High Pressure Sintering of Nano-Size γ-Al2O3

In recent years nano crystalline materials have been paid much attention because they have a variety of interesting and novel physical properties. Research on the sintering of nano crystalline ceramics has focused on the problem of achieving high densities(> 95% of theoretical) without excessive grain growth. Dense and fine grained alumina ceramics are widely used in practical applications, because of mechanical, electrical and optical properties. The mechanical strength, dielectric properties and transparency are strongly affected by the microstructure of alumina ceramics such as porosity, grain size and their distribution. (Fig.1)


Introduction
In recent years nano crystalline materials have been paid much attention because they have a variety of interesting and novel physical properties.
Research on the sintering of nano crystalline ceramics has focused on the problem of achieving high densities(> 95% of theoretical) without excessive grain growth.
Dense and fine grained alumina ceramics are widely used in practical applications, because of mechanical, electrical and optical properties. The mechanical strength, dielectric properties and transparency are strongly affected by the microstructure of alumina ceramics such as porosity, grain size and their distribution. (Fig.1) For example, the transparency and mechanical strength of alumina were improved by decreasing the grain size, and the residul porosity less than 0.05% was required for obtaining a high transmission of light."in [2]" Alumina ceramics with sub micrometer microstructure obtained by pressureless sintering have been widely studied.
It is difficult to obtain a fully dense ceramic with nanocrystalline grain size. Because phase transformation sequences that occur during the conventional sintering process at atmospheric (ambient ) pressure: γ Al 2 O 3 ( 750 0 C) → δ Al 2 O 3 ( 900 0 C) → θ Al 2 O 3 (> 1000 0 C) → α Al 2 O 3 The transition paths and temperatures vary depending on the particle size, chemical homogeneity, heating rate, and water vapor pressure.
The transformation from θ -Al 2 O 3 to α -Al 2 O 3 involves a chance in the oxygen sublattice from cubic close packing to hexagonal close packing and generally requires temperatures above 1100 0 C.
As a result of the volume reduction and low real nucleation density (10 8 -10 11 nuclei /cm 3 ) during the transformation, the α -Al 2 O 3 colonies recede from the matrix and the microstructure develops into vermicular morphology containing larger scale interconnected porosity. (Fig. 2) The temperature required for the densification of this vermicular microstructure is over 1600 0 C. to obtain dense, fine grained α-Al 2 O 3 at low temperatures, the scale of the vermicular microstructure must be minimized "in [4 ]".
Pressure-assisted densification provide to obtain nanometric grains in fully sintered compacts.
The effect of pressure on nucleation and growth of the α-phase is the critical subject because the final grain size after sintering depends on these factors.
Low pressures create a smaller number of nucleation sites where the transformation begin.. These nuclei grow very fast forming vermicular clusters. The clusters stop growing when they touch on each other.

Low pressure
Vermicular microstructure High pressure equiaxed microstructure During High pressure each particle can act as an original nucleation site and transformation can occur within each particle, so that avoiding the formation of vermicular structure. In other words, high pressure can create a sufficient number of nucleation sites to prevent the formation of the vermicular structure.
Modifications in the structure, by the application of high pressure, have been investigated by several groups "in [5][6][7]". They have reported applying pressure decreases the thermodynamic energy barrier and kinetic energy barrier required for nucleation and causes the phase transformation to shift to a lower temperature. Figure 3 shows that the start of transformation from γ-to α -Al 2 O 3 temperature decreases from about 1075 0 C at 1 atmosphere, to around 800 0 C at 1 GPa, and to 640 0 C at 2.5 GPa; 560 0 C at 5.5 GPa and to 460 0 C at 8 GPa.
Grain growth is limited by the low sintering temperature and variation of nucleation events in the γ phase at high pressure creates a nanoscale α grain size "in [7 ]".
In this work the required conditions were examined to produce high-density nano-Al 2 O 3 ceramics using high pressure and different sintering temperatures. The effect of various sintering conditions on the properties of sintered sample such as microstructure and relative density were discussed.

