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[WORLD_PM2024_Manuscript_K.Morita_C000244.doc](https://mdr.nims.go.jp/filesets/cc9266f7-d235-4f72-8e86-e6fe535d8ced/download)

## Creator

[Koji Morita](https://orcid.org/0000-0001-6040-7054), [Ji-Hwoan Lee](https://orcid.org/0000-0001-9413-0110)

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[Creative Commons BY-NC-ND Attribution-NonCommercial-NoDerivs 4.0 International](https://creativecommons.org/licenses/by-nc-nd/4.0/)

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[Effect of Electric Field/Current on Sintering Behavior of Yttria during SPS Processing](https://mdr.nims.go.jp/datasets/52d0a963-5786-4d71-965c-34298af4a67b)

## Fulltext

Preparation of Papers in Two-Column FormatEffect of Electric Field/Current on Sintering Behavior of Yttria during SPS processingKoji MORITA1,2( and Ji-Hwoan LEE1,31 Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan.2 Department of Materials Science and Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 891-0395, Japan.3 Department of Space and Defense Materials, Extreme Materials Institute, Korea Institute of Materials Science (KIMS), 797 Changwon-daero, Changwon, Gyeongsangnam-do 51508, Korea.Abstract　Effect of the electric current on the densification behavior of yttria (Y2O3) and zirconia (3Y-ZrO2) powders during the spark-plasma-sintering (SPS) process has been examined. The powders were densified by the SPS device under two types of die set-up; one is a conductive normal set-up, in which the powders loaded into a graphite die were sandwiched by carbon papers, another is an insulation set-up, in which the powders were sandwiched by BN powder in the die. For the ZrO2(3Y) powder, the shrinkage is independent of the die set-ups. Whereas for the Y2O3 powder, on the other hand, it was apparently accelerated during the initial and intermediate sintering stages and completed at more than 100 oC lower temperature in the normal die set-up than in the insulation set-up. It is expected that the DC pulsed current applied during the SPS processing enhanced the sintering of the Y2O3 powder. This suggests that during the SPS processing, applied pulsed electric field/current is likely to contribute to the sintering of Y2O3 ceramics.Keywords: Spark-Plasma-Sintering (SPS), Current Effect, Yttria, Zirconia.IntroductionSpark plasma sintering (SPS) processing is known as an advanced sintering technique and has widely been utilized in the densification of various materials1-6). This is because the SPS technique can suppress grain coarsening during the densification by reducing the processing time and temperature, and hence, would be a promising method to achieve fine-grained and dense materials. Hence, the SPS technique has widely been applied to the densification of several ceramic powders and succeeded to fabricate dense ceramics enough to show optical transmission in MgO, Al2O3, MgAl2O4, Y2O3, YAG etc7-16). Reducing the grain size in the dense ceramics has also been expected as a promising method to attain excellent mechanical properties, such as strength and hardness.It has generally been explained that for the SPS processing, spark and/or plasma, which were generated between the powders due to the applied pulsed DC electric field/current as schematically shown in Fig. 1, has been reported to enhance the sintering behavior of various materials6,17,18). For conductive materials such as metallic alloys, the applied pulsed DC electric field/current might indeed contribute to the densification by passing the current into the material and its composites (II in Fig. 1) during the SPS processing6). For non-conductive materials such as oxide ceramics, however, the DC electric field/current effects on the sintering remain unclear and are still under discussion. For the SPS processing of the insulating materials, some studies argued that since the current bypasses the sample and flows only through the graphite die (I in Fig. 1), plasma and/or spark do not generate in the powder compacts, and hence, no current effect contributes to its densification19,20).In order to examine the effects of the field/current effects on the densification of ceramics, therefore, the field/current effects in the SPS technique were examined in yttria (Y2O3) and zirconia (ZrO2) ceramic powders as reference materials by comparing the shrinkage behavior using conductive and insulation sintering die set-ups.1mGraphitePunchSampleGraphiteDiePuressurePulsedCurrentPowderplasma       spark PowderIIIIExperimental ProceduresCommercially available high purity powders of Y2O3 (BB-type, Shin-Etsu Chemical Co., Ltd., Japan) and ZrO2 containing 3mol% Y2O3 (TZ-3Y, Tosoh Co., Ltd., Japan) was used as starting materials. The densification behavior of the starting powders was examined with a SPS machine (LABOX, Sinter Land Inc., Japan). The powders were loaded into a graphite mold with an inner diameter of 10 mm, as shown in Fig. 2.21) The inside of the graphite mold was covered with a graphite paper. In order to examine the current effects on the densification behavior, the Y2O3 and ZrO2(3Y) powders were separated from graphite punches by placing carbon papers or BN powders on both sides of the powders to isolate from the graphite punches. Hereafter, these