Scripta Materialia 57 (2007) 607–610

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Spark plasma sintering of transparent alumina

Byung-Nam Kim,* Keijiro Hiraga, Koji Morita and Hidehiro Yoshida

National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

Received 18 May 2007; revised 21 May 2007; accepted 4 June 2007Available online 6 July 2007

Transparent alumina with a fine grain size (0.27 lm) was obtained by controlling the heating rate during spark plasma sinteringprocessing. The alumina sintered at 1150 �C with a heating rate of 8 �C/min has a residual porosity of 0.03% and an in-line trans-mission of 47% for a wavelength of 640 nm. We show that a low heating rate has an effect on the densification and transparency ofalumina for sintering at 1150 �C.� 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Sintering; Optical properties; Ceramics; Heating rate

To attain transparency in alumina ceramics, two ap-proaches have been conducted in order to reduce thesource of light scattering and absorption, such as grainboundaries and pores. One approach is sintering at veryhigh temperatures (>1700 �C) for a long time, whichmainly reduces the area of the grain boundaries [1].High-temperature sintering, however, causes severegrain growth, and hence, poor mechanical properties.Furthermore, the in-line transmission of sinteredcoarse-grained alumina is typically lower than 10% [1].The low in-line transmission probably results fromresidual pores which are not sufficiently eliminated byconventional pressureless sintering. Such alumina istranslucent, not transparent. The other approach ishot isostatic pressing (HIP) at low temperatures (1200–1300 �C), which sufficiently eliminates residual poreswith suppressed grain growth [2–5]. By using HIP, theporosity can easily be reduced to lower than 0.05%and the grain size can be suppressed to less than 1 lm.The fully dense and fine-grained alumina has goodmechanical properties and the in-line transmission ex-ceeds typically 50%. This is the reason why many studieshave recently been conducted on transparent aluminaprepared by using HIP [2–5].

Another available candidate for obtaining transpar-ent alumina is spark plasma sintering (SPS). Since SPShas the advantage of a rapid heating rate, it has widelybeen used to sinter dense and fine-grained alumina atlow temperatures within a short time [6–10]. Risbud

1359-6462/$ - see front matter � 2007 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2007.06.009

* Corresponding author. E-mail: kim.byung-nam@nims.go.jp

et al. [6] sintered alumina with a relative density of99.2% and a grain size of 0.65 lm at 1150 �C. At thesame temperature (1150 �C), Zhan et al. [8] obtained ahigher density (99.8%) and a smaller grain size(0.35 lm) by using a a-Al2O3 nanopowder with a sizeof 50 nm. A fully dense (relative density � 100%)alumina with a grain size of 0.5 lm was obtained at1200 �C by Shen et al. [9]. Their heating rates were veryhigh (P150 �C/min) and the holding time at the sinter-ing temperature was very short (3–10 min). The shortheating time at low temperatures significantly suppressesgrain growth, and in the case of alumina (a non-conduc-tor), rapid densification can proceed by easy shear-slid-ing between small powder particles under appliedmechanical pressure. By using SPS, Chaim et al. [11] re-cently obtained a transparent magnesia at low tempera-tures (�800 �C).

Unfortunately, however, transparency has not beenreported in those aluminas prepared by SPS, althoughthe full density and the fine grain size are comparableto those of HIP transparent alumina. This is probablydue to a few residual pores in the SPS-sintered alumina.According to the analysis of Apetz and Bruggen [5],‘‘only 0.1% of residual porosity can completely deterio-rate the transparency’’. For a high transmission of light,the porosity should be reduced to less than 0.05% [4]. Inthe present study, we report on a transparent aluminaprepared by SPS. We show that residual pores in alu-mina can be eliminated sufficiently by controlling theheating rate during SPS processing.

Commercial a-Al2O3 powder (TM-DAR, TaimeiChemicals Co. Ltd., Japan), with a purity of 99.99%

sevier Ltd. All rights reserved.

608 B.-N. Kim et al. / Scripta Materialia 57 (2007) 607–610

and an average particle size of 0.15 lm, was used in thisstudy. As-received powder was heated directly, withoutspecial treatment or additives, to 1150 �C under a uniax-ial pressure of 80 MPa using a spark plasma sinteringmachine (SPS-1050, Sumitomo). Two heating rates wereapplied; one is 100 �C/min from 600 �C to 1150 �C, andthe other is 25 �C/min from 600 �C to 1000 �C followedby 8 �C/min to 1150 �C. The temperature was measuredwith an optical pyrometer focused on the non-throughhole (1 mm in diameter and 2 mm in depth) of a graphitedie. After holding for 20 min at the sintering tempera-ture and subsequent annealing at 1000 �C for 10 min,we obtained a sintered disk with a diameter of 30 mmand a thickness of 3 mm. The mechanical pressure wasunloaded between the sintering and the annealingprocedure.

