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Sintering study of ITO using a ZnO-doped and microwave hybrid sintering approach Sintering study of ITO using a ZnO-doped and microwave hybrid sintering approach
Chen, Di; Jiang, Chongxi; Sun, Hongliang; Feng, Bo; Lu, Xiong; Weng, Jie; Wang, Jianxin
2014-03-01 00:00:00
Article history: ITO ceramics with full densities are difficult to achieve using conventional heating because of the Received 24 October 2013 volatilization property of both indium oxide (In O ) and tin oxide (SnO ) at high temperatures. In our 2 3 2 Received in revised form present study, we proposed to use a ZnO-doped and microwave hybrid-sintering approach to prepare for 10 December 2013 ITO ceramics with full densities under normal atmospheric condition. The investigation on the effect of Accepted 8 January 2014 the content of ZnO on the densification and resistivity of the ITO ceramics showed that as the ZnO content Available online 5 February 2014 increased, the relative density of the ceramics increased while the weight loss and grain size decreased. The resistivity of the ceramics initially decreased by increasing the ZnO content but increased when the Keywords: content of ZnO was more than 9.09 wt.%. Employing this logic, a relative density approaching 99% of Indium tin oxide the theoretical density was obtained and the sintering time required was just 25 min. The obtained ITO Microwave sintering ceramics were pure ITO phase and had the lowest resistivity and the relative density of 98.1% when the Zinc oxide Densification content of ZnO was 9.09 wt.%. This hybrid sintering approach might open a new avenue in the fabrication of ITO ceramics with high densities. © 2014 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved. 1. Introduction difficult to achieve because of the volatilization property of both indium oxide (In O ) and tin oxide (SnO ) at high temperatures. 2 3 2 ITO (Tin-doped indium oxide) films are widely used as trans- Normally, the relative density of sintered ITO ceramics obtained by parent electrodes for display devices and transparent coatings for using conventional sintering in air under normal atmospheric con- solar-energy heat mirrors due to its excellent properties such as ditions is in the range of 62–65% [7]. To achieve more dense ITO 4 −1 −1 high electrical conductivity (10 cm ) and high transparency ceramics, some approaches such as hot press, hot isostatic press- (85–90%) to visible light [1,2]. Various methods can be used to fab- ing (HIP) and pressureless sintering under pure oxygen conditions ricate ITO films; among them, magnetron sputtering is one of the have been used [8]. Nevertheless, vacuum hot press and hot iso- best techniques, which utilizes ITO targets [3,4]. Studies show that static pressing (HIP) are very expensive and have low production the properties of sputtering targets can greatly influence sputter- efficiency while pressureless sintering under pure oxygen condi- ing efficiency and quality of the sputtered films [5]. Dense targets tions takes more than 10 h from the temperature-rise period to the are advantageous for increasing deposition rate and obtaining a end of heat preservation and additionally, sintering in an oxidizing more stable resistivity of the deposited films [6]. However, the atmosphere is dangerous and requires an expensive and complex densification of ITO ceramics with near-to-theoretical density is system. Recently, microwave sintering has received increasing atten- tion, because of its desirable advantages, such as reduced activation energy [9–12,17], lower sintering temperature [12,13], enhanced Corresponding author. Fax: +86 28 87634649. diffusion process [14–17], very rapid heating rate [18] and so on. E-mail addresses: j.wang63@gmail.com, jwang@swjtu.cn (J. Wang). Microwave heating is a process in which the materials can cou- Peer review under responsibility of The Ceramic Society of Japan and the Korean ple with microwaves, absorb the electromagnetic energy and then Ceramic Society. transform into heat within the sample volume itself [19–21]. This is quite different from the conventional methods where heat is generated by external heating elements and then is transferred to the samples via radiation, conduction and convection. It was reported that ITO ceramics has 98% relative density by microwave 2187-0764 © 2014 The Ceramic Society of Japan and the Korean Ceramic Society. sintering at 1600 C for 1.5 h under oxygen atmosphere. The sinter- Production and hosting by Elsevier B.V. All rights reserved. ing time of ITO was drastically decreased. However, as mentioned http://dx.doi.org/10.1016/j.jascer.2014.01.003 58 D. Chen et al. / Journal of Asian Ceramic Societies 2 (2014) 57–63 above, sintering the ITO targets in pure oxygen environment is dangerous. Furthermore, the addition of ZnO to ITO was used to increase the relative density of ITO using conventional sintering under normal atmospheric conditions and a density up to 92% has been achieved [22]. The study shows that the addition of ZnO into ITO has no any negative effect on the optical properties of the sputtered films at all [22–25]. Considering the advantages of ZnO-doped sintering and microwave sintering, in the present study, ITO ceramics would be sintered by a ZnO-doped and microwave hybrid sintering approach in air under normal atmospheric conditions and the effect of the amount of ZnO on the densification and resistivity of the ceram- ics would be investigated so that high-density ITO ceramics would obtained. 