Sintering of Transparent Yttria Ceramics in Oxygen Atmosphere

Yihua Huang, Dongliang Jiang,*,w Jingxian Zhang, Qingling Lin, and Zhengren Huang

The State Key Laboratory of High Performance Ceramics and SuperfineMicrostructure, Shanghai Institute of Ceramics,Shanghai 200050, China

A novel method is reported for the preparation of transparentpolycrystalline yttria ceramics in oxygen atmosphere. Zirconiaand other additives were added to control the grain growth.Pores can be eliminated clearly at a temperatureo16501C withthe grain size around 1 lm. The grain growth kinetics and themechanisms controlling grain growth were studied. Sintering inoxygen atmosphere is beneficial for making samples with abig size at low cost and avoiding the posttreatment of samplessintered in vacuum or hydrogen atmosphere.

I. Introduction

Transparent polycrystalline yttria ceramics have aroused muchinterest in recent years. Yttria has many potential applicationssuch as missile domes1 and bulb envelopes2 for the advantage ofits optical transparency, high melting temperature, and highcorrosion resistance. Specially, yttria is a promising host mate-rial for the solid-state laser,3 because its thermal conductivity is alittle higher than YAG (13.6 and 11 W � (m �K)�1, respectively),4

which makes it capable for enduring higher temperature duringworking.

The crystal structure of yttria belongs to C-type rare-earthoxide. And oxygen vacancies and interstitials are the majordefects existing in pure yttria.5–8 Similar to fluorite-structureoxides, a large number of aliovalent cations can be dissolvedinto yttria. These kind of dopants like Zr41 will lead to the for-mation of oxygen interstitials.9,10 According to Kingery et al.,11

the diffusion of oxygen anion is much faster than that of yttriumcation in yttria. Hence, the yttrium cation interstitial [Yi] diffu-sion is the rate-control step for grain-boundary migration.

In order to fabricate high-transparency yttria ceramics, poresshould be eliminated as much as possible.12 If the grain size islarge, it will be difficult to remove pore because there are lessgrain boundaries for pores release. Our aim was to find a morefeasible route to prepare transparent ceramics with reducedgrain size to effectively remove the pores and reduce the fabri-cation cost.

Sintering in vacuum13 and hydrogen14 atmosphere are thetwo traditional routes for sintering transparent yttria ceramics.Hydrogen molecules are small enough and can be dissolved intothe yttria crystalline, and hence the hydrogen gas can be elim-inated from the sample during the sintering process. But thesintering temperature is about 1001C higher than that sintered invacuum. There is nearly no gas in the pores in vacuum sintering,and hence pores can also be removed easily. But samples sin-tered in vacuum are black because of the formation of anoxygen-deficiency phase. And samples need to be postannealedin air for a long time after sintering.

The first transparent yttria ceramic was reported by Lefeverand Matsho15 in 1967. They used hot pressing at 9501C invacuum under 70 MPa with LiF as sintering additive. Gresko-vich and Chernoch16 reported the method of a sintering yttriatransparent ceramics in hydrogen atmosphere with ThO2 as asintering additive in 1973. Rhodes14 reported two-step sinteringin hydrogen atmosphere for La-doped yttria transparent ceram-ics in 1981, the material reached theoretical total transmittances,but the sintering temperature is420001C. In 1998; Saito et al.17

fabricated transparent yttria ceramics at low temperature (about17001C) using carbonate-derived powder. They used vacuumsintering to remove most of the pores. The transmittance of a1-mm-thick sample is around 18% at 400 nm. And the averageparticle size is around 20 mm.

Recently, a combination of vacuum sintering and hot isostat-ic pressing was proposed byMouzon et al.18 They found that theopen porosity of samples after vacuum sintering was tolerable,while after HIP treatment, the pore can be removed completely.The grain size was around 30 mm. An inline transmittance of43% at 400 nm was found for a 2.5-mm-thick sample. Seri-valsatit et al.19,20 reported the first nanograined highly trans-parent yttria ceramics sintering from HIP after two-stepsintering.21 The average grain size is about 300 nm.

As far as we know, it is difficult to sinter yttria transparentceramics in air. This is mainly because the size of nitrogen atomsin air is too big and it will be difficult for them to be dissolvedinto the yttria crystal lattice. But oxygen molecule is smaller andhas a high diffusion coefficient at high temperature in yttriaceramics. According to Chen and Chen’s10 report, high oxygenpartial pressure would depress the grain-boundary mobility,which can eliminate more pores during sintering. If yttria canbe sintered in oxygen atmosphere, then the process will be moreconvenient and the cost for sintering will be largely reduced.Based on this knowledge, we reported our recent work on thesintering of yttria in oxygen atmosphere.

