Co-reporter:Hidehiro Yoshida, Koji Morita, Byung-Nam Kim, Yoshio Sakka, Takahisa Yamamoto
Acta Materialia 2016 Volume 106() pp:344-352
Publication Date(Web):March 2016
DOI:10.1016/j.actamat.2016.01.037
Abstract
1 mol% Ni2+-doped Y2O3 shows flash-sintering at DC fields greater than 300 V/cm. When compared to the undoped Y2O3, the onset temperature for the flash event is reduced by about 200 °C due to the Ni-doping. The electric conductivity during the sintering experiments for the Ni-doped Y2O3 begins to increase above those of the undoped Y2O3 at temperatures greater than 700 °C, then steeply rises corresponding to the flash event. Electron energy loss spectrometry (EELS) measurements indicated that Y2O3 is highly reduced by the combination of the Ni-doping and flash-sintering as well as by conventional sintering in a reducing atmosphere. The applied electric field must involve the formation of oxygen anion vacancies and liberated electrons, and consequently, enhances the diffusional mass transport and electronic conduction in the Y2O3. The enhanced flash-sintering in the Ni-doped Y2O3 must be attributed to an avalanche of the vacancy-electron pairs facilitated by the Ni-doping.
Co-reporter:Koji Morita, Byung-Nam Kim, Hidehiro Yoshida, Keijiro Hiraga, Yoshio Sakka
Journal of the European Ceramic Society 2016 Volume 36(Issue 12) pp:2961-2968
Publication Date(Web):September 2016
DOI:10.1016/j.jeurceramsoc.2015.11.010
In order to discuss the influence of carbon contamination on the transmission, the effect of pre- and post-annealing treatments was investigated in the spark-plasma-sintered (SPSed) MgAl2O4 spinel. During the SPS process, the carbon phases transformed from CO32−, which is pre-existing in the powder, remains along grain junctions and caused discoloration. The pre-annealing of the starting powder can reduce the impurities and improve in-line transmission Tin by 10%. Post-annealing can remove the discoloration from the spinel, but it changes the spinel to whitish color with the annealing temperature and degraded Tin. This degradation can be explained by the increase of a scattering coefficient caused by the pore. During the post-annealing, the carbon phases generate high pressure CO/CO2 gas by reacting with oxygen and forms many pores along the grain boundaries. This suggests that reducing the carbon contamination is important for attaining highly transparent spinel by the SPS processing.
Co-reporter:Koji Morita, Byung-Nam Kim, Hidehiro Yoshida, Keijiro Hiraga, Yoshio Sakka
Acta Materialia 2015 Volume 84() pp:9-19
Publication Date(Web):1 February 2015
DOI:10.1016/j.actamat.2014.10.030
Abstract
Discoloration of spark-plasma-sintered spinel was investigated by spectroscopic techniques and transmission electron microscopy. The discoloration is explained by the combination of carbon contaminations and lattice defects (color centers), which are introduced in the spinel matrix depending on the spark-plasma-sintering conditions. For low heating rates of ⩽10 °C min−1, the trace carbonate CO3 pre-existing in the starting powder remained as glassy carbon within the matrix, irrespective of the sintering temperature. For a high heating rate of ⩾50 °C min−1, additional carbon contamination occurred by evaporating the carbon phases from the carbon papers and graphite dies during the heating process, and tended to be enhanced by the increasing heating rate. The present data indicate that the color center (F+-center) may be generated by the formation of oxygen vacancies, which are mainly introduced by dislocation motion depending on the sintering conditions. Since the rate of sintering, namely the deformation rate, increased with the heating rate, the concentration of the dislocation-related color centers increased with the heating rate, but decreased with the sintering temperature due to the bleaching of the oxygen vacancies. For the present spinel, the discoloration due to the carbon contamination and the formation of F+-centers deteriorates the light transmission, depending on the sintering conditions.
Co-reporter:Koji Morita, Byung-Nam Kim, Hidehiro Yoshida, Haibin Zhang, Keijiro Hiraga, Yoshio Sakka
Journal of the European Ceramic Society 2012 Volume 32(Issue 10) pp:2303-2309
Publication Date(Web):August 2012
DOI:10.1016/j.jeurceramsoc.2012.02.016
An increase in the loading temperature during SPS processing can reduce the residual porosity in a spinel and thus attain a high transmission even at the high heating rate of 100 °C/min. This suggests that load controlling is an important factor as well as the heating rate and sintering temperature. Although the transmission is lower than the maximum value attained at the low heating rates of <10 °C/min, the loading schedule optimization enables utilization of the high heating rate processing that is a primary advantage of the SPS technique.
Co-reporter:K. Morita, K. Hiraga, B.-N. Kim
Acta Materialia 2007 Volume 55(Issue 13) pp:4517-4526
Publication Date(Web):August 2007
DOI:10.1016/j.actamat.2007.04.016
Abstract
The high-strain-rate superplasticity of a ZrO2–spinel composite exhibits a sigmoidal strain rate–stress relationship. At high strain rates, a strain rate sensitivity of m ≈ 0.25 can be ascribed to the intervention of plastic deformation. At intermediate and low strain rates, the decreasing m-value from 0.5 to 0.25 with decreasing strain rate can be ascribed to a single flow mechanism of grain boundary sliding (GBS) accompanied by a threshold stress for intragranular dislocation nucleation. At intermediate strain rates, the apparent activation energy has a value of lattice diffusion of cations, suggesting that the rate of deformation is controlled by the rate of recovery of the dislocations, which contribute to the relaxation of stress concentrations exerted within ZrO2 grains by GBS.
Co-reporter:K. Morita, K. Hiraga, B.-N. Kim, H. Yoshida, Y. Sakka
Scripta Materialia 2005 Volume 53(Issue 9) pp:1007-1012
Publication Date(Web):November 2005
DOI:10.1016/j.scriptamat.2005.07.008
A nanocrystalline ZrO2–spinel composite with an average grain size of smaller than 100 nm can be synthesized by high-energy ball-milling followed by spark plasma sintering. Nanocrystallization can strengthen the ZrO2–spinel composite by a factor of 2.0–2.5; the maximum flexural strength of the nanocrystalline composite reached ≈2200 MPa.
Co-reporter:K. Morita, K. Hiraga
Scripta Materialia 2003 Volume 48(Issue 9) pp:1403-1407
Publication Date(Web):May 2003
DOI:10.1016/S1359-6462(03)00017-4
This paper replies to a comment by Balasubramanian and Langdon on our studies of yttria-stabilized zirconia. Their conclusion conflicting with experimental facts may arise from contradictions in their analysis. Microstructural aspects indicate that the deformation occurs through grain boundary sliding rather than interface-controlled Coble creep.