die set-ups utilized GraphitepunchCarbon paperorBN powderpowderGraphitepunchGraphiteMoldCarbonFeltCarbon paperCarbon paperorBN powder -1.2-1-0.8-0.6-0.4-0.200.2500 700 900 1100 1300Temperature,  Ts/ ℃Apparent Displacement,  Lapp/  mmBN : insulationC : conductiveZrO2(3Y)the carbon paper and BN powder will be referred to as conductive and insulation die set-ups, respectively. The SPS process was carried out at 950-1250 oC with a heating rate of 25 oC/min and a dwelling time of 10 min under a uniaxial pressure of 80 MPa. The outside of the mold was covered with a thermal insulator carbon felt to suppress any heat losses from the graphite die surface. During the SPS processing, the SPS temperature Ts was controlled by monitoring the surface temperature of the graphite die using an optical pyrometer. The densification behavior was evaluated by continuously recording the relative displacement L of the moving ram of the SPS machine. Since the displacement value during the SPS processing includes a component of the thermal expansion of the whole system of the graphite die and specimen, the apparent displacement, Lapp, does not necessarily represent only on the densification behavior of the powder. Thus, the displacement caused only by the densification component of the powder, Ldens, was evaluated by correcting the thermal expansion component Lexp, which was separately measured by a dummy test at the same SPS processing.The microstructures of the SPS proceeded samples were examined by the scanning electron microscope (FE-SEM; S-4800, Hitachi High-Tech. Co., Ltd., Hitachi, Japan). For the SEM observations of low density specimens of ρt ( 95%, the microstructure was observed at fracture surface. For the SEM observations of high density specimens of ρt = 98.8%, the surface of the specimen was mechanically removed and mirror-polished with a colloidal silica suspension, and then thermal-etched at 1100˚C for 30 min in air to more clearly reveal the grain boundaries.Results and DiscussionFig. 3(a) shows the densification behavior of Y2O3 during the SPS processing tested at 950 ℃ under the conductive die set-up. Since the SPS temperature Ts was controlled by measuring the die surface using an optical pyrometer, Ts is plotted at (570 ℃, which is the lower limit of the optical pyrometer. From the apparent displacement Lapp, it can be confirmed that the Y2O3 powder shows three regions in the shrinkage behavior depending on the temperature Ts. First, Lapp shows a rapid shrinkage with the temperature rises to 600°C, and second, follows to a plateau region from 600 to 750°C. Finally, it shows a gradual densification at above 750°C as the temperature increases and completes at around 950°C.Since the apparent displacement Lapp is the sum of the displacement caused by the powder densification Ldens and the thermal expansion Lexp of the whole system, the true displacement behavior Ldens, which is caused only by powder densification, should be 5005506006507007508008509009501000-3.0-2.5-2.0-1.5-1.0-0.50.00.51.01.50 500 1000 1500 2000Temperature,  Ts/  oCTime,  t /  secDisplacements,  Lapp, Ldens,Lexp/  mmLappLexpLdensTs(a)-1.1-1-0.9-0.8-0.7-0.6-0.5-0.4-0.3-0.2-0.10500 600 700 800 900 1000 1100 1200①②③Temperature,  Ts/ ℃BN : insulationCorrected Displacement,  L(mm)(b)C : conductiveexpansionshrinkageY2O3evaluated by subtracting the thermal expansion component Lexp from the apparent value Lapp. The true displacement behavior caused only by the densification Ldens is plotted after correcting the effect of the thermal expansion in Fig. 3(a). The correction does not change the general trend of the densification behavior and can be divided into three regions. The corrected densification behavior Ldens of the Y2O3 powder is re-plotted as a function of the temperature Ts in Fig. 3(b). For comparison, the densification behavior Ldens obtained under the insulation die set-up is also plotted by gray symbols in Fig. 3(b) after correcting the thermal expansion component Lexp. Although the densification behavior can be divided into three regions irrespective of the conductive and insulation die set-ups, the densification curve is apparently different depending on the die set-ups. As shown by the arrows in Fig. 3(b), the conductive set-up shows rapid shrinkage immediately after the start of heating up to 600℃ (①) and in the temperature range above 800℃ (②), and the shrinkage is completed around 935℃ (③). On the other hand, for the insulation set-up with the BN powder, although the similar shrinkage also occurs at in the temperature ranges of 600℃ (①) and 800℃ (②), it relatively slower as compared to those of the conductive set-up. Particularly for the insulation set-up, the gradual shrinkage occurs even above 935℃, which is the densified temperature of the conductive die set-up, and gradually continues up to around 1060℃ (③).It can be assumed from the results that this different shrinkage behavior might occur due to the difference in the die set-up. If the sample temperature was different depending on the die set-up, this may be reflected in the different shrinkage behavior. However, this is not the case of the present results. This is because since the processing time - thermal expansion component relationship, t-Lexp, almost