Highly Transparent Pure Alumina Fabricated by High-Pressure SparkPlasma Sintering

Salvatore Grasso,z,y Byung-Nam Kim,y Chunfeng Hu,y Giovanni Maizza,z and Yoshio Sakka*,w,z,y

zGraduate School of Pure and Applied Sciences, University of Tsukuba, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

yWorld Premier International Research Center Initiative (WPI Initiative) on Materials Nanoarchitronics (MANA) andNano Ceramics Center, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

zDipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24,I-10129 Torino, Italy

Highly transparent pure alumina with an average grain size of200 nm was fabricated by means of high-pressure spark plasmasintering. The alumina sintered either at 9501 or at 10001Cfor 10 min under an applied pressure of 500 MPa had an in-linetransmission of about 64% for a wavelength of 645 nm.The application of high pressure allowed to obtain highly trans-parent full dense alumina at low temperatures with noconsiderable grain growth.

I. Introduction

HOT isostatic pressing (HIP) at temperatures of 11501–13001C and pressure as high as 200 MPa has been tradi-

tionally used1,2 to attain highly transparent pure alumina. UsingHIP, the porosity can easily be lowered below 0.05% and thegrain size can be limited to o1 mm. The tri-axial pressure ap-plication in HIP, together with the optimized powder processingroute, allowed to obtain fully dense and highly transparent purealumina with a grain size of about 300 nm and an in-line trans-mittance of up to 71%.1 Generally, the in-line transmittance ofHIPed pure alumina exceeds 50%.1,2

Recently, spark plasma sintering (SPS) has been used to pro-duce transparent/translucent alumina.3–7 Kim et al.,4 in the caseof alumina sintered at 11501C under an applied pressure of 80MPa, obtained an in-line transmittance of 47% at a wavelengthof 640 nm. As SPS typically operates with punches and die ingraphite, the maximum pressure in the SPS method is generallylimited by the compressive strength of the material used for thedies, which is 140 MPa for high-strength graphite.8–10

In 2006, Anselmi-Tamburini et al.11 reported a method forpreparing dense functional oxide compacts with a crystallite sizein the range of 10–20 nm. They used relatively short thermalcycles (o10 min) coupled with a progressive increase in pressure(up to 1 GPa) to promote a high degree of compaction and verylimited grain growth. However, the method patented by Ans-elmi-Tamburini et al.12 has not yet used for the production offully dense and transparent alumina compacts. In this study, we

propose a new method for the fabrication of highly transparentalumina using high-pressure SPS.

II. Experimental Procedure

Commercial a-Al2O3 powder (TM-DAR, Taimei Chemicals Co.Ltd., Tokyo, Japan), with a purity of 99.99%13 and an averageparticle size of 0.2 mm measured by SEM. Scanning electronmicroscope (SEM) images of the as-received powder showedthat spherical particles are agglomerated to form clusters of 5–50mm average size. As specified by the manufacturer, the averageBET specific surface area is 14.5 m2/g.

The as-received powder was sintered at 9001, 9501, and10001C, without any treatment or additives, under a uniaxialpressure of 500 MPa using an SPS machine (SPS-1050, Sumi-tomo, Kawasaki, Japan). The high-pressure device is sketched inFig. 1. This device allows one to apply a pressure higher than500 MPa on a sample 5 mm in diameter and about 2.5 mmthick. In a typical sintering experiment, 0.18–0.2 g of aluminapowder was poured into the die. The device is composed of anouter and an inner graphite die, and the powder is pressed be-tween the two WC punches between two intermediated WCdisks. Apart from the protective disks and punches, remainingdevices were made entirely of graphite. The temperature wasmeasured accurately using a pyrometer focused on the die sur-face of the inner die (i.e., 0.75 cm far from the sample). Agraphite felt was used to reduce the heat loss by radiation. Thepowder was heated from room temperature up to 7001 C for 10min, subsequently, up to the sintering temperature (i.e. 9001,9501, and 10001C). The dwelling time was 10 min and pressurewas raised just before the holding. Heating was conducted usinga sequence consisting of 12 DC pulses (40.8 ms) followed by zerocurrent for 6.8 ms. During the entire duration of the experi-ments, the current intensity was below 1000 A and the voltagedrop between the cooled rams was below 4 V.

