Study of Large-Scale Aluminium-Doped Zinc Oxide Ceramic Targets Prepared by Slip Casting

Aluminium-doped zinc oxide (AZO) ceramic green compacts at a size of 170 × 340 × 17mm have been prepared by slip casting. An AZO slurry with good fluidity and viscosity was obtained when 1.2 wt% of dispersant was added, which indicates that the slurry prepared with micrometre particles is more suitable for slip casting. The densification and electrical properties of the AZO targets prepared by particles with different sizes were investigated after sintering from 1380 to 1460C.The results showed that the relative density of the AZO target prepared with 45 nmparticles could reach nearly 98% at 1380C; the resistivity of the AZO target prepared with nanometre particles could be as low as 1.6 × 10Ω⋅cm at 1400C; and the average transmittance of the AZO film prepared with nanometre particles could reach a maximum value of 93.73% in the visible region at 250W. In this study, the correlation between the density, resistivity, and grain growth of AZO ceramic green compacts was studied.

There are several methods of preparing AZO films, including magnetron sputtering, pulsed laser deposition (PLD), sol-gel, and chemical vapour deposition.However, only magnetron sputtering has been used to prepare largescale films with high quality because of its high deposition rate and low deposition temperature [11][12][13][14].Because sputtering is the main method of preparing AZO films, fabricating the sputtering target has become the most important step of the method [15].
The main methods used to prepare sputtering targets in industry include pressureless sintering after mould pressing, cold isostatic pressing (CIP), hot pressed sintering, and hot isostatic pressing (HIP).However, cracks always occur when these methods are used to fabricate large-sized ceramic targets.Thus, slip casting has become one of the most useful methods in the ceramic industry of preparing homogeneous and dense green bodies with large sizes and complex shapes [16][17][18].
In the slip casting process, particle dispersion is the main factor affecting the viscosity and homogeneity of the slurry.Because of the small size effect, surface effect, and quantum size effect, nanometre particles may significantly impact the preparation of ceramics, including the sintering temperature, holding time, and density.A number of studies have reported on the sintering and grain growth of ZnO and Al-doped ZnO, including the studies of Roy et al. [19] and Han et al. [20].However, the preparation of large-sized AZO targets for the industry has rarely been studied.
In this paper, we studied the effect of dispersant concentration on AZO slurries for slip casting prepared using different particle sizes and explored the effect of nanometre particles on the density, microstructure, and electrical properties of the AZO target.The transmittance of AZO films deposited at different sputtering powers was also investigated.

Experimental
Commercial ZnO and Al 2 O 3 powders with 99.99% purity and average particle sizes of 45, 120, 230, and 510 nm were used.Ammonium polyacrylate and Arabic gum were used as the dispersant and binder, respectively.All of the chemical reagents used in the experiments were analytically pure.Dispersant (0.2-2.0 wt%) and binder (0.8 wt%) were first added to distilled water to prepare an aqueous solution, and the amounts of the dispersant and binder used in the experiment were based on the dry powder weight.Subsequently, 2 wt% Al 2 O 3 was mixed with 98 wt% ZnO powders and added to the aqueous solution.After ball milling with Al 2 O 3 milling balls in a vacuum for 12 h, white Al 2 O 3 -doped ZnO (AZO) slurries with good fluidity and viscosity could be obtained.The ball to powder ratio was 2 : 1.The prepared slurries were poured gently into the plaster mould and placed at room temperature for 24 hours.The solid slurries then were dried at 80 ∘ C for 1 week to remove the water.After thermal treatment at 500 ∘ C for 48 hours, AZO green compacts at a size of 170 × 340 × 17 mm were finally obtained.
The green compacts were heated at 1 ∘ C/min to the sintering temperatures (1380, 1400, 1420, 1440, and 1460 ∘ C) for 2 h and cooled to room temperature in the furnace.
The AZO films were deposited on the glass substrates using the sintered compacts by magnetron sputtering at different working powers (150-300 W).The thickness of the film was 300 nm, and the working pressure was 1.0 × 10 −2 torr.
The viscosity of the slurries was measured by a digital rotational viscometer (NDJ-5S, Jingtian, Shanghai, China) at a shear rate of 6.3 s −1 .The sintered densities of these samples were measured by Archimedes' method in distilled water.The theoretical densities of ZnO and Al 2 O 3 were determined to be 5.6 and 3.98 g/cm 3 , respectively.Thus, the theoretical density of AZO was calculated to be 5.56 g/cm 3 .The resistivity of the sintered targets was tested by a four-point probe method (ST2258A, Jingge Electronic Co., Suzhou, China); the microstructure of the fractures was tested using a scanning electron microscope (1530vp, Leo Co., Germany); and the optical transmission spectra were obtained using a UV-vis spectrophotometer (V570, JASCO, Japan).