overlapped between the conductive and insulation die set-ups, the sintering temperatures might be almost the same or the difference should be negligibly small. In addition, since the gradual shrinkage obtained in the insulation die set-up is likely to be the conventional sintering behavior, the rapid shrinkage behavior obtained in the conductive die set-up would be unusual. For the insulation die set-up, the pulsed electric current applied during the SPS processing would be interrupted by the BN powder, whereas for the conductive die set-up, it would pass through the sample and accelerate the sintering of the Y2O3 powder though the generation of the spark/plasma is still unclear from this test.Fig. 4 shows the microstructures obtained after the sintering at 950 ℃ and 1060 ℃ under the conductive and insulation die set-ups, respectively, at which the shrinkage is almost completed. For the conductive die set-up, although Y2O3 maintains the fine grain size, it exhibits almost pore free dense microstructure even at the lower temperature of 950 ℃. In contrast, for the insulation die set-up, it shows larger grain size due to a higher sintering temperature of 1060 ℃. Nevertheless, the densification is not completed yet and many pores can be observed to remain in the multiple grain junctions though the shrinkage is almost completed similar to that of the conductive die set-up. This suggests that the current flow caused by the conductive die set-up is likely to accelerate the densification of Y2O3 during the SPS processing. Yoshida et al,22,23) also reported that the densification of Y2O3 was accelerated under the SPS processing and completed at lower temperatures. From TEM-EELS analysis, the improved sinterability is explained by the enhanced diffusivity that arises from defect reactions activated by the SPS processing.The shrinkage behavior shows different feature depending on the material. Fig. 5 shows the Lapp-Ts relationship obtained by ZrO2(3Y) under the conductive and insulation die set-ups. Although the SPS processing was carried out in the similar way to that of Y2O3, ZrO2(3Y) shows almost similar shrinkage behavior between the conductive and insulation die set-ups within experimental error. This result in ZrO2(3Y) indicates two important insights. First, the sintering temperature should be almost the same irrespective of the conductive and insulation die set-ups, and second, the current effect during the SPS processing would change with the powder nature and can accelerate the sintering of Y2O3 at least, but not of ZrO2(3Y). This work cannot argue the generation of the spark/plasma, but suggests that the SPS processing contributes to accelerating the sintering of certain ceramics. Fig. 5 Comparison of the densification behavior Lapp of ZrO2(3Y) under the conductive and insulation die set-ups plotted as a function of the temperature Ts. 21) ConclusionIn this study, the effect of the current on the densification behavior of Y2O3 and ZrO2(3Y) powders during the SPS processing has been examined under the conductive and insulation die set-ups. The results obtained in this work can be summarized as follows: 1) The densification behavior during the SPS processing changes with the materials depending on the die set-ups. For the ZrO2 powder, the densification is independent of the die set-ups and shows almost the same behavior.2) For the Y2O3 powder, on the other hand, the densification depends on the die set-ups. Under the conductive die set-up, it is apparently accelerated in the initial and intermediate sintering stages, and then, completed at more than 100 oC lower temperature as compared to that of the insulation die set-up.3) Under the conductive die set-up, Y2O3 shows almost pore-free dense microstructure even at 950 oC, which is more than 100 oC lower temperature of the insulation die set-up.4) This suggests that during the SPS processing, the applied DC pulsed electric current is likely to contribute to the acceleration of the sintering behavior of Y2O3 ceramics. AcknowledgementsThe author appreciates to the financial supports by Core Research for Evolutionary Science and Technology (CREST) (JPMJCR1996) from Japan Science and Technology Agency (JST) and by Japan Society for the Promotion of Science (JSPS) KAKENHI (20H02444) Japan.References1) Z. A. Munir, U. Anselmi-Tamburini, M. Ohyanagi: J. Mater. Sci., 41 (2006) 763-777.2) R. Orrù, R. Licheri, A. M. Locci, A. Cincotti, G. Cao: Mat. 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Soc., 94 (2011) 3301-3307.Fig. 1 Schematic diagram of Spark Plasma Sintering (SPS) process and DC Pulsed current flow through the particle surfaces.Fig. 2 Schematic explanation of a graphite die set up for the SPS processing21).Fig. 3 Densification behavior of Y2O3 during the SPS processing tested at 950 ℃. (a) Displacements Lapp, Lexp and Ldens, which is tested under the conductive die set-up, plotted as a function of processing time t. (b) Comparison of the corrected densification behavior Ldens under the conductive and insulation die set-ups 21).�Fig. 4 Typical SEM images taken by Y2O3 densified at (a) 950 ℃ under the conductive die set-up and (b) 1060 ℃ under the insulation die set-ups21).( corresponding author, E-mail: MORITA.Koji@nims.go.jp