The sintered sample was machined to a disk of 5 mm diam-eter with a thickness of 1 mm and mirror polished carefully onboth sides using a diamond slurry. The final thickness of thesample was 0.8 mm. The in-line transmission was measured inthe wavelength range of 0.24–1.6 mm using a double-beam spec-trophotometer (SolidSpec-3700DUV, Shimadzu, Kyoto, Japan)by inserting an aperture (3 mm diameter) in front of the detectorin order to allow the detection of only the specularly transmittedportion of the incident light beam. The distance between thesample and the detector was about 55 cm.

The microstructure of the fracture surfaces was observed us-ing a scanning electron microscope (SEM) (JSM-7100, Jeol,Tokyo, Japan). The porosity was measured on the SEM images

J. Groza—contributing editor

This work was supported by World Premier International Research Center Initiative(WPI Initiative), MEXT, Japan.

*Member, The American Ceramic SocietywAuthor to whom correspondence should be addressed. e-mail: sakka.yoshio@nims.-

go.jp

Manuscript No. 27605. Received February 24, 2010; approved March 15, 2010.

Journal

J. Am. Ceram. Soc., 93 [9] 2460–2462 (2010)

DOI: 10.1111/j.1551-2916.2010.03811.x

r 2010 The American Ceramic Society

2460

taken at a magnification of 10000. We did not measure the ab-solute density because conventional techniques such as the Ar-chimedes method are insensitive to extremely low porosity.

III. Results and Discussion

Unlike the sintering device developed by Anselmi-Tamburiniet al.11,12 which includes electrically insulating SiC punches (i.e.Good Fellow SiC with an electrical resistivity of 102–103 O � cm),the configuration of the present die shown in Fig. 1 uses elec-trically conductive punches and protective disks made up ofbinderless tungsten carbide WC with an electrical resistivity of20 10�6 O � cm. Because of the high electrical resistivity of thepunches, the configuration developed by Anselmi-Tamburiniand colleagues is more similar to a high-pressure hot pressingrather than to high-pressure SPS.

At present, although the sintering mechanisms of other oxidepowders under SPS conditions are not completely known, elec-tric current may induce some effect not limited to Joule heatingduring sintering.14–16 Recently, Langer et al.14 compared hotpressing (HP) and SPS technique for the alumina powder (TM-DAR). The sample geometry, the heating schedule, the appliedpressure, and the atmosphere were the same for both sinteringmethods. The results showed that at a given constant time,SPSed samples achieved a higher density compared with HPedones. The electroplasticity effect on fine-grained alumina wasinvestigated by Campbell et al.15 and Conrad et al.16 They re-ported that an electric field of 300 V/cm had a significant influ-ence on the plastic deformation at 14501 and 16001C. In fact, theelectric field reduced the flow stress up to 70% in the flow stressand in general led to an increase in elongation. As shown inFig. 1, we decided to use electrically conductive punches becauseelectric current may enhance densification.

Figure 2 shows a photograph of 5-mm-diameter aluminasintered at 9501C and 500 MPa. The text, the images, and theholograms can be clearly seen through the sample positionedeither on the top or 1.2 cm above the text. As shown in Fig. 3,the in-line transmission of the 0.8 mm sample sintered at 9501and 10001C, for a wavelength of 645 nm, were 63.3% and 64%,respectively. The transparency of the samples sintered at 9501

and 10001C are nearly the same. The sample sintered at 9001Cwas opaque and consequently its transparency was not mea-sured. Figure 3 shows a comparison of the data reported in theliterature for transparent alumina for a wavelength of 645 and asample thickness of 0.8 mm.

The in-line transmittance value obtained in the present workis only lower than that reported by Apetz et al.,1 but it is greaterthan other’s reported in the literature.2–7 Apetz and colleaguesprepared fine-grained samples by slip or pressure casting ofAl2O3 powder with a mean particle size of 150 nm. They sinteredthe alumina samples at a temperature of 11501–12501C for 2 h toobtain closed porosity. Finally, the samples were subjected toHIP at 12001–14001C and 200 MPa for 2 h in argon.