2. Experimental procedure Fig. 1. XRD patterns for the initial ITO powders and the prepared ITO ceramics 2.1. Preparation of ITO green bodies doped with various ZnO contents. The powders of ITO (99.9%, In O :SnO = 9:1 wt.%) and ZnO (AR 2 3 2 99.9%) were purchased from CNMC Ningxia Orient Group Co., Ltd., 9.09 wt.% ZnO were indexed to the XRD patterns of the bixbyite China and Sigma–Aldrich, USA, respectively. structure of ITO (JCPDS File No. 89-4596). For the doped samples, ITO powders were mixed with ZnO powders, and the investi- no peaks corresponding to ZnO and Zn In O were observed k 2 3+k gated weight percentage of which was 0%, 4.76%, 7.41%, 9.09% and while increasing ZnO content up to 9.09 wt.%. Nevertheless, some 16.7%, respectively. An organic binder (3 wt.% of polyvinyl butyral peaks corresponding to the phase of Zn In O could be detected in 3 2 6 (PVB) dissolved in ethanol) was added to improve the compact- the sintered samples doped with 16.7 wt.% ZnO. Only the sintered ing behavior. The mixed powders were wet ball milled in ethanol samples doped with 9.09 wt.% ZnO among all the doped samples in a PTFE jar using agate balls for 12 h, and then dried in an oven displayed pure ITO phase with a bixbyite structure, indicating that 2+ 4+ 3+ at 110 C for 8 h and whetted in an agate mortar. After whetting, Zn and Sn substituted for In within the In O lattice. In addi- 2 3 the obtained powders were uniaxially pressed at 300 MPa in a tion, some very weak characteristic peaks of SnO with a rutile steel die into pellets (20 mm in diameter, ∼4 mm in thickness). The structure were observed in the sintered samples doped with 0 wt.%, compacted pellets were then annealed at 500 C for 2 h to remove 4.76 wt.%, 7.41 wt.% and 16.7 wt.% ZnO, respectively. A systemic organic binder. All the pellets had green body densities with a range shift of the positions for (4 0 0), (4 4 0) and (6 2 2) peaks toward of 46–47% of TD. higher angle side of 2 was observed in the samples doped with ZnO compared with pure ITO. The XRD peaks of ITO for the lattice planes with bigger absolute values of miller indices had a bigger 2.2. Microwave sintering shift toward high angle side of 2 and that meanwhile, the more the ZnO content, the bigger the shift for the same XRD peaks of ITO. Microwave sintering was carried out in a microwave furnace This kind of shift might be associated with a decrease in the lattice with a silicon carbide ceramic crucible and alumina insulation in 2+ parameter caused by Zn substitution [22]. it (MKE 0.8/2.45–0.6/5.8, Linn High Therm GmbH, Germany). The Fig. 2 shows the images of the ITO ceramics sintered by using used microwave frequency and power were 2.45 Hz and 800 W, microwave sintering for 25 min in air and by using conventional respectively. The temperature was measured using a pyrometer. sintering for 8 h in air. It can be seen that the sample sintered with Microwave heating lasted 25 min and then cooled down natu- conventional approach was still yellow in appearance whereas it rally. The maximum temperature was 1358 C during the process of was dark gray after microwave sintering, which was the same color sintering. like the targets for industrial use. It can be seen that the sample 2.3. Characterization The densities of the obtained ITO ceramics were measured using the Archimedes method. The phases of the samples were exam- ined by an X-ray diffractometer (XRD) (X pert Pro MPD, Philips, Netherlands). The microstructure and morphology of the ceram- ics were characterized by a scanning electron microscopy (SEM) (Quanta 200, Philips, Netherlands). The electrical resistivity was measured at room temperature using a Automatic Four-Point Probe device (Model 280, Four Dimensions Inc., USA). 3. Results and discussion X-ray diffraction (XRD) patterns for the initial powders and the prepared ITO ceramics doped with various contents of ZnO are shown in Fig. 1. It can be seen that all the X-ray diffraction peaks Fig. 2. Image of ITO ceramics before and after microwave sintering and conventional for both the initial powders and the sintered samples doped with sintering. D. Chen et al. / Journal of Asian Ceramic Societies 2 (2014) 57–63 59 boundaries for the sintered ITO ceramics doped with high content of ZnO. The sintered ITO ceramics containing ZnO presented denser structures compared with the sintered ITO ceramics without ZnO. It can be seen that some pores with a size of about 3–5 m existed at boundaries for the sintered ITO ceramics doped without ZnO while they were seldom found in the