Experimental procedure
The   The impurity content of the initial powder was given in Table 1, in accordance with the analytical certificate supplied.
The powder was first preheated in the air at 700 0 C for 3 h for the removal of the binder. After preheating, the pure γ-Al 2 O 3 powder containing no additives was uniaxially cold pressed at 20 MPa into cylinders 20mm in diameter and 10mm in height. All the green compacts were pellets of 4 g. Green compact was encapsulated in a cube die made of pyrophyllite. All the high-pressure sintering experiments were carried out in a cubic anvil apparatus. High mechanical pressure was applied on all six faces of the die concurrently.
Additionaly 10 wt% TiO 2 was added by sol-gel to inhibited the grain growth.
The TIP was hydrolyzed by addition of water. The ethanol was removed in a rotating evaporator at 65 0 C and powders were dried in a furnace at 90 0 C for 24 hour.After drying powder mixture was decomposed into oxide at 400 0 C.
Phase analysis of the sintered samples was carried out by X-ray diffraction (XRD). Grain sizes were estimated from high-resolution scanning electron micrographs taken from fracture surfaces and micro hardness was determined on the polished surfaces using an applied load of 500 g.   An alumina hydrate, AlO(OH), phase ( Fig. 8-9) was found in the samples sintered at 5 and 7 GPa and 500 0 C. This phase is caused by trapped water or surface OH groups which forms the hydrate phase during low temperature sintering.

Effect of temperature on the microstructure and density at 5 GPa
As can be seen in Fig 10, the morphology of the sintered samples at high temperatures were different from those sintered at lower temperatures.
The grain size of sintered sample increases with increasing sintering temperature and the grain size distribution was fairly wide at especially high sintering temperature.
Grain Growth was started at 900 0 C and abnormal grain growth was observed at the higher sintering temperature (1200 0 C) as shown in Fig. 10 (c ), ( d ).
Previous experimental results indicate that abnormal grain growth in commercially pure alumina is strongly correlated with presence of impurities. The minimum concentration for AGG over 300 ppm silicon or 30 ppm calcium. "in [8]" This impurities are believed to form glassy films in the grain boundaries and somehow to catalyze AGG.
The sintered density generally increases with the sintering temperature for all samples.
From Figure 11, it is noted that the relative density first increased rapidly, than climbed slowly from 1000 to 1500 0 C.  At 1500 0 C. the relative density of sintered sample is reached a highest value of 98.1% when the pressure is 5GPa.
Due to significant grain growth, no complete densification could be reached with initial γ-Al 2 O 3 powder

Effect of pressure on the grain size of sintered samples
The microstructure of sintered samples obtained from γ -Al 2 O 3 powders at 1000 0 C and 5-7 GPa for 15 minute were examined. Representative SEM's of fracture surfaces are shown in figure 12. As shown in Fig. 12 the overall grain size for the sample sintered at 1000 0 C at 7GPa appeared coarser than the sample sintered at the lower pressures (5 GPa) ( for the same sintering temperature and time of 15 minutes).  As shown in Fig. 13 the microstructure of the Al 2 O 3 ceramics sintered at high pressure (7GPa) and high temperature (1000 0 C) for 1 minute is obviously different from the ceramic microstructure sintered at the same pressure and temperature for 15 minutes (see in Fig. 12 (b)).
While Fig.13 contains much smaller fine grains, coarser grains are visible on the microstructure shown in Fig.12 ( b), showing the considerable effect of sintering time on the sintered grain size.
This could be attributed to the higher input energy in the system at high pressure and high temperature conditions, thus the final stage in sintering was reached quickly as was the grain growth regime. Thus for high pressure high temperature conditions, either the sintering time or the temperature should be reduced to prevent grain growth.
The effect of the sintering time on the microstructure can also be seen from the pictures shown in Fig.14. Sintering γ-Al 2 O 3 at 900 0 C at 7GPa for 15 min. resulted in obtaining the opaque sample while a translucent sample was obtained with a sintering time of 5 min. as shown in Figs.14 (a and b, respectively. The opaque structure is considered as a result of abnormal grain growth of alumina, as shown in Fig.14 (a). But when the sintering temperature is lowered to 500 o C, a translucent alumina with the finest grain size and the gamma form is evident using a sintering time of 5 min. as shown in Fig.14 (c).

