characterization of green-emitting translucent zinc oxide ceramics prepared via spark plasma...

Characterization of Green-Emitting Translucent ZincOxide Ceramics Prepared Via Spark Plasma Sintering

Mei Hongw and Daniela Fredrick

Department of Chemical Engineering and Materials Science, University of California, Davis, California95616

David M. Devito and Jane Y. Howe

Oak Ridge National Laboratory, Center for Radiation Detection Materials and Systems, Oak Ridge,Tennessee 37831

Xiaocheng Yang and Nancy C. Giles

Department of Physics, West Virginia University, Morgantown, West Virginia 26506

John S. Neal

Center for Radiation Detection Materials and Systems, Oak Ridge National Laboratory, Oak Ridge,Tennessee 37831

Zuhair A. Munir*,**

Department of Chemical Engineering and Materials Science, University of California, Davis, California95616

Int. J. Appl. Ceram. Technol., 8 [4] 725–733 (2011)DOI:10.1111/j.1744-7402.2010.02527.x

Ceramic Product Development and Commercialization

r 2010 The American Ceramic Society

Research supported by the DOE Office of Nonproliferation Research and Engineering in the National Nuclear Security Administration (NNSA), U.S. Department of Energy under contract

DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.wPresent address: Lonza Guangzhou Research and Development Center, Guangzhou 511455, China.

*[email protected]��Fellow, The American Ceramic Society.

mailto:[email protected]

Translucent, green-emitting zinc oxide (ZnO) bodies, 19 mm in diameter and 0.72 mm in thickness, have been preparedvia spark plasma sintering method. The consolidation of ZnO powders was investigated over the temperature range of550–10501C and the pressure range of 55–530 MPa. Samples sintered at temperatures 48501C and pressures of B120 MPawere translucent and had densities of B100%. Samples sintered at 9501C and 130 MPa showed a higher maximumtransmittance than the samples sintered at higher or lower temperatures or pressures, with an excellent in-line transmission of70% in the IR region around 2330 nm. The dense ZnO ceramics exhibited a strong green emission and a weak ultravioletemission, and the relative intensity of the green emission increased with increasing sintering temperature.

Introduction

Zinc oxide (ZnO) has received considerable atten-tion in recent years because of its potential applicationin a variety of devices. These include flat-panel displays,transparent conducting electrodes in solar cells, chem-ical sensors, surface acoustic wave and LED devices, andothers.1–3 The consolidation of powders of this oxidehas been investigated by several methods. Wang et al.4

sintered ZnO by microwave heating and reported theproduct to contain large, highly faceted (40 mm long),rod-shaped grains. However, no indication of the rela-tive density of the sintered oxide was provided. Ina similar study, Birnboim et al.5 investigated the effectof the microwave frequency on the sintering of ZnO. Itwas shown that the frequency had an effect on the den-sity when sintering was carried out at temperatureslower than 10001C; a lower frequency resulted in ahigher density. However, when sintering was carried outat T410001C, the frequency had no effect and, in fact,there was no advantage for microwave sintering relativeto pressureless sintering. The highest relative density,obtained at 11501C, was 98%. Recently, attempts havebeen made to sinter nanopowders of ZnO and in nearlyall cases a significant grain growth occured.6–8 Cai et al.6

synthesized B35 nm pure ZnO nanopowders by solvo-thermal synthesis but these grains grew to a size ofB3 mm using pressureless sintering at 9501C. The sinte-ring study of nanocrystalline undoped ZnO powdershas been conducted over a wide range of temperaturesand holding times, but the grain size increased to themicrometric region for all the dense sintered samples.7

A two-step approach still could not prevent the signifi-cant grain growth of ZnO nanopowders during sinte-ring.8 However, Gao and colleagues reported that98.5% dense ZnO with a grain size of B100 nm canbe prepared by sintering at 5501C by the spark plasmasintering (SPS) method.9,10