sintered ITO ceramics doped with ZnO, especially when the content of ZnO was high. It can also be seen that all the sintered ITO ceramics displayed uniform grain size cross the whole fracture surfaces. A systemic decrease in grain size was found as ZnO content increased as shown in Fig. 4 and Table 1. The average grain size was 8.54 m for the sintered ITO ceramics without ZnO while it was between 5.16 m and 6.96 m for the sintered ITO ceramics doped with ZnO. In order to compare with the microstructures of the ITO sam- ples sintered with conventional approach, SEM micrographs of the fractured surfaces of the ITO ceramics sintered at 1450 C with con- ventional approach were shown in Fig. 5. It can be seen that the ITO Fig. 3. Relative densities and weight losses of the sintered ITO ceramics as a function ceramics sintered with conventional approach displayed an uneven of ZnO content. distribution of pores and grains from the edge to the center. Densi- fication presented a gradient structure with a dense edge but a porous core in sintered ITO ceramics. As expected, small grains sintered by microwave heating shrank at least 5 mm in diameter, compared with the unsintered ones. However, the sample sintered existed in the cores. However, it can be noted that large grains were not detected in the edge sections but in the middle sections of by conventional heating shrank only 2 mm in diameter. The theoretical density and relative density of ZnO-doped ITO ITO ceramics. Obviously, two different microstructures caused by two different sintering methods were associated with two different ceramics by microwave sintering for 25 min in air are shown in Table 1. From Table 1 and Fig. 3, it can be seen that the theoretical densification mechanisms. This will be discussed later. density of the ceramics decreased with increasing ZnO content due Fig. 6 shows electrical resistivity of ITO ceramics versus the ZnO content. Resistivity presented a decreasing tendency as the con- to the fact that the density of ZnO (5.606 g/cm ) is smaller than ITO (7.15 g/cm ). The relative density of the sintered samples increased tent of ZnO increased before the content reached 16.7 wt.%. This might be associated with densification and grain size, because high with increasing ZnO content as has been expected. The samples without ZnO had the lowest relative density among all the sintered densification and small grain size and hence more boundaries can improve charge transport. As a result, the electrical resistivity of samples, which was 91.0% of the theoretical density. The highest relative density, approaching 99% of the theoretical density, was ITO would be reduced due to the improvement of charge transport. When the content of ZnO was more than 9.09 wt.%, densification found in the samples doped with 16.7 wt.% ZnO. In the meanwhile, it can be seen that the weight loss of the sin- and grain size would not change significantly as shown in Fig. 4 and Table 1. However, due to higher electrical resistivity for ZnO tered bodies also changed with increasing the ZnO content in Fig. 3. It decreased with increasing the ZnO content. A large amount of compared with ITO, the increased content of ZnO would result in weight loss was found when the doped relative amount of ZnO was an increase of electrical resistivity of ITO as detected in the ITO less than 9.09 wt.%. Once when the doped relative amount of ZnO ceramics doped with 16.7 wt.% ZnO. was more than 9.09 wt.% did the weight loss curve of the sintered Normally, densification and microstructures are generally determined by heating methods and temperature profiles caused bodies as function of the doped relative amount of ZnO become flat. From the relative density curve of the sintered bodies as a function by them. However, for the materials which have a property of volatilization at high temperatures, such as indium oxide and tin of the doped relative amount of ZnO in Fig. 3, it can be seen that the lower the weight loss of the sintered bodies, the higher the relative oxide, densification and microstructures are not only affected by heating methods and temperature profiles but also controlled by density of the sintered bodies, and vice versa. SEM examination was used to investigate the microstructures of volatilization. Volatilization results in a decrease in densification and retards grain growth, thus leading to an inverse effect on den- the fractured surfaces of the sintered ITO ceramics and the average grain size was determined using the linear intercept method. Both sification and grain growth. Because the direction of mass transport for densification caused the core and edge sections of the fracture surfaces of the sintered ITO ceramics with various ZnO contents are shown in Fig. 4. All the by conventional heating is the opposite of volatilization in Fig. 7(a), ITO ceramics sintered with the microwave heating approach pre- it is difficult for materials with a very high sintering temperature sented a uniform distribution of pores and grains. It can be noted and a property