersive x ray diffractometry study with an ultrahigh energy synchrotron probe
Journal of Applied Physics, 2013Co-Authors: E K Akdogan, I şavkliyildiz, Hulya Bicer, William A Paxton, F Toksoy, Zhong Zhong, Thomas TsakalakosAbstract:Nonisothermal Densification in 8% yttria doped zirconia (8YSZ) particulate matter of 250 nm median particle size was studied under 215 V/cm dc electric field and 9 °C/min heating rate, using time-resolved in-situ high temperature energy dispersive x-ray diffractometry with a polychromatic 200 keV synchrotron probe. Densification occurred in the 876–905 °C range, which resulted in 97% of the theoretical density. No local melting at particle-particle contacts was observed in scanning electron micrographs, implying Densification was due to solid state mass transport processes. The maximum current draw at 905 °C was 3 A, corresponding to instantaneous absorbed power density of 570 W/cm3. Densification of 8YSZ was accompanied by anomalous elastic volume expansions of the unit cell by 0.45% and 2.80% at 847 °C and 905 °C, respectively. The anomalous expansion at 905 °C at which maximum Densification was observed is characterized by three stages: (I) linear stage, (II) anomalous stage, and (III) anelastic recovery stage. The Densification in stage I (184 s) and II (15 s) was completed in 199 s, while anelastic relaxation in stage III lasted 130 s. The residual strains (e) at room temperature, as computed from tetragonal (112) and (211) reflections, are e(112) = 0.05% and e(211) = 0.13%, respectively. Time dependence of (211) and (112) peak widths (β) show a decrease with both exhibiting a singularity at 905 °C. An anisotropy in (112) and (211) peak widths of {β(112)/β(211)} = (3:1) magnitude was observed. No phase transformation occurred at 905 °C as verified from diffraction spectra on both sides of the singularity, i.e., the unit cell symmetry remains tetragonal. We attribute the reduction in Densification temperature and time to ultrafast ambipolar diffusion of species arising from the superposition of mass fluxes due to Fickian diffusion, thermodiffusion (Soret effect), and electromigration, which in turn are a consequence of a superposition of chemical, temperature, and electrical potential gradients. On the other hand, we propose defect pile-up at particle-particle contacts and subsequent tunneling as a mechanism creating the “burst-mode” discontinuous Densification at the singularities observed at 847 and 905 °C.
Randall M German - One of the best experts on this subject based on the ideXlab platform.
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Densification and shape retention in supersolidus liquid phase sintering
Acta Materialia, 1999Co-Authors: Randall M German, Randall M GermanAbstract:Abstract Rapid sintering Densification of relatively large prealloyed powders is possible by exceeding the solidus temperature in an approach termed supersolidus liquid phase sintering. This process is often limited by a narrow processing window for attaining Densification without distortion. Densification and distortion both reflect viscous flow responses of the solid–liquid mixtures. In the present research, a control microstructure softening parameter ζ , which combines the effects of grain size, liquid volume fraction, and contiguity, is proposed to separate Densification and distortion events. This microstructure parameter has two critical values, ζ densif and ζ distort , for Densification and shape loss, respectively. It must be smaller than ζ distort to avoid shape distortion, while for Densification, it must be larger than ζ densif . As ζ densif is smaller than ζ distort , there is window of sintering conditions where Densification is achieved without accompanying shape loss. Understanding this parameter provides a means to design alloys and processing cycles for net shape, full density components by supersolidus liquid phase sintering.
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supersolidus liquid phase sintering of prealloyed powders
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 1997Co-Authors: Randall M GermanAbstract:A model is derived for the sintering Densification of prealloyed particles that form internal liquids when heated over the solidus temperature. The model considers the powder size, composition, and microstructure, as well as the processing conditions of green density, heating rate, maximum temperature, hold time, and atmosphere. Internal liquid forms and spreads to create an interparticle capillary bond that induces Densification during sintering. Densification is delayed until the particles achieve a mushy state due to grain boundary wetting by the internal liquid. This loss of rigidity and concomitant Densification of the semisolid particles depends on the grain size and liquid quantity. Viscous flow is the assumed Densification mechanism, where both viscosity and yield strength vary with the liquid content and particle microstructure. Densification predictions are compared to experimental data, giving agreement with previously reported rapid changes in sintered density over narrow temperature ranges. The model is tested using data from steels and tool steels of varying carbon contents, as well as boron-doped stainless steel, bronze, and two nickel-based alloys.