One important aspect associated with the sinteringof ZnO relates to the formation of optically transparentsamples. Transparent ceramics have recently acquired ahigh degree of interest due to their device applicableproperties, such as sensors, electrodes, and armor.Luminescent and transparent ceramic samples have a largepotential for many applications in the field of photonics.Translucent bulk ZnO ceramics were obtained by pres-sureless sintering with the aid of a special additive(H3PO4) but no transmittance data were reported.11 Inthe majority of research on the optical properties, how-ever, the focus has been on one-dimensional nanostruc-tures and thin films of ZnO.12 This is probably related tothe difficulty in obtaining highly dense polycrystallineZnO ceramics. Attempts to achieve such a goal have beenmade using the SPS method to consolidate ZnO pow-ders.9,10,13,14 SPS is an effective process to consolidatedifficult-to-sinter materials in a short time at a low tem-perature.15 Other than thin film samples,16 there appearsto be only one investigation that has focused on the trans-mittance of bulk ZnO sintered by the SPS method.13

Kougo and Yashikawa investigated the transmittance oflarge-grained (up to 100mm) ZnO samples and reporteda value of a few percentages.

Another important aspect of bulk ZnO ceramics istheir photoluminescence (PL) properties. The PL spec-tra of sintered nanocrystalline ZnO ceramics showed astrong green emission and a negligible ultraviolet (UV)emission.10 These nanocrystalline ZnO ceramics withgrain size of B100 nm and high density (98.5%) weresintered by SPS at a low temperature of 5501C.9,10

Their green emission property, which was attributed tothe vacuum environment of the SPS process, wasdeemed advantageous for potential use in green-display-ing devices.10 However, no transmittance data or prop-erties regarding transparency were reported for thesegreen-emitting ZnO ceramics. In this paper, we presentthe results of an investigation on the effect of sintering

726 International Journal of Applied Ceramic Technology—Hong, et al. Vol. 8, No. 4, 2011

conditions on the microstructure, transmittance, and PLproperties of ZnO bodies consolidated by the SPSmethod in a wide sintering temperature and pressurerange. The dense ZnO samples showed a high in-linetransmittance of 70% and a strong green emission.

Experimental Procedures

ZnO powders with a purity of 99.9995% (metalsbasis) and a grain size in the range 50–500 nm werepurchased from Alfa Aesar (Ward Hill, MA). Powdersof ZnO, 1.25 g, were placed inside a graphite die withan internal diameter of 19 mm, which was then posi-tioned in the SPS apparatus (Model 2050, SumitomoCoal Mining, Tokyo, Japan). The apparatus utilizespulsed high DC current along with uniaxial pressure toconsolidate the powders in the die. The pulses were3.3 ms in length, with a maximum voltage in the rangeof 0–10 V and a peak current of several thousand am-peres. The pulse pattern used in this work was 12-2, sig-nifying the application of 12 consecutive pulses followedby 2 missing pulses. For sintering under higher pressures(up to 530 MPa), a double-acting die with SiC spacerswas used, as has been described in earlier publications.17,18

In this case, 0.25 g of ZnO powder was used. The samplesproduced from the high-pressure die had a diameter of5 mm. A heating rate of 2001C/min was used for all sam-ples. The temperature was recorded by a K-type thermo-couple that was inserted into the die (B2 mm from thesample).

For sintering temperatures above 8001C, a holdtime of 5 min was used; for lower temperatures, the holdtime was 6 min. Moderate uniaxial pressures of 35 MPafor the low-pressure die and 150 MPa for the high-pres-sure die were used as the initial conditions. Once thesample was close to the hold temperature, the pressurewas increased to the final value. After that, the samplewas held at the final temperature and final pressure for5–6 min, then the pressure was rapidly released and theheating power was turned off. The entire densificationprocess including the time for the temperature rampingwas no longer than 10 min. A vacuum of B10 Pa wasmaintained during all SPS runs. The sample was thenallowed to cool naturally in the system and removed forcharacterization. Typical temperature and pressure pro-files for an SPS experiment are shown in Fig. 1.