II. Experimental Procedure

(1) Powder Preparation and Experiment Process

Yttria powder and appropriate amount of Zirconium nitratepentahydrate (99.99%, Shanghai YueKai New Materials Co.,Ltd., Shanghai, China) were dissolved in nitric acid, and thendiluted with suitable deionized water to prepare the 0.3M nitratesolution, which was used as the mother solution. Precursorswere prepared by adding ammonia solution (2M) to the mothersolution at a rate of 3 mL/min with stirring. When the pH valueof the system reached 8, titration was stopped followed by a 3 haging. Then the gel was washed with deionized water for severaltimes to remove the byproducts. After washing, the gel wasdried by freeze-drying (FD-1A-50, Beijing Boyikang ExperimentInstrument Co., Ltd., Beijing, China). The precursors were thencalcined at 10001C. The BET specific surface area is 8.2 m2/g.The mean particle size of the powder is about 70 nm.

The powder was uniaxially compacted into disks in+20 mmsteel die at 40 MPa and then isostatically pressed at a pressure of

J. Ballato—contributing editor

*Member, The American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: dljiang@sunm.shcnc.

ac.cn

Manuscript No. 27762. Received April 1, 2010; approved May 17, 2010.

Journal

J. Am. Ceram. Soc., 93 [10] 2964–2967 (2010)

DOI: 10.1111/j.1551-2916.2010.03940.x

r 2010 The American Ceramic Society

2964

200 MPa. Specimens were sintered in oxygen atmosphere. Then,the samples were polished and etched for characterizations.

(2) Characterizations

The specific surface area of the calcined yttria powder wasdetermined by the BET method. A Guinier-Hagg camera(XDC-1000, Stockholm, Sweden) was used to precisely charac-terize the lattice parameters of yttria. Mirror-polished sampleswere thermally etched at 15001C for 30 min for grain size mea-surement. EPMA (Model JXA-8100, JEOL, Tokyo, Japan) wasused to determine the grain size, and at least 300 grains weremeasured to obtain the mean size. The grain sizes were calcu-lated by the linear-intercept method using equation G5 1.5 L,where G is the average grain size and L is the average interceptlength. Mirror-polished samples on both surfaces were used tomeasure the optical transmittance (Model U-2800 Spectropho-tometer, Hitachi, Tokyo, Japan).

III. Results and Discussion

It is well known that sintering atmosphere is important forobtaining transparent ceramics; for example, it is quite difficultto make yttria ceramics transparent in air. The fracture surfaceof yttria ceramics sintered at 16501C in air is shown in Fig. 1.The intergrain pores can be easily observed. There is 79% ni-trogen in air, and its size (1.71 A) is too big to pass through theyttria lattice. In addition, the gas trapped in the pore will makethe pressure high enough to balance the material transport.Therefore, it is difficult to eliminate this kind of pore in thesubsequent sintering process. Hence, nitrogen is the main prob-lem for the sintering of yttria transparent ceramics in air.

Sintering in hydrogen atmosphere is well-accepted for thepreparation of transparent ceramics, because the size of hydro-gen molecule is small enough to pass through the crystal lattice.Because oxygen can be ionized into O2� ion at high temperaturein oxides, it may indicate that oxygen atoms can also passthrough the crystal lattice during sintering as the size of O2� ionis small. Fig. 2 shows the transparent yttria ceramics with 2 at.%Zr sintered in oxygen atmosphere. The sample is 1 mm thick,standing on the paper. Words behind it can be read clearly. Thetransmittance spectra are shown in Fig. 3. The inline transmit-tance reaches 61% at 800 nm for 2 at.% Zr-doped yttriaceramics. The sintering temperature is at 16501C and the grainsize is only around 2 mm, which is lower than that reported in theliterature.17 As far as we know, it is the first report about yttriatransparent ceramics sintered in oxygen atmosphere.

It is found that Zr is an effective sintering additive for sup-pressing the grain growth. Fig. 4 shows the microstructure of theetched surface of yttria samples with and without Zr dopingsintered in oxygen at 16501C. Fig. 4 a1, a2, and a3 are micro-structures of samples without Zr doping, the holding times are 0,1, and 3 h, and their grain sizes are 2.47, 3.47, and 4.60 mm,respectively. Fig. 3 b1, b2, and b3 are microstructures of sampleswith 0.5 at.% Zr doping, holding time are 0, 1, and 3 h, theirgrain sizes are 0.87, 1.83, and 2.25 mm, respectively. The grainsize of samples with Zr is smaller than that without Zr at thesame sintering conditions. No pores (either intra or inter pores)were detected in samples with Zr doping. While inter pores canbe observed in the pure yttria samples. The longer the hold time,the bigger is the pore size. Sample b1 is translucent with nano-sized grains. It is promising to obtain nanograined transparentyttria ceramics in a new way. Samples b2 and b3 are transparent.The inline transmittance for 1-mm-thick samples with 0.5 at.%Zr reaches 80% at 800 nm wavelength (shown in Fig. 3). HighZr doping would cause more scattering center and make thetransparency decrease. Besides Zr, it is found that lanthanumand other sintering additives are also suitable for sintering yttriaceramics in oxygen atmosphere.