Results and Discussion
3.1.Viscosity.In general, a lower slurry viscosity is beneficial for slip casting.Figure 1 shows the effect of dispersant concentration on the viscosity of AZO slurries with different particle sizes.The viscosity was higher because of the absence of dispersant and decreased as the dispersant concentration rose from 0.2 to 1.2 wt%.When the amount of dispersant exceeded 1.2 wt%, the viscosity of slurries exhibited nearly constant value.This result indicates that the addition of 1.2 wt% of the dispersant is the optimum concentration.The main reason for this trend was that when the amount of dispersant was adequate to cover the particle surface, the dispersant worked as an electrolyte, thus increasing the repulsive electrostatic force, which leads to the separation of particles and decreased viscosity [21].However, the viscosity of the slurry prepared with smaller particles was higher, suggesting that the slurry  with micrometre particles was more suitable for slip casting, which may have been caused by the small size effect and the surface effect of the nanoparticles.Aggregation of the particles reduced the surface in contact with the dispersant, which impaired the effect of the dispersant as an electrolyte.

Densification and Microstructural
Coarsening.Table 1 reveals the relative densities of AZO green compacts prepared in different particle sizes (45-510 nm) before sintering.The densities changed slightly from 61.5% to 58.6% with the increase of particle sizes.Figure 2 shows the fracture morphology of AZO green compacts prepared in different particle sizes.What we could see in pictures is the particle packing of ZnO and Al 2 O 3 powders.The particles in nanometre size aggregated closer than that in micrometre size due to the small size effect and the surface effect of the nanoparticles.The pores of green compact in nanoparticles were small and rare.With the increase of particle sizes from 45 to 510 nm, the number and size of pores in green compacts also increased.It may be because the effect of flocculation in slurries prepared with micrometre particles inhibited the particle movement, resulting in the pore expansion in particle packing process.
The green densities then decreased with the pore expansion.Figure 3 shows the relative densities of AZO targets sintered at different temperatures of 1380, 1400, 1420, 1440, and 1460 ∘ C. Unlike the green compacts, the sintered densities of different samples decreased markedly with increasing particle size at 1380 ∘ C. The relative density of 45 nm sample could reach 98%, but it only reached 92.7% for the 510 nm sample.The big gap in densities for samples in different particle sizes decreased with increasing sintering temperature.The relativity densities of 230 and 510 nm samples increased with increasing sintering temperature from 1380 ∘ C to 1460 ∘ C, while the relative densities of 45 and 120 nm samples decreased from 97.8% and 98% to 96.5% and 97.5%, respectively, with the sintering temperature increased from 1440 ∘ C to 1460 ∘ C. The main reason for the densification of target in nanometre might be the surface effect of the nanoparticles.The densification process was related to the evolution of pores [22].Figure 4   With the sintering temperature increased, the vapour pressure inside the closed pores became higher than the driving force for densification, which led to the expansion of pores and decreased density.The grain size reached to 30 m.