of volatilization to achieve high densification. As a that there were lots of grains with transcrystalline ruptures on the result, conventional heating leads to a structure with a dense shell and a porous core for these materials during sintering as shown fracture surfaces in the sintered ITO ceramics doped with high con- tent of ZnO while they were not found on the fracture surfaces in Fig. 7(a). Compared with conventional heating which relies on thermal sintered ITO ceramics doped without ZnO and with low content of ZnO, indicating that there would have been a high bond strength of conduction and radiation to transport heat from the surface of Table 1 Relative densities and average grain sizes of the microwave-sintered ITO ceramics with various ZnO contents. ZnO content 0 wt.% 4.76 wt.% 7.41 wt.% 9.09 wt.% 16.7 wt.% Theoretical density (g/cm ) of ZnO-doped ITO ceramics 7.15 7.05 7.01 6.98 6.84 Relative density (91.0 ± 0.25)% (95.7 ± 0.25)% (97.6 ± 0.30)% (98.1 ± 0.25)% (98.7 ± 0.30)% Average grain size (m) 8.54 ± 0.61 6.96 ± 0.55 5.87 ± 0.50 5.28 ± 0.46 5.16 ± 0.59 60 D. Chen et al. / Journal of Asian Ceramic Societies 2 (2014) 57–63 Fig. 4. SEM micrographs of the fractured surfaces (edge and central sections) of the microwave-sintered ITO ceramics with various ZnO contents: 0 wt.%, 4.76 wt.%, 7.41 wt.%, 9.09 wt.% and 16.7 wt.%. D. Chen et al. / Journal of Asian Ceramic Societies 2 (2014) 57–63 61 Fig. 5. SEM micrographs of a fractured surface of the ITO ceramics sintered at 1450 C for 8 h using conventional heating: (a) edge section, (b) middle section, and (c) the center. the surface because the surrounding air remains cooler than in the body [14,26,27]. This inverse temperature profile allows the den- sification direction to be the same with the volatilization direction in Fig. 7(b); hence, it is easier to remove pores out of sintered bodies compared with conventional sintering. It is possible for materials to achieve full density by using microwave heating if the level of microwave power is high enough to allow the “microwave effect” [13,18,26,28] to create a much higher rate for densification in a short sintering time than that for volatilization. However, it is still difficult to achieve full density only by using pure microwave heating under a low level of microwave power. The obtained relative density reached 91% only for the ceramics sintered by microwave sintering under the low level of microwave power (800 W), while there was a significant increase in density compared with conventional sintering. A similar result was also obtained and the obtained relative density was 92% when only ZnO- doped sintering was used [22]. The 92% density seems to be the limiting value of the relative density obtained via a single approach whatever it is conventional sintering or pure microwave sintering Fig. 6. Resistivity of the sintered ITO ceramics versus ZnO content. under a low level of microwave power and or ZnO-doped sinter- ing. In our present study, a hybrid approach, combining microwave sintering with ZnO-doped sintering, was used to achieve high den- the ceramic to the center of the body, because a distinguishing sification. Because it combined the advantages of both microwave feature of microwave heating is its volumetric nature caused by sintering and ZnO-doped sintering, an ITO ceramic material with a direct depositing into the ceramic via the interaction between relative density of 98.7% has been obtained. Obviously, the hybrid microwaves and materials, it can result in the creation of an approach has a large advantage over conventional sintering or pure inverse temperature profile with time, i.e. a hotter interior than Fig. 7. Schematic diagram showing possible densification process and direction during conventional sintering (a) and microwave sintering (b). The black dots represent the pores in the samples, whereas yellow represents the region with low density and green the one with high density. 62 D. Chen et al. / Journal of Asian Ceramic Societies 2 (2014) 57–63 Fig. 8. Linear EDS analysis showing Zn dispersed at ITO grain boundaries or between ITO grains. microwave sintering under a low level of microwave power and or the neck region can be much larger than the average field in the ZnO-doped sintering. We think that the doped ZnO might play a material, up to 10 times that of the externally applied field. The crucial role on inhibiting the volatilization of In and Sn and grain field in the neck region can be even higher, up to 30 times larger growth during sintering. This has been proved by the fact that grain than the applied field. The net result is that the local absorbed size decreases and density increases with increasing ZnO content. energy in this region can be some 500 times larger than the aver- Linear EDS analysis and XRD analysis showed that Zn in the form age absorbed energy [32]. The results from Wang et al. [26] can fit of ZnO or Zn In O was dispersed at ITO grain boundaries or this theory well, which showed that the most pronounced effect k 2 3+k