Alexandre Maitre - One of the best experts on this subject based on the ideXlab platform.
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new approach of the evolution of Densification mechanisms during spark plasma sintering application to zirconium oxy carbide ceramics
Scripta Materialia, 2015Co-Authors: Guy Antou, Nicolas Pradeilles, Mathieu Gendre, Alexandre MaitreAbstract:The kinetics of spark plasma sintered ZrC0.94O0.05 ceramics have been investigated. A change of Densification mechanism appears during the intermediate and final sintering stages. During this last stage, the deformation mechanism is similar to the one involved during creep of dense ZrC0.94O0.05 ceramics. The comparison of Densification and creep strain rates seems to show that no specific effects strongly enhance strain rate during the final Densification stage of Spark Plasma Sintering.
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a study of the Densification mechanisms during spark plasma sintering of zirconium oxy carbide powders
Acta Materialia, 2010Co-Authors: Mathieu Gendre, Alexandre Maitre, Gilles TrolliardAbstract:Zirconium oxycarbide powders with controlled composition ZrC0.94O0.05 were synthesized using the carboreduction of zirconia. They were further subjected to spark plasma sintering (SPS) under several applied loads (25, 50, 100 MPa). The Densification mechanism of zirconium oxycarbide powders during the SPS was studied. An analytical model derived from creep deformation studies of ceramics was successfully applied to determine the mechanisms involved during the final stage of Densification. These mechanisms were elucidated by evaluating the stress exponent (n) and the apparent activation energy (Ea) from the Densification rate law. It was concluded that at low macroscopic applied stress (25 MPa), an intergranular glide mechanism (n 6 2) governs the Densification process, while a dislocation motion mechanism (nP 3) operates at higher applied load (100 MPa). Transmission electron microscopy observations confirm theses results. The samples treated at low applied stress appear almost free of dislocations, whereas samples sintered at high applied stress present a high dislocation density, forming sub-grain boundaries. High values of apparent activation energy (e.g. 687-774 kJ mol1) are reached irrespective of the applied load, indicating that both mechanisms mentioned above are assisted by the zirconium lattice diffusion which thus appears to be the rate-limiting step for Densification.
Mathieu Gendre - One of the best experts on this subject based on the ideXlab platform.
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new approach of the evolution of Densification mechanisms during spark plasma sintering application to zirconium oxy carbide ceramics
Scripta Materialia, 2015Co-Authors: Guy Antou, Nicolas Pradeilles, Mathieu Gendre, Alexandre MaitreAbstract:The kinetics of spark plasma sintered ZrC0.94O0.05 ceramics have been investigated. A change of Densification mechanism appears during the intermediate and final sintering stages. During this last stage, the deformation mechanism is similar to the one involved during creep of dense ZrC0.94O0.05 ceramics. The comparison of Densification and creep strain rates seems to show that no specific effects strongly enhance strain rate during the final Densification stage of Spark Plasma Sintering.
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a study of the Densification mechanisms during spark plasma sintering of zirconium oxy carbide powders
Acta Materialia, 2010Co-Authors: Mathieu Gendre, Alexandre Maitre, Gilles TrolliardAbstract:Zirconium oxycarbide powders with controlled composition ZrC0.94O0.05 were synthesized using the carboreduction of zirconia. They were further subjected to spark plasma sintering (SPS) under several applied loads (25, 50, 100 MPa). The Densification mechanism of zirconium oxycarbide powders during the SPS was studied. An analytical model derived from creep deformation studies of ceramics was successfully applied to determine the mechanisms involved during the final stage of Densification. These mechanisms were elucidated by evaluating the stress exponent (n) and the apparent activation energy (Ea) from the Densification rate law. It was concluded that at low macroscopic applied stress (25 MPa), an intergranular glide mechanism (n 6 2) governs the Densification process, while a dislocation motion mechanism (nP 3) operates at higher applied load (100 MPa). Transmission electron microscopy observations confirm theses results. The samples treated at low applied stress appear almost free of dislocations, whereas samples sintered at high applied stress present a high dislocation density, forming sub-grain boundaries. High values of apparent activation energy (e.g. 687-774 kJ mol1) are reached irrespective of the applied load, indicating that both mechanisms mentioned above are assisted by the zirconium lattice diffusion which thus appears to be the rate-limiting step for Densification.