Resistivity.
As shown in Figure 5, the resistivity of the 510 nm sample at 1380 ∘ C was the highest at 6.5 × 10 −3 Ω⋅cm, whereas that of the 45 nm sample was only 1.8 × 10 −3 Ω⋅cm.
These results indicate that a huge reduction in the resistivity of the AZO targets occurred from micrometre-to nanometre-sized particles.With increasing sintering temperature, the resistivity of the different samples decreased to a value of approximately 2.0 × 10 −3 Ω⋅cm at 1440 ∘ C. With further sintering, the resistivity values of the 45 and 120 nm samples were raised to 2.5 × 10 −3 and 2.4 × 10 −3 Ω⋅cm, respectively, whereas the values of the 230 and 510 nm samples decreased to 1.9 × 10 −3 and 2 × 10 −3 Ω⋅cm, respectively.A comparison of Figure 3 with Figure 5 shows that the change between the density and resistivity of the AZO targets prepared with different particle sizes was similar.The resistivity decreased with increases in density primarily because the grain growth reduced the number of pores with increases in density at the grain boundary.The increased density and changes in the pores reduced the scanning of electrons by the pores in the target, thus resulting in the fast movement of electrons along the grain boundary.However, when the sintering temperature reached a relatively high value (1440 ∘ C), the elevated internal vapour stress leads to the growth of the pores, which accelerated the scattering of the electrons and decreased the resistivity.Figure 3 shows that  the resistivity of the nanometre samples remains at a relatively low standard (1.6-2.5 Ω⋅cm) over the entire sintering process, which indicates that AZO targets with good electrical properties fabricated by nanometre-sized particles could be achieved at a relatively low temperature (1380 ∘ C).

AZO Film.
Table 2 lists the parameters of the AZO films deposited at different working powers.The films were prepared by using the 45 nm AZO compact sintered at 1420 ∘ C. As shown in Table 2, the transmittance and sheet resistance cannot be tested because of the absence of a complete film when the power is 100 W. As the working power increased, the deposition rate increased but the transmittance changed slightly.Due to the formation of complete film, the sheet resistance decreased from 200.42 Ω/sq to 52.83 Ω/sq. Figure 6 shows the optical transmission spectrum of AZO films deposited at different working powers.The average transmittance of the AZO films prepared at different working powers remained at approximately 90% in the wavelength range of 400-800 nm.As the working power increased, the average transmittance in the visible region increased slightly and reached a maximum value of 93.73% at 250 W, which corresponds to the results found in a study by Chen et al. [23].
Additionally, there was obvious interference in the spectrum, which indicates that the surface of the AZO films was smooth and homogeneous.

Conclusions
(1) The viscosities of AZO slurries with different particle sizes for slip casting were studied.A well-dispersed AZO slurry could be obtained when 1.2 wt% of the dispersant was added.
The viscosity increased slightly with excess dispersant and decreased with increasing particle size.The slurry prepared by micrometre-sized particles was more suitable for slip casting.
(2) The relative density of the AZO targets prepared by nanometre-sized particles was higher than that prepared by the micrometre-sized particles at a lower sintering temperature.The relative density of the nanometre-sized target reached nearly 98% at 1380 ∘ C and remained at a high level (97.9-98.2%)from 1380 to 1420 ∘ C.
(3) The variety of resistivity at different sintering temperatures was comparable with the change of density for the sintered AZO targets.The resistivity values of the nanometre target were much lower and more stable than that of targets prepared by micrometre-sized particles.
(4) Sputtering power had no obvious effect on the transmittance of the AZO film.The average transmittance of the AZO film prepared by nanometre-sized particles reached a maximum value of 93.73% in the visible region at 250 W.

Figure 1 :
Figure 1: Effect of dispersant concentration on the viscosity of AZO slurries with different particle sizes of 45, 120, 230, and 510 nm.
presents SEM micrographs of the AZO targets prepared with 45 nm particles sintered at (a) 1380 ∘ C, (b) 1400 ∘ C, (c) 1420 ∘ C, or (d) 1460 ∘ C. It shows the grain size increased with the rise of sintering temperature and the evolution of pores inside the target.As shown in Figure 4(a), the grain size was only 8 m at 1380 ∘ C, and a number of small pores were observed at the grain boundary.It was at initial stage for the sintering process; the driving force for sintering was low.Because of the high surface active energy, the particles in nanometre aggregated faster than those in micrometre, resulting in the high density (97.9%) at a low sintering temperature (1380 ∘ C).

Figure 6 :
Figure 6: Transmittance spectrum for AZO films deposited at different working powers.

Table 1 :
Relative densities of AZO green compacts.

Table 2 :
Deposition parameters of the AZO films.