The morphology of the purchased ZnO powderswas observed using a transmission electron microscope

(TEM, Hitachi HF-3300, Tokyo, Japan). Densities ofthe sintered samples were determined using both the Ar-chimedes and the geometric–gravimetric methods. Therelative density of the ceramics was calculated based on thetheoretical density of 5.606 g/cm3 for ZnO.19 Themicrostructure of the samples was determined by ahigh-resolution scanning electron microscope (SEM, Phi-lips XL30s, Eindhoven, The Netherlands) with no con-ductive coating. To determine the average grain size, atleast 100 grains in the SEM images were measured man-ually using the AnalySIS software (Soft Imaging System,Lakewood, CO). Optical transmission measurements inthe wavelength region between 300 and 7000 nm wereperformed on a Cary UV/Visible/Near-IR spectropho-tometer (Varian, Shropshire, U.K.) and a Nexus 870FTIR (Thermo-Nicolet, Madison, WI).20 Room temper-ature PL measurements were performed using above-bandgap excitation at 325 nm from a He–Cd laser (LiCONiX,Santa Clara, CA). The unfocused laser beam was directedonto the sample surface at an incident angle of about 451and yielding a power density of approximately 40 mW/cm2. The PL signal was collected normal to the samplesurface by a fiber-coupled spectrograph system with CCDdetector (Oriel, Stratford, CT).

Results and Discussion

Powder Characterization

Figure 2 shows a TEM image of the starting com-mercial ZnO powder. The faceted particles are nano-metric to submicrometric crystals having a crystal sizerange of 50–500 nm. Some cube-shaped particles are

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www.ceramics.org/ACT Characterization of Green-Emitting Translucent ZnO Ceramics 727

evident, but most particles are slightly elongated. Aportion of the powder was annealed in air at 3001Cfor 4 h, resulting in no measurable weight loss or dis-cernable color change.

Effect of Sintering Temperature

The effect of sintering temperature on the micro-structure of samples consolidated over the range of 550–10501C is shown by the SEM images of Fig. 3, whichare at different magnifications as indicated by the ac-companying scales. As can be seen from these images,the consolidation process was not complete (as evi-denced by significant porosity) when the sintering tem-perature was 5501C or 6251C. When the sinteringtemperature was increased to 7001C, the microstruc-ture exhibited polyhedral grain morphology withwell-defined grain boundaries. The faceted grains areprimarily hexagonal in shape, consistent with the wurt-zite structure of ZnO.21 Along with a change in the grainmorphology, there is a significant increase in the grainsize; while the grains in the sample sintered at 6251Cwere o200 nm, the average grain size for the samplesintered at 7001C was about 1.2 mm. The increase ingrain size with sintering temperature continues, and atthe highest temperature, 10501C, the average grain sizeis B8mm, with the largest grains being B15mm.

The quantitative sintering behavior of ZnO, shownin Fig. 4, demonstrates the effect of temperature on thesample density and grain size. The grain size increasedsignificantly from the nanocrystalline to the microcrys-talline range when the sintering temperature increasedfrom 6251C to 7001C, and continued to increase withincreasing temperature, reaching an average value ofB8 mm. These results are consistent with those reportedby Gao et al.9 In addition, Fig. 4 shows the relativedensities of samples sintered at 6251C or lower to be85% or less, but increases to 98.5% as the temperatureis increased to 7001C. The density increases further withincreasing temperature, reaching a value of 100% at9501C. These results show that sintering at 8501C andhigher (under a pressure of B120 MPa and a hold timeof 5–6 min) will result in a fully consolidated ZnO. Thisis in contrast to the work of Gao et al.9, in which theyreported the sintering of nanocrystalline ZnO powder toa maximal density of 98.5% at 5501C with a resultinggrain size of B100 nm. The difference between thepresent results and those reported earlier may be theconsequence of the initial powder morphology: theparticles used by Gao and colleagues9,10 were smaller(10–20 nm) and had a spherical shape, whereas thecommercial powder used in this study had a muchlarger grain size and a faceted grain morphology.