Influnce of TiO 2 additives on alumina microstructure
As you can see Fig. 16 The secondary phase precipitates mostly at along the grain boundaries (white phase)..The existence of this secondary phase reduces the driving force for grain growth by pinning effect.
It can be shown that TiO 2 additive leads to finer grain size after pressure assisted sintered at low and high temperature for same sintering pressure and time. ( Fig 17 -18) From figure 17 -18 it is obvious that average grain size of TiO 2 doped sample is finer than that of undoped alumina sintered at the same temperature and pressure for same holding time.
However TiO 2 addition did not has significant effect on the density of sintered samples  Although Translucent Alumina is obtained with the alumina sample at 6GPa and 600 o C for 15 min ( Fig 19 (a) ). it is not possible to obtained translucent alumina with the alumina containing 10 wt% TiO 2 sample at same sintering conditions (Fig 19 (b) )

Effect of pressure and sintering time on the hardness of sintered samples
Sintering time has a strong effect on the grain size and hence the hardness. For example, a sample sintered at 1000 o C (at 7 GPa) for 15 min. shows a hardness value of 8.30 GPa while a sample sintered at the same temperature for 1 min. provides a hardness value of 11.46 GPa.
The increment in hardness depending on the decrease in sintering time can be correlated with the grain size as shown by the SEM micrographs in Figs.12 b and 13. As shown in Fig. 12b, the sample sintered at 1000 o C for 15 min. (the applied pressure is at 7 GPa )contains much bigger alumina grains (the main grain size is about 5 microns) compared with sample in Fig.13 that was sintered at the same temperature and pressure for 1 min (the main grain size is about 200 nm).
Using applied pressures of 7 GPa and 5 GPa and lower sintering temperature of 600 0 C, translucent α-alumina could be obtained with the hardness value of 13.89 and 13.35 GPa, respectively.
The highest hardness value of 20.31 GPa is achieved for the TiO 2 doped sample sintered at 700 o C for 15 min (the applied pressure is at 5 GPa) due to presence of very fine alumina grains with the main grain size of < 100 nm, as shown in Fig.18 b.

Conclusions
In the present work, the sintering behavior of a γ-Al 2 O 3 powder subjected to different pressure, temperature and time conditions is examined.
Applying pressure drops γ-α Al 2 O 3 transformation temperature and increase nucleation rate making it possible to obtain nano grain size sintered sample.
It was also found that, when using nano-size starting powder, the sintering time should be optimised in order to control the final sintered grain size under the same sintering temperature and applied pressure.
Chemical impurity in initial powder is very important to reduce grain growth during sintering.
To produce lower than 100 nm sintered sample it is very important to eliminate the hydrates & impurities before sintering.
TiO 2 additive in initial γ-Al 2 O 3 leads to finer grain size after pressure assisted sintered Using applied pressures of 5-7 GPa and lower sintering temperature of 500-600 0 C, translucent α-alumina could be obtained Translucency can be controlled by increasing the applied pressure from 5 GPa to 7 GPa for a sample sintered at 700 o C for 15 min. or decreasing the sintering temperature and time. As the sintering pressure is decreased, the sintering temperature also needs to be decreased in order to obtain translucent samples.
The hardness of the sintered 10 wt % TiO 2 doped Al 2 O 3 nanocomposites were higher than undoped Al 2 O 3 at same sintering conditions.
The highest hardness value of 20.31 GPa is achieved for the TiO 2 doped sample sintered at 700 o C for 15 min. (the applied pressure is at 5 GPa)