between ITO grains in Fig. 8, which could inhibit the volatilization occurred during the intermediate stage of sintering, which is when of In and Sn and also would retard grain growth efficaciously. This the neck region dominates densification, and their results demon- effect became more pronounced with increasing the ZnO content. strate clearly the effect of particle size on the magnitude of the Compared with microwave sintering, during conventional sin- effect. As predicted by the ponderomotive theory, finer particles, tering process, due to the volatilization of components In and which will have smaller and more neck regions, show an enhanced Sn consisting of ITO at high temperatures, they kept moving out microwave effect compared to larger particles. Their results gen- of the sintered bodies to the exterior surfaces. As the amount erally showed that higher microwave power levels, finer particle of the volatilized In and Sn increased, they would redeposit on sizes, and, particularly, greater microwave absorption can result in the surfaces and reformed grains on the surfaces via a process of greater enhancement [26]. Wang et al. [26] study also indicated classical nucleation and crystal growth. In the meanwhile, due to that, as a good microwave absorption material, ZnO was seen to the volatilization of components In and Sn consisting of ITO, the have a great “microwave effect” in microwave sintering. grains in the center sections of sintered bodies became smaller and In the present study, the experiment results show that both fac- smaller with the volatilization and the pores got bigger and bigger. tors of dopant of ZnO and “microwave effect” played a crucial role As a result, conventional heating led also to a microstructure with on enhancing the densification of ITO ceramics. For the first factor, small grains in the edge sections and smaller small grains in the cen- it has been widely accepted that dopant may enhance the densifi- ter sections and big grains in the middle sections as shown in Fig. 5. cation mechanism if suitable additives are chosen. In the study of The microwave processing of ceramics has been demonstrated the influence of TiO additives on the sintering behavior of In O , 2 2 3 to enhance sintering and grain growth. The enhanced mass trans- Nadaud et al. found that TiO doping caused an increase of sintering port and solid state reaction rates during the processing of a variety density and limited grain growth acting by a second-phase mecha- of ceramic, glass, polymer, and other organic and inorganic mate- nism and also hindering the decomposition rate of In O by means 2 3 rials, including lower sintering or reaction temperatures, as well of precipitation of TiO at the grain boundary, thus resulting in as accelerated kinetics for a wide range of processes in these increased grain boundary diffusion at reduced diffusion activation materials and reduced activation energies, and these two aspects energies [33]. In our present study, the enhancement mechanism have broadly been called the “microwave effect”. The latter may of ZnO should be similar as described before. Another factor is therefore be categorized as a “nonthermal” phenomenon. Accord- microwave effect, which might also be correlated to the dopant 2+ ing to the theory of the so-called ponderomotive driving forces of ZnO in the present study. The presence of Zn in substitutional [29], microwaves can induce an additional (electric) driving force, positions can lead to the formation of defects like oxygen vacancies. which specifies an enhanced diffusion in ionic solids. Freeman The following reaction could describe a possible formation process et al. [30] published the results of conductivity measurements of oxygen vacancies: made on sodium chloride single crystals under microwave and •• In O 2 3 X non-microwave conditions. Their results indicated that it was the 2ZnO −→ 2Zn + V + 2O (1) In O driving force for diffusion that was enhanced by the application of microwaves. Similar conclusions were drawn by Wroe and Rowley Accompanied by the formation of interstitial atoms and vacan- [31] in their UK-based study on the sintering of partially stabi- cies, each interstitial-vacancy pair or double-vacancy point defect lized zirconia. They found that an enhancement in densification can be regarded as a polarized dipole, its damped vibration can when using microwaves was consistent with a dependence on the lead to dielectric losses, then causing the absorption of microwave electric field experienced by the material. Both these experimen- [34], which is beneficial for the “microwave effect” in sintering and tal results support the ponderomotive theory first suggested by hence for the enhancement of densification. As a result, ZnO doping Rybakov and Semenov [29]. Additionally, previous studies by Birn- will enhance the “microwave effect” and further promote the den- boim et al. [32] showed the very strong influence of the ceramic sification of ITO in sintering. 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