The ZnO ceramics sintered at 7001C and highertemperatures were translucent. Typical images areshown in Fig. 5, which are for ZnO ceramics sinteredat 700–10501C and B120 MPa. These samples had athickness between 0.2 and 0.9 mm and a diameter of19 mm. Printed black words can be clearly seen throughthese samples (Fig. 5). The measured transmittancevalue in the visible region, however, was o10%. Anexample of these transmittance values in the visiblerange would be for a wavelength of 632.8 nm. Thetransmittance for the sample sintered at 7001C was only0.41% for a piece with a thickness of 0.86 mm, and thetransmittance value was 1.8% for the sample sintered at8501C with a thickness of 0.71 mm. The transmittancewas 2.6% for 9501C with 0.78 mm, and 3.5% for10501C with 0.81 mm. These values are consistentwith the photographs shown in Fig. 5, confirmingthat the sample sintered at 7001C was not dense. Awide range of wavelength from 300 to 6700 nm wasexplored, and Fig. 6 shows the transmittance of differentZnO ceramics sintered at B120 MPa and 700–10501Cas a function of wavelength. The sample sintered at9501C exhibited high transmittance values and was

Fig. 2. Transmission electron microscope image of Puratronic zincoxide purchased from Alfa Aesar.


transparent in the IR region. The maximum transmit-tance was 70%, which occurred at a wavelength between2100 and 2400 nm. Samples sintered at other temper-atures, however, did not show such a high transmittanceunder the same conditions. The effect of grain size andsample density were both examined for these interestingoptical properties. When the grains are big (larger than20 mm), transmittance can be improved by increasingthe grain size.22 For small grains, the in-line transmis-sion increases with a decrease in grain size,23,24 and alight scattering model was proposed to explain this phe-

nomenon.23 Based on the result of transmission opticalproperties of polycrystalline alumina with submicrom-eter grains, Hayashi et al.25 concluded that improve-ment of optical transmittance can be achieved byreducing the crystal grains to smaller than 1 mm aswell as by the conventional method of growing themto larger than 20–30 mm. The translucent ZnO ceramicsobtained in this study had an average grain size from 1.2to 8 mm, and thus it is possible that higher transmittancecould be obtained if the final grain size can be carefullycontrolled. It has been recognized that high density is of

Fig. 3. Scanning electron microscope images of fracture surfaces of zinc oxide ceramics prepared by spark plasma sintering at a sinteringpressure of 120710 MPa and a sintering temperature of (a) 5501C, (b) 6251C, (c) 7001C, (d) 8501C, (e) 9501C, and (f) 10501C.


primary importance for transparent ceramics, due to thelight diffraction caused by micropores in sintered bod-ies. In view of the density curve shown in Fig. 4, thesample sintered at 9501C was slightly denser than theother samples. Therefore, the sample sintered at 9501C

probably had less porosity than the other samples. In-creasing the sintering temperature to 10501C may nothave discernibly affected the overall density but may

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Fig. 4. Relative density and average grain size as a function ofsintering temperature of zinc oxide samples sintered at120710 MPa for 5–6 min.

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Fig. 5. Appearance of the translucent bulk zinc oxide bodies sintered at 120710 MPa and (a) 7001C with a thickness of 0.86 nm;(b) 8501C with a thickness of 0.71 nm; (c) 9501C with a thickness of 0.72 nm; and (d) 10501C with a thickness of 0.20–0.81 nm; all thesamples have a diameter of 19 mm.

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Fig. 6. Transmittance as a function of wavelength of the zincoxide samples sintered at 120710 MPa and (a) 7001C with athickness of 0.86 nm; (b) 8501C with a thickness of 0.71 nm;(c) 9501C with a thickness of 0.72 nm; and (d) 10501C with athickness of 0.20–0.81 nm.


have resulted in pore formation due to evaporation. Thevapor pressure of ZnO26 at 9501C is 0.12 Pa, but be-comes 1.20 Pa at 10501C. Other investigators have notedthe effect of vaporization on the sintering of ZnO. Hyneset al.27 observed a decrease in the density of ZnO whensamples were sintered at temperatures higher than 8001C.Similar observations were reported by Senda and Bradt.28

Thermogravimetric analysis by Hynes et al.27 showedthat weight loss becomes significant above about 9001C.It was suggested that the weight loss is related to the lossof Zn.10,27 In view of the amount of weight loss (4% at10001C) and the dissociative congruent sublimation ofZnO,29 it is not likely that the observation is related to theloss of Zn atoms only.