However, in Apetz’s work, the manufacturer of the aluminapowder as well as the morphology of the initial powder were notdisclosed. Instead, Krell et al.2 used the same powder as thatused in the present work (TM-DAR). An in-line transmission aslow as 55%was obtained by sintering the powder at 1280 for 2 hfollowed by HIP(200 M Pa) at 12001C for 15 h.

With respect to the HIP method of Apetz et al.1 and Krellet al.,2 the SPS method proposed in the present work is ex-tremely fast and simple. Because it did not require any powderdeagglomeration/preparation, the as-received powder couldsimply be poured into the die and the holding time in SPS wasjust 10 min against the 4–17 h of HIP.1,2

In the present work, the transparency is superior to the oneobtained by SPS at 80MPa.3–7 The highest in-line transmittanceof 47%, in the case of samples sintered under 80 MPa wasobtained using SPS with a heating rate of 21C/min (i.e., sinteringtime sintering about 5 h), by Kim and colleagues.The data for Kim et al.,4 are measured for a 0.88-mm-thicksample for a wavelength of 640 nm. Aman et al.7 by optimizingthe green shaping process obtained a real in-line transmittance

Fig. 2. Photograph of the transparent alumina sample (a) on top of thetext and (b) 1.2 cm above the text. The sample was sintered at 9501C for10 min under an applied pressure of 500 MPa. Samples are 0.8 mm thickand polished on both sides. The text was not retro illuminated.

Fig. 3. In-line transmission of the alumina sintered at 9501 and 10001Cfor 10 min under an applied pressure of 500 MPa. The real in-line trans-mission of alumina samples is given for a wavelength of 645 nm and asample thickness of 0.8 mm. The sintering method and the applied pres-sure are also reported.

Fig. 1. Schematic of the high-pressure SPS device. The pure aluminapowder was compressed between two electrically conductive binderlessWC punches pushed by the WC protective disks. The temperature wasmeasured by a pyrometer at the inner die surface (i.e., 0.75 cm far fromthe sample).

September 2010 Rapid Communications of the American Ceramic Society 2461

of 40%. More recently, Stuer et al.6 investigated the effect ofMg, Y, and La doping on the transparency of SPSed alumina; inthe case of pure alumina, the transparency was below 5%.

Figure 4 compares the micrograph of TM-DAR powder withthe fracture surface of sintered samples at 10001C under 500MPa. As shown in Fig. 4(a), the initial powder is heavily ag-glomerated, the particle size is distributed between 150 and 250nm, and the initial particles are roughly spherical particle withan average particle size of 200 nm. Figure 4(b) shows the trans-granular fracture surface of sintered samples, and the particlesize is nearly the same as that of the initial powder. Bernard-Granger et al.17 and Langer et al.14 studied the grain size/relativedensity trajectory for TM-DAR powder in the case of pressure-less sintering and SPS, respectively. They reported no graingrowth at a higher temperature with a dwelling time greater thanthat reported here (i.e., pressureless sintered at 12201C for 21min17 or in the case of powder sintered at 11001C under 50 MPawith a dwelling time of 1 h14). However, the resulting relativedensities were 83% and 81.5%, respectively. In the present in-vestigation, by increasing the pressure up to 500 MPa, fullydense compacts were obtained instead with a grain size of 200nm and porosity below 0.05%.

In comparison with conventional HIP1,2 and SPS,3–7 high-pressure SPS led to three significant results. First, full densealumina with no considerable grain growth was sintered at atemperature 2001C lower.1–7 Second, the dwelling time wasreduced dramatically to 10 min. Third, the samples sinteredstarting from pure alumina powder without any preparationpossessed a real in-line transmittance of 64%. The high value ofthe real in-line transmittance was attributed to both the fine mi-crostructure and the low porosity level. A further improvement

of the transparency may be provided by a strong magnetic fieldalignment technique18 because alumina monocrystal shows an-isotropic transparency with respect to the crystallographic di-rection.

References

1R. Apetz and M. P. B. Bruggen, ‘‘Transparent Alumina: A Light-ScatteringModel,’’ J. Am. Ceram. Soc., 86, 480–6 (2003).

2A. Krell, P. Blank, H. Ma, and T. Hutzler, ‘‘Transparent Sintered Corundumwith High Hardness and Strength,’’ J. Am. Ceram. Soc., 86, 12–8 (2003).