The center of the sintered body was machined to a tileof 10 mm · 10 mm with a thickness of 1 mm and mirror-polished carefully on both sides using diamond slurry.The final thickness of the sample is 0.88 mm. The in-linetransmission was measured in the wavelength range0.24–1.6 lm using a double-beam spectrophotometer(SolidSpec-3700DUV, Shimadzu) equipped with anintegrating sphere. The distance between the sampleand the detector is about 55 cm.

The microstructure was observed on the specimensurfaces, which had been polished and thermally etchedat 1050 �C for 1 h, using a scanning electron microscope(SEM) (JSM-6500, JEOL). The porosity was measuredon the SEM images taken at a magnification of10,000. We did not measure the absolute density becauseconventional techniques such as the Archimedes methodare insensitive to extremely low porosity. The grain sizewas measured by obtaining the average cross sectionarea per grain and assuming spherical grains. The mea-sured grain size is an apparent one, so that it was multi-plied by 1.225 to determine the true grain size [5].

SPS is one of the powerful tools for obtaining fullydense and fine-grained alumina. During SPS processing,since the DC pulses of high energy heat the die andpowder directly, high heating rates are available. Forthis reason, most researchers tried to sinter aluminaceramics at a heating rate of typically >50 �C/min [6–10]. For small sample sizes, even a heating rate of>500 �C/min is not difficult to apply. However, the pres-ent study reveals that transparent alumina with a fulldensity can be obtained at much lower heating rates,as demonstrated in Figure 1. For a wavelength of0.64 lm, the in-line transmission of the alumina sinteredat a heating rate of 8 �C/min is 47% (transparent), while

Figure 1. Alumina ceramics sintered by SPS at a heating rate of100 �C/min (left) and 8 �C/min (right). Both samples are 0.88 mmthick, and 3 mm above the text. A definition of transparency is that‘objects on the other side may be distinctly seen’.

that sintered at a rate of 100 �C/min is only 0.2%(opaque).

Krell et al. [4] and Apetz and Bruggen [5] obtained atransparent alumina by using HIP, and measured the in-line transmission for a sample thickness of 0.8 mm. Thein-line transmission T for an arbitrary sample thicknesst can be estimated by the relation:

T 2 ¼ ð1� RÞ T 1

ð1� RÞ

� �t2=t1

; ð1Þ

where R is the reflection loss for two alumina surfaces(=0.14). Calculating for a sample thickness of 0.8 mm,we obtain an in-line transmission of 50% in the presenttransparent alumina. Although the transmission is lowerthan the best one (71%) reported for alumina [5], it iscomparable to that prepared by HIP at 1200 �C for12 h [4]. Hence, it is demonstrated that SPS can yieldhighly transparent alumina in a time much shorter thanHIP.

High transparency indicates a high density or lowporosity. The porosity measured for the alumina sin-tered at a heating rate of 100 �C/min and 8 �C/min is0.59% and 0.03%, respectively. The distribution of resid-ual pores is shown in Figure 2. In the transparentalumina, fine pores smaller than 100 nm remain sparsely

Figure 2. Microstructure showing the distribution of residual pores inthe alumina sintered at a heating rate of (a) 100 �C/min and (b) 8 �C/min. Arrows in (b) represent small residual pores.

Figure 3. Microstructure of the transparent alumina sintered at aheating rate of 8 �C/min.

Figure 4. In-line transmission of the transparent alumina comparedwith the theoretical prediction for zero porosity.

B.-N. Kim et al. / Scripta Materialia 57 (2007) 607–610 609

distributed, whereas in the opaque one, numerous poreslarger than 100 nm are frequently observed. Despite theidentical powder and sintering temperature, the trans-parency and the porosity of samples prepared by SPSare quite different depending on the heating rate. Thetransparency completely deteriorated owing to a resid-ual porosity of 0.59%.