Room-temperature PL spectra of the as-preparedZnO ceramics are shown in Fig. 7; the intensity of thePL was normalized for clearer sample-to-sample com-parison. The spectra from samples sintered at differenttemperatures have common features. All have a stronggreen emission at 510–540 nm and a weak UV emissionat 370–390 nm, and are similar to those reported byWang and Gao10 for their nanocrystalline ZnO ceram-ics pressed by SPS. Compared with the samples sinteredat higher temperatures, the green emission for the sam-ple sintered at 7001C was slightly red-shifted and hada broad tail in the orange-red region. Orange-red emis-sion at B626 nm in ZnO nanorods was attributed tooxygen interstitial.30 As temperature increases, the shift

is no longer evident, with the implication that the con-centration of oxygen interstitials decreases. This wouldbe consistent with the influence of the atmosphere in-side the SPS. The graphite of the dies produces a highlyreducing atmosphere as has been observed in the sinte-ring of other oxides, such as yttria-stabilized zirconia.31

In agreement with published results, the intensity of UVemission is relatively weak, o9% of the intensity of thegreen emission, and becomes weaker when the samplewas sintered at a higher temperature. The UV emissionof ZnO crystals is cited as an indication of high crys-talline quality (low defect) samples32 and is attributed tothe near-edge annihilation of free excitons.33 The greenemission, on the other hand, has been associated withdefects, which until recently have been identified as sin-gly charged oxygen vacancies. More recent work, how-ever, has cast doubt on this conclusion and thus thenature of the defects responsible for green emission isdebatable.34 Although the type of defect responsible forthe green emission has not been conclusively identified,there is considerable evidence that defects acting asgreen luminescence centers are contained in the vicin-ity of grain boundaries rather than being distributedevenly throughout the samples.34,35 The strong greenemission of the samples pressed by SPS probably resultsfrom the creation of oxygen vacancies due to the highlyreducing environment in this method, as pointed outabove. Increasing the sintering temperature increases

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Fig. 7. Room-temperature photoluminescence spectra of zinc oxide ceramics prepared by spark plasma sintering at 120710 MPa for5–6 min and different sintering temperatures; the insert shows the zoomed-in spectra in the ultraviolet region.


this tendency, creating more oxygen vacancies. There-fore, increasing the sintering temperature shifted thedefect emission from the red-orange to the green emis-sion and decreased the relative intensity of the band-edge UV emission.

Effect of Sintering Pressure

The sintering pressure also has a pronounced influ-ence on the final properties of the sintered bodies. Ourprevious experience was that an increase in the applied

pressure in the SPS results in an increase in density for thesame sintering temperature.15 However, sintering pressureshigher than 100 MPa are seldom used for pressing ZnObodies. In an attempt to achieve higher density, five ZnOsamples were sintered at 8501C for 5 min at sinteringpressures ranging from 55 up to 530 MPa. The samplesintered at 55 MPa had a relative density of 96%. Increas-ing the sintering pressure to 130 MPa increased the sampledensity to 100%, but further increase in the pressure led tothe formation of visible cracks in the sample, as can be seenfrom their microstructures shown in Fig. 8. Therefore, the

Fig. 8. Scanning electron microscope images of the fracture surface of zinc oxide ceramics prepared by spark plasma sintering at a sinteringtemperature of 8501C and a sintering pressure of (a) 55 MPa, (b) 130 MPa, (c) 350 MPa, (d) 450 MPa, and (e) 530 MPa.


samples sintered at 350 MPa and higher pressures werevisibly less translucent than the samples sintered at130 MPa. The enhancement of sintering driving force byincreasing the sintering pressure is therefore only beneficialwhen the pressure is not too high to cause crack formationin the sample. In this case for ZnO, the best samples, interms of density, microstructure, and optical transparency,were obtained at a medium pressure of B130 MPa.

Summary

Dense, polycrystalline ZnO samples were preparedby the SPS method. The effects of sintering temperatureand pressure on the sample density, microstructure, andoptical properties were investigated. Samples with a rela-tive density higher than 99% were obtained by sintering attemperatures higher than 8501C and with a moderatepressure of B120 MPa. The dense samples were all trans-lucent. Samples sintered at 9501C under a pressure of130 MPa had a maximum in-line transmittance of 70% ata wavelength of around 2300 nm. All the samples exhib-ited a strong green emission and a weak UV emission, andthe relative intensity of the green emission increased withincreasing sintering temperature.

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