Tiechui Yuan - One of the best experts on this subject based on the ideXlab platform.
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Effect of low-melting-point sintering aid on Densification mechanisms of boron carbide during spark plasma sintering
Scripta Materialia, 2019Co-Authors: Mei Zhang, Ruidi Li, Tiechui Yuan, Xiao FengAbstract:Abstract A Densification creep model was proposed to analyze the Densification mechanism involved in spark plasma sintering (SPS) of boron carbide containing low-melting-point sintering aid. This model incorporated the effect of the sintering aid on relative density as well as the actual internal stress during SPS of boron carbide ceramics. The internal stress bias caused by sintering aid was considered and a more accurate effective stress was used to investigate the kinetics during SPS. The proposed model explains why an increase of sintering aid amount decreases the sintering activation energy and promotes Densification.
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improving the Densification of indium tin oxide targets via secondary cold isostatic pressing and oxygen exchange treatments
Scripta Materialia, 2018Co-Authors: Tiechui Yuan, Ruidi Li, Jingwei HuangAbstract:Abstract Traditional preparation technology is difficult to achieve near full densified indium tin oxide targets with thickness more than 10 mm. By introducing an anti-Densification sintering model into studying the Densification behavior of In2O3-SnO2 mixed powders, we proposed secondary cold isostatic pressing and oxygen exchange treatments to improve the Densification of the targets by experiment. It is found that the application of secondary cold isostatic pressing and oxygen exchange treatments can increase the Densification of targets and reduce the number and size of pores in sintered bodies effectively. Besides, two mechanism models were introduced to reveal the underlying causes.
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direct current enhanced Densification kinetics during spark plasma sintering of tungsten powder
Scripta Materialia, 2018Co-Authors: Shenghua Deng, Ruidi Li, Mei Zhang, Tiechui Yuan, Kechao ZhouAbstract:Abstract Rapid Densification associated with spark plasma sintering has been well documented; however theoretical understanding is still very limited. In this work we proposed a Densification creep model to explicitly elucidate the effect of current density on Densification kinetics, by taking electromigration into account. This new model was then validated experimentally. It is found that an increased current density decreases Densification activation energy (energy barrier), thus accelerating powder Densification. In essence, the rapid Densification is caused by direct current that accelerates mass transport and increases dislocation mobility.
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spark plasma sintering of pure tungsten powder Densification kinetics and grain growth
Powder Technology, 2017Co-Authors: Shenghua Deng, Ruidi Li, Tiechui Yuan, Fanhao Zeng, Xiang ZhouAbstract:Abstract The Densification kinetics and grain growth behavior of an undoped tungsten powder during spark plasma sintering (SPS) were investigated under the pressure of 40 MPa and constant heating rate of 100 °C min− 1. Two stages of the sintering process were clearly identified: Densification without grain growth at the low temperatures (1200–1450 °C) and grain growth without much further Densification at higher temperatures (1500–2000 °C). A creep model was applied to determine the Densification mechanisms involved in the Densification stage, which can be elucidated by evaluating the stress exponent (n) and the apparent activation energy (Qd) from the Densification rate law. It shows that a boundary diffusion governs the Densification process at low effective compaction stresses (n = 1.5, Qd = 140.57 ± 12 kJ mol− 1), while grain boundary diffusion and dislocation motion both operate at higher effective compaction stresses (n = 3, Qd = 302.48 ± 24 kJ mol− 1), which is confirmed by transmission electron microscopy observation. During the final-stage of sintering, the fast grain growth mechanism was suggested as surface diffusion.