According to the following equation for grain growth10:

Gm � Gm0 ¼ 2Mg ðt� t0Þ

where G0 is the reference grain size at time t0, G is the averagegrain size at time t, and g is the grain-boundary energy (g is 0.3J/m2 according to literatures).M is the grain-boundary mobility.

The best fit for m is 2 in our experimental range. Fig. 5 showsthe grain growth kinetics of yttria with 0.5 at.% Zr sintered at15501, 16001, and 16501C in oxygen atmosphere. The slope ofthe fitting line reflects the rate of grain growth at the definedtemperature.

According to grain growth kinetics shown in Fig. 5, the grain-boundary mobility is determined and presented in Fig. 6. It isfound that pure yttria sintered in oxygen has the lower grain-

Fig. 1. Microstructure of the fracture surface of yttria ceramics sinteredin air at 16501C for 1 h.

Fig. 2. Polycrystalline yttria ceramics (2 at.% Zr) sintered in oxygenatmosphere. The sample is standing on the paper.

Fig. 3. The inline transmittance of yttria transparent ceramics sinteredfrom oxygen atmosphere.

October 2010 Rapid Communications of the American Ceramic Society 2965

boundary mobility than that sintered in air. This is due to thehigh oxygen pressure suppressing the [Yi] concentration, whichis the rate-control factor for grain growth. And the grain-bound-ary mobility of samples with Zr decreases considerably than thatof without Zr, because Zr will introduce more [Oi] in yttria,which also leads to the decrease in [Yi]. The mobility of undopedand 1 at.% Zr-doped yttria sintered in air is higher than Chenand Chen’s10 results. The difference may be caused by the differ-ent initial powders. In Chen’s work, monosized yttria powderswere used. According to Sordelet and Akinc,22 a uniform start-ing microstructure would retard the rapid grain growth at acertain extent, and hence the mobility is lower. Further study isneeded.

It is observed that the slopes of the four lines are similar. Thismight suggest that the mechanism or ‘‘grain growth accelera-tion’’ behind them is the same. The grain growth is controlled by

[Yi] diffusion. The modified equation for the apparent activationenergy is given as

RTK ¼ K0e�Q=RT

where R is the universal gas constant, T is the absolute temper-ature, K is a rate constant depending on the mechanism con-trolling grain growth, K0 a constant, and Q is the apparentactivation energy for grain growth. The value is about 630 kJ/mol calculated from the data in Fig. 6, which is almost equal tothe sum of [Yi] formation energy (587.6 kJ/mol)10 and diffusionenergy (69.4 kJ/mol).23 This further confirms that the graingrowth is controlled by [Yi] diffusion.

Oxygen atmosphere sintering method has some advantagesover other conventional sintering methods. The sintering tem-perature is at about 16001–16501C, which is the lowest for yttriatransparent ceramics except for HIP method from our knowl-edge.3,14,17 The grain size is about 1–2 mm, much lower thanLefever and Matsho’s15 report (about 100 mm). It is also lowerthan Eilers’s report24 (21.5 mm); in their work, they tried to useair sintering in combination with HIPing to reduce the grainsize. Sintering in oxygen is another practicable method in fab-ricating small-grained transparent ceramics. And the inlinetransmittance is comparable with Serivalsatit et al.’s report.19,20

In experiments, it is found that carbon residue (especially fromthe wet forming sample) can be avoided during oxygen sintering,which would prevent the ceramics from being transparency invacuum sintering. And compared with vacuum sintering, longtime annealing is not necessary. In addition, it is more feasible formaking ceramics parts with big size and high transparency at lowcost. Further work in this area is necessary.

IV. Conclusions

Sintering in oxygen atmosphere is a new route for yttria trans-parent ceramics. The samples can be transparent after sinteringat about 16001C, and the grain size is smaller comparedwith that sintered from other conventional methods. It wasfound that Yi diffusion controls the grain growth and the dens-ification at middle-final sintering stage. The presence of oxygencould reduce grain-boundary mobility probably due to thereduction of [Yi] concentration. And the addition of Zr couldalso reduce the grain-boundary mobility presumably by thesame mechanism.

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Fig. 6. Temperature dependence of grain-boundary mobility of yttriawith and without Zr, sintered in air and in oxygen.

Fig. 4. The etched surface of yttria samples with (b) and without (a) Zrdoping sintered in oxygen at 16501C. Holding times are 0 h for a1, b1;1 h for a2, b2; and 3 h for a3, b3.

Fig. 5. Grain growth kinetics of yttria with 0.5 at.% Zr at 15501, 16001,and 16501C.

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