Effect of Cobalt Oxide On the Sintering and Grain Growth of Al 2 O 3-YAG Composite Nanopowder

In the present study, the effect of cobalt oxide on the densification of Al2O3-YAG composite nanopowder was investigated. An amorphous nanopowder was synthesized and crystallized to Al2O3-YAG after heat-treatment via a solid-state reaction. The average particle size of heat-treated powder at 800 °C was about 80 nm. Cobalt oxide improved the sintering rate of Al2O3-YAG composite nanopowders and promoted grain boundary mobility. Cobalt oxide doping increased the ratio of densification/grain growth rate. The activation energy for grain growth decreased from 590 to 485 kJ/mol by adding cobalt oxide to the composite nanopowder.


Introduction
Alumina is one of the most widely used ceramic because of its good mechanical properties such as hardness, good corrosion resistance and high temperature insulation.YAG (Yttrium aluminum garnet, Y 3 Al 5 O 12 ) is an important ceramic material because of its wide applications in laser industry and in electronic devices.The combination of YAG with alumina leads to development of composite materials for structural applications with excellent mechanical properties.It is well known that both fracture toughness and strength of alumina ceramics are improved significantly by the dispersion of YAG particles [1][2][3][4].
The sintering of Al 2 O 3 -YAG composite requires high temperatures and long times [5].Three main approaches are applied to reduce the thermal budget.First approach is application of alternative consolidation techniques, such as hot pressing or spark plasma sintering.Applying an external pressure increases the driving force for densification.Li and Gao [4] hot-pressed Al 2 O 3 -25 vol% YAG powder in N 2 to nearly full density at 1400°C for 1h.The second approach is the use of nanopowders.In nano-scale form, as the powder particle size decreases, the amount of lattice defects, grain boundary and surface area increase, results in densification at lower temperatures [6,7].The third approach is addition of the dopants to accelerate diffusion processes.It is well known that ceramic microstructures and its mechanical properties strongly depend on the presence of dopants [8].For instance, MgO promotes sintering and controls grain growth in alumina due to a solid-solution pinning mechanism [9].Various investigations revealed that cobalt oxide is a suitable additive for densification of Ce 0.8 Gd 0.2 O 2-δ (CGO) material.It has been reported by Kleinlogel and Gauckler [10] that the sintering temperature can be decreased by 400 °C for CGO with 2 mol% cobalt oxide addition.Zhang et.al. [11] have reported the reduction in sintering temperature of CGO by 200 °C with just 0.5 at.% of cobalt oxide addition.Zhang et.al. [12] showed that a small amount of cobalt oxide doping reduces sintering temperature of CeO 2 .Satapathy [13] sintered YAG using cobalt oxide as a sintering aid and showed that cobalt oxide reduces sintering temperature.
The purpose of this study was to investigate the effect of cobalt oxide on densification, grain growth and microstructure of Al 2 O 3 -YAG composite nanopowder prepared by an aqueous sol-gel method.

Experimental procedure
For preparing of Al 2 O 3 -7wt.%YAGcomposite the raw materials used were AlCl 3 •6H 2 O (Merck, >99.9%),Al powder (M.A. University, >99.6%) and Y 2 O 3 (Sigma-Aldrich, >99.9%).The Al powder had a spherical shape with an average diameter about 38 μm.At first Y 2 O 3 was dissolved in aqueous HCl.The main solution was prepared by dissolving yttrium oxide solution, AlCl 3 •6H 2 O and Al powder into the deionized water.This precursor solution was continuously stirred at 100 °C to completely dissolve the starting materials and then placed in an oven at 60 °C for 3 hrs.The viscosity of the solution gradually increased and finally a rigid gel obtained.The gel was dried at 80 °C for 48 hrs and calcined at 600 °C.
To study the effect of cobalt oxide on sintering behavior and microstructure of the Al 2 O 3 -YAG composite nanopowder, Co 3 O 4 powder (Merck, >99.9%) was added to HCl solution.The solution was continuously stirred at 100 °C for 4 hrs to completely dissolve Co 3 O 4 powder.The amount of cobalt oxide was 0.1 wt.%.Then the obtained composite nanopowders were added to this solution.The slurry was dried by means of magnetic stirring at 80 °C and calcined in air at 800 °C for 30 min in order to eliminate the rest of volatile component.In order to reduce the amount of hard agglomerates, the calcined nanopowder was wet-planetary milled using a high dense alumina jar and high pure alumina balls for 1 hrs.The obtained powder was cold isostatically pressed at 200 MPa and sintered in air at various temperatures.The heating rate was set at 6 °C/min.The soaking time at the maximum temperature was 3 hrs.The sintered bodies were cooled in the furnace.
The crystalline structure of the powders was determined by X-ray diffraction using Philips X-pert model with Cu Kα radiation.Crystallite size was estimated from the broadening of the X-ray diffraction peaks, using the Scherrer equation [14].The microstructures of powders and sintered specimens were observed by scanning electron microscopy (SEM, XL30-Phillips).An image analyzer was used to measure the mean grain size of the samples.The densities of the sintered specimens were measured by the Archimedes method.