3B. Kim, K. Hiraga, K. Morita, and H. Yoshida, ‘‘Effects of Heating Rate onMicrostructure and Transparency of Spark-Plasma-Sintered Alumina,’’ J. Eur.Ceram. Soc., 29, 323–7 (2009).

4B. Kim, K. Hiraga, K. Morita, and H. Yoshida, ‘‘Spark Plasma Sintering ofTransparent Alumina,’’ Scr. Mater., 57, 607–10 (2007).

5B. Kim, K. Hiraga, K. Morita, H. Yoshida, T. Miyazaki, and Y. Kagawa,‘‘Microstructure and Optical Properties of Transparent Alumina,’’ Acta Mat., 57,1319–26 (2009).

6M. Stuer, Z. Zhao, U. Aschauer, and P. Bowen, ‘‘Transparent PolycrystallineAlumina Using Spark Plasma Sintering: Effect of Mg, Y and La Doping,’’ J. Eur.Ceram. Soc., 30, 1335–43 (2010).

7Y. Aman, V. Garnie, and E. Djurado, ‘‘Influence of Green State Processes onthe Sintering Behaviour and the Subsequent Optical Properties of Spark PlasmaSintered Alumina,’’ J. Eur. Ceram. Soc., 29, 3363–70 (2009).

8G. Suarez, Y. Sakka, T. S. Suzuki, T. Uchikoshi, X. Zhu, and E. F. Aglietti,‘‘Effect of Starting Powders on the Sintering of Nanostructured ZrO2 Ceramics byColloidal Processing,’’ Sci. Tech. Adv. Mater., 10, 02500 (2009).

9H. Borodianska, O. Vasylkiv, and Y. Sakka, ‘‘Nanoreactor Engineering andSpark Plasma Sintering of Gd20Ce80O1.90 Nanopowders,’’ J. Nanosci. Nanotech.,8, 3077–84 (2006).

10S. Grasso, Y. Sakka, and G. Maizza, ‘‘Electric Current Activated/AssistedSintering (ECAS): A Review of Patents 1906–2008,’’ Sci. Tech. Adv. Mat., 10,053001 (2009).

11U. Anselmi-Tamburini, J. E. Garay, and Z. A. Munir, ‘‘Fast Low-Temper-ature Consolidation of Bulk Nanometric Ceramic Materials,’’ Scripta Mater., 54,823–8 (2006).

12U. Anselmi-Tamburini, J. E. Garay, and Z. A. Munir, International PatentPublication Number WO 2006/113354 A2 (2006).

13K. Nakane, Y. Uwamino, H. Morikawa, A. Tsuge, and T. Ishizuka,‘‘Determination of Trace Impurities in High-Purity Aluminium Oxide by HighResolution Inductively Coupled Plasma Mass Spectrometry,’’ Anal. Chim. Acta,369, 79–85 (1998).

14J. Langer, M. J. Hoffmann, and O. Guillon, ‘‘Direct Comparison BetweenHot Pressing and Electric Field-Assisted Sintering of Submicron Alumina,’’ ActaMater., 57, 5454–65 (2009).

15J. Campbell, Y. Fahmy, and H. Conrad, ‘‘Influence of an Electric Field on thePlastic Deformation of Fine-Grained Al2O3,’’Metall. Mater. Trans A, 30, 2817–23(1999).

16H. Conrad, ‘‘Electroplasticity in Metals and Ceramics,’’ Mater. Sci. Eng. A,287, 276–87 (2000).

17G. Bernard-Granger, N. Monchalin, and C. Guizard, ‘‘Sintering of CeramicPowders: Determination of the Densification and Grain Growth Mechanismsfrom the Grain Size/Relative Density Trajectory,’’ Scr. Mater., 57, 137–40(2007).

18Y. Sakka, T. S. Suzuki, and T. Uchikoshi, ‘‘Fabrication and Some Propertiesof Textured Alumina-Related Compounds by Colloidal Processing in High-Mag-netic Field and Sintering,’’ J. Eur. Ceram. Soc., 28, 935–42 (2008). &

Fig. 4. FESEM of (a) as-received TM-DAR alumina powder and(b) fracture surface of sintered samples at 10001C under 500 MPa for10 min.

2462 Rapid Communications of the American Ceramic Society Vol. 93, No. 9