The effect of the heating rate on densification duringSPS was examined by Zhou et al. [10] using the samealumina powder used in this study. Contrary to the pres-ent results, however, they reported faster densification ata higher heating rate at around 1150 �C: a relativedensity of 76.6% at 50 �C/min and 91.3% at 300 �C/min. The reason for the opposite results is unknown.Differences in the pressure, sample size and sinteringtime from the present study may provide a clue to theexplanation. In particular, their holding time at the sin-tering temperature is zero. As soon as the temperaturereached 1150 �C, they shut off the electric current andcooled down the sample, whereas we held it for 20 minat that temperature. Studies on densification at the finalstage of sintering with and without the holding time mayelucidate the difference. Furthermore, the densificationbehavior may be changed at much low heating rates(<10 �C/min). We are now examining the effect of theheating rate on densification in more detail.

The average grain sizes are 0.55 lm and 0.27 lm forthe opaque and transparent alumina, respectively. De-spite a higher heating rate (100 �C/min) or a shorterheating time, the grain size of the opaque alumina isabout twice as large as that of the transparent one. Inthe case of heating up to 1150 �C, Zhou et al. [10] ob-tained a similar result: 0.17 lm at 50 �C/min and0.31 lm at 300 �C/min. This is an unexpected result, be-cause it is natural to consider that a shorter heating timeyields smaller grains, as also reported by Stanciu et al.[7] and Zhou et al. [10] for sintering temperatures of>1150 �C. In the present case, for heating at a rate of100 �C/min, the graphite die was temporarily over-heated up to 1194 �C owing to the large volume andthe overheating time was about 2 min in total. Owingto the high grain growth rate of pure alumina, the over-heating may cause the larger grain size in the opaquealumina. In actual, Zhou et al. [10] reported that thegrain size at a lower heating rate is larger for the alu-mina heated up to >1150 �C, while it is smaller for1100–1150 �C. This indicates that the grain growth rateof pure alumina may increase rapidly at >1150 �C.However, even when overheating was avoided at1150 �C, a larger grain size was obtained at a higherheating rate [10]. Hence, overheating at around1150 �C may not have significant effect on the twice aslarge grain size. The main reason should be found inthe inherent nature of SPS processing, such as masstransport enhanced by electric current, thermal gradientin sample, and so on.

The microstructure of the present transparent alu-mina is shown in Figure 3. The grain size 0.27 lm inthe transparent alumina is at a minimum level comparedwith the reported fully dense aluminas [1–10]. For HIPtransparent aluminas, the grain size is 0.3–1.0 lm. De-spite the smaller grain size, however, the in-line trans-mission of the present transparent alumina is lower

than that of the HIP ones. The theoretical in-line trans-mission was calculated by Apetz and Bruggen [5], on thebasis of Rayleigh–Gans–Debye theory. When light scat-tering occurs only at grain boundaries (zero porosity),the theoretical in-line transmission T is represented as

T ¼ ð1� RÞ exp � 3p2dDn2t

2k2

� �; ð2Þ

where d is the grain size, Dn is the refractive indexdifference (=0.0053) and k is the wavelength of the inci-dent light. Calculating Eq. (2) with d = 0.27 lm, t =0.88 mm and k = 640 nm, we obtain an in-line transmis-sion of 68%. Therefore, we consider that a residualporosity of 0.03% in the present transparent aluminadecreased the in-line transmission by 21%. Althoughwe do not quantitatively evaluate the effect of the resi-dual porosity on the transmission, we understand thatthe deterioration is reasonable, referring to the analysisof Apetz and Bruggen [5]. Reducing the residual poros-ity has more of an effect on transparency than reducingthe grain size in submicrometer-grained alumina.

The dependence of the in-line transmission on thewavelength of light is shown in Figure 4. As expected,the in-line transmission approaches the value of

610 B.-N. Kim et al. / Scripta Materialia 57 (2007) 607–610

sapphire (86%) with increasing wavelength, whereas itdecreases rapidly with decreasing wavelength, particu-larly in the ultraviolet region (k < 400 nm). Figure 4 alsoshows the theoretical prediction of Eq. (2) for zeroporosity. Compared with the theoretical prediction,the experimental in-line transmission is entirely lower.As noted above, this is mainly due to the residual poros-ity. At all wavelengths examined, the residual porescaused the transparency to deteriorate.

We demonstrated that transparent alumina with anin-line transmission of 47% can be obtained by control-ling the heating rate during SPS processing. Contrary tothe existing research on SPS of alumina [7,10], full den-sification and fine grains (0.27 lm) were attained at lowheating rate (8 �C/min). The lower in-line transmissioncompared with the theoretical prediction was explainedby the residual porosity (0.03%) in the alumina.

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