Results and discussion
In sol-gel method, some alternative reactions create the final product.The synthesis mechanism may be described by the following reactions.Yttrium oxide first reacts with HCl and forms yttrium chloride (equation 1).
Then aluminum chloride hexahydrate and yttrium chloride may be hydrolyzed to produce the sol (equations ( 2) and ( 3)). 4) shows that Al powder reacts with HCl to produce aluminum chloride and hydrogen gas and therefore Al can be used as a source of AlCl 3 .showed the formation of nanoparticles on the surface of alumina by dispersing of alumina powder in ytrria solution [3].
By increasing the calcination temperature up to 950 ºC, Co 3 O 4 converts to CoO by equation ( 6) and it can be mentioned that CoO is stable above 950 ºC [12].The melting temperature of CoO is about 1833 ºC.[15] ↑ + → 1 presents XRD patterns of the calcined powders at various temperatures.It can be observed that the heat treated powder at 500 °C does not have any peaks and seems to be amorphous.By increasing the heat treatment temperature to 800 °C, some transition phases of alumina are detected (η and θ-Al 2 O 3 ).The average crystallite size of this powder was determined by means of the X-ray line-broadening method to be 45 nm at 800 °C.YAlO 3 (YAP) and α-Al 2 O 3 are formed at 1200 °C.It seems that yttrium reacts with alumina, as the heat-treatment temperature increases, to produce YAP [16].YAlO 3 are transformed to YAG at 1300 °C.There are no peaks related to presence of cobalt components such as cobalt oxide in XRD analysis of these specimens.It indicates that the amount of cobalt oxide in the material is negligible.Fig. 2 illustrates SEM micrograph of powder heat-treated at 800 °C for 30 min.Most of the particles are in the range of 60-120 nm and spherical in shape.Considering that the powder has been synthesized by wet soft chemistry, the uniform size and shape are achieved.The formation of spherical particles have also been reported in other material systems during wet chemical synthesis, and one plausible reason is "contact recrystallization" [17] through which the ultrafine primary crystallites are cemented together to form bigger spheres.The effect of cobalt oxide doping level on grain size and sintered density of Al 2 O 3 -YAG composite is shown in Fig. 3.The sintering plots of all samples illustrate a sigmoidal shape.The densification rate increases dramatically with the temperature rise up to 1300 °C.
It can be seen that a small amount of cobalt oxide doping increases both the grain size and sintered density.As shown for many high performance ceramics, the strength of the sintered ceramic can be increased by decreasing the porosity and processing related flaws [18][19][20].Satapathy [13] reported that small amount of cobalt oxide can lie within the solubility limit of the material.It is known that formation of solid solution improves the densification.The samples exhibit fast rate of grain growth at final stage of sintering (>90% theoretical density), which is likely due to the gradual loss of pore drag on grain boundary migration [21].It is possible to use a parameter to describe the relevant rates of densification and grain growth.Ikegami et al. [22] proposed a model that depicts the densification of a compact powder along with simultaneous grain growth in the initial and intermediate stages of sintering, using the relative sintered density, ρ, and the average grain size, D, according to the following equation: where ρ 0 and D 0 are constants which denote the green density of the compact powder and the initial grain size, respectively, and k is a parameter that can be used to indicate the relative ratio between densification and grain growth rates.The sintering results were analyzed in the intermediate stage of sintering and the results are presented in Fig. 4. As can be seen, cobalt oxide doping increases the densification/grain growth rate ratio.The grain growth kinetic is deduced by analyzing the grain size as a function of annealing time in accordance with the classical theory for grain growth [23]: where D is the average grain size at time, t, D 0 is the average grain size at time, t=0, n is the growth exponent, K is a rate constant, K 0 is a pre-exponential constant, Q is the activation energy of grain growth, R is the ideal gas constant and T is the absolute temperature of sintering.Taking D 0 as the average grain size at 1500 °C and (t -t 0 ) as the soaking time at the sintering temperature (4 h), we observed that grain growth of the samples follows the parabolic law (n=2).Fig. 5 shows (D 2 -D 0 2 ) against 1/T for samples, with activation energies indicated in the figure .It clearly shows that the doped-sample exhibits lower activation energy for grain growth, which may be used to explain why cobalt oxide doping leads to a grain growth.Fig. 6 presents the SEM images of the specimens sintered at 1500 °C for 4hrs.It can be seen, cobalt oxide-doped sample is very dense since practically little pores are trapped along grains and grain boundaries but in undoped sample there are some trapped pores along the grain boundaries.In addition the final microstructure of undoped composite shows nearly regular shape of Al 2 O 3 and YAG grains.In the case of doped composite, microstructure evolution shows irregular shape of the grains.

Conclusions
In the present study the effect of cobalt oxide on the densification of Al 2 O 3 -YAG composite nanopowder was investigated.An amorphous nanopowder was prepared and it was crystallized to Al 2 O 3 -YAG after heat-treatment via a solid state reaction.Small amount of cobalt oxide doping enhanced the densification rate and promoted the grain growth of Al 2 O 3 -YAG nanopowder.It was also found that cobalt oxide-doping increased the ratio of densification/grain growth rate.The final microstructure of doped composites showed irregular shape of grains with lower porosity than undoped samples.The activation energy for grain growth decreased from 590 to 485 kJ/mol by adding cobalt oxide to composite nanopowders.

Fig. 4 .
Fig. 4. Log-log plot showing the linear relationship between ρ/1-ρ and the average grain size during the intermediate stage of sintering for undoped and cobalt oxide -doped Al 2 O 3 -YAG composites.
Adding composite nanopowders to this solution and subsequent calcination of the mixture, cause to form the Co 3 O 4 particles on the surface of powders.Torrecillas et al.